Methods and reagents for discovering and using mammalian melanocortin receptor agonists and antagonists to modulate feeding behavior in animals
The present invention provides recombinant expression constructs comprising nucleic acid encoding mammalian melanocortin receptors, and mammalian cells into which said recombinant expression constructs have been introduced that express functional mammalian melanocortin receptors. The invention provides a panel of such transformed mammalian cells expressing melanocortin receptors for screening compounds for receptor agonist and antagonist activity. The invention also provides methods for using such panels of melanocortin receptor-expressing mammalian cells to specifically detect and identify agonists and antagonists for each melanocortin receptor, as well as patterns of agonist and antagonist activity of said compounds for the class of melanocortin receptors. Such screening methods provide a means for identifying compounds with patterns of melanocortin agonist and antagonist activity which is associated with the capacity to influence or modify metabolism and behavior, particularly feeding behavior.
[0001] This application is a continuation-in-part of U.S. Ser. No. 08/466,906, filed Jun. 6, 1995, which is a divisional of U.S. Ser. No. 07/886,979, filed Apr. 10, 1992, now U.S. Pat. No. 5,532,347, issued Jul. 2, 1996. This application is also a continuation-in-part of U.S. Ser. No. 08/478,992, filed Jun. 7, 1995, which is a divisional of U.S. Ser. No. 08/077,673, filed Jun. 15, 1993, which is a divisional of U.S. Ser. No. 07/866,560, filed Apr. 10, 1992, now U.S. Pat. No. 5,280,112, issued Jan. 18, 1994. This application is also a continuation-in-part of U.S. Ser. No. 08/044,812, filed Apr. 8, 1993.
BACKGROUND OF THE INVENTION[0003] 1. Field of the Invention
[0004] The present invention relates to the cloning, expression and functional characterization of mammalian melanocortin receptor genes. The invention provides nucleic acid encoding mammalian melanocortin receptors, recombinant expression constructs comprising said nucleic acid, and mammalian cells into which said recombinant expression constructs have been introduced, and that express functional mammalian melanocortin receptors. The invention also provides a panel of such transformed mammalian cells' expressing melanocortin receptors for screening compounds for receptor agonist and antagonist activity. The invention provides methods for using such panels of melanocortin receptor-expressing mammalian cells to specifically detect and identify agonists and antagonists for each melanocortin receptor, as well as patterns of agonist and antagonist activity of said compounds for the class of melanocortin receptors. Such screening methods provide a means for identifying compounds with patterns of melanocortin agonist and antagonist activity which is associated with the capacity to influence or modify physiological function and behavior, particularly metabolism and feeding behavior.
[0005] 2. Background of the Invention
[0006] The proopiomelanocortin (POMC) gene product is processed to produce a large number of biologically active peptides. Two of these peptides, &agr;-melanocyte stimulating hormone (&agr;MSH), and adrenocorticotropic hormone (ACTH) have well understood roles in control of melanocyte and adrenocortical function, respectively. Both of these hormones are also found in a variety of forms with unknown functions, for example, &ggr;-melanocyte stimulating hormone (&ggr;MSH), which has little or no ability to stimulate pigmentation (Ling et al., 1979, Life Sci. 25: 1773-1780; Slominski et al., 1992, Life Sci. 50: 1103-1108). A melanocortin receptor gene specific for each of the &agr;MSH, ACTH and &ggr;MSH hormones has been discovered by some of the present inventors (see U.S. Pat. Nos. 5,280,112, 5,532,347 and U.S. application Ser. No. 08/044,812, incorporated by reference herein). In addition, two other melanocortin receptor genes have been discovered by some of the present inventors (see Lu et al, 1994, Nature 371: 799-802; Mountjoy et al, 1994, Molec. Endocrinol. 8: 1298-1308) and others (see Gantz et al., 1993, J. Biol. Chem. 268: 15174-15179 and Labbe et al., 1994, Biochem. 33: 4543-4549).
[0007] Along with the well-recognized activities of &agr;MSH in melanocytes and ACTH in adrenal and pituitary glands, the melanocortin peptides also have a diverse array of biological activities in other tissues, including the brain and immune system, and bind to specific receptors in these tissues with a distinct pharmacology (see, Hanneman et al., in Peptide Hormone as Prohormones, G. Martinez, ed. (Ellis Horwood Ltd.: Chichester, UK) pp. 53-82; DeWied & Jolles, 1982, Physiol. Rev. 62: 976-1059 for reviews). A complete understanding of these peptides and their diverse biological activities requires the isolation and characterization of their corresponding receptors. Some biochemical studies have been reported in the prior art.
[0008] Shimuze, 1985, Yale J. Biol. Med. 58: 561-570 discusses the physiology of melanocyte stimulating hormone.
[0009] Tatro & Reichlin, 1987, Endocrinology 121: 1900-1907 disclose that MSH receptors are widely distributed in rodent tissues.
[0010] Sola et al., 1989, J. Biol. Chem. 264: 14277-14280 disclose the molecular weight characterization of mouse and human MSH receptors linked to radioactively and photoaffinity labeled MSH analogues.
[0011] Siegrist et al., 1991, J. Receptor Res. 11: 323-331 disclose the quantification of receptors on mouse melanoma tissue by receptor autoradiography.
[0012] Cone & Mountjoy, U.S. patent application Ser. No. 07/866,979, filed Apr. 10, 1992, disclose the isolation of human and mouse &agr;-MSH receptor genes and uses thereof (incorporated herein by reference).
[0013] Cone & Mountjoy, U.S. patent application Ser. No. 07/866,560, filed Apr. 10, 1992, disclose the isolation of human and bovine ACTH receptor genes and uses thereof (incorporated herein by reference).
[0014] Mountjoy et al., 1992, Science 257: 1248-1251 disclose the isolation of cDNAs encoding mammalian ACTH and MSH receptor proteins.
[0015] POMC neurons are present in only two regions of the brain, the arcuate nucleus of the hypothalamus, and the nucleus of the solitary tract of the brain stem. Neurons from both sites project to a number of hypothalamic nuclei known to be important in feeding behavior, including the paraventricular nucleus, lateral hypothalamic area, and ventromedial hypothalamic nucleus. While previous reports have claimed both stimulatory and inhibitory effects of &agr;-MSH on feeding behavior (see Shimizu et al., 1989, Life Sci. 45: 543-552; Tsujii et al., 1989, Brian Res. Bull. 23: 165-169), knowledge of specific melanocortin receptors, their location within the central nervous system and the necessary pharmacological tools were not sufficiently developed at that time to allow the resolution of this issue. The present inventors have shown herein that a novel antagonist of the MC-3 and MC-4 melanocortin receptors can substantially increase food consumption in animals engaged in normal or fast-induced feeding behavior. This is consistent with expression of both MC-3 and MC-4 receptor mRNAs at these sites in in situ hybridization studies (Roselli-Rehfuss et al., 1993, Proc. Natl. Acad. Sci. USA 90: 8856-8860; Mountjoy et al., 1994, Molec. Endocrinol. 8: 1298-1308). Moreover, the regulation of arcuate nucleus POMC gene expression is consistent with an inhibitory role for POMC in feeding behavior. POMC mRNA levels are decreased following a fast (Bergendahl et al., 1992, Neuroendocrinol. 56: 913-920; Brady et al., 1990, Neuroendocrinol. 52: 441447), and a significant diurnal variation in POMC mRNA levels in the arcuate nucleus is seen in rat, with the nadir occurring around the onset of nighttime feeding at 1800 hrs (Steiner et al., 1994, FASEB J. 8: 479-488).
[0016] Thus, the experimental evidence indicates that POMC neurons play an important role in tonic inhibition of feeding behavior, wherein obesity results from a chronic disruption of this inhibitory tone by antagonism of central melanocortin receptors in at least one animal model (agouti).
[0017] These results reveal for the first time a need in the art for a means for characterizing mammalian melanocortin receptor agonists and antagonists in vitro for the development of compounds that affect feeding behavior in animals.
SUMMARY OF THE INVENTION[0018] The present invention provides a biological screening system for identifying and characterizing compounds that are agonists or antagonists of mammalian melanocortin receptors. The biological screening system of the invention comprises a panel of transformed mammalian cells comprising a recombinant expression construct encoding a mammalian melanocortin receptor, and expressing said receptor thereby. The invention provides such a panel of transformed mammalian cells wherein the panel comprises cells expressing each of type of mammalian melanocortin receptor. Thus, the invention also provides nucleic acid encoding mammalian melanocortin receptors, recombinant expression constructs comprising said nucleic acid, and mammalian cells into which said recombinant expression constructs have been introduced, and that express functional mammalian melanocortin receptors. Methods for using such panels of melanocortin receptor-expressing mammalian cells to specifically detect and identify agonists and antagonists for each melanocortin receptor, as well as patterns of agonist and antagonist activity of said compounds for the class of melanocortin receptors, are also provided. Such screening methods provide a means for identifying compounds with patterns of melanocortin agonist and antagonist activity which is associated with the capacity to influence or modify metabolism and behavior in an animal, particularly feeding behavior.
[0019] Thus, the invention provides in a first aspect a biological screening panel for determining the melanocortin receptor agonist/antagonist profile of a test compound. The panel comprises a first mammalian cell comprising a recombinant expression construct encoding a mammalian melanocortin receptor that is the &agr;-MSH (MC-1) receptor. The panel also comprises a second mammalian cell comprising a recombinant expression construct encoding a mammalian melanocortin receptor that is the ACTH (MC-2) receptor. The panel also comprises a third mammalian cell comprising a recombinant expression construct encoding a mammalian melanocortin receptor that is the MC-3 receptor. The panel also comprises a fourth mammalian cell comprising a recombinant expression construct encoding a mammalian melanocortin receptor that is the MC-4 receptor. The panel also comprises a fifth mammalian cell comprising a recombinant expression construct encoding a mammalian melanocortin receptor that is the MC-5 receptor. As provided by the invention, each mammalian cell expresses the melanocortin receptor encoded by the recombinant expression construct comprising said cell.
[0020] In preferred embodiments, the melanocortin receptors encoded by the recombinant expression constructs comprising the transformed mammalian cells comprising the panel are mouse MC-1 receptor (SEQ ID Nos.: 3 and 4); human MC-1 receptor (SEQ ID Nos.: 5 and 6), human MC-2 (ACTH) receptor (SEQ ID Nos.: 7 and 8), bovine MC-2 receptor (SEQ ID Nos.: 9 and 10), rat MC-3 receptor (SEQ ID Nos.: 11 and 12), human MC-4 receptor (SEQ ID Nos.: 15 and 16) and human MC-5 receptor (SEQ ID Nos.: 17 and 18).
[0021] In a second aspect, the invention provides a method for using the melanocortin receptor panel to identify and characterize test compounds as melanocortin receptor agonists and/or antagonists. In this embodiment, the method provided by the invention identifies a melanocortin receptor agonist, and comprises the steps of contacting each of the cells of the panel with a test compound to be characterized as an agonist of a mammalian melanocortin receptor and detecting binding of the test compound to each of the mammalian melanocortin receptors by assaying for a metabolite produced in the cells that bind the compound. In a preferred embodiment, the detected metabolite is cAMP.
[0022] In a preferred embodiment of this method, each of the cells of the panel of mammalian cells expressing mammalian melanocortin receptors further comprises a recombinant expression construct encoding a cyclic AMP responsive element (CRE) transcription factor binding site that is operatively linked to a nucleic acid sequence encoding a protein capable of producing a detectable metabolite. In preferred embodiments, said protein is &bgr;-galactosidase, most preferably encoded by a nucleic acid comprising the recombinant expression construct identified as pCRE/&bgr;-galactosidase (as disclosed in Chen et al., 1994, Analyt. Biochem. 226: 349-354). As provided by the invention, expression of the protein that produces the detectable metabolite is dependent on binding of the test compound to the melanocortin receptor expressed by each cell in the panel and the intracellular production of cAMP as a result. In this embodiment, cAMP production results in expression of a protein capable of producing a detectable metabolite, the protein most preferably being &bgr;-galactosidase. In preferred embodiments, the detectable metabolite absorbs light to produce a colored product. Thus, this embodiment of the invention provides a panel of melanocortin receptor-expressing cells whereby melanocortin hormone binding results in the production of a colored product in proportion to the extent of cAMP production in the cell as a result of hormone receptor binding.
[0023] In another embodiment of this aspect of the invention is provided a method for characterizing a compound as an antagonist of a mammalian melanocortin receptor. In this embodiment, the method comprises the steps of contacting each of the cells of the panel with an agonist of the mammalian melanocortin receptor in an amount sufficient to produce a detectable amount of a metabolite produced in the cells that bind the agonist, in the presence or absence of a test compound to be characterized as an antagonist of a mammalian melanocortin receptor, and detecting the amount of the metabolite produced in each cell in the panel in the presence of the test compound with the amount of the metabolite produced in each cell in the panel in the absence. As provided by the assay, inhibition of the production of the detectable metabolite is used as an indication that the tested compound is a melanocortin receptor antagonist, which is further characterized quantitatively by the extent of said inhibition.,
[0024] In a preferred embodiment of this method, each of the cells of the panel of mammalian cells expressing mammalian melanocortin receptors further comprises a recombinant expression construct encoding a cyclic AMP responsive element (CRE) transcription factor binding site that is operatively linked to a nucleic acid sequence encoding a protein capable of producing a detectable metabolite. In preferred embodiments, said protein is &bgr;-galactosidase, most preferably encoded by a nucleic acid comprising the recombinant expression construct identified as pCRE/&bgr;-galactosidase. As provided by the invention, expression of the protein that produces the detectable metabolite is dependent on binding of the test compound to the melanocortin receptor expressed by each cell in the panel. In preferred embodiments, the detectable metabolite absorbs light to produce a colored product. Thus, this embodiment of the invention provides a panel of melanocortin receptor-expressing cells whereby melanocortin hormone binding results in the production of a colored product in proportion to the extent of cAMP production in the cell as a result of hormone receptor binding.
[0025] The invention also provides melanocortin receptor agonists identified by the methods and using the screening panel of the invention. In preferred embodiments, the agonist is an agonist of the MC-3 mammalian melanocortin receptor. In preferred embodiments, the agonist is an agonist of the MC-4 mammalian melanocortin receptor.
[0026] The invention provides melanocortin receptor antagonists identified by the methods and using the screening panel of the invention. In preferred embodiments, the antagonist is an antagonist of the MC-3 mammalian melanocortin receptor. In preferred embodiments, the antagonist is an antagonist of the MC-4 mammalian melanocortin receptor.
[0027] The invention also provides methods for characterizing mammalian melanocortin receptor agonists for the capacity to modify or influence metabolism and feeding behavior in an animal. In a first aspect, the invention provides a method for characterizing melanocortin receptor MC-3 or MC-4 agonists as inhibitors of feeding behavior in an animal, the method comprising the steps of providing food to an animal that has been deprived of food for at least 12 hours, with or without administering to the animal an MC-3 or MC-4 receptor agonist of the invention, and comparing the amount of food eaten by the animal after administration of the MC-3 or MC-4 receptor agonist with the amount of food eaten by the animal without administration of the MC-3 or MC-4 receptor agonist.
[0028] In another aspect, the invention provides a method for characterizing a melanocortin MC-3 or MC-4 receptor antagonist as a stimulator of feeding behavior in an animal. In this embodiment, the method comprises the steps of providing food to an animal not deprived of food for at least 12 hours, with or without administering to the animal an MC-3 or MC-4 receptor antagonist, immediately prior to the onset of darkness or nighttime, and comparing the amount of food eaten by the animal after administration of the MC-3 or MC-4 receptor antagonist with the amount of food eaten by the animal without administration of the MC-3 or MC-4 receptor antagonist.
[0029] Thus, the invention also provides methods for using certain of the melanocortin receptor agonists and antagonists for modifying feeding behavior in an animal. In a first aspect, the invention provides a method for stimulating feeding in an animal, the method comprising administering to the animal an MC-3 or MC-4 receptor antagonist. In a preferred embodiment, the antagonists are administered systemically. In additional embodiments, the antagonists are administered intracerebroventricularly.
[0030] In another aspect, the invention provides a method for inhibiting feeding in an animal, the method comprising administering to the animal an MC-3 or MC-4 receptor agonist. In a preferred embodiment, the agonists are administered systemically. In additional embodiments, the agonists are administered intracerebroventricularly.
[0031] In yet another aspect, the invention provides mammalian melanocortin receptor agonists having the general formula:
A-B-C-D-E-F-G-amide
[0032] wherein A is an aliphatic amino acid residue, including for example Leu, Ile, Nle and Met, as well as analogues and substituted derivatives thereof; B is an acidic amino acid residue, including for example Asp and Glu; C is a basic amino acid residue, such as His; D is an aromatic amino acid residue having a d-conformation, including d-Phe, d-Tyr and substituted derivatives thereof; E is a basic amino acid residue, for example Arg, Lys, homoArg, homoLys, and analogues or substituted derivatives thereof; F is Trp or substituted derivatives thereof; and G is Lys, homoLys or a substituted derivative thereof In the peptide embodiments of the melanocortin receptor agonists of the invention, the peptide is cyclized by the formation of an amide bond between the side chain carboxyl group of the Asp or Glu residue at position B in the peptide, and the side chain amino group of the Lys or homoLys residue at position G. In preferred embodiments, the melanocortin receptor agonists of the invention are agonists of the MC-3 or MC-4 receptor.
[0033] The invention also provides mammalian melanocortin receptor antagonists having the general formula:
A-B-C-D-E-F-G-amide
[0034] wherein A is an aliphatic amino acid residue, including for example Leu, Ile, Nle and Met, as well as analogues and substituted derivatives thereof; B is an acidic amino acid residue, including for example Asp and Glu; C is a basic amino acid residue, such as His; D is an aromatic amino acid residue having a d-conformation, including d-Nal and substituted derivatives thereof; E is a basic amino acid residue, for example Arg, Lys, homoArg, homoLys, and analogues or substituted derivatives thereof; F is Trp or substituted derivatives thereof; and G is Lys, homoLys or a substituted derivative thereof. In the peptide embodiments of the melanocortin receptor antagonists of the invention, the peptide is cyclized by the formation of an amide bond between the side chain carboxyl group of the Asp or Glu residue at position B in the peptide, and the side chain amino group of the Lys or homoLys residue at position G. In preferred embodiments, the melanocortin receptor antagonists of the invention are agonists of the MC-3 or MC-4 receptor.
[0035] It is an advantage of the present invention that it provides an in vitro screening method for characterizing compounds having melanocortin receptor activities that relate to feeding behavior in animals. Specifically, the invention advantageously provides means and methods for identifying compounds having melanocortin receptor agonist and/or antagonist activity that have been associated with either stimulating or inhibiting feeding behavior when administered to an animal. The invention thus provides an economical first step in screening compounds for the capacity to affect feeding behavior, including synthetic, peptidomimetic or organomimetic derivatives of melanocortin receptor agonists or antagonists as disclosed herein or elsewhere.
[0036] Specific preferred embodiments of the present invention will become evident from the following more detailed description of certain preferred embodiments and the claims.
DESCRIPTION OF THE DRAWINGS[0037] FIGS. 1A and 1B illustrate the nucleotide (SEQ ID No. 3) and amino acid (SEQ ID No.: 4) sequence of the mouse melanocyte stimulating hormone receptor gene.
[0038] FIGS. 2A and 2B illustrate the nucleotide (SEQ ID No. 5) and amino acid (SEQ ID No. 6) sequence of the human melanocyte stimulating hormone receptor gene.
[0039] FIGS. 3A through 3C illustrate the nucleotide (SEQ ID No. 7) and amino acid (SEQ ID No. 8) sequence of the human adrenocorticotropic hormone receptor gene.
[0040] FIGS. 4A and 4B illustrate the nucleotide (SEQ ID No. 9) and amino acid (SEQ ID No. 10) sequence of the bovine adrenocorticotropic hormone receptor gene.
[0041] FIGS. 5A and 5B illustrate the nucleotide (SEQ ID No:11) and amino acid (SEQ ID No.:12) sequences of the rat melanocortin-3 receptor.
[0042] FIGS. 6A through 6C illustrate the nucleotide (SEQ ID No. 15) and amino acid (SEQ ID No. 16) sequence of the human melanocortin 4 receptor gene.
[0043] FIGS. 7A and 7B illustrate the nucleotide (SEQ ID No:17) and amino acid (SEQ ID No.:18) sequences of the rat melanocortin-5 receptor gene.
[0044] FIG. 8 shows a graph of intracellular cAMP accumulation resulting from melanocyte stimulating hormone receptor agonist binding in human 293 cells transfected with a MSH receptor-encoding recombinant expression construct.
[0045] FIG. 9 illustrates the cAMP response of mouse Y1 cells to binding of melanocortin peptides to human melanocortin-2 (ACTH) receptor, as measured using the &bgr;-galactosidase assay described in Example 4.
[0046] FIG. 10 illustrates the results of competition binding experiments of melanocortin peptides to cells expressing a recombinant expression construct encoding the rat melanocortin-3 receptor.
[0047] FIGS. 11A through 11C illustrate the results of experiment showing intracellular cAMP accumulation caused by receptor-ligand binding in human 293 cells expressing the MC-3 receptor.
[0048] FIG. 12 shows a graph of intracellular cAMP accumulation resulting from peptides to human melanocortin-4 receptor agonist binding in human 293 cells transfected with a MC-4 receptor-encoding recombinant expression construct.
[0049] FIG. 13 illustrates the results of cAMP accumulation (AC) and cAMP-dependent &bgr;-galactosidase (&bgr;-gal) assays of melanocortin peptide binding to a rat melanocortin-5 receptor.
[0050] FIG. 14 illustrates the structure of the pCRE/&bgr;-gal plasmid.
[0051] FIG. 15 illustrates the results of the &bgr;-galactosidase-coupled, colorimetric melanocortin receptor binding assay using cells expressing each of the MC-1, MC-3, MC-4 or MC-5 receptors and contacted with &agr;MSH or a variety of &agr;MSH analogues.
[0052] FIG. 16 shows the results of the &bgr;-galactosidase-coupled, colorimetric melanocortin receptor binding assay to determine antagonist activity of melanocortin analogues SHU9119 and SHU8914 in cells expressing each of the melanocortin receptors MC-1, MC-3, MC-4 and MC-5.
[0053] FIG. 17 shows the results of classic competition binding assays using the melanocortin analogues SHU9119 and SHU8914 at the MC3-R and MC-4 R receptors.
[0054] FIGS. 18A and 18B shows the results of cAMP accumulation experiments (performed using the &bgr;-galactosidase assay of Example 4) for mouse MC-4 receptor (FIG. 18A) and rat MC-5 receptor (FIG. 18B).
[0055] FIGS. 19A through 19C show the effect on food intake of intracerebroventricular administration of melanocortin analogue SHU9119 in mice.
[0056] FIGS. 20A through 20C show the effect on food intake of intracerebroventricular administration of melanocortin analogue MTII in mice.
[0057] FIG. 20D shows the effect on locomotor activity of intracerebroventricular administration of melanocortin analogue MTII in mice.
[0058] FIGS. 21A through 21D show the effect on food intake of intracerebroventricular administration of melanocortin analogue MTII in mice.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS[0059] The term “melanocortin receptor” as used herein reference to proteins having the biological activity of any of the disclosed melanocortin receptors, including the MC-1 (SEQ ID Nos.: 3, 4, 5 and 6), MC-2 (ACTH; SEQ ID Nos.: 7, 8,9 and 10), MC-3 (SEQ ID Nos.: 11 and 12), MC-4 (SEQ ID Nos.: 15 and 16) or MC-5 (SEQ ID Nos.: 17 and 18) receptors, as well as naturally-occurring and genetically-engineered allelic variations in these sequences.
[0060] Cloned nucleic acid provided by the present invention may encode MC receptor protein of any species of origin, including, for example, mouse, rat, rabbit, cat, and human, but preferably the nucleic acid provided by the invention encodes MC receptors of mammalian, most preferably rodent and human, origin.
[0061] The production of proteins such as the MC receptors from cloned genes by genetic engineering means is well known in this art. The discussion which follows is accordingly intended as an overview of this field, and is not intended to reflect the full state of the art.
[0062] DNA which encodes MC receptors may be obtained, in view of the instant disclosure, by chemical synthesis, by screening reverse transcripts of mRNA from appropriate cells or cell line cultures, by screening genomic libraries from appropriate cells, or by combinations of these procedures, as illustrated below. Screening of mRNA or genomic DNA may be carried out with oligonucleotide probes generated from the MC receptor gene sequence information provided herein. Probes may be labeled with a detectable group such as a fluorescent group, a radioactive atom or a chemiluminescent group in accordance with know procedures and used in conventional hybridization assays, as described in greater detail in the Examples below. In the alternative, MC receptor gene sequences may be obtained by use of the polymerase chain reaction (PCR) procedure, with the PCR oligonucleotide primers being produced from the MC receptor gene sequences provided herein. See U.S. Pat. Nos. 4,683,195 to Mullis et al. and 4,683,202 to Mullis.
[0063] MC receptor proteins may be synthesized in host cells transformed with a recombinant expression construct comprising a nucleic acid encoding each of the receptors disclosed herein. Such a recombinant expression construct can also be comprised of a vector that is a replicable DNA construct. Vectors are used herein either to amplify DNA encoding an MC receptor and/or to express DNA which encodes an MC receptor. For the purposes of this invention, a recombinant expression construct is a replicable DNA construct in which a DNA sequence encoding an MC receptor is operably linked to suitable control sequences capable of effecting the expression of the receptor in a suitable host cell. The need for such control sequences will vary depending upon the host selected and the transformation method chosen. Generally, control sequences include a transcriptional promoter, an optional operator sequence to control transcription, a sequence encoding suitable mRNA ribosomal binding sites, and sequences which control the termination of transcription and translation. Amplification vectors do not require expression control domains. All that is needed is the ability to replicate in a host, usually conferred by an origin of replication, and a selection gene to facilitate recognition of transformants. See, Sambrook et al., 1990, Molecular Cloning: A Laboratory Manual (Cold Spring Harbor Press: New York).
[0064] Also specifically provided by the invention are reporter expression constructs comprising a nucleic acid encoding a protein capable of expressing a detectable phenotype, such as the production of a detectable reporter molecule, in a cell expressing the construct. Such constructs can be used for producing recombinant mammalian cell lines in which the reporter construct is stably expressed. Most preferably, however, the reporter construct is provided and used to induce transient expression over an experimental period of from about 18 to 96 hrs in which detection of the reporter protein-produced detectable metabolite comprises an assay. Such reporter expression constructs are also provided wherein induction of expression of the reporter construct is controlled by a responsive element operatively linked to the coding sequence of the reporter protein, so that expression is induced only upon proper stimulation of the responsive element. Exemplary of such a responsive element is a cAMP responsive element (CRE), which induces expression of the reporter protein as a result of an increase in intracellular cAMP concentration. In the context of the present invention, such a stimulus is associated with melanocortin receptor binding, so that a reporter construct comprising one or more CREs is induced to express the reporter protein upon binding of a receptor agonist to a MC receptor in a recombinantly transformed mammalian cell. Production and use of such a reporter construct is illustrated below in Example 5.
[0065] Vectors useful for practicing the present invention include plasmids, viruses (including phage), retroviruses, and integratable DNA fragments (i.e., fragments integratable into the host genome by homologous recombination). The vector replicates and functions independently of the host genome, or may, in some instances, integrate into the genome itself. Suitable vectors will contain replicon and control sequences which are derived from species compatible with the intended expression host. A preferred vector is the plasmid pcDNA/neo I. Transformed host cells are cells which have been transformed or transfected with recombinant expression constructs made using recombinant DNA techniques and comprising mammalian MC receptor-encoding sequences. Preferred host cells are human 293 cells. Preferred host cells for the MC-2 (ACTH) receptor are Y1 cells (subclone OS3 or Y6). Transformed host cells are chosen that ordinarily express functional MC receptor protein introduced using the recombinant expression construct. When expressed, the mammalian MC receptor protein will typically be located in the host cell membrane. See, Sambrook et al., ibid.
[0066] Cultures of cells derived from multicellular organisms are a desirable host for recombinant MC receptor protein synthesis. In principal, any higher eukaryotic cell culture is workable, whether from vertebrate or invertebrate culture. However, mammalian cells are preferred, as illustrated in the Examples. Propagation of such cells in cell culture has become a routine procedure. See Tissue Culture, Academic Press, Kruse & Patterson, editors (1973). Examples of useful host cell lines are human 293 cells, VERO and HeLa cells, Chinese hamster ovary (CHO) cell lines, mouse Y1 (subclone OS3), and WI138, BHK, COS-7, CV, and MDCK cell lines. Human 293 cells are preferred.
[0067] Cells expressing mammalian MC receptor proteins made from cloned genes in accordance with the present invention may be used for screening agonist and antagonist compounds for MC receptor activity. Competitive binding assays are well known in the art and are described in the Examples below. Such assays are useful for drug screening of MC receptor agonist and antagonist compounds, as detected in receptor binding assays as described below.
[0068] One particular use of such screening assays are for developing drugs and other compounds useful in modifying or changing feeding behavior in mammals. The invention provides an assay system, comprising a panel of recombinant mammalian cells, heterologously expressing each of the MC receptors disclosed herein, wherein the panel is constructed of at least one cell line expressing an MC receptor. The invention provides such panels also comprising a detection means for detecting receptor agonist or antagonist binding, such as the reporter expression constructs described herein, using direct binding and competition binding assays as described in the Examples below. In the use of this panel, each MC receptor is assayed for agonist or antagonist patterns of binding a test compound, and a characteristic pattern of binding for all MC receptors is thereby determined for each test compound. This pattern is then compared with known MC receptor agonists and antagonists to identify new compounds having a pattern of receptor binding activity associated with a particular behavioral or physiological effect.
[0069] For example, provided herein is experimental evidence that MC-3 or MC-4 receptor antagonists are capable of stimulating feeding in hungry animals, and that MC-3 or MC-4 agonists are capable of inhibiting feeding in animals otherwise stimulated to eat. The invention provides an in vitro assay to characterize MC-3 and MC-4 agonists/antagonists as a preliminary and economical step towards developing feeding behavior-modulating drugs for use in vivo.
[0070] These results on feeding behavior in vivo have been obtained with certain MC receptor binding analogues, SHU9119 and MTII. These compounds have the following chemical structure: 1 2
[0071] (An additional analogue, SHU8914, has been tested in in vitro receptor binding assays; this compound is identical in structure to SHU9119, except that the amino acid analogue at position 7 is para-iodo-d-phenylalanine.)
[0072] Generally, those skilled in the art will recognize that peptides as described herein may be modified by a variety of chemical techniques to produce compounds having essentially the same activity as the unmodified peptide, and optionally having other desirable properties. For example, carboxylic acid groups of the peptide, whether carboxyl-terminal or sidechain, may be provided in the form of a salt of a pharmaceutically-acceptable cation or esterified to form a C1-C16 ester, or converted to an amide of formula NR1R2 wherein R1 and R2 are each independently H or C1-C16 alkyl, or combined to form a heterocyclic ring, such as 5- or 6-membered. Amino groups of the peptide, whether amino-terminal or sidechain, may be in the form of a pharmaceutically-acceptable acid addition salt, such as the HCl, HBr, acetic, benzoic, toluene sulfonic, maleic, tartaric and other organic salts, or may be modified to C1-C16 alkyl or dialkyl amino or further converted to an amide. Hydroxyl groups of the peptide sidechain may be converted to C1-C16 alkoxy or to a C1-C16 ester using well-recognized techniques. Phenyl and phenolic rings of the peptide sidechain may be substituted with one or more halogen atoms, such as fluorine, chlorine, bromine or iodine, or with C1-C16 alkyl, C1-C16 alkoxy, carboxylic acids and esters thereof, or amides of such carboxylic acids. Methylene groups of the peptide sidechains can be extended to homologous C2-C4 alkylenes. Thiols can be protected with any one of a number of well-recognized protecting groups, such as acetamide groups. Those skilled in the art will also recognize methods for introducing cyclic structures into the peptides of this invention to select and provide conformational constraints to the structure that result in enhanced binding and/or stability. For example, a carboxyl-terminal or amino-terminal cysteine residue can be added to the peptide, so that when oxidized the peptide will contain a disulfide bond, thereby generating a cyclic peptide. Other peptide cyclizing methods include the formation of thioethers and carboxyl- and amino-terminal amides and esters.
[0073] Peptidomimetic and organomimetic embodiments are also hereby explicitly declared to be within the scope of the present invention, whereby the three-dimensional arrangement of the chemical constituents of such peptido- and organomimetics mimic the three-dimensional arrangement of the peptide backbone and component amino acid sidechains in the peptide, resulting in such peptido- and organomimetics of the peptides of this invention having substantial biological activity. It is implied that a pharmacophore exists for the receptor agonist and antagonist properties of these and related MC receptor binding analogues. A pharmacophore is an idealized; three-dimensional definition of the structural requirements for biological activity. Peptido- and organomimetics can be designed to fit each pharmacophore with current computer modeling software (computer aided drug design). MC receptor binding analogues derived using such software and comprising peptido- and organomimetics of SHU9119 and MTII and related analogues are within the scope of the claimed invention.
[0074] The MC receptor binding analogues, in particular those analogues that are MC-3 or MC4 receptor agonists or antagonists are provided to be used in methods of influencing, modifying or changing feeding behavior in mammals in vivo. Specific examples of uses for the MC receptor binding analogues of the invention include but are not limited to treatment of eating disorders such as anorexia and obesity, and other pathological weight and eating-related disorders. Other examples are failure to thrive disorders and disease-related cachexia, such as occurs in cancer patients. Also within the scope of the analogues of the invention is use for enhancing appearance, athletic ability, or adjuvant to other therapies to treat disorders such as high blood pressure, high serum cholesterol, vascular and heart disease, stroke, kidney disease, diabetes and other metabolic disorders.
[0075] The Examples which follow are illustrative of specific embodiments of the invention, and various uses thereof They set forth for explanatory purposes only, and are not to be taken as limiting the invention.
EXAMPLE 1 Isolation of an &agr;MSH Receptor Probe by Random PCR Amplification of Human Melanoma cDNA Using Degenerate Oligonucleotide Primers[0076] In order to clone novel G-protein coupled receptors, cDNA prepared from RNA from human melanoma cells was used as template for a polymerase chain reaction (PCR)-based random cloning experiment. PCR was performed using a pair of degenerate oligonucleotide primers corresponding to the putative third and sixth transmembrane regions of G-protein coupled receptors (Libert et al., 1989, Science 244: 569-72; Zhou et al., 1990, Nature 3: 76-80). The PCR products obtained in this experiment were characterized by nucleotide sequencing. Two novel sequences representing novel G-protein-coupled receptors were identified.
[0077] PCR amplification was performed as follows. Total RNA was isolated from a human melanoma tumor sample by the guanidinium thiocyanate method (Chirgwin et al., 1979, Biochemistry 18: 5294-5299). Double-stranded cDNA was synthesized from total RNA with murine reverse transcriptase (BRL, Gaithersburg, Md.) by oligo-dT priming (Sambrook et al., ibid.). The melanoma cDNA mixture was then subjected to 45 cycles of PCR amplification using 500 picomoles of degenerate oligonucleotide primers having the following sequence: 1 Primer III (sense): (SEQ ID NO:1) GAGTCGACCTGTG(C/T)G(C/T)(C/G)AT(C/T)(A/G)CIIT(G/T) GAC(C/A)G(C/G)TAC and Primer VI (antisense): (SEQ ID NO:2) CAGAATTCAG(T/A)AGGGCAICCAGCAGAI(G/C)(G/A)(T/C)GAA
[0078] in 100 &mgr;l of a solution containing 50 mM Tris-HCl (pH 8.3), 2.5 mM MgCl2, 0.01% gelatin, 200 &mgr;M each dNTP, and 2.5 units of Taq polymerase (Saiki et al., 1988, Science 239: 487-491). These primers were commercially synthesized by Research Genetics Inc. (Huntsville, Ala.). Each PCR amplification cycle consisted of incubations at 94° C. for 1 min (denaturation), 45° C. for 2 min (annealing), and 72° C. for 2 min (extension).
[0079] Amplified products of the PCR reaction were extracted with phenol/chloroform and precipitated with ethanol. After digestion with EcoRI and SalI, the PCR products were separated on a 1.2% agarose gel. A slice of this gel, corresponding to PCR products of 300 basepairs (bp) in size, was cut out and purified using glass beads and sodium iodide, and the insert was then cloned into a pBKS cloning vector (Stratagene, LaJolla, Calif.).
[0080] A total of 172 of such pBKS clones containing inserts were sequenced using Sequenase (U.S. Biochemical Corp., Cleveland, Ohio) by the dideoxynucleotide chain termination method (Sanger et al., 1977, Proc. Natl. Acad. Sci. USA 74: 5463-5467). Two types of sequences homologous to other G-protein coupled receptors were identified.
EXAMPLE 2A[0081] Isolation of a Mouse &agr;MSH (MC-1) Receptor cDNA
[0082] Probes isolated in Example 1 was used to screen a Cloudman melanoma cDNA library in order to isolate a full-length cDNA corresponding to the cloned probe. One clone was isolated from a library of 5×106 clones screened as described below. This clone contained an insert of 2.6 kilobases (kb). The nucleotide sequence of the complete coding region was determined (see co-owned U.S. Pat. No. 5,532,347, incorporated by reference); a portion of this cDNA comprising the coding region was sequenced and is shown in FIGS. 1A and 1B (SEQ ID Nos: 3 & 4).
EXAMPLE 2B[0083] Isolation of a Human &agr;MSH (MC-1) Receptor cDNA
[0084] In order to isolate a human counterpart of the murine melanocyte &agr;MSH receptor gene disclosed in Example 2A and co-owned U.S. Pat. No. 5,532,347, a human genomic library was screened at high stringency (50% formamide, 42° C.) using the human PCR fragments isolated as described in Example 1. A genomic clone was determined to encode an human MSH receptor (SEQ ID NO:5.; FIGS. 2A and 2B). The human MSH receptor has a predicted amino acid sequence (SEQ ID NO:6) that is 75% identical and colinear with the mouse &agr;MSH receptor cDNA sequence. The predicted molecular weight of the human MSHR is 34.7 kD.
EXAMPLE 2C[0085] Isolation of a Human ACTH (MC-2) Receptor cDNA
[0086] For cloning the ACTH receptor (MC-2), a human genomic library was screened at high stringency (50% formamide, 1M NaCl, 50 nM Tris-HCl, pH 7.5, 0.1% sodium pyrophosphate, 0.2% sodium dodecyl sulfate, 100 &mgr;g/ml salmon sperm DNA, 10× Denhardt's solution, 42° C.), using the human PCR fragments isolated as described in Example 1 herein and U.S. Pat. No. 5,280,112, incorporated by reference. A genomic clone was isolated that encodes a highly related G-coupled receptor protein (SEQ ID No:7 and FIGS. 3A and 3B). The predicted amino acid sequence (SEQ ID NO:8) of this clone is 39% identical and also colinear, excluding the third intracellular loop and carboxy-terminal tail, with the human MSH receptor gene product. The predicted molecular weight of this putative ACTHIR is 33.9 kilodaltons (kD). This clone was identified as encoding an MC-2 receptor based on its high degree of homology to the murine and human MSH receptors, and the pattern of expression in different tissue types, as described in Example 3 in U.S. Pat. No. 5,280,112, incorporated by reference herein.
EXAMPLE 2D[0087] Isolation of a Bovine ACTH (MC-2) Receptor cDNA
[0088] A bovine genomic DNA clone encoding the bovine counterpart of the MC-2 (ACTH) receptor was isolated from a bovine genomic library, essentially as described in Example 2C above, and its nucleotide sequence determined (as shown in FIGS. 4A and 4B; SEQ ID Nos:9 & 10).
EXAMPLE 2E[0089] Isolation of a Rat &ggr;-MSH (MC-3) Receptor cDNA
[0090] The mouse &agr;MSH receptor cDNA isolated as described in Example 2A and co-owned U.S. Pat. No. 5,532,347 was used to screen a rat hypothalamus cDNA library at low stringency (30% formamide, 5×SSC, 0.1% sodium pyrophosphate, 0.2% sodium dodecyl sulfate, 1100 &mgr;g/ml salmon sperm DNA, and 10% Denhardt's solution) at 42° C. for 18 h. A 1 kb cDNA clone was isolated and sequenced as described in co-owned U.S. Pat. No. 5,532,347, and this clone used to re-screen the rat hypothalamus cDNA library at high stringency (same conditions as above except that formamide was present at 45%). A cDNA clone approximately 2.0 kb in length was isolated and analyzed as described in co-pending U.S. application Ser. No. 08/044,812, incorporated by reference; a portion of this cDNA comprising the coding region was sequenced and is shown in FIGS. 5A and 5B (SEQ ID Nos:11 & 12).
EXAMPLE 2F[0091] Isolation of a Human MC4 Receptor DNA
[0092] For cloning the MC-4 receptor, a human genomic library was screened at moderate stringency (40% formamide, IM NaCl, 50 mM Tris-HCl, pH 7.5, 0.1% sodium pyrophosphate, 0.2% sodium dodecyl sulfate, 100 &mgr;g/ml salmon sperm DNA, 10× Denhardt's solution, 42° C.), using rat PCR fragments isolated as described in Example 1 herein, with the exception that the following primers were used for PCR: 2 Primer II (sense): (SEQ ID NO:13) GAGTCGACC(A/G)CCCATGTA(C/T)T(AGT)(C/T)TTCATCTG and Primer VII (antisense): (SEQ ID NO:14) CAGAATTCGGAA(A/G)GC(A/G)TA(G/T)ATGA(A/G)GGGGTC
[0093] A genomic clone was isolated that encodes a highly related G-coupled receptor protein (SEQ ID NO:15 and FIGS. 6A and 6B) on a 1.9 kb HindIII fragment. The predicted amino acid sequence (SEQ ID NO:16) of this clone shares 55-61% sequence identity with human MC-3 and MC-5 receptors, and 46-47% sequence identity with the human MC-1 and MC-2 (ACTH) receptor.
EXAMPLE 2G[0094] Isolation of a Mouse MC-5 Receptor DNA
[0095] One million clones from a mouse 129SVJ genomic library comprising 5,000,000 clones in the &lgr;FixII vector (Stratagene) were screened at low stringency (hybridization in 40% formamide at 42° C., washing performed in 0.5×SSC at 60° C., as described above in Example 2E) using radiolabeled probes from the rat MC-3 and MC-4 receptors, as described in Examples 2E and 2F). Positively-hybridizing clones were isolated and sequenced, and the sequences obtained were compared to previously-isolated melanocortin receptor clones. One clone, comprising a previously-unknown sequence, was determined to encode the MC-5 melanocortin receptor. The nucleotide and amino acid sequences of this receptor are shown in FIGS. 7A and 7B (SEQ ID No. 17 & 18).
EXAMPLE 3 Construction of a Recombinant Expression Construct, DNA Transfection and Functional Expression of the MCR Gene Products[0096] In order to produce recombinant mammalian cells expressing each of the melanocortin receptors of Example 2, cDNA or the coding exons from genomic DNA from each receptor was cloned into a mammalian expression construct, the resulting recombinant expression construct transfected into human 293 cells, and cell lines generated that expressed the melanocortin receptor proteins in cellular membranes at the cell surface.
[0097] The mouse &agr;MSH receptor was cloned by excising the entire coding region of the &agr;MSHR (MC-1) cDNA insert comprising a 20.1 kb fragment and subcloning this fragment into the BamHI/XhoI sites of pcDNAI/neo expression vector (Invitrogen, San Diego, Calif.). The resulting plasmid was prepared in large-scale through one cycle of CsCl gradient ultracentrifugation, and 20 &mgr;g of the plasmid transfected into each 100 mm dish of 293 cells using the calcium phosphate method (see Chen & Okayama, 1987, Mol. Cell. Biol. 7: 2745-2752). After transfection, cells were cultured in DMEM media supplemented with 10% calf serum in a 3% CO2 atmosphere at 37° C. Selection was performed with neomycin (G418; GIBCO) at a concentration of 1000 &mgr;g/1 mL; selection was started 72 hr after transfection and continued for 3 weeks.
[0098] The &agr;MSHR is known to couple to G-proteins and thereby activate adenyl cyclase, increasing intracellular levels of cAMP (see Buckley & Ramachandran, 1981, Proc. Natl. Acad. Sci. USA 78: 7431-7435; Grahame-Smith et al., 1967, J. Biol. Chem 242: 5535-5541; Mertz & Catt, 1991, Proc. Natl. Acad. Sci. USA 88: 8525-8529; Pawalek et al., 1976, Invest. Dermatol. 66: 200-209). This property of cells expressing the &agr;MSH receptor was used analyze expression of the &agr;MSH receptor in cell colonies transfected with the expression vectors described herein as follows. Cells (˜1×106) were plated in 6-well dishes, washed once with DMEM containing 1% bovine serum albumin (BSA) and 0.5 mM IBMX (a phosphodiesterase inhibitor), then incubated for 45 minutes at 37° C. with varying concentrations of the melanotropic peptides &agr;MSH, &bgr;MSH, &ggr;MSH, the MSH peptide analogues Nle4, D-Phe7-&agr;MSH (NDP-MSH), and ACTH. Following hormone treatment, the cells were washed twice with phosphate buffered saline and intracellular cAMP extracted by lysing the cells with 1 mL of 60% ethanol. Intracellular cAMP concentrations were determined using an assay (Amersham) which measures the ability of cAMP to displace [8-3H] cAMP from a high affinity cAMP binding protein (see Gilman, 1970, Proc. Natl. Acad. Sci. USA 67: 305-312).
[0099] The results of these experiments are shown in FIG. 8. The abscissa indicates the concentration of each hormone and the ordinate indicates the percentage of basal intracellular cAMP concentration achieved by each treatment. Points indicate the mean of duplicate incubations; the standard error did not exceed 15% for any data point. None of the peptides tested induced any change in intracellular cAMP in cells containing the vector alone. Cells expressing the murine &agr;MSH receptor responded to melanotropic peptides with a 2-3 fold elevation of intracellular cAMP, similar to levels of cAMP induced by these peptides in the Cloudman cell line (see Pawalek, 1985, Yale J Biol. Med. 58: 571-578). The EC50 values determined for &agr;MSH (2.0×10−9M), ACTH (8.0×10−9M) and the superpotent MSH analogue NDP-MSH (2.8×1011M) correspond closely to reported values (see Tatro et al., 1990, Cancer Res. 50: 1237-1242). As expected, the &bgr;MSH peptide had an EC50 value comparable to &agr;MSH, while &ggr;MSH had little or no activity (see Slominski et al., 1992, Life Sci. 50: 1103-1108), confirming the identity of this receptor as a melanocyte &agr;MSH receptor.
[0100] A similar series of experiments were performed using mouse Y1 cells (subclone OS3; Schimmer et al., 1995, J. Cell. Physiol. 163: 164-171) expressing the human and bovine MC-2 (ACTH) receptor clones of Examples 2C and 2D. These results are shown in FIG. 9, where the extent of cAMP responsive element-linked &bgr;-galactosidase activity (see Example 4, below) is shown with increasing concentrations of ACTH.
[0101] The entire coding region of the MC-3 receptor cDNA insert, obtained as described above in the co-pending U.S. Ser. No. 08/044,812, was contained in a 2.0 kb restriction enzyme digestion fragment and was cloned into the BamHI/AhoI sites of pcDNA/neo I expression vector (Invitrogen, San Diego, Calif.). The resulting plasmid was prepared in large-scale through one cycle of CsCl gradient ultracentrifugation and 20 &mgr;g pcDNA/MC-3 receptor DNA were transfected into each 100 mm dish of 293 cells using the calcium phosphate method (see Chen & Okayama, 1987, Mol. Cell. Biol. 7: 2745-2752). After transfection, cells were cultured in DMEM media supplemented with 10% calf serum in a 3% CO2 atmosphere at 37° C. Selection was performed with neomycin (G418; GIBCO) at a concentration of 1000 &mgr;g/mL; selection was started 72 h after transfection and continued for 3 weeks.
[0102] Specific binding of melanocortin peptides to cells expressing the MC-3 receptor was demonstrated by competition experiments using 125I-labeled Nle4-D-Phe7-&agr;-MSH (NDP-MSH, as described in Tatro et al., 1990, Cancer Res. 50: 1237-1242). Suspended cells (2×105) were incubated at 37° C. with 500,000 cpm of labeled peptide for 10 min in binding, buffer (Ham's F10 media plus 10 mM HEPES, pH 7.2, 0.25% bovine serum albumin, 500K IU/mL aprotinin, 100 &mgr;g/mL bacitracin and 1 mM 1,10-phenanthroline) in the presence or absence of the indicated concentrations of peptides. Maximum labeling was achieved within 10 min.
[0103] The results of these experiments are shown in FIG. 10. Labeled NDP-MSH binding to cells expressing the MC-3 receptor, produced as described above, is inhibited by competition with unlabeled peptides known to be melanocortin receptor agonists, having a relative order of potency as follows:
[0104] NDP-MSH>&ggr;-MSH>&agr;-MSH>ACTH4-10>>>ORG2766.
[0105] Approximate Ki values derived from this experiment are as shown in Table I: TABLE I 3 TABLE I Agonist Ki (approx) NDP-MSH 2 × 10−8 &ggr;-MSH 5 × 10−8 &agr;-MSH 1 × 10−7 ACTH4-10 8 × 10−5
[0106] cAMP production assays as described above were also used to analyze expression of MC3-R in cells transfected with the expression vectors described herein as follows. Cells (˜5×106) were plated in 6-well dishes, washed once with DMEM containing 1% bovine serum albumin (BSA) and 0.5 mM IBMX (a phosphodiesterase inhibitor), then incubated for 1 h at 37° C. with varying concentrations of the melanotropic peptides &agr;MSH, &ggr;3MSH, &ggr;MSH, the MSH peptide analogues Nle4-D-Phe7-&agr;MSH (NDP-MSH), ACTH4-10 and ACTH1-39 Following hormone treatment, the cells were washed twice with phosphate buffered saline and intracellular cAMP extracted by lysing the cells with 1 mL of 60% ethanol. Intracellular cAMP concentrations were determined using an assay which measures the ability of cAMP to displace [8-3H],cAMP from a high affinity cAMP binding protein (see Gilman, 1979, Proc. Natl. Acad. Sci. USA 67: 305-312).
[0107] The results of these experiments are shown in FIGS. 11A through 11C. The abscissa indicates the concentration of each hormone and the ordinate indicates the percentage of basal intracellular cAMP concentration achieved by each treatment. Points indicate the mean of duplicate incubations; the standard error did not exceed 15% for any data point. Panel A depicts the results of experiments using peptides found in vivo; Panel B depicts results found with &ggr;-MSH variants; and Panel C shows results of synthetic melanocortin analogues. None of the peptides tested induced any change in intracellular cAMP in cells containing the vector alone. Cells expressing rat MC3-R responded strongly to every melanotropic peptide containing the MSH code sequence His-Phe-Arg-Trp, with up to a 60-fold elevation of intracellular cAMP levels. EC50 values ranged from 1-50 nM. The most potent ligand and the one having the lowest EC50 was found to be &ggr;MSH. The order of potency for the naturally occurring melanocortins was found to be:
[0108] &ggr;2-MSH=&ggr;MSH>&agr;MSH=ACTH1-39>&ggr;3-MSH>des-acetyl-&agr;MSH>ACTH4-10.
[0109] Ec50 values for these compounds are shown in Table II: 4 TABLE II Agonist Ec50 NDP-MSH 1 × 10−9 &ggr;1-MSH 3 × 10−9 &ggr;2-MSH 3 × 10−9 &agr;-MSH 4 × 10−9 ACTH1-39 4 × 10−9 &ggr;3-MSH 6 × 10−9 desacetyl-&agr;MSH 8 × 10−9 ACTH4-10 1 × 10−7
[0110] Additionally, a synthetic melanocortin peptide (ORG2766), known to have the greatest activity in vivo in stimulation of retention of learned behavior and in stimulation of neural regeneration, was unable to stimulate MC3-R-mediated cAMP production, and was also inactive as an antagonist. The results strongly indicate that this peptide does not bind to MC3-R protein.
[0111] The MC-4 receptor was cloned in a 1.9 kb HindIII genomic DNA fragment after PCR amplification of a lambda phage clone into pcDNAI/Neo (Invitrogen). This plasmid was stably introduced into human 293 cells by calcium phosphate co-precipitation using standard techniques, and plasmid-containing cells selected in G418 containing media. Specificity of receptor-hormone binding was assayed using adenylate cylcase activity as described above. The MC-4 receptor was found to couple to adenylate cyclase activity having the following pattern of agonist affinity:
[0112] NDP-MSH>des-acetyl-&agr;-MSH>/=ACTH1-39>/=&agr;-MSH>>&ggr;2-MSH=ACTH4-10
[0113] whereas the synthetic ACTH4-9 analogue ORG2766 showed no detectable binding to the MC-4 receptor. The results of adenylate cyclase activity assays are shown in FIG. 12. EC50 values for each of the tested MC-4 receptor agonists are as shown in Table III: 5 TABLE III Agonist Ec50 NDP-MSH 1.1 × 10−11 M desacetyl-&agr;MSH 4.9 × 10−10 M ACTH1-39 6.8 × 10−10 M &agr;-MSH 1.5 × 10−9 M &ggr;2-MSH >10−7 M ACTH4-10 >10−7 M
[0114] A 1.6 kb ApaI-HindIII fragment comprising the entire coding sequence of the mouse MC-5 melanocortin receptor disclosed in Example 2G above was cloned into the pcDNA/neo expression vector (Invitrogen) after PCR amplification of the lambda phage clone. This plasmid was stably introduced into human 293 cells by calcium phosphate co-precipitation using standard techniques, and plasmid-containing cells selected in G418 containing media. Specificity of receptor-hormone binding was assayed using adenylate cylcase activity as described above. The MC-5 receptor was found to couple to adenylate cyclase activity having the following pattern of agonist affinity:
[0115] &agr;-MSH>&bgr;MSH>>&ggr;-MSH
[0116] The results of adenylate cyclase activity assays (AC) and cAMP-dependent &bgr;-galactosidase (&bgr;-gal) assay are shown in FIG. 13. EC50 values for each of the tested MC-5 receptor agonists are: &agr;-MSH=1.7×10−1M; and &bgr;MSH 5×10−9M.
EXAMPLE 4 Melanocortin Analogue Binding to Mammalian Melanocortin Receptors[0117] Recombinant cells prepared as described above in Example 3 were used to characterize receptor binding of two melanocortin analogues comprising cyclic lactam heptapeptides.
[0118] The melanocortin receptor analogue SHU9119 has the following chemical structure: 3
Ac-Nle4-cyclo(Asp5, D-Nal(2)7, Lys10) &agr;MSH-(4-10)-amide[0119] The melanocortin receptor analogue MTII has the following chemical structure: 4
[0120] These analogues were prepared as described in Hruby et al. (1995, J. Med. Chem. 38: 3454-3461).
[0121] These analogues were tested for melanocortin receptor binding using a colorimetric assay system developed by some of the instant inventors (Chen et al., 1995, Analyt. Biochem. 226: 349-354) as follows. A series of concatamers of the synthetic oligonucleotide: 6 5′-GAATTCGACGTCACAGTATGACGGCCATGG-3′ (SEQ ID No:19)
[0122] was produced by self-annealing and ligation and a tandem tetramer obtained. This fragment was cloned upstream of a fragment of the human vasoactive intestinal peptide (−93−+152; SEQ ID No. 13; see Fink et al., 1988, Proc. Natl. Acad. Sci. USA 85: 6662-6666). This promoter was then cloned upstream of the &bgr;-galactosidase gene from E. coli. The resulting plasmid construct is shown in FIG. 14.
[0123] Transient transfection of the pCRE/&bgr;-gal plasmid described above was performed as follows. Cells at between 40-60% confluency (corresponding to about 1.5 million cells/6 cm tissue culture plate) were incubated with Opti-MEM (Gibco BRL, Long Island, N.Y.) and then contacted with a pCRE/&bgr;-gal-lipofectin complex which was prepared as follows. 3 &mgr;g plasmid DNA and 20 &mgr;L lipofectin reagent (Gibco) were each diluted into 0.5 mL Opti-MEM media and then mixed together. This mixture was incubated at room temperature for 15-20 min., and then the mixture (1 mL) added to each 6 cm plate. Transfected plates were incubated at 37° C. for 5-24 h, after which the plates were washed and incubated with DMEM media (Gibco) and the cells split equally into a 96-well culture plate.
[0124] To assay melanocortin receptor analogue binding, human 293 cells expressing each of the melanocortin receptors MC-1, MC-3, MC-4 and MC-5, and mouse Y1 cells expressing the MC-2 receptor, were transiently transfected with pCRE/&bgr;-gal as described above and assayed as follows. Two days after transfection, cells were stimulated with hormones specific for each receptor or hormone analogue by incubation for 6 h at 37° C. with a mixture comprising 10-12-10−6M hormone or analogue, 0.1 mg/mL bovine serum albumin and 0.1 mM isobutylmethylxanthine in DMEM. The effect of hormone or analogue binding was determined by &bgr;-galactosidase assay according to the method of Felgner et al. (1994, J. Biol. Chem. 269: 2550-2561). Briefly, media was aspirated from culture wells and 50 &mgr;L lysis buffer (0.25M Tris-HCl, pH 8/0.1% Triton-X100) added to each well. Cell lysis was enhanced by one round of freezing and thawing the cell/lysis buffer mixture. 10 &mgr;L aliquots were sampled from each well for protein determination using a commercially-available assay (BioRad, Hercules, Calif.). The remaining 40 &mgr;L from each well was diluted with 40 &mgr;L phosphate buffered saline/0.5% BSA and 150 &mgr;L substrate buffer (60 mM sodium phosphate/1 mM MgCl2/10 mM KCl/5 mM &bgr;-mercaptoethanol/200 &mgr;g/mL o-nitrophenyl-&bgr;-D-galactopyranoside) added. Plates were incubated at 37° C. for 1 h and then absorbance at 405 nm determined using a 96-well plate reader (Molecular Devices, Sunnyvale, Calif.). A series of two-fold dilutions from 20 ng of purified &bgr;-galactosidase protein (Sigma Chemical Co, St. Louis, Mo.) were assayed in parallel in each experiment to enable conversion of OD405 to known quantity of P-galactosidase protein.
[0125] The results of these experiments are shown in FIG. 15. This Figure shows the results of the &bgr;-galactosidase assay described above using cells expressing each of the MC-1, MC-3, MC-4 or MC-5 receptors and contacted with &agr;MSH or a variety of &agr;MSH analogues, including SHU9119. These results showed that SHU9119 had relatively weak agonist activity for both the human MC-3 and MC-4 receptors.
[0126] These results demonstrated the development of a colorimetric assay for cAMP accumulation as the result of melanocortin receptor binding to agonists and antagonists.
[0127] The action of MTII, SHU9119, and the endogenous mouse agouti peptide as agonists or antagonists of rodent MC receptors was determined by examining their ability to elevate intracellular cAMP in 293 cell lines expressing the rat MC3-R or mouse MC4-R (expressed as IC50 values representing ligand concentrations required for half-maximal inhibition of binding of 125I-[Nle4,D-Phe7]&agr;-MSH tracer). Agonist/antagonist activity was also shown by demonstrating inhibition of cAMP elevation by the potent &agr;-MSH analogue [Nle4, D-Phe7]&agr;-MSH, using either a cAMP-responsive &bgr;-galactosidase reporter construct as described above, or by direct adenyl cyclase assay as described in Example 3 (wherein EC50 values represent ligand concentration required for half-maximal activation of a cAMP-responsive &bgr;-galactosidase reporter). Competition binding experiments were determined as the amount of radioactivity bound in the presence of 5×10−6M cold [Nle4, D-Phe7]&agr;-MSH, and was typically 3-5% of total counts bound.
[0128] In these experiments, murine agouti peptide was produced using a baculovirus system as described by Lu et al., (1994, Nature 371: 799-802), with the modification that the agouti peptide was purified from baculovirus supernatants by 0.6M NaCl step elution from an EconoS cation exchange column (BioRad). Agouti peptide used in these assays was approximately 60% pure.
[0129] Competition binding assays were performed to determine whether SHU9119 had antagonist activity towards &agr;MSH binding to either the MC-3 or MC-4 receptors. These assays were performed as follows. Human 293 cells (100,000 cells/well in 24-well plates) expressing either the MC-3 or MC-4 receptors prepared as described above were incubated with a solution of 1 mg/mL BSA in PBS containing 100,000 cpm (3.1×10−10M [125I](Nle4, D-Phe7)&agr;MSH and varying concentrations of &agr;MSH, (Nle4, D-Phe)&agr;MSH or SHU9119. Cells were incubated for 30 min at 37° C., washed twice with PBS-BSA, lysed with 0.5 mL 0.5N NaOH, and counted using a &ggr;-counter to quantitate the amount of bound [125I](Nle4, D-Phe7)&agr;MSH. Control experiments showed non-specific binding to occur at about 3-5% levels, and this was taken into account when analyzing the experimental results.
[0130] The SHU9119 analogue was found to be a potent antagonist of both the human MC-3 and MC-4 receptors, as shown in FIG. 16. These assays showed pA2 values of 8.3 and 9.3 for the human MC-3 and MC4 receptors, respectively, as determined using the method of Schild (1947, Brit. J. Pharmacol. 2: 189-206). In contrast, no significant alteration in IC50 values was detected in binding experiments using this analogue with either the MC-3 or MC-4 receptors (FIG. 17).
[0131] The activity of the MTII analogue was also assayed for melanocortin receptor agonist activity. These results are shown in FIGS. 18A and 18B, and confirmed that the MTII, analogue is a specific agonist of the MC-3 and MC-4 receptors.
[0132] Specific competition of [Nle4, D-Phe7]&agr;-MSH binding to the rat MC-3 receptor by agouti peptide was observed, although accurate IC50 values could not be determined because the peptide preparation was not homogeneous (results not shown). Specific competition of &agr;MSH activation of human MC4-R by agouti was previously disclosed (Lu et al., 1994, Nature 371: 799-802).
EXAMPLE 5 Feeding Behavior Effect of Melanocortin Analogue Binding in Brain[0133] The results shown in Example 4 above suggested a role in the regulation of feeding behavior in mammalian brain for MC receptor agonists and antagonists, in view of the antagonist properties of the agouti peptide at the MC-3 and MC-4 receptors. The agouti peptide was known to cause obesity when expressed ectopically in the mouse, and has been found to be a high affinity antagonist of the melanocyte stimulating hormone receptor (MC1-R) and of the hypothalamic MC4 receptor (see Lu et al., ibid.). The former activity explained the inhibitory effect of the agouti peptide on eumelanin pigment synthesis. Similarly, it was hypothesized by the inventors that agouti causes obesity in mice by antagonizing hypothalamic MC4 receptors. The cyclic melanocortin analogue, SHU9119, having been shown herein and elsewhere (Hruby et al.) to be a specific, high affinity antagonist of the central MC-3 and MC4 receptors, was tested for the effect of direct administration to mouse brain on feeding behavior in the animals. Intracerebroventricular (ICV) administration of SHU9119 was performed to avoid any complications caused by inhibition of peptide traverse of the blood-brain barrier.
[0134] Briefly, male C57B1/6J mice (18-29 g) were maintained on a normal 12 hr/12 hr light dark cycle with food (Purina mouse chow) and water ad libitum. Animals were housed individually for 24 hrs, distributed into experimental and control groups, avoiding any bias as a function of prior weight, then injected with vehicle or vehicle plus drug just prior to the onset of a 12 hr light or dark cycle. Fasted animals were deprived of food from 18:00 to 10:30 hrs to stimulate feeding during the daytime experimental period. Animals were lightly anesthetized with halothane, and administered into one lateral ventricle 2 &mgr;L of a solution of artificial cerebrospinal fluid alone (acsf, comprising 130 mM NaCl, 27 mM NaHCO3, 1.2 mM Na2HPO4, 0.3 mM NaH2PO4, 0.5 mM Na2SO4, 1.0 mM CaCl2, 1.0 mM MgCl2, and 2.5 mM KCl), or 6 nmol SHU9119 in acsf Freehand injections were performed as described by Laursen and Belknap (1986, J. Pharmacol. Methods 16: 355-357) with some modifications. A 10 &mgr;l luertip syringe (Hamilton 701LT) was fitted with a 0.5 inch 27 gauge needle. Stiff Tygon tubing was slipped over the needle to expose 3 mM of the needle tip. The syringe was held at a 45° angle from the front of the skull with the bevel facing up. The coronal suture was found by lightly rubbing the needle over the skull. Maintaining the 45° angle, the needle was then inserted 1-2 mm lateral to the midline, using only mild pressure to insert and remove the needle. The compounds indicated in a 2 &mgr;l volume of acsf were administered slowly over approximately 15 seconds, and the needle removed after 35 seconds. Animals were allowed to recover from anesthesia and placed into a cage containing a premeasured quantity of food pellets in a spill-free cup. Moribund animals were not included in the study.
[0135] Stimulation of feeding by intracerebroventricular administration of the melanocortin antagonist SHU9119 is shown in FIGS. 19A through 19C. Curves show cumulative food intake as a function of time following administration of the substances shown. FIG. 19A shows stimulation of feeding by administration of SHU9119 just prior to lights off(19:00 hrs) to C57B1/6J mice fed ad libidum. FIG. 19B, in contrast, shows no effect of morning (10:00 hrs) SHU9119 administration in C57B1/6J mice fed ad libidum. FIG. 19C illustrates stimulation of daytime feeding by SHU9119 administration in fasted C57B1/6J mice. In deriving the data points comprising these Figures, food remaining was briefly removed and weighted at the time intervals indicated. Data points indicate the mean and bars indicate standard error. Significance of the effect over time was determined by ANOVA with repeated measures. Significance of drug effects at individual time points was determined by two-way ANOVA, and is indicated in each Figure (***=P<0.001, **=P<0.01, *=P<0.05).
[0136] These results demonstrated that ICV administration of SHU9119 into one lateral ventricle of the C57B1/6J mouse just prior to lights out led to a mean 60% increase in food intake over 12 hrs (FIG. 19A; P<0.005). In contrast, daytime food intake in animals fed ad libidum was not stimulated by administration of SHU9119 (FIG. 19B). SHU9119-treatment did, however, significantly stimulate daytime food intake in animals fasted for 16 hrs prior to the experiment (FIG. 19C; P<0.001). Stimulation of feeding was evident at approximately two hrs post-treatment, and continued for 12 hrs, to produce a mean 34% in food intake relative to vehicle-injected controls.
[0137] These results supported the hypothesis that agouti or SHU9119 stimulate feeding by antagonizing MC receptors in the central nervous system. To further test this hypothesis, a series of experiments were performed wherein MC receptor agonists were administered to animals primed by fasting to eat, to determine whether feeding in such animals could be inhibited by the MC receptor agonists. Animals were induced to feed by food deprivation for 16 h prior to ICV administration of the non-specific melanocortin agonist MTII. In these experiments, ICV injections in male C57B1/6J mice (20-30 g) and the measurement of food intake were as described above.
[0138] Results of these experiments are shown in FIGS. 20A through 20C. In comparison to vehicle-injected animals, MTII was found to produce a potent inhibition of feeding within one hour after administration (FIG. 20A) in a dose-responsive manner. Food intake was significantly inhibited for up to four hours following administration (P<0.001) at the highest dose administered (3 nmol), and decreased food intake continued for the next four hours with normal rates of food intake resuming at about 8 hours after treatment. This dose-responsive inhibition of feeding had an IC50 at the two hour time point of approximately 0.5 nmol (FIG. 20C). However, inhibition of feeding with 3 nmol MTII was completely blocked by co-administration of 6 nmol SHU9119 (FIG. 20B; P<0.001), demonstrating that the effect results specifically from agonist binding to the MC4 and/or MC-3 receptor.
[0139] Locomotor assays were performed to determine whether the effects on feeding behavior observed in these mice were secondary to generalized behavioral effects caused by administration of these melanocortin analogues. The effects of MTII on locomotor activity were tested by placing vehicle or MTII-treated mice in sound and light-proof cages containing multiple light beam detectors. These assays were performed by first injecting 3 nmol MTII or acsf as described above. At three hours (2:45-3:25) post-injection, 12 mice were placed into 12 separate boxes containing multiple infrared light sources and photodetectors. The boxes were contained within separate ventilated light and sound attenuating chambers (Coulbourn model E10-20). Disruption of the infrared beams, with a 10 msec. resolution, was tallied independently for each one minute time period in each cage. The results of these assays are shown in FIG. 20D. Data points indicate the mean total activity (# of light breaks) for 6 animals in each experimental group. Four way Anova statistical analysis was used to analyze the data, and indicated an absence of a significant difference among the two groups.
[0140] Inhibition of feeding by MTII could not be explained by any apparent behavioral abnormalities, or any effect on arousal or locomotor activity. MTII-treated animals appeared alert and exhibited no unusual behavior relative to controls. At approximately three hours after ICV administration, MTII-treated animals exhibited locomotor activity that was indistinguishable from vehicle-treated animals (FIG. 20D). The higher initial activity, indicative of exploratory behavior, and continued locomotion over a 15 min period was indistinguishable between the two groups, indicating that the inhibition of feeding was not due to decreased locomotion or decreased arousal.
[0141] The administration of MTII also inhibited food intake in three other models of hyperphagia: the C57B1/6J-Lepob mouse, C57BI/6J-AY mouse, and NPY-injected C57B1/6J mouse. FIG. 21A shows inhibition of feeding by intracerebroventricular administration of MTII in AY mice (females, 19-28 gms). FIG. 21B shows inhibition of feeding by intracerebroventricular administration of MTII in C57B 1/6J mice (females, 21-25 gm) stimulated to feed by co-administration of neuropeptide Y. FIG. 21C shows inhibition of feeding by intracerebroventricular administration of the MTII in ob/ob mice (females, 48-69 gms). FIG. 21D shows inhibition of feeding in ob/ob mice intraperitoneal administration of MTII (females, 40-45 gms). ICV injections and measurement of food intake was performed as described above, with the exception of NPY treated animals, which were not fasted prior to experimentation. Animals treated intraperitoneally received 100 &mgr;l of a 1 mM solution of MTII in saline, and vehicle injections consisted of the same value of saline alone. Significance indicated for individual time points, determined as described above, was for 3 nmol MTH vs. acsf(FIG. 21A), 1.18 nmol NPY vs. 1.18 nmol NPY+3 nmol MTII (FIG. 21B), 3 nmol MTII vs. acsf (FIG. 21C), and 100 nmol MTII vs. saline (FIG. 21D).
[0142] The hyperphagia in these models can be clearly seen by comparing the 12 hr food intake following a fast in vehicle-injected C57B1/6J (2.4 g, FIG. 19A), C57B1/6J-AY (3.7 g, FIG. 21A) and C57B1/6J-Lepob (3.7 g. FIG. 21C) animals. As expected, MTII treatment inhibited food intake following a 16 hr fast in the C57B1/6J-AY mouse (FIG. 21A; P<0.05). Interestingly, while food intake for the first four hours is significantly inhibited relative to vehicle-injected animals, it is also significantly less inhibited in the C57B1/6J-AY animal than in the C57B1/6J animal given the same 3 nmol dose (compare, FIG. 20A versus FIG. 21A, 1-4 hrs; P<0.001). The decreased effectiveness of the agonist in the presence of the AY allele is consistent with the proposal that his allele results in chronic expression of agouti peptide melanocortin antagonist in the brain.
[0143] MTII, upon co-administration, also significantly inhibited the profound stimulation of feeding induced by neuropeptide Y (NPY), measured over a three hr period (FIG. 21C; P<0.005). Co-administration of an approximately two-fold molar excess of MTII produced a 74% inhibition of NPY-stimulated food intake at the three hour time point.
[0144] Finally, MTII also inhibited hyperphagia due to absence of leptin in the C57B1/6J-Lepob mouse (FIG. 21C; P<0.001). MTII blocked feeding as potently (FIG. 20A), in contrast to the less potent inhibition described above for the C57B1/6J-AY mouse.
[0145] The C57B/16J-Lepob animal was also used to test the ability of MTII to regulate feeding when administered peripherally. Moderate doses (100 nmol) of MTII were inhibited feeding in the C57B1/6J-Lepob mouse (P<0.00) while low doses (10 nmol) did not (date not shown). The kinetics were similar to those seen with ICV administration, with a potent inhibition of feeding for the first four hours. The 100-fold higher dose required peripherally, as well as the similar kinetics suggest a primarily central mechanism of action of MTII.
[0146] These data show that melanocortinergic neurons exert a tonic inhibition of feeding behavior, and that disruption of this signal leads to hyperphagia. With regard to the recently-discovered leptin hormone made by adipocytes, which is generally expressed at elevated levels in obese humans and rodents (such as the C57B1/6J-Lepob animal), the regulatory defect is understood to be an incapacity to respond properly to the leptin hormone signal. The instant results indicate that the melanocortins act independently, or physiologically “downstream,” from the leptin hormone/receptor interaction, because it has been shown herein that melanocortin receptor agonists can potently inhibit feeding in the C57B1/6J-Lepob animal.
[0147] These results suggest that MC receptor agonists and antagonists can affect mammalian feeding behavior, and provide a means for determining candidate compounds for the development of effective pharmacological products directed towards alleviating such human ailments as obesity, anorexia and cachexia.
EXAMPLE 6 Use of MC Receptor-Expressing Recombinant Cells for Screening Compounds that Affect Feeding Behavior in Mammals[0148] The results obtained in Example 6 indicated that cells expressing a variety of mammalian melanocortin receptors are useful for characterizing compounds as a first step towards developing MC receptor agonists and antagonists for controlling feeding behavior in mammals, particularly obesity and overweight disorders in general, as well as anorexia, cachexia and other failure-to-thrive disorders.
[0149] A panel of mammalian melanocortin receptor-expressing recombinant cells are provided as described above in Example 3, wherein each member of the panel comprises appropriate mammalian cells, such as human 293 cells, comprising a recombinant expression construct encoding the MC-1, MC-2 (ACTH), MC-3, MC-4 or MC-5 receptor, the panel constructed to comprise cells functionally expressing each of these MC receptor proteins.
[0150] The panel is used as follows. Receptor agonist activity is assayed by transient or stable expression of a protein which produces a metabolite reporter molecule in response to receptor binding by any of the MC receptor proteins. An example of such a reporter system is the recombinant expression construct described in Example 4, wherein cAMP responsive elements (CREs) are operatively linked to bacterially-derived &bgr;-galactosidase (&bgr;-gal) gene. In the event of receptor binding, cAMP is produced in the mammalian cell, and the CRE induces &bgr;-gal expression. When co-incubated with a colorless substrate for P-gal, receptor binding results in conversion of the colorless substrate to a blue-colored product, which can be easily scored visually or spectrophotometrically. Alternative reporter genes, such a luciferase, can also be used as reporter systems, provided that expression of the reporter molecule-producing protein is functionally linked to receptor binding of a test compound. Alternatively, cAMP production resulting from MC receptor binding can also be measured directly. Additionally, the cell panel or membranes from these cells can be used for direct radioligand binding assays.
[0151] Assay panels are arranged so that agonist activity can be identified, quantitated and correlated with expression of each MC receptor. Automated assays using such panels are also envisioned, whereby the qualitative and quantitative detection of a reporter metabolite is detected in an array (such as a 96-well tissue culture plate) and the data collected and assembled into a computer database or other analytical program.
[0152] Antagonist activity is detected by a modification of the above assay. In this assay, the inhibition of production of an amount of a known receptor agonist, specific for each receptor, is assayed in the presence of a putative antagonist compound. Production of metabolite reporter molecules and their qualitative and quantitative detection is achieved as described above, and the specificity and potency of each antagonist compound characterized with regard to the degree of inhibition achieved for each receptor.
[0153] In view of the instant disclosure, MC-3/MC4 receptor antagonists are expected to be useful to inhibit food intake in a hungry animal, and MC-3/MC-4 receptor agonists are expected to be useful to increase food intake in an animal. Alternative patterns of feeding behavior associated with different patterns of MC receptor agonist/antagonist activity can be determined using this assay.
[0154] Compounds having agonist or antagonist activity with the MC-3 or MC-4 receptors detected using this assay are further screened in vivo to determine whether the observed receptor binding activity results in modification of feeding behavior when administered to an animal. In these assays, the MC receptor binding compounds detected using the assay are administered intracerebroventricularly as described above in Example 5 to animals after an overnight fast, to waking animals, or to animals that are not otherwise primed to be hungry. Feeding and locomotor activity is monitored in these animals, and compounds affecting eating behavior (either by inhibiting feeding in otherwise hungry animals or stimulating feeding in otherwise sated animals) are selected for further development.
[0155] In addition, systemic administration of compounds found to be active by ICV administration assays is used to screen such compounds for the ability to cross the blood-brain barrier. Such compounds are also useful as templates for modifications aimed at increasing the availability of these compounds in the brain after systemic administration, for increasing bioactivity, or both.
[0156] It should be understood that the foregoing disclosure emphasizes certain specific embodiments of the invention and that all modifications or alternatives equivalent thereto are within the spirit and scope of the invention as set forth in the appended claims.
Claims
1. A compound having the general formula:
- A-B-C-D-E-F-G-amide
- wherein
- A is Leu, Ile, Met, or isosteres thereof;
- B is Asp, Glu, or substituted analogues thereof;
- C is His or isosteres thereof;
- D is d-Phe, d-Tyr or isosteres thereof;
- E is Arg, Lys, homoArg, homoLys, or isosteres thereof;
- F is Trp or isosteres thereof;
- G is Lys, homoLys or isosteres thereof;
- and wherein the peptide is cyclized by the formation of an amide bond between the side chain carboxyl group of the Asp or Glu residue at position B in the peptide, and the side chain amino group of the Lys or homoLys residue at position G, and wherein the compound is a mammmalian melanocortin MC-3 or MC-4 receptor agonist.
2. A compound having the general formula:
- A-B-C-D-E-F-G-amide
- wherein
- A is Leu, Ile, Met, or isosteres thereof;
- B is Asp, Glu, or isosteres thereof;
- C is His, or isosteres thereof;
- D is d-Nal, or isosteres thereof;
- E is Arg, Lys, homoArg, homoLys, or isosteres thereof;
- F is Trp, or isosteres thereof;
- G is Lys, homoLys, or isosteres thereof;
- and wherein the peptide is cyclized by the formation of an amide bond between the side chain carboxyl group of the Asp or Glu residue at position B in the peptide, and the side chain amino group of the Lys or homoLys residue at position G, and wherein the compound is a mammalian melanocortin MC-3 or MC-4 receptor antagonist.
Type: Application
Filed: Nov 5, 2002
Publication Date: Jun 5, 2003
Inventors: Roger D. Cone (Oregon City, OR), Wei Fan (Portland, OR), Bruce A. Boston (Lake Oswego, OR), Robert A. Kesterton (Portland, OR), Dongsi Lu (Beaverton, OR), Wenbiao Chen (Portland, OR)
Application Number: 10288160
International Classification: A61K038/08; C07K007/06;
