Abstract: The invention relates to vectors and mammalian cells in a system useful for switching on or switching off gene expression in response to pH and gaseous carbon dioxide, taking advantage of signal transduction of the human proton-activated cell-surface receptor TDAG8, OGR1 and/or GPR4 and related receptors to chimeric promoters containing cAMP-response elements. The systems according to the invention can be used to precisely monitor culture pH within the physiologic range, or also to adjust expression by gaseous C02 which shifts the C02-bicarbonate balance towards hydrogen ions. Production of biopharmaceuticals of engineered cells cultivated in a bioreactor setup can be induced by gaseous C02. Also, implanting engineered cells provide novel treatment strategies for acidosis-related disorders and type 1 diabetes.