PHYSIOLOGICAL REGULATION OF GASTRIN GENE EXPRESSION

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Gastrin secreted from antral G cells is the major hormonal regulator of gastric acid secretion. The secretion of gastrin is regulated by stimuli in the gastric lumen acting on the G cell's apical surface and by neurotransmitters released from mucosal nerves acting on the basal surface. Gastric secretions contain many potent regulatory peptides such as Epidermal Growth Factor (EGF). Preliminary studies have shown that EGF stimulates gastrin gene transcription. Mucosal nerves release Acetylcholine (ACH) and Gastrin Releasing Peptide (GRP) which directly stimulate G cells. Gastrin secretion from G cells is also under negative feedback inhibition from gastric acid, this being mediated by the paracrine action of the inhibitory peptide Somatostatin. The proposed studies evaluate the effects of these G cell stimuli (EGF, GRP and ACH) on gastrin gene transcription in gastrin expressing cell lines that have been identified in preliminary studies. The intracellular mechanism through which these extracellular stimuli regulate gastrin gene transcription will also be examined. Preliminary studies indicate that changes in intracellular calcium regulate gastrin gene expression whereas the other second messengers of hormone action, cAMP and Diacylglycerol, had no effect. The role of extracellular calcium flux through voltage sensitive membrane channels in regulating gastrin gene expression will be assessed. To further elucidate the intracellular events regulating gastrin gene transcription, cell lines derived from another gastrin expressing tissue (the pituitary) will also be studied. GH4 cells expressing an integrated gastrin reporter gene will be stimulated with EGF, GRP and ACH to compare effects on gastrin gene transcription with those of other genes such as prolactin. Preliminary studies also show that somatostatin directly inhibits gastrin gene transcription. The intracellular mechanism through which somatostatin inhibits gastrin gene expression will be studied, especially the role of membrane hyperpolarization induced by Gi protein and the consequent decrease in intracellular calcium. The cis acting sequences in the gastrin gene which confer responsiveness to these stimuli will be defined by transient expression assays of deletion mutants. Once identified, the gastrin response element(s) will be examined for transcriptional enhancer properties.