Molecular Mechanisms for Growth Factor Action of Gastrin

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Although characterized as a stimulant of gastric acid secretion, the peptide hormone gastrin also exerts growth-promotingeffects on normal and malignantgastrointestinal tissues. While numerous studies have elucidated the mechanisms that are responsible for the stimulatory effect of gastrin on gastric acid secretion,the molecular events that mediate the growth factor action of gastrin have been only partially characterized. Gastrin stimulatesthe growth of rat pancreatic adenocarcinoma cells (AR4-2J) through activationof the ERKs and of the early response gene c-fos. In addition,we have recently observed that gastrin is a potent inhibitor ofcellular apoptosis. Accordingly, we speculate that one of the physiological functions of gastrin is to stimulate both cellular growth and survival. The overall goal of this applicationwill be to dissect the molecular mechanisms that mediate the growth promoting and anti-apoptotic effects of gastrin using pancreatic cancer cells and gastric parietal cells, as our models. In the first specific aim, we will investigate the effect of gastrin on the expression of the early response genes c-fos. For these experiments we will take advantageof luciferase reporter gene constructs comprisingseveral DNA-regulatory elements present in the promoters of the c-fos gene. In particular, we will study the function of the ERKs and of p38 kinase in the regulation of c-fos transcription. Additional experiments will be focused on the role of the small GTP-bindingproteins Ras, Rac, and Cdc 42 in gastrin signalingto c-fos and in the regulation of AR4-2J cell growth. For these experiments we will take advantageof adenoviral mediated gene transfer of dominantnegative mutants of Ras, Rac, and Cdc 42 into the AR4-2J cells, in order to achieve high levels of expression of these GTP-ases. In the second specific aim, we will investigatethe signal transductionpathwaysresponsible for the anti-apoptotic effect of gastrin in both the AR4-2J cells and in the gastric parietal cells. These studies will be focused on the molecular mechanism that regulate gastrin induction of the protein kinase AKT which is known to promote cell survivalin multiple systems. The role of AKT kinase in the anti-apoptotic effect of gastrin will beassessed by adenoviral mediated gene transfer of dominant negative mutantsof AKT. Through these studies, we hope to shed new knowledgenot only in the areas of physiology and cellular and molecular biology, but also in the arena of clinical medicine as, we believe that our studies may contributeto the rationale for application of selective gastrin agonists and antagonistsas therapeutic agents in treatmentof human diseases.