FUNCTIONAL-ROLE OF EXTRACELLULAR SIGNAL-REGULATED PROTEIN-KINASES IN GASTRIC-ACID SECRETION

Epidermal growth factor (EGF) has acute inhibitory and chronic stimula tory effects on gastric acid secretion. Because a cascade of intracell ular events culminating in the activation of a family of serine-threon ine protein kinases called extracellular signal-regulated protein kina ses (ERKs) is known to mediate the actions of EGF, we undertook studie s to explore the functional role of the ERKs in gastric acid secretion . ERK2 was immunoprecipitated from cell lysates of highly purified (>9 5%) gastric canine parietal cells, and its activity was quantified usi ng in-gel kinase assays. Of the primary gastric secretagogues, carbach ol was the most potent inducer of ERK2 activity. Gastrin and EGF had w eaker stimulatory effects, whereas no induction was noted in response to histamine. The effect of carbachol appeared to be independent of Ca 2+ signaling. PD-98059, a selective inhibitor of the upstream ERK acti vator mitogen-activated protein kinase/ERK kinase, dose-dependently in hibited both carbachol-and EGF-stimulated ERK2 activity, with a maxima l effect observed between 50 and 100 mu M. ERKs activation is required for induction of the early gene c-fos via phosphorylation of the tran scription factor Elk-l which binds to the c-ibs serum response element (SRE). Carbachol stimulated a two-to threefold induction of luciferas e activity in cultured parietal cells transfected with either a SRE-lu ciferase reporter plasmid or with a chimeric GAL4-ElkC expression vect or and the 5xGAL-luciferase reporter plasmid. To examine the significa nce of ERK activation in gastric acid secretion, we tested the effect of PD-98059 on carbachol-stimulated uptake of C-14-labeled aminopyrine (AP). Acute inhibition of the ERKs by PD-98059 led to a small increas e in AP uptake and a complete reversal of the acute inhibitory effect of EGF on AP uptake induced by either carbachol or histamine. In contr ast, exposure of the cells to PD-98059 for 16 h led to a reversal of t he chronic stimulatory effect of EGF on AP uptake induced by carbachol . Our data led us to conclude that carbachol induces a cascade of even ts in parietal cells that results in ERK activation. Although the acut e effect of the ERKs on gastric acid secretion appears to be inhibitor y, the activation of transcription factors and of early gene expressio n could be responsible for its chronic stimulatory effects.