Functional role of extracellular signal-regulated protein kinases in gastric acid secretion.
Epidermal growth factor (EGF) has acute inhibitory and chronic stimulatory effects on gastric acid secretion. Because a cascade of intracellular events culminating in the activation of a family of serine-threonine protein kinases called extracellular signal-regulated protein kinases (ERKs) is known to mediate the actions of EGF, we undertook studies to explore the functional role of the ERKs in gastric acid secretion. ERK2 was immunoprecipitated from cell lysates of highly purified (> 95%) gastric canine parietal cells, and its activity was quantified using in-gel kinase assays. Of the primary gastric secretagogues, carbachol was the most potent inducer of ERK2 activity. Gastrin and EGF had weaker stimulatory effects, whereas no induction was noted in response to histamine. The effect of carbachol appeared to be independent of Ca2+ signaling. PD-98059, a selective inhibitor of the upstream ERK activator mitogen-activated protein kinase/ERK kinase, dose-dependently inhibited both carbachol- and EGF-stimulated ERK2 activity, with a maximal effect observed between 50 and 100 microM. ERKs activation is required for induction of the early gene c-fos via phosphorylation of the transcription factor Elk-1 which binds to the c-fos serum response element (SRE). Carbachol stimulated a two- to threefold induction of luciferase activity in cultured parietal cells transfected with either a SRE-luciferase reporter plasmid or with a chimeric GAL4-ElkC expression vector and the 5 x GAL-luciferase reporter plasmid. To examine the significance of ERK activation in gastric acid secretion, we tested the effect of PD-98059 on carbachol-stimulated uptake of 14C-labeled aminopyrine (AP). Acute inhibition of the ERKs by PD-98059 led to a small increase in AP uptake and a complete reversal of the acute inhibitory effect of EGF on AP uptake induced by either carbachol or histamine. In contrast, exposure of the cells to PD-98059 for 16 h led to a reversal of the chronic stimulatory effect of EGF on AP uptake induced by carbachol. Our data led us to conclude that carbachol induces a cascade of events in parietal cells that results in ERK activation. Although the acute effect of the ERKs on gastric acid secretion appears to be inhibitory, the activation of transcription factors and of early gene expression could be responsible for its chronic stimulatory effects.