CARBACHOL STIMULATES TRANSACTIVATION OF EPIDERMAL GROWTH-FACTOR RECEPTOR AND MITOGEN-ACTIVATED PROTEIN-KINASE IN T-84 CELLS - IMPLICATIONS FOR CARBACHOL-STIMULATED CHLORIDE SECRETION

We have examined the role of tyrosine phosphorylation in regulation of calcium-dependent chloride secretion across T-84 colonic epithelial c ells. The calcium-mediated agonist carbachol (CCh, 100 mu M) stimulate d a time dependent increase in tyrosine phosphorylation of a range of proteins (with molecular masses ranging up to 180 kDa) in T-84 cells. The tyrosine kinase inhibitor, genistein (5 mu M), significantly poten tiated chloride secretory responses to CCh, indicating a role for CCh- stimulated tyrosine phosphorylation in negative regulation of CCh-stim ulated secretory responses. Further studies revealed that CCh stimulat ed an increase in both phosphorylation and activity of the extracellul ar signal-regulated kinase (ERK) isoforms of mitogen-activated protein kinase, Chloride secretory responses to CCh were also potentiated by the mitogen-activated protein kinase inhibitor, PD98059 (20 mu M), Pho sphorylation of ERK in response to CCh was mimicked by the protein kin ase C (PKC) activator, phorbol myristate acetate (100 nM), but was not altered by the PKC inhibitor GF 109203X (1 mu M). ERK phosphorylation was also induced by epidermal growth factor (EGF) (100 ng/ml), Immuno precipitation/Western blot studies revealed that CCh stimulated tyrosi ne phosphorylation of the EGF receptor (EGFr) and increased co immunop recipitation of the adapter proteins, Shc and Grb2, with the EGFr, An inhibitor of EGFr phosphorylation, tyrphostin AG1478 (1 mu M), reverse d CCh-stimulated phosphorylation of both EGFr and ERK, Tyrphostin AG14 78 also potentiated chloride secretory responses to CCh, We conclude t hat CCh activates ERK in T-84 cells via a mechanism involving transact ivation of the EGFr, and that this pathway constitutes an inhibitory s ignaling pathway by which chloride secretory responses to CCh may be n egatively regulated.