DISTINCT SIGNALING MEDIATES CHLORIDE SECRETION INDUCED BY TUMOR PROMOTER BILE-SALTS AND PHORBOL ESTERS IN HUMAN COLONIC CELLS

Tumor promoter phorbol esters directly activate protein kinase C (PKC). Similarly tumor promoter bile salts activate PKC in the human colon epithelial cell line HT-29 cl.19A, as well as in platelets. Bile salts were also able to directly activate the purified brain enzyme. We designed experiments to investigate the effects of bile salts on Cl- secretion in HT-29cl.19A cell monolayers and the involvement of PKC in this process. Sodium deoxycholate (DOG) gradually evoked a slow increase of the short-circuit current (I-sc) in the Ussing chamber-mounted cell monolayers. By contrast, 12-O-tetradecanoylphorbol-13-acetate (TPA) elicited a large increase in I-sc which rapidly plateaued. The I-sc response was representative of enhanced Cl- secretion, since it was abrogated in Cl--free medium, and was prevented by Cl- channel blocker diphenylamine-2-carbonylate as well as by bumetanide, a Na+/K+/Cl- transporter inhibitor. Sequential applications of TPA and DOC at suboptimal doses elicited additive effects. Neither DOC nor TPA increased the levels of cAMP in these cells. However DOG, at variance with TPA, increased the intracellular content of inositol trisphosphate, presumably due to triggering of the phosphoinositide pathway. From our studies and others, it is suggested that DOG-induced unspecific activation of some membrane-associated proteins, elicits a rise in Ca2+ from extracellular and internal sources which combines its effects with those due to PKC activation.