Involvement of c-Src and protein kinase Cδ in the inhibition of Cl-/OH- exchange activity in Caco-2 cells by serotonin

Serotonin (5-hydroxytryptamine (5-HT)) is an important neurotransmitter and intercellular messenger regulating various gastrointestinal functions, including electrolyte transport. To date, however, no information is available with respect to its effects on the human intestinal apical anion exchanger Cl-/OH- (HCO3-). The present studies were therefore undertaken to examine the direct effects of serotonin on OH- gradient-driven 4,4 '-diisothiocyanato-stilbene-2,2 '-disulfonic acid-sensitive Cl-36(-) uptake utilizing the post-confluent transformed human intestinal epithelial cell line Caco-2. Our results demonstrate that serotonin inhibits Cl-/OH- exchange activity in Caco-2 cells via both tyrosine kinase and Ca2+-independent protein kinase C delta-mediated pathways involving either 5-HT3 or 5-HT4 receptor subtype. The data consistent with our inference are as follows. (i) The short term treatment of cells with 5-HT (0.1 mu M) for 15-60 min significantly decreased C-/OH- exchange (50-70%, p < 0.05). (ii) The specific agonists for 5-HT3, m-chlorophenylbiguanide, and 5-HT4, 3-(4-allylpiperazin-1-yl)-2-quinoxaline chloronitrile, mimicked the effects of serotonin. (iii) Tropisetron dual inhibitor for both the 5-HT3/4 receptor subtypes significantly blocked the inhibition, whereas specific 5-HT3 (Y-25130) or 5-HT4 receptor (RS39604) antagonist failed to block the inhibitory effects of 5-HT. (iv) The Ca2+ chelator 1,2-bis(2-aminophenoxy) ethane-N,N,N ',N '- tetraacetic acid tetra( acetoxymethyl ester) had no effect on the serotonin-induced inhibition. (v) The specific protein kinase C (PKC) inhibitors chelerythrine chloride or calphostin C completely blocked the inhibition by 5-HT. (vi) The specific inhibitor for PKC delta, rottlerin, significantly blocked the inhibition by 5-HT. (vii) The specific tyrosine kinase inhibitor, herbimycin, or Src family kinase inhibitor, PP1, abolished the 5-HT-mediated inhibition of Cl-/OH- exchange activity. (viii) 5-HT stimulated tyrosine phosphorylation of c-Src kinase and PKC delta.