CYCLIC-AMP AND CHLORIDE-DEPENDENT REGULATION OF THE APICAL CONSTITUTIVE SECRETORY PATHWAY IN COLONIC EPITHELIAL-CELLS

Epithelial cells of the colonic crypt engage in cAMP-mediated fluid an d electrolyte secretion. In addition to participating in electrolyte t ransport, colonic crypt cells also synthesize and secrete a number of proteins and peptides that play a crucial role in mucosal homeostasis. In the present study me show that cAMP regulates not only electrolyte secretion but also polarized protein secretion in a tissue culture mo del of colonic crypt cells. We found that apical but not basolateral p rotein secretion was stimulated by a physiological activator of the cA MP pathway, vasoactive intestinal peptide, as well as by a cell-permea nt analogue of cAMP (8-(4-chlorophenylthio)cAMP) at concentrations as low as 12.5 mu M. Based on several criteria, we determined that the re gulation of protein secretion by cAMP in HT29-CL19A cells occurs via s timulation of constitutive membrane traffic from the trans-Golgi netwo rk (TGN) to the apical cell surface, In addition, the regulation of ap ical protein secretion by cAMP was Cl--dependent with cAMP inhibiting rather than stimulating secretion in Cl--depleted cells. The locus of cAMP action on the secretory pathway is at least in part at the level of the TGN, where it stimulates the sialylation of alpha 1-antitrypsin (i.e. one of the identified secretory proteins) in addition to the tr affic of secretory proteins from the TGN to the apical cell surface. W e propose that a cyclic AMP and Cl--dependent regulation of TGN acidif ication could modulate both sialylation and secretory vesicle budding at the TGN.