Protein kinase C-epsilon is the likely mediator of mucin exocytosis in human colonic cell lines.

The phorbol ester, phorbol 12-myristate 13-acetate (PMA), induces mucin secretion in the colonic tumor cell line T84 in a Ca(2+)-independent manner. To determine whether a specific protein kinase C (PKC) isoform is involved in colonic cells, we compared PMA-dependent mucin secretion by three human colonic tumor cell lines (T84, HT-29/A1, and LS 180) with the expression of PKC isoforms alpha, beta, delta, epsilon, and zeta, previously identified in human colon (L. A. Davidson, Y. H. Jiang, J. D. Derr, H. Aukema, J. R. Lupton, and R. S. Chapkin. Arch. Biochem. Biophys. 312:547-553, 1994). In each cell line PMA (10(-7) M) caused mucin secretion within 30 min. PMA-dependent mucin secretion was three to four times greater from HT-29/A1 and T84 cells than from LS 180 cells. All three-cell lines contained mRNA for PKC-alpha, PKC-epsilon, and PKC-zeta but not PKC-beta or -delta. Each cell line also expressed PKC-alpha, -epsilon, and -zeta protein. PKC-epsilon expression (mRNA and protein) was three to four times greater in HT-29/A1 and T84 cells than in LS 180 cells, correlating with PMA-responsive mucin secretion, whereas all cell lines contained similar levels of PKC-alpha mRNA and protein. When cells were stimulated by PMA, only PKC-epsilon was translocated from cytosol to membrane fractions early enough to stimulate mucin secretion. Because PKC-epsilon is also a Ca(2+)-independent isoform, it is likely to mediate mucin exocytosis in colonic cells.