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We investigated the expression, distribution, and activation parameters of protein kinase C (PKC) isozymes in isolated rat parotid acinar cells. By analyzing cellular extracts by western blot analysis and for isozyme-specific RNA, the Ca(2+)-independent PKC-delta, -epsilon, and -zeta were detected in the cytosolic, particulate (plasma membrane), and nuclear fractions of unstimulated cells, whereas the Ca(2+)-dependent PKC-alpha was confined to the cytosolic and particulate fractions. The expressed isozymes showed distinct responses to phorbol 12-myristate 13-acetate (PMA), thymeleatoxin, and cell surface receptor agonists with respect to translocation from cytosol to particulate fraction and nucleus, as well as sensitivity to down-regulation caused by prolonged exposure to PMA (3-20 hr). The marked susceptibility to down-regulation displayed by PKC-alpha and -delta was accompanied by an enhanced secretory response to norepinephrine as compared with control cells. Further, the selective PKC inhibitors Ro 31-8220 and CGP 41,251 also produced a concentration-dependent enhancement of norepinephrine-induced amylase secretion. Our findings suggest that PKC-alpha or -delta plays a negative modulatory role, rather than an obligatory role, in amylase secretion. Also, the localization and redistribution of PKC-epsilon and -delta to the nucleus by PKC activators imply that one or both of these isozymes may regulate such processes as cellular proliferation and/or differentiation. |
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