ROLES OF PKC ISOFORMS IN COLONIC CARCINOGENESIS

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Protein kinase C (PKC), family of lipid-dependent serine/threonine kinases, is intimately involved in cell growth. Rat colonocytes express the Ca2+-dependent isoforms, PKC-alpha and -BetaII, and Ca2+-independent isoforms, PKC-delta, -epsilon, and zeta. Changes in PKC occur in colonic premalignancy in humans and experimental animals, suggesting a pathogenic role for these kinases. The specific isoforms involved are, however, unknown. Azoxymethane (AOM), a colonic carcinogen, with a predictable premalignant phase and high tumor incidence, decreased PKC-alpha, -delta and -zeta, while increasing PKC-BetaII in rodent tumors. As identical alterations in specific PKC isoforms occur in human colonic carcinomas and AOM-induced tumors, the AOM model is well suited to examine the roles of PKC isozymes in human colonic carcinogenesis. Bile salts, known activators of PKC, have been found to promote colonic tumor formation. Recently, however, two bile salts were found to exert differential effects on the incidence of AOM-induced tumors, with cholate promoting and ursodeoxycholate inhibiting the tumorigenic actions of AOM. Concomitantly, supplementation with these bile acids caused opposite effects on PKC-BetaII and -zeta in AOM-induced tumors, increasing their particulate-association and decreasing their expression in the cholate, but not ursodeoxycholate group, implicating them in the differential effects of bile salts in colonic carcinogenesis. We hypothesize that alterations in PKC-alpha, -BetaII, -delta and/or - zeta, lead to specific changes in their downstream effectors [Raf1, MAP kinase kinase (MAPKK) and MAP kinase (MAPK)] that are involved in the pathogenesis of AOM-induced tumors. To explore this hypothesis, we will investigate alterations in these signal transduction mediators during the premalignant stage of this model. Furthermore, we speculate that the effects of dietary bile salts on colonic carcinogenesis in the AOM model are mediated by their differential effects on PKC-BetaII and/or -zeta. The proposed studies will, therefore, address the following specific aims: I. To investigate the roles of specific isoforms of PKC, particularly alpha, BetaII, delta and zeta, and downstream effectors of PKC, including Raf1, MAPKK and MAPK, in AOM-induced colonic carcinogenesis. II. To investigate the roles of specific isoforms of PKC, particularly BetaII and zeta, and their downstream effectors, in the ability of bile salts to promote or inhibit AOM-induced colonic tumors. III. To begin to define the roles of specific isoforms of PKC involved in colonic carcinogenesis by characterizing their functional consequences on cell growth and tumorigenicity. Stable transfectants of Caco-2 cells, expressing sense or antisense cDNAs for targeted PKC isoforms, particularly PKC-~alpha, BetaII, -delta and -zeta, will be examined for phenotypic changes in cell growth, and alterations in downstream effectors of PKC.