MECHANISM OF ACTION OF CHEMOPROTECTIVE URSODEOXYCHOLATE IN THE AZOXYMETHANE MODEL OF RAT COLONIC CARCINOGENESIS - POTENTIAL ROLES OF PROTEIN-KINASE-C-ALPHA, PROTEIN-KINASE-BETA(II) AND PROTEIN-KINASE-ZETA(1)

Several lines of evidence from our laboratory and others indicate that epigenetic alterations in protein kinase C (PKC) are involved in colo nic carcinogenesis in both man and experimental animals. Furthermore, bile salts, known activators of PKC, have also been implicated in colo nic tumor development. Recently, however, our laboratory has demonstra ted that, whereas dietary cholic acid increased the occurrence of azox ymethane (AOM)-induced rat colonic tumors, ursodeoxycholic acid was as sociated with a significant protective effect. In the present studies, we therefore examined changes in PKC isoforms that accompanied AOM-in duced tumor formation and investigated whether the chemopromotional an d/or chemopreventional actions of these supplemental dietary bile salt s involved changes in specific isoforms of PKC. Rats treated with vehi cle (saline) or AOM and maintained on bile salt unsupplemented or supp lemented diets were used to isolate control colonocytes and carcinogen -induced tumors, which were then subjected to subcellular fractionatio n. The homogenates and subcellular fractions mere then probed for indi vidual PKC isoforms by quantitative Western blotting using isoform-spe cific antibodies. Normal rat colonocytes expressed PKC-alpha, -beta(II ), -delta, -epsilon, and -zeta. AOM, in unsupplemented or cholate-supp lemented groups, caused significant down-regulation of PKC-alpha, -del ta and -zeta and up-regulation of PKC-beta(II) while increasing partic ulate PKC-alpha, -beta(II), and -zeta in carcinogen-induced tumors com pared to normal colonocytes. Dietary supplementation with ursodeoxycho hc acid, in marked contrast to these groups, prevented the changes in the subcellular distributions of PKC-alpha, -beta(II), and -zeta, and preserved the expression of PKC-zeta in AOM-induced tumors. These stud ies suggest that changes in specific isoforms of PKC (particularly, PC K-alpha, -beta(II), -delta, and/or -zeta) are involved in colonic mali gnant transformation in the AOM model but do not account for the chemo promotional actions of cholic acid in this model. furthermore, the abi lity of ursodeoxycholic acid to block AOM-induced increases in particu late PKC-alpha, -beta(II), and -zeta, and/or inhibit down-regulation o f PKC-zeta, may contribute to the chemopreventive effects of this bile acid.