Mechanism of Action of Chemoprotective Ursodeoxycholate in the Azoxymethane Model of Rat Colonic Carcinogenesis: Potential Roles of Protein Kinase C-α, -β_II, and -ξ¹

Several lines of evidence from our laboratory and others indicate that epigenetic alterations in protein kinase C (PKC) are involved in colonic carcinogenesis in both man and experimental animals. Furthermore, bile salts, known activators of PKC, have also been implicated in colonic tumor development. Recently, however, our laboratory has demonstrated that, whereas dietary cholic acid increased the occurrence of azoxymethane (AOM)-induced rat colonic tumors, ursodeoxycholic acid was associated with a significant protective effect. In the present studies, we therefore examined changes in PKC isoforms that accompanied AOM-induced tumor formation and investigated whether the chemopromotional and/or chemopreventional actions of these supplemental dietary bile salts involved changes in specific isoforms of PKC. Rats treated with vehicle (saline) or AOM and maintained on bile salt unsupplemented or supplemented diets were used to isolate control colonocytes and carcinogen-induced tumors, which were then subjected to subcellular fractionation. The homogenates and subcellular fractions were then probed for individual PKC isoforms by quantitative Western blotting using isoform-specific antibodies. Normal rat colonocytes expressed PKC-α, -β_II, -δ, -ε, and -ξ. AOM, in unsupplemented or cholate-supplemented groups, caused significant down-regulation of PKC-α, -δ and -ξ and up-regulation of PKC-β_II, while increasing particulate PKC-α, -β_II, and -ξ in carcinogen-induced tumors compared to normal colonocytes. Dietary supplementation with ursodeoxycholic acid, in marked contrast to these groups, prevented the changes in the subcellular distributions of PKC-α, -β_II, and -ξ, and preserved the expression of PKC-ξ in AOM-induced tumors. These studies suggest that changes in specific isoforms of PKC (particularly, PKC-α, -β_II, -δ, and/or -ξ) are involved in colonic malignant transformation in the AOM model but do not account for the chemopromotional actions of cholic acid in this model. Furthermore, the ability of ursodeoxycholic acid to block AOM-induced increases in particulate PKC-α, -β_II, and -ξ, and/or inhibit down-regulation of PKC-ξ, may contribute to the chemopreventive effects of this bile acid.