Bile acids, non‐phorbol‐ester‐type tumor promoters, stimulate the phosphorylation of protein kinase C substrates in human platelets and colon cell line HT29

Abstract

Protein kinase C (PKC) is the target for a number of tumor promoters. The mechanism underlying the promoting effects of bile acids in colorectal cancer is not understood. We report that sodium deoxycholate (DOC) triggered activation of PKC in physiological conditions. The biphasic effects of DOC upon PKC activation were Ca(2+)-stimulated and did not require phosphatidylserine (PtdSer) as phospholipid co-factor. The optimal rate of activation was obtained at 0.4 mM DOC and reached approximately half the maximal rate of activation obtained in the presence of PtdSer. Similarly to PtdSer, DOC supported diacylglycerol- as well as phorbol-ester-mediated PKC activation. The reciprocal effects of PtdSer and DOC upon PKC in either 0.5 mM CaCl2 or 0.5 mM EGTA suggest that DOC interacts with the phospholipid-binding domain to elicit PKC activation. DOC-supported enzyme activation exhibited substrate specificity different from that of PtdSer-supported enzyme activation. All tested primary and secondary bile acids activated PKC to various extents, with DOC being the most potent. We suggest that amphipathic bile acids acting in a PtdSer-like manner provide the hydrophobic environment required for PKC activation. Treatment of 32P-labeled platelets and colonic cells HT29 Cl.19A with DOC enhanced the phosphorylation of endogenous substrates for PKC. Colonic cells responsive at 50 microM DOC, appeared to be 10-fold more sensitive than platelets. We suggest that direct or indirect activation of PKC by bile acids may account for the promoting effects of these non-phorbol-ester-type tumor promoters.

Opposing effects of butyrate and bile acids on apoptosis of human colon adenoma cells: differential activation of PKC and MAP kinases

“…While bile acids are not mutagenic, they have been shown to act as tumour promoters in animal studies (Narisawa et al, 1974;Mahmoud et al, 1999) and PKC has been identified as their molecular target (Pongracz et al, 1995). We confirm here that the secondary bile acid UDCA was able to activate PKC in whole cells (Huang et al, 1992) and show that this action was restricted to the classical PKC isoenzyme PKC-a. Although UDCA was the focus of this study, we have also shown that other bile acids can inhibit butyrate-induced apoptosis, including CDCA (McMillan et al, 2000), and this bile acid also activates PKC-a (data not shown).…”

“…The tumour-promoting effects of bile acids have been ascribed predominantly to their potent activation of protein kinase C (PKC) (Huang et al, 1992). PKC is a family of 11 isoenzymes that are differentially regulated and play specific roles in the control of cell proliferation, differentiation and apoptosis (Clemens and Trayner, 1992;Deacon et al, 1997).…”

Dihydroxy Bile Acids Activate the Transcription of Cyclooxygenase-2

“…The protein-DNA complex that formed was separated on a 4% polyacrylamide gel as described under "Experimental Procedures." cholic acid or conjugated bile acids (32). The present data therefore suggest a plausible mechanism to explain the promotion of esophageal (42)(43)(44) and colon (24 -27) cancer by bile acids: induction of COX-2.…”

“…Bile acids are known tumor promoters that activate PKC, which may be the mechanism of bile acid-induced carcinogenesis (31)(32)(33). Dihydroxy bile acids are more effective in activating PKC than mono-and trihydroxy bile acids (32).…”

Deoxycholate induces DNA damage and apoptosis in human colon epithelial cells expressing either mutant or wild-type p53

“…Other studies have focused on the ability of secondary bile acids to act as tumor promoters by stimulating signal transduction and cancer-promoting genes. Activation of protein kinase C, which is a common event triggered by many tumor promoters, was reported in cultured colon cells that were exposed to deoxycholic acid [12]. Additionally, activation of the transcription factors, AP-1 [13] and NF-κB [14], as well as the NF-κB -responsive cyclooxygenase-2 (COX-2) gene [15], has been reported.…”