RAT HEPATOMA L35 CELLS, A LIVER-DIFFERENTIATED CELL-LINE, DISPLAY RESISTANCE TO BILE-ACID REPRESSION OF CHOLESTEROL 7-ALPHA-HYDROXYLASE

A stable hepatoma cell line (L35 cells) showing an activation of the c holesterol 7 alpha-hydroxylase gene (CYP7) that had been silent in the parental hepatoma cell line (H35 cells) was used to examine the influ ence of bile acids on its gene expression and activity. L35 cells were found to concentrate taurocholate from the culture medium, without an y significant effect on the expression of 7 alpha-hydroxylase. At phys iologic levels (up to 100 mu M), CYP7 mRNA expression was not represse d by any bile acid. At supra-physiologic levels (1 mM), the more hydro phobic dihydroxy bile acids, taurodeoxycholate and taurochenodeoxychol ate, decreased CYP7 mRNA without decreasing the relative abundance of beta-actin mRNA. Similar results were obtained by culturing cells with sodium dodecylsulfate (50 mu M). The medium of L35 cells treated with either taurochenodeoxycholate (1 mM), taurodeoxycholate (1 mM), or so dium dodecylsulfate (50 mu M) contained significantly greater activiti es of two cytosolic enzymes, lactate dehydrogenase and phosphoglucose isomerase, indicating a cytotoxic response. Activation of protein kina se C by phorbol esters decreased the expression of 7 alpha-hydroxylase mRNA without evidence of cytotoxicity; therefore, the inability of L3 5 cells to show bile acid repression cannot be ascribed to a lack of a n effect by this secondary messenger system. In addition, insulin decr eased and dexamethasone increased 7 alpha-hydroxylase mRNA without inc reasing the release of the cytoplasmic enzyme markers. The combined da ta suggest that L35 cells are resistant to repression of CYP7 gene exp ression by bile acids, but display physiologic expression to hormones and protein kinase C activation.