Hepatocellular protein kinase C activation by bile acids: implications for regulation of cholesterol 7a-hydroxylase
Stravitz, R.Todd; Rao, Y.P.; Vlahcevic, Z.Reno
American Journal of Physiology 271: 93-G303
ISSN/ISBN: 0363-6143 Accession: 009866823
We have recently shown that taurocholate (TCA) represses the transcriptional activity of cholesterol 7a-hydroxylase, the rate-limiting enzyme of the bile acid biosynthetic pathway, through a protein kinase C (PKC)-dependent mechanism in primary cultures of rat hepatocytes. The present studies sought to determine the mechanisms by which bile acids activate hepatic PKC activity and the consequences of this activation on isoform distribution and cholesterol 7a-hydroxylase mRNA levels. TCA (12.5-100 mM for 15 min) increased membrane-associated "classic" isoenzyme cPKC-a and "novel" isoenzymes nPKC-d, and nPKC by two- to sixfold. Membrane-associated PKC progressively increased, and cytosolic PKC decreased, for 1 h after the addition of TCA (50 mM); after 24 h whole cell cPKC-a, nPKC-d, and nPKC were downregulated by 35-55% compared with untreated controls. In a reconstituted assay system, TCA or taurodeoxycholate (10-100 mM) increased calcium-dependent and -independent PKC activity by three- and fourfold, respectively. Taurine-conjugated bile acids stimulated PKC activity in proportion to their hydrophobicity index (r = 0.99). Finally, cholesterol 7a-hydroxylase mRNA was repressed >75% by phorbol 12-myristate 13-acetate (100 nM for 3 h), a nonselective activator of PKC isoforms. In contrast, selective cPKC-a activation with thymeleatoxin (100 nM for 3 h) had no significant effect on cholesterol 7a-hydroxylase mRNA levels. We conclude that bile acids activate hepatocellular PKC, resulting in sequential redistribution and downregulation of calcium-dependent and -independent isoforms. The calcium-independent PKC isoforms may mediate the repression of cholesterol 7a-hydroxylase mRNA by TCA. Reprinted by permission of the publisher.