Hepatocellular protein kinase c activation by bile acids:
implications for regulation of cholesterol 7[alpha]-hydroxylase.
Stravitz, R. Todd, Yi-Ping Rao, Z. Reno Vlahcevic, Emily C. Gurley, W.
David Jarvis, and Phillip B. Hylemon.
Department of Medicine, Divisions of Gastroenterology and
Hematology/Oncology, and Department of Microbiology/Immunology,
Medical College of Virginia, Virginia Commonwealth University, and
McGuire Veterans Affairs Medical Center, Richmond, VA 23298
APStracts 3:0045G, 1996.
We have recently shown that taurocholate represses the transcriptional
activity of cholesterol 7[alpha]-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 7[alpha]-hydroxylase mRNA levels. Taurocholate (12.5-100
[mu]M for 15 min) increased membrane-associated cPKC[alpha],
nPKC[delta], and nPKC by 2-6-fold. Membrane-associated PKC
progressively increased, and cytosolic PKC decreased, for 1 h after
the addition of TCA (50 [mu]M); after 24 h, whole-cell cPKC[alpha],
nPKC[delta], and nPKC , was down-regulated by 35-55% compared to
untreated controls. In a reconstituted assay system, taurocholate or
taurodeoxycholate (10-100 [mu]M) increased calcium-dependent and
-independent PKC activity by 3-fold and 4-fold, respectively. Taurine
-conjugated bile acids stimulated PKC activity in proportion to their
hydrophobicity index (r=0.99). Finally, cholesterol 7[alpha]
-hydroxylase mRNA was repressed >75% by phorbol 12-myristate, 13
-acetate (100 nM for 3 h), an non-selective activator of PKC isoforms.
In contrast, selective cPKC[alpha] activation with thymeleatoxin (100
nM for 3 h) had no significant effect on cholesterol 7[alpha]
-hydroxylase mRNA levels. We conclude that bile acids activate
hepatocellular PKC, resulting in sequential redistribution and down
-regulation of calcium-dependent and -independent isoforms. The
calcium-independent PKC isoforms may mediate the repression of
cholesterol 7[alpha]-hydroxylase mRNA by taurocholate.
Received 10 October 1995; accepted in final form 25 January 1996.
APS Manuscript Number G439-5.
Article publication pending Am. J. Physiol. (Gastrointest. Liver
Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 13 March 96