Differential regulation of hepatic triglyceride lipase and 3-hydroxy-3-methylglutaryl-CoA reductase gene expression in a human hepatoma cell line, HepG2.

S J Busch,R L Barnhart,G A Martin,M A Flanagan,R L Jackson

doi:10.1016/s0021-9258(18)45729-8

Abstract

The level of hepatic triglyceride lipase (H-TGL) synthesis and secretion was examined in response to changes in cholesterol biosynthesis in the human hepatoma cell line HepG2. Cells were first fed a lipoprotein-deficient serum-supplemented medium to eliminate exogenous cholesterol. Mevinolin, a 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase inhibitor, was then added at a concentration (37 microM) which inhibited cholesterol biosynthesis by greater than 85% and decreased total cell cholesterol from 36.1 to 27.4 micrograms/ml of cell protein. Mevinolin treatment caused a 4.9 +/- 0.8-fold increase in the amount of H-TGL activity secreted into the medium, a 1.8 +/- 0.4-fold rise in H-TGL-specific mRNA, and a concurrent 14-fold increase in HMG-CoA reductase mRNA. Addition of 1 mM mevalonic acid to normal or mevinolin-treated cells raised the cellular cholesterol content and decreased the amount of secreted H-TGL activity to levels below control values. Mevalonic acid also prevented mevinolin-induction of H-TGL and HMG-CoA reductase mRNA, suggesting a common regulatory step for H-TGL and HMG-CoA reductase. Exposure of cells to mevinolin and 25-hydroxycholesterol together resulted in a marked repression of HMG-CoA reductase mRNA levels, whereas these conditions further enhanced the secretion of H-TGL activity and the expression of H-TGL mRNA. These results demonstrate a differential role for 25-hydroxycholesterol in the regulation of H-TGL and HMG-CoA reductase expression.

Highlights

The level of hepatic triglyceride lipase (H-TGL) synthesis and secretion was examined in response to changes in cholesterol biosynthesis in the human hepatoma cell line HepGZ
The incorporation of [14C]acetate into de rzouo-synthesized cholesterol in HepG2 cells increased by 113% in cells fed with LPDS-supplemented medium in place of fetal bovine serum supplemented medium demonstrating that lipoprotein deprivation induces cholesterol biosynthesis in these cells
Cells were treated with 37 pM mevinolin for 24 h; this caused an inhibition of cholesterol biosynthesis by 85.1 f 2.1%

Summary

Differential Regulation of Hepatic Triglyceride Lipase and

The level of hepatic triglyceride lipase (H-TGL) synthesis and secretion was examined in response to changes in cholesterol biosynthesis in the human hepatoma cell line HepGZ. Exposure of cells to mevinolin and 25-hydroxycholesterol together resulted in a marked repression of HMG-CoA reductase mRNA levels, whereas these conditions further enhanced the secretion of H-TGL activity and the expression of H-TGL mRNA. The receptormediated uptake of LDL is regulated by the cellular content of cholesterol and, like HMG-CoA reductase, LDL receptor gene transcription is repressed as cellular cholesterol increases [5]. Inhibition of cholesterol biosynthesis with mevinOlin, a potent inhibitor of HMG-CoA reductase [6], results in a marked increase in the expression of the genes for both HMG-CoA reductase and the LDL receptor. The fact that H-TGL can contribute to cellular cholesterol content may suggest that it is regulated to HMG-CoA reductase and the LDL receptor. Under conditions which perturb cellular cholesterol homeostasis, we demonstrate that both H-TGL mRNA levels and secreted enzymatic activities are affected

PROCEDURES

RESULTS

TABLE I

HMGCoA Reductase

DISCUSSION

HMGCoA Reductose s