Calcium ionophore treatment impairs the sterol-mediated suppression of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, 3-hydroxy-3-methylglutaryl-coenzyme A synthase, and farnesyl diphosphate synthetase.

Abstract

We report that the sterol-mediated suppression of the mRNA levels of three cholesterogenic enzymes, 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, HMG-CoA synthase, and farnesyl diphosphate (FPP) synthetase is partially overcome by the calcium ionophore A23187. Addition of A23187 to the human monocytic leukemia cell line THP-1 in the presence of fetal calf serum led to rapid increases in mRNA concentration of up to 40-fold for HMG-CoA synthase and 15-fold for HMG-CoA reductase with little or no change in FPP synthetase mRNA levels. Treatment of HepG2 cells with A23187 resulted in approximately 2-4-fold increases in the mRNA levels for these three enzymes. The increases in HMG-CoA synthase and HMG-CoA reductase mRNAs were maximal after treatment of THP-1 cells with 10 micrograms/ml A23187 for 3 h. The stimulation was blocked by actinomycin D but not by cycloheximide treatment. Ionophore treatment had no effect on the half-lives of the mRNAs for HMG-CoA reductase and HMG-CoA synthase. Surprisingly, the addition of A23187 to THP-1 cells incubated in the presence of 25-hydroxycholesterol and mevalonic acid also led to significant increases in the mRNA levels for HMG-CoA reductase and HMG-CoA synthase. Finally, the stimulation of these mRNA levels by A23187 was reduced in cells in which protein kinase C had been inactivated by preincubation of the cells with a phorbol ester. Taken together, these data suggest that A23187 treatment results in increased transcription of HMG-CoA reductase, HMG-CoA synthase, and, in some cell types, FPP synthetase by a mechanism that does not involve de novo protein synthesis. We speculate that A23187 treatment results in the modification of a trans-acting factor(s) which is common for the transcription of all these genes.