Transcriptional and posttranscriptional regulation of rat hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase by thyroid hormones.

The mechanisms by which thyroid hormones increase hepatic 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase mRNA levels were investigated in hypophysectomized rats. Feeding these rats a diet supplemented with 0.5% desiccated porcine thyroid powder resulted in a 5-fold increase in the rate of transcription of the HMG-CoA reductase gene as measured by in vitro "run-on" transcription assays in isolated rat liver nuclei. Time courses of change in reductase mRNA, showing the kinetics of approach to new steady-state levels, indicate that reductase mRNA is also 4-6-fold more stable in thyroid hormone-treated animals than in non-treated animals. Reductase mRNA decayed with a half-life of 2.5 h when mevinolin, a potent inhibitor of HMG-CoA reductase, and colestipol, a bile acid sequesterant, were removed from the diet of hypophysectomized rats. When these drugs were removed from the diet of thyroid hormone-treated hypophysectomized rats, reductase mRNA decayed with a half-life of 15 h. Treating rats with only mevinolin and colestipol increased reductase mRNA levels without stabilizing the mRNA. Administration of cycloheximide to thyroid hormone treated rats rapidly decreased HMG-CoA reductase mRNA levels by destabilizing reductase mRNA and decreasing reductase gene transcription. Cycloheximide treatment had no effect on beta-actin gene transcription or steady state levels of beta-actin mRNA. These results suggest that a short-lived protein(s) may mediate the transcriptional and post-transcriptional effects of thyroid hormones on HMG-CoA reductase mRNA levels.