The Regulatory Effects Of Thyroid Hormone On The Activity Of 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase

Abstract

The effects of 3, 3', 5-triiodothyronine (T₃) on 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity were evaluated in the C100 baby hamster kidney cell line. Cells cultured in Minimal Essential Medium (MEM) were supplemented with 10% thyroid hormone-depleted fetal bovine serum (THDS-MEM) and had a 70.1% lower level of HMG-CoA reductase activity than the cells grown in a medium supplemented with fetal bovine serum (FBS). When T₃ was added to THDS-MEM, the reduction of the reductase activity was blocked in a dose-dependent manner. In the cells grown in THDS-MEM for 48 hours, T₃ (10^-6 M) treatment rapidly increased HMG-CoA reductase activity, achieving the control level six hours after treatment. Such effects of T₃ were blocked by actinomycin D (5 μg/ml) or cycloheximide (10 μg/ml). The transcriptional rate of the HMG-CoA reductase gene did not change significantly regardless of the presence of T₃, while T₃ inhibited the 25-hydroxycholesterol-mediated decay of the reductase mRNA significantly. Our results show that T₃-dependent regulation of HMG-CoA reductase activity, via the de novo synthesis of the reductase enzyme, seems to be mediated at least partially by the stabilization of HMG-CoA reductase mRNA.