Triiodothyronine increases rat apolipoprotein A‐I synthesis and alters high‐density lipoprotein composition in vivo

Abstract

The effects of altered serum 3,3′,5‐triiodothyronine levels on rat lipoprotein metabolism were examined. Daily injections of the hormone (50 μg/100 g body mass) over a period of six days led to an increase of 6.4‐fold in the hepatic mRNA level for apolipoprotein(apo)A‐I, and a 21% increase in serum apoA‐I levels. 12 h after a single injection of 3,3′,5‐triiodothyronine the rate of [14C]leucine incorporation into apoA‐I increased 2.1 fold. Conversely, in hypothyroid rats there was a decrease in hepatic mRNA levels for apoA‐I and a decreased rate of [14C]leucine incorporation into apoA‐I. The increase in hepatic apoA‐I mRNA levels following 3,3′,5‐triiodothyronine treatment occurred prior to significant changes in serum triacylglycerol levels. High‐density lipoprotein (HDL) particles isolated from the serum of hyperthyroid rats were smaller and enriched in apoA‐I compared to apoA‐IV and apoE. Similar changes in HDL composition were observed following in vitro incubations of normal rat serum with purified rat apoA‐I. The results suggest that during altered thyroid status, changes in serum HDL size and composition occur in association with significant changes in apoA‐I gene expression. Copyright © 1990, Wiley Blackwell. All rights reserved