Thyroid hormone modulates apolipoprotein B gene expression in HepG2 cells.

We have investigated the modulation of apolipoprotein B gene expression in HepG2 cells by thyroid hormone. ApoB secretion rate in serum-free media was found to be significantly increased in the presence of the hormone in long-term cultures (48 h, 37%). This stimulatory effect was dose-dependent. The mechanisms underlying the stimulatory effect of triiodothyronine on apoB production were investigated. Triiodothyronine increased apoB mRNA levels by about 25-36% as determined by slot- and Northern-blot analysis of total RNA. ApoB synthesis rate was also found to be increased both in in vivo pulse-chase experiments (61%) and in in vitro translation studies (54.5%). Despite the 54.5-61% increase in apoB synthesis with triiodothyronine, only a 30% increase in apoB secretion was noted suggesting that part of the increase in the intracellular apoB pool may be lost by degradation. Overall, apoB gene expression appears to be modulated by thyroid hormone at both transcriptional and posttranscriptional levels.