Regulation of hepatic triglyceride lipase by thyroid hormone in HepG2 cells

Hypothyroidism has been reported to be associated with reduced hepatic triglyceride lipase (HTGL) activity. In order to understand the molecular mechanism by which thyroid hormone regulates HTGL activity, effects of triiodothyronine (T3) on HTGL activity, mRNA level, transcription run-on activity, and protein synthetic rate were studied in HepG2 cells. HepG2 cells treated with 1 nM T3 showed an increase in HTGL activity that was first detected at 24 h; HTGL activity continued to increase at 36 h and stayed at the elevated level at 48 and 60 h. At maximal stimulation (48 h), T3-treated cells had the following HTGL activities: 155% in spontaneously released (SR) and 224% in heparin-releasable (HR) HTGL activities (mean levels compared to control). Stimulation of HTGL activity by T3 was dose-dependent and saturable. There was, however, no change in HTGL mRNA level throughout the course of T3 treatment. The effects of T3 were reduced when transcription was blocked by actinomycin D (mean level compared to actinomycin D treatment in the absence of T3: 109% in SR and 127% in HR activities) or translation was blocked by cycloheximide (127% in SR and 122% in HR activities), but HTGL activities were still significantly higher than control. Nuclear run-on assays indicate that T3 did not change the rate of transcription of the HTGL gene. We further determined the rate of HTGL synthesis by measuring the amount of [35S]methionine incorporated into newly synthesized HTGL immunoprecipitated by a monospecific anti-human HTGL antibody. We found that the T3-stimulated increase in HTGL activity was not accompanied by any change in the rate of HTGL biosynthesis. Our experimental data indicate that the T3 stimulation of HTGL activity in HepG2 cells is mediated at posttranscriptional and posttranslational levels. The partial but significant inhibition of the T3 stimulation of HTGL activity by actinomycin D and cycloheximide suggests that the effects of T3 may be mediated by other cellular processes that are more directly regulated by the hormone. This study represents the initial report on the mechanism of HTGL activation by physiological concentrations of thyroid hormone.