Induction of malic enzyme in explants of human adipose tissue by L-3,5,3'-triiodothyronine.

The influence of L-3,5,3'-triiodothyronine (triiodothyronine), dexamethasone and insulin on malic enzyme was investigated in cultures of explants of adipose tissue from mammae of healthy female subjects. The enzyme was present mainly in adipose tissue; connective tissue possessed very low activity. After preincubation of explants of adipose tissue from five different subjects for 96 h to deplete endogenous thyroid hormones addition of triiodothyronine (final concentration 1.0 microM) stimulated enzyme activity by 16 to 72%. Dexamethasone (170 nM) increased enzyme activity by 32%; a combination of triiodothyronine and dexamethasone did not lead to an increase beyond that observed for dexamethasone alone. Insulin (final concentration 0.7 I. U./ml) alone stimulated malic enzyme activity minimally, but together with triiodothyronine stimulation was additive. Simultaneous treatment of explants with actinomycin D or cycloheximide and triiodothyronine abolished the stimulatory influence of the latter on malic enzyme. These results suggest that human white adipose tissue is a suitable model to study the effects of triiodothyronine on lipid metabolism. The triiodothyronine stimulated malic enzyme may play an important role in providing reducing equivalents for lipogenesis in human adipose tissue.