Nuclear triiodothyronine receptor of human adipose tissue

Abstract

l-Triiodothyronine induces malic enzyme in explants from human adipose tissue. Consequently, we looked for the presence of receptors for l-triiodothyronine in nuclei isolated from human adipose tissue. The binding of 125I-triiodothyronine by the nuclei was time- and temperature-dependent. At 37°C binding reached a steady state after 60 minutes. Dithiothreitol enhanced total binding and suppressed nonspecific binding. Scatchard analysis showed the presence of a single class of binding sites. The apparent association constant, K a, was 0.13 ± 0.03 × 10 10 M −1, the maximal binding capacity 2.20 ± 0.81 pmol/mg DNA (mean ± SD, n = 7) and the number of binding sites 8,000/nucleus. l-Triiodothyronine and d-triiodothyronine had equal affinity to the nuclear receptor; triiodothyroacetic acid had three times higher affinity. l- and d-thyroxine had 8% and 12%, respectively, and tetraiodothyroacetic acid had 19% affinity compared to that of l-triiodothyronine. Reverse triiodothyronine was a weak competitor. Digestion of nuclei with micrococcal nuclease abolished specific binding. These results show that nuclei from human white adipose tissue possess high affinity receptors for l-triiodothyronine, which are associated with nuclear chromatin. It is likely that induction of malic enzyme in human adipose tissue by l-triiodothyronine is mediated by the nuclear receptors.