Triiodothyronine regulates distribution of thyroid hormone receptors by activating AMP-activated protein kinase in 3T3-L1 adipocytes and induces uncoupling protein-1 expression

The purposes of this study were to examine whether thermogenesis in 3T3-L1 adipocytes is related to variations in thyroid hormone receptors (TRs) that are differently regulated by triiodothyronine (T3), and the possible role of AMP-activated protein (AMPK) in thermogenesis after cell differentiation. Differentiated 3T3-L1 adipocytes were maintained under four conditions: normal control group, T3 treatment group, AMPK agonist (5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside) treatment group, and T3 and AMPK inhibitor (Compound C) treatment group. Real-time polymerase chain reaction was then performed to evaluate the changes in TR alpha and TR beta mRNA levels in the cells, as well as marker genes for brown adipose tissue including uncoupling protein (UCP)-1 and Cidea. Western blotting was carried out for the cells to detect the expressions of TR alpha, TR beta, and AMPK protein levels. After T3 treatment, the mRNA and protein levels of TR alpha decreased compared with the control group, while TR beta mRNA and protein levels increased markedly at the same time. We also found elevated mRNA levels of UCP-1 and Cidea after exposure to T3. However, the distribution of TRs was reversed by Compound C. AMPK protein levels were clearly activated by T3. Our results suggest that the distribution of TRs is related to thermogenesis, and AMPK may participate in the alterations.