Triiodothyronine and Adipose Conversion of OB17Preadipocytes: Binding to High Affinity Sites and Effects on Fatty Acid Synthetizing and Esterifying Enzymes

Abstract

Cells of a preadipocyte clonal line (ob 17) isolated from epididymal fat pad of ob/ob mouse possess high-affinity binding sites for triiodothyronine. A single class of sites was found on growing and early confluent cells (K_D 0.14 ± 0.025 nM; 5 000 ± 600 sites per cell). A two-fold increase in the number of T₃ binding sites occurs during adipose conversion, with no significant change in K_D values. The order of potency of structural analogs to compete with ¹²⁵I-T₃ is in favor of nuclear binding sites. A correlation was obtained between this order of potency and the ability of the analogs, included on a long-term basis to confluent cells, to increase ¹⁴C-acetate incorporation into lipids, suggesting an enhancement of de novo fatty acid synthesis. This hypothesis was supported by increased activity levels of fatty acid synthetase after chronic exposure to 1.5 nM triiodothyronine. Under these conditions activity levels of acid:CoA ligase and glycerol-3-phosphate dehydroge-nase were also increased significantly. Inclusion of bromodeoxyuridine as a differentiation-blocking agent in the culture medium of growing cells decreases drastically the T₃ effects, favoring the role of the latter hormone as amplifier of specific phenotypes expressed during adipose conversion. These results show that ob₁₇ cell line should be an useful tool to study the role of thyroid hormones in the regulation of fatty acid synthesis and esterification in adipose cells.