TRIIODOTHYRONINE INDUCES THE EXPRESSION OF THE UNCOUPLING PROTEIN IN LONG-TERM FETAL-RAT BROWN ADIPOCYTE PRIMARY CULTURES - ROLE OF NUCLEAR THYROID-HORMONE RECEPTOR EXPRESSION

Fetal rat brown adipocytes at the beginning of culture showed a minimal T-3 nuclear binding capacity and very low expression of the c-erbA genes, with low steady state levels of the mRNA forms beta-type and 5.5- and 2.6-kilobase (kb) alpha-types. The levels of these mRNA species increased after 7 days in culture in the presence of thyroid hormone-depleted serum; however, no significant increase in nuclear T-3-binding capacity was observed. The addition of T-3 incremented the abundance of the c-erbA beta-mRNA, induced the appearance of the 6.6-kb c-erbA alpha-mRNA and increased the nuclear T-3-binding capacity by 30-fold. Parallel analysis of the uncoupling protein (UCP) mRNA, immunoreactive UCP, and functional UCP (detected by its ability to bind CDP) demonstrated that T-3 induced the expression of the UCP gene in long term treated fetal brown adipocytes in culture. The presence of noradrenaline, a positive control for UCP expression, increased only the level of expression of the 5.5-kb c-erbA alpha-mRNA, but values for nuclear T-3-binding capacity similar to those obtained in T-3-treated cells were observed. Simultaneous addition of both hormones gave a pattern of expression of the c-erbA-mRNA forms similar to those obtained with these agents individually, and no further increment in their separate effects was observed on the nuclear T-3-binding capacity and UCP expression.