Nuclear Triiodothyronine Receptor in Differentiating Preadipocytes Cloned from Obese and Lean Mice*

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Abstract
The nuclear T3 [triiodothyronine] receptor was characterized in 2 T3-responsive preadipocyte cell lines cloned from the epididymal fat pads of ob/ob adult mice (ob-17 cells) and their lean counterpart (HGFu cells). Isolated nuclei from confluent or differentiating cells bound [125I]T3 to one class of high affinity sites exhibiting kinetic properties, stereospecificity, and salt extractibility of the nuclear T3 receptor. The solubilized T3 binding sites behave like the hepatic nuclear T3 receptor considering physicochemical and DNA binding properties. At confluence, no significant difference could be detected in equilibrium apparent affinity constant (Ka) and maximum binding capacity (MBC) for T3 whether nuclei were prepared from cells originating from ob/ob mice [Ka: 1.7 .+-. (SE) 0.5 .RTM. 1010) M-1; MBC: 432 .+-. 29 fmol/mg DNA] or from lean mice (Ka: 1.6 .+-. 0.2 .times. 1010) M-1; MBC: 487 .+-. 39 fmol/mg DNA). MBC values were in the range found in several T3-responsive tissues. The primary defect in ob/ob mice is probably not at the level of the nuclear T3 receptor. Furthermore, during differentiation into adipose cells in both cell series, and roughly paralleling the amplifying effect of T3 on several lipogenic enzymes in the course of their development, the nuclear T3 receptor concentration significantly increased, attaining about twice the initial values after completion of the differentiation without any significant change in the affinity for T3.