Triiodothyronine (T3)-induced down-regulation of the nuclear T3 receptor in mouse preadipocyte cell lines.

As previously reported, preadipocytes cloned from the epididymal fat of lean or genetically obese mice (HGFu and ob 17, respectively) contain the nuclear T3 receptor. The number of receptor sites was similar in confluent cells of both lines and approximately doubled during adipocyte differentiation. T3 added to the culture medium increased triacylglycerol synthesis. T3 also increased fatty acid synthase specific activity, relative synthesis rate, and relative mRNA content (1.5- to 2.5-fold). Optimal responses were obtained at 1.5 nm. This study shows that under the same culture conditions in both cell lines, 1.5 nm T3 decreased the receptor concentration with no significant change in the affinity for T3. The receptor depletion was time dependent, rapid, stable in the presence of T3, and reversible in less than 24 h after its withdrawal. Receptor depletion was also dependent on T3 concentration and close to maximum at 1.5 nm T3 [45.1 ± 2.7% (±se) of the data values without T3; n = 14]. A linear relationshi...