Expression of leptin and β3-adrenergic receptors in rat adipose tissue in altered thyroid states

The level of leptin [the obese (ob) gene product] mRNA is markedly elevated in hypothyroid male rats. The administration of tri-iodothyronine (T₃) to hypothyroid rats resulted in a 40% decrease in leptin mRNA at 8 h. This decrease in leptin mRNA was associated with a parallel decline in circulating leptin levels of about 50% at 24 h. Conversely, β₃-adrenergic receptor mRNA levels were markedly decreased in epididymal adipose tissue from hypothyroid rats. T₃ administration resulted in a 147% increase at 12 h in β₃-adrenergic receptor mRNA. There was a corresponding increase due to T₃ in the lipolytic response to the specific β₃-adrenergic agonist CL 316,243 that paralleled the increase in β₃-adrenergic receptor mRNA. T₃-mediated changes in leptin and β₃-adrenergic receptor mRNAs were blocked by cycloheximide, suggesting the involvement of short-lived proteins in these effects. The present results indicate that T₃ has opposite effects to those of insulin on the white adipose tissue of rats with respect to leptin mRNA expression.