Effect of tri-iodothyronine on leptin release and leptin mRNA accumulation in rat adipose tissue

Leptin, the product of the obese gene, is produced by white adipocytes. The release of leptin, as well as leptin mRNA content, was enhanced in adipocytes isolated from hypothyroid rats. The administration of tri-iodothyronine (T-3 ) 8 h before death inhibited leptin release by adipocytes incubated for 6 or 24 h. Direct addition of T-3 to pieces of adipose tissue enhanced the loss of leptin mRNA seen over 24 h in the presence of dexamethasone plus the beta(3)-adrenergic agonist Cl 316,243. In contrast, if pieces of adipose tissue were incubated with dexamethasone plus insulin, enhanced the T-3 accumulation of leptin mRNA. These results indicate that T-3 enhances net adipocyte leptin mRNA accumulation in a condition that approximates the fed state (presence of insulin) but inhibits leptin mRNA accumulation in a condition that approximates the fasted state (absence of insulin).