Evidence that triiodothyronine decreases rat serum leptin concentration by down-regulation of leptin gene expression in white adipose tissue

Conflicting results have been reported regarding the effect of triiodothyronine (T-3) on serum leptin and adipose tissue leptin gene expression in human and animals. The aim of the present study was to evaluate the effect of administration of increasing doses of T-3 on serum leptin concentration and on leptin mRNA abundance in white adipose tissue of rats. The results presented in this paper indicate that administration of single different doses of T-3 to cuthyroid rats resulted dose dependent increases of serum total T-3 concentrations which are associated with a decrease in white adipose tissue leptin mRNA level. The leptin mRNA level in white adipose tissue was negatively correlated with serum total T-3 concentration (r=-0.8, p < 0.001). Like white adipose tissue leptin mRNA level, serum leptin concentration decreased after T-3 administration, and was also negatively correlated with the serum T-3 concentration (r=-0.8, p < 0.001). In contrast, administration of T-3 to the same rats led to a significant increase in white adipose tissue expression of the malic enzyme gene (malic enzyme activity and malic enzyme mRNA level), a known target gene for T-3. The results indicate that T-3 exerts a selective inhibitory effect on white adipose tissue leptin gene expression in vivo. A conclusion is that T-3 decreases rat serum leptin concentration by down-regulation of leptin gene expression in white adipose tissue. (c) 2006 Elsevier Inc. All rights reserved.