RESUMO
UNLABELLED: Transgenic mice expressing dominant-negative retinoic acid receptor (RAR) α specifically in the liver exhibit steatohepatitis, which leads to the development of liver tumors. Although the cause of steatohepatitis in these mice is unknown, diminished hepatic expression of insulin-like growth factor-1 suggests that insulin resistance may be involved. In the present study, we examined the effects of retinoids on insulin resistance in mice to gain further insight into the mechanisms responsible for this condition. Dietary administration of all-trans-retinoic acid (ATRA) significantly improved insulin sensitivity in C57BL/6J mice, which served as a model for high-fat, high-fructose diet-induced nonalcoholic fatty liver disease (NAFLD). The same effect was observed in genetically insulin-resistant KK-A(y) mice, occurring in concert with activation of leptin-signaling pathway proteins, including signal transducer and activator of transcription 3 (STAT3) and Janus kinase 2. However, such an effect was not observed in leptin-deficient ob/ob mice. ATRA treatment significantly up-regulated leptin receptor (LEPR) expression in the livers of NAFLD mice. In agreement with these observations, in vitro experiments showed that in the presence of leptin, ATRA directly induced LEPR gene expression through RARα, resulting in enhancement of STAT3 and insulin-induced insulin receptor substrate 1 phosphorylation. A selective RARα/ß agonist, Am80, also enhanced hepatic LEPR expression and STAT3 phosphorylation and ameliorated insulin resistance in KK-A(y) mice. CONCLUSION: We discovered an unrecognized mechanism of retinoid action for the activation of hepatic leptin signaling, which resulted in enhanced insulin sensitivity in two mouse models of insulin resistance. Our data suggest that retinoids might have potential for treating NAFLD associated with insulin resistance.