[Retinoids as promising treatment for non-alcoholic fatty liver disease]

Non-alcoholic fatty liver disease (NAFLD) is frequently associated with insulin resistance, suggesting its crucial role in the development and progression of NAFLD. We used a mouse model of high-fat, high-fructose (HFHFr) diet-induced NAFLD to examine the effects of retinoids on insulin resistance. Dietary administration of all-trans-retinoic acid (ATRA) significantly improved insulin sensitivity in C57BL/6J mice fed the HFHFr diet, and in KK-Ay mice but not in the leptin-deficient ob/ob mice. ATRA treatment significantly upregulated hepatic leptin receptor (LEPR) expression. In agreement with these observations, in vitro experiments showed ATRA directly induced LEPR gene expression through RARα. In the livers of C57BL/6J mice administered ATRA, insulin receptor substrate-1 (IRS1) was activated concomitantly with the phosphorylation of Janus kinase-2 and signal transducer and activator of transcription-3 (STAT3). STAT3 phosphorylation was also observed in KK-Ay but not in ob/ob mice. In in vitro experiments, ATRA significantly enhanced insulin-induced IRS1 tyrosine phosphorylation solely in the presence of leptin. A selective RARα/β agonist, tamibarotene, also enhanced hepatic LEPR expression, STAT3 phosphorylation, and ameliorated insulin resistance in KK-Ay mice. We discovered an unrecognized mechanism of retinoid action for the activation of hepatic leptin signaling, which resulted in enhancing insulin sensitivity in two mouse models of insulin resistance. Moreover, we also found that retinoids attenuate hepatic iron overload and iron-induced oxidative stress, which have recently emerged as an important factor for the development and progression of insulin resistance. Our data suggest that retinoids might have potential for treating NAFLD associated with insulin resistance.