All‐trans‐retinoic acid ameliorates hepatic steatosis in mice by a novel transcriptional cascade

SHP, an orphan nuclear hormone receptor, functions as a corepressor and antagonizes coactivator recruitment to target transcription factors with which it interacts (1, 2). Molecular and genetic studies have suggested that SHP also plays a role in obesity and diabetes (2-6). Supporting this, deletion or overexpression of SHP gene in mice disrupts lipid and glucose homeostasis (7-10). Congenic C57BL/6 SHP−/− mice show a more insulin resistant phenotype in response to a western diet (WestD), but are protected against development of hepatic steatosis and obesity (7, 11). The protection against steatosis was attributed to enhancement of hepatic β-oxidation and reduced expression of fatty acid synthetic genes. Gene expression profiling revealed that PPARγ was markedly down-regulated in SHP−/− mouse liver regardless of type of diets, suggesting that the effect of SHP on PPARγ expression is not a secondary complication of fat accumulation. PPARγ expression is low in normal livers, but is strongly induced in models of nonalcoholic fatty liver diseases (NAFLD). Given its role in adipogenesis, this induction has been considered a key factor contributing to fat accumulation in the livers of animal models and human patients associated with obesity and diabetes (12-16). Indeed, adenoviral overexpression of PPARγ in the liver is sufficient to drive marked steatosis (17-19), and liver specific deletion of PPARγ strongly decreases steatosis in the ob/ob background (13) and high fat-fed mice (14, 15, 18). One proposed mechanism for PPARγ-dependent fat accumulation in ob/ob mice is direct induction of fat specific factor 27 (Fsp27), a lipid droplet binding protein promoting lipid accumulation in adipocytes (20). As observed in white adipose tissue, deletion of Fsp27 decreases the size of lipid droplets in liver and increases lipolysis and fatty acid oxidation due to increased accessibility of lipolytic enzymes and increased availability of non esterified free fatty acids (FFA) without affecting FFA uptake, triglyceride (TG) export, and de novo TG synthesis (20, 21). Thus, decreased PPARγ expression in SHP−/− liver (7, 11) may protect the mice from development of hepatic steatosis at least in part by decreasing Fsp27 expression. Hes6 is a novel member of the family of mammalian homologues of Drosophila hairy and enhancer of split, and was initially identified as an inhibitor of its relative Hes1, a basic helix-loop-helix transcription factor regulating neuronal and muscle differentiation negatively (22). Like SHP, Hes6 alone cannot bind directly to DNA, but represses target gene transcription through protein-protein interaction. Intriguingly, Hes6 was recently identified as a repressor of hepatic PPARγ expression via inhibition of HNF4α transactivation and its expression is directly regulated by HNF4α, which is one of the direct targets of SHP-mediated repression (2, 23). These results suggest that Hes6 may function as a mediator in the regulation of fatty acid synthesis by SHP. In the present study, we discovered a novel transcriptional cascade consisting of RAR, Hes6, HNF4α, and PPARγ in the regulation of hepatic fat mobilization. In the proposed transcriptional cascade, SHP and atRA coordinately regulates transcription of Hes6, and Hes6 subsequently suppresses Pparγ2 expression via repression of HNF4α transcriptional activity. We confirmed the physiologic effects of atRA and RAR on fat mobilization via the cascade using mouse models of fatty liver disease.