Nonalcoholic fatty liver disease (NAFLD) is characterized by hepatic steatosis, which may progress to nonalcoholic steatohepatitis (NASH) with or without fibrosis. Interestingly, steatosis, even without coexisting fibrosis, may facilitate development of hepatocellular carcinoma. While the exact mechanisms underlying the pathogenesis of hepatic steatosis and its progression to NASH remain poorly understood, nuclear receptor signaling is now in the spotlight of NAFLD research.
Insulin resistance (IR) and hepatic fatty acid turnover are sentinel features of the metabolic syndrome and contribute to NASH progression. IR as well as fatty acid metabolism are regulated in part by the actions of peroxisome proliferator activated receptors (PPARs). PPARα induces enzymes involved in fatty acid oxidation as well as gluconeogenesis. Fibrates act as ligands of PPARα, but their clinical benefits in the treatment of NAFLD have been rather disappointing. PPARγ expression is induced in steatotic livers, and upon activation with glitazones PPARγ improves IR and hepatic steatosis. However, no effects on the clinical course of NASH, especially the progression of fibrosis, were observed. Oxysterols are known activators of the liver X receptors (LXR), which control hepatic lipogenesis and LXR knockout protects obese mice from development of hepatic steatosis. Other nuclear receptors involved in hepatic steatosis include xenobiotic sensors (CAR and PXR), the vitamin D receptor (VDR), hepatocyte nuclear factor 4 alpha (HNF4α), and the farnesoid X receptor (FXR).
The bile acid receptor FXR and its downstream targets emerge as important mediators in hepatic steatosis. FXR transcriptionally activates PPARα and thus induces fatty acid oxidation and hepatic glucose metabolism.[9, 10] Bile acids activate FXR and are ligands for the G-protein-coupled receptor TGR5, which acts as an important mediator in systemic energy homeostasis. FXR knockout mice not only develop hepatic steatosis, but also marked systemic hyperlipidemia, which might be explained in part by interactions between FXR, TGR5, and fibroblast growth factors (FGFs) 15/19 and 21.3 In patients with type 2 diabetes, obeticholic acid treatment, which activates FXR, improves IR as well as surrogate markers for hepatic inflammation and fibrogenesis. FXR furthermore activates small heterodimer partner (SHP), which modulates hepatic lipid export, uptake, and synthesis via repression of microsomal triglyceride transfer protein (MTP) as well as interaction with other nuclear receptors (LRH-1, PPARγ). Interestingly, FXR-mediated SHP activation appears to be disrupted in obese individuals. SHP polymorphisms might contribute to IR and development of obesity and NASH. Mechanistically, SHP is known to inhibit HNF4α as well as retinoid X receptor (RXR) transactivation.
In this issue of Hepatology, Kim et al. crosslink SHP with the retinoic acid receptor (RAR), which upon activation acts as a transcriptional activator of Hes6 (Fig. 1). Conversely, SHP represses Hes6, that itself represses HNF4α activation, and thus PPARγ activity. This study was based on the observation that SHP knockout mice are protected from high-fat diet-induced steatosis and obesity, despite an increase in IR, compared to controls. This was in part explained by transcriptional repression of PPARα by SHP. Interestingly, PPARγ was also down-regulated in these mice. Thus, the authors aimed to focus on the mechanisms linking SHP to PPARγ expression. Array data suggested Hes6, a known repressor of PPARγ expression via inhibition of HNF4α, to account for SHP effects on PPARγ. This theory was supported by SHP overexpression studies and in vivo observations.
Analysis of the promoter region of Hes6 revealed transcriptional activation upon binding of RAR/RXR heterodimers, which could be abolished by SHP activation. In fact, the clinical relevance of this RAR/SHP interaction in Hes6 transcriptional activation was further supported by the observation that long-term treatment with all-trans-retinoic acid (atRA) alleviates hepatic steatosis in high-fat diet-fed mice. In vitro and mouse studies confirmed the effects of atRA treatment on mRNA expression of Hes6, PPARγ2, and its downstream target Fsp27. Complementarily, blood glucose levels, body weight, and hepatic steatosis were reduced in mice treated with atRA for 7 days and atRA appeared to increase brown adipocyte function. To verify these effects, hepatic overexpression of adenoviral RAR led to suppression of blood glucose levels and reduced hepatic triglyceride content in vivo. In mice, overexpressing RAR in hepatocytes, Hes6 mRNA expression was up-regulated, while PPARγ2 and its downstream targets were down-regulated.
Specific activation of RAR adds a novel aspect of nuclear receptor interaction to the field of NAFLD research. In addition to emerging data, supporting the importance of bile acids as mediators in glucose and lipid metabolism, specifically the revelation of the RAR/SHP interrelation by Kim et al., gives insight into the complexity of nuclear receptor signaling in hepatic steatosis. As with Hes6, various transcription factors and other mediators have previously been identified to interfere with nuclear receptor signaling.[21-23] However, the availability of a specific agonist for RAR and the crosslink to FXR signaling makes this mechanism tempting. Nevertheless, further studies need to be conducted before a clinical application of atRA in NAFLD might be considered. This should include work in different models for NASH to investigate if the reduction of steatosis may also ameliorate its sequelae (i.e., inflammation and fibrosis). Given the complexity of networks in nuclear receptor signaling, it remains unknown how stimulation of different nuclear receptors might interfere with other pathways. These include effects on hepatic lipid and glucose metabolism as well as fundamental pathways and mediators involved in hepatocyte growth, regeneration, and tumorigenesis.
Lars P. Bechmann, M.D. and Ali Canbay, M.D.
Department of Gastroenterology and Hepatology University Hospital, University Duisburg-Essen Essen, Germany