SummaryNon-alcoholic fatty liver disease (NAFLD) is one of the most common liver diseases, but its underlying mechanism is poorly understood. Here we show that hepatocyte nuclear factor 4α (HNF4α), a liver-enriched nuclear hormone receptor, is markedly inhibited whereas miR-34a is highly induced in patients with non-alcoholic steatohepatitis, diabetic mice and mice fed a high fat diet. miR-34a is essential for HNF4α expression and regulates triglyceride accumulation in human and murine hepatocytes. miR-34a inhibits very low-density lipoprotein secretion and promotes liver steatosis and hypolipidemia in an HNF4α-dependent manner. As a result, increased miR-34a or reduced HNF4α expression in the liver attenuates the development of atherosclerosis in Apoe−/− or Ldlr−/− mice. These data indicate that the miR-34a-HNF4α pathway is activated under common conditions of metabolic stress and may have a role in the pathogenesis of NAFLD and in regulating plasma lipoprotein metabolism. Targeting this pathway may represent a novel approach for the treatment of NAFLD.

Objective Hepatic miR-34a expression is elevated in diet-induced or genetically obese mice and patients with non-alcoholic steatohepatitis (NASH), yet hepatocyte miR-34a's role in the progression of non-alcoholic fatty liver disease (NAFLD) from non-alcoholic fatty liver (NAFL) to NASH remains to be elucidated. Methods Mice overexpressing or deficient in hepatocyte miR-34a and control mice were fed a diet enriched in fats, cholesterol, and fructose (HFCF) to induce NASH. C57BL/6 mice with NASH were treated with an miR-34a inhibitor or a scramble control oligo. The effect of miR-34a on the development, progression, and reversal of NAFLD was determined. Results The hepatocyte-specific expression of miR-34a aggravated HFCF diet-induced NAFLD. In contrast, germline or adult-onset deletion of hepatocyte miR-34a attenuated the development and progression of NAFLD. In addition, pharmacological inhibition of miR-34a reversed HFCF diet-induced steatohepatitis. Mechanistically, hepatocyte miR-34a regulated the development and progression of NAFLD by inducing lipid absorption, lipogenesis, inflammation, and apoptosis but inhibiting fatty acid oxidation. Conclusions Hepatocyte miR-34a is an important regulator in the development and progression of NAFLD. MiR-34a may be a useful target for treating NAFLD.

Atherosclerotic cardiovascular disease is a leading cause of death in the western world. Increased plasma triglyceride and cholesterol levels are major risk factors for this disease. Carboxylesterase 1 (Ces1/Ces1g) has been shown to play a role in metabolic control. So far, the role of mouse Ces1/Ces1g deficiency in atherosclerosis is not elucidated. We generated Ces1/Ces1g −/− mice. Compared to wild-type mice, Ces1/Ces1g −/− mice had reduced plasma cholesterol levels. We then generated Ces1g −/− Ldlr −/− double knockout (DKO) mice, which were fed a Western diet for 16 weeks. Compared to Ldlr −/− mice, DKO mice displayed decreased plasma cholesterol and TG levels and reduced atherosclerotic lesions. Interestingly, knockdown of hepatic Ces1/Ces1g in Apoe −/− mice resulted in hyperlipidemia and exacerbated Western diet-induced atherogenesis. Mechanistically, global inactivation of Ces1/Ces1g inhibited intestinal cholesterol and fat absorption and Niemann-Pick C1 like 1 expression, and increased macrophage cholesterol efflux by inducing ATP-binding cassette subfamily A member 1 (ABCA1) and ABCG1. Ces1/Ces1g ablation also promoted M2 macrophage polarization and induced hepatic cholesterol 7α-hydroxylase and sterol 12α-hydroxylase expression. In conclusion, global loss of Ces1/Ces1g protects against the development of atherosclerosis by inhibiting intestinal cholesterol and triglyceride absorption and promoting macrophage cholesterol efflux.

Activating transcription factor 3 (ATF3) has been shown to play an important role in HDL metabolism; yet, the role of hepatocytic ATF3 in the development of steatohepatitis remains elusive. Here we show that adenoassociated virus-mediated overexpression of human ATF3 in hepatocytes prevents diet-induced steatohepatitis in C57BL/6 mice and reverses steatohepatitis in db/db mice. Conversely, global or hepatocyte-specific loss of ATF3 aggravates diet-induced steatohepatitis. Mechanistically, hepatocytic ATF3 induces hepatic lipolysis and fatty acid oxidation and inhibits inflammation and apoptosis. We further show that hepatocyte nuclear factor 4α (HNF4α) is required for ATF3 to improve steatohepatitis. Thus, the current study indicates that ATF3 protects against steatohepatitis through, at least in part, hepatic HNF4α. Targeting hepatic ATF3 may be useful for treatment of steatohepatitis.

All-trans retinoic acid (AtRA) is an active metabolite of vitamin A that influences many biological processes in development, differentiation, and metabolism. AtRA functions through activation of retinoid acid receptors (RARs).AtRA is shown to ameliorate hepatic steatosis, but the underlying mechanism is not well understood. In this study, we investigated the role of hepatocyte RAR alpha (RARα) in mediating the effect of AtRA on hepatosteatosis in mice. Hepatocyte-specific Rarα −/− (L-Rarα −/− ) mice and their control mice were fed a chow diet, high-fat diet (HFD), or a high-fat/cholesterol/fructose (HFCF) diet. Some of the mice were also treated with AtRA. Loss of hepatocyte RARα-induced hepatosteatosis in chow-fed aged mice and HFD-fed mice. AtRA prevented and reversed HFCF diet-induced obesity and hepatosteatosis in the control mice but not in L-Rarα −/− mice. Furthermore, AtRA reduced hepatocyte fatty acid uptake and lipid droplet formation, dependent on hepatocyte RARα. Our data suggest that hepatocyte RARα plays an important role in preventing hepatosteatosis and mediates AtRA's effects on diet-induced hepatosteatosis.