Nuclear receptor corepressors non-canonically drive glucocorticoid receptor-dependent activation of hepatic gluconeogenesis

Nuclear receptor corepressors (NCoRs) function in multiprotein complexes containing histone deacetylase 3 (HDAC3) to alter transcriptional output primarily through repressive chromatin remodelling at target loci1-5. In the liver, loss of HDAC3 causes a marked hepatosteatosis largely because of de-repression of genes involved in lipid metabolism6,7; however, the individual roles and contribution of other complex members to hepatic and systemic metabolic regulation are unclear. Here we show that adult loss of both NCoR1 and NCoR2 (double knockout (KO)) in hepatocytes phenocopied the hepatomegalic fatty liver phenotype of HDAC3 KO. In addition, double KO livers exhibited a dramatic reduction in glycogen storage and gluconeogenic gene expression that was not observed with hepatic KO of individual NCoRs or HDAC3, resulting in profound fasting hypoglycaemia. This surprising HDAC3-independent activation function of NCoR1 and NCoR2 is due to an unexpected loss of chromatin accessibility on deletion of NCoRs that prevented glucocorticoid receptor binding and stimulatory effect on gluconeogenic genes. These studies reveal an unanticipated, non-canonical activation function of NCoRs that is required for metabolic health.

Krüppel-like factor 10 protects against metabolic dysfunction-associated steatohepatitis by regulating HNF4α-mediated metabolic pathways

BACKGROUND

Krüppel-like factor 10 (KLF10), a zinc finger transcription factor, plays a pivotal role in modulating TGF-β-mediated cellular processes such as growth, apoptosis, and differentiation. Recent studies have implicated KLF10 in regulating lipid metabolism and glucose homeostasis. This study aimed to elucidate the precise role of hepatic KLF10 in developing metabolic dysfunction-associated steatohepatitis (MASH) in diet-induced obese mice.

METHODS

We investigated hepatic KLF10 expression under metabolic stress and the effects of overexpression or ablation of hepatic KLF10 on MASH development and lipidemia. We also determined whether hepatocyte nuclear factor 4α (HNF4α) mediated the metabolic effects of KLF10.

RESULTS

Hepatic KLF10 was downregulated in MASH patients and genetically or diet-induced obese mice. AAV8-mediated overexpression of KLF10 in hepatocytes prevented Western diet-induced hypercholesterolemia and steatohepatitis, whereas inactivation of hepatocyte KLF10 aggravated Western diet-induced steatohepatitis. Mechanistically, KLF10 reduced hepatic triglyceride and free fatty acid levels by inducing lipolysis and fatty acid oxidation and inhibiting lipogenesis, and reducing hepatic cholesterol levels by promoting bile acid synthesis. KLF10 highly induced HNF4α expression by directly binding to its promoter. The beneficial effect of KLF10 on MASH development was abolished in mice lacking hepatocyte HNF4α. In addition, the inactivation of KLF10 in hepatic stellate cells exacerbated Western diet-induced liver fibrosis by activating the TGF-β/SMAD2/3 pathway.

CONCLUSIONS

Our data collectively suggest that the transcription factor KLF10 plays a hepatoprotective role in MASH development by inducing HNF4α. Targeting hepatic KLF10 may offer a promising strategy for treating MASH.

Adipocyte retinoic acid receptor α prevents obesity and steatohepatitis by regulating energy expenditure and lipogenesis

OBJECTIVE

The adipose tissue-liver axis is a major regulator of the pathogenesis of nonalcoholic fatty liver disease (NAFLD). Retinoic acid signaling plays an important role in development and metabolism. However, little is known about the role of adipose retinoic acid signaling in the development of obesity-associated NAFLD. In this work, the aim was to investigate whether and how retinoic acid receptor alpha (RARα) regulated the development of obesity and NAFLD.

METHODS

RARα expression in adipose tissue of db/db or ob/ob mice was determined. Rarαfl/fl mice and adipocyte-specific Rarα-/- (RarαAdi-/- ) mice were fed a chow diet for 1 year or high-fat diet (HFD) for 20 weeks. Primary adipocytes and primary hepatocytes were co-cultured. Metabolic regulation and inflammatory response were characterized.

RESULTS

RARα expression was reduced in adipose tissue of db/db or ob/ob mice. RarαAdi-/- mice had increased obesity and steatohepatitis (NASH) when fed a chow diet or HFD. Loss of adipocyte RARα induced lipogenesis and inflammation in adipose tissue and the liver and reduced thermogenesis. In the co-culture studies, loss of RARα in adipocytes induced inflammatory and lipogenic programs in hepatocytes.

CONCLUSIONS

The data demonstrate that RARα in adipocytes prevents obesity and NASH via inhibiting lipogenesis and inflammation and inducing energy expenditure.

Retinoic Acid Signaling in Fatty Liver Disease

Retinoic acid (RA) is a metabolite of vitamin A and is essential for development and growth as well as cellular metabolism. Through genomic and nongenomic actions, RA regulates a variety of physiological functions. Dysregulation of RA signaling is associated with many diseases. Targeting RA signaling has been proven valuable to human health. All-trans retinoic acid (AtRA) and anthracycline-based chemotherapy are the standard treatment of acute promyelocytic leukemia (APL). Both human and animal studies have shown a significant relationship between RA signaling and the development and progression of nonalcoholic fatty liver disease (NAFLD). In this review article, we will first summarize vitamin A metabolism and then focus on the role of RA signaling in NAFLD. AtRA inhibits the development and progression of NAFLD via regulating lipid metabolism, inflammation, thermogenesis, etc.

Hepatocyte Sirtuin 6 Protects against Atherosclerosis and Steatohepatitis by Regulating Lipid Homeostasis

Histone deacetylase Sirtuin 6 (SIRT6) regulates many biological processes. SIRT6 is known to regulate hepatic lipid metabolism and inhibit the development of nonalcoholic fatty liver disease (NAFLD). We aimed to investigate the role of hepatocyte SIRT6 in the development of atherosclerosis and further characterize the mechanism underlying SIRT6's effect on NAFLD. Ldlr-/- mice overexpressing or lacking hepatocyte SIRT6 were fed a Western diet for 16 weeks. The role of hepatic SIRT6 in the development of nonalcoholic steatohepatitis (NASH), atherosclerosis, and obesity was investigated. We also investigated whether p53 participates in the pathogenesis of NAFLD in mice overexpressing hepatic SIRT6. Our data show that loss of hepatocyte SIRT6 aggravated the development of NAFLD, atherosclerosis, and obesity in Ldlr-/- mice, whereas adeno-associated virus (AAV)-mediated overexpression of human SIRT6 in the liver had opposite effects. Mechanistically, hepatocyte SIRT6 likely inhibited the development of NAFLD by inhibiting lipogenesis, lipid droplet formation, and p53 signaling. Hepatocyte SIRT6 also likely inhibited the development of atherosclerosis by inhibiting intestinal lipid absorption and hepatic VLDL secretion. Hepatic SIRT6 also increased energy expenditure. In conclusion, our data indicate that hepatocyte SIRT6 protects against atherosclerosis, NAFLD, and obesity by regulating lipid metabolism in the liver and intestine.

Retinoic Acid Receptor Alpha (RARα) in Macrophages Protects from Diet-Induced Atherosclerosis in Mice

Retinoic acid signaling plays an important role in regulating lipid metabolism and inflammation. However, the role of retinoic acid receptor alpha (RARα) in atherosclerosis remains to be determined. In the current study, we investigated the role of macrophage RARα in the development of atherosclerosis. Macrophages isolated from myeloid-specific Rarα-/- (RarαMac-/-) mice showed increased lipid accumulation and inflammation and reduced cholesterol efflux compared to Rarαfl/fl (control) mice. All-trans retinoic acid (AtRA) induced ATP-binding cassette subfamily A member 1 (Abca1) and Abcg1 expression and cholesterol efflux in both RarαMac-/- mice and Rarαfl/fl mice. In Ldlr-/- mice, myeloid ablation of RARα significantly reduced macrophage Abca1 and Abcg1 expression and cholesterol efflux, induced inflammatory genes, and aggravated Western diet-induced atherosclerosis. Our data demonstrate that macrophage RARα protects against atherosclerosis, likely via inducing cholesterol efflux and inhibiting inflammation.

Hepatic retinoic acid receptor alpha mediates all-trans retinoic acid's effect on diet-induced hepatosteatosis

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.