High-Fat Diet-Induced DeSUMOylation of E4BP4 Promotes Lipid Droplet Biogenesis and Liver Steatosis in Mice

Dysregulated lipid droplet accumulation has been identified as one of the main contributors to liver steatosis during nonalcoholic fatty liver disease (NAFLD). However, the underlying molecular mechanisms for excessive lipid droplet formation in the liver remain largely unknown. In the current study, hepatic E4 promoter-binding protein 4 (E4BP4) plays a critical role in promoting lipid droplet formation and liver steatosis in a high-fat diet (HFD)-induced NAFLD mouse model. Hepatic E4bp4 deficiency (E4bp4-LKO) protects mice from HFD-induced liver steatosis independently of obesity and insulin resistance. Our microarray study showed a markedly reduced expression of lipid droplet binding genes, such as Fsp27, in the liver of E4bp4-LKO mice. E4BP4 is both necessary and sufficient to activate Fsp27 expression and lipid droplet formation in primary mouse hepatocytes. Overexpression of Fsp27 increased lipid droplets and triglycerides in E4bp4-LKO primary mouse hepatocytes and restored hepatic steatosis in HFD-fed E4bp4-LKO mice. Mechanistically, E4BP4 enhances the transactivation of Fsp27 by CREBH in hepatocytes. Furthermore, E4BP4 is modified by SUMOylation, and HFD feeding induces deSUMOylation of hepatic E4BP4. SUMOylation of five lysine residues of E4BP4 is critical for the downregulation of Fsp27 and lipid droplets by cAMP signaling in hepatocytes. Taken together, this study revealed that E4BP4 drives liver steatosis in HFD-fed mice through its regulation of lipid droplet binding proteins. Our study also highlights the critical role of deSUMOylation of hepatic E4BP4 in promoting NAFLD.

A specialized Hsp90 co-chaperone network regulates steroid hormone receptor response to ligand

Heat shock protein-90 (Hsp90) chaperone machinery is involved in the stability and activity of its client proteins. The chaperone function of Hsp90 is regulated by co-chaperones and post-translational modifications. Although structural evidence exists for Hsp90 interaction with clients, our understanding of the impact of Hsp90 chaperone function toward client activity in cells remains elusive. Here, we dissect the impact of recently identified higher eukaryotic co-chaperones, FNIP1/2 (FNIPs) and Tsc1, toward Hsp90 client activity. Our data show that Tsc1 and FNIP2 form mutually exclusive complexes with FNIP1, and that unlike Tsc1, FNIP1/2 interact with the catalytic residue of Hsp90. Functionally, these co-chaperone complexes increase the affinity of the steroid hormone receptors glucocorticoid receptor and estrogen receptor to their ligands in vivo. We provide a model for the responsiveness of the steroid hormone receptor activation upon ligand binding as a consequence of their association with specific Hsp90:co-chaperone subpopulations.

Hepatic E4BP4 induction promotes lipid accumulation by suppressing AMPK signaling in response to chemical or diet-induced ER stress

Prolonged ER stress has been known to be one of the major drivers of impaired lipid homeostasis during the pathogenesis of non-alcoholic liver disease (NAFLD). However, the downstream mediators of ER stress pathway in promoting lipid accumulation remain poorly understood. Here, we present data showing the b-ZIP transcription factor E4BP4 in both the hepatocytes and the mouse liver is potently induced by the chemical ER stress inducer tunicamycin or by high-fat, low-methionine, and choline-deficient (HFLMCD) diet. We showed that such an induction is partially dependent on CHOP, a known mediator of ER stress and requires the E-box element of the E4bp4 promoter. Tunicamycin promotes the lipid droplet formation and alters lipid metabolic gene expression in primary mouse hepatocytes from E4bp4^flox/flox but not E4bp4 liver-specific KO (E4bp4-LKO) mice. Compared with E4bp4^flox/flox mice, E4bp4-LKO female mice exhibit reduced liver lipid accumulation and partially improved liver function after 10-week HFLMCD diet feeding. Mechanistically, we observed elevated AMPK activity and the AMPKβ1 abundance in the liver of E4bp4-LKO mice. We have evidence supporting that E4BP4 may suppress the AMPK activity via promoting the AMPKβ1 ubiquitination and degradation. Furthermore, acute depletion of the Ampkβ1 subunit restores lipid droplet formation in E4bp4-LKO primary mouse hepatocytes. Our study highlighted hepatic E4BP4 as a key factor linking ER stress and lipid accumulation in the liver. Targeting E4BP4 in the liver may be a novel therapeutic avenue for treating NAFLD.

The hepatic BMAL1/AKT/lipogenesis axis protects against alcoholic liver disease in mice via promoting PPARα pathway

Alcohol liver disease (ALD) is one of the major chronic liver diseases worldwide, ranging from fatty liver, alcoholic hepatitis, cirrhosis, and potentially, hepatocellular carcinoma. Epidemiological studies suggest a potential link between ALD and impaired circadian rhythms, but the role of hepatic circadian proteins in the pathogenesis of ALD remains unknown. Here we show that the circadian clock protein BMAL1 in hepatocytes is both necessary and sufficient to protect mice from ALD. Ethanol diet-fed mice with liver-specific knockout (Bmal1-LKO) or depletion of Bmal1 develop more severe liver steatosis and injury as well as a simultaneous suppression of both de novo lipogenesis and fatty acid oxidation, which can be rescued by the supplementation of synthetic PPARα ligands. Restoring de novo lipogenesis in the liver of Bmal1-LKO mice by constitutively active AKT not only elevates hepatic fatty acid oxidation but also alleviates ethanol-induced fatty liver and liver injury. Furthermore, hepatic over-expression of lipogenic transcription factor ChREBP, but not SREBP-1c, in the liver of Bmal1-LKO mice also increases fatty acid oxidation and partially reduces ethanol-induced fatty liver and liver injury. Conclusion: we identified a protective role of BMAL1 in hepatocytes against ALD. The protective action of BMAL1 during alcohol consumption depends on its ability to couple ChREBP-induced de novo lipogenesis with PPARα-mediated fatty oxidation. (Hepatology 2018).