Mice deficient small heterodimer partner (SHP) are protected from diet induced hepatic steatosis due to increased fatty acid oxidation and decreased lipogenesis. The decreased lipogenesis appears to be a direct consequence of very low expression of peroxisome proliferator activated receptor gamma 2 (PPARγ2), a potent lipogenic transcription factor, in the SHP−/− liver. The current study focuses on the identification of a SHP dependent regulatory cascade that controls PPARγ2 gene expression, thereby regulating hepatic fat accumulation. Illumina BeadChip array and real-time polymerase chain reaction were used to identify genes responsible for the linkage between SHP and PPARγ2 using hepatic RNAs isolated from SHP−/− and SHP-overexpressing mice. The initial efforts identify that hairy and enhancer of split 6 (Hes6), a novel transcriptional repressor, is an important mediator of the regulation of PPARγ2 transcription by SHP. The Hes6 promoter is specifically activated by the retinoic acid receptor (RAR) in response to its natural agonist ligand all-trans retinoic acid (atRA), and is repressed by SHP. Hes6 subsequently represses hepatocyte nuclear factor 4 alpha (HNF4α) activated-PPARγ2 gene expression via direct inhibition of the HNF4α transcriptional activity. Furthermore, we provide evidences that atRA treatment or adenovirus-mediated RARα overexpression significantly reduced hepatic fat accumulation in obese mouse models as observed in earlier studies and the beneficial effect is achieved via the proposed transcriptional cascade. Conclusions Our study describes a novel transcriptional regulatory cascade controlling hepatic lipid metabolism that identifies retinoic acid signaling as a new therapeutic approach to non-alcoholic fatty liver diseases.

Peroxisome proliferator-activated receptor gamma (PPARγ or PPARG) is a ligand-activated transcription factor belonging to the nuclear hormone receptor superfamily. It plays a master role in the differentiation and proliferation of adipose tissues. It has two major isoforms, PPARγ1 and PPARγ2, encoded from a single gene using two separate promoters and alternative splicing. Among them, PPARγ2 is most abundantly expressed in adipocytes and plays major adipogenic and lipogenic roles in the tissue. Furthermore, it has been shown that PPARγ2 is also expressed in the liver, specifically in hepatocytes, and its expression level positively correlates with fat accumulation induced by pathological conditions such as obesity and diabetes. Knockout of the hepatic Pparg gene ameliorates hepatic steatosis induced by diet or genetic manipulations. Transcriptional activation of Pparg in the liver induces the adipogenic program to store fatty acids in lipid droplets as observed in adipocytes. Understanding how the hepatic Pparg gene expression is regulated will help develop preventative and therapeutic treatments for non-alcoholic fatty liver disease (NAFLD). Due to the potential adverse effect of hepatic Pparg gene deletion on peripheral tissue functions, therapeutic interventions that target PPARγ for fatty liver diseases require fine-tuning of this gene’s expression and transcriptional activity.

Bile acid-CoA: amino acid N-acyltransferase (BAAT) catalyzes bile acid conjugation, the last step in bile acid synthesis. BAAT gene mutation in humans results in hypercholanemia, growth retardation, and fat-soluble vitamin insufficiency. The current study investigated the physiological function of BAAT in bile acid and lipid metabolism using Baat −/− mice. The bile acid composition and hepatic gene expression were analyzed in 10-week-old Baat −/− mice. They were also challenged with a westernized diet (WD) for additional 15 weeks to assess the role of BAAT in bile acid, lipid, and glucose metabolism. Comprehensive lab animal monitoring system and cecal 16S ribosomal RNA gene sequencing were used to evaluate the energy metabolism and microbiome structure of the mice, respectively. In Baat −/− mice, hepatic bile acids were mostly unconjugated and their levels were significantly increased compared with wild-type mice. Bile acid polyhydroxylation was markedly upregulated to detoxify unconjugated bile acid accumulated in Baat −/− mice.Although the level of serum marker of bile acid synthesis, 7α-hydroxy-4cholesten-3-one, was higher in Baat −/− mice, their bile acid pool size was smaller. When fed a WD, the Baat −/− mice showed a compromised body

Deficiency of the orphan nuclear hormone receptor small heterodimer partner (SHP, NR0B2) protects mice from diet-induced hepatic steatosis, in part, via repression of peroxisome proliferator-activated receptor (PPAR)-γ2 (Pparg2) gene expression. Alcoholic fatty liver diseases (AFLD) share many common pathophysiological features with non-AFLD. To study the role of SHP and PPARγ2 in AFLD, we used a strategy of chronic ethanol feeding plus a single binge ethanol feeding to challenge wild-type (WT) and SHP-null (SHP(-/-)) mice with ethanol. The ethanol feeding induced liver fat accumulation and mRNA expression of hepatic Pparg2 in WT mice, which suggests that a high level of PPARγ2 is a common driving force for fat accumulation induced by ethanol or a high-fat diet. Interestingly, ethanol-fed SHP(-/-) mice displayed hepatic fat accumulation similar to that of ethanol-fed WT mice, even though their Pparg2 expression level remained lower. Mortality of SHP(-/-) mice after ethanol binge feeding was significantly reduced and their acetaldehyde dehydrogenase (Aldh2) mRNA level was higher than that of their WT counterparts. After an intoxicating dose of ethanol, SHP(-/-) mice exhibited faster blood ethanol clearance and earlier wake-up time than WT mice. Higher blood acetate, the end product of ethanol metabolism, and lower acetaldehyde levels were evident in the ethanol-challenged SHP(-/-) than WT mice. Ethanol-induced inflammatory responses and lipid peroxidation were also lower in SHP(-/-) mice. The current data show faster ethanol catabolism and extra fat storage through conversion of acetate to acetyl-CoA before its release into the circulation in this ethanol-feeding model in SHP(-/-) mice.

Background Bile acid‐CoA: amino acid N‐acyltransferase (BAAT) is the enzyme which is responsible for bile acid (BA) conjugation with glycine and taurine in the final step of bile acid synthesis in humans. More than 98% of BA conjugation occurs in the liver. BAAT deficiency affects lipid homeostasis in the whole body. BA conjugation is essential to improve fat solubility and counteract passive diffusion in intestine by reducing their pKa values. Thus, the absence of conjugated BAs in the duodenum will affect the absorption of dietary fat and lipid soluble vitamins. Aim We aimed to investigate the physiological effects of BAAT deficiency on the mice in diet‐induced obesity using BAAT knockout (BAAT KO) mice. Methods Male BAAT KO mice and their littermate WT control mice were fed a WD for 15 weeks. LC‐MS were used to analyze BAs in the gallbladder. Body weight, EchoMRI analysis, CLAMS, gene expression, and plasma analysis were used to discern the severity of fat malabsorption that is associated with the deficiency of BAAT when mice were challenged with a WD. Results Total BA levels in serum, liver, and urine were induced in KO mice. Levels of unconjugated BAs were significantly higher and taurine conjugated but not glycine conjugated BAs were almost completely absent in the gallbladders of KO mice. Cyp7a1, Bsep, Abca1a, Abcc4, Papss2, and Sult2a1, genes involved in BA metabolism, were strongly upregulated in the livers of KO mice. Triglyceride levels in the liver exhibited a significant reduction while fecal total lipids levels were increased in KO mice compared with controls after WD challenge. CLAMS data displays an increased in the respiratory exchange ratio and food intake in KO mice when they were on WD. Their BW gains were also significantly impaired during gestation period. Conclusion BAAT KO mice fail to gain body weight either during gestation or when they consumed a WD due to the defect in fat absorption, which is caused by conjugated bile acid deficiency. BAATKO mice is a novel model to study bile acid and lipid metabolism. Support or Funding Information This work was supported by a bridge fund (Y.K.L) from NEOMED.