Hepatic Ago2 Regulates PPARα for Oxidative Metabolism Linked to Glycemic Control in Obesity and Post Bariatric Surgery.

Argonaute 2 (Ago2) is the main component of the RNA-induced silencing complex. We recently showed that liver-specific Ago2-deficiency in mice (L-Ago2 knockout [KO] mice) enhances mitochondrial oxidation and alleviates obesity-associated pathophysiology. However, the precise mechanisms behind the role of hepatic Ago2 in regulating the mitochondrial oxidation associated with glucose metabolism are still unclear. Here, we show that hepatic Ago2 regulates the function of peroxisome proliferator-activated receptor α (PPARα) for oxidative metabolism. In both genetically and diet-induced severe obese conditions, L-Ago2 KO mice developed obesity and hepatic steatosis but exhibited improved glucose metabolism accompanied by lowered expression levels of pathologic microRNAs (miRNAs), including miR-802, miR-103/107, and miR-152, and enhanced expression of PPARα and its target genes regulating oxidative metabolism in the liver. We then investigated the role of hepatic Ago2 in the outcomes of vertical sleeve gastrectomy (VSG) in which PPARα plays a crucial role in a drastic transcription reprogram associated with improved glycemia post VSG. Whereas VSG reduced body weight and improved fatty liver in wild-type mice, these effects were not observed in hepatic Ago2-deficient mice. Conversely, glucose metabolism was improved in a hepatic Ago2-dependent manner post VSG. Treating Ago2-deficient primary hepatocytes with WY-14643, a PPARα agonist, showed that Ago2-deficiency enhances sensitivity to WY-14643 and increases expression of PPARα target genes and mitochondrial oxidation. Our findings suggest that hepatic Ago2 function is intrinsically associated with PPARα that links Ago2-mediated RNA silencing with mitochondrial functions for oxidation and obesity-associated pathophysiology.

Assessment of the role of FGF15 in mediating the metabolic outcomes of murine Vertical Sleeve Gastrectomy (VSG).

Vertical sleeve gastrectomy (VSG) is the best current therapy for remission of obesity and its co-morbidities. It is understood to alter the enterohepatic circulation of bile acids in vivo. Fibroblast growth factor 19 (FGF19) in human and its murine orthologue Fgf15 plays a pivotal role in this bile acid driven enterohepatic signaling. The present study evaluated the metabolic outcomes of VSG in Fgf15 deficient mice. 6-8 weeks old male wildtype mice (WT) and Fgf15 deficient mice (KO) were fed a high fat diet (HFD) for 8 weeks. At 8th week of diet, both WT and KO mice were randomly distributed to VSG or sham surgery. Post-surgery, mice were observed for 8 weeks while fed a HFD and then euthanized to collect tissues for experimental analysis. Fgf15 deficient (KO) mice lost weight post VSG, but glucose tolerance in KO mice did not improve post VSG compared to WT mice. Enteroids derived from WT and KO mice proliferated with bile acid exposure in vitro. Post VSG both WT and KO mice had similarly altered bile acid enterohepatic flux, however Fgf15 deficient mice post VSG had increased hepatic accumulation of free and esterified cholesterol leading to lipotoxicity related ER stress, inflammasome activation, and increased Fgf21 expression. Intact Fgf15 mediated enterohepatic bile acid signaling, but not changes in bile acid flux, appear to be important for the metabolic improvements post-murine bariatric surgery. These novel data introduce a potential point of distinction between bile acids acting as ligands compared to their canonical downstream signaling pathways.

Bile Acid Signaling: Mechanism for Bariatric Surgery, Cure for NASH?

Bariatric surgery is the most effective and durable treatment option for obesity today. More importantly, beyond weight loss, bariatric procedures have many advantageous metabolic effects including reversal of obesity-related liver disease--nonalcoholic steatohepatitis (NASH). NASH is an important comorbidity of obesity given that it is a precursor to the development of liver cirrhosis that may necessitate liver transplantation in the long run. Simultaneously, we and others have observed increased serum bile acids in humans and animals that undergo bariatric surgery. Specifically, our preclinical studies have included experimental procedures such as 'ileal transposition' or bile diversion and established procedures such as Roux-en-Y gastric bypass and the adjustable gastric band. Importantly, these effects are not simply the result of weight loss since our data show that the resolution of NASH and increase in serum bile acids are not seen in rodents that lose an equivalent amount of weight via food restriction. In particular, we have studied the role of altered bile acid signaling, in the potent impact of a bariatric procedure termed 'vertical sleeve gastrectomy' (VSG). In this review we focus on the mechanisms of NASH resolution and weight loss after VSG surgery. We highlight the fact that bariatric surgeries can be used as 'laboratories' to dissect the mechanisms by which these procedures work to improve obesity and fatty liver disease. We describe key bile acid signaling elements that may provide potential therapeutic targets for 'bariatric-mimetic technologies' that could produce benefits similar to bariatric surgery--but without the surgery!

The role of small heterodimer partner in nonalcoholic fatty liver disease improvement after sleeve gastrectomy in mice.

OBJECTIVE: Bile acids (BA) are elevated after vertical sleeve gastrectomy (VSG) and farnesoid-X-receptor (FXR) is critical to the success of murine VSG. BA downregulate hepatic lipogenesis by activating the FXR-small heterodimer partner (SHP) pathway. The role of SHP in fatty liver disease improvement after VSG was tested. METHODS: Wild type (WT), SHP liver transgenic (SHP-Tg), and SHP knockout (SHP-KO) high-fat diet (HFD) fed mice underwent either VSG or Sham surgery. Body weight, BA level and composition, steatosis, and BA metabolism gene expression were evaluated. RESULTS: Obese WT mice post-VSG lost weight, reduced steatosis, decreased plasma alanine aminotransferase (ALT), had more BA absorptive ileal area, and elevated serum BA. Obese SHP-Tg mice post-VSG also lost weight and had decreased steatosis. SHP-KO mice were however resistant to steatosis despite weight gain on a HFD. Further SHP-KO mice that underwent VSG lost weight, but developed hepatic inflammation and had increased ALT. CONCLUSIONS: VSG produces weight loss independent of SHP status. SHP ablation creates a proinflammatory phenotype which is exacerbated after VSG despite weight loss. These inflammatory alterations are possibly related to factors extrinsic to a direct manifestation of NASH.

Activation of beta-catenin in dendritic cells regulates immunity versus tolerance in the intestine.

Dendritic cells (DCs) play a vital role in initiating robust immunity against pathogens as well as maintaining immunological tolerance to self antigens. However, the intracellular signaling networks that program DCs to become tolerogenic remain unknown. We report here that the Wnt-beta-catenin signaling in intestinal dendritic cells regulates the balance between inflammatory versus regulatory responses in the gut. beta-catenin in intestinal dendritic cells was required for the expression of anti-inflammatory mediators such as retinoic acid-metabolizing enzymes, interleukin-10, and transforming growth factor-beta, and the stimulation of regulatory T cell induction while suppressing inflammatory effector T cells. Furthermore, ablation of beta-catenin expression in DCs enhanced inflammatory responses and disease in a mouse model of inflammatory bowel disease. Thus, beta-catenin signaling programs DCs to a tolerogenic state, limiting the inflammatory response.

Salmonella flagellin induces bystander activation of splenic dendritic cells and hinders bacterial replication in vivo.

Bacterial flagellin is a target of innate and adaptive immune responses during Salmonella infection. Intravenous injection of Salmonella flagellin into C57BL/6 mice induced rapid IL-6 production and increased expression of activation markers by splenic dendritic cells. CD11b(+), CD8alpha(+), and plasmacytoid dendritic cells each increased expression of CD86 and CD40 in response to flagellin stimulation, although CD11b(+) dendritic cells were more sensitive than the other subsets. In addition, flagellin caused the rapid redistribution of dendritic cells from the red pulp and marginal zone of the spleen into the T cell area of the white pulp. Purified splenic dendritic cells did not respond directly to flagellin, indicating that flagellin-mediated activation of splenic dendritic cells occurs via bystander activation. IL-6 production, increased expression of activation markers, and dendritic cell redistribution in the spleen were dependent on MyD88 expression by bone marrow-derived cells. Avoiding this innate immune response to flagellin is important for bacterial survival, because Salmonella-overexpressing recombinant flagellin was highly attenuated in vivo. These data indicate that flagellin-mediated activation of dendritic cells is rapid, mediated by bystander activation, and highly deleterious to bacterial survival.