Preclinical models indicate roles for adiponectin in the mai
Preclinical models indicate roles for adiponectin in the maintenance of hepatic lipid metabolism. Adiponectin overexpression prevents accumulation of triglycerides or the deleterious lipid metabolites diacylglycerols or ceramides [5,8]. Direct manipulation of adiponectin expression demonstrates a potential causal relationship between adiponectin signaling and steatosis . Conversely, genetic ablation of adiponectin in leptin-deficient (ob/ob) mice further exacerbates hepatic triglyceride accumulation . Hepatic steatosis is just an initial step in the progression of non-alcoholic steatohepatitis (NASH), and can ultimately enhance risk for cirrhosis and hepatocarcinoma. Adiponectin null mice develop fibrotic steatohepatitis and adenomas when maintained on high fat diets for 48 weeks  but not in response to shorter-term diet administration [13,35]. Administration of recombinant adiponectin in rodents results in beneficial effects on lipid metabolism, such as enhancing lipid clearance and increasing fatty angiopoietin oxidation in muscle and liver [30,33]. Delivery of adenoviral adiponectin can attenuate hepatomegaly, steatosis, and liver injury in mice with NASH . Our studies suggest that adiponectins' anti-steatotic actions occur by local effects of ceramide-lowering in the liver, where targeted degradation of ceramides with acid ceramidase decreases hepatic lipid uptake and lipogenic gene expression . Additionally, adiponectins' effects locally within the adipocyte contribute to enhanced lipid clearance mediated locally via AdipoR1 and AdipoR2, protecting non-adipocytes from excess lipid exposure. Studies with genetic manipulation of adiponectin receptors show similar trends as adiponectin itself, and human single nucleotide polymorphisms in AdipoR1 or AdipoR2 are significantly associated with hepatic triglyceride accumulation [9,22] and enhanced risk for cirrhosis . Adenoviral-mediated overexpression of either adiponectin receptor is sufficient to stimulate lipid oxidation and diminish hepatic triglyceride content . By contrast, mice lacking both isoforms of the adiponectin receptors display elevated hepatic triglyceride accumulation. Our data suggest that activation of either isoform of adiponectin receptor will decrease hepatic steatosis and provide glycemic benefit. These studies provide strong preclinical evidence that adiponectin receptor agonism may be a powerful therapeutic tool. We show here for the first time that AdipoRon stimulates a ceramidase activity and can activate the adiponectin receptors to induce its metabolic beneficial effects when adiponectin is not present. Importantly, the actions of adiponectin receptors critically rely on adiponectin for their functional lowering of ceramides and subsequent improvements in glucose and lipid homeostasis. This study adds strong in vivo genetic support that AdipoRs are indeed the cognate receptors that convey adiponectin actions. This challenges our previous notion that these receptors indeed have a strong basal activity and suggest that it is very difficult to eliminate adiponectin from culture systems where it is present in FBS-containing medium, which is enriched in bovine adiponectin. Collectively, our data suggest that the lowering of ceramides is a critical player in adiponectin-induced improvements in non-alcoholic fatty liver disease and hepatic insulin resistance in mice with diet-induced obesity. Our data also demonstrate that the induction of AC activity is insufficient to rescue metabolic syndrome in ob/ob mice, while AdipoR2 overexpression is sufficient. Several key differences in these two approaches have been noted. First, adiponectin receptors lower more chain lengths of ceramides with diet induced obesity, suggesting that it is a potential novel therapeutic target. Additionally, AdipoR-induced ceramidase activity cleaves additional species like deoxyceramides, which may be more cytotoxic because of the limited pathways for metabolizing these alanine (rather than serine) derived sphingolipids. Unlike AC overexpression in the liver, AdipoR2 overexpression results in a significant increase in hepatic S1P levels. S1P has been shown in the past to induce AMPK, a major downstream effector of adiponectin . Thus, a possible explanation of why AdipoR2 overexpression is capable of rescuing ob/ob phenotype is that it is an essential initiator of the broad spectrum of adiponectin action. Thus, orally active small-molecule adiponectin receptor agonists, such as AdipoRon, could potentially be an important therapeutic agent for patients with non-alcoholic fatty liver disease, diabetes, or other comorbidities of the metabolic syndrome .