Brown fat activates uncoupled respiration to defend against cold and contributes to systemic metabolic homeostasis. To date, the metabolic action of brown fat has been primarily attributed to its role in fuel oxidation and uncoupling protein 1 (UCP1)-mediated thermogenesis. Whether brown fat engages other tissues through secreted factors remains largely unexplored. Here we show that Neuregulin 4 (Nrg4), a member of the EGF family of extracellular ligands, is highly expressed in adipose tissues, enriched in brown fat, and markedly increased during brown adipocyte differentiation. Adipose tissue Nrg4 expression was reduced in rodent and human obesity. Gain- and loss-of-function studies in mice demonstrated that Nrg4 protects against diet-induced insulin resistance and hepatic steatosis through attenuating hepatic lipogenic signaling. Mechanistically, Nrg4 activates ErbB3/ErbB4 signaling in hepatocytes and negatively regulates de novo lipogenesis mediated by LXR/SREBP1c in a cell-autonomous manner. These results establish Nrg4 as a brown fat-enriched endocrine factor with therapeutic potential for the treatment of obesity-associated disorders, including type 2 diabetes and non-alcoholic fatty liver disease.

Recent studies suggest that adipose tissue hypoxia (ATH) may contribute to endocrine dysfunction in adipose tissue of obese mice. In this study, we examined hypoxia's effects on metabolism in adipocytes. We determined the dynamic relationship of ATH and adiposity in ob/ob mice. The interstitial oxygen pressure (Po(2)) was monitored in the epididymal fat pads for ATH. During weight gain from 39.5 to 55.5 g, Po(2) declined from 34.8 to 20.1 mmHg, which are 40-60% lower than those in the lean mice. Insulin receptor-beta (IRbeta) and insulin receptor substrate-1 (IRS-1) were decreased in the adipose tissue of obese mice, and the alteration was observed in 3T3-L1 adipocytes after hypoxia (1% oxygen) treatment. Insulin-induced glucose uptake and Akt Ser(473) phosphorylation was blocked by hypoxia in the adipocytes. This effect of hypoxia exhibited cell type specificity, as it was not observed in L6 myotubes and betaTC6 cells. In response to hypoxia, free fatty acid (FFA) uptake was reduced and lipolysis was increased in 3T3-L1 adipocytes. The molecular mechanism of decreased fatty acid uptake may be related to inhibition of fatty acid transporters (FATP1 and CD36) and transcription factors (PPARgamma and C/EBPalpha) by hypoxia. The hypoxia-induced lipolysis was observed in vivo after femoral arterial clamp. Necrosis and apoptosis were induced by hypoxia in 3T3-L1 adipocytes. These data suggest that ATH may promote FFA release and inhibit glucose uptake in adipocytes by inhibition of the insulin-signaling pathway and induction of cell death.

Background Mitochondrial DNA (MT-DNA) are intrinsically inflammatory nucleic acids released by damaged solid organs. Whether circulating cell-free MT-DNA quantitation could be used to predict the risk of poor COVID-19 outcomes remains undetermined. Methods We measured circulating MT-DNA levels in prospectively collected, cell-free plasma samples from 97 subjects with COVID-19 at hospital presentation. Our primary outcome was mortality. Intensive care unit (ICU) admission, intubation, vasopressor, and renal replacement therapy requirements were secondary outcomes. Multivariate regression analysis determined whether MT-DNA levels were independent of other reported COVID-19 risk factors. Receiver operating characteristic and area under the curve assessments were used to compare MT-DNA levels with established and emerging inflammatory markers of COVID-19. Results Circulating MT-DNA levels were highly elevated in patients who eventually died or required ICU admission, intubation, vasopressor use, or renal replacement therapy. Multivariate regression revealed that high circulating MT-DNA was an independent risk factor for these outcomes after adjusting for age, sex, and comorbidities. We also found that circulating MT-DNA levels had a similar or superior area under the curve when compared against clinically established measures of inflammation and emerging markers currently of interest as investigational targets for COVID-19 therapy. Conclusion These results show that high circulating MT-DNA levels are a potential early indicator for poor COVID-19 outcomes. Funding Washington University Institute of Clinical Translational Sciences COVID-19 Research Program and Washington University Institute of Clinical Translational Sciences (ICTS) NIH grant UL1TR002345.

ObjectiveAn increase in circulating branched-chain amino acids (BCAA) is associated with insulin resistance. Adipose tissue is a potentially important site for BCAA metabolism. We evaluated whether monomethyl branched chain fatty acids (mmBCFA) in adipose tissue, which are likely derived from BCAA catabolism, are associated with insulin sensitivity.Design and MethodsInsulin-stimulated glucose disposal was determined by using the hyperinsulinemic-euglycemic clamp procedure with stable isotope glucose tracer infusion, in 9 lean and 9 obese subjects, and in a separate group of 9 obese subjects before and 1 year after Roux-en-Y gastric bypass (RYGB) surgery (38% weight loss). Adipose tissue mmBCFA content was measured in tissue biopsies taken in the basal state.ResultsTotal adipose tissue mmBCFA content was ~30% lower in obese than lean subjects (P = 0.02), and increased by ~65% after weight loss in the RYGB group (P = 0.01). Adipose tissue mmBCFA content correlated positively with skeletal muscle insulin sensitivity (R2 = 35%, P = 0.01, n = 18).ConclusionsThese results demonstrate a novel association between adipose tissue mmBCFA content and obesity-related insulin resistance. Additional studies are needed to determine whether the association between adipose tissue mmBCFA and muscle insulin sensitivity is causal or a simple association.

CD36 has been linked to the etiology of insulin resistance and inflammation. We explored its function in regulating adipose tissue lipolysis, which influences fat accumulation by liver and muscle and overall metabolism. Knockdown of CD36 in differentiated 3T3-L1 adipocytes decreased lipolysis in response to 10 μM of the β-adrenergic agonist isoproterenol (by 42%), 10 μM of the adenyl cyclase activator forskolin (by 32%), and 500 μM of the phosphodiesterase (PDE) inhibitor isobutylmethylxanthine (by 33%). All three treatments in the knockdown adipocytes were associated with significant decreases of cAMP levels and of the hormone-sensitive lipase (HSL) and perilipin phosphorylation. An important role for PDE was supported by the lack of inhibition of the lipolysis induced by the poorly hydrolyzable dibutyryl cAMP analog. An additional contributory mechanism was diminished activation of the Src-ERK1/2 pathway. Regulation of lipolysis and lipolytic signaling by CD36 was reproduced with adipose tissue from CD36(-/-) mice. The importance of surface CD36 in this regulation was suggested by the finding that the plasma membrane-impermeable CD36 inhibitor sulfo-N-succinimidyl oleate (20 μM) decreased lipolysis. Interestingly, isoproterenol induced CD36 internalization, and this process was blocked by HSL inhibition, suggesting feedback regulation of adipocyte lipolysis via CD36 trafficking.