ASK1 inhibits browning of white adipose tissue in obesity

Increasing energy expenditure via induction of adipose tissue browning has become an appealing strategy to treat obesity and associated metabolic complications. Herein, we identify adipocyte-expressed apoptosis signal-regulating kinase 1 (ASK1) as regulator of adipose tissue browning. High fat diet-fed adipocyte-specific ASK1 knockout mice reveal increased UCP1 protein levels in inguinal adipose tissue concomitant with elevated energy expenditure, reduced obesity and ameliorated glucose tolerance compared to control littermates. In addition, ASK1-depletion blunts LPS-mediated downregulation of isoproterenol-induced UCP1 in subcutaneous fat both in vitro and in vivo. Conversely, adipocyte-specific ASK1 overexpression in chow-fed mice attenuates cold-induced UCP1 protein levels in inguinal fat. Mechanistically, ASK1 phosphorylates interferon regulatory factor 3 (IRF3) resulting in reduced Ucp1 expression. Taken together, our studies unravel a role of ASK1 in mediating the inhibitory effect of caloric surplus or LPS-treatment on adipose tissue browning. Adipocyte ASK1 might be a pharmacological target to combat obesity and associated morbidities

Metabolic reconstitution of germ-free mice by a gnotobiotic microbiota varies over the circadian cycle.

The capacity of the intestinal microbiota to degrade otherwise indigestible diet components is known to greatly improve the recovery of energy from food. This has led to the hypothesis that increased digestive efficiency may underlie the contribution of the microbiota to obesity. OligoMM12-colonized gnotobiotic mice have a consistently higher fat mass than germ-free (GF) or fully colonized counterparts. We therefore investigated their food intake, digestion efficiency, energy expenditure, and respiratory quotient using a novel isolator-housed metabolic cage system, which allows long-term measurements without contamination risk. This demonstrated that microbiota-released calories are perfectly balanced by decreased food intake in fully colonized versus gnotobiotic OligoMM12 and GF mice fed a standard chow diet, i.e., microbiota-released calories can in fact be well integrated into appetite control. We also observed no significant difference in energy expenditure after normalization by lean mass between the different microbiota groups, suggesting that cumulative small differences in energy balance, or altered energy storage, must underlie fat accumulation in OligoMM12 mice. Consistent with altered energy storage, major differences were observed in the type of respiratory substrates used in metabolism over the circadian cycle: In GF mice, the respiratory exchange ratio (RER) was consistently lower than that of fully colonized mice at all times of day, indicative of more reliance on fat and less on glucose metabolism. Intriguingly, the RER of OligoMM12-colonized gnotobiotic mice phenocopied fully colonized mice during the dark (active/eating) phase but phenocopied GF mice during the light (fasting/resting) phase. Further, OligoMM12-colonized mice showed a GF-like drop in liver glycogen storage during the light phase and both liver and plasma metabolomes of OligoMM12 mice clustered closely with GF mice. This implies the existence of microbiota functions that are required to maintain normal host metabolism during the resting/fasting phase of circadian cycle and which are absent in the OligoMM12 consortium

Response-to Letter-to-the-editor: “A low-carbohydrate diet induces hepatic insulin resistance and metabolic associated fatty liver disease in mice”

ISSN:2212-877

A short bout of HFD promotes long-lasting hepatic lipid accumulation

A short bout of high fat diet (HFD) impairs glucose tolerance and induces hepatic steatosis in mice. Here, we aimed to elaborate on long-lasting effects of short-term high fat feeding. As expected, one week of HFD significantly impaired glucose tolerance. Intriguingly, recovery feeding with a standard rodent diet for 8 weeks did not fully normalize glucose tolerance. In addition, mice exposed to a short bout of HFD revealed significantly increased liver fat accumulation paralleled by elevated portal free fatty acid levels after 8 weeks of recovery feeding compared to exclusively chow-fed littermates. In conclusion, a short bout of HFD has long-lasting effects on hepatic lipid accumulation and glucose tolerance

Mesenteric Fat Lipolysis Mediates Obesity-associated Hepatic Steatosis and Insulin Resistance

Hepatic steatosis and insulin resistance are among the most prevalent metabolic disorders and tightly associated with obesity and type 2 diabetes. However, underlying mechanism linking obesity to hepatic lipid accumulation and insulin resistance are incompletely understood. Glycoprotein 130 (gp130) is the common signal transducer of all interleukin 6 (IL-6) cytokines. Herein, we provide evidence that gp130-mediated adipose tissue lipolysis promotes hepatic steatosis and insulin resistance. In obese mice, adipocyte-specific gp130 deletion reduced basal lipolysis and enhanced insulin's ability to suppress lipolysis from mesenteric but not epididymal adipocytes. Consistently, free fatty acid levels were reduced in portal but not in systemic circulation of obese knockout mice. Importantly, adipocyte-specific gp130 knockout mice were protected from high fat diet (HFD)-induced hepatic steatosis as well as insulin resistance. In humans, omental but not subcutaneous IL-6 mRNA expression correlated positively with liver lipid accumulation (r=0.31; p<0.05) and negatively with euglycemic clamp glucose infusion rate (r=-0.28; p<0.05). Our results demonstrate that IL-6 cytokine-induced lipolysis may be restricted to mesenteric WAT and that it contributes to hepatic insulin resistance and steatosis. Therefore, blocking IL-6 cytokine signaling in (mesenteric) adipocytes may be a novel approach to blunt detrimental fat-liver crosstalk in obesity

A low-carbohydrate diet induces hepatic insulin resistance and metabolic associated fatty liver disease in mice

Objectives: Metabolic-associated fatty liver disease (MAFLD) is the most common chronic liver disease that can range from hepatic steatosis to non-alcoholic steatohepatitis (NASH), which can lead to fibrosis and cirrhosis. Recently, ketogenic diet (KD), a low carbohydrate diet, gained popularity as a weight-loss approach, although it has been reported to induce hepatic insulin resistance and steatosis in animal model systems via an undefined mechanism. Herein, we investigated the KD metabolic benefits and its contribution to the pathogenesis of NASH. Methods: Using metabolic, biochemical and omics approaches, we identified the effects of a KD on NASH and investigated the mechanisms by which KD induces hepatic insulin resistance and steatosis. Results: We demonstrate that KD can induce fibrosis and NASH regardless of body weight loss compared to high-fat diet (HFD) fed mice at thermoneutrality. At ambient temperature (23  C), KD-fed mice develop a severe hepatic injury, inflammation, and steatosis. In addition, KD increases liver cholesterol, IL-6, and p-JNK and aggravates diet induced-glucose intolerance and hepatic insulin resistance compared to HFD. Pharmacological inhibition of IL-6 and JNK reverses KD-induced glucose intolerance, and hepatic steatosis and restores insulin sensitivity. Conclusions: Our studies uncover a new mechanism for KD-induced hepatic insulin resistance and NASH potentially via IL-6-JNK signaling and provide a new NASH mouse model

A Genetic Model to Study the Contribution of Brown and Brite Adipocytes to Metabolism

UCP1-dependent thermogenesis is studied to define new strategies to ameliorate obesity and type 2 diabetes; however, animal models are mostly limited to germline mutations of UCP1, which can effect adaptive changes in UCP1-independent pathways. We develop an inducible mouse model for the sequential ablation of UCP1+ brown and brite/beige adipocytes in adult mice. We demonstrate that activated brown adipocytes can increase systemic energy expenditure (EE) by 30%, while the contribution of brite/beige UCP1+ cells is <5%. Notably, UCP1+ adipocytes do not contribute to circulating FGF21 levels, either at room temperature or after cold exposure. We demonstrate that the FGF21-mediated effects on EE and glucose homeostasis are partially dependent on the presence of UCP1+ cells, while the effect on weight loss is not. In conclusion, acute UCP1+ cell deletion may be a useful model to study the impact of brown and brite/beige adipocytes on metabolism.ISSN:2666-3864ISSN:2211-124

Obesity-Induced Increase in Cystatin C Alleviates Tissue Inflammation

We recently demonstrated that removal of one kidney (uninephrectomy; UniNx) in mice reduced high fat-diet (HFD)-induced adipose tissue inflammation thereby improving adipose tissue and hepatic insulin sensitivity. Of note, circulating cystatin C (CysC) levels were increased in UniNx compared to sham-operated mice. Importantly, CysC may have anti-inflammatory properties, and circulating CysC levels were reported to positively correlate with obesity in humans and as shown herein in HFD-fed mice. However, the causal relationship of such observation remains unclear. HFD feeding of CysC-deficient (CysC KO) mice deteriorated obesity-associated adipose tissue inflammation and dysfunction, as assessed by pro-inflammatory macrophage accumulation. In addition, mRNA expression of pro-inflammatory mediators was increased, whereas markers of adipocyte differentiation were decreased. Similarly to findings in adipose tissue, expression of pro-inflammatory cytokines was increased in liver and skeletal muscle of CysC KO mice. In line, HFD-induced hepatic insulin resistance and impairment of glucose tolerance were further aggravated in knockout mice. Consistently, chow-fed CysC KO mice were more susceptible to lipopolysaccharide (LPS)-induced adipose tissue inflammation. In people with obesity, circulating CysC levels correlated negatively with adipose tissue Hif1α as well as IL-6 mRNA expression. Moreover, healthy (i.e. insulin-sensitive) subjects with obesity depicted significantly higher mRNA expression of CysC in white adipose tissue. In conclusion, CysC is upregulated under obesity conditions and thereby counteracts inflammation of peripheral insulin-sensitive tissues and, thus, obesity-associated deterioration of glucose metabolism

Liver ASK1 protects from non-alcoholic fatty liver disease and fibrosis

Non‐alcoholic fatty liver disease (NAFLD) is strongly associated with obesity and may progress to non‐alcoholic steatohepatitis (NASH) and liver fibrosis. The deficit of pharmacological therapies for the latter mainly results from an incomplete understanding of involved pathological mechanisms. Herein, we identify apoptosis signal‐regulating kinase 1 (ASK1) as a suppressor of NASH and fibrosis formation. High‐fat diet‐fed and aged chow‐fed liver‐specific ASK1‐knockout mice develop a higher degree of hepatic steatosis, inflammation, and fibrosis compared to controls. In addition, pharmacological inhibition of ASK1 increased hepatic lipid accumulation in wild‐type mice. In line, liver‐specific ASK1 overexpression protected mice from the development of high‐fat diet‐induced hepatic steatosis and carbon tetrachloride‐induced fibrosis. Mechanistically, ASK1 depletion blunts autophagy, thereby enhancing lipid droplet accumulation and liver fibrosis. In human livers of lean and obese subjects, ASK1 expression correlated negatively with liver fat content and NASH scores, but positively with markers for autophagy. Taken together, ASK1 may be a novel therapeutic target to tackle NAFLD and liver fibrosis.ISSN:1757-4676ISSN:1757-468