Hye Lim Noh

A major role of insulin in promoting obesity- associated adipose tissue inflammation

Objective Adipose tissue (AT) inflammation is associated with systemic insulin resistance and hyperinsulinemia in obese rodents and humans. A longstanding concept is that hyperinsulinemia may promote systemic insulin resistance through downregulation of its receptor on target tissues. Here we tested the novel hypothesis that insulin also impairs systemic insulin sensitivity by specifically enhancing adipose inflammation. Methods Circulating insulin levels were reduced by about 50% in diet-induced and genetically obese mice by treatments with diazoxide or streptozotocin, respectively. We then examined AT crown-like structures, macrophage markers and pro-inflammatory cytokine expression in AT. AT lipogenesis and systemic insulin sensitivity was also monitored. Conversely, insulin was infused into lean mice to determine its affects on the above parameters. Results Lowering circulating insulin levels in obese mice by streptozotocin treatment decreased macrophage content in AT, enhancing insulin stimulated Akt phosphorylation and de novo lipogenesis (DNL). Moreover, responsiveness of blood glucose levels to injected insulin was improved by streptozotocin and diazoxide treatments of obese mice without changes in body weight. Remarkably, even in lean mice, infusion of insulin under constant euglycemic conditions stimulated expression of cytokines in AT. Consistent with these findings, insulin treatment of 3T3-L1 adipocytes caused a 10-fold increase in CCL2 mRNA levels within 6 h, which was blocked by the ERK inhibitor PD98059. Conclusion Taken together, these results indicate that obesity-associated hyperinsulinemia unexpectedly drives AT inflammation in obese mice, which in turn contributes to factors that suppress insulin-stimulated adipocyte DNL and systemic insulin sensitivity.

ChREBP regulates fructose-induced glucose production independently of insulin signaling

Obese, insulin-resistant states are characterized by a paradoxical pathogenic condition in which the liver appears to be selectively insulin resistant. Specifically, insulin fails to suppress glucose production, yet successfully stimulates de novo lipogenesis. The mechanisms underlying this dysregulation remain controversial. Here, we hypothesized that carbohydrate-responsive element-binding protein (ChREBP), a transcriptional activator of glycolytic and lipogenic genes, plays a central role in this paradox. Administration of fructose increased hepatic hexose-phosphate levels, activated ChREBP, and caused glucose intolerance, hyperinsulinemia, hypertriglyceridemia, and hepatic steatosis in mice. Activation of ChREBP was required for the increased expression of glycolytic and lipogenic genes as well as glucose-6-phosphatase (G6pc) that was associated with the effects of fructose administration. We found that fructose-induced G6PC activity is a major determinant of hepatic glucose production and reduces hepatic glucose-6-phosphate levels to complete a homeostatic loop. Moreover, fructose activated ChREBP and induced G6pc in the absence of Foxo1a, indicating that carbohydrate-induced activation of ChREBP and G6PC dominates over the suppressive effects of insulin to enhance glucose production. This ChREBP/G6PC signaling axis is conserved in humans. Together, these findings support a carbohydrate-mediated, ChREBP-driven mechanism that contributes to hepatic insulin resistance.

Tenomodulin promotes human adipocyte differentiation and beneficial visceral adipose tissue expansion

Proper regulation of energy storage in adipose tissue is crucial for maintaining insulin sensitivity and molecules contributing to this process have not been fully revealed. Here we show that type II transmembrane protein tenomodulin (TNMD) is upregulated in adipose tissue of insulin-resistant versus insulin-sensitive individuals, who were matched for body mass index (BMI). TNMD expression increases in human preadipocytes during differentiation, whereas silencing TNMD blocks adipogenesis. Upon high-fat diet feeding, transgenic mice overexpressing Tnmd develop increased epididymal white adipose tissue (eWAT) mass, and preadipocytes derived from Tnmd transgenic mice display greater proliferation, consistent with elevated adipogenesis. In Tnmd transgenic mice, lipogenic genes are upregulated in eWAT, as is Ucp1 in brown fat, while liver triglyceride accumulation is attenuated. Despite expanded eWAT, transgenic animals display improved systemic insulin sensitivity, decreased collagen deposition and inflammation in eWAT, and increased insulin stimulation of Akt phosphorylation. Our data suggest that TNMD acts as a protective factor in visceral adipose tissue to alleviate insulin resistance in obesity.

PI3-kinase mutation linked to insulin and growth factor resistance in vivo

The phosphatidylinositol 3-kinase (PI3K) signaling pathway is central to the action of insulin and many growth factors. Heterozygous mutations in the gene encoding the p85α regulatory subunit of PI3K (PIK3R1) have been identified in patients with SHORT syndrome - a disorder characterized by short stature, partial lipodystrophy, and insulin resistance. Here, we evaluated whether SHORT syndrome-associated PIK3R1 mutations account for the pathophysiology that underlies the abnormalities by generating knockin mice that are heterozygous for the Pik3r1Arg649Trp mutation, which is homologous to the mutation found in the majority of affected individuals. Similar to the patients, mutant mice exhibited a reduction in body weight and length, partial lipodystrophy, and systemic insulin resistance. These derangements were associated with a reduced capacity of insulin and other growth factors to activate PI3K in liver, muscle, and fat; marked insulin resistance in liver and fat of mutation-harboring animals; and insulin resistance in vitro in cells derived from these mice. In addition, mutant mice displayed defective insulin secretion and GLP-1 action on islets in vivo and in vitro. These data demonstrate the ability of this heterozygous mutation to alter PI3K activity in vivo and the central role of PI3K in insulin/growth factor action, adipocyte function, and glucose metabolism.

Sclerostin influences body composition by regulating catabolic and anabolic metabolism in adipocytes

Significance Sclerostin exerts profound control over skeletal metabolism by regulating the osteoanabolic Wnt/β-catenin signaling pathway. In this study, we demonstrate that in addition to a dramatic increase in bone mass, Sost−/− mice as well as those treated with a sclerostin-neutralizing antibody exhibit a reduction in white adipose tissue mass and are protected from high fat diet feeding. This effect is associated with an increase in fatty acid oxidation and reduced de novo fatty acid synthesis in adipocytes due to increased Wnt/β-catenin signaling. Sclerostin has traditionally been thought of as a local inhibitor of bone acquisition that antagonizes the profound osteoanabolic capacity of activated Wnt/β-catenin signaling, but serum sclerostin levels in humans exhibit a correlation with impairments in several metabolic parameters. These data, together with the increased production of sclerostin in mouse models of type 2 diabetes, suggest an endocrine function. To determine whether sclerostin contributes to the coordination of whole-body metabolism, we examined body composition, glucose homeostasis, and fatty acid metabolism in Sost−/− mice as well as mice that overproduce sclerostin as a result of adeno-associated virus expression from the liver. Here, we show that in addition to dramatic increases in bone volume, Sost−/− mice exhibit a reduction in adipose tissue accumulation in association with increased insulin sensitivity. Sclerostin overproduction results in the opposite metabolic phenotype due to adipocyte hypertrophy. Additionally, Sost−/− mice and those administered a sclerostin-neutralizing antibody are resistant to obesogenic diet-induced disturbances in metabolism. This effect appears to be the result of sclerostin’s effects on Wnt signaling and metabolism in white adipose tissue. Since adipocytes do not produce sclerostin, these findings suggest an unexplored endocrine function for sclerostin that facilitates communication between the skeleton and adipose tissue.

Altered Interleukin-10 Signaling in Skeletal Muscle Regulates Obesity-Mediated Inflammation and Insulin Resistance

ABSTRACT Skeletal muscle insulin resistance is a major characteristic of obesity and type 2 diabetes. Although obesity-mediated inflammation is causally associated with insulin resistance, the underlying mechanism is unclear. Here, we examined the effects of chronic obesity in mice with muscle-specific overexpression of interleukin-10 (MIL10). After 16 weeks of a high-fat diet (HFD), MIL10 mice became markedly obese but showed improved insulin action compared to that of wild-type mice, which was largely due to increased glucose metabolism and reduced inflammation in skeletal muscle. Since leptin regulates inflammation, the beneficial effects of interleukin-10 (IL-10) were further examined in leptin-deficient ob/ob mice. Muscle-specific overexpression of IL-10 in ob/ob mice (MCK-IL10ob/ob) did not affect spontaneous obesity, but MCK-IL10ob/ob mice showed increased glucose turnover compared to that in ob/ob mice. Last, mice with muscle-specific ablation of IL-10 receptor (M-IL10R−/−) were generated to determine whether IL-10 signaling in skeletal muscle is involved in IL-10 effects on glucose metabolism. After an HFD, M-IL10R−/− mice developed insulin resistance with reduced glucose metabolism compared to that in wild-type mice. Overall, these results demonstrate IL-10 effects to attenuate obesity-mediated inflammation and improve insulin sensitivity in skeletal muscle, and our findings implicate a potential therapeutic role of anti-inflammatory cytokines in treating insulin resistance and type 2 diabetes.

IL-10 prevents aging-associated inflammation and insulin resistance in skeletal muscle.

Altered energy balance and insulin resistance are important characteristics of aging. Skeletal muscle is a major site of glucose disposal, and the role of aging‐associated inflammation in skeletal muscle insulin resistance remains unclear. To investigate, we examined glucose metabolism in 18‐mo‐old transgenic mice with muscle‐specific overexpression of IL‐10 (MIL10) and in wild‐type mice during hyperinsulinemic‐euglycemic clamping. Despite similar fat mass and energy balance, MIL10 mice were protected from aging‐associated insulin resistance with significant increases in glucose infusion rates, whole‐body glucose turnover, and skeletal muscle glucose uptake (~60%; P <0.05), as compared to age‐matched WT mice. This protective effect was associated with decreased muscle inflammation, but no changes in adipose tissue inflammation in aging MIL10 mice. These results demonstrate the importance of skeletal muscle inflammation in aging‐mediated insulin resistance, and our findings further implicate a potential therapeutic role of anti‐inflammatory cytokine in the treatment of aging‐mediated insulin resistance.—Dagdeviren, S., Jung, D. Y., Friedline, R. H., Noh, H. L., Kim, J. H., Patel, P. R., Tsitsilianos, N., Inashima, K., Tran, D. A., Hu, X., Loubato, M. M., Craige, S. M., Kwon, J. Y., Lee, K. W., Kim, J. K. IL‐10 prevents aging‐associated inflammation and insulin resistance in skeletal muscle. FASEB J. 31, 701–710 (2017). http://www.fasebj.org

Nocturnal activation of melatonin receptor type 1 signaling modulates diurnal insulin sensitivity via regulation of PI3K activity

Recent genetic studies have highlighted the potential involvement of melatonin receptor 1 (MT1) and melatonin receptor 2 (MT2) in the pathogenesis of type 2 diabetes. Here, we report that mice lacking MT1 (MT1 KO) tend to accumulate more fat mass than WT mice and exhibit marked systemic insulin resistance. Additional experiments revealed that the main insulin signaling pathway affected by the loss of MT1 was the activation of phosphatidylinositol‐3‐kinase (PI3K). Transcripts of both catalytic and regulatory subunits of PI3K were strongly downregulated within MT1 KO mice. Moreover, the suppression of nocturnal melatonin levels within WT mice, by exposing mice to constant light, resulted in impaired PI3K activity and insulin resistance during the day, similar to what was observed in MT1 KO mice. Inversely, administration of melatonin to WT mice exposed to constant light was sufficient and necessary to restore insulin‐mediated PI3K activity and insulin sensitivity. Hence, our data demonstrate that the activation of MT1 signaling at night modulates insulin sensitivity during the day via the regulation of the PI3K transcription and activity. Lastly, we provide evidence that decreased expression of MTNR1A (MT1) in the liver of diabetic individuals is associated with poorly controlled diabetes.

Endoplasmic reticulum chaperone GRP78 regulates macrophage function and insulin resistance in diet-induced obesity

Obesity‐mediated inflammation is a major cause of insulin resistance, and macrophages play an important role in this process. The 78‐kDa glucose‐regulated protein (GRP78) is a major endoplasmic reticulum chaperone that modulates unfolded protein response (UPR), and mice with GRP78 heterozygosity were resistant to diet‐induced obesity. Here, we show that mice with macrophage‐selective ablation of GRP78 (Lyz‐GRP78−/−)are protected from skeletal muscle insulin resistance without changes in obesity compared with wild‐type mice after 9 wk of high‐fat diet. GRP78‐deficient macrophages demonstrated adapted UPR with up‐regulation of activating transcription factor (ATF)‐4 and M2‐polarization markers. Diet‐induced adipose tissue inflammation was reduced, and bone marrow‐derived macrophages from Lyz‐GRP78−/− mice demonstrated a selective increase in IL‐6 expression. Serum IL‐13 levels were elevated by >4‐fold in Lyz‐GRP78−/− mice, and IL‐6 stimulated the myocyte expression of IL‐13 and IL‐13 receptor. Lastly, recombinant IL‐13 acutely increased glucose metabolism in Lyz‐GRP78−/− mice. Taken together, our data indicate that GRP78 deficiency activates UPR by increasing ATF‐4, and promotes M2‐polarization of macrophages with a selective increase in IL‐6 secretion. Macrophage‐derived IL‐6 stimulates the myocyte expression of IL‐13 and regulates muscle glucose metabolism in a paracrine manner. Thus, our findings identify a novel crosstalk between macrophages and skeletal muscle in the modulation of obesity‐mediated insulin resistance.— Kim, J. H., Lee, E., Friedline, R. H., Suk, S., Jung, D. Y., Dagdeviren, S., Hu, X., Inashima, K., Noh, H. L., Kwon, J. Y., Nambu, A., Huh, J. R., Han, M. S., Davis, R. J., Lee, A. S., Lee, K. W., Kim, J. K. Endoplasmic reticulum chaperone GRP78 regulates macrophage function and insulin resistance in diet‐induced obesity. FASEB J. 32, 2292–2304 (2018). www.fasebj.org

DNA Repair Enzyme Ogg1 Regulates Hepatic Insulin Resistance in High-Fat Diet-Fed Obese Mice

Mitochondrial DNA (mtDNA) damage has been implicated in the development of insulin resistance. The mtDNA is highly specialized and encodes for proteins essential for energy metabolism. Also, mtDNA damage heightens mitochondrial oxidative stress, which is very critical for insulin resistance. OGG1 (8-oxoguanine DNA glycosylase-1) is a DNA glycosylase mediating the first step in the base excision repair which removes 7,8-dihydro-8-oxoguanine (8-oxoG) and repairs oxidized nuclear and mitochondrial DNA. Previous studies showed that Ogg1 deficiency results in an increased susceptibility to high fat diet (HFD)-induced obesity, metabolic dysfunction and insulin resistance in mice, suggesting a crucial role of Ogg1 in glucose metabolism. In the current study, we performed a 2-hour hyperinsulinemic-euglycemic clamp to measure tissue-specific insulin sensitivity in wild type (WT) and Ogg1-/- (KO) mice after chronic feeding of low fat diet (LFD as controls) or HFD. On LFD, both WT and KO mice showed comparable body weight and insulin sensitivity. After 16 weeks of HFD, the KO mice were more obese than WT mice with significant increases in whole body fat mass. There was a strong trend of increased insulin resistance with lower glucose infusion rates and whole body glucose turnover and glycogen synthesis in HFD-fed KO mice compared to HFD-fed WT mice. Hepatic insulin action was significantly lower in the HFD-fed KO mice which was consistent with recent evidence showing Ogg1 regulation of hepatic gluconeogenesis in the fed state. This is the first evidence demonstrating that Ogg1 contributes to HFD-induced insulin resistance in liver. Our findings suggest that protecting mtDNA from damage might be crucial to prevent insulin resistance and further identify therapeutic strategies for stimulating OGG1 as potential treatment of insulin resistance. Disclosure L. Yuzefovych: None. M. Schuler: None. H. Noh: None. S. Suk: None. J.K. Kim: None. L. Rachek: None.