Saswata Talukdar

GPR120 Is an Omega-3 Fatty Acid Receptor Mediating Potent Anti-inflammatory and Insulin-Sensitizing Effects

Omega-3 fatty acids (omega-3 FAs), DHA and EPA, exert anti-inflammatory effects, but the mechanisms are poorly understood. Here, we show that the G protein-coupled receptor 120 (GPR120) functions as an omega-3 FA receptor/sensor. Stimulation of GPR120 with omega-3 FAs or a chemical agonist causes broad anti-inflammatory effects in monocytic RAW 264.7 cells and in primary intraperitoneal macrophages. All of these effects are abrogated by GPR120 knockdown. Since chronic macrophage-mediated tissue inflammation is a key mechanism for insulin resistance in obesity, we fed obese WT and GPR120 knockout mice a high-fat diet with or without omega-3 FA supplementation. The omega-3 FA treatment inhibited inflammation and enhanced systemic insulin sensitivity in WT mice, but was without effect in GPR120 knockout mice. In conclusion, GPR120 is a functional omega-3 FA receptor/sensor and mediates potent insulin sensitizing and antidiabetic effects in vivo by repressing macrophage-induced tissue inflammation.

Neutrophils mediate insulin resistance in mice fed a high-fat diet through secreted elastase

Chronic low-grade adipose tissue and liver inflammation is a major cause of systemic insulin resistance and is a key component of the low degree of insulin sensitivity that exists in obesity and type 2 diabetes. Immune cells, such as macrophages, T cells, B cells, mast cells and eosinophils, have all been implicated as having a role in this process. Neutrophils are typically the first immune cells to respond to inflammation and can exacerbate the chronic inflammatory state by helping to recruit macrophages and by interacting with antigen-presenting cells. Neutrophils secrete several proteases, one of which is neutrophil elastase, which can promote inflammatory responses in several disease models. Here we show that treatment of hepatocytes with neutrophil elastase causes cellular insulin resistance and that deletion of neutrophil elastase in high-fat-diet–induced obese (DIO) mice leads to less tissue inflammation that is associated with lower adipose tissue neutrophil and macrophage content. These changes are accompanied by improved glucose tolerance and increased insulin sensitivity. Taken together, we show that neutrophils can be added to the extensive repertoire of immune cells that participate in inflammation-induced metabolic disease.

Inflammation Is Necessary for Long-Term but Not Short-Term High-Fat Diet–Induced Insulin Resistance

OBJECTIVE Tissue inflammation is a key factor underlying insulin resistance in established obesity. Several models of immuno-compromised mice are protected from obesity-induced insulin resistance. However, it is unanswered whether inflammation triggers systemic insulin resistance or vice versa in obesity. The purpose of this study was to assess these questions. RESEARCH DESIGN AND METHODS We fed a high-fat diet (HFD) to wild-type mice and three different immuno-compromised mouse models (lymphocyte-deficient Rag1 knockout, macrophage-depleted, and hematopoietic cell-specific Jun NH2-terminal kinase–deficient mice) and measured the time course of changes in macrophage content, inflammatory markers, and lipid accumulation in adipose tissue, liver, and skeletal muscle along with systemic insulin sensitivity. RESULTS In wild-type mice, body weight and adipose tissue mass, as well as insulin resistance, were clearly increased by 3 days of HFD. Concurrently, in the short-term HFD period inflammation was selectively elevated in adipose tissue. Interestingly, however, all three immuno-compromised mouse models were not protected from insulin resistance induced by the short-term HFD. On the other hand, lipid content was markedly increased in liver and skeletal muscle at day 3 of HFD. CONCLUSIONS These data suggest that the initial stage of HFD-induced insulin resistance is independent of inflammation, whereas the more chronic state of insulin resistance in established obesity is largely mediated by macrophage-induced proinflammatory actions. The early-onset insulin resistance during HFD feeding is more likely related to acute tissue lipid overload.

Increased Macrophage Migration Into Adipose Tissue in Obese Mice

Macrophage-mediated inflammation is a key component of insulin resistance; however, the initial events of monocyte migration to become tissue macrophages remain poorly understood. We report a new method to quantitate in vivo macrophage tracking (i.e., blood monocytes from donor mice) labeled ex vivo with fluorescent PKH26 dye and injected into recipient mice. Labeled monocytes appear as adipose, liver, and splenic macrophages, peaking in 1–2 days. When CCR2 KO monocytes are injected into wild-type (WT) recipients, or WT monocytes given to MCP-1 KO recipients, adipose tissue macrophage (ATM) accumulation is reduced by ~40%, whereas hepatic macrophage content is decreased by ~80%. Using WT donor cells, ATM accumulation is several-fold greater in obese recipient mice compared with lean mice, regardless of the source of donor monocytes. After their appearance in adipose tissue, ATMs progressively polarize from the M2- to the M1-like state in obesity. In summary, the CCR2/MCP-1 system is a contributory factor to monocyte migration into adipose tissue and is the dominant signal controlling the appearance of recruited macrophages in the liver. Monocytes from obese mice are not programmed to become inflammatory ATMs but rather the increased proinflammatory ATM accumulation in obesity is in response to tissue signals.

Brain PPAR-γ promotes obesity and is required for the insulin–sensitizing effect of thiazolidinediones

In adipose tissue, muscle, liver and macrophages, signaling by the nuclear receptor peroxisome proliferator–activated receptor-γ (PPAR-γ) is a determinant of insulin sensitivity and this receptor mediates the insulin–sensitizing effects of thiazolidinediones (TZDs). As PPAR-γ is also expressed in neurons, we generated mice with neuron-specific Pparg knockout (Pparg brain knockout (BKO)) to determine whether neuronal PPAR-γ signaling contributes to either weight gain or insulin sensitivity. During high-fat diet (HFD) feeding, food intake was reduced and energy expenditure increased in Pparg-BKO mice compared to Ppargf/f mice, resulting in reduced weight gain. Pparg-BKO mice also responded better to leptin administration than Ppargf/f mice. When treated with the TZD rosiglitazone, Pparg-BKO mice were resistant to rosiglitazone-induced hyperphagia and weight gain and, relative to rosiglitazone-treated Ppargf/f mice, experienced only a marginal improvement in glucose metabolism. Hyperinsulinemic euglycemic clamp studies showed that the increase in hepatic insulin sensitivity induced by rosiglitazone treatment during HFD feeding was completely abolished in Pparg-BKO mice, an effect associated with the failure of rosiglitazone to improve liver insulin receptor signal transduction. We conclude that excess weight gain induced by HFD feeding depends in part on the effect of neuronal PPAR-γ signaling to limit thermogenesis and increase food intake. Neuronal PPAR-γ signaling is also required for the hepatic insulin sensitizing effects of TZDs.

Adipocyte NCoR Knockout Decreases PPARγ Phosphorylation and Enhances PPARγ Activity and Insulin Sensitivity

Insulin resistance, tissue inflammation, and adipose tissue dysfunction are features of obesity and Type 2 diabetes. We generated adipocyte-specific Nuclear Receptor Corepressor (NCoR) knockout (AKO) mice to investigate the function of NCoR in adipocyte biology, glucose and insulin homeostasis. Despite increased obesity, glucose tolerance was improved in AKO mice, and clamp studies demonstrated enhanced insulin sensitivity in liver, muscle, and fat. Adipose tissue macrophage infiltration and inflammation were also decreased. PPARγ response genes were upregulated in adipose tissue from AKO mice and CDK5-mediated PPARγ ser-273 phosphorylation was reduced, creating a constitutively active PPARγ state. This identifies NCoR as an adaptor protein that enhances the ability of CDK5 to associate with and phosphorylate PPARγ. The dominant function of adipocyte NCoR is to transrepress PPARγ and promote PPARγ ser-273 phosphorylation, such that NCoR deletion leads to adipogenesis, reduced inflammation, and enhanced systemic insulin sensitivity, phenocopying the TZD-treated state.

Targeting GPR120 and other fatty acid sensing GPCRs ameliorates insulin resistance and inflammatory diseases

The past decade has seen great progress in the understanding of the molecular pharmacology, physiological function and therapeutic potential of G-protein-coupled receptors (GPCRs). Free fatty acids (FFAs) have been demonstrated to act as ligands of several GPCRs including GPR40, GPR43, GPR84, GPR119 and GPR120. We have recently shown that GPR120 acts as a physiological receptor of ω3 fatty acids in macrophages and adipocytes, which mediate potent anti-inflammatory and insulin sensitizing effects. The important role GPR120 plays in the control of inflammation raises the possibility that targeting this receptor could have therapeutic potential in many inflammatory diseases including obesity and type 2 diabetes. In this review paper, we discuss lipid-sensing GPCRs and highlight potential outcomes of targeting such receptors in ameliorating disease.

Maintenance of Metabolic Homeostasis by Sestrin2 and Sestrin3

Chronic activation of mammalian target of rapamycin complex 1 (mTORC1) and p70 S6 kinase (S6K) in response to hypernutrition contributes to obesity-associated metabolic pathologies, including hepatosteatosis and insulin resistance. Sestrins are stress-inducible proteins that activate AMP-activated protein kinase (AMPK) and suppress mTORC1-S6K activity, but their role in mammalian physiology and metabolism has not been investigated. We show that Sestrin2--encoded by the Sesn2 locus, whose expression is induced upon hypernutrition--maintains metabolic homeostasis in liver of obese mice. Sesn2 ablation exacerbates obesity-induced mTORC1-S6K activation, glucose intolerance, insulin resistance, and hepatosteatosis, all of which are reversed by AMPK activation. Furthermore, concomitant ablation of Sesn2 and Sesn3 provokes hepatic mTORC1-S6K activation and insulin resistance even in the absence of nutritional overload and obesity. These results demonstrate an important homeostatic function for the stress-inducible Sestrin protein family in the control of mammalian lipid and glucose metabolism.

LTB4 promotes insulin resistance in obese mice by acting on macrophages, hepatocytes and myocytes

Insulin resistance results from several pathophysiologic mechanisms, including chronic tissue inflammation and defective insulin signaling. We found that liver, muscle and adipose tissue exhibit higher levels of the chemotactic eicosanoid LTB4 in obese high-fat diet (HFD)–fed mice. Inhibition of the LTB4 receptor Ltb4r1, through either genetic or pharmacologic loss of function, led to an anti-inflammatory phenotype with protection from insulin resistance and hepatic steatosis. In vitro treatment with LTB4 directly enhanced macrophage chemotaxis, stimulated inflammatory pathways, reduced insulin-stimulated glucose uptake in L6 myocytes, and impaired insulin-mediated suppression of hepatic glucose output in primary mouse hepatocytes. This was accompanied by lower insulin-stimulated Akt phosphorylation and higher Irs-1/2 serine phosphorylation, and all of these events were dependent on Gαi and Jnk1, two downstream mediators of Ltb4r1 signaling. These observations elucidate a novel role of the LTB4–Ltb4r1 signaling pathway in hepatocyte and myocyte insulin resistance, and they show that in vivo inhibition of Ltb4r1 leads to robust insulin-sensitizing effects.Chronic inflammation is a key component of obesity–induced insulin resistance and plays a central role in metabolic disease. In this study, we found that the major insulin target tissues, liver, muscle and adipose tissue exhibit increased levels of the chemotactic eicosanoid LTB4 in obese high fat diet (HFD) mice compared to lean chow fed mice. Inhibition of the LTB4 receptor, Ltb4r1, through either genetic or pharmacologic loss of function results in an anti–inflammatory phenotype with protection from systemic insulin resistance and hepatic steatosis in the setting of both HFD–induced and genetic obesity. Importantly, in vitro treatment with LTB4 directly enhanced macrophage chemotaxis, stimulated inflammatory pathways in macrophages, promoted de novo hepatic lipogenesis, decreased insulin stimulated glucose uptake in L6 myocytes, increased gluconeogenesis, and impaired insulin–mediated suppression of hepatic glucose output (HGO) in primary mouse hepatocytes. This was accompanied by decreased insulin stimulated Akt phosphorylation and increased Irs1 and Irs2 serine phosphorylation and all of these events were Gαi and Jnk dependent. Taken together, these observations elucidate a novel role of LTB4/Ltb4r1 in the etiology of insulin resistance in hepatocytes and myocytes, and shows that in vivo inhibition of Ltb4r1 leads to robust insulin sensitizing effects.

A Long-Acting FGF21 Molecule, PF-05231023, Decreases Body Weight and Improves Lipid Profile in Non-human Primates and Type 2 Diabetic Subjects

FGF21 plays a central role in energy, lipid, and glucose homeostasis. To characterize the pharmacologic effects of FGF21, we administered a long-acting FGF21 analog, PF-05231023, to obese cynomolgus monkeys. PF-05231023 caused a marked decrease in food intake that led to reduced body weight. To assess the effects of PF-05231023 in humans, we conducted a placebo-controlled, multiple ascending-dose study in overweight/obese subjects with type 2 diabetes. PF-05231023 treatment resulted in a significant decrease in body weight, improved plasma lipoprotein profile, and increased adiponectin levels. Importantly, there were no significant effects of PF-05231023 on glycemic control. PF-05231023 treatment led to dose-dependent changes in multiple markers of bone formation and resorption and elevated insulin-like growth factor 1. The favorable effects of PF-05231023 on body weight support further evaluation of this molecule for the treatment of obesity. Longer studies are needed to assess potential direct effects of FGF21 on bone in humans.