Gautam Bandyopadhyay

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.

Muscle-specific Pparg deletion causes insulin resistance

Thiazolidinediones (TZDs) are insulin-sensitizing drugs and are potent agonists of the nuclear peroxisome proliferator-activated receptor-γ (PPAR-γ). Although muscle is the major organ responsible for insulin-stimulated glucose disposal, PPAR-γ is more highly expressed in adipose tissue than in muscle. To address this issue, we used the Cre-loxP system to knock out Pparg, the gene encoding PPAR-γ, in mouse skeletal muscle. As early as 4 months of age, mice with targeted disruption of PPAR-γ in muscle showed glucose intolerance and progressive insulin resistance. Using the hyperinsulinemic-euglycemic clamp technique, the in vivo insulin-stimulated glucose disposal rate (IS-GDR) was reduced by ∼80% and was unchanged by 3 weeks of TZD treatment. These effects reveal a crucial role for muscle PPAR-γ in the maintenance of skeletal muscle insulin action, the etiology of insulin resistance and the action of TZDs.

Protein Kinase C-ζ as a Downstream Effector of Phosphatidylinositol 3-Kinase during Insulin Stimulation in Rat Adipocytes POTENTIAL ROLE IN GLUCOSE TRANSPORT

Insulin provoked rapid increases in enzyme activity of immunoprecipitable protein kinase C-ζ (PKC-ζ) in rat adipocytes. Concomitantly, insulin provoked increases in32P labeling of PKC-ζ both in intact adipocytes and during in vitro assay of immunoprecipitated PKC-ζ; the latter probably reflected autophosphorylation, as it was inhibited by the PKC-ζ pseudosubstrate. Insulin-induced activation of immunoprecipitable PKC-ζ was inhibited by LY294002 and wortmannin; this suggested dependence upon phosphatidylinositol (PI) 3-kinase. Accordingly, activation of PI 3-kinase by a pYXXM-containing peptide in vitro resulted in a wortmannin-inhibitable increase in immunoprecipitable PKC-ζ enzyme activity. Also, PI-3,4-(PO4)2, PI-3,4,5-(PO4)3, and PI-4,5-(PO4)2 directly stimulated enzyme activity and autophosphoralytion in control PKC-ζ immunoprecipitates to levels observed in insulin-treated PKC-ζ immunoprecipitates. In studies of glucose transport, inhibition of immunoprecipitated PKC-ζ enzyme activity in vitro by both the PKC-ζ pseudosubstrate and RO 31-8220 correlated well with inhibition of insulin-stimulated glucose transport in intact adipocytes. Also, in adipocytes transiently expressing hemagglutinin antigen-tagged GLUT4, co-transfection of wild-type or constitutive PKC-ζ stimulated hemagglutinin antigen-GLUT4 translocation, whereas dominant-negative PKC-ζ partially inhibited it. Our findings suggest that insulin activates PKC-ζ through PI 3-kinase, and PKC-ζ may act as a downstream effector of PI 3-kinase and contribute to the activation of GLUT4 translocation.

Hematopoietic Cell-Specific Deletion of Toll-like Receptor 4 Ameliorates Hepatic and Adipose Tissue Insulin Resistance in High-Fat-Fed Mice

Chronic low-grade inflammation, particularly in adipose tissue, is an important modulator of obesity-induced insulin resistance. The Toll-like receptor 4 (Tlr4) is a key initiator of inflammatory responses in macrophages. We performed bone marrow transplantation (BMT) of Tlr4lps-del or control C57Bl/10J donor cells into irradiated wild-type C57Bl6 recipient mice to generate hematopoietic cell-specific Tlr4 deletion mutant (BMT-Tlr4(-/-)) and control (BMT-WT) mice. After 16 weeks of a high-fat diet (HFD), BMT-WT mice developed obesity, hyperinsulinemia, glucose intolerance, and insulin resistance. In contrast, BMT-Tlr4(-/-) mice became obese but did not develop fasting hyperinsulinemia and had improved hepatic and adipose insulin sensitivity during euglycemic clamp studies, compared to HFD BMT-WT controls. HFD BMT-Tlr4(-/-) mice also showed markedly reduced adipose tissue inflammatory markers and macrophage content. In summary, our results indicate that Tlr4 signaling in hematopoietic-derived cells is important for the development of hepatic and adipose tissue insulin resistance in obese mice.

Activation of Protein Kinase C (α, β, and ζ) by Insulin in 3T3/L1 Cells TRANSFECTION STUDIES SUGGEST A ROLE FOR PKC-η IN GLUCOSE TRANSPORT

We presently studied (a) insulin effects on protein kinase C (PKC) and (b) effects of transfection-induced, stable expression of PKC isoforms on glucose transport in 3T3/L1 cells. In both fibroblasts and adipocytes, insulin provoked increases in membrane PKC enzyme activity and membrane levels of PKC-α and PKC-β. However, insulin-induced increases in PKC enzyme activity were apparent in both non-down-regulated adipocytes and adipocytes that were down-regulated by overnight treatment with 5 μM phorbol ester, which largely depletes PKC-α, PKC-β, and PKC-ε, but not PKC-η. Moreover, insulin provoked increases in the enzyme activity of immunoprecipitable PKC-η. In transfection studies, stable overexpression of wild-type or constitutively active forms of PKC-α, PKC-β1, and PKC-β2 failed to influence basal or insulin-stimulated glucose transport (2-deoxyglucose uptake) in fibroblasts and adipocytes, despite inhibiting insulin effects on glycogen synthesis. In contrast, stable overexpression of wild-type PKC-η increased, and a dominant-negative mutant form of PKC-η decreased, basal and insulin-stimulated glucose transport in fibroblasts and adipocytes. These findings suggested that: (a) insulin activates PKC-η, as well as PKC-α and β; and (b) PKC-η is required for, and may contribute to, insulin effects on glucose transport in 3T3/L1 cells.

Increased Malonyl-CoA Levels in Muscle From Obese and Type 2 Diabetic Subjects Lead to Decreased Fatty Acid Oxidation and Increased Lipogenesis; Thiazolidinedione Treatment Reverses These Defects

Increased accumulation of fatty acids and their derivatives can impair insulin-stimulated glucose disposal by skeletal muscle. To characterize the nature of the defects in lipid metabolism and to evaluate the effects of thiazolidinedione treatment, we analyzed the levels of triacylglycerol, long-chain fatty acyl-coA, malonyl-CoA, fatty acid oxidation, AMP-activated protein kinase (AMPK), acetyl-CoA carboxylase (ACC), malonyl-CoA decarboxylase, and fatty acid transport proteins in muscle biopsies from nondiabetic lean, obese, and type 2 subjects before and after an euglycemic-hyperinsulinemic clamp as well as pre–and post–3-month rosiglitazone treatment. We observed that low AMPK and high ACC activities resulted in elevation of malonyl-CoA levels and lower fatty acid oxidation rates. These conditions, along with the basal higher expression levels of fatty acid transporters, led accumulation of long-chain fatty acyl-coA and triacylglycerol in insulin-resistant muscle. During the insulin infusion, muscle fatty acid oxidation was reduced to a greater extent in the lean compared with the insulin-resistant subjects. In contrast, isolated muscle mitochondria from the type 2 subjects exhibited a greater rate of fatty acid oxidation compared with the lean group. All of these abnormalities in the type 2 diabetic group were reversed by rosiglitazone treatment. In conclusion, these studies have shown that elevated malonyl-CoA levels and decreased fatty acid oxidation are key abnormalities in insulin-resistant muscle, and, in type 2 diabetic patients, thiazolidinedione treatment can reverse these abnormalities.

SIRT1 inhibits inflammatory pathways in macrophages and modulates insulin sensitivity.

Chronic inflammation is an important etiology underlying obesity-related disorders such as insulin resistance and type 2 diabetes, and recent findings indicate that the macrophage can be the initiating cell type responsible for this chronic inflammatory state. The mammalian silent information regulator 2 homolog SIRT1 modulates several physiological processes important for life span, and a potential role of SIRT1 in the regulation of insulin sensitivity has been shown. However, with respect to inflammation, the role of SIRT1 in regulating the proinflammatory pathway within macrophages is poorly understood. Here, we show that knockdown of SIRT1 in the mouse macrophage RAW264.7 cell line and in intraperitoneal macrophages broadly activates the JNK and IKK inflammatory pathways and increases LPS-stimulated TNFalpha secretion. Moreover, gene expression profiles reveal that SIRT1 knockdown leads to an increase in inflammatory gene expression. We also demonstrate that SIRT1 activators inhibit LPS-stimulated inflammatory pathways, as well as secretion of TNFalpha, in a SIRT1-dependent manner in RAW264.7 cells and in primary intraperitoneal macrophages. Treatment of Zucker fatty rats with a SIRT1 activator leads to greatly improved glucose tolerance, reduced hyperinsulinemia, and enhanced systemic insulin sensitivity during glucose clamp studies. These in vivo insulin-sensitizing effects were accompanied by a reduction in tissue inflammation markers and a decrease in the adipose tissue macrophage proinflammatory state, fully consistent with the in vitro effects of SIRT1 in macrophages. In conclusion, these results define a novel role for SIRT1 as an important regulator of macrophage inflammatory responses in the context of insulin resistance and raise the possibility that targeting of SIRT1 might be a useful strategy for treating the inflammatory component of metabolic diseases.

Insulin Activates Protein Kinases C-ζ and C-λ by an Autophosphorylation-dependent Mechanism and Stimulates Their Translocation to GLUT4 Vesicles and Other Membrane Fractions in Rat Adipocytes

In rat adipocytes, insulin provoked rapid increases in (a) endogenous immunoprecipitable combined protein kinase C (PKC)-ζ/λ activity in plasma membranes and microsomes and (b) immunoreactive PKC-ζ and PKC-λ in GLUT4 vesicles. Activity and autophosphorylation of immunoprecipitable epitope-tagged PKC-ζ and PKC-λ were also increased by insulinin situ and phosphatidylinositol 3,4,5-(PO4)3 (PIP3) in vitro. Because phosphoinositide-dependent kinase-1 (PDK-1) is required for phosphorylation of activation loops of PKC-ζ and protein kinase B, we compared their activation. Both RO 31-8220 and myristoylated PKC-ζ pseudosubstrate blocked insulin-induced activation and autophosphorylation of PKC-ζ/λ but did not inhibit PDK-1-dependent (a) protein kinase B phosphorylation/activation or (b) threonine 410 phosphorylation in the activation loop of PKC-ζ. Also, insulinin situ and PIP3 in vitro activated and stimulated autophosphorylation of a PKC-ζ mutant, in which threonine 410 is replaced by glutamate (but not by an inactivating alanine) and cannot be activated by PDK-1. Surprisingly, insulin activated a truncated PKC-ζ that lacks the regulatory (presumably PIP3-binding) domain; this may reflect PIP3effects on PDK-1 or transphosphorylation by endogenous full-length PKC-ζ. Our findings suggest that insulin activates both PKC-ζ and PKC-λ in plasma membranes, microsomes, and GLUT4 vesicles by a mechanism requiring increases in PIP3, PDK-1-dependent phosphorylation of activation loop sites in PKC-ζ and λ, and subsequent autophosphorylation and/or transphosphorylation.

Evidence for Involvement of Protein Kinase C (PKC)-ζ and Noninvolvement of Diacylglycerol-Sensitive PKCs in Insulin-Stimulated Glucose Transport in L6 Myotubes1

We examined the question of whether insulin activates protein kinase C (PKC)-zeta in L6 myotubes, and the dependence of this activation on phosphatidylinositol (PI) 3-kinase. We also evaluated a number of issues that are relevant to the question of whether diacylglycerol (DAG)-dependent PKCs or DAG-insensitive PKCs, such as PKC-zeta, are more likely to play a role in insulin-stimulated glucose transport in L6 myotubes and other insulin-sensitive cell types. We found that insulin increased the enzyme activity of immunoprecipitable PKC-zeta in L6 myotubes, and this effect was blocked by PI 3-kinase inhibitors, wortmannin and LY294002; this suggested that PKC-zeta operates downstream of PI 3-kinase during insulin action. We also found that treatment of L6 myotubes with 5 microM tetradecanoyl phorbol-13-acetate (TPA) for 24 h led to 80-100% losses of all DAG-dependent PKCs (alpha, beta1, beta2, delta, epsilon) and TPA-stimulated glucose transport (2-deoxyglucose uptake); in contrast, there was full retention of PKC-zeta, as well as insulin-stimulated glucose transport and translocation of GLUT4 and GLUT1 to the plasma membrane. Unlike what has been reported in BC3H-1 myocytes, TPA treatment did not elicit increases in PKCbeta2 messenger RNA or protein in L6 myotubes, and selective retention of this PKC isoform could not explain the retention of insulin effects on glucose transport after prolonged TPA treatment. Of further interest, TPA acutely activated membrane-associated PI 3-kinase in L6 myotubes, and acute effects of TPA on glucose transport were inhibited, not only by the PKC inhibitor, LY379196, but also by both wortmannin and LY294002; this suggested that DAG-sensitive PKCs activate glucose transport through cross-talk with phosphatidylinositol (PI) 3-kinase, rather than directly through PKC. Also, the cell-permeable, myristoylated PKC-zeta pseudosubstrate inhibited insulin-stimulated glucose transport both in non-down-regulated and PKC-depleted (TPA-treated) L6 myotubes; thus, the PKC-zeta pseudosubstrate appeared to inhibit a protein kinase that is required for insulin-stimulated glucose transport but is distinct from DAG-sensitive PKCs. In keeping with the latter dissociation of DAG-sensitive PKCs and insulin-stimulated glucose transport, LY379196, which inhibits PKC-beta (preferentially) and other DAG-sensitive PKCs at relatively low concentrations, inhibited insulin-stimulated glucose transport only at much higher concentrations, not only in L6 myotubes, but also in rat adipocytes, BC3H-1 myocytes, 3T3/L1 adipocytes and rat soleus muscles. Finally, stable and transient expression of a kinase-inactive PKC-zeta inhibited basal and insulin-stimulated glucose transport in L6 myotubes. Collectively, our findings suggest that, whereas PKC-zeta is a reasonable candidate to participate in insulin stimulation of glucose transport, DAG-sensitive PKCs are unlikely participants.

Activation of the ERK Pathway and Atypical Protein Kinase C Isoforms in Exercise- and Aminoimidazole-4-carboxamide- 1-β-d-riboside (AICAR)-stimulated Glucose Transport

Exercise increases glucose transport in muscle by activating 5′-AMP-activated protein kinase (AMPK), but subsequent events are unclear. Presently, we examined the possibility that AMPK increases glucose transport through atypical protein kinase Cs (aPKCs) by activating proline-rich tyrosine kinase-2 (PYK2), ERK pathway components, and phospholipase D (PLD). In mice, treadmill exercise rapidly activated ERK and aPKCs in mouse vastus lateralis muscles. In rat extensor digitorum longus (EDL) muscles, (a) AMPK activator, 5-aminoimidazole-4-carboxamide-1-β-d-riboside (AICAR), activated PYK2, ERK and aPKCs; (b) effects of AICAR on ERK and aPKCs were blocked by tyrosine kinase inhibitor, genistein, and MEK1 inhibitor, PD98059; and (c) effects of AICAR on aPKCs and 2-deoxyglucose (2-DOG) uptake were inhibited by genistein, PD98059, and PLD-inhibitor, 1-butanol. Similarly, in L6 myotubes, (a) AICAR activated PYK2, ERK, PLD, and aPKCs; (b) effects of AICAR on ERK were inhibited by genistein, PD98059, and expression of dominant-negative PYK2; (c) effects of AICAR on PLD were inhibited by MEK1 inhibitor UO126; (d) effects of AICAR on aPKCs were inhibited by genistein, PD98059, 1-butanol, and expression of dominant-negative forms of PYK2, GRB2, SOS, RAS, RAF, and ERK; and (e) effects of AICAR on 2DOG uptake/GLUT4 translocation were inhibited by genistein, PD98059, UO126, 1-butanol, cell-permeable myristoylated PKC-ζ pseudosubstrate, and expression of kinase-inactive RAF, ERK, and PKC-ζ. AMPK activator dinitrophenol had effects on ERK, aPKCs, and 2-DOG uptake similar to those of AICAR. Our findings suggest that effects of exercise on glucose transport that are dependent on AMPK are mediated via PYK2, the ERK pathway, PLD, and aPKCs.