Salicylic acid reverses phorbol 12-myristate-13-acetate (PMA)- and tumor necrosis factor alpha (TNFalpha)-induced insulin receptor substrate 1 (IRS1) serine 307 phosphorylation and insulin resistance in human embryonic kidney 293 (HEK293) cells

Development of a Crystallographic Screening to Identify Sudan Virus VP40 Ligands

The matrix protein VP40 of the highly pathogenic Sudan virus (genus Orthoebolavirus) is a multifunctional protein responsible for the recruitment of viral nucleocapsids to the plasma membrane and the budding of infectious virions. In addition to its role in assembly, VP40 also downregulates viral genome replication and transcription. VP40's existence in various homo-oligomeric states is presumed to underpin its diverse functional capabilities during the viral life cycle. Given the absence of licensed therapeutics targeting the Sudan virus, our study focused on inhibiting VP40 dimers, the structural precursors to critical higher-order oligomers, as a novel antiviral strategy. We have established a crystallographic screening pipeline for the identification of small-molecule fragments capable of binding to VP40. Dimeric VP40 of the Sudan virus was recombinantly expressed in bacteria, purified, crystallized, and soaked in a solution of 96 different preselected fragments. Salicylic acid was identified as a crystallographic hit with clear electron density in the pocket between the N- and the C-termini of the VP40 dimer. The binding interaction is predominantly coordinated by amino acid residues leucine 158 (L158) and arginine 214 (R214), which are key in stabilizing salicylic acid within the binding pocket. While salicylic acid displayed minimal impact on the functional aspects of VP40, we delved deeper into characterizing the druggability of the identified binding pocket. We analyzed the influence of residues L158 and R214 on the formation of virus-like particles and viral RNA synthesis. Site-directed mutagenesis of these residues to alanine markedly affected both VP40's budding activity and its effect on viral RNA synthesis, underscoring the potential of the salicylic acid binding pocket as a drug target. In summary, our findings lay the foundation for structure-guided drug design to provide lead compounds against Sudan virus VP40.

Salsalate: a pleotropic anti-inflammatory drug in the treatment of diabetes, obesity, and metabolic diseases

Type two Diabetes Mellitus (T2DM) is a rising epidemic. Available therapeutic strategies have provided glycaemic control via HbA1c reduction but fail to provide clinically meaningful reduction in microvascular and macrovascular (cardiac, renal, ophthalmological, and neurological) complications. Inflammation is strongly linked to the pathogenesis of T2DM. Underlying inflammatory mechanisms include oxidative stress, endoplasmic reticulum stress amyloid deposition in the pancreas, lipotoxicity, and glucotoxicity. Molecular signalling mechanisms in chronic inflammation linked to obesity and diabetes include JANK, NF-kB, and AMPK pathways. These activated pathways lead to a production of various inflammatory cytokines, such as Interleukin (IL-6), tumor necrosis factor (TNF)-alpha, and C-reactive protein (CRP), which create a chronic low-grade inflammation and ultimately dysregulation of glucose homeostasis in the liver, skeletal muscle, and smooth muscle. Anti-inflammatory agents are being tested as anti-diabetic agents such as the IL-1b antagonist, Anakinra, the IL-1b inhibitor, Canakinuma, the IL-6 antagonists such as Tocilizumab, Rapamycin (Everolimus), and the IKK-beta kinase inhibitor, Salsalate. Salsalate is a century old safe anti-inflammatory drug used in the treatment of arthritis. Long-term safety and efficacy of Salsalate in the treatment of T2DM have been evaluated, which showed improved fasting plasma glucose and reduced HbA1C levels as well as reduced pro-inflammatory markers in T2DM patients. Current publication summarizes the literature review of pathophysiology of role of inflammation in T2DM and clinical efficacy and safety of Salsalate in the treatment of T2DM.

Fenofibrate alleviates insulin resistance by reducing tissue inflammation in obese ovariectomized mice

BACKGROUND

Fenofibrate is a hypolipidemic peroxisome proliferator-activated receptor α (PPARα) agonist used clinically to reduce hypercholesterolemia and hypertriglyceridemia.

OBJECTIVE

We investigated the effects of fenofibrate on insulin resistance and tissue inflammation in a high-fat diet (HFD)-fed ovariectomized (OVX) C57BL/6J mice, a mouse model of obese postmenopausal women.

METHODS

Female OVX mice were randomly divided into 3 groups and received a low-fat diet, an HFD, or an HFD supplemented with 0.05% (w/w) fenofibrate for 9 weeks. Parameters of insulin resistance and tissue inflammation were measured using blood analysis, histological analysis, immunohistochemistry, and quantitative real-time polymerase chain reaction.

RESULTS

When fenofibrate was administered to HFD-fed OVX mice for 9 weeks, we observed reductions in body weight gain, adipose tissue mass, and the size of visceral adipocytes without the change of food intake. Fenofibrate improved mild hyperglycemia, severe hyperinsulinemia, and glucose tolerance in these mice. It also reduced pancreatic islet size and insulin-positive β-cell area to levels similar to those in OVX mice fed a low-fat diet. Concomitantly, administration of fenofibrate not only suppressed pancreatic lipid accumulation but also decreased CD68-positive macrophages in both the pancreas and visceral adipose tissue. Treatment with fenofibrate reduced tumor necrosis factor α (TNFα) mRNA levels in adipose tissue and lowered serum TNFα levels.

CONCLUSION

These results suggest that fenofibrate treatment attenuates insulin resistance in part by reducing tissue inflammation and TNFα expression in HFD-fed OVX mice.

Anti-leukemic principle(s) from Momordica charantia seeds induce differentiation of HL-60 cells through ERK/MAPK signalling pathway

Myeloid leukemia is one of the major causes of deaths among elderly with very poor prognosis. Due to the adverse effects of existing chemotherapeutic agents, plant-derived components are being screened for their anti-leukemic potential. Momordica charantia (bitter gourd) possesses a variety of therapeutic activities. We have earlier demonstrated anti-leukemic activity of acetone extract of M. charantia seeds. The present study reports purification of differentiation inducing principle(s) from further fractionated seed extract (hexane fraction of the acetone extract, Mc2-Ac-hex) using HL-60 cells. Out of the 5 peak fractions (P1-P5) obtained from normal phase HPLC of the Mc2-Ac-hex, only peak fraction 3 (P3) induced differentiation of HL-60 cells as evident from an increase in NBT-positive cells and increased expression of cell surface marker CD11b. The P3 differentiated the HL-60 cells to granulocytic lineage, established by increased CD15 (granulocytic cell surface marker) expression in the treated cells. Further, possible molecular mechanism and the signalling pathway involved in the differentiation of HL-60 cells were also investigated. Use of specific signalling pathway inhibitors in the differentiation study, and proteome array analysis of the treated cells collectively revealed the involvement the of ERK/MAPK mediated pathway. Partial characterization of the P3 by GC-MS analysis revealed the presence of dibutyl phthalate, and derivatives of 2,5-dihydrofuran to be the highest among the 5 identified compounds. This study thus demonstrated that purified differentiation-inducing principle(s) from M. charantia seed extract induce HL-60 cells to granulocytic lineage through ERK/MAPK signalling pathway.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10616-022-00547-x.

Glycyrrhetinic Acid Improves Insulin-Response Pathway by Regulating the Balance between the Ras/MAPK and PI3K/Akt Pathways

Glycyrrhetinic acid (GA), a bioactive component in the human diet, has been reported to improve hyperglycemia, dyslipidemia, insulin resistance and obesity in rats with metabolic syndrome. However, GA-specific target proteins and the mechanisms involved in the downstream signaling and cross-talk to improve insulin sensitivity have not been fully elucidated. In this study, the potential targets of GA were identified by chemical proteomics strategies using serial GA probes for target fishing and cell molecular imaging. Intracellular enzyme activity evaluation and insulin resistance models were used for validating the function of the target proteins on the downstream insulin signaling pathways. Collectively, our data demonstrate that GA improved the insulin-responsive pathway and glucose consumption levels via multiple diabetogenic factors that activated the insulin signaling pathway in HepG2 cells. GA improved Glucose transporter 4(GLUT4) expression by targeting the Ras protein to regulate the mitogen-activated protein kinase (MAPK) pathway. GA exhibited a strong inhibitory effect on IRS1ser307 phosphorylation in cells treated with the Protein kinase C (PKC) activator Phorbol 12-myristate 13-acetate (PMA.) Consistently, IRS1ser307 phosphorylation was also inhibited by GA in Free fatty acid (FFA)-treated HepG2 cells. GA also inhibited the PMA-induced phosphorylation of IκB kinase α/β (IKKα/β), c-Jun N-terminal kinase (JNK) and p38 proteins (P38), suggesting that IKKα/β, JNK and P38 activation is dependent on PKC activity.

Plant hormones: a fungal point of view

Most classical plant hormones are also produced by pathogenic and symbiotic fungi. The way in which these molecules favour the invasion of plant tissues and the development of fungi inside plant tissues is still largely unknown. In this review, we examine the different roles of such hormone production by pathogenic fungi. Converging evidence suggests that these fungal-derived molecules have potentially two modes of action: (i) they may perturb plant processes, either positively or negatively, to favour invasion and nutrient uptake; and (ii) they may also act as signals for the fungi themselves to engage appropriate developmental and physiological processes adapted to their environment. Indirect evidence suggests that abscisic acid, gibberellic acid and ethylene produced by fungi participate in pathogenicity. There is now evidence that auxin and cytokinins could be positive regulators required for virulence. Further research should establish whether or not fungal-derived hormones act like other fungal effectors.

Therapeutic potential of the dual peroxisome proliferator activated receptor (PPAR)α/γ agonist aleglitazar in attenuating TNF-α-mediated inflammation and insulin resistance in human adipocytes

Adipose tissue inflammation is a mechanistic link between obesity and its related sequelae, including insulin resistance and type 2 diabetes. Dual ligands of peroxisome proliferator activated receptor (PPAR)α and γ, combining in a single molecule the metabolic and inflammatory-regulatory properties of α and γ agonists, have been proposed as a promising therapeutic strategy to antagonize adipose tissue inflammation. Here we investigated the effects of the dual PPARα/γ agonist aleglitazar on human adipocytes challenged with inflammatory stimuli. Human Simpson-Golabi-Behmel syndrome (SGBS) adipocytes were treated with aleglitazar or - for comparison - the selective agonists for PPARα or γ fenofibrate or rosiglitazone, respectively, for 24h before stimulation with TNF-α. Aleglitazar, at concentrations as low as 10nmol/L, providing the half-maximal transcriptional activation of both PPARα and PPARγ, reduced the stimulated expression of several pro-inflammatory mediators including interleukin (IL)-6, the chemokine CXC-L10, and monocyte chemoattractant protein (MCP)-1. Correspondingly, media from adipocytes treated with aleglitazar reduced monocyte migration, consistent with suppression of MCP-1 secretion. Under the same conditions, aleglitazar also reversed the TNF-α-mediated suppression of insulin-stimulated ser473 Akt phosphorylation and decreased the TNF-α-induced ser312 IRS1 phosphorylation, two major switches in insulin-mediated metabolic activities, restoring glucose uptake in insulin-resistant adipocytes. Such effects were similar to those obtainable with a combination of single PPARα and γ agonists. In conclusion, aleglitazar reduces inflammatory activation and dysfunction in insulin signaling in activated adipocytes, properties that may benefit diabetic and obese patients. The effect of aleglitazar was consistent with dual PPARα and γ agonism, but with no evidence of synergism.

Sustained Action of Ceramide on the Insulin Signaling Pathway in Muscle Cells: IMPLICATION OF THE DOUBLE-STRANDED RNA-ACTIVATED PROTEIN KINASE

In vivo, ectopic accumulation of fatty acids in muscles leads to alterations in insulin signaling at both the IRS1 and Akt steps. However, in vitro treatments with saturated fatty acids or their derivative ceramide demonstrate an effect only at the Akt step. In this study, we adapted our experimental procedures to mimic the in vivo situation and show that the double-stranded RNA-dependent protein kinase (PKR) is involved in the long-term effects of saturated fatty acids on IRS1. C2C12 or human muscle cells were incubated with palmitate or directly with ceramide for short or long periods, and insulin signaling pathway activity was evaluated. PKR involvement was assessed through pharmacological and genetic studies. Short-term treatments of myotubes with palmitate, a ceramide precursor, or directly with ceramide induce an inhibition of Akt, whereas prolonged periods of treatment show an additive inhibition of insulin signaling through increased IRS1 serine 307 phosphorylation. PKR mRNA, protein, and phosphorylation are increased in insulin-resistant muscles. When PKR activity is reduced (siRNA or a pharmacological inhibitor), serine phosphorylation of IRS1 is reduced, and insulin-induced phosphorylation of Akt is improved. Finally, we show that JNK mediates ceramide-activated PKR inhibitory action on IRS1. Together, in the long term, our results show that ceramide acts at two distinct levels of the insulin signaling pathway (IRS1 and Akt). PKR, which is induced by both inflammation signals and ceramide, could play a major role in the development of insulin resistance in muscle cells.

Polyactives: controlled and sustained bioactive release via hydrolytic degradation

Significant and promising advances have been made in the polymer field for controlled and sustained bioactive delivery. Traditionally, small molecule bioactives have been physically incorporated into biodegradable polymers; however, chemical incorporation allows for higher drug loading, more controlled release, and enhanced processability. Moreover, the advent of bioactive-containing monomer polymerization and hydrolytic biodegradability allows for tunable bioactive loading without yielding a polymer residue. In this review, we highlight the chemical incorporation of different bioactive classes into novel biodegradable and biocompatible polymers. The polymer design, synthesis, and formulation are summarized in addition to the evaluation of bioactivity retention upon release via in vitro and in vivo studies.

Osmostress-induced apoptosis in Xenopus oocytes: role of stress protein kinases, calpains and Smac/DIABLO

Hyperosmotic shock induces cytochrome c release and caspase-3 activation in Xenopus oocytes, but the regulators and signaling pathways involved are not well characterized. Here we show that hyperosmotic shock induces rapid calpain activation and high levels of Smac/DIABLO release from the mitochondria before significant amounts of cytochrome c are released to promote caspase-3 activation. Calpain inhibitors or EGTA microinjection delays osmostress-induced apoptosis, and blockage of Smac/DIABLO with antibodies markedly reduces cytochrome c release and caspase-3 activation. Hyperosmotic shock also activates the p38 and JNK signaling pathways very quickly. Simultaneous inhibition of both p38 and JNK pathways reduces osmostress-induced apoptosis, while sustained activation of these kinases accelerates the release of cytochrome c and caspase-3 activation. Therefore, at least four different pathways early induced by osmostress converge on the mitochondria to trigger apoptosis. Deciphering the mechanisms of hyperosmotic shock-induced apoptosis gives insight for potential treatments of human diseases that are caused by perturbations in fluid osmolarity.

Injectable microspheres for extended delivery of bioactive insulin and salicylic acid

Developing methods for insulin delivery continues to be of great translational research interest as insulin remains one of the most effective and commonly used treatments for diabetes. Bolus insulin injection at frequent intervals or insulin-loaded pumps used to treat diabetic patients have drawbacks including highly uneven kinetics, low patient compliance, enhanced chances of infections and disease transmission, and device fouling. This study evaluates the in vivo effects of insulin-loaded, salicylate-based, biocompatible, biodegradable polymeric microspheres that gradually release salicylic acid and insulin simultaneously. The study is predicated on the knowledge that such a continuous delivery system can release insulin over an extended period of time and overcome the aforementioned issues. Additionally, salicylic acid reduces insulin resistance in type-2 diabetic patients. In this work, we observed that insulin and salicylic acid were detected in serum over an extended period of time (at least 12 h and 4 days, respectively), and mice receiving insulin-loaded microspheres had a blood glucose reduction time frame ⩾12 times that of bolus insulin administration.

Hibiscus sabdariffa polyphenols alleviate insulin resistance and renal epithelial to mesenchymal transition: a novel action mechanism mediated by type 4 dipeptidyl peptidase

The epithelial to mesenchymal transition (EMT) is important in renal fibrosis. Ser307 phosphorylation of insulin receptor substrate-1 (IRS-1 (S307)) is a hallmark of insulin resistance. We report that polyphenol extracts of Hibiscus sabdariffa (HPE) ameliorate diabetic nephropathy and EMT. Recently it has been observed that type 4 dipeptidyl peptidase (DPP-4) inhibitor linagliptin is effective for treating type 2 diabetes and albuminuria. We investigated if DPP-4 and insulin resistance are involved in renal EMT and explored the role of HPE. In high glucose-stimulated tubular cells, HPE, like linagliptin, inhibited DPP-4 activation, thereby regulating vimentin (EMT marker) and IRS-1 (S307). IRS-1 knockdown revealed its essential role in mediating downstream EMT. In type 2 diabetic rats, pIRS-1 (S307) abundantly surrounds the tubular region, with increased vimentin in kidney. Both the expressions were reduced by HPE. In conclusion, HPE exerts effects similar to those of linagliptin, which improves insulin resistance and EMT, and could be an adjuvant to prevent diabetic nephropathy.

A PKC-SHP1 signaling axis desensitizes Fcγ receptor signaling by reducing the tyrosine phosphorylation of CBL and regulates FcγR mediated phagocytosis

BACKGROUND

Fcγ receptors mediate important biological signals in myeloid cells including the ingestion of microorganisms through a process of phagocytosis. It is well-known that Fcγ receptor (FcγR) crosslinking induces the tyrosine phosphorylation of CBL which is associated with FcγR mediated phagocytosis, however how signaling molecules coordinate to desensitize these receptors is unclear. An investigation of the mechanisms involved in receptor desensitization will provide new insight into potential mechanisms by which signaling molecules may downregulate tyrosine phosphorylation dependent signaling events to terminate important signaling processes.

RESULTS

Using the U937IF cell line, we observed that FcγR1 crosslinking induces the tyrosine phosphorylation of CBL, which is maximal at 5 min. followed by a kinetic pattern of dephosphorylation. An investigation of the mechanisms involved in receptor desensitization revealed that pretreatment of U937IF or J774 cells with PMA followed by Fcγ receptor crosslinking results in the reduced tyrosine phosphorylation of CBL and the abrogation of downstream signals, such as CBL-CRKL binding, Rac-GTP activation and the phagocytic response. Pretreatment of J774 cells with GF109203X, a PKC inhibitor was observed to block dephosphorylation of CBL and rescued the phagocytic response. We demonstrate that the PKC induced desensitization of FcγR/ phagocytosis is associated with the inactivation of Rac-GTP, which is deactivated in a hematopoietic specific phosphatase SHP1 dependent manner following ITAM stimulation. The effect of PKC on FcγR signaling is augmented by the transfection of catalytically active SHP1 and not by the transfection of catalytic dead SHP1 (C124S).

CONCLUSIONS

Our results suggest a functional model by which PKC interacts with SHP1 to affect the phosphorylation state of CBL, the activation state of Rac and the negative regulation of ITAM signaling i.e. Fcγ receptor mediated phagocytosis. These findings suggest a mechanism for Fcγ receptor desensitization by which a serine-threonine kinase e.g. PKC downregulates tyrosine phosphorylation dependent signaling events via the reduced tyrosine phosphorylation of the complex adapter protein, CBL.

Non-steroidal anti-inflammatory drugs activate NADPH oxidase in adipocytes and raise the H2O2 pool to prevent cAMP-stimulated protein kinase a activation and inhibit lipolysis

BACKGROUND

Non-steroidal anti-inflammatory drugs (NSAIDs) -aspirin, naproxen, nimesulide, and piroxicam- lowered activation of type II cAMP-dependent protein kinase A (PKA-II) in isolated rat adipocytes, decreasing adrenaline- and dibutyryl cAMP (Bt2cAMP)-stimulated lipolysis. The molecular bases of insulin-like actions of NSAID were studied.

RESULTS

Based on the reported inhibition of lipolysis by H2O2, catalase was successfully used to block NSAID inhibitory action on Bt2cAMP-stimulated lipolysis. NSAID, at (sub)micromolar range, induced an H2O2 burst in rat adipocyte plasma membranes and in whole adipocytes. NSAID-mediated rise of H2O2 was abrogated in adipocyte plasma membranes by: diphenyleneiodonium, an inhibitor of NADPH oxidase (NOX); the NOX4 antibody; and cytochrome c, trapping the NOX-formed superoxide. These three compounds prevented the inhibition of Bt2cAMP-stimulated lipolysis by NSAIDs. Inhibition of aquaporin-mediated H2O2 transport with AgNO3 in adipocytes allowed NOX activation but prevented the lipolysis inhibition promoted by NSAID: i.e., once synthesized, H2O2 must reach the lipolytic machinery. Since insulin inhibits adrenaline-stimulated lipolysis, the effect of aspirin on isoproterenol-stimulated lipolysis in rat adipocytes was studied. As expected, isoproterenol-mediated lipolysis was blunted by both insulin and aspirin.

CONCLUSIONS

NSAIDs activate NOX4 in adipocytes to produce H2O2, which impairs cAMP-dependent PKA-II activation, thus preventing isoproterenol-activated lipolysis. H2O2 signaling in adipocytes is a novel and important cyclooxygenase-independent effect of NSAID.

PPARγ as a therapeutic target in diabetic nephropathy and other renal diseases

PURPOSE OF REVIEW

Peroxisome proliferator-activated receptor gamma (PPARγ) is a ligand-activated nuclear transcription factor that regulates many important physiological processes including glucose and lipid metabolism, energy homeostasis, cell proliferation, inflammation, immunity and reproduction. The current review aims to summarize and discuss recent findings evaluating the protective effects of PPARγ against kidney diseases with a focus on diabetic nephropathy. We will also delineate the potential underlying mechanisms.

RECENT FINDINGS

PPARγ plays important roles in renal physiology and pathophysiology. Agonists of PPARγ exert protective effects against various kidney diseases including diabetic nephropathy, ischemic renal injury, IgA nephropathy, chemotherapy-associated kidney damage, polycystic kidney diseases and age-related kidney diseases via both systemic and renal actions.

SUMMARY

PPARγ agonists are effective in delaying and even preventing the progression of many renal diseases, especially diabetic nephropathy. PPARγ may represent a promising target for the treatment of renal diseases.

Suppression of free fatty acid-induced insulin resistance by phytopolyphenols in C2C12 mouse skeletal muscle cells

It was reported that increased plasma levels of free fatty acids (FFAs) are associated with profound insulin resistance in skeletal muscle and may also play a critical role in the insulin resistance of obesity and type 2 diabetes mellitus. Skeletal muscle is the major site for insulin-stimulated glucose uptake and is involved in energy regulation and homeostasis. In this study, we used 12-O-tetradecanoylphorbol 13-acetate (TPA), a protein kinase C (PKC) activator, and palmitate to induce insulin resistance in C2C12 mouse skeletal muscle cells. Our data show that epigallocatechin gallate (EGCG) and curcumin treatment reduce insulin receptor substrate-1 (IRS-1) Ser307 phosphorylation, and curcumin is more potent to increase Akt phosphorylation in TPA induction. Moreover, we found that after 5 h of palmitate incubation, epicatechin gallate (ECG) can suppress IRS-1 Ser307 phosphorylation and significantly promote Akt, ERK1/2, p38 MAPK, and AMP-activated protein kinase activation. With a longer incubation with palmitate, IRS-1 exhibited a dramatic depletion, and treatment with EGCG, ECG, and curcumin could reverse IRS-1 expression, Akt phosphorylation, and MAPK signaling cascade activation and improve glucose uptake in C2C12 skeletal muscle cells, especially ECG and curcumin. In addition, treatment with these polyphenols can suppress acetyl-CoA carboxylase activation, but only EGCG could inhibit lipid accumulation in the intracellular site. These findings may suggest that curcumin shows the best capacity to improve FFA-induced insulin resistance than the other two, and ECG was more effective than EGCG in attenuating insulin resistance.

Insulin receptor substrate 2 expression and involvement in neuronal insulin resistance in diabetic neuropathy

Insulin signaling depends on tyrosine phosphorylation of insulin receptor substrates (IRSs) to mediate downstream effects; however, elevated serine phosphorylation of IRS impairs insulin signaling. Here, we investigated IRS protein expression patterns in dorsal root ganglia (DRG) of mice and whether their signaling was affected by diabetes. Both IRS1 and IRS2 are expressed in DRG; however, IRS2 appears to be the prevalent isoform and is expressed by many DRG neuronal subtypes. Phosphorylation of Ser(731)IRS2 was significantly elevated in DRG neurons from type 1 and type 2 diabetic mice. Additionally, Akt activation and neurite outgrowth in response to insulin were significantly decreased in DRG cultures from diabetic ob/ob mice. These results suggest that DRG neurons express IRS proteins that are altered by diabetes similar to other peripheral tissues, and insulin signaling downstream of the insulin receptor may be impaired in sensory neurons and contribute to the pathogenesis of diabetic neuropathy.

Inflammatory signaling in skeletal muscle insulin resistance: green signal for nutritional intervention?

PURPOSE OF REVIEW

To review the evidence implying a role of inflammatory signaling pathways, specifically nuclear factor-κB and c-Jun NH2-terminal kinase, in fatty acid-induced skeletal muscle insulin resistance and to discuss the potential of dietary interventions to interfere with these processes.

RECENT FINDINGS

Fatty acids can induce skeletal muscle insulin resistance via inflammatory signaling after binding Toll-like receptors at the cell membrane of muscle cells or after accumulating as intramyocellular lipid metabolites. In both processes, activation of intracellular inflammatory signaling is involved. The majority of literature addressing the causality of muscle nuclear factor-κB activation in skeletal muscle insulin resistance suggests that insulin resistance does not require muscle nuclear factor-κB activation. Recently, strong evidence was given that c-Jun NH2-terminal kinase signaling is an important inflammatory pathway involved in skeletal muscle insulin resistance. Furthermore, it is well established that proinflammatory cytokines originating from the enlarged adipose tissue or from activated adipose tissue macrophages can cause muscle insulin resistance. Recently, also macrophages resided in the muscle have been proposed to play an important role in muscle insulin resistance. Because of their anti-inflammatory characteristics, several dietary components like polyphenols may be interesting candidates for manipulating skeletal muscle insulin resistance.

SUMMARY

Several dietary components, like polyphenols, have been reported to interfere with inflammatory signaling. To test whether these compounds can be used to prevent or reverse insulin resistance, well controlled human intervention studies have to be designed.

Palmitate-induced skeletal muscle insulin resistance does not require NF-κB activation

Palmitate activates the NF-κB pathway, and induces accumulation of lipid metabolites and insulin resistance in skeletal muscle cells. Little information is available whether and how these processes are causally related. Therefore, the objectives were to investigate whether intra-cellular lipid metabolites are involved in FA-induced NF-κB activation and/or insulin resistance in skeletal muscle and to investigate whether FA-induced insulin resistance and NF-κB activation are causally related. Inhibiting DGAT or CPT-1 by using, respectively, amidepsine or etomoxir increased DAG accumulation and sensitized myotubes to palmitate-induced insulin resistance. While co-incubation of palmitate with etomoxir increased NF-κB transactivation, co-incubation with amidepsine did not, indicating that DAG accumulation is associated with insulin resistance but not with NF-κB activation. Furthermore, pharmacological or genetic inhibition of the NF-κB pathway could not prevent palmitate-induced insulin resistance. In conclusion, we have demonstrated that activation of the NF-κB pathway is not required for palmitate-induced insulin resistance in skeletal muscle cells.

Potential role of salicylates in type 2 diabetes

OBJECTIVE

To review the evidence base supporting the use of salicylates for glucose level control in patients with type 2 diabetes and provide a comprehensive review of available information describing the potential role of salicylates and, in particular, salsalate, for glucose control in type 2 diabetes prevention and treatment.

DATA SOURCES

A literature search using MEDLINE (1966-March 2010), PubMed, and Google Scholar was conducted using the search terms salicylates, salicylic acid, aspirin, salsalate, acetylsalicylic acid, insulin, glucose, glycemic control, diabetes, hyperglycemia, and nuclear factor. The bibliographies of identified articles were reviewed for additional citations.

STUDY SELECTION AND DATA EXTRACTION

All relevant English-language information on the pharmacology, efficacy, and safety of salicylates for glucose control related to insulin resistance or diabetes prevention were reviewed. Preclinical data, clinical trials, and case reports were identified, evaluated, and included in this systematic review.

DATA SYNTHESIS

Treatment of inflammation may be a potential novel strategy in prevention and treatment of type 2 diabetes, in which the body is resistant to the effects of insulin. Previous and recent studies reveal a possible relationship between inflammation and obesity. The chronic activation of the immune system due to low-grade inflammation was found in several studies to be associated with obesity, and this, in turn, can promote development of insulin resistance and impaired glucose tolerance. Administration of salicylates was shown over a century ago to lower glucose levels in patients with diabetes. Many in vitro and in vivo pharmacologic studies have demonstrated a glucose-lowering effect of salicylates. Salicylates, especially salsalate, were found in several clinical studies and case reports to be potential agents for diabetes treatment with a favorable safety profile. Although these studies had inherent limitations, such as small numbers of patients and short duration, the vast majority showed significant glucose-lowering effects. A large randomized trial, the National Institute of Diabetes and Digestive and Kidney Diseases-sponsored Targeting Inflammation with Salsalate in Type 2 Diabetes (TINSAL-T2D) trial, recently concluded that salsalate lowers hemoglobin A(1c) levels and improves glycemic control in patients with type 2 diabetes.

CONCLUSIONS

Salicylates, especially salsalate, appear to be a promising treatment option for prevention or treatment of diabetes by lowering glucose levels. More extensive studies are needed to confirm the mechanisms involved and whether the effects are sustainable with continued administration of these agents. Further studies are warranted.

JNK1 and IKKbeta: molecular links between obesity and metabolic dysfunction

Inflammation is thought to underlie the pathogenesis of many chronic diseases. It is now established that obesity results in a state of chronic low-grade inflammation thought to contribute to several metabolic disorders, including insulin resistance and pancreatic islet dysfunction. The protein kinases JNK1 and IKKbeta have been found to serve as critical molecular links between obesity, metabolic inflammation, and disorders of glucose homeostasis. The precise mechanisms of these linkages are still being investigated. However, as we discuss here, JNK1 and IKKbeta are activated by almost all forms of metabolic stress that have been implicated in insulin resistance or islet dysfunction. Furthermore, both JNK1 and IKKbeta are critically involved in the promotion of diet-induced obesity, metabolic inflammation, insulin resistance, and beta-cell dysfunction. Understanding the molecular mechanisms by which JNK1 and IKKbeta mediate obesity-induced metabolic stress is likely to be of importance for the development of new treatments for a variety of obesity-associated diseases.

Aspirin attenuates insulin resistance in muscle of diet-induced obese rats by inhibiting inducible nitric oxide synthase production and S-nitrosylation of IRbeta/IRS-1 and Akt

AIM/HYPOTHESIS

High-dose aspirin treatment improves fasting and postprandial hyperglycaemia in patients with type 2 diabetes, as well as in animal models of insulin resistance associated with obesity and sepsis. In this study, we investigated the effects of aspirin treatment on inducible nitric oxide synthase (iNOS)-mediated insulin resistance and on S-nitrosylation of insulin receptor (IR)-beta, IRS-1 and protein kinase B (Akt) in the muscle of diet-induced obese rats and also in iNos (also known as Nos2)-/- mice on high fat diet.

METHODS

Aspirin (120 mg kg-1 day-1 for 2 days) or iNOS inhibitor (L-NIL; 80 mg/kg body weight) were administered to diet-induced obese rats or mice and iNOS production and insulin signalling were investigated. S-nitrosylation of IRbeta/IRS-1 and Akt was investigated using the biotin switch method.

RESULTS

iNOS protein levels increased in the muscle of diet-induced obese rats, associated with an increase in S-nitrosylation of IRbeta, IRS-1 and Akt. These alterations were reversed by aspirin treatment, in parallel with an improvement in insulin signalling and sensitivity, as measured by insulin tolerance test and glucose clamp. Conversely, while aspirin reversed the increased phosphorylation of IkappaB kinase beta and c-Jun amino-terminal kinase, as well as IRS-1 serine phosphorylation in diet-induced obese rats and iNos -/- mice on high-fat diet, these alterations were not associated with the improvement of insulin action induced by this drug.

CONCLUSIONS/INTERPRETATION

Our data demonstrate that aspirin treatment not only reduces iNOS protein levels, but also S-nitrosylation of IRbeta, IRS-1 and Akt. These changes are associated with improved insulin resistance and signalling, suggesting a novel mechanism of insulin sensitisation evoked by aspirin treatment.

Sensing the insulin signaling pathway with an antibody array

The development of insulin resistance and type 2 diabetes is determined by various factors, including defects within the insulin signaling pathway. Mediators of insulin resistance operate through activation of various protein kinase C isoforms, IκB kinase β (IKKβ), and/or c-Jun N-terminal kinase, and subsequent inhibition of the proximal insulin signaling pathway via the insulin receptor substrate 1 and Akt. These mechanisms are still largely unresolved because of the complexity of the molecular events. In this study, an expression and activation state profiling of multiple known key signaling biomolecules involved in insulin metabolic and mitogenic signaling pathways was evaluated using a phosphospecific antibody array platform. The results of the arrayed antibodies were verified by the multiplexed bead array assay and conventional Western blot analysis, and confirmed the well-known inhibitory effects of phorbol esters on insulin signaling pathway activation. Of interest, the increase in protein kinase C signaling responses with phorbol esters was associated with activation of the lipid phosphatase PTEN and a 27 kDa HSP. Thus, this insulin signaling antibody array provides a powerful and effective way to investigate the mechanism of insulin resistance and likely assist the development of innovative therapeutic drugs for type 2 diabetes.

TNFalpha activation of PKCdelta, mediated by NFkappaB and ER stress, cross-talks with the insulin signaling cascade

TNFalpha plays key roles in the regulation of inflammation, cell death, and proliferation and its signaling cascade cross-talks with the insulin signaling cascade. PKCdelta, a novel PKC isoform, is known to participate in proximal TNFalpha signaling events. However, it has remained unclear whether PKCdelta plays a role in distal TNFalpha signaling events. Here we demonstrate that PKCdelta is activated by TNFalpha in a delayed fashion that is temporally associated with JNK activation. To investigate the signaling pathways activating PKCdelta and JNK, we used pharmacological and genetic inhibitors of NFkappaB. We found that inhibition of NFkappaB attenuated PKCdelta and JNK activations. Further analysis revealed that ER stress contributes to TNFalpha-stimulated PKCdelta and JNK activations. To investigate the role of PKCdelta in TNFalpha action, we used 29-mer shRNAs to silence PKCdelta expression. A reduction of ~90% in PKCdelta protein levels reduced TNFalpha-stimulated stress kinase activation, including JNK. Further, PKCdelta was necessary for thapsigargin-stimulated JNK activation. Because thapsigargin is a potent inducer of ER stress, we determined whether PKCdelta was necessary for induction of the UPR. Indeed, a reduction in PKCdelta protein levels reduced thapsigargin-stimulated CHOP induction, a hallmark of the UPR, but not BiP/GRP78 induction, suggesting that PKCdelta does not globally regulate the UPR. Next, the role of PKCdelta in TNFalpha mediated cross-talk with the insulin signaling pathway was investigated in cells expressing human IRS-1 and a 29-mer shRNA to silence PKCdelta expression. We found that a reduction in PKCdelta protein levels reversed the TNFalpha-mediated reduction in insulin-stimulated IRS-1 Tyr phosphorylation, Akt activation, and glycogen synthesis. In addition, TNFalpha-stimulated IRS protein Ser/Thr phosphorylation and degradation were blocked. Our results indicate that: 1) NFkappaB and ER stress contribute in part to PKCdelta activation; 2) PKCdelta plays a key role in the propagation of the TNFalpha signal; and 3) PKCdelta contributes to TNFalpha-induced inhibition of insulin signaling events.

Specificity of insulin signalling in human skeletal muscle as revealed by small interfering RNA

Insulin action on metabolically active tissues is a complex process involving positive and negative feedback regulation to control whole body glucose homeostasis. At the cellular level, glucose and lipid metabolism, as well as protein synthesis, are controlled through canonical insulin signalling cascades. The discovery of small interfering RNA (siRNA) allows for the molecular dissection of critical components of the regulation of metabolic and gene regulatory events in insulin-sensitive tissues. The application of siRNA to tissues of human origin allows for the molecular dissection of the mechanism(s) regulating glucose and lipid metabolism. Penetration of the pathways controlling insulin action in human tissue may aid in discovery efforts to develop diabetes prevention and treatment strategies. This review will focus on the use of siRNA to validate critical regulators controlling insulin action in human skeletal muscle, a key organ important for the control of whole body insulin-mediated glucose uptake and metabolism.

Feed-forward signaling of TNF-alpha and NF-kappaB via IKK-beta pathway contributes to insulin resistance and coronary arteriolar dysfunction in type 2 diabetic mice

We hypothesized that the interaction between tumor necrosis factor-alpha (TNF-alpha)/nuclear factor-kappaB (NF-kappaB) via the activation of IKK-beta may amplify one another, resulting in the evolution of vascular disease and insulin resistance associated with diabetes. To test this hypothesis, endothelium-dependent (ACh) and -independent (sodium nitroprusside) vasodilation of isolated, pressurized coronary arterioles from mLepr(db) (heterozygote, normal), Lepr(db) (homozygote, diabetic), and Lepr(db) mice null for TNF-alpha (db(TNF-)/db(TNF-)) were examined. Although the dilation of vessels to sodium nitroprusside was not different between Lepr(db) and mLepr(db) mice, the dilation to ACh was reduced in Lepr(db) mice. The NF-kappaB antagonist MG-132 or the IKK-beta inhibitor sodium salicylate (NaSal) partially restored nitric oxide-mediated endothelium-dependent coronary arteriolar dilation in Lepr(db) mice, but the responses in mLepr(db) mice were unaffected. The protein expression of IKK-alpha and IKK-beta were higher in Lepr(db) than in mLepr(db) mice; the expression of IKK-beta, but not the expression of IKK-alpha, was attenuated by MG-132, the antioxidant apocynin, or the genetic deletion of TNF-alpha in diabetic mice. Lepr(db) mice showed an increased insulin resistance, but NaSal improved insulin sensitivity. The protein expression of TNF-alpha and NF-kappaB and the protein modification of phosphorylated (p)-IKK-beta and p-JNK were greater in Lepr(db) mice, but NaSal attenuated TNF-alpha, NF-kappaB, p-IKK-beta, and p-JNK in Lepr(db) mice. The ratio of p-insulin receptor substrate (IRS)-1 at Ser307 to IRS-1 was elevated in Lepr(db) compared with mLepr(db) mice; both NaSal and the JNK inhibitor SP-600125 reduced the p-IRS-1-to-IRS-1 ratio in Lepr(db) mice. MG-132 or the neutralization of TNF-alpha reduced superoxide production in Lepr(db) mice. In conclusion, our results indicate that the interaction between NF-kappaB and TNF-alpha signaling induces the activation of IKK-beta and amplifies oxidative stress, leading to endothelial dysfunction in type 2 diabetes.

Role of PPARgamma in renoprotection in Type 2 diabetes: molecular mechanisms and therapeutic potential

DN (diabetic nephropathy) is a chronic disease characterized by proteinuria, glomerular hypertrophy, decreased glomerular filtration and renal fibrosis with loss of renal function. DN is the leading cause of ESRD (end-stage renal disease), accounting for millions of deaths worldwide. TZDs (thiazolidinediones) are synthetic ligands of PPARgamma (peroxisome-proliferator-activated receptor gamma), which is involved in many important physiological processes, including adipose differentiation, lipid and glucose metabolism, energy homoeostasis, cell proliferation, inflammation, reproduction and renoprotection. A large body of research over the past decade has revealed that, in addition to their insulin-sensitizing effects, TZDs play an important role in delaying and preventing the progression of chronic kidney disease in Type 2 diabetes. Although PPARgamma activation by TZDs is in general considered beneficial for the amelioration of diabetic renal complications in Type 2 diabetes, the underlying mechanism(s) remains only partially characterized. In this review, we summarize and discuss recent findings regarding the renoprotective effects of PPARgamma in Type 2 diabetes and the potential underlying mechanisms.

S6K directly phosphorylates IRS-1 on Ser-270 to promote insulin resistance in response to TNF-(alpha) signaling through IKK2

S6K1 (p70S6K) is a serine kinase downstream from Akt in the insulin signaling pathway that is involved in negative feedback regulation of insulin action. S6K1 is also activated by TNF-alpha, a pro-inflammatory cytokine. However, its role remains to be characterized. In the current study, we elucidated a mechanism for S6K1 to mediate TNF-alpha-induced insulin resistance in adipocytes and hepatocytes. S6K1 was phosphorylated at Thr-389 in response to TNF-alpha. This led to phosphorylation of IRS-1 by S6K1 at multiple serine residues including Ser-270, Ser-307, Ser-636, and Ser-1101 in human IRS-1 (Ser-265, Ser-302, Ser-632, and Ser-1097, in rodent IRS-1). Direct phosphorylation of these sites by S6K1 was observed in an in vitro kinase assay using purified IRS-1 and S6K1. Phosphorylation of all these serines was increased in the adipose tissue of obese mice. RNAi knockdown demonstrated an important role for S6K1 in mediating TNF-alpha-induced IRS-1 inhibition that led to impaired insulin-stimulated glucose uptake in adipocytes. A point mutant of IRS-1 (S270A) impaired association of IRS-1 with S6K1 resulting in diminished phosphorylation of IRS-1 at three other S6K1 phosphorylation sites (Ser-307, Ser-636, and Ser-1101). Expression of a dominant negative S6K1 mutant prevented TNF-induced Ser-270 phosphorylation and IRS-1 protein degradation. Moreover, in IKK2 (but not IKK1)-null cells, TNF-alpha treatment did not result in Thr-389 phosphorylation of S6K1. We present a new mechanism for TNF-alpha to induce insulin resistance that involves activation of S6K by an IKK2-dependent pathway. S6K directly phosphorylates IRS-1 on multiple serine residues to inhibit insulin signaling.

Interplay and effects of temporal changes in the phosphorylation state of serine-302, -307, and -318 of insulin receptor substrate-1 on insulin action in skeletal muscle cells

Transduction of the insulin signal is mediated by multisite Tyr and Ser/Thr phosphorylation of the insulin receptor substrates (IRSs). Previous studies on the function of single-site phosphorylation, particularly phosphorylation of Ser-302, -307, and -318 of IRS-1, showed attenuating as well as enhancing effects on insulin action. In this study we investigated a possible cross talk of these opposedly acting serine residues in insulin-stimulated skeletal muscle cells by monitoring phosphorylation kinetics, and applying loss of function, gain of function, and combination mutants of IRS-1. The phosphorylation at Ser-302 was rapid and transient, followed first by Ser-318 phosphorylation and later by phosphorylation of Ser-307, which remained elevated for 120 min. Mutation of Ser-302 to alanine clearly reduced the subsequent protein kinase C-zeta-mediated Ser-318 phosphorylation. The Ser-307 phosphorylation was independent of Ser-302 and/or Ser-318 phosphorylation status. The functional consequences of these phosphorylation patterns were studied by the expression of IRS-1 mutants. The E302A307E318 mutant simulating the early phosphorylation pattern resulted in a significant increase in Akt and glycogen synthase kinase 3 phosphorylation. Furthermore, glucose uptake was enhanced. Because the down-regulation of the insulin signal was not affected, this phosphorylation pattern seems to be involved in the enhancement but not in the termination of the insulin signal. This enhancing effect was completely absent when Ser-302 was unphosphorylated and Ser-307 was phosphorylated as simulated by the A302E307E318 mutant. Phospho-Ser-318, sequentially phosphorylated at least by protein kinase C-zeta and a mammalian target of rapamycin/raptor-dependent kinase, was part of the positive as well as of the subsequent negative phosphorylation pattern. Thus we conclude that insulin stimulation temporally generates different phosphorylation statuses of the same residues that exert different functions in insulin signaling.

siRNA-mediated reduction of inhibitor of nuclear factor-kappaB kinase prevents tumor necrosis factor-alpha-induced insulin resistance in human skeletal muscle

OBJECTIVE

Proinflammatory cytokines contribute to systemic low-grade inflammation and insulin resistance. Tumor necrosis factor (TNF)-alpha impedes insulin signaling in insulin target tissues. We determined the role of inhibitor of nuclear factor-kappaB kinase (IKK)beta in TNF-alpha-induced impairments in insulin signaling and glucose metabolism in skeletal muscle.

RESEARCH DESIGN AND METHODS

Small interfering RNA (siRNA) was used to silence IKKbeta gene expression in primary human skeletal muscle myotubes from nondiabetic subjects. siRNA gene silencing reduced IKKbeta protein expression 73% (P < 0.05). Myotubes were incubated in the absence or presence of insulin and/or TNF-alpha, and effects of IKKbeta silencing on insulin signaling and glucose metabolism were determined.

RESULTS

Insulin increased glucose uptake 1.7-fold (P < 0.05) and glucose incorporation into glycogen 3.8-fold (P < 0.05) in myotubes from nondiabetic subjects. TNF-alpha exposure fully impaired insulin-mediated glucose uptake and metabolism. IKKbeta siRNA protected against TNF-alpha-induced impairments in glucose metabolism, since insulin-induced increases in glucose uptake (1.5-fold; P < 0.05) and glycogen synthesis (3.5-fold; P < 0.05) were restored. Conversely, TNF-alpha-induced increases in insulin receptor substrate-1 serine phosphorylation (Ser(312)), Jun NH(2)-terminal kinase phosphorylation, and extracellular signal-related kinase-1/2 mitogen-activated protein kinase (MAPK) phosphorylation were unaltered by siRNA-mediated IKKbeta reduction. siRNA-mediated IKKbeta reduction prevented TNF-alpha-induced insulin resistance on Akt Ser(473) and Thr(308) phosphorylation and phosphorylation of the 160-kDa Akt substrate AS160. IKKbeta silencing had no effect on cell differentiation. Finally, mRNA expression of GLUT1 or GLUT4 and protein expression of MAPK kinase kinase kinase isoform 4 (MAP4K4) was unaltered by IKKbeta siRNA.

CONCLUSIONS

IKKbeta silencing prevents TNF-alpha-induced impairments in insulin action on Akt phosphorylation and glucose uptake and metabolism in human skeletal muscle.

Secretion of atherogenic risk factor apolipoprotein B-100 is increased by a potential mechanism of JNK/PKC-mediated insulin resistance in liver cells

Apolipoprotein B-100 (ApoB) is the main protein of the atherogenic lipoproteins and plasma ApoB levels reflect the total numbers of atherogenic lipoproteins. Induction of insulin resistance was accompanied by a considerable rise in the production of hepatic very low density lipoprotein (VLDL) containing ApoB and triglyceride. Increased plasma levels of ApoB and triglyceride in VLDL are common characteristics of the dyslipidemia associated with insulin resistance and type 2 diabetes mellitus. Thus, we investigate whether phorbol 12-myristate-13-acetate (PMA)-induced insulin resistance affects the increase of ApoB secretion. PMA increased ApoB secretion and transcriptional level of microsomal triglyceride transfer protein (MTP). PMA treatment also resulted in increase of insulin receptor substrate 1 (IRS1) serine312 (Ser312) and serine1101 (Ser1101) phosphorylation and induction of IRS1 degradation. Additionally, PMA induced activation of c-jun N-terminal kinase (JNK) and protein kinase C (PKC) isoforms (alpha, betaI, delta, zeta, theta), and reduced AKT8 virus oncogene cellular homolog (AKT) activation in a time dependent manner. PMA-induced ApoB secretion, MTP promoter activities, and IRS1 degradation was significantly decreased by treatment of JNK and PKCs inhibitors. Orthovanadate, a potent tyrosine phosphatase inhibitor, increased tyrosine phosphorylation of IRS1 and decreased ApoB secretion of Chang liver cells although PMA was co-treated. From the results, it was concluded that PMA-induced insulin resistance, through induction of serine phosphorylation of IRS1 mediated by activated JNK and PKCs, increases ApoB secretion in Chang liver cells.

Regulating insulin signaling and beta-cell function through IRS proteins

Diabetes mellitus is a complex disorder that arises from various causes, including dysregulated glucose sensing and impaired insulin secretion (maturity onset diabetes of youth, MODY), autoimmune-mediated beta-cell destruction (type 1), or insufficient compensation for peripheral insulin resistance (type 2). Type 2 diabetes is the most prevalent form that usually occurs at middle age; it afflicts more than 30 million people over the age of 65, but is appearing with greater frequency in children and adolescents. Dysregulated insulin signaling exacerbated by chronic hyperglycemia promotes a cohort of systemic disorders--including dyslipidemia, hypertension, cardiovascular disease, and female infertility. Understanding the molecular basis of insulin resistance can prevent these disorders and their inevitable progression to type 2 diabetes.

Ceramide-activated protein phosphatase involvement in insulin resistance via Akt, serine/arginine-rich protein 40, and ribonucleic acid splicing in L6 skeletal muscle cells

Elevated TNFalpha levels are associated with insulin resistance, but the molecular mechanisms linking cytokine signaling to impaired insulin function remain elusive. We previously demonstrated a role for Akt in insulin regulation of protein kinase CbetaII alternative splicing through phosphorylation of serine/arginine-rich protein 40, a required mechanism for insulin-stimulated glucose uptake. We hypothesized that TNFalpha attenuated insulin signaling by dephosphorylating Akt and its targets via ceramide-activated protein phosphatase. Western blot analysis of L6 cell lysates demonstrated impaired insulin-stimulated phosphorylation of Akt, serine/arginine-rich protein 40, and glycogen synthase kinase 3beta in response to TNFalpha and the short chain C6 ceramide analog. TNFalpha increased serine/threonine phosphatase activity of protein phosphatase 1 (PP1) in response to C6, but not insulin, suggesting a ceramide-specific effect. Myriocin, an inhibitor of de novo ceramide synthesis, blocked stimulation of the PP1 activity. Ceramide species measurement by liquid chromatography-mass spectrometry showed consistent increases in C24:1 and C16 ceramides. Effects of TNFalpha and C6 on insulin-stimulated phosphorylation of glycogen synthase kinase 3beta were prevented by myriocin and tautomycin, a PP1 inhibitor, further implicating a de novo ceramide-PP1 pathway. Alternative splicing assays demonstrated that TNFalpha abolished insulin-mediated inclusion of the protein kinase CbetaII exon. Collectively, our work demonstrates a role for PP1-like ceramide-activated protein phosphatase in mediating TNFalpha effects blocking insulin phosphorylation cascades involved in glycogen metabolism and alternative splicing.

Preadministration of high-dose salicylates, suppressors of NF-kappaB activation, may increase the chemosensitivity of many cancers: an example of proapoptotic signal modulation therapy

NF-kappaB activity is elevated in a high proportion of cancers, particularly advanced cancers that have been treated previously. Cytotoxic treatment selects for such up-regulation inasmuch as NF-kappaB promotes transcription of a large number of proteins that inhibit both the intrinsic and extrinsic pathways of apoptosis; NF-kappaB also boosts expression of mdr1, which expels many drugs from cells. Indeed, high NF-kappaB activity appears to be largely responsible for the chemo- and radioresistance of many cancers. Thus, agents that suppress NF-kappaB activity should be useful as adjuvants to cytotoxic cancer therapy. Of the compounds that are known to be NF-kappaB antagonists, the most practical for current use may be the nonsteroidal anti-inflammatory drugs aspirin, salicylic acid, and sulindac, each of which binds to and inhibits Ikappa kinase- beta, a central mediator of NF-kappa activation; the low millimolar plasma concentrations of salicylate required for effective inhibition of this kinase in vivo can be achieved with high-dose regimens traditionally used to manage rheumatic disorders. The gastrointestinal toxicity of such regimens could be minimized by using salsalate or enteric-coated sodium salicy-late or by administering misoprostol in conjunction with aspirin therapy. Presumably, best results would be seen if these agents were administered for several days prior to a course of chemo- or radiotherapy, continuing throughout the course. This concept should first be tested in nude mice bearing xenografts of chemoresistant human tumors known to have elevated NF-kappa activity. Ultimately, more complex adjuvant regimens can be envisioned in which salicylates are used in conjunction with other NF-kappa antagonists and/or agents that target other mediators of down-regulated apoptosis in cancer, such as Stat3; coadministration of salicylate and organic selenium may have intriguing potential in this regard. These strategies may also have potential as adjuvants to metronomic chemotherapy, which seeks to suppress angio-genesis by targeting cycling endothelial cells in tumors.

Differential effects of IRS1 phosphorylated on Ser307 or Ser632 in the induction of insulin resistance by oxidative stress

AIMS/HYPOTHESIS

Induction of stress kinases leading to serine hyperphosphorylation of IRS1 may link oxidative stress to insulin resistance. The aim of this study was to investigate the roles of the phosphorylated serine residues Ser307 and Ser632, two sites implicated in the inhibition of IRS1 function in insulin signalling.

MATERIALS AND METHODS

Fao hepatoma cells were exposed to an H(2)O(2)-generating system, and antibodies against the two phosphorylated serine residues were used for immunoprecipitation, immunoblot and immunofluorescence analyses.

RESULTS

Exposure to approximately 50 mumol/l H(2)O(2) for 2 h resulted in IRS1 phosphorylation on both Ser307 and Ser632, concomitant with activation of inhibitor kappa kinase beta (IKKbeta) and c-Jun kinase (JNK). Immunoprecipitation studies revealed that the maximum overlap between phospho (p) Ser307-IRS1 and pSer632-IRS1 was 20%, and confocal microscopy suggested distinct localisations of IRS1 molecules phosphorylated on either site. Although pSer307-IRS1 showed decreased insulin-induced tyrosine phosphorylation and interaction with phosphatidylinositol 3-kinase (PI3K) in response to insulin, pSer632-IRS1 molecules were normally tyrosine-phosphorylated and exhibited typical associated PI3K activity. Salicylic acid and SP600125 partially inhibited IKKbeta and JNK, respectively, which indicated distinct roles for these two kinases in the phosphorylation of IRS1 at the two serine sites. Protection against oxidation-mediated impairment in insulin-induced phosphorylation of protein kinase B/Akt and in glycogen synthesis was achieved only by combining salicylic acid and SP600125.

CONCLUSIONS/INTERPRETATION

These results suggest that pSer307-IRS1 and pSer632-IRS1 may define two minimally overlapping pools of IRS1 in response to oxidative stress, contributing differentially to insulin resistance. A combination of stress kinase inhibitors is required to protect against insulin resistance and IRS1 hyperphosphorylation induced by oxidative stress.

Rac upregulates tissue inhibitor of metalloproteinase-1 expression by redox-dependent activation of extracellular signal-regulated kinase signaling

The Rho-like GTPase Rac regulates distinct actin cytoskeleton changes required for adhesion, migration and invasion of cells. Tiam1 specifically activates Rac, and Rac has been shown to affect several signaling pathways in a partly cell-type-specific manner. Recently, we demonstrated that Rac activation inhibits Matrigel invasion of human carcinoma cells by transcriptional upregulation of tissue inhibitor of metalloproteinase-1. The purpose of the present study was to identify key mediators of Tiam1/Rac-induced tissue inhibitor of metalloproteinase-1 expression. Mutational analysis of the human tissue inhibitor of metalloproteinase-1 promoter revealed a major role for a distinct activating protein-1 site at -92/-86 and a minor role for an adjacent polyoma enhancer A3 site. Moreover, Rac activation induced the generation of reactive oxygen species and subsequent reactive oxygen species-dependent activation of extracellular signal-regulated kinase 1,2. In contrast, c-Jun N-terminal kinase and p38 mitogen-activated protein kinase activities were not affected. In line with this, Tiam1/Rac-induced tissue inhibitor of metalloproteinase-1 expression as well as Tiam1/Rac-induced binding of nuclear extracts to the activating protein-1 site at -92/-86 were inhibited by catalase and by specific inhibitors of the extracellular signal-related kinase-1,2 activators, mitogen-activated protein kinase kinase-1 and mitogen-activated protein kinase kinase-2 (PD098059, U0126). In conclusion, Rac-induced transcriptional upregulation of tissue inhibitor of metalloproteinase-1 is mediated by reactive oxygen species-dependent activation of extracellular signal-related kinase-1,2 and by transcription factors of the activating protein-1 family.

Enhancement of insulin-induced PI3K/Akt/GSK-3beta and ERK signaling by neuronal nicotinic receptor/PKC-alpha/ERK pathway: up-regulation of IRS-1/-2 mRNA and protein in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells treated with nicotine (10 microm for 24 h), phosphorylation of Akt, glycogen synthase kinase-3beta (GSK-3beta) and extracellular signal-regulated kinase (ERK)1/2 induced by insulin (100 nm for 10 min) was enhanced by approximately 62%, without altering levels of these protein kinases. Nicotine produced time (> 12 h)- and concentration (EC(50) 3.6 and 13 microm)-dependent increases in insulin receptor substrate (IRS)-1 and IRS-2 levels by approximately 125 and 105%, without altering cell surface density of insulin receptors. In these cells, insulin-induced tyrosine phosphorylation of IRS-1/IRS-2 and recruitment of phosphoinositide 3-kinase (PI3K) to IRS-1/IRS-2 were augmented by approximately 63%. The increase in IRS-1/IRS-2 levels induced by nicotine was prevented by nicotinic acetylcholine receptor (nAChR) antagonists, the Ca(2+) chelator 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetra-acetic acid tetrakis-acetoxymethyl ester, cycloheximide or actinomycin D. Nicotine increased IRS-1 and IRS-2 mRNA levels by approximately 57 and approximately 50%, and this was prevented by conventional protein kinase C (cPKC) inhibitor Gö6976, or ERK kinase inhibitors PD98059 and U0126. Nicotine phosphorylated cPKC-alpha, thereby increasing phosphorylation of ERK1/ERK2, as demonstrated by using Gö6976, PD98059 or U0126. Selective activation of cPKC-alpha by thymeleatoxin mimicked these effects of nicotine. Thus, stimulation of nAChRs up-regulated expression of IRS-1/IRS-2 via Ca(2+)-dependent sequential activation of cPKC-alpha and ERK, and enhanced insulin-induced PI3K/Akt/GSK-3beta and ERK signaling pathways.

Induction of IL-8 expression by human herpesvirus 8 encoded vFLIP K13 via NF-kappaB activation

Human herpesvirus 8 (HHV-8) encodes a viral FLICE inhibitory protein (vFLIP), called K13, with homology to the prodomain of caspase 8. K13 has been postulated to protect virally infected cells against death receptor-induced apoptosis. We report that K13 leads to constitutive upregulation of IL-8 secretion by transcriptional upregulation of its promoter. K13-induced IL-8 promoter activation is dependent on an intact NF-kappaB-binding site and is associated with increased binding of classical NF-kappaB pathway subunits p65, c-Rel and p50, respectively. IL-8 production is defective in K13 mutants defective in classical NF-kappaB activation and is blocked by genetic and pharmacological inhibitors of this pathway. In contrast, K13 failed to activate the JNK/AP-1 pathway and deletion of AP-1-binding site in the IL-8 promoter or use of a specific JNK inhibitor had only a partial effect on K13-induced IL-8 promoter activation. Collectively, above results demonstrate that K13 is a major mediator of IL-8 production and therapeutic agents targeting K13-induced NF-kappaB pathway may have a role in the treatment of conditions in which HHV-8-induced IL-8 production plays a pathogenic role.

Ciliary neurotrophic factor prevents acute lipid-induced insulin resistance by attenuating ceramide accumulation and phosphorylation of c-Jun N-terminal kinase in peripheral tissues

Ciliary neurotrophic factor (CNTF) is a member of the gp130 receptor cytokine family recently identified as an antiobesity agent in rodents and humans by mechanisms that remain unclear. We investigated the impact of acute CNTF treatment on insulin action in the presence of lipid oversupply. To avoid confounding effects of long-term high-fat feeding or genetic manipulation on whole-body insulin sensitivity, we performed a 2-h Intralipid infusion (20% heparinized Intralipid) with or without recombinant CNTF pretreatment (Axokine 0.3 mg/kg), followed by a 2-h hyperinsulinemic-euglycemic clamp (12 mU/kg.min) in fasted, male Wistar rats. Acute Intralipid infusion increased plasma free fatty acid levels from 1.0 +/- 0.1 to 2.5 +/- 0.3 mM, which subsequently caused reductions in skeletal muscle (insulin-stimulated glucose disposal rate) and liver (hepatic glucose production) insulin sensitivity by 30 and 45%, respectively. CNTF pretreatment completely prevented the lipid-mediated reduction in insulin-stimulated glucose disposal rate and the blunted suppression of hepatic glucose production by insulin. Although lipid infusion increased triacylglycerol and ceramide accumulation and phosphorylation of mixed linage kinase 3 and c-Jun N-terminal kinase 1 in skeletal muscle, CNTF pretreatment prevented these lipid-induced effects. Alterations in hepatic and muscle insulin signal transduction as well as phosphorylation of c-Jun N-terminal kinase 1/2 paralleled alterations in insulin sensitivity. These data support the use of CNTF as a potential therapeutic means to combat lipid-induced insulin resistance.

PKCtheta is a key player in the development of insulin resistance

Activation of PKCtheta is associated with lipid-induced insulin resistance and PKCtheta knockout mice are protected from the lipid-induced defects. However, the exact mechanism by which PKCtheta contributes to insulin resistance is not known. To investigate whether an increase in PKCtheta expression leads to insulin resistance, C2C12 skeletal muscle cells were transfected with PKCtheta DNA and treated with different concentrations of insulin for 10 min. PKCtheta overexpression induced reduction of IRS-1 protein levels with a decrease in insulin-induced p85 binding to IRS-1, phosphorylation of PKB and its substrates, p70 and GSK3. Pretreatment of these cells with GF-109203X (a non-specific PKC inhibitor, IC50 for PKCtheta = 10 nM) recovered insulin signaling. PKCtheta was found to be expressed in liver and treatment of human hepatoma cells (HepG2) with high insulin and glucose resulted in an increase in PKCtheta expression that correlated with a decrease in IRS-1 protein levels and the development of insulin resistance. Reduction of PKCtheta expression using RNAi technology significantly inhibited the degradation of IRS-1 and enhanced insulin-induced IRS-1 tyrosine phosphorylation, p85 association to IRS-1 and PKB phosphorylation. In conclusion, by overexpressing PKCtheta or using RNAi technology to downregulate PKCtheta, we have demonstrated that PKCtheta has a key role in the development of insulin resistance. These findings suggest that PKCtheta mediates not only insulin resistance in muscle but also in liver, which may contribute to the development of whole body insulin resistance and diabetes.

Acetylsalicylic acid improves lipid-induced insulin resistance in healthy men

CONTEXT AND OBJECTIVE

Insulin resistance is a central feature of type 2 diabetes. Salicylates prevent lipid-induced insulin resistance in rodents by interrupting inflammatory pathways. We therefore investigated whether salicylates reduce lipid-induced insulin resistance in humans by affecting inflammatory pathways as reflected by serum adipocytokines.

PARTICIPANTS AND INTERVENTION

Ten healthy men were included in a crossover intervention study. Four euglycemic-hyperinsulinemic clamps were performed, one without pretreatment, one with prior 2-h lipid infusion, one after pretreatment with 4 g acetylsalicylic acid (ASA), and one with ASA pretreatment and prior lipid infusion.

MAIN OUTCOME MEASURE

Lipid-induced insulin resistance was quantified by the euglycemic-hyperinsulinemic clamp technique running at least 2 h. Repeated-measures ANOVA on two factors was used for comparison, and results were Bonferroni adjusted for multiple measurements. ASA effects on serum adipocytokines were addressed by comparing the areas under the curves.

RESULTS

Glucose infusion rate (M value) of the control clamp without pretreatment was 6.3 (+/- 0.6) mg/kg.min. ASA pretreatment did not change glucose infusion rates (P = 0.6). Lipid infusion significantly decreased the M value to 4.1 (+/- 0.6) mg/kg.min (P = 0.008). After ASA pretreatment and lipid infusion, the M value was 4.8 (+/- 0.7) mg/kg.min and was significantly improved, compared with the lipid-only clamp (P = 0.036 after Bonferroni's adjustment). General biomarkers of inflammatory processes (IL-6, C-reactive protein), the insulin-sensitizing mediator adiponectin, and circulating adiponectin oligomers were unchanged by ASA pretreatment.

CONCLUSIONS

ASA pretreatment attenuated lipid-induced insulin resistance in healthy humans. This acute insulin-sensitizing effect of ASA was unrelated to changes of circulating inflammatory markers.

PKC-mediated modulation of L-type calcium channels may contribute to fat-induced insulin resistance

Increased intracellular free calcium [Ca2+]i has been noted in adipocytes, platelets, and leukocytes of subjects with insulin resistance syndrome or allied disorders. In rodent studies, measures which increase [Ca2+]i in adipocytes and skeletal muscle are associated with impaired insulin signaling, attributable at least in part to diminished ability of insulin to activate phosphoserine phosphatase-1 (PP-1). In fat-fed insulin resistant rats, pre-treatment with a drug that selectively chelates intracellular calcium eliminates about half of the decrement in insulin-stimulated glucose uptake induced by fat feeding; since this chelator does not influence the insulin sensitivity of chow-fed rats, it is reasonable to suspect that fat feeding boosts [Ca2+]i in skeletal muscle, and that this effect is partially responsible for the associated reduction in insulin sensitivity. Clinical insulin resistance is associated with increased levels of triglycerides and other fatty acid metabolites in muscle fibers; this can give rise to diacylglycerol-mediated activation of PKC, which in turn compromises insulin signaling by triggering kinase cascades that phosphorylate IRS-1 on key serine residues. Yet there is also evidence that, in skeletal muscle, PKC activity up-regulates the function of L-type calcium channels, increasing their maximal conductance while left-shifting their voltage dependence. Thus, the PKC activation associated with fat overexposure might be expected to boost basal [Ca2+]i in skeletal muscle, potentially impeding insulin-mediated activation of PP-1. This hypothesis is consistent with several clinical studies demonstrating that long-acting inhibitors of L-type calcium channels can improve insulin sensitivity in overweight hypertensives; it should be readily testable in rodent models of fat-induced insulin resistance. Since parathyroid hormone can act on adipocytes and muscle to boost [Ca2+]i, mild secondary hyperparathyroidism associated with low calcium intakes and poor vitamin D status may contribute to insulin resistance, consistent with certain clinical and epidemiological findings. Magnesium, often thought of as a mild calcium antagonist, appears to have favorable effects on insulin sensitivity and risk for diabetes, and recent evidence indicates that increases of intracellular magnesium within the physiological range can diminish calcium influx through phosphorylated L-type calcium channels. It will be of interest to determine whether calcium antagonism does indeed underlie the favorable influence of good magnesium status on insulin function. A report that chromium picolinate can induce the plasmalemmal Ca2+-ATPase in smooth muscle cells, raises the possibility that modulation of calcium transport might play a role in the insulin-sensitizing efficacy of bioactive chromium.

Physiological concentrations of insulin induce endothelin-dependent vasoconstriction of skeletal muscle resistance arteries in the presence of tumor necrosis factor-alpha dependence on c-Jun N-terminal kinase

OBJECTIVE

Tumor necrosis factor-alpha (TNF-alpha) has been linked to obesity-related insulin resistance and impaired endothelium-dependent vasodilatation, but the mechanisms have not been elucidated. To investigate whether TNF-alpha directly impairs insulin-mediated vasoreactivity in skeletal muscle resistance arteries and the role of c-Jun N-terminal kinase (JNK) in this interference.

METHODS AND RESULTS

Insulin-mediated vasoreactivity of isolated resistance arteries of the rat cremaster muscle to insulin (4 to 3400 microU/mL) was studied in the absence and presence of TNF-alpha (10 ng/mL). Although insulin or TNF-alpha alone did not affect arterial diameter, insulin induced dose-dependent vasoconstriction of cremaster resistance arteries in the presence of TNF-alpha, (-12+/-1% at 272 microU/mL). Blocking endothelin receptors in the absence of TNF-alpha uncovered insulin-mediated vasodilatation (18+/-6% at 272 microU/mL) but not in the presence of TNF-alpha (2+/-2% at 272 microU/mL), showing that TNF-alpha inhibits vasodilator effects of insulin. Using digital imaging microscopy, we discovered that TNF-alpha activates JNK in arterial endothelium, visible as an increase in phosphorylated JNK. Moreover, inhibition of JNK with the cell-permeable peptide inhibitor L-JNKI abolished insulin-mediated vasoconstriction in the presence of TNF-alpha, showing that JNK is required for interaction between TNF-alpha and insulin.

CONCLUSIONS

TNF-alpha inhibits vasodilator but not vasoconstrictor effects of insulin in skeletal muscle resistance arteries, resulting in insulin-mediated vasoconstriction in the presence of TNF-alpha. This effect of TNF-alpha is critically dependent on TNF-alpha-mediated activation of JNK.

Induction of heat shock proteins may combat insulin resistance

The molecular mechanism responsible for obesity-associated insulin resistance has been partially clarified: increased fatty acid levels in muscle fibers promote diacylglycerol synthesis, which activates certain isoforms of protein kinase C (PKC). This in turn triggers a kinase cascade which activates both IkappaB kinase-beta (IKK-beta) and c-Jun N-terminal kinase (JNK), each of which can phosphorylate a key serine residue in IRS-1, rendering it a poor substrate for the activated insulin receptor. Heat shock proteins Hsp27 and Hsp72 have the potential to prevent the activation of IKK-beta and JNK, respectively; this suggests that induction of heat shock proteins may blunt the adverse impact of fat overexposure on insulin function. Indeed, bimoclomol--a heat shock protein co-inducer being developed for treatment of diabetic neuropathy--and lipoic acid--suspected to be a heat shock protein inducer--have each demonstrated favorable effects on the insulin sensitivity of obese rodents, and parenteral lipoic acid is reported to improve the insulin sensitivity of type 2 diabetics. Moreover, there is reason to believe that heat shock protein induction may have a favorable impact on the microvascular complications of diabetes, and on the increased risk for macrovascular disease associated with diabetes and insulin resistance syndrome. Heat shock protein induction may also have potential for preventing or treating neurodegenerative disorders, controlling inflammation, and possibly even slowing the aging process. The possible complementarity of bimoclomol and lipoic acid for heat shock protein induction should be assessed, and further efforts to identify well-tolerated agents active in this regard are warranted.

Proposed mechanisms for the induction of insulin resistance by oxidative stress

In diabetes (type 1 and type 2), increased flux of free fatty acids and glucose is associated with increased mitochondrial reactive oxygen species (ROS) production and, as a consequence, increased oxidative stress. ROS have been shown to activate various cellular stress-sensitive pathways, which can interfere with cellular signaling pathways. Exposure of different cell lines to micromolar concentrations of hydrogen peroxide leads to the activation of stress kinases such as c-Jun N-terminal kinase, p38, I kappaB kinase, and extracellular receptor kinase 1/2. This activation is accompanied by a down-regulation of the cellular response to insulin, leading to a reduced ability of insulin to promote glucose uptake, and glycogen and protein synthesis. The mechanisms leading to this down-regulation in oxidized cells are complicated, involving increased serine/threonine phosphorylation of insulin receptor substrate-1 (IRS1), impaired insulin-stimulated redistribution of IRS1 and phosphatidylinositol-kinase between cytosol and low-density microsomal fraction, followed by a reduced protein kinase-B phosphorylation and GLUT4 translocation to the plasma membrane. In addition, prolonged exposure to ROS affects transcription of glucose transporters: whereas the level of GLUT1 is increased, GLUT4 level is reduced. As can be expected, administration of antioxidants such as lipoic acid in oxidized cells, in animal models of diabetes, and in type 2 diabetes shows improved insulin sensitivity. Thus, oxidative stress is presently accepted as a likely causative factor in the development of insulin resistance.

Mechanism of feedback regulation of insulin receptor substrate-1 phosphorylation in primary adipocytes

Serine and threonine phosphorylation of IRS-1 (insulin receptor substrate-1) has been reported to decrease its ability to be tyrosine-phosphorylated by the insulin receptor. Insulin itself may negatively regulate tyrosine phosphorylation of IRS-1 through a PI3K (phosphoinositide 3-kinase)-dependent feedback pathway. In the present study, we examined the regulation and role of IRS-1 serine phosphorylation in the modulation of IRS-1 tyrosine phosphorylation in physiologically relevant cells, namely freshly isolated primary adipocytes. We show that insulin-stimulated phosphorylation of Ser312 and Ser616 in IRS-1 was relatively slow, with maximal phosphorylation achieved after 20 and 5 min respectively. The effect of insulin on phosphorylation of both these sites required the activation of PI3K and the MAPKs (mitogen-activated protein kinases) ERK1/2 (extracellular-signal-regulated kinase 1 and 2), but not the activation of mTOR (mammalian target of rapamycin)/p70S6 kinase, JNK (c-Jun N-terminal kinase) or p38MAPK. Although inhibition of PI3K and ERK1/2 both substantially decreased insulin-stimulated phosphorylation of Ser312 and Ser616, only wortmannin enhanced insulin-stimulated tyrosine phosphorylation of IRS-1. Furthermore, inhibition of mTOR/p70S6 kinase, JNK or p38MAPK had no effect on insulin-stimulated IRS-1 tyrosine phosphorylation. The differential effect of inhibition of ERK1/2 on insulin-stimulated IRS-1 phosphorylation of Ser312/Ser616 and tyrosine indicates that these events are independent of each other and that phosphorylation of Ser312/Ser616 is not responsible for the negative regulation of IRS-1 tyrosine phosphorylation mediated by PI3K in primary adipocytes.

Influence of the crosstalk between growth hormone and insulin signalling on the modulation of insulin sensitivity

Growth hormone (GH) is an important modulator of insulin sensitivity. Multiple mechanisms appear to be involved in this modulatory effect. GH does not interact directly with the insulin receptor (IR), but conditions of GH excess are associated in general with hyperinsulinemia that induces a reduction of IR levels and impairment of its kinase activity. Several post-receptor events are shared between GH and insulin. This signaling crosstalk could be involved in the diabetogenic effects of GH. The utilization of animal models of GH excess, deficiency or resistance provided evidence that the signaling pathway leading to stimulation of the phosphatidylinositol 3-kinase (PI3K)/Akt cascade is an important site of regulation, and pointed to the liver as the major site of GH-induced insulin resistance. In skeletal muscle, GH-induced insulin resistance might involve an increase in the amount of the p85 subunit of PI3K that plays a negative role in insulin signalling. GH also reduces insulin sensitivity by enhancing events that negatively modulate insulin signaling such as stimulation of serine phosphorylation of IRS-1, which prevents its recruitment to the IR and induction of the suppressor of cytokine signalling (SOCS)-1 and SOCS-3 which modulate the signalling potential of the IRS proteins. In addition, GH has been shown to decrease the expression of the insulin-sensitizing adipo-cytokines adiponectin and visfatin. Finally, genetic manipulation of mice indicated that whereas GH plays a major role in reducing insulin sensitivity, circulating IGF-I also participates in the control of insulin sensitivity and plays an important role in the hormonal balance between GH and insulin.

Metabolic and vascular effects of tumor necrosis factor-alpha blockade with etanercept in obese patients with type 2 diabetes

OBJECTIVE

The pro-inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) impairs insulin action in insulin-sensitive tissues, such as fat, muscle and endothelium, and causes endothelial dysfunction. We hypothesized that TNF-alpha blockade with etanercept could reverse vascular and metabolic insulin resistance.

METHOD AND RESULTS

Twenty obese patients with type 2 diabetes were randomized to etanercept treatment (25 mg subcutaneously twice weekly for 4 weeks) or used as controls in an open parallel study. Forearm blood flow and glucose uptake were measured during intra-arterial infusions of serotonin, sodium nitroprusside and insulin co-infused with serotonin. Beta-cell function was assessed with oral and intra-venous glucose tolerance tests and whole-body insulin sensitivity by hyperinsulinemic euglycemic clamps. Plasma levels of C-reactive protein and interleukin-6 decreased significantly with etanercept (C-reactive protein from 9.9 +/- 3.1 to 4.8 +/- 1.4 mg l(-1), p = 0.04; interleukin-6 from 3.1 +/- 0.4 to 1.9 +/- 0.2 ng l(-1), p = 0.03). Vasodilatory responses to serotonin and sodium nitroprusside infusions remained unchanged. Insulin effect on vasodilatation and on whole-body and forearm glucose uptake remained unchanged as well. Beta-cell function tended to improve.

CONCLUSION

Although short-term etanercept treatment had a significant beneficial effect on systemic inflammatory markers, no improvement of vascular or metabolic insulin sensitivity was observed.

Insulin-independent pathways mediating glucose uptake in hindlimb-suspended skeletal muscle

Insulin resistance accompanies atrophy in slow-twitch skeletal muscles such as the soleus. Using a rat hindlimb suspension model of atrophy, we have previously shown that an upregulation of JNK occurs in atrophic muscles and correlates with the degradation of insulin receptor substrate-1 (IRS-1) (Hilder TL, Tou JC, Grindeland RF, Wade CE, and Graves LM. FEBS Lett 553: 63-67, 2003), suggesting that insulin-dependent glucose uptake may be impaired. However, during atrophy, these muscles preferentially use carbohydrates as a fuel source. To investigate this apparent dichotomy, we examined insulin-independent pathways involved in glucose uptake following a 2- to 13-wk hindlimb suspension regimen. JNK activity was elevated throughout the time course, and IRS-1 was degraded as early as 2 wk. AMP-activated protein kinase (AMPK) activity was significantly higher in atrophic soleus muscle, as were the activities of the ERK1/2 and p38 MAPKs. As a comparison, we examined the kinase activity in solei of rats exposed to hypergravity conditions (2 G). IRS-1 phosphorylation, protein, and AMPK activity were not affected by 2 G, demonstrating that these changes were only observed in soleus muscle from hindlimb-suspended animals. To further examine the effect of AMPK activation on glucose uptake, C2C12 myotubes were treated with the AMPK activator metformin and then challenged with the JNK activator anisomycin. While anisomycin reduced insulin-stimulated glucose uptake to control levels, metformin significantly increased glucose uptake in the presence of anisomycin and was independent of insulin. Taken together, these results suggest that AMPK may be an important mediator of insulin-independent glucose uptake in soleus during skeletal muscle atrophy.