Insulin Action in Skeletal Muscle

MondoA drives muscle lipid accumulation and insulin resistance

Obesity-related insulin resistance is associated with intramyocellular lipid accumulation in skeletal muscle. We hypothesized that in contrast to current dogma, this linkage is related to an upstream mechanism that coordinately regulates both processes. We demonstrate that the muscle-enriched transcription factor MondoA is glucose/fructose responsive in human skeletal myotubes and directs the transcription of genes in cellular metabolic pathways involved in diversion of energy substrate from a catabolic fate into nutrient storage pathways including fatty acid desaturation and elongation, triacylglyeride (TAG) biosynthesis, glycogen storage, and hexosamine biosynthesis. MondoA also reduces myocyte glucose uptake by suppressing insulin signaling. Mice with muscle-specific MondoA deficiency were partially protected from insulin resistance and muscle TAG accumulation in the context of diet-induced obesity. These results identify MondoA as a nutrient-regulated transcription factor that under normal physiological conditions serves a dynamic checkpoint function to prevent excess energy substrate flux into muscle catabolic pathways when myocyte nutrient balance is positive. However, in conditions of chronic caloric excess, this mechanism becomes persistently activated leading to progressive myocyte lipid storage and insulin resistance.

Increased Mitochondrial Protein Levels and Bioenergetics in the Musculus Rectus Femoris of Wfs1-Deficient Mice

Wfs1 deficiency leads to a progressive loss of plasma insulin concentration, which should reduce the consumption of glucose in insulin-dependent tissues, causing a variety of changes in intracellular energy metabolism. Our objective here was to assess the changes in the amount and function of mitochondrial proteins in different muscles of Wfs1-deficient mice. Mitochondrial functions were assayed by high-resolution oxygraphy of permeabilized muscle fibers; the protein amount was evaluated by liquid chromatography tandem mass spectrometry (LC/MS/MS) analysis and mRNA levels of the uncoupler proteins UCP2 and UCP3 by real-time PCR; and citrate synthase (CS) activity was determined spectrophotometrically in muscle homogenates. Compared to controls, there were no changes in proton leak and citrate synthase activity in the heart and m. soleus tissues of Wfs1-deficient mice, but significantly higher levels of both of these factors were observed in the m. rectus femoris; mitochondrial proteins and mRNA of UCP2 were also higher in the m. rectus femoris. ADP-stimulated state 3 respiration was lower in the m. soleus, remained unchanged in the heart, and was higher in the m. rectus femoris. The mitochondrial protein amount and activity are higher in Wfs1-deficient mice, as are mitochondrial proton leak and oxygen consumption in m. rectus femoris. These changes in muscle metabolism may be important for identifying the mechanisms responsible for Wolfram syndrome and diabetes.

MondoA coordinately regulates skeletal myocyte lipid homeostasis and insulin signaling

Intramuscular lipid accumulation is a common manifestation of chronic caloric excess and obesity that is strongly associated with insulin resistance. The mechanistic links between lipid accumulation in myocytes and insulin resistance are not completely understood. In this work, we used a high-throughput chemical biology screen to identify a small-molecule probe, SBI-477, that coordinately inhibited triacylglyceride (TAG) synthesis and enhanced basal glucose uptake in human skeletal myocytes. We then determined that SBI-477 stimulated insulin signaling by deactivating the transcription factor MondoA, leading to reduced expression of the insulin pathway suppressors thioredoxin-interacting protein (TXNIP) and arrestin domain-containing 4 (ARRDC4). Depleting MondoA in myocytes reproduced the effects of SBI-477 on glucose uptake and myocyte lipid accumulation. Furthermore, an analog of SBI-477 suppressed TXNIP expression, reduced muscle and liver TAG levels, enhanced insulin signaling, and improved glucose tolerance in mice fed a high-fat diet. These results identify a key role for MondoA-directed programs in the coordinated control of myocyte lipid balance and insulin signaling and suggest that this pathway may have potential as a therapeutic target for insulin resistance and lipotoxicity.

Role of protein kinase C in pitavastatin-induced human paraoxonase I expression in Huh7 cells

We have demonstrated that pitavastatin, a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, enhanced human serum paraoxonase (PON1) gene promoter activity and that protein kinase C (PKC) activated PON1 expression through Sp1 in cultured HepG2 cells. We investigated whether PKC was involved in pitavastatin-induced PON1 expression. PON1 gene promoter activity was assessed by a reporter gene assay using cultured Huh7 cells. PON1 protein expression and PKC activation were measured by Western blotting. The binding activity of Sp1 to the PON1 gene upstream was analyzed by electrophoretic mobility shift assay. Both PON1 gene promoter activity and PON1 protein expression were elevated by pitavastatin stimulation. The effects of pitavastatin on PON1 promoter activity and PON1 protein expression were attenuated by both bisindolylmaleimide IX (Ro-31-8220) and bisindolylmaleimide I. Electrophoretic mobility shift assay showed that pitavastatin increased the Sp1-PON1 DNA binding, and this effect was attenuated by Ro-31-8220. Pitavastatin activated atypical PKC, but never conventional or novel PKC. Myristoylated pseudosubstrate peptide inhibitor of PKCzeta abolished the pitavastatin-increased PON1 promoter activity; however, calphostin C and Gö6976 (PKC inhibitors except for PKCzeta) did not influence the promoter activity. In addition, an overexpression of dominant negative form of PKCzeta expression vector obviously decreased pitavastatin-induced PON1 promoter activation. These observations suggest that pitavastatin activates PKC, especially PKCzeta isoform, which increases the binding intensity of Sp1 to PON1 DNA promoter responsible for enhanced transcription of PON1 gene and increased PON1 protein expression in Huh7 cells.

In skeletal muscle advanced glycation end products (AGEs) inhibit insulin action and induce the formation of multimolecular complexes including the receptor for AGEs

Chronic hyperglycemia promotes insulin resistance at least in part by increasing the formation of advanced glycation end products (AGEs). We have previously shown that in L6 myotubes human glycated albumin (HGA) induces insulin resistance by activating protein kinase Calpha (PKCalpha). Here we show that HGA-induced PKCalpha activation is mediated by Src. Coprecipitation experiments showed that Src interacts with both the receptor for AGE (RAGE) and PKCalpha in HGA-treated L6 cells. A direct interaction of PKCalpha with Src and insulin receptor substrate-1 (IRS-1) has also been detected. In addition, silencing of IRS-1 expression abolished HGA-induced RAGE-PKCalpha co-precipitation. AGEs were able to induce insulin resistance also in vivo, as insulin tolerance tests revealed a significant impairment of insulin sensitivity in C57/BL6 mice fed a high AGEs diet (HAD). In tibialis muscle of HAD-fed mice, insulin-induced glucose uptake and protein kinase B phosphorylation were reduced. This was paralleled by a 2.5-fold increase in PKCalpha activity. Similarly to in vitro observations, Src phosphorylation was increased in tibialis muscle of HAD-fed mice, and co-precipitation experiments showed that Src interacts with both RAGE and PKCalpha. These results indicate that AGEs impairment of insulin action in the muscle might be mediated by the formation of a multimolecular complex including RAGE/IRS-1/Src and PKCalpha.

Roles of Sp1 and protein kinase C in regulation of human serum paraoxonase 1 (PON1) gene transcription in HepG2 cells

Human serum paraoxonase 1 (PON1) is associated with high-density lipoprotein, and inhibits oxidative modification of low-density lipoprotein in vitro. Therefore, PON1 is supposed to protect against atherosclerosis in vivo. In this study, we investigated the direct effect of Sp1 on PON1 transcription in HepG2 cells using a reporter gene assay. A deletion analysis of the PON1 upstream region revealed that dominant promoter elements were present within a sequence between -269 and -97bp, which contained a consensus binding site for Sp1, and an electrophoretic mobility shift analysis (EMSA) indicated the Sp1 binding to the upstream sequence. In accordance with this, overexpression of Sp1 dramatically enhanced PON1 promoter activity, and the Sp1 inhibitor mithramycin inhibited Sp1-induced promoter activation in a dose-dependent manner. The basal promoter activity was also enhanced by phorbol 12-myristate 13-acetate (PMA), and synergistic promoter activation was observed when Sp1-transfected cells were treated with PMA. The PMA-induced promoter activation was inhibited by mithramycin. In addition, overexpression of the dominant negative version of PKCalpha or zeta, significantly reduced PON1 promoter activity. These data suggest that Sp1 acts as a positive regulator of PON1 transcription, and that an interaction between Sp1 and PKC is a key mechanism for the effect of Sp1 on PON1 transcription.

Alterations in insulin receptor signalling in the rat epitrochlearis muscle upon cessation of voluntary exercise

The major purpose of this study was to elucidate mechanisms by which decreasing enhanced physical activity induces decreased insulin sensitivity in skeletal muscle. Rats with access to voluntary running wheels for 3 weeks had their wheels locked for 5 h (WL5), 29 h (WL29), or 53 h (WL53); a separate group of rats never had wheel access (sedentary, SED). Relative to WL5, submaximal insulin-stimulated 2-deoxyglucose uptake into the epitrochlearis muscle was lower in WL53 and SED. Insulin binding, insulin receptor beta-subunit (IRbeta) protein level, submaximal insulin-stimulated IRbeta tyrosine phosphorylation, and glucose transporter-4 protein level were each lower in both WL53 and SED than in WL5 and WL29. Akt/protein kinase B Ser(473) phosphorylation was lower in WL53 and SED than in WL5. Protein levels of protein tyrosine phosphatase-1B, Src homology phosphatase-2, and protein kinase C- did not vary among groups. The amount of protein tyrosine phosphatase-1B, Src homology phosphatase-2, and protein kinase C- associated with IRbeta in insulin-stimulated muscle also did not differ among the four groups. The mean of SED and WL53 had a significantly higher IRbeta-associated protein tyrosine phosphatase-1B than the mean of WL5 and WL29. The enclosure of multiple changes (decreases in insulin binding, IRbeta protein, IRbeta tyrosine phosphorylation, and glucose transporter-4 protein) in the epitrochlearis muscle within the 29th to 53rd hour after cessation of voluntary wheel running raises the possibility that a single regulatory event could be responsible for the coordinated decrease.

Human glycated albumin affects glucose metabolism in L6 skeletal muscle cells by impairing insulin-induced insulin receptor substrate (IRS) signaling through a protein kinase C alpha-mediated mechanism

Nonenzymatic glycation is increased in diabetes and leads to increased levels of glycated proteins. Most studies have focused on the role of glycation products in vascular complications. Here, we have investigated the action of human glycated albumin (HGA) on insulin signaling in L6 skeletal muscle cells. Exposure of these cells to HGA inhibited insulin-stimulated glucose uptake and glycogen synthase activity by 95 and 80%, respectively. These effects were time- and dose-dependent, reaching a maximum after 12 h incubation with 0.1 mg/ml HGA. In contrast, exposure of the cells to HGA had no effect on thymidine incorporation. Further, HGA reduced insulin-stimulated serine phosphorylation of PKB and GSK3, but did not alter ERK1/2 activation. HGA did not affect either insulin receptor kinase activity or insulin-induced Shc phosphorylation on tyrosine. In contrast, insulin-dependent IRS-1 and IRS-2 tyrosine phosphorylation was severely reduced in cells preincubated with HGA for 24 h. Insulin-stimulated association of PI3K with IRS-1 and IRS-2, and PI3K activity were reduced by HGA in parallel with the changes in IRS tyrosine phosphorylation, while Grb2-IRS association was unchanged. In L6 myotubes, exposure to HGA increased PKC activity by 2-fold resulting in a similar increase in Ser/Thr phosphorylation of IRS-1 and IRS-2. These phosphorylations were blocked by the PKC inhibitor bisindolylmaleimide (BDM). BDM also blocked the action of HGA on insulin-stimulated PKB and GSK3 alpha. Simultaneously, BDM rescued insulin-stimulation of glucose uptake and glycogen synthase activity in cells exposed to HGA. The use of antibodies specific to PKC isoforms shows that this effect appears to be mediated by activated PKC alpha, independent of reactive oxygen species production. In summary, in L6 skeletal muscle cells, exposure to HGA leads to insulin resistance selectively in glucose metabolism with no effect on growth-related pathways regulated by the hormone.