In skeletal muscle, glucose storage and oxidation are differentially impaired by the IR1152 mutant receptor

Metabolic-sensing of the skeletal muscle clock coordinates fuel oxidation

Circadian clock confers temporal control in metabolism, with its disruption leading to the development of insulin resistance. Metabolic substrate utilization in skeletal muscle is coordinated with diurnal nutrient cycles. However, whether the molecular clock is involved in this coordination is largely unknown. Using a myocyte-selective genetic ablation mouse model of the essential clock activator Bmal1, here we identify muscle-intrinsic clock as a sensor of feeding cues to orchestrate skeletal muscle oxidation required for global nutrient flux. Bmal1 in skeletal muscle responds robustly to feeding in vivo and insulin induces its expression. Muscle Bmal1 deficiency impaired the transcriptional control of glucose metabolic pathway, resulting in markedly attenuated glucose utilization and fasting hyperglycemia. Notably, the loss of Bmal1 response to feeding abolished fasting-to-feeding metabolic fuel switch from fatty acids to glucose in skeletal muscle, leading to the activation of energy-sensing pathways for fatty acid oxidation. These altered metabolic substrate oxidations in Bmal1-deficient muscle ultimately depleted circulating lipid levels that prevented hepatic steatosis. Collectively, our findings highlight the key role of the metabolic-sensing function of skeletal muscle clock in partitioning nutrient flux between muscle and liver to maintain whole-body lipid and glucose homeostasis.

Validation of mouse phosphoprotein enriched in astrocyte 15 (mPEA15) expressing transgenic pig as a potential model in diabetes translational research

The present study aimed to investigate the characteristics of mPEA15 expressing transgenic pig (TG pig) as a potential model for diabetes. Expression analysis confirmed the ubiquitous expression of mPEA15 in TG pigs at F4. Oral glucose tolerance test results showed that restoration of normal glucose levels was significantly delayed in the TG pigs when compared with that in the wild-type pigs (WT pigs). Primary skeletal muscle cells isolated from TG pigs demonstrated reduced glucose uptake and reduced GLUT4 translocation to the plasma membrane in response to insulin treatment. Combined, these results suggest that mPEA15 expressing pigs has a glucose intolerance and insulin resistance which are known to mediate the pathophysiology of type 2 diabetes mellitus. Thus, mPEA15 transgenic pigs would serve as a promising model for diabetes translational research.

Low-Dose Bisphenol-A Impairs Adipogenesis and Generates Dysfunctional 3T3-L1 Adipocytes

Environmental endocrine disruptors (EDCs), including bisphenol-A (BPA), have been recently involved in obesity and diabetes by dysregulating adipose tissue function. Our aim was to examine whether prolonged exposure to low doses of BPA could affect adipogenesis and adipocyte metabolic functions. Therefore, 3T3-L1 pre-adipocytes were cultured for three weeks with BPA 1 nM to mimic human environmental exposure. We evaluated BPA effect on cell proliferation, differentiation, gene expression and adipocyte metabolic function. BPA significantly increased pre-adipocyte proliferation (p<0.01). In 3T3-L1 adipocytes differentiated in the presence of BPA, the expression of Peroxisome proliferator-activated receptor gamma (PPARγ), Fatty Acid Binding Protein 4/Adipocyte Protein 2 (FABP4/AP2) and CCAAT/enhancer binding protein (C/EBPα) was increased by 3.5, 1.5 and 3 folds, respectively. Mature adipocytes also showed a significant increase in lipid accumulation (p<0.05) and alterations of insulin action, with significant reduction in insulin-stimulated glucose utilization (p<0.001). Moreover, in mature adipocytes, mRNA levels of Leptin, interleukin-6 (IL6) and interferon-γ (IFNγ) were significantly increased (p<0.05). In conclusion, BPA prolonged exposure at low doses, consistent with those found in the environment, may affect adipocyte differentiation program, enhancing pre-adipocyte proliferation and anticipating the expression of the master genes involved in lipid/glucose metabolism. The resulting adipocytes are hypertrophic, with impaired insulin signaling, reduced glucose utilization and increased pro-inflammatory cytokine expression. Thus, these data supported the hypothesis that BPA exposure, during critical stages of adipose tissue development, may cause adipocyte metabolic dysfunction and inflammation, thereby increasing the risk of developing obesity-related diseases.

PED/PEA-15 inhibits hydrogen peroxide-induced apoptosis in Ins-1E pancreatic beta-cells via PLD-1

The small scaffold protein PED/PEA-15 is involved in several different physiologic and pathologic processes, such as cell proliferation and survival, diabetes and cancer. PED/PEA-15 exerts an anti-apoptotic function due to its ability to interfere with both extrinsic and intrinsic apoptotic pathways in different cell types. Recent evidence shows that mice overexpressing PED/PEA-15 present larger pancreatic islets and increased beta-cells mass. In the present work we investigated PED/PEA-15 role in hydrogen peroxide-induced apoptosis in Ins-1E beta-cells. In pancreatic islets isolated from Tg_PED/PEA-15 mice hydrogen peroxide-induced DNA fragmentation was lower compared to WT islets. TUNEL analysis showed that PED/PEA-15 overexpression increases the viability of Ins-1E beta-cells and enhances their resistance to apoptosis induced by hydrogen peroxide exposure. The activity of caspase-3 and the cleavage of PARP-1 were markedly reduced in Ins-1E cells overexpressing PED/PEA-15 (Ins-1E_PED/PEA-15). In parallel, we observed a decrease of the mRNA levels of pro-apoptotic genes Bcl-xS and Bad. In contrast, the expression of the anti-apoptotic gene Bcl-xL was enhanced. Accordingly, DNA fragmentation was higher in control cells compared to Ins-1E_PED/PEA-15 cells. Interestingly, the preincubation with propranolol, an inhibitor of the pathway of PLD-1, a known interactor of PED/PEA-15, responsible for its deleterious effects on glucose tolerance, abolishes the antiapoptotic effects of PED/PEA-15 overexpression in Ins-1E beta-cells. The same results have been obtained by inhibiting PED/PEA-15 interaction with PLD-1 in Ins-1E_PED/PEA-15. These results show that PED/PEA-15 overexpression is sufficient to block hydrogen peroxide-induced apoptosis in Ins-1E cells through a PLD-1 mediated mechanism.

Bisphenol-A impairs insulin action and up-regulates inflammatory pathways in human subcutaneous adipocytes and 3T3-L1 cells

Current evidence indicates that chemical pollutants may interfere with the homeostatic control of nutrient metabolism, thereby contributing to the increased prevalence of metabolic disorders. Bisphenol-A (BPA) is a lipophilic compound contained in plastic which is considered a candidate for impairing energy and glucose metabolism. We have investigated the impact of low doses of BPA on adipocyte metabolic functions. Human adipocytes derived from subcutaneous adipose tissue and differentiated 3T3-L1 cells were incubated with BPA, in order to evaluate the effect on glucose utilization, insulin sensitivity and cytokine secretion. Treatment with 1nM BPA significantly inhibited insulin-stimulated glucose utilization, without grossly interfering with adipocyte differentiation. Accordingly, mRNA levels of the adipogenic markers PPARγ and GLUT4 were unchanged upon BPA exposure. BPA treatment also impaired insulin-activated receptor phosphorylation and signaling. Moreover, adipocyte incubation with BPA was accompanied by increased release of IL-6 and IFN-γ, as assessed by multiplex ELISA assays, and by activation of JNK, STAT3 and NFkB pathways. Treatment of the cells with the JNK inhibitor SP600125 almost fully reverted BPA effect on insulin signaling and glucose utilization. In conclusion, low doses of BPA interfere with inflammatory/insulin signaling pathways, leading to impairment of adipose cell function.

PED/PEA-15 induces autophagy and mediates TGF-beta1 effect on muscle cell differentiation

TGF-beta1 has been shown to induce autophagy in certain cells but whether and how this action is exerted in muscle and whether this activity relates to TGF-beta1 control of muscle cell differentiation remains unknown. Here, we show that expression of the autophagy-promoting protein phosphoprotein enriched in diabetes/phosphoprotein enriched in astrocytes (PED/PEA-15) progressively declines during L6 and C2C12 skeletal muscle cell differentiation. PED/PEA-15 underwent rapid induction upon TGF-beta1 exposure of L6 and C2C12 myoblasts, accompanied by impaired differentiation into mature myotubes. TGF-beta1 also induced autophagy in the L6 and C2C12 cells through a PP2A/FoxO1-mediated mechanism. Both the TGF-beta1 effect on differentiation and that on autophagy were blocked by specific PED/PEA-15 ShRNAs. Myoblasts stably overexpressing PED/PEA-15 did not differentiate and showed markedly enhanced autophagy. In these same cells, the autophagy inhibitor 3-methyladenine rescued TGF-beta1 effect on both autophagy and myogenesis, indicating that PED/PEA-15 mediates TGF-beta1 effects in muscle. Muscles from transgenic mice overexpressing PED/PEA-15 featured a significant number of atrophic fibers, accompanied by increased light chain 3 (LC3)II to LC3I ratio and reduced PP2A/FoxO1 phosphorylation. Interestingly, these mice showed significantly impaired locomotor activity compared with their non-transgenic littermates. TGF-beta1 causes transcriptional upregulation of the autophagy-promoting gene PED/PEA-15, which in turn is capable to induce atrophic responses in skeletal muscle in vivo.

Effect of oral sebacic Acid on postprandial glycemia, insulinemia, and glucose rate of appearance in type 2 diabetes

OBJECTIVE

Dicarboxylic acids are natural products with the potential of being an alternate dietary source of energy. We aimed to evaluate the effect of sebacic acid (a 10-carbon dicarboxylic acid; C10) ingestion on postprandial glycemia and glucose rate of appearance (Ra) in healthy and type 2 diabetic subjects. Furthermore, the effect of C10 on insulin-mediated glucose uptake and on GLUT4 expression was assessed in L6 muscle cells in vitro.

RESEARCH DESIGN AND METHODS

Subjects ingested a mixed meal (50% carbohydrates, 15% proteins, and 35% lipids) containing 0 g (control) or 10 g C10 in addition to the meal or 23 g C10 as a substitute of fats.

RESULTS

In type 2 diabetic subjects, the incremental glucose area under the curve (AUC) decreased by 42% (P<0.05) and 70% (P<0.05) in the 10 g C10 and 23 g C10 groups, respectively. At the largest amounts used, C10 reduced the glucose AUC in healthy volunteers also. When fats were substituted with 23 g C10, AUC of Ra was significantly reduced on the order of 18% (P<0.05) in both healthy and diabetic subjects. The insulin-dependent glucose uptake by L6 cells was increased in the presence of C10 (38.7±10.3 vs. 11.4±5.4%; P=0.026). This increase was associated with a 1.7-fold raise of GLUT4.

CONCLUSIONS

Sebacic acid significantly reduced hyperglycemia after a meal in type 2 diabetic subjects. This beneficial effect was associated with a reduction in glucose Ra, probably due to lowered hepatic glucose output and increased peripheral glucose disposal.

The p65 subunit of NF-kappaB binds to PGC-1alpha, linking inflammation and metabolic disturbances in cardiac cells

AIMS

Nuclear factor-kappaB (NF-kappaB) is a transcription factor induced by a wide range of stimuli, including hyperglycaemia and pro-inflammatory cytokines. It is associated with cardiac hypertrophy and heart failure. It was previously reported that the NF-kappaB-mediated inhibition of proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) might explain the shift in glucose metabolism during cardiac pathological processes induced by pro-inflammatory stimuli, although the specific mechanisms remain to be elucidated. We addressed the specific mechanisms by which exposure to tumour necrosis factor-alpha (TNF-alpha) results in PGC-1alpha down-regulation in cardiac cells and, as a consequence, in the metabolic dysregulation that underlies heart dysfunction and failure.

METHODS AND RESULTS

By using coimmunoprecipitation studies, we report for the first time that the p65 subunit of NF-kappaB is constitutively bound to PGC-1alpha in human cardiac cells and also in mouse heart, and that NF-kappaB activation by TNF-alpha exposure increases this binding. Overexpression and gene silencing analyses demonstrated that the main factor limiting the degree of this association is p65, because only the modulation of this protein modified the physical interaction. Our data show that the increased physical interaction between p65 and PGC-1alpha after NF-kappaB activation is responsible for the reduction in PGC-1alpha expression and subsequent dysregulation of glucose oxidation.

CONCLUSION

On the basis of these data, we propose that p65 directly represses PGC-1alpha activity in cardiac cells, thereby leading to a reduction in pyruvate dehydrogenase kinase 4 (PDK4) expression and the subsequent increase in glucose oxidation observed during the proinflammatory state.

Selective disruption of insulin-like growth factor-1 (IGF-1) signaling via phosphoinositide-dependent kinase-1 prevents the protective effect of IGF-1 on human cancer cell death

Insulin-like growth factor-1 (IGF-1) signaling system exerts a broad antiapoptotic function and plays a crucial role in resistance to anticancer therapies. Exposure of MCF-7 breast cancer cells to IGF-1 rapidly and transiently induced tyrosine phosphorylation and activation of phosphoinositide-dependent kinase-1 (PDK1). This was paralleled by Akt/protein kinase B and protein kinase C-zeta phosphorylation, at Thr(308) and Thr(410), respectively. IGF-1 treatment also enhanced PDK1 interaction with IGF-1 receptor (IGF-1R) in intact MCF-7 cells. Pulldown assays revealed that PDK1 bound IGF-1R in vitro and that the region encompassing amino acids 51-359 of PDK1 was necessary for the interaction. Synthetic peptides corresponding to IGF-1R C terminus amino acids 1295-1337 (C43) and to PDK1 amino acids 114-141 reduced in vitro IGF-1R/PDK1 interaction in a concentration-dependent manner. Loading of fluoresceinated-C43 (fluorescein isothiocyanate (FITC)-C43) into MCF-7 cells significantly reduced IGF-1R/PDK1 interaction and phosphorylation of PDK1 substrates. Moreover, FITC-C43 intracellular loading reverted the protective effect of IGF-1 on growth factor deprivation-induced cell death. Finally, the inhibition of IGF-1R/PDK1 interaction and signaling by FITC-C43 was accompanied by 2-fold enhanced killing capacity of cetuximab in human GEO colon adenocarcinoma cells and was sufficient to restore cell death in cetuximab-resistant cell clones. Thus, disruption of PDK1 interaction with IGF-1R reduces IGF-1 survival effects in cancer cells and may enhance cell death by anticancer agents.

TNF-alpha reduces PGC-1alpha expression through NF-kappaB and p38 MAPK leading to increased glucose oxidation in a human cardiac cell model

AIMS

Inflammatory responses in the heart that are driven by sustained increases in cytokines have been associated with several pathological processes, including cardiac hypertrophy and heart failure. Emerging data suggest a link between cardiomyopathy and myocardial metabolism dysregulation. To further elucidate the relationship between a pro-inflammatory profile and cardiac metabolism dysregulation, a human cell line of cardiac origin, AC16, was treated with tumour necrosis factor-alpha (TNF-alpha).

METHODS AND RESULTS

Exposure of AC16 cells to TNF-alpha inhibited the expression of peroxisome proliferator-activated receptor coactivator 1alpha (PGC-1alpha), an upstream regulator of lipid and glucose oxidative metabolism. Studies performed with cardiac-specific transgenic mice (Mus musculus) overexpressing TNF-alpha, which have been well characterized as a model of cytokine-induced cardiomyopathy, also displayed reduced PGC-1alpha expression in the heart compared with that of control mice. The mechanism by which TNF-alpha reduced PGC-1alpha expression in vitro appeared to be largely mediated via both p38 mitogen-activated protein kinase and nuclear factor-kappaB pathways. PGC-1alpha downregulation resulted in an increase in glucose oxidation rate, which involved a reduction in pyruvate dehydrogenase kinase 4 expression and depended on the DNA-binding activity of both peroxisome proliferator-activated receptor beta/delta and estrogen-related receptor alpha transcription factors.

CONCLUSION

These results point to PGC-1alpha downregulation as a potential contributor to cardiac dysfunction and heart failure in metabolic disorders with an inflammatory background.

Tyrosine phosphorylation of phosphoinositide-dependent kinase 1 by the insulin receptor is necessary for insulin metabolic signaling

In L6 myoblasts, insulin receptors with deletion of the C-terminal 43 amino acids (IR(Delta43)) exhibited normal autophosphorylation and IRS-1/2 tyrosine phosphorylation. The L6 cells expressing IR(Delta43) (L6(IRDelta43)) also showed no insulin effect on glucose uptake and glycogen synthase, accompanied by a >80% decrease in insulin induction of 3-phosphoinositide-dependent protein kinase 1 (PDK-1) activity and tyrosine phosphorylation and of protein kinase B (PKB) phosphorylation at Thr(308). Insulin induced the phosphatidylinositol 3 kinase-dependent coprecipitation of PDK-1 with wild-type IR (IR(WT)), but not IR(Delta43). Based on overlay blotting, PDK-1 directly bound IR(WT), but not IR(Delta43). Insulin-activated IR(WT), and not IR(Delta43), phosphorylated PDK-1 at tyrosines 9, 373, and 376. The IR C-terminal 43-amino-acid peptide (C-terminal peptide) inhibited in vitro PDK-1 tyrosine phosphorylation by the IR. Tyr-->Phe substitution prevented this inhibitory action. In the L6(hIR) cells, the C-terminal peptide coprecipitated with PDK-1 in an insulin-stimulated fashion. This peptide simultaneously impaired the insulin effect on PDK-1 coprecipitation with IR(WT), on PDK-1 tyrosine phosphorylation, on PKB phosphorylation at Thr(308), and on glucose uptake. Upon insulin exposure, PDK-1 membrane persistence was significantly reduced in L6(IRDelta43) compared to control cells. In L6 cells expressing IR(WT), the C-terminal peptide also impaired insulin-dependent PDK-1 membrane persistence. Thus, PDK-1 directly binds to the insulin receptor, followed by PDK-1 activation and insulin metabolic effects.

PGC-1alpha coactivates PDK4 gene expression via the orphan nuclear receptor ERRalpha: a mechanism for transcriptional control of muscle glucose metabolism

The transcriptional coactivator PGC-1alpha is a key regulator of energy metabolism, yet little is known about its role in control of substrate selection. We found that physiological stimuli known to induce PGC-1alpha expression in skeletal muscle coordinately upregulate the expression of pyruvate dehydrogenase kinase 4 (PDK4), a negative regulator of glucose oxidation. Forced expression of PGC-1alpha in C(2)C(12) myotubes induced PDK4 mRNA and protein expression. PGC-1alpha-mediated activation of PDK4 expression was shown to occur at the transcriptional level and was mapped to a putative nuclear receptor binding site. Gel shift assays demonstrated that the PGC-1alpha-responsive element bound the estrogen-related receptor alpha (ERRalpha), a recently identified component of the PGC-1alpha signaling pathway. In addition, PGC-1alpha was shown to activate ERRalpha expression. Chromatin immunoprecipitation assays confirmed that PGC-1alpha and ERRalpha occupied the mPDK4 promoter in C(2)C(12) myotubes. Additionally, transfection studies using ERRalpha-null primary fibroblasts demonstrated that ERRalpha is required for PGC-1alpha-mediated activation of the mPDK4 promoter. As predicted by the effects of PGC-1alpha on PDK4 gene transcription, overexpression of PGC-1alpha in C(2)C(12) myotubes decreased glucose oxidation rates. These results identify the PDK4 gene as a new PGC-1alpha/ERRalpha target and suggest a mechanism whereby PGC-1alpha exerts reciprocal inhibitory influences on glucose catabolism while increasing alternate mitochondrial oxidative pathways in skeletal muscle.

A novel syndrome of autosomal-dominant hyperinsulinemic hypoglycemia linked to a mutation in the human insulin receptor gene

Recently, various subtypes of familial hyperinsulinemic hypoglycemia with an autosomal-dominant inheritance have been etiologically characterized. In the present study, we have delineated the genetics and metabolic phenotype of a novel form of hypoglycemia in a large pedigree with an apparent autosomal-dominant transmission. After initial investigations of the proband, her mother, and a sister, the study was extended to 19 family members in three generations. Glucose tolerance was assessed by a 5-h oral glucose tolerance test (OGTT) and insulin sensitivity by euglycemic-hyperinsulinemic clamp in six affected family members and six control subjects. To identify the genetic cause of hypoglycemia, linkage analysis and mutation analysis of genomic DNA from all family members were performed. All affected family members were characterized by postprandial hypoglycemia, fasting hyperinsulinemia, and an elevated serum insulin-to-C-peptide ratio. The 5-h OGTT demonstrated hyperinsulinemic hypoglycemia, and the clamp studies showed reduced insulin sensitivity and clearance of serum insulin in affected family members compared with control subjects. Linkage analysis and subsequent mutation screening revealed a missense mutation (Arg1174Gln) in the tyrosine kinase domain of the insulin receptor gene that cosegregated with the disease phenotype (logarithm of odds [LOD] score 3.21). In conclusion, we report a novel syndrome of autosomal-dominant hyperinsulinemic hypoglycemia. The findings demonstrate the coexistence of severe postprandial hypoglycemia, insulin resistance, and impaired insulin clearance and suggest that hypoglycemia should be considered as a phenotype linked to heterozygote mutations in the insulin receptor gene.

Protein Kinase C-ζ and Protein Kinase B Regulate Distinct Steps of Insulin Endocytosis and Intracellular Sorting

We have investigated the molecular mechanisms regulating insulin internalization and intracellular sorting. Insulin internalization was decreased by 50% upon incubation of the cells with the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY294002. PI3K inhibition also reduced insulin degradation and intact insulin release by 50 and 75%, respectively. Insulin internalization was reduced by antisense inhibition of protein kinase C-zeta (PKCzeta) expression and by overexpression of a dominant negative PKCzeta mutant (DN-PKCzeta). Conversely, overexpression of PKCzeta increased insulin internalization as a function of the PKCzeta levels achieved in the cells. Expression of wild-type protein kinase B (PKB)-alpha or of a constitutively active form (myr-PKB) did not significantly alter insulin internalization and degradation but produced a 100% increase of intact insulin release. Inhibition of PKB by a dominant negative mutant (DN-PKB) or by the pharmacological inhibitor ML-9 reduced intact insulin release by 75% with no effect on internalization and degradation. In addition, overexpression of Rab5 completely rescued the effect of PKCzeta inhibition on insulin internalization but not that of PKB inhibition on intact insulin recycling. Indeed, PKCzeta bound to and activated Rab5. Thus, PI3K controls different steps within the insulin endocytic itinerary. PKCzeta appears to mediate the PI3K effect on insulin internalization in a Rab5-dependent manner, whereas PKB directs intracellular sorting toward intact insulin release.

Activation and mitochondrial translocation of protein kinase Cdelta are necessary for insulin stimulation of pyruvate dehydrogenase complex activity in muscle and liver cells

In L6 skeletal muscle cells and immortalized hepatocytes, insulin induced a 2-fold increase in the activity of the pyruvate dehydrogenase (PDH) complex. This effect was almost completely blocked by the protein kinase C (PKC) delta inhibitor Rottlerin and by PKCdelta antisense oligonucleotides. At variance, overexpression of wild-type PKCdelta or of an active PKCdelta mutant induced PDH complex activity in both L6 and liver cells. Insulin stimulation of the activity of the PDH complex was accompanied by a 2.5-fold increase in PDH phosphatases 1 and 2 (PDP1/2) activity with no change in the activity of PDH kinase. PKCdelta antisense blocked insulin activation of PDP1/2, the same as with PDH. In insulin-exposed cells, PDP1/2 activation was paralleled by activation and mitochondrial translocation of PKCdelta, as revealed by cell subfractionation and confocal microscopy studies. The mitochondrial translocation of PKCdelta, like its activation, was prevented by Rottlerin. In extracts from insulin-stimulated cells, PKCdelta co-precipitated with PDP1/2. PKCdelta also bound to PDP1/2 in overlay blots, suggesting that direct PKCdelta-PDP interaction may occur in vivo as well. In intact cells, insulin exposure determined PDP1/2 phosphorylation, which was specifically prevented by PKCdelta antisense. PKCdelta also phosphorylated PDP in vitro, followed by PDP1/2 activation. Thus, in muscle and liver cells, insulin causes activation and mitochondrial translocation of PKCdelta, accompanied by PDP phosphorylation and activation. These events are necessary for insulin activation of the PDH complex in these cells.

Surfing the insulin signaling web

The diverse biological actions of insulin and insulin-like growth factor I (IGF-I) are initiated by binding of the polypeptides to their respective cell surface tyrosine kinase receptors. These activated receptors phosphorylate a series of endogenous substrates on tyrosine, amongst which the insulin receptor substrate (IRS) proteins are the best characterized. Their phosphotyrosine-containing motifs become binding sites for Src homology 2 (SH2) domains on proteins such as SH2 domain-containing protein-tyrosine-phosphatase (SHP)-2/Syp, growth factor receptor bound-2 protein, (Grb-2), and phosphatidyl inositol 3 kinase (PI3 kinase), which participate in activation of specific signaling cascades. However, the IRS molecules are not only platforms for signaling molecules, they also orchestrate the generation of signal specificity, integration of signals induced by several extracellular stimuli, and signal termination and modulation. An extensive review is beyond the scope of the present article, which will be centered on our own contribution and reflect our biases.

Insulin receptor substrate-2 phosphorylation is necessary for protein kinase C zeta activation by insulin in L6hIR cells

We have investigated glycogen synthase (GS) activation in L6hIR cells expressing a peptide corresponding to the kinase regulatory loop binding domain of insulin receptor substrate-2 (IRS-2) (KRLB). In several clones of these cells (B2, F4), insulin-dependent binding of the KRLB to insulin receptors was accompanied by a block of IRS-2, but not IRS-1, phosphorylation, and insulin receptor binding. GS activation by insulin was also inhibited by >70% in these cells (p < 0.001). The impairment of GS activation was paralleled by a similarly sized inhibition of glycogen synthase kinase 3 alpha (GSK3 alpha) and GSK3 beta inactivation by insulin with no change in protein phosphatase 1 activity. PDK1 (a phosphatidylinositol trisphosphate-dependent kinase) and Akt/protein kinase B (PKB) activation by insulin showed no difference in B2, F4, and in control L6hIR cells. At variance, insulin did not activate PKC zeta in B2 and F4 cells. In L6hIR, inhibition of PKC zeta activity by either a PKC zeta antisense or a dominant negative mutant also reduced by 75% insulin inactivation of GSK3 alpha and -beta (p < 0.001) and insulin stimulation of GS (p < 0.002), similar to Akt/PKB inhibition. In L6hIR, insulin induced protein kinase C zeta (PKC zeta) co-precipitation with GSK3 alpha and beta. PKC zeta also phosphorylated GSK3 alpha and -beta. Alone, these events did not significantly affect GSK3 alpha and -beta activities. Inhibition of PKC zeta activity, however, reduced Akt/PKB phosphorylation of the key serine sites on GSK3 alpha and -beta by >80% (p < 0.001) and prevented full GSK3 inactivation by insulin. Thus, IRS-2, not IRS-1, signals insulin activation of GS in the L6hIR skeletal muscle cells. In these cells, insulin inhibition of GSK3 alpha and -beta requires dual phosphorylation by both Akt/PKB and PKC zeta.

Protein kinase C (PKC)-alpha activation inhibits PKC-zeta and mediates the action of PED/PEA-15 on glucose transport in the L6 skeletal muscle cells

Overexpression of the PED/PEA-15 protein in muscle and adipose cells increases glucose transport and impairs further insulin induction. Like glucose transport, protein kinase C (PKC)-alpha and -beta are also constitutively activated and are not further stimulatable by insulin in L6 skeletal muscle cells overexpressing PED (L6(PED)). PKC-zeta features no basal change but completely loses insulin sensitivity in L6(PED). In these cells, blockage of PKC-alpha and -beta additively returns 2-deoxy-D-glucose (2-DG) uptake to the levels of cells expressing only endogenous PED (L6(WT)). Blockage of PKC-alpha and -beta also restores insulin activation of PKC-zeta in L6(PED) cells, with that of PKC-alpha sixfold more effective than PKC-beta. Similar effects on 2-DG uptake and PKC-zeta were also achieved by 50-fold overexpression of PKC-zeta in L6(PED). In L6(WT), fivefold overexpression of PKC-alpha or -beta increases basal 2-DG uptake and impairs further insulin induction with no effect on insulin receptor or insulin receptor substrate phosphorylation. In these cells, overexpression of PKC-alpha blocks insulin induction of PKC-zeta activity. PKC-beta is 10-fold less effective than PKC-alpha in inhibiting PKC-zeta stimulation. Expression of the dominant-negative K(281)-->W PKC-zeta mutant simultaneously inhibits insulin activation of PKC-zeta and 2-DG uptake in the L6(WT) cells. We conclude that activation of classic PKCs, mainly PKC-alpha, inhibits PKC-zeta and may mediate the action of PED on glucose uptake in L6 skeletal muscle cells.

In L6 skeletal muscle cells, glucose induces cytosolic translocation of protein kinase C-alpha and trans-activates the insulin receptor kinase

In L6 skeletal muscle cells expressing human insulin receptors (L6(hIR)), exposure to 25 mM glucose for 3 min induced a rapid 3-fold increase in GLUT1 and GLUT4 membrane translocation and glucose uptake. The high glucose concentration also activated the insulin receptor kinase toward the endogenous insulin receptor substrates (IRS)-1 and IRS-2. At variance, in L6 cells expressing kinase-deficient insulin receptors, the exposure to 25 mM glucose elicited no effect on glucose disposal. In the L6(hIR) cells, the acute effect of glucose on insulin receptor kinase was paralleled by a 2-fold decrease in both the membrane and the insulin receptor co-precipitated protein kinase C (PKC) activities and a 3-fold decrease in receptor Ser/Thr phosphorylation. Western blotting of the receptor precipitates with isoform-specific PKC antibodies revealed that the glucose-induced decrease in membrane- and receptor-associated PKC activities was accounted for by dissociation of PKCalpha but not of PKCbeta or -delta. This decrease in PKCalpha was paralleled by a similarly sized increase in cytosolic PKCalpha. In intact L6(hIR) cells, inhibition of PKCalpha expression by using a specific antisense oligonucleotide caused a 3-fold increase in IRS phosphorylation by the insulin receptor. This effect was independent of insulin and accompanied by a 2.5-fold increase in glucose disposal by the cells. Thus, in the L6 skeletal muscle cells, glucose acutely regulates its own utilization through the insulin signaling system, independent of insulin. Glucose autoregulation appears to involve PKCalpha dissociation from the insulin receptor and its cytosolic translocation.

Differential role of insulin receptor substrate (IRS)-1 and IRS-2 in L6 skeletal muscle cells expressing the Arg1152 --> Gln insulin receptor

In L6 muscle cells expressing the Arg1152 --> Gln insulin receptor (Mut), basal tyrosine phosphorylation of insulin receptor substrate (IRS)-1 was increased by 35% compared with wild-type cells (WT). Upon exposure to insulin, IRS-1 phosphorylation increased by 12-fold in both the Mut and WT cells. IRS-2 was constitutively phosphorylated in Mut cells and not further phosphorylated by insulin. The maximal phosphorylation of IRS-2 in basal Mut cells was paralleled by a 4-fold increased binding of the kinase regulatory loop binding domain of IRS-2 to the Arg1152 --> Gln receptor. Grb2 and phosphatidylinositol 3-kinase association to IRS-1 and IRS-2 reflected the phosphorylation levels of the two IRSs. Mitogen-activated protein kinase activation and [3H]thymidine incorporation closely correlated with IRS-1 phosphorylation in Mut and WT cells, while glycogen synthesis and synthase activity correlated with IRS-2 phosphorylation. The Arg1152 --> Gln mutant did not signal Shc phosphorylation or Shc-Grb2 association in intact L6 cells, while binding Shc in a yeast two-hybrid system and phosphorylating Shc in vitro. Thus, IRS-2 appears to mediate insulin regulation of glucose storage in Mut cells, while insulin-stimulated mitogenesis correlates with the activation of the IRS-1/mitogen-activated protein kinase pathway in these cells. IRS-1 and Shc-mediated mitogenesis may be redundant in muscle cells.

PED/PEA-15 gene controls glucose transport and is overexpressed in type 2 diabetes mellitus

We have used differential display to identify genes whose expression is altered in type 2 diabetes thus contributing to its pathogenesis. One mRNA is overexpressed in fibroblasts from type 2 diabetics compared with non-diabetic individuals, as well as in skeletal muscle and adipose tissues, two major sites of insulin resistance in type 2 diabetes. The levels of the protein encoded by this mRNA are also elevated in type 2 diabetic tissues; thus, we named it PED for phosphoprotein enriched in diabetes. PED cloning shows that it encodes a 15 kDa phosphoprotein identical to the protein kinase C (PKC) substrate PEA-15. The PED gene maps on human chromosome 1q21-22. Transfection of PED/PEA-15 in differentiating L6 skeletal muscle cells increases the content of Glut1 transporters on the plasma membrane and inhibits insulin-stimulated glucose transport and cell-surface recruitment of Glut4, the major insulin-sensitive glucose transporter. These effects of PED overexpression are reversed by blocking PKC activity. Overexpression of the PED/PEA-15 gene may contribute to insulin resistance in glucose uptake in type 2 diabetes.

Evidence that IRS-2 phosphorylation is required for insulin action in hepatocytes

Insulin receptor substrates (IRSs) are tyrosine-phosphorylated following stimulation with insulin, insulin-like growth factors (IGFs), and interleukins. A key question is whether different IRSs play different roles to mediate insulin's metabolic and growth-promoting effects. In a novel system of insulin receptor-deficient hepatocytes, insulin fails to (i) stimulate glucose phosphorylation, (ii) enhance glycogen synthesis, (iii) suppress glucose production, and (iv) promote mitogenesis. However, insulin's ability to induce IRS-1 and gab-1 phosphorylation and binding to phosphatidylinositol (PI) 3-kinase is unaffected, by virtue of the compensatory actions of IGF-1 receptors. In contrast, phosphorylation of IRS-2 and generation of IRS-2/PI 3-kinase complexes are markedly reduced. Thus, absence of insulin receptors selectively reduces IRS-2, but not IRS-1 phosphorylation, and the impairment of IRS-2 activation is associated with lack of insulin effects. To address whether phosphorylation of additional IRSs is also affected, we analyzed phosphotyrosine-containing proteins in PI 3-kinase immunoprecipitates from insulin-treated cells. However, these experiments indicate that IRS-1 and IRS-2 are the main PI 3-kinase-bound proteins in hepatocytes. These data identify IRS-2 as the main effector of both the metabolic and growth-promoting actions of insulin through PI 3-kinase in hepatocytes, and IRS-1 as the main substrate mediating the mitogenic actions of IGF-1 receptors.