p38 MAPK activation upregulates proinflammatory pathways in skeletal muscle cells from insulin-resistant type 2 diabetic patients.

Skeletal muscle is the key site of peripheral insulin resistance in type 2 diabetes. Insulin-stimulated glucose uptake is decreased in differentiated diabetic cultured myotubes, which is in keeping with a retained genetic/epigenetic defect of insulin action. We investigated differences in gene expression during differentiation between diabetic and control muscle cell cultures. Microarray analysis was performed using skeletal muscle cell cultures established from type 2 diabetic patients with a family history of type 2 diabetes and clinical evidence of marked insulin resistance and nondiabetic control subjects with no family history of diabetes. Genes and pathways upregulated with differentiation in the diabetic cultures, compared with controls, were identified using Gene Spring and Gene Set Enrichment Analysis. Gene sets upregulated in diabetic myotubes were associated predominantly with inflammation. p38 MAPK was identified as a key regulator of the expression of these proinflammatory gene sets, and p38 MAPK activation was found to be increased in the diabetic vs. control myotubes. Although inhibition of p38 MAPK activity decreased cytokine gene expression from the cultured diabetic myotubes significantly, it did not improve insulin-stimulated glucose uptake. Increased cytokine expression driven by increased p38 MAPK activation is a key feature of cultured myotubes derived from insulin-resistant type 2 diabetic patients. p38 MAPK inhibition decreased cytokine expression but did not affect the retained defect of impaired insulin action in the diabetic muscle cells.

IGF-I, IGF-II, and Insulin Stimulate Different Gene Expression Responses through Binding to the IGF-I Receptor.

Insulin and the insulin-like growth factors (IGF)-I and -II are closely related peptides important for regulation of metabolism, growth, differentiation, and development. The IGFs exert their main effects through the IGF-I receptor. Although the insulin receptor is the main physiological receptor for insulin, this peptide hormone can also bind at higher concentrations to the IGF-I receptor and exert effects through it. We used microarray gene expression profiling to investigate the gene expression regulated by IGF-I, IGF-II, and insulin after stimulation of the IGF-I receptor. Fibroblasts from mice, knockout for IGF-II and the IGF-II/cation-independent mannose-6-phosphate receptor, and expressing functional IGF-I but no insulin receptors, were stimulated for 4 h with equipotent saturating concentrations of insulin, IGF-I, and IGF-II. Each ligand specifically regulated a group of transcripts that was not regulated by the other two ligands. Many of the functions and pathways these regulated genes were involved in, were consistent with the known biological effects of these ligands. The differences in gene expression might therefore account for some of the different biological effects of insulin, IGF-I, and IGF-II. This work adds to the evidence that not only the affinity of a ligand determines its biological response, but also its nature, even through the same receptor.

Cross-talk between insulin and Wnt signaling in preadipocytes: role of Wnt co-receptor low density lipoprotein receptor-related protein-5 (LRP5).

Disturbed Wnt signaling has been implicated in numerous diseases, including type 2 diabetes and the metabolic syndrome. In the present study, we have investigated cross-talk between insulin and Wnt signaling pathways using preadipocytes with and without knockdown of the Wnt co-receptors LRP5 and LRP6 and with and without knock-out of insulin and IGF-1 receptors. We find that Wnt stimulation leads to phosphorylation of insulin signaling key mediators, including Akt, GSK3ß, and ERK1/2, although with a lower fold stimulation and slower time course than observed for insulin. These Wnt effects are insulin/IGF-1 receptor-dependent and are lost in insulin/IGF-1 receptor double knock-out cells. Conversely, in LRP5 knockdown preadipocytes, insulin-induced phosphorylation of IRS1, Akt, GSK3ß, and ERK1/2 is highly reduced. This effect is specific to insulin, as compared with IGF-1, stimulation and appears to be due to an inducible interaction between LRP5 and the insulin receptor as demonstrated by co-immunoprecipitation. These data demonstrate that Wnt and insulin signaling pathways exhibit cross-talk at multiple levels. Wnt induces phosphorylation of Akt, ERK1/2, and GSK3ß, and this is dependent on insulin/IGF-1 receptors. Insulin signaling also involves the Wnt co-receptor LRP5, which has a positive effect on insulin signaling. Thus, altered Wnt and LRP5 activity can serve as modifiers of insulin action and insulin resistance in the pathophysiology of diabetes and metabolic syndrome.

Gene expression in skeletal muscle biopsies from people with type 2 diabetes and relatives: differential regulation of insulin signaling pathways.

BACKGROUND: Gene expression alterations have previously been associated with type 2 diabetes, however whether these changes are primary causes or secondary effects of type 2 diabetes is not known. As healthy first degree relatives of people with type 2 diabetes have an increased risk of developing type 2 diabetes, they provide a good model in the search for primary causes of the disease. METHODS/PRINCIPAL FINDINGS: We determined gene expression profiles in skeletal muscle biopsies from Caucasian males with type 2 diabetes, healthy first degree relatives, and healthy controls. Gene expression was measured using Affymetrix Human Genome U133 Plus 2.0 Arrays covering the entire human genome. These arrays have not previously been used for this type of study. We show for the first time that genes involved in insulin signaling are significantly upregulated in first degree relatives and significantly downregulated in people with type 2 diabetes. On the individual gene level, 11 genes showed altered expression levels in first degree relatives compared to controls, among others KIF1B and GDF8 (myostatin). LDHB was found to have a decreased expression in both groups compared to controls. CONCLUSIONS/SIGNIFICANCE: We hypothesize that increased expression of insulin signaling molecules in first degree relatives of people with type 2 diabetes, work in concert with increased levels of insulin as a compensatory mechanism, counter-acting otherwise reduced insulin signaling activity, protecting these individuals from severe insulin resistance. This compensation is lost in people with type 2 diabetes where expression of insulin signaling molecules is reduced.

Insulin-like growth factor I (IGF-I) is a more potent regulator of gene expression than insulin in primary human myoblasts and myotubes.

Conventionally, insulin is believed to induce a metabolic response, and IGF-I a mitogenic/differentiation response in vivo. However, several studies indicate that the roles of insulin and IGF-I may not be that easy to separate. In this study, insulin and IGF-I specificity in terms of gene regulation was investigated in primary human skeletal muscle cells before and after differentiation. Cell cultures were treated with 100 nM insulin, IGF-I or nothing for 4h, and gene expression was subsequently determined using the Affymetrix microarray platform. Insulin and IGF-I receptor levels were determined by qRT-PCR and by radioligand binding assays. In primary myoblasts, insulin did not have any significant effect on gene expression, whereas IGF-I regulated 229 genes. In primary myotubes, insulin regulated 105 genes, whereas IGF-I regulated 697 genes. Additionally, 99 genes were found to be differentially regulated by insulin and IGF-I in a direct comparison. The majority of these genes were specifically regulated by IGF-I, 16 genes were regulated by both ligands, and no genes were regulated by only insulin. The microarray results correlated with low levels of insulin receptors compared to IGF-I receptors as determined by radioligand binding assays. In the myotubes, we did not identify any ligand specificity in terms of functional categories. The major difference between the two ligands was their respective potencies in gene regulation, which was higher for IGF-I than for insulin. This was true for genes involved in both mitogenic and metabolic regulations. The data suggest that IGF-I is a more important metabolic regulator in skeletal muscle than previously estimated.

Activation of the insulin receptor (IR) by insulin and a synthetic peptide has different effects on gene expression in IR-transfected L6 myoblasts.

Single-chain peptides have been recently produced that display either mimetic or antagonistic properties against the insulin and IGF-1 (insulin-like growth factor 1) receptors. We have shown previously that the insulin mimetic peptide S597 leads to significant differences in receptor activation and initiation of downstream signalling cascades despite similar binding affinity and in vivo hypoglycaemic potency. It is still unclear how two ligands can initiate different signalling responses through the IR (insulin receptor). To investigate further how the activation of the IR by insulin and S597 differentially activates post-receptor signalling, we studied the gene expression profile in response to IR activation by either insulin or S597 using microarray technology. We found striking differences between the patterns induced by these two ligands. Most remarkable was that almost half of the genes differentially regulated by insulin and S597 were involved in cell proliferation and growth. Insulin either selectively regulated the expression of these genes or was a more potent regulator. Furthermore, we found that half of the differentially regulated genes interact with the genes involved with the MAPK (mitogen-activated protein kinase) pathway. These findings support our signalling results obtained previously and confirm that the main difference between S597 and insulin stimulation resides in the activation of the MAPK pathway. In conclusion, we show that insulin and S597 acting via the same receptor differentially affect gene expression in cells, resulting in a different mitogenicity of the two ligands, a finding which has critical therapeutic implications.

Structural biology of insulin and IGF-1 receptors.

The insulin and IGF-1 receptors are members of the superfamily of receptor tyrosine kinases (RTKs). Many of these have been implicated in human cancers due to amplification, overexpression or somatic mutations of the gene. Congenital mutations of the RTKs are implicated in a growing number of inherited syndromes. Unlike most RTKs that are single-chain monomeric transmembrane polypeptides, the insulin and IGF-1 receptors are dimers made of two extracellular alpha subunits and two transmembrane beta subunits containing the tyrosine kinase domain. The alpha subunits contain the ligand binding sites, of which at least three subdomains have been mapped by photoaffinity cross-linking, alanine-scanning mutagenesis or minimized receptor constructs. All RTKs are dimeric or oligomeric in the ligand-activated form, a mechanism that allows for transphosphorylation of the kinase domains and triggers the signalling cascade. The residues of insulin involved in receptor binding have been mapped by alanine-scanning mutagenesis. They form at least two major epitopes that partially overlap with the dimer- and hexamer-forming surfaces of the insulin molecule, and we propose that insulin is using those surfaces to cross-link the receptor alpha subunits. This mechanism provides a structural basis for negative cooperativity in binding, and probably also operates in the IGF-receptor interaction.