SEARCH · PubMed Health

Results for “Insulin Receptor Substrate Proteins”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 recordsLinked to original sources

PubMed2000

Structural and functional characterization of insulin receptor substrate proteins and the molecular mechanisms of their interaction with insulin superfamily tyrosine kinase receptors and effector proteins.

The literature data on the role of IRS1/IRS2 proteins, endogenous substrates for insulin receptor tyrosine kinase, in transduction of signals generated by insulin superfamily peptides (insulin, insulin-like growth factor) were analyzed. The molecular mechanisms of the functional coupling of IRS proteins with peptide receptors possessing a tyrosine kinase activity and SH2 domain-containing proteins (phosphatidylinositol 3-kinase, Grb2 adaptor protein, protein phosphotyrosine phosphatase) were discussed. The structural and functional properties of IRS proteins (distribution of functional domains and sites for tyrosine phosphorylation; conservatism of amino acid sequences) were characterized. The data on the alternative pathways of transduction of signals which are generated by insulin and related peptides and do not involve IRS proteins were analyzed. These pathways are realized through Shc proteins or via direct interaction between receptors and SH2 proteins. Amino acid sequences of IRS proteins and insulin superfamily tyrosine kinase receptors were compared. The homologous regions in IRS proteins and receptors, which can be responsible for their coupling with phosphatidylinositol 3-kinase and protein phosphotyrosine phosphatases, were identified.

Amino Acid Sequence↗

PubMed2000

Release of insulin receptor substrate proteins from an intracellular complex coincides with the development of insulin resistance.

Insulin receptor substrate (IRS) proteins are major substrates of the insulin receptor (IR). IRS-1 associates with an insoluble multiprotein complex, possibly the cytoskeleton, in adipocytes. This localization may facilitate interaction with the IR at the cell surface. In the present study, we examined the hypothesis that the release of IRS proteins from this location may be a mechanism for insulin desensitization. We show that a second IRS protein, IRS-2, is associated with a multiprotein complex in adipocytes with similar characteristics to the IRS-1 complex. Insulin treatment (15-60 min) caused the release of IRS-1 and IRS-2 from this complex (high speed pellet; HSP) into the cytosol, whereas the level of tyrosyl-phosphorylated IRS proteins remained constant. Chronic insulin treatment resulted in a dramatic reduction in IRS-1 and IRS-2 in the HSP, eventually (>2 h) leading to IRS protein degradation and decreased levels of tyrosyl-phosphorylated IRS proteins. Okadaic acid, which rapidly induces insulin resistance in adipocytes independently of IR function, caused an almost quantitative release of IRS-1 into the cytosol commensurate with a significant reduction in tyrosyl-phosphorylated IRS proteins. Platelet-derived growth factor, a factor known to compromise insulin signaling, caused a more moderate release of IRS proteins from the HSP. Collectively, these results suggest that the assembly of IRS-1/IRS-2 into a multiprotein complex facilitates coupling to the IR and that the regulated release from this location may represent a novel mechanism of insulin resistance.

3T3 Cells↗

PubMed2001

Extension of life-span by loss of CHICO, a Drosophila insulin receptor substrate protein.

The Drosophila melanogaster gene chico encodes an insulin receptor substrate that functions in an insulin/insulin-like growth factor (IGF) signaling pathway. In the nematode Caenorhabditis elegans, insulin/IGF signaling regulates adult longevity. We found that mutation of chico extends fruit fly median life-span by up to 48% in homozygotes and 36% in heterozygotes. Extension of life-span was not a result of impaired oogenesis in chico females, nor was it consistently correlated with increased stress resistance. The dwarf phenotype of chico homozygotes was also unnecessary for extension of life-span. The role of insulin/IGF signaling in regulating animal aging is therefore evolutionarily conserved.

Aging↗

PubMed2000

Different subcellular localization and phosphoinositides binding of insulin receptor substrate protein pleckstrin homology domains.

Insulin evokes diverse biological effects through receptor-mediated tyrosine phosphorylation of the insulin receptor substrate (IRS) proteins. Here, we show that, in vitro, the IRS-1, -2 and -3 pleckstrin homology (PH) domains bind with different specificities to the 3-phosphorylated phosphoinositides. In fact, the IRS-1 PH domain binds preferentially to phosphatidylinositol 3,4,5-trisphosphate (PtdIns-3,4,5-P3), the IRS-2 PH domain to phosphatidylinositol 3,4-bisphosphate (PtdIns-3,4-P2), and the IRS-3 PH domain to phosphatidylinositol 3-phosphate. When expressed in NIH-IR fibroblasts and L6 myocytes, the IRS-1 and -2 PH domains tagged with green fluorescent protein (GFP) are localized exclusively in the cytoplasm. Stimulation with insulin causes a translocation of the GFP-IRS-1 and -2 PH domains to the plasma membrane within 3-5 min. This translocation is blocked by the phosphatidylinositol 3-kinase (PI 3-K) inhibitors, wortmannin and LY294002, suggesting that this event is PI 3-K dependent. Interestingly, platelet-derived growth factor (PDGF) did not induce translocation of the IRS-1 and -2 PH domains to the plasma membrane, indicating the existence of specificity for insulin. In contrast, the GFP-IRS-3 PH domain is constitutively localized to the plasma membrane. These results reveal a differential regulation of the IRS PH domains and a novel positive feedback loop in which PI 3-K functions as both an upstream regulator and a downstream effector of IRS-1 and -2 signaling.

Animals↗

PubMed2004

Insulin receptor substrate proteins and diabetes.

The discovery of insulin receptor substrate (IRS) proteins and their role to link cell surface receptors to the intracellular signaling cascades is a key step to understanding insulin and insulin-like growth factor (IGF) action. Moreover, IRS-proteins coordinate signals from the insulin and IGF receptor tyrosine kinases with those generated by proinflammatory cytokines and nutrients. The IRS2-branch of the insulin/IGF signaling cascade has an important role in both peripheral insulin response and pancreatic beta-cell growth and function. Dysregulation of IRS2 signaling in mice causes the failure of compensatory hyperinsulinemia during peripheral insulin resistance. IRS protein signaling is down regulated by serine phosphorylation or proteasome-mediated degradation, which might be an important mechanism of insulin resistance during acute injury and infection, or chronic stress associated with aging or obesity. Understanding the regulation and signaling by IRS1 and IRS2 in cell growth, metabolism and survival will reveal new strategies to prevent or cure diabetes and other metabolic diseases.

Animals↗

PubMed2003

Developmental and hormonal signals dramatically alter the localization and abundance of insulin receptor substrate proteins in the mammary gland.

Insulin receptor substrates (IRS) are central integrators of hormone, cytokine, and growth factor signaling. IRS proteins can be phosphorylated by a number of signaling pathways critical to normal mammary gland development. Studies in transgenic mice that overexpress IGF-I in the mammary gland suggested that IRS expression is important in the regulation of normal postlactational mammary involution. The goal of these studies was to examine IRS expression in the mouse mammary gland and determine the importance of IRS-1 to mammary development in the virgin mouse. IRS-1 and -2 show distinct patterns of protein expression in the virgin mouse mammary gland, and protein abundance is dramatically increased during pregnancy and lactation, but rapidly lost during involution. Consistent with hormone regulation, IRS-1 protein levels are reduced by ovariectomy, induced by combined treatment with estrogen and progesterone, and vary considerably throughout the estrous cycle. These changes occur without similar changes in mRNA levels, suggesting posttranscriptional control. Mammary glands from IRS-1 null mice have smaller fat pads than wild-type controls, but this reduction is proportional to the overall reduction in body size. Development of the mammary duct (terminal endbuds and branch points) is not altered by the loss of IRS-1, and pregnancy-induced proliferation is not changed. These data indicate that IRS undergo complex developmental and hormonal regulation in the mammary gland, and that IRS-1 is more likely to regulate mammary function in lactating mice than in virgin or pregnant mice.

Adipose Tissue↗

PubMed1998

A novel insulin receptor substrate protein, xIRS-u, potentiates insulin signaling: functional importance of its pleckstrin homology domain.

A novel Xenopus insulin receptor substrate cDNA was isolated by hybridization screening using the rat insulin receptor substrate-1 (IRS-1) cDNA as a probe. The xIRS-u cDNA encodes an open reading frame of 1003 amino acids including a putative amino-terminal pleckstrin homology (PH) domain and phosphotyrosine-binding (PTB) domain. The carboxy terminus of xIRS-u contains several potential Src homology 2 (SH2)-binding sites, five of which are in the context of YM/LXM (presumptive binding sites for phosphatidylinositol 3-kinase). It also contains a putative binding site for Grb2 (YINID). Pair-wise amino acid sequence comparisons with the previously identified xIRS-1 and the four members of the mammalian IRS family (1 through 4) indicated that xIRS-u has similar overall sequence homology (33-45% identity) to all mammalian IRS proteins. In contrast, the previously isolated xIRS-1 is particularly similar (67% identical) to IRS-1 and considerably less similar (31-46%) to the other IRS family members (2 through 4). xIRS-u is also distinct from xIRS-1, having an overall sequence identity of 47%. These sequence analyses suggest that xIRS-u is a novel member of the IRS family rather than a Xenopus homolog of an existing member. Microinjection of mRNA encoding a Myc-tagged xIRS-u into Xenopus oocytes resulted in the expression of a 120-kDa protein (including 5 copies of the 13-amino acid Myc tag). The injection of xIRS-u mRNA accelerated insulin-induced MAP kinase activation with a concomitant acceleration of insulin-induced oocyte maturation. An aminoterminal deletion of the PH domain (xIRS-u deltaPH) significantly reduced the ability of xIRS-u to potentiate insulin signaling. In contrast to the full-length protein, injection of xIRS-u (1-299), which encoded the PH and PTB domain, or xIRS-u (1-170), which encoded only the PH domain, blocked insulin signaling in Xenopus oocytes. Finally, xIRS-u (119-299), which had a truncated PH domain and an intact PTB domain, had no effect on insulin signaling. This is the first report that the PH domain of an IRS protein can function in a dominant negative manner to inhibit insulin signaling.

Adaptor Proteins, Signal Transducing↗

PubMed2001

Insulin receptor substrate proteins and neuroendocrine function.

A family of insulin receptor substrate (IRS) proteins mediates the pleiotropic effects of insulin and insulin-like growth factor 1 (IGF-1) on cellular function by recruiting several intracellular signalling networks. Conventional murine knockout strategies have started to reveal distinct physiological roles for the IRS proteins. Deletion of Irs1 produces a mild metabolic phenotype with compensated insulin resistance but also causes marked growth retardation. In contrast, mice lacking IRS-2 display nearly normal growth but develop diabetes owing to a combination of peripheral insulin resistance and beta-cell failure. As well as the classical metabolic events regulated by insulin signalling pathways, studies in lower organisms have implicated insulin/IGF-1 signalling pathways in the control of food intake and reproductive function. Our analysis of IRS-2 knock-out mice shows that female mice are infertile owing to defects in the hypothalamus, pituitary and gonad. IRS-2(-/-) mice have small, anovulatory ovaries with reduced numbers of follicles. Levels of the pituitary hormones luteinizing hormone and prolactin and gonadal steroids are low in these animals. Pituitaries of IRS-2(-/-) animals are decreased in size and contain reduced numbers of gonadotrophs. Additionally, IRS-2(-/-) females display increased food intake and develop obesity, despite elevated leptin levels, suggesting abnormalities in hypothalamic function. Coupled with recent observations that brain-specific deletion of the insulin receptor causes a similar phenotype, these findings implicate IRS signalling pathways in the neuroendocrine regulation of reproduction and energy homeostasis.

Animals↗

PubMed1996

The Drosophila insulin receptor activates multiple signaling pathways but requires insulin receptor substrate proteins for DNA synthesis.

The Drosophila insulin receptor (DIR) contains a 368-amino-acid COOH-terminal extension that contains several tyrosine phosphorylation sites in YXXM motifs. This extension is absent from the human insulin receptor but resembles a region in insulin receptor substrate (IRS) proteins which binds to the phosphatidylinositol (PI) 3-kinase and mediates mitogenesis. The function of a chimeric DIR containing the human insulin receptor binding domain (hDIR) was investigated in 32D cells, which contain few insulin receptors and no IRS proteins. Insulin stimulated tyrosine autophosphorylation of the human insulin receptor and hDIR, and both receptors mediated tyrosine phosphorylation of Shc and activated mitogen-activated protein kinase. IRS-1 was required by the human insulin receptor to activate PI 3-kinase and p70s6k, whereas hDIR associated with PI 3-kinase and activated p70s6k without IRS-1. However, both receptors required IRS-1 to mediate insulin-stimulated mitogenesis. These data demonstrate that the DIR possesses additional signaling capabilities compared with its mammalian counterpart but still requires IRS-1 for the complete insulin response in mammalian cells.

Amino Acid Sequence↗

PubMed2004

Suppressor of cytokine signaling 1 (SOCS-1) and SOCS-3 cause insulin resistance through inhibition of tyrosine phosphorylation of insulin receptor substrate proteins by discrete mechanisms.

Insulin resistance is a pathophysiological component of type 2 diabetes and obesity and also occurs in states of stress, infection, and inflammation associated with an upregulation of cytokines. Here we show that in both obesity and lipopolysaccharide (LPS)-induced endotoxemia there is an increase in suppressor of cytokine signaling (SOCS) proteins, SOCS-1 and SOCS-3, in liver, muscle, and, to a lesser extent, fat. In concordance with these increases by LPS, tyrosine phosphorylation of the insulin receptor (IR) is partially impaired and phosphorylation of the insulin receptor substrate (IRS) proteins is almost completely suppressed. Direct overexpression of SOCS-3 in liver by adenoviral-mediated gene transfer markedly decreases tyrosine phosphorylation of both IRS-1 and IRS-2, while SOCS-1 overexpression preferentially inhibits IRS-2 phosphorylation. Neither affects IR phosphorylation, although both SOCS-1 and SOCS-3 bind to the insulin receptor in vivo in an insulin-dependent fashion. Experiments with cultured cells expressing mutant insulin receptors reveal that SOCS-3 binds to Tyr960 of IR, a key residue for the recognition of IRS-1 and IRS-2, whereas SOCS-1 binds to the domain in the catalytic loop essential for IRS-2 recognition in vitro. Moreover, overexpression of either SOCS-1 or SOCS-3 attenuates insulin-induced glycogen synthesis in L6 myotubes and activation of glucose uptake in 3T3L1 adipocytes. By contrast, a reduction of SOCS-1 or SOCS-3 by antisense treatment partially restores tumor necrosis factor alpha-induced downregulation of tyrosine phosphorylation of IRS proteins in 3T3L1 adipocytes. These data indicate that SOCS-1 and SOCS-3 act as negative regulators in insulin signaling and serve as one of the missing links between insulin resistance and cytokine signaling.

Animals↗

PubMed2000

Association of insulin receptor substrate proteins with Bcl-2 and their effects on its phosphorylation and antiapoptotic function.

Insulin receptor substrate (IRS) proteins are docking proteins that couple growth factor receptors to various effector molecules, including phosphoinositide-3 kinase, Grb-2, Syp, and Nck. Here we show that IRS-1 associates with the loop domain of Bcl-2 and synergistically up-regulates antiapoptotic function of Bcl-2. IRS-2 but not IRS-3 binds to Bcl-2, and IRS-1 associates with Bcl-XL but not with Bax or Bik. Overexpression of IRS-1 suppresses phosphorylation of Bcl-2 induced by stimulation with insulin, and the hypophosphorylation may lead to its enhanced antiapoptotic activity. The binding site for Bcl-2 is located on the carboxyl half-domain of IRS-1. IRS-3, which lacks the corresponding region, dominant-negatively abrogates the survival effects of IRS-1 and Bcl-2. For the antiapoptotic activity of IRS-1, binding to Bcl-2 is more critical than activating phosphoinositide-3 kinase. Our results indicate that IRS proteins transmit signals from the insulin receptor to Bcl-2, thus regulating cell survival probably through regulating phosphorylation of Bcl-2.

Animals↗

PubMedDate not supplied

[Sequence homology in the primary structures of tyrosine kinase receptors of insulin superfamily and protein substrates of insulin type I and type II receptors].

Ligand-activated tyrosine kinase receptors of insulin superfamily peptides can realize the signal transduction to SH2-proteins (phosphatidylinositol 3-kinase, PI3K), protein phosphotyrosine phosphatase (PPTP), GRB2-adaptor protein in two pathways: 1) with participation of specific proteins--insulin receptor substrates 1 and 2 (IRS1/IRS2); and 2) direct interaction between receptors and SH2-proteins (without IRS-proteins). Consequently, structural related determinants, which are responsible for the interaction with SH2-proteins, must be present in the receptor and IRS molecules. The comparative analysis of amino acid sequences (AAS) of human receptors of insulin, insulin-like growth factor-I and insulin-related peptide and AAS of IRS1/IRS2 proteins allow one to identify for the first time the long homologous regions in their primary structures. After alignment of AAS of the regions, the sited-targets for tyrosine phosphorylation, most important for functional activity of tyrosine kinase receptors and IRS proteins, coincided with each other. These results show that some homologous regions can have similar function. Thus, the regions can involve in coupling the receptors and IRS-proteins with SH2-proteins, such as PI3K, PPTP, GRB2-adaptor protein. It is also possible that the homologous regions of tyrosine kinase receptors and IR1/IRS2 proteins mediate the interaction between their proteins.

Adaptor Proteins, Signal Transducing↗

PubMed2004

Identification of insulin receptor substrate proteins as key molecules for the TbetaR-V/LRP-1-mediated growth inhibitory signaling cascade in epithelial and myeloid cells.

The type V TGF-beta receptor (TbetaR-V) mediates IGF-independent growth inhibition by IGFBP-3 and mediates growth inhibition by TGF-beta1 in concert with the other TGF-beta receptor types. TbetaR-V was recently found to be identical to LRP-1. Here we find that insulin and (Q3A4Y15L16) IGF-I (an IGF-I analog that has a low affinity for IGFBP-3) antagonize growth inhibition by IGFBP-3 in mink lung epithelial cells (Mv1Lu cells) stimulated by serum. In these cells, IGFBP-3 induces serine-specific dephosphorylation of IRS-1 and IRS-2. The IGFBP-3-induced dephosphorylation of IRS-2 is prevented by cotreatment of cells with insulin, (Q3A4Y15L16) IGF-I, or TbetaR-V/LRP-1 antagonists. The magnitude of the IRS-2 dephosphorylation induced by IGFBP-3 positively correlates with the degree of growth inhibition by IGFBP-3 in Mv1Lu cells and mutant cells derived from Mv1Lu cells. Stable transfection of murine 32D myeloid cells (which lack endogenous IRS proteins and are insensitive to growth inhibition by IGFBP-3) with IRS-1 or IRS-2 cDNA confers sensitivity to growth inhibition by IGFBP-3; this IRS-mediated growth inhibition can be completely reversed by insulin in 32D cells stably expressing IRS-2 and the insulin receptor. These results suggest that IRS-1 and IRS-2 are key molecules for the TbetaR-V/LRP-1-mediated growth inhibitory signaling cascade.

Animals↗

PubMed1997

Insulin receptor substrate proteins create a link between the tyrosine phosphorylation cascade and the Ca2+-ATPases in muscle and heart.

Following phosphorylation by the insulin receptor kinase, the insulin receptor substrates (IRS)-1 and IRS-2 bind to and activate several Src homology 2 (SH2) domain proteins. To identify novel proteins that interact with IRS proteins in muscle, a human skeletal muscle cDNA expression library was created in the lambdaEXlox system and probed with baculovirus-produced and tyrosine-phosphorylated human IRS-1. One clone of the 10 clones which was positive through three rounds of screening represented the C terminus of the human homologue of the adult fast twitch skeletal muscle Ca2+-ATPase (SERCA1) including the cytoplasmic tail and part of transmembrane region 10. Western blot analysis of extracts of rat muscle demonstrated co-immunoprecipitation of both IRS-1 and IRS-2 with the skeletal muscle Ca2+-ATPase (SERCA1) and the cardiac muscle isoform (SERCA2). In both cases, injection of insulin stimulated a 2- to 6-fold increase in association of which was maximal within 5 min. In primary cultures of aortic smooth muscle cells and C2C12 cells, the insulin-stimulated interaction between IRS proteins and SERCA1 and -2 was dose-dependent with a maximum induction at 100 nM insulin. This interaction was confirmed in a "pull down" experiment using a glutathione S-transferase fusion protein containing the C terminus of the human SERCA isoform and phosphorylated IRS-1 in vitro and could be blocked by a FLVRES-like domain peptide present in the human SERCA sequence. Affinity chromatography of phosphopeptide libraries using the glutathione S-transferase fusion protein of the C terminus of SERCA1 indicated a consensus sequence for binding of XpYGSS; this is identical to potential tyrosine phosphorylation sites at position 431 of human IRS-1 and at position 500 of human IRS-2. In streptozotocin diabetic rats the interaction between IRS proteins and SERCA1 in skeletal muscle and SERCA2 in cardiac muscle was significantly reduced. Taken together, these results indicate that the IRS proteins bind to the Ca2+-ATPase of the sarcoplasmic reticulum in an insulin-regulated fashion, thus creating a potential link between the tyrosine phosphorylation cascade and effects of insulin on calcium.

Animals↗

PubMed1999

Imidapril, an angiotensin-converting enzyme inhibitor, improves insulin sensitivity by enhancing signal transduction via insulin receptor substrate proteins and improving vascular resistance in the Zucker fatty rat.

Angiotensin-converting enzyme (ACE) inhibitors are antihypertensive agents, that inhibit the conversion of angiotensin I to angiotensin II, resulting in smooth-muscle relaxation and a reduction of vascular resistance. Recently, it has been suggested that ACE inhibitors improve insulin resistance in diabetic patients. To investigate the effect of an ACE inhibitor on insulin sensitivity, insulin signaling, and circulation, imidapril was administered orally or intraduodenally to Zucker fatty rats. Oral administration of imidapril improved insulin sensitivity based on the results of an oral glucose tolerance test (OGTT) and a decrease in urinary glucose secretion. Phosphatidylinositol 3-kinase (PI 3-kinase) activity associated with hepatic insulin receptor substrate-1 (IRS-1) in the insulin-stimulated condition was significantly enhanced 110% without a significant alteration in tyrosine phosphorylation of IRS-1 in the imidapril-treated group. In muscle, IRS-1 tyrosine phosphorylation and PI 3-kinase activity associated with IRS-1 in the insulin-stimulated condition were enhanced 70% and 20%, respectively, in the imidapril-treated group. In contrast, an alteration of the IRS-2 pathway was observed only in liver; a significant insulin-induced increase in the IRS-2-associated PI 3-kinase over the basal level was observed in the imidapril-treated group but not in the control. In addition, treatment with imidapril was shown to significantly reduce blood pressure and increase blood flow in the liver and muscle. These results suggest that the ACE inhibitor imidapril may improve insulin sensitivity not only by acting directly on the insulin signaling pathway but also by increasing blood flow in tissues via normalization of vascular resistance, a major cause of hypertension.

Angiotensin-Converting Enzyme Inhibitors↗

PubMed2003

Novel espin actin-bundling proteins are localized to Purkinje cell dendritic spines and bind the Src homology 3 adapter protein insulin receptor substrate p53.

We identified a group of actin-binding-bundling proteins that are expressed in cerebellar Purkinje cells (PCs) but are not detected in other neurons of the CNS. These proteins are novel isoforms of the actin-bundling protein espin that arise through the use of a unique site for transcriptional initiation and differential splicing. Light and electron microscopic localization studies demonstrated that these espin isoforms are enriched in the dendritic spines of PCs. They were detected in the head and neck and in association with the postsynaptic density (PSD) of dendritic spines in synaptic contact with parallel or climbing fibers. They were also highly enriched in PSD fractions isolated from cerebellum. The PC espins efficiently bound and bundled actin filaments in vitro, and these activities were not inhibited by Ca2+. When expressed in transfected neuronal cell lines, the PC espins colocalized with actin filaments and elicited the formation of coarse cytoplasmic actin bundles. The insulin receptor substrate p53 (IRSp53), an Src homology 3 (SH3) adapter protein and regulator of the actin cytoskeleton, was identified as an espin-binding protein in yeast two-hybrid screens. Cotransfection studies and pull-down assays showed that this interaction was direct and required the N-terminal proline-rich peptide of the PC espins. Thus, the PC espins exhibit the properties of modular actin-bundling proteins with the potential to influence the organization and dynamics of the actin cytoskeleton in PC dendritic spines and to participate in multiprotein complexes involving SH3 domain-containing proteins, such as IRSp53.

Actin Cytoskeleton↗

PubMed2005

Modulation of insulin signalling by insulin sensitizers.

Insulin resistance is a hallmark of Type II diabetes. It is well documented that insulin sensitizers such as peroxisome-proliferator-activated receptor gamma agonists and aspirin improve insulin action in vivo. The detailed mechanisms by which the insulin sensitizers promote insulin signalling, however, are not completely understood and remain somewhat controversial. In the present review, we summarize our studies attempting to explore the molecular mechanisms underlying the effects of insulin sensitizers in cells and in animal models of insulin resistance. In 3T3-L1 adipocytes and/or in HEK-293 cells stably expressing recombinant IRS1 protein (insulin receptor substrate protein 1), the peroxisome-proliferator-activated receptor gamma agonist rosiglitazone and aspirin promote insulin signalling by decreasing inhibitory IRS1 serine phosphorylation. Increased IRS1 Ser-307 phosphorylation and concomitant decreased insulin signalling as measured by insulin-stimulated IRS1 tyrosine phosphorylation and Akt threonine phosphorylation were observed in adipose tissues of Zucker obese rats compared with lean control rats. Treatment with rosiglitazone for 24 and 48 h increased insulin signalling and decreased IRS1 Ser-307 phosphorylation concomitantly. Treatment of the Zucker obese rats with rosiglitazone for 24 h also reversed the high circulating levels of free fatty acids, which have been shown to correlate with increased IRS1 serine phosphorylation. Taken together, the results suggest that IRS1 inhibitory serine phosphorylation is a key component of insulin resistance and its reversal may be physiologically relevant to insulin sensitization in vivo.

Animals↗

PubMed2000

Thrombopoietin induces phosphoinositol 3-kinase activation through SHP2, Gab, and insulin receptor substrate proteins in BAF3 cells and primary murine megakaryocytes.

Thrombopoietin (TPO) is a recently characterized member of the hematopoietic growth factor family that serves as the primary regulator of megakaryocyte (MK) and platelet production. The hormone acts by binding to the Mpl receptor, the product of the cellular proto-oncogene c-mpl. Although many downstream signaling targets of TPO have been identified in cell lines, primary MKs, and platelets, the molecular mechanism(s) by which many of these molecules are activated remains uncertain. In this report we demonstrate that the TPO-induced activation of phosphoinositol 3-kinase (PI3K), a signaling intermediate vital for cellular survival and proliferation, occurs through its association with inducible signaling complexes in both BaF3 cells engineered to express Mpl (BaF3/Mpl) and in primary murine MKs. Although a direct association between PI3K and Mpl could not be demonstrated, we found that several proteins, including SHP2, Gab2, and IRS2, undergo phosphorylation and association in BaF3/Mpl cells in response to TPO stimulation, complexes that recruit and enhance the enzymatic activity of PI3K. To verify the physiological relevance of the complex, SHP2-Gab2 association was disrupted by overexpressing a dominant negative SHP2 construct. TPO-induced Akt phosphorylation was significantly decreased in transfected cells suggesting an important role of SHP2 in the complex to enhance PI3K activity. In primary murine MKs, TPO also induced phosphorylation of SHP2, its association with p85 and enhanced PI3K activity, but in contrast to the results in cell lines, neither Gab2 nor IRS2 are phosphorylated in MKs. Instead, a 100-kDa tyrosine-phosphorylated protein (pp100) co-immunoprecipitated with the regulatory subunit of PI3K. These findings support a model where PI3K activity is dependent on its recruitment into TPO-induced multiphosphoprotein complexes, implicate the existence of a scaffolding protein in primary MKs distinct from the known Gab and IRS proteins, and suggest that, in contrast to erythroid progenitor cells that employ Gab1 in PI3K signaling complexes, utilization of an alternate member of the Gab/IRS family could be responsible for specificity in TPO signaling.

Adaptor Proteins, Signal Transducing↗