Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.7.11.24 (mitogen-activated protein kinase)
 95,810 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We have identified two new human genes that encode proteins with tandem pleckstrin homology-phosphotyrosine binding (PH-PTB) domains at their amino termini. Because the other known PH-PTB proteins (insulin receptor substrates: IRS-1, IRS-2, IRS-3, and IRS-4, and the downstream of kinases: DOK-1, DOK-2, and DOK-3) are substrates of insulin and insulin-like growth factor (IGF)-1 receptors, we asked whether these new proteins, termed IRS5/DOK4 and IRS6/DOK5, might also have roles in insulin and IGF-1 signaling. Northern analyses indicate that IRS5/DOK4 is ubiquitously expressed but most abundant in kidney and liver. IRS6/DOK5 expression is highest in skeletal muscle. Both proteins are tyrosine-phosphorylated in response to insulin and IGF-1 in transfected cells, although the kinetics differ. Insulin receptor-phosphorylated IRS5/DOK4 associates with RasGAP, Crk, Src, and Fyn, but not phosphatidylinositol 3-kinase p85, Grb2, SHP-2, Nck, or phospholipase Cgamma Src homology 2 domains, and activates MAPK in cells. IRS6/DOK5 neither associates with these Src homology 2 domains nor activates MAPK. IRS5/DOK4 and IRS6/DOK5 represent two new signaling proteins with potential roles in insulin and IGF-1 action.
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PMID:Two new substrates in insulin signaling, IRS5/DOK4 and IRS6/DOK5. 1273 Feb 41

Insulin receptor substrate-1 (IRS-1) plays an essential role in mediating the insulin signals that trigger mitogenesis, lipid synthesis, and uncoupling protein-1 gene expression in mouse brown adipocytes. Expression of IRS-3 is restricted mainly to white adipose tissue; expression of this IRS protein is virtually absent in brown adipocytes. We have tested the capacity of IRS-3 to mediate insulin actions in IRS-1-deficient brown adipocytes. Thus, we expressed exogenous IRS-3 in immortalized IRS-1-/- brown adipocytes at a level comparable with that of endogenous IRS-3 in white adipose tissue. Under these conditions, IRS-3 signaling in response to insulin was observed, as revealed by tyrosine phosphorylation of IRS-3, and the activation of phosphatidylinositol (PI) 3-kinase associated with this recombinant protein. However, although insulin promoted the association of Grb-2 with recombinant IRS-3 in IRS-1-/- cells, the exogenous expression of this IRS family member failed to activate p42/44 MAPK and mitogenesis in brown adipocytes lacking IRS-1. Downstream of PI 3-kinase, IRS-3 expression restored insulin-induced Akt phosphorylation, which is impaired by the lack of IRS-1 signaling. Whereas the generation of IRS-3 signals enhanced adipocyte determination and differentiation-dependent factor 1/sterol regulatory element-binding protein (ADD-1/SREBP-1c) and fatty acid synthase mRNA and protein expression, activation of this pathway was unable to reconstitute CCAAT/enhancer-binding protein alpha and uncoupling protein-1 transactivation and gene expression in response to insulin. Similar results were obtained following insulin-like growth factor-I stimulation. In brown adipocytes expressing the IRS-3F4 mutant, the association of the p85alpha regulatory subunit via Src homology 2 binding was lost, but insulin nevertheless induced PI 3-kinase activity and Akt phosphorylation in a wortmannin-dependent manner. In contrast, activation of IRS-3F4 signaling failed to restore the induction of ADD-1/SREBP-1c and fatty acid synthase gene expression in IRS-1-deficient brown adipocytes. These studies demonstrate that recombinant IRS-3 may reconstitute some, but not all, of the signals required for insulin action in brown adipocytes. Thus, our data further implicate a unique role for IRS-1 in triggering insulin action in brown adipocytes.
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PMID:Role of IRS-3 in the insulin signaling of IRS-1-deficient brown adipocytes. 1294 2

IGFs are potent mitogens that play a crucial role in cell proliferation and/or differentiation and tumorigenesis. Insulin receptor substrate-1 (IRS-1) is a key protein in the IGF signaling pathway in the estrogen-dependent MCF-7 breast carcinoma cell line. In this study, three growth factors [fibroblast growth factor (FGF), epidermal growth factor (EGF), and platelet-derived growth factor (PDGF)] were tested for their ability to modulate IRS-1 protein expression and the IGF-I signaling pathway. FGF and, to a lesser extent, EGF were found to increase IRS-1 protein, whereas PDGF had no effect. This indicates that growth factors can specifically modulate IRS-1 protein content. The increases provoked by EGF and FGF were dependent on the MAPK signaling pathway but independent of phosphatidylinositol 3-kinase (PI 3-kinase) signaling and required de novo protein synthesis. We noted that the kinetics of MAPK activation was continuous in response to FGF but transient in response to EGF. In addition, transfection of cells with a constitutively active form of MAPK kinase, which results in continuous MAPK activity, increased IRS-1 expression. Taken together, these results suggest that stimulation of IRS-1 expression was therefore stronger when MAPK activity was sustained. Pretreatment of cells with EGF, FGF, or PDGF for 24 h reduced IGF-I-induced tyrosine phosphorylation per molecule of IRS-1. However, IGF-I-induced PI 3-kinase activity was decreased by 24 h of pretreatment with EGF or PDGF but not with FGF. Our results therefore demonstrate that different growth factors are capable of specifically modulating the IGF-I signaling via IRS-1. They further suggest that the FGF-induced increase in IRS-1 counterbalances the inhibition of IRS-1 tyrosine phosphorylation to allow normal stimulation of IGF-I-induced PI 3-kinase activity.
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PMID:Growth factor-specific regulation of insulin receptor substrate-1 expression in MCF-7 breast carcinoma cells: effects on the insulin-like growth factor signaling pathway. 1296 57

Insulin receptor substrate (IRS)-1 and IRS-2 are the major substrates that mediate insulin action. Insulin itself regulates the expression of the IRS protein in the liver, but the underlying mechanisms of IRS-1 and IRS-2 regulation are not fully understood. Here we report that insulin suppressed the expression of both IRS-1 and IRS-2 proteins in Fao hepatoma cells. The decrease in IRS-1 protein occurred via proteasomal degradation without any change in IRS-1 mRNA, whereas the insulin-induced suppression of IRS-2 protein was associated with a parallel decrease in IRS-2 mRNA without changing IRS-2 mRNA half-life. The insulin-induced suppression of IRS-2 mRNA and protein was blocked by the phosphatidylinositol (PI) 3-kinase inhibitor, LY294002, but not by the MAP kinase-ERK kinase (MEK) inhibitor, PD098059. Inhibition of Akt by overexpression of dominant-negative Akt also caused complete attenuation of the insulin-induced decrease in IRS-2 protein and partial attenuation of its mRNA down-regulation. Some nuclear proteins bound to the insulin response element (IRE) sequence on the IRS-2 gene in an insulin-dependent manner in vitro, and the binding was also blocked by the PI 3-kinase inhibitor. Reporter gene assay showed that insulin suppressed the activity of both human and rat IRS-2 gene promoters through the IRE in a PI 3-kinase-dependent manner. Our results indicate that insulin regulates IRS-1 and IRS-2 through different mechanisms and that insulin represses IRS-2 gene expression via a PI 3-kinase/Akt pathway.
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PMID:Insulin down-regulates insulin receptor substrate-2 expression through the phosphatidylinositol 3-kinase/Akt pathway. 1459 77

The molecular signaling mechanisms by which insulin leads to increased glucose transport and metabolism and gene expression are not completely elucidated. We have characterized the nature of insulin signaling defects in skeletal muscle from Type 2 diabetic patients. Insulin receptor substrate (IRS-1) phosphorylation, phosphatidylinositol (PI) 3-kinase activity, and glucose transport activity are impaired as a consequence of functional defects, whereas insulin receptor tyrosine phosphorylation, mitogen-activated protein kinase (MAPK) phosphorylation, and glycogen synthase activity are normal. Using biotinylated photoaffinity labeling, we have shown that reduced cell surface GLUT4 levels can explain glucose transport defects in skeletal muscle from Type 2 diabetic patients under insulin-stimulated conditions. Current work is focused on mechanisms behind insulin-dependent and insulin-independent regulation of glucose uptake. We have recently determined the independent effects of insulin and hypoxia/AICAR exposure on glucose transport and cell surface GLUT4 content in skeletal muscle from nondiabetic and Type 2 diabetic subjects. Hypoxia and AICAR increase glucose transport via an insulin-independent mechanism involving activation of 5'-AMP-activated kinase (AMPK). AMPK signaling is intact, because 5-aminoimidazole-4-carboxamide 1-beta-D-ribonucleoside (AICAR) increased AMPK and acetyl-CoA carboxylase (ACC) phosphorylation to a similar extent in Type 2 diabetic and nondiabetic subjects. However, AICAR responses on glucose uptake were impaired. Our studies highlight important AMPK-dependent and independent pathways in the regulation of GLUT4 and glucose transport activity in insulin resistant skeletal muscle. Understanding signaling mechanisms to downstream metabolic responses may provide valuable clues to a future therapy for Type 2 diabetes.
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PMID:Sending the signal: molecular mechanisms regulating glucose uptake. 1523 28

Insulin receptor substrate-1 (IRS-1) mediates signaling from the insulin-like growth factor type-I receptor. We found that all-trans retinoic acid (RA) decreases IRS-1 protein levels in MCF-7, T47-D, and ZR75.1 breast cancer cells, which are growth arrested by RA, but not in the RA-resistant MDA-MB-231 and MDA-MB-468 cells. Based on prior reports of ubiquitin-mediated degradation of IRS-1, we investigated the ubiquitination of IRS-1 in RA-treated breast cancer cells. Two proteasome inhibitors, MG-132 and lactacystin, blocked the RA-mediated degradation of IRS-1, and RA increased ubiquitination of IRS-1 in the RA-sensitive breast cancer cells. In addition, we found that RA increases serine phosphorylation of IRS-1. To elucidate the signaling pathway responsible for this phosphorylation event, pharmacologic inhibitors were used. Two PKC inhibitors, but not a MAPK inhibitor, blocked the RA-induced degradation and serine phosphorylation of IRS-1. We demonstrate that RA activates PKC-delta in the sensitive, but not in the resistant cells, with a time course that is consistent with the RA-induced decrease of IRS-1. We also show that: (1) RA-activated PKC-delta phosphorylates IRS-1 in vitro, (2) PKC-delta and IRS-1 interact in RA-treated cells, and (3) mutation of three PKC-delta serine sites in IRS-1 to alanines results in no RA-induced in vitro phosphorylation of IRS-1. Together, these results indicate that RA regulates IRS-1 levels by the ubiquitin-proteasome pathway, involving a PKC-sensitive mechanism.
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PMID:Retinoic acid mediates degradation of IRS-1 by the ubiquitin-proteasome pathway, via a PKC-dependant mechanism. 1551 86

The effect of insulin and growth factor mediated signaling to gene regulation was investigated in cultured fibroblasts of a patient with a premature aging syndrome (metageria) and severe insulin resistance. Insulin receptor structure and function as well as major pathways activated by insulin, i.e. phosphatidyl inositol-3 kinase (PI-3 K) cascade or mitogen-activated protein kinase (MAPK) cascades, were functional. Inducibility of the proto-oncogene cfos, a representative endpoint of signaling pathways related to gene expression, by growth factors or insulin was reduced in patient cells. This reduced induction persisted in cfos promoter reporter gene studies indicating that the post receptor defect is localized proximal to the cfos promoter itself. Abundances of the transcription factors Elk-1 and SRF being major players in coupling of MAPKs to cfos promoter activation were not altered. However, basal and inducible phosphorylation of Elk-1 was impaired. In addition, basal and stimulated transcriptional activity mediated by Elk-1 was almost abolished in patient cells. Therefore these results identify a post receptor defect in cFos induction, which appears to be related to a functional alteration of Elk-1. A possible relation of this signal transduction defect to the specific premature aging syndrome remains to be elucidated.
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PMID:Reduced phosphorylation of transcription factor Elk-1 in cultured fibroblasts of a patient with premature aging syndrome and insulin resistance. 1577 1

Insulin receptor (IR) may play an essential role in the development of beta-cell mass in the mouse pancreas. To further define the function of this signaling system in beta-cell development, we generated IR-deficient beta-cell lines. Fetal pancreata were dissected from mice harboring a floxed allele of the insulin receptor (IRLoxP) and used to isolate islets. These islets were infected with a retrovirus to express simian virus 40 large T antigen, a strategy for establishing beta-cell lines (beta-IRLoxP). Subsequently, these cells were infected with adenovirus encoding cre recombinase to delete insulin receptor (beta-IR(-/-)). beta-Cells expressed insulin and Pdx-1 mRNA in response to glucose. In beta-IRLoxP beta-cells, p44/p42 MAPK and phosphatidylinositol 3 kinase pathways, mammalian target of rapamycin (mTOR), and p70S(6)K phosphorylation and beta-cell proliferation were stimulated in response to insulin. Wortmannin or PD98059 had no effect on insulin-mediated mTOR/p70S(6)K signaling and the corresponding mitogenic response. However, the presence of both inhibitors totally impaired these signaling pathways and mitogenesis in response to insulin. Rapamycin completely blocked insulin-activated mTOR/p70S(6)K signaling and mitogenesis. Interestingly, in beta-IR(-/-) beta-cells, glucose failed to stimulate phosphatidylinositol 3 kinase activity but induced p44/p42 MAPKs and mTOR/p70S(6)K phosphorylation and beta-cell mitogenesis. PD98059, but not wortmannin, inhibited glucose-induced mTOR/p70S(6)K signaling and mitogenesis in those cells. Finally, rapamycin blocked glucose-mediated mitogenesis of beta-IR(-/-) cells. In conclusion, independently of glucose, insulin can mediate mitogenesis in fetal pancreatic beta-cell lines. However, in the absence of the insulin receptor, glucose induces beta-cell mitogenesis.
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PMID:Differential mitogenic signaling in insulin receptor-deficient fetal pancreatic beta-cells. 1639 89

Insulin resistance in polycystic ovary syndrome (PCOS) results from a postbinding defect in signaling. Insulin receptor and insulin receptor substrate (IRS)-1 serine hyperphosphorylation by an unidentified kinase(s) contributes to this defect. We investigated whether insulin resistance is selective, affecting metabolic but not mitogenic pathways, in skeletal muscle as it is in cultured skin fibroblasts in PCOS. Extracellular signal-regulated kinase (ERK)1/2 activation was increased in skeletal muscle tissue and in cultured myotubes basally and in response to insulin in women with PCOS compared with control women. Mitogen-activated/extracellular signal-regulated kinase kinase (MEK)1/2 was also activated in PCOS, whereas p38 mitogen-activated protein kinase phosphorylation and signaling from the insulin receptor to Grb2 was similar in both groups. The activity of p21Ras was decreased and Raf-1 abundance increased in PCOS, suggesting that altered mitogenic signaling began at this level. MEK1/2 inhibition reduced IRS-1 Ser312 phosphorylation and increased IRS-1 association with the p85 subunit of phosphatidylinositol 3-kinase in both groups. We conclude that in PCOS skeletal muscle, 1) mitogenic signaling is enhanced in vivo and in culture, 2) ERK1/2 activation inhibits association of IRS-1 with p85 via IRS-1 Ser312 phosphorylation, and 3) ERK1/2 activation may play a role in normal feedback of insulin signaling and contribute to resistance to insulin's metabolic actions in PCOS.
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PMID:Enhanced mitogenic signaling in skeletal muscle of women with polycystic ovary syndrome. 1650 39

In this study, we evaluated the activation of various insulin signaling molecules in human fat in vivo and compared signaling reactions in visceral and subcutaneous fat depots. Paired abdominal omental and subcutaneous fat biopsies were obtained from nonobese subjects with normal insulin sensitivity under basal conditions and 6 and 30 min following administration of intravenous insulin. Insulin receptor phosphorylation was more intense and rapid and insulin receptor protein content was greater in omental than in subcutaneous adipose tissue (P < 0.05). Insulin-induced phosphorylation of Akt also occurred to a greater extent and earlier in omental than in subcutaneous fat (P < 0.05) in the absence of significant changes in Akt protein content. Accordingly, phosphorylation of the Akt substrate glycogen synthase kinase-3 was more responsive to insulin stimulation in omental fat. Protein content of extracellular signal-regulated kinase (ERK)-1/2 was threefold higher in omental than in subcutaneous fat (P < 0.05), and ERK phosphorylation showed an early 6-min peak in omental fat, in contrast with a more gradual increase observed in subcutaneous fat. In conclusion, the adipocyte insulin signaling system of omental fat shows greater and earlier responses to insulin than that of subcutaneous fat. These findings may contribute to explain the biological diversity of the two fat depots.
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PMID:Insulin signaling in human visceral and subcutaneous adipose tissue in vivo. 1656 16


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