Insulin Receptor Substrate 1

Cloning and Expression

Sun et al. (1991) isolated cDNAs encoding a 160- to 185-kD phosphotyrosyl protein that was a substrate of insulin receptor tyrosine kinase and a putative participant in insulin (INS; 176730) signaling. This protein, designated insulin receptor substrate-1 (IRS1), was found in a variety of insulin-responsive cells and tissues.

Stoffel et al. (1993) cloned the IRS1 gene from a human male placenta library.

Gene Function

Sun et al. (1991) found that IRS1 exhibited no intrinsic enzyme activity. They suggested it serves as a docking protein involved in binding and activating other signal transduction molecules after being phosphorylated on tyrosine by insulin receptor kinase.

Binding of insulin to its receptor induces phosphorylation of the cytosolic substrates IRS1 and IRS2 (600797), which associate with several Src homology-2 (SH2) domain-containing proteins. To identify unique IRS1-binding proteins, Ogihara et al. (1997) screened a human heart cDNA library with recombinant IRS1. They obtained 2 isoforms of the 14-3-3 (YWHA) protein family, namely 14-3-3-epsilon (YWHAE; 605066) and 14-3-3-zeta (YWHAZ; 601288). 14-3-3 protein has been shown to associate with IRS1 in L6 myotubes, HepG2 hepatoma cells, Chinese hamster ovary cells, and bovine brain tissue. IRS2, a protein structurally similar to IRS1, was also shown to form a complex with 14-3-3 protein using a baculovirus expression system. The amount of 14-3-3 protein associated with IRS1 was not affected by insulin stimulation but was increased significantly by treatment with okadaic acid, a potent serine/threonine phosphatase inhibitor. Peptide inhibition experiments using phosphoserine-containing peptides of IRS1 revealed that IRS1 contains 3 putative binding sites for 14-3-3 protein. Among these 3, the motif around serine-270 is located in the phosphotyrosine-binding (PTB) domain of IRS1, which is responsible for the interaction with the insulin receptor (INSR; 147670). Indeed, a truncated mutant of IRS1 consisting of only the PTB domain retained the capacity to bind to 14-3-3 protein in vivo. Ogihara et al. (1997) investigated the effect of 14-3-3 protein binding on the insulin-induced phosphorylation of IRS1. Phosphoamino acid analysis revealed that IRS1 coimmunoprecipitated with anti-14-3-3 antibody is weakly phosphorylated after insulin stimulation, on tyrosine as well as serine residues, compared with IRS1 immunoprecipitated with anti-IRS1 antibody. Thus, the authors suggested that the association with 14-3-3 protein may play a role in the regulation of insulin sensitivity by interrupting the association between the insulin receptor and IRS1.

Cell size is strongly dependent on ribosome biogenesis, which is controlled by RNA polymerase I (see 602000). The activity of this polymerase is modulated by a complex of proteins, including UBTF (600673). From experiments with mouse embryonic fibroblasts, Drakas et al. (2004) presented evidence that a nuclear complex forms between IRS1, UBTF, and phosphatidylinositol 3-kinase (PI3K; see 171834), leading to the serine phosphorylation of UBF1 and regulation of rRNA synthesis.

Shi et al. (2007) found that expression of microRNA-145 (MIRN145; 611795) in a human colorectal adenocarcinoma cell line downregulated IRS1 translation, but did not alter IRS1 mRNA levels. Downregulation of IRS1 translation by MIRN145 required the 3-prime UTR of IRS1 mRNA. Treatment of cells with MIRN145 caused growth arrest comparable to use of small interfering RNA directed against IRS1.

Using chromatin immunoprecipitation and reporter gene assays, Wu et al. (2008) showed that nuclear Irs1 bound and activated the Myc (190080), cyclin D1 (CCND1; 168461), and ribosomal DNA promoters in mouse fibroblasts in response to Igf1. In the absence of nuclear translocation, Irs1 did not localize to these promoters, and their activation was dramatically reduced. Deletion of the phosphotyrosine-binding domain of Irs1 abolished its ability to activate the Myc and cyclin D1 promoters. Activation of Myc and cyclin D1 promoters by nuclear Irs1 did not require PI3K activity.

Using poly(A) RT-PCR, Zhang et al. (2008) found that microRNA-126 (MIR126; 611767) was downregulated in human embryonic kidney (HEK293) cells and breast cancer cell lines. Overexpression of MIR126 inhibited cell growth in HEK293 and breast cancer cells by suppressing cycle cycle progression from G0/G1 to S phase. Zhang et al. (2008) identified a complementary site for MIR126 in the 3-prime UTR of IRS1, and in vitro luciferase assays confirmed that MIR126 targeted IRS1. Overexpression of MIR126 significantly decreased IRS1 protein, but not IRS1 mRNA. Knockdown of IRS1 vi short hairpin RNA decreased cell growth in HEK293 and breast cancer cells, recapitulating the effect of MIR126 overexpression.

In mouse and human lung adenocarcinoma (211980) cells, Houghton et al. (2010) found neutrophil elastase (ELANE; 130130) directly induced tumor cell proliferation at physiologic levels by gaining access to an endosomal compartment within tumor cells, where it degraded IRS1. Degradation of IRS1 was associated with increased interaction between PI3K and the potent mitogen PDGFR (173410), skewing the PI3K axis toward tumor cell proliferation. The findings identified IRS1 as a key regulator of PI3K within malignant cells.

Song et al. (2013) showed in mice that muscle-specific mitsugumin-53 (MG53; 613288) mediates the degradation of the insulin receptor and Irs1, and when upregulated causes metabolic syndrome featuring insulin resistance, obesity, hypertension, and dyslipidemia. Mg53 expression is markedly elevated in models of insulin resistance, and Mg53 overexpression suffices to trigger muscle insulin resistance and metabolic syndrome sequentially. Conversely, ablation of Mg53 prevents diet-induced metabolic syndrome by preserving the insulin receptor, Irs1, and insulin signaling integrity. Mechanistically, Mg53 acts as an E3 ligase targeting the insulin receptor and Irs1 for ubiquitin-dependent degradation, comprising a central mechanism controlling insulin signal strength in skeletal muscle. Song et al. (2013) concluded that these findings defined MG53 as a novel therapeutic target for treating metabolic disorders and associated cardiovascular complications.

Mapping

Using DNA from a human-hamster somatic cell hybrid panel and PCR, Stoffel et al. (1993) mapped the IRS1 gene to chromosome 2. By fluorescence in situ hybridization, they regionalized the assignment to 2q35-q36.1. Using a genomic clone for fluorescence in situ hybridization, Nishiyama et al. (1994) localized the IRS1 gene to 2q36. Araki et al. (1993) likewise mapped the gene to this region and showed that the homologous gene is on mouse chromosome 1. Stoffel et al. (1993) also identified a simple tandem repeat DNA polymorphism useful for genetic studies. Because of its central role in the signal transduction pathway, IRS1 is a candidate for the site of the defect in insulin action seen in patients with noninsulin-dependent diabetes mellitus (NIDDM; 125853).

Molecular Genetics

Laakso et al. (1994) investigated the frequency and clinical significance of variants in the coding region of the IRS1 gene in patients with NIDDM. Applying single-strand conformation polymorphism (SSCP) analysis, they found 3 amino acid substitutions among 40 Finnish patients with typical NIDDM: gly81 to arg, ser892 to gly, and gly971 to arg (147545.0002). Almind et al. (1993) had suggested that the ala512-to-pro and gly971-to-arg polymorphisms of the IRS1 gene are common in Danish patients with NIDDM. Of the 3 amino acid substitutions observed by Laakso et al. (1994), they found ser892 to gly the most 'interesting' since it abolishes one of the potential serine phosphorylation sites which is located immediately N-terminal to the only SH2-binding site of growth factor receptor-bound protein (GRB2; 108355) and thus could potentially influence some aspects of signal transduction and metabolic response to insulin. GRB2 is a protein that associates with IRS1 upon insulin-induced phosphorylation. The ser892-to-gly substitution may influence the binding of GRB2 to IRS1 and the activation of downstream insulin signaling proteins.

Rung et al. (2009) used genomewide association data from 1,376 French individuals to identify 16,360 SNPs nominally associated with type 2 diabetes and studied these SNPs in an independent sample of 4,977 French individuals. They then selected the 28 best hits for replication in 7,698 Dutch subjects and identified 4 SNPs showing strong association with type 2 diabetes. One of these, rs2943641 (P = 9.3 x 10(-12), odds ratio = 1.19), was located 500 kb upstream of the IRS1 gene. Unlike previously reported type 2 diabetes risk loci, which predominantly associate with impaired beta cell function, the C allele of rs2943641 was associated with insulin resistance and hyperinsulinemia in 14,358 French, Danish, and Finnish participants from population-based cohorts; this allele was also associated with reduced basal levels of IRS1 protein and decreased insulin induction of IRS1-associated phosphatidylinositol-3-hydroxykinase activity in human skeletal muscle biopsies. Rung et al. (2009) noted that rs2943641 and the common IRS1 missense polymorphism G972R (147545.0002) lie 567 kb apart and were not in linkage disequilibrium. Further analysis suggested that rs2943641 and G972R may independently influence insulin sensitivity and type 2 diabetes risk.

Animal Model

Insulin resistance is often associated with atherosclerotic diseases in subjects with obesity and impaired glucose tolerance. Abe et al. (1998) studied female mice homozygous for targeted disruption of the Irs1 gene and female wildtype mice that were offspring of heterozygous mice. In this nonobese animal model of insulin resistance, they found that blood pressure and plasma triglyceride levels were significantly higher than in normal mice. Impaired endothelium-dependent vascular relaxation was also observed in these mice. Furthermore, lipoprotein lipase activity was lower than in normal mice, implicating impaired lipolysis in the increase in plasma triglyceride levels under insulin-resistant conditions. Thus, insulin resistance plays an important role in the clustering of coronary risk factors that may accelerate the progression of atherosclerosis in subjects with insulin resistance.

Bohni et al. (1999) showed that chico, a Drosophila homolog of the vertebrate IRS gene family, plays an essential role in the control of cell size and growth. Animals mutant for chico were less than half the size of wildtype flies, owing to fewer and smaller cells. In mosaic animals, chico homozygous cells grew slower than their heterozygous sibs, showed an autonomous reduction in cell size, and formed organs of reduced size. Although chico flies were smaller, they showed an almost 2-fold increase in lipid levels.

Clancy et al. (2001) found that mutation of chico extends fruit fly 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.

In a review, Myers et al. (1994) pointed out that since disruption of the gene encoding Irs1 in mice is not lethal, there must be other molecules that the insulin receptor can use to regulate critical metabolic pathways.

Kulkarni et al. (1999) found that freshly isolated islets from Irs1 knockout mice and SV40-transformed Irs1-deficient beta-cell lines exhibited marked insulin secretory defects in response to glucose and arginine. Furthermore, insulin expression was reduced by about 2-fold in the Irs1-null islets and beta-cell lines, and this defect could be partially restored by transfecting the cells with Irs1. These data provided evidence for an important role of IRS1 in islet function and for a novel functional link between the insulin signaling and insulin secretion pathways.

Type II diabetes (NIDDM) is characterized by abnormalities of insulin action in muscle, adipose tissue, and liver and by altered beta-cell function. To analyze the role of the insulin signaling pathway in these processes, Kido et al. (2000) generated mice with combined heterozygous null mutations in the insulin receptor, insulin receptor substrate-1, and/or insulin receptor substrate-2 (Irs2; 600797). Diabetes developed in 40% of animals heterozygous for all 3 null mutations, 20% of those heterozygous for the Insr/Irs1 null mutations, 17% of those heterozygous for the Insr/Irs2 mutations, and 5% of those heterozygous for the null mutation of Insr only. Although combined heterozygosity for Insr/Irs1 null mutations and Insr/Irs2 null mutations resulted in a similar number of diabetic mice, there were significant differences in the underlying metabolic abnormalities. Mice of the Insr/Ins1 double heterozygosity developed severe insulin resistance in skeletal muscle and liver, with compensatory beta-cell hyperplasia. In contrast, mice of the Insr/Ins2 double heterozygosity developed severe insulin resistance in liver, mild insulin resistance in skeletal muscle, and modest beta-cell hyperplasia. Triple heterozygotes developed severe insulin resistance in skeletal muscle and liver and marked beta-cell hyperplasia. These data indicated tissue-specific differences in the IRSs to mediate insulin action, with Irs1 playing a prominent role in skeletal muscle and Irs2 in liver. They also provided a practical demonstration of the polygenic and genetically heterogeneous interactions underlying the inheritance of type II diabetes.

Mice homozygous for lack of the IRS1 gene show severe osteopenia with low bone turnover. IRS1 is expressed in osteoblasts, but not in osteoclasts, of wildtype mice. Ogata et al. (2000) showed that osteoblasts from homozygous deficient mice treated with insulin or IGF1 (147440) failed to induce tyrosine phosphorylation of cellular proteins and showed reduced proliferation and differentiation. Osteoclastogenesis in the coculture of hemopoietic cells and osteoblasts depended on IRS1 expression in osteoblasts and could not be rescued by IRS1 expression in hemopoietic cells in the presence not only of IGF1 but also 1,25(OH)2D3. Ogata et al. (2000) concluded that IRS1 deficiency in osteoblasts impairs osteoblast proliferation, differentiation, and support of osteoclastogenesis, resulting in low-turnover osteopenia. Ogata et al. (2000) concluded further that osteoblastic IRS1 is essential for maintaining bone turnover, because it mediates signaling by IGF1 and insulin and, they proposed, also by other factors, such as 1,25(OH)2D3.

Using hyperinsulinemic-euglycemic clamps, Kim et al. (2004) demonstrated that skeletal muscle and hepatic insulin action did not differ between wildtype and Pkc-theta (600448) null mice. A 5-hour lipid infusion decreased insulin-stimulated skeletal muscle glucose uptake in the wildtype mice that was associated with 40 to 50% decreases in insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1 and IRS1-associated PI3K activity. In contrast, Pkc-theta inactivation prevented fat-induced defects in insulin signaling and glucose transport in skeletal muscle. Kim et al. (2004) concluded that PKC-theta is a crucial component mediating fat-induced insulin resistance in skeletal muscle.

History

The article by Taniguchi et al. (2005) reporting results of the knockdown of Irs1, Irs2, or both in mice was retracted 'at the request of the corresponding author' because of duplications found in some of the figures.