Insulin receptor substrate 1(IRS-1) is a signaling adapter
protein that in humans is encoded by the IRS1gene.[5] It is a 131 kDa protein with amino acid sequence of 1242 residues.[6] It contains a single
pleckstrin homology (PH) domain at the N-terminus and a
PTB domain ca. 40 residues downstream of this, followed by a poorly conserved C-terminus tail.[7] Together with
IRS2, IRS3 (pseudogene) and
IRS4, it is homologous to the Drosophila protein chico, whose disruption extends the median lifespan of flies up to 48%.[8] Similarly, Irs1 mutant
mice experience moderate life extension and delayed age-related pathologies.[9]
Function
Insulin receptor substrate 1 plays a key role in transmitting signals from the
insulin receptor (IR) and
insulin-like growth factor 1 receptor (IGF-1) to intracellular pathways
PI3K /
Akt and
ErkMAP kinase pathways. Tyrosine phosphorylation of IRS-1 by insulin receptor (IR) introduces multiple binding sites for proteins bearing SH2 homology domain, such as PI3K, Grb-2/Sos complex and
SHP2. PI3K, involved in interaction with IRS-1, produces
PIP3, which, in turn, recruits Akt kinase. Further, Akt kinase is activated via phosphorylation of its T308 residue and analogous sites in
PKC by
PDK1. This phosphorylation is absent in tissues lacking IRS-1. The cascade is followed by glucose uptake. Formation of the Grb-2/Sos complex, also known as the RAS guanine nucleotide exchange factor complex, results in ERK1/2 activation. IRS-1 signal transduction may be inhibited by SHP2 in some tissues.[7]
Tyrosinephosphorylation of the insulin receptors or IGF-1 receptors, upon extracellular
ligand binding, induces the cytoplasmic binding of IRS-1 to these receptors, through its
PTB domains. Multiple tyrosine residues of IRS-1 itself are then phosphorylated by these receptors. This enables IRS-1 to activate several signalling pathways, including the
PI3K pathway and the
MAP kinase pathway.
An alternative multi-site phosphorylation of Serine/Threonine in IRS-1 regulates insulin signaling positively and negatively. C-terminal region contains most of the phosphorylation sites of the protein. The C-terminal tail is not structured, therefore the mechanisms of regulation of IRS-1 by phosphorylation still remain unclear. It has been shown that
TNFα causes insulin resistance and multi-site S/T phosphorylation, which results in block of interaction between IRS-1 and juxtamembrane domain peptide, thus converting IRS-1 into an inactive state.[7]
IRS-1 plays important biological function for both
metabolic and
mitogenic (growth promoting) pathways: mice deficient of IRS1 have only a mild diabetic
phenotype, but a pronounced growth impairment, i.e., IRS-1
knockout mice only reach 50% of the weight of normal mice.
Regulation
The cellular protein levels of IRS-1 are regulated by the
Cullin-7 E3
ubiquitin ligase, which targets IRS-1 for
ubiquitin mediated degradation by the
proteasome.[10] Different Serine phosphorylation of IRS-1, caused by various molecules, such as
fatty acids, TNFα and
AMPK, has different effects on the protein, but most of these effects include cellular re-localization, conformational and steric changes. These processes lead to decrease in Tyrosine phosphorylation by insulin receptors and diminished PI3K recruitment. Altogether, these mechanisms stimulate IRS-1 degradation and insulin resistance. Other inhibitory pathways include
SOCS proteins and
O-GlcNAcylation of IRS-1. SOCS proteins act by binding to IR and by interfering with IR phosphorylation of IRS-1, therefore attenuating insulin signaling. They can also bind to
JAK, causing a subsequent decrease in IRS-1 tyrosine phosphorylation. During insulin resistance induced by
hyperglycemia, glucose accumulates in tissues as its
hexosamine metabolite
UDP-GlcNAc. This metabolite if present in high amounts leads to O-GlcNAc protein modifications. IRS-1 can undergo this modification, which results in its phosphorylation and functional suppression.[11]
Interactions
IRS1 has been shown to
interact (also concerted activity[12]) with:
IRS-1, as a signalling adapter protein, is able to integrate different signalling cascades, which indicates its possible role in cancer progression.[36] IRS-1 protein is known to be involved in various types of cancer, including
colorectal,[37]lung,[38]prostate and
breast cancer.[39] IRS-1 integrates signalling from
insulin receptor (InsR),
insulin-like growth factor-1 receptor (IGF1R) and many other cytokine receptors and is elevated in
β-catenin induced cells. Some evidence shows that
TCF/LEF-β-catenin complexes directly regulate IRS-1. IRS-1 is required for maintenance of neoplasmic phenotype in
adenomatous polyposis coli (APC) - mutated cells, it is also needed for transformation in ectopically expressing oncogenic β-catenin cells. IRS-1 dominant-negative mutant functions as
tumor suppressor, whereas ectopic IRS-1 stimulates oncogenic transformation. IRS-1 is upregulated in colorectal cancers (CRC) with elevated levels of β-catenin,
c-MYC, InsRβ and IGF1R. IRS-1 promotes CRC metastasis to the liver.[37] Decreased apoptosis of crypt stem cells is associated with colon cancer risk. Reduced expression of IRS-1 in
Apc (min/+) mutated mice shows increased irradiation-induced apoptosis in crypt. Deficiency in IRS-1 - partial (+/-) or absolute (-/-) - in Apc (min/+) mice demonstrates reduced amount of tumors comparing to IRS-1 (+/+)/ Apc (min/+) mice.[40]
In lung
adenocarcinoma cell line
A549 overexpression of IRS-1 leads to reduced growth. Tumor infiltrating
neutrophils have recently been thought to adjust tumor growth and invasiveness.
Neutrophil elastase is shown to degrade IRS-1 by gaining access to endosomal compartment of carcinoma cell. IRS-1 degradation induces cell proliferation in mouse and human adenocarcinomas. Ablation of IRS-1 alters downstream signalling through
phosphatidylinositol-3 kinase (PI3K), causing an increased interaction of it with
platelet-derived growth factor receptor (PDGFR). Therefore, IRS-1 acts as major regulator of PI3K in lung adenocarcinoma.[38]
Some evidence shows role of IRS-1 in
hepatocellular carcinoma (HCC). In rat model, IRS-1 focal overexpression is associated with early events of hepatocarcinogenesis. During progression of preneoplastic foci into hepatocellular carcinomas expression of IRS-1 gradually decreases, which is characterises a metabolic shift heading towards malignant neoplastic phenotype.[41] Transgenic mice, co-expressing IRS-1 and
hepatitis Bx (
HBx) protein, demonstrate higher rate of hepatocellular
displasia that results in HCC development. Expressed alone, IRS-1 and HBx are not sufficient to induce neoplastic alterations in the liver, though their paired expression switches on IN/IRS-1/
MAPK and
Wnt/β-catenin cascades, causing HCC transformation.[42]
LNCaP prostate cancer cells increase cell adhesion and diminish cell motility via
IGF-1 independent mechanism, when IRS-1 is ectopically expressed in the cells. These effects are mediated by PI3K. Uncanonical phosphorylation of Serine 612 by PI3K of IRS-1 protein is due to hyper-activation of
Akt/PKB pathway in LNCaP. IRS-1 interacts with
integrin α5β1, activating an alternative signalling cascade. This cascade results in decreased cell motility opposing to IGF-1 - dependent mechanism. Loss of IRS-1 expression and
PTEN mutations in LNCaP cells could promote metastasis.[43]Ex vivo studies of IRS-1 involvement in prostate cancer show ambiguous results. Down-regulation of IGF1R in bone marrow
biopsies of metastatic prostate cancer goes along with down-regulation of IRS-1 and significant reduction of PTEN in 3 out of 12 cases. Most of the tumors still express IRS-1 and IGF1R during progression of the metastatic disease.[44]
IRS-1 has a functional role in breast cancer progression and metastasis. Overexpression of PTEN in
MCF-7 epithelial breast cancer cells inhibits cell growth by inhibiting MAPK pathway.
ERK phosphorylation through IRS-1/
Grb-2/
Sos pathway is inhibited by phosphatase activity of PTEN. PTEN does not have effect on IRS-1 independent MAPK activation. When treated with
insulin, ectopic expression of PTEN in MCF-7 suppresses IRS-1/Grb-2/Sos complex formation due to differential phosphorylation of IRS-1.[45] Overexpression of IRS-1 has been linked to
antiestrogen resistance and hormone independence in breast cancer.
Tamoxifen (TAM) inhibits IRS-1 function, therefore suppressing IRS-1/PI3K signalling cascade in
estrogen receptor positive (ER+) MCF-7 cell line. IRS-1
siRNA is able to reduce IRS-1 transcript level, thereby reducing protein expression in MCF-7 ER+ cells. Reduction of IRS-1 leads to decreased survival of these cells. siRNA treatment effects are additive to effects of TAM treatment.[46] IGFRs and estrogen coaction facilitates growth in different breast cancer cell lines, however amplification of IGF1R signalling can abrogate need of estrogen for transformation and growth of MCF-7 cells. IRS-1 overexpression in breast cancer cells decreased estrogen requirements. This decrease is dependent on IRS-1 levels in the cells.[47]Estradiol enhances expression of IRS-1 and activity of ERK1/2 and PI3K/Akt pathways in MCF-7 and
CHO cells transfected with mouse IRS-1
promoter. Estradiol acts directly on IRS-1
regulatory sequences and positively regulates IRS-1 mRNA production.[48] Decreased anchorage- dependent/independent cell growth and initiation of cell death under low growth factor and estrogen conditions are observed in MCF-7 cells with down-regulated IRS-1.[49]mir126 is underexpressed in breast cancer cells. mir126 targets IRS-1 at transcriptional level and inhibits transition from G1/G0 phase to S phase during cell cycle in
HEK293 and MCF-7 cells.[50]Transgenic mice overexpressing IRS-1 develop metastatic breast cancer. The tumors demonstrate squamous differentiation which is associated with β-catenin pathway. IRS-1 interacts with β-catenin both in vitro and in vivo.[51] IRS-1 and its homologue
IRS-2 play distinct roles in breast cancer progression and metastasis. Overexpression of either one is sufficient to cause tumorogenesis in vivo. Frequency of lung metastasis in IRS-1 deficient tumor is elevated opposing to IRS-2 deficient tumor, where it is decreased. Basically, IRS-2 has a positive impact on metastasis of breast cancer whereas a stronger metastatic potential is observed when IRS-1 is down-regulated.[citation needed] IRS-1 is strongly expressed in
ductal carcinoma in situ, when IRS-2 is elevated in invasive tumors. Increased IRS-1 makes MCF-7 cells susceptible to specific chemotherapeutic agents, such as
taxol,
etoposide, and
vincristine. Therefore, IRS-1 can be a good pointer of specific drug therapies effectiveness for breast cancer treatment.[52]
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