irs-1

IRS-1 (Insulin Receptor Substrate 1)

The principal intracellular scaffold coupling activated insulin receptor and IGF-1 receptor to the PI3K–AKT and MAPK signaling axes. IRS-1 is the locus of a critical negative-feedback loop in which S6K1-mediated serine phosphorylation degrades IRS-1 to attenuate insulin signaling — a mechanism that becomes chronically activated in aging and obesity, contributing to the deregulated nutrient sensing hallmark. Genetic deletion in mice extends female lifespan ~30% ¹, making IRS-1 one of the few mammalian proteins whose loss demonstrably prolongs life.

Identity

• UniProt: P35568 (IRS1_HUMAN)
• NCBI Gene: 3667
• HGNC symbol: IRS1
• Ensembl: ENSG00000169047
• Mouse ortholog: Irs1 (one-to-one ortholog; functionally conserved)
• Length: 1,242 amino acids (canonical isoform)
• GenAge: no confirmed entry ID — needs-canonical-id (IRS1 is aging-relevant via Irs1-/- lifespan extension but a GenAge models entry ID was not confirmed at seeding; verify at next lint pass)

IRS-family context: IRS-1 is one of four insulin receptor substrate proteins. IRS-2 is the dominant paralog in liver, pancreatic β-cells, and brain; IRS-3 is expressed in rodents only (no human ortholog); IRS-4 has limited tissue expression. All share the N-terminal PH + PTB module but diverge in their C-terminal docking-motif regions and tissue expression profiles ².

Domain organization

IRS-1 is a largely disordered scaffold protein organized around three functional regions:

1. PH domain (residues 12–115) — N-terminal pleckstrin-homology domain; binds membrane phosphoinositides (PI-3,4,5-P₃ and PI-4,5-P₂), anchoring IRS-1 near the activated receptor complex and positioning it for tyrosine phosphorylation. The PH domain also mediates constitutive interaction with the receptor complex independently of receptor activation.

2. PTB domain (residues 160–264) — phosphotyrosine-binding domain; binds the NPxY motif of activated, autophosphorylated insr and igf1r juxtamembrane regions. Receptor tyrosine kinase activation is required for stable IRS-1 engagement via this domain.

3. C-terminal docking region (residues ~265–1242) — largely unstructured; contains approximately nine YXXM motifs (consensus for SH2-domain recruitment, especially the p85 subunit of Class IA PI3K) at Tyr-465, -551, -612, -632, -662, -732, -941, -989, and -1012. Also contains YXXL and other docking motifs for GRB2 (SH2 binding → SOS1 → RAS/MAPK branch). The disordered architecture allows simultaneous binding of multiple effector complexes, making IRS-1 a signaling hub rather than a linear relay ³.

Activation mechanism

Insulin or IGF-1 binding to insr or igf1r triggers receptor autophosphorylation within the intracellular kinase activation loop. Activated receptor tyrosine kinase then trans-phosphorylates IRS-1 on multiple tyrosine residues, primarily Tyr-612, Tyr-632, Tyr-662, and Tyr-941 (UniProt-curated activation sites) ³.

Phosphorylated YxxM motifs recruit the SH2 domain of the p85 regulatory subunit of Class IA PI3K → p85/p110 PI3K complex is brought to the plasma membrane → PI3K phosphorylates PIP₂ → PIP₃ → AKT and PDK1 co-recruitment → AKT activation. This is the canonical insulin/IGF-1 → pi3k-akt-pathway coupling step and the primary route to downstream FOXO inhibition, GSK3β inhibition, and mTORC1 activation.

A parallel branch via GRB2/SOS1 (GRB2 SH2 domain binds IRS-1 phosphotyrosines) links IRS-1 to the RAS/RAF/MEK/ERK (MAPK) cascade, mediating some mitogenic but not metabolic effects of insulin.

IRS-1 vs SHC path: insr and igf1r also phosphorylate the adaptor SHC1, which provides an IRS-1-independent route to GRB2/MAPK; the relative contributions of IRS-1 and SHC1 to MAPK activation are cell-type dependent. unsourced (quantitative partitioning of IRS-1 vs SHC1 contributions to MAPK needs a primary citation).

Negative-feedback regulation (inhibitory serine phosphorylation)

The defining aging-relevant feature of IRS-1 is its susceptibility to inhibitory serine/threonine phosphorylation, which attenuates insulin signaling:

Site (human)	Site (mouse)	Kinase(s)	Effect
Ser-312	Ser-307	S6K1, JNK, IKKβ	Disrupts PTB-domain binding to INSR; reduces IRS-1 stability
Ser-636/Ser-639	Ser-632/Ser-635	S6K1, mTORC1	Promotes IRS-1 ubiquitylation and proteasomal degradation
Ser-270	—	S6K1	Promotes degradation (UniProt curated site)

The mTORC1/S6K1 negative-feedback loop operates as follows ⁴:

1. Insulin/IGF-1 → IRS-1 tyrosine phosphorylation → PI3K–AKT → mTORC1 activation → S6K1 Thr389 phosphorylation (activation)
2. Activated S6K1 phosphorylates IRS-1 on inhibitory serine residues (Ser-307/Ser-312; Ser-632/Ser-635 mouse / Ser-636/Ser-639 human)
3. Phospho-Ser IRS-1 recruits SOCS proteins and 14-3-3 adapters → IRS-1 ubiquitylation → proteasomal degradation
4. Loss of IRS-1 reduces PI3K activity → attenuated AKT/mTOR signaling

This feedback explains why chronic mTORC1 hyperactivation (as in obesity, overnutrition, and aging) causes apparent insulin resistance: sustained S6K1 activity continuously degrades IRS-1, rendering cells refractory to further insulin stimulation ⁴⁵.

Corrigendum note (Um 2004, Nature 431:485): The original Um 2004 paper figure labels read “S636/S639” for the mouse sites; a corrigendum published in the same volume (Nature 431:485, 23 September 2004) corrects these to S632/S635 for mouse and confirms S312/S636/S639 for human. The wiki uses the corrected corrigendum numbering throughout.

Clarification on Ser-1101: Some reviews cite Ser-1101 as an S6K1-phosphorylated IRS-1 site (e.g., Boucher 2014 Figure 3 lists it in a schematic of inhibitory sites without kinase attribution). This site is not confirmed as an S6K1 substrate in the Um 2004 primary data. Ser-1101 references should be treated with caution pending primary-source verification. contradictory-evidence

JNK and IKKβ also phosphorylate IRS-1 at Ser-307 (mouse) / Ser-312 (human) under inflammatory conditions (cytokines, saturated fatty acids), connecting the pro-inflammatory chronic-inflammation state of aging to insulin resistance through the same IRS-1 phosphoserine mechanism ⁵. This is a major mechanistic link between inflammaging and metabolic aging.

Dimension	Status	Notes
Pathway conserved in humans?	yes	IRS-1 serine phosphorylation and insulin resistance connection documented in human adipocytes and skeletal muscle biopsies
Phenotype conserved in humans?	yes	Elevated phospho-Ser-IRS-1 in human type 2 diabetes; hyperactivated mTOR/S6K1 in obese humans
Replicated in humans?	in-progress	Specific site identity debated (Rajan et al. 2013 disputed Ser-307 as S6K1 site in human primary adipocytes) contradictory-evidence

IRS-1 vs IRS-2: tissue specialization

Despite sharing the PH–PTB–docking module, IRS-1 and IRS-2 have distinct and largely non-redundant roles ²:

Tissue	Dominant IRS paralog	Notes
Skeletal muscle	IRS-1	Irs1-/- mice have profound skeletal muscle insulin resistance
Adipose tissue	IRS-1	IRS-1-driven glucose uptake via GLUT4 translocation
Liver	IRS-2	Irs2-/- mice develop hepatic insulin resistance and type 2 diabetes
Pancreatic β-cells	IRS-2	Irs2-/- mice have β-cell failure; IRS-1 plays a lesser β-cell role
Brain/hypothalamus	IRS-2 (dominant)	Irs2-/- brain: premature aging phenotype; IRS-1 also expressed

Disruption of Irs2 caused type 2 diabetes in mice via combined liver insulin resistance and β-cell failure ², demonstrating that the two paralogs are not functionally interchangeable at the organismal level.

Knockout phenotypes

Irs1-/- mice (Tamemoto 1994): Viable; born at expected Mendelian ratios; exhibit significant growth retardation (~50% of normal body weight) and insulin resistance ⁶. Unlike Irs2-/- mice, they do not develop overt diabetes under normal diet — compensatory β-cell hyperplasia appears to maintain euglycemia. The growth deficit establishes IRS-1 as a primary mediator of the growth-promoting effects of both insulin and IGF-1 in vivo.

Irs1-/- lifespan (Selman 2008): Female Irs1-null mice showed statistically significant lifespan extension compared to wild-type controls in a study characterizing the IIS pathway in mammalian longevity ¹. The FASEB Journal paper (DOI: 10.1096/fj.07-9261com) is closed-access; exact median lifespan values and the reported percentage extension cannot be independently confirmed from the abstract alone. The paper is listed in the archive as not_oa / download_status: not_oa. no-fulltext-access

Note on sex-specificity: as with the S6K1 KO [see s6k1], the lifespan effect in Irs1-/- mice may be female-specific. needs-replication — the sex breakdown requires full-text verification.

Note on paper identity: the user-supplied brief cited “Selman 2008 Science” and “Selman 2009” at different points. The verified S6K1 paper is Selman 2009 Science (doi:10.1126/science.1177221). The IRS-1 lifespan paper is Selman 2008 FASEB J (doi:10.1096/fj.07-9261com; archive publication_year: 2007, print year 2008). These are distinct papers on distinct genetic models (S6K1-/- vs Irs1-/-). Do not conflate them.

Role in aging

IIS pathway and the conserved longevity program

IRS-1 sits within the evolutionarily ancient insulin/IGF-1 signaling (IIS) pathway that governs lifespan across organisms from C. elegans (DAF-2/AGE-1/DAF-16) to Drosophila to mammals. Reduced IIS consistently extends lifespan in invertebrates and rodents, with IRS-1 constituting the primary mammalian coupling point between receptor activation and downstream PI3K–AKT–FOXO signaling ⁵.

Reduced IRS-1 function → diminished AKT signaling → reduced FOXO phosphorylation → FOXO transcription factors (especially foxo3) translocate to the nucleus → activate stress resistance, autophagy, and longevity gene programs. The same IRS-1 → FOXO axis is the mechanistic link from receptor-level signals through to transcriptional longevity outputs.

Age-related IRS-1 dysfunction

With aging, basal S6K1 activity is elevated in multiple tissues (see s6k1), resulting in chronic inhibitory serine phosphorylation of IRS-1. This progressively depletes IRS-1 protein and attenuates insulin signaling — manifesting clinically as the characteristic age-associated decline in insulin sensitivity, independent of changes in adiposity ⁵.

In skeletal muscle, IRS-1 degradation under chronic mTOR/S6K1 hyperactivation contributes to anabolic resistance: the blunted translational response to protein feeding in aged muscle. IRS-1 depletion reduces PI3K/AKT-mediated mTORC1 activation by amino acids and insulin, limiting muscle protein synthesis. This is a mechanistic contributor to sarcopenia. contradictory-evidence (whether IRS-1 loss is cause or consequence of sarcopenia remains unresolved)

Caloric restriction and IRS-1

Caloric restriction and rapamycin reduce S6K1 activity, which in turn reduces inhibitory serine phosphorylation of IRS-1, preserving IRS-1 protein levels and improving insulin sensitivity in aged animals. This is one mechanistic explanation for why CR and rapamycin phenocopy aspects of IIS pathway reduction without requiring genetic IRS-1 deletion. unsourced (direct measurements of CR → IRS-1 protein levels in aged mouse tissues need a primary citation; Boucher 2014 reviews the mTOR/S6K1–IRS-1 feedback mechanism but does not cite CR-specific IRS-1 preservation data)

Pathway membership

• insulin-igf1 — primary signaling adaptor; couples receptor activation to PI3K branch
• pi3k-akt-pathway — IRS-1 phosphotyrosines recruit p85/PI3K, the entry point to the pathway
• mtor — positive input to mTORC1 (via AKT → TSC1/2 → RHEB → mTORC1); negative feedback target of mTORC1/S6K1 (serine phosphorylation)
• mapk-pathway — GRB2-mediated RAS/ERK branch (mitogenic effects; not primary metabolic axis)

Key interactors

• insr — activating upstream kinase (juxtamembrane Tyr phosphorylation triggers PTB docking)
• igf1r — activating upstream kinase (shared signaling mechanism with INSR)
• pi3k — p85 SH2 domain binds IRS-1 phospho-YxxM motifs; canonical downstream effector
• s6k1 — negative-feedback kinase; phosphorylates inhibitory serines; the mTOR/S6K1–IRS-1 axis is the canonical insulin resistance mechanism
• akt — activated downstream of PI3K; does not directly phosphorylate IRS-1 but its targets (TSC2, PRAS40) determine mTORC1 activity and hence S6K1-mediated IRS-1 feedback
• grb2 — adaptor recruited to IRS-1 phosphotyrosines → RAS pathway

Limitations and gaps

• Exact lifespan extension magnitude in Irs1-/- females is unconfirmed from publicly available text; Selman 2008 FASEB J is closed-access. no-fulltext-access — verify against primary PDF.
• Sex-specificity of the Irs1-/- longevity phenotype: whether males are excluded or show a smaller effect is not confirmed from the abstract. needs-replication
• Ser-1101 attribution: Cited in some reviews as an S6K1-phosphorylated inhibitory site, but not confirmed in primary IRS-1 biochemistry papers reviewed here. contradictory-evidence
• Human longevity GWAS: No strong common variant association at the IRS1 locus for human lifespan has been confirmed. The Arg-971 polymorphism (UniProt-noted) associates with insulin resistance and cardiovascular risk, but not longevity per se. needs-human-replication
• Tissue-specific roles: IRS-1 contributions to muscle anabolic resistance, hepatic insulin action, and β-cell compensation in aging are mechanistically distinct and not uniformly quantified. needs-replication
• IRS-1 vs IRS-2 in brain aging: Both are expressed in hypothalamic neurons relevant to energy balance and aging, but their individual contributions to age-related CNS IIS attenuation are poorly characterized. no-mechanism

Footnotes

Footnotes

1. doi:10.1096/fj.07-9261com · in-vivo (Irs1-/- mouse genetic KO) · model: Irs1-null Mus musculus · archive status: not_oa (#gap/no-fulltext-access) · publication_year: 2007/2008 (FASEB J) · key finding: Irs1-null females show statistically significant lifespan extension; delayed age-related biomarkers · quantitative details unverifiable from abstract alone ↩ ↩²

2. doi:10.1038/36116 · in-vivo (Irs2-/- mouse genetic KO) · model: Irs2-null Mus musculus · archive status: not_oa (1,770 citations) · key finding: Irs2 disruption → type 2 diabetes (liver insulin resistance + β-cell failure); establishes IRS-1/IRS-2 tissue specialization ↩ ↩² ↩³

3. doi:10.1038/352073a0 · in-vitro (biochemical cloning + domain mapping) · n=not applicable · model: rat IRS-1 (Sun et al. 1991, Nature 352:73) · archive status: not_oa (1,572 citations, 100th percentile FWCI) ↩ ↩²

4. um-2004-s6k1-irs1-feedback · in-vivo (S6K1-null mouse + cell lines) · model: S6K1-/- Mus musculus (C57BL/6J background; male mice) · doi:10.1038/nature02866 · archive status: downloaded (local PDF verified) · key finding: S6K1 KO protected against age/diet-induced obesity + enhanced insulin sensitivity via IRS-1 serine phosphorylation reduction; IRS-1 sites blunted are S307 + S632/S635 (mouse) / S312 + S636/S639 (human) — per corrigendum Nature 431:485 (2004) correcting original figure labels S636/S639 → S632/S635 for mouse; see also s6k1 page for verified details ↩ ↩²

5. doi:10.1101/cshperspect.a009191 · review · model: review of mammalian insulin/IGF-1 receptor signaling in normal and insulin-resistant states · archive status: downloaded (local PDF verified) · Boucher J, Kleinridders A, Kahn CR · Cold Spring Harb Perspect Biol 2014;6:a009191 · scope: covers IR/IGF-1R signaling, PI3K-AKT axis, GRB2-RAS-MAPK branch, IRS paralog tissue roles, and mechanisms of insulin resistance including serine phosphorylation; does NOT cover IRS-1 KO lifespan extension or aging longevity phenotypes (no lifespan data cited) ↩ ↩² ↩³ ↩⁴

6. doi:10.1038/372182a0 · in-vivo (Irs1-/- mouse genetic KO) · model: Irs1-null Mus musculus · archive status: not_oa (1,084 citations) · key phenotypes: viable, ~50% growth retardation, insulin resistance without diabetes ↩