Insulin / IGF-1 signaling pathway (IIS)

The insulin / IGF-1 signaling (IIS) pathway is the canonical longevity pathway in aging biology. Its discovery as a longevity regulator — Kenyon et al. 1993 showing that C. elegans daf-2 mutants live approximately twice as long as wild-type ¹ — launched modern molecular gerontology. IIS is the most phylogenetically ancient nutrient/growth-factor signaling axis in metazoans, and its reduction is associated with lifespan extension in every major model organism tested.

The pathway integrates insulin and IGF-1 receptor signals through a shared PI3K–AKT intracellular axis. Its two primary aging-relevant outputs are: (1) activation of mTORC1 (anabolic, pro-aging when chronically elevated), and (2) inhibition of FOXO transcription factors (when disinhibited, FOXO drives stress resistance, proteostasis, and longevity programs).

Naming note: This page covers the IIS pathway. Cross-links: mtor (direct downstream effector, has its own pathway page); ampk (antagonist, inhibits mTORC1 and activates FOXO in parallel).

Receptor tier: INSR, IGF1R, and IRS adapters

Two receptor tyrosine kinases initiate IIS signaling in mammals:

Receptor	Gene	Primary ligand	Key roles in aging
Insulin receptor	INSR (IR)	Insulin	Metabolic regulation; glucose uptake in muscle/fat; liver anabolism
IGF-1 receptor	IGF1R	IGF-1 (also insulin at high concentrations)	Growth, cell survival; reduced signaling in long-lived mice and centenarians

Both receptors auto-phosphorylate upon ligand binding and recruit the insulin receptor substrate (IRS) adapter family (IRS1–4 in mammals; IRS1 dominant in muscle/fat; IRS2 dominant in liver and brain). The IRS proteins serve as docking scaffolds, linking receptor phosphotyrosines to downstream SH2-containing proteins — most critically, the p85/p110 regulatory/catalytic subunits of PI3K.

The IRS proteins also receive negative feedback from mTORC1-activated S6K1, which phosphorylates IRS-1 on serine residues, inhibiting it — a negative-feedback loop that attenuates insulin sensitivity under chronically anabolic conditions.

Core intracellular cascade

Insulin / IGF-1
       ↓
INSR / IGF1R (receptor tyrosine kinase activation)
       ↓
IRS1/2 (adapter scaffold, phosphotyrosine docking)
       ↓
PI3K (generates PIP3 at plasma membrane)
       ↓
AKT (PDK1 + mTORC2 fully activate AKT)
      ↙          ↘
TSC1/TSC2        FOXO1/3/4
(AKT phosphorylates &    (AKT phosphorylates →
inhibits → releases      cytoplasmic retention
RHEB → mTORC1 ↑)        → transcription program ↓)

AKT (protein kinase B; three isoforms AKT1/2/3) is the central effector kinase. It requires dual phosphorylation — PDK1 on Thr308 and mTORC2 on Ser473 — for full activation. AKT then bifurcates to the two outputs with opposing aging significance:

TSC1/TSC2 inhibition → mTORC1 activation — AKT phosphorylates TSC2 (Ser939, Thr1462), inactivating the TSC1/TSC2 GAP complex; this releases RHEB-GTP at the lysosome to activate mTORC1. High IIS = high mTORC1 = suppressed autophagy, elevated protein synthesis. This branch is pro-aging when constitutively active. (See mtor for full mTORC1 biology.)
FOXO inhibition → reduced longevity transcription — AKT phosphorylates FOXO1/3/4 on three conserved serine/threonine residues, promoting 14-3-3 binding and cytoplasmic sequestration. When IIS is reduced, FOXO factors translocate to the nucleus and drive expression of stress-response genes (superoxide dismutases, catalase, heat-shock proteins, autophagy genes, cell-cycle arrest factors). FOXO nuclear activity is the molecular correlate of most IIS longevity phenotypes.

PTEN (phosphatase and tensin homolog) antagonizes PI3K by dephosphorylating PIP3 → PIP2, attenuating AKT activation. PTEN is a major tumor suppressor and a key determinant of basal pathway tone.

FOXO transcription factors in aging

The FOXO family (FOXO1, FOXO3, FOXO4, FOXO6 in humans; DAF-16 in C. elegans; dFOXO in Drosophila) is the primary downstream effector of IIS longevity effects:

FOXO3 is the most-studied longevity-associated FOXO in humans. Multiple SNPs in the FOXO3 locus are associated with exceptional longevity across independent cohorts (Hawaiian Japanese ², European, Ashkenazi Jewish, Chinese, Italian centenarians).
FOXO target genes include: SOD2 (mitochondrial superoxide dismutase), GADD45 (DNA damage response), BNIP3 (mitophagy), BECN1 (autophagy initiation), CCND1 (cyclin D1, growth suppression), PCSK9 (cholesterol metabolism), and many proteostasis genes.
FOXO nuclear activity is post-translationally regulated not just by IIS/AKT, but also by ampk (AMPK directly phosphorylates and activates FOXO), JNK (stress → FOXO nuclear import), and class III HDAC SIRTs (SIRT1/2 deacetylate and activate FOXO).

Cross-pathway connections

Connection	Direction	Mechanism
IIS → mtor	Downstream (activating)	AKT phosphorylates TSC2 → RHEB-GTP → mTORC1 activation
ampk → IIS outputs	Antagonistic/parallel	AMPK phosphorylates TSC2 (activating, not inhibiting) → mTORC1 ↓; AMPK also directly activates FOXO
mTORC1 → IIS	Negative feedback	S6K1 (mTORC1 substrate) phosphorylates IRS-1 Ser → IRS-1 degradation → insulin resistance
IIS → ampk	Indirect suppression	High IIS/mTOR = replete energy state → low AMP:ATP → AMPK less active

This feedback topology means that chronically high IIS drives mTORC1 hyperactivation (suppressing autophagy) while simultaneously creating IRS-1 serine phosphorylation — contributing to the insulin resistance of aging.

Role in aging: the longevity story across organisms

Reduced IIS extends lifespan in every major model organism tested. The magnitude varies dramatically (see “Magnitude attenuation” below).

C. elegans — the founding result

Kenyon et al. 1993: daf-2 (insulin receptor ortholog) mutant worms live approximately 2× as long as wild-type Bristol N2 ¹. This was the first demonstration that a single-gene mutation could double animal lifespan — transforming aging from an intractable biological mystery into a tractable molecular problem.

The mechanism: reduced DAF-2 → reduced AGE-1 (PI3K ortholog) → PIP3 ↓ → DAF-16 (FOXO ortholog) nuclear translocation → longevity gene expression. DAF-18 (PTEN ortholog) also regulates pathway tone. The daf-16 deletion epistasis result (removing DAF-16 blocks daf-2 longevity) established that FOXO is the essential downstream mediator ¹.

Worm genotype	Lifespan relative to WT	Notes
`daf-2` (lf)	~2×	Seminal Kenyon 1993 result ¹
`daf-16` (lf)	slightly shorter	FOXO null; epistatic to `daf-2` ¹
`age-1` (lf)	~1.5×	PI3K ortholog [^gap/needs-replication]
`daf-2; daf-16`	WT	Confirms FOXO requirement

Drosophila melanogaster — IRS and FOXO

chico (IRS ortholog) null mutants are long-lived (~40% extension in homozygous females, ~11% in heterozy-gous) [^gap/needs-replication — chico DOI not verified in archive]. needs-replication
InR (insulin receptor ortholog) hypomorphs or overexpression of dFOXO in fat body extend lifespan.
dFOXO is both necessary and sufficient for much of the lifespan extension from reduced IIS in flies.

mus-musculus — GH/IGF1 axis and dwarf mice

In mice, IIS reduction has been achieved through several genetic models — all producing lifespan extension:

Model	Mechanism	Lifespan extension	Notes
Ames dwarf (Prop1^df/df)	GH/PRL/TSH-deficient pituitary	~50–65% [^gap/needs-replication]	GH → IGF-1 axis abolished; multiple hormonal changes
Snell dwarf (Pit1^dw/dw)	GH/PRL/TSH-deficient pituitary	~40% [^gap/needs-replication]	Similar to Ames
GHR knockout (GHR^-/-, Laron dwarfs)	No GH receptor → IGF-1 ↓	~25–55% in various studies	Cleanest evidence for GH/IGF1 axis
IGF1R+/- heterozygous (Igf1r^+/-)	Haploinsufficiency of IGF-1 receptor	26% overall (P<0.02); 33% females (P<0.001); 15.9% males (NS) ³	Sex-specific; 129/Sv background; cleaner than total GH loss
Brain-specific IRS-2 knockout	IRS-2 deletion in neurons	~14% (females) ⁴	Tissue-restricted; highlights brain IIS relevance; figure unverifiable against PDF — paper closed-access no-fulltext-access
Fat-specific INSR knockout (FIRKO)	Insulin receptor deleted in adipose	~18% [^gap/needs-replication]	Fat-restricted; metabolic phenotype distinct from systemic KO

Key caveat for GHR/Ames/Snell models: these are pituitary dwarfs with pleiotropic hormonal changes (reduced GH, IGF-1, thyroid hormones, prolactin) and altered body composition, thermogenesis, and cancer rates. They are not pure IIS experiments. The IGF1R heterozygous and tissue-specific models are cleaner.

homo-sapiens — centenarian evidence (correlational)

FOXO3 SNPs (rs2802292, rs13217795, others): multiple population-based studies show FOXO3 genotype associated with exceptional longevity. Willcox et al. 2008: nested case-control (n=615: 213 cases who survived to ≥95y, 402 controls who died ~77y) drawn from the Honolulu Heart Program/HAAS cohort of Hawaiian Japanese men; FOXO3A rs2802292 homozygous minor allele (GG) vs major (TT) OR=2.75 (P=0.0007) for longevity to 95+ ². Replicated across European, Japanese, Italian, Chinese centenarian cohorts. needs-replication (independent replication studies exist but not all cited here)
IGF1R polymorphisms: Suh et al. 2008 (Ashkenazi Jewish centenarian cohort, n=79 female centenarians screened vs n=161 controls) identified two heterozygous nonsynonymous missense variants in IGF1R — Ala-37-Thr and Arg-407-His — enriched in centenarians vs controls (P=0.04); carriers showed reduced IGFIR levels and reduced IGF-I-stimulated AKT phosphorylation in immortalized lymphocytes, indicating partial loss of IGF-1 signaling ⁵. needs-replication
Circulating IGF-1 levels and longevity in humans: relationship is complex and possibly U-shaped; very low IGF-1 in old age may reflect frailty rather than protected longevity. Requires careful separation of cause/effect.

Magnitude attenuation: the central translation problem

The IIS pathway is conserved; the longevity magnitude is not. This is one of the most important epistemic caveats in aging biology:

Organism	Intervention	Lifespan extension
C. elegans	`daf-2` null (worm ~80–90% functional loss)	~100% (2×)
Drosophila	`chico` null (IRS ortholog)	~40% (females)
Mus musculus	IGF1R+/- (50% receptor haploinsufficiency)	26% overall; 33% females, 15.9% males (NS) ³
Mus musculus	GHR-/- (indirect IGF-1 axis)	~25–55%
homo-sapiens	FOXO3 longevity SNPs	Increased odds ratio for 95+ survival; Willcox 2008: OR=2.75 (GG vs TT, rs2802292) in Hawaiian Japanese men ²; OR for heterozygous (TG vs TT) = 1.91 (P=0.0003)
homo-sapiens	IGF1R variants in centenarians	Correlational; no lifespan data

Why does magnitude attenuate? Several non-exclusive hypotheses:

Genetic buffering: redundant pathways compensate for IIS reduction in long-lived mammals with slower metabolic rates.
Pleiotropy of complete knockouts: worm daf-2 null is highly pleiotropic (smaller, sterile under some conditions, stress-resistant); the “longevity” signal includes many indirect effects not present in partial IIS reduction.
Body size scaling: larger organisms have proportionally lower metabolic rates; the marginal gain from further IIS reduction may be smaller.
FOXO dose-response: partial IIS reduction in humans may insufficiently activate FOXO to drive longevity programs.

See _extrapolation-guide for the full rubric and see the #gap/needs-human-replication convention for model-organism claims.

Extrapolation assessment for IIS lifespan extension:

Dimension	Status
Pathway conserved in humans?	yes — IIS components are human-orthologous (INSR, IGF1R, IRS1/2, AKT, FOXO3)
Phenotype conserved in humans?	partial — FOXO3 SNPs associate with longevity; direct lifespan extension undemonstrated
Replicated in humans?	no (lifespan) / in-progress (surrogate endpoints)

Therapeutic implications

No approved pharmacological IIS inhibitor is used for longevity in humans. Several indirect angles are under investigation:

Growth hormone suppression / GH axis blockade: studied in Laron syndrome patients (GHR mutations) as a natural human model; these individuals appear resistant to certain age-related cancers and diabetes despite short stature.
IGF1R antagonism: monoclonal antibodies (e.g., ganitumab, teprotumumab — oncology indications) reduce IGF-1 signaling. Not tested for longevity.
Dietary protein restriction / low-protein diet: reduces IGF-1 circulating levels in humans (unlike caloric restriction alone in primates). Mechanism parallels reduced IIS. Evidence for longevity in humans is observational.
Indirect via mTOR: rapamycin and rapalogs suppress the shared mTORC1 effector arm; see mtor for detail.
Klotho (sKlotho): the secreted Klotho ectodomain inhibits IGF-1R and INSR at the cell surface, blunting IIS without a receptor-level genetic change. Klotho transgenic OE mice live ~20-30% longer; KL-VS heterozygote humans show longevity association. Klotho declines with age and CKD — see klotho for full evidence profile.

needs-human-replication — No controlled human trial has demonstrated IIS-pathway reduction extends human lifespan or health-span by a reproducible magnitude. All human evidence is GWAS/cohort/centenarian-study correlational.

Canonical database IDs

Database	Entry	Notes
KEGG	`hsa04910`	Insulin signaling pathway (human)
KEGG	`hsa04150`	mTOR signaling (contains IIS → mTOR branch)
Reactome	`R-HSA-74752`	Signaling by Insulin receptor
Reactome	`R-HSA-2404192`	Signaling by Type 1 IGF1 receptor (IGF1R)
WikiPathways	`WP481`	Insulin signaling

Limitations and gaps

needs-human-replication — All lifespan data is from model organisms; human evidence is GWAS and centenarian cohort studies only.
contradictory-evidence — Circulating IGF-1 levels show complex, possibly non-monotonic, relationship with longevity in humans. Some studies find low IGF-1 associated with frailty; others find FOXO3 genotype (which may indicate reduced IIS) associated with longevity.
Suh 2008 IGF1R centenarian variant paper verified against primary source PDF (doi:10.1073/pnas.0705467105); n=79 centenarians screened; two nonsynonymous IGF1R variants (Ala-37-Thr, Arg-407-His) enriched in centenarians, P=0.04. Small cohort; needs-replication.
The sex differences in the mouse IGF1R+/- result (females long-lived, males not) are unexplained and flag that IIS biology is sexually dimorphic, possibly through interaction with sex steroids. no-mechanism

Footnotes

doi:10.1038/366461a0 · in-vivo · model: C. elegans (daf-2 mutant) · ~2× lifespan extension · citation_percentile: 100 (Nature 1993, 3379 citations) ↩ ↩² ↩³ ↩⁴ ↩⁵
doi:10.1073/pnas.0801030105 · nested case-control · n=615 men (213 cases ≥95y; 402 controls ~77y; drawn from Honolulu Heart Program/HAAS) · model: homo-sapiens (Hawaiian Japanese) · FOXO3A rs2802292 GG vs TT OR=2.75 (95% CI 1.51–5.02, P=0.0007) for longevity to 95+ · PNAS 2008 ↩ ↩² ↩³
doi:10.1038/nature01298 · in-vivo · genetic model · model: Igf1r+/- mice (129/Sv background) · 26% mean lifespan extension overall (P<0.02, Cox); 33% in females (P<0.001), 15.9% in males (NS) · local PDF available ↩ ↩²
doi:10.1126/science.1142179 · in-vivo · genetic model · model: brain-specific IRS-2 knockout mice · ~14% lifespan extension (females) · Science 2007 ↩
doi:10.1073/pnas.0705467105 · case-control · n=79 female centenarians screened (full cohort: 286 female + 98 male centenarians; controls n=312) · model: homo-sapiens (Ashkenazi Jewish) · IGF1R Ala-37-Thr and Arg-407-His variants enriched in centenarians P=0.04; carriers show reduced IGFIR activity in lymphocytes · PNAS 2008 ↩

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