Background

The GH/IGF-1 axis shows a U-shaped mortality curve, with low and high IGF-1 linked to increased cancer, cardiovascular, and all-cause death [2]. IGF-1 drives tumorigenesis in colorectal, breast, prostate, and thyroid cancers [3][4], yet low IGF-1 correlates with impaired cognition and metabolic frailty [7][9]. Metformin reduces cancer incidence in models, but its effect varies with baseline IGF-1 [9].

Hypothesis

Individuals possess a tissue-specific equilibrium between IGF-1R isoform A (fetal, high affinity) and isoform B (adult, metabolic) that determines the IGF-1 concentration at which downstream signaling switches from predominantly anabolic to pro‑senescent. The optimal circulating IGF-1 range for longevity is thus the concentration that maintains isoform B‑driven metabolic signaling in muscle and liver while keeping isoform A‑mediated mitogenic activity below a tumorigenic threshold in epithelial tissues.

Mechanistic Rationale

1. Isoform switching – Alternative splicing of IGF-1R exon produces IGF-1R-A and IGF-1R-B. IGF-1R-A preferentially activates MAPK/ERK pathways, whereas IGF-1R-B favors PI3K-AKT signaling linked to insulin sensitivity [1]. Age-related changes in splicing factors (e.g., SRSF1, hnRNP A1) shift the A/B ratio toward isoform A in epithelial compartments, raising cancer risk even at moderate IGF-1 levels.

2. Metformin’s action – Metformin activates AMPK, which phosphorylates the splicing regulator SRSF1, promoting exon inclusion that favors IGF-1R-B expression [8]. Simultaneously, metformin increases hepatic IGFBP-1 production, lowering free IGF-1 and preferentially attenuating IGF-1R-A signaling [9].

3. Threshold model – When free IGF-1 falls below the concentration needed to saturate IGF-1R-B in muscle/liver, AKT signaling drops, causing insulin resistance and sarcopenia. When free IGF-1 exceeds the level that drives IGF-1R-A-mediated ERK activation in epithelia, mitogenic signaling rises, increasing cancer incidence. The window between these two thresholds defines the individual optimal IGF-1 range.

Testable Predictions

• In older adults, the IGF-1R-A/IGF-1R-B ratio in peripheral blood mononuclear cells will correlate positively with cancer risk and negatively with muscle strength, independent of total IGF-1 concentration.
• Metformin treatment will decrease the IGF-1R-A/IGF-1R-B ratio and raise IGFBP-1 levels, shifting participants with high baseline IGF-1 toward the lower end of the U-curve and those with low baseline IGF-1 toward the higher end, thereby narrowing individual mortality risk.
• Pharmacologic inhibition of IGF-1R-A (using an isoform-selective antibody) will mimic metformin’s cancer-protective effect without altering glucose homeostasis, confirming isoform-specific mediation.

Experimental Design

1. Cohort: 500 community-dwelling adults aged 60-80, stratified by baseline IGF-1 tertiles.
2. Measures: serum IGF-1, IGFBP-1, free IGF-1, IGF-1R-A/IGF-1R-B ratio (via isoform-specific qPCR on PBMCs), muscle strength (hand-grip), cognitive score (MoCA), and cancer incidence over 3 years.
3. Intervention: double-blind, placebo-controlled metformin 1500 mg daily vs. placebo for 12 months.
4. Analyses: mixed-effects models testing whether changes in the isoform ratio mediate the effect of metformin on cancer markers and metabolic outcomes, moderated by baseline IGF-1.

Potential Outcomes

• If the isoform ratio predicts risk and metformin normalizes it across IGF-1 strata, the hypothesis is supported, indicating that personalized IGF-1 targeting should focus on restoring isoform balance rather than absolute hormone levels.
• If metformin alters IGF-1 but not the isoform ratio, or if the ratio does not relate to outcomes, the hypothesis is falsified, suggesting that other mechanisms dominate the U-shaped curve.

By tying IGF-1 concentration to tissue-specific receptor splicing and metformin’s AMPK-dependent splicing modulation, this framework offers a concrete, falsifiable path to resolve the GH/IGF-1 paradox and guide precision interventions for aging.