Late‑Life Hepatic IGF‑1 Suppression as a Gerotherapeutic Strategy to Uncouple Longevity Benefits from Somatopause‑Associated Frailty

Background The somatopause‑related decline in GH/IGF‑1 correlates with frailty, sarcopenia and low bone density, yet genetically reduced GH/IGF‑1 signaling extends lifespan and protects against cancer, diabetes and dementia [1]. This paradox suggests that the source and timing of IGF‑1 reduction matter: endocrine (hepatic) IGF‑1 drives systemic aging pathways, whereas local IGF‑1 in muscle and bone supports tissue maintenance [5].

Hypothesis Selective suppression of hepatic IGF‑1 production in late life will recapitulate the longevity and metabolic advantages of low IGF‑1 signaling while preserving peripheral IGF‑1‑dependent musculoskeletal function, thereby attenuating frailty despite low circulating IGF‑1.

Mechanistic Rationale

1. Hepatic IGF‑1 constitutes ~75 % of circulating IGF‑1 and exerts endocrine effects that promote insulin resistance via increased hepatic gluconeogenesis and adipose lipolysis [5].
2. Muscle‑derived (autocrine/paracrine) IGF‑1 activates Akt/mTOR locally, stimulating protein synthesis and satellite cell activity, which is critical for resisting sarcopenia [2].
3. Late‑life hepatic IGF‑1 knock‑down reduces endocrine IGF‑1 without affecting intramuscular IGF‑1 synthesis, thereby lowering systemic aging signals while retaining anabolic support for muscle and bone.
4. This dichotomy mirrors the phenotype of liver‑specific IGF‑1 knockout mice, which show extended lifespan and improved insulin sensitivity despite normal muscle IGF‑1 levels (see supporting data in [[5]]).

Testable Predictions

• Late‑life hepatic IGF‑1 reduction will increase median lifespan by ≥15 % in mice compared with controls.
• Treated mice will exhibit lower fasting insulin and HOMA‑IR, reflecting enhanced insulin sensitivity.
• Frailty index (grip strength, gait speed, activity) will not differ significantly from age‑matched controls, indicating preserved musculoskeletal function.
• Muscle IGF‑1 mRNA and protein levels will remain unchanged, while serum IGF‑1 drops ≤30 % of baseline.

Experimental Design

• Use 18‑month‑old C57BL/6J mice (equivalent to ~60 human years).
• Administer a liver‑targeted IGF‑1 antisense oligonucleotide (ASO) or AAV8‑shIGF‑1 subcutaneously weekly for 12 weeks; control group receives scrambled ASO.
• Monitor serum IGF‑1, insulin, glucose monthly; perform intraperitoneal glucose tolerance test (IP‑GTT) at 3‑month intervals.
• Assess frailty every 2 months using a validated mouse frailty index (including grip strength, gait speed, coat state, activity).
• At natural death, record lifespan; harvest muscle, liver, bone for IGF‑1 expression, Akt phosphorylation, and histology.

Potential Outcomes and Implications If hepatic IGF‑1 suppression extends lifespan without worsening frailty, it would support the notion that somatopause‑related IGF‑1 decline is maladaptive only when it reflects a generalized tissue‑wide deficit. Conversely, if frailty worsens, the hypothesis would be refuted, indicating that circulating IGF‑1 is required for musculoskeletal health even in late life. Positive results would justify clinical trials of liver‑directed IGF‑1 silencing (e.g., GalNAc‑conjugated siRNA) in older adults as a gerotherapeutic that separates longevity benefits from sarcopenic risk.