Hypothesis

Acarbose increases colonic propionate, which travels to the liver and triggers a sex‑specific rise in fibroblast growth factor 21 (FGF21) in males. Elevated hepatic FGF21 suppresses hepatic IGF‑1 production, lowering circulating IGF‑1 activity and thereby reducing neoplastic progression and extending lifespan. In females, estrogen‑mediated repression of FGF21 blunts this cascade, explaining the weaker acarbose response.

Mechanistic Rationale

• Propionate activates hepatic GPR41/43 receptors, leading to cAMP‑PKA signaling that upregulates FGF21 transcription. [https://pmc.ncbi.nlm.nih.gov/articles/PMC6567620/] shows propionate correlates with longevity.
• FGF21 inhibits IGF‑1 signaling via upregulation of IGF‑binding proteins and downregulation of IGF‑1 receptor expression, a pathway known to modulate cancer risk and aging. [https://pmc.ncbi.nlm.nih.gov/articles/PMC3954939/] reported acarbose extends lifespan mainly by delaying neoplasia.
• Sex differences arise because estrogen receptor α directly binds the FGF21 promoter and represses its transcription in hepatocytes, an effect observed in female mice. [https://lsom.uthscsa.edu/molecular-medicine/2020/02/20/acarbose-improved-survival-for-apc-min-mice/] notes acarbose improves survival without reducing tumor burden, consistent with a systemic hormonal milieu shift rather than direct antitumor activity.

Testable Predictions

1. Male mice treated with acarbose will show a ≥2‑fold increase in hepatic FGF21 mRNA and protein relative to vehicle; females will show no significant change.
2. Pharmacological blockade of hepatic GPR41 (using a selective antagonist) will abolish the acarbose‑induced FGF21 rise and the associated lifespan extension in males.
3. Liver‑specific overexpression of FGF21 in females will recapitulate the male‑like lifespan extension under acarbose treatment.
4. Circulating IGF‑1 levels will drop proportionally to hepatic FGF21 increase in acarbose‑treated males, correlating with delayed neoplastic lesions.
5. IGF‑1 receptor inhibition (e.g., with picropodophyllin) will not add further lifespan benefit on top of acarbose in males, indicating a shared pathway.

Experimental Approach

• Use cohorts of male and female C57BL/6 mice, administer acarbose (200 ppm in diet) or control for 18 months.
• Measure fecal propionate (GC‑MS), hepatic FGF21 (qPCR, ELISA), serum IGF‑1 (ELISA), and IGF‑1R phosphorylation (Western blot) at 6, 12, and 18 months.
• Track survival and cause of death (necropsy, histopathology for neoplasms).
• Intervention arms: GPR41 antagonist (e.g., GLPG0974) co‑administered; hepatic FGF21 overexpression via AAV8‑TBG‑FGF21; liver‑specific FGF21 knockout.
• Statistical analysis: Cox proportional hazards for survival; ANOVA for molecular endpoints; mediation analysis to test whether FGF21 mediates the effect of propionate on IGF‑1 and lifespan.

Falsifiability

If acarbose treatment does not elevate hepatic FGF21 in males, or if blocking GPR41/FGF21 fails to diminish the lifespan benefit, the hypothesis is refuted. Likewise, if female mice with forced hepatic FGF21 overexpression do not show acarbose‑like lifespan extension, the sex‑specific mechanism is unsupported.

Broader Implications

This model reframes acarbose’s action not as a generic metabolic slowdown but as a targeted endocrine signal that exploits an evolutionarily conserved IGF‑1/FGF21 axis, whose sex‑dependent regulation explains observed dimorphism. It suggests that longevity interventions may be optimized by coordinating gut‑derived metabolites with liver endocrine nodes, rather than attempting to override a putative programmed aging mechanism.