Hypothesis\n\nAcarbose extends lifespan in male HET3 mice by activating a gut‑liver axis that hinges on colonic propionate signaling through the G‑protein‑coupled receptor GPR41 on hepatocytes, thereby stimulating hepatic FGF21 production. This cascade is potentiated by male‑specific androgen receptor (AR) co‑activation, which augments FGF21 transcriptional response, explaining the pronounced sex dimorphism.\n\n## Mechanistic Rationale\n\n1. Colonial carbohydrate load → propionate surge – Acarbose‑mediated α‑glucosidase inhibition shunts starch to the colon, where Megasphaera and other saccharolytic bacteria ferment it to propionate (2, 3).\n2. Propionate → GPR41 → FGF21 – Propionate binds GPR41 (FFAR3) expressed on hepatocytes, triggering a cAMP‑PKA cascade that up‑regulates Fgf21 transcription (5).\n3. Androgen receptor synergy – In males, circulating testosterone activates AR, which physically interacts with the PPARα co‑activator complex on the Fgf21 promoter, increasing transcriptional output beyond that achieved by propionate alone (4). Females lack sufficient androgenic tone, resulting in a blunted FGF21 response.\n4. FGF21 systemic effects – Elevated circulating FGF21 improves insulin sensitivity, promotes autophagy, and suppresses mTORC1 signaling, collectively extending lifespan (1).\n\n## Testable Predictions\n\n- Prediction 1: Germ‑free mice colonized with a propionate‑producing consortium (e.g., Megasphaera elsdenii) will recapitulate the male‑specific lifespan extension of acarbose, whereas colonization with a non‑propionate producer will not.\n- Prediction 2: Hepatocyte‑specific Gpr41 knockout will abolish acarbose‑induced hepatic FGF21 rise and the longevity benefit in males, but will have minimal effect in females.\n- Prediction 3: Androgen receptor antagonism (using flutamide) in male mice will reduce acarbose‑evoked FGF21 elevation to female‑like levels and erase the sex difference in lifespan.\n- Prediction 4: Pharmacologic FGF21 neutralization will suppress the lifespan extension conferred by acarbose in males, confirming FGF21 as the downstream effector.\n\n## Experimental Design (brief)\n\n- Groups: Male and female HET3 mice (n=30 per group) receiving: (i) control diet, (ii) acarbose (high dose, high starch), (iii) acarbose + hepatocyte‑specific Gpr41 KO (via Alb‑Cre), (iv) acarbose + AR antagonist, (v) acarbose + anti‑FGF21 antibody.\n- Readouts: Survival curves, fecal SCFA quantification (GC‑MS), hepatic Gpr41 signaling (p‑PKA, p‑CREB), hepatic FGF21 mRNA/protein, serum FGF21, metabolic phenotyping (GTt, ITT), autophagy markers (LC3‑II/I, p62) in liver.\n- Statistical analysis: Cox proportional hazards model for survival; two‑way ANOVA for biochemical endpoints; significance set at p<0.05.\n\nIf the data show that removing any step of the propionate‑GPR41‑FGF21 axis (microbiota, receptor, AR co‑activation, or FGF21 itself) abolishes the male‑specific longevity effect while leaving females unchanged, the hypothesis will be supported. Conversely, persistence of lifespan extension despite these manipulations would falsify the model and point to alternative mechanisms.
