Hypothesis

Rapamycin extends lifespan by mimicking scarcity, but its effect on the humoral coagulation arm is liver‑specific and conditional on nutrient status. We propose that mTORC1 inhibition in hepatocytes de‑represses FOXO1, which then suppresses transcription of Factor VIII (F8) and von Willebrand factor (VWF) genes, lowering circulating procoagulant proteins only when intracellular amino acid levels are low. Under fed conditions, compensatory activation of SREBP‑1c restores F8/VWF expression, preserving a prothrombotic milieu despite improved platelet function.

Mechanistic Rationale

1. FOXO‑driven repression – In starvation, AMPK activates FOXO1, which binds insulin‑response elements in the F8 and VWF promoters, reducing their expression [2][3]. Rapamycin‑induced mTORC1 inhibition similarly activates FOXO1 via reduced S6K‑mediated phosphorylation.
2. Nutrient‑dependent override – High glucose/insulin stimulates SREBP‑1c, which recruits co‑activators to the same promoters, counteracting FOXO repression. This creates a bistable switch: low nutrients → low F8/VWF; high nutrients → high F8/VWF.
3. Platelet‑centric effects remain – Rapamycin’s anti‑thrombotic actions on platelets (PKCδ/ε, Rac1/S6K1, autophagy) are independent of liver signaling [1][4][5], explaining why platelet function improves while plasma coagulation factors may not.

Testable Predictions

• Prediction 1: Liver‑specific Raptor knockout (L‑RaptorKO) mice will show decreased plasma F8 and VWF levels and prolonged thrombin generation time (TGT) only when maintained on a 24‑hour fast, but not on ad libitum high‑carb diet.
• Prediction 2: Pharmacological activation of SREBP‑1c (e.g., via LXR agonist GW3965) in rapamycin‑treated mice will restore F8/VWF to youthful levels without altering platelet PKCδ phosphorylation.
• Prediction 3: Chromatin immunoprecipitation (ChIP) in isolated hepatocytes will reveal increased FOXO1 binding to the F8/VWF promoters after rapamycin treatment under low‑amino‑acid media, and decreased binding when media is supplemented with leucine.
• Prediction 4: Thrombin‑generation assays using plasma from L‑RaptorKO mice fed ad libitum will resemble aged wild‑type controls, confirming that improved platelet function does not equate to systemic anticoagulation.

Experimental Approach

• Generate L‑RaptorKO mice (Alb‑Cre; Raptor^fl/fl) and littermate controls.
• Assign cohorts to ad libitum, 24‑h fast, or high‑fat diet for 8 weeks.
• Measure plasma F8, VWF, D‑dimer, and perform calibrated automated thrombography (CAT).
• Assess platelet activation (P‑selectin, PAC‑1) and autophagy (LC3‑II) as per prior protocols [1][4][5].
• Perform ChIP‑qPCR for FOXO1 and SREBP‑1c on F8/VWF promoters.
• Use GW3965 or siRNA to modulate SREBP‑1c in a subset.

Falsifiability

If liver‑specific mTORC1 inhibition fails to alter F8/VWF irrespective of nutritional state, or if SREBP‑1c manipulation does not rescue factor levels, the FOXO‑SREBP bistable model is falsified. Conversely, observing the predicted nutrient‑dependent shifts would support the hypothesis that rapamycin’s longevity benefit stems from a simulated scarcity signal that does not fully reprogram the hepatic coagulation program, thereby limiting its capacity to achieve longevity escape velocity.

References

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC5097363/ [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC6611156/ [3] https://www.aging-us.com/article/100070/text [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC3175787/ [5] https://pmc.ncbi.nlm.nih.gov/articles/PMC8111939/