Hypothesis

Rapamycin extends lifespan by generating transient, p38MAPK‑Jmjd3 driven loss of H3K27me3 at the CDKN2A/B locus, producing pulsed p16INK4a expression that activates senescent surveillance without establishing a permanent senescent state. In contrast, caloric restriction maintains persistent PRC2 occupancy and high H3K27me3, keeping p16INK4a repressed. This difference predicts that rapamycin‑treated tissues will show oscillatory chromatin states correlated with feeding cycles, whereas CR‑treated tissues will display stable repression.

Mechanistic rationale

mTORC1 activity supports EZH2 translation and PRC2 stability [2]. Acute rapamycin withdrawal (or pulsatile dosing) reduces EZH2 synthesis, allowing pre‑existing H3K27me3 to be diluted during DNA replication. Concurrently, rapamycin‑induced metabolic stress activates p38MAPK, which phosphorylates and activates the H3K27me3 demethylase Jmjd3 [1][3]. The resulting epigenetic window permits short‑lived p16INK4a transcription. Because mTORC1 remains inhibited between pulses, newly synthesized p16INK4a protein is rapidly cleared via autophagy, preventing accumulation of a stable SASP. Caloric restriction, by lowering insulin/IGF signaling without directly inhibiting mTORC1 translation control, preserves EZH2 levels and PRC2 chromatin binding, thereby sustaining H3K27me3 and keeping p16INK4a off [2][5].

Testable predictions

1. Chromatin dynamics – In mouse liver or adipose, ChIP‑qPCR for H3K27me3 and EZH2 will show a 2‑4 h dip after each rapamycin dose, followed by recovery within 24 h; CR mice will exhibit a flat, high baseline across the same interval.
2. Expression pulses – Nascent RNA‑seq (EU‑labeling) will detect p16INK4a transcriptional bursts coinciding with H3K27me3 loss in rapamycin cohorts, absent in CR.
3. Functional requirement – Genetic or pharmacological inhibition of p38MAPK or Jmjd3 will blunt the rapamycin‑induced H3K27me3 loss and abolish improvements in glucose tolerance and grip strength, while CR benefits remain intact.
4. SASP avoidance – Rapamycin‑treated mice will show low circulating IL‑6 and CXCL10 despite p16INK4a pulses, whereas chronic p16INK4a elevation (e.g., in p16‑overexpressors) yields high SASP markers.

Falsifiability

If rapamycin fails to produce detectable, cyclical reductions in H3K27me3 at CDKN2A/B, or if blocking p38MAPK/Jmjd3 does not diminish its healthspan effects, the hypothesis is refuted. Conversely, observing stable H3K27me3 repression under rapamycin would support an alternative model of direct epigenetic reprogramming.