Hypothesis

Hormetic stressors do not merely signal imminent threat; they trigger a self‑reinforcing epigenetic program that depends on mitochondrial export of citrate to generate nuclear acetyl‑CoA for H3K27ac deposition. When this metabolic‑epigenetic loop is constitutively engaged—e.g., by enhancing ATP‑citrate lyase (ACLY) activity or supplying cell‑permeable acetyl‑CoA precursors—cells maintain a "maintenance mode" that confers stress resistance and lifespan extension without any external hormetic challenge. Conversely, blocking mitochondrial citrate export or nuclear acetyl‑CoA production will abolish the durable H3K27ac marks and longevity benefits of hormesis, proving that the epigenetic memory, not the stress perception, is the causal longevity mechanism.

Mechanistic Rationale

1. Stress‑induced AMPK activation → phosphorylates NUP50, promoting its nuclear import (, ref [2]).
2. Nuclear NUP50 scaffolds CBP‑1, a histone acetyltransferase, to promoters of antioxidant and lipid‑catabolism genes (e.g., sod‑3, gpx‑6/7), leading to H3K27ac (ref [2]).
3. H3K27ac stability requires a steady supply of acetyl‑CoA. In many models, mitochondrial citrate exported via the citrate carrier (CIC) is cleaved by ACLY to produce cytosolic acetyl‑CoA, which then enters the nucleus (ref [5]).
4. Hormetic stress transiently boosts mitochondrial activity and citrate efflux, creating a pulse of acetyl‑CoA that fuels the initial H3K27ac wave. If this pulse is followed by sustained elevated ACLY activity (or exogenous acetyl‑CoA donors), the acetyl‑CoA pool remains high enough to maintain the acetylation mark, locking in the repair program.
5. Genetic bypass: Overexpressing mitochondrial fusion genes extends lifespan (ref [4]), likely by enhancing citrate export and thus acetyl‑CoA availability, independent of external stress.

Testable Predictions

• Prediction 1: In C. elegans, pharmacological activation of ACLY (e.g., with ACY‑1215) or supplementation with cell‑permeable acetyl‑CoA will reproduce the H3K27ac pattern at sod‑3/gpx‑6/7 promoters and extend lifespan by ~20 % without exposure to heat, hypoxia, or irradiation.
• Prediction 2: RNAi‑mediated knockdown of the mitochondrial citrate carrier (CIC) or ACLY will block the persistence of H3K27ac after a hormetic hypoxia pre‑conditioning regimen and abolish the associated lifespan extension, despite normal acute stress‑response signaling.
• Prediction 3: Overexpressing NUP50 fused to a constitutively nuclear localization signal will rescue longevity in ACLY‑deficient worms only when acetyl‑CoA levels are artificially elevated (e.g., via acetate supplementation), demonstrating that NUP50 nuclear import is necessary but insufficient without the metabolic substrate.
• Prediction 4: Metabolomic profiling will show a significant increase in nuclear acetyl‑CoA concentrations in hormetically treated wild‑type worms; this increase will be absent in CIC‑ or ACLY‑knockdown animals, correlating directly with H3K27ac levels (ChIP‑qPCR) and survival curves.

Experimental Outline (for C. elegans)

1. Strains: Wild‑type N2; acly‑1 RNAi; cit‑1.1 (CIC) RNAi; nup‑50 overexpressor; nup‑50;acly‑1 double manip.
2. Treatments: (a) Control, (b) Mild hypoxia (0.5 % O₂, 4 h), (c) ACLY activator, (d) Acetyl‑CoA ester supplement, (e) Hypoxia + RNAi/activator combos.
3. Readouts: Lifespan assays; western blot for phospho‑AMPK; immunofluorescence for nuclear NUP50; ChIP‑qPCR for H3K27ac at sod‑3/gpx‑6/7; LC‑MS for nuclear acetyl‑CoA.
4. Analysis: Compare lifespan and epigenetic marks across conditions; test for interaction effects (e.g., does ACLY activation rescue lifespan in cit‑1.1 RNAi?).

Falsifiability

If ACLY activation or acetyl‑CoA supplementation fails to extend lifespan or sustain H3K27ac in the absence of external stressors, or if citrate carrier knockdown does not diminish the longevity benefits of hormesis, the hypothesis is refuted. Likewise, if nuclear acetyl‑coa levels remain unchanged upon hormetic treatment yet H3K27ac persists, the proposed metabolic link would be untenable.

Broader Implication

This work reframes hormesis not as a generic alarm system but as a metabolically gated epigenetic switch. By decoupling the switch from stress‑induced signaling and wiring it directly to mitochondrial citrate flux, we reveal a route to program durable cellular repair—potentially yielding geroprotective interventions that operate in a truly low‑stress, physiological environment.