Hypothesis: Repeated, low‑amplitude hormetic stresses generate brief, synchronized ERK1/2 nuclear pulses that transiently overcome DUSP‑mediated cytoplasmic retention and re‑establish normal nucleocytoplasmic shuttling, thereby silencing SASP and reducing p21/p16 expression. If these pulses are insufficient or mistimed, ERK remains cytoplasmic and hormesis merely fuels a compensatory threat state.

Mechanistic rationale: In senescent cells, sustained MEK activity fails to drive ERK nuclear import because phosphorylated ERK is sequestered by cytoplasmic anchors such as PEA‑15 and phosphorylated DUSPs that dephosphorylate ERK upon nuclear entry [1][3]. Hormetic stimuli (e.g., intermittent fasting, mild heat shock) activate upstream RTKs and produce oscillatory MEK activity that can create short windows where ERK‑phosphatase activity is low enough for phosphorylated ERK to accumulate in the nucleus before being exported again [4]. A nuclear ERK pulse can phosphorylate ELK1 and other transcription factors that induce immediate‑early genes (e.g., Fos, Jun) and, importantly, induce DUSP6 expression in a delayed negative feedback loop that ultimately reduces cytoplasmic ERK activity and permits a reset of the shuttling equilibrium [2]. Concurrently, nuclear ERK can phosphorylate and inhibit NF‑κB p65, attenuating SASP transcription [5]. Thus, the hormetic benefit depends on achieving a critical pulse amplitude and duration that flips the ERK localization switch from a cytoplasmic "senescence lock" to a transient nuclear "repair mode".

Testable predictions: (1) In human fibroblasts rendered senescent by irradiation, a single bout of 2 h glucose restriction will produce a measurable increase in nuclear p‑ERK1/2 within 15 min, detectable by subcellular fractionation and immunoblot, followed by a return to cytoplasmic localization by 2 h. (2) Repeating this glucose restriction every 24 h for five days will reduce SASP IL‑6 and IL‑8 secretion by >40 % and lower p21^CIP1 and p16^INK4a mRNA levels compared with untreated senescent controls, without increasing apoptosis. (3) Pharmacological blockade of ERK nuclear import (using a selective ERK‑PEA‑15 interaction inhibitor) will abolish the SASP‑suppressive effect of hormesis, confirming that nuclear ERK is required. (4) If hormetic pulses are too weak or too infrequent (e.g., 30 min glucose restriction every 48 h), nuclear p‑ERK will remain at baseline and SASP will be unchanged or slightly increased, demonstrating a failure to reset the switch.

Experimental approach: Use immunofluorescence quantification of nuclear versus cytoplasmic p‑ERK1/2 in senescent IMR‑90 cells after defined hormetic intervals. Combine with RNA‑seq for SASP components and qPCR for CDK inhibitors. Include controls with MEK inhibitor (U0126) to verify ERK dependence and with DUSP6 siRNA to test the role of delayed negative feedback. Apoptosis will be assessed by caspase‑3/7 activity to ensure outcomes are not due to selective cell death.

Falsifiability: If repeated hormetic stresses consistently fail to produce detectable nuclear p‑ERK pulses, or if nuclear ERK pulses occur without any reduction in SASP or CDK inhibitor expression, the hypothesis is refuted. Likewise, if blocking nuclear ERK import does not diminish hormetic benefits, the proposed mechanism is invalid.

This framework reinterprets hormesis not as a generic stress‑response but as a precise temporal intervention that can temporarily unlock the ERK‑driven senescence lock, offering a falsifiable route to distinguish true rejuvenation from mere threat compensation.