Core Hypothesis

Hormetic stressors only promote longevity when they generate transient ERK1/2 activity that drives bona fide damage repair. In aged cells, age‑dependent downregulation of the ERK phosphatase DUSP6 shifts the same mild stressor into a sustained ERK signal, which activates senescence programs without reducing accumulated molecular damage. Thus, hormesis fails as a longevity mechanism in older organisms not because the stress is insufficient, but because the signaling threshold for transient vs. sustained ERK is altered.

Mechanistic Rationale

• ERK dynamics dictate outcome: Transient ERK pulses (seconds‑minutes) promote proliferation and activate repair pathways (e.g., NRF2‑mediated antioxidant transcription, autophagy initiation). Sustained ERK (hours‑days) drives p21/CDKN2A expression, reinforcing senescence (1).
• Aged‑specific phosphatase loss: DUSP6 (MKP‑3) preferentially dephosphorylates nuclear ERK. Studies show DUSP6 expression declines with age in fibroblasts and immune cells (2). Reduced DUSP6 prolongs nuclear ERK residence, converting a hormetic pulse into a sustained signal.
• Feedback loop with p21: Sustained ERK stabilizes p21, which further inhibits cyclin‑dependent kinases and promotes a senescent state that suppresses DUSP6 transcription (3). This creates a self‑reinforcing loop that locks ERK in the active, senescent‑promoting mode.
• Repair uncoupled: When ERK is sustained, downstream effectors such as ELK1 favor transcription of SASP components rather than DNA‑repair genes. Consequently, markers like γH2AX or 8‑oxo‑dG remain unchanged despite transient reductions in SA‑β‑gal or p16 expression.

Testable Predictions

1. Phosphatase rescue: Overexpressing DUSP6 in aged human fibroblasts subjected to a hormetic stimulus (e.g., 2 h intermittent fasting‑mimetic serum) will restore transient ERK dynamics (measured by live‑cell ERK‑KTR reporter) and reduce DNA damage markers (γH2AX foci, comet assay) compared with vector controls.
2. Pharmacological inhibition: Treating aged cells with a selective ERK inhibitor (e.g., SCH772984) during the hormetic window will block the sustained ERK phase, allowing transient ERK to prevail and resulting in measurable clearance of oxidized proteins (Western blot for carbonylated proteins) and improved proteostasis (LC3‑II turnover).
3. In vivo validation: Old mice subjected to intermittent fasting will show increased nuclear ERK persistence in liver tissue (immunofluorescence) and elevated p21; co‑administration of a DUSP6‑activating compound (e.g., benzoyl‑cAMP) will shift ERK to transient, decrease hepatic 8‑oxo‑dG, and improve survival relative to fasting alone.
4. Single‑cell resolution: scRNA‑seq coupled with phospho‑ERK flow cytometry will reveal a bimodal distribution of ERK activity in aged populations after hormesis; the high‑ERK cluster will express senescence signatures (p21, SASP) without enrichment of repair gene sets (OGG1, XRCC1).

Falsifiability

If experiments demonstrate that hormetic interventions in aged cells consistently produce transient ERK dynamics and concomitant reductions in DNA/protein damage irrespective of DUSP6 levels, the hypothesis would be falsified. Conversely, if DUSP6 manipulation fails to alter ERK duration or damage outcomes, the proposed mechanistic link would be rejected.

Broader Implications

This framework reframes hormesis not as a universal anti‑aging switch but as a context‑dependent signal whose translation into repair hinges on the cellular capacity to restrain ERK activity. It suggests that longevity strategies should combine mild stressors with agents that preserve or restore ERK phosphatase function—particularly in aged or disease‑prone tissues—to convert threat signaling into genuine maintenance.