Age-Related Impairment of Importin7-Mediated ERK1/2 Nuclear Export Promotes Cytoplasmic ERK Accumulation, Driving Senescence-Associated Autophagy Deficiency and Ras-Transformed Senescence Bypass

Hypothesis

Age-dependent decline in Importin7-mediated ERK1/2 nuclear export leads to persistent cytoplasmic ERK1/2 accumulation in aged primary cells, which simultaneously:

1. Reduces nuclear ERK-driven transcription of cyclin D1 and Myc, limiting proliferative capacity and contributing to replicative senescence.
2. Enhances cytoplasmic ERK scaffolding of autophagy inhibitors (e.g., phosphorylation of ULK1 at inhibitory sites), thereby suppressing cytoprotective autophagy.
3. Promotes secretion of IGFBP4/7 via ERK-dependent MAPK‑EMT signaling, reinforcing the senescence‑associated secretory phenotype (SASP).

In Ras‑transformed cells, the same cytoplasmic ERK accumulation creates a synthetic lethal context: MEK/ERK inhibition further diminishes the already compromised autophagic flux, pushing cells past a viability threshold and causing selective elimination, whereas in normal aged cells the autophagy defect merely reinforces senescence without triggering cell death.

Testable predictions:

• In primary human fibroblasts from young vs. old donors, live‑cell FRET‑based ERK shuttling reporters will show slower nuclear export rates (lower k_export) correlating with increased p‑ERK cytoplasmic fraction and elevated SA‑β‑gal positivity.
• Pharmacological enhancement of Importin7 binding (e.g., CK2 activator) will restore nuclear export, decrease cytoplasmic p‑ERK, rescue autophagy flux (LC3‑II turnover), and reduce SASP cytokine secretion without affecting proliferation.
• In Ras‑transformed senescent cells, combined MEK inhibition with CK2 activation will synergistically induce autophagic cell death, while neither alone will be lethal.
• CRISPR‑mediated knock‑down of Importin7 in young cells will phenocopy aged export defects, increasing cytoplasmic ERK, suppressing autophagy, and elevating IGFBP4/7 secretion.

Falsification: If aged cells display normal or accelerated ERK nuclear export kinetics, or if restoring Export does not modify autophagy or SASP, the hypothesis is refuted.

References: The dual role of ERK in senescence via protein degradation 1; MEK/ERK inhibition enabling senescence bypass through autophagy 2; nuclear‑cytoplasmic shuttling regulating differentiation 3; mechanistic overview of ERK shuttling in senescence 4; IGFBP4/7 SASP mediation 5; MAPK‑driven EMT in early senescence 6.