ASK1-MKK7-JNK Kinetics Dictate AP-1 Dimer Switch to JunD/FosL1, Locking Senescent SASP via Mitochondrial ROS‑Dependent Feed‑Forward Loop

Aria Boateng (DPhil)· Mar 26, 2026

Mechanism: Sustained JNK signaling, driven by mitochondrial ROS, shifts AP-1 dimer composition to JunD/FosL1, which locks in SASP expression. Readout: Readout: Inhibiting ASK1 or scavenging ROS reduces JunD/FosL1 dimers and lowers the SASP Score, while maintaining early c-Jun/c-Fos activity.

Hypothesis

Sustained ASK1‑MKK7‑JNK signaling, driven by mitochondrial ROS that persists beyond the first hour of stress, rewires AP‑1 dimer composition toward JunD/FosL1 heterodimers, which then preferentially bind and sustain transcription of a core SASP subset (IL‑6, IL‑8, MMP‑3) in senescent cells.

Mechanistic Rationale

Kinetic switch – Rapid JNK activation via GCKR/TRAF2 phosphorylates c‑Jun (Ser63/73) favoring c‑Jun/c‑Fos dimers that drive immediate‑early genes.
ROS‑dependent ASK1 – Mitochondrial ROS that accumulates over 20 min–1 h sustains ASK1 activity, leading to prolonged MKK7‑JNK signaling.
Dimer bias – Chronic JNK activity favors phosphorylation of JunD and FosL1 (which have lower turnover and distinct docking sites) over c‑Jun, shifting AP‑1 heterogeneity.
SASP lock‑in – JunD/FosL1‑AP‑1 binds composite sites with NF‑κB and C/EBPβ at SASP promoters, creating a feed‑forward loop that maintains SASP despite attenuation of upstream stress signals.
Mitochondrial dependence – Eliminating mitochondria (e.g., via Parkin overexpression) or scavenging ROS with MitoQ should prevent the JunD/FosL1 shift and attenuate SASP, even if acute JNK spikes occur.

Testable Predictions

Prediction 1: In cells undergoing senescence (e.g., IR‑induced or oncogenic RAS), phospho‑JNK levels will show a biphasic pattern: an early peak (<15 min) followed by a sustained plateau (>2 h) that correlates with mitochondrial ROS measured by MitoSOX.
Prediction 2: ChIP‑seq for JunD and FosL1 will reveal enriched binding at SASP gene promoters (IL6, CXCL8, MMP3) only after the sustained JNK phase, whereas c‑Jun/c‑Fos binding will be transient and lost after 6 h.
Prediction 3: Genetic ablation of ASK1 or pharmacological inhibition (e.g., GS‑444217) will block the sustained JNK phase, prevent JunD/FosL1 dimer accumulation, and reduce SASP secretion without affecting the early c‑Jun‑c‑Fos response.
Prediction 4: Forced expression of a phospho‑mimetic JunD (Ser‑→‑Asp) will rescue SASP expression in ASK1‑deficient senescent cells, indicating that JunD activation is downstream of sustained JNK.

Experimental Approach

Use human diploid fibroblasts (IMR‑90) and induce senescence via 10 Gy ionizing radiation or inducible HRAS^G12V.
Measure mitochondrial ROS (MitoSOX), phospho‑JNK (Thr183/Tyr185) by western blot and live‑cell FRET biosensor at 0, 0.5, 2, 6, 24 h.
Perform co‑immunoprecipitation followed by mass spectrometry to quantify AP‑1 dimer isoforms across time points.
Conduct ChIP‑seq for JunD, FosL1, c‑Jun, c‑Fra‑1 at 2 h and 24 h.
Apply ASK1 KO (CRISPR) or GSK‑690693, MKK7 siRNA, and MitoQ treatment; assess SASP cytokine secretion by ELISA and p16^INK4a/p21^CIP1 expression.
Rescue experiments with JunD‑S/D phospho‑mimetic plasmid.

Potential Outcomes and Falsifiability

If sustained JNK, JunD/FosL1 enrichment, and SASP maintenance are all abolished by ASK1 inhibition or MitoQ while early JNK/c‑Jun activity remains, the hypothesis is supported.
If JunD/FosL1 binding does not increase despite prolonged JNK activity, or SASP persists despite loss of JunD/FosL1, the hypothesis is falsified, indicating that alternative AP‑1 complexes or JNK‑independent pathways drive SASP lock‑in.

Hypothesis

Mechanistic Rationale

Kinetic switch – Rapid JNK activation via GCKR/TRAF2 phosphorylates c‑Jun (Ser63/73) favoring c‑Jun/c‑Fos dimers that drive immediate‑early genes.

ROS‑dependent ASK1 – Mitochondrial ROS that accumulates over 20 min–1 h sustains ASK1 activity, leading to prolonged MKK7‑JNK signaling.

Dimer bias – Chronic JNK activity favors phosphorylation of JunD and FosL1 (which have lower turnover and distinct docking sites) over c‑Jun, shifting AP‑1 heterogeneity.

SASP lock‑in – JunD/FosL1‑AP‑1 binds composite sites with NF‑κB and C/EBPβ at SASP promoters, creating a feed‑forward loop that maintains SASP despite attenuation of upstream stress signals.

Mitochondrial dependence – Eliminating mitochondria (e.g., via Parkin overexpression) or scavenging ROS with MitoQ should prevent the JunD/FosL1 shift and attenuate SASP, even if acute JNK spikes occur.

Testable Predictions

Prediction 1: In cells undergoing senescence (e.g., IR‑induced or oncogenic RAS), phospho‑JNK levels will show a biphasic pattern: an early peak (<15 min) followed by a sustained plateau (>2 h) that correlates with mitochondrial ROS measured by MitoSOX.

Prediction 2: ChIP‑seq for JunD and FosL1 will reveal enriched binding at SASP gene promoters (IL6, CXCL8, MMP3) only after the sustained JNK phase, whereas c‑Jun/c‑Fos binding will be transient and lost after 6 h.

Prediction 3: Genetic ablation of ASK1 or pharmacological inhibition (e.g., GS‑444217) will block the sustained JNK phase, prevent JunD/FosL1 dimer accumulation, and reduce SASP secretion without affecting the early c‑Jun‑c‑Fos response.

Prediction 4: Forced expression of a phospho‑mimetic JunD (Ser‑→‑Asp) will rescue SASP expression in ASK1‑deficient senescent cells, indicating that JunD activation is downstream of sustained JNK.

Experimental Approach

Use human diploid fibroblasts (IMR‑90) and induce senescence via 10 Gy ionizing radiation or inducible HRAS^G12V.

Measure mitochondrial ROS (MitoSOX), phospho‑JNK (Thr183/Tyr185) by western blot and live‑cell FRET biosensor at 0, 0.5, 2, 6, 24 h.

Perform co‑immunoprecipitation followed by mass spectrometry to quantify AP‑1 dimer isoforms across time points.

Conduct ChIP‑seq for JunD, FosL1, c‑Jun, c‑Fra‑1 at 2 h and 24 h.

Apply ASK1 KO (CRISPR) or GSK‑690693, MKK7 siRNA, and MitoQ treatment; assess SASP cytokine secretion by ELISA and p16^INK4a/p21^CIP1 expression.

Rescue experiments with JunD‑S/D phospho‑mimetic plasmid.

Potential Outcomes and Falsifiability

If sustained JNK, JunD/FosL1 enrichment, and SASP maintenance are all abolished by ASK1 inhibition or MitoQ while early JNK/c‑Jun activity remains, the hypothesis is supported.

If JunD/FosL1 binding does not increase despite prolonged JNK activity, or SASP persists despite loss of JunD/FosL1, the hypothesis is falsified, indicating that alternative AP‑1 complexes or JNK‑independent pathways drive SASP lock‑in.