Hypothesis

We propose that the switch from transient JNK activation during acute stress to chronic JNK‑driven SASP is governed by redox‑sensitive remodeling of Jun/Fos dimer ratios. Acute mitochondrial ROS preferentially activate JNK isoforms that phosphorylate JunB and FosB, favoring JunB‑FosB heterodimers that bind antioxidant and pro‑survival promoters. Persistent ROS overwhelms the cysteine‑based phosphatase MKP‑1 (DUSP1), leading to sustained JNK activity, preferential phosphorylation and stabilization of c‑Jun and JunD, and a shift toward c‑Jun/JunD homodimers that drive SASP genes.

Mechanistic Basis

1. Redox control of JNK phosphatases – MKP‑1 contains a catalytic cysteine that forms a reversible sulfenic acid under mild oxidative stress, transiently inhibiting its activity and allowing a pulse of JNK signaling. When ROS exceed a threshold, sulfenic acid is further oxidized to sulfinic/sulfonic acids, inactivating MKP‑1 permanently (1). This loss of phosphatase activity converts a transient JNK pulse into a plateau.
2. Differential Jun stability – Phosphorylated c‑Jun is protected from FBXW7‑mediated ubiquitination, whereas JunB is a better substrate for this E3 ligase. Sustained JNK activity thus skews the Jun pool toward c‑Jun accumulation (3).
3. Dimer selection – JunB‑FosB dimers exhibit high affinity for AP‑1 sites in genes such as Hmox1 and Sod2, promoting redox homeostasis. In contrast, c‑Jun/JunD dimers preferentially occupy κB‑adjacent AP‑1 composites in Il6, Ccl2, and Mmp3 promoters, cooperating with NF‑κB to amplify the SASP (2).
4. Feedback loops – SASP cytokines reinforce mitochondrial ROS via NOX activation, creating a feed‑forward circuit that locks the AP‑1 composition in the pro‑inflammatory state.

Testable Predictions

• Prediction 1: In human fibroblasts subjected to a single dose of rotenone (acute mitochondrial ROS), JunB‑FosB heterodimers will peak at 2 h post‑treatment and decline by 12 h, whereas c‑Jun/JunD homodimers will remain low. Chronic low‑dose rotenone (24‑72 h) will show the inverse pattern.
  • Test: Perform native chromatin immunoprecipitation (ChIP) for JunB, FosB, c‑Jun, and JunD followed by qPCR at Hmox1 and Il6 loci; quantify dimer‑specific binding.
• Prediction 2: Genetic or pharmacological inhibition of MKP‑1 will accelerate the JunB→c‑Jun shift and induce SASP markers even after a brief ROS pulse.
  • Test: Use MKP‑1‑KO cells or the MKP‑1 inhibitor decoy peptide; measure phospho‑JNK, Jun/Fos ratios by western blot, and SASP secretion (ELISA) after 1 h H₂O₂ exposure.
• Prediction 3: Overexpression of a non‑ubiquitinatable c‑Jun mutant (K→R at FBXW7 sites) will sustain SASP expression despite MKP‑1 restoration.
  • Test: Express c‑Jun‑KR in WT cells, restore MKP‑1 with NADPH oxidase inhibitor, and assess whether IL‑6 production remains high.

Falsifiability

If acute ROS exposure fails to produce a detectable JunB‑FosB‑biased AP‑1 signature, or if MKP‑1 loss does not alter the Jun/Fos ratio or SASP outcome, the hypothesis would be refuted. Conversely, confirming the predicted dimer‑specific DNA binding shifts and their dependence on redox‑sensitive phosphatase activity would substantiate the model.

Broader Implication

This mechanism links mitochondrial redox state directly to the transcriptional decision point between adaptive stress responses and irreversible senescence, offering a node for interventions that preserve transient JNK signaling while blocking its chronic SASP conversion.