Hypothesis

Chronic JNK/AP-1 signaling directly represses the NAD+ biosynthetic gene NAMPT in senescent cells, turning NAD+ decline into an active transcriptional program rather than a passive consequence of metabolic wear.

Mechanistic rationale

• Sustained JNK phosphorylates c‑Jun, enabling AP‑1 dimers to bind canonical TGACTCA sites in the NAMPT promoter (2).
• In senescence, oxidative stress–inhibited MKP5 prolongs JNK activity, increasing nuclear AP‑1 occupancy (2).
• Mitochondrial ROS oxidizes the catalytic cysteine of MKP5, rendering it inactive and thereby sustaining JNK phosphorylation (3).
• AP‑1 recruits HDAC1/2 and the co‑repressor Sin3A, leading to histone deacetylation and compact chromatin at the NAMPT locus.
• Falling NAD+ diminishes SIRT1‑mediated deacetylation of histones at the NAMPT promoter, making chromatin more permissive for AP‑1 binding and creating a feed‑forward loop.
• Reduced NAPRT and NMNAT expression follow, lowering NAD+ synthesis and amplifying the SASP via the HMGA–NAMPT–NAD+ axis described previously (1).

Testable predictions

1. ChIP‑seq for c‑Jun in irradiated or oncogene‑induced senescent fibroblasts will show enriched peaks at the NAMPT promoter compared with proliferating controls (2).
2. Luciferase reporters containing the wild‑type NAMPT promoter will lose activity upon AP‑1 over‑expression, whereas mutation of the AP‑1 site will rescue expression.
3. Pharmacologic JNK inhibition (SP600125) or MKP5 over‑expression will increase NAMPT mRNA, NAD+ levels, and decrease SASP cytokines (MCP‑1, ICAM‑1) in senescent cells.
4. Conversely, forced AP‑1 activation in young cells will suppress NAMPT, drop NAD+, and induce a senescence‑like SASP without DNA damage.
5. Supplementing NAD+ precursors (NR, NMN) in JNK‑active senescent cells will not reduce SASP unless JNK/AP‑1 is concurrently inhibited, predicting a paradoxical increase in inflammatory output if NAD+ is raised alone.

Falsifiability

• No detectable AP‑1 binding to the NAMPT promoter in senescent chromatin.
• Manipulating AP‑1 activity fails to alter NAMPT transcription or NAD+ levels.
• NAD+ boosting lowers SASP markers irrespective of JNK/AP‑1 status.

Implications

Reframing NAD+ loss as a JNK/AP‑1‑driven transcriptional downshift suggests that anti‑aging strategies must pair NAD+ repletion with JNK pathway inhibition to avoid fueling the proinflammatory SASP.