The chronic interferon signature in aging tissues isn't just inflammation—it's enforcing a metabolic checkpoint. We propose that sustained, low-grade IFN-γ signaling actively suppresses NAD+ biosynthesis through STAT-mediated epigenetic repression of rate-limiting enzymes, creating a deliberate trade-off that sacrifices metabolic capacity to contain genomic instability.

Mechanistic Basis

1. STAT proteins as epigenetic repressors of NAD+ machinery: Chronic IFN-γ activates STAT1/3, which don't just induce ISGs—they recruit HDACs and HMTs to promoters of key NAD+ biosynthetic genes [PMC6966445]. Specifically, we hypothesize STAT complexes bind to NAMPT (rate-limiting enzyme in NAD+ salvage pathway) and NMNAT promoters, depositing H3K27me3 marks via EZH2 recruitment. This creates a transcriptional bottleneck that directly reduces NAD+ availability.

2. Metabolic coordination beyond simple downregulation: The aged heart's correlation between IFN-γ signature and dampened oxidative phosphorylation [Cardiovasc Res 2023] suggests this isn't random. IFN-driven repression likely targets specific NAD+-consuming enzymes: PARPs (DNA repair) and sirtuins (chromatin remodeling) may be differentially regulated compared to metabolic dehydrogenases. This creates a "hierarchy of sacrifice"—the cell preserves glycolytic ATP production while cutting NAD+-dependent epigenomic maintenance.

3. Bypassing JAK-STAT brakes via TE-driven feedback: The persistent TE-cGAS-STING axis generates constant low-level IFN production [PMC9261325], creating a feedforward loop that overwhelms SOCS-mediated negative feedback [PMC11565648]. Each retrotransposition event introduces new IFN triggers, maintaining STAT activation despite inhibitory mechanisms. The system isn't stuck—it's being actively rebooted.

Novel Synthesis: The Checkpoint Model

This represents a cellular "austerity program" with specific logic:

• Trigger: Genomic instability/TE activation signals danger
• Response: IFN-γ → STAT → epigenetic repression of NAD+ biosynthesis
• Consequence: Reduced NAD+ → decreased PARP/SIRT activity → less DNA repair capacity but also less futile repair of unfixable damage
• Trade-off: Metabolic downshift limits replicative potential but prevents propagation of damaged genomes

This reframes NAD+ decline from passive decay to active checkpoint enforcement. The cell isn't failing to maintain NAD+—it's deliberately restricting the epigenomic maintenance budget when genomic integrity cannot be guaranteed.

Testable Predictions

1. Genetic: STAT1/3 knockout in aged mice should increase NAMPT expression, raise NAD+ levels, but also increase micronuclei formation and aneuploidy rates.

2. Pharmacological: JAK inhibitors (e.g., ruxolitinib) in aged cardiomyocytes should restore NAD+ within 72 hours while simultaneously increasing γH2AX foci (DNA damage markers).

3. Epigenomic: ChIP-seq in aged vs young hepatocytes should reveal STAT3 enrichment at NAMPT promoter coinciding with H3K27me3 deposition—reversible by IFN-γ neutralization.

4. Metabolic flux: 13C-tracing in chronically IFN-γ treated cells should show preferential NAD+ allocation to glycolysis versus nuclear sirtuins, measurable via subcellular NAD+ biosensors.

This model predicts that NAD+ supplementation alone may be insufficient or even detrimental in aging—if the checkpoint remains active, exogenous NAD+ could fuel repair of irreparable damage. Therapeutic success might require dual targeting: IFN signaling to release the brake plus NAD+ precursors to rebuild capacity.

The implications extend to inflammaging generally: chronic low-grade inflammation may represent not dysfunction but a costly, damage-containment strategy that trades tissue function for cancer suppression—a bargain that ultimately fails when senescence burden overwhelms the system.