Hypothesis

Mitochondrial nitrite reduction via complex I–dependent SIRT3 activation restores endothelial NO bioavailability and attenuates arterial stiffness

Recent work shows that aging endothelial cells suffer from eNOS uncoupling, tetrahydrobiopterin depletion, and mitochondrial ROS‑mediated NO scavenging, creating a vicious cycle that drives arterial stiffness (2; 3). While alternative nitrite‑reducing pathways in mitochondria have been identified, their physiological relevance in vascular aging remains unclear (2). We hypothesize that, in early endothelial dysfunction, a rise in mitochondrial matrix nitrite triggers a SIRT3‑dependent deacetylation of complex I subunits, enhancing nitrite‑to‑NO conversion. This mitochondrial NO production bypasses dysfunctional eNOS, locally elevates NO, and reduces peroxynitrite formation by competing with superoxide for nitrite. The increased NO S‑nitrosylates and activates eNOS promoter via cGMP‑PKG signaling, promoting BH4 synthesis and recoupling of eNOS. Simultaneously, NO‑mediated S‑nitrosylation of matrix metalloproteinase‑2 (MMP‑2) inhibits excessive collagen cross‑linking, thereby limiting extracellular matrix stiffening. Consequently, arterial pulse wave velocity (PWV) declines, breaking the feed‑forward loop between stiffness and eNOS inhibition.

Testable Predictions

1. In human endothelial cells exposed to oxidative stress (e.g., tert‑butyl hydroperoxide), supplementation with sodium nitrite (0.5 mM) will increase mitochondrial NO production measurable by mitochondrial‑targeted DAF‑FM fluorescence, an effect abolished by SIRT3 knockdown or the complex I inhibitor rotenone.
2. Mice with endothelial‑specific SIRT3 overexpression will exhibit lower carotid‑femoral PWV and higher plasma nitrite/nitrate ratios compared with wild‑type littermates after 12 months of aging, despite similar eNOS expression levels.
3. Pharmacologic inhibition of mitochondrial nitrite reduction (using mito‑TEMPO to scavenge matrix superoxide) will blunt the PWV‑lowering effect of nitrite supplementation in old mice, confirming that the benefit depends on mitochondrial NO rather than cytosolic pathways.
4. Circulating nitro‑tyrosine levels will inversely correlate with mitochondrial NO signals in peripheral blood mononuclear cells from a cohort of individuals stratified by cfPWV (>11 m/s vs ≤11 m/s), providing a translational biomarker.

Falsifiability If nitrite supplementation fails to raise mitochondrial NO or improve PWV in SIRT3‑deficient endothelium, or if mitochondrial NO scavengers do not affect stiffness despite nitrite treatment, the hypothesis would be refuted. Conversely, a positive outcome would support a novel mito‑nitrite‑SIRT3 axis as a therapeutic target to decouple arterial stiffness from endothelial senescence, addressing the open question of whether stiffness reduction improves clinical outcomes beyond CVD (1).