Hypothesis

In senescent endothelial cells, selective autophagy cargo receptors do not lose function broadly; instead, they undergo a re‑programming of substrate priority that spares oxidized eNOS complexes while preferentially degrading damaged mitochondria. This selective failure allows oxidized eNOS to accumulate, activate Src‑Akt signaling, and amplify ICAM‑1‑mediated inflammation.

Rationale

• Autophagy receptors (p62/SQSTM1, NBR1, OPTN, NDP52) exhibit ligand‑specific affinities that can be shifted by post‑translational modifications (e.g., p62 phosphorylation at Ser403)【p62 oligomerization enhances cargo targeting】.
• Senescent endothelium shows eNOS uncoupling via arginase‑I‑mediated L‑arginine depletion, generating oxidized eNOS that becomes ubiquitinated【eNOS uncoupling via arginase‑I】.
• Oxidized eNOS, when not cleared, can directly stimulate Src‑Akt pathways that increase ICAM‑1 adhesiveness【Src‑mediated eNOS‑ICAM-1 link】.
• Mitophagy is impaired in senescence, yet mitochondria are still turned over, suggesting a receptor‑driven choice rather than a global block【impared mitophagy in senescence】.

Thus, the hierarchy of cargo selection—not the capacity of the autophagic machinery—may dictate which toxic species accumulate and trigger the inflammatory phenotype.

Novel Mechanistic Insight

We propose that p62 acts as a molecular switch: under basal conditions, its ubiquitin‑associated (UBA) domain binds oxidized eNOS with high affinity, targeting it for autophagic degradation. In senescence, increased ROS activates kinases (e.g., TBK1) that phosphorylate p62 at Ser403, altering its conformation to favor LC3‑interacting region (LIR) exposure and strengthening its interaction with ubiquitinated mitochondrial proteins (e.g., PINK1/Parkin substrates). Consequently, p62 is sequestered toward mitophagy, leaving oxidized eNOS to escape degradation. The accumulating oxidized eNOS then acts as a scaffold for Src activation, propagating ICAM‑1 expression and endothelial leukocyte adhesion.

Testable Predictions

1. p62 phosphorylation state correlates with cargo preference – phospho‑deficient (S403A) p62 will preferentially bind oxidized eNOS, while phospho‑mimetic (S403D/E) p62 will show increased binding to mitochondrial ubiquitinated proteins.
2. Restoring oxidized eNOS clearance rescues the inflammatory phenotype – overexpression of S403A p62 in senescent endothelial cells will reduce oxidized eNOS levels, decrease Src‑Akt activity, and lower ICAM‑1 expression without globally altering autophagy flux.
3. Blocking p62‑mitochondria interaction exacerbates senescence – expression of a p62 LIR mutant that cannot bind LC3 will shift p62 back toward oxidized eNOS, reducing mitochondrial clearance and increasing ROS, but will not affect ICAM‑1 if oxidized eNOS is the dominant driver.
4. Pharmacological modulation of p62 oligomerization alters cargo hierarchy – small molecules that promote p62 oligomerization (e.g., the compound described in [4]) will enhance overall cargo targeting; however, if the senescence‑induced phosphorylation bias persists, oxidized eNOS will still be poorly cleared, indicating that oligomerization alone is insufficient to reset hierarchy.

Experimental Approach (brief)

• Cell model: Human umbilical vein endothelial cells (HUVECs) induced to senesce via repeated passaging or oxidative stress (H₂O₂).
• Manipulations: CRISPR‑knock‑in of p62 S403A/D, siRNA against p62, overexpression of WT or mutant p62, treatment with p62 oligomerizer.
• Readouts:
  • Co‑immunoprecipitation and quantitative mass spectrometry to assess p62 binding partners (oxidized eNOS vs mitochondrial ubiquitinated proteins).
  • Autophagy flux (LC3‑II/I, p62 turnover) via immunoblotting and mRFP‑GFP‑LC3 assay.
  • Oxidized eNOS levels (biotin‑switch assay), Src‑Akt phosphorylation (p‑Src, p‑Akt), ICAM‑1 surface expression (flow cytometry).
  • Functional assays: leukocyte adhesion under flow, NO production (DAF‑FM), mitochondrial ROS (MitoSOX).
• Falsification: If altering p62 phosphorylation or its cargo bias does not change oxidized eNOS levels or ICAM‑1 expression (while autophagy flux remains unchanged), the hypothesis that selective cargo prioritization drives senescence‑associated inflammation is falsified.

Implications

Confirming this hierarchy‑centric model would reposition selective autophagy receptors as signaling nodes that integrate metabolic stress with inflammatory outputs, opening therapeutic avenues to modulate receptor specificity (e.g., phospho‑specific p62 inhibitors) rather than attempting global autophagy activation in vascular aging.