Active Autophagy Suppression as a Lysosomal Stress‑Feedback Loop in Aging

Hypothesis

Aged cells actively suppress autophagy to avert lysosomal overload when degradation capacity wanes. Persistent SASP signaling activates NF‑κB, which induces miR‑34a that represses TFEB, curtailing lysosomal biogenesis. The ensuing lysosomal stress triggers a compensatory rise in mTORC1 activity and Rubicon, blocking autophagosome formation and fusion. Thus autophagy inhibition is a calibrated protective response, not a mere failure.

Mechanistic Model

1. SASP → NF‑κB → miR‑34a: Senescent cells secrete IL‑6 and TNFα that engage JAK/STAT3 and NF‑κB pathways [6]. NF‑κB drives transcription of the microRNA miR‑34a.
2. miR‑34a ⟂ TFEB: miR‑34a binds the 3′‑UTR of TFEB mRNA, reducing its translation and lysosomal gene expression (LAMP1, cathepsins).
3. Lysosomal stress → mTORC1 ↑ & Rubicon ↑: Diminished lysosomal function is sensed via the mTORC1‑lysosome axis, reactivating mTORC1 even under low nutrients [2] and upregulating Rubicon, which stalls autophagosome‑lysosome fusion [3].
4. Feedback to SASP: Impaired autophagy elevates cytosolic DNA and ROS, reinforcing the senescent phenotype and SASP output [1], thereby locking the loop.

Testable Predictions

• Prediction 1: In aged tissues, TFEB protein levels will be inversely correlated with miR‑34a abundance and positively correlated with lysosomal markers (LAMP1, cathepsin B).
• Prediction 2: Pharmacological inhibition of miR‑34a (antagomiR) in old mice will restore TFEB activity, increase lysosomal biogenesis, and reduce mTORC1‑dependent phosphorylation of ULK1 despite the presence of senescent cells.
• Prediction 3: Inducing lysosomal damage (e.g., with Leu‑Leu‑OMe) in young fibroblasts will raise miR‑34a, suppress TFEB, and elevate Rubicon and p‑S6K, mimicking the aged autophagy‑suppressed state.
• Prediction 4: Clearing senescent cells with ABT‑263 will lower SASP, decrease miR‑34a, and rescue TFEB‑driven lysosomal gene expression, thereby reactivating flux even when mTORC1 remains active.

Experimental Approach

Measure: qPCR for miR‑34a, Western blot for TFEB, LAMP1, phospho‑ULK1 (Ser757) and phospho‑S6K; immunofluorescence for LC3‑II puncta with and without bafilomycin A1; flow cytometry for senescent markers (p16^INK4a^, SA‑β‑gal). Intervene: antagomiR‑34a, TFEB overexpression via AAV, lysosomal stabilizer (trehalose), senolytic ABT‑263. Readout: autophagic flux (LC3‑II turnover), lysosomal capacity (DQ‑BSA assay), senescence burden.

Potential Implications

If validated, this model reframes autophagy decline as a homeostatic safeguard rather than degenerative decay. Therapeutic strategies that solely activate autophagy (e.g., rapamycin) may overshoot lysosomal capacity and exacerbate damage; instead, combining lysosomal enhancement with SASP inhibition could re‑establish a balanced turnover in aged tissues.