Hypothesis

Senescent cells export a soluble SASP component that directly activates mTORC1 in neighboring non‑senescent cells, thereby suppressing autophagy beyond the senescent compartment and establishing a self‑propagating suppression field that explains the systemic decline of autophagy in aged tissues.

Mechanistic Basis

• Senescent cells exhibit chronic mTORC1 hyperactivity that sustains SASP production via NF‑κB signaling [3].
• Recent data show that certain SASP factors (e.g., IL‑6, IGF‑BP7) can engage receptors on nearby cells and trigger PI3K‑Akt‑mTORC1 signaling [4].
• mTORC1 activation in recipient cells phosphorylates ULK1/ATG13 and retains TFEB in the cytoplasm, blocking autophagosome formation and lysosomal biogenesis [1].
• This creates a paracrine loop where autophagy suppression in bystander cells raises intracellular amino acids (through basal autophagy of their own proteins), further fueling mTORC1 activity and making the signal starvation‑insensitive.

Novel Insight

The field hypothesis posits that the SASP is not merely a biomarker of senescence but an active signaling agent that reprograms nutrient‑sensing pathways in adjacent cells. This extends the known intracellular autophagy‑SASP feedback [2, 5] to a tissue‑level communication network.

Testable Predictions

1. Conditioned medium transfer: Media from cultured senescent fibroblasts will increase phospho‑S6K (mTORC1 read‑out) and decrease LC3‑II/I ratio in naïve epithelial cells; effects will be blocked by neutralizing antibodies against IL‑6 or IGF‑BP7.
2. In vivo spatial correlation: In aged mouse liver, regions with high SASP factor concentration (detected by immunofluorescence) will show reduced TFEB nuclear localization and lower autophagic flux (measured by mCherry‑GFP‑LC3 reporter) in nearby CD‑negative hepatocytes.
3. Genetic interruption: Mice with hepatocyte‑specific Raptor knockout will resist SASP‑mediated autophagy suppression, exhibiting higher lysosomal activity and lower inflammatory markers despite normal senescent cell burden.
4. Pharmacological rescue: Treatment of aged tissue explants with rapamycin or an IL‑6R antibody will restore autophagic flux in non‑senescent cells without clearing senescent cells.

Experimental Design (outline)

• In vitro: Isolate senescent human fibroblasts (irradiation‑induced), collect conditioned medium, apply to primary kidney epithelial cells. Measure p‑S6K, nuclear TFEB, LC3 turnover, and amino‑acid levels. Use siRNA against IL‑6R and IGF‑BP7R to dissect contributions.
• In vivo: Employ p16‑3MR mice to label senescent cells; inject AAV‑mCherry‑GFP‑LC3 to monitor flux. Perform immunofluorescence for SASP factors and p‑S6K at varying distances from p16‑positive cells. Treat cohorts with rapamycin or anti‑IL‑6R and quantify flux changes.
• Readouts: Western blot, flow cytometry for lysosomal markers (LAMP1), Seahorse assay for mitochondrial function, ELISA for SASP cytokines, and histological scoring of tissue integrity.

Implications

If validated, this hypothesis would redefine autophagy decline in aging as a communicable, signal‑driven process rather than a cell‑autonomous failure. It suggests that targeting SASP‑mTORC1 paracrine signaling could rejuvenate tissue‑wide autophagy, potentially delaying multiple age‑related pathologies without needing to eliminate senescent cells.