Hypothesis

Indole-3-propionic acid (IPA) shifts autophagic substrate selection from lipid droplets to mitochondria, turning a siege‑like rationing response into a targeted quality‑control upgrade.

Mechanistic rationale

• IPA activates the aryl hydrocarbon receptor (AhR) in intestinal and peripheral cells.
• AhR signaling promotes phosphorylation of ULK1 at Ser467 (a site distinct from the canonical AMPK/mTOR sites) via Src family kinases.
• This modification changes the affinity of the ULK1‑ATG13‑FIP200 complex for autophagy receptors, favoring binding to NIX/BNIP3L over p62.
• Consequently, mitophagy is preferentially induced while lipophagy is restrained, even when cellular ATP is low.

Testable predictions

1. In cultured hepatocytes treated with IPA under starvation, the ratio of mito‑Keima (mitophagy) to BODIPY‑lipid droplet (lipophagy) flux will increase ≥2‑fold compared with untreated controls.
2. AhR knockout or ULK1‑S467A mutant cells will fail to show this shift, confirming the pathway.
3. Feeding IPA to aged mice will raise mitophagy markers (e.g., PINK1‑Parkin colocalization) in liver and muscle without altering total autophagic flux measured by LC3‑II turnover.
4. Lifespan extension observed with IPA supplementation will be attenuated in mice with liver‑specific Atg7 deletion, linking the phenotype to altered autophagic substrate choice.

Experimental outline

• Cell model: HepG2 cells cultured in EBSS ± 10 µM IPA, with or without AhR antagonist CH‑223191.
• Readouts: mt‑Keima Red/Green ratio, BODIPY‑493/503 lipid droplet imaging, Seahorse OCR, immunoblot for phospho‑ULK1 (Ser467) and LC3‑II.
• In vivo: C57BL/6J mice 20 months old receive IPA (10 mg/kg/day) or vehicle for 12 weeks; tissues harvested for mitophagy (mito‑Keima transgenic reporters) and lipid droplet quantification (Oil Red O).
• Longevity cohort: Parallel survival study; additional groups with liver‑specific Atg7 KO to test dependence.

If IPA redirects autophagy toward mitochondrial quality control rather than bulk lipid catabolism, it would demonstrate that the 'siege' response can be rewired to improve cellular fitness, challenging the view that autophagy activation merely prolongs survival under scarcity.