Hypothesis

Gut‑derived indole‑3‑propionic acid (IPA) acts as a microbial metabolite that tunes the autophagic rationing system, redirecting flux toward specific metabolites that sustain redox balance and mitochondrial function during prolonged nutrient siege.

Mechanistic Rationale

• IPA is a high‑affinity ligand for the aryl hydrocarbon receptor (AhR) AhR activation by indoles.
• AhR signaling can increase intracellular NAD⁺ via induction of NAMPT, thereby activating SIRT1 deacetylase activity SIRT1‑NAD⁺ link.
• SIRT1 deacetylates and activates AMPK while simultaneously inhibiting mTORC1 through TSC2 phosphorylation AMPK/mTORC1 cross‑talk.
• Activated AMPK phosphorylates ULK1, initiating autophagy, but also phosphorylates specific autophagy receptors (e.g., p62, NIX) that favor selective turnover of damaged mitochondria and glycolytic enzymes selective autophagy substrates.
• Consequently, IPA‑driven AhR‑SIRT1‑AMPK axis biases autophagic recycling toward NADH‑producing substrates (e.g., glutamate, aspartate) and away from bulk protein, optimizing the siege economy for redox homeostasis.

Testable Predictions

1. IPA treatment will increase autophagic flux (LC3‑II turnover) in wild‑type cells but not in AhR‑knockout or SIRT1‑deficient lines.
2. Stable‑isotope tracing (¹³C‑glucose, ¹⁵N‑glutamine) will show a higher proportion of label in TCA intermediates and glutathione when IPA is present, indicating preferential recycling of redox‑supporting metabolites.
3. Mitochondrial ROS production will decrease under starvation plus IPA, correlating with enhanced mitophagy (mt‑Keima assay).
4. Rescue experiments: Adding an AMPK inhibitor (Compound C) will abolish IPA‑induced shifts in metabolite labeling, confirming AMPK as the downstream effector.

Experimental Design

• Cell models: HepG2 and primary hepatocytes; parallel lines with CRISPR‑mediated AhR or SIRT1 knockout.
• Conditions: EBSS starvation ± 10 µM IPA (physiological colonic concentration) for 4 h; include bafilomycin A1 to block lysosomal degradation for flux measurement.
• Readouts: Western blot for LC3‑II/p62, mt‑Keima flow cytometry for mitophagy, Seahorse OCR/ECAR for mitochondrial function, GSH/GSSG ratio, and LC‑MS/MS metabolomics of intracellular metabolites with isotope labeling.
• Analysis: Compare flux and labeling patterns between genotypes and treatments; statistical significance set at p < 0.05 (ANOVA with Tukey post‑hoc).

If IPA fails to alter autophagic selectivity or metabolite prioritization in AhR‑ or SIRT1‑deficient cells, the hypothesis is falsified. Conversely, a consistent shift toward redox‑supporting recycling would support the notion that microbial metabolites directly engage the siege‑rationing machinery, extending autophagy beyond mere housekeeping to a microbiota‑modulated metabolic triage system.