Hypothesis

Circulating indole-3-propionic acid (IPA) mediates the relationship between gut microbial fldC abundance and epigenetic age acceleration (GrimAge) through AhR‑driven hepatic NAD+ biosynthesis, with the mediation effect modulated by sex and APOE genotype.

Mechanistic rationale

IPA is a high‑affinity ligand for the aryl hydrocarbon receptor (AhR) and pregnane X receptor (PXR) [https://www.science.org/doi/10.1126/sciadv.adw8410]. Activation of AhR in hepatocytes up‑regulates the NAD+ salvage enzyme NAMPT, increasing intracellular NAD+ levels that support sirtuin‑dependent deacetylation of epigenetic regulators (e.g., SIRT1, SIRT6) [https://doi.org/10.1038/s41467-020-15119-w]. Higher NAD+ availability thus attenuates age‑related DNA methylation drift, lowering GrimAge acceleration. Conversely, low hepatic AhR signaling reduces NAD+, permitting heightened activity of DNA methyltransferases and accelerated epigenetic aging.

The gut bacterium Clostridium sporogenes produces IPA via the fldC enzyme; its abundance predicts serum IPA (r=0.80 in mice) [https://www.science.org/doi/10.1126/sciadv.adw8410]. In humans, we expect a similar correlation, linking microbiome composition to circulating IPA.

Sex differences arise because estrogen modulates AhR expression; males exhibit higher basal AhR activity in liver, making them more sensitive to IPA‑driven NAD+ boost, whereas females may rely more on estrogen‑mediated pathways, attenuating the mediation effect [https://pubmed.ncbi.nlm.nih.gov/41540161/]. APOE4 carriers show impaired hepatic lipid handling and heightened oxidative stress, which could blunt AhR‑NAMPT coupling, weakening the IPA‑mediated protection.

Testable predictions

1. In a diverse adult cohort (n ≥ 1,000) with matched serum IPA (LC‑MS/MS), shotgun metagenomic quantification of fldC, and GrimAge calculated from DNA methylation, serum IPA will positively correlate with fldC abundance (r > 0.6) and negatively correlate with GrimAge acceleration (β < 0).
2. Mediation analysis will show that the indirect effect of fldC on GrimAge via IPA accounts for ≥30 % of the total effect (bootstrap CI not crossing zero).
3. The indirect effect will be significantly stronger in males than females (interaction p < 0.05) and weaker in APOE4 carriers compared with non‑carriers (interaction p < 0.05).
4. Experimental manipulation: treating primary human hepatocytes with physiologic IPA (1–10 µM) will increase NAMPT mRNA and NAD+ levels in an AhR‑dependent manner (blocked by AhR antagonist CH‑223191) and reduce DNMT1 activity, measurable via fluorometric assay.

Falsifiability

If serum IPA shows no correlation with fldC abundance, or if IPA does not mediate the fldC‑GrimAge relationship after adjusting for confounders (BMI, diet, antibiotics), the hypothesis is refuted. Likewise, if AhR inhibition does not abolish IPA‑induced NAD+ elevation in hepatocytes, the proposed mechanistic link is invalid.

Relevance

Confirming IPA as a quantitative mediator would elevate it from an observational correlate to an actionable biomarker and therapeutic target, informing microbiome‑based interventions aimed at slowing epigenetic aging.