Hypothesis

Core idea: Age‑associated loss of BSH‑T3 activity is driven by oxidative post‑translational modification (e.g., S‑glutathionylation) of the enzyme in low‑abundance anaerobes, which reduces secondary bile acid synthesis and increases hepatic primary bile acids (TCDCA/GCDCA). These accumulated primary bile acids act as endogenous agonists of the G‑protein coupled bile acid receptor TGR5 on Kupffer cells, triggering NLRP3 inflammasome activation and NF‑κB‑mediated inflammation. The resulting inflammatory milieu suppresses hepatic FXR signaling through phosphorylation of FXR (Serine‑273) and induction of the corepressor SHP, creating a feed‑forward loop that exacerbates the primary‑to‑secondary bile acid shift and impairs hepatocyte regeneration.

Sex‑specific modulation: Estradiol enhances hepatic antioxidant capacity (via Nrf2) in young females, protecting BSH‑T3 from oxidation; with menopause, declining estradiol lifts this protection, leading to a sharper rise in oxidized BSH‑T3 and thus a greater hepatic primary bile acid burden in aged females, consistent with the observed sex bias.

Testable predictions:

1. In aged mouse livers, BSH‑T3 protein from fecal microbiota will show higher levels of S‑glutathionylation (detected by biotin‑switch assay) compared with young mice, without changes in bsh‑T3 mRNA abundance.
2. Pharmacological inhibition of TGR5 (e.g., with SBI‑115) or scavenging of hepatic ROS (N‑acetylcysteine) in aged mice will restore secondary bile acid ratios, decrease Kupffer cell inflammasome activation (reduced caspase‑1 cleavage, IL‑1β), and reactivate FXR target genes (Foxm1b, Shp).
3. Ovariectomized young female mice treated with estradiol will exhibit lower BSH‑T3 oxidation and higher secondary bile acids than vehicle‑treated ovariectomized controls, mimicking the protective effect seen in intact young females.
4. Conversely, male mice receiving chronic low‑dose estradiol will show reduced BSH‑T3 oxidation and improved FXR signaling, indicating that the sex difference is hormonally mediated rather than chromosomal.

Falsifiability: If BSH‑T3 oxidation does not increase with age, or if modulating TGR5/ROS fails to alter bile acid composition or FXR activity despite confirmed target engagement, the hypothesis would be refuted. Similarly, if estradiol manipulation does not affect BSH‑T3 oxidation status in females, the sex‑specific mechanism would be unsupported.

Implications: This positions oxidative enzyme inactivation as a mechanistic bridge between microbiome dysfunction, bile acid signaling, and inflammatory suppression of FXR, suggesting that combinatorial therapies targeting microbial enzyme redox state (e.g., microbiome‑directed antioxidants) and hepatic inflammatory pathways could restore FXR‑dependent regeneration without the metabolic side effects seen with chronic FXR agonism.

References

• [1] https://www.tandfonline.com/doi/full/10.1080/19490976.2020.1763770
• [2] https://pubs.acs.org/doi/10.1021/acscentsci.9b00147
• [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC3033699/
• [4] https://pubmed.ncbi.nlm.nih.gov/36803569/
• [5] https://pmc.ncbi.nlm.nih.gov/articles/PMC8156739/
• [6] https://pmc.ncbi.nlm.nih.gov/articles/PMC6649552/