Hypothesis

Aging‑associated expansion of the BSH‑T3 phylotype in males increases 7α‑dehydroxylation of chenodeoxycholic acid to lithocholic acid (LCA), which acts as a selective FXR antagonist in hepatocytes. This antagonism amplifies inflammation and OXPHOS decline, whereas in females the dominant BSH‑T1 maintains deconjugation without generating antagonistic secondary BAs, preserving FXR signaling.

Mechanistic Rationale

• BSH‑T3 exhibits higher activity toward taurine‑conjugated chenodeoxycholic acid (T‑CDCA) compared with other phylotypes, favoring release of free CDCA that is subsequently 7α‑dehydroxylated by gut anaerobes to LCA.[1]
• LCA binds FXR with low affinity but recruits corepressors (NCOR1, SMRT) that blunt transcriptional activation of FXR targets such as Shp and Fgf15.[2]
• Male aging microbiota shows a bloom of Blautia and Roseburia that harbor the T3 gene cluster, while female aging microbiota favors Bifidobacterium expressing T1, which preferentially deconjugates glyco‑cholic acids, yielding less lithogenic substrates.[1]
• The resulting hepatic FXR antagonism reduces PGC‑1α expression, compromising mitochondrial OXPHOS and promoting ROS‑driven NF‑κB activation.

Testable Predictions

1. In old male mice, fecal LCA concentrations will correlate positively with hepatic FXR antagonist activity and inversely with Shp mRNA levels; this relationship will be absent in old females.
2. Selective suppression of BSH‑T3 using CRISPR‑phage or a T3‑specific inhibitor will lower fecal LCA, restore FXR target expression, and improve mitochondrial respiration in aged male livers.
3. Transplantation of aged male microbiota into germ‑free young recipients will recapitulate the LCA‑FXR antagonism phenotype only when the donor community retains a high T3 abundance.
4. In humans, serum LCA/CDCA ratio will be higher in aging men with elevated hepatic inflammation scores and will predict progression from NAFLD to HCC independently of traditional risk factors.

Experimental Design

• Mouse cohort: C57BL/6J males and females, 20 months old; groups: (i) untreated, (ii) BSH‑T3 knockdown via oral delivery of phagemid targeting t3 gene, (iii) control phagemid.
• Readouts: fecal BA profiling (LC‑MS/MS), hepatic FXR antagonist assay (reporter hepatoma cells), qPCR of Shp, Fgf15, Pgc‑1α, mitochondrial respiration (Seahorse), histology for inflammation and fibrosis.
• Human validation: Analyze existing metagenomic and serum BA datasets from the AGING‑LIVER consortium; correlate BSH‑T3 abundance (metagenomic reads per kilobase per million) with LCA/CDCA ratio, serum ALT/AST, and histologic NAFLD activity score.

If BSH‑T3–driven LCA production is indeed a sex‑specific FXR antagonistic mechanism, targeting this node should decouple microbiota aging from hepatic metabolic decline, offering a precision‑biotic strategy to avert NAFLD‑to‑HCC progression in aging males.