A gut microbiota–derived metabolite may contribute to the progression of primary sclerosing cholangitis by promoting bile duct inflammation and fibrosis. Higher circulating levels of imidazole propionate were associated with poorer transplant-free survival, while experimental studies identified a signaling pathway through which the metabolite may promote biliary injury.
Investigators combined human metabolomic analyses with microbiome profiling and experimental studies in patient-derived cholangiocytes and mice. The discovery analysis included plasma samples from 132 patients with primary sclerosing cholangitis (PSC) prior to liver transplantation, 29 patients following transplantation, and 32 healthy controls. Findings were subsequently evaluated using targeted metabolomics in independent Norwegian cohorts and external cohorts from Sweden and the US. A longitudinal cohort included 567 samples from 158 patients sampled prior to and following liver transplantation. The investigators evaluated microbial metabolites associated with PSC, transplant-free survival, and persistence following transplantation, as well as recurrent PSC and experimental measures of cholangiocyte activation, inflammation, fibrosis, and signaling.
Of 1,108 metabolites evaluated in the discovery analysis, 261 were associated with PSC after adjustment for potential confounders. Of these, 38 were also associated with transplant-free survival after adjustment for sex and disease severity, and 9 remained candidates after authors restricted the analysis to metabolites whose concentrations did not return to healthy-control levels following liver transplantation. Imidazole propionate (ImP) emerged as the most consistently selected candidate during bootstrap validation.
Targeted analyses confirmed higher circulating ImP concentrations among patients with PSC, with the finding replicated across independent cohorts. Higher ImP was consistently associated with reduced transplant-free survival across all 4 cross-sectional PSC cohorts. The association was most apparent among patients with ImP concentrations in the highest tertile.
In the longitudinal transplant cohort, ImP concentrations remained elevated following liver transplantation rather than returning toward concentrations observed in healthy controls. Higher concentrations were also associated with recurrent PSC. Each doubling of ImP was associated with an 18% higher hazard of recurrence, while concentrations above the median were associated with nearly twice the hazard of recurrent disease.
ImP elevations were more pronounced in PSC earlier in the disease course. Median serum concentrations were 19.1 nM among patients in the pooled Norwegian PSC cohort compared with 10.5 nM among patients with inflammatory bowel disease without PSC. Concentrations were also lower among patients with primary biliary cholangitis and metabolic dysfunction–associated steatotic liver disease. However, ImP concentrations converged across liver diseases at advanced stages, suggesting that factors associated with advanced liver disease may also influence circulating concentrations. ImP was also associated with lower gut microbial diversity among patients with PSC.
Experimental studies provided evidence for a potential mechanism underlying these clinical associations. Exposure to ImP increased proinflammatory and profibrotic factors in cholangiocyte organoids derived from patients with PSC. In mice, chronic ImP exposure increased portal inflammation, liver fibrosis, and ductular reaction. Investigators also identified mammalian target of rapamycin complex 1 signaling downstream of p38 as part of the pathway linking ImP exposure with biliary injury.
Genetic deletion of p38 gamma and p38 delta further supported the proposed mechanism. In mice lacking these proteins, ImP did not increase mammalian target of rapamycin complex 1 activation or worsen fibrosis, inflammation, or ductular reaction, indicating that the observed ImP-driven biliary injury in the mouse models depended on p38 gamma and p38 delta and functional mammalian target of rapamycin complex 1 signaling.
Several limitations temper the findings. The human analyses were observational and therefore cannot establish that ImP causes PSC progression. The authors characterized the histologic effect observed in the mouse models as modest, and the ImP concentrations used in vitro were high and not directly translatable to human exposure. They also noted that PSC develops over decades and likely reflects interactions among multiple genetic, environmental, and immune factors, suggesting that chronic ImP exposure may act in concert with other contributors rather than independently.
Taken together, the findings suggest that increased gut microbiota–derived ImP and subsequent p38 signaling may represent 1 pathway linking gut dysbiosis with biliary injury in PSC and could provide targets for future therapeutic investigation.
“We have identified ImP as a circulating metabolite associated with PSC transplant-free survival,” wrote lead study authors Antonio Molinaro, of the Wallenberg Laboratory at the University of Gothenburg, and colleagues. “By demonstrating that ImP can activate cholangiocytes ex vivo and in vivo and elucidating the molecular pathways involved, we highlight that the development of treatments that can reduce production of ImP or inhibit ImP-induced cellular signalling may represent an important step forward in this orphan disease.”
Disclosures: Fredrik Bäckhed reported being a cofounder and shareholder of Implexion Pharma and Roxbiosens and receiving research funding from BioGaia. Antonio Molinaro, Alba Carreras, and Katharina R. Beck reported shares in Implexion Pharma. The other authors reported no competing interests.
Source: Nature Metabolism