Identification of gut bacterial factors linked to altered host susceptibility to acetaminophen-induced liver injury.

Acetaminophen (APAP) overdose causes acute liver injury. We previously demonstrated that differential gut microbiota modulates the host susceptibility to APAP-induced liver injury in mice and subsequently identified 16 metabolites associated with altered susceptibility to APAP hepatotoxicity. This study aims to examine whether these metabolites modulate APAP hepatotoxicity. In primary mouse hepatocytes cultured in high-galactose medium, phenylpropionic acid (PPA) and 3-(4-hydroxyphenyl)-propionic acid (HPPA) significantly decreased APAP-induced cell necrosis whereas 5-aminovaleric acid (5-AV) worsens it. In mice, PPA (in drinking water at 4 % w/v) for four weeks and HPPA (intraperitoneal injection, 50 mg/kg 2 hours prior to APAP) significantly alleviated the APAP-induced liver injury. PPA, HPPA and 5-AV were rarely detected, if any, in the tissue (portal vein serum or liver) of germ-free mice, suggesting that that gut microbial production is a major determinant of host exposure to these metabolites. The biosynthetic pathways for these metabolites have been reported in bacteria. Indeed, mouse cecal bacteria readily converted L-phenylalanine, L-tyrosine and L-proline to PPA, HPPA and 5-AV, respectively. Bacterial conversion of L-Phe to PPA is known to be mediated by the phenylalanine reductive metabolism pathway (called "Fld pathway") or phenylalanine ammonia lyase. Examination of the deuterium-labeled metabolites (d -PPA and d -PPA; produced via Fld pathway and phenylalanine ammonia lyase, respectively) after incubating mouse cecal bacteria with deuterium-labeled L-Phe (d -Phe) revealed that Fld pathway is the main route of PPA production from L-Phe in mouse cecum. Interestingly, previously reported bacterial genes in Fld pathway were not detected in the mouse cecum. Also, the in vitro rate of d -PPA production by the cecal bacteria did not correlate with the differential levels of basal PPA levels in mouse cecum contents. The bacterial genes encoding the Fld enzymes as well as factors modulating its activities are remained to be identified. Taken together, this study presents multiple gut bacterial factors that are involved in gut microbiota's modulation on APAP-induced hepatotoxicity. Our next step is to study the underlying mechanisms for altered susceptibility to APAP hepatotoxicity by these metabolites.