Protocatechuic Acid as a Phenolic Intermediate to Ameliorate Non-alcoholic Fatty Liver Disease by Inhibiting Enterococcus Faecalis in Mice

Abstract

Objective: To assess the protective effects and mechanisms of cyanidin 3-glucoside (C3G) and its major phenolic metabolites including protocatechuic acid (PCA), phloroglucinaldehyde (PGA), vanillic acid (VA) and ferulic acid (FA) on non-alcoholic fatty liver disease (NAFLD). Design: First, C57BL/6J mice were fed a low-fat diet (LFD), high-fat diet (HFD) only, or HFD containing 0.4% C3G, PCA, PGA, VA or FA for 12 weeks to understand their protective effects against NAFLD. Next, the dose-dependent effect and mechanism of PCA against NAFLD were studied in a mouse model by supplementing 0.025% or 0.1% PCA in HFD. Eventually, the bacteriostatic effect of PCA on Enterococcus faecalis was confirmed in vitro, and the role of Enterococcus faecalis in NAFLD was discovered by fecal microbiota transplantation (FMT) experiment.Results: As the major phenolic metabolite of C3G, PCA dose-dependently reduced intraperitoneal and hepatic fat deposition, with alleviated insulin resistance, hepatic inflammation and lipid peroxidation. Characterization of gut microbiota indicated that PCA decreased the Firmicutes/Bacteroidetes ratio mainly by reducing the relative abundance of Roseburia, Intestinibacter and Enterococcus. Lipidomics and correlation analysis revealed that Enterococcus had a significant correlation with the changed lipid metabolites by PCA. In vitro bacteriostatic experiment showed that PCA has a direct inhibitory effect against Enterococcus faecalis, which has been proved to accelerate NAFLD by fecal microbiota transplantation.Conclusion: This study demonstrated the first time that PCA can protect against NAFLD through inhibiting Enterococcus faecalis.