Melatonin Ameliorates Cadmium‐Induced Liver Fibrosis Via Modulating Gut Microbiota and Bile Acid Metabolism

ABSTRACT Cadmium (Cd) is a widespread environmental contaminant with high toxicity to human health. Melatonin has been shown to improve Cd‐induced liver damage. However, its mechanism has not yet been elucidated. In this study, we aimed to investigate the effects of melatonin on Cd‐induced liver damage and fibrosis. A combination of 16S rRNA gene sequencing and mass spectrometry‐based metabolomics was adopted to investigate changes in the gut microbiome and its metabolites on the regulation of melatonin in Cd‐induced liver injury and fibrosis of mice. Further, nonabsorbable antibiotics, a fecal microbiota transplantation (FMT) program and intestine‐specific farnesoid X receptor (FXR) knockout mice were employed to explore the mechanism of melatonin (MT) on liver injury and fibrosis in Cd treated mice. MT significantly improved hepatic inflammation, bile duct hyperplasia, liver damage, and liver fibrosis, with a notable decrease in liver bile acid levels in Cd‐exposed mice. MT treatment remodeled the gut microbiota, improved gut barrier function, and reduced the production of gut‐derived lipopolysaccharide (LPS). MT significantly decreased the intestinal tauro‐β‐muricholic acid levels, which are known as FXR antagonists. Notably, MT prominently activated the intestinal FXR signaling, subsequently inhibiting liver bile acid synthesis and decreasing hepatic inflammation in Cd‐exposed mice. However, MT could not ameliorate Cd‐induced liver damage and fibrosis in Abx‐treated mice. Conversely, MT still exerted a protective effect on Cd‐induced liver damage and fibrosis in FMT mice. Interestingly, MT failed to reverse liver damage and fibrosis in Cd‐exposed intestinal epithelial cell‐specific FXR gene knockout mice, indicating that intestinal FXR signaling mediated the protective effect of MT treatment. MT improves Cd‐induced liver damage and fibrosis through reshaping the intestinal flora, activating the intestinal FXR‐mediated suppression of liver bile acid synthesis and reducing LPS leakage in mice.