Vitamin D deficiency exacerbates alcohol-related liver injury via gut barrier disruption and hepatic overload of endotoxin

Endogenous lipopolysaccharide (LPS) that translocates via the disrupted intestinal barrier plays an essential role in the progression of alcohol-related liver disease (ALD). Vitamin D deficiency is observed in ALD, and it participates in regulating gut barrier function. The current study aimed to examine the association between vitamin D deficiency and endotoxemia in patients with ALD-related cirrhosis. Moreover, the effect of vitamin D deficiency on ethanol (EtOH)- and carbon tetrachloride (CCl₄)-induced liver injury relevant to gut barrier disruption in mice was investigated. Patients with ALD-related cirrhosis (Child-Pugh Class A/B/C; n = 56/15/7) had lower 25(OH)D levels and higher endotoxin activities than non-drinking healthy controls (n = 19). The serum 25(OH)D levels were found to be negatively correlated with endotoxin activity (R = -0.481, p < 0.0001). The EtOH/CCl₄-treated mice developed hepatic inflammation and fibrosis, which were significantly enhanced by vitamin D-deficient diet. Vitamin D deficiency enhanced gut hyperpermeability by inhibiting the intestinal expressions of tight junction proteins including ZO-1, occludin, and claudin-2/5/12/15 in the EtOH/CCl₄-treated mice. Consequently, it promoted the accumulation of lipid peroxidases, increased the expression of NADPH oxidases, and induced Kupffer cell infiltration and LPS/Toll-like Receptor 4 signaling-mediated proinflammatory response. Based on the in vitro assay, vitamin D-mediated vitamin D receptor activation inhibited EtOH-stimulated paracellular permeability and the downregulation of tight junction proteins via the upregulation of caudal-type homeobox 1 in Caco-2 cells. Hence, vitamin D deficiency exacerbates the pathogenesis of ALD via gut barrier disruption and hepatic overload of LPS.