NOX4 promotes mucosal barrier injury in inflammatory bowel disease by mediating macrophages M1 polarization through ROS.

NADPH oxidase 4 (NOX4) plays an important role in transporting electrons in the mitochondrial respiratory chain, which is also one major source of ROS. This study investigates the mechanism by which NOX4 promotes the M1 polarization of intestinal macrophages in inflammatory bowel disease (IBD) through ROS. Dextran sulfate sodium (DSS) was used to induce the inflammatory bowel disease (IBD) in wild-type (C57BL/6N, WT) and NOX4 knockout (C57BL/6N-NOX4em1cyagen, KO) mice. Body weights of mice were dynamically monitored and the disease active index (DAI) scores were assessed. H&E staining was performed to examine pathological changes, and immunohistochemical (IHC) staining was conducted to measure the expressions of TJ proteins (ZO-1, Occludin) and CD11c. Tissue ROS labeling was accomplished with ROS probe. More ucosal permeability was assessed by FITC-D. Tissue inflammatory cytokines were detected by enzyme-linked immunosorbent assay (ELISA), while the expressions of TJ proteins (ZO-1, Occludin) were measured through Western Blotting. After NOX4 inhibitor pretreatment of intestinal macrophages in vitro, polarization was induced by lipopolysaccharide (LPS) and IFN-γ, followed by determination of polarization degree. The polarized intestinal macrophages were co-cultured with Caco-2 cells, and their effect on the monolayer cell permeability was evaluated. DSS can induce the intestinal inflammation and mucosal barrier injury in mice. Besides, it can enhance the FITC-D permeability, reduce the TJ protein levels, and promote the CD11c and ROS expressions. In KO mice, intestinal inflammation was alleviated and barrier permeability was reduced. Moreover, the TJ protein levels were higher than those of WT mice, while the CD11c and ROS were down-regulated. In WT mice, the intestinal inflammation and barrier permeability could also be reduced after treatment with NOX4 inhibitor. Overexpression of NOX4 in intestinal macrophages could promote the macrophage M1 polarization while improving the barrier integrity of Caco-2 monolayer cells. NOX4 is capable of promoting M1 polarization of intestinal macrophages through ROS, thereby further aggravating the intestinal inflammation and mucosal barrier injury in IBD. NOX4 has potential as a novel therapeutic target for IBD.