E3 Ligase FBXW7 Promotes Inflammatory Bowel Disease By Regulating The Function Of Intestinal Macrophages

Chronic and progressive inflammation is the key pathogenesis of inflammatory boweldisease（IBD）including Crohn’s disease（CD）and ulcerative colitis（UC）,which involve dysregulation of the genetic,environmental,or microbial factors and immune responses.Macrophage-induced innate immune response within the intestinal mucosa is the first line of defense against the invading pathogens.Macrophages are abundant in the intestinal lamina propria,and mainly include two phenotypically distinct subsets,CX3CR1^hi resident macrophages and CX3CR1^intmonocyte-derived mononuclear phagocytes（MPhs）.CX3CR1^hi resident macrophages are leading players in the maintenance of intestinal homeostasis and immune tolerance.CX3CR1^intMPhs which play an important role in secreting inflammatory factors and proinflammatory mediators,mainly derived from Ly6C^hi monocytes under inflammatory circumstances and involved in the regulation of inflammation and infections.CX3CR1^int MPhs enhances IBD progression by promoting the secretion of inflammatory cytokines and mediators in inflammatory microenvironment.Resident macrophages and inflammatory mononuclear phagocytes（MPhs）as distinct subsets with functional plasticity in the intestine are critically involved in the pathology of inflammatory bowel diseases（IBDs）,the mechanism of which remains incompletely understood.F-box and WD repeat domain-containing 7（FBXW7）is a component of the SKP1,CUL1,and F-box protein type ubiquitin ligase（SCF）complex.Fbxw7 mutations have been identified in various types of cancers.FBXW7 is also reported to regulate lipid metabolism,target osteogenic and chondrogenic transcriptional factors,interact with parkin,and play important roles in Parkinson’s disease.Our previous study showed that FBXW7 is critical for promoting innate antiviral immunity by mediating the ubiquitination of SHP2.However,the function and mechanisms of FBXW7 in inflammation responses have not been clarified.In the present study,we explored the role of FBXW7 in macrophages in regulating the inflammatory bowel disease.In this study,we found that FBXW7 expression was markedly increased in inflamedintestinal tissues in the peripheral blood monocytes from patients with UC or CD.Fbxw7 deficiency in myeloid cells reduced inflammation and disease severity in the colitis mouse model induced by dextran sodium sulfate（DSS）or 2,6,4-trinitrobenzene sulfonic acid（TNBS）,showed significantly alleviated body weight loss,diarrhea,and rectal bleeding.Moreover,myeloid FBXW7 deficiency mice(Lys M⁺Fbxw7^f/f)displayed significantly longer colons and improved survival rates compared with littermate control(Fbxw7^f/f).Markedly decreased expression of CCL2 and CCL7 in CX3CR1^hi macrophages from Lys M⁺Fbxw7^f/f colitis mice revealed by transcriptome sequencing.Furthermore,Fbxw7 deficiency resulted in the reduced accumulation of CX3CR1^int proinflammatory MPhs in colitis-affected colon tissue.The protective effects in myeloid Fbxw7 deficient mice disappears after depletion of macrophages in intestinal tissue.Mice that received adeno-associated virus-sh FBXW7（AAV-sh FBXW7）showed significantly improved survival rates and alleviation of colitis induced by DSS or spontaneous colitis mice with Il10 gene knockout,further confirming the effect of FBXW7 on promoting intestinal inflammation.To clarify the mechanism involved in the regulation of colitis by FBXW7,we demonstrated that FBXW7 suppressed H3K27me3 modification on the promoters of the chemokines Ccl2 and Ccl7,and promoted CCL2 and CCL7 expression in colonic lamina propria（CLP）macrophages via ubiquitination and degradation of the histone-lysine N-methyltransferase enhancer of zeste homolog 2（EZH2）.These results proved that FBXW7 positively regulates the progression of Inflammatory bowel disease through an epigenetic mechanism.In summary,we identified a critical role of FBXW7 in the regulation of colitis byinducing Ccl2 and Ccl7 expression in resident macrophages and promoting the accumulation of proinflammatory MPhs.FBXW7/EZH2/CCL2/CCL7 is an important pathway for shaping the status and activity of colon-resident macrophages and inflammatory MPhs in the colitis setting.Our results indicate that FBXW7 may be a predictive marker of IBD severity and targeting FBXW7 might represent a potential therapeutic approach for the treatment of intestinal inflammation.