Smad7 interrupts TGF-β signaling in intestinal macrophages and promotes inflammatory activation of these cells during necrotizing enterocolitis

Abstract

Background Necrotizing enterocolitis (NEC) is an inflammatory bowel necrosis of premature infants. Based on our recent findings of increased Smad7 expression in surgically-resected bowel affected by NEC, we hypothesized that NEC macrophages undergo inflammatory activation because increased Smad7 expression renders these cells resistant to normal, gut-specific, transforming growth factor (TGF)-β-mediated suppression of inflammatory pathways. Methods We used surgically-resected human NEC tissue, murine models of NEC-like injury, bone marrow-derived and intestinal macrophages, and RAW264.7 cells. Smad7 and IκB kinase-beta (IKK-β) were measured by quantitative polymerase chain reaction (qPCR), Western blots, and immunohistochemistry. Promoter activation was confirmed in luciferase reporter and chromatin immunoprecipitation assays. Results NEC macrophages showed increased Smad7 expression, particularly in areas with severe tissue damage and high bacterial load. LPS-induced Smad7 expression suppressed TGF-β signaling and augmented NF-κB activation and cytokine production in macrophages. Smad7-mediated NF-κB activated was likely mediated via increased expression of IKK-β, which, further increased Smad7 expression in a feed-forward loop. We show that Smad7 induced IKK-β expression through direct binding to the IKK-β promoter and its transcriptional activation. Conclusions Smad7 expression in NEC macrophages interrupts TGF-β signaling and promotes NF-κB-mediated inflammatory signaling in these cells through increased expression of IKK-β.

Necrotizing enterocolitis: new insights into pathogenesis and mechanisms

“…96). On the other hand, macrophages present within NEC injury lesions are characterized by a highly inflammatory phenotype, resulting from increased expression of mothers against decapentaplegic homologue 7, an inhibitor of transforming growth factor β2 signalling 97,98 . Further studies have suggested a role for impaired Paneth cell function in the development of NEC 99 .…”

Trinitrobenzene sulfonic acid-induced intestinal injury in neonatal mice activates transcriptional networks similar to those seen in human necrotizing enterocolitis

“…Considering this developmental predilection, and because clinical antecedents of NEC seem too diverse, we have argued that the pathoanatomy of NEC represents a generic injury response of the intestine during a certain developmental epoch, rather than mirroring specific causal mechanism(s) (6, 7). To investigate this hypothesis, we induced inflammatory intestinal injury in 10-day-old murine pups by using 2, 4, 6-trinitrobenzene sulfonic acid (TNBS) as a non-specific immunological insult that causes mucosal injury in the presence of the intestinal microflora (6).…”

“…To investigate this hypothesis, we induced inflammatory intestinal injury in 10-day-old murine pups by using 2, 4, 6-trinitrobenzene sulfonic acid (TNBS) as a non-specific immunological insult that causes mucosal injury in the presence of the intestinal microflora (6). TNBS-induced murine neonatal enterocolitis was marked by monocyte/macrophage-rich infiltrates and extensive necrosis, and therefore, showed a strong histopathological resemblance to human NEC (6, 7). …”

Smad7 as a Target for Immunomodulation Strategy in Inflammatory Bowel Diseases

“…Studies in other systems have recently shown that expression of Smad7 in antigen presenting cells, such as dendritic cells and macrophages, can facilitate the progression of destructive inflammatory responses and induction of Smad7 seems to rely on bacterial-derived stimuli [47,48]. Therefore, experimental work is still needed to better characterize which cell types over-express Smad7 in the different phases of the natural history of CD and UC and which factors account for such an induction.…”