Selenium modulation of gut epithelial cell stress responses

Abstract

Selenium (Se) is an essential micronutrient necessary for human health. In humans, Se de ciency has been associated with in ammatory bowel disease (IBD) and increased risk of certain cancers, including colorectal cancer. Se has well established antioxidant and anti-in ammatory properties which are medi- ated, in part, though the actions of the selenoproteins, in which Se is present in the form of the amino acid selenocysteine (Sec). The cells of the gastrointestinal tract are exposed to stresses from pro-oxidative and hypoxic conditions, which have been suggested to be involved in the pathogenesis and pathology of IBD. Further characteristics of IBD are inappropriate immune responses of the gut epithelial cells to the gut microbiota. Thus, to help explain the roles of Se in IBD, it is important to understand the modulatory e ects of Se on the cell innate immune responses following challenge of intestinal epithelial cells with pathogen-associated molecular patterns (PAMPs), as well as oxidative and hypoxic stresses. The present work aimed to assess the roles of Se and the selenoproteins, SelH and TR1, in the responses of Caco-2 cell, modelling the gut epithelium, to hypoxia and infection, the latter replicated by challenge with S. typhimurium agellin. To investigate the responses of gut cells to low Se and PAMPs, undi erentiated Caco-2 cells with either supplemented with Se (40 nM selenite) or depleted of Se for 72 h before challenging with agellin (F) (500 ng/mL). The gene expression of the pro-in ammatory cytokines IL-8 and TNF- were measured in addition to the genes encoding the antimicrobial peptides (AMPs) hBD1 and hBD2. Data showed that Se depletion signi cantly a ected hBD1 expression (0.88-fold increase, P < 0.05), but that Se depletion plus F signi cantly increased the induced expression of all genes (IL-8: 1.68-fold, P < 0.001; TNF- : 0.71-fold, P < 0.001; hBD2: 1.74-fold, P < 0.001) compared with the Se supplemented cells. F and Se depletion were also associated with a signi cant increase in expression of TR1 (F: 1.68-fold, P < 0.001; Se depletion: 0.33-fold, P < 0.01) and GPX2 (F: 3-fold, P < 0.001; Se depletion: 11-fold, P < 0.001), but a signi cant decrease due to Se depletion in SelH (62 %, P < 0.001) and GPX1 (47 %, P < 0.001). The selenoprotein TR1 is an antioxidant enzyme and the primary regulator of the thioredoxin system (TXN), which has previously been shown to regulate immune responses. Knockdown of TR1 expression resulted in the reduced agellin-induced expression of IL-8 (40 %, P < 0.001), TNF-a (45 %, P < 0.01), hBD1 (40 %, P < 0.01) and hBD2 (45 %, P < 0.001). These data suggested that Se, through TR1, is involved in regulating the expression of agellin-induced immune e ectors. The selenoprotein SelH has also been suggested to have antioxidant functions. Knockdown of SelH was associated with the increased expression of the oxidative stress-associated genes NQO1 (0.41-fold, P < 0.001), and HMOX1 (1.78-fold, P < 0.001), supporting a role for SelH in the expression of oxidative stress-associated genes. The role of Se, through SelH and oxidative stress, in regulating the gut responses to agellin, has been discussed. The Caco-2 cell model is more representative of intestinal epithelial cells in vivo, when the cells are di erentiated and placed in a gaseous environment re ecting the oxygen gradient of the gut. Thus the F challenge experiments using di erentiated Caco-2 cells were repeated using a dualoxic environment. Interestingly, no potentiation of gene expression relating to the pro-in ammatory agents IL-8 and TNF- , and the defensins hBD1 and hBD2 was observed. These data suggested that the dualoxic environment completely diminished the e ects of Se depletion on the expression of immune e ectors IL-8, TNF- , hBD2 and hBD1, following agellin challenge. These data suggested the e ects of Se in more physiologically relevant intestinal epithelial cell models, more representative of the in vivo state, are required.