Differential effects of selenium and knock-down of glutathione peroxidases on TNFα and flagellin inflammatory responses in gut epithelial cells
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Selenium (Se) is essential for human health. Despite evidence that Se intake affects inflammatory responses, the mechanisms by which Se and the selenoproteins modulate inflammatory signalling, especially in the gut, are not yet defined. The aim of this work was to assess effects of altered Se supply and knock-down of individual selenoproteins on NF-κB activation in gut epithelial cells. Caco-2 cells were stably transfected with gene constructs expressing luciferase linked either to three upstream NF-κB response elements and a TATA box or only a TATA box. TNFα and flagellin activated NF-κB-dependent luciferase activity and increased IL-8 expression. Se depletion decreased expression of glutathione peroxidase1 (GPX1) and selenoproteins H and W and increased TNFα-stimulated luciferase activity, endogenous IL-8 expression and reactive oxygen species (ROS) production. These effects were not mimicked by independent knock-down of either GPX1, selenoprotein H or W; indeed, GPX1 knock-down lowered TNFα-induced NF-κB activation and did not affect ROS levels. GPX4 knock-down decreased NF-κB activation by flagellin but not by TNFα. We hypothesise that Se depletion alters the pattern of expression of multiple selenoproteins that in turn increases ROS and modulates NF-κB activation in epithelial cells, but that the effect of GPX1 knock-down is ROS-independent.
KeywordsSelenoprotein Inflammatory signalling NF-κB GPx1 GPx4 Selenium
The micronutrient selenium (Se) is essential for human health (Bellinger et al. 2009; Fairweather-Tait et al. 2011). Low Se status has been reported to be associated with increased risk of colorectal cancer (CRC) whereas in contrast higher Se intake and status are associated with lower risk of colonic adenoma recurrence (Russo et al. 1997; Jacobs et al. 2004; Rayman 2005; Peters and Takata 2008). Supplementation with 200 μg Se per day resulted in reduced CRC mortality, especially in those individuals with low Se status prior to supplementation (Clark et al. 1996). However, the biochemical and cellular mechanisms that link Se intake gut epithelial cell function and carcinogenesis are poorly understood.
In humans, Se is incorporated as the amino acid selenocysteine into ~25 selenoproteins (Bellinger et al. 2009) that play important roles in cell protection mechanisms. The selenoproteins include a family of glutathione peroxidases (GPx1, GPx2 and GPx4) that protect cells from reactive oxygen species (ROS), the thioredoxin reductases (TR) that function in redox control (Brigelius-Flohé 2006; Arnér 2009) and members of a novel series of thioredoxin-like proteins that have been proposed to have antioxidant functions (Bellinger et al. 2009). There is evidence that Se has anti-inflammatory effects in macrophages and immune cells (Hoffmann and Berry 2008; Vunta et al. 2008) and Se supplementation has been found to modulate activation of the transcription factor NF-κB, which plays a pivotal role in regulation of inflammatory pathways (Jeong et al. 2002; Zamamiri-Davis et al. 2002; Prabhu et al. 2002; Gasparian et al. 2002, Christensen et al. 2007; Vunta et al. 2007).
However, critically, the in vitro effects of Se supplementation on NF-κB signalling are distinct in different cell lines. Thus, for example, supplementation of the cell culture medium lowered activation of NF-κB in human bronchial and prostate cells (Gasparian et al. 2002, Christensen et al. 2007) but had no effect on NF-κB translocation in human endothelial cells and led to increased NF-κB translocation and increased NF-κB response in macrophages (Prabhu et al. 2002; Vunta et al. 2007). These observations correspond closely to the suggestion that in vivo NF-κB pathways are regulated in a cell-type dependent mechanism (Smale 2011). Gut epithelial cells such as the Caco-2 cell line respond to bacterial and host inflammatory challenges by activation of NF-κB pathways (Kelly et al. 2004), but the NF-κB signalling response of such cells to altered Se supply or changes in selenoprotein expression are not known. Indeed, the effects of Se on inflammatory signalling in the gut and the roles of selenoproteins, especially in the epithelial cells, have not yet been defined.
However, transcriptomic analysis of samples from a recent mouse experiment examined the response of mouse colon gene expression to sub-optimal Se intake and identified the expression of genes associated with the NF-κB signalling pathway as being sensitive to dietary Se (Kipp et al. 2009). In view of the links between gut inflammation and susceptibility to colorectal cancer (Klampfer 2011), and the possible benefits of increased Se intake (Russo et al. 1997; Jacobs et al. 2004; Rayman 2005; Bellinger et al. 2009; Fairweather-Tait et al. 2011), it is important to elucidate whether Se supply affects NF-κB signalling in gut epithelial cells. The aim of the present work was to use an in vitro model of the gut epithelium to test the hypothesis that alteration of Se supply modulates NF-κB activation in response to host and microbial factors, and the to test whether the effects are due to changes in selenoprotein expression.
Materials and methods
Human colon adenocarcinoma Caco-2 cells were grown at 37°C in a 5% CO2 atmosphere in Dulbecco’s modified Eagle’s medium (with 4.5 g/l glucose and Glutamax) supplemented with 10% (v/v) foetal calf serum (Sigma, Poole, UK) and 1% (v/v) (100 μg/ml) penicillin–streptomycin. For siRNA knockdown experiments, cells were grown in the same medium. In Se depletion/supplementation experiments, cells were transferred to serum-free medium containing 0.1% (v/v) penicillin–streptomycin, 1% (v/v) (100 units/ml) non-essential amino acids (Gibco), insulin (5 μg/ml) and transferrin (5 μg/ml) without (selenium-deficient medium) or supplemented with 7 ng/ml sodium selenite (equivalent to 40 nM; selenium-repleted medium), as described previously (Pagmantidis et al. 2005; Crosley et al. 2007). Cells were cultured in Se-deficient or Se-supplemented medium for 3 days and then tested for responses to treatment with either 20 ng/ml TNFα (Sigma) for or 100 ng/ml Salmonella typhimurium flagellin for 0–8 h.
Luciferase constructs and transfection
Luciferase constructs were obtained by modification of a previously established construct (Carlsen et al. 2002), which contains three NFκB binding sites and a TATA box linked to a luciferase coding sequence (CDS). The entire 3× NF-κB-TATA box-luciferase CDS sequence was isolated by Hind III/ApaI restriction digestion and ligated into pBLUE-TOPO plasmid vector (Invitrogen). The control construct was obtained by isolation of TATA box-luciferase CDS sequence by BamHI/ApaI restriction digestion and fusion with pBLUE-TOPO vector. The correct insertion of the 3× NF-κB-luciferase and TATA-luciferase sequences were verified by sequencing (data not shown). Stable transfections of Caco-2 cells (1 × 106 cells/transfection) with luciferase constructs (test or control) used 2–6 μg of plasmid DNA combined with 5 μl Lipofectamine™ 2000 (Invitrogen). Stably transfected cells were selected in medium containing 750 μg/ml G418 (Sigma) for 6 weeks (medium changed every 2 days) and harvested as a mixed population.
Knock-down of expression of selenoprotein genes GPX1, GPX4, selenoprotein W (SELW) and selenoprotein H (SELH) was achieved by transient transfection of Caco-2 cells with siRNA duplexes (Ambion), specific to the mRNAs of these targets The relevant siRNA sequences were: SELH sense GCGACCGUUGUUAUCGAGCAUUGCA and antisense UGCAAUGCUCGAUAACAACGGUCGC; GPX1 sense GGUACUACUUAUCGAGA AUUU and antisense AUUCUCGAUAAGUAGUACCUU; SELW sense CCACCGGGUUCUUUGAAGUGAUGGU and antisense ACCAUCA CUUCAAAGAACCCGGUGG GPX4 sense UUCGAUAUGUUCAGCAAGAUU and antisense UCUUGCUGAACAUAUCGAAUU negative control sense GUUCAAUAUUAUCAAGCGGUU and antisense CCGCUUGAUAAUAUUGAACUU. According to Ambion’s specifications, 5 × 105 cells were grown in 2 ml of serum-free medium supplemented with 5% (v/v) foetal calf serum in the absence of antibiotics. A siRNA/transfection reagent complex was formed at 37°C by combining siRNA oligomer (30–45 nM) with 5 μl (2 μg/ml) Lipofectamine™ 2000 transfection reagent (Invitrogen) in 0.5 ml Optimem medium (Gibco), and this was applied to cells for 3 days until they were harvested. Control cells were transfected with a non-specific ‘scrambled’ siRNA duplex (Ambion).
NFκB activation in Caco-2 cells was determined by measuring luciferase activity in the reporter cells stably transfected with the 3× NF-κB-luciferase or TATA-luciferase constructs. Cells were washed twice in 1× PBS, mixed with 1× Reporter lysis buffer (Promega), frozen and thawed once three times, and lysed by vortexing vigorously for 15 s. Cell lysate was harvested as the supernatant fluid after centrifugation at 12,000g for 2 min at 4°C. Twenty microliters of cell lysate was mixed with 80 μl of 5× Luciferase assay reagent (Promega) and luciferase activity was measured on a TD-20e luminometer (Turner Designs). Protein concentration was quantified using the bicinchoninic acid protein assay (Sigma). Specific luciferase activity was calculated as luciferase activity (relative light units; RLU) per mg protein lysate.
Primers used for RTPCR analysis of GPX1, SELW, SELH, GPX4, GPX2, GAPDH and IL8 transcripts
5′-CGA TAC GCT GAG TGT GGT TTG C-3′
5′-CAT TTC CCA GGA TGC CCT TG-3′
5′-TGA AGG TCG GAG TCA ACG GAT TTG-3′
5′-CAT GTA AAC CAT GTA GTT GAG GTC-3′
5′-CAG TCG GTG TAT GCC TTC TCG-3′
5′-TGT CAG GCT CGA TGT CAA TG-3′
5′-GGC TTT CAT TGC CAA GTC CTT C-3′
5′-CTA TAT GGC AAC TTT AAG GAG GCG C-3′
5′-GTT TAT TGT GGC GCT TGA GGC-3′
5′-GAA CAT CAG GGA AAG ACC ACC-3′
5′-GCT TCC AGT AAA GGT GAA CCC G-3′
5′-ACC CAA ATC TCC CTA CGA CAG G-3′
5′-ATG ACT TCC AAG CTG GCC GTG GCT-3′
5′-TCT CAG CCC TCT TCA AAA ACT TCT C-3′
Total cellular ROS levels were determined using the Image-iT™ LIVE Green Reactive Oxygen Species (ROS) Detection Kit (Invitrogen) according to manufacturer’s specifications. A total of 6 × 104 or 3 × 104 cells were seeded in each well of a 96-well plate and grown in Se−/Se+ medium, or treated with GPX1 siRNA/control siRNA, respectively. Cells were washed with 1× HBSS and incubated with 50 μl staining solution containing 25 μM 5-(and-6)-carboxy-2′,7′-dichlorodihydrofluorescein diacetate (carboxy-H2DCFDA) and 1 μM Hoechst 33342 (control staining of nucleic acid) for 30 min at 37°C. Cells were washed with 1× HBSS and ROS levels measured at 495/529 nm (for carboxy-H2DCFDA) and 350/461 nm (for Hoechst 33342) wavelengths on a BMG LABtech Fluostar Omega luminometer.
Total protein was extracted from Caco-2 cells by washing twice with ice-cold PBS, resuspension in PBS containing protease inhibitor, centrifugation at 150 rpm, 4°C for 5 min and then lysis in 25 mM HEPES buffer (pH 7.6) containing 3 mM MgCl2, 40 mM KCL, 2 mM DTT, 5% glycerol and 0.5% NP40. Protein concentration was determined using the Bradford protein assay procedure. Twenty micrograms of protein was subjected to SDS-PAGE, transferred to a PVDF membrane (ROCHE), and incubated overnight in PBS containing 5% dried milk, 0.05% Tween20 and primary antibody (Anti-GPx1 from AbCam diluted 1/500, Anti-GPx4 from Labfrontier, Korea diluted 1/500, monoclonal Anti-βactin from Sigma diluted 1/5,000). After four washes in PBS containing 0.05% Tween20, the membrane was incubated with secondary antibody (polyclonal Anti-rabbit for Anti-GPx1 and Anti-GPx4, polyclonal Anti-mouse for Anti-βactin) linked to horseradish peroxidase (from Sigma, UK diluted 1/5000) for 1 h, washed 4 times and bound antibody detected by chemiluminescence.
Groups were compared by Mann–Whitney U-test using SPSS 17.0 software.
TNFα and flagellin activate NF-κB signalling in Caco-2 cells
Se depletion activates NF-κB signalling in Caco-2 cells
GPX1 knock-down compromises TNFα-mediated activation of NF-κB signalling
On the contrary, transfection with 80 pmol siRNA against GPX1 resulted in approximately 60% decrease in GPx1 expression and a 20% decrease in the response of luciferase activity to TNFα stimulation in NF-κB-luciferase cells compared with the addition of a scrambled control siRNA (see Fig. 5c). Quantification of IL-8 expression after TNFα stimulation of Caco-2 cells showed a reduced cytokine response after GPX1-knock-down compared with the response in cells treated with the control scrambled siRNA (Fig. 5d). It thus appears that GPx1 is required for the full NF-κB response to TNFα.
GPX4 knock-down compromises flagellin-mediated activation of NF-κB signalling
Results from both luciferase reporter assays and measurement of endogenous IL-8 expression showed that both TNFα and flagellin elicited an activation of NF-κB signalling in the gut epithelial Caco-2 cell line. This confirms earlier data showing that these enterocytic cells activate inflammatory pathways in response to such molecules (Kelly et al. 2004). In addition, the present data show that the response of NF-κB activation to TNFα was modulated by Se supply such that Se depletion led to increased luciferase activity in cells expressing the reporter driven by NF-κB response elements and to increased expression of endogenous IL-8, a chemokine that is released from epithelial cells and is key in determining neutrophil chemotaxis (Eckmann et al. 1993). These observations are consistent with recent transcriptomic data that suggest that a marginal dietary Se deficiency in the mouse causes altered expression of genes in inflammatory NF-κB signalling pathways in the colon (Kipp et al. 2009) but in addition provide the first direct evidence that NF-κB activation in a gut epithelial cell line is sensitive to Se depletion/supplementation. Moreover, Se depletion affected the response of NF-κB activation to TNFα, but not the response to bacterial flagellin, indicating modulation of responses to endogenous inflammatory mediators rather than modulation of the responses to exogenous bacterial activators via Toll-like receptors.
Earlier studies have shown that alterations in Se supply lead to changes in activation of the transcription factor NF-κB but that the effects differ between cell lines, with increased NF-κB response to Se supplementation in macrophages (Zamamiri-Davis et al. 2002; Prabhu et al. 2002; Youn et al. 2008) but lower activation of NF-κB in human bronchial and prostate cells (Jeong et al. 2002; Gasparian et al. 2002; Christensen et al. 2007). The present results show that in gut epithelial cell lines, the effect of Se on the NF-κB response to TNFα is comparable to that in bronchial and prostate epithelial cell lines and distinct from that found in macrophages, a finding consistent with the view that regulation of NF-κB differs between epithelial and immune cell types (Pasparakis 2009; Smale 2011).
Expression of GPX1, SELH and SELW in the mouse colon and Caco-2 cells has been shown to be highly sensitive to Se depletion (Pagmantidis et al. 2005; Kipp et al. 2009). However, knock-down of expression of these selenoprotein genes either failed to affect the response to of NF-κB activation to 20 ng/ml TNFα (SELH, SELW) or in the case of GPX1 knock-down, it caused a reduction in the NF-κB-dependent response. Indeed, the data in Figs. 3 and 5 show opposite effects of Se depletion and GPX1 knock-down on the NF-κB response despite both treatments lowering GPX1 expression (see Fig. 2; Pagmantidis et al. 2005). Thus, the effect of Se deficiency on NF-κB activation cannot be accounted for by a reduction of expression of SELH, SELW or GPX1 individually.
Measurement of total Caco-2 cell ROS levels indicated that Se depletion resulted in an increase in ROS levels, but there was no increase after GPX1 knock-down. A key difference between GPX1 knock-down and Se depletion is that Se depletion alters the expression of multiple selenoproteins (Fig. 2), but that knock-down of GPX1 does not (Fig. 7). Thus, after GPX1 knock-down when Se supply is adequate, expression of these other selenoproteins is expected to be maintained and we speculate that their activity can compensate for the lower GPX1 expression; as a result, any elevation of a particular species of oxidative signal, such as superoxide radicals, will be transient and rapidly corrected, whereas in Se-depleted cells, the rise in ROS cannot be corrected. Superoxide radicals are known to modulate inflammatory signalling pathways, and activation of NF-κB is redox sensitive (Schreck et al. 1991; Takada et al. 2003; Brigelius-Flohé et al. 2004; Kabe et al. 2005) with modulation of hydrogen peroxide clearance affecting IKK activity (Li et al. 2001). Therefore, our hypothesis is that in gut epithelial cells, the TNFα-induced, IKK-mediated activation of NF-κB following Se depletion is modulated by increased superoxide levels as a result of Se supply altering expression of multiple selenoproteins.
In contrast, when Se supply was adequate, knock-down of GPX1 expression led to lower NF-κB activation in response to TNFα but had no effect on baseline ROS levels. This suggests that GPx1 can have a ROS-independent effect on TNFα-induced activation of NF-κB. ROS-independent activation of NF-κB has been reported in other cell types (Legrand-Poels et al. 1997), and knock-out of GPX1 has opposite effects on ROS-dependent and reactive nitrogen species-dependent signalling pathways (Fu et al. 2001). In addition, GPX4 knock-down affected activation of NF-κB by bacterial flagellin, suggesting that changes in GPx4 expression modulate interactions between bacterial microbiota and the host. Thus, the results indicate that in Caco-2 cells, both GPx1 and GPx4 are both required for full response of NF-κB, but it appears that GPx1 modulates pathways operating through the TNFα receptor and GPx4 pathways through Toll-like receptor 5 in response to flagellin. These differences in response of NF-κB activation after GPX1 and GPX4 knock-down may reflect their different products derived from lipid peroxides (Seiler et al. 2008).
In conclusion, the present results indicate that Se depletion affects the response of NF-κB activation to TNFα, but not the response to bacterial flagellin, and that GPX1 knock-down and GPX4 knock-down have distinct effects on activation of NF-κB. We hypothesise that Se depletion affects the pattern of expression of multiple selenoproteins and that the combined changes in expression lead to increased superoxide levels that in turn modulate NF-κB signalling. In contrast, the effect of GPX1 knock-down alone is ROS-independent. The complex roles of selenoproteins in modulating these inflammatory pathways is further illustrated by the finding that knock-down of GPX4 lowers flagellin-induced activation of NF-κB but has no effect on TNFα-induced activation. Effects of Se on inflammatory signalling in macrophages have been linked both to effects of 15-deoxy-delta12,14 prostaglandin J2 and to Toll-like receptor pathways and IκB-kinaseß (Vunta et al. 2007; Youn et al. 2008), and so future work on gut epithelial cell lines should explore the possible links between Se, selenoproteins, pathways of activation of NF-κB and eicosanoid metabolism but bearing in mind that regulation of NF-κB differs between epithelial and immune cell types (Pasparakis 2009). Since gut inflammation has been found to be associated with subsequent tumour development (Klampfer 2011), these effects of Se supply and selenoproteins on inflammatory pathways in gut epithelial cells provide a potential mechanistic link between the effects of Se intake on susceptibility to colorectal cancer, inflammation and carcinogenesis.
We thank the Wellcome Trust for financial support (ref: 081380) and Professor Rune Blomhoff (Oslo University, Norway) for the kind gift of the vector used as a source of NFκB-luciferase sequences for the DNA cloning. We thank Chris Blackwell for carrying out the Western blotting.
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