Background
Salmonella spp. remain a major public health problem for the whole world. A better understanding of host defense mechanisms of these food-borne pathogens is a prerequisite to design efficient strategies that could reduce the use of antimicrobial agents and drug-resistant Salmonellosis.
Recent studies highlight the importance of sphingolipids in regulation of bacterial infections [
1,
2]. Sphingolipids are important for innate immune response to eliminate infected pathogens and play a crucial role in infectious diseases [
3]. On the other hand, sphingolipids are involved in the regulation of autophagy [
4] and might potentially be novel targets for therapeutic intervention in human diseases [
5]. For example, sphingolipid synthesis is involved in autophagy in
Saccharomyces cerevisiae [
6]. Nucleotide-binding oligomerization domain-containing protein 2 (NOD2) recruiting autophagy-related protein 16-like 1 (ATG16L1) to the plasma membrane is critical for the autophagic response to invasive bacteria [
7,
8]. Additionally, Voss, et al. reported that NOD2 served as an intracellular pattern recognition receptor to enhance host defense by inducing the production of antimicrobial peptides such as human beta-defensin-2 (hBD-2) [
9]. Human beta-defensin-2 an antimicrobial peptide induced in various epithelia (e.g. skin, respiratory tract, digestive tract, and genitourinary tract) upon extracellular as well as intracellular bacterial challenge, exhibits a broad spectrum of antimicrobial activity and has been demonstrated to kill bacteria in vivo [
10], suggesting that it is an important in host defense against microbes. Sphingolipids and cholesterol act in concert to form raft nanodomains and contribute to Akt/PKB plasma membrane recruitment and activation [
11]. Nevertheless, they did not specify the action of sphingolipids and cholesterol.
Salmonella protects epithelial cells from apoptosis by activation of Akt [
12] to form
Salmonella-containing vacuoles (SCVs), thus escaping from autophagy [
13]. Recently, it was observed plasma membrane cholesterol plays a critical role on
Salmonella-induced autophagy [
14]. However, the effects of membrane sphingolipids on
Salmonella-induced autophagy and hBD-2 in intestinal epithelial cells (IECs) have not been investigated before. It is mandatory to exploit the exact effects of the membrane sphingolipids in IECs infected by
Salmonella. In the present work, we examine if membrane sphingolipids play a crucial role on the
Salmonella-induced autophagy and hBD-2 response in IECs via NOD2.
Discussion
Recent studies have begun to implicate sphingolipids in regulation of bacterial infections [
1,
2]. Increasing evidence indicates the potential of autophagy in controlling infections by directing intracellular or ingested pathogens to lysosomes leading to their destruction [
23]. NOD2 and ATG16L1 recruited to the plasma membrane at bacterial entry are critical for the autophagic response to invasive bacteria [
7,
8]. The activated autophagy of epithelial cells by NOD2 and Atg16L1 [
24] increased killing of
Salmonella [
25]. Inhibition of
de novo sphingolipid synthesis with myriocin suppressed the membranous recruitment of NOD2 and Atg16L1 (Fig.
3), resulting in reduced autophagic process of
Salmonella-infected IECs (Fig.
1) and impaired clearance of the intracellular bacteria (Fig.
2). It suggests sphingolipids recruiting NOD2 and Atg16L1 into plasma membrane of IECs infected by
Salmonella infection contribute to autophagic clearance of invading
Salmonella. It is compatible with the report [
26] that fumonisin B1, a ceramide synthases inhibitor, increases intestinal colonization by pathogenic
Escherichia coli in pigs. The abnormalities in the handling of intracellular bacteria through autophagy might play a role in Crohn’s disease pathogenesis [
24,
27,
28]. The Crohn’s disease associated ATG16L1 coding variant shows impairment of the capture of internalized
Salmonella within autophagosomes [
28]. Studies with experimental animals have shown that feeding sphingolipids inhibits colon carcinogenesis and atherosclerosis [
29], suggesting that sphingolipids represent a “functional” constituent of food. The drugs or foods enhancing membrane sphingolipids may induce autophagic clearance of invading pathogens and lower the risk of Crohn’s disease.
NOD2 was reported to serve as an intracellular pattern recognition receptor to enhance host defense by inducing the production of antimicrobial peptide hBD-2 [
9]. An increased risk of inflammatory bowel disease (IBD) following enteric infections with
Salmonella was reported [
30]. Dysregulation of NOD2-mediated defense against invasive pathogens may play an important role on the pathogenesis of IBD [
31]. Colonic hBD2 was dysregulated at mRNA and protein level in IBD [
32]. The antimicrobial dysfunction (e.g. hBD-2) of intestinal epithelial cells to enteric bacteria (e.g. pathogenic adherent-invasive
E. coli (AIEC)) in patients with NOD2 mutants predispose particularly to ileal involvement in Crohn’s Disease [
27]. Children with Crohn’s Disease showed a lower expression of hBD-2 in the inflamed terminal ileum and ascending colon [
33]. This study illustrating inhibition of
de novo sphingolipid synthesis with myriocin suppressed
Salmonella-induced NOD2-mediated hBD-2 expression (Fig.
4), links the relationship between membrane sphingolipids and inflammatory bowel diseases. In contrast, the synthetic sphingosine analog of myriocin FTY720 leads to a specific down-regulation of proinflammatory signals while simultaneously inducing functional activity of CD4
+CD25
+ Treg [
34]. FTY720 was suggested to offer a promising new therapeutic strategy for the treatment of IBD. Thus, the role of membrane sphingolipids on inflammatory bowel disease is deserved to be investigated in vivo.
Sphingolipid and cholesterol-based structures called membrane rafts [
35], has received much attention in the last few years and are believed to be important structures for the regulation of many biological and pathological processes. These structures attract signaling proteins and allow these proteins to move to new locations for subsequent signaling [
36]. It was recently reported [
11] that nanodomains play a crucial role in triggering the PI3 K/Akt signaling pathway. However, they did not specify the action of sphingolipids and cholesterol.
Salmonella-induced cholesterol accumulation in
Salmonella-containing vacuoles (SCVs) activates PI3 K/Akt signaling [
37] and subsequently result in an anti-apoptosis [
12] and anti-inflammatory signal [
22], both of which may contribute to the invasiveness of
Salmonella in IECs [
37]. As demonstrated in this study that inhibition of
de novo sphingolipid synthesis with myriocin enhanced the phosphorylation of Akt but suppressed the activation of ERK, membrane sphingolipids in IECs infected by
Salmonella may play a contrary role on SCV formation to membrane cholesterol [
22,
37]. The suppression of phosphorylated Akt may contribute to apoptosis of SCV leading to damaged SCV, which is directed for autophagic clearance. Likewise, although PI3 K/Akt was involved in membrane cholesterol-induced anti-inflammatory response but it is not involved in the effect of depletion of sphingolipids on
Salmonella-induced hBD-2 expression (Figs.
4c,
6), which is mediated by NOD2 (Fig.
4b). It is very important for the pathogenesis of
Salmonella infection or innate immunity of the host to defense against the invasive bacteria by regulating the homeostasis between cholesterol and sphingolipids [
38]. It is a novel and promising finding that provides therapeutic strategy to enhance sphingolipids beyond cholesterol, in order to enrich innate immunity to
Salmonella infection.
Conclusion
Altogether, we demonstrated inhibition of
de novo sphingolipid synthesis by myriocin, in one way, enhanced the activation of Akt which is important to maintain intact SCV; and in another way, interfered with the recruitment of NOD2 and ATG16L1 into membrane resulting in impairment of autophagy and suppression of hBD-2. It suggests membrane sphingolipids at the entry site of
Salmonella recruited NOD2 and ATG16L1 to the membrane, inhibit activation of Akt signaling, resulting in the autophagy of the apoptotic SCV and enhanced NOD2-mediated hBD-2 expression. Because manipulating sphingolipids in host cells affects infection and inflammation by the pathogens, pharmacological agents aiming to regulate sphingolipids or diet enriched with sphingolipids [
29] could be potentially used in the treatment of infectious or inflammatory bowel diseases in the future.
Methods
Reagents
Stock solutions of myriocin and fumonisin B1 (FB1) were prepared as follows: myriocin 1 mg/ml in methanol; FB1, 10 mM in DMSO (all from Sigma, St. Louis, MO).
Bacterial strain
The wild-type
Salmonella enterica serovar Typhimurium (
S. Typhimurium) strain used in this work was SL1344. The preparation of
Salmonella inoculum has been described previously [
14,
39].
Cell culture and infection
Caco-2, and SW480 cells were purchased from the American Type Culture Collection (Manassas, VA, USA) and were cultured as described previously [
14,
37,
39] or as recommended by the manufacturer.
Inhibition of de novo sphingolipid synthesis
To inhibit sphingolipids synthesis, cells were allowed to attach overnight and were then cultivated in DMEM containing 10 µM myriocin for 48 h as previously described [
16]. Cells were stained with Rhodamine 123 (10 μg/ml, 10 min) to confirm that the treatment with inhibitors was not lethal. For lipid analysis, lipids from roughly 2 × 10
6 cells were extracted and were resolved by two sequential thin layer chromatography (TLC) runs according to Poole et al. [
16] with some modification.
Cell fractionation
Cytosolic and membrane fractions were prepared as previously described [
14,
37,
39] with some modification. The protein concentration of extracts was normalized before analysis.
Western blotting
Equal amounts of protein were analyzed by immunoblot as previously described [
14,
37,
39]. The transferred membranes were probed with primary antibodies against phosphorylated (p-)Akt (Cell Signaling, Beverly, MA), (p-)ERK (Santa Cruz Biotechnology, Santa Cruz, CA) or (p-)JNK (New England BioLabs, Beverly, MA), or anti-ATG16L1, LC3B (Cell Signaling, Beverly, MA) or NOD2 (Cayman Chemical, Ann Arbor, MI). After washes, the membranes were incubated with appropriate horseradish peroxidase-associated secondary antibodies before signals were visualized with the enhanced chemiluminescence detection system (Amersham Bioscience).
hBD-2 assay
After treatment or infection, the culture media were collected and analyzed for hBD-2 by enzyme-linked immunosorbent assay (ELISA) as manufacturer’ s instructions with some modification [
40].
Real-time Reverse Transcription PCR
Total RNA was prepared from control or infected cells. Transcripts were amplified after reverse transcription with random hexamers using the GeneAmp kit (Roche, Nutley, NJ). Real-time reverse transcription-PCR analyses were performed in a fluorescence temperature cycler (LightCycler; Roche Diagnostics) as described previously [
14,
37,
39]. For hBD-2, the primers were as follows: forward, 5′-ATCAGCCATGAGGGTCTTGT-3′ and reverse, 5′-GAGACCACAGGTGCCAATTT-3′. For NOD2, the primers were as follows: forward, 5′-AGCCATTGTCAGGAGGCTC-3′ and reverse, 5′-CGTCTCTGCTCCATCATAGG-3′. All quantifications were normalized to the housekeeping gene glyceraldehyde-3-phosphate dehydrogenase. Relative expression is given as a ratio between target gene expression and glyceraldehyde-3-phosphate dehydrogenase expression.
Immunofluorescence analysis
After infection and treatment, the cultured SW480 cells were fixed, permeabilized and incubated with rabbit anti-LC3B (Cell Signaling Technology, Danvers, MA) (1:250). Secondary antibody was goat anti-rabbit IgG conjugated with Alexa Fluor 594 fluorochrome (Invitrogen Molecular Probes, Eugene, OR, USA). Nuclei were counterstained with fluorescent dye Hoechst (Sigma Aldrich, St. Louis, MO, USA). The percentage of cells with endogenous LC3 punctae was determined by counting the number of positively staining cells from 100 randomly chosen cells from three separate experiments.
RNA interference (RNAi) in SW480 cells
RNAi experiments in SW480 cells were done as described previously [
14,
39], including control nonsilencing small interference RNA (siRNA), NOD2siRNA and different siRNAs targeting Atg16L1 (sequence 1: sense GAGUUGUCUUCAGCCCUGAUGGCAG, antisense CUGCCAUCAGGGCUGAAGACAACUC; sequence 2: sense GGCUCUGCUGAGGGCUCUCUGUAUA, antisense UAUACAGAGAGCCCUCAGCAGAGCC; sequence 3: sense CAAGGGUUCCCUAUCUGGCAGUAAU, antisense AUUACUGCCAGAUAGGGAACCCUUG (sequence were purchased from Invitrogen Corporation, Carlsbad, CA, USA). Briefly, cultured SW480 cells were transfected with chemically synthesized siRNA to silence NOD2 and Atg16L1, respectively. Immunoblotting were performed to examine the efficiency of protein knockdown. For the SW480 cells, 20 nM of each siRNA was transfected 48–96 h before
S. typhimurium infection.
Gentamicin protection assay
SW480 cells were pre-treated and infected, then gentamicin protection assay were undertaken as described previously [
37].
Cell viability and morphologic features
Representative cell populations from each condition were examined under light microscopy. No significant change was noted under any condition. Cells were stained with Rhodamine 123 (Santa Cruz Biotechnology, Santa Cruz, CA) to confirm that the treatment with inhibitors was not lethal.
Statistical analysis
All experiments were carried out at least three times with similar results. Statistical significance was determined using the student’s t-test.
Ethics statement
This was an entirely in vitro study that was approved by the Chang Gung University Biosafety Committee.