Background
The adenoma-carcinoma sequence indicates that the accumulation of genetic and epigenetic alterations induces malignant epithelial cell proliferation, leading to colorectal cancer (CRC) [
1]. Recent studies suggest that gut microbial dysbiosis results in colitis which contributes to initiation and development of CRC [
2]. It is hardly to obtain samples from a same patient who went through the process from inflammatory bowel disease (IBD) to CRC, so distribution of gut bacteria in on-tumor sites and off-tumor sites of CRC patients is usually used to investigate bacteria associated with initiation of CRC by promoting inflammation. Deep sequencing-based 16S rRNA profiling can detect biodiversity of gut microbiome and reveal the relationship between gut microbiome and sporadic CRC. Microbial metabolites, such as butyrate and hydrogen sulfide [
3,
4], as well as bacterial pathogen-associated molecular patterns (PAMP), such as lipopolysaccharide (LPS) [
2], are known for triggering proinflammatory cascades that lead to adenoma-carcinoma sequence.
A variety of gut bacteria is more abundant on CRC tissue surface than on normal large intestinal surface. For example,
Fusobacterium nucleatum in tumor samples shows high abundance but is almost absent from normal intestinal surface [
5]. However,
F. nucleatum may not promote CRC [
6], because its high abundance in tumor samples may be caused by alterations of gut microenvironment [
7].
F. nucleatum does not exhibit pro-tumorigenic or proinflammatory activities in preclinical models of colon carcinogenesis [
6] and has high abundance in CRC samples, but not colorectal carcinoma [
8]. Some bacteria, like
F. nucleatum, cannot breach the intact healthy colon wall and colonize. However, they have a competitive advantage in tumor environment with rupture and bleeding of the colonic epithelium [
7]. Thus, these bacteria are ‘passenger’ bacteria based on the ‘driver-passenger’ model proposed by Tjalsma et al. [
9].
‘Driver’ bacteria initiate CRC by inducing IBD in epithelial cells. Meanwhile, they are gradually substituted by ‘passenger’ bacteria because gut microenvironment changes during tumor development. Many ‘driver’ bacteria are prone to be ignored if their abundances in CRC tissue are used as the only criterion. For example,
Bacteroides fragilis is a potential ‘driver’ bacterium. It is a common colonic commensal which colonizes the majority of human guts. A subset of them is identified as enterotoxigenic
B. fragilis (ETBF) which secrets a 20 kDa metalloprotease toxin (BFT). BFT cleaves E-cadherin, stimulates cell proliferation, and promotes IBD [
10,
11]. Furthermore, CRC microbiomes analysis showed that
Bacteroides are less abundant in CRC tissue than in adjacent non-malignant tissue [
5]. Thus, as a potential ‘driver’ bacterium,
B. fragilis secrets a toxin that may contribute to IBD that leads to CRC initiation, but it has relatively low growth competition on CRC epithelium. Bacterial taxon markers associated with off-tumor sites may serve as potential driver bacteria for CRC initiation [
9]. Further identification and characterization of driver bacteria and their pathogenic activities will pave the way to CRC prevention therapies.
To identify CRC-associated gut microbiome profiles and potential driver bacteria that initiate CRC, we performed 16S ribosomal RNA gene sequencing on gut mucosal microbiome of paired samples of tumor and tumor-adjacent mucosae, off-tumor sites and healthy controls. The gut microbiome of tumor mucosae, but not that of tumor-adjacent mucosae and healthy control, shows lower alpha-diversity and distinct bacterial taxa compared with that at off-tumor sites. Linear discriminant analysis Effect Size (LEfSe) revealed specific bacterial taxa associated with tumor and tumor-adjacent mucosae, off-tumor sites and healthy controls, respectively. Moreover, the relative abundance of Eubacteriaceae is higher on off-tumor sites than tumor mucosae. Using Eubacterium rectale as an example, we showed the effect of E. rectale LPS on colon epithelium cells in vitro and that E. rectale promotes IBD in vivo.
Materials and methods
Reagents
All cell culture media, trypsin and antibiotics were purchased from Gibco (Grand Island, NY, USA), and FBS was purchased from HyClone (Logan, UT, USA). DAPI, lysosyme, proteinase K, DNase and RNase were purchased from Sigma-Aldrich (St Louis, MO, USA). Immobilon membranes were purchased from Merck Millipore (Bedford, MA, USA). ECL Plus substrate was purchased from CWBio (Beijing, China). Nuclear and cytoplasmic protein extraction kit was purchased from Beyotime (Shanghai, China). ZR Fungal/Bacterial DNA Kit was purchased from Zymo Research Corp. (Irvine, CA, USA). Qualitative fecal occult blood detection kit was purchased from Beijing Leagene Biotechnology Co., Ltd. (Beijing, China). Limulus Amebocyte Lysate (catalogs: T7572) was purchased from Solarbio (Beijing, China).
Rabbit anti-nuclear factor kappa B (anti-NF-κΒ) p65 antibody (catalogs: SAB4502610), rabbit anti-Histone H3 antibody (catalogs: SAB4500354), rabbit anti-IKKα antibody (catalogs: SAB4500257), rabbit anti-IκBα antibody (catalogs: SAB1305978), rabbit anti-β-actin antibody (catalogs: SAB2100037), goat anti-rabbit IgG-peroxidase (catalogs: A0545) and goat anti-rabbit IgG FITC (catalogs: AP132F) were purchased from Sigma-Aldrich (St Louis, MO, USA).
Sample collection and preparation
Intestinal microflora samples were collected from 75 patients and 26 health people in Tianjin Union Medical Center. Cotton swab was used to dip on intestinal surface of colorectal cancer tissues collected from patients. Three distinct locations from the same tissue of a given cancer patient were collected, including tumor position (T), para-tumor epithelia position (P) and normal epithelia position (N). Intestinal microflora samples of healthy people were collected when people were diagnosed for colonoscopy and determined as healthy people (H). These samples were rinsed in 1 ml physiological saline. Then, 200 μl of these solutions were used for Bacterial DNA extraction. Bacterial DNA of these samples was extracted using ZR Fungal/Bacterial DNA Kit according to the manufacturer’s instructions.
16S RNA sequencing and bioinformatic analysis
The 16S ribosomal RNA amplicon libraries were constructed according to the Illumina manufactory manual. Briefly, the following primers were used to amplify the V3-V4 region of 16S rRNA gene: forward primer, 5′TCGTCGGCAGCGTCAGATGTGTATAAGAGACAGCCTACGGGNGGCWGCAG and reverse primer 5′GTCTCGTGGGCTCGGAGATGTGTATAAGAGACAGGACTACHVGGGTATCTAATCC.
The amplified DNA library was subsequently purified using AMPure XP beads (Beckman Coulter, USA) and quantified using Quant-iT PicoGreen dsDNA assay kit (Thermo Fisher, USA). The paired-end sequencing reads (2 × 300 bp) were generated on Illumina MiSeq platform, according to the Illumina standard protocol. Quality control and filtering of raw sequences were carried out using FastQC (
https://www.bioinformatics.babraham.ac.uk/projects/fastqc/). The filtered paired-end reads were assembled using PandaSeq [
33]. The assembled sequences were loaded on QIIME pipeline (qiime.org) [
34] for de novo OTU picking, taxonomic assignment, and diversity analyses. Usearch was used within QIIME to detect and remove de novo chimera. Rarefaction was performed using alpha_rarefaction.py in QIIME pipeline. De novo OTUs were picked based on sequence similarity (97%) within the assembled sequences. Next, taxonomy was assigned to OTU representative sequences that were picked for each OTU. To identify differential abundance of bacterial phylo associated with specific sites of CRC patients and healthy people, we applied LEfSe [
35].
Bacterial strains and culture conditions
E. rectale ATCC 33656 and F. nucleatum subsp. Nucleatum ATCC 25586 were both purchased from Biobw (Beijing, China). E. rectale was grown anaerobically in PYG medium (ATCC medium 1527) composing of 0.5% peptone, 1% yeast extract, 0.5% tryptone, 0.01% resazurin (Sigma), 0.0008% MgSO4, 0.0008% anhydrous CaCl2, 0.004% K2HPO4, 0.004% KH2PO4, 0.002% NaCl, 0.04% NaHCO3, 0.0001% (v/v) Vitamin K1 (Sigma), 0.05% L-cysteine-HCL (Sigma). The pH was adjusted to 7.0. F. nucleatum ATCC 25586 was maintained anaerobically in Columbia broth (BD Biosciences). PYG medium and Columbia broth were both boiled to remove oxygen and placed in anaerobic bottles. Argon gas was added to the bottles. Then the bottles were sealed by a cap with a rubber septum before being autoclaved for 20 min at 121 °C.
Mouse models of bacterial colonization
Colitis was experimentally induced by administration of 2.5% Dextran sodium sulfate (DSS) in the drinking water of 7-week-old Balb/c mice for 7 days. Afterwards, the mice were allowed to drink water without DSS until the end of experiment. After 2 days of DSS drinking, mice were inoculated with E. rectale or F. nucleatum by coloclysis with 1 × 107 colony forming units (CFU) in 200 μl PBS. Briefly, intraperitoneal injection of 4% chloral hydrate (10 μl/g) was used to deeply anesthetize mice. The mice were kept to prone position. Colon was inserted with paraffin oil applied plastic hose (2 mm internal diameter) through anus. When the top of hose was inserted 4 cm from anus, the mice were kept to handstand. The hose was pulled out after bacterium was injected. Anus was pressed with cotton swab within 1 min, and the mice were put back to cage. Mice were separated into 6 groups: (Water) control group, water drinking and no bacterial inoculation; (DSS) DSS drinking and no bacterial inoculation; (E. rectale + water) water drinking and E. rectale inoculation; (F. nucleatum + water) water drinking and F. nucleatum inoculation; (E. rectale + DSS) DSS drinking and E. rectale inoculation; (F. nucleatum + DSS) DSS drinking and F. nucleatum inoculation. Each group consisted of five mice. The mice were monitored and weighed daily until they had lost > 20% of their initial body weight, And upon being sacrificed, the colon and spleen were both harvested for determination of colon length, histological examination and spleen weight. Stool was collected and measured daily for the presence of occult and gross blood. Occult blood was measured by a qualitative fecal occult blood detection kit.
Histological examination
Harvested colons were cleaned in physiological saline solution to remove fecal residue. After 24 h of fixation in 10% buffered formalin, the colons were embedded in paraffin and sectioned. Then the tissues were stained with hematoxylin and eosin (H&E). Images of histology slides were taken by an Olympus IX51 microscope. Histology of colon was scored on a scale of 0–5 where 0 = normal and 5 = severe inflammation and complete loss of surface and crypt epithelium.
Isolation of bacterial endotoxins
Endotoxic LPSs were isolated from bacteria using the hot phenol-water method as previously described [
36]. In brief, bacterial pellets were digested with lysozyme. Then, cell lysate was incubated with DNase and RNase to digest nucleic acids. 90% phenol extraction procedure was used. The water layer was dialyzed against deionized water and digested with proteinase K. Then the solution was dialyzed again. The purity of the phenol extract was tested by the detection of nucleic acid (260 nm), coomassie brilliant blue stained protein (595 nm). Isolated LPSs were also detected by SDS-PAGE. Finally, LPSs were lyophilized using Freeze Dry System (FD-1A-80, BILON Ltd., Shanghai, China). The biological effectiveness of isolated LPSs was tested in Limulus Amebocyte Lysate assay according to the manufacturer’s instructions.
LPS treatment to human normal colon epithelial cells
The human normal colon epithelial cell lines NCM460 and HCoEpiC were both purchased from Tong Pai Technology, Inc. (Shanghai, China). These cells were cultured in RPMI 1640 medium supplemented with 10% FBS, 100 μg/mL streptomycin, and 100 U/mL penicillin. LPS was dissolved in RPMI 1640 medium and vortexed for 10 min by ultrasonic bath (Shenglan Ltd., Jiangsu, China) before use. A concentration series of LPS dilutions was used to treat cells for 2.5 h.
Western blotting
For detection of NFκΒ p65 expression in nuclei, nuclear proteins were isolated from harvested cells using a nuclear and cytoplasmic protein extraction kit and following the manufacturer’s protocol. Western blotting was carried out as previously described [
37]. Briefly, protein samples were suspended in SDS loading buffer and boiling for 5 min. Then 10 μg protein was separated by SDS-PAGE and transferred to immobilon membranes by semi-dry blotting method. The membranes were probed with antibodies using standard techniques. Finally, the protein bands were visualized using ECL Plus and exposed film. Each assay was carried out in triplicate.
Immunocytochemistry analysis
Immunofluorescence staining was used to analyze the location of NFκΒ p65 expression. The procedure was carried out as previously described [
37]. In brief, cells were washed with PBS for three times, fixed in 10% formalin at room temperature for 20 min, treated with 0.5% Triton X-100 for 5 min at 4 °C and blocked with 5% normal goat serum overnight at 4 °C. Then the slides were incubated with primary antibody for 1 h at 37 °C. After washing with PBS, the slides were incubated with FITC-conjugated secondary antibody and DAPI simultaneously for 1 h, at 37 °C. Subsequently, the slides were washed with PBS and then sealed. The slides were photographed immediately using a fluorescence microscope (Olympus, Tokyo, Japan).
Statistical analysis
All data represent mean ± standard deviation. Statistical analysis was carried out by Student’s t test using SPSS software. P value < 0.05 was considered statistically significant.
Discussion
According to the ‘driver-passenger’ model proposed by Tjalsma et al. [
9], CRC ‘driver’ bacteria may not preferentially inhabit on-tumor sites, because many ‘passenger’ bacteria have high adaptability on-tumor sites and colonize most of the on-tumor sites niches. Conversely, the off-tumor site niches may be maintained similar to that on healthy intestinal tract. Thus, ‘driver’ bacteria may still be present in considerable abundance in off-tumor sites during CRC tumorigenesis [
19]. We found that
E. rectale distribution in our samples fit the characteristic of ‘driver’ bacteria, leading to our further investigation on the contribution of
E. rectale to colitis in vitro and in vivo.
F. nucleatum is anaerobic Gram-negative bacterium, which is an adherent and invasive species. It is very suitable to survive in the broken colon wall and has high abundance in IBD [
20] or CRC [
6] samples. Because
F. nucleatum is a potential ‘passenger’ bacterium [
9], it is used as control bacterium in this study. Many reports found that
F. nucleatum has higher abundance in CRC patients biopsy or fecal samples measured by 16S ribosomal RNA profiling [
21,
22]. Our results also exhibited that abundance of
F. nucleatum increased in CRC patient samples, especially in on-tumor sites.
E. rectale is an anaerobic Gram-positive bacterium and is one of the most abundant bacterial species in human fecal samples. Some reports showed that
E. rectale is a major species for butyrate formation, which is the preferred energy source for colonocytes and benefits for colon health [
23], but butyrate also promotes CRC by stimulating the transformation of colonic epithelial cells and inducing aberrant proliferation [
2].
E. rectale is reported to be significantly less present [
24] in ulcerative colitis (UC) patients, which implies that
E. rectale may initiate colitis that leads to CRC initiation if
E. rectale is a ‘driver’ bacterium. According to this report,
E. rectale seems to function as probiotic, but some recent studies that focused on distribution of intestinal microflora in CRC samples implied that
E. rectale may be a potential ‘driver’ bacterium [
7,
9]. Butyrate-producing bacteria, including
Eubacterium, were found to be less abundant in CRC fecal microbiota measured by 16S ribosomal RNA profiling [
12]. To our best knowledge, the studies about
E. rectale abundance in CRC samples measured by 16S ribosomal RNA profiling are lacking. There are only a few reports [
22] on which other species in the genus
Eubacterium were measured by 16S ribosomal RNA profiling in CRC samples, such as
E. hallii,
E. oxidoreducens and
E. ruminantium, which exhibited different trend of abundance changes in carcinogenesis process, suggested that different species in the genus
Eubacterium have different adaptabilities to the on-tumor site niche. Our 16S ribosomal RNA profiling also showed that
E. rectale has considerable density in healthy samples and higher abundance in off-tumor sites, but abundance of
E. rectale declined in on-tumor sites significantly. Thus, we further investigated whether
E. rectale contributes to CRC initiation by promoting inflammation in vitro and in vivo.
Cancer is a disease mainly caused by genetic mutations, but inflammation can temporarily bypass the mutation requirement for tumor initiation. Besides mutations, IBD is an important risk factor for the development of CRC. Two of the most common forms of IBD are Crohn’s disease (CD) and ulcerative colitis (UC). For example, Crohn's disease (CD) increases cumulative risk of colitis associated cancer (CAC) by up to 8% [
25], and Ulcerative colitis (UC) up to 18–20% risk of CAC [
26]. Wnt/β-catenin, which is a critical pathway to regulate normal and malignant cell proliferation, is activated by mutation in over 90% of sporadic CRC [
27], which including adenomatous polyposis coli (APC). But several inflammatory pathways, including NF-κΒ, can induce Wnt/β-catenin activation without any mutations in APC [
28].
Some of the symbiotic or commensal intestinal bacteria in human beings and mice have been found as conditionally pathogenic, which induces various forms of IBD. For example,
Helicobacter spp. infection in Il10
−/− mice usually induces IBD and consequently CAC development [
29]. Under normal conditions, intestinal bacteria are separated from immune system by complete protective epithelial barrier. When intestinal epithelial barrier is breakdown by some factors, for example DSS, commensal flora translocates to interact with immune system, which causes IBD development. Many potential ‘driver’ bacteria induce IBD, such as
Bacteroides fragilis [
10,
11] and
Clostridium difficile [
30]. Several reports show that abundance of
Fusobacterium nucleatum [
20] increases and
E. rectale declines [
14] in UC patients. Although
F. nucleatum is also enriched in CRC samples and appears to be the most common passenger bacterium [
5], this does not constitute sufficient evidence to confirm that it plays an active part in IBD or CRC progression [
7]. And the relation between
E. rectale and IBD promotion is also still unknown.
Conclusion
In our study, we investigated the relation between
E. rectale and IBD promotion by utilizing the well-established dextran sulfate sodium (DSS) model of colitis in Balb/c mouse. This model was widely used to mimic characteristics of IBD, induce acute colitis and sustain chronic levels of inflammation [
31]. This model has been further verified by applications of several therapeutic agents used to treat human IBD [
32]. We found that
E. rectale but not
F. nucleatum prolonged and/or worsened the severity of DSS-induced colitis, and
E. rectale also activated inflammatory factor, NF-κΒ, in normal colon epithelial cells. Combined with
E. rectale distribution in our samples, our findings suggest that
E. rectale is a ‘driver’ bacterium and contribute to CRC initiation by promoting colitis.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.