Immunoregulatory mechanisms underlying prevention of colitis-associated colorectal cancer by probiotic bacteria

Abstract

Background: Inflammatory bowel disease (IBD) increases the risk of colorectal cancer. Probiotic bacteria produce immunoregulatory metabolites in vitro such as conjugated linoleic acid (CLA), a polyunsaturated fatty acid with potent anti-carcinogenic effects. This study aimed to investigate the cellular and molecular mechanisms underlying the efficacy of probiotic bacteria in mouse models of cancer.

Methodology/principal findings: The immune modulatory mechanisms of VSL#3 probiotic bacteria and CLA were investigated in mouse models of inflammation-driven colorectal cancer. Colonic specimens were collected for histopathology, gene expression and flow cytometry analyses. Immune cell subsets in the mesenteric lymph nodes (MLN), spleen and colonic lamina propria lymphocytes (LPL) were phenotypically and functionally characterized. Mice treated with CLA or VSL#3 recovered faster from the acute inflammatory phase of disease and had lower disease severity in the chronic, tumor-bearing phase of disease. Adenoma and adenocarcinoma formation was also diminished by both treatments. VSL#3 increased the mRNA expression of TNF-α, angiostatin and PPAR γ whereas CLA decreased COX-2 levels. Moreover, VSL#3-treated mice had increased IL-17 expression in MLN CD4+ T cells and accumulation of Treg LPL and memory CD4+ T cells.

Conclusions/significance: Both CLA and VSL#3 suppressed colon carcinogenesis, although VSL#3 showed greater anti-carcinogenic and anti-inflammatory activities than CLA. Mechanistically, CLA modulated expression of COX-2 levels in the colonic mucosa, whereas VSL#3 targeted regulatory mucosal CD4+ T cell responses.

Figure 1. Effect of VSL#3 and dietary conjugated linoleic acid (CLA) supplementation on experimental azoxymethane-induced colorectal cancer.

C57BL/6J mice (n = 60) were treated with the VSL#3 probiotic (n = 20), CLA-supplemented (1 g/100 g) (n = 20) or control diets (n = 20) for 32 days and challenged i.p. with azoxymethane (10 mg/kg) followed by 2% dextran sodium sulfate (DSS) in the drinking water for 7 days to induce colitis-associated colorectal cancer (CRC). The disease activity index, a composite score reflecting clinical signs of the disease (i.e. perianal soiling, rectal bleeding, diarrhea, and piloerection) was assessed daily for mice undergoing the DSS challenge (A). Mice were euthanized on day 68. Colon and mesenteric lymph nodes (MLN) (B&C) were macroscopically scored for inflammation. Data are represented as mean ± standard error. Points with an asterisk are significantly different when compared to the control group (P<0.05).

Figure 2. Effect of VSL#3 and dietary conjugated linoleic acid (CLA) supplementation on experimental Helicobacter typhlonius-induced colorectal cancer.

Bacterial-free 129/SvEv and IL-10 gene deficient (IL-10−/−) 129/SvEv mice in a 129/SvEv background (n = 60) were treated with the VSL#3 probiotic (n = 20), CLA-supplemented (1 g/100 g) (n = 20) or control diets (n = 20) for 32 days and then were infected with H. typhlonius in order to develop experimental colorectal cancer associated with colitis. The disease activity index, a composite score reflecting clinical signs of the disease, was assessed daily for mice undergoing the DSS challenge (A). Colon, spleen and mesenteric lymph nodes (MLN) (B–D) were macroscopically scored for inflammation. Data are represented as mean ± standard error. Points with an asterisk are significantly different when compared to the control group (P<0.05).

Figure 3. Effect of the CLA and VSL#3 treatment on colon histopathology on experimental azoxymethane-induced colorectal cancer.

C57BL/6J mice (n = 60) were treated with the VSL#3 probiotic (n = 20), CLA-supplemented (1 g/100 g) (n = 20) or control diets (n = 20) for 32 days and challenged i.p. with azoxymethane (10 mg/kg) followed by 2% dextran sodium sulfate (DSS) in the drinking water for 7 days to induce colitis-associated colorectal cancer (CRC). Mice were euthanized on day 68. All specimens underwent blinded histological examination and were scored 1–4 on leukocyte infiltration (A), and mucosal wall thickening (B), adenocarcinomas (C) and adenomas (D). Data are represented as mean ± standard error. Points with an asterisk are significantly different when compared to the control group (P<0.05).

Figure 4. Effect of the CLA and VSL#3 treatment on colon histopathology on experimental Helicobacter typhlonius-induced colorectal cancer.

Bacterial-free 129/SvEv and IL-10 gene deficient (IL-10−/−) 129/SvEv mice in a 129/SvEv background (n = 60) were treated with the VSL#3 probiotic (n = 20), CLA-supplemented (1 g/100 g) (n = 20) or control diets (n = 20) for 32 days and then were infected with H. typhlonius in order to develop experimental colorectal cancer associated with colitis. After the necropsy, all specimens underwent blinded histological examination and were scored 1–4 on mucosal wall thickening (A), leukocyte infiltration (B), adenomas (C) and adenocarcinomas (D). Data are represented as mean ± standard error. Points with an asterisk are significantly different when compared to the control group (P<0.05).

Figure 5. VSL#3 and conjugated linoleic acid (CLA) modulate colonic gene expression.

C57BL/6J mice (n = 60) were treated with the VSL#3 probiotic (n = 20), CLA-supplemented (1 g/100 g) (n = 20) or control diets (n = 20) for 32 days and challenged i.p. with azoxymethane (10 mg/kg) followed by 2% dextran sodium sulfate (DSS) in the drinking water for 7 days to induce colitis-associated colorectal cancer (CRC). Mice were euthanized on day 68. Expression of CD36 (A), cyclooxygenase 2 (COX2) (B), peroxisome proliferator-activated receptor γ (PPAR γ) (C) and angiostatin (D) were assessed by real-time quantitative PCR. Data are represented as mean ± standard error. Points with an asterisk are significantly different when compared to the control group (P<0.05).

Figure 6. VSL#3 and conjugated linoleic acid (CLA) modulate immune cell subsets in mesenteric lymph nodes (MLN), colonic lamina propria lymphocytes (LPL) and spleen.

C57BL/6J mice (n = 60) were treated with the VSL#3 probiotic (n = 20), CLA-supplemented (1 g/100 g) (n = 20) or control diets (n = 20) for 32 days and challenged i.p. with azoxymethane (10 mg/kg) followed by 2% dextran sodium sulfate (DSS) in the drinking water for 7 days to induce colitis-associated colorectal cancer (CRC). Mice were euthanized on day 68. MLN (A–C), spleen (D) and LPL (E–F) from wild type mice were immunophenotyped to identify immune cells subsets by flow cytometry. Data are represented as mean ± standard error. Points with an asterisk are significantly different when compared to the control group (P<0.05).