Conjugated linoleic acid ameliorates inflammation-induced colorectal cancer in mice through activation of PPARgamma

Abstract

Conjugated linoleic acid (CLA) exerts a protective effect on experimental inflammatory bowel disease and shows promise as a chemopreventive agent against colorectal cancer (CRC) in mice, although the mechanisms by which it exerts its beneficial effects against malignancies in the gut are not completely understood. Mice lacking PPARgamma in immune and epithelial cells and PPARgamma-expressing littermates were fed either control or CLA-supplemented (1 g CLA/100 g) diets to determine the role of PPARgamma in inflammation-induced CRC. To induce tumor formation and colitis, mice were treated with azoxymethane and then challenged with 2% dextran sodium sulfate, respectively. Dietary CLA ameliorated disease activity, decreased colitis, and prevented adenocarcinoma formation in the PPARgamma-expressing floxed mice but not in the tissue-specific PPARgamma-null mice. Dietary CLA supplementation significantly decreased the percentages of macrophages in the mesenteric lymph nodes (MLN) regardless of the genotype and increased regulatory T cell numbers in MLN of PPARgamma-expressing, but not in the tissue-specific, PPARgamma-null mice. Colonic tumor necrosis factor-alpha mRNA expression was significantly suppressed in CLA-fed, PPARgamma-expressing mice. This study suggests CLA ameliorates colitis and prevents tumor formation in part through a PPARgamma-dependent mechanism.

FIGURE 1

Effect of CLA on PPARγ reporter activity in Raw 264.7 macrophages (A) and SW480 IEC (B). In A, macrophages were stimulated with 2.5, 5, or 10 μmol/L CLA for 24 h. In B, IEC were stimulated with CLA for 4 or 24 h. Rosiglitazone (Ros) was used in both experiments as a positive control to stimulate PPARγ activity. Values are means + SEM, n = 6. Means without a common letter differ, P < 0.05.

FIGURE 2

Effect of dietary CLA on disease activity index (DAI) in PPARγ floxed (A) and tissue-specific PPARγ null mice (B) at 56, 98, 112, and 126 d, and on the multiplicity of adenocarcinomas (C) and adenomas (D) in PPARγ floxed (black bars) and tissue-specific PPARγ null mice (white bars) on d 126. Values are means + SEM, n = 10. Means at a time with an asterisk differ, P < 0.05.

FIGURE 3

Effect of dietary CLA on the percentages of macrophages (A) and Treg (B) in MLN from PPARγ floxed (black bars) and PPARγ null (white bars) mice. Values are means + SEM, n = 10. Means without a common letter differ, P < 0.05.

FIGURE 4

Effect of dietary CLA on lymphocytic/macrophage infiltration into colonic lamina propria of PPARγ floxed (black bars) and PPARγ null (white bars) mice. Values are means + SEM, n = 10. Means without a common letter differ, P < 0.05.

FIGURE 5

Effect of dietary CLA on Ly6c (red) and F4/80 (green) staining in the colonic lamina propria from PPARγ floxed (black bars) and PPARγ null mice (white bars). (A) Representative photomicrographs illustrating colocalization of F4/80 and Ly6c at an original magnification of 200×. (B,C) The numbers of F4/80- and Ly6c-positive cells, respectively. Values are means + SEM, n = 4.

FIGURE 6

Effect of dietary CLA on colonic expression of TNFα, CD36, and IL-4 in PPARγ floxed (black bars) and PPARγ null (white bars) mice. Values are means + SEM, n = 10. Means without a common letter differ, P < 0.05.