PPAR-γ ligand promotes the growth of APC-mutated HT-29 human colon cancer cells in vitro and in vivo

PPAR-γ has been known to induce suppression, differentiation and reversal of malignant changes in colon cancer in vitro. However, there are several reports that PPAR-γ ligands enhance colon polyp development in APC^min mice in vivo. These contradictory results have not yet been thoroughly explained. To explain the contradictory results, we analyzed the effects of different concentrations of the PPAR-γ agonist, 15-deoxy-D12, 14-prostaglandin (15-d Δ PGJ2) and pioglitazone, on APC gene-mutated colon cancer cell lines (HT-29). We measured cell growth and suppression by cell count and MTT assay and analyzed the expression of β-catenin and c-Myc protein by Western blot. In addition, we inoculated HT-29 cells into APC^min mice to compare tumor size. High concentrations (10–100 μM/L 15-d Δ PGJ2 and pioglitazone) of PPAR-γ ligand suppressed growth, while low concentrations (0.01–1 μM/L 15-d Δ PGJ2 and pioglitazone) of PPAR-γ ligand promoted growth. In particular, the effects of 0.1 μM/L 15-d Δ PGJ2 and pioglitazone on cell growth were statistically significant (P = 0.003, P = 0.001, respectively). Tumor growth was associated with an increase in β-catenin and c-Myc expression. The growth of xenograft tumors was greater in PPAR-γ ligand-treated mice than in control mice (control vs day 14: P = 0.024, control vs day 28: P = 0.007). The expression of β-catenin and c-Myc protein were also elevated in PPAR-γ-treated mouse tissues. PPAR-γ ligand can promote the growth of APC-mutated HT-29 colon cancer cells in vitro and in vivo. In addition, the tumor promoting effect seems to be associated with an increase in β-catenin and c-Myc expression. We think that well-controlled clinical trials should be conducted to confirm our results and to verify clinical applications.