Activation of PPAR γ in colon tumor cell lines by oxidized metabolites of linoleic acid, endogenous ligands for PPAR γ

Abstract

The nuclear hormone receptor peroxisome proliferator-activated receptor (PPAR) γ plays an important role in the differentiation of intestinal cells and other tissues. Realtime PCR examination of PPAR mRNA for γ1, γ2 and γ3, in Caco-2 and HCT-116 colon cell lines showed that γ3 is the most abundant message in both lines. Treatment of Caco-2 cells with sodium butyrate, which induces cell differentiation, also leads to an increase in all three PPAR mRNAs. In contrast, treatment of HCT-116 cells with sodium butyrate, which does not lead to differentiation of these cells, causes a decrease in the amount of all three PPAR mRNAs. Furthermore, the amount of PPAR mRNA is greater in Caco-2 cells than in HCT-116 cells at all times examined. As several oxidative metabolites of linoleic acid, including 13-hydroxyoctadecadienoic acid (13-HODE) and 13-oxooctadecadienoic acid (13-OXO) have been shown to bind PPAR, and there is a strong positive correlation between enzymes for metabolism of linoleate oxidation products, intestinal cell differentiation and the distribution of PPAR, we also performed a detailed investigation of the activation of PPAR γ by 13-HODE and 13-OXO. For these experiments, Caco-2 and HCT-116 cells were transfected with constructs containing PPAR γ1 or γ2 then a PPRE-luc reporter construct. Exposure of transfected cells to micromolar concentrations of 13-HODE or 13-OXO produced concentration-dependent increases in luciferase activity. In addition, the two linoleate metabolites activate endogenous PPAR in these cell lines transfected with only PPRE-luc. The data substantiate the contention that oxidation products of linoleic acid are metabolically produced endogenous ligands for PPAR γ and that PPAR γ plays an important role in the differentiation of intestinal cells.