Inverse Relationship between 15-Lipoxygenase-2 and PPAR-γ Gene Expression in Normal Epithelia Compared with Tumor Epithelia

Abstract15-Lipoxygenase-2 (15-LOX-2) synthesizes 15-S-hydroxyeicosatetraenoic acid (15-S-HETE), an endogenous ligand for the nuclear receptor, peroxisome proliferator-activated receptor-γ (PPAR-γ). Several studies have described an inverse relationship between 15-LOX-2 and PPAR-γ expression in normal versus tumor samples. To systematically determine if this is a ubiquitous phenomenon, we used a variety of epithelial and nonepithelial cells and some tissues to further evaluate the extent of this inverse relationship. The levels of mRNA or protein were measured by reverse transcriptase polymerase chain reaction or Western gray level intensity, whereas distribution was determined by in situ hybridization or immunofluorescence. 15-S-HETE was measured by liquid chromatography/tandem mass spectrometry. Normal epithelial cells/samples generally expressed high levels of 15-LOX-2 along with the enzyme product 15-S-HETE, but both levels were reduced in cancer cells/samples. In contrast, most cancer cells expressed high levels of PPAR-γ mRNA and protein, which were absent from normal epithelial cells. Overall, the inverse relationship between these two genes was primarily restricted to epithelial samples. Forced expression of PPAR-γ reduced 15-LOX-2 protein levels in normal cells, whereas forced expression of 15-LOX-2 in tumor cells suppressed PPAR-y protein levels. These results suggest that feedback mechanisms may contribute to the loss of 15-LOX-2 pathway components, which coincide with an increase in PPAR-γ in many epithelial cancers