Upregulation of proline oxidase transcriptional activity by peroxisome proliferator-activated receptor-gamma (PPAR-γ)

4350 Proline oxidase (POX), also known as proline dehydrogenase, is a mitochondrial membrane-bound enzyme catalyzing the conversion of proline to pyrroline-5-carboxylate coupled to the transfer of electrons to cytochrome c . We and others have shown that POX is a p53-induced gene and hyperexpression of POX in colon cancer cells is sufficient for initiating the apoptotic cascade. Despite the demonstrated role of POX in p53-mediated growth inhibition and apoptosis, the molecular mechanisms regulating its expression are not defined. To gain an understanding of these mechanisms, we cloned the human POX promoter region and produced a POX promoter/luciferase reporter construct. By transient transfection studies using expression constructs of several transcription factors in human colon cancer (LoVo) cells and human embryonic kidney (HEK293) cells, we character-ized mechanisms involved in POX expression. Analysis of the promoter nucleotide sequence revealed a number of putative transcription factor-binding sites including C/EBP, AP-1, Sp1, RAR, PPAR, RXR and others which became the basis for our initial studies. In LoVo cells, several transcription factors (PEA3, p300, c-Jun, c-Fos, Fra 1, Jun-D) were modestly active in POX expression, but the most active was PPAR-γ. We found a similar response in HEK 293 cells in which expression of PPAR-γ resulted in a seven- to ten-fold activation of the POX promoter. This effect of PPAR-γ was further enhanced by PPAR-γ ligands, troglitazone and ciglitizone, in a concentration-dependent manner. Peroxisome proliferator-activated receptors (PPARs) have received recent attention because these nuclear hormone receptors, which function as ligand-activated transcription factors regulating lipid metabolism and homeostasis, are involved in genetic control of many cellular processes. Recent reports by others suggest that PPAR-γ ligands induce apoptotic cell death in colon cancer cells and suppress colon carcinogenesis in mice. Our current results establish a significant role for PPAR-γ in regulation of POX expression and suggest that POX is a target gene of PPAR-γ mediating the apoptotic process.