Proline oxidase, encoded by p53-induced gene-6, catalyzes the generation of proline-dependent reactive oxygen species

The p53-dependent initiation of apoptosis is accompanied by the induction o f proline oxidase (POX), a mitochondrial enzyme catalyzing the conversion o f proline to pyrroline-5-carboxylate with the concomitant transfer of elect rons to cytochrome c, However, the contribution of increased POX activity t o apoptosis, if any, remains unknown, Using Adriamycin to initiate p53-depe ndent apoptosis, we showed that the expression of POX is up-regulated in a time- and dose-dependent manner in a human colon cancer cell line (LoVo), I n cells expressing POX, the addition of proline increases reactive oxygen s pecies (ROS) generation in a concentration-dependent manner; glutamate, a d ownstream product of proline oxidation, had no effect, Induction of POX was dependent on the p53 status of the cell. In the conditionally immortalized murine colonic epithelial cell line YAMC. where the p53 phenotype can be m odulated by temperature, proline oxidase expression and ROS production coul d only be induced when the cells were phenotypically p53-positive. To confi rm that the observed ROS production was not secondary to some other effect of p53, we also conditionally expressed POX in a p53-negative colon cancer line, Again, we found a proline-dependent ROS increase with POX expression, We hypothesize that proline oxidation supports the generation of KOS by do nating reducing potential to an electron transport chain altered either by p53-dependent mechanisms or by overexpression of POX.