Proline plays a special role in cancer metabolism. Proline oxidase (POX), a.k.a. proline dehydrogenase (PRODH), is among a few genes induced rapidly and robustly by P53, the tumor suppressor. Ectopic expression of POX under control of tet-off promoter initiated mitochondrial apoptosis. The mechanism activated by POX is mediated by its production of ROS. In immunodeficient mice, POX overexpression markedly retarded growth of xenograft tumors. In human tumors of the digestive tract and kidney, POX was markedly decreased, suggesting that the suppressive effect of POX was downregulated. This was not due to POX gene mutations or hypermethylation. Instead, a microRNA, miR-23b*, expressed at high levels in tumors, was a potent inhibitor of POX expression. Furthermore, antagomirs of miR-23b* reversed the downregulated expression of POX and its tumor-suppressive effect, thereby providing a therapeutic strategy. POX not only responds to genotoxic stress, but also to inflammatory and metabolic stress. Depending on microenvironmental and temporal factors, POX can mediate oppositely-directed responses-programmed cell death, on the one hand, and survival, on the other.