Proline oxidase produces superoxide which activates both limbs of the apoptotic pathway. Abbreviations: TZDs, thiazolidinediones; TRAIL, tumor necrosis factor related apoptosis-inducing ligand; DR5, death receptor 5, PARP, polyadenoribosyl polymerase; MEK, MAP kinase kinase; ERK, extracellular-signal regulated MAP kinase.