Proline oxidase, a proapoptotic gene, is induced by troglitazone: evidence for both peroxisome proliferator-activated receptor gamma-dependent and -independent mechanisms

Proline oxidase (POX) is a redox enzyme localized in the mitochondrial inner membrane. We and others have shown that POX is a p53-induced gene that can mediate apoptosis through generation of reactive oxygen species (ROS). The peroxisome proliferator-activated receptor gamma (PPARgamma) ligand troglitazone was found to activate the POX promoter in colon cancer cells. PPARgamma ligands have been reported to induce apoptosis in a variety of cancer cells. In HCT116 cells expressing a wild-type PPARgamma, troglitazone enhanced the binding of PPARgamma to PPAR-responsive element in the POX promoter and increased endogenous POX expression. Blocking of PPARgamma activation either by antagonist GW9662 or deletion of PPAR-responsive element in the POX promoter only partially decreased the POX promoter activation in response to troglitazone, indicating also the involvement of PPARgamma-independent mechanisms. Further, troglitazone also induced p53 protein expression in HCT116 cells, which may be the possible mechanism for PPARgamma-independent POX activation, since POX has been shown to be a downstream mediator in p53-induced apoptosis. In HCT15 cells, with both mutant p53 and mutant PPARgamma, there was no effect of troglitazone on POX activation, whereas in HT29 cells, with a mutant p53 and wild type PPARgamma, increased activation was observed by ligand stimulation, indicating that both PPARgamma-dependent and -independent mechanisms are involved in the troglitazone-induced POX expression. A time- and dose-dependent increase in POX catalytic activity was obtained in HCT116 cells treated with troglitazone with a concomitant increase in the production of intracellular ROS. Our results suggest that the induction of apoptosis by troglitazone may, at least in part, be mediated by targeting POX gene expression for generation of ROS by POX both by PPARgamma-dependent and -independent mechanisms.