Tumor suppressor activities of proline oxidase
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Proline oxidase (POX) is a p53-regulated redox gene that mediates tumor suppression by inducing cell cycle arrest and proline-dependent apoptosis. A degenerate p53 consensus binding site in the POX promoter allows the gene to respond to p53. POX functions as a mitochondrial tumor suppressor by inducing intrinsic and extrinsic apoptosis, mediating a G2 cell cycle arrest, and reducing hypoxia-inducible factor (HIF) signaling. Consistent with its role as a tumor suppressor, POX expression levels are frequently much lower in renal, colorectal, pancreatic, liver, and stomach tumor tissues than in their normal tissue counterparts. The loss of POX is not due to genetic or epigenetic mechanisms but instead occurs through upregulation of the mir-23b microRNA, which suppresses translation of POX mRNA by binding to its 3' untranslated region. P53-induced POX generates reactive oxygen species (ROS) through oxidation of proline. Pyrroline-5- carboxylate, the product of proline oxidation, is recycled back to proline withredox transfers or converted to glutamate and a-ketoglutarate; the latter enhances the prolyl hydroxylation of HIF and its proteasomal degradation. POX mediates the activation of the NFAT transcription factor by p53 through activation of the calcium/calmodulin-dependent phosphatase, calcineurin. POX-induced ROS mobilizes calcium from intracellular stores to activate calcineurin, which then activates the NFAT transcription factor to modulate gene expression and apoptosis. The initiation of calcineurinregulated transcription factor pathways may affect gene expression events important in not only apoptosis but other p53-regulated physiological processes. POX also mediates p53-independent signaling pathways. Peroxisome proliferator-activated receptor ligands mediate apoptosis in human lung cancers by inducing POX. Upregulation of POX expression dramatically decreases phosphorylation of MEK and ERK, which is partially reversed by expression of manganese superoxide dismutase. Overexpression of a constitutively active form of MEK suppressed POX-induced apoptosis, suggesting the involvement of MEK/ERK signaling and further confirming the role of ROS/superoxides in POX-induced apoptosis. Etoposide, a conventional anticancer drug, upregulates p53, POX, NFAT, and ROS, and induces apoptosis, in responsive tumor cells. The involvement of POX in both p53-dependent and -independent tumor suppressor pathways makes it an attractive strategic target for cancer therapy in a variety of human cancers. 2013 Nova Science Publishers, Inc. All rights reserved.
