Proline oxidase, a p53-induced gene, targets COX-2/PGE2 signaling to induce apoptosis and inhibit tumor growth in colorectal cancers

Abstract

Proline oxidase (POX), a flavoenzyme localized at the inner mitochondrial membrane, catalyzes the first step of proline degradation by converting proline to pyrroline-5-carboxylate (P5C). POX is markedly elevated during p53-induced apoptosis and generates proline-dependent reactive oxygen species (ROS), specifically superoxide radicals, to induce apoptosis through both mitochondrial and death receptor pathways. These previous studies also showed suppression of the mitogen-activated protein kinase pathway leading us to broaden our exploration of proliferative signaling. In our current report, we used DLD-1 colorectal cancer cells stably transfected with the POX gene under the control of a tetracycline-inducible promoter and found that three pathways which cross talk with each other were downregulated by POX: the cyclooxygenase-2 (COX-2) pathway, the epidermal growth factor receptor (EGFR) pathway and the Wnt/bcatenin pathway. First, POX markedly reduced COX-2 expression, suppressed the production of prostaglandin E2 (PGE 2 ) and importantly, the growth inhibition by POX was partially reversed by treatment with PGE 2. Phosphorylation of EGFR was decreased with POX expression and the addition of EGF partially reversed the POXdependent downregulation of COX-2. Wnt/b-catenin signaling was decreased by POX in that phosphorylation of glycogen synthase kinase-3b (GSK-3b) was decreased on the one hand and phosphorylation of b-catenin was increased on the other. There changes led to decreased accumulation of b-catenin and decreased b-catenin/TCF/ LEF-mediated transcription. Our newly described POXmediated suppression of proliferative signaling together with the previously reported induction of apoptosis suggested that POX could function as a tumor suppressor. Indeed, in human colorectal tissue samples, immunohistochemically-monitored POX was dramatically decreased in tumors compared with normal counterparts. Thus, POX metabolism of substrate proline affects multiple signaling pathways, modulating both apoptosis and tumor growth, and could be an attractive target to metabolically control the cancer phenotypes.

Determination of common urine substances as an assay for improving prostate carcinoma diagnostics

“…microRNA miR-23b* an inhibitor of POX is highly expressed in different types of tumors (35,36). Due to the lack of POX, the conversion of proline to pyrroline-5-carboxylate (P5C) is altered (37), and subsequently the amount of proline in urine is increased. Based on these facts and the results showing increased levels of proline compared to controls, proline appears to be a biomolecule with the potential to enlarge the spectrum of diagnostic tools for CaP.…”

MiRNA and Proline Metabolism in Cancer

“…Further investigation on a variety of cancer tissues along with normal tissue counterparts including kidney, bladder, stomach, colon and rectum, liver, pancreas, breast, prostate, ovary, brain, lung, skin, etc., showed that 61% of all tumors had decreased expression of PRODH/POX compared to normal tissues, especially the tumor from kidney and digestive tract [46,47,62], suggesting tumor could eliminate the tumor suppressor roles of PRODH/POX. Suppression of PRODH/POX was more significant in kidney and digestive tract.…”

“…The inhibition of MEK/ERK pathway is involved in PRODH/POX-induced apoptosis. Secondly, PRODH/POX markedly reduces the expression of cyclooxygenase-2 (COX-2), and thus suppresses the production of prostaglandin E2 (PGE2) [47]. The addition of PGE2 partially reverses the apoptosis and inhibits tumor growth induced by PRODH/POX.…”

“…Activating mutants and overexpression of EGFR signaling contributes to carcinogenesis of various tumors by inducing cell proliferation and counteracting apoptosis [51]. Fourthly, Wnt/ -catenin signaling is decreased by PRODH/POX [47]. Constitutive activation of this signaling pathway is found in many human cancers, which regulates proliferation, differentiation and cell fate [52].…”

Cancer Cell Metabolism

“…At extremely high levels of ROS, p53 initiates a failsafe program of apoptosis, maximizing ROS levels through the mitochondrial apoptosis pathway to ensure cell death (Liu et al 2005(Liu et al , 2008(Liu et al , 2009). In the absence of functional p53, this defense mechanism is lost and ROS inexorably rise, fostering mutagenesis and transformation.…”