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/beta-catenin 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 POX-dependent downregulation of COX-2. Wnt/beta-catenin signaling was decreased by POX in that phosphorylation of glycogen synthase kinase-3beta (GSK-3beta) was decreased on the one hand and phosphorylation of beta-catenin was increased on the other. There changes led to decreased accumulation of beta-catenin and decreased beta-catenin/TCF/LEF-mediated transcription. Our newly described POX-mediated 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.