The induction of proline oxidase accompanying p53-dependent induction of apoptosis suggests that the proline metabolic pathway plays a role in programmed cell death. We are studying this metabolic pathway at the level of : a) Proline oxidase - its induction and its role in apoptosis, b) Pyrroline 5-carboxylate (P5C) and P5C Reductase in cell signaling and c) Ornithine aminotransferase in embryonic development. a) Proline oxidase - its induction and its role in apoptosis. We have focused on proline oxidase, the enzyme which catalyzes the conversion of proline to pyrroline 5-carboxylate. The enzyme is bound to mitochondrial inner membranes and donates electrons to complex II. Recently others have shown that proline oxidase (POX) is one of only 14 genes (from 7202 genes monitored by SAGE) highly induced accompanying p53-dependent induction of apoptosis in colorectal tumor cells. To investigate the role of POX, we first used RT-PCR to examine several cell types and the stimuli which will result in POX induction. In human colorectal cancer (LoVo) cells, mouse colonic epithelial cells transformed by temperature sensitive SV40 (YAMC), as well as a variety of carcinoma cell lines, we found that serum starvation as well as cytotoxic drugs induced proline oxidase in a p53-dependent fashion. We proposed that proline oxidase may contribute to the metabolic events accompanying apoptosis by generating reactive oxygen species (ROS). This possibility was examined in LoVo and YAMC cells by using a fluorescent probe and laser cytometry to monitor ROS. Importantly, we included DLD1-POX, a p53-negative colon tumor cell line stably transfected with POX. Interestingly, the addition of proline markedly increased ROS generation in a concentration-dependent manner but only when POX was expressed. Glutamate, on the other hand, was without effect. We are currently identifying other proline-modulated events downstream in the apoptosis pathway. b) Pyrroline 5-carboxylate (P5C) as a signaling and regulatory molecule. Pyrroline 5-carboxylate (P5C), the product of proline oxidase, ornithine aminotransferase and glutamate synthase has been shown to have regulatory activities. Recent work suggests that P5C reductase (P5CR) may play a role in receptor-mediated regulation. The two known isozymes of P5CR may serve distinct functions. P5CR1 is a NADH-dependent enzyme regulated to produce proline whereas P5CR2 is a NADPH-dependent enzyme coupled with redox-related regulation. In lymphocytes activated by lectins, it appears that P5CR2 is induced. We are defining the interaction of P5CR2 with membrane receptors. c) Ornithine aminotransferase in embryonic development. Ornithine aminotransferase (OAT) converts ornithine to P5C and inherited deficiencies of this enzyme results in degeneration of the neuroretina. Using Xenopus embryos as a model, we have shown that OAT is expressed during a critical stage in neural development. In situ hybridization with Xenoopus OAT cDNA shows that expression of OAT correlates with critical stages of neural development. Since the pathway for neurodevelopment has been characterized, the interaction of OAT or its products in modulating this pathway is a promising area of research.
