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James M. Phang, M.D.
Metabolic Mechanisms for Modulating the Cancer Susceptible Phenotype
The underlying theme of the laboratory is to elucidate metabolic mechanisms for regulating proliferation, apoptosis, autophagy and other stress-related responses. The importance of metabolic regulation in cancer has been emphasized and recent advances have focused on epigenetic mechanisms which may be sensors for metabolic reprogramming. From the early work of Warburg, metabolic research in cancer has focused on core metabolism (glucose) and on its shift to aerobic glycolysis. Investigators generally agree that the endpoint of this reprogramming is to provide building blocks for increasing cell mass. Recent studies have revealed parametabolic regulation contributed by the metabolism of nonessential amino acids. Glutamine, glutamate, serine, glycine, and aspartate - each plays a special role in cancer metabolism. Our laboratory has emphasized studies of proline, the unique, secondary proteinogenic amino (imino) acid. We have shown that the degradation of proline is a special source of redox signaling. On the other hand, the biosynthesis of proline functions to maintain redox homeostasis and provide redox cycling to maximize growth.
The Proline Regulatory Axis: Metabolic Reprogramming in Cancer
Glutamic-gamma-semialdehyde (GSA), an open-chain molecule is in equilibrium with its cyclized tautomer, pyrroline-5-carboxylate (P5C). They are the direct oxidation products of proline. GSA and P5C are at the center of intermediary metabolism with pathways interconnecting proline, glutamate and arginine. GSA/P5C is the obligate intermediate in the transfer of carbons between the TCA and urea cycles. In earlier studies, a proline cycle was identified which transfers redox potential between cytosol and mitochondria. More recently, the regulation of the pathway with metabolic reprogramming suggests that it plays an important role in carcinogenesis. The proline degradative enzyme, proline dehydrogenase (PRODH), a.k.a. proline oxidase (POX) is upregulated by p53, PPARgamma and AMPK and downregulated by miR-23b* and proto-oncogene c-MYC. Tightly bound to mitochondrial inner membranes, PRODH/POX transfers a pair of electrons through complex II to Coenzyme Q to generate ATP. Importantly, proline-derived ROS are generated by complex III for redox signaling. The endpoint of this signaling, however, depends on context. Induced by p53 or PPARgamma, the ROS from PRODH/POX block cell cycle progression and initiate apoptosis. But with nutrient deprivation or hypoxia, PRODH is upregulated by AMPK and the ROS signaling activates prosurvival autophagy. Recent studies suggest that these redox features of PRODH/POX are enabled by the interactions with the structure and functions of complex II while the ROS are generated by complex III. A working hypothesis is that in many signaling mechanisms dependent on ROS, PRODH/POX is the source of the ROS.
In contrast to proline degradation, the proline biosynthetic enzymes are upregulated by the proto-oncogene c-MYC. The utilization of glutamine is increased by upregulation of glutaminase. By upregulating the enzymes of proline biosynthesis, MYC also makes proline an important product of glutamine. The reprogramming provides parametabolic regulation for recycling of pyridine nucleotides. Knockdown of proline biosynthetic enzymes markedly inhibits MYC-activated glycolysis. Thus, proline degradation and biosynthesis provide redox signaling for epigenetic programming and for maintenance of redox homeostasis. Our discovery of the proline regulatory axis led others to show its critical signaling function in their experimental systems including: 1) the prolongation of survival with impaired insulin/IGF-1 signaling in C. elegans and mouse embryonic fibroblasts: 2) the induction of adipose triglyceride lipase with nutrient deprivation; and 3) the activation of the 'embryonic-stem-cell-to-mesenchymal-like transition.'
The elucidation of the unique role of the mechanisms by which PRODH/POX functions to generate redox for signaling, and its regulation by a variety of stress-related mechanisms suggest that the proline axis is critical for metabolic epigenetics. In our current studies, we are identifying the targets for proline-derived ROS signaling. A number of transcriptional factors are dependent on ROS for translocation into the nucleus. Additionally, the metabolic interlocks formed by the proline biosynthetic pathway may be involved in the production of critical substrates and also to maintain redox homeostasis. These studies may lead to novel metabolic targets for prevention or treatment of cancer.
This page was last updated on 11/19/2013.