Metabolic Mechanisms for Modulating the Cancer Susceptible Phenotype

The underlying theme of our laboratory was 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 advances have focused on epigenetic mechanisms that 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. Studies revealed that parametabolic regulation contributed to the metabolism of nonessential amino acids (glutamine, glutamate, serine, glycine and aspartate) and that each amino acid plays a special role in cancer metabolism. Our laboratory emphasized studies of proline, the unique, secondary proteinogenic amino (imino) acid. We showed 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). These 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 tricarboxylic acid (TCA) and urea cycles. In earlier studies, a proline cycle was identified that transfers redox potential between the cytosol and mitochondria. The regulation of this pathway with metabolic reprogramming suggested that it played 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 the 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 adenosine triphosphate (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. Studies suggested that these redox features of PRODH/POX were enabled by the interactions with the structure and functions of complex II, while the ROS were 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 the 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/insulin-like growth factor 1 (IGF-1) signaling in Caenorhabditis 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 studies, we identified the targets for proline-derived ROS signaling. A number of transcriptional factors were 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 the maintenance of redox homeostasis. These studies may lead to novel metabolic targets for the prevention or treatment of cancer.