The Janus-like role of proline metabolism in cancer

Abstract

The metabolism of the non-essential amino acid L-proline is emerging as a key pathway in the metabolic rewiring that sustains cancer cells proliferation, survival and metastatic spread. Pyrroline-5-carboxylate reductase (PYCR) and proline dehydrogenase (PRODH) enzymes, which catalyze the last step in proline biosynthesis and the first step of its catabolism, respectively, have been extensively associated with the progression of several malignancies, and have been exposed as potential targets for anticancer drug development. As investigations into the links between proline metabolism and cancer accumulate, the complexity, and sometimes contradictory nature of this interaction emerge. It is clear that the role of proline metabolism enzymes in cancer depends on tumor type, with different cancers and cancer-related phenotypes displaying different dependencies on these enzymes. Unexpectedly, the outcome of rewiring proline metabolism also differs between conditions of nutrient and oxygen limitation. Here, we provide a comprehensive review of proline metabolism in cancer; we collate the experimental evidence that links proline metabolism with the different aspects of cancer progression and critically discuss the potential mechanisms involved.

Keywords: Cancer; Cancer metabolism.

Fig. 1. The proline metabolic pathway.

A The proline metabolic pathway. The NEAA L-proline is formed through reduction of precursor P5C that is obtained via two pathways, from glutamine in the mitochondria and from ornithine in the cytosol. B Chemical structures of the proline metabolic pathway intermediates. Note the secondary amine group in proline is different from that of other amino acids. Produced in Chemdraw.

Fig. 2. Detailed 3D structure of the human PYCR1 enzyme.

A Overview of space-filling model of PYCR1 (PDB ID 5UAV, ref. ). PYCR1 forms a pentamer of dimers, with individual dimer subunits colored blue and red, respectively. B Zoomed view of the NADPH binding site and the active site. NADPH is bound to the N-terminal Rossmann-fold dinucleotide-binding domain, which forms the outer part of the protomer, whereas the P5C/proline analog L-tetrahydrofuroic acid (THFA) interacts with the C-terminal dimerization domains of both protomers. NADPH and THFA are shown as sticks with dark gray and light gray carbons, respectively. C Close-up cartoon of the active site of PYCR1. Residues within 4 Å of THFA or the nicotinamide moiety of NADPH are shown as sticks, water oxygens are shown as red spheres, and hydrogen bonds are highlighted with purple dashes. The substrate analog is mainly stabilized by a hydrogen bond network with the backbone and side-chain atoms of Ser233 and Thr238 and 2 water molecules. The images were prepared using PyMOL 2.3.5 (ref. ).

Fig. 3. Proline metabolism is regulated by many cellular pathways and influence key cellular signaling nodes.

A Regulation of the proline metabolic pathway is supported by the activity of numerous signaling pathways (→). Feedback inhibition loops (--|) also contribute to regulate the rate of the proline metabolism pathway. B The proline metabolic pathway enzymes regulate signaling activity of numerous pathways both positively (→) and negatively (--|).

Fig. 4. Proline metabolism in the regulation of cell survial and redox balance.

A PRODH has been implicated in the activation of apoptosis via both the intrinsic and extrinsic pathways. Activation of the intrinsic pathway by reactive oxygen species (ROS) is mediated by the p53-induced transcriptional activation of genes, including PRODH1. In the intrinsic pathway, the BH3-only proteins, such as NOXA and PUMA, inactivate the pro-survival proteins such as BCL-2 and lead to the release of pro-apoptotic proteins such as BAX. This allows the permeabilization of the mitochondrial outer membrane. Cytochrome c is released from the mitochondria and stimulates assembly of the apoptosome containing APAF-1 and pro-caspase 9. The apoptosome leads to the activation of caspase 9, which, in turn, activates the effector caspase 3 resulting in apoptosis. The extrinsic pathway of apoptosis is initiated by the activation of the death receptor of the TNF superfamily, such FAS and TRAIL. Receptor engagement and trimerization lead to the recruitment and activation of caspase-8 in the death inducing signaling complex (DISC), which in turn activates the effector caspase 3, resulting in apoptosis. PRODH1 has been shown to induce expression of the death receptor 5 (DR5) and its ligand TRAIL through stimulation of the nuclear factor of activated T cells (NFAT) transcription factor. B Schematic representation of the proposed proline cycle. L-proline is oxidized to P5C by PRODH1 in the mitochondria. P5C is then reduced back to L-proline through the action of the PYCR3 enzyme in the cytosol, a reaction coupled with the oxidation of NADPH. The cycle shuttles electrons from the cytosol to the mitochondria and stimulates flux through the PPP via oxidation of NADPH. In this way, the proline cycle can potentially contribute to nucleotide biosynthesis.