Person:

Mullarky, Edouard

Cancer cells exhibit metabolic dependencies that distinguish them from their normal counterparts1. Among these addictions is an increased utilization of the amino acid glutamine (Gln) to fuel anabolic processes2. Indeed, the spectrum of Gln-dependent tumors and the mechanisms whereby Gln supports cancer metabolism remain areas of active investigation. Here we report the identification of a non-canonical pathway of Gln utilization in human pancreatic ductal adenocarcinoma (PDAC) cells that is required for tumor growth. While most cells utilize glutamate dehydrogenase (GLUD1) to convert Gln-derived glutamate (Glu) into α-ketoglutarate in the mitochondria to fuel the tricarboxylic acid (TCA) cycle, PDAC relies on a distinct pathway to fuel the TCA cycle such that Gln-derived aspartate is transported into the cytoplasm where it can be converted into oxaloacetate (OAA) by aspartate transaminase (GOT1). Subsequently, this OAA is converted into malate and then pyruvate, ostensibly increasing the NADPH/NADP+ ratio which can potentially maintain the cellular redox state. Importantly, PDAC cells are strongly dependent on this series of reactions, as Gln deprivation or genetic inhibition of any enzyme in this pathway leads to an increase in reactive oxygen species and a reduction in reduced glutathione. Moreover, knockdown of any component enzyme in this series of reactions also results in a pronounced suppression of PDAC growth in vitro and in vivo. Furthermore, we establish that the reprogramming of Gln metabolism is mediated by oncogenic Kras, the signature genetic alteration in PDAC, via the transcriptional upregulation and repression of key metabolic enzymes in this pathway. The essentiality of this pathway in PDAC and the fact that it is dispensable in normal cells may provide novel therapeutic approaches to treat these refractory tumors.

Cancer cells are known to reprogram their metabolism in order to promote growth and proliferation. The amino acid serine is utilized in a plethora of anabolic reactions and supports the synthesis of all three major macromolecular classes: proteins, lipids, and nucleic acids. Serine can either be synthesized de novo via the phosphoserine pathway or imported from the extracellular space via amino acid transporters. The gene encoding the enzyme 3-phosphoglycerate dehydrogenase (PHGDH), which catalyzes the first committed step of the phosphoserine pathway, is focally amplified in human cancers suggesting that it is pro-tumorigenic. Cancer cell lines that harbor PHGDH amplifications, or over express PHGDH independently of amplification, are uniquely sensitive to genetic ablation of the pathway. In contrast, cancer cell lines that express little PHGDH, and instead rely on serine import, are resistant to genetic ablation of the pathway. Given these observations, we speculated that PHGDH might be a clinically interesting target in oncology and sought to develop small molecule inhibitors of PHGDH in order to provide tool compounds with which to study the biology of PHGDH and evaluate the efficacy of inhibiting serine synthesis in cancers. In order to identify inhibitors of PHGDH an in vitro enzymatic assay was developed and libraries of drug-like small molecules were screened. Hit compounds were validated in biochemical assays to determine potency and selectivity for PHGDH. Selected compounds were tested on cells for their ability to inhibit de novo serine synthesis and one lead, CBR-5884, was identified. CBR-5884 was selectively toxic to PHGDH amplified or overexpressing cancer cells but had no effect on cells that express little PHGDH. Mechanistically, CBR-5884 was found to be a non-competitive inhibitor that showed a time dependent onset of inhibition and disrupted the oligomerization state of PHGDH. These results provide a proof-of-concept for targeting PHGDH and suggest that inhibiting PHGDH in cancers addicted to serine synthesis is a potentially viable targeted therapy option.