Abstract PR05: Aspartate metabolism links the urea cycle with nucleic acid synthesis in cancerous proliferation

Argininosuccinate synthase (ASS1) is a urea cycle cytosolic enzyme that conjugates aspartate transported across the mitochondria and citrulline. In the liver, this is a critical step in conversion of nitrogenous waste to urea, whereas in most other tissues, it is the penultimate step in arginine synthesis. Citrullinemia is a urea cycle disorder caused by germline mutations that lead to decreased flux through ASS1. Citrullinemia type I (CTLN I) is caused by ASS1 deficiency and citrullinemia type II (CTLN II) is caused by deficiency in mitochondrial aspartate transporter Citrin. In contrast to the established role of ASS1 in ureagenesis, it was found to be somatically silenced in multiple cancers for which the purpose is unknown. Whereas ASS1 silencing renders the tumors auxotrophic for arginine, we hypothesized that down-regulation of ASS1 has an arginine-independent survival effect by redirecting of aspartate towards pyrimidine synthesis. Supported by computational modeling and using multiple methodologies including studies of fibroblasts from patients with CTLN I and CTLN II, cancer cells, clinical data, robust informatics analysis of multiple tumors, we show that ASS1 is a key regulator of the mitochondria-derived aspartate flux. Silencing of ASS1, leads to preferential diversion of aspartate away from the synthesis of arginine and urea to pyrimidine synthesis. Decreasing aspartate flux to pyrimidine synthesis by either expressing ASS1 in cancer cells that have endogenous silencing, or by blocking the transport of aspartate through the mitochondrial membrane by inhibiting Citrin, decreases cell proliferation due to decreased nucleic acid synthesis. Our results demonstrate that ASS1 silencing is a novel mechanism to support nucleic acid synthesis in cancers and provides the first metabolic link between the urea cycle enzymes and pyrimidine synthesis. Citation Format: Shiran Rabinovich, Keren Yizhak, Qin Sun, Alexander Brandis, Daniel Helbling, David Dimmock, Sandesh Nagamani, Eytan Ruppin, Ayelet Erez. Aspartate metabolism links the urea cycle with nucleic acid synthesis in cancerous proliferation. [abstract]. In: Proceedings of the AACR Special Conference: Metabolism and Cancer; Jun 7-10, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(1_Suppl):Abstract nr PR05.