title: Diversion of aspartate in ASS1-deficient tumours fosters de novo pyrimidine synthesis.
abstract: Cancer cells hijack and remodel existing metabolic pathways for their benefit. Argininosuccinate synthase (ASS1) is a urea cycle enzyme that is essential in the conversion of nitrogen from ammonia and aspartate to urea. A decrease in nitrogen flux through ASS1 in the liver causes the urea cycle disorder citrullinaemia. In contrast to the well-studied consequences of loss of ASS1 activity on ureagenesis, the purpose of its somatic silencing in multiple cancers is largely unknown. Here we show that decreased activity of ASS1 in cancers supports proliferation by facilitating pyrimidine synthesis via CAD (carbamoyl-phosphate synthase 2, aspartate transcarbamylase, and dihydroorotase complex) activation. Our studies were initiated by delineating the consequences of loss of ASS1 activity in humans with two types of citrullinaemia. We find that in citrullinaemia type I (CTLN I), which is caused by deficiency of ASS1, there is increased pyrimidine synthesis and proliferation compared with citrullinaemia type II (CTLN II), in which there is decreased substrate availability for ASS1 caused by deficiency of the aspartate transporter citrin. Building on these results, we demonstrate that ASS1 deficiency in cancer increases cytosolic aspartate levels, which increases CAD activation by upregulating its substrate availability and by increasing its phosphorylation by S6K1 through the mammalian target of rapamycin (mTOR) pathway. Decreasing CAD activity by blocking citrin, the mTOR signalling, or pyrimidine synthesis decreases proliferation and thus may serve as a therapeutic strategy in multiple cancers where ASS1 is downregulated. Our results demonstrate that ASS1 downregulation is a novel mechanism supporting cancerous proliferation, and they provide a metabolic link between the urea cycle enzymes and pyrimidine synthesis.
authors.affiliation: Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel.
authors.forename: Shiran
authors.initials: S
authors.lastname: Rabinovich
authors.affiliation: Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel.
authors.forename: Lital
authors.initials: L
authors.lastname: Adler
authors.affiliation: The Blavatnik School of Computer Science, Tel-Aviv University, Tel-Aviv.
authors.forename: Keren
authors.initials: K
authors.lastname: Yizhak
authors.affiliation: Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel.
authors.forename: Alona
authors.initials: A
authors.lastname: Sarver
authors.affiliation: Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel.
authors.forename: Alon
authors.initials: A
authors.lastname: Silberman
authors.affiliation: Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel.
authors.forename: Shani
authors.initials: S
authors.lastname: Agron
authors.affiliation: Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel.
authors.forename: Noa
authors.initials: N
authors.lastname: Stettner
authors.affiliation: Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas.
authors.forename: Qin
authors.initials: Q
authors.lastname: Sun
authors.affiliation: Biological services, Weizmann Institute of Science, Rehovot, Israel.
authors.forename: Alexander
authors.initials: A
authors.lastname: Brandis
authors.affiliation: Human and Molecular Genetic and Biochemistry center, Medical College Wisconsin, Milwaukee, Wisconsin.
authors.forename: Daniel
authors.initials: D
authors.lastname: Helbling
authors.affiliation: Genetic and Metabolic Center, Hadassah Medical Center, Jerusalem, Israel.
authors.forename: Stanley
authors.initials: S
authors.lastname: Korman
authors.affiliation: Department of Molecular Cell Biology, Weizmann Institute of Science, Rehovot, Israel.
authors.forename: Shalev
authors.initials: S
authors.lastname: Itzkovitz
authors.affiliation: Human and Molecular Genetic and Biochemistry center, Medical College Wisconsin, Milwaukee, Wisconsin.
authors.forename: David
authors.initials: D
authors.lastname: Dimmock
authors.affiliation: Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel.
authors.forename: Igor
authors.initials: I
authors.lastname: Ulitsky
authors.affiliation: Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas.|Texas Children's Hospital, Houston, TX, USA.
authors.forename: Sandesh Cs
authors.initials: SC
authors.lastname: Nagamani
authors.affiliation: The Blavatnik School of Computer Science, Tel-Aviv University, Tel-Aviv.|The Sackler School of Medicine, Tel-Aviv University, Tel-Aviv.|Center for Bioinformatics and Computational Biology & Dept. of Computer Science, University of Maryland, College Park, MD.
authors.forename: Eytan
authors.initials: E
authors.lastname: Ruppin
authors.affiliation: Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel.
authors.forename: Ayelet
authors.initials: A
authors.lastname: Erez
mesh_terms: D000818:Animals; D001124:Argininosuccinate Synthase / Q000172:deficiency* / Q000378:metabolism; D001221:Aspartate Carbamoyltransferase / Q000378:metabolism; D001224:Aspartic Acid / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D002223:Carbamoyl-Phosphate Synthase (Glutamine-Hydrolyzing) / Q000378:metabolism; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D020159:Citrullinemia / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D004080:Dihydroorotase / Q000378:metabolism; D015536:Down-Regulation; D004789:Enzyme Activation; D006801:Humans; D008297:Male; D051379:Mice; D016513:Mice, SCID; D009369:Neoplasms / Q000201:enzymology / Q000378:metabolism* / Q000473:pathology; D027361:Organic Anion Transporters / Q000037:antagonists & inhibitors / Q000378:metabolism; D010766:Phosphorylation; D011743:Pyrimidines / Q000096:biosynthesis*; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism
chemical_list: D002135:Calcium-Binding Proteins; D027361:Organic Anion Transporters; D011743:Pyrimidines; C008822:citrin; D001224:Aspartic Acid; D001221:Aspartate Carbamoyltransferase; D058570:TOR Serine-Threonine Kinases; D004080:Dihydroorotase; D001124:Argininosuccinate Synthase; D002223:Carbamoyl-Phosphate Synthase (Glutamine-Hydrolyzing); C030986:pyrimidine