Urea cycle metabolite fuels leukemia growth by switching on a purine-recycling enzyme

Cancer cells are famous for rewiring their chemistry, but one of the most heavily studied metabolic programs in tumours, the urea cycle, has long been treated mostly as a bystander in blood cancers. A study published in Nature Metabolism now argues that it is anything but. A team led by Gen Li, Zhijun He and Peng Jiang at Tsinghua University, working with collaborators at Huazhong University of Science and Technology, China Medical University and Hubei University of Technology, reports that in B cell acute lymphoblastic leukaemia (B-ALL) driven by the BCR–ABL tyrosine kinase, a urea cycle intermediate called argininosuccinate (ASA) acts as a signalling molecule that supercharges purine nucleoside production, and that the resulting nucleosides feed the very pathways leukaemia cells use to burn carbon and grow.

The story begins with an untargeted look at metabolism. Using liquid chromatography–mass spectrometry, the researchers compared urea cycle metabolites in Philadelphia chromosome-positive B-ALL cells, which carry the t(9;22) translocation that fuses BCR to ABL, with metabolites in healthy human B cells freshly isolated from peripheral blood. The leukaemia cells stood out for elevated levels of ASA and altered handling of citrulline and arginine, the substrates and products that bracket the ASA step of the cycle. Urea cycle gene expression profiles in patient-derived cells and in the Sup-B15 and Nalm21 B-ALL lines reinforced the picture that this pathway, generally associated with the liver’s ammonia detoxification, was being co-opted in malignant B cells.

Why would BCR–ABL care about a urea cycle enzyme? The answer the authors provide is a phosphorylation event. Through mass spectrometric mapping of tyrosine-phosphorylated proteins, they identified argininosuccinate synthase 1 (ASS1), the enzyme that condenses citrulline and aspartate into ASA, as a direct substrate of the BCR–ABL kinase. Four high-confidence tyrosine residues were detected, and mutation analysis pinpointed tyrosine 282 (Y282) as the functionally important site. In vitro, purified GST-tagged Abl kinase phosphorylated wild-type ASS1 but not an ASS1-Y282F mutant, and in cells the interaction was verified by co-immunoprecipitation in both directions. Treatment with the kinase inhibitor STI571, better known as imatinib, reduced ASS1 phosphorylation, tying the modification directly to BCR–ABL activity.

Notably, Y282 phosphorylation was not a quirk of BCR–ABL alone. When the team co-expressed ASS1 with other fusion tyrosine kinases implicated in haematological malignancies, including ALK, JAK2, FGFR and PDGFR, ASS1 phosphorylation and enzymatic activity rose in each case, and kinase inhibitors lowered urea cycle metabolite levels in a chronic myelogenous leukaemia line and in anaplastic large cell lymphoma cells. This suggests that ASS1 phosphorylation at Y282 is a convergent metabolic endpoint of oncogenic tyrosine kinase signalling, one that boosts the enzyme’s capacity to convert citrulline into ASA.

What does the extra ASA actually do? The key experiments traced the fate of isotopically labelled citrulline in leukaemia cells. When B-ALL cells were fed [13C5]citrulline, labelled carbon flowed through ASA into arginine, confirming active flux through ASS1. Cells expressing a phosphorylation-deficient ASS1-Y282F mutant accumulated less ASA and arginine than cells with the wild-type enzyme. The team then depleted ASS1 with short hairpin RNAs and performed metabolomic profiling of BCR–ABL-positive BAF3 cells recovered from mouse bloodstreams. The most striking changes appeared in purine metabolism: nucleotides and nucleosides dropped when ASS1 was silenced, implicating the urea cycle intermediate in maintaining purine pools.

The mechanistic link turned out to be a direct protein–metabolite interaction. ASA, the authors show, binds cytosolic 5′-nucleotidase II (NT5C2), an enzyme that removes phosphate groups from nucleoside monophosphates to yield nucleosides such as inosine, adenosine and guanosine. Molecular docking and molecular dynamics simulations, performed by Mingjie Liu and Xiaojing He’s groups, indicated that ASA localizes to the effector site of NT5C2 and stabilizes helix A, a structural element (residues Gly355–Glu364) that gates the enzyme’s allosterically activated state. Enzyme kinetics assays confirmed that ASA enhances NT5C2’s nucleotidase activity, whereas arginine and adenylosuccinate did not reproduce the effect. In other words, ASA is not merely a passive intermediate here; it is a small-molecule activator that tells NT5C2 to convert purine nucleotides into nucleosides.

Those nucleosides, it turns out, are not waste products but fuel. When leukaemia cells were incubated with [ribose-13C5]inosine or [ribose-13C5]adenosine, label from the ribose moiety appeared in glycolytic intermediates, tricarboxylic acid (TCA) cycle metabolites and the pentose phosphate pathway, even when glucose was abundant. The purine nucleosides serve as a carbon source for central carbon metabolism under glucose-replete conditions, an unexpected contribution given that nucleoside salvage is usually framed in terms of nitrogenous bases and nucleotide pools rather than carbon economics. Earlier work had shown that ribose salvaged from uridine can fuel glucose-restricted pancreatic cancer; this study extends the concept to purine nucleosides in glucose-rich leukaemia, where they complement rather than replace glycolysis.

The physiological importance of the axis became clear in vivo. Loss of ASS1 or NT5C2 disrupted central carbon metabolism and inhibited leukaemia progression in mouse models, and supplementing the animals’ leukaemia cells with inosine or adenosine rescued the metabolic and growth defects. In Nalm21-based xenografts, an ASS1-Y282F mutant supported less disease burden than wild-type ASS1, and inosine administration partially restored phenotypes suppressed by imatinib, consistent with the nucleosides acting downstream of the kinase. Serum measurements in leukaemia-bearing mice showed elevated adenosine, inosine and cytidine compared with healthy controls, hinting that the pathway shapes the systemic nucleoside environment as well as the intracellular one.

The study also maps the transport machinery that moves these metabolites across the membrane. Uptake experiments with doubly labelled [13C6,15N4]ASA identified SLC13A2 and SLC13A3 as candidate ASA transporters, while citrulline import depended on SLC7A5, the amino acid transporter also known as LAT1, and was blocked by the LAT1 inhibitor JPH203. These assignments sharpen the metabolic wiring diagram and point to additional potential intervention points.

The findings carry a double significance for leukaemia biology and therapy. First, they identify an ASA–NT5C2 signalling axis that directly links tyrosine kinase activity, urea cycle dysregulation and purine metabolism, adding a concrete molecular mechanism to the growing appreciation of metabolites as signalling molecules. Second, they highlight purine nucleosides as a carbon source that tyrosine kinase-driven leukaemias exploit, and NT5C2 as an enzymatic node essential to that exploitation. NT5C2 is already notorious in the clinic because acquired mutations in relapsed lymphoblastic leukaemia confer resistance to thiopurine chemotherapy by altering the enzyme; this work now gives the protein a second, more fundamental role in leukaemia metabolism that is independent of treatment selection. Whether targeting the ASS1–NT5C2 axis can be translated into combination strategies with existing kinase inhibitors will require further preclinical and clinical evaluation, but the study establishes a clear metabolic vulnerability: cut off the urea cycle’s signalling output, and the leukaemia cell’s carbon supply begins to starve.

Subject of Research: The role of ASS1-derived argininosuccinate in promoting purine nucleoside synthesis and central carbon metabolism in B cell acute lymphoblastic leukaemia.

Article Title: ASS1-derived argininosuccinate promotes purine nucleoside synthesis in B cell acute lymphoblastic leukaemia

Article References: Li, G., He, Z., Liu, M., Zhao, J., Zhu, H., Zhou, C., Wang, Z., He, X., Tang, J., & Jiang, P. (2026). ASS1-derived argininosuccinate promotes purine nucleoside synthesis in B cell acute lymphoblastic leukaemia. Nature Metabolism, 8(9), 1871-1887. https://doi.org/10.1038/s42255-026-01586-w

Image Credits: AI Generated

DOI: 10.1038/s42255-026-01586-w

Keywords: ASS1, argininosuccinate, NT5C2, BCR-ABL, B-ALL, urea cycle, purine metabolism, cancer metabolism, leukaemia, tyrosine kinase, nucleosides, Nature Metabolism