Ammonia generated from glutamate metabolism enhances the proliferation of breast cancer cells.
Major finding: Ammonia generated from glutamate metabolism enhances the proliferation of breast cancer cells.
Mechanism: GDH catalyzes reductive amination to assimilate ammonia for the generation of amino acids.
Impact: Ammonia is a nitrogen source for amino acid synthesis in breast tumors, not merely a toxic by-product.
In rapidly proliferating cells, increased nutrient consumption generates excess metabolic waste. In the tumor microenvironment, this can lead to accumulation of lactate and ammonia. Ammonia has been considered a toxic by-product, and it is not clear how tumor cells handle the high levels of ammonia generated by amino acid catabolism. Spinelli and colleagues hypothesized that ammonia might be incorporated into central metabolism. To test this hypothesis, metabolic tracing analysis with hydrophilic interaction LC/MS-MS was performed to determine the fate of ammonia generated from metabolism of labeled glutamine [15N(amide)-glutamine]. In estrogen receptor (ER)–positive breast cancer cells, 15N(amide)-glutamine was incorporated into asparagine and nucleotides, and labeled nitrogens were also unexpectedly present in proline, aspartate, branched chain amino acids, and glutamate, which had not been previously linked to glutamine metabolism. The labeled nitrogen was freed as ammonia before incorporation into these glutamine derivatives. Glutaminase converts glutamine to glutamate and ammonia, and treatment with the glutaminase inhibitor Bis-2-(5-phenylacetamido-1,3,4-thiadiazol-2-yl)ethyl sulfide (BPTES) reduced the production of labeled glutamate, proline, and aspartate, whereas nucleotides and asparagine remained labeled, as they are involved in direct glutamine metabolism. Mechanistically, glutamate dehydrogenase (GDH) catalyzed reductive amination to recycle ammonia generated from glutaminolysis into the generation of glutamate and downstream metabolites including proline and aspartate. Ammonia was not toxic to breast cancer cells in vitro, and the addition of ammonia resulted in a GDH-dependent increase in proliferation. In ER+ breast cancer xenografts, intraperitoneal injection of 15NH4Cl resulted in labeled proline and aspartate in the tumor, whereas the liver assimilated ammonia via the urea cycle. Further, depletion of GDH in tumor xenografts suppressed tumor growth in vivo. Altogether, these findings suggest that ammonia accumulation in the tumor microenvironment promotes the growth of breast cancer cells by serving as a nitrogen source for amino acid synthesis.
Spinelli JB, Yoon H, Ringel AE, Jeanfavre S, Clish CB, Haigis MC, et al. Metabolic recycling of ammonia via glutamate dehydrogenase supports breast cancer biomass. Science 2017 Oct 12 [Epub ahead of print].
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