Glutamine Metabolism Is Altered in Myeloproliferative Neoplasms and Represents a Potential Novel Therapeutic Target

Cancer cells exhibit metabolic reprogramming to ensure sufficient production of macromolecules used to promote cell growth and proliferation. Previous studies have shown that proliferating cancer cells rely significantly on glutamine metabolism as a source of nitrogen and carbon to facilitate nucleotide and lipid biosynthesis, respectively, and to generate metabolites and/or maintain redox homeostasis. Despite this phenomenon being well characterized in many different solid and blood cancer types, very few studies have investigated the metabolic landscape of myeloproliferative neoplasms (MPNs). Glutamine catabolism begins via the uptake of glutamine through sodium-dependent transporters such as SLC1A5, and its transport to the mitochondria where it then undergoes deamination by glutaminase (GLS1) to produce glutamate. This conversion by GLS1 is the rate limiting step in the central glutaminolysis axis, making it a preferential point of study and therapeutic targeting. Indeed, many studies have explored the glutaminolysis pathway in a variety of cancers to decipher potential targets for treatment, but these studies have been extremely limited in MPNs and the potential mechanisms with respect to MPN driver mutations (CALR, JAK2, MPL) and their differential utilization of glutamine have yet to be fully explored. One previous study demonstrated that in JAK2V617F mutant-Ba/F3 cells, GLS1 expression was significantly increased and that GLS1 inhibition served as a promising intervention to suppress colony formation in patient samples but not healthy donors. To more comprehensively decipher the role of glutamine metabolism in MPNs driven by all three driver mutations ( CALR, JAK2, and MPL), we performed RNA-sequencing analysis on peripheral blood mononuclear cells (PMBCs) from MPN patients and found that genes involved in glutamine metabolism are significantly altered across the board. In contrast to the previous study mentioned above, our data demonstrate that GLS1 is not significantly up-regulated, and that MPN PBMCs are not sensitive to GLS1 inhibition. Rather, we characterized another key enzyme in glutamine metabolism, glutamine synthetase (GS) as a molecular and metabolic dependency in MPNs. GS catalyzes the reversion of glutamate into glutamine and serves to regulate nitrogen metabolism, support nucleotide biosynthesis, and most importantly synthesize glutamine to satiate the demand caused by the cancer cells' affinity for high glutamine levels. We found that GS is significantly up-regulated at the mRNA, protein, and activity level in MPNs driven by all three driver mutations, and that genetic and pharmacological inactivation of GS abrogates MPN cell proliferation in vitro. In summary, we have demonstrated that GLS1 may not be an effective therapeutic target for MPNs, and instead characterize GS as a novel dependency and potential point of therapeutic intervention.