The Glutaminase Activity of L-Asparaginase Mediates Suppression of Asns Upregulation

L-asparaginase (ASNase) is a standard component of treatment regimens used for acute lymphoblastic leukemia and is being tested against other cancer types, including acute myeloid leukemia, lymphoma, and pancreatic cancer. We and others have reported that the anticancer activity of ASNase requires the enzyme's glutaminase activity, but the underlying glutaminase-mediated mechanism(s) that lead to leukemia cell death are unknown. Glutamine, the most abundant amino acid in the blood, is known for pleiotropic roles in numerous biological pathways, including energy metabolism, redox metabolism, nucleotide anabolism, and amino acid anabolism. Many cancer cells have been found to reprogram their metabolic pathways to become highly dependent on glutamine for survival and proliferation. Glutaminase (GLS/GLS2)-mediated conversion of glutamine to glutamic acid provides the latter as a substrate for conversion to α‐ketoglutarate by transaminases or glutamate dehydrogenases (GLUD1/GLUD2) to fuel the TCA cycle. Consequently, targeting glutamine metabolism has become an attractive strategy for anticancer therapy.

The enzyme asparagine synthetase (ASNS) mediates resistance to ASNase through synthesis of asparagine. ASNS is expressed in most cell types, and its expression is upregulated in response to a wide variety of cell stresses, including amino acid limitation and endoplasmic reticulum stress. We and others have shown that ASNS-positive leukemia cells capable of synthesizing asparagine de novo are less responsive than ASNS-negative leukemia cells to ASNase therapy (Chan et al., Blood, 2014). Moreover, ASNase resistance has been associated with elevated ASNS expression. In fact, we have shown that ASNS expression is a predictive marker of the in vitro response of leukemia cell lines and some solid tumor cell types to ASNase. The expression of ASNS in most cells in the body poses a serious challenge for successful therapy with ASNase; for example, production of asparagine by the liver and cells (e.g., mesenchymal stem cells and adipocytes) of the tumor microenvironment may contribute significantly to ASNase resistance in vivo.

Here we used the high-glutaminase E. chrysanthemi ASNase (Erwinaze®), wild-type E. coli ASNase (ASNase^WT), and the glutaminase-deficient E. coli mutant, ASNase^Q59L, as models of high, medium-, and low-glutaminase, respectively, to explore ASNase glutaminase activity-mediated mechanisms of leukemia cell death. Unexpectedly, we found that increasing glutaminase activity caused an increase in the suppression of ASNS upregulation in vitro (Figure 1A). In NSG mice injected with luciferase-labeled Sup-B15 cells, single-agent ASNase^WT yielded a durable response approximating cure, whereas glutaminase-deficient ASNase^Q59L yielded a complete response but with recurrence. Together, the results suggest that ASNase glutaminase activity is associated with suppression of ASNS upregulation, making durable, single-agent anticancer activity easier to achieve. Overall, the results provide new insight into the mechanism of action of ASNase.

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