The inhibition of lymphocyte blastogenesis by asparaginase: critical role of glutamine in both T and B lymphocyte transformation

The glutaminase activity of L-asparaginase is not required for anticancer activity against ASNS-negative cells

L-Asparaginase (L-ASP) is a key component of therapy for acute lymphoblastic leukemia. Its mechanism of action, however, is still poorly understood, in part because of its dual asparaginase and glutaminase activities. Here, we show that L-ASP's glutaminase activity is not always required for the enzyme's anticancer effect. We first used molecular dynamics simulations of the clinically standard Escherichia coli L-ASP to predict what mutated forms could be engineered to retain activity against asparagine but not glutamine. Dynamic mapping of enzyme substrate contacts identified Q59 as a promising mutagenesis target for that purpose. Saturation mutagenesis followed by enzymatic screening identified Q59L as a variant that retains asparaginase activity but shows undetectable glutaminase activity. Unlike wild-type L-ASP, Q59L is inactive against cancer cells that express measurable asparagine synthetase (ASNS). Q59L is potently active, however, against ASNS-negative cells. Those observations indicate that the glutaminase activity of L-ASP is necessary for anticancer activity against ASNS-positive cell types but not ASNS-negative cell types. Because the clinical toxicity of L-ASP is thought to stem from its glutaminase activity, these findings suggest the hypothesis that glutaminase-negative variants of L-ASP would provide larger therapeutic indices than wild-type L-ASP for ASNS-negative cancers.

Targeted metabolomic analysis of amino acid response to L-asparaginase in adherent cells

L-asparaginase (L-ASP) is a therapeutic enzyme used clinically for the treatment of childhood acute lymphoblastic leukemia. L-ASP's anticancer activity is believed to be associated primarily with depletion of asparagine, but secondary glutaminase activity has also been implicated in its anticancer mechanism of action. To investigate the effects of L-ASP on amino acid metabolism, we have developed an LC-MS/MS metabolomics platform for high-throughput quantitation of 29 metabolites, including all 20 proteinogenic amino acids, 6 metabolically related amino acid derivatives (ornithine, citrulline, sarcosine, taurine, hypotaurine, and cystine), and 3 polyamines (putrescince, spermidine, and spermine) in adherent cultured cells. When we examined the response of OVCAR-8 ovarian cancer cells in culture to L-ASP, asparagine was depleted from the medium within seconds. Interestingly, intracellular asparagine was also depleted rapidly, and the mechanism was suggested to involve rapid export of intracellular asparagine followed by rapid conversion to aspartic acid by L-ASP. We also found that L-ASP-induced cell death was more closely associated with glutamine concentration than with asparagine concentration. Time-course analysis revealed the dynamics of amino acid metabolism after feeding cells with fresh medium. Overall, this study provides new insight into L-ASP's mechanism of action, and the optimized analytical method can be extended, with only slight modification, to other metabolically active amino acids, related compounds, and a range of cultured cell types.

Metabolic reprogramming and metabolic dependency in T cells

Upon activation, quiescent naive T cells undergo a growth phase followed by massive clonal expansion and differentiation that are essential for appropriate immune defense and regulation. Accumulation of cell biomass during the initial growth and rapid proliferation during the expansion phase is associated with dramatically increased bioenergetic and biosynthetic demands. This not only requires a metabolic rewiring during the transition between resting and activation but also 'addicts' active T cells to certain metabolic pathways in ways that naive and memory T cells are not. We consider such addiction in terms of the biological effects of deprivation of metabolic substrates or inhibition of specific pathways in T cells. In this review, we illustrate the relevant metabolic pathways revealed by recent metabolic flux analysis and discuss the consequences of metabolic intervention on specific metabolic pathways in T lymphocytes.

The eIF2 kinase GCN2 is essential for the murine immune system to adapt to amino acid deprivation by asparaginase

Amino acid starvation by asparaginase (ASNase) enhances phosphorylation of eukaryotic initiation factor 2 (eIF2) by general control nonderepressible 2 (GCN2) kinase, leading to reduced global mRNA translation rates. This conserves energy and allows cells time to reprogram stress-related gene expression to alleviate cell injury. This study addressed the importance of GCN2 for the immune system to adapt to amino acid starvation by ASNase. GCN2(+/+) and GCN2(-/-) mice were injected once daily with ASNase or saline for up to 7 d. In both thymus and spleen, activation of amino acid stress response genes to ASNase, such as asparagine synthetase and CAAT enhancer binding protein homologous protein, required GCN2. ASNase reduced food intake and body weight in both genotypes, but spleen and thymus wet weights and total cell numbers in thymus, spleen, bone marrow, and mesenteric lymph nodes were less in GCN2(-/-) mice treated with ASNase (genotype x ASNase, P < 0.05). In the thymus, GCN2(-/-) mice treated with ASNase demonstrated enhanced apoptosis and fewer cells in all subpopulations examined (CD3+, CD4-8-, CD4+8+, CD4+8-, CD4-8+) compared with GCN2(+/+) mice treated with ASNase (genotype x ASNase, P < 0.05). In the spleen, GCN2 deletion magnified ASNase-induced reductions in CD4+ T cells, CD8+ T cells, CD19+ B cells, and CD11b+ leukocytes (genotype x ASNase, P < 0.05). These results indicate that loss of GCN2 enhances immunosuppression by ASNase and that this eIF2 kinase is broadly required for amino acid stress management in the immune system.

Asparagine synthetase as a causal, predictive biomarker for L-asparaginase activity in ovarian cancer cells

L-Asparaginase (l-ASP), a bacterial enzyme used since the 1970s to treat acute lymphoblastic leukemia, selectively starves cells that cannot synthesize sufficient asparagine for their own needs. Molecular profiling of the NCI-60 cancer cell lines using five different microarray platforms showed strong negative correlations of asparagine synthetase (ASNS) expression and DNA copy number with sensitivity to l-ASP in the leukemia and ovarian cancer cell subsets. To assess whether the ovarian relationship is causal, we used RNA interference to silence ASNS in three ovarian lines and observed 4- to 5-fold potentiation of sensitivity to l-ASP with two of the lines. For OVCAR-8, the line that expresses the least ASNS, the potentiation was >500-fold. Significantly, that potentiation was >700-fold in the multidrug-resistant derivative OVCAR-8/ADR, showing that the causal relationship between ASNS expression and l-ASP activity survives development of classical multidrug resistance. Tissue microarrays confirmed low ASNS expression in a subset of clinical ovarian cancers as well as other tumor types. Overall, this pharmacogenomic/pharmacoproteomic study suggests the use of l-ASP for treatment of a subset of ovarian cancers (and perhaps other tumor types), with ASNS as a biomarker for patient selection.

Glutamine deprivation-mediated cell shrinkage induces ligand-independent CD95 receptor signaling and apoptosis

Cell shrinkage and loss of cell viability by apoptosis have been examined in cultured CD95(Fas/Apo-1)-expressing leukemia-derived CEM and HL-60 cells subjected to acute deprivation of glutamine, a major compatible osmolyte engaged in cell volume control. Glutamine deprivation-mediated cell shrinkage promoted a ligand-independent activation of the CD95-mediated apoptotic pathway. Cell transfection with plasmids expressing FADD-DN or v-Flip viral proteins pointed to a functional clustering of CD95 receptors at the cell surface with activation of the 'extrinsic pathway' caspase cascade. Accordingly, cell shrinkage did not induce apoptosis in CD95 receptor-negative lymphoma L1210 cells. Replacement of glutamine with surrogate compatible osmolytes counteracted cell volume decrement and protected the CD95-expressing cells from apoptosis. A glutamine deprivation-dependent cell shrinkage with activation of the CD95-mediated pathway was also observed when asparaginase was added to the medium. Asparagine depletion had no role in this process. The cell-size shrinkage-dependent apoptosis induced by glutamine restriction in CD95-expressing leukemic cells may therefore be of clinical relevance in amidohydrolase enzyme therapies.

Glutamine utilization in activated lymphocytes from rats receiving endotoxin

BACKGROUND

A beneficial effect of supplemental glutamine for lymphocyte function in patients under metabolic stress has been suggested. Nevertheless, it is not clear how glutamine is used by lymphocytes when under stress. This time course study investigated the effect of endotoxin-induced stress on in vitro glutamine utilization and glutamine-dependent proliferation of activated lymphocytes.

METHODS

Metabolic stress was modeled by intraperitoneal (ip) administration of endotoxin (5 mg/kg body wt) to rats. Control animals were injected with sterile saline. Cervical lymph node lymphocytes collected from animals 6, 12, 24, and 48 h following injection were activated with concanavalin A. Proliferation of these activated lymphocytes in the presence of 0.1-2 mM glutamine was determined. The glutamine utilization rate and glutaminase activity in the activated lymphocytes were also determined.

RESULTS

The proliferation rate of lymphocytes was not affected by ip administration of endotoxin 6 h following the insult, however, 12, 24, and 48 h following the insult, the maximal response was suppressed (P < 0.05). In addition, at 12, 24, and 48 h, the concentration of glutamine for the maximal response of lymphocytes was lower than that for the control group (P < 0.05). Throughout the investigation period, both the glutamine utilization rate and glutaminase activity in the activated lymphocytes were decreased time-dependently.

CONCLUSION

The present study demonstrates that glutamine utilization by lymphocytes under a mitogenic challenge in vitro is significantly decreased in the late period after endotoxin injection. This is at least partly due to decreased glutaminase activity and is associated with decreased proliferation rate of mitogen-activated lymphocytes.

L-Asparaginase inhibits the rapamycin-targeted signaling pathway

L-Asparaginase is widely used in the treatment of acute lymphoblastic leukemia. L-Asparaginase preparation derived from E. coli converts asparagine (Asn) and glutamine (Gln) to aspartate (Asp) and glutamate (Glu), respectively, and causes rapid depletion of Asn and Gln. It thus suppresses growth of malignant cells that are more dependent on an exogenous source of Asn and Gln than are normal cells. It remains unclear, however, which signaling events in leukemic cells are affected by L-asparaginase. Recently, amino acid sufficiency has been demonstrated to selectively regulate p70 S6 kinase (p70(s6k)) and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), both of which are targeted by the anti-proliferative drug rapamycin. Here we demonstrate that addition of L-asparaginase to human leukemic cells inhibits activity of p70(s6k) and phosphorylation of 4E-BP1, but not activities of other cell growth-related serine/threonine kinases. The rate and kinetics of p70(s6k) inhibition by L-asparaginase were comparable to those seen by deprivation of Asn and/or Gln from cell culture media, suggesting that the effect of L-asparaginase on p70(s6k) is explained by depletion of Asn and/or Gln. Moreover, L-Asparaginase as well as rapamycin selectively suppressed synthesis of ribosomal proteins at the level of mRNA translation. These data indicate that L-asparaginase and rapamycin target a common signaling pathway in leukemic cells.