Flow Cytometric Detection of Asparagine Synthetase Protein in Leukemia Cells; Indication for L-Asparaginase Therapy

Modern clinical treatments of childhood acute lymphoblastic leukemia (ALL) employ enzyme-based methods for depletion of blood asparagine in combination with standard chemotherapeutic agents. L-asparaginase (L-asp) therapy causes depletion of plasma asparagine followed by the loss of intracellular asparagine. Due to the lack of a rapid up-regulation of asparagine synthetase (ASNS) protein content in ALL cells, they are preferentially killed by L-asp. Elevated expression of ASNS within the leukemia cells causes decreased sensitivity to L-asp. The proof of ASNS deficiency in leukemia cells is considered to predictive for effectiveness of L-asp even in acute myeloid leukemia (AML) other than ALL patients. The establishment of quantitative estimation of ASNS protein content would be useful for the L-asp treatment in leukemia therapy.

Objective: Our aim was to set up a flow cytometry system to check ASNS deficiency in leukemia cells and to investigate the sensitivity to L-Asp and the ASNS expression in AML leukemia cells.

Methods: AML (KG-1, HL-60, U937) and ALL (MOLT-4, RS4;11) and CML (K562) cell lines were grown in RPMI1640 medium with 10% FCS. Primary leukemic cells from the peripheral blood or bone marrow of 20 AML patients were harvested on EDTA and isolated by Ficoll density gradient within 72h. ASNS expression was evaluated by cytosolic flow cytometry with Z5808 McAb (Hybridoma 31: 325-332.2012) and expressed as a ΔMFI(Difference of Mean Fluorescence Intensity(MFI) between by Z5808 and isotypic control) or MFI ratio(MFI by Z5808/MFI by isotypic control). When a sufficient amount of leukemic cells was available, sensitivity to L-asp (expressed as an IC50 - concentration inhibiting 50% of cell viability) was evaluated in vitro by incubating various concentrations of E. coliL-asp with the cells and by measuring the cell viability with a counting kit (WST1 viability assay) at day 3.

Results: Determination of IC50 for the HL-60 (⊿MFI 48 ± 8.01, MFI ratio 1.77 ± 0.03) and U937 (⊿MFI 16.7 ± 0.47, MFI ratio 1.19 ± 0.02) demonstrated that these cells were equally sensitive to L-asp than the ALL cell line MOLT-4 in vitro (0.37 and 0.02IU/mL versus 0.15 IU/mL, respectively). K562 and KG-1 (⊿MFI 135.7 ± 5.66, MFI ratio 2.48 ± 0.09) cells with the highest ASNS expression exhibited resistance to L-asp (>10 IU/ml). Both of ASNS Expression by ⊿MFI and MFI ratio was inversely correlated with L-asp sensitivity judging from cell line studies. Judging from cell line study, the threshold for ASNS protein expression effective for L-asp treatment was considered to be <25 for ⊿MFI and <1.8 for MFI ratio respectively. Fresh leukemia cases contained three ALLs, Ph1ALL, and 13 AML; M0, 1; M1, 3; M2, 2; M4, 1; M5, 3; M7, 3; and Acute Mixed lineage leukemia, 2 cases. IC50 determination was possible on 11/20 patients. Four displayed a high sensitivity to L-asp (IC50 < 0.01 IU/mL) whereas two displayed resistant to L-asp (IC50 >10 IU/mL) Remaining 5 patients were a moderate sensitivity (IC50 < 0.5 IU/mL). ASNS Expression in ALL was almost near zero. ASNS expression in fresh AML was low in all cases except for one M2 and one M4 cases. Indeed, at high ASNS expression, these cases were resistant in vitro to L-asp. The patients with blasts sensitive to L-asp had mainly M1, M5 or M7 AML. A good inverse correlation between ASNS expression and sensitivity to L-asp was observed also in primary leukemic cells from AML patients.

Conclusions: We demonstrated here that some of AML cell lines with low ASNS expression are more sensitive to L-asp than leukemia cells with high ASNS expression. Also, fresh AML cells with low ASNS expression are more sensitive to L-asp than with high ASNS expression. 11/13AML or two AMLL cases were supposed to be effective for L-asp. Plasma asparagine depletion by L-asp in selected patients having low ASNS may be a promising therapeutic approach even for AML.

This work was supported by Japan Society for the Promotion of Science Grant-in-Aid for Scientific Research (C) KAKENHI Grant Number 24590713.