BCR-ABL chronic myeloid leukemia (CML) and acute lymphoblastic leukemia (ALL) have recently received much attention as model diseases for the effectiveness of targeted drug therapy. The ABL kinase inhibitor imatinib mesylate has revolutionized treatment of CML. However, a serious drawback to this specific treatment is the emergence of drug resistance. Protein tyrosine phosphatase-1B (PTP1B) has been demonstrated to block the BCR-ABL mediated transformation of hematopoietic cells in vitro and in vivo. Therefore the impact of PTP1B function on imatinib-response of the BCR-ABL positive cell line K562 and an imatinib-resistant subclone of the cell line SupB15 was investigated.

K562 cells were transfected with wild-type PTP1B (K562/PTP1B), a substrate-trapping dominant-negative mutant PTP1B (K562/D181A), or an empty vector (K562/mock), cultured in the presence or absence of imatinib (0.25–1.0 μM) and analyzed for proliferation, apoptosis and differentiation. K562/PTP1B and K562/D181A cells displayed 1.6-fold higher and 13-fold lower phosphatase activity than K562/mock cells, resp. In both K562/mock and K562/PTP1B cells, 0.5 μM imatinib induced growth arrest, an increase of cells in sub-G1 apoptotic phase (36.9±2.0% and 38.6±3.8%, resp.) and a decrease of cells in S/G2/M phases (18.9±1.1% and 23.7±2.7%, resp.) as determined by FACS analysis of propidium iodide stained cells. Imatinib-induced apoptosis was further confirmed by Annexin V staining and Western blot analysis showing increased active caspase-3 and -8 protein levels and induction of poly ADP-Ribose Polymerase (PARP) cleavage in both cells. Low doses of imatinib (0.25 μM) promoted erythroid differentiation of these cells as evaluated by enhanced expression of glycophorin A. Conversely, K562/D181A cells were significantly less sensitive to imatinib-induced growth arrest (sub-G1: 25±3.9 %, S/G2/M: 34.2±1.4 %) apoptosis, and promotion of erythroid differentiation. Inhibition of PTP1B phosphatase activity by the PTP1B inhibitor bis(N,N-Dimethylhydroxamido)hydroxooxovanadate (DHMV) decreased imatinib-induced apoptosis in wild-type K562 cells (46.7% vs. 28.3% annexin V positive cells; p<0,01). Moreover, the imatinib-resistant subclone of the ALL cell line SupB15, which had been generated in the presence of 1μM imatinib, showed significantly lower PTP1B activity (64.3±5.6%, p<0.05) as compared with imatinib-sensitive wild type cells. In conclusion, our data demonstrate that functional PTP1B is required for imatinib-induced apoptosis, growth arrest, and differentiation. Alterations of PTP1B gene expression and function might contribute to imatinib-resistence and should therefore be assessed in patients with BCR-ABL positive chronic and acute leukaemia who developed restance to imatinib.

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