Homozygous JAK2 V617P Gain-of-Function Mutation Is Responsible for Constitutive JAK2/STAT5 Activation and Proliferation of HEL.92.1.7. Cell Line.

Abstract

HEL (Human erythroleukemia) is a growth factor independent erythroleukemic cell line established from the bone marrow of a patient with relapsed Hodgkin disease after autologous bone marrow transplantation (Martin P & Papayannopoulou T: Science 1982; 216:1233–1235). HEL cells display a block in differentiation at the level of common erythroid-megakaryocytic progenitor and have been commonly used as a model to study erythroid and megakaryocytic differentiation. Liu RY et al. ( Blood 1999; 93:2369–2379) reported that constitutive activation of JAK2 tyrosine kinase in Dami/HEL cell line correlated with factor independent growth. Recently, several groups of investigators described a point mutation V617P in the autoinhibitory regulatory (JH2) domain of JAK2 tyrosine kinase in patients with myeloproliferative disorders (MPD). This acquired mutation results in constitutive activation of the JAK2/STAT5 pathway in hematopoietic cells. To date, the V617P mutation represents the most common causative genetic defect detected in patients with Philadelphia chromosome negative MPD. Interestingly, rare cases of patients with myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) were found to have the same mutation. We tested several hematopoietic cell lines including HL-60, HEL, KG-1, K-562, TF-1, U937 and UT-7 for the presence of V617P mutation using RFLP and automatic DNA sequencing. We found the homozygous mutation V617P only in HEL cells. This cell line has a very complex karyotype suggesting that a defect of multiple oncogenes or antioncogenes could be responsible for sustained cell proliferation. We employed the JAK2 inhibitor, AG490 at concentrations ranging from 0 to 100 mM and measured proliferation of HEL and control K562 cell lines using MTT assay. We observed dose-dependent inhibition of HEL cell proliferation with low micromolar concentrations of AG490 in contrast to K562 control cell line, harboring bcr-abl fusion tyrosine kinase. IC50 was 13 mM for HEL cells and 65 mM for control K562 cells, respectively. Low concentrations of AG490 significantly decreased phosphorylation of JAK2 and STAT5 in HEL cells but not in control K562 cell line, where bcr-abl activates separately JAK2 and STAT5 pathway. Exposure of HEL cells to AG490 [0–50mM] induced apoptosis as measured by annexin V labeling in a concentration dependent fashion. No significant increase in apoptosis was detected in K562 cells using similar concentrations of the JAK2 inhibitor. Overall, these results suggest that the mechanism of proliferation of the HEL cell line is driven by constitutive activation of JAK2 tyrosine kinase secondary to V617P gain-of-function mutation. Our data provides further evidence that the JAK2/STAT5 intracellular signaling pathway is preserved in this cell line. Thus, HEL cells can serve as a model to test novel JAK2 specific inhibitors in preclinical studies.