Constitutive activation of the JAK2/STAT5 signal transduction pathway correlates with growth factor independence of megakaryocytic leukemic cell lines

The factor-independent Dami/HEL and Meg-Ol and factor-dependent Mo7e leukem ic cell lines were used as models to investigate JAK/STAT signal transducti on pathways in leukemic cell proliferation, Although Dami/HEL and Meg-Ol ce ll proliferation in vitro was independent of and unresponsive to exogenous cytokines including granulocyte-macrophage colony-stimulating factor (GM-CS F), interleukin-3 (IL-3), IL-6, thrombopoietin (TPO), and tumor necrosis fa ctor-alpha (TNF-alpha), the growth of Mo7e cells was dependent on hematopoi etic growth factors. When these cell lines were cultured in medium without cytokines, a constitutively activated STAT-like DNA-binding factor was dete cted in nuclear extracts from both Dami/HEL and Meg-Ol cells. However, the STAT-like factor was not detectable in untreated Mo7e cells, but was activa ted transiently in Mo7e cells in response to cytokine treatments. The const itutively activated and cytokine-induced STAT-like DNA-binding factor in th ese three cell lines was identified as STATE by oligonucleotide competition gel mobility assays and by specific anti-STAT antibody gel supershift assa ys. Constitutive activation of JAK2 also was detected in the factor-indepen dent cell lines, but not in Mo7e cells without cytokine exposure. Meg-Ol ce lls express a p185 BCR/ABL oncogene, which may be responsible for the const itutive activation of STATE. Dami/HEL cells do not express the BCR/ABL onco gene, but increased constitutive phosphorylation of Raf-l oncoprotein was d etected. In cytokine bioassays using growth factor-dependent Mo7e and TF-1 cells as targets, conditioned media from Dami/HEL and Meg-01 cells did not show stimulatory effects on cell proliferation. Our results indicate that t he constitutive activation of JAK2/STAT5 correlates with the factor-indepen dent growth of Dami/HEL and Meg-01 cells. The constitutive activation of JA K2/STAT5 in Dami/HEL cells is triggered by a mechanism other than autocrine cytokines or the BCR/ABL oncoprotein. (C) 1999 by The American Society of Hematology.