K562-R As a Model to Study Jak2 in a Non Bcr-Abl Addicted Cell Line

Abstract 4412

Chronic myeloid leukemia (CML) is a hematological disease caused by the fusion protein Bcr-Abl tyrosine kinase. Development of the tyrosine kinase inhibitor Imatinib Mesylate (IM) has significantly improved the long-term survival of early stage CML patients. However, occurrence of drug resistance, permanence of residual disease and recurrence of active leukemia if IM is discontinued, remain problems awaiting solution. Therefore, new therapeutic strategies aimed at targeting alternative signaling pathways or CML progenitor cells that survive IM treatment are needed. We have previously shown that Janus kinase 2 (Jak2) is activated in Bcr-Abl+ cells. We have demonstrated that reduction of Jak2 activity by the Jak2-specific inhibitor TG101209 (TG) or by genetic knock down (Jak2 shRNA and siRNA) in Bcr-Abl+ cell lines, IM-resistant cells and CML blast crisis cell lines resulted in reduced levels of phosphorylation of Tyr177 and of total Bcr-Abl protein. Jak2 inhibition results in diminished activation of the Ras, PI-3 kinase pathways and reduced levels of pTyrSTAT5 (Samanta et al., Leukemia 2011). During these studies we observed that K562 cells and IM-resistant cell line K562-R had different susceptibility to the effect of TG, with K562-R showing increase sensitivity to lower concentrations of TG resulting in faster destabilization of the Bcr-Abl protein. Based on these observations, we hypothesized that increased sensitivity of the K562-R cells was due to the different state of activation of Jak2. In addition, based on recent studies by (Dawson et al., Nat 2009) and by (Rinaldi et al., Blood 2010) we also hypothesized that different levels of Jak2 activation may influence the localization of Jak2 in the cell. We used cell fractionation and western blotting analysis to show that in K562-R cells, active Jak2 is mostly localized in the nucleus with a minor pool found in the cytoplasm. In K562 cells, active Jak2 is equally distributed in both cytoplasmic and nuclear compartment. In addition, immuno-fluorescence confocal analysis of total Jak2 distribution in K562 shows a very organized localization of Jak2 at one pole of the cells but this organization is lost in K562-R and total Jak2 appears uniformly distributed within the cell. K562-R cells were isolated as a Bcr-Abl independent IM resistant cell line that expressed high levels of activated Lyn kinase (Donato et al., Blood 2003). We used K562-R as a model to study the role of Jak2 in a non Bcr-Abl addicted cell line. Since we have previously published that Jak2 up-regulates Lyn kinase activity (Samanta et al., Oncogene 2009), we propose that the higher activation of Jak2 in K562-R is the main driver of oncogenicity and IM resistance and that this cell line may be used to model the role of Jak2 in a cell that is not Bcr-Abl “addicted.”