The Jak2V617F Oncogene Associated with Polycythemia Vera Regulates G1/S-Phase Transition.

The V617F activating point mutation in Jak2 has recently been detected in a high proportion of patients with the myeloproliferative disorders polycythemia vera, essential thrombocythemia, and idiopathic myelofibrosis. Using the Jak2V617F-mutant erythroid leukemia cell line HEL as a model, potential mechanisms that contribute to transformation were investigated. Inhibition of Jak2V617F with a small molecule kinase inhibitor reduced cell growth of HEL cells in a dose dependent manner with an IC₅₀ of 300 nM. This inhibition of growth was associated with a G1 cell cycle arrest, with minimal or delayed apoptosis. The major Jak2 target in normal hematopoietic cells, STAT5, was found to be activated by Jak2V617F. Treatment of the cells with either a Jak2 kinase inhibitor, or with a Jak2-targeted siRNA, decreased STAT5 activation, and also resulted in decreased expression of cyclin D2 and increased expression of p27^Kip. Of interest, we found that Jak2V617F induced high levels of reactive oxygen species (ROS), an activity associated with several other tyrosine kinase oncogenes. Expression of a constitutively active form of STAT5 by itself was capable reducing expression of p27^Kip and increasing production of ROS, suggesting that each of these signaling events are downstream of STAT5. Additionally, treatment of HEL cells with the anti-oxidant N-acetylcystein increased expression of p27^Kip, suggesting that Jak2V617F regulates cell cycle progression at least in part through STAT5 activation of ROS, and ROS regulation of p27^Kip. Cell growth of HEL cells was found to be blocked by anti-oxidants. Overall, our results suggest that constitutive activation of Jak2 contributes to a transforming phenotype and therefore hints at novel targets for drug development that may aid traditional therapy.