P067 JAK2 V617F mutation causes cytokine-independent growth and tumorigenesis – Roles of STAT5 and c-Myc

Abstract

Introduction JAK2 is an essential signal transducer for various cytokine signaling including IL-3 or Epo. Mutation at V617F of JAK2 promotes cell growth and prevents apoptosis which causes a variety of myeloproliferative neoplasms (MPNs). How the JAK2 V617F mutation causes dysregulated proliferation, resistance to anti-tumor drugs and tumorigenesis in nude mice is to be investigated. Methods Ba/F3 cells harboring JAK2 V617F were established to study the constitutive activation of JAK2 and cytokine-independent growth. Tumorigenesis was tested by the inoculation of JAK2 V617F mutant in nude mice. DNA array was done to search genes expressed predominantly by JAK2 V617F and STAT5. Results (1) JAK2 V617F mutant caused aberrant activation of STAT5 and Akt when EpoR was expressed, which is critical for the JAK2 V617F-induced anti-apoptotic and oncogenic activities. (2) In search of genes induced by JAK2 V617F mutant, we observed significant induction of c-Myc. (3) While the phosphorylation at Thr58 by GSK-3b induced the proteasome-mediated degradation of c-Myc, GSK-3b was inactivated in JAK2 V617F mutant, which resulted in the enhancement of c-Myc expression. The enforced expression of c-Myc accelerated not only the cell proliferation but also apoptotic cell death; however, the inhibition of GSK-3b completely inhibited c-Myc-induced apoptosis. (4) JAK2 V617F mutant also induced expression of a target gene of c-Myc, ornithine decarboxylase (ODC) known as a rate-limiting enzyme in polyamine biosynthesis. An ODC inhibitor, difluoromethylornithine (DFMO) prevented the proliferation of the JAK2 V617F mutant cells. Furthermore, administration of DFMO effectively delayed tumor formation in nude mice, resulting in a prolonged survival. Conclusion JAK2 V617F mutant caused aberrant activation of Akt and STAT5 pathways. ODC expression induced by c-Myc is critical for JAK2 V617F mutant-driven transformation and that targeted disruption of c-Myc-ODC axis may provide therapeutic utility for the treatment of MPNs. STAT5 and c-Myc have critical roles for JAK2 V617F mutant-induced tumorigenesis, which help clarify the mechanism how JAK2 V617F mutant causes MPNs.