The JAK2 V617F mutant -mediated aberrant signaling pathway is a hallmark of myeloproliferative neoplasms (MPNs). Although cytokine-inducible Src homology 2 protein (CIS) and suppressors of cytokine signaling (SOCS) are negative regulators of the JAK-STAT pathway, the functional role of CIS/SOCS family members in the JAK2 V617F mutant -induced oncogenic signaling pathway has not yet been elucidated. In this study, we found that the expression of CIS and SOCS1 was induced through the activation of signal transducer and activator of transcription 5 (STAT5) in not only the cells stimulated with Epo or IL -3 but also the cells transformed by the JAK2 V617F mutant. Cell proliferation and tumor formation in nude mice induced by the JAK2 V617F mutant were significantly enhanced when the expression of CIS was silenced using an RNA interference technique, whereas the knockdown of SOCS1 had no effect. The enforced expression of CIS caused apoptotic cell death in the transformed by JAK2 V617F mutant and drastically inhibited the JAK2 V617F mutant -induced tumor formation. CIS interacted with phosphorylated EpoR at Y401, which was critical for the activation of STAT5 and ERK. Whereas the activation of STAT5 and ERK in the transformed cells by JAK2 V617F mutant was increased by the knockdown of CIS, the enforced expression of CIS reduced the activation of these molecules. Furthermore, these anti -tumor effects of CIS required the function of SH2 domain and its tyrosine phosphorylation at Y253. We herein elucidated the mechanism by which CIS functions as a novel type of tumor suppressor in JAK2 V617F mutant -induced tumorigenesis. (C) 2016 Elsevier Inc. All rights reserved.