Stat3 Negatively Regulates Myeloproliferative Neoplasm Induced By Jak2V617F

The JAK2V617F mutation has been found in most patients with Ph-negative myeloproliferative neoplasms (MPNs) including polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF). Expression of JAK2V617F results in constitutive activation of several signaling molecules/pathways, such as Stat5, Stat3, Akt and Erk. Unraveling the contribution of these signaling pathways in MPNs will improve our understanding of the pathogenesis of MPNs and allow us to develop more effective targeted therapies.

We have previously reported the generation of a conditional Jak2V617F knock-in mouse, which exhibits all the clinical features of human PV. Using this mouse model, we have demonstrated that Stat5 is absolutely required for the pathogenesis of PV induced by Jak2V617F. However, the contribution of other signaling molecules activated by Jak2V617F in the development and progression of MPNs still remains elusive. Stat3, a member of the family of signal transducer and activator of transcription (Stat), is often found activated in solid tumors and hematologic malignancies including MPNs. Although Stat3 is known to play a tumor-promoting function in various human malignancies, recent studies also have found a tumor suppressive function of Stat3 in certain malignancies. For instance, Stat3 negatively regulates BRAFV600E-induced thyroid tumorigenesis (Couto et al., Pro Natl Acad Sci USA 2012) or suppresses PTEN loss-induced malignant transformation of astrocytes (Iglesia et al., Genes Dev 2008). Thus, Stat3 can positively or negatively regulate cell growth and tumor progression. Here, we sought to determine the role of Stat3 in Jak2V617F-evoked MPN using conditional Stat3 knock-out (Stat3 floxed) and Jak2V617F knock-in mice.

Whereas expression of Jak2V617F resulted an increase in red blood cells (RBC), hemoglobin, hematocrit, white blood cells (WBC), neutrophils and platelets in the peripheral blood of the Jak2V617F knock-in mice, deletion of Stat3 did not cause any significant change in RBC, hemoglobin, hematocrit and platelet numbers in Jak2V617F knock-in mice. Strikingly, Stat3 deficiency significantly increased nertrophil counts in mice expressing Jak2V617F. Flow cytometric analysis showed that deletion of Stat3 increased the hematopoietic stem cell (HSC) compartments (LSK, LT-HSC, ST-HSC) and GMP populations in the bone marrow (BM) and spleens of mice expressing Jak2V617F. However, MEP population was unaffected by Stat3 deletion. Cell cycle analysis using Hoechst/Pyronin Y staining revealed that Jak2V617F expression alone resulted in increased cycling of HSC-enriched LSK cells, and Stat3-deficiency further enhanced the cycling of Jak2V617F-expressing LSK cells. Stat3-deficiency also caused a marked expansion of Gr-1⁺/Mac-1⁺ population in the BM and spleens of mice expressing Jak2V617F. As a consequence, CD71⁺/Ter119⁺ population was proportionally reduced in Stat3-deficient Jak2V617F-expressing mice BM. Histopathologic analysis showed marked increase in granulocytes in the BM and spleens of Stat3-deficient Jak2V617F-expressing mice compared with mice expressing Jak2V617F. Stat3-deficient Jak2V617F-expressing mice also exhibited marked infiltration of neutrophils in their livers. Furthermore, deletion of Stat3 significantly reduced the survival of Jak2V617F knock-in mice. Together, these results suggest a negative role for Stat3 in Jak2V617F-induced MPN. Thus, Stat3 may not be a suitable therapeutic target for treatment of PV and other JAK2V617F-positive MPNs.