Search

The constitutively active JAK2 V617F mutant is the major determinant of human myeloproliferative neoplasms (MPNs). We show that coexpression of murine JAK2 V617F and the murine thrombopoietin (Tpo) receptor (TpoR, c-MPL) in hematopoietic cell lines or heterozygous knock-in of JAK2 V617F in mice leads to down-modulation of TpoR levels. Enhanced TpoR ubiquitinylation, proteasomal degradation, reduced recycling, and maturation are induced by the constitutive JAK2 V617F activity. These effects can be prevented in cell lines by JAK2 and proteasome inhibitors. Restoration of TpoR levels by inhibitors could be detected in platelets from JAK2 inhibitor-treated myelofibrosis patients that express the JAK2 V617F mutant, and in platelets from JAK2 V617F knock-in mice that were treated in vivo with JAK2 or proteasome inhibitors. In addition, we show that Tpo can induce both proliferative and antiproliferative effects via TpoR at low and high JAK2 activation levels, respectively, or on expression of JAK2 V617F. The antiproliferative signaling and receptor down-modulation by JAK2 V617F were dependent on signaling via TpoR cytosolic tyrosine 626. We propose that selection against TpoR antiproliferative signaling occurs by TpoR down-modulation and that restoration of down-modulated TpoR levels could become a biomarker for the treatment of MPNs.

Thrombopoietin (TPO) via signaling through its cognate receptor MPL is a key cytokine involved in the regulation of megakaryocyte differentiation leading to platelet production. Mature megakaryocytes are polyploid cells that have arrested DNA replication and cellular proliferation but continue sustained protein synthesis. Here, we show that TPO induces cell-cycle arrest in the megakaryocytic UT7-MPL cell line by the activation of the ERK/MAPK pathway, induction of p21CIP transcription, and senescence markers through EGR1 activation. A similar senescence-like process was also detected in normal primary postmitotic megakaryocytes. In contrast, senescence was not observed in malignant megakaryocytes derived from primary myelofibrosis patients (a form of chronic myeloid hemopathy). Our data indicate that polyploid mature megakaryocytes receive signals from TPO to arrest cell proliferation and enter a senescent-like state. An escape from this physiological process may be associated with certain myeloproliferative neoplasms leading to abnormal megakaryocytic proliferation.