JAK2 and MPL protein levels determine TPO-induced megakaryocyte proliferation vs differentiation

Abstract

• We propose that megakaryopoiesis is regulated by the expression levels of the TPO receptor MPL and the associated tyrosine kinase JAK2.• This model could explain why suboptimal doses of JAK2 inhibitors can induce a paradoxical increase in platelet production.Megakaryopoiesis is a 2-step differentiation process, regulated by thrombopoietin (TPO), on binding to its cognate receptor myeloproliferative leukemia (MPL). This receptor associates with intracytoplasmic tyrosine kinases, essentially janus kinase 2 (JAK2), which regulates MPL stability and cell-surface expression, and mediates TPO-induced signal transduction. We demonstrate that JAK2 and MPL mediate TPO-induced proliferation arrest and megakaryocytic differentiation of the human megakaryoblastic leukemia cell line UT7-MPL. A decrease in JAK2 or MPL protein expression, and JAK2 chemical inhibition, suppress this antiproliferative action of TPO. The expression of JAK2 and MPL, which progressively increases along normal human megakaryopoiesis, is decreased in platelets of patients diagnosed with JAK2-or MPL-mutated essential thrombocytemia and primary myelofibrosis, 2 myeloproliferative neoplasms in which megakaryocytes (MKs) proliferate excessively. Finally, low doses of JAK2 chemical inhibitors are shown to induce a paradoxical increase in MK production, both in vitro and in vivo. We propose that JAK2 and MPL expression levels regulate megakaryocytic proliferation vs differentiation in both normal and pathological conditions, and that JAK2 chemical inhibitors could promote a paradoxical thrombocytosis when used at suboptimal doses. (Blood. 2014;124(13): 2104-2115

Emergence of MPLW515 mutation in a patient with CALR deletion: Evidence of secondary acquisition of MPL mutation in the CALR clone

“…This could be explained by the fact that the MPL homozygous sub‐clone emerged recently and that this clone size will increase in the future. An alternative hypothesis could be in accordance with our previous study suggesting that the higher the MPL signalling, the lower the proliferative advantage . In this case, the homozygous mutated MPL clone would not have proliferative advantage compared with the heterozygous MPL mutated clone.…”

The thrombopoietin receptor, MPL, is critical for development of a JAK2V617F-induced myeloproliferative neoplasm

“…44 Similarly, using UT7-MPL cell lines, Besancenot et al showed that low levels of MPL expression increased TPO-stimulated proliferation, whereas low doses of JAK2 chemical inhibitors caused increased platelet production in mice. 45 It would appear that some of these data contradict our findings that reduced Mpl expression prevents JAK2V 617 F-mediated myeloproliferation. However, there are a number of differences between the studies that may explain the varying results.…”

Ruxolitinib- but not fedratinib-induced extreme thrombocytosis: the combination therapy with hydroxyurea and ruxolitinib is effective in reducing platelet count and splenomegaly/constitutional symptoms

“…Additionally, some studies suggest that an incomplete JAK2 inhibition and JAK2/MPL deletion in megakaryocytes and platelets may induce a paradoxical increase in platelet production [8][9][10]. Accordingly, we can hypothesize that in our patient, quantitative/qualitative abnormalities of the MPL receptor/signaling pathway may occur in megakaryocytecommitted progenitor cells and/or in megakaryocytes, resulting in stimulation of platelet production and thrombocytosis when ruxolitinib-driven JAK2 inhibition contributes to pathway deregulation.…”