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Thrombocytosis is associated with inflammation, and certain inflammatory cytokines, including IFN-γ, stimulate megakaryocyte and platelet production. However, the roles of IFN-γ and its downstream effector STAT1 in megakaryocyte development are poorly understood. We previously reported that STAT1 expression was significantly downregulated in Gata1-knockdown murine megakaryocytes, which also have impaired terminal maturation. Here, we show that ectopic expression of STAT1, or its target effector IRF-1, rescued multiple defects in Gata1-deficient megakaryopoiesis in mice, inducing polyploidization and expression of a subset of platelet-expressing genes. Enforced expression of STAT1, IRF-1, or GATA-1 enhanced phosphorylation of STAT1, STAT3, and STAT5 in cultured Gata1-deficient murine megakaryocytes, with concomitant megakaryocyte maturation. In contrast, enhanced thrombopoietin signaling, conferred by enforced expression of constitutively active JAK2 or c-MPL, induced phosphorylation of STAT3 and STAT5, but not STAT1, and failed to rescue megakaryocyte maturation. Finally, megakaryocytes from Stat1 -/-mice were defective in polyploidization. Together, these findings reveal a unique role for STAT1 in megakaryopoiesis and provide new insights into how GATA-1 regulates this process. Our studies elucidate potential mechanisms by which various inflammatory disorders can cause elevated platelet counts.

SUMMARY Chuvash polycythemia (CP) is a rare congenital form of polycythemia caused by homozygous R200W and H191D mutations in the von Hippel-Lindau (VHL) gene whose gene product is the principal negative regulator of hypoxia-inducible factor. However, the molecular mechanisms underlying some of the hallmark features of CP such as hypersensitivity to erythropoietin are unclear. Here, we show that VHL directly binds suppressor of cytokine signalling 1 (SOCS1) to form a heterodimeric E3 ligase that targets phosphorylated (p)JAK2 for ubiquitin-mediated destruction. In contrast, CP-associated VHL mutants have altered affinity for SOCS1 and fail to engage and degrade pJAK2. Systemic administration of a highly selective JAK2 inhibitor, TG101209, reverses the disease phenotype in vhlR200W/R200W knock-in mice, a model that faithfully recapitulates human CP. These results reveal VHL as a SOCS1-cooperative negative regulator of JAK2 and provide compelling biochemical and preclinical evidence for JAK2- targeted therapy in CP patients.

Background: A subset of juvenile myelomonocytic leukemia (JMML) patients harbor mutations in the E3 ubiquitin ligase CBL. Results: CBL mutations result in increased GM-CSFR phosphorylation, elevated JAK2 and LYN levels, and enhanced survival. Conclusion: CBL JMML mutants display hypersensitive GM-CSF signaling that can be modulated via inhibition of JAK2 and/or SRC kinases. Significance: Mutation of CBL in JMML is associated with altered GM-CSF function.

Body fluid analysis on the Sysmex XE-2100 series automated hematology analyzer was evaluated at 4 hospitals (Baylor University Medical Center, Dallas, TX; St. John's Mercy, St. Louis, MO; Carle Clinic, Urbana, IL; and ACL Laboratories, West Allis, WI, USA). The total nucleated cell and red blood cell (RBC) counts of 493 samples were obtained with the Sysmex XE-2100 automated hematology analyzer and compared with results obtained by manual chamber counting. Seventy-eight samples were not suitable for evaluation because of the presence of clots, crystals, error messages related to white blood cell (WBC) and RBC parameters, and so on. Pearson correlation coefficients for the WBC parameter were 0.99 for cerebrospinal fluid, 0.95 for serous fluid, 0.99 for synovial fluid, and 0.99 for samples of combined body fluids. Ninety-six samples were used to compare RBC counting methods because these samples had RBC counts greater than 0.01 10 6/mL. The Pearson correlation coefficients for the RBC parameter were 0.96 for cerebrospinal fluid, 0.97 for serous fluid, 0.97 for synovial fluid, and 0.97 for samples of combined body fluids. Carryover, precision, and linearity studies also performed for WBC and RBC counts yielded very good results.

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Erythropoietin (EPO) is the primary cytokine regulator of erythropoiesis, stimulating growth, preventing apoptosis, and promoting differentiation of red blood cell progenitors. The critical importance of EPO, EPO receptor (EPO-R) and JAK2 to erythropoiesis is demonstrated by the fatal embryonic anemia that develops in EPO, EPO-R or JAK2 knockout mice. Intracellular signal transduction pathways regulating growth, differentiation and cell survival downstream of the EPO-R and JAK2 are well documented. However, activation of the EPO-R is transient and down regulated by several negative regulators including tyrosine phosphatases, inositol phosphatases and ubiquitin ligases. The negative regulator, Cbl, has been implicated as a tumour suppressor in murine sarcoma, B cell leukemia, and erythroleukemia. More recently, Cbl was found in a de novo form of acute myeloid leukemia and has been implicated in the formation of gastric tumours. Cbl is known to bind, ubiquitinate, and downregulate signaling from numerous activated hematopoietic and non-hematopoietic receptors. The discovery that Cbl is a target of EPO-dependent tyrosine phosphorylation, together with the finding that the EPO-R is ubiquitinated in vivo, led us to hypothesize that Cbl deficiency leads to altered murine erythropoiesis. Resting C57Bl/6 Cbl-/- mice display normal hematologic parameters with the exception of an increased platelet count. However, Cbl deficient mice respond to phenylhydrazine-mediated anemia with increased reticulocyte production and hematocrit recovery. The hypersensitivity of Cbl deficient mice to anemia may be explained by a three-fold enhancement of splenic colony forming unit-erythroid (CFU-E) and an overall increase in burst forming unit-erythroid (BFU-E) and CFU-E. Furthermore, the elevated sensitivity of Cbl deficient mice to anemia is echoed by increased EPO-R and protein kinase B (PKB)/Akt phosphorylation in splenic erythroblasts at high levels of EPO stimulation. Erythrocyte differentiation was examined by monitoring the expression of the erythroid markers CD71 (Transferrin Receptor) and Ter119. Cbl deficient mice do not have significantly more proerythroblasts than wild-type mice. Interestingly, Cbl deficient mice show impaired erythroid maturation with a 1.7 fold decrease in orthochromatophilic erythroblast levels, elevated erythroid apoptosis in the bone marrow, and no compensation by splenic erythroblast production. These data (as well as earlier studies from our laboratory with STAT1-/- and SHIP-1-/- mice) illustrate the remarkable ability of the spleen to compensate for alterations in bone marrow erythropoiesis. It also suggests that Cbl regulates pathway(s) associated with regulation of erythroid cell survival through regulation of the PI 3 kinase, PKB/Akt signaling cascade.