Apoptosis of erythroid precursors under stimulation with thrombopoietin: contribution to megakaryocytic lineage choice

STI571 inhibits growth and adhesion of human mast cells in culture

Stem cell factor (SCF)/c-kit system is critical for human mast cell development. We thus examined the effects of STI571, an inhibitor of the c-kit tyrosine kinase receptor, on the proliferation and function of human mast cells. STI571 at concentrations of 10(-6) M or higher almost completely abolished the SCF-dependent progeny generation from cord blood-derived cultured mast cells through an inhibition of the tyrosine phosphorylation of c-kit. The compound also suppressed the early phase of mast cell development. The extinction of mast cell growth induced by STI571 may be due largely to apoptosis according to the flow cytometric analysis and gel electrophoresis. Two-hour exposure to STI571 that failed to influence the total viable cell number suppressed adhesion of the cells to fibronectin in the presence of SCF without altering the expressions of integrin molecules. Our results may provide a fundamental insight for the clinical application of STI571 in allergic disorders.

Effects of anoxia on megakaryocyte progenitors derived from cord blood CD34pos cells

BACKGROUND

Severe hypoxic insults to the fetus and neonate are associated with the development of thrombocytopenia. The thrombocytopenia in some cases is the result of disseminated intravascular coagulation, but that mechanism fails to account for all, perhaps the majority, of cases.

OBJECTIVE

We hypothesized that human fetal megakaryocyte (Mk) progenitors are directly adversely affected by transient anoxia.

DESIGN AND METHODS

To test this, we isolated CD34pos cells from the umbilical cord blood of 10 healthy term neonates, and exposed these to 0% or 20% O2 for 24 h, with or without recombinant thrombopoietin (rTpo, 50 ng/mL). After 24 h, a portion of the CD34pos cells were harvested for flow cytometric evaluation of apoptosis. The remaining cells were cultured for an additional 10-12 days, under normoxic conditions, in a collagen-based serum-free system containing rTpo, IL-3, and IL-6. In this way, we sought to determine the effect of transient anoxia on clonogenic capacity of Mk progenitors.

RESULTS

Contrary to our hypothesis, anoxia did not increase either apoptosis or cell death of the CD34pos cells. The addition of rTpo was protective, with a significant decrease in apoptosis and cell death (P < 0.0001), and an increase in the number of Mk colonies cultured (P = 0.04). There was no difference between the normoxic and anoxic groups in proliferative potential of the Mk progenitor cells.

CONCLUSIONS

The thrombocytopenia observed in neonates following an acute hypoxic event is not likely due to a direct deleterious effect of hypoxia on Mk progenitors.

Dynamic DNA methylation change in the CpG island region of p15 during human myeloid development

We examined the kenetics of p15 methylation and expression during myeloid development. We treated human cord blood CD34+ cells with either GM-CSF alone or in combination with stem cell factor and followed methylation at this locus using bisulfite genomic sequencing. CD34+ cells were found to be either fully methylated or completely unmethylated at 27 CpG dinucleotide sites in exon 1 and at 18 CpG sites in the promoter region of the p15 gene. A time-course study showed that the percentage of the allelic methylation of p15 CpG island increased to approximately 50% to 60% until 7 days after cytokine stimulation, then decreased to less than 10% after 21 days. The methylation was also observed in bone marrow CD34+ cells exposed to GM-CSF. p15 expression varied inversely with methylation. Expression was negligible or at low levels until 14 days, after which it increased substantially. The frequency of myeloid colony-forming cells in the progeny decreased and myeloid-specific markers increased in the later stages. Based on our observations on cells grown with GM-CSF and 5-aza-2'-deoxycytidine, DNA methylation of the p15 promoter region CpG island appears to be associated with proliferation rather than differentiation of normal human myeloid progenitors.

Expansion of megakaryocyte precursors and stem cells from umbilical cord blood CD34+ cells in collagen and liquid culture media

Umbilical cord blood (UCB) is now commonly used as a source of stem cells for hematopoietic reconstitution following myeloablative therapy in patients with a variety of diseases. Although UCB is a rich source of stem cells, platelet engraftment occurs at a median of 71 days which is significantly prolonged compared to allogeneic bone marrow. The number of megakaryocyte (MK) precursors in stem cell harvests appears to correlate inversely with the time to platelet engraftment. In an effort to increase the number of platelet precursors, we cultured CD34-selected cord blood mononuclear cells (MNC) in serum-free collagen medium with numerous cytokine combinations. The cells were cultured with four cytokines: interleukin-3 (IL-3), thrombopoietin (TPO), stem cell factor (SCF), and Flt-3); five cytokines, IL-3, TPO, SCF, Flt-3 plus granulocyte-macrophage colony-stimulating factor (GM-CSF), or erythropoietin (Epo); or all six cytokines in combination. After 16 days, significant expansion of MK precursors (CD41(+)) and stem cells (CD34(+) and AC133(+) cells) were seen in cells cultured in IL-3, TPO, SCF, and Flt-3 with or without GM-CSF compared to the combinations that contained Epo (p < 0.05). Similar studies were performed using liquid culture medium, and after 14 days the number of MNCs, CD34(+), AC133(+), CD41(+), and CD61(+) cells were higher in the UCB cells cultured in IL-3, TPO, SCF, and Flt-3 compared to those cultured with those four cytokines plus GM-CSF. These results demonstrate that UCB stem cells can be effectively expanded ex vivo and enriched with platelet precursors using TPO, SCF, Flt-3, and IL-3, whereas the addition of Epo and GM-CSF is unnecessary.

Apoptosis and megakaryocytic differentiation during ex vivo expansion of human cord blood CD34+ cells using thrombopoietin

Thrombopoietin (TPO), the primary regulator of megakaryocytopoiesis, plays important roles in early haematopoiesis. Previously, we have demonstrated that TPO induces a characteristic pattern of apoptosis during ex vivo expansion of cord blood (CB) CD34+ cells. In this study, we have demonstrated that the TPO-induced apoptotic cells belong to the megakaryocytic (MK) lineage and that initially expanding MK progenitors declined along with the appearance of TPO-induced apoptosis. Human CB CD34+ cells were expanded in serum-free conditions with TPO. Multidimensional flow cytometry using simultaneous measurement of apoptosis and immunophenotyping showed that the TPO-induced apoptotic cells appeared in CD61+ fractions. Immunocytochemical analysis of the fluorescent activated cell-sorted fractions showed that the apoptosis-associated CD44low fraction expressed CD61. Clonogenic assay revealed 7.4 +/- 0.50-fold increase of total megakaryocyte colony-forming units (CFU-MKs) during the initial 9 d. Thereafter, the number of CFU-MKs decreased in parallel with the increase of apoptosis. When the MK colonies were subdivided according to size, the proportion of large colonies progressively decreased, while that of medium and small colonies increased. In particular, from d 6 small colonies became predominant. These results suggested that the MK progenitors matured as they expanded during ex vivo expansion with TPO and then proceeded to apoptosis.

Induction of megakaryocytic differentiation in primary human erythroblasts: a physiological basis for leukemic lineage plasticity

In myelodysplasias and acute myeloid leukemias, abnormalities in erythroid development often parallel abnormalities in megakaryocytic development. Erythroleukemic cells in particular have been shown to possess the potential to undergo megakaryocytic differentiation in response to a variety of stimuli. Whether or not such lineage plasticity occurs as a consequence of the leukemic phenotype has not previously been addressed. In this study, highly purified primary human erythroid progenitors were subjected to stimuli known to induce megakaryocytic differentiation in erythroleukemic cells. Remarkably, the primary erythroid progenitors rapidly responded with morphological and immunophenotypic evidence of megakaryocytic differentiation, equivalent to that seen in erythroleukemic cells. Even erythroblasts expressing high levels of hemoglobin manifested partial megakaryocytic differentiation. These results indicate that the lineage plasticity observed in erythroleukemic cells reflects an intrinsic property of cells in the erythroid lineage rather than an epiphenomenon of leukemic transformation.