Enforced expression of KDR receptor promotes proliferation, survival and megakaryocytic differentiation of TF1 progenitor cell line.

Vascular endothelial growth factor (VEGF) receptor-2/kinase insert domain-containing receptor (KDR) is expressed in primitive hematopoietic cells, in megakaryocytes and platelets. In primitive hematopoiesis KDR mediates cell survival via autocrine VEGF, while its effect on cell growth and differentiation has not been elucidated. We induced enforced KDR expression in the granulocyte macrophage-colony-stimulating factor (GM-CSF)-dependent TF1 progenitor cell line (TF1-KDR), treated the cells with VEGF and analyzed their response. In GM-CSF-deprived cells, VEGF induces cell proliferation and protection against apoptosis, followed by enhanced expression of megakaryocytic (MK) markers. Combined with GM-CSF, VEGF induces a mild proliferative stimulus, followed by cell adherence, accumulation in G0/G1, massive MK differentiation and Fas-mediated apoptosis. Accordingly, we observed that MK-differentiating cells, derived from hematopoietic progenitors, produce VEGF, express KDR, inhibition of which reduces MK differentiation, indicating a key role of KDR in megakaryopoiesis. In conclusion, TF1-KDR cells provide a reliable model to investigate the biochemical and molecular mechanisms underlying hematopoietic progenitor proliferation, survival and MK differentiation.

Ectopic expression of interferon regulatory factor-1 potentiates granulocytic differentiation.

Numerous transcription factors allow haematopoietic cells to respond to lineage- and stage-specific cytokines and to act as their effectors. It is increasingly evident that the interferon regulatory factor-1 (IRF-1) transcription factor can selectively regulate different sets of genes depending on the cell type and/or the nature of cellular stimuli, evoking distinct responses in each. In the present study, we investigated mechanisms underlying the differentiation-inducing properties of granulocytic colony-stimulating factor (G-CSF) and whether IRF transcription factors are functionally relevant in myeloid differentiation. Both normal human progenitors and murine 32Dcl3 myeloblasts induced to differentiate along the granulocytic pathway showed an up-regulation of IRF-1 expression. Ectopic expression of IRF-1 did not abrogate the growth factor requirement of 32Dcl3 cells, although a small percentage of cells that survived cytokine deprivation differentiated fully to neutrophils. Moreover, in the presence of G-CSF, granulocytic differentiation of IRF-1-expressing cells was accelerated, as assessed by morphology and expression of specific differentiation markers. Down-modulation of c-Myb protein and direct stimulation of lysozyme promoter activity by IRF-1 were also observed. Conversely, constitutive expression of IRF-2, a repressor of IRF-1 transcriptional activity, completely abrogated the G-CSF-induced neutrophilic maturation. We conclude that IRF-1 exerts a pivotal role in granulocytic differentiation and that its induction by G-CSF represents a limiting step in the early events of differentiation.