The α1-adrenergic receptor antagonists, benoxathian and prazosin, induce apoptosis and a switch towards megakaryocytic differentiation in human erythroleukemia cells

The erythroleukemia cell lines K562 and human erythroleukemia (HEL) are established models to study erythroid and megakaryocytic differentiation in vitro. In this study, we show that the α1-adrenergic antagonists, benoxathian and prazosin, inhibit the proliferation and induce apoptosis in K562 and HEL cells. Furthermore, both tested substances induced the expression of the megakaryocytic marker CD41a, whereas the expression of the erythroid marker glycophorin-a was decreased or unchanged. Even though the expression of differentiation markers was similar after benoxathian and prazosin treatment in both cell lines, endomitosis of erythroleukemia cells was observed only after prazosin treatment. So far, benoxathian and prazosin are the first described extracellular ligands, which cause megakaryocytic differentiation in K562 and HEL cells. In summary, these results indicate a possible role of α1-adrenergic receptor signaling in the regulation of erythroid and megakaryocytic differentiation, even though the receptor dependence of the observed effects needs further investigation. Konrad Schauenstein deceased at May 22, 2007.     

Receptor tyrosine kinase, EphB4 (HTK), accelerates differentiation of select human hematopoietic cells

EphB4 (HTK) and its ligand, ephrinB2, are critical for angiogenesis and result in fatal abnormalities of capillary formation in null mice. EphB4 was originally identified in human bone marrow CD34(+) cells by us and has since been reported to be expressed in erythroid progenitors, whereas the ligand ephrinB2 is expressed in bone marrow stromal cells. Reasoning that the developmental relationship between angiogenesis and hematopoiesis implies common regulatory molecules, we assessed whether EphB4 signaling influences the function and phenotype of primitive human hematopoietic cells. Ectopically expressed EphB4 in cell lines of restricted differentiation potential promoted megakaryocytic differentiation, but not granulocytic or monocytic differentiation. Primary cord blood CD34(+) cells transduced with EphB4 resulted in the elevated expression of megakaryocytic and erythroid specific markers, consistent with EphB4 selectively enhancing some lineage-committed progenitors. In less mature cells, EphB4 depleted primitive cells, as measured by long-term culture-initiating cells or CD34(+)CD38(-) cell numbers, and increased progenitor cells of multiple cell types. Effects of ectopic EphB4 expression could be abrogated by either targeted mutations of select tyrosine residues or by the tyrosine kinase inhibitor, genistein. These data indicate that EphB4 accelerates the differentiation of primitive cells in a nonlineage-restricted manner but alters only select progenitor populations, influencing lineages linked by common ancestry with endothelial cells. EphB4 enforces preferential megakaryocytic and erythroid differentiation and may be a molecular bridge between angiogenesis and hematopoiesis.     

Megakaryocyte growth and development factor-induced proliferation and differentiation are regulated by the mitogen-activated protein kinase pathway in primitive cord blood hematopoietic progenitors.

In several erythroleukemia cell lines, activation of mitogen-activated protein kinases (MAPK) by phorbol esters or megakaryocyte growth and development factor (MGDF) is required for induction of megakaryocytic phenotype and growth arrest. To support this model, we have examined the effect of a specific inhibitor of this pathway (PD98059) on human CD34(+) hematopoietic progenitors isolated from cord blood (CB), induced to differentiate along the megakaryocytic lineage in liquid cultures supplemented with rhuMGDF. RhuMGDF induced a sustained activation of MAPK in megakaryocytes and this activation was completely inhibited in the presence of low concentrations of PD98059 (6 to 10 micromol/L). At this concentration, PD98059 induced an increase in cell proliferation, resulting in accumulation of viable cells and a prolongation of the life time of the cultures. This increase correlated with an increase in DNA synthesis rather than with a reduction in apoptosis. This effect was combined with developmental changes indicative of delayed megakaryocytic differentiation: (1) PD98059-treated cells tended to retain markers of immature progenitors as shown by the increased proportion of both CD34(+) and CD41(+)CD34(+) cells. (2) PD98059-treated cultures were greatly enriched in immature blasts cells. (3) PD98059 increased megakaryocytic progenitors able to form colonies in semisolid assays. Thus, the MAPK pathway, although not required for megakaryocyte formation, seems to be involved in the transition from proliferation to maturation in megakaryocytes. Inhibition of MAPK activation also led to an increase in the number and size of erythroid colonies without affecting granulocyte/macrophage progenitor numbers suggesting that, in addition to the megakaryocytic lineage, the MAPK pathway could play a role in erythroid lineage differentiation.     

Erythroid gene expression is differentially regulated by erythropoietin, haemin and delta-aminolaevulinic acid in UT-7 cells

Erythropoietin (Epo) is essential for the later stages of erythropoiesis, acting to promote cell survival and proliferation, but its role in differentiation remains to be defined. The UT-7 cell line exhibits both erythroid and megakaryocytic characteristics and can be induced to differentiate along the erythroid pathway by Epo or the megakaryocytic pathway by phorbol myristic acetate. We have compared the effects of Epo and the chemical inducers, delta-aminolaevulinic acid (delta-ALA) and haemin on the differentiation capacity of UT-7 cells. Epo alone promoted relatively early events in erythroid maturation, without significant changes in haemoglobin production or morphology. GATA-2 and c-myb were down-regulated by Epo, and GATA-2 was further down-modulated by the inducers. Conversely, SCL expression was up-regulated by Epo and further increased by haemin and delta-ALA. Epo caused an increase in the proportion of cells expressing cell surface glycophorin A (GPA) and up-regulated beta- and gamma-globin by several fold. Both haemin and delta-ALA caused a de novo increase in alpha-globin expression as well as enhancing Epo-induced beta-globin expression, leading to a marked increase in haemoglobin production. These results suggest that haemoglobin production in UT-7 cells is limited by a deficiency of erythroid-specific aminolaevulinic acid synthase (ALAS-E) activity or globin synthesis as a consequence of their immaturity as a multipotential cell line.     

Surface antigenic profile and globin phenotype of two new human erythroleukemia lines: characterization and interpretations

Detailed characterization of the composite phenotype of two newly established erythroleukemia lines (OCIM1, OCIM2) shows that these lines share many of their erythroid markers (ie, surface antigens and globin program) as well as several of their nonerythroid properties (myeloid/monocytic/megakaryocytic) with the two known erythroleukemia lines (K562, HEL). In addition, each displays novel and instructive features. We argue that the surface and globin phenotype of all erythroleukemia lines is nonrandom and that it may be of physiologic relevance; it could represent the most prevalent phenotype of cells transformed by leukemia in vivo, and it raises the possibility that cells with similar potentials exist transiently during normal hematopoietic differentiation before their irreversible commitment to a single lineage. As such, these cells demonstrate a greater phenotypic adaptability in vitro than do their single lineage-committed counterparts since they can differentiate toward more than one lineage.     

Use of a monoclonal antibody (GA3) to demonstrate lineage restricted O- glycosylation on leukosialin during terminal erythroid differentiation

A murine monoclonal antibody (GA3) obtained by immunizing mice with cells of the human erythroleukemic cell line K562 is shown to define a 105 kilodalton (kd) membrane antigen on K562 cells that is restricted within the hematopoietic system to the erythroid lineage and to a minor population of CD3, CD4 positive T lymphocytes. Cocapping studies and immunoprecipitation experiments performed with GA3 and L10, an anti- sialophorin monoclonal antibody reacting with leukosialin (Gp 105) on K562 cells, demonstrate that the antigen detected by GA3 on K562 cells is identical to leukosialin. Neuraminidase treatment but not tunicamycin treatment of K562 cells abolishes the expression of the GA3- epitope without affecting the L10-epitope thus providing evidence that terminal sialic acid present on O-linked oligosaccharide chains on Gp 105 is essential for the expression of the GA3-epitope. Further analysis by flow cytometry and immune panning experiments performed on bone marrow cells with GA3 or L10 demonstrate that, in contrast to L10, which reacts with all types of hematopoietic progenitors, the epitope recognized by GA3 is restricted to the erythroid lineage, and appears during erythroid differentiation before glycophorin A on the earliest morphologically recognizable erythroid precursor, the proerythroblast. Our results therefore suggest that O-linked oligosaccharides on leukosialin express lineage restricted and even maturation restricted antigenic structures that might serve as cell lineage specific markers.