Antagonism between interleukin 3 and erythropoietin in mice with azidothymidine-induced anemia and in bone marrow endothelial cells

Erythropoietic and non-erythropoietic functions of erythropoietin in mouse models

As the basic function of erythropoietin (Epo) is stimulation of red blood cell production, systemic overexpression of Epo results in erythrocytosis. The patho-physiological consequences of chronically elevated red blood cell counts have been studied in Epo overexpressing mice. Genetically modified mice, however, have also played an important role in discovering multiple additional functions of Epo besides stimulating erythrocyte production. Non-erythropoietic functions of Epo are widespread and play a role in organogenesis during early embryonic development and in tissue protection in ischaemic diseases. Future work in the field will most likely focus on these additional functions of Epo, which have great clinical potential.

Biological activities and molecular interactions of the C-terminal residue of thrombospondin-4, an epitome of acidic amphipathic peptides

C21, the C-terminal residue of thrombospondin-4 (TSP-4), was identified as a peptide growth factor during an investigation concerning erythropoietin-dependent, erythroid stimulating factors of endothelial origin. It is active in cultures of several human hematopoietic stem cells, skin fibroblasts and kidney epithelial cells and stimulates red cell formation in anemic mice. A method of affinity chromatography in the presence of high concentrations of Triton X-100, previously developed for identifying proteins associated with the TSP-1 receptor CD47, was utilized for the detection of C21 binding molecules and their detergent-resistant, associated partners. These experiments helped to delineate two different mechanisms of C21 action, which are compatible with its cell proliferating activity. As a cell matrix peptide, C21 binds to the osteopontin receptor CD44 and could act as an osteopontin antagonist, preventing the inhibition of primitive hematopoietic stem cell proliferation. TSP-1, another matrix protein, binds to C21 and could indirectly act as an antagonist, by shunting C21-CD44 interactions. The second mechanism is a direct effect of C21 on cell proliferation. The extremely rapid internalization and nuclear localization of the peptide could be explained by CD44-mediated internalization, followed by a microtubule-mediated transport towards the nucleus, or, eventually, direct membrane insertion. These alternative hypotheses are supported by previously observed membrane insertion of similar synthetic and viral acidic amphipathic peptides, the presence of microtubule-associated protein 1B (MAP1B) and dynactin in the triton-soluble complexes associated with C21 and the presence in such complexes of dual compartment proteins for nuclei and plasma membranes, such as MAP1B, AHNAK and CD44.

Inhibition of cation channels and suicidal death of human erythrocytes by zidovudine

Zidovudine, a drug widely used in the treatment of AIDS, has been shown to influence cytosolic calcium activity in HIV-infected lymphocytes. Thus, zidovudine may modify the activity of Ca(2+)-permeable ion channels. In erythrocytes, activation of Ca(2+)-permeable cation channels stimulates eryptosis, the suicidal erythrocyte death. Eryptosis is characterized by cell shrinkage (apparent from a decrease of forward scatter) and phosphatidylserine (PS) exposure (apparent from annexin V-binding) at the erythrocyte surface. Triggers of eryptosis include isotonic cell shrinkage (Cl(-) replacement by gluconate), energy depletion (removal of glucose) or exposure to a variety of drugs including azathioprine. The present study explored, whether zidovudine influences the activity of erythrocytic Ca(2+)-permeable cation channels and eryptosis. Whole-cell patch-clamp recordings indeed revealed that zidovudine blocked the Ca(2+)-permeable cation channels activated by Cl(-) removal. In the presence of Cl(-) and glucose, the percentage of annexin V-binding cells was low and not significantly modified by the presence of zidovudine. Both, Cl(-) removal and glucose depletion increased annexin V-binding and decreased forward scatter, effects significantly blunted by zidovudine (2 microg/ml). According to Fluo3 fluorescence, zidovudine (2 microg/ml) did not significantly modify cytosolic Ca(2+) concentration under control conditions, but significantly blunted the increase in cytosolic Ca(2+) activity following glucose depletion. Furthermore, zidovudine significantly inhibited azathioprine-induced eryptosis. The present observations disclose a completely novel effect of zidovudine, i.e. its inhibitory influence on Ca(2+) entry and subsequent suicidal erythrocyte death during isotonic cell shrinkage or energy depletion.

Comparison of the effects of serpin A1, a recombinant serpin A1-IGF chimera and serpin A1 C-terminal peptide on wound healing

Serpin A1 (alpha1-antitrypsin, alpha1-proteinase inhibitor), a potent neutrophil elastase inhibitor, has therapeutic potential as a wound-healing agent. We compared the in vitro wound-healing action of serpin A1-IGF, a recombinant fusion protein of serpin A1(M351E-M358L) and insulin-like growth factor I with that observed in the presence of natural serpin A1 or A1-C26, the synthetic C-terminal 26 residue peptide of serpin A1, previously shown to have mitogenic and antiviral activities. All agents reduced wound sizes in monolayers of the kidney epithelial cell line LLC-PK1 and in primary cultures of human skin fibroblasts. Wound reduction in primary human keratinocytes was only observed with the serpin A1-IGF chimera. None of the factors stimulated cell proliferation using a colorimetric assay, with the exception of the serpin A1-IGF chimera, which caused a significant increase of cell proliferation and thymidine incorporation in human skin fibroblasts. However, wound healing by the A1-IGF chimera was reduced in keratinocytes in the presence of mitomycin C, suggesting a role of cell proliferation in wound reduction. The hydrophobic A1-C26 peptide significantly increased the production of collagen I in skin fibroblasts, an appealing asset for skin care applications.

Thrombospondin 1, produced by endothelial cells under the action of erythropoietin, stimulates thymidine incorporation into erythroid cells and counteracts the inhibitory action of insulin-like growth factor binding protein 3

The nature of erythropoietin (EPO)-dependent, erythroid cell regulatory factors secreted by endothelial cells is largely unknown. The production of thrombospondin 1 (TSP-1) and insulin-like growth factor binding protein 3 (IGFBP-3) is increased in cultures of human umbilical vein endothelial cells (HUVEC) incubated with erythropoietin (EPO). Simultaneous incubation of HUVEC with EPO and interleukin 3 (IL-3) resulted in a decreased production, suggesting that both TSP-1 and IGFBP-3 belong to the EPO- and IL-3-dependent erythroid regulatory factors previously described in cultures of bone marrow endothelial cells. TSP-1 and TSP-1 derived synthetic peptides based on the CD36 and CD47 binding sites of TSPs increased thymidine incorporation into bovine erythroid cells of fetal liver. IGBBP-3 inhibited thymidine incorporation in the same cells. Preincubation of erythroid cells with TSP-1 eliminated the inhibitory activity of IGFBP-3. We suggest that EPO-dependent, endothelial-derived TSP-1 may play a positive role in red cell production by acting directly on erythroid cells, stimulating DNA synthesis and preventing the inhibitory action of IGFBP-3.

The C-terminal peptide of thrombospondin-4 stimulates erythroid cell proliferation

Erythropoietin (EPO) stimulates the production of small erythroid cell stimulating factors (molecular weight <5 kDa) in cultures of bone marrow endothelial cells. We identified a fragment of thrombospondin-4 (TSP-4) as an EPO-stimulated protein in endothelial cell lysates. Pre-incubation of the low molecular weight fractions from supernatants of EPO-treated umbilical cord endothelial cells (HUVEC) with antibodies against the C-terminal residues of TSP-1,2 and TSP-4 decreased the erythroid cell stimulating activity. The C-terminal TSP-1 section corresponding to a molecular weight lower than 6 kDa has the integrin-associated protein binding motif VVM. The corresponding TSP-4 fragment, lacking the three residue sequence VVM, has a distinctive acidic peptide comprising the last 21 amino acids (C21) with the characteristics of an amphipathic helix. C21 stimulated thymidine incorporation into bovine erythroid cells, increased cell numbers in cultures of cord blood CD36+ erythroid precursors and skin fibroblasts, and decreased HUVEC proliferation. SC21, a homologous peptide of identical amino acid composition but with interchanged residues, was non-amphipathic and had no erythroid cell stimulating activity.

The fusion of IGF I with stromal cell-derived factor I or alpha1 proteinase inhibitor alters their mitogenic or chemotactic activities while keeping their ability to inhibit HIV-1-gp120 binding

It has been previously reported that insulin-like growth factor I (IGF I) decreases in AIDS patients with wasting, a condition that is partially prevented by combined IGF I growth hormone therapy. By generating bifunctional proteins of IGF I and stromal cell-derived factor 1alpha (SDF-1alpha) or alpha1 proteinase inhibitor (API), two proteins known to prevent HIV infection, it may be possible to improve the therapeutic effectiveness of these compounds for the treatment of AIDS-mediated wasting. SDF-1alpha or the M351E-M358L mutant of API were attached at the C-terminal end of IGF I and synthesized by a stable insect cell expression technique. The IGF I-SDF-1alpha chimera reduced the enhancement of thymidine incorporation into bovine fetal erythroid cells observed in the presence of insect cell produced IGF I alone. It also decreased the SDF-1 and IGF I-stimulated hematopoietic cell migration, without losing the capacity to compete with the binding of HIV-1 (IIIB)-surface glycoprotein gp120. The IGF I-API chimera displayed the same mitogenic activity and a similar, but lower chemotactic activity than IGF I in the assays mentioned above. It had a comparable anti-elastase activity to that observed with a previously described IGF II-API fusion protein with the single mutation M351E. The binding of gp120 to a murine hematopoietic cell line was stimulated by human neutrophil elastase (25-100 nM) and inhibited by IGF I-API. In conclusion, the linkage of IGF I with SDF-1 or API can alter some biological functions of the single components of the chimera while keeping their ability to compete with HIV-1-gp120 binding.

The improved survival of hematopoietic cells cultured with a fusion protein of insulin-like growth factor II (IGF-II) and interleukin 3 (IL-3) is associated with increases in Bcl-xL and phosphatidylinositol-3 kinase activity

We compared the antiapoptotic activity of a recombinant chimera of insulin-like growth factor II (IGF-II) and interleukin (IL)-3 with the corresponding equimolar mixture of the individual components based on changes in several factors associated with survival in the CD34+ human hematopoietic cell line TF-1. Propidium iodide-stained cells analyzed by fluorescein-activated cell sorter indicated that the chimera was more effective than the corresponding equimolar mixture in decreasing the amounts of apoptotic cells and increasing the proportion of cells in the S-phase of the cell cycle. The chimera was more effective in increasing the antiapoptotic protein Bclx(L) and produced a significant increase in signal transducer and activator of transcription-5 phosphorylation and in phosphatidylinositol-3 kinase (PI-3K) activity. The PI-3K inhibitor LY294002 specifically inhibited cell survival in the presence of the chimera, suggesting a key role of this enzyme in the potentiation of survival caused by the linkage of IGF and IL-3. This potentiation of survival and its preferential inhibition by LY294002 were also observed in a nontransformed, primary culture of human umbilical cord endothelial cells.