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The endothelium was the first non-hematopoietic tissue to be identified as a physiological target for erythropoietin (EPO). EPO is involved in recruitment and mobilization of endothelial progenitors and stimulates the production of erythroid cell regulatory factors in endothelial cells. Production of these EPO-dependent factors is inhibited by IL-3 in vitro. Furthermore, EPO-dependent red cell formation in anemic mice is equally repressed by IL-3. The number of IL-3 receptors on endothelial cells increases in chronic inflammation and IL-3 may be one of the inflammatory cytokines, together with TNF-alpha, IFN-gamma or IL-6, which prevents optimal red cell formation in many patients with kidney failure receiving high doses of EPO. These patients could benefit from the administration of some of the EPO-stimulated endothelial factors, such as C21 (the C-terminal segment thrombospondin-4), thrombospondin-1 and chaperonin 10, because these proteins bypass EPO receptors and signaling pathways that are usually compromised in EPO resistance. C21 stimulates red cell formation in anemic mice, increases human hematopoietic cell proliferation in vitro and could eventually fight inflammation, because it is an osteopontin antagonist. Thrombospondin-1 prevents inflammation, stimulates erythroblast proliferation and counteracts IGFBP-3-mediated erythroid inhibition. Finally, chaperonin 10 stimulates hemoglobin synthesis and has anti-inflammatory properties through the inhibition of Toll-like receptor signaling pathways., (Copyright 2010 Elsevier Ltd. All rights reserved.)

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.

Azidothymidine (AZT)-induced anemia in mice can be reversed by the administration of IGF-IL-3 (fusion protein of insulin-like growth factor II (IGF II) and interleukin 3). Although interleukin 3 (IL-3) and erythropoietin (EPO) are known to act synergistically on hematopoietic cell proliferation in vitro, injection of IGF-IL-3 and EPO in AZT-treated mice resulted in a reduction of red cells and an increase of plasma EPO levels as compared to animals treated with IGF-IL-3 or EPO alone. We tested the hypothesis that the antagonistic effect of IL-3 and EPO on erythroid cells may be mediated by endothelial cells. Bovine liver erythroid cells were cultured on monolayers of human bone marrow endothelial cells previously treated with EPO and IGF-IL-3. There was a significant reduction of thymidine incorporation into both erythroid and endothelial cells in cultures pre-treated with IGF-IL-3 and EPO. Endothelial cell culture supernatants separated by ultrafiltration and ultracentrifugation from cells treated with EPO and IL-3 significantly reduced thymidine incorporation into erythroid cells as compared to identical fractions obtained from the media of cells cultured with EPO alone. These results suggest that endothelial cells treated simultaneously with EPO and IL-3 have a negative effect on erythroid cell production., (Copyright 2002 Elsevier Science Ltd. All rights reserved.)