Erythropoietin-dependent endothelial proteins: potential use against erythropoietin resistance.

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.