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.