Abstract
The regulatory control of human erythropoiesis through a purified protein, activin A, was examined. Previous studies using mixed populations of bone marrow cells suggested that activin A has an indirect effect on cellular proliferation and DNA synthesis of erythroid progenitors through the mediation of accessory cells. In present studies, the cultures of purified erythroid progenitors were used to examine the effect of activin A on globin gene expression. Human erythroid burst-forming units (BFU-E) were partially purified from peripheral blood, and after 8 days of culture the cells generated consisted mainly of erythroid colony-forming units (CFU-E). It was found that the subsequent 7-day cultures of these purified progenitors yielded similar numbers and size distributions of erythroid colonies, regardless of the presence of activin A in the cultures. In addition, these erythroid progenitor cells were responsive, in terms of stimulation of DNA synthesis, to the addition of erythropoietin, but not to treatment by activin A. Therefore, once the erythroid progenitors are depleted of accessory cells, activin A has little effect on both the proliferation and the DNA synthesis of these progenitors. However, when these purified erythroid progenitors were cultured in the presence of activin A, the levels of all alpha, beta, and epsilon globin transcripts and hemoglobins were significantly increased. In addition, disuccinimidyl suberate was found to chemically cross-link 125I-activin A to cell surface binding proteins (45 to 54 Kd) in both purified erythroid progenitors and K562 cells. The labeling of these binding proteins was specifically inhibited by the presence of unlabeled activin A, but not transforming growth factor-beta. These results suggest that, in addition to its indirect effect on DNA synthesis and cellular proliferation of erythroid progenitors, activin A directly affects the levels of globin mRNAs and hemoglobins in developing human erythroid cells through its specific surface binding receptor(s).
