Interleukin 3 opposes the action of negative regulatory protein (NRP) and of transforming growth factor-beta (TGF-beta) in their inhibition of DNA synthesis of the erythroid stem cell BFU-E.

DNA synthesis of the early erythropoietic progenitor cell (erythroid burst-forming unit, BFU-E) is inhibited by a growth factor that we have called negative regulatory protein (NRP). This protein appears to act during the S-phase of the cell cycle and to be specific to the BFU-E. It is nontoxic and its action is readily reversible by washing the cells. Erythropoietic burst formation by BFU-E in culture is promoted by interleukin 3 (IL-3). In the present work, using the hydroxyurea suicide assay method, we investigated the effects on DNA synthesis of exposing BFU-E of mouse bone marrow in vitro to NRP, IL-3, and combinations of NRP and IL-3. We found that the action of NRP on BFU-E DNA synthesis was opposed by IL-3 within the 45 min that it took to carry out the cell suicide assay. We also studied transforming growth factor-beta (TGF-beta) and found that its action on DNA synthesis of BFU-E was identical to that of NRP in time scale, reversibility, and opposition by IL-3, although the two have different molecular properties. According to the competence-progression model, regulation of cell proliferation occurs at two sites: 1) the G0 to G1 transition, where competence factors (e.g., platelet-derived growth factor [PDGF] and interleukin 1 [IL-1]) act, and 2) during G1-phase, where progression factors (e.g., interleukin 2 [IL-2] and IL-3) act. Our data indicate that cell proliferation may also be regulated at a third site, during S-phase. Here, the decision as to whether or not DNA synthesis will proceed appears to depend on a balance between positive and negative regulatory signals.