L-triiodothyronine augments erythropoietic growth factor release from peripheral blood and bone marrow leukocytes

To investigate cellular mechanisms involved in thyroid hormone stimulation of erythropoiesis, we studied the response of erythroid burst-forming unit (BFU-E) proliferation to L-triiodothyronine (L-T3) in a serum-free culture system. When added directly to culture, L-T3 stimulates erythroid burst formation by normal human bone marrow cells. In contrast, granulocyte-macrophage colony formation is unaffected. Enhancement of erythroid burst formation by L-T3 required the presence of nylon wool adherent and/or B-4 antigen-positive light-density marrow populations. Addition of other erythropoietic factors including platelet-derived growth factor and insulinlike growth factor II did not abrogate this apparent cellular requirement. Pulse exposure of marrow and peripheral blood mononuclear cells (greater than 95% lymphocytes) to L-T3 accelerates the release of a soluble factor that augments BFU-E proliferation into serum-free liquid culture medium. Time-course studies show that this factor appears in conditioned medium (CM) coincidentally with erythroid burst-promoting activity (BPA). Furthermore, incubation of CM with an antibody known to react with and adsorb BPA from solution removes the inducible mitogen. Biochemical analysis of CM prepared from unexposed and L-T3 pulse-exposed cells indicates that the rate of protein appearance is accelerated by L-T3 in a fashion that immediately precedes growth factor release and that several polypeptides are quantitatively increased. We conclude that unlike erythropoietin, which is mitogenic for progenitor cells directly, L-T3 enhances BFU-E proliferation indirectly by augmenting the release of soluble BPA-like molecules from accessory cells in culture.