REGULATION OF MYELOID GROWTH AND DIFFERENTIATION BY THE INSULIN-LIKE GROWTH-FACTOR-I RECEPTOR

Flow cytometry was used to examine the expression of type I insulin-li ke growth factor receptors (IGF-IR) on three type of human hematopoiet ic cells that represent different stages of myeloid lineage developmen t. Both HL-60 (promyeloid) and U-937 (monocytic) cells express abundan t IGF-IR protein (>79% cells positive for the IGF-IR), whereas KG-1 my eloblasts express negligible levels of IGF-IR (<1% IGF-IR-positive cel ls). Exogenous IGF-I, IGF-II, and an IGF-I analog that binds poorly to IGF-binding protein-3 (des-IGF-I) increased DNA synthesis of HL-60 an d U-937 cells in a dose-dependent (1-25 ng/ml) fashion by 2- to 4-fold in serum-free medium, whereas KG-1 cells did not respond to any of th ese growth factors. The IGF-induced increase in proliferation of HL-60 promyeloid cells was inhibited by soluble IGF-binding protein-3 (500 ng/ml) when these cells were stimulated with 10 ng/ml of either IGF-I (53 +/- 8%) or IGF-II (59 +/- 8%), but not with des-IGF-I (3 +/- 1%). In contrast, the anti-IGF-IR monoclonal antibody (mAb; alpha IR-3) inh ibited the DNA synthesis caused bu 10 ng/ml exogenous IGF-I (67 +/- 6% ), IGF-II (72 +/- 8%), and des-IGF-1 (82 +/- 9%). Proliferation of KG- 1 myeloblasts, however, was neither stimulated by the IGF's nor inhibi ted by the anti-IGF-IR mAb. In the absence of exogenous IGF-I, the mAb directed against the IGF-IR significantly suppressed basal DNA synthe sis of HL-60 promyeloid (72 +/- 5%) and U-935 monocytic (39 +/- 7%) ce lls, but did not affect DNA synthesis of KG-1 myeloblasts (8 +/- 1%) c ompared to all isotype-matched control mAb. Similarly, the alpha IR-3 mAb abrogated vitamin D-3-induced differentiation of the HL-60 cells i nto macrophages in serum-free medium, as assessed by expression of the leucam surface protein. CD11b. As the alpha IR-3 mAb inhibits DNA syn thesis in the presence and absence of exogenous IGF-I on receptor-bear ing cells, but not IGF-IR-negative cells, these data demonstrate that both endocrine and autocrine IGF-I are potent growth factors in human myeloid cells where expression of the surface receptor, rather than th e ligand, is the critical control element. More importantly, these dat a support the hypothesis that autocrine IGF-I may play a significant r ole in the differentiation of promyeloid cells into macrophages.