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- like growth factor receptors (IGF-IR) on three types of human hematopoietic cells that represent different stages of myeloid lineage development. Both HL-60 (promyeloid) and U-937 (monocytic) cells express abundant IGF-IR protein (>79% cells positive for the IGF-IR), whereas KG-1 myeloblasts express negligible levels of IGF-IR (> 1% IGF-IR-positive cells). 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 and 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 these 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 (13 ± 1%). In contrast, the anti-IGF-IR monoclonal antibody (mAb; αIR-3) inhibited the DNA synthesis caused by 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 IGFs nor inhibited by the anti-IGF-IR mAb. In the absence of exogenous IGF-1, the mAb directed against the IGF-IR significantly suppressed basal DNA synthesis of HL-60 promyeloid (72 ± 5%) and U-937 monocytic (39 ± 7%) cells, but did not affect DNA synthesis of KG-1 myeloblasts (8 ± 1%) compared to an isotype-matched control mAb. Similarly, the αIR-3 mAb abrogated vitamin D₃-induced differentiation of the HL-60 cells into macrophages in serum-free medium, as assessed by expression of the leucam surface protein, CD11b. As the αIR-3 mAb inhibits DNA synthesis in the presence and absence of exogenous IGF-I on receptor-bearing 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 the ligand, is the critical control element. More importantly, these data support the hypothesis that autocrine IGF-I may play a significant role in the differentiation of promyeloid cells into macrophages.