ACTIVATION OF PHOSPHATIDYLINOSITOL 3'-KINASE BY INSULIN-LIKE GROWTH-FACTOR-I RESCUES PROMYELOID CELLS FROM APOPTOSIS AND PERMITS THEIR DIFFERENTIATION INTO GRANULOCYTES

Insulin-like growth factor-I (IGF-I) promotes cell division and preven ts programmed cell death in hemopoietic progenitors. Human HL-60 promy eloid cells differentiate toward the granulocytic lineage when stimula ted with retinoic acid (RA) in serum-containing medium. When deprived of serum, however, we found that these cells differentiate poorly in t he presence of RA, as assessed by expression of the alpha subunit of t he beta(2) integrin heterodimer, CD11b/CD18. However, when ICF-I is ad ded to RA-treated cells, the proportion of CD11b-positive cells increa ses to a level similar to that in RA-treated cells cultured in serum-c ontaining medium. Cells treated with RA alone not only differentiate p oorly but also undergo apoptosis, as assessed by flow cytometry using propidium iodide and HO33342. In serum-free medium, one-third of RA-tr eated cells become apoptotic compared with only 5% apoptotic cells in the absence of RA. However, addition of ICF-I to RA-treated cells prev ents the appearance of this apoptotic population and increases phospha tidylinositol 3'-kinase (PI 3-kinase) activity by fivefold. Wortmannin , a PI 3-kinase inhibitor, potently decreases this ICF-I-induced lipid kinase activity, blocks the ability of ICF-I to prevent apoptosis, an d inhibits IGf-I-enhanced CD11b expression. These data demonstrate tha t ICF-I acts on RA-treated progenitors to promote their differentiatio n along the granulocytic lineage. ICF-I acts by rescuing these cells f rom apoptotic cell death via a downstream pathway that is dependent up on PI 3-kinase.