The Role of Igf-1 and Il-4 in Myeloid Progenitor Cell Survival

176 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1997.The development of myeloid progenitor cells requires paracrine growth factors to mediate the proliferation, differentiation and survival of these cells. In this thesis, we demonstrate that IGF-I and IL-4, a cytokine produced by activated T-lymphocytes, enhance the survival of CSF-stimulated progenitor cells. One particular enzyme activated by both IGF-I and IL-4 is phosphatidylinositol 3$\sp\prime$-kinase, a lipid kinase which phosphorylates phosphatidylinositol on its 3$\sp\prime$ carbon. We demonstrate that activation of this enzyme is critical for IGF-I- and IL-4-mediated inhibition of apoptosis in CSF-deprived myeloid progenitor cells. Although IL-3 effectively induces PI 3-kinase activity, this is not required for IL-3-stimulated survival of FDCP cells. These data suggest that at least two distinct pathways enhance the survival of myeloid cells, one that is dependent upon PI 3-kinase and one that involves another mechanism. We also clearly demonstrate that the downstream mediator of PI 3-kinase-stimulated survival in IGF-I- and IL-4-treated cells is the anti-apoptotic protein Bcl-2. In IL-3-deprived FDCP cells the levels of Bcl-2 decrease in a time-dependent fashion, and this is negatively correlated with an increase in the apoptotic population of these cells. Treatment of IL-3-deprived FDCP cells with IGF-I or IL-4 increases the expression of Bcl-2 protein relative to cells incubated in medium alone. This enhanced expression of Bcl-2 is dependent upon IGF-I- or IL-4-activated PI 3-kinase. Treatment of these cells with either wortmannin or LY294002, two potent inhibitors of PI 3-kinase activity, abrogates the ability of IGF-I or IL-4 to maintain the expression of Bcl-2 in FDCP cells. As expected neither inhibitor affected the ability of IL-3 to maintain Bcl-2 expression or inhibit apoptosis. These data confirm our initial findings which demonstrate that two distinct ligand-dependent mechanisms are involved in mediating the survival of myeloid progenitor cells. We also demonstrate that S6-kinase, an enzyme which mediates the downstream proliferative signal of PI 3-kinase, does not transduce IGF-I- or IL-4-stimulated inhibition of apoptosis. The expression of the pro-apoptotic protein, Bax, does not change even after IL-3 withdrawal for 24 h, and is not regulated by IL-3, IGF-I or IL-4 in the presence or absence of PI 3-kinase inhibitors. These data confirm previous studies that demonstrate cell survival is regulated by relative Bcl-2 and Bax ratios. Collectively, these data demonstrate that IGF-I and IL-4 enhance myeloid cell survival by increasing the expression of Bcl-2 via activation of PI 3-kinase, while IL-3 utilizes an alternative signaling pathway.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD