Dissociation of apoptosis from proliferation, protein kinase B activation,and BAD phosphorylation in interleukin-3-mediated phosphoinositide 3-kinase signaling

Interleukin-3 (IL-3) acts as both a growth and survival factor for many hem opoietic cells. IL-3 treatment of responsive cells leads to the rapid and t ransient activation of Class I-A phosphoinositide-3-kinases (PI3Ks) and the serine/threonine kinase Akt/protein kinase B (PKB) and phosphorylation of BAD. Each of these molecules has been implicated in anti-apoptotic signalin g in a wide range of cells. Using regulated expression of dominant-negative p85 (Delta p85) in stably transfected IL-3-dependent BaF/3 cells, we have specifically investigated the role of class I-A PI3K in IL-3 signaling. The major functional consequence of Delta p85 expression in these cells is a h ighly reproducible, dramatic reduction in IL-3-induced proliferation. Expre ssion of Delta p85 reduces IL-3-induced PKB phosphorylation and activation and phosphorylation of BAD dramatically, to levels seen in unstimulated cel ls. Despite these reductions, the levels of apoptosis observed in the same cells are very low and do not account for the reduction in IL-3-dependent p roliferation we observe. These results show that Delta p85 inhibits both PK B activity and BAD phosphorylation without significantly affecting levels o f apoptosis, suggesting that there are targets other than PKB and BAD that can transmit survival signals in these cells. Our data indicate that the pr ime target for PI3K action in IL-3 signaling is at the level of regulation of proliferation.