DISSOCIATION OF CYTOKINE-INDUCED PHOSPHORYLATION OF BAD AND ACTIVATION OF PKB AKT - INVOLVEMENT OF MEK UPSTREAM OF BAD PHOSPHORYLATION/

The phosphatidylinositol 3-kinase (PI3K)signaling pathway has emerged as an important component of cytokine-mediated survival of hemopoietic cells. Recently, the protein kinase PKB/akt (referred to here as PKB) has been identified as a downstream target of PI3K necessary for surv ival, PKB has also been implicated in the phosphorylation of Bad, pote ntially linking the survival effects of cytokines with the Bcl-2 famil y. We have shown that granulocyte/macrophage colony-stimulating factor (GM-CSF) maintains survival in the absence of PI3K activity, and we n ow show that when PKB activation is also completely blocked, GM-CSF is still able to stimulate phosphorylation of Bad. Interleukin 3 (IL-3), on the other hand, requires PI3K for survival, and blocking PI3K part ially inhibited Bad phosphorylation, IL-4, unique among the cytokines in that it lacks the ability to activate the p21ras-mitogen-activated protein kinase (MAPK) cascade, was found to activate PKB and promote c ell survival, but it did not stimulate Bad phosphorylation, Finally, a lthough our data suggest that the MAPK pathway is not required for inh ibition of apoptosis, we provide evidence that phosphorylation of Bad may be occurring via a MAPK/ERK kinase (MEK)-dependent pathway. Togeth er, these results demonstrate that although PI3K may contribute to pho sphorylation of Bad in some instances, there is at least one other PI3 K-independent pathway involved, possibly via activation of MEK. Our da ta also suggest that although phosphorylation of Bad may be one means by which cytokines can inhibit apoptosis, it may be neither sufficient nor necessary for the survival effect.