Cooperation between STAT5 and phosphatidylinositol 3-kinase in the IL-3-dependent survival of a bone marrow derived cell line

Cytokine-dependent activation of distinct signaling pathways is a common sc heme thought to be required for the subsequent programmation into cell prol iferation and survival. The PI 3-kinase/Akt, Ras/MAP kinase, Ras/NFIL3 and JAK/STAT pathways have been shown to participate in cytokine mediated suppr ession of apoptosis in various cell types. However the relative importance of these signaling pathways seems to depend on the cellular context. In sev eral cases, individual inhibition of each pathway is not sufficient to comp letely abrogate cytokine mediated cell survival suggesting that cooperation en these pathways is required. Here we showed individual inhibition of STA T5, PI 3-kinase or MEK activities did not or weakly affected the IL-3 depen dent survival of the bone marrow derived Ba/F3 cell line. However, the simu ltaneous inhibition of STAT5 and PI 3-kinase activities but not that of STA T5 and MEK reduced the IL-3 dependent survival of Ba/F3, Analysis of the ex pression of the Bcl-2 members indicated that phosphorylation of Bad and Bcl -x expression which are respectively regulated by the PI 3-kinase/Akt pathw ay and STAT5 probably explain this cooperation. Furthermore, me showed by c o-immunoprecipitation studies and pull down experiments with fusion protein s encoding the GST-SH2 domains of p85 that STAT5 in its phosphorylated form interacts with the p85 subunit of the PI 3-kinase, These results indicate that the activations of STAT5 and the PI 3-kinase by IL-3 in Ba/F3 cells ar e tightly connected and cooperate to mediate IL-3-dependent suppression of apoptosis by modulating Bad phosphorylation and Bcl-x expression.