Anti-apoptotic function of Rac in hematopoietic cells

The small GTP-binding protein Rac plays a pivotal role in the regulation of diverse physiological events including reorganization of the actin cytoskeleton, cell cycle progression, and transformation. Here we show an anti-apoptotic effect of Rac in interleukin-3-dependent murine hematopoietic BaF3 cells. Activated Rac(G12V), when ectopically expressed in BaF3 cells, rendered the cells resistant to apoptosis upon interleukin-3 deprivation, while activated mutants of Rho and Cdc42 displayed no significant anti-apoptotic effect. In contrast to activated Ras, which also supports cell survival in the absence of interleukin-3, Rac required fetal bovine serum for the prevention of cell death. The involvement of phosphatidylinositol 3-kinase downstream of Rac was demonstrated by the inhibition of Rac-induced cell survival by wortmannin and LY294002 and the presence of phosphatidylinositol kinase activity in the Rac immunoprecipitate, Furthermore, the serine/threonine kinase Akt was stimulated by activated Rac and fetal bovine serum in a synergistic manner. Rac-induced Akt activation was mediated by phosphorylation of threonine-308 and serine-473, In addition to the phosphatidylinositol 3-kinase/Akt pathway, the p38 mitogen-activated protein kinase pathway was crucial for Rac-dependent survival, whereas p38 mitogen-activated protein kinase was not implicated in Ras-induced anti-apoptotic signaling. These findings provide evidence for the involvement of Rac in survival signaling of hematopoietic cells.