Involvement of 3-phosphoinositide-dependent protein kinase-1 in the MEK/MAPK signal transduction pathway

The phosphatidylinositide-3-OH kinase/3-phosphoinositide-dependent protein kinase-1 (PDK1)/Akt and the Raf/mitogen-activated protein kinase (MAPK/ERK) kinase (MEK)/mitogen-activated protein kinase ( MAPK) pathways have central roles in the regulation of cell survival and proliferation. Despite their importance, however, the cross-talk between these two pathways has not been fully understood. Here we report that PDK1 promotes MAPK activation in a MEK-dependent manner. In vitro kinase assay revealed that the direct targets of PDK1 in the MAPK pathway were the upstream MAPK kinases MEK1 and MEK2. The identified PDK1 phosphorylation sites in MEK1 and MEK2 are Ser(222) and Ser(226), respectively, and are known to be essential for full activation. To date, these sites are thought to be phosphorylated by Raf kinases. However, PDK1 gene silencing using small interference RNA demonstrates that PDK1 is associated with maintaining the steady-state phosphorylated MEK level and cell growth. The small interference RNA-mediated down-regulation of PDK1 attenuated maximum MEK and MAPK activities but could not prolong MAPK signaling duration. Stable and transient expression of constitutively active MEK1 overcame these effects. Our results suggest a novel cross-talk between the phosphatidylinositide-3-OH kinase/PDK1/Akt pathway and the Raf/MEK/MAPK pathway.