Constitutively activated AKT-1 is vital for the survival of human monocyte-differentiated macrophages: Role of Mcl-1, independent of nuclear factor (NF)-κB, bad, or caspase activation

Recent data from mice deficient for phosphatase and tensin homologue deleted from chromosome 10 or src homology 2 domain-containing 5′ inositol phosphatase, phosphatases that negatively regulate the phosphatidylinositol 3-kinase (PI3K) pathway, revealed an increased number of macrophages in these animals, suggesting an essential role for the PI3K pathway for macrophage survival. Here, we focused on the role of the PI3K-regulated serine/threonine kinase Akt-1 in modulating macrophage survival. Akt-1 was constitutively activated in human macrophages and addition of the PI3K inhibitor, LY294002, suppressed the activation of Akt-1 and induced cell death. Furthermore, suppression of Akt-1 by inhibition of PI3K or a dominant negative (DN) Akt-1 resulted in loss of mitochondrial transmembrane potential, activation of caspases-9 and -3, and DNA fragmentation. The effects of PI3K inhibition were reversed by the ectopic expression of constitutively activated Akt-1 or Bcl-x_L. Inhibition of PI3K/Akt-1 pathway either by LY294002 or DN Akt-1 had no effect on the constitutive or inducible activation of nuclear factor (NF)-κB in human macrophages. However, after inhibition of the PI3K/Akt-1 pathway, a marked decrease in the expression of the antiapoptotic molecule Mcl-1, but not other Bcl-2 family members was observed, and Mcl-1 rescued macrophages from LY294002-induced cell death. Further, inhibition of Mcl-1 by antisense oligonucleotides, also resulted in macrophage apoptosis. Thus, our findings demonstrate that the constitutive activation of Akt-1 regulates macrophage survival through Mcl-1, which is independent of caspases, NF-κB, or Bad.