Colony-stimulating factor-1-induced oscillations in phosphatidylinositol-3 kinase/AKT are required for caspase activation in monocytes undergoing differentiation into macrophages.

The differentiation of human peripheral blood monocytes into resident macrophages is driven by colony-stimulating factor-1 (CSF-1), which upon interaction with CSF-1 receptor (CSF-1R) induces within minutes the phosphorylation of its cytoplasmic tyrosine residues and the activation of multiple signaling complexes. Caspase-8 and -3 are activated at day 2 to 3 and contribute to macrophage differentiation, for example, through cleavage of nucleophosmin. Here, we show that the phosphatidylinositol-3 kinase and the downstream serine/threonine kinase AKT connect CSF-1R activation to caspase-8 cleavage. Most importantly, we demonstrate that successive waves of AKT activation with increasing amplitude and duration are required to provoke the formation of the caspase-8-activating molecular platform. CSF-1 and its receptor are both required for oscillations in AKT activation to occur, and expression of a constitutively active AKT mutant prevents the macrophage differentiation process. The extracellular receptor kinase 1/2 pathway is activated with a coordinated oscillatory kinetics in a CSF-1R-dependent manner but plays an accessory role in caspase activation and nucleophosmin cleavage. Altogether, CSF-1 stimulation activates a molecular clock that involves phosphatidylinositol-3 kinase and AKT to promote caspase activation. This oscillatory signaling pathway, which is coordinated with extracellular receptor kinase 1/2 oscillatory activation, involves CSF-1 and CSF-1R and controls the terminal differentiation of macrophages.