Caspase-8 prevents sustained activation of NF-kappaB in monocytes undergoing macrophagic differentiation

Caspases have demonstrated several nonapoptotic functions including a role in the differentiation of specific cell types. Here, we show that Caspase-8 is the upstream enzyme in the proteolytic Caspase cascade whose activation is required for the differentiation of peripheral-blood monocytes into macrophages. On macrophage colony-stimulating factor (M-CSF) exposure, Caspase-8 associates with the adaptor protein Fas-associated death domain (FADD), the serine/threonine kinase receptor-interacting protein 1 (RIP1) and the long isoform of FLICE-inhibitory protein FLIP. Overexpression of FADD accelerates the differentiation process that does not involve any death receptor. Active Caspase-8 cleaves RIP1, which prevents sustained NF-kappaB activation, and activates downstream caspases. Together these data identify a role for Caspase-8 in monocytes undergoing macrophagic differentiation, that is, the enzyme activated in an atypical complex down-regulates NF-kappaB activity through RIP1 cleavage.