The flip side of FLIP

Two major pathways regulate apoptosis induction in mammalian cells. In the extrinsic pathway, apoptosis is induced through specialized surface receptors, whereas in the intrinsic pathway, apoptosis is induced from within the cell, mainly through activation of mitochondria. Shortly after the major mediators of the extrinsic apoptosis pathway, the initiator caspases-8 and -10, were identified, c-FLIP [FLICE-like inhibitory protein; FLICE is FADD (Fas-associated death domain protein)-like interleukin-1beta-converting enzyme], a catalytically inactive caspase-8/-10 homologue, was reported. Whether this structure acts as an inhibitor or promoter of the extrinsic apoptosis pathway has been the subject of much debate. In this issue of the Biochemical Journal, Boatright et al. provide further evidence for the long splice form of c-FLIP (c-FLIP(L)) being an activator of caspase-8/-10, and demonstrate that the resulting heterodimer is enzymically active with a substrate specificity identical with that of the caspase-8 homodimer. Our understanding of the regulators of the extrinsic apoptosis signalling pathway biochemically may provide the means to design drugs to correct the imbalance between apoptosis and proliferation, as found in many diseases.