At physiological levels, cellular FLICE-inhibitory protein (c-FLIPL) — a caspase homologue that is thought to be an inhibitor of death-receptor-mediated apoptosis — is actually an activator of apoptosis, according to a report in The EMBO Journal, which describes an entirely new mechanism of cell-death regulation.
After ligand binding, death receptors such as FAS recruit the death adaptor FAS-associated via death domain (FADD) and procaspase-8 to form a death-inducing signalling complex (DISC). The activation of procaspase-8 in the DISC triggers a cascade of caspase activity that leads to cell death. c-FLIPL, which is highly homologous to procaspase-8 but has no protease activity, is also recruited to the DISC, but its role is controversial. The overexpression of c-FLIPL can inhibit apoptosis, but this effect has not been shown at physiological levels of expression and there is some evidence that c-FLIPL can, in fact, promote apoptosis.
As c-FLIPL is known to be recruited to the DISC, Chang and co-workers asked if dimerization of procaspase-8 with c-FLIPL might have a role in its activation. An inducible dimerization system was used to pair-up procaspase-8 and c-FLIPL in vitro and in transfected cells. The cleavage and activation of procaspase-8 in vitro was markedly enhanced after dimerization with c-FLIPL compared with procaspase-8 homodimerization. Also, enhancement of procaspase-8 processing by c-FLIPL in the DISC was observed using cell lines that stably or inducibly express c-FLIPL. This indicates that c-FLIPL can catalyse the processing and activation of procaspase-8.
By transfecting HeLa cells with various amounts of c-FLIPL DNA, and using stable and inducible transfectants, the authors show that the cell-death-sensitizing effects of c-FLIPL are highly dependent on expression level; maximum sensitivity to FAS-mediated apoptosis was seen when levels of exogenous c-FLIPL approximated physiological endogenous levels. So, it is probable that c-FLIPL functions in vivo to promote death-receptor-mediated apoptosis.
But, the level of endogenous c-FLIPL is typically only 1% of that of procaspase-8. How can such a small amount of c-FLIPL regulate death-receptor-mediated apoptosis? When the authors quantified the amounts of c-FLIPL and procaspase-8 in the cytosol and DISCs of FAS-stimulated cells, they found that the amount of c-FLIPL is increased 18-fold in the DISCs, so that the ratio of c-FLIPL to procaspase-8 is approximately 1 to 5.
This new dimension to c-FLIPL makes it an attractive target for therapies that seek to inhibit or promote death-receptor-mediated cell death.
ORIGINAL RESEARCH PAPER
Chang, D. W. et al. c-FLIPL is a dual function regulator for caspase-8 activation and CD95-mediated apoptosis. EMBO J. 21, 3704–3714 (2002)
Thome, M. & Tschopp, J. Regulation of lymphocyte proliferation and death by FLIP. Nature Rev. Immunol. 1, 50–58 (2001)