Description
Background: Cellular FLIP (FLICE inhibitory protein) is a regulator of apoptosis that has various names, such as c-FLIP (1), Casper (2), CLARP (3), FLAME (4), I-FLICE (5), MRIT (6), CASH (7), and Usurpin (8). FLIP is expressed as two alternative splice isoforms, FLIP short (FLIPS) and FLIP long (FLIPL). FLIPS contains two death effector domains (DEDs) like those found on the death receptor adaptor protein FADD and the pro-domain of caspase-8. FLIPL shares significant homology with caspase-8 (FLICE), and contains an additional death effector domain, but FLIPL lacks the catalytic active site of the caspases and does not have protease activity. Both FLIP isoforms have been reported to interact with FADD and pro-caspase-8. The role of FLIP in apoptosis is controversial as some research studies have reported it to be anti-apoptotic, while others claim that it is pro-apoptotic. Overexpression of FLIPL can lead to caspase-8 heterodimers that produce an active protease, resulting in apoptosis. However, at physiological levels, it is thought that the binding of FLIP to the DED of FADD results in inhibition of caspase-8 processing. Reduction of FLIP by siRNA or gene targeting sensitizes cells to death receptor-mediated apoptosis. FLIP has also been implicated in the resistance of cancer cells to apoptosis and is upregulated in some cancer types including Hodgkin's lymphoma and ovarian and colon carcinomas (9)