Research Article: Life and Death Decisions in the CD95 System: Main Pro-and Anti-Apoptotic Modulators

Date Published: April , 2009

Publisher: A.I. Gordeyev

Author(s): Inna N. Lavrik, Peter H. Krammer.



Apoptosis is common to all multicellular organisms. Apoptosis can be triggered by the extrinsic (death receptor (DR)) or the intrinsic (mitochondrial) death pathways. CD95 (APO-1/Fas) is a prototypic member of the DR family. This review is focused on the mechanisms of CD95 (APO-1/Fas)-mediated apoptosis and the role in the apoptosis of the death effector domain (DED)-containing proteins: pro-apoptotic protein procaspase-8 and anti-apoptotic protein c-FLIP. Gaining insights into these processes will improve our understanding of the pathogenesis of diseases such as cancer, autoimmunity and AIDS, and will open new approaches to rational treatment strategies.

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CD95 (also called APO-1; Fas; fas antigen; tumor necrosis factor receptor superfamily member 6, TNFRSF6 or apoptosis antigen 1, APT1) is a member of the death receptor (DR) family, a subfamily of the tumor necrosis factor receptor superfamily (1). All members of the DR family are characterized by a cytoplasmic region termed the Death Domain (DD) (2; 3). DD are 80-100 amino acid long motifs involved in the transduction of the apoptotic signal. The DD belongs to the so-called ‘death domain-fold superfamily’. This superfamily comprises the death domain (DD), the death effector domain (DED), and the caspase recruitment domain (CARD). Each of these motifs interacts with other proteins through homotypic interactions. All members of the DD-fold superfamily are characterized by similar structures that comprise six or seven antiparallel amphipatic α-helices.

Procaspase-8 (FLICE, MACHα, Mch5) belongs to the family of caspases (7; 10). Caspases, a family of cysteinyl aspartate specific proteases, are synthesized as zymogens with a prodomain of variable length followed by a large subunit (p20) and a small subunit (p10). Caspases are activated through proteolysis at specific aspartate (D) residues that are located between the prodomain, the p20, and p10 subunits (Fig. 2) (11). This results in the generation of mature active caspases that consist of heterotetramers p202-p102. Subsequently, active caspases specifically process various substrates that are involved in apoptosis and inflammation. Depending on their function and the structure of the prodomain, caspases are divided into initiator caspases and effector caspases and are typically divided into three major groups (Fig. 3) (11). The caspases with large prodomains are referred to as inflammatory caspases (group I) and initiator of apoptosis caspases (group II), while caspases with a short prodomain of 20-30 amino acids are named effector caspases (group III).

c-FLIP, also known as FLAME-1/I-FLICE/CASPER/CASH/MRIT/CLARP/Usurpin, is a well-described inhibitor of DR-mediated apoptosis. The current view on c-FLIP proteins is shown in figure 5. Five c-FLIP proteins have been characterized so far: three c-FLIP isoforms and two cleavage products (9; 20-24). The three c-FLIP isoforms comprise: Long (L), Short (S), and Raji (R), e.g. c-FLIPL, c-FLIPS, and c-FLIPR, respectively (Fig. 5). All three isoforms possess two DED domains and thereby bind to the DISC. In this way, the short FLIP isoforms, c-FLIPS, and c-FLIPR block procaspase-8 activation and apoptosis. The role of the long c-FLIP isoform, c-FLIPL, at the DISC is controversial. It has been shown that depending upon its concentration at the DISC it can act either as an anti-apoptotic molecule, functioning in a way analogous to c-FLIPS, or as a pro-apoptotic molecule, facilitating the activation of procaspase-8 at the DISC (25; 26). This pro-apoptotic role is in agreement with the phenotype of c-FLIP-deficient mice, which are characterized by heart failure and death at embryonic day 11 (27).

DED proteins procaspase-8 and c-FLIP play a central role in the regulation of DR-induced apoptosis and might also induce the NF-κB pathway (6). Regulation of DR-induced apoptosis by procaspase-8 and c-FLIP occurs at the DISC. (Fig. 6. left side). Procaspase-8 is activated at the DISC inducing the apoptotic process, while this activation can be inhibited by all reported c-FLIP proteins. The only exception is the c-FLIPL isoform, which might induce procaspase-8 activation when expressed at low concentrations and block procaspase-8 activation when expressed at high concentrations. Therefore, procaspase-8 at the DISC has a pro-apoptotic role and c-FLIP proteins, except for the c-FLIPL isoform, possess an anti-apoptotic function.