c-FLIP, a master anti-apoptotic regulator

Abstract

Cellular FLICE (FADD-like IL-1β-converting enzyme)-inhibitory protein (c-FLIP) is a master anti-apoptotic regulator and resistance factor that suppresses tumor necrosis factor-α (TNF-α), Fas-L, and TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis, as well as apoptosis triggered by chemotherapy agents in malignant cells. c-FLIP is expressed as long (c-FLIP(L)), short (c-FLIP(S)), and c-FLIP(R) splice variants in human cells. c-FLIP binds to FADD and/or caspase-8 or -10 and TRAIL receptor 5 (DR5) in a ligand-dependent and -independent fashion and forms an apoptosis inhibitory complex (AIC). This interaction in turn prevents death-inducing signaling complex (DISC) formation and subsequent activation of the caspase cascade. c-FLIP(L) and c-FLIP(S) are also known to have multifunctional roles in various signaling pathways, as well as activating and/or upregulating several cytoprotective and pro-survival signaling proteins including Akt, ERK, and NF-kB. Upregulation of c-FLIP has been found in various tumor types, and its silencing has been shown to restore apoptosis triggered by cytokines and various chemotherapeutic agents. Hence, c-FLIP is an important target for cancer therapy. For example, small interfering RNAs (siRNAs) that specifically knockdown the expression of c-FLIP(L) in diverse human cancer cell lines augmented TRAIL-induced DISC recruitment and increased the efficacy of chemotherapeutic agents, thereby enhancing effector caspase stimulation and apoptosis. Moreover, small molecules causing degradation of c-FLIP as well as decreasing mRNA and protein levels of c-FLIP(L) and c-FLIP(S) splice variants have been found, and much effort is focused on developing other c-FLIP-targeted cancer therapies. This review focuses on (1) the anti-apoptotic role of c-FLIP splice variants in preventing apoptosis and inducing cytokine and chemotherapy drug resistance, (2) the molecular mechanisms and factors that regulate c-FLIP expression, and (3) modulation of c-FLIP expression and function to eliminate cancer cells or increase the efficacy of anticancer agents. This article is part of a Special Issue entitled "Apoptosis: Four Decades Later".

Fig. 1

Apoptosis pathways and roles of c-FLIP in preventing apoptosis. TRAIL interaction with DR4 and DR5 transduces the death receptor (extrinsic) and mithochondrial apoptosis signaling pathways through activation of caspases-8 and -10 (see the text for detailed information). c-FLIP isoforms are major anti-apoptotic proteins that suppress caspase-8 and -10 activation, and therefore prevent the downstream apoptosis cascade

Fig. 2

Structures of c-FLIP variants. Three c-FLIP variants, c-FLIP_L, c-FLIP_S, and c-FLIP_R, contain two death effector domains (DEDs) at their N termini. In addition to two DEDs, c-FLIP_L contains a large (p20) and a small (p12) caspase-like domain without catalytic activity. c-FLIP_S and c-FLIP_R consist of two DEDs and a small C terminus. Depending on its cellular level at the DISC, c-FLIP_L may act as an anti-apoptotic or pro-apoptotic factor [9, 16, 18]. c-FLIP_S, c-FLIP_S, c-FLIP_R, and two cleavage products of c-FLIP (p43-FLIP and p22-FLIP) act as anti-apoptotic proteins [9, 16, 18]. p43-FLIP and p22-FLIP are generated from c-FLIP_L by procaspase-8 cleavage at D376 [9] (adapted from [9, 16, 18])

Fig. 3

Multifunctional roles of c-FLIP on various signaling pathways. As discussed in the text, in addition to its functional role in inhibiting apoptosis by binding to procaspases-8 and -10 and inhibiting their activation, c-FLIP activates various anti-apoptotic and cell survival signaling pathways, leading to proliferation and cell survival