Death receptors as targets in cancer

Abstract

Anti-tumour therapies based on the use pro-apoptotic receptor agonists, including TNF-related apoptosis-inducing ligand (TRAIL) or monoclonal antibodies targeting TRAIL-R1 or TRAIL-R2, have been disappointing so far, despite clear evidence of clinical activity and lack of adverse events for the vast majority of these compounds, whether combined or not with conventional or targeted anti-cancer therapies. This brief review aims at discussing the possible reasons for the lack of apparent success of these therapeutic approaches and at providing hints in order to rationally design optimal protocols based on our current understanding of TRAIL signalling regulation or resistance for future clinical trials.

Linked articles: This article is part of a themed section on Emerging Therapeutic Aspects in Oncology. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.169.issue-8.

Keywords: Fas; TNF-receptor superfamily; TRAIL; apoptosis; resistance; therapy; tumour targeting.

Figure 1

Simplified schematic representation of Fas ligand, TNF-α and TRAIL-induced signalling: (A) Binding of TNF-α to TNF-RI induces the formation of a membrane bound-complex, composed of RIPK1, TRAF-2, TRADD, IAPs, LUBAC and IKKs that mainly triggers NF-κB activation and cell survival through the transcriptional regulation of the caspase-8 inhibitor c-FLIP. From complex I, a pro-apoptotic cytosolic complex (complex II) is generated, containing the initiator caspase-8 and the adaptor protein FADD. Complex II formation is partly regulated by the linear ubiquitin chain assembly complex, LUBAC, which contains HOIP, HOIL-1 and Sharpin and its pro-apoptotic function is inhibited by c-FLIP. Since the vast majority of cells are proficient for NF-kB activation, upon TNF-α stimulation, TNF-RI fails most of the time to trigger apoptosis. (B) TRAIL- and Fas ligand are potent apoptotic inducers. Binding of these ligands to their cognate pro-apoptotic receptors, TRAIL-R1 or TRAIL-R2 and Fas, respectively, trigger the formation of a membrane complex coined DISC, in which the adaptor protein FADD and the pro-caspase-8/-10 are recruited allowing strong caspase activation and apoptosis triggering. Apoptosis induced by these ligands is either induced through direct caspase-8-mediated caspase-3 activation or through an amplification loop involving the mitochondria and the cleavage of the BH3-only protein Bid by caspase-8.

Figure 2

Fas ligand and TRAIL recombinant proteins or derivatives assessed in clinical trials: Schematic representation of (A) Fas ligand, TRAIL recombinant preparations and TRAIL agonistic monoclonal antibodies assessed in clinical trials and (B) the four main receptors to which TRAIL can bind, namely, TRAIL-R1, TRAIL-R2, TRAIL-R3 and TRAIL-R4. (C) Table representing potential pro-apoptotic capabilities of TRAIL preparations or monoclonal antibodies targeting TRAIL-R1 or TRAIL-R2 in tumour cells expressing variable amounts of TRAIL receptors. Receptors represented in light colour indicate poor engagement of the apoptotic machinery. S stands for potentially sensitive tumour cells; R, potentially resistant tumour cells; S/R, cells potentially sensitive or resistant to TRAIL-induced apoptosis depending on the selective or preferential engagement by TRAIL-R1 or TRAIL-R2. (D) Schematic representation of receptor complex formation and apoptosis induced by TRAIL recombinant preparations or monoclonal antibodies targeting TRAIL-R1 or TRAIL-R2. In all cases, overexpression of Bcl-2 family anti-apoptotic members or c-FLIP by tumour cells may impair TRAIL-induced cell death. TRAIL-R3 or TRAIL-R4 expression by tumour cells on the other hand can only impair apoptosis-induced by recombinant TRAIL preparations.

Figure 3

Differential TRAIL-induced apoptosis following combined versus sequential chemotherapy. (A) Schematic representation of the treatment protocols used panel B. (B) TRAIL-induced apoptosis in HCT116 WT cells (empty squares), HCT116 Bax deficient (Bax-/-) (grey squares) or HCT16 Bax deficient expressing ectopically TRAIL-R4 [Bax-/-(TRAIL-R4)] cells (black squares), stimulated either sequentially with etoposide (VP16) or simultaneously (combo). For sequential treatments, cells were first incubated for 3 h in the presence of 10 μM VP16, washed, allowed to recover at 37°C for 45 h and then stimulated with 500 ng·mL⁻¹ TRAIL for 6 h. Alternatively, cells were stimulated simultaneously with TRAIL and VP16 (combo), or with single agents for 24 or 48 h respectively. Apoptosis was measured by Hoechst staining. (C) Schematic representation of the treatment protocols used panel D. (D) Apoptosis induced by TRAIL, 5-FU or sequential treatments associating 5-FU and TRAIL in HeLa WT cells (empty squares) or HeLa cells expressing TRAIL-R4 ectopically (empty red squares), TRAIL-R4 and Bcl-2 (grey squares) or TRAIL-R4 and c-FLIP (black squares). HeLa cells were stimulated or not for 72 h with 1 μM 5-FU, then treated or not with 500 ng·mL⁻¹ TRAIL for 6 h and apoptosis was monitored by Hoechst staining. Modified from Morizot et al. (2011).