Getting TRAIL back on track for cancer therapy

Abstract

Unlike other members of the TNF superfamily, the TNF-related apoptosis-inducing ligand (TRAIL, also known as Apo2L) possesses the unique capacity to induce apoptosis selectively in cancer cells in vitro and in vivo. This exciting discovery provided the basis for the development of TRAIL-receptor agonists (TRAs), which have demonstrated robust anticancer activity in a number of preclinical studies. Subsequently initiated clinical trials testing TRAs demonstrated, on the one hand, broad tolerability but revealed, on the other, that therapeutic benefit was rather limited. Several factors that are likely to account for TRAs' sobering clinical performance have since been identified. First, because of initial concerns over potential hepatotoxicity, TRAs with relatively weak agonistic activity were selected to enter clinical trials. Second, although TRAIL can induce apoptosis in several cancer cell lines, it has now emerged that many others, and importantly, most primary cancer cells are resistant to TRAIL monotherapy. Third, so far patients enrolled in TRA-employing clinical trials were not selected for likelihood of benefitting from a TRA-comprising therapy on the basis of a valid(ated) biomarker. This review summarizes and discusses the results achieved so far in TRA-employing clinical trials in the light of these three shortcomings. By integrating recent insight on apoptotic and non-apoptotic TRAIL signaling in cancer cells, we propose approaches to introduce novel, revised TRAIL-based therapeutic concepts into the cancer clinic. These include (i) the use of recently developed highly active TRAs, (ii) the addition of efficient, but cancer-cell-selective TRAIL-sensitizing agents to overcome TRAIL resistance and (iii) employing proteomic profiling to uncover resistance mechanisms. We envisage that this shall enable the design of effective TRA-comprising therapeutic concepts for individual cancer patients in the future.

Figure 1

Overview of the TRAIL-R system in humans. TRAIL can bind to four membrane-bound and to one soluble receptor. TRAIL-R1 (DR4) and TRAIL-R2 (DR5) can induce apoptosis via their DDs. In contrast, TRAIL-R3 (DcR1), TRAIL-R4 (DcR2) and the soluble receptor osteoprotegerin (OPG) have been suggested to impair TRAIL-induced apoptosis as they are capable of binding to TRAIL but lack a functional DD required for apoptosis induction. TRAIL-R3 is as glycosyl-phosphatidyl-inositol-anchored protein that completely lacks an intracellular domain. TRAIL-R4 is inserted in the membrane via a transmembrane domain but only expresses a truncated death domain, which is incapable of inducing apoptosis

Figure 2

The current model of TRAIL-induced DISC formation. Upon binding of trimerized TRAIL to TRAIL-R1/2, the adaptor molecule FADD is recruited via homotypic DD interaction. Subsequently, FADD recruits pro-caspase-8/10 molecules via their respective DEDs. These pro-caspases are cleaved and activated at the DISC, initiating the apoptosis signaling cascade. The E3 ligase Cullin3 has been shown to stabilize DISC formation by polyubiquitination of caspase-8. Different forms of cFLIP can inhibit DISC formation by competing with caspase-8/10 for binding to FADD. TRAF2 has been suggested to negatively regulate DISC activity by promoting K48-linked ubiquitination and subsequent proteasomal degradation of caspase-8

Figure 3

TRAIL-induced apoptosis. In type-I cells, DISC-activated caspase-8 is sufficient to directly cleave and activate the downstream effector caspase-3. In type-II cells, activation of the mitochondrial apoptosis pathway, mediated by caspase-8-dependent cleavage of Bid, is required to achieve effector caspase activation. Truncated Bid (tBid) translocates to the mitochondria where it induces Bax/Bak-mediated mitochondrial outer membrane permeabilization (MOMP). MOMP releases pro-apoptotic factors such as cytochrome-c and Smac/DIABLO into the cytosol. Cytosolic cytochrome-c aggregates with Apaf-1 and procaspase-9 to form the apoptosome, the activation platform for caspase-9. Active caspase-9 cleaves, and thereby activates, downstream effector caspases including caspase-3. The TRAIL-induced apoptosis cascade is inhibited at various levels: (i) at the DISC, cFLIP competes with caspase-8 for binding to FADD; (ii) at the mitochondria, anti-apoptotic Bcl-2 family members like Bcl-2, Bcl-xL and Mcl-1 inhibit pro-apoptotic Bax- and Bak-mediated MOMP and (iii) at the level of effector caspases, XIAP inhibits them by direct interaction. Hence, together with the extent of DISC-generated caspase-8 activity, the ratio of the expression of caspase-3 to XIAP is crucial for categorization of a particular cell line as type-I or type-II

Figure 4

Progress of TRA in clinical trials. Schematic representation of the different TRA for which results of clinical trials have been reported. Cancer entities in which the different TRAs have been tested and the respective phase of clinical testing are shown