TRAIL on trial: preclinical advances in cancer therapy

Abstract

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand, or TRAIL, is a promising anticancer agent as it can induce apoptosis in a wide range of cancers whilst generally sparing non-malignant cells. However, the translation of TRAIL into the clinic has been confounded by its short half-life, inadequate delivery methods, and TRAIL-resistant cancer cell populations. In this review, we discuss how TRAIL has been functionalized to diversify its traditional tumor-killing role and novel strategies to facilitate its effective deployment in preclinical cancer models. The successes and failures of the most recent clinical trials using TRAIL agonists are highlighted and we provide a perspective for improving its clinical implementation.

Keywords: TRAIL; apoptosis; cancer; stem cells; targeted therapy.

Figure 1

Organization of human TRAIL

Figure 2. Apoptosis overview

Apoptosis occurs through two pathways. The extrinsic apoptosis pathway occurs independently of p53 and requires the binding of pro-apoptotic ligands such as TRAIL. TRAIL can bind to four membrane-bound receptors (TRAIL-R1-4) and one soluble receptor (OPG). TRAIL-R1 and TRAIL-R2 contain a cytoplasmic death domain (DD) through which TRAIL can transmit an apoptotic signal. TRAIL-R3 and TRAIL-R4 also bind TRAIL, but because of the absence of a DD they are unable to initiate signaling. For this reason they are called decoy receptors. Binding of TRAIL to TRAIL-R1/2 leads to recruitment of the adaptor FADD and initiator procaspase-8 and 10 to rapidly form the DISC. Procaspase-8 and 10 are cleaved to form caspase-8 and 10 which activate effector caspase-3, 6 and 7 committing the cell to apoptose. At the DISC, variants of a protease-deficient caspase homolog called cFLIP inhibit the activation of caspase-8 and thus the propagation of apoptotic signaling. The intrinsic apoptosis pathway is initiated in response to cell stress such as chemotherapy and radiotherapy. This results in DNA damage causing the p53 oncogene to activate the pro-apoptotic Bcl-2 family proteins BAK and BAX. Pro-apoptotic proteins cytochrome c and Smac/DIABLO are released from mitochondria. Cytochrome c forms a protein complex, the apoptosome, with APAF-1 and activates caspase-9 which in turn activates effector caspase-3, 6 and 7 resulting in apoptosis. Smac/DIABLO inhibits apoptosis-inhibiting proteins such as XIAP, thus amplifying apoptotic signaling. APAF-1, Apoptotic protease activating factor-1; Bcl-2, B cell chronic lymphocytic leukaemia/lymphoma 2; BAK, Bcl-2 homologous antagonist/killer; BAX, Bcl-2-associated protein; FADD, Fas-associated death domain; cFLIP, cellular FLICE- inhibitory protein; DISC, Death-inducing signaling complex; SMAC, second mitochondria-derived activator of caspase; XIAP, X-linked inhibitor of apoptosis protein.

Figure 3. Applications of TRAIL

TRAIL has been modified in a variety of ways to enhance its efficacy and diversify its applications

Figure 4. Mouse neural stem cells engineered to secrete nanobody-TRAIL prolong the life of mice bearing GBM tumors

(A) Schematic representation of lentiviral transfer vectors comprising eGFP, anti-EGFR nanobodies (LV-ENb2) and cytotoxic TRAIL (LV-ENb2-TRAIL). (B) ELISA showing EGFR competition by ENb2-TRAIL. (C) Western blot analysis showing inhibition of EGFR and downstream signaling via the AKT and MAPK pathways on serum-starved Her14 cells incubated with ENb2-TRAIL. (D) Photomicrographs of H&E stained sections and tumor volumes from the brains of GBM-bearing mice treated with NSC-GFP, NSC-ENb2, and NSC-ENb2-TRAIL. (E) Kaplan-Meier survival curves of mice bearing established tumors and implanted with NSC expressing GFP, ENb2 or ENb2-TRAIL intratumorally (n=5 per group). (F) Representative fluorescent images and plot showing the number of GBM8-mCherry invading cells 7 days after NSC-GFP, NSC-ENb2, and NSC-ENb2-TRAIL treatment. Data represented as mean ± SEM, * denotes P< 0.05, Student’s t test. White lines indicate the wall of the lateral ventricle. eGFP, Enhanced green fluorescent protein. Adapted with permission from[70].