Should We Keep Walking along the Trail for Pancreatic Cancer Treatment? Revisiting TNF-Related Apoptosis-Inducing Ligand for Anticancer Therapy

Abstract

Despite recent advances in oncology, diagnosis, and therapy, treatment of pancreatic ductal adenocarcinoma (PDAC) is still exceedingly challenging. PDAC remains the fourth leading cause of cancer-related deaths worldwide. Poor prognosis is due to the aggressive growth behavior with early invasion and distant metastasis, chemoresistance, and a current lack of adequate screening methods for early detection. Consequently, novel therapeutic approaches are urgently needed. Many hopes for cancer treatment have been placed in the death ligand tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) since it was reported to induce apoptosis selectively in tumor cells in vitro and in vivo. TRAIL triggers apoptosis through binding of the trans-membrane death receptors TRAIL receptor 1 (TRAIL-R1) also death receptor 4 (DR4) and TRAIL receptor 2 (TRAIL-R2) also death receptor 5 (DR5) thereby inducing the formation of the death-inducing signaling complex (DISC) and activation of the apoptotic cascade. Unlike chemotherapeutics, TRAIL was shown to be able to induce apoptosis in a p53-independent manner, making TRAIL a promising anticancer approach for p53-mutated tumors. These cancer-selective traits of TRAIL led to the development of TRAIL-R agonists, categorized into either recombinant variants of TRAIL or agonistic antibodies against TRAIL-R1 or TRAIL-R2. However, clinical trials making use of these agonists in various tumor entities including pancreatic cancer were disappointing so far. This is thought to be caused by TRAIL resistance of numerous primary tumor cells, an insufficient agonistic activity of the drug candidates tested, and a lack of suitable biomarkers for patient stratification. Nevertheless, recently gained knowledge on the biology of the TRAIL-TRAIL-R system might now provide the chance to overcome intrinsic or acquired resistance against TRAIL and TRAIL-R agonists. In this review, we summarize the status quo of clinical studies involving TRAIL-R agonists for the treatment of pancreatic cancer and critically discuss the suitability of utilizing the TRAIL-TRAIL-R system for successful treatment.

Keywords: TRAIL; TRAIL-R agonists; pancreatic adenocarcinoma.

Figure 1

The human and murine tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-TRAIL-receptor (TRAIL-R) system. TRAIL-TRAIL-R-system in humans. Human tumor-necrosis-factor-related apoptosis-inducing ligand (TRAIL) can engage four membrane-bound TRAIL-Rs and one soluble receptor, while two of these receptors can promote apoptosis (TRAIL-R1 and TRAIL-R2) and three might serve as decoys. TRAIL-R1 and TRAIL-2 express an intracellular domain comprising a death domain (DD), required for apoptosis induction. TRAIL-R2 occurs as short or long isoform, distinguishable by the absence or presence of a single TAPE (threonine/alanine/proline/glutamine) domain. TRAIL-R3 is attached to the membrane via a glycosylphosphatidylinositol (GPI) anchor and expresses five TAPE domains, but no intracellular domain. TRAIL-R4 merely expresses a truncated DD incapable of transmitting an apoptotic signal. Osteoprotegerin (OPG) represents the soluble receptor with low affinity for TRAIL. Cysteine-rich domains of the receptors are crucial for ligand binding. TRAIL-TRAIL-R-system in mice. Mice express four receptors for TRAIL, while mTRAIL-R is homologous to human TRAIL-R1 and TRAIL-R2. mDcTRAIL-R1 and mDc-TRAIL-R2 differ from the human forms. mOPG is the soluble receptor for TRAIL in the murine system. MPD, membrane-proximal domain.

Figure 2

Apoptotic signaling. (a) Tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-induced extrinsic apoptosis. Upon TRAIL binding to TRAIL receptor 1 (TRAIL-R1) and/or TRAIL receptor 2 (TRAIL-R2) the death-inducing signaling complex (DISC) is formed. In type I cells, the signal of DISC-activated caspase-8 is sufficient to activate downstream effector caspase-3 and therefore apoptosis, whereas the DISC signal needs amplification by mitochondrial apoptosis via caspase-8-dependent cleavage of the B cell lymphoma 2 (Bcl-2) homology domain 3 (BH3) interacting domain death agonist (Bid) in type II cells. Truncated Bid (tBid) translocates to the mitochondria and triggers the Bcl-2-family members Bcl-2-associated X protein (Bax) and Bcl-2 homologous antagonist killer (Bak) in the mitochondrial outer membrane (MOM), resulting in its permeabilization (MOMP) and eventually cytochrome c and second mitochondrial activator of caspases/direct inhibitor of apoptosis-binding protein with low pI (Smac/DIABLO) release. Apoptotic protease activating factor-1 (Apaf-1) complexes with cytochrome c and caspase-9 to build the apoptosome promoting effector caspases-3, -6 and -7. (b) Intrinsic death signaling. In response to stress signals, p53 becomes activated thereby triggering BH3-only proteins resulting in MOMP. In type I cells, DISC formation is sufficient to activate caspase-8 for activation of the executioner caspase-3. In type II cells, DISC formation is enhanced by MOMP to neutralize the caspase-inhibitory protein X-linked inhibitor of apoptosis protein (XIAP). XIAP binds to and potently inhibits caspase-3, -7 and -9. Several apoptosis regulation mechanisms are known: Fas-associated death domain (FADD)-like IL-1β-converting enzyme (FLICE)-inhibitory protein (c-FLIP) and caspase-8 compete for binding to FADD. Thus caspase-8 activation is repressed. In type II cells, binding of Bcl-2, B-cell lymphoma-extra-large (Bcl-XL) or induced myeloid leukemia cell differentiation protein (Mcl-1) to Bax and Bak, converts them into their inactive forms. Smac/DIABLO binds to and antagonizes XIAP and thereby ensures potent activation of caspases-3, -7 and -9.

Figure 3

TRAIL-mediated non-apoptotic signaling. TRAIL binding induces assembly of a second cytosolic complex, retaining TNF superfamily receptor 6 (Fas)-associated death domain (FADD) and caspase-8 and recruiting receptor-interacting protein kinase 1 (RIPK1), tumor necrosis factor (TNF) receptor-associated factor 2 (TRAF2), and nuclear factor kappa light chain enhancer of activated B cells (NF-κB) essential modifier (NEMO). Downstream of caspase-8, TRAF2 recruits cellular inhibitor of apoptosis protein 1/2 (cIAP1/2) which in turn leads to the ubiquitination of RIPK1 and therefore recruitment linear ubiquitin chain assembly complex (LUBAC), that attaches linear poly-ubiquitin chains on RIPK1 [167]. RIPK1 is compulsory for the stimulation of tyrosine-protein kinase Rous sarcoma oncogene cellular homolog (SRC) and signal transducer and activator of transcription 3 (STAT3) which are responsible for pushing migration and invasion. Complex I and complex II trigger NF-κB, p38 mitogen-activated protein kinase (p38 MAPK), JUN N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK). LUBAC acts in both complexes, alleviating caspase-8 activation and enabling recruitment of the Inhibitor of κB (IκB) kinase (IKK) complex, and consequently activation of NF-κB. In case of inhibited caspase activation, the necrosome is built by the interaction of RIPK1 and RIPK3. Independently of FADD and complex I and II, the membrane-proximal domain (MPD) of TRAIL receptor 2 (TRAIL-R2) induces Rat sarcoma (Ras)-related C3 botulinum toxin substrate 1 (Rac1) activation to promote migration and invasion. TRAIL-R2 is also present in the nucleus and interacts with ribonucleoprotein complexes attributed to the maturation of microRNAs (miRNAs) of the let-7 family which interact with and inhibit mRNAs of various regulators of mitogenic pathways such as Ras and avian myelocytomatosis virus oncogene cellular homolog (c-Myc) thereby promoting proliferation. DD: death domain; DED: death effector domain; ER: endoplasmic reticulum; GTP: guanosine-5′-triphosphate; Ub: ubiquitin; DGCR8: DiGeorge critical region 8; P: phosphorylated.