A guide to ferroptosis in cancer.

Ferroptosis is a newly identified iron-dependent type of regulated cell death that can also be regarded as death caused by the specific collapse of the lipid antioxidant defence machinery. Ferroptosis has gained increasing attention as a potential therapeutic strategy for therapy-resistant cancer types. However, many ferroptosis-inducing small molecules do not reach the pharmacokinetic requirements for their effective clinical use yet. Nevertheless, their clinical optimization is under development. In this review, we summarize the current understanding of molecular pathways regulating ferroptosis, how cells protect themselves from the induction of ferroptotic cell death, and how a better understanding of cancer cell metabolism can represent vulnerabilities for ferroptosis-based therapies. Lastly, we discuss the context-dependent effect of ferroptosis on various cell types within the tumor microenvironment and address controversies on how tissue ferroptosis might impact systemic cancer immunity in a paracrine manner.

Elevated FSP1 protects KRAS-mutated cells from ferroptosis during tumor initiation.

Oncogenic KRAS is the key driver oncogene for several of the most aggressive human cancers. One key feature of oncogenic KRAS expression is an early increase in cellular reactive oxygen species (ROS) which promotes cellular transformation if cells manage to escape cell death, mechanisms of which remain incompletely understood. Here, we identify that expression of oncogenic as compared to WT KRAS in isogenic cellular systems renders cells more resistant to ferroptosis, a recently described type of regulated necrosis. Mechanistically, we find that cells with mutant KRAS show a specific lack of ferroptosis-induced lipid peroxidation. Interestingly, KRAS-mutant cells upregulate expression of ferroptosis suppressor protein 1 (FSP1). Indeed, elevated levels of FSP1 in KRAS-mutant cells are responsible for mediating ferroptosis resistance and FSP1 is upregulated as a consequence of MAPK and NRF2 pathway activation downstream of KRAS. Strikingly, FSP1 activity promotes cellular transformation in soft agar and its overexpression is sufficient to promote spheroid growth in 3D in KRAS WT cells. Moreover, FSP1 expression and its activity in ferroptosis inhibition accelerates tumor onset of KRAS WT cells in the absence of oncogenic KRAS in vivo. Consequently, we find that pharmacological induction of ferroptosis in pancreatic organoids derived from the LsL-KRASG12D expressing mouse model is only effective in combination with FSP1 inhibition. Lastly, FSP1 is upregulated in non-small cell lung cancer (NSCLC), colorectal cancer (CRC) and pancreatic ductal adenocarcinoma (PDAC) as compared to the respective normal tissue of origin and correlates with NRF2 expression in PDAC patient datasets. Based on these data, we propose that KRAS-mutant cells must navigate a ferroptosis checkpoint by upregulating FSP1 during tumor establishment. Consequently, ferroptosis-inducing therapy should be combined with FSP1 inhibitors for efficient therapy of KRAS-mutant cancers.

Extrinsic cell death pathway plasticity: a driver of clonal evolution in cancer?

Human cancers are known to adhere to basic evolutionary principles. During their journey from early transformation to metastatic disease, cancer cell populations have proven to be remarkably adaptive to different forms of intra- and extracellular selective pressure, including nutrient scarcity, oxidative stress, and anti-cancer immunity. Adaption may be achieved via the expansion of clones bearing driver mutations that optimize cellular fitness in response to the specific selective scenario, e.g., mutations facilitating evasion of cell death, immune evasion or increased proliferation despite growth suppression, all of which constitute well-established hallmarks of cancer. While great progress concerning the prevention, diagnosis and treatment of clinically apparent disease has been made over the last 50 years, the mechanisms underlying cellular adaption under selective pressure via the immune system during early carcinogenesis and its influence on cancer cell fate or disease severity remain to be clarified. For instance, evasion of cell death is generally accepted as a hallmark of cancer, yet recent decades have revealed that the extrinsic cell death machinery triggered by immune effector cells is composed of an astonishingly complex network of interacting-and sometimes compensating-modes of cell death, whose role in selective processes during early carcinogenesis remains obscure. Based upon recent advances in cell death research, here we propose a concept of cell death pathway plasticity in time shaping cancer evolution prior to treatment in an effort to offer new perspectives on how cancer cell fate may be determined by cell death pathway plasticity during early carcinogenesis.

Potent pro-apoptotic combination therapy is highly effective in a broad range of cancers.

Primary or acquired therapy resistance is a major obstacle to the effective treatment of cancer. Resistance to apoptosis has long been thought to contribute to therapy resistance. We show here that recombinant TRAIL and CDK9 inhibition cooperate in killing cells derived from a broad range of cancers, importantly without inducing detectable adverse events. Remarkably, the combination of TRAIL with CDK9 inhibition was also highly effective on cancers resistant to both, standard-of-care chemotherapy and various targeted therapeutic approaches. Dynamic BH3 profiling revealed that, mechanistically, combining TRAIL with CDK9 inhibition induced a drastic increase in the mitochondrial priming of cancer cells. Intriguingly, this increase occurred irrespective of whether the cancer cells were sensitive or resistant to chemo- or targeted therapy. We conclude that this pro-apoptotic combination therapy has the potential to serve as a highly effective new treatment option for a variety of different cancers. Notably, this includes cancers that are resistant to currently available treatment modalities.

Non-neuroendocrine differentiation generates a ferroptosis-prone lipidome in small cell lung cancer (SCLC).

Our recent study revealed that non-neuroendocrine small cell lung cancer (SCLC) is sensitive to the induction of ferroptosis due to upregulation of ether lipid synthesis. While neuroendocrine SCLC is ferroptosis resistant, it acquires addiction to the thioredoxin pathway. Combined redox pathway targeting therefore achieves efficient anti-tumor activity in heterogenous SCLC.

MAPK-pathway inhibition mediates inflammatory reprogramming and sensitizes tumors to targeted activation of innate immunity sensor RIG-I.

Kinase inhibitors suppress the growth of oncogene driven cancer but also enforce the selection of treatment resistant cells that are thought to promote tumor relapse in patients. Here, we report transcriptomic and functional genomics analyses of cells and tumors within their microenvironment across different genotypes that persist during kinase inhibitor treatment. We uncover a conserved, MAPK/IRF1-mediated inflammatory response in tumors that undergo stemness- and senescence-associated reprogramming. In these tumor cells, activation of the innate immunity sensor RIG-I via its agonist IVT4, triggers an interferon and a pro-apoptotic response that synergize with concomitant kinase inhibition. In humanized lung cancer xenografts and a syngeneic Egfr-driven lung cancer model these effects translate into reduction of exhausted CD8+ T cells and robust tumor shrinkage. Overall, the mechanistic understanding of MAPK/IRF1-mediated intratumoral reprogramming may ultimately prolong the efficacy of targeted drugs in genetically defined cancer patients.

TRAIL-receptor 2-a novel negative regulator of p53.

TNF-related apoptosis-inducing ligand (TRAIL) receptor 2 (TRAIL-R2) can induce apoptosis in cancer cells upon crosslinking by TRAIL. However, TRAIL-R2 is highly expressed by many cancers suggesting pro-tumor functions. Indeed, TRAIL/TRAIL-R2 also activate pro-inflammatory pathways enhancing tumor cell invasion, migration, and proliferation. In addition, nuclear TRAIL-R2 (nTRAIL-R2) promotes malignancy by inhibiting miRNA let-7-maturation. Here, we show that TRAIL-R2 interacts with the tumor suppressor protein p53 in the nucleus, assigning a novel pro-tumor function to TRAIL-R2. Knockdown of TRAIL-R2 in p53 wild-type cells increases the half-life of p53 and the expression of its target genes, whereas its re-expression decreases p53 protein levels. Interestingly, TRAIL-R2 also interacts with promyelocytic leukemia protein (PML), a major regulator of p53 stability. PML-nuclear bodies are also the main sites of TRAIL-R2/p53 co-localization. Notably, knockdown or destruction of PML abolishes the TRAIL-R2-mediated regulation of p53 levels. In summary, our finding that nTRAIL-R2 facilitates p53 degradation and thereby negatively regulates p53 target gene expression provides insight into an oncogenic role of TRAIL-R2 in tumorigenesis that particularly manifests in p53 wild-type tumors.

Ferroptosis response segregates small cell lung cancer (SCLC) neuroendocrine subtypes.

Loss of TP53 and RB1 in treatment-naïve small cell lung cancer (SCLC) suggests selective pressure to inactivate cell death pathways prior to therapy. Yet, which of these pathways remain available in treatment-naïve SCLC is unknown. Here, through systemic analysis of cell death pathway availability in treatment-naïve SCLC, we identify non-neuroendocrine (NE) SCLC to be vulnerable to ferroptosis through subtype-specific lipidome remodeling. While NE SCLC is ferroptosis resistant, it acquires selective addiction to the TRX anti-oxidant pathway. In experimental settings of non-NE/NE intratumoral heterogeneity, non-NE or NE populations are selectively depleted by ferroptosis or TRX pathway inhibition, respectively. Preventing subtype plasticity observed under single pathway targeting, combined treatment kills established non-NE and NE tumors in xenografts, genetically engineered mouse models of SCLC and patient-derived cells, and identifies a patient subset with drastically improved overall survival. These findings reveal cell death pathway mining as a means to identify rational combination therapies for SCLC.

Dynasore Blocks Ferroptosis through Combined Modulation of Iron Uptake and Inhibition of Mitochondrial Respiration.

Ferroptosis is a form of regulated necrosis characterized by a chain-reaction of detrimental membrane lipid peroxidation following collapse of glutathione peroxidase 4 (Gpx4) activity. This lipid peroxidation is catalyzed by labile ferric iron. Therefore, iron import mediated via transferrin receptors and both, enzymatic and non-enzymatic iron-dependent radical formation are crucial prerequisites for the execution of ferroptosis. Intriguingly, the dynamin inhibitor dynasore, which has been shown to block transferrin receptor endocytosis, can protect from ischemia/reperfusion injury as well as neuronal cell death following spinal cord injury. Yet, it is unknown how dynasore exerts these cell death-protective effects. Using small interfering RNA suppression, lipid reactive oxygen species (ROS), iron tracers and bona fide inducers of ferroptosis, we find that dynasore treatment in lung adenocarcinoma and neuronal cell lines strongly protects these from ferroptosis. Surprisingly, while the dynasore targets dynamin 1 and 2 promote extracellular iron uptake, their silencing was not sufficient to block ferroptosis suggesting that this route of extracellular iron uptake is dispensable for acute induction of ferroptosis and dynasore must have an additional off-target activity mediating full ferroptosis protection. Instead, in intact cells, dynasore inhibited mitochondrial respiration and thereby mitochondrial ROS production which can feed into detrimental lipid peroxidation and ferroptotic cell death in the presence of labile iron. In addition, in cell free systems, dynasore showed radical scavenger properties and acted as a broadly active antioxidant which is superior to N-acetylcysteine (NAC) in blocking ferroptosis. Thus, dynasore can function as a highly active inhibitor of ROS-driven types of cell death via combined modulation of the iron pool and inhibition of general ROS by simultaneously blocking two routes required for ROS and lipid-ROS driven cell death, respectively. These data have important implications for the interpretation of studies observing tissue-protective effects of this dynamin inhibitor as well as raise awareness that off-target ROS scavenging activities of small molecules used to interrogate the ferroptosis pathway should be taken into consideration.

Friend or Foe: S100 Proteins in Cancer.

S100 proteins are widely expressed small molecular EF-hand calcium-binding proteins of vertebrates, which are involved in numerous cellular processes, such as Ca2+ homeostasis, proliferation, apoptosis, differentiation, and inflammation. Although the complex network of S100 signalling is by far not fully deciphered, several S100 family members could be linked to a variety of diseases, such as inflammatory disorders, neurological diseases, and also cancer. The research of the past decades revealed that S100 proteins play a crucial role in the development and progression of many cancer types, such as breast cancer, lung cancer, and melanoma. Hence, S100 family members have also been shown to be promising diagnostic markers and possible novel targets for therapy. However, the current knowledge of S100 proteins is limited and more attention to this unique group of proteins is needed. Therefore, this review article summarises S100 proteins and their relation in different cancer types, while also providing an overview of novel therapeutic strategies for targeting S100 proteins for cancer treatment.

An unexpected turn of fortune: targeting TRAIL-Rs in KRAS-driven cancer.

Twenty-one percent of all human cancers bear constitutively activating mutations in the proto-oncogene KRAS. This incidence is substantially higher in some of the most inherently therapy-resistant cancers including 30% of non-small cell lung cancers (NSCLC), 50% of colorectal cancers, and 95% of pancreatic ductal adenocarcinomas (PDAC). Importantly, survival of patients with KRAS-mutated PDAC and NSCLC has not significantly improved since the 1970s highlighting an urgent need to re-examine how oncogenic KRAS influences cell death signaling outputs. Interestingly, cancers expressing oncogenic KRAS manage to escape antitumor immunity via upregulation of programmed cell death 1 ligand 1 (PD-L1). Recently, the development of next-generation KRASG12C-selective inhibitors has shown therapeutic efficacy by triggering antitumor immunity. Yet, clinical trials testing immune checkpoint blockade in KRAS-mutated cancers have yielded disappointing results suggesting other, additional means endow these tumors with the capacity to escape immune recognition. Intriguingly, oncogenic KRAS reprograms regulated cell death pathways triggered by death receptors of the tumor necrosis factor (TNF) receptor superfamily. Perverting the course of their intended function, KRAS-mutated cancers use endogenous TNF-related apoptosis-inducing ligand (TRAIL) and its receptor(s) to promote tumor growth and metastases. Yet, endogenous TRAIL-TRAIL-receptor signaling can be therapeutically targeted and, excitingly, this may not only counteract oncogenic KRAS-driven cancer cell migration, invasion, and metastasis, but also the immunosuppressive reprogramming of the tumor microenvironment it causes. Here, we provide a concise summary of the current literature on oncogenic KRAS-mediated reprogramming of cell death signaling and antitumor immunity with the aim to open novel perspectives on combinatorial treatment strategies involving death receptor targeting.

Ferroptosis in Cancer Cell Biology.

A major hallmark of cancer is successful evasion of regulated forms of cell death. Ferroptosis is a recently discovered type of regulated necrosis which, unlike apoptosis or necroptosis, is independent of caspase activity and receptor-interacting protein 1 (RIPK1) kinase activity. Instead, ferroptotic cells die following iron-dependent lipid peroxidation, a process which is antagonised by glutathione peroxidase 4 (GPX4) and ferroptosis suppressor protein 1 (FSP1). Importantly, tumour cells escaping other forms of cell death have been suggested to maintain or acquire sensitivity to ferroptosis. Therefore, therapeutic exploitation of ferroptosis in cancer has received increasing attention. Here, we systematically review current literature on ferroptosis signalling, cross-signalling to cellular metabolism in cancer and a potential role for ferroptosis in tumour suppression and tumour immunology. By summarising current findings on cell biology relevant to ferroptosis in cancer, we aim to point out new conceptual avenues for utilising ferroptosis in systemic treatment approaches for cancer.

TRAIL Induces Nuclear Translocation and Chromatin Localization of TRAIL Death Receptors.

Binding of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) to the plasma membrane TRAIL-R1/-R2 selectively kills tumor cells. This discovery led to evaluation of TRAIL-R1/-R2 as targets for anti-cancer therapy, yet the corresponding clinical trials were disappointing. Meanwhile, it emerged that many cancer cells are TRAIL-resistant and that TRAIL-R1/-R2-triggering may lead to tumor-promoting effects. Intriguingly, recent studies uncovered specific functions of long ignored intracellular TRAIL-R1/-R2, with tumor-promoting functions of nuclear (n)TRAIL-R2 as the regulator of let-7-maturation. As nuclear trafficking of TRAIL-Rs is not well understood, we addressed this issue in our present study. Cell surface biotinylation and tracking of biotinylated proteins in intracellular compartments revealed that nTRAIL-Rs originate from the plasma membrane. Nuclear TRAIL-Rs-trafficking is a fast process, requiring clathrin-dependent endocytosis and it is TRAIL-dependent. Immunoprecipitation and immunofluorescence approaches revealed an interaction of nTRAIL-R2 with the nucleo-cytoplasmic shuttle protein Exportin-1/CRM-1. Mutation of a putative nuclear export sequence (NES) in TRAIL-R2 or the inhibition of CRM-1 by Leptomycin-B resulted in the nuclear accumulation of TRAIL-R2. In addition, TRAIL-R1 and TRAIL-R2 constitutively localize to chromatin, which is strongly enhanced by TRAIL-treatment. Our data highlight the novel role for surface-activated TRAIL-Rs by direct trafficking and signaling into the nucleus, a previously unknown signaling principle for cell surface receptors that belong to the TNF-superfamily.

Heterogeneous intracellular TRAIL-receptor distribution predicts poor outcome in breast cancer patients.

Upon ligand binding, plasma membrane-located TNF-related apoptosis-inducing ligand (TRAIL)-receptors 1 and 2 induce apoptosis as well as cancer-promoting signaling in cancer cells. TRAIL-R3 and TRAIL-R4 are believed to negatively regulate TRAIL-mediated apoptosis. Intracellular localization of TRAIL-receptors, as observed in many tumor cells, has been associated with oncogenic features, which are distinct from membrane-associated TRAIL-R signaling. Here, analyzing a panel of 354 breast cancer specimens, we found that an unfavorable outcome correlating with cancer-promoting properties of TRAIL-R1, TRAIL-R2, and TRAIL-R4 was most significantly defined by their intracellular distribution and mutual co-expression. A nuclear or cytoplasmic heterogeneous expression pattern correlated with markedly decreased overall survival and discriminated high-risk breast cancer patients from low-risk patients with a homogeneous distribution of expression, i.e., nuclear and cytoplasmic expression. The homogeneous TRAIL-R expression was associated with favorable breast cancer surrogate markers corresponding with excellent survival prognoses at 5 years after diagnosis (hazard ratio, 0.043) and over the complete course of follow-up (hazard ratio, 0.098; both p < 0.001). No associations with specific intrinsic breast cancer subtypes were found. Our data suggest that the determination of intracellular co-expression patterns of TRAIL-R1, TRAIL-R2, and TRAIL-R4 provides an innovative and robust method for risk stratification in breast cancer patients beyond conventional prognostic markers. KEY MESSAGES: A total of 70% of breast cancer specimens show comparably high levels of intracellular TRAIL-Rs. Nuclear or cytoplasmic TRAIL-R co-expression occurs in the majority of tumors. A total of 25% of tumors show a heterogeneous expression of cytoplasmic or nuclear TRAIL-Rs. Patients with a heterogeneous TRAIL-R expression present with poor prognoses. Additive TRAIL-R-based risk stratification comprises different breast cancer subtypes.

Cystine/glutamate antiporter xCT (SLC7A11) facilitates oncogenic RAS transformation by preserving intracellular redox balance.

The RAS family of proto-oncogenes are among the most commonly mutated genes in human cancers and predict poor clinical outcome. Several mechanisms underlying oncogenic RAS transformation are well documented, including constitutive signaling through the RAF-MEK-ERK proproliferative pathway as well as the PI3K-AKT prosurvival pathway. Notably, control of redox balance has also been proposed to contribute to RAS transformation. However, how homeostasis between reactive oxygen species (ROS) and antioxidants, which have opposing effects in the cell, ultimately influence RAS-mediated transformation and tumor progression is still a matter of debate and the mechanisms involved have not been fully elucidated. Here, we show that oncogenic KRAS protects fibroblasts from oxidative stress by enhancing intracellular GSH levels. Using a whole transcriptome approach, we discovered that this is attributable to transcriptional up-regulation of xCT, the gene encoding the cystine/glutamate antiporter. This is in line with the function of xCT, which mediates the uptake of cystine, a precursor for GSH biosynthesis. Moreover, our results reveal that the ETS-1 transcription factor downstream of the RAS-RAF-MEK-ERK signaling cascade directly transactivates the xCT promoter in synergy with the ATF4 endoplasmic reticulum stress-associated transcription factor. Strikingly, xCT was found to be essential for oncogenic KRAS-mediated transformation in vitro and in vivo by mitigating oxidative stress, as knockdown of xCT strongly impaired growth of tumor xenografts established from KRAS-transformed cells. Overall, this study uncovers a mechanism by which oncogenic RAS preserves intracellular redox balance and identifies an unexpected role for xCT in supporting RAS-induced transformation and tumorigenicity.

TRAILblazing Strategies for Cancer Treatment.

In the late 1990s, tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), a member of the TNF-family, started receiving much attention for its potential in cancer therapy, due to its capacity to induce apoptosis selectively in tumour cells in vivo. TRAIL binds to its membrane-bound death receptors TRAIL-R1 (DR4) and TRAIL-R2 (DR5) inducing the formation of a death-inducing signalling complex (DISC) thereby activating the apoptotic cascade. The ability of TRAIL to also induce apoptosis independently of p53 makes TRAIL a promising anticancer agent, especially in p53-mutated tumour entities. Thus, several so-called TRAIL receptor agonists (TRAs) were developed. Unfortunately, clinical testing of these TRAs did not reveal any significant anticancer activity, presumably due to inherent or acquired TRAIL resistance of most primary tumour cells. Since the potential power of TRAIL-based therapies still lies in TRAIL's explicit cancer cell-selectivity, a desirable approach going forward for TRAIL-based cancer therapy is the identification of substances that sensitise tumour cells for TRAIL-induced apoptosis while sparing normal cells. Numerous of such TRAIL-sensitising strategies have been identified within the last decades. However, many of these approaches have not been verified in animal models, and therefore potential toxicity of these approaches has not been taken into consideration. Here, we critically summarise and discuss the status quo of TRAIL signalling in cancer cells and strategies to force tumour cells into undergoing apoptosis triggered by TRAIL as a cancer therapeutic approach. Moreover, we provide an overview and outlook on innovative and promising future TRAIL-based therapeutic strategies.

The Emerging Role of Cyclin-Dependent Kinases (CDKs) in Pancreatic Ductal Adenocarcinoma.

The family of cyclin-dependent kinases (CDKs) has critical functions in cell cycle regulation and controlling of transcriptional elongation. Moreover, dysregulated CDKs have been linked to cancer initiation and progression. Pharmacological CDK inhibition has recently emerged as a novel and promising approach in cancer therapy. This idea is of particular interest to combat pancreatic ductal adenocarcinoma (PDAC), a cancer entity with a dismal prognosis which is owed mainly to PDAC's resistance to conventional therapies. Here, we review the current knowledge of CDK biology, its role in cancer and the therapeutic potential to target CDKs as a novel treatment strategy for PDAC.

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

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.

Exploring the TRAILs less travelled: TRAIL in cancer biology and therapy.

The discovery that the tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) can induce apoptosis of cancer cells without causing toxicity in mice has led to the in-depth study of pro-apoptotic TRAIL receptor (TRAIL-R) signalling and the development of biotherapeutic drug candidates that activate TRAIL-Rs. The outcome of clinical trials with these TRAIL-R agonists has, however, been disappointing so far. Recent evidence indicates that many cancers, in addition to being TRAIL resistant, use the endogenous TRAIL-TRAIL-R system to their own advantage. However, novel insight on two fronts - how resistance of cancer cells to TRAIL-based pro-apoptotic therapies might be overcome, and how the pro-tumorigenic effects of endogenous TRAIL might be countered - gives reasonable hope that the TRAIL system can be harnessed to treat cancer. In this Review we assess the status quo of our understanding of the biology of the TRAIL-TRAIL-R system - as well as the gaps therein - and discuss the opportunities and challenges in effectively targeting this pathway.