The role of Ubiquitination in Apoptosis and Necroptosis

Cell death pathways have evolved to maintain tissue homoeostasis and eliminate potentially harmful cells from within an organism, such as cells with damaged DNA that could lead to cancer. Apoptosis, known to eliminate cells in a predominantly non-inflammatory manner, is controlled by two main branches, the intrinsic and extrinsic apoptotic pathways. While the intrinsic pathway is regulated by the Bcl-2 family members, the extrinsic pathway is controlled by the Death receptors, members of the tumour necrosis factor (TNF) receptor superfamily. Death receptors can also activate a pro-inflammatory type of cell death, necroptosis, when Caspase-8 is inhibited. Apoptotic pathways are known to be tightly regulated by post-translational modifications, especially by ubiquitination. This review discusses research on ubiquitination-mediated regulation of apoptotic signalling. Additionally, the emerging importance of ubiquitination in regulating necroptosis is discussed.

RALB GTPase: a critical regulator of DR5 expression and TRAIL sensitivity in KRAS mutant colorectal cancer

RAS mutant (MT) metastatic colorectal cancer (mCRC) is resistant to MEK1/2 inhibition and remains a difficult-to-treat group. Therefore, there is an unmet need for novel treatment options for RASMT mCRC. RALA and RALB GTPases function downstream of RAS and have been found to be key regulators of several cell functions implicated in KRAS-driven tumorigenesis. However, their role as regulators of the apoptotic machinery remains to be elucidated. Here, we found that inhibition of RALB expression, but not RALA, resulted in Caspase-8-dependent cell death in KRASMT CRC cells, which was not further increased following MEK1/2 inhibition. Proteomic analysis and mechanistic studies revealed that RALB depletion induced a marked upregulation of the pro-apoptotic cell surface TRAIL Death Receptor 5 (DR5) (also known as TRAIL-R2), primarily through modulating DR5 protein lysosomal degradation. Moreover, DR5 knockdown or knockout attenuated siRALB-induced apoptosis, confirming the role of the extrinsic apoptotic pathway as a regulator of siRALB-induced cell death. Importantly, TRAIL treatment resulted in the association of RALB with the death-inducing signalling complex (DISC) and targeting RALB using pharmacologic inhibition or RNAi approaches triggered a potent increase in TRAIL-induced cell death in KRASMT CRC cells. Significantly, high RALB mRNA levels were found in the poor prognostic Colorectal Cancer Intrinsic Subtypes (CRIS)-B CRC subgroup. Collectively, this study provides to our knowledge the first evidence for a role for RALB in apoptotic priming and suggests that RALB inhibition may be a promising strategy to improve response to TRAIL treatment in poor prognostic RASMT CRIS-B CRC.

Abstract 5220: Development of first-in-class small molecule inhibitors of FLIP which activate caspase-8, the nodal regulator of apoptosis, necroptosis and pyroptosis

Caspase-8 is the initiator caspase for the extrinsic apoptotic pathway and now recognized as the molecular “switch” that controls 3 major forms of programmed cell death: apoptosis, necroptosis and pyroptosis. As such, methods to selectively activate caspase-8 in appropriate disease contexts represent an exciting new therapeutic paradigm. In cancer, evasion of cell death is a fundamental cause of resistance to therapy, prompting the development of therapeutics that reactivate cell death pathways, with the first of these (targeting the intrinsic apoptotic pathway) recently clinically approved. The apoptosis modulator FLIP is a non-redundant inhibitor of caspase-8 activation in the extrinsic apoptotic pathway and is the only human pseudo-caspase. FLIP is frequently overexpressed in a number of cancers and leukemias and, through its modulation of caspase-8, has been shown to be a major mediator of resistance to standard-of-care chemotherapies, targeted therapies and radiotherapy. Moreover, by regulating caspase-8 activity, FLIP is a key determinant of cell death induced by immune effector cells. Thus, targeting FLIP's interaction with caspase-8 represents a unique therapeutic approach for enhancing standard-of-care anti-cancer therapies and promoting anti-tumor immunity.

We have identified a series of potent (nM), selective small molecule inhibitors which, by disrupting the binding of FLIP to effector proteins, are able to activate caspase-8-dependent apoptosis in multiple cancer cell lines. These protein-protein interaction inhibitors, which are relatively low MW and have favorable drug-like properties, have potent in vitro activity as single agents in several clinically-relevant settings, including KRAS mutant non-small cell lung cancer (NSCLC). Moreover, these FLIP inhibitors have been shown to potentiate the effects of standard-of-care chemotherapeutics and targeted anti-cancer agents in pre-clinical models. Recently, FLIP has emerged as a key mediator of resistance to the 3rd generation EGFR inhibitor Osimertinib in EGFR mutant NSCLC. We found that Osimertinib-resistant EGFR mutant NSCLC cells express elevated levels of FLIP and are more sensitive to FLIP inhibitors. Most importantly, compared to treatment with Osimertinib alone, co-treatment with Osimertinib and FLIP inhibitors prevented tumor regrowth in vivo after treatment cessation. Similar effects have been observed in vitro with the MEK inhibitor Selumetinib in KRAS mutant NSCLC models.

In summary, we have identified novel, drug-like activators of caspase-8 with a unique mechanism-of-action and successfully completed proof-of-concept in vivo efficacy studies which have strong translational relevance to the clinical situation in EGFR mutant NSCLC, where despite excellent response rates to Osimertinib, patient relapse is inevitable. Moreover, these compounds have potential for broad application in treatment of several cancers both as single agents and in combination with standard-of-care therapy and other clinically-relevant targeted agents.

Citation Format: Daniel B. Longley, Catherine Higgins, Jennifer Fox, Jamie Z. Roberts, Ray Boffey, Sophie Williams, Trevor Perrior, Martin J. Page, Tim Harrison. Development of first-in-class small molecule inhibitors of FLIP which activate caspase-8, the nodal regulator of apoptosis, necroptosis and pyroptosis [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5220.

Abstract 2409: FLIP(L) determines colon cancer cell fate following p53 activation

Background: How p53 differentially activates cell cycle arrest versus cell death remains poorly understood. Understanding the mechanisms governing the switch from p53-induced cell-fates is important for optimising efficacy of p53-activating therapies, such as DNA damaging chemotherapy and radiotherapy. HDAC inhibitors (HDACi) have potential to enhance p53 induced cell death through enhancing p53 activation and altering regulation of other cell death regulatory proteins.

Methods: We utilised a panel of matched p53 wild-type and deficient colorectal cancer cell line models to the potential for HDACi to augment cell death induced by direct and indirect p53 activating agents. A number of molecular (Western blot, RT-PCR), phenotypic (cell death) and functional genomic (RNA-seq, CRISPR, ChIP-seq) analyses were used to investigate the importance of p53 and its downstream transcriptional programs.

Results: Here we report that upregulation of canonical pro-apoptotic p53 target genes in colon cancer cells imposes critical dependence on the long splice form of the caspase-8 regulator FLIP (FLIP(L)), which we identify as a direct p53 transcriptional target. Inhibiting FLIP(L) expression with siRNA or Class-I HDAC inhibitors promotes apoptosis in response to p53 activation by the MDM2 inhibitor Nutlin-3A, which otherwise predominantly induces cell-cycle arrest. When FLIP(L) upregulation is inhibited, apoptosis is induced in response to p53 activation via a ligand-independent TRAIL-R2/caspase-8 complex, which is distinct from the ligand-dependent DISC. Notably, FLIP(L) depletion inhibits p53-induced expression of the cell cycle regulator p21 and enhances p53-mediated upregulation of PUMA, with the latter activating mitochondrial-mediated apoptosis in FLIP(L)-depleted, Nutlin-3A-treated cells lacking TRAIL-R2/caspase-8.

Conclusion: Acute p53-mediated transcriptional upregulation of FLIP(L) plays an unexpected nodal role in determining cell fate following p53 activation. This is mediated through two previously undescribed mechanisms, preventing apoptosis by a ligand-independent TRAIL-R2 complex and by suppressing expression of pro-apoptotic PUMA. Which, importantly imposes a critical dependence on FLIP(L) which can be overcome through combinations with class-I HDAC inhibitors such as Entinostat.

Citation Format: Andrea Lees, Alexander J. McIntyre J. McIntyre, Nyree T. Crawford, Fiammetta Falcone, Chris McCann, Gerard P. Quinn, Jamie Z. Roberts, Thomas Sessler, Peter F. Gallagher, Gemma M. Gregg, Katherine McAllister, Kirsty M. McLaughlin, Wendy L. Allen, Caitriona Holohan, Laurence J. Egan, Aideen E. Ryan, Melissa Labonte-Wilson, Phillip D. Dunne, Mark Wappett, Vicky M. Coye, Patrick G. Johnston, Emma M. Kerr, Daniel B. Longley, Simon S. McDade. FLIP(L) determines colon cancer cell fate following p53 activation [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2409.

The pseudo-caspase FLIP(L) regulates cell fate following p53 activation

p53 is the most frequently mutated, well-studied tumor-suppressor gene, yet the molecular basis of the switch from p53-induced cell-cycle arrest to apoptosis remains poorly understood. Using a combination of transcriptomics and functional genomics, we unexpectedly identified a nodal role for the caspase-8 paralog and only human pseudo-caspase, FLIP(L), in regulating this switch. Moreover, we identify FLIP(L) as a direct p53 transcriptional target gene that is rapidly up-regulated in response to Nutlin-3A, an MDM2 inhibitor that potently activates p53. Genetically or pharmacologically inhibiting expression of FLIP(L) using siRNA or entinostat (a clinically relevant class-I HDAC inhibitor) efficiently promoted apoptosis in colorectal cancer cells in response to Nutlin-3A, which otherwise predominantly induced cell-cycle arrest. Enhanced apoptosis was also observed when entinostat was combined with clinically relevant, p53-activating chemotherapy in vitro, and this translated into enhanced in vivo efficacy. Mechanistically, FLIP(L) inhibited p53-induced apoptosis by blocking activation of caspase-8 by the TRAIL-R2/DR5 death receptor; notably, this activation was not dependent on receptor engagement by its ligand, TRAIL. In the absence of caspase-8, another of its paralogs, caspase-10 (also transcriptionally up-regulated by p53), induced apoptosis in Nutlin-3A-treated, FLIP(L)-depleted cells, albeit to a lesser extent than in caspase-8-proficient cells. FLIP(L) depletion also modulated transcription of canonical p53 target genes, suppressing p53-induced expression of the cell-cycle regulator p21 and enhancing p53-induced up-regulation of proapoptotic PUMA. Thus, even in the absence of caspase-8/10, FLIP(L) silencing promoted p53-induced apoptosis by enhancing PUMA expression. Thus, we report unexpected, therapeutically relevant roles for FLIP(L) in determining cell fate following p53 activation.

Pevonedistat (MLN4924): mechanism of cell death induction and therapeutic potential in colorectal cancer

Pevonedistat (MLN4924), a selective inhibitor of the NEDD8-activating enzyme E1 regulatory subunit (NAE1), has demonstrated significant therapeutic potential in several malignancies. Although multiple mechanisms-of-action have been identified, how MLN4924 induces cell death and its potential as a combinatorial agent with standard-of-care (SoC) chemotherapy in colorectal cancer (CRC) remains largely undefined. In an effort to understand MLN4924-induced cell death in CRC, we identified p53 as an important mediator of the apoptotic response to MLN4924. We also identified roles for the extrinsic (TRAIL-R2/caspase-8) and intrinsic (BAX/BAK) apoptotic pathways in mediating the apoptotic effects of MLN4924 in CRC cells, as well as a role for BID, which modulates a cross-talk between these pathways. Depletion of the anti-apoptotic protein FLIP, which we identify as a novel mediator of resistance to MLN4924, enhanced apoptosis in a p53-, TRAIL-R2/DR5-, and caspase-8-dependent manner. Notably, TRAIL-R2 was involved in potentiating the apoptotic response to MLN4924 in the absence of FLIP, in a ligand-independent manner. Moreoever, when paired with SoC chemotherapies, MLN4924 demonstrated synergy with the irinotecan metabolite SN38. The cell death induced by MLN4924/SN38 combination was dependent on activation of mitochondria through BAX/BAK, but in a p53-independent manner, an important observation given the high frequency of TP53 mutation(s) in advanced CRC. These results uncover mechanisms of cell death induced by MLN4924 and suggest that this second-generation proteostasis-disrupting agent may have its most widespread activity in CRC, in combination with irinotecan-containing treatment regimens.

The SCFSkp2 ubiquitin ligase complex modulates TRAIL-R2-induced apoptosis by regulating FLIP(L)

TRAIL-R2 (DR5) is a clinically-relevant therapeutic target and a key target for immune effector cells. Herein, we identify a novel interaction between TRAIL-R2 and the Skp1-Cullin-1-F-box (SCF) Cullin-Ring E3 Ubiquitin Ligase complex containing Skp2 (SCF^Skp2). We find that SCF^Skp2 can interact with both TRAIL-R2’s pre-ligand association complex (PLAC) and ligand-activated death-inducing signalling complex (DISC). Moreover, Cullin-1 interacts with TRAIL-R2 in its active NEDDylated form. Inhibiting Cullin-1’s DISC recruitment using the NEDDylation inhibitor MLN4924 (Pevonedistat) or siRNA increased apoptosis induction in response to TRAIL. This correlated with enhanced levels of the caspase-8 regulator FLIP at the TRAIL-R2 DISC, particularly the long splice form, FLIP(L). We subsequently found that FLIP(L) (but not FLIP(S), caspase-8, nor the other core DISC component FADD) interacts with Cullin-1 and Skp2. Importantly, this interaction is enhanced when FLIP(L) is in its DISC-associated, C-terminally truncated p43-form. Prevention of FLIP(L) processing to its p43-form stabilises the protein, suggesting that by enhancing its interaction with SCF^Skp2, cleavage to the p43-form is a critical step in FLIP(L) turnover. In support of this, we found that silencing any of the components of the SCF^Skp2 complex inhibits FLIP ubiquitination, while overexpressing Cullin-1/Skp2 enhances its ubiquitination in a NEDDylation-dependent manner. DISC recruitment of TRAF2, previously identified as an E3 ligase for caspase-8 at the DISC, was also enhanced when Cullin-1’s recruitment was inhibited, although its interaction with Cullin-1 was found to be mediated indirectly via FLIP(L). Notably, the interaction of p43-FLIP(L) with Cullin-1 disrupts its ability to interact with FADD, caspase-8 and TRAF2. Collectively, our results suggest that processing of FLIP(L) to p43-FLIP(L) at the TRAIL-R2 DISC enhances its interaction with co-localised SCF^Skp2, leading to disruption of p43-FLIP(L)’s interactions with other DISC components and promoting its ubiquitination and degradation, thereby modulating TRAIL-R2-mediated apoptosis.

A revised model of TRAIL-R2 DISC assembly explains how FLIP(L) can inhibit or promote apoptosis

The long FLIP splice form FLIP(L) can act as both an inhibitor and promoter of caspase‐8 at death‐inducing signalling complexes (DISCs) formed by death receptors such as TRAIL‐R2 and related intracellular complexes such as the ripoptosome. Herein, we describe a revised DISC assembly model that explains how FLIP(L) can have these opposite effects by defining the stoichiometry (with respect to caspase‐8) at which it converts from being anti‐ to pro‐apoptotic at the DISC. We also show that in the complete absence of FLIP(L), procaspase‐8 activation at the TRAIL‐R2 DISC has significantly slower kinetics, although ultimately the extent of apoptosis is significantly greater. This revised model of DISC assembly also explains why FLIP's recruitment to the TRAIL‐R2 DISC is impaired in the absence of caspase‐8 despite showing that it can interact with the DISC adaptor protein FADD and why the short FLIP splice form FLIP(S) is the more potent inhibitor of DISC‐mediated apoptosis.

Abstract 704: RALB GTPase: A critical regulator of DR5 cell surface expression and TRAIL sensitivity in RAS mutant colorectal cancer

RAS mutant (MT) metastatic colorectal cancer (mCRC) remains a difficult-to-treat group. RALA and RALB GTPases function downstream of RAS and have been found to be key regulators of several cell functions and implicated in KRAS-driven tumourigenesis. However, their role as regulators of the apoptotic machinery remains to be elucidated. Here, we found that inhibition of RALB expression, but not RALA, resulted in caspase 8-dependent cell death in KRASMT cells, that was not further increased following MEK1/2 inhibition. Proteomics analysis and mechanistic studies revealed that RALB inhibition induces marked upregulation of the pro-apoptotic cell-surface TRAIL Death Receptor 5 (DR5) receptor in KRASMT CRC. Moreover, DR5 knockout also attenuated siRALB-induced apoptosis, confirming the role of the extrinsic apoptotic pathway as regulator of siRALB-induced cell death. Importantly, TRAIL treatment resulted in acute association of RALB with the death-inducing signalling complex (DISC) and targeting RALB using pharmacologic inhibition or RNAi approaches, resulted in a potent increase in TRAIL-induced cell death in RASMT CRC. Significantly, high RALB mRNA levels were found in the poor prognostic CRIS-B CRC subgroup. Collectively, this study provides the first evidence for a role of RALB in apoptotic priming and that RALB inhibition may be a promising strategy to improve response to TRAIL treatment in poor prognostic RASMT CRC.

Citation Format: Hajrah Khawaja, Jamie Z. Roberts, Arman Javadi, Paul O'Reilly, Darragh McArt, Wendy L. Allen, Markus Morrison, Richard Kennedy, Nicolas Vitale, Patrick G. Johnston, Tim Harrison, Daniel B. Longley, Emma Evergren, Sandra Van Schaeybroeck. RALB GTPase: A critical regulator of DR5 cell surface expression and TRAIL sensitivity in RAS mutant colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 704.