Development of novel drugs targeting inhibitors of apoptosis

Abstract

Future OncologyVol. 5, No. 2 EditorialFree AccessDevelopment of novel drugs targeting inhibitors of apoptosisJohn A Flygare & Domagoj VucicJohn A FlygareDepartment of Medicinal Chemistry, Genentech, Inc., South San Francisco, CA, 94080, USA. Search for more papers by this authorEmail the corresponding author at flygare.john@gene.com & Domagoj Vucic† Author for correspondenceDepartment of Protein Engineering, Genentech, Inc., South San Francisco, CA, 94080, USA. Search for more papers by this authorEmail the corresponding author at vucic.domagoj@gene.comPublished Online:16 Mar 2009https://doi.org/10.2217/14796694.5.2.141AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareShare onFacebookTwitterLinkedInReddit Figure 1. Mechanistic role of IAP antagonists in apoptotic signaling.IAP antagonists stimulate ubiquitin ligase activity of c-IAPs leading to auto-ubiquitination and proteasome-mediated degradation of c-IAPs. Subsequent NF-κB activation induces expression of TNF-α, which engages TNF receptor 1 to trigger proapoptotic signaling events. This apoptotic activity is normally suppressed by c-IAPs. However, elimination of c-IAPs through IAP antagonist-induced autoubiquitination alters the balance to favor the apoptotic outcome.c-AIP: Cellular inhibitor of apoptosis; IAP: Inhibitor of apoptosis.Regulation of cellular survival is one of the most important features that impacts development and homeostasis of multicellular organisms [1]. Disregulation of cellular death can alter the survival of defective and/or normal cells and contribute to a number of human diseases, such as cancer, neurodegeneration and immunodeficiency [2]. Inhibitor of apoptosis (IAP) proteins are crucial components of these processes because of their functional importance in the regulation of cell death and their elevated expression in diseased tissues [3]. IAPs were originally identified in baculoviruses and subsequently IAP homologs have been discovered in invertebrates and vertebrates [4]. Eight human IAP family members include neuronal apoptosis inhibitory protein (NAIP), cellular IAP 1 and 2 (c-IAP1 and 2), X chromosome-linked IAP (XIAP), survivin, Apollon/Bruce, melanoma IAP (ML-IAP), and IAP-like protein 2 (ILP2) [4]. IAP proteins contain one to three signature baculovirus IAP repeat (BIR) domains, and most of them also possess ubiquitin ligase RING domains [5,6].Two major pathways are used for the execution of apoptosis or programmed cell death. The extrinsic apoptotic pathway is initiated by the binding of death ligands to their specific cell-surface receptors and results in the assembly of the death receptor-signaling complex [7]. The intrinsic or mitochondrial pathway is initiated by irradiation, growth factor withdrawal, or other intracellular stresses and leads to disruption of internal cellular integrity [8]. Both of these two pathways lead to activation of caspases, cysteine-dependent aspartyl-specific proteases that are effectors of apoptotic signaling [9].XIAP is the best-studied IAP protein owing to its prominence as the only true endogenous inhibitor of caspases [9]. XIAP inhibits caspases 3 and 7 using the linker region between BIR1 and BIR2 as well as the BIR2 domain, while inhibition of caspase-9 relies on the binding of the BIR3 domain to an amino-terminal IAP-binding motif (IBM) of partially processed caspase-9 [10]. Caspase-inhibitory activity of XIAP is blocked by second mitochondrial activator of caspases (Smac) [11,12]. During induction of apoptosis, Smac undergoes proteolyic processing resulting in its release from mitochondria into the cytoplasm where it can bind BIR2 and BIR3 of XIAP and antagonize XIAP via an exposed IBM [11,12]. Some IAP proteins, such as ML-IAP, have a high affinity for Smac but do not interact strongly with caspases [13]. Instead, ML-IAP is able to compete with XIAP for Smac binding and thereby alleviate the antagonism of XIAP [13]. c-IAP1 and 2 are components of TNF receptor (TNFR) complexes where they modulate apoptotic signaling and caspase-8 activation [14]. In addition, c-IAP1 and 2 ubiquitinate RIP1 to promote the TNF-α-induced canonical NF-κB pathway, while in the noncanonical NF-κB pathway, c-IAP1 and 2 ubiquitinate NF-κB-inducing kinase (NIK), causing its proteasomal degradation and inhibition of NF-κB signaling [15–20]. Survivin and some other IAP proteins, such as Bruce, appear to modulate cell survival through the regulation of the cell cycle and proliferation [21].IAP proteins are expressed at elevated levels in the majority of human malignancies, which makes them attractive targets for developing a novel class of cancer therapeutics [3]. In addition, overexpression of IAPs in human cancers has been shown to suppress apoptosis induced by a variety of stimuli [3,22]. For example, ML-IAP is a potent anti-apoptotic protein that is upregulated in a number of melanomas, but not expressed in most normal adult tissues [3,22]. Genetic loci for c-IAP1 and 2 are also the target of genetic amplification, and in the case of c-IAP2 genetic translocation [23]. These aberrations are associated with enhanced tumorigenicity and with development of inflammation-associated tumors [3,23]. Thus, the contribution of IAP proteins to the development of human malignancies stems from their elevated expression levels combined with inhibition of cell death and promotion of survival signaling pathways (e.g., NF-κB).It has been firmly established that the interaction between Smac, the endogenous IAP antagonist, and IAP proteins is limited to the AVPI tetrapeptide at the amino-terminus of mature, processed Smac [24,25]. Efforts to identify small-molecule antagonists of IAPs have targeted the BIR3 domains of XIAP, c-IAP1 and 2, or the single BIR domain of ML-IAP with a peptidomimetic approach designed to mimic the AVPI motif [15,26]. Two classes of these mimics have been reported, monovalent and bivalent [15,26]. The monovalent IAP antagonist compounds are small molecules designed to mimic one AVPI tetrapeptide, while the bivalent compounds comprise two of the AVPI mimetics linked together with a chemical spacer. In general, monovalent antagonists bind with highest affinity to the BIR3 domain of IAP proteins and with moderate affinities to XIAP BIR2. In the case of XIAP, the bivalent antagonists can simultaneously interact with both the BIR2 and BIR3 domains, thus enabling efficient elimination of XIAP-mediated caspase inhibition [19,27]. Perhaps as a result of this added functional property, bivalent antagonists are often 10–100-fold more potent than their corresponding monomeric components in cell death induction and inhibition of tumor growth [17,19,28,29]. Alternative strategies for targeting IAP proteins involve small molecule-based inhibition of XIAP BIR2-caspase-3 interaction, antisense oligonucleotides and antigen-based vaccination [26].The monovalent and bivalent antagonists can disrupt the association between multiple IAP proteins and Smac as well as XIAP and caspases. Binding of IAP antagonists results in a dramatic induction of c-IAP auto-ubiquitination activity and rapid proteasomal degradation of the c-IAP proteins [17,19,29]. Besides neutralizing these anti-apoptotic proteins, the IAP antagonists activate NF-κB pathways and induce cell death that is dependent on TNF signaling and de novo protein biosynthesis [17,19,29,30]. IAP antagonists induce apoptotic cell death with characteristic caspase activation in a variety of human cancer cell lines and inhibit tumor growth in a number of xenograft cancer models [15,26]. The ultimate test for these compounds and the validation of targeting this cell death pathway will be human clinical trials. At the moment, two compounds have entered Phase I clinical trials, GDC-0152 from Genentech, Inc., and AEG408/HGS1029 from Aegera/Human Genome Science [31,32]. These trials will explore the applicability of monovalent and bivalent IAP antagonists for treatment of human malignancies and pave the way for future clinical investigations of IAP-regulated apoptotic pathways.Although active as single agents, the efficacy of small-molecule IAP antagonists may be most effective when used in combination with other agents designed to initiate apoptosis signaling. In cellular and in vivo assays, Smac-mimicking peptides and IAP antagonist compounds sensitize various cancer cell lines and xenograft models towards death receptor- or chemotherapy-induced apoptosis. In addition, IAP antagonists might also combine with antiproliferative and anti-angiogenesis agents, thus expanding the repertoire of treatment options. Clinical trials designed to test these combinations should explore potential synergistic interactions between IAP antagonists and other pro-apoptotic or anti-proliferative agents in order to provide novel anti-tumor therapeutic opportunities. It will also be exceedingly important to identify the patient population(s) that will benefit the most from this therapeutic approach. Recent progress in the mechanistic understanding of IAP antagonism, combined with the advancements of molecular diagnostic techniques, should enable these goals and bring significant benefit to cancer patients.AcknowledgmentsWe thank Wayne J Fairbrother for critical reading of the manuscript.Financial & competing interests disclosureBoth authors are employees and shareholders of Genentech, Inc. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.No writing assistance was utilized in the production of this manuscript.Papers of special note have been highlighted as: ▪ of interest ▪▪ of considerable interestBibliography1 Steller H: Mechanisms and genes of cellular suicide. Science267(5203),1445–1449 (1995).Crossref, Medline, CAS, Google Scholar2 Reed JC: Apoptosis-based therapies. Nat. Rev. Drug Discov.1(2),111–121 (2002).Crossref, Medline, CAS, Google Scholar3 Hunter AM, LaCasse EC, Korneluk RG: The inhibitors of apoptosis (IAPs) as cancer targets. Apoptosis12(9),1543–1568 (2007).Crossref, Medline, CAS, Google Scholar4 Salvesen GS, Duckett CS: IAP proteins: blocking the road to death’s door. Nat. Rev. Mol. Cell Biol.3(6),401–410 (2002).▪ Summarizes the discovery of inhibitors of apoptosis (IAPs).Crossref, Medline, CAS, Google Scholar5 Miller LK: An exegesis of IAPs: salvation and surprises from BIR motifs. Trends Cell. Biol.9(8),323–328 (1999).▪ Summarizes the discovery of IAPs.Crossref, Medline, CAS, Google Scholar6 Vaux DL, Silke J: IAPs, RINGs and ubiquitylation. Nat. Rev. Mol. Cell Biol.6(4),287–297 (2005).Crossref, Medline, CAS, Google Scholar7 Ashkenazi A, Dixit VM: Death receptors: signaling and modulation. Science281(5381),1305–1308 (1998).Crossref, Medline, CAS, Google Scholar8 Kaufmann SH, Vaux DL: Alterations in the apoptotic machinery and their potential role in anticancer drug resistance. Oncogene22(47),7414–7430 (2003).Crossref, Medline, CAS, Google Scholar9 Salvesen GS, Abrams JM: Caspase activation – stepping on the gas or releasing the brakes? Lessons from humans and flies. Oncogene23(16),2774–2784 (2004).Crossref, Medline, CAS, Google Scholar10 Eckelman BP, Salvesen GS, Scott FL: Human inhibitor of apoptosis proteins: why XIAP is the black sheep of the family. EMBO Rep.7(10),988–994 (2006).Crossref, Medline, CAS, Google Scholar11 Du C, Fang M, Li Y, Li L, Wang X: Smac, a mitochondrial protein that promotes cytochrome c-dependent caspase activation by eliminating IAP inhibition. Cell102(1),33–42 (2000).▪ Presents initial discovery of second mitochondrial activator of caspases (Smac).Crossref, Medline, CAS, Google Scholar12 Verhagen AM, Ekert PG, Pakusch M et al.: Identification of DIABLO, a mammalian protein that promotes apoptosis by binding to and antagonizing IAP proteins. Cell102(1),43–53 (2000).▪ Presents initial discovery of direct IAP binding protein with low pI (DIABLO).Crossref, Medline, CAS, Google Scholar13 Vucic D, Franklin MC, Wallweber HJ et al.: Engineering ML-IAP to produce an extraordinarily potent caspase 9 inhibitor: implications for Smac-dependent anti-apoptotic activity of ML-IAP. Biochem J.385(Pt 1),11–20 (2005).Crossref, Medline, CAS, Google Scholar14 Chen G, Goeddel DV: TNF-R1 signaling: a beautiful pathway. Science296(5573),1634–1635 (2002).Crossref, Medline, CAS, Google Scholar15 LaCasse EC, Mahoney DJ, Cheung HH, Plenchette S, Baird S, Korneluk RG: IAP-targeted therapies for cancer. Oncogene27(48),6252–6275 (2008).Crossref, Medline, CAS, Google Scholar16 Varfolomeev E, Vucic D: (Un)expected roles of c-IAPs in apoptotic and NF-κB signaling pathways. Cell Cycle7(11),1511–1521 (2008).Crossref, Medline, CAS, Google Scholar17 Bertrand MJ, Milutinovic S, Dickson KM et al.: cIAP1 and cIAP2 facilitate cancer cell survival by functioning as E3 ligases that promote RIP1 ubiquitination. Mol. Cell30(6),689–700 (2008).Crossref, Medline, CAS, Google Scholar18 Mahoney DJ, Cheung HH, Mrad RL et al.: Both cIAP1 and cIAP2 regulate TNFα-mediated NF-kappaB activation. Proc. Natl Acad. Sci. USA105(33),11778–11783 (2008).Crossref, Medline, CAS, Google Scholar19 Varfolomeev E, Blankenship JW, Wayson SM et al.: IAP antagonists induce autoubiquitination of c-IAPs, NF-κB activation, and TNFα-dependent apoptosis. Cell131(4),669–681 (2007).▪▪ Describes IAP antagonist-stimulated proteasomal degradation of c-IAP proteins and activation of NF-κB pathways and TNF-dependent cell deathCrossref, Medline, CAS, Google Scholar20 Varfolomeev E, Goncharov T, Fedorova AV et al.: c-IAP1 and c-IAP2 are critical mediators of tumor necrosis factor α (TNFα)-induced NF-κB activation. J. Biol. Chem.283(36),24295–24299 (2008).Crossref, Medline, CAS, Google Scholar21 Verhagen AM, Coulson EJ, Vaux DL: Inhibitor of apoptosis proteins and their relatives: IAPs and other BIRPs. Genome Biol.2(7), REVIEWS3009 (2001).Crossref, Medline, Google Scholar22 Vucic D: Targeting IAP (inhibitor of apoptosis) proteins for therapeutic intervention in tumors. Curr. Cancer Drug Targets8(2),110–117 (2008).Crossref, Medline, CAS, Google Scholar23 Isaacson PG: Update on MALT lymphomas. Best Pract. Res. Clin. Haematol.18(1),57–68 (2005).Crossref, Medline, CAS, Google Scholar24 Liu Z, Sun C, Olejniczak ET et al.: Structural basis for binding of Smac/DIABLO to the XIAP BIR3 domain. Nature408(6815),1004–1008 (2000).▪ Provides structural evidence for the binding between IAP proteins and Smac.Crossref, Medline, CAS, Google Scholar25 Wu G, Chai J, Suber TL et al.: Structural basis of IAP recognition by Smac/DIABLO. Nature408(6815),1008–1012 (2000).▪ Provides structural evidence for the binding between IAP proteins and Smac.Crossref, Medline, CAS, Google Scholar26 Vucic D, Fairbrother WJ: The inhibitor of apoptosis proteins as therapeutic targets in cancer. Clin. Cancer Res.13(20),5995–6000 (2007).Crossref, Medline, CAS, Google Scholar27 Gao Z, Tian Y, Wang J et al.: A dimeric Smac/diablo peptide directly relieves caspase-3 inhibition by XIAP. Dynamic and cooperative regulation of XIAP by Smac/Diablo. J. Biol. Chem.282(42),30718–30727 (2007).Crossref, Medline, CAS, Google Scholar28 Petersen SL, Wang L, Yalcin-Chin A et al.: Autocrine TNFα signaling renders human cancer cells susceptible to smac-mimetic-induced apoptosis. Cancer Cell12(5),445–456 (2007).▪▪ Describes dependence of IAP antagonist-stimulated cell death on TNF signaling.Crossref, Medline, CAS, Google Scholar29 Vince JE, Wong WW, Khan N et al.: IAP antagonists target cIAP1 to induce TNFα-dependent apoptosis. Cell131(4),682–693 (2007).▪▪ Describes IAP antagonist-stimulated proteasomal degradation of c-IAP proteins and activation of NF-κB pathways and TNF-dependent cell death.Crossref, Medline, CAS, Google Scholar30 Gaither A, Porter D, Yao Y et al.: A Smac mimetic rescue screen reveals roles for inhibitor of apoptosis proteins in tumor necrosis factor-α signaling. Cancer Res.67(24),11493–11498 (2007).▪▪ Describes dependence of IAP antagonist-stimulated cell death on TNF signaling.Crossref, Medline, CAS, Google Scholar31 Call JA, Eckhardt SG, Camidge DR: Targeted manipulation of apoptosis in cancer treatment. Lancet Oncol.9(10),1002–1011 (2008).Crossref, Medline, CAS, Google Scholar32 Gillard JW: (X)IAP inhibition and apoptosis induction: explaining the remarkable synergy with death receptor agonists. Presented at: University of Ulm Collaborative Research Center International Symposium. 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The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.No writing assistance was utilized in the production of this manuscript.PDF download