8 Death Receptor Mutations
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Chapter 8 / Death Receptor Mutations 149 8 Death Receptor Mutations Sug Hyung Lee, MD, PhD, Nam Jin Yoo, MD, PhD, and Jung Young Lee, MD, PhD, SUMMARY It is generally believed that human cancers may arise as the result of an accumulation of mutations in genes and subsequent clonal selection of variant progeny with increas- ingly aggressive behaviors. Also, among the remarkable advances in our understanding in cancer biology is the realization that apoptosis has a profound effect on the malignant phenotypes. Along with these, compelling evidence indicates that somatic mutations in the genes encoding apoptosis-related proteins contribute to either development or pro- gression of human cancers. In this chapter, we present an overview of the death receptor pathway and its dysregulation in cancers. We then review the current knowledge of death receptor mutations that have been detected in humans. INTRODUCTION Programmed cell death through apoptosis plays a fundamental role in a variety of physiological processes, and its deregulation contributes to many diseases, including autoimmunity, cancer, acquisition of drug resistance in tumors, stroke, progression of some degenerative diseases, and acquired immunodeficiency syndrome (AIDS) (1–4). Apoptosis is an active cell-suicide process executed by a cascade of molecular events involving a number of membrane receptors and cytoplasmic proteins (1–4). Although many pathways for activating caspases may exist, only two, the intrinsic pathway and the extrinsic pathway, have been demonstrated in detail (3). The extrinsic pathway can be induced by members of the tumor necrosis factor (TNF) receptor family, such as TNF receptor 1 (TNFR1) and Fas (5–7). These proteins recruit adaptor proteins, including FADD, to their cytosolic death domains, which then bind caspases-8 and -10 (8–12). The intrinsic pathway can be induced by release of cytochrome c from mitochondria (13–15). In the cytosol, cytochrome c binds and activates apaf-1, allowing it to bind and activate caspase-9 (14,15). Active initiator caspases of the extrinsic pathway (caspases-8 and -10) From: Cancer Drug Discovery and Development: Death Receptors in Cancer Therapy Edited by: W. S. El-Deiry © Humana Press Inc., Totowa, NJ 149 150 Lee, Yoo, and Lee and the intrinsic pathway (caspase-9) have been shown to directly cleave and activate the effector protease, caspase-3 (16,17). Also, though commonly viewed as separate path- ways and capable of functioning independently, cross-talk can occur between these path- ways at multiple levels, depending on the repertoire of apoptosis-modulating proteins expressed (18–21). APOPTOSIS AND CANCER Several lines of evidence indicate that tumorigenesis is a multistep process and that these steps reflect genetic alterations that drive the progressive transformation of normal cells into malignant phenotypes (22). The genomes of tumor cells are invariably altered at multiple sites, having suffered disruption through lesions as subtle as point mutations and as obvious as changes in chromosome complement (23). In the multistep tumorigen- esis model, mutations in key cellular genes produce a series of acquired capabilities that allow the developing cancer cell to grow unchecked in the absence of growth-stimulating signals, while overcoming growth-inhibitory signals and host immune responses (22– 25). They also allow the tumor to replicate indefinitely, maintain an oxygen and nutrient supply, and invade adjacent and distant tissues (26–29). Finally, the ability of cells to evade apoptosis is also an essential hallmark of cancer (22,30,31). Since the discovery of bcl-2 as an oncogene that promotes cell survival, it has been widely acknowledged that antiapoptotic genetic lesions are necessary for tumors to arise (32,33). Clonal expansion and tumor growth are the results of the deregulation of intrinsic proliferation (cell division) and cell death (apoptosis). The evidence is mounting, from studies in mouse models and cultured cells, as well as from descriptive analyses of tumor tissues along the multistep carcinogenesis (22,23). Enhanced cell survival is needed at several steps during tumorigenesis: deregulated oncogene expression not only leads to accelerated proliferation, but concomitantly induces apoptosis, which needs to be sup- pressed for the transformed cell to survive and multiply (34). The tumor cells can persist in a hostile environment. For example, sufficient nutrition for every tumor cell becomes restricted; starvation of tumor cells from cytokines usually leads to apoptotic cell death (26,27). Finally, defective apoptosis facilitates metastasis (22). To metastasize, a tumor cell must acquire the ability to survive in the bloodstream and invade a foreign tissue. Normally, this process is prevented by the propensity of epithelial cells to die in suspen- sion, or in the absence of appropriate tissue survival. During metastasis, cancer cells can ignore restraining signals from neighbors and survive detachment from the extracellular matrix (29). Hence, loss of apoptosis can impact tumor development, progression, and metastasis. Loss of apoptosis is also a significant impediment to anticancer therapy. It is now well established that anticancer agents induce apoptosis, and the disruption of apoptotic machineries reduces treatment sensitivity (35). The mutations that favored tumor development dampen the response to chemotherapy, and treatment might select more refractory clones. DEATH RECEPTORS Cell surface death receptors transmit an apoptosis signal on binding of a specific death ligand (2). The best known family of death receptors is represented by tumor necrosis factor receptors (TNFRs), Fas (CD95, Apo-1), and TNF-related apoptosis-inducing Chapter 8 / Death Receptor Mutations 151 ligand receptors (TRAIL-Rs) (36-41). The receptors’ ligands comprise another related family that includes TNF, CD95 ligand (FasL/CD95L), and TRAIL (2). Each of the ligands is synthesized as a membrane-associated protein and shares a characteristic 150- amino-acid region towards the C-terminus by which each ligand interacts with its cognate receptor (2). For the most part, these ligands exist as trimeric or multimeric membrane- bound proteins that may function to induce receptor aggregation. The Fas-FasL system has been recognized as a major pathway for the induction of apoptosis in cells and tissues (1,2). Fas is a member of the death receptor subfamily of the TNFR superfamily (37–39). Ligation of Fas by either agonistic antibody or by its natural ligand transmits a death signal to the target cells, potentially triggering apoptosis. Fas has three cystein-rich extracellular domains and an intracellular death domain (DD) essential for signaling (37–39). The death domain, a name deriving from its ability to recruit downstream effectors that can induce apoptosis, is present in the cytoplasmic tail of all death receptors (1). The death domain is a protein interaction module consisting of a compact bundle of six _-helices (42). Death domains bind each other, probably forming oligomers of unknown stoichiom- etry. Stimulation of Fas results in aggregation of its intracellular death domain, leading to the recruitment of two key signaling proteins that, together with the receptor, form the death-inducing signaling complex (DISC) (43). FADD/MORT-1 (8) couples through its C-terminal death domain to crosslinked Fas receptors and recruits caspase-8 (9) through its N-terminal death effector domain (DED) to the DISC. Caspase-10 (Mch4/FLICE2) is a caspase homologous to caspase-8 and is present as an inactive proenzyme, comprising a prodomain that contains two DEDs to allow caspase-10 to interact with the DED of FADD and a catalytic protease domain that can be further processed to give a large and a small subunit (44). The local aggregation of the procaspases-8 and -10 is sufficient to allow autoprocessing or transprocessing to produce active caspases-8 and -10, which can subsequently activate downstream executioners, such as caspases 3 and 7 (44). Five receptors have been identified for TRAIL, including two apoptosis-inducing receptors (death receptor 4/TRAIL-R1 and DR5/TRICK-2/TRAIL-R2/KILLER-DR7), two decoy receptors (DcRs) (TRID/DcR1 and TRUNDD/DcR2), and osteo-progeterin. TRAIL induces apoptosis through DR4 and DR5 requiring FADD, caspase-8 and caspase- 10, just like CD95-mediated cell killing (45). DEATH RECEPTOR MUTATIONS Fas, DR5, and DR4 are widely expressed in normal and neoplastic cells (46,47), but the expression of these proteins does not necessarily predict susceptibility to killing (48,49). This can reflect the presence of inhibiting mechanisms of death receptor-medi- ated apoptosis. Fas-mediated apoptosis can be blocked by several mechanisms, including the production of soluble Fas (50), the lack of cell-surface Fas expression (51–53), the overexpression of inhibitory proteins in signal transduction pathways such as Fas-asso- ciated phosphatase-1 (54) and FLICE- inhibitory protein (FLIP) (55), and the mutation of the primary structure of Fas (56–65). TRAIL-induced apoptosis can be blocked by several mechanisms, including the expression of decoy receptors for TRAIL (10), the loss of TRAIL receptor expression (5), the overexpression of inhibitory proteins in signal transduction pathways such as FLIP (7), and the mutation of the primary structure of DR4 and DR5 (11). 152 Lee, Yoo, and Lee Germline Mutation of Death Receptors The consequences of naturally occurring mutants of Fas/FasL have been well demon- strated in both mice and humans. lpr and gld are mutations in mice of Fas and FasL, respectively (56,66). Because the Fas/FasL system is involved in the apoptotic process that occurs during cell muturation, lpr and gld mutations result in the development of lymphadenopathy and autoimmune diseases in the mice (67). To date, two lpr mutations are known: lpr and lprcg. The mouse Fas gene consists of over 70kb, and is split by 9 exons (56). The restriction mapping of the Fas gene from lpr mice has revealed the insertion of an early transposable element (ETn) of 5.4 kb in intron 2. The ETn is a mouse endogenous retrovirus, of which about 1,000 copies can be found in the mouse genome.