Peter Liston

The inhibitors of apoptosis: there is more to life than Bcl2

The inhibitor of apoptosis (IAP) genes constitute a highly conserved family found in organisms as diverse as insects and mammals. These genes encode proteins that directly bind and inhibit caspases, and thus play a critical role in deciding cell fate. The IAPs are in turn regulated by endogenous proteins (second mitochondrial activator of caspases and Omi) that are released from the mitochondria during apoptosis. Overexpression of the IAPs, particularly the X-chromosome-linked IAP, has been shown to be protective in a variety of experimental animal models of human neurodegenerative diseases. Furthermore, overexpression of one or more of the IAPs in cancer cell lines and primary tumor samples appears to be a frequent event. IAP gene amplification and translocation events provide genetic evidence that further strengthens the case for classifying the IAPs as oncogenes. Therapeutic strategies that interfere with IAP expression or function are under investigation as an adjuvant to conventional chemotherapy- and radiation-based cancer therapy. This paper reviews the structure and function of the IAP family members and their inhibitors, and surveys the available evidence for IAP dysregulation in cancer.

Identification of XAF1 as an antagonist of XIAP anti-Caspase activity.

The inhibitors of apoptosis (IAPs) suppress apoptosis through the inhibition of the caspase cascade and thus are key proteins in the control of cell death. Here we have isolated the protein XIAP-associated factor 1 (XAF1) on the basis of its ability to bind XIAP, a member of the IAP family. XIAP suppresses caspase activation and cell death in vitro, and XAF1 antagonizes these XIAP activities. Expression of XAF1 triggers a redistribution of XIAP from the cytosol to the nucleus. XAF1 is ubiquitously expressed in normal tissues, but is present at low or undetectable levels in many different cancer cell lines. Loss of control over apoptotic signalling is now recognized as a critical event in the development of cancer. Our results indicate that XAF1 may be important in mediating the apoptosis resistance of cancer cells.

Elevation of neuronal expression of NAIP reduces ischemic damage in the rat hippocampus.

We show here that transient forebrain ischemia selectively elevates levels of neuronal apoptosis inhibitory protein (NAIP) in rat neurons that are resistant to the injurious effects of this treatment. This observation suggests that increasing NAIP levels may confer protection against ischemic cell death. Consistent with this proposal, we demonstrate that two other treatments that increase neuronal NAIP levels, systemic administration of the bacterial alkaloid K2S2a and intracerebral injection of an adenovirus vector capable of overexpressing NAIP in vivo, reduce ischemic damage in the rat hippocampus. Taken together, these findings suggest that NAIP may play a key role in conferring resistance to ischemic damage and that treatments that elevate neuronal levels of this antiapoptotic protein may have utility in the treatment of stroke.

Human Ovarian Cancer and Cisplatin Resistance: Possible Role of Inhibitor of Apoptosis Proteins1

The inhibitor of apoptosis proteins (IAPs) constitutes a family of highly conserved apoptosis suppressor proteins that were originally identified in baculoviruses. Although IAP homologs have recently been demonstrated to suppress apoptosis in mammalian cells, their expression and role in human ovarian epithelial cancer and chemotherapy resistance are unknown. In the present study we used cisplatin-sensitive and -resistant human ovarian surface epithelial (hOSE) cancer cell lines and adenoviral antisense and sense complementary DNA expression to examine the role of IAP in the regulation of apoptosis in human ovarian cancer cells and chemoresistance. Antisense down-regulation of X-linked inhibitor of apoptosis protein (Xiap), but not human inhibitor of apoptosis protein-2 (Hiap-2), induced apoptosis in cisplatin-sensitive and, to a lesser extent, in -resistant cells. Cisplatin consistently decreased Xiap content and induced apoptosis in the cisplatin-sensitive, but not cisplatin-resistant, cells. Hiap-2 expression was either unaffected or inhibited to a lesser extent. The inhibition of IAP protein expression and induction of apoptosis by cisplatin was time and concentration dependent. Infection of cisplatin-sensitive cells with adenoviral sense Xiap complementary DNA resulted in overexpression of Xiap and markedly attenuated the ability of cisplatin to induce apoptosis. Immunohistochemical localization of the IAPs in hOSE tumors demonstrated the presence of Xiap and Hiap-2, with their levels being highest in proliferative, but not apoptotic, epithelial cells. These studies indicate that Xiap is an important element in the control of ovarian tumor growth and may be a point of regulation for cisplatin in the induction of apoptosis. These results suggest that the ability of cisplatin to down-regulate Xiap content may be an important determinant of chemosensitivity in hOSE cancer. (Endocrinology 142: 370–380, 2001) H OVARIAN surface epithelial cancer (hOSE) ranks fifth among the most common female cancers and is the leading cause of death from gynecologic malignancy in the western world. Although the clinical and histological prognostic factors (e.g. tumor grade and surgical stage) are well understood, less is known about the biological process that leads to uncontrolled cellular growth. The control of cell number during tissue growth is thought to be the result of a balance of cell proliferation and cell death. Whereas cisplatin is currently a front line chemotherapeutic agent for ovarian epithelial cancer, chemoresistance remains a major barrier to successful therapy. Ovarian epithelial cancer cell apoptosis has been demonstrated to be involved in cisplatin-induced cellular responses (1, 2). The action of cisplatin is thought to involve the formation of interand intrastrand DNA cross-links (3), although the events leading to cell death after cisplatin treatment are unclear. Understanding the molecular mechanism by which this drug induces cell death should provide a fundamental approach for increasing the sensitivity of cells to this anti-cancer agent. Apoptosis plays an important role in the maintenance of physiological homeostasis in response to stimuli that indicate that a cell is potentially harmful or abnormal (4–6). When the apoptosis machinery fails, abnormal cells can survive, and unopposed tissue growth, as in the case of cancer, can result. Thus, it is conceivable that carcinomas may be caused or promoted in part by factors inhibiting cell death. In this regard, considerable work has been focused on the role of bcl-2 (7–9) as a negative regulator of apoptotic cell death (10). In ovarian cancer, expression of the bcl-2 gene is an important modulator of drug-induced apoptosis and a potential determinant of chemoresistance (8) and survival (9). However, evidence also indicates that bcl-2 overexpression or mutation cannot adequately account for the etiology of existing ovarian cancer (7, 11), suggesting that other cell survival factors may also be involved in this complex

Granzyme B-Induced Apoptosis Requires Both Direct Caspase Activation and Relief of Caspase Inhibition

Cytotoxic lymphocytes employ Granzyme B as a potent initiator of apoptosis to cleave and activate effector caspases. Unexpectedly, cells transfected with Bcl-2 were resistant to granzyme B-induced killing, suggesting that a mitochondrial pathway was critical. Utilizing cells expressing a dominant-negative caspase 9, the current study demonstrated that caspase activation via the apoptosome was not required. Indeed, cleavage of caspase 3 to p20 still occurred in Bcl-2-transfectants but processing to p17 was blocked. This blockade was recapitulated by the Inhibitor-of-Apoptosis-Protein XIAP and relieved by Smac/DIABLO. Thus granzyme B mediates direct cleavage of caspase 3 and also activates mitochondrial disruption, resulting in the release of proapoptotic proteins that suppress caspase inhibition. Engagement of both pathways is critical for granzyme-induced killing.

Degradation of survivin by the X-linked inhibitor of apoptosis (XIAP)-XAF1 complex.

X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1) is a putative tumor suppressor in which expression is significantly reduced in human cancer cell lines and primary tumors. The proapoptotic effects of XAF1 have been attributed to both caspase-dependent and -independent means. In particular, XAF1 reverses the anti-caspase activity of XIAP, a physiological inhibitor of apoptosis. We further investigated the function of XAF1 by examining its relationship with other IAPs. Immunoprecipitation studies indicate that XAF1 binds to XIAP, cIAP1, cIAP2, Livin, TsIAP, and NAIP but not Survivin, an IAP that prevents mitotic catastrophe and in which antiapoptotic activity is exerted through direct XIAP interaction and stabilization. We found that overexpressed XAF1 down-regulates the protein expression of Survivin. Under these conditions, Survivin expression was restored in the presence of the proteasome inhibitor MG132 or a XIAP RING mutant that is defective in ubiquitin-protein isopeptide ligase (E3) activity, suggesting that XAF1 interaction activates E3 activity of XIAP and targets Survivin by direct ubiquitination. In addition, RNA interference targeting endogenous XIAP protected Survivin degradation by XAF1. Furthermore, interferon-β-mediated XAF1 induction promoted formation of an endogenous XIAP-XAF1-Survivin complex. This complex facilitated Survivin degradation, which was prevented in XAF1-/- stable clones. Altogether, our study demonstrates that XAF1 mediates Survivin down-regulation through a complex containing XIAP, supporting dual roles for XAF1 in apoptosis and mitotic catastrophe.

Expression and biological activity of X-linked inhibitor of apoptosis (XIAP) in human malignant glioma

The inhibitor-of-apoptosis (IAP) proteins are a novel family of antiapoptotic proteins that are thought to inhibit cell death via direct inhibition of caspases. Here, we report that human malignant glioma cell lines express XIAP, HIAP-1 and HIAP-2 mRNA and proteins. NAIP was not expressed. IAP proteins were not cleaved during CD95 ligand (CD95L)-induced apoptosis, and loss of IAP protein expression was not responsible for the potentiation of CD95L-induced apoptosis when protein synthesis was inhibited. LN-18 cells are highly sensitive to CD95-mediated apoptosis, whereas LN-229 cells require co-exposure to CD95L and a protein synthesis inhibitor, CHX, to acquire sensitivity to apoptosis. Adenoviral XIAP gene transfer blocked caspase 8 and 3 processing in both cell lines in the absence of CHX. Apoptosis was blocked in the absence and in the presence of CHX. However, XIAP failed to block caspase 8 processing in LN-229 cells in the presence of CHX. There was considerable overlap of the effects of XIAP on caspase processing with those of BCL-2 and the viral caspase inhibitor crm-A. These data define complex regulatory mechanisms for CD95-mediated apoptosis in glioma cells and indicate that there may be a distinct pathway of death receptor-mediated apoptosis that is readily activated when protein synthesis is inhibited. The constitutive expression of natural caspase inhibitors may play a role in the resistance of these cells to apoptotic stimuli that directly target caspases, including radiochemotherapy and immune-mediated tumor cell lysis.

IAPs are essential for GDNF-mediated neuroprotective effects in injured motor neurons in vivo

During embryonic development, and in certain neurodegenerative diseases, neurons die by apoptosis. A new family of anti-apoptotic proteins, termed inhibitors of apoptosis (IAP), suppresses apoptosis through the direct inhibition of caspases. The anti-apoptotic activity of IAPs is inhibited by second mitochondria-derived activator of caspase (Smac)/DIABLO and XAF1 (ref. 8). IAPs, as well as neurotrophic factors, can protect degenerating neurons both in vivo and in vitro. However, the downstream targets of neurotrophic factors have not yet been identified. Here, we demonstrate that XIAP and NAIP, but not HIAP2, are directly involved in the intracellular response to glial cell-derived neurotrophic factor (GDNF). In newborn rats, GDNF regulates endogenous levels of XIAP and NAIP in motor neurons after sciatic nerve axotomy. The inhibition of XIAP or NAIP activity prevents GDNF-mediated neuroprotective effects. These results suggest that XIAP and NAIP are essential for intracellular signalling of GDNF in motor neuron survival.

The X-linked inhibitor of apoptosis (XIAP) prevents cell death in axotomized CNS neurons in vivo.

The inhibition of neuronal apoptosis in acute traumatic and ischemic injuries as well as in long term neurodegenerative disorders like spinal muscular atrophy and possibly Alzheimers disease is a fundamental requirement for a therapeutic strategy. In this study we used an established in vivo model system of induction of neuronal apoptosis in the CNS to evaluate the properties of the X-linked inhibitor of apoptosis protein (XIAP) to inhibit secondary cell death after axonal lesions. We used adenoviral vectors to transduce retinal ganglion cells after axotomy of the optic nerve of adult rats. Vector application was performed at the optic nerve stump so that only the lesioned retinal neurons could be transduced. We found XIAP to be as effective as the viral broad spectrum caspase inhibitor protein p35. These findings suggest that axotomized RGCs degenerate through class II caspase activity and furthermore offer the possibility of using mammalian XIAP protein to inhibit neuronal apoptosis as a basis for a regenerative therapy in the CNS. Cell Death and Differentiation (2000) 7, 815–824

IAP family proteins delay motoneuron cell death in vivo

Neuronal apoptosis inhibitory protein (NAIP), and human inhibitors of apoptosis 1 and 2 (HIAP1 and HIAP2) are three members of the mammalian family of antiapoptosis proteins called ‘inhibitors of apoptosis’ (IAP). These molecules can prevent apoptosis in vitro and the over‐expression of NAIP can decrease ischemic damage in the hippocampus. The goal of our experiments was to determine whether administration of NAIP, HIAP1 and HAIP2 could rescue motoneurons following axotomy of a peripheral nerve. In young rats, an adenoviral gene transfer technique was used to deliver and express these proteins in motoneurons; a fluorescent tracer was simultaneously added as a means for quantitatively assessing the rescue of fluorescently labelled motoneurons in serial sections of the lumbar spinal cord. Control experiments using adenoviral vectors (adv) expressing the lacZ gene showed that 14% of the sciatic motoneuron pool could be transfected indicating the existence of a subpopulation of spinal motoneurons susceptible to this class of viral vectors. The administration of an adv‐NAIP, adv‐HIAP1 and adv‐HIAP2 rescued 30–40% of motoneurons at one week after sciatic axotomy. The efficiency of these proteins was similar to that of two neurotrophic factors, ciliary neurotrophic factor and brain‐derived neurotrophic factor, administrated by the same viral technique. The effect of the IAP proteins on motoneuron survival decreased with time but was still present after 4 weeks postaxotomy; the duration of the response was dependent upon the viral titre. These experiments demonstrate that IAP family proteins can prevent motoneuron cell death in vivo and may offer a new therapeutic approach for motoneuron diseases.