Apoptosis caused by p53-induced protein with death domain (PIDD) depends on the death adapter protein RAIDD

The PIDDosome, DNA-damage-induced apoptosis and beyond

P53-induced protein with a death domain (PIDD) was cloned as a death domain (DD)-containing protein whose expression is induced by p53. It was later described as the core of a molecular platform-activating caspase-2, named the PIDDosome. These first results pointed towards a role for PIDD in apoptosis, in response to DNA damage. Identification of new PIDDosome complexes involved in DNA repair and nuclear factor-κB signaling challenged this early concept. PIDD functions are growing as new complexes and new interaction partners are being discovered, and as additional functions are being revealed. A fascinating feature of PIDD lies within its complex and tight regulation mechanisms, which allow the molecule to fine-tune its different functions: from transcriptional regulation to the expression of different isoforms, and from the interaction with regulatory proteins to an ingenious post-translational cleavage mechanism generating various active fragments with specific functions. Further studies still need to be carried out to provide answers to many unresolved issues and to reconcile conflicting results. This review aims at providing an overview of the current PIDD knowledge status.

Caspase-2: the orphan caspase

Despite an abundance of literature on the role of caspase-2 in apoptosis, there exists much controversy about this protease making it difficult to place caspase-2 correctly in the apoptotic cascade, and hence its role in apoptosis remains unclear. The identification of the PIDDosome as a signaling platform for caspase-2 activation prompted intense investigation into the true role of this orphan caspase. What has emerged is the idea that caspase-2 may not be mandatory for apoptosis and that activation of this caspase in response to some forms of stress has other effects on the cell such as regulation of cell cycle progression. This idea is particularly relevent to the discovery that caspase-2 may act as a tumor suppressor. Here, we discuss the proposed mechanisms through which caspase-2 signals, in particular those involving PIDD, and their impact on cellular fate.

Identification of senescence-associated genes and their networks under oxidative stress by the analysis of Bach1

Cellular senescence is induced in response to DNA damage, caused by genotoxic stresses, including oxidative stress, and serves as a barrier against malignant transformation. Tumor-suppressor protein p53 induces genes critical for implementing cellular senescence. However, the identities of p53 target genes and other regulators that achieve senescence under oxidative stress remain to be elucidated. Effector genes for oxidative stress-induced cellular senescence were sought, based on the fact that transcription factor Bach1 inhibits this response by impeding the transcriptional activity of p53. pRb became hypophosphorylated more rapidly in Bach1-deficient MEFs than in wild-type cells, suggesting that pRb activation was involved in their senescence. Bach1-deficient MEFs bypassed the senescence state when the expression of a subset of p53 target genes, including p21, Pai1, Noxa, and Perp, was simultaneously reduced by using RNAi. Combined knockdown of p21 and pRb resulted in vigorous re-proliferation. These results suggest that oxidative stress-induced cellular senescence is registered by multiple p53 target genes, which arrest proliferation redundantly, in part by activating pRb. Our elucidations contrast with previous reports describing monopolistic regulations of senescence by single p53 target genes.

PIDD4, a novel PIDD isoform without the LRR domain, can independently induce cell apoptosis in cytoplasm

PIDD1 (P53-induced death domain) is a pro-apoptotic gene which can be induced by p53. So far, three alternative splicing products of human PIDD gene have been identified. Here we report a new splicing variant of this gene and named it PIDD4. The coding sequence of PIDD4 contains intron 3 and a 60 bp insert at the 5' of exon 3. Each insertion has an in-frame stop codon, which makes PIDD4 get translated from exon 5 then. Therefore, PIDD4 protein lacks the 32 KD N-terminal peptide, missing the LRR domain found in the other three isoforms. In this study, we have shown that the expression of PIDD4 is also regulated by p53, and as PIDD2, it is not expressed in heart either. Moreover, PIDD4 is the only isoform which is expressed in skeletal muscle. This isoform mainly localizes in the cytoplasm, and produces a relatively higher proportion of PIDD-CC fragment. Overexpression of PIDD4 independently promotes apoptosis.

Regulation of PIDD auto-proteolysis and activity by the molecular chaperone Hsp90

In response to DNA damage, p53-induced protein with a death domain (PIDD) forms a complex called the PIDDosome, which either consists of PIDD, RIP-associated protein with a death domain and caspase-2, forming a platform for the activation of caspase-2, or contains PIDD, RIP1 and NEMO, important for NF-κB activation. PIDDosome activation is dependent on auto-processing of PIDD at two different sites, generating the fragments PIDD-C and PIDD-CC. Despite constitutive cleavage, endogenous PIDD remains inactive. In this study, we screened for novel PIDD regulators and identified heat shock protein 90 (Hsp90) as a major effector in both PIDD protein maturation and activation. Hsp90, together with p23, binds PIDD and inhibition of Hsp90 activity with geldanamycin efficiently disrupts this association and impairs PIDD auto-processing. Consequently, both PIDD-mediated NF-κB and caspase-2 activation are abrogated. Interestingly, PIDDosome formation itself is associated with Hsp90 release. Characterisation of cytoplasmic and nuclear pools of PIDD showed that active PIDD accumulates in the nucleus and that only cytoplasmic PIDD is bound to Hsp90. Finally, heat shock induces Hsp90 release from PIDD and PIDD nuclear translocation. Thus, Hsp90 has a major role in controlling PIDD functional activity.

The role of caspase-2 in stress-induced apoptosis

Caspase-2 is the most evolutionarily conserved of all the caspases, yet it has a poorly defined role in apoptotic pathways. This is mainly due to a dearth of techniques to determine the activation status of caspase-2 and the lack of an abnormal phenotype in caspase-2 deficient mice. Nevertheless, emerging evidence suggests that caspase-2 may have important functions in a number of stress-induced cell death pathways, in cell cycle maintenance and regulation of tumour progression. This review discusses recent advances that have been made to help elucidate the true role of this elusive caspase and the potential contribution of caspase-2 to the pathology of human diseases including cancer.

DNA damage- and stress-induced apoptosis occurs independently of PIDD

The p53-induced protein with a death domain, PIDD, was identified as a p53 target gene whose main role is to execute apoptosis in a p53-dependent manner. To investigate the physiological role of PIDD in apoptosis, we generated PIDD-deficient mice. Here, we report that, although PIDD expression is inducible upon DNA damage, PIDD-deficient mice undergo apoptosis normally not only in response to DNA damage, but also in response to various p53-independent stress signals and to death receptor (DR) engagement. This indicates that PIDD is not required for DNA damage-, stress-, and DR-induced apoptosis. Also, in the absence of PIDD, both caspase-2 processing and activation occur in response to DNA damage. Our findings demonstrate that PIDD does not play an essential role for all p53-mediated or p53-independent apoptotic pathways.

Hexokinase II detachment from the mitochondria potentiates cisplatin induced cytotoxicity through a caspase-2 dependent mechanism

Cancer cells are frequently glycolytic and overexpress hexokinase II (HXK II). In cancer cells, the majority of hexokinase II is localized to the mitochondria through interaction with the voltage dependent anion channel (VDAC). Disruption in the binding of hexokinase II to the mitochondria has been shown to promote mitochondrial injury provoked by pro-apoptotic proteins. The present study demonstrates that cisplatin induces the PIDD (p53 induced protein with a death domain) dependent activation of caspase-2. In turn, caspase-2 cleaves and activates Bid, resulting in the oligomerization of Bak and the release of cytochrome c. Notably, the detachment of hexokinase II from the mitochondria markedly potentiates the onset of caspase-2 induced mitochondrial damage, thus resulting in a synergistic induction of cisplatin induced cytotoxicity.

Impaired TNFalpha-induced A20 expression in E1A/Ras-transformed cells

Background:

Tumour necrosis factor (TNF) is capable of activating the cell death pathway, and has been implicated in killing transformed cells. However, TNF also activates survival signals, including NF-κB activation and the subsequent expression of anti-apoptotic genes, leading to protection against TNF toxicity.

Methods:

In this study, we show that, although untransformed mouse embryonic fibroblasts (MEFs) were resistant to TNF killing, E1A/Ras-transformed MEFs were susceptible to extensive apoptosis induced by TNF. The key factors for determining TNF sensitivity were explored by comparing wild-type and E1A/Ras-transformed MEFs.

Results:

TNF signalling to NF-κB and to its target genes such as IκBα seemed to be mostly intact in E1A/Ras-transformed cells. Instead, the induction of A20 was completely abolished in E1A/Ras-transformed MEFs, although A20 is known to be NF-κB dependent. Reintroduction of A20 into E1A/Ras-transformed MEFs rescued these cells from TNF-induced death and reduced the formation of the FADD/caspase-8 complex. This impaired A20 induction in E1A/Ras MEFs was not because of the stabilisation of p53 or a defective TNF-induced p38 and Jun N-terminal kinase (JNK) signalling. Consistently, we found a reduced A20 promoter activity but normal NF-κB activity in TNF-treated E1A/Ras MEFs. However, Bcl-3 seemed to have a role in the transactivation of the A20 promoter in E1A/Ras cells.

Conclusions:

Our results suggest that specific inhibition of certain survival factors, such as A20, may determine the sensitivity to TNF-induced apoptosis in transformed cells such as E1A/Ras MEFs.

Caspase-2: controversial killer or checkpoint controller?

The caspases are an evolutionarily conserved family of cysteine proteases, with essential roles in apoptosis or inflammation. Caspase-2 was the second caspase to be cloned and it resembles the prototypical nematode caspase CED-3 more closely than any other mammalian protein. An absence of caspase-2-specific reagents and the subtle phenotype of caspase-2-deficient mice have hampered definition of the physiological role of caspase-2 and identification of factors regulating its activity. Although some data implicate caspase-2 in apoptotic pathways, a link with apoptosis has been less firmly established for caspase-2 than for some other caspases. Emerging evidence suggests that caspase-2 regulates the cell cycle and may act as a tumour suppressor. This article critically reviews the current state of knowledge regarding the biochemistry and biology of this controversial caspase.

Restraint of apoptosis during mitosis through interdomain phosphorylation of caspase-2

The apoptotic initiator caspase-2 has been implicated in oocyte death, in DNA damage- and heat shock-induced death, and in mitotic catastrophe. We show here that the mitosis-promoting kinase, cdk1-cyclin B1, suppresses apoptosis upstream of mitochondrial cytochrome c release by phosphorylating caspase-2 within an evolutionarily conserved sequence at Ser 340. Phosphorylation of this residue, situated in the caspase-2 interdomain, prevents caspase-2 activation. S340 was susceptible to phosphatase 1 dephosphorylation, and an interaction between phosphatase 1 and caspase-2 detected during interphase was lost in mitosis. Expression of S340A non-phosphorylatable caspase-2 abrogated mitotic suppression of caspase-2 and apoptosis in various settings, including oocytes induced to undergo cdk1-dependent maturation. Moreover, U2OS cells treated with nocodazole were found to undergo mitotic catastrophe more readily when endogenous caspase-2 was replaced with the S340A mutant to lift mitotic inhibition. These data demonstrate that for apoptotic stimuli transduced by caspase-2, cell death is prevented during mitosis through the inhibitory phosphorylation of caspase-2 and suggest that under conditions of mitotic arrest, cdk1-cyclin B1 activity must be overcome for apoptosis to occur.

Oxidative stress and mitochondrial dysfunction as determinants of ischemic neuronal death and survival

Mitochondria are the powerhouse of the cell. Their primary physiological function is to generate adenosine triphosphate through oxidative phosphorylation via the electron transport chain. Reactive oxygen species generated from mitochondria have been implicated in acute brain injuries such as stroke and neurodegeneration. Recent studies have shown that mitochondrially-formed oxidants are mediators of molecular signaling, which is implicated in the mitochondria-dependent apoptotic pathway that involves pro- and antiapoptotic protein binding, the release of cytochrome c, and transcription-independent p53 signaling, leading to neuronal death. Oxidative stress and the redox state of ischemic neurons are also implicated in the signaling pathway that involves phosphatidylinositol 3-kinase/Akt and downstream signaling, which lead to neuronal survival. Genetically modified mice or rats that over-express or are deficient in superoxide dismutase have provided strong evidence in support of the role of mitochondrial dysfunction and oxidative stress as determinants of neuronal death/survival after stroke and neurodegeneration.

Caspase-2 activation in the absence of PIDDosome formation

PIDD (p53-induced protein with a death domain [DD]), together with the bipartite adapter protein RAIDD (receptor-interacting protein-associated ICH-1/CED-3 homologous protein with a DD), is implicated in the activation of pro–caspase-2 in a high molecular weight complex called the PIDDosome during apoptosis induction after DNA damage. To investigate the role of PIDD in cell death initiation, we generated PIDD-deficient mice. Processing of caspase-2 is readily detected in the absence of PIDDosome formation in primary lymphocytes. Although caspase-2 processing is delayed in simian virus 40–immortalized pidd^−/− mouse embryonic fibroblasts, it still depends on loss of mitochondrial integrity and effector caspase activation. Consistently, apoptosis occurs normally in all cell types analyzed, suggesting alternative biological roles for caspase-2 after DNA damage. Because loss of either PIDD or its adapter molecule RAIDD did not affect subcellular localization, nuclear translocation, or caspase-2 activation in high molecular weight complexes, we suggest that at least one alternative PIDDosome-independent mechanism of caspase-2 activation exists in mammals in response to DNA damage.

DISC-mediated activation of caspase-2 in DNA damage-induced apoptosis

The tumor suppressor p53 protein supports growth arrest and is able to induce apoptosis, a signaling cascade regulated by sequential activation of caspases. Mechanisms that lead from p53 to activation of individual initiator caspases are still unclear. The present model for caspase-2 activation includes PIDDosome complex formation. However, in certain experimental models, elimination of complex constituents PIDD or RAIDD did not significantly influence caspase-2 activation, suggesting the existence of an alternative activation platform for caspase-2. Here we have investigated the link between p53 and caspase-2 in further detail and report that the latter is able to utilize the CD95 DISC as an activation platform. The recruitment of caspase-8 to this complex is required for activation of caspase-2. In the experimental system used, the DISC is formed through a distinct, p53-dependent upregulation of CD95. Moreover, we show that caspase-2 and -8 cleave Bid, and that both act simultaneously upstream of mitochondrial cytochrome c release. Finally, a direct interaction between the two caspases and the ability of caspase-8 to cleave caspase-2 are demonstrated. Thus, the observed functional link between caspase-8 and -2 within the DISC represents an alternative mechanism to the PIDDosome for caspase-2 activation in response to DNA damage.

The enigma of caspase-2: the laymen's view

Proteolysis of cellular substrates by caspases (cysteine-dependent aspartate-specific proteases) is one of the hallmarks of apoptotic cell death. Although the activation of apoptotic caspases is considered a 'late-stage' event in apoptosis signaling, past the commitment stage, one caspase family member, caspase-2, splits the cell death community into half - those searching for evidence of an apical initiator function of this molecule and those considering it as an amplifier of the apoptotic caspase cascade, at best, if relevant for apoptosis at all. This review screens past and present biochemical as well as genetic evidence for caspase-2 function in cell death signaling and beyond.

Mechanisms of neuronal death in disease: defining the models and the players

Dysregulation of life and death at the cellular level leads to a variety of diseases. In the nervous system, aberrant neuronal death is an outstanding feature of neurodegenerative diseases. Since the discovery of the caspase family of proteases, much effort has been made to determine how caspases function in disease, including neurodegenerative diseases. Although many papers have been published examining caspases in neuronal death and disease, the pathways have not been fully clarified. In the present review, we examine the potential players in the death pathways, the current tools for examining these players and the models for studying neurological disease. Alzheimer's disease, the most common neurodegenerative disorder, and cerebral ischaemia, the most common cause of neurological death, are used to illustrate our current understanding of death signalling in neurodegenerative diseases. A better understanding of the neuronal death pathways would provide targets for the development of therapeutic interventions for these diseases.

The PIDDosome mediates delayed death of hippocampal CA1 neurons after transient global cerebral ischemia in rats

A brief period of global brain ischemia, such as that induced by cardiac arrest or cardiopulmonary bypass surgery, causes cell death in vulnerable hippocampal CA1 pyramidal neurons days after reperfusion. Although numerous factors have been suggested to account for this phenomenon, the mechanisms underlying it are poorly understood. We describe a cell death signal called the PIDDosome, a protein complex of p53-induced protein with a death domain (PIDD), receptor-interacting protein-associated ICH-1/CED-3 homologous protein with a death domain (RAIDD), and procaspase-2. We induced 5 min of transient global cerebral ischemia (tGCI) using bilateral common carotid artery occlusion with hypotension. Western blot analysis showed that expression of twice-cleaved fragment of PIDD (PIDD-CC) increased in the cytosolic fraction of the hippocampal CA1 subregion and preceded procaspase-2 activation after tGCI. Caspase-2 cleaved Bid in brain homogenates. Co-immunoprecipitation and immunofluorescent studies demonstrated that PIDD-CC, RAIDD, and procaspase-2 were co-localized and bound directly, which indicates the formation of the PIDD death domain complex. Furthermore, we tested inhibition of PIDD expression by using small interfering RNA (siRNA) treatment that was initiated 48 h before tGCI. Administration of siRNA against PIDD decreased not only expression of PIDD-CC, but also activation of procaspase-2 and Bid, resulting in a decrease in histological neuronal damage and DNA fragmentation in the hippocampal CA1 subregion after tGCI. These results imply that PIDD plays an important role in procaspase-2 activation and delayed CA1 neuronal death after tGCI. We propose that PIDD is a hypothetical molecular target for therapy against neuronal death after tGCI.

Advances in regulation mechanism of hepatocyte apoptosis in nonalcoholic fatty liver disease

The pathogenic mechanism of nonalcoholic fatty liver disease (NAFLD) still remains unclear. In recent years, many studies indicate that abnormal hepatocyte apoptosis exists in NAFLD, confirming the close relationship between NAFLD and hepatocyte apoptosis. The regulation of cell apoptosis includes two: positive or negative. In this paper, we review the research advances in the regulation of hepatocyte apoptosis during the pathogenesis of NAFLD.

No death without life: vital functions of apoptotic effectors

As a result of the genetic experiments performed in Caenorhabditis elegans, it has been tacitly assumed that the core proteins of the 'apoptotic machinery' (CED-3, -4, -9 and EGL-1) would be solely involved in cell death regulation/execution and would not exert any functions outside of the cell death realm. However, multiple studies indicate that the mammalian orthologs of these C. elegans proteins (i.e. caspases, Apaf-1 and multidomain proteins of the Bcl-2 family) participate in cell death-unrelated processes. Similarly, loss-of-function mutations of ced-4 compromise the mitotic arrest of DNA-damaged germline cells from adult nematodes, even in a context in which the apoptotic machinery is inoperative (for instance due to mutations of egl-1 or ced-3). Moreover, EGL-1 is required for the activation of autophagy in starved nematodes. Finally, the depletion of caspase-independent death effectors, such as apoptosis-inducing factor (AIF) and endonuclease G, provokes cell death-independent consequences, both in mammals and in yeast (Saccharomyces cerevisiae). These results corroborate the conjecture that any kind of protein that has previously been specifically implicated in apoptosis might have a phylogenetically conserved apoptosis-unrelated function, most likely as part of an adaptive response to cellular stress.

A role for caspase 2 and PIDD in the process of p53-mediated apoptosis

When treated with some DNA-damaging agents, human tumor-derived H1299 cells expressing inducible versions of wild-type or mutant p53 with inactive transactivation domain I (p53(Q22/S23)) undergo apoptosis as evidenced by cytochrome c release, nuclear fragmentation, and sub-G1 DNA content. Apoptosis induced by p53(Q22/S23) is relatively slow, however, and key downstream effector caspases are not activated. Nevertheless, with either version of p53, caspase 2 activation is required for release of cytochrome c and cell death. Remarkably, although p53(Q22/S23) is known to be defective in transcriptional activation of numerous p53 target genes, it can induce expression of proapoptotic targets including PIDD and AIP1 at least to the same extent as wild-type p53. Furthermore, RNAi silencing of PIDD, previously shown to be required for caspase 2 activation, suppresses apoptosis by both wild-type p53 and p53(Q22/S23). Thus, the initial stage of DNA damage-facilitated, p53-mediated apoptosis occurs by a PIDD- and caspase 2-dependent mechanism, and p53's full transcriptional regulatory functions may be required only for events that are downstream of cytochrome c release.

HSP110, caspase-3 and -9 expression in physiological apoptosis and apoptosis induced by in vivo embryonic exposition to all-trans retinoic acid or irradiation during early mouse eye development

Apoptosis is an essential physiological process in embryonic development. In the developing eye of vertebrates, three periods of developmental apoptosis can be distinguished: early, intermediate and later. Within the apoptosis pathway, caspases play a crucial role. It has also been shown that HSP110 may have a potential role in apoptosis. The aim of this research was to study the expression of HSP110, caspase-3 and -9 in physiological, retinoic- or irradiation-induced apoptosis during early eye development. Seven pregnant C57Bl/6J mice received 80 mg kg(-1) of all-trans retinoic acid mixed with sesame oil. Seven pregnant NMRI mice received 2 Gy irradiation at the same gestational day. Control mice of both strains (seven mice of each) were not submitted to any treatment. Embryos were harvested at 3, 6, 12 and 24 h after exposition, fixed, dehydrated and embedded. Coronal sections (5 microm) were made. Slide staining occurred alternatively using anti-caspase-3, anti-caspase-9 and anti-HSP110 immunohistochemistry. HSP110 and caspase-3 expression presented similar topographic and chronological patterns, whereas expression of HSP110 was more precocious in retinoic acid-treated embryos. After retinoic exposure, caspase-3- and HSP110-positive cells were increased in the region of the optic vesicle. By contrast, after irradiation, caspase-3- and HSP110-positive cells were noticeably increased in the optic vesicle, peri-optical mesoderm but less in lens placode. HSP110 was expressed before caspase-3. By contrast, caspase-9 was expressed by a very small number of cells in the optic vesicle either under physiological or under teratogenic conditions. Thus, it seems that activation of caspase-9 is dispensable in early eye developmental apoptosis.

Requirement of Apaf-1 for mitochondrial events and the cleavage or activation of all procaspases during genotoxic stress-induced apoptosis

Sequential activation of caspases is critical for the execution of apoptosis. Recent evidence suggests caspase 2 is a significant upstream caspase capable of initiating mitochondrial events, such as the release of cytochrome c. In particular, in vitro studies using recombinant proteins have shown that cleaved caspase 2 can induce mitochondrial outer membrane permeabilization directly or by cleaving the BH3-only protein BID (BH3 interacting domain death agonist). However, whether interchain cleavage or activation of procaspase 2 occurs prior to Apaf-1-mediated procaspase 9 activation under more natural conditions remains unresolved. In the present study, we show that Apaf-1-deficient Jurkat T-lymphocytes and mouse embryonic fibroblasts were highly resistant to DNA-damage-induced apoptosis and failed to cleave or activate any apoptotic procaspase, including caspase 2. Significantly, drug-induced cytochrome c release and loss of mitochondrial membrane potential were inhibited in cells lacking Apaf-1. By comparison, procaspase proteolysis and apoptosis were only delayed slightly in Apaf-1-deficient Jurkat cells upon treatment with anti-Fas antibody. Our data support a model in which Apaf-1 is necessary for the cleavage or activation of all procaspases and the promotion of mitochondrial apoptotic events induced by genotoxic drugs.

The death domain superfamily in intracellular signaling of apoptosis and inflammation

The death domain (DD) superfamily comprising the death domain (DD) subfamily, the death effector domain (DED) subfamily, the caspase recruitment domain (CARD) subfamily, and the pyrin domain (PYD) subfamily is one of the largest domain superfamilies. By mediating homotypic interactions within each domain subfamily, these proteins play important roles in the assembly and activation of apoptotic and inflammatory complexes. In this chapter, we review the molecular complexes assembled by these proteins, the structural and biochemical features of these domains, and the molecular interactions mediated by them. By analyzing the potential molecular basis for the function of these domains, we hope to provide a comprehensive understanding of the function, structure, interaction, and evolution of this important family of domains.

The expression of p53-induced protein with death domain (Pidd) and apoptosis in oral squamous cell carcinoma

The Pidd (p53-induced protein with death domain) gene was shown to be induced by the tumour suppressor p53 and to mediate p53-dependent apoptosis in mouse and human cells, through interactions with components of both the mitochondrial and the death receptor signalling pathways. To study the role of Pidd in clinical tumours, we measured its expression by quantitative reverse transcription-PCR in microdissected oral squamous cell carcinomas (OSCC) with and without p53 mutation. Tumour cell apoptosis was assessed by in situ terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling. Tumour proliferation was assessed by immunohistochemical staining for the Ki-67 antigen. We found a wide range of Pidd expression among OSCC. Statistical analysis revealed an association between Pidd expression and apoptotic index (Mann-Whitney test, P<0.001), consistent with a role of Pidd in apoptosis in this tumour type. Furthermore, we showed a positive correlation between apoptotic index and proliferative index that has not been previously described for OSCC. There was no correlation between Pidd expression and the p53 mutation status of these tumours, suggesting that Pidd expression may be regulated by p53-independent mechanisms. Further characterisation of these molecular defects in the control of proliferation and apoptosis should help in developing treatments that target OSCC according to their biological properties.

The role of connexins in controlling cell growth and gene expression

The purpose of this paper is to provide a brief overview of current thinking on the role of connexins, in particular Cx43, in growth regulation, and a more detailed discussion as to potential mechanisms involved with an emphasis on gene expression. While the precise molecular mechanism by which connexins can affect the growth of normal or tumor cells remains elusive, a number of exciting reports have expanded our understanding and are presented in some detail. Thus, we will discuss (Section 2): the role of protein-protein interactions in integrating connexins into multiple signal transduction pathways; phosphorylation at specific sites and reversal of growth inhibition; the role of the carboxy-terminal regulatory domain as a signaling molecule. Some of our latest work on the potential functions of endogenously produced carboxy-terminal fragments of Cx43 are also presented (Section 3). Finally, Section 4 will pay tribute to the rapidly emerging realization that connexins such as Cx43 and Cx32 exert important and extensive effects on gene expression, particularly those genes linked to growth regulation.

Commuting (to) suicide: an update on nucleocytoplasmic transport in apoptosis

Commuting is the process of travelling between a place of residence and a place of work. In the context of biology, this expression evokes the continuous movement of macromolecules between different compartments of a eukaryotic cell. Transport in and out of the nucleus is a major example of intracellular commuting. This article discusses recent findings that substantiate the emerging link between nucleocytoplasmic transport and the signalling and execution of cell death.

Mitochondrial membrane permeabilization in cell death

Irrespective of the morphological features of end-stage cell death (that may be apoptotic, necrotic, autophagic, or mitotic), mitochondrial membrane permeabilization (MMP) is frequently the decisive event that delimits the frontier between survival and death. Thus mitochondrial membranes constitute the battleground on which opposing signals combat to seal the cell's fate. Local players that determine the propensity to MMP include the pro- and antiapoptotic members of the Bcl-2 family, proteins from the mitochondrialpermeability transition pore complex, as well as a plethora of interacting partners including mitochondrial lipids. Intermediate metabolites, redox processes, sphingolipids, ion gradients, transcription factors, as well as kinases and phosphatases link lethal and vital signals emanating from distinct subcellular compartments to mitochondria. Thus mitochondria integrate a variety of proapoptotic signals. Once MMP has been induced, it causes the release of catabolic hydrolases and activators of such enzymes (including those of caspases) from mitochondria. These catabolic enzymes as well as the cessation of the bioenergetic and redox functions of mitochondria finally lead to cell death, meaning that mitochondria coordinate the late stage of cellular demise. Pathological cell death induced by ischemia/reperfusion, intoxication with xenobiotics, neurodegenerative diseases, or viral infection also relies on MMP as a critical event. The inhibition of MMP constitutes an important strategy for the pharmaceutical prevention of unwarranted cell death. Conversely, induction of MMP in tumor cells constitutes the goal of anticancer chemotherapy.

Autoproteolysis of PIDD marks the bifurcation between pro-death caspase-2 and pro-survival NF-kappaB pathway

Upon DNA damage, a complex called the PIDDosome is formed and either signals NF-kappaB activation and thus cell survival or alternatively triggers caspase-2 activation and apoptosis. PIDD (p53-induced protein with a death domain) is constitutively processed giving rise to a 48-kDa N-terminal fragment containing the leucine-rich repeats (LRRs, PIDD-N) and a 51-kDa C-terminal fragment containing the death domain (DD, PIDD-C). The latter undergoes further cleavage resulting in a 37-kDa fragment (PIDD-CC). Here we show that processing occurs at S446 (generating PIDD-C) and S588 (generating PIDD-CC) by an auto-processing mechanism similar to that found in the nuclear pore protein Nup98/96 and inteins. Auto-cleavage of PIDD determines the outcome of the downstream signaling events. Whereas initially formed PIDD-C mediates the activation of NF-kappaB via the recruitment of RIP1 and NEMO, subsequent formation of PIDD-CC causes caspase-2 activation and thus cell death. A non-cleavable PIDD mutant is unable to translocate from the cytoplasm to the nucleus and loses both activities. In this way, auto-proteolysis of PIDD might participate in the orchestration of the DNA damage-induced life and death signaling pathways.

Upon intracellular processing, the C-terminal death domain-containing fragment of the p53-inducible PIDD/LRDD protein translocates to the nucleoli and interacts with nucleolin

The p53-inducible and death domain-containing PIDD/LRDD protein has been described as an adaptor protein, which forms large protein complexes with RAIDD, another death domain-containing protein, leading to recruitment, and activation of the initiator caspase-2, and p53-mediated apoptosis. Here, we describe in further detail the proteolytic processing of PIDD/LRDD that occurs in healthy cells before induction of apoptosis. We could demonstrate that the C-terminal fragment containing the PIDD death domain shuttles into the nucleoli. This translocation is mediated by or leads to the interaction of the PIDD death domain with nucleolin, a protein important for rRNA processing within nucleoli and possibly involved in the DNA damage response. Ectopically expressed LRDD and endogenous nucleolin co-localized within the nucleoli, and overexpression of both full-length LRDD and the LRDD death domain sensitized cells for UV-induced apoptosis. When expressed alone, the PIDD/LRDD death domain tended to form large filamentous structures resembling so-called death filaments. The functional consequences of the identified PIDD/nucleolin interaction remain to be elucidated, but may be related to a recently discovered new role for PIDD in the activation of NF-kappaB upon genotoxic stress.

Disruption of the endoplasmic reticulum and increases in cytoplasmic calcium are early events in cell death induced by the natural triterpenoid Asiatic acid

Triterpenoids are a novel class of compounds being investigated as potential therapeutic agents for the treatment of prostate cancer and other malignancies. Asiatic acid (AA) is a member of the ursane family of triterpenoids and has anticancer activity, but its mechanism of action is not completely understood. To investigate its mechanism of action, PPC-1 prostate cancer cells were treated with AA at increasing concentrations and times. AA induced rapid caspase-dependent and independent cell death that peaked within 8 h of treatment. AA-induced death was associated with early activation of caspases 2, 3, and 8, but not caspase 9. Within 2.5 h of treatment, release of calcium from intracellular stores and dilatation of the endoplasmic reticulum was observed. Thus, disruption of the endoplasmic reticulum and alterations in calcium homeostasis are early events in AA-induced death.

Heat Shock Induces Apoptosis Independently of Any Known Initiator Caspase-activating Complex

Adaptive responses to mild heat shock are among the most widely conserved and studied in nature. More intense heat shock, however, induces apoptosis through mechanisms that remain largely unknown. Herein, we present evidence that heat shock activates an apical protease that stimulates mitochondrial outer membrane permeabilization and processing of the effector caspase-3 in a benzyloxycarbonyl-VAD-fluoromethyl ketone (polycaspase inhibitor)- and Bcl-2-inhibitable manner. Surprisingly, however, neither FADD.caspase-8 nor RAIDD.caspase-2 PIDDosome (p53-induced protein with a death domain) complexes were detected in dying cells, and neither of these initiator caspases nor the endoplasmic reticulum stress-activated caspases-4/12 were required for mitochondrial outer membrane permeabilization. Similarly, although cytochrome c was released from mitochondria following heat shock, functional Apaf-1.caspase-9 apoptosome complexes were not formed, and caspase-9 was not essential for the activation of caspase-3 or the induction of apoptosis. Thus, heat shock does not require any of the known initiator caspases or their activating complexes to promote apoptotic cell death but instead relies upon the activation of an apparently novel apical protease with caspase-like activity.

Functional connection between p53 and caspase-2 is essential for apoptosis induced by DNA damage

Recent findings have established caspase-2 as an important apical regulator in apoptotic pathways leading from DNA damage to release of mitochondrial cytochrome c and subsequent activation of effector caspases. Yet, the molecular map connecting the embarking stimuli of genotoxic stress with caspase-2 activation remains to be elucidated. Here, we address the question of potential caspase-2 regulators by examining 5-fluorouracil (5-FU)-induced apoptosis in wild-type and p53-deficient human colon carcinoma cells. Apoptosis was observed only in p53(+/+) cells and was preceded by caspase-2 activation. Hence, although no direct interaction between p53 and caspase-2 was observed in the cell system used, our data clearly demonstrate that a functional connection between these two proteins is essential for initiation of the 5-FU-induced apoptotic process. Proposed mediators of caspase-2 activation include PIDDosome complex proteins PIDD and RAIDD. Surprisingly, the presence of a complex encompassing at least RAIDD, PIDD and caspase-2 was verified in both p53(+/+) and p53(-/-) cells, also in the absence of 5-FU treatment. Thus, our results confirm the participation of PIDD and RAIDD in PIDDosome complex formation but question their role as sole mediators of caspase-2 activation. This assumption was further supported by siRNA transfections targeting PIDD or RAIDD. In conclusion, our findings support the hypothesis of p53 as an upstream regulator of caspase activity and provide data concerning caspase-2 processing mechanisms. As suppression of caspase-2 expression in 5-FU-treated cells also affects the level of the p53 protein, possibilities of a reciprocal interaction between these proteins are discussed.

Dissecting p53-dependent apoptosis

The complexity of the p53 protein, coupled with the vast cellular responses to p53, is simply astonishing. As new isoforms, functional domains and protein-protein interactions are described; each morsel of information forces us to think (and re-think) about how it 'fits' into the current p53 paradigm. One aspect of p53 signaling that is under refinement is the mechanism(s) leading to apoptosis. Here we discuss what is known about p53-induced apoptosis, what proteins and protein-protein interactions are responsible for regulating apoptosis, how can this cascade be genetically dissected, and what pharmacological tools are available to modulate p53-dependent apoptosis. While everything may not comfortably fit into our understanding of p53, all of these data will certainly broaden our viewpoint on the complexity and significance of the p53-induced apoptotic pathway. Here, our discussion is primarily focused on the works presented at the 12th International p53 Workshop, except where appropriate background is required.

In situ trapping of activated initiator caspases reveals a role for caspase-2 in heat shock-induced apoptosis

Activation of 'initiator' (or 'apical') caspases-2, -8 or -9 (refs 1-3) is crucial for induction of apoptosis. These caspases function to activate executioner caspapses that, in turn, orchestrate apoptotic cell death. Here, we show that a cell-permeable, biotinylated pan-caspase inhibitor (bVAD-fmk) both inhibited and 'trapped' the apical caspase activated when apoptosis was triggered. As expected, only caspase-8 was trapped in response to ligation of death receptors, whereas only caspase-9 was trapped in response to a variety of other apoptosis-inducing agents. Caspase-2 was exclusively activated in heat shock-induced apoptosis. This activation of caspase-2 was also observed in cells protected from heat-shock-induced apoptosis by Bcl-2 or Bcl-xL. Reduced sensitivity to heat-shock-induced death was observed in caspase-2(-/-) cells. Furthermore, cells lacking the adapter molecule RAIDD failed to activate caspase-2 after heat shock treatment and showed resistance to apoptosis in this setting. This approach unambiguously identifies the apical caspase activated in response to apoptotic stimuli, and establishes caspase-2 as a proximal mediator of heat shock-induced apoptosis.