Tumor-suppressing function of caspase-2 requires catalytic site Cys-320 and site Ser-139 in mice

Caspase-2 protects against ferroptotic cell death

Caspase-2, one of the most evolutionarily conserved members of the caspase family, is an important regulator of the cellular response to oxidative stress. Given that ferroptosis is suppressed by antioxidant defense pathways, such as that involving selenoenzyme glutathione peroxidase 4 (GPX4), we hypothesized that caspase-2 may play a role in regulating ferroptosis. This study provides the first demonstration of an important and unprecedented function of caspase-2 in protecting cancer cells from undergoing ferroptotic cell death. Specifically, we show that depletion of caspase-2 leads to the downregulation of stress response genes including SESN2, HMOX1, SLC7A11, and sensitizes mutant-p53 cancer cells to cell death induced by various ferroptosis-inducing compounds. Importantly, the canonical catalytic activity of caspase-2 is not required for its role and suggests that caspase-2 regulates ferroptosis via non-proteolytic interaction with other proteins. Using an unbiased BioID proteomics screen, we identified novel caspase-2 interacting proteins (including heat shock proteins and co-chaperones) that regulate cellular responses to stress. Finally, we demonstrate that caspase-2 limits chaperone-mediated autophagic degradation of GPX4 to promote the survival of mutant-p53 cancer cells. In conclusion, we document a novel role for caspase-2 as a negative regulator of ferroptosis in cells with mutant p53. Our results provide evidence for a novel function of caspase-2 in cell death regulation and open potential new avenues to exploit ferroptosis in cancer therapy.

Unveiling caspase-2 regulation by non-coding RNAs

Non-coding RNAs (ncRNAs) are a group of RNA molecules, such as small nucleolar RNAs, circular RNAs (circRNAs), microRNAs (miRNAs) and long-noncoding RNAs (ncRNAs), that do not encode proteins. Although their biofunctions are not well-understood, many regulatory ncRNAs appear to be highly involved in regulating the transcription and translation of several genes that have essential biological roles including cell differentiation, cell death, metabolism, tumorigenesis and so on. A growing number of studies have revealed the associations between dysregulated ncRNAs and caspases involved in cell death in numerous human diseases. As one of the initiator and executor caspases, caspase-2 is the most evolutionally conserved caspase in mammals, exerting both apoptotic and non-apoptotic functions. A great deal of studies has shown the involvement of caspase-2 as a tumor suppressor in multiple oncogene-driven cancers, and yet a comprehensive understanding of its biological roles remains largely unknown. In this review, we highlight a compilation of studies focused on the interaction between caspase-2 and miRNAs/lncRNAs in the context of different diseases in order to deepen our knowledge on the regulatory biofunctions of caspase-2 and, furthermore, provide more insight into understanding the role that ncRNAs/caspase-2 axis plays in the development of human diseases.

The caspase-2 substrate p54nrb exhibits a multifaceted role in tumor cell death susceptibility via gene regulatory functions

Caspase-2 represents an evolutionary conserved caspase, which plays a role in genotoxic stress-induced apoptosis, ageing-related metabolic changes, and in deleting aneuploid cells in tumors. Genetic deletion of caspase-2 leads to increased tumor susceptibility in vivo. The exact downstream signaling mechanism by which caspase-2 accomplishes its specific tumor suppressor functions is not clear. Caspase-2, uniquely among caspases, resides in the nucleus and other cellular compartments. In this study, we identify a nuclear caspase-2 specific substrate, p54nrb, which is selectively cleaved by caspase-2 at D422, leading to disruption of the C-terminal site, the putative DNA binding region of the protein. P54nrb is an RNA and DNA binding protein, which plays a role in RNA editing, transport, and transcriptional regulation of genes. Overexpression of p54nrb is observed in several human tumor types, such as cervix adenocarcinoma, melanoma, and colon carcinoma. In contrast, the loss of p54nrb in tumor cell lines leads to increased cell death susceptibility and striking decrease in tumorigenic potential. By employing high resolution quantitative proteomics, we demonstrate that the loss/cleavage of p54nrb results in altered expression of oncogenic genes, among which the downregulation of the tumorigenic protease cathepsin-Z and the anti-apoptotic gelsolin can be detected universally across three tumor cell types, including adenocarcinoma, melanoma and colon carcinoma. Finally, we demonstrate that p54nrb interacts with cathepsin-Z and gelsolin DNA, but not RNA. Taken together, this study uncovers a so far not understood mechanism of caspase-2 tumor suppressor function in human tumor cells.

Caspase-2 regulates S-phase cell cycle events to protect from DNA damage accumulation independent of apoptosis

In addition to its classical role in apoptosis, accumulating evidence suggests that caspase-2 has non-apoptotic functions, including regulation of cell division. Loss of caspase-2 is known to increase proliferation rates but how caspase-2 is regulating this process is currently unclear. We show that caspase-2 is activated in dividing cells in G1-phase of the cell cycle. In the absence of caspase-2, cells exhibit numerous S-phase defects including delayed exit from S-phase, defects in repair of chromosomal aberrations during S-phase, and increased DNA damage following S-phase arrest. In addition, caspase-2-deficient cells have a higher frequency of stalled replication forks, decreased DNA fiber length, and impeded progression of DNA replication tracts. This indicates that caspase-2 protects from replication stress and promotes replication fork protection to maintain genomic stability. These functions are independent of the pro-apoptotic function of caspase-2 because blocking caspase-2-induced cell death had no effect on cell division, DNA damage-induced cell cycle arrest, or DNA damage. Thus, our data supports a model where caspase-2 regulates cell cycle and DNA repair events to protect from the accumulation of DNA damage independently of its pro-apoptotic function.

Caspase-2 Substrates: To Apoptosis, Cell Cycle Control, and Beyond

Caspase-2 belongs to the caspase family of proteins responsible for essential cellular functions including apoptosis and inflammation. Uniquely, caspase-2 has been identified as a tumor suppressor, but how it regulates this function is still unknown. For many years, caspase-2 has been considered an “orphan” caspase because, although it is able to induce apoptosis, there is an abundance of conflicting evidence that questions its necessity for apoptosis. Recent evidence supports that caspase-2 has non-apoptotic functions in the cell cycle and protection from genomic instability. It is unclear how caspase-2 regulates these opposing functions, which has made the mechanism of tumor suppression by caspase-2 difficult to determine. As a protease, caspase-2 likely exerts its functions by proteolytic cleavage of cellular substrates. This review highlights the known substrates of caspase-2 with a special focus on their functional relevance to caspase-2’s role as a tumor suppressor.

The Role of Caspase-2 in Regulating Cell Fate

Caspase-2 is the most evolutionarily conserved member of the mammalian caspase family and has been implicated in both apoptotic and non-apoptotic signaling pathways, including tumor suppression, cell cycle regulation, and DNA repair. A myriad of signaling molecules is associated with the tight regulation of caspase-2 to mediate multiple cellular processes far beyond apoptotic cell death. This review provides a comprehensive overview of the literature pertaining to possible sophisticated molecular mechanisms underlying the multifaceted process of caspase-2 activation and to highlight its interplay between factors that promote or suppress apoptosis in a complicated regulatory network that determines the fate of a cell from its birth and throughout its life.

Targeting apoptotic caspases in cancer

Members of the caspase family of proteases play essential roles in the initiation and execution of apoptosis. These caspases are divided into two groups: the initiator caspases (caspase-2, -8, -9 and -10), which are the first to be activated in response to a signal, and the executioner caspases (caspase-3, -6, and -7) that carry out the demolition phase of apoptosis. Many conventional cancer therapies induce apoptosis to remove the cancer cell by engaging these caspases indirectly. Newer therapeutic applications have been designed, including those that specifically activate individual caspases using gene therapy approaches and small molecules that repress natural inhibitors of caspases already present in the cell. For such approaches to have maximal clinical efficacy, emerging insights into non-apoptotic roles of these caspases need to be considered. This review will discuss the roles of caspases as safeguards against cancer in the context of the advantages and potential limitations of targeting apoptotic caspases for the treatment of cancer.

Differential roles of protease isoforms in the tumor microenvironment

Alternative splicing of precursor mRNA is a key mediator of gene expression regulation leading to greater diversity of the proteome in complex organisms. Systematic sequencing of the human genome and transcriptome has led to our understanding of how alternative splicing of critical genes leads to multiple pathological conditions such as cancer. For many years, proteases were known only for their roles as proteolytic enzymes, acting to regulate/process proteins associated with diverse cellular functions. However, the differential expression and altered function of various protease isoforms, such as (i) anti-apoptotic activities, (ii) mediating intercellular adhesion, and (iii) modifying the extracellular matrix, are evidence of their specific contribution towards shaping the tumor microenvironment. Revealing the alternative splicing of protease genes and characterization of their protein products/isoforms with distinct and opposing functions creates a platform to understand how protease isoforms contribute to specific cancer hallmarks. Here, in this review, we address cancer-specific isoforms produced by the alternative splicing of proteases and their distinctive roles in the tumor microenvironment.

Identification of TRIM25 as a Negative Regulator of Caspase-2 Expression Reveals a Novel Target for Sensitizing Colon Carcinoma Cells to Intrinsic Apoptosis

Colorectal cancer (CRC) is one of the most common cancers that is characterized by a high mortality due to the strong metastatic potential of the primary tumor and the high rate of therapy resistance. Hereby, evasion of apoptosis is the primary underlying cause of reduced sensitivity of tumor cells to chemo- and radiotherapy. Using RNA affinity chromatography, we identified the tripartite motif-containing protein 25 (TRIM25) as a bona fide caspase-2 mRNA-binding protein in colon carcinoma cells. Loss-of-function and gain-of-function approaches revealed that TRIM25 attenuates the protein levels of caspase-2 without significantly affecting caspase-2 mRNA levels. In addition, experiments with cycloheximide revealed that TRIM25 does not affect the protein stability of caspase-2. Furthermore, silencing of TRIM25 induced a significant redistribution of caspase-2 transcripts from RNP particles to translational active polysomes, indicating that TRIM25 negatively interferes with caspase-2 translation. Functionally, the elevation in caspase-2 upon TRIM25 depletion significantly increased the sensitivity of colorectal cells to drug-induced intrinsic apoptosis as implicated by increased caspase-3 cleavage and cytochrome c release. Importantly, the apoptosis-sensitizing effects by transient TRIM25 knockdown were rescued by concomitant silencing of caspase-2, demonstrating a critical role of caspase-2. Inhibition of caspase-2 by TRIM25 implies a survival mechanism that critically contributes to chemotherapeutic drug resistance in CRC.

Inhibition of Caspase-2 Translation by the mRNA Binding Protein HuR: A Novel Path of Therapy Resistance in Colon Carcinoma Cells?

An increased expression and cytoplasmic abundance of the ubiquitous RNA binding protein human antigen R (HuR) is critically implicated in the dysregulated control of post- transcriptional gene expression during colorectal cancer development and is frequently associated with a high grade of malignancy and therapy resistance. Regardless of the fact that HuR elicits a broad cell survival program by increasing the stability of mRNAs coding for prominent anti-apoptotic factors, recent data suggest that HuR is critically involved in the regulation of translation, particularly, in the internal ribosome entry site (IRES) controlled translation of cell death regulatory proteins. Accordingly, data from human colon carcinoma cells revealed that HuR maintains constitutively reduced protein and activity levels of caspase-2 through negative interference with IRES-mediated translation. This review covers recent advances in the understanding of mechanisms underlying HuR's modulatory activity on IRES-triggered translation. With respect to the unique regulatory features of caspase-2 and its multiple roles (e.g., in DNA-damage-induced apoptosis, cell cycle regulation and maintenance of genomic stability), the pathophysiological consequences of negative caspase-2 regulation by HuR and its impact on therapy resistance of colorectal cancers will be discussed in detail. The negative HuR-caspase-2 axis may offer a novel target for tumor sensitizing therapies.

Transcriptome profiling of caspase-2 deficient EμMyc and Th-MYCN mouse tumors identifies distinct putative roles for caspase-2 in neuronal differentiation and immune signaling

Caspase-2 is a highly conserved cysteine protease with roles in apoptosis and tumor suppression. Our recent findings have also demonstrated that the tumor suppression function of caspase-2 is context specific. In particular, while caspase-2 deficiency augments lymphoma development in the EμMyc mouse model, it leads to delayed neuroblastoma development in Th-MYCN mice. However, it is unclear how caspase-2 mediates these differential outcomes. Here we utilized RNA sequencing to define the transcriptomic changes caused by caspase-2 (Casp2^−/−) deficiency in tumors from EμMyc and Th-MYCN mice. We describe key changes in both lymphoma and neuroblastoma-associated genes and identified differential expression of the EGF-like domain-containing gene, Megf6, in the two tumor types that may contribute to tumor outcome following loss of Casp2. We identified a panel of genes with altered expression in Th-MYCN/Casp2^−/− tumors that are strongly associated with neuroblastoma outcome, with roles in melanogenesis, Wnt and Hippo pathway signaling, that also contribute to neuronal differentiation. In contrast, we found that key changes in gene expression in the EμMyc/Casp2^−/− tumors, are associated with increased immune signaling and T-cell infiltration previously associated with more aggressive lymphoma progression. In addition, Rap1 signaling pathway was uniquely enriched in Casp2 deficient EμMyc tumors. Our findings suggest that Casp2 deficiency augments immune signaling pathways that may be in turn, enhance lymphomagenesis. Overall, our study has identified new genes and pathways that contribute to the caspase-2 tumor suppressor function and highlight distinct roles for caspase-2 in different tissues.

14-3-3 protein masks the nuclear localization sequence of caspase-2

Caspase-2 is an apical protease responsible for the proteolysis of cellular substrates directly involved in mediating apoptotic signaling cascades. Caspase-2 activation is inhibited by phosphorylation followed by binding to the scaffolding protein 14-3-3, which recognizes two phosphoserines located in the linker between the caspase recruitment domain and the p19 domains of the caspase-2 zymogen. However, the structural details of this interaction and the exact role of 14-3-3 in the regulation of caspase-2 activation remain unclear. Moreover, the caspase-2 region with both 14-3-3-binding motifs also contains the nuclear localization sequence (NLS), thus suggesting that 14-3-3 binding may regulate the subcellular localization of caspase-2. Here, we report a structural analysis of the 14-3-3ζ:caspase-2 complex using a combined approach based on small angle X-ray scattering, NMR, chemical cross-linking, and fluorescence spectroscopy. The structural model proposed in this study suggests that phosphorylated caspase-2 and 14-3-3ζ form a compact and rigid complex in which the p19 and the p12 domains of caspase-2 are positioned within the central channel of the 14-3-3 dimer and stabilized through interactions with the C-terminal helices of both 14-3-3ζ protomers. In this conformation, the surface of the p12 domain, which is involved in caspase-2 activation by dimerization, is sterically occluded by the 14-3-3 dimer, thereby likely preventing caspase-2 activation. In addition, 14-3-3 protein binding to caspase-2 masks its NLS. Therefore, our results suggest that 14-3-3 protein binding to caspase-2 may play a key role in regulating caspase-2 activation. DATABASE: The atomic coordinates and structure factors have been deposited in the Protein Data Bank, www.ww pdb.org (PDB ID codes 6GKF and 6GKG).

A caspase-2-RFXANK interaction and its implication for MHC class II expression

Despite recent achievements implicating caspase-2 in tumor suppression, the enzyme stands out from the apoptotic caspase family as a factor whose function requires further clarification. To specify enzyme characteristics through the definition of interacting proteins in apoptotic or non-apoptotic settings, a yeast 2-hybrid (Y2H) screen was performed using the full-length protein as bait. The current report describes the analysis of a captured prey and putative novel caspase-2 interacting factor, the regulatory factor X-associated ankyrin-containing protein (RFXANK), previously associated with CIITA, the transactivator regulating cell-type specificity and inducibility of MHC class II gene expression. The interaction between caspase-2 and RFXANK was verified by co-immunoprecipitations using both exogenous and endogenous proteins, where the latter approach suggested that binding of the components occurs in the cytoplasm. Cellular co-localization was confirmed by transfection of fluorescently conjugated proteins. Enhanced caspase-2 processing in RFXANK-overexpressing HEK293T cells treated with chemotherapeutic agents further supported Y2H data. Yet, no distinct differences with respect to MHC class II expression were observed in plasma membranes of antigen-presenting cells derived from wild type and caspase-2^-/- mice. In contrast, increased levels of the total MHC class II protein was evident in protein lysates from caspase-2 RNAi-silenced leukemia cell lines and B-cells isolated from gene-targeted mice. Together, these data identify a novel caspase-2-interacting factor, RFXANK, and indicate a potential non-apoptotic role for the enzyme in the control of MHC class II gene regulation.

microRNA-149 targets caspase-2 in glioma progression

Malignant gliomas are the most common form of intrinsic primary brain tumors worldwide. Alterations in microRNAs play a role in highly invasive malignant glioma, but detail mechanism still unknown. In this study, the role and mechanism of microRNA-149 (miR-149) in glioma are investigated. We show that miR-149 is expressed at substantially higher levels in glioma than in normal tissues. Stable overexpression of miR-149 augments potent prosurvival activity, as evidenced by promotion of cell viability, inhibition of apoptosis, and induced xenografted tumor growth in vivo. We further show that Caspase-2 is identified as a functional target of miR-149 and expression of caspase-2 is inversely associated with miR-149 in vitro. In addition, miR-149 promotes tumor survival in the U87-MG and A172 cell lines and it targets caspase-2 via inactivation of the p53 and p21 pathways. There results support a special role for miR-149 by targeting Caspase-2 to impact on p53 signaling pathway. We speculate that miR-149 has distinct biological functions in p53 wild type cells and p53 mutation cells, and the mechanisms involved remain to be explored in future. Our study suggests that targeting miR-149 may be a novel therapy strategy for treating p53 wild type glioma tumors in humans.

Human procaspase-2 phosphorylation at both S139 and S164 is required for 14-3-3 binding

Procaspase-2 phosphorylation at several residues prevents its activation and blocks apoptosis. This process involves procaspase-2 phosphorylation at S164 and its binding to the scaffolding protein 14-3-3. However, bioinformatics analysis has suggested that a second phosphoserine-containing motif may also be required for 14-3-3 binding. In this study, we show that human procaspase-2 interaction with 14-3-3 is governed by phosphorylation at both S139 and S164. Using biochemical and biophysical approaches, we show that doubly phosphorylated procaspase-2 and 14-3-3 form an equimolar complex with a dissociation constant in the nanomolar range. Furthermore, our data indicate that other regions of procaspase-2, in addition to phosphorylation motifs, may be involved in the interaction with 14-3-3.

Caspase-2: an orphan enzyme out of the shadows

Caspase-2 has been embodied as an initiator or executioner protease in diverse apoptotic scenarios. However, accumulating evidence is challenging this view, pertaining to its true role. The enzyme's catalytic activity is currently implicated in various functions required for correct cell proliferation, such as counteracting genomic instability, as well as suppressing tumorigenesis. Here, apart from summarizing the latest observations in caspase-2-related research, we make an attempt to reconcile these findings and discuss their implications for future directions.

Caspase-2-mediated cell death is required for deleting aneuploid cells

Caspase-2, one of the most evolutionarily conserved of the caspase family, has been implicated in maintenance of chromosomal stability and tumour suppression. Caspase-2 deficient (Casp2^−/−) mice develop normally but show premature ageing-related traits and when challenged by certain stressors, succumb to enhanced tumour development and aneuploidy. To test how caspase-2 protects against chromosomal instability, we utilized an ex vivo system for aneuploidy where primary splenocytes from Casp2^−/− mice were exposed to anti-mitotic drugs and followed up by live cell imaging. Our data show that caspase-2 is required for deleting mitotically aberrant cells. Acute silencing of caspase-2 in cultured human cells recapitulated these results. We further generated Casp2^C320S mutant mice to demonstrate that caspase-2 catalytic activity is essential for its function in limiting aneuploidy. Our results provide direct evidence that the apoptotic activity of caspase-2 is necessary for deleting cells with mitotic aberrations to limit aneuploidy.

Molecular Cell Biology of Apoptosis and Necroptosis in Cancer

Cell death is a major mechanism to eliminate cells in which DNA is damaged, organelles are stressed, or oncogenes are overexpressed, all events that would otherwise predispose cells to oncogenic transformation. The pathways that initiate and execute cell death are complex, genetically encoded, and subject to significant regulation. Consequently, while these pathways are often mutated in malignancy, there is considerable interest in inducing cell death in tumor cells as therapy. This chapter addresses our current understanding of molecular mechanisms contributing to two cell death pathways, apoptotic cell death and necroptosis, a regulated form of necrotic cell death. Apoptosis can be induced by a wide variety of signals, leading to protease activation that dismantles the cell. We discuss the physiological importance of each apoptosis pathway and summarize their known roles in cancer suppression and the current efforts at targeting each pathway therapeutically. The intricate mechanistic link between death receptor-mediated apoptosis and necroptosis is described, as well as the potential opportunities for utilizing necroptosis in the treatment of malignancy.

An Inhibitor of PIDDosome Formation

The PIDDosome-PIDD-RAIDD-caspase-2 complex-is a proapoptotic caspase-activation platform of elusive significance. DNA damage can initiate complex assembly via ATM phosphorylation of the PIDD death domain (DD), which enables RAIDD recruitment to PIDD. In contrast, the mechanisms limiting PIDDosome formation have remained unclear. We identify the mitotic checkpoint factor BubR1 as a direct PIDDosome inhibitor, acting in a noncanonical role independent of Mad2. Following its phosphorylation by ATM at DNA breaks, "primed" PIDD relocates to kinetochores via a direct interaction with BubR1. BubR1 binds the PIDD DD, competes with RAIDD recruitment, and negates PIDDosome-mediated apoptosis after ionizing radiation. The PIDDosome thus sequentially integrates DNA damage and mitotic checkpoint signals to decide cell fate in response to genotoxic stress. We further show that by sequestering PIDD at the kinetochore, BubR1 acts to delay PIDDosome formation until the next cycle, defining a new mechanism by which cells evade apoptosis during mitosis.

The tumor-modulatory effects of Caspase-2 and Pidd1 do not require the scaffold protein Raidd

The receptor-interacting protein-associated ICH-1/CED-3 homologous protein with a death domain (RAIDD/CRADD) functions as a dual adaptor and is a constituent of different multi-protein complexes implicated in the regulation of inflammation and cell death. Within the PIDDosome complex, RAIDD connects the cell death-related protease, Caspase-2, with the p53-induced protein with a death domain 1 (PIDD1). As such, RAIDD has been implicated in DNA-damage-induced apoptosis as well as in tumorigenesis. As loss of Caspase-2 leads to an acceleration of tumor onset in the Eμ-Myc mouse lymphoma model, whereas loss of Pidd1 actually delays onset of this disease, we set out to interrogate the role of Raidd in cancer in more detail. Our data obtained analyzing Eμ-Myc/Raidd^−/− mice indicate that Raidd is unable to protect from c-Myc-driven lymphomagenesis. Similarly, we failed to observe a modulatory effect of Raidd deficiency on DNA-damage-driven cancer. The role of Caspase-2 as a tumor suppressor and that of Pidd1 as a tumor promoter can therefore be uncoupled from their ability to interact with the Raidd scaffold, pointing toward the existence of alternative signaling modules engaging these two proteins in this context.

A novel anti-proliferative role of HMGA2 in induction of apoptosis through caspase 2 in primary human fibroblast cells

The HMGA2 (high-mobility group AT-hook) protein has previously been shown as an oncoprotein, whereas ectopic expression of HMGA2 is found to induce growth arrest in primary cells. The precise mechanisms underlying this phenomenon remain to be unravelled. In the present study, we determined that HMGA2 was able to induce apoptosis in WI38 primary human cells. We show that WI38 cells expressing high level of HMGA2 were arrested at G2/M phase and exhibited apoptotic nuclear phenotypes. Meanwhile, the cleaved caspase 3 (cysteine aspartic acid-specific protease 3) was detected 8 days after HMGA2 overexpression. Flow cytometric analysis confirmed that the ratio of cells undergoing apoptosis increased dramatically. Concurrently, other major apoptotic markers were also detected, including the up-regulation of p53, Bax and cleaved caspase 9, down-regulation of Bcl-2; as well as release of cytochrome c from the mitochondria. We further demonstrate that the shRNA (small-hairpin RNA)-mediated Apaf1 (apoptotic protease activating factor 1) silencing partially rescued the HMGA2-induced apoptosis, which was accompanied by the decrease of cleaved caspase-3 level and a decline of cell death ratio. Our results also reveal that γH2A was accumulated in nuclei during the HMGA2-induced apoptosis along with the up-regulation of cleaved caspase 2, suggesting that the HMGA2-induced apoptosis was dependent on the pathway of DNA damage. Overall, the present study unravelled a novel function of HMGA2 in induction of apoptosis in human primary cell lines, and provided clues for clarification of the mechanistic action of HMGA2 in addition to its function as an oncoprotein.

Caspase-2 impacts lung tumorigenesis and chemotherapy response in vivo

Caspase-2 is an atypical caspase that regulates apoptosis, cell cycle arrest and genome maintenance, although the mechanisms are not well understood. Caspase-2 has also been implicated in chemotherapy response in lung cancer, but this function has not been addressed in vivo. Here we show that Caspase-2 functions as a tumor suppressor in Kras-driven lung cancer in vivo. Loss of Caspase-2 leads to enhanced tumor proliferation and progression. Despite being more histologically advanced, Caspase-2-deficient tumors are sensitive to chemotherapy and exhibit a significant reduction in tumor volume following repeated treatment. However, Caspase-2-deficient tumors rapidly rebound from chemotherapy with enhanced proliferation, ultimately hindering long-term therapeutic benefit. In response to DNA damage, Caspase-2 cleaves and inhibits Mdm2 and thereby promotes the stability of the tumor-suppressor p53. Caspase-2 expression levels are significantly reduced in human lung tumors with wild-type p53, in agreement with the model whereby Caspase-2 functions through Mdm2/p53 regulation. Consistently, p53 target genes including p21, cyclin G1 and Msh2 are reduced in Caspase-2-deficient tumors. Finally, we show that phosphorylation of p53-induced protein with a death domain 1 leads to Caspase-2-mediated cleavage of Mdm2, directly impacting p53 levels, activity and chemotherapy response. Together, these studies elucidate a Caspase-2-p53 signaling network that impacts lung tumorigenesis and chemotherapy response in vivo.

Analysis of the minimal specificity of caspase-2 and identification of Ac-VDTTD-AFC as a caspase-2-selective peptide substrate

Caspase-2 is an evolutionarily conserved but enigmatic protease whose biological role remains poorly understood. To date, research into the functions of caspase-2 has been hampered by an absence of reagents that can distinguish its activity from that of the downstream apoptotic caspase, caspase-3. Identification of protein substrates of caspase-2 that are efficiently cleaved within cells may also provide clues to the role of this protease. We used a yeast-based transcriptional reporter system to define the minimal substrate specificity of caspase-2. The resulting profile enabled the identification of candidate novel caspase-2 substrates. Caspase-2 cleaved one of these proteins, the cancer-associated transcription factor Runx1, although with relatively low efficiency. A fluorogenic peptide was derived from the sequence most efficiently cleaved in the context of the transcriptional reporter. This peptide, Ac-VDTTD-AFC, was efficiently cleaved by purified caspase-2 and auto-activating caspase-2 in mammalian cells, and exhibited better selectivity for caspase-2 relative to caspase-3 than reagents that are currently available. We suggest that this reagent, used in parallel with the traditional caspase-3 substrate Ac-DEVD-AFC, will enable researchers to monitor caspase-2 activity in cell lysates and may assist in the determination of stimuli that activate caspase-2 in vivo.

Induced cell death pathway post photodynamic therapy using a metallophthalocyanine photosensitizer in breast cancer cells

OBJECTIVE

Zinc phthalocyanine (ZnPcSmix) was used as the photosensitizer (PS) in this study to investigate the cell death patterns as a result of photodynamic therapy (PDT) in a breast cancer cell line (MCF-7) in vitro using a 680 nm diode laser at a fluence of 5 J/cm(2).

BACKGROUND

PDT is a noninvasive form of cancer therapy, successfully applied for the treatment of various cancer types.

METHODS

Flow cytometry using Annexin V-fluorescein isothiocyanate (FITC), a cell death immunosorbent assay (ELISA), and gene expression analysis following ZnPcSmix mediated PDT were performed to determine the induced cell death pathways.

RESULTS

The apoptotic cells abounded after the treatment, nuclear fragmentation was seen as oligonucleosomal degradation and increased expression of the B-cell lymphoma 2 (Bcl-2), DNA fragmentation factor alpha (DFFA1), and caspase 2 (CASP2) genes, indicated that apoptosis is the main induced mode of cell death.

CONCLUSIONS

ZnPcSmix mediated PDT led to an apoptotic cell death pathway and the PS used showed its ability to stimulate and initiate programmed cell death.

Caspase-2 as a tumour suppressor

Ever since its discovery 20 years ago, caspase-2 has been enigmatic and its function somewhat controversial. Although many in vitro studies suggested that caspase-2 was important for apoptosis, demonstrating an in vivo cell death role for this caspase has been more problematic, with caspase-2-deficient mice showing limited, tissue-specific cell death defects. Recent results from different laboratories suggest that at least one of its physiological roles in animals is to protect against cellular stress and transformation. As such, loss of caspase-2 augments tumorigenesis in some mouse models of cancer, assigning a tumour suppressor function to this enigmatic caspase. This review focuses on this seemingly non-apoptotic function of caspase-2 as a tumour suppressor and reconciles some of the recent findings in the field.

A novel cell lysis approach reveals that caspase-2 rapidly translocates from the nucleus to the cytoplasm in response to apoptotic stimuli

Unlike other caspases, caspase-2 appears to be a nuclear protein although immunocytochemical studies have suggested that it may also be localized to the cytosol and golgi. Where and how caspase-2 is activated in response to apoptotic signals is not clear. Earlier immunocytochemistry studies suggest that caspase-2 is activated in the nucleus and through cleavage of BID leads to increased mitochondrial permeability. More recent studies using bimolecular fluorescence complementation found that caspase-2 oligomerization that leads to activation only occurs in the cytoplasm. Thus, apoptotic signals may lead to activation of caspase-2 which may already reside in the cytoplasm or lead to release of nuclear caspase-2 to the extra-nuclear cytoplasmic compartment. It has not been possible to study release of nuclear caspase-2 to the cytoplasm by cell fractionation studies since cell lysis is known to release nuclear caspase-2 to the extra-nuclear fraction. This is similar to what is known about unliganded nuclear estrogen receptor-α (ERα ) when cells are disrupted. In this study we found that pre-treatment of cells with N-ethylmaleimide (NEM), which alkylates cysteine thiol groups in proteins, completely prevents redistribution of caspase-2 and ERα from the nucleus to the extra-nuclear fraction when cells are lysed. Using this approach we provide evidence that apoptotic signals rapidly leads to a shift of caspase-2 from the nucleus to the extra-nuclear fraction, which precedes the detection of apoptosis. These findings are consistent with a model where apoptotic signals lead to a rapid shift of caspase-2 from the nucleus to the cytoplasm where activation occurs.

Importance of transcript levels of caspase-2 isoforms S and L for breast carcinoma progression

Aim: A role of caspase-2 in chemotherapy-induced apoptosis has been suggested. Our study aimed to evaluate the prognostic and predictive importance of caspase-2 isoforms in breast cancer patients. Materials & methods: Caspase-2L and -2S transcript levels were determined in paired tumor and non-malignant control tissues from 64 patients after neoadjuvant chemotherapy and 100 pretreatment patients (general set) by real-time PCR with absolute quantification. Results: Low but statistically significant upregulation of caspase-2L in tumor versus control tissues was observed in both sets. Significant associations of the levels of caspase-2L, -2S or S/L ratio with clinical prognostic factors were observed. However, none of these associations were confirmed in both sets. Levels of caspase-2 isoforms or the S/L ratio did not significantly associate with progression-free survival in the general set or with chemotherapy response in the neoadjuvant set. Conclusion: Our results suggest that the role of caspase-2 isoforms in the progression of breast cancer may considerably differ between pre- and post-chemotherapy patients.

PIDD death-domain phosphorylation by ATM controls prodeath versus prosurvival PIDDosome signaling

Biochemical evidence implicates the death-domain (DD) protein PIDD as a molecular switch capable of signaling cell survival or death in response to genotoxic stress. PIDD activity is determined by binding-partner selection at its DD: whereas recruitment of RIP1 triggers prosurvival NF-κB signaling, recruitment of RAIDD activates proapoptotic caspase-2 via PIDDosome formation. However, it remains unclear how interactor selection, and thus fate decision, is regulated at the PIDD platform. We show that the PIDDosome functions in the "Chk1-suppressed" apoptotic response to DNA damage, a conserved ATM/ATR-caspase-2 pathway antagonized by Chk1. In this pathway, ATM phosphorylates PIDD on Thr788 within the DD. This phosphorylation is necessary and sufficient for RAIDD binding and caspase-2 activation. Conversely, nonphosphorylatable PIDD fails to bind RAIDD or activate caspase-2, and engages prosurvival RIP1 instead. Thus, ATM phosphorylation of the PIDD DD enables a binary switch through which cells elect to survive or die upon DNA injury.