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

Caspase-2 as a master regulator of genomic stability

Genomic instability underlies genesis and the development of various types of cancer. During tumorigenesis, cancer initiating cells assume a set of features, which allow them to survive and proliferate. Different mutations and chromosomal alterations promote a selection of the most aggressive cancer clones that worsen the prognosis of the disease. Despite that caspase-2 was described as a protease fulfilling an initiator and an effector function in apoptosis, it has recently been discovered to play an important role in the maintenance of genomic integrity and normal chromosome configuration. This protein is able to stabilize p53 and affect the level of transcription factors, which activates cell response to oxidative stress. Here we focus on the discussion on the mechanism(s) of how caspase-2 regulates genomic stability and decreases tumorigenesis.

BMP5 silencing inhibits chondrocyte senescence and apoptosis as well as osteoarthritis progression in mice

In this study, we using the in vivo destabilization of the medial meniscus (DMM) mouse model to investigate the role of bone morphogenetic protein 5 (BMP5) in osteoarthritis (OA) progression mediated via chondrocyte senescence and apoptosis. BMP5 expression was significantly higher in knee articular cartilage tissues of OA patients and DMM model mice than the corresponding controls. The Osteoarthritis Research Society International scores based on histological staining of knee articular cartilage sections were lower in DMM mice where BMP5 was knocked down in chondrocytes than the corresponding controls 4 weeks after DMM surgery. DMM mice with BMP5-deficient chondrocytes showed reduced levels of matrix-degrading enzymes such as MMP13 and ADAMTS5 as well as reduced cartilage destruction. BMP5 knockdown also decreased chondrocyte apoptosis and senescence by suppressing the activation of p38 and ERK MAP kinases. These findings demonstrate that BMP5 silencing inhibits chondrocyte senescence and apoptosis as well as OA progression by downregulating activity in the p38/ERK signaling pathway.

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.

Transcriptome Analyses of the Anti-Proliferative Effects of 20(S)-Ginsenoside Rh2 on HepG2 Cells

20(S)-ginsenoside Rh2 (Rh2), a well-known protopanaxadiol-type ginsenoside from Panax ginseng has especially gained attention for its anticancer activities on various types of human cancer cells. However, the molecular mechanism through which Rh2 promotes apoptosis in hepatocellular carcinoma (HePG2) cells is not known at the transcriptome level. Rh2 can specifically inhibit the proliferation of HePG2 in a dose- and time-dependent manner. Moreover, Rh2 can significantly increase the apoptosis which was related with an increase in protein expression levels of caspase-3, caspase-6, and poly (ADP-ribose) polymerase. Comparison of RNA-seq transcriptome profiles from control group and Rh2-treated group yielded a list of 2116 genes whose expression was significantly affected, which includes 971 up-regulated genes and 1145 down-regulated genes. The differentially expressed genes in p53 signaling pathway and DNA replication may have closely relationships to the cells apoptosis caused by Rh2 treatment. The results of qPCR validation showed that dynamic changes in mRNA, such as CDKN1A, CCND2, PMAIP1, GTSE1, and TP73.

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.

Role of Cell Death in Toxicology: Does It Matter How Cells Die?

My research activity started with studies on drug metabolism in rat liver microsomes in the early 1960s. The CO-binding pigment (cytochrome P450) had been discovered a few years earlier and was subsequently found to be involved in steroid hydroxylation in adrenal cortex microsomes. Our early studies suggested that it also participated in the oxidative demethylation of drugs catalyzed by liver microsomes, and that prior treatment of the animals with phenobarbital caused increased levels of the hemoprotein in the liver, and similarly enhanced rates of drug metabolism. Subsequent studies of cytochrome P450-mediated metabolism of toxic drugs in freshly isolated rat hepatocytes characterized critical cellular defense systems and identified mechanisms by which accumulating toxic metabolites could damage and kill the cells. These studies revealed that multiple types of cell death could result from the toxic injury, and that it is important to know which type of cell death results from the toxic injury.

Inhibition of IRES-dependent translation of caspase-2 by HuR confers chemotherapeutic drug resistance in colon carcinoma cells

HuR plays an important role in tumor cell survival mainly through posttranscriptional upregulation of prominent anti-apoptotic genes. In addition, HuR can inhibit the translation of pro-apoptotic factors as we could previously report for caspase-2. Here, we investigated the mechanisms of caspase-2 suppression by HuR and its contribution to chemotherapeutic drug resistance of colon carcinoma cells. In accordance with the significant drug-induced increase in cytoplasmic HuR abundance, doxorubicin and paclitaxel increased the interaction of cytoplasmic HuR with the 5ʹuntranslated region (5ʹUTR) of caspase-2 as shown by RNA pull down assay. Experiments with bicistronic reporter genes furthermore indicate the presence of an internal ribosome entry site (IRES) within the caspase-2-5ʹUTR. Luciferase activity was suppressed either by chemotherapeutic drugs or ectopic expression of HuR. IRES-driven luciferase activity was significantly increased upon siRNA-mediated knockdown of HuR implicating an inhibitory effect of HuR on caspase-2 translation which is further reinforced by chemotherapeutic drugs. Comparison of RNA-binding affinities of recombinant HuR to two fragments of the caspase-2-5ʹUTR by EMSA revealed a critical HuR-binding site residing between nucleotides 111 and 241 of caspase-2-5ʹUTR. Mapping of critical RNA binding domains within HuR revealed that a fusion of RNA recognition motif 2 (RRM2) plus the hinge region confers a full caspase-2-5ʹUTR-binding. Functionally, knockdown of HuR significantly increased the sensitivity of colon cancer cells to drug-induced apoptosis. Importantly, the apoptosis sensitizing effects by HuR knockdown were rescued after silencing of caspase-2. The negative caspase-2 regulation by HuR offers a novel therapeutic target for sensitizing colon carcinoma cells to drug-induced apoptosis.

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.

E1A enhances cellular sensitivity to DNA-damage-induced apoptosis through PIDD-dependent caspase-2 activation

Expression of the adenoviral protein, E1A, sensitizes mammalian cells to a wide variety of apoptosis-inducing agents through multiple cellular pathways. For example, E1A sensitizes cells to apoptosis induced by TNF-superfamily members by inhibiting NF-kappa B (NF-κB)-dependent gene expression. In contrast, E1A sensitization to nitric oxide, an inducer of the intrinsic apoptotic pathway, is not dependent upon repression of NF-κB-dependent transcription but rather is dependent upon caspase-2 activation. The latter observation suggested that E1A-induced enhancement of caspase-2 activation might be a critical factor in cellular sensitization to other intrinsic apoptosis pathway-inducing agents. Etoposide and gemcitabine are two DNA damaging agents that induce intrinsic apoptosis. Here we report that E1A-induced sensitization to both of these agents, like NO, is independent of NF-κB activation but dependent on caspase-2 activation. The results show that caspase-2 is a key mitochondrial-injuring caspase during etoposide and gemcitabine-induced apoptosis of E1A-positive cells, and that caspase-2 is required for induction of caspase-3 activity by both chemotherapeutic agents. Expression of PIDD was required for caspase-2 activation, mitochondrial injury and enhanced apoptotic cell death. Furthermore, E1A-enhanced sensitivity to injury-induced apoptosis required PIDD cleavage to PIDD-CC. These results define the PIDD/caspase-2 pathway as a key apical, mitochondrial-injuring mechanism in E1A-induced sensitivity of mammalian cells to chemotherapeutic agents.

5-Fluorouracil-induced RNA stress engages a TRAIL-DISC-dependent apoptosis axis facilitated by p53

Despite recent advances in targeted therapeutics, administration of 5-fluorouracil (5-FU) remains a common clinical strategy for post-surgical treatment of solid tumors. Although it has been proposed that RNA metabolism is disturbed by 5-FU treatment, the key cytotoxic response is believed to be enzymatic inhibition of thymidylate synthase resulting in nucleotide pool disproportions. An operating p53 tumor suppressor signaling network is in many cases essential for the efficiency of chemotherapy, and malfunctions within this system remain a clinical obstacle. Since the fate of chemotherapy-insensitive tumor cells is rarely described, we performed a comparative analysis of 5-FU toxicity in p53-deficient cells and conclude that p53 acts as a facilitator rather than a gatekeeper of cell death. Although p53 can act as a regulator of several cellular stress responses, no rerouting of cell death mode was observed in absence of the tumor suppressor. Thus, the final death outcome of 5-FU-treated p53^−/− cells is demonstrated to be caspase-dependent, but due to a slow pace, accumulation of mitochondrial reactive oxygen species contributes to necrotic characteristics. The oligomerization status of the p53 target gene DR5 is determined as a significant limiting factor for the initiation of caspase activity in an intracellular TRAIL-dependent manner. Using several experimental approaches, we further conclude that RNA- rather than DNA-related stress follows by caspase activation irrespectively of p53 status. A distinct 5-FU-induced stress mechanism is thereby functionally connected to a successive and discrete cell death signaling pathway. Finally, we provide evidence that silencing of PARP-1 function may be an approach to specifically target p53-deficient cells in 5-FU combinatorial treatment strategies. Together, our results disclose details of impaired cell death signaling engaged as a consequence of 5-FU chemotherapy. Obtained data will contribute to the comprehension of factors restraining 5-FU efficiency, and by excluding DNA as the main stress target in some cell types they propose alternatives to currently used and suggested synergistic treatment regimens.

Crocetin exploits p53-induced death domain (PIDD) and FAS-associated death domain (FADD) proteins to induce apoptosis in colorectal cancer

Tumor suppressor p53 preserves the genomic integrity by restricting anomaly at the gene level. The hotspots for mutation in half of all colon cancers reside in p53. Hence, in a p53-mutated cellular milieu targeting cancer cells may be achievable by targeting the paralogue(s) of p53. Here we have shown the effectiveness of crocetin, a dietary component, in inducing apoptosis of colon cancer cells with varying p53 status. In wild-type p53-expressing cancer cells, p53 in one hand transactivates BAX and in parallel up-regulates p53-induced death domain protein (PIDD) that in turn cleaves and activates BID through caspase-2. Both BAX and t-BID converge at mitochondria to alter the transmembrane potential thereby leading to caspase-9 and caspase-3-mediated apoptosis. In contrast, in functional p53-impaired cells, this phytochemical exploits p53-paralogue p73, which up-regulates FAS to cleave BID through FAS-FADD-caspase-8-pathway. These findings not only underline the phenomenon of functional switch-over from p53 to p73 in p53-impaired condition, but also validate p73 as a promising and potential target for cancer therapy in absence of functional p53.

Targeting the Mitotic Catastrophe Signaling Pathway in Cancer

Mitotic catastrophe, as defined in 2012 by the International Nomenclature Committee on Cell Death, is a bona fide intrinsic oncosuppressive mechanism that senses mitotic failure and responds by driving a cell to an irreversible antiproliferative fate of death or senescence. Thus, failed mitotic catastrophe can promote the unrestrained growth of defective cells, thereby representing a major gateway to tumour development. Furthermore, the activation of mitotic catastrophe offers significant therapeutic advantage which has been exploited in the action of conventional and targeted anticancer agents. Yet, despite its importance in tumour prevention and treatment, the molecular mechanism of mitotic catastrophe is not well understood. A better understanding of the signals that determine cell fate following failed or defective mitosis will reveal new opportunities to selectively target and enhance the programme for therapeutic benefit and reveal biomarkers to predict patient response. This review is focused on the molecular mechanism of mitotic catastrophe induction and signalling and highlights current strategies to exploit the process in cancer therapy.

Mixed-ligand ruthenium polypyridyl complexes as apoptosis inducers in cancer cells, the cellular translocation and the important role of ROS-mediated signaling

Ruthenium (Ru) polypyridyl complexes have emerged as leading players among the potential metal-based candidates for cancer treatment. However, the roles of cellular translocation in their action mechanisms remain elusive. Herein we present the synthesis and characterization of a series of ruthenium (Ru) complexes containing phenanthroline derivatives with varying lipophilicities, and examine their mechanism of anticancer action. Results showed that increasing the lipophilicity of complexes can enhance the rates of cellular uptake. The in vitro anticancer efficacy of these complexes depended on the levels of ROS overproduction, rather than on cellular Ru uptake levels. The introduction of a phenolic group on the ligand effectively enhanced their intracellular ROS generation and anticancer activities. In particular, complex 4, with an ortho-phenolic group on the ligand, exhibited better selectivity between cancer and normal cells in comparison with cisplatin. Notably, complex 4 entered the cancer cells partially through transferrin receptor-mediated endocytosis, and then it translocated from lysosomes to the mitochondria, where it activated mitochondrial dysfunction by regulation of Bcl-2 family proteins, thus leading to intracellular ROS overproduction. Excess ROS amplified apoptotic signals by activating many downstream pathways such as p53 and MAPK pathways to promote cell apoptosis. Overall, this study provides a drug design strategy for discovery of Ru-based apoptosis inducers, and elucidates the intracellular translocation of these complexes.

Cell death controlling complexes and their potential therapeutic role

Programmed cell death plays a central role in the regulation of homeostasis and development of multicellular organisms. Deregulation of programmed cell death is connected to a number of disorders, including cancer and autoimmune diseases. Initiation of cell death occurs in the multiprotein complexes or high molecular weight platforms. Composition, structure, and molecular interactions within these platforms influence the cellular decision toward life or death and, therefore, define the induction of a particular cell death program. Here, we discuss in detail the key cell-death complexes-including DISC, complex II, and TNFRI complex I/II, and the necrosome, RIPoptosome, apoptosome, and PIDDosome-that control apoptosis or necroptosis pathways as well as their regulation. The possibility of their pharmacological targeting leading to the development of new strategies of interference with cell death programs via control of the high molecular weight platforms will be discussed.

Hepatitis B virus core protein inhibits Fas-mediated apoptosis of hepatoma cells via regulation of mFas/FasL and sFas expression

Hepatitis B virus core protein (HBc) has been implicated in hepatocarcinogenesis through several mechanisms. Resistance of hepatitis B virus (HBV)-infected hepatocytes to apoptosis is considered one of the major contributors to the progression of chronic hepatitis to cirrhosis and ultimately to hepatocellular carcinoma. The Fas receptor/ligand (Fas/FasL) system plays a prominent role in hepatocyte death during HBV infection. Here we report that HBc mediates resistance of hepatoma cells to agonistic anti-Fas antibody (CH11)-induced apoptosis. When HBc was introduced into human hepatoma cells, the cells became resistant to CH11 cytotoxicity in a p53-dependent manner. HBc significantly down-regulated the expression of p53, total Fas, and membrane-bound Fas at the mRNA and protein levels and reduced FasL mRNA expression. In contrast, HBc up-regulated the expression of soluble forms of Fas by increasing Fas alternative mRNA splicing. Mechanistically, HBc-mediated Fas alternative mRNA splicing was associated with up-regulation of polypyrimidine tract-binding protein 1 and down-regulation of Fas-activated serine/threonine kinase. These results indicated that HBc may prevent hepatocytes from Fas-induced apoptosis by the dual effects of reducing the expression of the proapoptotic form of Fas and enhancing the expression of the antiapoptotic form of the receptor, which may contribute to the survival and persistence of infected hepatocytes during chronic infection.

Senescence and apoptosis: dueling or complementary cell fates?

In response to a variety of stresses, mammalian cells undergo a persistent proliferative arrest known as cellular senescence. Many senescence-inducing stressors are potentially oncogenic, strengthening the notion that senescence evolved alongside apoptosis to suppress tumorigenesis. In contrast to apoptosis, senescent cells are stably viable and have the potential to influence neighboring cells through secreted soluble factors, which are collectively known as the senescence-associated secretory phenotype (SASP). However, the SASP has been associated with structural and functional tissue and organ deterioration and may even have tumor-promoting effects, raising the interesting evolutionary question of why apoptosis failed to outcompete senescence as a superior cell fate option. Here, we discuss the advantages that the senescence program may have over apoptosis as a tumor protective mechanism, as well as non-neoplastic functions that may have contributed to its evolution. We also review emerging evidence for the idea that senescent cells are present transiently early in life and are largely beneficial for development, regeneration and homeostasis, and only in advanced age do senescent cells accumulate to an organism's detriment.

In vitro anticancer activity of loquat tea by inducing apoptosis in human leukemia cells

Fresh loquat leaves have been used as folk health herb in Asian countries for long time, although the evidence supporting their functions is still minimal. This study aimed to clarify the chemopreventive effect of loquat tea extract (LTE) by investigating the inhibition on proliferation, and underlying mechanisms in human promyelocytic leukemia cells (HL-60). LTE inhibited proliferation of HL-60 in a dose-dependent manner. Molecular data showed that the isolated fraction of LTE induced apoptosis of HL-60 as characterized by DNA fragmentation; activation of caspase-3, -8, and -9; and inactivation of poly(ADP)ribose polymerase. Moreover, LTE fraction increased the ratio of pro-apoptotic Bcl-2-associated X protein (Bax)/anti-apoptotic myeloid cell leukemia 1 (Mcl-1) that caused mitochondrial membrane potential loss and cytochrome c released to cytosol. Thus, our data indicate that LTE might induce apoptosis in HL-60 cells through a mitochondrial dysfunction pathway. These findings enhance our understanding for chemopreventive function of loquat tea.

5-Fluorouracil-induced apoptosis in colorectal cancer cells is caspase-9-dependent and mediated by activation of protein kinase C-δ

Elucidation of the molecular mechanisms by which 5-fluorouracil (5-FU) induces apoptosis is required in order to understand the resistance of colorectal cancer (CRC) cells to 5-FU. In the current study, 5-FU-induced apoptosis was assessed using the propidium iodide method. Involvement of protein kinase C (PKC) was assessed by evaluating the extent of their activation in CRC, following treatment with 5-FU, using biochemical inhibitors and western blot analysis. The results revealed that 5-FU induces varying degrees of apoptosis in CRC cells; HCT116 cells were identified to be the most sensitive cells and SW480 were the least sensitive. In addition, 5-FU-induced apoptosis was caspase-dependent as it appeared to be initiated by caspase-9. Furthermore, PKCɛ was marginally expressed in CRC cells and no changes were observed in the levels of cleavage or phosphorylation following treatment with 5-FU. The treatment of HCT116 cells with 5-FU increased the expression, phosphorylation and cleavage of PKCδ. The inhibition of PKCδ was found to significantly inhibit 5-FU-induced apoptosis. These results indicated that 5-FU induces apoptosis in CRC by the activation of PKCδ and caspase-9. In addition, the levels of PKCδ activation may determine the sensitivity of CRC to 5-FU.

Metyrapone, an inhibitor of cytochrome oxidases, does not affect viability in a neuroblastoma cell model of bilirubin toxicity

Background

Unconjugated hyperbilirubinemia may cause brain damage in infants, and globally remains a source of neonatal morbidity and mortality. A significant inter-individual variability in vulnerability to bilirubin toxicity remains largely unexplained. An enzyme located in mitochondria oxidizes bilirubin. We hypothesized that inhibiting bilirubin oxidation in human neuronal cell cultures exposed to bilirubin would increase cell death.

Methods

The ability of mitochondrial membranes from CHP-212 human neuroblastoma cells to oxidize bilirubin was verified by spectrophotometry. Intact cells in culture were exposed to bilirubin (75 μM) with or without metyrapone (250 μM) for 24 h, stained with Annexin-V and Propidium iodide and analyzed for apoptosis and necrosis by flow cytometry.

Results

Bilirubin caused a significant reduction of viability, from 84 ± 2.0% (mean ± SEM) vs 67 ± 2.7% (p < 0.05), but adding metyrapone to the bilirubin-exposed cells did not further impact cell viability. Metyrapone alone did not influence cell viability.

Conclusion

Herein we have shown that metyrapone does not increase cell death in neuroblastoma cells in culture exposed to bilirubin. Our results question the relationship between the oxidative mechanism evaluated by spectrophotometry and cell viability. Our findings add to the discussion on whether bilirubin oxidation represents a potentially important protective mechanism in neurons challenged by hyperbilirubinemia.

•

Neonatal jaundice may cause brain damage in newborn infants.

•

Oxidation of bilirubin in the brain has been proposed as a protective mechanism.

•

In a cell model, blocking the enzyme responsible for bilirubin oxidation did not increase toxicity.

Ubiquitin-like (UBX)-domain-containing protein, UBXN2A, promotes cell death by interfering with the p53-Mortalin interactions in colon cancer cells

Mortalin (mot-2) induces inactivation of the tumor suppressor p53's transcriptional and apoptotic functions by cytoplasmic sequestration of p53 in select cancers. The mot-2-dependent cytoprotective function enables cancer cells to support malignant transformation. Abrogating the p53-mot-2 interaction can control or slow down the growth of cancer cells. In this study, we report the discovery of a ubiquitin-like (UBX)-domain-containing protein, UBXN2A, which binds to mot-2 and consequently inhibits the binding between mot-2 and p53. Genetic analysis showed that UBXN2A binds to mot-2's substrate binding domain, and it partly overlaps p53's binding site indicating UBXN2A and p53 likely bind to mot-2 competitively. By binding to mot-2, UBXN2A releases p53 from cytosolic sequestration, rescuing the tumor suppressor functions of p53. Biochemical analysis and functional assays showed that the overexpression of UBXN2A and the functional consequences of unsequestered p53 trigger p53-dependent apoptosis. Cells expressing shRNA against UBXN2A showed the opposite effect of that seen with UBXN2A overexpression. The expression of UBXN2A and its apoptotic effects were not observed in normal colonic epithelial cells and p53-/- colon cancer cells. Finally, significant reduction in tumor volume in a xenograft mouse model in response to UBXN2A expression was verified in vivo. Our results introduce UBXN2A as a home defense response protein, which can reconstitute inactive p53-dependent apoptotic pathways. Inhibition of mot-2-p53 interaction by UBXN2A is an attractive therapeutic strategy in mot-2-elevated tumors.

Death of p53-defective cells triggered by forced mitotic entry in the presence of DNA damage is not uniquely dependent on Caspase-2 or the PIDDosome

Much effort has been put in the discovery of ways to selectively kill p53-deficient tumor cells and targeting cell cycle checkpoint pathways has revealed promising candidates. Studies in zebrafish and human cell lines suggested that the DNA damage response kinase, checkpoint kinase 1 (Chk1), not only regulates onset of mitosis but also cell death in response to DNA damage in the absence of p53. This effect reportedly relies on ataxia telangiectasia mutated (ATM)-dependent and PIDDosome-mediated activation of Caspase-2. However, we show that genetic ablation of PIDDosome components in mice does not affect cell death in response to γ-irradiation. Furthermore, Chk1 inhibition largely failed to sensitize normal and malignant cells from p53^−/− mice toward DNA damaging agents, and p53 status did not affect the death-inducing activity of DNA damage after Chk1 inhibition in human cancer cells. These observations argue against cross-species conservation of a Chk1-controlled cell survival pathway demanding further investigation of the molecular machinery responsible for cell death elicited by forced mitotic entry in the presence of DNA damage in different cell types and model organisms.

Caspase-2 promotes cytoskeleton protein degradation during apoptotic cell death

The caspase family of proteases cleaves large number of proteins resulting in major morphological and biochemical changes during apoptosis. Yet, only a few of these proteins have been reported to selectively cleaved by caspase-2. Numerous observations link caspase-2 to the disruption of the cytoskeleton, although it remains elusive whether any of the cytoskeleton proteins serve as bona fide substrates for caspase-2. Here, we undertook an unbiased proteomic approach to address this question. By differential proteome analysis using two-dimensional gel electrophoresis, we identified four cytoskeleton proteins that were degraded upon treatment with active recombinant caspase-2 in vitro. These proteins were degraded in a caspase-2-dependent manner during apoptosis induced by DNA damage, cytoskeleton disruption or endoplasmic reticulum stress. Hence, degradation of these cytoskeleton proteins was blunted by siRNA targeting of caspase-2 and when caspase-2 activity was pharmacologically inhibited. However, none of these proteins was cleaved directly by caspase-2. Instead, we provide evidence that in cells exposed to apoptotic stimuli, caspase-2 probed these proteins for proteasomal degradation. Taken together, our results depict a new role for caspase-2 in the regulation of the level of cytoskeleton proteins during apoptosis.

Cellular mechanisms controlling caspase activation and function

Caspases are the primary drivers of apoptotic cell death, cleaving cellular proteins that are critical for dismantling the dying cell. Initially translated as inactive zymogenic precursors, caspases are activated in response to a variety of cell death stimuli. In addition to factors required for their direct activation (e.g., dimerizing adaptor proteins in the case of initiator caspases that lie at the apex of apoptotic signaling cascades), caspases are regulated by a variety of cellular factors in a myriad of physiological and pathological settings. For example, caspases may be modified posttranslationally (e.g., by phosphorylation or ubiquitylation) or through interaction of modulatory factors with either the zymogenic or active form of a caspase, altering its activation and/or activity. These regulatory events may inhibit or enhance enzymatic activity or may affect activity toward particular cellular substrates. Finally, there is emerging literature to suggest that caspases can participate in a variety of cellular processes unrelated to apoptotic cell death. In these settings, it is particularly important that caspases are maintained under stringent control to avoid inadvertent cell death. It is likely that continued examination of these processes will reveal new mechanisms of caspase regulation with implications well beyond control of apoptotic cell death.

The antiangiogenic compound aeroplysinin-1 induces apoptosis in endothelial cells by activating the mitochondrial pathway

Aeroplysinin-1 is a brominated metabolite extracted from the marine sponge Aplysina aerophoba that has been previously characterized by our group as a potent antiangiogenic compound in vitro and in vivo. In this work, we provide evidence of a selective induction of apoptosis by aeroplysinin-1 in endothelial cells. Studies on the nuclear morphology of treated cells revealed that aeroplysinin-1 induces chromatin condensation and nuclear fragmentation, and it increases the percentage of cells with sub-diploid DNA content in endothelial, but not in HCT-116, human colon carcinoma and HT-1080 human fibrosarcoma cells. Treatment of endothelial cells with aeroplysinin-1 induces activation of caspases-2, -3, -8 and -9, as well as the cleavage of apoptotic substrates, such as poly (ADP-ribose) polymerase and lamin-A in a caspase-dependent mechanism. Our data indicate a relevant role of the mitochondria in the apoptogenic activity of this compound. The observation that aeroplysinin-1 prevents the phosphorylation of Bad relates to the mitochondria-mediated induction of apoptosis by this compound.

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.

Degradomics reveals that cleavage specificity profiles of caspase-2 and effector caspases are alike

Caspase-2 is considered an initiator caspase because its long prodomain contains a CARD domain that allows its recruitment and activation in several complexes by homotypic death domain-fold interactions. Because little is known about the function and specificity of caspase-2 and its physiological substrates, we compared the cleavage specificity profile of recombinant human caspase-2 with those of caspase-3 and -7 by analyzing cell lysates using N-terminal COmbined FRActional DIagonal Chromatography (COFRADIC). Substrate analysis of the 68 cleavage sites identified in 61 proteins revealed that the protease specificities of human caspases-2, -3, and -7 largely overlap, revealing the DEVD↓G consensus cleavage sequence. We confirmed that Asp(563) in eukaryotic translation initiation factor 4B (eIF4B) is a cleavage site preferred by caspase-2 not only in COFRADIC setup but also upon co-expression in HEK 293T cells. These results demonstrate that activated human caspase-2 shares remarkably overlapping protease specificity with the prototype apoptotic executioner caspases-3 and -7, suggesting that caspase-2 could function as a proapoptotic caspase once released from the activating complex.

PIDDosome-independent tumor suppression by Caspase-2

The PIDDosome, a multiprotein complex constituted of the ‘p53-induced protein with a death domain (PIDD), ‘receptor-interacting protein (RIP)-associated ICH-1/CED-3 homologous protein with a death domain' (RAIDD) and pro-Caspase-2 has been defined as an activating platform for this apoptosis-related protease. PIDD has been implicated in p53-mediated cell death in response to DNA damage but also in DNA repair and nuclear factor kappa-light-chain enhancer (NF-κB) activation upon genotoxic stress, together with RIP-1 kinase and Nemo/IKKγ. As all these cellular responses are critical for tumor suppression and deregulated expression of individual PIDDosome components has been noted in human cancer, we investigated their role in oncogenesis induced by DNA damage or oncogenic stress in gene-ablated mice. We observed that Pidd or Caspase-2 failed to suppress lymphoma formation triggered by γ-irradiation or 3-methylcholanthrene-driven fibrosarcoma development. In contrast, Caspase-2 showed tumor suppressive capacity in response to aberrant c-Myc expression, which did not rely on PIDD, the BH3-only protein Bid (BH3 interacting domain death agonist) or the death receptor ligand Trail (TNF-related apoptosis-inducing ligand), but associated with reduced rates of p53 loss and increased extranodal dissemination of tumor cells. In contrast, Pidd deficiency associated with abnormal M-phase progression and delayed disease onset, indicating that both proteins are differentially engaged upon oncogenic stress triggered by c-Myc, leading to opposing effects on tumor-free survival.

Folic acid supplementation increases survival and modulates high risk HPV-induced phenotypes in oral squamous cell carcinoma cells and correlates with p53 mRNA transcriptional down-regulation

BACKGROUND

Although the primary risk factors for developing oral cancers are well understood, less is known about the relationship among the secondary factors that may modulate the progression of oral cancers, such as high-risk human papillomavirus (HPV) infection and folic acid (FA) supplementation. This study examined high-risk HPV and FA supplementation effects, both singly and in combination, to modulate the proliferative phenotypes of the oral cancer cell lines CAL27, SCC25 and SCC15.

RESULTS

Using a comprehensive series of integrated in vitro assays, distinct effects of HPV infection and FA supplementation were observed. Both high-risk HPV strains 16 and 18 induced robust growth-stimulating effects in CAL27 and normal HGF-1 cells, although strain-specific responses were observed in SCC25 and SCC15 cells. Differential effects were also observed with FA administration, which significantly altered the growth rate of the oral cancer cell lines CAL27, SCC15, and SCC25, but not HGF-1 cells. Unlike HPV, FA administration induced broad, general increases in cell viability among all cell lines that were associated with p53 mRNA transcriptional down-regulation. None of these cell lines were found to harbor the common C677T mutation in methylenetetrahydrofolate reductase (MTHFR), which can reduce FA availability and may increase oral cancer risk.

CONCLUSION

Increased FA utilization and DNA hypermethylation are common features of oral cancers, and in these cell lines, specifically. The results of this study provide further evidence that FA antimetabolites, such as Fluorouracil (f5U or 5-FU) and Raltitrexed, may be alternative therapies for tumors resistant to other therapies. Moreover, since the incidence of oral HPV infection has been increasing, and can influence oral cancer growth, the relationship between FA bioavailability and concomitant HPV infection must be elucidated. This study is among the first pre-clinical studies to evaluate FA- and HPV-induced effects in oral cancers, both separately and in combination, which provides additional rationale for clinical screening of HPV infection prior to treatment.

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

The multifunctional caspase-2 protein is involved in apoptosis, NF-κB regulation, and tumor suppression in mice. However, the mechanisms of caspase-2 responsible for tumor suppression remain unclear. Here we identified two sites of caspase-2, the catalytic Cys-320 site and the Ser-139 site, to be important for suppression of cellular transformation and tumorigenesis. Using SV40- and K-Ras-transformed caspase-2 KO mouse embryonic fibroblast cells reconstituted with expression of wild-type, catalytic dead (C320A), or Ser-139 (S139A) mutant caspase-2, we demonstrated that similar to caspase-2 deficiency, when Cys-320 and Ser-139 were mutated, caspase-2 lost its ability to inhibit cellular transformation and tumorigenesis. These mutant cells exhibited enhanced cell proliferation, elevated clonogenic activity, accelerated anchorage-independent growth, and transformation and were highly tumorigenic, rapidly producing large tumors in athymic nude mice. Investigation into the underlying mechanism showed that these two residues are needed for caspase-2 to suppress NF-κB activity, promote apoptosis, and sustain the G(2)/M checkpoint following DNA damage induction. In addition, tumors in nude mice derived from the two mutant cell lines had higher constitutive NF-κB activity and elevated expression of NF-κB targets of antiapoptotic proteins Bcl-xL, XIAP, and cIAP2. A reduction in caspase-2 mRNA was associated with multiple types of cancers in patients. Together, these observations suggest the combined functions of caspase-2 in suppressing NF-κB activation, promoting apoptosis, and sustaining G(2)/M checkpoint contribute to caspase-2 tumor-suppressing function and that caspase-2 may also impact tumor suppression in humans. These findings provide insight into tumor suppression at the cross-roads of apoptosis, cell cycle checkpoint, and NF-κB pathways.

Chromosomal breaks during mitotic catastrophe trigger γH2AX-ATM-p53-mediated apoptosis

Although the cause and outcome of mitotic catastrophe (MC) has been thoroughly investigated, precisely how the ensuing lethality is regulated during or following this process and what signals are involved remain unknown. Moreover, the mechanism of the decision of cell death modalities following MC is still not well characterised. We demonstrate here a crucial role of the γH2AX-ATM-p53 pathway in the regulation of the apoptotic outcome of MC resulting from cells entering mitosis with damaged DNA. In addition to p53 deficiency, the depletion of ATM (ataxia telangiectasia mutated), but not ATR (ataxia telangiectasia and Rad3-related protein), protected against apoptosis and shifted cell death towards necrosis. Activation of this pathway is triggered by the augmented chromosomal damage acquired during anaphase in doxorubicin-treated cells lacking 14-3-3σ (also known as epithelial cell marker protein-1 or stratifin). Moreover, cells that enter mitosis with damaged DNA encounter segregation problems because of their abnormal chromosomes, leading to defects in mitotic exit, and they therefore accumulate in G1 phase. These multi- or micronucleated cells are prevented from cycling again in a p53- and p21-dependent manner, and subsequently die. Because increased chromosomal damage resulting in extensive H2AX phosphorylation appears to be a direct cause of catastrophic mitosis, our results describe a mechanism that involves generation of additional DNA damage during MC to eliminate chromosomally unstable cells.

P53-induced protein with a death domain (PIDD): master of puppets?

P53-induced protein with a death domain (PIDD) has been described as primary p53 target gene, induced upon DNA damage. More than 10 years after its discovery, its physiological role in the DNA damage response remains enigmatic, as it seems to be able to execute life-death decisions in vitro, yet genetic ablation in mice failed to reveal an obvious phenotype. Nonetheless, evidence is accumulating that it contributes to the fine-tuning of the DNA-damage response by orchestrating critical processes such as caspase activation or nuclear factor κB translocation and can also exert additional nuclear functions, for example, the modulation of translesion synthesis. In this review, we aim to integrate these observations and propose possible unexplored functions of PIDD.

Caspases and cancer

Evasion of apoptosis is considered to be one of the hallmarks of human cancers. This cell death modality is executed by caspases and several upstream regulatory factors, which direct their proteolytic activity, have been defined as either tumor suppressors or oncogenes. Often these regulatory factors, in addition to being potent apoptosis inducers, function in cell survival or repair signaling pathways in response to cellular stress. Thus, loss of function in a distinct regulatory mechanism does not necessarily mean that tumor formation is due to apoptosis malfunction resulting from insufficient caspase activation. Although each caspase has been assigned a distinct role in apoptosis, some redundancy with respect to their regulatory functions and substrate recognition is evident. Jointly, these proteases could be considered to possess solid tumor suppressor function, but what is the evidence that deregulation of specific caspases per se induces inappropriate cell survival, leading to enhanced tumorigenic potential? This question will be addressed in this review, which covers basic molecular mechanisms derived from in vitro analyses and emphasizes new insights that have emerged from in vivo and clinical studies.

Caspase-2 is required for DNA damage-induced expression of the CDK inhibitor p21(WAF1/CIP1)

Although caspase-2 represents the most conserved caspase across species and was the second caspase identified, its precise function remains enigmatic. In several cell types we show that knockdown of caspase-2 specifically impaired DNA damage-induced p21 expression, whereas overexpression of a caspase-2 mutant increased p21 levels. Caspase-2 did not influence p21 mRNA transcription; moreover, various inhibitors targeting proteasomal or non-proteasomal proteases, including caspases, could not restore p21 protein levels following knockdown of caspase-2. As, however, silencing of caspase-2 impaired exogenous expression of p21 constructs containing 3'-UTR sequences, our results strongly indicate that caspase-2 regulates p21 expression at the translational level. Intriguingly, unlike depletion of caspase-2, which prevented p21 expression and thereby reverted the γ-IR-induced senescent phenotype of wild-type HCT116 colon carcinoma cells into apoptosis, knockdown of none of the caspase-2-interacting components RAIDD, RIP or DNA-PKcs was able to mimic these processes. Together, our data suggest that this novel role of caspase-2 as a translational regulator of p21 expression occurs not only independently of its enzymatic activity but also does not require known caspase-2-activating platforms.

Cell death by the quinoxaline dioxide DCQ in human colon cancer cells is enhanced under hypoxia and is independent of p53 and p21

INTRODUCTION

We have shown that the radio sensitizer DCQ enhances sensitivity of HCT116 human colon cancer cells to hypoxia. However, it is not known whether the p53 or p21 genes influence cellular response to DCQ. In this study, we used HCT116 that are either wildtype for p53 and p21, null for p53 or null for p21 to understand the role of these genes in DCQ toxicity.

METHODS

HCT116 cells were exposed to DCQ and incubated under normoxia or hypoxia and the viability, colony forming ability, DNA damage and apoptotic responses of these cells was determined, in addition to the modulation of HIF-1α and of p53, p21, caspase-2, and of the ataxia telangiectasia mutated (ATM) target PIDD-C.

RESULTS

DCQ decreased colony forming ability and viability of all HCT116 cells to a greater extent under hypoxia than normoxia and the p21-/-cell line was most sensitive. Cells had different HIF-1α responses to hypoxia and/or drug treatment. In p53+/+, DCQ significantly inhibited the hypoxia-induced increases in HIF-1α protein, in contrast to the absence of a significant HIF-1α increase or modulation by DCQ in p21-/- cells. In p53-/- cells, 10 μM DCQ significantly reduced HIF-1α expression, especially under hypoxia, despite the constitutive expression of this protein in control cells. Higher DCQ doses induced PreG1-phase increase and apoptosis, however, lower doses caused mitotic catastrophe. In p53+/+ cells, apoptosis correlated with the increased expression of the pro-apoptotic caspase-2 and inhibition of the pro-survival protein PIDD-C. Exposure of p53+/+ cells to DCQ induced single strand breaks and triggered the activation of the nuclear kinase ATM by phosphorylation at Ser-1981 in all cell cycle phases. On the other hand, no drug toxicity to normal FHs74 Int human intestinal cell line was observed.

CONCLUSIONS

Collectively, our findings indicate that DCQ reduces the colony survival of HCT116 and induces apoptosis even in cells that are null for p53 or p21, which makes it a molecule of clinical significance, since many resistant colon tumors harbor mutations in p53.

Cranberry and grape seed extracts inhibit the proliferative phenotype of oral squamous cell carcinomas

Proanthocyanidins, compounds highly concentrated in dietary fruits, such as cranberries and grapes, demonstrate significant cancer prevention potential against many types of cancer. The objective of this study was to evaluate cranberry and grape seed extracts to quantitate and compare their anti-proliferative effects on the most common type of oral cancer, oral squamous cell carcinoma. Using two well-characterized oral squamous cell carcinoma cell lines, CAL27 and SCC25, assays were performed to evaluate the effects of cranberry and grape seed extract on phenotypic behaviors of these oral cancers. The proliferation of both oral cancer cell lines was significantly inhibited by the administration of cranberry and grape seed extracts, in a dose-dependent manner. In addition, key regulators of apoptosis, caspase-2 and caspase-8, were concomitantly up-regulated by these treatments. However, cranberry and grape seed extracts elicited differential effects on cell adhesion, cell morphology, and cell cycle regulatory pathways. This study represents one of the first comparative investigations of cranberry and grape seed extracts and their anti-proliferative effects on oral cancers. Previous findings using purified proanthocyanidin from grape seed extract demonstrated more prominent growth inhibition, as well as apoptosis-inducing, properties on CAL27 cells. These observations provide evidence that cranberry and grape seed extracts not only inhibit oral cancer proliferation but also that the mechanism of this inhibition may function by triggering key apoptotic regulators in these cell lines. This information will be of benefit to researchers interested in elucidating which dietary components are central to mechanisms involved in the mediation of oral carcinogenesis and progression.

Dz13, a c-jun DNAzyme, is a potent inducer of caspase-2 activation

Signaling pathways for caspase-2-mediated apoptosis are poorly defined. This is partially due to a lack of a reproducible stimulus to trigger caspase-2 activation. We present the oligonucleotide Dz13, a DNA enzyme that cleaves c-Jun mRNA and is capable of inhibiting various model tumors in mice, which potently induces caspase-2 resulting in apoptosis in a panel of tumor cell lines. Dz13-mediated cell death occurred even in the absence of known caspase-2 molecular partners in p53-induced protein with a death domain, RIP-associated Ich-1/CED homologous protein with death domain, or DNA-dependent protein kinase catalytic subunit, or other caspases in cell lines of breast cancer, prostate cancer, osteosarcoma, and liposarcoma. z-VDVAD-fmk, caspase-2(-/-) mouse embryonic fibroblasts and siRNA silencing of caspase-2 in tumor cells abrogated Dz13-mediated cell death. In an orthotopic tumor model, expression of caspase-2 increased as the tumor metastasized and caspase-2 expression was sporadic in patient tumor specimens. These findings provide hope that Dz13, and other agents that evoke activation of caspase-2, may be therapeutic clinically.