The essential role of evasion from cell death in cancer

Genome-wide prediction and validation of peptides that bind human prosurvival Bcl-2 proteins

Programmed cell death is regulated by interactions between pro-apoptotic and prosurvival members of the Bcl-2 family. Pro-apoptotic family members contain a weakly conserved BH3 motif that can adopt an alpha-helical structure and bind to a groove on prosurvival partners Bcl-x_L, Bcl-w, Bcl-2, Mcl-1 and Bfl-1. Peptides corresponding to roughly 13 reported BH3 motifs have been verified to bind in this manner. Due to their short lengths and low sequence conservation, BH3 motifs are not detected using standard sequence-based bioinformatics approaches. Thus, it is possible that many additional proteins harbor BH3-like sequences that can mediate interactions with the Bcl-2 family. In this work, we used structure-based and data-based Bcl-2 interaction models to find new BH3-like peptides in the human proteome. We used peptide SPOT arrays to test candidate peptides for interaction with one or more of the prosurvival proteins Bcl-x_L, Bcl-w, Bcl-2, Mcl-1 and Bfl-1. For the 36 most promising array candidates, we quantified binding to all five human receptors using direct and competition binding assays in solution. All 36 peptides showed evidence of interaction with at least one prosurvival protein, and 22 peptides bound at least one prosurvival protein with a dissociation constant between 1 and 500 nM; many peptides had specificity profiles not previously observed. We also screened the full-length parent proteins of a subset of array-tested peptides for binding to Bcl-x_L and Mcl-1. Finally, we used the peptide binding data, in conjunction with previously reported interactions, to assess the affinity and specificity prediction performance of different models.

Bcl-2 family proteins regulate key cell death vs. survival decisions and are implicated in the development of many cancers. To understand the roles of Bcl-2 family proteins in both normal and diseased cells, it is important to map the interaction network of the family. Low sequence conservation in known Bcl-2 interaction motifs precludes easy identification of possible binding partners, but we developed computational models based on structure and experimental mutation data that show good predictive performance. We used our models to search the human proteome for new Bcl-2 interaction partners. We predicted and experimentally validated more than twice as many tight-binding peptides as were previously known.

The first endogenous herpesvirus, identified in the tarsier genome, and novel sequences from primate rhadinoviruses and lymphocryptoviruses

Herpesviridae is a diverse family of large and complex pathogens whose genomes are extremely difficult to sequence. This is particularly true for clinical samples, and if the virus, host, or both genomes are being sequenced for the first time. Although herpesviruses are known to occasionally integrate in host genomes, and can also be inherited in a Mendelian fashion, they are notably absent from the genomic fossil record comprised of endogenous viral elements (EVEs). Here, we combine paleovirological and metagenomic approaches to both explore the constituent viral diversity of mammalian genomes and search for endogenous herpesviruses. We describe the first endogenous herpesvirus from the genome of the Philippine tarsier, belonging to the Roseolovirus genus, and characterize its highly defective genome that is integrated and flanked by unambiguous host DNA. From a draft assembly of the aye-aye genome, we use bioinformatic tools to reveal over 100,000 bp of a novel rhadinovirus that is the first lemur gammaherpesvirus, closely related to Kaposi's sarcoma-associated virus. We also identify 58 genes of Pan paniscus lymphocryptovirus 1, the bonobo equivalent of human Epstein-Barr virus. For each of the viruses, we postulate gene function via comparative analysis to known viral relatives. Most notably, the evidence from gene content and phylogenetics suggests that the aye-aye sequences represent the most basal known rhadinovirus, and indicates that tumorigenic herpesviruses have been infecting primates since their emergence in the late Cretaceous. Overall, these data show that a genomic fossil record of herpesviruses exists despite their extremely large genomes, and expands the known diversity of Herpesviridae, which will aid the characterization of pathogenesis. Our analytical approach illustrates the benefit of intersecting evolutionary approaches with metagenomics, genetics and paleovirology.

Herpesviridae is a family of DNA viruses that have characteristically large and complex genomes. This defining feature is also responsible for bioinformatic challenges that complicate herpesvirus genomics, and why an endogenous herpesvirus remains elusive. Given that several species of herpesvirus are clinically relevant to humans, there is a pressing demand for techniques capable of generating and managing large quantities of herpesvirus genome data. This is coupled with a need to explore herpesvirus diversity in order to understand pathogenesis within an evolutionary context. Lessons from the study of ancient viral integrations have also highlighted the need to include information offered by paleoviruses. Using perspectives from paleovirology and metagenomics, we identify three herpesviruses within the genome data of their primate hosts, including the first endogenous herpesvirus. All three viruses are closely related to important human pathogens and two of them are entirely new species. Both comparative molecular biology and evolutionary analysis were applied to examine our results for their clinical relevance. Furthermore, we demonstrate how this analytical approach was also used for the data collection itself, by treating nucleotide databases in their entirety as a single metagenomic resource.

Small molecules as pro-apoptotic anticancer agents

The quest for potent and selective targeted therapies in anticancer research is taking advantage of apoptosis-related mechanisms of action to identify a number of novel clinical candidates. This review is chemically focused on small molecules and deals with five target families that influence caspase-dependent apoptosis: caspase-3, Bcl-2 and IAP protein family members, p53 and the proteasome. Each target class is briefly described at first in terms of its involvement and relevance in tumor initiation and progression. Drug candidates currently undergoing clinical trials are then presented for each target class, followed by a quick summary of target-modulating chemotypes that have appeared in patent literature since 2006. Finally, future trends likely to become significant in apoptosis-targeted cancer therapies are presented and discussed.

Targeting of MCL-1 kills MYC-driven mouse and human lymphomas even when they bear mutations in p53

The transcriptional regulator c-MYC is abnormally overexpressed in many human cancers. Evasion from apoptosis is critical for cancer development, particularly c-MYC-driven cancers. We explored which anti-apoptotic BCL-2 family member (expressed under endogenous regulation) is essential to sustain c-MYC-driven lymphoma growth to reveal which should be targeted for cancer therapy. Remarkably, inducible Cre-mediated deletion of even a single Mcl-1 allele substantially impaired the growth of c-MYC-driven mouse lymphomas. Mutations in p53 could diminish but not obviate the dependency of c-MYC-driven mouse lymphomas on MCL-1. Importantly, targeting of MCL-1 killed c-MYC-driven human Burkitt lymphoma cells, even those bearing mutations in p53. Given that loss of one allele of Mcl-1 is well tolerated in healthy tissues, our results suggest that therapeutic targeting of MCL-1 would be an attractive therapeutic strategy for MYC-driven cancers.

Synthesis and antiproliferative activity of novel selenoester derivatives

A series of 31 new selenoesters were synthesized and their cytotoxic activity was evaluated against a prostate cancer cell line (PC-3). The most active compounds were also tested against three tumoural cell lines (MCF-7, A-549 and HT-29) and one non-tumour prostate cell line (RWPE-1). Thirteen compounds showed significant activity towards all tumour cells investigated, and some of them were even more potent than etoposide and cisplatin, which were used as reference drugs. Because of their pronounced potency and/or selectivity, four analogues (5, 21, 28 and 30), were selected in order to assess their redox properties related to a possible redox modulating activity. The glutathione peroxidase (GPx) assay showed slight activity for compound 30 and the 2,2-diphenyl-1-picrylhydrazyl-(DPPH) assay showed a weak activity for compounds 5 and 28. The present results revealed that analogues 5, 21, 28 and 30 might serve as a useful starting point for the design of improved anti-tumour agents.

LRP5 negatively regulates differentiation of monocytes through abrogation of Wnt signalling

Molecular changes involved in cell differentiation are only partially known. Circulating inflammatory cells need to differentiate to perform specialized functions in target tissues. Here, we hypothesized that low-density lipoprotein receptor–related protein 5 (LRP5) is involved, through its participation in the canonical Wnt/β-catenin signalling, in the differentiation process of monocytic cells. To this aim, we characterized differentiation mechanisms of HL60 cells and primary human monocytes. We show that silencing the LRP5 gene increased differentiation of HL60 cells and human monocytes, suggesting that LRP5 signalling abrogates differentiation. We demonstrate that the mechanisms behind this blockade include sequestration of β-catenin at the cellular membrane, inhibition of the Wnt signalling and increase of apoptosis. We further demonstrate the involvement of LRP5 and the Wnt/β-catenin signalling in the process because cellular differentiation can be rescued by the addition of downstream Wnt target genes to the monocytic cells.

Targeting BCL2 family in human myeloid dendritic cells: a challenge to cure diseases with chronic inflammations associated with bone loss

Rheumatoid arthritis (RA) and Langerhans cell histiocytosis (LCH) are common and rare diseases, respectively. They associate myeloid cell recruitment and survival in inflammatory conditions with tissue destruction and bone resorption. Manipulating dendritic cell (DC), and, especially, regulating their half-life and fusion, is a challenge. Indeed, these myeloid cells display pathogenic roles in both diseases and may be an important source of precursors for differentiation of osteoclasts, the bone-resorbing multinucleated giant cells. We have recently documented that the proinflammatory cytokine IL-17A regulates long-term survival of DC by inducing BCL2A1 expression, in addition to the constitutive MCL1 expression. We summarize bibliography of the BCL2 family members and their therapeutic targeting, with a special emphasis on MCL1 and BCL2A1, discussing their potential impact on RA and LCH. Our recent knowledge in the survival pathway, which is activated to perform DC fusion in the presence of IL-17A, suggests that targeting MCL1 and BCL2A1 in infiltrating DC may affect the clinical outcomes in RA and LCH. The development of new therapies, interfering with MCL1 and BCL2A1 expression, to target long-term surviving inflammatory DC should be translated into preclinical studies with the aim to increase the well-being of patients with RA and LCH.

Long non-coding RNA GAS5 regulates apoptosis in prostate cancer cell lines

While the role of small non-coding RNAs, such as miRNAs, in apoptosis control is well established, long non-coding RNAs (lncRNAs) have received less attention. Growth Arrest-Specific 5 (GAS5) encodes multiple snoRNAs within its introns, while exonic sequences produce lncRNA which can act as a riborepressor of the glucocorticoid and related receptors. GAS5 negatively regulates the survival of lymphoid and breast cells, and is aberrantly expressed in several cancers. Although cellular GAS5 levels decline as prostate cancer cells acquire castration-resistance, the influence of GAS5 on prostate cell survival has not been determined. To address this question, prostate cell lines were transfected with GAS5-encoding plasmids or GAS5 siRNAs, and cell survival was assessed. Basal apoptosis increased, and cell survival decreased, after transfection of 22Rv1 cells with plasmids encoding GAS5 transcripts, including mature GAS5 lncRNA. Similar effects were observed in PC-3 cells. In stable clones of 22Rv1, cell death correlated strongly with cellular GAS5 levels. Induction of 22Rv1 cell death by UV-C irradiation and chemotherapeutic drugs was augmented in cells transiently transfected with GAS5 constructs, and attenuated following down-regulation of GAS5 expression. Again, in these experiments, cell death was strongly correlated with cellular GAS5 levels. Thus, GAS5 promotes the apoptosis of prostate cells, and exonic sequence, i.e. GAS5 lncRNA, is sufficient to mediate this activity. Abnormally low levels of GAS5 expression may therefore reduce the effectiveness of chemotherapeutic agents. Although several lncRNAs have recently been shown to control cell survival, this is the first report of a death-promoting lncRNA in prostate cells.

A novel interaction of nucleophosmin with BCL2-associated X protein regulating death evasion and drug sensitivity in human hepatoma cells

Death evasion is crucial for both carcinogenesis and resistance to anticancer therapies. Recently, we identified nucleophosmin (NPM) as a key factor counteracting death stimuli in human hepatocellular carcinoma (HCC) cells. Here we report the identification of a novel NPM-BCL2-associated X protein (BAX) pathway orchestrating death evasion in human HCC cells. Silencing of NPM expression significantly sensitized HCC cells-particularly those bearing inactivated p53 gene (Huh7, Hep3B, and Mahlavu)-to ultraviolet irradiation, mitomycin C, doxorubicin, cisplatin, sorafenib, and lapatinib. This sensitizing effect was not changed further, as p53 expression had been simultaneously silenced. Following cell stress, NPM and BAX were induced and exported out of the nucleoli and nucleus, respectively. BAX was translocated to cytoplasm in cells with relatively high NPM level, or accumulated in the mitochondria in cells with relatively low NPM level and undergoing apoptosis. Subcellular fractionation revealed that silencing of NPM expression greatly enhanced mitochondrial translocation and oligomerization of BAX in Huh7 and Mahlavu cells. In situ proximity ligation assays and reciprocal co-immunoprecipitation revealed a direct interaction between NPM and BAX in the cytoplasm. Silencing of BAX expression abolished the sensitization effect exerted by silencing of NPM in HCC cells. Clinically, up-regulation of NPM was significantly associated with advanced tumor stage and poor prognosis.

CONCLUSION

By directly blockading BAX mitochondrial translocation and activation, NPM helps human HCC cells evade death induction independently of p53-mediated cell death. Silencing of NPM significantly sensitized HCC cells to anticancer therapies. NPM is a potential cotarget in combination with other therapies for HCC, particularly those that harbor inactivated p53 gene. Our findings are of clinical significance because NPM up-regulation and p53 mutations are usually found in advanced human cancers, including HCC.

Mitochondrial quality control: impact on aging and life span - a mini-review

A fundamental impact of mitochondria on biological aging has been suggested decades ago. One prominent theory explains aging as the result of the age-related accumulation of random molecular damage of biomolecules resulting from the reaction of reactive oxygen species, the majority of which are generated in mitochondria. Although this concept appeared to be very attractive and strongly influenced aging research, in recent years more and more data accumulated which seem to contradict this theory. However, since these data are derived from reductionist approaches and do not integrate the various components and pathways which are affected as a result of a primary experimental intervention, they are prone to misinterpretation and have to be taken with some caution. Here, after a general introduction of mitochondrial function, we discuss the relevance of various pathways which are involved in keeping mitochondria functional over time. Moreover, we provide examples which emphasize the importance of a critical interpretation of experimental data and the necessity for a holistic analysis of the aging process. The success of such a systems biology approach is strongly dependent on the development of methods for data mining and an efficient analysis and modeling of the huge data sets that are raised.

The presence of HIV-1 Tat protein second exon delays fas protein-mediated apoptosis in CD4+ T lymphocytes: a potential mechanism for persistent viral production

HIV-1 replication is efficiently controlled by the regulator protein Tat (101 amino acids) and codified by two exons, although the first exon (1-72 amino acids) is sufficient for this process. Tat can be released to the extracellular medium, acting as a soluble pro-apoptotic factor in neighboring cells. However, HIV-1-infected CD4(+) T lymphocytes show a higher resistance to apoptosis. We observed that the intracellular expression of Tat delayed FasL-mediated apoptosis in both peripheral blood lymphocytes and Jurkat cells, as it is an essential pathway to control T cell homeostasis during immune activation. Jurkat-Tat cells showed impairment in the activation of caspase-8, deficient release of mitochondrial cytochrome c, and delayed activation of both caspase-9 and -3. This protection was due to a profound deregulation of proteins that stabilized the mitochondrial membrane integrity, such as heat shock proteins, prohibitin, or nucleophosmin, as well as to the up-regulation of NF-κB-dependent anti-apoptotic proteins, such as BCL2, c-FLIPS, XIAP, and C-IAP2. These effects were observed in Jurkat expressing full-length Tat (Jurkat-Tat101) but not in Jurkat expressing the first exon of Tat (Jurkat-Tat72), proving that the second exon, and particularly the NF-κB-related motif ESKKKVE, was necessary for Tat-mediated protection against FasL apoptosis. Accordingly, the protection exerted by Tat was independent of its function as a regulator of both viral transcription and elongation. Moreover, these data proved that HIV-1 could have developed strategies to delay FasL-mediated apoptosis in infected CD4(+) T lymphocytes through the expression of Tat, thus favoring the persistent replication of HIV-1 in infected T cells.

TPEN induces apoptosis independently of zinc chelator activity in a model of acute lymphoblastic leukemia and ex vivo acute leukemia cells through oxidative stress and mitochondria caspase-3- and AIF-dependent pathways

Acute lymphoblastic leukemia is still an incurable disease with resistance to therapy developing in the majority of patients. We investigated the effect of TPEN, an intracellular zinc chelator, in Jurkat and in ex vivo acute lymphoblastic leukemia (ALL) cells resistant to chemotherapy. Changes of nuclei morphology, reactive oxygen species generation, presence of hypodiploid cells, phosphatidylserine translocation, mitochondrial membrane depolarization, immunohistochemical identification of cell death signalling molecules, and pharmacological inhibition were assayed to detect the apoptotic cell death pathways. We found that TPEN induces apoptosis in both types of cells by a molecular oxidative stress pathway involving O(2)(•-) > H(2)O(2) >> NF-κB (JNK/c-Jun) >p53> loss ΔΨ(m)> caspase-3, AIF > chromatin condensation/DNA fragmentation. Interestingly, TPEN induced apoptosis independently of glucose; leukemic cells are therefore devoid of survival capacity by metabolic resistance to treatment. Most importantly, TPEN cytotoxic effect can eventually be regulated by the antioxidant N-acetyl-cysteine and zinc ions. Our data suggest that TPEN can be used as a potential therapeutic prooxidant agent against refractory leukemia. These data contribute to understanding the importance of oxidative stress in the treatment of ALL.

Defective DNA repair systems and the development of breast and prostate cancer (review)

Genetic defects in DNA repair and DNA damage response genes often lead to an increase in cancer incidence. The role of defects is also associated with the modulation of hormone signaling pathways. A number of studies have suggested a role for estrogen in the regulation of DNA repair activity. Furthermore, mutations or epigenetic silencing in DNA repair genes have been associated with the sensitivity of cancers to hormonal therapy. The molecular basis for the progression of cancers from hormone-dependent to hormone-independent remains a critical issue in the management of these types of cancer. In the present review, we aimed to summarize the function of DNA repair molecules from the viewpoint of carcinogenesis and hormone-related cell modulation, providing a comprehensive view of the molecular mechanisms by which hormones may exert their effects on the regulation of tumor progression.

Integrin/Fak/Src-mediated regulation of cell survival and anoikis in human intestinal epithelial crypt cells: selective engagement and roles of PI3-K isoform complexes

In human intestinal epithelial crypt (HIEC) cells, the PI3-K/Akt-1 pathway is crucial for the promotion of cell survival and suppression of anoikis. Class I PI3-K consists of a complex formed by a catalytic (C) and regulatory (R) subunit. Three R (p85α, β, and p55γ) and four C (p110α, β, γ and δ) isoforms are known. Herein, we analyzed the expression of PI3-K isoforms in HIEC cells and determined their roles in cell survival, as well as in the β1 integrin/Fak/Src-mediated suppression of anoikis. We report that: (1) the predominant PI3-K complexes expressed by HIEC cells are p110α/p85β and p110α/p55γ; (2) the inhibition and/or siRNA-mediated expression silencing of p110α, but not that of p110β, γ or δ, results in Akt-1 down-activation and consequent apoptosis; (3) the expression silencing of p85β or p55γ, but not that of p85α, likewise induces Akt-1 down-activation and apoptosis; however, the impact of a loss of p55γ on both Akt-1 activation and cell survival is significantly greater than that from the loss of p85β; and (4) both the p110α/p85β and p110α/p55γ complexes are engaged by β1 integrin/Fak/Src signaling; however, the engagement of p110α/p85β is primarily Src-dependent, whereas that of p110α/p55γ is primarily Fak-dependent (but Src-independent). Hence, HIEC cells selectively express PI3-K isoform complexes, translating into distinct roles in Akt-1 activation and cell survival, as well as in a selective engagement by Fak and/or Src within the context of β1 integrin/Fak/Src-mediated suppression of anoikis.

Carbon monoxide mediates the anti-apoptotic effects of heme oxygenase-1 in medulloblastoma DAOY cells via K+ channel inhibition

Tumor cell survival and proliferation is attributable in part to suppression of apoptotic pathways, yet the mechanisms by which cancer cells resist apoptosis are not fully understood. Many cancer cells constitutively express heme oxygenase-1 (HO-1), which catabolizes heme to generate biliverdin, Fe(2+), and carbon monoxide (CO). These breakdown products may play a role in the ability of cancer cells to suppress apoptotic signals. K(+) channels also play a crucial role in apoptosis, permitting K(+) efflux which is required to initiate caspase activation. Here, we demonstrate that HO-1 is constitutively expressed in human medulloblastoma tissue, and can be induced in the medulloblastoma cell line DAOY either chemically or by hypoxia. Induction of HO-1 markedly increases the resistance of DAOY cells to oxidant-induced apoptosis. This effect was mimicked by exogenous application of the heme degradation product CO. Furthermore we demonstrate the presence of the pro-apoptotic K(+) channel, Kv2.1, in both human medulloblastoma tissue and DAOY cells. CO inhibited the voltage-gated K(+) currents in DAOY cells, and largely reversed the oxidant-induced increase in K(+) channel activity. p38 MAPK inhibition prevented the oxidant-induced increase of K(+) channel activity in DAOY cells, and enhanced their resistance to apoptosis. Our findings suggest that CO-mediated inhibition of K(+) channels represents an important mechanism by which HO-1 can increase the resistance to apoptosis of medulloblastoma cells, and support the idea that HO-1 inhibition may enhance the effectiveness of current chemo- and radiotherapies.

Investigation of mechanisms underlying inhibition of apoptosis-related signaling inhibited by survivin in human cholangiocarcinoma cells

AIM: To explore the mechanisms by which survivin regulates apoptosis-related signaling in human cholangiocarcinoma cells.

METHODS: SiRNA targeting the survivin gene and control siRNA were constructed and transfected into human cholangiocarcinoma cell line QBC939. The knockdown of survivin expression in QBC939 cells was confirmed by Western blot. Apoptosis rate was evaluated by flow cytometry. Capase-3 activity was determined using a commercial kit, and expression of caspase-3, caspase-9 and procaspase-9 in QBC939 cells was detected by Western blot.

RESULTS: Transfection of siRNA targeting the survivin gene significantly inhibited survivin expression in QBC939 cells (P < 0.05). Inhibition of survivin significantly increased apoptosis rate (18.9% ± 2.3%, P < 0.05) and caspase-3 activity (0.83 ± 0.15, P < 0.01), up-regulated the expression of caspase-3 and caspase-9 (both P < 0.05), and down-regulated the expression of procaspase-9 (P < 0.05) in QBC939 cells. No significant differences were observed in the above parameters between non-transfected QBC939 cells and cells transfected with control siRNA (all P > 0.05).

CONCLUSION: Survivin inhibits apoptosis of cholangiocarcinoma cells possibly by inhibiting caspase-3 and caspase-9 activities through activating procaspase-9.