Induction of apoptosis and mitosis inhibition by degraded DNA lipotransfection mimicking genotoxic drug effects

Genotoxic damage induces cell cycle arrest and/or apoptosis by activation of p53 oncosuppressor protein. A number of anticancer drugs are genotoxic and their damaging effect upon cells is mediated by this mechanism. Microinjection of defined DNA species directly into nucleus has been reported previously to activate p53 and inhibit cell cycle. Here, we demonstrate that simple addition of heterogeneous degraded DNA to cultured cells (Rat-1 fibroblasts) in combination with lipotransfecting agent DOTAP leads to apoptosis induction and mitosis inhibition by a molecular mechanism which mimics that of the cellular response to genotoxic anticancer agents. Indeed, both cellular effects induced by lipotransfected degraded DNA (essentially, heterogeneous small DNA fragments) are associated to p53 activation and modulated by two apoptosis-related genes, such as bcl-2 and c-myc, which also modulate the apoptotic threshold to anticancer agents. Here we raise the hypothesis of exogenous DNA segment lipotransfection as possible new tool for anticancer therapy.     

Requirements for effective inhibition of immunostimulatory CpG motifs by neutralizing motifs

The DNA of bacteria and many viruses contain unmethylated CpG dinucleotides in particular sequence contexts that activate vertebrate immune cells. A subset of these CpG motifs was previously found to oppose the effects of immunostimulatory (CpG-S) motifs and has been termed neutralizing (CpG-N) motifs. Here we show that oligodeoxynucleotides (ODNs) composed of clusters of CpG-N motifs could partially inhibit the induction of interleukin-12 (IK-12) from mouse spleen cells by ODN containing CpG-S motifs. However, non-CpG-containing ODN were also inhibitory, suggesting that neutralization of CpG-S ODNs by CpG-N ODNs in trans was nonspecific. Neutralization of CpG-S motifs by CpG-N motifs in cis was specific, but the degree of inhibition was strongly dependent on the particular CpG-S motif being neutralized, with motifs having an A residue 5' to the CG being much more resistant to inhibition than motifs having a T residue 5' to the CG. The degree of inhibition was dependent on the spacing between the CpG-S and CpG-N motifs, with the ability to neutralize inversely correlating with distance. In addition, whereas ODNs containing extended clusters of CpG-N motifs were nonstimulatory, isolated CpG-N motifs remained stimulatory in most sequence contexts. Finally, CpG-N ODNs were shown to be nonstimulatory when instilled into the lungs of BALB/c mice, but the ability of CpG-N motifs to neutralize CpG-S motifs in cis was not observed. These results show that there are precise and fairly complex interactions between immunostimulatory and inhibitory sequence motifs that govern whether a given DNA is able to activate the vertebrate immune system.     

Modulation of CpG oligodeoxynucleotide-mediated immune stimulation by locked nucleic acid (LNA)

Locked nucleic acid (LNA) is an RNA derivative that when introduced into oligodeoxynucleotides (ODN), mediates high efficacy and stability. CpG ODNs are potent immune stimulators and are recognized by toll-like receptor-9 (TLR9). Some phosphorothioate antisense ODNs bearing CpG dinucleotides have been shown to possess immune modulatory capacities. We investigated the effects of LNA substitutions on immune stimulation mediated by antisense ODN G3139 or CpG ODN 2006. LNA ODNs were tested for their ability to stimulate cytokine secretion from human immune cells or TLR9-dependent signaling. Phosphorothioate chimeric LNA/DNA antisense ODNs with phosphodiester-linked LNA nucleobases at both ends showed a marked decrease of immune modulation with an increasing number of 3' and 5' LNA bases. In addition, guanosine-LNA and cytosine-LNA or simply cytosine-LNA substitutions in the CpG dinucleotides of ODN 2006 led to strong decrease or near complete loss of immune modulation. TLR9-mediated signaling was similarly affected. These data indicate that increasing amounts of LNA residues in the flanks or substitutions of CpG nucleobases with LNA reduce or eliminate the immune stimulatory effects of CpG-containing phosphorothioate ODN.     

Essential postmitochondrial function of p53 uncovered in DNA damage-induced apoptosis in neurons

In postmitotic sympathetic neurons, unlike most mitotic cells, death by apoptosis requires not only the release of cytochrome c from the mitochondria, but also an additional step to relieve X-linked inhibitor of apoptosis protein (XIAP)'s inhibition of caspases. Here, we examined the mechanism by which XIAP is inactivated following DNA damage and found that it is achieved by a mechanism completely different from that following apoptosis by nerve growth factor (NGF) deprivation. NGF deprivation relieves XIAP by selectively degrading it, whereas DNA damage overcomes XIAP via a p53-mediated induction of Apaf-1. Unlike wild-type neurons, p53-deficient neurons fail to overcome XIAP and remain resistant to cytochrome c after DNA damage. Restoring Apaf-1 induction in p53-deficient neurons is sufficient to overcome XIAP and sensitize cells to cytochrome c. Although a role for p53 in apoptosis upstream of cytochrome c release has been well established, this study uncovers an additional, essential role for p53 in regulating caspase activation downstream of mitochondria following DNA damage in neurons.     

Polyoma virus middle T and small t antigens cooperate to antagonize p53-induced cell cycle arrest and apoptosis.

Wild-type p53 triggers two distinct biological responses, cell cycle arrest and apoptosis. Several small DNA tumor viruses encode proteins that bind p53 and thus block the function of p53. This probably reflects the need of these viruses to prevent p53-induced cell cycle arrest and apoptosis to allow viral DNA replication. Unlike SV40 large T, polyoma virus large T does not bind p53, and it is still unclear how polyoma virus blocks p53 function. To address this question, we transfected polyoma virus middle T or small t alone or middle T and small t together into J3D mouse T-lymphoma cells carrying temperature-sensitive p53 (ts p53). Induction of wild-type p53 by temperature shift to 32 degrees C triggered both G1 cell cycle arrest and apoptosis in parental J3D-ts p53 cells. In contrast, J3D-ts p53 cells coexpressing middle T and small t showed only a weak G1 cell cycle arrest response after induction of wild-type p53 at 32 degrees C. Fluorescence-activated cell sorter analysis revealed that nearly half of the middle T-expressing cells, 30% of the small t-expressing cells, and a majority of the cells coexpressing middle T and small t were resistant to p53-induced apoptosis. The phosphatidylinositol 3-kinase inhibitor wortmannin partially abrogated the protective effect of middle T but not small t on p53-induced apoptosis, indicating that middle T prevents p53-induced apoptosis through the phosphatidylinositol 3-kinase signal transduction pathway. Our results thus establish a mechanism for polyoma virus-mediated inhibition of p53 function.     

Regulation of p53 translation and induction after DNA damage by ribosomal protein L26 and nucleolin

Increases in p53 protein levels after DNA damage have largely been attributed to an increase in the half-life of p53 protein. Here we demonstrate that increased translation of p53 mRNA is also a critical step in the induction of p53 protein in irradiated cells. Ribosomal protein L26 (RPL26) and nucleolin were found to bind to the 5' untranslated region (UTR) of p53 mRNA and to control p53 translation and induction after DNA damage. RPL26 preferentially binds to the 5'UTR after DNA damage, and its overexpression enhances association of p53 mRNA with heavier polysomes, increases the rate of p53 translation, induces G1 cell-cycle arrest, and augments irradiation-induced apoptosis. Opposite effects were seen when RPL26 expression was inhibited. In contrast, nucleolin overexpression suppresses p53 translation and induction after DNA damage, whereas nucleolin downregulation promotes p53 expression. These findings demonstrate the importance of increased translation of p53 in DNA-damage responses and suggest critical roles for RPL26 and nucleolin in affecting p53 induction.     

Signaling Crosstalk of FHIT,p53, and p38 in etoposide-induced apoptosis in MCF-7 cells

Fragile histidine trail (FHIT) is a tumor suppressor in response to DNA damage which has been deleted in various tumors. However, the signaling mechanisms and interactions of FHIT with regard to apoptotic proteins including p53 and p38 in the DNA damage-induced apoptosis are not well described. In the present study, we used etoposide-induced DNA damage in MCF-7 as a model to address these crosstalks. The time course study showed that the expression of FHIT, p53, and p38MAPK started after 1 hour following etoposide treatment. FHIT overexpression led to increase p53 expression, p38 activation, and augmented apoptosis following etoposide-induced DNA damage compared to wild-type cells. However, FHIT knockdown blocked p53 expression, delayed p38 activation, and completely inhibited etoposide-induced apoptosis. Inhibition of p38 activity prevented induction of p53, FHIT, and apoptosis in this model. Thus, activation of p38 upon etoposide treatment leads to increase in FHIT and p53 expression. In p53 knockdown MCF-7, the FHIT induction was hampered but p38 activation was induced in lower doses of etoposide. In p53 knockdown cells, inhibition of p38 induced FHIT expression and apoptosis. Our data demonstrated that the exposure of MCF-7 cells to etoposide increases apoptosis through a mechanism involving the activation of the p38-FHIT-p53 pathway. Moreover, our findings suggest signaling interaction for these pathways may represent a promising therapy for breast cancer.     

ODN 491, a novel antisense oligodeoxynucleotide that targets thymidylate synthase, exerts cell-specific effects in human tumor cell lines

Thymidylate synthase (TS) is essential for DNA replication and is a target for cancer chemotherapy. However, toxicity to normal cells and tumor cell drug resistance necessitate development of new therapeutic strategies. One such strategy is to use antisense (AS) technology to reduce TS mRNA and protein levels in treated cells. We have developed oligodeoxynucleotides (ODNs) that target different regions of TS mRNA, inhibit human tumor cell proliferation as single agents, and enhance cytotoxicity of clinically useful TS protein-targeting drugs. Here we describe ODN 491, a novel 20mer AS ODN complementary to a previously untargeted portion of the TS mRNA coding region. AS ODN 491 decreased TS mRNA levels to different degrees in a panel of human tumor-derived cell lines, and induced different physiological effects in a tumor cell line-dependent manner. ODN 491 (like AS TS ODN 83, previously shown to be effective) decreased TS protein levels in HeLa cells with a concomitant increase in sensitivity to TS-targeting chemotherapeutics. However (and contrary to HeLa cell response to an AS ODN 83), it did not, as a single agent, inhibit HeLa cell proliferation. In MCF-7 cells, ODN 491 treatment was less effective at reducing TS mRNA and did not reduce TS protein, nor did it enhance sensitivity to TS-targeting or other chemotherapeutics. Moreover, specifically in MCF-7 cells but not HeLa cells, ODN 491 as a single agent induced apoptosis. These data indicate that AS TS ODN 491 is an effective AS reagent targeting a novel TS mRNA region. However, treatment of tumor cell lines with AS TS ODNs targeting different TS mRNA regions results in a pattern of physiological effects that varies in a tumor cell line-specific fashion. In addition, the capacity of different AS TS ODNs to induce physiological effects does not correlate well with their capacity to reduce TS mRNA and/or protein and, further, depends on the region of TS mRNA selected for targeting. Recognition of tumor cell-specific and mRNA region-specific variability in response to AS TS ODNs will be important in designing AS TS ODNs for potential clinical use.     

Induction of apoptosis and stress response in ovarian carcinoma cell lines treated with ST1926, an atypical retinoid

To understand the molecular mechanisms mediating apoptosis induction by a novel atypical retinoid, ST1926, the cellular response to drug treatment was investigated in IGROV-1 ovarian carcinoma cells carrying wild-type p53 and a cisplatin-resistant p53 mutant subline (IGROV-1/Pt1). Despite a similar extent of drug-induced DNA strand breaks, the level of apoptosis was substantially higher in p53 wild-type cells. p53 activation and early upregulation of p53-target genes were consistent with p53-dependent apoptosis in IGROV-1 cells. Stress-activated protein kinases were activated in both cell lines in response to ST1926. This event and activation of AP-1 were more pronounced in IGROV-1/Pt1 cells, in which the modulation of DNA repair-associated genes suggests an increased ability to repair DNA damage. Inhibition of JNK or p38 stimulated ST1926-induced apoptosis only in IGROV-1 cells, whereas inhibition of ERKs enhanced apoptosis in both the cell lines. Such a pattern of cellular response and modulation of genes implicated in DNA damage response supports that the genotoxic stress is a critical event mediating drug-induced apoptosis. The results are consistent with apoptosis induction through p53-dependent and -independent pathways, regulated by MAP kinases, which likely play a protective role.     

Targeting Stat3 with G-quartet oligodeoxynucleotides in human cancer cells

Stat3 is an oncogene that is activated in many human cancer cells. Genetic approaches that disrupt Stat3 activity result in inhibition of cancer cell growth and enhanced cell apoptosis supporting the development of novel drugs targeting Stat3 for cancer therapy. G-quartet oligodeoxynucleotides (ODNs) were demonstrated to be potent inhibitors of Stat3 DNA binding activity in vitro with the G-quartet ODN, T40214, having an IC(50) of 7 microM. Computer-simulated docking studies indicated that G-quartet ODNs mainly interacted with the SH2 domain of Stat3 and were capable of inserting between the SH2 domains of Stat3 dimers bound to DNA. We demonstrated that the G-rich ODN T40214, which forms a G-quartet structure at intracellular but not extracellular K+ ion concentrations, is delivered efficiently into the cytoplasm and nucleus of cancer cells where it inhibited IL-6-stimulated Stat3 activation and suppressed Stat3-mediated upregulation of bcl-x and mcl-1 gene expression. Thus, G-quartet represents a new class of drug for targeting of Stat3 within cancer cells.     

Synaptic complex formation of two retrovirus DNA attachment sites by integrase: a fluorescence energy transfer study

The integration of retroviral DNA by the viral integrase (IN) into the host genome occurs via assembled preintegration complexes (PIC). We investigated this assembly process using purified IN and viral DNA oligodeoxynucleotide (ODN) substrates (93 bp in length) that were labeled with donor (Cy3) and acceptor fluorophores (Cy5). The fluorophores were attached to the 5' 2 bp overhangs of the terminal attachment (att) sites recognized by IN. Addition of IN to the assay mixture containing the fluorophore-labeled ODN resulted in synaptic complex formation at 14 degrees C with significant fluorescence resonance energy transfer (FRET) occurring between the fluorophores in close juxtaposition (from approximately 15 to 100 A). Subsequent integration assays at 37 degrees C with the same ODN (32P-labeled) demonstrated a direct association of a significant FRET signal with concerted insertion of the two ODNs into the circular DNA target, here termed full-site integration. FRET measurements (deltaF) show that IN binds to a particular set of 3' OH recessed substrates (type I) generating synaptic complexes capable of full-site integration that, as shown previously, exhibit IN mediated protection from DNaseI digestion up to approximately 20 bp from the ODN att ends. In contrast, IN also formed complexes with nonspecific DNA ends and loss-of-function att end substrates (type II) that had significantly lower deltaF values and were not capable of full-site integration, and lacked the DNaseI protection properties. The type II category may exemplify what is commonly understood as "nonspecific" binding by IN to DNA ends. Two IN mutants that exhibited little or no integration activity gave rise to the lower deltaF signals. Our FRET analysis provided the first direct physical evidence that IN forms synaptic complexes with two DNA att sites in vitro, yielding a complex that exhibits properties comparable to that of the PIC.     

An immunostimulatory DNA sequence from a probiotic strain of Bifidobacterium longum inhibits IgE production in vitro

The immunostimulatory oligodeoxynucleotide (ODN) BL07 (5'-GCGTCGGTTTCGGTGCTCAC-3') was identified from the genomic DNA of the probiotic strain Bifidobacterium longum BB536. ODN BL07 stimulated B-lymphocyte proliferation and induced interleukin-12 (IL-12) production in macrophage-like J774.1 cells. ODNs BL07 and BL07S (modified with phosphorothioate backbone) significantly inhibited immunoglobulin E (IgE) production and stimulated interferon-gamma (IFN-gamma) and IL-12 production, but did not affect IL-4 secretion in murine splenic cells of ovalbumin-primed BALB/c mice. These ODNs also significantly inhibited production of IgE in purified murine B cells in the presence of IL-4 and anti-CD40. The results suggest the potential of ODNs BL07 and BL07S in preventing IgE-related immune responses and the possible involvement of ODN BL07 in the antiallergic efficacy of B. longum BB536.     

CpG oligodeoxynucleotides modulate the osteoclastogenic activity of osteoblasts via Toll-like receptor 9

Regulation of osteoclastogenesis by lipopolysaccharide (LPS) is mediated via its interactions with toll-like receptor 4 (TLR4) on both osteoclast- and osteoblast-lineage cells. We have recently demonstrated that CpG oligodeoxynucleotides (CpG ODNs), known to mimic bacterial DNA, modulate osteoclastogenesis via interactions with osteoclast precursors. In the present study we characterize the interactions of CpG ODNs with osteoblasts, in comparison with LPS. We find that, similar to LPS, CpG ODNs modulate osteoclastogenesis in bone marrow cell/osteoblast co-cultures, although in a somewhat different pattern. Osteoblasts express receptors for both LPS and CpG ODN (TLR4 and TLR9, respectively). The osteoblastic TLR9 transmits signals into the cell as demonstrated by NFkappaB activation as well as by extracellular-regulated kinase (ERK) and p38 phosphorylation. Similar to LPS, CpG ODN increases in osteoblasts the expression of tumor necrosis factor (TNF)-alpha and macrophage-colony stimulating factor (M-CSF). The two TLR ligands do not affect osteoprotegerin expression in osteoblasts. CpG ODN does not significantly affect receptor activator of NFkappaB ligand (RANKL) expression, in contrast to LPS, which induces the expression of this molecule. In the co-cultures CpG ODN induces RANKL expression in osteoblasts as a result of the more efficient TNF-alpha induction. CpG ODN activity (modulation of osteoclastogenesis, gene expression, ERK and p38 phosphorylation, and nuclear translocation of NFkappaB) is specific, because the control oligodeoxynucleotide, not containing CpG, is inactive. Furthermore, these effects (unlike the LPS effects) are inhibited by chloroquine, suggesting a requirement for endosomal maturation/acidification, the classic CpG ODN mode of action. We conclude that CpG ODN, upon TLR9 ligation, induces osteoblasts osteoclastogenic activity.     

Structural characterization of the inhibitory DNA motif for the type A (D)-CpG-induced cytokine secretion and NK-cell lytic activity in mouse spleen cells

Oligodeoxynucleotides (ODN) with the CpG motif have been shown to be potent stimulators of innate immunity. A theoretical concern is that uncontrolled stimulation of the innate immune system through the TLR-9 receptor could induce, or worsen, some autoimmune diseases such as adjuvant arthritis or systemic lupus erythematosus. Safe therapeutic use of such ODN could be enhanced if one could regulate some of their stimulatory activities. We have designed a group of synthetic ODNs, which were able to inhibit the induction of NK lytic activity, IL-12p40 and IFN-gamma cytokine secretion by type A (D)-CpG-ODNs. Inhibition occurred in both DNA-sequence and dose-dependent fashion. Fifty percent inhibition was achieved with ~10-nM concentration of the most potent inhibitory ODNs. Delayed addition of these ODNs for up to 2 h was still able to profoundly affect CpG-induced IL-12p40 production at 18 h. Inhibitory DNA motif consists of two nucleotide triplets, a proximal pyrimidine-rich CCT sequence and a more distal GGG triplet. Optimal distance between these blocks is between three to five nucleotides. The linker sequence between the CCT and GGG blocks can additionally modify the activity of inhibitory ODNs, in both a positive and in negative way. When the order of CCT and GGG blocks is reversed, inhibition is completely lost. These findings suggest that CpG regulation of innate immunity can itself be regulated by particular motifs, which could be of therapeutic benefit in autoimmune diseases.     

P53 is necessary for the apoptotic response mediated by a transient increase of Ras activity

The tumor suppressor p53 eliminates cancer-prone cells via multiple mechanisms, including apoptosis. Ras elicits apoptosis in cells after protein kinase C (PKC) downregulation. However, the role of p53 in Ras-mediated apoptosis has not been fully investigated. Here, we demonstrate that mouse fibroblasts that express wild-type p53 are more susceptible to apoptosis elicited by PKC inhibition if Ras is transiently expressed or upregulated as opposed to stably expressed. In the latter case, p53 is frequently mutated. Transiently increased Ras activity induces Bax, and PKC inhibition augments this induction. Overexpression of E6 inactivates p53 and thereby suppresses both Bax induction and apoptosis. In contrast, Bax is not induced in stable ras transfectants, regardless of PKC inhibition. The data suggest that short- and long-term activation of Ras use a different mechanism(s) to initiate apoptosis. The status of p53 may contribute to such differences.