Transcription inhibition by platinum-DNA cross-links in live mammalian cells

Photochemistry and DNA photocleavage by a new unsupported dirhodium(II,II) complex

The new complex [Rh2(phen)2(CH3CN)6](BF4)4 (1) was synthesized and characterized in solution and its crystal structure was determined. Irradiation of 1 with visible light (λirr>590 nm) in water results in the release of two equatorial CH3CN ligands, CH3CNeq, as well as in the formation of mononuclear radical Rh(II) fragments stemming from the homolytic photocleavage of the metal-metal bond. The photoproducts, identified by electrospray ionization mass spectrometry, include [Rh(phen)(CH3CN)(OH)](+) and [Rh(phen)(CH3CN)(H2O)3(BF4)](+). The quantum yield for the photochemical transformation of 1 in H2O exceeds unity (Φ550 nm=1.38) indicative of dark reactions following the initial photoprocess. DNA photocleavage was observed for 1 (λirr>590 nm), whereas the complex is unreactive in the dark. This feature makes 1 a promising photodynamic therapy agent that does not operate via the production of singlet oxygen, 1O2.

Ototoxic Model of Oxaliplatin and Protection from Nicotinamide Adenine Dinucleotide

Oxaliplatin, an anticancer drug commonly used to treat colorectal cancer and other tumors, has a number of serious side effects, most notably neuropathy and ototoxicity. To gain insights into its ototoxic profile, oxaliplatin was applied to rat cochlear organ cultures. Consistent with it neurotoxic propensity, oxaliplatin selectively damaged nerve fibers at a very low dose 1 μM. In contrast, the dose required to damage hair cells and spiral ganglion neurons was 50 fold higher (50 μM). Oxailiplatin-induced cochlear lesions initially increased with dose, but unexpectedly decreased at very high doses. This non-linear dose response could be related to depressed oxaliplatin uptake via active transport mechanisms. Previous studies have demonstrated that axonal degeneration involves biologically active processes which can be greatly attenuated by nicotinamide adenine dinucleotide (NAD+). To determine if NAD+ would protect spiral ganglion axons and the hair cells from oxaliplatin damage, cochlear cultures were treated with oxaliplatin alone at doses of 10 μM or 50 μM respectively as controls or combined with 20 mM NAD+. Treatment with 10 μM oxaliplatin for 48 hours resulted in minor damage to auditory nerve fibers, but spared cochlear hair cells. However, when cochlear cultures were treated with 10 μM oxaliplatin plus 20 mM NAD+, most auditory nerve fibers were intact. 50 μM oxaliplatin destroyed most of spiral ganglion neurons and cochlear hair cells with apoptotic characteristics of cell fragmentations. However, 50 μM oxaliplatin plus 20 mM NAD+ treatment greatly reduced neuronal degenerations and hair cell missing. The results suggested that NAD+ provides significant protection against oxaliplatin-induced neurotoxicity and ototoxicity, which may be due to its actions of antioxidant, antiapoptosis, and energy supply.

Visualizing inhibition of nucleosome mobility and transcription by cisplatin-DNA interstrand crosslinks in live mammalian cells

Cisplatin is a widely used anticancer drug that acts by binding DNA and causing the formation of intrastrand and interstrand (ICL) crosslinks, but the precise downstream effects of the latter damage are not well understood. In this study, we investigated the influence of cisplatin ICLs on synthetic nucleosomes that were platinated in a site-specific manner in vitro and on gene transcription in live mammalian cells. Nucleosome core particles that we constructed contained site-specific cisplatin 5'-d(G*pC)/5'-d(G*pC) ICLs, where the asterisk denotes the platinated nucleoside, to examine the influence of platinum lesions on the dynamic behavior of nucleosomes in solution. A cisplatin ICL, but not a 1,2-d(GpG) crosslink, significantly inhibited ATP-independent histone octamer-DNA sliding. We also used a novel linearization-recircularization strategy described here to synthesize mammalian expression vectors containing site-specific cisplatin ICLs. Plasmid vectors were tested in live mammalian cells to study the transcription inhibition effects of cisplatin ICLs in the context of two different repair backgrounds. Cisplatin ICLs inhibit transcription as effectively as 1,2-d(GpG) crosslinks. We determined that nucleotide excision repair plays a key role in the removal of cisplatin ICLs, acting in a replication-independent fashion. We also found that loss of mismatch repair function dramatically attenuates the transcription inhibition effects by cisplatin ICLs but not 1,2-d(GpG) intrastrand crosslinks. Our results revealed the unique properties of cisplatin ICLs on nucleosome mobility and on transcription, and they defined how these adducts act in a manner completely different from that used for cisplatin 1,2-d(GpG) crosslinks. These new findings provide direct support for a role of ICLs in the pharmacologic activities of cisplatin, despite the lower frequency of their formation.

Phosphorylation of p68 RNA helicase by p38 MAP kinase contributes to colon cancer cells apoptosis induced by oxaliplatin

Background

We previously demonstrated that p68 phosphorylation at threonine residues correlates with cancer cell apoptosis under the treatments of TNF-α and TRAIL (Yang, L. Mol Cancer Res Vol 3, pp 355–63 2005).

Results

In this report, we characterized the role of p68 phosphorylation in apoptosis induction under the treatment of oxaliplatin in the colon cancer cells. Our data suggest that oxaliplatin treatment activates p38 MAP kinase, which subsequently phosphorylates p68 at T564 and/or T446. The phosphorylation of p68, at least partially, mediates the effects of the drug on apoptosis induction, as mutations at these two sites greatly reduce the cancer cell death.

Conclusion

Our studies reveal an important molecular mechanism that mediates the effects of anti-cancer drug, providing a potential strategy for improving cancer treatment.

Effects of 6-thioguanine and S6-methylthioguanine on transcription in vitro and in human cells

Thiopurine drugs are extensively used as chemotherapeutic agents in clinical practice, even though there is concern about the risk of therapy-related cancers. It has been previously suggested that the cytotoxicity of thiopurine drugs involves their metabolic activation, the resultant generation of 6-thioguanine ((S)G) and S(6)-methylthioguanine (S(6)mG) in DNA, and the futile mismatch repair triggered by replication-induced (S)G:T and S(6)mG:T mispairs. Disruption of transcription is known to be one of the major consequences of DNA damage induced by many antiviral and antitumor agents; however, it remains undefined how (S)G and S(6)mG compromise the efficiency and fidelity of transcription. Using our recently developed competitive transcription and adduct bypass assay, herein we examined the impact of (S)G and S(6)mG on transcription in vitro and in human cells. Our results revealed that, when situated on the transcribed strand, S(6)mG exhibited both inhibitory and mutagenic effects during transcription mediated by single-subunit T7 RNA polymerase or multisubunit human RNA polymerase II in vitro and in human cells. Moreover, we found that the impact of S(6)mG on transcriptional efficiency and fidelity is modulated by the transcription-coupled nucleotide excision repair capacity. In contrast, (S)G did not considerably compromise the efficiency or fidelity of transcription, and it was a poor substrate for NER. We propose that S(6)mG might contribute, at least in part, to thiopurine-mediated cytotoxicity through inhibition of transcription and to potential therapy-related carcinogenesis via transcriptional mutagenesis.

Repair of cisplatin-induced DNA interstrand crosslinks by a replication-independent pathway involving transcription-coupled repair and translesion synthesis

DNA interstrand crosslinks (ICLs) formed by antitumor agents, such as cisplatin or mitomycin C, are highly cytotoxic DNA lesions. Their repair is believed to be triggered primarily by the stalling of replication forks at ICLs in S-phase. There is, however, increasing evidence that ICL repair can also occur independently of replication. Using a reporter assay, we describe a pathway for the repair of cisplatin ICLs that depends on transcription-coupled nucleotide excision repair protein CSB, the general nucleotide excision repair factors XPA, XPF and XPG, but not the global genome nucleotide excision repair factor XPC. In this pathway, Rev1 and Polζ are involved in the error-free bypass of cisplatin ICLs. The requirement for CSB, Rev1 or Polζ is specific for the repair of ICLs, as the repair of cisplatin intrastrand crosslinks does not require these genes under identical conditions. We directly show that this pathway contributes to the removal of ICLs outside of S-phase. Finally, our studies reveal that defects in replication- and transcription-dependent pathways are additive in terms of cellular sensitivity to treatment with cisplatin or mitomycin C. We conclude that transcription- and replication-dependent pathways contribute to cellular survival following treatment with crosslinking agents.

Phenanthriplatin, a monofunctional DNA-binding platinum anticancer drug candidate with unusual potency and cellular activity profile

Monofunctional platinum(II) complexes of general formula cis-[Pt(NH(3))(2)(N-heterocycle)Cl]Cl bind DNA at a single site, inducing little distortion in the double helix. Despite this behavior, these compounds display significant antitumor properties, with a different spectrum of activity than that of classic bifunctional cross-linking agents like cisplatin. To discover the most potent monofunctional platinum(II) compounds, the N-heterocycle was systematically varied to generate a small library of new compounds, with guidance from the X-ray structure of RNA polymerase II (Pol II) stalled at a monofunctional pyriplatin-DNA adduct. In pyriplatin, the N-heterocycle is pyridine. The most effective complex evaluated was phenanthriplatin, cis-[Pt(NH(3))(2)(phenanthridine)Cl]NO(3), which exhibits significantly greater activity than the Food and Drug Administration-approved drugs cisplatin and oxaliplatin. Studies of phenanthriplatin in the National Cancer Institute 60-cell tumor panel screen revealed a spectrum of activity distinct from that of these clinically validated anticancer agents. The cellular uptake of phenanthriplatin is substantially greater than that of cisplatin and pyriplatin because of the hydrophobicity of the phenanthridine ligand. Phenanthriplatin binds more effectively to 5'-deoxyguanosine monophosphate than to N-acetyl methionine, whereas pyriplatin reacts equally well with both reagents. This chemistry supports DNA as a viable cellular target for phenanthriplatin and suggests that it may avoid cytoplasmic platinum scavengers with sulfur-donor ligands that convey drug resistance. With the use of globally platinated Gaussia luciferase vectors, we determined that phenanthriplatin inhibits transcription in live mammalian cells as effectively as cisplatin, despite its inability to form DNA cross-links.

Mitochondrial DNA damage and its consequences for mitochondrial gene expression

How mitochondria process DNA damage and whether a change in the steady-state level of mitochondrial DNA damage (mtDNA) contributes to mitochondrial dysfunction are questions that fuel burgeoning areas of research into aging and disease pathogenesis. Over the past decade, researchers have identified and measured various forms of endogenous and environmental mtDNA damage and have elucidated mtDNA repair pathways. Interestingly, mitochondria do not appear to contain the full range of DNA repair mechanisms that operate in the nucleus, although mtDNA contains types of damage that are targets of each nuclear DNA repair pathway. The reduced repair capacity may, in part, explain the high mutation frequency of the mitochondrial chromosome. Since mtDNA replication is dependent on transcription, mtDNA damage may alter mitochondrial gene expression at three levels: by causing DNA polymerase γ nucleotide incorporation errors leading to mutations, by interfering with the priming of mtDNA replication by the mitochondrial RNA polymerase, or by inducing transcriptional mutagenesis or premature transcript termination. This review summarizes our current knowledge of mtDNA damage, its repair, and its effects on mtDNA integrity and gene expression. This article is part of a special issue entitled: Mitochondrial Gene Expression.

A method for detecting genetic toxicity using the RNA synthesis response to DNA damage

To date, biological risk assessment studies of chemicals that induce DNA lesions have been primarily based on the action of DNA polymerases during replication. However, DNA lesions interfere not only with replication but also with transcription. Therefore, detecting the damaging effects of DNA lesions during transcription might be important for estimating the safety of chemical mutagens and carcinogens. However, methods to address these effects have not been developed. Here, we report a simple, non-isotopic method for determining the toxicity of chemical agents by visualizing transcription in a mammalian cell system. The method is based on the measurement of the incorporation of bromouridine (as the uridine analogue) into the nascent RNA during RNA synthesis inhibition (RSI) induced by the stalling of RNA polymerases at DNA lesions on the transcribed DNA strand, which triggers transcription-coupled nucleotide excision repair (TC-NER). When we tested chemical agents (camptothecin, etoposide, 4-nitroquinoline-1-oxide, mitomycin C, methyl methanesulfonate, and cisplatin) in HeLa cells by the method, RSI indicative of genomic toxicity was observed in the nucleoli of the tested cells. This procedure provides the following advantages: 1) it uses common, affordable mammalian cells (HeLa cells, WI38VA13 cells, human dermal fibroblasts, or Chinese hamster ovary cells) rather than genetically modified microorganisms; 2) it can be completed within approximately 8 hr after the cells are prepared because RNA polymerase responses during TC-NER are faster than other DNA damage responses (replication, recombination, and apoptosis); and 3) it is safe because it uses non-radioactive bromouridine and antibodies to detect RNA synthesis on undamaged transcribed DNA strands.

Transcription Inhibition by Organometallic Ruthenium - Arene Anticancer Complexes in Live Mammalian Cells

Organometallic ruthenium–arene RAPTA complexes, currently being actively pursued as potential anticancer agents, interact with intracellular biological targets to form covalent adducts. Because their mode of action is still unclear, we investigated their binding with DNA and the ability of ruthenated-DNA adducts to elicit cellular responses such as transcription inhibition and repair. To investigate the influence of the spectator arene ligands on RAPTA activity, a novel RAPTA complex containing the bulky 1,3,5-triisopropylbenzene ligand was synthesized and characterized. Transcription experiments carried out in live mammalian cells using ruthenated plasmid probes revealed that increasing steric bulk of the arene ligand did not improve its ability to arrest transcription.

Monofunctional platinum-DNA adducts are strong inhibitors of transcription and substrates for nucleotide excision repair in live mammalian cells

To overcome drug resistance and reduce the side effects of cisplatin, a widely used antineoplastic agent, major efforts have been made to develop next generation platinum-based anticancer drugs. Because cisplatin-DNA adducts block RNA polymerase II unless removed by transcription-coupled excision repair, compounds that react similarly but elude repair are desirable. The monofunctional platinum agent pyriplatin displays antitumor activity in mice, a cytotoxicity profile in cell cultures distinct from that of cisplatin, and a unique in vitro transcription inhibition mechanism. In this study, we incorporated pyriplatin globally or site specifically into luciferase reporter vectors to examine its transcription inhibition profiles in live mammalian cells. Monofunctional pyriplatin reacted with plasmid DNA as efficiently as bifunctional cisplatin and inhibited transcription as strongly as cisplatin in various mammalian cells. Using repair-defective nucleotide excision repair (NER)-, mismatch repair-, and single-strand break repair-deficient cells, we show that NER is mainly responsible for removal of pyriplatin-DNA adducts. These findings reveal that the mechanism by which pyriplatin generates its antitumor activity is very similar to that of cisplatin, despite the chemically different nature of their DNA adducts, further supporting a role for monofunctional platinum anticancer agents in human cancer therapy. This information also provides support for the validity of the proposed mechanism of action of cisplatin and provides a rational basis for the design of more potent platinum anticancer drug candidates using a monofunctional DNA-damaging strategy.

Targeting DNA damage and repair: embracing the pharmacological era for successful cancer therapy

DNA is under constant assault from genotoxic agents which creates different kinds of DNA damage. The precise replication of the genome and the continuous surveillance of its integrity are critical for survival and the avoidance of carcinogenesis. Cells have evolved an arsenal of repair pathways and cell cycle checkpoints to detect and repair DNA damage. When repair fails, typically cell cycle progression is halted and apoptosis is initiated. Here, we review the different sources and types of DNA damage including DNA replication stress and oxidative stress, the repair pathways that cells utilize to repair damaged DNA, and discuss their biological significance, especially with reference to cancer induction and cancer therapy. We also describe the main methodologies currently used for the detection of DNA damage with their strengths and limitations. We conclude with an outline as to how this information can be used to identify novel pharmacological targets for DNA repair pathways or enhancers of DNA damage to develop improved treatment strategies that will benefit cancer patients.

Metal to ligand charge transfer induced DNA photobinding in a Ru(II)-Pt(II) supramolecule using red light in the therapeutic window: a new mechanism for DNA modification

The Ru(II)-Pt(II) supramolecular complex, [(Ph(2)phen)(2)Ru(dpp)PtCl(2)](2+), displays a new mechanism for DNA modification: photobinding through a (3)MLCT excited state. Gel shift analysis, selective DNA precipitation, and DNA melting point experiments support efficient covalent DNA binding following visible light excitation.

Role of endonucleases XPF and XPG in nucleotide excision repair of platinated DNA and cisplatin/oxaliplatin cytotoxicity

Resistance of tumor cells to platinum anticancer agents poses a major problem in cancer chemotherapy. One of the mechanisms associated with platinum-based drug resistance is the enhanced capacity of the cell to carry out nucleotide excision repair (NER) on platinum-damaged DNA. Endonucleases XPF and XPG are critical components of NER, responsible for excising the damaged DNA strand to remove the DNA lesion. Here, we investigated possible consequences of down-regulation of XPF and XPG gene expression in osteosarcoma cancer cells (U2OS) and the impact on cellular transcription and DNA repair. We further evaluated the sensitivity of such cells toward the platinum anticancer drugs cisplatin and oxaliplatin.

Photogeneration and reactivity of naphthoquinone methides as purine selective DNA alkylating agents

A one-step protecting-group-free synthesis of both 6-hydroxy-naphthalene-2-carbaldehyde and the bifunctional binaphthalenyl derivative afforded 6-hydroxymethylnaphthalen-2-ol, 6-methylaminomethyl-naphthalen-2-ol, [(2-hydroxy-3-naphthyl)methyl]trimethyl ammonium iodide, and a small library of bifunctional binol analogues in good yields. Irradiation of naphthol quaternary ammonium salt and binol-derivatives (X = OH, NHR, NMe(3)(+), OCOCH(3), and L-proline) at 310 and 360 nm resulted in the photogeneration of the 2,6-naphthoquinone-6-methide (NQM) and binol quinone methide analogues (BQMs) by a water-mediated excited-state proton transfer (ESPT). The hydration, the mono- and bis-alkylation reactions of morpholine and 2-ethanethiol, as N and S prototype nucleophiles, by the transient NQM (λ(max) 310, 330 nm) and BQMs (λ(max) 360 nm) were investigated in water by product distribution analysis and laser flash photolysis (LFP). Both the photogeneration and the reactivity of NQM and BQMs exhibited striking differences. BQMs were at least 2 orders of magnitude more reactive than NQM, and they were generated much more efficiently from a greater variety of photoprecursors including the hydroxymethyl, quaternary ammonium salt and several binol-amino acids. On the contrary, the only efficient precursor of NQM was the quaternary ammonium salt. All water-soluble BQM precursors were further investigated for their ability to alkylate and cross-link plasmid DNA and oligonucleotides by gel electrophoresis: the BQMs were more efficient than the isomeric o-BQM (binol quinone methide analogue of 2,3-naphthoquinone-3-methide). Sequence analysis by gel electrophoresis, HPLC, and MS showed that the alkylation occurred at purines, with a preference for guanine. In particular, a BQM was able to alkylate N7 of guanines resulting in depurination at the oligonucleotide level, and ribose loss at the nucleotide level. The photoreactivity of BQM precursors translated into photocytotoxic and cytotoxic effects on two human cancer cell lines: in particular, one compound showed promising selectivity index on both cell lines.

Binding of kinetically inert metal ions to RNA: the case of platinum(II)

In this chapter several aspects of Pt(II) are highlighted that focus on the properties of Pt(II)-RNA adducts and the possibility that they influence RNA-based processes in cells. Cellular distribution of Pt(II) complexes results in significant platination of RNA, and localization studies find Pt(II) in the nucleus, nucleolus, and a distribution of other sites in cells. Treatment with Pt(II) compounds disrupts RNA-based processes including enzymatic processing, splicing, and translation, and this disruption may be indicative of structural changes to RNA or RNA-protein complexes. Several RNA-Pt(II) adducts have been characterized in vitro by biochemical and other methods. Evidence for Pt(II) binding in non-helical regions and for Pt(II) cross-linking of internal loops has been found. Although platinated sites have been identified, there currently exists very little in the way of detailed structural characterization of RNA-Pt(II) adducts. Some insight into the details of Pt(II) coordination to RNA, especially RNA helices, can be gained from DNA model systems. Many RNA structures, however, contain complex tertiary folds and common, purine-rich structural elements that present suitable Pt(II) nucleophiles in unique arrangements which may hold the potential for novel types of platinum-RNA adducts. Future research aimed at structural characterization of platinum-RNA adducts may provide further insights into platinum-nucleic acid binding motifs, and perhaps provide a rationale for the observed inhibition by Pt(II) complexes of splicing, translation, and enzymatic processing.