Proteins that specifically recognize cisplatin-damaged DNA: a clue to anticancer activity of cisplatin

Methanolic Phoenix dactylifera L. Extract Ameliorates Cisplatin-Induced Hepatic Injury in Male Rats

This study investigated the ameliorative potential of methanolic date flesh extract (MDFE) against cisplatin-induced hepatic injury. Twenty male rats (weighing 180-200 g) were allocated into four groups: control; date flesh (DF) group (oral 600 mg/kg MDFE for 21 days); Cis group (7.5 mg/kg i.p. at day 16); and date flesh/cisplatin (DF/Cis) group (oral 600 mg/kg MDFE for 21 days and 7.5 mg/kg i.p. at day 16). Hepatic biochemical parameters in sera, and inflammatory and oxidant/antioxidant hepatic biomarkers were estimated. Hepatic histological changes and the immunohistochemistry of cyclooxygenase-2 (COX-2), nuclear factor kappa B (NF-κB), and alpha smooth muscle actin (α-SMA) were assessed. Pretreatment with MDFE decreased Cis-triggered liver biochemical parameters, oxidative stress, inflammatory biomarkers, and histological damage. Moreover, MDFE treatment reduced Cis-induced hepatic NF-κB, COX-2, and α-SMA protein expression. MDFE exerted a hepatoprotective effect when used concomitantly with Cis. Its effect was mediated via its antioxidant and anti-inflammatory ingredients.

Homo and heterobimetallic palladium and platinum complexes bearing μ-diphosphane bridges involved in biological studies

Given the increasing reports of well-defined bimetallic molecular complexes as potential anticancer agents in the last decades, along with the prevalence of platinum in anticancer therapy, we report here a detailed survey of bimetallic platinum and palladium complexes investigated as potential anticancer agents. Specifically, we will concentrate on the synthesis, characterisation and biological (anticancer) studies of a sub-class of these agents, namely homo and heterobimetallic complexes bearing a bridging phosphane ligand of the type: [L_nM₁(μ-R₂P(CH₂)_nPR₂)M₂L_m] (where M₁ is platinum or palladium, M₂ is any other transition metal, R = alkyl or aryl substituents, L_n or L_m are co-ligands, n = 1-6). We will review the in vitro and in vivo activities and any mechanistic anticancer studies of these complexes with a view of trying to delineate patterns in biological activity and structure-activity relationships (SAR). We do not include the review of bimetallic complexes in this class that have not undergone any anticancer testing, nor those that have been involved in other biological investigations unrelated to cancer studies.

Cisplatin reacts with histone H1 and the adduct forms a ternary complex with DNA

Cisplatin is an anticancer drug widely used in clinics; it induces the apoptosis of cancer cells by targeting DNA. However, its interaction with proteins has been found to be crucial in modulating the pre and post-target activity. Nuclear DNA is tightly assembled with histone proteins to form nucleosomes in chromatin; this can impede the drug to access DNA. On the other hand, the linker histone H1 is considered 'the gate to nucleosomal DNA' due to its exposed location and dynamic conformation; therefore, this protein can influence the platination of DNA. In this study, we performed a reaction of cisplatin with histone H1 and investigated the interaction of the H1/cisplatin adduct with DNA. The reactions were conducted on the N-terminal domains of H1.4 (sequence 1-90, H1N90) and H1.0 (sequence 1-7, H1N7). The results show that H1 readily reacts with cisplatin and generates bidentate and tridentate adducts, with methionine and glutamate residues as the preferential binding sites. Chromatographic and NMR analyses show that the platination rate of H1 is slightly higher than that of DNA and the platinated H1 can form H1-cisplatin-DNA ternary complexes. Interestingly, cisplatin is more prone to form H1-Pt-DNA ternary complexes than trans-oriented platinum agents. The formation of H1-cisplatin-DNA ternary complexes and their preference for cis- over trans-oriented platinum agents suggest an important role of histone H1 in the mechanism of action of cisplatin.

"Head-to-head" double-hamburger-like structure of di-ruthenated d(GpG) adducts of mono-functional Ru-arene anticancer complexes

Guanine bases in DNA are targets for some Ru-arene anticancer complexes. We have investigated the structure of the novel di-ruthenated d(GpG) adduct Ru₂-GpG (where Ru = {(η⁶-biphenyl)-Ru(en)}²⁺ (1')) in aqueous solution. 2D NMR results indicate that there are two conformers, supported by modeling studies. The major conformer I is a novel double-hamburger-like structure with a "head-to-head" (HH) base arrangement involving hydrophobic interactions between neighboring arene rings, the first example of a HH d(GpG) adduct constructed by weak interactions. Hence there are significant differences compared to Pt-d(GpG) adducts formed by cisplatin. There is no obviously rigid bending for the major conformer I. The minor conformer II of Ru₂-GpG has a back-to-back structure, with two ruthenated guanine bases flipped away from each other. 19-23 base-pair oligodeoxyribonucleotides containing central TGGT sequences di-ruthenated by 1 show no directional bending, only slightly distorted di-ruthenated duplexes, consistent with the NMR data for conformer I. The structural differences and similarities of d(GpG) residues which are di-ruthenated or cross-linked by platination are discussed in the context of the biological activity of these metal complexes.

Design, synthesis and DNA interactions of a chimera between a platinum complex and an IHF mimicking peptide

Conjugation of metal complexes with peptide scaffolds possessing high DNA binding affinity has shown to modulate their biological activities and to enhance their interaction with DNA. In this work, a platinum complex/peptide chimera was synthesized based on a model of the Integration Host Factor (IHF), an architectural protein possessing sequence specific DNA binding and bending abilities through its interaction with a minor groove. The model peptide consists of a cyclic unit resembling the minor grove binding subdomain of IHF, a positively charged lysine dendrimer for electrostatic interactions with the DNA phosphate backbone and a flexible glycine linker tethering the two units. A norvaline derived artificial amino acid was designed to contain a dimethylethylenediamine as a bidentate platinum chelating unit, and introduced into the IHF mimicking peptides. The interaction of the chimeric peptides with various DNA sequences was studied by utilizing the following experiments: thermal melting studies, agarose gel electrophoresis for plasmid DNA unwinding experiments, and native and denaturing gel electrophoresis to visualize non-covalent and covalent peptide-DNA adducts, respectively. By incorporation of the platinum metal center within the model peptide mimicking IHF we have attempted to improve its specificity and DNA targeting ability, particularly towards those sequences containing adjacent guanine residues.

Silencing of high-mobility group box 2 (HMGB2) modulates cisplatin and 5-fluorouracil sensitivity in head and neck squamous cell carcinoma

Dysregulation of protein expression is associated with most diseases including cancer. MS-based proteomic analysis is widely employed as a tool to study protein dysregulation in cancers. Proteins that are differentially expressed in head and neck squamous cell carcinoma (HNSCC) cell lines compared to the normal oral cell line could serve as biomarkers for patient stratification. To understand the proteomic complexity in HNSCC, we carried out iTRAQ-based MS analysis on a panel of HNSCC cell lines in addition to a normal oral keratinocyte cell line. LC-MS/MS analysis of total proteome of the HNSCC cell lines led to the identification of 3263 proteins, of which 185 proteins were overexpressed and 190 proteins were downregulated more than twofold in at least two of the three HNSCC cell lines studied. Among the overexpressed proteins, 23 proteins were related to DNA replication and repair. These included high-mobility group box 2 (HMGB2) protein, which was overexpressed in all three HNSCC lines studied. Overexpression of HMGB2 has been reported in various cancers, yet its role in HNSCC remains unclear. Immunohistochemical labeling of HMGB2 in a panel of HNSCC tumors using tissue microarrays revealed overexpression in 77% (54 of 70) of tumors. The HMGB proteins are known to bind to DNA structure resulting from cisplatin-DNA adducts and affect the chemosensitivity of cells. We observed that siRNA-mediated silencing of HMGB2 increased the sensitivity of the HNSCC cell lines to cisplatin and 5-FU. We hypothesize that targeting HMGB2 could enhance the efficacy of existing chemotherapeutic regimens for treatment of HNSCC. All MS data have been deposited in the ProteomeXchange with identifier PXD000737 (http://proteomecentral.proteomexchange.org/dataset/PXD000737).

Current View in Platinum Drug Mechanisms of Peripheral Neurotoxicity

Peripheral neurotoxicity is the dose-limiting factor for clinical use of platinum derivatives, a class of anticancer drugs which includes cisplatin, carboplatin, and oxaliplatin. In particular cisplatin and oxaliplatin induce a severe peripheral neurotoxicity while carboplatin is less neurotoxic. The mechanisms proposed to explain these drugs' neurotoxicity are dorsal root ganglia alteration, oxidative stress involvement, and mitochondrial dysfunction. Oxaliplatin also causes an acute and reversible neuropathy, supposed to be due by transient dysfunction of the voltage-gated sodium channels of sensory neurons. Recent studies suggest that individual genetic variation may play a role in the pathogenesis of platinum drug neurotoxicity. Even though all these mechanisms have been investigated, the pathogenesis is far from clearly defined. In this review we will summarize the current knowledge and the most up-to-date hypotheses on the mechanisms of platinum drug-induced peripheral neurotoxicity.

Metal-based drugs

Metals have been considered for millennia to have medicinal values. With the advent of modern medicine, many metal-based drugs have proven to be highly effective in the clinic. Many different metal ions have shown activity against a range of diseases. The unique electronic structure of transition metals offers great versatility, not always seen in organic drugs, in terms of the ability to tune the properties of a given molecule. This review gives a brief overview of the most established therapeutic metals, and their more common applications, such as platinum-based anticancer drugs. New developments within the field of metallodrugs and novel strategies being employed to improve methods of delivery, are also discussed.

Monofunctional and higher-valent platinum anticancer agents

Platinum compounds represent one of the great success stories of metals in medicine. Following the serendipitous discovery of the anticancer activity of cisplatin by Rosenberg, a large number of cisplatin variants have been prepared and tested for their ability to kill cancer cells and inhibit tumor growth. These efforts continue today with increased realization that new strategies are needed to overcome issues of toxicity and resistance inherent to treatment by the approved platinum anticancer agents. One approach has been the use of so-called "non-traditional" platinum(II) and platinum(IV) compounds that violate the structure-activity relationships that governed platinum drug-development research for many years. Another is the use of specialized drug-delivery strategies. Here we describe recent developments from our laboratory involving monofunctional platinum(II) complexes together with a historical account of the manner by which we came to investigate these compounds and their relationship to previously studied molecules. We also discuss work carried out using platinum(IV) prodrugs and the development of nanoconstructs designed to deliver them in vivo.

Transplatin enhances effect of cisplatin on both single DNA molecules and live tumor cells

Cisplatin is the main platinum antitumor drug applied in clinical settings. However, its trans isomer, transplatin, is known to have an ineffective antitumor activity. Despite intensive studies in this field, the structural and biophysical properties of DNA molecules reacting with these two platinum complexes have not been fully elucidated. In the present study, we observed that transplatin made efficient cross-linking of DNA in the vicinity of cisplatin adducts. High-resolution atomic force microscopy studies revealed that the transplatin-induced cross-linkings of nucleotides flanking cisplatin adducts were characterized by kinked-loop structures with rod-like shapes of nanometer scales (∼10-60nm). The results were further confirmed by denaturing gel electrophoresis and single-molecule experiment using magnetic tweezers. In vivo studies revealed that transplatin and cisplatin co-treatment could induce a considerable amount of kinked loops with smaller sizes (∼15nm) in cellular DNA. Furthermore, compared with cisplatin treatment alone, the co-treatment resulted in enhanced cytotoxicity, increased amount of interstrand cross-links, and cell lesions more reluctant to cellular repair system. The results of the present study provide a new clue for understanding the stepwise reactions of DNA with platinum drugs and might serve as a basis for the development of a new antitumor strategy.

Structure-based DNA-targeting strategies with small molecule ligands for drug discovery

Nucleic acids are the molecular targets of many clinical anticancer drugs. However, compared with proteins, nucleic acids have traditionally attracted much less attention as drug targets in structure-based drug design, partially because limited structural information of nucleic acids complexed with potential drugs is available. Over the past several years, enormous progresses in nucleic acid crystallization, heavy-atom derivatization, phasing, and structural biology have been made. Many complicated nucleic acid structures have been determined, providing new insights into the molecular functions and interactions of nucleic acids, especially DNAs complexed with small molecule ligands. Thus, opportunities have been created to further discover nucleic acid-targeting drugs for disease treatments. This review focuses on the structure studies of DNAs complexed with small molecule ligands for discovering lead compounds, drug candidates, and/or therapeutics.

Cellular uptake and cytoplasm / DNA distribution of cisplatin and oxaliplatin and their liposomal formulation in human colorectal cancer cell HCT116

Liposomal formulations of cisplatin and oxaliplatin (Lipoplatin™ and Lipoxal™, respectively) were recently proposed to reduce systemic toxicity, while optimizing the anti-cancer effectiveness of these compounds. As the anti-neoplastic or radio-sensitizing activity of these drugs is attributed to their binding to DNA, we assessed the impact of the liposomal formulations on the time course of accumulation of these platinum compounds in the human colorectal cancer HCT116 cell lines and their distribution between cytoplasm and DNA. Their cytotoxicity was determined by colony formation assay. Intracellular platinum and platinum bound to DNA was measured by inductively coupled plasma mass spectrometry. Although, as a chemotherapeutic agent, cisplatin was as efficient as oxaliplatin after exposure for a short time, oxaliplatin and Lipoxal™ became more active than cisplatin against HCT116 cells after 24 h incubation. Lipoxal™ displayed a higher accumulation in the cytoplasm of HCT116 cells compared to free oxaliplatin, consistent with its proposed mechanism of fusion with the cell membrane. The distribution cytoplasm/DNA of free cisplatin and Lipoplatin™ were similar. Conversely, Lipoxal™ had a significantly different cytoplasm/DNA distribution from oxaliplatin: more than 95% of oxaliplatin transported by the liposome was trapped in the cytoplasm, even after 48 h incubation. Our study indicates that Lipoxal™ can largely improve the cellular uptake of oxaliplatin, but this was not followed by a similar increase in the DNA bound fraction.

Magnetic tweezers measurements of the nanomechanical properties of DNA in the presence of drugs

Herein, we study the nanomechanical characteristics of single DNA molecules in the presence of DNA binders, including intercalating agents (ethidium bromide and doxorubicin), a minor groove binder (netropsin) and a typical alkylating damaging agent (cisplatin). We have used magnetic tweezers manipulation techniques, which allow us to measure the contour and persistence lengths together with the bending and torsional properties of DNA. For each drug, the specific variations of the nanomechanical properties induced in the DNA have been compared. We observed that the presence of drugs causes a specific variation in the DNA extension, a shift in the natural twist and a modification of bending dependence on the imposed twist. By introducing a naive model, we have justified an anomalous correlation of torsion data observed in the presence of intercalators. Finally, a data analysis criterion for discriminating between different molecular interactions among DNA and drugs has been suggested.

Involvement of high mobility group B proteins in cisplatin-induced cytotoxicity in squamous cell carcinoma of skin

Cis-diamminedichloroplatinum (II) (cisplatin) is a well-known anticancer drug with high potency and efficacy against various types of human cancers. Although it is widely accepted that the mechanism of cisplatin action is via apoptosis, there is enough evidence to support that cisplatin-induced cell death also occurs by other nonapoptotic pathways. Nonhistone, high mobility group (HMG) proteins are known to bind cisplatin-damaged DNA, and we studied their expression during cisplatin-induced cell death using immunohistochemistry, Western blot, and RT-PCR. Results show that the cell death is primarily apoptotic during initial stages of cisplatin treatment of skin tumors, and there is only marginal increase in high mobility group B (HMGB) levels, indicating that HMGB are still bound to nucleus. However, extended treatment of skin tumors with cisplatin caused necrosis and showed significantly increased levels of HMGB, which suggests that HMGB thus released from nuclei act as cytokine and trigger inflammatory response leading to necrosis. Present results clearly indicate a strong association between HMGB proteins and cisplatin-induced cell death that is dominantly apoptotic or necrotic depending on the duration of cisplatin exposure. Because of their important implication in the outcome of cancer chemotherapy, HMGB proteins can be interesting therapeutic targets.

The effect of cellular environment and p53 status on the mode of action of the platinum derivative LA-12

In this study, we characterized the effects of LA-12 on tumor cell lines possessing wild type p53 and on p53-deficient/mutant cell lines and the results were compared to those obtained using cisplatin. We have determined changes of p53 levels, of its transcriptional activity, of its posttranscriptional modifications and the effect of the treatment on the cell cycle, on the induction of apoptosis and on gene expression. LA-12 induces weak accumulation of both transcriptionally active p53 tumor suppressor and of p21(WAF1/CIP1) protein. LA-12 and cisplatin also significantly differ in their effects on apoptosis and cell cycle and on gene expression spectra in studied cell lines. LA-12 induces higher apoptosis levels in comparison with those induced by cisplatin, especially in p53-deficient H1299 cells and in MCF-7DD cells with transcriptionally inactive p53. We suggest that LA-12-mediated apoptosis is not fully dependent on p53. This confirms the therapeutic potential of LA-12 as a more potent cytostatic agent for both tumor cells expressing wild type p53 and for p53-deficient or mutant cells.

Photoaffinity isolation and identification of proteins in cancer cell extracts that bind to platinum-modified DNA

The activity of the anticancer drug cisplatin is a consequence of its ability to bind DNA. Platinum adducts bend and unwind the DNA duplex, creating recognition sites for nuclear proteins. Following DNA damage recognition, the lesions will either be repaired, facilitating cell viability, or if repair is unsuccessful and the Pt adduct interrupts vital cellular functions, apoptosis will follow. With the use of the benzophenone-modified cisplatin analogue Pt-BP6, 25 bp DNA duplexes containing either a 1,2-d(G*pG*) intrastrand or a 1,3-d(G*pTpG*) intrastrand crosslink were synthesized, where the asterisks designate platinated nucleobases. Proteins having affinity for these platinated DNAs were photocrosslinked and identified in cervical, testicular, pancreatic and bone cancer-cell nuclear extracts. Proteins identified in this manner include the DNA repair factors RPA1, Ku70, Ku80, Msh2, DNA ligase III, PARP-1, and DNA-PKcs, as well as HMG-domain proteins HMGB1, HMGB2, HMGB3, and UBF1. The latter strongly associate with the 1,2-d(G*pG*) adduct and weakly or not at all with the 1,3-d(G*pTpG*) adduct. The nucleotide excision repair protein RPA1 was photocrosslinked only by the probe containing a 1,3-d(G*pTpG*) intrastrand crosslink. The affinity of PARP-1 for platinum-modified DNA was established using this type of probe for the first time. To ensure that the proteins were not photocrosslinked because of an affinity for DNA ends, a 90-base dumbbell probe modified with Pt-BP6 was investigated. Photocrosslinking experiments with this longer probe revealed the same proteins, as well as some additional proteins involved in chromatin remodeling, transcription, or repair. These findings reveal a more complete list of proteins involved in the early steps of the mechanism of action of the cisplatin and its close analogue carboplatin than previously was available.

Role of glutathione S-transferase P1-1 in the cellular detoxification of cisplatin

Cells expressing elevated levels of allelic variants of human glutathione S-transferase P1 (GSTP1) and/or efflux transporters, MRP1 or MRP2, were used to evaluate the role of GSTP1-1 in cisplatin resistance. These studies revealed that GSTP1-1 confers low-level resistance (1.4- to 1.7-fold) to cisplatin-induced cytotoxicity in MCF7 cells. However, expression of MRP1 (MCF7 cells) or MRP2 (HepG2 cells) failed to augment or potentiate GSTP1-1-mediated resistance in either cell line. To understand the mechanism by which variants of GSTP1-1 confer resistance to cisplatin, their relative abilities to catalyze conjugation of cisplatin with glutathione were examined. Enzymes encoded by all three alleles tested, GSTP1a (I(104)A(113)), GSTP1b (V(104)A(113)), and GSTP1c (V(104)V(113)), increased the formation rate of the mono-platinum/glutathione derivative of cisplatin with relative catalytic activities of 1.0 (GSTP1a-1a variant) and 1.8 to 1.9 (GSTP1b-1b and GSTP1c-1c variants). Although these data are consistent with the idea that very low level resistance to cisplatin may be conferred by GSTP1-1-mediated cisplatin/glutathione conjugation, two observations indicate that such catalysis plays a minor role in the protection from cisplatin toxicity. First, the rates of GSTP1-1-mediated conjugation are extremely slow (1.7-2.6 h(-1) at 25 degrees C). Second, despite an 80% to 90% increase in catalysis of cisplatin conjugation by GSTP1b-1b or GSTP1c-1c over GSTP1a-1a, we observed no discernable differences in relative resistances conferred by these alternative variants when expressed in MCF7 cells. We conclude that high-level cisplatin resistance attributed to GSTP1-1 in other studies is not likely due to catalysis of cisplatin conjugation but rather must be explained by other mechanisms, which may include GSTP1-mediated modulation of signaling pathways.

The anti-tumour agent, cisplatin, and its clinically ineffective isomer, transplatin, produce unique gene expression profiles in human cells

Cisplatin is a DNA-damaging anti-cancer agent that is widely used to treat a range of tumour types. Despite its clinical success, cisplatin treatment is still associated with a number of dose-limiting toxic side effects. The purpose of this study was to clarify the molecular events that are important in the anti-tumour activity of cisplatin, using gene expression profiling techniques. Currently, our incomplete understanding of this drug's mechanism of action hinders the development of more efficient and less harmful cisplatin-based chemotherapeutics. In this study the effect of cisplatin on gene expression in human foreskin fibroblasts has been investigated using human 19K oligonucleotide microarrays. In addition its clinically inactive isomer, transplatin, was also tested. Dualfluor microarray experiments comparing treated and untreated cells were performed in quadruplicate. Cisplatin treatment was shown to significantly up- or down-regulate a consistent subset of genes. Many of these genes responded similarly to treatment with transplatin, the therapeutically inactive isomer of cisplatin. However, a smaller proportion of these transcripts underwent differential expression changes in response to the two isomers. Some of these genes may constitute part of the DNA damage response induced by cisplatin that is critical for its anti-tumour activity. Ultimately, the identification of gene expression responses unique to clinically active compounds, like cisplatin, could thus greatly benefit the design and development of improved chemotherapeutics.

Substrate specificity and sequence-dependent activity of the Saccharomyces cerevisiae 3-methyladenine DNA glycosylase (Mag)

DNA glycosylases initiate base excision repair by first binding, then excising aberrant DNA bases. Saccharomyces cerevisiae encodes a 3-methyladenine (3MeA) DNA glycosylase, Mag, that recognizes 3MeA and various other DNA lesions including 1,N6-ethenoadenine (epsilon A), hypoxanthine (Hx) and abasic (AP) sites. In the present study, we explore the relative substrate specificity of Mag for these lesions and in addition, show that Mag also recognizes cisplatin cross-linked adducts, but does not catalyze their excision. Through competition binding and activity studies, we show that in the context of a random DNA sequence Mag binds epsilon A and AP-sites the most tightly, followed by the cross-linked 1,2-d(ApG) cisplatin adduct. While epsilon A binding and excision by Mag was robust in this sequence context, binding and excision of Hx was extremely poor. We further studied the recognition of epsilon A and Hx by Mag, when these lesions are present at different positions within A:T and G:C tracts. Overall, epsilon A was slightly less well excised from each position within the A:T and G:C tracts compared to excision from the random sequence, whereas Hx excision was greatly increased in these sequence contexts (by up to 7-fold) compared to the random sequence. However, given most sequence contexts, Mag had a clear preference for epsilon A relative to Hx, except in the TTXTT (X=epsilon A or Hx) sequence context from which Mag removed both lesions with almost equal efficiency. We discuss how DNA sequence context affects base excision by various 3MeA DNA glycosylases.

In vitro evaluation of dichloro-bis(pyrazole)palladium(II) and dichloro-bis(pyrazole)platinum(II) complexes as anticancer agents

INTRODUCTION

Cisplatin (cis-diamminedichloroplatinum) was first identified for its anti-bacterial activity, and was later also shown to be an efficient anticancer agent. However, the therapeutic use of this anticancer drug is somewhat limited by its toxic side effects, which include nephrotoxicity, nausea, and vomiting. Furthermore the development of drug-resistant tumours is commonly observed following therapy with cisplatin. Hence there is a need for improved platinum derived drugs to overcome these limitations.

AIMS

Apoptosis contributes significantly to the cytotoxic effects of anticancer agents such as cisplatin; therefore in this study the potential anticancer properties of a series of pyrazole palladium(II) and platinum(II) complexes, [(3,5-R(2)pz)(2)PdCl(2)] [R = H (1), R = Me (2)] and [(3,5-R(2)pz)(2)PtCl(2)] [R = H (3), R = Me (4)], were evaluated by assessment of their pro-apoptotic activity.

METHODS

The induction of apoptosis was measured in CHO cells by the detection of phosphatidylserine (PS) exposure using the annexin V and APOPercentage assays; DNA fragmentation using the Terminal deoxynucleotide transferase dUTP Nick End Labelling (TUNEL) assay; and the detection of activated caspase-3.

RESULTS

The platinum complexes were shown to be considerably more active than the palladium complexes, with complex 3 demonstrating the highest level of cytotoxic and pro-apoptotic activity. The LD(50) values for complex 3 and cisplatin were 20 and 70 microM, respectively, demonstrating that the cytotoxic activity for complex 3 was three times higher than for cisplatin. Various human cancer cell lines, including CaSki, HeLa, as well as the p53 mutant Jurkat T cell line were also shown to be susceptible to complex 3.

CONCLUSIONS

Collectively, this in vitro study provides insights into action of palladium and platinum complexes and demonstrates the potential use of these compounds, and in particular complex 3, in the development of new anticancer agents.

DNA modification with cisplatin affects sequence-specific DNA binding of p53 and p73 proteins in a target site-dependent manner

Proteins p53 and p73 act as transcription factors in cell cycle control, regulation of cell development and/or in apoptotic pathways. Both proteins bind to response elements (p53 DNA-binding sites), typically consisting of two copies of a motif RRRCWWGYYY. It has been demonstrated previously that DNA modification with the antitumor drug cisplatin inhibits p53 binding to a synthetic p53 DNA-binding site. Here we demonstrate that the effects of global DNA modification with cisplatin on binding of the p53 or p73 proteins to various p53 DNA-binding sites differed significantly, depending on the nucleotide sequence of the given target site. The relative sensitivities of protein-DNA binding to cisplatin DNA treatment correlated with the occurrence of sequence motifs forming stable bifunctional adducts with the drug (namely, GG and AG doublets) within the target sites. Binding of both proteins to mutated p53 DNA-binding sites from which these motifs had been eliminated was only negligibly affected by cisplatin treatment, suggesting that formation of the cisplatin adducts within the target sites was primarily responsible for inhibition of the p53 or p73 sequence-specific DNA binding. Distinct effects of cisplatin DNA modification on the recognition of different response elements by the p53 family proteins may have impacts on regulation pathways in cisplatin-treated cells.

Role of p21waf1/cip1 in effects of oxaliplatin in colorectal cancer cells

Clinical studies have shown that oxaliplatin, a novel platinum derivative, is a potent chemotherapeutic agent for colorectal cancer when combined with 5-fluorouracil and leucovorin. Although the toxic activity is based on covalent adducts between platinum and DNA, its actual biological behavior is mostly unknown. In an effort to explore the mechanism of tumor susceptibility to oxaliplatin, we examined the cytotoxic effects of oxaliplatin in colorectal cancer cell lines in reference to p53 gene status. Although p53 gene status did not clearly predict sensitivity to oxaliplatin, p53 wild-type cells including HCT116 were sensitive but HCT116 p53-/- were found to be resistant to oxaliplatin. Oxaliplatin caused strong p21waf1/cip1 induction and G0-G1 arrest in p53 wild-type cells, whereas cisplatin did not induce G0-G1 arrest. Assays using p53 wild but p21waf1/cip1 null HCT116 cells revealed that oxaliplatin did not show G0-G1 arrest and reduced growth-inhibitory effects, suggesting that p21waf1/cip1 may be a key element in oxaliplatin-treated p53 wild-type cells. Although HCT116 is DNA mismatch repair-deficient, a mismatch repair-proficient HCT116+ch3 cell line displayed similar responses with regard to p21waf1/cip1-mediated growth inhibition and G0-G1 arrest. In p53 mutant cells, on the other hand, oxaliplatin caused an abrupt transition from G1 to S phase and eventually resulted in G2-M arrest. This abrupt entry into S phase was associated with loss of the p21waf1/cip1 protein via proteasome-mediated degradation. These findings suggest that p21waf1/cip1 plays a role in oxaliplatin-mediated cell cycle and growth control in p53-dependent and -independent pathways.

Recognition of cisplatin-damaged DNA by p53 protein: Critical role of the p53 C-terminal domain

It was shown previously that the p53 protein can recognize DNA modified with antitumor agent cisplatin (cisPt-DNA). Here, we studied p53 binding to the cisPt-DNA using p53 deletion mutants and via modulation of the p53-DNA binding by changes of the protein redox state. Isolated p53 C-terminal domain (CTD) bound to the cisPt-DNA with a significantly higher affinity than to the unmodified DNA. On the other hand, p53 constructs involving the core domain but lacking the C-terminal DNA binding site (CTDBS) exhibited only small binding preference for the cisPt-DNA. Oxidation of cysteine residues within the CD of posttranslationally unmodified full length p53 did not affect its ability to recognize cisPt-DNA. Blocking of the p53 CTDBS by a monoclonal antibody Bp53-10.1 resulted in abolishment of the isolated CTD binding to the cisPt-DNA. Our results demonstrate a crucial role of the basic region of the p53 CTD (aa 363-382) in the cisPt-DNA recognition.

The antiproliferative agent MLN944 preferentially inhibits transcription

MLN944 is a novel compound currently being codeveloped by Millennium Pharmaceuticals and Xenova Ltd. as a cancer therapeutic and is in a phase I clinical trial for solid tumors. Although MLN944 was originally proposed to function as a topoisomerase I and II inhibitor, more recent data has shown that it is a DNA-intercalating agent that does not inhibit the catalytic activity of topoisomerase I or II. We show here that MLN944 inhibits incorporation of radiolabeled precursors into RNA preferentially over incorporation into DNA and protein in HCT116 and H460 cells. To determine if MLN944 inhibits transcription, a human RNA polymerase II in vitro transcription system was used. MLN944 inhibited initiation when added before or after the formation of preinitiation complexes and inhibited elongation at higher concentrations. The preferential inhibition of initiation differentiates MLN944 from actinomycin D, which more strongly inhibits elongation. Transcription of all RNA polymerases was inhibited in nuclei isolated from HeLa cells treated with low concentrations of MLN944. Our data are consistent with transcription as the target of the potent cytotoxic effects of MLN944.

Water-soluble platinum(II) complexes of diamine chelating ligands bearing amino-acid type substituents: the effect of the linked amino acid and the diamine chelate ring size on antitumor activity, and interactions with 5'-GMP and DNA

Six new Pt(II) complexes are described having the general formula PtCl(2)(LL), in which LL is a chelating diamine ligand bearing an amino acid as substituent. The amino acids chosen are l-alanine and its methyl ester, and l-phenylalanine. The compounds have been characterized using analytical and spectroscopic methods. The influence on the biological properties of the size of the chelate ring and the structure of the amino acid substituent has been studied. The effect of the presence of a carboxylic or carboxylate group on the amino acid C-terminus has also been determined. It is demonstrated by circular dichroism (CD) that the effect on the secondary structure of DNA induced by the six complexes differ from each other. In all cases, the interaction takes place at the N7 position of the purine bases, as shown by NMR monitoring. The general behavior of these platinum complexes, with one exception, is to uncoil the DNA from the B form to the C form. The interactions with 5'-GMP and DNA have been compared with their expected antitumour activity. The complexes with l-alanine and l-phenylalanine exhibit cytotoxic activity in HeLa and HL-60 cell lines, in a dose- and time-dependent manner. No cytotoxic activity of the methyl ester derivatives have been determined because of their low solubility in aqueous solution.

Effects of anticancer drugs on transcription factor–DNA interactions

DNA-interacting anticancer drugs are able to affect the propensity of DNA to interact with proteins through either reversible binding or covalent bond formation. The effect of the drugs on transcription factor interactions with DNA is reviewed. These effects can be classified as (i) competition between a drug and regulatory protein for target sequences; (ii) weakening of this interaction; (iii) enhancement of this interaction by chemical modification of the DNA and the creation of non-natural binding sites; and (iv) a 'suicide' mechanism, which is observed when a transcription factor induces changes in DNA structure, allowing a drug to bind to a target sequence. Several new strategies -- the antigene approach with oligonucleotides, peptide nucleic acids or locked nucleic acids, and sequence-specific polyamides -- are also reviewed.

Identification of promoters bound by c-Jun/ATF2 during rapid large-scale gene activation following genotoxic stress

The NH2-terminal Jun kinases (JNKs) function in diverse roles through phosphorylation and activation of AP-1 components including ATF2 and c-Jun. However, the genes that mediate these processes are poorly understood. A model phenotype characterized by rapid activation of Jun kinase and enhanced DNA repair following cisplatin treatment was examined using chromatin immunoprecipitation with antibodies against ATF2 and c-Jun or their phosphorylated forms and hybridization to promoter arrays. Following genotoxic stress, we identified 269 genes whose promoters are bound upon phosphorylation of ATF2 and c-Jun. Binding did not occur following treatment with transplatin or the JNK inhibitor SP600125 or JNK-specific siRNA. Of 89 known DNA repair genes represented on the array, 23 are specifically activated by cisplatin treatment within 3-6 hr. Thus, the genotoxic stress response occurs at least partly via activation of ATF2 and c-Jun, leading to large-scale coordinate gene expression dominated by genes of DNA repair.

Inhibition of cell growth, induction of apoptosis and mechanism of action of the novel platinum compound cis-diaminechloro-[2-(diethylamino) ethyl 4-amino-benzoate, N(4)]-chloride platinum (II) monohydrochloride monohydrate

Cis-diaminechloro-[2-(diethylamino) ethyl 4-amino-benzoate, N(4)]-chloride platinum (II) monohydrochloride monohydrate (DPR) is a new platinum triamine complex obtained from the synthesis of cisplatin and procaine. In this paper we analyzed, adopting a disease-oriented strategy, the tumour selectivity of this compound, its ability to induce apoptosis and its mechanism of interaction with DNA. The inhibition of cell proliferation was evaluated by the MTT assay using a panel of 51 tumour cell lines. Some of them were also evaluated for the induction of apoptosis by 4'-6-diamidine-2'-phenylindole (DAPI) staining, Western blot of p53 protein and agarose gel electrophoresis of ladder DNA. Finally, interstand cross-links (ISCL) were evaluated by ethidium bromide fluorescence technique. When evaluated by the MTT assay, DPR showed a high selective activity for neuroblastoma, small cell lung cancer (SCLC), ovarian cancer and leukemia cell lines. The comparison of mean graphs of DPR and cisplatin suggested that our compound possesses a mechanism of action similar to that, at least in part, of its parent compound. Moreover, DPR showed itself to be a good trigger of programmed cell death, as demonstrated by DAPI staining, activation of p53 protein and agarose gel electrophoresis of ladder DNA. Finally, the study of the formation of ISCLs demonstrated that DPR, despite being a monofunctional platinum compound, is able to form bifunctional adducts through the release of procaine residue. Data presented here suggest that DPR is an antitumour agent able to trigger apoptosis, and that it is endowed with a peculiar mechanism(s) of action and a special selective activity against two tumours, namely neuroblastoma and SCLC, which are still characterized by a low incidence of long-term survivors.

ZNF143 activates gene expression in response to DNA damage and binds to cisplatin-modified DNA

We have identified a cisplatin-inducible gene, the mitochondrial ribosomal protein S11 (MRP S11) gene, by means of mRNA differential display. Functional analysis of the MRP S11 promoter showed that a Staf binding site in the promoter is required for both basal promoter activity and cisplatin-inducible activity. We also found that Staf binding activity is significantly increased in nuclear extracts from cells treated with cisplatin. ZNF 143 and ZNF 76 are human homologues of the Xenopus transcriptional activator, Staf. ZNF 143 expression is induced by cisplatin but ZNF 76 expression is not. However, ZNF 143 expression is not induced by transplatin, which is clinically ineffective. ZNF143 is an inducible gene by other DNA damaging agents such as gamma-irradiation, etoposide and adriamycin. ZNF 143 also binds preferentially to cisplatin-modified DNA. These results suggest that ZNF 143 participates in cellular responses to DNA damage.

Multiple states of stalled T7 RNA polymerase at DNA lesions generated by platinum anticancer agents

Transcription inhibition by DNA adducts of cisplatin is considered to be one of the major routes by which this anticancer drug kills cancer cells. Stalled RNA polymerases at platinum-DNA lesions evoke various cellular responses such as nucleotide excision repair, polymerase degradation, and apoptosis. T7 RNA polymerase and site-specifically platinated DNA templates immobilized on a solid support were used to study stalled transcription elongation complexes. In vitro transcription studies were performed in both a promoter-dependent and -independent manner. An elongation complex is strongly blocked by cisplatin 1,2-intrastrand d(GpG) and 1,3-intrastrand d(GpTpG) cross-links located on the template strand. Polymerase action is inhibited at multiple sites in the vicinity of the platinum lesion, the nature of which can be altered by the choice and concentration of NTPs. The [(1R,2R-diaminocyclohexane)Pt]2+ DNA adducts formed by oxaliplatin, which carries a stereochemically more demanding spectator ligand than the ammine groups in cisplatin, also strongly block the polymerase with measurable differences compared with cis-[(NH3)2Pt]2+ lesions. Elongation complexes stopped at sites of platinum damage were isolated and characterized. The stalled polymerase can be dissociated from the DNA by subsequent polymerases initiated from the same template. We also discovered that a polymerase stalled at the platinum-DNA lesion can resume transcription after the platinum adduct is chemically removed from the template.

Unprecedented head-to-head conformers of d(GpG) bound to the antitumor active compound tetrakis (mu-carboxylato)dirhodium(II,II)

The N7/O6 equatorial binding interactions of the antitumor active complex Rh(2)(OAc)(4)(H(2)O)(2) (OAc(-) = CH(3)CO(2)(-)) with the DNA fragment d(GpG) have been unambiguously determined by NMR spectroscopy. Previous X-ray crystallographic determinations of the head-to-head (HH) and head-to-tail (HT) adducts of dirhodium tetraacetate with 9-ethylguanine (9-EtGH) revealed unprecedented bridging N7/O6 guanine nucleobases that span the Rh-Rh bond. The absence of N7 protonation at low pH and the notable increase in the acidity of N1-H (pK(a) approximately 5.7 as compared to 8.5 for N7 only bound platinum adducts), suggested by the pH dependence titrations of the purine H8 (1)H NMR resonances for Rh(2)(OAc)(2)(9-EtG)(2) and Rh(2)(OAc)(2-)[d(GpG)],are consistent with bidentate N7/O6 binding of the guanine nucleobases. The pK(a) values estimated for N1-H (de)protonation, from the pH dependence studies of the C6 and C2 (13)C NMR resonances for the Rh(2)(OAc)(2)(9-EtG)(2) isomers, concur with those derived from the H8 (1)H NMR resonance titrations. Comparison of the (13)C NMR resonances of C6 and C2 for the dirhodium adducts Rh(2)(OAc)(2)(9-EtG)(2) and Rh(2)(OAc)(2)[d(GpG)] with the corresponding resonances of the unbound ligands [at pH 7.0 for 9-EtGH and pH 8.0 for d(GpG)], shows substantial downfield shifts of Deltadelta approximately 11.0 and 6.0 ppm for C6 and C2, respectively; the latter shifts reflect the effect of O6 binding to the dirhodium centers and the ensuing enhancement in the acidity of N1-H. Intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum of Rh(2)(OAc)(2)[d(GpG)] indicate head-to-head arrangement of the guanine bases. The Rh(2)(OAc)(2)[d(GpG)] adduct exhibits two major right-handed conformers, HH1 R and HH2 R, with HH1 R being three times more abundant than the unusual HH2 R. Complete characterization of both adducts revealed repuckering of the 5'-G sugar rings to C3'-endo (N-type), retention of C2'-endo (S-type) conformation for the 3'-G sugar rings, and anti orientation with respect to the glycosyl bonds. The structural features obtained for Rh(2)(OAc)(2))[d(GpG)] by means of NMR spectroscopy are very similar to those for cis-[Pt(NH(3))(2))[d(GpG)]] and corroborate molecular modeling studies.

Role of the p38 MAPK pathway in cisplatin-based therapy

p38 MAPK has been implicated in the response to cancer therapy. To determine whether the activation of p38 MAPK could be specific to cancer therapy, we investigated the activation of p38 MAPK in response to several chemotherapeutic agents, such as cisplatin, doxorubicin and taxol in several human cell lines. Activation of p38 MAPK was measured after exposure to several chemotherapeutic agents, using specific phosphoantibodies. Only cisplatin was able to activate p38 MAPK in all the cell lines tested. Furthermore, other platinum compounds such as transplatin and platinum (IV) chloride can induce activation of p38 MAPK. The kinetics of this activation is a key event in the biological role of p38 MAPK in response to cisplatin, as we conclude from the differences observed after treatment with transplatin and cisplatin. The p38 MAPK activation is independent of the origin or genetic alterations of the cell lines and seems to be mediated through both upstream activators MKK6 and MKK3. Although the isoforms alpha/beta are mainly activated, we also demonstrated that other members of the p38 MAPK family were susceptible to activation by cisplatin when they were overexpressed in 293 T. Finally, pretreatment with specific inhibitors (SB 203580 and SKF 86002) induces a resistant phenotype in response to cisplatin. Furthermore, low activation of this SAPK pathway correlates with a resistant phenotype as demonstrated in our experimental model of head and neck cancer. Therefore, we conclude that the p38 MAPK pathway is a specific target for cisplatin-based therapy with clinical implications.

Physical interaction of tumour suppressor p53/p73 with CCAAT-binding transcription factor 2 (CTF2) and differential regulation of human high-mobility group 1 (HMG1) gene expression

The CCAAT-binding transcription factor (CTF)/nuclear factor I (NF-I) group of cellular DNA-binding proteins recognizes the sequence GCCAAT and is implicated in eukaryotic transcription, as well as DNA replication. Molecular analysis of human CTF/NF-I cDNA clones revealed multiple mRNA species that contain alternative coding regions, apparently as a result of differential splicing. Expression and functional analysis established that individual gene products can bind to GCCAAT recognition sites and serve as both promoter-selective transcriptional activators and initiation factors for DNA replication. The interaction between CTF2 and p53/p73 was shown to modulate their ability to regulate transcription of their respective target genes. In the present paper, we report that p53 down-regulates the activity of the high mobility group 1 (HMG1) gene promoter, whereas p73alpha up-regulates the activity of this promoter. Furthermore, CTF2 transactivates p53-induced p21 promoter activity, but inhibits p73alpha-induced p21 promoter activity. Using deletion mutants, we found that the DNA-binding domains of both p53 and p73alpha are required for physical interaction with CTF2 via the regions between amino acid residues 161 and 223, and 228 and 312 respectively. CTF2 enhances the DNA-binding activity of p53 and inhibits the DNA-binding activity of p73alpha. These results provide novel information on the functional interplay between CTF2 and p53/p73 as important determinants of their function in cell proliferation, apoptosis, DNA repair and cisplatin resistance.

Recognition of DNA modified by antitumor cisplatin by "latent" and "active" protein p53

Tumor suppressor protein p53 possesses two DNA-binding sites. One that is located within its core domain is responsible for sequence-specific DNA binding of the protein, non-specific binding to internal segments of single- or double-stranded DNA, and to certain kinds of non-B DNA structures. The other that is contained in the C-terminus of the protein binds to damaged DNA. Binding of active, latent, and in vitro-activated p53 protein to DNA fragments modified by antitumor cisplatin was studied using electrophoretic mobility shift assay in agarose gels and immunoblotting analysis. We found that both latent and active p53 forms bound to random sequences of DNA globally modified by cisplatin with a higher affinity than to unmodified DNA. Interestingly, the latent form exhibited a more pronounced selectivity for platinated DNA than the active p53. Consistently with this observation, the preference of the latent form for platinated DNA decreased as a consequence of the activation of latent p53 by phosphorylation at the protein kinase C site within its C-terminus or by binding of the monoclonal antibody Bp53-10.1. Competition experiments involving a 20-bp consensus sequence of p53 suggested that the p53 core domain was a primary binding site of the active p53 when it bound to DNA fragments lacking consensus sequence, but modified by cisplatin. In addition, the latent protein was found to selectively interact with DNA modified by cisplatin probably via its C-terminus.

DNA modifications by antitumor platinum and ruthenium compounds: their recognition and repair

The development of metal-based antitumor drugs has been stimulated by the clinical success of cis-diamminedichloroplatinum(II) (cisplatin) and its analogs and by the clinical trials of other platinum and ruthenium complexes with activity against resistant tumors and reduced toxicity including orally available platinum drugs. Broadening the spectrum of antitumor drugs depends on understanding existing agents with a view toward developing new modes of attack. It is therefore of great interest to understand the details of molecular and biochemical mechanisms underlying the biological efficacy of platinum and other transition-metal compounds. There is a large body of experimental evidence that the success of platinum complexes in killing tumor cells results from their ability to form various types of covalent adducts on DNA; thus, the research of DNA interactions of metal-based antitumor drugs has predominated. The present review summarizes current knowledge on DNA modifications by platinum and ruthenium complexes, their recognition by specific proteins, and repair. It also provides strong support for the view that either platinum or ruthenium drugs, which bind to DNA in a fundamentally different manner from that of 'classical' cisplatin, have altered pharmacological properties. The present article also demonstrates that this concept has already led to the synthesis of several new unconventional platinum or ruthenium antitumor compounds that violate the original structure-activity relationships.

Cisplatin sensitivity in Hmbg1-/- and Hmbg1+/+ mouse cells

The study presented here investigates the effect of HMGB1 knockout on the sensitivity of mouse embryonic fibroblasts treated with the anticancer drug cisplatin. We evaluated both the growth inhibition by cisplatin and cisplatin-induced cell death in the Hmgb1(-/-) cells and its wild-type counterpart. No significant differences were observed in the responses of these cells to cisplatin, indicating that HMGB1 does not play a significant role in modulating the cellular responses to cisplatin in this context. Since HMGB1 significantly enhances the cytotoxicity of cisplatin in other cells, these results illustrate the importance of cell type in determining the ability of this and probably other cisplatin-DNA-binding proteins to influence the efficacy of the drug.

DNA interstrand cross-links of the novel antitumor trinuclear platinum complex BBR3464. Conformation, recognition by high mobility group domain proteins, and nucleotide excision repair

The novel phase II antitumor polynuclear platinum drug BBR3464 ([(trans-PtCl(NH(3))(2))(2)(mu-trans-Pt(NH(3))(2)(NH(2)(CH(2))(6)NH(2))(2))](NO(3))(4)) forms intra- and interstrand cross-links (CLs) on DNA (which is the pharmacological target of platinum drugs). We examined first in our recent work how various intrastrand CLs of BBR3464 affect the conformation of DNA and its recognition by cellular components (Zehnulova, J., Kasparkova, J., Farrell, N., and Brabec, V. (2001) J. Biol. Chem. 276, 22191-22199). In the present work, we have extended the studies on the DNA interstrand CLs of this drug. The results have revealed that the interstrand CLs are preferentially formed between guanine residues separated by 2 base pairs in both the 3' --> 3' and 5' --> 5' directions. The major 1,4-interstrand CLs distort DNA, inducing a directional bending of the helix axis and local unwinding of the duplex. Although such distortions represent a potential structural motif for recognition by high mobility group proteins, these proteins do not recognize 1,4-interstrand CLs of BBR3464. On the other hand, in contrast to intrastrand adducts of BBR3464, 1,4-interstrand CLs are not removed from DNA by nucleotide excision repair. It has been suggested that interstrand CLs of BBR3464 could persist considerably longer in cells compared with intrastrand adducts, which would potentiate the toxicity of the interstrand lesions to tumors sensitive to this polynuclear drug.

Synergism between rViscumin and cisplatin is not dependent on ERCC-1 expression

Interactions between recombinant mistletoe lectin (rViscumin) and anticancer drugs were investigated in vitro. rViscumin enhanced the cytotoxic effects of vincristine, mafosfamide, idarubicin and cisplatin in the human leukemia cell lines K562 and KG1a. In human marrow progenitor cells, rViscumin inhibited colony formation and did not exert any protective activity against cisplatin-induced inhibition of clonogenicity. Quantitative real-time reverse transcription polymerase chain reaction analysis revealed that cisplatin treatment of K562 cells resulted in a 1.9-fold increase in mRNA expression of the nucleotide excision repair gene ERCC-1. This upregulation was not prevented when cells were post-incubated with rViscumin. Our study provides evidence that rViscumin is capable of enhancing cytotoxicity of anticancer agents in vitro. This synergism appears to be independent of transcriptional activity of DNA repair relevant genes.

Testis-specific HMG-domain protein alters the responses of cells to cisplatin

Cisplatin is an effective agent for the treatment of testicular cancer. In the present study with mouse testicular teratocarcinoma cell extracts, we observed a deficiency in nucleotide excision repair (NER) of a DNA probe bearing a cisplatin 1,2-d(GpG) intrastrand cross-link. In contrast, repair of the cisplatin 1,3-d(GpTpG) intrastrand cross-link was still active in these cell extracts. A current working hypothesis is that complexes of HMG-domain proteins with the major cisplatin 1,2-intrastrand cross-links could enhance cisplatin cytotoxicity by blocking repair of these lesions on the genome. The family of HMG-domain proteins include a testis-specific protein, tsHMG, which might account for the altered NER in testicular cells. To test this possibility, a human cervical carcinoma cell line (HeLa) was constructed which ectopically expressed tsHMG under the control of an inducible promoter. Microscopic examination of tsHMG expression and cisplatin-induced apoptosis on a cellular level revealed that the nuclear protein did indeed modulate the cytotoxic consequences of cisplatin treatment. Also, tsHMG enhanced transcription inhibition by cisplatin. These results reveal that an HMG-domain protein can affect cellular responses to cisplatin and may be relevant to the clinical observation that cancer cells in specific tissues are particularly sensitive to cisplatin.

DNA bending and unwinding due to the major 1,2-GG intrastrand cross-link formed by antitumor cis-diamminedichloroplatinum(II) are flanking-base independent

Antitumor cisplatin [cis-diamminedichloroplatinum(II)] forms on DNA predominantly intrastrand cross-links between neighboring purine residues. Several discoveries suggested that the toxicity of cisplatin originated from these lesions. The formation of 1,2-GG intrastrand cross-link of cisplatin leads to marked conformational alterations in DNA including a directional, rigid bend toward the major groove and local unwinding. These altered structures attract various cellular proteins. This phenomenon has been postulated to mediate antitumor properties of cisplatin. Importantly, the binding affinity of several proteins that specifically recognize 1,2-GG intrastrand cross-link to platinated DNA is modulated by the nature of the base pairs that immediately flank the platinated d(GpG) site. However, the influence of sequence context on DNA bending and unwinding due to the formation of the 1,2-GG intrastrand cross-link has not been extensively investigated. In the present study we have employed electrophoretic retardation (phasing) assay to analyze bending and unwinding induced by the single, site-specific 1,2-GG intrastrand cross-link immediately flanked by various bases formed by cisplatin in nine oligodeoxyribonucleotide duplexes. The results indicate that bending and unwinding of DNA as a consequence of the formation of the major adduct of cisplatin is, in the first approximation, independent of the base pairs flanking the platinated d(GpG) site.

Molecular aspects of resistance to antitumor platinum drugs

The processes by which cells develop resistance to antitumor platinum drugs have been the subject of intense research because resistance is a major obstacle for the clinical use of this class of drugs. It is therefore of great interest to understand the molecular and biochemical mechanisms that underlie resistance to platinum drugs and their biological effects. There is a large body of experimental evidence suggesting that the antitumor activity of platinum complexes stems from their ability to form on DNA various types of covalent adducts. As a result, research on DNA modifications by these drugs and their cellular processing has predominated. The resistance of tumor cells to platinum drugs has been attributed to several processes and an increased repair of platinum-DNA adducts is considered a most significant event. The present review summarizes recent insights into the effects of sulfur-containing compounds on DNA modifications by antitumor platinum complexes and how these modifications are repaired including how this repair is associated with their recognition by cellular, damaged-DNA binding-proteins. It strongly supports the view that changes in the structure of platinum drugs, resulting in DNA binding mode fundamentally different from that of "classical" cisplatin, will alter resistance pathways of platinum drugs, and may also modulate their pharmacological properties.