Drug resistance and DNA repair

Writers, readers, and erasers RNA modifications and drug resistance in cancer

Drug resistance in cancer cells significantly diminishes treatment efficacy, leading to recurrence and metastasis. A critical factor contributing to this resistance is the epigenetic alteration of gene expression via RNA modifications, such as N6-methyladenosine (m6A), N1-methyladenosine (m1A), 5-methylcytosine (m5C), 7-methylguanosine (m7G), pseudouridine (Ψ), and adenosine-to-inosine (A-to-I) editing. These modifications are pivotal in regulating RNA splicing, translation, transport, degradation, and stability. Governed by "writers," "readers," and "erasers," RNA modifications impact numerous biological processes and cancer progression, including cell proliferation, stemness, autophagy, invasion, and apoptosis. Aberrant RNA modifications can lead to drug resistance and adverse outcomes in various cancers. Thus, targeting RNA modification regulators offers a promising strategy for overcoming drug resistance and enhancing treatment efficacy. This review consolidates recent research on the role of prevalent RNA modifications in cancer drug resistance, with a focus on m6A, m1A, m5C, m7G, Ψ, and A-to-I editing. Additionally, it examines the regulatory mechanisms of RNA modifications linked to drug resistance in cancer and underscores the existing limitations in this field.

Tricarboxylic Acid Cycle Relationships with Non-Metabolic Processes: A Short Story with DNA Repair and Its Consequences on Cancer Therapy Resistance

Metabolic changes involving the tricarboxylic acid (TCA) cycle have been linked to different non-metabolic cell processes. Among them, apart from cancer and immunity, emerges the DNA damage response (DDR) and specifically DNA damage repair. The oncometabolites succinate, fumarate and 2-hydroxyglutarate (2HG) increase reactive oxygen species levels and create pseudohypoxia conditions that induce DNA damage and/or inhibit DNA repair. Additionally, by influencing DDR modulation, they establish direct relationships with DNA repair on at least four different pathways. The AlkB pathway deals with the removal of N-alkylation DNA and RNA damage that is inhibited by fumarate and 2HG. The MGMT pathway acts in the removal of O-alkylation DNA damage, and it is inhibited by the silencing of the MGMT gene promoter by 2HG and succinate. The other two pathways deal with the repair of double-strand breaks (DSBs) but with opposite effects: the FH pathway, which uses fumarate to help with the repair of this damage, and the chromatin remodeling pathway, in which oncometabolites inhibit its repair by impairing the homologous recombination repair (HRR) system. Since oncometabolites inhibit DNA repair, their removal from tumor cells will not always generate a positive response in cancer therapy. In fact, their presence contributes to longer survival and/or sensitization against tumor therapy in some cancer patients.

Insights into the critical role of the PXR in preventing carcinogenesis and chemotherapeutic drug resistance

Pregnane x receptor (PXR) as a nuclear receptor is well-established in drug metabolism, however, it has pleiotropic functions in regulating inflammatory responses, glucose metabolism, and protects normal cells against carcinogenesis. Most studies focus on its transcriptional regulation, however, PXR can regulate gene expression at the translational level. Emerging evidences have shown that PXR has a broad protein-protein interaction network, by which is implicated in the cross signaling pathways. Furthermore, the interactions between PXR and some critical proteins (e.g., p53, Tip60, p300/CBP-associated factor) in DNA damage pathway highlight its potential roles in this field. A thorough understanding of how PXR maintains genome stability and prevents carcinogenesis will help clinical diagnosis and finally benefit patients. Meanwhile, due to the regulation of CYP450 enzymes CYP3A4 and multidrug resistance protein 1 (MDR1), PXR contributes to chemotherapeutic drug resistance. It is worthy of note that the co-factor of PXR such as RXRα, also has contributions to this process, which makes the PXR-mediated drug resistance more complicated. Although single nucleotide polymorphisms (SNPs) vary between individuals, the amino acid substitution on exon of PXR finally affects PXR transcriptional activity. In this review, we have summarized the updated mechanisms that PXR protects the human body against carcinogenesis, and major contributions of PXR with its co-factors have made on multidrug resistance. Furthermore, we have also reviewed the current promising antagonist and their clinic applications in reversing chemoresistance. We believe our review will bring insight into PXR-targeted cancer therapy, enlighten the future study direction, and provide substantial evidence for the clinic in future.

piR-39980 mediates doxorubicin resistance in fibrosarcoma by regulating drug accumulation and DNA repair

Resistance to doxorubicin (DOX) is an obstacle to successful sarcoma treatment and a cause of tumor relapse, with the underlying molecular mechanism still unknown. PIWI-interacting RNAs (piRNAs) have been shown to enhance patient outcomes in cancers. However, there are few or no reports on piRNAs affecting chemotherapy in cancers, including fibrosarcoma. The current study aims to investigate the relationship between piR-39980 and DOX resistance and the underlying mechanisms. We reveal that piR-39980 is less expressed in DOX-resistant HT1080 (HT1080/DOX) fibrosarcoma cells. Our results show that inhibition of piR-39980 in parental HT1080 cells induces DOX resistance by attenuating intracellular DOX accumulation, DOX-induced apoptosis, and anti-proliferative effects. Its overexpression in HT1080/DOX cells, on the other hand, increases DOX sensitivity by promoting intracellular DOX accumulation, DNA damage, and apoptosis. The dual-luciferase reporter assay indicates that piR-39980 negatively regulates RRM2 and CYP1A2 via direct binding to their 3'UTRs. Furthermore, overexpressing RRM2 induces DOX resistance of HT1080 cells by rescuing DOX-induced DNA damage by promoting DNA repair, whereas CYP1A2 confers resistance by decreasing intracellular DOX accumulation, which piR-39980 restores. This study reveals that piR-39980 could reduce fibrosarcoma resistance to DOX by modulating RRM2 and CYP1A2, implying that piRNA can be used in combination with DOX.

Engineering in Medicine To Address the Challenge of Cancer Drug Resistance: From Micro- and Nanotechnologies to Computational and Mathematical Modeling

Drug resistance has profoundly limited the success of cancer treatment, driving relapse, metastasis, and mortality. Nearly all anticancer drugs and even novel immunotherapies, which recalibrate the immune system for tumor recognition and destruction, have succumbed to resistance development. Engineers have emerged across mechanical, physical, chemical, mathematical, and biological disciplines to address the challenge of drug resistance using a combination of interdisciplinary tools and skill sets. This review explores the developing, complex, and under-recognized role of engineering in medicine to address the multitude of challenges in cancer drug resistance. Looking through the "lens" of intrinsic, extrinsic, and drug-induced resistance (also referred to as "tolerance"), we will discuss three specific areas where active innovation is driving novel treatment paradigms: (1) nanotechnology, which has revolutionized drug delivery in desmoplastic tissues, harnessing physiochemical characteristics to destroy tumors through photothermal therapy and rationally designed nanostructures to circumvent cancer immunotherapy failures, (2) bioengineered tumor models, which have benefitted from microfluidics and mechanical engineering, creating a paradigm shift in physiologically relevant environments to predict clinical refractoriness and enabling platforms for screening drug combinations to thwart resistance at the individual patient level, and (3) computational and mathematical modeling, which blends in silico simulations with molecular and evolutionary principles to map mutational patterns and model interactions between cells that promote resistance. On the basis that engineering in medicine has resulted in discoveries in resistance biology and successfully translated to clinical strategies that improve outcomes, we suggest the proliferation of multidisciplinary science that embraces engineering.

Functions and properties of nuclear lncRNAs-from systematically mapping the interactomes of lncRNAs

Protein and DNA have been considered as the major components of chromatin. But beyond that, an increasing number of studies show that RNA occupies a large amount of chromatin and acts as a regulator of nuclear architecture. A significant fraction of long non-coding RNAs (lncRNAs) prefers to stay in the nucleus and cooperate with protein complexes to modulate epigenetic regulation, phase separation, compartment formation, and nuclear organization. An RNA strand also can invade into double-stranded DNA to form RNA:DNA hybrids (R-loops) in living cells, contributing to the regulation of gene expression and genomic instability. In this review, we discuss how nuclear lncRNAs orchestrate cellular processes through their interactions with proteins and DNA and summarize the recent genome-wide techniques to study the functions of lncRNAs by revealing their interactomes in vivo.

PFKFB3 blockade inhibits hepatocellular carcinoma growth by impairing DNA repair through AKT

Overexpression of 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFKFB3), a key molecule of glucose metabolism in cytoplasm, has been found in various tumors. Emerging evidence has suggested that PFKFB3 is also located in the nucleus and apparent in regulatory functions other than glycolysis. In this study, we found that PFKFB3 expression is associated with hepatocellular carcinoma (HCC) growth and located mainly in the nucleus of tumor cells. PFKFB3 overexpression was associated with large tumor size (p = 0.04) and poor survival of patients with HCC (p = 0.027). Knockdown of PFKFB3 inhibited HCC growth, not only by reducing glucose consumption but also by damaging the DNA repair function, leading to G2/M phase arrest and apoptosis. In animal studies, overexpression of PFKFB3 is associated with increased tumor growth. Mechanistically, PFKFB3 silencing decreased AKT phosphorylation and reduced the expression of ERCC1, which is an important DNA repair protein. Moreover, PFK15, a selective PFKFB3 inhibitor, significantly inhibited tumor growth in a xenograft model of human HCC. PFKFB3 is a potential novel target in the treatment of HCC.

Guardians of the mycobacterial genome: A review on DNA repair systems in Mycobacterium tuberculosis

The genomic integrity of Mycobacterium tuberculosis is continuously threatened by the harsh survival conditions inside host macrophages, due to immune and antibiotic stresses. Faithful genome maintenance and repair must be accomplished under stress for the bacillus to survive in the host, necessitating a robust DNA repair system. The importance of DNA repair systems in pathogenesis is well established. Previous examination of the M. tuberculosis genome revealed homologues of almost all the major DNA repair systems, i.e. nucleotide excision repair (NER), base excision repair (BER), homologous recombination (HR) and non-homologous end joining (NHEJ). However, recent developments in the field have pointed to the presence of novel proteins and pathways in mycobacteria. Homologues of archeal mismatch repair proteins were recently reported in mycobacteria, a pathway previously thought to be absent. RecBCD, the major nuclease-helicase enzymes involved in HR in E. coli, were implicated in the single-strand annealing (SSA) pathway. Novel roles of archeo-eukaryotic primase (AEP) polymerases, previously thought to be exclusive to NHEJ, have been reported in BER. Many new proteins with a probable role in DNA repair have also been discovered. It is now realized that the DNA repair systems in M. tuberculosis are highly evolved and have redundant backup mechanisms to mend the damage. This review is an attempt to summarize our current understanding of the DNA repair systems in M. tuberculosis.

A comparative analysis of the DNA recombination repair pathway in mycobacterial genomes

In prokaryotes, repair by homologous recombination provides a major means to reinstate the genetic information lost in DNA damage. Recombination repair pathway in mycobacteria has multiple differences as compared to that in Escherichia coli. Of about 20 proteins known to be involved in the pathway, a set of 9 proteins, namely, RecF, RecO, RecR, RecA, SSBa, RuvA, RuvB and RuvC was found to be indispensable among the 43 mycobacterial strains. A domain level analysis indicated that most domains involved in recombination repair are unique to these proteins and are present as single copies in the genomes. Synteny analysis reveals that the gene order of proteins involved in the pathway is not conserved, suggesting that they may be regulated differently in different species. Sequence conservation among the same protein from different strains suggests the importance of RecO-RecA and RecFOR-RecA presynaptic pathways in the repair of double strand-breaks and single strand-breaks respectively. New annotations obtained from the analysis, include identification of a protein with a probable Holliday junction binding role present in 41 mycobacterial genomes and that of a RecB-like nuclease, containing a cas4 domain, present in 42 genomes. New insights into the binding of small molecules to the relevant proteins are provided by binding pocket analysis using three dimensional structural models. Analysis of the various features of the recombination repair pathway, presented here, is likely to provide a framework for further exploring stress response and emergence of drug resistance in mycobacteria.

Predicting efficacies of anticancer drugs using single cell HaloChip assay

Single cell HaloChip assay can be used to assess DNA repair ability.

Nuclear-Targeting MSNs-Based Drug Delivery System: Global Gene Expression Analysis on the MDR-Overcoming Mechanisms

The biological mechanisms of nuclear-targeting mesoporous silica nanoparticles (MSNs)-based DDSs (DOX@NT-MSNs) in overcoming multidrug resistance of cancer cells are studied. It is interesting to find for the first time that DOX@NT-MSNs down-regulate the expression of apoptosis suppressor genes and inhibit DNA repair process by disturbing the p53 pathway.

hMSH2 expression is associated with paclitaxel resistance in ovarian carcinoma, and inhibition of hMSH2 expression in vitro restores paclitaxel sensitivity

The objective of the present study was to investigate the association between paclitaxel resistance, gene copy number, and gene expression in ovarian carcinoma, and to restore paclitaxel sensitivity in a paclitaxel-resistant ovarian carcinoma cell line by using hMSH2-targeting siRNA. Paclitaxel-resistant ovarian carcinoma cell lines OC3/TAX300 and OC3/TAX50 and their parental cell lines were analyzed by comparative genomic hybridization, and the expression levels of hMSH2 in ovarian carcinoma cell lines and tissues were determined. An siRNA targeted to hMSH2 mRNA was used to transfect a paclitaxel-resistant cell line. We assessed the morphological features, proliferation, and susceptibility to apoptosis of the transfected cells after paclitaxel treatment. Chromosome 2p21 (gene locus of hMSH2) was amplified in OC3/TAX300 cells. hMSH2 was overexpressed in 93.9 and 47.6% of paclitaxel-treated and untreated ovarian carcinoma tissue samples (P=0.0001), respectively. hMSH2 was overexpressed in 93.3 and 54.2% of low-differentiated and moderate-to-highly differentiated ovarian carcinoma tissue samples (P=0.0008), respectively. hMSH2 expression was inhibited in the OC3/TAX300 cells transfected with hMSH2 siRNA. hMSH2 siRNA increased paclitaxel sensitivity, inhibited OC3/TAX300 cell proliferation (G2/M arrest), and increased susceptibility to apoptosis. hMSH2 expression was upregulated in ovarian carcinoma cell lines and tissues after paclitaxel treatment. hMSH2 overexpression is related to paclitaxel resistance and poor prognosis. Inhibition of hMSH2 expression in vitro restores paclitaxel sensitivity in paclitaxel‑resistant ovarian carcinoma cell lines and indicates a new direction in adjuvant therapy for ovarian carcinoma.

Kinetic and Mechanistic Aspects of Ligand Substitution on cis- Diaqua (cis-1,2-Diaminocyclohexane)Platinum(II) with Three Glycine-Containing Dipeptides

The kinetics of the interaction of glycyl-L-valine(L1-L′H), glycyl-L-isoleucine (L2-L′H) and glycyl-L-glutamine(L3-L′H) with cis-[Pt( cis-dach)(OH2)2]2+ (dach = 1,2-diaminocyclohexane) have been studied spectrophotometrically as a function of [ cis-[Pt( cis-dach)(OH2)2]2+], [ligand], pH and temperature at constant ionic strength, where the complex exists predominantly as the diaqua species and the dipeptides as a zwitterion. The substitution reactions show two consecutive steps: the initial is the ligand-assisted anation and the subsequent step is chelation. The activation parameters for both steps were evaluated using Eyring's equation. The low DL H1 ≠ and large negative value of Δ S1 ≠ as well as Δ H2 ≠ and Δ S2 ≠ indicate an associative mode of activation for both the aqua ligand substitution processes. The products of the reactions have been characterised using IR and ESI-mass spectroscopic analysis. The title complex is already established as an anticancer drug; but its reactivity with all biomolecules will confirm its therapeutic activity (i.e. its efficacy versus toxicity).

The mechanism of guanine alkylation by nitrogen mustards: a computational study

The thermodynamics and kinetics for the monofunctional binding of nitrogen mustard class of anticancer drugs to purine bases of DNA were studied computationally using guanine and adenine as model substrates. Mechlorethamine and melphalan are used as model systems in order to better understand the difference in antitumor activity of aliphatic and aromatic mustards, respectively. In good agreement with experiments that suggested the accumulation of a reactive intermediate in the case of mechlorethamine, our model predicts a significant preference for the formation of corresponding aziridinium ion for mechlorethamine, while the formation of the aziridinium ion is not computed to be preferred when melphalan is used. Two effects are found that contribute to this difference. First, the ground state of the drug shows a highly delocalized lone pair on the amine nitrogen of the melphalan, which makes the subsequent cyclization more difficult. Second, because of the aromatic substituent connected to the amine nitrogen of melphalan, a large energy penalty has to be paid for solvation. A detailed study of energy profiles for the two-step mechanism for alkylation of guanine and adenine was performed. Alkylation of guanine is ∼6 kcal mol(-1) preferred over adenine, and the factors contributing to this preference were explained in our previous study of cisplatin binding to purine bases. A detailed analysis of energy profiles of mechlorethamine and melphalan binding to guanine and adenine are presented to provide an insight into rate limiting step and the difference in reactivity and stability of the intermediate in both nitrogen mustards, respectively.

No association of ERCC1 C8092A and T19007C polymorphisms to cancer risk: a meta-analysis

ERCC1 (excision repair cross complementation group 1) is a subunit of the nucleotide excision repair complex, which can perform DNA strand incision correction of DNA damage. Association studies on the ERCC1 polymorphisms (C8092A and T19007C) in cancer had shown conflicting results. We performed a meta-analysis from all eligible case-control studies to assess the purported associations. Overall, the 19007C allele (3 853 patients and 4 349 controls) showed no significant effect on cancer risk compared to 19007T allele (P=0.39, odds ratio (OR)=0.95; 95% confidence interval (CI) 0.85-1.06, P(heterogeneity)=0.001) in all subjects. Meta-analysis under other genetic contrasts did not reveal any significant association of T19007C to cancer in all subjects, Caucasians and Asians. The 19007C allele (2 279 patients and 2 808 controls) showed no significant effect on lung cancer risk compared to 19007T allele (P=0.72, OR=0.94, 95% CI 0.69-1.29, P(heterogeneity)=0.0001) in all subjects. No significant effect of 8092A allele (3 865 patients and 3 750 controls) on cancer risk in all subjects (P=0.85, OR=1.01, 95% CI 0.94-1.08, P(heterogeneity)=0.92) and in Caucasians and Asians compare to 8092C. No evidences of association of C8092A (501 patients and 620 controls) to squamous cell carcinoma were found. The accumulated evidence indicated ERCC1 T19007C and C8092A might not be risk factors for cancer. Significant between-study heterogeneity existed in T19007C, which arose from a study showing significant protecting effect of 19007C allele compare to 19007T allele in smokers. More studies based on larger, stratified case-control population should be required to further evaluate the role of ERCC1 C8092A and T19007C polymorphisms in different cancer, especially in smokers.

Relationship between DNA repair capacity and resistance to genotoxins in four human cell lines

We have developed fast, reliable and simple fluorescent method to assess and compare repair capacity of cells. To this end plasmid pEGFP containing the gene for the enhanced green fluorescent protein was damaged in vitro by genotoxic agents and introduced into cells by transfection. The repair capacity of the cells was determined from the number of fluorescent cells counted with a fluorescent microscope 24 h after transfection. The ability of four human tumor cell lines--HEK293, HeLa, Namalwa and K562 to repair DNA lesions inflicted by cis-diamminedichloroplatinum(II), UV light, 8-methoxypsoralen and 4',5'-8-trimethylpsoralen were determined and compared to the survival rates of the cells after treatment with the same genotoxic agents. In most but not all cases, there was a good correlation between repair capacity and cell survival. This finding indicates that the DNA repair capacity could be used as a biomarker in risk assessment and/or drug resistance assays.

Molecular mechanisms of programmed cell death

Programmed cell death and apoptosis have now been recognised as biological phenomena which are of fundamental importance to the integrity of organisms. What may have evolved as an altruistic defence against pathogen invasion in simple organisms is now a major regulatory mechanism in the development and maintenance of multi-cellular organisms. The classically defined morphological characteristics of apoptosis are now accompanied by a plethora of information regarding common biochemical and genetic mediators of programmed cell death. It is apparent that life and death decisions are taken by individual cells based on their interpretation of physiological signals, or their own self-assessment of internal damage. The knowledge that cell death is a genetically regulated process has highlighted an inherent potential for manipulation and offered new avenues for research into several diseases, and also productivity improvements in the biotechnology industry. This relatively "new frontier" in cell science has undoubtedly widened our perspectives and may provide novel strategies to expedite both medical and biotechnological research.

Mechanisms of cyclophosphamide resistance in a human myeloid leukemia cell line

The 4-hydroperoxycyclophosphamide (4HC)-resistant B5-180(3) subline of the cloned KBM-7/B5 cell line was developed as a model of induced cyclophosphamide resistance in human myeloid leukemia. Based on IC90 values, this subline was approximately 20-fold resistant to 4HC. Furthermore, it was significantly cross-resistant to phosphorodiamidic mustard (PM), whose cytotoxicity is independent of aldehyde dehydrogenase (ADH). Using alkaline elution we found that the resistant line had decreased initial levels of DNA interstrand cross-links (ISCs) following 4HC but not PM treatment. The resistant cells also appeared to remove ISCs from their DNA more rapidly than the parental cells. Our data therefore suggest that 4HC resistance in the B5-180(3) subline is multifactorial; ADH is an important mediator of its resistance to ISC induction by 4HC, while a second process, which may involve an increased ability to tolerate drug-induced DNA damage, appears to be important for its resistance to both 4HC and PM. The B5-180(3) cells were also cross-resistant to gamma-radiation (approximately 1.7-fold at a surviving fraction of 0.1); if generally applicable, such effects could have important clinical implications, since pretransplant total body irradiation is a major component of the eradication of leukemic cells.

The glutathione S-transferase supergene family: regulation of GST and the contribution of the isoenzymes to cancer chemoprotection and drug resistance

The glutathione S-transferases (GST) represent a major group of detoxification enzymes. All eukaryotic species possess multiple cytosolic and membrane-bound GST isoenzymes, each of which displays distinct catalytic as well as noncatalytic binding properties: the cytosolic enzymes are encoded by at least five distantly related gene families (designated class alpha, mu, pi, sigma, and theta GST), whereas the membrane-bound enzymes, microsomal GST and leukotriene C4 synthetase, are encoded by single genes and both have arisen separately from the soluble GST. Evidence suggests that the level of expression of GST is a crucial factor in determining the sensitivity of cells to a broad spectrum of toxic chemicals. In this article the biochemical functions of GST are described to show how individual isoenzymes contribute to resistance to carcinogens, antitumor drugs, environmental pollutants, and products of oxidative stress. A description of the mechanisms of transcriptional and posttranscriptional regulation of GST isoenzymes is provided to allow identification of factors that may modulate resistance to specific noxious chemicals. The most abundant mammalian GST are the class alpha, mu, and pi enzymes and their regulation has been studied in detail. The biological control of these families is complex as they exhibit sex-, age-, tissue-, species-, and tumor-specific patterns of expression. In addition, GST are regulated by a structurally diverse range of xenobiotics and, to date, at least 100 chemicals have been identified that induce GST; a significant number of these chemical inducers occur naturally and, as they are found as nonnutrient components in vegetables and citrus fruits, it is apparent that humans are likely to be exposed regularly to such compounds. Many inducers, but not all, effect transcriptional activation of GST genes through either the antioxidant-responsive element (ARE), the xenobiotic-responsive element (XRE), the GST P enhancer 1(GPE), or the glucocorticoid-responsive element (GRE). Barbiturates may transcriptionally activate GST through a Barbie box element. The involvement of the Ah-receptor, Maf, Nrl, Jun, Fos, and NF-kappa B in GST induction is discussed. Many of the compounds that induce GST are themselves substrates for these enzymes, or are metabolized (by cytochrome P-450 monooxygenases) to compounds that can serve as GST substrates, suggesting that GST induction represents part of an adaptive response mechanism to chemical stress caused by electrophiles. It also appears probable that GST are regulated in vivo by reactive oxygen species (ROS), because not only are some of the most potent inducers capable of generating free radicals by redox-cycling, but H2O2 has been shown to induce GST in plant and mammalian cells: induction of GST by ROS would appear to represent an adaptive response as these enzymes detoxify some of the toxic carbonyl-, peroxide-, and epoxide-containing metabolites produced within the cell by oxidative stress. Class alpha, mu, and pi GST isoenzymes are overexpressed in rat hepatic preneoplastic nodules and the increased levels of these enzymes are believed to contribute to the multidrug-resistant phenotype observed in these lesions. The majority of human tumors and human tumor cell lines express significant amounts of class pi GST. Cell lines selected in vitro for resistance to anticancer drugs frequently overexpress class pi GST, although overexpression of class alpha and mu isoenzymes is also often observed. The mechanisms responsible for overexpression of GST include transcriptional activation, stabilization of either mRNA or protein, and gene amplification. In humans, marked interindividual differences exist in the expression of class alpha, mu, and theta GST. The molecular basis for the variation in class alpha GST is not known. (ABSTRACT TRUNCATED)

O6-alkylguanine-DNA-alkyltransferase activity and nitrosourea sensitivity in human cancer cell lines

The DNA repair enzyme, O6-alkylguanine-DNA-alkyltransferase (ATase), is thought to be the principal mechanism controlling resistance to nitrosoureas and related alkylating agents. We compared the sensitivities of five human testis and five bladder tumour cell lines to two nitrosoureas (N-nitroso-N-methylurea (MNU) and mitozolomide) with cellular levels of ATase. Enzyme levels ranged from 3 to 206 fmol mg-1 protein (0.1 x 10(4) to 5.1 x 10(4) molecules/cell) in the testis lines and from 11 to 603 fmol mg-1 (0.4 x 10(4) to 9.1 x 10(4) molecules/cell) in the bladder lines. Based on IC50s in an MTT assay, the testis tumour cell lines were, on average, four times more sensitive to MNU and six times more sensitive to mitozolomide than the bladder cell lines. The cytotoxicities of MNU and mitozolomide were closely related (R = 0.9). In the testis cell lines ATase activity (molecules/cell) was related to IC50s for mitozolomide (R = 0.97) but not MNU (R = 0.78). In the bladder cell lines and overall, ATase activity correlated with cellular sensitivity to neither agent. Relatively high levels of resistance occurred in cells expressing low levels of ATase, and amongst cell lines expressing high levels of ATase, large differences in IC50s were observed. These results support the suggestion that resistance to nitrosoureas can be mediated by mechanisms other than ATase and that at relatively high levels of expression, ATase does not confer resistance in proportion to its activity.

Cisplatin resistance in human cancers

Cancer chemotherapeutic agents primarily act by damaging cellular DNA directly or indirectly. Tumor cells, in contrast to normal cells, respond to cisplatin with transient gene expression to protect and/or repair their chromosomes. Repeated cisplatin treatments results in a stable resistant cell line with enhanced gene expression but lacking gene amplification for the proteins that will limit cisplatin cytotoxicity. Recently, several new human cell lines have been characterized for cisplatin resistance. These cell lines have led to a better understanding of the molecular and biochemical basis of cisplatin resistance. The c-fos proto-oncogene, a master switch for turning on other genes in response to a wide range of stimuli, has been shown to play an important role in cisplatin resistance both in vitro and in patients. Based on these studies, new strategies have been developed to circumvent and/or exploit clinical cisplatin resistance.

Glutathione levels and chemosensitizing effects of buthionine sulfoximine in human malignant glioma cells

Biopsy samples and cultured cells derived from them were obtained from 39 patients with malignant glioma and were analyzed for 1) glutathione (GSH) content; 2) sensitivity to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and/or nitrogen mustard (HN2) treatment and 3) the effect of buthionine sulfoximine (BSO) treatment on BCNU and/or HN2 cytotoxicity. The average GSH concentration of biopsy specimens was lower than those of cultured cells (2.36 +/- 0.44 vs. 11.42 +/- 2.32 nmol/10(6) cells). While some of the tumor specimens were sensitive to either BCNU or HN2, the majority were resistant to both. However, 8 of 23 tumors tested showed enhanced sensitivity to BCNU following treatment with BSO. Five of 17 tumors were similarly sensitized to HN2 by BSO. These results suggest that BSO chemosensitization may be of value for certain patients and that screening assays may help identify treatment-sensitive individuals.

Clinical pharmacokinetics of cyclophosphamide

Cyclophosphamide has been in clinical use for the treatment of malignant disease for over 30 years. It remains one of the most useful anticancer agents, and is also widely used for its immunosuppressive properties. Cyclophosphamide is inactive until it undergoes hepatic transformation to form 4-hydroxycyclophosphamide, which then breaks down to form the ultimate alkylating agent, phosphoramide mustard. Sensitive and specific methods are now available for the measurement of cyclophosphamide, its metabolites and its stereoisomers in plasma and urine. The pharmacokinetics of cyclophosphamide have been understood for many years; those of the cytotoxic metabolites have been described more recently. The pharmacokinetics are not significantly altered in the presence of hepatic or renal insufficiency. As activity resides exclusively in the metabolites, whose pharmacokinetics are not predicted by those of the parent compound, correlations between cyclophosphamide pharmacokinetics and pharmacodynamics have not been demonstrated. Cyclophosphamide is used in doses that range from 1.5 to 60 mg/kg/day. A steep dose-response curve exists, and reductions in dose can lead to unfavourable outcomes. Myelosuppression is the dose-limiting toxicity, although in the setting of bone marrow transplantation, escalation beyond that dosage range is limited by cardiac toxicity. Longer term complications of cyclophosphamide therapy include infertility and an increased incidence of second malignancies. Cellular sensitivity to cyclophosphamide is a function of cellular thiol concentration, metabolism by aldehyde dehydrogenases to form inactive metabolites, and the ability of DNA to repair alkylated nucleotides. Whether alteration of these cellular functions will lead to further improvements in clinical outcomes is an area of active investigation.

Genetic mechanisms of drug resistance. A review

An overview of our present understanding of mechanisms of resistance against cytotoxic drugs is presented. Most of this understanding has come from studies on tumor cells made resistant in vitro, but there is reason to think that similar mechanisms are responsible for resistance in patients. After a brief overview of biochemical mechanisms of drug resistance, the types of mutations in tumor cells that can alter drug handling are discussed. Three examples of resistance are analysed in more detail: resistance to the folate analogue methotrexate; the multidrug resistance caused by increased levels of P-glycoprotein, which extrudes drugs from the cell; and resistance to alkylating agents.

Selective uptake of toxic nucleoside (125IUdR) by resistant cancer

We report uptake of a thymidine analogue 125-Iodine-5-iodo-2'-deoxyuridine (125IUdR) by nude mice bearing human xenografts of choriocarcinoma or colonic cancer. When 125IUdR was given alone, uptake by intestinal tissues was 5-10 times greater than by the tumours as measured by tissue gamma counting. This ratio was reversed when hydroxyurea or cytosine arabinoside were used as inhibitors of ribonucleotide reductase and were given in combination with 5-fluorouracil or methotrexate to inhibit thymidine synthesis shortly before injecting 125IUdR. Counting the radioactivity in tissues removed 24 hours after 125IUdR gave tumour to highest normal tissue ratios of up to 15:1, but the corresponding nuclear grain counts, which is probably a more reliable indicator of selective uptake into DNA, were in excess of 100:1. The addition of unlabelled IUdR to the regimen only reduced the uptake of 125IUdR when given in relatively large amounts. For this approach to be exploited it is concluded that the tumour must be resistant at the cell level to the inhibitor of DNA synthesis either de novo or as a result of prior exposure to it. This inhibitor can then be used to block uptake of the potentially toxic nucleoside analogue by normal renewal tissues while it is taken up by the resistant cancer cells. By inhibiting synthesis of the corresponding normal nucleosides with inhibitors to which the cancer cells are not resistant, incorporation of the toxic analogues into tumour DNA was enhanced. Although 125IUdR is a convenient agent for exploring this approach and is highly cytotoxic when incorporated in DNA, the clinical potential of reverse role chemotherapy probably lies with the development of toxic non-radioactive nucleoside analogues.

Biochemical basis of resistance to chemotherapy

Recent progress in the understanding of drug resistance has led to the discovery of new targets for chemotherapy. By attacking the molecules that make cancer cells insensitive to chemotherapy, it is hoped that drug-resistant disease will respond to treatment. This review describes some of the latest advances in understanding of the biochemistry of drug resistance. Following a general introduction four areas of topical interest are discussed: (1) multidrug resistance and P-glycoprotein, (2) glutathione and its related enzymes, (3) topoisomerase II and (4) DNA repair.

Immunocytochemical analysis of in vivo DNA modification

In the past decades a large number of DNA adducts induced in the intact animal by alkylating agents have been identified. The formation and repair of these adducts are important determinants, not only of mutagenesis, tumor initiation and DNA-mediated toxicity but probably also of tumor progression. Most studies on in vivo DNA modification have been performed on isolated bulk DNA. More recently, methods have been developed to study the distribution of DNA adducts at the level of either the individual gene or the individual cell. This paper reviews immunocytochemical methods to study the formation and repair of DNA adducts and other DNA modifications at the level of the individual cell. DNA modifications induced by alkylating agents and a variety of other agents including ultraviolet radiation, aromatic amines, polycyclic aromatic hydrocarbons and platinum anti-cancer drugs will be discussed. Up to now, immunocytochemical analysis of in vivo modified DNA has largely concentrated on experimental animals. These studies have revealed striking heterogeneities with regard to formation and/or repair of DNA adducts in tissues from rat, hamster and mouse. Immunocytochemical adduct analysis can be used to identify in a convenient, fast and detailed way cell types, cell stages and sites in which biological effects of the adducts might be expressed. More recently, immunocytochemical analysis of DNA adducts also proved to be feasible on in situ exposed human samples. A number of existing and potential applications in the field of chemical carcinogenesis, experimental chemotherapy and molecular epidemiology are discussed.

Resistance to alkylating agents and cisplatin: insights from ovarian carcinoma model systems

The curative potential of chemotherapy for ovarian cancer is frequently not realized due to platinum and alkylating agent resistance. Mechanisms which may contribute to the resistant phenotype include alterations in drug transport, increased levels of sulfhydryl molecules (and/or related enzymes), and enhanced DNA repair. We have developed several ovarian cancer cell lines resistant to platinum compounds and alkylating agents. Increased levels of glutathione and enhanced DNA repair are major determinants of chemoresistance in these cells. Modulation of these processes with buthionine sulfoximine (BSO), aphidicolin, arc-C, etc. partially reverses in vitro resistance. Similar clinical treatment strategies are under investigation.