Preferential attack of mitochondrial DNA by aflatoxin B1 during hepatocarcinogenesis

Toxic effects of copper-based antineoplastic drugs (Casiopeinas) on mitochondrial functions

To elucidate some of the subcellular and biochemical mechanisms of toxicity of metal-based antineoplastic drugs, mitochondria and cells were exposed to Casiopeinas), a new class of copper-based compounds with high antineoplastic activity. The rates of respiration and swelling, the H(+) gradient, and the activities of succinate (SDH) and 2-oxoglutarate dehydrogenases (2-OGDH) and ATPase were measured in mitochondria isolated from rat liver, kidney, heart, and hepatoma AS-30D. Also, oligomycin-sensitive respiration and ATP content in hepatoma AS-30D cells were determined. Casiopeinas) (CS) II-gly and III-i inhibited the rates of state 3 and uncoupled respiration in mitochondria. CS II was 10 times more potent than CS III. The sensitivity to CS II was 4-5-fold higher in mitochondria incubated with 2-OG than with succinate. Thus, at low concentrations (< or =10 nmol (mg protein)(-1); 10 microM), CS II disturbed mitochondrial functions only when 2-OG was present, due to a specific inhibition of 2-OGDH. At high concentrations (> or =15nmol (mg protein)(-1)), CS II-induced stimulation of basal respiration, followed by a strong inhibition, which correlated with K(+)-dependent swelling and cytochrome c release, respectively; K(+)-channel openers induce a similar mitochondrial response. Mitochondria from liver, kidney and hepatoma showed a similar sensitivity towards CS II, whereas heart mitochondria were more resistant. Oxidative phosphorylation and ATP content were also decreased in tumor cells by CS II. The data suggested that CS affected several different mitochondrial sites, bringing about inhibition of respiration and ATP synthesis, which could compromise energy-dependent processes such as cellular duplication.

Mitochondrial DNA repair and aging

The mitochondrial electron transport chain plays an important role in energy production in aerobic organisms and is also a significant source of reactive oxygen species that damage DNA, RNA and proteins in the cell. Oxidative damage to the mitochondrial DNA is implicated in various degenerative diseases, cancer and aging. The importance of mitochondrial ROS in age-related degenerative diseases is further strengthened by studies using animal models, Caenorhabditis elegans, Drosophila and yeast. Research in the last several years shows that mitochondrial DNA is more susceptible to various carcinogens and ROS when compared to nuclear DNA. DNA damage in mammalian mitochondria is repaired by base excision repair (BER). Studies have shown that mitochondria contain all the enzymes required for BER. Mitochondrial DNA damage, if not repaired, leads to disruption of electron transport chain and production of more ROS. This vicious cycle of ROS production and mtDNA damage ultimately leads to energy depletion in the cell and apoptosis.

Mitochondria-to-nucleus stress signaling induces phenotypic changes, tumor progression and cell invasion

Recently we showed that partial depletion of mitochondrial DNA (genetic stress) or treatment with mitochondrial-specific inhibitors (metabolic stress) induced a stress signaling that was associated with increased cytoplasmic-free Ca(2+) [Ca(2+)](c). In the present study we show that the mitochondria-to-nucleus stress signaling induces invasive phenotypes in otherwise non-invasive C2C12 myoblasts and human pulmonary carcinoma A549 cells. Tumor-specific markers cathepsin L and transforming growth factor beta (TGFbeta) are overexpressed in cells subjected to mitochondrial genetic as well as metabolic stress. C2C12 myoblasts subjected to stress showed 4- to 6-fold higher invasion through reconstituted Matrigel membrane as well as rat tracheal xenotransplants in Scid mice. Activation of Ca(2+)-dependent protein kinase C (PKC) under both genetic and metabolic stress conditions was associated with increased cathepsin L gene expression, which contributes to increased invasive property of cells. Reverted cells with approximately 70% of control cell mtDNA exhibited marker mRNA contents, cell morphology and invasive property closer to control cells. These results provide insights into a new pathway by which mitochondrial DNA and membrane damage can contribute to tumor progression and metastasis.

Doxorubicin-induced persistent oxidative stress to cardiac myocytes

We recently reported a cardioselective and cumulative oxidation of cardiac mitochondrial DNA (mtDNA) following subchronic administration of doxorubicin to rats. The mtDNA adducts persist for up to 5 weeks after cessation of doxorubicin treatment. Since the evidence suggests that this persistence of mtDNA adducts cannot be attributed to a lack of repair and replication, we investigated whether it might reflect a long-lasting stimulation of free radical-mediated adduct formation. Male Sprague-Dawley rats received weekly s.c. injections of either doxorubicin (2 mg/kg) or an equivalent volume of saline. Cardiac myocytes isolated from rats following 6 weekly injections of doxorubicin expressed a much higher rate of reactive oxygen species (ROS) formation compared to saline controls. This higher rate of ROS formation persisted for 5 weeks following the last injection. Associated with this was a persistent depression of GSH in heart tissue, while protein-thiol content was not markedly altered. These data suggest that the accumulation and persistence of oxidized mtDNA may be due, not to the stability of the adducts, but to some as yet undefined toxic lesion that causes long-lasting stimulation of ROS generation by doxorubicin. This persistent generation of ROS may contribute to the cumulative and irreversible cardiotoxicity observed clinically with the drug.

Mitochondria-to-nucleus stress signaling induces phenotypic changes, tumor progression and cell invasion

Recently we showed that partial depletion of mitochondrial DNA (genetic stress) or treatment with mitochondrial-specific inhibitors (metabolic stress) induced a stress signaling that was associated with increased cytoplasmic-free Ca(2+) [Ca(2+)](c). In the present study we show that the mitochondria-to-nucleus stress signaling induces invasive phenotypes in otherwise non-invasive C2C12 myoblasts and human pulmonary carcinoma A549 cells. Tumor-specific markers cathepsin L and transforming growth factor beta (TGFbeta) are overexpressed in cells subjected to mitochondrial genetic as well as metabolic stress. C2C12 myoblasts subjected to stress showed 4- to 6-fold higher invasion through reconstituted Matrigel membrane as well as rat tracheal xenotransplants in Scid mice. Activation of Ca(2+)-dependent protein kinase C (PKC) under both genetic and metabolic stress conditions was associated with increased cathepsin L gene expression, which contributes to increased invasive property of cells. Reverted cells with approximately 70% of control cell mtDNA exhibited marker mRNA contents, cell morphology and invasive property closer to control cells. These results provide insights into a new pathway by which mitochondrial DNA and membrane damage can contribute to tumor progression and metastasis.

Terminal branch haplotype analysis: a novel approach to investigate newly arisen variants of mitochondrial DNA in natural populations

The discrimination of recent mutational derivatives from ancestral variation is a critical antecedent to any effort which aims to identify the factors modulating the rates of origin and persistence of new mutants. We propose that newly arisen mtDNA variants, which we designate as terminal branch haplotypes (TBHs), can be recognized by joint sequencing and phylogenetic analysis. This study examined mtDNA diversity in natural populations of the brown bullhead (Ameiurus nebulosus) from four heavily contaminated sites and three relatively pristine locations. While sequence analysis of the mtDNA D-loop region revealed that TBHs were prevalent in these populations, contaminant exposure appeared to play a minor role in their generation. Instead, most TBHs likely arose due to spontaneous mutations with variation in their incidence among sites reflecting the impact of demographic factors.

Intraindividual sequence diversity and a hierarchical approach to the study of mitochondrial DNA mutations

Investigations of intraindividual sequence diversity in mtDNA are a key step in exploring the linkage between somatic mutations in mtDNA and mitochondrial genome evolution. This paper reports a directional cloning procedure enabling the isolation of multiple copies of the D-loop region of the mitochondrial genome from the fish Ameiurus nebulosus. Sequence analysis of 708 D-loop molecules revealed eight mutants, an average intraindividual mutation frequency of 1.12%. Three different types of mutations were detected but each derived from a single mutational event. By contrasting the spectrum of nucleotide variation at multiple biological levels, one can investigate the effects of spontaneous mutations on genome evolution. Such hierarchical analysis suggested shifts in the type and distribution of mtDNA (mitochondrial DNA) mutations at different biological levels, indicating the need to recognize three different rates of mtDNA sequence change from the cellular to population level.

Constitutive and inducible cytochromes P450 in rat lung mitochondria: xenobiotic induction, relative abundance, and catalytic properties

The presence of xenobiotic-inducible CYP1A1, 2B1/2, and 3A1/2 in rat lung mitochondria was investigated using mitochondrial preparations of defined purity. The mitochondrial P450 content in uninduced lung was 1.5-fold higher compared to microsomes. Administration of BNF induced the P450 contents by twofold in both mitochondrial and microsomal membrane fractions. BNF treatment induced EROD activity to about 40-fold in the microsomal fraction and 25-fold in the mitochondrial fraction. The microsomal induction was observed at 4 days of BNF treatment, while the mitochondrial induction required 10 days of treatment. Consistent with the activity profile, Western blot analysis showed the presence of CYP1A1 antibody reactive protein only in lung mitochondria from BNF-treated rats. BNF administration also caused a 50 to 80% reduction in the CYP2B1/2-associated PROD and BROD activities and CYP3A1/2-associated ERND activity in both mitochondria and microsomes. There was also a parallel reduction in the antibody reactive CYP2B1/2 and 3A1/2 proteins in both of these membrane fractions. Administration of DEX for 4 days induced mitochondrial and microsomal ERND activity by 1. 7- and 2.5-fold, respectively. Mitochondrial EROD activity was inhibited by antibodies to P450MT2, as well as Adx, but not by antibody against P450 reductase, indicating the mitochondrial localization of CYP1A1. Protease protection and alkaline extraction experiments indicated that CYP1A1 associated with lung mitochondria is localized inside the inner membrane and exists as a membrane extrinsic protein. In summary, this is probably the first report of inducible P450s in rat lung mitochondria, and our results suggest a possible functional role for these mitochondrial enzymes in xenobiotic metabolism.

Physiological role of the N-terminal processed P4501A1 targeted to mitochondria in erythromycin metabolism and reversal of erythromycin-mediated inhibition of mitochondrial protein synthesis

Recently, we showed that the major species of beta-naphthoflavone-inducible rat liver mitochondrial P450MT2 consists of N-terminal truncated microsomal P4501A1 (+33/1A1) and that the truncated enzyme exhibits different substrate specificity as compared with intact P4501A1. The results of the present study show that P450MT2 targeted to COS cell mitochondria by transient transfection of P4501A1 cDNA is localized inside the mitochondrial inner membrane in a membrane-extrinsic orientation. Co-expression with wild type P4501A1 and adrenodoxin (Adx) cDNAs resulted in 5-7-fold higher erythromycin N-demethylation (ERND) in the mitochondrial fraction but minimal changes in the microsomal fraction of transfected cells. Erythromycin, a potent inhibitor of bacterial and mitochondrial protein synthesis, caused 8-12-fold higher accumulation of CYP1A1 mRNA, preferential accumulation of P450MT2, and 5-6-fold higher ERND activity in the mitochondrial compartment of rat C6 glioma cells. Consistent with the increased mitochondrial ERND activity, co-expression with P4501A1 and Adx in COS cells rendered complete protection against erythromycin-mediated mitochondrial translation inhibition. Mutations that specifically affect the mitochondrial targeting of P4501A1 also abolished protection against mitochondrial translation inhibition. These results for the first time suggest a physiological function for the xenobiotic inducible cytochrome P4501A1 against drug-mediated mitochondrial toxicity.

Retrograde Ca2+ signaling in C2C12 skeletal myocytes in response to mitochondrial genetic and metabolic stress: a novel mode of inter-organelle crosstalk

We have investigated the mechanism of mitochondrial-nuclear crosstalk during cellular stress in mouse C2C12 myocytes. For this purpose, we used cells with reduced mitochondrial DNA (mtDNA) contents by ethidium bromide treatment or myocytes treated with known mitochondrial metabolic inhibitors, including carbonyl cyanide m-chlorophenylhydrazone (CCCP), antimycin, valinomycin and azide. Both genetic and metabolic stresses similarly affected mitochondrial membrane potential (Deltapsim) and electron transport-coupled ATP synthesis, which was also accompanied by an elevated steady-state cytosolic Ca2+ level ([Ca2+]i). The mitochondrial stress resulted in: (i) an enhanced expression of the sarcoplasmic reticular ryanodine receptor-1 (RyR-1), hence potentiating the Ca2+ release in response to its modulator, caffeine; (ii) enhanced levels of Ca2+-responsive factors calineurin, calcineurin-dependent NFATc (cytosolic counterpart of activated T-cell-specific nuclear factor) and c-Jun N-terminal kinase (JNK)-dependent ATF2 (activated transcription factor 2); (iii) reduced levels of transcription factor, NF-kappaB; and (iv) enhanced transcription of cytochrome oxidase Vb (COX Vb) subunit gene. These cellular changes, including the steady-state [Ca2+]i were normalized in genetically reverted cells which contain near-normal mtDNA levels. We propose that the mitochondria-to-nucleus stress signaling occurs through cytosolic [Ca2+]i changes, which are likely to be due to reduced ATP and Ca2+ efflux. Our results indicate that the mitochondrial stress signal affects a variety of cellular processes, in addition to mitochondrial membrane biogenesis.

Efficient repair of abasic sites in DNA by mitochondrial enzymes

Mutations in mitochondrial DNA (mtDNA) cause a variety of relatively rare human diseases and may contribute to the pathogenesis of other, more common degenerative diseases. This stimulates interest in the capacity of mitochondria to repair damage to mtDNA. Several recent studies have shown that some types of damage to mtDNA may be repaired, particularly if the lesions can be processed through a base excision mechanism that employs an abasic site as a common intermediate. In this paper, we demonstrate that a combination of enzymes purified from Xenopus laevis mitochondria efficiently repairs abasic sites in DNA. This repair pathway employs a mitochondrial class II apurinic/apyrimidinic (AP) endonuclease to cleave the DNA backbone on the 5' side of an abasic site. A deoxyribophosphodiesterase acts to remove the 5' sugar-phosphate residue left by AP endonuclease. mtDNA polymerase gamma fills the resulting 1-nucleotide gap. The remaining nick is sealed by an mtDNA ligase. We report the first extensive purification of mtDNA ligase as a 100-kDa enzyme that functions with an enzyme-adenylate intermediate and is capable of ligating oligo(dT) strands annealed to poly(rA). These properties together with preliminary immunological evidence suggest that mtDNA may be related to nuclear DNA ligase III.

Interaction of adrenodoxin with P4501A1 and its truncated form P450MT2 through different domains: differential modulation of enzyme activities

Recently we showed that the beta-naphthoflavone-inducible liver mitochondrial P450MT2 consists of two N-terminal truncated forms of the microsomal P4501A1, termed P450MT2a (+5/1A1) and MT2b (+33/1A1) [Addya et al. (1997) J. Cell Biol. 139, 589-599]. In the present study, we demonstrate that intact P4501A1 and the major mitochondrial form, P450MT2b (routinely referred to as P450MT2), show distinct substrate specificities and preference for different electron transport proteins. Enzyme reconstitution and spectral studies show that the wild-type adrenodoxin (Adx), but not the mutant Adx, binds to P450MT2 in a functionally productive manner (Kd = 0.6 microM) and induces a characteristic high-spin state. Adx binding to intact P4501A1 or +5/1A1 is less efficient as seen from spectral shift patterns (Kd = 1.8-2.0 microM) and reconstitution of enzyme activity. Use of Adx--Sepharose affinity matrix yielded < 90% pure P450MT2 (specific activity: 13.5 nmol/mg of protein) starting from a partially purified fraction of 10-15% purity, further demonstrating the specificity of P450MT2 and Adx interaction. Chemical cross-linking studies show that the bovine Adx forms heteroduplexes with both P450MT2 and intact P4501A1, though at different efficiencies. Our results show that Adx interacts with P450MT2 through its C-terminal acidic domain 2, while interaction with intact P4501A1 likely involves the N-terminal acidic domain 1. These results point to an interesting possibility that different electron transfer proteins may differently modulate the enzyme activity. Our results also demonstrate for the first time as to how a different mode of Adx interaction differently modulates the substrate specificities of the two P450 forms.

DNA repair of UV photoproducts and mutagenesis in human mitochondrial DNA

The induction and repair of DNA photolesions and mutations in the mitochondrial (mt) DNA of human cells in culture were analysed after cell exposure to UV-C light. The level of induction of cyclobutane pyrimidine dimers (CPD) in mitochondrial and nuclear DNA was comparable, while a higher frequency of pyrimidine (6-4) pyrimidone photoproducts (6-4 PP) was detected in mitochondrial than in nuclear DNA. Besides the known defect in CPD removal, mitochondria were shown to be deficient also in the excision of 6-4 PP. The effects of repair-defective conditions for the two major UV photolesions on mutagenesis was assessed by analysing the frequency and spectrum of spontaneous and UV-induced mutations by restriction site mutation (RSM) method in a restriction endonuclease site, NciI (5'CCCGG3') located within the coding sequence of the mitochondrial gene for tRNALeu. The spontaneous mutation frequency and spectrum at the NciI site of mitochondrial DNA was very similar to the RSM background mutation frequency (approximately 10(-5)) and type (predominantly GC>AT transitions at G1 of the NciI site). Conversely, an approximately tenfold increase over background mutation frequency was recorded after cell exposure to 20 J/m2. In this case, the majority of mutations were C>T transitions preferentially located on the non-transcribed DNA strand at C1 and C2 of the NciI site. This mutation spectrum is expected by UV mutagenesis. This is the first evidence of induction of mutations in mitochondrial DNA by treatment of human cells with a carcinogen.

Preferential oxidation of cardiac mitochondrial DNA following acute intoxication with doxorubicin

The purpose of this investigation was to determine whether acute doxorubicin intoxication causes a preferential accumulation of 8-hydroxydeoxyguanosine (8OHdG) adducts to mitochondrial DNA (mtDNA) as opposed to nuclear DNA (nDNA), particularly in cardiac tissue. Adult male rats received a single i.p. bolus of doxorubicin (15 mg/kg) and were killed 1-14 days later. Acute intoxication with doxorubicin caused a 2-fold greater increase in 8OHdG adducts to mtDNA compared to nDNA, the concentration of adducts to both nDNA and mtDNA being 20%-40% greater for heart as opposed to liver. For both tissues, the relative abundance of adducts was highest at the earliest time-point examined (24 h) and decreased to control values by 2 weeks. The temporal dilution of 8OHdG adducts was not the result of cell hyperplasia and was only partially due to amplification of the mitochondrial genome, most probably via an increase in DNA copy number rather than a stimulation of mitochondrial biogenesis.

Preferential formation and decreased removal of cisplatin-DNA adducts in Chinese hamster ovary cell mitochondrial DNA as compared to nuclear DNA

Levels of DNA adducts in Chinese hamster ovary (CHO) cells exposed to cis-diamminedichloroplatinum(II) (cisplatin) for 24 h, have been shown to be 4- to 6-fold higher in mitochondrial (mt) DNA as compared to nuclear (n) DNA (Olivero et al., Mutation Res., 346 (1995) 221). The aim of the present study was to understand if the preferential cisplatin binding in mtDNA is partially caused by lack of adduct removal in the mitochondria. Chinese hamster ovary cells were exposed for 6 h to 50 microM cisplatin, followed by incubation for 24 and 48 h in cisplatin-free medium. At the 30-h time point (6 h with cisplatin, 24 h without cisplatin), half of the cells from each plate were harvested and the remainder were cultured and harvested at 54 h (6 h with cisplatin, 48 h without cisplatin). The 30- and 54-h time points are called 'T30' and 'T54', respectively. Cisplatin-DNA adducts were measured in DNA from nuclear and mitochondrial fractions by dissociation-enhanced lanthanide fluoroimmunoassay (DELFIA), a sensitive competitive microtiter-based immunoassay utilizing antiserum elicited against cisplatin-modified DNA. An initial higher level of cisplatin-DNA adducts was observed in mtDNA when compared to nDNA, at T30. In addition, a lack of removal of adducts in mtDNA was demonstrated in cells at T54. Dilution of DNA adducts by DNA replication was documented in pulse-chase experiments that employed [3H]thymidine incorporation. Adduct removal by repair-related mechanisms was considered to comprise the difference between total DNA adduct removal and adduct removal related to DNA replication. The final results demonstrated that both, higher initial binding and lack of removal of cisplatin-DNA adducts appear to contribute to the preferential cisplatin-mtDNA binding observed in CHO cells.

DNA adducts and chronic degenerative disease. Pathogenetic relevance and implications in preventive medicine

Chronic degenerative diseases are the leading causes of death in developed countries. Their control is exceedingly difficult due to their multiplicity and diversity, the interconnection with a network of multiple risk factors and protective factors, the long latency and multistep pathogenesis, and the multifocal localization. Adducts to nuclear DNA are biomarkers evaluating the biologically effective dose, reflecting an enhanced risk of developing a mutation-related disease more realistically than the external exposure dose. The localization and accumulation of these promutagenic lesions in different organs are the composite result of several factors, including (a) toxicokinetics (first-pass effect); (b) local and distant metabolism; (c) efficiency and fidelity of DNA repair; and (d) cell proliferation rate. The last factor will affect not only the dilution of DNA adducts but also the possible evolution towards either destructive processes, such as emphysema or cardiomyopathies, or proliferative processes, such as benign or malignant tumors at various sites. They also include heart tumors affecting fetal myocytes after transplacental exposure to DNA-binding agents, blood vessel tumors, and atherosclerotic plaques. In this article, particular emphasis is given to molecular alterations in the heart, which is the preferential target for the formation of DNA adducts in smokers, and in human aorta, where an extensive molecular epidemiology project is documenting the systematic presence of adducts to the nuclear DNA of smooth muscle cells from atherosclerotic lesions, and their significant correlation with known atherogenic risk factors. Exocyclic DNA adducts resulting from lipid peroxidation, and age-related indigenous adducts (I-compounds) may also originate from endogenous sources, chronic infections and infestations, and inflammatory processes. Type II I-compounds are bulky DNA lesions resulting from oxidative stress, whereas type II-compounds are presumably normal DNA modifications, which display positive correlations with median life span and are decreased in cancer and other pathological conditions. Profiles of type II-compounds strongly depend on diet and are related to the antidegenerative effects of caloric/ dietary restriction. Even broader is the possible meaning of adducts to mitochondrial DNA, which have been detected in rodents exposed to genotoxic agents and complex mixtures, as well as in untreated rodents, in larger amounts when compared to the nuclear DNA of the same cells. Mutations in mitochondrial DNA increase the number of oxidative phosphorylation-defective cells, especially in energy-requiring postmitotic tissues such as brain, heart and skeletal muscle, thereby playing an important role in aging and a variety of chronic degenerative diseases. A decreased formation of DNA adducts is an indicator of reduced risk of developing the associated disease. Therefore, these molecular dosimeters can be used as biomarkers in the prevention of chronic degenerative diseases, pursued either by avoiding exposure to adduct-forming agents or by using chemopreventive agents. Interventions addressed to the human organism by means of dietary measures or pharmacological agents have encountered a broad consensus in the area of cardiovascular diseases, and are deserving a growing interest also in cancer prevention. The efficacy of chemopreventive agents can be assessed by evaluating inhibition of nuclear DNA or mitochondrial DNA adduct formation in vitro, in animal models, and in phase II clinical trials in high-risk individuals.

Localization of multiple forms of inducible cytochromes P450 in rat liver mitochondria: immunological characteristics and patterns of xenobiotic substrate metabolism

Hepatic mitochondria contain inducible cytochromes P450 that cross-react with antibodies to P4501A1/2 and 2B1/2. In the present study, we present evidence for the occurrence of additional P450 forms in rat liver mitochondria that cross-react with antibodies to microsomal P4503A1/2 and 2E1. Protease protection and also immunoelectron microscopy studies were carried out to support the mitochondrial location of the immunoreactive P450s. The solubility of immunoreactive proteins in 0.1 M Na2CO3 suggests that the mitochondrial P450 forms tested are not membrane-integral proteins. The mitochondrial-associated P450 forms are capable of metabolizing resorufin derivatives, erythromycin, and p-nitrophenol in an adrenodoxin- and adrenodoxin reductase-supported system. Treatment of rats with phenobarbital (PB) resulted in the induction of mitochondrial pentoxyresorufin O-deethylase (PROD), benzoxyresorufin O-deethylase (BROD), and erythromycin N-demethylase (ERND) activities by 17-, 23-, and 2-fold, respectively. These activities were inhibited by 33 to 64% by antibodies to P4502B1/2 and P4503A1/2. The induction of the above monooxygenase activities correlated with the levels of mitochondrial proteins cross-reacting with antibodies to P4502B1/2 and P4503A1/2 in PB-treated livers. Similarly, administration of beta-naphthoflavone (BNF) resulted in a marked elevation of O-deethylation of ethoxy-, benzoxy-, and methoxyresorufins and a 2-fold increase in ERND activity. Immunoblot and immunoinhibition experiments using P4501A1/2, P4502B1/2, P4503A1/2, and P4502E1 antibodies revealed the presence of P450 forms closely related to the microsomal inducible forms. Results of immunoinhibition studies, using antibodies to adrenodoxin and reconstitution of enzyme activity with purified P450 forms, suggested a role for the mitochondrial P450 in the metabolism of xenobiotic substrates. The purified mitochondrial P450s also exhibited overlapping substrate specificities for resorufin derivatives and erythromycin.

Mitochondrial DNA maintenance in vertebrates

The discovery that mutations in mitochondrial DNA (mtDNA) can be pathogenic in humans has increased interest in understanding mtDNA maintenance. The functional state of mtDNA requires a great number of factors for gene expression, DNA replication, and DNA repair. These processes are ultimately controlled by the cell nucleus, because the requisite proteins are all encoded by nuclear genes and imported into the mitochondrion. DNA replication and transcription are linked in vertebrate mitochondria because RNA transcripts initiated at the light-strand promoter are the primers for mtDNA replication at the heavy-strand origin. Study of this transcription-primed DNA replication mechanism has led to isolation of key factors involved in mtDNA replication and transcription and to elucidation of unique nucleic acid structures formed at this origin. Because features of a transcription-primed mechanism appear to be conserved in vertebrates, a general model for initiation of vertebrate heavy-strand DNA synthesis is proposed. In many organisms, mtDNA maintenance requires not only faithful mtDNA replication, but also mtDNA repair and recombination. The extent to which these latter two processes are involved in mtDNA maintenance in vertebrates is also appraised.

The role of the neurohormone melatonin as a buffer against macromolecular oxidative damage

This paper summarizes the recent findings which show that the neural hormone melatonin is a free radical scavenger and general antioxidant. When compared with other antioxidants melatonin seems to have greater efficacy in protecting against cellular oxidative stress. These findings illustrate that melatonin preserves macromolecules including DNA, protein and lipid from oxidative damage following the administration of the chemical carcinogen, safrole, after exposure to ionizing radiation, following glutathione depletion, and after administration of the free radical generating herbicide, paraquat. In vitro evidence shows that melatonin is a potent scavenger of the highly toxic hydroxyl radical and in vitro evidence suggests that melatonin is an important and powerful antioxidant. Considering its high lipophilicity and its non-toxic nature as well as its ability to readily cross the blood-brain barrier, the neurohormone melatonin may prove to be an effective and important molecule in the antioxidative defense system, especially in the central nervous system. Besides the ease with which melatonin enters the brain, neurons seem to accumulate readily this hormone.

Aflatoxin B1--its effects on an in vitro plant system

The phytotoxic effects of aflatoxin B1 (AFB1) on in vitro cultures of differentiating calli and regenerating plantlets of Nicotiana tabacum were assessed. Callus appeared more sensitive to the effects of AFB1, with fresh mass accumulation and callus chlorophyll levels affected at low (approximately 0.5 micrograms/ml) aflatoxin concentrations. Transmission electron microscopy revealed early deteriorative alterations in chloroplast morphology. Inhibitory effects of the toxin (up to and including 10 micrograms/ml) on callus fresh mass accumulation were reversed following a 3 week toxin-free recovery period. In tobacco plantlets, root and leaf development, and root and leaf mass were significantly inhibited in a dose-dependent fashion with increasing AFB1 concentration above 0.5 micrograms/ml. Inhibitory effects on plantlet root development were more pronounced that on leaf development.

Action of mitochondrial DNA polymerase gamma at sites of base loss or oxidative damage

Mitochondrial DNA is subject to oxidative damage generating 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxo-dG) residues and to spontaneous or induced base loss generating abasic sites. Synthetic oligonucleotides containing these lesions were prepared and used as templates to determine their effects on the action of Xenopus laevis DNA polymerase gamma. An analogue of an abasic site in DNA, tetrahydrofuran, was found to inhibit elongation by DNA polymerase gamma. When the DNA polymerase was able to complete translesional synthesis, a dA residue was incorporated opposite the abasic site. In contrast, elongation by DNA polymerase gamma was not inhibited by an 8-oxo-dG residue in the template strand. The polymerase inserted dA opposite 8-oxo-dG in approximately 27% of the extended products. The effects of these lesions on the 3'-->5' exonuclease proofreading activity of DNA polymerase gamma were also investigated. The 3'-->5' exonuclease activity excised any of the four normal bases positioned opposite either a tetrahydrofuran residue or 8-oxo-dG, suggesting that proofreading may not play a major role in avoiding misincorporation at abasic sites or 8-oxo-dG residues in the template. Thus, both of these lesions have the prospect of causing high rates of mutation during mtDNA replication.

Preferential binding of cisplatin to mitochondrial DNA of Chinese hamster ovary cells

Some chemical carcinogens localize preferentially in mitochondrial DNA (mtDNA) when compared with genomic DNA (gDNA). Here we compare the ability of cisplatin (cis-diamminedichloroplatinum[II]) to induce DNA adducts in both genomic and mtDNA of Chinese hamster ovary (CHO) cells in culture. Cytotoxicity was examined by cell survival 4, 8 and 24 h after exposure to 50 microM cisplatin. Cisplatin-DNA adducts were measured in DNA from nuclear and mitochondrial fractions by dissociation-enhanced lanthanide fluoroimmunoassay (DELFIA), a sensitive competitive microtiter-based immunoassay utilizing antiserum elicited against cisplatin-modified DNA. An additional comparison of cisplatin-DNA binding in both compartments was performed by immunoelectron microscopy using the cisplatin-DNA antiserum and colloidal gold. DELFIA analysis of cisplatin-DNA adducts in gDNA and mtDNA showed a six-fold higher incorporation of drug into mtDNA as compared to gDNA. Morphometric studies of colloidal gold distribution in photomicrographs of CHO cells showed mtDNA to contain a four-fold higher concentration of cisplatin as compared to nuclear DNA. Therefore, both methods demonstrated a preferential binding of cisplatin to mtDNA versus gDNA.

Cellular interactions and metabolism of aflatoxin: an update

The aflatoxins are a group of closely related mycotoxins that are widely distributed in nature. The most important of the group is aflatoxin B1 (AFB1), which has a range of biological activities, including acute toxicity, teratogenicity, mutagenicity and carcinogenicity. In order for AFB1 to exert its effects, it must be converted to its reactive epoxide by the action of the mixed function mono-oxygenase enzyme systems (cytochrome P450-dependent) in the tissues (in particular, the liver) of the affected animal. This epoxide is highly reactive and can form derivatives with several cellular macromolecules, including DNA, RNA and protein. Cytochrome P450 enzymes may additionally catalyse the hydroxylation (to AFQ1 and AFM1) and demethylation (to AFP1) of the parent AFB1 molecule, resulting in products less toxic than AFB1. Conjugation of AFB1 to glutathione (mediated by glutathione S-transferase) and its subsequent excretion is regarded as an important detoxification pathway in animals. Resistance to AFB1 toxicity has been interpreted in terms of levels and activities of these detoxifying pathways. This article reviews the multiple reactions and effects attributed to aflatoxin, with particular reference to the interaction of aflatoxin with nucleic acids and proteins, and the contribution this mycotoxin has in disease development and in the promotion of hepatocellular carcinoma (HCC). The anti-mutagenic properties of several dietary factors are also considered in this article. Undoubtedly, the most important aspect of aflatoxin action is its putative role in the development of human cancer, in particular, HCC. Recently, there has been a renewed interest in this aspect and experimental evidence is rapidly accumulating at the molecular level, indicating aflatoxin as an important consideration in the aetiology of human HCC.

Interaction of 7H-dibenzo[c,g]carbazole and its organspecific derivatives with hepatic mitochondrial and nuclear DNA in the mouse

The recent observation of a high level of adducts in mitochondrial DNA (mtDNA) of cells exposed to chemical carcinogens aroused new interest in the hypothesis that carcinogen-induced damage in mitochondria plays a role in one or more stages of carcinogenesis. In order to investigate whether differences in the metabolic activation of carcinogens have qualitative and quantitative effects on ml- and nuclear DNA (nuDNA) adduct formation, mice were exposed to the potent hepatocarcinogenic and sarcomagenic polycyclic hydrocarbon 7H-dibenzo[c,g]carbazole (DBC) and to three of its derivatives that show large differences in enzymatic activation: N-acetyl-DBC (N-AcDBC), which is carcinogenic for several tissues; 5,9-dimethyl-DBC (DiMeDBC), which is exclusively hepatocarcinogenic; and N-methyl-DBC (N-MeDBC), which is exclusively sarcomagenic. Adduct formation and toxic effects were measured over 48 hr. With a moderate 5 mumol/kg dose of DBC, the adduct level in liver 24 hr after treatment was always higher in nuDNA than in mtDNA; after 48 hr a substantial increase in the level of adducts in mtDNA was observed, with a parallel decrease in the level in nuDNA. With DiMeDBC, a 4.9-fold increase in mtDNA was seen at 48 hr, whereas, at the same dose, the non-hepatocarcinogenic N-MeDBC induced a very small number of adducts. In order to obtain a nearly identical level of adducts in nu- and mtDNA at 24 hr, the dose of DBC must be three times higher (15 mumol/kg); this and higher dose levels had a strong cytotoxic effect in liver cells. Qualitative differences in adduct distribution were observed on chromatograms of mtDNA and nuDNA, showing that the access to mtDNA is a complex process. Our results confirm that mouse liver mtDNA is a major target for DBC and its hepatocarcinogenic derivatives. The possible interference of genotoxic alterations in mtDNA with carcinogenic mechanisms is discussed.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Deoxyribonucleoside triphosphate levels: a critical factor in the maintenance of genetic stability

DNA precursor pool imbalances can elicit a variety of genetic effects and modulate the genotoxicity of certain DNA-damaging agents. These and other observations indicate that the control of DNA precursor concentrations is essential for the maintenance of genetic stability, and suggest that factors which offset this control may contribute to environmental mutagenesis and carcinogenesis. In this article, we review the biochemical and genetic mechanisms responsible for regulating the production and relative amounts of intracellular DNA precursors, describe the many outcomes of perturbations in DNA precursor levels, and discuss implications of such imbalances for sensitivity to DNA-damaging agents, population monitoring, and human diseases.

Aflatoxin B1-adduct formation in rat and human small bowel enterocytes

BACKGROUND/AIMS

Hepatic CYP3A enzymes have been implicated in the bioactivation of aflatoxin B1 (AFB1) to DNA binding metabolites. CYP3A enzymes are also abundant in the small bowel, and we therefore examined the ability of this tissue to form intracellular AFB1 adducts.

METHODS

Immunohistochemistry using a antibody to the stable AFB1-DNA adduct was performed on small bowel sections obtained from rats orally gavaged with AFB1 and on human small bowel biopsy specimens maintained in explant culture. 3H-AFB1 was instilled into a loop of small bowel of untreated rats and rats pretreated with the CYP3A inducer dexamethasone during vivisection. DNA was isolated from the loop 2 hours later and assayed for specific activity.

RESULTS

In both rats and humans, AFB1-adducts were detected exclusively in mature enterocytes in a pattern similar to the distribution of CYP3A enzymes. Induction of enterocyte CYP3A in rats resulted in an increase in enterocyte immunoreactive AFB1 adducts and in a 1.8-fold increase in 3H-AFB1-nucleic acid adducts (P = 0.01).

CONCLUSIONS

Intracellular AFB1 adducts are formed in the small intestine, and this reflects, at least in part, the catalytic activity of CYP3A enzymes. Because these AFB1 adducts should ultimately pass in stool, enterocyte CYP3A may represent a regulatable barrier to dietary aflatoxins.

Effects of spermine on formation of HGPRT- mutants induced by ethylmethanesulfonate, methylmethanesulfonate, and mitomycin C in V79 Chinese hamster cells

Spermine is a polyamine found in bacteria, animal, and plant tissues. It is involved in a variety of biological processes, and its interaction with DNA stabilizes the secondary structure of the double helix. Spermine is one of the first reported antimutagens, reducing the mutation rate in several prokaryotic test systems, while in eukaryotic organisms conflicting results have been obtained. In light of the significant antimutagenic effect of spermine, it is important to evaluate its activity in mammalian cells in culture. The present study was undertaken to evaluate the ability of spermine to suppress the level of HGPRT- mutants induced by ethylmethanesulfonate, methylmethanesulfonate, and mitomycin C. Spermine reduced the mutation frequency induced by ethylmethanesulfonate and methylmethanesulfonate but did not affect survival; with mitomycin C survival was reduced but mutation rate was not influenced.

Restriction fragment length polymorphism (RFLP) analysis provides evidence for a high degree of homology of mitochondrial DNAs from rat hepatomas versus normal rat livers

Where differences have been reported between tumor and normal mitochondrial DNA (mtDNA), they have generally involved limited modifications of the genome (Taira et al., Nucleic Acids Res. 11:1635, 1983; Shay and Werbin, Mutat. Res. 186:149, 1987). However, Corral et al. (Nucleic Acids Res. 16:10935, 1988; 17:5191, 1989) observed recombination between cytochrome oxidase subunit I (COI) and NADH dehydrogenase subunit 6 (ND6), two genes normally on opposite sides of the circular mitochondrial genome. In rat hepatoma mtDNA COI and ND6 were reported to be separated by only 230 base pairs (Corral et al., 1988, 1989). We have performed RFLP analysis on mtDNA from normal rat livers and rat hepatomas, using COI and ND6 probes. Additional experiments compared end-labeled DNA fragments produced by EcoRI and HindIII digestion of mtDNA. These studies failed to provide any evidence for genetic recombination in rat hepatoma mtDNA, even in the same cell line used by Corral et al. Rather, they support the conclusion that mtDNA from tumor and normal tissues exhibits a low degree of heterogeneity.

Mitochondrial function in neurodegeneration and ageing

The mitochondrial respiratory chain and oxidative phosphorylation system are responsible for the production of ATP by aerobic metabolism. Defects of the respiratory chain are increasingly recognised as important causes of human disease, and neurodegenerative disorders in particular. This article will seek to review the clinical and biochemical effects of respiratory chain defects, and summarise what is known about the molecular mechanisms that underlie them. Increasing age is also associated with a decline in mitochondrial function. The biochemical correlates of this dysfunction and the possible molecular defects that may cause it will also be reviewed.

Analysis of DNA damage and repair in murine leukemia L1210 cells using a quantitative polymerase chain reaction assay

The polymerase chain reaction (PCR) represents an alternative to the current methods for investigating DNA damage and repair in specific genomic segments. In theory, any DNA lesion which blocks Taq polymerase can be measured by this assay. We used quantitative PCR (QPCR) to determine the lesion frequencies produced by cisplatin and ultraviolet light (UV) in a 2.3 kilobase (kb) segment of mitochondrial DNA and a 2.6 kb segment of the DHFR gene in mouse leukemia L1210 cells. The frequency of UV-induced lesions increased linearly with dose, and was 0.58 lesions/10 kb/10 J/m2 in the mitochondrial DNA, and 0.37 lesions/10 kb/10 J/m2 in the DHFR gene. With cisplatin, the lesion frequency also increased linearly with dose, and was 0.17 lesions/10 kb/10 microM in the DHFR gene, and 0.07 lesions/10 kb/10 microM in mitochondrial DNA. This result is contrary to that of Murata et al., 1990 (1), in which mitochondrial DNA received greater cisplatin damage than did nuclear DNA. Using PCR to measure the repair of UV-induced lesions in the DHFR gene segment, we observed that less than 10% of the lesions were removed by 4 h, but over 70% of the lesions were removed by 8 h. Repair of 43% of UV-induced lesions in mitochondrial DNA was also observed during a 24 h period.

Brain mitochondrial DNA is not damaged by prolonged cardiac arrest or reperfusion

Postischemic reperfusion is known to cause iron-mediated peroxidation of polyunsaturated fatty acids in membranes, including mitochondrial membranes, in the brain cortex. Consequently, we tested the hypothesis that this radical-mediated damage would extend to DNA. Mitochondrial DNA (mtDNA) was chosen because of its presence at a known site of free radical formation, its sensitivity and ease of assay, and its known lack of any repair systems. In model experiments we utilized endonuclease III or piperidine to amplify topological form conversions in mtDNA damaged by in vitro reactions with hydroxyl radical. We then applied the amplified detection assays to dog brain mtDNA isolated after 2 or 8 h of reperfusion following a 20-min cardiac arrest. We found that ischemia and reperfusion caused no topological form conversions in mtDNA. Similarly, nucleotide incorporation by a gap-filling reaction showed no sensitivity to digestion of the mtDNA by exonuclease III, an enzyme known to remove blocked 3' termini at the site of radical-generated nicks. Furthermore, the recovery of mtDNA was similar in all experimental groups, suggesting that putatively damaged forms had not been removed by rapid degradation. Thus, despite mitochondrial membrane damage, brain mtDNA does not accumulate oxygen radical damage during postischemic brain reperfusion.

Recovery of the missing tumorigenicity in mitochondrial DNA-less HeLa cells by introduction of mitochondrial DNA from normal human cells

The role of mitochondrial DNA (mtDNA) in the expression of the transformed phenotype was examined using mtDNA-less HeLa cells. Complete depletion of mtDNA and its products in the mtDNA-less HeLa cell line, EB8, was confirmed by Southern blot analysis and by [35S]methionine labeling of mitochondrially synthesized polypeptides. The tumorigenicity of the EB8 cells was assayed by inoculation of 1 x 10(7) cells subcutaneously into the backs of nude mice. The results showed that the tumorigenicity of HeLa cells was lost in good correspondence with the loss of mtDNA. However, the growth of EB8 cells in culture was very poor compared with that of HeLa cells, indicating that the apparent loss of tumorigenicity in EB8 cells could possibly be due to poor growth of the cells. Introduction of mtDNA from normal human fibroblasts into EB8 cells restored both the missing tumorigenicity and growth of the EB8 cells. These observations could be interpreted to show that mtDNA is required for expression of tumorigenicity, but that mutational changes of the mtDNA are not required for modulation of the phenotype in our experiments.

Mitochondrial DNA damage by anticancer agents

Mitochondrial DNA (mtDNA) is susceptible to damage by a number of anticancer agents either directly or indirectly. This damage is of little consequence if only a few of the mtDNA molecules are damaged. However, multiple drug treatments could result in a significant effect on a cell's ability to survive. The differential effect of anticancer agents on either organ specific toxicities or selective tumor kill can be partially accounted for by differential mtDNA content of cells and on the basis of differential protective mechanisms within mitochondria of various organs or tumor tissue. The concept of damage to mitochondria, especially its genome, is a subject of active investigation in various laboratories. This area of research may provide mechanism(s) by which organ specific toxicities or tumor specific toxicities may be elaborated. Also, the concept of targeting tumor specific mitochondria and/or mtDNA by anticancer agents is very attractive but has not come to fruition due to a lack of understanding of the regulation of the genome in tumor cells. Future investigations in this arena will enhance our knowledge on the interaction between anticancer agents and extranuclear DNA.

Increased levels of mitochondrial DNA in an etoposide-resistant human monocytic leukaemia cell line (THP-1/E)

Electron microscopic observations of THP-1/E (an etoposide-resistant human monocytic leukaemia cell line) showed a remarkable change of mitochondrial structure. Mitochondria were swollen and cristae were relatively intact. There was no difference in the activity of cytochrome oxidase, an enzyme which contains three subunits coded by mitochondrial DNA (mtDNA) between THP-1/E and THP-1 (the parent cell of THP-1/E). No measurable quantitative change of mitochondrial RNA was observed, but the level of mtDNA in THP-1/E was increased by a factor of about 4 compared with that of mtDNA in THP-1. These results suggest that, on acquisition of resistance to etoposide, some factors affect mitochondria, change its morphology and amplify its DNA.

Restriction enzyme analysis of mitochondrial DNA in colorectal tumours

Total cellular DNA samples were isolated from 15 colorectal adenocarcinomas, 8 colon adenomas and their adjacent histologically normal colon mucosa. These DNA samples were digested separately with 13 different restriction endonucleases and analysed by Southern blot hybridization using a purified 32P-labelled human mtDNA probe. The fragment patterns from tumour mtDNA were compared to those from corresponding normal mtDNA. No evidence for large deletions, insertions, rearrangements or single base mutations in the detectable regions was detected. This suggests that other mechanisms may be responsible for the changes of colorectal tumour mitochondria.

Antineoplastic drug resistance and DNA repair

Important aspects of the DNA repair mechanisms in mammalian, and especially human, cells are reviewed. The DNA repair processes are essential in the maintenance of the integrity of the DNA and in the defense against cancer. It has recently been discovered that the DNA repair efficiency differs in different regions of the genome and that active genes are preferentially repaired. There is mounting evidence that DNA repair processes play a role in the development of drug resistance by tumor cells. We will discuss such data as well as further approaches to clarify the relationship between DNA repair and antineoplastic drug resistance. Specifically, there is an increasing need to investigate the intragenomic heterogeneity of DNA repair and correlate the repair efficiency in specific genes to aspects of drug resistance. We also discuss the therapeutic potential of inhibiting the DNA repair processes and thereby possibly overcoming drug resistance.

Bisfuranoid mycotoxins: their genotoxicity and carcinogenicity

Based on the mode of action of AFB1 and the activities of its biologically active intermediates, one may conclude that: 1. The mode of toxic action of the bisfuranoid mycotoxin is through epoxidation of the vinyl ether double bond of their dihydrobisfuran functionality. 2. The DNA and plasma albumin adducts formed in vivo may be useful in the molecular dosimetry of these environmental carcinogens. 3. There appears to be a linear correlation between the steady state levels of AFB1-FAPy-DNA adducts and the carcinogenicity of AFB1. Elucidation of the molecular basis of this correlation may shed light on the mechanism of AFB1-induced carcinogenesis. 4. Consistent appearance of AFB1-DNA adducts in the livers of liver cancer patients tested is supportive of the IARC conclusion that AFB1 is a human carcinogen involved in human primary liver cancer.