Mitochondrial DNA topoisomerase I from human platelets

Inhibition of mitochondrial calcium transporters alters adp-induced platelet responses

INTRODUCTION

ADP-stimulated elevation of cytosolic Ca2+ is an important effector mechanism for platelet activation. The rapidly elevating cytosolic Ca2+ is also transported to mitochondrial matrix via Mitochondrial Ca2+ Uniporter (MCU) and extruded via Na+/Ca2+/Li+ Exchanger (NCLX). However, the exact contribution of MCU and NCLX in ADP-mediated platelet responses remains incompletely understood.

METHODS AND RESULTS

The present study aimed to elucidate the role of mitochondrial Ca2+ transport in ADP-stimulated platelet responses by inhibition of MCU and NCLX with mitoxantrone (MTX) and CGP37157 (CGP), respectively. As these inhibitory strategies are reported to cause distinct effects on matrix Ca2+ concentration, we hypothesized to observe opposite impact of MTX and CGP on ADP-induced platelet responses. Platelet aggregation profiling was performed by microplate-based spectrophotometery while p-selectin externalization and integrin αIIbβ3 activation were analyzed by fluorescent immunolabeling using flow cytometery. Our results confirmed the expression of both MCU and NCLX mRNAs with relatively low abundance of NCLX in human platelets. In line with our hypothesis, MTX caused a dose-dependent inhibition of ADP-induced platelet aggregation without displaying any cytotoxicity. Likewise, ADP-induced p-selectin externalization and integrin αIIbβ3 activation was also significantly attenuated in MTX-treated platelets. Concordantly, inhibition of NCLX with CGP yielded an accelerated ADP-stimulated platelet aggregation which was associated with an elevation of p-selectin surface expression and αIIbβ3 activation.

CONCLUSION

Together, these findings uncover a vital and hitherto poorly characterized role of mitochondrial Ca2+ transporters in ADP-induced platelet activation.

Drug-induced hepatotoxicity in cancer patients - implication for treatment

INTRODUCTION

All anticancer drugs can cause idiosyncratic liver injury. Therefore, hepatoprotective agents assume particular importance to preserve liver function. Hepatic injury represents 10% of cases of acute hepatitis in adults; drug-related damage is still misjudged because of relative clinical underestimation and difficult differential diagnosis. Chemotherapeutic agents can produce liver toxicity through different pathways, resulting in different categories of liver injuries, but these drugs are not homogeneously hepatotoxic. Frequently, anticancer-induced hepatotoxicity is idiosyncratic and influenced by multiple factors.

AREAS COVERED

The aim of this paper is to perform a review of the literature regarding anticancer-induced liver toxicity. We described hepatotoxicity mechanisms of principal anticancer agents and respective dose reductions. Furthermore, we reviewed studies on hepatoprotectors and their optimal use. Tiopronin, magnesium isoglycyrrhizinate and S-Adenosylmethionine (AdoMet) demonstrated, in some small studies, a potential hepatoprotective activity.

EXPERT OPINION

Actually, in the literature only small experiences are reported. Even though hepatoprotective agents seem to be useful in the oncologic setting, the lack of well-designed prospective Phase III randomized controlled trials is a major limit in the introduction of hepatoprotectors in cancer patients and these kind of studies are warranted to support their use and to give further recommendations for the clinical practice.

Chemotherapy induced hepatotoxicity in metastatic colorectal cancer: a review of mechanisms and outcomes

Colorectal cancer remains one of the most common cancers worldwide. The treatment of metastatic disease has advanced considerably in the past 10 years both in terms of surgical technique and development of novel chemotherapeutic agents. The widespread use of multiple chemotherapeutic agents has lead to recognition of distinct patterns of hepatotoxicity associated with specific drugs. These side-effects have potential implications for both the patient and medical professional, but the underlying mechanisms involved in these conditions remains poorly understood. This review explores the mechanisms of action of the commonly used chemotherapeutic agents and the potential mechanisms for their hepatotoxicity. It is important that all medical professionals involved in the management of metastatic colorectal cancer understand the problems of hepatotoxicity and the impact they have on the patient.

Mitochondrial topoisomerase I sites in the regulatory D-loop region of mitochondrial DNA

Mitochondrial DNA (mtDNA) is required for mitochondrial activities because it encodes key proteins for oxidative phosphorylation and the production of cellular ATP. We previously reported the existence of a specific mitochondrial topoisomerase gene, Top1mt, in all vertebrates. The corresponding polypeptide contains an N-terminal mitochondrial targeting sequence and is otherwise highly homologous to the nuclear topoisomerase I (Top1). In this study, we provide biochemical evidence of the presence of an endogenous Top1mt polypeptide in human mitochondria. Using novel antibodies against Top1mt, we detected the corresponding 70 kDa polypeptide in mitochondria but not in nuclear fractions. This polypeptide could be trapped to form covalent complexes with mtDNA when mitochondria from human cells were treated with camptothecin. Mapping of Top1mt sites in the regulatory D-loop region of mtDNA in mitochondria revealed the presence of an asymmetric cluster of Top1mt sites confined to a 150 bp segment downstream from, and adjacent to, the site at which replication is prematurely terminated, generating an approximately 650-base (7S DNA) product that forms the mitochondrial D-loop. Moreover, we show that inhibition of Top1mt by camptothecin reduces the level of formation of the 7S DNA. These results suggest novel roles for Top1mt in regulating mtDNA replication.

A truncated form of DNA topoisomerase IIbeta associates with the mtDNA genome in mammalian mitochondria

Despite the likely requirement for a DNA topoisomerase II activity during synthesis of mitochondrial DNA in mammals, this activity has been very difficult to identify convincingly. The only DNA topoisomerase II activity conclusively demonstrated to be mitochondrial in origin is that of a type II activity found associated with the mitochondrial, kinetoplast DNA network in trypanosomatid protozoa [Melendy, T., Sheline, C., and Ray, D.S. (1988) Cell 55, 1083-1088; Shapiro, T.A., Klein, V.A., and Englund, P.A. (1989) J. Biol. Chem.264, 4173-4178]. In the present study, we report the discovery of a type DNA topoisomerase II activity in bovine mitochondria. Identified among mtDNA replicative proteins recovered from complexes of mtDNA and protein, the DNA topoisomerase relaxes a negatively, supercoiled DNA template in vitro, in a reaction that requires Mg2+ and ATP. The relaxation activity is inhibited by etoposide and other inhibitors of eucaryotic type II enzymes. The DNA topoisomerase II copurifies with mitochondria and directly associates with mtDNA, as indicated by sensitivity of some mtDNA circles in the isolated complex of mtDNA and protein to cleavage by etoposide. The purified activity can be assigned to a approximately 150-kDa protein, which is recognized by a polyclonal antibody made against the trypanosomal mitochondrial topo II enzyme. Mass spectrometry performed on peptides prepared from the approximately 150-kDa protein demonstrate that this bovine mitochondrial activity is a truncated version of DNA topoisomerase IIbeta, one of two DNA topoisomerase II activities known to exist in mammalian nuclei.

Dual localization of human DNA topoisomerase IIIalpha to mitochondria and nucleus

The human TOP3alpha gene encoding DNA topoisomerase IIIalpha (hTop3alpha) has two potential start codons for the synthesis of proteins 1,001 and 976 aa residues in length. The sequence of the N-terminal region of the 1,001-residue form resembles signal peptide sequences for mitochondrial import, and fluorescence microscopy shows that the addition of as few as the first 34 aa of the 1,001-residue form of hTop3alpha to a green fluorescent protein can direct the chimeric protein to mitochondria. Biochemical analyses of subcellular fractions of HeLa cells further demonstrate that a distinctive fraction of hTop3alpha is present inside mitochondria, as evidenced by its resistance to proteinase K. This fraction constitutes several percent of the enzyme in the nuclear fraction, suggesting that the distribution of the mitochondrial and nuclear forms of hTop3alpha is roughly in proportion to the DNA contents of these cellular compartments. The presence of a type IA DNA topoisomerase in the mitochondria of other eukaryotes is supported by an examination of the amino acid sequences of mouse and Drosophila DNA topoisomerase IIIalpha and Schizosaccharomyces pombe DNA topoisomerase III. Given the presence of at least one type IA DNA topoisomerase in all forms of life examined to date, the finding of a type IA enzyme in mitochondria further supports the notion of a key role of such enzymes in DNA transactions.

Role of the leukemia-associated transcription factor STAT3 in platelet physiology

Actinomycin D, a transcriptional inhibitor, was found to inhibit platelet potentiation by thrombopoietin (TPO), suggesting that TPO stimulation of platelets involves mitochondrial transcription. We sought to determine a possible role for leukemia-associated signal transducers and activators of transcription (STAT) proteins as mitochondrial transcription factors, focusing specifically on STAT3 in human platelets. We found TPO stimulation of platelets activated STAT3 in vitro, that STAT3 was present in platelet mitochondrial-rich fractions as determined by Western Blot analysis and was capable of binding to the regulatory D-loop region of human mitochondrial DNA upon activation. These results suggest that platelet signaling pathways activated by TPO may affect mitochondrial transcription via activation of STAT3.

Human mitochondrial topoisomerase I

Tension generated in the circular mitochondrial genome during replication and transcription points to the need for mtDNA topoisomerase activity. Here we report a 601-aa polypeptide highly homologous to nuclear topoisomerase I. The N-terminal domain of this novel topoisomerase contains a mitochondrial localization sequence and lacks a nuclear localization signal. Therefore, we refer to this polypeptide as top1mt. The pattern of top1mt expression matches the requirement for high mitochondrial activity in specific tissues. top1mt is a type IB topoisomerase that requires divalent metal (Ca(2+) or Mg(2+)) and alkaline pH for optimum activity. The TOP1mt gene is highly homologous to the nuclear TOP1 gene and consists of 14 exons. It is localized on human chromosome 8q24.3.

The mitochondrial genome: structure, transcription, translation and replication

Mitochondria play a central role in cellular energy provision. The organelles contain their own genome with a modified genetic code. The mammalian mitochondrial genome is transmitted exclusively through the female germ line. The human mitochondrial DNA (mtDNA) is a double-stranded, circular molecule of 16569 bp and contains 37 genes coding for two rRNAs, 22 tRNAs and 13 polypeptides. The mtDNA-encoded polypeptides are all subunits of enzyme complexes of the oxidative phosphorylation system. Mitochondria are not self-supporting entities but rely heavily for their functions on imported nuclear gene products. The basic mechanisms of mitochondrial gene expression have been solved. Cis-acting mtDNA sequences have been characterised by sequence comparisons, mapping studies and mutation analysis both in vitro and in patients harbouring mtDNA mutations. Characterisation of trans-acting factors has proven more difficult but several key enzymes involved in mtDNA replication, transcription and protein synthesis have now been biochemically identified and some have been cloned. These studies revealed that, although some factors may have an additional function elsewhere in the cell, most are unique to mitochondria. It is expected that cell cultures of patients with mitochondrial diseases will increasingly be used to address fundamental questions about mtDNA expression.

Metabolism in human cells of the D and L enantiomers of the carbocyclic analog of 2'-deoxyguanosine: substrate activity with deoxycytidine kinase, mitochondrial deoxyguanosine kinase, and 5'-nucleotidase

The carbocyclic analog of 2'-deoxyguanosine (CdG) has broad-spectrum antiviral activity. Because of recent observations with other nucleoside analogs that biological activity may be associated the L enantiomer rather than, as expected, with the D enantiomer, we have studied the metabolism of both enantiomers of CdG to identify the enzymes responsible for the phosphorylation of CdG in noninfected and virally infected human and duck cells. We have examined the enantiomers as substrates for each of the cellular enzymes known to catalyze phosphorylation of deoxyguanosine. Both enantiomers of CdG were substrates for deoxycytidine kinase (EC 2.7.1.74) from MOLT-4 cells, 5'-nucleotidase (EC 3.1.3.5) from HEp-2 cells, and mitochondrial deoxyguanosine kinase (EC 2.7.1.113) from human platelets and CEM cells. For both deoxycytidine kinase and mitochondrial deoxyguanosine kinase, the L enantiomer was the better substrate. Even though the D enantiomer was the preferred substrate with 5'-nucleotidase, the rate of phosphorylation of the L enantiomer was substantial. The phosphorylation of D-CdG in MRC-5 cells was greatly stimulated by infection with human cytomegalovirus. The fact that the phosphorylation of D-CdG was stimulated by mycophenolic acid and was not affected by deoxycytidine suggested that 5'-nucleotidase was the enzyme primarily responsible for its metabolism in virally infected cells. D-CdG was extensively phosphorylated in duck hepatocytes, and its phosphorylation was not affected by infection with duck hepatitis B virus. These results are of importance in understanding the mode of action of D-CdG and related analogs and in the design of new biologically active analogs.

Mitochondrial DNA topoisomerase I of Saccharomyces cerevisiae

A mitochondrial DNA topoisomerase (type I, ATP-independent) can be biochemically distinguished from the nuclear enzyme DNA topoisomerase I. This conclusion is based on the subcellular localization of the mitochondrial enzyme, its optimal reaction conditions and sensitivity to enzyme inhibitors. Unlike its nuclear counterpart, the mitochondrial DNA topoisomerase exhibits an absolute requirement for a divalent cation (Mg2+ and Ca2+ work equally well in vitro). In addition, it is slightly more sensitive to monovalent salts, with optimal activity obtained in 50-100 mM KCl. The mitochondrial enzyme is equally active at pH 7.5 or pH 9.5, but unlike its nuclear equivalent, is inactivated at higher pH values. The mitochondrial DNA topoisomerase is sensitive to coumermycin, berenil, camptothecin and 2,2,5,5-tetramethyl-4-imidazolidinone, a chemical that has no inhibitory effect on DNA topoisomerase I. Immunoblotting studies show that mitochondrial DNA topoisomerase activity is associated with a polypeptide (M(r) approximately 79,000) that cross-reacts with the antiserum against DNA topoisomerase I. Thus, the mitochondrial DNA topoisomerase may be derived by the differential expression of the DNA topoisomerase I gene or from the expression of a gene that is homologous to the DNA topoisomerase I gene.

Mammalian mitochondrial DNA topoisomerase I preferentially relaxes supercoils in plasmids containing specific mitochondrial DNA sequences

Selected regions of mammalian mitochondrial DNA (mtDNA) were inserted into pGEM plasmid vectors and used as substrates in a kinetic analysis of the highly purified bovine mitochondrial type I topoisomerase. Recombinant plasmids containing the bovine mtDNA heavy and light strand origins of replication (pZT-Hori and pZT-Lori, respectively), a major transcription termination region (pZT-Term) and a portion of cytochrome b gene (pZT-Cytb) were prepared. Southern hybridization using probes specific for either control or mtDNA-containing plasmid indicated a relative preference by the mitochondrial topoisomerase I to relax supercoils in pZT-Hori and pZT-Term. Quantitative determination of kinetic parameters derived from double-reciprocal Lineweaver-Burk plots showed that recombinant plasmids containing the heavy and light strand origins and the transcription termination region were preferentially relaxed by the mitochondrial enzyme with Km values 2.3- to 3.3-fold lower than controls. The Km values for pZT-Hori, pZT-Lori and pZT-Term were 21.0 +/- 0.9 microM, 25.2 +/- 1.0 microM and 17.0 +/- 0.8 microM, respectively, while those for control plasmids were 57.5 +/- 2.1 microM and 56.3 +/- 2.3 microM. pZT-Cytb was not preferentially relaxed compared to the control plasmid (Km = 53.4 +/- 2.0 microM vs. 56.3 +/- 2.3 microM, respectively) indicating that mitochondrial topoisomerase I preferentially interacts with certain mtDNA sequences but not others. Identical experiments with the purified nuclear enzyme did not differentiate between control or mtDNA containing plasmids.

Evidence for a nucleotide-dependent topoisomerase activity from yeast mitochondria

Yeast mitochondria were found to contain a novel topoisomerase-like activity which required nucleoside di- or tri-phosphates as a cofactor. ADP supported activity as effectively as ATP and the optimal concentration for each was approximately 20 microM. None of the other standard ribo- or deoxyrib-onucleotides could fully substitute for either ADP or ATP. The non-hydrolyzable ATP analogs, adenosine-5'-0-(3-thiotriphosphate) (ATP-gamma-S), adenylyl (beta,gamma-methylene) (AMP-PCP), and andenyl-imidodiphosphate (AMP-PNP) also supported activity suggesting that the nucleotide cofactor regulated topoisomerase activity rather than serving as an energy donor in the reaction. The mitochondrial topoisomerase activity relaxed both positively and negatively supercoiled DNA. It was not inhibited by concentrations of ethidium bromide up to 2 micrograms/ml nor by either nalidixic or oxolinic acids; novobiocin, coumermycin, and berenil inhibited the activity. Genetic and biochemical analysis of the mitochondrial topoisomerase activity indicated that it was not encoded by the nuclear TOP1, TOP2, and TOP3 genes.

Immunological identification of human platelet mitochondrial DNA topoisomerase I

A nuclear human blood platelets have been used to study mitochondrial topoisomerase activity in the absence of nuclear contamination. Previous work utilizing this novel system demonstrated that platelet mitochondria contain type-I topoisomerase (Kosovsky, M.J. and Soslau, G. (1991) Biochim. Biophys. Acta 1078, 56-62). The present work has demonstrated that mitochondrial topoisomerase activity was inhibited by the specific topoisomerase-I inhibitor, topotecan, yet was not affected by a specific topoisomerase-II inhibitor, VM-26. These results confirm that platelet mitochondria contain topoisomerase I, yet do not contain a detectable level of topoisomerase-II activity. It has been demonstrated for the first time that antibodies directed against nuclear topo I cross-react with mitochondria topo I. Furthermore, immunoblot analysis of platelet mitochondrial proteins, in conjunction with renaturation studies, has led to the identification of a catalytically active 60-kDa form of mitochondrial topoisomerase I.