ANT2 expression under hypoxic conditions produces opposite cell-cycle behavior in 143B and HepG2 cancer cells

Under hypoxic conditions, mitochondrial ATP production ceases, leaving cells entirely dependent on their glycolytic metabolism. The cytoplasmic and intramitochondrial ATP/ADP ratios, partly controlled by the adenine nucleotide translocator (ANT), are drastically modified. In dividing and growing cells that have a predominantly glycolytic metabolism, the ANT isoform 2, which has kinetic properties allowing ATP import into mitochondria, is over-expressed in comparison to control cells. We studied the cellular metabolic and proliferative response to hypoxia in two transformed human cell lines with different metabolic backgrounds: HepG2 and 143B, and in their rho(o) derivatives, i.e., cells with no mitochondrial DNA. Transformed 143B and rho(o) cells continued their proliferation whereas HepG2 cells, with a more differentiated phenotype, arrested their cell-cycle at the G(1)/S checkpoint. Hypoxia induced an increase in glycolytic activity, correlated to an induction of VEGF and hexokinase II (HK II) expression. Thus, according to their tumorigenicity, transformed cells may adopt one of two distinct behaviors to support hypoxic stress, i.e., proliferation or quiescence. Our study links the constitutive glycolytic activity and ANT2 expression levels of transformed cells with the loss of cell-cycle control after oxygen deprivation. ATP import by ANT2 allows cells to maintain their mitochondrial integrity while acquiring insensitivity to any alterations in the proteins involved in oxidative phosphorylation. This loss of cell dependence on oxidative metabolism is an important factor in the development of tumors.

Mitochondrial diseases preferentially involve proteins with prokaryote homologues

The comparison of each of the 393 nuclear-encoded human mitochondrial proteins annotated in the SwissProt databank with 256,953 proteins from 94 prokaryote species showed that two thirds of the mitochondrial proteome were homologous with prokaryotic proteins, whereas one third was not. Prokaryotic mitochondrial proteins differ markedly from eukaryotic proteins, particularly in regard to their size, localization, function, and mitochondrial-targeting N-terminal sequence. Remarkably, the majority of nuclear genes implicated in respiratory chain mitochondrial diseases were found to be of prokaryotic ancestry. Our study indicates that the investigation of the co-evolution of eukaryotic and prokaryotic mitochondrial proteins should lead to a better understanding of mitochondrial diseases.

Maternal smoking is associated with mitochondrial DNA depletion and respiratory chain complex III deficiency in placenta

Maternal smoking during pregnancy is often associated with a decrease in placental function, which might lead to intrauterine growth retardation. Because tobacco is known to alter the mitochondrial respiratory function in cardiomyocytes and lung tissue, we hypothesized that placental mitochondrial function could be altered by maternal smoking. Placental mitochondria from 9 smoking and 19 nonsmoking mothers were isolated by differential centrifugation. Mitochondrial oxygen consumption was measured by polarography, and the enzymatic activity of each complex of the electron transport chain was assessed by spectrophotometry. In addition, the relative content in mitochondrial DNA (mtDNA) was determined by real-time quantitative PCR in placentas from seven smoking and seven nonsmoking mothers. We observed a 29% reduction in the enzymatic activity of complex III in the placental mitochondria from smokers compared with nonsmokers (P = 0.03). The relative content of mtDNA (with respect to the beta-globin gene) was reduced by 37% in the placental tissue from smokers compared with nonsmokers (P < 0.02). Both the enzymatic activity of complex III and mtDNA content were inversely related with the daily consumption of cigarettes, and mtDNA content was correlated with cord blood insulin-like growth factor-binding protein-3 (r = 0.74, P < 0.01), a marker of fetal growth. These results show that maternal smoking is associated with placental mitochondrial dysfunction, which might contribute to restricted fetal growth by limiting energy availability in cells.

Low oocyte mitochondrial DNA content in ovarian insufficiency

BACKGROUND

Mitochondrial biogenesis and bioenergetics play an important role in oocyte maturation and embryo development. We have investigated the relationship between defective mitochondrial biogenesis and the lack of oocyte maturity observed during IVF procedures with patients suffering from ovarian dystrophy and ovarian insufficiency.

METHODS

We used real-time quantitative PCR to quantify mitochondrial DNA (mtDNA) in 116 oocytes obtained from 47 women undergoing the ICSI procedure. We compared the mtDNA content of oocytes from women with a normal ovarian profile with that of oocytes from women with ovarian dystrophy and ovarian insufficiency.

RESULTS

We found an average of 256,000 +/- 213,000 mitochondrial genomes per cell. The mean mtDNA copy number was not significantly different in ovarian dystrophy compared with controls, but it was significantly lower in oocytes from women with ovarian insufficiency (100,000 +/- 99,000, P < 0.0001).

CONCLUSIONS

Our results suggest that low mtDNA content is associated with the impaired oocyte quality observed in ovarian insufficiency.

Decreased expression of thyrotropin receptor gene suggests a high-risk subgroup for oncocytic adenoma

OBJECTIVE

The malignancy of thyroid oncocytic tumours, or oncocytomas, is higher than that of follicular tumours. The aim of this study was to investigate the role of thyroid-specific genes in oncocytic tumours and papillary carcinomas.

DESIGN AND METHODS

We compared 29 oncocytic tumours with 12 papillary carcinomas. Real-time quantitative PCR was used to measure the expression of thyroid-specific differentiation markers (thyrotrophin-stimulation hormone receptor (TSHR), thyroglobulin (TG) and Na(+)/I(-) symporter (NIS)), transcription factors (thyroid transcription factor-1 (TTF-1) and paired box gene-8 (PAX8)) and nuclear receptors (peroxisome proliferator-activated receptor (PPARgamma1) and thyroid hormone receptor (TRbeta1)) involved in thyroid carcinogenesis.

RESULTS

TSHR, TTF-1 and TRbeta1 levels were significantly lower in oncocytic tumours than in papillary carcinomas, as a result of specific biological changes in oncocytic tumours. However, PAX8 and PPARgamma1 did not seem to be involved in the process. Applying the criterion of the underexpression of the thyroid-specific differentiation markers, TSHR, TG and NIS, we classified the oncocytic tumours and papillary carcinomas into three groups. In the first, all three markers were underexpressed; in the second, TSHR was normal while TG and NIS were underexpressed; and in the third, only NIS was underexpressed. The expression patterns revealed that 13 of the 24 oncocytic adenomas underexpressing TSHR in our study, as did four of the five oncocytic carcinomas.

CONCLUSION

Cases of oncocytic adenoma associated with low levels of TSHR could be putative oncocytic carcinomas and should therefore receive adequate follow-up [corrected].

Transcriptional profiling reveals coordinated up-regulation of oxidative metabolism genes in thyroid oncocytic tumors

Oncocytomas are large cell tumors characterized by an abnormal proliferation of mitochondria. To investigate this phenomenon in thyroid oncocytomas, we determined gene expression profiles of 87 samples using microarrays of 6720 PCR products from cDNA clones. Samples included 29 thyroid oncocytomas and six papillary carcinomas, the remainder representing other thyroid pathologies or mitochondria-rich tumor samples, normal thyroid samples, and two thyroid cell lines. Hierarchical clustering and supervised analysis identified two specific oncocytic clusters and 163 distinctly regulated genes between oncocytoma and normal thyroid. Differential expression of five selected genes (APOD, BCL-2, COX, CTSB, and MAP2) was confirmed by immunohistochemistry. The two specific oncocytic clusters were rich in mitochondrial genes and revealed coordinated expression of nuclear and mitochondrial respiratory chain genes. We also observed the up-regulation of genes involved in mitochondrial biogenesis, such as nuclear respiratory factor 1 and the endothelial nitric oxide synthase. Several oxidative metabolism genes were overexpressed in oncocytomas, including those from the tricarboxylic acid cycle (MDH1) and cytosolic glycolysis (GAPD, ENO1, and GPI). On the contrary, the lactate dehydrogenase A gene, involved in anaerobic metabolism, was down-regulated. Our results suggest that, unlike a large number of solid tumors, thyroid oncocytomas produce energy through an aerobic pathway.

PGC-1-related coactivator and targets are upregulated in thyroid oncocytoma

Thyroid oncocytomas are tumors characterized by dense mitochondrial accumulation, the cause of which is currently unknown. Members of the PGC-1 coactivator family have been identified as important mediators of mitochondrial biogenesis because of their ability to activate nuclear genes encoding mitochondrial proteins. We have investigated the influence of the PGC-1 related coactivator (PRC) on the high mitochondrial content observed in oncocytoma by quantifying the transcripts of PRC, the nuclear respiratory factor 1 (NRF-1) and the mitochondrial transcription factor A (TFAM), in 30 oncocytic tumors and corresponding normal tissues. The three genes studied were found to be significantly overexpressed in thyroid oncocytomas, concomitantly with an increase in cytochrome oxidase activity and mitochondrial DNA (mtDNA) content. However, no mtDNA variant in the D-loop region appeared to be involved in oncocytic development. We conclude that overexpression of the PRC pathway is responsible for mitochondrial proliferation in the context of thyroid oncocytoma.

Fourteen novel OPA1 mutations in autosomal dominant optic atrophy including two de novo mutations in sporadic optic atrophy

The OPA1 gene, encoding a dynamin-related GTPase that plays a role in mitochondrial biogenesis, is implicated in most cases of autosomal dominant optic atrophy (ADOA). Sixty-nine pathogenic OPA1 mutations have been reported so far. Most of these are truncating mutations located in the GTPase domain coding region (exons 8-16) and at the 3'-end (exons 27-28). We screened 44 patients with typical ADOA using PCR-sequencing. We also tested 20 sporadic cases of bilateral optic atrophy compatible with ADOA. Of the 18 OPA1 mutations found, 14 have never been previously reported. The novel mutations include one nonsense mutation, 3 missense mutations, 6 deletions, one insertion and 3 exon-skipping mutations. Two of these are de novo mutations, which were found in 2 patients with sporadic optic atrophy. The recurrent c.2708_2711delTTAG mutation was found in 2 patients with a severe congenital presentation of the disease. These results suggest that screening for OPA1 gene mutations may be useful for patients with optic atrophy who have no affected relatives, or when the presentation of the disease is atypical as in the case of early onset optic atrophy.

Increased sperm mitochondrial DNA content in male infertility

BACKGROUND

There is increasing evidence that mitochondrial DNA (mtDNA) anomalies in sperm may lead to infertility. Point mutations, deletions and the presence of a specific mtDNA haplogroup have been associated with poor sperm quality, but little attention has been paid to the role of mtDNA content.

METHODS

Using density gradient separation and swim-up methods, we selected motile sperm from 32 normal and 35 abnormal sperm samples. The mtDNA/beta-globin gene ratio was determined by real-time quantitative PCR.

RESULTS

The average mtDNA/beta-globin ratio of sperm collected from 100% density layers was 1.4 for normal sperm, 6.1 for sperm samples presenting at least one abnormal criterion [among the three criteria established by World Health Organization (1999), i.e. sperm count, motility and morphology], and 9.1 for sperm samples presenting two or more of these abnormal criteria. These differences are very highly significant (P < 0.0001). The mtDNA numbers were also much greater in sperm collected from the 40% density gradient layers (mean: 17.1, P < 0.001), known to contain the most abnormal sperm of the sperm samples, than in those collected from the 100% layers known to contain sperm with the best fertilizing ability.

CONCLUSION

Our results showed significant mtDNA amplification in sperm collected from abnormal sperm samples.

Leigh-like encephalopathy complicating Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy is a mitochondrial disease caused by point mutations in mitochondrial DNA. It usually presents as severe bilateral visual loss in young adults. We report on a neurological disorder resembling Leigh syndrome, which complicated Leber's hereditary optic neuropathy in three unrelated male patients harboring mitochondrial DNA mutations at nucleotide positions 3460, 14459, and 14484, respectively. This Leigh-like encephalopathy appears to be associated with a much more severe outcome than isolated Leber's hereditary optic neuropathy.

Oxygen consumption and expression of the adenine nucleotide translocator in cells lacking mitochondrial DNA

It has been shown previously that human rho degrees cells, deprived of mitochondrial DNA and consequently of functional oxidative phosphorylation, maintain a mitochondrial membrane potential, which is necessary for their growth. The goal of our study was to determine the precise origin of this membrane potential in three rho degrees cell lines originating from the human HepG2, 143B, and HeLa S3 cell lines. Residual cyanide-sensitive oxygen consumption suggests the persistence of residual mitochondrial respiratory chain activity, about 8% of that of the corresponding parental cells. The fluorescence emitted by the three rho degrees cell lines in the presence of a mitochondrial specific fluorochrome was partially reduced by a protonophore, suggesting the existence of a proton gradient. The mitochondrial membrane potential is maintained both by a residual proton gradient (up to 45 to 50% of the potential) and by other ion movements such as the glycolytic ATP(4-) to mitochondrial ADP(3-) exchange. The ANT2 gene, encoding isoform 2 of the adenine nucleotide translocator, is overexpressed in rho degrees HepG2 and 143B cells strongly dependent on glycolytic ATP synthesis, as compared to the corresponding parental cells, which present a more oxidative metabolism. In rho degrees HeLa S3 cells, originating from the HeLa S3 cell line, which already displays a glycolytic energy status, ANT2 gene expression was not higher as in parental cells. Mitochondrial oxygen consumption and ANT2 gene overexpression vary in opposite ways and this suggests that these two parameters have complementary roles in the maintenance of the mitochondrial membrane potential in rho degrees cells.

Structure and chromosomal distribution of human mitochondrial pseudogenes

Nuclear mitochondrial pseudogenes (Numts) have been found in the genome of many eukaryote species, including humans. Using a BLAST approach, we found 1105 DNA sequences homologous to mitochondrial DNA (mtDNA) in the August 2001 Goldenpath human genome database. We assembled these sequences manually into 286 pseudogenes on the basis of single insertion events and constructed a chromosomal map of these Numts. Some pseudogenes appeared highly modified, containing inversions, deletions, duplications, and displaced sequences. In the case of four randomly selected Numts, we used PCR tests on cells lacking mtDNA to ensure that our technique was free from genome-sequencing artifacts. Furthermore, phylogenetic investigation suggested that one Numt, apparently inserted into the nuclear genome 25-30 million years ago, had been duplicated at least 10 times in various chromosomes during the course of evolution. Thus, these pseudogenes should be very useful in the study of ancient mtDNA and nuclear genome evolution.

Total and structured water in cancer: an NMR experimental study of serum and tissues in DMBA-induced OF1 mice

Total water and structured water (fraction of total water which remains unfrozen below the transition point from the semisolid to solid state) were characterized by 1H NMR relaxometry in the sera and tissues of 3 groups of 30 female mice (C, H and L) receiving a single administration of DMBA and different diets. Mice given the diet H, containing the highest proportion of saturated fatty acids and processed starch, and the lowest phytochemicals content, presented the highest tumor incidence (lymphoma). This allowed 3 subgroups to be defined: subnormal (SN), small (T+) and large tumor (T++). Spin-lattice relaxation times of total water (Tlobs) in the sera and tissues did not significantly differ between C, H and L groups, and SN, T+ and T++ subgroups. In T+ mice, a decrease in the relative amount of structured water was noticed in the serum, liver and heart, while changes in the temperature dependence of the Tl of structured water (Tlsw) were observed between -21 degrees C and -42 degrees C. These results suggest a moderate increase in the rotational mobility of structured water molecules in the serum and the heart, and a pronounced decrease in the liver. Likewise, the modification of the Tlsvv temperature dependence curve's shape tends to confirm the existence of important conformational changes in the macromolecular assemblies, which markedly affect the properties of structured water, especially in the earliest stage of cancer development.

Mitochondrial DNA content affects the fertilizability of human oocytes

Mitochondrial DNA content varies considerably in oocytes, even when collected from the same patient. In the present study, real-time quantitative polymerase chain reaction analysis of 113 unfertilized oocytes obtained from 43 patients revealed an average of 193,000 (range: 20,000 to 598,000) mitochondrial genomes per cell. We compared several groups of oocytes to investigate the relationship between mitochondrial DNA content and fertilizability. The average mitochondrial DNA copy number was significantly lower in cohorts suffering from fertilization failure compared to cohorts with a normal rate of fertilization. In addition, the mitochondrial copy number of oocytes from patients with fertilization failure due to unknown causes was significantly lower than that of oocytes from patients in which IVF failure was due mainly to a severe sperm defect. The lower mtDNA copy number could be due to defective cytoplasmic maturation of oocytes. We conclude that low mitochondrial DNA content, due to inadequate mitochondrial biogenesis or cytoplasmic maturation, may adversely affect oocyte fertilizability.

A conserved N-terminal sequence targets human DAP3 to mitochondria

Human DAP3 (death-associated protein-3) has been identified as an essential positive mediator of programmed cell death. Structure-function studies have shown previously the N-terminal extremity of the protein to be required in apoptosis induction. Analysis of human DAP3 gene structure predicted 13 exons and subsequent targeting prediction by two software packages (MITOPROT and TargetP) gave a high probability for mitochondrial targeting. The predicted N-terminal targeting structure was also found in the mouse, Drosophila, and C. elegans orthologues with a strong sequence homology between mouse and human. Secondary structure analyses identified alpha-helical structures typical of mitochondrial target peptides. To confirm experimentally this targeting we constructed a fusion protein with N-terminal human DAP3 upstream of enhanced green fluorescent protein (EGFP). Confocal analysis of transfected human fibroblasts clearly demonstrated EGFP localization exclusive to mitochondria. The positioning of this key apoptotic factor at the heart of the mitochondrial pathway provides exciting insight into its role in programmed cell death.

No evidence of thyrotropin receptor and G(s alpha) gene mutation in high iodine uptake thyroid carcinoma

Usually, thyroid carcinoma presents as a cold nodule on radioiodine scintigraphy. High-uptake nodules on iodine thyroid scans are associated with an exceedingly low incidence of malignancy. Only 29 cases of carcinomas appearing as hot or warm nodules have as yet been reported. From 1993 to 1999, we have observed eight similar cases (4 hot and 4 warm thyroid nodules) suggesting that thyroid carcinomas may not be as rare as usually considered in these circumstances. Four tumors were available for molecular analysis on paraffin-embedded sections. Because no mutations were found in the whole coding portions of thyrotropin-receptor (TSH-R) gene and fragments encompassing the mutational hot spots of the G(s alpha) gene, it is unlikely that activating mutations of the TSH-R or G(s alpha) genes were involved in these carcinomas.

mtDNA haplogroup J: a contributing factor of optic neuritis

Optic neuritis frequently occurs in multiple sclerosis (MS), and shares several similarities with the optic neuritis of Leber's hereditary optic neuropathy (LHON), which is mainly due to maternally transmitted mitochondrial DNA (mtDNA) mutations. Our report shows for the first time that a mitochondrial DNA background could influence the clinical expression of MS. One European mtDNA haplogroup was found only in MS patients with optic neuritis but not in MS patients without visual symptoms. Therefore, we hypothesize that mtDNA haplogroup J might constitute a risk factor for optic neuritis occurrence when it is coincidentally associated with MS, but not be a risk factor for developing MS per se as suggested previously.

Long PCR analysis of human gamete mtDNA suggests defective mitochondrial maintenance in spermatozoa and supports the bottleneck theory for oocytes

The long PCR and the Southern blot techniques were used to study mitochondrial DNA (mtDNA) in 94 sperm samples, and in 35 oocytes collected from 12 women. The sperm samples were classified in two sets: 37 samples from normal subjects, and 57 samples from patients with oligoasthenospermia. In both sets, most of the spermatozoan mitochondria had multiple mtDNA deletions. The rate of mtDNA mutation, which appears unexpectedly high, considering the short life span of the spermatozoa, may be due to impaired maintenance during differentiation. In contrast, despite the long life span of oocytes and the extended meiotic period, oocyte mitochondria showed few mtDNA rearrangements. However, mitochondria in oocytes from a given donor revealed considerable mutational heterogeneity. This supports the bottleneck theory of rapid segregation of mtDNA genotypes during early oogenesis. The long PCR technique, which allows analysis of the entire mitochondrial genome, provides new information on mtDNA instability in human gametes. Our findings suggest that mtDNA maintenance differs in the two types of gametes.

Oligoasthenospermia associated with multiple mitochondrial DNA rearrangements

A patient who wished to be treated for infertility by intracytoplasmic sperm injection (ICSI) was referred to our group for assessment. Upon clinical examination, a ptosis (partial closure of the eyelid) was noted, and histology revealed ragged red fibres in the skeletal muscle. Southern blot analysis of spermatozoa and skeletal muscle revealed the presence of multiple mitochondrial DNA deletions. This kind of rearrangement may be of nuclear origin since three nuclear loci have been ascribed to multiple mitochondrial DNA deletions in humans. Since mitochondrial DNA is maternally transmitted, the use of ICSI was feasible. However, an alteration of nuclear gene product affecting the integrity of mitochondrial DNA, and thus sperm mobility, might be transmitted to the offspring with the risk of developing a mitochondrial DNA disease.

Male infertility associated with multiple mitochondrial DNA rearrangements

Male sterility results from a number of characterized exogenous or genetic dysfunctions preventing normal differentiation into mobile spermatozoa. This may now be overcome by intra cytoplasmic sperm injection (ICSI). This practice does not require mobile, or even mature spermatozoa for in vitro fecondation. However, a functional respiratory chain, partly encoded by the mitochondrial DNA (mtDNA), is required for the mobility of the spermatozoa. We report the case of an infertile patient who wished to procreate. ICSI was proposed but he displayed multiple mtDNA deletions of possible nuclear origin in the spermatozoa and in the deltoid muscle. Even though mtDNA is maternally inherited, the possibility of a nuclear-driven mutation affecting the integrity of the mtDNA should be taken into account when ICSI is to be performed. Together with recent genetic in vitro manipulations in mammals, our data point to the importance of studying the mtDNA structure in human spermatozoa, and the potential risks of these non-natural practices for procreation.

Association of deletion and homoplasmic point mutation of the mitochondrial DNA in an ocular myopathy

The mitochondrial DNA of a 41 year old patient with ocular myopathy was explored. We found a deletion of 3540 base pair in about 50% of the mitochondrial genomes associated with a homoplasmic point mutation. The mutation at nucleotide pair 7444 converts stop codon AGA into lysine codon AAA (human mitochondrial genetic code). The synergistic effect between two point mutations has already been described in mitochondrial pathology but this is the first time that an association between a deletion and a point mutation is shown.

Epitope mapping of human thyroid peroxidase defined seven epitopes recognized by sera from patients with thyroid pathologies

Human thyroid peroxidase (hTPO) is the major component of the microsomal antigen. In almost cases, antibodies against this protein are found in sera from patients with autoimmune thyroid diseases. Overlapping cDNAs which correspond to the complete hTPO mRNA obtained from a thyroid library or by polymerase chain reaction were cloned and expressed as fusion proteins in a prokaryotic vector. Seven antigenic determinants between 21 and 49 amino acids were defined by cloning, subcloning of the immunoreactive regions and screenings with the two rabbit polyclonal anti hTPO antibodies. This study confirms the antigenic nature of the sequences 70-160 and 590-675 but above all refines the localization of three shorter distinct antigenic peptides corresponding to the sequences 68-105, 106-126 and 574-621. Moreover, four other determinants were characterized on the sequences 233-277, 467-515, 641-685 and 701-730. Analysis of sera from patients with autoimmune thyroid diseases (AITD) against the seven immunoreactive peptides confirms the heterogeneous nature of autoantibodies to hTPO.

A major human thyroglobulin epitope defined with monoclonal antibodies is mainly recognized by human autoantibodies

The antigenic nature of 15 anti-human thyroglobulin (hTg) monoclonal antibody (mAb) epitopes was studied by two different approaches. First, we tested two successive protease-digest products of hTg. Only four mAb from the same cluster of reactivity recognized a low-molecular weight peptide, the other mAb only bound native hTg or high-molecular weight digest fractions. Second, these 15 mAb were used to immunoscreen hTg expression libraries. Only the same four mAb revealed immunoreactive clones corresponding to region 1149-1295 on the hTg primary sequence. After subcloning, this antigenic determinant was reduced to a 102-amino acid peptide (hTg region 1149-1250). The two different methodologies were coherent and complementary, and demonstrated that hTg sequence 1149-1250 is the target for this cluster of four mAb. Moreover, anti-hTg autoantibodies which cross-reacted with these mAb bound the 102-amino acid peptide. This epitope was the one most frequently detected by sera from autoimmune thyroid disease. The data confirm the presence of an immunodominant domain in the central part of the hTg molecule and suggest that this mAb epitope may be a powerful probe for the diagnosis of autoimmune thyroid disorders.