Mitochondria-associated ribonucleic acid of the HeLa cell. Effect of ethidium bromide on the synthesis of ribosomal and 4S ribonucleic acid

TRMT2B is responsible for both tRNA and rRNA m5U-methylation in human mitochondria

RNA species play host to a plethora of post-transcriptional modifications which together make up the epitranscriptome. 5-methyluridine (m⁵U) is one of the most common modifications made to cellular RNA, where it is found almost ubiquitously in bacterial and eukaryotic cytosolic tRNAs at position 54. Here, we demonstrate that m⁵U54 in human mitochondrial tRNAs is catalysed by the nuclear-encoded enzyme TRMT2B, and that its repertoire of substrates is expanded to ribosomal RNAs, catalysing m⁵U429 in 12S rRNA. We show that TRMT2B is not essential for viability in human cells and that knocking-out the gene shows no obvious phenotype with regards to RNA stability, mitochondrial translation, or cellular growth.

The 7472insC mitochondrial DNA mutation impairs the synthesis and extent of aminoacylation of tRNASer(UCN) but not its structure or rate of turnover

The 7472insC mitochondrial DNA mutation in the tRNA(Ser(UCN)) gene is associated with sensorineural deafness combined, in some patients, with a wider neurological syndrome. In cultured cybrid cells it causes a 70% decrease in tRNA(Ser(UCN)) abundance and mild respiratory impairment, previously suggested to be due to decreased tRNA stability. When mitochondrial transcription was blocked by ethidium bromide treatment, the half-life of the mutant tRNA was not significantly different from that of wild-type tRNA(Ser(UCN)). Over-expression of mitochondrial translational elongation factor EF-Tu also had no effect on the mutant phenotype. However, during recovery from prolonged ethidium bromide treatment, the synthesis of the mutant tRNA(Ser(UCN)) was specifically impaired, without polarity effects on downstream tRNAs of the light strand transcription unit. We infer that the mutation acts posttranscriptionally to decrease tRNA(Ser(UCN)) abundance by affecting its synthesis rather than its stability. The extent of aminoacylation of the mutant tRNA was also decreased by approximately 25%. In contrast, the mutation had no detectable effect on tRNA(Ser(UCN)) base modification or structure other than the insertion of an extra guanosine templated by the mutation, which was structurally protected from nuclease digestion like the surrounding nucleotides. These findings indicate a common molecular process underlying sensorineural deafness caused by mitochondrial tRNA(Ser(UCN)) mutations.

Role of mitochondria and calcium ions in tributyltin-induced gene regulatory pathways

Tributyltin (TBT) salts are potent skin irritants both in humans and rodents. Data in the literature indicate mitochondria as target of TBT effects. Here, we investigate the early intracellular molecular events that follow TBT treatment and the relevance of calcium ions and mitochondria in gene-regulatory signaling pathways. Confluent HEL30 cells were treated with increasing doses of TBT (0-5 microM). At different times thereafter, the level of intracellular Ca2+, the cellular oxidative activity, nuclear factor-kappaB (NF-kappaB) activation, and IL-1alpha production were measured. TBT induced a dose-related increase of intracellular Ca2+ that reached the plateau 4 min following treatment. The increase of intracellular Ca2+ was followed by an increase in cellular oxidative activity as measured by DCFH oxidation (15 min) that preceded NF-kappaB activation (30 min) and IL-1alpha production (4 hr). All these events can be almost completely abrogated by BAPTA, an intracellular Ca2+ chelator. Furthermore, the modulation of cellular oxidative activity induced by TBT observed with rotenone, an inhibitor of the electron entry from complex I to ubiquinone, or after prolonged treatment with ethidium bromide, an inhibitor of mitochondrial DNA and RNA synthesis, indicates mitochondria as an important intracellular source of reactive oxygen species. These findings indicate the rise in intracellular Ca2+ as the starting event and indicate the role of mitochondria as the source of second messenger molecules essential for TBT-induced NF-kappaB activation and IL-1alpha production.

Role of mitochondria in tributyltin-induced interleukin-1alpha production in murine keratinocytes

Tributyltin (TBT) salts are well known skin irritants in rodents and humans. TBT induced both the intracellular production of interleukin-1alpha (IL-1alpha) and its release into culture medium in a murine keratinocyte cell line (HEL30). Here, we report that mitochondria are important for TBT-induced IL-1alpha production. Confluent cells were treated with increasing concentrations of TBT (0--2.5 microM) or dimethylsulfoxide as vehicle control. At different times thereafter (0--24 h), nuclear extracts were analyzed for nuclear factor-(kappa)B (NF-(kappa)B) binding activity by electrophoretic mobility shift assay, and the released and cell-associated IL-1alpha was measured by enzyme-linked immunosorbent assay. TBT induced a direct and concentration-related activation of NF-(kappa)B, which peaked at 2 h and was blocked by pyrrolidinedithiocarbamate, a potent NF-(kappa)B inhibitor, and rotenone, an inhibitor of the electron entry from complex I to ubiquinone. Rotenone also induced a concentration-related inhibition of IL-1alpha synthesis induced by TBT, but rotenone did not completely abrogate TBT-induced IL-1alpha production, which suggests that other transcription factors may be involved in IL-1alpha production. Prolonged treatment with ethidium bromide, an inhibitor of mitochondrial DNA and RNA synthesis, was used to partially deplete cells of functional mitochondria. After 5 d of treatment, mitochondrial conversion of tetrazolium bromide to formazan was reduced by 50%, and IL-1alpha release was decreased by 65%, whereas no induction of intracellular IL-1alpha was observed. This effect was not due to inhibition of protein synthesis, because identical incorporation of [3H]leucine into protein in control and ethidium bromide-treated cells was identical. This impairment of mitochondrial metabolism inhibited NF-(kappa)B activation by TBT. These findings indicate that mitochondria may be the source of second messenger molecules important for TBT-induced IL-1alpha production.

Molecular analysis of the muscle pathology associated with mitochondrial DNA deletions

Large-scale deletions of mitochondrial DNA (mtDNA) are associated with a subgroup of mitochondrial encephalomyopathies. We studied seven patients with Kearns-Sayre syndrome or isolated ocular myopathy who harboured a sub-population of partially-deleted mitochondrial genomes in skeletal muscle. Variable cytochrome c oxidase (COX) deficiencies and reduction of mitochondrially-encoded polypeptides were found in affected muscle fibres, but while many COX-deficient fibres had increased levels of mutant mtDNA, they almost invariably had reduced levels of normal mtDNA. Our results suggest that a specific ratio between mutant and wild-type mitochondrial genomes is the most important determinant of a focal respiratory chain deficiency, even though absolute copy numbers may vary widely.

Control and virus-transformed baby hamster kidney cells resistant to ethidium bromide. I. Characterization and the respiratory enzymes

Cell lines resistant to ethidium bromide have been developed from cultured mammalian BHK21/C13 cells and these same cells transformed by Rous sarcoma virus (C13/B4). Cells resistant to 2 micrograms ethidium bromide per milliliter have been cloned. One clone of the control and one of the virus-transformed cell lines has been employed for characterization. The resistant cells, in the presence of 2 micrograms ethidium bromide/ml, grow at approximately the same rate as the untreated parental cells. The control cells possess a "normal" karyotype (44 chromosomes), while the corresponding ethidium bromide mutant has a reduced chromosome number of 41 and a number of translocations. The mitochondria displayed morphological alterations compared to the parental lines during the transition phase prior to the isolation of the ethidium bromide-resistant cells. The mitochondria of the ethidium bromide-resistant mutants appear somewhat enlarged with a normal morphology. The effect of ethidium bromide on selected respiratory enzymes in normal and virus-transformed ethidium bromide-resistant baby hamster kidney cells was determined. Ethidium bromide-resistant cells exhibited a depressed level of cytochrome aa3. This depression could not be reversed by growth in ethidium bromide-free media. Ethidium bromide-resistant cells possessed the same cytochrome b, c, and c1 levels per cell as their corresponding parental lines. Purified mitochondria isolated from virus-transformed ethidium bromide-resistant cells exhibited a depression in cytochrome oxidase-specific activity, while the ethidium bromide-resistant control cells did not. All cell lines studied showed a depression in NADH-ferricyanide and NADH-cytochrome c reductase-specific activities relative to their parental BHK21/C13 cells. No increase was observed in virus-transformed ethidium bromide-resistant cells. Ethidium bromide-resistant control cells exhibited a two-fold increase in oligomycin-insensitive adenosine triphosphatase activity relative to their parental cells. All of the cell lines studied possessed equivalent oligomycin-sensitive adenosine triphosphatase-specific activity except for the virus-transformed, dye-resistant mutant, whose activity was increased.

Stimulation of 2-deoxy-d-glucose transport in control and virus-transformed cells by ethidium bromide

Recently we demonstrated that ethidium bromide altered the plasma and subcellular membrane glycoproteins in control and virus transformed cells. It is reported here that ethidium bromide also stimulated the membrane associated process of sugar transport. The KM of the virus transformed cells and the ethidium bromide treated cells is the same as that of the control cells while the maximum velocity as compared to the control cells is significantly increased. The transport of 2-deoxyl-D-glucose was inhibited by glucose, cytochalasin B and neuraminidase but was unaffected by variations in cell density or pH of the incubation medium.

Effects of ethidium bromide on the cytochrome content and ultrastructure of L cell mitochondria

Ethidium bromide intercalates between the bases of native DNA, resulting in several biological anomalies. The effects of ethidium bromide on the mitochondria of cultured mouse L cells were studied. At a concentration of 1 microg ethidium bromide/ml it was observed that concentrations of cytochromes a + a(3) and b decreased, a + a(3) more rapidly than b. In contrast, the concentration of cytochromes c(1) and c increased or remained the same as in control cells. Concomitant with the decrease of cytochromes a + a(3) and b was an enlargement of the mitochondria and a reduction in the cristae. The cristae that remained were abnormally organized. After prolonged treatment with ethidium bromide a second population of small, more normally organized mitochondria was apparent. These effects of ethidium bromide could be reversed.

A paracrystalline inclusion in Neurospora crassa. Induction by ethidium and acridine, isolation, and characterization

A paracrystal indistinguishable from the one which occurs in the mitochondrial mutant abnormal-1 can be induced in wild-type Neurospora crassa after growth in either ethidium or euflavine. This paracrystal has been isolated and partially characterized. It appears to be composed of a single polypeptide (mol wt 68,000) which can be reversibly crystallized and dissociated by changes in the pH and ionic strength. When aggregated, the polypeptide forms oligomers which are arranged end-to-end into fibers. During the characterization of the polypeptide, it was found that the polypeptide's electrophoretic and immunological properties could be used as assays. Using these methods it was found that the polypeptide normally accumulates in a soluble form in the cytoplasm of wild-type Neurospora at the end of the log-phase of growth.

Abnormal DNA patterns in animal mitochondria: ethidium bromide-induced breakdown of closed circular DNA and conditions leading to oligomer accumulation

Treatment of cultured mouse fibroblasts (L-cells) and hamster kidney cells with ethidium bromide was found to inhibit the incorporation of [(3)H]thymidine into mitochondrial closed-circular DNA, but not into nuclear DNA. The specific activity of nuclear DNA in treated cells was higher than in control cells. Treatment also led to breakdown of closed-circular mitochondrial DNA, greatly enlarged mitochondrial profiles with few or no cristae, and a higher yield of mitochondrial protein per cell.A significant increase in the content of circular DNA dimers and oligomers of mitochondria was observed after treatment of L-cells with cycloheximide and puromycin or treatment of chick fibroblasts with cycloheximide, and in hamster kidney cells transformed with polyoma virus. Little or no effects on dimer and oligomer content were found in L-cells treated with vinblastine, colcemid, rifampicin, chloramphenicol, or mengovirus, in Rous-transformed hamster cells, or in cuprizone-induced giant mitochondria of rat liver.

Mitochondrial RNA synthesis during mitosis

HeLa cells arrested in metaphase synthesized relatively normal amounts of mitochondrial RNA, while little RNA synthesis associated with the nucleus was detected. The RNA synthesized resembled the portion of mitochondrial RNA sensitive to ethidium bromide in interphase cells, with major peaks at 21, 12, and 4S. Unlike that in interphase cells, RNA synthesis in the mitoclhonidrial fraction of mitotic cells was completely inhibited by ethidium bromide.