Respiration-defective Chinese hamster cell mutants containing low levels of NADH-ubiquinone reductase and cytochrome c oxidase

Regulation of hypoxia-inducible factor is preserved in the absence of a functioning mitochondrial respiratory chain

Hypoxia-inducible factor (HIF) mediates a large number of transcriptional responses to hypoxia and has an important role in processes that include angiogenesis and erythropoiesis. The HIF DNA binding complex consists of 2 basic-helix-loop-helix PAS proteins designated alpha and beta subunits. Regulation occurs principally through the alpha subunits, which are stabilized and activated in hypoxia. Although substantial evidence implicates reactive oxygen species (ROS) in the regulatory process, the precise mechanisms remain unclear. Mitochondria are an important source of ROS, and in one model it has been proposed that hypoxia increases the generation of ROS at complex III in the mitochondrion and that this signal acts through a transduction pathway to stabilize HIF-1alpha and to activate HIF. To test this model the induction of the HIF-1alpha subunit and the HIF target gene, glucose-transporter-1, was examined in a variety of mutant cells that lacked mitochondrial DNA (rho0) or had other genetic defects in mitochondrial respiration. HIF induction by hypoxia was essentially normal in all cells tested. Hydrogen peroxide production was measured by the luminol/peroxidase method and found to be reduced in rho0 versus wild-type cells and reduced by hypoxia in both rho0 and wild-type cells. Furthermore, concentrations of rotenone that maximally inhibited respiration did not affect HIF activation by hypoxia. These data do not support the model outlined above and indicate that a functional respiratory chain is not necessary for the regulation of HIF by oxygen.

Nuclear inheritance of a gene affecting mitochondrial gene expression

Due to a deficiency in mitochondrial protein synthesis, Chinese hamster lung (CHL) cell mutant Gal- 32 does not grow in galactose or fructose. This report examines the nuclear or cytoplasmic inheritance of this single, recessive mutation. In a control experiment, fusion of Gal+TGSTK- cells with Gal- 32TGRTK+ cells resulted in tetraploid hybrids (as verified by karyotyping) that were selected in hypoxanthine/aminopterin/thymidine medium. The majority (2/3) of the control hybrids grew in galactose as expected since Gal+ is dominant over Gal-. Fusion of Rhodamine 6-G treated Gal+TGSTK- cells with Gal- 32TGRTK+ cells resulted in Rhodamine 6-G-tetraploid hybrids that were selected in hypoxanthine/aminopterin/thymidine medium. The majority (7/12) of the Rhodamine 6-G-hybrids grew in galactose as expected for a nuclearly encoded gene considering that Rhodamine 6-G interferes with transmission of mtDNA but not nuclear DNA. Therefore, these results are compelling in their demonstration of the nuclear origin of the Gal- 32 mutation.

A procedure for selecting mammalian cells with an impairment in oxidative phosphorylation

The mitochondrial encephalomyopathies in man are characterized by heterogeneous defects leading to an impairment in the pathway of aerobic energy production. As a means of investigating the molecular and genetic mechanisms underlying these disorders we have developed a procedure for selecting mammalian cell lines with features resembling the human pathological phenotypes. The principle of the selection is the use of a fluorescent amphiphilic dye, 2,4-(dimethylamino)-1-styrylmethylpyridiniumiodine, a cation showing two main features. Firstly, it is accumulated by mitochondria to an extent correlated with the magnitude of the electrochemical gradient of protons across the mitochondrial inner membrane. Secondly, upon irradiation with UV light, it gives rise to formation of free radicals, which inflict damage to the cell. Mutant cells with an impairment in oxidative phosphorylation will have more chance to survive than wild type cells. The selection procedure was applied to a stock of mutagenized Chinese hamster ovary cells. After subcloning of the cells which survived the selection procedure, twenty-six independent clones were isolated. Eighteen of the clones had a partial deficiency of cytochrome c oxidase ranging from 30 to 60% of the activity in control cells. The properties of two of the clones are described. One clone has been cultured under non-selective conditions for at least 12 months with retention of the partial deficiency of cytochrome c oxidase.

Fatal infantile mitochondrial cardiomyopathy and myopathy with heterogeneous tissue expression of combined respiratory chain deficiencies

A 5-month-old boy died of progressive heart failure that started at the age of 3 months. Autopsy revealed a mitochondrial cardiomyopathy and a mitochondrial myopathy of the limb muscle and diaphragm. Cytochemically random defects of cytochrome c oxidase were visualized by light and electron microscopy in the diaphragm and especially the heart muscle, the limb muscle showing a diffuse attenuation whereas the liver and kidneys reacted normally. The activities of NADH-dehydrogenase (complex I) and cytochrome c oxidase (complex IV) were severely diminished (20% residual activity of controls) in the skeletal and heart muscle. In the heart, succinate cytochrome c reductase (complex II/III) was additionally decreased to the same degree. Loss of cytochrome c oxidase activity was based on a reduction of both mitochondrial and nuclear derived subunits in the heart and diaphragm as revealed by immunohistochemical analysis, whereas the limb muscle showed a normal immunoreactive protein content. The results illustrate heterogeneous tissue expression of respiratory chain enzyme defects and demonstrate that a cardiomyopathy may be the leading presentation of a mitochondrial disorder in early infancy.

Six unidentified reading frames of human mitochondrial DNA encode components of the respiratory-chain NADH dehydrogenase

The products of six unidentified reading frames of human mitochondrial DNA are precipitated from a mitochondrial lysate by antibodies against highly purified native beef heart NADH-ubiquinone oxidoreductase (complex I). These products are enriched greatly in a human submitochondrial fraction enriched in NADH-Q1 and NADH-K3Fe(CN)6 oxidoreductase activities. We conclude that the six reading frames encode components of the respiratory-chain NADH dehydrogenase.

Biochemical genetics of the mammalian oxidative phosphorylation system: analysis of the difference in the sensitivity of various Chinese hamster cell lines to inhibitors of the mitochondrial ATP synthase complex

Seven different Chinese hamster cell lines were found to vary greatly in their sensitivity to inhibitors of the mitochondrial ATPase. In plating-efficiency experiments, Chinese hamster lung V79 and bone marrow M3-1 cells were approximately 10,000-fold more resistant to oligomycin, 100-fold more resistant to efrapeptin, and 10-fold more resistant to ossamycin and leucinostatin than were ovary CHO or peritoneal B14 cells. In vitro experiments indicated that the increased resistance of V79 versus CHO cells to these inhibitors was due to an increased resistance of the mitochondrial ATPase. Heat-inactivation experiments indicated that there was a difference in the structure of the mitochondrial ATPase of V79 and CHO cells. Genetic experiments indicated that the difference in the sensitivity of V79 and CHO cells to inhibitors of the ATPase and the difference in the structure of the mitochondrial ATPase of V79 and CHO cells was due to a difference in both a nuclear and a cytoplasmic gene.