Sodium arsenite induces ATP depletion and mitochondrial damage in HeLa cells

Our present results show that treatment with sodium arsenite apparently decreases cellular ATP levels in a dose- and time-dependent manner in HeLa S-3 cells. The reduction in ATP induced by sodium arsenite was possibly through mitochondrial damage, since treatment with sodium arsenite resulted in reduction of rhodamine 123 accumulation and disruption of the structure of the cristae in mitochondria. However, all of these changes could be reversed by removing sodium arsenite from the culture medium. The levels of ATP depletion were correlated with the killing effects of sodium arsenite in HeLa S-3 cells.     

Analysis of gene expression profiles in HeLa cells in response to overexpression or siRNA-mediated depletion of NASP

Background  

NASP (Nuclear Autoantigenic Sperm Protein) is a linker histone chaperone required for normal cell division. Changes in NASP expression significantly affect cell growth and development; loss of gene function results in embryonic lethality. However, the mechanism by which NASP exerts its effects in the cell cycle is not understood. To understand the pathways and networks that may involve NASP function, we evaluated gene expression in HeLa cells in which NASP was either overexpressed or depleted by siRNA.     

Rapid response of protein synthesis to insulin in 3T3 cells: Effects of protein kinase C depletion and differences from the response to serum repletion

Quiescent 3T3 cells grown in media containing 4% foetal calf serum showed different responses to insulin and to serum repletion (to 12%). Insulin stimulated protein synthesis within 1 h and this early response was insensitive to actinomycin D. The later insulin response showed progressive sensitivity to actinomycin D. The serum response was slower, not occurring until 1 h, and was inhibited by actinomycin D. Depletion of cell protein kinase C by pre-treatment with phorbol ester caused a total block of the immediate response to insulin but had little effect on the response to serum or the later response to insulin. Acute phorbol ester treatment stimulated protein synthesis.     

Cytoplasmic transfer of platelet mtDNA from elderly patients with Parkinson's disease to mtDNA-less HeLa cells restores complete mitochondrial respiratory function

For determination of whether platelet mtDNA in patients with Parkinson's disease (PD) possesses some lesions to reduce respiratory enzyme activities, platelet mtDNA was transferred into mtDNA-less (rho0) HeLa cells from aged PD patients and age-matched normal subjects, since their activities were controlled by both mitochondrial and nuclear genomes. The resultant mtDNA-repopulated cybrid clones containing the HeLa nuclear genome as a common background were used for comparison of respiratory enzyme activities. Remarkable variations of the enzyme activities were observed in the cybrid clones, irrespective of whether their mtDNA was transferred from normal subjects or PD patients, and some of them showed 20% reduction of average activities. Thus, the mtDNA mutations responsible for inducing 20% reduction should be polymorphic rather than pathogenic. On the other hand, pathogenic control cybrid clones possessing mtDNA mutations from patients with mitochondrial disorders showed significant and specific decline of respiratory enzyme complex I activity beyond the normal range of the variations. These observations warrant reassessment of the conventional concept that complex I activity in platelets of PD patients is defective due to mtDNA mutations.     

Nuclear complementation restores mtDNA levels in cultured cells from a patient with mtDNA depletion.

We have studied cultured skin fibroblasts from a patient with a fatal mitochondrial disease manifesting soon after birth. These fibroblasts were found to grow only in the presence of pyruvate and uridine, a characteristic of cells lacking mtDNA (rho0 cells). Southern blot and PCR analyses confirmed that the patient's fibroblasts contained less than 2% of control levels of mtDNA. Biochemical analyses indicated that the activities of all the respiratory-chain enzymes were severely decreased in mitochondria isolated from these fibroblasts. In order to elucidate the underlying molecular defect, cell fusions were performed between enucleated fibroblasts from this patient and a human-derived rho0 cell line (rho0 A549.B2). The resulting cybrids were plated in medium lacking pyruvate and uridine, to select for the restoration of respiratory-chain function. Complementation was observed between the nuclear genome of the rho0 A549.B2 cells and the mtDNA of the patient's cells, restoring mtDNA levels and respiratory-chain function in the cybrid cells. These results indicate that mtDNA depletion in our patient is under the control of the nuclear genome.     

Exercise-induced muscle glycogen depletion and repletion in diabetic rats.

The purpose of this experiment was to examine glycogen depletion in muscles of chronic diabetic rats during treadmill running of moderate intensity and glycogen repletion following the exercise bouts. Diabetes was induced with a single intravenous injection of streptozotocin (70 mg × kg^?1). Glycogen concentrations in muscles from diabetic and normal animals were determined at rest, after running either 10 or 30 min at 23 m × min^?1 (5% incline), or 2, 4, or 8 hr following 30 min of running at the same speed and incline. With the exception of soleus muscle after 30 min of running, there were no differences in muscle glycogen contents between normal and diabetic rats before exercise, immediately after exercise, or during the recovery period. All muscles showed a significant loss of glycogen during exercise, and most muscles had completely restored their glycogen by 2 hr following exercise. Blood lactate concentrations were also similar for normal and diabetic rats at rest and after exercise. It is concluded that the diabetic condition studied in this experiment did not significantly alter muscle glycogen metabolism during exercise of moderate intensity or during recovery from the activity.     

mtDNA depletion with variable tissue expression: a novel genetic abnormality in mitochondrial diseases.

We studied two related infants with a fatal mitochondrial disease, affecting muscle in one and liver in the other. Quantitative analysis revealed a severe depletion of mtDNA in affected tissues. This genetic abnormality was also observed in muscle of an unrelated infant with myopathy and in muscle and kidney of a fourth child with myopathy and nephropathy. Biochemistry, immunohistochemistry, and in situ hybridization showed that the depletion of mtDNA in muscle fibers was correlated with a respiratory chain defect and with lack of mitochondrially translated proteins. Although the differential tissue involvement in these infants suggests mtDNA heteroplasmy, sequence analysis of mtDNA replication origins did not reveal any abnormality that could account for the low copy number.     

Human mitochondrial pyrophosphatase: cDNA cloning and analysis of the gene in patients with mtDNA depletion syndromes

Pyrophosphatases (PPases) catalyze the hydrolysis of inorganic pyrophosphate generated in several cellular enzymatic reactions. A novel human pyrophosphatase cDNA encoding a 334-amino-acid protein approximately 60% identical to the previously identified human cytosolic PPase was cloned and characterized. The novel enzyme, named PPase-2, was enzymatically active and catalyzed hydrolysis of pyrophosphate at a rate similar to that of the previously identified PPase-1. A functional mitochondrial import signal sequence was identified in the N-terminus of PPase-2, which targeted the enzyme to the mitochondrial matrix. The human pyrophosphatase 2 gene (PPase-2) was mapped to chromosome 4q25 and the 1.4-kb mRNA was ubiquitously expressed in human tissues, with highest levels in muscle, liver, and kidney. The yeast homologue of the mitochondrial PPase-2 is required for mitochondrial DNA maintenance and yeast cells lacking the enzyme exhibit mitochondrial DNA depletion. We sequenced the PPA2 gene in 13 patients with mitochondrial DNA depletion syndromes (MDS) of unknown cause to determine if mutations in the PPA2 gene of these patients were associated with this disease. No pathogenic mutations were identified in the PPA2 gene of these patients and we found no evidence that PPA2 gene mutations are a common cause of MDS in humans.     

Ribonucleotides in closed circular mitochondrial DNA from HeLa cells

Closed circular mitochondrial DNA from HeLa cells is sensitive to both alkali and ribonucleases. The kinetics of ring opening in alkali suggest at least two classes of molecules. One class undergoes rapid breakdown, ultimately to fragments smaller than unit length, in contrast to the second class, which is more resistant to alkaline cleavage and is converted in large part to unit length single strands. Ribonucleases A, T₁ and H relax the supercoiled molecules, indicating that the alkali susceptibility is due to the presence of ribonucleotides in the DNA. By comparison with the rate of hydrolysis of RNA, the alkali-resistant class of mitochondrial DNA molecules is estimated to contain approximately 3 ribonucleotides and the alkalisensitive class 10–18.