Mika Hayakawa

Age-associated oxygen damage and mutations in mitochondrial DNA in human hearts

Some mutations in mitochondrial DNA (mtDNA) causing a number of neuromuscular diseases are suggested to arise spontaneously during the life of an individual. To substantiate the extent and the rate of these somatic mutations, mtDNA specimens from post-mortem human heart muscles of subjects in differing age groups were hydrolyzed. 8-Hydroxy-deoxyguanosine (8-OH-dG), a hydroxyl-radical adduct of deoxyguanosine, in mtDNA, was quantitatively determined using a micro high-performance liquid chromatography/mass spectrometry system. In each specimen, the mtDNA with a 7.4 kilo base-pair deletion was quantified by the kinetic polymerase chain reaction method. In association with age, the 8-OH-dG content accumulated exponentially up to 1.5% with a correlative increase in the content of the deleted mtDNA up to 7%. Clear correlation between the 8-OH-dG content in mtDNA and the population of mtDNA with a deletion (r = 0.93, P < 0.01) gives insight into the mechanism for the generation of a large deletion. These results indicate that accumulation of somatically acquired oxygen damage together with age-associated mutations in mtDNA which lead to bioenergetic deficiency and the heart muscle weakness are inevitable in human life.

Age-associated accumulation of 8-hydroxydeoxyguanosine in mitochondrial DNA of human diaphragm

This is the first report that age-associated accumulation of 8-hydroxydeoxyguanosine (8-OH-dG) does occur in human mitochondrial DNA (mtDNA) in muscle of diaphragm. We extracted mtDNA from human diaphragm muscles from differing age groups, and determined the amount of 8-OH-dG by ultramicro-high performance liquid chromatography/mass-spectrometry system. With the same specimen, multiple deletions of mtDNA were detected by electrophoresis after amplification by the polymerase chain reaction method. In subjects below age 55, the level of 8-OH-dG in mtDNA was below 0.02% of the total deoxyguanosine (dG), whereas, in subjects over age 65, the level of 8-OH-dG increased with age at a rate of ca. 0.25% per 10 years, reaching 0.51% at age 85. Moreover, a concomitant increase in multiple deletions was detected with the increase in age. These results suggest that, in younger diaphragms, replication of mtDNA dilutes out 8-OH-dG being not detectable. In the elderly subjects aged over 65, the replication rate might be slowed down leading to the accumulation of 8-OH-dG in mtDNA, which would accelerate the age-associated multiple deletions of mtDNA observed among the subjects.

Massive conversion of guanosine to 8-hydroxy-guanosine in mouse liver mitochondrial DNA by administration of azidothymidine.

As typical mitochondrial myopathy has been reported to be expressed among many patients with AIDS treated with long-term azidothymidine (AZT) therapy, we examined changes in mouse liver mitochondrial DNA (mtDNA) after 4-week administration of AZT. Even below 1/10th the dose given to the patients (AZT, 1 mg/kg/day), 25% of the total deoxyguanosine (dG) was converted to be 8-hydroxy-deoxyguanosine (8-OH-dG). 38% of the total dG was converted to 8-OH-dG with AZT 5 mg/kg/day. In vitro, the conversion of dG to 8-OH-dG was demonstrated by incubating mtDNA in the oxygen radical producing system containing NADH and KCN treated mitochondrial inner membrane. Thus it is concluded that, by lack of repairing system, damaged mtDNA with AZT results in impaired mitochondrial respiratory chain causing oxygen radicals which are responsible for 8-OH-dG formation. These results suggest that the oxygen damage of mtDNA is the primary cause of mitochondrial myopathy with AZT therapy.

Age-associated damage in mitochondrial function in rat hearts

The aim of this study is to elucidate effects of aging on mitochondrial function and mitochondrial DNA (mtDNA) in rat heart and liver. The activities of complex I and complex IV of heart mitochondria of rats aged 100 weeks decreased significantly by 31% and 22%, respectively, compared with those of rats aged 7 weeks. No significant changes were observed in these two parameters in rats aged 7 weeks and aged 55 weeks. There were no significant differences in the specific activities of complex II and complex III among the age groups of 7, 55, and 100 weeks. The mtDNA content decreased by 58% in rats aged 100 weeks compared with that in rats aged 7 weeks. Content of 8-hydroxydeoxyguanosine (8-OH-dG), an oxidative product of deoxyguanosine (dG), increased by 130% in rats aged 100 weeks compared with that in rats aged 7 weeks. No significant changes were observed in these parameters between rats aged 7 weeks and 55 weeks. In contrast to heart mtDNA, these age-dependent changes were not observed in liver mitochondria at rats aged up to 100 weeks. From our results, age-associated decline in mitochondrial function might play an important role in cell aging, particularly in postmitotic cells such as heart muscle, and accumulation of oxidative damage to mtDNA might be involved in this mechanism.

Adverse effects of anti-tumor drug, cisplatin, on rat kidney mitochondria: Disturbances in glutathione peroxidase activity

This study was designed to clarify mechanisms responsible for cisplatin-induced nephrotoxicity together with the effect of selenium. Rats were divided into 3 groups: the cisplatin group; cisplatin (60 mg/kg) was administered intraperitoneally once, the cisplatin + Se group; cisplatin (60 mg/kg) once, and selenious acid (10 mumol/kg) were administered intraperitoneally once a day for 5 consecutive days, the control group; untreated. In each group, mitochondrial respiratory function, enzymic activities in mitochondrial respiratory chain and glutathione peroxidase, and plasma creatinine and BUN contents were measured. In the cisplatin group, decreases in mitochondrial respiratory function, enzymic activities in the respiratory chain and glutathione peroxidase, and increases in plasma creatinine and BUN contents were observed compared with the control group, while the cisplatin + Se group lessened these impairments. These results suggested that cisplatin-induced nephrotoxicity was closely related to mitochondrial dysfunction through the impairment of glutathione peroxidase. This toxicity might be ascribed to free radical mediated-injury. We propose here that, with selenium, higher dose administration of cisplatin to patients might be applicable.

Age-associated damage in mitochondrial DNA in human hearts

Damage to mitochondrial DNA seems to be involved in the etiology of age-associated degenerative diseases. The aim of this study is to elucidate effects of aging on human mitochondrial DNA. 8-Hydroxy-deoxyguanosine, a product of free radical damage to deoxyguanosine, is reported to cause random point mutations. In human mitochondrial DNA, 8-hydroxy-deoxyguanosine increased exponentially with age, and the population of mitochondrial DNA with deletion increased also exponentially with age. Furthermore, a clear correlation existed between the accumulation of 8-hydroxy-deoxyguanosine and that of mitochondrial DNA with deletion. We also determined the effects of aging on rat mitochondrial function together with 8-hydroxy-deoxyguanosine content in mitochondrial DNA. The activities of complexes I and IV of the mitochondrial electron transport chain decreased significantly in rats aged 100 weeks compared with those in rats aged 7 weeks. A concomitant increase in 8-hydroxy-deoxyguanosine was observed in mitochondrial DNA of rats aged 100 weeks. From our results, it is concluded that the age-associated accumulation of somatically acquired oxygen damage together with deletions in mitochondrial DNA might be important contributors to the deterioration of cardiac function associated with age.

Quantitative analysis of age-associated accumulation of mitochondrial DNA with deletion in human hearts

We have demonstrated that myocardial mitochondrial DNA (mtDNA) with a 7,436 base-pair deletion existed in all subjects that were over 70 years old. Since each mitochondrion has two or three copies of its own DNA, quantitative analysis is required for the evaluation of the role of mtDNA with deletions in the age-related deterioration of cardiac performance. For this purpose, the kinetic polymerase chain reaction (PCR) method developed in our laboratory was used in this investigation to determine myocardial mtDNA with this 7,436 base-pair deletion in human cadavers of various ages. The mtDNA population with this deletion increased exponentially with age [log f (% of deleted mtDNA) = -3.136 + 0.0454 x age, r = 0.95, P less than 0.01)], and was estimated at 3% and 9% in subjects of age 80 and 90, respectively. The deleted portion encodes 7 subunits of the mitochondrial ATP production system, and a population of mtDNA with this deletion over a certain threshold might induce a significant deterioration of cardiac energy metabolism. Cardiac function is known to deteriorate with age, and an increase in the population of mtDNA with deletion is likely to be an important contributing factor to aged heart (presbycardia).

Leukotoxin, a linoleate epoxide: Its implication in the late death of patients with extensive burns

Burn death based on circulatory shock is often encountered after recovery from primary shock in patients with deep and extensive burns,i.e., late death. Several toxic substances have been proposed, however, the responsible substance remains obscure. Since we have found leukotoxin, a highly cytotoxic linoleate epoxide biosynthesized by neutrophils, in the burned skin, in the present study we determined plasma leukotoxin concentrations in various degree of 30 burn patients. C-reactive protein and circulatory white blood cells were also measured. A significantly high mortality rate of patients with extensive burns (burn surface area over 70%) was observed compared with that in patients with burn surface area under 70%, and significantly high leukotoxin concentrations were observed within a week, and 3 weeks after the thermal injury in patients with extensive burns compared with those in patients with burn surface area under 70%. There were two peaks of plasma leukotoxin concentrations,i.e., the early phase (within 1 week) and the late phase (over 1 week) in patients with extensive burns. Plasma leukotoxin concentrations significantly correlated with burn surface area in the early phase, and similar correlations were observed in the late phase. A significantly high mortality rate (61%) of patients with peak leukotoxin concentrations over 30 nmol/ml was observed compared with 8% for those below 30 nmol/ml. Plasma leukotoxin concentration correlated significantly to C-reactive protein concentration, log (leukotoxin nmol/ml)=0.042×C-reactive protein (mg/dl)+0.74, (r=0.83,P<0.01) in the late phase. From these results, it is concluded that leukotoxin is produced in patients with burns particularly in the late phase of extensive burns, and leukotoxin might play an important role in the tissue destructive procedure associated with severe burns.

Accumulation of Deletions and Point Mutations in Mitochondrial Genome in Degenerative Diseases

Accumulation of various mutations in the mitochondrial genome is proposed as an important contributor to aging and degenerative diseases. Extensive fragmentation of mtDNA was detected in association with increased 8-hydroxydeoxyguanosine content in the heart mitochondrial DNA (mtDNA) from a patient with premature aging and mitochondrial cardiomyopathy, who carried a mutation within the mitochondrial tRNA(Asp) gene. This result suggests that damage to mtDNA by hydroxyl radical and accumulation of deleted mtDNA can be accelerated by a specific mitochondrial genotype. Similarly, extensive fragmentation of mtDNA was also detected in cultured cells exposed to a high oxygen concentration atmosphere, implying that mtDNA is vulnerable to reactive oxygen species. To clarify the role of point mutations accumulated in mtDNA, we examined the sequence heterogeneity of mtDNA in the skeletal muscle of a MELAS patient who carried a mutation within the mitochondrial tRNA(leu)(UUR) gene. The analysis revealed that the frequency of mutant clones in the MELAS muscle was significantly higher than those in an age-matched control muscle and a control placenta. Some of these nucleotide substitutions were missense and nonsense mutations, which potentially have deleterious effects on the mitochondrial function. The frequency of nucleotide substitutions in the striatum of three patients with Parkinsons disease was also significantly higher than that in control tissues. We also observed increased protein modification by 4-hydroxy-2-nonenal, a lipid peroxidation by-product, in Parkinsons disease. These results suggests that a vicious cycle contributes to the progression of degenerative process. In this cycle, first a primary mitochondrial mutation(s) induces a mitochondrial respiratory defect, which increases the leakage of reactive oxygen species (ROS) from the respiratory chain. Then the ROS would trigger accumulation of secondary mtDNA mutations in postmitotic cells, leading to further aggravation of mitochondrial respiratory defects and increased production of ROS and lipid peroxides from mitochondria, and thus resulting in degeneration of cellular components.

Leukotoxin, 9, 10-epoxy-12-octadecenoate, causes cardiac failure in dogs

An epoxy derivative of linoleate, 9, 10-epoxy-12-octadecenoate, was demonstrated to be biosynthesized by leukocytes, thus nominated as leukotoxin. Its chemical structure was determined by gas-chromatography/mass spectrometry and nuclear magnetic resonance measurements. When it was injected intravenously, 15 mg/kg, canine heart showed signs of a typical cardiac failure; viz. Aortic flow started to drop immediately after the injection, and fell to 22% of the original at 40 min after the injection. At that point, systolic aortic pressure dropped to 35%, diastolic aortic pressure to 23%, and electronically differentiated maximal rate of left ventricular pressure rise (LV dp/dt) to 29%. All of experimental dogs died 40 to 50 min after the injection. On the contrary, administration of linoleic acid (15 mg/kg) did not affect these hemodynamical parameters. Therefore, leukotoxin seems to be an important factor to the genesis of heart failure.