Age-associated increase in 8-oxo-deoxyguanosine glycosylase/AP lyase activity in rat mitochondria

The mitochondrial theory of aging postulates that organisms age due to the accumulation of DNA damage and mutations in the multiple mitochondrial genomes, leading to mitochondrial dysfunction. Among the wide variety of DNA damage, 8-oxo-deoxyguanosine (8-oxo-dG) has received the most attention due to its mutagenicity and because of the possible correlation between its accumulation and pathological processes like cancer, degenerative diseases and aging. Although still controversial, many studies show that 8-oxo-dG accumulates with age in the mitochondrial (mt) DNA. However, little is known about the processing of this lesion and no study has yet examined whether mtDNA repair changes with age. Here, we report the first study on age-related changes in mtDNA repair, accomplished by assessing the cleavage activity of mitochondrial extracts towards an 8-oxo-dG-containing substrate. In this study, mitochondria obtained from rat heart and liver were used. We find that this enzymatic activity is higher in 12 and 23 month-old rats than in 6 month-old rats, in both liver and heart extracts. These mitochondrial extracts also cleave oligonucleotides containing a U:A mismatch, at the uracil position, reflecting the combined action of mitochondrial uracil DNA glycosylase (mtUDG) and mitochondrial apurinic/apyrimidinic (AP) endonucleases. The mtUDG activity did not change with age in liver mitochondria, but there was a small increase in activity from 6 to 23 months in rat heart extracts, after normalization to citrate synthase activity. Endonuclease G activity, measured by a plasmid relaxation assay, did not show any age-associated change in liver, but there was a significant decrease from 6 to 23 months in heart mitochondria. Our results suggest that the mitochondrial capacity to repair 8-oxo-dG, the main oxidative base damage suggested to accumulate with age in mtDNA, does not decrease, but rather increases with age. The specific increase in 8-oxo-dG endonuclease activity, rather than a general up-regulation of DNA repair in mitochondria, suggests an induction of the 8-oxo-dG-specific repair pathway with age.

Age-associated change in mitochondrial DNA damage

There is an age-associated decline in the mitochondrial function of the Wistar rat heart. Previous reports from this lab have shown a decrease in mitochondrial cytochrome c oxidase (COX) activity associated with a reduction in COX gene and protein expression and a similar decrease in the rate of mitochondrial protein synthesis. Damage to mitochondrial DNA may contribute to this decline. Using the HPLC-Coularray system (ESA, USA), we measured levels of nuclear and mitochondrial 8-oxo-2'-deoxyguanosine (8-oxodG) from 6-month (young) and 23-month-old (senescent) rat liver DNA. We measured the sensitivity of the technique by damaging calf thymus DNA with photoactivated methylene blue for 30s up to 2h. The levels of damage were linear over the entire time course including the shorter times which showed levels comparable to those expected in liver. For the liver data, 8-oxodG was reported as a fraction of 2-deoxyguanosine (2-dG). There was no change in the levels of 8-oxodG levels in the nuclear DNA from 6 to 23-months of age. However, the levels of 8-oxodG increased 2.5-fold in the mitochondrial DNA with age. At 6 months, the level of 8-oxodG in mtDNA was 5-fold higher than nuclear and increased to approximately 12-fold higher by 23 months of age. These findings agree with other reports showing an age-associated increase in levels of mtDNA damage; however, the degree to which it increases is smaller. Such damage to the mitochondrial DNA may contribute to the age-associated decline in mitochondrial function.

Age-associated changes in mitochondrial mRNA expression and translation in the Wistar rat heart

The purpose of this research is to determine possible causes and mechanisms involved in the age-associated decline in mitochondrial activity. We have focused on cytochrome c oxidase because it is comprised of both nuclear and mitochondrial-encoded subunits and may provide some insight into the coordination of the two genomes. In agreement with previous reports, we show an approximate 30% decrease in cardiac cytochrome c oxidase activity at 24 months compared to 6 months with no change in the activity of the nuclear encoded citrate synthase of the mitochondrial matrix. The rate of the mitochondrial protein synthesis as shown by [35S]methionine incorporation decreased approximately 35% in the 24-month-old rat compared to the 6-month-old rat. The decrease in protein synthesis was associated with a 30-50% reduction in the levels of most individually radiolabeled translation products including the COX subunits and specifically, a 23% decrease in COX1 protein steady-state levels according to Western analysis. Similarly, there was a decrease in the mRNA steady-state levels of both nuclear and mitochondrial-encoded subunits of cytochrome c oxidase. These results suggest that a number of different mechanisms are involved in the age-associated decrease in heart mitochondrial activity and these are discussed.

An oxidative damage-specific endonuclease from rat liver mitochondria

Reactive oxygen species have been shown to generate mutagenic lesions in DNA. One of the most abundant lesions in both nuclear and mitochondrial DNA is 7,8-dihydro-8-oxoguanine (8-oxoG). We report here the partial purification and characterization of a mitochondrial oxidative damage endonuclease (mtODE) from rat liver that recognizes and incises at 8-oxoG and abasic sites in duplex DNA. Rat liver mitochondria were purified by differential and Percoll gradient centrifugation, and mtODE was extracted from Triton X-100-solubilized mitochondria. Incision activity was measured using a radiolabeled double-stranded DNA oligonucleotide containing a unique 8-oxoG, and reaction products were separated by polyacrylamide gel electrophoresis. Gel filtration chromatography predicts mtODE's molecular mass to be between 25 and 30 kDa. mtODE has a monovalent cation optimum between 50 and 100 mM KCl and a pH optimum between 7.5 and 8. mtODE does not require any co-factors and is active in the presence of 5 mM EDTA. It is specific for 8-oxoG and preferentially incises at 8-oxoG:C base pairs. mtODE is a putative 8-oxoG glycosylase/lyase enzyme, because it can be covalently linked to the 8-oxoG oligonucleotide by sodium borohydride reduction. Comparison of mtODE's activity with other known 8-oxoG glycosylases/lyases and mitochondrial enzymes reveals that this may be a novel protein.

Increased carnitine palmitoyltransferase in cardiac myocytes is mediated by insulin growth factor I

The mitochondrial carnitine palmitoyltransferase (CPT) system is composed of two proteins, CPT-I and -II, which, together with carnitine acylcarnitine translocase, are involved in the transport of fatty acids into the mitochondrial matrix for beta-oxidation. In the liver, CPT-I and its inhibition by malonyl-CoA are sensitive to hormonal (10(-9) M) levels of insulin; however, a similar effect of insulin on heart CPT is controversial. In cultured neonatal rat cardiac myocytes, tissue culture concentrations (1.7 microM) of insulin increase CPT and cytochrome oxidase activities as well as mitochondrial protein synthesis, suggesting that a growth mechanism may be involved. Because insulin at high concentrations may interact with the insulin-like growth factor (IGF-I) receptor, the consequences of insulin's action on heart cells in culture may be mediated through the IGF pathway. Consistent with an IGF-mediated pathway for the effect of insulin, incorporation of radioactivity into immunoprecipitated CPT-II from insulin-treated cardiac myocytes is dramatically increased over control cells. The amount of immunoreactive CPT-I is also increased in insulin-treated cells. Moreover, an IGF-I analogue that inhibits the autophosphorylation of the IGF-I receptor blunts the insulin-mediated increase in CPT-I and -II activities by > 70%. At low physiologically relevant concentrations (10 ng/ml), IGF-I significantly increases the activities of both CPT-I and -II, and the IGF-I analogue eliminates the IGF-I response. This is the first study to suggest involvement of the IGF-I pathway in the regulation of mitochondrial CPT synthesis and activities in the heart.

Insulin-associated changes in carnitine palmitoyltransferase in cultured neonatal rat cardiac myocytes

Insulin increases the synthesis of mitochondrial proteins in the isolated perfused heart and total cell protein synthesis in neonatal cardiac myocytes. Since carnitine-dependent fatty acid oxidation is modulated by insulin in a variety of tissues, the effects of 1.7 microM insulin on the mitochondrial enzyme(s), carnitine palmitoyltransferase (malonyl-CoA-sensitive CPT-I and the matrix-facing CPT-II), were studied in neonatal rat cardiac myocytes cultured in the absence of serum. Following incubation in serum-free medium, there is a four-fold increase in the I50 of CPT-I for malonyl-CoA (3.8 microM) compared to cells cultured in serum-free medium to which insulin has been added (I50 = 0.8 microM). CPT-I activity in the insulin-supplemented, serum-free cultures is 57% higher (P < 0.002) than CPT-I activity in cells cultured in the absence of insulin; CPT-II activity is also significantly increased (P < 0.01) in the presence of insulin. Since CPT-II is an inner membrane protein, the CPT response to insulin may be coordinately regulated with other mitochondrial activities. Similar to CPT, cytochrome oxidase activity of cardiac myocytes in serum-free medium is increased 33% by insulin. Consistent with this finding, both CPT-II and cytochrome oxidase mRNA expression is elevated over control in the presence of insulin. CPT-II activity increases significantly only at very high insulin concentrations (1.7 microM), suggesting a role for insulin-like growth factor pathway. When myocytes are cultured in the presence of 1.7 microM insulin and then transferred to an insulin-free medium, subsequent addition of insulin does not stimulate uptake of deoxyglucose. These results suggest that the response of CPT to insulin is mediated by insulin-like growth factor activity and not by cellular glucose availability. The response of CPT to insulin does not appear to be mediated by the protein kinase C pathway since CPT-II activity is not reduced by the protein kinase C inhibitor, chelerythrine. Insulin significantly increases protein synthesis in the neonatal cardiac myocyte and in isolated mitochondria by increasing incorporation of labelled amino acid into total myocyte and/or mitochondrial protein. The degradation rate of radiolabelled protein in cardiac myocytes cultured in the presence of insulin is not different from that of insulin-deprived cells. The data suggest that insulin can affect the activity and expression of mitochondrial proteins, e.g., CPT, through the insulin-like growth factor-I pathway in neonatal cardiac myocytes.

Mitochondrial metabolism and substrate competition in the aging Fischer rat heart

OBJECTIVE

The objective was to examine mitochondrial oxidative metabolism of long chain fatty acids and to compare it with glucose uptake and the generation of pressure-volume work in hearts from mature and aged rats.

METHODS

Hearts from mature (8 to 15 months of age) and old (28 to 30 months) Fischer 344 rats were perfused as working hearts with either 10 mM glucose or glucose plus 1 mM oleic acid (2% bovine serum albumin) and rates of glucose extraction were determined. Hearts were subjected to a stepwise increase in work load. In separate experiments, mitochondria were isolated from mature and old rat hearts and assayed for respiratory function, carnitine exchange, carnitine palmitoyltransferase activities, and phospholipid content.

RESULTS

Although there were no differences in peak work attained between the mature and old rats in the presence of either glucose alone or glucose plus oleic acid, glucose utilisation was significantly decreased by oleate in the mature animals only. No significant changes in either glutamate or succinate (+rotenone) supported respiration were found in heart mitochondria isolated from old rats compared with mature animals. In agreement with prior studies with the Wistar rat model of aging, significant decrements in the rates of palmitoylcarnitine oxidation and carnitine exchange were apparent in the old Fischer animals. A significant lowering of heart mitochondrial carnitine palmitoyltransferase I activity was also found in the old animals. A decrease in the amounts of carnitine loaded in mitochondria from old animals is consistent with reduced carnitine content in both mitochondria and whole hearts from aged Wistar and Fischer rats. A significant (23%) reduction in heart mitochondrial cardiolipin content from 30 month old Fischer rats suggests that this phospholipid may also contribute to the lower rates of carnitine and acylcarnitine transport across the mitochondrial inner membrane.

CONCLUSION

The limitation in the delivery of fatty acyl units to beta oxidation as measured in isolated heart mitochondria from old rats has a physiological correlate in the intact heart. The well documented suppression of glucose oxidation by fatty acids seen in the adult rat heart is not seen in old hearts, supporting the in vitro finding of decreased oxidation of palmitoylcarnitine with senescence.

Evidence for malonyl-CoA-sensitive carnitine acyl-CoA transferase activity in sarcoplasmic reticulum of canine heart

The formation of palmitoylcarnitine is catalyzed by carnitine palmitoyl-transferase (CPT-I) and this catalysis is the first committed step in beta-oxidation. The malonyl-CoA-inhibited isoform appears to be distinct from latent (CPT-II) activity, which is localized to the matrix side of the mitochondrial inner membrane. Sarcoplasmic reticulum from canine cardiac muscle was fractionated on a discontinuous sucrose density gradient into three major bands, all of which contained Ca(2+)-ATPase activity. Only the fraction that banded at a concentration of 38% surcrose was slightly contaminated by mitochondria. Peroxisomal uricase was low or absent in fractionated SR. All sarcoplasmic reticulum fractions contained malonyl-CoA-sensitive medium- (COT) and long-chain (CPT) carnitine acyltransferase activities. CPT activity decreased in sarcoplasmic reticulum when Triton X-100 was present. Carnitine acyltransferase activities were inactivated by preincubating the sarcoplasmic reticulum with the sulfhydryl reagent, 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB). In contrast, mitochondrial CPT-II activity was stable in the presence of DTNB and activated by Triton X-100. Western blots of mitochondria and sarcoplasmic reticulum fractions showed that the mitochondrial fractions reacted with antibody to mitochondrial CPT-II but not with SR protein when both were added at comparable specific activities. The data suggest that cardiac SR contains a unique malonyl-CoA-sensitive isoform of CPT, and that synthesis of acylcarnitine may occur in the microenvironment of Ca2+ transport, where the extent of production of acylcarnitine is controlled by cardiac acetyl-CoA carboxylase activity.

Vaccine choice and program participation rates when two hepatitis B vaccines are offered

Participation rates of health care workers in voluntary free hepatitis B virus immunization programs are 35% to 40%. University teaching hospital employees at risk for hepatitis B virus and presenting for immunization were surveyed as to vaccine preference. Both plasma-derived and recombinant hepatitis B virus vaccines were available. During a 10-month period, 173 health care workers enrolled in the study. One hundred seventeen received recombinant vaccine, and 56 received plasma-derived vaccine; 66 were immunized postexposure. Concern of a relationship of human immunodeficiency virus to hepatitis B virus plasma-derived vaccine was acknowledged by a small number of health care workers as important in vaccine selection. Recombinant hepatitis B virus vaccine rapidly and substantially supplanted plasma-derived vaccine but did not increase program participation. We suspect that mandatory immunization or proof of immunity will be necessary if hepatitis B virus protection rates in health care workers are to improve.

Enhanced calcium uptake of cardiac sarcoplasmic reticulum in exercise-trained old rats

To determine whether endurance exercise training initiated during senescence improves the slower calcium transport of cardiac sarcoplasmic reticulum in senescent rats, three groups of male Fischer 344 rats were used: sedentary mature (SM), 11-12 mo of age; sedentary old (SO), 23-24 mo of age; and exercised old (EO), 23-24 mo of age. The EO rats ran on a motor-driven treadmill 5 days/wk for 8-10 wk while two other groups remained sedentary. The contraction duration of isometrically contracting papillary muscle was prolonged in the SO rats compared with the SM group, resulting from both a longer time-to-peak tension and a longer half-relaxation time. Chronic exercise improved the papillary muscle contractile function of the EO group to that observed in the younger SM rats. More importantly, the exercise regime ameliorated the slower oxalate-supported, ATP-dependent calcium transport observed in the SO group so that the EO and SM groups were the same. In contrast, the calcium-activated myosin adenosinetriphosphatase activity, which was slower in the SO group, was not altered after exercise training, so that both senescent groups were slower than the SM rats. Thus the increased calcium transport by the cardiac sarcoplasmic reticulum may be one of the potential mechanisms underlying the improvement of myocardial performance with chronic exercise initiated during senescence.