Increased uptake and retention of rhodamine 123 by mitochondria of old human fibroblasts

Quantitative analysis of intracellular calcium and mitochondrial kinetic fluorescence changes in GSNO-induced thymocyte early apoptosis

A fluorescence microscopy imaging technique was applied to observe the single-cell kinetic changes of intracellular Ca(2+) concentration ([Ca(2+)](i)) and mitochondrial membrane potential (ΔΨ(m)) during the early stage of S-nitrosoglutathione (GSNO)-induced thymocytes apoptosis. The kinetic features of [Ca(2+)](i) and ΔΨ(m) were quantitatively analyzed and compared by fitting the fluorescence intensity data. The mathematical parameter, inflection point which indicated the time point when [Ca(2+)](i) or ΔΨ(m) changed the most rapidly, was proposed to analyze the fitting curve. The results revealed that the inflection point of [Ca(2+)](i) always appeared prior to that of ΔΨ(m) during apoptosis induced by a certain GSNO concentration. Both the [Ca(2+)](i) and ΔΨ(m) changed in a GSNO concentration-dependent manner. Another parameter, half-max effect point was also employed and displayed the similar results. Such quantitative analyses of real-time observations at the single-cell level are useful for interpreting the sequence of the biological events operating in GSNO-induced thymocyte apoptosis.

Sustained inhibition of oxidative phosphorylation impairs cell proliferation and induces premature senescence in human fibroblasts

The mitochondrial theory of aging predicts that functional alterations in mitochondria contribute to the aging process. Whereas this hypothesis implicates increased production of reactive oxygen species (ROS) as a driving force of the aging process, little is known about molecular mechanisms by which mitochondrial impairment might contribute to aging. Using cellular senescence as a model for human aging, we have recently reported partial uncoupling of the respiratory chain in senescent human fibroblasts. In the present communication, we address a potential cause-effect relationship between mitochondrial impairment and the appearance of a senescence-like phenotype in young cells. We found that treatment by antimycin A delays proliferation and induces premature senescence in a subset of the cells, associated with increased reactive oxygen species (ROS) production. Quenching of ROS by antioxidants did however not restore proliferation capacity nor prevent premature senescence. Premature senescence is also induced upon chronic exposure to oligomycin, irrespective of ROS production, and oligomycin treatment induced the up-regulation of the cdk inhibitors p16, p21 and p27, which are also up-regulated in replicative senescence. Thus, besides the well-established influence of ROS on proliferation and senescence, a reduction in the level of oxidative phosphorylation is causally related to reduced cell proliferation and the induction of premature senescence.

Mitochondrial function in fibroblasts with aging in culture and/or Alzheimer's disease

Mitochondrial membrane potentials (MMP) reflect the functional state of the mitochondria within cells. Our recently published method provides a quantitative estimate of the MMP of populations of mitochondrial-like particles (MLP) within living cells at 37 degrees C using the combination of conventional fluorescence microscopy, 3D-deconvolution and exhaustive photon reassignment (EPR). Although the method does not provide an absolute measure of MMP, these relative MMP allow direct comparison between various mitochondria in cells at various ages in culture and in different cell lines from multiple patients. Fibroblasts lines from four Alzheimer's disease (AD) patients bearing the presenilin-1 (PS-1) mutation and four appropriate controls were evaluated at different ages in culture. The results showed a large variation in the relative MMP, cell size and sum of relative MMP of all MLP within each cell or within each cell line. Nevertheless, combining the values of relative MMP for the cell lines in each group revealed changes in distribution with age in culture. The relative MMP decreased while the cell sizes and sum of relative MMP within each cell increased with age in fibroblasts. Values did not differ between controls and the AD patients bearing PS-1 mutation at any age in cultures. This new, sensitive and quantitative estimate of relative MMP indicates that under non-stressed conditions relative MMP change with aging in culture, but relative MMP do not differ between controls and AD subjects.

Senescence-associated changes in respiration and oxidative phosphorylation in primary human fibroblasts

Limitation of lifespan in replicative senescence is related to oxidative stress, which is probably both the cause and consequence of impaired mitochondrial respiratory function. The respiration of senescent human diploid fibroblasts was analysed by high-resolution respirometry. To rule out cell-cycle effects, proliferating and growth-arrested young fibroblasts were used as controls. Uncoupled respiration, as normalized to citrate synthase activity, remained unchanged, reflecting a constant capacity of the respiratory chain. Oligomycin-inhibited respiration, however, was significantly increased in mitochondria of senescent cells, indicating a lower coupling of electron transport with phosphorylation. In contrast, growth-arrested young fibroblasts exhibited a higher coupling state compared with proliferating controls. In intact cells, partial uncoupling may lead to either decreased oxidative ATP production or a compensatory increase in routine respiration. To distinguish between these alternatives, we subtracted oligomycin-inhibited respiration from routine respiration, which allowed us to determine the part of respiratory activity coupled with ATP production. Despite substantial differences in the respiratory control ratio, ranging from 4 to 11 in the different experimental groups, a fixed proportion of respiratory capacity was maintained for coupled oxidative phosphorylation in all the experimental groups. This finding indicates that the senescent cells fully compensate for increased proton leakage by enhanced electron-transport activity in the routine state. These results provide a new insight into age-associated defects in mitochondrial function and compensatory mechanisms in intact cells.

Iron deficiency and iron excess damage mitochondria and mitochondrial DNA in rats

Approximately two billion people, mainly women and children, are iron deficient. Two studies examined the effects of iron deficiency and supplementation on rats. In study 1, mitochondrial functional parameters and mitochondrial DNA (mtDNA) damage were assayed in iron-deficient (< or =5 microg/day) and iron-normal (800 microg/day) rats and in both groups after daily high-iron supplementation (8,000 microg/day) for 34 days. This dose is equivalent to the daily dose commonly given to iron-deficient humans. Iron-deficient rats had lower liver mitochondrial respiratory control ratios and increased levels of oxidants in polymorphonuclear-leukocytes, as assayed by dichlorofluorescein (P < 0.05). Rhodamine 123 fluorescence of polymorphonuclear-leukocytes also increased (P < 0.05). Lowered respiratory control ratios were found in daily high-iron-supplemented rats regardless of the previous iron status (P < 0.05). mtDNA damage was observed in both iron-deficient rats and rats receiving daily high-iron supplementation, compared with iron-normal rats (P < 0.05). Study 2 compared iron-deficient rats given high doses of iron (8,000 microg) either daily or every third day and found that rats given iron supplements every third day had less mtDNA damage on the second and third day after the last dose compared to daily high iron doses. Both inadequate and excessive iron (10 x nutritional need) cause significant mitochondrial malfunction. Although excess iron has been known to cause oxidative damage, the observation of oxidant-induced damage to mitochondria from iron deficiency has been unrecognized previously. Untreated iron deficiency, as well as excessive-iron supplementation, are deleterious and emphasize the importance of maintaining optimal iron intake.

N-t-Butyl hydroxylamine is an antioxidant that reverses age-related changes in mitochondria in vivo and in vitro

N-t-butyl hydroxylamine (NtBHA) delays senescence-dependent changes in human lung fibroblasts (IMR90) (Atamna et al., J. Biol. Chem. 275, 6741-6748). The current study examines the effect of NtBHA on mitochondria in old and young rats and human primary fibroblasts (IMR90). In NtBHA-treated rats, the age-dependent decline in food consumption and ambulatory activity was reversed without affecting body weight. The respiratory control ratio of mitochondria from liver of old rats improved after feeding NtBHA. These findings suggest that NtBHA improved mitochondrial function in vivo. The age-dependent increase in proteins with thiol-mixed disulfides was significantly lower in old rats treated with NtBHA. NtBHA was effective only in old rats; no significant effect was observed in young rats. In IMR90 cells, NtBHA delayed senescence-associated changes in mitochondria and cellular senescence induced by maintaining the cells under suboptimal levels of growth factors. Proteasomal activity was also higher in cells treated with NtBHA than in untreated cells. NtBHA accumulates in cells 10- to 15-fold the extracellular concentration and is maintained by mitochondrial NADH. NtBHA is an antioxidant that is recycled by mitochondrial electron transport chain and prevents radical-induced toxicity to mitochondria.

Age-related changes in the mitochondrial depolarization induced by oxidative injury in human peripheral blood leukocytes

Aging is associated with impaired immunity and reduced host defenses. Mitochondrial bioenergetic dysfunctions and reduced antioxidative ability of immunocompetent cells may contribute to this phenomenon. In this study, 60 healthy volunteers of different age groups donated their blood after overnight fasting. Leukocytes were subjected to oxidative injuries by exposure to t-butylhydroperoxide, and were labeled with fluorochromes for measuring mitochondria transmembrane potential (delta psi m), membrane peroxidation and mitochondrial oxidant formation. delta psi m declined after t-butylhydroperoxide exposure, and the change was more prominent in leukocytes from older individuals. Cyclosporin A partly restored delta psi m, implying the contributing role of mitochondrial permeability transition pores. The mitochondrial depolarization was accompanied by increased oxidant formation and oxidation of pyridine nucleotides, which were more prominent in older subjects. The results support the view that the bioenergetic functions of mitochondria are more susceptible to oxidative injury in aged individuals. The decreased ability of leukocytes to resist oxidative stress may contribute to immunosenescence in humans.

Mitochondrial decay in hepatocytes from old rats: membrane potential declines, heterogeneity and oxidants increase

Mitochondrial function during aging was assessed in isolated rat hepatocytes to avoid the problem of differential lysis when old, fragile mitochondria are isolated. Rhodamine 123, a fluorescent dye that accumulates in mitochondria on the basis of their membrane potential, was used as a probe to determine whether this key function is affected by aging. A marked fluorescent heterogeneity was observed in hepatocytes from old (20-28 months) but not young (3-5 months) rats, suggesting age-associated alterations in mitochondrial membrane potential, the driving force for ATP synthesis. Three distinct cell subpopulations were separated by centrifugal elutriation; each exhibited a unique rhodamine 123 fluorescence pattern, with the largest population from old rats having significantly lower fluorescence than that seen in young rats. This apparent age-associated alteration in mitochondrial membrane potential was confirmed by measurements with radioactive tetraphenylphosphonium bromide. Cells from young rats had a calculated membrane potential of -154 mV, in contrast to that of the three subpopulations from old rats of -70 mV (the largest population), -93 mV, and -154 mV. Production of oxidants was examined using 2',7'dichlorofluorescin, a dye that forms a fluorescent product upon oxidation. The largest cell subpopulation and a minor one from old animals produced significantly more oxidants than cells from young rats. To investigate the molecular cause(s) for the heterogeneity, we determined the levels of an age-associated mtDNA deletion. No significant differences were seen in the three subpopulations, indicating that the mitochondrial decay is due to other mutations, epigenetic changes, or both.

Relationship between cellular accumulation of rhodamine 123 (R123) and cytotoxicity in B16 melanoma cells

Rhodamine 123 (R123) is a mitochondria-specific prototype anticancer agent because its target is the energy-producing mechanism of the cell. The goal of this study was to investigate the relationship between intracellular R123 accumulation and cytotoxicity in a R123-sensitive cell line (RS) and a R123-resistant subline (RR) that we developed. Cytotoxicity after exposure to R123 (0-60 micrograms/ml) was assessed using the clonogenic assay. Intracellular R123 was extracted with acid-alcohol and measured by fluorimetry. The rate of R123 accumulation over 1 hr was significantly higher (P less than 0.0001) for RS cells (4.65 +/- 0.39 micrograms/min/10(6) cells) than for RR cells (1.29 +/- 0.24 micrograms/min/10(6) cells). R123 accumulation in RS cells was strongly correlated (r = 0.80; P less than 0.0001) with cytotoxicity. Treatment of RR cells with verapamil (100 microM) reversed R123 resistance. The resulting dose-survival curve was identical to the dose-response curve of RS cells treated with R123 alone. Cellular content of R123 in RR cells treated with verapamil increased to a level similar to that of RS cells and correlated with cytotoxicity. These data suggest that cytotoxicity of R123 in B16 cells results from increased cellular accumulation of R123.