Autofluorescent structures in cultured WI-38 cells

The concept of telomeric non-reciprocal recombination (TENOR) applied to human fibroblasts grown in serial cultures: concordance with genealogical data

Since the discovery of the limited life span of human fibroblasts some 50 years ago, many genealogical studies have been undertaken to describe growth kinetics of fibroblasts in serial cultures by their individual division behavior. It is now accepted that proliferation capacities of human fibroblasts strongly depend on their telomere lengths and integrity. Telomeres shorten with each replication round, and there is a direct correlation between cell division capacity and telomere lengths; that is, the consumption of disposable telomeric DNA repeats during cell divisions progresses until critically short telomeres determining the replicative senescence of the cells are present. Recently, we have suggested that telomeres in fibroblasts can also become elongated during DNA replication by telomeric non-reciprocal recombination (TENOR). Here we discuss genealogical data collected over the last decades as well as more recent findings on the telomere-driven replicative senescence process, and we summarize both to give an integrated picture.

Antioxidant status in J774A.1 macrophage cell line during chronic exposure to glycated serum

Advanced glycation end-products (AGEs) are linked to aging and correlated diseases. The aim of present study was to evaluate oxidative stress related parameters in J774A.1 murine macrophage cells during chronic exposure to a subtoxic concentration of AGE (5% ribose-glycated serum (GS)) and subsequently for 48 h to a higher dose (10% GS). No effects on cell viability were evident in either experimental condition. During chronic treatment, glycative markers (free and bound pentosidine) increased significantly in intra- and extracellular environments, but the production and release of thiobarbituric acid reactive substances (TBARs), as an index of lipid peroxidation, underwent a time-dependent decrease. Exposure to 10% GS evidenced that glycative markers rose further, while TBARs elicited a cellular defence against oxidative stress. Nonadapted cultures showed an accumulation of AGEs, a marked oxidative stress, and a loss of viability. During 10% GS exposure, reduced glutathione levels in adapted cultures remained constant, as did the oxidized glutathione to reduced glutathione ratio, while nonadapted cells showed a markedly increased redox ratio. A constant increase of heat shock protein 70 (HSP70) mRNA was observed in all experimental conditions. On the contrary, HSP70 expression became undetectable for a longer exposure time; this could be due to the direct involvement of HSP70 in the refolding of damaged proteins. Our findings suggest an adaptive response of macrophages to subtoxic doses of AGE, which could constitute an important factor in the spread of damage to other cellular types during aging.Key words: in vitro cytotoxicity, AGE, pentosidine, glycoxidation, oxidative stress, TBARs.

Comparison of Colony-Formation Efficiency of Bovine Fetal Fibroblast Cell Lines Cultured with Low Oxygen, Hydrocortisone, L-Carnosine, bFGF, or Different Levels of FBS

A comparison of colony-formation efficiency (CFE) was made between six independent bovine fetal fibroblast (BFF) cell lines used in somatic cell nuclear transfer. Variation in CFE was assessed under different culture conditions. The conditions examined were ambient atmosphere (approximately 20% oxygen) culture versus 5% oxygen culture, three levels of fetal bovine serum (FBS) in the medium (5%, 10% or 20%), and the amendment of 10% FBS medium with basic fibroblast growth factor (1 ng/mL), L-carnosine (20 mM), or hydrocortisone (1 microM). The six BFF cell lines showed significant differences from one another in CFE. No significant difference in CFE was found with reduced oxygen culture. L-Carnosine also had no significant effect on CFE. A FBS concentration of 10% was found to produce the best overall CFE. Hydrocortisone treatment reduced the size of colonies although the number of colonies formed was not affected. Basic FGF increased the size of colonies but the number of colonies formed was not affected. The results showed that different BFF cell lines varied significantly in their CFE. Also, some medium supplements or culture conditions that have shown positive CFE effects on the fibroblasts of other species failed to show significant positive CFE effects on the BFF cell lines tested.

Damaging effects of advanced glycation end-products in the murine macrophage cell line J774A.1

The interaction of reducing sugars, such as aldose, with proteins and the subsequent molecular rearrangements, produces irreversible advanced glycation end-products (AGEs), a heterogeneous class of non-enzymatic glycated proteins or lipids. AGEs form cross-links, trap macromolecules and release reactive oxygen intermediates. AGEs are linked to aging, and increase in several related diseases. The aim of this study was to assess, in a murine macrophage cell line, J774A.1, the effects of 48 h of exposure to glycated serum containing a known amount of pentosidine, a well-known AGE found in the plasma and tissues of diabetic and uremic subjects. Fetal bovine serum was incubated with ribose (50 mM) for 7 days at 37 degrees C to obtain about 10 nmol/ml of pentosidine. The cytotoxic parameters studied were cell morphology and viability by neutral red uptake, lactate dehydrogenase release and tetrazolium salt test. In the medium and in the intracellular compartment, bound and free pentosidine were evaluated by HPLC, as sensitive and specific glycative markers, and thiobarbituric acid reactive substances (TBARs), as index of the extent of lipid peroxidation. Our results confirm that macrophages are able to take up pentosidine. It is conceivable that bound pentosidine is degraded and free pentosidine is released inside the cell and then into the medium. The AGE increase in the medium was combined with an increase in TBARs, meaning that an oxidative stress occurred; marked cytotoxic effects were observed, and were followed by the release of free pentosidine and TBARs into the culture medium.

Cell-associated pentosidine as a marker of aging in human diploid cells in vitro and in vivo

Cellular aging is characterized by alterations at both the morphological and molecular levels, some of which are decreased mitotic rate, increased cytoplasmic vacuolization, and changes in intrinsic cellular constituents (Stanulis-Praeger, 1987. Mech. Ageing Dev. 38, 1-48). In the present investigation, glycoxidation is studied as a marker for cellular aging by measuring cell-associated pentosidine levels in human skin fibroblasts as a function of replicative life span and in human peripheral blood T lymphocytes as a function of chronological age. Fibroblasts were isolated from culture by detachment/centrifugation while lymphocytes were isolated from blood by a Ficoll-Paque/Lympho-Kwik T-Cell Prep technique. Pentosidine levels were measured in acid-hydrolyzed cell pellet suspensions by high-pressure liquid chromatography. Results show that pentosidine was detected in early and late cultured reticular and papillary fibroblasts. Pentosidine, expressed as either protein, DNA, or cell number, significantly (P < 0.0006) increased with in vitro passage and was significantly (P < 0.01) related to cell proliferation as measured by cell density and cell doublings per day during culture. Cell-associated pentosidine was measured in T lymphocytes isolated from healthy, diabetic, and uremic individuals. In healthy controls, levels significantly (P < 0.0003) increased with age. In uremic individuals, a large variation was observed with many values above the 95% confidence intervals determined for controls. Since a previous study showed that plasma pentosidine in healthy subjects does not increase with age, these results suggest that cellular turnover perhaps coupled to a deterioration in cellular anti-glycoxidation defensive mechanisms play a substantial role in explaining increased pentosidine concentrations during cellular aging.

Spermine prevent iron accumulation and depress lipofuscin accumulation in cultured myocardial cells

The present work demonstrates that spermine prevents aging in cultured myocytes exposed to oxidative stress. It is found that physiological levels of spermine reduce lipofuscin accumulation with 20%, and that the antioxidative effect compares with vitamin E. By autometallography we also demonstrate that spermine prevent accumulation of free iron in the myocytes, probably by acting as a chelating agent. The effect compares to that of deferoxamine. These data provide additional insight into the antioxidative mechanism of spermine, and suggest that spermine may prevent diseases related to the Fenton reaction, as well as retard aging reactions.

Indenoindole depresses lipofuscin formation in cultured neonatal rat myocardial cells

Lipofuscin accumulation in cultured rat myocardial cells is considered an index of intra-lysosomal oxidative reactions and was registered by autofluorescence measurements. Lipofuscinogenesis in secondary lysosomes is thought to be a consequence of Fenton reactions, and is much enhanced by oxidative stress obtained by culturing the cells in an atmosphere containing 40% oxygen. The influence of the synthetic antioxidant indenoindole (DHII), as compared to control cells, was a dose-responsive depression of lipofuscinogenesis to a degree of 19% and 17% with 20 microM DHII and to 25% and 23% with 40 microM DHII after 7 and 14 days in culture, respectively. This demonstrates a significant quenching of oxidative stress and suggests the therapeutic value of DHII and related antioxidants in protecting against oxygen radical-related diseases. It is also suggested that neonatal cardiac myocytes in culture are a suitable model system for the evaluation of oxygen radical-induced myocardial damage.

Oxidants and antioxidants in proliferative senescence

In terms of the amount of experimental research it has generated the free radical theory of ageing is one of the most popular hypotheses to explain this ubiquitous phenomenon. From the theory two postulates were derived: either cellular defence mechanisms against free radical-dependent oxidants deteriorate during ageing of cells, or essential, unrepairable damages are imparted to the cell by oxidants regardless of the activity of antioxidant defence systems. The many reports dealing with a putative breakdown in antioxidant defence systems failed to positively support this postulate. However, a minor depletion in cellular glutathione by exposure to a model lipophilic peroxide led to a significant decrement in DNA and protein synthesis. In other words, the glutathione redox cycle is intrinsically fallible with respect to defending the cellular DNA replication system against this model lipophilic peroxide. Interestingly, after ageing in culture cells a partial uncoupling of the NADPH-producing and -consuming systems tends to take place. Experiments involving the addition of antioxidants to the culture medium have failed to significantly extend the lifespan of cultured diploid somatic cells. The level of antioxidants appears to be a modulator rather than a primary determinant of cellular ageing in culture. Several lines of evidence suggest that DNA damages accumulate during ageing of the organism, but no oxidant-related DNA damage has been pinpointed in the cultured cell system. Human mutants with defects in antioxidant enzymes have not shown conclusive signs of accelerated ageing. Cells from patients with Werner's syndrome (progeria of the adult), on the other hand, do not suffer from a defect in their antioxidant defence system, nor do they accumulate more than normal amounts of autofluorescent products resulting from lipid peroxidation. The recent finding that Werner's syndrome constitutes a mutator phenotype may prompt the comparison of oxidant- and ageing-related mutation spectra in order to investigate a mutational theory of ageing as a new derivative from the free radical hypothesis.

Biochemical, genetic and ultrastructural defects in a mitochondrial mutant (ER-3) of Neurospora crassa with senescence phenotype

The structural and functional abnormalities in a new respiratory deficient, mitochondrial senescence mutant ER-3 of Neurospora crassa are described. The mitochondrial mutant, which grows at a rate of only 10% of that of the wild type, was found deficient in all three cytochromes, and completely lacking in cytochromes aa3. Cytochrome oxidase activity in the mutant mitochondria was only about 5% of the wild type mitochondria. However, the total whole cell respiration rate of the mutant was 33% greater than that of the wild type, while the cyanide-resistant respiration rates were equal. The results of inhibitor studies clearly demonstrate that the mutant possesses a defect in one or more components of the terminal oxidase. Electron microscopic examination of whole cell sections and subsequent morphometric analysis revealed a significant (33%) reduction in membrane surface density of mitochondrial cristae in the mutant as compared with the wild type. Results of genetic and heterokaryon analyses indicate the location of mutation (ER-3) in the mitochondrial DNA. It is concluded that the senescence mutant ER-3 possesses a defect in the terminal portion of the mitochondrial respiratory apparatus. These results are consistent with previous analyses of mitochondrial DNA populations, and support the notion that obligately aerobic eukaryotic cells deficient in mitochondrial respiration necessarily exist as a result of stable heteroplasmosis and that defects in mitochondria lead to senescence in Neurospora mutant ER-3.

The waste-product theory of aging: transformation to unlimited growth in cell cultures

A differential equation governing intracellular waste content is solved numerically to determine the circumstances under which the growth of an in vitro cell population is limited. Parameter values derived from data on human glial cell cultures are employed. It is assumed that a) waste accumulation depresses the rate of cellular reproduction and b) intracellular waste is diluted by cell division, but is not otherwise eliminated. Population size depends upon two parameters: the rate of waste production and the rate of cell division in the absence of waste. If the rate of waste production is sufficient, the population size approaches an asymptote as in phase III growth in vitro. If a lower rate of waste production allows the cells to outmultiply the waste, growth is unlimited as in a transformed cell population. The asymptotic population size and the threshold for unlimited growth are remarkably sensitive to small changes in the values of the two rate parameters unless the ratio of their values is constant. This suggests that there may be a cellular mechanism that relates the waste production and cell division rates.

Cellular senescence revisited: a review

The field of cellular senescence (cytogerontology) is reviewed. The historical precedence for investigation in this field is summarized, and placed in the context of more recent studies of the regulation of cellular proliferation and differentiation. The now-classical embryonic lung fibroblast model is compared to models utilizing other cell types as well as cells from donors of different ages and phenotypes. Modulation of cellular senescence by growth factors, hormones, and genetic manipulation is contrasted, but newer studies in oncogene involvement are omitted. A current consensus would include the view that the life span of normal diploid cells in culture is limited, is under genetic control, and is capable of being modified. Finally, embryonic cells aging in vitro share certain characteristics with early passage cells derived from donors of increasing age.

Oxidative damage to DNA and replicative lifespan in cultured adrenocortical cells

Oxidative damage to DNA in cultured bovine adrenocortical cells was investigated by exposing cells to a sublethal concentration (10 microM) of cumene hydroperoxide under conditions previously shown to be deficient in the biological antioxidants selenium and alpha-tocopherol (vitamin E). DNA prepared from cells incubated for 4 h with 10 microM cumene hydroperoxide had a greater fraction showing resistance to S1 nuclease after denaturation and reassociation to a log C0t of -3. Cross-linking by cumene hydroperoxide was abolished in cells that had been grown in the presence of 20 nM selenite or 1 microM alpha-tocopherol for 96 h prior to peroxide addition, whereas such cells remained susceptible to cross-linking by nitrogen mustard. Extensive strand breaks in DNA from peroxide-treated cells as assessed by alkaline sucrose gradient centrifugation were greatly reduced in cells grown in selenite or alpha-tocopherol. Despite the evidence of damage to DNA, cumene hydroperoxide was not detectably mutagenic, in contrast to 5 microM methylnitronitrosoguanidine (MNNG), when assessed as the incidence of resistance to 25 microM ouabain. We confirmed that cumene hydroperoxide at greater than 10 microM lowers cloning efficiency and that this is largely prevented by selenite or alpha-tocopherol. Additionally, selenite or alpha-tocopherol produced increased clonogenicity in cells not incubated with peroxide. To examine effects of the biological antioxidants on replicative lifespan, cells were grown continuously in fetal bovine serum (FBS), fibroblast growth factor (FGF), and selenite or alpha-tocopherol. Selenium increased replicative lifespan by 10-20% and alpha-tocopherol by 22-30%. Levels of DNA cross-links and strand breaks did not differ under any circumstances between early (second) passage and late (30th) passage cells. The experiments on replicative potential were all performed in the presence of FGF. When FGF was omitted from the culture medium, replicative lifespan was reduced by 85%. We conclude that types of damage to DNA resulting from peroxide exposure are not present in cells under standard culture conditions at early or late stages of the lifespan. Other work has noted a relationship between clonogenicity and replicative lifespan; thus, the increase in cloning efficiency seen with selenium and alpha-tocopherol may cause the observed slight increase in replicative lifespan. Oxidative damage does not appear to be a major determinant of cellular senescence in adrenocortical cells.

Autofluorescence and ageing: changes in ribosome accuracy and lysosome [corrected] function

Autofluorescence increased with decreasing proliferative capacity of fibroblast cultures. It also increased rapidly when cultures were treated with the aminoglycosides, streptomycin, paromomycin and G418, as well as with ammonium chloride. All these compounds are known to accumulate in lysosomes. Paromomycin is known to reduce proliferative capacity and G418 was found to have a similar effect. As these aminoglycosides accumulate in the lysosomes, promote errors in protein synthesis and reduce proliferative capacity this has led us to suggest that such compounds may cause a loss of homeostasis between erroneous protein synthesis and lysosome function. The increase in autofluorescence may be a consequence of reduced lysosome function arising from lack of control of the intralysosomal pH.

The waste-product theory of aging: cell division rate as a function of waste volume

The rate of cell division is calculated as a function of waste product volume in U-787CG human diploid glial cells grown in vitro. The calculation is based on two earlier mathematical models. One is a compartmental analysis in which cell division rate is obtained from data on the fraction of cells which become sterile as the passage level increases. A second model is used to calculate the amount of waste per cell from the observed rate of waste accumulation in a non-dividing population and from the division rate calculated with the use of the first model. Results from the two models are correlated to obtain the desired function relating cell division rate to waste volume. If cellular aging is taken to mean loss of the ability of cells to divide, and if, as in the waste-product theory, this loss is attributed to waste accumulation, the calculated results show that aging is evident at waste levels well below those at which non-dividing populations can survive. Thus the process of cell division may be much more sensitive to waste accumulation than other cellular processes needed for the maintenance of life.

Accumulation of a high molecular weight glycoprotein during in vitro ageing and contact inhibition of growth

A 240 000 molecular weight protein was found to accumulate in sorted autofluorescent (AF) cells, and during growth inhibition and in vitro ageing of cultures of human skin fibroblasts. Vitamin E, a lipophilic free radical scavenger which suppressed completely the formation of cellular autofluorescence, did not affect the accumulation of this protein. So, this accumulation is not related to cellular autofluorescence and lipid peroxidation, the major cause of this autofluorescence. This protein was also found in cells from a patient with the Spielmeyer-Vogt syndrome with a high percentage of maximal lifespan (MLS), while it was completely absent from all cells of a patient with Werner's syndrome. On two-dimensional gel electrophoresis the protein showed a heterogeneous acidic isoelectric point (IEP) of around 5.3. Neuraminidase treatment caused the IEP of this protein to shift towards a less acidic pH value (5.85). Upon differential centrifugation of a cell homogenate the protein was found to be located in the microsomal pellet and the cytosol. Chromatography on gelatin-sepharose revealed that the protein was not fibronectin. It is concluded that in human skin fibroblasts a high molecular weight glycoprotein accumulates as a result of impaired proliferation and that this accumulation is not related to cellular lipid peroxidation.

Glutathione content of cultured human fibroblasts during in vitro ageing

The glutathione level of cultured human fibroblasts was determined with a micromodification of a spectrophotometric glutathione cycling method. There was a slight increase in reduced glutathione (GSH) content during in vitro ageing of normal human fibroblasts. Fibroblasts from patients with Werner's syndrome or ceroid lipofuscinosis (Spielmeyer-Vogt syndrome) and healthy individuals exhibited similar patterns of GSH levels during in vitro ageing. The GSH content of non-proliferating confluent cultures of normal fibroblasts and of proliferating normal fibroblasts was identical. Moreover, autofluorescent "aged" cells isolated by cell sorting did not differ in GSH content from the non-autofluorescent cells in the same culture. It was concluded that the GSH content does not play a role in in vitro ageing, nor in the accumulation of autofluorescent material in human skin fibroblasts.

Cell sorting and microchemistry of cultured human fibroblasts: applications in genetics and aging research

Cell sorting is a way to isolate viable subpopulations of cells present in a mixture. The drawback of the isolation method is the shortage of material for subsequent biochemical determinations. We have employed a combination of (ultra-) microchemistry and cell sorting to overcome this problem. The methods enable determinations of protein and several enzyme activities on triton extracts of 5,000-10,000 sorted cells. In addition, using ultramicromethods we could determine enzyme activity in single sorted cells. This combination of methods is used for clinical genetic studies on heterozygote detection in Fabry's disease, an X-linked genetic disease. Moreover, microchemistry is used to study enzyme activities in sorted autofluorescent "aged" fibroblasts.

Autofluorescence as an index of ageing in human fibroblasts in culture

An exponential increase in autofluorescence (AF) during serial passaging of human diploid fibroblasts, strain MRC-5, was observed using a fluorescence-activated cell sorter. Skin fibroblasts from patients with premature-ageing syndromes had high levels of AF, whereas virus-transformed cell lines had negligible amounts of AF.

Reaction of malonaldehyde with mitochondrial membranes

Malonaldehyde formed by lipid oxidation is regarded as a main crosslinker in the formation of natural age pigment. To elucidate the mechanism of pigment formation the reaction of malonaldehyde with biomembranes using fluorescence spectroscopy has been studied. Rat liver mitochondrial ghosts or bovine serum albumin were reacted with malonaldehyde. In both cases two main fluorescence changes were observed: protein fluorescence decreased to 50% of its initial value in about two hours; aminoiminopropene fluorescence reached a maximum at a much slower rate. The kinetics support a two-step reaction hypothesis. First, malonaldehyde reacts with protein quenching its fluorescence. Next fluorescent interprotein aminoiminopropene (AIP) crosslinks are formed. The fluorescence lifetime value of the induced AIP fluorophore was shown to be similar to the lifetime of naturally occurring age pigment previously reported for mitochondrial ghosts prepared from aged animals (5.4 ns +/- 0.3 and 5.9 ns +/- 0.6, respectively).

The sensitivity to hyperbaric oxygen of human diploid fibroblasts during ageing in vitro

The effect of hyperbaric oxygen on human diploid fibroblasts, WI-38, at various passages was examined. Their colony-forming ability deteriorates on exposure to hyperbaric oxygen. The cellular injury is not due to the high pressure per se, but to oxygen toxicity, since compressed nitrogen has no effect on the cells. When the cells beyond mid-40 population doublings (lifespan completed more than about 75%) are exposed to 50 atm of oxygen, there is a great decrease in the efficiency of the colony formation. A double exposure to 75 atm of oxygen inhibits the growth of the late passage cells more strongly than that of the early passage cells. These results suggest that the late passage cells are more sensitive to oxygen than the early passage cells.

Superoxide dismutase specific activities in cultured human diploid cells of various donor ages

It has been postulated that superoxide dismutase (SOD) protects cells from free radical-induced damage. In these experiments SOD specific activity was measured as established human diploid cell lines from various donor ages progressed through their in vitro lifespan. Significant elevations in activity occurred during the in vitro lifespans of cells from fetal and newborn donors, but no change in activity was detected during the lifespan of cells from an adult donor. In addition, a direct relationship between enzyme activity and donor age was detected with the following relative activities: adult greater than newborn greater than fetal. The possible relationship between these findings and the free radical theory of aging is discussed.

The effect of oxygen and vitamin E on the lifespan of human diploid cells in vitro

Human diploid cells (WI-38) were serially subcultivated at partial pressures of oxygen (Po2) ranging from 5.6 mm Hg to 608 mm Hg. At a Po2 of 5.6 mm Hg, the number of doublings to phase out was less than that of control cells at a Po2 of 137 mm Hg. Cultures grown at Po2's of 24, 49, or 137 mm Hg grew at the same rate and phased out after a similar number of population doublings. Population lifespan was markedly shortened by chronic exposure to elevated Po2's, a phenomenon that was, in part, reversible. d-1-alpha-Tocopherol (10 microgram/ml or 100 microgram/ml) homogenized into the medium at each weekly subcultivation did not extend the lifespan of cells at reduced, ambient, or elevated oxygen tensions. These results indicate that neither oxygen toxicity nor free radical reactions play a significant role in limiting the lifespan of WI-38 cells grown in vitro under ambient oxygen tensions (Po2 137 mm Hg).

Lipid pigment formation in cultures of chick hepatocytes. Effects of aging and of oxidizable agents

Tissue cultures of hepatocytes were studied for the formation of lipid pigments as related to aging of the cultures and of the addition to them of: 1) colloidal iron; 2) L-ascorbic acid; and 3) tyrosine. The presence of lipid pigments was gauged by their sudanophilia after a light solvent extraction and yellow autofluorescence. It has been found that all the three agents enhanced pigmentogenesis. It has been concluded that both pro-oxidants and antioxidants may enhance lipid pigment formation: the first by enhancing lipid oxidation, the second by enhancing copolymerization which terminates the oxidative chain.

Ageing of Neurospora crassa. IV. Induction of senescence in wild type by dietary amino acid analogs and reversal by antioxidants and membrane stabilizers

The extensional growth rate of wild-type 74A8 N. crassa in the presence of various concentrations of 19 amino acid analogs was measured. Seven analogs were not inhibitory at concentrations in the range of one to 10mM. Of the remaining 12 analogs, nine inhibited growth in a novel way. The kinetics of growth in the presence of these analogs at 30 degrees were characterized by seven sequential phases: (1)lag; (2) acceleration of growth rate; (3) steady-state growth rate; (4) exponential rate of decline of growth rate; (5) no growth or growth rate less than or equal to 0.1 mm h-1; (6) accleration of growth rate; and (7) steady state. At 33 degrees, phases 6 and 7 did not occur and irreparable death of the clones occurred. The mechanism by which the clones acquired resistance at 30 degrees appeared to involve a combination of physiological adaptation and cellular selection. Dietary application of either free radical scavengers or surface-active membrane 'stabilizers' alleviated or prevented the inhibition and deterioration of growth rate which occurred in the presence of the nine amino acid analogs. Culture with either 4-fluorophenylalanine or ethionine led to an increase of the activities of antioxygenic enzymes glutathione peroxidase, glutathione reductase and superoxide dismutase. The amino acid analogs that cause senescence and death of growing cells are known to be incorporated into proteins and such proteins are generally abnormal. Because a substantial fraction of cellular protein occurs in membranes and the proteins synthesized by mitochondria are exclusively intrinsic membrane proteins, we suggest that a primary consequence of errors in protein synthesis is the production of faulty membranes. The deterioration of such membranes with associated lipid autoxidation and free radical production proceeding as a chain reaction at an exponential rate may in itself contribute to the exponential rate of cellular deterioration which is characteristic of the ageing process. According to this hypothesis, dietary membrane stabilizers, free radical scavengers and antioxygenic enzymes protect cells from error catastrophy arising from the chain of events leading from membrane deterioration.

Ageing of Neurospora crassa. I. Evidence for the free radical theory of ageing from studies of a natural-death mutant

A recessive mutant of Neurospora crassa, called natural-death, is characterized by a decreasing clonal growth potential under all nutritional conditions and the irreversible cessation of growth. The primary molecular defect of this mutant is not known. Evidence presented here, based upon measurements of the activities and thermolabilities of several enzymes, suggests that faulty protein synthesis is probably not a cause of the senescence and death of the mutant, as suggested by Lewis and Holliday (Nature, 228 (1970) 877). Three lines of evidence indicate that lipid autoxidation and associated free radical reactions contribute to the senescence and death of this mutant: (1) The relative times before the onset of senescence and death of mutant clones in the last 40% of their chronological life-span were prolonged 2 to 3-fold by either dietary antioxidants or selenite and the total life-span was increased by 40% to 80%. These compounds also alleviated the senescent morphology and enhanced biomass production; (2) Senescing clones accumulated a green fluorescent pigment in situ, but dietary antioxidant nordihydroguaiaretic acid prevented this accumulation. The fluorescent pigment exhibited the spectral properties of lipofuscin, an end product of lipid autoxidation; (3) Relative to wild type, mycelial extracts of the mutant exhibited a 2 to 4-fold excess of activities of the antioxygenic enzymes superoxide dismutase, glutathione peroxidase and glutathione reductase. We briefly review: (1) the roles of antioxygenic enzymes and antioxidants in their protection against cellular damage from lipid autoxidation and free radical reactions; and (2) the major lines of evidence which appear to support a form of the free radical theory of ageing, encompassing the interrelated processes of membrane deterioration, lipid autoxidation and deleterious free radical reactions as the major causes of cellular deterioration.