The levels of 8-oxo-2-deoxyguanosine (oxo8dG) in DNA isolated from tissues of rodents (male F344 rats, male B6D2F1 mice, male C57BL͞6 mice, and female C57BL͞6 mice) of various ages were measured using sodium iodide to prevent oxidative damage to DNA during DNA isolation. Oxo8dG was measured in nuclear DNA (nDNA) isolated from liver, heart, brain, kidney, skeletal muscle, and spleen and in mitochondrial DNA (mtDNA) isolated from liver. We observed a significant increase in oxo8dG levels in nDNA with age in all tissues and strains of rodents studied. The age-related increase in oxo8dG in nDNA from old mice was shown not to the result of the tissue's reduced ability to remove the oxo8dG lesion. Rather, the increase in oxo8dG levels appears to arise from an age-related increase in the sensitivity of these tissues to oxidative stress. We also observed an age-related increase in oxo8dG in mtDNA isolated from the livers of the rats and mice. Dietary restriction, which is known to retard aging and increase the lifespan of rodents, was shown to significantly reduce the agerelated accumulation of oxo8dG levels in nDNA in all tissues of male B6D23F1 mice and in most tissues of male F344 rats. Our study also showed that dietary restriction prevented the age-related increase in oxo8dG levels in mtDNA isolated from the livers of both rats and mice. T he oxidative stress hypothesis of aging (or the free radical hypothesis as it was first proposed) is currently one of the most popular explanations for how aging occurs at the biochemical level. The basic tenet of the oxidative stress hypothesis is that the age-related loss of physiological function and aging are because of the progressive and irreversible accumulation of oxidative damage (1). Over the past decade, the oxidative stress hypothesis of aging has gained wide acceptance because numerous studies have shown a strong correlation between increasing age and the accumulation of oxidative damage to cellular macromolecules (2, 3) and because the increased survival observed with dietary restriction has been correlated to reduced oxidative damage (3,4). It also appears that certain types of pathological lesions that arise with age are associated with increased levels of oxidative damage to cellular macromolecules (5, 6).In 1990, Ames' laboratory reported the first data on the effect of aging on DNA oxidation (7). They observed a significant (Ϸ2-fold) increase in 8-oxo-2-deoxyguanosine (oxo8dG) levels in nuclear DNA (nDNA) isolated from liver, kidney, and intestine of male rats between 2 and 24 months of age. Later, Ames et al. (5) reported that the levels of oxo8dG in mitochondrial DNA (mtDNA) isolated from male rat liver increased 2-to 3-fold with age. Since 1990, a number of research groups have observed an age-related increase in the level of oxo8dG in both nDNA and mtDNA in a variety of tissues of rats and mice (8).On the other hand, many investigators have been unable to detect a significant increase in DNA oxidation in rodent tissues with increasing age (9). The most lik...

The phenotypically similar hamster mutants irs1 and irs1SF exhibit high spontaneous chromosome instability and broad-spectrum mutagen sensitivity, including extreme sensitivity to DNA cross-linking agents. The human XRCC2 and XRCC3 genes, which functionally complement irs1 and irs1SF, respectively, were previously mapped in somatic cell hybrids. Characterization of these genes and sequence alignments reveal that XRCC2 and XRCC3 are members of an emerging family of Rad51-related proteins that likely participate in homologous recombination to maintain chromosome stability and repair DNA damage. XRCC3 is shown to interact directly with HsRad51, and like Rad55 and Rad57 in yeast, may cooperate with HsRad51 during recombinational repair. Analysis of the XRCC2 mutation in irs1 implies that XRCC2's function is not essential for viability in cultured hamster cells.

Glutathione peroxidase 4 (Gpx4) is uniquely involved in the detoxification of oxidative damage to membrane lipids. Our previous studies showed that Gpx4 is essential for mouse survival and that Gpx4 deficiency makes cells vulnerable to oxidative injury. In the present study, we generated two lines of transgenic mice overexpressing Gpx4 (Tg(GPX4) mice) using a genomic clone containing the human GPX4 gene. Both lines of Tg-(GPX4) mice, Tg5 and Tg6, had elevated levels of Gpx4 (mRNA and protein) in all tissues investigated, and overexpression of Gpx4 did not cause alterations in activities of glutathione peroxidase 1, catalase, Cu/Zn superoxide dismutase, and manganese superoxide dismutase. The human GPX4 transgene rescued the lethal phenotype of null mutation of the mouse Gpx4 gene, indicating that the transgene can replace the essential role of mouse Gpx4 in mouse development. Cell death induced by t-butylhydroperoxide and diquat was significantly less in murine embryonic fibroblasts from Tg(GPX4) mice compared with wild type mice. Liver damage and lipid peroxidation induced by diquat were reduced significantly in Tg(GPX4) mice. In addition, diquat-induced apoptosis was decreased in Tg(GPX4) mice, as evidenced by attenuated caspase-3 activation and reduced cytochrome c release from mitochondria. These data demonstrate that Gpx4 plays a role in vivo in the mechanism of apoptosis induced by oxidative stress that most likely occurs through oxidative damage to mitochondrial phospholipids such as cardiolipin.Reactive oxygen species (ROS), 1 such as superoxide and hydrogen peroxide, are constantly generated in aerobic organisms during normal respiration. In addition, environmental factors (such as ionizing radiation) and pathological compounds (such as ␤-amyloid in Alzheimer's disease) can generate ROS. Although ROS at physiological concentrations may be required for normal cell function, excessive production of ROS can be detrimental to cells, because ROS can cause oxidative damage to lipids, protein, and DNA. Polyunsaturated fatty acids, which are found predominantly in cellular membranes, are especially vulnerable to attack by ROS because of the high concentration of allylic hydrogens in their structure (1). The resulting lipid hydroperoxides can affect membrane fluidity and the function of membrane proteins. In addition, lipid hydroperoxides can undergo iron-mediated, one-electron reduction and oxygenation to form epoxyallylic peroxyl radicals, which trigger a chain reaction of free radical-mediated lipid peroxidation (2). The end-products of lipid peroxidation are reactive aldehydes such as 4-hydroxyl nonenal and malondialdehyde, many of which are highly toxic to cells (3). In addition, reactive aldehydes generated by lipid peroxidation can attack other cellular targets, such as proteins and DNA, thereby propagating the initial damage in cellular membranes to other macromolecules. Because lipid hydroperoxides formed in membranes are an important component of ROS generation in vivo, their detoxification appears to ...

Five percent of live-born human offspring will have a genetic disorder. Of these, 20% are because of germ-line de novo mutations. Several genetic diseases, such as neurofibromatosis and Duchenne muscular dystrophy, are associated with a high percentage of de novo germ-line mutations. Until recently, a direct analysis of spontaneous mutation frequencies in mammalian germ cells has been prevented by technical limitations. We have measured spontaneous mutation frequencies in a lacI transgene by using enriched populations of specific spermatogenic cell types. Similar to previously published results, we observed a lower mutation frequency for seminiferous tubule cell preparations, which contain all stages of spermatogenesis, relative to somatic tissues. We made the unexpected observation of a decline in mutation frequency during spermatogenesis, such that the mutation frequencies of type B spermatogonia and all subsequent stages of spermatogenesis are lower than the frequency for primitive type A spermatogonia. In addition, spermatogenic cells from old mice have significantly increased mutation frequencies compared with spermatogenic cells from young or middle-aged mice. Finally, the mutation frequency was observed to increase during spermiogenesis in postreplicative cell types when spermatogenic cells were obtained from old mice.From a genetic perspective, germ cells are profoundly different from somatic cells because they carry the genetic information that will direct the development of the next generation, not simply the next daughter cell. Thus, safeguarding the integrity of germ-line DNA might provide evolutionary advantages. Indeed, in mice, mutation frequencies obtained from mixed populations of germ cells are lower than for somatic tissues (1). This was demonstrated by using a transgenic system in which the bacteriophage genome carrying the lacI repressor gene and the ␣lacZ gene from the prokaryotic lac operon was introduced into the mouse genome as a transgene. DNA was recovered from genomic DNA preparations by packaging and used to infect a strain of Escherichia coli carrying a lacZ (␤-galactosidase) gene, but lacking a functional lacI gene. Mutation of the lacI gene renders a blue plaque on agarose containing the chromogenic substrate 5-bromo-4-chloro-3-indolyl-␤-D-galactopyranoside (X-gal). In contrast, other studies using a lacI transgenic mouse (2) or a lacZ transgenic mouse (3) did not report a significant difference in mutation frequencies for spermatogenic cells compared with somatic cells.Although provocative, interpretation of the results demonstrating a lower mutation frequency for male germ cells is complicated by the fact that adult seminiferous tubules contain a mixture of spermatogenic cell types encompassing all stages of spermatogenesis. Spermatogonia serve as the stem cells for spermatogenesis and undergo mitotic divisions that give rise to cells that will either retain their identity as spermatogonia to maintain the stem cell population or enter meiosis to become primary spermatocy...