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LOWERING
OF THE RATE OF GENE DAMAGE |
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Department of Genetics and Medical
Genetics, University of Wisconsin, 5302B Genetics building,
445 Henry Mall, Madison, WI 53706, USA.
To examine molecular events associated
with aging and its retardation by caloric restriction
(CR), we have employed high-density oligonucleotide microarrays
to define transcriptional patterns in mouse tissues, including
skeletal muscle, brain, heart, and adipose. Aging results
in a differential gene expression pattern specific to
each tissue, and most alterations can be completely or
partially prevented by CR. Transcriptional patterns of
tissues from calorie-restricted animals suggest that CR
retards the aging process by reducing endogenous damage
and by inducing metabolic shifts associated with specific
transcriptional profiles. These studies demonstrate that
DNA microarrays can be used in aging research to generate
panels of hundreds of transcriptional biomarkers, providing
a new tool to measure biological age on a tissue-specific
basis and to evaluate interventions designed to mimic
the effects of CR.
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Department of Biochemistry, University
of California, Riverside, CA 92521, USA.
It is widely held that caloric
restriction (CR) extends lifespan by preventing or reducing
the age-related accumulation of irreversible molecular
damage. In contrast, our results suggest that CR can act
rapidly to begin life and health span extension, and that
its rapid genomic effects are closely linked to its health
effects. We found that CR begins to extend lifespan and
reduce cancer as a cause of death within 8 weeks in older
mice, apparently by reducing the rate of tumor growth.
Further, 8 weeks of CR progressively reproduces nearly
three quarters of the genomic effects of long-term CR
(LTCR) in liver. Fewer of the genomic effects of LTCR
are rapidly reproduced by the initiation of CR in the
heart, but the changes produced are keys to cardiovascular
health. Thus, the genomic effects of CR may be established
more rapidly in mitotic than in postmitotic tissues. Most
of the genomic effects of LTCR dissipate 8 weeks after
switching to a control diet. Consistent with these results,
others have shown that acute CR rapidly and reversibly
reduces the short-term risk of death in Drosophila to
that of LTCR treated flies. Further, in late adulthood,
acute CR partially or completely reverses age-related
alterations of liver, brain and heart proteins. CR also
rapidly and reversibly mitigates biomarkers of aging in
adult rhesus macaques and humans. These data argue that
highly conserved mechanisms for the rapid and reversible
enhancement of life- and health-span exist for mitotic
and postmitotic tissues.
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Burzynski Clinic, 9432 Old Katy Road,
Houston, TX 77055, USA.
According to the author's theory
of gene silencing, the key process in aging involves reduced
expression of a number of genes. Silencing of genes has
a complex mechanism, which involves methylation of DNA,
histone modification and chromatin remodeling. In addition
to deacetylation of the histones and methylation of DNA,
recently described RNAi mechanism could initiate formation
of silenced chromatin. Hypermethylation of the promoter
will silence the gene. Genome-wide hypomethylation will
induce genomic instability, amplification of oncogenes
and also silencing of the genes through RNAi mechanism.
Studies by different groups, conducted in yeast, worms,
flies and mice, confirmed substantial changes in gene
expression in aging. Among them, the most important was
silencing of tumor suppressors and other genes involved
in the control of cell cycle, apoptosis, detoxification,
and cholesterol metabolism. There was also increased expression
of the smaller group of oncogenes and other genes which
are associated with typical diseases of old age. Caloric
restriction normalizes expression of a substantial percentage
of these genes. Animal studies confirmed importance of
caloric restriction, which decreases signaling through
the IGF-1/AKT pathway and expression of gene p53. These
studies, however, cannot be directly applied to human
aging. It is proposed that age management therapy should
attempt to normalize gene expression in the older population
to the level typical for young adults. This would require
activation of silenced genes and normalization of overexpressed
genes. Caloric restriction and exercise are helpful in
decreasing the activity of important oncogenes and activation
of silenced tumor suppressors, and may have a positive
impact, not only on aging, but also on prevention of cancer.
Dietary supplements containing phytochemicals should normalize
increased expression of oncogenes. Examples are: genistein
and EGCG, which effect signaling through the IGF-1/AKT
pathway and resveratrol and limonen, which do so through
the RAS pathway. A group of amino acid derivatives and
organic acids of animal and human origin should activate
silenced tumor suppressor genes (Aminocare A10, Aminocare
Extra). Among them 3-phenylacetylamino-2, 6-piperidinedione
intercalates specifically with DNA and protects sequences
of tumor suppressor genes, which are vulnerable to the
effects of carcinogens. Phenylacetate activates p53 and
p21 through inhibition of methyltransferase and farnesylation
of the RAS protein. Phenylbutyrate activates tumor suppressor
genes through inhibition of histone deacetylation. Phenylacetylglutamine
decreases genomic instability and expression of oncogenes
and promotes apoptosis. The application of DNA microarray
techniques to human studies should provide more information
about differences in gene expression in different age
groups and help design more effective age management regimens.
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BioMarker Pharmaceuticals, Incorporated,
900 East Hamilton Avenue, Campbell, CA 95008, USA.
Caloric restriction (CR), the consumption
of fewer calories while avoiding malnutrition, decelerates
the rate of aging and the development of age-related diseases.
CR has been viewed as less effective in older animals
and as acting incrementally to slow or prevent age-related
changes in gene expression. Here we demonstrate that CR
initiated in 19-month-old mice begins within 2 months
to increase the mean time to death by 42% and increase
mean and maximum lifespans by 4.7 (P = 0.000017) and 6.0
months (P = 0.000056), respectively. The rate of age-associated
mortality was decreased 3.1-fold. Between the first and
second breakpoints in the CR survival curve (between 21
and 31 months of age), tumors as a cause of death decreased
from 80% to 67% (P = 0.012). Genome-wide microarray analysis
of hepatic RNA from old control mice switched to CR for
2, 4, and 8 weeks showed a rapid and progressive shift
toward the gene expression profile produced by long-term
CR. This shift took place in the time frame required to
induce the health and longevity effects of CR. Shifting
from long-term CR to a control diet, which returns animals
to the control rate of aging, reversed 90% of the gene
expression effects of long-term CR within 8 weeks. These
results suggest a cause-and-effect relationship between
the rate of aging and the CR-associated gene expression
biomarkers. Therefore, therapeutics mimicking the gene-expression
biomarkers of CR may reproduce its physiological effects.
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Endocrinology Division, Mayo Clinic,
Rochester, Minnesota 55905, USA.
Rodent skeletal muscle mitochondrial
DNA has been shown to be a potential site of oxidative
damage during aging. Caloric restriction (CR) is reported
to reduce oxidative stress and prolong life expectancy
in rodents. Gene expression profiling and measurement
of mitochondrial ATP production capacity were performed
in skeletal muscle of male rats after feeding them either
a control diet or calorie-restricted diet (60% of control
diet) for 36 wk to determine the potential mechanism of
the beneficial effects of CR. CR enhanced the transcripts
of genes involved in reactive oxygen free radical scavenging
function, tissue development, and energy metabolism while
decreasing expression of those genes involved in signal
transduction, stress response, and structural and contractile
proteins. Real-time PCR measurements confirmed the changes
in transcript levels of cytochrome-c oxidase III, superoxide
dismutase (SOD)1, and SOD2 that were noted by the microarray
approach. Mitochondrial ATP production and citrate synthase
were unaltered by the dietary changes. We conclude that
CR alters transcript levels of several genes in skeletal
muscle and that mitochondrial function in skeletal muscle
remains unaltered by the dietary intervention. Alterations
in transcripts of many genes involved in reactive oxygen
scavenging function may contribute to the increase in
longevity reported with CR.
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Department of Nutrition & Food
Science, Wayne State University, Detroit, Michigan 48202,
USA.
Caloric restriction (CR) without
malnutrition is the only experimental manipulation that
has consistently been shown to increase the mean and maximum
lifespan of laboratory rodents. It has been suggested
that CR extends the longevity of rodents and reduces the
incidence of age-related pathological lesions by reducing
the levels of DNA damage and mutations that accumulate
with age within a cells genome. This hypothesis is attractive
because the integrity of the genome is essential to a
cell/organism and because it is supported by the observations
that both cancer and immunological defects, which increase
significantly with age and are delayed by CR, are associated
with changes in DNA damage. However, all the evidence
supporting the premise that the accumulation of DNA damage/mutations
plays a role in aging and CR is correlative, i.e., the
anti-aging action of CR-fed rodents is correlated with
decreased DNA damage and mutation and increased DNA repair
capacity. Therefore, additional experiments are required
which employ more accurate assays of the DNA repair pathways
as well as genetically engineered animal models to establish
the role of specific DNA repair pathways and/or enzymes
in the anti-aging action of CR. In this paper, we review
the proposed mechanisms of DNA damage/repair while providing
insight into current research that may assist in "unlocking"
the mechanisms behind the life-prolonging effect of CR.
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