The free radical theory of aging (FRTA) states that organisms age because cells accumulate free radical damage over time. A free radical is any atom or molecule that has a single unpaired electron in an outer shell. While a few free radicals such as melanin are not chemically reactive, most biologically relevant free radicals are highly reactive. For most biological structures, free radical damage is closely associated with oxidative damage. Antioxidants are reducing agents, and limit oxidative damage to biological structures by passivating them from free radicals.
Strictly speaking, the free radical theory is only concerned with free radicals such as superoxide ( O2− ), but it has since been expanded to encompass oxidative damage from other reactive oxygen species such as hydrogen peroxide (H2O2), or peroxynitrite (OONO−).
Denham Harman first proposed the free radical theory of aging in the 1950s, and in the 1970s extended the idea to implicate mitochondrial production of reactive oxygen species.
In some model organisms, such as yeast and Drosophila, there is evidence that reducing oxidative damage can extend lifespan. However, in mice, only 1 of the 18 genetic alterations (SOD-1 deletion) that block antioxidant defenses, shortened lifespan. Similarly, in roundworms (Caenorhabditis elegans), blocking the production of the naturally occurring antioxidant superoxide dismutase has recently been shown to increase lifespan. Whether reducing oxidative damage below normal levels is sufficient to extend lifespan remains an open and controversial question.
Strictly speaking, the free radical theory is only concerned with free radicals such as superoxide ( O2− ), but it has since been expanded to encompass oxidative damage from other reactive oxygen species such as hydrogen peroxide (H2O2), or peroxynitrite (OONO−).
Denham Harman first proposed the free radical theory of aging in the 1950s, and in the 1970s extended the idea to implicate mitochondrial production of reactive oxygen species.
In some model organisms, such as yeast and Drosophila, there is evidence that reducing oxidative damage can extend lifespan. However, in mice, only 1 of the 18 genetic alterations (SOD-1 deletion) that block antioxidant defenses, shortened lifespan. Similarly, in roundworms (Caenorhabditis elegans), blocking the production of the naturally occurring antioxidant superoxide dismutase has recently been shown to increase lifespan. Whether reducing oxidative damage below normal levels is sufficient to extend lifespan remains an open and controversial question.
Background
The free radical theory of aging was conceived by Denham Harman in the 1950s, when prevailing scientific opinion held that free radicals were too unstable to exist in biological systems. This was also before anyone invoked free radicals as a cause of degenerative diseases. Two sources inspired Harman: 1) the rate of living theory,
which holds that lifespan is an inverse function of metabolic rate
which in turn is proportional to oxygen consumption, and 2) Rebbeca
Gershman's observation that hyperbaric oxygen toxicity and radiation toxicity could be explained by the same underlying phenomenon: oxygen free radicals.
Noting that radiation causes "mutation, cancer and aging", Harman
argued that oxygen free radicals produced during normal respiration
would cause cumulative damage which would eventually lead to organismal
loss of functionality, and ultimately death.
In later years, the free radical theory was expanded to include not only aging per se, but also age-related diseases. Free radical damage within cells has been linked to a range of disorders including cancer, arthritis, atherosclerosis, Alzheimer's disease, and diabetes.
There has been some evidence to suggest that free radicals and some
reactive nitrogen species trigger and increase cell death mechanisms
within the body such as apoptosis and in extreme cases necrosis.
In 1972, Harman modified his original theory. In its current form, this theory proposes that reactive oxygen species that are produced in the mitochondria, causes damage to certain macromolecules including lipids, proteins and most importantly mitochondrial DNA. This damage then causes mutations which lead to an increase of ROS production and greatly enhance the accumulation of free radicals within cells. This mitochondrial theory has been more widely accepted that it could play a major role in contributing to the aging process.
Since Harman first proposed the free radical theory of aging,
there have been continual modifications and extensions to his original
theory.
Processes
In chemistry, a free radical is any atom, molecule, or ion with an unpaired valence electron
Free radicals are atoms or molecules containing unpaired electrons. Electrons normally exist in pairs in specific orbitals in atoms or molecules.
Free radicals, which contain only a single electron in any orbital, are
usually unstable toward losing or picking up an extra electron, so that
all electrons in the atom or molecule will be paired.
Note that the unpaired electron does not imply charge - free radicals can be positively charged, negatively charged, or neutral.
Damage occurs when the free radical encounters another molecule
and seeks to find another electron to pair its unpaired electron. The
free radical often pulls an electron off a neighboring molecule, causing
the affected molecule to become a free radical itself. The new free
radical can then pull an electron off the next molecule, and a chemical chain reaction of radical production occurs.
The free radicals produced in such reactions often terminate by
removing an electron from a molecule which becomes changed or cannot
function without it, especially in biology. Such an event causes damage
to the molecule, and thus to the cell that contains it (since the
molecule often becomes dysfunctional).
The chain reaction caused by free radicals can lead to
cross-linking of atomic structures. In cases where the free
radical-induced chain reaction involves base pair molecules in a strand of DNA, the DNA can become cross-linked.
DNA cross-linking can in turn lead to various effects of aging, especially cancer. Other cross-linking can occur between fat and protein molecules, which leads to wrinkles. Free radicals can oxidize LDL, and this is a key event in the formation of plaque in arteries, leading to heart disease and stroke. These are examples of how the free-radical theory of aging has been used to neatly "explain" the origin of many chronic diseases.
Free radicals that are thought to be involved in the process of aging include superoxide and nitric oxide.
Specifically, an increase in superoxide affects aging whereas a
decrease in nitric oxide formation, or its bioavailability, does the
same.
Antioxidants
are helpful in reducing and preventing damage from free radical
reactions because of their ability to donate electrons which neutralize
the radical without forming another. Ascorbic acid,
for example, can lose an electron to a free radical and remain stable
itself by passing its unstable electron around the antioxidant molecule.
This has led to the hypothesis that large amounts of antioxidants,
with their ability to decrease the numbers of free radicals, might
lessen the radical damage causing chronic diseases, and even radical
damage responsible for aging.
Evidence
Numerous studies have demonstrated a role for free radicals
in the aging process and thus tentatively support the free radical
theory of aging. Studies have shown a significant increase in superoxide radical (SOR) formation and lipid peroxidation in aging rats. Chung et al. suggest ROS production increases with age and indicated the conversion of XDH to XOD may be an important contributing factor. This was supported by a study that showed superoxide production by xanthine oxidase and NO synthase in mesenteric arteries was higher in older rats than young ones.
Hamilton et al. examined the similarities in impaired endothelial function in hypertension and aging in humans and found a significant overproduction of superoxide in both. This finding is supported by a 2007 study which found that endothelial oxidative stress develops with aging in healthy men and is related to reductions in endothelium-dependant dilation.
Furthermore, a study using cultured smooth muscle cells displayed
increased reactive oxygen species (ROS) in cells derived from older
mice. These findings were supported by a second study using Leydig cells isolated from the testes of young and old rats.
The Choksi et al. experiment with Ames dwarf (DW) mice suggests
the lower levels of endogenous ROS production in DW mice may be a factor
in their resistance to oxidative stress and long life. Lener et al. suggest Nox4 activity increases oxidative damage in human umbilical vein endothelial cells via superoxide overproduction.
Furthermore, Rodriguez-Manas et al. found endothelial dysfunction in
human vessels is due to the collective effect of vascular inflammation
and oxidative stress.
Sasaki et al. reported superoxide-dependent chemiluminescence was inversely proportionate to maximum lifespan in mice, Wistar rats, and pigeons. They suggest ROS signalling may be a determinant in the aging process. Mendoza-Nunez et al. propose an age of 60 years or older may be linked with increased oxidative stress. Miyazawa found mitochondrial superoxide anion production can lead to organ atrophy and dysfunction via mitochondrial- mediated apoptosis. In addition, they suggest mitochondrial superoxide anion plays an essential part in aging. Lund et al. demonstrated the role of endogenous extracellular superoxide dismutase in protecting against endothelial dysfunction during the aging process using mice.
Modifications of the free radical theory of aging
One
of the main criticisms of the free radical theory of aging is directed
at the suggestion that free radicals are responsible for the damage of biomolecules, thus being a major reason for cellular senescence and organismal aging. Several modifications have been proposed to integrate current research into the overall theory.
Mitochondrial theory of aging
Major sources of Reactive oxygen species in living systems
Mitochondrial theory of aging was first proposed in 1978, and shortly thereafter the Mitochondrial free radical theory of aging was introduced in 1980.
The theory implicates the mitochondria as the chief target of radical
damage, since there is a known chemical mechanism by which mitochondria
can produce Reactive oxygen species (ROS), mitochondrial components such as mtDNA
are not as well protected as nuclear DNA, and by studies comparing
damage to nuclear and mtDNA that demonstrate higher levels of radical
damage on the mitochondrial molecules. Electrons may escape from metabolic processes in the mitochondria like the Electron transport chain, and these electrons may in turn react with water to form ROS such as the superoxide radical, or via an indirect route the hydroxyl radical.
These radicals then damage the mitochondria's DNA and proteins, and
these damage components in turn are more liable to produce ROS
byproducts. Thus a positive feedback loop
of oxidative stress is established that, over time, can lead to the
deterioration of cells and later organs and the entire body.
This theory has been widely debated and it is still unclear how ROS induced mtDNA mutations develop.
Conte et al. suggest iron-substituted zinc fingers may generate free
radicals due the zinc finger proximity to DNA and thus lead to DNA
damage.
Afanas'ev suggests the superoxide dismutation activity of CuZnSOD
demonstrates an important link between life span and free radicals.
The link between CuZnSOD and life span was demonstrated by Perez et al.
who indicated mice life span was affected by the deletion of the Sod1
gene which encodes CuZnSOD.
Contrary to the usually observed association between
mitochondrial ROS (mtROS) and a decline in longevity, Yee et al.
recently observed increased longevity mediated by mtROS signaling in an
apoptosis pathway. This serves to support the possibility that observed
correlations between ROS damage and aging are not necessarily indicative
of the causal involvement of ROS in the aging process but are more
likely due to their modulating signal transduction pathways that are
part of cellular responses to the aging process.
Epigenetic oxidative redox shift (EORS) theory of aging
Brewer proposed a theory which integrates the free radical theory of aging with the insulin signalling effects in aging. Brewer’s theory suggests "sedentary behavior associated with age triggers an oxidized redox shift and impaired mitochondrial function". This mitochondrial impairment leads to more sedentary behaviour and accelerated aging.
Metabolic stability theory of aging
The
metabolic stability theory of aging suggests it is the cells ability to
maintain stable concentration of ROS which is the primary determinant
of lifespan.
This theory criticizes the free radical theory because it ignores that
ROS are specific signalling molecules which are necessary for
maintaining normal cell functions.
Mitohormesis
Oxidative stress may promote life expectancy of Caenorhabditis elegans by inducing a secondary response to initially increased levels of reactive oxygen species. This observation was initially named mitohormesis, or mitochondrial hormesis on a purely hypothetical basis. In mammals, the question of the net effect of reactive oxygen species on aging is even less clear. Recent epidemiological
findings support the process of mitohormesis in humans, and even
suggest that the intake of exogenous antioxidants may increase disease prevalence
in humans (according to the theory, because they prevent the
stimulation of the organism's natural response to the oxidant compounds
which not only neutralizes them but provides other benefits as well).
Effects of calorie restriction
Studies have demonstrated that calorie restriction displays positive
effects on the lifespan of organisms even though it is accompanied by
increases in oxidative stress. Many studies suggest this may be due to anti-oxidative action, oxidative stress suppression, or oxidative stress resistance
which occurs in calorie restriction. Fontana et al. suggest calorie
restriction influenced numerous signal pathways through the reduction of
insulin-like growth factor I (IGF-1). Additionally they suggest antioxidant SOD and catalase are involved in the inhibition of this nutrient signalling pathway.
The increase in life expectancy observed during some calorie
restriction studies which can occur with lack of decreases or even
increases in O2 consumption is often inferred as opposing the mitochondrial free radical theory of aging. However, Barja showed significant decreases in mitochondrial oxygen radical production (per unit of O2 consumed) occur during dietary restriction, aerobic exercise, chronic exercise, and hyperthyroidism.
Additionally, mitochondrial oxygen radical generation is lower in
long-lived birds than in short-lived mammals of comparable body size and
metabolic rate. Thus, mitochondrial ROS production must be regulated independently of O2 consumption in a variety of species, tissues and physiologic states.
Challenges to the free radical theory of aging
Naked Mole-rat
The naked mole-rat is a long-lived (32 years) rodent. As reviewed by Lewis et al., (2013), levels of reactive oxygen species (ROS) production in the naked mole rat are similar to that of another rodent, the relatively short-lived mouse
(4 years). They concluded that it is not oxidative stress that
modulates health-span and longevity in these rodents, but rather other
cytoprotective mechanisms that allow animals to deal with high levels of
oxidative damage and stress.
In the naked mole-rat, a likely important cytoprotective mechanism
that could provide longevity assurance is elevated expression of DNA repair genes involved in several key DNA repair pathways. Compared with the mouse, the naked mole rat had significantly higher
expression levels of genes essential for the DNA repair pathways of DNA mismatch repair, non-homologous end joining and base excision repair.
Birds
Among birds, parrots live about 5-times longer than quail. Reactive oxygen species (ROS)
production in heart, skeletal muscle, liver and intact erythrocytes was
found to be similar in parrots and quail and showed no correspondence
with longevity difference. These findings were concluded to cast doubt on the robustness of the oxidative stress theory of aging.
