Aging and Reactive oxygen Species

Aging: What is it?  Aging, has been termed generally as a progressive decline in the ability of a physiological process after the reproductive phase of life.

Theories of aging  Genetic theories Claim that aging is genetically determined Claim that aging is genetically determined  Damage-accumulating theories Claim that aging is a result of accumulating cellular damage to proteins, membranes… Claim that aging is a result of accumulating cellular damage to proteins, membranes… A possible cause of cellular damage could be reactive oxygen species. A possible cause of cellular damage could be reactive oxygen species.

Genetic theories of aging propose that aging is a continuation of the process of development and differentiation, and is a sequence of events encoded into the genome. Some genes that alter or regulate life span (age-1, LAG1, RAS1 and RAS2) were identified in worm (C. elegans), fly (Drosophila) and fungi (yeast and Neurospora). Human disorders of premature aging (e.g., Werner’s syndrome) were linked to specific genes encoding, for example, enzymes such as helicase. Aging was linked to the length of telomeres and to telomerase activity in aging cells. We will not discuss further the theory of genetic aging in this presentation. Genetic theories

Damage-accumulating theories Damage-accumulating theories propose that aging is caused by progressive accumulation of cellular damage (different theories propose different damages, see table below). The accumulation of damaged molecules in cells is thought to result over time due to failure of repair and maintenance systems.

Basis for The Free Radical Theory Pearl 1920s Basis for The Free Radical Theory Pearl 1920s  Pearl proposed “The rate of living” hypothesis. –This hypothesis directly links the metabolic rate with the life expectancy of an organism. –It suggested that there was a precise mechanism that linked metabolism with life span. –The hypothesis stated that there is a vital cellular element that was somehow consumed at the same rate as the overall metabolic rate. When this unknown, vital element was exhausted death occurred.

Many experimental evidences support the metabolic-life span theory. The graph below shows an example of an experiment performed with housefly.

Free Radical Theory of Aging Harman 1956  Normal aging results from random deleterious damage to tissues by free radicals.  Because many ROS are not radicals, the theory should be named “The Oxidative Damage Theory of Aging”

Oxidative damage: A common link between all aging theories 1.ROS are produced during normal metabolism. Studies have shown accumulated damage to DNA, protein and membrane. 2.Enhanced ROS production may be a consequence of genes selected because they confer benefits in early life, however, would cause enhanced ROS production during aging. 3.Deterioration of tissues with age may lead to increaser ROS production. 4.Link to metabolic activity. Large animals consume less oxygen per unit of body mass than small animals and they live longer. 5.Insects consume much more oxygen when flying than at rest and resting insects live longer (see Figure 1). Continue in next slide

6. Most oxygen consumed by aerobic eukaryotes is used in the mitochondria. Mitochondrial DNA rapidly accumulates mutations with age. 7. Longer lived organisms were found to contain more efficient ROS scavenging systems (see correlation table below). Continue in next slide

8. In certain species with similar metabolic rates, but with different lifespans, the rate of mitochondrial ROS production correlates with longevity (decreased rate in longer lifespan species). 9. Senescence can be induced by ROS application to cells. 10. Caloric restriction in rodents is associated with decreased level of oxidative damage to DNA.

Mitochondrial ROS generation appears to be a common link in aging of different organisms

Potential targets of ROS within cells that may determine the rate of aging.

Testing the hypothesis: One way to test the oxidative damage theory of aging is to apply antioxidants. In lower organisms a positive effect was found (see Figure below), however in mammals the effects of antioxidants on aging are very small or zero.

Testing the hypothesis: Another way to test the hypothesis is to use transgenic organisms. Experiments with transgenic organisms are ongoing. Some evidance from Drosophila has shown that overexpression of SOD and catalase together improved lifespan by 30-40% and decreased oxidative damage. Intersotingly, overexpression of SOD or catalase did not improve lifespan. Extended lifespan mutants of C. elegans show high tolerance to ROS. One of these mutants (age-1) also shows enahnced expression of SOD and catalase. It is possible that the extended life mutations cause activation of ROS scavenging mechanisms.

Many different factors could affect ROS metabolism. These could change due to genetic alterations and could be linked to aging (see table below).

Conclusions The oxidative damage theory seems to link all other aging theories. It holds for now. The only experimentally proven treatment that can improve lifespan in mammals (and most likely humans) is restricted calorie intake!!! This presentation is based on the following book: Halliwell B, Gutteridge JMC (1999) Free radicals in biology and medicine. 3 rd Edition. Oxford University Press.