On the other hand, exercise necessarily increases a person's rate of metabolism, as food is processed to provide energy expended through exercise. An inevitable side-effect of metabolism is the production of reactive oxygen species (ROS) and "free radicals" that can damage DNA and other cellular constituents. This cellular damage can lead to either cancer or accelerated aging due to cell senescence and cell death.
The paradox, then, is that the health benefits of exercise do not seem to be canceled out by the side-effects of higher rates of metabolism. It's an important issue not just for humans who are trying to stay healthy, but even more important in animals like birds that may need to expend energy continuously over significant periods of time.
So what's going on here? Perhaps this research has some of the answer:
The effect of marathon on mRNA expression of anti-apoptotic and pro-apoptotic proteins and sirtuins family in male recreational long-distance runners
A large body of evidence shows that a single bout of strenuous exercise induces oxidative stress in circulating human lymphocytes leading to lipid peroxidation, DNA damage, mitochondrial perturbations, and protein oxidation.
In our research, we investigated the effect of physical load on the extent of apoptosis in primary cells derived from blood samples of sixteen healthy amateur runners after marathon (a.m.).
Blood samples were collected from ten healthy amateur runners peripheral blood mononuclear cells (PBMCs) were isolated from whole blood and bcl-2, bax, heat shock protein (HSP)70, Cu-Zn superoxide dismutase (SOD), Mn-SOD, inducible nitric oxide synthase (i-NOS), SIRT1, SIRT3 and SIRT4 (Sirtuins) RNA levels were determined by Northern Blot analysis. Strenuous physical load significantly increased HSP70, HSP32, Mn-SOD, Cu-Zn SOD, iNOS, GADD45, bcl-2, forkhead box O (FOXO3A) and SIRT1 expression after the marathon, while decreasing bax, SIRT3 and SIRT4 expression (P < 0.0001).
These data suggest that the physiological load imposed in amateur runners during marathon attenuates the extent of apoptosis and may interfere with sirtuin expression.
There are two main findings here, related to apoptosis and sirtuin expression. Let's take them in order.
Apoptosis is a form of programmed cell death that has several purposes. The invocation of a cell's apopotosis program isn't necessarily an indication that something is wrong. For example, it occurs normally during embryonic development. Early in the development process embryos of all tetrapods have tissues between what will become the fingers and toes of their hands and feet. But since animals that have left an aquatic environment are usually better off without this extra tissue, evolution has led to signals at a certain stage of embryonic development that cause apoptosis in the cells of the relevant tissue. This is an example of what's known as the "extrinsic" apoptotic pathway.
But for our present purposes there's a second pathway – the "intrinsic" pathway – which is used whenever a cell either detects internal damage (usually to its DNA) or some stressful condition, such as an excessive level of reactive oxygen species. A ROS is a chemically-reactive molecule containing oxygen, including what are sometimes called "free radicals".
This condition of excess ROS is called oxidative stress. It can occur for various reasons, including exposure to high levels of heat or ultraviolet radiation – or abnormally rapid cell metabolism due to vigorous exercise. Cells recognize the condition of oxidative stress indirectly though signaling involving various other molecules that are produced in response to particular ROS molecules. Among such indicators are proteins called heat shock proteins. Two members of this family that were measured in the research under discussion were HSP70 and HSP32.
Signals of oxidative stress trigger the second, "intrinsic" apoptotic pathway, which involves a cell's energy-producing organelles, the mitochondria. The main players in the intrinsic pathway are proteins called, generically, "caspases" – short for "cysteine-rich aspartate proteases". Caspases are enzymes that cleave proteins at aspartate units. (Cysteine and aspartate are two of the 21 amino acids that normally make up proteins.)
Caspases are fairly active enzymes, so they don't ordinarily occur at significant concentrations within cells. Instead, they are produced when needed from other protein enzymes called procaspases. One of these, procaspase-9 is found normally within mitochondria, along with another protein, cytochrome c. Most of the time these proteins are confined within the mitochondria. However, under certain conditions some channels in a mitochondrion's membrane can open and allow the release of procaspase-9 and cytochrome c. Once these proteins enter the cytosol (cell fluid) outside a mitochondrion, they can team up with another protein (Apaf-1: "apoptotic protease activating factor 1") to convert the procaspase-9 into the caspase known as caspase-9. The latter is an active enzyme that leads to the production of other caspases, with cell apoptosis as the eventual result.
Since a cell does not want to have apoptosis going on normally, the process must be tightly regulated. This is done (partly) by another pair of proteins, Bcl-2 and Bax. These two proteins have structural similarities and are considered to be in the same family, the Bcl-2 family. They are always present in the cytosol, and the relative concentration between Bcl-2 and Bax is what controls whether mitochondrial membrane channels will allow release of procaspase-9 and cytochrome c. If the ratio favors Bcl-2, the channels are essentially closed – the normal case – but if the ratio favors Bax, the channels open... and apoptosis may follow.
The present research measured the levels of certain proteins in 10 individuals before and after a marathon run. (The measurement was done indirectly by measuring levels of mRNA transcripts of the associated genes.) A key finding was that the ratio of Bcl-2 to Bax shifted in favor of Bcl-2 from the before to the after measurement. In other words, there was an anti-apoptotic effect, which countered the pro-apoptotic effects of ROS molecules produced by vigorous exercise. Although ROS levels were not measured (since there was no corresponding mRNA), levels of superoxide dismutase (SOD) antioxidants (Mn-SOD and Cu-Zn-SOD) increased after the marathons, reflecting ROS production.
Analysis of the results indicates that apoptosis actually was inhibited, though less in some experimental subjects than others. An increase in levels of procaspase-9 was not observed. Further, in 7 of the 10 experimental subjects, there was little evidence of DNA fragmentation (a consequence of apoptosis). In the other 3 subjects, there was some evidence of DNA fragmentation, but also smaller changes in the Bcl-2 to Bax ratios.
Most interestingly, there was a significant positive correlation in after marathon measurements between levels of Bcl-2 and both HSP70 and HSP32. This suggests that the expected increases of HSP70 and HSP32 may play some part in increased Bcl-2 levels. There was also a positive correlation post-marathon between HSP70 and Mn-SOD levels.
These findings, especially given the small sample size, certainly aren't conclusive. But, as the paper says, "Here, we have found a significant relationship between HSP70 and bcl-2 RNA ... following marathon, but the underlying cellular and molecular mechanisms involved in this [sic] exercise induced adaptations in apoptosis and HSP70 are unknown and require further investigation."
Expression of the sirtuins SIRT1, SIRT3, and SIRT4 pre- and post-marathon were also measured. (We've discussed the sirtuins on a number of occasions.) There's an extensive history of research on SIRT1, concerning its connections with such things as cellular metabolism, cell survival under stress, and antioxidant activity. Research on other sirtuins like SIRT3 and SIRT4 is less extensive. However, members of this family have various things in common. All are enzymes. SIRT1 and SIRT3 are histone deacetylases (HDACs), so have epigenetic roles in affecting gene expression. SIRT3 and SIRT4 occur in mitochondria.
Although it's possible to make various speculations about how sirtuins could be involved with apoptosis and metabolic consequences of exercise, not all that much is known about specific molecular mechanisms. Nevertheless, it's interesting that the present research does show an effect of strenuous exercise on SIRT1, SIRT3, and SIRT4 expression. The paper notes that "the RNA contents of SIRT1 increased substantially in the group after marathon.... On the other hand, the RNA contents of SIRT3 and SIRT4 decreased in the group after marathon."
Further research into these connections could be very interesting.
|Marfe, G., Tafani, M., Pucci, B., Di Stefano, C., Indelicato, M., Andreoli, A., Russo, M., Sinibaldi-Salimei, P., & Manzi, V. (2010). The effect of marathon on mRNA expression of anti-apoptotic and pro-apoptotic proteins and sirtuins family in male recreational long-distance runners BMC Physiology, 10 (1) DOI: 10.1186/1472-6793-10-7|
Running a marathon halts cellular suicide (5/11/10)
Articles related to sirtuins:
Sirtuin proteins (11/16/07)
The discovery of sirtuins, part 1 (11/17/07)
The discovery of sirtuins, part 2 (11/20/07)
Sirtuin news (1/21/08)
SIRT1 and cancer (10/26/08)
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