Reactive Oxygen Species and Apoptosis

Reactive Oxygen Species and Apoptosis

By Caleb Galindo

Normal functions and mechanism of Apoptosis

Traditionally, much of the field of science has focused on the realm of life and how to preserve it, especially in the medical field. Contrary to the long tradition, recent studies in apoptosis have lured the attention of many researchers to fully understand the beneficial implications of controlled cell death, or apoptosis (Renehan, Booth, and Potten 2001). Apoptosis describes the controlled, orchestrated collapse of a cell characterized by membrane blebbing, cell shrinkage, condensation of chromatin, and fragmentation of DNA followed by rapid engulfment of the corpse by neighboring cells (Renehan, Booth, and Potten 2001). Specifically within the focus of this page, the emphasis is reactive oxygen species (ROS) as a means to elicit apoptosis within the cell. Below is a general schematic of what apoptosis looks like within an unspecified cell. For a more technical definition of ROSs and ROIs, the home page goes into more detail.

http://science.howstuffworks.com/life/cellular-microscopic/apoptosis.html

Figure 1.  General schematic summarizing the process of a cell undergoing apoptosis (unspecified cell type). Apoptosis can arise from a variety of different pathways, though we focus on ROS as being the stimulus. Overall, the process of apoptosis is controlled and blebbing/shrinkage occurs so that precious cellular materials are conserved and do not interfere with neighboring cells. Primarily, macrophages engulf the packages of the cell to clean the area and prevent inflammation (Renehan, Booth, and Potten 2001).

Many fields such as aging, embryogenesis, homestasis studies, and human disease are just a few examples of areas where apoptosis yields promising implications in better understanding each respective mechanism (Renehan, Booth, and Potten 2001). For example in the development of humans, a primary function of apoptosis is during intrauterine development though it also helps to sculpture organ shape and shape hands by "carving" tissues and interdigital webs of fingers and toes prior to birth(Renehan, Booth, and Potten 2001). In addition, both the nervous system and the immune system become functional systems via overproduction of cells followed by the apoptotic death of those that fail to establish functional synaptic connections or productive antigen specificities, respectively (Renehan, Booth, and Potten 2001). Within development alone, it has been found that apoptosis functions in a variety of pathways that are essential to progression into maturity.

Even after maturity has been reached, apoptosis is needed as well for maintaining homeostasis. In adulthood, roughly 10 billion cells die daily simply to maintain balance with the numbers of new cells arising from the body's stem cell populations (Renehan, Booth, and Potten 2001). Even though a vast number of cells are regulated by apoptosis, programmed cell death is still heavily regulated within specific tissues and cell types (Renehan, Booth, and Potten 2001). The same general mechanisms of apoptosis serve to destroy old or damaged cells (Renehan, Booth, and Potten 2001). Depending on tissue type, 'old' cells may be defined relatively. For example, skin tissue undergoes cell death quite quickly compared to neural tissue. The specificity of apoptosis in regards to tissue is a rough illustration of how complex apoptosis is within a single organism.

 Apoptosis vs Necrosis

In general, necrosis (as opposed to apoptosis) often occurs when a cell undergoes damage from an external force, such as blunt injury, a poinsonous substances, exposure to infectious agents, or even lack of blood (Perrone, Tan, and Dawes 2008). A simple example of necrosis from lack of blood is the deterioration of cardiac muscle or brain tissue following a heart attack or stroke, respectively. When cells die from necrosis, it is typically considered more "messy" since the cellular components are not as orderly handled. Often times, the death causes inflammation that can cause further distress or injury within the body. It is important to note that it is sometimes difficult to distinguish cells undergoing necrosis compared to apoptosis since the tissue will vary in how well it responds to the death of a cell. Therefore, there is a vague continuum between apoptosis and necrosis. 

A key marker is apoptosis is distinguished from death via necrosis by the absence of an associated inflammatory response (Kerr et al., 1972). These observations were made by Kerr et al as early as 1972,1 but their importance was underestimated for many years. A useful analogy for apoptosis is a relatively civil process in comparison to the chaotic response elicited by necrosis.
As mentioned before, the cleanup is much easier for cells that have undergone apoptosis since the cellular components are packaged neatly as shown in Figure 1.The ease of cleanup is a good demonstration of why apoptosis is sometimes referred to as programmed cell death, because the process of apoptosis follows a controlled and predictable routine.

https://theartofmed.wordpress.com/2015/05/29/pathologic-cell-injury-and-cell-death-ii-necrosis

Figure 2.  A demonstration of the difference between a cell that is subject to necrosis vs a cell that undergoes apoptosis. The illustration gives a good general distinction in the difference of organization for the cellular components after the cell dies as well as the general causes for the respective pathways. On the left, necrosis is generally caused by abrupt cell injury and the resulting cell death is not properly destroyed for effective salvaging and cleanup. On the right, apoptosis may arise from either old age or cellular signals from neighbors and the resulting death is handled efficiently as cellular components are kept organized within membrane blebs.

How ROS initiate Apoptosis

At relatively higher doses of ROS, death of a proportion of cells in the population occurs initially by apoptosis, but at extreme doses by necrosis, thus again illustrating that cell death may be perceived on a continuum between apoptosis and necrosis (Perrone, Tan, and Dawes 2008). Many studies show ROS generation is an early and possibly regulatory step in many of the apoptotic processes. For example,it has been demonstrated that yeast cells aging either chronologically or replicatively accumulate ROS and undergo apoptotic death and, indicating that oxidative stress defense plays a major role in governing ageing-induced apoptosis (Perrone, Tan, and Dawes 2008).Though it has long been known that age plays a large role in the initiation of apoptosis, studies like this suggest that ROS production from inefficient or old cells serve as an important initiator in the mechanism.

The following is the mechanism description by Perrone, Tan, and Dawes of the various ROS molecular species in the propagation of oxidative damage within a given cell;

"The main ROS generated from oxygen in cells include the superoxide anion, which is relatively abundant in cells and mainly generated from the leakage of electrons from the mitochondrial respiratory chain as a normal consequence of aerobic respiration. The superoxide anion is generally not strongly reactive, but can react directly with some proteins. Hydrogen peroxide is produced during the detoxification of superoxide anion catalysed by superoxide dismutases. Hydrogen peroxide can readily cross most biological membranes, including the mitochondrial membranes. While it is relatively unreactive, it has deleterious effects in the cytoplasm through its conversion to the extremely reactive hydroxyl radical.The hydroxy radical reacts indiscriminately with most metabolites and macromolecules, in many cases generating other radicals in the process. In cells that can produce nitric oxide, the nitric oxide radical can react with the superoxide anion to generate a range of reactive nitrogen species including the fairly reactive peroxynitrite (ONOO−) which is more reactive than H2O2 in oxidising thiols and damaging some proteins."

Perrone, Tan, and Dawes 2008

Because ROS molecules act as primary messengers as well as having secondary reactive molecules, the study of all the effects caused by ROS production is a difficult field of inquiry. Many of the products of these reactions act indiscriminately within the cell and so the effects show up in a variety of cellular organelles, making tracing reactions back to the original cause no simple task. Therefore, ROS molecules have large mechanism implications of understanding apoptosis as a whole but future research will be a challenge given the nature of the molecules.

Once a cell has undergone enough oxidative stress, the mitochondrial release of cytochrome c is thought to play a major role in apoptosis.Currently this theory is still under investigation however it sheds some light on the general general role that the mitochondria plays in initiation apoptosis. Since the mitochondria commonly produces relatively large amounts of ROS molecules, it is natural that it should be further investigated to understand ROS-induced apoptosis as a whole.

https://www.quora.com/Do-cyanide-and-azide-bind-to-cytochrome-c

 Figure 3. The structure of cytochrome c, an electron carrier between Complex III and Complex IV.

This video is a good summary of how cytochrome c plays a role in the whole of apoptosis.

Conclusion 

Reactive oxygen species are widely generated in biological cells, found throughout a variety of tissue types. Intracellular production of active oxygen species such as hydroxy radicals, superoxide anions and hydrogen peroxide within the mitochondria represent a small subgroup of cellular conditions associated with the arrest of cell proliferation (Mates and Sanchez 2000). Therapeutically,  the regulation of gene expression by means of oxidants, antioxidants and the redox state remains as a promising  approach in manipulation of a cell's decision to enter or refrain from apoptosis (Mates and Sanchez 2000). Interestingly, several anticarcinogenic agents have been shown to inhibit reactive oxygen species production and oxidative DNA damage, inhibiting tumour promotion. thus suggesting that the lack of apoptotic regulation led to a cancerous cell type (Mates and Sanchez 2000). In conclusion, oxidative stress has been shown to be heavily involved in both apoptosis and the progression of cancerous cells. From the data collected thus far, it can be concluded that free radical reactions may be increased in cancerous cells and oxidant scavenging systems may serve highly useful therapeutic roles in the treatment of cancer in the future. 

Because of the nature of ROS and how fleeting they are, more research is needed to better understand their roles in apoptosis. While many studies have given good insight, it is still poorly understood how ROS exposure will affect a given cell, which is of major therapeutic concern if apoptosis is the key. Apoptosis is therefore a unique facet in the discussion of pros and cons because of its central role in the health of the whole organism. While it is deleterious in inappropriate doses, perhaps control of its role in apoptosis could be of powerful therapeutic use for a variety of different human diseases.