Written by Julie Bick, Ph.D.
With modern medicine, we now understand how cellular health is foundational for overall health and well-being. Every organ, tissue, and system rely on the proper functioning of individual cells. One of the crucial factors that can disrupt this delicate balance is known as oxidative stress (Betteridge, 2000). This phenomenon, often considered a double-edged sword, plays a pivotal role in the body's cellular health. Several processes such as cell signaling, immune system defenses including phagocytosis, apoptosis and drug metabolism all rely on the generation of reactive oxygen species (ROS), however, as we’ll see in this blog, ROS are potentially damaging and the oxidative stress that they can induce can be highly detrimental. This dual nature underscores the complexity of oxidative stress and its pivotal role in health and disease (Pizzo et. al. 2017).
To comprehend the implications of oxidative stress on cellular health, it is important to initially understand the fundamental concepts surrounding it. Oxidative stress arises from an imbalance between the production of ROS and the body's ability to neutralize and detoxify these harmful molecules. ROS, including free radicals like superoxide and hydroxyl radicals, are natural byproducts of cellular metabolism. These molecules have unpaired electrons that make them highly reactive. While they serve essential roles in various physiological processes, an excess can result in oxidative stress, triggering a cascade of detrimental effects on cellular structures and functions.
One of the earliest described impacts of oxidative stress is as a potent driver of DNA damage, directly affecting the integrity of the genetic code (Cooke et. al. 2003). The effect of ROS on DNA molecules includes base pair damage, strand breaks, and even crosslinking between DNA strands. One common form of oxidative DNA damage involves the modification of DNA bases. For instance, guanine is particularly susceptible to oxidation, resulting in the formation of 8-oxo-7,8-dihydroguanine (8-oxoG). This modification can lead to mispairing during DNA replication, potentially causing mutations. The cumulative impact of oxidative stress on the genome is a critical aspect of cellular health, influencing the body's ability to maintain proper functionality. In the long term, oxidative stress increases the frequency of DNA mutations and structural distortions in the genome that contribute to cellular dysfunction and senescence, that all contribute to the potential development of various diseases, including cardiovascular diseases, neurodegeneration, and cancer.
Proteins mediate the pathways of cellular function, and like DNA, proteins are susceptible to oxidative stress, although for proteins this damage is associated with reactive amino acid residues (Berlett and Stadtman, 1997). Cysteine residues in proteins are highly susceptible to oxidation. The thiol group (-SH) in cysteine can be oxidized to form disulfide bonds, leading to significant changes in protein conformation and function. Moreover, cysteine residues can undergo more severe oxidation, forming sulfenic acid, sulfinic acid, or sulfonic acid, which can alter protein structure irreversibly. Methionine residues also contain a sulfur atom that is vulnerable to oxidation. Oxidation of methionine can lead to the formation of methionine sulfoxide, affecting protein structure and potentially impairing function. Histidine and Tyrosine residues can also be oxidized, leading to changes in protein charge and potentially influencing their functions. Furthermore, ROS can induce cross-linking between amino acid residues in proteins, forming covalent bonds between adjacent protein molecules. This process can result in the formation of protein aggregates, leading to loss of solubility and functional impairment. This is believed to be one of the underlying mechanisms for the development of amyloid plaques and tau tangles (neurofibrillary tangles NFT) in neurodegenerative diseases (Mattson, 2004). Oxidative stress can also induce protein fragmentation, breaking down large proteins into smaller fragments. This can alter the protein's function and disrupt cellular processes that depend on intact proteins. This dysfunction can extend to enzymes, receptors, and signaling molecules, creating a domino effect that extends through cellular processes and signaling pathways.
Proteins damaged by oxidative stress may be targeted for degradation by the ubiquitin-proteasome system. However, if the rate of protein damage overwhelms the system's capacity and damaged proteins can accumulate and contribute to cellular dysfunction. Furthermore, oxidative stress can impair the function of molecular chaperones, leading to the accumulation of misfolded or unfolded proteins, which also can have detrimental effects on cellular homeostasis.
Cell membranes, composed of lipids, are another primary target for oxidative stress-induced damage. Lipid peroxidation is a chain reaction that is initiated by ROS and results in the degradation of lipid molecules within cell membranes (Su et. al. 2019). ROS, including superoxide anions and hydroxyl radicals, react with polyunsaturated fatty acids (PUFAs) present in lipid molecules, leading to the formation of lipid radicals, starting the chain reaction of lipid peroxidation that produces lipid hydroperoxides. This compromises membrane integrity, leading to altered permeability and functionality. The destabilization of cell membranes has profound implications for cellular health, affecting vital processes such as nutrient transport and signal transduction. In addition, lipid peroxidation generates reactive aldehydes such as malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) that can covalently modify proteins and nucleic acids. The end products of lipid peroxidation include various oxidized lipids, such as oxysterols and isoprostanes. These oxidized lipids can have cytotoxic effects and contribute to inflammation and oxidative stress in the surrounding cellular environment.
Cells have several mechanisms to detect and respond to lipid peroxidation, including the activation of transcription factors like nuclear factor-κB (NF-κB) and activator protein-1 (AP-1) that lead to the expression of genes involved in inflammation and antioxidant defense. However, lipid peroxidation is a positive feedback loop of oxidative stress, and both have been implicated in various pathological conditions including neurodegenerative and cardiovascular diseases, as well as inflammatory disorders. The accumulation of oxidized lipids can contribute to the progression of these diseases and their levels can be monitored to assess disease progression.
Oxidative stress significantly impacts mitochondria, the powerhouse of the cell responsible for energy production. The intricate relationship between oxidative stress and mitochondrial function is a crucial aspect of cellular health. One reason for this is that mitochondria are both sources and targets of ROS. During normal cellular respiration in the electron transport chain (ETC) and ATP synthesis, a small percentage of electrons leak, leading to the production of superoxide radicals. Under conditions of oxidative stress, the rate of ROS production within mitochondria can increase significantly through disruption of their membrane integrity and the breakdown of their internal antioxidant mechanisms.
Uniquely among cellular organelles, mitochondria have their own DNA (mtDNA); this is highly susceptible to oxidative damage due to its proximity to the inner mitochondrial membrane where ROS are generated. Oxidative stress can lead to mutations and deletions in mtDNA, compromising the integrity of the mitochondrial genome. The significance of this was recently highlighted in a Nature Cancer publication, in which tumors with high levels of mutations within the mtDNA we shown to be up to two and a half times more likely to respond to immunotherapy with drugs such as nivolumab; in essence mutations with the mtDNA appear to sensitize tumors to checkpoint blockade and improve the efficacy of checkpoint inhibitors such as anti-PD-L1 and PD-L2 immunotherapies (Mahmood, et. al. 2924).
Severe mitochondrial damage may lead to the release of cytochrome c from the mitochondrial intermembrane space into the cytoplasm. Cytochrome c release is a key event in apoptosis, initiating the caspase cascade and programmed cell death. Under extreme conditions, mitophagy, the selective degradation of damaged mitochondria by autophagy, is triggered as a protective response to eliminate dysfunctional mitochondria. However, under chronic oxidative stress, the balance between mitophagy and mitochondrial damage may be disrupted, leading to the accumulation of damaged mitochondria. These impacts various additional cellular functions including calcium homeostasis within cells, leading to abnormal accumulation of calcium within mitochondria. Excessive calcium levels can further contribute to mitochondrial dysfunction.
As with lipid peroxidation, a positive feedback loop can be established whereby damaged mitochondria can contribute to increased oxidative stress. Dysfunctional mitochondria release more ROS into the cell, exacerbating oxidative damage to cellular components beyond the mitochondria itself. Mitochondrial dysfunction resulting from oxidative stress is implicated in various diseases, including neurodegenerative disorders (e.g., Alzheimer's and Parkinson's), cardiovascular diseases, metabolic disorders, and age-related conditions. The role of mitochondria in cellular energy production and their susceptibility to oxidative stress make them critical players in disease pathology.
Mitochondria possess their own antioxidant defenses, including enzymes like superoxide dismutase and glutathione peroxidase. These defenses work to neutralize ROS generated within the mitochondria directly and help disrupt the escalation of ROS damage. Cellular nutrient-sensing pathways, such as the AMP-activated protein kinase (AMPK) pathway, play a role in regulating mitochondrial responses to oxidative stress, and activation of these pathways appears to enhance the ability of cells to initiate mitochondrial biogenesis and replace damaged mitochondria with new, functional ones. This involves the synthesis of new mitochondrial components to maintain a healthy mitochondrial population. As such, pathways involved in mitochondrial biogenesis hold promise for mitigating the detrimental effects of oxidative stress on cellular health and the pathogenesis of associated diseases.
While oxidative stress poses a significant threat to cellular health, the human body is equipped with sophisticated defense systems to counteract its deleterious effects. Antioxidant defenses, comprising enzymes such as superoxide dismutase, catalase, and glutathione peroxidase, along with non-enzymatic antioxidants like vitamins C and E, act as the body's frontline guardians against oxidative stress.
Antioxidant enzymes play a pivotal role in neutralizing ROS and preventing their harmful effects within different cells and cellular compartments. Superoxide dismutase (SOD) converts superoxide radicals into hydrogen peroxide, while catalase and glutathione peroxidase (GPx) further break down hydrogen peroxide into harmless water and oxygen. Within mitochondria catalase (CAT) activity works to maintain the mitochondrial membrane potential and has an anti-apoptotic effect. CAT activity requires NADPH and levels of this molecule are maintained by the thioredoxin and glutathione systems working together. Collectively these enzymatic defenses form a robust network that collaboratively works to maintain cellular balance. Nonenzymatic antioxidants such as vitamins A, C and E, along with other small molecules like glutathione, coenzyme Q10 and melatonin act as non-enzymatic antioxidants (Su et. al. 2019). These molecules donate electrons to neutralize free radicals, preventing them from causing damage to cellular components. The intricate interplay between enzymatic and non-enzymatic antioxidants is essential for the body's resilience against oxidative stress.
The food we consume plays a central role in modulating oxidative stress. A diet rich in both water-soluble and fat-soluble antioxidants, derived from fruits, vegetables, and whole grains, provides the body with the necessary tools to combat excess ROS. On the contrary, a diet high in processed foods and lacking essential nutrients can exacerbate oxidative stress and contribute to cellular damage resulting in poor health. The potential therapeutic role of antioxidant supplementation in mitigating oxidative stress has been the subject of extensive research. However, there is a still a great deal of confusion around the effectiveness of nutritional supplements such as antioxidants, and there are calls for more FDA oversight. Studies examining the effect of nutritional supplements are often poorly designed, and don’t consider key factors such as population differences, the baseline nutritional status of participants, or even the quality or doses of the active components. The effectiveness of antioxidants in specific contexts remains an active area of investigation. This ongoing research and a commitment to transparency in reporting study results are essential for advancing our understanding of the role of antioxidant supplements in health and disease.
We’ve all heard of this before but smoking, excessive alcohol consumption, and sedentary lifestyles are also clearly associated with increased oxidative stress and accelerated aging. Some early observational studies suggested a potential protective effect of antioxidant vitamins, such as vitamin C, vitamin E, and beta-carotene, against certain cancers, including lung cancer among smokers, and these findings fueled interest in antioxidant supplementation as a preventive measure. However, subsequent large-scale clinical trials, including the Alpha-Tocopherol Beta-Carotene Cancer Prevention (ATBC) trial (Albanes et. al. 1996) and the Beta-Carotene and Retinol Efficacy Trial (CARET) (Omenn et. al. 1996), yielded unexpected results. Both trials found that beta-carotene supplementation, either alone or in combination with other antioxidants, did not reduce lung cancer risk and, in some cases, was associated with an increased risk, particularly among smokers. Based on current evidence, organizations such as the American Cancer Society and the U.S. Preventive Services Task Force do not recommend the routine use of antioxidant supplements for cancer prevention, including among smokers. Instead, they emphasize the importance of obtaining antioxidants from a balanced diet rich in fruits, vegetables, and other nutrient-dense foods.
Oxidative stress, with its intricate interplay of ROS generation and antioxidant defenses, stands as a critical determinant of cellular health and therefore overall health. Understanding the mechanisms through which oxidative stress impacts DNA, proteins, and lipids provides insights into its role in various diseases and the aging process. While the body possesses a formidable defense arsenal, environmental factors and lifestyle choices can tip the balance towards oxidative damage. Exploring interventions, both through antioxidant supplementation and lifestyle modifications, offers avenues for promoting cellular resilience and overall well-being. As we continue to unravel the complexities of oxidative stress, the quest for innovative therapeutic strategies continues, aiming to harness the delicate equilibrium that defines cellular health.
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