Review
Markers of oxidant stress that are clinically relevant in aging and age-related disease

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Abstract

Despite the long held hypothesis that oxidant stress results in accumulated oxidative damage to cellular macromolecules and subsequently to aging and age-related chronic disease, it has been difficult to consistently define and specifically identify markers of oxidant stress that are consistently and directly linked to age and disease status. Inflammation because it is also linked to oxidant stress, aging, and chronic disease also plays an important role in understanding the clinical implications of oxidant stress and relevant markers. Much attention has focused on identifying specific markers of oxidative stress and inflammation that could be measured in easily accessible tissues and fluids (lymphocytes, plasma, serum). The purpose of this review is to discuss markers of oxidant stress used in the field as biomarkers of aging and age-related diseases, highlighting differences observed by race when data is available. We highlight DNA, RNA, protein, and lipid oxidation as measures of oxidative stress, as well as other well-characterized markers of oxidative damage and inflammation and discuss their strengths and limitations. We present the current state of the literature reporting use of these markers in studies of human cohorts in relation to age and age-related disease and also with a special emphasis on differences observed by race when relevant.

Highlights

► A review of the current status of oxidative stress markers in aging and age-related diseases. ► This review discusses the use of different methodologies to assess oxidative stress levels. ► Particular focus on the oxidation of DNA, RNA and protein in human cohort studies with respect to aging.

Introduction

The specific role of oxidative stress in aging and in the development of age-related disease is an area of active investigation but the exact mechanisms that may define this complex relationship are unclear (Voss and Siems, 2006). Understanding this complex relationship may provide potential biomarkers of oxidative stress, which can be objectively measured as indicators of normal and pathologic processes that result in age-related disease and decrement in cellular function associated with aging.

The Harman Free Radical Theory of Aging states that accumulation of oxidative damage to DNA and other cellular components and tissues over the lifespan leads to aging, disease and death (Harman, 1956). Free radicals can be formed exogenously by environmental sources including ionizing radiation, ultraviolet light, and pollutants (cigarette smoke, emissions from automobiles or factories, asbestos). These sources of oxidative stress, in addition to endogenous enzymatic sources, such as cellular respiration, cell signaling, and inflammation, result in oxidative damage in a biologically relevant manner (Mateos and Bravo, 2007).

Oxidant stress has been defined as an alteration in the balance between the production of reactive oxygen species (ROS) (free radicals) and the antioxidant defense system in place to counter them (Halliwell, 1994). Free radicals as well as ROS are any chemical species with unpaired electrons and are produced through many sources including the environment (from ozone and nitrogen dioxide) and many varied biological and biochemical processes (both deliberately and as by-products). Common examples of free radicals and ROS are the hydroxyl radical (OH), the superoxide anion (O2), nitric oxide (NO), and transition metals.

Free radicals are neutralized by the anti-oxidant system. This system functions at the cellular, membrane, and extracellular levels to protect against free radical attack. Components that comprise this system include members of the catalase, peroxidase, and dismutase families as well as the glutathione system, including superoxide dismutase (SOD) (converts superoxide to hydrogen peroxide), catalase (removes hydrogen peroxide), and glutathione peroxidase (removes hydrogen peroxide generated by SOD).

In addition to the endogenous enzyme antioxidant system, additional antioxidants play a key role in defense against ROS including vitamins A, C, and E. These antioxidants are classified into two groups: those that are hydrophobic (Vitamins A and E) protect membranes from free radical attack and those that are hydrophilic (Vitamin C) interact with free radicals in the blood and cytosol (Sies, 1997) to neutralize the free radicals that are formed.

Free radicals can cause damage in DNA that can potentially lead to mutagenesis and thus cellular transformation and uncontrolled proliferation. In addition, oxidant stress is thought to contribute to the development of human diseases including but not limited to Alzheimer's disease (AD) (Butterfield, 2006, Christen, 2000, Halliwell, 2006, Nunomura et al., 2006, Perry et al., 2002), cardiovascular disease (Aviram, 2000), atherosclerosis (Parthasarathy et al., 2008, Stocker and Keaney, 2004), Parkinsons disease (PD) (Wood-Kaczmar et al., 2006), rheumatoid arthritis (Hitchon and El-Gabalawy, 2004), diabetes (Dav et al., 2005, Giugliano et al., 1996), and motor neuron diseases (Cookson and Shaw, 1999). In this review we address commonly used markers of oxidant stress as they are related to aging and age-related diseases only in the context of human studies. These human cohort studies are summarized in Table 1, Table 2 where we have highlighted the disease state examined, the methodology and the significant findings.

Section snippets

DNA oxidative lesions as a measure of oxidant stress

DNA is a highly susceptible target of free radicals, resulting in oxidation of DNA bases and the ribose sugar ring leading to sites of base loss and strand breaks. The rate of damage to DNA by free radicals is estimated to be 1000 to 1,000,000 hits per day in a single cell (Ames et al., 1993). The accumulation of free radical DNA damage can be a lethal event for an organism. An increased baseline level of DNA oxidation (single strand breaks (SSBs) and oxidative base damage) is associated with

RNA oxidative lesions as a measure of oxidant stress

The human genome consists of more than 21,000 genes, encompassing only 1.5% of the total bases of the genome, yet approximately 80% of the DNA is transcribed into RNA (Birney et al., 2007). Recent data indicates that many of the non-gene encoding but transcribed RNAs have functional roles; however, the focus on nucleic acid oxidation research has been centered on DNA. The vast number and types of RNAs in the body provide a sizable target for ROS. There are many reasons that RNA is more prone to

Protein oxidation as a measure of oxidant stress

Another target of free radical attack is proteins, which can be both oxidized and cross-linked. These alterations to proteins can result in inhibition of function of various cellular proteins, in some cases permanently. It is estimated that 30–50% of cellular proteins are altered or dysfunctional in cells of older animals due to free radicals (Levine and Stadtman, 2001, Stadtman, 1995). Furthermore, oxidation of proteins has been demonstrated to affect the enzymatic activity of certain proteins

Oxidant stress of red blood cells (RBCs)

Red blood cells are the most abundant cells in the human body and primarily act to transport O2 and CO2 between the lungs and tissues of the body. RBCs pass through the lungs approximately once per minute, where they are exposed to conditions of oxidative stress. A unique feature of RBCs is an effective antioxidant system that protects these cells as well as other organs and tissues from free radical attack. However, despite the presence of an antioxidant system, RBCs are highly susceptible to

Glutathione (GSH)

The glutathione system is pivotal and thought to be a critical safeguard in the cellular defense against oxidative stress. GSH plays a central role as a non-enzymatic antioxidant and functions to eliminate peroxides and maintain the thiol/disulfide redox state of proteins that are critical for proper biological function. GSH also maintains the redox state of ascorbate enhancing its function as a non-enzymatic antioxidant (Jones, 2006).

Several laboratories have reported that reduced glutathione

Inflammation as a form of oxidant stress

In the format of this review, we are unable to present a complete review of inflammation. However, we feel that it is important to mention inflammation because of its association with oxidative stress and with aging and age-related diseases.

Inflammation is the body's reaction to both endogenous and exogenous harmful stimuli and the initiation of the healing process, which can be classified into two types, acute and chronic. Chronic inflammation has cellular side effects, including production of

Closing remarks and perspectives

In the more than 50 years since the introduction of the Harman Free Radical Theory on Aging, great strides have been made to understand the role of free radicals in human health and disease. Our schematic model (Fig. 1) highlights pathology, age, race and genotype (non-modifiable risk factors) as well as modifiable risk factors (diet, socioeconomic status, environment and behavior) may interact with oxidant stress and inflammatory processes to contribute to aging and age-related disease. These

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