Current Medicinal Chemistry - Volume 8, Issue 7, 2001

The age-related neurodegenerative diseases exemplified by Alzheimer's disease (AD), Lewy body diseases such as Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease are characterized by the deposition of abnormal forms of specific proteins in the brain. Although several factors appear to underlie the pathological depositions, the cause of neuronal death in each disease appears to be multifactorial. In this regard, evidence in each case for a role of oxidative stress is provided by the finding that the pathological deposits are immunoreactive to antibodies recognizing protein side-chains modified either directly by reactive oxygen or nitrogen species, or by products of lipid peroxidation or glycoxidation. Although the source(s) of increased oxidative damage are not entirely clear, the findings of increased localization of redox-active transition metals in the brain regions most affected is consistent with their contribution to oxidative stress. It is tempting to speculate that free radical oxygen chemistry plays a pathogenetic role in all these neurodegenerative conditions, though it is as yet undetermined what types of oxidative damage occur early in pathogenesis, and what types are secondary manifestations of dying neurons. Delineation of the profile of oxidative damage in each disease will provide clues to how the specific neuronal populations are differentially affected by the individual disease conditions.

Hypoxia is a feature of some regions of many tumours, ischaemic events, and arthritis. Drugs activated in hypoxia have wide potential application, particularly in overcoming the resistance of hypoxic tumour cells to radiotherapy. Key features of such drugs include redox properties appropriate for activation by reductase enzymes (typically flavoproteins), and oxygen-sensitive reduction chemistry such that normal levels of oxygen inhibit or reverse reduction. In many cases this selectivity is achieved by a fast, free-radical reaction in which the drug radical (often an obligate intermediate in drug reduction) reduces oxygen to form superoxide radicals and thus ‘futile cycles’ the drug in normoxic tissues. However, this enhances cellular oxidative stress, which may be linked to normal tissue toxicity. Appropriate redox properties are found with nitroarene, quinone, or aromatic N-oxide moieties. A particularly promising and versatile exploitation of bioreductive activation is for reduction of such ‘triggers’ to activate release of an ‘effector’, an agent that can obviously be active against diverse conditions associated with hypoxia. The same approach can also be used in diagnosis of hypoxia. Much information concerning the reactions of intermediates in drug action and the quantitative prediction of redox properties of analogues has been accrued. Drug design can be mechanism-led, with the wealth of literature quantifying redox properties of drug candidates a rich source of potential new leads. There is a clear appreciation of the kinetic factors that limit drug efficacy or selectivity. Thus the potential for rapid expansion of these concepts to diverse diseases is considerable.

Reactive oxygen (ROS) and reactive nitrogen species (RNS) produced in vivo at levels that cannot be dealt with adequately by endogenous antioxidant systems can lead to the damage of lipids, proteins, carbohydrates and nucleic acids. Oxidative modification of these molecules by toxic levels of ROS and RNS represents an extreme event that can lead to deleterious consequences such as loss of function. More recently, however, interest has focused on the formation of these species at sub-toxic levels and their potential to act as biological signal molecules. Subtoxic ROS and RNS production can lead to alterations in cellular and extracellular redox state, and it is such alterations that have been shown to signal changes in cell functions. By the use of a variety of cell types it has been shown that numerous cellular processes including gene expression can be regulated by subtle changes in redox balance Examples of this include the activation of certain nuclear transcription factors, and the determination of cellular fate by apoptosis or necrosis. Cellular redox balance is, under normal circumstances, probably under genetic control and maintained by an array of enzymatic systems that ensure that overall reducing conditions prevail. Thiols, by virtue of their ability to be reversibly oxidised, are recognised as key components involved in the maintenance of redox balance. Additionally, increasing evidence suggests that thiol groups located on various molecules act as redox sensitive switches thereby providing a common trigger for a variety of ROS and RNS mediated signalling events. In this review we discuss a number of cellular processes in which ROS and RNS have been implicated in redox signalling mechanisms. Particular attention has been paid to the importance of thiols and thiol-containing molecules in these processes.

For more than half a century, numerous proposals have been advanced for the mode of action of carcinogens. This review presents a wide array of evidence that implicates oxidative stress (OS) in many aspects of oncology, including: formation of reactive oxygen species (ROS) by the major classes of carcinogens (as well as minor ones), cancer stages, oncogene activation, aging, genetic and infectious illnesses, nutrition, and the role of antioxidants (AOs). Although diverse origins pertain, inclu-ding both endogenous and exogenous agents, ROS are frequently generated by redox cycling via electron transfer (ET) groups, e.g., quinones (or phenolic precursors), metal complexes (or complexors), aromatic nitro compounds (or reduced products), and conjugated imines (or iminium species). We believe it is not coincidental that these functionalities are often found in carcinogens or their metabolites. The pervasive aspects of DNA binding by ultimate carcinogens, and mutations caused by ROS are treated. Often, ROS are implicated in more conventional rationales, such as oncogenes. A multi-faceted approach to mechanisms appears to be the most logical. The OS unifying theme represents an approach which is able to rationalize the diverse data associated with carcinogenesis. Because this theoretical framework aids in the understanding of cancer initiation, it can serve as a useful tool in combating cancer, particularly in relation to prevention. Significantly, the electron transfer - oxidative stress (ET-OS) scenario can also be applied to many drug categories, toxins, enzymes, and hormones.

In order to ascertain the role of dietary flavonoids as antioxidants in vivo it is necessary to understand the chemical nature of the absorbed forms in the circulation in vivo and how the multiplicity of research findings in vitro reflect the bioactivity of flavonoids in vivo. Only when we gain adequate information on the circulating forms can we begin to understand the targeting to the tissues, whether flavonoids cross the blood-brain barrier, for example, and in what forms. Flavonoids are powerful antioxidants in vitro, but their overall function in vivo has yet to be clarified, whether antioxidant, anti-inflammatory, enzyme inhibitor, enzyme inducer, inhibitor of cell division, or some other role. It should also be emphasised that the reducing properties of flavonoids might contribute to redox regulation in cells, independently of their antioxidant properties, and thus might protect against cell ageing, for example, by working together with the intracellular reductant network. To gain understanding of these issues the factors influencing the absorption of flavonoids in the gastrointestinal tract needs to be established, namely the questions of: de-glycosylation before absorption, conjugation in the small intestine through glucuronidation, sulphation or methylation etc, metabolism and degradation in the colon to smaller phenolic molecules. The forms in which they circulate in vivo will influence their polarity and, thus, their localization and bioactivities in vivo. Finally if antioxidant activities are important, the elucidation of how such properties in vitro relate to the potential for conjugates and metabolites in vivo to act as antioxidants is required. The absorbed flavonoid components might function in the aqueous phase (like vitamin C) or in the lipophilic milieu (as vitamin E) in vivo. This will depend on their polarity properties on uptake, how they are metabolised on absorption, and their resulting structural forms in the circulation.

Parkinson's disease occurs in 1percent of people over the age of 65 when about 60percent of the dopaminergic neurons in the substantia nigra of the midbrain are lost. Dopaminergic neurons appear to die by a process of apoptosis that is induced by oxidative stress. Oxygen radicals abstract hydrogen from DNA forming DNA radicals that lead to DNA fragmentation, activation of DNA protective mechanisms, NAD depletion and apoptosis. Oxygen radicals can be formed in dopaminergic neurons by redox cycling of MPP+ , the active metabolite of MPTP. This redox cycling mechanism involves the reduction of MPP+ by a number of enzymes, especially flavin containing enzymes, some of which are found in mitochondria. Tyrosine hydroxylase is present in all dopaminergic neurons and is responsible for the synthesis of dopamine. However, tyrosine hydroxylase can form oxygen radicals in a redox mechanism involving its cofactor, tetrahydrobiopterin. Dopamine may be oxidized by monoamine oxidase to form oxygen radicals and 3,4-dihydroxyphenylacetaldehyde. This aldehyde may be oxidized by aldehyde dehydrogenase with the formation of oxygen radicals and 3,4-dihydroxyphenylacetic acid. The redox mechanisms of oxygen radical formation by MPTP, tyrosine hydroxylase, monoamine oxidase and aldehyde dehydrogenase will be discussed. Possible clinical applications of these mechanisms will be briefly presented.

Oxidative stress in brain is emerging as a potential causal factor in aging and age-related neurodegenerative disorders. Brain tissue from living patients is difficult to acquire; hence, animal models of aging and age-related neurodegenerative disorders, though not perfect models, have provided tissue to study the role of oxidative stress in these disorders. In this review, the central role of oxidative damage in brain in models of accelerated aging (progeria and Werner's syndrome) and the age-related neurodegenerative disorders, Alzheimer's disease and Huntington's disease, will be presented and evaluated. To the extent that the animal models faithfully mirror their respective disorders, and based on the totality of the studies, it is apparent that oxidative stress, the excess of free radicals over the means of scavenging these harmful agents, may play critical roles in the molecular basis of accelerated aging, Alzheimer's disease, and Huntington's disease.

Increased aerobic metabolism during exercise is a potential source of oxidative stress. In muscle, mitochondria are one important source of reactive intermediates that include superoxide,hydrogen peroxide, and possibly hydroxyl radical . The recent discovery that mitochondria may generate nitric oxide also has implications for oxidant production and mitochondrial function. In this review, we critically examine the concept that production of reactive intermediates increases during exercise. Because the health benefits of regular exercise are well-documented, we also examine adaptations to exercise that may decrease oxidative stress. These include increased antioxidant defenses, reduced basal production of oxidants, and reduction of radical leak during oxidative phosphorylation.

Redox cycling of catecholestrogen metabolites between quinone and catechol forms is a mechanism of generating potentially mutagenic oxygen radicals in estrogen-induced carcinogenesis. Consistent with this concept, multiple forms of oxygen radical-generated DNA damage are induced by estrogen in cell-free systems, in cells in culture and in rodents prone to estrogen-induced cancer. Metal ions, specifically iron, are necessary for the production of hydroxy radicals. Iron has not received much attention in discussions of estrogen-induced carcinogenesis and human hormone-associated cancer, and is the focus of this review. An elevated dietary iron intake enhances the incidence of carcinogen-induced mammary cancer in rats and estrogen-induced kidney tumors in Syrian hamsters. Estrogen administration increases iron accumulation in hamsters and facilitates iron uptake by cells in culture. In humans, elevated body iron storage has been shown to increase the risk of several cancers including breast cancer. A role of iron in hormone-associated cancer in humans offers attractive routes for cancer prevention by regulating metal ion metabolism and interfering with iron accumulation in tissues.

Impairment of normal spermatogenesis and sperm function are the most common causes of male factor infertility. Abnormal sperm function is difficult to evaluate and treat. There is a lack of understanding of the factors contributing to normal and abnormal sperm function leading to infertility. Many recent studies indicate that oxygen-derived free radicals induce damage to spermatozoa. The excessive generation of these reactive oxygen species (superoxide, hydroxyl, nitric oxide, peroxide, peroxynitrile) by immature and abnormal spermatozoa and by contaminating leukocytes associated with genitourinary tract inflammation have been identified with idiopathic male infertility. Mammalian sper-matozoa membranes are rich in polyunsaturated fatty acids. This makes them very susceptible to oxygen-induced damage, which is mediated by lipid peroxidation. In a normal situation, the antioxidant mechanisms present in the reproductive tissues and their secretions are likely to quench these reactive oxygen species (ROS) and protect against oxidative damage to gonadal cells and mature spermatozoa. During chronic disease states, aging, toxin exposure, or genitourinary infection / inflammation, these cellular antioxidant mechanisms downplay and create a situation called oxidative stress. Thus, a balance between ROS generation and antioxidant capacity plays a critical role in the pathophysiology of disease state. Recent efforts towards the development of new reliable assays to evaluate this oxidative stress status have resulted in the establishment of ROS-TAC score. Such assessment of oxidative stress status (OSS) may help in designing newer modes of male factor infertility treatment by suitable antioxidants.

Reproductive toxicity has been a topic of increasing interest and concern in recent years, generating controversy in association with danger to humans and other living things. A veritable host of chemicals is known to be involved, encompassing a wide variety of classes, both organic and inorganic. Exposure is pervasive and virtually unavoidable due to contamination of air, water, ground, food, beverages, drugs, and household items. The corresponding adverse effects on reproduction are numerous. There is uncertainty regarding mode of action although various theories have been advanced, e.g., disruption of the CNS, DNA attack, enzyme inhibition, interference with hormonal action, and insult to membranes and proteins. This review provides extensive evidence for involvement of oxidative stress (OS) and electron transfer (ET) as a unifying theme. Successful application is made to all of the main classes of toxins, in addition to large numbers of miscellaneous types. We believe it is not coincidental that the vast majority of these substances incorporate ET functionalities (quinone, metal complex, ArNO 2 , or conjugated iminium) either per se or in metabolites, potentially giving rise to reactive oxygen species (ROS) by redox cycling. Some categories, e.g., peroxides and radiation, appear to generate ROS by non-ET routes. For completeness, other theories are also addressed; a multifaceted approach appears to be the most logical. Our framework should increase understanding and contribute to preventative measures, such as use of antioxidants (AOs). The ET-OS theory has recently been used as the central theme by us in reviews of biomechanisms involved with anti-infective drugs, anticancer agents, and carcinogens (see text).