At the end of the 1980s, it was clearly demonstrated that cells produce nitric oxide and that this gaseous molecule is involved in the regulation of the cardiovascular, immune and nervous systems, rather than simply being a toxic pollutant. In the CNS, nitric oxide has an array of functions, such as the regulation of synaptic plasticity, the sleep-wake cycle and hormone secretion. Particularly interesting is the role of nitric oxide as a Janus molecule in the cell death or survival mechanisms in brain cells. In fact, physiological amounts of this gas are neuroprotective, whereas higher concentrations are clearly neurotoxic.

Despite the capacity of chaperones and other homeostatic components to restore folding equilibrium, cells appear poorly adapted for chronic oxidative stress that increases in cancer and in metabolic and neurodegenerative diseases. Modulation of endogenous cellular defense mechanisms represents an innovative approach to therapeutic intervention in diseases causing chronic tissue damage, such as in neurodegeneration. This article introduces the concept of hormesis and its applications to the field of neuroprotection. It is argued that the hormetic dose response provides the central underpinning of neuroprotective responses, providing a framework for explaining the common quantitative features of their dose-response relationships, their mechanistic foundations, and their relationship to the concept of biological plasticity, as well as providing a key insight for improving the accuracy of the therapeutic dose of pharmaceutical agents within the highly heterogeneous human population. This article describes in mechanistic detail how hormetic dose responses are mediated for endogenous cellular defense pathways, including sirtuin and Nrf2 and related pathways that integrate adaptive stress responses in the prevention of neurodegenerative diseases. Particular attention is given to the emerging role of nitric oxide, carbon monoxide, and hydrogen sulfide gases in hormetic-based neuroprotection and their relationship to membrane radical dynamics and mitochondrial redox signaling.

Modulation of endogenous cellular defense mechanisms represents an innovative approach to therapeutic intervention in diseases causing chronic tissue damage, such as in neurodegeneration. This paper introduces the emerging role of exogenous molecules in hormetic-based neuroprotection and the mitochondrial redox signaling concept of hormesis and its applications to the field of neuroprotection and longevity. Maintenance of optimal long-term health conditions is accomplished by a complex network of longevity assurance processes that are controlled by vitagenes, a group of genes involved in preserving cellular homeostasis during stressful conditions. Vitagenes encode for heat shock proteins (Hsp) Hsp32, Hsp70, the thioredoxin and the sirtuin protein systems. Dietary antioxidants, such as polyphenols and L-carnitine/acetyl-L-carnitine, have recently been demonstrated to be neuroprotective through the activation of hormetic pathways, including vitagenes. Hormesis provides the central underpinning of neuroprotective responses, providing a framework for explaining the common quantitative features of their dose response relationships, their mechanistic foundations, their relationship to the concept of biological plasticity as well as providing a key insight for improving the accuracy of the therapeutic dose of pharmaceutical agents within the highly heterogeneous human population. This paper describes in mechanistic detail how hormetic dose responses are mediated for endogenous cellular defense pathways including sirtuin, Nrfs and related pathways that integrate adaptive stress responses in the prevention of neurodegenerative diseases. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.

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The predominant molecular symptom of aging is the accumulation of altered gene products. Moreover, several conditions including protein, lipid or glucose oxidation disrupt redox homeostasis and lead to accumulation of unfolded or misfolded proteins in the aging brain. Alzheimer's and Parkinson's diseases or Friedreich ataxia are neurological diseases sharing, as a common denominator, production of abnormal proteins, mitochondrial dysfunction and oxidative stress, which contribute to the pathogenesis of these so called "protein conformational diseases". The central nervous system has evolved the conserved mechanism of unfolded protein response to cope with the accumulation of misfolded proteins. As one of the main intracellular redox systems involved in neuroprotection, the vitagene system is emerging as a neurohormetic potential target for novel cytoprotective interventions. Vitagenes encode for cytoprotective heat shock proteins (Hsp) Hsp70 and heme oxygenase-1, as well as thioredoxin reductase and sirtuins. Nutritional studies show that ageing in animals can be significantly influenced by dietary restriction. Thus, the impact of dietary factors on health and longevity is an increasingly appreciated area of research. Reducing energy intake by controlled caloric restriction or intermittent fasting increases lifespan and protects various tissues against disease. Genetics has revealed that ageing may be controlled by changes in intracellular NAD/NADH ratio regulating sirtuin, a group of proteins linked to aging, metabolism and stress tolerance in several organisms. Recent findings suggest that several phytochemicals exhibit biphasic dose responses on cells with low doses activating signaling pathways that result in increased expression of vitagenes encoding survival proteins, as in the case of the Keap1/Nrf2/ARE pathway activated by curcumin and NAD/NADH-sirtuin-1 activated by resveratrol. Consistently, the neuroprotective roles of dietary antioxidants including curcumin, acetyl-L-carnitine and carnosine have been demonstrated through the activation of these redox-sensitive intracellular pathways. Although the notion that stress proteins are neuroprotective is broadly accepted, still much work needs to be done in order to associate neuroprotection with specific pattern of stress responses. In this review the importance of vitagenes in the cellular stress response and the potential use of dietary antioxidants in the prevention and treatment of neurodegenerative disorders is discussed.

Heme oxygenase-1 (HO-1) is a redox-sensitive inducible protein that provides efficient cytoprotection against oxidative stress. Curcumin, a polyphenolic natural compound that possesses anti-tumor and anti-inflammatory properties, has been reported recently to induce potently HO-1 expression in vascular endothelial cells (Free Rad Biol Med 28:1303-1312. Here, we extend our previous findings by showing that caffeic acid phenethyl ester (CAPE), another plant-derived phenolic agent, markedly increases heme oxygenase activity and HO-1 protein in astrocytes. The effect seems to be related to the peculiar chemical structures of curcumin and CAPE, because analogous antioxidants containing only portions of these two molecules were totally ineffective. At a final concentration of 30 M, both curcumin and CAPE maximally up-regulated heme oxygenase activity while promoting marked cytotoxicity at higher concentrations (50 -100 M). Similar results were obtained with Curcumin-95, a mixture of curcuminoids commonly used as a dietary supplement. Incubation of astrocytes with curcumin or CAPE at concentrations that promoted maximal heme oxygenase activity resulted in an early increase in reduced glutathione followed by a significant elevation in oxidized glutathione contents. A curcumin-mediated increase in heme oxygenase activity was not affected by the glutathione precursor and thiol donor N-acetyl-L-cysteine. These data suggest that regulation of HO-1 expression by polyphenolic compounds is evoked by a distinctive mechanism which is not necessarily linked to changes in glutathione but might depend on redox signals sustained by specific and targeted sulfydryl groups. This study identifies a novel class of natural substances that could be used for therapeutic purposes as potent inducers of HO-1 in the protection of tissues against inflammatory and neurodegenerative conditions. Heme oxygenase-1 (HO-1) is a ubiquitous and redox-sensitive inducible stress protein (Motterlini et al., 2002). In mammals, the crucial participation of HO-1 gene expression in alleviating organ dysfunction and counteracting metabolic disorders is supported by consistent reports showing a protective role for the products of the enzymatic activity of HO-1. Heme serves as a substrate for HO-1 in the formation of carbon monoxide, free ferrous iron, and biliverdin; the latter is rapidly converted to bilirubin by biliverdin reductase (Choi and Alam, 1996;. A substantial body of evidence demonstrates that increased carbon monoxide and bilirubin effectively contribute to modulate important physiological processes within the cardiovascular, immune, and nervous systems. These include the regulation of vessel tone (Motterlini et al., 1998), inhibition of platelet aggregation (Durante and Schafer, 1998), and prevention of cell death and tissue injury (Clark et al., 2000b). The overall concept emerging from these and other studies is that the induction of HO-1 is an essential step in the cellular adaptation to stress inflicted by pathological events.Apart from ...

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This article discusses selected aspects of secondary brain injury after trauma and outlines key mechanisms associated with toxicity, oxidative stress, inflammation, and necrosis. Finally, this review discusses the role of different oxidants and presents potential clinically relevant molecular targets that could be harnessed to treat secondary injury associated with brain trauma.