Nitrative and oxidative stress in toxicology and disease

Abstract

Persistent inflammation and the generation of reactive oxygen and nitrogen species play pivotal roles in tissue injury during disease pathogenesis and as a reaction to toxicant exposures. The associated oxidative and nitrative stress promote diverse pathologic reactions including neurodegenerative disorders, atherosclerosis, chronic inflammation, cancer, and premature labor and stillbirth. These effects occur via sustained inflammation, cellular proliferation and cytotoxicity and via induction of a proangiogenic environment. For example, exposure to the ubiquitous air pollutant ozone leads to generation of reactive oxygen and nitrogen species in lung macrophages that play a key role in subsequent tissue damage. Similarly, studies indicate that genes involved in regulating oxidative stress are altered by anesthetic treatment resulting in brain injury, most notable during development. In addition to a role in tissue injury in the brain, inflammation, and oxidative stress are implicated in Parkinson's disease, a neurodegenerative disease characterized by the loss of dopamine neurons. Recent data suggest a mechanistic link between oxidative stress and elevated levels of 3,4-dihydroxyphenylacetaldehyde, a neurotoxin endogenous to dopamine neurons. These findings have significant implications for development of therapeutics and identification of novel biomarkers for Parkinson's disease pathogenesis. Oxidative and nitrative stress is also thought to play a role in creating the proinflammatory microenvironment associated with the aggressive phenotype of inflammatory breast cancer. An understanding of fundamental concepts of oxidative and nitrative stress can underpin a rational plan of treatment for diseases and toxicities associated with excessive production of reactive oxygen and nitrogen species.

FIG. 1.

A schematic depicting the impact of NO and ROS on the intrinsic and extrinsic pathways that regulate apoptosis.

FIG. 2.

Cellular compartmentation of GSH and GSSG and metabolism of oxidants. Extracellular, intracellular, mitochondrial, and nuclear compartments are indicated, but other subcompartmental distinctions are not precluded. Although simplified, the schemes shown illustrate the fractal properties of branching in H₂O₂ disposition, as between GPx and other thiol/disulfide-dependent pathways, and Fe-chelate–directed reactions that are not reflected by thiol/disulfide reactions. Also illustrated is that cellular oxidation capacities are based on O₂ and reduction capacities ultimately rely upon food, with NADPH as a key intermediary for any estimates of a global or ‘the’ redox status of a cell. Other concepts to consider are that H₂O₂ could react with Fe²⁺ to form hydroxyl radicals which may lead to lipid peroxides, but perhaps not directly to LOH. DNA (both nuclear and mitochondrial) may also be a target for such radicals. Abbreviations: CoASH, coenzyme A; CoASSG, mixed disulfide of CoASH with GSH; PSH, protein thiol, PSSG, mixed disulfide with GSSG; GPx, glutathione peroxidase; GR, GSSG reductase; ALF, alveolar lining fluid; LOH and LOOH, lipid hydroxy acids and hydroperoxides, respectively.

FIG. 3.

A working model of NMDA antagonist-induced neuronal cell death. Excessive activation of upregulated NMDA receptors results in a calcium overload that exceeds the buffering capacity of the mitochondria and interferes with electron transport yielding ROS. This in turn causes dissociation and nuclear translocation of transcription proteins such as NF-κB that bind to genes such as P53 and Bcl-XL. The downregulation of Bcl-XL combined with an increase in Bax, diminishes the formation of antiapoptotic Bax/Bcl-XL heterodimers in favor of proapoptotic Bax/Bax homodimers.

FIG. 4.

Exposure to toxic levels of ozone results in alveolar epithelial damage. Macrophages responding to tissue injury release TNF-α which downregulates caveolin-1. This leads to release of signaling molecules such as PI3K, activation of NF-κB and upregulation of iNOS. RNS generated as a consequence contribute to ozone-induced lung injury.

FIG. 5.

Elucidation of IBC-related genes involved in oxidative and nitrative stress and associated inflammation. (A) PTGHS2/COX-2 mRNA is upregulated in SUM149 and SUM190 IBC cells compared with MCF-7 cells. (B) PGE₂ production by SUM149 and SUM190 cells. (C) Western blot of COX-2 and EP4 proteins in SUM149 and SUM190 IBC tumor cells. (D) Inhibition of anchorage independent growth in soft agar by SUM149 IBC cells by EP4 antagonist and EP4 shRNA.

FIG. 6.

Schematic of DA catabolism. DA catabolism yields DOPAL, which undergoes oxidation to an acid (DOPAC) or reduction to an alcohol (DOPET) as a minor pathway. Increased cytosolic DA or impairment of aldehyde metabolism yields elevated levels of the reactive and toxic intermediate, DOPAL.