Diverse functions of eukaryotic cells are optimized by organization of compatible chemistries into distinct compartments defined by the structures of lipid-containing membranes, multiprotein complexes and oligomeric structures of saccharides and nucleic acids. This structural and chemical organization is coordinated, in part, through cysteine residues of proteins which undergo reversible oxidation-reduction and serve as chemical/structural transducing elements. The central thiol/ disulfide redox couples, thioredoxin-1, thioredoxin-2, GSH/GSSG and cysteine/cystine (Cys/CySS), are not in equilibrium with each other and are maintained at distinct, non-equilibrium potentials in mitochondria, nuclei, the secretory pathway and the extracellular space. Mitochondria contain the most reducing compartment, have the highest rates of electron transfer and are highly sensitive to oxidation. Nuclei also have more reduced redox potentials but are relatively resistant to oxidation. The secretory pathway contains oxidative systems which introduce disulfides into proteins for export. The cytoplasm contains few metabolic oxidases and this maintains an environment for redox signaling dependent upon NADPH oxidases and NO synthases. Extracellular compartments are maintained at stable oxidizing potentials. Controlled changes in cytoplasmic GSH/GSSG redox potential are associated with functional state, varying with proliferation, differentiation and apoptosis. Variation in extracellular Cys/CySS redox potential is also associated with proliferation, cell adhesion and apoptosis. Thus, cellular redox biology is inseparable from redox compartmentalization. Further elucidation of the redox control networks within compartments will improve the mechanistic understanding of cell functions and their disruption in disease.

Understanding the dynamics of redox elements in biologic systems remains a major challenge for redox signaling and oxidative stress research. Central redox elements include evolutionarily conserved subsets of cysteines and methionines of proteins which function as sulfur switches and labile reactive oxygen species (ROS) and reactive nitrogen species (RNS) which function in redox signaling. The sulfur switches depend upon redox environments in which rates of oxidation are balanced with rates of reduction through the thioredoxins, glutathione/glutathione disulfide and cysteine/cystine redox couples. These central couples, which we term redox control nodes, are maintained at stable but non-equilibrium steady states, are largely independently regulated in different subcellular compartments and are quasi-independent from each other within compartments. Disruption of the redox control nodes can differentially affect sulfur switches, thereby creating a diversity of oxidative stress responses. Systems biology provides approaches to address the complexity of these responses. In the present review, we summarize thiol/disulfide pathway, redox potential and rate information as a basis for kinetic modeling of sulfur switches. The summary identifies gaps in knowledge especially related to redox communication between compartments, definition of redox pathways and discrimination between types of sulfur switches. A formulation for kinetic modeling of GSH/GSSG redox control indicates that systems biology could encourage novel therapeutic approaches to protect against oxidative stress by identifying specific redox-sensitive sites which could be targeted for intervention.

New methods to measure thiol oxidation show that redox compartmentation functions as a mechanism for specificity in redox signaling and oxidative stress. Redox Western analysis and redox-sensitive green fluorescent proteins provide means to quantify thiol/disulfide redox changes in specific subcellular compartments. Analyses using these techniques show that the relative redox states from most reducing to most oxidizing are mitochondria > nuclei > cytoplasm > endoplasmic reticulum > extracellular space. Mitochondrial thiols are an important target of oxidant-induced apoptosis and necrosis and are especially vulnerable to oxidation because of the relatively alkaline pH. Maintenance of a relatively reduced nuclear redox state is critical for transcription factor binding in transcriptional activation in response to oxidative stress. The new methods are applicable to a broad range of experimental systems and their use will provide improved understanding of the pharmacologic and toxicologic actions of drugs and toxicants.

Shear stress differentially regulates production of many vasoactive factors at the level of gene expression in endothelial cells that may be mediated by mitogenactivated protein kinases, including extracellular signal-regulated kinase (ERK) and N-terminal Jun kinase (JNK). Here we show, using bovine aortic endothelial cells (BAEC), that shear stress differentially regulates ERK and JNK by mechanisms involving G i2 and pertussis toxin (PTx)-insensitive G-protein-dependent pathways, respectively. Shear activated ERK with a rapid, biphasic time course (maximum by 5 min and basal by 30-min shear exposure) and force dependence (minimum and maximum at 1 and 10 dyn/cm 2 shear stress, respectively). PTx treatment prevented shear-dependent activation of ERK1/2, consistent with a G i -dependent mechanism. In contrast, JNK activity was maximally turned on by a threshold level of shear force (0.5 dyn/ cm 2 or higher) with a much slower and prolonged time course (requiring at least 30 min to 4 h) than that of ERK. Also, PTx had no effect on shear-dependent activation of JNK. To further define the shear-sensitive ERK and JNK pathways, vectors expressing hemagglutinin epitope-tagged ERK (HA-ERK) or HA-JNK were cotransfected with other vectors by using adenoviruspolylysine in BAEC. Expression of the mutant ␣ i2 (G203), antisense G␣ i2 and a dominant negative Ras (N17Ras) prevented shear-dependent activation of HA-ERK, while that of ␣ i2 (G204) and antisense ␣ i3 did not. Expression of a G␤/␥ scavenger, the carboxyl terminus of ␤-adrenergic receptor kinase (␤ARK-ct), and N17Ras inhibited sheardependent activation of HA-JNK. Treatment of BAEC with genistein prevented shear-dependent activation of ERK and JNK, indicating the essential role of tyrosine kinase(s) in both ERK and JNK pathways. These results provide evidence that 1) G i2 -protein, Ras, and tyrosine kinase(s) are upstream regulators of shear-dependent activation of ERK and 2) that shear-dependent activation of JNK is regulated by mechanisms involving G␤/␥, Ras, and tyrosine kinase(s).Endothelial cells lining the inner vessel wall are in direct contact with flowing blood, which generates a frictional force, hemodynamic shear stress, acting on the surface of the endothelium. Hemodynamic shear stress controls vascular tone, vessel wall remodeling, interaction of blood cells with endothelium, coagulation, and fibrinolysis (1). The focal pattern of atherosclerotic lesions in areas of low and/or unstable shear stress further highlights the importance of shear stress in the atherogenic process (2, 3). Endothelial cells play a key role in shear-dependent vascular changes, sensing shear stress by an unidentified mechanoreceptor(s) followed by production of autocrine and paracrine factors (1). For example, hemodynamic shear stress selectively and differentially regulates production of intercellular adhesion molecule-1, vascular cell adhesion molecule-1, platelet-derived growth factor-B, basic fibroblast growth factor, transforming growth factor ␤-1, tissue plasminogen activator,...