Redox outside the box: linking extracellular redox remodeling with intracellular redox metabolism

Abstract

Aerobic organisms generate reactive oxygen species as metabolic side products and must achieve a delicate balance between using them for signaling cellular functions and protecting against collateral damage. Small molecule (e.g. glutathione and cysteine)- and protein (e.g. thioredoxin)-based buffers regulate the ambient redox potentials in the various intracellular compartments, influence the status of redox-sensitive macromolecules, and protect against oxidative stress. Less well appreciated is the fact that the redox potential of the extracellular compartment is also carefully regulated and is dynamic. Changes in intracellular metabolism alter the redox poise in the extracellular compartment, and these are correlated with cellular processes such as proliferation, differentiation, and death. In this minireview, the mechanism of extracellular redox remodeling due to intracellular sulfur metabolism is discussed in the context of various cell-cell communication paradigms.

FIGURE 1.

Changes in redox potentials and cellular function. A, the steady-state cytoplasmic redox potentials for the three major thiol-based buffers in differentiated cells are shown inside the box, whereas the extracellular redox potentials for the GSH/GSSG and cysteine/cystine redox couple are shown outside. Shifts in the reductive and oxidative directions are associated with proliferation and apoptosis, respectively. Trx_red/Trx_ox, reduced/oxidized thioredoxin. B, the GSH/cysteine cycle connects intra- and extracellular GSH and cysteine pools. Cysteine and cystine can be transported via Na⁺-dependent and Na⁺-independent transporters. Although the mechanisms of cystine reduction are not fully understood, it is reduced in the intracellular milieu, e.g. by GSH or thioredoxin. Alternatively, cysteine can be produced from homocysteine, which is a product of the methionine cycle, via the transsulfuration pathway catalyzed by cystathionine β-synthase (CBS) and γ-cystathionase (CSE). Cysteine is incorporated into GSH via the actions of γ-glutamylcysteine ligase (GCL) and glutathione synthetase (GS). The transmembrane GSH/cysteine cycle, shown in red, results in circulation of intracellular GSH to the outside, where it is cleaved by the actions of γ-GT and a dipeptidase (DP). X and X-CH₃ represent a methyl group acceptor and a methylated acceptor, respectively.

FIGURE 2.

Intercellular redox signaling and metabolite changes. A, redox remodeling during T cell activation. Induction of the x_C⁻ transporter in dendritic cells leads to increased consumption of cystine, which, via the γ-glutamyl cycle, is converted to extracellular cysteine, which is imported by T cells and used in GSH biosynthesis. The reductive shift in the extracellular redox potential during T cell activation has been documented to increase the proportion of thiols on exofacial membrane protein domains in dendritic cells and T cells. B, activated T cells secrete large quantities of glutamate that are efficiently cleared via the X_AG⁻ transporter on astrocytes. In turn, this activates the x_C⁻ transporter, stimulating cystine consumption and GSH synthesis, secretion, and cleavage. Cysteine derived from extracellular GSH is taken up by neurons and converted to GSH and serves a neuroprotective function. Although the status of membrane protein thiols under these co-culture conditions has not been characterized, the reductive shift in the extracellular compartment is expected to increase the proportion of thiols versus disulfides and may be important in initiating or curtailing signaling pathways. The upward and downward red arrows indicate increases and decreases, respectively.

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