Redoxins as gatekeepers of the transcriptional oxidative stress response

Abstract

Transcription factors control the rate of transcription of genetic information from DNA to messenger RNA, by binding specific DNA sequences in promoter regions. Transcriptional gene control is a rate-limiting process that is tightly regulated and based on transient environmental signals which are translated into long-term changes in gene transcription. Post-translational modifications (PTMs) on transcription factors by phosphorylation or acetylation have profound effects not only on sub-cellular localization but also on substrate specificity through changes in DNA binding capacity. During times of cellular stress, specific transcription factors are in place to help protect the cell from damage by initiating the transcription of antioxidant response genes. Here we discuss PTMs caused by reactive oxygen species (ROS), such as H₂O₂, that can expeditiously regulate the activation of transcription factors involved in the oxidative stress response. Part of this rapid regulation are proteins involved in H₂O₂-related reduction and oxidation (redox) reactions such as redoxins, H₂O₂ scavengers described to interact with transcription factors. Redoxins have highly reactive cysteines of rate constants around 6-10^-1 s^-1 that engage in nucleophilic substitution of a thiol-disulfide with another thiol in inter-disulfide exchange reactions. We propose here that H₂O₂ signal transduction induced inter-disulfide exchange reactions between redoxin cysteines and cysteine thiols of transcription factors to allow for rapid and precise on and off switching of transcription factor activity. Thus, redoxins are essential modulators of stress response pathways beyond H₂O₂ scavenging capacity.

Fig. 1

Formation and elimination of reactive oxygen species. ROS are chemically reactive chemical species that contain oxygen. The reduction of oxygen (O₂) produces superoxide, which in the presence of hydrogen can further dismutate to hydrogen peroxide (H₂O₂) (I)·H₂O₂, in turn, may be partially reduced to hydroxyl radical (∙OH) or fully reduced to water (H₂O) (II)·H₂O₂ can react with ∙O₂^- (Haber Weiss reaction, Fig. 1, III) or ferrous iron (Fe²⁺) (Fenton reaction, Fig. 1, IV) and hydroxyl radicals (∙OH) and hydroxide ions (HO^-) are generated.

Fig. 2

Stepwise oxidation of cysteine residues. Oxidation of the sulfur atom within a cysteine residue can result in the stepwise formation of a reactive cysteine thiolate, sulfenic acid, sulfinic acid, and finally sulfonic acid. With the aid of reducing systems including NADPH, as well as thioredoxin reductase (TRXR) or glutathione reductase (GR), the oxidation reactions leading to disulfide formation are reversible. On the other hand, oxidation to sulfinic (not for peroxiredoxins) or sulfonic acid is irreversible.

Fig. 3

Thiol-disulfide exchange. A and B. A simplified model shows the thiolate group (nucleophile) on protein R to attack a sulfur atom of the protein disulfide bond of R′ and R′′. This creates a temporary "tri-thiol" ion (B) and ultimately displaces the other sulfur atom in the disulfide (protein R′′). This reaction leads in turn to the formation of a new disulfide bond between proteins R and R′, and a new thiolate ion (nucleophile) on protein R′′.

Fig. 4

Neutralization of ROS by different antioxidants. SOD: Superoxide dismutase; Cat: Catalase; GPX: Glutathione peroxidase; GSH: Glutathione; TRX: Thioredoxin; PRDX: Peroxiredoxin.

Fig. 5

H₂O₂ sensing by OxyR. In its reduced form OxyR binds to DNA promoter regions of target genes preventing transcription. After oxidation of the Cys199 of OxyR by H2O2, sulfenic acid forms that attacks Cys208 to form an intradisulfide. These structural changes in OxyR allow in turn recruitment of RNA polymerase and activation of transcription. The Cys199-Cys208 bond is then reduced by GRX1, which is regenerated by GSH.

Fig. 6

Redoxin induced transcription factor activation and inactivation. (A) The redoxin, PRDX2, oxidizes STAT3 via a thiol-disulfide exchange. (B) Based on (A) the following model for PRDX1 regulation of FOXO3 is proposed: PRDX1 dimer forms an oligomer with free FOXO3 via disulfide bonds.

Fig. 7

Redox-regulation of NF-kB by TRX. In resting cells, p50 Cys62 is highly oxidized in the cytoplasm but is rapidly reduced by TRX once NF-κB has migrated into the nucleus. After the reduction of Cys62, a zinc ion replaces the inter-molecular disulfide bridge and dissociates NF-kB from TRX leading to NF-kB DNA binding and activity. Recombinant p50 forms a homodimer involving Cys62 of another p50 protein. However, a biological role of this homodimer remains elusive. Therefore, it is entirely possible that Cys62 oxidation spurs disulfide-based heterodimers with p65 or even another protein.