Redox homeostasis: The Golden Mean of healthy living

Abstract

The notion that electrophiles serve as messengers in cell signaling is now widely accepted. Nonetheless, major issues restrain acceptance of redox homeostasis and redox signaling as components of maintenance of a normal physiological steady state. The first is that redox signaling requires sudden switching on of oxidant production and bypassing of antioxidant mechanisms rather than a continuous process that, like other signaling mechanisms, can be smoothly turned up or down. The second is the misperception that reactions in redox signaling involve "reactive oxygen species" rather than reaction of specific electrophiles with specific protein thiolates. The third is that hormesis provides protection against oxidants by increasing cellular defense or repair mechanisms rather than by specifically addressing the offset of redox homeostasis. Instead, we propose that both oxidant and antioxidant signaling are main features of redox homeostasis. As the redox shift is rapidly reversed by feedback reactions, homeostasis is maintained by continuous signaling for production and elimination of electrophiles and nucleophiles. Redox homeostasis, which is the maintenance of nucleophilic tone, accounts for a healthy physiological steady state. Electrophiles and nucleophiles are not intrinsically harmful or protective, and redox homeostasis is an essential feature of both the response to challenges and subsequent feedback. While the balance between oxidants and nucleophiles is preserved in redox homeostasis, oxidative stress provokes the establishment of a new radically altered redox steady state. The popular belief that scavenging free radicals by antioxidants has a beneficial effect is wishful thinking. We propose, instead, that continuous feedback preserves nucleophilic tone and that this is supported by redox active nutritional phytochemicals. These nonessential compounds, by activating Nrf2, mimic the effect of endogenously produced electrophiles (parahormesis). In summary, while hormesis, although globally protective, results in setting up of a new phenotype, parahormesis contributes to health by favoring maintenance of homeostasis.

Keywords: Hormesis; Kinetics; Oxidative stress; Phytochemicals; Signaling; Thiols.

Graphical abstract

Fig. 1

The “Golden Mean” of redox homeostasis. A steady-state redox status of the ensemble of redox couples is maintained by metabolic fluxes and redox feedback where electrophiles produced by aerobic life stressors activate the mechanism reestablishing nucleophilic tone. Parahormesis refers to nonessential compounds that support the redox feedback loop activating the nucleophilic response. As long as homeostasis is maintained, there is not a phenotypic switch. In contrast, a phenotypic switch occurs in adaptation when a stable offset of homeostasis takes place. In the scheme two opposing examples of stable offset of redox homeostasis are illustrated; one refers to a more oxidizing environment (e.g. oxidative stress) facilitating cell death, while the other refers to a dramatically more reducing environment (e.g., constitutive activation of Nrf2), facilitating survival. Both conditions are part of pathological phenotypes.

Fig. 2

Electrophilic activation of Nrf2. When cysteines in Keap1 are modified by electrophiles, Nrf2 escapes Keap1 assisted ubiquitinylation and subsequent proteasomal degradation and can translocate to the nucleus , , . Phosphorylation of Nrf2 by PKCδ and Akt assists in the nuclear translocation of Nrf2, where it increases transcription of genes through binding to the EpRE (also called ARE) element , . PKCδ, and Akt are also activated by hydroperoxides and other electrophiles , , , , . The proteins that are Nrf2 partners in binding to EpRE include Mafs G/F/K , c-Jun , c-Fos and Fra1 , c-Maf , JunD , , but little attention has been paid to whether any of their interactions with Nrf2 involve redox signaling. Similarly, little attention has been paid to the possibility of redox regulation of Bach1 and Nrf1 that compete with Nrf2 for binding to EpRE . Furthermore, potential redox regulation of c-Myc inhibition of transcription through its binding to nuclear Nrf2 and acceleration of Nrf2 degradation and the stabilizing influence of p21 have yet to be explored in detail. All of these aspects may be part of the redox homeostasis involved with Nrf2. Additional redox-related activation of Nrf2 may occur during stress conditions through the sequestration of Keap1 by p62 such as occurs during dysregulation of autophagy .

Fig. 3

Hydroperoxide activation of NF-κB activation. NF-κB is composed of homo- and heterodimers of p50, p52, p65 (RelA), c-Rel, and RelB. Only p65, c-Rel, and RelB have transcriptional activation domains. IκB family members bind NF-κB dimers in the cytosol until IκB is phosphorylated and degraded. IκB isoforms differ among cell types in dependence upon the IKK complex for their phosphorylation. Two serine kinases, IKKα and IKKβ, and a modulator, NEMO (also called IKKγ) form the IKK complex that phosphorylates IκB. For an extensive description of NF-κB regulation not focusing on redox regulation, the reader is referred to some recent reviews , . Early in the investigation of how H₂O₂ activated NF-κB, it became obvious that tyrosine phosphorylation was involved. H₂O₂, lipid hydroperoxides, and other electrophilic lipid peroxidation products, can directly activate members of the Src kinase family, Src, Syk, and Lyc and the closely related ZAP leading to activation of the tyrosine kinase Abl and serine kinase PKCδ that activate PKD. In an alternative pathway, Src or Syk directly phosphorylate IκB on a tyrosine residue, bypassing IKK activation. One of the more intriguing proposed targets in H₂O₂ activation of NF-κB is SHIP-1, an inositol-5-phosphatase. SHIP-1 activation by H₂O₂ has been shown to lead to the activation of the classical IKK-dependent pathway for NF-κB activation. SHIP-1 activity depends on its SH2 domain, which is of course, the classic target of tyrosine phosphorylation and in a study unrelated to NF-κB activation, SHIP-1 activity was demonstrated to be through its tyrosine phosphorylation by Lyn, another member of the Src kinase family . Other possible pathways for NF-κB activation involving the Src kinases may exist; however, the activation of the Src kinases seems to be the key step through which most studies suggest H₂O₂ activates NF-κB. Although it is not clear exactly how and where NF-κB is oxidized, the redox chaperone activity of APE/Ref-1 is required to restore the ability of NF-κB to bind to DNA in the nucleus .

Fig. 4

Activation of NF-κB-dependent cytokine production through lipid peroxidation. When non-cytotoxic levels of iron-laden particles interact with macrophages, the resulting lipid peroxidation in the area of the plasma membrane with which the particle interacts, produces a stress response that activates NF-κB and pro-inflammatory cytokine production. Lipid raft disruption caused by minor lipid peroxidation, results in the release of calcium from annexin 6 leading to the activation of phosphatidylcholine specific phospholipase C (PC-PLC) activation that produces diacylglycerol (DAG). DAG then activates acidic sphingomyelinase (ASM) that produces ceramide. Ceramide then activates ceramide-activated protein kinase (CAPK), which phosphorylates IκB allowing NF-κB to migrate to the nucleus. The PC-PLC-dependent pathway can also be stimulated by endotoxin, again representing a non-physiological stress response , .

Fig. 5

Redox activation of JNK/AP-1. H₂O₂ activates JNK through the Prdx1 catalyzed oxidation of Trx bound to ASK1. The Trx dissociates from ASK1, allowing the kinase to dimerize and phosphorylate the dual specificity (ser/thr and tyr phosphorylating) mitogen activated kinase kinase, MKK4. MKK4 in turn phosphorylates and activates JNK1 or JNK2. JNK then phosphorylates the c-Jun transcription factor, which pairs with another member (X) of the Jun/Fos family of transcription factors forming an AP-1 complex that binds to the TRE element of many AP-1-regulated genes. As with NF-κB (Fig. 3), AP-1 appears to require reduction in the nucleus by the redox chaperone activity of APE/Ref-1 to bind to DNA , .