Keap1 Dependent Regulation of Nrf2

The Nrf2 protein is rapidly turned over; it has a short half-life under basal conditions (7–15 min) which is increased to 30–100 min in the presence of inducers [44] and [45]. The Nrf2 protein stability is primarily regulated by Keap1 [46] (Fig. 4). The ETGE and DLG motifs within Nrf2 interact with Keap1. Keap1 is a substrate adaptor protein for a Cullin 3 (Cul3)-dependent E3 ubiquitin ligase complex; hence binding of Keap1 with Nrf2 mediates ubiquitination and subsequent proteasomal degradation of Nrf2 [41], [46], [47], [48], [49] and [50].

 

Fig. 4. 

Keap1 dependent regulation of Nrf2. Under basal conditions, Nrf2 is sequestered in the cytosol by a Keap1 homodimer, which facilitates the ubiquitination and proteasomal degradation of Nrf2. Inducers react with specific cysteine residues in Keap1, leading to the release of Nrf2 and allowing its nuclear translocation. In the nucleus, Nrf2 heterodimerizes with small Maf proteins and binds to the antioxidant response element (ARE), activating the expression of a battery of cytoprotective genes.

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Sulfhydryl groups within Keap1 act as sensors for electrophiles and oxidants [51]. In the presence of reactive oxygen species, critical cysteine residues in Keap1 become oxidized leading to a conformational change of Keap1, which prevents its binding to Nrf2. As a consequence, Nrf2 ubiquitination and degradation is stopped, and its nuclear translocation promoted.

Among the 27 cysteine residues within Keap1, Cys151 seems to be the most critical sensor residue which mediates the formation of an intermolecular disulfide – Cys151–Cys151 – between two Keap1 molecules. Thus, in response to a diverse array of stimuli, redox-sensitive residues, especially Cys151 within Keap1, can be covalently modified, allowing Nrf2 to evade Keap1-mediated ubiquitination [52] and [53]. As a result, Nrf2 accumulates in the nucleus and heterodimerizes with a small Maf protein. The Nrf2-small Maf heterodimer binds to regulatory gene regions known as ARE sites with a consensus sequence 5′-TGACnnnGC-3′, also called electrophile response elements (EpRE) [54]. Substances able to induce a response at the ARE are derived from a variety of sources, including phytochemicals and derivatives such as genistein, quercetin, curcumin and sulforaphane; therapeutics such as oltipraz, auranofin and acetaminophen; environmental agents like paraquat and metals and endogenous inducers such as nitric oxide, nitro-fatty acids, hydrogen peroxide and 4-hydroxynonenal [55]. Nrf2 target genes mainly

 

Berberine;

 

BBR also activated nuclear erythroid 2-related factor 2 (Nrf2),

the key antioxidative transcription factor, which is accompanied with up-regulation of hemeoxygenase-1 (HO-1).

Furthermore, BBR markedly enhanced nerve growth factor (NGF) expression and promoted neurite outgrowth in high glucose-treated cells.

 

To further determine the role of the Nrf2 in BBR neuroprotection, RNA interference directed against Nrf2 was used. Results indicated Nrf2 siRNA abolished BBR-induced HO-1, NGF, neurite outgrowth and ROS decrease.

 

In conclusion, BBR attenuated high glucose-induced neurotoxicity, and we are the first to reveal this novel mechanism of BBR as an Nrf2 activator against glucose neurotoxicity, providing another potential therapeutic use of BBR on the treatment of diabetic complications.