The role of Nrf2 in periodontal disease by regulating lipid peroxidation, inflammation and apoptosis

Abstract

Nuclear factor E2-related factor 2(Nrf2) is a transcription factor that mainly regulates oxidative stress in the body. It initiates the expression of several downstream antioxidants, anti-inflammatory proteins and detoxification enzymes through the Kelch-like ECH-associating protein 1 (Keap1) -nuclear factor E2-related factor 2(Nrf2) -antioxidant response element (ARE) signaling pathway. Its anti-apoptosis, anti-oxidative stress and anti-inflammatory effects have gradually become the focus of periodontal disease research in recent years. In this paper, the structure and function of Nrf2 pathway and its mechanism of action in the treatment of periodontitis in recent years were analyzed and summarized, so as to further clarify the relationship between Nrf2 pathway and oxidative stress in the occurrence and development of periodontitis, and to provide ideas for the development of new treatment drugs targeting Nrf2 pathway.

Keywords: Nrf2; apoptosis; bone homeostasis; lipid peroxidation; periodontal disease.

Figure 1

The domain structure of Nrf2 and Keap1. Keap1 has 5 domains. BTB domain mediates the homodimerization of Keap1 and its binding with Cullin3(Cul3), in which cysteine residue (C151) senses electrophilic compounds and mediates the dissociation of Nrf2 and Keap1; IVR region is related to the stability of Nrf2, and participates in the ubiquitination degradation of Nrf2 under non-oxidative stress. Cys 273 and Cys 288 on it are necessary to inhibit Nrf2; DGR domain mediates the binding of Keap1 to Nrf2. Nrf2 contains six domains, namely Neh1-6. The Neh1 domain contains a bZip motif, which mediates the dimerization of Nrf2 and sMaf, promotes the binding of Nrf2 and ARE, and is responsible for DNA recognition.; The Neh2 domain contains ETGE and DLG motifs required for interaction with Keap1; Neh3-5 acts as a trans-activation domain; The Neh6 domain contains DSGIS and DSAPGS motifs which can be recognized by β-TrCP linker protein.

Figure 2

Regulation of Nrf2. (A) Canonical pathways regulated by Nrf2. Keap1 binds to Nrf2 in the form of dimers and mediates its ubiquitination degradation to maintain low levels of Nrf2 in the cytoplasm; under stress conditions, Nrf2 can dissociate from Keap1 and translocate to the nucleus to interact with AREs. After binding, the transcription of antioxidant enzymes is promoted. (B) Non-Keap1-mediated pathways. P62/SQSTM1 is an important selective autophagy protein. After binding to Keap1, it guides it into the autophagosome, leading to the autophagic degradation of Keap1; P21 interferes with the Keap1-Nrf2 interaction by binding to Keap1 and contributes to the positive regulation of Nrf2; p38 MAPK and GSK-3β can phosphorylate Nrf2 at Ser215, Ser408 and Ser577 and Ser334-338 of Neh6 domain, respectively, to facilitate its degradation; β-TrCP recognizes DSGIS and DSAPGS motifs to bind to Nrf2 and mediates its ubiquitination.

Figure 3

Mechanism of Nrf2 inhibiting lipid peroxidation. The cystine/glutamic acid reverse transporter on the cell membrane, namely, system xc-, is responsible for the uptake of cystine and the excretion of glutamic acid. The ingested cystine provides the raw material for intracellular GSH synthesis. Solute carrier family 7 member 11 (SLC7A11) is a part of this system, and SLC7A11 is a transcription target of Nrf2. As a cofactor of GPX4, GSH can cooperate with GPX4 to exert anti-lipid peroxidation effect. Many integrases involved in glutathione synthesis and metabolism are under the control of Nrf2, such as glutamate-cysteine ligase (GCL) and glutathione synthase (GSS). In addition, Nrf2 can stimulate the expression of GPX4.

Figure 4

Nrf2 can inhibit cell apoptosis by reducing intracellular ROS; Nrf2 can indirectly inhibit cell apoptosis by participating in mitochondrial biogenesis; Nrf2 can also inhibit apoptosis by directly mediating the transcription of anti-apoptotic factors.

Figure 5

Regulation of Nrf2 in inflammation. The production of ROS can induce high expression of pro-inflammatory cytokines such as IL-1β and IL-6. However, Nrf2 not only reduces the production of pro-inflammatory cytokines through its antioxidant effect, but also directly inhibits the expression of inflammation-related genes. iNOS can be induced to express in various inflammatory diseases and play the role of inflammatory mediators. The downstream factor of Nrf2, HO-1, can negatively regulate iNOS through CO-mediated iNOS inactivation and ferrous iron inhibiting iNOS transcription. NF-κB is an important transcription factor involved in regulating the expression of inflammation-related genes in the body. There is a crosstalk between the Nrf2 pathway and NF-κB, and Nrf2 can inhibit its activation. As the most typical inflammasome in pyroptosis, NLRP3 can promote the maturation and secretion of a variety of inflammatory mediators, and produce an inflammatory response. However, Nrf2 pathway can reduce the inflammatory response by inhibiting the activation of NLRP3 inflammasome. Nrf2 can reverse the transformation of pro-inflammatory M1 macrophages into anti-inflammatory M2 macrophages. Nrf2 can enhance the proliferation and differentiation potential of mesenchymal stem cells, thereby increasing the number of exosomes and promoting tissue repair. In turn, exosomes can promote the expression of Nrf2 and exert antioxidant effects to reduce inflammation and oxidative stress.

Figure 6

Regulation of bone homeostasis by Nrf2. (A) Regulation of osteoblasts by Nrf2. Oxidative stress hinders the proliferation and differentiation of osteoblasts, resulting in osteoblast dysfunction. Nrf2 is able to reduce peroxidation through its antioxidant capacity, thereby reducing osteoblast damage and maintaining its function. Runx2 is a transcription factor necessary for osteoblast differentiation, and overexpression of Nrf2 and its downstream factor (HO-1) interacts with Runx2; it can also directly inhibit the expression of many key genes regulating osteogenic differentiation and mineralization to negatively regulate osteogenic differentiation. (B) Regulation of osteoclasts by Nrf2. RANKL regulates the mature differentiation of osteoclasts by binding to RANK on the surface of osteoclast precursor cells. ROS can directly or indirectly promote the differentiation of osteoclast precursor cells into osteoclasts. The Nrf2/ARE signaling pathway ultimately inhibits the activity of the key osteoclast transcription factor NFATC1 by regulating the production of ROS and the expression level of NF-κB and other related signaling pathways, thereby inhibiting the differentiation of osteoclasts. Meanwhile, Nrf2 can also indirectly promote the formation of osteoclasts by inhibiting the secretion of OPG by osteoblasts and increasing the ratio of RANKL/OPG.