Contribution of Nrf2 Modulation to the Mechanism of Action of Analgesic and Anti-inflammatory Drugs in Pre-clinical and Clinical Stages

Abstract

Despite the progress that has occurred in recent years in the development of therapies to treat painful and inflammatory diseases, there is still a need for effective and potent analgesics and anti-inflammatory drugs. It has long been known that several types of antioxidants also possess analgesic and anti-inflammatory properties, indicating a strong relationship between inflammation and oxidative stress. Understanding the underlying mechanisms of action of anti-inflammatory and analgesic drugs, as well as essential targets in disease physiopathology, is essential to the development of novel therapeutic strategies. The Nuclear factor-2 erythroid related factor-2 (Nrf2) is a transcription factor that regulates cellular redox status through endogenous antioxidant systems with simultaneous anti-inflammatory activity. This review summarizes the molecular mechanisms and pharmacological actions screened that link analgesic, anti-inflammatory, natural products, and other therapies to Nrf2 as a regulatory system based on emerging evidences from experimental disease models and new clinical trial data.

Keywords: Keap1; Nrf2; analgesic; anti-inflammatory; antioxidant; inflammation; oxidative stress; pain.

FIGURE 1

Intracellular signaling pathways that regulate Nrf2. In basal conditions, (A) Nrf2 is sequestered in the cytosol by Keap1 by two motifs (ETGE and DLG), which are essential to recruit Nrf2. Keap1 works as a dimeric redox sensitive substrate adaptor for cullin-based E3 ubiquitin ligase, which inhibits the transcriptional activity of Nrf2 via ubiquitination and proteasomal degradation. This signaling is known as canonical pathway. (B) Alternatively, Nrf2 is also regulated by a non-canonical pathway. The phosphorylation of Nrf2 by GSK-3 facilitates its recognition by β-TrCP, leading to Cul1-mediated ubiquitination, followed by Nrf2 proteasome degradation. Under oxidative stress or pathological conditions, (C,D) Keap1-CUL3 ubiquitin E3 ligase activity decreases and Nrf2 dissociates from Keap1. Nrf2 translocates to the nucleus and heterodimerizes with small musculoaponeurotic fibrosarcoma (Maf) protein and binds to DNA and other transcription partners to setting up a nuclear complex with the ubiquitin-conjugating enzyme UbcM2. These nuclear complexes formed with Nrf2 induce the expression of the ARE-gene battery, such as: NQO1, HMOX1, GCL, GSTs, CAT, SOD, and thioredoxin UDP-glucuronosyltransferase. (E) The multifunctional protein p62 and LC3 acts by sequestration of Keap1, which culminates in its autophagic degradation. As a consequence, Nrf2 can translocate to the nucleus and activate ARE.

FIGURE 2

Molecular mechanisms of drugs that modulate Nrf2 activity. (A) Group of drugs that increase Nrf2 biding to DNA and/or, Nrf2-Maf affinity to ARE region and/or modulate histone acetyltransferase (HDAC). (B,C) Drugs modulating the disruption of Keap1/Nrf2 complex; either (B) by protein–protein interaction though Michael addition reaction and/or, (C) increasing Keap1 degradation through proteasome or autophagic pathways. (D) Drug targeting the degradation of Nrf2 via proteasome by increasing the GSK-3/β-TrCP signaling and reducing Nrf2 activity.

FIGURE 3

Role of drugs acting via Nrf2 in cancer. (A) In normal cells, Nrf2 activity is regulated by canonical and non-canonical pathways, which in the absence of oxidative stress, culminate in Nrf2 proteossomal degradation. (B) Enhancing Nrf2 in premalignant and early malignant cells is important to prevent cancer development, specially by low doses of drugs capable of inducing phase II enzymes and antioxidant proteins expression. (C) Otherwise, in malignant cells the enhancement of Nrf2 activity caused by mutations such as Kras, Bras and Myc, can protect tumors from the cytotoxic effects of reactive oxygen species (ROS) induced by chemotherapy. However, the effects of drugs that act via Nrf2 at intermediate and chemotherapy stages still need investigation. Overall, the effects of Nrf2 in cancer depend on the biological development stage of tumor cells.