Cruciferous vegetables—such as broccoli, cabbage, kale, and radishes—are widely hailed for health benefits like cancer prevention, immune enhancement, cardiovascular protection, and anti-aging. Behind these benefits lies a key signaling pathway: Nrf2.
01 Introduction to the Nrf2 Signaling Pathway
As a critical transcription factor in the body, Nrf2 (Nuclear Factor Erythroid 2-Related Factor 2) activates over 250 protective genes and mediates the body’s most important antioxidant pathway. It is a focus of anti-aging research, alongside well-known pathways like AMPK, mTOR, FOXO, and sirtuins.
Based on the free radical theory of aging, uncontrolled excess free radicals damage cell membranes, mitochondria, and DNA. Nrf2 addresses this and offers unique advantages:
- Unlike "one-for-one" antioxidants (e.g., vitamin C, vitamin E, coenzyme Q10), Nrf2 regulates an entire antioxidant signaling network, boosting the expression of antioxidant proteins (SOD, CAT, GSH, etc.) for far greater efficiency.
- Nrf2 also participates in detoxification, inflammation inhibition, metabolism regulation, and radiation protection. It converts cellular stress into adaptive protective responses—for example, upregulating glutathione S-transferase to detoxify exogenous toxins when cells face harmful substances[1].
02 Molecular Regulation of Nrf2
Nrf2 must be activated to exert its functions:
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Under normal physiological conditions: Nrf2 is anchored in the cytoplasm by its inhibitor, Keap1 (Kelch-like ECH-Associated Protein 1). Keap1 acts as a substrate for the E3 ubiquitin ligase complex, promoting Nrf2 ubiquitination and rapid degradation by the proteasome—keeping Nrf2 levels very low[2].
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Under oxidative stress or other stimuli: Multiple mechanisms activate Nrf2:
- Classic Keap1-Nrf2-ARE Axis: Reactive oxygen species (ROS) inhibit Keap1 and hydrolase activity, causing Nrf2 to dissociate from Keap1, translocate into the nucleus, form a heterodimer with MAF proteins, and bind to the ARE (Antioxidant Response Element) sequence in DNA—activating transcription of target genes[3].
- Nrf2 Self-Activation: Nrf2 contains nuclear localization/export signals (e.g., an oxidation-sensitive nuclear export signal in the Neh5 domain), allowing it to respond to external signals and enter the nucleus independently of Keap1 to perform transcriptional functions[4].
- Positive Regulation by p21/p62: p21 (a cell cycle regulatory protein) competes with Keap1 for Nrf2 binding, inhibiting Keap1-dependent Nrf2 ubiquitination and increasing Nrf2 levels[5]. p62 (a ubiquitin-binding protein) induces Nrf2, regulating Nrf2-dependent protein synthesis, autophagy-related protein degradation, and protein homeostasis[6].
- Oncogene-Mediated Nrf2 Upregulation: Oncogenes (K-Ras, B-Raf, Myc) activate Nrf2 transcription via the Mek-Erk-Jun pathway and enhance Nrf2 protein stability—triggering Nrf2’s antioxidant program[7].
Activated Nrf2 binds to DNA response elements (e.g., AREs, EPRES) to regulate gene expression and induce broad biological effects.
03 Physiological Functions of Nrf2
Nrf2 is a multifunctional transcription factor with effects including antioxidant defense, anti-inflammation, detoxification, DNA repair, metabolism regulation, and autophagy promotion. Its key target genes and functions are summarized below (genes marked with ↓ are downregulated by Nrf2):
| Gene | Protein | Key Role | Physiological Function |
|---|---|---|---|
| GCLC | Glutamate-cysteine ligase catalytic subunit | Rate-limiting enzyme for glutathione synthesis | Maintains cellular redox homeostasis |
| GCLM | Glutamate-cysteine ligase modifier subunit | Assists glutathione synthesis | Maintains cellular redox homeostasis |
| GPX2 | Glutathione peroxidase 2 | Detoxifies H₂O₂ | Maintains cellular redox homeostasis |
| GSTA1 | Glutathione S-transferase A1 | Detoxifies electrophiles; metabolizes bilirubin and anticancer drugs | Phase II drug metabolism, cellular protection |
| NQO1 | NAD(P)H quinone oxidoreductase 1 | Reduces quinones to hydroquinones; prevents radical formation | Phase I drug metabolism |
| HMOX1 | Heme oxygenase 1 | Breaks down heme into biliverdin during heme catabolism | Heme metabolism |
| PPARγ | Peroxisome proliferator-activated receptor γ | Regulates adipocyte differentiation and glucose homeostasis | Lipid mobilization, fatty acid oxidation, glucose metabolism |
| P62/SQSTM1 | Selective autophagy adapter protein | Facilitates autophagosome formation and degradation | Autophagy, immunity |
Nrf2 activation also improves chronic diseases (metabolic disorders, respiratory/gastrointestinal diseases, cardiovascular diseases, neurodegenerative diseases). However, its role in cancer is dual:
- Tumor suppression: Nrf2 target genes mediate DNA repair and xenobiotic metabolism, preventing cancer initiation and carcinogen accumulation.
- Tumor promotion: Cancer cells enhance Nrf2 activity to create a survival-friendly microenvironment—making Nrf2 a key target for anticancer drugs[8].
04 Nrf2 Activation
Nrf2 is "inactive" under normal conditions; low-level oxidative stress triggers its activation (Keap1 dissociation → Nrf2 nuclear translocation → ARE binding → gene transcription).
Lifestyle & Dietary Activation
- Lifestyle: Calorie restriction and exercise induce low-level stress to activate Nrf2.
- Dietary Compounds: Plant polyphenols, terpenes, organosulfur compounds, n-3 polyunsaturated fatty acids, and carotenoids activate Nrf2. Sulforaphane (SFN)—found in broccoli and other cruciferous vegetables—is the most potent Nrf2 activator. However, scientists note that ~3 kg of raw broccoli per day would be needed to potentially reap health benefits.
Pharmaceutical Activation & Clinical Trials
Many Nrf2-activating drugs (more accurately "Keap1 inhibitors," as they interact with Keap1’s cysteine residues) are in clinical trials for various indications. Key examples:
| Compound | Mechanism | Indication | Clinical Phase | Identifier |
|---|---|---|---|---|
| Bardoxolone-methyl | Electrophilic modification of Keap1-Cys-151 | Chronic kidney disease, advanced solid tumors | Phase 2/3 | NCT01351675 |
| Dimethyl fumarate | Electrophilic modification of Keap1-Cys-151 | Multiple sclerosis, rheumatoid arthritis | Phase 2 | NCT00810836 |
| Oltipraz | Electrophilic modification of Keap1-Cys-151 | Non-alcoholic steatohepatitis, lung cancer | Phase 3 | NCT02068339 |
| Sulforaphane | Electrophilic modification of Keap1-Cys-151 | Schizophrenia, melanoma | Phase 2/3 | NCT02880462 |
| Curcumin | Electrophilic modification of Keap1-Cys-151 | Type 2 diabetes, prostate cancer | Phase 2/3 | NCT03262363 |
References
[1] Taguchi K, Motohashi H, Yamamoto M. Molecular mechanisms of the Keap1–Nrf2 pathway in stress response and cancer evolution. Genes Cells. 2011;16:123–140.
[2] Ma L, Liu X, Zhao Y, et al. Ginkgolide B reduces LOX-1 expression by inhibiting Akt phosphorylation and increasing Sirt1 expression in oxidized LDL-stimulated human umbilical vein endothelial cells. PLoS ONE. 2013;8(9):e74769.
[3] Ma J, Cai H, Wu T, et al. PALB2 interacts with Keap1 to promote Nrf2 nuclear accumulation and function. Mol Cell Biol. 2012;32(8):1506–1517.
[4] Jiang ZY, Chu HX, Xi MY, et al. Insight into the intermolecular recognition mechanism between Keap1 and IKKβ. PLoS ONE. 2013;8(9):e75076.
[5] Chen W, Sun Z, Wang XJ, et al. Direct interaction between Nrf2 and p21(Cip1/WAF1) upregulates the Nrf2-mediated antioxidant response. Mol Cell. 2009;34(6):663–673.
[6] Komatsu M, Kurokawa H, Waguri S, et al. The selective autophagy substrate p62 activates the stress responsive transcription factor Nrf2 through inactivation of Keap1. Nat Cell Biol. 2010;12(3):213–223.
[7] DeNicola GM, Karreth FA, Humpton TJ, et al. Oncogene-induced Nrf2 transcription promotes ROS detoxification and tumorigenesis. Nature. 2011;475(7354):106–109.
[8] Shelton P, Jaiswal AK. The transcription factor NF-E2-related factor 2 (Nrf2): a protooncogene? FASEB J. 2013;27(2):414–423.
[9] Kensler TW, et al. Modulation of the metabolism of airborne pollutants by glucoraphanin-rich broccoli sprout beverages. Carcinogenesis. 2012;33:101–107.
[10] Shureiqi I, Baron JA. Curcumin chemoprevention: the long road to clinical translation. Cancer Prev Res. 2011;4:296–298.
[11] Linker RA, et al. Fumaric acid esters exert neuroprotective effects via Nrf2 activation. Brain. 2011;134:678–692.
[12] Pergola PE, et al. Bardoxolone methyl and kidney function in CKD with type 2 diabetes. N Engl J Med. 2011;365:327–336.
[13] Palsamy P, Subramanian S. Resveratrol protects diabetic kidney via Nrf2-Keap1 signaling. Biochim Biophys Acta. 2011;1812:719–731.
[2] Ma L, Liu X, Zhao Y, et al. Ginkgolide B reduces LOX-1 expression by inhibiting Akt phosphorylation and increasing Sirt1 expression in oxidized LDL-stimulated human umbilical vein endothelial cells. PLoS ONE. 2013;8(9):e74769.
[3] Ma J, Cai H, Wu T, et al. PALB2 interacts with Keap1 to promote Nrf2 nuclear accumulation and function. Mol Cell Biol. 2012;32(8):1506–1517.
[4] Jiang ZY, Chu HX, Xi MY, et al. Insight into the intermolecular recognition mechanism between Keap1 and IKKβ. PLoS ONE. 2013;8(9):e75076.
[5] Chen W, Sun Z, Wang XJ, et al. Direct interaction between Nrf2 and p21(Cip1/WAF1) upregulates the Nrf2-mediated antioxidant response. Mol Cell. 2009;34(6):663–673.
[6] Komatsu M, Kurokawa H, Waguri S, et al. The selective autophagy substrate p62 activates the stress responsive transcription factor Nrf2 through inactivation of Keap1. Nat Cell Biol. 2010;12(3):213–223.
[7] DeNicola GM, Karreth FA, Humpton TJ, et al. Oncogene-induced Nrf2 transcription promotes ROS detoxification and tumorigenesis. Nature. 2011;475(7354):106–109.
[8] Shelton P, Jaiswal AK. The transcription factor NF-E2-related factor 2 (Nrf2): a protooncogene? FASEB J. 2013;27(2):414–423.
[9] Kensler TW, et al. Modulation of the metabolism of airborne pollutants by glucoraphanin-rich broccoli sprout beverages. Carcinogenesis. 2012;33:101–107.
[10] Shureiqi I, Baron JA. Curcumin chemoprevention: the long road to clinical translation. Cancer Prev Res. 2011;4:296–298.
[11] Linker RA, et al. Fumaric acid esters exert neuroprotective effects via Nrf2 activation. Brain. 2011;134:678–692.
[12] Pergola PE, et al. Bardoxolone methyl and kidney function in CKD with type 2 diabetes. N Engl J Med. 2011;365:327–336.
[13] Palsamy P, Subramanian S. Resveratrol protects diabetic kidney via Nrf2-Keap1 signaling. Biochim Biophys Acta. 2011;1812:719–731.