Nuclear factor (erythroid-derived 2)-like 2 (NFE2L2) is a novel therapeutic target for diabetic complications

Abstract

Diabetes is a leading cause of death and disability. In 2004, 3.4 million people worldwide died of symptoms relating to high blood sugar. Diabetic complications are caused by organ damage resulting from long-term exposure to high blood sugar, and include diseases such as heart failure, kidney failure, vision loss and neuropathy. The transcription factor nuclear factor (erythroid-derived 2)-like 2 (NFE2L2, also known as NRF2) is an important component of the intracellular antioxidant machinery and a target for treatment of diabetic complications. This article reviews the role of NFE2L2 in diabetic complications with a focus on diabetic nephropathy, cardiomyopathy, neuropathy and retinopathy. Activation of NFE2L2 protects against oxidative stress in vitro and in vivo, and represents an important target for prophylaxis and treatment of diabetic complications. NFE2L2 has potential clinical applications for diabetic patients in the near future.

Introduction

Diabetes is characterized by abnormally high levels of sugar in the blood, and is a leading cause of death and disability. ¹ In 2004, 3.4 million people worldwide died of symptoms relating to high blood sugar and this number is predicted to double by 2030. ² Diabetes is classified by underlying cause: type 1 diabetes is an autoimmune disease (usually occurring in children) that destroys pancreatic β-cells, rendering them unable to produce insulin; type 2 diabetes (>90% of all cases, most common in obese adults) is characterised by insulin resistance or defects in insulin secretion.^3,4 High blood sugar in type 2 diabetes has several underlying causes, including deficiencies in insulin secretion and activity. ⁵ Uncontrolled high blood sugar can lead to serious medical complications that are related to microvascular and macrovascular damage. These complications are listed in Table 1.^6–8 Additional long-term complications of diabetes include skin problems, digestive disorders, sexual dysfunction, and problems with teeth and gums.

Table 1.

Complications of diabetes.

Microvascular disease	Diabetic nephropathy
	Diabetic neuropathy
	Cardiomyopathy
	Retinopathy
Macrovascular disease	Coronary artery disease
	Diabetic myonecrosis
	Peripheral vascular disease
	Stroke

Nuclear factor (erythroid-derived 2)-like 2 (NFE2L2, also known as NRF2; encoded by the NFE2L2 gene), is a transcription factor of the basic region leucine zipper family, ⁹ widely acknowledged to be a master regulator of redox balance in the cellular cytoprotective response.^9,10 Under unstressed conditions, NFE2L2 is sequestered in the cytoplasm by the repressor protein Kelch-like ECH (erythroid cell-derived protein with CNC homology)-associated protein 1 (KEAP1) and targeted for proteasomal degradation. ¹¹ During periods of oxidative stress, NFE2L2 is released from sequestration in the cytoplasm and is translocated to the nucleus, where it activates transcription of a large battery of genes by binding to antioxidant response elements (AREs) in upstream promoter regions.^12,13 The activation of NFE2L2 leads to the production of cytoprotective proteins including NAD(P)H dehydrogenase, quinone 1 (NQO1), glutamate–cysteine ligase (GCLC) and hemeoxygenase-1 (HMOX1).^14–17 The antioxidant response provided by the NFE2L2 and KEAP1–NFE2L2/ARE signalling pathways protect the lungs, liver, and digestive, neural and cardiovascular systems.^18–20

Studies have suggested a role for NFE2L2 in the treatment of diabetes. ²¹ The aim of the current review article was to evaluate the importance of NFE2L2 in the symptoms and complications of diabetes.

Diabetic nephropathy

Diabetic nephropathy is a progressive kidney disease caused by angiopathy of glomerular capillaries. ²² The pathogenesis is unclear, but may include the aldose–reductase pathway, advanced glycation, protein kinase C activity, inflammation and overactivation of poly ADP-ribose polymerase. ²³ It is known, however, that hyperglycaemia is the primary initiating factor, ²⁴ and hyperglycaemia-induced renal damage is associated with excessive production of reactive oxygen species (ROS).^25,26

Nuclear factor (erythroid-derived 2)-like 2 is a major defence against oxidative stress, regulating and activating the intracellular antioxidation system to neutralize ROS and maintain redox homeostasis. ²⁷ Studies have indicated protective effects of NFE2L2 in diabetes, both in vitro and in vivo,^28,29 partially mediated via inhibition of transforming growth factor-β1 (TGF-β1) and reduction of extracellular matrix production. ³⁰ In addition, bardoxolone methyl (a synthetic inducer of NFE2L2) increased kidney function in patients with diabetes. ³¹ NFE2L2 was shown to have both a protective role and therapeutic potential in a mouse model of diabetic nephropathy. ³² The NFE2L2 activators sulphoraphane and cinnamic aldehyde ameliorated albuminuria and minimized renal damage, and the resulting Nfe2l2 activity negatively regulated TGF-β1, extracellular matrix production and cyclin-dependent kinase inhibitor 1 A (Cdkn1a) levels. ³¹

Diabetic neuropathy

Diabetic neuropathy is characterized by a network of interrelated metabolic, neurotrophic and vascular defects, and is one of the most common complications of long term diabetes. ³² The pathogenic deficits of diabetic neuropathy include inflammation, changes in the aldose–reductase pathway and oxidative stress. ³³ The expression of Nfe2l2 and Hmox1 is downregulated in the sciatic nerve of diabetic animals compared with normal control animals. ³⁴ In addition, Nfe2l2-mediated Hmox1 expression reduced formalin-induced inflammatory pain, indicating a possible role in preventing sensorimotor alteration at peripheral sites. ³⁴ The NFE2L2 pathway has been implicated in diabetic neuropathy as a major regulator of antioxidant gene expression, including upregulation of the thioredoxin system. ³⁵

Diabetic cardiomyopathy

Diabetic cardiomyopathy is characterized by ventricular dilatation, myocyte hypertrophy, prominent interstitial fibrosis and decreased or preserved systolic function in the presence of diastolic dysfunction.^36–38 Vascular oxidative/nitrosative stress, endothelial activation and increased endothelial cell death are hallmarks of cardiovascular ageing that are involved in multiple diabetic complications. The development of treatment approaches that can simultaneously target several of these pathophysiological processes is therefore desirable. ³⁹ Oxidative stress is implicated in many chronic diseases including Alzheimer’s, diabetes and coronary artery disease.^40–43 Phytochemical treatment induced NFE2L2 nuclear localization and antioxidant protein production (HMOX1, superoxide dismutase [SOD1], NQO1 and glutathione reductase [GSR]) in human coronary artery endothelial cells. ⁴⁴ In addition, the NFE2L2 activator resveratrol extended cell lifespan in vitro, attenuated myocardial ischemic–reperfusion injury and atherosclerosis, and had vasoprotective effects in rodent models of metabolic diseases.^45,46 Resveratrol increased the transcriptional activity of NFE2L2 in cultured human coronary arterial endothelial cells, upregulating target genes such as NQO1, GCLC and HMOX1. ⁴⁵ The attenuation of hyperglycaemia-induced mitochondrial and cellular oxidative stress by resveratrol treatment was significantly reversed by knockdown of NFE2L2 or overexpression of KEAP1, known to inactivate NFE2L2.^47,48

Diabetic retinopathy

Diabetic retinopathy is widely believed to result from diabetes-induced oxidative stress.^49,50 Retinal blood vessels are small and easily damaged by high blood glucose and elevated blood pressure. ⁵¹ NFE2L2 has a protective function in the retina.^52,53 Nfe2l2 knockout mice develop age-dependent degeneration in the retinal pigment epithelium, indicating that Nfe2l2 deficiency may induce retinal disease. ⁵² In addition, NFE2L2 was found to protect the retina from hyperoxia-induced oxidative stress during a specific time window shared with angiogenesis. ³⁵ Studies in Nfe2l2 knockout mice revealed a cytoprotective effect of Nfe2l2 in response to retinal ischemia–reperfusion injury, suggesting that pharmacological induction of Nfe2l2 could be a new therapeutic strategy for ischemia–reperfusion and other retinal diseases. ⁵³

Studies have demonstrated that NFE2L2 regulates antioxidant genes via ARE binding. ⁵⁴ The stimulation of NFE2L2 nuclear translocation and the subsequent increase in antioxidant proteins may represent new treatment strategies for diabetic retinopathy. ⁵⁵ Such potential treatments include structurally diverse agents, with chemically versatile cytoprotective properties, that act as a defence against toxic metabolites and xenobiotics. ⁵⁵ HMOX1, an enzyme with potent anti-inflammatory, antioxidant and antiproliferative effects, is an important positive modulator of NFE2L2/ARE-dependent signalling that can counteract diabetic retinopathy-mediated injuries in retinal neurons and vascular endothelial cells. ⁵⁶

Other agents include electrophiles and oxidants such as zinc, sulphoraphane, oltipraz and dimethyl fumarate. These induce the expression of detoxification genes, increase glutathione synthesis and protect against oxidative injury in cultured human retinal pigment epithelial cells, via modulation of NFE2L2.^57–60 A molecule with NFE2L2 stimulatory activity has been described as a prophylactic and therapeutic agent for diabetic retinopathy and drusen formation in age-related macular degeneration. ⁶¹

Bone and joint disorders

Patients with diabetes are often at increased risk of bone and joint disorders including Charcot foot, osteoarthritis, osteoporosis, frozen shoulder (adhesive capsulitis) and hand disorders.^62,63

Oxidative stress plays a key role in joint destruction. The role of NFE2L2 has been investigated in Nfe2l2 knockout mice, with the data providing strong evidence that oxidative stress is significantly involved in cartilage degradation in experimental arthritis, and a functional Nfe2l2 gene is a major requirement for limiting cartilage destruction. ⁶⁴ In addition, Nfe2l2 deficiency accelerated the effector phase of arthritis and aggravated joint disease, suggesting that Nfe2l2 may be a therapeutic target for arthritis. ⁶⁵ Although the relationship between diabetes and bone and joint problems is not fully understood, NFE2L2 may be a therapeutic target for diabetes-related bone and joint disease.

Conclusions

Activation of NFE2L2 represents an important therapeutic target for diabetic nephropathy, neuropathy, cardiomyopathy and retinopathy. The antioxidant effects of NFE2L2 may also be useful for prophylaxis of diabetic complications. NFE2L2 has potential clinical applications for diabetic patients in the near future.