Dialing Down Inflammation: The Role of Nrf2

Inflammation is a complex process that occurs when tissues become infected or otherwise injured by physical damage or exposure to irritants.  The first goal of inflammation is to limit the extent of damage and, in the case of infection, eliminate the cause of injury.  Infected and injured tissues send signals to recruit immune cells, such as macrophages, to the sites of injury.  Once the immune cells arrive, they produce reactive oxygen species (ROS), which help proinflammatory transcription factors, such as NF-κB, to turn on proinflammatory cytokine genes.  The ROS and proinflammatory cytokines are then able to coordinate the attack on infectious agents and irritants to prevent further injury. 

After the battle is over, it is essential that the inflammatory response is turned off and tissues are repaired.  In fact, the resolution of inflammation is equally as important as the initiation of inflammation.  In many chronic diseases, failure to resolve inflammation leads to the persistent production of cytokines and ROS, which causes chronic (long-term) inflammation and ultimately leads to permanent tissue damage and loss of organ function. 

Nrf2 – Master Regulator of the Antioxidant Response

Nuclear factor erythroid 2-related factor (Nrf2) plays an important role in the resolution of inflammation.  Nrf2 is a transcription factor that is often thought of as the “master” regulator of the antioxidant response as it turns on genes that protect the cell during periods of oxidative stress.  Oxidative stress is an imbalance that occurs when the levels of oxidants, such as ROS, are too high or the ability of the natural antioxidant defense system to remove excess ROS is impaired.  High levels of ROS and oxidative stress damage the cell and increase inflammation, which can lead to cell death and damage neighboring tissues. 

It has been widely accepted that Nrf2 suppresses inflammation indirectly by turning on antioxidant genes, which eliminate ROS and subsequently reduce inflammation.  However, some studies have hinted that Nrf2 may also directly affect genes involved in inflammation.1,2  To better understand the role of Nrf2 in inflammation, Dr. Masayuki Yamamoto, the scientist who first discovered the Nrf2 regulatory system3,4, and researchers in his lab, conducted a study to examine whether Nrf2 directly turns off proinflammatory cytokines, or if its effects on inflammation are merely a consequence of an increased antioxidant response.5

Nrf2 – Suppressor of Inflammation

In this study, published by Kobayashi, et al., in Nature Communications, researchers found that exposing macrophages to an inflammatory irritant turned on many genes that make proinflammatory cytokines.  However, when Nrf2 was activated in macrophages, the irritant was no longer able to turn on these genes. 

In contrast to the accepted theory that Nrf2 turns off proinflammatory genes by first turning on antioxidant genes that reduce ROS levels, researchers in the Yamamoto lab showed that Nrf2 turns off proinflammatory genes and turns on antioxidant genes at the same time. These findings support the idea that Nrf2 could influence proinflammatory genes outside of its role in the antioxidant response.

Since Nrf2 is a transcription factor that binds to DNA to turn genes on and off, the researchers went on to ask whether Nrf2 binds to DNA near these proinflammatory genes. They found that this was indeed the case—Nrf2 directly associated with proinflammatory genes.  Previous studies have shown that when Nrf2 turns on antioxidant genes, it does so by binding to a specific DNA sequence called the antioxidant response element (ARE).  However, researchers in the Yamamoto lab discovered that Nrf2 appears to turn off proinflammatory genes in a completely different way.  They found that Nrf2 turned off the expression of two proinflammatory cytokines, IL-6 and IL-1β, by blocking the recruitment of the transcriptional machinery that is required for genes to be turned on.  They also showed that Nrf2 may block proinflammatory gene expression by associating with proinflammatory transcription factors, such as NF-κB, and interfering with their activity. 

Nrf2 – Potential Target for the Treatment of Chronic Diseases

Not only were the researchers in the Yamamoto lab the first to show that Nrf2 directly blocks proinflammatory genes, they also demonstrated that the Nrf2 pathway may be a target for the development of new treatments for chronic inflammatory diseases.  IL-6 is a key player in the development of multiple sclerosis and other inflammatory diseases.  The Yamamoto lab found that Nrf2 activation turned off IL-6 gene expression in a nonclinical model of multiple sclerosis.  Importantly, they also found that Nrf2 activation reduced the symptoms of multiple sclerosis in this model. 

In many chronic diseases, the activity of Nrf2 is often inadequate, which may render the cell unable to turn off the processes that lead to chronic inflammation.  The results from this study suggest that activation of Nrf2 may be a potential approach for the treatment of acute and chronic diseases that have an inflammatory component. 

Nrf2 and Inflammation

To fight or not to fight. This is the question faced by the immune system every day. Effective inflammatory responses must occur in response to real threats, in the right tissue, and at the right time. A misguided, misplaced, or mistimed inflammatory response can have serious consequences.  So how does the immune system respond to threats but also support the timely resolution of inflammation? Researchers are working to answer complex questions like these.

What we do know is that inflammation remains a major driver of disease progression in many conditions, and that nuclear factor erythroid 2-related factor 2 (Nrf2) is a key player in the resolution of inflammation. Nrf2 normalizes mitochondrial metabolism, steering cells away from the inflammatory state and restoring their normal function.  Nrf2 activity results in the neutralization of reactive oxygen species (ROS), which helps to suppress proinflammatory signaling.  Notably, these examples position inflammation as a secondary threat that Nrf2 neutralizes indirectly.

In the reviews below and in posts to come, we will dissect additional contributions of Nrf2 in facilitating the resolution of inflammation, including important examples of how Nrf2 can directly suppress key proinflammatory mediators.

The diverse roles of Nrf2 in regulating inflammation are reviewed here.1 This review highlights important Nrf2 roles, including crosstalk with the NF-κB pathway, regulation of the NLRP3 inflammasome, and impact on the recruitment of immune cells.

Another review on Nrf2 as a therapeutic target for chronic disease provides valuable insights regarding the role of Nrf2 in resolving inflammation, as well as the broader landscape of diseases that may be impacted by Nrf2 function.2

Nrf2 and Oxidative Stress

As I touched upon in my last post, in the face of a threat, mitochondria become immune responders rather than energy producers. 1 Instead of using oxygen within the mitochondria to efficiently produce energy, mitochondria reprogram their metabolism to increase the production of reactive oxygen species (ROS) and amplify the inflammatory response.

ROS production is part of the inflammatory response, and it is critical to a healthy immune system. ROS are also important chemical signals that act as messengers within the cell to regulate many normal processes such as cell death, mitochondrial health, and metabolism. However, problems arise when ROS linger past their welcome and begin to damage the tissues they are trying to protect. For this reason, ROS levels must be carefully regulated by antioxidant defense systems that actively neutralize harmful molecules, such as hydrogen peroxide and superoxide.  Poor regulation results in high ROS levels and leads the cell into a state of oxidative stress.

Oxidative stress occurs when cells contain more ROS than their antioxidant defense systems can handle.  Excess ROS can cause problems in two ways: 1) they directly damage the cell by attacking its membranes, proteins, and DNA; and 2) they act as a signal to recruit inflammatory immune cells. As oxidative stress is a major driver of inflammation, excess ROS and proinflammatory molecules tend to go hand-in-hand and together characterize a variety of chronic diseases.

Nrf2 (nuclear factor erythroid 2-related factor 2) is a key coordinator of the cellular response to stress and activates antioxidant defense systems to reduce ROS levels. Nrf2 turns on many genes that play a role in restoring balance, including components of the glutathione and thioredoxin systems, antioxidant enzymes, and proteins that regulate iron metabolism. 2,3

Researchers have shown that when Nrf2 levels are too low, symptoms in a variety of disease models are more severe. In contrast, boosting Nrf2 reduces ROS levels and associated inflammation. Based on these observations, increasing Nrf2 activity represents a promising approach to treating certain diseases.4

See the following links for additional information on Nrf2 and oxidative stress:

  • The strategy of targeting Nrf2 to ameliorate the oxidative stress underlying chronic kidney disease (CKD) is reviewed here.5 The review highlights the contribution of impaired Nrf2 activity in CKD-associated oxidative stress.
  • The role of Nrf2 in neurodegenerative diseases like Friedreich’s ataxia and Alzheimer’s disease is reviewed here.6 The review underscores the ability of Nrf2 to ameliorate oxidative stress as well as other underlying features of neurological diseases.

Mitochondria Beyond ATP Production: Working with Nrf2 to Coordinate the Cell’s Response to Stress

Mitochondria are often called the “powerhouses” of the cell, for their ability to efficiently use the air we breathe and the food we eat and package it into molecules called adenosine triphosphate (ATP)—the currency it takes to get a job done in the cell. As the primary supplier of ATP, the mitochondria keep our hearts beating, neurons firing, and muscles moving.

With Great Power Comes Great Responsibility

Recent scientific discoveries have shown mitochondria in a new light—as regulators not only of ATP supply but where it is used. Like balancing a budget, the cell must manage its resources and prioritize its activities in response to the situation or environment it is in. How does a cell know when to turn on and off cellular processes? The mitochondria are vigilant sensors that help the cell monitor environmental cues and changing energy demands. They actively influence the function and fate of cells by determining how resources are used.

In addition to producing energy, mitochondria help the cell break down dietary sugar, fat, and protein into smaller building blocks that can be used for other purposes. These other purposes include some of the most central aspects of a cell’s life, including activating the immune system and regulating cell death. As we age, cells increasingly struggle to extract energy, metabolize nutrients, or generate a robust immune response. Importantly, as mitochondrial function becomes less efficient over the course of aging, we all become increasingly susceptible to chronic diseases. It may come as no surprise, then, that defects in mitochondrial function have been observed across a wide variety of diseases, from well-known diseases like diabetes and cancer, to the orphan diseases that Reata is studying.

ATP: The Cellular Currency That Can’t Buy Everything

The survival of a cell depends on its ability to respond quickly and robustly to stress—be it injury, invasion, or infection. In certain immune cells, for example, mitochondria are one of the main switchboards for this response, turning on and off many processes that are adjusted in response to a stressor. When doing so, mitochondria shift metabolism to divert resources that would have been used to make ATP, to instead produce molecules that activate and amplify the inflammatory response. When this happens, ATP production by the mitochondria decreases. As one example, mitochondria modify some of the oxygen that would have been used to make ATP to instead form reactive oxygen species (ROS). Although ROS can directly tackle intruders, like bacteria, and promote inflammation to help swiftly eliminate the threat, they are notorious for their damaging side effects.

This metabolic shift is meant to be a temporary diversion. Once the threat has been eliminated, it is critical that the cells return to homeostasis—a balanced state where inflammatory processes are turned off, ROS are neutralized, and mitochondrial metabolism returns to ATP production. In many chronic, autoimmune, and genetic diseases, this “off switch” fails, and these damaging processes stay on, ultimately leading to tissue damage and the potential loss of organ function.1

Nrf2 and the Restoration of Cellular Health

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a key coordinator of the return to homeostasis. Several molecules that are naturally produced during the resolution of inflammation are activators of Nrf2, underscoring the importance of a well-timed Nrf2 response in this restorative process.

Though Nrf2 is known to reduce ROS and suppress inflammation, its roles in restoring mitochondrial function on the road to recovery are more recent discoveries.  Nrf2 increases efficient ATP production, helps remove damaged mitochondria, and increases the production of new healthy mitochondria. 2,3  By clearing the damaging byproducts of inflammation and supporting mitochondrial function, Nrf2 improves the ability of the cell to recover from cellular stress, and is invaluable to overall cellular health.

Helping Mitochondria Help Cells Better

As the major suppliers of ATP, mitochondria remain invested in using resources wisely over the course of a stress response. In response to a threat, mitochondria help the cell reprioritize activities to ensure destruction of the threat and survival of the cell.  Accomplishing this requires a temporary shift toward inflammation and ROS production at the expense of efficient ATP production. To recover, the cell activates Nrf2 to restore proper mitochondrial function, reduce oxidative stress, and clear inflammation.

Reata’s lead investigational drugs were designed to harness the power of Nrf2 by increasing its activity. These Nrf2 activators are currently being evaluated in clinical trials for the treatment of patients with diseases that have key features—mitochondrial dysfunction, oxidative stress, and inflammation—in common. As improving mitochondrial function emerges as a promising therapeutic strategy, we believe the complex relationship between Nrf2 and mitochondria holds great promise for the treatment of disease.

  1. Gilroy, D. & De Maeyer, R. New insights into the resolution of inflammation. Seminars in Immunology (2015). doi:10.1016/j.smim.2015.05.003
  2. Holmström, K. M. et al. Nrf2 impacts cellular bioenergetics by controlling substrate availability for mitochondrial respiration. Biol. Open (2013). doi:10.1242/bio.20134853
  3. Dinkova-Kostova, A. T. & Abramov, A. Y. The emerging role of Nrf2 in mitochondrial function. Free Radical Biology and Medicine (2015). doi:10.1016/j.freeradbiomed.2015.04.036