Article at a Glance
- When our cells use energy, the end product is the production of free radicals.
- An excess of free radicals create oxidative stress, which has been linked to many chronic health conditions.
- The Nrf2 pathway, is a genetic pathway that turns on over 200 genes, many of which are related to detoxification, such as GSTP1. In health, Nrf2 is activated by oxidative stress, but there are also nutrients that can activate Nrf2.
- Among the most potent activators of Nrf2 is sulforaphane, a compound found in great abundance in broccoli sprouts.
We’ve discussed oxidative stress and free radicals in various posts on the Gene Food blog, often focusing on specific enzymes produced by the body such as superoxide dismutase 2 (SOD2), or specific beneficial nutrients such as glutathione. These direct acting enzymes and nutrients are great at fighting the fire of oxidative stress, but where this is a chronic condition, understanding how best to activate these antioxidants in the body is of great interest.
Before we dive into the importance of the Nrf2 pathway, we’ll go over a little primer on what happens when our bodies generate energy at the cellular level.
Cellular energy generation produces free radicals
Most compounds and molecules in our body exist in a relatively stable state, this is why we need enzymes (think of them as molecular power tools) to break them down or turn them into other more useful molecules.
However, some reactions in the body lead to the formation of molecules called free radicals (including reactive oxygen species and peroxides), most notably those involved in energy generation within the cell. Put another way, the byproduct of energy metabolism are damaging free radicals.
These free radicals are chemically unstable, and highly reactive. Two common examples are O2– sometimes called superoxide and H2O2 or hydrogen peroxide. O2 is the normal stable form of oxygen, however when involved in some reactions in the body O2– is formed. That little – sign means that an extra electron has been added. Superoxide is therefore highly reactive as it is trying to find a way to lose that extra electron, and the only way to do that is to transfer it to another molecule or compound.
This is where antioxidants step in, it’s their function to mop up these highly reactive species making sure they don’t interact with any of the important molecules in a cell. This mopping up occurs constantly in the body, however, when antioxidant function in the body becomes overwhelmed by free radicals, oxidative stress can occur (R).
It is important to understand that free radicals in themselves are not bad, their formation is part and parcel of cellular energy generation, and they serve a vital role in other functions such as bacterial killing (R). So your aim should never be to remove free radicals, but rather ensure your system is in balance.
Oxidative stress and disease
In the short term, oxidative stress is unlikely to be harmful, as whilst the body may be temporarily overwhelmed it will usually respond and clear any harmful compounds.
However chronic oxidative stress is associated with several diseases including Parkinson’s disease (R), Alzheimer’s disease (R), cardiovascular disease (R) and several cancers (R). The exact impact is hard to pin down due to the long time scales involved in the development of these diseases, but it is an area of very active research.
Other major areas of interest are dietary and environmental exposures. At a base level things like alcohol and tobacco or a meat rich diet can increase oxidative stress. However, these are relatively easy to avoid and if you’re reading this blog chances are you’re already making positive health choices. There are other factors however which are more difficult to avoid such as air pollution (R), excessive exposure to pesticides (R), herbicides (R) or heavy metals (R). In these cases chronic oxidative stress can be generated, leaving existing anti-oxidant defences overwhelmed.
Fighting chronic oxidative stress
So we know that molecules such as glutathione are great at mopping up free radicals on a one to one basis, and that enzymes such as the SOD or glutathione peroxidase (GPX) enzyme families can rapidly reduce oxidative stress. But what happens in a chronic system where these defenses have been overwhelmed?
Nuclear factor erythroid 2 [NF-E2]-related factor 2, known by the much easier to remember acronym Nrf2 is a transcription factor which is a major regulator of cell protective, including antioxidant, responses (R). Transcription factors are a class of protein which bind to DNA and induce the expression of particular genes, in the case of Nrf2, these are potent antioxidants such as NAD(P)H:quinone oxidoreductase 1 (NQO1) (R) and glutathione S-transferases (GSTs), and many others, as shown in the table below (R).
Table 1 – Genes activated by NRF2. Adapted from: Baird, L. and Dinkova-Kostova, A.T. Arch. Toxicol. 85,241-272 (2011)
Nrf2 in health
In health, Nrf2 exists in the cytosol of a cell, or the region outside of the nucleus where it cannot interact with DNA. It is held here by another protein called Keap1, which prevents it moving into the nucleus (R).
Keap1 contains several sensors for reactive oxygen species, along with receptor regions for other cell proteins associated with cell stress (R). If these receptors are activated then Nrf2 is released, and can pass into the nucleus (R).
Once within the nucleus Nrf2 binds to a region of DNA known as an “Antioxidant Response Element” or ARE (R). These ARE elements are closely associated with the genes for NQO1 and the GSTs (and many other genes) as discussed above, and when Nrf2 binds it induces the production of active proteins from these genes. Both of these proteins exhibit potent antioxidant capacity and can rapidly clear oxidative stress.
Figure 1 – Nrf2 activation. Adapted from: May, O.L. Cayman Chemical (2012)
The important role for Nrf2 has been shown by several studies with a particular focus on animal models. Mice which have been genetically engineered to lack Nrf2 protein are considerably more sensitive to chemical carcinogens, and also chemical toxins leading to increased inflammation in the lung and brain, all major markers of oxidative stress.
Foods and supplements that activate Nrf2
While oxidative stress alone is able to activate Nrf2, its effect is greatly enhanced by the presence of certain chemical compounds.
The simplest of these compounds were identified as 1,4-benzenediol (hydroquinone), tert-butylhydroquinone, and 1,2-benzendiol (catechol), and more complex examples include the isothiocyanate sulforaphane from broccoli seed extract, curcumin from the turmeric plant and carotenoids such as zeaxanthin and lutein. All are complex in name and structure, but they share a common function in that they support increased and prolonged expression of Nrf2 gene targets (R, R).
Issues with Nrf2 activation
Whilst Nrf2 activation is undoubtedly beneficial in the vast majority of cases, there are some instances where it may prove harmful. Once again using a mouse model, this time lacking Keap1, meaning that Nrf2 would be constantly activated, researchers demonstrated that severe health effects would develop (R). Additionally, mutations in the Nrf2 gene in human studies have been associated with an increased risk of cancer, with the authors proposing that constance Nrf2 activity, prevents anti-cancer targeted oxidative stress, allowing cancer cells to survive (R).
It is unlikely that dietary supplementation would ever induce such strong effects but it’s a useful example of how oxidative stress isn’t always bad. Importantly there are no currently reported SNPs in Nrf2 which impact on its function. A lack of variation in genes like this typically points to how important they are to life, however an alternative option is that Nrf2 has not been widely investigated.
The take home message in this instance is quite simple. Nrf2 is a potent modulator of antioxidant response and can rapidly target oxidative stressors. While Nrf2 responds to oxidative stress directly, certain key nutrients can improve this effect, the exact source of these nutrients varies widely but dark green and leafy vegetables (think spinach, kale and broccoli, especially the seed extract), or reddy vegetables and spices (saffron, tumeric or paprika) are a particularly rich source. Importantly these foods are also typically rich in direct antioxidants and display numerous other health benefits, so they are a great way to improve overall health.