The Aryl Hydrocarbon receptor and intestinal immunity: an overview of ligands derived from microbial metabolism and food
Mooij, F.A. de
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Layman’s summary Everybody knows that eating vegetables and overall healthy food is beneficial for your health. In recent years advertisers also started to tell us that products such as Yakult© that contain bacteria can improve the wellbeing of our intestines1. This is why a lot of research has been going towards finding the mechanisms behind these healthy bacteria and the beneficial effects of healthy food. It is now suggested that there could be a link between these two. Our bodies are built of cells that obtain their energy from food. In our intestines we have a lot of bacteria that helps us digest our food. This may sound weird, but in contrast to bacteria in other parts of our body, bacteria in the intestine are not harmful as long as there is a state of symbiosis. The cells of our intestines are not able to use all the food that we eat because they lack the machinery to make little pieces of it that can get absorbed into the cells. That’s where our resident bacteria come in. They have different machinery that help us process certain types of food into the small pieces we need. When these small pieces of food are taken up by the intestinal cells, they can start processes within the cells by binding to receptors. For instance, this could lead to the development and activation of the immune system. This can help to keep the balance between the intestine and the resident bacteria. Recent research has shown us that an important receptor in this process is the Aryl Hydrocarbon Receptor (AhR). The AhR can assist in protecting our intestines from harm by activating part of our immune system. They do so by activating the cells to produce signaling molecules, one of the most important ones is interleukin 22 (IL-22). This IL-22 is on its own able to bind its own specific receptor on other cells thereby amplifying the signal and activating these cells to start secreting small proteins such as defensins. As the name suggests, these defensins are small proteins that can kill the bacteria. With the secretion of defensins the amount of bacteria can be controlled, hereby keeping the balance to provide symbiosis. Next to the activation of cells to produce defensins, the activation of the AhR also leads to the development of other cells from our immune system, thereby making sure that we are well protected in the long run. As described, via activation of the AhR we can modulate the health of our intestines. It is therefore very interesting to know how we can activate this receptor. It is now suggested that the small molecules that bind the AhR can come from our food. For instance, tryptophan, which is an amino acid, gets cut by the gut bacteria into kynurenine which is then able to bind and activate the AhR. A particular group of bacteria, the lactobacilli, are very able to contribute to this process. Additionally, small molecules derived from food such as broccoli and cabbage have been shown to activate the AhR. One of these molecules is known as indole-3-carbinol. In conclusion, activating the AhR in the intestines could improve the health of our intestines. Molecules from our food could trigger this process. This provides a mechanism to explain that consumption of certain types of food, or supplementing our food with certain bacteria strains, has the potential to increase our health.