AGEs are recognized to have detrimental effects on several physiological processes within the human body through two main mechanisms. The first mechanism, as elaborated in the preceding posts, involves a direct process in which they bind to proteins and lipids to modify them and further deepen and expand the modification through cross-linking, which is the formation of chemical bridges between nearby proteins. The second mechanism is through a specific receptor that binds to AGEs on various cell surfaces. We will look at the second mechanism in this post.
Various ligands binding to RAGE, including AGEs
RAGE, which stands for Receptor AGEs, was first identified in 1992 when it was isolated from bovine lungs.[1] The name was given due to its role as an AGEs receptor and research was focused mainly on its interaction with AGEs. However, ongoing studies have revealed that RAGE binds not only to AGEs but also to various ligands, including the S100 protein family (linked to inflammation/immune response), Amyloid-β (associated with Alzheimer's pathology), HMGB1 (High mobility group box 1, involved in inflammation/immune response), and Mac-1 (CD11b/CD18, related to immune cells). These ligands represent numerous bonding partners for RAGE. AGEs still remain the most well-known ligand.
As a pattern recognition receptor and a member of the immunoglobulin superfamily, RAGE is present in various cell tissues throughout the body, such as endothelial cells, nerve cells, immune cells, and epithelial cells lining the skin and internal surfaces. As a transmembrane receptor, it extends both externally and internally across the cell membrane.
3 domains of RAGE (structural organization)
1. Extracellular Domains
Among the three regions that make up RAGE, this is the part of RAGE located outside the cell and is responsible for recognizing and binding specific ligands or molecules present in the extracellular environment. There is a V(Variable) type immunoglobulin domain and C1(constant-1) and C2(constant-2) type immunoglobulin domains. V is a hand-like part that captures and binds various ligand molecules, including AGEs and S100 proteins. C1 and C2 are subdomains that act like a framework to help RAGE remain stable and robust. These are all located outside the cell.
2. Transmembrane Domain
This region spans the cell membrane and acts like a peg that anchors RAGE to the cell surface, connecting the inside and outside of the cell. It is not directly related to ligand binding.
3. Intracellular Domain
The cytoplasmic tail is located inside the cell after passing through the cell membrane. When a ligand is bound, this cytoplasmic tail activates signaling pathways within the cell, leading to a cellular response. Different ligands can activate different signaling pathways.
Each domain has a specific purpose: the extracellular domain identifies and binds ligands and contributes to overall structural stability; the transmembrane domain anchors RAGE to the cell membrane, and the intracellular domain transmits signals after ligand binding to initiate the cellular response.Structural organization of RAGE [2]
Structural organization of RAGE [2]
sRAGE(soluble RAGE): Water-soluble RAGE
sRAGE is a soluble form of RAGE that is produced by a process called alternative splicing, a mechanism by which various combinations of exons (coding regions) within a gene are included or excluded during the creation of messenger RNA (mRNA).
While RAGE typically possesses all three domains mentioned above, sRAGE lacks the second transmembrane domain. As a result, sRAGE is secreted into the extracellular space and freely circulates in bodily fluids such as blood (see the picture above). In addition, sRAGE cannot play a role in signal transmission because it lacks the third intracellular domain that carries out intracellular signal transduction. Instead, sRAGE can act as a decoy by competing with regular RAGE for ligand binding, intercepting the binding of ligands to RAGE and effectively blocking signal transmission. This decoy effect is expected to prevent RAGE from binding with ligands and disrupting signal transduction [3].
If AGEs cannot bind to RAGE, they cannot enter the cells. Some medications contain sRAGE, such as antihypertensive drugs (calcium channel blockers, angiotensin-converting enzyme inhibitors, angiotensin II receptor blockers), antidiabetic drugs (thiazolidinedione), and cholesterol-lowering drugs (statins). Their suppressive effects on RAGE expression can play a part in preventing various diabetic complications by preventing AGE from binding to RAGE [4]
In fact, sRAGE has been shown to successfully prevent or reverse the signaling effects of RAGE such as diabetic atherosclerosis and impaired wound healing, colitis, amyloid-β penetration of the blood-brain barrier, and tumor cell migration and invasion.[5]
IMPACT OF RAGE
1. Inflammatory response
When RAGE binds to various ligands including AGEs, the cytoplasmic tail interacts with intracellular signaling molecules, activating signaling pathways within the cell and promoting immune responses and inflammation.
One major signaling pathway is the activation of nuclear factor-kappa B (NF-κB), a key transcription factor involved in inflammation.[6] When RAGE is activated, NF-κB moves into the nucleus and promotes the expression of genes involved in inflammation and immune responses. This leads to increased secretion of cytokines such as interleukin-6 (IL-6) and tumor necrosis factor, further amplifying and overheating the inflammatory process.
NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) is a protein complex that plays a crucial role in regulating immune responses to infection and inflammation. It is also a transcription factor that regulates the expression of genes involved in inflammatory and immune responses, cell proliferation, and apoptosis(cell death). It is present in all cell types and is generally found in an inactive state in the cytoplasm. However, when activated by various stimuli such as bacterial or viral antigens, cytokines, or stress, it moves into the cell nucleus and binds to specific DNA sequences to initiate the transcription of target genes. Therefore, if these processes are not properly regulated, it can lead to inflammatory conditions.
As we know, the inflammatory response is part of the human body's defense mechanism to protect itself from harmful substances, and it is natural and beneficial to us. However, if the RAGE-mediated inflammatory signaling pathway is chronically activated and uncontrolled or persists for a long time, it can eventually contribute to tissue damage and the development of various conditions, including diabetic complications, neurodegenerative diseases (such as Alzheimer's disease), cardiovascular diseases, and cancer.
2. Oxidative stress
It also activates NADPH oxidase. This leads to the production of reactive oxygen species (ROS) by generating significant amounts of superoxide radicals, anions, and hydroxyl radicals, which increases oxidative stress and can eventually cause tissue and cell damage. Because of its highly reactive nature, ROS can damage cellular components and cause inflammatory reactions. In fact, the expression of NADPH oxidase and the level of ROS in the brain have been found to increase in neurodegenerative disorders such as Parkinson's disease and Alzheimer's disease. [7]
Future use plans for RAGE
Understanding the role of RAGE is important to get insight into the mechanism of the AGEs related pathophysiology, and it also opens up possibilities for developing therapeutic strategies that utilize the interaction between RAGE and its ligands. For example, specific inhibitors or antagonists could be developed to block the activation of RAGE and alleviate inflammation and immune responses. Additionally, by measuring the amount of RAGE or identifying its condition, it is possible to diagnose the presence or absence of a disease related to RAGE and monitor its progress. In other words, RAGE could potentially be utilized as a diagnostic marker for its related diseases.
Research is already underway to inhibit or block the signaling pathways of RAGE for the treatment of AGEs-related diseases [8]. However, since RAGE also plays important roles under normal physiological conditions, it is emphasized that future studies need to carefully investigate the advantages and disadvantages of RAGE-targeted therapies, as well as the long-term effects of RAGE blockade in humans.
[References]
[1] RAGE (Receptor for Advanced Glycation Endproducts), RAGE Ligands, and their role in Cancer and Inflammation
[2] Receptor for Advanced Glycation End Products (RAGE): A Pivotal Hub in Immune Diseases
[3] Pathophysiology of RAGE in inflammatory diseases
[5] Purification and Characterization of Mouse Soluble Receptor for Advanced Glycation End Products (sRAGE)
[7] Enhanced expression of RAGE/NADPH oxidase signaling pathway and increased level of oxidative stress in brains of rats with chronic fluorosis and the protective effects of blockers
[8]Targeting receptors of advanced glycation end products (RAGE): Preventing diabetes induced cancer and diabetic complications
Structural Basis for Ligand Recognition and Activation of RAGE
: Ligands, Three Domains, Impacts on Health, and Therapeutic Usage){kind=link}