The surface receptor for advanced glycosylation end-products (RAGE) and its soluble (sRAGE) and endogenous secretory (EN-RAGE) forms belong to the superfamily of toll-like receptors and play important roles in inflammation and autoimmunity, directly or through binding with advanced glycosylation end-products (AGE) and advanced oxidation protein products (AOPP).
Oxidation is a physiological and necessary part of the complex network of physical and chemical phenomena related to life. Oxidative reactions are involved, for example, in the biological chain of aerobic metabolism, in the biosynthesis of small and big molecules, in signal transduction between the outside and inside of cells as well as between the different cell compartments, and in the defense mechanisms used by immune cells [1][2]. However, quantitatively excessive and too long oxidation phenomena are linked to cellular suffering and destruction, which result, at the organism level, in several inflammatory, neoplastic, and degenerative diseases [3]. In this typically hormetic situation, where the beneficial or detrimental effect of reactive oxygen species (ROS) and electrophiles depends on their amount [4], it is not surprising that cells and living organisms have developed several enzymatic and non-enzymatic homeostatic mechanisms to adaptively maintain the delicate and dynamic oxidoreductive (redox) equilibrium within the limits of the so-called “eustress”, as opposed to the aforementioned detrimental imbalance, to which the term “oxidative stress” is commonly referred [5].
In normal conditions, ROS can be generated by the mitochondrial electron transport chain, proteins of the peroxisomal and endoplasmic reticulum, the Fenton reaction, as well as specific enzymes, like cyclooxygenases, nicotinamide adenine dinucleotide phosphate oxidases, lipoxygenases, and xanthine oxidases [6]. External factors, like smoking, eating/drinking habits or lifestyle, may also influence the production of ROS. Environmental variables, and the control of their effects on the redox status of cells, are particularly important in the case of skin, which is the outermost and largest organ of the human body and is structured to be an active interface structure, with multiple connections with the main homeostatic systems of the organism. The major environmental cause of ROS production in the skin is ultraviolet (UV) radiations, mainly derived from sun exposure. Many cutaneous cromophores may directly absorb UV and generate ROS: DNA and other nucleic acids, urocanic acid, tryptophan, tyrosine, NADH (reduced form of nicotinamide adenine dinucleotide), quinones, flavins, and porphyrins mostly absorb ultraviolet B (UVB), while trans-urocanic acid, melanin precursors, and riboflavin absorb ultraviolet A (UVA). Some authors also demonstrated that electromagnetic radiations with lower energy, i.e., visible and infrared light, may induce ROS formation in the skin [7]. Pollutants, like airborne particulate matter or ozone, may increase ROS and/or deplete cutaneous antioxidants [8]. To counterbalance oxidative stress, many antioxidants are available in the skin, including vitamins C and E, beta-carotene, ferritin, uric acid, coenzyme Q, glutathione, glutathione peroxidases, catalase, heme oxygenase, and superoxide dismutase [9].
However generated, excessive ROS may induce cell and organ suffering through multiple mechanisms, such as telomere shortening, DNA damage, mutations of mitochondrial DNA, microRNA (dys)regulation, alteration of matrix metalloproteinases and signal transduction pathways, modifications of the extracellular matrix, chronic inflammation (clinically evident or subclinical), protein oxidation, vascular alterations, and accumulation of advanced glycation end products (AGEs).
AGEs are biomolecules of various nature (proteins, lipids, nucleic acids) that are abnormally covalently bound by glucose or fructose, with the final effect of inhibition of their function. The term “glycation” is used to distinguish this process from glycosilation, a physiological enzyme-mediated binding of sugars that occurs at specific sites of target molecules to enable their functions [10]. Scientific interest in AGEs has grown rapidly in the past decades: published papers went from less than 10 per year in 1991 to about one thousand per year in the last seven years (2014–2020). The reason for this interest is their involvement in a wide and yet incompletely explored range of diseases, as well as biologic processes that are physiological but can be significantly influenced by extrinsic factors [11]. In the latter group, a process of dermatological interest is skin aging, which depends on time but can be enhanced by ROS production, UV exposure, pollutants, lifestyle, and/or personal habits. Among cutaneous molecules, glycated elastin is subject to abnormal aggregation and unusually interacts with lysozyme, glycated collagen resists to the proteolytic action of matrix metalloproteinases, and vimentin undergoes perinuclear accumulation, reducing the cellular contractile capacity; altogether, these modifications result in the tissue stiffening and reduced elasticity that are typically observed in aged skin [11]. Other than having direct detrimental effects, AGEs induce further alterations typical of skin aging through interaction with their receptor, RAGE (Receptor for Advanced Glycation End products) [12].
Discovered by Neeper et al. in 1992 [13], RAGE is a receptor that belongs to the superfamily of toll-like receptors and consists of two cytoplasmatic domains and a variable extracellular domain. Both domains are present in the surface receptor form, while the former domain is missing in the soluble form of the receptor (sRAGE) and the endogenous secretory form (EN-RAGE). In addition to AGEs, which are its canonical ligands, RAGE is also able to bind a number of other endogenous and exogenous molecules. Endogenous ligands belong to the group of DAMPs (Damage-Associated Molecular Patterns) or alarmins, molecules that are released by cells under stress conditions and after their death, are involved in the host’s defense immune system and act, through RAGE, as promoters of inflammation and cytokine production [14]. These alternative RAGE ligands include Advanced Oxidation Protein Products (AOPP) [15], markers of oxidative stress generated by the interaction of plasma proteins, mainly albumin, with chlorinated oxidants, like chloramines and hypochlorous acid [16]. This group of markers includes dityrosine, pentosidine, and carbonyl-containing protein products [17]. AOPP seem to play a role mostly in acute processes of response to inflammatory insults and are emerging as crucial factors in the development of a great deal of skin diseases with acute manifestations, such as systemic lupus erythematosus, vitiligo, psoriasis, and Behcet’s disease, even though other studies involving chronic and tumoral diseases (such as mastocytosis and melanoma) have shown an increase of their circulating levels in patients’ sera [18].
Other known alternative RAGE ligands are S100A12 (S100 calcium-binding protein A12, which is also known as “extracellular newly identified RAGE binding protein” or calgranulin C), High-Mobility Group Box 1 (HMGB1), phosphatidylserine on apoptotic cells, the complement protein C1q, DNA, and Gram negative bacterial lipopolysaccharide (LPS) [15]. Additionally, in other studies, binding to RAGE has been inferred, although without direct demonstration, for diesel particulate matter, oxidized low density lipoprotein, and extracellular heat shock protein 70 [15].
The membrane isoform of RAGE is implicated in the pathways of NFAT (nuclear factor of activated T-cells), NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells), STAT3 (signal transducer and activator of transcription 3), and CREB (cAMP response element-binding protein) transcription factors, which are involved in the autophagy and apoptosis processes. RAGE has been found in a large number of cell types, which include, among others, endothelial cells, neural cells, cardiac myocytes, mesangial cells, as well as cells of the immune system such as monocytes, macrophages, and T lymphocytes [19]. In the skin, RAGE is highly expressed, with variable patterns that depend on age and sun exposure [20]. In detail, in fetal skin, RAGE is located in the upper epidermis and in few endothelial cells within the dermis; in young skin, it is present in the superficial and middle epidermis and in the papillary dermis; in aged skin, it can be found in the middle and basal part of the epidermis and in the reticular dermis. Chronically sun-exposed skin shows patchy distribution of RAGE in the epidermis and increased expression of RAGE on cells of the papillary dermis. For which concerns the type of cells, RAGE was mainly expressed on fibroblasts, dendrocytes, and endothelial cells, while the number of RAGE-positive CD45 lymphocytes is low [20].
The soluble form of RAGE should act as a decoy receptor, preventing the furthering of the pro-inflammatory cascade, in which RAGE plays a role. The role of sRAGE as a diagnostic marker for disease activity has been debated, since its levels might increase (as a way for the body itself to try to shut down inflammatory processes) in several diseases, but results have been inconclusive.
As of now, we know that RAGE plays a role in a plethora of other physiological and pathological processes, including diabetes, osteoporosis and cardiovascular diseases. Over the course of time, many different factors, including oxidative stress, were studied to explain the pathophysiology of many skin diseases characterized by chronic inflammation and frequent relapses, such as atopic dermatitis [21][22][23], psoriasis [24][25], lichen planus [26][27], vitiligo [28][29], or discoid lupus erythematosus [30]. It is well known that the clinical expression of such diseases depends on a variety of factors, both inner and outer, but, as of yet, one of these factors, namely RAGE, seems to play an important role in a context of continuous inflammatory and autoimmune stimuli, thus prompting further studies in order to complete the picture we have of such diseases.
As outlined before, there is great scientific interest and research about the importance of RAGE in all fields of medicine. Dermatology is a peculiar discipline, which includes purely cutaneous diseases and cutaneous manifestations of internal or multi-organ diseases, and dermatological research on RAGE reflects this fact, because available resources were spread over multiple studies concerning different conditions within the discipline, producing a remarkable but polyhedric amount of data. The aim of this review is to organize the currently available knowledge about the role of RAGE in the macrocosm represented by skin diseases to better understand immunopathological pathways involved and possible therapeutic options.
This entry was performed by browsing the PubMed database (
), searching for articles labeled with both the MeSH (Medical Subject Headings) terms “RAGE” and “Skin”, with the search string
. The initial search produced 39 articles, of which nine were excluded from this review by title and abstract only, and three were excluded after reading them entirely. One additional article was found and included after a search in the PubMed database with the two MeSH terms
and
, for a grand total of 28 articles. The articles included in this review showed a direct link between RAGE and various skin conditions, because RAGE was either a prominent feature in prolonging the inflammatory response or a primary cause of it. The excluded articles were reviews, did not concern a direct link between RAGE and skin, or did not provide enough information about materials and methods. Due to the great heterogeneity of materials, methods, and outcomes of the above cited studies, a meta-analysis was not deemed appropriate.
The RAGE induced pathway has also an important role in melanogenesis, through ligation with AGEs. The phosphorylated form of CREB, which is part of the RAGE downstream cascade, activates the CRE (cAMP response elements) consensus motifs of MITF (microphthalmia-associated transcription factor) and, consequently, tyrosinases, boosting melanin production in melanocytes [55]. On the other hand, Cui et al. demonstrated that RAGE plays an important role in the pathogenesis of vitiligo [56] and ubiquinone-induced irritation of melanocytes [57]. In vitiligo-affected patients, RAGE, out of the three PRRs (pattern recognition receptors) that bind HMGB1 (the others being the toll-like receptors TLR2 and TLR4), is the only one upregulated, and its blockade significantly reduces the levels of IL-8 and CXCL16 [chemokine (C-X-C motif) ligand 16] in keratinocytes. The authors investigated the in vitro levels of RAGE and HMGB1 after hydrogen peroxide was added to the culture: indeed, the levels of both HMGB1 and RAGE increased. Because of these results, questions arise about the possible effects of pro-oxidative therapies such as UVB phototherapy, in which there is an activation of melanocytic stem cells and down-regulation of self-directed T cells, opposed to the UV-induced, pro-inflammatory release of HMGB1 by keratinocytes [56].
The current knowledge about the RAGE receptor is very limited. Given the great deal of roles that this molecule plays in different types of skin diseases, what emerges from these studies is that it would be convenient to push forward the research using live models, in order to better understand the role of RAGE in a more complete and complex environment. The great deal of skin diseases which involve both AGEs and AOPPs can provide different opportunities to study the different roles played by RAGE. It is our proposition that the two most interesting diseases that can prove useful on such note would be systemic lupus erythematosus and systemic scleroderma, since both diseases have characteristic phases (acute, subacute, and chronic), which can better define the already discussed roles of AGE (mostly in chronic conditions) and AOPP (most prominent in acute processes) and their interaction with RAGE.
Another key feature emerging from these studies is that the soluble form of RAGE (sRAGE) can prove to be a potential diagnostic tool to better asses the disease activity in various skin diseases, mostly chronic, and its topical form could be used as an alternative treatment option when other topicals are not available or could provide significant side effects. Indeed, it could be interesting to perform compared studies with the current golden standard, topical therapy and sRAGE. On the other hand, based on the current knowledge about this receptor, a systemic sRAGE treatment, although fascinating, is likely to be difficult, since RAGE does not play a single role as an inflammatory receptor but as a remodeling one as well.
Finally, a potential target therapy with antibodies, anti-RAGE, as a systemic treatment, seems to be an interesting perspective for a plethora of diseases, even though further studies must be conducted. This approach could prove useful especially in those diseases where the accumulation of ROS, presence of alarmins, and overall inflammatory state could be a greater management challenge than the first pathogenetic cause, or in those acute diseases where there is a need of a preemptive stop to a self-directed harmful process (such as small vessels diseases).