Review 28 July 2023, and 1 College of Pharmacy, Gachon University, 191 Hambakmoe-ro, Yeonsu-gu, Incheon 21936, Republic of Korea 2 Gachon Pharmaceutical Research Institute, Gachon University, 191 Hambakmoe-ro, Yeonsu-gu, Incheon 21936, Republic of Korea Author to whom correspondence should be addressed. These authors contributed equally to this work.
Melanocortins play crucial roles in regulating the stress response, inflammation, and skin pigmentation. In this review, we focus on the melanocortin 1 receptor (MC1R), a G protein-coupled receptor primarily known for regulating skin pigmentation and exhibiting anti-inflammatory effects. First, we provide an overview of the structure, signaling pathways, and related diseases of MC1R. Next, we discuss the potential therapeutic use of synthetic peptides and small molecule modulators of MC1R, highlighting the development of various drugs that enhance stability through amino acid sequence modifications and small molecule drugs to overcome limitations associated with peptide characteristics. Notably, MC1R-targeted drugs have applications beyond skin pigmentation-related diseases, which predominantly affect MC1R in melanocytes. These drugs can also be useful in treating inflammatory diseases with MC1R expression present in various cells. Our review underscores the potential of MC1R-targeted drugs to treat a wide range of diseases and encourages further research in this area.
Melanocortins are a group of hormones that play crucial roles in regulating various physiological processes, including stress response, inflammation, and skin pigmentation. These hormones are polypeptides derived from pro-opiomelanocortin (POMC) and include adrenocorticotropin hormone (ACTH), α-, β-, and γ-melanocyte stimulating hormone (α-, β-, and γ-MSH) [1,2]. While the functions of ACTH [3] and α-MSH are well-established in skin pigmentation [4], anti-inflammatory [5], and microbicidal characteristics [6], the role of β-MSH and γ-MSH are less well understood. Some studies suggested that they possess anti-inflammatory properties [7,8,9,10].
The functions of melanocortin peptides are mediated by G protein-coupled receptors (GPCRs), primarily the stimulatory G protein (Gs) [11]. The melanocortin receptor family is the smallest member of class A, a rhodopsin-like family of GPCRs, and consists of five isotypes (MC1R, MC2R, MC3R, MC4R, and MC5R) with varied tissue expression and functions [12]. MC1R, mainly expressed in both melanocytes and leukocytes, enhances UV resistance and anti-inflammatory signaling when activated. MC2R is found in the adrenal cortex, and both MC3R and MC4R, which are reported to control food intake and sexual function, are largely present in the CNS. MC5R, located in the brain and skeletal muscle, has an exocrine role. The low sequence homology (40–60%) among the five receptors explains the lack of ligand selectivity [13,14,15,16].
The MC1R is a well-known receptor for α-MSH expressed in the skin and hair follicles, where it controls pigmentation. However, recent studies have revealed that MC1R is also expressed in various other cell types that are susceptible to the anti-inflammatory effects of melanocortins [17,18]. Thus, understanding the regulation of MC1R could potentially lead to the development of novel therapeutic strategies. This review article provides a brief overview of the structure, signaling pathway, and related diseases associated with MC1R and discusses the potential of synthetic peptide modulators and small molecule modulators of MC1R as therapeutic agents. Through a detailed examination of MC1R and its modulators, this review aims to provide insights into the potential clinical implications of modulating MC1R activity.
The human MC1R is primarily found on melanocytes and malignant melanoma cells and consists of 317 amino acids [19,20,21]. While the normal expression of MC1R protein is low, melanocytes express approximately 700 protein units, with slightly higher amounts found on melanoma cells [22,23]. MC1R is a GPCR with seven α-helical transmembrane (TM) domains, an N-linked glycosylation site at the external N-terminus, a palmitoylation site at the intracellular C-terminus, and a DRY motif at the junction of the third TM domain. Unlike other GPCRs, the first and second extracellular domains of the MC receptor subfamily lack one or two cysteines, while the fourth and fifth TM domains lack proline. The intracellular and transmembrane domains of MC1R regulate adenylyl cyclase connections and signaling, while the extracellular and transmembrane domains interact with MC1R ligands.
The extracellular N-terminal tail serves as a signal anchor and plays a crucial role in ligand affinity. A conserved cysteine residue at the intersection of the N-terminus and the first TM domain is crucial for receptor function [24,25,26,27,28]. The C-terminus is involved in receptor interactions with the G protein at the plasma membrane, as well as protein trafficking from the endoplasmic reticulum to the plasma membrane. Also, C-terminus affects desensitization, internalization, and the plasma membrane location of the receptor [29,30,31,32,33].
The intracellular and extracellular loops (ils and els, respectively) lie between the transmembrane domains of the melanocortin 1 receptor. These loops share conserved sequences with other MC receptors. Despite being smaller than most GPCRs, MC1R els are essential for constitutive basal signaling activity. Mutations in this area affect binding affinity, as the els of the MC1R interact with ligands. Due to conserved proline and cysteine residues, els appear to be essential for melanocortin affinity. Similarly, MC1R ils play a crucial role in Gs protein binding and have phosphorylation sites that impact signal modulation, internalization, and receptor cycling [25,27,28,31,34].
Recently, the Cryo-electron microscopy (Cryo-EM) structure of MC1R and the MC1R–Gs complexes bound to the endogenous hormone α-MSH, the marketed drug afamelanotide, and a synthetic agonist called SHU9119 were determined [35]. These findings may pave the way for resolving the lack of selectivity in MC drug discovery.
The melanocortin 1 receptor (MC1R) is a receptor that forms a complex with heterotrimeric G proteins. When agonistic ligands bind to MC1R, the Gαs protein is separated, and MC1R activates adenylyl cyclase, which leads to the production of cAMP, a crucial second messenger that regulates many cellular processes. In melanocytes, cAMP activates protein kinase A (PKA) and triggers downstream signaling pathways that activate different effector pathways, including the CREB and MITF networks. These pathways lead to the increased expression of tyrosinase and dopachrome tautomerase, two enzymes that are involved in melanin synthesis, resulting in the production of melanin. The melanin produced is then transmitted to nearby keratinocytes, creating a protective layer that improves the skin’s ability to prevent further UV damage. Moreover, the increase in cAMP levels in melanocytes enhances antioxidant defenses and accelerates nucleotide excision repair (NER), which is vital for safeguarding the skin from UV damage (Figure 1) [36,37,38,39,40,41,42,43].
Figure 1. Signaling pathway of MC1R in melanocytes.
Moreover, it is also reported that there is a cAMP-independent pathway mediated by cKIT, which is not discussed in this review. This pathway activates the extracellular signal-regulated protein kinases 1 and 2 (ERK 1/2) by triggering the NRAS-BRAF-MEK-ERK cascade and active ERKs can phosphorylate MITF. Furthermore, it has been suggested that the activation of AKT downstream of α-MSH may also occur via cKIT [44].
Furthermore, the MC1R signaling axis has an anti-inflammatory effect through downstream pathways. When α-MSH binds to MC1R, it triggers the production of intracellular cAMP, which activates protein kinases C and A, leading to the activation of the MAPK and JAK-STAT pathways. These pathways prevent IκB degradation and activate CREB, a transcription factor that regulates anti-inflammatory mediators such as IB and IL-10. In addition, protein kinase activation enhances the levels of cytoplasmic IκB, inhibiting the expression of downstream pro-inflammatory genes, including IL-1, TNF-a, IL-6, IL-8, and IL-12, iNOS, and adhesion molecules (ICAM-1, VCAM-1, and MMPs) (Figure 2) [45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60].
Figure 2. The role of MC1R signaling in anti-inflammatory effects.
After UV damage, melanocortins such as α-MSH and ACTH are induced in the skin, and these ligands protect the skin by binding to MC1R and triggering changes in melanocytes that enhance their resistance to UV (Figure 3) [87,88,89]. However, the MC1R can also be affected by other ligands, such as agouti signaling protein (ASIP) and β-defensin 3 (βD3), which can have a significant impact on MC1R signaling [90,91,92,93]. Melanocortins stimulate MC1R signaling, while ASIP inhibits MC1R signaling and reduces cAMP levels [94,95]. In contrast, βD3 has little effect on signaling but can act as a competitive inhibitor, interfering with the binding of α-MSH or ASIP [96,97]. Synthetic modulators of MC1R, including both peptides and small molecules, have been reported, and now some of these modulators will be introduced.
Figure 3. Structure of α-MSH.
As previously explained, the endogenous ligands for melanocortin receptors are ACTH and α-, β-, γ-MSH. The most important finding in early drug development was that the amino acid sequence, His6-Phe-Arg-Trp9, was the key sequence needed to activate the melanocortin receptor [98]. And the sequence was modified to solve the instability, which was a problem of α-MSH, leading to the development of [Nle4, D-Phe7]-α-MSH (melanotan I) [99,100]. Additionally, studies on the cyclization of peptides were conducted to obtain melanotan II using NDP-MSH as a scaffold [101,102]. With these findings, efforts are currently underway to produce selective and stable peptides on the melanocortin 1 receptor, such as PL-8177 [67]. Among them, we would like to introduce melanotan I and II, which are key substances in the development of peptide drugs (Figure 4).
Figure 4. Structure of melanotan I.
Melanotan I is an early analog of α-MSH that acts as a non-selective agonist of melanocortin receptors and stimulates melanin production. During the development of peptide-based drugs, researchers focused on the common sequence of MSH, namely His6-Phe-Arg-Trp9, and further studies were conducted on this sequence (Figure 4) [98]. Eventually, in the frog skin bioassay, it was observed that Ac(Acetyl)-α-MSH7-10-NH2 and Ac-α-MSH6-8-NH2 had no activity, while Ac-α-MSH6-9-NH2 did. This led to the discovery that the minimal sequence required for the biological activity of α-MSH is His6-Phe-Arg-Trp9 [99]. Furthermore, to increase the stability of the initial α-MSH analog, Sawyer et al. replaced Phe at position 7 with D-Phe and Methionine at position 4 with the amino acid norleucine to prevent oxidation of methionine, as highlighted in green ball. These changes resulted in the production of [Nle4, D-Phe7]-α-MSH (NDP-MSH, melanotan I), a peptide with higher potency and stability than α-MSH [100].
When melanotan I activates MC1R, cAMP is produced, and it activates microphthalmia transcription factor (MITF) expression, which induces the expression of enzymes for eumelanin production. This process increases the production of eumelanin in melanocytes. In addition, melanotan I activates tyrosinase and induces an increase in eumelanin content in melanocytes [103,104,105].
Afamelanotide, well-known as the international nonproprietary name of melanotan I, has been used in patients with erythropoietic protoporphyria (EPP) since 2019. EPP is a disease that causes abnormal hemoglobin synthesis in red blood cells, which can cause skin damage even with a little sunlight. When afamelanotide binds to MC1R, it activates melanocytes and stimulates eumelanin synthesis. Eumelanin protects against UV light by absorbing and scattering it, scavenging free radicals and reactive oxygen species, and acting as a neutral density filter capable of decreasing transmission of all wavelengths of light [106,107]. Recent studies have shed light on the binding structure of MC1R and ligands, while no information has yet been disclosed about the binding structure of receptors and peptides. Afamelanotide, the compound that was switched from Phe7 to D-Phe7 in α-MSH, has an extra hydrogen bond with the TM2 domain in D-Phe7. It has been demonstrated that afamelanotide has a higher affinity for receptors than α-MSH, as evidenced by its capacity to make cAMP, which is superior to α-MSH, as determined by various mutants (Figure 5) [35].
Figure 5. Interactions of afamelanotide (melanotan I) with MC1R domains.
Melanotan II is a cyclic peptide that is derived from melanotan I (Figure 6). In particular, the peptide is modified by replacing Glu5 with Asp and Gly10 with Lys. Previous studies by Obeidi and Hadley have demonstrated that cyclic peptides are more stable and potent than linear peptides. Their research investigated the efficacy and stability of a linear form of Ac-[Nle4, D-Phe7, Lys10]-α-MSH4-10-NH2 and a cyclic form, in which Asp5 and Lys10 were linked. The findings of their bioassay demonstrated that cyclic peptides exhibited greater potency and stability than linear peptides [101,102]. Melanotan II has been shown to activate melanocytes as an MC1R agonist, but it is weaker than melanotan I. However, melanotan II was proven to have a stronger sexual effect [108]. It has been reported to induce penile erections in male rats and sexual inspiration [109,110] and increases the sexual activities of female rats [111]. Bremelanotide (PT-141), an acid derivative of melanotan II, acts as an agonist of the MC1R
