Receptor · Melanocortin 1 receptor
MC1R — the pigment switch
The receptor on melanocytes that decides between protective brown-black eumelanin and reddish pheomelanin. Its weak variants explain red hair and fair skin — and carry an independent melanoma risk.
- Tissue
- Melanocytes (skin & hair); immune cells
- Ligands
- α-MSH, ACTH (agonists); agouti signalling protein (antagonist)
- Function
- Switches melanocytes from red/yellow pheomelanin toward brown/black eumelanin — the master control of pigmentation and UV response.
- When it fails
- Loss-of-function variants drive red hair, fair skin, and raised melanoma risk.
MC1R is the melanocortin system's contribution to how you look — and to how well your skin defends itself against the sun. Sitting on the surface of melanocytes, it is the master switch that sets which kind of melanin those cells produce.[1]
The pigment switch
Melanocytes can make two broad classes of pigment: eumelanin, the brown-black pigment that absorbs UV and protects DNA, andpheomelanin, a reddish-yellow pigment that is a poor sunscreen and can itself generate damaging free radicals. When α-MSH (or ACTH) activates MC1R, the receptor — coupled to Gs — raises cyclic AMP, drives the pigment master-regulator MITF, and shifts production toward eumelanin.[2] When MC1R signalling is weak, the balance tips toward pheomelanin: the visible result is red or blond hair, fair skin that burns rather than tans, and freckling.[2]
The system also has a built-in brake. Agouti signalling protein antagonises MC1R, lowering its activity — the same mechanism that produces banded coat colour in other mammals.[1]
The "red hair" variants
MC1R is one of the most variable genes in the human genome. A handful of strongly penetrant loss-of-function alleles — classically R151C, R160W, and D294H — are associated with the red-hair, fair-skin phenotype. These variants largely fail because the mutant receptor is mis-trafficked and under-expressed at the cell surface, so less signal gets through even when α-MSH is present.[3]
Why it matters: skin cancer risk
Carrying red-hair-colour MC1R variants raises the risk of cutaneous melanoma and of non-melanoma skin cancers — and the effect persists even after accounting for fair skin and red hair, pointing to a contribution beyond visible pigmentation.[2] Part of the explanation is that pheomelanin amplifies UV-induced DNA damage, and may even contribute to melanoma formation through oxidative damage in the relative absence of UV.[2] Strong MC1R signalling, by contrast, supports not just eumelanin synthesis but also DNA repair and antioxidant defences in melanocytes.[2]
Beyond pigment
MC1R is not confined to the skin. It is also expressed on immune cells, where melanocortin signalling is broadly anti-inflammatory — one reason the receptor is of interest well outside dermatology.[4] Therapeutically, the α-MSH analogue afamelanotide is an MC1R agonist used to raise protective eumelanin in erythropoietic protoporphyria; see thetherapeutics section for that entry, and theoverview pillar for how MC1R fits the wider system.
References
- 1.Cone RD. Studies on the physiological functions of the melanocortin system. Endocr Rev. 27(7):736–49. 2006. link ↗
- 2.Nasti TH, Timares L. MC1R, eumelanin and pheomelanin: their role in determining the susceptibility to skin cancer. Photochem Photobiol. 91(1):188–200. 2015. link ↗
- 3.Beaumont KA, et al. Altered cell surface expression of human MC1R variant receptor alleles associated with red hair and skin cancer risk. Hum Mol Genet. 14(15):2145–54. 2005. link ↗
- 4.Cooray SN, Clark AJL. The melanocortin receptors and their accessory proteins. Front Endocrinol (Lausanne). 4:9. 2013. link ↗