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Disorders of Pigmentation
Published in Ayşe Serap Karadağ, Lawrence Charles Parish, Jordan V. Wang, Roxburgh's Common Skin Diseases, 2022
Michael Joseph Lavery, Charles Cathcart, Hasan Aksoy
Melanin is derived from phenylalanine and passes through several steps before forming pheomelanin, eumelanin, or neuromelanin (Figure 25.1). Pheomelanin produces yellow-red pigment, and eumelanin produces brown-black pigment. The process of determining which pigment is produced is controlled by the melanocortin-1 receptor (MC1R). A loss of function in this receptor leads to increased production of pheomelanin and can increase the risk of melanoma from ultra-violet radiation. Genetic variations in the MC1R gene have been identified in malignant melanoma and oculocutaneous albinism.
Ocular Tumors
Published in Ching-Yu Cheng, Tien Yin Wong, Ophthalmic Epidemiology, 2022
Vishal Raval, Alexander Melendez, Hansell Soto, Alléxya Affonso, Rubens Belfort Neto, Arun D. Singh
It has been reported that iris color is a risk factor for the development of uveal melanoma.45,47 The most affected eye colors are light-color irises while brown-colored eyes are the least affected.45,49 Recent studies have determined the potential link of higher risk to the difference of melanin molecules in the iris. Eumelanin and pheomelanin both are present in the iris; the distinction lies with the ratio of these types of melanin. Lighter-colored eyes contain a higher degree of pheomelanin and lower amounts of eumelanin, while in darker eyes the distribution is opposite.50 Pheomelanin is more phototoxic than eumelanin, causing an increased propensity of developing radical oxygen species and decreased antioxidant potential when exposed to light, both of which can cause damage to DNA.51,52 Additionally, recent studies have identified three single-nucleotide polymorphisms in pigment genes HERC2, OCA4, and IRF4, which are known risk factors linked tocutaneous melanoma; however, further studies are needed to confirm these observations reading uveal melanoma.53
Hair Coloring
Published in Dale H. Johnson, Hair and Hair Care, 2018
Pheomelanin pigments are largely responsible for the red and yellow colors of hair and feathers. While they are obviously different in color from the eumelanins, they are also characterized by a significant solubility in aqueous or aqueous alkaline systems. The pheomelanins also show, by elemental analysis, considerable sulfur content in contrast to the eumelanins. Typically, the sulfur:nitrogen ratio is about 1:2. Evidence from chemical degradative studies suggests that pheomelanins contain benzothiazine units such as [8]. Pheomelanins appear to be more stable toward chemical and photochemical bleaching than eumelanins (4).
Effect of ultraviolet radiation on the Nrf2 signaling pathway in skin cells
Published in International Journal of Radiation Biology, 2021
Alena Ryšavá, Jitka Vostálová, Alena Rajnochová Svobodová
Melanocytes are the second most abundant epidermal element (1–2%), situated in the basal layer and with an origin in neural crests. They are small, rounded cells with numerous long cytoplasmic appendixes that penetrate the epidermal stratum basale and stratum spinosum (Figure 1) (Kanitakis 2002; Mescher 2016). Melanocytes synthesize the melanin pigments, in particular eumelanin, pheomelanin, and mixed melanin, which participate in skin coloring and represent one protection mechanism against solar radiation. Synthesis of melanin arises from L-tyrosine and requires the enzyme tyrosinase (Slominski et al. 2004, Slominski, Zmijewski et al. 2012). Melanin has the ability to scatter UV rays and absorb UV, visible and infrared radiation as well as scavenge reactive oxygen and nitrogen species (ROS, RNS) that have been produced. However, melanin synthesis itself generates ROS, especially the synthesis of lighter variant pheomelanin (Smit et al. 2008). Moreover, pheomelanin can undergo photosensitization that results in the generation of superoxide radicals, hydroxyl radicals and hydrogen peroxide (Slominski et al. 2004). From this point of view, pheomelanin also has pro-oxidative properties and can participate in photoaging and carcinogenic processes (see below). In addition to melanin synthesis, melanocytes secrete a wide range of signal molecules such as cytokines (tumor necrosis factor-α (TNF-α), interleukins), catecholamines and nitric oxide in response to a number of stimuli (including UV radiation) and, via these molecules, influence other epidermal and dermal cells (Svobodová and Vostálová 2010).
Afamelanotide for prevention of phototoxicity in erythropoietic protoporphyria
Published in Expert Review of Clinical Pharmacology, 2021
Debby Wensink, Margreet A.E.M. Wagenmakers, Janneke G. Langendonk
First, we start with a short introduction on the physiological mechanism of action of α-melanocyte-stimulating hormone (α-MSH), to clarify the working mechanism of afamelanotide thereafter. Natural α-MSH is a 13 amino acid peptide that is only produced in small amounts by the pars distalis of the pituitary gland and by extra pituitary cells including keratinocytes, this is in contrast to most mammalian species where it is only synthesized by the anterior pituitary gland. Alpha-MSH is an endogenous peptide hormone and neuropeptide of the melanocortin family, it stimulates melanogenesis via the melanocortin-1 receptor (MC1R) and tyrosinase activation, the rate-limiting enzyme in this pathway. This results in eumelanin production, which accounts for the skin-darkening effect of α-MSH [47]. In the human skin, melanogenesis is initiated by exposure to UV radiation, also causing the skin to darken. α-MSH has besides this skin-darkening effect due to activation of eumelanogeneisis, also an anti-inflammatory effect [48]. There are three basic types of melanin: eumelanin, pheomelanin, and neuromelanin. Eumelanin is photoprotective, while pheomelanin may contribute to UV-induced skin damage, this is related to its potential to generate free radicals in response to UV [49]. Individuals with a predominance of pheomelanin, a less protective yellow-red pigment, in their skin and/or a reduced ability to produce eumelanin, will hardly tan and are at risk from UV-related damage, like skin cancer. Previous studies have shown that eumelanin is preferentially produced when α-MSH stimulates melanogenesis in human melanocytes [50]. Besides the increase in total synthesis, MC1R activation also results in a switch from pheomelanin, to eumelanin production, a more protective brown-black pigment [51]. The MC1R gene has a lot of polymorphisms, some of them associated with variation in pigmentation phenotypes within human populations [52]. MC1R activity is related to different genotypes, a homozygous low-activity genotype will result in a phenotype featuring fair skin, red hair, freckling, and an inability to tan [53].