The pigmentary system: new interpretations of old data

Hypothesis: Possible Role for the Melatonin Receptor in Vitiligo: Discussion Paper

A new unifying hypothesis for the aetiology of vitiligo is proposed, in which we postulate that the final destruction of melanocytes in vitiligo results from a cascade of reactions initiated by a disregulation of melanogenesis, caused by activation of the melatonin receptor. These events result in the high and uncontrolled production of free radicals and toxic products of melanogenesis which sequentially damage or destroy melanocytes and keratinocytes, provoke an autoimmune response against exposed intracellular or altered cell surface antigens, and increase the propensity of melanocytes to undergo malignant transformation.

Human skin pigmentation: melanocytes modulate skin color in response to stress

All organisms, from simple invertebrates to complex human beings, exist in different colors and patterns, which arise from the unique distribution of pigments throughout the body. Pigmentation is highly heritable, being regulated by genetic, environmental, and endocrine factors that modulate the amount, type, and distribution of melanins in the skin, hair, and eyes. In addition to its roles in camouflage, heat regulation, and cosmetic variation, melanin protects against UV radiation and thus is an important defense system in human skin against harmful factors. Being the largest organ of the body that is always under the influence of internal and external factors, the skin often reacts to those agents by modifying the constitutive pigmentation pattern. The focus of this review is to provide an updated overview of important physiological and biological factors that increase pigmentation and the mechanisms by which they do so. We consider endocrine factors that induce temporary (e.g., during pregnancy) or permanent (e.g., during aging) changes in skin color, environmental factors (e.g., UV), certain drugs, and chemical compounds, etc. Understanding the mechanisms by which different factors and compounds induce melanogenesis is of great interest pharmaceutically (as therapy for pigmentary diseases) and cosmeceutically (e.g., to design tanning products with potential to reduce skin cancer risk).

Contribution of melanogenic proteins to the heterogeneous pigmentation of human melanocytes

Human melanocytes from individuals with different skin types, as well as from the skin of the same individual, are heterogeneous in their melanin content. This heterogeneity may be attributed to differences in the activity and expression of the three melanogenic proteins: tyrosinase and tyrosinase-related proteins 1 and 2 (gp75 and DOPAchrome tautomerase, respectively), which in turn are affected by certain regulatory factors. Established melanocyte strains that exhibited intrinsic melanogenic heterogeneity could be separated into subpopulations according to density and melanin content by Percoll density gradient centrifugation. The least melanotic subpopulation consisted of melanocytes that contained an active tyrosinase enzyme and a low amount of melanin. Tyrosinase activity and the quantities of tyrosinase enzyme, tyrosinase-related protein-1 and DOPAchrome tautomerase gradually increased with increased melanin content and Percoll density of the isolated melanocyte subpopulations. We have found a direct correlation between melanin content, tyrosinase activity and the expression of the three melanogenic proteins in melanocyte strains established from different skin types. Addition of the two epidermal cytokines, tumor necrosis factor-alpha or interleukin-1 alpha, to cultures of human melanocytes from different skin types caused decreased proliferation, tyrosinase activity and expression of tyrosinase, tyrosinase-related protein-1 and DOPAchrome tautomerase. Similar results were obtained when Percoll-derived melanocyte subpopulations were treated with tumor necrosis factor-alpha and interleukin-1 alpha. These results indicate that the variation in melanin content in human melanocytes is due to differences in the activity and expression of the melanogenic proteins, which are influenced by autocrine and paracrine factors.

Mucosal melanotic macule, reactive type: a simulation of melanoma

Two patients with spontaneously resolving hyperpigmented patches of the oral mucosa are reported. Histologically, lesions in both cases showed large pigment-laden dendritic cells singly dispersed at all levels of the epidermis, which simulated in situ melanoma. The absence of such cells in the basal layer of the epidermis served to histologically distinguish these lesions from a biologically malignant melanocytic tumor. Spontaneous resolution in both patients suggests that the condition is reactive. We propose the term mucosal melanotic macule, reactive type, for this lesion, which has been described previously under the misnomer oral melanoacanthoma.