Pharmacologic Induction of Epidermal Melanin and Protection Against Sunburn in a Humanized Mouse Model

Abstract

Fairness of skin, UV sensitivity and skin cancer risk all correlate with the physiologic function of the melanocortin 1 receptor, a G s -coupled signaling protein found on the surface of melanocytes. Mc1r stimulates adenylyl cyclase and cAMP production which, in turn, up-regulates melanocytic production of melanin in the skin. In order to study the mechanisms by which Mc1r signaling protects the skin against UV injury, this study relies on a mouse model with "humanized skin" based on epidermal expression of stem cell factor (Scf). K14-Scf transgenic mice retain melanocytes in the epidermis and therefore have the ability to deposit melanin in the epidermis. In this animal model, wild type Mc1r status results in robust deposition of black eumelanin pigment and a UV-protected phenotype. In contrast, K14-Scf animals with defective Mc1r signaling ability exhibit a red/blonde pigmentation, very little eumelanin in the skin and a UV-sensitive phenotype. Reasoning that eumelanin deposition might be enhanced by topical agents that mimic Mc1r signaling, we found that direct application of forskolin extract to the skin of Mc1r-defective fair-skinned mice resulted in robust eumelanin induction and UV protection 1 . Here we describe the method for preparing and applying a forskolin-containing natural root extract to K14-Scf fair-skinned mice and report a method for measuring UV sensitivity by determining minimal erythematous dose (MED). Using this animal model, it is possible to study how epidermal cAMP induction and melanization of the skin affect physiologic responses to UV exposure. Video LinkThe video component of this article can be found at

Ultraviolet Radiation, Aging and the Skin: Prevention of Damage by Topical cAMP Manipulation

“…This important physiologic pathway is a natural UV-protective response to protect the skin against further UV insult after an initial UV exposure. The ability of the skin to tan depends on the function and activity of the cutaneous melanocortin 1 receptor (MC1R) signaling pathway [ 51 , 52 , 53 , 54 , 55 ] ( Figure 2 ).…”

Photosensitivity of murine skin greatly depends on the genetic background: clinically relevant dose as a new measure to replace minimal erythema dose in mouse studies

“…In case of fur‐coated animals, hair was removed 24 h before UVB irradiations with and electric clipper and chemical depilation as described before . Irradiations were performed with a Multitester SBC LT 400 (MEDlight GmbH, Herford, Germany), which operates with a high pressure UV lamp (Osram, HTC 400‐221 R7s) and is routinely used for clinical phototesting.…”

Paracrine regulation of melanocyte genomic stability: a focus on nucleotide excision repair

“…While targeting approaches that limit cAMP induction to melanocytes is important because of the potential for off‐target effects, this approach may not benefit individuals harboring loss‐of‐function mutations in MC1R as signaling through their MC1Rs is blunted. As these persons are among those of highest risk of melanoma in the general population (Kennedy et al., ; Valverde et al., ), a broader therapeutic strategy may be needed such as topical application of agents that increase cAMP levels in skin cells through either adenylyl cyclase activation or phosphodiesterase inhibition both of which have been validated in an Mc1r ‐defective UV‐sensitive animal model of the fair‐skinned humans with respect to pigmentation (Amaro‐Ortiz et al., ; D'Orazio et al., ; Khaled et al., ; Spry et al., ). Indeed, we have already shown in proof‐of‐principle preclinical experiments that topical application of forskolin enhanced clearance of UV photodamage in the skin (Jarrett et al., ).…”