Distinct Melanogenic Response of Human Melanocytes in Mono‐culture, in Co‐Culture with Keratinocytes and in Reconstructed Epidermis, to UV Exposure

Abstract

Striking differences are observed in the melanogenic response of normal human melanocytes to UVA and UVB irradiation depending on culture conditions and the presence of keratinocytes. Exposure of melanocytes co-cultured with keratinocytes to UVB irradiation triggered, already at low doses (5 mJ/cm2), an increase in melanin synthesis whereas in melanocyte mono-cultures, UVB doses up to 50 mJ/cm2 had no melanogenic effect. Unlike UVB, UVA exposure caused the same melanogenic response in both mono- and co-cultures. Removing certain keratinocyte growth factors from the co-culture medium abolished the melanogenic response to UVB, but not to UVA exposure. When integrated into the basal layer of a reconstructed human epidermis, human melanocytes similarly reacted to UVA and UVB irradiation as in vivo by increasing their production and transfer of melanin to the neighboring keratinocytes which resulted in a noticeable tanning of the reconstructed epidermis. The presence of a dense stratum corneum, known to scatter and absorb UV light, is responsible for higher minimal UVB and UVA doses required to trigger a melanogenic response in the reconstructed epidermis compared to keratinocyte-melanocyte co-cultures. Furthermore, an immediate tanning response was observed in the pigmented epidermis following UVA irradiation. From these results we conclude that: (i) keratinocytes play an important role in mediating UVB-induced pigmentation, (ii) UVA-induced pigmentation is the result of a rather direct effect on melanocytes and (iii) reconstructed pigmented epidermis is the most appropriate model to study UV-induced pigmentation in vitro.

A Melanocyte–Keratinocyte Coculture Model to Assess Regulators of Pigmentation in Vitro

“…However, that melanocyte culture model does not reflect the effects of putative bioactive compounds on keratinocytes (or other cell types) that might indirectly affect melanocyte function (such as might occur in the skin). In recent years, various pigmented human skin equivalents have been reconstituted and used to evaluate melanocyte responses to environmental stimuli (9,20,22,(32)(33)(34)(35)(36). Although such skin equivalents more effectively mimic the physiologic situation of normal human skin, they are not suitable for extensive screening of potential melanogenic compounds due to their expense and the time and effort required to set up and maintain them.…”

MicroRNA 340 Is Involved in UVB-Induced Dendrite Formation through the Regulation of RhoA Expression in Melanocytes

“…Studies have shown that UV irradiation induces the production of melanogenic stimulators such as nitric oxide, endothelin 1,␣-melanocyte-stimulating hormone, adrenocorticotropic hormone, and prostaglandin E2 (5,6,7). In vivo reflectance confocal imaging of murine skin following exposure to UVB light highlighted its effect on the dendricity of melanocytes (8).…”

UV light phototransduction activates transient receptor potential A1 ion channels in human melanocytes

“…Although UVB exposure results in a delayed pigmentation response that occurs many hours to days later (22), brief UVA exposure leads to a rapid and transient response within minutes via a largely unknown mechanism (23,24). Because UVA penetrates skin more deeply than UVB, HEMs, located in the basal layer of the epidermis, are exposed to significantly more UVA than UVB (25,26). Because our UVR-stimulation source contains ∼90% UVA and ∼10% UVB, we compared the photocurrent amplitude in response to 250 mJ/cm 2 UVR to the components 25 mJ/cm 2 UVB or 225 mJ/cm 2 UVA.…”