Regulation of Pigmentation in Human Epidermal Melanocytes by Functional High-Affinity β-Melanocyte-Stimulating Hormone/Melanocortin-4 Receptor Signaling

Abstract

To date, the principal receptor considered to regulate human pigmentation is the melanocortin-1 receptor (MC1-R) via induction of the cAMP/protein kinase A pathway by the melanocortins α-MSH and ACTH. In this context, it is noteworthy that β-MSH can also induce melanogenesis, although it has a low affinity for the MC1-R, whereas the preferred receptor for this melanocortin is the MC4-R. Because β-MSH is present in the epidermal compartment, it was of interest to ascertain whether functioning MC4-Rs are present in human epidermal keratinocytes and melanocytes. Our results provide evidence that the MC4-R is expressed in situ and in vitro throughout the human epidermis at the mRNA and protein level using RT-PCR, Western blotting, and double immunofluorescence staining. Moreover, radioligand binding studies yielded high-affinity receptors for β-MSH on epidermal melanocytes (3600 receptors per cell), undifferentiated keratinocytes (7200 receptors per cell), and differentiated keratinocytes (72,700 receptors per cell), indicating that MC4-R expression correlates with epidermal differentiation. Importantly, increased melanogenesis after stimulation of the β-MSH/cAMP/microphthalmia-associated transcription factor/tyrosinase cascade proved the functionality of this signal in melanocytes, which was attenuated in the presence of the specific MC4-R antagonist HS014. In summary, our results imply an important role for the β-MSH/MC4-R cascade in human melanocyte biology, although the function and purpose of this signal in keratinocytes needs further elucidation.Although a high affinity MC4-R signal in human epidermal melanocytes (3,600 receptors/cell) stimulates melanogenesis via the β-MSH /cAMP/MITF/tyrosinase cascade, the purpose of this signal in keratinocytes remains unclear.