CCN3 controls proliferation and localization of human melanocytes in epidermis

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Melanocytes are located within the basal layer of human epidermis, where they attach to the basement membrane and extend dendrites to surrounding keratinocytes for transfer of pigment containing melanosomes. Their functions are regulated by keratinocytes through as yet unknown mechanisms. We have now identified CCN3 (NOV, nephroblastoma overexpressed), which is a member of the matricellular protein family as critical for cell to cell regulation. Melanocytes upregulated CCN3 when exposed to pro-inflammatory cytokines such as IL-1β and TNF-α and by co-culture with keratinocytes. CCN3 overexpression or addition of recombinant protein to medium led to growth inhibition whereas knockdown stimulated proliferation. Knockdown dysregulated localization of melanocytes in the epidermis, which migrated to the supra basal layers similar to atypical nevus cells or they invaded into the dermis. CCN3 overexpression led to strong adhesion of melanocytes to basement membrane associated with collagen IV, despite the lack of adhesive properties of this protein. The mechanism for CCN3 regulation of melanocytes growth and adhesion is due to its upregulation of DDR1, a receptor tyrosine kinase that acts as collagen IV adhesive receptor. Knockdown of DDR1 led to decreased collagen IV adhesion of melanocytes and increased proliferation similar to CCN3 knockdown. These results demonstrate for the first time the intricate intercellular communication between keratinocytes and melanocytes for maintenance of normal epithelial homeostasis and point to its dysregulation during the earliest step of transformation.