Keratinocytes and dermal factors activate CRABP-I in melanocytes

Recognition that cellular retinoic acid binding protein (CRABP)-I and CRABP-II are found in different cell types has provided additional support for the presumably divergent roles of these two proteins in mediating retinoic acid (RA) effects in human skin. CRABP-II is expressed in fibroblasts and keratinocytes, and CRABP-I in as yet unidentified cells, possibly epidermal melanocytes. Recently, we demonstrated that each of these RA-binding proteins in human skin possesses two classes of binding sites, possibly related to the state of phosphorylation of the proteins. We now characterize the cutaneous origin of CRABP-I further using an anion-exchange HPLC assay that allows effective separation of the two proteins in human skin, and a fluorescent in situ hybridization technique. We report that CRABP-I is expressed in isolated melanocytes at the mRNA level, although under these circumstances the protein has minimal RA-binding activity, and that keratinocytic and dermal influences are required for CRABP-I activity in melanocytes. This melanocyte origin for CRABP-I and the improvement by RA of the irregular hyperpigmentation associated with photoaging led us to examine the effects of RA using various cellular associations, from conventional pure cultures of melanocytes grown on plastic dishes to a pigmented skin equivalent consisting of melanocytes and keratinocytes grown on a dermal equivalent. We established that the inhibitory effects of RA on melanogenesis do not result from a direct effect on melanocytes alone but also involve keratinocytes and dermal influence. These data expand our understanding of cell-to-cell signaling in cutaneous pigmentation, and strongly suggest a role for CRABP-I in mediating RA effects on melanogenesis.