Expression of loricrin is negatively controlled by retinoic acid in human epidermis reconstructed in vitro

In epidermis, the last steps of keratinocyte differentiation are characterized by the covalent cross-linking of cornified envelope precursors such as involucrin and loricrin, a hydrophobic protein recently described in mouse and human epidermis. In situ hybridization of normal human skin sections with a human loricrin cRNA probe and immunolabeling with an antiserum directed against a synthetic peptide corresponding to the carboxyterminus of human loricrin revealed the presence of loricrin transcripts and protein in the granular layers of epidermis. In human epidermis reconstructed in vitro by growing keratinocytes on dermal equivalents, loricrin and loricrin mRNAs were also restricted to granular cells, but their amounts seemed higher than in epidermis from skin biopsies. The reactivities for both loricrin and loricrin mRNAs were abolished by a treatment of the cultures with a retinoic acid concentration (10(-6) M) provoking a complete inhibition of terminal epidermal differentiation (parakeratosis). Thus, the regulation of loricrin synthesis is different from that of another envelope precursor, involucrin, which does not seem to be significantly modulated by retinoic acid. Together with the well-documented inhibition of epidermal transglutaminase by retinoic acid, our results provide a molecular basis for the inhibition of cornified envelope formation by retinoic acid.