Retinoic Acid inhibits epidermal differentiation when applied topically on the stratum corneum of epidermis formed in vitro by human keratinocytes grown on defined medium

There is no doubt that retinoids have profound physiological and pharmacologic effects on epidermal differentiation and maintenance. However, the response of keratinocytes to retinoic acid (RA) is strikingly different whether they are cultured in vitro or whether epidermis is treated topically in vivo. In vitro, RA is able to inhibit completely the appearance of several epidermal-specific markers and to induce the ectopic appearance of several markers of nonkeratinized stratified epithelia, suggesting that a metaplasia has occurred. On the contrary, RA applied in vivo does not inhibit keratinization but provokes an increase of several markers of epidermal differentiation, while still inducing the expression of markers of nonkeratinized epithelia. The work presented in this paper shows that these different behaviors of the keratinocytes in vivo and in vitro are genuine, and not due to the fact that in vitro retinoids were added to the culture medium ("systemically"), before keratinization of the cultures, and in the presence of fetal calf serum and dermal components, while in vivo they were applied topically on the stratum corneum. We show that the "in vitro behavior" persists when retinoids are applied topically on the stratum corneum of keratinocyte cultures performed in defined medium on inert filters, suggesting that there are unexplored crucial regulatory pathways involved in the cutaneous response to retinoids in vivo.