RETINOIC ACID REGULATES ORAL EPITHELIAL DIFFERENTIATION BY 2 MECHANISMS

The effect of retinoic acid (RA) concentration on differentiation of o ral keratinocytes and the influence of fibroblasts on RA-dependent reg ulation were investigated in a lifted culture system, Keratinocyte dif ferentiation was assessed by morphology, immunohistochemistry and immu noblotting, Filaggrin/profilaggrin and keratin 1 were used as biochemi cal markers for cornified epithelium and keratins 13 and 19 as markers for noncornified epithelium, Cultured oral keratinocytes in RA-free c onditions differentiated in a manner that closely resembled the differ entiation pattern of gingival epithelia in vivo. Increasing RA concent rations altered the in vivo-like terminal differentiation of oral kera tinocytes by disruption of organized stratification, inhibition of fil aggrin/profilaggrin and K1 expression, and stimulation of K13 and K19 expression. Differentiation of keratinocytes from both cornified and n oncornified regions of the oral cavity varied in a similar manner in r esponse to added RA, with the exception of K19 expression, K19 was con sistently expressed at higher levels in keratinocytes originating from noncornified epithelia as compared to those from cornified epithelia, The level of RA regulation was ultimately dependent on the type of fi broblasts underlying the epithelial cells, Homologous fibroblasts rend ered the oral keratinocytes less sensitive to the effects of RA than s kin fibroblasts. In addition, at a given RA concentration, fibroblasts from cornified oral mucosa potentiated keratinocyte expression of RA sensitive markers of keratinization as compared to the influence exert ed by fibroblasts originating from noncornified oral mucosa, These res ults indicate that the RA regulation of oral epithelial differentiatio n is mediated by two separate mechanisms: a direct, RA concentration-d ependent effect, and an indirect, fibroblast-mediated effect.