Background and objective: It has been reported that retinoic acid disintegrates the desmosome formation of squamous epithelium, resulting in inhibition of stratification. In contrast, it is not known whether retinoic acid influences the integration of tight junctions. Therefore, our objective of this study is to disclose effects of retinoic acid on the formation and maintenance of tight junction.
Material and methods: In the present study, the alteration of expression of tight junction constituent proteins and keratin peptides in immortalized oral mucosal epithelial cells (GE1) induced by 1 microm retinoic acid was analyzed by immunofluorescence, electron microscopy and reverse transcription-polymerase chain reaction (RT-PCR).
Results: The stratifying GE1 cells expressed claudin-1, claudin-4, claudin-5, occludin and zonula occludens 1 in the control culture. The RT-PCR showed that retinoic acid significantly reduced the expression of claudin-1 mRNA, whereas it dramatically enhanced expression of claudin-4 mRNA. Immunofluorescence showed that claudin-1 was present at cell-to-cell contact sites in the flattened uppermost layers of the control culture. In the culture with retinoic acid, the flattened uppermost cells were absent and there claudin-1 was less present, but claudin-4 was prominently present in all layers. Claudin-5 was present in a variety of patterns, regardless of the presence of retinoic acid. Along with the change of claudin species, the expressions of keratin 7, keratin 8 and keratin 18, as markers for the simple epithelium, were clearly stimulated by retinoic acid.
Conclusion: Retinoic acid changed the expression of tight junction constituent molecules, such as claudin-1 and claudin-4, in oral keratinocytes. These findings suggest that long-term application of retinoids in clinical therapy should be carefully performed.