Stimulatory effect of fibroblast‐derived prostaglandin E2 on keratinocyte stratification in the skin equivalent

Epidermal–dermal interaction plays important roles in physiological events such as wound healing. In this study, we examined a double paracrine mechanism between keratinocytes and fibroblasts through interleukin‐1 (IL‐1) and an IL‐1‐induced inflammatory mediator prostaglandin E₂ (PGE₂) using the skin equivalent. The epidermal layer of the skin equivalent expressed high levels of IL‐1α mRNA (IL1A mRNA) and relatively low levels of IL‐1β mRNA (IL1B mRNA). IL1A mRNA was not detected in fibroblasts. Fibroblasts also expressed low but not negligible levels of IL1B mRNA only in the presence of keratinocytes. Expression of prostaglandin‐endoperoxide synthase 2 mRNA (PTGS2 mRNA) and production of PGE₂ in three‐dimensionally cultured fibroblasts were noticeably stimulated by co‐culture with keratinocytes, whereas PTGS2 mRNA expression in the epidermal layer was very low. In addition, hydroxyprostaglandin dehydrogenase 15‐(NAD) mRNA was highly expressed in keratinocytes but not in fibroblasts, and exogenous IL‐1β stimulated PTGS2 mRNA expression in the dermal equivalent. The thickness of the epidermal layer and the number of MKI67‐positive keratinocytes in the skin equivalent were decreased by treatment with indomethacin, and the decrease recovered when exogenous PGE₂ was added. These results indicate that keratinocytes stimulate their own proliferation through a double paracrine mechanism mediated by IL‐1 and PGE₂.