Several studies have suggested that murine and human keratinocytes respond differently to phorbol 12-myristate 13-acetate (PMA). Using an in vitro assay, we found that in contrast to its effect on murine skin, PMA did not induce ornithine decarboxylase (ODC) activity in human skin biopsies. To explore the signalling induced by PMA and to determine whether an in vitro culture system could be used to predict biological activity of retinoids in human keratinocytes, we studied a simian virus 40 (SV40)-transformed human keratinocyte cell line. Epidermal growth factor (EGF) stimulates ODC activity and increases the steady-state level of ODC mRNA in a dose- and time-dependent manner in these cells [Prystowsky, Clevenger and Zheng (1993) Exp. Dermatol. 2, 125–132]. In this report, 10-10 M–10-7 M PMA induced ODC mRNA and enzyme synthesis at 7 h, but did not significantly induce ODC activity and inhibited the EGF induction of ODC activity. To explore the mechanism whereby PMA interferred with EGF signalling, the effect of PMA on EGF binding to its cell-surface receptor was studied; acute treatment with PMA (within 7 h) decreased EGF binding to 41–57% of the baseline level. In contrast, chronic treatment with PMA (24 h) increased EGF binding to 156% of the baseline level and was associated with an increase in quantity of EGF receptor protein. Protein kinase C (PKC) activation correlated with the acute decrease in EGF binding following PMA treatment. In summary, PMA induced ODC mRNA and ODC enzyme synthesis, while steady-state levels of immunoprecipitable ODC enzyme protein and ODC activity were not increased, demonstrating possible increased turnover of ODC enzyme protein. Additionally, PMA inhibited the induction of ODC by EGF through decreased EGF binding, possibly mediated by PKC activation. Finally treatment of the keratinocytes with retinoids including etretinate, Ro13-7410, etarotene, Ro40-8757, 13-cis-retinoic acid, and acitretin blocked the PMA induction of ODC mRNA, suggesting this in vitro model could be a valuable screening assay for predicting biological activity in humans.

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