Induction of phase I and phase II enzyme activities by UVB and xenobiotics in normal human keratinocytes.

The function of the skin is to provide a barrier for protection against the external environment. Relatively little is known about the overall role of CYP450 in the metabolism of xenobiotics or endogenous cellular compounds in the skin. The aim of this study was to analyse the expression and induction of several drugs metabolizing enzymes. To this purpose both phase I and phase II enzymes activities were studied in normal human keratinocytes in culture after exposure to UVB radiation and to classical cytochrome inducers: β-naphthoflavone (BNF), 3-methylcholanthrene (MC), phenobarbital (PB). We investigated 7-ethoxyresorufìn O-deethylase (EROD) and 7-pentoxyresorufìn O-depenthylase activities (PROD). Normal human keratinocytes were cultured with mitomycin-treated 3T3 cells in Dulbecco’s modified Eagle’s medium/Ham’s F12. At confluence cells were incubated with inducers or irradiated with different doses of UVB. The microsomal fraction was studied by western-blot analysis. The MC-induced EROD activity was up to fourfold higher when compared with BNF induced activity. UVB exposure resulted in a dose-dependent (25-75 mJ) and time dependent (6-24 h) induction of CYP450 1A1. Immunoblotting assay showed expression for CYP450 1B1 not only keratinocytes but also in melanocytes. Proadifen, an inhibitor of CYP450-monooxygenase, led to a significant decrease in EROD activity. The results of the present study clearly show that irradiation with UVB is capable of modifying the activity of CYP450 isoenzymes in keratinocytes. The phase II enzyme gluthatione S transferase activity (GST) was induced by UVB and PB. These experimental findings stress the value of epidermal cell culture for pharmaco-toxicological studies of topical agents used in dermatology.