Induction of cytochrome P450 enzyme activity by UVB and xenobiotics in normal human keratinocites.

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 involved in either phase I or phase II reactions, in proliferanting human keratinocytes after exposure to UVB radiation and to three classical cytochrome inducers such as: 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 confluency 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 4 fold 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 for both keratinocytes and 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.