INHIBITION OF ORNITHINE DECARBOXYLASE ACTIVITY AND CELL-PROLIFERATIONBY ULTRAVIOLET-B RADIATION IN EGF-STIMULATED CULTURED HUMAN EPIDERMAL-KERATINOCYTES

Irradiation of EGF-stimulated human keratinocytes in vitro with ultrav iolet B (UVB) radiation inhibited both ornithine decarboxylase (ODC) a ctivity and cellular proliferation. A dose-dependent reduction in ODC activity occurred in primary cultures of adult facial keratinocytes an d neonatal foreskin keratinocytes, and in an SV40-transformed keratino cyte cell line derived from neonatal foreskin. When SV40-transformed k eratinocytes were treated with epidermal growth factor (EGF), ODC acti vity was induced up to 21 times in the absence of ultraviolet radiatio n. However, pre-treatment with UVB significantly reduced the EGF induc tion of ODC. For example, 85% less ODC activity was observed in cultur es treated with EGF (10 ng/ml) plus 2.5 mJ/cm2 of UVB than cultures tr eated with EGF alone. To assess the effect of UVB on cell proliferatio n, normal human epidermal keratinocytes grown in medium containing EGF were irradiated with 5 and 10 mj/cm2 UVB. At days 3 and 5 post-irradi ation a significant (up to 78%) decrease in proliferation was observed . Nevertheless, the mean proportion of viable to dead cells remained s imilar in both UVB-treated and non-irradiated cell cultures. Northern blot analysis of total RNA isolated from irradiated and sham-irradiate d cultures showed that UVB caused approximately a one third reduction in steady-state ODC mRNA levels in EGF-stimulated keratinocyte culture s. Because ODC is an enzyme required for cell proliferation, we propos e that the UVB-induced decrease in cell proliferation may result at le ast in part from UVB inhibition of ODC mRNA accumulation and reduced e nzyme activity.