LONG-WAVE ULTRAVIOLET-RADIATION INDUCES PROTEIN-KINASE-C IN NORMAL HUMAN KERATINOCYTES

Skin tumor promotion by phorbol eater is believed to be mediated by th e phospholipid-dependent ser/thr kinase, protein kinase C (PKC). Long- wave ultraviolet radiation (320-400 nm, UVA), which has also been show n to promote skin tumors, induces elevated levels of PKC in murine fib roblasts, suggesting that UVA may promote the development of basal and squamous cell skin cancers by a mechanism involving PKC. To examine U VA effects on PKC in a model relevant to skin, we maintained normal hu man epidermal keratinocytes (NHEK) in serum-free medium and exposed th e cultured cells to various doses of UVA or to the phorbol ester, 12-O -tetradecanoyrphorbol-13-acetate (TPA). Fifty minutes after exposure t o UVA (5-20 J/cm(2)), PKC activity was elevated up to three-fold in NH EK cytosolic fractions, and membrane-associated PKC activity was eleva ted up to two-fold by UVA. The TPA treatment induced a 10-fold increas e in membrane-associate PKC activity only. Immunoblot analysis suggest ed that a UVA-induced increase in PKC protein occurred. Both UVA and T PA reduced the cell number by 50-75% in the first 24-48 h; however, ir radiated cultures began to recover at 72 h post-UVA due to an increase d proliferative rate beginning after 48 h. Treatment with TPA induced a high lever of differentiation as measured by cornified envelope form ation. Ultraviolet A irradiation exposure was not followed by increase d differentiation. These findings suggest that acute UVA exposure elev ates PKC activity in human keratinocytes and may act through PKC to pr omote actinic skin cancer. The molecular mechanism is like to differ f rom that of the phorbol eaters, however.