Nerve growth factor protects human keratinocytes from ultraviolet-B-induced apoptosis

Ultraviolet radiation is a potent inducer of apoptosis, whereas autocrine n erve growth factor protects human keratinocytes from programmed cell death. To evaluate the role of nerve growth factor in the mechanisms of ultraviol et B-induced apoptosis, cultured human keratinocytes were ultraviolet B irr adiated following pretreatment with K252, a specific inhibitor of the tyros ine kinase high-affinity nerve growth factor receptor. Here we report that the addition of K252 significantly enhanced keratinocyte apoptosis. We then transfected normal human keratinocytes with pNUT-hNGF, Nerve growth factor overexpressing keratinocytes secreted the highest amounts of nerve growth factor in culture supernatants, were more viable, and had a higher rate of proliferation than mock-transfected cells. Whereas ultraviolet B radiation downregulated nerve growth factor mRNA and protein as well as the tyrosine kinase high-affinity nerve growth factor receptor in normal keratinocytes, it failed to do so in nerve growth factor-transfected cells. Moreover, nerv e growth factor overexpressing keratinocytes were partially resistant to ap optosis induced by increasing doses of ultraviolet B at 24 and 48 h. These results indicate that downregulation of nerve growth factor function plays an important part in the mechanisms of ultraviolet B-induced apoptosis in h uman keratinocytes. In addition, ultraviolet B caused a decrease in BCL-2 a nd BCL-x(L) expression in mock-transfected keratinocytes, but not in nerve growth factor overexpressing cells. Finally, nerve growth factor prevented the cleavage of the enzyme poly(ADP-ribose) polymerase induced in human ker atinocytes by ultraviolet B, These results are consistent with a model wher eby the autocrine nerve growth factor protects human keratinocytes from ult raviolet B-induced apoptosis by maintaining constant levels of BCL-2 and BC L-x(L), which in turn might block caspase activation.