ULTRAVIOLET-B-INDUCED APOPTOSIS OF KERATINOCYTES - EVIDENCE FOR PARTIAL INVOLVEMENT OF TUMOR-NECROSIS-FACTOR-ALPHA IN THE FORMATION OF SUNBURN CELLS

Irradiation with ultraviolet (UV) B radiation results in the formation of apoptotic keratinocytes called sunburn cells. Recently, it was dem onstrated that keratinocytes can release tumor necrosis factor-alpha ( TNF-alpha), which is known to cause apoptosis in particular cells. In addition, it has been shown that UVB light induces the release of TNF- alpha by keratinocytes and that keratinocytes express the 55-kD recept or for TNF-alpha. Therefore, we investigated whether TNF-alpha is invo lved in UV-induced apoptosis of keratinocytes. Normal human keratinocy tes and HaCaT cells were exposed to UVB light, and apoptosis was exami ned by nick translation evaluated by fluorescence-activated cell sorte r analysis. UVB induced apoptosis in a dose-dependent manner, which wa s confirmed by electron microscopy. Addition of a polyclonal antibody directed against human TNF-alpha immediately after UVB exposure was ab le to reduce DNA fragmentation. However, it was not possible to rescue all cells from apoptosis. To prove whether TNF-alpha is also involved in vivo in UVB-induced apoptosis of keratinocytes, Balb/c mice were e xposed to UVB on their abdomens, skin biopsies were performed 24 h lat er, and sunburn cells were counted. A single dose of 2000 J/m(2) cause d a significant induction of sunburn cells. Subcutaneous injection of a polyclonal antibody directed against murine TNF-alpha immediately af ter UVB treatment resulted in a significant but incomplete reduction o f sunburn cells, whereas injection of a rabbit IgG as a control had no effect. In both the in vitro and in vivo systems, application of reco mbinant TNF-alpha alone either to untreated keratinocytes or into norm al murine skin did not induce sunburn cells. Thus, these data demonstr ate that TNF-alpha is involved in UVB-induced apoptosis, but by itself is not able to induce sunburn cells. This further supports the notion that UVB-induced apoptosis of keratinocytes is a multifactorial event .