RAS-INDEPENDENT ACTIVATION OF REL-FAMILY TRANSCRIPTION FACTORS BY UVBAND TPA IN CULTURED KERATINOCYTES

UV irradiation of mammalian cells results in the activation of transcr iption factors which mediate induction of early response genes designe d to repair and minimise the damage sustained by the cell. Evidence fr om studies in HeLa cells suggest that UVC regulates NF-kappa B activit y via tyrosine kinases and activation of Ras and Raf kinase. In this s tudy we have used a previously characterized TPA-responsive element (V LTRE) that binds Rel/NF-kappa B proteins and a Ras-responsive element (B10 RRE) to analyse the signalling pathway in UVB-stimulated gene tra nscription in cultured keratinocytes, We demonstrate that the tumour p romoters TPA and WB use different signalling intermediates to activate different sets of Rel/NF-kappa B proteins. UVB transactivation is ind ependent of PKC activity but dependent on tyrosine kinase activity whe reas TPA stimulation requires PKC but not tyrosine kinase activity. Fu rthermore, neither UVB- nor TPA-transactivation is mediated through p2 1 Ras but both stimuli are dependent on a functional Raf protein. A co nstitutively active Raf-1 kinase however, was unable to induce transac tivation through VLTRE. Thus, Raf has an essential but permissive role in UVB activation of Rel proteins. These findings demonstrate that ke ratinocytes contain a novel Ras-independent pathway for induction of R el mediated transcription.