Mouse 3T3 fibroblasts lacking c-fos were employed to demonstrate an essential function of the UV-inducible transcription factor AP-1 (Fos/Jun) in the response to the cytotoxic effects of short-wavelength ultraviolet (UVC) radiation. Clonogenic survival and proliferation of cells lacking c-fos were drastically reduced following UV irradiation, This UV hypersensitivity manifests itself primarily in increased cell death, partly by apoptosis, and prolonged recovery time from UV-induced cell cycle arrest; Go-culture with wild-type cells did not ameliorate the hypersensitivity of mutant cells, Transcriptional induction of the c-Fos target genes collagenase I, stromelysin-l and stromelysin-2 by UV is almost absent in cells lacking c-fos which correlates with a reduced UV induction of AP-1 DNA-binding and transactivation activity, The repair of UV-induced DNA lesions was not affected, as shown by unscheduled DNA synthesis and host cell reactivation assays. These data demonstrate that c-Fos is involved in a novel protective function other than DNA repair against the harmful consequences of UVC.
