Sunlight-induced killing of nondividing human cells in culture

Nondividing populations of human diploid fibroblasts that are DNA excision repair proficient and repair deficient were exposed to mid-day summer sunlight and their survival determined based on their ability to remain attached to a culture vessel surface. Whereas mid- and far-UV wavelengths and sunlamp radiation cause a gradual degeneration and detachment of cells in a dose-dependent manner, sunlight does not promote cell killing in repair proficient cells. Detachment of repair deficient cells is promoted to a limited extent but only at sunlight exposure times that are low with respect to the amount of DNA damage induced. Repair proficient and deficient cells exposed to sunlight for longer times do not detach. Pyrimidine dimer levels in these sunlight irradiated cells were great enough to have promoted detachment had these levels been induced by UV (254 nm) alone. Other photodamage induced by these exposures evidently inhibits the dimer induced cell degeneration that leads to cell detachment. It was concluded that pyrimidine dimers are responsible for cell killing at short sunlight exposure times (<40 min) but that at longer exposures (>80 min) cells are killed by a different mechanism. (author)