Immediate and delayed molecular response of human keratinocytes to solar-simulated irradiation

The clinical and histologic events in human skin after a single ultraviolet exposure are well documented; and both in vitro and in vivo studies have established immunologic and mutagenic effects of ultraviolet rays on cells. However, little is known about the ultraviolet damage/repair response at the level of gene induction. This study examines the genetic response in cultured human keratinocytes exposed to a single solar-simulated ultraviolet dose and harvested at 1, 4, 8, 24, and 48 hours postirradiation for northern blot analysis using c-DNA probes to well studied stress related genes, heat shock protein 70 and ubiquitin; to a recently cloned gene associated with growth arrest and DNA damage (gadd 153); to c-myc and c-fos, two proto-oncogenes known to be associated with cell growth and differentiation; and to glyceraldehyde phosphate dehydrogenase, known to be involved in cellular intermediary metabolism, gadd 153 and c-fos mRNA levels are increased early (1-4 hours) in keratinocytes, at a time when the cells appear indistinguishable from nonirradiated controls. At the same early time points, the c-myc mRNA level is decreased. Heat shock protein 70 and ubiquitin transcripts are detectable under basal conditions but are not increased by ultraviolet exposure; although at later time points (24-48 hours), when there is morphologic evidence of cell damage and downregulation of the otherwise constitutively expressed gene glyceraldehyde phosphate dehydrogenase, their unchanged steady state levels of mRNA constitute relative over expression. Irradiation of cells with the same solar-simulated ultraviolet dose filtered to remove the shortest and most damaging wavelengths results in a similar pattern of early gene induction, even though cell growth and survival are unaffected. These data suggest that two classes of genes participate in the response to ultraviolet rays: early genes whose mRNA levels are increased (gadd 153, c-fos) or decreased (c-myc) before or independent of any morphologic evidence of cell damage and late genes that are relatively increased (heat shock protein 70, ubiquitin) or decreased (glyceraldehyde phosphate dehydrogenase) in expression when morphologic cell damage is present. The early events may be a direct response to ultraviolet irradiation, whereas the late events may represent a secondary response to stress.