561 Time-dependence of UVB induced cellular mechanisms in cultured human keratinocytes

Abstract

UVB-induced cyclobutane pyrimidine dimers (CPDs) are considered to be the main cause of acute sunburn and epidermal carcinogenesis. In humans, these lesions are repaired by nucleotide excision repair, but marsupials and lower organisms present photolyase enzyme which rapidly removes CPDs in a visible light-dependent process (photoreactivation). Previously we established an in vitro pseudouridin-modified mRNA encoding CPD-specific photolyase transfection on human keratinocyte cell lines, which was proved to be a proper method to avoid UVB-induced apoptosis. According to clinical experiences, there is a need for a treatment to diminish the deleterious effects of sunburn when UVB injury has already occurred and the first symptoms appear. Our aim was to determine the time interval after UVB exposure when keratinocyte apoptosis is still preventable. Normal human epidermal keratinocytes were transfected with lipofectamine-complexed, in vitro transcribed mRNA encoding CPD-specific photolyase. Cells were irradiated with 60 mJ/cm2 UVB. At 0, 6, 8 or 12 hours after UVB cells were either exposed to visible light (photoreactivation) or kept in the dark. Viability was measured by Annexin V and propidium iodide dual staining followed by flow cytometry, the relative amount of intracellular CPDs was analyzed by CPD-specific ELISA. Photolyase-mediated CPD removal restored cell viability close to the baseline conditions 0 to 6 hours after UVB treatment. The effect of CPD removal on keratinocyte survival began to decrease 8 hours after the UVB damage. In mRNA-transfected and photoreactivated cells 80% of CPDs were removed at 6 hours post-irradiation. 8 to 12 hours after UVB 40-60% of the CPDs were eliminated. Our results suggest that UV-induced keratinocyte apoptosis can be prevented within 6-8 hours after the UVB injury by the elimination of CPD photolesions. After that point the effect of CPD removal on cell viability is less feasible.