The cytoprotective effects of exogenous DNA fragments

Ultraviolet irradiation of normal human keratinocytes induces a cytotoxic effect. The chromophore for this effect is believed to be genomic DNA. However, DNA damage is known to be repaired in UVB irradiated keratinocytes. The trigger for this DNA repair is potentially damaged DNA itself. To test the hypothesis that damaged DNA can induce the host cell's own DNA repair mechanism, we treated the keratinocytes with the damaged DNA and evaluated its cytoprotective effects. We have observed that fragmented calf thymus DNA irradiated and damaged with a UVC light can induce a protective effect in cultured human keratinocytes. Keratinocytes treated with UVC damaged DNA fragments are less susceptible to UVB irradiation-induced cell death as measured by neutral red uptake. Unirradiated exogenous DNA did not induce this protective effect. Similar protective effects can be seen with irradiated salmon sperm DNA. UVC damaged DNA fragments induced 60% increase in protection in human HaCaT keratinocyte in culture to the cell death induced by UVB. Similar protection was observed with UVC irradiated oligothymidylic acid (dT3-dT5) which increased the survival of human HaCaT keratinocytes after UVB irradiated by 50%. Isolated mononucleotides, irradiated or not, do not increase UVB survivability. Cellular DNA synthesis was greatly inhibited by UVB, becoming undetectable at 40 mJ/cm(2). Exogenous treatment with damaged fragments causes immediate and significant inhibition of total cellular DNA synthesis. This inhibition was dose dependent. Cells that undergo damage to their DNA are known to inhibit endogenous DNA synthesis via p53 suppressor gene activation. This is believed to allow them sufficient time to repair the host DNA. The cellular response to exogenous damaged DNA may be a similar mechanism.