The Cytoprotective Effects of Exogenous DNA Fragments

Abstract

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/cm². 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.

Normal Human Keratinocytes Pretreated with Damaged mRNA Are Resistant to UVB Damage and Exhibit Increased DNA Repair

“…Treatment of normal human keratinocytes with UVC-irradiated rabbit globin mRNA 24 h before and after UVB exposure increased the survival of the normal human keratinocyte cells. This effect is similar to that observed for damaged DNA fragments [4], but damaged mRNA appears more effective than DNA. Keratinocytes required treatment with 1 mg/ml of damaged DNA fragments to get the same cytoprotective effects as 1 Ìg/ml of damaged mRNA fragments.…”

“…Previously, it has been reported that UV-damaged DNA fragments provide a cytoprotective effect to mammalian cells [3,4,[7][8][9]11]. Here, we report on the first observations that UV-damaged mRNA fragments have a similar property.…”

“…If this response is present in mammalian cells it may be precipitated by the addition of exogenous, damaged DNA [3,4]. This may explain the early observations of a protective effect of DNA in other systems [3,4,7,8,9,11]. Djord- 13 jecic et al [7] and Miletic et al [8] were the first to report the enhancement of cell survival after UV and X irradiation with external DNA treatments.…”

“…This DNA repair mechanism in prokaryotes is inducible and called the SOS response in prokaryotes and the SOS-like response in mammalian cells [2]. If this response is present in mammalian cells it may be precipitated by the addition of exogenous, damaged DNA [3,4]. This may explain the early observations of a protective effect of DNA in other systems [3,4,7,8,9,11].…”

“…However, these authors used undamaged DNA and did not specifically investigate the effects of damaged DNA on protecting cells. It has been suggested that mammalian cells may recognize damaged DNA and mount a repair response [3,4].…”