Similarity in the Effect of Caffeine on DNA Synthesis after UV Irradiation between Xeroderma Pigmentosum Variant Cells and Mouse Cells

The effect of UV irradiation on the rate of DNA synthesis was compared among normal human, xeroderma pigmentosum (XP, group A and variant) and mouse cells with and without caffeine in the culture medium after UV irradiation. At the same levels of survival, approximately 37%, all cells showed reduction in the rate of synthesis 0–3 h after UV irradiation followed by a recovery to normal or near‐normal level 12 h later. In the presence of caffeine, no change in the recovery patterns was observed in normal human and XP A cells. XP variant cells and mouse cells showed little or no recovery in the presence of caffeine even after 12 h, when full recovery was obtained without caffeine. XP variant and mouse cells appear to have a common response in that post‐irradiation treatment with caffeine inhibits reinitiation of UV‐reduced DNA replication. Enhancement by caffeine of UV‐killing in XP variant and mouse cells may he due to the retarded resumption of DNA replication.