DNA damage by the enediyne C-1027 results in the inhibition of DNA replication by loss of replication protein a function and activation of DNA-dependent protein kinase

Treatment of cells with the enediyne C-1027 is highly efficient at inducing single- and double-strand DNA breaks. This agent is highly cytotoxic when used at picomolar levels over a period of days. For this study, C-1027 has been used at higher levels for a much shorter time period to look at early cellular responses to DNA strand breaks. Extracts from cells treated with C -1027 for as little as 2 h are deficient in SV40 DNA replication activity. Treatment with low levels of C-1027 (1-3 nM) does not result in the presenc e of a replication inhibitor in cell extracts, but they are deficient in re plication protein A (RPA) function. Extracts from cells treated with high l evels of C-1027 (10 nM) do show the presence of a trans-acting inhibitor of DNA replication. The deficiency in RPA in extracts from cells treated with low levels of C-1027 can be fully complemented by the addition of exogenou s RPA, and may be due to a C-1027-induced decrease in the extractability of RPA. This decrease in the extractability of RPA correlates with the appear ance of many extraction-resistant intranuclear RPA foci. The trans-acting i nhibitor of DNA replication induced by treatment of cells with high levels of C-1027 (10 nM) is DNA-dependent protein kinase (DNA-PK). DNA-PK is activ ated by the presence of DNA fragments induced by C-1027 treatment, and can be abrogated by removal of the DNA fragments. Although it is activated by D NA damage and phosphorylates RPA, DNA-PK is not required for either RPA foc alization or loss of RPA replication activity.