Regulation of hypoxanthine DNA glycosylase in normal human and Bloom's syndrome fibroblasts.

The regulation of the base excision repair enzyme hypoxanthine DNA glycosylase was examined in normal human skin fibroblasts (NHS) and fibroblasts from a patient with Bloom's syndrome. Using randomly proliferating cells and those synchronized at specific intervals in the cell cycle, enzyme levels were shown to become elevated severalfold in a proliferation-associated manner. In NHS synchronized in G0 by serum deprivation or in G1 by isoleucine deprivation, maximal enzyme levels were reached prior to maximal rates of DNA synthesis. In Bloom's syndrome cells synchronized in this manner, these two activities were coincident. Cells synchronized at the G1-S border by hydroxyurea exhibit an initial wave of DNA synthesis upon removal of the drug. The cells then undergo another DNA synthetic cycle climaxing 18-21 h after release. Maximal hypoxanthine glycosylase activity of hydroxyurea-synchronized Bloom's cells was observed during the second round of DNA synthesis. However, in NHS the peak of enzyme activity was observed as early as 9 h prior to the second round of DNA synthesis. To determine if hypoxanthine glycosylase could be induced in the absence of DNA synthesis, serum-synchronized NHS were released in the presence of hydroxyurea. The results showed that inhibition of DNA synthesis did not diminish glycosylase induction which demonstrated that DNA replication was not required for glycosylase induction.