Does hydroxyurea inhibit DNA replication in mouse cells by more than one mechanism?

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Hydroxyurea is a known inhibitor of DNA synthesis, primarily acting by inhibiting ribonucleoside diphosphate reductase.
• The precise mechanism by which hydroxyurea affects DNA replication, particularly in later stages, remains incompletely understood.

Purpose of the Study:

• To investigate the effect of hydroxyurea on cell-free DNA synthesis in lysed mouse cells.
• To elucidate the mechanism by which hydroxyurea impacts DNA replication processing and intermediate formation.

Main Methods:

• Cell-free DNA synthesis assays were conducted using lysed mouse cell cultures.
• In vitro reactions were treated with N-ethylmaleimide, hydroxyurea, and fluorodeoxyuridine.
• Analysis of DNA synthesis products was performed using alkaline density gradients.

Main Results:

• Hydroxyurea added after cell lysis did not inhibit DNA synthesis, but pre-treatment of cells with hydroxyurea markedly reduced subsequent cell-free DNA synthesis.
• Alkaline density gradient analysis revealed retarded ligation of primary DNA synthesis products and accumulation of small fragments in hydroxyurea-pretreated cells.
• These effects were observed despite the presence of all necessary triphosphate precursors in the in vitro incubation mixture.

Conclusions:

• Hydroxyurea interferes with the processing and ligation of early DNA replication products, preventing the formation of longer DNA intermediates.
• The observed effect suggests a hydroxyurea-mediated mechanism independent of its classical inhibition of ribonucleoside diphosphate reductase or an unknown function of this enzyme in later replication steps.
• This finding helps explain hydroxyurea's cell cycle block in early S phase and relative insensitivity of DNA repair synthesis to the drug.