Separate effect of hydroxyurea on the initiation and elongation of DNA synthesis in BHK cells.

BHK21/Cl1 cells, starved for 30 h in serum deficient medium and treated for 15 h with 1 mM hydroxyorea (HO) in order to obtain a synchronous cell population in the G1/S-boundary, incorporate a residual proportion of 3H-thymidine (dThd). This residual incorporation is due to semiconservative synthesis and may not be reduced by increasing the drug concentration without affecting the reversion capacity of the cells proportionally. As shown by autoradiographic analysis, the residual DNA synthesis does not correspond to 3H-dThd incorporation within a small number of resistant cells, but is located in the nuclei of a high proportion of cells with reduced density of silver grains. After treatment with 0.05 mM HU, however, the incorporation of 3H-dThd increases considerably over the control values. The determination of the radioactivity incorporated by microgram DNA corresponding to nuclei in S phase indicates that this concentration of HU is also able to reduce the rate of DNA polymerization. Kinetic data on the appearance of this increased 3H-dThd incorporation and on the accumulation of labelled nuclei in cells growing at random and labelled continuously with the radioactive DNA precursor indicate that HU stimulates the cells to enter the S phase. The reported results are consistent with a mechanism of action of HU which affects initiation and elongation of DNA chains separately.