Varying Effects of Deoxynucleosides on the Incorporation of Thymidine into the DNA of Monolayer and Suspension Cultures

Deoxyuridine will normally suppress the incorporation of subsequently added tritiated thymidine into the DNA thy-mine of cells grown in suspension culture to less than 10%. Conversely, deoxyuridine appears to stimulate thymidine incorporation, at times approaching thousandfold excess, when the cells are maintained as mon-olayers. This stimulation is time, temperature, and, to a lesser extent, pH dependent, but is not directly related to the density of the monolayer. Disruption of the monolayer reverts the cells to normal deoxyuridine suppression of thymidine incorporation into DNA. This study extends earlier evidence of variations in de novo and salvage DNA synthesis between suspension and monolayer cell cultures. We caution the equating of data obtained with suspension cell systems to solid tumor analysis.