Mechanism of Growth Inhibition of H.Ep. #1 Cells by 5-Fluorodeoxycytidine and 5-Fluorodeoxyuridine

Studies related to the mechanism of growth inhibition of H.Ep. #1 cells, a line derived from a human tumor, by 5-fluorodeoxyuridine (FUDR) and 5-fluorodeoxycytidine (FCDR) have been described. The reversal of growth inhibition (in the presence of completely inhibitory concentrations of FUDR and FCDR) when thymidine or 5-methyldeoxycytidine were added to the medium was followed by daily observation of clonal development. Addition of 5-bromodeoxyuridine in the presence of FUDR or FCDR permitted an initial doubling of cell numbers followed by complete growth inhibition. Tracer studies with orotic acid-C14 and thymidine-H3 in the presence of FCDR and thymidine (as reverser) showed substantial suppression of de novo synthesis of the DNA thymine moiety accompanied by enhancement in utilization of the preformed thymine moiety and relatively little effect on the specific activity of the RNA pyrimidines and DNA cytosine. During exposure of cells to FUDR or FCDR for 24 hours in the absence of thymidine the utilization of orotic acid for DNA pyrimidines was markedly depressed, whereas that for the RNA pyrimidines was not altered appreciably. During this period RNA and protein content per cell increased by approximately 60–80 per cent, while DNA per cell remained practically constant. These results in mammalian cells resemble unbalanced growth phenomena previously observed in bacterial systems.