Effects of thymidine analogs on Syrian hamster melanoma cells: Phenotypes arising from selection for analog resistance

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Abstract
In order to compare the biological effects of different thymidine (dT) analogs, two unusual cell lines (B-4 and HAB) previously isolated from a Syrian hamster melanoma line by selection with 5-bromodeoxyuridine (BrdU) were analyzed for their response to other analogs. B-4 cells require high concentrations of BrdU for optimal growth, and it was seen that the requirement for BrdU could be satisfied partially by 5-chlorodeoxyuridine (CldU) but not by the other dT analogs tested. HAB cells are able to grow with all the dT residues in nuclear DNA replaced by BrdU, and it was found that they could also grow with essentially all the dT residues in nuclear DNA replaced by CldU but not by other analogs. New cell lines resistant to 100 µM concentrations of CldU, 5-iododeoxyuridine (IdU), and 5-hydroxymethyldeoxyuridine (HMdU) were isolated from the melanoma line and tested for cross-resistance to the other dT analogs. A high level of cross-resistance was observed only with BrdU and CldU. The ability of the cell lines resistant to BrdU, CldU, and IdU to incorporate these analogs into nuclear DNA also was determined. BrdU and CldU were incorporated efficiently by all of the lines tested, but the IdU-resistant cells seemed to preferentially exclude IdU.