Characterization of pyrazofurin-resistant HeLa cells with amplification of UMP synthase gene

Three different phenotypes have been characterized in HeLa cells that have been selected for resistance to pyrazofurin, a potent inhibitor of the de novo pyrimidine biosynthetic enzyme UMP synthase. All of the resistant cell lines had a coordinate increase in UMP synthase activity, UMP synthase-specific mRNA, and UMP synthase gene sequences. In one of the resistant cell lines, the amplification of the UMP synthase gene is associated with a stable phenotype. There is no decrease in UMP synthase gene copy number or UMP synthase activity when these cells are grown for over six months in the absence of pyrazofurin. Another resistant cell line that has a higher level of gene amplification when grown in the presence of pyrazofurin loses its elevated UMP synthase activity and amplified DNA sequences with growth in the absence of the drug. A third cell line that possessed a moderate level of UMP synthase gene amplification is tenfold more resistant to pyrazofurin than the cell line with the highest level of amplification. The extraordinary level of resistance is due to a decreased level of activity for the enzyme adenosine kinase that is required for the conversion of pyrazofurin to its inhibitory monophosphate form.