Mechanisms of Resistance and Collateral Sensitivity in Cultured Human MCF-7 Carcinoma Cells Resistant to Pyrazofurin

Abstract

Pyrazofurin resistant human breast carcinoma (MCF-7) cells and a subline of clones were developed for studies aimed at elucidating the mechanisms involved in resistance to the drug. Enzyme assays indicate that the resistant cells and clones exhibit increases in the activities of orotate phosphoribosyl transferase and orotidylate decarboxylase, pyrazofurin's target enzyme. The appearance of several resistant clones with no increased enzyme activities may indicate a deficiency in the levels of adenosine kinase present. Enzyme kinetic studies suggest that there is no change in the structure of the target enzyme which could have caused resistance. The resistant cells show increased sensitivity to 5-FU. The incorporation of 5-FU into cellular RNA was compared in resistant and the wild type cells from which they were derived using radiolabels. The incorporation was greatly increased in the resistant cells. The conversion of 5-FU to 5-FUMP was also studied by employing an enzyme assay and separation via high pressure liquid chromatography. The conversion is linear with the reaction time and requires the presence of PRPP. These experiments indicate that collateral sensitivity to 5-FU is created by the increased transferase activity in pyrazofurin resistant cells.