Microsatellite Instability, Apoptosis, and Loss of p53 Function in Drug-resistant Tumor Cells

Abstract

We have examined microsatellite instability and loss of p53 function in human tumor cell line models of acquired anticancer drug resistance. We observe acquisition of an RER⁺ phenotype in cell lines selected for resistance to cisplatin or doxorubicin. The majority of independent cisplatinresistant sublines are RER⁺, whereas the parental line shows no evidence of microsatellite instability. Microsatellite mutations in random, nonselected subclones of a cisplatin-resistant line are observed in the absence of further drug exposure, suggesting that the RER⁺ phenotype is a stable phenotype rather than being transiently induced by DNA damage. Furthermore, a cisplatin-resistant derivative shows reduction in a G:T mismatch recognition activity compared to the parental line. Independent lines selected by multiple exposure to cisplatin show resistance factors of up to 5-fold by clonogenic assay and have reduced cisplatin-induced apoptosis. The resistant lines that are RER⁺ show evidence of loss of p53-dependent functions, as measured by loss of radiation-induced G₁ arrest and reduced CIP1 mRNA. Induced loss of p53 function by transfection of mutant TP53 does not cause a detectable RER⁺ phenotype. We speculate that tolerance of DNA damage and expansion of cells with an RER⁺ phenotype may select for reduced ability to engage apoptosis and loss of p53 function.