NORMAL P53 STATUS AND FUNCTION DESPITE THE DEVELOPMENT OF DRUG-RESISTANCE IN HUMAN BREAST-CANCER CELLS

Loss of or mutations in p53 protein have been shown to decrease both r adio- and chemosensitivity. The present study assessed the p53 gene st atus, ability to arrest in G(1) of the cell cycle, the functionality o f the p53 transduction pathway, and apoptosis following treatment with radiation in a series of drug-resistant human breast cancer cells to determine whether p53 alterations occur during the development of drug resistance. We used 13 sublines derived from MCF-7, ZR75B, and T47D c ells, which were resistant to doxorubicin, paclitaxel, vinblastine, ci splatin, etoposide, and amsacrine. Eleven of 12 drug-resistant subline s retained the parental p53 gene status, as determined by sequence ana lysis and functional yeast assay; only one subline was found to have a cquired a mutation in the p53 gene. The MCF-7 TH subline was found to both acquire mutated p53 and to have major changes in p53 protein expr ession and function. In 12 other drug-resistant sublines, the G(1) che ckpoint was conserved or only slightly impaired. A normal accumulation of p53, p21(Cip1/Waf1), and Mdm2 proteins and hypophosphorylation of Rb protein occurred in response to radiation with only small differenc es noted in the kinetics of p53 and p21(Cip1/Waf1) induction. Increase d susceptibility to apoptosis was found in the ZR75B drug-resistant su blines, whereas no evidence for apoptosis was observed in the ZR75B, M CF-7, and T47D parentals and the MCF-7 and T47D drug-resistant subline s. This effect could not be explained by alterations in bcl-2 or bar e xpression. Our results demonstrate that alterations in: (a) p53 gene s tatus; (b) ability to arrest in G(1); (c) induction of p53 protein and p53-dependent genes; and (d) decreased activation of apoptosis is not a requirement for the onset of drug resistance. The function of p53 a ppears to be dissociated from drug resistance in our model system.