FIG. 2.

Expression of MDR1 mRNA in MCF-7 cell sublines. Total cellular RNA (10 μg) was electrophoresed, transferred, hybridized with ³²P-labeled MDR1 cDNA, and processed for autoradiography. A glyceraldehyde-3-phosphate dehydrogenase (GA3PDH) probe was utilized to normalize RNA load (lower panel). Final concentrations of drugs were as follows: doxorubicin, 2 μg/ml (3 μM); doxycycline, 100 μg/ml (208 μM); cefoperazone, 2 mg/ml (3 mM); and piperacillin, 8 mg/ml (14 mM). Solubility problems prevented further increases in cefoperazone and piperacillin concentrations. The duration of drug exposure was approximately 12 weeks for doxorubicin and doxycycline and 16 weeks for cefoperazone and piperacillin.