FIG. 3.

Immunoblot analysis of P-gp expression in cell lysates from MCF-7 sublines. Cell extracts (50 μg of protein) were electrophoresed, transferred, and immunoblotted with monoclonal (C219) anti-human P-gp antibody. A monoclonal antibody recognizing actin (45 kDa) was used to normalize protein load. The analysis was repeated twice with protein extracts from separate cell lysates. 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. Numbers to the left of the top panel are molecular masses in kilodaltons.