Determining MDR1P‐glycoprotein expression in breast cancer

Abstract

The mechanism of chemotherapy resistance in breast cancer is unresolved. MDRI/P-glycoprotein (P-Gp) over-expression confers multidrug resistance in vitro and might play a role in clinical breast cancer. Studies using clinical samples have yielded conflicting results. MDRI/P-Gp mRNA expression was determined relative to the expression in normal human liver using TaqMan real-time RT-PCR (corrected for expression of the housekeeping gene PBGD). Immunohistochemistry (IHC) was performed with monoclonal antibodies against P-Gp (JSB I, C219), The positive control was SW1573/2R160, the intermediate control SW1573 and the negative control GLC4/ADR, We assayed 9 breast cancer cell lines by RT-PCR and IHC, 52 carcinoma samples by RT-PCR and 168 samples by IHC, SW1573/2R160 contained high levels of MDRI/P-Gp mRNA (1.0, equal to liver) and showed strong membranous staining. Expression of MDRI/P-Gp mRNA in SW1573 (0.05) and GLC4/ADR (3.2 x 10(-5)) was not detectable by IHC. Very low levels of MDRI/P-Gp mRNA were measured in breast-cancer cell lines (mean 3.1 x 10(-4), range I to 12 x 10(-4)), but P-Gp was not detected by IHC. In 25 specimens from chemotherapy-naive patients, MDRI/P-Gp mRNA levels varied from I to 11 x 10(-2) (mean 3.9 x 10(-2)). In sections of 80 chemotherapy-naive tumors, no membrane bound staining was observed in the tumor cells, Tumors of 27 anthracycline-treated patients had comparable MDRI/P-Gp mRNA expression levels (mean 5.4 x 10(-2)). P-Gp was undetectable in 88 tumor samples of patients who had received anthracycline-based chemotherapy. In breast cancer, MDRI/P-Gp mRNA is low or absent and P-Gp levels in cancer cells are too low to detect by IHC. Chemotherapy exposure does not result in detectable MDRI/P-Gp over-expression. (C) 2001 Wiley-Liss, Inc.