Atypical multidrug resistance: Breast cancer resistance protein messenger RNA expression in mitroxantrone-selected cell lines

Background: Human cancer cell lines grown in the presence of the cytotoxic agent mitoxantrone frequently develop resistance associated with a reductio n in intracellular drug accumulation without increased expression of the kn own drug resistance transporters P-glycoprotein and multidrug resistance pr otein (also known as multidrug resistance-associated protein). Breast cance r resistance protein (BCRP) is a recently described adenosine triphosphate- binding cassette transporter associated with resistance to mitoxantrone and anthracyclines. This study was undertaken to test the prevalence of BCRP o verexpression in cell lines selected for growth in the presence of mitoxant rone. Methods: Total cellular RNA or poly A(+) RNA and genomic DNA were iso lated from parental and drug-selected cell fines. Expression of BCRP messen ger RNA (mRNA) and amplification of the BCRP gene were analyzed by northern and Southern blot hybridization, respectively, Results: A variety of drug- resistant human cancer cell lines derived by selection with mitoxantrone ma rkedly overexpressed BCRP mRNA; these cell lines included sublines of human breast carcinoma (MCF-7), colon carcinoma (S1 and HT29), gastric carcinoma (EPG85-257), fibrosarcoma (EPF86-079), and myeloma (8226) origins, Analysi s of genomic DNA from BCRP-overexpressing MCF-7/MX cells demonstrated that the BCRP gene was also amplified in these cells. Conclusions: Overexpressio n of BCRP mRNA is frequently observed in multidrug-resistant cell lines sel ected with mitoxantrone, suggesting that BCRP is likely to be a major cellu lar defense mechanism elicited in response to exposure to this drug. It is likely that BCRP is the putative "mitoxantrone transporter" hypothesized to be present in these cell lines.