Subcellular localization and distribution of the breast cancer resistance protein transporter in normal human tissues

High expression of the Breast Cancer Resistance Protein (BCRP) gene has bee n shown to be involved in resistance to chemotherapeutic drugs. Knowledge o f the localization of BCRP protein in normal tissues may help unravel the n ormal function of this protein. Therefore, we characterized the tissue dist ribution and cellular localization of BCRP in frozen sections of normal hum an tissues. For this purpose, we used the recently described monoclonal ant ibody BXP-34 and another independently developed monoclonal antibody direct ed against BCRP, BXP-21. Both monoclonal antibodies show specific BCRP plas ma membrane staining on cytospins obtained from topotecan- or mitoxantrone- selected cell lines, as well as from BCRP-transfected cell lines. Immunopre cipitation experiments using either BXP-21 or BXP-34 yielded a clear M-r 72 ,000 BCRP band from BCRP-overespressing tumor cells. In the topotecan-selec ted Tg and mitoxantrone-selected MX3 tumor cell lines, BCRP turned out to b e differentially glycosylated. In contrast to BXP-33, BXP-21 is able to det ect the M-r 72,000 BCRP protein on immunoblots and is reactive with BCRP in formalin-fixed, paraffin-embedded tissues. Using BXP-21 and BXP34, promine nt staining of BCRP was observed in placental syncytiotrophoblasts, in the epithelium of the small intestine and colon, in the liver canalicular membr ane, and in ducts and lobules of the breast. Furthermore, BCRP was present in veinous and capillary endothelium, but not in arterial endothelium in al l of the tissues investigated. In the tissues studied, the mRNA levels of B CRP were assessed using reverse transcription-PCR, and these corresponded w ith the levels of BCRP protein estimated from immunohistochemical staining. The presence of BCRP at the placental syncytiotrophoblasts is consistent w ith the hypothesis of a protective role of BCRP far the fetus. The apical l ocalization in the epithelium of the small intestine and colon indicates a possible role of BCRP in the regulation of the uptake of p.o. administered BCRP substrates by back-transport of substrate drug entering from the gut l umen. Therefore, it may be useful to attempt to modulate the uptake of p.o, delivered BCRP substrates, e.g, topotecan or irinotecan, by using a BCRP i nhibitor. Clinical trials testing this hypothesis have been initiated in ou r institute.