Sterol Transport by the Human Breast Cancer Resistance Protein (ABCG2) Expressed in Lactococcus lactis

Abstract

The emergence of multidrug resistant cancer cells is a serious problem in the chemotherapeutic treatment of human tumors. In mammals, multidrug resistance based on the active extrusion of cytotoxic drugs from the cell is mediated by several members of the ATP-binding cassette (ABC) 1 superfamily. These include the multidrug resistance P-glycoprotein MDR1 (also termed ABCB1) and the protein MRP1 (multidrug resistance-associated protein 1, also termed ABCC1) (1). The breast cancer resistance protein (BCRP, also termed MXR, ABCP, or ABCG2) is one of the more recently discovered ABC multidrug transporters in human cancer cells. BCRP confers resistance on cells to (i) toxic ions such as rhodamine 123, (ii) anticancer agents including mitoxantrone and the anthracyclines daunomycin and doxorubicin, and (iii) the camptothecins topotecan and SN-38 (2-7). Overexpression of BCRP has been observed in several human cancer cell lines selected for drug resistance (2, 5, 8) as well as in tumor samples of cancer patients (9 -11). Recently, fumitremorgin C (FTC), a novel chemosensitizing agent, was identified and shown to reverse drug resistance in human BCRP-expressing cancer cells by inhibiting BCRP-mediated drug transport (12).BCRP is a 655-amino acid, 72.1-kDa protein and is the second member of the G subfamily of ABC transporters. Members of the G subfamily are all half-transporters and include among others (i) the Drosophila white, brown, and scarlet proteins, which are involved in the transport of eye pigment (13); (ii) ABCG1, which is thought to be involved in the transport of cholesterol and phospholipids (14); and (iii) heterodimeric ABCG5/ABCG8, which has been implicated in the transport of cholesterol and plant sterols (15). In contrast to P-glycoprotein MDR1 and MRP1, which are full size transporters, BCRP most likely functions as a homodimer (16).In normal tissue, high expression of the BCRP is found in stem cells (17), epithelial cells of small and large intestines, ducts and lobules of the breast, endothelial cells of veins and capillaries (18), and synchitiotrophoblastic cells of the placenta (19). The localization of BCRP suggests that it could have a potential role in protection against toxins. The recent observation in BCRP knock-out mice that BCRP protects against a chlorophyll-derived dietary phototoxin and protoporphyria is consistent with this notion (20).Previously, we have characterized the molecular basis of the drug specificity of LmrA, a half-transporter homologue of human P-glycoprotein MDR1, in the Gram-positive bacterium Lactococcus lactis (21,22). To allow a detailed comparison of BCRP and LmrA, human BCRP was functionally expressed in L. lactis using the nisin A-induced expression system that is used for the expression of LmrA. BCRP was active as an ATPdependent multidrug transporter in L. lactis and was able to interact with sterols. We conclude that the substrate specificity of BCRP partly overlaps with that proposed for ABCG1 and ABCG5/ABCG8. Our observations may suggest a physiological role f...

Association of the BCRP C421A polymorphism with nonpapillary renal cell carcinoma

“…They also demonstrated that the mean BCRP protein level in the placenta was lower in homozygotes for the A421 allele than in those for the C421 allele and that heterozygotes had an intermediate level. 26 Although the molecular mechanism for the relationship between BCRP and carcinogenesis remains to be resolved, BCRP transports a wide variety of substances other than anticancer drugs, including fluorescent dyes, 27 sterols, 28 phytoestrogens, 29 chlorophyll derivatives 30 and folate. 31 Pheophorbide a, a chlorophyll derivative, was shown to have antimutagenic activity and to inhibit tumor cell growth.…”

The 44-kDa Pim-1 Kinase Phosphorylates BCRP/ABCG2 and Thereby Promotes Its Multimerization and Drug-resistant Activity in Human Prostate Cancer Cells

“…In addition to androgen, a large set of BCRP substrates has been identified including chemotherapeutic agents, fluorescent dyes, as well as chemical toxicants (16). BCRP overexpression has been detected in a variety of mitoxantrone or other chemotherapeutic agents selected cell lines (17,18). The molecular mechanism of BCRP in drug efflux has been well studied in breast cancer cells.…”

Identification of Intra- and Intermolecular Disulfide Bridges in the Multidrug Resistance Transporter ABCG2

“…Although a physiological function of ABCG2 remains to be established, the tissue distribution suggests a role of the protein in the protection against xenobiotics. Lately, ABCG2 has been found in relation to stem cells (15,16) and sterol transport (17).…”