Breast Cancer Resistance Protein Exports Sulfated Estrogens but Not Free Estrogens

Abstract

Breast cancer resistance protein (BCRP), an ATP-binding cassette transporter, confers resistance to a series of anticancer reagents such as mitoxantrone, 7-ethyl-10-hydroxycamptothecin, and topotecan. We reported previously that estrone and 17␤-estradiol reverse BCRP-mediated multidrug resistance. In the present study, we demonstrate that BCRP exports estrogen metabolites. First, we generated BCRP-transduced LLC-PK1 (LLC/BCRP) cells, in which exogenous BCRP is expressed in the apical membrane, and investigated transcellular transport of 3 H-labeled compounds using cells plated on microporous filter membranes. The basal-to-apical transport (excretion) of mitoxantrone, estrone, and 17␤-estradiol was greater in LLC/ BCRP cells than in LLC-PK1 cells. Thin-layer chromatography of transported steroids revealed that the transport of estrone and 17␤-estradiol was independent of BCRP expression. Alternatively, increased excretion of estrone sulfate and 17␤-estradiol sulfate was observed in LLC/BCRP cells. BCRP inhibitors completely inhibited the increased excretion of sulfated estrogens across the apical membrane. Conversion of estrogens into their sulfate conjugates was similar between LLC/BCRP and LLC-PK1 cells, suggesting that the increased excretion of estrogen sulfates was attributable to BCRP-mediated transport. Next, the uptake of 3 H-labeled compounds in membrane vesicles from BCRP-transduced K562 (K562/BCRP) cells was investigated.3 H-labeled estrone sulfate, but not 3 H-labeled estrone or 17␤-estradiol, was taken up by membrane vesicles from K562/BCRP cells, and this was ATP-dependent. Additionally, BCRP inhibitors suppressed the transport of estrone sulfate in membrane vesicles from K562/BCRP cells. These results suggest that BCRP does not transport either free estrone or 17␤-estradiol but exports sulfate conjugates of these estrogens.Breast cancer resistance protein (BCRP) is a member of the ATP-binding cassette transporter G family and is also known as ABCG2 or ABCP. BCRP mediates concurrent resistance to such chemotherapeutic agents as mitoxantrone (MXR), SN-38 (an active metabolite of CPT-11), and topotecan, presumably by pumping these reagents out of the cell, thereby resulting in concentrations lower than cytotoxic levels (Allikmets et al., 1998;Doyle et al

Human Breast Cancer Resistance Protein: Interactions with Steroid Drugs, Hormones, the Dietary Carcinogen 2-Amino-1-methyl-6-phenylimidazo(4,5-b)pyridine, and Transport of Cimetidine

“…The latter authors also suggested that 17␤-estradiol is a transported substrate of human BCRP. This conclusion, however, conflicts with the report of Imai et al (2003) that found that BCRP transports only sulfated conjugates of estrone and 17␤-estradiol but not free estrogens.…”

The PI3K/Akt inhibitor LY294002 reverses BCRP-mediated drug resistance without affecting BCRP translocation

“…BCRP transports certain chlorophyll metabolites, steroid metabolites and xenobiotics and is presumed to play a protective role against toxic substances in the maternal-placental barrier, the digestive tract and the blood-testis barrier (12)(13)(14)(15). BCRP is a molecular determinant of side-population (SP) cells, which are enriched in various stem cells (16).…”

Subcellular localization of the ABCG2 transporter in normal and malignant human gallbladder epithelium

“…12,13 ABCG2 is a half-transporter, exhibiting a broad substrate specificity for various anticancer agents, including mitoxantrone, anthracyclines, and camptothecins. 14,15,16 It is implicated in the transport of endogenous compounds, for example, sulfate conjugates of estrogen, 17 and its expression can be modified by estrogen. 18 Expression of the transporter also protects against a certain form of porphyria characterized by severe phototoxicity, which was observed in ABCG2 knockout mice after exposure to a diet rich in the chlorophyll-derived phototoxin pheophorbide and UV light.…”