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¹ Department of Biopharmaceutical Sciences, University of Illinois at Chicago, Chicago, Illinois and ² University of Maryland Greenebaum Cancer Center and School of Medicine, and the Baltimore VA Medical Center, Baltimore, Maryland

Requests for reprints: William T. Beck, Department of Biopharmaceutical Sciences, University of Illinois at Chicago, MC865, 833 South Wood Street, Chicago, IL 60612. Phone: 312-996-0888; Fax: 312-996-0098. E-mail: wtbeck{at}iuc.edu

Overexpression of the breast cancer resistance protein (BCRP/ABCG2) confers multidrug resistance (MDR) to tumor cells and often limits the efficacy of chemotherapy. To circumvent BCRP-mediated MDR, a common approach is the use of potent and specific inhibitors of BCRP transport such as fumitremorgin C, novobiocin, and GF120918. Here, we evaluated a new approach using RNA interference for the specific knockdown of BCRP. We designed and synthesized small interfering RNA (siRNA) using T7 RNA polymerase and showed that siRNAs markedly down-regulated both exogenous and endogenous expression of BCRP. As a functional consequence, knockdown of BCRP by siRNAs increased the sensitivity of human choriocarcinoma BeWo cells to mitoxantrone and topotecan by 10.5- and 8.2-fold, respectively. Using flow cytometry, we found that introduction of siRNAs also enhanced the intracellular accumulation of topotecan. We have previously identified an estrogen response element in the BCRP promoter and have shown that 17ß-estradiol increased BCRP mRNA expression. Furthermore, in the present study, we found that expression of BCRP protein was inducible by 17ß-estradiol and that this effect was ameliorated by the introduction of siRNAs. These studies indicate that siRNAs could modulate MDR in vitro and may present a new approach to overcome BCRP-mediated drug resistance.

Key Words: ABCG2 • BCRP • estrogen response element • drug resistance • RNA interference

Grant support: National Cancer Institute grants CA30103 and CA40570(W.T. Beck), VA Merit Review grant (D.D. Ross), and University of Illinois at Chicago.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely toindicate this fact.

Received 8/30/04; revised 10/ 1/04; accepted 10/18/04.