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Original Article

Cytoplasmic Confinement of Breast Cancer Resistance Protein (BCRP/ABCG2) as a Novel Mechanism of Adaptation to Short-Term Folate Deprivation

Department of Biology, Technion-Israel Institute of Technology, Haifa, Israel (I.I., Y.G.A.); and Department of Rheumatology, VU University Medical Center, Amsterdam, The Netherlands (G.J.)

Abstract

The unique capability of breast cancer resistance protein (BCRP/ABCG2) to export mono-, di-, and triglutamates of folates should limit cellular proliferation under conditions of folate deprivation, particularly upon BCRP overexpression. Here, we explored the mode of adaptation of BCRP-overexpressing cells to short-term folate deprivation. MCF-7/MR cells grown in high folate medium (2.3 µM folic acid) containing mitoxantrone had 62% of their overexpressed BCRP in the plasma membrane and only 38% in the cytoplasm. In contrast, cells grown for 2 weeks in folic acid-free medium followed by an adaptation week in low folate medium (1 nM folic acid) had 86% of BCRP in the cytoplasm and only 14% in the plasma membrane. Unlike BCRP, various transmembrane proteins retained their normal plasma membrane localization in folate-deprived cells. Folate deprivation was also associated with a 3-fold decrease in BCRP and multidrug resistance protein 1 (MRP1/ABCC1) levels. Confocal microscopy with folate-deprived cells revealed that cytoplasmic BCRP colocalized with calnexin, an established endoplasmic reticulum resident. The loss of BCRP from the plasma membrane in folate-deprived cells consistently resulted in a 4.5-fold increase in [³H]folic acid accumulation relative to MCF-7/MR cells. Hence, cellular adaptation to shortterm folate deprivation results in a selective confinement of BCRP to the cytoplasm along with a moderate decrease in BCRP and MRP1 levels aimed at preserving the poor intracellular folate pools. These results constitute a novel mechanism of cellular adaptation to short-term folate deprivation and provide further support to the possible role of BCRP in the maintenance of cellular folate homeostasis.

Received for publication October 14, 2004.

Accepted for publication January 18, 2005.

Address correspondence to: Dr. Yehuda G. Assaraf, Department of Biology, Technion-Israel Institute of Technology, Haifa 32000, Israel. E-mail: assaraf{at}tx.technion.ac.il

This article has been cited by other articles:

				I. Ifergan, G. Jansen, and Y. G. Assaraf The Reduced Folate Carrier (RFC) Is Cytotoxic to Cells under Conditions of Severe Folate Deprivation: RFC AS A DOUBLE EDGED SWORD IN FOLATE HOMEOSTASIS J. Biol. Chem., July 25, 2008; 283(30): 20687 - 20695. [Abstract] [Full Text] [PDF]

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