title: Cytoplasmic confinement of breast cancer resistance protein (BCRP/ABCG2) as a novel mechanism of adaptation to short-term folate deprivation.
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 muM 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 [(3)H]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.
authors.affiliation: Department of Biology, Technion-Israel Institute of Technology, Haifa 32000, Israel.
authors.forename: Ilan
authors.initials: I
authors.lastname: Ifergan
authors.forename: Gerrit
authors.initials: G
authors.lastname: Jansen
authors.forename: Yehuda G
authors.initials: YG
authors.lastname: Assaraf
mesh_terms: D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D000222:Adaptation, Physiological; D001943:Breast Neoplasms / Q000378:metabolism*; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005260:Female; D005492:Folic Acid / Q000378:metabolism; D005494:Folic Acid Deficiency / Q000378:metabolism*; D006031:Glycosylation; D006801:Humans; D008565:Membrane Proteins / Q000378:metabolism; D009363:Neoplasm Proteins / Q000378:metabolism*
chemical_list: C116614:ABCG2 protein, human; D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters; D008565:Membrane Proteins; D009363:Neoplasm Proteins; D005492:Folic Acid