title: Novel 5' untranslated region variants of BCRP mRNA are differentially expressed in drug-selected cancer cells and in normal human tissues: implications for drug resistance, tissue-specific expression, and alternative promoter usage.
abstract: To investigate transcriptional activation of the breast cancer resistance protein gene (BCRP/ABCG2), we examined the 5' untranslated region of BCRP mRNA in cell lines with high BCRP transcriptional activity and in normal tissues. Human choriocarcinoma cells with high endogenous BCRP expression (JAR and BeWo) and human cancer cells (MCF-7 and Igrov1) and their BCRP-overexpressing, drug-selected, multidrug-resistant derivatives (MCF-7/AdrVp, Igrov1/MX3, and Igrov1/T8) were studied. Rapid amplification of 5'-cDNA ends-PCR (5'RACE-PCR) revealed at least three novel forms of the untranslated exon 1 (E1a, E1b, and E1c) that are spliced to a common exon 2, with differential expression of these splice variants in the drug-selected cell lines. Additionally, sequence analysis of the 5'RACE-PCR products revealed multiple transcriptional start sites for each variant, particularly in the drug-selected cells. The E1c isoform predominated in drug-selected MCF-7 cell lines and was translated more efficiently in MCF-7 cells than the E1a isoform. Varying patterns of expression of the exon 1 isoforms were observed in a variety of human tissues, suggesting that tissue-specific alternative promoters of BCRP exist. In summary, we find that BCRP overexpression in the drug-selected cells is accompanied by multiple transcriptional start sites and predominance of the more efficiently translated E1c isoform. The exon 1 variation we observe suggests that alternative promoters of the BCRP gene exist.
authors.affiliation: The Program in Experimental Therapeutics, Marlene and Stewart Greenebaum Cancer Center; Departments of Medicine and Microbiology, University of Maryland School of Medicine, 655 West Baltimore Street, Baltimore, MD 21201, USA. tnakanishi@som.umaryland.edu
authors.forename: Takeo
authors.initials: T
authors.lastname: Nakanishi
authors.forename: Kimberly J
authors.initials: KJ
authors.lastname: Bailey-Dell
authors.forename: Bret A
authors.initials: BA
authors.lastname: Hassel
authors.forename: Ken
authors.initials: K
authors.lastname: Shiozawa
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Sullivan
authors.forename: Joel
authors.initials: J
authors.lastname: Turner
authors.forename: Douglas D
authors.initials: DD
authors.lastname: Ross
mesh_terms: D020121:5' Untranslated Regions / Q000235:genetics* / Q000378:metabolism; D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters / Q000096:biosynthesis / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D001943:Breast Neoplasms / Q000188:drug therapy / Q000235:genetics* / Q000378:metabolism; D045744:Cell Line, Tumor; D002822:Choriocarcinoma / Q000188:drug therapy / Q000235:genetics / Q000378:metabolism; D005091:Exons; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D008969:Molecular Sequence Data; D009363:Neoplasm Proteins / Q000096:biosynthesis / Q000235:genetics* / Q000378:metabolism; D010051:Ovarian Neoplasms / Q000188:drug therapy / Q000235:genetics* / Q000378:metabolism; D011401:Promoter Regions, Genetic; D014176:Protein Biosynthesis; D020033:Protein Isoforms; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics*; D015533:Transcriptional Activation; D015854:Up-Regulation
chemical_list: D020121:5' Untranslated Regions; C116614:ABCG2 protein, human; D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters; D009363:Neoplasm Proteins; D020033:Protein Isoforms; D012333:RNA, Messenger