title: Functional characterization of the ABCG2 5' non-coding exon variants: Stem cell specificity, translation efficiency and the influence of drug selection.
abstract: ABCG2 is a multidrug transporter with wide substrate specificity, and is believed to protect several cell types from various xenobiotics and endobiotics. This "guardian" function is important in numerous cell types and tissue barriers but becomes disadvantageous by being responsible for the multidrug resistance phenotype in certain tumor cells. ABCG2 regulation at the protein level has already been extensively studied, however, regulation at the mRNA level, especially the functional role of the various 5' untranslated exon variants (5' UTRs) has been elusive. In the present work, we describe a comprehensive characterization of four ABCG2 mRNA variants with different exon 1 sequences, investigate drug inducibility, stem cell specificity, mRNA stability, and translation efficiency. Although certain variants (E1B and E1C) are considered as "constitutive" mRNA isoforms, we show that chemotoxic drugs significantly alter the expression pattern of distinct ABCG2 mRNA isoforms. When examining human embryonic stem cell lines, we provide evidence that variant E1A has an expression pattern coupled to undifferentiated stem cell stage, as its transcript level is regulated parallel to mRNAs of Oct4 and Nanog pluripotency marker genes. When characterizing the four exon 1 variants we found no significant differences in terms of mRNA stabilities and half-lives of the isoforms. In contrast, variant E1U showed markedly lower translation efficiency both at the total protein level or regarding the functional presence in the plasma membrane. Taken together, these results indicate that the different 5' UTR variants play an important role in cell type specific regulation and fine tuning of ABCG2 expression.
authors.affiliation: Institute of Enzymology, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Budapest, Hungary.
authors.forename: Sára
authors.initials: S
authors.lastname: Sándor
authors.affiliation: Institute of Enzymology, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Budapest, Hungary.
authors.forename: Theodora
authors.initials: T
authors.lastname: Jordanidisz
authors.affiliation: Institute of Enzymology, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Budapest, Hungary.
authors.forename: Anita
authors.initials: A
authors.lastname: Schamberger
authors.affiliation: Institute of Enzymology, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Budapest, Hungary.
authors.forename: György
authors.initials: G
authors.lastname: Várady
authors.affiliation: Institute of Enzymology, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Budapest, Hungary.
authors.forename: Zsuzsa
authors.initials: Z
authors.lastname: Erdei
authors.affiliation: Institute of Enzymology, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Budapest, Hungary.
authors.forename: Ágota
authors.initials: Á
authors.lastname: Apáti
authors.affiliation: Institute of Enzymology, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Budapest, Hungary; Department of Biophysics and Radiation Biology, Semmelweis University, MTA-SE Molecular Biophysics Research Group, Budapest, Hungary.
authors.forename: Balázs
authors.initials: B
authors.lastname: Sarkadi
authors.affiliation: Institute of Enzymology, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Budapest, Hungary. Electronic address: orban.tamas@ttk.mta.hu.
authors.forename: Tamás I
authors.initials: TI
authors.lastname: Orbán
mesh_terms: D020121:5' Untranslated Regions / Q000235:genetics; D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters / Q000235:genetics*; D000818:Animals; D002478:Cells, Cultured; D018432:Drug Resistance, Multiple / Q000235:genetics*; D005091:Exons / Q000235:genetics; D057809:HEK293 Cells; D006801:Humans; D061986:MCF-7 Cells; D051379:Mice; D009363:Neoplasm Proteins / Q000235:genetics*; D009928:Organ Specificity / Q000235:genetics; D011110:Polymorphism, Genetic; D013234:Stem Cells / Q000502:physiology*
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
keywords: 5′ UTR; ABCG2; Alternative promoters; MCF-7; Noncoding exons; Stem cell