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

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, multidrugresistant 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 I 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 I variation we observe suggests that alternative promoters of the BCRP gene exist.