Expression of mouse Abcg2 mRNA during hematopoiesis is regulated by alternative use of multiple leader exons and promoters.

ABCG2 encodes a transmembrane transporter associated with multidrug resistance in various cancer cells. ABCG2 is also highly expressed in hematopoietic stem cells (HSCs) and is down-regulated in most committed progenitors, whereas expression is sharply up-regulated during erythroid differentiation. The mechanisms for regulation of ABCG2 expression in hematopoietic cells are poorly understood. We have recently identified three novel leader exons (termed E1A, E1B, and E1C) located in the 5'-untranslated region of mouse Abcg2 mRNA by data base searches and reverse transcription-PCR. In a mouse erythroid cell line, reverse transcription-PCR analysis showed that the transcript containing E1B exon was the only isoform detected. Consistently, the E1B-containing transcript was the predominant isoform of Abcg2 mRNA in primary Ter119+ erythroid cells from mouse bone marrow as well as in mouse fetal liver cells. In contrast, the E1A-containing transcript was highly expressed in c-Kit+, Sca-1+, Lin- (KSL) bone marrow cells, especially in CD34- KSL fraction, which is highly enriched for repopulating HSCs. The differential expression pattern of Abcg2 mRNA isoforms in mouse HSCs and erythroid cells was confirmed by 5'-rapid amplification of cDNA ends, indicating that at least two different promoters control mouse Abcg2 transcription during hematopoiesis. Promoter functional assays using EGFP as reporter gene demonstrated that the E1A 5'-flanking region had promoter activity, which contains multiple putative hematopoietic transcription factor binding sites. In summary, our data show that the expression of Abcg2 during hematopoiesis is transcriptionally regulated by alternative use of multiple leader exons and promoters in a developmental stage-specific manner.