Bcrp1 transcription in mouse testis is controlled by a promoter upstream of a novel first exon (E1U) regulated by steroidogenic factor-1.

Alternative promoter usage is typically associated with mRNAs with differing first exons that contain or consist entirely of a 5' untranslated region. The murine Bcrp1 (Abcg2) transporter has three alternative promoters associated with mRNAs containing alternative untranslated first exons designated as E1A, E1B, and E1C. The E1B promoter regulates Bcrp1 transcription in mouse intestine. Here, we report the identification and characterization of a novel Bcrp1 promoter and first exon, E1U, located upstream from the other Bcrp1 promoters/first exons, which is the predominant alternative promoter utilized in murine testis. Using in silico analysis we identified a putative steroidogenic factor-1 (SF-1) response element that was unique to the Bcrp1 E1U alternative promoter. Overexpression of SF-1 in murine TM4 Sertoli cells enhanced Bcrp1 E1U mRNA expression and increased Bcrp1 E1U alternative promoter activity in a reporter assay, whereas mutation of the SF-1 binding site totally eliminated Bcrp1 E1U alternative promoter activity. Moreover, expression of Bcrp1 E1U and total mRNA and Bcrp1 protein was markedly diminished in the testes from adult Sertoli cell-specific SF-1 knockout mice, in comparison to the testes from wild-type mice. Binding of SF-1 to the SF-1 response element in the E1U promoter was demonstrated by chromatin immunoprecipitation assays. In conclusion, nuclear transcription factor SF-1 is involved with the regulation of a novel promoter of Bcrp1 that governs transcription of the E1U mRNA isoform in mice. The present study furthers understanding of the complex regulation of Bcrp1 expression in specific tissues of a mammalian model.