Estrogen receptor isoform-specific regulation of the retinoblastoma-binding protein 1 (RBBP1) gene: Roles of AF1 and enhancer elements

Estrogen (E2) is involved in mediating many important functions relevant to osteoblast biology through the actions of the estrogen receptors (ER) α and β. To further understand the mechanisms of ER-specific regulation, we used microarray and reverse transcription-PCR analyses of E2-treated U2OS-ERα or -ERβ cells and identified retinoblastoma-binding protein 1 (RBBP1) as a major E2-regulated gene. RBBP1 is a retinoblastoma cofactor involved in the control of osteoblastic proliferation. Although RBBP1 mRNA levels rapidly increased after 2 h of E2 treatment in both U2OS-ER-expressing lines, a sustained induction was only observed in U2OS-ERα cells. Examination of the RBBP1 genomic sequence revealed an ER response element and a Sp1 site located within the first intron. Chromatin immunoprecipitation analyses demonstrated that E2-dependent ERα binding to the intron 1 enhancer region was constitutive, whereas ERβ binding was transient, consistent with the mRNA time course. Interestingly, transient transfection and receptor mutational studies revealed that RBBP1 induction by ERα only requires the Sp1 site, whereas ERβ utilizes both the Sp1 and estrogen response elements binding sites for maximal E2-dependent activation. Stable U2OS transfectants containing a deletion of the ERα activation function 1 (AF1) resulted in a temporal mRNA induction profile similar to that of wild type ERβ. Further, overexpression and chromatin immunoprecipitation analyses also demonstrated that E2-dependent RBBP1 induction is SRC2-dependent for both ER isoforms. These results describe an E2-dependent, ER isoform-specific transcriptional activation of the RBBP1 gene, which in part, is explained by the differential activity of ER AF1 and enhancer element binding.