A NOVEL, CELL-TYPE-SPECIFIC MECHANISM FOR ESTROGEN RECEPTOR-MEDIATED GENE ACTIVATION IN THE ABSENCE OF AN ESTROGEN-RESPONSIVE ELEMENT

The estrogen receptor (ER) typically activates gene transcription by b inding to estrogen-responsive elements (EREs). The brain creatine kina se (BCK) promoter is responsive to estrogen but contains no ERE-relate d sequence. To investigate the mechanism of estrogen induction, we hav e introduced the estrogen receptor into HeLa cells and primary rat car diomyocytes and fibroblasts along with 195 bp of BCK promoter linked t o a chloramphenicol acetyltransferase (CAT) reporter gene. A 10-fold s timulation of CAT activity was observed in the presence of beta-estrad iol in both HeLa and rat primary fibroblasts, but no induction was obs erved in primary rat cardiomyocytes. In contrast, a control vitellogen in gene construct which contains a typical ERE was induced in an ER-de pendent manner in all cell types studied. Estrogen induction in HeLa w as not sensitive to cycloheximide and was blocked by the ER antagonist s tamoxifen arid ICI 164,384. Analysis of 5' deletion and linker-scann ing mutations indicates sequences between bp -45 and -75 including a T A-rich sequence and a CCAAT sequence to be crucial for stimulation of the BCK promoter by the ER. BCK estrogen induction is dependent on the DNA-binding domain and transactivation domain TAF2 of the ER. However , direct DNA binding is probably not required. Taken together, these r esults suggest a novel mechanism for ER-mediated gene activation. This mechanism is consensus ERE independent and cell type specific and req uires interactions between the ER and molecules capable of interacting with the BCK promoter TA-rich region.