IN-VITRO INTERACTION OF UTERINE ESTROGEN-RECEPTOR WITH THE ESTROGEN RESPONSE ELEMENT PRESENT IN THE 3'-FLANKING REGION OF THE MURINE C-FOS PROTOONCOGENE

Estradiol treatment rapidly stimulates transcription of the c-fos prot ooncogene in the rodent uterus, and transfection analysis previously i dentified an estrogen response element (ERE) in the 3'-flanking region of the murine gene with the sequence GGTCAnnnCAGCC. We now report tha t endogenous estrogen receptor (ER) obtained from either mouse or rat uterus binds to this 3'-ERE. Unoccupied receptor, receptor occupied wi th estradiol, and receptor occupied with the antiestrogen tamoxifen al l bind to this element, and the binding of receptor exhibits strict se quence specificity. By using a competition binding assay, the affinity of the ER for the c-fos-ERE is estimated to be approximately an order of magnitude less than the affinity for the consensus ERE (GGTCAnnnTG ACC) found in the Xenopus and chicken vitellogenin genes. Differences in the electrophoretic mobilities of the c-fos and vitellogenin EREs b ound to the ER in band-shift assays also suggest subtle structural dif ferences in the two complexes. Mutations in either half-site of the c- fos -ERE destroy ER binding, suggesting that the receptor binds to thi s sequence as either a homo- or heterodimer. The 3'-fos-ERE region exh ibits some homologies to both AP1 and AP2 consensus sites, but neither AP1-like proteins present in uterine extracts nor recombinant AP2 bin d this protooncogene sequence. The finding that the ERE present in the 3'-region of the murine c-fos gene interacts with receptors present i n the mouse and rat uterus supports a role for this element in the phy siological regulation of c-fos expression in the uterus by estrogens.