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 protooncogene in the rodent uterus, and transfection analysis previously identified an estrogen response element (ERE) in the 3′-flanking region of the murine gene with the sequence GGTCAnnnCAGCC. We now report that endogenous estrogen receptor (ER) obtained from either mouse or rat uterus binds to this 3′-ERE. Unoccupied receptor, receptor occupied with estradiol, and receptor occupied with the antiestrogen tamoxifen all bind to this element, and the binding of receptor exhibits strict sequence 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 (GGTCAnnnTGACC) found in the Xenopus and chicken vitellogenin genes. Differences in the electrophoretic mobilities of the c-fos and vitellogenin EREs bound to the ER in band-shift assays also suggest subtle structural differences in the two complexes. Mutations in either half-site of the c-fos-ERE destroy ER binding, suggesting that the receptor binds to this sequence as either a homo- or heterodimer. The 3′-fos-ERE region exhibits some homologies to both AP1 and AP2 consensus sites, but neither AP1-like proteins present in uterine extracts nor recombinant AP2 bind this protooncogene sequence. The finding that the ERE present in the 3′-region of the murine c-fos gene interacts with receptors present in the mouse and rat uterus supports a role for this element in the physiological regulation of c-fos expression in the uterus by estrogens.