INVOLVEMENT OF THE CODING SEQUENCE FOR THE ESTROGEN-RECEPTOR GENE IN AUTOLOGOUS LIGAND-DEPENDENT DOWN-REGULATION

The estrogen receptor (ER) appears to be down-regulated by its own lig and in some estrogen (E2)-responsive tissues as well as in cell lines such as MCF-7 and GH3. Surprisingly, we observed ER down-regulation in a newly constructed E2-responsive cell line (Rat1+ER), in which expre ssion of the coding region of the ER cDNA was driven by the Moloney mu rine leukemia virus long terminal repeat. We present evidence that the coding region of the ER cDNA, but not the Moloney murine leukemia vir us long terminal repeat, possesses a sequence(s) necessary for ER down -regulation. The observed down-regulation occurs at ligand-binding and protein, as well as mRNA, levels. Marked decreases in both protein an d mRNA levels were observed as early as 3 h after E2 treatment. Furthe rmore, maximal down-regulation occurred by 18-24 h with ligand-binding , and mRNA levels reached approximately 20% that of controls. ER down- regulation in Rat1+ER cells is only partially inhibited by the presenc e of cycloheximide and therefore suggests a direct participation of th e ER in this process. E2 does not appear to influence the stability of the ER transcript, which implies that negative regulation is occurrin g at the transcriptional level. Finally, since we can demonstrate ER b inding to a portion of the cDNA sequence, we propose a mechanism where by ER binding to its putative negative element leads to transcriptiona l repression of the upstream promoter.