Anomalous Behavior of Protein Synthesis Inhibitors on the Turnover of the Estrogen Receptor as Measured by Density Labeling*

Estrogen receptor half-life has been previously estimated to be as great as 5 days in uterine tissue (using protein synthesis inhibitors) and as short as 4 h in MCF-7 breast cancer cells (using dense amino acid incorporation). Using the density shift technique we have demonstrated that the half-life of the estrogen receptor is 2.1 h in primary uterine cell cultures. However, the protein synthesis inhibitors, cycloheximide and puromycin, in conjunction with the density shift technique, block this rapid turnover rate. Presence of steroid does not markedly alter the turnover rate. Dense amino acid incorporation did not appear to alter either the turnover rate or the ability of receptor to undergo ligand-induced transformation.