Estrogen Regulation of Protein Synthesis in the Immature Rat Uterus: The Analysis of Proteins Released into the Medium duringin VitroIncubation*

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Abstract
Immature rats were treated with estradiol (E2) or other steroids before their uteri were removed and incubated under in vitro conditions in the presence of [35S]methionine. The analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the radiolabeled proteins synthesized and released into the incubation medium demonstrate that E2 regulates the appearance of two proteins. These two proteins have mol wt of 115,000 and 65,000. The concentrations of proteins in the medium increases linearly with time, suggesting that they may be secreted. These two proteins were not produced by several other tissues in response to E2 and appear to be specific to the uterus. They also appear to be increased only by estrogens (E2 > estrone > estriol) and not by other steroids tested. They are increased in response to a single injection within 6 h, and the maximal concentration of proteins occurs approximately 24 h after E2 administration. The protein concentrations have essentially returned to control values by 72 h after hormone injection. The kinetics of the induction is the same for both proteins, suggesting that their increase may be coordinated. Based on the tissue and hormone specificity of the increase in the 115,000- and the 65,000-dalton proteins, they may serve as reliable markers for the study of the uterine response to E2.