Estrogen priming is necessary for progesterone action, and it was demonstrated that the priming increases progesterone-receptor in the cytoplasm of the rabbit uterus. However, it is not yet known whether estrogen priming results in some changes of the nuclear constituent for progesterone action or not, while progesterone-receptor complex enters into the nucleus and binds to the chromatin acceptor site. The estrogen priming effect on the nucleus for the mechanism of action of progesterone is the subject of this paper.
The 274,200 x g supernatant of rabbit uterine homogenate was used as cytosol. The nuclei and chromatins were prepared by the method in Table 1. The cytosol labeled with 3H-progesterone was incubated with nuclei obtained from estrogen primed or castrated rabbit and then nuclear radioactivity was counted. Estrogen priming increased the nonspecific binding sites of progesterone-cytosol complex but did not increase the specific binding sites in the nuclei as in Fig. 2.
Uterine cytosol of castrated rabbit gives progesterone-5S binding and that of estrogen primed rabbit gives progesterone-8S and 5S bindings as in Fig. 1. 3H-progesterone-8S was fractionated as in Fig.1, and it was then incubated with the uterine chromatins obtained from the castrated or primed rabbits. Radioactivities of these chromatins were counted. Progesterone-8S complexes bound to both estrogen primed and estrogen non-primed chromatins in almost equal amount per DNA as in Fig. 3. It is indicated that the chromatin acceptor sites (per DNA) for progesterone-receptor complex is equal in spite of estrogen priming. The activity of RNA synthesis by progesterone-8S on the uterine chromatins of primed and non-primed rabbits was almost same in amount per DNA as in Fig. 4.
These results indicate that the function of estrogen priming on the mechanism of action of progesterone is the synthesis of progesterone-receptor and the increase of nonspecific binding sites of progesterone·macromolecule (non-specific binding protein) in the nucleus.
