Effect of progesterone on the activity of occupied nuclear estrogen receptor in vitro.

In previous studies, we have demonstrated that progesterone administration in vivo can selectively alter estrogen receptor levels and distribution in the rat anterior pituitary. The present study represents an attempt to extend these observations to an in vitro model. Cytosolic and nuclear preparations of uterine homogenates from ovariectomized adult rats were shown to be capable of temperature-dependent estrogen-mediated receptor activation and translocation from cytosol to nuclei upon recombination. Addition of progesterone to isolated cytosol did not diminish estrogen receptor binding capacity over at least a 2 h period at 22 degrees C. Preincubation of the subcellular fractions with progesterone, followed by removal of free progesterone prior to cytosol-nuclear recombination, resulted in dramatic reduction in nuclear estrogen receptor activity. This action was equally apparent whether progesterone was introduced to the cytosolic or nuclear fraction, and was confined to the steroid-occupied subpopulation of nuclear receptors. The ability of this in vitro system to mimic the estrogen receptor-suppressive effect of progesterone provides a good model in which to analyze the biochemical basis for a direct estrogen-inhibitory effect of progesterone on estrogen action.