Baseline and luteinizing hormone-releasing hormone-perturbed patterns of estrogen receptor distribution in anterior pituitary cell nuclei

We have previously demonstrated that LHRH elicits a direct and dramatic elevation of nuclear estradiol receptor (ER_n) levels in the anterior pituitary of young adult female rats. We now describe the effect of LHRH on subpopulations of ERn in the anterior pituitary. Intact purified pituitary nuclei were prepared from adult ovariectomized rats primed with estradiol (E₂) and incubated with or without (control) 100 pmol LHRH/pituitary equivalent for 30 rhin at 37 C. The nuclei were subjected to salt extraction, and the number of occupied and unoccupied specific E₂-binding sites in the salt-soluble and saltresistant fractions of nuclei were measured. In the control pituitary nuclei, 70% of ER_n were in the salt-soluble fraction, of which the great majority were occupied by endogenous steroid. The remaining ER_n in the salt-resistant fraction consisted of an almost equal distribution of free and occupied sites. On preincubation of the nuclei in the absence of LHRH at 37 C for 30 min, a 40% decrease in the total number of ER_n was observed, which reflected primarily a significant decrease in the number of salt-soluble ER_n. Incubation of the nuclei in the presence of LHRH led to the expected increase in total ER_n levels, and this was traced to a dramatic and significant increase in the saltresistant forms of ER_n, while number of salt-soluble ER_n was not significantly changed from the control level. Triton X-100 treatment of nonextracted nuclei had no effect on control or LHRH-induced levels of ER_n. Salt-resistant ER_n were subjected to DNase and RNase treatment, and the majority of the specific binding sites (70%) remained resistant to the digestion. The enzyme-resistant forms increased significantly in the presence of LHRH, while the enzyme-soluble forms did not change significantly. It is clear from these studies that LHRH affects a specific subpopulation of ER_n, which appears to be an integral part of the protein matrix of the nuclei. These observations pose new questions about the mechanism of peptide hormone action and advance our understanding of the molecular basis for LHRH-E2 interactions in regulation of reproductive functions.