Pituitary Sex Steroid Receptors: Localization and Function.

The pituitary contains estrogen receptor (ER), progesterone receptor (PR), and androgen receptor (AR). In accordance with immunocytochemistry, it is agreed that sex hormone receptors reside into the nucleus. All three receptors are found predominantly in gonadotrophs and lactotrophs, and less frequently in other cell types. ER plays a major role in prolactin (PRL) production and lactotroph proliferation, and protracted estrogen administration induces lactotroph hyperplasia and adenoma in rodents. Most research on PR and AR is focused on their role in the fine-tuning of gonadotropin secretion during estrous cycle. Contrary to the effect in nontumorous pituitary, estrogens can inhibit the proliferation of transplantable rat pituitary tumors and of cell lines derived from them. In humans, despite the presence of ER in all types of adenohypophysial tumors, the role of estrogen in tumor cell proliferation is still unclear. Few results indicate that tumor growth is stimulated by estrogen, and inhibited by progesterone and androgen. Novel data reveal that steroid hormones can act directly on plasma membrane or via other receptors, and interact with growth factors, oncogenes, and other transcription factors. The mechanisms by which steroid hormones control cell proliferation remain a major challenge for future research.