Expression of membrane-associated ERalpha in lactotropes and somatotropes.

S. ZARATE; G. JAITA; V. ZALDIVAR; D. RADL; G. EIJO; J. FERRARIS; D. PISERA; A. SEILICOVICH

Carefree

Conferencia; FASEB Summer reasearch Conferences, Extra-nuclear ateroid receptors: integration with multiple signaling pathways; 2008

Rapid actions of estrogens are elicited through estrogen receptors (ERs) associated to the plasma membrane, which trigger several intracellular signaling cascades. The activation of signal transduction pathways are cell-context specific and have profound implications in estrogen regulation of different cell functions such as differentiation, proliferation or death. In the rat, anterior pituitary cells follow a cyclic pattern of proliferation and death throughout the estrous cycle, in which lactotropes are the cells with the highest turnover. Although estradiol induces hyperprolactinemia and lactotrope proliferation, the rate of anterior pituitary cell apoptosis has a peak at proestrous, when estrogen levels are the highest. Previous studies from our laboratory showed that both a membrane-impermeant 17b-estradiol conjugate and the nongenotropic signaling activator estren induce a rapid apoptotic action in anterior pituitary cells, lactotropes and somatotropes, an effect completely abrogated by ICI 182,780, the pure antagonist of classical ERs. Although a great bulk of evidence recognizes the existence of ERs at or near the plasma membrane, their identity is still a matter of controversy. In the present study, we investigated the expression of a membrane form of the classical ERa (mERa) on the surface of anterior pituitary (AP) cells, lactotropes and somatotropes from female Wistar rats killed at random or selected stages of the estrous cycle. In order to achieve this aim, indirect immunofluorescence assays for mERa were performed on dispersed, fixed, non-permeabilized AP cells previously immunostained for prolactin (PRL) or growth hormone (GH). The cells were then analyzed by flow cytometry using a FACScan. Our results show that in dispersed cells from female rats killed at random stages of the estrous cycle, mERa is expressed in 21.5 ± 1.0 % of total AP cells. Within the lactotrope population, mERa is expressed in 41.5 ± 6.6 % of PRL-bearing cells whereas within the somatotrope population mERa is expressed in 12.7 ± 2.8 % of GH-bearing cells. When we studied the expression of mERa on the surface of these cells from rats killed at proestrus (P) or diestrus (D), there was no difference in the percentage of mERa in total AP cells (P: 24.4 ± 1.5 %; D: 20.8 ± 0.5 %) or GH-poducing cells (P: 15.3 ± 1.4 %; D: 20.0 ± 0.1 %) between these two stages of the estrous cycle. However, mERa expression in PRL-producing cells was higher at proestrus than at diestrus (P: 38.2 ± 0.9 %; D: 26.2 ± 2.4 % p<0.05, Student t test). Taken together, these results show the presence of a membrane version of ERa on the surface of anterior pituitary cells, GH- and PRL-producing cells. This membrane receptor seems to be differentially expressed throughout the estrous cycle only in PRL-bearing cells, which are cells with the highest turnover in the anterior pituitary gland. The presence of mERs would allow AP cells to rapidly and accurately assess changes in hormonal levels and respond to them. In this way, AP cells and especially lactotropes could change their sensitivity to proapoptotic or mitotic stimuli depending on the pattern of circulating gonadal hormones, as occurs throughout the estrous cycle. Since we did not dual label secretory cells, we cannot rule out the possibility that at least part of the mERa positive lactotropes and somatotropes are in fact bihormonal mammosomatotropes, or even multihormonal cells that may express ERs.