Endocrine Abstracts

In the pituitary, estrogens regulate several cell functions via the activation of membrane-associated estrogen receptors (mERs). We have previously reported that both 17β-estradiol (E₂) and membrane-impermeable E2-BSA induce rapid apoptotic actions in anterior pituitary (AP) cells, lactotropes and somatotropes through the activation of mERs.

In the present study, we investigated the involvement of mERα in the apoptotic action of E2 in lactotropes. We also studied the expression of mERα variants in AP cells during the estrous cycle and the regulation of this receptor expression by E2.

Cultured anterior pituitary cells from ovariectomized (OVX) Wistar rats were incubated with E2-BSA (10⁻⁹ M) in the presence of an ERα selective antagonist, MPP (10⁻⁷ M), for 120 min. Apoptotic lactotropes were identified by TUNEL and immunocytochemistry for prolactin. The expression of mERα isoforms was studied in AP cells from rats at proestrus (P) or diestrus I (D) by surface biotinylation and Western Blot. The expression of mERα was also studied in AP cells from OVX rats incubated with E2 (10⁻⁸ M) for 0–120 min.

E2-BSA increased the percentage of TUNEL-positive lactotropes (P<0.001, χ²) but failed to induce apoptosis in lactotropes coincubated with MPP. Three mERα isoforms of 66, 39 and 20–24 kDa were detected in AP cells. mERα66 expression was higher at P vs D (P<0.05), whereas mERα20-24 was lower (P<0.05). E2 increased the expression of mERα66 and mERα39 (P<0.05, ANOVA) but not of mERα20-24 at 30 min. Intracellular ERα isoforms remained unchanged during the estrous cycle or after E2 incubation.

Our results suggest that E2 induces the rapid trafficking of mERα isoforms to the plasma membrane of AP cells. At proestrus, the high circulating E2 levels could participate in the transient expression of mERα66, which may be involved in the apoptotic action of E2 in the pituitary at this stage of the estrous cycle.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This work was supported, however funding details unavailable.