Cooperative effect of E₂ and FGF2 on lactotroph proliferation triggered by signaling initiated at the plasma membrane.

In the present work, we investigated the effect of 17β-estradiol (E₂) and basic fibroblast growth factor 2 (FGF2) on the lactotroph cell-proliferative response and the related membrane-initiated signaling pathway. Anterior pituitary mixed-cell cultures of random, cycling 3-mo-old female rats were treated with 10 nM E₂, E₂ membrane-impermeable conjugated BSA (E₂-BSA), PPT (ERα agonist), and DPN (ERβ agonist) alone or combined with FGF2 (10 ng/ml) for 30 min or 4 h. Although our results showed that the uptake of BrdU into the nucleus of lactotrophs was not modified by E₂ or FGF2 alone, a significant increase in the lactotroph uptake of BrdU was observed after E₂/FGF2 coincubation, with this effect being mimicked by PPT/FGF2. These proliferative effects were blocked by ICI 182,780 or PD-98059. The involvement of membrane ER in the proliferative response of prolactin cells induced by the steroid and FGF2 coincubation was confirmed using E₂-BSA, and the association between ERα and FGF receptor was observed after E₂/FGF2 treatment by immunoprecipitation. A significant increase in the ERK1/2 expression was noted after E₂, E₂-BSA, PPT, and FGF2 alone, which was more noticeable after E₂-BSA/FGF2, E₂/FGF2, or PPT/FGF2 treatments. This study provides evidence that E₂ and FGF2 exert a cooperative effect on the lactotroph proliferation principally by signaling initiated at the plasma membrane triggering a genomic effect mediated by MEK/ERK1/2, a common signaling pathway, that finally regulates the lactotroph population, thus contributing to pituitary plasticity.