Antagonic effects of oestradiol in interaction with IGF-1 on proliferation of lactotroph cells in vitro

Involvement of MEK/ERK1/2 and PI3K/Akt pathways in the refractory behavior of GH3B6 pituitary tumor cells to the inhibitory effect of TGFβ1

Pituitary tumor cells have a poor response to the growth inhibitory effect of TGFβ1, possibly resulting from the cross talk of TGFβ/Smads signal with other signaling pathways, an undescribed mechanism in these tumoral cells. To address this hypothesis, we investigated whether the mitogen-activated extracellular signal-regulated kinase (MEK)/ERK1/2 and phosphoinositide-3 kinase/protein kinase B (PI3K/Akt) pathways were able to regulate the antimitogenic effect of TGFβ1 on GH3B6 cells. TGFβ1 treatment decreased the cell proliferation and induced an activation of mothers against decapentaplegic homolog 2/3 (Smad2/3), effects that were potentiated by MEK and PI3K inhibitors, thus indicating the existence of a cross talk between TGFβ1/Smad with the MEK/ERK1/2 or PI3K/Akt pathways. In addition, through immunoprecipitation assays, a direct interaction was observed between Smad2/3-ERK1/2 and Smad2/3-Akt, which decreased when the GH3B6 cells were incubated with TGFβ1 in the presence of MEK or PI3K inhibitors, thereby suggesting that the ERK1/2- and Akt-activated states were involved. These Smad2/3-ERK1/2 and Smad2/3-Akt associations were also confirmed by confocal and transmission electron microscopy. These findings indicate that the TGFβ1-antimitogenic effect in GH3B6 cells was attenuated by the MEK/ERK1/2 and PI3K/Akt pathways via modulating Smad2/3 phosphorylation. This molecular mechanism could explain in part the refractory behavior of pituitary tumor cells to the inhibitory effect of TGFβ1.

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.

17β-Estradiol stimulates the translocation of endogenous estrogen receptor α at the plasma membrane of normal anterior pituitary cells

In the present work we aimed at identifying ERα in the plasma membrane of normal anterior pituitary cells and investigated if 17β-estradiol was able to induce their subcellular redistribution. Our results show that about 8% of anterior pituitary cells expressed ERα in the plasma membrane, with the geometrical mean fluorescence intensity being increased after steroid hormone treatment. 17β-Estradiol and the selective ERα agonist PPT induced an increase of ERα expression in the plasma membrane and activated the PKCα/ERK 1/2 pathway in a time-course not compatible with genomic actions, thus supporting the notion of membrane-initiated effects. These findings suggest that 17β-estradiol stimulates the translocation of endogenous ERα to the plasma membrane, consequently modulating this ER pool and leading to cellular biological effects in normal anterior pituitary gland.

17β-oestradiol acts as a negative modulator of insulin-induced lactotroph cell proliferation through oestrogen receptor α, via nitric oxide/guanylyl cyclase/cGMP

OBJECTIVES

17β-oestradiol interacts with growth factors to modulate lactotroph cell population. However, contribution of isoforms of the oestrogen receptor in these activities is not fully understood. In the present study, we have established participation of α and β oestrogen receptors in effects of 17β-oestradiol on lactotroph proliferation induced by insulin and shown involvement of the NO/sGC/cGMP pathway.

MATERIALS AND METHODS

Cell cultures were prepared from anterior pituitaries of female rats to evaluate lactotroph cell proliferation using bromodeoxyuridine (BrdUrd) detection, protein expression by western blotting and cGMP by enzyme immunoassay.

RESULTS

In serum-free conditions, 17β-oestradiol and α and β oestrogen receptor agonists (PPT and DPN) failed to increase numbers of lactotroph cells undergoing mitosis. Co-incubation of 17β-oestradiol/insulin and PPT/insulin significantly decreased lactotroph mitogenic activity promoted by insulin alone. Both ICI 182780 and NOS inhibitors (L-NMMA and L-NAME) induced reversal of the anti-proliferative effect promoted by 17β-oestradiol/insulin and PPT/insulin. Moreover, 17β-oestradiol, PPT and insulin increased sGC α1 protein expression and inhibited β1, whereas co-incubation of 17β-oestradiol/insulin or PPT/insulin induced increases of the two isoforms α1 and β1. 17β-oestradiol and insulin reduced cGMP production, while 17β-oestradiol/insulin co-incubation increased this cyclic nucleotide.

CONCLUSIONS

Our results suggest that 17β-oestradiol is capable of arresting lactotroph proliferation induced by insulin through ER α with participation of the signalling NO/sGC/cGMP pathway.

Differential involvement of insulin-like growth factor-I and estrogen on prolactin cells in the mouse anterior pituitary

Estrogen and insulin-like growth factor-I (IGF-I) stimulate prolactin (PRL) production, release and proliferation of PRL-producing cells (PRL cells) in the anterior pituitary. PRL cells in adult estrogen receptor alpha (ERalpha) knockout (alphaERKO) mice and IGF-I knockout (IGF-IKO) mice are decreased considerably in number. To investigate a correlation between 17beta-estradiol (E2) and IGF-I on PRL production, IGF-I wild-type (WT) or IGF-IKO mice were ovariectomized at day 8 and the number of PRL cells was examined at days 20 and 60. Although PRL cell number at day 20 and WT or IGF-IKO mice ovariectomized at day 8 was similar to that in intact WT or IGF-IKO mice, PRL cells in adult WT or IGF-IKO mice ovariectomized at day 8 were significantly decreased as compared with those in intact WT or IGF-IKO mice. Therefore, estrogen is essential for PRL cell differentiation between days 20 and 60, regardless of IGF-I. While PRL cells in WT ovariectomized mice increased from days 20 to 60, those in IGF-IKO ovariectomized mice did not increase, suggesting that IGF-I modified PRL cell differentiation after day 20. ICI 182,780 (anti-estrogen) treatment canceled an increase of PRL cells in 30-day-old ovariectomized WT mice, indicating that the presence of ERalpha is important. The number of PRL cells in alphaERKO mice was similar to that in WT mice at day 20; however, PRL cells in alphaERKO mice at day 60 were not increased in number from day 20, supporting the idea that estrogen is essential for PRL cell differentiation after day 20. Finally, the percentage of PRL cells in IGF-IKO mice was decreased as compared with that in WT mice at day 20; therefore, IGF-I affects PRL cells before day 20. In conclusion, PRL cell differentiation is differently regulated by E2 and IGF-I depending on the age.

Involvement of insulin-like growth factor-I for the regulation of prolactin synthesis by estrogen and postnatal proliferation of lactotrophs in the mouse anterior pituitary

Estradiol (E2) stimulates not only secretion of prolactin (PRL) and proliferation of PRL-producing cells (PRL cells) in the anterior pituitary, but also the expression of growth factors. In insulin-like growth factor-I (IGF-I) knockout (KO) mice, the number of PRL cells is decreased and administration of IGF-I does not increase either the number of PRL cells or plasma PRL levels, indicating that IGF-I plays a pivotal role in PRL cells. The effect of E2 on PRL cells in KO mice was investigated by immunohistochemistry and real-time RT-PCR. The number of PRL cells in KO mice was significantly lower than in the wild-type (WT) control mice. E2 increased the PRL mRNA in WT and KO mice; however, an increase of PRL mRNA in KO was less than that in WT. In addition, no vasoactive intestinal peptide (VIP)-immunoreactive cells were found in KO mice, therefore IGF-I is essential for VIP expression. To investigate the roles of IGF-I on PRL cells in the postnatal development, double-immunostaining with PRL and BrdU was performed in WT and KO mice from days 5-20. The percentages of PRL cells and BrdU-labeled cells in the anterior pituitary of KO mice were lower than in WT mice. Thus, IGF-I may be responsible for proliferation and differentiation of PRL cells in this postnatal period. Differentiation and the proliferation of PRL cells are controlled by IGF-I during the postnatal development, and IGF may be a mediator of E2 action through VIP induction in PRL cells of adults.

Activation of PKC epsilon induces lactotroph proliferation through ERK1/2 in response to phorbol ester

The aim of this investigation was to contribute to current knowledge about intracellular mechanisms that are involved in lactotroph cell proliferation, by evaluating the role of PKCalpha, PKCepsilon and extracellular-signal regulated kinase (ERK) 1/2 in response to phorbol 12-myristate13-acetate (PMA). In primary pituitary cultures, the activation of protein kinase C (PKC) by PMA for 15 min stimulated lactotroph proliferation; whereas a prolonged activation for 3-8h diminished this proliferative effect. The use of PMA for 15 min-activated PKCepsilon and ERK1/2, whereas incubation with PMA for 3 h induced PKCalpha activation and attenuated the PMA-triggered phosphorylation of ERK1/2. The following inhibitors: PKCs (bisindolylmaleimide I), PKCepsilon (epsilonV1 peptide) and ERK1/2 (PD98059) prevented the mitogenic activity induced by PMA for 15 min. Lactotroph cells stimulated with PMA for 15 min showed a translocation of PKCepsilon to membrane compartment and nucleus. These results thus establish that PKCepsilon plays an essential role in the lactotroph proliferation induced by PMA by triggering signals that involve ERK1/2 activation.

Insulin-like growth factor I (IGF-I) and its receptor (IGF-1R) in the rat anterior pituitary

Few and controversial results exist on the cellular sites of insulin-like growth factor (IGF)-I synthesis and the type 1 IGF receptor (IGF-1R) in mammalian anterior pituitary. Thus, the present study analysed IGF-I and the IGF-1R in rat pituitary. Reverse transcription-polymerase chain reaction revealed IGF-I and IGF-1R mRNA expression in pituitary. The sequences of both were identical to the corresponding sequences in other rat organs. In situ hybridization localized IGF-I mRNA in endocrine cells. The majority of the growth hormone (GH) cells and numerous adrenocorticotropic hormone (ACTH) cells exhibited IGF-1R-immunoreactivity at the cell membrane. At lower densities, IGF-1 receptors were also present at the other hormone-producing cell types, indicating a physiological impact of IGF-I for all endocrine cells. IGF-I-immunoreactivity was located constantly in almost all ACTH-immunoreactive cells. At the ultrastructural level, IGF-I-immunoreactivity was confined to secretory granules in co-existence with ACTH-immunoreactivity, indicating a concomitant release of both hormones. Occasionally, IGF-I-immunoreactivity was detected in an interindividually varying number of GH cells. In some individuals, weak IGF-I-immunoreactions were also detected also in follicle-stimulating hormone and luteinizing hormone cells. Thus, IGF-I seems to be produced as a constituent in ACTH cells, possibly indicating its particular importance in stress response. Generally, IGF-I from the endocrine cells may regulate synthesis and/or release of hormones in an autocrine/paracrine manner as well as prevent apoptosis and stimulate proliferation. Production of IGF-I in GH cells may depend on the physiological status, most likely the serum IGF-I level. IGF-I released from GH cells may suppress GH synthesis and/or release by an autocrine feedback mechanism in addition to the endocrine route.

The histochemistry and cell biology vade mecum: a review of 2005–2006

The procurement of new knowledge and understanding in the ever expanding discipline of cell biology continues to advance at a breakneck pace. The progress in discerning the physiology of cells and tissues in health and disease has been driven to a large extent by the continued development of new probes and imaging techniques. The recent introduction of semi-conductor quantum dots as stable, specific markers for both fluorescence light microscopy and electron microscopy, as well as a virtual treasure-trove of new fluorescent proteins, has in conjunction with newly introduced spectral imaging systems, opened vistas into the seemingly unlimited possibilities for experimental design. Although it oftentimes proves difficult to predict what the future will hold with respect to advances in disciplines such as cell biology and histochemistry, it is facile to look back on what has already occurred. In this spirit, this review will highlight some advancements made in these areas in the past 2 years.

Ultrastructural immunolocalization of IGF-1 and insulin receptors in rat pituitary culture: evidence of a functional interaction between gonadotroph and lactotroph cells

We have investigated the expression of receptors for insulin and insulin-like growth factor 1 (IGF-1) in rat pituitary cells in vitro and examined the morphological and proliferative changes induced in adenohypophyseal cells by insulin and IGF-1. The proliferation of lactotrophs was determined by double-immunostaining for bromodeoxyuridine and prolactin. Incubation with insulin (10, 100 or 1000 ng/ml) or IGF-1 (5, 30 or 100 ng/ml) for 48 or 72 h significantly increased the number of lactotrophs undergoing mitosis. Co-incubation of insulin or IGF-1 with genistein (25 microM), an inhibitor of the tyrosine kinase receptor, reduced the proliferation of lactotrophs elicited by the hormone and the growth factor. The receptors for insulin and IGF-1 were localized in intact pituitary cells by ultrastructural immunocytochemistry with the colloidal gold-protein A technique. Gonadotrophs expressed both receptors, specific labelling being restricted to this cell type. Electron-microscopical observations of pituitary cell cultures incubated with insulin or IGF-1 revealed gonadotroph cells exhibiting the fine-structural features of enhanced protein synthetic activity. These findings suggest that both insulin and IGF-1 are able to induce the proliferation of lactotrophs through an indirect mechanism mediated by a factor synthesized by gonadotroph cells, in addition to stimulating the biosynthetic activity of the gonadotroph in a direct manner.

Recent progress in histochemistry and cell biology: the state of the art 2005

Advances in the field of histochemistry, a multidisciplinary area including the detection, localization and functional characterization of molecules in single cells and complex tissues, often drives the attainment of new knowledge in the broadly defined discipline of cell biology. These two disciplines, histochemistry and cell biology, have been joined in this journal to facilitate the flow of information with celerity from technical advancement in histochemical procedures, to their utilization in experimental models. This review summarizes advancements in these fields during the past year.