Regulation of pituitary DNA synthesis during different reproductive states in the female rat: role of estrogens and prolactin

Variations in the adenohypophysis of the expression of proliferating cellular nuclear antigen, oestrogen and androgen receptors in relation to gonadal steroids during pregnancy of viscachas (Lagostomus maximus maximus)

Viscachas are native rodents of South America that present a long pregnancy of ~154 days. In this work, we analysed variations in the expression of proliferating cellular nuclear antigen, oestrogen and androgen receptors (ERα and AR) in pituitary pars distalis (PD) and pars tuberalis (PT) in relation to oestradiol and testosterone serum levels in non-pregnant and pregnant viscachas. In PD, cell proliferation increased with pregnancy and lactotrophs proliferated during mid-pregnancy (MP). ERα nuclear-immunoreactive cells (ERαn-ir) were maximal in late pregnancy and AR expression did not vary during pregnancy. In PT, cell proliferation and AR expression increased during pregnancy, but ERα expression was very scarce. The immunostaining pattern of receptors was different in PD and PT. The peak of serum oestradiol and testosterone occurred during MP. Our results suggest that cell proliferation and gonadal receptors might be differentially regulated in the pituitary by oestradiol and testosterone during viscacha pregnancy.

The expansion of adult stem/progenitor cells and their marker expression fluctuations are linked with pituitary plastic adaptation during gestation and lactancy

Extensive evidence has revealed variations in the number of hormone-producing cells in the pituitary gland, which occur under physiological conditions such as gestation and lactancy. It has been proposed that new hormone-producing cells differentiate from stem cells. However, exactly how and when this takes place is not clear. In this work, we used immunoelectron microscopy to identify adult pituitary stem/progenitor cells (SC/P) localized in the marginal zone (MZ), and additionally, we detected GFRa2-, Sox2-, and Sox9-positive cells in the adenoparenchyma (AP) by fluorescence microscopy. Then, we evaluated fluctuations of SC/P mRNA and protein level markers in MZ and AP during gestation and lactancy. An upregulation in stemness markers was shown at term of gestation (AT) in MZ, whereas there were more progenitor cell markers in the middle of gestation and active lactancy. Concerning committed cell markers, we detected a rise in AP at beginning of lactancy (d1L). We performed a BrdU uptake analysis in MZ and AP cells. The highest level of BrdU uptake was observed in MZ AT cells, whereas in AP this was detected in d1L, followed by a decrease in both the MZ and AP. Finally, we detected double immunostaining for BrdU-GFRa2 in MZ AT cells and BrdU-Sox9 in the AP d1L cells. Taken together, we hypothesize that the expansion of the SC/P niche took place mainly in MZ from pituitary rats in AT and d1L. These results suggest that the SC niche actively participates in pituitary plasticity during these reproductive states, contributing to the origin of hormone cell populations.

Estradiol partially recapitulates murine pituitary cell cycle response to pregnancy

Because pregnancy and estrogens both induce pituitary lactotroph hyperplasia, we assessed the expression of pituitary cell cycle regulators in two models of murine pituitary hyperplasia. Female mice were assessed during nonpregnancy, pregnancy, day of delivery, and postpartum. We also implanted estradiol (E(2)) pellets in female mice and studied them for 2.5 months. Pituitary weight in female mice increased 2-fold after E(2) administration and 1.4-fold at day of delivery, compared with placebo-treated or nonpregnant females. Pituitary proliferation, as assessed by proliferating cell nuclear antigen and/or Ki-67 staining, increased dramatically during both mid-late pregnancy and E(2) administration, and lactotroph hyperplasia was also observed. Pregnancy induced pituitary cell cycle proliferative and inhibitory responses at the G(1)/S checkpoint. Differential cell cycle regulator expression included cyclin-dependent kinase inhibitors, p21(Cip1), p27(Kip1), and cyclin D1. Pituitary cell cycle responses to E(2) administration partially recapitulated those effects observed at mid-late pregnancy, coincident with elevated circulating mouse E(2), including increased expression of proliferating cell nuclear antigen, Ki-67, p15(INK4b), and p21(Cip1). Nuclear localization of pituitary p21(Cip1) was demonstrated at mid-late pregnancy but not during E(2) administration, suggesting a cell cycle inhibitory role for p21(Cip1) in pregnancy, yet a possible proproliferative role during E(2) administration. Most observed cell cycle protein alterations were reversed postpartum. Murine pituitary meets the demand for prolactin during lactation associated with induction of both cell proliferative and inhibitory pathways, mediated, at least partially, by estradiol.

Differential regulation of prolactin release and lactotrope proliferation during pregnancy, lactation and the estrous cycle

The proestrous surge of prolactin (PRL) secretion and subsequent proliferation of lactotropes at estrus have been suggested to be induced by a common hypothalamic hormone. We investigated changes in lactotrope proliferation at other reproductive stages of female rats when PRL secretion was stimulated. To assess proliferative activity of lactotropes, incorporation of 5-bromo-2'-deoxyuridine (BrdU) into DNA was measured by double immunostaining for PRL and BrdU. BrdU-labeling indices, determined by BrdU injections at 10.00 h, revealed low levels of proliferative activity of lactotropes at the reproductive stages including diestrus, days 6 and 13 of pregnancy, and day 6 of lactation while high levels were detected on estrus and the day of parturition. When BrdU-labeling indices were determined at 3-hour intervals through day 6 of pregnancy to find an increase in lactotrope proliferation which might occur at times other than 10.00 h, proliferative activity of lactotropes remained at low levels with a slight increase in the afternoon. Such a diurnal change as observed in early pregnancy was not detected on day 13 of pregnancy. In contrast, short-interval determinations of BrdU-labeling indices during a period from day 20 of pregnancy to day 2 of lactation revealed a marked increase in proliferative activity on the day of parturition with a peak at 18.00 h, which was comparable to that observed at estrus. To investigate involvement of ovarian steroids in suppression of lactotrope proliferation as observed during early pregnancy and lactation, ovariectomized and pup-removed lactating rats were given one of treatment combinations of estradiol and suckling. In pup-removed lactating rats, estradiol treatment alone induced neither a PRL surge nor an increase in BrdU-labeling indices. Suckling stimuli, which were effective in increasing serum PRL concentrations irrespective of estradiol treatment, elicited a marked increase in BrdU-labeling indices in the presence of estradiol but not in its absence. These results suggest that proliferative activity of rat lactotropes does not necessarily correlate with PRL secretion during pregnancy and lactation. In contrast to PRL release, lactotrope proliferation requires both a hypophysiotropic stimulatory input from the hypothalamus and a sensitizing action of estradiol, an observation which may account for the fact that proliferation does not occur during pregnancy and lactation in spite of elevated PRL release.

Estrogen down-regulates mRNA encoding the exocytotic protein SNAP-25 in the rat pituitary gland

Exocytosis is dependent on specific proteins that are located at the secretory granule membrane, in the cytoplasm or at the plasma membrane. The mRNA expression of synaptosomal-associated protein of 25 kDa (SNAP-25) isoforms SNAP-25a and SNAP-25b, vesicle associated membrane protein (VAMP) 2, mammalian homologue of unc-18 (munc-18) and Hrs-2 was studied in the pituitary of ovariectomized rats after subcutaneous insertion of capsules containing estrogen or placebo using in situ hybridization. Estrogen treatment (0.25 mg estradiol) significantly decreased SNAP-25a (32%; 10%) and SNAP-25b (25%; 22%) mRNA levels in the anterior and intermediate lobes, respectively, whereas VAMP-2, munc-18 and Hrs-2 mRNA levels remained unchanged. The results suggest that estrogen selectively regulates SNAP-25 transcription in the pituitary gland, but leaves VAMP-2, munc-18 and Hrs-2 mRNA levels unaffected.

A comparison of the temporal effects of estradiol and diethylstilbestrol on pituitary content of DNA, prolactin mRNA and prolactin and on serum prolactin levels in ovariectomized Holtzman rats

Ovariectomized Holtzman rats implanted with Silastic capsules of estradiol-17 beta (E2) or diethylstilbestrol (DES) were decapitated at intervals between 4 hours and 10 days of steroid treatment and the anterior pituitaries were collected and analyzed for DNA, prolactin mRNA, and prolactin content. Serum prolactin was also determined. Untreated ovariectomized rats decapitated at 10 days after ovariectomy served as controls. Pituitary DNA levels were not consistently affected by either steroid until 10 days of treatment when levels approximately doubled. In contrast both steroids increased prolactin mRNA and pituitary and serum prolactin levels within 4-8 hours of treatment. Prolactin mRNA levels increased throughout the 10 day period. Pituitary prolactin levels paralleled mRNA levels until day 7 then decreased significantly (P < 0.05) at day 10. Serum prolactin levels increased at 4 and 8 hours, decreased slightly at 24 hours only to increase dramatically at 48 hours. Then at 4 and 7 days of treatment serum levels decreased to the levels seen at 24 hours, followed by a significant (P < 0.05) increase at day 10. Both steroids were equipotent in their effects on serum prolactin; however, DES was significantly (P < 0.05) more potent in increasing pituitary prolactin content than was E2; whereas, E2 was more potent in increasing prolactin mRNA and pituitary DNA. It is concluded that both natural and synthetic estrogens have rapid and sustained, although not equivalent, actions on the levels of prolactin mRNA in the rat. Further, pituitary DNA and pituitary and serum prolactin do not consistently follow the temporal pattern of change seen for prolactin gene expression.

Growth and cell cycle regulation of mRNA levels in GH3 cells

In the rat anterior pituitary gland, estrogen increases both prolactin (PRL) mRNA levels and stimulates the proliferation of PRL-producing cells. The temporal sequence of these events suggests that PRL gene expression may be coordinated with cell proliferation. We investigated the relationship between cell cycle progression and the accumulation of the PRL mRNA, as well as several other mRNAs, in the rat pituitary tumor GH3 cell line. Serum-deprived cells progressed from G0 to S phase in 20-24 h following serum stimulation. During this time, beta-actin mRNA levels increased 7-fold in 5 h, then returned to basal levels prior to the beginning of S phase. Histone H1 mRNA levels increased approximately 3-fold as cells entered S phase. These data are consistent with the cell cycle-dependent regulation of beta-actin and histone H1 gene expression reported for other cell types. Glucocorticoid receptor mRNA levels were barely detectable in serum-deprived cells but rapidly increased 3- to 5-fold following serum stimulation. This increase resulted in glucocorticoid receptor mRNA levels that were equivalent to those seen in cells maintained in serum-containing medium, suggesting that serum factors regulate glucocorticoid receptor gene expression. In contrast to these changes in gene expression, the levels of PRL and growth hormone (GH) mRNAs gradually increased 2-fold while the cells progressed through G1 phase. Similarly, in cells synchronized to progress through S and G2 phases following aphidicolin treatment, histone H1 gene expression showed a specific increase in S phase cells, whereas PRL and GH mRNA levels changed little with cell cycle progression. These results indicate that the levels of PRL and GH mRNAs are not regulated in a cell cycle-dependent manner. When changes in estrogen responsiveness were determined during the cell cycle, we found that estradiol treatment was capable of increasing PRL mRNA accumulation independent of cell cycle progression and cell cycle distribution in synchronized GH3 cells. These results support the hypothesis that the hormonal regulation of PRL gene expression is not significantly affected by cell growth.