Anterior pituitary cells: getting to know their neighbors

Fibromodulin Expression in Folliculostellate Cells and Pericytes Is Promoted by TGFβ Signaling in Rat Anterior Pituitary Gland

Fibromodulin belongs to the family of small leucine-rich proteoglycans (SLRPs), an active component of extracellular matrix. It directly binds collagens to promote fibrillogenesis and also binds transforming growth factor-beta (TGFβ) to antagonize its actions. Our previous studies of rat anterior pituitary gland revealed that fibromodulin is expressed in folliculostellate cells and pericytes. Although our recent study showed that TGFβ2 secreted from folliculostellate cells induces collagen synthesis in pericytes, the involvement of fibromodulin in TGFβ2-mediated collagen regulation has not been studied. The present study examined the effect of TGFβ2 on fibromodulin synthesis in rat anterior pituitary gland. In situ hybridization for TGFβ receptor II and immunohistological techniques revealed the presence of TGFβ receptor II in folliculostellate cells and pericytes. To confirm canonical TGFβ intracellular signaling, Smad2 immunocytochemistry was performed. Nuclear translocation of Smad2 was observed in folliculostellate cells and pericytes after TGFβ2 treatment. TGFβ2 strongly enhanced fibromodulin mRNA and protein expressions, and TGFβ2-induced mRNA expression was completely blocked by TGFβ receptor I inhibitor (SB431542). These results suggest that folliculostellate cells and pericytes exhibit canonical TGFβ2 signaling, which is associated with fibromodulin production. Thus, this is the first report to show that TGFβ signaling regulates the endogenous TGFβ antagonist fibromodulin in the gland.

Laminin and collagen modulate expression of the small leucine-rich proteoglycan fibromodulin in rat anterior pituitary gland

The anterior pituitary is a complex organ consisting of five types of hormone-producing cells, non–hormone-producing cells such as folliculostellate (FS) cells and vascular cells (endothelial cells and pericytes). We have previously shown that FS cells and pericytes produce fibromodulin, a small leucine-rich proteoglycan (SLRP). SLRPs are major proteoglycans of the extracellular matrix (ECM) and are important in regulating cell signaling pathways and ECM assembly. However, the mechanism regulating fibromodulin expression in the anterior pituitary has not been elucidated. Here, we investigate whether fibromodulin expression is modulated by major anterior pituitary ECM components such as laminin and type I collagen. Using transgenic rats expressing green fluorescent protein (GFP) specifically in FS cells, we examine fibromodulin expression in GFP-positive (FS cells) and GFP-negative cells (e.g., pericytes, endocrine cells and endothelial cells). Immunostaining and Western blot analysis were used to assess protein expression in the presence and absence of laminin or type I collagen. We confirmed fibromodulin expression in the pituitary and observed the up-regulation of fibromodulin in FS cells in the presence of ECM components. However, neither laminin nor type I collagen affected expression in GFP-negative cells. This suggests that laminin and type I collagen support the function of FS cells by increasing fibromodulin protein expression in the anterior pituitary.

Distinct pools of cAMP centre on different isoforms of adenylyl cyclase in pituitary-derived GH3B6 cells

Microdomains have been proposed to explain specificity in the myriad of possible cellular targets of cAMP. Local differences in cAMP levels can be generated by phosphodiesterases, which control the diffusion of cAMP. Here, we address the possibility that adenylyl cyclases, the source of cAMP, can be primary architects of such microdomains. Distinctly regulated adenylyl cyclases often contribute to total cAMP levels in endogenous cellular settings, making it virtually impossible to determine the contribution of a specific isoform. To investigate cAMP dynamics with high precision at the single-isoform level, we developed a targeted version of Epac2-camps, a cAMP sensor, in which the sensor was tagged to a catalytically inactive version of the Ca(2+)-stimulable adenylyl cyclase 8 (AC8). This sensor, and less stringently targeted versions of Epac2-camps, revealed opposite regulation of cAMP synthesis in response to Ca(2+) in GH(3)B(6) pituitary cells. Ca(2+) release triggered by thyrotropin-releasing hormone stimulated the minor endogenous AC8 species. cAMP levels were decreased by inhibition of AC5 and AC6, and simultaneous activation of phosphodiesterases, in different compartments of the same cell. These findings demonstrate the existence of distinct adenylyl-cyclase-centered cAMP microdomains in live cells and open the door to their molecular micro-dissection.

From galactorrhea to osteopenia: rethinking serotonin-prolactin interactions

The widespread use of the selective serotonin reuptake inhibitors (SSRIs) has been accompanied by numerous reports describing a potential association with hyperprolactinemia. Antipsychotics are commonly known to elevate serum prolactin (PRL) through blockade of dopamine receptors in the pituitary. However, there is little awareness of the mechanisms by which SSRIs stimulate PRL release. Hyperprolactinemia may result in overt symptoms such as galactorrhea, which may be accompanied by impaired fertility. Long-term clinical sequelae include decreased bone density and the possibility of an increased risk of breast cancer. Through literature review, we explore the possible pathways involved in serotonin-induced PRL release. While the classic mechanism of antipsychotic-induced hyperprolactinemia directly involves dopamine cells in the tuberoinfundibular pathway, SSRIs may act on this system indirectly through GABAergic neurons. Alternate pathways involve serotonin stimulation of vasoactive intestinal peptide (VIP) and oxytocin (OT) release. We conclude with a comprehensive review of clinical sequelae associated with hyperprolactinemia, and the potential role of SSRIs in this phenomenon.

Differentially expressed nucleolar TGF-beta1 target (DENTT) in mouse development

Differentially expressed nucleolar TGF-beta1 target (DENTT) is a recently identified gene whose mRNA is differentially affected by TGF-beta1 in TGF-beta1-responsive human lung cancer cells and who is a new member of the TSPY/TSPY-like/SET/NAP-1 (TTSN) protein superfamily. Here, we report that mouse DENTT mRNA contains a 2031-bp open reading frame that encodes a predicted polypeptide of 677-amino acids with a relative molecular mass of 77,671 Da. The mouse and human DENTT sequences show 77% and 78% homology at the nucleotide and amino acid level, respectively. Mouse DENTT is predicted to be a nuclear protein with two nuclear localization signals (NLS), two coiled-coil regions, and a domain that shows significant identity to a region that defines the TTSN superfamily. Green fluorescent protein (GFP)-tagged full-length mouse DENTT transfected into COS-7 cells showed localization predominantly in the nucleolus. Reverse transcription-polymerase chain reaction amplification, Northern hybridization, and Western blot analyses showed expression of mouse DENTT mRNA and protein throughout mouse embryogenesis. Immunohistochemical staining analysis showed that DENTT is expressed in multiple tissues in a defined spatiotemporal pattern during mouse embryogenesis. The heart and primitive brain were the first organs of the embryo that showed immunoreactivity for the DENTT antibody by day 8 of development (E8). In the developing mouse brain, the choroid plexus was intensely stained for DENTT in all stages of development. The spinal cord and dorsal root ganglia were also positive for DENTT staining beginning in the 11-day-old embryo (E11), where homogeneous immunostaining was observed throughout the developing neurons. By day 16 of development (E16), only a small subset of the neuronal population in the spinal cord and dorsal root ganglia was positively stained for DENTT. DENTT immunoreactivity increased steadily with maturation as the differentiation of cartilage and osteoblasts proceeded and reached a maximum in the growth plate during endochondral ossification. DENTT expression was also detected in multiple rodent cell types in vitro, including mouse F9 embryonal carcinoma (EC) cells. Addition of retinoic acid or sodium butyrate to F9 EC cells showed a rapid decrease in expression of DENTT protein occurring by 1 hr that continued to decrease to almost undetectable levels after 24 hr. Cotransfection of full-length mouse DENTT expression plasmid with 3TPLux or COL7A1Luc Luciferase reporter plasmids into F9 EC cells significantly increased the level of 3TPLux reporter transcription while decreasing the level of COL7A1Luc reporter transcription, suggesting that DENTT may play multiple roles in modulating transcriptional responses. These findings suggest new roles for the TTSN superfamily during embryogenesis and differentiation.

Expression of differentially expressed nucleolar transforming growth factor-beta1 target (DENTT) in adult mouse tissues

Differentially expressed nucleolar TGF-beta1 target (DENTT) is a novel member of the TSPY/TSPY-L/SET/NAP-1 (TTSN) superfamily that we have previously identified in human lung cancer cells. Here, we have investigated the expression of this protein in the adult mouse. By Western analysis, DENTT is highly expressed in the pituitary gland and moderately in the adrenals, brain, testis, and ovary. Immunohistochemical staining analysis for DENTT showed differential cytoplasmic and nuclear staining patterns in several cell types. The pituitary gland showed the highest level of immunostaining for DENTT, with strong cytoplasmic immunoreactivity in the anterior lobe, moderate levels in the posterior lobe, and a few cells showing nuclear staining in the intermediate lobe. In contrast, the intermediate lobe of the pituitary showed intense cytoplasmic staining for TGF-beta1. Nuclear and cytoplasmic staining for DENTT was present in the islets of Langerhans in the pancreas. Cytoplasmic staining for DENTT was particularly intense in the cortex of the adrenal gland, whereas the medulla showed weak nuclear staining. In the nervous system, the choroid plexus showed the highest immunoreactivity, with cortical motoneurons and Purkinje cells having relatively high levels of staining for DENTT as well. DENTT immunoreactivity was found in Leydig interstitial cells, Sertoli cells, and primary spermatocytes in the testis. In the female reproductive system, DENTT immunoreactivity was present in oocytes, thecal cells, and corpora lutea. The bronchial epithelium of the lung showed moderate levels of staining for DENTT localized to the cell nucleus. Additionally, three rodent pituitary cell lines (AtT20, GH3, and alphaT3-1, representing corticotropes, lactotropes, and gonadotropes, respectively) showed expression of DENTT. Addition of TGF-beta1 or serum to AtT20 cells increased DENTT protein production by 4 hr and, after reaching maximal levels at 2.4-fold above basal level by 8 hr, decreased, whereas no more than a 1.5-fold increase in DENTT protein occurred in GH3 or alphaT3-1 cells. Transient transfection studies showed that ectopic DENTT expression significantly increased the level of p3TP-Lux reporter transcription in AtT20 cells, but not in GH3 or alphaT3-1 cells. Interestingly, addition of TGF-beta1 had no significant effect on the ability of DENTT expression to influence p3TP-Lux reporter transcription in AtT20 cells. This report is the first detailed immunohistochemical examination of a member of the TTSN superfamily in the adult mouse. Expression of DENTT in endocrine tissues, nervous system, lung, oocytes, and thecal cells, in addition to the testis, suggests new roles for the TTSN superfamily. The differential patterns of expression of DENTT and TGF-beta1 in some tissues, including the pituitary, suggest that other factors are likely to be regulators of DENTT besides TGF-beta1.

Effect of cell density on hormonal secretion from human pituitary adenomas in vitro

Cell density effects were investigated on tumorous hormonal secretion from 10 pituitary adenomas: 3 somatotrophinomas secreting GH and PRL; 7 gonadotrophinomas, 3 co-secreted both FSH and LH, all 7 secreted LH. Enzymatically dispersed tissue was plated out in 24-well plates at 5 x 10(5), 10(5), 5 x 10(4) and 10(4) cells/well in serum-free media. Media were collected weekly for 2 weeks.

RESULTS

In 3 of 3 somatotrophinomas, GH and PRL secretion was higher (p < 0.05) at both week 1 and 2 from 10(4) cells/well, but similar at other cell densities. In all 3 gonadotrophinomas, the FSH secretory rate was highest at 5 x 10(5) cells/well which fell as cell density decreased. Conversely, in 7 of 7 gonadotrophinomas the LH secretory rate was highest at 10(4) cells/well (p < 0.01) which fell as cell density increased.

CONCLUSION

These data suggest that paracrine factors may modulate tumorous GH, PRL, FSH and LH secretion, and show that FSH and LH secretion vary inversely as cell density increases.

Pathophysiological role of the cytokine network in the anterior pituitary gland

Recent evidence has demonstrated that cytokines and other growth factors act in the anterior pituitary gland. Using the traditional criteria employed to determine autocrine or paracrine functions our review shows that, in addition to their role as lymphocyte messengers, certain cytokines are autocrine or paracrine regulators of anterior pituitary function and growth. The cytokines known to regulate and/or be expressed in the anterior pituitary include the inflammatory cytokine family (IL-1 and its endogenous antagonist, IL-1ra; TNF-alpha, and IL-6), the Th1-cytokines (IL-2 and IFN-gamma), and other cytokines such as LIF, MIF, and TGF-beta. This review examines at the cellular, molecular, and physiological levels whether: (1) each cytokine alters some aspect of pituitary physiology; (2) receptors for the cytokine are expressed in the gland; and (3) the cytokine is produced in the anterior pituitary. Should physiological stimuli regulate pituitary cytokine production, this would constitute additional proof of their autocrine/paracrine role. In this context, we analyze in this review the current literature on the actions of cytokines known to regulate anterior pituitary hormone secretion, selecting the in vivo studies that support the direct action of the cytokine in the anterior pituitary. Further support for direct regulatory action is provided by in vitro studies, in explant cultures or pituitary cell lines. The cytokine receptors that have been demonstrated in the pituitary of several species are also discussed. The endogenous production of the homologous cytokines and the regulation of this expression are analyzed. The evidence indicating that cytokines also regulate the growth and proliferation of pituitary cells is reviewed. This action is particularly important since it suggests that intrinsically produced cytokines may play a role in the pathogenesis of pituitary adenomas. The complex cell to cell communication involved in the action of these factors is discussed.

Transforming growth factor beta1 is a paracrine inhibitor of prolactin gene expression

We have shown previously that rat mammotropes produce an activity that suppresses PRL gene expression by neighboring mammotropes. Here, we tested the hypothesis that this mammotrope-derived inhibitor is transforming growth factor-beta1 (TGFbeta1). To this end, we pursued a two-pronged strategy wherein we added exogenous TGFbeta1 to primary cultures of anterior pituitary cells transfected with a rat PRL-luc construct. Measurement of luciferase activity by luminometry of extracts revealed that administration of TGFbeta1, over a range of doses shown by others to be secreted by cultures of pituitary cells, caused a significant (P < 0.05) suppression of PRL gene expression. In contrast, immunoremoval of secreted TGFbeta1 led to an elevation of PRL promoter-driven reporter activity in these cultures. In a subsequent study, we repeated these experiments with a single cell model in an attempt to determine the demographics of the cellular responses. Accordingly, we transfected (via microinjection) individual mammotropes with the rat PRL-luc construct; exposed them to TGFbeta1, its neutralizing antibody, or respective controls; and then assessed PRL gene expression in "real-time" by quantification of photons emitted by the living cells after exposure to the substrate luciferin. Our results revealed that 1) TGFbeta1 inhibited PRL gene expression in all mammotrope studied; 2) only a subgroup of mammotropes (approximately 23%) was relieved of TGFbeta1 inhibition by antibody treatment; and 3) the growth factor exerted its inhibitory effect via a paracrine, as opposed to an autocrine, mechanism. These findings identify TGFbeta1 as the paracrine agent that exerts a tonic inhibitory influence over PRL gene expression in mammotropes.

Effects of cellular interactions on calcium dynamics in prolactin-secreting cells

Signals derived from other pituitary cells can have a dramatic effect on PRL gene expression and secretion by mammotropes. However, the intracellular mechanisms by which these effects are manifested on the target cell remain unexplored. Inasmuch as calcium is a key modulator of both gene expression and hormone export in mammotropes, we evaluated the effects of cell to cell contact vs. specific cellular interactions on calcium dynamics within these cells. This was accomplished by digital-imaging fluorescence microscopy of fura-2 in pituitary cells that were isolated in culture (singles) or adjoining one other cell (doublets). After calcium imaging, we then subjected cells to immunocytochemistry for PRL. Doublets were further categorized into mammotropes attached to another mammotrope (M-M) or to a nonmammotrope (M-nonM). We then calculated and compared Mean [Ca2+]i values as well as Oscillation Indices (which reflect the oscillatory behavior of cells) in singles and doublets and found that they were not different (P > 0.05). However, the phenotype of the adjoining cell had a profound influence on both of these calcium parameters, such that the presence of one mammotrope could consistently decrease (P < 0.05) the Mean [Ca2+]i value (39.17 +/- 3.83 vs. 56.24 +/- 5.56 in M-nonM) and Oscillation Index (10.19 +/- 1.76 vs. 21.21 +/- 3.73 in M-nonM) of its neighboring counterpart. A more detailed analysis of oscillatory patterns in these cells revealed that nonoscillators were more abundant in M-M (23%) than in M-nonM (12%) doublets. Taken together, our results indicate that PRL-secreting cells convey a signal that dampens the oscillatory behavior of neighboring mammotropes. Thus, it appears that it is the phenotype rather than the physical presence of a neighbor that controls intercellular regulation of calcium dynamics among mammotropes.

Synaptoid contacts between gland cells of the anterior pituitary of the rat

BACKGROUND

There is evidence suggesting cross-talk among gland cells of the anterior pituitary. We had reported a rare form of synaptoid contact between corticotrophs in the anterior pituitary of the dog. We then found similar synaptoid contacts with different characteristics in the rat, as described in the present article.

METHODS

Male Sprague-Dawley rats were used. The anterior pituitaries were prepared for ultrastructural study of substance P immunoreactivity of the anterior pituitary. Routine preembedding immunohistochemical staining was conducted, the sections were embedded in Epon 812 (Serva Feinbiochemica, Heidelberg, New York), and ultrathin sections were prepared.

RESULTS

In the anterior pituitary of the rat, synaptoid contacts were found between corticotrophs and lactotrophs. They appeared very close to typical synapses in the central nervous system, aside from evident weakness of presynaptic density.

CONCLUSIONS

The presence of synaptoid contacts suggests a form of cross-talk between the gland cells in the anterior pituitary of the rat.

Immunoreactive TSH cells in the pituitary of female middle-aged rats after treatment with estradiol or calcium

The pituitary TSH cell structure of middle-aged (14-month-old) female Wistar rats chronically treated with estradiol dipropionate (EDP), calcium glucoheptonate (Ca) or with the combination of both was studied. TSH-producing cells were examined in the pituitary pars distalis using rabbit anti-rat beta-thyrotropin (TSH) serum and peroxidase-antiperoxidase immunohistochemistry. A stereological method for the determination of morphometric changes of the volume of TSH cells and nuclei as well as of their number and relative volume densities was used. All examined morphometric parameters in treated animals showed a significant decrease in comparison with immunoreactive TSH cells of the controls; the most significant decrease was observed in EDP-treated rats. These results together with structural features of immunoreactive TSH cells in the pituitary of middle-aged rats after chronic application of EDP or Ca indicate that both compounds inhibit these cells.

Growth hormone. A paracrine growth factor?

A number of tissues, including the brain, pituitary, immune system, placenta, mammary gland, and testis, may be self-contained units of GH regulation, production, and action. The production of GH and GH-releasing factors outside the hypothalamo-pituitary axis complements, rather than replaces, the traditional endocrine interactions between GH-releasing factors, GH, and its target tissues.

Interleukin-2 (IL-2) and IL-6 regulate c-fos protooncogene expression in human pituitary adenoma explants

We have previously shown that interleukin-2 (IL-2) and IL-6, which are expressed in the anterior pituitary, affect anterior pituitary cell proliferation in normal rats and cell lines. Here we examined their effects on the c-fos expression by human anterior pituitary adenomas. Adenoma cells in culture do not express c-fos mRNA. In adenoma explants, however, c-fos expression was detected and was regulated by IL-2 or IL-6. In different tumors (ACTH-, PRL-, GH-secreting and non functioning adenomas), these interleukins had inhibitory or stimulatory effects but the kind of response does not seem to be associated to tumor type or size. Using blocking antibodies, we observed that intrinsic IL-2 and IL-6 regulate c-fos expression in the same way. Our data suggest that IL-2 and IL-6 are not only involved in the regulation of pituitary adenoma function but may also, given the role of c-fos in cell proliferation, be implicated in the development of human pituitary adenomas.

Differential desensitization response of the neonatal and adult rat somatotroph to growth hormone-releasing hormone and phorbol ester

Elevated levels of circulating growth hormone (GH) in the perinatal animal may be caused in part by relative resistance to the desensitizing effects of GH secretagogues. We compared the effects of 4-day exposure of primary pituitary cell cultures from adult male and 2-day-old rats to GH-releasing hormone (GHRH; 10 nM) or 12-O-tetradecanoyl-phorbol-13-acetate (TPA; 1 microM) on subsequent acute GH response to these secretagogues. Prolonged exposure to GHRH reduced subsequent GHRH-induced GH release from pituitary cells of both age groups, but the reduction in GH response was significantly less in neonates than adults. In addition, GH secretion from neonatal pituitaries rose progressively during each day of GHRH exposure, to reach levels almost 7 times basal; by contrast, GH secretion from adult pituitaries increased only transiently and then declined. Prolonged exposure to TPA reduced the subsequent GH response to TPA equally in neonates and adults, but differentially affected the GH response to GHRH; TPA exposure reduced the GH response to GHRH in neonates, but not in adults. These data suggest a fundamental difference between the GH regulatory processes of neonatal and adult pituitaries. The ability of the somatotroph to exhibit attenuated GH response on exposure to secretagogue is developmentally regulated, and relative resistance of the immature somatotroph to homologous desensitization by GHRH may contribute to elevated serum GH levels during the perinatal period.