Activation of anterior pituitary folliculo-stellate cells in the formation of estrogen-induced prolactin-secreting tumors

The role of the tumour microenvironment in the angiogenesis of pituitary tumours

PURPOSE

Angiogenesis has been studied in pituitary neuroendocrine tumours (PitNETs), but the role of the tumour microenvironment (TME) in regulating PitNET angiogenesis remains unknown. We aimed to characterise the role of TME components in determining the angiogenetic PitNET profile, focusing on immune cells and tumour-derived cytokines.

METHODS

Immune cells were studied by immunohistochemistry in 24 human PitNETs (16 non-functioning-PitNETs (NF-PitNETs) and 8 somatotrophinomas): macrophages (CD68, CD163, HLA-DR), cytotoxic (CD8) and T helper (CD4) lymphocytes, regulatory T cells (FOXP3), B cells (CD20) and neutrophils (neutrophil elastase); endothelial cells were assessed with CD31. Five normal pituitaries (NP) were included for comparison. Microvessel density and vascular morphology were estimated with ImageJ. The cytokine secretome from these PitNETs were assessed on culture supernatants using a multiplex immunoassay panel.

RESULTS

Microvessel density/area was higher in NP than PitNETs, which also had rounder and more regular vessels. NF-PitNETs had vessels of increased calibre compared to somatotrophinomas. The M2:M1 macrophage ratio correlated with microvessel area. PitNETs with more CD4+ T cells had higher microvessel area, while tumours with more FOXP3+ cells were associated with lower microvessel density. PitNETs with more B cells had rounder vessels. Of the 42 PitNET-derived cytokines studied, CCL2, CXCL10 and CX3CL1 correlated with microvessel density and vessel architecture parameters.

CONCLUSIONS

M2 macrophages appear to play a role in PitNET neovascularisation, while B, CD4+ and FOXP3+ lymphocytes, as well as non-cellular TME elements such as CCL2, CXCL10 and CX3CL1, may also modulate the angiogenesis of PitNETs.

The tumour microenvironment of pituitary neuroendocrine tumours

The tumour microenvironment (TME) includes a variety of non-neoplastic cells and non-cellular elements such as cytokines, growth factors and enzymes surrounding tumour cells. The TME emerged as a key modulator of tumour initiation, progression and invasion, with extensive data available in many cancers, but little is known in pituitary tumours. However, the understanding of the TME of pituitary tumours has advanced thanks to active research in this field over the last decade. Different immune and stromal cell subpopulations, and several cytokines, growth factors and matrix remodelling enzymes, have been characterised in pituitary tumours. Studying the TME in pituitary tumours may lead to a better understanding of tumourigenic mechanisms, identification of biomarkers useful to predict aggressive disease, and development of novel therapies. This review summarises the current knowledge on the different TME cellular/non-cellular elements in pituitary tumours and provides an overview of their role in tumourigenesis, biological behaviour and clinical outcomes.

The anterior pituitary gap junctions: potential targets for toxicants

The anterior pituitary regulates endocrine organs and physiological activities in the body. Environmental pollutants and drugs deleterious to the endocrine system may affect anterior pituitary activity through direct action on anterior pituitary cells. Within the gland, endocrine and folliculostellate cells are organized into and function as individual tridimensional networks, each network regulating its activity by coordinating the connected cells' responses to physiological or pathological cues. The gap junctions connecting endocrine cells and/or folliculostellate cells allow transmission of information among cells that is necessary for adequate network function. Toxicants may affect gap junctions as well as the physiology of the anterior pituitary. However, whether toxicants effects on anterior pituitary hormone secretion involve gap junctions is unknown. The folliculostellate cell gap junctions are sensitive to hormones, cytokines and growth factors. These cells may be an interesting experimental model for evaluating whether toxicants target anterior pituitary gap junctions.

Ultrastructural changes in lactotrophs and folliculo-stellate cells in the ovine pituitary during the annual reproductive cycle

In seasonal mammals living in temperate zones, photoperiod regulates prolactin secretion, such that prolactin plasma concentrations peak during the summer months and are lowest during the winter. In sheep, a short-day breeder, circulating prolactin has important modulatory effects on the reproductive system via inhibitory actions on pituitary gonadotrophs and hypothalamic gonadotrophin-releasing hormone release. The exact cellular mechanisms that account for the chronic hypersecretion of prolactin during the summer is not known, although evidence supports an intrapituitary mechanism regulated by melatonin. Folliculo-stellate (FS) cells are non-endocrine cells that play a crucial role in paracrine communication within the pituitary and produce factors controlling prolactin and gonadotrophin release. The present study examined the morphology of the FS and lactotroph cell populations and their distribution in the sheep pituitary during the annual reproductive cycle. Ovine pituitary glands were collected in the winter (breeding season; BS) and summer (nonbreeding season; NBS) and were prepared for quantitative electron microscopy to assess the effects of season on FS and lactotroph cell density, morphology and distribution, as well as on junctional contacts between cells. It was found that lactotrophs in the NBS are larger in size and contain more numerous PRL granules than lactotrophs in the BS. FS cells were also larger in the NBS compared to BS and showed altered morphology such that, in the BS, long cell processes surrounded clusters of adjacent secretory cells. Although no significant change in the number of junctions was observed between lactotrophs and FS cells, or lactotrophs and gonadotrophs, there was a significant increase in the number of adherens junctions between lactotrophs and between FS cells. These findings demonstrate seasonal plasticity in the morphology of lactotrophs and FS cells that reflect changes in PRL secretion.

Differences between rat strains in the development of PRL-secreting pituitary tumors with long-term estrogen treatment: In vitro insulin-like growth factor-1-induced lactotroph proliferation and gene expression are affected in Wistar-Kyoto rats with low estrogen-susceptibility

There are differences in the susceptibility of rat strains to pituitary growth and lactotroph proliferation caused by long-term treatment with estrogens. To investigate the pituitary mechanism for this strain difference in estrogen-induced lactotroph proliferation, we compared the abilities of 17-β estradiol (E2) and insulin-like growth factor-1 (IGF-1) to modulate lactotroph proliferation and gene expression in vitro in Wistar and Wistar-Kyoto (WKY) rats. These two strains of rats have a high and very low susceptibility to estrogen, respectively. Long-term in vivo treatment with E2 was confirmed to markedly increase pituitary weight and lactotroph proliferation in ovariectomized Wistar, but not in WKY rats. Pituitary lactotrophs in primary cultures showed similar proliferative responsiveness to the culture condition-dependent, stimulatory and inhibitory actions of E2 in both strains. The only difference in lactotroph proliferation in vitro was a lower response to IGF-1 in WKY cells compared with Wistar cells. This difference in proliferation was associated with strain differences in IGF-1-induced gene expression in Wistar and WKY cultured cells. Of the genes tested, IGF-1-induced expression of the Wnt4, Stc1, Mybl1, and Myc genes was attenuated or abolished in WKY cells. These results suggest that the proliferative response to estrogen in lactotrophs in primary culture does not reflect the proliferative response to long-term estrogen treatment observed in vivo in Wistar and WKY rats. The strain difference in proliferation and gene expression to IGF-1 may be implicated in the variable degree of susceptibility for lactotroph proliferation observed in different strains of rats following long-term estrogen treatment.

Cellular and molecular specificity of pituitary gland physiology

The anterior pituitary gland has the ability to respond to complex signals derived from central and peripheral systems. Perception of these signals and their integration are mediated by cell interactions and cross-talk of multiple signaling transduction pathways and transcriptional regulatory networks that cooperate for hormone secretion, cell plasticity, and ultimately specific pituitary responses that are essential for an appropriate physiological response. We discuss the physiopathological and molecular mechanisms related to this integrative regulatory system of the anterior pituitary gland and how it contributes to modulate the gland functions and impacts on body homeostasis.

Non-hormonal cell types in the pituitary candidating for stem cell

Hormone balances in the body are primarily governed by the hypothalamus-pituitary system. For its pivotal role, the pituitary gland relies on an assortment of different hormone-producing cell types, the proportions of which dynamically change in response to fluctuating endocrine demands. Mechanisms of pituitary cellular plasticity are at present far from understood, and may include proliferation and transdifferentiation of hormonal cells. Whether new cells also originate by recruitment from stem cells is unsettled, although this idea has frequently been proposed. Here, I will review these data by focusing on the non-hormonal cell types that have been advanced as candidates for the pituitary stem cell position.

Estrogen actions on lactotroph proliferation are independent of a paracrine interaction with other pituitary cell types: a study using lactotroph-enriched cells

The mitogenic action of estrogen on estrogen-responsive tissues is suggested to be mediated by paracrine growth factors secreted from neighboring estrogen receptor-positive cells. Using pituitary lactotrophs in primary culture, on which estrogen exerts both mitogenic and antimitogenic actions in a cell context-dependent manner, we investigated whether a paracrine cell-to-cell interaction with other pituitary cell types was required for estrogen action. In pituitary cells, enriched for lactotrophs by 85% using differential sedimentation on a discontinuous Percoll gradient, 17beta-estradiol (E2) showed an antimitogenic action on lactotrophs in the presence of IGF-I, which was similar to that in control unenriched cells. Mitogenic actions were also seen in lactotroph-enriched cells when E2 was administered alone, in combination with serum, or in combination with the adenylate cyclase activator forskolin. Similar results were obtained in 90% lactotroph-enriched cells collected by fluorescence-activated cell sorting from transgenic rats expressing enhanced green fluorescent protein under the control of the prolactin promoter. The putative role of basic fibroblast growth factor (bFGF) as a paracrine factor mediating the mitogenic action of estrogen was not supported by the results that: 1) bFGF inhibited lactotroph proliferation; 2) immunoneutralization of bFGF failed to block E2-induced proliferation; and 3) cellular bFGF levels were not altered by E2 treatment. These results suggest that the antimitogenic and mitogenic actions of estrogen on lactotrophs do not require paracrine signals from other pituitary cell types and that estrogen directly influences lactotroph proliferation.

Stem cells in the postnatal pituitary?

Tissue-specific stem cells are uncovered in a growing number of organs by their molecular expression profile and their potential for self-renewal, multipotent differentiation and tissue regeneration. Whether the pituitary gland also contains a pool of versatile 'master' cells that drive homeostatic, plastic and regenerative cell ontogenesis is at present unknown. Here, I will give an overview of data that may lend support to the existence of stem cells in the postnatal pituitary. During the many decades of pituitary research, various approaches have been used to hunt for the pituitary stem cells. Transplantation and regeneration studies advanced chromophobes as possible source of new hormonal cells. Clonogenicity approaches identified pituitary cells that clonally expand to floating spheres, or to colonies in adherent cell cultures. Behavioural characteristics and changes of marginal, follicular and folliculostellate cells during defined developmental and (patho-)physiological conditions have been interpreted as indicative of a stem cell role. Expression of potential stem cell markers like nestin, as well as topographical localization in the marginal zone around the cleft has also been considered to designate pituitary stem cells. Finally, a 'side population' was recently identified in the postnatal pituitary which in many other tissues represents a stem cell-enriched fraction. Taken together, in the course of the long-standing study of the pituitary, several arguments have been presented to support the existence of stem cells, and multiple cell types have been placed in the spotlight as possible candidates. However, none of these cells has until now unequivocally been shown to meet all quintessential characteristics of stem cells.

Modulation of GJA1 turnover and intercellular communication by proinflammatory cytokines in the anterior pituitary folliculostellate cell line TtT/GF

Our previous studies have advanced the idea that the folliculostellate cell GJA1 (gap junction membrane channel protein alpha1; previously known as connexin 43)-mediated gap junctions contribute to the establishment of an intercellular network that regulates the paracrine messages and the endocrine response within the anterior pituitary. The folliculostellate cells are targets for growth factors and cytokines that modulate hormone secretion. Proinflammatory cytokines modulate the cell-to-cell communication in many tissues of the body. The present study measured the effect of the proinflammatory cytokines tumor necrosis factor and interleukin-1 on the GJA1-mediated intercellular communication, specifically the expression, localization, degradation, and phosphorylation status of GJA1 in the folliculostellate cell line TtT/GF. The GJA1 localized to the plasma membrane and to minute cytoplasmic vesicles in the perinuclear area. Using different antibodies that recognize distinctly the nonphosphorylated from the phosphorylated forms of GJA1, we showed that nonphosphorylated GJA1 in Ser-368 (NP-GJA1) localized chiefly in the cytoplasm, whereas GJA1 phosphorylated in Ser-368 (P-GJA1) localized to the plasma membrane in controls. The cytokine treatment transiently increased 1) GJA1, NP-GJA1, and P-GJA1 levels; 2) NP-GJA1 and P-GJA1 degradation by both the lysosomal and proteasomal pathways; and 3) cell-to-cell communication in TtT/GF cells. The results suggest that the cytokine-evoked, transient enhancement of folliculostellate cell-mediated intercellular communication contributes to the coordination of the response among folliculostellate cells.

History and perspectives of pituitary folliculo-stellate cell research

Historically, the study of folliculo-stellate (FS) cells of the anterior pituitary dates back to the onset of electron microscopical observation of the pituitary gland. The morphological and electrophysiological characteristics, topographical distribution and contribution to intercellular junctions of these FS cells have been instrumental to the understanding of their putative function. Moreover, many studies have documented the role of FS cells as a source of newly discovered peptides, growth factors and cytokines. Quantitative immunohistochemical observation of FS cells in situ and functional in vitro studies, using either cultured FS cells or cells from an immortalized FS cell line, forwarded the notion of immunophenotypical and functional heterogeneity of the FS cell group. Double immunolabeling with a classical FS cell marker (S-100 protein) and with major histocompatibility complex class II markers characteristic for dendritic cells (DC) have shown a considerable overlap of FS cells with DC. The latter cells are immunocompetent cells belonging to the mononuclear phagocyte system. In this review, the FS cell heterogeneity is discussed with respect to the question of their embryological origin and developmental fate and with respect to the physiological relevance of functionally heterogeneous subpopulations. Recent findings of a myeloid origin of part of the interstitial cells of the anterior pituitary are confronted by other developmental paradigms of pituitary cell differentiation. The possibility that FS cells represent an adult stem cell population of the pituitary is critically examined. Also the physiological role of FS cells in the interferon-gamma- and nitric oxide-mediated effects on pituitary hormone secretion is discussed. New approaches for the study of this enigmatic cell group using immortalized cell lines and new markers for an hitherto unrecognized pituitary cell population, the so-called 'side population', are evaluated.

Mediation of basic fibroblast growth factor-induced lactotropic cell proliferation by Src-Ras-mitogen-activated protein kinase p44/42 signaling

Basic fibroblast growth factor (bFGF), which is secreted from folliculostellate cells in the anterior pituitary, is known to be involved in the communication between folliculostellate cells and lactotropes during estradiol-induced lactotropic cell proliferation. We studied the role of MAPK p44/42 in bFGF-regulated cell proliferation using enriched lactotropes and the lactotrope-derived PR1 cell line. In cell cultures, bFGF increased cell proliferation of PR1 cells and enriched lactotropes. In both of these cell populations, bFGF also increased phosphorylation of MAPK p44/42. U0126, an inhibitor of MAPK p44/42, blocked the bFGF-induced activation of MAPK p44/42 as well as the bFGF-induced cell proliferation of enriched lactotropes and PR1 cells. Treatment of PR1 cells with bFGF increased the activity of Ras p21, whereas overexpression of a dominant negative mutant of Ras p21 abrogated the bFGF-induced activation of MAPK p44/42 in these cells. Furthermore, the Src kinase inhibitor PP1 suppressed bFGF-induced activation of MAPK p44/42 in both enriched lactotropes and PR1 cells. The Src kinase inhibitor PP1 also reduced bFGF activation of Ras p21 and cell proliferation in PR1 cells. On the other hand, the bFGF-induced activation of MAPK p44/42 in enriched lactotropes and PR1 cells was not affected by protein kinase C inhibitors. These data suggest that bFGF induction of lactotropic cell proliferation is possibly mediated by activation of Src kinase, Ras p21, and MAPK p44/42.

Folliculostellate cells determine the susceptibility of lactotropes to estradiol's mitogenic action

Estradiol is known to increase lactotropic cell proliferation, but estradiol susceptibility varies among human populations and among various strains of rats. We had reported that folliculostellate (FS) cells regulate estradiol's mitogenic action on lactotropes; therefore, we studied their role in determining the susceptibility to estradiol in a high estradiol-responsive rat strain, Fischer 344 (F344), and in a low-responsive strain, Sprague Dawley (SD). Determination of total S-100-positive FS cells in the pituitary revealed that F344 rats have significantly more FS cells than do SD rats. Estradiol treatment did not change the number of FS cells in both F344 and SD rats. When cotransplanted with F344 pituitaries under the kidney capsule or cocultured with F344-derived lactotropes in vitro, FS cells derived from F344 rats increased estradiol's mitogenic action. They also increased estradiol's mitogenic action on SD-derived lactotropes in primary cultures. However, SD-derived FS cells failed to increase estrogen's action on F344- or SD-derived lactotropes. The levels of basic fibroblast growth factor production and secretion by TGF-beta 3 and estradiol were much higher in F344-derived FS cells than in SD-derived FS cells. However, the lactotropes' growth response to basic fibroblast growth factor was similar in both strains. These data suggest that cell-cell interaction between FS cells and lactotropes regulates estradiol's mitogenic action on lactotropes and also determines lactotrope susceptibility to the steroid.

Angiogenesis in endocrine tumors

Angiogenesis is the process of new blood vessel development from preexisting vasculature. Although vascular endothelium is usually quiescent in the adult, active angiogenesis has been shown to be an important process for new vessel formation, tumor growth, progression, and spread. The angiogenic phenotype depends on the balance of proangiogenic growth factors such as vascular endothelial growth factor (VEGF) and inhibitors, as well as interactions with the extracellular matrix, allowing for endothelial migration. Endocrine glands are typically vascular organs, and their blood supply is essential for normal function and tight control of hormone feedback loops. In addition to metabolic factors such as hypoxia, the process of angiogenesis is also regulated by hormonal changes such as increased estrogen, IGF-I, and TSH levels. By measuring microvascular density, differences in angiogenesis have been related to differences in tumor behavior, and similar techniques have been applied to both benign and malignant endocrine tumors with the aim of identification of tumors that subsequently behave in an aggressive fashion. In contrast to other tumor types, pituitary tumors are less vascular than normal pituitary tissue, although the mechanism for this observation is not known. A relationship between angiogenesis and tumor size, tumor invasiveness, and aggressiveness has been shown in some pituitary tumor types, but not in others. There are few reports on the role of microvascular density or angiogenic factors in adrenal tumors. The mechanism of the vascular tumors, which include adrenomedullary tumors, found in patients with Von Hippel Lindau disease has been well characterized, and clinical trials of antiangiogenic therapy are currently being performed in patients with Von Hippel Lindau disease. Thyroid tumors are more vascular than normal thyroid tissue, and there is a clear correlation between increased VEGF expression and more aggressive thyroid tumor behavior and metastasis. Although parathyroid tissue induces angiogenesis when autotransplanted and PTH regulates both VEGF and MMP expression, there are few studies of angiogenesis and angiogenic factors in parathyroid tumors. An understanding of the balance of angiogenesis in these vascular tumors and mechanisms of vascular control may assist in therapeutic decisions and allow appropriately targeted treatment.

Ultrastructural aspects of the follicular cells of the pars tuberalis in bats related to the seasonal cycle

The topography and structure of the follicular cells and the follicular cavity of the hypophyseal pars tuberalis (PT) were studied in adult hibernating bats (Pipistrellus pipistrellus and Rhinolophus ferrumequinum) of both sexes, during the annual seasonal cycle and the reproductive cycle. The follicular cells were found to be organized around a central cavity. They showed a polyhedral shape and apical microvilli protruding into central cavities. During hibernation, the follicular cells showed active cytoplasmic organelles, clusters of glycogen particles, and lipid droplets. In the supranuclear cytoplasm, 9+2 type cilia, some dense bodies, microvesicular vacuoles, and thin actin-like filaments (rather scarce during autumn) were detected. The contents of the follicular cavity showed well-defined ultrastructural seasonal characteristics, with a colloid-like aspect during awakening and a strongly granular aspect during autumn oestrus and mating. Positive staining for PAS and paraldehyde fuchsin, and a marked reaction to lectins PHA-L4, MAM, and RCA 60 suggested the presence of sialo-glycoproteins in the follicular cavities. Both follicular and endocrine PT-specific cells appeared to mark the boundary of follicular cavities. This finding suggests that the follicular cavity contents are comprised of both types of cells, rather than by cell fragmentation or degeneration products.

Oestrogen regulates neurofilament expression in a subset of anterior pituitary cells of the adult female rat

It is the prevailing view that the neurohypophysis derives from neural crest while the pituitary's anterior lobe is of ectodermal origin. However, it has been recently suggested that anterior pituitary cells could have in part neuro-ectodermal origin, and thus should express specific neuronal markers. This issue was examined previously with conflicting results. The present study attempts to clarify the question of whether or not neuronal markers are expressed in the adenohypophysis. Using quantitative immunofluorescence, we have positively identified a subset of anterior pituitary cells, which express immunoreactivity for neuronal markers, including 68 kDa neurofilament (NF68). Interestingly, we noticed that the expression of NF68 is sexually dimorphic (i.e. neurofilament-positive cells are more abundant in sexually mature female rats). In addition, NF68 expression in female rats increases during ontogenic development and reaches a plateau level after puberty. Thereafter, it displays plastic changes along the oestrous cycle, with the maximum of neurofilament expression at oestrus and the minimum at proestrus. NF68 immunoreactivity was examined after ovariectomy, oestradiol replacement and treatment with an specific oestrogen receptor antagonist. Bilateral ovariectomy induced a significant reduction in the number of NF68-positive cells. This effect was completely prevented by treatment of ovariectomized rats with oestradiol. When intact female rats were treated with the anti-oestrogen tamoxifen, a drastic decrease in NF68 expression in anterior pituitary cells was observed. Furthermore, oestradiol administration in castrated male rats increased NF68 immunoreactivity. Double-immunolabelling experiments provided evidence that pituitary cells expressing neuronal traits correspond to subsets of lactotrophs, somatotrophs, thyrotrophs and gonadotrophs. It remains to be established if NF68 induction in the pituitary is due to direct and/or indirect effects of oestrogens. Also, the possible functional role of this subset of NF68-positive anterior pituitary cells in the female rat remains to be examined.

Transforming growth factor-beta regulation of estradiol-induced prolactinomas

Prolactin-secreting adenomas (prolactinomas) are the most prevalent form of pituitary tumors in humans, and increased tumor growth under estrogenic influence in female patients is often of clinical concern. Extensive experimental work has uncovered the roles of estrogen receptors and various growth-regulatory peptides in estradiol action on lactotropes. However, it is only recently that we are beginning to gain insight into how these growth factors interact to regulate estradiol action on lactotrope cell proliferation. Recent studies have identified the regulatory role of TGF-beta-related peptides in estradiol action on lactotropes. Additionally, these studies determined that TGF-beta and FGF interact to facilitate the communication between lactotropes and folliculostellate cells that is necessary for the mitogenic action of estradiol. This review addresses the role of estradiol in prolactinoma formation and summarizes data that support a novel concept: Alterations in the expression and action of TGF-beta isoforms are crucial steps in estradiol-induced tumorigenesis.

Biochemical and morphological diversity among folliculo-stellate cells of the mink (Mustela vison) anterior pituitary

The folliculo-stellate (FS) cells are agranular cells of the anterior pituitary whose origin and function are still a matter of debate. This study examined the presence, topography, and morphological characteristics of FS cells in the mink anterior pituitary throughout the annual reproductive cycle. The S-100 protein was used as a FS cell marker. Immunoperoxidase labeling on tissue sections demonstrated the presence of two types of S-100 positive cells. Type 1 cells were stellate-shaped cells whose nuclei were localized near the center of pituitary follicles. In this type, S-100 labeling was strong in anterior pituitary sections obtained during spring, a period characterized by high prolactin pituitary content and low gonadotropin pituitary content. Type 2 cells were rounded cells occupying the periphery of the follicles. During periods of low prolactin pituitary content and high gonadotropin anterior content the type 2 S-100 positive cells formed aggregates of several cells. The total number of S-100 positive cells was constant during these two periods of the annual reproductive cycle, suggesting that type 1 and type 2 may reflect different morphological and physiological states of the same cell. Of the two subunits, alpha and beta, that, combined, form three different dimeric S-100 proteins, mink FS cells expressed mostly the beta subunit. FS cells also expressed the glial fibrillary acidic protein (GFAP). In culture, 8 +/- 3% of anterior pituitary cells were S-100 positive. Cultured S-100 cells were elongated, polygonal, or rounded. The S-100 labeling accumulated in the cytoplasm around and within the nucleus, whereas it was weak in pseudopods and large cytoplasmic vacuoles. The presence of pseudopods suggests that cultured FS cells could migrate. The vacuoles may be related to the phagocytic activity ascribed to these cells. Some FS cells presented membrane blebbing and peripheral vesicles that were immunopositive for S-100 and that may indicate a secretory activity. Cultured FS cells possessed actin filaments organized as a peripheral network; a few actin cables were also observed running across the cytoplasm. Pseudopods depicted a highly organized actin network. The microtubules of FS cells expanded throughout the cytoplasm. The intermediate filaments expressed by cultured FS cells were GFAP and vimentin. GFAP labeling was punctate and vimentin was organized as filaments. All cultured S-100 cells were positive for vimentin, suggesting a mesenchymal origin for the cells, and all cultured S-100 positive cells were positive for GFAP, suggesting a neuroectodermal origin. In conclusion, S-100 positive cells are heterogeneous with respect to cell shape and expression of S-100 subunits in the mink anterior pituitary. The presence of morphologically different S-100 positive cells is modified in accordance with the endocrine status of the animal, suggesting that FS cells may be involved in the modulation of the anterior pituitary endocrine activity in the mink.

Transforming growth factor-beta3 stimulates lactotrope cell growth by increasing basic fibroblast growth factor from folliculo-stellate cells

Recently, we have shown that transforming growth factor-beta3 (TGFbeta3) mediates estradiol's mitogenic action in primary cultures of mixed anterior pituitary cells. In some cell types, TGFbeta isoforms stimulate cell proliferation via a paracrine mechanism by increasing growth stimulatory peptide growth factors. Whether such a mechanism exists in pituitary cell culture was examined in the studies presented here. The data demonstrate that unlike the response of lactotropes in mixed pituitary cultures, cultures of enriched lactotropes, obtained by Percoll gradient separation, did not proliferate in response to TGFbeta3 treatment. The lactotropic cells of the RC-4B/C cell line, a cell line that contains all of the hormone-secreting cell types of the anterior pituitary but is devoid of folliculo-stellate (FS) cells, did not proliferate in response to TGFbeta3 unless RC-4B/C cells were cocultured with FS cells. Enriched lactotropes cocultured with FS cells also demonstrated a proliferative response to TGFbeta3. Media collected from FS cells treated with TGFbeta3 stimulated the proliferation of lactotropes in enriched cultures. TGFbeta3 increased the release of basic fibroblast growth factor from FS cells. Immunoneutralization of basic fibroblast growth factor in FS cell-conditioned medium inhibited the growth stimulatory action on lactotropes. These data provide evidence for a novel mechanism of TGFbeta3 action involving cell-to-cell interaction in the anterior pituitary between lactotropes and FS cells during estrogen-induced mitogenesis.

Regulation and role of intrapituitary IL-6 production by folliculostellate cells

Interleukin-6, mainly produced by monocytes and macrophages is known to influence the secretion of anterior pituitary hormones and is, therefore, considered to play an important role in the interaction between the immune system and the endocrine system. However, IL-6 represents not only a lymphocyte message but is also produced within the anterior pituitary. Folliculostellate (FS) cells have been identified as the source of the intrapituitary IL-6 production in the normal pituitary, whereas in pituitary adenomas IL-6 is produced by the tumor cells themselves. The present review summarizes the knowledge about the regulation of the intrapituitary IL-6 synthesis and release in FS cells. Moreover, the possible roles of the intrinsic IL-6 production for function and growth of normal and adenomatous endocrine pituitary cells are discussed.

Cell-specific induction of c-fos expression in the pituitary gland by estrogen

Estrogens regulate many functions of pituitary lactotrophs, including PRL gene expression, release, storage, and cellular proliferation. The mechanism by which estrogens exert such a variety of functions is poorly understood. In the uterus, estrogens rapidly and transiently induce the expression of the immediate early genes c-fos and c-jun in specific cell types. The Fos/Jun proteins form the activating protein-1 (AP1) transcription factor that mediates ligand-activated cell proliferation, differentiation, and secretion. Here we used Fischer 344 (F344) rats that develop hyperprolactinemia and prolactinomas in response to estrogens. The objectives were to: 1) determine whether estrogen induces c-fos expression in the pituitary gland and identify the responsive cells; 2) compare the dynamics of c-fos induction in the pituitary and uterus; and 3) examine the temporal relationship between c-fos expression and PRL release. Ovariectomized F344 rats were injected with 1 microg estradiol and killed at different times thereafter. Pituitaries were subjected to in situ hybridization for c-fos and immunostaining for selected pituitary cells. Estradiol stimulated c-fos expression in lactotrophs and folliculo-stellate cells within the anterior lobe without affecting either the intermediate or neural lobes. In a second experiment, c-fos messenger RNA levels were measured by solution hybridization in anterior pituitaries and uteri from estradiol-treated rats. Trunk blood was analyzed for PRL by RIA. The estrogen-induced c-fos rise in the uterus was rapid, robust, and transient, whereas that in the anterior pituitary was delayed, lower, and sustained. The profile of serum PRL levels resembles that of c-fos induction in the anterior pituitary. We conclude that: 1) both lactotrophs and folliculo-stellate cells increase c-fos expression in response to estrogens; 2) induction of c-fos expression may mediate some estrogenic effects on PRL synthesis and release and lactotroph proliferation in F344 rats; and 3) the atypical dynamics of c-fos induction in the pituitary could be due to indirect effects of estrogens on PRL-regulating factors within the hypothalamo-pituitary complex as well as to pituitary-specific estrogen receptor isoforms, coactivators, or repressors.

Basic fibroblast growth factor within endothelial cells during vascularization of the anterior pituitary

BACKGROUND

Basic fibroblast growth factor (FGF-2), a potent angiogenic peptide, is known to be present in gonadotropes of the anterior pituitary parenchyma of rats and mice, and has been isolated from endothelial cells of many organs. Its localization within endothelial cells has not been determined, nor the mechanisms by which it might be released from endothelial cells during normal organogenesis.

METHODS

Localization of FGF within endothelial cells of the anterior pituitary was accomplished by immunocytochemistry and studied by light- and electron microscopy. Capillaries within the anterior pituitary were studied in fetal rats from day 15 to term, and in adult rats.

RESULTS

At the onset stages of vascularization (15-18 days fetal), the cytoplasm of the endothelial cells of many of the invading, immature capillaries (thick-walled with few or no fenestrations) was intensely immunopositive for FGF. Immunoprecipitate-filled blebs and slender cytoplasmic processes projected from the endothelial cells into the presumptive pericapillary space and toward the parenchymal cells. As gestation progressed (19-20 day fetal), and an increasing number of capillaries acquired the features characteristic of capillaries in the anterior pituitary of adult animals, i.e., thin-walled and fenestrated, there were fewer capillaries demonstrating immunopositivity for FGF. Foci of released FGF, i.e., extracellular, were occasionally evident within the presumptive pericapillary spaces throughout gestation. By comparison, capillaries of the anterior pituitary of adult rats did not contain immunostainable FGF in their cytoplasm, nor were any blebs and/or processes filled with immunoprecipitate evident. However capillaries did reveal an immunopositive enhancement of their lumenal and ablumenal surfaces.

CONCLUSIONS

During vascularization of the anterior pituitary, FGF within the cytoplasm of endothelial cells is released from blebs and/or processes of endothelial cells, and after the capillary bed is stabilized postnatally, these characteristics of vascularization are absent.

Interleukin-6 stimulates cell proliferation of rat pituitary clonal cell lines in vitro

We investigated the effect of recombinant human IL-6 (rhIL-6) on cell proliferation using the MtT/E rat pituitary tumor cell line, which was recently established by Inoue et al. This cell line expresses the homeodomain protein Pit-1/GHF 1 and does not produce any significant amount of pituitary hormones, but retains its tumorigenicity by back-transplantation into rats, resulting in production of prolactin. MtT/E cells were seeded into Falcon 24-well plates at a density of 2 x 10(4) cells/well in a cultured medium, containing 10% horse serum and 2.5% fetal bovine, with test drug. After four-days (12 days for the time-course study) incubations, the cells were counted using a hemocytometer. Incubation for 4 days with rhIL-6 caused concentration-dependent stimulation of MtT/E cell growth and [3H]-thymidine incorporation into MtT/E cells. Addition of 20 ng/ml rhIL-6 to the culture medium stimulated MtT/E cell growth in a time-dependent manner, withdrawal of rhIL-6 from the culture medium reduced MtT/E cell growth, and re-addition of rhIL-6 to the culture medium again stimulated MtT/E cell growth. Among the cytokines tested, granulocyte colony-stimulating factor (rh G-CSF) also showed a slight but significant mitogenic activity on the MtT/E cells. Analysis of 125I-rhIL-6 binding to the MtT/E cells indicated a dissociation constant of 0.953 x 10(-9) mmol/l and the presence of 968 binding sites per cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Pituitary Folliculo-Stellate-Like Cells Stimulate Somatotroic Pituitary Tumor Growth in Nude Mice

A new cell line (TtT/GF) established from a murine pituitary thyrotropic tumor having characteristics similar to those of pituitary folliculo-stellate cell (FS cell) was implanted into nude mice together with cells from a rat pituitary somatotrophic tumor cell line (MtT/S) to determine whether the former enhances pituitary tumor growth. For as long as 2-3 mo after implantation, MtT/S cells implanted either alone or together with fibroblasts formed either no tumors or only very small tumors in the nude mice. In contrast, all of the nude mice that had received MtT/S cells implanted together with TtT/GF cells developed large tumors. Furthermore, the mice bearing the MtT/S and TtT/GF implants showed a significantly higher body weight and serum growth hormone level than those bearing only MtT/S cells or a combination of MtT/S cells and fibroblasts. The TtT/GF cell line itself had no tumorigenicity during the experimental period. Therefore, the TtT/GF cell line as a model of FS cells enhanced pituitary endocrine cell tumor formation. Additionally, immunocytochemistry showed that TtT/GF cells positive for glial fibrillary acidic protein (GFAP) or S-100 protein were present in the parenchymatous tissue elements or connective tissue surrounding the tumor nests. In the parenchymatous tissue, the TtT/GF cells exhibited a stellate appearance and surrounded neighboring tumor cells with their long cell processes. These results suggest that TtT/GF cells can serve as a model for pituitary FS cells, and are capable of stimulating pituitary tumor growth either by modifying the microenvironment or producing growth factors.

Development of the vasculature of the anterior pituitary: ontogeny of basic fibroblast growth factor

This study correlates the ontogeny of basic fibroblast growth factor (FGF) with the development of the vasculature of the anterior pituitary (AP) in two strains of rat, Sprague-Dawley (SD) and Fischer 344 (F344). Immunolocalization of FGF was followed from the first appearance of Rathke's pouch (RP) in 12-day (12d) fetuses, through each day of fetal development, and in 5, 20, and 50d postnatal female rats. In addition, the ontogeny of folliculo-stellate cells (FSC) is described, since previous studies suggested that these unique cells might function as phagocytes in the regulation of FGF. In both rat strains, vascularization of the AP commenced in 16d fetuses. In 15-20d fetuses, dense foci of immunopositivity for extracellular FGF were apparent at sites of capillary penetration adjacent to partially disrupted, immature gonadotropes. Localization of FGF was first detected in immature gonadotropes in 18d fetuses and persisted in the cytosol of a subpopulation of gonadotropes thereafter. In 15d fetuses, FGF was localized within the cytosol intimately associated with the peripheral-facing plasma membranes of all cells of the adenohypophysis, and persisted to variable degrees in later fetal stages. Localization of FGF within nuclei of AP parenchymal cells was evident only in 16-17d fetuses. Although the ontogeny of FGF and vascularization of the AP was very similar in both rat strains, the ontogeny of FSC differed markedly. In both strains, follicular lumens contained FGF during late fetal and early postnatal development. However, both electron microscopy and immunostaining for S-100 marker protein revealed that the postero-lateral edges of the AP of F344 rats often lacked FSC when compared to SD rats, a situation which could compromise regulation of FGF by FSC at the AP periphery in that strain, and thereby contribute to the neovascularization from systemic blood vessels known to occur in that strain during prolactinoma formation.

Fate of degenerating lactotrophs in rat pituitary gland after interruption of lactation: a histochemical and immunocytochemical study

In the pituitary gland of pregnant and lactating rats a striking proliferation of lactotrophs occurs to meet the increased demands for prolactin. Following interruption of lactation the redundant lactotrophs undergo a massive degeneration until pre-pregnant proportions are re-established. Immunocytochemical detection of prolactin allows the recognition of degenerating lactotrophs until advanced stages of degeneration and leads to the conclusion that this process is autolytic in nature. Histochemistry of acid phosphatase reveals a remarkable accumulation of this enzyme in Golgi cisternae and lysosomes. At later stages of degeneration the acid phosphatase spreads throughout the entire cell. The presence of increased numbers of necrotic cells appears to activate phagocytosis of stellate cells and, to a lesser extent, of follicular cells. Stellate cells responsible for the secondary processing of cell residues are isolated cells characterized by a prominent oval nucleus and an electron-lucent cytoplasm with scarce organelles and extensive cytoplasmic processes. They appear as scavenger cells engulfing cell remnants and debris. Immunocytochemistry of S-100 protein discloses differential staining of two types of cell, one forming clusters of 2-4 cells with faint immunoreactivity, while the other type consists of isolated cells with a stellate profile and stronger labelling to S-100 protein.

S-100 protein immunopositivity in human nontumorous hypophyses and pituitary adenomas

The presence, distribution, and morphological appearance of S-100 protein-immunoreactive cells in the human hypophysis were studied by immunocytochemistry. One hundred and twelve nonadenomatous pituitaries from fetuses to adults and pituitaries affected by several lesions including metastases, acute infarcts, and lymphocytic hypophysitis, as well as 115 pituitary adenomas were examined.S-100 protein immunoreactivity was detected in neurohy-pophyseal pituicytes and stellate cells of the pars distalis from 5 months following birth. In adults, S-100 protein-immunopositive cells displayed a preferential topographical association with growth hormone-, follicle-stimulating hormone-, luteinizing hormone-, and alpha-sub-unit-immunoreactive cells and with capillary walls. Colloid-containing follicles were mainly lined by hormone-containing cells, although scattered S-100 protein-immunoreactive processes or cell bodies were also observed forming their walls.No major changes in S-100 protein-immunoreactive cells were observed in the pituitary parenchyma bordering metastatic, inflammatory, necrotic, or adenomatous tissues. Eighteen of 115 pituitary adenomas contained a variable number of S-100 protein-immunoreactive cells. No preferential association of these cells with any type of pituitary adenoma was found.We propose that S-100 protein expression in the nontumorous adenohypophysis and pituitary adenomas may constitute a dynamic process and that S-100 protein-positive cells may constitute a heterogeneous cell population composed of pure, fully differentiated stellate cells and of transdifferentiated follicular cells.

Is cellular disruption the mechanism of release of basic fibroblast growth factor from anterior pituitary gonadotropes?

Adult female Fischer 344 (F344) and Sprague-Dawley (SD) rats, intact and ovariectomized (10-30d), have been used for immunolocalization of basic fibroblast growth factor (FGF). Tissues were selected from three specific sites (postero-lateral, lateral wing, and anterior wedge) of the periphery of the anterior pituitary (AP) carefully maintaining the association between the gland and the highly vascular meningeal connective tissue which envelopes it. In both rat strains, most of the periphery of the AP was characterized by intact parenchymal cells delimited from the meningeal connective tissue by an intact basal lamina. However, foci also were evident in which parenchymal cells projected directly into the connective tissue without a basal lamina intervening. These zones, designated the Non-Delimited Peripheral Parenchyma (NDPP), were present minimally in control rats, but were more numerous in ovariectomized rats. Profiles of focally disrupted gonadotropes were evident within the NDPP of 20-30d ovariectomized rats juxtaposed against intact, granulated parenchymal cells. Partially disrupted gonadotropes also were evident within the peripheral parenchyma within approximately 100 mu of the edge, and occasionally the disruptions resulted in an association of neighboring gonadotropes as a syncytium. FGF was localized only within the cytosol of gonadotropes, i.e., cells immunopositive for LH-beta and FSH-beta subunits. Gonadotropes nearer to the edges of the AP, especially the postero-lateral edge, were the most intensely stained. Electron microscopy and immunostaining for S-100 protein, a marker for folliculo-stellate cells (FSC), demonstrated that in intact and ovariectomized SD rats FSC were present in all peripheral zones of the AP, whereas portions of the postero-lateral periphery of the AP of intact and ovariectomized F344 rats often lacked FSC. We propose that FGF may be released from the cytosol of gonadotropes by a mechanism of cellular disruption. FGF released at peripheral sites of the AP would be well-positioned to stimulate angiogenesis from systemic blood vessels within the meninges. Since FSC are known phagocytes within the AP, their consistent presence in the periphery of the AP of SD rats may help regulate the effects of the released FGF, and by contrast, their absence in F344 rats may intensify or prolong the effects of released FGF. Such differences may underlie the higher incidence of pituitary tumors in F344 rats.

Folliculostellate cells in pituitary adenomas: Studies of hormonal profile and tumor vascularity

The hormonal immunoreactivity and vascularity of pituitary adenomas containing folliculostellate (FS) cells have been compared with those of tumors in which such cells were not identified. FS cells were present in variable numbers in 36 of 92 tumors. Adenomas immunoreactive for growth hormone (GH), adrenocorticotropic hormone (ACTH), or prolactin (PRL) contained FS cells in 40-50% of cases. Those immunoreactive for glycoprotein hormones and alphasubunit contained FS cells in 67-85% of cases, a statistically significant correlation. When alpha-subunit was also present in GH-, GH/PRL-, and ACTH-immunoreactive tumors, a higher proportion contained FS cells (57-91%). These data suggest a correlation between the presence of FS cells and glycoprotein immunoreactivity in pituitary adenomas. Vascular channels identified by the binding of the lectinUlex europaeus were quantified in the two types of tumors. Those containing FS cells were not more vascular than those without FS cells, which suggests that FS cells do not play a significant role in the regulation of intratumoraf vascularization in human pituitary adenomas.

Folliculostellate cells of the pituitary

In addition to the hormone-producing granulated cells, the so-called folliculostellate (FS) cells of the adenohypophysis represent a population of nongranulated cells extensively described in a large number of species. They show distinctive morphological features including a star shape with thin cytoplasmic projections extending between granulated cells and well-developed junctional complexes. FS cells are joined together surrounding irregular microcavities and project microvilli into the lumina. The immunocytochemical localization of S-100 protein, glial fibrillary acidic protein, and vimentin constitutes a reliable and easy method for investigating their presence and distribution in the normal pituitary gland and in pituitary adenomas. Although the expression of glial cell markers raised the hypothesis of a neuroectodermal origin of FS cells, most evidence supports that they derive from the epithelium of the Rathke's pouch, as do granulated adenohypophyseal cells. Morphological studies indicate that FS cells are involved in phagocytosis and possess sustentacular functions. Investigations using cell cultures show that FS cells play important roles in the paracrine regulation of adenohypophy-seal secretion by their ability to liberate several growth factors and regulate the ionic composition of the extracellular fluid. Further research using novel immunocytochemical markers and ceil culture techniques may clarify the origin and the role of this enigmatic cell type in the normal and pathological pituitary gland.

The fine structure of the S-100 protein positive cells in the rat pituitary gland: an immunoelectron microscopic study

We examined the fine structure of rat folliculo-stellate (FS) cells of the anterior pituitary and pituicytes of the neural lobe using immunoelectron microscopy for S-100 protein. The study proves that S-100 protein is solely expressed in the FS cells and pituicytes. However, the ultrastructural differences between these two cell types demonstrated in this study may argue against the hypothesis that the FS cells and the pituicytes have a common origin.

Ultrastructural morphology of folliculo-stellate cells in human pituitary adenomas

Fifty-six pituitary adenomas were studied by electron microscopy in a search for the presence of folliculo-stellate cells (FSCs) with the aim of evaluating their prevalence and ultrastructural morphology. FSCs were scattered in two adenomas (one oncocytoma and one densely granulated GH cell adenoma) and were numerous in a sparsely granulated GH cell adenoma; their overall prevalence was 5.4%. Ultrastructural examination of the three neoplasms revealed that FSCs were hypertrophic element with abundant cytoplasm and organelles (in contrast to FSCs of the normal pituitary) and no obvious signs of neoplastic transformation. Junctional complexes between FSCs were similar to those described in the normal gland. Numerous follicular structures were lined by FSCs. FSCs in pituitary adenomas are probably nonneoplastic, reactive cells showing signs of hyperactivity, similar to FSCs found during pituitary hypersecretion and in estrogen-induced tumor.

Development of the vasculature of the pars distalis in a tumor-susceptible strain of rat (Fischer 344) differs from vascular development in a non-tumor-susceptible strain (Lewis)

The development of the vasculature of the pars distalis of two strains of rat, Fischer 344 (F344) and Lewis (LEW), was followed in 16-day (16d) and 20-day (20d) fetuses, and in 1-day (1d), 5d, 20d, 50d, and 6-month-old females. No differences in the two strains were apparent in 16d fetuses; and the capillaries that were present were immature, i.e., tall, non-fenestrated endothelial cells, and were surrounded by poorly delineated pericapillary spaces. Immature capillaries also were predominant in 20d fetuses of both strains. Agranular folliculo-stellate cells were identifiable, projecting endfeet to the parenchymal basal lamina in 20d F344 fetuses, but not in LEW fetuses. Postnatally, the capillaries of LEW rats became progressively more thin-walled and fenestrated, and were surrounded by a pericapillary space that was well delimited by basal laminae at 20d. In 50d and 6-month LEW rats, capillaries were intact and surrounded by well-defined pericapillary spaces. By comparison in F344 rats, the capillaries remained more immature even in 50d rats and older. In addition, in F344 rats focal disruptions in endothelial cells and disruptions in parenchymal and capillary basal laminae were present in all postnatal stages, and a dramatic accumulation of plasma was evident within the pericapillary spaces at 20d. Endfeet processes of folliculo-stellate cells were abundant at the parenchymal basal lamina of 1d and 5d F344 neonates, but only rarely were identified in LEW neonates. Some activation of folliculo-stellate cells, i.e., increased numbers of lysosomes and dilated endoplasmic reticulum, was present in 50d F344 rats. Connective-tissue cells within the pericapillary space also were numerous and activated in F344 rats. Discrete gaps in the parenchymal basal lamina were evident subjacent to the folliculo-stellate cell endfeet in F344 rats but not in LEW rats. The vascular bed of F344 rats differs in its development from that of LEW rats. Characteristic of the F344 strain is a persistence of more immature capillaries, an inherent vascular fragility, and an activated state of folliculo-stellate cells.

New perspectives in the function of pituitary folliculo-stellate cells

Classical morphological studies of the folliculo-stellate (FS) cells of the anterior pituitary have suggested that these cells play roles as supporting cells, in metabolism and in macromolecular transport. Over the last 10 years the details of their activity in both trophic and catabolic processes has been clarified, and recent work has demonstrated several transport systems in these cells. Various novel peptides with growth factor or cytokine activity have been identified in FS cells and/or FS cell conditioned media. These recent functional experiments confirm and extend previous morphological and experimental studies, and in addition open new perspectives on the physiological roles of FS cells.

Immunohistochemical demonstration of alpha1-antichymotrypsin and alpha1 -antitrypsin in the normal pituitary gland and its tumors

The immunohistochemical distribution of the protease inhibitors alpha₁-antichymotrypsin (alpha₁-ACT) and alpha₁-antitrypsin (alpha₁-AT) has been documented in the normal human pituitary gland and in a series of pituitary tumors. In normal gland, alpha₁-ACT was localized mainly in the dendritic folliculostellate cells, identified by immunopositivity for S 100 protein. A minority of endocrine cells also stained in 3 of 10 autopsy glands. Folliculostellate cells were identified in 11 of 28 tumors, and again, the distribution of alpha₁-ACT positivity corresponded to these cells. In 4 cases, there was staining of a small minority of tumor cells. Alpha₁-AT was localized to colloid in the microfollicles of the anterior lobe. In I normal gland, there was granular staining of endocrine cells. Alpha₁-AT was present in 5 tumors, in microfollicles and in scattered endocrine cells in 2 adenomas. These data would support a physiological role for alpha₁-ACT and alpha₁-AT in the pituitary gland. Their differing distribution might reflect different functions.