FOXO Transcription Factors Are Required for Normal Somatotrope Function and Growth.

Understanding the molecular mechanisms underlying pituitary organogenesis and function is essential for improving therapeutics and molecular diagnoses for hypopituitarism. We previously found that deletion of the forkhead factor, Foxo1, in the pituitary gland early in development delays somatotrope differentiation. While these mice grow normally, they have reduced growth hormone expression and free serum insulin-like growth factor-1 (IGF1) levels, suggesting a defect in somatotrope function. FOXO factors show functional redundancy in other tissues, so we deleted both Foxo1 and its closely related family member, Foxo3, from the primordial pituitary. We find that this results in a significant reduction in growth. Consistent with this, male and female mice in which both genes have been deleted in the pituitary gland (dKO) exhibit reduced pituitary growth hormone expression and serum IGF1 levels. Expression of the somatotrope differentiation factor, Neurod4, is reduced in these mice. This suggests a mechanism underlying proper somatotrope function is the regulation of Neurod4 expression by FOXO factors. Additionally, dKO mice have reduced Lhb expression and females also have reduced Fshb and Prl expression. These studies reveal FOXO transcription factors as important regulators of pituitary gland function.

IGSF1 Deficiency Results in Human and Murine Somatotrope Neurosecretory Hyperfunction.

CONTEXT: The X-linked immunoglobulin superfamily, member 1 (IGSF1), gene is highly expressed in the hypothalamus and in pituitary cells of the POU1F1 lineage. Human loss-of-function mutations in IGSF1 cause central hypothyroidism, hypoprolactinemia, and macroorchidism. Additionally, most affected adults exhibit higher than average IGF-1 levels and anecdotal reports describe acromegaloid features in older subjects. However, somatotrope function has not yet been formally evaluated in this condition. OBJECTIVE: We aimed to evaluate the role of IGSF1 in human and murine somatotrope function. PATIENTS, DESIGN, AND SETTING: We evaluated 21 adult males harboring hemizygous IGSF1 loss-of-function mutations for features of GH excess, in an academic clinical setting. MAIN OUTCOME MEASURES: We compared biochemical and tissue markers of GH excess in patients and controls, including 24-hour GH profile studies in 7 patients. Parallel studies were undertaken in male Igsf1-deficient mice and wild-type littermates. RESULTS: IGSF1-deficient adult male patients demonstrated acromegaloid facial features with increased head circumference as well as increased finger soft-tissue thickness. Median serum IGF-1 concentrations were elevated, and 24-hour GH profile studies confirmed 2- to 3-fold increased median basal, pulsatile, and total GH secretion. Male Igsf1-deficient mice also demonstrated features of GH excess with increased lean mass, organ size, and skeletal dimensions and elevated mean circulating IGF-1 and pituitary GH levels. CONCLUSIONS: We demonstrate somatotrope neurosecretory hyperfunction in IGSF1-deficient humans and mice. These observations define a hitherto uncharacterized role for IGSF1 in somatotropes and indicate that patients with IGSF1 mutations should be evaluated for long-term consequences of increased GH exposure.

Glucocorticoid receptor involvement in goose (Anas cygnoides) pituitary somatotroph differentiation induced by glucocorticoids during embryonic development.

1. In this study, geese (Anas cygnoides) embryonic pituitary cells were cultured in vitro to determine if glucocorticoids could induce growth hormone (GH) expression and to investigate the molecular mechanisms involved in this process. 2. On embryonic day 15 (e15) and e20 the pituitary cells were treated with corticosterone (CORT), membrane impermeable bovine serum albumin-conjugate corticosterone (CORT-BSA), dexamethasone (DEX), and a glucocorticoid receptor (GR) antagonist (RU486) to detect responsiveness of somatotrophs to glucocorticoids. 3. Treatment with CORT, CORT-BSA, and DEX for as little as 6 h increased the percentage of GH-positive cells (P < 0.01) and increased GH mRNA expression (P < 0.01) in e15 goose pituitary cells compared to the control. CORT significantly increased the level of GH protein secreted from cultured e15 goose embryonic pituitary cells, and CORT-BSA increased GH secretion from e20 goose embryonic pituitary cells. 4. A significant increase was observed in the glucocorticoid receptor in GR transcription levels (P < 0.01) with CORT, CORT-BSA, and DEX treatment. Furthermore, the CORT-stimulated GH mRNA expression was completely negated by pre-treatment with RU486. 5. These findings demonstrate that glucocorticoids can stimulate somatotroph differentiation in vitro, as characterised by enhanced GH protein secretion andmRNA expression in cultured geese embryonic pituitary cells. The membrane GR was involved in pituitary somatotroph differentiation induced by glucocorticoids during the embryonic development of geese.

Biology of the somatotroph axis (after the pituitary).

Normal growth requires that pituitary-secreted growth hormone (GH) bind to its specific receptor and activate a complex signaling cascade, leaving to production of insulin-like growth factor-I (IGF-I), which, in turn, activates its own receptor (IGF1R). The GH receptor (GHR) is preformed as a dimer and is transported in a nonligand bound state to the cell surface. Binding of GH to the GHR dimer, results in a conformational change of the dimer, activation of the intracellular Janus Kinase 2 (JAK2) and phosphorylation of signal transducer and activator of transcription (STAT) 5B. Phosphorylated STAT5B dimers are then translocated to the nucleus, where they transcriptionally activate multiple genes, including those for IGF-I, IGF binding protein-3 and the acid-labile subunit (ALS).

Mild pituitary phenotype in 3- and 12-month-old Aip-deficient male mice.

Germline mutations in the aryl hydrocarbon receptor-interacting protein (AIP) gene predispose humans to pituitary adenomas, particularly of the somatotroph lineage. Mice with global heterozygous inactivation of Aip (Aip(+/-)) also develop pituitary adenomas but differ from AIP-mutated patients by the high penetrance of pituitary disease. The endocrine phenotype of these mice is unknown. The aim of this study was to determine the endocrine phenotype of Aip(+/-) mice by assessing the somatic growth, ultradian pattern of GH secretion and IGF1 concentrations of longitudinally followed male mice at 3 and 12 months of age. As the early stages of pituitary tumorigenesis are controversial, we also studied the pituitary histology and somatotroph cell proliferation in these mice. Aip(+/-) mice did not develop gigantism but exhibited a leaner phenotype than wild-type mice. Analysis of GH pulsatility by deconvolution in 12-month-old Aip(+/-) mice showed a mild increase in total GH secretion, a conserved GH pulsatility pattern, but a normal IGF1 concentration. No pituitary adenomas were detected up to 12 months of age. An increased ex vivo response to GHRH of pituitary explants from 3-month-old Aip(+/-) mice, together with areas of enlarged acini identified on reticulin staining in the pituitary of some Aip(+/-) mice, was suggestive of somatotroph hyperplasia. Global heterozygous Aip deficiency in mice is accompanied by subtle increase in GH secretion, which does not result in gigantism. The absence of pituitary adenomas in 12-month-old Aip(+/-) mice in our experimental conditions demonstrates the important phenotypic variability of this congenic mouse model.

Altered somatotroph feedback regulation improves metabolic efficiency and limits adipose deposition in male mice.

Several transgenic mouse models with disruption in the growth hormone (GH) axis support the role of GH in augmenting metabolic homeostasis. Specifically, interest has focused on GH's lipolytic properties and ability to affect adipose deposition. Furthermore, both GH and insulin growth factor 1 (IGF-1) may also play a direct or indirect role in adipose development. The somatotroph insulin-like growth factor-1 receptor knockout (SIGFRKO) mouse with only a modest increase in serum GH and IGF-1 demonstrates less adipose tissue than controls. In order to characterize the metabolic phenotype of SIGFRKO mice, histologic analysis of fat depots confirmed a smaller average diameter of adipocytes in the SIGFRKO mice compared to controls. These changes were accompanied by an increase in lipolytic gene expression in fat depots. Indirect calorimetry performed on 6-8week old male mice and again at 25weeks of age demonstrated that SIGFRKO mice, at both ages, had a higher VO2 and increased energy expenditure when compared with controls. The calculated respiratory exchange ratio (RER) was lower in the younger SIGFRKO mice compared to controls. No differences in food consumption or in either ambulatory or total activity were seen between SIGFRKO and control mice in either age group. These studies highlight the role of GH in adipose deposition and its influence on the expression of lipolytic genes resulting in an altered metabolic state, thus providing a mechanism for the decrease in weight gain seen in the SIGFRKO mouse model.

Dose-dependent dual role of PIT-1 (POU1F1) in somatolactotroph cell proliferation and apoptosis.

To test the role of wtPIT-1 (PITWT) or PIT-1 (R271W) (PIT271) in somatolactotroph cells, we established, using inducible lentiviral vectors, sublines of GH4C1 somatotroph cells that allow the blockade of the expression of endogenous PIT-1 and/or the expression of PITWT or PIT271, a dominant negative mutant of PIT-1 responsible for Combined Pituitary Hormone Deficiency in patients. Blocking expression of endogenous PIT-1 induced a marked decrease of cell proliferation. Overexpressing PITWT twofold led also to a dose-dependent decrease of cell proliferation that was accompanied by cell death. Expression of PIT271 induced a strong dose-dependent decrease of cell proliferation accompanied by a very pronounced cell death. These actions of PIT271 are independent of its interaction/competition with endogenous PIT-1, as they were unchanged when expression of endogenous PIT-1 was blocked. All these actions are specific for somatolactotroph cells, and could not be observed in heterologous cells. Cell death induced by PITWT or by PIT271 was accompanied by DNA fragmentation, but was not inhibited by inhibitors of caspases, autophagy or necrosis, suggesting that this cell death is a caspase-independent apoptosis. Altogether, our results indicate that under normal conditions PIT-1 is important for the maintenance of cell proliferation, while when expressed at supra-normal levels it induces cell death. Through this dual action, PIT-1 may play a role in the expansion/regression cycles of pituitary lactotroph population during and after lactation. Our results also demonstrate that the so-called "dominant-negative" action of PIT271 is independent of its competition with PIT-1 or a blockade of the actions of the latter, and are actions specific to this mutant variant of PIT-1.

Melatonin regulates somatotrope and lactotrope function through common and distinct signaling pathways in cultured primary pituitary cells from female primates.

Melatonin (MT) is secreted by the pineal gland and exhibits a striking circadian rhythm in its release. Depending on the species studied, some pituitary hormones also display marked circadian/seasonal patterns and rhythms of secretion. However, the precise relationship between MT and pituitary function remains controversial, and studies focusing on the direct role of MT in normal pituitary cells are limited to nonprimate species. Here, adult normal primate (baboons) primary pituitary cell cultures were used to determine the direct impact of MT on the functioning of all pituitary cell types from the pars distalis. MT increased GH and prolactin (PRL) expression/release in a dose- and time-dependent fashion, a response that was blocked by somatostatin. However, MT did not significantly affect ACTH, FSH, LH, or TSH expression/release. MT did not alter GHRH- or ghrelin-induced GH and/or PRL secretions, suggesting that MT may activate similar signaling pathways as ghrelin/GHRH. The effects of MT on GH/PRL release, which are likely mediated through MT1 receptor, involve both common (adenylyl cyclase/protein kinase A/extracellular calcium-channels) and distinct (phospholipase C/intracellular calcium-channels) signaling pathways. Actions of MT on pituitary cells also included regulation of the expression of other key components for the control of somatotrope/lactotrope function (GHRH, ghrelin, and somatostatin receptors). These results show, for the first time in a primate model, that MT directly regulates somatotrope/lactotrope function, thereby lending support to the notion that the actions of MT on these cells might substantially contribute to the define daily patterns of GH and PRL observed in primates and perhaps in humans.

Seasonal effects of GnIH on basal and GnRH-induced goldfish somatotrope functions.

To understand how gonadotropin-inhibitory hormone (GnIH) regulates goldfish GH cell functions, we monitored GH release and expression during early, mid-, and/or late gonadal recrudescence. In vivo and in vitro responses to goldfish (g) GnIH were different, indicating direct action at the level of pituitary, as well as interactions with other neuroendocrine factors involved in GH regulation. Injection of gGnIH consistently reduced basal serum GH levels but elevated pituitary gh mRNA levels, indicating potential dissociation of GH release and synthesis. Goldfish GnRH (sGnRH and cGnRHII) injection differentially stimulated serum GH and pituitary gh mRNA levels with some seasonal differences; these responses were reduced by gGnIH. In contrast, in vitro application of gGnIH during 24-h static incubation of goldfish pituitary cells generally elevated basal GH release and attenuated sGnRH-induced changes in gh mRNA, while suppressing basal gh mRNA levels at mid- and late recrudescence but elevating them at early recrudescence. gGnIH attenuated the GH release responses to sGnRH during static incubation at early, but not at mid- and late recrudescence. In cell column perifusion experiments examining short-term GH release, gGnIH reduced the cGnRHII- and sGnRH-stimulated secretion at late recrudescence but inhibited tha action of cGnRHII only during mid-recrudescence. Interestingly, a reduction of basal GH release upon perifusion with gGnIH during late recrudescence was followed by a rebound increase in GH release upon gGnIH removal. These results indicate that gGnIH exerts complex effects on basal and GnRH-stimulated goldfish GH cell functions and can differentially affect GH release and mRNA expression in a seasonal reproductive manner.

Adipocyte versus pituitary leptin in the regulation of pituitary hormones: somatotropes develop normally in the absence of circulating leptin.

Leptin is a cytokine produced by white fat cells, skeletal muscle, the placenta, and the pituitary gland among other tissues. Best known for its role in regulating appetite and energy expenditure, leptin is produced largely by and in proportion to white fat cells. Leptin is also important to the maintenance and function of the GH cells of the pituitary. This was shown when the deletion of leptin receptors on somatotropes caused decreased numbers of GH cells, decreased circulating GH, and adult-onset obesity. To determine the source of leptin most vital to GH cells and other pituitary cell types, we compared two different leptin knockout models with Cre-lox technology. The global Lep-null model is like the ob/ob mouse, whereby only the entire exon 3 is deleted. The selective adipocyte-Lep-null model lacks adipocyte leptin but retains pituitary leptin, allowing us to investigate the pituitary as a potential source of circulating leptin. Male and female mice lacking adipocyte leptin (Adipocyte-lep-null) did not produce any detectable circulating leptin and were infertile, suggesting that the pituitary does not contribute to serum levels. In the presence of only pituitary leptin, however, these same mutants were able to maintain somatotrope numbers and GH mRNA levels. Serum GH trended low, but values were not significant. However, hypothalamic GHRH mRNA was significantly reduced in these animals. Other serum hormone and pituitary mRNA differences were observed, some of which varied from previous results reported in ob/ob animals. Whereas pituitary leptin is capable of maintaining somatotrope numbers and GH mRNA production, the decreased hypothalamic GHRH mRNA and low (but not significant) serum GH levels indicate an important role for adipocyte leptin in the regulation of GH secretion in the mouse. Thus, normal GH secretion may require the coordinated actions of both adipocyte and pituitary leptin.

Ghrelin restoration of function in vitro in somatotropes from male mice lacking the Janus kinase (JAK)-binding site of the leptin receptor.

Deletion of the signaling domain of leptin receptors selectively in somatotropes, with Cre-loxP technology, reduced the percentage of immunolabeled GH cells and serum GH. We hypothesized that the deficit occurred when leptin's postnatal surge failed to stimulate an expansion in the cell population. To learn more about the deficiency in GH cells, we tested their expression of GHRH receptors and GH mRNA and the restorative potential of secretagogue stimulation in vitro. In freshly plated dissociated pituitary cells from control male mice, GHRH alone (0.3 nM) increased the percentage of immunolabeled GH cells from 27 ± 0.05% (vehicle) to 42 ± 1.8% (P < .002) and the secretion of GH 1.8-3×. Deletion mutant pituitary cells showed a 40% reduction in percentages of immunolabeled GH cells (16.7 ± 0.4%), which correlated with a 47% reduction in basal GH levels (50 ng/mL control; 26.7 ng/mL mutants P = .01). A 50% reduction in the percentage of mutant cells expressing GHRH receptors (to 12%) correlated with no or reduced responses to GHRH. Ghrelin alone (10 nM) stimulated more GH cells in mutants (from 16.7-23%). When added with 1-3 nM GHRH, ghrelin restored GH cell percentages and GH secretion to levels similar to those of stimulated controls. Counts of somatotropes labeled for GH mRNA confirmed normal percentages of somatotropes in the population. These discoveries suggest that leptin may optimize somatotrope function by facilitating expression of membrane GHRH receptors and the production or maintenance of GH stores.

Anterior pituitary cell networks.

Both endocrine and non-endocrine cells of the pituitary gland are organized into structural and functional networks which are formed during embryonic development but which may be modified throughout life. Structural mapping of the various endocrine cell types has highlighted the existence of distinct network motifs and relationships with the vasculature which may relate to temporal differences in their output. Functional characterization of the network activity of growth hormone and prolactin cells has revealed a role for cell organization in gene regulation, the plasticity of pituitary hormone output and remarkably the ability to memorize altered demand. As such, the description of these endocrine cell networks alters the concept of the pituitary from a gland which simply responds to external regulation to that of an oscillator which may memorize information and constantly adapt its coordinated networks' responses to the flow of hypothalamic inputs.

Regenerative capacity of the adult pituitary: multiple mechanisms of lactotrope restoration after transgenic ablation.

In a recent study, we showed that the adult pituitary gland is capable of regenerating transgenically ablated growth hormone-producing (GH(+)) somatotropes. Here, we investigated whether the gland's regenerative capacity is more general and also applies to the other major hormonal cell type, the prolactin-producing (PRL(+)) lactotropes. We set up the transgenic PRLCre/inducible diphtheria toxin receptor (iDTR) mouse model, in which the PRL promoter drives expression of Cre that induces DTR in lactotropes. Injection of female mice with DT for different periods causes a gradual ablation of PRL(+) cells, reaching a maximum of 70% after 10-day DT treatment. During the following weeks, lactotropes progressively reappear achieving a 60% restoration after 6 weeks. The Sox2(+) stem/progenitor cell compartment displays a prompt reaction to the DT-triggered cell ablation injury, including expansion of the marginal-zone niche and coexpression of PRL, the latter only very rarely observed in control pituitary. Throughout the regeneration period (2-6 weeks), Sox2(+) as well as double Sox2(+)/PRL(+) cells continue to be more abundant than in control pituitary. In addition to this stem cell reaction, surviving or newborn lactotropes increase their proliferative activity, and bihormonal PRL(+)/GH(+) cells become detectable suggesting somatotrope-to-lactotrope transdifferentiation. In conclusion, the adult pituitary gland is capable of restoring lactotrope cells after destruction, further confirming its regenerative competence. Repair of lactotropes appears to be driven by a combination of mechanisms, including recruitment from stem cells, proliferation of lactotropes, and transdifferentiation of somatotropes.

The hidden but positive role for glucocorticoids in the regulation of growth hormone-producing cells.

Growth hormone (GH) is a prominent metabolic factor that is targeted by glucocorticoids; however, their role in GH production remains controversial. This is explained in part by discrepancies between in vitro and in vivo, short-term versus long-term exposure and even species-specific effects. The prevailing view, however, is that glucocorticoids are negative modulators of growth and GH production. An examination of recent findings from elegant avian and gene ablation in mice studies as well as clinical case reports, suggests this is not the case. The evidence suggests that the effect of glucocorticoids on growth and GH production can be uncoupled, and reveals they play a crucial and positive role in maturation of functional somatotrophs, the GH-producing cells of the anterior pituitary. Here, we provide an overview and insights into the possible roles of glucocorticoids in the development of somatotrophs before birth as well as regulation of GH production in infancy (neonatal) and adulthood (postnatal). A fully functional glucocorticoid-signaling pathway appears to be required for establishment of somatotrophs before birth, and glucocorticoids continue to be required for maintenance of GH production in the newborn. There is evidence to suggest progenitor somatotrophs may persist after birth, and perhaps account for the ability of glucocorticoid therapy to correct some cases of GH deficiency as a result of compromised glucocorticoid signaling. Finally, there is support for positive regulation of avian, murine and human GH gene activation and/or expression by glucocorticoids, however, there appears to be no common mechanism and the contribution of direct versus indirect effects remains unclear. Thus, our observations reveal a largely hidden face of glucocorticoids, specifically, a positive role in somatotroph development and GH gene activation/expression, which may enable us to better understand the differential effect of glucocorticoids on growth and GH production in human studies.

The evaluation of estradiol and leptin action on the activity of the somatotropic and gonadotropic axes in peripubertal female rats.

OBJECTIVE: Available data suggest that estrogens and leptin play a role in the control of the pubertal process. In humans and some mammal species the increase of the activity of gonadotropic axis accompanies the decrease in the rate of growth at puberty. The effect of 17ß-estradiol and/or leptin administration on the somatotropic and gonadotropic axes was studied using prepubertal female rats as an animal model. MATERIAL AND METHODS: Prepubertal female rats received estradiol/saline, estradiol/leptin, oil/leptin or oil/saline (vehicles) respectively. The changes of growth rate, and serum 17ß-estradiol, leptin, GH, IGF-I and gonadotropins levels as well as LHRH and estrogen receptor (ER) concentrations in the medial basal hypothalamus (MBH) and the pituitary were determined. All hormones concentrations were measured by radioimmunoassay and ER by radioligand methods . RESULTS: In estradiol and/or leptin treated animals noticeable reduction of rate of growth was found. The decrease of growth in response to estradiol treatment accompanied the increase GH level and the decrease of IGF-I concentration in the circulation. Both hormones operating together activated reproductive axis, what was manifested by a significant increase of LHRH abundant in the hypothalamus as well as elevated LH and FSH levels in the circulation. In these rats a significant decrease of the estrogen receptor concentrations in the pituitary was observed. CONCLUSION: The role of estradiol and leptin in the control of growth and reproduction seems to overlap only partially. Estradiol plays a significant role in the activation of the reproductive axis, and leptin takes part as a permissive factor in pubertal process.

The absence of somatotroph proliferation during continuous stress is a result of the lack of extracellular signal-regulated kinase 1/2 activation.

The integrity of homeostasis can be affected by chronic stress, and hyposomatotropism is evident in chronic stress-associated illnesses. In the present study, we demonstrated that a continuous stress (CS) severely affected somatotrophs among hormone-secreting cells in the anterior lobe (AL) of the pituitary by using a rat CS model. Among AL cells, the proliferation of somatotrophs was almost entirely suppressed in rats that had 3-5 days of CS (5dCS), although other hormone-secreting cells continued to proliferate. The cell size of somatotrophs was reduced at 5dCS (P<0.01), the number of secretory granules was increased at 3dCS (P<0.01) and serum growth hormone (GH) was on declining trend during 1 to 5dCS, suggesting that GH release was inhibited. GH-releasing hormone (GHRH) mRNA level in the arcuate nucleus was transiently decreased, whereas its receptor expression in the AL was significantly increased in CS rats. When 5dCS rats were injected with GHRH, transient GH secretion was observed, whereas proliferation of somatotrophs did not occur. The GHRH administration failed to stimulate extracellular signal-regulated kinase (ERK) 1/2 phosphorylation and the nuclear translocation of ERK in somatotrophs. These results suggest that somatotrophs of 5dCS rats expressed sufficient GHRH receptor, which could transfer a signal for GH release. However, the GHRH-induced proliferation signal was blocked somewhere between the receptor and ERK1/2. Because significant increase of corticosterone in the initial stage (the 1-3dCS) was observed in this model, the corticosterone may affect the signalling. Although the mechanism underlying the blockage of the proliferation signal in somatotrophs under CS remains unclear, these somatotrophic disorder, suggesting that the present animal model may be useful for understanding the molecular mechanisms of chronic stress-associated illnesses.

The effects of angiotensin peptides and angiotensin receptor antagonists on the cell growth and angiogenic activity of GH3 lactosomatotroph cells in vitro.

The local renin-angiotensin system (RAS) is present in the pituitary gland, and inhibitory effects of angiotensins on the lactosomatotroph (GH3) cell growth have been revealed. The aim of this study was to examine the influence of various angiotensin peptides and angiotensin AT1, AT2, and AT4 receptors antagonists on the cell proliferation, viability, and VEGF secretion in pituitary lactosomatotroph GH3 cell culture in order to identify receptors involved in antiproliferative effects of angiotensins on GH3 tumor cells. Cell viability and proliferation using Mosmann method and BrdU incorporation during DNA synthesis, and VEGF secretion using ELISA assay were estimated. The inhibitory effects of ang II, ang IV, and ang 5-8 on the cell viability and BrdU incorporation in GH3 culture were not abolished by AT1, AT2, and AT4 receptors antagonists. Ang II, as well as ang III and ang IV at lower concentrations stimulated the secretion of VEGF in GH3 cell culture. The secretion of VEGF was inhibited by ang III and ang IV at higher concentrations. AT1 and AT2 receptors antagonists prevented the proangiogenic effects of ang II. Ang II, ang IV, and ang 5-8 decrease the cell number and proliferation in GH3 cell culture independently of the AT1, AT2, and AT4 receptors. These peptides affect also secretion of VEGF in culture examined. Both the AT1 and AT2 receptors appear to mediate the proangiogenic effects of ang II.

Influence of membrane ion channel in pituitary somatotrophs by hypothalamic regulators.

The secretion of growth hormone (GH) from somatotrophs located within the anterior pituitary gland is stimulated by endogenous hypothalamic growth hormone-releasing hormone (GHRH) and the GH secretagogue (GHS) ghrelin, and inhibited by somatotropin-releasing inhibitory factor (SRIF, also known as somatostatin). These factors bind to specific G-protein-coupled receptors on the cell membrane, and directly or indirectly modify the properties of ion channels and second messenger systems. Ultimately this results in a change in intracellular free Ca(2+) concentration ([Ca(2+)](i)) and the secretion of GH. Somatotrophs possess a variety of ion channels on their membranes, and modification of these ion channels, especially Ca(2+), K(+), and Na(+) channels, is tightly linked to intracellular Ca(2+) levels and therefore hormone secretion. Various issues regarding receptor distribution, role of ion channels, alteration of membrane potential, and involvement of intracellular signaling system in the control of GH secretion are discussed in this review. In particular, this work will focus on ion channels and [Ca(2+)](i) in somatotrophs.

Calcium and other signalling pathways in neuroendocrine regulation of somatotroph functions.

Relative to mammals, the neuroendocrine control of pituitary growth hormone (GH) secretion and synthesis in teleost fish involves numerous stimulatory and inhibitory regulators, many of which are delivered to the somatotrophs via direct innervation. Among teleosts, how multifactorial regulation of somatotroph functions are mediated at the level of post-receptor signalling is best characterized in goldfish. Supplemented with recent findings, this review focuses on the known intracellular signal transduction mechanisms mediating the ligand- and function-specific actions in multifactorial control of GH release and synthesis, as well as basal GH secretion, in goldfish somatotrophs. These include membrane voltage-sensitive ion channels, Na(+)/H(+) antiport, Ca(2+) signalling, multiple pharmacologically distinct intracellular Ca(2+) stores, cAMP/PKA, PKC, nitric oxide, cGMP, MEK/ERK and PI3K. Signalling pathways mediating the major neuroendocrine regulators of mammalian somatotrophs, as well as those in other major teleost study model systems are also briefly highlighted. Interestingly, unlike mammals, spontaneous action potential firings are not observed in goldfish somatotrophs in culture. Furthermore, three goldfish brain somatostatin forms directly affect pituitary GH secretion via ligand-specific actions on membrane ion channels and intracellular Ca(2+) levels, as well as exert isoform-specific action on basal and stimulated GH mRNA expression, suggesting the importance of somatostatins other than somatostatin-14.