The neuropeptide somatostatin (SRIF) is an important modulator of neurotransmission in the central nervous system and acts as a potent inhibitor of hormone and exocrine secretion. In addition, SRIF regulates cell proliferation in normal and tumorous tissues. The six somatostatin receptor subtypes (sst1, sst2A, sst2B, sst3, sst4, and sst5), which belong to the G protein-coupled receptor (GPCR) family, share a common molecular topology: a hydrophobic core of seven transmembrane-spanning α-helices, three intracellular loops, three extracellular loops, an amino-terminus outside the cell, and a carboxyl-terminus inside the cell. For most of the GPCRs, intracytosolic sequences, and more particularly the C-terminus, are believed to interact with proteins that are mandatory for either exporting neosynthesized receptor, anchoring receptor at the plasma membrane, internalization, recycling, or degradation after ligand binding. Accordingly, most of the SRIF receptors can traffic not only in vitro within different cell types but also in vivo. A picture of the pathways and proteins involved in these processes is beginning to emerge.
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- Abstract: Pituitary x
- Abstract: Brain x
- Abstract: Tumours x
- Abstract: Hypothalamus x
- Abstract: Kisspeptin x
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- Abstract: Cushing's x
- Abstract: NETs x
- Abstract: Paraganglioma x
- Abstract: Vasopressin x
- Abstract: neuroendocrine x
- Abstract: Growth x
- Abstract: somatostatin x
Zsolt Csaba, Stéphane Peineau and Pascal Dournaud
Aspasia-Athina Volakaki, Daniel Lafkas, Eva Kassi, Andrew V Schally, Athanasios G Papavassiliou and Hippokratis Kiaris
GHRH, besides its neuroendocrine action in controlling the release of GH from the pituitary, stimulates the growth of various cancers in vivo and in vitro by direct mechanism(s). However, the molecular mechanism that mediates these proliferative effects of GHRH in extrapituitary tissues remains poorly characterized. In the present study, we investigated whether the tumor suppressor p21/waf1 is involved in the mediation of the proliferative effects of GHRH in A549 human lung cancer epithelial cells. Exposure of A549 cells to the GHRH antagonist JMR-132 caused a significant inhibition in the rate of cell proliferation. In A549 cells, GHRH suppressed while JMR-132 increased the levels of p21 expression in a dose-dependent manner. This suggests that GHRH could regulate p21 levels. We then evaluated whether p21 is required in A549 cells for the regulation of cell proliferation by GHRH. To this end, we knocked-down p21 expression in A549 cells by siRNA and assessed the effects of antagonist JMR-132 on cell proliferation. We found that the loss of p21 expression abolished the anti-proliferative effects of JMR-132. Suppression of p21 expression by siRNA in human HT29 colon cancer cells and non-transformed mouse osteoblasts KS483 also blocked the anti-proliferative effects of JMR-132 suggesting that the regulation of cell proliferation by GHRH is p21 dependent. These results shed light on the molecular mechanism of action of GHRH antagonists in tumor tissues and suggest that the antineoplastic activity of GHRH antagonists could be considered for the treatment of cancers expressing p21.
Raul M Luque, Manuel D Gahete, Rudy J Valentine and Rhonda D Kineman
In humans, circulating GH levels are increased in catabolic states and suppressed in obesity. In both extremes, normalization of the metabolic environment normalizes GH release, leading to the conclusion that changes in metabolic hormones and/or metabolites promote changes in GH synthesis and release. Metabolic regulation of GH secretion can be mediated centrally by modulation of hypothalamic GHRH and somatostatin input to the pituitary and/or by direct regulation of pituitary somatotrope function. Although data are available showing glucocorticoids, free fatty acids (FFA), IGF-I, and insulin have direct effects on rat somatotrope function, little information is available regarding the direct pituitary effects of these metabolic factors in primates. Therefore, this study examined the effects of glucocorticoids (dexamethasone (0.1–100 nM) and hydrocortisone (10 nM)), FFA (oleic and linoleic acid, 100 and 400 μM each), IGF-I (0.5–50 nM), and insulin (0.5–50 nM) on GH release and GH, GHRH-receptor (GHRH-R) and ghrelin-receptor (GHS-R) mRNA levels, in primary pituitary cell cultures of baboons (Papio anubis) after 24 h treatment. A commercial ELISA kit was used to determine the amount of GH released into the media, while quantitative real-time reverse transcription-PCR was used to determine mRNA levels. To design species-specific primers for baboon GH, GHRH-R, GHS-R, insulin receptor (INSR), IGF-I receptor (IGF-IR), pituitary-specific transcription factor-1 (Pit-1), and cyclophilin A (used as a housekeeping gene) cDNA, sequence data for each baboon transcript were obtained and this data were submitted to Genbank. Glucocorticoids, FFA, insulin and IGF-I treatment did not significantly alter the expression of Pit-1, a transcription factor essential for normal somatotrope development and function. However, as previously reported in the rat, glucocorticoids increased, while FFA, IGF-I and insulin decreased GH release in baboon pituitary cell cultures, where changes in GH release were reflected by comparable changes in GH mRNA levels. In addition, glucocorticoids increased, while FFA, IGF-I and insulin decreased the expression of the GH stimulatory receptors, GHRH-R and GHS-R, without significantly altering cyclophilin A mRNA levels. A role of insulin/INSR pathway, independent of IGF-I, in regulating pituitary function is supported by the fact that (1) IGF-I and insulin significantly suppressed somatotrope function at doses (0.5 and 5 nM respectively) not anticipated to activate their respective receptors, and (2) the baboon pituitary expresses INSR mRNA at levels comparable to or greater than that of tissues commonly considered as insulin sensitive (i.e. liver, skeletal muscle, and fat). Taken together, these results demonstrate that metabolic factors can directly modulate primate somatotrope function through regulating GH synthesis and release, as well as mediating the expression of receptors important in central (GHRH) and systemic (ghrelin) regulation of GH secretion.
C J Newton, T Trapp, U Pagotto, U Renner, R Buric and G K Stalla
The GH3 pituitary cell line has been used to investigate the role of the oestrogen receptor (ER) as a modulator of mitogenic signals in tumour cells in the absence of exogenous oestrogen. Using a chemically defined, serum- and oestrogen-free medium, we have demonstrated that the pure steroidal anti-oestrogens ICI 182780 and ICI 164384 are capable of blocking growth by more than 50% after 5 days of culture. Studies with conditioned medium have indicated that the basal growth is due to the secretion of autocrine growth stimulatory substances. Under serum- and oestrogen-free conditions, insulin and IGF-I increased the growth rate of these cells by twofold over a 5-day treatment period, and this effect was also blocked by the anti-oestrogens ICI 182780 and ICI 164384 (50% of maximum inhibition at 0·6 and 6 nM respectively). To explore the potential mechanism by which the ER apparently facilitates the growth factor effects under oestrogen-free conditions, GH3 cells were transiently transfected with a plasmid reporter containing the vitellogenin oestrogen response element (ΔMTV-ERE-LUC). We have shown that as well as oestradiol (OE2), insulin and IGF-I induce luciferase activity by between two- and sevenfold (four experiments), and these effects were completely blocked by ICI 182780. In contrast, growth factors and OE2 were unable to induce luciferase expression when transfections were performed with a plasmid reporter lacking the oestrogen response element. The studies presented here strongly suggest that, in the absence of oestrogen, the ER in these pituitary tumour cells has a role in growth, as peptide factors are able to induce its conversion to a state which is capable of up-regulating the transcription of key growth-promoting genes.
B Houston, B H Thorp and D W Burt
Longitudinal bone growth occurs in the epiphyseal growth plate and is regulated by a network of paracrine and autocrine interactions. Bone morphogenetic proteins (BMPs) are a family of growth factors whose potent osteogenic properties suggest that they may play an important role within this network, but direct evidence for this is lacking. To address this question, a cDNA encoding chick BMP-7 was cloned from a chick embryo cDNA library. Sequence homology and evolutionary arguments strongly suggested that we had cloned the chicken BMP-7 homologue. Using a reverse transcription-PCR assay, BMP-7 expression was readily detected in bone, growth plate cartilage, brain and heart, and was just detectable in liver, skeletal muscle and adipose tissue. In contrast to the pattern of BMP-7 expression in the rat and mouse, no BMP-7 expression was detected in the chick kidney. In situ hybridization was used to locate the site of BMP-7 expression more precisely within the growth plate. BMP-7 expression was confined to hypertrophic chondrocytes adjacent to and at the tips of the metaphyseal vessels. No expression was detected in the reserve zone or in proliferating chondrocytes. These results point to a specific role for BMP-7 in the growth plate, possibly in osteoblast activation or as a chemotactic agent for the metaphyseal vessels.
Shilpa Thakur, Brianna Daley and Joanna Klubo-Gwiezdzinska
Incidence of endocrine cancers is rising every year. Over the last decade, evidence has accumulated that demonstrates the anti-cancer effects of an anti-diabetic drug, metformin, in endocrine malignancies. We performed a literature review utilizing the PubMed, Medline and clinicaltrials.gov databases using the keyword ‘metformin’ plus the following terms: ‘thyroid cancer’, ‘thyroid nodules’, ‘parathyroid’, ‘hyperparathyroidism’, ‘adrenal adenoma’, ‘Cushing syndrome’, ‘hyperaldosteronism’, ‘adrenocortical cancer’, ‘neuroendocrine tumor (NET)’, ‘pancreatic NET (pNET)’, ‘carcinoid’, ‘pituitary adenoma’, ‘pituitary neuroendocrine tumor (PitNET)’, ‘prolactinoma’, ‘pheochromocytoma/paraganglioma’. We found 37 studies describing the preclinical and clinical role of metformin in endocrine tumors. The available epidemiological data show an association between exposure of metformin and lower incidence of thyroid cancer and pNETs in diabetic patients. Metformin treatment has been associated with better response to cancer therapy in thyroid cancer and pNETs. Preclinical evidence suggests that the primary direct mechanisms of metformin action include inhibition of mitochondrial oxidative phosphorylation via inhibition of both mitochondrial complex I and mitochondrial glycerophosphate dehydrogenase, leading to metabolic stress. Decreased ATP production leads to an activation of a cellular energy sensor, AMPK, and subsequent downregulation of mTOR signaling pathway, which is associated with decreased cellular proliferation. We also describe several AMPK-independent mechanisms of metformin action, as well as the indirect mechanisms targeting insulin resistance. Overall, repositioning of metformin has emerged as a promising strategy for adjuvant therapy of endocrine tumors. The mechanisms of synergy between metformin and other anti-cancer agents need to be elucidated further to guide well-designed prospective trials on combination therapies in endocrine malignancies.
M J Waters, H N Hoang, D P Fairlie, R A Pelekanos and R J Brown
It has been 75 years since Evans and Long identified a somatic growth-promoting substance in pituitary extracts, yet it is only in the last 20 years that the molecular basis for this action has been established. Three key elements in this elucidation were the cloning of the GH receptor, the identification of Janus kinase (JAK) 2 as the receptor-associated tyrosine kinase, and the delineation of signal transduction and activators of transcription (STAT) 5a/b as the key transcription factor(s) activated by JAK2. The interaction between these three elements results in enhanced postnatal growth and is the subject of this review. We describe a new model for GH receptor activation based on subunit rotation within a constitutive dimer, together with the phenotype and hepatic transcript profile of mice with targeted knockins to the receptor cytoplasmic domain. These support a central role for STAT5a/b in postnatal growth.
Simona Volpi, Ying Liu and Greti Aguilera
Previous studies show that binding of nuclear proteins to GAGA repeats (GAGA box) in the vasopressin type 1b receptor (V1bR) promoter is essential for transcriptional initiation of the gene. To determine whether increased vasopressin (VP) during stress activates V1bR expression through the GAGA box, we examined the effects of VP on GAGA binding activity and on the ability of the V1bR promoter to recruit RNA polymerase in the hypothalamic cell line, H32. In chromatin immunoprecipitation assays, VP induced RNA polymerase II recruitment by the wild type V1bR promoter but not by a construct with the major GAGA box deletion. VP (10 min) also increased binding of nuclear proteins to radiolabeled GAGA oligonucleotides in electromobility shift assays. VP-induced GAGA binding activity was potentiated by the protein kinase C inhibitor, calphostin C, and was prevented by the MEK inhibitor, UO126, and the epidermal growth factor receptor (EGFR) inhibitor, AG1478, suggesting that VP activates GAGA binding through transactivation of the EGFR. This was confirmed by western blot experiments showing rapid increases in phospho ERK after incubation with VP, an effect that was potentiated by calphostin C and inhibited by UO12 and AG1478, as well as by the ability of VP to phosphorylate the EGFR. Using receptor selective VP analogs we showed that both V1aR and V1bR subtypes can mediate GAGA binding activation in H32 cells. This study demonstrates that VP stimulates GAGA binding to the V1bR promoter through transactivation of the EGFR and MAP kinase. The data support the hypothesis that VP contributes to pituitary V1bR upregulation during stress through GAGA binding-mediated transcriptional activation.
Andreas Hoeflich and Maximilian Bielohuby
The adrenal gland influences a multitude of processes during stress response, but also potently affects the immune system, glucose metabolism, electrolyte or water homeostasis, and cardiovascular functions. According to the present understanding, the adrenal cortex is tightly controlled by the hypothalamic–pituitary–adrenal axis. This axis involves hypothalamic CRH and pituitary ACTH which determine processes of adrenocortical growth and function. However, control of the adrenal gland comprises a plethora of additional endogenous or exogenous factors. Among those are diverse hormones, psychosocial parameters, physiological stress, secondary plant products, or even environmental pollutants. In the present review, we summarize the current view of endocrine growth control in the adrenal gland. We then discuss intracellular mechanisms of adrenal growth control and focus on extracellular signal regulated kinases 1/2 (ERK1/2), which have been demonstrated to be controlled by not only ACTH or angiotensin II, but also by a large number of additional effectors. On the basis of these multiple exogenous or endogenous factors which impact on the adrenal gland through ERK1/2 activity, we speculate on a mechanism by which ERK1/2 act as a central integrative growth regulatory elements in the adrenal gland.
Niamh Cosgrave, Arnold D K Hill and Leonie S Young
Survivin has emerged as a unique regulator of cell death through its response to growth factors, such as basic fibroblast growth factor (bFGF), which we have previously shown to be mitogen-activated protein kinase (MAPK) dependent. The transcriptional complex myc/max is an oncogene that lies downstream of the MAPK pathway, suggesting a possible role in survivin’s regulation. In this study, we investigated the ability of bFGF to induce signalling of the MAPK effector transcription factor c-myc in human breast cancer. Treatment of SK-BR-3 breast cancer cell line with growth factor induced survivin expression and recruitment of c-myc to its response element in the promoter region of the target gene survivin as demonstrated by electromobility shift analysis and chromatin immunoprecipitation assays. The promoter region of survivin was assessed using bioinformatic techniques and DNA footprinting. Overexpression of c-myc increased survivin protein expression. This effect was eliminated when siRNA against c-myc was transfected into the cells. c-Myc drove transcriptional activity of survivin when transfected into SK-BR-3 cells with a luciferase reporter vector harbouring the c-myc response element specific for survivin. Using confocal fluorescent microscopy, myc was located to the nucleus of breast tumour epithelial cells and was found to be significantly associated with survivin (P < 0.0001). These data provide evidence that growth factors can signal through the transcription factor c-myc in human breast cancer. They also indicate a role for c-myc in the transcriptional regulation of survivin in breast cancer.