The two hypothalamic hormones, GH-releasing hormone (GHRH) and somatostatin (SRIF), are known to regulate GH secretion. However, the effects of these hormones on GH gene expression are not completely clear, partly because of the lack of appropriate host cells maintaining the original characteristics of the somatotroph. Since MtT/S, a pure somatotroph cell line, has become available, the effects of GHRH and SRIF on GH gene transcription have been studied using a subclone of MtT/S (MtT/SGL), in which the GH gene 5'-promoter-luciferase fusion gene was stably incorporated. The expression of GHRH receptor and SRIF receptor subtypes was also studied by RT-PCR. The results showed that MtT/SGL cells intrinsically expressed the functional GHRH receptor and all of the SRIF receptor subtypes. The expression of GHRH receptor was markedly enhanced by glucocorticoid pretreatment and, in the presence of corticosterone and 3-isobutyl-1-methylxanthine, GHRH (at or above 100 pM) stimulated GH gene 5'-promoter activity in a dose-dependent manner. On the other hand, SRIF (100 nM) significantly antagonized the effect of GHRH, which was completely reversed by pretreatment with pertussis toxin (50 ng/ml). Taken together, the present data indicated that both GHRH and SRIF are involved in the transcriptional regulation of the GH gene, and that the effect of SRIF is mediated through pertussis toxin-sensitive G protein. The MtT/SGL cell line is a good in vitro model for studying the molecular mechanisms of GH gene transcription by GHRH and/or SRIF.
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- Abstract: Pituitary x
- Abstract: Brain x
- Abstract: Tumours x
- Abstract: Hypothalamus x
- Abstract: Kisspeptin x
- Abstract: ACTH x
- Abstract: TSH x
- Abstract: Cushing's x
- Abstract: NETs x
- Abstract: Paraganglioma x
- Abstract: Vasopressin x
- Abstract: neuroendocrine x
- Abstract: Growth x
- Abstract: somatostatin x
M Morishita, Y Iwasaki, A Onishi, M Asai, N Mutsuga, M Yoshida, Y Oiso, K Inoue and T Murohara
D. Bretherton-Watt, M. A. Ghatei, S. Legon, H. Jamal, K. Suda and S. R. Bloom
Islet amyloid polypeptide (IAPP) in the pancreas of the spontaneously diabetic (BB) Wistar rat was examined by radioimmunoassay, and IAPP mRNA levels were determined by Northern blotting. IAPP-like immunoreactivity in the diabetic rat pancreas was found to be significantly depleted compared with control (non-diabetic) BB rats (85·9±5 pmol/g in control rats, n = 8, vs 8·97 ± 0·9 pmol/g in diabetic rats, n=5; mean ± s.e.m.). A similar change in insulin concentrations was found, although insulin was present in approximately 100-fold greater amounts than IAPP. Chromatography of the IAPP immunoreactivity revealed a single molecular form, corresponding to synthetic IAPP. Northern blot analysis of pancreatic RNA (n = 4) revealed that IAPP mRNA in the diabetic group was depleted to 22% of the signal intensity in the control group. Insulin mRNA was dramatically reduced to only 4% of the control group and, in contrast, somatostatin was relatively unaffected, with the diabetic group retaining 86% of signal compared with the controls.
This animal model of insulin-dependent diabetes results from severe autoimmune destruction of the β cell. The extremely low levels of both insulin and its messenger RNA are in agreement with this. These results demonstrate that this pathological state is also associated with a loss of IAPP from the pancreas. Insulin-dependent diabetes is associated with a range of metabolic disturbances. It is possible that the concomitant depletion of IAPP may be a contributory factor in exacerbating the condition.
Andreas Petri, Jonas Ahnfelt-Rønne, Klaus Stensgaard Frederiksen, David George Edwards, Dennis Madsen, Palle Serup, Jan Fleckner and R Scott Heller
To understand the molecular mechanisms regulating pancreatic endocrine development and function, pancreatic gene expression was compared between Ngn3-deficient mice and littermate controls on embryonic days 13 and 15. Microarray analysis identified 504 genes with significant differences in expression. Fifty-two of these showed at least twofold reduction in Ngn3 knockouts compared to controls. Many of them were previously described to be involved in endocrine development and function. Among the genes not previously characterized were Rhomboid veinlet-like 4, genes involved in tetrahydrobiopterin biosynthesis and the Iroquois-type homeobox gene Irx1, the latter was selected for further investigation. In situ hybridisation demonstrated that two Iroquois genes, Irx1 and Irx2, were expressed in pancreatic endoderm of wild-type, but not Ngn3 mutant embryos. Furthermore, ectopic Ngn3 induced prominent Irx2 expression in chicken endoderm. Co-labelling established that Irx1 and Irx2 mRNA is located to glucagon-, but not insulin- or somatostatin-producing cells in mice and chicken. These data suggest that Irx1 and Irx2 serve an evolutionary conserved role in the regulation of α-cell-specific gene expression.
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.
SA Butler, P Laidler, Porter JR, AT Kicman, T Chard, DA Cowan and RK Iles
The free beta-subunit of human chorionic gonadotrophin (hCGbeta) is well recognised as a product of many epithelial tumours. Recently, it has been shown that this ectopic production may have a functional relationship to tumour growth. The growth-promoting activity of hCGbeta may be explained by its structural similarity to a family of growth factors which all contain the same distinct topological fold known as the cystine-knot motif. Since the other members of this family all exhibit their activities as homo- and heterodimers, it is possible that the same may be true for hCGbeta. Using size-exclusion chromatography, low stringency SDS-PAGE and matrix assisted laser desorption/ionisation (MALDI) time-of-flight (TOF) mass spectrometry (MS) we have shown that pure preparations of hCGbeta contain hCGbetabeta homodimers. Size-exclusion chromatography revealed asymmetric elution profiles with a forward peak corresponding to the size-exclusion characteristic of a globular protein with an approximate mass of 44-54 kDa and a late shoulder centered around an elution position expected for a globular protein of approximately 29 kDa. Two immunoreactive hCGbeta species, of approximately 32 and 64 kDa, were clearly resolved by SDS-PAGE and Western blotting. When analysed by MALDI-TOF MS a |mf23 kDa monomer and a |mf46 kDa dimer were identified. Formation of hCGbetabeta homodimers is consistent with the behaviour of other cystine-knot growth factors and strengthens the inclusion of the glycoprotein hormones within this superfamily. It has yet to be determined whether it is this dimeric molecular species that is responsible for growth-promoting activity of hCGbeta preparations in tumours.
Dellel Rezgui, Christopher Williams, Sharon A Savage, Stuart N Prince, Oliver J Zaccheo, E Yvonne Jones, Matthew P Crump and A Bassim Hassan
The mannose 6-phosphate/IGF 2 receptor (IGF2R) is comprised of 15 extra-cellular domains that bind IGF2 and mannose 6-phosphate ligands. IGF2R transports ligands from the Golgi to the pre-lysosomal compartment and thereafter to and from the cell surface. IGF2R regulates growth, placental development, tumour suppression and signalling. The ligand IGF2 is implicated in the growth phenotype, where IGF2R normally limits bioavailability, such that loss and gain of IGF2R results in increased and reduced growth respectively. The IGF2R exon 34 (5002A>G) polymorphism (rs629849) of the IGF2 specific binding domain has been correlated with impaired childhood growth (A/A homozygotes). We evaluated the function of the Gly1619Arg non-synonymous amino acid modification of domain 11. NMR and X-ray crystallography structures located 1619 remote from the ligand binding region of domain 11. Arg1619 was located close to the fibronectin type II (FnII) domain of domain 13, previously implicated as a modifier of IGF2 ligand binding through indirect interaction with the AB loop of the binding cleft. However, comparison of binding kinetics of IGF2R, Gly1619 and Arg1619 to either IGF2 or mannose 6-phosphate revealed no differences in ‘on’ and ‘off’ rates. Quantitative PCR, 35S pulse chase and flow cytometry failed to demonstrate altered gene expression, protein half-life and cell membrane distribution, suggesting the polymorphism had no direct effect on receptor function. Intronic polymorphisms were identified which may be in linkage disequilibrium with rs629849 in certain populations. Other potential IGF2R polymorphisms may account for the correlation with childhood growth, warranting further functional evaluation.
Pilar Argente-Arizón, Santiago Guerra-Cantera, Luis Miguel Garcia-Segura, Jesús Argente and Julie A Chowen
The search for new strategies and drugs to abate the current obesity epidemic has led to the intensification of research aimed at understanding the neuroendocrine control of appetite and energy expenditure. This intensified investigation of metabolic control has also included the study of how glial cells participate in this process. Glia, the most abundant cell type in the central nervous system, perform a wide spectrum of functions and are vital for the correct functioning of neurons and neuronal circuits. Current evidence indicates that hypothalamic glia, in particular astrocytes, tanycytes and microglia, are involved in both physiological and pathophysiological mechanisms of appetite and metabolic control, at least in part by regulating the signals reaching metabolic neuronal circuits. Glia transport nutrients, hormones and neurotransmitters; they secrete growth factors, hormones, cytokines and gliotransmitters and are a source of neuroprogenitor cells. These functions are regulated, as glia also respond to numerous hormones and nutrients, with the lack of specific hormonal signaling in hypothalamic astrocytes disrupting metabolic homeostasis. Here, we review some of the more recent advances in the role of glial cells in metabolic control, with a special emphasis on the differences between glial cell responses in males and females.
Soojin Kim, Daksh Thaper, Samir Bidnur, Paul Toren, Shusuke Akamatsu, Jennifer L Bishop, Colin Colins, Sepideh Vahid and Amina Zoubeidi
Neuroendocrine (NE) differentiation of advanced prostate adenocarcinoma following androgen receptor (AR) axis-directed therapy is becoming increasingly recognized. Several models of this transdifferentiation provide insight into its molecular pathogenesis and have highlighted the placental gene PEG10 for further study. Using our unique model of enzalutamide resistance (ENZR) and NE differentiation, we studied PEG10/AR interplay in enzalutamide treatment-resistant cell lines 42DENZR and 42FENZR compared to LNCaP and castration-resistant 16DCRPC cells. ENZR cell lines with positive terminal NE marker status also displayed higher baseline expression of PEG10 compared to LNCaP and 16DCRPC. Antagonism of AR activity increased PEG10 expression followed by an increase in terminal NE markers. Conversely, stimulating AR activity via androgen supplementation reversed PEG10 and NE marker expression in a time and dose-dependent manner. These results were supported by human data showing that PEG10 expression is highest in NEPC and that AR-dependent gene, PSA, is negatively correlated with PEG10 in adenocarcinoma. Further, ChIP assay confirmed binding of activated AR to the PEG10 enhancer, decreasing PEG10 expression. While PEG10 did not drive NEPC, its knockdown reduced NE markers in our cell lines. Moreover, PEG10 knockdown in vitro- and in vivo-attenuated tumor growth. Overall, these observations indicate that PEG10 is an AR-repressed gene which modulates NE markers in ENZR cells and targeting PEG10 in advanced prostate cancer with NE features is a rational and viable option.
J C Pascall
Steroid hormones generally mediate their effects by interacting with specific receptors which then bind to defined DNA sequences in the regulatory regions of target genes to activate expression (see Gronmeyer (1992) and references therein). However, the post-transcriptional regulation of gene expression by steroid hormones is also well documented (see Nielsen & Shapiro 1990). Indeed steroid hormones were amongst the first agents to be demonstrated to play a role in mRNA stabilization (Palmiter & Carey 1974). For example, glucocorticoid hormones have been shown to enhance the stability of growth hormone mRNA (Paek & Axel 1987), testosterone has been reported to induce changes in the poly(A) tail length of the mRNA encoding cystatin-related protein (Vercaeren et al. 1992) and testosterone and/or oestrogen induce changes in the poly(A) tail length of the vasopressin mRNA (Carter & Murphy 1993) associated with changes in mRNA accumulation. However, it is still unclear how steroids mediate these
Shereen Ezzat and Sylvia L Asa
In this review, we cover the evidence implicating Ikaros as a key factor whose transcriptional actions and chromatin remodeling properties determine the fate of hypothalamic neuroendocrine and pituitary cell population expansion. We propose that the governing mechanisms involved in the regulation and action of Ikaros are of importance during developmental as well as neoplastic transitions.