Constitutive Somatostatin Receptor Subtype-3 Signaling Suppresses Growth Hormone Synthesis

Somatostatin signals through somatostatin receptor subtypes (SSTR) 2 and 5 to attenuate GH secretion. Although expressed in normal pituitary glands and in GH-secreting pituitary tumors, SSTR3 function was unclear, and we have now determined the role of SSTR3 in somatotroph function. Stable rat pituitary tumor cell (GC) transfectants of human SSTR3 (GpSSTR3WT) showed suppression of rat (r) GH promoter activity, GH mRNA expression, and secreted GH concordant with suppressed cAMP/protein kinase A (PKA) signaling. In contrast, cAMP levels and GH expression were unchanged in cells expressing a mutant SSTR3 DRY motif (GpSSTR3R141A). GH expression was rescued by treatment of GpSSTR3WT with forskolin and 8-bromo-cAMP. GpSSTR3WT exhibited activation of glycogen synthase kinase3-β (GSK3-β), a PKA substrate, which was also reversed by 8-Bromo-cAMP treatment. Moreover, SSTR3-dependent GH transcriptional inhibition was rescued by inhibition of GSK3-β. GpSSTR3WT exhibited elevated Pit-1 serine phosphorylation and decreased Pit-1 occupancy of the rGH promoter with sustained Pit-1 expression. GSK3-β and Pit-1 physically interacted with each other, indicating that Pit-1 may be a GSK3-β phosphorylation substrate. In conclusion, constitutive SSTR3 activity mediates transcriptional repression of GH through cAMP/PKA, leading to subsequent activation of GSK3-β and increased Pit-1 phosphorylation and ultimately attenuating Pit-1 binding to the rGH promoter.

[1]  Jen Q. Pan,et al.  Discovery of Potent and Highly Selective Inhibitors of GSK3b , 2014 .

[2]  S. Melmed,et al.  Constitutive somatostatin receptor subtype 2 activity attenuates GH synthesis. , 2013, Endocrinology.

[3]  Felipe F Casanueva,et al.  Sirt1 inhibits the transcription factor CREB to regulate pituitary growth hormone synthesis , 2013, FASEB journal : official publication of the Federation of American Societies for Experimental Biology.

[4]  U. Kumar,et al.  Coexpression of human somatostatin receptor-2 (SSTR2) and SSTR3 modulates antiproliferative signaling and apoptosis , 2012, Journal of molecular signaling.

[5]  J. Woodgett,et al.  Glycogen Synthase Kinase-3 in Neurological Diseases , 2012 .

[6]  James Robert Woodgett,et al.  Molecular Neuroscience Review Article Gsk-3: Functional Insights from Cell Biology and Animal Models , 2022 .

[7]  C. Sutherland.  What Are the bona fide GSK3 Substrates? , 2011, International journal of Alzheimer's disease.

[8]  M. Buchfelder,et al.  CRH and SRIF Have Opposite Effects on the Wnt/β-Catenin Signalling Pathway Through PKA/GSK-3β in Corticotroph Pituitary Cells , 2010, Cancer investigation.

[9]  S. Melmed,et al.  Pituitary somatostatin receptor signaling , 2010, Trends in Endocrinology & Metabolism.

[10]  Dianqing Wu,et al.  GSK3: a multifaceted kinase in Wnt signaling. , 2010, Trends in biochemical sciences.

[11]  P. Michael Conn,et al.  Constitutive activity in receptors and other proteins , 2010 .

[12]  S. Melmed,et al.  Constitutive activity of somatostatin receptor subtypes. , 2010, Methods in enzymology.

[13]  A. Gutierrez-Hartmann,et al.  A Pit-1 threonine 220 phosphomimic reduces binding to monomeric DNA sites to inhibit Ras and estrogen stimulation of the prolactin gene promoter. , 2010, Molecular endocrinology.

[14]  S. Melmed.  Acromegaly pathogenesis and treatment. , 2009, The Journal of clinical investigation.

[15]  S. Nonogaki,et al.  Acromegaly: correlation between expression of somatostatin receptor subtypes and response to octreotide-lar treatment , 2009, Pituitary.

[16]  D. Figarella-Branger,et al.  Somatostatin receptor sst2 decreases cell viability and hormonal hypersecretion and reverses octreotide resistance of human pituitary adenomas. , 2008, Cancer research.

[17]  U. Hochgeschwender,et al.  Null mutant mouse models of somatostatin and cortistatin, and their receptors , 2008, Molecular and Cellular Endocrinology.

[18]  R. Kineman,et al.  Quantitative analysis of somatostatin receptor subtypes (1-5) gene expression levels in somatotropinomas and correlation to in vivo hormonal and tumor volume responses to treatment with octreotide LAR. , 2008, European journal of endocrinology.

[19]  D. Clemmons,et al.  Modifying IGF1 activity: an approach to treat endocrine disorders, atherosclerosis and cancer , 2007, Nature Reviews Drug Discovery.

[20]  S. Melmed,et al.  Selective regulation of somatostatin receptor subtype signaling: evidence for constitutive receptor activation. , 2007, Molecular endocrinology.

[21]  Valérie Capra,et al.  The Highly Conserved DRY Motif of Class A G Protein-Coupled Receptors: Beyond the Ground State , 2007, Molecular Pharmacology.

[22]  T. Hori,et al.  Expression of type 5 somatostatin receptor in TSH-secreting pituitary adenomas: a possible marker for predicting long-term response to octreotide therapy. , 2007, Endocrine journal.

[23]  Rob Leurs,et al.  Pharmacogenomic and structural analysis of constitutive g protein-coupled receptor activity. , 2007, Annual review of pharmacology and toxicology.

[24]  R. Kineman,et al.  Quantitative analysis of somatostatin receptor subtype (SSTR1-5) gene expression levels in somatotropinomas and non-functioning pituitary adenomas. , 2007, European journal of endocrinology.

[25]  P. Lochhead,et al.  A chaperone-dependent GSK3beta transitional intermediate mediates activation-loop autophosphorylation. , 2006, Molecular cell.

[26]  S. Rhodes,et al.  Transcriptional control during mammalian anterior pituitary development. , 2003, Gene.

[27]  R. Seifert,et al.  Constitutive activity of G-protein-coupled receptors: cause of disease and common property of wild-type receptors , 2002, Naunyn-Schmiedeberg's Archives of Pharmacology.

[28]  R. Jope,et al.  The multifaceted roles of glycogen synthase kinase 3β in cellular signaling , 2001, Progress in Neurobiology.

[29]  J. Shabb.  Physiological substrates of cAMP-dependent protein kinase. , 2001, Chemical reviews.

[30]  R. Paylor,et al.  Impairment in motor learning of somatostatin null mutant mice , 2001, Brain Research.

[31]  M. Low,et al.  Somatostatin is required for masculinization of growth hormone-regulated hepatic gene expression but not of somatic growth. , 2001, The Journal of clinical investigation.

[32]  P. Sassone-Corsi,et al.  Cyclic AMP signalling. , 2001, Journal of cell science.

[33]  S. Schulz,et al.  Homo- and Heterodimerization of Somatostatin Receptor Subtypes , 2001, The Journal of Biological Chemistry.

[34]  Y. Patel.  Somatostatin and Its Receptor Family , 1999, Frontiers in Neuroendocrinology.

[35]  S. Melmed,et al.  Somatostatin receptor (SSTR) subtype-selective analogues differentially suppress in vitro growth hormone and prolactin in human pituitary adenomas. Novel potential therapy for functional pituitary tumors. , 1997, The Journal of clinical investigation.

[36]  J. Tentler,et al.  Somatostatin acts by inhibiting the cyclic 3',5'-adenosine monophosphate (cAMP)/protein kinase A pathway, cAMP response element-binding protein (CREB) phosphorylation, and CREB transcription potency. , 1997, Molecular endocrinology.

[37]  C. Tuggle,et al.  Control of growth hormone synthesis. , 1996, Domestic animal endocrinology.

[38]  C. Diéguez,et al.  Regulation of the pituitary-specific transcription factor GHF-1/Pit-1 messenger ribonucleic acid levels by growth hormone-secretagogues in rat anterior pituitary cells in monolayer culture. , 1995, Endocrinology.

[39]  C. Bancroft,et al.  A Pit-1 phosphorylation mutant can mediate both basal and induced prolactin and growth hormone promoter activity. , 1994, Molecular endocrinology.

[40]  P. Cohen,et al.  Inactivation of glycogen synthase kinase-3 beta by phosphorylation: new kinase connections in insulin and growth-factor signalling. , 1993, The Biochemical journal.

[41]  F. Wondisford,et al.  Hormonal regulation of the thyrotropin beta-subunit gene by phosphorylation of the pituitary-specific transcription factor Pit-1. , 1992, Proceedings of the National Academy of Sciences of the United States of America.

[42]  M. Wegner,et al.  Variable effects of phosphorylation of pit-1 dictated by the DNA response elements , 1991 .

[43]  M. Karin,et al.  Regulation of the pituitary-specific homeobox gene GHF1 by cell-autonomous and environmental cues , 1990, Nature.

[44]  J. W. Tanner,et al.  Modulation of growth hormone (GH) secretion and GH mRNA levels by GH-releasing factor, somatostatin and secretagogues in cultured bovine adenohypophysial cells. , 1990, The Journal of endocrinology.

[45]  H. Samuels,et al.  Identification of an adenosine 3',5'-monophosphate (cAMP)-responsive region in the rat growth hormone gene: evidence for independent and synergistic effects of cAMP and thyroid hormone on gene expression. , 1989, Molecular endocrinology.

[46]  M. Karin,et al.  Induction of human growth hormone promoter activity by the adenosine 3',5'-monophosphate pathway involves a novel responsive element. , 1989, Molecular endocrinology.

[47]  J. Harney,et al.  Multihormonal regulation of the human, rat, and bovine growth hormone promoters: differential effects of 3',5'-cyclic adenosine monophosphate, thyroid hormone, and glucocorticoids. , 1988, Molecular endocrinology.

[48]  L. Bilezikjian,et al.  Stimulation of adenosine 3',5'-monophosphate production by growth hormone-releasing factor and its inhibition by somatostatin in anterior pituitary cells in vitro. , 1983, Endocrinology.

[49]  J. Axelrod,et al.  Somatostatin inhibits multireceptor stimulation of cyclic AMP formation and corticotropin secretion in mouse pituitary tumor cells. , 1982, Proceedings of the National Academy of Sciences of the United States of America.