Selective regulation of somatostatin receptor subtype signaling: evidence for constitutive receptor activation.
Anterior pituitary hormone secretion is under tonic suppression by hypothalamic somatostatin signaling through somatostatin receptor subtypes (SSTs). Because some hormonal axes are known to be abnormally regulated by ligand-independent constitutively active G protein-coupled receptors, we tested pituitary SSTs for selective constitutive signaling. We therefore differentially silenced endogenous SST2, SST3, and SST5 in somatostatin-sensitive ACTH-secreting mouse AtT-20 pituitary corticotroph cells using small inhibitory RNA (siRNA) and analyzed downstream SSTs-regulated pathways. Transfection with siRNA reduced specific receptor subtype mRNA expression up to 82%. Specificity of receptor silencing was validated against negative controls with different gene-selective siRNAs, concordance of mRNA and cAMP changes, reduced potency of receptor-selective agonists, and phenotype rescue by overexpression of the silenced receptor. Mouse SST3 > SST5 > SST2 knockdown increased basal cAMP accumulation (up to 200%) and ACTH secretion (up to 60%). SST2- and SST5-selective agonist potencies were reduced by SST3- and SST5-silencing, respectively. SST5 > SST2 = SST3 silencing also increased basal levels of ERK1/2 phosphorylation. SST3- and SST5-knockdown increased cAMP was only partially blocked by pertussis toxin. The results show that SST2, SST3, and SST5 exhibit constitutive activity in mouse pituitary corticotroph cells, restraining adenylate cyclase and MAPK activation and ACTH secretion. SST3 mainly inhibits cAMP accumulation and ACTH secretion, whereas SST5 predominantly suppresses MAPK pathway activation. Therefore, SST receptor subtypes control pituitary cell function not only through somatostatin binding to variably expressed cell membrane receptor subtypes, but also by differential ligand-independent receptor-selective constitutive action.
Ben-Shlomo
Anat
A
Department of Medicine, Cedars Sinai Research Institute, David Geffen School of Medicine at UCLA, Los Angeles, California 90048, USA.
Pichurin
Oxana
O
Barshop
Nicole J
NJ
Wawrowsky
Kolja A
KA
Taylor
John
J
Culler
Michael D
MD
Chesnokova
Vera
V
Liu
Ning-Ai
NA
Melmed
Shlomo
S
D000818:Animals; D002460:Cell Line; D052680:Corticotrophs / Q000187:drug effects / Q000378:metabolism*; D000242:Cyclic AMP / Q000378:metabolism; D051379:Mice; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D048052:Mitogen-Activated Protein Kinase 3 / Q000378:metabolism; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism; D034741:RNA, Small Interfering / Q000494:pharmacology; D017481:Receptors, Somatostatin / Q000819:agonists* / Q000037:antagonists & inhibitors / Q000235:genetics; D015398:Signal Transduction
D034741:RNA, Small Interfering; D017481:Receptors, Somatostatin; C103631:somatostatin receptor 3; C106140:somatostatin receptor sst2A; C094573:somatostatin receptor 5; D000242:Cyclic AMP; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020929:Mitogen-Activated Protein Kinase Kinases