Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UMLS:C0043167 (pertussis)
 19,595 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Sodium ions have been shown to reduce the binding of agonists to a number of G protein-linked receptors. They are believed to do so by interacting with aspartate residues in the second membrane-spanning region of these receptors to cause G protein uncoupling, resulting in a diminished affinity of the receptors for agonists. To investigate Na+ regulation of agonist binding to somatostatin receptors, Na+ was tested for its effect on the binding of agonists to cloned somatostatin receptor type 1 (SSTR1) and somatostatin receptor type 2 (SSTR2) stably expressed in Chinese hamster ovary cells. Na+ reduced agonist binding to SSTR2 but not to SSTR1. Because high affinity agonist binding to SSTR1 does not depend on G protein coupling but agonist binding to SSTR2 is reduced by guanosine-5'-(beta, gamma-imido)triphosphate and pertussis toxin treatment, the selective Na+ effect on SSTR2 is consistent with previous findings with other receptors showing that Na+ uncouples receptors from G proteins, thereby reducing the affinity of the receptors for agonists. Conversion of Asp89 to Asn89 in SSTR2 resulted in a mutant receptor whose affinity for agonists was not altered by Na+, indicating that Asp89 is involved in mediating the effects of Na+ on agonist binding to SSTR2. However, the affinities of the mutant and wild-type receptors for somatostatin were the same, and both guanosine-5'-O-(gamma-thio)triphosphate and pertussis toxin treatment reduced agonist binding to the mutant and wild-type receptors. These findings differ from the results of similar mutagenesis studies on other G protein-linked receptors, in that the mutant and wild-type SSTR2 forms associate with G proteins in similar ways. These results indicate that Asp89 acts in a novel manner to regulate agonist binding and G protein interaction with SSTR2.
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PMID:Mutation of an aspartate at residue 89 in somatostatin receptor subtype 2 prevents Na+ regulation of agonist binding but does not alter receptor-G protein association. 810 84

Neuroblastoma is the most common extracranial solid tumor of children. Neuroblastoma tumors derive from the neural crest and synthesize neurotransmitters including the neuropeptide somatostatin. This study was designed to characterize somatostatin receptors both in primary neuroblastoma tumors and in two neuroblastoma cell lines, SKNSH and IMR32. Somatostatin receptors were identified in 6 of 7 Stage I and II compared to 7 of 19 Stage III and IV tumors. Down-regulation of somatostatin receptor binding was observed in five tumors during disease progression. A lack of high affinity binding of somatostatin was identified as a poor prognostic indicator; negative binding correlated with advanced disease and death. Somatostatin receptor binding was observed in the IMR32 cell line, but not in the SKNSH cell line. IMR32 cells demonstrated a single class of high affinity binding sites for both somatostatin and a synthetic analogue, octreotide (Kd 0.16 +/- 0.05 nM and 0.89 +/- 0.23 nM, respectively). Somatostatin and octreotide inhibited both vasoactive intestinal peptide-mediated and forskolin-mediated cyclic AMP accumulation in IMR32 cells. Somatostatin and octreotide inhibition of signal transduction was attenuated by pretreatment of the cells with pertussis toxin. Octreotide inhibited proliferation of IMR32 cells by 70% in a 6-day culture. In contrast, octreotide did not exhibit high affinity binding in SKNSH cells and had no effect on cyclic AMP accumulation or on proliferation in SKNSH cells. Together, these data indicate that octreotide interacts with high affinity somatostatin receptors to modulate signal transduction and regulate proliferation in neuroblastoma cell lines. These data also suggest that somatostatin receptor expression may be an independent prognostic factor in primary neuroblastoma tumors.
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PMID:Characterization of somatostatin receptors on human neuroblastoma tumors. 812 88

Somatostatin regulates diverse cellular effectors, including adenylyl cyclase, ion channels, and ion exchangers. We expressed two somatostatin receptor subtypes, SSTR1 and SSTR2, stably in mouse fibroblast Ltk- cells and transiently in human embryonic kidney HEK293 cells to investigate subtype-specific pharmacological and functional properties. The effects of GTP gamma S and pertussis toxin on [125I-Tyr11]somatostatin-14 binding indicated that SSTR2 may couple exclusively to pertussis toxin-sensitive G proteins, whereas SSTR1 may couple to both pertussis-sensitive and -insensitive G proteins. When expressed either stably or transiently, both receptor subtypes mediated somatostatin inhibition of cAMP accumulation by a pertussis toxin-sensitive mechanism. In contrast, only SSTR1 mediated somatostatin inhibition of Na(+)-H+ exchange activity, and this action was insensitive to pertussis toxin. We generated two chimeric receptors by replacing sequential residues of SSTR2 with cognate sequences of SSTR1 to identify molecular determinants unique to SSTR1 that may confer coupling to the exchanger. SSTCR4 included a SSTR1 segment encompassing determinants within the fifth and sixth hydrophobic domains and the entire third cytoplasmic loop, while SSTCR5 contained a SSTR1 segment spanning the second through sixth hydrophobic domains, including both second and third cytoplasmic loops. Although both chimeric receptors mediated somatostatin inhibition of cAMP accumulation, only SSTCR5 mediated the inhibition of Na(+)-H+ exchange activity, and this effect was pertussis-insensitive. These findings demonstrate both pharmacological and functional differences between SSTR1 and SSTR2. The ability of SSTR1 to selectively attenuate Na(+)-H+ exchange activity requires determinants outside the third cytoplasmic domain.
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PMID:Subtype-specific signaling mechanisms of somatostatin receptors SSTR1 and SSTR2. 814 17

Using a polymerase chain reaction approach, we have studied the expression of somatostatin receptor (SSTR) subtypes in the GH3 rat pituitary cell line, a well established in vitro model for the cellular effects of somatostatin. We found that the previously identified SSTR1 and SSTR2 are the major subtypes expressed in this cell line. No other SSTR subtype was detected by our analysis. Northern blots confirmed that both subtypes, but not SSTR3, are expressed in GH3 cells. We studied the functional expression of both SSTR subtypes by transfection of their cDNAs into human embryonic kidney 293 cells. We found that somatostatin inhibited cAMP accumulation in human embryonic kidney 293 cells only when cells were transfected with either SSTR1 or SSTR2. This inhibition was blocked by treatment of the transfected cells with pertussis toxin, demonstrating that it is mediated by G proteins sensitive to this toxin. In addition, we provide pharmacological evidence that the endogenous SSTR2 subtype mediates inhibition of cAMP accumulation in intact GH3 cells. Our results contradict previous reports that concluded thsat neither SSTR1 nor SSTR2 is involved in inhibition of adenylyl cyclase. The reasons for this apparent contradiction are discussed. We conclude that both SSTR1 and SSTR2 are capable of coupling to pertussis toxin-sensitive G proteins to inhibit adenylyl cyclase.
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PMID:Pituitary cell line GH3 expresses two somatostatin receptor subtypes that inhibit adenylyl cyclase: functional expression of rat somatostatin receptor subtypes 1 and 2 in human embryonic kidney 293 cells. 814 27

Somatostatin has a modulatory role in regulating the membrane conductance in hippocampal neurons. To examine the signal transducing molecules involved in this process, we isolated the cDNA encoding the dominant rat hippocampal somatostatin receptor, SSTR4. Distribution of SSTR4 in the adult central nervous system was restricted to the hippocampus, cerebral cortex, striatum, hypothalamus, and thalamus, as determined by Northern blot analysis and in situ hybridization. In SSTR4-expressing Chinese hamster ovary cells, SSTR4 was functionally coupled not only to inhibition of adenylate cyclase, but also to activation of both arachidonate release and mitogen-activated protein (MAP) kinase cascade, with similar ED50 values. All of these pathways, including both MAP kinase kinase and MAP kinase activation, were completely blocked by pretreatment with pertussis toxin. On the other hand, neither inositol 1,4,5-trisphosphate synthesis nor intracellular Ca2+ mobilization was induced upon SSTR4 stimulation. These data indicate that the hippocampal functions of somatostatin might be mediated through diverse but selective second messenger systems activated via SSTR4 and reveal an unsuspected coupling of a neuronal SSTR subtype to a mitogenic signaling pathway. SSTR4, in addition, provides a useful system to study the Ca(2+)-independent, Gi-dependent (pertussis toxin-sensitive) pathway of MAP kinase activation.
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PMID:Functional coupling of SSTR4, a major hippocampal somatostatin receptor, to adenylate cyclase inhibition, arachidonate release and activation of the mitogen-activated protein kinase cascade. 817 84

The somatostatin receptor subtypes SSTR2 and SSTR5 mediate distinct endocrine and exocrine functions of somatostatin and may also be involved in mediating the neuromodulatory actions of somatostatin in the brain. To investigate whether these receptors couple to voltage-sensitive Ca2+ channels, SSTR2 and SSTR5 selective agonists were tested for their effects on AtT-20 cells using whole cell patch clamp techniques. The SSTR2 selective agonist MK 678 inhibited Ca2+ currents in AtT-20 cells. The effects of MK 678 were reversible and blocked by pertussis toxin pretreatment, suggesting that SSTR2 couples to the L-type Ca2+ channels via G proteins. Other SSTR2-selective agonists, including BIM 23027 and NC8-12, were able to inhibit the Ca2+ currents in these cells. The SSTR5 selective agonist BIM 23052 also inhibited the Ca2+ currents in these cells and this effect was reversible and blocked by pertussis toxin treatment. The ability of SSTR5 to mediate inhibition of the Ca2+ current was greatly attenuated by pretreatment with the SSTR5-selective agonist BIM 23052, whereas SSTR2-mediated inhibition of the Ca2+ current was not altered by pretreatment with the SSTR2-selective agonist MK 678. Thus, the SSTR2 and SSTR5 couplings to the Ca2+ current are differentially regulated. The peptide L362,855, which we previously have shown to have high affinity for the cloned SSTR5, had minimal effects on Ca2+ currents in AtT-20 cells at concentrations up to 100 nM and did not alter the ability of MK 678 to inhibit Ca2+ currents. However, it completely antagonized the effects of the SSTR5-selective agonist BIM 23052 on the Ca2+ currents. L362,855 is an antagonist/partial agonist at SSTR5 since it can reduce Ca2+ currents in these cells at concentrations above 100 nM. L362,855 is also an antagonist/partial agonist at the cloned rat SSTR5 expressed in CHO cells since it is able to block the inhibition of cAMP accumulation induced by somatostatin at concentrations below 100 nM but at higher concentrations can inhibit cAMP formation itself. Structural analysis of L362,855 reveals that only a single hydroxyl group at residue seven in the peptide is needed to convert the compound from an antagonist/partial agonist to a full agonist at SSTR5. These studies reveal that two different somatostatin receptor subtypes, SSTR2 and SSTR5, can mediate the inhibition of an L-type Ca2+ channel in AtT-20 cells by somatostatin. The receptor subtype responses can be distinguished by selective agonists and antagonists and are regulated differently by agonist pretreatment. The inhibition of Ca2+ influx into endocrine cells and neurons may be a major cellular mechanism by which somatostatin modulates hormone and neurotransmitter release. Our results reveal that at least two receptor subtypes can mediate this cellular response.
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PMID:Somatostatin receptor subtypes SSTR2 and SSTR5 couple negatively to an L-type Ca2+ current in the pituitary cell line AtT-20. 868 11

The alpha 2A-adrenergic receptor (alpha 2AAR), via its interaction with the pertussis toxin-sensitive Gi/G(o) class of G proteins, modulates multiple effector systems, including inhibition of adenylyl cyclase and Ca2+ channels and activation of K+ channels. Mutation of a membrane-embedded aspartate residue, highly conserved among G protein-coupled receptors, in the alpha 2AAR to asparagine (D79N alpha 2AAR) results in selective uncoupling of the receptor to K+ currents but retention of inhibition of cAMP production and of voltage-sensitive Ca2+ currents when expressed in AtT20 anterior pituitary cells in culture. It is known that attenuation of cAMP synthesis alone cannot account for alpha 2AAR suppression of stimulus-secretion coupling; thus, the D79N alpha 2AAR provides a unique tool with which to assess the relative contribution of K+ current activation and Ca2+ current suppression in mediating the cellular responses of alpha 2AAR. The wild-type alpha 2AAR suppresses basal and secretagogue-evoked adrenocorticotropic hormone (ACTH) release in a manner indistinguishable from response to the endogenous somatostatin receptor. In contrast, the D79N alpha 2AAR does not attenuate basal ACTH release and is only partially effective in suppressing ACTH secretion evoked by the secretagogue isoproterenol. Regulation of ACTH release evoked by 8-bromo-cAMP, which bypasses receptor regulation of cAMP synthesis, suggests that attenuation of cAMP production, although not sufficient for inhibition of ACTH secretion, nevertheless participates in a functionally relevant manner. Taken together, the present findings indicate that alpha 2AAR-mediated suppression of neuropeptide secretion requires concomitant regulation of K+ and Ca2+ currents in parallel with attenuation of cAMP production.
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PMID:Genetic evidence for involvement of multiple effector systems in alpha 2A-adrenergic receptor inhibition of stimulus-secretion coupling. 870 Jan 25

Effector coupling of somatostatin receptor subtypes sst1 and sst2 was examined in a reconstituted system. Forskolin-stimulated cyclic adenosine monophosphate (cAMP) formation was inhibited 66% by somatostatin (SRIF-14) in CHO cells expressing somatostatin receptor 1(sst1) (CHO-SR1), but not sst2, in a dose-dependent manner with an ED50 of 1 x 10(-9) mol/L SRIF-14. The inhibition was blocked by pertussis toxin (PTX), indicating that sst1 is coupled to adenylyl cyclase via PTX-sensitive Gi protein. In CHO cells, Gi alpha 2 and Gi alpha 3 mRNAs were detected. In adenylyl cyclase assays, 1 mumol/L SRIF-14 caused a 16% inhibition of forskolin-stimulated adenyly cyclase activity. Preincubation with Gi alpha 3, but not Gi alpha 1/Gi alpha 2, antiserum blocked this inhibition. By contrast, sst2 is coupled to adenylyl cyclase via Gi alpha 1. In cells expressing sst2 with Gi alpha 1(CHO-SR2G1), SRIF-14 significantly inhibited forskolin-stimulated cAMP formation by 53% and with an ED50 at 4 x 10(-9)mmol/L SRIF-14, which was completely blocked by PTX; ED50 values for sst1 and sst2 agree with the IC50 values in binding assays. In CHO-SR1, the rank of potency of agonists affecting adenyl cyclase was SRIF-14 = SRIF-28 > RC 160 > SMS 201-995. In CHO-SR2G1, the rank was RC-160 > SRIF-14 = SRIF-28 > SMS 201-995.
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PMID:Effector coupling of somatostatin receptor subtypes on human endocrine tumors. 876 78

The signal transduction pathways regulated by somatostatin receptor subtype 1 (sst1) have been difficult to define because of the variability observed when this receptor is expressed in different cell types by transfection and because pharmacological approaches are inadequate to distinguish sst1 receptor subtypes. To study the sst1 receptor in its endogenous environment, we developed a polyclonal antibody to a 15-amino acid peptide corresponding to a unique sequence in the receptor carboxyl terminus. The peptide antibody routinely precipitated 70% of the soluble [125I-Tyr11]somatostatin/receptor complex prepared from Chinese hamster ovary-K1 cells expressing the sst1 receptor but precipitated < 1% of the complex from cells expressing other sst receptor subtypes. Photoaffinity-labeled sst1 receptor was also specially immunoprecipitated and migrated as a broad 60-kDa band on sodium dodecyl sulfate polyacrylamide gels. The observation that sst receptors from GH4C1 pituitary cells were immunoprecipitated by the antibody and that receptors from AR4-2J pancreatic acinar cells were not indicated that only the former expressed sst1 receptor protein. Because reverse transcription-polymerase chain reaction showed that GH4C1 cells contained both sst1 and sst2 receptor mRNA, immunoprecipitation permitted the sst1 receptor to be separated from the other receptors present. Two observations showed that G proteins were coprecipitated with sst1 receptors from GH4C1 cells. First, pertussis toxin pretreatment markedly decreased hormone binding in the immunoprecipitate. Second, the addition of 20 microM guanosine-5'-(gamma-thio)triphosphate to the immunoprecipitated [125I-Tyr11]somatostatin/receptor complex stimulated the rate of dissociation of bound ligand by 10-fold. Interestingly, however, the dissociation rate of approximately 30% of the ligand/receptor complex was unaffected by guanosine-5'-(gamma-thio)triphosphate. In summary, we have developed an sst1 receptor-specific antibody and used it to show that sst1 receptors endogenously expressed in GH4C1 pituitary cells couple primarily to pertussis toxin-sensitive G proteins. Furthermore, these receptors exist in two distinct high affinity states distinguished by their GTP sensitivity.
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PMID:Development and use of a receptor antibody to characterize the interaction between somatostatin receptor subtype 1 and G proteins. 884 99

1. Receptor-dependent internalization of somatostatin (SRIF) agonists has been a matter of controversy probably because [125I]Tyr11-SRIF-14 is rapidly degraded. We have studied the internalization of a stable somatostatin analogue, [125I]-BIM-23027, in a neuronal cell line, Neuro2A, which natively expresses somatostatin sst2 receptors. 2. Incubation of Neuro2A cells with [125I]-BIM-23027 at 37 degrees C resulted in a time-dependent internalization of the ligand, which reached a maximum at 30 min. Acid-washing showed that cell-surface binding of the ligand accounted for only 34% of total binding at this time. Internalization was dramatically reduced at 15 degrees C. 3. Internalization of [125I]-BIM-23027 was prevented by inclusion of unlabelled somatostatin receptor agonists in a concentration-dependent manner. The IC50 values for inhibition of [125I]-BIM-23027 internalization were approximately 100 fold lower than for inhibition of [125I]-BIM-23027 binding to membrane homogenates but followed the same rank order of potencies. 4. Disruption of G-protein coupling by treatment with pertussis toxin caused a 60% reduction in internalization of ligand. A combination of antimycin (50 nM) and deoxyglucose (50 mM) pretreatment, which leads to a depletion of cellular ATP, decreased internalization of [125I]-BIM-23027 by 66% of control and increased the proportion of surface-bound ligand. Hypertonic sucrose, which prevents clathrin-mediated endocytosis, reversibly abolished the internalization of ligand without increasing the proportion bound at the cell surface. 5. After internalization of [125I]-BIM-23027, approximately half of the ligand was recycled back to the extracellular medium within 20 min at 37 degrees C. This finding suggests that the intracellular content of [125I]-BIM-23027 reaches a steady state which is determined by the rates of both internalization and recycling of the ligand. In contrast to studies in which the internalization of [125I]-Tyr11-SRIF-14 was examined, neither internalized nor recycled [125I]-BIM-23027 was degraded to its component amino acids. 6. These findings indicate that the somatostatin agonist, [125I]-BIM-23027, is internalized in a receptor-dependent manner which involves clathrin-coated pits in Neuro2A cells. Furthermore, much of the internalized ligand is rapidly recycled back to the extracellular medium without undergoing significant degradation.
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PMID:Somatostatin receptors in Neuro2A neuroblastoma cells: ligand internalization. 911 98


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