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Effector coupling of somatostatin receptor subtypes on human endocrine tumors
A Kubota1, Y Yamada, S Kagimoto
1Department of Metabolism and Clinical Nutrition, Kyoto University, Japan.
Abstract:
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.
Insights
Somatostatin receptor 1 (sst1) couples to adenylyl cyclase via Gi protein, while sst2 couples via Gi alpha 1. This reveals distinct signaling pathways for somatostatin receptor subtypes.
Area of Science:
- Molecular Pharmacology
- G protein-coupled receptors
- Signal Transduction
Background:
- Somatostatin receptors (sstr) are G protein-coupled receptors involved in various physiological processes.
- Understanding the specific effector coupling of sstr subtypes is crucial for elucidating their distinct signaling mechanisms.
- Previous studies suggested differential coupling, but the precise Gi protein involvement remained to be fully elucidated.
Purpose of the Study:
- To investigate the effector coupling mechanisms of somatostatin receptor subtypes sst1 and sst2 in a reconstituted system.
- To determine the specific Gi protein subtypes involved in sst1 and sst2-mediated inhibition of adenylyl cyclase activity.
- To compare the signaling pathways utilized by sst1 and sst2.
Main Methods:
- Functional assays were performed in Chinese Hamster Ovary (CHO) cells expressing either sst1 or sst2.
- Forskolin-stimulated cyclic adenosine monophosphate (cAMP) formation was measured in the presence of somatostatin (SRIF-14) and various agonists.
- Pertussis toxin (PTX) treatment and specific Gi alpha subunit antisera were used to identify the G protein involvement.
Main Results:
- Sst1 significantly inhibited forskolin-stimulated cAMP formation in a dose-dependent manner, an effect blocked by PTX, indicating coupling to PTX-sensitive Gi protein.
- Sst1-mediated inhibition was specifically blocked by Gi alpha 3 antiserum, suggesting coupling via Gi alpha 3.
- Sst2 significantly inhibited cAMP formation via Gi alpha 1, as demonstrated by PTX blockade and experiments with sst2 co-expressed with Gi alpha 1.
Conclusions:
- Somatostatin receptor subtype 1 (sst1) couples to adenylyl cyclase through a PTX-sensitive Gi protein, likely Gi alpha 3.
- Somatostatin receptor subtype 2 (sst2) couples to adenylyl cyclase via Gi alpha 1.
- These findings highlight distinct effector coupling pathways for sst1 and sst2, contributing to their differential physiological roles.