Abstract
We have discovered a novel compound, J-2156 [(1′S, 2S)-4-amino-N-(1′-carbamoyl-2′-phenylethyl)-2-(4″-methyl-1″-naphthalenesulfonylamino)butanamide], that belongs to a new class of somatostatin receptor ligands. J-2156 binds with nanomolar affinity to the human somatostatin receptor subtype 4 and is over 400-fold subtype-selective against the other somatostatin receptors. When evaluated in a [35S]guanosine-5′-O-(3-thio) triphosphate binding assay, J-2156 elicited a response 2 to 3 times as large as that of somatostatin-28 and somatostatin-14. That somatostatin-14 is clearly not a maximally efficacious agonist could be verified by demonstrating that it displays the typical behavior of a partial agonist when tested against J-2156. Increasing concentrations of somatostatin-14 cause a concentration-dependent rightward shift of the dose-response curves for J-2156, without affecting its maximal response. This lack of reduction of the maximal response and the fact that the superior efficacy of J-2156 is detected in membranes argue against desensitization and internalization as possible explanations for the superior efficacy of J-2156. More likely is that somatostatin-14 and J-2156 stabilize distinct receptor conformations that differ in their ability to interact with G-proteins. In a cyclic AMP assay, J-2156, somatostatin-28, and somatostatin-14 all act as full agonists. However, this outcome is most likely due to the presence of a receptor reserve in the cyclic AMP assay since there is a large gain of apparent potency in the cyclic AMP assay and the gain is larger for J-2156 than for somatostatin. We conclude that the endogenous ligands somatostatin-14 and somatostatin-28 do not define maximal agonism on the human somatostatin receptor subtype 4 and that J-2156 represents a so-called superagonist.
Footnotes
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This work was supported by the National Technology Agency of Finland (Tekes).
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doi:10.1124/jpet.104.075531.
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ABBREVIATIONS: SRIF, somatotropin release-inhibiting factor; GPCR, G-protein-coupled receptor; sst1-5, somatostatin receptor subtypes 1 through 5; h sst1-5, human sst1-5; J-2156, (1′S, 2S)-4-amino-N-(1′-carbamoyl-2′-phenylethyl)-2-(4″-methyl-1″-naphthalenesulfonylamino)butanamide; DMF, N,N-dimethylformamide; DCM, dichloromethane; Fmoc, 9-fluorenylmethoxycarbonyl; Boc, tert-butyloxycarbonyl; CHO, Chinese hamster ovary; 125I-LTT-SRIF-28, (125I-Tyr)-[Leu8,DTrp22]-somatostatin-28; [35S]GTPγS, [35S]guanosine-5′-O-(3-thio)triphosphate.
- Received August 2, 2004.
- Accepted August 27, 2004.
- The American Society for Pharmacology and Experimental Therapeutics
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