Binding properties of somatostatin receptor subtypes

Abstract 

In the past few years, five different somatostatin (SRIF) receptor subtypes (sst_1–5) have been identified, which form a distinct group in the superfamily of G-protein—coupled receptors. The naturally occurring somatostatins SRIF-28, SRIF-25, and SRIF-14 all reveal high-affinity binding for sst_1–5. In contrast, short synthetic analogs that are in clinical use, such as SMS 201–995, RC 160, or BIM 23014, primarily interact with the sst₂ subtype. Some SRIF analogs were previously reported to be selective for one SRIF receptor subtype, eg, the sst₂ (MK 678), the sst₃ (BIM 23056), or the sst₅ (BIM 23052, L362–L855) subtype. However, when we studied the binding affinities of these SRIF analogs for human (h) sst_1–5 expressed in either CHO or COS-1 cells, we were unable to confirm these previously reported selectivities. The absence of sst antagonists is a major drawback for investigating the functional role of each sst subtype. We used site-directed mutagenesis to identify amino acids that determine ligand specificity for sst₂. A single Ser305 to Phe mutation in TM VII increased the affinity of hsst₁ for SMS 201–995 nearly 100-fold, and when Gln291 was also exchanged to Asn in TM VII of hsst₁, almost full sst₂-like binding of SMS 201–995 was obtained. These data may aid in the design and synthesis of new selective type sst ligands. We have identified the expression of sst subtypes in nonclassical SRIF target tissue such as the lung. The pK_i values for SRIF and various SRIF analogs in rat lung tissue preparations were in close correlation with those obtained for CHO cells expressing the sst₄ subtype. Furthermore, reverse transcriptase polymerase chain reaction (RT-PCR) experiments revealed the predominant expression of mRNA specific for sst₄ in mouse, rat, and human lung tissue, confirmed by autoradiographies of rat lung. No specific binding for [¹²⁵I]Tyr³-SMS 201–995 was detected, since SMS 201–995 has low affinity for sst₄. In contrast, specific binding of [¹²⁵I]SRIF-28 to rat lung sections was demonstrated, which could be displaced by unlabelled SRIF-14 and SRIF-28, indicating specific, high affinity binding of this radioligand to sst₄ receptors.

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