The cloning of a family of SRIF receptors confirmed that subtypes of these receptors exist and are expressed in the body (1,6). Bell and his associates (7,8) cloned the first three SRIF receptors and named them sstr1, sstr2, and sstr3. Berelowitz and his associates (9) cloned the fourth receptor and O'Carroll (10) cloned the fifth receptor. The five receptors have approx 40% amino-acid sequence similarity, with highest similarity in the transmembrane spanning regions (1,6). They are dissimilar in amino-acid sequence from any other receptors except the opiate receptors (11). Interestingly, some SRIF analogs, such as octreotide, bind potently to opiate receptors (12) and the selective mu opiate antagonist CTOP was designed based on the structure of octreotide (12), suggesting that the structural similarities of these two families of receptors may afford some functional similarities.
The genes of the five receptors are localized to different chromosomes (1). Both RNA analysis and in situ hybridization studies have shown that the mRNA for the different receptors have distinct but overlapping distributions (1,6). Most notably, all five receptor mRNAs are expressed in the anterior pituitary, although at different levels.
The pharmacological properties of each receptor have been characterized following their expression in different tumor cell lines. All five receptors bind SRIF and SRIF 28 with high affinities (14,15). Only sstr5 shows a preferential affinity for SRIF 28 over SRIF. Both sstrl and sstr4 have low affinity for most synthetic analogs of SRIF. This similarity is consistent with the unusually high amino-acid sequence similarity of these two receptor subtypes (1). No subtype-selective ligand has been identified for sstr4. However recent studies have identified the peptide des-AA1,2,5-[DTrp8, IAMP9]SRIF as a selective ligand for sstrl (16). The peptide binds to sstrl with an affinity in the 5-10 nM range, whereas it interacts with the other cloned receptors with much lower affinities. As a result, des-AA1,2,5-[DTrp8,IAMP9]SRIF and its tyrosine analog, which can be iodinated, may be useful for detecting expression of sstrl in tissues and for activating this receptor to determine its biological functions.
Several different SRIF analogs were identified that selectively bind to sstr2. The octapeptide NC8-12 and the hexapeptide BIM 23027 have over 100-fold higher affinity for sstr2 than the other receptors and can be used to selectively detect and activate this receptor (14,15). The analogs octreotide and MK 678 also bind potently to sstr2, although they do have some crossreactivity with sstr3 and sstr5.
No selective analogs have been agreed upon for sstr3, but the linear peptide BIM 23052 has been reported to have some selectivity for rat sstr5 (14,15) and functional studies by Coy and his associates (17,18) suggest that this peptide can be used to identify selective functions of rat sstr5. This peptide does not show selectivity for human sstr5. In fact, rat and human sstr5 show considerable species variations in amino acid sequence and major differences in ligand selectivities (19).
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