Somatostatin receptors, an expanding gene family: cloning and functional characterization of human SSTR3, a protein coupled to adenylyl cyclase.

Abstract

We previously reported the cloning of two distinct somatostatin receptor (SSTR) subtypes, SSTR1 and SSTR2. Although both SSTR1 and SSTR2 bound somatostatin specifically and with high affinity, neither was coupled to adenylyl cyclase, a major cellular effector of somatostatin's actions. Here we report the cloning and functional characterization of a third member of the SSTR family. Human SSTR3 is a protein of 418 amino acids and has 45% and 46% identity with human SSTR1 and SSTR2, respectively. RNA blotting studies showed that SSTR3 mRNA could be readily detected in brain and pancreatic islets. The pharmacological properties of human SSTR3 were characterized by transiently expressing the human SSTR3 gene in COS-1 cells. Membranes from cells expressing human SSTR3 bound the somatostatin agonist [125I]CGP 23996 specifically and with high affinity, with a rank order of potency of somatostatin-28 = CGP 23996 > somatostatin-14 > SMS-201-995. Studies using cells transiently coexpressing the human dopamine D1 receptor and human SSTR3 showed that somatostatin was able to inhibit dopamine-stimulated cAMP formation in a dose-dependent manner, indicating that SSTR3 was functionally coupled to adenylyl cyclase. These results indicate that the diverse biological effects of somatostatin are mediated by a family of receptor with distinct, but overlapping, tissue distributions, unique pharmacological properties, and potentially different functions.

MOLECULAR EVOLUTION OF GPCRS: Somatostatin/urotensin II receptors

“…Already the first report on molecular cloning and functional characterization had described two receptor subtypes in both humans and mice, SSTR1 (STT 1 ) and SSTR2 (SST 2 ) (Yamada et al 1992a). The discovery of SSTR3 (SST 3 ) ensued later the same year (Yamada et al 1992b) and the following year that of the mammalian SSTR4 (SST 4 ) and SSTR5 (SST 5 ) receptors was reported (Yamada et al 1993). Thus, SS1 was rapidly found to have one of the largest receptor families for any neuronal or endocrine peptide in mammals, with as many as five subtypes in all placental mammals that have been investigated in detail.…”

Octreotide

“…1A). [8][9][10][11] The genes for these subtypes are located on different chromosomes, suggesting different functions in different organs (Table 1). Indeed, distinct but often overlapping patterns in the expression of these subtypes have been demonstrated.…”

“…There have been no reports of the loss of the suppressive effect of octreotide on growth hormone secretion in patients with acromegaly, even after more than 10 years of uninterrupted therapy. 38,43 The high cost of treatment with octreotide *Data on the five subtypes were obtained from the following reports: subtypes 1 and 2, Yamada et al 8 ; subtype 3, Yamada et al 9 ; subtype 4, Bruno et al 10 ; and subtype 5, O'Carroll et al 11 †IC 50 denotes the concentration required for 50 percent inhibition of the binding of 125 I-labeled somatostatin to the cloned subtype, as expressed in Chinese hamster-ovary or transformed African green-monkey COS kidney cells. Data were obtained from Patel and Srikant.…”

Somatostatin receptor subtypes sst1, sst2, sst3 and sst5expression in human pituitary, gastroentero-pancreatic and mammary tumors

“…The protocol followed was essentially that described in detail by Reubi et al (1994). Fortyeight-base-long oligonucleotides complementary to the bases coding for amino acids 2-17 and 252-267 of the human sst 1 mRNA sequence (Yamada et al, 1992b), 31-46 and 237-252 of the human sst 2 mRNA sequence (Yamada et al, 1992b), 228-243 and 366-381 of the human sst 3 mRNA sequence (Yamada et al, 1992a) and 2-17, 327-342 and 349-364 of the human sst 5 mRNA sequence (Yamada et al, 1993) were synthesized and purified on a 20% polyacrylamide-8 M urea sequencing gel (Microsynth, Balgach, Switzerland).…”