Conserved Motifs in Somatostatin, D2-dopamine, and α2B-Adrenergic Receptors for Inhibiting the Na-H Exchanger, NHE1

Abstract

Receptor subtypes within families of G proteincoupled receptors that are activated by similar ligands can regulate distinct intracellular effectors. We identified conserved motifs within intracellular domains 2 and 3 of selective subtypes of several G protein-coupled receptor families that confer coupling to the Na-H exchanger, NHE1. A T(s,p)V motif within intracellular domain 2 and a QQ(r) motif within intracellular domain 3 are shared by the somatostatin receptor subtypes SSTR1, -3, and -4, which couple to the inhibition of NHE1, but not by SSTR2 and -5, which do not signal to NHE1. Only the collective substitution of cognate SSTR2 residues with these two motifs conferred the ability of mutant SSTR2 to inhibit NHE1. Both motifs are present in D 2 -dopamine receptors, which inhibit NHE1, and in ␣ 2B -adrenergic receptors, which couple to the inhibition of NHE1, but not in ␣ 2A -adrenergic receptors, which do not regulate NHE1. These findings indicate that motifs shared by different subfamilies of G protein-coupled receptors, but not necessarily by receptor subtypes within a subfamily, can confer coupling to a common effector.Members of the superfamily of G protein-coupled receptors (GPCRs) 1 share a heptahelical transmembrane topology and similar mechanisms for transducing signals through trimeric G proteins to intracellular effectors. Subfamilies of GPCRs, distinguished by sequence identity, generally include several receptor isoforms or subtypes that can be activated by similar ligands. Although receptor subtypes within a GPCR subfamily often regulate similar intracellular effectors, they can also signal to distinct effectors and signaling networks through a process termed receptor subtype-specific signaling. Using the subfamily of somatostatin receptors as a model, the objective of this study was to determine a molecular basis for subtypespecific signaling by GPCRs.The somatostatin receptor family includes five subtypes (SSTR1 to SSTR5) (1-5) that belong to the class A group of GPCRs. Each member is activated by the neuroendocrine peptide somatostatin-14 (SST). SST inhibits diverse cell functions such as hormone secretion, neurotransmitter release, smooth muscle contractility, and cell proliferation. These cellular effects are mediated by coupling of the five SSTR subtypes to similar as well as distinct effectors and signaling networks. All five subtypes couple to the inhibition of adenylyl cyclase (6) and the activation of tyrosine phosphatases (7-13). Different SSTR subtypes, however, also regulate distinct effectors. SSTR2 to -5, but not SSTR1, activate the GIRK1 inwardly rectifying K ϩ channel (14), SSTR1 and -2 inhibit voltage-activated Ca 2ϩ channels (15-17), SSTR2 and -5 stimulate phospholipase C activity (8, 18), and only SSTR4 has been shown to stimulate phospholipase A 2 (19).SST acting at endogenous SSTRs in enteric endocrine cells (20) and hepatic cells (21) also inhibits the activity of the ubiquitously expressed Na-H exchanger NHE1. When heterologously expressed in fibroblasts, SSTR1, but not S...

Dopamine Receptor Signaling

“…It is of interest that several GPCR subtypes, including endogenous D2-like receptors in primary lactotrophs, mediate pertussis toxin-insensitive and cyclic AMP-independent inhibition of Na þ /H þ exchanger activity (132,244). Lin et al (244) have proposed that a potential mechanism for this inhibition is G 12 competition for the interaction between G 13 and p115RhoGEF. Na þ /H þ exchange is also inhibited by D1-like receptors by both cyclic AMP-dependent and -independent mechanisms (115,245).…”

“…Like GPCR activation of phospholipase D, D2 receptor activation of Na þ /H þ exchange is insensitive to pertussis toxin in some cell lines (133), and the G protein G 13 can activate the exchanger via Rho (243). It is of interest that several GPCR subtypes, including endogenous D2-like receptors in primary lactotrophs, mediate pertussis toxin-insensitive and cyclic AMP-independent inhibition of Na þ /H þ exchanger activity (132,244). Lin et al (244) have proposed that a potential mechanism for this inhibition is G 12 competition for the interaction between G 13 and p115RhoGEF.…”

Antitumor effects of somatostatin

“…Consensus motifs T/S/P-V (intracellular loop 2) and Q-Q/R (intracellular loop 3) of sst1, sst3, or sst4 directly interact with NHE1. Interestingly these motifs are absent in sst2 and 5, which do not mediate inhibition of NHE1 (Lin et al, 2003). Somatostatin-induced apoptosis can be signalled through sst3 and sst2.…”

“…These compounds will probably improve the diagnosis and treatment of tumors, which express somatostatin receptors other than sst2. In addition, novel strategies based on sst2 receptor gene Hou et al 1994;Smalley et al, 1999;Weckbecker et al, 2003;Ferjoux et al, 2003;Lin et al 2003;Arena et al, 2005Arena et al, , 2007Cordelier et al 2006;Theodoropoulou et al 2007;.Guillermet-Guibert et al 2007.…”

Effect of NHE1 antisense gene transfection on the biological behavior of SGC-7901 human gastric carcinoma cells

“…The different subtypes have different number and distribution, different pharmacological properties, and are regulated by different factors. NHE1, a house-keeping gene, located at 1p [1] with mRNA of 3.8 Kb, is found in nearly all human tissues, and serves to remove excessive H + in the cytoplasm by the Na + -H + exchange system, resulting in the maintenance of a stable pH i [11][12][13][14] . Glycolysis of tumor cells may result in the production of large quantities of lactic acid and H + .…”