Native somatostatin sst2 and sst5 receptors functionally coupled to Gi/o-protein, but not to the serum response element in AtT-20 mouse tumour corticotrophs.

Abstract

Of the five cloned somatostatin (SRIF: somatotropin release inhibitory factor) receptors (sst1-5), only sst2 and sst5 receptors appear to be endogenously expressed and functionally active in AtT-20 mouse anterior pituitary tumour cells. In this study, the presence and the functional coupling of SRIF receptors to G-protein in AtT-20 cells was evaluated by receptor autoradiography and guanosine-5'-Omicron-(3-[35S]thio)-triphosphate ([35S]GTPgammaS) binding, respectively. In addition, transcriptional effects via the serum response element (SRE) were assessed in AtT-20-SRE-luci cells, engineered to express constitutively SRE upstream of the luciferase reporter gene. [125I]LTT-SRIF-28, [125I]CGP 23996 and [125I]Tyr3-octreotide binding illustrates the high level of sst2/5 receptor in AtT-20 cell membranes. SRIF-14 and SRIF-28 produced a concentration-dependent increase in [35S]GTPgammaS binding (pEC50=6.72 and 7.45; Emax=79 and 74.9, respectively) which was completely abolished by pertussis toxin. sst2/5 receptor-selective ligands caused a concentration-dependent increase in [35S]GTPgammaS binding (pEC50=7.74-5.84; Emax=76.6-20.2) while sst1/3/4 receptor-selective ligands were devoid of activity. The binding profiles of [125I]LTT-SRIF-28 and the inhibition of cAMP accumulation correlated highly significantly with their corresponding [35S]GTPgammaS binding profiles (r=0.862 and 0.874, respectively). The effects of the sst2 receptor-preferring agonists Tyr3-octreotide and BIM 23027 on [35S]GTPgammaS binding, but not those of SRIF-14 and the sst5/1 receptor selective-agonist L-817,818, were competitively antagonised by the sst2 receptor antagonist d-Tyr8-CYN 154806 (pKB=7.36 and 7.72, respectively; slope factors not significantly different from unity). In AtT-20-SRE-luci cells, which carry a SRE-luciferase construct functioning in a very efficient manner, SRIF and its analogues did not affect luciferase activity. Taken together, these results demonstrate that in AtT-20 cells the expression of sst2 and sst5 receptors fit with their functional coupling to G(i/o)-proteins. The pharmacological implications of the existence of different ligand/receptor complexes are discussed. However, the intracellular pathways coupled to the activation of sst2 and sst5 receptors appear not to modulate the SRE-mediated transcriptional activity, suggesting that SRIF effects on gene expression coupled to mechanisms that have promoters other than SRE.