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

Of the five cloned somatostatin (SRIF: somatotropin release inhibitory factor) receptors (sst(1-5)), only sst(2) and sst(5) 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'-Omega-(3-[S-35]thio)-triphosphate ([S-35]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. [I-125]LTT-SRIF-28, [I-125]CGP 23996 and [I-125]Tyr(3)-octreotide binding illustrates the high level of sst(2/5) receptor in AtT-20 cell membranes. SRIF-14 and SRIF-28 produced a concentration-dependent increase in [S-35]GTPgammaS binding (pEC(50) = 6.72 and 7.45; E-max = 79 and 74.9, respectively) which was completely abolished by pertussis toxin. sst(2/5) receptor-selective ligands caused a concentration-dependent increase in [S-35]GTPgammaS binding (pEC(50) = 7.74-5.84; E-max = 76.6-20.2) while sst(1/3/4) receptor-selective ligands were devoid of activity. The binding profiles of [I-125]LTT-SRIF-28 and the inhibition of cAMP accumulation correlated highly significantly with their corresponding [S-35]GTPgammaS binding profiles (r=0.862 and 0.874, respectively). The effects of the sst(2) receptor-preferring agonists Tyr(3)-octreotide and BIM 23027 on [S-35]GTPgammaS binding, but not those of SRIF-14 and the sst(5/1) receptor selective-agonist L-817,818, were competitively antagonised by the sst(2) receptor antagonist D-Tyr(8)-CYN 154806 (pK(B) = 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 sst(2) and sst(5) 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 sst(2) and sst(5) 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.