Prolonged activation of extracellular signal-regulated kinase by a protein kinase C-dependent and N17Ras-insensitive mechanism mediates the proliferative response of G(i/o)-coupled somatostatin sst(4) receptors.

The human sst(4) receptor, recombinantly expressed in Chinese hamster ovary cells, mediates proliferative activity of the peptide hormone somatostatin. This effect was shown to involve activation of pertussis toxin-sensitive G proteins and was inhibited by overexpression of the betagamma-sequestrant, transducin. Somatostatin-induced proliferation was abolished by the MEK1 inhibitor, PD 98059, whereas the Src inhibitor, PP1, had no effect. A marked increase was observed in the phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1 and ERK2) 10 min after sst(4) receptor activation, which was blocked by pertussis toxin, decreased by PP1 and the betagamma-sequestrant, but unaffected by PD 98059. In contrast, the somatostatin-induced phosphorylation of ERK obtained at 4 h, although sensitive to both pertussis toxin and transducin, was unaffected by PP1 but ablated by PD 98059. Protein kinase C inhibition also abolished this somatostatin-induced sustained phosphorylation of ERK, together with the associated increase in cell proliferation. Expression of dominant negative Ras (N17) failed to significantly reduce the proliferative effect mediated by the sst(4) receptor but markedly attenuated the acute phase of the somatostatin-induced phosphorylation of ERK obtained at 10 min. In contrast, the phosphorylation induced at 4 h was unaffected. We conclude that ERK activation by G(i/o)-coupled sst(4) receptors involves a Src and Ras-dependent acute phase, but the proliferative response is dependent upon the prolonged ERK-induced activity, mediated by protein kinase C.     

Heterodimerization of somatostatin and opioid receptors cross-modulates phosphorylation, internalization, and desensitization

Heterodimerization has been shown to modulate the ligand binding, signaling, and trafficking properties of G protein-coupled receptors. However, to what extent heterodimerization may alter agonist-induced phosphorylation and desensitization of these receptors has not been documented. We have recently shown that heterodimerization of sst(2A) and sst(3) somatostatin receptors results in inactivation of sst(3) receptor function (Pfeiffer, M., Koch, T., Schr?der, H., Klutzny, M., Kirscht, S., Kreienkamp, H. J., H?llt, V., and Schulz, S. (2001) J. Biol. Chem. 276, 14027-14036). Here we examine dimerization of the sst(2A) somatostatin receptor and the mu-opioid receptor, members of closely related G protein-coupled receptor families. In coimmunoprecipitation studies using differentially epitope-tagged receptors, we provide direct evidence for heterodimerization of sst(2A) and MOR1 in human embryonic kidney 293 cells. Unlike heteromeric assembly of sst(2A) and sst(3), sst(2A)-MOR1 heterodimerization did not substantially alter the ligand binding or coupling properties of these receptors. However, exposure of the sst(2A)-MOR1 heterodimer to the sst(2A)-selective ligand L-779,976 induced phosphorylation, internalization, and desensitization of sst(2A) as well as MOR1. Similarly, exposure of the sst(2A)-MOR1 heterodimer to the mu-selective ligand [d-Ala(2),Me-Phe(4),Gly(5)-ol]enkephalin induced phosphorylation and desensitization of both MOR1 and sst(2A) but not internalization of sst(2A). Cross-phosphorylation and cross-desensitization of the sst(2A)-MOR1 heterodimer were selective; they were neither observed with the sst(2A)-sst(3) heterodimer nor with the endogenously expressed lysophosphatidic acid receptor. Heterodimerization may thus represent a novel regulatory mechanism that could either restrict or enhance phosphorylation and desensitization of G protein-coupled receptors.     

Receptor isoforms mediate opposing proliferative effects through gbetagamma-activated p38 or Akt pathways

The opposing effects on proliferation mediated by G-protein-coupled receptor isoforms differing in their COOH termini could be correlated with the abilities of the receptors to differentially activate p38, implicated in apoptotic events, or phosphatidylinositol 3-kinase (PI 3-K), which provides a source of survival signals. These contrasting growth responses of the somatostatin sst(2) receptor isoforms, which couple to identical Galpha subunit pools (Galpha(i3) > Galpha(i2) > Galpha(0)), were both inhibited following betagamma sequestration. The sst(2(a)) receptor-mediated ATF-2 activation and inhibition of proliferation induced by basic fibroblast growth factor (bFGF) were dependent on prolonged phosphorylation of p38. In contrast, cell proliferation and the associated transient phosphorylation of Akt and p70(rsk) induced by sst(2(b)) receptors were blocked by the PI 3-K inhibitor LY 294002. Stimulation with bFGF alone had no effect on the activity of either p38 or Akt but markedly enhanced p38 phosphorylation mediated by sst(2(a)) receptors, suggesting that a complex interplay exists between the transduction cascades activated by these distinct receptor types. In addition, although all receptors mediated a sustained activation of extracellular signal-regulated kinases (ERK1 and ERK2), induction of the tumor suppressor p21(cip1) was detected only following amplification of ERK and p38 phosphorylation by concomitant bFGF and sst(2(a)) receptor activation. Expression of constitutively active Akt in the presence of a p38 inhibitor enabled a proliferative response to be detected in sst(2(a)) receptor-expressing cells. These findings demonstrate that the duration of activation and a critical balance between the mitogen-activated protein kinase and PI 3-K pathways are important for controlling cell proliferation and that the COOH termini of the sst(2) receptor isoforms may determine the selection of appropriate betagamma-pairings necessary for interaction with distinct kinase cascades.     

sst2 Somatostatin receptor inhibits cell proliferation through Ras-, Rap1-, and B-Raf-dependent ERK2 activation

The G protein-coupled sst2 somatostatin receptor is a critical negative regulator of cell proliferation. sstII prevents growth factor-induced cell proliferation through activation of the tyrosine phosphatase SHP-1 leading to induction of the cyclin-dependent kinase inhibitor p27Kip1. Here, we investigate the signaling molecules linking sst2 to p27Kip1. In Chinese hamster ovary-DG-44 cells stably expressing sst2 (CHO/sst2), the somatostatin analogue RC-160 transiently stimulates ERK2 activity and potentiates insulin-stimulated ERK2 activity. RC-160 also stimulates ERK2 activity in pancreatic acini isolated from normal mice, which endogenously express sst2, but has no effect in pancreatic acini derived from sst2 knock-out mice. RC-160-induced p27Kip1 up-regulation and inhibition of insulin-dependent cell proliferation are both prevented by pretreatment of CHO/sst2 cells with the MEK1/2 inhibitor PD98059. In addition, using dominant negative mutants, we show that sst2-mediated ERK2 stimulation is dependent on the pertussis toxin-sensitive Gi/o protein, the tyrosine kinase Src, both small G proteins Ras and Rap1, and the MEK kinase B-Raf but is independent of Raf-1. Phosphatidylinositol 3-kinase (PI3K) and both tyrosine phosphatases, SHP-1 and SHP-2, are required upstream of Ras and Rap1. Taken together, our results identify a novel mechanism whereby a Gi/o protein-coupled receptor inhibits cell proliferation by stimulating ERK signaling via a SHP-1-SHP-2-PI3K/Ras-Rap1/B-Raf/MEK pathway.     

Protein kinase C activation stimulates the phosphorylation and internalization of the sst2A somatostatin receptor

The sst2A receptor is expressed in the endocrine, gastrointestinal, and neuronal systems as well as in many hormone-sensitive tumors. This receptor is rapidly internalized and phosphorylated in growth hormone-R2 pituitary cells following somatostatin binding (Hipkin, R. W., Friedman, J., Clark, R. B., Eppler, C. M., and Schonbrunn, A. (1997) J. Biol. Chem. 272, 13869-13876). The protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate (PMA), also stimulates sst2A phosphorylation. Here we examine the mechanisms and consequences of PMA and agonist-induced sst2A phosphorylation. Like somatostatin, both PMA and bombesin increased sst2A receptor phosphorylation within 2 min. The PKC inhibitor GF109203X blocked PMA- and bombesin- stimulated sst2A phosphorylation, whereas stimulation by the somatostatin analog SMS 201-995 was unaffected. Agonist and PMA each stimulated phosphorylation in two receptor domains, the third intracellular loop and the C-terminal tail. Functionally, PMA dramatically increased the internalization of the sst2A receptor-ligand complex. This PMA stimulation was blocked by GF109203X, whereas basal internalization was unaffected. However, neither basal nor PMA-stimulated internalization was altered by pertussis toxin, whereas both were blocked by hypertonic sucrose. Therefore PKC activation and agonist binding stimulate sst2A phosphorylation by distinct mechanisms, and PKC potentiates internalization of the sst2A receptor via clathrin-coated pits. Thus, hormonal stimulation of PKC-coupled receptors may provide a mechanism for regulating the inhibitory actions of somatostatin in target tissue.     

Agonist-induced phosphorylation of somatostatin receptor subtype 1 (sst1). Relationship to desensitization and internalization

The sst1 somatostatin (SRIF) receptor subtype is widely expressed in the endocrine, gastrointestinal, and neuronal systems as well as in hormone-sensitive tumors, yet little is known about its regulation. Here we investigated the desensitization, internalization, and phosphorylation of sst1 expressed in CHO-K1 cells. Treatment of cells with 100 nm SRIF for 30 min reduced maximal SRIF inhibition of adenylyl cyclase from 40 to 10%. This desensitization was rapid (t(12) < 2 min) and dependent on agonist concentration (EC(50) = 2 nm). However, internalization of receptor-bound ligand occurred slowly (t(12) > 180 min). Incubation of cells with SRIF also caused a rapid (t(12) < 2 min) increase in sst1 receptor phosphorylation in a dose-dependent manner (EC(50) = 1.3 nm), as determined in a mobility shift phosphorylation assay. Receptor phosphorylation was not affected by pertussis toxin, indicating a requirement for receptor occupancy rather than signaling. The protein kinase C activator, phorbol 12-myristate 13-acetate also stimulated sst1 receptor phosphorylation whereas forskolin did not. Both agonist- and phorbol 12-myristate 13-acetate-stimulated receptor phosphorylation occurred mainly on serine. These studies are the first to demonstrate phosphorylation of the sst1 receptor and suggest that phosphorylation mediated uncoupling, rather than sequestration, leads to its desensitization.     

Homologous and heterologous regulation of somatostatin receptor 2

We previously demonstrated that phosphorylation of somatostatin receptor 2A (sst2A) is rapidly increased in transfected cells both by agonist and by the protein kinase C (PKC) activator phorbol myristate acetate (PMA). Here, we investigate whether PKC-mediated receptor phosphorylation is involved in the homologous or heterologous regulation of endogenous sst2 receptors in AR42J pancreatic acinar cells upon stimulation by agonist or by cholecystokinin (CCK) or bombesin (BBS). Somatostatin, PMA, CCK, and BBS all increased sst2A receptor phosphorylation 5- to 10-fold within minutes. Somatostatin binding also caused rapid internalization of the ligand-receptor complex, and PMA, CCK, and BBS all stimulated this internalization further. Additionally, sst2 receptor-mediated inhibition of adenylyl cyclase was desensitized by all treatments. Somatostatin, as well as peptidic (SMS201-995) and nonpeptidic (L-779,976) sst2 receptor agonists increased the EC(50) for somatostatin inhibition 20-fold. In contrast, pretreatment with BBS, CCK, or PMA caused a modest 2-fold increase in the EC(50) for cyclase inhibition. Whereas the PKC inhibitor GF109203X abolished sst2A receptor phosphorylation by CCK, BBS, and PMA, it did not alter the effect of somatostatin, demonstrating that these reactions were catalyzed by different kinases. Consistent with a functional role for PKC-mediated receptor phosphorylation, GF109203X prevented PMA stimulation of sst2 receptor internalization. Surprisingly, however, GF109203X did not inhibit BBS and CCK stimulation of sst2A receptor endocytosis. These results demonstrate that homologous and heterologous hormones induce sst2A receptor phosphorylation by PKC-independent and -dependent mechanisms, respectively, and produce distinct effects on receptor signaling and internalization. In addition, the heterologous hormones also modulate sst2 receptor internalization by a novel mechanism that is independent of receptor phosphorylation.     

Differential temporal and spatial regulation of somatostatin receptor phosphorylation and dephosphorylation

The G(i)-coupled somatostatin 2A receptor (sst2A) mediates many of the neuromodulatory and neuroendocrine actions of somatostatin (SS) and is targeted by the SS analogs used to treat neuroendocrine tumors. As for other G protein-coupled receptors, agonists stimulate sst2A receptor phosphorylation on multiple residues, and phosphorylation at different sites has distinct effects on receptor internalization and uncoupling. To elucidate the spatial and temporal regulation of sst2A receptor phosphorylation, we examined agonist-stimulated phosphorylation of multiple receptor GPCR kinase sites using phospho-site-specific antibodies. SS increased receptor phosphorylation sequentially, first on Ser-341/343 and then on Thr-353/354, followed by receptor internalization. Reversal of receptor phosphorylation was determined by the duration of prior agonist exposure. In acutely stimulated cells, in which most receptors remained on the cell surface, dephosphorylation occurred only on Thr-353/354. In contrast, both Ser-341/343 and Thr-353/354 were rapidly dephosphorylated when cells were stimulated long enough to allow receptor internalization before agonist removal. Consistent with these observations, dephosphorylation of Thr-353/354 was not affected by either hypertonic sucrose or dynasore, which prevent receptor internalization, whereas dephosphorylation of Ser-341/343 was completely blocked. An okadaic acid- and fostriecin-sensitive phosphatase catalyzed the dephosphorylation of Thr-353/354 both intracellularly and at the cell surface. In contrast, dephosphorylation of Ser-341/343 was insensitive to these inhibitors. Our results show that the phosphorylation and dephosphorylation of neighboring GPCR kinase sites in the sst2A receptor are subject to differential spatial and temporal regulation. Thus, the pattern of receptor phosphorylation is determined by the duration of agonist stimulation and compartment-specific enzymatic activity.     

A transplantable phosphorylation probe for direct assessment of G protein-coupled receptor activation

The newly developed multireceptor somatostatin analogs pasireotide (SOM230), octreotide and somatoprim (DG3173) have primarily been characterized according to their binding profiles. However, their ability to activate individual somatostatin receptor subtypes (sst) has not been directly assessed so far. Here, we transplanted the carboxyl-terminal phosphorylation motif of the sst(2) receptor to other somatostatin receptors and assessed receptor activation using a set of three phosphosite-specific antibodies. Our comparative analysis revealed unexpected efficacy profiles for pasireotide, octreotide and somatoprim. Pasireotide was able to activate sst(3) and sst(5) receptors but was only a partial agonist at the sst(2) receptor. Octreotide exhibited potent agonistic properties at the sst(2) receptor but produced very little sst(5) receptor activation. Like octreotide, somatoprim was a full agonist at the sst(2) receptor. Unlike octreotide, somatoprim was also a potent agonist at the sst(5) receptor. Together, we propose the application of a phosphorylation probe for direct assessment of G protein-coupled receptor activation and demonstrate its utility in the pharmacological characterization of novel somatostatin analogs.     

Angiotensin II phosphorylation of extracellular signal-regulated kinases in rat anterior pituitary cells

We studied the effects of ANG II on extracellular signal-regulated kinase (ERK)1/2 phosphorylation in rat pituitary cells. ANG II increased ERK phosphorylation in a time- and concentration-dependent way. Maximum effect was obtained at 5 min at a concentration of 10-100 nM. The effect of 100 nM ANG II was blocked by the AT1 antagonist DUP-753, by the phospholipase C (PLC) inhibitor U-73122, and by the MAPK kinase (MEK) antagonist PD-98059. The ANG II-induced increase in phosphorylated (p)ERK was insensitive to pertussis toxin blockade and PKC depletion or inhibition. The effect was also abrogated by chelating intracellular calcium with BAPTA-AM or TMB-8 by depleting intracellular calcium stores with a 30-min pretreatment with EGTA and by pretreatment with herbimycin A and PP1, two c-Src tyrosine kinase inhibitors. It was attenuated by AG-1478, an inhibitor of epidermal growth factor receptor (EGFR) activation. Therefore, in the rat pituitary, the increase of pERK is a Gq- and PLC-dependent process, which involves an increase in intracellular calcium and activation of a c-Src tyrosine kinase, transactivation of the EGFR, and the activation of MEK. Finally, the response of ERK activation by ANG II is altered in hyperplastic pituitary cells, in which calcium mobilization evoked by ANG II is also modified.     

Somatostatin down-regulates the expression and release of endozepines from cultured rat astrocytes via distinct receptor subtypes

Endozepines, a family of regulatory peptides related to diazepam-binding inhibitor (DBI), are synthesized and released by astroglial cells. Because rat astrocytes express various subtypes of somatostatin receptors (sst), we have investigated the effect of somatostatin on DBI mRNA level and endozepine secretion in rat astrocytes in secondary culture. Somatostatin reduced in a concentration-dependent manner the level of DBI mRNA in cultured astrocytes. This inhibitory effect was mimicked by the selective sst4 receptor agonist L803-087 but not by the selective sst1, sst2 and sst3 receptor agonists L779-591, L779-976 and L797-778, respectively. Somatostatin was unable to further reduce DBI mRNA level in the presence of the MEK inhibitor U0126. Somatostatin and the sst1, sst2 and sst4 receptor agonists induced a concentration-dependent inhibition of endozepine release. Somatostatin and the sst1, sst2 and sst4 receptor agonists also inhibited cAMP formation dose-dependently. In addition, somatostatin reduced forskolin-induced endozepine release. H89 mimicked the inhibitory effect of somatostatin on endozepine secretion. In contrast the PLC inhibitor U73122, the PKC activator PMA and the PKC inhibitor calphostin C had no effect on somatostatin-induced inhibition of endozepine release. The present data demonstrate that somatostatin reduces DBI mRNA level mainly through activation of sst4 receptors negatively coupled to the MAPK pathway, and inhibits endozepine release through activation of sst1, sst2 and sst4 receptors negatively coupled to the adenylyl cyclase/PKA pathway.     

Insulin reverses growth hormone-induced homologous desensitization

Growth hormone (GH) is secreted in a pulsatile pattern to promote body growth and metabolism. GH exerts its function by activating several signaling pathways, including JAK2/STAT and MEK/ERK. ERK1/2 activation by GH plays important roles in gene expression, cell proliferation, and growth. We previously reported that in rat H4IIE hepatoma cells after an initial GH exposure, a second GH exposure induces STAT5 phosphorylation but not ERK1/2 phosphorylation (Ji, S., Frank, S. J., and Messina, J. L. (2002) J. Biol. Chem. 277, 28384-28393). In this study the mechanisms underlying GH-induced homologous desensitization were investigated. A second GH exposure activated the signaling intermediates upstream of MEK/ERK, including JAK2, Ras, and Raf-1. This correlated with recovery of GH receptor levels, but was insufficient for GH-induced phosphorylation of MEK1/2 and ERK1/2. Insulin restored the ability of a second GH exposure to induce phosphorylation of MEK1/2 and ERK1/2 without altering GH receptor levels or GH-induced phosphorylation/activation of JAK2 and Raf-1. GH and insulin synergized in promoting cell proliferation. Further investigation suggested that insulin increased the amount of MEK bound to KSR (kinase suppressor of Ras) and restored GH-induced tyrosine phosphorylation of KSR. Previous GH exposure also induced desensitization of STAT1 and STAT3 phosphorylation, but this desensitization was not reversed by insulin. Thus, insulin-regulated resensitization of GH signaling may be necessary to reset the complete response to GH after a normal, physiologic pulse of GH.     

Ligand-dependent mechanisms of sst2A receptor trafficking: role of site-specific phosphorylation and receptor activation in the actions of biased somatostatin agonists

The somatostatin receptor subtype 2A (sst2A) mediates many of somatostatin's neuroendocrine actions and is the primary therapeutic target for the stable somatostatin analogs used to inhibit hormone secretion by pituitary and gastroenteropancreatic tumors. Two new multireceptor targeting somatostatin analogs currently under clinical investigation, the multisomatostatin receptor agonist cyclo-[diaminoethylcarbamoyl-HydroxyPro-Phenylglycine-D-Trp-Lys-(4-O-benzyl)Tyr-Phe] (SOM230) (Pasireotide) and pan-somatostatin receptor agonist Tyr-cyclo-[D-diaminobutyric acid-Arg-Phe-Phe-D-Trp-Lys-Thr-Phe] (KE108), behave as functionally selective ligands at the sst2A receptor, mimicking some of somatostatin's actions but antagonizing others. Further, SOM230 and KE108 are less able to induce receptor internalization than somatostatin, indicating that they exhibit functional selectivity for receptor regulation as well as signaling. Here, we identify agonist-specific differences in the molecular events regulating sst2A receptor endocytosis. SOM230 and KE108 were less potent and less effective than somatostatin at stimulating sst2A receptor phosphorylation at two pairs of residues, Ser341/343 and Thr353/354. Only the pattern of Thr353/354 phosphorylation correlated with receptor internalization, consistent with the known importance of Thr phosphorylation for sst2A receptor endocytosis. As expected, arrestin recruitment to membrane receptors was reduced with SOM230 and KE108. In addition, both receptor dephosphorylation and receptor recycling occurred more rapidly with SOM230 and KE108 than with somatostatin. Surprisingly, however, SOM230 and KE108 also altered sst2A internalization in a phosphorylation-independent manner, because these analogs were less effective than somatostatin at stimulating the endocytosis of a phosphorylation-negative receptor mutant. These results show that the decreased receptor internalization produced by SOM230 and KE108 compared with somatostatin result from phosphorylation-independent effects as well as reduced site-specific receptor phosphorylation and receptor-arrestin association.     

Ras/ERK1/2-mediated STAT3 Ser727 phosphorylation by familial medullary thyroid carcinoma-associated RET mutants induces full activation of STAT3 and is required for c-fos promoter activation, cell mitogenicity, and transformation

The precise role of STAT3 Ser(727) phosphorylation in RET-mediated cell transformation and oncogenesis is not well understood. In this study, we have shown that familial medullary thyroid carcinoma (FMTC) mutants RET(Y791F) and RET(S891A) induced, in addition to Tyr(705) phosphorylation, constitutive STAT3 Ser(727) phosphorylation. Using inhibitors and dominant negative constructs, we have demonstrated that RET(Y791F) and RET(S891A) induce STAT3 Ser(727) phosphorylation via a canonical Ras/ERK1/2 pathway and that integration of the Ras/ERK1/2/ELK-1 and STAT3 pathways was required for up-regulation of the c-fos promoter by FMTC-RET. Moreover, inhibition of ERK1/2 had a more severe effect on cell proliferation and cell phenotype in HEK293 cells expressing RET(S891A) compared with control and RET(WT)-transfected cells. The transforming activity of RET(Y791F) and RET(S891A) in NIH-3T3 cells was also inhibited by U0126, indicating a role of the ERK1/2 pathway in RET-mediated transformation. To investigate the biological significance of Ras/ERK1/2-induced STAT3 Ser(727) phosphorylation for cell proliferation and transformation, N-Ras-transformed NIH-3T3 cells were employed. These cells displayed elevated levels of activated ERK1/2 and Ser(727)-phosphorylated STAT3, which were inhibited by treatment with U0126. Importantly, overexpression of STAT3, in which the Ser(727) was mutated into Ala (STAT3(S727A)), rescued the transformed phenotype of N-Ras-transformed cells. Immunohistochemistry in tumor samples from FMTC patients showed strong nuclear staining of phosphorylated ERK1/2 and Ser(727) STAT3. These data show that FMTC-RET mutants activate a Ras/ERK1/2/STAT3 Ser(727) pathway, which plays an important role in cell mitogenicity and transformation.     

Somatostatin increases mitogen-induced IL-2 secretion and proliferation of human jurkat T cells via sst3 receptor isotype

The neuropeptide somatostatin (SRIF) modulates normal and leukemia T cell proliferation. However, neither molecular isotypes of receptors nor mechanisms involved in these somatostatin actions have been elucidated as yet. Here we show by using RT-PCR approach that mitogen-activated leukemia T cells (Jurkat) express mRNA for a single somatostatin receptor, sst3. This mRNA is apparently translated into protein since specific somatostatin binding sites (K_I1 = 78 ± 3 pM) were detected in semipurified plasma membrane preparations by using ¹²⁵I-Tyr¹-SRIF14 as a radioligand. Moreover, somatostatin inhibits adenylyl cyclase activity with similar efficiency (IC₅₀ = 23 ± 4 pM) thus strongly suggesting a functional coupling of sst3 receptor to this transduction pathway. The involvement of sst3 receptor in immuno-modulatory actions of somatostatin was assessed by analysis of neuropeptide effects on IL-2 secretion and on proliferation of mitogen-activated Jurkat cells. Our data show that in the concentrations comprised between 10 pM and 10 nM, somatostatin potentiates IL-2 secretion. This effect is correlated with somatostatin-dependent increase of Jurkat cell proliferation since the EC₅₀ concentrations for both actions were almost identical (EC₅₀ = 22 ± 9 pM and EC₅₀ = 12 ± 1 pM for IL-2 secretion and proliferation, respectively). Altogether, these data strongly suggest that in mitogen-activated Jurkat cells, somatostatin increases cell proliferation through the increase of IL-2 secretion via a functional sst3 receptor negatively coupled to the adenylyl cyclase pathway. J. Cell. Biochem. 68:62–73, 1998. © 1998 Wiley-Liss, Inc.