Stimulation of tyrosine phosphatase and inhibition of cell proliferation by somatostatin analogues: mediation by human somatostatin receptor subtypes SSTR1 and SSTR2

The effects of somatostatin analogues RC-160 and SMS-201-995 on tyrosine phosphatase and cell proliferation were investigated in COS-7 and NIH 3T3 cells expressing human somatostatin receptor subtype 1 or 2 (SSTR1 or SSTR2). Binding experiments were performed on membranes from COS-7 cells expressing human SSTR1 or SSTR2 using 125I-labeled [Tyr11]S-14 or [Tyr3]SMS-201-995, respectively. The somatostatin analogues RC-160 and SMS-201-995 exhibited low affinity for SSTR1 (IC50 of 0.43 and 1.5 microM, respectively) and high affinity for SSTR2 (IC50 of 0.27 and 0.19 nM). Addition of these analogues to cells expressing either SSTR1 or SSTR2 did not result in an inhibition of adenylate cyclase activity. In SSTR2-expressing cells, both analogues induced a rapid stimulation of a tyrosine phosphatase activity (EC50: RC-160, 2 pM; SMS-201-995, 6 pM) and an inhibition of serum-stimulated proliferation (EC50: RC-160, 6.3 pM; SMS-201-995, 12 pM). In SSTR1-expressing cells, only RC-160 induced stimulation of a tyrosine phosphatase activity. Both analogues caused an inhibition of cell proliferation at a concentration higher than 10 nM in accordance with their affinities for the SSTR1 receptor subtype. A good correlation between the affinities of RC-160 and SMS-201-995 for each receptor subtype and their potencies to inhibit cell proliferation suggests the involvement of these receptors in cell growth regulation. Tyrosine phosphatase was stimulated by both these analogues in SSTR2 and by RC-160 in SSTR1 at affinities similar to their ability to inhibit growth and bind to receptors, implicating tyrosine phosphatase as a transducer of the growth inhibition signal. We also found that mRNAs of receptor subtypes were variably expressed in different pancreatic and colon cancer cell lines, indicating the necessity of a precise analysis of receptor subtypes in target tissues before therapy with analogues.

P2y(12), a new platelet ADP receptor, target of clopidogrel

The binding characteristics of (33)P-2MeS-ADP, a stable analogue of ADP, were determined on CHO cells transfected with the human P2Y(12) receptor, a novel purinergic receptor. These transfected CHO cells displayed a strong affinity for (33)P-2MeS-ADP, the binding characteristics of which corresponded in all points to those observed on platelets. In particular, this receptor recognised purines with the following order of potency: 2MeS-ADP = 2MeS-ATP > ADP = ATPgammaS = ATP >> UTP, a binding profile which is similar to that obtained in platelets. The binding of (33)P-2MeS-ADP was antagonised by pCMPS but not by MRS2179 and FSBA, antagonists of P2Y(1) and aggregin, respectively. Moreover, the binding of (33)P-2MeS-ADP to these cells was strongly and irreversibly inhibited by the active metabolite of clopidogrel with a potency which was consistent with that observed for this compound on platelets. Like in platelets, 2MeS-ADP induced adenylyl cyclase down-regulation in these P2Y(12) transfected CHO cells, an effect which was absent in the corresponding non-transfected cells. As already shown in platelets, the active metabolite of clopidogrel antagonised 2MeS-ADP-induced inhibition of adenylyl cyclase on transfected cells. Our results confirm that P2Y(12) is the previously called "platelet P2t(AC)" receptor and show that this receptor is antagonised by the active metabolite of clopidogrel.

The tyrosine phosphatase SHP-1 associates with the sst2 somatostatin receptor and is an essential component of sst2-mediated inhibitory growth signaling

Activation of the somatostatin receptor sst2, a member of the Gi protein-coupled receptor family, results in the stimulation of a protein-tyrosine phosphatase activity involved in the sst2-mediated growth inhibitory signal. Here, we report that SHP-1, a cytoplasmic protein-tyrosine phosphatase containing two Src homology 2 domains constitutively associated with sst2 as evidence by coprecipitation of SHP-1 protein with sst2, in Chinese hamster ovary cells coexpressing sst2 and SHP-1. Activation of sst2 by somatostatin resulted in a rapid dissociation of SHP-1 from sst2 accompanied by an increase of SHP-1 activity. SHP-1 was phosphorylated on tyrosine in control cells and somatostatin induced a rapid and transient dephosphorylation on tyrosine residues of the enzyme. Stimulation of SHP-1 activity by somatostatin was abolished by pertussis toxin pretreatment of cells. Gialpha3 was specifically immunoprecipitated by anti-sst2 and anti-SHP-1 antibodies, and somatostatin induced a rapid dissociation of Gialpha3 from sst2, suggesting that Gialpha3 may be involved in the sst2.SHP-1 complexes. Finally, somatostatin inhibited the proliferation of cells coexpressing sst2 and SHP-1, and this effect was suppressed in cells coexpressing sst2 and the catalytic inactive SHP-1 (C453S mutant). Our data identify SHP-1 as the tyrosine phosphatase associated with sst2 and demonstrate that this enzyme may be an initial key transducer of the antimitogenic signaling mediated by sst2.

Characterization of the antiproliferative signal mediated by the somatostatin receptor subtype sst5

We investigated cell proliferation modulated by cholecystokinin (CCK) and somatostatin analogue RC-160 in CHO cells bearing endogenous CCKA receptors and stably transfected by human subtype sst5 somatostatin receptor. CCK stimulated cell proliferation of CHO cells. This effect was suppressed by inhibitor of the soluble guanylate cyclase, LY 83583, the inhibitor of the cGMP dependent kinases, KT 5823, and the inhibitor of mitogen-activated protein (MAP) kinase kinase, PD 98059. CCK treatment induced an increase of intracellular cGMP concentrations, but concomitant addition of LY 83583 virtually suppressed this increase. CCK also activated both phosphorylation and activity of p42-MAP kinase; these effects were inhibited by KT 5823. All the effects of CCK depended on a pertussis toxin-dependent G protein. Somatostatin analogue RC-160 inhibited CCK-induced stimulation of cell proliferation but it did not potentiate the suppressive effect of the inhibitors LY 83583 and KT 5823. RC-160 inhibited both CCK-induced intracellular cGMP formation as well as activation of p42-MAP kinase phosphorylation and activity. This inhibitory effect was observed at doses of RC-160 similar to those necessary to occupy the sst5 recombinant receptor and to inhibit CCK-induced cell proliferation. We conclude that, in CHO cells, the proliferation and the MAP kinase signaling cascade depend on a cGMP-dependent pathway. These effects are positively regulated by CCK and negatively influenced by RC-160, interacting through CCKA and sst5 receptors, respectively. These studies provide a characterization of the antiproliferative signal mediated by sst5 receptor.

sst2 somatostatin receptor mediates negative regulation of insulin receptor signaling through the tyrosine phosphatase SHP-1

We have previously reported in Chinese hamster ovary (CHO) cells expressing sst2 that activation of the sst2 somatostatin receptor inhibits insulin-induced cell proliferation by a mechanism involving stimulation of a tyrosine phosphatase activity. Here we show that the tyrosine phosphatase SHP-1 was associated with the insulin receptor (IR) at the basal level. Activation of IR by insulin resulted in a rapid and transient increase of tyrosine phosphorylation of IR, its substrates IRS-1 and Shc, and also of SHP-1. This was then followed by a rapid dephosphorylation of these molecules, which was related to the insulin-induced increase of SHP-1 association to IR and of SHP-1 activity. On the other hand, addition to insulin of the somatostatin analogue, RC160, resulted in a higher and faster increase of SHP-1 association to IR directly correlated with an inhibition of phosphorylation of IR and its substrates, IRS-1 and Shc. RC160 also induced a higher and more sustained increase in SHP-1 activity. Furthermore, RC160 completely suppressed the effect of insulin on SHP-1 phosphorylation. Finally, in CHO cells coexpressing sst2 and a catalytically inactive mutant SHP-1, insulin as well as RC160 could no longer stimulate SHP-1 activity. Overexpression of the SHP-1 mutant prevented the insulin-induced signaling to be terminated by dephosphorylation of IR, suppressed the inhibitory effect of RC160 on insulin-induced IR phosphorylation, and abolished the cell proliferation modulation by insulin and RC160. Our results suggest that SHP-1 plays a role in negatively modulating insulin signaling by association with IR. Furthermore, somatostatin inhibits the insulin-induced mitogenic signal by accelerating and amplifying the effect of SHP-1 on the termination of the insulin signaling pathway.

Gene therapy for pancreatic carcinoma: local and distant antitumor effects after somatostatin receptor sst2 gene transfer

Human pancreatic adenocarcinomas lose the ability to express sst2, the somatostatin receptor, which mediates the antiproliferative effect of somatostatin. Reintroducing sst2 into human pancreatic cancer cells by stable expression evokes an autocrine negative feedback loop leading to a constitutive activation of the sst2 gene and an inhibition of cell proliferation and tumorigenicity. In vivo studies have been conducted in athymic mice to investigate the antitumor bystander effects resulting from the transfer of the sst2 gene into human pancreatic cancer cell line BxPC-3. In mixing experiments, a local bystander effect was observed: mixed tumors containing a ratio of sst2-expressing cells to control cells of 25:75, 50:50, and 75:25 grew with a time delay of 31, 44, and 50 days, respectively, when compared with control tumors derived from control cells. Tumors containing 100% sst2-expressing cells remained quiescent for up to 80 days. A significant increase in apoptosis and a decrease in the Ki67 index were detected in mixed and sst2 tumor when compared with control tumors. In combined experiments, mice were separately xenografted with control cells on one flank and with sst2-expressing cells on the other flank. A distant antitumor effect was induced: growth of control tumors was delayed by 33 days, the Ki67 index decreased significantly, and apoptosis increased when compared with control tumors that grew alone. The distant bystander effect may be explained in part by a significant increase in serum somatostatin-like immunoreactivity levels resulting from the autocrine feedback loop produced by sst2-expressing cells and inducing an upregulation of the type 1 somatostatin receptor, sst1, which also mediates the antiproliferative effect of somatostatin. In conclusion, the local and distant antitumor bystander effects obtained in this experimental model suggest that sst2 gene transfer may represent a new therapy for pancreatic cancer.

Induction of a negative autocrine loop by expression of sst2 somatostatin receptor in NIH 3T3 cells

The somatostatin receptor subtype sst2 mediates both activation of a tyrosine phosphatase activity and inhibition of cell proliferation induced by somatostatin analogues. In the absence of exogenous ligand, expression of sst2 in NIH 3T3 cells resulted in inhibition of cell growth. Polymerase chain reaction coupled to reverse transcription demonstrated that expression of sst2 in NIH 3T3 cells stimulated the expression of preprosomatostatin mRNA accompanied by a production of immunoreactive somatostatin-like peptide which corresponded predominantly to somatostatin 14. Moreover anti-somatostatin antibodies suppressed sst2-promoted inhibition of cell proliferation. Inhibition of cell proliferation associated with increased secretion of somatostatin-like immunoreactivity was also observed after expression of sst2 in human pancreatic tumor cells BxPC3 devoid of endogenous receptors. In addition, expression of sst2 in NIH 3T3 cells was associated with constitutive activation of tyrosine phosphatase PTP1C that resulted from enhanced expression of the protein. Blocking of PTP1C tyrosine phosphatase activity with orthovanadate or that of PTP1C protein with antisense PTP1C oligonucleotides decreased the sst2-induced inhibition of cell proliferation. These results, taken together, show that expression of sst2 in NIH 3T3 cells generated a negative autocrine loop by stimulating sst2 ligand production and amplifying PTP1C sst2-transducer. Sst2/ligand may function as a determinant factor involved in the negative growth control of cells.

Molecular mechanisms of antiproliferative effect of somatostatin: involvement of a tyrosine phosphatase

A protein of 66 kd immunoreactive to anti-tyrosine phosphatase (PTP1C) antibodies coeluted with, and so may be associated with, somatostatin receptors (ssts) from rat pancreatic membranes. Also, anti-PTP1C antibodies immunoprecipitated functional ssts from pancreatic membranes, suggesting a PTP1C protein can associate with ssts at the membrane level. Somatostatin analog RC 160 had good affinity for sst2,3 and sst5 (IC50 = 0.2, 0.1, and 21 nmol/L) and low affinity for sst1 and sst4 (IC50 = 200 and 620 nmol/L), and induced rapid dose-dependent stimulation of PTP activity (maximal at 1 nmol/L and half maximal at 5 pmol/L) in NIH3T3 and CHO cells expressing sst2, with similar results for sst1, but no stimulation with sst3,4 or sst5. Treatment of cells expressing sst2 with RC 160 for 24 hours inhibited serum- or growth factor-induced cell proliferation dose-dependently (maximal at 1 nmol/L, half maximal at 6 to 53 pmol/L RC 160). In cells expressing sst1, weak inhibition of fibroblast growth factor 2-induced NIH3T3 cell proliferation was provoked by somatostatin analogs (> 10 nmol/L). The good correlation between inhibition of somatostatin binding, stimulation of PTP activity, and inhibition of cell proliferation implicates a PTP in growth inhibition mediated by sst2 and sst1.

A tyrosine phosphatase is associated with the somatostatin receptor

Regulation of tyrosine phosphorylation is thought to be an essential step in signal transduction mechanisms that mediate cellular responses. In pancreatic tumour cells we demonstrated that somatostatin analogues inhibited cell proliferation and stimulated a membrane protein tyrosine phosphatase (PTP) activity at concentrations at which they bind to the somatostatin receptor. To elucidate the role of PTP in the signal transduction pathway activated by somatostatin receptors we first studied the interaction of PTP with the somatostatin receptor at the membrane. We purified somatostatin receptors by immunoaffinity from pancreatic membranes that strongly expressed the type 2 somatostatin receptor sstr2. We identified the receptor as an 87 kDa protein. We demonstrated that a PTP activity co-purified with somatostatin receptors. The PTP was identified as a 66 kDa protein immunoreactive to antibodies against SHPTP1. These antibodies immunoprecipitated somatostatin receptors either occupied or unoccupied by ligand indicating that SHPTP1 is associated with somatostatin receptors. We then expressed sstr2A in monkey kidney COS-7 cells and mouse NIH/3T3 fibroblasts and demonstrated that somatostatin analogues (RC 160, octreotide and BIM 23014) which exhibited high affinity for sstr2 stimulated a PTP activity and inhibited cell proliferation in proportion to their affinities for sstr2. Under the same conditions these analogues have no effect on the growth of cells expressing sstr1. All these results suggest that a PTP related to SHPTP1 is associated with somatostatin receptors and may be involved in the negative growth signal promoted by sstr2.

sst2 somatostatin receptor expression reverses tumorigenicity of human pancreatic cancer cells

Among the five cloned somatostatin receptor subtypes (sst1 to sst5), sst2 mediates the antiproliferative effect of somatostatin analogues in vitro. Somatostatin analogues have been shown to inhibit cell growth in vitro and in vivo in pancreatic cancer models that expressed sst2. We recently demonstrated the loss of sst2 gene expression in human pancreatic adenocarcinomas and most of the derived pancreatic cancer cell lines. In the present study, we corrected the sst2 defect in human pancreatic cancer BxPC-3 and Capan-1 cells by stable transfection with human sst2 cDNA. In the absence of exogenous ligand, both BxPC-3 and Capan-1 cells expressing sst2 showed a significant reduction in cell growth. This inhibitory effect was blocked by treatment with antiserum to somatostatin. sst2-expressing cells produced somatostatin-like immunoreactivity that mainly corresponded to somatostatin 14, indicating the induction of a negative autocrine loop. In other respects, sst2 expression in Capan-1 cells induced a significant reduction of clonogenicity in soft agar. Moreover, a significantly reduced (Capan-1 cells) or suppressed (BxPC-3 cells) tumor growth in athymic nude mice was observed. The reversal of tumorigenicity induced by the restoration of sst2 expression suggests that the loss of sst2 contributes to the malignancy of human pancreatic cancers.

Up-regulation of somatostatin receptors by epidermal growth factor and gastrin in pancreatic cancer cells

Interactions between growth factor receptor systems may be important in the regulation of cell growth. The proliferation of pancreatic tumor AR42J cells has been shown to be stimulated by Epidermal growth factor (EGF) and gastrin and inhibited by somatostatin. To analyze the interaction between these different peptides, we explored the influence of EGF and gastrin on the somatostatin receptors. Treatment of AR42J cells with 10 nM EGF or gastrin for 24 hr increased specific binding of [125I] Tyr3SMS to 131 and 147% of that in control cells, respectively. The effect of peptides on [125I]Tyr3SMS binding was time- and dose-dependent, with half-maximal effect at 0.2 +/- 0.03 nM EGF and 0.3 +/- 0.15 nM gastrin. Scatchard plots revealed an increase in somatostatin receptor number of 27 and 80% after 48 hr of treatment with EGF and gastrin, respectively, without any change in receptor affinity. The increase in somatostatin receptor density was accompanied by the enhancement of biological responses to somatostatin. In cells pretreated with EGF or gastrin, the potency of somatostatin for inhibiting vasoactive intestinal peptide-stimulated cAMP content was increased 2-fold as that of somatostatin analog, SMS, for inhibiting cell proliferation. Furthermore, the efficiency of SMS as antiproliferative agent was greatly increased. Vasoactive intestinal peptide or forskolin did not modify [125I]Tyr3SMS binding of control or treated cells. The phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) did not affect [125I]Tyr3SMS binding. On the other hand, cycloheximide completely blocked the increase in [125I]Tyr3SMS binding induced by EGF and gastrin.(ABSTRACT TRUNCATED AT 250 WORDS)