Suppression of superoxide release from human monocytes by somatostatin-related peptides.

Somatostatin and octreotide share with vasoactive intestinal peptide the property of having an inhibitory effect on leukocyte functions. While there are studies reporting the inhibitory effect of the latter on respiratory burst in human monocytes, no such reports are available about similar inhibitory effects of the former. The aim of the present study was to investigate such effects of somatostatin and octreotide on human monocytes. Release of superoxide anion from monocytes was measured by superoxide dismutase-inhibitable reduction of cytochrome c in vitro. Somatostatin 1-14, somatostatin 1-28 and octreotide inhibited release of superoxide anion from stimulated monocytes. Formylpeptide-stimulated reduction of cytochrome c was inhibited by 1 mumol/l of octreotide and somatostatin 1-14 by about 50% and 35%, respectively. The effect was dose-dependent with half-maximal effective peptide concentrations at about 10 nmol/l. Somatostatin 1-28, which is the major form found in circulating plasma, also antagonized formylpeptide-stimulated respiratory burst activity; when directly compared to the effect of 1 mumol/l of somatostatin 1-14, somatostatin 1-28 was significantly more active (P less than 0.05). Our observations suggest that somatostatin-related peptides have a regulatory role in oxygen radical metabolism and a mediator role in the neuro-immune axis.     

The importance of presurgical somatostatin analogue therapy in acromegaly

Different types of treatment, including surgery, medical therapy and radiotherapy, are possible in achieving control of acromegaly. Of the medical therapies available, somatostatin analogues are effective in the majority of patients and can induce pituitary tumour shrinkage. The rationale and outcome of somatostatin analogue treatment before surgery in patients with acromegaly is briefly presented. In summary, the benefits of somatostatin analogues given preoperatively should be considered carefully as optimisation of cardiovascular, respiratory and metabolic functions is clinically relevant for perioperative morbidity. Somatostatin analogues also induce significant shrinkage of GH-secreting pituitary tumours, although this does not seem to be helpful in terms of improved surgical outcome.     

Reciprocal paracrine pathways link atrial natriuretic peptide and somatostatin secretion in the antrum of the stomach

Atrial natriuretic peptide (ANP) as well as its receptor, NPR-A, have been identified in gastric antral mucosa, suggesting that ANP may act in a paracrine fashion to regulate gastric secretion. In the present study, we have superfused antral mucosal segments obtained from rat stomach to examine the paracrine pathways linking ANP and somatostatin secretion in this region.ANP (0.1 pM to 0.1 microM) caused a concentration-dependent increase in somatostatin secretion (EC(50), 0.3 nM). The somatostatin response to ANP was unaffected by the axonal blocker tetrodotoxin but abolished by addition of the selective NPR-A antagonist, anantin. Anantin alone inhibited somatostatin secretion by 18+/-3% (P<0.005), implying that endogenous ANP, acting via the NPR-A receptor, stimulates somatostatin secretion. Somatostatin (1 pM to 1 microM) caused a concentration-dependent decrease in ANP secretion (EC(50), 0.7 nM) that was abolished by addition of the somatostatin subtype 2 receptor (sst2) antagonist, PRL2903. Neutralization of ambient somatostatin with somatostatin antibody (final dilution 1:200) increased basal ANP secretion by 70+/-8% (P<001), implying that endogenous somatostatin inhibits ANP secretion. We conclude that antral ANP and somatostatin secretion are linked by paracrine feedback pathways: endogenous ANP, acting via the NPR-A receptor, stimulates somatostatin secretion, and endogenous somatostatin, acting via the sst2 receptor, inhibits ANP secretion.     

Localization of serotonin, cholecystokinin and somatostatin immunoreactivity in the lower respiratory tract of embryonic, foetal and postnatal sheep.

The lower respiratory tract of the sheep was studied by light-microscopical immunocytochemistry for serotonin, cholecystokinin, somatostatin, bombesin and calcitonin during different periods of lung development; embryonic, foetal and postnatal. At embryonic period only intraepithelial serotonin-containing cells as solitary neuroendocrine cells (NEC) and neuroepithelial bodies (NEB) were found. At foetal stages, immunoreactive cells to serotonin, cholecystokinin and somatostatin were observed in airway epithelium, as solitary NEC and NEBs, and in autonomic intrapulmonary ganglia as single or clusters of small intensely-fluorescent (SIF) cells. In postnatal sheep, serotonin- and cholecystokinin-containing cells were found within airway mucosa as solitary NECs and NEBs. No immunoreactive cells were observed with antiserum to bombesin and calcitonin. Quantitative studies showed that serotonin was the predominant substance, and that solitary neuroendocrine cells were more numerous in distal conducting airways and at foetal stages.     

Somatostatin-induced inhibition of insulin secretion from isolated islets of rat pancreas in presence of glucagon.

In order to study the oeffect of somatostatin on the endocrine pancreas directly, islets isolated from rat pancreas by collagenase were incubated for 2 hrs 1) at 50 and 200 mg/100 ml glucose in the absence and presence of somatostatin (1, 10 and 100 mg/ml) and2) at 200 mg/100 ml glucose together with glucagon (5 mug/ml), with or without somatostatin (100 ng/ml). Immunologically measurable insulin was determined in the incubation media at 0, 1 and 2 hrs. Insulin release was not statistically affected by any concentration stomatostatin. On the other hand, somatostatin exerted a significant inhibitory action on glucagon-potentiated insulin secretion (mean +/- SEM, mu1/2 hrs/10 islets: glucose and glucagon: 1253 +/- 92; glucose, glucagon and somatostatin: 786 +/- 76). The insulin output in th epresence of glucose, glucagon and somatostatin was also significantly smaller than in thepresence of glucose alone (1104 +/- 126) or of glucose and somatostatin (1061 +/- 122). The failure of somatostatin to affect glucose-stimulated release of insulin from isolated islets contrasts its inhibitory action on insulin secretion as observed in the isolated perfused pancreas and in vivo. This discrepancy might be ascribed to the isolation procedure using collagenase. However, somatostatin inhibited glucagon-potentiated insulin secretion in isolated islets which resulted in even lower insulin levels than obtained in the parallel experiments without glucagon. It is concluded that the hormone of the alpha cells, or the cyclic AMP system, might play a part in the machanism of somatostatin-induced inhibition of insulin release from the beta-cell.     

Endotoxin administration increases hypothalamic somatostatin mRNA through nitric oxide release

Acute inflammation induced by endotoxin (LPS) administration inhibits insulin-like growth factor (IGF-I) and growth hormone (GH) secretion. The aim of this study was to elucidate the role of glucocorticoids and nitric oxide (NO) in the effect of LPS on hypothalamic somatostatin gene expression. Adult male Wistar rats were injected with different doses of LPS (5, 10 and 100 microg/kg). Rats received two i.p. injections of LPS (at 17:30 and 8:30 h the following day) and were killed 4 h after the second injection. LPS administration at the dose of 100 microg/kg increased the hypothalamic somatostatin mRNA content, as well as the serum concentrations of corticosterone. Glucocorticoids do not seem to be involved in LPS-induced increase in hypothalamic somatostatin mRNA since adrenalectomy did not prevent this effect. In order to analyze the possible effect of NO, aminoguanidine, an inducible nitric oxide synthase inhibitor, was injected (100 mg/kg s.c.) simultaneously with LPS injection. Aminoguanidine administration did not modify somatostatin mRNA in saline injected rats, but it prevented LPS-induced increase in hypothalamic somatostatin mRNA. These data suggest that the stimulatory effect of endotoxin on hypothalamic somatostatin gene expression is not mediated by glucocorticoids, but instead by the increase in NO release.     

Binding of somatostatin to guinea-pig pancreatic membranes: regulation by ions

Somatostatin receptors were characterized on guinea-pig pancreatic acini membranes using 125I-[Tyr11] somatostatin 14 as a radioligand. In 0.1 mM Ca2+ buffer the binding was saturable and slowly reversible, exhibiting a single class of high affinity binding sites (KD = 0.15 +/- 0.03 nM) with a maximal binding capacity (B max) of 178 +/- 18 fmol/mg protein. In 30 nM) free Ca2+ buffer, the binding was highly reversible. Affinity and B max were decreased by about 2-fold. Ca2+ exhibited an EC50 of 2.4 +/- 0.9 microM to potentiate the binding of somatostatin. Na+, but not K+, inhibited the binding: Bmax was decreased with no change in affinity. Somatostatin analogs inhibited the binding of 125I-[Tyr11] somatostatin 14. The relative potencies were: somatostatin 14 greater than somatostatin 28 = [Nle8]somatostatin 28 greater than [D Tryp8, D Cys14]somatostatin 14.     

Somatostatin receptor 1 (SSTR1)-mediated inhibition of cell proliferation correlates with the activation of the MAP kinase cascade: role of the phosphotyrosine phosphatase SHP-2.

The mitogen activated protein (MAP) kinase cascade represents one of the major regulator of cell growth by hormones and growth factors. However, although the activation of this intracellular pathway has been often regarded as mediator of cell proliferation, in many cell types the increase in MAP kinase (also called extra-cellular signal regulated kinase: ERK) activity may result in cell growth arrest, depending on the length or the intensity of the stimulation. In this review we examine recent data concerning the effects of somatostatin on the MAP kinase cascade through one of its major receptor subtype, the somatostatin receptor 1 (SSTR1), stably expressed in CHO-K1 cells. Somatostatin inhibits the proliferative effects of basic FGF (bFGF) in CHO-SSTR1 cell line. However, in these cells, somatostatin robustly activates the MAP kinase and augments bFGF-induced stimulation of ERK. We show that the activation of ERK via SSTR1 is mediated by the betagamma subunit of a pertussis toxin-sensitive G-protein and requires both the small G protein Ras and the serine/threonine kinase Raf-1. Moreover the phosphatidyl inositol-3kinase and the cytosolic tyrosine kinase c-src participate in the signal transduction regulated by SSTRI to activate ERK, as well as it is involved the protein tyrosine phosphatase (PTP) SHP-2. Previous studies have suggested that somatostatin-stimulated PTP activity mediates the growth inhibitory actions of somatostatin, in CHO-SSTR1 cells. Thus, the activation of SHP-2 by SSTR1 may mediate the antiproliferative activity of somatostatin. SHP-2 may. in turn, regulate the activity of kinases upstream of ERK that require tyrosine dephosphorylation to be activated, such as c-src. Finally, the synergism between somatostatin and bFGF in the activation of ERK results in an increased expression of the cyclin-dependent kinase inhibitor p21cip/WAF1 as molecular effector of the antiproliferative activity of somatostatin.     

Somatostatin induces rapid contraction of neuroendocrine cells

The peptide hormone somatostatin, as well as the somatostatin analog octreotide, induces rapid morphological changes in neuroendocrine cells. The effect can be detected in less than 2 min: retraction fibers are formed, cells round up and cell-cell contacts are broken. Somatostatin-dependent cell contraction is inhibited by Y-27632, indicating that this effect is dependent on Rho kinase. In BON1 cells, the somatostatin-induced inhibition of forskolin-induced secretion of chromogranin A is not blocked by Y-27632. It is therefore concluded that the inhibitory effect of somatostatin in forskolin-stimulated cells is not dependent on cell contraction.     

Effect of somatostatin infusion on VIP-induced transport changes in the human jejunum

This study was designed to elucidate the mechanism by which somatostatin administration ameliorates or abolishes diarrhea in pancreatic cholera syndrome (PCS). Absorption (or secretion) of water and electrolytes was measured in 30-cm segments of jejunum of 18 healthy volunteers in whom PCS was mimicked by intravenous infusion of VIP. Using the triple-lumen tube technique, the intestine was perfused with a plasma-like electrolyte solution while administering intravenous saline (control), VIP (400 pmol/kg/hr), somatostatin (5000 pmol/kg/hr), or VIP plus somatostatin. VIP infusion abolished water and electrolyte absorption and somatostatin had no effect on these VIP-induced transport changes regardless of whether somatostatin infusion was started before or after VIP infusion. Somatostatin infusion had no effect on VIP plasma concentration when elevated by intravenous VIP infusion (control: 10 +/- 1 pmol/l; during VIP infusion: 108 +/- 6). In a patient with pancreatic cholera syndrome identical perfusion experiments showed jejunal water secretion (93 ml/30 cm/hr) which changed to absorption (65 ml/30 cm/hr) when somatostatin was infused (5000 pmol/kg/hr). Plasma VIP concentration fell from 145 to 74 pmol/l (normal less than 50) during somatostatin infusion. Stool weight fell from 3722 g to 819 g per 24 hours when somatostatin was given at a dose of 2500 pmol/kg/hr for two days. Our observations in healthy subjects show that somatostatin has no effect on intestinal transport at the mucosal level when circulating VIP concentration is elevated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Somatostatin and the interdigestive migrating motor complex in man

The relationship between somatostatin and the interdigestive migrating motility complex (MMC) was determined in human volunteers. Motor activity was monitored manometrically by means of seven perfused catheters: four in the stomach, one in the duodenum, two in the jejunum. Blood samples were drawn every 10 min and radioimmunoassayed for motilin, pancreatic polypeptide and somatostatin. In four volunteers two activity fronts (AF) were recorded and somatostatin levels correlated to the manometric data. The start of an AF in the upper duodenum was accompanied by somatostatin peaks. Peak values, taken as the mean of the levels in the sample obtained after the start of an AF, the preceding sample and the next one, averaged 32 +/- 4 pM compared to 12 +/- 2 pM in the remaining period. In four volunteers somatostatin was infused in doses of 1.2, 2.4 and 4.8 pM/kg per min over three consecutive periods of 90 min, causing dose-dependent increments in plasma somatostatin levels of 7, 32 and 76 pM. In all volunteers and for all doses all gastric activity was completely inhibited. In the intestine phase 2 was abolished but phase 3 stimulated: during somatostatin infusion phase 3 occurred with an interval of 39 +/- 6 min. Motilin and PP levels were decreased. As the two lowest infusion doses caused increases in somatostatin levels that might be considered as physiological, somatostatin seems to have a physiological role in the regulation of the migrating motor complex. We propose that it facilitates the progressing of the activity front into the small intestine.     

Somatostatin receptors in the gastrointestinal tract in health and disease.

The multiple actions of somatostatin are mediated by specific membrane-bound receptors present in all somatostatin target tissues, such as brain, pituitary, pancreas, and gastrointestinal tract. Three different types of tissues in the human gastrointestinal tract express somatostatin receptors: (1) the gastrointestinal mucosa, (2) the peripheral nervous system, and (3) the gut-associated lymphoid tissue, where the receptors are preferentially located in germinal centers. In all these cases, somatostatin binding is of high affinity and specific for bioactive somatostatin analogs. Somatostatin receptors are also expressed in pathological states, particularly in neuroendocrine tumors of the gastrointestinal tract. Ninety percent of the carcinoids and a majority of islet-cell carcinomas, including their metastases, usually have a high density of somatostatin receptors. Only 10 percent of the colorectal carcinomas and none of the exocrine pancreatic carcinomas, however, contain somatostatin receptors. The somatostatin receptors in tumors are identified with in vitro binding methods or with in vivo imaging techniques; the latter allow the precise localization of the tumors and their metastases in the patients. Since somatostatin receptors in gastroenteropancreatic tumors are functional, their identification can be used to assess the therapeutic efficacy of octreotide to inhibit excessive hormone release in the patients.     

Evidence for Ascending and Descending Intraspinal as Well as Primary Sensory Somatostatin Projections in the Rat Spinal Cord

Somatostatin distribution was measured quantitatively in the rat spinal cord by radioimmunoassay. Rostro-caudally, somatostatin content was about 50% higher in lumbar-sacral cord than in cervical or thoracic levels. The dorso-ventral distribution is more uneven: somatostatin is highest in the dorsal horn, where the peptide is 15 times as concentrated as it is in the ventral white matter, the region of lowest concentration. However, measurable amounts of the peptide were found in all regions studied. Dorsal root ganglionectomy decreased somatostatin levels in the dorsal cord, supporting the previously proposed role for this peptide as a primary sensory neurotransmitter or modulator; but somatostatin content also was decreased both rostral and caudal to spinal transection, indicating the presence of ascending and descending somatostatin pathways within the spinal cord. Brain levels did not change. Met-enkephalin and substance P were also measured after the above surgical manipulations. Met-enkephalin content was not altered and substance P content was lowered significantly only after ganglionectomy. Although this study confirms the primary sensory neuron as the origin of a part of spinal cord somatostatin, it further indicates the presence of ascending and descending somatostatin pathways within the rat spinal cord.     

Solubilization and characterization of guinea-pig pancreatic somatostatin receptors

The solubilization of somatostatin receptors from guinea-pig pancreas by different non-denaturing detergents was investigated after stabilization of the receptors by prior binding of 125I-[Tyr11]somatostatin or its analogue 125I-[Leu8,DTrp22,Tyr25]somatostatin 28, to pancreatic plasma membranes. The somatostatin-receptor complexes were solubilized in a high yield by Zwittergent 3-14 (3-[tetradecyldimethylammonio]-1-propanesulfonate), a zwitterionic detergent. Other detergents, digitonin, Triton X-100, Chaps (3-[cholamidopropyldimethylammonio]-1-propanesulfonate) and octyl beta-D-glycopyranoside, achieved only partial solubilization. The recovery of receptor complexes was increased by glycerol. In order to characterize solubilized somatostatin-receptor complexes, membranes receptors were covalently labelled using N-5-azido-2-nitrobenzoyloxysuccinimide as cross-linking reagent before solubilization. Gel filtration chromatography analysis resulted in the identification of a major protein component of apparent Mr = 93,000 which interacted with the two radioligands. In addition, a similar component of Mr = 88,000 was characterized after analysis by SDS-PAGE of membrane receptors covalently cross-linked with 125I-[Leu8,DTrp22,Tyr25]somatostatin 28 by different heterobifunctional reagents: N-5-azido-2-nitrobenzoyloxysuccinimide, N-hydroxysuccinimidyl 4-azidobenzoate, N-succinimidyl 6-(4'-azido-2'-nitrophenylamino)hexanoate. Optimal cross-linking results were obtained with N-5-azido-2-nitrobenzoyloxysuccinimide. The solubilized somatostatin-receptor complex was adsorbed to wheat-germ agglutinin-agarose column and eluted by specific sugars. We concluded that the guinea-pig pancreatic somatostatin receptor in the membrane and in the non-denaturing detergent solution behaves as a protein monomer of apparent Mr approximately 85,000-90,000. The somatostatin receptor is a glycoprotein which contains complex-type carbohydrate chains.     

Somatostatin receptors in normal and tumoral tissue

Somatostatin receptors have been visualized with autoradiography and characterised biochemically in various somatostatin target tissues, such as brain, pituitary, pancreas and gastrointestinal tract, where they are likely to mediate the somatostatin actions. With the same methods, somatostatin receptors have been detected also in tumors originating from somatostatin target tissues: high receptor incidence is found in GH-producing pituitary adenomas as well as in some hormone-producing gastrointestinal tumors. These tumors are often highly responsive to somatostatin analogs in vivo. Among brain tumors, meningiomas usually contain a high density of receptors, suggesting a novel function for somatostatin in the human meninges. Among other human tumors tested, prostate, ovarian and endometrial carcinomas were free of receptors whereas 3 out of 39 mammary tumors contained somatostatin receptors.     

A bioactive somatostatin analog without a type II' beta-turn: synthesis and conformational analysis in solution.

A cyclic somatostatin analog [structure: see text] (1) has been synthesized. Biological assays show that this compound has strong binding affinities to somatostatin hsst2 and hsst5 receptor subtypes (5.2 and 1.2 nM, respectively, and modest affinity to hsst4 (41.1 nM)). Our conformational analysis carried out in DMSO-d6 indicates that this compound exists as two structures arising from the trans and cis configurations of the peptide bond between Phe7 and N-alkylated Gly8. However, neither conformer exhibits a type II' beta-turn. This is the first report of a potent bioactive somatostatin analog that does not exhibit a type II' beta-turn in solution. Molecular dynamics simulations (500 ps) carried out at 300 K indicate that the backbone of compound 1 is more flexible than other cyclic somatostatin analogs formed by disulfide bonds.     

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.     

Distribution and molecular forms of peptides containing somatostatin immunodeterminants in extracts from the entire gastrointestinal tract of man and pig

Specimens from human porcine mucosal and muscular tissue from the entire gastrointestinal tract were extracted in acid ethanol, subjected to chromatography and analysed for somatostatin-like immunoreactivity by region-specific radioimmunoassays. The concentration of somatostatin-like immunoreactivity from man and pig ranged from 1.13 +/- 0.37 to 101.15 +/- 33.93 pmol/g wet weight, and from 7.64 to 159.48 +/- 23.79 pmol/g wet weight, respectively. In both species the highest concentrations were found in the jejunum. The immunoreactivity in intestinal mucosal extracts was distributed among four major peaks, two of which were identified by HPLC as somatostatin 1-28 and somatostatin 1-14, respectively. A peak of approx. 10 kDa was resolved by ion exchange plus HPLC into three components, two containing at least part of the somatostatin 1-14 sequence as well as (part of) the somatostatin 1-28(1-14) sequence (but differing in charge), the third containing only the 1-28(1-14) sequence. These peptides probably represent uncleaved and partially cleaved prosomatostatin. The fourth component to be identified by gel filtration was slightly larger than somatostatin 1-14. Extracts from the antrum, the pancreas and from muscular tissues contained almost exclusively somatostatin 1-14, and very little somatostatin 1-28, indicating that the somatostatin precursor is processed differently at these sites. Furthermore, extracts of porcine gastric antrum, analysed for somatostatin 1-28(1-14) immunoreactivity, showed two immunoreactive forms in the mucosa and three major forms in the muscular layers.     

Preliminary evaluation of neuroendocrine cells in the respiratory tract in rats with experimental uremia

The goal of this study was to investigate the influence of experimetally induced chronic renal failure on endocrine cells in the respiratory tract in rats. After 30 days of uremia, the fragments of rat lungs were collected. Paraffin sections were stained using H+E, silver impregnation and immunohistochemistry with specific antibodies against calcitonin (CT), synaptophysin (SY), somatostatin (ST), and neuron-specific enolase (NSE). A large number of endocrine cells with a strong calcitonin immunoreactivity were observed in the respiratory tract of rats with experimental uremia, as compared with the control group. Other immunoreactions were weakened.