A somatostatin receptor negatively coupled to adenylate cyclase in the human gastric cell line HGT-1

Somatostatin analogs stimulate DNA-dependent protein kinase activity in human gastric tumoral cell-line HGT1

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) phosphorylates, in the presence of double-stranded DNA, several transcription-, replication- and repair -factors. Its interaction with the DNA-binding regulatory component Ku (p86-/p70-Ku) is required for stabilization and activity. We have previously shown that p86-Ku behaves as a specific receptor for the growth inhibitory tetradecapeptide, somatostatin. In this work, we investigate a possible regulation by somatostatin analogs, of DNA-PK activity in the human gastric tumoral HGT1/clone6 cell-line. We demonstrate that a 48 h-preincubation of cells with octreotide or RC-160, stimulates DNA-PK activity by 8 and 10 fold with ED50s of 1 and 0.1 nM, respectively. These stimulations appearing only after 3 h were inhibited by cycloheximide. They were not observed in a cell clone which was transfected by a cDNA encoding p86-Ku antisense. This study demonstrates the existence of a new somatostatin signaling pathway involving the stimulation of DNA-PK activity.

Efficacy of octreotide in diarrhoea due to Vibrio cholerae: a randomized, controlled trial

Although octreotide, a long-acting analogue of somatostatin, is currently used in the treatment of chronic secretory diarrhoea due to various causes, its role in the management of acute secretory diarrhoea is not well established. In the present study, therefore, the therapeutic value of octreotide in the management of cholera, a classical example of acute secretory diarrhoea, was investigated. During an outbreak of cholera, patients admitted with acute secretory diarrhoea of < or = 24 h duration and a purging rate > 100 ml/h were enrolled on the study and randomly assigned to octreotide (N = 17) and control (N = 16) groups. All 33 patients received intravenous fluid replacement and antibiotic treatment (200 mg ofloxacin twice daily for 3 days, by mouth). Each patient in the octreotide group was also given a subcutaneous injection containing 100 micrograms octreotide every 8 h for a maximum of six doses. The stool output of each patient was recorded every hour until there had been none for an hour, which was taken as the endpoint. Mean (S.D.) total stool output was lower [6.56 (3.7) v. 9.7 (6.5) litres] and the mean (S.D.) duration of diarrhoea after admission was shorter [32.9 (15.6) v. 47.8 (22.3); P < 0.05] in the octreotide group than in the control group. However, as both groups generally had similar purging rates, the higher volume of stools from the control group was simply the result of the longer period of diarrhoea in this group. Octreotide therefore only decreased the duration of diarrhoea in the cholera patients.

Messenger RNA expression of somatostatin receptor subtypes in human and rat gastric mucosae

In several tissues including gastric mucosa, somatostatin displays various biological effects. Five seven-transmembrane-domain somatostatin receptor subtypes (SSTR1-5) have been recently cloned and only SSTR1 has been shown to be present in the human stomach. We used the polymerase chain reaction on reverse transcripts (RT-PCR) to characterize further the SSTR's mRNAs in human and rat gastric mucosae and in the human gastric tumoral cell-line HGTL. The SSTR1-5's mRNAs were found in both human fundic and antral mucosae as well as in the HGT1 cell and rat antrum. The four SSTR2-5's mRNA's but not SSTR1's were detected in the rat fundic mucosa. Furthermore, the use of rat isolated and purified fundic mucosal cells allowed us to localize SSTR2-5 in the parietal cell-enriched fraction, whereas SSTR2 and SSTR5 were the only subtypes found in the endocrine cell-enriched fraction. These results are the first to demonstrate the presence of five SSTR's mRNA subtypes in the stomach.

Receptors for gut regulatory peptides

Receptors for regulatory peptides (hormones or neurotransmitters) play a pivotal role in the ability of cells to taste the rich neuroendocrine environment of the gut. Recognition of low concentration of peptides with a high specificity and translation of the peptide-receptor interaction into a biological response through different signalling pathways (adenylyl cyclase-cAMP or phospholipase C-phosphatidylinositol) are crucial properties of receptors. While many new receptors have been identified and thereafter characterized functionally during the 1980s, molecular biology now emerges as the privileged way for the structural characterization and discovery of receptors. Different strategies of receptor cloning have been developed which may or may not require prior receptor purification. Among cloning strategies that do not require receptor purification, homology screening of cDNA libraries, expression of receptor cDNA or mRNA in Xenopus laevis oocytes or in COS cells, and the polymerase chain reaction method achieved great success, e.g. cloning of receptors for cholecystokinin, gastrin, glucagon-like peptide 1, gastrin-releasing peptide/bombesin, neuromedin K, neuropeptide Y, neurotensin, opioids, secretin, somatostatin, substance K, substance P and vasoactive intestinal peptide. All these receptors belong to the superfamily of G-protein-coupled receptors which consist of a single polypeptide chain (350-450 amino acids) with seven transmembrane segments, an N-terminal extracellular domain and a C-terminal cytoplasmic domain. In this chapter, we have detailed the properties of three receptors which play an important role in digestive tract physiology and illustrate various signal transduction pathways: pancreatic beta-cell galanin receptors which mediate inhibition of insulin release and intestinal epithelial receptors for vasoactive intestinal peptide and peptide YY, which mediate the stimulation and inhibition of water and electrolyte secretion, respectively.

Evidence for autocrine growth stimulation by a gastrin/CCK-like peptide of the gastric cancer HGT-1 cell line

Gastrin has been shown to promote the growth of some colonic tumor cell lines. To evaluate the involvement of this hormone in the proliferation of gastric tumors, we studied the effects of gastrin/CCK-receptor antagonists (L365,260 and L364,718), proglumide and C terminal-specific gastrin antibodies on the human gastric adenocarcinoma cell line HGT-1. L365,260, but not L364,718, dose-dependently inhibited cell proliferation (72% after 4 days at 10 nM) and [3H]thymidine incorporation (68% after 2 days at 10 nM) in serum-free medium. No cytotoxic effects of proglumide or L365,260 on this cell line were detected. Proglumide inhibited cell proliferation in serum-free medium (40% and 66.5% after 2 and 4 days of treatment; IC50 = 1.4 mM) and in 5% fetal calf serum (FCS)-supplemented medium (30% and 22% after 2 and 4 days of treatment; IC50 = 3.25 mM). [3H]Thymidine incorporation was also inhibited by proglumide in serum-free medium (IC50 = 2.3 mM) and 5% FCS-supplemented medium (IC50 = 3.35 mM). Gastrin did not induce cell proliferation or increase [3H]thymidine incorporation and no high-affinity gastrin binding sites were observed. However, C terminal-specific gastrin antibodies, even at low concentration, caused a dramatic decrease in both cell number (IC50 = 1:4000 antiserum dilution) and [3H]thymidine incorporation (IC50 = 1:400 antiserum dilution) in the HGT-1 cell line. In addition, immunofluorescence analysis revealed that these antibodies specifically bind HGT-1 cells and radioimmunoassay analysis confirms the presence of gastrin/CCK-like peptide in cell extracts.(ABSTRACT TRUNCATED AT 250 WORDS)

Octreotide (SMS 201-995) inhibits the growth of colon peritoneal carcinomatosis in BDIX rats

In order to assess the effect of octreotide, a somatostatin analogue, on the growth of colon peritoneal carcinomatosis, 20 BDIX rats were injected i.p. with 1 x 10(6) colon cancer cells (DHD/K12 tumor cell line) and received octreotide, 65 micrograms/kg s.c. every 12 h (n = 10) or saline (n = 10) for 42 days, starting 3 days after tumor cell injection. Animals were killed at the end of the treatment. The mean volume of ascites was lower in the octreotide group (33.7 +/- 7.6 ml), than in the control group (67.5 +/- 16.3 ml; P < 0.05). The extent of peritoneal carcinomatosis (in five classes according to a previously published classification) was lower in the octreotide group (P < 0.05). Cell proliferation, using the BrdU technique, was markedly inhibited by octreotide (labeling index of tumor cells: 17.0 +/- 0.6% vs. 26.3 +/- 2.2% in controls, P < 0.001). No significant decrease in labeling index was observed in normal colonic mucosa. Two subtypes of somatostatin receptors were found in all tumors, using the 30F3 monoclonal antireceptor antibody. KD and Bmax values were not significantly different in the octreotide and control groups: high affinity, low capacity receptors (KD = 1.4 x 10(-10) M and 0.7 x 10(-10) M, respectively; Bmax = 3.8 and 2.9 pmol/mg protein, respectively); low affinity, high capacity receptors (KD = 1 +/- 0.2 x 10(-9) M and 5.5 +/- 0.05 x 10(-10) M, respectively; Bmax = 27.8 +/- 0.1 and 22.8 +/- 0.05 pmol/mg protein, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of PGE2, misoprostol, and enprostil on guinea pig enterocyte adenylate cyclase. Clinical implications

Prostaglandins of the E series (PGE) are known to stimulate intestinal water and electrolyte secretions via the activation of the enterocyte adenylate cyclase. Their methylated synthetic analogs misoprostol and enprostil induced diarrhea in 5-13% of the patients in most clinical studies. In order to elucidate the role of PGE-adenylate cyclase interaction in these phenomena, we studied the stimulation of adenylate cyclase by native prostaglandin E2 (PGE2) and synthetic PGE analogs on isolated guinea pig intestinal epithelial cells. PGE2 stimulation of adenylate cyclase was dose-dependent, reaching a maximum for 3 x 10(-4) M, with an EC50 of 3.7 x 10(-6) M. The Hill analysis of the concentration-response curve gave a straight line, with a slope close to 1. The effect of PGE2 was strictly additive to that of 10(-5) M forskolin, whereas it was decreased in terms of potency by 10(-9) M cholera toxin. Somatostatin-14 markedly inhibited PGE2 stimulation by 37% and 45% with 10(-9) M and 10(-6) M, respectively. The two PGE methylated analogs misoprostol and enprostil were less potent than PGE2 in stimulating adenylate cyclase in our model.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatostatin inhibition of phosphoinositides turnover in isolated rat acinar pancreatic cells: interaction with bombesin

The effects of somatostatin-14 and bombesin on [3H]inositol phosphate accumulation were studied in 24 h myo-[3H]inositol-prelabeled cultured rat acinar cells. Bombesin, 10 nM, stimulated basal formation of phosphatidyl monophosphate (InsP1), phosphatidyl 4,5-biphosphate (InsP2) and inositol 1,4,5-triphosphate (InsP3) by 128 +/- 5.2%, 147 +/- 10% and 155 +/- 5%, respectively. At 5 s, the ED50 value for InsP3 stimulation was 0.70 +/- 0.2 nM. This stimulation was partly blocked (64 +/- 0.04% inhibition) by 10 ng/ml Bordetella pertussis toxin. In contrast to bombesin, somatostatin, 10 nM, inhibited basal InsP1, InsP2 and InsP3 formation. At 5 s, the inhibition degree for InsP3 was 18 +/- 2.5% and the IC50s values 1 +/- 0.09 nM, 1 +/- 0.12 nM and 0.07 +/- 0.005 nM for InsP1, InsP2 and InsP3, respectively. Bombesin-stimulated InsP3 formation was also inhibited by somatostatin. At 5 s, the inhibition degree was 85 +/- 3.5% at 10 nM and the IC50 value, 0.10 +/- 0.05 nM. Furthermore, somatostatin inhibition of bombesin stimulation was partly blocked (66 +/- 4% inhibition) by Bordetella pertussis toxin. These data therefore suggest that the acinar pancreatic cells contain a somatostatin receptor exerting a negative control on basal and bombesin receptor-stimulated phosphatidyl inositol turnover.

Molecular characterization of the solubilized receptor of somatostatin from rat pancreatic acinar membranes

The somatostatin receptors on rat pancreatic acinar membranes were demonstrated by use of a radioiodinated (125I-) analogue of somatostatin (SMS 204-090 or [Tyr3]SMS). The tracer was found to bind to the receptor with a Kd of 58 pM. The number of sites detected by this tracer (4.7 pmol/mg of protein) was 5-10 times higher than the number of sites previously found with other tracers. Since the level of non-specific binding was also very low as compared with findings with other tracers, 125I-204-090 might be of interest in future attempts to characterize the somatostatin receptors in the pancreas. The prelabelled membranes were solubilized with 1% CHAPS, and the solubilized complexes were found to adsorb to wheat-germ-agglutinin-coupled agarose, from which they could be eluted with 4 mM-triacetylchitotriose. The complexes within this eluate were shown by gel filtration on Trisacryl GF-2000 to have an Mr of about 400,000. The dissociation of the complexes was augmented both within the membranes as well as in the solubilized state by incubation with the GTP analogue guanosine 5'-[gamma-thio]triphosphate, indicating that the complexes are probably functionally linked to a guanine-nucleotide-binding regulatory protein. After SDS/slab-gel electrophoresis and autoradiography of cross-linked complexes after treatment with the heterobifunctional reagent N-5-azido-2-nitrobenzoyloxysuccinimide, a broad band occurred at approximately Mr 90,000 both in the membranes and in the eluates of complexes after lectin-adsorption chromatography. We conclude that the augmentation of the number of detectable sites for binding of somatostatin, as well as the very low level of non-specific binding obtained by the use of 125I-[Tyr3]SMS as tracer, has made it possible for us to demonstrate the solubilization of the somatostatin receptor in conjunction with its ligand and a GTP-binding regulatory protein, and we have succeeded in cross-linking 125I-[Tyr3]SMS to a binding subunit of Mr 90,000 in the membranes and in demonstrating the presence of the same labelled binding subunit within complexes solubilized and chromatographed on a lectin column before cross-linking.