Solubilization and immunopurification of a somatostatin receptor from the human gastric tumoral 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.

Development of selective antibodies against the human somatostatin receptor subtypes sst1-sst5

Antisera selective for five somatostatin receptor subtypes, human sst1-sst5, were raised in rabbits. C-terminal parts of human sst1-sst5 receptors were expressed as fusion proteins with glutathione S-transferase. Fusion proteins were affinity-purified and used for raising polyclonal antibodies. In Western blot analysis, all five antisera were tested on preparations of mammalian cell lines transfected with human sst1-sst5, respectively. sst1 antiserum reacted with a broad band of 53-72 kDa. A band of 71-95 kDa was detected with the antiserum raised against sst2, 65-85 kDa with sst3 antiserum, 45 kDa with sst4 antiserum and 52-66 kDa with sst5 antiserum. No cross-reactivity could be detected to any of the other four somatostatin receptor subtypes. Enzymatical deglycosylation of the receptors revealed that sst1, sst2, sst5 and possibly sst3 in this system are subjected to N-linked glycosylation, whereas sst4 is not. Two of the antisera (sst2 and sst5) were used for immunohistochemical localization of the receptors. sst2 and sst5 antisera labeled neurons in e.g. the amygdaloid complex, hippocampus, fascia dentata and the neocortex in rat and monkey tissue. This is the first report on antisera against all five somatostatin receptor subtypes and the first immunohistochemical visualization of sst5 receptors in the mammalian brain.

Somatostatin receptors in the central nervous system

Somatostatin was first identified chemically in 1973, since when much has been established about its synthesis, storage and release. It has important physiological actions, including a tonic inhibitory effect on growth hormone release from the pituitary. It has other central actions which are not well understood but recent cloning studies have identified at least five different types of cell membrane receptor for somatostatin. The identification of their genes has allowed studies on the distribution of the receptor transcripts in the central nervous system where they show distinct patterns of distribution, although there is evidence to indicate that more than one receptor type can co-exist in a single neuronal cell. Receptor selective radioligands and antibodies are being developed to further probe the exact location of the receptor proteins. This will lead to a better understanding of the functional role of these receptors in the brain and the prospect of determining the role, if any, of somatostatin in CNS disorders and the identification of potentially useful medicines.

Bombesin activation of phospholipase C beta 1 in rat acinar pancreatic cells involves the pertussis toxin-sensitive G alpha i3 protein

Bombesin stimulation of inositol 1,4,5-trisphosphate (Ins P3) formation in rat sonicated pancreatic acinar cells was inhibited by an antibody directed against the pertussis toxin (PTX)-sensitive GTP-binding G alpha i3 protein but not by an anti-G alpha q-11 antibody. After solubilization and gel filtration, [125I-Tyr4]bombesin binding sites were recovered in a peak of protein of 67 approximately 90 kDa with a maximal enrichment corresponding to a molecular mass of 83-kDa. Results obtained from the non-hydrolysable GTP analog guanosine-5'-[gamma-thio]triphosphate (GTP gamma S) binding, PTX-stimulated ADP-ribosylation and immunoblotting showed that the 83-kDa fraction contained the G alpha i3 protein but not the G alpha q-11 protein. Furthermore, GTP gamma S increased the bombesin binding dissociation constant (KD) from 0.32 to 0.60 nM, while the anti-G alpha i3 antibody decreased the maximal binding capacity (Bmax) from 50 to 25 fmol/mg protein without affecting the KD. Mixing solubilized bombesin binding sites with a phospholipase C (PLC) preparation from rat pancreas reconstituted a bombesin-stimulated PLC activity which was markedly inhibited by the anti-G alpha i3 antibody but unaffected by the anti-G alpha q-11 antibody. In addition, this stimulation was inhibited by an anti-PLC beta 1 antibody. This result supports the involvement of the PLC beta 1 isoform in bombesin receptor activation.

Stimulated human gastric tumor cells (HGT) fail to synthesize eicosanoids

HGT cells are a human gastric tumor cell line. Preliminary data have shown that HGT cells incorporate exogenous arachidonic acid (AA) in their membrane lipids. However, we found that HGT cells are unable to produce significant amounts of AA metabolites after stimulation with calcium ionophore A23187. Furthermore, no lipoxygenase activity was detected in crude HGT cell extracts by employing an assay monitoring the in vitro utilization of linoleic acid. The meaning of these results is discussed in respect of the role of eicosanoids during cell proliferation.

Nonsurgical closure of esophago-respiratory fistulas: role for the somatostatin analogue octreotide acetate?

Esophago-respiratory fistulas (ERF) do not close spontaneously and are uniformly fatal. A somatostatin analogue (octreotide acetate) was used in three consecutive patients to promote the closure of ERF. In 2 patients with esophageal cancer, treatment with octreotide acetate was associated with fistula closure in 30 and 46 days, respectively. In a third patient with virally-induced ERF, treatment was associated with improvement of the inflammation of the fistula before the patient's death from pulmonary aspiration after 40 days of treatment. These preliminary observations suggest that octreotide acetate treatment of ERF should receive further investigative scrutiny.

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.

Characterization and purification of the solubilized pituitary adenylate-cyclase-activating polypeptide-1 receptor from porcine brain using a biotinylated ligand

A specific receptor for the brain-gut neuropeptide pituitary adenylate-cyclase-activating polypeptide (PACAP-1 receptor) was solubilized with Chapso from porcine brain plasma membranes and purified. Binding of 125I-PACAP(1-27) to the solubilized material was reversed equipotently by unlabeled PACAP(1-27) and PACAP(1-38). Soluble receptors retained the binding affinities and specificities of the plasma membrane fraction. Scatchard analysis of equilibrium-binding data indicated the existence of a single high-affinity binding site (Kd = 0.23 nM, Bmax = 1.2 pmol/mg protein). Binding of 125I-PACAP(1-27) to solubilized receptors was not affected by guanosine nucleotides, suggesting that solubilization dissociates the PACAP-1 receptor/guanosine-nucleotide-binding protein complex. Affinity cross-linking of 125I-PACAP(1-27) to soluble PACAP-1 receptors identified a specifically labeled 60-kDa protein. Enzymic deglycosylation of soluble affinity-labeled receptors reduced the apparent molecular mass by 10 kDa. The solubilized receptor glycoprotein was purified 4-5-fold by lectin-adsorption chromatography on wheatgerm agglutinin immobilized on agarose. S-Biotinyl[Ala28-34, Cys35]PACAP(1-35) was synthesized, immobilized on streptavidin-coated magnetic Sepharose beads and used to further affinity-purify wheatgerm-agglutinin-eluted receptor material. This more than 6000-fold enriched PACAP-1-receptor-preparation retained single-class high-affinity binding and consisted of an almost homogenous 55-60-kDa protein identified by silver staining. In conclusion, we established a rapid method for purification of PACAP-1 receptors, allowing further studies to be performed by protein chemistry.

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)

Rat heart smooth muscle cells express high and low affinity receptors for somatostatin-14, which are involved in regulation of cell proliferation

We demonstrate that rat heart coronary artery smooth muscle cells express specific binding sites (receptors) for somatostatin-14. The sigmoidal shape kinetics of the somatostatin-14 binding by the cells suggests the presence of either an allosteric binding site or binding sites with different affinities towards the ligand. Scatchard analysis reveals presence of high affinity (Kd = 0.039 x 10(-9)M) and low affinity (Kd = 0.602 x 10(-9)M) binding sites. Somatostatin-14 and Angiopeptin can displace each other from the binding sites. The concentrations of Angiopeptin required to displace 28%-48% of bound somatostatin-14 (10(-9)M) are in the range of 10(-4)-10(-3)M. The concentrations of somatostatin-14 required to displace 8-27% of bound Angiopeptin (10(-6)M) are in the range of 10(-6)-10(-5)M. Thus, somatostatin-14 seems to possess much higher binding affinity than Angiopeptin. Binding of somatostatin-14 and Angiopeptin to rat smooth muscle cells triggers intracellular event(s) leading to inhibition of smooth muscle cell proliferation. Exposure of smooth muscle cells to somatostatin-14 and Angiopeptin decreases amount of phosphorylated tyrosine residues. The effect of somatostatin-14 and Angiopeptin on the expression of phosphotyrosine precedes and is most likely responsible, at least in part, for the inhibition of smooth muscle cell proliferation. This demonstrates that rat heart smooth muscle cells express physiologically active receptor(s) for somatostatin-14.

Partial amino acid sequence of a somatostatin receptor isolated from GH4C1 pituitary cells

A somatostatin receptor isolated from GH4C1 rat pituitary tumor-derived cells was cleaved with cyanogen bromide or cyanogen bromide+trypsin to obtain sequenceable fragments. Five unique amino acid sequences ranging from 6 to 27 amino acid residues were obtained. The sequence was identical to sequence recently reported for one of two somatostatin receptors cloned from human pancreas [Yamada et al., (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 251-255] except for a single valine to isoleucine substitution. This is the first report of amino acid sequence from a purified somatostatin receptor.

Pharmacological and biochemical characteristics of partially purified GABAB receptor

Pharmacological and biochemical characteristics of the partially purified gamma-aminobutyric acid (GABA)B receptor using baclofen affinity column chromatography have been examined. The Scatchard analysis of [3H]GABA binding to the purified GABAB receptor showed a linear relationship and the KD and Bmax values were 60 nM and 118 pmol/mg of protein, respectively. Although GTP and Mg2+ did not affect on the GABAB receptor binding, Ca2+ significantly increased [3H]GABA binding to the purified GABAB receptor in a dose-dependent manner and showed its maximum effect at 2 mM. The enhancement of the binding by Ca2+ was found to be due to the increase of Bmax by the Scatchard analysis. The treatments with pronase and trypsin significantly decreased the binding of [3H]GABA, but phospholipase A2 had no significant effect on the binding. In addition, treatment with glycosidases such as glycopeptidase A and beta-galactosidase significantly decreased the binding of [3H]GABA to the purified GABAB receptor. These results suggest that purification of the solubilized GABAB receptor by the affinity column chromatography may result in the functional uncoupling of GABAB receptor with GTP-binding protein. Furthermore, the present results suggest that cerebral GABAB receptor may be a glycoprotein and membrane phospholipids susceptible to phospholipase A2 treatment may not be involved in the exhibition of the binding activity.

VIP receptors from porcine liver: high yield solubilization in a GTP-insensitive form

Vasoactive intestinal peptide (VIP) receptors were solubilized from porcine liver membranes using CHAPS. The binding of 125I-VIP to solubilized receptors was reversible, saturable and specific. Scatchard analysis indicated the presence of one binding site with a Kd of 6.5 +/- 0.3 nM and a Bmax of 1.20 +/- 0.15 pmol/mg protein. Solubilized and membrane-bound receptors displayed the same pharmacological profile since VIP and VIP-related peptides inhibited 125I-VIP binding to both receptor preparations with the same rank order of potency e.g. VIP greater than helodermin greater than rat GRF greater than rat PHI greater than secretin greater than human GRF. GTP inhibited 125I-VIP binding to membrane-bound receptors but not to solubilized receptors supporting functional uncoupling of VIP receptor and G protein during solubilization. Affinity labeling of solubilized and membrane-bound VIP receptors with 125I-VIP revealed the presence of a single molecular component with Mr 55,000 in both cases. It is concluded that VIP receptors from porcine liver can be solubilized with a good yield, in a GTP-insentive, G protein-free form. This represents a major advance towards the purification of VIP receptors.

Mechanism of action of somatostatin: an overview of receptor function and studies of the molecular characterization and purification of somatostatin receptor proteins

To determine whether somatostatin receptor subtypes arise from molecular heterogeneity of the receptor protein, we have cross-linked the putative receptor in normal rat tissues and in AtT-20 and GH3 cells, both chemically with SS-14, SS-28 and Tyr3 SMS ligands, as well as by photoaffinity labeling with an azido derivative of Tyr3 SMS (EE 581). Three prominent somatostatin receptor proteins of 58-kDa, 32-kDa, and 27-kDa size have been identified. These proteins exhibit a tissue-specific distribution, ligand selectivity, and relative preference for SS-14 and SS-28 binding, and thus qualify as somatostatin receptor subtypes. Using EE 581 as a photoaffinity probe, the 58-kDa and 32-kDa proteins have been purified to homogeneity from brain and AtT-20 cells by successive SDS-PAGE. The 58-kDa form has been trypsinized and amino acid sequence data obtained from four tryptic fragments. With the help of synthetic oligonucleotides derived from these sequences, work is currently in progress to clone the 58-kDa protein to elucidate its complete sequence, its expression, and its functional relationship to the somatostatin receptor and its pharmacological subtypes.