Membrane receptors in the gastrointestinal tract

The Role of Somatostatin and Octreotide in Bowel Obstruction: Pre-Clinic Al and Clinical Results

Malignant bowel obstruction is a common complication in patients with advanced abdominal or pelvic cancer. Whereas surgery should be considered in all cases of malignant bowel obstruction, many advanced and terminal cancer patients are considered unfit for surgery. In such patients with a short life expectancy, gastrointestinal symptoms such as nausea, vomiting, continuous and/or colicky pain, can be controlled by using a pharmacologic approach made up of analgesics, antiemetics and antisecretory drugs, without the use of a venting nasogastric tube. Among the antisecretory drugs, octreotide has been shown to reduce nausea and vomiting in bowel-obstructed patients owing to a reduction of gastrointestinal secretions, thus allowing in most patients removal of the nasogastric tube and the associated distress. Preclinical and clinical studies that demonstrated the role of somatostatin and octreotide in bowel obstruction are reviewed.

Pharmacological treatment of bowel obstruction in cancer patients

INTRODUCTION

Malignant bowel obstruction (MBO) is a common complication of advanced cancer, occurring most frequently in gynaecological and colorectal cancer. Its management remains complex and variable. This is in part due to the lack of evidence-based guidelines for the clinicians involved. Although surgery should be considered the primary treatment, this may not be feasible in patients with a poor performance status or advanced disease. Advances have been made in the medical management of MBO which can lead to a considerable improvement in symptom management and overall quality of life.

AREAS COVERED

This review emphasizes the importance of a prompt diagnosis of MBO with early introduction of pharmacological agents to optimize symptom control. The authors summarize the treatment options available for bowel obstruction in those patients for whom surgical intervention is not a feasible option. The authors also explore the complexities involved in the introduction of parenteral hydration and total parenteral nutrition in this group of patients.

EXPERT OPINION

It is not always easy to distinguish reversible from irreversible bowel obstruction. Early and aggressive management with the introduction of pharmacological agents including corticosteroids, octreotide and anti-cholinergic agents have the potential to maintain bowel patency, and allow for more rapid recovery of bowel transit. A combination of analgesics, anti-emetics and anti-cholinergics with or without anti-secretory agents can successfully improve symptom control in patients with irreversible bowel obstruction.

Role of octreotide, scopolamine butylbromide, and hydration in symptom control of patients with inoperable bowel obstruction and nasogastric tubes: a prospective randomized trial

Bowel obstruction may be an inoperable complication in patients with end-stage cancer. Scopolamine butylbromide (SB) and octreotide (OCT) have been successfully used with the aim of reducing gastrointestinal (GI) secretions to avoid placement of a nasogastric tube (NGT); however, there have been no comparative studies concerning the efficacy of these drugs. Furthermore, there is little information about the role played by parenteral hydration in symptom control of these patients. In a prospective trial that involved all 17 inoperable bowel-obstructed patients presenting to our services with a decompressive NGT, patients were randomized to OCT 0.3 mg/day or SB 60 mg/day for 3 days through a continuous subcutaneous infusion. Clinical data, survival time, and the time interval from the first diagnosis of cancer to the onset of inoperable bowel obstruction were noted. The intensity of pain, nausea, dry mouth, thirst, dyspnea, feeling of abdominal distension, and drowsiness were assessed by means of a verbal scale before starting treatment with the drugs under study (T0) and then daily for 3 days (T1, T2, T3). Moreover, daily information was collected regarding the quantity of GI secretions through the NGT, the oral intake of fluids, the quantity of parenteral hydration, and the analgesic therapy used. The NGT could be removed in all 10 home care and in 3 hospitalized patients without changing the dosage of the drugs. OCT significantly reduced the amount of GI secretions at T2 (P = 0.016) and T3 (P = 0.020). Compared to the home care patients, the hospitalized patients received significantly more parenteral hydration (P = 0.0005) and drank more fluids (P = 0.025). There was no difference in the daily thirst and dry mouth intensity in relation to the amount of parenteral hydration or the treatment provided (OCT or SB). Independent of antisecretory treatment, the patients receiving less parenteral hydration presented significantly more nausea (T0 P = 0.002; T1 P = 0.001; T2 P = 0.003; T3 P = 0.001) and drowsiness at T3 (P < 0.5). Pain relief was obtained in all 17 patients and only two patients required an increase in morphine dose at T1. All patients with inoperable malignant bowel obstruction should undergo treatment with antisecretory drugs so as to evaluate the possibility of removing the NGT. When a more rapid reduction in GI secretions is desired, OCT should be considered as the first choice drug. Parenteral hydration over 500 ml/day may reduce nausea and drowsiness.

The alpha isoform of protein kinase C inhibits histamine-stimulated adenylate cyclase activity in a particulate fraction of the human gastric cancer cell line HGT-1

The isoform of protein kinase C responsible for the inhibition of histamine-stimulated adenylate cyclase by the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), has been investigated in a particulate fraction prepared from the human gastric cancer cell line HGT-1. The alpha and epsilon isoforms of protein kinase C were detected in HGT-1 cells and in a 40,000 x g particulate fraction by immunoblotting procedures. The inhibitory effect of TPA on histamine-stimulated adenylate cyclase was enhanced by the presence of Ca2+, but decreased in a concentration-dependent manner by anti-peptide antibody to protein kinase C alpha, but not to protein kinase C epsilon. Addition of Ca2+ and TPA to the 40,000 x g particulate fraction stimulated the phosphorylation of the protein kinase C substrate myelin basic peptide 4-14. Protein kinase C alpha is probably the isoform responsible for inhibition of histamine-stimulated adenylate cyclase in HGT-1 cells.

Protein kinase C inhibits cyclic adenosine monophosphate generation by histamine and truncated glucagon like peptide 1 in the human gastric cancer cell line HGT-1

The HGT-1 gastric cancer cell line was used to determine the actions of protein kinase C on the stimulation of adenylate cyclase by the human histamine H2 receptor, and the receptors for gastric inhibitory polypeptide and truncated glucagon like peptide 1 (TGLP-1). Suspensions of HGT-1 cells were preincubated with the activator of protein kinase C, 12-O-tetradecanoylphorbol 13-acetate (TPA, 100 nmol/l), for 10 minutes. The subsequent cyclic adenosine monophosphate (AMP) response to 0.5 mmol/l histamine or 100 nmol/l TGLP-1 was reduced by comparison with control cells preincubated in the absence of TPA. The cyclic AMP response to 100 nmol/l gastric inhibitory polypeptide was enhanced by preincubation with TPA, while the responses to cholera toxin and forskolin were unaffected. Preincubation with pertussis toxin prevented the enhancement of the gastric inhibitory polypeptide response by TPA, suggesting an involvement of an inhibitory guanine nucleotide regulatory subunit of the Gi class, but did not change the inhibition of histamine stimulation. In conclusion, activation of protein kinase C produces a specific inhibition of the effects of histamine and TGLP-1 on adenylate cyclase activity in a human gastric cancer cell line by acting at a site close to their receptors.

Processing and characterization of the low density lipoprotein receptor in the human colonic carcinoma cell subclone HT29-18: a potential pathway for delivering therapeutic drugs and genes

Low density lipoprotein (LDL) processing has been investigated in the subcloned human colonic carcinoma cell line HT29-18. LDL binding at 4 degrees C was a saturable process in relation to time and LDL concentration. The Kd for LDL binding was 11 micrograms/ml. ApoE-free HDL3 or acetylated LDL did not significantly compete with 125I-LDL binding, up to 500 micrograms/ml. 125I-LDL binding was decreased by 70% in HT29-18 cells preincubated for 24 hours in culture medium containing 100 micrograms/ml unlabelled LDL. Ligand blotting studies performed on HT29-18 homogenates using colloidal gold labelled LDL indicated the presence of one autoradiographic band corresponding to an apparent molecular weight of 130 kDa, which is consistent with the previously reported molecular weight of the LDL receptor in human fibroblasts. At 37 degrees C, 125I-LDL was actively internalized by HT29-18 cells and lysosomal degradation occurred as demonstrated by the inhibitory effect of chloroquine. LDL uptake and degradation by HT29-18 cells also resulted in a marked decrease in endogenous sterol synthesis. These data demonstrate that the HT29-18 human cancerous intestinal cells are able to specifically bind and internalize LDL, and that LDL processing results in down-regulation of sterol biosynthesis. Thus, intestinal epithelial cells possess specific LDL receptors that can be exploited to accomplish drug delivery and gene transfer via the receptor-mediated endocytosis pathway.

Functional expression of VIP receptors in normal, immortalized and transformed mammary epithelial cells

The effect of VIP and its related peptides on cAMP production has been characterized: 1) in long term culture of normal human mammary epithelial cells (HMEC); 2) in immortalized and transformed ST cell lines established from normal HMEC after genomic insertion of the large T oncogene of SV40; 3) in the spontaneously immortalized HC-11 cells, a clone isolated from the mouse mammary epithelial cells COMMA-1D, described to exhibit normal morphogenesis in vivo and functional differentiation in vitro. Basal cAMP levels were increased 1.5- to 8.7-fold in mammary epithelial cells (p less than 0.001-0.05), with a potency EC50 = 0.02-0.6 nM VIP. The pharmacological specificity of the VIP receptors coupled to cAMP generation was established according to the following potency sequence: VIP greater than PACAP-38 greater than helodermin greater than PHM, PHV greater than helospectin 1 much greater than hpGRF, secretin in HMEC, VIP greater than PACAP-38 greater than helodermin greater than helospectin 1, PHM, PHV greater than hpGRF greater than secretin in S1T3 cells, and VIP, PHI, helodermin greater than PHV greater than rhGRF greater than secretin in HC-11 cells. Our data demonstrate the presence of functional, highly sensitive and specific VIP receptors in normal, immortalized and transformed mammary epithelial cells, suggesting a regulatory role for this neuropeptide on the growth, differentiation and function in normal and neoplastic breast tissue.

Continuous subcutaneous infusion of hyoscine butylbromide reduces secretions in patients with gastrointestinal obstruction

We describe the use of hyoscine butylbromide as a subcutaneous infusion in 3 patients with inoperable malignant bowel obstruction. An objective reduction of drainage from the gastrointestinal tract was observed with the hyoscine butylbromide infusion (60-120 mg/day). We suggest that this effect can be useful in the palliative treatment of vomiting in inoperable bowel obstruction.

Functional and structural characterization of the secretin receptors in rat gastric glands: desensitization and glycoprotein nature

We have documented and characterized the down-regulation of the 125I-secretin binding sites and the associated desensitization of the secretin receptor-cAMP system in rat gastric glands. Secretin induced a rapid decrease of the high-affinity 125I-secretin binding sites with t1/2 = 30 min at 37 degrees C. Half-maximal down-regulation and desensitization occurred at 10(-9) M secretin, a physiological concentration corresponding to the half-maximal activation of the secretin receptor. The Scatchard parameters of the low-affinity 125I-secretin binding sites were unaffected by the pretreatment. This desensitization is heterologous in view of the loss of responsiveness to the truncated glucagon-like peptide 1 (TGLP-1), and pharmacologically selective since the secretin-related analogue VIP (10(-7) M) does not alter the secretin-induced cAMP generation in rat gastric glands. The glycoprotein nature of the secretin receptor has also been demonstrated using WGA-agarose affinity chromatography of the solubilized 125I-secretin receptor complex.

Transfection of fetal rat intestinal epithelial cells by viral oncogenes: establishment and characterization of the E1A-immortalized SLC-11 cell line

Intestinal epithelial cells from 19-day-old rat fetuses underwent electropermeabilization and were successfully transfected by three recombinant plasmids containing the cloned oncogenes from the human adenovirus type 2 early region E1A (SLC-11 cells) and polyoma virus and simian virus 40 large T tumor antigens (SLC-21 and SLC-41 cells). SLC-11 cells were propagated for 21 months in culture (current passage, 76; doubling time, 17 hr) and were immortalized by E1A, as shown by RNA transfer blot (Northern blot) analysis and indirect immunofluorescence of the nuclear oncoproteins. These cells were not tumorigenic in either athymic nude mice or syngeneic Wistar rats and showed a nearly normal karyotype with minimal chromosomal changes. The immortalized epithelial cell line SLC-11 retained several of the phenotypes observed in the parent cells of the intestinal mucosa, including cytoplasmic villin, cytokeratins, enkephalinase, and cell surface receptors sensitive to vasoactive intestinal peptide. It is concluded that immortal SLC-11 cells are a suitable model for studying the proliferation and differentiation of epithelial intestinal cells and analyzing cancer progression in the gastrointestinal tract.

Differential regulation of membrane receptors sensitive to histamine (H2-type), isoproterenol (beta 2-type) and glucagon-like peptides by the somatostatin analogue Sandostatin in rat gastric glands

The potential use of somatostatin-14 and its long lasting analogue Sandostatin as antiulcer agents led us to study the functional properties of these peptides on the histamine H2-receptor H2R adenylate cyclase system in gastric glands isolated from the rat fundus. The action of the two peptides has also been compared on membrane receptors sensitive to isoproterenol and the truncated glucagon-like peptide TGLP-1. The data indicate that somatostatins inhibit selectively H2R and TGLP-1 receptor activity with similar potencies and kinetics, suggesting that the two peptides share the same receptor pool coupled with the Gi subunits of adenylate cyclase. Somatostatin-14 and Sandostatin have no evident action on the beta 2-type adrenergic receptor beta 2R. Therefore, the higher potency of Sandostatin compared to somatostatin-14 in inhibiting acid secretion is probably related to an increased stability of the analogue in vivo.

Pharmacological control of the human gastric histamine H2 receptor by famotidine: comparison with H1, H2 and H3 receptor agonists and antagonists

Histamine 0.1 microM-0.1 mM increased adenylate cyclase activity five- to ten-fold in human fundic membranes, with a potency Ka = 3 microM. The histamine dose-response curve was mimicked by the H3 receptor agonist (R) alpha-MeHA, but at 100 times lower potency, Ka = 0.3 mM. Histamine-induced adenylate cyclase activation was abolished by H2, H1 and H3 receptor antagonists, according to the following order of potency IC50: famotidine (0.3 microM) greater than triprolidine (0.1 mM) thioperamide (2 mM), respectively. Famotidine has no action on membrane components activating the adenylate cyclase system, including the Gs subunit of the enzyme stimulated by forskolin and cell surface receptors sensitive to isoproterenol (beta 2-type), PGE2 and VIP. The Schild plot was linear for famotidine (P less than 0.01) with a regression coefficient r = 0.678. The slope of the regression line was 0.64 and differs from unity. Accordingly, famotidine showed a slow onset of inhibition and dissociation from the H2 receptor in human cancerous HGT-1 cells. The results demonstrate that famotidine is a potent and selective H2 receptor antagonist with uncompetitive actions in human gastric mucosa. Consequently, famotidine might be a suitable drug with long-lasting actions in the treatment of Zollinger-Ellison syndrome. The results also confirm and extend the previous observations that (R) alpha-MeHA and thioperamide are two selective ligands at histamine H3 receptor sites. In the human gastric mucosa, these drugs are respectively 330 and 6700 times less potent than histamine and famotidine on the adenylate cyclase system. The possible involvement of histamine H3 receptors in the regulation of gastric secretion is proposed.

Expression of membrane receptors and (proto)oncogenes during the ontogenic development and neoplastic transformation of the intestinal mucosa

The functional relationship between membrane receptors involved in signal transduction and (proto) oncogene expression has been explored during the ontogenic development and differentiation of the intestinal mucosa in man and rat. The present review develops detailed picture of the current understanding of some mechanisms underlying growth and function of normal, immortalized and cancerous intestinal epithelial cells.

Expression of histamine and vasoactive intestinal peptide (VIP) receptors in immortalized rat fetal intestinal cells

Four intestinal cell lines derived from rat fetuses at 19 days of gestation were successfully propagated after electroporation in the presence of different recombinant DNAs containing the viral oncogenes E1A from Adenovirus 5 and large T from SV40 or Polyoma. These immortalized intestinal cells, designated SLC, possess several properties observed in the parent cells of this tissue, including the expression of cytoplasmic villin, enkephalinase and retention of VIP receptors. In contrast, histamine elevated cAMP levels in the SLC cell lines only. The data suggest that the transfection of fetal rat intestinal cells by E1A and large T is associated with the induction of functional histamine receptors coupled with the Gs/Gi regulatory proteins of adenylate cyclase.