Charles D. Ulrich

Secretin and vasoactive intestinal peptide receptors: Members of a unique family of G protein-coupled receptors

The superfamily of guanine nucleotide–binding protein (G protein)-coupled receptors represents the largest group of receptor molecules yet identified. They bind ligands as structurally diverse as photons, odorants, biogenic amines, peptides, and large glycoproteins. This entire spectrum of natural agonists is accommodated by members of the largest, most extensively studied, and best understood family of such receptors, the rhodopsin/ b-adrenergic receptor family. Recently, another distinct family within this superfamily was identified.1 Known as the secretin receptor family, this family shares few of the specific sequence motifs of the rhodopsin/b-adrenergic receptor family and has less than 12% sequence homology with it, but still possesses the same predicted molecular architecture with seven transmembrane helices and appears to signal via a similar sequence of molecular events (Figure 1).1 Among the members of this family are receptors for two extremely important regulators of pancreatic, biliary, and gastrointestinal physiology: secretin and vasoactive intestinal polypeptide (VIP). Because of their clear physiological relevance to gastroenterology, they are the focus of this review. Also included in the secretin receptor family are receptors for pituitary adenylate cyclase–activating polypeptide (PACAP),2 gastric inhibitory polypeptide (GIP),3 glucagon,4 glucagon-like peptide 1,5 calcitonin,6 calcitonin gene-related peptide,7 parathyroid hormone,8 growth hormone–releasing factor (GHRF),9 corticotropin-releasing factor,10 insect diuretic hormone,11 and a few orphan receptors or more distantly related molecules. These receptors share substantial homology with each other, and all bind moderately large peptides possessing diffuse pharmacophoric domains. Some of these receptors are probably important in digestive physiology, but our current understanding of them does not suggest the presence of unique themes distinct from those that are developed in the following discussion. Biochemical and Molecular Characterization of Gastrointestinal Hormone Receptors

Abnormal processing of the human cholecystokinin receptor gene in association with gallstones and obesity

BACKGROUND & AIMSnCholesterol gallstone disease and obesity are often associated and share the potential, yet unreported, common etiology of cholecystokinin (CCK) dysfunction. While cloning the human CCK-A receptor complementary DNA (cDNA), we found predominance of a 262-base pair coding region deletion in a cDNA library prepared from a patient with this phenotype. The aim of this study was to determine the abundance, functional significance, and mechanism for generating this gene product.nnnMETHODSnRelative abundance of CCK receptor gene products was determined using polymerase chain reaction and hybridization analysis. Constructs were expressed in COS cells and studied for radioligand binding and intracellular calcium responses. A human genomic clone for this receptor was sequenced, and the critical regions were compared with those of the patient.nnnRESULTSnNinety-three percent of the patients CCK receptor transcripts contained the 262-base pair deletion, whereas only 1.5% +/- 0.9% of control patients had the deletion. This encoded a receptor that did not bind or signal. The deletion corresponded with the third exon; however, this sequence and flanking introns were normal in the patient.nnnCONCLUSIONSnAbnormality of processing an apparently normal CCK receptor gene yields the predominant product with an absent third exon and encoding a nonfunctional receptor, probably reflecting a defective trans-acting splicing factor. An atypical lariat region in the third intron may explain the presence of small amounts of this product in control patients.

Endoscopic pancreatic duct stenting to treat pancreatic ascites

BACKGROUNDnManagement of pancreatic ascites with conservative medical therapy or surgery has met with limited success. Decompression of the pancreatic ductal system through transpapillary stent placement, an alternative strategy, has been reported in only a handful of cases of pancreatic ascites.nnnMETHODSnWe reviewed all cases from 1994 to 1997 in which patients with pancreatic ascites underwent an endoscopic retrograde pancreatogram documenting pancreatic duct disruption with subsequent placement of a transpapillary pancreatic duct stent. Clinical end points were resolution of ascites and need for surgery.nnnRESULTSnThere were 8 cases of pancreatic ascites in which a 5F or 7F transpapillary pancreatic duct stent was placed as the initial drainage procedure. Pancreatic ascites resolved in 7 of 8 patients (88%) within 6 weeks. Ascites resolved in the eighth patient, a poor candidate for surgery, following placement of a 5 mm expandable metallic pancreatic stent. No infections, alterations in ductal morphology, or other complications related to stent placement were noted. There was no recurrence of pancreatic ascites or duct disruption at a mean follow-up of 14 months.nnnCONCLUSIONSnOur experience doubles the number of reported cases in which transpapillary pancreatic stent placement safely obviated the need for surgical intervention in the setting of pancreatic ascites. This therapeutic endoscopic intervention should be seriously considered in the initial management of patients with pancreatic ascites.

Intrinsic photoaffinity labeling of native and recombinant rat pancreatic secretin receptors

BACKGROUNDnStructural characterization of pancreatic secretin receptors has been limited by difficulties in generating suitable radioligands and obtaining sufficient substrate. The aims of this study were to design, synthesize, and characterize high affinity radiolabeled analogues of secretin suitable for intrinsic photoaffinity labeling and to clone, express, and characterize the recombinant rat pancreatic secretin receptor.nnnMETHODSnThe ability of synthetic analogues to stimulate amylase secretion by pancreatic acini was studied. Receptor complementary DNA (cDNA) was cloned by screening a rat pancreatic library with a probe based on the sequence of a neural cell secretin-binding protein. Competition binding and affinity labeling were performed with membranes prepared from rat pancreas and transfected cells.nnnRESULTSnTwo probes were fully efficacious secretagogues, which bound in a specific, high-affinity, rapid, and temperature-dependent manner. Only ([125I]Tyr10, pNO2-Phe22) rat secretin 27 covalently labeled a 50,000-62,000-molecular weight pancreatic membrane protein, with labeling inhibited in a concentration-dependent manner by secretin but not vasoactive intestinal polypeptide. Hybridization screening yielded a full-length cDNA identical to the neural clone. Photoaffinity labeling of this recombinant receptor identified a 57,000-62,000-molecular weight protein with specificity similar to that of native pancreas. Both native and recombinant receptors migrated at a molecular weight of 42,000 after endoglycosidase F deglycosylation.nnnCONCLUSIONSnThis study provides evidence for the molecular identity of the pancreatic secretin receptor and presents a novel probe important in structural characterization of its agonist-binding domain.

PANCREATIC CANCER SURVEILLANCE IN A HIGH-RISK COHORT: Is It Worth the Cost?

Pancreatic adenocarcinoma is the 10th most common malignancy and 4th largest cancer killer in adults. Earlier tumor detection through screening of high risk groups, presumably to increase the percentage of cases resectable for cure in these cohorts, has emerged as a prominent strategy to combat this disease. This article examines the feasibility of this strategy in patients with hereditary pancreatic cancer (HPC) and hereditary pancreatitis (HP). Because of a variety of factors, specific cost projections for screening with HPC kindreds are problematic at best. Patients with HP exhibit a 53-fold increased risk of pancreatic cancer, with a cumulative risk of 40% by age 70. The authors discuss the modalities available to screen this cohort and subsequently perform a theoretical cost analysis. The authors findings suggest that screening has the potential to be cost-effective only in hereditary pancreatitis patients = 50 years-of-age. The most cost-effective option will likely combine an initial serologic test with high sensitivity and a subsequent serologic or pancreatic juice test with sufficient specificity to act as a gatekeeper to imaging with endoscopic ultrasound (EUS). Banking of blood and pancreatic juice samples should be mandatory in any screening protocol. The lower tumor yield in other high-risk groups (e.g., non-hereditary chronic pancreatitis) will effectively preclude the use of such screening protocols. The vast majority of patients will continue to present with unresectable disease.

How cystic fibrosis affects pancreatic ductal bicarbonate secretion

Pancreatic bicarbonate secretion is impaired in patients with cystic fibrosis. This article reviews recent advances in bicarbonate dependent transporters in pancreatic duct cells and discusses their regulation in cystic fibrosis.

GROWTH FACTORS, RECEPTORS, AND MOLECULAR ALTERATIONS IN PANCREATIC CANCER: Putting It All Together

Because of the dismal prognosis of advanced ductal pancreatic adenocarcinoma, recent investigational strategies have focused on improved detection and therapeutic intervention in early-stage pancreatic cancer. The obvious cost constraints of screening populations at risk but with a low tumor yield will restrict screening protocols to only the highest risk groups (hereditary pancreatitis = age 50, certain hereditary pancreatic cancer kindreds). The vast majority of patients, either lacking or exhibiting an inherited predisposition to pancreatic cancer, will continue to present with disease not resectable for cure. The authors believe that the best hope for these patients lies in the further delineation of the integrative pathophysiology driving tumor growth; this would facilitate the future development of a computer program or other modality that would predict the dominant pathways driving the growth and spread of each tumor based on its molecular profile. This article reviews the authors current knowledge regarding the growth factors, receptors, and molecular alterations driving uncontrolled proliferation, local invasion, and metastatic spread of these tumors. The current and potential contributions of studies in cohorts with an inherited predisposition to pancreatic cancer to this pathophysiologic model are also discussed. The future strategy for incorporating this information into a working pathophysiologic road map with clinical relevance is subsequently outlined.