Biosynthesis of pancreatic polypeptide. Identification of a precursor and a co-synthesized product

Illuminating the neuropeptide Y4 receptor and its ligand pancreatic polypeptide from a structural, functional, and therapeutic perspective

The neuropeptide Y4 receptor (Y4R), a rhodopsin-like G protein-coupled receptor (GPCR) and the hormone pancreatic polypeptide (PP) are members of the neuropeptide Y family consisting of four receptors (Y1R, Y2R, Y4R, Y5R) and three highly homologous peptide ligands (neuropeptide Y, peptide YY, PP). In this family, the Y4R is of particular interest as it is the only subtype with high affinity to PP over NPY. The Y4R, as a mediator of PP signaling, has a pivotal role in appetite regulation and energy homeostasis, offering potential avenues for the treatment of metabolic disorders such as obesity. PP as anorexigenic peptide is released postprandial from the pancreas in response to food intake, induces satiety signals and contributes to hamper excessive food intake. Moreover, this system was also described to be associated with different types of cancer: overexpression of Y4R have been found in human adenocarcinoma cells, while elevated levels of PP are related to the development of pancreatic endocrine tumors. The pharmacological relevance of the Y4R advanced the search for potent and selective ligands for this receptor subtype, which will be significantly progressed through the elucidation of the active state PP-Y4R cryo-EM structure. This review summarizes the development of novel PP-derived ligands, like Obinepitide as dual Y2R/Y4R agonist in clinical trials or UR-AK86c as small hexapeptide agonist with picomolar affinity, as well as the first allosteric modulators that selectively target the Y4R, e.g. VU0506013 as potent Y4R positive allosteric modulator or (S)-VU0637120 as allosteric antagonist. Here, we provide valuable insights into the complex physiological functions of the Y4R and PP and the pharmacological relevance of the system in appetite regulation to open up new avenues for the development of tool compounds for targeted therapies with potential applications in metabolic disorders.

Patterns of immunoreactive pancreatic polypeptide in human plasma

Pancreatic polypeptide (PP) is synthesized as an amino-terminal moiety of a precursor peptide and is released into plasma during stimulation as an amidated hormone (PP1-36). The purpose of this investigation was to ascertain the immunoreactive forms of PP in human plasma using HPLC chromatographic technique. Plasma was obtained from five normal volunteers under various postprandial intervals and from the Blood Bank. PP in each plasma sample was processed for HPLC analysis by immunoprecipitation and/or immunoaffinity extractions. Migration patterns of PP-forms were identified under isocratic elution. This study shows that human plasma contains four distinct immunoreactive (IR) forms of PP during stimulation by a protein-rich meal. These forms are PP1-36 (peak 4), PP3-36 (peak 3) and unidentified material migrating as peak 2 and peak 1. The corresponding migration constants were Kav 0.828 +/- 0.04, Kav 0.790 +/- 0.003, Kav 0.570 +/- 0.009 and Kav 0.409 +/- 0.007, respectively. The predominant fasting from of IR PP chromatographed as peak 1, while peaks 2 and 4 were reduced in amplitude. The 1 h and 3 h postprandial chromatograms of HPLC profiles of plasma PP were similar in shape but lower in relative magnitude and amplitude. The authenticity of peak 4 as the migration of native PP1-36 was confirmed using purified IR native PP1-36 extracted from human pancreas. Partial amino acid sequence analysis of PP peak 3 revealed deletions of two N-terminal amino acid residues. The chemical identities of peaks 1 and 2 are unknown but appear to differ from PP in peaks 3 and 4 by virtue of their migration profiles. It is concluded that there are at least four distinct IR forms of PP in human plasma. Native PP1-36 accounts for less than 1% of total PP after an overnight fast and is about 1/3 of total postprandial IR plasma PP. Discernment of the nature and etiology of forms of PP in plasma may provide a new understanding of the role of PP in mammalian physiology.

Different splice site utilization generates diversity between the rat and human pancreatic polypeptide precursors

Pancreatic polypeptide is derived from a polyprotein precursor molecule. Although the amino acid sequences specifying the signal peptide and pancreatic polypeptide are well conserved between the rat and the human, the carboxy-terminal amino acid sequences of the precursors are highly divergent. To better understand the molecular basis of the divergence between the rat and human C-terminal peptides, we have determined the nucleotide sequence of the rat pancreatic polypeptide gene. A comparison between the primary structures of the rat and human genes reveals that the heterogeneity of the C-terminal peptides can be explained in large part by a frameshift mutation and the utilization of an alternative splice donor site in the third exon of the rat gene. As a consequence of the displaced splice site, part of the third exon of the rate gene is homologous to the sequence in the third intron of the human gene. Our results suggest that the rat and human pancreatic polypeptide genes arose from a common ancestral gene, and that differences in the C-terminal domains of the precursor reflect less strict evolutionary constraints than those imposed upon the amino-terminal domains of the precursor.

Expression of human pancreatic polypeptide precursors from a dicistronic mRNA in mammalian cells

The ability of eukaryotic ribosomes to reinitiate translation at downstream AUG codons on polycistronic mRNAs was used to select transfected CHO clones that secreted a precursor to the human pancreatic polypeptide (PP). In the in vitro constructed transcription unit, a viral promoter directed the synthesis of a dicistronic mRNA. The PP cDNA was placed in the 5'-part of this transcript, while a DHFR cDNA was placed 3' to the PP. This dicistronic expression unit was transfected into CHO cells, and methotrexate-resistant colonies were selected. RNA-blots verified that the PP precursor and DHFR were expressed from the same dicistronic mRNA. The CHO cells synthesized the hormone precursor and secreted it through the constitutive secretory pathway.

Biosynthesis of pancreatic islet hormones

We have outlined the various strategies used to characterize the precursors of three pancreatic islet hormones--somatostatin, pancreatic polypeptide and VIP. In each case, isolation of the cDNA clones was facilitated by the use of gastrointestinal tissues that were extremely rich in specific mRNA. Characterization of the structures of the precursors is clearly only the first step in understanding the regulation of pancreatic hormone biosynthesis. It is likely that the availability of the cDNA clones will allow us to define the actual mechanisms underlying hormone production within the pancreas.

Antibodies to pancreatic hormones in diabetics treated before 1964 and after 1974

The prevalence and titer of antibodies to pancreatic hormones in 21 diabetics treated with USP insulin preparations before 1964 (Group 1) and in another 21 diabetics treated with more highly purified insulin preparations after 1974 (Group 2) were determined. All subjects in Group 1 had antibodies to insulin, 11 had antibodies to pancreatic polypeptide (PP) and 3 to glucagon. In Group 2, 17 had antibodies to insulin, 5 to PP, and 1 to glucagon. Antibodies to somatostatin (SRIF) were not detectable in either group. Group 1 subjects had higher titers of antibodies to insulin, glucagon, and PP than Group 2 subjects. Of note, the Group 1 subjects with the highest insulin antibodies also had the greatest prevalence and highest titers of antibodies to glucagon and PP. In order to address the question of possible differences in immune responsivity between Group 1 and 2 subjects, antibody titers to 4 ubiquitous viral agents were determined. There was no significant difference in viral antibody titers between the groups.

The processing of peptide precursors. 'Proline-directed arginyl cleavage' and other monobasic processing mechanisms

The classical conversion site in precursors of regulatory peptides is a sequence of two basic amino acids. During recent years, however, a group of monobasic cleavage sites has emerged. In certain cell systems it has been shown that the monobasic cleavage mechanism is both a specific mechanism which only attacks a particular basic residue, and a distinct mechanism which can be separated from the dibasic cleaving mechanism within the same cell. The vast majority of monobasic cleavages occur at single arginines although cleavage after a lysine residue has also been demonstrated. There is no 'consensus sequence' of amino acids surrounding the single basic residue which is the apparent signal for proteolytic processing. However, in approximately one third of the cases, a proline residue is found either just before or just after the basic residue. On the basis of this 'proline-directed arginyl cleavage' it is discussed how the conformation of the peptide backbone might be important for this type of cleavage. Finally, it is suggested that tissue-specific expression of different processing enzymes, e.g. dibasic and monobasic specific forms, might explain the tissue-specific processing of precursors like the pro-opiomelanocortin and the CKK and somatostatin precursor.

Characterization of immunoreactive forms of pancreatic polypeptide in islet cell tumors using antisera with different regional specificities

Tumor tissue from a heterogeneous group of patients with clinically and histologically diagnosed islet cell tumors has been assessed for pancreatic polypeptide (PP) content using radioimmunoassay. Immunoreactive forms of PP, obtained by gel filtration of tissue extracts, were detected using antisera with different regional specificities. Normal pancreatic tissue contained one form of PP coeluting with human PP whereas, tumor tissue from patients with insulinoma, Zollinger-Ellison, and WDHA (watery diarrhea hypokalemic achlorhydric) syndromes contained, in addition, higher molecular weight forms of immunoreactive PP. Furthermore, a lower molecular weight form was detected in tumor tissue from a Zollinger-Ellison syndrome patient. The high molecular weight forms of immunoreactive PP were not recognized using a C-terminal-specific antiserum, whereas the smaller form was only detected using this antiserum. The variation in cross-reactivity observed with two antisera in this study emphasizes the limitations of using a single antiserum to detect molecular forms of peptide hormones, particularly those in islet cell tumors.

Immunoreactive pancreatic polypeptide components in plasma from normal subjects and patients with chronic renal failure in basal and postprandial conditions

Gel filtration of plasma from normal subjects and patients with chronic renal failure (CRF) yielded four immunoreactive human pancreatic polypeptide (IRhPP) peaks of approximately greater than 20,000, 10,000, 4200, and 2000 daltons. In normal subjects, the basal plasma distribution was 36 +/- 2%, 16 +/- 3%, 25 +/- 1% and 23 +/- 2%, respectively. In the CRF patients, the total plasma IRhPP was close to ten times higher than in the normal group. This difference was due to an increase in IRhPP10000 and IRhPP4200 plasma levels. After a standard breakfast, plasma IRhPP10000 and IRhPP4200 levels increased in the normal subjects and in the CRF patients, while the levels of the other two plasma components were not affected by the meal. In a patient with multiple endocrine adenomatosis type 1, the basal plasma levels of IRhPP10000 and IRhPP4200 were, respectively, 41 and 44 times higher than the mean value in the normal subjects; these fractions appeared to be further increased after breakfast. In trying to characterize the IRhPP10000, it was found that treatment with urea, guanadium hydrochloride, and acetic acid (pH 2.2) did not alter its molecular size, whereas limited trypsin treatment was able to destroy part of the immunoreactivity and to produce greater than 20,000-dalton and 4200-dalton immunoreactive materials. Plasma IRhPP10000 seems to be a co-secretory product of the PP cell, and the kidney plays a role in its catabolism as well as in that of the IRhPP4200 component. The IRhPP10000 component may correspond to a PP precursor.

Absence of hepatic extraction of pancreatic polypeptide in conscious dogs

In 15 conscious dogs basal portal vein pancreatic polypeptide (PP) (306 +/- 8 pg/ml) exceeded both hepatic vein PP (255 +/- 8 pg/ml) and arterial PP (244 +/- 9 pg/ml) and increased rapidly 10 min after oral glucose administration. In contrast to oral glucose, intraportal glucose infusion decreased PP levels from 45 min until the end of the infusion. Meat ingestion rapidly and promptly increased PP. During the basal state, hepatic extraction of total immunoreactive PP was 10 +/- 4%, not significantly different from zero, of the 150 +/- 14 ng/min presented to the liver. Fractional hepatic extraction of PP did not change after oral glucose, meat ingestion, or intraportal glucose infusion. Chromatographic analysis showed at least four different components of immunoreactive PP. The first peak eluted with the void volume, the second peak between the void volume and authentic PP, and the third peak coincided with authentic PP. When present, the fourth peak eluted after authentic PP. More than half of the immunoreactive PP in the basal state eluted with authentic PP, whereas about one-third was found in the second peak. The marked alterations in PP after meat, oral glucose, or intraportal glucose and atropine reflected primarily changes in the third peak. The fractional hepatic extraction of all the components was very similar.

Pancreatic polypeptide: a unique model for vagal control of endocrine systems

Pancreatic polypeptide (PP) is a hormone synthesized only in the duodenal pancreas where the PP cell is the dominating endocrine cell type. The secretion of PP is regulated by food-intake and by plasma glucose--in both cases through vagal cholinergic mechanisms. In vitro cholinergic stimulation is 4-10 times as potent as any other stimulatory mechanism, e.g. beta-adrenergic stimulation. Although other agents such as gastrointestinal hormones and neuropeptides are potent stimulators of PP secretion in vivo, their action is totally eliminated by blockade of the muscarinic receptor and in several cases also abolished by vagotomy. Furthermore, these peptides have no or only a weak effect in vitro. The stimulation of PP secretion by hypoglycemia and inhibition by hyperglycemia is mediated also through an efferent vagal mechanism. Under extreme hypoglycemia the stimulation of PP release becomes partially atropine resistant, although still totally dependent on the vagus; conceivably the release of other transmitters, like e.g. VIP, is activated under these circumstances. In the basal state, PP secretion is under oscillating cholinergic tone. Thus, the secretion of PP is unique in the way that the cholinergic, vagal stimulation is not only the most powerful stimulatory mechanism, but also the key through which other mechanisms act. PP secretion can be used, e.g. as a sensitive indicator of autonomic disorders in patients with diabetes.

Pancreatic polypeptide-like material in nerves and endocrine cells of the rat

A region-specific antiserum (AbS11) raised against the carboxyl-terminal hexapeptide of pancreatic polypeptide has been employed to measure rat pancreatic polypeptide specifically and to demonstrate apparent immunoreactivity in nerves and in endocrine cells outside the pancreas. The concentration of pancreatic polypeptide in the head of the rat pancreas measured with AbS11 (176 +/- 47 pmol/g) was 750 fold higher than that measured with a conventional anti-bPP antiserum (0.23 +/- 0.08 pmol/g). Column chromatographs of rat pancreatic extracts demonstrated two peaks of immunoreactivity both eluting after the porcine pancreatic polypeptide standard. AbS11 also detected specific immunoreactivity in rat brain (470 fmol/g) which went undetected in convention assays. Although immunohistochemical studies with AbS11 and human pancreatic polypeptide antiserum demonstrated immunoreactivity in the same population of pancreatic endocrine cells, immunoreactive nerve fibres and enteroglucagon cells were only demonstrable with AbS11. These studies demonstrate that the carboxyl terminus of rat pancreatic polypeptide is immunochemically similar to that of higher mammals. Furthermore, neural and extrapancreatic endocrine variants of this peptide share an immunochemical determinant contained within the carboxyl-terminal hexapeptide.

Localization of vasoactive intestinal polypeptide- and avian pancreatic polypeptide-like immunoreactivity in the Golgi apparatus of peripheral neurons

Using the indirect immunofluorescence technique of Coons and collaborators, vasoactive intestinal polypeptide (VIP)-like immunoreactivity (VIP-LI) and avian pancreatic polypeptide (APP)-like immunoreactivity (APP-LI) was observed in certain neurons of the peripheral nervous system of the cat. In the cell bodies in the cat sympathetic ganglia a strong to very strong VIP-LI or APP-LI was observed with a distribution resembling that of the Golgi apparatus. In addition, a weaker immunoreactivity was seen diffusely in the cytoplasm. After photography, the sections processed for immunohistochemistry were stained with the thiamine pyrophosphatase technique of Novikoff and Goldfischer. The latter technique is assumed to be a specific marker for the Golgi complex. In all cases it was found that the strong peptide immunoreactivity and the thiamine pyrophosphatase activity had an identical distribution. Thus, one pool of these peptides appears to be localized to the Golgi apparatus. Yet another pool is localized to other components, such as vesicles.

A new in vitro model for studies of pancreatic polypeptide secretion and biochemistry

A new model tissue (pseudoislet) is described for studies of pancreatic polypeptide (PP) secretion and biochemistry. It consists of islet-like aggregates of canine pancreatic endocrine cells which are formed and maintained on tissue culture. Immunocytochemical staining revealed that pseudoislets prepared from the duodenal end of the pancreas contained a predominance (40-60%) of F cells (the PP secreting cell). Also present were 10-25% exocrine cells and an equal proportion of A, B and D cells. Several studies were conducted to characterize the pseudoislets' capacity to secrete PP. Basal rates of PP release and the concentration of PP per pseudoislet remained constant during four weeks of culture. Stimulation at weekly intervals by carbachol (0.1 mM) resulted in a stable secretory rate for 2 weeks, that declined progressively at weeks 3 and 4. When studied in a perfusion system, carbachol-stimulated PP release occurred in a biphasic pattern, similar to the well-recognized biphasic release of insulin from perifused rat islets. Dose-response curves of four cholinergic agonists revealed clear differences in secretagogue activity. Acetylcholine and methacholine were found to be equipotent, followed in order of potency by carbachol and bethanechol. These histologic and secretory data show that canine pseudoislets are healthy tissues composed of a high proportion of F cells which secrete PP in response to cholinergic stimulation. The data suggest that the cultured canine pseudoislet model provides an excellent system useful in studies of PP secretion and biosynthesis.

[The endocrine cells of the digestive system (author's transl)]

Endocrine cells occur in the digestive system as micro-organs (islets of Langerhans) or scattered throughout the epithelium of the gastrointestinal tract ("diffuse endocrine epithelial organ" of Feyrter). These gastro-entero-pancreatic (GEP) endocrine cells synthesize--in addition to serotonin--a great variety of polypeptide hormones, which regulate both carbohydrate metabolism and digestive processes. The present review deals mainly with cytology and cytochemistry of GEP endocrine cells. A synopsis is presented of the 19 endocrine cell types identified to date, which includes their update nomenclature and their anatomical distribution pattern. Morphological-functional aspects of cell biology, pathology, and cytogenesis of theses cells and their position within superimposed systems (APUD cells, paraneurons) are discussed.

Altered amidation of pancreatic polypeptide in cultured dog islet tissue

Three forms of immunoreactive pancreatic polypeptide (PPI) were detected in extracts of cultured dog pancreatic PP cells: PPI of (1) larger apparent molecular weight than PP, (2) similar apparent molecular weight but different isoelectric point than PP, and (3) identical apparent molecular weight and isoelectric point with PP. Dog pancreatic endocrine cells in culture were labeled biosynthetically with tritiated amino acids, and extracted proteins were fractionated by sodium dodecyl sulfate gel electrophoresis. A total of 97% of the PPI migrated like PP itself while about 3% of the PPI migrated like proteins up to 7200 molecular weight. PPI migrating like PP was analyzed further by isoelectric focusing and was found to occur in a neutral form like PP and a more acidic form. Peptide mapping of neutral and acidic PPI forms showed that both were like PP with the exception that the C-terminal [3H]tyrosine-containing peptide was a peptide with a net negative charge of 1 arising from a peptide extension of one or a few amino acids. The acidic form of PP was also shown to occur in pancreas extracts. However, neutral PPI was 90% of the total PPI in the pancreas extracts while the converse was true of culture extracts. We conclude that culturing the PP cell affects the efficiency of the process of amidation, that acidic PP could be either biosynthetic precursor or end product, and that the existence of the larger PP form(s) signals (signal) the possible production of yet other peptides by the PP cell.