Pancreastatin--a novel regulatory peptide?

Role and function of granin proteins in diabetes mellitus

The granin glycoprotein family consists of nine acidic proteins; chromogranin A (CgA), chromogranin B (CgB), and secretogranin II–VIII. They are produced by a wide range of neuronal, neuroendocrine, and endocrine cells throughout the human body. Their major intracellular function is to sort peptides and proteins into secretory granules, but their cleavage products also take part in the extracellular regulation of diverse biological processes. The contribution of granins to carbohydrate metabolism and diabetes mellitus is a recent research area. CgA is associated with glucose homeostasis and the progression of type 1 diabetes. WE-14, CgA_10-19, and CgA_43-52 are peptide derivates of CgA, and act as CD4⁺ or CD8⁺ autoantigens in type 1 diabetes, whereas pancreastatin (PST) and catestatin have regulatory effects in carbohydrate metabolism. Furthermore, PST is related to gestational and type 2 diabetes. CgB has a crucial role in physiological insulin secretion. Secretogranins II and III have angiogenic activity in diabetic retinopathy (DR), and are novel targets in recent DR studies. Ongoing studies are beginning to investigate the potential use of granin derivatives as drugs to treat diabetes based on the divergent relationships between granins and different types of diabetes.

Chromogranin A and its fragments in cardiovascular, immunometabolic, and cancer regulation

Chromogranin A (CgA)-the index member of the chromogranin/secretogranin secretory protein family-is ubiquitously distributed in endocrine, neuroendocrine, and immune cells. Elevated levels of CgA-related polypeptides, consisting of full-length molecules and fragments, are detected in the blood of patients suffering from neuroendocrine tumors, heart failure, renal failure, hypertension, rheumatoid arthritis, and inflammatory bowel disease. Full-length CgA and various CgA-derived peptides, including vasostatin-1, pancreastatin, catestatin, and serpinin, are expressed at different relative levels in normal and pathological conditions and exert diverse, and sometime opposite, biological functions. For example, CgA is overexpressed in genetic hypertension, whereas catestatin is diminished. In rodents, the administration of catestatin decreases hypertension, cardiac contractility, obesity, atherosclerosis, and inflammation, and it improves insulin sensitivity. By contrast, pancreastatin is elevated in diabetic patients, and the administration of this peptide to obese mice decreases insulin sensitivity and increases inflammation. CgA and the N-terminal fragment of vasostatin-1 can enhance the endothelial barrier function, exert antiangiogenic effects, and inhibit tumor growth in animal models, whereas CgA fragments lacking the CgA C-terminal region promote angiogenesis and tumor growth. Overall, the CgA system, consisting of full-length CgA and its fragments, is emerging as an important and complex player in cardiovascular, immunometabolic, and cancer regulation.

Granin-derived peptides

The granin family comprises altogether 7 different proteins originating from the diffuse neuroendocrine system and elements of the central and peripheral nervous systems. The family is dominated by three uniquely acidic members, namely chromogranin A (CgA), chromogranin B (CgB) and secretogranin II (SgII). Since the late 1980s it has become evident that these proteins are proteolytically processed, intragranularly and/or extracellularly into a range of biologically active peptides; a number of them with regulatory properties of physiological and/or pathophysiological significance. The aim of this comprehensive overview is to provide an up-to-date insight into the distribution and properties of the well established granin-derived peptides and their putative roles in homeostatic regulations. Hence, focus is directed to peptides derived from the three main granins, e.g. to the chromogranin A derived vasostatins, betagranins, pancreastatin and catestatins, the chromogranin B-derived secretolytin and the secretogranin II-derived secretoneurin (SN). In addition, the distribution and properties of the chromogranin A-derived peptides prochromacin, chromofungin, WE14, parastatin, GE-25 and serpinins, the CgB-peptide PE-11 and the SgII-peptides EM66 and manserin will also be commented on. Finally, the opposing effects of the CgA-derived vasostatin-I and catestatin and the SgII-derived peptide SN on the integrity of the vasculature, myocardial contractility, angiogenesis in wound healing, inflammatory conditions and tumors will be discussed.

Chromogranin A Regulation of Obesity and Peripheral Insulin Sensitivity

Chromogranin A (CgA) is a prohormone and granulogenic factor in endocrine and neuroendocrine tissues, as well as in neurons, and has a regulated secretory pathway. The intracellular functions of CgA include the initiation and regulation of dense-core granule biogenesis and sequestration of hormones in neuroendocrine cells. This protein is co-stored and co-released with secreted hormones. The extracellular functions of CgA include the generation of bioactive peptides, such as pancreastatin (PST), vasostatin, WE14, catestatin (CST), and serpinin. CgA knockout mice (Chga-KO) display: (i) hypertension with increased plasma catecholamines, (ii) obesity, (iii) improved hepatic insulin sensitivity, and (iv) muscle insulin resistance. These findings suggest that individual CgA-derived peptides may regulate different physiological functions. Indeed, additional studies have revealed that the pro-inflammatory PST influences insulin sensitivity and glucose tolerance, whereas CST alleviates adiposity and hypertension. This review will focus on the different metabolic roles of PST and CST peptides in insulin-sensitive and insulin-resistant models, and their potential use as therapeutic targets.

Pancreastatin is an endogenous peptide that regulates glucose homeostasis

Pancreastatin (PST) is a regulatory peptide containing 49 amino acids, first isolated from porcine pancreas. Intracellular and extracellular processing of the prohormone Chromogranin A (Chga) results various bioactive peptides of which PST has dysglycemic activity. PST regulates glucose, lipid, and protein metabolism in liver and adipose tissues. It also regulates the secretion of leptin and expression of leptin and uncoupling protein 2 in adipose tissue. In Chga knockout mice, PST induces gluconeogenesis in the liver. PST reduces glucose uptake in mice hepatocytes and adipocytes. In rat hepatocytes, PST induces glycogenolysis and glycolysis and inhibits glycogen synthesis. In rat adipocytes, PST inhibits lactate production and lipogenesis. These metabolic effects are confirmed in humans. In the dual signaling mechanism of PST receptor, mostly PST activates Gαq/11 protein leads to the activation of phospholipase C β3-isoform, therefore increasing cytoplasmic free calcium and stimulating protein kinase C. PST inhibits the cell growth in rat HTC hepatoma cells, mediated by nitric oxide and cyclic GMP production. Elevated levels of PST correlating with catecholamines have been found in gestational diabetes and essential hypertension. Rise in the blood PST level in Type 2 diabetes suggests that PST is a negative regulator of insulin sensitivity and glucose homeostasis.

Chromogranins A and B and secretogranin II as prohormones for regulatory peptides from the diffuse neuroendocrine system

Chromogranin A (CgA), chromogranin B (CgB), and secretogranin II (SgII) belong to a family of uniquely acidic secretory proteins in elements of the diffuse neuroendocrine system. These "granins" are characterized by numerous pairs of basic amino acids as potential sites for intra- and extragranular processing. In response to adequate stimuli, the granins are coreleased with neurotransmitters and hormones and appear in the circulation as potential modulators of homeostatic processes. This review is directed towards functional aspects of the secreted CgA, CgB, and SgII and their biologically active sequences. Widely different effects and targets have been reported for granin-derived peptides. So far, the CgA peptides vasostatin-I, pancreastatin, and catestatin, the CgB peptides CgB(1-41) and secretolytin, and the SgII peptide secretoneurin are the most likely candidates for granin-derived regulatory peptides. Most of their effects fit into patterns of direct or indirect modulations of major functions, in particular associated with inflammatory conditions.

The granin family of uniquely acidic proteins of the diffuse neuroendocrine system: comparative and functional aspects

The chromogranins A (CgA) and B (CgB) and secretogranin II (SgII) constitute the main members of a family of uniquely acidic secretory proteins in elements of the diffuse neuroendocrine system. These genetically distinct proteins, CgA, CgB, SgII and the less well known secretogranins III-VII are collectively referred to as 'granins' and characterised by numerous pairs of basic amino acids as potential cleavage sites for processing by the co-stored prohormone converting enzymes PC 1/3 and PC2. This review is directed towards comparative and functional aspects of the granins with emphasis on their phylogenetically conserved sequences. Recent developments provide ample evidence of widely different effects and targets for the intact granins and their derived peptides, intracellularly in the directed trafficking of storage components during granule maturation and extracellularly in autocrine, paracrine and endocrine interactions. Most of the effects assigned to the granin derived peptides fit into patterns of direct or indirect inhibitory modulations of major functions. So far, peptides derived from CgA (vasostatins, chromacin, pancreastatin, WE-14, catestatin and parastatin), CgB (secretolytin) and SgII (secretoneurin) are the most likely candidates for granin-derived regulatory peptides, of postulated relevance not only for homeostatic processes, but also for tissue assembly and repair, inflammatory responses and the first line of defence against invading microorganisms.

Pancreastatin-like immunoreactivity in globular dystrophic neurites of senile plaques in brains of patients with Alzheimer's disease

The immunohistochemical localization of pancreastatin (PST) was examined in brains of Alzheimer's disease (AD) and control cases using three different antisera to PST, and was compared with the staining for chromogranin A (CgA), the precursor of PST. In control brains, CgA-like immunoreactivity was observed in the cytoplasm and fibers of certain neuronal populations, which were not immunostained with any of the PST antisera. In AD brains, dystrophic neurites of globular shape located in senile plaques were immunostained with each of the PST antisera, as well as with the CgA antibody. PST-positive and CgA-positive dystrophic neurites showed similar profiles. The present study indicates that CgA is probably cleaved to produce PST in some globular dystrophic neurites in senile plaques.

Pancreastatin action in the liver: dual coupling to different G proteins

Pancreastatin is a 49 amino acid peptide first isolated, purified and characterized from porcine pancreas. Its biological activity in different tissues can be assigned to the C-terminal part of the molecule. Pancreastatin has a prohormonal precursor, chromogranin A, which is a glycoprotein present in neuroendocrine cells, including the endocrine pancreas. We have been interested in pancreastatin action in the liver. We found that pancreastatin has a glycogenolytic effect in the hepatocyte both in vivo and in vitro. We then studied and characterized the specific pancreastatin receptor in the rat liver plasma membrane, as well as the specific signal transduction. This receptor appears to be coupled to two different G proteins. A pertussis toxin-insensitive G proteins leads to the activation of phospholipase C, and therefore mediates the glycogenolytic effect in the liver by increasing cytoplasmic free calcium and stimulating protein kinase C. The role of cyclic GMP in the action of pancreastatin is not known yet, although it seems to regulate negatively the activation of phospholipase C. The precise mechanism by which pancreastatin stimulates guanylate cyclase activity remains to be studied.

Plasma pancreastatin-like immunoreactivity correlates with plasma norepinephrine levels in essential hypertension

Pancreastatin (PST), a 49 amino acid peptide originally isolated from porcine pancreas, is derived from chromogranin A (Cg A), an acidic protein co-released with catecholamines from sympathetic nerve terminals and chromaffin cells. Extracellular processing of Cg A yields PST as well as other biological active peptides. Measurement of Cg A and PST-like immunoreactivity (PST-LI) has been used to investigate patients with pheochromocytoma and other neuroendocrine neoplasia. Some studies have found increased plasma norepinephrine (NE) levels in essential hypertension. We therefore measured venous plasma PST-LI and catecholamines in patients with essential hypertension. We employed a radioimmunoassay developed with commercially available reagents for measuring plasma PST-like immunoreactivity, and HPLC with electrochemical detection for measurement of plasma catecholamines. The correlation of PST-LI with epinephrine (E) was very weak. However, its correlation with NE was highly significant. Thus, venous plasma PST-LI immunoreactivity may reflect sympathetic nerve activity in essential hypertension.

Pancreastatin activates protein kinase C by stimulating the formation of 1,2-diacylglycerol in rat hepatocytes

We describe here the stimulation by pancreastatin of 1,2-diacylglycerol production and protein kinase C activity in liver plasma membrane and isolated hepatocytes. The dose-dependency for the stimulation of both processes was similar to the recently described pattern of glucose output and cytosolic Ca2+ transients produced by pancreastatin. The time course of diacylglycerol production at 30 degrees C showed a rapid increase within 5 min, reaching a maximum at 10 min. Protein kinase C from hepatocytes was dependent on Ca2+ and phosphatidylserine. Neither the pancreastatin-stimulated diacylglycerol production nor the activation of protein kinase C was affected by pretreatment with pertussis toxin. However, the presence of GTP partially inhibited this pancreastatin stimulation of 1,2-diacylglycerol in a dose-dependent manner, although GTP alone stimulates diacylglycerol accumulation. This inhibitory effect of GTP on pancreastatin stimulation of diacylglycerol synthesis was completely abolished by the pretreatment with pertussis toxin. In conclusion, this study provides evidence that pancreastatin stimulates the formation of 1,2-diacylglycerol by a pertussis-toxin-independent mechanism, which may be responsible for the pancreastatin activation of protein kinase C.

Pancreastatin activates pertussis toxin-sensitive guanylate cyclase and pertussis toxin-insensitive phospholipase C in rat liver membranes

We have recently found the calcium dependent glycogenolytic effect of a pancreastatin on rat hepatocytes and the mobilization of intracellular calcium. To further investigate the mechanism of action of pancreastatin on liver we have studied its effect on guanylate cyclase, adenylate cyclase, and phospholipase C, and we have explored the possible involvement of GTP binding proteins by measuring GTPase activity as well as the effect of pertussis toxin treatment of plasma liver membranes on the pancreastatin stimulated GTPase activity and the production of cyclic GMP and myo-inositol 1,4,5-triphosphate. Pancreastatin stimulated GTPase activity of rat liver membranes about 25% over basal. The concentration dependency curve showed that maximal stimulation was achieved at 10(-7)M pancreastatin (EC50 = 3 nM). This stimulation was partially inhibited by treatment of the membranes with pertussis toxin. The effect of pancreastatin on guanylate cyclase and phospholipase C were examined by measuring the production of cyclic GMP and myo-inositol 1,4,5-triphosphate respectively. Pancreastatin increased the basal activity of guanylate cyclase to a maximum of 2.5-fold the unstimulated activity at 30 degrees C, in a time- and dose-dependent manner, reaching the maximal stimulation above control with 10(-7) M pancreastatin at 10 min (EC50 = 0.6 nM). This effect was completely abolished when rat liver membranes had been ADP-ribosylated with pertussis toxin. On the other hand, adenylate cyclase activity was not affected by pancreastatin. Phospholipase C activity of rat liver membranes was rapidly stimulated (within 2-5 min) at 30 degrees C by 10(-7) M pancreastatin, reaching a maximum at 15 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Pancreastatin molecular forms in normal human plasma

Circulating molecular forms with pancreastatin (PST)-like immunoreactivity in plasma from normal subjects were examined. An immunoreactive form corresponding to a human PST-like sequence [human chromogranin-A-(250-301)] (hPST-52) and a larger form (mol wt 15-21 kDa) were detected by gel filtration of plasma from normal subjects. On high performance liquid chromatography, predominant immunoreactive forms coeluted with the three larger forms which were purified from the xenograft of human pancreatic islet cell carcinoma cell line QGP-1N cells and with synthetic hPST-52. The fraction containing larger forms purified from xenograft of QGP-1N cells had biological activity equivalent to that of hPST-52 on the inhibition of pancreatic exocrine secretion. These results suggest that the larger molecular forms as well as hPST-52 may be physiologically important circulating forms of PST in human.

Posttranslational processing of proenkephalins and chromogranins/secretogranins

Posttranslational processing of peptide-precursors is nowadays believed to play an important role in the functioning of neurons and endocrine cells. Both proenkephalins and chromogranins/secretogranins are considered as precursor molecules in these tissues, resulting in posttranslationally formed degradation products with potential biological activities. Among the proteins and peptides of neuronal and endocrine secretory granules, the enkephalins and enkephalin-containing peptides have been most extensively studied. The characterization of the post-translationally formed degradation products of the proenkephalins have enabled the understanding of their processing pathway. Chromogranins/secretogranins represent a group of acidic glycoproteins, contained within hormone storage granules. The biochemistry, biogenesis and molecular properties of these proteins have already been studied for 25 years. The chromogranins/secretogranins have a widespread distribution throughout the neuroendocrine system, the adrenal medullary chromaffin granules being the major source of these storage components. Recent data provide evidence for a precursor role for all members of the chromogranins/secretogranins family although also several other functions have been proposed. In this review, some of the methods applied to study proteolytic processing are described. In addition, the posttranslational processing of chromogranins/secretogranins and proenkephalins, especially the biochemical aspects, will be discussed and compared. Recent exciting developments on the generation and identification of potential physiologically active fragments will be covered.

Chromostatin, a chromogranin A-derived bioactive peptide, is present in human pancreatic insulin (beta) cells

Chromogranin A (CGA) is a secretory protein present in the adrenal medulla and in a variety of endocrine organs. This protein may serve as precursor for pancreastatin (PST) and for other biologically active peptides. Recently, chromostatin (CST), a CGA derivative, has been identified that possesses high biological activity. The cellular distribution of CST in various endocrine organs is completely unknown. Using immunohistochemistry on plastic sections, we investigated the occurrence and cellular distribution of CST, PST, and CGA in human endocrine pancreas of healthy and diseased states and in the adrenal medulla. In the normal and diabetic pancreas, CST immunoreactivity was localized exclusively in beta cells, which were mostly unreactive for PST and CGA. Both latter peptides were confined mainly to glucagon (alpha) cells. Insulinoma cells displayed strong insulin, PST, and CGA immunoreactivities, but they were faintly immunoreactive for CST or unreactive. Adrenal chromaffin cells exhibited strong immunoreactivity for CGA but lacked CST and PST immunoreactivities. Based on the peculiar distributive pattern of CST, PST, and CGA, we suggest that CGA is differentially processed in chromaffin and islet tissues and in insulinoma cells. The unique cellular localization of CST in the endocrine pancreas of normal and pathological conditions may indicate that CST is involved in beta-cell function.