A high-molecular IGF-2 immunoreactive peptide (pro-IGF-2?) in the insulin cells of the islets of Langerhans in pancreas of man and rat

New Peptides as Potential Players in the Crosstalk Between the Brain and Obesity, Metabolic and Cardiovascular Diseases

According to the World Health Organization report published in 2016, 650 million people worldwide suffer from obesity, almost three times more than in 1975. Obesity is defined as excessive fat accumulation which may impair health with non-communicable diseases such as diabetes, cardiovascular diseases (hypertension, coronary artery disease, stroke), and some cancers. Despite medical advances, cardiovascular complications are still the leading causes of death arising from obesity. Excessive fat accumulation is caused by the imbalance between energy intake and expenditure. The pathogenesis of this process is complex and not fully understood, but current research is focused on the role of the complex crosstalk between the central nervous system (CNS), neuroendocrine and immune system including the autonomic nervous system, adipose tissue, digestive and cardiovascular systems. Additionally, special attention has been paid to newly discovered substances: neuropeptide 26RFa, preptin, and adropin. It was shown that the above peptides are synthesized both in numerous structures of the CNS and in many peripheral organs and tissues, such as the heart, adipose tissue, and the gastrointestinal tract. Recently, particular attention has been paid to the role of the presented peptides in the pathogenesis of obesity, metabolic and cardiovascular system diseases. This review summarizes the role of newly investigated peptides in the crosstalk between brain and peripheral organs in the pathogenesis of obesity, metabolic, and cardiovascular diseases.

Insulin-like growth factor-II and bioactive proteins containing a part of the E-domain of pro-insulin-like growth factor-II

Insulin-like growth factor (IGF)-II is considered to function as an important fetal growth factor, which is structurally and functionally related to IGF-I and proinsulin. At least in vitro, IGF-II actions are mediated through the IGF-I receptor and to a lesser extent the insulin receptor. After birth, the function of IGF-II is less clear although in adults the serum level of IGF-II exceeds that of IGF-I several fold. The IGF-II gene is maternally imprinted, with exception of the liver and several parts of the brain, where it is expressed from both alleles. The regulation, organization, and translation of the IGF-II gene is complex, with five different putative promotors leading to a range of noncoding and coding mRNAs. The 180-amino acid pre-pro-IGF-II translation product can be divided into five domains and include a N-terminal signal peptide of 24 amino acid residues, the 67 amino acid long mature protein, and an 89 residues extension at the COOH terminus, designated as the E-domain. After removal of the signal peptide, the processing of pro-IGF-II into mature IGF-II requires various steps including glycosylation of the E-domain followed by the action of endo-proteases. Several of these processing intermediates can be found in the human circulation. There is increasing evidence that, besides IGF-II, several incompletely processed precursor forms of the protein, and even a 34-amino acid peptide (preptin) derived from the E-domain of pro-IGF-II, exhibit distinct biological activities. This review will focus on the current insights regarding the specific roles of the latter proteins in cancer, glucose homeostasis, and bone physiology. To address this topic clearly in the right context, a concise overview of the biological and biochemical properties of IGF-II and several relevant aspects of the IGF system will be provided.

Epigenetic aspects on therapy development for gastroenteropancreatic neuroendocrine tumors

The understanding of epigenetic modifications in gastroenteropancreatic neuroendocrine tumors is a novel and still small field. Activation of the insulin-like growth factor 2 gene locus by loss of imprinting is a classical epigenetic alteration frequently observed in insulinoma. Inactivation of the MEN1 gene, commonly involved in endocrine pancreatic tumors, impairs the association with mixed lineage leukemia involved in histone H3K4me3 methylation. In addition, promising effects on tumor phenotypes such as growth, apoptosis, cell cycle arrest, and expression of neuroendocrine markers have been obtained in vitro for inhibitors of DNA methyltransferase (azacytidine) and histone deacetylation (butyrate, valproic acid, trichostatin A and MS-275). The frequent need for complementary treatments in addition to surgery in this tumor entity supports further efforts in the development and application of drugs acting at general as well as more specific epigenetic alterations.

A novel two-chain IGF-II-derived peptide from purified β-cell granules

OBJECTIVE

Insulin-like growth factor II (IGF-II) is a potent mitogen that regulates prenatal growth and development in both humans and rodents. Its role in post-natal life is less clear although immunohistochemical studies have observed IGF-II-like immunoreactivity (IGF-II-LI) associated with insulin-producing pancreatic β-cells. Here we isolated secretory granules from a β-cell line, βTC6-F7, and characterized the nature of the IGF-II-LI located therein.

DESIGN

Secretory granules were isolated from cultured mouse βTC6-F7 cells by ultracentrifugation. Granule protein content was separated by reversed-phase HPLC, and assayed for IGF-II (radioimmunoassay) prior to identification by gas-phase NH(2)-terminal sequencing and MALDI-TOF MS. Effects of glucose incorporation into muscle glycogen were determined by incubating with isolated rat soleus muscle strips.

RESULTS

βTC6-F7 cells contained 60 ± 8 pmol of IGF-II-LI per 10⁶ cells compared to 340 ± 44 pmol insulin-LI per 10⁶ cells. IGF-II immunoreactive fractions were found to contain an IGF-II-like molecule with a molecular mass of 6847.6 Da. The protein was found to be a two-chain insulin-like product of Igf2 that corresponds to mouse des(37-40)IGF-II, which we termed 'vesiculin'. This molecule was also detectable in βTC6-F7 cells by intact-cell mass spectrometry. Mouse vesiculin evoked concentration-dependent stimulation of muscle glycogen synthesis ex vivo with an EC(50) value of 131 nM ± 1.35.

CONCLUSIONS

Vesiculin, des(37-40)IGF-II, is a novel two-chain insulin-like hormone and the major "IGF-II-like" peptide found in purified mouse βTC6-F7 secretory granules. It stimulated ex vivo muscle glycogen synthesis with an efficacy greater than or equal to the intrinsic potency of IGF-II when compared to insulin derived from the same species.

Hypoglycemia associated with the production of insulin-like growth factor II in a pancreatic islet cell tumor: a case report

An insulinoma is characterized by endogenous hyperinsulinemia and hypoglycemia. However, it has been reported that insulinomas with normal levels of plasma insulin and a normal insulin to glucose ratio occur in patients with hypoglycemia. Although overproduction of Insulin-like growth factor II (IGF-II) by non-islet cell tumors such as large mesenchymal tumors, can cause hypoglycemia, no cases of circulating plasma IGF-II from an islet cell tumor contributing to hypoglycemia have been reported. We report here a rare case of a pancreatic islet tumor in a patient with hypoglycemia that was associated with increased plasma IGF-II, which returned to normal after tumor resection.

Neuroendocrine cell markers for pancreatic islets and tumors

The authors review the application of a variety of neuroendocrine cell markers to identify pancreatic islet cells and tumors. In the past, several empiric histochemical techniques had been used to demonstrate neuroendocrine cells, particularly the Grimelius argyrophilic stain. The development of immunohistochemistry made it possible to demonstrate specific cell products such as regulatory peptides, thus allowing the classification of pancreatic neuroendocrine tumors with a view to clinical symptoms. However, it is not always possible to visualize regulatory peptides in these tumors. It is therefore important to use broad-spectrum neuroendocrine cell markers to identify the neuroendocrine nature. These markers are proteins localized in the secretory granules (core- or membrane-related), in the cytosol, or in the cellular membrane. The markers most commonly used in routine histopathology are the secretory granule proteins chromogranin A and synaptophysin and the cytosolic enzyme neuronspecific enolase. Other new markers (e.g., synaptic vesicle protein 2) are of general diagnostic value. Region-specific antibodies to chromogranin A can be valuable in differentiating between benign and malignant neuroendocrine tumors. Some markers may be related to the functioning characteristics of pancreatic neuroendocrine tumors, such as prohormone convertases. In addition, markers giving further complementary information have been identified, such as five somatostatin receptor subtypes, the expression of which varies markedly in pancreatic neuroendocrine tumors. Antibodies against all somatostatin receptor subtypes are now commercially available, and immunohistochemical investigation of its expression should be routinely applied when considering treatment with somatostatin analogs.

Biology of insulin-like growth factors in development

Insulin-like growth factors (IGFs) provide essential signals for the control of embryonic and postnatal development in vertebrate species. In mammals, IGFs act through and are regulated by a system of receptors, binding proteins, and related proteases. In each of the many tissues dependent on this family of growth factors, this system generates a complex interaction specific to the tissue concerned. Studies carried out over the last decade, mostly with transgenic and gene knockout mouse models, have demonstrated considerable variety in the cell type-specific and developmental stage-specific functions of IGF signals. Brain, muscle, bone, cartilage, pancreas, ovary, skin, and fat tissue have been identified as major in vivo targets for IGFs. Concentrating on several of these organ systems, we review here phenotypic analyses of mice with genetically modified IGF systems. Much progress has also been made in understanding the specific intracellular signaling cascades initiated by the binding of circulating IGFs to their cognate receptor. We also summarize the most relevant aspects of this research. Considerable efforts are currently focused on deciphering the functional specificities of intracellular pathways, particularly the molecular mechanisms by which cells distinguish growth-stimulating insulin-like signals from metabolic insulin signals. Finally, there is a growing body of evidence implicating IGF signaling in lifespan control, and it has recently been shown that this function has been conserved throughout evolution. Very rapid progress in this domain seems to indicate that longevity may be subject to IGF-dependent neuroendocrine regulation and that certain periods of the life cycle may be particularly important in the determination of individual lifespan.

Biology of kidney cells: ontogeny-recapitulating phylogeny

Biology of kidney cells can be used as a model for further understanding of ontogeny-recapitulating phylogeny. The common and species-specific structural and functional relationship between blood capillaries and the environment via a filtration barrier of nephrons is a biological phenomenon resulting from renal cell memory acquired through evolution. Genetically programmed development, a subsequent series of gene expression, and inductive interactions played a key role in differentiation and maintenance of specific activities of kidneys in birds and mammals. Various environmental factors may alter kidney development and specific activities at the levels of gene expression, repression, or derepression, and defensive mechanisms involved in reaction to risk factors are developed. Autoimmunity and cancerogenesis are closely dependent on a variety of environmental agents, such as antigens originating from infections with some viruses and toxins, or irradiation, advanced industrialization, and progress of civilization. As a result of gene mutation, delation, rearrangement, and/or susceptibility to different agents, renal cell memory is altered. Instead of cell-specific activities, the abilities for regeneration, and other genetically programmed activities, the genesis of kidney diseases are common. Balkan endemic nephropathy, as regional disease, is an important example of the role, of environmental agents, at the level of genes. Research programs on molecular genetics will contribute to our efforts both to prevent infections and to elucidate the genesis, diagnosis, prognosis, prevention, and therapy of kidney diseases.

Preptin derived from proinsulin-like growth factor II (proIGF-II) is secreted from pancreatic islet beta-cells and enhances insulin secretion

Pancreatic islet beta-cells secrete the hormones insulin, amylin and pancreastatin. To search for further beta-cell hormones, we purified peptides from secretory granules isolated from cultured murine beta TC6-F7 beta-cells. We identified a 34-amino-acid peptide (3948 Da), corresponding to Asp(69)-Leu(102) of the proinsulin-like growth factor II E-peptide, which we have termed 'preptin'. Preptin, is present in islet beta-cells and undergoes glucose-mediated co-secretion with insulin. Synthetic preptin increases insulin secretion from glucose-stimulated beta TC6-F7 cells in a concentration-dependent and saturable manner. Preptin infusion into the isolated, perfused rat pancreas increases the second phase of glucose-mediated insulin secretion by 30%, while anti-preptin immunoglobulin infusion decreases the first and second phases of insulin secretion by 29 and 26% respectively. These findings suggest that preptin is a physiological amplifier of glucose-mediated insulin secretion.

The phylogeny of the insulin-like growth factors

The insulin-like growth factors are major regulators of growth and development in mammals and their presence in lower vertebrates suggests that they played a similarly fundamental role throughout vertebrate evolution. While originally perceived simply as mediators of growth hormone, on-going research in mammals has revealed several hierarchical layers of complexity in the regulation of ligand bioavailability and signal transduction. Our understanding of the biological role and mechanisms of action of these important growth factors in mammals patently requires further elucidation of the IGF hormone system in the simple model systems that can be found in lower vertebrates and protochordates. This review contrasts our knowledge of the IGF hormone system in mammalian and nonmammalian models through comparison of tissue and developmental distributions and gene structures of IGF system components in different taxa. We also discuss the evolutionary origins of the system components and their possible evolutionary pathways.

Apoptosis in the pancreatic islet cells of the neonatal rat is associated with a reduced expression of insulin-like growth factor II that may act as a survival factor

Islet cell ontogeny will define adult beta-cell mass and will consist of a balance of islet cell birth and death. We have investigated the ontogeny of factors that may be related to developmental apoptosis in the islets, insulin-like growth factor II (IGF-II) and inducible nitric oxide synthase (iNOS), in pancreata of young Wistar rats. Pancreata were collected from rats of 21 days gestation to 29 days postnatal age. In situ hybridization and immunohistochemistry showed that IGF-II was expressed and present in fetal and neonatal islet cells, but declined rapidly 2 weeks after birth. Little IGF-I was associated with fetal or postnatal islets. Apoptosis in islet cells was visualized by molecular histochemistry for DNA breakage in tissue sections. Apoptosis was low in the fetus, but increased in incidence postnatally so that 13% of islet cells were undergoing apoptosis on postnatal day 14, with the incidence declining thereafter. Immunohistochemistry for iNOS showed that it was expressed within beta-cells and was most abundant 12 days after birth. When islets were isolated from rat pancreata 20-22 days after birth, islet cell viability, DNA synthetic rate, and insulin release were reduced after incubation with interleukin-1beta, tumor necrosis factor, or interferon-gamma. An increased rate of islet cell survival was found after simultaneous incubation with IGF-I or -II. Cytokine-mediated islet cell death involved the induction of apoptosis. Islets isolated from neonatal rats were not killed after exposure to these cytokines at the same concentrations, but cytokine-induced cell death was seen when neonatal islets were incubated with a neutralizing antibody against IGF-II. These experiments show that a peak of islet cell apoptosis that is maximal in the rat pancreas 14 days after birth is temporally associated with a fall in the islet cell expression of IGF-II. IGF-II was shown to function as an islet survival factor in vitro. The induction of islet cell apoptosis in vivo may involve an increased expression of iNOS within beta-cells.

Ultrastructural localization of insulin-like growth factor-2 (IGF-2) to the secretory granules of insulin cells: a study in normal and diabetic (GK) rats

By using biochemical and light-microscopical techniques, insulin-like growth factor-2 (IGF-2) has recently been found in adult pancreas, co-localized immunohistochemically with insulin in the islet B-cells. The purpose of this study was to trace IGF-2 immunoreactivity (IR) at the ultrastructural level in normal and diabetic Goto-Kakizaki (GK) rats. Using a pre-embedding technique and immuno-gold-silver staining, IGF-2 antibody binding was localized exclusively to the halo of a subset of secretory beta-granules in normal rats. Insulin IR occurred more frequently in the granules. GK rats had, in addition to normal-looking islets, some islets with irregular shape and an increased amount of fibrous tissue, so-called "starfish-shaped" islets. In these, beta-granules were usually found, but most of the B-cells were also occupied by large, usually electron-translucent vesicles, some resembling crinophagic bodies, i.e., the sign of intracellular degradation of secretory granules. In starfish-shaped islets, IGF-2 IR was localized to the halo of beta-granules, as in GK islets with normal appearance. Occasionally, IGF-2 IR was also found in the cytoplasm and even in adjacent fibroblasts. Insulin IR was restricted to beta-granules. Because the lysosomes have IGF-2 receptors, the presence of IGF-2 peptide in secretory granules could explain why some granules are guided to lysosomes for degradation.

High-molecular weight IGF-2 expression in a haemangiopericytoma associated with hypoglycaemia

Spontaneous hypoglycaemia is usually caused by an insulin-producing islet-cell tumour of the pancreas. Rarely, it can be caused by non-islet cell tumours. Most of the tumours are of mesenchymal type, large, and slowly growing. One representative is haemangiopericytoma (HAP). The present report describes a case of a large recurrent retroperitoneal HAP associated with severe hypoglycaemia. Blood serum insulin and proinsulin concentrations were low. By means of acid-gel chromatography and dot-blot techniques, an increased amount of a high-molecular-weight IGF-2 peptide was found. By using antigen retrieval procedures, IGF-2-immunoreactive tumour cells were found in specimens of the recent tumour recurrence-but not in the original. When the in situ hybridization technique was used it could be shown that IGF-2 mRNA labelling had already occurred in the original tumour specimen, 11 years before the onset of hypoglycaemic symptoms. These observations confirm the hypothesized hypoglycaemic effects of high-molecular-weight (HMW) IGF-2, but also point to the presence of a prolonged compensation of this effect. A literature review, based on 17 similar cases of haemangiopericytoma with hypoglycaemia, is presented. Our observation and findings in the literature review support the idea that non-islet-cell tumour hypoglycaemia is caused by an overproduction of a HMW IGF-2 peptide. The insulin-like effect is mediated via non-specific binding to the insulin receptors. To anticipate patients at risk of developing this kind of hypoglycaemia, the histopathological investigation should include not only immunohistochemical analyses of the presence of IGF-2 peptide, but also in situ hybridization of the IGF-2 mRNA expression.

Functional active receptors for insulin-like growth factors-I (IGF-I) and IGF-II on insulin-, glucagon-, and somatostatin-producing cells

Insulin-like growth factors I and II (IGF-I and IGF-II) are expressed at high levels in the endocrine pancreas during development and tissue regeneration. However, their effects at the endocrine pancreas are poorly understood. We searched for receptors of IGF-I and IGF-II and possible biological effects on clonal insulin-secreting (HIT), glucagon-secreting (INR1G9), and somatostatin-secreting (RIN 1027 B2) cell lines. Our data showed that HIT cells and RIN 1027 B2 cells express specific type I and type 11 IGF receptors. INR1G9 cells possess type II IGF receptors and IGF-I binding sites with the same affinity for both IGF-I and IGF-II. In HIT cells, insulin secretion was not influenced by either peptide. Proinsulin gene transcription was stimulated by IGF-II but not by IGF-I. IGF-I potently inhibited proglucagon gene transcription and glucagon secretion in INR1G9 cells, whereas IGF-II only inhibited glucagon release. In RIN 1027 B2 cells, IGF-I but not IGF-II increased somatostatin output, whereas both stimulated somatostatin gene expression. These data demonstrate the presence of classic type I and type II IGF receptors on insulin-, glucagon-, and somatostatin-secreting cells. Both peptides may be important regulators of endocrine pancreatic function in terms of islet hormone release and gene expression. Therefore, both peptides may be involved in the regulation of intraislet cellular homeostasis.

Insulin-like growth factor-II gene expression in a rat insulin-producing beta-cell line (INS-1) is regulated by glucose

A highly differentiated rat glucose-responsive insulin producing cell line INS-1 expresses high levels of insulin-like growth factor-II (IGF-II). Basal levels of IGF-II gene mRNA were expressed in cells cultured at 1-6 mmol/l glucose. At glucose concentrations of 10-20 mmol/l, IGF-II mRNA was increased more than threefold after 44 h of incubation. Levels of IGF-II mRNA in INS-1 cells incubated at 5.6 and 20 mmol/l glucose in the presence of 4 micrograms/ml actinomycin D are comparable and are not reduced during 20 h of treatment, indicating the high stability of IGF-II mRNA in this cell line. From the three rat IGF-II promoters, promoter 3 is by far the most active in INS-1 cells. The IGF-II promoter 3 activity and IGF-II mRNA production at high glucose concentrations increased threefold over their respective levels at low glucose concentration, suggesting that the glucose-induced IGF-II gene expression in this beta-cell line might be transcriptionally controlled. The up-regulation of IGF-II mRNA by glucose was not due to the increased intracellular cyclic AMP levels or protein kinase C activation. A protein kinase C activator had no effect on IGF-II gene expression, and an adenylate cyclase activator (forskolin), suppressed the stimulatory effects of glucose on the IGF-II mRNA. Under all the experimental conditions examined, the IGF-II and insulin genes were differentially regulated in INS-1 cells. The IGF-II gene expression and DNA synthesis, however, were regulated in parallel, suggesting that these two cellular activities are closely associated.

IGF-2-like peptides are present in insulin cells of the elasmobranchian endocrine pancreas: an immunohistochemical and chromatographic study

Evidence for the presence of peptides, related to insulin-like growth factor 2 (IGF-2), has been obtained in the endocrine pancreas of the elasmobranchian species Raja clavata, the sting ray. By radioimmunoassay, IGF-2-like immunoreactivity was detected in Raja pancreas extract. Further characterization of this activity by acid gel chromatography revealed two distinct peaks of IGF-2-like immunoreactivity with apparent molecular weights of approximately 8.2 kDa and 4.5 kDa. Using the same IGF-2 antibody as well as antisera specific for mammalian IGF-1, insulin, glucagon, somatostatin and pancreatic polypeptide in double immunofluorescence studies, IGF-2-like immunoreactivity was located exclusively in insulin-immunoreactive cells. In contrast, IGF-1-like immunoreactivity was mainly observed in somatostatin- and glucagon-immunoreactive cells. A varying proportion (0-70%) of insulin-immunoreactive cells, however, displayed both IGF-1- and IGF-2-like immunoreactivity. Absorption studies indicated that the IGF-2-like peptides in Raja are different from mammalian and submammalian insulin and mammalian IGF-1, but similar to mammalian IGF-2. Thus, IGF-2-like peptides seem to occur during evolution as early as the phylogenetic development of the elasmobranchians. Furthermore, the results indicate a particularly conservative evolution of the islet IGF-2 system.