The branching of insulin-like growth factor 1 and insulin: an immunohistochemical analysis during phylogeny

A hypothesis on paradoxical privileged portal vein metastasis of hepatocellular carcinoma. Can organ evolution shed light on patterns of human pathology, and vice versa?

Unlike other carcinomas, hepatocellular carcinoma (HCC) metastasizes to distant organs relatively rarely. In contrast, it routinely metastasizes to liver vasculature/liver, affecting portal veins 3-10 times more often than hepatic veins. This portal metastatic predominance is traditionally rationalized within the model of a reverse portal flow, due to accompanying liver cirrhosis. However, this intuitive model is not coherent with facts: 1) reverse portal flow occurs in fewer than 10% of cirrhotic patients, while portal metastasis occurs in 30-100% of HCC cases, and 2) portal vein prevalence of HCC metastasis is also characteristic of HCC in non-cirrhotic livers. Therefore, we must assume that the route for HCC metastatic dissemination is the same as for other carcinomas: systemic dissemination via the draining vessel, i.e., via the hepatic vein. In this light, portal prevalence versus hepatic vein of HCC metastasis appears as a puzzling pattern, particularly in cases when portal HCC metastases have appeared as the sole manifestation of HCC. Considering that other GI carcinomas (colorectal, pancreatic, gastric and small bowel) invariably disseminate via portal vein, but very rarely form portal metastasis, portal prevalence of HCC metastasis appears as a paradox. However, nature does not contradict itself; it is rather our wrong assumptions that create paradoxes. The 'portal paradox' becomes a logical event within the hypothesis that the formation of the unique portal venous system preceded the appearance of liver in evolution of chordates. The analysis suggests that the appearance of the portal venous system, supplying hormones and growth factors of pancreatic family, which includes insulin, glucagon, somatostatin, and pancreatic polypeptide (HGFPF) to midgut diverticulum in the early evolution of chordates (in an Amphioxus-like ancestral animal), promoted differentiation of enterocytes into hepatocytes and their further evolution to the liver of vertebrates. These promotional-dependent interactions are conserved in the vertebrate lineage. I hypothesize that selective homing and proliferation of malignant hepatocytes (i.e., HCC cells) in the portal vein environment are due to a uniquely high concentration of HGFPF in portal blood. HGFPF are also necessary for liver function and renewal and are significantly extracted by hepatocytes from passing blood, creating a concentration gradient of HGFPF between the portal blood and hepatic vein outflow, making post-liver vasculature and remote organs less favorable spaces for HCC growth. It also suggested that the portal vein environment (i.e., HGFPF) promotes the differentiation of more aggressive HCC clones from already-seeded portal metastases, explaining the worse outcome of HCC with the portal metastatic pattern. The analysis also offers new hypothesis on the phylogenetic origin of the hepatic diverticulum of cephalochordates, with certain implications for the modeling of the chordate phylogeny.

Privileged portal metastasis of hepatocellular carcinoma in light of the coevolution of a visceral portal system and liver in the chordate lineage: a search for therapeutic targets

Hepatocellular carcinoma (HCC) disseminates systemically, but metastases occur in distant organs only in minority of patients, whereas HCC routinely metastasizes to liver and its vessels. HCC cells disseminate via hepatic veins, but portal veins are affected by metastasis more frequently than are hepatic veins, and correlates with poor prognosis. In this review, I suggest that privileged HCC portal metastasis occurs because of high levels of pancreatic family hormones and growth factors (PHGFs) in the portal blood. The analysis suggests that the appearance of the portal system carrying PHGFs in the evolution of invertebrate chordate (Amphioxus) led to the evolution of the liver in vertebrate; given that the portal pattern of HCC metastasis and selection of more-aggressive clones are PHGF dependent, PHGFs and their ligands constitute therapeutic targets.

The effect of experimentally induced chronic hyperglycaemia on serum and pancreatic insulin, pancreatic islet IGF-I and plasma and urinary ketones in the domestic cat (Felis felis)

Like in humans, diabetes mellitus is on the rise in cats. Feline diabetes is a suitable model for human type-2 diabetes. We investigated magnitude and timing of insulin suppression with induced hyperglycaemia and its relationship to plasma and urinary ketones and to pancreatic islet insulin. IGF-I is under discussion as a protective mechanism but little is known about its role in diabetes in general and its distinct localisation in feline pancreatic islets in particular. Thirteen healthy, adult cats were allocated to 2 groups and infused with glucose to maintain their blood glucose at a high or moderate concentration for 42 days resulting in insulin secretion suppression. After initial increase, insulin levels declined to baseline but were still detectable in the blood at a very low level after 6 weeks of glucose infusion and then increased after a 3 week recovery period. While IGF-I in healthy cats was primarily located in glucagon cells, in hyperglycaemia-challenge IGF-I was pronounced in the β-cells 3 weeks after ceasation of infusion. Six/8 cats developing glucose toxicity became ketonuric after 3-4 weeks. Gross lipaemia occurred approx 1 week prior to ketonuria. Ketonuric cats required 1-2 weeks of insulin therapy after-infusion until β-cell recovery. In conclusion, ketosis and hyperlipidaemia are likely to occur in diabetic cats with glucose at 30 mmol/L, especially after ≥2 weeks. Three weeks after ceasation of infusions, clinical and morphological recovery occurred. We propose a local protective effect of IGF-I to support survival and insulin production in the hyperglycaemic state and recovery period.

Influences of the environment on the endocrine and paracrine fish growth hormone-insulin-like growth factor-I system

Insulin-like growth factor-I (IGF-I) is a key component of the complex system that regulates differentiation, development, growth and reproduction of fishes. The IGF-I gene is mainly expressed in the liver that represents the principal source of endocrine IGF-I but also in numerous other organs where the hormone most probably acts in an autocrine-paracrine manner. The primary stimulus for synthesis and release of IGF-I is growth hormone (GH) from the anterior pituitary. Thus, in analogy to mammals, it is usual to speak of a fish 'GH-IGF-I axis'. The GH-IGF-I system is affected by changes in the environment and probably represents a target of endocrine disrupting compounds (EDC) that impair many physiological processes in fishes. Thus, the review deals with the influences of changes in different environmental factors, such as food availability, temperature, photoperiod, season, salinity and EDCs, on GH gene expression in pituitary, IGF-I gene expression in liver and extrahepatic sites and the physiological effects resulting from the evoked alterations in endocrine and local IGF-I. Environmental influences certainly interact with each other but for convenience of the reader they will be dealt with in separate sections. Current trends in GH-IGF-I research are analysed and future focuses are suggested at the end of the sections.

Expression, mitogenic activity and regulation by growth hormone of growth hormone/insulin-like growth factor in Branchiostoma belcheri

The growth hormone (GH)/insulin-like growth factor (IGF) axis is unique to all the vertebrate species but its evolutionary origin is ill-defined. We therefore cloned a cDNA encoding Branchiostoma belcheri IGF (BbIGF). BbIGF was expressed in a tissue-specific manner, with the most abundant expression in the hepatic caecum, the putative liver precursor. The recombinant BbIGF expressed in vitro showed mitogenic activity capable of stimulating cell proliferation in the flounder gill, a characteristic of vertebrate IGF. Quantitative real-time polymerase chain reaction demonstrated that the recombinant rat GH was able to induce a significant up-regulation of BbIGF expression in the hepatic caecum. Moreover, Western blotting revealed the presence of a molecule similar to rat GH receptor in the hepatic caecum. These results suggest that BbIGF expression is inducible by exogenous mammalian GH, suggesting the presence of a GH/IGF axis in B. belcheri. The relationship between BbIFG expression and the origin of the vertebrate liver is discussed.

Organ-specific expression of IGF-I during early development of bony fish as revealed in the tilapia, Oreochromis niloticus, by in situ hybridization and immunohistochemistry: indication for the particular importance of local IGF-I

The cellular sites of insulin-like growth factor I (IGF-I) synthesis in the early developing tilapia (0-140 days post fertilization, DPF) were investigated. IGF-I mRNA and peptide appeared in liver as early as 4 DPF and in gastro-intestinal epithelial cells between 5-9 DPF. In exocrine pancreas, the expression of IGF-I started at 4 DPF and continued until 90 DPF. IGF-I production was detected in islets at 6 DPF in non-insulin cells and occurred throughout life. In renal tubules and ducts, IGF-I production started at 8 DPF. IGF-I production in chondrocytes had its onset at 4 DPF, was more pronounced in growing regions and was also found in adults. IGF-I mRNA and peptide appeared in the cytoplasm of skeletal muscle cells at 4 DPF. In gill chloride cells, IGF-I production started at 6 DPF. At 13 DPF, IGF-I was detected in cardiac myocytes. IGF-I-producing epidermal cells appeared at 5 DPF. In brain and ganglia, IGF-I was expressed in virtually all neurones from 6 to 29 DPF, their number decreasing with age. Neurosecretory IGF-I-immunoreactive axons were first seen in the neurohypophysis around 17 DPF. Endocrine cells of the adenohypophysis exhibited IGF-I mRNA at 28 DPF and IGF-I immunoreactivity at 40 DPF. Thus, IGF-I appeared early (4-5 DPF), first in liver, the main source of endocrine IGF-I, and then in organs involved in growth or metabolism. The expression of IGF-I was more pronounced during development than in juvenile and adult life. Local IGF-I therefore seems to have a high functional impact in early growth, metabolism and organogenesis.

Insulin-like growth factor signaling in fish

The insulin-like growth factor (IGF) system plays a central role in the neuroendocrine regulation of growth in all vertebrates. Evidence from studies in a variety of vertebrate species suggest that this growth factor complex, composed of ligands, receptors, and high-affinity binding proteins, evolved early during vertebrate evolution. Among nonmammalian vertebrates, IGF signaling has been studied most extensively in fish, particularly teleosts of commercial importance. The unique life history characteristics associated with their primarily aquatic existence has fortuitously led to the identification of novel functions of the IGF system that are not evident from studies in mammals and other tetrapod vertebrates. Furthermore, the emergence of the zebrafish as a preferred model for development genetics has spawned progress in determining the requirements for IGF signaling during vertebrate embryonic development. This review is intended as a summary of our understanding of IGF signaling, as revealed through research into the expression, function, and evolution of IGF ligands, receptors, and binding proteins in fish.

Unprecedented Multiplicity of Ig Transmembrane and Secretory mRNA Forms in the Cartilaginous Fish

In most jawed vertebrates including cartilaginous fish, membrane-bound IgM is expressed as a five Ig superfamily (Igsf)-domain H chain attached to a transmembrane (Tm) region. Heretofore, bony fish IgM was the one exception with IgM mRNA spliced to produce a four-domain Tm H chain. We now demonstrate that the Tm and secretory (Sec) mRNAs of the novel cartilaginous fish Ig isotypes, IgW and IgNAR, are present in multiple forms, most likely generated by alternative splicing. In the nurse shark, Ginglymostoma cirratum, and horn shark, Heterodontus francisci, alternative splicing of Tm exons to the second or the fourth constant (C(H)) exons produces two distinct IgW Tm cDNAs. Although the seven-domain IgW Sec cDNA form contains a canonical secretory tail shared with IgM, IgNAR, and IgA, we report a three-domain cDNA form of shark IgW (IgW(short)) having an unusual Sec tail, which is orthologous to skate IgX(short) cDNA. The IgW and IgW(short) Sec transcripts are restricted in their tissue distribution and expression levels vary among individual sharks, with all forms expressed early in ontogeny. IgNAR mRNA is alternatively spliced to produce a truncated four-domain Tm cDNA and a second Tm cDNA is expressed identical in Igsf domains as the Sec form. PBL is enriched in the Tm cDNA of these Igs. These molecular data suggest that cartilaginous fish have augmented their humoral immune repertoire by diversifying the sizes of their Ig isotypes. Furthermore, these Tm cDNAs are prototypical and the truncated variants may translate as more stable protein at the cell surface.

Localization of IGF-I, IGF-I receptor, and IGFBP-2 in developing Umbrina cirrosa (Pisces: Osteichthyes)

In this study, the distribution of IGF-I, IGF type I receptor (IGF-IR), and IGF-binding protein 2 (IGFBP-2) was investigated during larval and post-larval developmental stages of the shi drum (Umbrina cirrosa) by immunohistochemistry using antisera raised against Sparus aurata IGF-I and IGF-IR, and against mouse IGFBP-2. Immunoreactivity of the mitogenic marker PCNA (proliferating cell nuclear antigen) was used for assessment of cellular proliferation. Distribution of IGF-I mRNA was studied by in situ hybridization. IGF-I immunoreactivity was detected in liver and developing intestine already in 1-5 day post-hatching larvae. From day 11, immunostaining in the intestine was evident in the enterocytes of the anterior intestine and in the apical zone of the epithelium of developing posterior intestine. Positive reaction with IGF-I antibody was also detected in chondrocytes, in the epithelium of the skin, gills and in the central nervous system (CNS), and lateral muscle. At hatching IGF-IR immunoreactivity was already detectable in developing CNS, notochord, and skin. From day 6 immunostaining was evident in the olfactory epithelium, in eyes and from day 11 in the developing olfactory bulbs and CNS. Positive reaction with IGF-IR antibody was also detected in chondrocytes, in the epithelium of the skin, gills, heart, and in the lateral muscle. Immunoreactive IGFBP-2, as detected by anti-mouse IGFBP-2 antiserum, exhibited generally a similar distribution pattern to that of IGF-I and IGF-IR. In situ hybridization, which has been performed by using riboprobes from S. aurata cDNA, revealed IGF-I mRNA in skeletal musculature, liver, and CNS. These data strongly suggest a role for the IGF system during development and growth of U. cirrosa.

Ontogenetic and phylogenetic development of the endocrine pancreas (islet organ) in fish

The morphology of the gastroenteropancreatic (GEP) system of fish was reviewed with the objective of providing the phylogenetic and ontogenetic development of the system in this vertebrate group, which includes agnathans and gnathostome cartilaginous, actinoptyerygian, and sarcopterygian fish. Particular emphasis is placed on the fish homolog of the endocrine pancreas of other vertebrates, which is referred to as the islet organ. The one-hormone islet organ (B cells) of larval lampreys is the most basic pattern seen among a free-living vertebrate, with the two-hormone islet organ (B and D cells) of hagfish and the three-hormone islet organ (B, D, and F cells) of adult lampreys implying a phylogenetic trend toward the classic four-hormone islet tissue (B, D, F, and A cells) in most other fish. An earlier stage in the development of this phylogenetic sequence in vertebrates may have been the restriction of islet-type hormones to the alimentary canal, like that seen in protochordates. The relationship of the islet organ to exocrine pancreatic tissue, or its equivalent, is variable among bony, cartilaginous, and agnathan fishes and is likely a manifestation of the early divergence of these piscine groups. Variations in pancreatic morphology between individuals of subgroups within both the lamprey and chondrichthyan taxa are consistent with their evolutionary distance. A comparison of the distribution and degree of concentration of the components of the islet organ among teleosts indicates a diffuse distribution of relatively small islets in the generalized euteleosts and the tendency for the concentration into Brockmann bodies of large (principal) islets (with or without secondary islets) in the more derived forms. The holostean actinopterygians (Amiiformes and Semiontiformes) share with the basal teleosts (osteoglossomorphs, elopomorphs) the diffuse arrangement of the components of the islet organ that is seen in generalized euteleosts. Since principal islets are also present in adult lampreys the question arises whether principal islets are a derived or a generalized feature among teleosts. There is a paucity of studies on the ontogeny of the GEP system in fish but it has been noted that the timing of the appearance of the islet cell types parallels the time that they appear during phylogeny; the theory of recapitulation has been revisited. It is stressed that the lamprey life cycle provides a good opportunity for studying the development of the GEP system. There are now several markers of cell differentiation in the mammalian endocrine pancreas which would be useful for investigating the development of the islet organ and cells of the remaining GEP system in fish.

Immunocytochemical characterization of the pancreatic islet cells of the Nile Tilapia (Oreochromis niloticus)

The cellular composition and topography of the pancreatic islet of Oreochromis niloticus, now known to be a donor source for islet xenotransplantation studies, were characterized. Whole tilapia islets were harvested using an enzymatic method and then further digested into single-cell preparations. Cell cytospin preparations of islet cells and paraffin sections of whole islets were stained using antisera against tilapia insulin, human glucagon, salmon somatostatin-25 (SST-25), human somatostatin-14 (SST-14), and salmon peptide tyrosine-tyrosine (PYY) using the immunoperoxidase method. Cell counts, performed on cytospin preparations using a Quantimet 570 computerized image analysis system, revealed that O. niloticus islets contained 78% endocrine cells and 22% immunonegative cells (i. e., mainly nucleated erythrocytes and rare tissue eosinophils). The proportions of immunopositive endocrine cell types were: 42.3% insulin immunopositive cells, 11.5% glucagon immunopositive cells, 23.1% SST-25 immunopositive cells, 21.8% SST-14 immunopositive cells, and 1.3% PYY immunopositive cells. Islet cell topography was evaluated using histologic sections of whole endocrine pancreata including large, medium, and small islets. Round to polygonal insulin immunopositive cells with round central nuclei were distributed in clusters throughout both the principal and the smaller islets. Elongate SST-14 immunopositive cells were closely associated with the clusters of insulin immunopositive cells; both were surrounded by SST-25 immunopositive cells, which were similar in shape to the insulin immunopositive cells. There were elongate glucagon immunopositive cells throughout the islets, whereas the PYY immunopositive cells were restricted to the periphery and to channels of fibrovascular connective tissue penetrating the islets.

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.

IGF-I in the bony fish Cottus scorpius: cDNA, expression and differential localization in brain and islets

The cDNA encoding prepro-insulin-like growth factor (IGF)-I of a teleost, Cottus scorpius, (Scorpaeniformes) was established from liver by RT-PCR and molecular cloning. Typically, the deduced 184 amino acid protein contains a signal peptide, B-, C-, A-, D- and E-domains and all residues necessary for maintenance of tertiary structure. C. scorpius IGF-I shares only approximately 57% identity with C. scorpius insulin in the A-domain and 7% in the B-domain. RT-PCR followed by Southern blotting revealed a transcript in liver, pancreatic islets, stomach, small and large intestine, kidney, gill, testis, ovary, heart and brain indicating paracrine/autocrine actions of locally produced IGF-I. IGF-I- and insulin-immunoreactivities coexisted in the islets, but did not in other sites such as brain. Thus, in contrast to other bony fish, sculpin insulin cells most probably produce IGF-I. The results also challenge the current model of the IGF/insulin evolution.

An immunohistochemical and morphometric analysis of insulin, insulin-like growth factor I, glucagon, somatostatin, and PP in the development of the gastro-entero-pancreatic system of Xenopus laevis

The ontogeny of the classical islet hormones insulin (INS), glucagon (GLUC), somatostatin (SOM), and pancreatic polypeptide (PP) as well as insulin-like growth factor I (IGF-I) in the gastro-entero-pancreatic (GEP) system of Xenopus laevis (stages 41-66) was studied using double immunofluorescence and morphometric analysis. As early as stage 41, clustered INS-immunoreactive (-IR) and isolated GLUC-IR cells occurred in the pancreas. The first SOM-IR cells appeared at stage 43, followed by PP-IR cells at stage 46. About 79% of the PP immunoreactivity was confined to a subpopulation of the GLUC-IR cells. Both the GLUC/PP-IR cells and the PP-IR cells were located in a distinct area of the pancreas. The first islets occurred in premetamorphosis (around stage 50) and comprised mainly INS-IR and GLUC-IR cells. The majority of SOM-IR, PP-IR, and GLUC/PP-IR cells was dispersed. The numbers of hormone cells remained quite constant until the end of prometamorphosis (stage 58). Around stages 60-62, the islets were partly disintegrated and the numbers of islet cells slightly decreased. At stage 63, the cell number began to increase and reached the levels typical for the adult around stage 66. After metamorphic climax, the islets were reformed. In the gastrointestinal tract, transient INS-IR cells occurred prior to the adaptation of the gastrointestinal tract to feeding (stages 41-44) and during metamorphosis when there is remodeling of the gastrointestinal tract (stages 60-63). Therefore, INS released from the transient mucosal INS-IR cells may be involved in the temporary proliferation of mucosal epithelial cells. The first GLUC-IR and SOM-IR cells were seen at stage 41. PP-IR cells followed at stage 46. In contrast to the islets, GLUC-IR and PP-IR cells constituted different cell populations. Around stage 46, the first IGF-I immunoreactions appeared in the GEP-system. In pancreas, IGF-I immunoreactivity was found in the GLUC/PP-IR, cells (85-99%) but was absent from INS-IR, GLUC-IR, and SOM-IR cells. The IGF-I-IR gastro-entero-endocrine cells, however, seemed to contain none of the classical islet hormones.

Localization of IGF-I and IGF-I receptor mRNA in Sparus aurata larvae

Studies of the ontogeny of IGF-I mRNA during embryonic and larval development of the gilthead sea bream Sparus aurata showed its expression during these early developmental stages. The present study applies in situ hybridization to localize IGF-I and IGF receptor mRNAs in 16-day larvae of S. aurata. Paraffin sections were hybridized with homologous RNA probes labeled by [35S]UTP. IGF-I mRNA expression was found mainly in chondrocytes, in both the gill arches and cranial cartilage, in skeletal muscle, in the brain, in the pancreas, in the retina, and in the epithelial cells surrounding the lens. A strong positive reaction for IGF receptor mRNA was found in skeletal muscle, in the pancreas, and in the lymphoid tissue found in the intertubular tissue of the kidney. Signals were less intense in brain and chondrocytes. It is suggested that in teleosts, as in higher vertebrates, IGF-I may be involved in the regulation of tissue growth and differentiation in an autocrine/paracrine manner.

Insulin-like growth factor I in the teleost Oreochromis mossambicus, the tilapia: gene sequence, tissue expression, and cellular localization

Using reverse transcription-PCR and molecular cloning, the complementary DNA sequence encoding preproinsulin-like growth factor I (IGF-I) of a teleost, the tilapia (Oreochromis mossambicus) was established from liver. At the amino acid level, tilapia IGF-I shows all residues necessary for the maintenance of tertiary structure and shares about 80% identity with IGF-I from other teleosts. The B and A domains of tilapia IGF-I show more than 90% homology to those of other teleosts and 86-93% to those of human. However, in contrast to salmonids, the C domain of tilapia is truncated. Reverse transcription-PCR analysis followed by Southern blotting with an internal probe specific for tilapia IGF-I indicated a transcript in liver, pancreas, gut, kidney, head kidney, gill, ovary, testis, eye, and brain. In correlation, parenchymal cells were identified as likely local production sites by the use of immunohistochemistry. IGF-I immunoreactivity was confined to D cells in pancreatic islets, gastroentero-endocrine cells, cells of renal proximal tubules, interrenal cells of the head kidney, gill chondrocytes, chloride cells of the gill epithelium, granulosa cells in the ovary, spermatocytes and Sertoli cells in testis, and neurons in retina and brain. The local production of IGF-I in multiple organs of the tilapia indicates paracrine/autocrine actions of IGF-I involved in organ-specific functions. The results further demonstrate that the primary structure of IGF-I, especially in the B and A domains, is highly conserved during phylogeny.

Localization and expression of insulin-like growth factor in the teleost retina

Teleost fish retinas continue to add neurons throughout life, and evidence from in vitro experiments have implicated insulin-like growth factors (IGFs) in this process. To discover whether these factors are expressed in vivo, we have examined their expression in the cichlid fish, Haplochromis burtoni. Three lines of evidence show that IGFs are present in the fish retina. An IGF-I specific antibody, sm 1.2, binds preferentially to the retinal outer plexiform layer, in areas of cone photoreceptor synaptic endings. Northern blots of mRNA hybridized with riboprobes from trout IGF-I and IGF-II genes revealed transcripts of approximately 6.5 and 4.9 kb, respectively. The IGF-I probe detected an additional transcript of 1.2 kb in liver but not in retinal mRNA. In situ hybridization with digoxigenin-labeled riboprobes revealed that the IGF gene product is localized in the cone photoreceptors. These results show that cone photoreceptors are the source of IGFs in the fish retina, consistent with the hypothesis that IGFs play a role in regulating production of new neurons in the teleost retina.

Ontogeny of IGF-1 and the classical islet hormones in the turbot, Scophthalmus maximus

The ontogeny of the endocrine pancreas of a teleost, the turbot (Scophthalmus maximus), was investigated by the use of double immunofluorescence. Clustered insulin (INS)-immunoreactive (IR) cells were observed on the first day after hatching. During the following days, the islet largely increased in size and some smaller islets appeared. All islets consisted only of INS-IR cells. Between day 5 (onset of exogenous feeding) and 7, somatostatin (SOM) and glucagon (GLUC) cells appeared. In the large (principal) islet, the SOM-IR cells intermingled with the INS-IR cells. In the secondary islets, they occurred at the periphery. The GLUC-IR cells were located at the periphery in all islets. Subsequently, two-four additional small principal islets appeared. At day 11, pancreatic polypeptide (PP)-IR cells were present in principal islets and secondary islets. Starting with day 11, in all islets, insulin-like growth factor 1 (IGF-1) immunoreactivity was localized in numerous PP-IR cells and GLUC-IR and some SOM-IR cells. It also occurred in enteroendocrine cells that seemed to contain none of the classical islet hormones. The early appearance of INS correlates with its key role in the regulation of fish protein and lipid metabolism. Islet-derived IGF-1 might inhibit the regulation of INS secretion in a paracrine manner and may be highly involved in growth-promoting processes.

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.