Characterization of insulin-like growth factor binding proteins of cultured rat astroglial and neuronal cells

Survival of Purkinje Cells in Cerebellar Cultures is Increased by Insulin-like Growth Factor I

Insulin-like growth factor I (IGF-I) is a trophic factor for both neurons and glia. Its presence in the developing and adult cerebellum suggests a role for this growth factor in this area of the brain. Recently, we have described the existence of an IGF-I-containing pathway in afferents of Purkinje neurons arising from the inferior olive. In addition, IGF-I receptors are present in the molecular layer of the cerebellar cortex. These observations prompted us to investigate whether the Purkinje cell is a target for IGF-I. Addition of IGF-I to rat cerebellar cultures produced a 7-fold increase in the number of Purkinje cells (calbindin-positive) together with an increase in the calbindin content of the cultures. IGF-I also doubled the number of surviving neurons and produced a moderate, non-significant increase in [3H]thymidine incorporation by the cultures. On the other hand, basic fibroblast growth factor (bFGF), which is also present in the cerebellum, produced a dramatic increase in both the proportion of astrocytes and in the mitotic activity of the cultures, without affecting neuron survival. We conclude that IGF-I is a specific promoter of Purkinje cell survival and that its effects differ from those produced by bFGF in fetal cerebellar cultures. These findings reinforce our hypothesis that the Purkinje cell is a target neuron for IGF-I action in the developing cerebellum.

Overexpression of insulin-like growth factor-binding protein-2 in C6 glioma cells results in conditional alteration of cellular growth

To examine the relationship between the expression of insulin-like growth factor (IGF)-binding protein-2 (IGFBP-2) and cell growth in a cell type with a defined IGF/IGFBP system, an ovine IGFBP-2 complementary DNA was overexpressed in C6 glioma cells. C6 cells produce IGFBP-3, IGFBP-4, a negligible amount of IGFBP-2, and IGF-I. An ovine IGFBP-2 complementary DNA was transfected into C6 cells, and nine colonies that stably expressed variable levels of IGFBP-2 messenger RNA were selected. Synthesis of corresponding levels of IGFBP-2 was confirmed by ligand blot and immunoblot analyses of conditioned media. Three clones exhibited significantly reduced growth rates, and the remainder showed growth rates similar to those of the wild-type C6 cells. The clones, which overexpressed high levels of IGFBP-2 and IGF-I, had growth rates similar to the wild-type cells, whereas the three clones that overexpressed IGFBP-2 without a concomitant increase in IGF-I had reduced growth rates. In addition, a cell-associated IGFBP was identified in the slow growing clones, but not in the wild-type or the fast growing clones. This cell-associated IGFBP was deduced to be IGFBP-5 based on its molecular size, detection of IGFBP-5 messenger RNA only in slow growing clones, and competition of its binding by heparin. Growth of the slow growing clone, C6BP2-1, could not be overcome by the addition of exogenous IGF-I, suggesting that the cell-associated IGFBP-5 was the dominant regulator of IGF action. These observations suggested that 1) in C6 glioma cells cellular growth is altered by a disturbance in the equilibrium between IGF-I and IGFBPs and/or the functional properties of the IGFBPs; and 2) C6 cells may have a limited capacity to modulate IGF/IGFBP expression in response to changes in endogenous expression of IGFBPs. Endogenous regulation of the balance between IGFs and IGFBPs may be a model of regulation of cellular growth in tumor cells.

Insulin-like growth factors and IGF binding proteins in cyst fluid from patients with craniopharyngioma prior to intracavitary irradiation with Yttrium and thereafter

Aim-To examine a series of cyst fluid samples from patients with craniopharyngioma at various stages of treatment in order to evaluate the use of insulin-like growth factors (IGFs) and IGF binding proteins as tumour markers or indicators of successful treatment, or both.Methods-Cyst fluid samples were obtained by stereotactic puncture prior to the intracavitary application of (90)Yttrium and at subsequent occasions. Analysis was performed by gel chromatography, radio-immunoassays, binding studies, and sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) with subsequent western blotting.Results-IGF-I, -II and IGF binding protein-1 concentrations were measured in three craniopharyngioma cyst fluid samples. Immunoreactive IGF-I and IGF binding protein-1 concentrations in these three samples were between 6 and 29 ng/ml, and 17 and 48 ng/ml, respectively. In contrast, the IGF-II concentrations measured in 19 cyst fluid samples from seven patients with craniopharyngioma at various stages of treatment were much higher at 25-671 ng/ml. SDS-PAGE and subsequent western blotting using [(125)I]IGF-II as the ligand gave bands with estimated molecular weights of 330, 220, 135, 96, 46, 43, 34, 29, and 13.5 kDa in one adult, and identical bands at 220, 41.5, 37.5, 32, and 19 kDa in three cyst fluid samples from three children with craniopharyngioma.Conclusions-These results suggest that IGFs and IGF binding proteins are secreted by craniopharyngiomas and that they may alter the growth characteristics of these tumours. Furthermore, the distinct pattern of IGF binding protein sizes might be used as a tool for the differential diagnosis of tumours of the central nervous system.

The cerebral expression of plasma protein genes in different species

The cerebrospinal fluid (CSF) contains the same proteins as blood plasma, but with a different pattern of concentrations. Protein concentrations in CSF are much lower than those in blood. CSF proteins are derived from blood or synthesized within the brain. The choroid plexus is an important source of CSF proteins. Transthyretin is the protein most abundantly synthesized and secreted by choroid plexus. It determines the distribution of thyroxine in the cerebral compartment. Synthesis of transthyretin first evolved in the brain, then later it became a plasma protein synthesized in the liver. Other proteins secreted by choroid plexus are serum retinol-binding protein, transferrin, caeruloplasmin, insulin-like growth factors, insulin-like growth factor binding proteins, cystatin C, alpha 1-antichymotrypsin, alpha 2-macroglobulin, prothrombin, beta 2-microglobulin and prostaglandin D synthetase. Species differences in expression of the genes for these proteins are outlined, and their developmental pattern, regulation and roles in the cerebral extracellular compartment are discussed.

The insulin-like growth factor I system in the rat cerebellum: developmental regulation and role in neuronal survival and differentiation

The developmental regulation of insulin-like growth factor I (IGF-I), its receptor, and its binding proteins (IGFBPs) was studied in the rat cerebellum. All the components of the IGF-I system were detectable in the cerebellum at least by embryonic day 19. Levels of IGF-I receptor and its mRNA were highest at perinatal ages and steadily decrease thereafter, although a partial recovery in IGF-I receptor mRNA was found in adults. Levels of IGF-I and its mRNA also peaked at early ages, although immunoreactive IGF-I showed a second peak during adulthood. Finally, levels of IGFBPs were also highest at early postnatal ages and abruptly decreased thereafter to reach lower adult levels. Since highest levels of the different components of the IGF-I system were found at periods of active cellular growth and differentiation we also examined possible trophic effects of IGF-I on developing cerebellar cells in vitro. We found a dose-dependent effect of IGF-I on neuron survival together with a specific increase of the two main neurotransmitters used by cerebellar neurons, GABA and glutamate. Analysis of cerebellar cultures by combined in vitro autoradiography and immunocytochemistry with cell-specific markers indicated that both Purkinje cells (calbindin-positive) and other neurons (neurofilament-positive) contain IGF-I binding sites. These results extend previous observations on a developmental regulation of the IGF-I system in the cerebellum and reinforce the notion of a physiologically relevant trophic role of IGF-I in cerebellar development.

Alterations in the synthesis of insulin-like growth factor binding proteins and insulin-like growth factors in rat C6 glioma cells transfected with a gap junction connexin43 cDNA

When C6 glioma cells are stably transfected with a connexin43 cDNA and gap junctions are increased, the rate of cellular proliferation is decreased. To determine if this phenomenon is related to alterations in IGFBP and IGF synthesis, we have compared IGFBPs and IGFs in the conditioned media from primary rat astroglia, C6, and transfected C6 clones Cx43-13 (high expresser), and Cx43-12 and Cx43-14 (intermediate expressers). Primary astroglia produced IGFBP-2 (34 kDa) and IGFBP-3 (40-45 kDa). C6 cells synthesized high levels of IGFBP-3 and low levels of IGFBP-2, and a 24 kDa IGFBP (IGFBP-4). Cx43-13 cells did not synthesize IGFBP-3, but produced low levels of IGFBP-2 and high levels of IGFBP-4. Cx43-12 and Cx43-14 secreted IGFBP profiles similar to the parent C6 line, but with reduced levels of IGFBP-2. The lack of IGFBP-3 in Cx43-13 cells was not due to the presence of proteases. Northern analysis showed IGFBP-2 mRNA to be readily detectable only in the primary astroglia. IGFBP-3 mRNA was detected in the primary astroglia, C6, Cx43-12 and Cx43-14, but not in Cx43-13. In contrast, IGFBP-4 mRNA was readily detected only in the Cx43-13. IGF-II concentrations in the media were low to undetectable for both C6 and transfected cells. IGF-I concentrations were significantly lower in the media from transfected cells compared to the C6 cells. Stable mRNA levels for IGF-I were lower in transfected cells, with the lowest levels observed in the Cx43-13 cells. Although C6 cells did not respond mitogenically to exogenous IGF-I or IGF-II, Cx43-13 cells responded to IGF-I or IGF-II in a dose dependent manner. Conditioned media from Cx43-13 cells decreased the DNA synthesis of C6 cells, and this effect could be reversed by the addition of IGF-II. The decreased synthesis of the autocrine/paracrine growth factor IGF-I together with decreased levels of a positive modulator IGFBP-3, and the increased levels of a negative modulator IGFBP-4 in the extracellular milieu, may be responsible for the reduced proliferative capacity in cells expressing abundant connexin43.

Expression of insulin-like growth factor binding protein-4 (IGFBP-4) by rat neural cells--comparison to other IGFBPs

We recently isolated and characterized the 24 kDa and N-glycosylated 28 kDa insulin-like growth factor binding protein-4 (rIGFBP-4) from the B104s rat neuronal cell line (Endocrinology, 129 (1991) 1009-1115). To examine the prevalence of IGFBP-4 secretion by cells of neural origin, we assessed the expression of IGFBP-4 in different neural cell types using ligand blotting, immunoblotting and blot hybridization with relevant cDNAs. A specific IGFBP-4 antibody raised against a synthetic 20 amino acid peptide was used for immunologic recognition. In all the neural cells tested (B104s, C6 astrocytoma, primary neonatal astrocytes and primary fetal neurons), IGFBP-4 was definitively identified by immunoblotting. Blot hybridization using a rat cDNA probe revealed expression of IGFBP-4 mRNA transcripts by all these cells. Using a combination of the same techniques, expression of IGFBP-1, -2, and -3 were also examined. The B104s cells secreted primarily IGFBP-4; C6 cells secreted predominantly IGFBP-3 and small amount of IGFBP-4; both primary neonatal astrocytes and fetal neurons secreted IGFBP-2 as the major IGFBP accompanied by a small quantity of IGFBP-4. IGFBP-1 was not identified in any of the cell media. When probed with the respective IGFBP cDNAs, the mRNA abundance generally reflected the media IGFBP content. The expression of IGFBP-4 mRNA in vivo was examined as well and compared to that of IGFBP-1 and IGFBP-2. Transcripts for both IGFBP-2 and IGFBP-4 were found in all gross anatomical divisions of the rat brain from embryonic day 15 until adulthood, whereas IGFBP-1 was not detected at any time. IGFBP-4 mRNA tended to be more abundant at the youngest ages whereas IGFBP-2 increased during development. These data indicate that IGFBP-4 is produce by a variety of neural cell types and suggest that it may play a role in brain development.

Insulin-like growth factor (IGF)-I, -II and IGF-binding proteins in the cyst fluid of a patient with astrocytoma

Insulin-like growth factor (IGF)-I, -II and IGF-binding proteins (IGFBPs) were demonstrated in the cyst fluid of a patient with a hypothalamic astrocytoma. The astrocytoma cyst fluid was subjected to gel chromatography at low pH and the IGF-I and IGF-II levels were measured by specific radioimmunoassays. Immunoreactive IGF-I and IGF-II levels were 19 ng/ml and 78 ng/ml respectively. Several-fold higher IGF-II values were obtained when cyst fluid was not extracted or was extracted with acid ethanol before radioimmunoassay analysis. The immunoreactive IGFBP-1 concentration was 26 ng/ml. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and subsequent Western ligand blotting with [125I]IGF-II revealed bands at approximately 200, 34.5, 29.5, 24 and 21 kD as visualized by autoradiography. Binding studies demonstrated that these binding proteins bind specifically [125I]IGF-I and [125I]IGF-II. These observations suggest that IGFs as well as IGF-binding proteins are produced by astrocytoma cells and may act in a paracrine or autocrine fashion capable of modulating the growth of astrocytoma tumours.

Expression of insulin-like growth factor I by astrocytes in response to injury

Astrocytes are known to express several growth factors in response to injury and neurological disease. Insulin-like growth factor I (IGF-I) induces astrocytes to divide in vitro and is expressed by developing, but not adult astrocytes both in vivo and in vitro. We tested whether IGF-I is re-expressed by reactive astrocytes in response to injury. We found that astrocytes surrounding the lesioned parenchyma after introduction of a cannula through the cerebral cortex, hippocampus and midbrain contain high levels of immunoreactive IGF-I, as determined by immunocytochemistry using a highly sensitive and specific anti-IGF-I monoclonal antibody. Interestingly, the contralateral hippocampus also contained IGF-I positive astrocytes although in substantial lower numbers. Intact animals showed no detectable IGF-I immunoreactivity in astrocytes. IGF-I was detected at the first time point tested after the lesion was made, 1 week, and for at least 1 month thereafter. Reactive astrocytes expressing high levels of glial fibrillary acidic protein were found in a much wider distribution all along the lesioned area and beyond. We conclude that mechanical injury of the brain induces a specific pattern of expression of IGF-I by a subpopulation of astrocytes. These findings suggest that IGF-I is participating in the response of astrocytes to injury.

Glucose, insulin, and insulin-like growth factor I regulate the glycogen content of astroglia-rich primary cultures

The glycogen content of astroglia-rich primary cultures derived from the brains of newborn rats depends on the concentration of glucose in the culture medium. After administration of culture medium lacking glucose, the glycogen content decreases with a half-time of 7 min. Readdition of glucose results in replenishment of the glycogen stores within 2-3 h, but fully only if glucose is present in a concentration of at least 4 mM. Insulin, or the more potent insulin-like growth factor I, increases the content of glycogen approximately 1.7-fold, with the half-maximal effects being attained at concentrations of 10 and 0.5 nM, respectively. These results suggest that (a) glucose or a metabolite of it and (b) insulin-like growth factor I or a closely related peptide, but not insulin, are likely to be physiological regulators of the level of glycogen in astrocytes.

Climbing fiber deafferentation reduces insulin-like growth factor I (IGF-I) content in cerebellum

The presence of insulin-like growth factor I (IGF-I) and its mRNA in adult rat cerebellum has recently been documented. Previous immunocytochemical studies showed prominent IGF-I-like staining in fibers around Purkinje cell somas. To determine the origin of this IGF-I input to the Purkinje cell we destroyed the inferior olivary complex by either 3-acetylpiridine administration or electrolytical lesions. In both types of lesions we found a similar significant depletion of IGF-I levels in cerebellum (40-50% of controls). No changes were found in cerebellar IGF-I receptors. These results suggest that almost half of the IGF-I content in cerebellum is provided by climbing fiber afferents arising from the inferior olivary complex.

Ontogeny of insulin-like growth factor I, its receptor, and its binding proteins in the rat hypothalamus

A role for the insulin-like growth factors (IGFs) in brain growth and differentiation has recently been suggested. In previous studies on fetal hypothalamic cells we found a trophic influence of IGF-I on in vitro survival and differentiation of both neurons and glia. We have now investigated the expression of IGF-I, its receptor and its binding proteins in the rat hypothalamus to determine whether endogenous IGF-I might serve as a trophic factor during development of this brain area. Both IGF-I receptors and IGF-I binding proteins showed marked developmental stage-dependent variations. Thus, IGF-I receptors as measured by both binding and cross-linking techniques, were highest during fetal life and steadily decreased thereafter to reach low adult levels. Changes in receptor numbers rather than in its affinity constant accounted for the differences seen in binding activity during development. In addition, we found 3 different IGF-I binding proteins (IGFBPs) of apparent Mr of 24, 29 and 32 kDa respectively, whose levels also showed a specific developmental pattern. Highest levels of the 29 and 32 kDA IGFBPs were found in fetal and early postnatal life, whereas levels of the 24 kDa form were highest in young adults. Changes in the concentration of IGFBPs rather than in their affinities for IGF-I accounted for the different binding capacities found. Using a specific IGF-I radioimmunoassay we found that IGF-I-like immunoreactivity (IGF-I-li) levels had no direct correlation with developmental stage. IGF-I-li levels oscillated with no apparent trend throughout development of the hypothalamus.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequence analysis, expression and chromosomal localization of a gene, isolated from a subtracted human retina cDNA library, that encodes an insulin-like growth factor binding protein (IGFBP2)

The metabolic functions of insulin-like growth factors (IGFs) I and II are modulated by a family of binding proteins which are present in biological fluids and are synthesized by a variety of cell types. A cDNA clone, isolated at random from a subtracted human retina library, has been identified to code for a novel IGF-binding protein (IGFBP2) by its sequence homology to the peptide sequence of IGF binding proteins purified from bovine MDBK and rat BRL-3A cells. The complete nucleotide sequence of the IGFBP2 cDNA is 1406 bp long, contains 66% G-Cs and an open reading frame of 328 amino acids with a putative signal or pro-peptide of 39 residues. The mature polypeptide of 289 amino acids has 18 cysteines, a putative ATP-binding site and an RGD tripeptide. The 1.4 kb IGFBP2 transcript is expressed in several human tissues including fetal eye and fetal brain, but not in the human lymphoblastoid cell line against which the retinal cDNA library was subtracted. In situ hybridization to sections of mouse retina localized the mRNA for IGFBP2 primarily in the outer nuclear layer of photoreceptors. Southern blot analysis of DNA from human x rodent and mouse x rodent somatic cell hybrids assigned the gene for IGFBP2 to human chromosome 2q33-qter and mouse chromosome 1 in a known conserved syntenic region.

FGFs stimulate IGF binding protein synthesis without affecting IGF synthesis in rat astroblasts in primary culture

The production of insulin-like growth factor (IGF)-I and -II and their binding proteins (BPs) has been studied in new-born rat astroblasts at confluency in primary culture. Under the influence of fibroblast growth factors (FGFs) (acidic and basic), the morphology of the astroblasts was altered, 125I-deoxyuridine incorporation was increased, and glutamine synthetase activity was stimulated. IGF production and IGF mRNA expression remained unchanged. Production of the 32 kDa BP (IGFBP-2), the sole or predominant form under base-line conditions, was enhanced and the 43-39 kDa forms (IGFBP-3) appeared or were increased. Epidermal growth factor (EGF) also stimulated production of these BPs, whereas thrombin and db-cAMP had no effect. Our data suggest that a relationship exists between FGF-induced maturation of astroblasts and the forms of BP they produce. The data also indicate that some factors may act specifically on BP synthesis, without affecting IGF synthesis, and in this way play a role in regulating the bioavailability of the IGFs.

Isolation of an inhibitory insulin-like growth factor (IGF) binding protein from bone cell-conditioned medium: a potential local regulator of IGF action

Inhibitory insulin-like growth factor binding protein (In-IGF-BP) has been purified to homogeneity from medium conditioned by TE89 human osteosarcoma cells by two different methods using Sephadex G-100 gel filtration, FPLC Mono Q ion-exchange, HPLC C4 reverse-phase, HPLC CN reverse-phase, and affinity chromatographies. In-IGF-BP thus purified appeared to be homogeneous and unique by the following criteria. (i) N-terminal sequence analysis yielded a unique sequence (Asp-Glu-Ala-Ile-His-Cys-Pro-Pro-Glu-Ser-Glu-Ala-Lys-Leu-Ala). (ii) Amino acid composition of In-IGF-BP revealed marked differences with the amino acid compositions of other known BPs. (iii) In-IGF-BP exhibited a single band with a molecular mass of 25 kDa under reducing conditions on sodium dodecyl sulfate/polyacrylamide gels. IGF-I and IGF-II but not insulin displaced the binding of 125I-labeled IGF-I or 125I-labeled IGF-II binding to In-IGF-BP. In-IGF-BP inhibited basal, IGF-stimulated bone cell proliferation and serum-stimulated bone cell proliferation. Forskolin increased synthesis of In-IGF-BP in TE85 human osteosarcoma cells in a dose-dependent manner. Based on these findings, we conclude that In-IGF-BP is a protein that has a unique sequence and significant biological actions on bone cells.