Phylogeny of the insulin-like growth factors (IGFs) and receptors: a molecular approach

Radiolabeled hormones in insulin research, a minireview

Preparation of both ¹²⁵ I-labeled insulin and insulin-like growth factor 1 (IGF-1) was critical because it enabled a detailed characterization of binding properties of these important hormones towards their cognate transmembrane receptors. Binding modes of hundreds of hormone derivatives were analyzed using competition radioligand binding assays. This effort has resulted in development of six insulin analogs that are today clinically used for the treatment of diabetes. Here, we will briefly summarize a history of insulin research employing iodinated hormones.

The IGF-II-Insulin Receptor Isoform-A Autocrine Signal in Cancer: Actionable Perspectives

Insulin receptor overexpression is a common event in human cancer. Its overexpression is associated with a relative increase in the expression of its isoform A (IR^A), a shorter variant lacking 11 aa in the extracellular domain, conferring high affinity for the binding of IGF-II along with added intracellular signaling specificity for this ligand. Since IGF-II is secreted by the vast majority of malignant solid cancers, where it establishes autocrine stimuli, the co-expression of IGF-II and IR^A in cancer provides specific advantages such as apoptosis escape, growth, and proliferation to those cancers bearing such a co-expression pattern. However, little is known about the exact role of this autocrine ligand–receptor system in sustaining cancer malignant features such as angiogenesis, invasion, and metastasis. The recent finding that the overexpression of angiogenic receptor kinase EphB4 along with VEGF-A is tightly dependent on the IGF-II/IR^A autocrine system independently of IGFIR provided new perspectives for all malignant IGF2omas (those aggressive solid cancers secreting IGF-II). The present review provides an updated view of the IGF system in cancer, focusing on the biology of the autocrine IGF-II/IR^A ligand–receptor axis and supporting its underscored role as a malignant-switch checkpoint target.

Below the Surface: IGF-1R Therapeutic Targeting and Its Endocytic Journey

Ligand-activated plasma membrane receptors follow pathways of endocytosis through the endosomal sorting apparatus. Receptors cluster in clathrin-coated pits that bud inwards and enter the cell as clathrin-coated vesicles. These vesicles travel through the acidic endosome whereby receptors and ligands are sorted to be either recycled or degraded. The traditional paradigm postulated that the endocytosis role lay in signal termination through the removal of the receptor from the cell surface. It is now becoming clear that the internalization process governs more than receptor signal cessation and instead reigns over the entire spatial and temporal wiring of receptor signaling. Governing the localization, the post-translational modifications, and the scaffolding of receptors and downstream signal components established the endosomal platform as the master regulator of receptor function. Confinement of components within or between distinct organelles means that the endosome instructs the cell on how to interpret and translate the signal emanating from any given receptor complex into biological effects. This review explores this emerging paradigm with respect to the cancer-relevant insulin-like growth factor type 1 receptor (IGF-1R) and discusses how this perspective could inform future targeting strategies.

Neurotropic and modulatory effects of insulin-like growth factor II in Aplysia

Insulin-like growth factor II (IGF2) enhances memory in rodents via the mannose-6-phosphate receptor (M6PR), but the underlying mechanisms remain poorly understood. We found that human IGF2 produces an enhancement of both synaptic transmission and neurite outgrowth in the marine mollusk Aplysia californica. These findings were unexpected since Aplysia lack the mammal-specific affinity between insulin-like ligands and M6PR. Surprisingly, this effect was observed in parallel with a suppression of neuronal excitability in a well-understood circuit that supports several temporally and mechanistically distinct forms of memory in the defensive withdrawal reflex, suggesting functional coordination between excitability and memory formation. We hypothesize that these effects represent behavioral adaptations to feeding that are mediated by the endogenous Aplysia insulin-like system. Indeed, the exogenous application of a single recombinant insulin-like peptide cloned from the Aplysia CNS cDNA replicated both the enhancement of synaptic transmission, the reduction of excitability, and promoted clearance of glucose from the hemolymph, a hallmark of bona fide insulin action.

'Beauty and the beast' in infection: How immune-endocrine interactions regulate systemic metabolism in the context of infection

The immune and endocrine systems ensure two vital functions in the body. The immune system protects us from lethal pathogens, whereas the endocrine system ensures proper metabolic function of peripheral organs by regulating systemic homeostasis. These two systems were long thought to operate independently. The immune system uses cytokines and immune receptors, whereas the endocrine system uses hormones to regulate metabolism. However, recent findings show that the immune and endocrine systems closely interact, especially regarding regulation of glucose metabolism. In response to pathogen encounter, cytokines modify responsiveness of peripheral organs to endocrine signals, resulting in altered levels of blood hormones such as insulin, which promotes the ability of the body to fight infection. Here we provide an overview of recent literature describing various mechanisms, which the immune system utilizes to modify endocrine regulation of systemic metabolism. Moreover, we will describe how these immune-endocrine interactions derail in the context of obesity. From a clinical perspective we will elaborate how infection and obesity aggravate the development of metabolic diseases such as diabetes mellitus type 2 in humans. In summary, this review provides a comprehensive overview of immune-induced changes in systemic metabolism following infection, with a focus on regulation of glucose metabolism.

Data for evolutive analysis of insulin related peptides in bilaterian species

In bilaterian species, the amino acid sequence conservation between Insulin related peptides is relatively low except for the cysteine residues involved in the disulphide bonds. In the A chain, the conserved cystein residues are included in a signature motif. Investigating the variations in this motif would give insight into the phylogenetic history of the family. The table presented in this paper contains a large set of insulin-related peptides in bilateral phylogenetic groups (deuterostomian, ecdysozoan, lophotrochozoan). NCBI databases in silico wide screening combined with bibliographic researches provided a framework for identifying and categorising the structural characteristics of these insulin related peptides. The dataset includes NCBI IDs of each sequence with hyperlinks to FASTA format. Moreover, the structural type (α, β or γ), the A chain motif, the total number of cysteins, the C peptide cleavage mode and the potential additional domains (D or E) are specified for each sequence. The data are associated with the research article "Molecular evolution and functional characterisation of insulin-related peptides in molluscs: contributions of Crassostrea gigas genomic and transcriptomic-wide screening" [1]. The table presented here can be found at http://dx.doi.org/10.17632/w4gr8zcpk5.4#file-21c0f6a5-a3e3-4a15-86e0-e5a696458866.

Anti-insulin-like growth factor therapy in breast cancer

Early preclinical and population data suggested a role for the type I insulin-like growth factor receptor (IGF1R) in the regulation of breast cancer growth and survival. To target this pathway, multiple monoclonal antibodies and tyrosine kinase inhibitors were developed and tested in clinical trials. While some of the early clinical trials suggested a benefit for these drugs, none of the attempts showed improved outcomes when compared to conventional therapy. This failure of the IGF1R inhibitors was pronounced in breast cancer; multiple trials testing IGF1R inhibition in estrogen receptor-positive breast cancer were conducted, none showed benefit. This review will evaluate the rationale for IGF1R inhibition, discuss results of the clinical trials and suggest a path forward.

The regulation of reproductive neuroendocrine function by insulin and insulin-like growth factor-1 (IGF-1)

The mammalian reproductive hormone axis regulates gonadal steroid hormone levels and gonadal function essential for reproduction. The neuroendocrine control of the axis integrates signals from a wide array of inputs. The regulatory pathways important for mediating these inputs have been the subject of numerous studies. One class of proteins that have been shown to mediate metabolic and growth signals to the CNS includes Insulin and IGF-1. These proteins are structurally related and can exert endocrine and growth factor like action via related receptor tyrosine kinases. The role that insulin and IGF-1 play in controlling the hypothalamus and pituitary and their role in regulating puberty and nutritional control of reproduction has been studied extensively. This review summarizes the in vitro and in vivo models that have been used to study these neuroendocrine structures and the influence of these growth factors on neuroendocrine control of reproduction.

Involvement of IGF-II receptors in the antioxidant and neuroprotective effects of IGF-II on adult cortical neuronal cultures

Insulin-like growth factor-II (IGF-II) is a naturally occurring peptide that exerts known pleiotropic effects ranging from metabolic modulation to cellular development, growth and survival. IGF-II triggers its actions by binding to and activating IGF (IGF-I and IGF-II) receptors. In this study, we assessed the neuroprotective effect of IGF-II on corticosterone-induced oxidative damage in adult cortical neuronal cultures and the role of IGF-II receptors in this effect. We provide evidence that treatment with IGF-II alleviates the glucocorticoid-induced toxicity to neuronal cultures, and this neuroprotective effect occurred due to a decrease in reactive oxygen species (ROS) production and a return of the antioxidant status to normal levels. IGF-II acts via not only the regulation of synthesis and/or activity of antioxidant enzymes, especially manganese superoxide dismutase, but also the restoration of mitochondrial cytochrome c oxidase activity and mitochondrial membrane potential. Although the antioxidant effect of IGF-I receptor activation has been widely reported, the involvement of the IGF-II receptor in these processes has not been clearly defined. The present report is the first evidence describing the involvement of IGF-II receptors in redox homeostasis. IGF-II may therefore contribute to the mechanisms of neuroprotection by acting as an antioxidant, reducing the neurodegeneration induced by oxidative insults. These results open the field to new pharmacological approaches to the treatment of diseases involving imbalanced redox homeostasis. In this study, we demonstrated that the antioxidant effect of IGF-II is at least partially mediated by IGF-II receptors.

Anabolic effects of IGF-1 signaling on the skeleton

This review focuses on the anabolic effects of IGF-1 signaling on the skeleton, emphasizing the requirement for IGF-1 signaling in normal bone formation and remodeling. We first discuss the genomic context, splicing variants, and species conservation of the IGF-1 locus. The modulation of IGF-1 action by growth hormone (GH) is then reviewed while also discussing the current model which takes into account the GH-independent actions of IGF-1. Next, the skeletal phenotypes of IGF-1-deficient animals are described in both embryonic and postnatal stages of development, which include severe dwarfism and an undermineralized skeleton. We then highlight two mechanisms by which IGF-1 exerts its anabolic action on the skeleton. Firstly, the role of IGF-1 signaling in the modulation of anabolic effects of parathyroid hormone (PTH) on bone will be discussed, presenting in vitro and in vivo studies that establish this concept and the proposed underlying molecular mechanisms involving Indian hedgehog (Ihh) and the ephrins. Secondly, the crosstalk of IGF-1 signaling with mechanosensing pathways will be discussed, beginning with the observation that animals subjected to skeletal unloading by hindlimb elevation are unable to mitigate cessation of bone growth despite infusion with IGF-1 and the failure of IGF-1 to activate its receptor in bone marrow stromal cell cultures from unloaded bone. Disrupted crosstalk between IGF-1 signaling and the integrin mechanotransduction pathways is discussed as one of the potential mechanisms for this IGF-1 resistance. Next, emerging paradigms on bone-muscle crosstalk are examined, focusing on the potential role of IGF-1 signaling in modulating such interactions. Finally, we present a future outlook on IGF research.

New insulin-like growth factor (IGF)-precursor sequences from mammalian genomes: the molecular evolution of IGFs and associated peptides in primates

The insulin-like growth factors (IGF-I and IGF-II) and insulin are related proteins that play an important role in regulation of metabolism and growth. In mammals these proteins are generally strongly conserved, though the sequence of insulin underwent periods of rapid change during the evolution of hystricomorph rodents and new-world monkeys (NWM). The availability of genomic sequence information for a number of mammals provides gene sequences for insulin and IGF precursors from several new species, and this has been used here to study the evolution of these proteins in primates. The sequence of insulin is strongly conserved in primates except for the branch leading to NWM - the sequence of marmoset insulin confirms the episode of rapid evolution in this lineage. Strongly conserved sequences are also seen for IGF-I and IGF-II, though for IGF-I (but not IGF-II) the marmoset sequence again shows an episode of fairly rapid evolution, paralleling the changes seen in insulin. Thus in NWM the sequences of insulin and IGF-I show a co-evolution that may reflect a coordinated change in the functional properties of these two molecules. The other components of the insulin and IGF precursors (signal peptides, E-domains of IGFs, insulin C-peptide) are much less strongly conserved, though to a variable extent. Signal peptides are generally quite variable, but the sequence encoding the N-terminal region of the unusually long signal peptide of IGF-I is strongly conserved, suggesting specific function(s), at least partly associated with nucleotide rather than protein sequence. The Ea domain of proIGF-I and the N-terminal end of the E-domain of proIGF-II are quite strongly conserved, which accords with reports of a biologically active peptide (preptin) derived from the latter. However, the C-terminal parts of the Eb and Ec domains of proIGF-I (produced by alternative splicing) are very variable, which is of interest in view of reports of peptides with important biological activities deriving from these regions.

Polymorphisms of the IGF1R gene and their genetic effects on chicken early growth and carcass traits

BACKGROUND

The insulin-like growth factor I receptor (IGF1R) has an important effect on growth, carcass, and meat quality traits in many species. However, few studies on associations of the IGF1R gene with growth and carcass traits have been reported in chickens. The objectives of the present study were to study the associations of the IGF1R gene with chicken early growth and carcass traits using a neutral test, variation scan of the gene, genetic diversity, linkage disequilibrium and association analyses.

RESULTS

The tree generated from the amino acid sequences of 15 species showed that the IGF1R gene was conservative in the whole evolution among the mammalian animals and chickens. In a total of 10,818 bp of sequence, 70 single nucleotide polymorphisms were identified in the chicken IGF1R gene. The allelic and genotypic frequency distribution, genetic diversity and linkage disequilibrium of 18 single nucleotide polymorphisms (SNPs) in the Xinghua and White Recessive Rock chickens showed that six of them were possibly associated with growth traits. Association analyses showed that the A17299834G SNP was significantly associated with chicken carcass body weight, eviscerated weight with giblets, eviscerated weight, body weights at 28, 35, and 56 d of age, leg length at 56 d of age, and daily weight gain at 0-4 weeks. The haplotypes of the A17307750G and A17307494G were associated with early growth traits. The haplotypes of the A17299834G and C17293932T were significantly associated with most of the early growth traits and carcass traits.

CONCLUSION

There were rich polymorphisms in the chicken IGF1R gene. Several SNPs associated with chicken early growth traits and carcass traits were identified in the IGF1R gene by genetic diversity, linkage disequilibrium, and association analyses in the present study.

Muscle-specific overexpression of the type 1 IGF receptor results in myoblast-independent muscle hypertrophy via PI3K, and not calcineurin, signaling

The insulin-like growth factors (IGF-I and IGF-II), working through the type 1 IGF receptor (IGF-1R), are key mediators of skeletal muscle fiber growth and hypertrophy. These processes are largely dependent on stimulation of proliferation and differentiation of muscle precursor cells, termed myoblasts. It has not been rigorously determined whether the IGFs can also mediate skeletal muscle hypertrophy in a myoblast-independent fashion. Similarly, although the phosphatidylinositol 3-kinase (PI3K) and calcineurin signaling pathways have been implicated in skeletal muscle hypertrophy, these pathways are also involved in skeletal myoblast differentiation. To determine whether the IGFs can stimulate skeletal muscle hypertrophy in a myoblast-independent fashion, we developed and validated a retroviral expression vector that mediated overexpression of the human IGF-1R in rat L6 skeletal myotubes (immature muscle fibers), but not in myoblasts. L6 myotubes transduced with this vector accumulated significantly higher amounts of myofibrillar proteins, in a ligand- and receptor-dependent manner, than controls and demonstrated significantly increased rates of protein synthesis. Stimulation of myotube hypertrophy was independent of myoblast contributions, inasmuch as these cultures did not exhibit increased levels of myoblast proliferation or differentiation. Experiments with PI3K and calcineurin inhibitors indicated that myoblast-independent myotube hypertrophy was mediated by PI3K, but not calcineurin, signaling. This study demonstrates that IGF can mediate skeletal muscle hypertrophy in a myoblast-independent fashion and suggests that muscle-specific overexpression of the IGF-1R or stimulation of its signaling pathways could be used to develop strategies to ameliorate muscle wasting without stimulating proliferative pathways leading to carcinogenesis or other pathological sequelae.

Autoradiographic and immunohistochemical localization of insulin-like growth factor-I receptor binding sites in brain of the brown trout, Salmo trutta

Insulin-like growth factor-I (IGF-I), a peptide closely related to insulin, is known to play crucial roles in brain development. While the central sites of action of IGF-I in higher vertebrates are now well established, surprisingly little is known in the teleost model where the brain undergoes continual, indeterminate, growth. In this study, we have mapped the distribution of putative IGF-I receptor (IGF-IR) binding sites in the brain of the brown trout using both ligand binding in vitro autoradiography and immunohistochemistry. The presence of IGF binding proteins (IGFBPs) was further studied by competitive inhibition using unlabelled IGF-I and des-(1-3)-IGF-I. In both juvenile and adult trout brain, [125I]IGF-I binding was highest in cerebellum and optic tectum, both regions of the teleost brain known to grow the most actively throughout life. At the cellular level, IGF-IR immunoreactivity was confirmed on cell bodies and dendrites, particularly of larger presumptive neurons including purkinje cells and dendritic fibres throughout the molecular layer of the cerebellum. Abundant IGF-IR expression in hypothalamic regions may further be related to neuron growth while a possible hypophysiotropic role will require further investigation. Competitive inhibition studies employing des-(1-3)-IGF-I also suggest IGFBPs are present in all regions exhibiting high [125I]IGF-I ligand binding and confirms the presence of this important regulatory component of the IGF-I system in the teleost brain. The importance of the IGF-I system in brain development, particularly in regions such as the cerebellum, together with the continual lifetime growth of the fish central nervous system, suggest the teleost brain is an extremely useful site for studying the actions of IGF-I in relation to neuron proliferation, growth, and survival in an adult brain.

Isolation and biological characterization of a 6-kDa protein from hepatopancreas of lobster Panulirus argus with insulin-like effects

A protein with insulin-like effects was isolated from the hepatopancreas of the lobster Panulirus argus following a classic method for mammalian insulin purification from the pancreas. After acid-alcoholic extraction and ethanol-ether precipitation followed by molecular filtration chromatography, a protein with an apparent molecular weight of 6 kDa was isolated. This protein is characterized by its ability to interact with anti-insulin antibodies and by mimicking insulin actions as the stimulation of glucose oxidation to CO(2) and lipogenesis in isolated rat adipocytes. In addition, this insulin immunoreactive protein (IIP) was able to stimulate the autophosphorylation of the insulin receptor present in rat adipocyte plasma membranes, in a dose-dependent manner. The immunological and biochemical results obtained are consistent with the hypothesis that protein(s) with insulin-like effects occur in the digestive gland of the lobster P. argus and may be of significance to control metabolic and growth related processes in crustaceans.

Growth factors regulate phototransduction in retinal rods by modulating cyclic nucleotide-gated channels through dephosphorylation of a specific tyrosine residue

Illumination of vertebrate rod photoreceptors leads to a decrease in the cytoplasmic cGMP concentration and closure of cyclic nucleotide-gated (CNG) channels. Except for Ca(2+), which plays a negative feedback role in adaptation, and 11-cis-retinal, supplied by the retinal pigment epithelium, all of the biochemical machinery of phototransduction is thought to be contained within rod outer segments without involvement of extrinsic regulatory molecules. Here we show that insulin-like growth factor-I (IGF-I), a paracrine factor released from the retinal pigment epithelium, alters phototransduction by rapidly increasing the cGMP sensitivity of CNG channels. The IGF-I-signaling pathway ultimately involves a protein tyrosine phosphatase that catalyzes dephosphorylation of a specific residue in the alpha-subunit of the rod CNG channel protein. IGF-I conjointly accelerates the kinetics and increases the amplitude of the light response, distinct from events that accompany adaptation. These effects of IGF-I could result from the enhancement of the cGMP sensitivity of CNG channels. Hence, in addition to long-term control of development and survival of rods, growth factors regulate phototransduction in the short term by modulating CNG channels.

Receptors for insulin-like growth factor-I (IGF-I) predominate over insulin receptors in skeletal muscle throughout the life cycle of brown trout, Salmo trutta

Insulin and IGF-I binding has been studied in brown trout (Salmo trutta) wheat germ agglutinin semipurified receptors from embryos (organogenesis), larvae (yolk sac), juveniles (2.98 +/- 0.21 g bw) and adults (111.6 +/- 6.92 and 522 +/- 53 g bw). Embryos and larvae were sampled at 5 and 12 weeks after fertilization (December 1999 and February 2000) and juvenile and adults were taken simultaneously (July 1999) and under the same feeding conditions to minimize potential nutritional and seasonal effects. Insulin receptor number was maximal at 12 weeks (144 fmol/mg glycoprotein) and progressively decreased in subsequent samplings. No alterations in affinity were detected (K(d) range, 0.21-0.32 nM) and changes in number of receptor paralleled changes in total specific binding. IGF-I receptor number was highest at 5 weeks (1044 fmol/mg) and was significantly higher than values for insulin in all samplings. The affinity of IGF-I receptor did not change (K(d) range, 0.11-0.18 nM) but was consistently higher than that for the insulin receptor. A more rapid decrease of IGF-I binding and receptor number was found with age. However, the ratio of insulin/IGF-I binding established in 12-week-old larvae (0.18 +/- 0.01) was thereafter maintained at very similar values in juveniles and adults (0.15-0.17). Tyrosine kinase activity (TKA) for insulin receptors ranged between 136 and 183% and there were no significant changes with age. For the IGF-I receptor, TKA ranged from 174 to 281% and was significantly higher in 5-week-old larvae coincident with the highest levels of receptor number and declined gradually in parallel with binding levels. In conclusion, the greater abundance of IGF-I receptors during embryonic and larval development is maintained throughout juvenile and adult stages. This would suggest a key role for IGF-I in the growth and metabolism of trout muscle.

Insulin-like growth factor binding proteins in cerebrospinal fluid during human development and aging

We analyzed samples of insulin-like growth factor binding proteins (IGFBPs) in human cerebrospinal fluid (CSF) in neurologically normal patients from one day after birth to age 76 years. CSF samples were separated on SDS-PAGE and then transferred to nitrocellulose membranes where IGFBPs were detected by Western ligand blot using [(125)I]-IGF-II, confirming other reports where we found the presence of IGFBP-2, 3, 4, 5. The 34 kDa IGFBP-2 was present in all samples, and progressively decreased with age. A broad 28- to 30-kDa IGFBP band, having the appearance of IGFBP-5, was triphasic: faint during infancy, barely detectable at 6 months, but intense in adult and aged individuals. The 24-kDa IGFBP-4 band was only seen in neonatal CSF samples, while the IGFBP-3 doublet gradually increased during aging. Thus, these present results show that IGFBP-2, 3, 4 and 5 in CSF are developmentally regulated, suggesting roles for these molecules in the development of the nervous system.

Appearance of insulin and insulin-like growth factor-I (IGF-I) receptors throughout the ontogeny of brown trout (Salmo trutta fario)

Insulin and IGF-I receptors were characterized in glycoprotein fractions prepared by affinity chromatography from different developmental stages of brown trout. The specificity of insulin and IGF-I binding was demonstrated by crossed-competition assays: unlabelled insulin displaced bound radiolabelled insulin at concentrations 45-fold lower than unlabelled IGF-I, whilst unlabelled IGF-I displaced bound radiolabelled IGF-I at concentrations 2,000-fold lower than unlabelled insulin. The affinity of these receptors did not change significantly during trout development. Insulin-specific binding was detectable 3 weeks after spawning, after which it increased to a maximum in fry weighing 0.4 g, and decreased progressively to adult levels. IGF-I specific binding was detectable in newly laid eggs and increased to a maximum during organogenesis in eyed eggs. It then decreased progressively during subsequent stages of development to adult levels. The apparent molecular weight (Mr) of the alpha-subunit of brown trout insulin and IGF-I receptors was smaller than that of the alpha-subunit of the rat insulin receptor. Receptor tyrosine kinase activity was stimulated in a dose-dependent manner by insulin and IGF-I. Insulin and IGF-I stimulated tyrosine kinase activity and reached a maximum of 201 +/- 17.6 and 240 +/- 29.6% of basal phosphorylation, respectively.

Isolation and characterization of tilapia (Oreochromis mossambicus) insulin-like growth factors gene and proximal promoter region

To understand the molecular mechanism which controls the transcription of the insulin-like growth factors (IGFs) gene, we have cloned and sequenced the cDNA for the proximal promoter region of the tilapia IGFs gene and have characterized its activity by chloramphenicol acetyltransferase (CAT) transient transfected expression assays. Tilapia (Oreochromis mossambicus) IGF-I cDNA (549 bp) was amplified by PCR from single-stranded cDNA of growth hormone (GH)-induced liver RNA using a pair of oligonucleotides specific for fish IGF-I as amplification primers. Tilapia IGF-I and IGF-II 5' termini were analyzed by rapid amplification of cDNA 5' ends (5'RACE). Analysis of the 5'RACE results revealed two transcription start sites in IGF-I and one transcription start site in IGF-II. Different fragments of the 5' flanking region were transfected into human lung adenocarcinoma cells. In the cell line, maximum promoter activity was located in the distal 657 basepairs of the IGF-I 5' flanking region and in the distal 450 basepairs of the IGF-II 5' flanking region. The in vivo actions of the IGFs promoter on developmental stage expression were investigated further in transgenic zebrafish in which an IGFs promoter-driven green fluorescent protein (GFP) encoding the cDNA transgene was microinjected into embryos. Morphologic and RT-PCR studies of the transgenic zebrafish indicated that IGF-I promoter-driven GFP transcripts appeared for the first time in the 1-K-cell stage and the IGF-II promoter-driven GFP transcripts appeared for the first time in the 32-cell stage. Fluorescent (GFP) distribution was apparent within 48 h in IGF-II-transgenic zebrafish embryos, especially in eye, muscle, corpuscle, floor plate, horizontal myoseptum, yolk sac extension, and yolk sac. These results indicate that the IGF-I and IGF-II promoters are active in tissue and in a development-specific manner. Our findings also indicate that the IGF-II promoter influences the growth of fish embryos earlier than does IGF-I, and IGF-II has higher levels of expression than does IGF-I. These results suggest that the IGF-II promoter plays a growth factor role in teleost embryo development.

Characterization of the insulin-like growth factor type 1 receptor messenger in two teleost species

The insulin-like growth factor type 1 receptor (IGF-1R) is a tyrosine kinase which plays essential role in the regulation of growth and development. In this study, we have cloned cDNAs encoding the tyrosine kinase domain of the IGF-1R from two species of fish. The turbot and trout nucleotide sequences share 82% identity. Moreover, the deduced polypeptides are also highly conserved (> 90% identity) compared with the IGF-1R sequences described in other vertebrates, particularly within domains involved in the catalytic activity and in the transduction pathway. Northern blot analyses have revealed a unique 13-kb mRNA transcript. Using an RT-PCR approach, we have also shown that the polyadenylation status seems to vary according to the developmental stage in turbot: polyadenylated in oocytes and in the first larval stages, the mRNA becomes undetectable in the polyadenylated fraction in later stages or in adult somatic tissues. These results suggest that IGF-1R mRNA undergoes complex post-transcriptional regulation.

Insulin and insulin-like growth factor-I receptors in fish brain

We investigated insulin and insulin-like growth factor-I (IGF-I) receptor-binding and receptor intrinsic tyrosine kinase activity in the brain of carp (Cyprinus carpio) and trout (Salmo trutta fario). Glycoprotein fractions of semi-purified receptors were prepared by WGA-agarose affinity chromatography. Insulin receptors were found in the brains of both fish species investigated. Carp and trout brain preparations bound, respectively (per 50 micrograms glycoprotein), with 6.0 +/- 1.5% and 8.0 +/- 2.0% of 125I-labeled insulin added to the assay. Insulin binding was specific: much higher quantity of IGF-I (EC50 165 +/- 11 nM for carp and 88.0 +/- 6 nM for trout receptors) than insulin (EC50 0.26 +/- 0.04 nM for carp and 0.25 +/- 0.02 nM for trout) was necessary to displace bound insulin tracer. In preparations of brain receptors, IGF-I binding (52.8 +/- 6.5% in carp brain and 55.0 +/- 13.0% in trout brain) surpassed insulin binding several fold. IGF-I bound to the brain receptors with high affinity (Kd for carp was 0.13 +/- 0.06 nM and for trout 0.22 +/- 0.11 nM) and specificity. Although IGF-I binding could be displaced with insulin, EC50 were 660 +/- 51 nM for carp and 1557 +/- 194 nM for trout. Both ligands stimulated phosphorylation of exogenous substrates in a dose-dependent manner. Carp brain receptors were not significantly different from trout receptors with respect to basal phosphotransferase activities (250.0 +/- 50.0 fm P/mg glycoprotein in carp and 330.0 +/- 120.0 fm P/mg glycoprotein in trout). In both species IGF-I caused higher maximal stimulation (308.0 +/- 36.0% and 270.0 +/- 39%, for carp and trout, respectively) than insulin (250.0 +/- 13.0% and 209.0 +/- 6.0%, for carp and trout, respectively).

The ovarian insulin and insulin-like growth factor system with an emphasis on domestic animals

Insulin and insulin-like growth factors (IGFs) have direct effects on cultured ovarian cells. These effects include stimulation of granulosa cell mitogenesis, granulosa and luteal cell progesterone production, and thecal cell androgen production and appear similar among species. However, species differences exist with regard to insulin and IGF-I effects on granulosa cell estradiol production. In addition to endocrine effects of insulin and IGFs, IGFs are produced by granulosa, thecal, and luteal cells, allowing for an intraovarian autocrine and paracrine system. Granulosa, thecal, and luteal cells contain receptors for insulin and IGFs, and these receptors appear to mediate the effects of insulin and IGFs. Adding to the complexity of the regulatory role of IGFs is the presence of IGF-binding proteins (IGFBPs) within the ovary. These IGFBPs are produced by granulosa, thecal, and luteal cells, and their production is hormonally regulated. Evidence for a coherent mechanism by which insulin, IGFs, and IGFBPs interact and regulate ovarian function in vivo has yet to be found.

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.

Isolation of a second nonallelic insulin-like growth factor I gene from the salmon genome

We have characterized a second nonallelic insulin-like growth factor-I (IGF-I) gene in the chum salmon (Oncorhynchus keta) genome. This gene, IGF-I.2, differs from the previously described chum salmon IGF-I gene, IGF-I.1, in the E peptide-coding portion of exon 3; specifically, the IGF-I.2 gene lacks one codon present in the IGF-I gene and contains two potential splice donor sites at the 3' end of exon 3 rather than the single, more distal site present in the IGF-I.1 gene. The expression of these two IGF-I genes could give rise to as many as six IGF-I mRNA species, each of which would encode a unique E-peptide moiety of the IGF-I prohormone. Thus, the presence of multiple, distinct IGF genes adds an additional level of complexity to IGF-I gene expression and IGF-I biosynthesis in salmon.

The insulin-like growth factor system as a target in breast cancer

Evidence from several experimental systems has shown that the insulin-like growth factors (IGFs) can stimulate breast cancer proliferation. Since IGF action is mediated by interaction with specific cell surface receptors, interruption of these signalling pathways could result in inhibition of cellular growth. In all extracellular fluids, the IGFs are associated with high affinity binding proteins, the IGFBPs can bind the IGFs and prevent receptor activation, and thus might have a role in a targeted approach to breast cancer therapy. Here we present our studies using IGFBP-1 to inhibit growth of the breast cancer cell line MCF-7.