Isolation of the human insulin-like growth factor genes: insulin-like growth factor II and insulin genes are contiguous

Hypoglycemia mediated by paraneoplastic production of Insulin like growth factor-2 from a malignant renal solitary fibrous tumor - clinical case and literature review

BACKGROUND

Hypoglycemic episodes are infrequent in individuals without a history of diabetes mellitus or bariatric surgery. When hypoglycemia does occur in such individuals, an uncommon but important diagnosis to consider is non-islet cell tumor hypoglycemia (NICTH). We report a case of NICTH associated with paraneoplastic insulin-like growth factor-2 (IGF-2) production and review current relevant medical literature.

CASE PRESENTATION

A 60 year old male with no relevant past medical history was referred to the endocrinology clinic with 18 month history of episodic hypoglycemic symptoms and, on one occasion was noted to have a fingerstick glucose of 36 mg/dL while having symptoms of hypoglycemia. Basic laboratory evaluation was unrevealing. Further evaluation however showed an elevated serum IGF-2 level at 2215 ng/mL (reference range 411-1248 ng/mL). Imaging demonstrated a large right suprarenal mass. A right nephrectomy with resection of the mass demonstrated a malignant solitary fibrous tumor. Post resection, the patient's IGF-2 levels normalized and hypoglycemic symptoms resolved.

CONCLUSION

Due to the structural and biochemical homology between IGF-2 and insulin, elevated levels of IGF-2 can result in hypoglycemia. A posttranslational precursor to IGF-2 known as "big IGF" also possesses biologic activity. Review of recent reported cases of NICTH identified widespread anatomic locations and varied pathologic diagnoses of tumors associated with paraneoplastic production of IGF-2 causing hypoglycemia. Definitive management of hypoglycemia associated with paraneoplastic production of IGF-2 consists of resection of the tumor responsible for IGF-2 production. Accumulating literature provides a firm basis for routine IGF-2 laboratory evaluation in patients presenting with spontaneous hypoglycemia with no readily apparent cause.

Tumors, IGF-2, and hypoglycemia: insights from the clinic, the laboratory, and the historical archive

Tumors of mesenchymal and epithelial origin produce IGF-2, which activates pathways in the tumors. In a minority of patients, the tumors (hepatomas, fibromas, and fibrosarcomas are the most common among many) release into the circulation enough IGF-2-related peptides to mimic the fasting hypoglycemia characteristic of patients with insulin-producing islet-cell tumors. Rarely, markedly elevated IGF-2 levels produce somatic changes suggestive of acromegaly. Typically, the elevated IGF-2 levels are associated with suppressed plasma levels of insulin, IGF-1, and GH. Complicating the pathophysiology are the IGF binding proteins (IGFBPs) that can bind IGF-2 and IGF-1, modifying hormone metabolism and action. IGFBP concentrations are often altered in the presence of these tumors. At the cellular level, the 3 hormone-related ligands, IGF-2, IGF-1, and insulin, all bind to 4 (or more) types of IGF-1 receptor (IGF-1R) and insulin receptor (IR). Each receptor has its own characteristic affinity for each ligand, a tyrosine kinase, and overlapping profiles of action in the target cells. The IGF-2R, in addition to binding mannose-6-phosphate-containing proteins, provides an IGF-2 degradation pathway. Recent evidence suggests IGF-2R involvement also in signal transduction. Surgery, the treatment of choice, can produce a cure. For patients not cured by surgery, multiple therapies exist, for the tumor and for hypoglycemia. Potential future therapeutic approaches are sketched. From 1910 to 1930, hypoglycemia, insulin, insulinomas, and non-islet-cell tumors were recognized. The latter third of the century witnessed the emergence of the immunoassay for insulin; the IGFs, their binding proteins, and assays to measure them; and receptors for the insulin-related peptides as well as the intracellular pathways beyond the receptor. In closing, we replace non-islet-cell tumor hypoglycemia, an outdated and misleading label, with IGF-2-oma, self-explanatory and consistent with names of other hormone-secreting tumors.

Promoter-specific expression and imprint status of marsupial IGF2

In mice and humans, IGF2 has multiple promoters to maintain its complex tissue- and developmental stage-specific imprinting and expression. IGF2 is also imprinted in marsupials, but little is known about its promoter region. In this study, three IGF2 transcripts were isolated from placental and liver samples of the tammar wallaby, Macropus eugenii. Each transcript contained a unique 5' untranslated region, orthologous to the non-coding exons derived from promoters P1–P3 in the human and mouse IGF2 locus. The expression of tammar IGF2 was predominantly from the P2 promoter, similar to humans. Expression of IGF2 was higher in pouch young than in the adult and imprinting was highly tissue and developmental-stage specific. Interestingly, while IGF2 was expressed throughout the placenta, imprinting seemed to be restricted to the vascular, trilaminar region. In addition, IGF2 was monoallelically expressed in the adult mammary gland while in the liver it switched from monoalleleic expression in the pouch young to biallelic in the adult. These data suggest a complex mode of IGF2 regulation in marsupials as seen in eutherian mammals. The conservation of the IGF2 promoters suggests they originated before the divergence of marsupials and eutherians, and have been selectively maintained for at least 160 million years.

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

OBJECTIVE

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

DESIGN

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

RESULTS

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

CONCLUSIONS

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

Alteration of the insulin-like growth factor axis during in vitro differentiation of the human osteosarcoma cell line HOS 58

The insulin-like growth factors I and II (IGF-I, IGF-II), their receptors, and high affinity binding proteins (IGFBPs) represent a family of cellular modulators that play essential roles in the development and differentiation of cells and tissues including the skeleton. Recently, the human osteosarcoma cell line HOS 58 cells were used as an in vitro model of osteoblast differentiation characterized by (i) a rapid proliferation rate in low-density cells that decreased continuously with time of culture and (ii) an increasing secretion of matrix proteins during their in vitro differentiation. In the present paper, HOS 58 cells with low cell density at early time points of the in vitro differentiation (i) displayed a low expression of IGF-I and -II; (ii) synthesized low levels of IGFBP-2, -3, -4, and -5, but (iii) showed high expression levels of both the type I and II IGF receptors. During the in vitro differentiation of HOS 58 cells, IGF-I and -II expressions increased continuously in parallel with an upregulation of IGFBP-2, -3, -4, and -5 whereas the IGF-I receptor and IGF-II/M6P receptor mRNA were downregulated. In conclusion, the high proliferative activity in low cell density HOS 58 cells was associated with high mRNA levels of the IGF-IR, but low concentrations of IGFBP-2. The rate of proliferation of HOS 58 cells continuously decreased during cultivation in parallel with a decline in IGF-IR expression, but increase of mitoinhibitory IGFBP-2. These data are indicative for a role of the IGF axis during the in vitro differentiation of HOS 58 cells.

Regulation of DAF-2 receptor signaling by human insulin and ins-1, a member of the unusually large and diverse C. elegans insulin gene family

The activity of the DAF-2 insulin-like receptor is required for Caenorhabditis elegans reproductive growth and normal adult life span. Informatic analysis identified 37 C. elegans genes predicted to encode insulin-like peptides. Many of these genes are divergent insulin superfamily members, and many are clustered, indicating recent diversification of the family. The ins genes are primarily expressed in neurons, including sensory neurons, a subset of which are required for reproductive development. Structural predictions and likely C-peptide cleavage sites typical of mammalian insulins suggest that ins-1 is most closely related to insulin. Overexpression of ins-1, or expression of human insulin under the control of ins-1 regulatory sequences, causes partially penetrant arrest at the dauer stage and enhances dauer arrest in weak daf-2 mutants, suggesting that INS-1 and human insulin antagonize DAF-2 insulin-like signaling. A deletion of the ins-1 coding region does not enhance or suppress dauer arrest, indicating a functional redundancy among the 37 ins genes. Of five other ins genes tested, the only other one bearing a predicted C peptide also antagonizes daf-2 signaling, whereas four ins genes without a C peptide do not, indicating functional diversity within the ins family.

Insulin-like growth factors and ovarian physiology

OBJECTIVE

To review the available information regarding the roles of insulin-like growth factor (IGF)-IGF binding protein (IGFBP) system in ovarian physiology.

DESIGN

Studies that specifically relate to the roles of ovarian folliculogenesis, oocyte maturation, and ovulation were identified through the literature and Medline searches.

RESULTS

Numerous actions of the IGFs have been demonstrated in the ovary, including an enhancement of cell proliferation, aromatase activity, and progesterone biosynthesis. The ovarian IGF system, comprised of IGF-I and IGF-II peptides, IGFBPs and IGF receptors, plays a significant role in the process of follicular development. In addition, IGF-I stimulates the meiotic maturation of follicle-enclosed oocytes in vitro via the IGF-I receptors. IGFBP-3 significantly inhibit gonadotropin-induced ovulation and oocyte maturation by neutralizing endogenously produced IGF-I. Thus, the intraovarian IGF-IGFBP system play a significant role in the processes of follicular development, oocyte maturation, and ovulation.

CONCLUSION

IGF-IGFBP systems have autocrine/paracrine regulatory actions in ovarian physiology. The disturbance of the IGF-IGFBP system in human ovaries may lead to an ovulation, disorders of androgen excess, and infertility.

Transgenic fish and its application in basic and applied research

Since 1985, transgenic fish have been successfully produced by microinjecting or electroporating desired foreign DNA into unfertilized or newly fertilized eggs using many different fish species. More recently, transgenic fish have also been produced by infecting newly fertilized eggs with pantropic, defective retroviral vectors carrying desired foreign DNA. These transgenic fish can serve as excellent experimental models for basic scientific investigations as well as in biotechnological applications. In this paper, we will review the current status of the transgenic fish research and its potential application in basic and applied research.

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.

Cloning of a full-length insulin-like growth factor-I complementary DNA in the goldfish liver and ovary and development of a quantitative PCR method for its measurement

Five forms of insulin-like growth factor-I (IGF-I) complementary DNA (cDNA) were isolated by PCR from goldfish liver and ovary, using primers based on common carp IGF-I sequence. In the goldfish liver, we cloned and sequenced three IGF-I forms (1,2, and 3), and elucidated the full-length cDNA sequence using the 5'-and 3'-RACE. Two IGF-I forms (1 and 2) were cloned from the goldfish ovary and were found to have differences with respect to both size and nucleotide sequence compared to liver IGF-I. The entire liver IGF-I form 1 sequence was found to be 833 nucleotides long, containing a 483-nucleotide open reading from encoding 161 amino acids. The deduced amino acid sequence of the mature peptide was compared to IGF-I sequences of other vertebrates, and found to have 97 and 93% similarity to carp and salmon IGF-I, respectively, In this study we also developed a competitive quantitative PCR method and demonstrated an increase in IGF-I expression following treatments with growth hormone or gonadotropin-releasing hormone in the goldfish liver.

The INS 5' variable number of tandem repeats is associated with IGF2 expression in humans

The minisatellite DNA polymorphism consisting of a variable number of tandem repeats (VNTR) at the human INS (insulin gene) 5'-flanking region has demonstrated allelic effects on insulin gene transcription in vitro and has been associated with the level of insulin gene expression in vivo. We now show that this VNTR also has effects on the nearby insulin-like growth factor II gene (IGF2) in human placenta in vivo and in the HepG2 hepatoma cell line in vitro. We show that higher steady-state IGF2 mRNA levels are associated with shorter alleles (class I) than the longer class III alleles in term placentae. In vitro, reporter gene activity was greater from reporter gene constructs with IGF2 promoter 3 in the presence of class I alleles than from those with class III. Taken together with the documented transcriptional effects on the insulin gene, we propose that the VNTR may act as a long range control element affecting the expression of both INS and IGF2. The localization of a type 1 diabetes susceptibility locus (IDDM2) to the VNTR itself suggests that either or both of these genes may be involved in the biologic effects of IDDM2.

Molecular regulation of insulin-like growth factor-I and its principal binding protein, IGFBP-3

The insulin-like growth factors (IGFs) have diverse anabolic cellular functions, and structure similar to that of proinsulin. The distribution of IGFs and their receptors in a wide variety of organs and tissues enables the IGFs to exert endocrine, paracrine, and autocrine effects on cell proliferation and differentiation, caloric storage, and skeletal elongation. IGF-I exhibits particular metabolic responsiveness, and circulating IGF-I originates predominantly in the liver. Hepatic IGF-I production is controlled at the level of gene transcription, and transcripts are initiated largely in exon 1. Hepatic IGF-I gene transcription is reduced in conditions of protein malnutrition and diabetes mellitus, and our laboratory has used in vitro transcription to study mechanisms related to diabetes. We find that the presence of sequences downstream from the major transcription initiation sites in exon 1 is necessary for the diabetes-induced decrease in IGF-I transcription. Six nuclear factor binding sites have been identified within the exon 1 downstream region, and footprint sites III and V appear to be necessary for metabolic regulation; region V probes exhibit a decrease in nuclear factor binding with hepatic nuclear extracts from diabetic animals. IGFs in biological fluids are associated with IGF binding proteins, and IGFs circulate as a 150-kDa complex that consists of an IGF, an IGFBP-3, and an acid-labile subunit. Circulating IGFBP-3 originates mainly in hepatic nonparenchymal cells, where IGF-I increases IGFBP-3 mRNA stability, but insulin increases IGFBP-3 gene transcription. Regulation of IGFBP-3 gene transcription by insulin appears to be mediated by an insulin-responsive element, which recognizes insulin-responsive nuclear factors in both gel mobility shift assays and southwestern blots. Studies of mechanisms underlying the modulation of IGF-I and IGFBP-3 gene transcription, and identification of critical nuclear proteins involved, should lead to new understanding of the role and regulation of these important growth factors in diabetes mellitus and other metabolic disorders.

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

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

Spatial and temporal changes in the insulin-like growth factor (IGF) axis indicate autocrine/paracrine actions of IGF-I within wounds of the rat brain

A precise role for insulin-like growth factors (IGFs), IGF-binding proteins (IGFBPs), and IGF-receptors (IGF-Rs) in damaged central nervous system (CNS) tissue has not been elucidated, although their expression in the ischemic brain has been demonstrated. However, little is known of IGF responses after CNS trauma. In this study, we have used ribonuclease protection assay, in situ hybridization, and immunohistochemistry to demonstrate that IGF-I, IGFBPs, and IGF-1R expression alters in response to a penetrating CNS injury. Within penetrant cerebral wounds in the acute phase of the response (1-7 days post lesion; dpl), increased levels of IGF-I, IGFBP-1, -2, -3, -6, and IGF-1R protein were localized to injury responsive astrocytes, neurons and cells of the monocyte lineage. IGF-I, IGFBP-2, and 3 showed a congruency in sites of messenger RNA (mRNA) and peptide expression, with IGF-I and IGFBP-2 mRNA expression predominating. IGF-I, IGFBP-1, and IGFBP-3 protein were also associated with the microvascular endothelium, which was accompanied by increased levels of IGFBP-3 mRNA. These early changes in IGFBP expression probably facilitate IGF-I action. Later in the wounding response (7-14 dpl), the expression of IGFBP-4 and IGFBP-5 peaked within astrocytes and neurons, with IGFBP-5 mRNA being specifically localized to the glia limitans within the wound, suggesting an inhibitory role for these proteins, down-regulating the effects of IGF-I chronically. Our evidence suggests that within penetrating CNS wounds, IGF-I acts in an autocrine/paracrine manner to regulate cellular responses, with its spatial and temporal availability being modulated by the differential presence of stimulatory vs. inhibitory IGFBPs.

Pancreatic development and maturation of the islet B cell. Studies of pluripotent islet cultures

Pancreas organogenesis is a highly regulated process, in which two anlage evaginate from the primitive gut. They later fuse, and, under the influence of the surrounding mesenchyme, the mature organ develops, being mainly composed of ductal, exocrine and endocrine compartments. Early buds are characterized by a branching morphogenesis of the ductal epithelium from which endocrine and exocrine precursor cells bud to eventually form the two other compartments. The three compartments are thought to be of common endodermal origin; in contrast to earlier hypotheses, which suggested that the endocrine compartment was of neuroectodermal origin. It is thus generally believed that the pancreatic endocrine-lineage possesses the ability to mature along a differentiation pathway that shares many characteristics with those of neuronal differentiation. During recent years, studies of insulin-gene regulation and, in particular, the tissue-specific transcriptional control of insulin-gene activity have provided information on pancreas development in general. The present review summarizes these findings, with a special focus on our own studies on pluripotent endocrine cultures of rat pancreas.

Two pairs of male monozygotic twins discordant for Wiedemann-Beckwith syndrome

Wiedemann-Beckwith syndrome (WBS) is a congenital anomaly syndrome which classically consists of exomphalos, macroglossia, and gigantism. The syndrome is also associated with a variety of minor anomalies and affected individuals have an increased risk of developing rare embryonal cell tumors. To date, 15 monozygotic (MZ) twin pairs have been reported of which 13 are discordant for WBS. All except one pair of the discordant WBS twin pairs have been female. We report two pairs of male MZ twins, each discordant for WBS.

Cloning and expression of murine high molecular mass heat shock proteins, HSP105

We have shown that the 105-kDa heat shock protein (HSP105) and the 42 degrees C-specific heat shock protein (42 degrees C-HSP) constitute high molecular mass heat shock proteins. To elucidate the structure of these heat shock proteins, we have screened a cDNA library constructed with poly(A)+ RNA derived from mouse FM3A cells preheated at 42 degrees C for 2 h using an antibody against murine HSP105. Two full-length cDNA clones were obtained: the pB105-1 insert encoded an 858-amino acid protein, and the pB105-2 insert encoded an 814-amino acid protein and lacked 44 amino acids found in pB105-1. The two clones contained the amino acid sequence found in the 17-kDa polypeptide fragments from HSP105 and 42 degrees C-HSP by lysylendopeptidase digestion. In vitro translation products of the RNA transcripts from pB105-1 and pB105-2 migrated to the same positions of HSP105 and 42 degrees C-HSP, respectively, on SDS-polyacrylamide gel electrophoresis. Northern blot analysis showed that the transcript was approximately 4 kilobases in murine FM3A cells and was strongly induced by heat shock and by treatment with arsenite or an amino acid analog. By reverse transcription-polymerase chain reaction analysis using primers by which deletion of 132 nucleotides in pB105-2 could be detected, the polymerase chain reaction product corresponding to pB105-2 was increased only after heat shock at 42 degrees C, whereas the product corresponding to pB105-1 was induced by heat shock at either 42 or 45 degrees C and also by other stresses. Thus, the cDNA clones pB105-1 and pB105-2 encode HSP105 and 42 degrees C-HSP, respectively, and HSP105 and 42 degrees C-HSP (a short form of HSP105) are suggested to be produced by alternative splicing. Here, HSP105 and 42 degrees C-HSP are renamed HSP105 alpha and HSP105 beta, respectively. A protein sequence homology search revealed that HSP105 shares 54, 34, and 25% amino acid identity with human HSP70RY, the sea urchin egg receptor for sperm, and murine inducible HSP70, respectively. Furthermore, by Northern blot analysis, HSP105 mRNA was revealed to be present in most murine tissues and to be highly expressed in the brain.

Analysis of insulin-like growth factors (IGF)-I, and -II, type II IGF receptor and IGF-binding protein-2 mRNA and peptide levels in normal and nephrectomized rat kidney

Immunocytochemistry, in situ hybridization, and radioimmunoassay were employed to examine the cellular distribution of mRNAs and proteins for IGF-I, II, IGF-II/M6P receptor, IGFBP2 as well as the levels of IGF-I and II in normal and unilaterally nephrectomized (Nx) adult rat kidneys. A similar distribution of immunoreactive IGF-I, and -II as well as IGF-II/M6P receptor was found in the principal cells of the cortical collecting duct and in all cells of the inner medullary collecting duct. In addition, immunostainable IGF-I and IGF-II/M6P receptor were noted in some inner medullary loops of Henle, while IGFBP2 was seen in the collecting ducts and loops of Henle of the inner medullar and the renal vasculature of all animals. By comparison, in situ hybridization revealed IGF-I mRNA only in the medullary thick ascending limbs while IGF-II mRNA was localized to the wall of the renal microvasculature in all kidneys. IGFBP2 mRNA was localized to the renal corpuscle and to inner medullary interstitial cells of all kidneys. These data suggest that renal IGF-I and IGFBP2 are synthesized at upstream sites along the nephron and then transported downstream for interaction with IGF receptors. Following nephrectomy, the renal levels of IGF-I peptide and mRNA were elevated at both 5 and 33 days post-nephrectomy, supporting a potential functional role for IGF-I in stimulating the structural and functional recovery in compensatory hypertrophy.

Molecular genetics of Wilms' tumour

Wilms' tumour, or nephroblastoma, is an embryonal malignancy of the kidney with an incidence of approximately 1 in 10,000 live births. It occurs in both sporadic and familial forms, but only 1% of Wilms' tumour patients have a positive family history. The molecular genetics of Wilms' tumour have been the subject of extensive research and at least three genes (WT1, WT2, WT3) have been implicated. WT1 has been mapped to 11p13, and it has been suggested that loss or inactivation of a tumour-suppressor gene at 11p13 might be a primary event in the development of Wilms' tumour. The WT2 gene maps to 11p15 in the region of the Beckwith-Wiedemann locus. The WT3 locus is likely to be located to chromosome 16q. The understanding of the molecular genetics of Wilms' tumour is reviewed briefly.

Identification of a large genomic region in UV-irradiated human cells which has fewer cyclobutane pyrimidine dimers than most genomic regions

Size separation after UV-endonuclease digestion of DNA from UV-irradiated human cells using denaturing conditions fractionates the genome based on cyclobutane pyrimidine dimer content. We have examined the largest molecules available (50-80 kb; about 5% of the DNA) after fractionation and those of average size (5-15 kb) for content of some specific genes. We find that the largest molecules are not a representative sampling of the genome. Three contiguous genes located in a G+C-rich isochore (tyrosine hydroxylase, insulin, insulin-like growth factor II) have concentrations two to three times greater in the largest molecules. This shows that this genomic region has fewer pyrimidine dimers than most other genomic regions. In contrast, the beta-actin genomic region, which has a similar G+C content, has an equal concentration in both fractions as do the p53 and beta-globin genomic regions, which are A+T-rich. These data show that DNA damage in the form of cyclobutane pyrimidine dimers occurs with different probabilities in specific isochores. Part of the reason may be the relative G+C content, but other factors must play a significant role. We also report that the transcriptionally inactive insulin region is repaired at the genome-overall rate in normal cells and is not repaired in xeroderma pigmentosum complementation group C cells.

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

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

Quantification of myotrophin from spontaneously hypertensive and normal rat hearts

A novel peptide, myotrophin, has been isolated, purified, cloned, and sequenced from the hearts of spontaneously hypertensive rats (SHR) and from dilated cardiomyopathic human heart tissue. Myotrophin accelerates myocyte growth by stimulating protein synthesis (not by altering myocardial cell division). Our successive studies were conducted to evaluate the pathophysiological significance of myotrophin; a solid-phase radioimmunoassay technique was developed for quantifying the protein in hypertrophied and normal hearts. Specific antipeptide antibody was raised in rabbits against a peptide that represents a selected amino acid sequence of a 17-amino acid myotrophin segment by using the multiple antigenic peptide technique. The specificity of the antibody was evaluated by determining the affinity constant after constructing the Scatchard plot obtained from the ratio of bound to free myotrophin against bound myotrophin. The value obtained was 2.61 x 10(7) L/mol. The specificity was further demonstrated by Western blot analysis, in which a single protein band was obtained in the region of 12 kD. Pretreatment of the antibody with myotrophin completely blocked the binding sites, because no protein band was detected on the immunoblot. The antibody prevented the myocardial protein synthesis induced by myotrophin as revealed by the blockage of the stimulation of [3H]leucine incorporation into myocyte protein. Quantification of myotrophin from different heart tissues was achieved by Western blot and dot blot analyses. Amounts of myotrophin present in different dots were determined by using a video image analyzer. The level of myotrophin in the embryonic tissue was found to be similar in male normal and SHR hearts.(ABSTRACT TRUNCATED AT 250 WORDS)

Southern hybridization analyses of somatic cell hybrids reveal that human BB1 is a member of a multigene family dispersed throughout the human genome and appears to be linked to the human G25K genes

The hBB1 RNA (2.3 kb in length) encodes a 57-amino-acid protein whose production is essential for cellular transit of G1 phase of the cell cycle (Moats-Staats et al., 1994). Homology searches of GenBank and EMBL revealed that bases 1-234 of the hBB1 cDNA were 97% homologous to the 3' terminal 234 bases of the g25K cDNA (bases 940-1,175, Shinjo et al., 1990) the human homolog of the yeast cdc42 cDNA. We have used the techniques of the long-range polymerase chain reaction (PCR) (Barnes, 1994) and Southern hybridization analyses of a somatic cell hybrid panel to investigate hBB1 gene structure and to determine whether the hBB1 gene(s) overlaps the g25K gene. These studies have demonstrated that the hBB1 RNA is encoded by a gene family composed of eight members that is dispersed throughout the human genome localizing under high-stringency conditions to chromosomes 1, 4, 7, 8, and 20. We have also determined that two hBB1 gene(s) and two g25K gene(s) map to similar-size Bam HI restriction fragments on chromosomes 4 and 7.

Epigenetic changes encompassing the IGF2/H19 locus associated with relaxation of IGF2 imprinting and silencing of H19 in Wilms tumor

In most tissues IGF2 is expressed from the paternal allele while H19 is expressed from the maternal allele. We have previously shown that in some Wilms tumors the maternal IGF2 imprint is relaxed such that the gene is expressed biallelically. We have now investigated this subset of tumors further and found that biallelic expression of IGF2 was associated with undetectable or very low levels of H19 expression. The relaxation of IGF2 imprinting in Wilms tumors also involved a concomitant reversal in the patterns of DNA methylation of the maternally inherited IGF2 and H19 alleles. Furthermore, the only specific methylation changes that occurred in tumors with relaxation of IGF2 imprinting were solely restricted to the maternal IGF2 and H19 alleles. These data suggest that there has been an acquisition of a paternal epigenotype in these tumors as the result of a pathologic disruption in the normal imprinting of the IGF2 and H19 genes.

DNA recognition by splicing variants of the Wilms' tumor suppressor, WT1

The Wilms' tumor suppressor, WT1, is a zinc finger transcriptional regulator which exists as multiple forms owing to alternative mRNA splicing. The most abundant splicing variants contain a nine-nucleotide insertion encoding lysine, threonine, and serine (KTS) in the H-C link region between the third and fourth WT1 zinc fingers which disrupts binding to a previously defined WT1-EGR1 binding site. We have identified WT1[+KTS] binding sites in the insulin-like growth factor II gene and show that WT1[+KTS] represses transcription from the insulin-like growth factor II P3 promoter. The highest affinity WT1[+KTS] DNA binding sites included nucleotide contacts involving all four WT1 zinc fingers. We also found that different subsets of three WT1 zinc fingers could bind to distinct DNA recognition elements. A tumor-associated, WT1 finger 3 deletion mutant was shown to bind to juxtaposed nucleotide triplets for the remaining zinc fingers 1, 2, and 4. The characterization of novel WT1 DNA recognition elements adds a new level of complexity to the potential gene regulatory activity of WT1. The results also present the possibility that altered DNA recognition by the dominant WT1 zinc finger 3 deletion mutant may contribute to tumorigenesis.

DNA recognition by splicing variants of the Wilms' tumor suppressor, WT1

The Wilms' tumor suppressor, WT1, is a zinc finger transcriptional regulator which exists as multiple forms owing to alternative mRNA splicing. The most abundant splicing variants contain a nine-nucleotide insertion encoding lysine, threonine, and serine (KTS) in the H-C link region between the third and fourth WT1 zinc fingers which disrupts binding to a previously defined WT1-EGR1 binding site. We have identified WT1[+KTS] binding sites in the insulin-like growth factor II gene and show that WT1[+KTS] represses transcription from the insulin-like growth factor II P3 promoter. The highest affinity WT1[+KTS] DNA binding sites included nucleotide contacts involving all four WT1 zinc fingers. We also found that different subsets of three WT1 zinc fingers could bind to distinct DNA recognition elements. A tumor-associated, WT1 finger 3 deletion mutant was shown to bind to juxtaposed nucleotide triplets for the remaining zinc fingers 1, 2, and 4. The characterization of novel WT1 DNA recognition elements adds a new level of complexity to the potential gene regulatory activity of WT1. The results also present the possibility that altered DNA recognition by the dominant WT1 zinc finger 3 deletion mutant may contribute to tumorigenesis.

Cloning and characterization of a novel RNA involved in cellular growth regulation

During the course of antisense oligodeoxynucleotide (oligo) inhibition experiments investigating the role of insulin-like growth factor I (IGF-I) in the WI-38 cell cycle, we found that a sense-strand oligo (S oligo), used as a control, inhibited DNA synthesis 90 to 95%. S1 nuclease protection assays demonstrated that this S oligo formed intracellular duplexes with WI-38 RNA, and Northern (RNA) hybridization analyses demonstrated specific hybridization of this 32P-labeled S oligo to 1.8-, 2.3-, and 3.2-kb RNAs. We have cloned and sequenced a 2,251-bp cDNA, designated BB1, corresponding to the 2.3-kb RNA. Decoding of the BB1 cDNA sequence reveals several open reading frames arranged in a motif similar to that seen in proteins subject to translational control mechanisms. Homology searches of nucleic acid and protein data bases reveal no significant homology of BB1 with known sequences other than a 234-bp region in the BB1 5' untranslated region that shared 97% homology with a region in the 3' untranslated region of the human cdc42 mRNA. S1 nuclease protection analyses performed with IGF-I gene fragments and computer homology searches demonstrated that the BB1 RNA does not derive from transcription from the opposite strand of the IGF-I gene. Northern hybridization analyses of RNA extracted from serum-starved HeLa S3 cells demonstrated that steady-state BB1 RNA levels increased upon serum growth stimulation, with steady-state levels peaking 4 h after release from the block induced by serum starvation. Antisense oligo inhibition experiments using specific BB1 antisense oligos targeted to the putative open reading frames of the BB1 RNA reduce DNA synthesis of HeLa S3 cells to 15% of control levels, indicating that the BB1 RNA is essential for cell cycle traversal and, as such, encodes a growth-reguLating gene product.

Cloning and characterization of a novel RNA involved in cellular growth regulation

During the course of antisense oligodeoxynucleotide (oligo) inhibition experiments investigating the role of insulin-like growth factor I (IGF-I) in the WI-38 cell cycle, we found that a sense-strand oligo (S oligo), used as a control, inhibited DNA synthesis 90 to 95%. S1 nuclease protection assays demonstrated that this S oligo formed intracellular duplexes with WI-38 RNA, and Northern (RNA) hybridization analyses demonstrated specific hybridization of this 32P-labeled S oligo to 1.8-, 2.3-, and 3.2-kb RNAs. We have cloned and sequenced a 2,251-bp cDNA, designated BB1, corresponding to the 2.3-kb RNA. Decoding of the BB1 cDNA sequence reveals several open reading frames arranged in a motif similar to that seen in proteins subject to translational control mechanisms. Homology searches of nucleic acid and protein data bases reveal no significant homology of BB1 with known sequences other than a 234-bp region in the BB1 5' untranslated region that shared 97% homology with a region in the 3' untranslated region of the human cdc42 mRNA. S1 nuclease protection analyses performed with IGF-I gene fragments and computer homology searches demonstrated that the BB1 RNA does not derive from transcription from the opposite strand of the IGF-I gene. Northern hybridization analyses of RNA extracted from serum-starved HeLa S3 cells demonstrated that steady-state BB1 RNA levels increased upon serum growth stimulation, with steady-state levels peaking 4 h after release from the block induced by serum starvation. Antisense oligo inhibition experiments using specific BB1 antisense oligos targeted to the putative open reading frames of the BB1 RNA reduce DNA synthesis of HeLa S3 cells to 15% of control levels, indicating that the BB1 RNA is essential for cell cycle traversal and, as such, encodes a growth-reguLating gene product.

Disruption of insulin-like growth factor 2 imprinting in Beckwith-Wiedemann syndrome

To study insulin-like growth factor 2 (IGF2) imprinting in BWS (Beckwith-Wiedemann syndrome, an overgrowth syndrome associated with Wilms and other embryonal tumours), we examined allele-specific expression using an Apal polymorphism in the 3' untranslated region of IGF2. Four of six BWS fibroblast strains demonstrated biallelic expression, as did the tongue tissue from one of these patients. Paternal heterodisomy was excluded for all BWS patients with biallelic expression, suggesting strongly that the BWS phenotype in some patients involves disruption of IGF2 imprinting. Constitutional loss of IGF2 imprinting in a subgroup of our BWS patients, and recent reports of loss of imprinting in sporadic Wilms tumour, further strengthens the view that IGF2 overexpression plays an important role in somatic overgrowth and the development of embryonal tumours.

Parental imprinting of an Igf-2 transgene

As a consequence of parental imprinting in mice, the paternal allele encoding insulin-like growth factor-II (IGF-II) is expressed, whereas the maternal allele is silent in most tissues. To examine whether cis-acting sequences involved in imprinting are located in the vicinity of the Igf-2 gene, we have constructed mouse transgenic lines and studied the expression of a 30 kb rat Igf-2 transgene, in which the coding region has been replaced with the lacZ reporter sequence. Chromatin position effects and/or absence of long-range regulatory elements seem to have affected tissue-specific expression in the transgenic mice. However, in one of six expressing lines, staining of embryos for beta-galactosidase activity was detected in a minor subset of tissues normally transcribing the endogenous homolog, but only when the transgene was transmitted paternally. This transgene was integrated into mouse chromosome 19, which is apparently free of imprinted loci. Although the possibility that the Igf-2 transgene was inserted into an as yet unidentified imprinted locus is discussed, a more likely interpretation of our results is that the transgene carries at least a portion of its own imprinting signal, because it consists of the genomic sequences of a locus already known to be imprinted and maintains the correct imprinting mode.

Immunohistochemical localization of insulin-like growth factor 1 and 2 in the endocrine pancreas of rat, dog, and man, and their coexistence with classical islet hormones

Immunohistochemical techniques were used to study the occurrence and distribution of insulin-like growth factor 1 (IGF-1) and IGF-2 in the pancreas of man, dog, and rat and their possible coexistence with insulin (INS), glucagon (GLUC), somatostatin (SOM) and pancreatic polypeptide (PP). All control experiments, including pre-absorption of the antisera with synthetic peptide hormones, indicated the specificity of the immunoreactions obtained. In all species investigated, IGF-2-immunoreactivity occurred exclusively in INS-immunoreactive cells as was found by the use of consecutive sections and double immunofluorescence on identical sections. In contrast, IGF-1-immunoreactivity co-existed with GLUC-immunoreactivity. In man, singular SOM-immunoreactive cells also contained IGF-1-immunoreactivity. Thus, IGF-1 and IGF-2 can be localized by means of immunohistochemistry in the mammalian pancreas, and can be shown to occur in different islet cell populations. It is presumed that IGF-1 derived from A-cells and/or D-cells acts on the B-cells in a paracrine manner. The co-existence of IGF-2-immunoreactivity and INS-immunoreactivity in the human, rat, and dog endocrine pancreas indicates that mammalian IGF-2 and INS genes are regulated simultaneously.

Allele-specific replication timing of imprinted gene regions

Several lines of evidence suggest that the paternal and maternal genomes may have different expression patterns in the developing organism and this has been confirmed by the identification of endogenous genes that are parentally imprinted in the mouse. Little is known about the precise mechanisms involved in the process, but structural differences between the two alleles must somehow provide cis-acting signals for directing parental-specific transcription. Cell-cycle replication time is one parameter that has been shown to be associated with both tissue-specific gene expression and the allele-specific transcription patterns of the X chromosomes in female cells. For this reason we have examined the replication timing patterns for the chromosomal regions containing the imprinted genes Igf2, Igf2r, H19 and Snrpn in the mouse. At all of these sites, and their corresponding positions in the human genome, the two homologous alleles replicate asynchronously and it is always the paternal allele that is early-replicating. Thus imprinted genes appear to be embedded in large DNA domains with differential replication patterns, which may provide a structural imprint for parental identity.

Susceptibility to insulin dependent diabetes mellitus maps to a 4.1 kb segment of DNA spanning the insulin gene and associated VNTR

Recent studies have demonstrated that a locus at 11p15.5 confers susceptibility to insulin dependent diabetes mellitus (IDDM). This locus has been shown to lie within a 19 kb region. We present a detailed sequence comparison of the predominant haplotypes found in this region in a population of French Caucasian IDDM patients and controls. Identification of polymorphisms both associated and unassociated with IDDM has allowed us to define further the region of association to 4.1 kb. Ten polymorphisms within this region are in strong linkage disequilibrium with each other and extend across the insulin gene locus and the variable number tandem repeat (VNTR) situated immediately 5' to the insulin gene. These represent a set of candidate disease polymorphisms one or more of which may account for the susceptibility to IDDM.

Parental genomic imprinting of the human IGF2 gene

The mouse igf2 gene, coding for the insulin-like growth factor II (IGF-II) is parentally imprinted, only the gene copy derived from the father is expressed. To know whether IGF2, the human homologue, is also imprinted, we used an ApaI polymorphism at the 3' untranslated region in order to distinguish between mRNA derived from each copy of the gene in placentae from heterozygote human fetuses, studied after careful removal of the decidua. Six term and two pre-term placentae of heterozygotes were studied, and in each case the cDNA contained only one of the two alleles present in the genomic DNA. In three cases the mother was homozygous for the non-expressed allele, allowing assignment of paternal origin to the transcribed gene copy. We conclude that, as in the mouse, human IGF2 is parentally imprinted.

DNA polymorphisms in the human tyrosine hydroxylase/insulin/insulin-like growth factor II chromosomal region in relation to glucose and insulin responses

The feasibility of disease association studies using polymorphic DNA markers in the tyrosine hydroxylase/insulin/insulin-like growth factor II chromosomal region was indicated by a high degree of linkage disequilibrium found in haplotypes. Haplotypes were resolved in the parents from Scandinavian nuclear families by studying the segregation of eight DNA polymorphisms. Comparison of observed vs expected frequencies of haplotypes, as well as pairwise measures of linkage disequilibrium, indicated a high degree of linkage disequilibrium. Five restriction fragment length polymorphisms linked to the tyrosine hydroxylase/insulin/insulin growth factor II region of chromosome 11 were investigated in relation to Type 2 (non-insulin-dependent) diabetes mellitus, and to glucose and insulin responses to glucose infusion in healthy subjects. No significant differences in genotype frequencies between Type 2 diabetic (n = 53) and healthy subjects (n = 106) were found. A significant association (p < 0.001) was initially found between genotypes defined by a PstI polymorphism located 5' of the tyrosine hydroxylase gene and the early glucose response to a standardized glucose infusion test in healthy subjects. However, a follow-up study of 112 healthy individuals failed to confirm this finding.

The third IGF-II promoter specifies transcription of three transcripts out of five in human placenta

Insulin-like growth factor-II (IGF-II) mRNA exists as multiple transcript size classes, such as 6.0, 5.3, 4.9, 3.2, and 2.2 kb mRNAs in various human tissues. Three different promoters, 2 different polyadenylation sites, and alternative splicing are involved in producing these multiple transcripts. Initiation of transcription at the 3 different promoters results in multiple mRNAs which contain identical coding regions but different 5'-untranslated regions (5'-UTRs). The first promoter is thought to direct expression of 5.3 kb mRNA in adult human liver. The second promoter region directs expression of 6.0, 3.2, and 2.2 kb mRNAs in human fetal tissues and several adult nonliver tissues. The third promoter specifies transcription of a 4.9 kb mRNA in various tissues. We isolated and sequenced a cDNA clone (pIGF-II-1-70) from a human placental cDNA library, which contains the IGF-II coding region and the 5'-UTR associated with the third promoter. By using a 5'-UTR-specific probe from the clone, we found that this third 5'-UTR is contained in the IGF-II mRNA of 2.2 kb and is absent in the 3.2 kb IGF-II mRNA. We also found an 0.9 kb transcript expressed in placenta, which hybridized strongly to the third 5'-UTR specific probe but not to IGF-II coding region probes. This finding might indicate the existence of an mRNA encoding an IGF-II-associated peptide.

Physical linkage of two mammalian imprinted genes, H19 and insulin-like growth factor 2

Parental imprinting is a phenomenon in mammals whereby the maternal and paternal alleles of a gene are differentially expressed. Three murine genes have been shown to display this type of allele-specific expression. Two of them, insulin-like growth factor-2 (Igf-2) and H19, map to the distal end of mouse chromosome 7, but are imprinted in opposite directions. Pulsed-field gel electrophoresis and large-fragment DNA cloning were utilized to establish a physical map that includes H19 and Igf-2. Igf-2 lies approximately 90 kilobases of DNA 5' to H19, in the same transcriptional orientation. This physical proximity is conserved in humans, based on pulsed-field gel analysis. We conclude that H19 and Igf-2 constitute an imprinted domain.

Molecular analysis of patients with Wiedemann-Beckwith syndrome. I. Gene dosage on the short arm of chromosome 11

Wiedemann-Beckwith syndrome (WBS) is characterised by a specific group of congenital malformations associated with an increased concurrent risk for development of a defined group of childhood neoplasms. The mode of inheritance is complex, but recently compiled family data suggest that it is an autosomal dominant trait of varying expression. It has previously been suggested that major rearrangements on the short arm of chromosome 11 may be involved in the aetiology of the disease, particularly in the region of the insulin like growth factor II (IGF-II) gene (11p15.5). This gene is thought to be parentally imprinted in the mouse and it has been suggested that in the human, duplication of the non-imprinted locus in WBS patient might lead to diploid expression of the gene and consequent general hyperplasia. This model predicts that there should be both frequent and parental origin specific duplication of the IGF-II gene in the patients. It was the aim of this study to examine the IGF-II locus and its surrounding chromosomal environment for such lesions in a large number of WBS patients. Using restriction fragment length polymorphism analysis for four linked markers on 11p and genomic clones internal to the IGF-II locus we could find no evidence of alteration or amplification of this area in any of the 11 patients investigated. In one patient who developed a Wilms tumour we could find no evidence for loss of any material on the short arm of chromosome 11 as reported previously.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular analysis of patients with Wiedemann-Beckwith syndrome. II. Paternally derived disomies of chromosome 11

In Wiedemann-Beckwith syndrome (WBS) a putative disease gene resides at the tip of the short arm of chromosome 11 in the region of the insulin growth like factor II (IGF-II) gene. Whilst changes in gene dosage in this area do not appear to be common in the syndrome, in familial cases the lesion appears to be dominant only when inherited through the female line. We undertook to examine the parental origin of the copies of chromosome 11 in a large group of WBS patients using a series of restriction fragment length polymorphisms (RFLPs) on 11p, and report here that in one sporadic case of WBS out of 14 both copies of chromosome 11 are derived from the father and are present in a normal dosage. This suggests that at least one mode of expression of the lesion is modified by genomic imprinting.

Functional analysis of the rat insulin-like growth factor I gene and identification of an IGF-I gene promoter

Insulin-like growth factor I (IGF-I) mediates many of the systemic growth-promoting effects of growth hormone and also functions as a locally acting growth stimulator. In mammals, IGF-I gene expression is complicated, as the gene is transcribed and processed into multiple mRNAs (ranging in length from less than 1 to nearly 7.5 kb) that encode at least two protein precursors. As a step toward understanding the regulation of IGF-I, we report the complete organization of the rat IGF-I gene, including identification of the structural determinants for all IGF-I mRNA species, and an initial functional analysis of its promoters. The gene is composed of 6 exons distributed over nearly 80 kb of chromosomal DNA and is structurally heterogeneous. Several transcription start sites were identified within IGF-I exons 1 and 2, adjacent to presumptive promoters 1 and 2, respectively, and at least three polyadenylation sites were mapped to exon 6. To test promoter function, fusion genes were constructed linking fragments of IGF-I DNA to a reporter plasmid. Chimeric genes containing at least 395 bp of DNA from the 5'-flanking region of exon 1 enhanced luciferase activity after transfection into the IGF-I-producing SK-N-MC cell line, while fusion plasmids containing up to 1,300 bp of DNA from the 5'-flanking region of exon 2 were inactive. Relative levels of IGF-I mRNAs containing exons 1 or 2 varied among different rat tissues, although in response to acute or chronic growth hormone treatment both classes of transcripts were induced coordinately in rat liver. These observations represent the first thorough characterization of a mammalian IGF-I gene, and provide a starting point for defining the mechanisms by which growth hormone and other trophic factors regulate IGF-I gene expression.

The insulin-like growth factor family of ligands, receptors, and binding proteins

The insulin-like growth factors (IGFs) have important roles in normal cellular growth and development. The IGFs have also been implicated in regulation of tumor cell growth. Two ligands, IGF-I and IGF-II, have been identified that are expressed in both fetal and adult tissues. They interact with at least two specific cell surface receptors. The type I IGF receptor is homologous to the insulin receptor in structure and has tyrosine kinase activity. The type II receptor is identical to the mannose-6-phosphate receptor known to be important in the trafficking of lysosomal enzymes; its role in IGF signal transduction is not clear. Furthermore, a hybrid receptor composed of subunits from the insulin receptor and the type I IGF receptor have been identified. In addition to these receptors, six different IGF binding proteins have been identified, which modulate the activity of the IGFs in various ways. Thus, there is great potential for complex interactions between the family members that could ultimately regulate normal and neoplastic cell growth.