Radioligand assays for insulin-like growth factor II

Biophysical and biological properties of naturally occurring high molecular weight insulin-like growth factor II variants

A soluble form of the insulin-like growth factor II/mannose 6-phosphate receptor (sIGF-II/MPR) is present in fetal bovine serum and carries mature 7.5-kDa insulin-like growth factor II (IGF-II) and at least 12 different high molecular weight (Mr) IGF-II isoforms (Valenzano, K. J., Remmler, J., and Lobel, P. (1995) J. Biol. Chem. 270, 16441-16448). In this study, we used gel filtration and anion exchange chromatographies to resolve the isoforms into eight fractions that were characterized with respect to their biochemical, biophysical, and biological properties. Each fraction contained one to three major protein species with apparent sizes ranging from 11 to 17 kDa by SDS-polyacrylamide gel electrophoresis. The 11-kDa species contains no post-translational modifications and consists of an extended IGF-II backbone terminating at Gly-87. The remaining high Mr IGF-II isoforms are also composed of an 87-amino acid IGF-II peptide backbone but contain increasing amounts of sialated, O-linked sugars. Plasmon resonance spectroscopy experiments revealed that all the high Mr isoforms and mature 7.5-kDa IGF-II bound to immobilized recombinant soluble human IGF-I receptor, recombinant human IGF-binding protein 1, and sIGF-II/MPR with similar kinetics. In addition, radiolabeled tracer experiments demonstrated that both mature and high Mr IGF-II isoforms have similar binding profiles in fetal bovine serum and have similar affinities for IGF-II-binding proteins secreted from human fibroblasts. Finally, the biological activity of high Mr IGF-II was shown to be similar to or slightly better than mature IGF-II in stimulating amino acid uptake in fibroblasts and in inducing myoblast differentiation.

Overexpression of insulin-like growth factor-II induces accelerated myoblast differentiation

Previous studies have shown that exogenous insulin-like growth factors (IGFs) can stimulate the terminal differentiation of skeletal myoblasts in culture and have established a correlation between the rate and the extent of IGF-II secretion by muscle cell lines and the rate of biochemical and morphological differentiation. To investigate the hypothesis that autocrine secretion of IGF-II plays a critical role in stimulating spontaneous myogenic differentiation in vitro, we have established C2 muscle cell lines that stably express a mouse IGF-II cDNA under control of the strong, constitutively active Moloney sarcoma virus promoter, enabling us to study directly the effects of IGF-II overproduction. Similar to observations with other muscle cell lines, IGF-II overexpressing myoblasts proliferated normally in growth medium containing 20% fetal serum, but they underwent enhanced differentiation compared with controls when incubated in low-serum differentiation medium. Accelerated differentiation of IGF-II overexpressing C2 cells was preceded by the rapid induction of myogenin mRNA and protein expression (within 1 h, compared with 24-48 h in controls) and was accompanied by an enhanced proportion of the retinoblastoma protein in an underphosphrylated and potentially active form, by a marked increase in activity of the muscle-specific enzyme, creatine phosphokinase, by extensive myotube formation by 48 h, and by elevated secretion of IGF binding protein-5 when compared with controls. These results confirm a role for IGF-II as an autocrine/paracrine differentiation factor for skeletal myoblasts, and they define a model cell system that will be useful in determining the biochemical mechanisms of IGF action in cellular differentiation.

Insulin-like growth factor-II is an autocrine survival factor for differentiating myoblasts

Recent studies indicate that insulin-like growth factor-II (IGF-II) acts as an autocrine differentiation factor for skeletal myoblasts in culture. IGF-II mRNA and protein are induced as early events in muscle differentiation, and the rate and extent of IGF-II secretion correlate with both biochemical and morphological differentiation. Here we show that IGF-II also functions as an essential survival factor during the transition from proliferating to differentiating myoblasts. Stably transfected C2 muscle cell lines were established in which a mouse IGF-II cDNA was expressed in the antisense orientation relative to the constitutively active Moloney sarcoma virus promoter. IGF-II antisense cells proliferated normally in growth medium containing 20% serum but underwent rapid death when placed in low serum differentiation medium. Death was accompanied by characteristic markers of apoptosis with more than 90% of cells showing DNA fragmentation within 12-16 h. Myoblast death was prevented by IGF-I, des [1-3] IGF-I, IGF-II, and insulin with a dose potency consistent with activation of the IGF-I receptor; death also could be blocked by the protein synthesis inhibitor, cycloheximide. Exogenous IGFs additionally stimulated passage through a single cell cycle and subsequently induced terminal differentiation. Cell survival and cell cycle progression also were enhanced by fibroblast growth factor-2 and platelet-derived growth factor-bb, but these peptides did not promote differentiation. Our results define a novel system for studying apoptotic cell death and its prevention by growth factors, underscore the importance of IGF action in minimizing inappropriate cell death, and indicate that shared signal transduction pathways may mediate myoblast survival in vitro.

Expression of IGF-II and IGF binding proteins in differentiating human intestinal Caco-2 cells

The mitogenic and metabolic effects of insulin-like growth factor-II (IGF-II) can be modulated by six distinct IGF binding proteins (IGFBPs). As a first step toward understanding the role of IGFs and their binding proteins in intestinal epithelial cell differentiation, the expression of IGF-II and IGFBPs was characterized in the human colon adenocarcinoma Caco-2 cell line. Northern blot analysis revealed two IGF-II transcripts of 5.4 and 4.5 kb, and ribonuclease protection assays indicated that IGF-II mRNA levels are regulated during Caco-2 differentiation. A specific radioimmunoassay detected IGF-II in serum-free conditioned medium, the level of which was three- to fivefold higher in proliferating cells than in differentiated cells. Immunoprecipitation and ligand blot analyses of conditioned medium demonstrated that IGFBP-2, IGFBP-3, IGFBP-4, and IGFBP-6 are synthesized by Caco-2 cells, with IGFBP-2 and IGFBP-4 being the major IGFBPs secreted, and that the levels of IGFBP-2 and IGFBP-6 decreased as differentiation proceeded. These results indicate that the expression of IGF-II, IGFBP-2, and IGFBP-6 is regulated in a differentiation-dependent manner in Caco-2 cells.

Insulin-like growth factor-I (IGF-I) and IGF-binding proteins blood serum levels in women with early- and late-stage breast cancer: mutual relationship and possible correlations with patients' hormonal status

The pathogenesis and progression of breast cancer involve complex interactions between hormones and polypeptide growth factors such as insulin-like growth factor-I (IGF-I). IGF-I has been found in stromal fibroblasts derived from malignant and benign breast tissue and it is a mitogen for several breast cancer cell lines. It circulates bound to specific high-affinity binding proteins, which could act as either positive or negative modulators of tumorigenesis. This study has been addressed to characterize IGF-I and its binding proteins in the serum of 85 unselected patients with early breast cancer. The IGF-I concentration was assessed by radioimmunoassay of 69 out of 85 samples before and after dissociation of the IGF-I and IGF-binding protein (IGF-BP) complex whereas IGF-BP of all 85 sera were analyzed by Western ligand blotting; estradiol and progesterone were measured by radioimmunoassay in native serum samples. In our study no differences in IGF-I serum levels between pre- and post-menopausal patients were observed. Patients with higher estradiol and progesterone serum levels did not present different IGF-I concentrations compared to patients with lower serum levels. Furthermore, IGF-I median values were not found to depend on estrogen receptor (ER) status. A heterogeneous quali-quantitative molecular pattern of binding proteins was detected: IGF-BP3 and IGF-BP1 were the most and the least expressed respectively. No correlations between ER status, or parameters related to the hormonal status, and IGF-I or binding proteins expression were observed. No significant differences in IGF-I concentration and IGF-BP expression were observed between cancer patients and a control group matched for age and menopausal status. Finally, preliminary collection of 20 sera derived from patients with late breast cancer was analyzed for IGF-I and its binding proteins content.

Soluble insulin-like growth factor II/mannose 6-phosphate receptor carries multiple high molecular weight forms of insulin-like growth factor II in fetal bovine serum

We have characterized a soluble form of the insulin-like growth factor II/mannose 6-phosphate receptor (sIGF-II/MPR) and bound ligands from bovine serum. Fetal serum contained 2-8 mg/liter sIGF-II/MPR. Affinity-purified receptor isolated by adsorption to phosphomannan-agarose and elution with mannose 6-phosphate contained nearly stoichiometric amounts of bound 7.5-kDa IGF-II. In addition, at least 12 distinct 12-20-kDa proteins immunologically related to IGF-II also copurified with receptor. Receptor was separated from its associated ligands by acidification and gel filtration chromatography. Sequence analysis revealed that the 12-20-kDa proteins have the same amino termini as mature 7.5-kDa IGF-II. Protease and glycosidase treatments revealed that the different high molecular weight IGF-II species contain an identical COOH-terminal extension that is differentially glycosylated with O-linked sugars. Radiolabeled tracer experiments demonstrated that the sIGF-II/MPR carries approximately 1/4 of the IGF-II in fetal bovine serum. These results support a significant role for sIGF-II/MPR in the transport of circulating IGF-II isoforms during development.

Loss of the imprinted IGF2/cation-independent mannose 6-phosphate receptor results in fetal overgrowth and perinatal lethality

Murine embryos that inherit a nonfunctional insulin-like growth factor-II/cation-independent mannose 6-phosphate receptor (Igf2r) gene from their fathers are viable and develop normally into adults. However, the majority of mice inheriting the same mutated allele from their mothers die around birth, as a consequence of major cardiac abnormalities. These mice do not express IGF2R in their tissues, are 25-30% larger than their normal siblings, have elevated levels of circulating IGF2 and IGF-binding proteins, and exhibit a slight kink in their tails. These results show that Igf2r is paternally imprinted and reveal that the receptor is crucial for regulating normal fetal growth, circulating levels of IGF2, and heart development.

Immunolocalization and expression of insulin-like growth factor I (IGF-I) in the mammary gland during rat gestation and lactation

We have studied the physiology and tissue expression of IGF-I and IGF-BP3 in pregnant and lactating rats. Specific assays (radioimmunoassays and a binding protein assay) were used to measure serum IGF-I, IGF-II, and IGF-BP levels. IGF-I and IGF-BP3 expression levels were determined in mammary gland and liver by slot-blotting. A sensitive and IGF-I-specific ribonuclease (RNAse) protection assay was further used to detect RNAs transcribed from the IGF-I gene. In the first half of pregnancy, the maternal serum IGF-I concentration rises while the IGF-BP level decreases. This may modify IGF-I availability, thus promoting rapid tissue growth and differentiation. In the second half of pregnancy, the mean serum IGF-I concentration falls sharply from 1140 +/- 150 ng/ml at seven days of pregnancy to 470 +/- 85 ng/ml at 20 days. Post-partum, serum IGF-I increases back to the level obtained in non-pregnant controls within 5 days. Serum levels of IGF-BP, during the same two periods, follow a similar pattern, decrease during pregnancy and increase after parturition. No IGF-II was detected at any time. From the onset of pregnancy to term, IGF-I gene expression in the mammary gland diminishes. In the liver, on the other hand, expression increases during very early pregnancy and diminishes thereafter, remaining below the level measured in non-pregnant animals from mid-pregnancy to term. The pattern of IGF-BP3 expression followed was similar in both organs, with a decrease during gestation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of insulin and 2-deoxy-D-glucose administration on CNS IGF-II content

In order to determine whether effects of insulin on central nervous system (CNS) insulin-like growth factor II (IGF-II) content are direct or whether they are due to insulin-induced glucoprivation, short-term (2 h) time course studies were conducted utilizing the glucoprivic agent 2-deoxy-D-glucose (2-DG). Insulin (2 U/kg b.wt.), 2-DG (500 mg/kg b.wt.) or vehicle were administered to Sprague-Dawley rats (350-450 kg) and groups of animals were killed at time 0 and 15, 30, 45, 60, and 120 min following injection. Specific hypothalamic and hindbrain regions obtained by microdissection, were analyzed for IGF-II content by RIA. Insulin and 2-DG exerted similar effects on IGF-II content in the dorsomedial hypothalamus and the paraventricular nucleus. These data suggest that IGF-II was primarily regulated by glucoprivation in these regions. Only 2-DG altered ventromedial hypothalamic IGF-II content, and opposite responses to the two agents were observed in the arcuate nucleus. This uncoupling of IGF-II response suggests that differences in peripheral vs. central signals for IGF-II secretion may be involved. The vagal complex was responsive only to insulin injection indicating a specific response that may be tied to sensory vagal function.

Serum "big insulin-like growth factor II" from patients with tumor hypoglycemia lacks normal E-domain O-linked glycosylation, a possible determinant of normal propeptide processing

The insulin-like growth factor II (IGF-II) gene is overexpressed in many mesenchymal tumors and can lead to non-islet-cell tumor hypoglycemia (NICTH). ProIGF-II consists of the 67 aa of IGF-II with a carboxyl 89-aa extension, the E domain. A derivative of proIGF-II containing only the first 21 aa of the E domain [proIGF-II-(E1-21)] has been isolated by others from normal serum and has O-linked glycosylation. We found that the "big IGF-II" of normal serum, as detected by an RIA directed against residues 1-21 of the E domain of proIGF-II, was reduced in size by treatment with neuraminidase and O-glycosidase. The big IGF-II, which is greatly increased in NICTH sera, was unaffected by neuraminidase and O-glycosidase treatment. We have also shown that big IGF-II from normal serum is retained by jacalin lectin columns and that big IGF-II from NICTH serum was not retained, indicating that it lacked O-glycosylation. Normal O-linked glycosylation may be required for proper peptidase processing of proIGF-II. The lack of normal O-linked glycosylation by tumors may explain the predominance of big IGF-II in NICTH sera. In normal serum, most of the IGF-II is present in a 150-kDa ternary complex with IGF-II binding protein (IGFBP) 3 and alpha subunit. In NICTH serum, however, the complexes carrying big IGF-II are < 50 kDa. We investigated whether big IGF-II of NICTH was responsible for this abnormality. Tumor big IGF-II and IGF-II were equally effective in forming the 150-kDa complex with purified IGFBP-3 and 125I-labeled alpha subunit. Both 125I-labeled IGF-II and 125I-labeled proIGF-II-(E1-21), when incubated with normal serum, formed the 150-kDa complex as detected by Superose 12 exclusion chromatography. We conclude that the nonglycosylated big IGF-II of NICTH serum can form normal complexes with serum IGFBPs. The defective binding in NICTH is attributable to defective IGFBP-3 binding.

Intracerebroventricular injection of insulin or glucose alters insulin-like growth factor II (IGF-II) concentrations in specific hypothalamic nuclei

Peripherally administered insulin has been shown to alter content and gene expression of hypothalamic insulin-like growth factor II (IGF-II) in a region specific manner (Lauterio, TJ. et al., Endocrinology, 126 (1990) 392-398. The objective of this experiment was to determine whether central administration of insulin can modulate hypothalamic IGF-II peptide content. Male Sprague-Dawley rats were implanted with lateral ventricular cannulae and allowed to recover from surgery one week prior to injection. At that point animals were remotely injected with one of the following: (1) synthetic cerebral spinal fluid vehicle (sCSF); (2) 2 mIU porcine insulin (I); (3) glucose (100 mg%) only. Animals were decapitated 30 min after injections and brains were quickly removed, frozen and dissected into specific hypothalamic regions for IGF-II analysis by RIA. Insulin increased IGF-II content in the ventromedial hypothalamic region by 80% (P < 0.001) and paraventricular nucleus by 30% (P < 0.01) compared to sCSF or glucose treatment. Arcuate nucleus and neurointermediary lobe pituitary IGF-II content was decreased with insulin treatment compared to controls (P < 0.01). Insulin had no effect on IGF-II concentrations in the dorsomedial or lateral hypothalamic regions or in the supraoptic and suprachiasmatic nuclei. Peripheral concentrations of glucose, insulin and IGF-II were unaffected by any treatment. Results show that insulin which reaches the brain can alter IGF-II levels in specific regions of the hypothalamus and suggests a possible role for IGF-II in insulin mediated changes in metabolism or hypothalamic hormone secretion.

Tumor hypoglycemia: relationship to high molecular weight insulin-like growth factor-II

The mechanism of tumor-associated hypoglycemia was examined in 11 patients with hepatocellular carcinoma, 6 of whom presented with severe hypoglycemia and 5 in whom plasma glucose was persistently normal. Serum insulin levels in the hypoglycemic patients were low. Although total serum insulin-like growth factor II (IGF-II) levels in both groups of tumor patients were lower than normal, tumor tissue from hypoglycemic patients contained levels of IGF-II mRNA that were 10-20-fold higher than those present in normal liver. IGF-II immunoreactivity consisted in all cases of a mixture of both higher molecular weight forms and material having the character of IGF-II itself. The former comprised a greater proportion of total IGF-II, in patients with hypoglycemia. Studies to characterize the interactions of IGF-II with serum proteins showed that (a) the radiolabeled peptide bound to an approximately 40,000-D protein in sera from both hypoglycemic patients and normal subjects, (b) sera from hypoglycemic patients and normal subjects had similar capacity to bind the radiolabeled peptide, and (c) the apparent affinities of serum binding proteins for IGF-II were the same for both hypoglycemic patients and normal subjects. Whereas, acid extracted, tumor-derived IGF-II immunoreactive peptides with low or intermediate molecular weights bound to serum proteins in a manner indistinguishable from that of IGF-II itself, the highest molecular weight IGF-II immunoreactive peptide exhibited negligible ability to compete for radiolabeled ligand binding to serum proteins. The low affinity of serum binding proteins for this component suggests that high molecular weight IGF-II immunoreactivity might circulate free and be available for interaction with cell-surface receptors.

The effect of ovarian function on insulin-like growth factor I plasma levels and hepatic IGF-I mRNA levels in diabetic rats treated with insulin

When insulin was administered to streptozotocin-induced diabetic female rats, the percentage of glycohemoglobin, growth rate, ovulatory cycle, uterus to body weight ratio, and insulin-like growth factor (IGF-I) level returned to near normal. In untreated diabetic rats there were no normal estrous cycles, and hepatic IGF-I mRNA (7.94 +/- 1.02 O.D. units per micrograms total RNA) levels were significantly lower than the control or insulin-treated groups in proestrus (16.47 +/- 0.91 and 17.15 +/- 1.84, respectively). Insulin therapy restored the hypothalamic-pituitary-ovarian axis with the reinstitution of normal estrous cycles. Plasma IGF-I levels were highest in non-diabetic proestrous animals (277 +/- 36.9 ng/ml), significantly higher than IGF-I levels in insulin-treated diabetic rats in diestrus (174 +/- 23.1 ng/ml), non-diabetic diestrus rats (165 +/- 18.4 ng/ml) and untreated diabetic rats (135 +/- 19.7 ng/ml). Plasma IGF-I levels were elevated in insulin-treated diabetic rats in proestrus (221 +/- 78.3 ng/ml), however this was not significantly different from any other group. The increases observed in plasma IGF-I and hepatic IGF-I mRNA after insulin therapy correlate with the normalization of sex hormone secretion. Though this study does not prove a causal relationship between restoration of ovarian function and normalization of circulating IGF-I levels, a relationship has been established, as evidenced by higher levels of IGF-I in both the control and insulin-treated diabetic proestrous groups when compared to the diestrus groups.

Human growth hormone stimulates liver regeneration in rats

To study the effect of human growth hormone (hGH) on liver regeneration in rats, 200 micrograms hGH was administered to partial hepatectomized rats twice a day for three days. The bw of hGH-treated rats was higher than that in untreated rats. After three day administration, the liver weight was 3.18 +/- 0.13 g, significant higher than that of untreated rats (2.68 +/- 0.17 g). Human GH also stimulated the mitosis in the liver. Serum insulin-like growth factor I (IGF-I) and albumin levels were significantly increased and urea nitrogen levels were significantly decreased in hGH-treated rats compared with those in untreated rats. When 120 micrograms/day IGF-I was continuously administered to partial hepatectomized rats for three days, the bw and the liver weight were not higher than those of controls. These data indicate that hGH directly stimulates liver regeneration and recover liver dysfunction in rats.

Coordinate expression of insulin-like growth factor II and its receptor during muscle differentiation

The role of polypeptide growth factors in promoting muscle differentiation is uncharacterized. We have used a fusing skeletal muscle cell line, C2, to examine the endogenous expression of one peptide, insulin-like growth factor II (IGF-II), and its receptor during differentiation. The synthesis of IGF-II is low during proliferation of myoblasts; IGF-II mRNA can be detected only through use of a highly sensitive solution-hybridization assay. Competition binding studies reveal that the IGF-II receptor is similarly nonabundant in myoblasts. During differentiation IGF-II mRNA rises rapidly. A nearly 4-fold increase is seen within 16 hr of onset of the differentiation process, and levels are 25 times higher than those in myoblasts by 96 hr, when myotubes have formed and muscle-specific alpha-actin mRNAs are synthesized. IGF-II accumulates in conditioned culture medium with similar kinetics. The expression of IGF-II receptors on the cell surface increases almost 6-fold 24 hr after the onset of differentiation and remains high. These studies suggest that IGF-II and its receptor are coordinately regulated during myogenic differentiation in C2 cells and that IGF-II may be an autocrine factor for skeletal muscle.

Production of IGF-II-related peptide by an anaplastic cell line (AT-3) established from the Dunning prostatic carcinoma of rats

AT-3 cells, one of anaplastic cell lines established from the Dunning prostatic carcinoma of rats, were able to grow under serum-free conditions in a state of suspension detached from a substratum. Radioimmunoassay using monoclonal antibody against rat insulin-like growth factor II (IGF-II) revealed the presence of IGF-II-related peptide in acid-ethanol extracts of lyophilized serum-free media conditioned by AT-3 cells. The peptide contents in the culture media increased with increase in cell number; 71 ng at 3.0 X 10(6) cells and 449 ng at 4.6 X 10(7) cells. IGF-II-related peptide was hardly detectable in acid-ethanol extracts of AT-3 cells harvested after 13-days culture. These results indicate that AT-3 cells produce IGF-II-related peptide and may release it into the culture media.