Insulin-like growth factor II binding to the type I somatomedin receptor. Evidence for two high affinity binding sites

Cell cycle control by the insulin-like growth factor signal: at the crossroad between cell growth and mitotic regulation

In proliferating cells and tissues a number of checkpoints (G1/S and G2/M) preceding cell division (M-phase) require the signal provided by growth factors present in serum. IGFs (I and II) have been demonstrated to constitute key intrinsic components of the peptidic active fraction of mammalian serum. In vivo genetic ablation studies have shown that the cellular signal triggered by the IGFs through their cellular receptors represents a non-replaceable requirement for cell growth and cell cycle progression. Retroactive and current evaluation of published literature sheds light on the intracellular circuitry activated by these factors providing us with a better picture of the pleiotropic mechanistic actions by which IGFs regulate both cell size and mitogenesis under developmental growth as well as in malignant proliferation. The present work aims to summarize the cumulative knowledge learned from the IGF ligands/receptors and their intracellular signaling transducers towards control of cell size and cell-cycle with particular focus to their actionable circuits in human cancer. Furthermore, we bring novel perspectives on key functional discriminants of the IGF growth-mitogenic pathway allowing re-evaluation on some of its signal components based upon established evidences.

IGF-I binding to the IGF-I receptor is affected by contaminants in commercial BSA: the contaminants are proteins with IGF-I binding properties

OBJECTIVE

To determine whether different albumins have an effect on IGF-I binding assays.

METHODS

We have studied the effect of five different albumins in plate antibody capture binding assay. For IGF-IR studies the IGF-IR specific antibody 24-31 was used and for IR/IGF-IR hybrid receptors the IR specific antibody 83-7 was used. Binding to IGF-IR was studied by displacement of (125)I-IGF-I with IGF-I in the absence or presence of 0.1%, 0.5% or 1% (w/v) albumin. The IR/IGF-IR hybrid receptors were studied in the presence of 0.5% (w/v) of HSA A-1887 and BSA A-7888 and with IGF-I or insulin displacement of (125)I-IGF-I. The albumins used were purchased from Sigma-Aldrich. Two batches of albumins from each catalog number were tested. The albumins were: HSA A-1887, BSA A-4503, BSA A-6003, BSA A-7030, and BSA A-7888. Contaminants in the albumins were characterized as proteins with IGF-I binding properties by cross-linking to (125)I-IGF-I and SDS-page analysis.

RESULTS

BSA A-4503, A-7030 and A-7888 from Sigma-Aldrich contain proteins with IGF-I binding properties. These contaminants increased the determined EC50 for displacement of (125)I-IGF-I from IGF-IR up to 40-fold in a BSA dependent manner. The presence of BSA-7888 in binding experiments increased the determined EC50 for IR/IGF-IR hybrid receptors 8-16-fold.

CONCLUSIONS

When IGF-I is characterized with respect to the effect on living cells and on binding to potential receptors unspecific binding to surfaces is often prevented by the addition of albumin in the assay. Here we report that when binding to the classical IGF-IR and IR/IGF-IR hybrid receptors are studied the measured EC50 values can be albumin dependent if it is contaminated with proteins with IGF-I binding properties. The free IGF-I concentration will be lower than estimated. Thus, the contaminated BSA preparations result in artifacts leading to misinterpretations and underestimation of the effect of IGF-I. Our results provide one possible explanation as to why different laboratories report different EC50 values for IGF-I.

Disruption of growth hormone secretion alters Ca2+ current density and expression of Ca2+ channel and insulin-like growth factor genes in rat atria

The influence of the growth hormone (GH)-insulin-like growth factor I (IGF-I) axis on expression of low-voltage-activated (LVA) Ca2+ current in atrial tissue was investigated using spontaneous dwarf (SpDwf) rats, a mutant strain that lacks GH. Atrial myocytes from SpDwf rats express LVA and high-voltage-activated (HVA) Ca2+ currents and the Ca2+ channel alpha1-subunit genes CaV1.2, CaV2.3, CaV3.1, and CaV3.2. LVA current density decreases significantly beginning at, or shortly after, birth in normal animals; however, its density is maintained in SpDwf rats at 1 pA/pF for > or =12 wk after birth. The abundance of mRNAs encoding CaV2.3 and CaV3.2 declines with advancing age in normal atrial development, yet expression of CaV2.3 mRNA remains significantly elevated in older SpDwf animals. Quantitation of local transcript levels for mRNAs encoding IGF-I and IGF-I receptor (IGF-IR) also reveals significant differences in expression of these transcripts in atrial tissue of SpDwf animals compared with controls. In SpDwf rats, the abundance of IGF-IR mRNA remains elevated at many postnatal ages, whereas mRNA encoding IGF-I is maintained only in older animals. Physiological concentrations of IGF-I cause two- to threefold increases in LVA current density in primary cultures of atrial myocytes, and this effect is blocked by an antisense oligonucleotide targeting the IGF-IR. Thus disruption of GH production in SpDwf animals alters expression of atrial LVA Ca2+ channel and IGF genes as well as postnatal regulation of LVA Ca2+ current density, most likely acting through compensatory mechanisms via the local IGF-IR.

Mapping of the insulin-like growth factor II binding site of the Type I insulin-like growth factor receptor by alanine scanning mutagenesis

The Type I insulin-like growth factor receptor is a physiological receptor for insulin-like growth factor II (IGF-II). To characterize the molecular basis of the receptor's ligand binding properties, we have examined the effects of alanine mutations of residues in the ligand binding site of the receptor on its affinity for IGF-II. The functional epitope for IGF-II comprises residues in the N-terminal L1 domain and residues at the C-terminus of the alpha subunit. Cysteine rich domain residues do not appear to be critical for IGF-II binding.

Effects of recombinant insulin-like growth factor-binding protein-4 on bone formation parameters in mice

Insulin-like growth factor (IGF)-binding protein-4 (IGFBP-4), one of the most abundant IGFBPs produced by bone cells, is a potent inhibitor of IGF actions in vitro. To evaluate the modulation of IGF actions on bone formation in vivo by IGFBP-4, we produced intact and fragment (50- to 100-fold reduced IGF affinity) forms of BP-4 and examined their local and systemic effects using biochemical markers. Local administration of IGF-I over the right parietal bone significantly increased bone extract alkaline phosphatase activity; this was completely blocked by an equimolar dose of intact IGFBP-4, but not IGFBP-4 fragment. A single sc administration of IGF-I (2 microg/g BW) significantly increased bone formation markers in both serum and skeletal extracts; surprisingly, so did intact IGFBP-4, but not fragment IGFBP-4. Subcutaneous administration of an equimolar dose of IGFBP-4 along with IGF-I did not significantly block the IGF-I effect. Administration of intact IGFBP-4 significantly increased the serum 50-kDa IGF pool and decreased the 150-kDa IGF pool without significantly changing total IGF-I. We postulate that the increase in the 50-kDa IGF pool might enhance IGFs bioavailability via a mechanism involving IGFBP-4-specific protease. This study demonstrates for the first time that a single local administration of IGFBP-4 inhibits IGF-I-induced increases in bone formation, whereas systemic administration of IGFBP-4 alone increases serum levels of bone formation markers.

Hammerhead ribozyme-mediated cleavage of the human insulin-like growth factor-II ribonucleic acid in vitro and in prostate cancer cells

Insulin-like growth factor (IGF)-II plays an important role in fetal growth and development. IGFs are potent mitogens for a variety of cancer cells. A paracrine/autocrine role of IGF-II in the growth of breast and prostate cancer cells has been suggested. To test the role of IGF-II in cancer cell growth, hammerhead ribozymes targeted to human IGF-II RNA were constructed. Single (R)- and double (RR)-ribozymes were catalytically active in vitro whereas mutant ribozymes (M or MM) did not cleave IGF-II RNA. RR was more active than R. In human prostate cancer PC-3 cells, both R and RR similarly suppressed IGF-II messenger RNA (mRNA) levels (approximately 40%) compared with the level in parental or M-expressing PC-3 cells. Polymerase II and III promoter-driven R similarly suppressed IGF-II mRNA levels. Suppression of IGF-II mRNA levels by R was associated with suppression of IGF-II protein levels. R- (or RR-) expressing PC-3 cells did not grow under serum-starved conditions and showed prolonged doubling times in the presence of 10% FCS compared with those of parental or M-expressing cells. These results substantiated that IGF-II plays a critical role in prostate cancer cell growth.

Inhibition of insulin receptor activation by insulin-like growth factor binding proteins

The insulin-like growth factors (IGFs) are transported by a family of high-affinity binding proteins (IGFBPs) that protect IGFs from degradation, limit their binding to IGF receptors, and modulate IGF actions. The six classical IGFBPs have been believed to have no affinity for insulin. We now demonstrate that IGFBP-7/mac25, a newly identified member of the IGFBP superfamily that binds IGFs specifically with low affinity is a high-affinity insulin binding protein. IGFBP-7 blocks insulin binding to the insulin receptor and thereby inhibiting the earliest steps in insulin action, such as autophosphorylation of the insulin receptor beta subunit and phosphorylation of IRS-1, indicating that IGFBP-7 is a functional insulin-binding protein. The affinity of other IGFBPs for insulin can be enhanced by modifications that disrupt disulfide bonds or remove the conserved COOH terminus. Like IGFBP-7, an NH2-terminal fragment of IGFBP-3 (IGFBP-3((1-87))), also binds insulin with high affinity and blocks insulin action. IGFBPs with enhanced affinity for insulin might contribute to the insulin resistance of pregnancy, type II diabetes mellitus, and other pathological conditions.

Spatial polarization of insulin-like growth factor receptors on the human syncytiotrophoblast

The expression of IGF receptors on the maternal-facing, microvillous membrane (MVM) surface and the fetal-facing, basal membrane (BM) surface of the syncytiotrophoblast was studied using standard ligand binding assays, covalent cross-linking techniques, and immunoblot analysis. Scatchard analysis of [125I]IGF-I and -II binding revealed the presence of both high and low affinity binding sites associated with each membrane preparation that did not clearly distinguish between the two membrane preparations. Cross-linking analysis, however, demonstrated type I and type II IGF receptors associated primarily with MVM, suggesting that nonreceptor binding sites may contribute to total membrane binding. Ligand blot analysis revealed that BM are uniquely associated with 29- and 24-kD IGF binding proteins (IGFBPs). [125I]QAYL-IGF-I, having reduced affinity for IGFBPs, was therefore used to study receptor-specific binding. Approximately 5-fold more type I IGF receptors were shown to be associated with MVM than BM by Scatchard and cross-linking analyses. This was confirmed by immunoblot analysis. By contrast, immunoblot analysis revealed approximately 50-100% more type II IGF receptor protein associated with BM, whereas cross-linking to [125I]IGF-II revealed a MVM predominance. In the presence of 5 mM mannose 6-phosphate, however, a substantial increase in [125I]IGF-II cross-linked to the type II IGF receptor was observed in BM but not MVM consistent with immunoblot analysis. These data demonstrate that type 1 and unoccupied type II IGF receptors are expressed primarily on the maternal-facing. MVM surface of the syncytiotrophoblast.

Type I insulin-like growth factor receptors in the BeWo choriocarcinoma cell (b30 clone) during cell differentiation

The expression of insulin-like growth factor (IGF) receptors in the differentiating human trophoblast was studied using the b30 clone of the BeWo choriocarcinoma cell line (BeWob30) as a model system. This clonally derived cell line differentiates over 48-72 h, in culture, to form syncytiotrophoblasts when intracellular cAMP levels are elevated by exposure to 100 microM forskolin (FSK). IGF receptors were studied at various times during the differentiation process by measuring the specific binding of [125I]-IGF-I and [125I]-IGF-II to attached cells. First, [125I]-IGF-I bound to a single class of binding sites in the untreated cells (KD approximately 1-2 x 10-10 M) that exhibited binding specificity characteristic of the type I IGF receptor (IGF-I > or = IGF-II > > Insulin). FSK treatment resulted in a two- to threefold increase in the number of these binding sites. Increased receptor expression was observed as early as 24 h after FSK treatment and remained elevated for at least 72 h. Next, [125I]-IGF-II bound to two classes of binding sites in the untreated cells, a high-affinity (KD approximately 2.5 x 10(-10) M), low-capacity site and a low-affinity (KD approximately 6 x 10(-9) M), high-capacity site. The Bmax and KD of the high-affinity site suggested that it represented the type I IGF receptor. Competition studies revealed that 15-20 per cent of total [125I]-IGF-II binding only was sensitive to IGF-I competition in the untreated cells. After FSK treatment, however, unlabelled IGF-I inhibited 60-70 per cent of specific [125I]-IGF-II binding. Scatchard analysis revealed a two- to fourfold increase in the number of both binding sites with no change in their respective binding affinities. Cross-linking analysis demonstrated that [125I]-IGF-II bound to two structurally distinct binding sites in the untreated BeWob30 cell consistent with both the type I and II IGF receptors. After FSK treatment, however, there was an increase in the relative amount of [125I]-IGF-II associated with the higher affinity type I IGF receptor. The BeWob30 cells expressed no insulin receptors at any stage of differentiation. These data demonstrate that the BeWob30 choriocarcinoma cell line expresses both type I and II IGF receptors. Induction of cell differentiation is associated with an increase in type I IGF receptors expressed at the cell surface. These receptors bind IGF-II with high-affinity, providing additional binding capacity for locally available IGF-II. These data are consistent with specific roles for the type I IGF receptor in regulating differentiated trophoblast cell function. Furthermore, the early rise in type I IGF receptor number suggests they may play a regulatory role in the differentiation process itself.

Expression of insulin-like growth factor (IGF)-I receptors, IGF-II/cation-independent mannose 6-phosphate receptors (CI-MPRs), and cation-dependent MPRs in polarized human intestinal Caco-2 cells

We have analyzed the surface distribution and functional expression of the insulin-like growth factor I (IGF-I) receptor and the IGF-II/cation-independent mannose 6-phosphate (IGF-II/CI-MPR) in the polarized human colon adenocarcinoma cell line, Caco 2. Domain-selective biotinylation of the apical and basolateral surfaces of Caco-2 cells grown on filter supports revealed a 3-4-fold enrichment of these receptors on basolateral membranes. In addition, the biotinylation studies revealed the presence of the cation-dependent MPR on both membrane surfaces, with a 3.4-fold enrichment on basolateral membranes. Binding of 125I-IGF-I at 4 degrees C confirmed similar higher levels of expression of the IGF-I receptor at the basolateral surface than at the apical surface. Cell surface-specific binding of the iodinated lysosomal enzyme beta-glucuronidase was detected at 4 degrees C on both plasma membrane domains. However, significant uptake of beta-glucuronidase at 37 degrees C was observed only from the basolateral surface. These results indicate that the MPRs and the IGF-I receptor are expressed in a polarized fashion in Caco-2 cells and that the IGF-II/CI-MPR present on apical membranes, unlike the IGF-II/CI-MPR expressed on the basolateral surface, is not functional in endocytosing lysosomal enzymes.

N-terminal deletion mutants of insulin-like growth factor-II (IGF-II) show Thr7 and Leu8 important for binding to insulin and IGF-I receptors and Leu8 critical for all IGF-II functions

To define the role of the N-terminal region of insulin-like growth factor-II (IGF-II) in its binding to insulin and IGF receptors, deletion mutants des-(1-5)-, des-(1-7)-, and des-(1-8)-recombinant (r) IGF-II, and the Gly8 for Leu substitution mutant of rIGF-II were prepared by site-directed mutagenesis, expressed in Escherichia coli, and purified. The binding affinity and mitogenic activity of these rIGF-II mutants as well as commercially available des-(1-6)-rIGF-II were analyzed. While the relative affinity of des-(1-5)- and des-(1-6)-rIGF-II for purified human insulin and IGF-I receptors remained at > or = 50% levels of that of rIGF-II, the affinity of des-(1-7)-rIGF-II decreased to approximately 10% and approximately 3%, respectively, of that of rIGF-II. When the octapeptide including Leu8 was removed prior to the Cys9-Cys47 intrachain bond, the relative affinity of this deletion mutant, des-(1-8)-rIGF-II, for these receptors dramatically decreased to < 1% of that of rIGF-II. Substituting Gly8 for Leu in rIGF-II decreased the affinity of this mutant for the IGF-I and insulin receptors to about the same extent. These results suggest that the side chains of Thr7 and Leu8 may play an important role in retaining all of the IGF-II functions. Decreases in the relative affinity for binding of the mutants to these receptors paralleled the decreases in their mitogenic potency for cultured Balb/c 3T3 cells. Although the relative affinity of des-(1-8)- or [Gly8]rIGF-II for rat IGF-II/CIM6-P (cation-independent mannose 6-phosphate) receptors was also < 1% of that of rIGF-II, the relative affinities of des-(1-5)-, des-(1-6)-, and des-(1-7)-rIGF-II for these receptors was significantly greater than that of rIGF-II. These results clearly demonstrate that Thr7 and Leu8 are important for binding to insulin and IGF-I receptors and Leu8 is critical for expression of all IGF-II functions.

IGF-II is more active than IGF-I in stimulating L6A1 myogenesis: greater mitogenic actions of IGF-I delay differentiation

Mitogens are generally thought to inhibit myogenesis, and many cell biologists have found it hard to interpret observations that the insulin-like growth factors (IGFs) stimulate both proliferation and differentiation of muscle cells in culture. Our previous studies suggested that the Type I IGF receptor mediates these actions. However, IGF-II and insulin treatment caused myoblasts to differentiate much more extensively, suggesting that more complex mechanisms may be involved. Here we present evidence that the greater mitogenic activity of IGF-I (compared to IGF-II and insulin) delays L6A1 myoblast differentiation. Under conditions in which the mitogenic actions of IGF-I are suppressed, the stimulation of myogenesis by IGF-I approached that by IGF-II: (1) in L6A1 cultures plated at a higher cell density; (2) in L6A1 cultures in which cell proliferation was inhibited by cytosine arabinoside or aphidicolin; and (3) in cultures of primary human muscle cells, which exhibit a smaller mitogenic response to IGF-I. Further evidence that the Type I receptor plays a major role in relaying the signal for differentiation was obtained by using IGF-I and IGF-II analogs. Analogs which have reduced affinity for the Type I receptor showed a dramatic decrease in activity, while an analog with increased affinity for the Type II receptor was no more active than native IGF-I. Our results indicate that both mitogenic and myogenic actions of IGF-I are mediated by the Type I receptor. We conclude that IGF-I delays the onset of myogenesis as a result of its mitogenic actions, and only subsequently stimulates myogenesis. These observations reconcile the apparent conflict between our results with the IGFs and other investigators' reports of effects of other mitogens.

Insulin-like growth factor II mRNA, peptides, and receptors in a thoracopulmonary malignant small round cell tumor

Insulin-like growth factor-(IGF) II and IGF-I and IGF-II/mannose 6-phosphate receptors were expressed in a thoracopulmonary malignant small round cell tumor (MSRCT) from a 14-year-old boy. Northern analysis showed that the MSRCT expresses multiple IGF-II mRNA of 6.0, 4.8, 4.2, and 2.2 kilobase from promoters P3 and P4 of the human IGF-II gene. Chromatography and radioimmunoassay revealed two forms of IGF-II with molecular masses of 7.5 kilodalton (kDa) and 10 kDa, corresponding to mature IGF-II and IGF-II with a C-terminal extension, in concentrations of 61 and 41 ng/g/tumor tissue, respectively. By a combined reverse transcription-polymerase chain reaction analysis, the authors also show that the MSRCT expresses IGF-I and IGF-II/mannose 6-phosphate receptor mRNA. The plasma concentration of IGF-II was 600 ng/ml and within the normal range of serum IGF-II. IGF binding proteins (IGFBP) of 41.5, 38.5, 34, 30, and 24 kDa were present in serum. Compared with normal plasma from healthy subjects and an age-matched group of boys, the level of the 41.5, 38.5, and 30 kDa IGFBP were approximately 3-fold elevated. The authors conclude that transcription of the IGF-II gene leads to the production of significant amounts of 10 kDa IGF-II and 7.5 kDa IGF-II. IGF-II may stimulate the proliferation of MSRCT by interaction with IGF-I receptors on the cells.

Human IM-9 lymphoblasts as a model of the growth hormone-insulin-like growth factor axis: gene expression, and interactions of ligands with receptors and binding proteins

Human IM-9 lymphoblasts bind growth hormone (hGH) and insulin-like growth factors (IGFs). We have systematically examined the IM-9 cells as a valuable model of the interaction of hGH and the IGFs at the cellular level. Cells were cultured in medium with 10% serum and for a subset of experiments cultured in serum-free medium. Binding of [125I]hGH and [125I]IGF-I and -II to intact IM-9 cells was measured: unlabeled hGH inhibited binding of [125I]hGH (half max. 20 ng/ml). Binding of [125I]IGF-I was inhibited by IGF-I (half max. 7.5 ng/ml), IGF-II (half max. 60 ng/ml), and insulin and anti IGF-I receptor antibody (alpha IR3). [125I]IGF-II was inhibited by IGF-II (half max. 15 ng/ml), IGF-I (half max. 500 ng/ml), insulin (half max. 250 ng/ml) but not by alpha IR3. Crosslinking experiments with [125I]IGF-II and DSS as the crosslinking agent and analysis of radioligand-receptor complexes by SDS-PAGE under reducing conditions revealed that [125I]IGF-II bound to a 250 kDa and a 135 kDa receptor species. The latter possibly represents an insulin-type receptor whereas the 250 kDa species had the characteristics of the IGF-II/M6P receptor. When IM-9 cell conditioned medium was analyzed in ligand blotting experiments with either [125I]IGF-I or -II a 30 kDa IGFBP species was detected on the autoradiographs. Also, IGF-II immunoreactivity (approx. 1 ng/ml medium) was measured in the cell conditioned medium using an IGF-BP blocked RIA employing [125I]IGF-II. In a subset of experiments IM-9 cells were homogenized in 4 M guanidinium-thiocyanate and RNA extracted in 5.7 M CsCl. Denatured RNA was electrophoresed on 0.8% agarose gels and transferred to a nylon membrane, fixed and the blots hybridized with cDNA probes. Probes were labeled with [32P]dCTP using a random prime labeling procedure: a Pst I 700 bp fragment of the human IGF-I cDNA, a 554 bp Pst I-Sal I fragment of the IGF-II cDNA, a 614 bp Pst I fragment of the IGF-I receptor cDNA and a 663 bp Pst I fragment of the IGF-II/M6P receptor. Autoradiographs of Northern blots showed specific hybridization with the IGF-I probe at 3.7 kb and with the IGF-II probe at 5.3 kb. No signal was detected with the IGF-I receptor cDNA probe. Hybridization with the IGF-II/M6P receptor probe yielded a 9 kb RNA species.(ABSTRACT TRUNCATED AT 400 WORDS)

Insulinlike growth factors and binding proteins in colon cancer

Insulinlike growth factors (IGFs) express anabolic and mitogenic activity on wide variety of cells. Besides endocrine effects, IGFs have major autocrine and paracrine effects on many cellular functions. Two factors that significantly affect the extent of cellular response to IGFs include the membrane receptors for IGFs and the soluble binding proteins (BPs), which modulate the action of IGFs at the receptor level. IGFs, IGF receptors, and IGFs and their BPs (IGF-BPs) thus constitute three components of the IGF system. A role of IGFs in the transformation and proliferation of cancer cells has become increasingly evident in the past few years. Studies from several laboratories show that all three components of the IGF system may play an important role in the proliferation of colon cancers. It was recently shown that the relative expression of IGFs and IGF/BPs may critically control the metastatic potential of colon cancers. The purpose of this article is to summarize our current knowledge of the IGF system and to present support for a significant role of IGFs in the initiation and growth of colon cancers. The expression and structural aspects of IGFs, their receptors, and BPs are outlined first, followed by a discussion of the role of IGFs in gastrointestinal functions and in colon cancers.

Epidermal growth factor receptor expression in three different human endometrial cancer cell lines

BACKGROUND

The aim of the current study was to establish the characteristics of the epidermal growth factor (EGF) receptor in endometrial cell lines to determine the possible relation of EGF to endometrial cancer.

METHODS

Three different cell lines were used: RL95-2 (derived from a moderately differentiated adenosquamous carcinoma), HEC-I-A (from a moderately differentiated adenocarcinoma), and KLE (from a poorly differentiated adenocarcinoma). The binding of (125-I) EGF to these cell lines and the stimulatory effect of EGF on (3H) thymidine incorporation into DNA were examined.

RESULTS

EGF receptor was present in all three cell lines. The binding of 125-I-labeled EGF was saturable and of high affinity. Scatchard analysis of the competitive binding data for KLE, HEC-I-A, and RL95-2 revealed linear plots, indicating a single class of binding sites with almost identical equilibrium dissociation constants (0.34 nM, 0.23 nM, and 0.20 nM, respectively). Other peptides, such as insulin-like growth factor (IGF) I and II and insulin, did not compete for the receptor. RL95-2 cells bound significantly more EGF (P < 0.005) than did the HEC-I-A and KLE cell lines. EGF increased 3H-thymidine incorporation in all three cell lines.

CONCLUSION

Because EGF receptors are expressed by all three cell lines at markedly different levels and because EGF stimulates 3H-thymidine incorporation into DNA in the three cell lines, the current study suggests that EGF may play a role in the promotion of growth endometrial adenocarcinoma.

Insulin-like growth factor I: action and receptor characterization in human prostate cancer cell lines

The role of insulin-like growth factor I (IGF-I) in the growth and development of prostate cancer was studied using established human prostate cancer cell lines. Under steroid and growth factor-free culture conditions, IGF-I significantly stimulated the androgen-independent cell lines PC-3 and DU-145 to incorporate [3H]thymidine into DNA, while the androgen-dependent cell line, LNCaP, was not affected. However, in the presence of dihydrotestosterone (DHT), DNA synthesis of LNCaP cells was stimulated by IGF-I in a dose-dependent manner. None of the cell lines tested secreted an immunoreactive level of IGF-I into their conditioned medium. Characterization of receptors by ligand binding assays revealed that all prostate cancer cell lines tested express specific binding sites for IGF-I with similar dissociation constants (0.23-0.39 nM). Crosslinking studies supported the suggestion that 125I-IGF-I was bound to a receptor on these cells. The IGF-I receptor concentrations of androgen-independent cell lines were significantly higher than those of the androgen-dependent cell line. Androgen appeared to affect neither the expression of IGF-I receptors nor the secretion of IGF-I. The results suggest that IGF-I may play an important role in stimulating the growth and progression of prostate cancer.

Preferential binding of insulin-like growth factor-II (IGF-II) to a putative alpha 2 beta 2 IGF-II receptor type in C2 myoblasts

We have studied insulin-like-growth-factor (IGF) binding in two subclones of the C2 myogenic cell line. In the permissive parental subclone, myoblasts differentiate spontaneously into myotubes in medium supplemented with fetal calf serum. Unlike permissive myoblasts, inducible myoblasts require high concentrations of insulin (1.6 microM) or lower concentrations of IGF-I (25 nM) to differentiate, and expression of MyoD1 is not constitutive. IGF receptors were studied in microsomal membranes of proliferating and quiescent myoblasts and myotubes. IGF-II binding was also studied in inducible myoblasts transfected with the MyoD1 cDNA (clone EP5). Both inducible and permissive cells exhibited a single class of binding sites with similar affinity for IGF-I (Kd 0.8-1.2 nM). Affinity cross-linking of [125I]IGF-I to microsomal membranes, under reducing conditions, revealed a binding moiety with an apparent molecular mass of 130 kDa in permissive cells and 140 kDa in inducible cells, which corresponded to the alpha subunit of the IGF-I receptor. In permissive quiescent myoblasts, linear Scatchard plots suggested that [125I]IGF-II bound to a single class of binding sites (Kd 0.6 nM) compatible with binding to the IGF-II/M6P receptor. This was confirmed by affinity cross-linking experiments showing a labeled complex with an apparent molecular mass of 260 kDa and 220 kDa when studied under reducing and non-reducing conditions, respectively. In contrast, competitive inhibition of [125I]IGF-II binding to inducible quiescent myoblasts generated curvilinear Scatchard plots which could be resolved into two single classes of binding sites. One of them corresponded to the IGF-II/M6P receptor (Kd 0.2 nM) as evidenced by cross-linking experiments. The second was the binding site of highest affinity (Kd 0.04 nM) which was less inhibited by IGF-I than by IGF-II and was not inhibited by insulin. It migrated in SDS/PAGE at a position equivalent a molecular mass of 140 kDa, under reducing conditions, and at approximately 300 kDa, under non-reducing conditions. The labeling of this atypical binding moiety was not inhibited by anti(IGF-II/M6P-receptor) immunoglobulin. It was also observed in permissive and inducible myoblasts at proliferating stage. It was absent for permissive quiescent myoblasts and from permissive and inducible myotubes. Forced expression of MyoD1 in inducible cells (EP5 cells) dramatically reduced [125I]IGF-II binding to this atypical receptor. It emerges from these experiments that C2 cells express a putative alpha 2 beta 2 IGF-II receptor structurally related to the insulin/IGF-I receptor family. It is present in myoblasts but not in myotubes.(ABSTRACT TRUNCATED AT 400 WORDS)

Evaluation of growth hormone secretion and treatment

The secretion of growth hormone (GH) is regulated by a complex system that includes both neurotransmitters and feedback by hormonal and metabolic substrates. Over the last few years it has been recognized that GH release varies over a wide spectrum from deficient to excessive secretion. The diagnosis of GH deficiency is based on a combination of anthropometric and clinical signs on the one hand and an inadequate stimulated and/or spontaneous GH secretion on the other. There is no distinct boundary between deficient and sufficient GH secretion. The cut-off limit for normal GH release is accordingly relative and has increased over the past decade from 5 to 10 micrograms/l. The effect of GH therapy on growth can be evaluated only after treatment for at least 6 months. There is, therefore, an indisputable need for methods that would reflect growth response soon after the start of treatment. There are several promising biochemical candidates, e.g. the aminoterminal propeptide of type III procollagen, the carboxyterminal propeptide of procollagen I and the bone Gla-protein, which may turn out to be useful early indicators of the growth response to long-term GH therapy.

The N-terminal hexapeptide fragment of IGF II stimulates thymidine incorporation into fibroblasts

We synthesized the N-terminal hexapeptide fragment of IGF II to study potential binding to NMDA receptors in analogy to the N-terminal tripeptide of IGF I. The amino acid sequence of the hexapeptide is furthermore identical with the C-terminal sequence of the casiragua insulin B chain. The hexapeptide did not bind to the NMDA receptors, but was found to promote [3H]-thymidine incorporation into fibroblasts at concentrations of 10(-8) - 10(-5) M in a dose-dependent manner. Since [125I]-hexapeptide did not bind to IGF receptors, indirect competition studies using either labelled IGFs or insulin had to be used. The competition of hexapeptide at a concentration of 10(-5) M with labelled IGF I or II was about equal to that of 10(-9) M IGF I or II. IGF receptors were apparently up-regulated by the hexapeptide, as has also been described for insulin. When using casiragua insulin as labelled ligand, IGF II and casiragua insulin competed with equal potency, whereas the hexapeptide at 10(-7) M caused an apparent up-regulation of the casiragua insulin binding sites. Our results that the hexapeptide stimulates [3H]-thymidine incorporation and up-regulates IGF II and casiragua insulin binding sites may be connected to one or several of the following findings: the hystricomorph insulins--of which the casiragua insulin is a member--stimulate DNA synthesis to a greater extent than other insulins; the insulin and type 1 IGF receptor binding regions are localized predominantly in the C-terminal region of the insulin B chain; and the "cooperative" site regulating the affinity of the insulin receptor is also located in the C-terminal region of the insulin B chain. Further experiments will be needed to clarify the exact mechanism.

Type I and type II insulin-like growth factor receptors and their function in human Ewing's sarcoma cells

Binding studies using recombinant human 125I-labelled insulin-like growth factor I ([125I]IGF-I) revealed IGF-I receptors in three Ewing's sarcoma cell lines with Kd ranging from 74 x 10(-12) M to 100 x 10(-12) M and Bmax = 36-63 fmol/mg cell protein. [125I]IGF-I binding was displaced by IGF-I, IGF-II and insulin with IC50 values of 1.5 nM, 6.3 nM and 0.7 microM respectively. Recombinant human [125I]IGF-II radioligand-binding assays in the cell lines disclosed specific binding sites for IGF-II with Kd = (110-175) x 10(-12) M and Bmax varying from 21 fmol/mg to 72 fmol/mg cell protein. Neither IGF-I nor insulin displaced [125I]IGF-II binding. IGF-I was found to increase basal glucose transport by maximally 1.5 times with EC50 = 0.9 nM IGF-I. The efficacy and potency of IGF-II on glucose uptake were comparable to those of IGF-I whereas insulin was ineffective. IGF-I and IGF-II also provoked stimulation of glycogen synthesis in Ewing's sarcoma cells. The maximal glycogenic response was reached at 0.01 microM IGF-I and 0.1 microM IGF-II, the EC50 value being approximately 1 nM IGF-I and 2 nM IGF-II. Insulin did not significantly influence glycogen formation. IGF-I and IGF-II but not insulin increased DNA synthesis in Ewing's sarcoma cells. The maximal mitogenic response was obtained with 10 nM IGF-I or IGF-II with an EC50 value of about 0.7 nM for both peptides. alpha-IR-3, a monoclonal antibody specific for the IGF type I receptor, effectively blocked IGF-I- and IGF-II-mediated metabolic responses. In conclusion, the data show that IGF-I and IGF-II induce rapid and long-term biological responses in Ewing's sarcoma cells exclusively through interaction with IGF type I receptors.

Insulin/IGF-1 hybrid receptors: implications for the dominant-negative phenotype in syndromes of insulin resistance

Classical insulin and IGF-1 receptors are alpha 2 beta 2 heterotetrameric complexes synthesized from two identical alpha beta half-receptor precursors. Recent data strongly suggests, however, that nonidentical alpha beta half-receptor precursors can assemble to generate hybrid holoreceptor species both in vivo and in vitro. This review focuses primarily on two types of hybrid receptors. The first type is an insulin/IGF-1 hybrid receptor generated by the association of an alpha beta insulin half-receptor with an alpha beta IGF-1 half-receptor. The second type is one formed from a wildtype (kinase-active) insulin or IGF-1 alpha beta half-receptor and a mutant (kinase-inactive) insulin alpha beta half-receptor. Although the functional properties of insulin/IGF-1 hybrid receptors have not yet been completely defined, wildtype/mutant hybrid receptors are essentially substrate kinase inactive. These data indicate that the mutant alpha beta half-receptor exerts a transdominant inhibition upon the wildtype alpha beta half-receptor within the alpha 2 beta 2 holoreceptor complex. This defect in substrate kinase activity may contribute to the molecular defect underlying some syndromes of severe insulin resistance and diabetes. Heterozygous individuals expressing both wildtype and mutant tyrosine kinase-defective insulin receptor precursors demonstrate varying degrees of insulin resistance and diabetes. In addition, cell lines which express both endogenous wildtype and transfected kinase-defective insulin receptors display markedly decreased insulin and IGF-1 sensitivity and responsiveness. Formation of hybrid receptors which results in premature termination of insulin signal transduction may be one mechanism underlying the observation that kinase-inactive receptors inhibit the function of native receptors.

Effects of insulin-like growth factors (IGFs) and IGF receptor antibodies on the proliferation of human breast cancer cells

It has been shown previously that MCF-7 cells proliferate in response to nanomolar concentrations of IGF-I and IGF-II. It has also been reported that the actions of both peptides are mediated through the IGF-I receptor. To further characterize these observations, we used MCF-7 and Hs578T cell lines in the serum-free/phenol red-free system developed by Ogasawara and Sibarsku, 1988. Cell proliferation was studied in the presence of insulin, IGF-I and -II and a series of growth factor receptor antibodies. No effect was observed on Hs578T cell proliferation with any of the growth factors. However, MCF-7 cells were stimulated 4-5 fold with IGF-I and insulin, while IGF-II was only slightly less potent. alpha IR3, a monoclonal antibody directed against the IGF-I receptor, was stimulatory when added alone. However, alpha IR3 blocked approximately 50% of the IGF-I response, only 5% of the insulin response, and did not block the IGF-II effect on cell proliferation. These data suggest that alpha IR3 and IGF-I are acting as agonists through the IGF-I receptor, but that insulin and IGF-II are acting through other receptors. Two different IGF-II/M-6-P receptor antibodies and an insulin receptor antibody failed to significantly block IGF-II actions. All three antibodies were stimulatory when added alone. beta-gal inhibited 27% of the IGF-II response and had no effect when added alone. Since beta-gal decreases the binding affinity of the IGF-II/M-6-P receptor for IGF-II and does not bind to the IGF-I or insulin receptor, these data suggest the possibility that IGF-II mitogenic action is mediated through the IGF-II/M-6-P receptor. In summary, these data indicate that nanomolar concentration of insulin, IGF-I and IGF-II are potent mitogens in MCF-7 cells and can potentially stimulate cell proliferation through all three receptors.

The molecular and cellular biology of insulin-like growth factor II

Insulin-like growth factor II (IGF-II) is a 67 amino acid polypeptide that belongs to the family of insulin-like peptides. The IGF-II gene is coupled to the insulin gene and paternally imprinted. Multiple IGF-II mRNAs with identical coding regions and 3' untranslated regions (UTRs) but different 5' UTRs are generated from 3 promoters. The transcripts are translationally discriminated and inactivated by a specific endonucleolytic cleavage in their 3' UTR. These features may be important in the control of IGF-II production. IGF-II functions in an auto- and paracrine manner and binds to two types of receptors. The IGF-I receptor that is a tyrosine kinase and closely related with the insulin receptor and the IGF-II/mannose 6-phosphate (IGF-II/Man 6-P) receptor that is identical with the cation-independent mannose 6-phosphate receptor. The mitogenic and metabolic actions of IGF-II are propagated by the IGF-I receptor. In contrast, the IGF-II/Man 6-P receptor, that target lysosomal enzymes from the Golgi apparatus or the plasma membrane to the lysosomes, mediates the rapid internalization and degradation of IGF-II. IGF-II is expressed at high levels during foetal life and it is a major growth factor for the foetus in rodents. The developmental profiles and tissue distribution of the IGF-I and the maternally imprinted IGF-II/Man 6-P receptors both parallel that of IGF-II. In this scenario IGF-II promotes the growth of the embryo through the IGF-I receptor, whereas the IGF-II/Man 6-P receptor balance the activity by controlling the extracellular level of IGF-II.

[Principles and clinical significance of insulin-like growth factors/somatomedins]

The Insulin-like Growth Factors (IGFs) or Somatomedins are polypeptide growth factors which are similar to insulin in respect to their aminoacid sequence, structure and biologic activities. The IGFs bind to high affinity receptors which are present on many cells and in many tissues. In the circulation the IGFs are bound to transport (binding) proteins (IGF-BPs). In this review the physiologic role, the basic chemistry and the gene expression of this family of growth factors is summarized systematically. The pathophysiology of growth disorders, diabetes mellitus, malnutrition, liver and kidney disease in relation to the IGFs as well as the therapeutic and diagnostic potentials of these peptides are discussed in detail.

Monoclonal antibody alpha IR-3 inhibits the ability of insulin-like growth factor II to stimulate a signal from the type I receptor without inhibiting its binding

We have previously shown that the protein encoded by a human insulin-like growth factor I (IGF-I) receptor cDNA binds both IGF-I and II with high affinity. In the present studies, we show that a monoclonal antibody to the IGF-I receptor, alpha IR-3, inhibits the binding of IGF-I but not IGF-II to the expressed receptor in intact cells and after solubilization. Surprisingly, this monoclonal antibody inhibits the ability of both IGF-I and II to stimulate thymidine synthesis in cells with the expressed receptor. Moreover, this antibody inhibits the ability of both IGF-I and II to stimulate the kinase activity of the IGF-I receptor in intact cells. These results indicate that alpha IR-3 binds to the IGF-I receptor in such a way that it does not inhibit the binding of IGF-II but does inhibit the subsequent ability of the receptor to be activated to transmit a signal.

Interactions of growth factors present in bone matrix with bone cells: effects on DNA synthesis and alkaline phosphatase

It has been shown that bone cells produce and secrete several growth factors (GFs) which are also found in the bone matrix. To investigate the role of these growth factors in bone cell metabolism, we compared the effects of different factors separately and in combination with respect to osteoblastic cell proliferation and differentiation. While basic fibroblast GF (FGF), transforming GF beta-1 (TGF beta), and platelet-derived GF (PDGF) enhance DNA synthesis, they had the opposite effect on alkaline phosphatase (ALP) activity in cell extracts: FGF, TGF beta, and PDGF inhibited cell ALP but strongly stimulated DNA synthesis. The IGFs had little effect on cell ALP but increased the release of ALP into the conditioned medium. In mitogenic tests of combinations of GFs, most had at least additive effects at low concentrations, and FGF, TGF beta, and IGF2 produced synergistic effects. Evidence is presented for (1) the modulation of the effects of one GF by the action of other GF, (2) synergistic interactions between FGF, TGF beta, and IGF2, and (3) a possible role for the observed interactions among GF for the mitogenic effect of human bone extract.

Long-term cultivation of cryopreserved human fetal brain cells in a chemically defined medium

Conditions for long-term cultivation of human fetal brain cells in a chemically defined medium were established using cryopreserved brain fragments obtained from legal abortions. Tissue of the same gestational age was pooled and the cells cultured in a fully defined medium containing insulin-like growth factors (IGF I and II). Primary cultures were kept for 2-4 weeks and secondary or tertiary cultures could be maintained for 3 months. The cultures were characterized by morphological, electrophysiological and biochemical methods. Glial cells were predominant during the first two weeks of culture. In later stages of cultivation, glial cells diminished in number and most cells were neuronal. Voltage-dependent Na+ channels were recorded from neurons. Biochemical studies indicated that the fetal brain cells contained and secreted immunoreactive somatostatin as well as the tachykinins, substance P and neurokinin A. Cultures grown in IGF II- or nerve growth factor-containing medium expressed increased choline acetyltransferase activity.

Growth factors and the regulation of pre- and postnatal growth

Peptide growth factors represent a largely paracrine level of intercellular communication that is basic to the process of life. Growth factors are present in the ovum and are amongst the first products expressed by the embryonic genome. They function as both signals and progression factors for embryonic tissue growth, induction, differentiation, maturation and function. While a widespread tissue expression is demonstrable during fetal development, and in certain postnatal tissues such as the epiphyseal growth plate, growth factor presence in the adult is restricted to tissues sharing rapid cellular turnover such as ovary. However, a transient re-expression of peptide growth factors occurs during adult tissue repair. In addition to mitogenic peptides such as IGFs or EGF, the family of growth factors also includes physiological growth inhibitors such as TGF beta and certain neuropeptides. Insulin is mitogenic in the early embryo and evidence is presented to support a continuation of this role, under defined nutritional conditions, in late gestation. The importance of insulin to pre- and postnatal growth has prompted an expanding literature dealing with the interactions of nutrients, hormones and growth factors during the growth and functional maturation of the islets of Langerhans. While the expression of growth factors in the early embryo is apparently autonomous, some, such as IGFs, become increasingly dependent on nutrient, insulin and GH availability during fetal development and in childhood growth. This has resulted in circulating IGF I and II determinations becoming useful diagnostic markers of endocrine-based growth disorder and nitrogen balance.

Enhancement of erythropoiesis in vitro by human growth hormone is mediated by insulin-like growth factor I

Insulin-like growth factor I (IGF-I) is the presumed paracrine or autocrine growth-promoting mediator of growth hormone in peripheral tissues. In order to evaluate the role of IGF-I as mediator of human growth hormone (hGH) in erythropoiesis, we compared the effects of both peptides upon in vitro colony formation by primitive (BFU-E) and relatively mature (CFU-E) human erythroid precursors. Biosynthetic IGF-I (2 ng/ml) and hGH (25 ng/ml) induced a significant increase in the growth of both BFU-E and CFU-E. BFU-E growth was maximally enhanced by 6 ng/ml IGF-I and by 50 ng/ml hGH, resulting in an increase in burst numbers of 62 +/- 12% and 52 +/- 12%, respectively. Maximal enhancement of CFU-E growth was detected at higher concentrations of IGF-I (20 ng/ml) and hGH (150 ng/ml), with respective increases of 121 +/- 35% and 137 +/- 18% in colony numbers. Enhancement of bone marrow and peripheral blood erythroid progenitor cell growth by hGH required the presence of monocytes and was abrogated by specific monoclonal antibodies directed against IGF-I membrane receptors. The in vitro growth-promoting effect of hGH upon human erythroid precursors thus appears to be mediated by paracrine IGF-I.

Functional properties of an isolated alpha beta heterodimeric human placenta insulin-like growth factor 1 receptor complex

Treatment of human placenta membranes at pH 8.5 in the presence of 2.0 mM dithiothreitol (DTT) for 5 min, followed by the simultaneous removal of the DTT and pH adjustment to pH 7.6, resulted in the formation of a functional alpha beta heterodimeric insulin-like growth factor 1 (IGF-1) receptor complex from the native alpha 2 beta 2 heterotetrameric disulfide-linked state. The membrane-bound alpha beta heterodimeric complex displayed similar curvilinear 125I-IGF-1 equilibrium binding compared to the alpha 2 beta 2 heterotetrameric complex. Triton X-100 solubilization of the alkaline pH and DTT-pretreated placenta membranes, followed by Bio-Gel A-1.5m gel filtration chromatography, was found to effectively separate the alpha 2 beta 2 heterotetrameric and alpha beta heterodimeric IGF-1 receptor species, 125I-IGF-1 binding to both the isolated alpha 2 beta 2 heterotetrameric and alpha beta heterodimeric complexes demonstrated a marked straightening of the Scatchard plots, compared to the placenta membrane-bound IGF-1 receptors, with a 2-fold increase in the high-affinity binding component. Similar to the membrane-bound IGF-1 receptor species, the 125I-IGF-1 binding properties between the alpha 2 beta 2 heterotetrameric and alpha beta heterodimeric complexes were not significantly different. IGF-1 stimulation of IGF-1 receptor autophosphorylation indicated that the ligand-dependent activation of alpha beta heterodimeric protein kinase activity occurred concomitant with the reassociation into a covalent alpha 2 beta 2 heterotetrameric state.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential regulation of insulin-like growth factor-II receptors in rat hepatocytes and hepatoma cells

This study compares the regulation of IGF-II receptors in three rat hepatoma lines, HTC, H-35 and 5123tc, and primary rat hepatocytes. In all cell types [125I]IGF-II bound solely to a species of approximately 250 kDa. Cell surface IGF-II receptors in hepatoma cells had slightly lower affinities (1-2 liters/nmol) than in hepatocytes (4 liters/nmol), but slightly higher IGF-I cross-reactivity (2-4% compared to 1% in hepatocytes). In confluent cultures, the three hepatoma lines expressed 5- to 15-fold more cell-surface receptors per cell than hepatocytes. However, while hepatocyte receptors showed marked inverse density-dependence, increasing over 6-fold between dense (3 x 10(5) cells/3.8 cm2) and sparse (0.16 x 10(5) cells/3.8 cm2) cultures, receptors in all hepatoma lines remained at a constant high level regardless of culture density. These distinct regulatory patterns resemble those described for growth-related functions in hepatocytes and hepatoma cells, and are thus consistent with a role for IGF-II receptors in liver cell proliferation.

Human blood-brain barrier insulin-like growth factor receptor

Insulin-like growth factor (IGF)-1 and IGF-2, may be important regulatory molecules in the CNS. Possible origins of IGFs in brain include either de novo synthesis or transport of circulating IGFs from blood into brain via receptor mediated transcytosis mechanisms at the brain capillary endothelial wall, ie, the blood-brain barrier (BBB). In the present studies, isolated human brain capillaries are used as an in vitro model system of the human BBB and the characteristics of IGF-1 or IGF-2 binding to this preparation were assessed. The total binding of IGF-2 at 37 degrees C exceeded 130% per mg protein and was threefold greater than the total binding for IGF-1. However, at 37 degrees C nonsaturable binding equaled total binding, suggesting that endocytosis is rate limiting at physiologic temperatures. Binding studies performed at 4 degrees C slowed endocytosis to a greater extent than membrane binding, and specific binding of either IGF-1 or IGF-2 was detectable. Scatchard plots for either peptide were linear and the molar dissociation constant of IGF-1 and IGF-2 binding was 2.1 +/- 0.4 and 1.1 +/- 0.1 nmol/L, respectively. Superphysiologic concentrations of porcine insulin inhibited the binding of both IGF-1 (ED50 = 2 micrograms/mL) and IGF-2 (ED50 = 0.5 microgram/mL). Affinity cross linking of 125I-IGF-1, 125I-IGF-2, and 125I-insulin to isolated human brain capillaries was performed using disuccinimidylsuberate (DSS). These studies revealed a 141 kd binding site for both IGF-1 and IGF-2, and a 133 kd binding site for insulin.(ABSTRACT TRUNCATED AT 250 WORDS)