Cloning and expression of human insulin-like growth factor binding protein 3

High-yield bacterial expression and structural characterization of recombinant human insulin-like growth factor binding protein-2

The diverse biological activities of the insulin-like growth factors (IGF-1 and IGF-2) are mediated by the IGF-1 receptor (IGF-1R). These actions are modulated by a family of six IGF-binding proteins (IGFBP-1-6; 22-31 kDa) that via high affinity binding to the IGFs (K(D) approximately 300-700 pM) both protect the IGFs in the circulation and attenuate IGF action by blocking their receptor access. In recent years, IGFBPs have been implicated in a variety of cancers. However, the structural basis of their interaction with IGFs and/or other proteins is not completely understood. A critical challenge in the structural characterization of full-length IGFBPs has been the difficulty in expressing these proteins at levels suitable for NMR/X-ray crystallography analysis. Here we describe the high-yield expression of full-length recombinant human IGFBP-2 (rhIGFBP-2) in Escherichia coli. Using a single step purification protocol, rhIGFBP-2 was obtained with >95% purity and structurally characterized using NMR spectroscopy. The protein was found to exist as a monomer at the high concentrations required for structural studies and to exist in a single conformation exhibiting a unique intra-molecular disulfide-bonding pattern. The protein retained full biologic activity. This study represents the first high-yield expression of wild-type recombinant human IGFBP-2 in E. coli and first structural characterization of a full-length IGFBP.

Expression and purification of biologically active IGF-binding proteins using the LCR/Mel expression system

The anabolic effects and bioavailability of insulin-like growth factors I and II (IGF-I, IGF-II) are regulated in part by a family of IGF-binding proteins (IGFBPs). There are six known members of the IGFBP family, which share distinct structural characteristics and functional activities. To study the binding properties of these proteins, we have expressed recombinant IGFBP-3 and IGFBP-4 using the LCR/Mel expression system. Using this system, we found that recombinant IGFBP-3 was secreted by Mel cells and had a glycosylation pattern similar to that of native IGFBP-3. Recombinant IGFBP-4 secreted from Mel cells had a molecular size identical to that of non-glycosylated native IGFBP-4. The binding kinetics of recombinant IGFBPs was measured using a solid-phase ligand-binding assay, an in vitro solution-binding assay, and a cellular proliferation assay. IGF-I bound with high affinity to recombinant IGFBP-3 and IGFBP-4 with K(D)s of <0.25 nmol. As reported for native IGFBPs, IGF-II bound with affinity higher than IGF-I to recombinant IGFBP-3 and IGFBP-4 (K(D) of <0.05 nmol). Recombinant IGFBP-3 and IGFBP-4 were found to inhibit the IGF-induced proliferation of an NIH3T3 cell line engineered to overexpress the IGF-I receptor. We have compared the binding kinetics of Mel cell-expressed IGFBPs with that of recombinant protein expressed in Escherichia coli and found them to be equivalent. Here, we show that the LCR/Mel expression system represents an effective route for expression of biologically active IGFBPs.

Factors regulating insulin-like growth factor binding protein-3 secretion from human hepatoma (HepG2) cells

Insulin-like growth factor (IGF) binding protein-3 (IGFBP-3) is a growth hormone (GH) dependent carrier of the IGFs in human serum. Apart from GH regulation the hormonal control of IGFBP-3 production is not well established and although the liver is considered to be the main source of circulating IGFBP-3, there are no in vitro studies of the effect of both insulin and IGFs on the IGFBP-3 produced in human hepatoma cells. The effect of sex hormones as well as cortisol has not been studied. To elucidate this we performed cell culture studies on HepG2 cells in the presence of various effectors. Insulin, IGF-I and IGF-II brought about a 1.5-2-fold enhancement of IGFBP-3 release at 7.5-30 nM concentrations. In contrast, cortisol decreased IGFBP-3 secretion by 30-40% whereas estradiol, tamoxifen and testosterone had no effect at physiological concentrations. We conclude that, in addition to GH, also insulin, IGF-I and IGF-II and glucocorticoids can modulate IGFBP-3 secretion by human hepatoma cells.

Calcium regulates the expression of insulin-like growth factor binding protein-3 by the human keratinocyte cell line HaCaT

The insulin-like growth factor (IGF) system is essential for epidermal homeostasis. Insulin-like growth factor binding protein 3 (IGFBP-3), a modulator of IGF action that also exhibits IGF-independent activity, is localized to selected keratinocytes in the basal epidermal layer and may thus contribute to keratinocyte differentiation. We have utilized the human keratinocyte cell line, HaCaT, to examine the effect of calcium on the regulation of components of the IGF system. Western ligand and northern blot analyses revealed secreted IGFBP-3 and IGFBP-3 mRNA were reduced by an elevation in calcium levels in the culture medium. At 1.0 and 1.2 mM CaCl2 culture conditions IGFBP-3 abundance was reduced to 36% +/- 1.6% and 26% +/- 7.1%, respectively, of that from cells grown at 0.03 mM CaCl2. IGFBP-3 mRNA levels in 0.7 mM and 1.2 mM CaCl2 were reduced to 46% +/- 17.4% and 24% +/- 4.6%, respectively, compared with IGFBP-3 mRNA levels at 0.03 mM CaCl2. The observed reduction of IGFBP-3 was not associated with IGFBP-3 proteolysis. In contrast IGF-I receptor protein and mRNA levels remained unchanged. The IGF-I stimulated proliferative response of HaCaT keratinocytes showed that under low (0.03 mM) and high (1.2 mM) CaCl2 conditions IGF-I at 100 and 1000 ng per ml similarly increased cell number 2.4- and 2.7-fold, respectively, with similar dose-response curves. HaCaT keratinocytes grown under medium (0.7 mM) and high (1.2 mM), but not low (0.03 mM), CaCl2 conditions for 21 d revealed an induction of profilaggrin mRNA, a marker of keratinocyte differentiation. These studies indicate that the exposure of HaCaT keratinocytes to elevated calcium levels is associated with a decline in IGFBP-3 but not IGF-I receptor levels. These findings suggest a potential mechanism for the distribution of IGFBP-3 in the epidermis, which may be involved in the process of keratinocyte differentiation.

Conservation of a growth hormone-responsive promoter element in the human and mouse acid-labile subunit genes

During extrauterine life, insulin-like growth factors (IGFs) circulate in a ternary serum complex with one IGF-binding protein-3 (IGFBP-3) or IGFBP-5 protein and with a single acid-labile subunit (ALS). GH increases levels of this ternary complex; in mice, this effect is achieved in part by the ability of GH to stimulate mouse ALS (mALS) transcription through an interferon-gamma-activated sequence-like element (GLE) in the mALS promoter. To begin studying how GH regulates human ALS (hALS) gene expression, we cloned the hALS gene and found that it spans approximately 3.3 kb of DNA at chromosomal region 16p13.3. The hALS gene has two exons separated by a 1235-bp intron, which is found at the identical site in rat and mouse ALS genes. Sequence analysis reveals that the hALS 5'-flanking sequence is homologous to the mALS promoter, and that the GH-responsive GLE in the mALS promoter is conserved in both sequence and location in the hALS gene. The region spanning from -755 to -4 bp 5' to the hALS ATG translation start codon directs expression of a luciferase reporter gene in primary rat hepatocytes, and GH increases reporter expression in the presence of the native, but not a mutant, GLE in the hALS promoter. These data suggest that GH stimulates hALS and mALS gene expression by a similar mechanism, which involves at least in part a conserved GLE in the ALS promoter.

Expression of insulin-like growth factor binding protein-3 (IGFBP-3) in human keratinocytes is regulated by EGF and TGFbeta1

Insulin-like growth factor-I (IGF-I) is essential for normal epidermal homeostasis; however, the role of IGF binding proteins (IGFBPs), regulators of IGF action, remains unclear. Here we examine the regulation of human keratinocyte-produced IGFBPs by epidermal growth factor (EGF), transforming growth factor beta 1 (TGFbeta1), and IGF-I, growth factors known to be active in skin. In the absence of added growth factors, IGFBP-3 was the major binding protein secreted into the medium by primary keratinocytes. Addition of EGF or TGFbeta1 to keratinocyte cultures resulted in a significant decrease in IGFBP-3 abundance in conditioned medium when compared with control, untreated cells. Specifically, EGF (50 ng/ml) and TGFbeta1 (50 ng/ml) reduced IGFBP-3 abundance to 15+/-6% and 22+/-9%, respectively. Using Northern blot analysis, we found EGF and TGFbeta1 (50 ng/ml) to reduce IGFBP-3 mRNA levels in keratinocytes to 51+/-12% and 50+/-38%, respectively, when compared with control, untreated cells. Treatment with IGF-I or its analogue des(1-3)IGF-I did not lead to any consistent change in IGFBP-3 abundance. However, both IGF-I and des(1-3)IGF-I at 100 ng/ml led to a modest increase in IGFBP-3 mRNA levels in keratinocytes, suggesting posttranscriptional regulation of IGFBP-3 abundance. We propose that local modulation of IGFBP-3 abundance may represent another level of regulation of growth factor action in the epidermis, where EGF and TGFbeta1 and possibly other local growth factors specifically regulate the availability of IGF-I to its keratinocyte receptors.

Characterization of insulin-like growth factor binding protein-3 (IGFBP-3) binding to human breast cancer cells: kinetics of IGFBP-3 binding and identification of receptor binding domain on the IGFBP-3 molecule

Insulin-like growth factor binding protein-3 (IGFBP-3) binds to specific membrane proteins located on human breast cancer cells, which may be responsible for mediating the IGF-independent growth inhibitory effects of IGFBP-3. In this study, we evaluated IGFBP-3 binding sites on breast cancer cell membranes by competitive binding studies with IGFBP-1 through -6 and various forms of IGFBP-3, including synthetic IGFBP-3 fragments. Scatchard analysis revealed the existence of high-affinity sites for IGFBP-3 in estrogen receptor-negative Hs578T human breast cancer cells (dissociation constant (Kd) = 8.19 +/- 0.97 x 10(-9) M and 4.92 +/- 1.51 x 10(5) binding sites/cell) and 30-fold fewer receptors in estrogen receptor-positive MCF-7 cells (Kd = 8.49 +/- 0.78 x 10(-9) M and 1.72 +/- 0.31 x 10(4) binding sites/cell), using a one-site model. These data demonstrate binding characteristics of typical receptor-ligand interactions, strongly suggesting an IGFBP-3:IGFBP-3 receptor interaction. Among IGFBPs, only IGFBP-5 showed weak competition, indicating that IGFBP-3 binding to breast cancer cell surfaces is specific and cannot be attributed to nonspecific interaction with glycosaminoglycans. This was confirmed by showing that synthetic IGFBP-3 peptides containing IGFBP-3 glycosaminoglycan-binding domains competed only weakly for IGFBP-3 binding to the cell surface. Rat IGFBP-3 was 20-fold less potent in its ability to compete with human IGFBP-3(Echerichia coli), as well as 10- to 20-fold less potent for cell growth inhibition than human IGFBP-3, suggesting the existence of species specificity in the interaction between IGFBP-3 and the IGFBP-3 receptor. When various IGFBP-3 fragments were evaluated for affinity for the IGFBP-3 receptor, only those fragments that contain the midregion of the IGFBP-3 molecule were able to inhibit 125I-IGFBP-3(Escherichia coli) binding, indicating that the midregion of the IGFBP-3 molecule is responsible for binding to its receptor. These observations demonstrate that specific, high-affinity IGFBP-3 receptors are located on breast cancer cell membranes. These receptors have properties that support the notion that they may mediate the IGF-independent inhibitory actions of IGFBP-3 in breast cancer cells.

Plasminogen binds the heparin-binding domain of insulin-like growth factor-binding protein-3

Limited proteolysis lowers affinity of insulin-like growth factor (IGF)-binding protein (IGFBP)-3 for bound IGFs, resulting in greater IGF bioavailability. Plasmin is one of many proteases that cleave IGFBP-3, and the plasmin system may regulate IGFBP-3 proteolysis and IGF bioavailability in cultured cells in vitro. A role for the plasmin system in IGFBP-3 proteolysis in vivo is suggested by data presented here showing that IGFBP-3 binds plasminogen (Pg; Glu-Pg) with a dissociation constant (Kd) ranging from 1.43 to 3.12 nM. IGF-I and Glu-Pg do not compete for IGFBP-3 binding; instead, the binary IGFBP-3/Glu-Pg complex binds IGF-I with high affinity (Kd = 0. 47 nM) to form a ternary complex. Competitive binding studies suggest that the kringle 1, 4, and 5 domains of Glu-Pg and the heparin-binding domain of IGFBP-3 participate in forming the IGFBP-3/Glu-Pg complex, and other studies show that Glu-Pg in this complex is activated at a normal rate by tissue Pg activator. Importantly, IGFBP-3/Glu-Pg complexes were detected in both human citrate plasma and serum, indicating that these complexes exist in vivo. Binding of IGFBP-3 to Glu-Pg in vivo suggests how Glu-Pg activation can specifically lead to IGFBP-3 proteolysis with subsequent release of IGFs to local target tissues.

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.

Estradiol and antiestrogens regulate a growth inhibitory insulin-like growth factor binding protein 3 autocrine loop in human breast cancer cells

MCF-7 human breast cancer cells are commonly used to model tissues responsive to estrogens and antiestrogens. We examined the effects of estradiol and the antiestrogen ICI 182780 on MCF-7 cell proliferation and insulin-like growth factor binding protein 3 (IGFBP-3) gene expression. ICI 182780-induced growth inhibition was associated with increased transcription of the IG-FBP-3 gene, increased IGFBP-3 mRNA abundance, and increased IGFBP-3 protein accumulation in the conditioned medium. The growth stimulatory effect of estradiol was associated with opposite effects, and the correlation between cellular proliferation and IGFBP-3 mRNA abundance was strong (r = -0.91). Recombinant IGFBP-3 inhibited basal and estradiol-stimulated MCF-7 cell proliferation, and an IGFBP-3 antisense oligodeoxynucleotide abolished antiestrogen-induced growth inhibition. These results provide evidence for an estradiol and antiestrogen-regulated IGFBP-3 growth inhibitory autocrine pathway in MCF-7 cells.

A keratinocyte cell line synthesizes a predominant insulin-like growth factor-binding protein (IGFBP-3) that modulates insulin-like growth factor-I action

Insulin-like growth factor-I (IGF-I) is an important regulator of epidermal proliferation and has been shown in vitro to be a powerful stimulator of keratinocyte growth. It is synthesized by fibroblasts in the dermis, along with several IGF-binding proteins (IGFBPs), which are known to modulate IGF-I responsiveness of virtually all tissues studied. Because it was not known how or in what form IGF-I produced in the dermis acts on epidermal keratinocytes in vivo, we investigated the possible role of IGFBPs in modulating the response of epidermal keratinocytes to IGF-1. We show here that tIGF-I, a non-IGFBP-binding analogue of IGF-1, is a more potent mitogenic stimulator of the keratinocyte cell line HaCaT than IGF-I, suggesting that keratinocytes produce IGFBPs that modulate their response to IGF-I. To confirm this and to identify which IGFBPs were produced, we analyzed HaCaT cell-conditioned medium and mRNA, with the following findings: HaCaT cells produce a major IGFBP, identified as IGFBP-3, and a minor 24-kD IGFBP, likely to be IGFBP-4. Northern analysis revealed a 2.6-kb IGFBP-3 mRNA; however, IGFBP-4 mRNA was not detectable. We conclude that production of predominantly IGFBP-3 by the HaCaT cell line modulates its sensitivity to IGF-I stimulation. Epidermal IGFBPs thus have a potential role in vivo in the interaction of dermis derived IGF-I with epidermal keratinocytes.

Cloning and sequence analysis of the alpha subunit of the cat flea sodium pump

We have cloned a cDNA that encodes the sodium pump alpha-subunit of the cat flea Ctenocephalides felis. The deduced amino acid sequence of 1037 residues is 90% homologous with the Drosophila alpha-subunit sequence and shows 75% similarity to the sheep kidney alpha-subunit. One characteristic unique to the insect polypeptides is an extended sequence at the N-terminus. Northern blot analysis of C. felis RNA detects two alpha-subunit transcripts with lengths of 3.8 and 4.4 kb. Sodium pump alpha-subunit sequences were readily detected in homogenates of flea midguts and in cDNA synthesized from midgut RNA.

Insulin-like growth factor-I induces hypertrophy with enhanced expression of muscle specific genes in cultured rat cardiomyocytes

BACKGROUND

Cardiac hypertrophy is commonly observed in acromegalic patients, in whom serum insulin-like growth factor-I (IGF-I) levels are elevated. In the present study, we examined whether IGF-I induces hypertrophy in cultured neonatal rat cardiomyocytes through its specific receptor and whether IGF binding protein-3 (IGFBP-3), which is a major circulating carrier protein for IGF-I, inhibits IGF-I-induced cardiac hypertrophy in vitro.

METHODS AND RESULTS

Because the response of cardiac hypertrophy is characterized by the induction of expression for muscle-specific genes, the effect of IGF-I on steady-state levels of mRNA for myosin light chain-2 (MLC-2) and troponin I and for skeletal and cardiac alpha-actin isoforms was evaluated by Northern blot analysis. IGF-I (10(-7) M) increased mRNA levels for MLC-2 and troponin I as early as 60 minutes with maximum levels by 6 hours, which were maintained for as long as 24 hours. IGF-I (10(-7) M) also increased transcripts for skeletal alpha-actin but not for cardiac alpha-actin. The cell size as evaluated morphometrically was almost doubled after 48-hour treatment with IGF-I. IGF-I induction of protein synthesis was dose dependent (10(-10) to 10(-7) M) with a maximal 2.2-fold increase seen at 10(-8) M. In contrast to the hypertrophic effect of IGF-I, growth hormone affected neither protein synthesis nor expression for muscle-specific genes. Binding study using 125I-IGF-I revealed the presence of specific binding sites for IGF-I in rat cardiomyocytes. IGFBP-3 induced a dose-dependent inhibition of protein synthesis stimulated by IGF-I; IGFBP-3 (10(-7) M) completely inhibited the [3H]leucine uptake stimulated by IGF-I (10(-8) M). IGFBP-3 similarly inhibited the IGF-I-stimulated gene expressions for MLC-2 and troponin I.

CONCLUSIONS

These results suggest that IGF-I directly causes cardiac hypertrophy and that its effect can be blocked by IGFBP-3.

Analysis of the primary structure of insulin-like growth factor binding protein-3 cDNA from Werner syndrome fibroblasts

The mRNA encoding insulin-like growth factor binding protein-3 (IGFBP-3) is equally overexpressed in late-passage (old) normal human diploid fibroblasts (HDF) and in HDF derived from individuals with the premature aging disorder Werner syndrome (WS), relative to early-passage (young) normal HDF. However, the accumulation of IGFBP-3 protein in medium conditioned by WS cells is substantially less than in medium of old cells. In an attempt to understand this disparity between mRNA levels and protein output, we determined the nucleotide sequence of IGFBP-3 cDNA isolated from a WS cDNA library derived from mRNA of WS HDF, and compared it to three published normal IGFBP-3 DNA sequences. In the open reading frame, our results differed from one of the three sequences by a glycine substitution for alanine at residue 32. Minor differences in the 3'-untranslated region between the WS cDNA sequence and all three of the normal DNA sequences were also detected as 12 individual base substitutions and one adenine insertion. Thus, dampened accumulation of IGFBP-3 in medium conditioned by WS cells is not due to significant alterations in the sequence of the cognate mRNA.

Purification and characterization of human recombinant insulin-like growth factor binding protein 3 expressed in Chinese hamster ovary cells

Recombinant human insulin-like growth factor binding protein 3 (hIGFBP-3) stably expressed in chinese hamster ovary cells (CHO cells) has been purified to homogeneity from serum-free culture media. The purified protein migrates as a doublet (45/43 kDa) upon SDS-PAGE. The purified recombinant hIGFBP-3 is fully active and binds one mole of IGF-I per mole of recombinant binding protein. When the transfected CHO cells are treated with tunicamycin a single 29 kDa hIGFBP-3 protein is observed. This expressed hIGFBP-3 protein maintains its ability to bind IGF-I. N-Glycanase treatment of the purified hIGFBP-3 protein results in a protein that migrates similar to E. coli-derived IGFBP-3 upon SDS-PAGE under reducing conditions (30 kDa). Carboxymethylation of hIGFBP-3 suggests that all 18 cysteines are involved in disulfide linkages. These results represent the first purification and characterization of recombinant hIGFBP-3 expressed in CHO cells.

Cloning and characterization of bovine insulin-like growth factor binding protein-3 (bIGFBP-3)

We report for the first time the isolation of a cDNA encoding the complete amino acid sequence for bovine growth hormone-dependent insulin-like growth factor binding protein-3 (bIGFBP-3). The deduced amino acid sequence from the cDNA revealed a mature polypeptide consisting of 264 amino acids and a 27 amino acid putative signal peptide. The amino acid sequence is over 80% homologous with human IGFBP-3 with complete conservation of the 18 cysteine residues and the 3 Asn-linked glycosylation sites. Between the two species there are 44 amino acid substitutions. Northern analysis of the bIGFBP-3 mRNA in bovine tissue revealed a single mRNA species of 1.65 kilobases.

Identification and molecular characterization of insulin-like growth factor binding proteins (IGFBP-1, -2, -3, -4, -5 and -6)

Six different insulin-like growth factor binding proteins (IGFBPs) have been identified by molecular cloning of their cDNAs from rat and human tissues and designated as IGFBP-1, -2, -3, -4, -5 and -6. The total number of amino acid residues for the mature rat BPs ranges from 201 for IGFBP-6 to 270 for IGFBP-2, while the human homologs range from 216 for IGFBP-6 to 289 for IGFBP-2. Except for IGFBP-6, all rat and human IGFBPs contain 18 homologous cysteines; twelve are located at the N-terminal and span approximately one-third of the total amino acid sequence, while the remaining six are distributed at the C-terminal and span the last one-third of the protein sequence. Both rat and human IGFBP-4 possess two extra cysteines at the mid-region of the molecule. By contrast, rat and human IGFBP-6 contain only 14 and 16 cysteines, respectively. Absence of the two and four cysteines in the N-terminal region in the human and rat IGFBP-6 resulted in the deletion of the invariant Gly-Cys-Gly-Cys-Cys sequence which is present in all the other five IGFBPs. Both rat and human IGFBP-3 possess multiple N-linked glycosylation sites at the mid-region of the molecule, which accounts for their apparent molecular size being larger than the calculated molecular weight, based on the amino acid sequence. One potential N-linked glycosylation site is located at the mid-region of rat and human IGFBP-4, whereas only human but not rat IGFBP-6 possesses one N-linked glycosylation site at the extreme C-terminal of the molecule. An RGD sequence is found in the C-terminal of IGFBP-1 and -2. In this short review, updated information on the structural identification and molecular cloning of the six IGFBPs will be presented. In addition, the potential regulation of the BPs at the transcriptional and translational levels will be discussed.