N-Linked glycosylation and sialylation of the acid-labile subunit. Role in complex formation with insulin-like growth factor (IGF)-binding protein-3 and the IGFs

Over 75% of the circulating insulin-like growth factors (IGF-I and -II) are bound in 140-kDa ternary complexes with IGF-binding protein-3 (IGFBP-3) and the 84-86-kDa acid-labile subunit (ALS), a glycoprotein containing 20 kDa of carbohydrate. The ternary complexes regulate IGF availability to the tissues. Since interactions of glycoproteins can be influenced by their glycan moieties, this study aimed to determine the role of ALS glycosylation in ternary complex formation. Complete deglycosylation abolished the ability of ALS to associate with IGFBP-3. To examine this further, seven recombinant ALS mutants each lacking one of the seven glycan attachment sites were expressed in CHO cells. All the mutants bound IGFBP-3, demonstrating that this interaction is not dependent on any single glycan chain. Enzymatic desialylation of ALS caused a shift in isoelectric point from 4.5 toward 7, demonstrating a substantial contribution of anionic charge by sialic acid. Ionic interactions are known to be involved in the association between ALS and IGFBP-3. Desialylation reduced the affinity of ALS for IGFBP-3. IGF complexes by 50-80%. Since serum protein glycosylation is often modified in disease states, the dependence of IGF ternary complex formation on the glycosylation state of ALS suggests a novel mechanism for regulation of IGF bioavailability.     

Mutations in the C-terminal part of insulin-like growth factor (IGF)-binding protein-1 result in dimer formation and loss of IGF binding capacity.

In an attempt to define domains in insulin-like growth factor (IGF)-binding protein-1 (IGFBP-1) that are involved in IGF binding, we subjected the carboxyl end of the coding region of IGFBP-1 cDNA to mutagenesis. Mutant cDNAs were isolated, characterized by sequencing, and cloned in an expression vector under control of the simian virus-40 (SV40) early promoter. The constructs were transfected into COS-1 cells, and the mutant proteins, secreted into the culture medium, were analyzed for IGF binding by ligand blotting. The results obtained show that deletion of the C-terminal 20 amino acids or introduction of frame-shifts in this region resulted in loss of IGF binding and for some mutants in the formation of dimeric IGFBP-1 molecules. These dimers are probably formed when cysteine-226 (Cys-226) is missing, and its putative partner is able to form intermolecular disulfide bonds. Site-directed mutagenesis demonstrated that most of the introduced point mutations in the C-terminal region did not affect IGF binding. Only mutation of Cys-226 to tyrosine completely abolished IGF binding, as did the introduction of a negatively charged amino acid in the vicinity of this residue. Again, dimers were observed, supporting that Cys-226 is essential for the conformation of IGFBP-1. In addition, our data suggest that an IGF-binding domain may be located in the vicinity of the intramolecular disulfide bond formed by Cys-226 and its putative partner.     

Specific amino acid substitutions determine the differential contribution of the N- and C-terminal domains of insulin-like growth factor (IGF)-binding protein-5 in binding IGF-I

We have previously reported that two highly conserved amino acids in the C-terminal domain of rat insulin-like growth factor-binding protein (IGFBP)-5, Gly(203) and Gln(209), are involved in binding to insulin-like growth factor (IGF)-1. Here we report that mutagenesis of both amino acids simultaneously (C-Term mutant) results in a cumulative effect and an even greater reduction in IGF-I binding: 30-fold measured by solution phase IGF binding assay and 10-fold by biosensor analysis. We compared these reductions in ligand binding to the effects of specific mutations of five amino acids in the N-terminal domain (N-Term mutant), which had previously been shown by others to cause a very large reduction in IGF-I binding (). Our results confirm this as the major IGF-binding site. To prove that the mutations in either N- or C-Term were specific for IGF-I binding, we carried out CD spectroscopy and showed that these alterations did not lead to gross conformational changes in protein structure for either mutant. Combining these mutations in both domains (N+C-Term mutant) has a cumulative effect and leads to a 126-fold reduction in IGF-I binding as measured by biosensor. Furthermore, the equivalent mutations in the C terminus of rat IGFBP-2 (C-Term 2) also results in a significant reduction in IGF-I binding, suggesting that the highly conserved Gly and Gln residues have a conserved IGF-I binding function in all six IGFBPs. Finally, although these residues lie within a major heparin-binding site in IGFBP-5 and -3, we also show that the mutations in C-Term have no effect on heparin binding.     

Cooperativity of the N- and C-terminal domains of insulin-like growth factor (IGF) binding protein 2 in IGF binding

A family of six insulin-like growth factor (IGF) binding proteins (IGFBP-1-6) binds IGF-I and IGF-II with high affinity and thus regulates their bioavailability and biological functions. IGFBPs consist of N- and C-terminal domains, which are highly conserved and cysteine-rich, joined by a variable linker domain. The role of the C-domain in IGF binding is not completely understood in that C-domain fragments have very low or even undetectable IGF binding affinity, but loss of the C-domain dramatically disrupts IGF binding by IGFBPs. We recently reported the solution structure and backbone dynamics of the C-domain of IGFBP-2 (C-BP-2) and identified a pH-dependent heparin binding site [Kuang, Z., Yao, S., Keizer, D. W., Wang, C. C., Bach, L. A., Forbes, B. E., Wallace, J. C., and Norton, R. S. (2006) Structure, dynamics and heparin binding of the C-terminal domain of insulin-like growth factor-binding protein-2 (IGFBP-2), J. Mol. Biol. 364, 690-704]. Here, we have analyzed the molecular interactions among the N-domain of IGFBP-2 (N-BP-2), C-BP-2, and IGFs using cross-linking and nuclear magnetic resonance (NMR) spectroscopy. The binding of C-BP-2 to the IGF-I.N-BP-2 binary complex was significantly stronger than the binding of C-BP-2 to IGF-I alone, switching from intermediate exchange to slow exchange on the NMR time scale. A conformational change or stabilization of the IGF-I Phe49-Leu54 region and the Phe49 aromatic ring upon binding to the N-domains, as well as an interdomain interaction between N-BP-2 and C-BP-2 (which is also detectable in the absence of ligand), may contribute to this cooperativity in IGF binding. Glycosaminoglycan binding by IGFBPs can affect their IGF binding although the effects appear to differ among different IGFBPs; here, we found that heparin bound to the IGF-I.N-BP-2.C-BP-2 ternary complex, but did not cause it to dissociate.     

C-terminal domain of insulin-like growth factor (IGF) binding protein-6: structure and interaction with IGF-II

IGFs are important mediators of growth. IGF binding proteins (IGFBPs) 1-6 regulate IGF actions and have IGF-independent actions. The C-terminal domains of IGFBPs contribute to high-affinity IGF binding and modulation of IGF actions and confer some IGF-independent properties, but understanding how they achieve this has been constrained by the lack of a three-dimensional structure. We therefore determined the solution structure of the C-domain of IGFBP-6 using nuclear magnetic resonance (NMR). The domain consists of a thyroglobulin type 1 fold comprising an alpha-helix followed by a loop, a three-stranded antiparallel beta-sheet incorporating a second loop, and finally a disulfide-bonded flexible third loop. The IGF-II binding site on the C-domain was identified by examining NMR spectral changes upon complex formation. It consists of a largely hydrophobic surface patch involving the alpha-helix, the first beta-strand, and the first and second loops. The site was confirmed by mutagenesis of several residues, which resulted in decreased IGF binding affinity. The IGF-II binding site lies adjacent to surfaces likely to be involved in glycosaminoglycan binding of IGFBPs, which might explain their decreased IGF affinity when bound to glycosaminoglycans, and nuclear localization. Our structure provides a framework for understanding the roles of IGFBP C-domains in modulating IGF actions and conferring IGF-independent actions, as well as ultimately for the development of therapeutic IGF inhibitors for diseases including cancer.     

Synthesis of the blood circulating C-terminal fragment of insulin-like growth factor (IGF)-binding protein-4 in its native conformation. Crystallization, heparin and IGF binding, and osteogenic activity

Insulin-like growth factor-binding proteins play a critical role in a wide variety of important physiological processes. It has been demonstrated that both an N-terminal and a C-terminal fragment of insulin-like growth factor-binding protein-4 exist and accumulate in the circulatory system, these fragments accounting for virtually the whole amino acid sequence of the protein. The circulating C-terminal fragment establishes three disulfide bridges, and the binding pattern of these has recently been defined. Here we show that the monodimensional 1H NMR spectrum of the C-terminal fragment is typical of a protein with a relatively close packed tertiary structure. This fragment can be produced in its native conformation in Escherichia coli, without the requirement of further refolding procedures, when synthesis is coupled to its secretion from the cell. The recombinant protein crystallizes with the unit cell parameters of a hexagonal system. Furthermore, it binds strongly to heparin, acquiring a well defined oligomeric structure that interacts with insulin-like growth factors, and promotes bone formation in cultures of murine calvariae.     

Structural determinants for binary and ternary complex formation between insulin-like growth factor-I (IGF-I) and IGF binding protein-3.

Structural analogs of recombinant human insulin-like growth factor-I (IGF-I), with alterations to each of the B, C, A, and D domains, have been tested for their ability to form binary complexes with IGF-binding protein-3 (IGFBP-3) and ternary complexes with IGFBP-3 and the acid-labile subunit (alpha-subunit). Two functionally distinct regions of IGF-I have been identified. The first, involving residues 3 and 4 and the alpha-helix between residues 8 and 18 of the B-domain, as well as residues 49-51 in the A-domain, appears important for IGFBP-3 binding, such that substitution of these residues results in decreased binary complex available for alpha-subunit binding. The second region, distal to the IGFBP-3-binding epitope and primarily involving the D-domain and B-domain near residue 24, with some involvement of the C-domain, appears slightly inhibitory to binary complex formation, such that analogs with a truncated D-domain or with a Gly4 bridge substituted for the C-domain show enhanced binding to IGFBP-3. However, binary complexes formed from these analogs bind the alpha-subunit with reduced affinity, the effect being most marked when substitution of the C-domain, or replacement of Tyr24, is superimposed on D-domain truncation. It is concluded that although the alpha-subunit does not itself bind IGF-I, its interaction with IGFBP-3 in the ternary complex is dependent on structural determinants on IGF-I distal to the IGFBP-3 binding domain.     

Understanding the mechanism of insulin and insulin-like growth factor (IGF) receptor activation by IGF-II

Background

Insulin-like growth factor-II (IGF-II) promotes cell proliferation and survival and plays an important role in normal fetal development and placental function. IGF-II binds both the insulin-like growth factor receptor (IGF-1R) and insulin receptor isoform A (IR-A) with high affinity. Interestingly both IGF-II and the IR-A are often upregulated in cancer and IGF-II acts via both receptors to promote cancer proliferation. There is relatively little known about the mechanism of ligand induced activation of the insulin (IR) and IGF-1R. The recently solved IR structure reveals a folded over dimer with two potential ligand binding pockets arising from residues on each receptor half. Site-directed mutagenesis has mapped receptor residues important for ligand binding to two separate sites within the ligand binding pocket and we have recently shown that the IGFs have two separate binding surfaces which interact with the receptor sites 1 and 2.

Methodology/Principal Findings

In this study we describe a series of partial IGF-1R and IR agonists generated by mutating Glu12 of IGF-II. By comparing receptor binding affinities, abilities to induce negative cooperativity and potencies in receptor activation, we provide evidence that residue Glu12 bridges the two receptor halves leading to receptor activation.

Conclusions/Significance

This study provides novel insight into the mechanism of receptor binding and activation by IGF-II, which may be important for the future development of inhibitors of its action for the treatment of cancer.     

NMR 15N relaxation of the insulin-like growth factor (IGF)-binding domain of IGF binding protein-5 (IGFBP-5) determined free in solution and in complex with IGF-II.

15N NMR relaxation rates of mini-IGFBP-5, an N-terminal insulin-like growth factor binding domain of the insulin-like growth factor binding protein 5 (IGFBP-5), were analysed at three field strengths using the Lipari-Szabo procedure (see below) and reduced spectral density methods. Isotropic and anisotropic Lipari-Szabo models were analysed and an analytical formula for the overall correlation time for anisotropic molecules is presented. Mini-IGFBP-5 was found to be mainly rigid on fast ps time scales except for 11 unstructured flexible residues at the C-terminus. The insulin-like growth factor binding loop in the apo-protein exhibits small amounts of flexibility on fast time scales (ps to ns) but several loop residues show significant exchange broadening. These loop residues display no exchange broadening in the complex of IGF-II/mini-IGFBP-5. The isotropic overall tumbling time in solution at 31 degrees C of mini-IGFBP-5 complexed to IGF-II is tauc = 18.4 +/- 0.2 ns indicating a strong tendency for aggregation.     

BIAcore analysis of bovine insulin-like growth factor (IGF)-binding protein-2 identifies major IGF binding site determinants in both the amino- and carboxyl-terminal domains

In the absence of a complete tertiary structure to define the molecular basis of the high affinity binding interaction between insulin-like growth factors (IGFs) and IGF-binding proteins (IGFBPs), we have investigated binding of IGFs by discrete amino-terminal domains (amino acid residues 1-93, 1-104, 1-132, and 1-185) and carboxyl-terminal domains (amino acid residues 96-279, 136-279, and 182-284) of bovine IGFBP-2 (bIGFBP-2). Both halves of bIGFBP-2 bound IGF-I and IGF-II in BIAcore studies, albeit with different affinities ((1-132)IGFBP-2, K(D) = 36.3 and 51.8 nm; (136-279)IGFBP-2HIS, K(D) = 23.8 and 16.3 nm, respectively). The amino-terminal half appears to contain components responsible for fast association. In contrast, IGF binding by the carboxyl-terminal fragment results in a more stable complex as reflected by its K(D). Furthermore, des(1-3)IGF-I and des(1-6)IGF-II exhibited reduced binding affinity to (1-279)IGFBP-2HIS, (1-132)IGFBP-2, and (136-279)IGFBP-2HIS biosensor surfaces compared with wild-type IGF. A charge reversal at positions 3 and 6 of IGF-I and IGF-II, respectively, affects binding interactions with the amino-terminal fragment and full-length bIGFBP-2 but not the carboxyl-terminal fragment.     

Partial IGF affinity of circulating N- and C-terminal fragments of human insulin-like growth factor binding protein-4 (IGFBP-4) and the disulfide bonding pattern of the C-terminal IGFBP-4 domain

Within the IGF axis, the insulin-like growth factor-binding proteins (IGFBPs) are known to play a pivotal role in cell proliferation and differentiation. Defined proteolysis of the IGFBPs is proposed to be an essential mechanism for regulating IGF bioavailability. The generated IGFBP fragments in part exhibit different IGF-dependent and -independent biological activities. Characterizing naturally occurring forms of IGFBPs in human plasma, we identified both a N- and a C-terminal fragment of IGFBP-4 by means of immunoreactivity screening. As a source for peptide isolation, we used large amounts of human hemofiltrate obtained from patients with chronic renal failure. Purification of the IGFBP-4 peptides from hemofiltrate was performed by consecutive cation-exchange and reverse-phase chromatographic steps. Mass spectrometric and sequence analysis revealed an M(r) of 13 233 for the purified N-terminal fragment spanning residues Asp(1)-Phe(122) of IGFBP-4 and an M(r) of 11 344 for the C-terminal fragment extending from Lys(136) to Glu(237). Proteolytic digestion and subsequent biochemical analysis showed that the six cysteines of the C-terminal IGFBP-4 fragment are linked between residues 153-183, 194-205, and 207-228 (disulfide bonding pattern, 1-2, 3-4, and 5-6). Plasmon resonance spectroscopy, ligand blot analysis, and saturation and displacement studies demonstrated a very low affinity of the C-terminal IGFBP-4 fragment for the IGFs (IGF-II, K(d) = 690 nM; IGF-I, K(d) > 60 nM), whereas the N-terminal fragment retained significant IGF binding properties (IGF-II, K(d) = 17 nM; IGF-I, K(d) = 5 nM). This study provides the first molecular characterization of circulating human IGFBP-4 fragments formed in vivo exhibiting an at least 5-fold decrease in the affinity of the N-terminal IGFBP-4 fragment for the IGFs and a very low IGF binding capacity of the C-terminal fragment.     

Effect of insulin-like growth factor (IGF)-I and Des (1-3) IGF-I on the level of IGF binding protein-3 and IGF binding protein-3 mRNA in cultured porcine embryonic muscle cells

Insulin-like growth factor binding protein (IGFBP)-3 effects proliferation and differentiation of numerous cell types by binding to insulin-like growth factors (IGF) and attenuating their activity or by directly affecting cells in an IGF-independent manner. Consequently, IGFBPs produced by specific cells may affect their differentiation and proliferation. In this study we show that embryonic porcine myogenic cells, unlike murine muscle cell lines, produce significant quantities of a binding protein immunologically identified as IGFBP-3. Nonfusing cells subcultured from highly fused porcine myogenic cell cultures do not produce detectable IGFBP-3 protein or mRNA, thus suggesting the IGFBP-3 is produced by muscle cells in the porcine myogenic cell cultures. Treatment of porcine myogenic cultures with 20 ng of IGF-I or 20 ng of Des (1-3) IGF-I/ml serum-free media for 24 h results in a threefold reduction in the level of IGFBP-3 in conditioned media. This reduction is not affected by cell density over a sixfold range. Additionally, treatment for 24 h with 20 ng of IGF-I/ml media results in a sevenfold decrease in the steady-state level of IGFBP-3 mRNA. This IGF-I-induced decrease in IGFBP-3 mRNA level appears to be relatively unique to myogenic cells. IGF-I treatment also causes a fourfold increase in the steady-state level of myogenin mRNA. This increase in myogenin mRNA suggests that, as expected, IGF-I treatment accelerates differentiation of myogenic cells. The simultaneous decrease in IGFBP-3 mRNA and protein that accompanies IGF-I-induced myogenin expression suggests that differentiation of myogenic cells may be preceded or accompanied by decreased production of IGFBP-3.     

Human osteoblast-derived insulin-like growth factor (IGF) binding protein-5 stimulates osteoblast mitogenesis and potentiates IGF action.

Insulin-like growth factor (IGF)-binding proteins (IGFBPs) either inhibit or enhance IGF-stimulated cellular effects. While inhibition occurs by sequestration of IGF from cell-surface receptors, the exact mechanism of IGF-enhancement remains undefined. Human osteoblast-like bone cells in culture secrete several IGF-binding proteins, one of which we have previously identified as IGFBP-5. In this study we purified a 23-kDa IGFBP-5 from cultures of human osteoblast-like cells using ligand affinity chromatography and reversed-phase high performance liquid chromatography and tested its bioactivity in serum-free cultures of normal mouse osteoblast-like cells. Binding studies with radioiodinated IGF showed similar and relatively low affinities for IGF-I and IGF-II consistent with a carboxyl truncated IGF-binding protein. Mitogenic assays demonstrated that the binding protein, when coincubated with IGF-I or -II, enhanced mitogenesis. This enhancement was unique from other binding proteins in not requiring a preincubation period or serum co-factors. Furthermore, the osteoblast-derived IGFBP-5 stimulated mitogenesis in the absence of exogenous or endogenous IGF. Using radioiodinated IGFBP-5 we found that the binding protein could associate with the osteoblast surface, an effect which did not require IGF nor an interaction with IGF receptors. We suggest that osteoblast-derived IGFBP-5 may stimulate osteoblast mitogenesis in at least two ways, by association with IGF and by a second pathway that is independent of IGF receptor activation.     

c-myb stimulates cell growth by regulation of insulin-like growth factor (IGF) and IGF-binding protein-3 in K562 leukemia cells

c-myb plays an important role in the regulation of cell growth and differentiation, and is highly expressed in immature hematopoietic cells. The human chronic myelogenous leukemia cell K562, highly expresses IGF-I, IGF-II, IGF-IR, and IGF-induced cellular proliferation is mediated by IGF-IR. To characterize the impact of c-myb on the IGF-IGFBP-3 axis in leukemia cells, we overexpressed c-myb using an adenovirus gene transfer system in K562 cells. The overexpression of c-myb induced cell proliferation, compared to control, and c-myb induced cell growth was inhibited by anti-IGF-IR antibodies. c-myb overexpression resulted in a significant increase in the expression of IGF-I, IGF-II, and IGF-IR, and a decrease in IGFBP-3 expression. By contrast, disruption of c-myb function by DN-myb overexpression resulted in significant reduction of IGF-I, IGF-II, IGF-IR, and elevation of IGFBP-3 expression. In addition, exogenous IGFBP-3 inhibited the proliferation of K562 cells, and c-myb induced cell growth was blocked by IGFBP-3 overexpression in a dose-dependent manner. The growth-promoting effects of c-myb were mediated through two major intracellular signaling pathways, Akt and Erk. Activation of Akt and Erk by c-myb was completely blocked by IGF-IR and IGFBP-3 antibodies. These findings suggest that c-myb stimulates cell growth, in part, by regulating expression of the components of IGF-IGFBP axis in K562 cells. In addition, disruption of c-myb function by DN-myb may provide a useful strategy for treatment of leukemia.     

Differential insulin-like growth factor (IGF)-independent interactions of IGF binding protein-3 and IGF binding protein-5 on apoptosis in human breast cancer cells. Involvement of the mitochondria

We have demonstrated previously in Hs578T cells that insulin-like growth factor binding protein (IGFBP)-3 can significantly accentuate ceramide (C2)-induced apoptosis, but has no effect on cell death induced by integrin detachment [using an arginine-glycine-aspartic acid (RGD)-containing peptide]. In contrast we found that IGFBP-5 could inhibit apoptosis induced by either C2 or integrin detachment. It is now clear that the mitochondria not only provide the energy required for cell viability, but can also play an important role during the commitment phase to apoptosis. We used a mitochondrial respiratory chain inhibitor, antimycin A, at both apoptotic and nonapoptotic doses to further investigate the IGF-independent actions of IGFBP-3 and IGFBP-5 on C2 and RGD-induced apoptosis in the Hs578T cells. Hs578T cells had one of three treatments. 1: They were incubated with increasing doses of antimycin A for 24 h. 2: They were coincubated with an apoptotic dose of either C2 or RGD together with a nonapoptotic dose of antimycin A for 24 h. 3: They were incubated with a binding protein (100 ng/ml) for 24 h followed by coincubation of the binding protein with an apoptotic dose of antimycin A for a further 24 h. Cell viability was assessed by trypan blue dye exclusion and MTT assay, and apoptosis was confirmed and measured by morphologic assessment and flow cytometry. We found that antimycin A initiated apoptosis at 10 micromol/L and above. We also demonstrated that a nonapoptotic dose of antimycin A (0.1 micromol/L) significantly inhibited C2-induced apoptosis, whereas it significantly accentuated RGD-induced cell death. In addition, we found that cell death induced by antimycin A can be accentuated by IGFBP-3 but is not affected by IGFBP-5. These data indicate that IGFBP-3 can directly enhance apoptosis triggered via the mitochondria; either directly by a mitochondrial inhibitor or by C2 (which we demonstrate to act via effects on the mitochondria in this model). IGFBP-5, however, appears to confer survival effects via a distinct pathway not involving the mitochondria.     

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.     

Role of epidermal growth factor receptor activation in regulating mucin synthesis

Healthy individuals have few goblet cells in their airways, but in patients with hypersecretory diseases goblet-cell upregulation results in mucus hypersecretion, airway plugging, and death. Multiple stimuli produce hypersecretion via epidermal growth factor receptor (EGFR) expression and activation, causing goblet-cell metaplasia from Clara cells by a process of cell differentiation. These cells are also believed to be the cells of origin of non-small-cell lung cancer, but this occurs via cell multiplication. The mechanisms that determine which pathway is chosen are critical but largely unknown. Although no effective therapy exists for hypersecretion at present, the EGFR cascade suggests methods for effective therapeutic intervention.     

Regulation of insulin-like growth factor (IGF) bioactivity by sequential proteolytic cleavage of IGF binding protein-4 and -5

The biological activity of IGF-I and -II is controlled by six binding proteins (IGFBPs), preventing the IGFs from interacting with the IGF receptor. Proteolytic cleavage of IGFBPs is one mechanism by which IGF can be released to bind the receptor. The IGFBPs are usually studied individually, although the presence of more than one of the IGFBPs in most tissues suggests a cooperative function. Thus, the IGFBPs are part of regulatory networks with proteolytic enzymes in one end and the IGF receptor in the other end. We have established a model system that allows analysis of the dynamics between IGF, IGFBP-4 and -5, the IGF receptor, and the proteolytic enzyme PAPP-A, which specifically cleaves both IGFBP-4 and -5. We demonstrate different mechanisms of IGF release from IGFBP-4 and -5: cooperative binding to IGF is observed for the proteolytic fragments of IGFBP-5, but not fragments of IGFBP-4. Furthermore, we find that PAPP-A-mediated IGF-dependent cleavage of IGFBP-4 is inhibited by IGFBP-5, which sequesters IGF from IGFBP-4, and that cleavage of both IGFBP-4 and -5 is required for the release of bioactive IGF. Finally, we show that cell surface-localized proteolysis of IGFBP-4 represents the final regulatory step of efficient IGF delivery to the receptor. Our data define a regulatory system in which molar ratios between the IGFBPs and IGF and between the different IGFBPs, sequential proteolytic cleavage of the IGFBPs, and surface association of the activating proteinase are key elements in the regulation of IGF receptor stimulation.     

Role of protein phosphatases in insulin-like growth factor II (IGF II)-stimulated mannose 6-phosphate/IGF II receptor redistribution.

The regulated expression of mannose 6-phosphate/insulin-like growth factor II (M6P/IGF II) receptors in plasma membranes has previously been shown to be accompanied by marked changes in the phosphorylation state of the receptors (Corvera, S., Folander, K., Clairmont, K. B., and Czech, M. P. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, 7567-7571). In the present study we show that protein phosphatase 2A dephosphorylates the human M6P/IGF II receptor in vitro. Incubation of human fibroblasts with okadaic acid, a specific inhibitor of this phosphatase, resulted in a depletion of M6P/IGF II receptors at the cell surface without affecting their internalization kinetics. The phosphorylation state of the remaining cell surface receptors was 3-fold increased. Thus, the endocytosis rate of M6P/IGF II receptors appears to be unaltered by increased phosphorylation. While the decreased cell surface expression of receptors was reversible upon removal of okadaic acid the IGF II-induced redistribution of M6P/IGF II receptors to the plasma membrane (Braulke, T., Tippmer, S., Neher, E., and von Figura, K. (1989) EMBO J. 8, 681-686) was irreversibly inhibited by the phosphatase inhibitor. Receptor redistribution in response to protein kinase C activation was not affected by okadaic acid. These results suggest that the cell surface expression of M6P/IGF II receptor can be regulated by phosphatase-dependent and -independent pathways. In addition, the phosphorylation state and the steady-state cell surface number of transferrin receptors were not affected by okadaic acid, whereas it impaired the IGF II-stimulated receptor redistribution similarly as for M6P/IGF II receptors. The data indicate that okadaic acid-sensitive protein phosphatases may play a general role in terms of IGF II-modulated receptor recycling.