Paradoxical actions of endogenous and exogenous insulin-like growth factor-binding protein-5 revealed by RNA interference analysis

Insulin-like growth factor-binding protein-5 (IGFBP-5) is abundantly expressed in bone cells. To determine the physiological role(s) of endogenous IGFBP-5 in regulating bone cell growth, differentiation, and survival, we used short double-stranded RNA (siRNA) to trigger RNA interference of IGFBP-5 in human osteosarcoma cells. The IGFBP-5 siRNA, targeting against a sequence unique to the IGFBP-5 middle domain, efficiently reduced IGFBP-5 mRNA and protein levels. The IGFBP-5 siRNA did not change the levels of IGFBP-4, a structurally related protein, or glyceraldehyde-3-phosphate dehydrogenase, a housekeeping gene. Knock-down of IGFBP-5 resulted in a significant increase in the number of transferase-mediated dUTP nick end labeling-positive cells and a decrease in a bone differentiation parameter (alkaline phosphatase activity) but had little effect on basal or insulin-like growth factor I-induced proliferation. Overexpression of a siRNA-resistant IGFBP-5 mutant in the IGFBP-5 knock-down cells restored the levels of survival to the control level; overexpression of IGFBP-4 or wild type IGFBP-5 had no such effect. Paradoxically, the addition of exogenous IGFBP-5 not only failed to rescue IGFBP-5 knock-down-induced apoptosis, it caused a further increase in apoptosis. Furthermore, the addition of exogenous IGFBP-5 alone increased apoptosis. This pro-apoptotic action of exogenous IGFBP-5 was abolished when IGF-I was added in excess, suggesting that exogenous IGFBP-5 increases apoptosis by binding to and inhibiting the activities of insulin-like growth factors. These results indicate that endogenous and exogenous IGFBP-5 exhibits opposing biological actions on cell survival and underscore the necessity and utility of studying IGFBP functions through loss-of-function approaches.

IGF-I increases IGFBP-5 and collagen alpha1(I) mRNAs by the MAPK pathway in rat intestinal smooth muscle cells

IGF-I is a potent fibrogenic growth factor that stimulates proliferation of intestinal smooth muscle cells and increases synthesis of collagen and IGF-I-binding proteins by the cells. These processes contribute to intestinal fibrosis that develops in patients with Crohn's disease and in Lewis-strain rats with experimental Crohn's disease. The aim of this study was to determine which early docking proteins are associated with IGF-I receptor signal transduction and which transduction pathway is involved in IGF-I-mediated gene regulation in intestinal smooth muscle cells. Primary cultures of smooth muscle cells isolated from the muscularis externa of the distal colon of Lewis rats were treated with IGF-I (100 ng/ml). Immunoprecipitation studies demonstrated that IGF-I stimulation resulted in tyrosine phosphorylation of IRS-1, IRS-2, and Shc. Coimmunoprecipitation demonstrated a close association between the IGF-I receptor and these three early docking proteins. Concurrent treatment with the MAPK inhibitor PD98059 (10 microM) resulted in an inhibition of the IGF-I-mediated increase in IGFBP-5 and collagen alpha(1)(I) mRNAs, while concurrent treatment with the phosphatidylinositol 3-kinase (PI3-K) inhibitor wortmannin (100 nM) had no effect. In additional experiments, cells were transiently transfected with adenoviral vectors dominantly expressing inactive mutant Akt or constitutively expressing wild-type Akt. In both cases, the IGF-I-mediated increase in collagen I protein did not differ from that observed in control cultures that had been transfected with an adenoviral vector carrying the LacZ reporter gene. These results suggest that the MAPK pathway is key to IGF-I-mediated gene regulation in intestinal smooth muscle cells, whereas data do not suggest a role for the Akt-dependent pathway in our system.

Insulin-like growth factor-binding protein 5 (Igfbp5) compromises survival, growth, muscle development, and fertility in mice

The insulin-like growth factors (IGFs) are essential for development; bioavailable IGF is tightly regulated by six related IGF-binding proteins (IGFBPs). Igfbp5 is the most conserved and is developmentally up-regulated in key lineages and pathologies; in vitro studies suggest that IGFBP-5 functions independently of IGF interaction. Genetic ablation of individual Igfbps has yielded limited phenotypes because of substantial compensation by remaining family members. Therefore, to reveal Igfbp5 actions in vivo, we generated lines of transgenic mice that ubiquitously overexpressed Igfbp5 from early development. Significantly increased neonatal mortality, reduced female fertility, whole-body growth inhibition, and retarded muscle development were observed in Igfbp5-overexpressing mice. The magnitude of the response in individual transgenic lines was positively correlated with Igfbp5 expression. Circulating IGFBP-5 concentrations increased a maximum of only 4-fold, total and free IGF-I concentrations increased up to 2-fold, and IGFBP-5 was detected in high M(r) complexes; however, no detectable decrease in the proportion of free IGF-I was observed. Thus, despite only modest changes in IGF and IGFBP concentrations, the Igfbp5-overexpressing mice displayed a phenotype more extreme than that observed for other Igfbp genetic models. Although growth retardation was obvious prenatally, maximal inhibition occurred postnatally before the onset of growth hormone-dependent growth, regardless of Igfbp5 expression level, revealing a period of sensitivity to IGFBP-5 during this important stage of tissue programming.

Genital skin fibroblasts (GF) of patients with androgen insensitivity syndrome express higher insulin-like growth factor binding protein (IGFBP)-2, -3 and -5 than GF of normally virilized males

BACKGROUND

We investigated the effects of androgens, estradiol (E2) and insulin-like growth factor (IGF)-I on IGF-II, insulin-like growth factor binding protein (IGFBP)-2, -3 and -5 and mRNA in genital fibroblasts (GF) from patients with complete androgen insensitivity (CAIS) and normally virilized males (C).

METHODS

Proteins were measured by specific RIA and Western ligand blot, and specific mRNA levels by RT-PCR normalized by GAPDH levels.

RESULTS

Secretion of IGF-II was lowered in CAIS (p<0.001) GF and by testosterone + IGF-I in C GF. Secretion of IGFBP-2 was higher (p<0.001) in CAIS GF and IGFBP-2 mRNA levels were increased by E2 in C GF (p<0.05). E2 stimulated IGFBP-2, -3 and -5 expression in CAIS GF. CAIS GF also secreted more IGFBP-3 (p<0.001) and accumulated 3-5 times more IGFBP-5 mRNA than C GF (p<0.001).

CONCLUSION

In contrast to C GF, the availability of IGF-II in CAIS GF is apparently decreased by two facts: by the decreased expression and by increased expression of IGFBP-2, -3 and -5. Furthermore, E2 and IGF-I modulate the expression of IGF-II and IGFBP in GF. This may play a role in the failure to develop male external genitals in CAIS patients.

Partitioning of IGFBP-5 actions in myogenesis: IGF-independent anti-apoptotic function

Igfbp5 is upregulated during the differentiation of several key cell lineages and in some tumours; the function of IGFBP-5 in these physiological and pathological situations is unknown. Since IGFBP-5 contains sequence motifs consistent with IGF-independent actions, the aim of these studies was to distinguish between IGF-dependent and -independent actions of IGFBP-5. Myc-tagged wild-type (termed wtIGFBP-5) and non-IGF binding mouse Igfbp5 (termed mutIGFBP-5) cDNAs were generated and used to transfect C2 myoblasts, a cell line that undergoes differentiation to myotubes in an IGF- and IGFBP-5-regulated manner. WtIGFBP-5, but not mutIGFBP-5, inhibited myogenesis, as assessed by cell morphology, MHC immunocytochemistry and caveolin 3 expression. However, both wt- and mutIGFBP-5 increased cell survival and decreased apoptosis, as indicated by decreased caspase-3 activity and cell surface annexin V binding. Further examination of apoptotic pathways revealed that wt- and mutIGFBP-5 ameliorated the increase in caspase-9 but not the modest increase in caspase-8 during myogenesis, suggesting that IGFBP-5 increased cell survival via inhibition of intrinsic cell death pathways in an IGF-independent manner. The relationship between IGF-II and IGFBP-5 was examined further by cotransfecting C2 myoblasts with antisense Igf2 (previously established to induce increased cell death) and Igfbp5; both wt- and mutIGFBP-5 conferred equivalent protection against the decreased cell survival and increased apoptosis. In conclusion, we have partitioned IGFBP-5 action in myogenesis into IGF-dependent inhibition of differentiation and IGF-independent cell survival. Our findings suggest that, by regulation of cell survival, IGFBP-5 has an autonomous role in the regulation of cell fate in development and in tumourigenesis.

Fibronectin Binds Insulin-like Growth Factor-binding Protein 5 and Abolishes Its Ligand-dependent Action on Cell Migration

Insulin-like growth factor-binding protein 5 (IGFBP-5) is a secreted protein that binds to insulin-like growth factors (IGFs) and modulates IGF actions on cell proliferation, differentiation, survival, and motility. IGFBP-5 also regulates these cellular events through IGF-independent mechanisms. To elucidate the molecular mechanisms governing these diverse actions of IGFBP-5, we screened a human cDNA library by a yeast two-hybrid system using IGFBP-5 as bait and identified fibronectin (FN) as a potential IGFBP-5-interacting partner. The complex formation of IGFBP-5 and FN was established by glutathione S-transferase pull-down, solution, and solid phase binding assays using glutathione S-transferase-IGFBP-5 and native IGFBP-5 in vitro and by co-immunoprecipitation in vivo. Binding assay using deletion mutants indicated that the IGFBP-5 C domain binds to the 10th and 11th type I repeats of FN. IGFBP-5 potentiated IGF-I-induced cell migration in FN-null, but not in wild-type, mouse embryonic cells. When FN was reintroduced either as an adhesive substrate or in solution to the FN-null cells, the potentiating effect of IGFBP-5 on IGF-I-induced cell migration was abolished. Binding of IGFBP-5 to FN had no effect on the ability of IGFBP-5 to bind IGF-I, but it increased the proteolytic degradation of IGFBP-5. Inhibition of IGFBP-5 proteolysis restored the potentiating effect of IGFBP-5. These results suggest that FN and IGFBP-5 bind to each other, and this binding negatively regulates the ligand-dependent action of IGFBP-5 by triggering IGFBP-5 proteolysis.

Effects of TGF-β2 on Immune Response–Related Gene Expression Profiles in the Human Corneal Endothelium

PURPOSE

To determine the effects of transforming growth factor (TGF)-beta2 on immune-response-related gene expression profiles in the stimulated human corneal endothelium (HCE).

METHODS

A human complementary DNA (cDNA) expression array analysis was used to investigate the effects of TGF-beta2 on cultured HCE incubated with interleukin (IL)-1alpha and tumor necrosis factor (TNF)-alpha. Gene-specific semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA) were performed to examine the gene expression patterns revealed by the cDNA expression array analysis. Moreover, the expression of newly identified genes in HCE was confirmed by RT-PCR in human donor corneas.

RESULTS

cDNA expression array analysis and semiquantitative RT-PCR revealed that TGF-beta2 downregulated the expression of IL-6, growth-related (Gro)-alpha (CXCL1), monocyte chemotactic protein (MCP)-1 (CCL2), granulocyte-colony stimulating factor (G-CSF), and insulin-like growth factor binding protein (IGFBP)-5 and upregulated the expression of tissue inhibitor of metalloproteinase (TIMP)-1. ELISA confirmed TGF-beta2-mediated changes in the expression of IL-6, CXCL1, CCL2, G-CSF, IGFBP-5, and TIMP-1 at the protein level. CXCL1, G-CSF, and IGFBP-5 mRNAs were detected for the first time in the HCE of donor corneas.

CONCLUSIONS

TGF-beta2 downregulates IL-6, CXCL1, CCL2, G-CSF, and IGFBP-5, and upregulates TIMP-1 in cultured HCE stimulated with proinflammatory cytokines, suggesting the immunomodulatory role of TGF-beta2 in the aqueous humor and the pathophysiological significance of TGF-beta2 in the anterior chamber of the eye.

Insulin-like growth factor binding protein-3 prevents retinoid receptor heterodimerization: implications for retinoic acid-sensitivity in human breast cancer cells

Insulin-like growth factor binding protein-3 (IGFBP-3) has both IGF-dependent and -independent effects on cell growth, which are frequently growth-inhibitory. Interestingly, the development of a more aggressive phenotype in breast cancer cells (BCCs) correlates positively with elevated expression of IGFBP-3 and is often associated with all-trans-retinoic acid (atRA)-resistance. IGFBP-3 was previously demonstrated to interact directly with retinoid X receptor (RXR). In this study we have shown that IGFBP-5 also interacts with RXR and that both IGFBPs interact with retinoic acid receptor (RAR). To investigate whether the presence of IGFBP-3 regulates breast cancer cell responsiveness to atRA, we immuno-neutralized the IGFBP-3 expressed by the atRA-resistant Hs578T and MDA-MB-231 BCCs (which express IGFBP-3 constitutively) and showed that they become more sensitive to the growth-inhibitory effects of atRA. Similarly, in Hs578T cells expressing a reporter gene under the control of an RAR response element (RARE), depletion of IGFBP-3 resulted in the induction of reporter gene expression in response to atRA. In investigating possible mechanisms for IGFBP-3 regulation of atRA-sensitivity, we found that IGFBP-3 blocked the formation of RAR:RXR heterodimers and disrupted the ligand-inducible receptor complex. Thus, IGFBP-3 has the potential to reduce the RARE-mediated transactivation of target genes and modulate the atRA-response in BCCs.

Follicular dynamics around the recruitment of the first follicular wave in the cow

The present study aimed to test the generally accepted view that a follicular wave starts with follicles newly recruited from the population smaller than 3 mm, which later compete for dominance. According to this view, subordinate follicles are expected to be too atretic to join the next follicular wave. Ten cows were ovariectomized shortly prior to the LH surge, thus around the start of the first follicular wave of the cycle. Per cow, on average, 14.4 follicles of >/=3 mm were dissected. Follicular health was determined on the basis of four parameters: 1) judgment of the degree of atresia by stereomicroscope, 2) incidence of apoptotic nuclei among the granulosa cells, 3) estradiol and progesterone concentrations, and 4) insulin-like growth factor-I (IGF-I) binding proteins (IGFBPs)-2, -4, and -5 concentrations in the follicular fluid. In addition to the preovulatory follicle, 3.1 other follicles, mainly sized 3-4.5 mm, were found to be healthy based on the proportion of apoptotic nuclei, and concentrations of estradiol/progesterone, and IGFBPs. The ability of these follicles to respond with growth on the preovulatory and periovulatory FSH surges was supported by a comparison to the follicular population of four cows 31-68 h after the LH surge. The present results point to an alteration of the view on the follicular wave. The larger follicles during the first days of the follicular wave are, in general, derived from follicles that also joined the previous wave. A portion of these growing follicles are estradiol active and compete for dominance. Other growing follicles lack estradiol production and are probably derived from rather atretic follicles. The first newly recruited follicles do not reach the size of 3 mm before 31 h after the preovulatory FSH surge. At that time, the larger follicles are already competing for dominance.

Regulation of vascular smooth muscle cell responses to insulin-like growth factor (IGF)-I by local IGF-binding proteins

Insulin-like growth factor (IGF)-I is a pleiotropic hormone that regulates vascular smooth muscle cell (VSMC) migration, proliferation, apoptosis, and differentiation. These actions are mediated by the IGF-I receptor. How activation of the same receptor by the same ligand leads to these diverse cellular responses is not well understood. Here we describe a novel mechanism specifying VSMC responses to IGF-I stimulation, distinctive for the pivotal roles of local IGF-binding proteins (IGFBPs). The role of local IGFBPs was indicated by comparing the activities of IGF-I and des-1-3-IGF-I, an IGF-I analog with reduced binding affinity to IGFBPs. Compared with IGF-I, des-1-3-IGF-I was more potent in stimulating DNA synthesis but much less potent in inducing directed migration of VSMCs. When the effects of individual IGFBPs were tested, IGFBP-2 and IGFBP-4 were found to inhibit IGF-I-stimulated DNA synthesis and migration. IGFBP-5 had an inhibitory effect on IGF-I-stimulated DNA synthesis, but it strongly potentiated IGF-I-induced VSMC migration. By using a non-IGF-binding IGFBP-5 mutant and an IGF-I-neutralizing antibody, it was demonstrated that IGFBP-5 also stimulates VSMC migration in an IGF-independent manner. This effect of IGFBP-5 was inhibited by soluble heparin and by treating cells with heparinase. Mutation of the heparin-binding motif of IGFBP-5 reduced its migration promoting activity. These findings suggest that local IGFBPs are important determinants of cellular responses to IGF-I stimulation, and a key player in this paradigm is IGFBP-5. IGFBP-5 not only modulates IGF-I actions, but it also stimulates cell migration by interacting with cell-surface heparan sulfate proteoglycans.

Zinc partitions insulin-like growth factors (IGFs) from soluble IGF binding protein (IGFBP)-5 to the cell surface receptors of BC3H-1 muscle cells

Zinc (Zn(2+)) is a multifunctional micronutrient. The list of functions for this micronutrient expanded with the recent discovery that Zn(2+) retains insulin-like growth factors binding proteins (IGFBPs) on the surface of cultured cells, lowers the affinity of cell-associated IGFBPs, and increases the affinity of the cell surface insulin-like growth factor (IGF)-type 1 receptor (IGF-1R). However, currently there is no information concerning the effect of Zn(2+) on soluble IGFBPs. In the current study, the soluble IGFBP-5 secreted by BC(3)H-1 cells is shown to bind approximately 50% more [(125)I]-IGF-II than [(125)I]-IGF-I at pH 7.4. Zn(2+) is shown to depress the binding of both IGF-I and IGF-II to soluble secreted IGFBP-5; [(125)I]-IGF-I binding is affected more so than [(125)I]-IGF-II binding. Zn(2+) acts by lowering the affinity (K(a)) of IGFBP-5 for the IGFs. Scatchard plots are non-linear indicating the presence of high and low affinity binding sites; Zn(2+) affects only binding to the high affinity site. In contrast, Zn(2+) increases the affinity by which either [(125)I]-IGF-I or [(125)I]-R(3)-IGF-I binds to the IGF-1R, but depresses [(125)I]-IGF-II binding to the IGF-type 2 receptor (IGF-2R) on BC(3)H-1 cells. By depressing the association of the IGFs with soluble IGFBPs, Zn(2+) is shown to repartition either [(125)I]-IGF-I or [(125)I]-IGF-II from soluble IGFBP-5 onto cell surface IGF receptors. Zn(2+) was active at physiological doses depressing IGF binding to IGFBP-5 and the IGF-2R at 15-20 microM. Hence, a novel mechanism is further characterized by which the trace micronutrient Zn(2+) could regulate IGF activity.

mRNA expression of components of the IGF system and of GH and insulin receptors in ovaries of broiler breeder hens fed ad libitum or restricted from 4 to 16 weeks of age

An evaluation of the insulin-like growth factors (IGF) system in the ovaries of heavy breeder hens during the growing phase was performed to measure the effect of feed restriction. The transcripts of IGF-I, IGF-II, IGF receptor (IGF-R), two IGF binding proteins (IGFBP-2 and IGFBP-5), GH receptor (GH-R) and insulin receptor (I-R) were measured by RT-PCR at 4, 8, 12 and 16 weeks of age in the ovaries of ad libitum fed and feed restricted hens. The transcripts of all genes screened were detected in all feed regime groups and at all ages sampled. However, no significant overall effects of feed restriction on these transcripts were found despite the marked difference in body growth (200% at 16 weeks of age). Similarly there were no significant age effects except for the expression of GH-R-that showed a decrease after 8 weeks of age (P<0.01). Although feed restriction seems to have a relatively limited effect on the expression of these components of the ovarian IGF system in juveniles, a more detailed determination is necessary to determine possible changes nearer to sexual maturity (16 weeks of age onwards). It is concluded that components of the IGF system, GH or insulin may not be involved in the mechanisms that, through restricted feeding, alter ovarian development and function during growth to enable better reproductive performance during lay.

Actions of insulin-like growth factor binding protein-5 (IGFBP-5) are potentially regulated by tissue kallikrein in rat brains

Tissue kallikrein (EC 3.4.21.35) that hydrolyzes kininogen and releases a physiologically active peptide, kinin, is found in rat brains. Insulin-like growth factor binding proteins (IGFBPs) that modulate IGF actions are also expressed in a variety of tissues including rat brains, and one of the major IGFBPs expressed in brain is known to be IGFBP-5, which is reported to be hydrolyzed in vitro by prostate-specific antigen (PSA) and gamma-nerve growth factor (gamma-NGF), both of which belong to the member of the kallikrein gene family. This study was designed to determine whether or not kallikrein has a potential to hydrolyze IGFBP-5 and their topographic proximity was investigated in rat brain using double immunohistochemical staining method. Immunohistochemically, IGFBP-5 positive cells were numerous and widespread in the cerebral cortex and belonged to neurons in the cell configuration. IGFBP-5 positive cells were negative for S-100 protein and were positive for betaIII tubulin, confirming them to be neurons. In addition, kallikrein positive cells were virtually all IGFBP-5 positive cells. IGFBP-5 was clearly hydrolyzed by kallikrein with cleavage sites of Arg188-Met189 and Arg136-Arg137 of IGFBP-5. Therefore, there is a possibility that kallikrein plays an important role in brain physiology, specifically in the neurons by regulating the actions of IGFBP-5 and IGF.

Possible involvement of IGF-1 receptor and IGF-binding protein in insulin-induced enhancement of noradrenaline response in diabetic rat aorta

1. We investigated the mechanisms underlying the changes in vascular contractile responsiveness induced by chronic treatment with insulin in controls and established streptozotocin (STZ)-induced diabetic rats. 2. The aortic contractile response to noradrenaline (NA) showed no significant difference between controls and diabetics, but it was significantly greater in insulin-treated diabetic rats than in the other groups. To investigate the mechanism, we examined the changes in NA-induced contractility following treatment with insulin and insulin-like growth factor-1 (IGF-1) in organ-cultured control and diabetic aortas. 3. The contractile response to NA in organ-cultured diabetic rat aortas treated with insulin (500 ng ml-1, 16 h) or IGF-1 (20 ng ml-1, 16 h) was significantly greater than the corresponding values for (a) diabetic rat aortas cultured in serum-free medium, and (b) control aortas incubated with insulin or IGF-1. Incubating control aortas with insulin or IGF-1 had no significant effect on the contraction induced by NA. 4. The expressions of the IGF-1 receptor mRNA and protein were increased in STZ-induced diabetic aortas and further increased in insulin-treated diabetics. The mRNA expressions of IGF-binding protein (IGFBP)-2 and IGFBP-3 were normal in diabetic aortas. In contrast, those of IGFBP-4 and IGFBP-5 were significantly decreased in diabetic aortas, and not restored by insulin treatment. 5. These results suggest that the insulin deficiency and chronic hyperinsulinemia in diabetes upregulate the IGF-1 receptor and downregulate IGFBP-4 and IGFBP-5 in the aorta. This may be a major cause of the increased vascular contractility induced by insulin administration and by hyperinsulinemia in established diabetes, resulting in hypertension.

Complex with a phage display-derived peptide provides insight into the function of insulin-like growth factor I

The dramatic improvement in the NMR spectra of insulin-like growth factor I (IGF-I) in the presence of a peptide identified from a phage display library has allowed for the first time the determination of a high-resolution solution structure for much of IGF-I. The three helices of IGF-I in this complex have an arrangement similar to that seen in high-resolution crystal structures of IGF-I and insulin, although there are differences in the conformation and precise location of helix 3. A cluster of hydrophobic and basic side chains within the turn-helix motif of the peptide contact a hydrophobic patch on helices 1 and 3 of IGF-I. The importance of this patch for tight binding was verified using alanine scanning mutagenesis of the peptide in two different phage display formats. Consistent with its antagonistic activity, the peptide binds to a region implicated by mutagenesis studies to be important for association with IGF binding proteins (IGFBPs). The ability of the peptide to also inhibit signaling has important implications for the manner in which IGF-I interacts with its receptor. Interestingly, the peptide uses the same binding site as detergent and a fragment of IGFBP-5 identified in other IGF-I complexes. The ligand-induced structural variability of helix 3 in these complexes suggests that exchange between such conformations may be the source of the dynamic nature of free IGF-I and likely has functional significance for the ability of IGF-I to recognize two signaling receptors and six binding proteins with high affinity.

Insulin-like growth factor-binding protein-5 inhibits the growth of human breast cancer cells in vitro and in vivo

The role of insulin-like growth factor-binding protein (IGFBP)-5 in human breast cancer cell growth is unclear. We determined the effects of IGFBP-5 expression on the growth of human breast cancer cell lines in vivo and in vitro. Expression of IGFBP-5, both by stable transfection and adenoviral-mediated infection, was inhibitory to the growth of MDA-MB-231 and Hs578T human breast cancer cells over a 13-day period. IGFBP-5 expression resulted in a G2/M cell cycle arrest and the induction of apoptosis in both cell lines, an effect that was abrogated in the presence of the broad-spectrum caspase inhibitor, z-VAD-fmk. IGFBP-5-induced apoptosis was associated with a transcriptional increase in expression of the proapoptotic regulator bax and decrease in the anti-apoptotic bcl-2 compared with vector controls. Secreted IGFBP-5 when added exogenously to breast cancer cells was not internalized and had no effect on cell growth or apoptosis, suggesting that IGFBP-5 may elicit its inhibitory effects via a novel, intracrine mechanism. In athymic nude mice, stable expression of IGFBP-5 significantly inhibited both the formation and growth of tumors derived from MDA-MB-231 cells. IGFBP-5-expressing tumors also had a significantly elevated level of bax mRNA and decreased levels of bcl-2 mRNA compared with vector tumors. These data suggest that IGFBP-5 is a potent growth inhibitor and proapoptotic agent in human breast cancer cells via modulation of cell cycle regulation and apoptotic mediators.

Zinc alters the kinetics of IGF-II binding to cell surface receptors and binding proteins

The growth of most tissues is markedly depressed as a result of zinc deficiency by uncharacterized mechanisms that clearly involve the insulin-like growth factor (IGF) system. Herein, we describe the mechanism by which zinc (Zn2+) maintains IGF-II in an active form by directly regulating IGF-II binding to IGF-binding proteins (IGFBPs) and the type 1 IGF receptor (IGF-1R). The specificity of Zn2+ effects was confirmed by using other cations that can (Cd2+ and Au3+) or cannot (La3+) mimic Zn2+ actions. Human fibroblasts, glioblastoma cells, and murine myoblasts were used to determine the kinetics of IGF-II binding to cell surface IGFBP-3, IGFBP-5, and the IGF-1R, respectively. Zn2+, Cd2+, and Au3+, but not La3+, decreased total binding and the affinity for [125I]IGF-II association with IGFBP-3 and IGFBP-5. These effects were a result of lowered rate of ligand association without affecting rate of dissociation. In contrast, Zn2+ enhanced [125I]IGF-II binding to the IGF-1R by enhancing the rate of ligand association and decreasing the rate of dissociation. Our previous work had shown that Zn2+ acts at physiological levels to alter IGF binding. Together with the current work, these findings imply that Zn2+ acts in vivo to prevent secreted IGF-II from binding to IGFBP-3 and IGFBP- 5, thus maintaining IGF-II in an "active state," i.e., readily available for IGF-1R association.

Hormonal control of IGF-binding protein (IGFBP)-5 and IGFBP-2 secretion during differentiation of the HC11 mouse mammary epithelial cell line

The mouse mammary epithelial cell line HC11 upregulates the synthesis of beta-casein (a differentiation marker) following treatment with the lactogenic hormone mix dexamethasone, insulin and prolactin (DIP). We demonstrate that the basal levels of IGF-binding protein (IGFBP)-5 secreted by undifferentiated HC11 cells are upregulated 10-fold during DIP-induced cellular differentiation whereas the level of the other IGFBP species secreted by HC11 cells (IGFBP-2) is downregulated during this process. As previously reported, the combination of all three of these hormones is required for synthesis of the differentiation marker beta-casein, whereas basal IGFBP-5 secretion is evident in the absence of any hormonal treatment and, unlike beta-casein, secretion of this protein can be stimulated by binary combinations of the hormones (although maximal levels of IGFBP-5 are achieved in the presence of all three lactogenic hormones). Additionally, levels of IGFBP-5 can be increased by DIP treatment under conditions (non-competency of HC11 cultures or presence of epidermal growth factor) where DIP treatment does not increase synthesis of beta-casein. For IGFBP-2, dexamethasone is a potent inhibitor of secretion whilst prolactin stimulated the secretion of this binding protein into the medium. For the IGFBP axis in HC11 cells we conclude that, although the levels of IGFBP-5 and -2 are influenced by the state of cellular differentiation, the hormonal regulation of the levels of these IGFBP species can be dissociated from the regulation of beta-casein synthesis. In a further series of experiments we demonstrate that IGF-I is able to replace insulin in the DIP lactogenic hormone mix and by the use of a specific IGF-I receptor blocking antibody indicate that the action of IGF-I is mediated through the cell surface IGF-I receptor and not by cross-reaction of IGF-I ligand at the insulin receptor. We discuss our data in the context of the potential role of the IGF axis in the process of cell differentiation and illustrate the significance of our findings in the context of the physiology and life cycle of the mammary epithelial cell.

Proteolysis of insulin-like growth factor binding proteins -4 and -5 in bovine follicular fluid: implications for ovarian follicular selection and dominance

Dominant follicles are characterized by low levels of low molecular weight IGF binding proteins (IGFBPs) and by proteolytic activity against IGFBP-4 and -5. To examine the hypothesis that proteolysis of IGFBP-4 and -5 plays a critical role in selection of the dominant follicle, we isolated follicles at various stages during the first wave of follicular development during the bovine estrous cycle, using ultrasonography to follow follicular growth. Ovariectomies were performed before divergence in follicular size (group 1; largest follicle, approximately 7 mm in diameter), at about the expected time of size divergence (group 2; largest follicle, approximately 8 mm) or after a dominant follicle was clearly present (group 3; largest follicle, > or =9 mm). The four largest follicles (F1-F4) were dissected and concentrations of steroids, IGFBPs and free IGF-I and levels of proteolytic activity for IGFBP-4 and -5 in the follicular fluid were determined. Follicles in group 1 did not differ significantly in size or estradiol concentrations, but levels of proteolytic activity against IGFBP-4 and -5 were higher in F1-F2 than in F3-F4. However, in group 2 the largest follicle (F1) had higher estradiol, free IGF-I, and IGFBP-4 and -5 proteolytic activity than F2-F4, whereas only slight (dissected) or no (ultrasound) differences in diameters could be detected. Differences between F1 and F2-F4 in diameter, estradiol, free IGF-I, and IGFBP-4 and -5 proteolytic activity were even greater in group 3. In addition, the hormonal regulation of IGFBP-4 and -5 proteolysis was evaluated in vivo by injecting heifers with small doses of recombinant bovine FSH to induce codominant follicles (group 4). The induced codominant follicles were larger and had higher IGFBP-4 and -5 proteolytic activity than subordinate follicles. The results suggest that follicular selection is a progression of changes starting with acquisition of an FSH-inducible IGFBP-4/-5 proteolytic activity, leading to an increase in intrafollicular concentration of free IGF-I that, in turn, synergizes with FSH to promote greater estradiol production by the follicle destined for dominance.

Structural and functional evidence for the interaction of insulin-like growth factors (IGFs) and IGF binding proteins with vitronectin

Previous studies demonstrated that IGF-II binds directly to vitronectin (VN), whereas IGF-I binds poorly. However, binding of VN to integrins has been demonstrated to be essential for a range of IGF-I-stimulated biological effects, including IGF binding protein (IGFBP)-5 production, IGF type-1 receptor autophosphorylation, and cell migration. Thus, we hypothesized that a link between IGF-I and VN must occur and may be mediated through IGFBPs. This was tested using competitive binding assays with VN and (125)iodine-labeled IGFs in the absence and presence of IGFBPs. IGFBP-4, IGFBP-5, and nonglycosylated IGFBP-3 were shown to significantly enhance binding of IGF-I to VN, whereas IGFBP-2 and glycosylated IGFBP-3 had a smaller effect. Furthermore, binding studies with analogs indicate that glycosylation status and the heparin-binding domain of IGFBP-3 are important in this interaction. To examine the functional significance of IGFs binding to VN, cell migration in MCF7 cells was measured and found to be enhanced when VN was prebound to IGF-I in the presence of IGFBP-5. The effect required IGF:IGFBP:VN complex formation; this was demonstrated by use of a non-IGFBP-binding IGF-I analog. Together, these data indicate the importance of IGFBPs in modulating IGF-I binding to VN and that this binding has functional consequences in cells.

Substrate specificity of the metalloproteinase pregnancy-associated plasma protein-A (PAPP-A) assessed by mutagenesis and analysis of synthetic peptides: substrate residues distant from the scissile bond are critical for proteolysis

Human pregnancy-associated plasma protein-A (PAPP-A) cleaves insulin-like growth factor (IGF) binding protein-4 (IGFBP-4), causing a dramatic reduction in its affinity for IGF-I and -II. Through this mechanism, PAPP-A is a regulator of IGF bioactivity in several systems, including the human ovary and the cardiovascular system. PAPP-A belongs to the metzincin superfamily of zinc metalloproteinases, and is the founding member of a fifth metzincin family, the pappalysins. Herein, we first determined that PAPP-A cleaves IGFBP-4 at a single site (Met-135/Lys-136), and we analysed the influence of ionic strength, pH and zinc ion concentration on the cleavage reaction. Secondly, we sought to delineate the role of substrate residues in PAPP-A-mediated cleavage by the construction and analysis of 30 IGFBP-4 mutants in which various residues were replaced by alanine, by the analysis of eight mutants of IGFBP-5 (found recently to be a second PAPP-A substrate), and by cleavage analysis of synthetic peptides derived from IGFBP-4. Our data reveal a complex mode of substrate recognition and/or binding, pointing at important roles for several basic residues located up to 16 residues N-terminal to the scissile bond. An unexpected parallel can be drawn with an intracellular enzyme, the mitochondrial processing peptidase, that may help us to understand properties of the pappalysins. Further, proteinase-resistant variants of IGFBP-4 and -5, presented here, will be useful tools for the study of proteolysis in cell-based systems, and our finding that a synthetic peptide can be cleaved by PAPP-A provides the basis for development of quantitative assays for the investigation of PAPP-A enzyme kinetics.

Quinoline-carboxylic acids are potent inhibitors that inhibit the binding of insulin-like growth factor (IGF) to IGF-binding proteins

4-benzylquinolines 5, based on a series of isoquinolines 1, were prepared and tested as inhibitors of the IGF/IGFBP-3 complex based on their ability to displace IGF-I from its binding to IGF-binding protein-3. SAR studies on the 6,7-dihydroxy moiety of the quinoline 5a showed that the catecol moiety could be replaced with other functional groups. Computational modeling of the 5a/mini-IGFBP-5 complex revealed the possible binding site of 5a on IGFBP-5.

Control of insulin-like growth factor binding protein-5 protease synthesis and secretion by human fibroblasts and porcine aortic smooth muscle cells

IGF binding protein-5 (IGFBP-5) is an important trophic factor for controlling the actions of IGF-I in human dermal fibroblasts and porcine aortic smooth muscle cells. When IGFBP-5 is associated with extracellular matrix, it acts to enhance the cell growth response to IGF-I. The amount of IGFBP-5 within the extracellular matrix is related in part to the amount that is present in conditioned medium, which is related to its rate of synthesis and degradation. A serine protease that degrades IGFBP-5 is present in the conditioned medium of both of these cell types. Because the IGFBP-5 protease activity that is secreted by fibroblasts has been shown to be due to the complement components C1r and C1s, these studies were undertaken to determine whether smooth muscle cells also secreted these proteases and to identify some of the factors that regulate their secretion by both cell types. Both smooth muscle cells and human fibroblasts were shown to release C1r and C1s into conditioned medium. Both C1r and C1s were detected as activated forms, as determined by SDS-PAGE using reducing conditions. The addition of increasing concentrations of either IL-1beta or TNFalpha resulted in increased synthesis of C1s by fibroblasts and smooth muscle cells, and they each increased C1r release. TNFalpha (50 ng/ml) and IL-1beta (20 ng/ml) resulted in maximum stimulation of release of both proteases. In contrast dexamethasone (10(-7) M) had no effect on C1s release and stimulated C1r release only by smooth muscle cells. To determine the physiological significance of this increase in C1r and C1s, the amount of IGFBP-5 protease activity that was present in conditioned medium was determined before and after exposure to TNFalpha, IL-beta, and dexamethasone. All three compounds resulted in an increase in the amount of IGFBP-5 proteolytic activity. Dexamethasone inhibited the release of C(1) inhibitor from fibroblasts, and this contributed to the net increase in proteolytic activity. TNFalpha inhibited the smooth muscle cell DNA synthesis response to IGF-I, but the effect of IGF-I was partially restored by the addition of C1 inhibitor. In conclusion, both C1r and C1s are released by cultured fibroblasts, and the release of each into fibroblast or porcine smooth muscle cells medium is stimulated by TNFalpha and IL-1beta. This increase results in a net increase in IGFBP-5 proteolysis, which has the potential to modify IGF-I and IGFBP-5 actions.

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.

Bovine lactoferrin binds to insulin-like growth factor-binding protein-3

Insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) has been shown to have IGF independent actions that appear to be mediated by specific IGFBP-3 binding proteins located on cell membranes. We show here using Western ligand blotting, a number of mammary membrane proteins that bind 125I-labeled rhIGFBP-3. Immunoprecipitation studies demonstrated that the >70 kDa protein was identified from bovine mammary microsomes as bovine lactoferrin (bLf). In addition to being a secretory protein, Lf is tightly associated with cellular membranes. Labeled rhIGFBP-3 was shown to bind to commercially purchased and processed apo- or holo-human or bLf, but not bovine transferrin (bTf). Binding of [125I]rhIGFBP-3 to other positively charged proteins was not detected nor was binding to rhIGFBP-5 or other mammary-secreted IGFBPs observed. Reciprocal specific binding of [125I]bLf to rhIGFBP-3 was shown, but [125I]bTf did not show binding to rhIGFBP-3. While [125I]rhIGF-II does not bind to bLf, unlabeled rhIGF-II was shown to compete with [125I]bLf for rhIGFBP-3 binding. More detailed analysis by dot blot showed that Lf competes (ED(50)=3 microg/ml) or displaces (ED(50)=1mg/ml) bound [125I]rhIGF-II from dot blotted rhIGFBP-3. In vitro studies with a bovine primary mammary epithelial cell culture showed that all-trans-retinoic acid stimulates the appearance of bovine IGFBP-3 and bLf in the conditioned media and that [125I]rhIGFBP-3 could be utilized to detect conditioned media bLf. These findings reveal a novel role for bLf, binding to IGFBP-3 and perhaps disassociating IGFBP-3:IGF when in high concentration.