Potent antitumor effects of the conditioned medium of bone marrow-derived mesenchymal stem cells via IGFBP-4

Cell transfer therapy using mesenchymal stem cells (MSCs) has pronounced therapeutic potential, but concerns remain about immune rejection, emboli formation, and promotion of tumor progression. Because the mode of action of MSCs highly relies on their paracrine effects through secretion of bioactive molecules, cell-free therapy using the conditioned medium (CM) of MSCs is an attractive option. However, the effects of MSC-CM on tumor progression have not been fully elucidated. Herein, we addressed this issue and investigated the possible underlying molecular mechanisms. The CM of MSCs derived from human bone marrow greatly inhibited the in vitro growth of several human tumor cell lines and the in vivo growth of the SCCVII murine squamous cell carcinoma cell line with reduced neovascularization. Exosomes in the MSC-CM were only partially involved in the inhibitory effects. The CM contained a variety of cytokines including insulin-like growth factor binding proteins (IGFBPs). Among them, IGFBP-4 greatly inhibited the in vitro growth of these tumors and angiogenesis, and immunodepletion of IGFBP-4 from the CM significantly reversed these effects. Of note, the CM greatly reduced the phosphorylation of AKT, ERK, IGF-1 receptor beta, and p38 MAPK in a partly IGFBP4-dependent manner, possibly through its binding to IGF-1/2 and blocking the signaling. The CM depleted of IGFBP-4 also reversed the inhibitory effects on in vivo tumor growth and neovascularization. Thus, MSC-CM has potent inhibitory effects on tumor growth and neovascularization in an IGFBP4-dependent manner, suggesting that cell-free therapy using MSC-CM could be a safer promising alternative for even cancer patients.

Insulin-like growth factor binding protein 4 inhibits proliferation of bone marrow mesenchymal stem cells and enhances growth of neurospheres derived from the stem cells

Insulin-like growth factor binding protein 4 (IGFBP-4) was reported to trigger cellular senescence and reduce cell growth of bone marrow mesenchymal stem cells (BMSCs), but its contribution to neurogenic differentiation of BMSCs remains unknown. In the present study, BMSCs were isolated from the femur and tibia of young rats to investigate effects of IGFBP-4 on BMSC proliferation and growth of neurospheres derived from BMSCs. Bone marrow mesenchymal stem cell proliferation was assessed using CCK-8 after treatment with IGFBP-4 or blockers of IGF-IR and β-catenin. Phosphorylation levels of Akt, Erk, and p38 in BMSCs were analysed by Western blotting. Bone marrow mesenchymal stem cells were induced into neural lineages in NeuroCult medium; the number and the size of BMSC-derived neurospheres were counted after treatment with IGFBP-4 or the blockers. It was shown that addition of IGFBP-4 inhibited BMSC proliferation and immunodepletion of IGFBP-4 increased the proliferation. The blockade of IGF-IR with AG1024 increased BMSC proliferation and reversed IGFBP-4-induced proliferation inhibition; however, blocking of β-catenin with FH535 did not. p-Erk was significantly decreased in IGFBP-4-treated BMSCs. IGFBP-4 promoted the growth of neurospheres derived from BMSCs, as manifested by the increases in the number and the size of the derived neurospheres. Both AG1024 and FH535 inhibited the formation of NeuroCult-induced neurospheres, but FH535 significantly inhibited the growth of neurospheres in NeuroCult medium with EGF, bFGF, and IGFBP-4. The data suggested that IGFBP-4 inhibits BMSC proliferation through IGF-IR pathway and promotes growth of BMSC-derived neurospheres via stabilizing β-catenin.

IGFBP-4 and -5 are expressed in first-trimester villi and differentially regulate the migration of HTR-8/SVneo cells

BACKGROUND

Adverse gestational outcomes such as preeclampsia (PE) and intrauterine growth restriction (IUGR) are associated with placental insufficiency. Normal placental development relies on the insulin-like growth factors -I and -II (IGF-I and -II), in part to stimulate trophoblast proliferation and extravillous trophoblast (EVT) migration. The insulin-like growth factor binding proteins (IGFBPs) modulate the bioavailability of IGFs in various ways, including sequestration, potentiation, and/or increase in half-life. The roles of IGFBP-4 and -5 in the placenta are unknown, despite consistent associations between pregnancy complications and the levels of two IGFBP-4 and/or -5 proteases, pregnancy-associated plasma protein -A and -A2 (PAPP-A and PAPP-A2). The primary objective of this study was to elucidate the effects of IGFBP-4 and -5 on IGF-I and IGF-II in a model of EVT migration. A related objective was to determine the timing and location of IGFBP-4 and -5 expression in the placental villi.

METHODS

We used wound healing assays to examine the effects of IGFBP-4 and -5 on the migration of HTR-8/SVneo cells following 4 hours of serum starvation and 24 hours of treatment. Localization of IGFBP-4, -5 and PAPP-A2 was assessed by immunohistochemical staining of first trimester placental sections.

RESULTS

2 nM IGF-I and -II each increased HTR-8/SVneo cell migration with IGF-I increasing migration significantly more than IGF-II. IGFBP-4 and -5 showed different levels of inhibition against IGF-I. 20 nM IGFBP-4 completely blocked the effects of 2 nM IGF-I, while 20 nM IGFBP-5 significantly reduced the effects of 2 nM IGF-I, but not to control levels. Either 20 nM IGFBP-4 or 20 nM IGFBP-5 completely blocked the effects of 2 nM IGF-II. Immunohistochemistry revealed co-localization of IGFBP-4, IGFBP-5 and PAPP-A2 in the syncytiotrophoblast layer of first trimester placental villi as early as 5 weeks of gestational age.

CONCLUSIONS

IGFBP-4 and -5 show different levels of inhibition on the migration-stimulating effects of IGF-I and IGF-II, suggesting different roles for PAPP-A and PAPP-A2. Moreover, co-localization of the pappalysins and their substrates within placental villi suggests undescribed roles of these molecules in early placental development.

[Expression of PAPP-A, IGF-I and their effects on the cytological functions of siRNA lentiviral vector of hCASMCs IGF-I after inflammatory factor]

OBJECTIVE

To evaluate expression of PAPP-A, IGF-I and the effect of TNF-alpha, IL-1beta on the cytological functions of hCASMCs with IGF-I gene silencing after inflammatory factor, in order to further study on the action of IGF axis hormone in the rupture of astable atheroxclerosis plaque.

METHODS

A RNA interference (RNAi) aimed at the gene of IGF-I was carried out to have an eukaryon transfection to hCASMCs. When the IGF- I-shRNA-hCASMC were treated by TNF-alpha, IL-1beta, IGFBP4, the expression of PAPP-A and IGF-I were detected with Western blot and ELISA. And then, the effect of TNF-alpha, IL-1beta, IGFBP4 on the proliferation of IGF-I-shRNA- hCASMC were assessed by MTT assay and changing in cell cycle and apoptosis were evaluated by using flow cytometry.

RESULTS

Significant positive expression of PAPP-A in hCASMCs which were treated by TNF-alpha + IL-1beta or TNF-alpha + IL-1beta + IGFBP4 in Blank control (CON), Negative control (NC) and RNAi group were observed, but lower expression in the RNAi group than that in CON and NC groups. However, there was no positive expression of PAPP-A in hCASMCs of CON, NC, RNAi group treating without TNF-alpha + IL-1beta or TNF-alpha + IL-1beta + IGFBP4. The expression of IGF-I in hCASMCs of CON, NC, RNAi group treated with TNF-alpha + IL-1beta + IGFBP4 were greater than that only with or without TNF-alpha + IL-1beta treatment. In RNAi group, the A570 decreased when the hCASMCs treated with TNF-alpha + IL-1beta + IGFBP4 and significant lower than that in hCASMCs treated with TNF-alpha + IL-1beta. When the hCASMCs were treated with TNF-alpha + IL-1beta or TNF-alpha + IL-1beta + IGFBP4, the rate of apoptosis significantly increased in RNAi group, which was significantly higher than that in CON group and NC group. In addition, in RNAi group, the rate of apoptosis in hCASMCs treated with TNF-alpha + IL-1beta + IGFBP4 was significant higher than that in hCASMCs treated only with TNF-alpha + IL-1beta.

CONCLUSION

When the IGF-I-shRNA-hCASMCs were stimulated by some inflammation factors, its proliferation decreased but the apoptosis enhanced. So the activated IGF-I level in local microebvironnment increased, which may cause the descend of cell proliferation and the increasing of apoptosis.

Intermittent high glucose concentrations reduce neuronal precursor survival by altering the IGF system: the involvement of the neuroprotective factor DHCR24 (Seladin-1)

The exposure of neurons to high glucose concentrations is considered a determinant of diabetic neuropathy, whereas members of the IGF system are neurotropic factors. Here, we investigated the effects of constant and intermittent high glucose concentrations on IGF1 and IGF-binding proteins (IGFBPs) in human neuroblast long-term cell cultures fetal neuroepithelial cells (FNC). These cells express the IGF1 receptor, and express and release in the culture medium IGFBP2, IGFBP4, and IGF1. The release of IGF1 was significantly increased by 17beta-estradiol (10 nM). IGF1 (100 nM) treatment determined a significant increase of IGFBP2 and a decrease of IGFBP4 release. In addition, IGF1 (1-100 nM) stimulated FNC cell proliferation in a dose-dependent manner. We hypothesized that this effect may be, at least partially, due to IGF1-induced up-regulation of the expression of the Alzheimer's disease related gene SELADIN-1 (now known as DHCR24 ), which acts as a pro-survival factor for neuronal cells. Conversely, the exposure to intermittent (20/10 mM), but not stable (20 mM), high glucose concentrations decreased the release of IGF1 and IGFBP2 in the culture medium and inhibited FNC growth by inducing apoptosis. The latter was prevented by the addition of IGF1 to the culture medium. Furthermore, high glucose concentrations reduced the expression of DHCR24. In conclusion, our results indicate for the first time that intermittent high glucose concentrations, similar to those observed in poorly controlled diabetic patients, may contribute to the development of diabetic neuropathy by interfering with the tropic effects exerted by the IGF system, and suggest the involvement of the neuroprotective factor DHCR24.

Insulin-like growth factor binding protein (IGFBP)-mediated hair cell survival on the mouse utricle exposed to neomycin: the roles of IGFBP-4 and IGFBP-5

CONCLUSION

This study suggests for the first time that 1) IGF-I, IGFBP-4, and -5 alone and IGF-I+IGFBP-5 mixture stimulated hair cell survival and prevented neomycin-induced hair cell loss in the sensory epithelial culture of mouse utricles, 2) When administered together, IGFBP-4 diminished the effect of IGF-I, 3) In P3-5 mice utricle, IGF-I, IGFBP-4, and IGFBP-5 are expressed in the cytoplasm of hair cells. And Insulin/IGF-I Receptor is expressed in the nucleus of hair cells.

OBJECTIVES

Several growth factors have been demonstrated to protect auditory sensory cells in vitro and in vivo from aminoglycoside toxicity. IGF-I is one of the most well-known mitogenic and protective substance working in the inner ear. However, there are no reports available regarding the function of IGFBPs in the inner ear. In the present study, the effects of IGFBP-4 and -5 on hair cell survival were investigated in mouse utriclular organ cultures.

MATERIALS AND METHODS

The amount of cellular damage and cell viability in vestibular organs were assessed by counting hair cells stained with a rhodamine-phalloidin probe. The expressions of IGFBP-4, IGFBP-5, IGF-IR, and IGF-I were localized by immunohistochemistry.

RESULTS

When treated with IGF-I, IGFBP-4, or IGFBP-5 for 24 h, explant culture showed hair cell survival rates of 136+/-18%, 140+/-15%, and 133+/-6%, respectively, compared to controls. Neomycin (1 mM) induced hair cell loss resulted in 45+/-17% of hair cell survival. However, pre-treatment of IGF-I, IGFBP-4, or -5 before neomycin insult showed survival rates of 113+/-14%, 98+/-8%, and 73+/-24%, respectively. Similar to IGF-I, IGFBP-4 and IGFBP-5 were significantly protective. IGFBP-4 and -5 immunoreactivities were observed in the cytoplasm of normal explanted vestibular hair cells as well as in the P3 mouse utricular hair cells in vivo.

Protease-resistant insulin-like growth factor (IGF)-binding protein-4 inhibits IGF-I actions and neointimal expansion in a porcine model of neointimal hyperplasia

IGF-I has been shown to play a role in the progression of atherosclerosis in experimental animal models. IGF-binding protein-4 (IGFBP-4) binds to IGF-I and prevents its association with receptors. Overexpression of a protease-resistant form of IGFBP-4 has been shown to inhibit the ability of IGF-I to stimulate normal smooth muscle cell growth in mice. Based on these observations, we prepared a protease-resistant form of IGFBP-4 and infused it into hypercholesterolemic pigs. Infusion of the protease-resistant mutant inhibited lesion development by 53.3 +/- 6.1% (n = 6; P < 0.01). Control vessels that received an equimolar concentration of IGF-I and the protease-resistant IGFBP-4 showed no reduction in lesion size compared with control lesions that were infused with vehicle. Infusion of a nonmutated form of IGFBP-4 did not significantly inhibit lesion development. Proliferating cell nuclear antigen analysis showed that the mutant IGFBP-4 appeared to inhibit cell proliferation. The area occupied by extracellular matrix was also reduced proportionally compared with total lesion area. Immunoblotting revealed that the mutant IGFBP-4 remained intact, whereas the wild-type IGFBP-4 that was infused was proteolytically cleaved. Further analysis of the lesions revealed that a marker protein, IGFBP-5, whose synthesis is stimulated by IGF-I, was decreased in the lesions that received the protease-resistant, IGFBP-4 mutant, whereas there was no change in lesions that received wild-type IGFBP-4 or the mutant protein plus IGF-I. These findings clearly illustrate that infusion of protease-resistant IGFBP-4 into the perilesion environment results in inhibition of cell proliferation and attenuation of the development of neointima. The findings support the hypothesis that inhibiting IGFBP-4 proteolysis in the lesion microenvironment could be an effective means for regulating neointimal expansion.

Enhanced proliferation of astrocytes from beta(2)-adrenergic receptor knockout mice is influenced by the IGF system

In the present study, we investigated the IGF system in neonatal astrocytes derived from mice with a targeted disruption of the beta-2 adrenergic receptor (beta(2)AR). beta(2)AR knockout astrocytes demonstrated higher proliferation rates and increased expression of the astrogliotic marker GFAP, as compared with wild-type cells. beta(2)AR deletion also regulated molecules of the IGF system. Although IGF-1 levels remained unaltered, IGF-2 and type 1 IGF receptor expression was increased in beta(2)AR knockout cells. Furthermore, conditioned medium from knockout astrocytes contained lower levels of IGF binding protein-2 and -4. Our data suggest a deficit of beta(2)AR on astrocytes, as previously reported in multiple sclerosis, may have implications on proliferative status of astrocytes, a feature that might be attributed to regulation of IGF mitogenic actions.

Targeted disruption of the pregnancy-associated plasma protein-A gene is associated with diminished smooth muscle cell response to insulin-like growth factor-I and resistance to neointimal hyperplasia after vascular injury

IGF-I is an important determinant of the vascular response to injury in large part through its ability to stimulate migration and proliferation of smooth muscle cells (SMCs). In this study, we used mice with targeted disruption of the pregnancy-associated plasma protein-A gene (PAPP-A-/-) and wild-type (WT) littermates to test the hypotheses that PAPP-A, a metalloproteinase that cleaves inhibitory IGF binding protein (IGFBP)-4, regulates vascular SMC responses to IGF-I in vitro and is critical for the development of vascular neointima after injury in vivo. Vascular SMCs from PAPP-A-/- mice lacked IGFBP-4 protease activity and failed to respond to treatment with IGF-I in the presence of IGFBP-4, whereas SMCs from WT mice with robust IGFBP-4 protease activity showed significant migratory and proliferative responses to IGF-I/IGFBP-4. For in vivo testing, PAPP-A-/- and WT mice underwent unilateral carotid ligation, a model of injury-induced neointimal hyperplasia. In WT mice, PAPP-A mRNA expression was markedly elevated 7 and 14 d after carotid ligation, associated with a progressive increase in neointimal hyperplasia and, in many cases, with complete occlusion of the vessel at 28 d. In contrast, PAPP-A-/- mice showed little evidence of progression resulting in a 75% reduction in neointimal area when compared with WT at 28 d. Cells staining for proliferating cell nuclear antigen were plentiful in the SMC-rich medial and neointimal areas of the injured WT vessel in stark contrast to the relatively few proliferating cells in the same areas of the PAPP-A-/- vessel. Expression of IGF-I and IGFBP-4 was similarly elevated in injured carotids from WT and PAPP-A-/- mice with no change in IGF-I receptor expression. IGFBP-5, an IGF-responsive gene, was increased 2-fold in WT but not in PAPP-A-/- carotids, suggesting reduced IGF activity in the absence of PAPP-A. Thus, PAPP-A-deficient mice are resistant to neointimal formation after injury, which may be explained in part by the ability of PAPP-A to enhance local IGF-I stimulation of vascular SMCs through proteolysis of IGFBP-4.

Insulin-like growth factor binding protein-4 (IGFBP-4) is a novel anti-angiogenic and anti-tumorigenic mediator secreted by dibutyryl cyclic AMP (dB-cAMP)-differentiated glioblastoma cells

cAMP has been shown to reverse the transformed phenotype of various cancer cells. Human glioblastoma U87MG cells exposed to 500 microM dB-cAMP for 6 days showed reduced proliferation, attenuated invasiveness, and inability to induce angiogenic responses in human brain endothelial cells (HBECs) grown in Matrigeltrade mark. VEGF was the principal mediator of angiogenic actions of U87MG conditioned media (CM), since VEGF neutralizing antibody completely inhibited U87MG-induced angiogenic responses and no detectable levels of IGF, bFGF, and PlGF were found in U87MG CM. VEGF release was induced ( approximately 20%) in dB-cAMP-treated U87MG cells, suggesting a simultaneous induction of anti-angiogenic mediators. Down-stream effectors of dB-cAMP actions in U87MG were investigated by microarray gene expression analysis. Detected increases in differentiation genes, staniocalcin-1 and Wnt-5a, and angiogenesis-related genes, PAI-1, SPARC, IGFBP-4, IGFBP-7, PAPP-A, and PRSS-11 in dB-cAMP-treated U87MG cells were validated by real-time PCR, Western blot, and/or ELISA. A subsequent series of experiments identified IGFBP-4 as the principal anti-angiogenic mediator secreted by glioblastoma cells in response to dB-cAMP. Human recombinant IGFBP-4 inhibited the angiogenic response of HBEC induced by U87MG CM, whereas anti-human IGFBP-4 antibody restored the pro-angiogenic activity of dB-cAMP-treated U87MG CM. Since neither U87MG nor HBEC cells secreted detectable levels of IGF-I, and there are no known cellular IGFBP-4 receptors, the anti-angiogenic effect of IGFBP-4 was likely IGF-I-independent and indirect. IGFBP-4 also antagonized angiogenic effects of VEGF(165), PlGF, and bFGF, and reduced U87MG colony formation in soft-agar. IGFBP-4 is a novel dB-cAMP-induced anti-angiogenic and anti-tumorigenic mediator that may be a promising candidate for glioblastoma therapy.

Biology of insulin-like growth factor binding protein-4 and its role in cancer (review)

Insulin-like growth factor binding protein-4 (IGFBP-4) is an important member of the insulin-like growth factor (IGF) system. The IGFBP-4 has three domains of which the N-terminal sequence is important for the binding of IGF. It acts as a transport protein for IGF-I and IGF-II and modulates their biological effects. There is increasing evidence that IGFBP-4 inhibits IGF-induced cellular growth both in vitro and in vivo. IGFBP-4 can also mediate its actions through a mechanism independent of IGFs. IGFBP-4 level and expression in various tissues are influenced by IGFBP protease, nutrition, several growth factors and hormones. Overexpression of IGFBP-4 in transgenic animal models causes reduced growth of organs containing smooth muscle. Most cancers express IGFBP-4 at levels which correlate with their state of differentiation. However, the effects of IGFBP-4 on tumor growth are uncertain. In vitro studies have shown that overexpression of IGFBP-4 inhibit the growth of some colon cancer cells. Overexpression of IGFBP-4 in vivo has been reported to decrease the growth of prostate cancer. The effect of altered expression of IGFBP-4 in vivo in colon and other cancers needs to be explored as locally available IGFs appear to stimulate mitogenesis.

Effect of insulin-like growth factors (IGF), FSH, and leptin on IGF-binding-protein mRNA expression in bovine granulosa and theca cells: quantitative detection by real-time PCR

To determine if insulin-like growth factor (IGF)-1 and -2, FSH, or leptin alter IGF-binding protein (IGFBP)-2, -3, -4, and -5 mRNA levels in bovine granulosa and (or) theca cells, granulosa and theca cells were collected from bovine ovarian follicles, plated for 48 h in 10% FCS and then treated for 24 h in serum-free medium containing various hormone treatments arranged in three different experiments. Amounts of IGFBP-2, -3, -4, and -5 mRNA were quantitated using fluorescent quantitative real-time RT-PCR. Neither 100 ng/ml of IGF-1 nor IGF-2 had an effect (P > 0.10) on IGFBP-2, -3, -4, or -5 mRNA levels in small-follicle (1-5 mm; Experiment 1) granulosa cells. In large-follicle (>7.9 mm; Experiment 2) granulosa cells, 100 ng/ml of IGF-1 increased (P < 0.05) IGFBP-2 mRNA levels above controls and 3 ng/ml of IGF-1; 100 ng/ml of IGF-1 also decreased (P < 0.10) IGFBP-5 mRNA levels compared to 3 ng/ml of IGF-1 or FSH or 100 ng/ml leptin, while 100 ng/ml of IGF-2 had no effect (P > 0.10) on IGFBP-2, -3, -4, and -5 mRNA levels (Experiment 2). At the doses tested, leptin and FSH had no effect (P > 0.10) on IGFBP-2, -3, -4, and -5 mRNA levels in large-follicle granulosa cells. In theca cells, IGF-2 decreased (P < 0.05) IGFBP-2 mRNA levels, but had no effect on IGFBP-3 or -4 mRNA expression (Exp. 3); IGF-1 did not affect (P > 0.10) thecal IGFBP-2, -3 or -4 mRNA levels. In contrast, IGF-1 but not IGF-2 increased (P < 0.01) thecal IGFBP-5 mRNA levels. Ligand blotting revealed that both IGF-1 and -2 increased IGFBP-2 and -5 (protein) and had no effect on IGFBP-3 (protein), whereas IGF-1 (but not IGF-2) increased IGFBP-4 (protein), suggesting IGFBP-2, -4, and -5 are post-transcriptionally regulated. These results suggest that expression of IGFBP-2, -3, -4, and -5 mRNA by granulosa and theca cells are differentially regulated by IGF-1 and -2, therefore discretely modulating the amount of bio-available IGFs to these cells depending upon the specific hormonal milieu.

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.

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.

Insulin-like growth factor (IGF)-independent effects of IGF binding protein-4 on human granulosa cell steroidogenesis

The ovarian insulin-like growth factor (IGF)/IGF binding protein (IGFBP) system operates to permit maximal stimulation of steroidogenesis in the dominant follicle. In atretic follicles, the predominant IGFBPs are IGFBP-2 and IGFBP-4, which appear to be selectively cleaved in healthy follicles. We have recently demonstrated potent inhibition by IGFBP-4 of both theca and granulosa cell steroid production. The degree to which the inhibition occurred suggested that it was greater than might be expected by sequestration of IGF alone. Our study was designed to test this idea. Granulosa cells were harvested from follicles dissected intact from patients undergoing total abdominal hysterectomy and bilateral salpingoophorectomy. Granulosa cells were incubated with or without gonadotropins and IGFBP-4 in the presence or absence of either the IGF type I receptor blocker alphaIR3 or excess IGFBP-3 to remove the effects of endogenous IGF action. Steroid accumulation in the medium was assessed. IGFBP-4 continued to exert potent inhibitory effects when the action of endogenous IGF was removed from the system, demonstrating that its actions are independent of IGF binding. There was no effect on cell metabolism, and the effects on steroidogenesis were reversible after IGFBP-4 removal from the culture medium. No similar effects were seen with IGFBP-2. These reasults are the first evidence of IGF-independent IGFBP-4 actions and the first evidence of IGF-independent actions of any IGFBPs in the ovary.

Target-derived cardiotrophin-1 and insulin-like growth factor-I promote neurite growth and survival of developing oculomotor neurons

Several trophic factors support the survival of developing motoneurons, but it is not known whether these factors act in a retrograde fashion from the motoneuron target muscle or are derived from other sources. Cardiotrophin-1 (CT-1) and the insulin-like growth factors (IGFs) are candidate target-derived motoneuron survival factors as both are expressed in muscle during naturally occurring motoneuron death and, applied systemically, support the survival of developing motoneurons. By using the embryonic chick oculomotor system, we show that CT-1 and IGF-I promote neurite outgrowth from E13-derived oculomotor explants and are retrogradely transported from muscle to nerve cell body in vivo, and injection of CT-1 or IGF-I into eye muscles increases motoneuron survival by 20 and 30%, respectively, as evidenced by calibrated stereological counting techniques. Pharmacological depletion of endogenous target-derived IGF-I in vivo reduces oculomotor neuron survival by up to 30% in a dose-dependent manner. These results significantly extend previous studies using systemic administration of trophic factors and are the first to demonstrate a target-derived retrograde mechanism of developing motoneuron survival factors.

Intact IGF-binding protein-4 and -5 and their respective fragments isolated from chronic renal failure serum differentially modulate IGF-I actions in cultured growth plate chondrocytes

Impairment of longitudinal growth among children with chronic renal failure (CRF) may be partly attributable to the inhibition of insulin-like growth factor (IGF) activity by an excess amount of high-affinity IGF-binding proteins (IGFBP). Elevated levels of immunoreactive IGFBP-4 in CRF serum are inversely correlated with the standardized heights of these children, whereas levels of IGFBP-5, which circulates mainly as proteolyzed fragments, are positively correlated with growth parameters. To delineate the respective effects of these IGFBP on growth cartilage, the biologic effects of intact and fragmented forms of IGFBP-4 and IGFBP-5 on rat growth plate chondrocytes in primary cultures were characterized. Intact IGFBP-4 and IGFBP-5 and the amino-terminal fragment IGFBP-5(1-169) were recombinant proteins; the carboxy-terminal fragments IGFBP-5(144-252) and IGFBP-4(136-237) and the amino-terminal fragment IGFBP-4(1-122) were purified to homogeneity from CRF hemofiltrates. Intact IGFBP-4 and, to a lesser extent, IGFBP-4(1-122) inhibited IGF-I-induced cell proliferation. In contrast, intact IGFBP-5 was stimulatory in the absence or presence of exogenous IGF-I, whereas the amino-terminal fragment IGFBP-5(1-169) was inhibitory. Studies on the mechanism by which IGFBP-4 and IGFBP-5 exert opposite effects on chondrocyte proliferation demonstrated that intact IGFBP-4 prevented the binding of (125)I-IGF-I to chondrocytes, whereas intact IGFBP-5 enhanced ligand binding and was able to bind specifically to the cell membrane. These data suggest that intact IGFBP-4 and, to a lesser extent, IGFBP-4(1-122) act exclusively as growth-inhibitory binding proteins in the growth cartilage. IGFBP-5, however, can either stimulate (if it remains intact) or inhibit (if amino-terminal forms predominate) IGF-I-stimulated chondrocyte proliferation.

Systemic administration of insulin-like growth factor (IGF)-binding protein-4 (IGFBP-4) increases bone formation parameters in mice by increasing IGF bioavailability via an IGFBP-4 protease-dependent mechanism

Insulin-like growth factor (IGF)-binding protein-4 (IGFBP-4) is a potent inhibitor of IGF actions in vitro. However, we found that systemic administration of IGFBP-4 at pharmacological doses caused a significant increase in bone formation parameters in mice by a mechanism that may involve increased IGF bioavailability via proteolysis of IGFBP-4. To evaluate the hypothesis that proteolysis of IGFBP-4 is essential for the stimulatory effects of systemically administered IGFBP-4, we produced wild-type, protease-resistant, and IGFBP-4 proteolytic fragments and evaluated their effects using biochemical markers. Protease-resistant IGFBP-4 was more potent than wild-type IGFBP-4 in inhibiting IGF-I-induced mouse osteoblast cell proliferation in vitro and in inhibiting IGF-I-induced increase in alkaline phosphatase (ALP) activity in bone extract after local administration in vivo. Systemic administration of wild-type IGFBP-4, but not protease-resistant IGFBP-4, increased serum osteocalcin, serum ALP, and ALP in skeletal extracts in a dose-dependent manner, with a maximal effect of 40% (P < 0.05) at 1.25 nmol/mouse. Systemic administration of wild-type, but not protease-resistant, IGFBP-4 increased free IGF-I levels in serum in normal mice. IGF-I, but not wild-type IGFBP-4, increased bone formation parameters in IGF-I-deficient mice. This study demonstrates that systemic administration of IGFBP-4 increases bone formation parameters in mice by increasing IGF bioavailability in the circulation via an IGFBP-4 protease-dependent mechanism.

A novel approach to reducing postoperative intraperitoneal adhesions through the inhibition of insulinlike growth factor I activity

HYPOTHESIS

Interference with insulinlike growth factor I (IGF-I) activity, both systemically and intraperitoneally, reduces postoperative intraperitoneal adhesion severity.

SETTING

Experimental animal model.

DESIGN, INTERVENTIONS, AND MAIN OUTCOME MEASURES

Adult female rats were subjected to hypophysectomy, sham hypophysectomy (control), IGF binding protein 4 (IGFBP-4) treatment, or albumin treatment (control). All rats underwent laparotomy and uterine horn abrasion with adjacent parietal peritoneal trauma for the purpose of creating postoperative intraperitoneal adhesions. Glucocorticoids and thyroid hormone were replaced in the hypophysectomy group. On postoperative day 10, rats were weighed, subjected to phlebotomy, and killed. Postmortem laparotomies were performed and blinded observers scored uterine-peritoneal adhesions on a 0 to 3 scoring system. Plasma IGFBP-4 levels and organ weights were measured in the IGFBP-4 and albumin treatment groups. Blood samples in all rats were analyzed for IGF-I levels.

RESULTS

Rats with low IGF-I levels (hypophysectomy) and inhibited IGF-I activity (IGFBP-4 treatment) formed significantly less severe adhesions than their control counterparts. As expected, rats in the hypophysectomy group displayed greater weight loss and lower plasma IGF-I levels than sham-treated rats. Rats treated with IGFBP-4 and those treated with albumin demonstrated no differences in body weight, organ weights, IGF-I levels, and IGFBP-4 levels.

CONCLUSIONS

Both the reduction of systemic IGF-I levels via hypophysectomy and the inhibition of local intraperitoneal IGF-I activity via IGFBP-4 treatment resulted in diminished postoperative adhesion severity. Treatment with IGFBP-4 may play a role in postoperative adhesion prophylaxis in the future.

Smooth muscle-targeted overexpression of insulin-like growth factor I results in enhanced vascular contractility

Insulin-like growth factor I (IGF-I) has been postulated to function as a vasodilator. We explored the vasoactive effects of chronic elevations of arterial IGF-I levels in SMP8-IGF-I mice, in which IGF-I is overexpressed in smooth muscle (SM) by means of a SM alpha-actin promoter. Denuded aortas from SMP8-IGF-I mice generated increased force in response to KCl or phenylephrine and had greater sensitivity to KCl depolarization. This is not due to desensitization of a SM NO pathway, as pretreatment with n-omega-nitro-L-arginine affected both wild-type and SMP8-IGF-I aortas to a similar degree. The increased contractility ex vivo is not associated with changes in heart rate or blood pressure. Total smooth muscle myosin heavy chain (SMHC) messenger RNA (mRNA) was greater in SMP8-IGF-I aortas, with preferential expression of SMHC-A. Reciprocal effects on contractility and SMHC mRNA were observed in SMP8-IGFBP-4 animals, in which IGF-binding protein-4 was overexpressed through the same promoter. Also, SM alpha-actin mRNA was increased in the aortas from SMP8-IGF-I mice. In summary, chronic arterial overexpression of IGF-I is associated with increased contractility. These effects differ from those seen after acute exposure to the growth factor and may relate to IGF-mediated changes in expression and relative isoform abundance of critical contractile proteins.

Evidence that IGF-binding protein-5 functions as a growth factor

Recent studies support the concept that IGF-binding protein-5 (IGFBP-5) stimulates bone formation, at least in part, via IGF-independent mechanisms. To evaluate this hypothesis further, we evaluated in vitro and in vivo effects of IGFBP-5 on bone formation parameters using the IGF-I knockout (KO) mouse. Treatment of serum-free cultures of osteoblast clones derived from IGF-I KO mice with recombinant human IGFBP-5 increased both proliferation and alkaline phosphatase (ALP) activity in a dose-dependent manner, an effect comparable to that seen with IGF-I. IGF-II levels from media conditioned by osteoblasts derived from IGF-I KO mouse were below those detectable by RIA. To eliminate possible actions of IGF-II, if any was produced by osteoblasts derived from IGF-I knockout mice, the IGFBP-5 effect was studied in the presence of exogenously added IGFBP-4, a potent inhibitor of IGF-II actions in bone cells. Addition of IGFBP-4 blocked IGF-I- but not IGFBP-5-induced cell proliferation in osteoblasts derived from IGF-I knockout mice. Consistent with in vitro results, a single local injection of IGFBP-5 to the outer periosteum of the parietal bone of IGF-I KO mice increased ALP activity and osteocalcin levels of calvarial bone extracts. The magnitudes of IGFBP-5-induced increases in ALP and osteocalcin in parietal bone extracts of IGF-I KO mice were comparable to those seen in C3H mice. In contrast to IGFBP-5, local administration of IGFBP-4 had no significant effect on bone formation in C3H and IGF-I KO mice. These results provide the first direct evidence to our knowledge that IGFBP-5 functions as a growth factor that stimulates its actions in part via an IGF-independent mechanism.

Insulin-like growth factor binding protein-4 expression is decreased by angiotensin II and thrombin in rat aortic vascular smooth muscle cells

Insulin-like growth factor-I (IGF-I) is a ubiquitous peptide that regulates cellular growth and differentiation and is involved in vascular proliferative responses. The effects of IGF-I are modulated by several IGF-I binding proteins (IGFBPs), including IGFBP-4, the main IGFBP produced by vascular smooth muscle cells (VSMCs). We have previously shown that angiotensin II (Ang II)-induced and thrombin-induced mitogenesis in VSMCs is dependent on autocrine IGF-I. In addition, we have demonstrated that IGF-I and IGFBP-4 mRNA levels are upregulated in the hypertensive aorta of abdominally coarcted rats, a high-renin hypertension model. To obtain further insight into the IGF-I system and to specifically study changes in IGFBP-4, a known inhibitor of IGF-I action, VSMCs were incubated with Ang II or thrombin. Compared with control, Ang II induced an 87+/-2% downregulation of IGFBP-4 mRNA levels at 24 hours, with a 61+/-6% decrease of IGFBP-4 levels, as determined by Western ligand blot analysis. Thrombin had the same depressor effects (87+/-2% for the mRNA levels and 61+/-3% for the protein levels). Ang II and thrombin coincubation with (125)I-IGFBP-4 in the conditioned media failed to reveal any increase in fragmentation, indicating that proteolytic cleavage of IGFBP-4 was not involved in the observed effects. Exogenous recombinant human IGFBP-4 decreased thrombin-induced DNA synthesis of human aortic VSMCs by 64%, whereas anti-IGFBP-4 antibody potentiated thrombin-induced DNA synthesis. These data suggest that downregulation of IGFBP-4 expression in VSMCs may play a critical role in vascular growth response to Ang II and thrombin in normal and diseased states, by increasing the bioavailability of IGF-I for its cell-surface receptor.

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

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

Studies on the role of human insulin-like growth factor-II (IGF-II)-dependent IGF binding protein (hIGFBP)-4 protease in human osteoblasts using protease-resistant IGFBP-4 analogs

To characterize the insulin-like growth factor binding protein-4 (IGFBP-4) protease produced by human osteoblasts (hOBs), we localized and determined the role of the proteolytic domains in human IGFBP-4 (hIGFBP-4) in modulating IGF-II actions. N-terminal amino acid sequence and mass spectrometric analyses of the 6xHis-tagged IGFBP-4 proteolytic fragments revealed that Met135-Lys136 was the only cleavage site recognized by the IGF-II-dependent IGFBP-4 protease produced by hOBs. This cleavage site was confirmed by the finding that deletion of His121 to Pro141 blocked proteolysis. However, unexpectedly, deletion of Pro94 to Gln119 containing no cleavage site had no effect on IGF-II binding activity but blocked proteolysis. Addition of the synthetic peptide corresponding to this region at concentrations of 250 or 1000 molar excess failed to block IGFBP-4 proteolysis. These data suggest that residues 94-119 may be involved in maintaining the IGFBP-4 conformation required to expose the cleavage site rather than being involved in direct protease-substrate binding. To determine the physiological significance of the IGF-II-dependent IGFBP-4 protease, we compared the effect of the wild-type IGFBP-4 and the protease-resistant IGFBP-4 analogs in blocking IGF-II-induced cell proliferation in normal hOBs, which produce IGFBP-4 protease, and MG63 cells, which do not produce IGFBP-4 protease. It was found that protease-resistant IGFBP-4 analogs were more potent than the wild-type protein in inhibiting IGF-II-induced cell proliferation in hOBs but not in MG63 cells. These data suggest that IGFBP-4 proteolytic fragments are not biologically active and that IGFBP-4 protease plays an important role in regulating IGFBP-4 bioavailability and consequently the mitogenic activity of IGFs in hOBs.

Mitogenic and antiapoptotic effects of insulin-like growth factor binding protein-6 in the human osteoblastic osteosarcoma cell line Saos-2/B-10

Insulin-like growth factor (IGF) I is a potent mitogen for human osteosarcoma cells such as the Saos-2/B-10 cell line. IGF binding proteins (IGFBPs) prevent stimulation of DNA synthesis by IGFs. In contrast to recombinant human (rh) IGFBP-2, -3, -4, and -5, 10-100 nM rhIGFBP-6 stimulated [(3)H]thymidine incorporation into DNA and multiplication of Saos-2/B-10 cells. Upon withdrawal of serum, 30 nM IGFBP-6 also decreased apoptosis (within 4 h) and increased protein content and sodium-dependent phosphate uptake (within 24 h), but less potently than IGF I. (125)I-labeled rhIGFBP-6 did not bind to the cells, and cold IGFBP-6 did not affect (125)I-labeled IGF I binding. Production of IGF I, IGF II, and IGFBP-6 by the cells or significant degradation of rhIGFBP-6 could not be detected within 24 h of incubation. Thus, among the rhIGFBPs tested, rhIGFBP-6 is unique in stimulating osteosarcoma cell growth. Furthermore, it has an antiapoptotic effect.

Insulin-like growth factor-II mediates the steroidogenic and growth promoting actions of follicle stimulating hormone on human ovarian pre-antral follicles cultured in vitro

FSH is important for ovarian antral follicle growth and steroidogenesis, processes in the human that are believed to be mediated by IGF-II. The objective of this study was to determine if human ovarian pre-antral follicles are also FSH- and IGF-II-responsive, since the clinical questions and mechanisms underlying the effects on the pre-antral follicle pool of exogenously administered gonadotropins for fertility therapy and elevated endogenous gonadotropins in the perimenopause remain unanswered. Class 2 preantral follicles were isolated from human premenopausal ovaries (n=6) and cultured in vitro with androstenedione and either no additives or with FSH or IGF-II. FSH (100 ng/mL) stimulated estradiol (E2) production by 3.58 +/- 0.4 fold over 48 hr, compared to controls without FSH. This effect was completely inhibited in the presence of the IGF-II antagonist, IGF binding protein-4 (IGFBP-4). IGF-II also stimulated E2 production by preantral follicles with doses as low as 1 ng/mL and within 24 hr of treatment. Maximal response of 3- to 9-fold above control was achieved with 100 ng/mL of IGF-II between 96-120 hr of culture. IGFBP-4 completely inhibited E2 production to basal levels. FSH stimulated IGF-II mRNA in pre-antral follicles about 4-fold, determined by RT-PCR. FSH also stimulated follicle growth, determined by light microscopy, 50-68% over 48 hr, compared to controls (P<0.001), a process that was inhibited in the presence of IGFBP-4. Cumulatively, these data support IGF-II as a mediator of FSH action on human preantral follicles.

A protease-resistant form of insulin-like growth factor (IGF) binding protein 4 inhibits IGF-1 actions

Smooth muscle cells (SMC) secrete a serine protease that cleaves insulin-like growth factor (IGF) binding protein (IGFBP)-4 into fragments that have low affinity for IGF-1. When IGFBP-4 is added to monolayer cultures of cell types that do not secrete this protease, IGF-1 stimulation of DNA synthesis is significantly inhibited. In contrast, if cell types that secrete this protease are used, IGFBP-4 is a much less potent inhibitor. These studies were conducted to determine whether proteolysis of IGFBP-4 accounted for its reduced capacity to inhibit IGF-1-stimulated DNA synthesis. The cleavage site in IGFBP-4 that the SMC protease uses was determined to be lysine120, histidine121. A protease-resistant mutant form of IGFBP-4 was prepared, expressed, purified, and tested for biologic activity using porcine SMC cultures. Addition of the protease-resistant mutant resulted in inhibition of DNA and cell migration responses to IGF-1. The inhibition was concentration dependent and was maximal when 500 ng/ml (20 nM) of the mutant was added with 20 ng/ml (2.8 nM) of IGF-1. When the mutant was added in the absence of IGF-1, it had no activity. The results show that cleavage of IGFBP-4 at lysine120, histidine121 results in inactivation of the ability of IGFBP-4 to bind to IGF-1. Creation of a mutant form of IGFBP-4 that was not cleaved by the protease resulted in inhibition of IGF-1-stimulated actions. The results suggest that IGFBP-4 can act as a potent inhibitor of the anabolic effects of IGF-1 and that the variables that regulate protease activity may indirectly regulate IGF-1 actions.

Modulation of insulin-like growth factor actions in L6A1 myoblasts by insulin-like growth factor binding protein (IGFBP)-4 and IGFBP-5: a dual role for IGFBP-5

We have previously shown that the insulin-like growth factors (IGFs) stimulate both proliferation and differentiation of skeletal muscle cells in culture, and that these actions in L6A1 muscle cells may be modulated by three secreted IGF binding proteins (IGFBPs), IGFBP-4, -5, and -6. Since we found that the temporal expression pattern of IGFBP-4 and IGFBP-5 differed dramatically during the transition from proliferating myoblasts to differentiated myotubes, we undertook the current study to examine the effects of purified IGFBP-4 and IGFBP-5 on IGF-stimulated actions in L6A1 muscle cells. As has been shown for other cell types, we found that IGFBP-4 had only inhibitory actions, inhibiting IGF-I and IGF-II-stimulated proliferation and differentiation. In contrast, IGFBP-5 exhibited both inhibitory and stimulatory actions. When added in the presence of 30 ng/ml IGF-I, IGFBP-5 (250 ng/ml) inhibited all markers of the early proliferative response: the tyrosine phosphorylation of the cytoplasmic signaling molecules IRS-1 and Shc, the activation of the MAP kinases, ERK1 and 2, the elevation of c-fos mRNA, the early inhibition of the elevation in myogenin mRNA, and the increase in cell number. In contrast, IGFBP-5 stimulated all aspects of the myogenic response to IGF-I: the later rise in myogenin mRNA, the elevation of creatine kinase activity, and the fusion of myoblasts into myotubes. This dual response to IGFBP-5 was greatest when it was added at a molar ratio of IGFBP-5 to IGF-I of 2:1. In contrast, when IGFBP-5 was added in the presence of IGF-II, it inhibited both proliferation and differentiation. Neither IGFBP had any effect when added in the presence of R3 IGF-I, an analog with substantially reduced affinity for IGFBPs. Our results suggest that the role of IGFBP-4 is mainly to sequester excess IGFs, and thus inhibit all actions. IGFBP-5, however, is capable of eliciting a dual response, possibly due to its unique ability to associate with the cell membrane.

Evidence for modulation of osteocalcin containing gamma-carboxyglutamic acid residues synthesis by insulin-like growth factor-I and vitamin K2 in human osteosarcoma cell line MG-63

The effect of insulin-like growth factor-I (IGF-I) and 2-methyl-3-all-trans-tetraphenyl-1,4-naphtoquinone (vitamin K2) on the synthesis of osteocalcin containing gamma-carboxyglutamic acid (Gla) residues which is the physiologically relevant form in bone metabolism was studied in cultured human osteoblast-like (MG-63) cells. Both IGF-I and vitamin K2 stimulated 1,25-dihydroxyvitamin D3 (1,25(OH)2D3)-induced osteocalcin containing Gla secretion in a concentration-dependent manner. This stimulatory effect of IGF-I and vitamin K2 was additive. Vitamin K2-enhanced osteocalcin containing Gla secretion was selectively suppressed by 3-(alpha-acetonyl-benzyl)-4-hydroxy-coumarin (warfarin). The stimulatory effect of IGF-I was completely abolished by the presence of cycloheximide; in contrast the effect of vitamin K2 was still observed in the presence of cycloheximide. Treatment of MG-63 cells with IGF-I caused an approximately 2.2-fold increase in osteocalcin mRNA levels (determined by reverse transcription-polymerase chain reaction). Vitamin K2 had no effect on either the stimulation of mRNA level by IGF-I or the basal level. IGF-I-stimulated osteocalcin containing Gla secretion was inhibited by one of its binding proteins (insulin-like growth factor binding protein-4) in a concentration-dependent manner. These findings suggest that the modes of action of IGF-I and vitamin K2 on 1.25(OH)2D3-induced osteocalcin containing Gla secretion in MG-63 cells are different.

Activin-induced inhibin alpha-subunit production by rat granulosa cells requires endogenous insulin-like growth factor-I

Inhibin-alpha subunit (Inh-alpha) gene expression is important for granulosa cell (GC) differentiation and prevention of ovarian tumorigenesis. Studies on Inh-alpha regulation have implicated activin and insulin-like growth factor-I (IGF-I) in the mechanisms of expression. Here we present evidence that endogenously produced IGF-I plays an obligatory role in activin-induced Inh-alpha production. Primary cultures of rat GC were incubated with increasing concentrations of various regulatory molecules, and the levels of Inh-alpha protein and its mRNA were measured in conditioned medium and cells, respectively. Recombinant activin A stimulated Inh-alpha expression, and the effects were dose- and time-dependent. The receptor tyrosine kinase inhibitor tyrphostin A23 caused a dose-dependent inhibition of activin-dependent Inh-alpha expression, whereas the inactive isomer, A63, had no effect. The stimulatory effect of activin was also blocked in a dose-dependent manner by added IGF binding protein-4 or -5, and the effects were reversed by IGF-I. Moreover, increasing concentrations of an anti-IGF-I antibody had a similar inhibitory effect on activin-stimulated Inh-alpha expression. Collectively, these results suggest, for the first time, that endogenously produced IGF-I is required for activin stimulation of Inh-alpha expression in cultured rat GC.

Potent inhibition of human ovarian steroidogenesis by insulin-like growth factor binding protein-4 (IGFBP-4)

Several studies have now documented the existence of IGFBPs in follicular fluid and their correlation with the health of the follicle. In particular, increased levels of IGFBP-4 have been reported in androgen-dominant atretic follicles and those from polycystic ovaries. The aim of this study was to elucidate the role of IGFBP-4 in ovarian steroidogenesis. Granulosa cells and theca tissue were incubated with or without LH or FSH in the presence or absence of IGFBP-4 (0.5-50 ng/ml). Inhibition by IGFBP-4 of estradiol production in the presence of testosterone alone was seen in three of four experiments. IGFBP-4 completely inhibited FSH-stimulated estradiol production in three experiments and caused 67% inhibition in a fourth. Similar results were obtained for theca, in which concurrent incubation with IGFBP-4 completely negated the stimulatory effects of LH on androstenedione production. The mechanism by which IGFBP-4 exerts these potent effects and the possibility that this may by IGF-independent are currently being investigated.

Endogenous insulin-like growth factor-I is obligatory for stimulation of rat inhibin alpha-subunit expression by follicle-stimulating hormone

Insulin-like growth factor-I (IGF-I) is essential for FSH-dependent steroidogenesis by rat granulosa cells (GC), but whether IGF-I is required for other FSH-dependent functions is unknown. To investigate the role of IGF-I in the mechanisms of FSH-stimulated inhibin alpha-subunit (Inh-alpha) production, rat GC were cultured with FSH, IGF-I, insulin-like growth factor-binding protein (IGFBP)-4, IGFBP-5, and/or anti-IGF-I antibody. Inh-alpha protein and mRNA levels were measured in conditioned medium and cells by Western immunoblotting and Northern analysis, respectively. Inh-alpha expression was increased by FSH (3.5-fold) and IGF-I (2.5-fold), and the effects were dose and time dependent. FSH stimulation of Inh-alpha was attenuated by IGFBP-4 or -5 in a dose-dependent fashion, and the effects were reversed by IGF-I. Anti-IGF-I antibody mimicked the inhibitory effects of IGFBP-4 and -5. Forskolin, cholera toxin, and 8-bromo-cAMP increased Inh-alpha production approximately 3.5-fold, and the effects were blocked by IGFBP-4 or -5. Increases in Inh-alpha by FSH, IGF-I, forskolin, cholera toxin, and 8-bromo-cAMP were totally blocked by the protein tyrosine kinase inhibitor, tyrphostin A23. In summary, these results suggest that the stimulation of Inh-alpha expression by FSH requires activation of protein tyrosine kinases by endogenously produced IGF-I. We propose that the IGF-I signaling is obligatory for FSH stimulation of Inh-alpha expression in rat GC.

The regulation of type-I collagen synthesis by insulin-like growth factor-I in human osteoblast-like SaOS-2 cells

The effect of insulin-like growth factor-I on collagen synthesis was studied using cultured human osteoblast-like SaOS-2 cells by measuring the incorporation of tritiated L-proline into immunoprecipitable type-I collagen. Tritiated L-proline incorporation into collagen was significantly stimulated by insulin-like growth factor-I in a time- and concentration-dependent manner. Unlabelled L-proline and alpha-(methylamino) isobutyric acid inhibited either the influx into cells, or the incorporation into collagen, of tritiated L-proline. The increase in incorporation of tritiated L-proline was significantly reduced by cycloheximide and actinomycin D. L-Proline incorporation into collagen was also stimulated by insulin-like growth factor-II, insulin-like growth factor-I analogues and insulin. The insulin-like growth factor-I-stimulated L-proline incorporation was inhibited by one of its binding proteins, insulin-like growth factor binding protein-4, in a concentration-dependent manner.

The regulation of L-proline transport by insulin-like growth factor-I in human osteoblast-like SaOS-2 cells

The effect of insulin-like growth factor-I on amino acid transport was studied by measuring the uptake of tritiated L-proline in the cultured human osteoblast-like SaOS-2 cells. The uptake of L-proline was supported by both transport system A, ASC and Gly and by Na+-dependent amino acid transport system A, and by Na+-independent system L. The initial rate of total L-proline uptake as a function of concentration showed saturation and obeyed Michaelis-Menten kinetics with Michaelis constant (Km) and maximum velocity (Vmax) values of 1.87 mM and 8.89 nmol x (mg protein)-1 x (3 min)-1, respectively. Na+-dependent L-proline uptake was significantly stimulated by insulin-like growth factor-I in a time- and concentration-dependent manner. Kinetic analysis showed that insulin-like growth factor-I enhanced transport activity by increasing the Vmax of transport without significant changes in the affinity (Km) of the carrier for the substrate. The increase in transport activity was significantly reduced by cycloheximide. The stimulated increment above basal L-proline uptake was completely inhibited by alpha-(methylamino) isobutyric acid, suggesting that only system A was affected by insulin-like growth factor-I. Na+-dependent L-proline uptake was also stimulated by insulin-like growth factor-II and insulin-like growth factor-I analogues. The insulin-like growth factor-I-stimulated L-proline uptake was inhibited by one of its binding protein, insulin-like growth factor binding protein-4, in a concentration-dependent manner.