Insulin-Like Growth Factor Binding Protein-5 in Physiology and Disease

Insulin-like growth factor (IGF) signaling is regulated by a conserved family of IGF binding proteins (IGFBPs) in vertebrates. Among the six distinct types of IGFBPs, IGFBP-5 is the most highly conserved across species and has the broadest range of biological activities. IGFBP-5 is expressed in diverse cell types, and its expression level is regulated by a variety of signaling pathways in different contexts. IGFBP-5 can exert a range of biological actions including prolonging the half-life of IGFs in the circulation, inhibition of IGF signaling by competing with the IGF-1 receptor for ligand binding, concentrating IGFs in certain cells and tissues, and potentiation of IGF signaling by delivery of IGFs to the IGF-1 receptor. IGFBP-5 also has IGF-independent activities and is even detected in the nucleus. Its broad biological activities make IGFBP-5 an excellent representative for understanding IGFBP functions. Despite its evolutionary conservation and numerous biological activities, knockout of IGFBP-5 in mice produced only a negligible phenotype. Recent research has begun to explain this paradox by demonstrating cell type-specific and physiological/pathological context-dependent roles for IGFBP-5. In this review, we survey and discuss what is currently known about IGFBP-5 in normal physiology and human disease. Based on recent in vivo genetic evidence, we suggest that IGFBP-5 is a multifunctional protein with the ability to act as a molecular switch to conditionally regulate IGF signaling.

Analysis of 17β-estradiol (E2) role in the regulation of corpus luteum function in pregnant rats: Involvement of IGFBP5 in the E2-mediated actions

BACKGROUND

In several species, considerably higher levels of estradiol-17 (E2) are synthesized in the CL. E2 has been suggested to participate in the regulation of luteal steroidogenesis and luteal cell morphology. In pregnant rats, several experiments have been carried out to examine the effects of inhibition of luteal E2 synthesis on CL structure and function.

METHODS

During days 12-15 of pregnancy in rats, luteal E2 was inhibited by way of daily oral administration of anastrozole (AI), a selective non-steroidal aromatase inhibitor, and experiments were also performed with E2 replacement i.e. AI+ E2 treatments. Luteal tissues from different treatment groups were subjected to microarray analysis and the differentially expressed genes in E2 treated group were further examined for expression of specific E2 responsive genes. Additional experiments were carried out employing recombinant growth hormone preparation and flutamide, an androgen receptor antagonist, to further address the specificity of E2 effects on the luteal tissue.

RESULTS

Microarray analysis of CL collected on day 16 of pregnancy post AI and AI+E2 treatments showed significantly lowered cyp19a1 expression, E2 levels and differential expression of a number of genes, and several of them were reversed in E2 replacement studies. From the differentially expressed genes, a number of E2 responsive genes were identified. In CL of AI pregnant rats, non-significant increase in expression of igf1, significant increase in igbp5, igf1r and decrease in expression of Erα were observed. In liver of AI treated rats, igf1 expression did not increase, but GH treatment significantly increased expression that was further increased with AI treatment. In CL of GH and AI+GH treated rats, expression of igfbp5 was higher. Administration of flutamide during days 12-15 of pregnancy resulted in non-significant increase in igfbp5 expression, however, combination of flutamide+AI treatments caused increased protein expression. Expression of few of the molecules in PI3K/Akt kinase pathway in different treatments was determined.

CONCLUSIONS

The results suggest a role for E2 in the regulation of luteal steroidogenesis, morphology and proliferation. igfbp5 was identified as one the E2 responsive genes with important role in the mediation of E2 actions such as E2-induced phosphorylation of PI3K/Akt kinase pathway.

Insulin-Like Growth Factor Binding Proteins--an Update

The insulin-like growth factor (IGF) system is essential for normal growth and development, and its perturbation is implicated in a number of diseases. IGF activity is finely regulated by a family of six high-affinity IGF binding proteins (IGFBPs). 1GFBPs usually inhibit IGF actions but may enhance them under certain conditions. Additionally, IGFBPs bind non-IGF ligands in the extracellular space, cell membrane, cytoplasm and nucleus, thereby modulating cell proliferation, survival and migration in an IGF-independent manner. IGFBP activity is regulated by transcriptional mechanisms as well as by post-translational modifications and proteolysis. Understanding the balance between the various actions of IGFBPs in vivo may lead to novel insights into disease processes and possible IGFBP-based therapeutics.

IGFBP-5 Promotes Fibrosis Independently of Its Translocation to the Nucleus and Its Interaction with Nucleolin and IGF

BACKGROUND

Insulin-like growth factor binding protein (IGFBP)-5 levels are increased in systemic sclerosis (SSc) skin and lung. We previously reported that IGFBP-5 is a pro-fibrotic factor that induces extracellular matrix (ECM) production and deposition. Since IGFBP-5 contains a nuclear localization signal (NLS) that facilitates its nuclear translocation, we sought to examine the role of nuclear translocation on the fibrotic activity of IGFBP-5 and identify IGFBP-5 binding partners relevant for its nuclear compartmentalization.

METHODS

We generated functional wild type IGFBP-5 and IGFBP-5 with a mutated NLS or a mutated IGF binding site. Abrogation of nuclear translocation in the NLS mutant was confirmed using immunofluorescence and immunoblotting of nuclear and cytoplasmic cellular extracts. Abrogation of IGF binding was confirmed using western ligand blot. The fibrotic activity of wild type and mutant IGFBP-5 was examined in vitro in primary human fibroblasts and ex vivo in human skin. We identified IGFBP-5 binding partners using immunoprecipitation and mass spectrometry. We examined the effect of nucleolin on IGFBP-5 localization and function via sequence-specific silencing in primary human fibroblasts.

RESULTS

Our results show that IGFBP-5-induced ECM production in vitro in primary human fibroblasts is independent of its nuclear translocation. The NLS-mutant also induced fibrosis ex vivo in human skin, thus confirming and extending the in vitro findings. Similar findings were obtained with the IGF-binding mutant. Nucleolin, a nucleolar protein that can serve as a nuclear receptor, was identified as an IGFBP-5 binding partner. Silencing nucleolin reduced IGFBP-5 translocation to the nucleus but did not block the ability of IGFBP-5 to induce ECM production and a fibrotic phenotype.

CONCLUSIONS

IGFBP-5 transport to the nucleus requires an intact NLS and nucleolin. However, nuclear translocation is not necessary for IGFBP-5 fibrotic activity; neither is IGF binding. Our data provide further insights into the role of cellular compartmentalization in IGFBP-5-induced fibrosis.

Association of porcine IGF binding protein-5 gene with meat quality

IGFBP-5 is a member of the IGF families. Using PCR-SSCP, genotypic and allelic frequencies were analyzed in 18 pig breeds (n = 600). The association between haplotypes and production performance was analyzed in a Jinhua x Pietrain population family (n = 212, total 24 traits). Two SNPs (T199C and G485A) within the gene were analyzed. The breeds had different genotypic and allelic frequencies. Typically, the Chinese native pig breeds carried a higher allele C and G frequency (over 50%) than those of the European pigs, and only Guangdong Large White and wild boar were at Hardy-Weinberg equilibrium. The pigs carrying the CG haplotype had higher hue, loin, and thigh pH1 values than pigs with the TA haplotype, and pigs with the TA haplotype had the lowest loin pH2 value and highest color-a value among the haplotypes. It is proposed that the IGFBP-5 gene is associated with the variation in meat quality, especially in pH value together with other QTLs on chromosome 15.

IGFBP-5 induces epithelial and fibroblast responses consistent with the fibrotic response

Fibrosis involves activation of fibroblasts, increased production of collagen and fibronectin and transdifferentiation into contractile myofibroblasts. The process resembles aspects of wound-healing but remains unresolved and can be life-threatening when manifest in the kidneys, lungs and liver, in particular. The causes are largely unknown, but recent suggestions that repetitive micro-injury results in the eventual failure of epithelial cell repair due to replicative senescence are gaining favour. This is consistent with the onset of fibrotic diseases in middle age. Because epithelial injury often involves blood loss, inflammatory responses associated with the fibrotic response have been considered as therapeutic targets. However, this has proved largely unsuccessful and focus is now switching to earlier events in the process. These include EMT (epithelial-mesenchymal transition) and fibroblast activation in the absence of inflammation. TGFbeta1 (transforming growth factor-beta1) induces both EMT and fibroblast activation and is considered to be a major pro-fibrotic factor. Recently, IGFBP-5 [IGF (insulin-like growth factor)-binding protein-5] has also been shown to induce similar effects on TGFbeta1, and is strongly implicated in the process of senescence. It also stimulates migration of peripheral blood mononuclear cells, implicating it in the inflammatory response. In this paper, we examine the evidence for a role of IGFBP-5 in fibrosis and highlight its structural relationship with other matrix proteins and growth factors also implicated in tissue remodelling.

Insulin-like growth factor binding protein-5 in osteogenesis: facilitator or inhibitor?

The insulin-like growth factors (IGFs) play a central role in controlling somatic growth in mammals and exert anabolic effects on most tissues, including bone. IGF action is mediated by the IGF-I receptor and additionally is regulated by six high-affinity IGF binding proteins (IGFBP-1 through IGFBP-6), of which IGFBP-4 and IGFBP-5 are most abundant in bone. The focus of this brief review is on the role of IGFBP-5 in bone biology. IGFBP-5 has been implicated as a pro-osteogenic factor in several studies but conversely has been shown to act as an inhibitor of bone formation, primarily by interfering with IGF actions on osteoblasts. These potentially contradictory effects of IGFBP-5 in bone are further complicated by observations indicating that IGFBP-5 additionally may function in an IGF-independent way, and may have been accentuated by differences in both experimental design and methodology among published studies. Suggestions are made for a more systematic approach to help discern the true roles of IGFBP-5 in bone physiology.

Delayed mammary gland involution in mice with mutation of the insulin-like growth factor binding protein 5 gene

IGFs (IGF-I and IGF-II) are essential for development, and their bioactivities are tightly regulated by six related IGF-binding proteins (IGFBPs). IGFBP-5 is the most highly conserved binding protein and is expressed in several key developmental lineages as well as in multiple adult tissues including the mammary gland. To explore IGFBP-5 actions in vivo, we produced IGFBP-5 knockout (KO) mice. Whole-body growth, selected organ weights, and body composition were essentially normal in IGFBP-5 KO mice, presumably because of substantial compensation by remaining IGFBP family members. The IGFBP-5 KO mice also exhibited normal mammary gland development and were capable of nursing their pups. We then directly evaluated the proposed role of IGFBP-5 in apoptosis and remodeling of mammary gland during involution. We found that the process of involution after forced weaning was delayed in IGFBP-5 KO mice, with both the appearance of apoptotic cells and the reappearance of adipocytes retarded in mutant mice, compared with controls. We also determined the effects of IGFBP-5 deletion on mammary gland development in pubertal females after ovariectomy and stimulation with estradiol/progesterone. In this paradigm, IGFBP-5 KO mammary glands exhibited enhanced alveolar bud formation consistent with enhanced IGF-I action. These results demonstrate that IGFBP-5, although not essential for normal growth, is required for normal mammary gland involution and can regulate mammary gland morphogenesis in response to hormone stimulation.

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

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

Insulin-like growth factor binding protein 5 induces skin fibrosis: A novel murine model for dermal fibrosis

OBJECTIVE

To determine the role of insulin-like growth factor binding protein 5 (IGFBP-5) in the development of skin fibrosis in vivo, by examining the effect of overexpression of IGFBP-5 in mouse skin.

METHODS

Wild-type C57BL/6J mice were injected subcutaneously with replication-deficient serotype 5 adenovirus expressing human IGFBP-3 (Ad3), IGFBP-5 (Ad5), or no complementary DNA (cAd). Mice were killed 3, 8, or 22 days postinjection. The dermal thickness and dermal collagen bundle thickness in skin sections were measured. The deposition of collagen in the extracellular matrix (ECM) was quantified using the Sircol assay. Expression of proliferating cell nuclear antigen (PCNA) and fibronectin, as determined by immunohistochemical analysis, was used to evaluate fibroblast activation, and vimentin and alpha-smooth muscle actin (alpha-SMA) were used to evaluate the fibroblast phenotype.

RESULTS

Adenovirally expressed IGFBP was detected in dermal fibroblasts, endothelial cells, epithelial cells, and muscle bundles in Ad3- and Ad5-injected mice. Increased collagen deposition, denser dermal connective tissue, and increased collagen bundle thickness were observed in IGFBP-5-overexpressing mice. Dermal thickness and collagen bundle thickness were significantly increased in Ad5-injected mice compared with cAd- and Ad3-injected mice. Treatment with Ad5 resulted in a dose-dependent increase in dermal and collagen bundle thickness. Increased deposition of collagen and fibronectin, increased numbers of PCNA-positive fibroblasts, as well as increased numbers of vimentin- and alpha-SMA-double-positive fibroblasts were detected in the dermis of IGFBP-5-overexpressing mouse skin.

CONCLUSION

IGFBP-5 is a key mediator of fibrosis. IGFBP-5 mediates its profibrotic effects through fibroblast activation, increased ECM deposition, and myofibroblastic transformation of dermal fibroblasts. Overexpression of IGFBP-5 provides a novel model for studying the pathogenesis of skin fibrosis in systemic sclerosis.

Osteoblast differentiation of NIH3T3 fibroblasts is associated with changes in the IGF-I/IGFBP expression pattern

Insulin-like growth factors (IGFs) and IGF-binding proteins (IGFBPs) are essential regulators for osteoblast proliferation and differentiation. It has been reported that Dexamethasone (Dex), an active glucocorticoid (GC) analogue, synergizes the stimulatory effect of 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) on osteoblast differentiation in the mouse fibroblastic cell line NIH3T3. I investigated whether this stimulatory effect is associated with changes in the expression pattern of the IGF/IGFBP system. Quantitative real-time PCR technology was used to quantify the gene expression levels of the IGF-system during osteoblast differentiation and in response to 1,25(OH)₂D₃ or Dex alone under serum-containing and serum-free culture conditions. Interestingly, NIH3T3 was shown to express high mRNA levels of IGF-I, IGF-II and IGFBP-5, and low levels of both IGFBP-2 and-6. During osteoblast differentiation (days 6-12), IGF-I mRNA was repressed by more than 60%, while the transcript of IGFBP-5 was markedly up-regulated, by more than 50-fold. Similarly, treatment with Dex alone resulted in a dose-and time-dependent increase in the expression of IGFBP-5 and a decrease in IGF-I mRNA. Treatment with 1,25(OH)₂D₃ alone increased the mRNA levels of IGF-I and IGFBP-6 by around 4-and 7-fold, respectively, in a dose-and time-dependent manner. In conclusion, my data demonstrated that osteoblast differentiation of NIH3T3 is associated with changes in the expression pattern of IGFs/IGFBPs, which are regulated by glucocorticoid in the presence of 1,25(OH)₂D₃. Modulation of the IGF/IGFBP levels by glucocorticoid might suggest important roles for the IGF-system in mediating the osteoblast differentiation of the NIH3T3 cell line.

Insulin-like growth factor-binding protein-5 activates plasminogen by interaction with tissue plasminogen activator, independently of its ability to bind to plasminogen activator inhibitor-1, insulin-like growth factor-I, or heparin

Transgenic mice expressing IGFBP-5 in the mammary gland exhibit increased cell death and plasmin generation. Because IGFBP-5 has been reported to bind to plasminogen activator inhibitor-1 (PAI-1), we determined the effects of this interaction in HC11 cells. PAI-1 prevented plasmin generation from plasminogen and inhibited cleavage of focal adhesions, expression of caspase 3, and cell death. IGFBP-5 could in turn prevent the effects of PAI-1. IGFBP-5 mutants with reduced affinity for IGF-I (N-term) or deficient in heparin binding (HEP- and C-term E and F) were also effective. This was surprising because IGFBP-5 reportedly interacts with PAI-1 via its heparin-binding domain. Biosensor analysis confirmed that, although wild-type IGFBP-5 and N-term both bound to PAI-1, the C-term E had greatly decreased interaction with PAI-1. This suggests that IGFBP-5 does not antagonize the actions of PAI-1 by a direct molecular interaction. In a cell-free system, using tissue plasminogen activator (tPA) and urokinase plasminogen activator (uPA) to activate plasminogen, PAI-1 inhibited plasmin generation induced by both activators, whereas IGFBP-5 prevented the effects of PAI-1 on tPA but not uPA. Furthermore, we noted that IGFBP-5 activated plasminogen to a greater extent than could be explained solely by inhibition of PAI-1, suggesting that IGFBP-5 could directly activate tPA. Indeed, IGFBP-5 and the C-term E and F were all able to enhance the activity of tPA but not uPA. These data demonstrate that IGFBP-5 can enhance the activity of tPA and that this can result in cell death induced by cleavage of focal adhesions. Thus IGFBP-5 can induce cell death by both sequestering IGF-I and enhancing plasmin generation.

Insulin-like growth factor-I (IGF-I) attenuates jejunal atrophy in association with increased expression of IGF-I binding protein-5 in parenterally fed mice

Total parenteral nutrition (TPN) induces dramatic mucosal hypoplasia in rat small intestine that is attenuated by insulin-like growth factor-I (IGF-I). Our aim was to determine the extent of TPN-induced intestinal atrophy and its response to infusion of IGF-I in mice. Male C57BL/6 mice (18-22 g) were maintained with TPN, TPN plus co-infusion of recombinant human IGF-I [2.5 mg/(kg . d)] or oral feeding for 5 d. Body weights did not differ among the groups although serum IGF-I was increased by 78% with IGF-I infusion. IGF-I prevented the significant 25% reduction in mass of the intact small intestine due to TPN compared with oral feeding. Greater TPN-induced atrophy was noted in duodenum and jejunum than ileum. Jejunal atrophy induced by TPN reflected significant decreases in muscularis mass and concentrations of protein and DNA; mucosal cellularity was not altered by TPN. TPN induced a significant decrease in jejunal muscularis width that was reversed by IGF-I with no differences in mucosal villus height and crypt depth. Local expression of IGF-I binding protein (IGFBP)-5 positively modulates the intestinotrophic effects of IGF-I. Jejunal atrophy due to TPN and growth due to IGF-I were directly associated with expression of IGFBP-5 mRNA. TPN decreased IGFBP-5 mRNA by 60% and IGF-I increased IGFBP-5 mRNA by 200% with no change in IGF-I mRNA compared with oral feeding. In summary, TPN induces significant 25% atrophy of the mouse small intestine that is attenuated by IGF-I in association with increased expression of IGFBP-5. Compared with rats, TPN-induced atrophy is less severe and occurs primarily in the jejunal muscularis layer in mice.

Insulin-like growth factor binding protein (IGFBP)-3 and IGFBP-5 mediate TGF-beta- and myostatin-induced suppression of proliferation in porcine embryonic myogenic cell cultures

We have previously shown that cultured porcine embryonic myogenic cells (PEMC) produce both insulin-like growth factor binding protein (IGFBP)-3 and IGFBP-5 and secrete these proteins into their media. Exogenously added recombinant porcine (rp) IGFBP-3 and rpIGFBP-5 act via IGF-dependent and IGF-independent mechanisms to suppress proliferation of PEMC cultures. Furthermore, immunoneutralization of endogenous IGFBP-3 and IGFBP-5 in the PEMC culture medium results in increased DNA synthesis rate suggesting that endogenous IGFBP-3 and IGFBP-5 suppress PEMC proliferation. TGF-beta superfamily members myostatin and TGF-beta1 have also been shown to suppress proliferation of myogenic cells, and treatment of cultured PEMC with either TGF-beta1 or myostatin significantly (P < 0.01) increases levels of IGFBP-3 and -5 mRNA. We have previously shown that immunoneutralization of IGFBP-3 decreases the proliferation-suppressing activity of TGF-beta1 and myostatin. Here, we show that immunoneutralization of IGFBP-5 also significantly (P < 0.05) decreases the DNA synthesis-suppressing activity of these molecules. Simultaneous immunoneutralization of both IGFBP-3 and IGFBP-5 in TGF-beta1 or myostatin-treated PEMC cultures restores Long-R3-IGF-I-stimulated DNA synthesis rates to 90% of the levels observed in control cultures receiving no TGF-beta1 or myostatin treatment (P < 0.05). Even though immunoneutralization of IGFBP-3 and -5 increased DNA synthesis rates in TGF-beta1 or myostatin-treated PEMC cultures, phosphosmad2 levels in these cultures were not affected. These findings strongly suggest that IGFBP-3 and IGFBP-5 affect processes downstream from receptor-mediated Smad phosphorylation that facilitate the ability of TGF-beta and myostatin to suppress proliferation of PEMC.

Insulin-like growth factor binding proteins initiate cell death and extracellular matrix remodeling in the mammary gland

We have demonstrated that insulin-like growth factor binding protein-5 (IGFBP-5) production by mammary epithelial cells increases dramatically during forced involution of the mammary gland in rats, mice and pigs. We proposed that growth hormone (GH) increases the survival factor IGF-I, whilst prolactin (PRL) enhances the effects of GH by decreasing the concentration of IGFBP-5, which would otherwise inhibit the actions of IGFs. To demonstrate a causal relationship between IGFBP-5 and cell death, we created transgenic mice expressing IGFBP-5, specifically, in the mammary gland. DNA content in the mammary glands of transgenic mice was decreased as early as day 10 of pregnancy. Mammary cell number and milk synthesis were both decreased by approximately 50% during the first 10 days of lactation. The concentrations of the pro-apoptotic molecule caspase-3 was increased in transgenic animals whilst the concentrations of two pro-survival molecules Bcl-2 and Bcl-x were both decreased. In order to examine whether IGFBP-5 acts by inhibiting the survival effect of IGF-I, we examined IGF receptor- and Akt-phoshorylation and showed that both were inhibited. These studies also indicated that the effects of IGFBP-5 could be mediated in part by IGF-independent effects involving potential interactions with components of the extracellular matrix involved in tissue remodeling, such as components of the plasminogen system, and the matrix metallo-proteinases (MMPs). Mammary development was normalised in transgenic mice by R3-IGF-I, an analogue of IGF-I which binds weakly to IGFBPs, although milk production was only partially restored. In contrast, treatment with prolactin was able to inhibit early involutionary processes in normal mice but was unable to prevent this in mice over-expressing IGFBP-5, although it was able to inhibit activation of MMPs. Thus, IGFBP-5 can simultaneously inhibit IGF action and activate the plasminogen system thereby coordinating cell death and tissue remodeling processes. The ability to separate these properties, using mutant IGFBPs, is currently under investigation.

Role of IGFBPs in the morphogenesis of lingual papillae

The expression of insulin-like growth factor binding proteins (IGFBPs) during the morphogenesis of lingual papillae of mice was examined by in situ hybridization. Among seven mouse IGFBPs, IGFBP-1, -6, and -7 mRNAs were not expressed in the tongue tissue. At E12, though no papillae have formed yet, IGFBP-2, -4, and -5 were expressed in the entire tongue epithelium. At E14, fungiform papillae appeared in the anterior region and circumvallate papillae were distinguished in the posterior region. Strong expression of IGFBP-5 was observed in the apical region of both fungiform and circumvallate papillae. At this stage, the epithelial elevation of filiform papillae was not clear; but IGFBP-5 was expressed in the apex. At E15, foliate papillae were distinguished and IGFBP-5 was expressed in the dorsal epithelium of ridges. In filiform papillae, IGFBP-3 was expressed in the core of the connective tissue. At E17, the expression of IGFBP-5 disappeared from the apical region of fungiform, filiform, foliate, and circumvallate papillae, whereas that of IGFBP-2 remained. This finding suggests that IGFBP-5 and -2 function to cause evagination of the epithelium into a raised structure. In the epithelium of trenches of foliate and circumvallate papillae, strong expression of IGFBP-4 was observed at E15 and E17. As previously suggested from a study on postnatal mice (Suzuki et al. J Comp Neurol 2005;482:74-84), IGFBP-4 acts in the epithelial invagination to form the trenches, grooves, or furrows of lingual papillae during development.

Overexpression of insulin-like growth factor binding protein-5 decreases osteoblastic function in vitro

Skeletal cells synthesize insulin-like growth factors (IGF) and their binding proteins (IGFBP). The effects of IGFBP-5 on bone cell growth and osteoblastic function are controversial, and transgenic mice overexpressing IGFBP-5 exhibit osteopenia. The mechanisms were not explored, and in this study, we investigated the effects of IGFBP-5 overexpression in MC3T3 cells in vitro. MC3T3 cells were transduced with a retroviral vector (pLPCX) or a vector directing IGFBP-5 transcription under the control of a constitutive promoter. Untreated MC3T3 cells expressed alkaline phosphatase, and osteocalcin mRNA 1 week after confluence and at 4 weeks of culture they formed mineralized nodules. IGFBP-5 overexpression delayed the appearance of alkaline phosphatase and osteocalcin mRNA, decreased type I collagen and osteopontin mRNA and alkaline phosphatase activity (APA), and inhibited the formation of mineralized nodules. IGFBP-5 caused a modest stimulation of DNA synthesis. In conclusion, overexpression of IGFBP-5 decreases osteoblastic function possibly by binding IGFs in the bone microenvironment.

The role of IGFBP-5 in mammary gland development and involution

Insulin-like growth factor-I (IGF-I) plays an important role as a survival factor during mammary gland development and remodelling during involution of the mature/lactating mammary gland, and elevated concentrations have been associated with increased risk of breast cancer. The actions of IGF-I are modulated by a family of binding proteins (IGFBPs) and we have shown that IGFBP-5 is associated with cell death in the mammary gland and more recently provided the first evidence that it is causally related to apoptosis of the mammary gland. A transgenic mouse expressing IGFBP-5 on a mammary-specific promoter led to impaired mammary development involving inhibition of IGF-signalling and involving members of the Bcl-2 family. Subsequent studies in vitro and in vivo using exogenous IGFBP-5 treatment have added support to this concept. Although the effects of IGFBP-5 did appear to involve inhibition of IGF action, a role for IGF-independent effects cannot be ruled out. Such IGF-independent effects involve potential interactions with components of the extracellular matrix involved in tissue remodelling including plasminogen activator inhibitor-1 (PAI-1). In addition, intracellular events involving nuclear localisation of IGFBP-5 have been shown to have the ability to inhibit cell proliferation. Thus, IGFBP-5 seems important for regulating both apoptosis and cell proliferation in the mammary gland during development and post-lactation involution.

Follicular development: the role of the follicular microenvironment in selection of the dominant follicle

The importance of endocrine signals in the regulation of follicular development has long been recognized. However, the follicular microenvironment also plays a critical role in determining follicular fate. This review summarizes our studies on the role of the intrafollicular IGF system in selection of the dominant follicle (DF) in cattle. During the bovine estrous cycle, the largest antral follicles develop in two or three successive waves of follicular recruitment and selection of a DF. High concentrations of estradiol in the follicular fluid are the hallmark of dominant and preovulatory follicles and are associated with lower concentrations of low molecular weight (MW) insulin-like growth factor binding proteins (IGFBP-2, -4, and -5), which can prevent binding of IGF to its receptor. Our studies have shown that dominant and preovulatory follicles also have much higher levels of an IGFBP-4/-5 protease activity, which is the bovine equivalent of the human IGFBP-4 protease, pregnancy-associated plasma protein-A (PAPP-A). Studies of follicles isolated just after the emergence of the DF showed that PAPP-A is present in the follicular fluid of the DF as soon as it can be detected as morphologically dominant. To examine whether higher levels of PAPP-A in one follicle of the cohort (the future DF) precedes morphological dominance, the four largest follicles were isolated from pairs of bovine ovaries obtained before one follicle of the cohort was significantly larger the others, around the time that one follicle was first detected as morphologically dominant and after dominance was well established. Analysis of the temporal sequence of changes in estradiol, low MW IGFBPs, free IGF, and PAPP-A in the follicular fluid suggested that an increase in PAPP-A is the earliest biochemical difference yet detected in the future DF and that follicular selection is the result of a progressive series of changes beginning with the acquisition of PAPP-A, which leads to a decrease in IGFBP-4 and -5 and an increase in free IGF, which synergizes with FSH to increase estradiol production. Co-dominant follicles, induced by injection of small doses of recombinant bovine (rb) FSH, both had levels of PAPP-A similar to the single DF of control heifers, supporting the hypothesized role of FSH in the induction of PAPP-A in the DF. Taken together, these results suggest a critical role for FSH-induced PAPP-A, and thus for free IGF, in the selection of the DF. In contrast, other experiments provided evidence for a deleterious effect of IGF on the initiation of bovine follicular growth and the survival of primordial and primary follicles in vitro. These results underscore the importance of the follicular microenvironment in determining follicular fate and indicate that its effects can be stage-specific.

Evidence That IGF Binding Protein-5 Functions as a Ligand-Independent Transcriptional Regulator in Vascular Smooth Muscle Cells

Insulin-like growth factor binding protein (IGFBP)-5 is a conserved protein synthesized and secreted by vascular smooth muscle cells (VSMCs). IGFBP-5 binds to extracellular IGFs and modulates IGF actions in regulating VSMC proliferation, migration, and survival. IGFBP-5 also stimulates VSMC migration through an IGF-independent mechanism, but the molecular basis underlying this ligand-independent action is unknown. In this study, we show that endogenous IGFBP-5 or transiently expressed IGFBP-5-EGFP, but not IGFBP-4-EGFP, is localized in the nuclei of VSMCs. Using a series of IGFBP-4/5 chimeras and IGFBP-5 points mutants, we demonstrated that the IGFBP-5 C-domain is necessary and sufficient for its nuclear localization, and residues K206, K208, K217, and K218 are particularly critical. Intriguingly, inhibition of protein secretion abolishes IGFBP-5 nuclear localization, suggesting the nuclear IGFBP-5 is derived from the secreted protein. When added exogenously, 125 I- or Cy3-labeled IGFBP-5 is capable of cellular entry and nuclear translocation. To identify potential transcriptional factor(s) that interact with IGFBP-5, a human aorta cDNA library was screened by a yeast two-hybrid screening strategy. Although this screen identified many extracellular and cytosolic proteins that are known to interact with IGFBP-5, no known transcription factors were found. Further motif analysis revealed that the IGFBP-5 N-domain contains a putative transactivation domain. When fused to GAL-4 DNA dinging domain and tested, the IGFBP-5 N-domain has strong transactivation activity. Mutation of the IGF binding domain or treatment of cells with IGF-I has little effect on transactivation activity. These results suggest that IGFBP-5 is localized in VSMC nucleus and possesses transcription-regulatory activity that is IGF independent. The full text of this article is available online at http://circres.ahajournals.org.

Modulation of the actions of IGFs by IGFBP-5 in the mammary gland

IGFBP-5 has been associated with cell death in a number of systems; recently, the first evidence that it is involved in apoptosis of the mammary gland has been provided by studies, both in vivo and in vitro, involving the addition of exogenous IGFBP-5 and from a transgenic mouse expressing IGFBP-5 on a mammary-specific promoter. These studies have indicated that the effects are mediated in part by inhibition of IGF-signalling and involving members of the Bcl-2 family, but a role for IGF-independent effects cannot be ruled out. These IGF-independent effects involve potential interactions with components of the extracellular matrix involved in tissue remodelling such as components of the plasminogen system. In addition, intracellular events involving nuclear localisation of IGFBP-5 have been shown to have potential to inhibit cell proliferation. IGFBP-5 binds to a considerable number of molecules in the extracellular matrix, but the specific roles of these interactions in modulating its biological effects are poorly understood. The development of IGFBP-5 mutants, with differential binding characteristics, will aid in elucidating the precise roles of IGFBP-5 and potentially offer new therapeutic approaches based on IGF-independent effects in addition to its classical role of modulating IGF actions.

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.

Role of Insulin-Like Growth Factor-1 Signaling in Dental Fibroblast Apoptosis

BACKGROUND

Studies have shown that periodontal ligament fibroblasts (PDLF) and gingival fibroblasts (GF) respond differently to growth factors in the repair and regeneration of periodontal tissues. The goal of this study was to determine the effects of insulin-like growth factor-1 (IGF-1) signaling on cell apoptosis in PDLF compared to GF.

METHODS

The levels of apoptosis were compared between cultured PDLF and GF by DNA fragmentation assay and trypan blue exclusion assay, either in the presence or absence of IGF-1. The transcript level of upstream signaling molecules, such as IGF binding protein-5 (IGFBP-5), IGF-1 receptor (IGF-1R), and phosphoinositide 3-kinase (PI3K), was studied using reverse transcription-polymerase chain reaction (RT-PCR). Furthermore, the role of IGFBP-5 in IGF-1 signaling was verified by annexin-V staining using flow cytometric analysis.

RESULTS

IGF-1 significantly inhibited the level of DNA fragmentation and decreased trypan blue-positive cells in PDLF compared to GF during serum deprivation. The mRNA expression of IGFBP-5, IGF-1R, and PI3K was constitutively upregulated in PDLF compared to GF. In the presence of exogenous IGFBP-5, the annexin-V-positive cells were significantly decreased in GF after IGF-1 stimulation.

CONCLUSIONS

The present study provides evidence that IGF-1 reduces apoptosis in cultured PDLF compared to GF. Upregulation of IGF-1R and PI3K in PDLF further suggests the activation of IGF signaling in PDLF. In addition, the anti-apoptotic effect of IGF-1 may be facilitated by the upregulation of IGFBP-5 in PDLF.

Free insulin-like growth factor (IGF)-I and IGF binding proteins 2, 5, and 6 in spinal motor neurons in amyotrophic lateral sclerosis

BACKGROUND

Insulin-like growth factor-I (IGF-I) is a potent survival factor for motor neurons and is being investigated as possible therapeutic agent for amyotrophic lateral sclerosis. However, very little information is available on the components of the IGF-I system in this disease. Insulin-like growth factor binding proteins (IGFBPs) play an important part in regulating the bioavailability of IGF-I.

METHODS

We investigated the components of the IGF-I system in spinal cord sections of ten patients with amyotrophic lateral sclerosis and in ten controls without neurological disease. IGF-I was studied by western immunoblotting. IGFBPs and IGF-I receptors were investigated by immunohistochemistry and Western immunoblotting.

FINDINGS

Total IGF-I in ventral horn homogenates did not differ between patients and controls. However, free IGF-I was 53% lower in patients than in controls. Compared with controls, immunoreactivity in the spinal motor neurons of patients with amyotrophic lateral sclerosis was 64% higher for IGFBP2, 46% higher for IGFBP5, and 33% higher for IGFBP6, with upregulation of IGF-I receptors. Immunoreactivity for IGFBPs1, 3, and 4 did not differ between patients and controls.

INTERPRETATION

In the ventral horns of patients, free IGF-I is reduced, which could be because of specific increases in IGFBPs 2, 5, and 6 in spinal motor neurons. This abnormality might have an important role in the processes leading to motor neuron death, and should be taken into account when developing treatments aimed to stimulate IGF-I receptors in motor neurons.

Insulin-like growth factor binding protein-5 (IGFBP-5) induces premature cell death in the mammary glands of transgenic mice

We have previously demonstrated that IGFBP-5 production by mammary epithelial cells increases dramatically during involution of the mammary gland. To demonstrate a causal relationship between IGFBP-5 and cell death we created transgenic mice expressing IGFBP-5 in the mammary gland using a mammary-specific promoter, β-lactoglobulin. DNA content in the mammary glands of transgenic mice was decreased as early as day 10 of pregnancy. Histological analysis indicated reduced numbers of alveolar end buds, with decreased ductal branching. Transgenic dams produced IGFBP-5 in their milk at concentrations similar to those achieved at the end of normal lactation. Mammary cell number and milk synthesis were both decreased by approximately 50% during the first 10 days of lactation. BrdU labelling was decreased, whereas DNA ladders were increased in transgenic animals on day 1 of lactation. On day 2 postpartum, the epithelial invasion of the mammary fat pad was clearly impaired in transgenic animals. The concentrations of the pro-apoptotic molecule caspase-3 and of plasmin were both increased in transgenic animals whilst the concentrations of 2 prosurvival molecules Bcl-2 and Bcl-xLwere both decreased. In order to examine whether IGFBP-5 acts by inhibiting the survival effect of IGF-I we examined IGF receptor phosphorylation and Akt phosphorylation and showed that both were inhibited. We attempted to “rescue” the transgenic phenotype by using growth hormone to increase endogenous IGF-I concentrations or by implanting minipumps delivering an IGF-1 analogue, R3-IGF-1, which binds weakly to IGFBP-5. Growth hormone treatment failed to affect mammary development suggesting that increased concentrations of endogenous IGF-1 are insufficient to overcome the high concentrations of IGFBP-5 produced by these transgenic animals. In contrast mammary development (gland weight and DNA content) was normalised by R3-IGF-I although milk production was only partially restored. This is the first demonstration that over-expression of IGFBP-5 can lead to; impaired mammary development, increased expression of the pro-apoptotic molecule caspase-3, increased plasmin generation and decreased expression of pro-survival molecules of the Bcl-2 family. It clearly demonstrates that IGF-I is an important developmental/survival factor for the mammary gland and, furthermore, this cell death programme may be utilised in a wide variety of tissues.

Insulin-like growth factor-binding protein-5 (IGFBP-5) stimulates growth and IGF-I secretion in human intestinal smooth muscle by Ras-dependent activation of p38 MAP kinase and Erk1/2 pathways

Insulin-like growth factor-binding protein-5 (IGFBP-5) and insulin-like growth factor-I (IGF-I) are produced by human intestinal smooth muscle cells. Endogenous IGF-I stimulates growth and increases IGFBP-5 secretion. IGFBP-5 augments the effects of IGF-I by facilitating interaction of IGF-I with the IGF-I receptor tyrosine kinase. Andress (Andress, D. L. (1998) Am. J. Physiol. 274, E744-E750) and Berfield et al. (Berfield, A. K., Andress, D. L., and Abrass, C. K. (2000) Kidney Int. 57, 1991-2003) have shown that in osteoblasts and kidney mesangial cells, IGFBP-5 stimulates proliferation and filopodia formation independently of IGF-I, presumably by activating a distinct IGFBP-5 receptor serine kinase. The present study determined whether IGFBP-5 exerts direct effects on growth in human intestinal smooth muscle cells and identified the intracellular signaling pathways involved. IGFBP-5 caused a concentration-dependent increase in [(3)H]thymidine incorporation and an increase in IGF-I secretion that occurred independently of IGF-I and the IGF-I receptor tyrosine kinase. IGFBP-5-induced phosphorylation of p38 MAP kinase, which was abolished by SB203580, or expression of a dominant negative Ras mutant, Ras(S17N), and phosphorylation of Erk1/2, which was abolished by a Raf1 kinase inhibitor, U1026, or expression of Ras(S17N). IGFBP-5-stimulated [(3)H]thymidine incorporation and IGF-I secretion were partly inhibited by SB203580 or U1026 and abolished by the combination of the two inhibitors or by expression of Ras(S17N). These data show that IGFBP-5 stimulates growth and IGF-I secretion in human intestinal smooth muscle cells by activation of p38 MAP kinase-dependent and Erk1/2-dependent pathways that are independent of IGF-I. A positive feedback mechanism therefore links IGFBP-5 and IGF-I secretion that reinforces their individual effects on growth.

IGF-binding protein-5: flexible player in the IGF system and effector on its own

The multiple activities of IGF-I and -II are modulated by a family of IGF-binding proteins (IGFBP-1 to -6). Although structurally related, each IGFBP has unique properties and exerts specific functions. IGFBP-5 is the most conserved IGFBP across species and was identified as an essential regulator of physiological processes in bone, kidney and mammary gland. In addition, IGFBP-5 appears to play a decisive role in the control of proliferation of specific tumour cell types. In many situations IGFBP5 exerts biological activities in the absence of IGFs, indicating the existence of IGF-independent actions. This concept was supported by the unexpected localisation of IGFBP-5 in the nucleus and the description of IGFBP-5-specific membrane-bound IGFBP-5 receptor(s). The scope of this review is to summarise the available information about the structure of IGFBP-5 and the regulation of its expression. Furthermore, the potential significance of IGFBP-5 in the regulation of physiological processes will be critically analysed in the light of recent experimental data.

Vitronectin binding to IGF binding protein-5 (IGFBP-5) alters IGFBP-5 modulation of IGF-I actions

IGF binding protein-5 (IGFBP-5) and vitronectin are matricellular proteins that are produced by smooth muscle cells and modulate their responsiveness to IGF-I. These studies were conducted to determine if vitronectin bound IGFBP-5 with high affinity and if this altered the ability of either protein to modify cellular responsiveness to IGF-I. Solution binding assays were used to determine that vitronectin bound to IGFBP-5 with high affinity. This binding was inhibitable with glycosaminoglycans. Synthetic peptides that contained four distinct regions of the IGFBP-5 sequence were used in competitive binding assays to determine the regions of IGFBP-5 that were necessary for vitronectin binding. The regions that mediated the interaction were determined to be between amino acids 201 and 218 and between amino acids 131 and 141. Mutation of specific basic residues within these regions resulted in significant reduction in vitronectin binding and residues R134, R136, K138, K139, R201, and K202 were shown to be the most important. When the combination of IGFBP-5 and IGF-I was added to smooth muscle cells that had been plated on a vitronectin-enriched matrix, the smooth muscle cell DNA synthesis and migration responses to IGF-I plus vitronectin were enhanced. In contrast, if mutant forms of IGFBP-5 that did not bind to vitronectin were used, the responses were decreased. These effects appeared to be due to modulation of IGF-I action because the addition of a mutant form of IGFBP-5 that did not bind to IGF-I had no additional effect over and above that noted with vitronectin alone. These findings suggest that localization of IGFBP-5 within the extracellular matrix by vitronectin results in modification of cellular responsiveness to IGF-I and that vitronectin may be an important component of the extracellular matrix that modulates IGFBP-5 actions.

Roles of insulin-like growth factor-I (IGF-I) and IGF-I binding protein-2 (IGFBP2) and -5 (IGFBP5) in developing chick limbs

Insulin-like growth factor-I (IGF-I) and the IGF-I binding proteins (IGFBPs) which modulate IGF-I action have been implicated in the development of the vertebrate limbs and skeleton. We have examined the distribution of IGF-I, IGFBP2 and IGFBP5 in developing chick limb buds and have investigated their functional roles and relationships during chick limb development. IGF-I and IGFBP2 are co-expressed throughout the lateral plate from which limbs form, although IGFBP2, unlike IGF-I, does not promote formation of rudimentary limb buds from non-limb-forming flank regions in vitro. During limb outgrowth, IGF-I is present in non-AER limb ectoderm, but little IGF-I is present in the AER itself, suggesting that restriction of endogenous IGF-I activity may be required for proper AER function. Consistent with this possibility, the ectoderm of mutant limbless and wingless wing buds, which fail to form an AER, continues to express IGF-I. We also found that the AER contains abundant IGFBP2 but that IGFBP2 is not present in limb subridge mesoderm. In contrast, IGFBP2 is present in the distal mesoderm of mutant limbless or wingless limb buds, which fail to grow out. This suggests that attenuation of IGFBP2 expression is controlled by the AER and that cessation of IGFBP2 expression may be necessary for the proliferation and suppression of differentiation of subridge mesoderm that is required for limb outgrowth to occur. Consistent with this possibility, we found that exogenous IGFBP2 inhibits the anti-differentiative activity of the AER in vitro. We also found that regions of cell death in the limb contain abundant IGF-I-immunoreactive cells, consistent with a role for IGF-I in apoptosis. During skeletogenesis, IGF-I and IGFBP2 are co-localized to the condensing central core of the limb, implicating these factors as potential regulators of the onset of chondrogenic differentiation. Intriguingly, we found that IGF-I and IGFBP2 have opposing effects on chondrogenesis, as IGF-I stimulates but IGFBP2 inhibits accumulation of cartilage matrix by micromass cultures in vitro. Long [R(3)] IGF-I, an analog of IGF-I that cannot bind IGFBPs, is more effective than IGF-I in stimulating matrix accumulation, consistent with a negative role for IGFBP2 in chondrogenesis. As the chondrocytes of the limb mature, IGF-I is present only in terminal hypertrophic chondrocytes, which undergo programmed cell death, while IGFBP2 becomes localized to prehypertrophic and hypertrophic chondrocytes, suggesting involvement in chondrocyte maturation. Consistent with this possibility, we found that exogenous IGFBP2 induces precocious expression of Indian hedgehog, a marker of prehypertrophy, in maturing chondrocytes in vitro. IGF-I and IGFBP2 are also present in the osteoblasts, clasts and nascent matrix of the long bones, consistent with roles in endochondral bone formation. Unlike in rodent limbs, IGFBP5 is not expressed by chick limb ectoderm or AER. IGFBP5 expression is highly localized to developing limb musculature and, later, to the developing skeletal elements where it is expressed by osteoblast precursers and osteoblasts. The results of this study suggest potential novel roles for IGF-I and IGFBP2 in several aspects of limb development including limb outgrowth and AER activity, programmed cell death, chondrogenesis and chondrocyte maturation.

Differential gene expression in premalignant human trophoblast: role of IGFBP-5

Tumorigenesis results from genetic alterations that occur in a stepwise manner giving rise to cells with increasingly cancer-like characteristics. We used in vitro propagated first trimester human extravillous trophoblast (EVT) cells to identify genetic changes responsible for the transition of the EVT from a normal to premalignant stage. The model used consisted of a normal invasive EVT (HTR8) cell line and its premalignant derivative (RSVT2/C) generated by transfection with the SV40 Tag and selected using a forced crisis regimen. RSVT2/C display increased proliferative, migratory and invasive behavior, unresponsiveness to anti-proliferative and anti-invasive signals of TGFbeta and a deficiency in gap junctional intercellular communication. These cells, however, were unable to form colonies on soft agar or tumors in nude mice and are thus defined as premalignant. Differential display revealed 18 gene sequences, 7 with unknown and 11 with known identity, showing altered expression between the normal HTR8 and premalignant RSVT2/C cell lines. The known sequences include the potential tumor suppressors insulin-like growth factor binding protein (IGFBP)-5 and fibronectin (FN) and potential protooncogenes such as chromokinesin (KIF4), alternative splicing factor (SF2), dynein, DNA polymerase epsilon (DNApol epsilon) and NF-kappaB activating kinase (NAK). The role of the remaining 4 genes upregulated in the premalignant EVT is presently unknown and these are FK506 binding protein (FKBP) 25, histone protein (HP1Hs)-gamma, nucleoporin (Nup) 155 and an 82 kDa acidic human protein. The functional role of IGFBP-5 was examined in the control of proliferation, migration and invasiveness of RSVT2/C cells measured in vitro. IGFBP-5 alone had no effect on these properties of RSVT2/C cells. Furthermore, unlike normal EVT cells, RSVT2/C cells exhibited refractoriness to the migration stimulating signals of IGF-II, which was explained by the loss or downregulation of the IGF type 2 receptor (IGF-R2). RSVT2/C cells, however, expressed the IGF type 1 receptor (IGF-R1) and responded to IGF-I by increased proliferation. This response was blocked with increasing concentrations of IGFBP-5. These results suggest that the loss of IGFBP-5 and possibly IGF-R2, both of which can sequester IGF-I from IGF-R1, permits unhindered proliferation of the premalignant EVT in an IGF-I rich environment of the fetal-maternal interface. The functions of the other differentially expressed genes, some of which are essential for cell cycle progression or cell survival require further investigation.

Insulin-like growth factors (IGF-I and IGF-II) inhibit C2 skeletal myoblast differentiation and enhance TNF alpha-induced apoptosis

IGF-I and IGF-II are thought to be unique in their ability to promote muscle cell differentiation. Murine C2 myoblasts differentiate when placed into low serum media (LSM), accompanied by increased IGF-II and IGF binding protein-5 (IGFBP-5) production. Addition of 20 ng/ml TNF alpha on transfer into LSM blocked differentiation, IGF-II and IGFBP-5 secretion and induced apoptosis. We, therefore, wished to assess whether IGFs could protect against the effects of TNF alpha. Neither inhibition of differentiation or induction of apoptosis was rescued by co-incubation with IGF-I or IGF-II. A lower dose of TNF alpha (1 ng/ml) while not inducing apoptosis still inhibited myoblast differentiation by 56% +/- 12, (P < 0.001), indicating that induction of apoptosis is not the sole mechanism by which TNF alpha inhibits myoblast differentiation. Addition of IGF-I or IGF-II alone reduced differentiation by 49% +/- 15 and 33% +/- 20, respectively, (P < 0.001), although neither induced apoptosis. For muscle cells to differentiate, they must arrest in G0. We established that addition of IGF-I, IGF-II or TNF alpha to the myoblasts promoted proliferation. The myoblasts could not exit the cell cycle as efficiently as controls and differentiation was thus reduced. Unexpectedly, co-incubation of IGF-I or IGF-II with 1 ng/ml TNF alpha enhanced the inhibition of differentiation and induced apoptosis. In the absence of apoptosis we show an association between IGF-induced inhibition of differentiation and increased IGFBP-5 secretion. These results indicate that the effects of the IGFs on muscle may depend on the cytokine environment. In the absence of TNF alpha, the IGFs delay differentiation and promote myoblast proliferation whereas in the presence of TNF alpha the IGFs induce apoptosis.

Insulin-like growth factor binding protein-5 (IGFBP-5) potentially regulates programmed cell death and plasminogen activation in the mammary gland

This study aims to investigate the mechanism by which prolactin and GH interact to maintain mammary epithelial cell function in the rat. IGF-I is an important survival factor for the mammary gland and we have demonstrated that the effects of GH and prolactin involve IGF-I. GH acts by increasing IGF-I whilst prolactin acts by inhibiting the expression of IGFBP-5 from the mammary epithelium. During mammary involution, when serum prolactin levels decline, IGFBP-5 expression is dramatically upregulated and it binds with high affinity to IGF-I preventing IGF-I interaction with the IGF-receptor and thus leading to epithelial cell apoptosis. We have identified a specific interaction of IGFBP-5 with alpha s2-casein. This milk protein has also been shown to bind plasminogen and its activator tissue-type plasminogen activator (tPA) leading to enhanced conversion of plasminogen to plasmin. Plasmin is an important initiator of re-modelling of the extracellular matrix during mammary involution. A potential interaction between the cell death and extracellular matrix remodelling is evident from the observation that IGFBP-5 binds to plasminogen activator inhibitor-I (PAI-1). We thus hypothesized that IGFBP-5 could activate cell death by sequestration of IGF-I and activate plasminogen cleavage by sequestering PAI-1. In support of this hypothesis we have shown that both prolactin and GH inhibit tPA activity and plasminogen activation in the involuting mammary gland. Our results suggest that GH and prolactin inhibit cell death and ECM remodelling via the IGF-axis and also indicate a novel role for the milk protein alpha s2-casein in this process. We have now established lines of transgenic mice expressing IGFBP-5 on the beta-lactoglobulin promoter to explore its function in greater detail.

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.

Estrogen modulates osteoblast proliferation and function regulated by parathyroid hormone in osteoblastic SaOS-2 cells: role of insulin-like growth factor (IGF)-I and IGF-binding protein-5

Although there is clinical evidence showing that combined therapy with parathyroid hormone (PTH) and estrogen is additively effective in increasing the bone mass of patients with osteoporosis, the mechanism of the interaction between these hormones remains unclear. The present study was performed to determine whether estrogen would affect osteoblast proliferation and function modulated by PTH in human osteoblastic SaOS-2 cells. Human PTH-(1-34) significantly inhibited [(3)H]thymidine (TdR) incorporation, which was attenuated by 24 h pretreatment with 10(-10) to 10(-7) M 17 beta-estradiol (17 beta-E(2)) in a concentration-dependent manner. PTH significantly stimulated alkaline phosphatase (ALP) activity, collagen synthesis and type-1 procollagen mRNA expression after pretreatment with 17 beta-E(2 )in these cells. Tamoxifen, an anti-estrogen, antagonized these 17 beta-E(2)-induced effects. Pretreatment with insulin-like growth factor-I (IGF-I) mimicked estrogen action, and coincubation of 3 microg/ml anti-IGF-I antibody antagonized the effects of 17 beta-E(2 )as well as those of IGF-I. In the presence of 17 beta-E(2 )pretreatment, PTH strongly stimulated IGF-binding protein (IGFBP)-5 mRNA expression in these cells, and recombinant IGFBP-5 increased type-1 procollagen mRNA expression and ALP activity. In conclusion, estrogen attenuates PTH-induced inhibition of osteoblast proliferation and PTH stimulates osteoblast function in the presence of estrogen pretreatment. IGF-I and/or IGFBP-5 seemed to be involved in the estrogen-induced modulation of PTH action on osteoblast proliferation and function.

Differential regulation by FSH and IGF-I of extracellular matrix IGFBP-5 in bovine granulosa cells: effect of association with the oocyte

Inhibition of insulin-like growth factor (IGF)-I induced DNA synthesis in bovine oocyte-cumulus complexes (OCCs) caused by follicle-stimulating hormone (FSH) has been linked to changes in the extracellular matrix which do not occur in mural granulosa cells (MGCs). We investigated regulation by IGF-I and FSH of secreted and extracellular matrix entrapped IGF-binding proteins. OCCs and MGCs from bovine ovaries were cultured in media supplemented with IGF-I and FSH for 24 h. Culture media and extracellular matrix were analysed for IGF-binding proteins by Western ligand blot and immunoblot and found to contain principally IGFBP-3 and -5. The combined treatment of IGF-I and FSH increased the concentration of IGFBP-3 in OCC and MGC conditioned media by 4- and 6-fold, respectively. Treatment of OCCs and not MGCs with IGF-I and FSH together increased extracellular matrix IGFBP-5 by 2.5-fold. The differential regulation of extracellular matrix IGFBP-5 in OCCs compared to MGCs suggest involvement of changes in the extracellular matrix brought about by IGF-I and FSH in overall regulation of IGF-I in the ovarian follicle.

Overexpression of insulin-like growth factor binding protein-5 helps accelerate progression to androgen-independence in the human prostate LNCaP tumor model through activation of phosphatidylinositol 3'-kinase pathway

Although insulin-like growth factor (IGF) binding protein-5 (IGFBP-5) is highly up-regulated in normal and malignant prostate tissues after androgen withdrawal, its functional role in castration-induced apoptosis and androgen-independent progression remains undefined. To analyze the functional significance of IGFBP-5 overexpression in IGF-I-mediated mitogenesis and progression to androgen-independence, IGFBP-5-overexpressing human androgen-dependent LNCaP prostate cancer cells were generated by stable transfection. The growth rates of IGFBP-5-transfected LNCaP cells were significantly faster, compared with either the parental or vector-only transfected LNCaP cells in both the presence and absence ofdihydrotestosterone. IGFBP-5-induced increases in LNCaP cell proliferation occurs through both IGF-I-dependent and -independent pathways, with corresponding increases in the cyclin D1 messenger RNA expression and the fraction of cells in S + G2/M phases of the cell cycle. Changes in Akt/protein kinase B, a downstream component of phosphatidylinositol 3'-kinase (PI3K) pathway, in the LNCaP sublines also paralleled changes in their growth rates. Although treatment with a PI3K inhibitor induced apoptosis in both control and IGFBP-5-overexpressing LNCaP cells, this PI3K inhibitor-induced apoptosis was prevented by exogenous IGF-I treatment only in IGFBP-5 transfectants, suggesting that IGFBP-5 overexpression can potentiate the antiapoptotic effects of IGF-I. Furthermore, tumor growth and serum prostate-specific antigen levels increased several fold faster in mice bearing IGFBP-5-transfected LNCaP tumors after castration, despite having similar tumor incidence and tumor growth rates with controls when grown in intact mice before castration. Collectively, these data suggest that IGFBP-5 overexpression in prostate cancer cells after castration is an adaptive cell survival mechanism that helps potentiate the antiapoptotic and mitogenic effects of IGF-I, thereby accelerating progression to androgen independence through activation of the PI3K-Akt/ protein kinase B signaling pathway.

Insulin-like growth factor binding proteins: IGF-dependent and -independent effects in the mammary gland

The insulin-like growth factor binding proteins (IGFBPs) are a family of proteins which bind to the IGFs with high affinity. Their expression within the mammary gland is species specific; it has thus been difficult to determine the biological roles of these binding proteins during lactation. In this article we propose a role for IGFBP-5 in the mammary gland involving the initiation of apoptosis induced by sequestration of IGF-1, an important survival factor for the mammary gland. We have shown that this binding protein retains its high affinity for IGF-1 and that it is present in extremely high concentrations compared with the growth factor. These observations make it likely that IGFBP-5 is capable of preventing interaction of IGF-1 with its receptor on the epithelial cells synthesizing milk. We have also demonstrated that IGFBP-5 interacts with alpha(s2)-casein and that this interaction implicates it in the regulation of plasminogen activation in the mammary gland. The generation of plasmin is a key initiating event in the remodeling of the extracellular matrix during mammary involution. As such, IGFBP-5 may play a key role in coordinating cell death and tissue remodeling processes. Many of the molecules involved in embryological development are also expressed in the developing and involuting mammary gland. We believe that our studies may offer mechanistic explanations for apoptotic events in a wide variety of tissues. We have recently shown that IGFBP-5 is apoptotic in the chick embryonic limb bud, adding further support to our belief that IGFBP-5 serves this function in the mammary gland. We hope to be able to explore the role of this binding protein in the mammary gland with a transgenic mouse model expressing IGFBP-5 on the beta-lactoglobulin promoter.

IGFBP-3 and IGFBP-5 production by human intestinal muscle: reciprocal regulation by endogenous TGF-beta1

Insulin-like growth factor-I (IGF-I)-mediated growth of cells can be modulated by specific IGF binding proteins (IGFBPs) that inhibit or augment IGF-I ligand-receptor interaction. IGFBP expression and production by human intestinal muscle cells in culture was characterized in rapidly growing cells (day 3 of culture), in confluent cells (day 7), and in postconfluent cells (day 14). RT-PCR analysis identified IGFBP-3, IGFBP-4, and IGFBP-5 mRNA during all three phases of growth. The production of IGFBP-3 and IGFBP-5 was regulated in reciprocal fashion. IGFBP-5 production was high on day 3 and decreased two- to fivefold by day 14, and IGFBP-3 production was low on day 3 and increased five- to eightfold by day 14. IGFBP-4 production remained constant. IGFBP-3 inhibited and IGFBP-5 augmented IGF-I-induced proliferation. IGFBP-3 and IGFBP-5 production was regulated in reciprocal fashion by transforming growth factor-beta1 (TGF-beta1). Immunoneutralization of endogenous TGF-beta1 decreased the production of IGFBP-3 and increased the production of IGFBP-5. Addition of exogenous recombinant human TGF-beta1 had the opposite effect. We conclude that the expression and time-dependent production of IGFBP-3, IGFBP-4, and IGFBP-5 and their regulation by endogenous TGF-beta1 represent mechanisms by which human intestinal muscle cells regulate autocrine IGF-I-mediated growth.

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.

A role for insulin-like growth factor binding protein 5 in the antiproliferative action of the antiestrogen ICI 182780

Insulin-like growth factors (IGFs) are potent mitogens for breast cancer cells. Although IGF-binding proteins (IGFBPs) are known to regulate access of IGFs to IGF receptors, their precise biological actions are poorly defined. We observed that the potent antiestrogen ICI 182780 (ICI) increased IGFBP-5 mRNA by 2-3-fold in 9,10-dimethyl-1,2-benzanthracene-induced mammary tumors in vivo. In vitro studies showed that growth inhibition of MCF-7 human breast cancer cells induced by ICI was associated with increased transcription of the IGFBP-5 gene, increased IGFBP-5 mRNA abundance, and increased IGFBP-5 protein accumulation in the conditioned medium. Growth stimulation following estradiol exposure was associated with opposite effects. An IGFBP-5 antisense oligodeoxynucleotide significantly decreased IGFBP-5 accumulation in conditioned media and enhanced MCF-7 cell DNA synthesis. Furthermore, this antisense oligodeoxynucleotide attenuated both antiestrogen-induced IGFBP-5 accumulation and antiestrogen-induced growth inhibition. These data demonstrate that estradiol down-regulates and ICI up-regulates an autocrine IGFBP-5 growth inhibitory pathway in MCF-7 cells and suggest that IGFBP-5 plays a role in modulation of proliferation of breast cancers by estrogens and antiestrogens.

The role of the insulin-like growth factor binding proteins and the IGFBP proteases in modulating IGF action

Over the past few years, there has been an explosion of data in the scientific literature regarding the various components of the IGF axis. IGFBPs and related molecules are now believed to be critical elements in numerous cellular processes and key factors in several disease states related to abnormal tissue and somatic growth. Recently, the BP-Prs were included in this complex system, and their importance is being unraveled. The upcoming years will undoubtedly bring even more information on the molecular biology of these key cellular regulators. These discoveries are likely to lead to better understanding of growth and cellular regulation and to development of novel therapeutic approaches to a variety of diseases.

Insulin-like growth factor-binding proteins produced by Vero cells, human oviductal cells and human endometrial cells, and the role of insulin-like growth factor-binding protein-3 in mouse embryo co-culture systems

Co-culturing embryos on helper cells can mimic the in-vivo environment, thereby enhancing embryo development in vitro. Insulin-like growth factors (IGF) and their binding proteins (IGFBP) also enhance embryo development. To investigate the kinds of IGFBP produced by various cell monolayers and the effects of IGFBP-3 on mouse embryo co-culture systems, 2-cell ICR mouse embryos were cultured in either human tubal fluid medium alone or in the presence of Vero cells, human oviductal cells or endometrial cells. The helper cells were analysed immunohistochemically to investigate the types of IGFBP produced by various cell monolayers. The concentrations of IGF-I and IGFBP-3 in media obtained from the culture of embryos alone, cells alone or cells plus embryos were determined by radioimmunoassays. On day 7, more blastocysts hatched in the co-culture groups (73% in the Vero cell group, 76% in the endometrial cell group and 74% in the oviductal cell group) than in the control group (43%) (P < 0.0001). The results of immunohistochemistry revealed that (i) all three cell groups produced a lot of IGFBP-1, -2 and -3, but only a little of IGFBP-4 and -5; and (ii) IGFBP-1, -2, and -3 were present in blastocysts in either the presence or absence of helper cells. The IGF-I secreted by cell monolayers or embryos was undetectable (detection limit 0.83 microg/l). The IGFBP-3 concentrations in media obtained from co-cultured embryos and cells were significantly higher than in media without embryos (median values in oviductal cell culture medium, 165 versus 127 microg/l, P = 0.04; median values in endometrial cell culture medium, 277.5 versus 183.5 microg/1, P = 0. 0002; median values in Vero cell culture medium, 219 versus 120 microg/l, P = 0.011). Although IGFBP-3 concentration in the medium that contained embryos alone was undetectable by radioimmunoassay (detection limit 1.1 microg/l), immunohistochemistry demonstrated the presence of IGFBP-3 in the embryos. Co-culture in systems in which there was an increased production of IGFBP-3 led to an improved development of mouse embryos. IGFBP can improve the binding of IGF to cell surface receptors of target tissue, and thus enhance the effect of limited IGF concentrations in promoting embryo development in a co-culture system. We conclude that Vero cells, human endometrial cells and oviductal cells produce IGFBP-1, -2, -3, -4 and -5. IGFBP-3 may play a role in embryotrophic potential by either regulating the action of IGF or directly enhancing embryo development.