Skeletal growth factors regulate the synthesis of insulin-like growth factor binding protein-5 in bone cell cultures

Pituitary Diseases and Bone

Neuroendocrinology of bone is a new area of research based on the evidence that pituitary hormones may directly modulate bone remodeling and metabolism. Skeletal fragility associated with high risk of fractures is a common complication of several pituitary diseases such as hypopituitarism, Cushing disease, acromegaly, and hyperprolactinemia. As in other forms of secondary osteoporosis, pituitary diseases generally affect bone quality more than bone quantity, and fractures may occur even in the presence of normal or low-normal bone mineral density as measured by dual-energy X-ray absorptiometry, making difficult the prediction of fractures in these clinical settings. Treatment of pituitary hormone excess and deficiency generally improves skeletal health, although some patients remain at high risk of fractures, and treatment with bone-active drugs may become mandatory. The aim of this review is to discuss the physiological, pathophysiological, and clinical insights of bone involvement in pituitary diseases.

Hierarchical scaffolds enhance osteogenic differentiation of human Wharton's jelly derived stem cells

Hierarchical structures, constituted by polymeric nano and microfibers, have been considered promising scaffolds for tissue engineering strategies, mainly because they mimic, in some way, the complexity and nanoscale detail observed in real organs. The chondrogenic potential of these scaffolds has been previously demonstrated, but their osteogenic potential is not yet corroborated. In order to assess if a hierarchical structure, with nanoscale details incorporated, is an improved scaffold for bone tissue regeneration, we evaluate cell adhesion, proliferation, and osteogenic differentiation of human Wharton's jelly derived stem cells (hWJSCs), seeded into hierarchical fibrous scaffolds. Biological data corroborates that hierarchical fibrous scaffolds show an enhanced cell entrapment when compared to rapid prototyped scaffolds without nanofibers. Furthermore, upregulation of bone specific genes and calcium phosphate deposition confirms the successful osteogenic differentiation of hWJSCs on these scaffolds. These results support our hypothesis that a scaffold with hierarchical structure, in conjugation with hWJSCs, represents a possible feasible strategy for bone tissue engineering applications.

IGF binding proteins in cancer: mechanistic and clinical insights

The six members of the family of insulin-like growth factor (IGF) binding proteins (IGFBPs) were originally characterized as passive reservoirs of circulating IGFs, but they are now understood to have many actions beyond their endocrine role in IGF transport. IGFBPs also function in the pericellular and intracellular compartments to regulate cell growth and survival - they interact with many proteins, in addition to their canonical ligands IGF-I and IGF-II. Intranuclear roles of IGFBPs in transcriptional regulation, induction of apoptosis and DNA damage repair point to their intimate involvement in tumour development, progression and resistance to treatment. Tissue or circulating IGFBPs might also be useful as prognostic biomarkers.

The role of liver-derived insulin-like growth factor-I

IGF-I is expressed in virtually every tissue of the body, but with much higher expression in the liver than in any other tissue. Studies using mice with liver-specific IGF-I knockout have demonstrated that liver-derived IGF-I, constituting a major part of circulating IGF-I, is an important endocrine factor involved in a variety of physiological and pathological processes. Detailed studies comparing the impact of liver-derived IGF-I and local bone-derived IGF-I demonstrate that both sources of IGF-I can stimulate longitudinal bone growth. We propose here that liver-derived circulating IGF-I and local bone-derived IGF-I to some extent have overlapping growth-promoting effects and might have the capacity to replace each other (= redundancy) in the maintenance of normal longitudinal bone growth. Importantly, and in contrast to the regulation of longitudinal bone growth, locally derived IGF-I cannot replace (= lack of redundancy) liver-derived IGF-I for the regulation of a large number of other parameters including GH secretion, cortical bone mass, kidney size, prostate size, peripheral vascular resistance, spatial memory, sodium retention, insulin sensitivity, liver size, sexually dimorphic liver functions, and progression of some tumors. It is clear that a major role of liver-derived IGF-I is to regulate GH secretion and that some, but not all, of the phenotypes in the liver-specific IGF-I knockout mice are indirect, mediated via the elevated GH levels. All of the described multiple endocrine effects of liver-derived IGF-I should be considered in the development of possible novel treatment strategies aimed at increasing or reducing endocrine IGF-I activity.

Growth hormone, insulin-like growth factors, and the skeleton

GH and IGF-I are important regulators of bone homeostasis and are central to the achievement of normal longitudinal bone growth and bone mass. Although GH may act directly on skeletal cells, most of its effects are mediated by IGF-I, which is present in the systemic circulation and is synthesized by peripheral tissues. The availability of IGF-I is regulated by IGF binding proteins. IGF-I enhances the differentiated function of the osteoblast and bone formation. Adult GH deficiency causes low bone turnover osteoporosis with high risk of vertebral and nonvertebral fractures, and the low bone mass can be partially reversed by GH replacement. Acromegaly is characterized by high bone turnover, which can lead to bone loss and vertebral fractures, particularly in patients with coexistent hypogonadism. GH and IGF-I secretion are decreased in aging individuals, and abnormalities in the GH/IGF-I axis play a role in the pathogenesis of the osteoporosis of anorexia nervosa and after glucocorticoid exposure.

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.

Characterization of human fetal osteoblasts by microarray analysis following stimulation with 58S bioactive gel-glass ionic dissolution products

Bioactive glasses dissolve upon immersion in culture medium, releasing their constitutive ions in solution. There is evidence suggesting that these ionic dissolution products influence osteoblast-specific processes. Here, we investigated the effect of 58S sol-gel-derived bioactive glass (60 mol % SiO2, 36 mol % CaO, 4 mol % P2O5) dissolution products on primary osteoblasts derived from human fetal long bone explant cultures (hFOBs). We used U133A human genome GeneChip oligonucleotide arrays to examine 22,283 transcripts and variants, which represent over 18,000 well-substantiated human genes. Hybridization of samples (biotinylated cRNA) derived from monolayer cultures of hFOBs on the arrays revealed that 10,571 transcripts were expressed by these cells, with high confidence. These included transcripts representing osteoblast-related genes coding for growth factors and their associated molecules or receptors, protein components of the extracellular matrix (ECM), enzymes involved in degradation of the ECM, transcription factors, and other important osteoblast-associated markers. A 24-h treatment with a single dosage of ionic products of sol-gel 58S dissolution induced the differential expression of a number of genes, including IL-6 signal transducer/gp130, ISGF-3/STAT1, HIF-1 responsive RTP801, ERK1 p44 MAPK (MAPK3), MAPKAPK2, IGF-I and IGFBP-5. The over 2-fold up-regulation of gp130 and MAPK3 and down-regulation of IGF-I were confirmed by real-time RT-PCR analysis. These data suggest that 58S ionic dissolution products possibly mediate the bioactive effect of 58S through components of the IGF system and MAPK signaling pathways.

Fibroblast growth factor signaling controlling osteoblast differentiation

Fibroblast growth factors (FGFs) play important roles in skeletal development and postnatal osteogenesis. FGF signaling controls bone formation by regulating the expression of various genes involved in osteoprogenitor cell replication, osteoblast differentiation and apoptosis. Recent genetic manipulation of FGF expression in mice and studies of the phenotype induced by gain-of-function mutations in FGF receptors in humans revealed the important role of FGF signaling in osteoblast function and differentiation. Additionally, cell biology studies allowed to identify some signaling pathways that are involved in the control of FGF actions in osteoblasts. This led to a better understanding of the functional role of FGF signaling in the control of gene expression in osteoblasts. The elucidation of molecular mechanisms by which FGF signaling promotes osteoblast gene expression and differentiation may help to find novel molecular targets and develop new therapeutic approaches to promote bone formation in human bone disorders.

Expression of insulin-like growth factor-binding protein 5 in neuroblastoma cells is regulated at the transcriptional level by c-Myb and B-Myb via direct and indirect mechanisms

Neuroblastoma (NB), a malignant childhood tumor deriving from the embryonic neural crest, is sensitive to the growth-stimulating effects of insulin-like growth factors (IGFs). Aggressive cases of this disease often acquire autocrine loops of IGF production, but the mechanisms through which the different components of the IGF axis are regulated in tumor cells remain unclear. Upon conditional expression of c-Myb in a NB cell line, we detected up-regulation of IGF1, IGF1 receptor, and insulin-like growth factor-binding protein 5 (IGFBP-5) expression. Analysis of the IGFBP-5 promoter revealed two potential Myb binding sites at position -59 to -54 (M1) and -429 to -424 (M2) from the transcription start site; both sites were bound by c-Myb and B-Myb in vitro and in vivo. Reporter assays carried out using the proximal region of the human IGFBP-5 promoter demonstrated that c-Myb and B-Myb enhanced transcription. However, site-directed mutagenesis and deletion of the Myb binding sites coupled with reporter assays revealed that M2 but not M1 was important for Myb-dependent transactivation of the IGFBP-5 promoter. The double mutant M1/M2 was still transactivated by c-Myb, suggesting the existence of Myb binding-independent mechanisms of IGFBP-5 promoter regulation. A constitutively active AKT transactivated the IGFBP-5 promoter, whereas the phosphatidylinositol 3-kinase inhibitor LY294002 suppressed it. Moreover, the kinase dead dominant negative K179M AKT mutant was able to inhibit transcription from the M2 and M1/M2 IGFBP-5 mutant promoters. Deletion analysis of the IGFBP-5 promoter revealed that the AKT-responsive region lies between nucleotides -334 and -83. Together, these data suggest that the Myb binding-independent transactivation of the IGFBP-5 promoter was due to the activation of the phosphatidylinositol 3-kinase/AKT pathway likely mediated by IGF1 receptor-dependent signals. Finally, IGFBP-5 was able to modulate proliferation of NB cells in a manner dependent on its concentration and on the presence of IGFs.

Expression of insulin-like growth factor binding proteins and collagen in experimental colitis in rats

OBJECTIVES AND DESIGN

Crohn's disease is complicated by smooth muscle hyperplasia and stricture formation. Insulin-like growth factor (IGF)-1 and insulin-like growth factor binding proteins (IGFBPs) may be involved in stimulating intestinal smooth muscle growth and collagen synthesis. Therefore, we investigated the expression of IGFBPs, collagen and collagenase activity in rat colitis and the effects of IGF-1 on IGFBP and collagen expression in rat colonic smooth muscle cells.

METHODS

Animals were sacrificed during a 4-week time course of 2,4,6-trinitrobenzenesulphonic acid (TNBS)-induced colitis. RNA from the animals' colons was blotted and hybridized with collagen-1 and IGFBP mRNA probes. Tissue proteins were screened for IGFBPs by Western ligand blotting. Collagenase activity was measured by zymography. Rat colonic smooth muscle cells in primary culture were incubated with IGF-1 then collagen-1, and IGFBP mRNAs and proteins were measured.

RESULTS

In the rat tissue, IGFBP-3 mRNA and protein were increased 2 h after induction of colitis. IGFBP-4 mRNA was elevated after 2 h and IGFBP-4 protein after 4 h. IGFBP-5 mRNA was upregulated after 2 h with a peak at 12 h. IGFBP-5 protein was upregulated after 1 h and reached a peak at 3 days. Collagen-1 mRNA was increased after 5 days. Collagenase levels were decreased after 1 h and returned to normal by 28 days. In rat colonic smooth muscle cells, IGF-1 increased collagen-1 and IGFBP-5 expression.

CONCLUSION

We demonstrated an upregulation of IGFBP and collagen expression and a downregulation of collagenase in rat colitis. In colonic smooth muscle cells, we found an upregulation of collagen-1 and IGFBP-5 following IGF-1 incubation. These results suggest an important role of IGF-1 in the collagen synthesis in colitis, mediated by IGFBPs.

The IGFBP-3 mRNA and protein levels are IGF-I-dependent and GH-independent in MG-63 human osteosarcoma cells

Growth Hormone (GH), Insulin-like Growth Factors (IGFs) and IGF-Binding Proteins which modulate the IGFs' bioavailability (e.g. IGFBP-3, -4, -5), are essential regulators of bone remodeling. In this study, MG-63 human osteosarcoma cells were used as a model system to investigate the mechanism(s) whereby IGF-I and GH control IGFBP-3 gene expression. Physiological concentrations of IGF-I (1-20 nM) induced a dose-dependent increase in the steady-state amount of IGFBP-3 mRNA (maximal stimulation: approximately 9-10-fold). This increase was detectable 3 h after the onset of IGF-I treatment, was enhanced over a 24 h period, then plateaued until at least 30 h. Consistently, a dose-dependent increase in IGFBP-3 secretion ( approximately 40-50-fold at IGF-I concentrations>/=16 nM) was observed by western ligand- and immuno-blot analysis of MG-63 cells conditioned medium, and its time course was similar to that observed for IGFBP-3 transcripts. IGFBP-3 mRNA stability (t(1/2) approximately 20 h) was identical in the presence or absence of IGF-I treatment. By contrast, human (h) GH treatment (24-72 h) of MG-63 cells did not increase IGFBP-3 secretion in the conditioned medium. Ectopic expression of recombinant rat GH-R resulted in hGH-enhanced expression of GH-responsive reporter gene constructs, but did not increase endogenous IGFBP-3 gene expression, suggesting that the GH unresponsiveness was not only due to the very low level of GH binding sites at the plasma membrane level. Altogether, these results support the conclusions that in MG-63 cells (i) transcriptional rather post-transcriptional mechanisms are involved in the IGF-I-induced increase of IGFBP-3; (ii) the abundance of GH-R is very low at the plasma membrane level; (iii) the dowstream GH-signaling cascade is fully functional in this human osteosarcoma cell line; and (iv) the endogenous IGFBP-3 gene is not responsive to hGH in human MG-63 osteosarcoma cells.

Identification of an osteogenic protein-1 (bone morphogenetic protein-7)-responsive element in the promoter of the rat insulin-like growth factor-binding protein-5 gene

Osteogenic protein-1 (OP-1), a member of the bone morphogenetic protein subfamily of the transforming growth factor-beta superfamily, induces new bone formation in vivo and regulates the expression of numerous growth factors. We previously showed that OP-1 down-regulates the transcription of the insulin-like growth factor-binding protein-5 (IGFBP-5) in primary cultures of fetal rat calvaria (FRC) cells. In the present study we identified, within the IGFBP-5 promoter, a 21-bp region that confers OP-1 responsiveness in FRC cells. Within this region lie three putative cis-acting regulatory elements, viz. a CAAT-like sequence, a CCAAT/enhancer-binding protein (C/EBPalpha)-like element, and a c-Myb or E-box-like motif. Mutations in the CAAT-like sequence reduced the promoter activity in both control and OP-1-treated cells, but did not abrogate the OP-1-induced down-regulation. Mutations in the C/EBPalpha-like element reduced the promoter activity in both control and OP-1-treated cells without significantly affecting the extent of down-regulation. Mutations in the putative c-Myb or E-box-like motif reduced the promoter activity in both the OP-1-treated and control cells and completely abolished the inhibitory effect of OP-1 on the IGFBP-5 promoter activity. Gel mobility shift analyses further showed specific interaction between nuclear protein(s) in FRC cells and the 21-bp region. OP-1 down-regulates the nuclear regulatory protein interaction with the 21-bp region by reducing either the cellular concentration of the regulatory protein(s) or the affinity of the regulatory protein(s) for the OP-1 responsive element. In conclusion, we identified an OP-1 response region in the rat IGFBP-5 promoter and further showed that OP-1 down-regulates the nuclear protein interaction with the response element(s).

Regulation of insulin-like growth factor binding protein-5 mRNA abundance in rat intestinal smooth muscle

IGF-I increases abundance of IGFBP-5 mRNA in rat intestinal smooth muscle cells (RISM), and IGFBP-5 protein in RISM conditioned media. The translational blocker, cycloheximide, decreased the abundance of IGFBP-5 mRNA to undetectable levels, suggesting that IGFBP-5 mRNA integrity is linked to protein synthesis. We studied the mechanism of IGF-I's effect on IGFBP-5 mRNA, and the role of cytoplasmic proteins in modulating IGFBP-5 mRNA abundance. Anisomycin, emetine, and puromycin abolished IGFBP-5 mRNA as seen with cycloheximide. Cycloheximide had a dose- and time-dependent effect on IGFBP-5 mRNA. IGF-I increased IGFBP-5 nuclear transcripts by reverse transcription-polymerase chain reaction (RT-PCR), suggesting that IGF-I acts at least partially by increasing IGFBP-5 mRNA transcription. Protein synthesis inhibitors did not affect IGFBP-5 nuclear transcripts, therefore, they affect only mature mRNA. The IGFBP-5 mRNA 3' and 5' UTRs were cloned and their sequences searched for adenosine-uridine rich elements (AUREs), elements shown to regulate RNA stability. RNA mobility gel shift assay showed two protein activities that bind to nt 922 to 2076 of the 3' UTR, a region that contains an AURE. One protein activity (BA2) was decreased in cytoplasmic extracts from cycloheximide-treated RISM. These data demonstrate that IGFBP-5 mRNA integrity is dependent on protein synthesis. The 3' UTR of IGFBP-5 contains elements shown to bind proteins important for RNA stability regulation. This region binds RISM cytoplasmic proteins, and may mediate the dramatic effect of cycloheximide on IGFBP-5 abundance. RNA-protein interactions may be important to IGFBP-5 mRNA stability and ultimately, to IGFBP-5 actions.

Effects of androgens on the insulin-like growth factor system in an androgen-responsive human osteoblastic cell line

Although androgens have significant effects on bone metabolism, the mediators of their effects are still unclear. As the insulin-like growth factors (IGFs) and IGF-binding proteins (IGFBPs) have important effects on osteoblast proliferation and differentiation, we examined androgen effects on the IGF system in a conditionally immortalized human fetal osteoblastic cell line, hFOB/AR-6, which displays a mature osteoblastic phenotype and physiological levels of functional androgen receptors. The nonaromatizable androgen, 5alpha-dihydrotestosterone (5alphaDHT), and testosterone, but not dehydroepiandrosterone, increased IGF-I messenger RNA (mRNA) levels up to 4-fold in a dose (10(-12)-10(-6) M)- and time (2-72 h)-dependent fashion. These changes were prevented by the specific androgen receptor antagonist, hydroxyflutamide. In addition, 5alpha-DHT decreased IGFBP-4 mRNA and protein levels by 2- and 4-fold, respectively, and increased IGFBP-2 and -3 mRNA and protein levels by 6- and 7-fold (for mRNA) and 3- and 5-fold (for protein), respectively. hFOB/AR-6 cells expressed the type-I IGF receptor, but this was not regulated by 5alphaDHT. 5alphaDHT and IGFBP-3 specifically increased hFOB/AR-6 cell proliferation, and a monoclonal antibody specific for IGF-I blocked this effect. Thus, androgens increase the expression of IGF-I, IGFBP-2, and IGFBP-3, but decrease levels of the inhibitory IGFBP-4 in an androgen-responsive human osteoblastic cell line. Our data are consistent with the hypothesis that the effects of androgen on bone cells may be mediated at least in part by increases in IGF-I production and by differential regulation of IGFBPs.

Novel aspects of the insulin-like growth factor binding proteins

The insulin-like growth factors (IGFs), IGF binding proteins (IGFBPs), and IGFBP proteases regulate somatic growth and cellular proliferation both in vivo and in vitro. IGFs are potent mitogens whose actions are determined by the availability of free IGFs to interact with IGF receptors. IGFBPs comprise a family of six proteins that bind IGFs with high affinity and specificity and thereby regulate IGF-dependent actions. IGFBPs have also recently emerged as IGF-independent regulators of cell growth. Several IGFBP association proteins have been discovered recently which can affect IGFBP action. Cleavage of IGFBPs by specific proteases modulates levels of free IGFs and IGFBPs and thereby their actions. IGFBP-related proteins (IGFBP-rPs) are an emerging group of proteins which bind IGFs with low affinity and also play important roles in cell growth and differentiation. The IGFBPs appear to have emerging roles in the mechanisms underlying human cancer. The GH-IGF-IGFBP axis is complex and powerful. Future research on its physiology promises exciting insights into cell biology as well as advancements in the treatment of a wide range of disease states including cancer, diabetes, vascular disease, asthma, and growth disorders.

The megencephaly mouse has disturbances in the insulin-like growth factor (IGF) system

Megencephaly, enlarged brain, is a major sign in several human neurological diseases. The mouse model for megencephaly, mceph/mceph, has an enlarged brain and a lowered body weight. In addition, it displays several neurological and motoric disturbances. Previous studies suggest that the brain enlargement results from hypertrophy of the brain cells, rather than hyperplasia. No structural abnormalities, edema or increased myelination have been found. In this study, a major imbalance in the mRNA expression of molecules in the insulin-like growth factor (IGF) system was found in brains of 9-10 weeks old mceph/mceph mice compared to +/+ wild-type mice. In mceph/mceph brains, we found upregulation of IGF binding proteins (BP)-2, -4, -5, and -6 mRNA, the regulating hormone transforming growth factor (TGF)beta1 mRNA and also a local downregulation of IGFBP-5 mRNA compared to wild-type brains by in situ hybridization. The altered expression of these mRNA species is colocalized in cerebral cortex, hippocampus, amygdala and piriform/entorhinal cortex. The mceph/mceph mice express less of the myelin component proteolipid protein (PLP) mRNA in corpus callosum. No expression difference of the housekeeping gene glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in brain or IGF system components in liver was found between mceph/mceph and wild-type mice. These data suggest that the IGF system has an important role in the excessive growth of the mceph/mceph brains.

Prostaglandin E1 and recombinant bone morphogenetic protein effect on strength of hydroxyapatite implants

Although combinations of hydroxyapatite (HAP) and bone morphogenetic protein (BMP) are expected to provide potent alternatives to autogenous bone grafts, it is still anticipated that substances that act synergistically with BMP will be found because the inducing potential of purified BMP in bone is not strong enough. We already have shown that prostaglandin (PG) E1 has a strong and dose-dependent synergistic effect on the osteoinductive activity induced by recombinant human (rh) BMP and that it enhances osteoconduction even when used alone. In this study, porous HAP rods were treated as follows: (1) without PGE1 or rhBMP (control group); (2) with varying concentrations of PGE1; and (3) with varying concentrations of PGE1 combined with 1 microg of rhBMP-2. The rods were subperiosteally implanted on the cranial bone of rabbits to evaluate the effect of these treatments on the mechanical strength of the implanted HAP rods. The HAP rods were removed 3, 6, or 9 weeks after implantation and subjected to mechanical strength determinations. The control group (no addition of BMP to the rods) showed no significant increase in three-point bending strength or in compression strength compared to pre-implantation. On the other hand, PGE1 combined with rhBMP had a strong and dose-dependent effect on the mechanical strength of HAP, increasing it significantly, especially compression strength. PGE1 also increased mechanical strength even when used alone. Histological examination revealed that PGE1, whether or not it was combined with rhBMP, increased bone formation into the pores of HAP and consequently increased the mechanical strength of porous HAP.

Insulin-like growth factor binding proteins: new proteins, new functions

The insulin-like growth factors (IGFs), IGF binding proteins (IGFBPs), and IGFBP proteases regulate somatic growth and cellular proliferation both in vivo and in vitro. IGFs are potent mitogens whose actions are determined by the availability of free IGFs to interact with IGF receptors. IGFBPs comprise a family of six proteins that bind IGFs with high affinity and specificity and thereby regulate IGF-dependent actions. IGFBPs have recently emerged as IGF-independent regulators of cell growth. Cleavage of IGFBPs by specific proteases modulate levels of free IGFs and IGFBPs and thereby their actions. IGFBP-related proteins (IGFBP-rPs) bind IGFs with low affinity and also play important roles in cell growth and differentiation. The GH-IGF-IGFBP axis is complex and powerful. Future research on its physiology promises exciting insights into cell biology as well as therapies for diseases such as cancer and diabetes mellitus.

Cellular actions of insulin-like growth factor binding proteins

The insulin-like growth factors (IGFs), insulin-like growth factor binding proteins (IGFBPs), and the IGFBP proteases are involved in the regulation of somatic growth and cellular proliferation both in vivo and in vitro. IGFs are potent mitogenic agents whose actions are determined by the availability of free IGFs to interact with the IGF receptors. IGFBPs comprise a family of proteins that bind IGFs with high affinity and specificity and thereby regulate IGF-dependent actions. IGFBPs have recently emerged as IGF-independent regulators of cell growth. Various IGFBP association proteins as well as cleavage of IGFBPs by specific proteases modulate levels of free IGFs and IGFBPs. The ubiquity and complexity of the IGF axis promise exciting discoveries and applications for the future.

Insulin-like growth factors sustain insulin-like growth factor-binding protein-5 expression in osteoblasts

Insulin-like growth factors (IGFs) I and II are considered to be autocrine regulators of bone cell function. Recently, we demonstrated that IGF-I induces IGF-binding protein-5 (IGFBP-5) expression in cultures of osteoblast-enriched cells from 22-day fetal rat calvariae (Ob cells). In the present study, we postulated that IGFs play an autocrine role in the maintenance of IGFBP-5 basal expression in Ob cells. IGFBP-2 and -3, at concentrations that bind endogenous IGFs, decreased IGFBP-5 mRNA levels, as determined by Northern blot analysis, and protein levels, as determined by Western immunoblots of extracellular matrix extracts of Ob cells. IGFBP-2 and -3 in excess inhibited IGFBP-5 heterogeneous nuclear RNA levels, as determined by RT-PCR, and did not alter the half-life of IGFBP-5 mRNA in transcriptionally arrested Ob cells. In conclusion, blocking endogenous IGFs in Ob cells represses IGFBP-5 expression, suggesting that IGFs are autocrine inducers of IGFBP-5 synthesis in osteoblasts.

Spaceflight modulates insulin-like growth factor binding proteins and glucocorticoid receptor in osteoblasts

Rat osteoblasts were cultured for 4 or 5 days during a Space Shuttle mission. After 20-h treatment with 1alpha,25-dihydroxyvitamin D3, conditioned media were harvested and cellular DNA and/or RNA were fixed on board. The insulin-like growth factor binding protein (IGF BP)-3 levels in the media were three- and tenfold higher than in ground controls on the fourth and fifth flight days, as quantitated by Western ligand blotting and radioimmunoassay, respectively. The increased IGF BP-3 protein levels correlated with two- to threefold elevation of IGF BP-3 mRNA levels, obtained by reverse transcription-polymerase chain reaction. The IGF BP-5 mRNA levels in flight cultures were 33-69% lower than in ground controls. The IGF BP-4 mRNA levels in flight cultures were 75% lower than in ground controls on the fifth day but were not different on the fourth day. The glucocorticoid receptor mRNA levels in flight cultures were increased by three- to eightfold on the fourth and fifth days compared with levels in ground controls. These data suggest potential mechanisms underlying spaceflight-induced osteopenia.

Osteogenic protein-1 regulates insulin-like growth factor-I (IGF-I), IGF-II, and IGF-binding protein-5 (IGFBP-5) gene expression in fetal rat calvaria cells by different mechanisms

Osteogenic protein-1 (OP-1 or BMP-7) stimulates new bone formation in vivo and induces cell proliferation and differentiation of osteoblasts in vitro. Previous studies from our laboratory revealed that OP-1 led to a two- to threefold increase in steady-state insulin-like growth factor-I (IGF-I) and IGF-II mRNA levels and a fivefold decrease in IGF-binding protein-5 (IGFBP-5) mRNA levels in primary cultures of fetal rat calvaria (FRC) cells. In the present study, we determined whether the effects of OP-1 were at the transcriptional or posttranscriptional level. OP-1 increased the half-life of the IGF-I mRNA from 6 to 17 h without changing the level of IGF-I nuclear pre-mRNA. In transiently transfected FRC cells, the luciferase activity driven by the -1122/+362 or the -133/+362 IGF-I exon 1 promoter fragment was not changed by OP-1. Similar results were observed using the -1500/+44 or -362/+44 IGF-I exon 2 promoter constructs. Effects of OP-1 on IGF-I mRNA were independent of cell division, as they remained elevated in the presence of hydroxyurea. Cycloheximide inhibited moderately the OP-1-induced increase in IGF-I mRNA, suggesting partial dependency on protein synthesis. On the other hand, the IGF-II nuclear pre-mRNA levels were increased by OP-1 but the half-life of the mature IGF-II mRNA was not affected. Effects of OP-1 on IGF-II mRNA were also independent of cell division, but were dependent on protein synthesis. OP-1 caused a 43-50% reduction in the level of IGFBP-5 nuclear pre-mRNA transcripts and a 40% decrease in the IGFBP-5 promoter activity in FRC cells transfected with the -1278/+1 IGFBP-5 promoter fragment. The half-life of the mature IGFBP-5 mRNA was not affected by OP-1. Hydroxyurea did not prevent the OP-1-induced reduction in IGFBP-5 mRNA. The level of IGFBP-5 mRNA was barely detectable in the presence of cycloheximide, and further suppressive effect of OP-1 on IGFBP-5 mRNA could not be determined. In conclusion, OP-1 regulates IGF-I gene expression at the posttranscriptional level, but regulates IGF-II and IGFBP-5 gene expression at the transcriptional level.

Interleukin-6 and its soluble receptor regulate the expression of insulin-like growth factor binding protein-5 in osteoblast cultures

Interleukin-6 (IL-6), a cytokine produced by bone cells, is known to influence bone resorption by stimulating the development of osteoclasts from precursor cells and to have mitogenic actions on osteoblastic cells. Insulin-like growth factors (IGFs) are important local regulators of bone formation, and IGF binding protein (IGFBP)-5 stimulates bone cell growth and enhances the effects of IGF-I. We tested the effects of IL-6 in the presence and absence of its soluble receptor (sIL-6R) on IGFBP-5 expression in cultures of osteoblast-enriched cells from 22-day-old fetal rat calvariae (Ob cells). When tested individually, IL-6 and sIL-6R had a modest stimulatory effect on IGFBP-5 messenger RNA (mRNA) levels. In contrast, when IL-6 and sIL-6R were tested in combination, they caused a considerable increase in IGFBP-5 mRNA levels, and IL-6 at 100 ng/ml and sIL-6R at 125 ng/ml increased IGFBP-5 transcripts by 5- to 7-fold after 24 h. The effect of IL-6 and sIL-6R on IGFBP-5 transcripts was not blocked by indomethacin, but cycloheximide markedly inhibited IGFBP-5 mRNA levels in control and treated cultures. IL-6 and sIL-6R did not modify the decay of IGFBP-5 mRNA in transcriptionally arrested Ob cells, and stimulated the rate of IGFBP-5 transcription as demonstrated by a nuclear run-on assay. IL-6 and sIL-6R did not increase intact IGFBP-5 levels in the extracellular matrix and increased IGFBP-5 fragments in the culture medium. Conditioned medium from Ob cells induced the proteolytic fragmentation of an IGFBP-5 standard, an effect that was accelerated and enhanced by conditioned medium from IL-6/sIL-6R-treated cultures and prevented by metalloprotease inhibitors. In conclusion, IL-6, in the presence of sIL-6R, stimulates IGFBP-5 mRNA expression in Ob cells by transcriptional mechanisms, and accelerates the fragmentation of the protein.

Growth factor regulation of insulin-like growth factor binding protein-6 expression in osteoblasts

Previously we have shown that transforming growth factor beta (TGF beta) 1, basic fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF) BB inhibit the synthesis of insulin-like growth factor (IGF) II, but their effects on IGF binding protein (IGFBP)-6 in osteoblast cultures are not known. IGFBP-6 binds IGF II with high affinity and prevents IGF II-mediated effects, so that a possible mode of regulating the IGF II available to bone cells would be by changing the levels of IGFBP-6. To enhance our understanding of the actions of growth factors on the IGF II axis in bone, we tested the effects of TGF beta 1, basic FGF, PDGF BB, IGF I, and IGF II on the expression of IGFBP-6 in cultures of osteoblast-enriched cells from 22 day fetal rat calvariae (Ob cells). Treatment of Ob cells with TGF beta 1 caused a time- and dose-dependent decrease in IGFBP-6 mRNA levels, as determined by Northern blot analysis. The effect was maximal after 48 h and observed with TGF beta 1 concentrations of 0.04 nM and higher. TGF beta 1 also decreased IGFBP-6 polypeptide levels in the medium, as determined by Western immunoblot analysis. Cycloheximide at 3.6 microM decreased IGFBP-6 transcripts and prevented the effect of TGF beta 1. The decay of IGFBP-6 mRNA in transcriptionally arrested Ob cells was not modified by TGF beta 1. In addition, TGF beta 1 decreased the rates of IGFBP-6 transcription as determined by a nuclear run-on assay. In contrast, basic FGF, PDGF BB, IGF I, and IGF II did not change IGFBP-6 mRNA levels in Ob cells. In conclusion, TGF beta 1 inhibits IGFBP-6 expression in Ob cells by transcriptional mechanisms. Since IGFBP-6 binds IGF II and prevents its effects on bone cells, decreased synthesis of IGFBP-6 induced by TGF beta 1 could be a local feedback mechanism to increase the amount of IGF II available in the bone microenvironment.

Insulin-like growth factor binding protein-3 and -5 are regulated by transforming growth factor-beta and retinoic acid in the human prostate adenocarcinoma cell line PC-3

The family of insulin-like growth factor binding proteins (IGFBPs) can affect cell proliferation by modulating the availability and bioactivity of insulin-like growth factors (IGFs), or by mechanisms independent of IGFs. To understand better the role(s) of IGFBPs in prostate growth and malignancy, we examined the regulation of IGFBPs in PC-3 cells, a human prostatic adenocarcinoma epithelial cell line that is androgen-insensitive. Both transforming growth factor-beta (TGF-beta) and retinoic acid (RA), known inhibitors of cellular proliferation, significantly changed the IGFBP profile in PC-3 cells. In cells that were treated with transforming growth factor beta-2 (TGF-beta 2) (0.5-10 ng/mL), IGFBP-3, and IGFBP-5 protein and mRNA increased in a time- and dose-dependent manner. At 10 ng/mL TGF-beta, IGFBP-3, and IGFBP-5 protein concentrations were 14- and 9-fold, respectively, over that of controls. Cells treated with RA (0-1 microM) also showed a time- and dose-dependent increase in IGFBP-3 protein and mRNA levels. However, in contrast to TGF-beta 2, high concentrations of RA (1 microM) negatively regulated IGFBP-5 expression, with IGFBP-5 mRNA levels downregulated to 20% of that of the control, and protein levels were decreased by 50%. Since both TGF-beta and RA increased IGFBP-3 expression and both are known to inhibit prostate cell growth, we speculate that the inhibition of growth is mediated, at least in part, by IGFBP-3.

Bone biology

Bone is a metabolically active and highly organized tissue consisting of a mineral phase of hydroxyapatite and amorphous calcium phosphate crystals deposited in an organic matrix. Bone has two main functions. It forms a rigid skeleton and has a central role in calcium and phosphate homeostasis. Bone modelling is the process associated with growth and re-shaping of bones in childhood and adolescence. This is distinguished from bone remodelling, which describes the lifelong process whereby skeletal tissue is continually being resorbed and replaced in order to maintain skeletal integrity, shape and mass. Bone remodelling is controlled by systemic hormones and cytokines and is an integral part of the calcium homeostatic system. The maintenance of a normal, healthy skeletal mass depends on interactions between osteoblasts, osteoclasts and constituents of the bone matrix to keep the process of bone resorption and formation in balance. The factors, local and systemic, which regulate these processes are discussed.

Influence of PDGF-BB on proliferation and transition through the MyoD-myogenin-MEF2A expression program during myogenesis in mouse C2 myoblasts

We have previously demonstrated that PDGF-BB enhances proliferation of C2 myoblasts. This has led us to examine whether the mitogenic influence of PDGF-BB in the C2 model correlates with modulation of specific steps associated with myogenic differentiation. C2 myoblasts transiting through these differentiation specific steps were monitored via immunocytochemistry. We show that the influence of PDGF on enhancing cell proliferation correlates with a delay in the emergence of cells positive for sarcomeric myosin. We further monitored the influence of PDGF-BB on differentiation steps preceding the emergence of myosin+ cells. We demonstrate that mononucleated C2 cells first express MyoD (MyoD+/myogenin- cells) and subsequently, myogenin. Cells negative for both MyoD and myogenin (the phenotype preceding the MyoD+ state) were present at all times in culture and comprised the majority, if not all, of the cells which responded mitogenically to PDGF. Additionally, the frequency of the MyoD+/myogenin+ cell phenotype was reduced in cultures receiving PDGF, suggesting that PDGF can modulate the transition of the cells into the myogenin+ state. We determined that many of the myogenin+ cells subsequently become MEF2A+ and this phenomenon is not influenced by PDGF-BB. FGF-2 also enhanced the proliferation of C2 myoblasts and suppressed the appearance of the myogenin+ cells, but did not influence the subsequent transition into the MEF2A+ state. The study raises the possibility that PDGF-BB and FGF-2 might delay the transition of the C2 cells into the MyoD+/myogenin+ state by depressing a paracrine signal that enhances differentiation.

Promotion of the osteogenetic activity of recombinant human bone morphogenetic protein by prostaglandin E1

We investigated the promotive effect of prostaglandin (PG) E1 on the osteogenetic activity of bone morphogenetic protein (BMP) using porous chemically synthesized hydroxyapatite ceramic (HAP) pellets as the carrier. After treating the pellets with recombinant human (rh) BMP-2 with or without PGE1, both of which were used at two different concentrations, they were inserted beneath the cranial periosteum of a rabbit. The degree of osteogenesis and osteoconductivity was then examined histopathologically as well as by means of an image-analyzing procedure. Results showed that there was extensive bone formation around the pellets as early as 3 weeks after insertion in the group, which received pellets treated with a relatively large amount of rhBMP alone as well as in the group receiving pellets treated with a small amount of rhBMP combined with PGE1. In the later group, the extent of bone formation was dose-dependent. Subsequent osteogenesis within the pores of the pellets in these groups slowly progressed over time, and by 9 weeks after the insertion, most of the pellet pores had filled with newly generated bone. In addition, the groups which received pellets treated with PGE1 alone also showed osteogenesis as demonstrated by osteoconduction, which was not observed in the group that received the phosphate buffered saline (PBS) treated pellets. The group that received pellets treated with a small amount of rhBMP plus a high concentration of PGE1 exhibited significantly greater bone induction than the group which received the pellets treated with a small amount rhBMP alone, and there was no statistically significant difference between the former group and the group that received a high rhBMP dose. These results clearly indicated that PGE1 has a strong and dose-dependent promotive effect on the osteogenetic activity of rhBMP and that it also promotes osteoconduction, even when used alone. This suggests that enormous reductions can be made in the amount of rhBMP administered when PGE1 is used in conjunction with porous HAP. This should prove to be very advantageous and practical in the clinical application of these materials.

Retinoic acid stimulates the transcription of insulin-like growth factor binding protein-6 in skeletal cells

Retinoic acid has important actions on cell differentiation and osteoblastic function, and some of these actions may be mediated by changes in the insulin-like growth factor (IGF) axis. Skeletal cells synthesize IGF I and II and the six known IGF binding proteins (IGFBP). IGFBP-6 binds IGF II with high affinity and prevents IGF II-mediated effects. In fibroblasts, IGFBP-6 levels are regulated by retinoic acid, and we postulated that retinoic acid may regulate IGF II in bone by altering IGFBP-6 synthesis. We examined the effect of retinoic acid on IGFBP-6 expression in cultures of osteoblast-enriched cells from 22-day fetal rat calvariae (Ob cells). Retinoic acid caused a time- and dose-dependent increase in IGFBP-6 mRNA levels, as determined by Northern blot analysis. The effect was maximal after 48 h of treatment and observed with retinoic acid at concentrations of 10 nM to 1 microM. Retinoic acid increased IGFBP-6 polypeptide levels in the culture medium, as determined by Western immunoblot analysis. Cycloheximide at 3.6 microM slightly decreased IGFBP-6 transcripts but did not prevent the stimulatory effect of retinoic acid. The decay of IGFBP-6 mRNA in transcriptionally arrested Ob cells was similar in control and retinoic acid-treated cells, and retinoic acid increased the rates of IGFBP-6 transcription, as determined by nuclear run on assays. In conclusion, retinoic acid enhances IGFBP-6 expression in Ob cells by transcriptional mechanisms. Since IGFBP-6 prevents the effects of IGF II, increased synthesis of IGFBP-6 could mediate selected actions of retinoic acid in bone.

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.

Cortisol inhibits the synthesis of insulin-like growth factor-binding protein-5 in bone cell cultures by transcriptional mechanisms

Glucocorticoids inhibit the synthesis of insulin-like growth factor-binding protein-5 (IGFBP-5) in osteoblasts, but the mechanisms involved are unknown. IGFBP-5 stimulates bone cell growth, and its inhibition by glucocorticoids may be relevant to the action of this binding protein on bone formation. We tested the effects of cortisol on IGFBP-5 expression in cultures of osteoblast-enriched cells from fetal rat calvariae (Ob cells). Cortisol decreased IGFBP-5 polypeptide levels in the extracellular matrix and caused a time- and dose-dependent decrease in IGFBP-5 mRNA. IGFBP-5 transcripts were markedly decreased by cycloheximide, and further suppressive effects of cortisol could not be determined. Cortisol did not modify the decay of IGFBP-5 mRNA in transcriptionally arrested Ob cells. Cortisol decreased IGFBP-5 hnRNA, the rate of IGFBP-5 transcription, and the activity of the murine IGFBP-5 promoter by 35% in transient transfection experiments. Deletion analysis showed that the region responsive to cortisol is from base pairs -70 to +22, and E-box-binding proteins or c-Myb-related nuclear factors may be involved in its regulation. In conclusion, cortisol inhibits IGFBP-5 transcription in Ob cells through the Myb-binding domain. This effect may be partly responsible for the effect of glucocorticoids on bone formation.

Promoter-dependent and -independent activation of insulin-like growth factor binding protein-5 gene expression by prostaglandin E2 in primary rat osteoblasts

Insulin-like growth factor (IGF) action is mediated by high affinity cell surface IGF receptors and modulated by a family of secreted IGF binding proteins (IGFBPs). IGFBP-5, the most conserved of six IGFBPs characterized to date, uniquely potentiates the anabolic actions of IGF-I for skeletal cells. In osteoblasts, IGFBP-5 production is stimulated by prostaglandin E2 (PGE2), a local factor that mediates certain effects induced by parathyroid hormone, cytokines such as interleukin-1 and transforming growth factor-beta, and mechanical strain. In this study, we show that transcriptional and post-transcriptional events initiated by PGE2 collaborate to enhance IGFBP-5 gene expression in primary fetal rat osteoblast cultures. PGE2 treatment stimulated up to a 7-fold rise in steady-state levels of IGFBP-5 mRNA throughout 32 h of incubation. Analysis of nascent IGFBP-5 mRNA suggested that PGE2 had only a modest stimulatory effect on IGFBP-5 gene transcription, and transient transfection studies with IGFBP-5 promoter-reporter genes confirmed that PGE2 enhanced promoter activity by approximately 2-fold. Similar stimulatory effects were seen with forskolin. A DNA fragment with only 51 base pairs of the 5'-flanking sequence retained hormonal responsiveness, which may be mediated by a binding site for transcription factor AP-2 located at positions -44 to -36 in the proximal IGFBP-5 promoter. Incubation of osteoblasts with the mRNA transcriptional inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole demonstrated that PGE2 enhanced IGFBP-5 mRNA stability by 2-fold, increasing the t1/2 from 9 to 18 h. The effects of PGE2 on steady-state IGFBP-5 transcripts were abrogated by preincubating cells with cycloheximide, indicating that the effects of PGE2 on both gene transcription and mRNA stability required ongoing protein synthesis. Therefore, both promoter-dependent and -independent pathways converge to enhance IGFBP-5 gene expression in response to PGE2 in osteoblasts.

Insulin-like growth factor-I (IGF-I) regulates IGF-binding protein-5 synthesis through transcriptional activation of the gene in aortic smooth muscle cells

Previous studies have shown that porcine aortic smooth muscle cells (SMCs) secrete two insulin-like growth factor-binding proteins (IGFBP), IGFBP-2 and -4, and that these IGFBPs modulate IGF-I-stimulated SMC proliferation and migration. In this study we demonstrate that porcine SMCs express IGFBP-5 mRNA and synthesize and secrete the protein. In this cell type, the biosynthesis of IGFBP-5 is up-regulated by IGF-I. This increase in IGFBP-5 synthesis is accompanied by an increase in the steady-state mRNA levels. The induction of IGFBP-5 mRNA by IGF-I is time- and dose-dependent and requires de novo protein synthesis. IGF-II and insulin also increase IGFBP-5 mRNA levels at high doses. An IGF-I analog with normal affinity for the IGF-I receptor but reduced affinity for IGFBPs evokes a similar increase. Another analog that binds to IGFBPs but not to the receptor has no effect, indicating that this effect of IGF-I is mediated through the IGF-I receptor. The IGF-I-induced IGFBP-5 gene expression is cell type-specific because IGF-I had no such effect in other cell types examined. Nuclear run-on assays revealed that IGF-I increased transcription rate of the IGFBP-5 gene, while IGF-I did not change the IGFBP-5 mRNA stability. Furthermore, the IGFBP-5 promoter was 3.5-fold more active in directing expression of the luciferase reporter gene in IGF-I-treated aortic SMCs as compared to control cells, whereas the luciferase activity remained the same in control- and IGF-I-treated fibroblasts. These results suggest that IGF-I up-regulates IGFBP-5 synthesis by transcriptionally activating the IGFBP-5 gene in aortic SMCs.

Cytokine regulation of bone cell differentiation

Systemic hormones and cytokines play important roles in regulating both osteoblast and osteoclast activity. These cytokines can have either positive or negative effects on the growth and differentiation of bone cells. These effects appear to be dependent on the model systems use to assess them, as well as the species tested. In the near future, other autocrine-paracrine factors will be identified that enhance osteoblast and osteoclast activity, and model systems should be available to further delineate their effects on cells in the osteoblast lineage. Use of transgenic mice with genes targeted to the osteoblast and osteoclast may further reveal the mechanisms responsible for the growth and differentiation of these cells, as well as produce immortalized cell lines that more accurately reflect the cell biology of the osteoclast and osteoblast in vivo.

Expression, effects, and fate of IGFBP-5 are different in normal and malignant osteoblastic cells

Normal osteoblasts from newborn rat calvaria and human osteosarcoma (Saos-2) cells express IGFBP-5 mRNA. IGF I increases IGFBP-5 mRNA levels in both cell types, whereas retinoic acid stimulates IGFBP-5 mRNA expression in calvaria but suppresses it in Saos-2 cells. IGFBP-5 mRNA expression is stimulated in normal bone cells by parathyroid hormone. 30 nM IGFBP-5 stimulates 3H-thymidine incorporation in calvaria (which produce IGF I contributing to basal proliferation in serum-free medium) but not in Saos-2 cells which produce little IGF I and IGF II. Among the 5 rhIGFBPs tested (IGFBP-2 to -6), only IGFBP-5 stimulates DNA synthesis in calvaria cells, and only IGFBP-6 in Saos-2 cells. RhIGFBP-5 displays a short half-life (approximately 30 min) in serum-free medium of calvaria cells and a long half-life (approximately 15 h) in the medium of Saos-2 cells. Fragments of 20 and 14 kDa accumulate in the media of both cell types. Intact (31 kDa) IGFBP-5 is associated and remains with the extracellular matrix of mature calvaria osteoblasts but not of Saos-2 cells. Among the IGFBPs produced IGFBP-5 is unique with regard to its marked affinity to matrix of normal bone cells, its short half-life when released, and its stimulatory effects on DNA synthesis.