Benefit of systemically administered rhIGF-I and rhIGF-I/IGFBP-3 on cancellous bone in ovariectomized rats

Global and Conditional Disruption of the Igf-I Gene in Osteoblasts and/or Chondrocytes Unveils Epiphyseal and Metaphyseal Bone-Specific Effects of IGF-I in Bone

To evaluate the relative importance of IGF-I expression in various cell types for endochondral ossification, we quantified the trabecular bone at the secondary spongiosa and epiphysis of the distal femur in 8-12-week-old male mice with a global knockout of the Igf-I gene, as well as the conditional deletion of Igf-I in osteoblasts, chondrocytes, and osteoblasts/chondrocytes and their corresponding wild-type control littermates. The osteoblast-, chondrocyte-, and osteoblast/chondrocyte-specific Igf-I conditional knockout mice were generated by crossing Igf-I floxed mice with Cre transgenic mice in which Cre expression is under the control of either the Col1α2 or Col2α1 promoter. We found that the global disruption of Igf-I resulted in 80% and 70% reductions in bone size, defined as total volume, at the secondary spongiosa and epiphysis of the distal femur, respectively. The abrogation of Igf-I in Col1α2-producing osteoblasts but not Col2α1-producing chondrocytes decreased bone size by 25% at both the secondary spongiosa and epiphysis. In comparison, the deletion of the Igf-I globally or specifically in osteoblasts or chondrocytes reduced trabecular bone mass by 25%. In contrast, the universal deletion of Igf-I in all cells, but not the conditional disruption of Igf-I in osteoblasts and/or chondrocytes reduced trabecular bone mass in the epiphysis. The reduced trabecular bone mass at the secondary spongiosa in osteoblast- and/or chondrocyte-specific Igf-I conditional knockout mice is caused by the reduced trabecular number and increased trabecular separation. Immunohistochemistry studies found that the expression levels of chondrocyte (COL10, MMP13) and osteoblast (BSP) markers were less in the secondary spongiosa and the epiphyses in the global Igf-I deletion mice. Our data indicate that local and endocrine Igf-I act pleiotropically and in a cell type- and bone compartment-dependent manner in bone.

Role of IGF1 and EFN-EPH signaling in skeletal metabolism

Insulin-like growth factor 1(IGF1) and ephrin ligand (EFN)-receptor (EPH) signaling are both crucial for bone cell function and skeletal development and maintenance. IGF1 signaling is the major mediator of growth hormone-induced bone growth, but a host of different signals and factors regulate IGF1 signaling at the systemic and local levels. Disruption of the Igf1 gene results in reduced peak bone mass in both experimental animal models and humans. Additionally, EFN-EPH signaling is a complex system which, particularly through cell-cell interactions, contributes to the development and differentiation of many bone cell types. Recent evidence has demonstrated several ways in which the IGF1 and EFN-EPH signaling pathways interact with and depend upon each other to regulate bone cell function. While much remains to be elucidated, the interaction between these two signaling pathways opens a vast array of new opportunities for investigation into the mechanisms of and potential therapies for skeletal conditions such as osteoporosis and fracture repair.

Skeletal effects of growth hormone and insulin-like growth factor-I therapy

The growth hormone/insulin-like growth factor (GH/IGF) axis is critically important for the regulation of bone formation, and deficiencies in this system have been shown to contribute to the development of osteoporosis and other diseases of low bone mass. The GH/IGF axis is regulated by a complex set of hormonal and local factors which can act to regulate this system at the level of the ligands, receptors, IGF binding proteins (IGFBPs), or IGFBP proteases. A combination of in vitro studies, transgenic animal models, and clinical human investigations has provided ample evidence of the importance of the endocrine and local actions of both GH and IGF-I, the two major components of the GH/IGF axis, in skeletal growth and maintenance. GH- and IGF-based therapies provide a useful avenue of approach for the prevention and treatment of diseases such as osteoporosis.

Increased linear bone growth by GH in the absence of SOCS2 is independent of IGF-1

Growth hormone (GH) signaling is essential for postnatal linear bone growth, but the relative importance of GHs actions on the liver and/or growth plate cartilage remains unclear. The importance of liver derived insulin like‐growth factor‐1 (IGF‐1) for endochondral growth has recently been challenged. Here, we investigate linear growth in Suppressor of Cytokine Signaling‐2 (SOCS2) knockout mice, which have enhanced growth despite normal systemic GH/IGF‐1 levels. Wild‐type embryonic ex vivo metatarsals failed to exhibit increased linear growth in response to GH, but displayed increased Socs2 transcript levels (P < 0.01). In the absence of SOCS2, GH treatment enhanced metatarsal linear growth over a 12 day period. Despite this increase, IGF‐1 transcript and protein levels were not increased in response to GH. In accordance with these data, IGF‐1 levels were unchanged in GH‐challenged postnatal Socs2^‐/‐ conditioned medium despite metatarsals showing enhanced linear growth. Growth‐plate Igf1 mRNA levels were not elevated in juvenile Socs2^‐/‐ mice. GH did however elevate IGF‐binding protein 3 levels in conditioned medium from GH challenged metatarsals and this was more apparent in Socs2^‐/‐ metatarsals. GH did not enhance the growth of Socs2^‐/‐ metatarsals when the IGF receptor was inhibited, suggesting that IGF receptor mediated mechanisms are required. IGF‐2 may be responsible as IGF‐2 promoted metatarsal growth and Igf2 expression was elevated in Socs2^‐/‐ (but not WT) metatarsals in response to GH. These studies emphasise the critical importance of SOCS2 in regulating GHs ability to promote bone growth. Also, GH appears to act directly on the metatarsals of Socs2^‐/‐ mice, promoting growth via a mechanism that is independent of IGF‐1. J. Cell. Physiol. 9999: 2796–2806, 2015. © 2015 Wiley Periodicals, Inc.

Effect of GH/IGF-1 on Bone Metabolism and Osteoporsosis

Background. Growth hormone (GH) and insulin-like growth factor (IGF-1) are fundamental in skeletal growth during puberty and bone health throughout life. GH increases tissue formation by acting directly and indirectly on target cells; IGF-1 is a critical mediator of bone growth. Clinical studies reporting the use of GH and IGF-1 in osteoporosis and fracture healing are outlined. Methods. A Pubmed search revealed 39 clinical studies reporting the effects of GH and IGF-1 administration on bone metabolism in osteopenic and osteoporotic human subjects and on bone healing in operated patients with normal GH secretion. Eighteen clinical studies considered the effect with GH treatment, fourteen studies reported the clinical effects with IGF-1 administration, and seven related to the GH/IGF-1 effect on bone healing. Results. Both GH and IGF-1 administration significantly increased bone resorption and bone formation in the most studies. GH/IGF-1 administration in patients with hip or tibial fractures resulted in increased bone healing, rapid clinical improvements. Some conflicting results were evidenced. Conclusions. GH and IGF-1 therapy has a significant anabolic effect. GH administration for the treatment of osteoporosis and bone fractures may greatly improve clinical outcome. GH interacts with sex steroids in the anabolic process. GH resistance process is considered.

Comparison of Tamoxifen and Testosterone Propionate in Male Rats: Differential Prevention of Orchidectomy Effects on Sex Organs, Bone Mass, Growth, and the Growth Hormone-IGF-I Axis

Testis dysfunction can weaken bone and reduce muscle mass as well as impair sexual function. Testosterone (T) therapy has useful effects on sex organs, bone, and muscle in T-deficient males, but prostate concerns can preclude T use in some men. Although estrogens or other drugs can protect bone in men, gynecomastia makes estrogens unappealing, and other drugs may also be undesirable in some cases. Selective estrogen receptor modulators (SERMs) inhibit estrogen-evoked sex organ growth but mimic estrogen effects on bone and cholesterol and are advantageous for some women. SERMs may also be useful in men who must avoid androgens. As a preclinical test of this idea, tamoxifen (a SERM) and testosterone propionate (TP, a classic androgen) were compared for their efficacy in preventing varied effects of orchidectomy (ORX) in adult male rats. ORX led to ventral prostate and seminal vesicle atrophy and decreases in somatic growth, proximal tibia bone mineral density (BMD), and serum growth hormone (GH) and insulin-like growth factor I (IGF-I). ORX also increased anterior pituitary glandular kallikrein, serum cholesterol, and body temperature. Pituitary prolactin (PRL) content was unaltered. ORX effects on sex organs, somatic growth, IGF-I, cholesterol, body temperature, and pituitary kallikrein were prevented by TP at 1 mg/kg (3 doses per week), but BMD and GH were unresponsive. ORX effects on BMD and GH were prevented by TP at 10 mg/kg, but this dose evoked supraphysiologic increases in sex organs and PRL, failed to restore somatic growth, and further reduced IGF-I. Tamoxifen (1 mg/kg daily) prevented ORX effects on BMD, GH, and cholesterol without altering basal or TP-induced sex organ growth and further reduced IGF-I and somatic growth. Tamoxifen did not alter basal PRL but blocked increases caused by TP at 10 mg/kg. In summary, tamoxifen prevented ORX effects on bone and cholesterol in male rats without affecting sex organs or PRL and might be useful for men who must avoid androgens. Unexpectedly, a TP dose that replicated testis effects on sex organs and other targets had no effect on BMD or GH, and a larger TP dose that restored BMD and GH was worse at replicating normal male physiology. In addition, correlation/regression results suggested that the GH-IGF-I axis contributes to changes in BMD.

Three-dimensional dynamic bone histomorphometry

Dynamic bone histomorphometry is the standard method for measuring bone remodeling at the level of individual events. Although dynamic bone histomorphometry is an invaluable tool for understanding osteoporosis and other metabolic bone diseases, the technique's two-dimensional nature requires the use of stereology and prevents measures of individual remodeling event number and size. Here, we used a novel three-dimensional fluorescence imaging technique to achieve measures of individual resorption cavities and formation events. We performed this three-dimensional histomorphometry approach using a common model of postmenopausal osteoporosis, the ovariectomized rat. The three-dimensional images demonstrated the spatial relationship between resorption cavities and formation events consistent with the hemiosteonal model of cancellous bone remodeling. Established ovariectomy was associated with significant increases in the number of resorption cavities per unit bone surface (2.38 ± 0.24 mm⁻² sham surgery versus 3.86 ± 0.35 mm⁻² bilateral ovariectomy [OVX], mean ± SD, p < 0.05) and total volume occupied by cavities per unit bone volume (0.38% ± 0.06% sham versus 1.12% ± 0.18% OVX, p < 0.001), but there was no difference in surface area per resorption cavity, maximum cavity depth, or cavity volume. In addition, we found that established ovariectomy is associated with increased size of bone formation events because of the merging of formation events (23,700 ± 6,890 µm² sham verusus 33,300 ± 7,950 µm² OVX). No differences in mineral apposition rate (determined in 3D) were associated with established ovariectomy. That established estrogen depletion is associated with increased number of remodeling events with only subtle changes in remodeling event size suggests that circulating estrogens may have their primary effect on the origination of new basic multicellular units with relatively little effect on the progression and termination of active remodeling events.

Insulin regulates GLUT1-mediated glucose transport in MG-63 human osteosarcoma cells

Osteosarcoma is the most common type of malignant bone cancer, accounting for 35% of primary bone malignancies. Because cancer cells utilize glucose as their primary energy substrate, the expression and regulation of glucose transporters (GLUT) may be important in tumor development and progression. GLUT expression has not been studied previously in human osteosarcoma cell lines. Furthermore, although insulin and insulin-like growth factor (IGF-I) play an important role in cell proliferation and tumor progression, the role of these hormones on GLUT expression and glucose uptake, and their possible relation to osteosarcoma, have also not been studied. We determined the effect of insulin and IGF-I on GLUT expression and glucose transport in three well-characterized human osteosarcoma cell lines (MG-63, SaOs-2, and U2-Os) using immunocytochemical, RT-PCR and functional kinetic analyses. Furthermore we also studied GLUT isoform expression in osteosarcoma primary tumors and metastases by in situ hybridization and immunohistochemical analyses. RT-PCR and immunostaining show that GLUT1 is the main isoform expressed in the cell lines and tissues studied, respectively. Immunocytochemical analysis shows that although insulin does not affect levels of GLUT1 expression it does induce a translocation of the transporter to the plasma membrane. This translocation is associated with increased transport of glucose into the cell. GLUT1 is the main glucose transporter expressed in osteosarcoma, furthermore, this transporter is regulated by insulin in human MG-63 cells. One possible mechanism through which insulin is involved in cancer progression is by increasing the amount of glucose available to the cancer cell.

Investigational anabolic therapies for osteoporosis

IMPORTANCE OF THE FIELD

Anabolic therapy, or stimulating the function of bone-forming osteoblasts, is the preferred pharmacological intervention for osteoporosis.

AREAS COVERED IN THIS REVIEW

We reviewed bone anabolic agents currently under active investigation. The bone anabolic potential of IGF-I and parathyroid hormone-related protein is discussed in the light of animal data and human studies. We also discuss the use of antagonists of the calcium-sensing receptor (calcilytics) as orally administered small molecules capable of transiently elevating serum parathyroid hormone (PTH). Further, we reviewed novel anabolic agents targeting members of the wingless tail (Wnt) signaling family that regulate bone formation including DKK-1, sclerostin, Thp1, and glycogen synthase kinase 3beta. We have also followed up on the promise shown by beta-blockers in modulating the activity of sympathetic nervous system, thus affecting bone anabolism. We give critical consideration to neutralizing the activity of activin A, a negative regulator of bone mass by soluble activin receptor IIA, as a strategy to promote bone formation.

WHAT THE READER WILL GAIN

Update on various strategies to promote osteoblast function currently under evaluation.

TAKE HOME MESSAGE

In spite of favorable results in experimental models, none of these strategies has yet achieved the ultimate goal of providing an alternative to injectable PTH, the sole anabolic therapy in clinical use.

Open-label trial of recombinant human insulin-like growth factor 1/recombinant human insulin-like growth factor binding protein 3 in myotonic dystrophy type 1

OBJECTIVE

To evaluate the safety and tolerability of recombinant human insulin-like growth factor 1 (rhIGF-1) complexed with IGF binding protein 3 (rhIGF-1/rhIGFBP-3) in patients with myotonic dystrophy type 1 (DM1).

DESIGN

Open-label dose-escalation clinical trial.

SETTING

University medical center.

PARTICIPANTS

Fifteen moderately affected ambulatory participants with genetically proven myotonic dystrophy type 1.

INTERVENTION

Participants received escalating dosages of subcutaneous rhIGF-1/rhIGFBP-3 for 24 weeks followed by a 16-week washout period.

MAIN OUTCOME MEASURES

Serial assessments of safety, muscle mass, muscle function, and metabolic state were performed. The primary outcome variable was the ability of participants to complete 24 weeks receiving rhIGF-1/ rhIGFBP-3 treatment.

RESULTS

All participants tolerated rhIGF-1/rhIGFBP-3. There were no significant changes in muscle strength or functional outcomes measures. Lean body muscle mass measured by dual-energy x-ray absorptiometry increased by 1.95 kg (P < .001) after treatment. Participants also experienced a mean reduction in triglyceride levels of 47 mg/dL (P = .002), a mean increase in HDL levels of 5.0 mg/dL (P = .03), a mean reduction in hemoglobin A(1c) levels of 0.15% (P = .03), and a mean increase in testosterone level (in men) of 203 ng/dL (P = .002) while taking rhIGF-1/rhIGFBP-3. Mild reactions at the injection site occurred (9 participants), as did mild transient hypoglycemia (3), lightheadedness (2), and transient papilledema (1).

CONCLUSIONS

Treatment with rhIGF-1/rhIGFBP-3 was generally well tolerated in patients with myotonic dystrophy type 1. Treatment with rhIGF-1/rhIGFBP-3 was associated with increased lean body mass and improvement in metabolism but not increased muscle strength or function. Larger randomized controlled trials would be needed to further evaluate the efficacy and safety of this medication in patients with neuromuscular disease.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT00233519.

RAGE supports parathyroid hormone-induced gains in femoral trabecular bone

Parathyroid hormone (PTH) restores bone mass to the osteopenic skeleton, but significant questions remain as to the underlying mechanisms. The receptor for advanced glycation end products (RAGE) is a multiligand receptor of the immunoglobulin superfamily; however, recent studies indicate a role in bone physiology. We investigated the significance of RAGE to hormone-induced increases in bone by treating 10-wk-old female Rage-knockout (KO) and wild-type (WT) mice with human PTH-(1-34) at 30 microg.kg(-1).day(-1) or vehicle control, 7 days/wk, for 7 wk. PTH produced equivalent relative gains in bone mineral density (BMD) and bone mineral content (BMC) throughout the skeleton in both genotypes. PTH-mediated relative increases in cortical area of the midshaft femur were not compromised in the null mice. However, the hormone-induced gain in femoral cancellous bone was significantly attenuated in Rage-KO mice. The loss of RAGE impaired PTH-mediated increases in femoral cancellous bone volume, connectivity density, and trabecular number but did not impact increases in trabecular thickness or decreases in trabecular spacing. Disabling RAGE reduced femoral expression of bone formation genes, but their relative PTH-responsiveness was not impaired. Neutralizing RAGE did not attenuate vertebral cancellous bone response to hormone. Rage-null mice exhibited an attenuated accrual rate of bone mass, with the exception of the spine, and an enhanced accrual rate of fat mass. We conclude that RAGE is necessary for key aspects of the skeleton's response to anabolic PTH. Specifically, RAGE is required for hormone-mediated improvement of femoral trabecular architecture but not intrinsically necessary for increasing cortical thickness.

Bone remodelling: its local regulation and the emergence of bone fragility

Bone modelling prevents the occurrence of damage by adapting bone structure - and hence bone strength - to its loading circumstances. Bone remodelling removes damage, when it inevitably occurs, in order to maintain bone strength. This cellular machinery is successful during growth, but fails during advancing age because of the development of a negative balance between the volumes of bone resorbed and formed during remodelling by the basic multicellular unit (BMU), high rates of remodelling during midlife in women and late in life in both sexes, and a decline in periosteal bone formation. together resulting in bone loss and structural decay each time a remodelling event occurs. The two steps in remodelling - resorption of a volume of bone by osteoclasts and formation of a comparable volume by osteoblasts - are sequential, but the regulatory events leading to these two fully differentiated functions are not. Reparative remodelling is initiated by damage producing osteocyte apoptosis, which signals the location of damage via the osteocyte canalicular system to endosteal lining cells which forms the canopy of a bone-remodelling compartment (BRC). Within the BRC, local recruitment of osteoblast precursors from the lining cells, the marrow and circulation, direct contact with osteoclast precursors, osteoclastogenesis and molecular cross-talk between precursors, mature cells, cells of the immune system, and products of the resorbed matrix, titrate the birth, work and lifespan of the cells of this multicellular remodelling machinery to either remove or form a net volume of bone appropriate to the mechanical requirements.

Mecasermin rinfabate: rhIGF-I/rhIGFBP-3 complex: iPLEX

BACKGROUND

Mecasermin rinfabate (iPLEX), comprising rhIGF-I complexed to rhIGFBP-3, was developed in an attempt to prolong the half-life of IGF-I and potentially reduce side effects. It is administered as a once-daily subcutaneous injection. Treatment with rhIGF-I has been explored in a number of growth and endocrine disorders.

OBJECTIVE

To review the published literature regarding the pharmacokinetics, safety profile and clinical efficacy of Mecasermin rinfabate.

METHODS

A comprehensive search via the NCBI PubMed portal was performed using the search terms rhIGF-I/rhIGFBP-3 complex, iPLEX and Somatokine.

RESULTS

The effects of Mecasermin rinfabate have been explored in a number of clinical situations including diabetes, severe insulin resistance, osteopaenia, burns and growth hormone insensitivity syndrome, with outcomes similar to those of rhIGF-I alone.

CONCLUSIONS

The biological effects of Mecasermin rinfabate are largely similar to those previously reported with rhIGF-I. There are little published data pertaining to pharmacokinetic properties in human subjects, and the side effect profile appears similar to that of rhIGF-I alone.

Contrasting bone effects of temporary versus permanent IGFBP administration in rodents

Transgenic animal technology has tremendously improved our current comprehension of IGFBP biology. The high potential of IGFBP transgenic mouse models is due to the fact that they mimic elevated serum IGFBP levels, which are diagnosed under the conditions of impaired growth or critical illness. In general, long term elevated levels of IGFBPs in transgenic mouse models almost exclusively resulted in inhibitory phenotypes e.g. of body or organ growth, indicating specific effects in different cell types. This holds especially for the distinct cellular populations present in the bone environment. After establishing transgenic mouse lines modelling permanent increases of IGFBPs, a second question now poses challenge to current functional genome analysis: what is the function of temporary exposure of a certain cell type to isolated IGFBPs? This question is particularly important due to the fact that elevated IGFBP expression is often found in a conditional fashion and in line with the contradictory findings after long or short term IGFBP exposure in rodent models. In order to understand the potential roles of the conditional increases of IGFBP expression, e.g. during illness, and to further study the adaptive or even therapeutic potential of IGFBPs for certain applications like osteoporosis, it is imperative to take a closer look also to the acute effects of the IGFBPs.

Effects of insulin-like growth factor binding proteins in bone -- a matter of cell and site

The actions of the insulin-like growth factor (IGF)-system are controlled by six IGF-binding proteins (IGFBPs). The IGFBPs are thought to affect local effects of IGF-I and IGF-II due to higher affinity if compared to IGF-I receptors and due to cell-type specific IGFBP expression patterns. It was found in IGFBP knockout models that the IGFBP family is functionally redundant. Thus, functional analysis of potential effects of IGFBPs is dependent on descriptive studies and models of IGFBP overexposure in vitro and in vivo. In the literature, the role of the IGFBPs for bone growth is highly controversial and, to date, no systematic look has been taken at IGFBPs resolving functional aspects of IGFBPs at levels of cell types and specific locations within bones. Since IGFBPs are thought to represent local modulators of the IGF actions and also exert IGF-independent effects, this approach is particularly reasonable on a physiological level. By sorting the huge number of in part controversial results on IGFBP effects in bone present in the literature for distinct cell types and bone sites it is possible to generate a focused, more specific and a less controversial picture of IGFBP functions in bone.

Histamine mediates osteoclastic resorption only during the acute phase of bone loss in ovariectomized rats

Short-term studies have shown that histamine is involved, via its H2 receptors (H2R), in the mediator network regulating trabecular bone loss in long bones of ovariectomized (OVX) rats. It is not known whether this effect of histamine persists over time or involves other skeletal sites. In this study, rats were maintained for 6 months postOVX and treated daily with saline or famotidine (10 mg kg(-1)), an H2R antagonist. At the end of the experimental period, femur trabecular bone mass was markedly decreased in OVX rats, whether or not they were treated with famotidine. In contrast, in the fourth lumbar vertebra, where bone loss starts later than in the femur, famotidine treatment attenuated the decline in trabecular bone volume, protected the trabecular architecture, maintained the thickness of the cortices and reduced the numbers of osteoclasts and tartrate-resistant acid phosphatase-positive preosteoclasts, whereas it had no influence on bone formation parameters. In vertebral bone marrow of OVX rats, the numbers of mast cells (MCs) and non-MC histamine-producing cells increased, while famotidine treatment significantly diminished both cell populations. These data show that H2R antagonism does not protect trabecular bone mass in the long term, and that short-term protection involves all bones. Histamine is involved during the early phase of strong osteoclastic resorption but not during the late phase of slower resorption, suggesting that different mediator networks control the two phases of destruction. Histamine would be part of the network mediating the early phase.

Insulin-like growth factor-I treatment of growth disorders, diabetes mellitus and insulin resistance

Insulin-like growth factor I (IGF-I) has many potential therapeutic uses because of its varied effects--growth promotion, insulin-like influence on glucose metabolism, and neuroprotection resulting from cell-proliferative and antiapoptotic properties--but they have not been investigated systematically in clinical situations. The growth-promoting effect of recombinant human IGF-I (rhIGF-I) in the extensively studied growth hormone insensitivity syndrome (GHIS; Laron syndrome) signifies an endocrine role for the GH-IGF system. The metabolism of (adult) patients with severe insulin resistance is improved by rhIGF-I, which--together with insulin therapy--also improves metabolic control in type 1 and 2 diabetes. Further studies on IGF-I metabolic effects and growing understanding of the IGF-I-IGF-binding protein system could open new therapeutic avenues.

Strontium ranelate improves bone resistance by increasing bone mass and improving architecture in intact female rats

Strontium ranelate given to intact rats at doses up to 900 mg/kg/day increases bone resistance, cortical and trabecular bone volume, micro-architecture, bone mass, and total ALP activity, thus indicating a bone-forming activity and an improvement of overall bone tissue quality.

INTRODUCTION

Various anti-osteoporotic agents are available for clinical use; however, there is still a need for drugs able to positively influence the coupling between bone formation and bone resorption to increase bone mass and bone strength. Strontium ranelate (PROTELOS), a new chemical entity containing stable strontium (Sr), was tested for its capacity to influence bone quality and quantity.

MATERIALS AND METHODS

The long-term effects of strontium ranelate on bone were investigated in intact female rats treated with various doses of strontium ranelate (0, 225, 450, and 900 mg/kg/day) for 2 years. In a second series of experiments, the effects of 625 mg/kg/day were evaluated in intact male and female rats for the same period of time. Bone mineral mass and mechanical properties were evaluated at various skeletal sites (vertebra and femur), and bone tissue micro-architecture was evaluated by static histomorphometry at the tibio-fibular junction (cortical bone) and at the tibia metaphysis (trabecular bone). Plasma total alkaline phosphatase (ALP) activity and serum levels of insulin-like growth factor-I (IGF-I) were also assessed.

RESULTS

In female rats treated with strontium ranelate over 2 years, dose-dependent increases of bone strength and bone mass of the vertebral body (containing a large proportion of trabecular bone) and of the midshaft femur (containing mainly cortical bone) were detected without change in bone stiffness. Similar effects were observed in males at the level of the vertebra. This increase in mechanical properties was associated with improvements of the micro-architecture as assessed by increases of trabecular and cortical bone volumes and trabecular number and thickness. Finally, plasma total ALP activity and IGF-I were also increased in treated animals, compatible with a bone-forming activity of strontium ranelate.

CONCLUSION

A long-term treatment with strontium ranelate in intact rats is very safe for bone and improves bone resistance by increasing bone mass and improving architecture while maintaining bone stiffness.

New frontiers in bone tissue engineering

No single scientific field can generate the ideal method of engineering bone. However, through collaboration and expansion of programs in bone tissue engineering, the right combination of materials, cells, growth factors, and methodology will come together for each clinical situation such that harvesting bone grafts will become obsolete. This article reviews the need for engineered bone and provides a historical perspective of bone engineering research, current research efforts, and the future direction of this work.

Perturbations in bone formation and resorption in insulin-like growth factor binding protein-3 transgenic mice

IGF-I and their binding proteins are important in bone health. Examination of BMD, osteoblast proliferation, and markers of bone resorption in transgenic mice that constitutively overexpress IGFBP-3 indicates that overexpression of IGFBP-3 increases osteoclast number and bone resorption, impairs osteoblast proliferation, and has a significant negative effect on bone formation.

INTRODUCTION

Low serum insulin-like growth factor I (IGF-I) levels correlate with an increased risk of osteoporotic fractures. Serum IGF-I is largely bound to IGF-binding protein-3 (IGFBP-3), which can inhibit IGF-I action and enhance delivery of IGF-I to tissues. Its role in bone biology is unclear.

METHODS

Bone mineral density (BMD), osteoblast proliferation, and markers of bone resorption were examined in transgenic (Tg) mice that constitutively overexpressed human IGFBP-3 cDNA driven by either the cytomegalovirus (CMV) or phosphoglycerate kinase (PGK) promoter.

RESULTS

Cultured calvarial osteoblasts from Tg mice expressed the transgene and grew more slowly than cells from wild-type (Wt) mice, and the mitogenic response to IGF-I was attenuated in osteoblasts from Tg mice. Total volumetric BMD and cortical BMD, measured in the femur using peripheral quantitative computed tomography (pQCT) were significantly reduced in both Tg mouse strains compared with Wt mice. PGKBP-3 Tg mice showed the most marked reduction in bone density. Osteocalcin levels were similar in Wt and CMVBP-3 Tg mice but were significantly reduced in PGKBP-3 Tg mice. Urinary deoxypyridinoline and osteoclast perimeter, markers of bone resorption, were significantly increased in both Tg mouse strains compared with Wt mice. Using double labeling with tetracycline, we demonstrated that pericortical and endocortical mineral apposition rate was significantly reduced in PGKBP-3 Tg mice compared with Wt mice.

CONCLUSIONS

These data show that overexpression of IGFBP-3 increases osteoclast number and bone resorption, impairs osteoblast proliferation, and has a significant negative effect on bone formation.

Insulin-like growth factor I effect on the number of osteoblast progenitors is impaired in ovariectomized mice

Because insulin-like growth factor (IGF) I is an important regulator of bone formation, we proposed the hypothesis that IGF-I could contribute in regulating the number of osteoblast progenitors (colony-forming unit fibroblast with ALP activity [CFU-F/ALP+]). To test ex vivo and in vivo effects of IGF-I on the number of CFU-F/ALP+, bone marrow cells (BMCs) derived from normal mice, growth hormone (GH)-deficient lit/lit mice, or ovariectomized (OVX) mice were cultured and the CFU-F/ALP+ number was counted. Ex vivo treatment of IGF-I increased the CFU-F/ALP+ number in a dose-dependent manner compared with vehicle-treated control cultures. The CFU-F/ALP+ number was decreased by 20% (p < 0.01; n = 7-9) in GH-deficient lit/lit mice compared with age-matched control mice. Four weeks after OVX or sham operation, IGF-I (2 microg/g body wt) or vehicle was administered twice on day 1, and 5 days later, BMCs were removed from the femur and cultured for 10 days (n = 9-10 per group). IGF-I administration increased the CFU-F/ALP+ number by 63% (p < 0.01) and 19% (NS), respectively, in sham-operated (sham) and OVX mice compared with the vehicle-treated control group. The serum IGF-I level was similar in OVX mice compared with sham mice; this finding is different from that found in rats in which OVX increases the serum IGF-I level. This study showed that IGF-I is an important regulator of osteoblast-progenitor number in the BMCs of mice both ex vivo and in vivo and that the IGF-I response to increase the number of osteoblast progenitors was impaired in OVX mice.

Growth factor delivery for bone tissue engineering

Bone is a dynamic tissue that undergoes significant turnover during the life cycle of an individual. Despite having a significant regenerative capability, trauma and other pathological scenarios commonly require therapeutic intervention to facilitate the healing process. Bone tissue engineering, where cellular and biological processes at a site are deliberately manipulated for a therapeutic outcome, offers a viable option for the treatment of skeletal diseases. In this review paper, we aim to provide a brief synopsis of cellular and molecular basis of bone formation that are pertinent to current efforts of bone healing. Different approaches for engineering bone tissue were presented with special emphasis on the use of soluble (diffusible) therapeutic agents to accelerate bone healing. The latter agents have been used for both local bone repair (i.e. introduction of agents directly to a site of repair) as well as systemic bone regeneration (i.e. delivery for regeneration throughout the skeletal system). Critical drug delivery and targeting issues pertinent for each mode of bone regeneration are provided. In addition, future challenges and opportunities in bone tissue engineering are proposed from the authors' perspective.

Subcutaneous administration of insulin-like growth factor (IGF)-II/IGF binding protein-2 complex stimulates bone formation and prevents loss of bone mineral density in a rat model of disuse osteoporosis

Elevated serum levels of insulin-like growth factor binding protein-2 (IGFBP-2) and a precursor form of IGF-II are associated with marked increases in bone formation and skeletal mass in patients with hepatitis C-associated osteosclerosis. In vitro studies indicate that IGF-II in complex with IGFBP-2 has high affinity for bone matrix and is able to stimulate osteoblast proliferation. The purpose of this study was to determine the ability of the IGF-II/IGFBP-2 complex to increase bone mass in vivo. Osteopenia of the femur was induced by unilateral sciatic neurectomy in rats. At the time of surgery, 14-day osmotic minipumps containing vehicle or 2 microg IGF-II+9 microg IGFBP-2/100g body weight/day were implanted subcutaneously in the neck. Bone mineral density (BMD) measurements were taken the day of surgery and 14 days later using a PIXImus small animal densitometer. Neurectomy of the right hindlimb resulted in a 9% decrease in right femur BMD (P<0.05 vs. baseline). This loss in BMD was completely prevented by treatment with IGF-II/IGFBP-2. On the control limb, there was no loss of BMD over the 14 days and IGF-II/IGFBP-2 treatment resulted in a 9% increase in left femur BMD (P<0.05). Bone histomorphometry indicated increases in endocortical and cancellous bone formation rates and in trabecular thickness. These results demonstrate that short-term administration of the IGF-II/IGFBP-2 complex can prevent loss of BMD associated with disuse osteoporosis and stimulate bone formation in adult rats. Furthermore, they provide proof of concept for a novel anabolic approach to increasing bone mass in humans with osteoporosis.

Musculoskeletal effects of the recombinant human IGF-I/IGF binding protein-3 complex in osteoporotic patients with proximal femoral fracture: a double-blind, placebo-controlled pilot study

The administration of recombinant human IGF-I complexed with its predominant binding protein IGF binding protein-3 (rhIGF-I/IGFBP-3) may allow the safe administration of higher doses of IGF-I than can be accomplished with rhIGF-I alone. The aim of this randomized, double-blind, placebo- controlled pilot study was to evaluate the short-term safety and musculoskeletal effects of rhIGF-I/IGFBP-3 in older women (aged 65-90 yr) with recent hip fracture. Within 72 h after the event, 30 patients received continuous administration of either placebo (n = 10), 0.5 mg/kg.d rhIGF-I/IGFBP-3 (n = 9), or 1 mg/kg.d rhIGF-I/IGFBP-3 (n = 11). Treatment was administered by sc infusion through a portable mini-pump for a total of 8 wk after hip fracture surgery, with patient follow-up to 6 months after surgery. Efficacy evaluations included a contralateral hip bone density determination, markers of bone turnover (including serum osteocalcin and urinary excretion of N-telopeptide), grip strength, and tests of functional ability. During the administration of rhIGF-I/IGFBP-3, mean serum levels of IGF-I significantly (P < 0.001) increased from 83 ng/ml to 289 ng/ml (0.5 mg/kg.d) and 393 ng/ml (1 mg/kg.d), respectively. Both doses were well tolerated, and no hypoglycemia or other therapy-induced side effects were observed. After an initial loss of hip bone density after hip fracture surgery, patients treated with 1 mg/kg.d rhIGF-I/IGFBP-3 regained a substantial portion of their femoral bone mass. At 6 months postfracture (4 months after the 2-month infusion), they showed a statistically not significant decrease from baseline in hip bone density (-2.6%, P = 0.53). Placebo-treated patients, on the other hand, failed to regain lost bone: at 6 months postfracture, bone density in the placebo group had declined by 6.1% (P = 0.04). Additionally, in patients treated with 1.0 mg/kg.d rhIGF-I/IGFBP-3, grip strength had increased from baseline by 11.4% by the end of the study (P = 0.04) whereas patients on placebo lost 11.6% from baseline (P = 0.16). This increase in muscle strength in the high-dose group was associated with a positive effect on functional recovery. We conclude that a 2-month infusion of rhIGF-I/IGFBP-3 in patients with recent hip fracture is feasible, safe, and well tolerated. Analyzing the effects on bone mass, muscle strength, and functional ability, we observed beneficial trends. In the context of a small exploratory study, these findings should be interpreted with caution, but they support the need for future trials to further assess the therapeutic potential of rhIGF-I/IGFBP-3 in elderly subjects with osteoporosis.

The skeletal structure of insulin-like growth factor I-deficient mice

The importance of insulin-like growth factor I (IGF-I) for growth is well established. However, the lack of IGF-I on the skeleton has not been examined thoroughly. Therefore, we analyzed the structural properties of bone from mice rendered IGF-I deficient by homologous recombination (knockout [k/o]) using histomorphometry, peripheral quantitative computerized tomography (pQCT), and microcomputerized tomography (muCT). The k/o mice were 24% the size of their wild-type littermates at the time of study (4 months). The k/o tibias were 28% and L1 vertebrae were 26% the size of wild-type bones. Bone formation rates (BFR) of k/o tibias were 27% that of the wild-type littermates. The k/o bones responded normally to growth hormone (GH; 1.7-fold increase) and supranormally to IGF-I (5.2-fold increase) with respect to BFR. Cortical thickness of the proximal tibia was reduced 17% in the k/o mouse. However, trabecular bone volume (bone volume/total volume [BV/TV]) was increased 23% (male mice) and 88% (female mice) in the k/o mice compared with wild-type controls as a result of increased connectivity, increased number, and decreased spacing of the trabeculae. These changes were either less or not found in L1. Thus, lack of IGF-I leads to the development of a bone structure, which, although smaller, appears more compact.

Use of mutagenesis to probe IGF-binding protein structure/function relationships

The IGF-binding proteins (IGFBPs) are multifunctional proteins that modulate IGF actions. To determine whether specific domains within these proteins account for specific functions, we and other laboratories have used in vitro mutagenesis. Prior experiments that used a variety of techniques had identified discrete regions within each protein that were proposed to account for specific functions. Alterations of these regions by substituting charged residues with neutral residues or hydrophobic residues with nonhydrophobic residues as well as domain swapping, i.e., substituting a domain from one specific form of IGFBP for the homologous domain in another form, has resulted in the elucidation of the functions of many of these specific sequences. Because the areas of protein sequence that are altered involve a limited number of amino acids, they generally do not alter the conformation of the entire protein; therefore, these specific substitutions can often be correlated with the functional changes that occur after mutagenesis. Mutants have been particularly useful for performing functional analyses in which the purified mutant protein is added to a biological test system. In some cases it has been possible to overexpress the mutagenized protein and determine whether the constitutively synthesized, mutant form of IGFBP has altered functional activity. These results have revealed that discrete regions of IGFBP sequence can mediate important and specific functional properties of these proteins.

The effect of ovariectomy and ovarian steroid treatment on growth hormone and insulin-like growth factor-I levels in the rat femur

Growth hormone (GH) and insulin-like growth factor-I (IGF-I) are known to play an important role in bone metabolism. The regulation of plasma levels of GH and IGF-I by ovarian steroids is well known, however, their effect on local GH and IGF-I is still unclear. In this study, we investigated the effect of ovariectomy and ovarian steroid treatment on the femur GH and IGF-I levels as well as on bone density in the rat. Nine month-old rats were ovariectomized (OVX) or sham-operated (SHAM) and 9 weeks after the surgery they were treated with daily s.c. injections of either 17beta-estradiol (OVX + E), progesterone (OVX + P), or vehicle (OVX + V) for another 10 weeks. GH and IGF-I levels in the femur extracts were measured by specific radioimmunoassay (RIA). Ovariectomy decreased GH and had no effect on IGF-I levels. Estradiol treatment increased femur GH and IGF-I levels compared to SHAM rats. Progesterone restored GH and increased IGF-I levels. Ovariectomy decreased, estrogen restored and progesterone partially restored femur bone density. Our results demonstrate that ovariectomy and ovarian steroids modulate the levels of GH and IGF-I in the bone of aged OVX rats. However, these effects appear to be limited to supraphysiological concentrations of estradiol and progesterone.

Distribution of chimeric IGF binding protein (IGFBP)-3 and IGFBP-4 in the rat heart: importance of C-terminal basic region

IGF binding proteins-3 and -4, whether given in the perfused rat heart or given iv in the intact animal, cross the microvascular endothelium of the heart and distribute in subendothelial tissues. IGF binding protein-3, like IGF-I/II, localizes in cardiac muscle, with lesser concentrations in CT elements. In contrast, IGFBP-4 preferentially localizes in CT. In this study, chimeric IGF binding proteins were prepared in which a basic 20-amino-acid C-terminal region of IGF binding protein-3 was switched with the homologous region of IGF binding protein-4, and vice-versa, to create IGF binding protein-3(4) and IGF binding protein-4(3). Perfused IGF binding protein-3(4) behaved like IGF binding protein-4, localizing in connective tissue elements, whereas IGF binding protein-4(3) now localized in cardiac muscle at concentrations identical to perfused IGF binding protein-3. To determine whether these small mutations altered the affinity of the chimera for cells, the ability of (125)I-IGF binding protein-3(4) and (125)I-IGF binding protein-4(3) to bind to microvascular endothelial cells was determined and compared with IGF binding protein-3. IGF binding protein-3(4) retained 15% of the binding capacity of IGF binding protein-3, whereas IGF binding protein-4(3) bound to microvessel endothelial cells with higher affinity and greater total binding than that of IGF binding protein-3. We conclude that small changes in the C-terminal basic domain of IGF binding protein-3 and the corresponding region of IGF binding protein-4 can alter their affinity for cultured cells and influence their tissue distribution in the rat heart.

Distraction osteogenesis of the craniofacial skeleton

Distraction osteogenesis is becoming the treatment of choice for the surgical correction of hypoplasias of the craniofacial skeleton. Its principle is based on the studies of Ilizarov, who showed that osteogenesis can be induced if bone is expanded (distracted) along its long axis at the rate of 1 mm per day. This process induces new bone formation along the vector of pull without requiring the use of a bone graft. The technique also provides the added benefit of expanding the overlying soft tissues, which are frequently deficient in these patients. This article reviews the authors' 11-year clinical and research experience with mandibular distraction osteogenesis. It highlights the indications and contraindications of the technique and emphasizes the critical role that basic science research has played in its evolution.

Age and ovariectomy impair both the normalization of mechanical properties and the accretion of mineral by the fracture callus in rats

The impact of age and ovariectomy on the healing of femoral fractures was studied in three groups of female rats at 8, 32 and 50 weeks of age at fracture. In the two older groups, the rats had been subjected to ovariectomy or sham surgery at random at 26 weeks of age. At fracture, all rats received unilateral intramedullary pinning of one femur and a middiaphyseal fracture. Rigidity and breaking load of the femora were evaluated at varying times up to 24 weeks after fracture induction by three-point bending to failure. Bone mineral density (BMD) was measured by dual-energy X-ray absorptiometry. In the youngest group, 8-week-old female rats regained normal femoral rigidity and breaking load by 4 weeks after fracture. They exceeded normal contralateral values by 8 weeks after fracture. In the middle group, at 32 weeks of age, fractures were induced, and the femora were harvested at 6 and 12 weeks after fracture. At 6 weeks after fracture there was partial restoration of rigidity and breaking load. At 12 weeks after fracture, only the sham-operated rats had regained normal biomechanical values in their fractured femora, while the fractured femora of the ovariectomized rats remained significantly lower in both rigidity and breaking load. In contrast, for the oldest group of rats, 50 weeks old at fracture, neither sham-operated nor ovariectomized rats regained normal rigidity or breaking load in their fractured femora within the 24 weeks in which they were studied. In all fractured bones, there was a significant increase in BMD over the contralateral intact femora due to the increased bone tissue and bone mineral in the fracture callus. Ovariectomy significantly reduced the BMD of the intact femora and also reduced the gain in BMD by the fractured femora. In conclusion, age and ovariectomy significantly impair the process of fracture healing in female rats as judged by measurements of rigidity and breaking load in three-point bending and by accretion of mineral into the fracture callus.

Anabolic effects of rhIGF-I/IGFBP-3 in vivo are influenced by thyroid status

BACKGROUND

There are close interrelationships between hormones that regulate bone formation and protein biosynthesis. For example, growth hormone and thyroid hormones can influence plasma levels of insulin-like growth factor-I (IGF-I). A patient's status regarding hormones other than IGF-I may thus indirectly modify the efficacy of IGF-I treatment. The aim of the current study was to determine if a statistical method could be used to identify key endocrine variables controlling an individual's response to IGF-I treatment.

DESIGN

Biochemical profiles from the somatotropic, thyroid and adrenal axes were determined on two separate occasions in two different study cohorts. Each cohort was divided into four groups: placebo, low, medium and high dose of recombinant human IGF-I/IGF binding protein-3 (rhIGF-I/IGFBP-3). The relative changes in collagen C-terminal peptide (CICP) levels were explained as a function of the basal endocrine profile of each treated individual. This relationship was further examined in an experimental rat model, where undernourished rats were rendered hypothyroid by propylthiouracil and subsequently treated with rhIGF-I/IGFBP-3.

RESULTS

The results of our statistical analysis in both cohorts indicated that each subject's response to rhIGF-I/IGFBP-3 administration was controlled in part by the individual's thyroid status prior to drug administration (r = 0.78 for both cohorts). The results from the animal study revealed that IGF-I treatment stimulated muscle protein synthesis by 35 +/- 9% (P = 0.05) in euthyroid rats but not in hypothyroid rats.

CONCLUSION

The relationship between endocrine axes is not simple. An improved understanding of the interactions between neuroendocrine systems may facilitate the design of efficient drug regimens in the treatment of diseases such as osteoporosis and muscle wasting.

Emerging anabolic treatments for osteoporosis

Therapy for osteoporosis is principally centered on the use of agents that block bone resorption and supplementation with vitamin D and calcium. Although these drugs are effective in reducing the risk of subsequent fractures, and modestly increasing bone density, most patients being treated for osteoporosis still have low bone mass and a greater risk of fracture. Anabolic agents stimulate bone formation, strength, and mass. In addition, there is emerging evidence that anabolic agents can reduce subsequent fracture risk. The two most promising agents, parathyroid hormone (PTH) and GH/IGF-I, act to increase osteoblast mediated bone formation. A review of the potential usefulness of PTH and GH/IGF-I is presented.

The complex of recombinant human insulin-like growth factor-I (rhIGF-I) and its binding protein-5 (IGFBP-5) induces local bone formation in murine calvariae and in rat cortical bone after local or systemic administration

The influence of recombinant human insulin-like growth factor-I (rhIGF-I), its binding protein-5 (IGFBP-5) or their equimolar complexes on calvarial osteogenesis was investigated by quantitative radiography and histomorphometry after local administration to adult mice or mature rats. The systemic effects of these proteins were investigated in aged Sprague-Dawley rats with regard to their ability to prevent or restore bone mass in ovariectomy induced osteopenia as assessed by radiography, dual-energy X-ray absorptiometry (DEXA) analyses, peripheral computerized tomography (pQCT) and mineral analyses after daily s.c. administration for 3 or 8 weeks following a bone depletion period of 8 weeks. Bone mass of murine calvariae was significantly increased in a dose-dependent manner by the complex 7 days after discontinuation of local administration for 19 days in mice, whereas IGF-I alone expressed only weak effects. IGFBP-5 alone was ineffective in this respect. In the same model, only the complex had a weak osteogenetic potential in 7 week or 5 month old rats. Systemic long-term treatment with the complex of rhIGF-I/IGFBP-5 (2.0/7.6 mg/kg/day, s.c.) for 8 weeks resulted in significantly increased cortical thickness, area and mineral density in femoral midshaft or tibial metaphysis suggesting periosteal bone formation. This was obviously related to increased muscle strength since these effects were parallelled by increased body weight. No effect on trabecular bone occurred as demonstrated by site-specific analyses (vertebrae, proximal tibia) using DEXA, pQCT and radiography. This selective action of rhIGF-I/IGFBP-5 on periosteal bone formation is unique for an IGFBP. Femoral ash and calcium content, both corrected for tissue volume, increased slightly. However, when the increase in cortical thickness and bone mass was corrected for bone size, the effects are nearly abolished, suggesting an additional effect of bone growth. This potential deserves further evaluation in order to differentiate between effects on cortical bone via muscle strength and lack of efficacy on trabecular bone balance.

IGF-I/IGFBP-3 binary complex modulates sepsis-induced inhibition of protein synthesis in skeletal muscle

The present study evaluated the ability of insulin-like growth factor I (IGF-I) complexed with IGF binding protein-3 (IGFBP-3) to modulate the sepsis-induced inhibition of protein synthesis in gastrocnemius. Beginning 16 h after the induction of sepsis, either the binary complex or saline was injected twice daily via a tail vein, with measurements made 3 and 5 days later. By day 3, sepsis had reduced plasma IGF-I concentrations approximately 50% in saline-treated rats. Administration of the binary complex provided exogenous IGF-I to compensate for the sepsis-induced diminished plasma IGF-I. Sepsis decreased rates of protein synthesis in gastrocnemius relative to controls by limiting translational efficiency. Treatment of septic rats with the binary complex for 5 days attenuated the sepsis-induced inhibition of protein synthesis and restored translational efficiency to control values. Assessment of potential mechanisms regulating translational efficiency showed that neither the sepsis-induced change in gastrocnemius content of eukaryotic initiation factor 2B (eIF2B), the amount of eIF4E associated with 4E binding protein-1 (4E-BP1), nor the phosphorylation state of 4E-BP1 or eIF4E were altered by the binary complex. Overall, the results are consistent with the hypothesis that decreases in plasma IGF-I are partially responsible for enhanced muscle catabolism during sepsis.

Changes in the insulin-like growth factor-system may contribute to in vitro age-related impaired osteoblast functions

Age-related bone loss is thought to be due to impaired osteoblast functions. Insulin-like growth factors (IGFs) have been shown to be important stimulators of bone formation and osteoblast activities in vitro and in vivo. We tested the hypothesis that in vitro osteoblast senescence is associated with changes in components of the IGF-system including IGF-I, IGF-II, IGF-binding proteins (IGFBPs) and IGFBP-specific proteases. We employed a human diploid osteoblast cell line obtained from trabecular bone explants and that exhibit typical characteristics of in vitro senescence during serial subculturing. Using a non-competitive reverse-transcriptase polymerase-chain reaction (RT-PCR) assay, we found that the constitutive level of IGF-I mRNA decreased progressively to 49.9 +/- 4.9% in old osteoblasts as compared to the levels found in the young cells. No age-related change was found in IGF-II steady-state mRNA levels. Changes in IGFBPs gene expression and protein production were assessed using Northern blot analysis and Western ligand blotting (WLB), respectively. IGFBP-3 mRNA levels decreased to 30% and protein production to 16% in aged osteoblasts as compared to levels found in young cells. We also found age-related decreases in mRNA levels of both IGFBP-4 and IGFBP-5 to 70% and 60% in aged osteoblasts, respectively, compared to young cells. While IGFBP-5 protein was not detected by WLB, IGFBP-4 protein production showed a biphasic change with 50% decrease in middle-aged cells and a subsequent increase in aged osteoblasts to levels similar to those in young osteoblasts. We found an age-related increase in the immunoreactive levels of IGFBP-4 protease, however, no detectable IGFBP-4 or IGFBP-3 protease activities in conditioned media from osteoblast cultures were observed. Our findings demonstrate that osteoblast aging is associated with impaired production of the stimulatory components of the IGF-system, that may be a mechanism contributing to age-related decline in osteoblast functions.

Evidence that the interaction between insulin-like growth factor (IGF)-II and IGF binding protein (IGFBP)-4 is essential for the action of the IGF-II-dependent IGFBP-4 protease

A variety of human cell types, including human osteoblasts (hOBs), produce an IGFBP-4 protease, which cleaves IGFBP-4 in the presence of IGF-II. Recently, the pregnancy-associated plasma protein (PAPP)-A has been determined to be the IGF-II-dependent IGFBP-4 protease produced by human fibroblasts. This study sought to define the mechanism by which IGF-II enhances IGFBP-4 proteolysis. Addition of PAPP-A antibody blocked the IGFBP-4 proteolytic activity in hOB conditioned medium (CM), suggesting that PAPP-A is the major IGFBP-4 protease in hOB CM. Pre-incubation of IGFBP-4 with IGF-II, followed by removal of unbound IGF-II, led to IGFBP-4 proteolysis without further requirement of the presence of IGF-II in the reaction. In contrast, prior incubation of the partially purified IGFBP-4 protease from either hOB CM or human pregnancy serum with IGF-II did not lead to IGFBP-4 proteolysis unless IGF-II was re-added to the assays. To further confirm that the interaction between IGF-II and IGFBP-4 is required for IGFBP-4 protease activity, we prepared IGFBP-4 mutants, which contained the intact cleavage site (Met135-Lys136) but lacked the IGF binding activity, by deleting the residues Leu72-His74 in the IGF binding domain or Cys183-Glu237 that contained an IGF binding enhancing motif. The IGFBP-4 protease was unable to cleave these IGFBP-4 mutants, regardless of whether or not IGF-II was present in the assay. Conversely, an IGFBP-4 mutant with His74 replaced by an Ala, which exhibited normal IGF binding activity, was effectively cleaved in the presence of IGF-II. Taken together, these findings provided strong evidence that the interaction between IGF-II and IGFBP-4, rather than the direct interaction between IGF-II and IGFBP-4 protease, is required for optimal IGFBP-4 proteolysis.

Effects of IGF-I treatment on osteopenia in rats with advanced liver cirrhosis

IGF-I is an anabolic hormone which has been reported to increase bone formation in several conditions of undernutrition. Advanced liver cirrhosis is associated with osteopenia and also with low serum levels of IGF-I. Previous results showed that low doses of IGF-I increase osteoblastic activity and decrease bone reabsorption in early liver cirrhosis. The aim of this study was to evaluate whether IGF-I-treatment also induces beneficial effect on osteopenia associated with advanced cirrhosis. Rats with ascitic cirrhosis were divided into two groups: group CI (n=10) which received saline and group CI+IGF (n=10) which were treated with IGF-I (2 microg/100 g bw x day, sc, during 21 days). Healthy controls which received saline were studied in parallel (CO n=10). On the 22nd day, the animals were sacrificed, and bone parameters were analyzed in femur. Posterior-anterior diameter was similar in all groups. No significant differences were observed in bone content of calcium, total proteins, collagen and hydroxyapatite in cirrhotic rats as compared with controls. However, CI rats showed significant reductions in total bone density (-13.5%, p<0.001) assessed by densitometry and radiological study. In CI+IGF rat bone density (assessed by densitometry) improved significantly as compared with CI animals. In summary, osteopenia characterized by loss of bone mass and preserved bone composition was found in rats with advanced cirrhosis induced by CCl4 and phenobarbital in drinking water. This bone disorder is partially restored by treatment with low doses of IGF-I during only three weeks. Thus, IGF-I could be considered as a possible therapy for osteopenia associated with advanced liver cirrhosis.

Gene expression of TGF-beta, TGF-beta receptor, and extracellular matrix proteins during membranous bone healing in rats

Poorly healing mandibular fractures and osteotomies can be troublesome complications of craniomaxillofacial trauma and reconstructive surgery. Gene therapy may offer ways of enhancing bone formation by altering the expression of desired growth factors and extracellular matrix molecules. The elucidation of suitable candidate genes for therapeutic intervention necessitates investigation of the endogenously expressed patterns of growth factors during normal (i.e., successful) fracture repair. Transforming growth factor beta1 (TGF-beta1), its receptor (Tbeta-RII), and the extracellular matrix proteins osteocalcin and type I collagen are thought to be important in long-bone (endochondral) formation, fracture healing, and osteoblast proliferation. However, the spatial and temporal expression patterns of these molecules during membranous bone repair remain unknown. In this study, 24 adult rats underwent mandibular osteotomy with rigid external fixation. In addition, four identically treated rats that underwent sham operation (i.e., no osteotomy) were used as controls. Four experimental animals were then killed at each time point (3, 5, 7, 9, 23, and 37 days after the procedure) to examine gene expression of TGF-beta1 and Tbeta-RII, osteocalcin, and type I collagen. Northern blot analysis was used to compare gene expression of these molecules in experimental animals with that in control animals (i.e., nonosteotomized; n = 4). In addition, TGF-beta1 and T-RII proteins were immunolocalized in an additional group of nine animals killed on postoperative days 3, 7, and 37. The results of Northern blot analysis demonstrated a moderate increase (1.7 times) in TGF-beta1 expression 7 days postoperatively; TGF-beta1 expression returned thereafter to near baseline levels. Tbeta-RII mRNA expression was downregulated shortly after osteotomy but then increased, reaching a peak of 1.8 times the baseline level on postoperative day 9. Osteocalcin mRNA expression was dramatically downregulated shortly after osteotomy and remained low during the early phases of fracture repair. Osteocalcin expression trended slowly upward as healing continued, reaching peak expression by day 37 (1.7 times the control level). In contrast, collagen type IalphaI mRNA expression was acutely downregulated shortly after osteotomy, peaked on postoperative days 5, and then decreased at later time points. Histologic samples from animals killed 3 days after osteotomy demonstrated TGF-beta1 protein localized to inflammatory cells and extracellular matrix within the fracture gap, periosteum, and peripheral soft tissues. On postoperative day 7, TGF-beta1 staining was predominantly localized to the osteotomized bone edges, periosteum, surrounding soft tissues, and residual inflammatory cells. By postoperative day 37, complete bony healing was observed, and TGF-beta1 staining was localized to the newly formed bone matrix and areas of remodeling. On postoperative day 3, Tbeta-RII immunostaining localized to inflammatory cells within the fracture gap, periosteal cells, and surrounding soft tissues. By day 7, Tbeta-RII staining localized to osteoblasts of the fracture gap but was most intense within osteoblasts and mesenchymal cells of the osteotomized bone edges. On postoperative day 37, Tbeta-RII protein was seen in osteocytes, osteoblasts, and the newly formed periosteum in the remodeling bone. These observations agree with those of previous in vivo studies of endochondral bone formation, growth, and healing. In addition, these results implicate TGF-beta1 biological activity in the regulation of osteoblast migration, differentiation, and proliferation during mandibular fracture repair. Furthermore, comparison of these data with gene expression during mandibular distraction osteogenesis may provide useful insights into the treatment of poorly healing fractures because distraction osteogenesis has been shown to be effective in the management of these difficult clinical cases.

In vitro studies of insulin-like growth factor I and bone

Insulin-like growth factor I (IGF-I) stimulates osteoblastic cells in culture to proliferate and to synthesize bone matrix proteins. However, local IGF-I action is modulated by a family of IGF-binding proteins (IGFBPs) that, in turn, are modified by specific IGFBP proteases. All these components of the IGF-I regulatory system are present and operative in bone cells in vitro.

Characterization of the inhibition of DNA synthesis in proliferating mink lung epithelial cells by insulin-like growth factor binding protein-3

Insulin-like growth factor binding protein-3 (IGFBP-3) can inhibit cell growth by directly interacting with cells, as well as by forming complexes with IGF-I and IGF-II that prevent their growth-promoting activity. The present study examines the mechanism of inhibition of DNA synthesis by IGFBP-3 in CCL64 mink lung epithelial cells. DNA synthesis was measured by the incorporation of 5-bromo-2'-deoxyuridine, using an immunocolorimetric assay. Recombinant human IGFBP-3 (rh[N109D,N172D]IGFBP-3) inhibited DNA synthesis in proliferating and quiescent CCL64 cells. Inhibition was abolished by co-incubation of IGFBP-3 with a 20% molar excess of Leu(60)-IGF-I, a biologically inactive IGF-I analogue that binds to IGFBP-3 but not to IGF-I receptors. DNA synthesis was not inhibited by incubation with a preformed 1:1 molar complex of Leu(60)-IGF-I and IGFBP-3, indicating that only free IGFBP-3 inhibits CCL64 DNA synthesis. Inhibition by IGFBP-3 is not due to the formation of biologically inactive complexes with free IGF, since endogenous IGFs could not be detected in CCL64 conditioned media; any IGFs that might have been present could only have existed in inactive complexes, since endogenous IGFBPs were present in excess; and biologically active IGFs were not displaced from endogenous IGFBP complexes by Leu(60)-IGF-I. After incubation with CCL64 cells, (125)I-IGFBP-3 was covalently cross-linked to a major thick similar400-kDa complex. This complex co-migrated with a complex formed after incubation with (125)I-labeled transforming growth factor-beta (TGF-beta) that has been designated the type V TGF-beta receptor. (125)I-IGFBP-3 binding to the thick similar400-kDa receptor was inhibited by co-incubation with unlabeled IGF-I or Leu(60)-IGF-I. The ability of Leu(60)-IGF-I to decrease both the inhibition of DNA synthesis by IGFBP-3 and IGFBP-3 binding to the thick similar400-kDa receptor is consistent with the hypothesis that the thick similar400-kDa IGFBP-3 receptor mediates the inhibition of CCL64 DNA synthesis by IGFBP-3.

Skeletal changes in rats bearing mammosomatotrophic pituitary tumors: a model of acromegaly with gonadal dysfunction

Growth hormone (GH) exerts potent effects on bone metabolism, resulting in an increased bone formation in animals and humans. Acromegaly has been associated with increased bone turnover, whereas the net effect of the increased bone metabolism has been obscured because patients with acromegaly are often associated with hypogonadism. We investigated changes in cortical and cancellous bone in adult rats implanted mammosomatotrophic pituitary tumor cells (GH3) as a model of acromegaly with gonadal dysfunction. Acromegaly model rats were prepared by implanting GH3 cells into female Wistar-Furth rats at 17 weeks of age. At 28 weeks of age, GH3-bearing rats (GH rats) showed very high serum GH levels and a moderate increase in serum prolactin levels, resulting in low circulating estradiol levels. The GH rats showed significant increases in body weight and in length and volume of both the femur and vertebral body. Bone mineral content values of either the midfemur or the whole lumbar body were significantly greater in the GH rats compared with littermate controls, while the areal bone mineral density values of the respective bones were not different between the two groups. The parameters of mechanical strength of the femur were significantly larger in the GH rats than in controls, whereas those of the lumbar vertebral body cylinder specimen were not different between the two groups. Respective normalized mechanical parameters of the femur and the vertebral body were the same in the GH rats as in controls. In the midfemur, the GH rats showed a significant increase in the total cross-sectional area without influencing the bone marrow area, resulting in an increase in the cortical bone area and the moment of inertia compared with controls. The indices of periosteal bone formation in the midfemur were greater in the GH rats compared with controls, but the endocortical bone formation and resorption were not different between the two groups. In the vertebral body cancellous bone, the GH rats had an increase in bone turnover rate, whereas the structural parameters were not different between the two groups. These results from GH3-bearing rats demonstrate that an excess of GH increases cortical bone mass in rats accompanied with estrogen deficiency, while no large effect on vertebral body cancellous bone mass is seen.

Synergistic effects of insulin-like growth factor-I and human chorionic gonadotropin in the rat ovary

Insulin and low doses of lutenizing hormone (LH) activity (human chorionic gonadotropin [hCG]) act synergistically in the rat to produce anovulation, large ovarian cysts, and elevated plasma androstenedione levels. Further, both insulin and insulin-like growth factor-I (IGF-I) affect the ability of gonadotropins to enhance both ovarian theca and granulosa cell function in vitro. The present series of experiments were performed to determine if recombinant human IGF-I (rhIGF-I) can act in a manner similar to insulin when combined with subovulatory doses of hCG in adult normally cycling rats. Fifty-four female Sprague-Dawley rats were randomly assigned to the following treatment groups at the age of 64 days: (A) vehicle alone (controls, phosphate-buffered saline containing 0.09% pig gelatin), (B) twice-daily subcutaneous injections of 0.5 to 3.0 U insulin, (C) twice-daily subcutaneous injections of 1.5 U hCG, (D) both insulin and hCG, (E) twice-daily subcutaneous injections of rhIGF-I (2.5 mg/kg/d), and (F) both hCG and rhIGF-I. After 22 days of treatment, the animals were killed on day 23, trunk blood was collected, and the ovaries were excised for histological study. Eight of 9 control rats and 5 or 6 of 9 rats treated with insulin, hCG, or rhIGF-I alone displayed normal estrus cycles throughout the in vivo treatment period as assessed by daily vaginal smears. In marked contrast, only 1 animal treated with hCG + insulin and 2 animals treated with hCG + rhIGF-I continued to display vaginal smears indicative of normal cycling. Multiple large ovarian follicular cysts were found only in these latter 2 groups (3 of 9 animals in each group). Mean serum testosterone levels were significantly elevated in animals receiving insulin + hCG (0.72 +/- 0.28 v 0.17 +/- 0.03 ng/mL in controls, P = .05). Mean serum androstenedione levels were significantly elevated in animals receiving hCG and animals receiving rhIGF-I + hCG (5.57 +/- 0.99 and 2.39 +/- 0.68 ng/mL, respectively, v0.14 +/- 0.14 ng/mL in controls, P< .01 and P< .05, respectively). We conclude that rhIGF-I and insulin act synergistically with subovulatory doses of hCG to disrupt normal reproductive cycling, elevate serum androgen concentrations, and induce large ovarian cysts in intact adult rats.

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

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

Recombinant human insulin-like growth factor-binding protein-5 stimulates bone formation parameters in vitro and in vivo

Insulin-like growth factor-binding protein-5 (rhIGFBP-5) is stored in bone and stimulates osteoblast cell proliferation in vitro. Bone formation is dependent on the number and activity of osteoblasts. We therefore evaluated the ability of recombinant human (rh) IGFBP-5 to increase osteoblast activity in vitro; both alkaline phosphatase (ALP) activity and osteocalcin levels showed a dose-dependent increase. In in vivo time-course studies, daily s.c. administration of 50 microg rhIGFBP-5/day/mouse significantly increased serum osteocalcin levels by day 7, and these levels were sustained through day 21. We further evaluated whether rhIGFBP-5 was as effective as IGF-I. Daily s.c. administration of rhIGFBP-5 (50 microg/day), IGF-I (13 microg/day), or IGF-I plus rhIGFBP-5 complex for 9 days increased serum osteocalcin levels by 58%, 65%, and 81% (P < 0.001 in all) and femoral bone extract ALP activity by 85% (P < 0.001), 29% (P < 0.05), and 13% (P = NS), respectively, and decreased carboxyl-terminal cross-linked telopeptide of type I collagen by 29% (P < 0.05), 20% (P = NS), and 12.5% (P = NS), respectively. One s.c. injection of rhIGFBP-5 (50 microg/mouse) increased serum osteocalcin and bone ALP activity by 21% (P < 0.05) and 27% (P < 0.02), respectively, after 5 days, but did not significantly increase serum IGF-I (1, 6, or 24 h/postinjection), suggesting that the effects of rhIGFBP-5 on bone are not mediated by increasing circulating IGF-I. Our data demonstrate that systemic administration of rhIGFBP-5, either alone or in combination with IGF-I, increases bone formation parameters in vivo.