Insulin receptor signaling in osteoblasts regulates postnatal bone acquisition and body composition

Global energy balance in mammals is controlled by the actions of circulating hormones that coordinate fuel production and utilization in metabolically active tissues. Bone-derived osteocalcin, in its undercarboxylated, hormonal form, regulates fat deposition and is a potent insulin secretagogue. Here, we show that insulin receptor (IR) signaling in osteoblasts controls osteoblast development and osteocalcin expression by suppressing the Runx2 inhibitor Twist2. Mice lacking IR in osteoblasts have low circulating undercarboxylated osteocalcin and reduced bone acquisition due to decreased bone formation and deficient numbers of osteoblasts. With age, these mice develop marked peripheral adiposity and hyperglycemia accompanied by severe glucose intolerance and insulin resistance. The metabolic abnormalities in these mice are improved by infusion of undercarboxylated osteocalcin. These results indicate the existence of a bone-pancreas endocrine loop through which insulin signaling in the osteoblast ensures osteoblast differentiation and stimulates osteocalcin production, which in turn regulates insulin sensitivity and pancreatic insulin secretion.

Role of HIF-1alpha in skeletal development

Angiogenesis and osteogenesis are tightly coupled during bone development and regeneration. Mesenchymal cells in the developing stroma elicit angiogenic signals to recruit new blood vessels into bone. Reciprocal signals, likely emanating from the incoming vascular endothelium, stimulate mesenchymal cell specification through additional interactions with cells within the vascular stem cell niche. The hypoxia-inducible factor-1 alpha (HIF-1) pathway has been identified as a key component in this process. We demonstrated that overexpression of HIF-1 in mature osteoblasts through disruption of the von Hippel-Lindau protein profoundly increases angiogenesis and osteogenesis; these processes appear to be coupled by cell nonautonomous mechanisms involving the action of vascular endothelial growth factor (VEGF) on the endothelial cells. The same occurred in the model of injury-mediated bone regeneration (distraction osteogenesis). Surprisingly, manipulation of HIF-1 does not influence angiogenesis of the skull bones, where earlier activation of HIF-1 in the condensing mesenchyme upregulates osterix during cranial bone formation.

Endogenous glucocorticoids decrease skeletal angiogenesis, vascularity, hydration, and strength in aged mice

Aging or glucocorticoid excess decrease bone strength more than bone mass in humans and mice, but an explanation for this mismatch remains elusive. We report that aging in C57BL/6 mice was associated with an increase in adrenal production of glucocorticoids as well as bone expression of 11beta-hydroxysteroid dehydrogenase (11beta-HSD) type 1, the enzyme that activates glucocorticoids. Aging also decreased the volume of the bone vasculature and solute transport from the peripheral circulation to the lacunar-canalicular system. The same changes were reproduced by pharmacologic hyperglucocorticoidism. Furthermore, mice in which osteoblasts and osteocytes were shielded from glucocorticoids via cell-specific transgenic expression of 11beta-HSD type 2, the enzyme that inactivates glucocorticoids, were protected from the adverse effects of aging on osteoblast and osteocyte apoptosis, bone formation rate and microarchitecture, crystallinity, vasculature volume, interstitial fluid, and strength. In addition, glucocorticoids suppressed angiogenesis in fetal metatarsals and hypoxia inducible factor-1alpha transcription and vascular endothelial growth factor production in osteoblasts and osteocytes. These results, together with the evidence that dehydration of bone decreases strength, reveal that endogenous glucocorticoids increase skeletal fragility in old age as a result of cell autonomous effects on osteoblasts and osteocytes leading to interconnected decrements in bone angiogenesis, vasculature volume, and osteocyte-lacunar-canalicular fluid.

Hypoxia-inducible factors 1alpha and 2alpha exert both distinct and overlapping functions in long bone development

The hypoxia-inducible factors have recently been identified as critical regulators of angiogenic-osteogenic coupling. Mice overexpressing HIFalpha subunits in osteoblasts produce abundant VEGF and develop extremely dense, highly vascularized long bones. In this study, we investigated the individual contributions of Hif-1alpha and Hif-2alpha in angiogenesis and osteogenesis by individually disrupting each Hifalpha gene in osteoblasts using the Cre-loxP method. Mice lacking Hif-1alpha demonstrated markedly decreased trabecular bone volume, reduced bone formation rate, and altered cortical bone architecture. By contrast, mice lacking Hif-2alpha had only a modest decrease in trabecular bone volume. Interestingly, long bone blood vessel development measured by angiography was decreased by a similar degree in both DeltaHif-1alpha and DeltaHif-2alpha mice suggesting a common role for these Hifalpha subunits in skeletal angiogenesis. In agreement with this idea, osteoblasts lacking either Hif-1alpha or Hif-2alpha had profound reductions in VEGF mRNA expression but only the loss of Hif-1alpha impaired osteoblast proliferation. These findings indicate that expression of both Hif-1alpha and Hif-2alpha by osteoblasts is required for long bone development. We propose that both Hif-1alpha and Hif-2alpha function through cell non-autonomous modes to promote vascularization of bone and that Hif-1alpha also promotes bone formation by exerting direct actions on the osteoblast.

A novel spontaneous mutation of Irs1 in mice results in hyperinsulinemia, reduced growth, low bone mass and impaired adipogenesis

A spontaneous mouse mutant, designated 'small' (sml), was recognized by reduced body size suggesting a defect in the IGF1/GH axis. The mutation was mapped to the chromosome 1 region containing Irs1, a viable candidate gene whose sequence revealed a single nucleotide deletion resulting in a premature stop codon. Despite normal mRNA levels in mutant and control littermate livers, western blot analysis revealed no detectable protein in mutant liver lysates. When compared with the control littermates, Irs1(sml)/Irs1(sml) (Irs1(sml/sml)) mice were small, lean, hearing impaired; had 20% less serum IGF1; were hyperinsulinemic; and were mildly insulin resistant. Irs1(sml/sml) mice had low bone mineral density, reduced trabecular and cortical thicknesses, and low bone formation rates, while osteoblast and osteoclast numbers were increased in the females but not different in the males compared with the Irs1(+/+) controls. In vitro, Irs1(sml/sml) bone marrow stromal cell cultures showed decreased alkaline phosphatase-positive colony forming units (pre-osteoblasts; CFU-AP+) and normal numbers of tartrate-resistant acid phosphatase-positive osteoclasts. Irs1(sml/sml) stromal cells treated with IGF1 exhibited a 50% decrease in AKT phosphorylation, indicative of defective downstream signaling. Similarities between engineered knockouts and the spontaneous mutation of Irs1(sml) were identified as well as significant differences with respect to heterozygosity and gender. In sum, we have identified a spontaneous mutation in the Irs1 gene associated with a major skeletal phenotype. Changes in the heterozygous Irs1(+)(/sml) mice raise the possibility that similar mutations in humans are associated with short stature or osteoporosis.

Deletion of IGF-I receptor (IGF-IR) in primary osteoblasts reduces GH-induced STAT5 signaling

GH promotes longitudinal growth and regulates multiple cellular functions in humans and animals. GH signals by binding to GH receptor (GHR) to activate the tyrosine kinase, Janus kinase 2 (JAK2), and downstream pathways including signal transducer and activator of transcription 5 (STAT5), thereby regulating expression of genes including IGF-I. GH exerts effects both directly and via IGF-I, which signals by activating the IGF-I receptor (IGF-IR). IGF-IR is a cell surface receptor that contains intrinsic tyrosine kinase activity within its intracellular domain. In this study, we examined the potential role of IGF-IR in facilitating GH-induced signal transduction, using mouse primary calvarial osteoblasts with Lox-P sites flanking both IGF-IR alleles. These cells respond to both GH and IGF-I and in vitro infection with an adenovirus that drives expression of Cre recombinase (Ad-Cre) dramatically reduces IGF-IR abundance without affecting the abundance of GHR, JAK2, STAT5, or ERK. Notably, infection with Ad-Cre, but not a control adenovirus, markedly inhibited acute GH-induced STAT5 activity (more than doubling the ED(50) and reducing the maximum activity by nearly 50%), while sparing GH-induced ERK activity, and markedly inhibited GH-induced transactivation of a STAT5-dependent luciferase reporter. The effect of Ad-Cre on GH signaling was specific, as platelet-derived growth factor-induced signaling was unaffected by Ad-Cre-mediated reduction of IGF-IR. Ad-Cre-mediated inhibition of GH signaling was reversed by adenoviral reexpression of IGF-IR, but not by infection with an adenovirus that drives expression of a hemagglutination-tagged somatostatin receptor, which drives expression of the unrelated somatostatin receptor, and Ad-Cre infection of nonfloxed osteoblasts did not affect GH signaling. Notably, infection with an adenovirus encoding a C-terminally truncated IGF-IR that lacks the tyrosine kinase domain partially rescued both acute GH-induced STAT5 activity and GH-induced IGF-I gene expression in cells in which endogenous IGF-IR was reduced. These data, in concert with our earlier findings that GH induces a GHR-JAK2-IGF-IR complex, suggest a novel function for IGF-IR. In addition to its role as a key IGF-I signal transducer, this receptor may directly facilitate acute GH signaling. The implications of these findings are discussed.

Mesenchymal stem cells expressing osteogenic and angiogenic factors synergistically enhance bone formation in a mouse model of segmental bone defect

The potential of mesenchymal stem cells (MSC) in tissue regeneration is increasingly gaining attention. There is now accumulating evidence that MSC make an important contribution to postnatal vasculogenesis. During bone development and fracture healing, vascularization is observed before bone formation. The present study determined the potential of MSC, transduced ex vivo with a recombinant adeno-associated virus 6 (rAAV6) encoding bone morphogenetic protein 2 (BMP2) and vascular endothelial growth factor (VEGF) in a mouse model of segmental bone defect created in the tibiae of athymic nude mice. Mouse MSC that were mock-transduced or transduced with rAAV6-BMP2:VEGF were systemically transplanted following radiographic confirmation of the osteotomy. Effects of the therapy were determined by enzyme-linked immunosorbent assay measurements for BMP2 and VEGF, dual-energy X-ray absorptiometry (DXA) for bone density, three-dimensional microcomputed tomography (microCT) for bone and capillary architecture, and histomorphometry for bone remodeling. Results of these analyses indicated enhanced bone formation in the group that received BMP2+VEGF-expressing MSC compared to other groups. The therapeutic effects were accompanied by increased vascularity and osteoblastogenesis, indicating its potential for effective use while treating difficult nonunion bone defects in humans.

Prolyl hydroxylase inhibitors increase neoangiogenesis and callus formation following femur fracture in mice

Skeletal trauma and impaired skeletal healing is commonly associated with diminished vascularity. Hypoxia inducible factor alpha (HIF-1) is a key transcription factor responsible for activating angiogenic factors during development and tissue repair. Small molecule inhibitors of the prolyl hydroxylase enzyme (PHD), the key enzyme responsible for degrading HIF-1, have been shown to activate HIF-1, and are effective in inducing angiogenesis. Here we examined the effects of several commercially available PHD inhibitors on bone marrow mesenchymal stromal cells (MSCs) in vitro and in a stabilized fracture model in vivo. Three PHD inhibitors [Desferrioxamine (DFO), L-mimosine (L-mim), and Dimethyloxalylglycine (DMOG)] effectively activated a HIF-1 target reporter, induced expression of vascular endothelial growth factor (VEGF) mRNA in vitro, and increased capillary sprouting in a functional angiogenesis assay. DFO and DMOG were applied by direct injection at the fracture site in a stabilized murine femur fracture model. PHD inhibition increased the vascularity at 14 days and increased callus size as assessed by microCT at 28 days. These results suggest that HIF activation is a viable approach to increase vascularity and bone formation following skeletal trauma.

Sustained BMP signaling in osteoblasts stimulates bone formation by promoting angiogenesis and osteoblast differentiation

Angiogenesis and bone formation are tightly coupled during the formation of the skeleton. Bone morphogenetic protein (BMP) signaling is required for both bone development and angiogenesis. We recently identified endosome-associated FYVE-domain protein (endofin) as a Smad anchor for BMP receptor activation. Endofin contains a protein-phosphatase pp1c binding domain, which negatively modulates BMP signals through dephosphorylation of the BMP type I receptor. A single point mutation of endofin (F872A) disrupts interaction between the catalytic subunit pp1c and sensitizes BMP signaling in vitro. To study the functional impact of this mutation in vivo, we targeted expression of an endofin (F872A) transgene to osteoblasts. Mice expressing this mutant transgene had increased levels of phosphorylated Smad1 in osteoblasts and showed increased bone formation. Trabecular bone volume was significantly increased in the transgenic mice compared with the wildtype littermates with corresponding increases in trabecular bone thickness and number. Interestingly, the transgenic mice also had a pronounced increase in the density of the bone vasculature measured using contrast-enhanced microCT imaging of Microfil-perfused bones. The vessel surface and volume were both increased in association with elevated levels of vascular endothelial growth factor (VEGF) in osteoblasts. Endothelial sprouting from the endofin (F872A) mutant embryonic metatarsals cultured ex vivo was increased compared with controls and was abolished by an addition of a VEGF neutralizing antibody. In conclusion, osteoblast targeted expression of a mutant endofin protein lacking the pp1c binding activity results in sustained signaling of the BMP type I receptor, which increases bone formation and skeletal angiogenesis.

Role of hypoxia-inducible factor-1alpha in angiogenic-osteogenic coupling

Angiogenesis and osteogenesis are tightly coupled during bone development and regeneration. The vasculature supplies oxygen to developing and regenerating bone and also delivers critical signals to the stroma that stimulate mesenchymal cell specification to promote bone formation. Recent studies suggest that the hypoxia-inducible factors (HIFs) are required for the initiation of the angiogenic-osteogenic cascade. Genetic manipulation of individual components of the HIF/vascular endothelial growth factor (VEGF) pathway in mice has provided clues to how coupling is achieved. In this article, we review the current understanding of the cellular and molecular mechanisms responsible for angiogenic-osteogenic coupling. We also briefly discuss the therapeutic manipulation of HIF and VEGF in skeletal repair. Such discoveries suggest promising approaches for the development of novel therapies to improve bone accretion and repair.

Estradiol and tamoxifen mediate rescue of the dominant-negative effects of estrogen response element-binding protein in vivo and in vitro

Biological responses to estrogens are dependent on the integrated actions of proteins, including the estrogen receptor (ER)-alpha, that regulate the transcription of estrogen response element (ERE)-containing target genes. We have identified a naturally occurring ERE antagonist, termed an ERE-binding protein (BP). To verify that ERE-BP can induce estradiol (E(2)) resistance in vivo, we generated transgenic mice that overexpress this protein in breast tissue. Female transgenic mice with high levels of ERE-BP were unable to lactate, and we hypothesized that this effect was dependent on the relative levels of ERE-BP and ERalpha ligand. To test this hypothesis, wild-type and ERE-BP-expressing female mice were implanted with capsules containing E(2), the selective estrogen receptor modulator tamoxifen, or placebo. Histological analysis of nonlactating mammary glands showed a 4.5-fold increase in gland branch number and 3.7-fold increase in ducts in ERE-BP mice treated with E(2) (7.5 mg, 21 d) compared with placebo-treated ERE-BP mice. Wild-type mice showed a 5.3-fold increase in branches and 1.4-fold increase in ducts under the same conditions. Similar results were obtained with tissue from lactating mice, in which tamoxifen also increased mammary gland branch number. Studies using ERE-BP-expressing MCF-7 breast cells showed that high doses of E(2) (1000 nM) restored normal ERalpha-chromatin interaction in these cells, whereas tamoxifen was able to achieve this effect at a dose of 10 nM. These data highlight the importance of ERE-BP as an attenuator of normal ERalpha signaling in vivo and further suggest that ERE-BP is a novel target for modulation by selective estrogen receptor modulators.

Smad7 stabilizes beta-catenin binding to E-cadherin complex and promotes cell-cell adhesion

Beta-catenin functions both as an adherens junction adhesion protein and as an essential mediator of the canonical Wnt signaling pathway. Wnts stabilize beta-catenin and promote its accumulation in the nucleus, where it regulates transcription of the target genes. Here we show that Smad7 promotes cell-cell adhesion by stabilizing beta-catenin and consequently increases the beta-catenin-E-cadherin complex level at the plasma membrane. A Smad7-Axin interaction disassociates GSK-3beta and beta-catenin from Axin, as well as inhibits the recruitment of Smurf2, an E3 ligase, to beta-catenin, thus protecting beta-catenin from phosphorylation and degradation. Smad7 increases the stabilized beta-catenin to form a complex with E-cadherin and stabilizes the E-cadherin-beta-catenin complex. Thereby, rather than being translocated to the nucleus for regulating the target gene transcription, Smad7-stabilized-beta-catenin is shunted to the E-cadherin complex to modulate cell-cell adhesion.

Oxidative stress induces vascular calcification through modulation of the osteogenic transcription factor Runx2 by AKT signaling

Oxidative stress plays a critical role in the pathogenesis of atherosclerosis including the formation of lipid laden macrophages and the development of inflammation. However, oxidative stress-induced molecular signaling that regulates the development of vascular calcification has not been investigated in depth. Osteogenic differentiation of vascular smooth muscle cells (VSMC) is critical in the development of calcification in atherosclerotic lesions. An important contributor to oxidative stress in atherosclerotic lesions is the formation of hydrogen peroxide from diverse sources in vascular cells. In this study we defined molecular signaling that is operative in the H2O2-induced VSMC calcification. We found that H2O2 promotes a phenotypic switch of VSMC from contractile to osteogenic phenotype. This response was associated with an increased expression and transactivity of Runx2, a key transcription factor for osteogenic differentiation. The essential role of Runx2 in oxidative stress-induced VSMC calcification was further confirmed by Runx2 depletion and overexpression. Inhibition of Runx2 using short hairpin RNA blocked VSMC calcification, and adenovirus-mediated overexpression of Runx2 alone induced VSMC calcification. Inhibition of H2O2-activated AKT signaling blocked VSMC calcification and Runx2 induction concurrently. This blockage did not cause VSMC apoptosis. Taken together, our data demonstrate a critical role for AKT-mediated induction of Runx2 in oxidative stress-induced VSMC calcification.

Aberrant Phex function in osteoblasts and osteocytes alone underlies murine X-linked hypophosphatemia

Patients with X-linked hypophosphatemia (XLH) and the hyp-mouse, a model of XLH characterized by a deletion in the Phex gene, manifest hypophosphatemia, renal phosphate wasting, and rickets/osteomalacia. Cloning of the PHEX/Phex gene and mutations in affected patients and hyp-mice established that alterations in PHEX/Phex expression underlie XLH. Although PHEX/Phex expression occurs primarily in osteoblast lineage cells, transgenic Phex expression in hyp-mouse osteoblasts fails to rescue the phenotype, suggesting that Phex expression at other sites underlies XLH. To establish whether abnormal Phex in osteoblasts and/or osteocytes alone generates the HYP phenotype, we created mice with a global Phex knockout (Cre-PhexDeltaflox/y mice) and conditional osteocalcin-promoted (OC-promoted) Phex inactivation in osteoblasts and osteocytes (OC-Cre-PhexDeltaflox/y). Serum phosphorus levels in Cre-PhexDeltaflox/y, OC-Cre-PhexDeltaflox/y, and hyp-mice were lower than those in normal mice. Kidney cell membrane phosphate transport in Cre-PhexDeltaflox/y, OC-Cre-PhexDeltaflox/y, and hyp-mice was likewise reduced compared with that in normal mice. Abnormal renal phosphate transport in Cre-PhexDeltaflox/y and OC-Cre-PhexDeltaflox/y mice was associated with increased bone production and serum FGF-23 levels and decreased kidney membrane type IIa sodium phosphate cotransporter protein, as was the case in hyp-mice. In addition, Cre-PhexDeltaflox/y, OC-Cre-PhexDeltaflox/y, and hyp-mice manifested comparable osteomalacia. These data provide evidence that aberrant Phex function in osteoblasts and/or osteocytes alone is sufficient to underlie the hyp-mouse phenotype.

Oxygen sensing and osteogenesis

Osteogenesis and angiogenesis are tightly coupled during bone formation and repair. Blood vessels not only carry oxygen and nutrients to the developing bone, but also play an active role in bone formation and remodeling by mediating the interaction between osteoblasts, osteocytes, osteoclasts, and vascular cells at a variety of levels. Tissue hypoxia is believed to be a major stimulus for angiogenesis by activating hypoxia-inducible factor alpha (HIFalpha) pathway, which is a central regulator of hypoxia adaptation in vertebrates. HIFalpha remains inactive under normoxic conditions through pVHL-mediated polyubiquitination and proteasomal degradation. Activation of the HIFalpha pathway by hypoxia triggers hypoxia-responsive gene expression, such as vascular endothelial growth factor (Vegf), which plays a critical role in angiogenesis, endochondral bone formation, and bone repair following fracture. Recent work from our laboratory has shown that osteoblasts use the HIFalpha pathway to sense reduced oxygen tension and transmit signals that impinge on angiogenic and osteogenic gene programs during bone formation. Using a genetic approach, we have demonstrated that overexpression of HIFalpha in mouse osteoblasts through disruption of Vhl results in profound increases in angiogenesis and osteogenesis, which appear to be mediated by cell nonautonomous mechanisms involving VEGF. These studies suggest that VEGF exerts many of its actions on bone indirectly by stimulation of angiogenesis. Whether or to what extent this angiogenic factor functions independent of endothelial cells remains to be determined.

Mode of Growth Hormone Action in Osteoblasts

Growth hormone (GH) affects bone size and mass in part through stimulating insulin-like growth factor type 1 (IGF-1) production in liver and bone. Whether GH acts independent of IGF-1 in bone remains unclear. To define the mode of GH action in bone, we have used a Cre/loxP system in which the type 1 IGF-1 receptor (Igf1r) has been disrupted specifically in osteoblasts in vitro and in vivo. Calvarial osteoblasts from mice homozygous for the floxed IGF-1R allele (IGF-1R(flox/flox)) were infected with adenoviral vectors expressing Cre. Disruption of IGF-1R mRNA (>90%) was accompanied by near elimination of IGF-1R protein but retention of GHR protein. GH-induced STAT5 activation was consistently greater in osteoblasts with an intact IGF-1R. Osteoblasts lacking IGF-1R retained GH-induced ERK and Akt phosphorylation and GH-stimulated IGF-1 and IGFBP-3 mRNA expression. GH-induced osteoblast proliferation was abolished by Cre-mediated disruption of the IGF-1R or co-incubation of cells with an IGF-1-neutralizing antibody. By contrast, GH inhibited apoptosis in osteoblasts lacking the IGF-1R. To examine the effects of GH on osteoblasts in vivo, mice wild type for the IGF-1R treated with GH subcutaneously for 7 days showed a doubling in the number of osteoblasts lining trabecular bone, whereas osteoblast numbers in similarly treated mice lacking the IGF-1R in osteoblasts were not significantly affected. These results indicate that although direct IGF-1R-independent actions of GH on osteoblast apoptosis can be demonstrated in vitro, IGF-1R is required for anabolic effects of GH in osteoblasts in vivo.

Conditional disruption of calcineurin B1 in osteoblasts increases bone formation and reduces bone resorption

We recently reported that the pharmacological inhibition of calcineurin (Cn) by low concentrations of cyclosporin A increases osteoblast differentiation in vitro and bone mass in vivo. To determine whether Cn exerts direct actions in osteoblasts, we generated mice lacking Cnb1 (Cn regulatory subunit) in osteoblasts (DeltaCnb1(OB)) using Cre-mediated recombination methods. Transgenic mice expressing Cre recombinase, driven by the human osteocalcin promoter, were crossed with homozygous mice that express loxP-flanked Cnb1 (Cnb1(f/f)). Microcomputed tomography analysis of tibiae at 3 months showed that DeltaCnb1(OB) mice had dramatic increases in bone mass compared with controls. Histomorphometric analyses showed significant increases in mineral apposition rate (67%), bone volume (32%), trabecular thickness (29%), and osteoblast numbers (68%) as well as a 40% decrease in osteoclast numbers as compared with the values from control mice. To delete Cnb1 in vitro, primary calvarial osteoblasts, harvested from Cnb1(f/f) mice, were infected with adenovirus expressing the Cre recombinase. Cre-expressing osteoblasts had a complete inhibition of Cnb1 protein levels but differentiated and mineralized more rapidly than control, green fluorescent protein-expressing cells. Deletion of Cnb1 increased expression of osteoprotegerin and decreased expression of RANKL. Co-culturing Cnb1-deficient osteoblasts with wild type osteoclasts demonstrated that osteoblasts lacking Cnb1 failed to support osteoclast differentiation in vitro. Taken together, our findings demonstrate that the inhibition of Cnb1 in osteoblasts increases bone mass by directly increasing osteoblast differentiation and indirectly decreasing osteoclastogenesis.

IGF-I receptor is required for the anabolic actions of parathyroid hormone on bone

We showed that the IGF-IR-null mutation in mature osteoblasts leads to less bone and decreased periosteal bone formation and impaired the stimulatory effects of PTH on osteoprogenitor cell proliferation and differentiation.

INTRODUCTION

This study was carried out to examine the role of IGF-I signaling in mediating the actions of PTH on bone.

MATERIALS AND METHODS

Three-month-old mice with an osteoblast-specific IGF-I receptor null mutation (IGF-IR OBKO) and their normal littermates were treated with vehicle or PTH (80 microg/kg body weight/d for 2 wk). Structural measurements of the proximal and midshaft of the tibia were made by microCT. Trabecular and cortical bone formation was measured by bone histomorphometry. Bone marrow stromal cells (BMSCs) were obtained to assess the effects of PTH on osteoprogenitor number and differentiation.

RESULTS

The fat-free weight of bone normalized to body weight (FFW/BW), bone volume (BV/TV), and cortical thickness (C.Th) in both proximal tibia and shaft were all less in the IGF-IR OBKO mice compared with controls. PTH decreased FFW/BW of the proximal tibia more substantially in controls than in IGF-IR OBKO mice. The increase in C.Th after PTH in the proximal tibia was comparable in both control and IGF-IR OBKO mice. Although trabecular and periosteal bone formation was markedly lower in the IGF-IR OBKO mice than in the control mice, endosteal bone formation was comparable in control and IGF-IR OBKO mice. PTH stimulated endosteal bone formation only in the control animals. Compared with BMSCs from control mice, BMSCs from IGF-IR OBKO mice showed equal alkaline phosphatase (ALP)(+) colonies on day 14, but fewer mineralized nodules on day 28. Administration of PTH increased the number of ALP(+) colonies and mineralized nodules on days 14 and 28 in BMSCs from control mice, but not in BMSCs from IGF-IR OBKO mice.

CONCLUSIONS

Our results indicate that the IGF-IR null mutation in mature osteoblasts leads to less bone and decreased bone formation, in part because of the requirement for the IGF-IR in mature osteoblasts to enable PTH to stimulate osteoprogenitor cell proliferation and differentiation.

The hypoxia-inducible factor alpha pathway couples angiogenesis to osteogenesis during skeletal development

Skeletal development and turnover occur in close spatial and temporal association with angiogenesis. Osteoblasts are ideally situated in bone to sense oxygen tension and respond to hypoxia by activating the hypoxia-inducible factor alpha (HIF alpha) pathway. Here we provide evidence that HIF alpha promotes angiogenesis and osteogenesis by elevating VEGF levels in osteoblasts. Mice overexpressing HIF alpha in osteoblasts through selective deletion of the von Hippel-Lindau gene (Vhl) expressed high levels of Vegf and developed extremely dense, heavily vascularized long bones. By contrast, mice lacking Hif1a in osteoblasts had the reverse skeletal phenotype of that of the Vhl mutants: long bones were significantly thinner and less vascularized than those of controls. Loss of Vhl in osteoblasts increased endothelial sprouting from the embryonic metatarsals in vitro but had little effect on osteoblast function in the absence of blood vessels. Mice lacking both Vhl and Hif1a had a bone phenotype intermediate between those of the single mutants, suggesting overlapping functions of HIFs in bone. These studies suggest that activation of the HIF alpha pathway in developing bone increases bone modeling events through cell-nonautonomous mechanisms to coordinate the timing, direction, and degree of new blood vessel formation in bone.

Disruption of the insulin-like growth factor type 1 receptor in osteoblasts enhances insulin signaling and action

Defective bone formation is common in patients with diabetes, suggesting that insulin normally exerts anabolic actions in bone. However, because insulin can cross-activate the insulin-like growth factor type 1 receptor (IGF-1R), which also functions in bone, it has been difficult to establish the direct (IGF-1-independent) actions of insulin in osteoblasts. To overcome this problem, we examined insulin signaling and action in primary osteoblasts engineered for conditional disruption of the IGF-1 receptor (DeltaIGF-1R). Calvarial osteoblasts from mice carrying floxed IGF-1R alleles were infected with adenoviral vectors expressing the Cre recombinase (Ad-Cre) or green fluorescent protein (Ad-GFP) as control. Disruption of IGF-1R mRNA (>90%) eliminated IGF-1R without affecting insulin receptor (IR) mRNA and protein expression and eliminated IGF-1R/IR hybrids. In DeltaIGF-1R osteoblasts, insulin signaling was markedly increased as evidenced by increased phosphorylation of insulin receptor substrate 1/2 and enhanced ERK/Akt activation. Microarray analysis of RNA samples from insulin-treated, DeltaIGF-1R osteoblasts revealed striking changes in several genes known to be downstream of ERK including Glut-1 and c-fos. Treatment of osteoblasts with insulin induced Glut-1 mRNA, increased 2-[1,2-(3)H]-deoxy-d-glucose uptake, and enhanced proliferation. Moreover, insulin treatment rescued the defective differentiation and mineralization of DeltaIGF-1R osteoblasts, suggesting that IR signaling can compensate, at least in part, for loss of IGF-1R signaling. We conclude that insulin exerts direct anabolic actions in osteoblasts by activation of its cognate receptor and that the strength of insulin-generated signals is tempered through interactions with the IGF-1R.

Lifelong accumulation of bone in mice lacking Pten in osteoblasts

Bone formation is carried out by the osteoblast, a mesenchymal cell whose lifespan and activity are regulated by growth factor signaling networks. Growth factors activate phosphatidylinositol 3-kinase (PI3K), which enhances cell survival and antagonizes apoptosis through activation of Akt/PKB. This process is negatively regulated by the Pten phosphatase, which inhibits the activity of PI3K. In this study, we investigated the effects of Akt activation in bone in vivo by conditionally disrupting the Pten gene in osteoblasts by using Cre-mediated recombination. Mice deficient in Pten in osteoblasts were of normal size but demonstrated a dramatic and progressively increasing bone mineral density throughout life. In vitro osteoblasts lacking Pten differentiated more rapidly than controls and exhibited greatly reduced apoptosis in association with markedly increased levels of phosphorylated Akt and activation of signaling pathways downstream of activated Akt. These findings support a critical role for this tumor-suppressor gene in regulating osteoblast lifespan and likely explain the skeletal abnormalities in patients carrying germ-line mutations of PTEN.