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 hypoxiainducible factor α (HIFα) pathway. Here we provide evidence that HIFα promotes angiogenesis and osteogenesis by elevating VEGF levels in osteoblasts. Mice overexpressing HIFα 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 HIFα 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. IntroductionThe development of the mammalian skeleton takes place in distinct phases involving the initial migration of cells to the site of future bone, condensation of mesenchymal cells, and finally the differentiation of progenitors into chondrocytes and osteoblasts. During intramembranous bone formation, which gives rise to the flat bones of the skull, mesenchymal cells differentiate directly into bone-forming osteoblasts. By contrast, in endochondral bone formation, bones are formed through a 2-stage mechanism that begins with the formation of a chondrocyte anlage, onto which osteoblasts then differentiate and deposit bone. Endochondral bone formation occurs in close spatial and temporal association and proximity to capillary invasion, suggesting that angiogenesis and osteogenesis are coupled.The initial signals for blood vessel invasion into bone are unknown, but tissue hypoxia is believed to be critical for commencement of the angiogenic cascade (1). Hypoxia triggers the changes in oxygen-regulated gene expression via the activation of the Per/Arnt/Sim (PAS)

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 exogenous 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 to control glucose homeostasis.

To examine the local actions of IGF signaling in skeletal tissue in a physiological context, we have used Cremediated recombination to disrupt selectively in mouse osteoblasts the gene encoding the type 1 IGF receptor (Igf1r). Mice carrying this bone-specific mutation were of normal size and weight but, in comparison with normal siblings, demonstrated a striking decrease in cancellous bone volume, connectivity, and trabecular number, and an increase in trabecular spacing. These abnormalities correlated with a striking decrease in the rate of mineralization of osteoid that occurred despite an unexpected osteoblast and osteoclast hyperactivity, detected from the significant increments in both osteoblast and erosion surfaces. Our findings indicate that IGF1 is essential for coupling matrix biosynthesis to sustained mineralization. This action is likely to be particularly important during the pubertal growth spurt when rapid bone formation and consolidation are required.Body size and linear bone growth in mammals is affected by cellular signaling pathways controlled by growth factors and hormones (1). In this regard, a major growth-promoting signaling system consisting of the insulin-like growth factors (IGF, 1 IGF1 and IGF2) and the type 1 IGF receptor (IGF1R) regulates embryonic growth, as shown by gene knockout experiments in mice (1). IGF1 acting through IGF1R also plays central roles in postnatal growth either independently or by mediating growth hormone functions (2). Signaling through the IGF1R tyrosine kinase receptor not only promotes cell proliferation, but also mediates anti-apoptotic actions (3, 4). The IGF system includes a second receptor (IGF2R) devoid of signaling properties, but serving IGF2 turnover, and at least six IGF-binding proteins (IGFBPs) of obscure functional significance (single and also some double mouse mutations ablating IGFBPs have not revealed as yet significant consequences in growth impairment). 2 The IGFs are produced locally in various tissues, including bones, and exert autocrine/paracrine functions, but they are also present in serum, mostly associated with IGFBPs. Whether the circulating IGFs act systemically as hormones is currently controversial (5, 6).A number of in vitro and in vivo studies are progressively unraveling the significance of the IGF system for skeletal development and metabolic control (for a review see Ref. 7). IGF1, by stimulating the proliferation of chondrocytes in the growth plate, plays an essential role in longitudinal bone growth (2) and is also involved in the formation of trabecular bone. In fact, chondrocytes and bone cells produce IGFs and express IGF1R (see for example Refs. 8 and 9). Studies using osteoblast culture systems have shown that IGF1 stimulates osteoblast proliferation, accelerates their differentiation, and enhances bone matrix production (10, 11). In addition, IGF1 is being recognized as a critical factor for bone cell survival (12)(13)(14). Finally, IGF1 also appears to regulate bone resorption, either directly or through its actio...

Mutations in the Wnt co-receptor LRP5 alter bone mass in humans, but the mechanisms responsible for Wnts actions in bone are unclear. To investigate the role of the classical Wnt signaling pathway in osteogenesis, we generated mice lacking the ␤-catenin or adenomatous polyposis coli (Apc) genes in osteoblasts. Loss of ␤-catenin produced severe osteopenia with striking increases in osteoclasts, whereas constitutive activation of ␤-catenin in the conditional Apc mutants resulted in dramatically increased bone deposition and a disappearance of osteoclasts. In vitro, osteoblasts lacking the ␤-catenin gene exhibited impaired maturation and mineralization with elevated expression of the osteoclast differentiation factor, receptor activated by nuclear factor-B ligand (RANKL), and diminished expression of the RANKL decoy receptor, osteoprotegerin. By contrast, Apc-deficient osteoblasts matured normally but demonstrated decreased expression of RANKL and increased osteoprotegerin. These findings suggest that Wnt/␤-catenin signaling in osteoblasts coordinates postnatal bone acquisition by controlling the differentiation and activity of both osteoblasts and osteoclasts.

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Decreased bone mass, osteoporosis, and increased fracture rates are common skeletal complications in patients with insulin-dependent diabetes mellitus (IDDM; type I diabetes). IDDM develops from little or no insulin production and is marked by elevated blood glucose levels and weight loss. In this study we use a streptozotocin-induced diabetic mouse model to examine the effect of type I diabetes on bone. Histology and microcomputed tomography demonstrate that adult diabetic mice, exhibiting increased plasma glucose and osmolality, have decreased trabecular bone mineral content compared with controls. Bone resorption could not completely account for this effect, because resorption markers (tartrate-resistant acid phosphatase 5b, urinary deoxypyridinoline excretion, and tartrate-resistant acid phosphatase 5 mRNA) are unchanged or reduced at 2 and/or 4 wk after diabetes induction. However, osteocalcin mRNA (a marker of late-stage osteoblast differentiation) and dynamic parameters of bone formation were decreased in diabetic tibias, whereas osteoblast number and runx2 and alkaline phosphatase mRNA levels did not differ. These findings suggest that the final stages of osteoblast maturation and function are suppressed. We also propose a second mechanism contributing to diabetic bone loss: increased marrow adiposity. This is supported by increased expression of adipocyte markers [peroxisome proliferator-activated receptor gamma2, resistin, and adipocyte fatty acid binding protein (alphaP2)] and the appearance of lipid-dense adipocytes in diabetic tibias. In contrast to bone marrow, adipose stores at other sites are depleted in diabetic mice, as indicated by decreased body, liver, and peripheral adipose tissue weights. These findings suggest that IDDM contributes to bone loss through changes in marrow composition resulting in decreased mature osteoblasts and increased adipose accumulation.

Abstract. Renal transplant recipients are at risk of developing bone abnormalities that result in bone loss and bone fractures. These are related to underlying renal osteodystrophy, hypophosphatemia, and immunosuppressive treatment regimen. Although bisphosphonates are useful in ameliorating bone mineral loss after transplantation, it is not known whether their use in renal transplant patients leads to excessive suppression of bone turnover and increased incidence of adynamic bone disease. A randomized, prospective, controlled, clinical trial was conducted using the bisphosphonate pamidronate intravenously in patients with new renal transplants. Treatment subjects (PAM) received pamidronate with vitamin D and calcium at baseline and at months 1, 2, 3, and 6. Control (CON) subjects received vitamin D and calcium only. During months 6 to 12, the subjects were observed without pamidronate treatment. Biochemical parameters of bone turnover were obtained monthly and, bone mineral density (BMD) was obtained at baseline and months 6 and 12. Bone biopsies for mineralized bone histology were obtained at baseline and at 6 mo in a subgroup of subjects who underwent scheduled living donor transplantation. PAM preserved bone mass at 6 and 12 mo as measured by bone densitometry and histomorphometry. CON had decreased vertebral BMD at 6 and 12 mo (4.8 Ϯ 0.08 and 6.1 Ϯ 0.09%, respectively). Biochemical parameters of bone turnover were similar in both groups at 6 and 12 mo. Bone histology revealed low turnover bone disease in 50% of the patients at baseline. At 6 mo, all of PAM had adynamic bone disease, whereas 50% of CON continued to have or developed decreased bone turnover. Pamidronate preserved vertebral BMD during treatment and 6 mo after cessation of treatment. Pamidronate treatment was associated with development of adynamic bone histology. Whether an improved BMD with adynamic bone histology is useful in maintaining long-term bone health in renal transplant recipients requires further study.

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.Akt ͉ bone acquisition ͉ osteoblast survival ͉ high bone mass T he development and maintenance of the mammalian skeleton are controlled by actions of morphogens and growth factors on bone cells. Bone formation is carried out by the osteoblast, a mesenchymal cell whose lifespan and activity are regulated by growth factor signaling networks (1, 2). Skeletal growth factors such as insulin-like growth factor-1 (IGF-1) affect osteoblast proliferation and lifespan by activating anti-apoptotic pathways, increasing cell proliferation, and influencing differentiation (3). A key control point in many anti-apoptotic pathways is the lipid kinase phosphatidylinositol (PI) 3-kinase (PI3K), which is activated in response to various extracellular signals (4, 5). On activation of growth factor receptor tyrosine kinases, a p85 regulatory subunit of PI3K is recruited to phosphorylated tyrosine residues. This action engages and activates a catalytic subunit (p110) and induces phosphorylation of the inositol ring of PI(4)P or PI(4,5)P 2 at the D position to generate the second messengers PI(3,4)P 2 and PI(3,4,5)P 3 (4, 5). A key downstream target of PI3K and PIP 3 is the serine-threonine Akt kinase family (also known as protein kinase B). PIP 3 generated in the plasma membrane recruits Akt and PI-dependent kinase 1 (PDK1) through an interaction between the PI and the Akt or PDK1 pleckstrin homology (PH) domains. Once recruited to the plasma membrane, Akt is phosphorylated and activated by PDK1.