The effects of antifracture therapies on the components of bone strength: assessment of fracture risk today and in the future.

Objective: To summarize the current knowledge regarding the impact of the most common antifracture medications on the various determinants of bone strength.

Methods: Relevant English-language articles acquired from Medline from 1966 to January 2005 were reviewed. Searches included the keywords bone AND 1 of the following: strength, remodeling, microcrack, structure, mineralization, collagen, organic, crystallinity, osteocyte, porosity, diameter, anisotropy, stress risers, or connectivity AND alendronate, estrogen, etidronate, hormone replacement therapy, parathyroid hormone, risedronate, OR teriparatide.

Exogenous 1,25-dihydroxyvitamin D3 exerts a skeletal anabolic effect and improves mineral ion homeostasis in mice that are homozygous for both the 1alpha-hydroxylase and parathyroid hormone null alleles.

1,25-Dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and PTH each modulate calcium and skeletal homeostasis. To identify 1,25(OH)(2)D(3)-mediated skeletal and mineral ion actions independent of PTH, double-knockout mice, which are homozygous for both the 1alpha-hydroxylase and PTH null alleles, were treated with 1,25(OH)(2)D(3), sc, from d 4 to 14 and compared with vehicle-treated animals. Serum calcium rose in 1,25(OH)(2)D(3)-treated double-knockout mice, and messenger RNA and protein levels of the renal calcium transporters TRPV5, calbindin-D(28K), calbindin-D(9K), and Na(+)/Ca(2+) exchanger 1 were up-regulated.

Parathyroid hormone for the treatment of osteoporosis: a systematic review.

Background: Human parathyroid hormone (hPTH)(1-34) was approved in 2004 for the treatment of severe osteoporosis. Members of the Osteoporosis Canada clinical guidelines committee conducted a systematic review of randomized controlled trials (RCTs) to assess the efficacy and safety of hPTH for fracture prevention in postmenopausal women and men with osteoporosis.

Methods: We searched MEDLINE, EMBASE, HTA, Current Contents and the Cochrane Controlled Trials Registry for published data from 1966 to February 2005.

The effect of cyclooxygenase-2 inhibitors on bone mineral density: results from the Canadian Multicentre Osteoporosis Study.

Introduction: The use of cyclooxygenase-2 (COX-2) inhibitors has been demonstrated to not only impair load-induced bone formation but also prevent menopause-associated bone loss. We hypothesized that COX-2 inhibitor use would be associated with increased bone mineral density (BMD) in postmenopausal women not using estrogen therapy and, conversely, with decreased BMD in men.

Methods: The Canadian Multicentre Osteoporosis Study is a longitudinal, randomly selected, population-based community cohort.

Expression of RANKL/RANK/OPG in primary and metastatic human prostate cancer as markers of disease stage and functional regulation.

Background: Late-stage prostate cancer patients are refractory to hormone therapy and exhibit a high propensity to develop skeletal metastasis. In this regard, the role of a novel cytokine system belonging to the tumor necrosis factor (TNF) family that is critical for osteoclastic osteolysis and that consists of receptor activator of NF-kappaB ligand (RANKL), its receptor (RANK), and decoy receptor osteoprotegerin (OPG) is of potential interest.

Methods: Reverse-transcriptase polymerase chain reaction (RT-PCR) and immunohistochemical analysis was used to examine the expression of RANKL, RANK, and OPG in human prostate cancer cell lines and in 89 archival samples of primary and metastatic (lymph nodes, skeleton) prostate cancer patients.

New insights into mineral and skeletal regulation by active forms of vitamin D.

Recent studies in mice using genetic approaches have shed new light on the physiological effects of 1,25-dihydroxyvitamin D (1,25(OH)(2)D) and the vitamin D receptor (VDR) in skeletal and mineral homeostasis, and on their interaction with calcium. These studies in mice with targeted deletion of the 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha(OH)ase), and of the VDR or of double mutants, have shown the discrete effects of calcium in inhibiting parathyroid hormone secretion and in enhancing bone mineralization, but overlapping effects of calcium and 1,25(OH)(2)D on inhibiting parathyroid growth and on normal development of the cartilaginous growth plate. The 1,25(OH)(2)D/VDR system is essential, however, in enhancing intestinal calcium absorption and in optimally increasing osteoclastic activation.

Exogenous PTH-related protein and PTH improve mineral and skeletal status in 25-hydroxyvitamin D-1alpha-hydroxylase and PTH double knockout mice.

Unlabelled: We examined the effect of NH2-terminal fragments of PTHrP and PTH in young mutant mice deficient in both PTH and 1,25-dihydroxyvitamin D. Both proteins prolonged murine survival by increasing serum calcium, apparently by enhancing renal calcium transporter expression. The dominant effect on the skeleton was an increase in both endochondral bone and appositional formation without increased bone resorption.

Osteoblast-derived PTHrP is a potent endogenous bone anabolic agent that modifies the therapeutic efficacy of administered PTH 1-34.

Mice heterozygous for targeted disruption of Pthrp exhibit, by 3 months of age, diminished bone volume and skeletal microarchitectural changes indicative of advanced osteoporosis. Impaired bone formation arising from decreased BM precursor cell recruitment and increased apoptotic death of osteoblastic cells was identified as the underlying mechanism for low bone mass. The osteoporotic phenotype was recapitulated in mice with osteoblast-specific targeted disruption of Pthrp, generated using Cre-LoxP technology, and defective bone formation was reaffirmed as the underlying etiology.

Does calcitriol have actions independent from the vitamin D receptor in maintaining skeletal and mineral homeostasis?

Purpose Of Review: Although the active metabolite of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)2D), is classically appreciated to exert its calcemic and other actions via interaction with the vitamin D receptor, thereby modulating gene transcription, some of its actions cannot be explained in this way when examined in vitro.

Recent Findings: Comparison of mouse models deleted for either the 25-hydroxyvitamin D-1alpha-hydroxylase enzyme (deficient in 1,25(OH)2D) or the vitamin D receptor or both has allowed an assessment of whether 1,25(OH)2D can function in the absence of the vitamin D receptor in vivo. The data indicated that calcium absorption required both the ligand and the receptor as did bone and cartilage remodeling.

Genetic models show that parathyroid hormone and 1,25-dihydroxyvitamin D3 play distinct and synergistic roles in postnatal mineral ion homeostasis and skeletal development.

In humans, loss-of-function mutations in parathyroid hormone (PTH) and 25-hydroxyvitamin D3-1alpha-hydroxylase [1alpha(OH)ase] genes lead to isolated hypoparathyroidism and vitamin D-dependent rickets type I, respectively. To better understand the relative contributions of PTH and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] to skeletal and calcium homeostasis, we compared mice with targeted disruption of the PTH or 1alpha(OH)ase genes to the double null mutants. Although PTH-/- and 1alpha(OH)ase-/- mice displayed only moderate hypocalcemia, PTH-/-1alpha(OH)ase-/- mice died of tetany with severe hypocalcemia by 3 weeks of age.

Regulation of parathyroid hormone-related peptide by estradiol: effect on tumor growth and metastasis in vitro and in vivo.

We evaluated the capacity of estradiol (E(2)) to regulate PTHrP production, cell growth, tumor growth, and metastasis to the skeleton in breast cancer. In estrogen receptor (ER)-negative human breast cancer cells, MDA-MB-231, and cells transfected with full-length cDNA encoding ER (S-30), E(2) caused a marked decrease in cell growth and PTHrP production, effects that were abrogated by anti-E(2) tamoxifen. E(2) also inhibited PTHrP promoter activity in S-30 cells.

Do cyclooxygenase-2 knockout mice have primary hyperparathyroidism?

The absence of cyclooxygenase-2 (COX-2) activity in vitro reduces differentiation of both bone-forming and bone-resorbing cells. To examine the balance of COX-2 effects on bone in vivo, we studied COX-2 knockout (KO) and wild-type (WT) mice. After weaning, KO mice died 4 times faster than WT mice, consistent with reports of progressive renal failure in KO mice.

Rosiglitazone impacts negatively on bone by promoting osteoblast/osteocyte apoptosis.

Thiazolidinediones (TZDs) increase peripheral tissue insulin sensitivity in patients with type 2 diabetes mellitus by activating the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma). In bone marrow stromal cell cultures and in vivo, activation of PPARgamma by high doses (20 mg/kg/day) of TZDs has been reported to alter stem cell differentiation by promoting commitment of progenitor cells to the adipocytic lineage while inhibiting osteoblastogenesis. Here, we have examined the in vivo effects of low-dose rosiglitazone (3 mg/kg/day) on bone, administered to mice by gavage for 90 days.

Up-regulation of Wnt-1 and beta-catenin production in patients with advanced metastatic prostate carcinoma: potential pathogenetic and prognostic implications.

Background: Wnt-1 and beta-catenin expression levels play an important role in several malignancies. The authors determined the level of production of Wnt-1 and beta-catenin in normal and malignant human prostate carcinoma cell lines. Surgical pathology specimens from primary prostatic adenocarcinoma, lymph node metastases, and skeletal metastases were used to establish a correlation between the level of Wnt-1/beta-catenin expression, Gleason score, serum prostate-specific antigen (PSA) status, and androgen receptor (AR) status.

Transgenic mice overexpressing human fibroblast growth factor 23 (R176Q) delineate a putative role for parathyroid hormone in renal phosphate wasting disorders.

Fibroblast growth factor 23 (FGF23) is a recently characterized protein likely involved in the regulation of serum phosphate homeostasis. Increased circulating levels of FGF23 have been reported in patients with renal phosphate-wasting disorders, but it is unclear whether FGF23 is the direct mediator responsible for the decreased phosphate transport at the proximal renal tubules and the altered vitamin D metabolism associated with these states. To examine this question, we generated transgenic mice expressing and secreting from the liver human FGF23 (R176Q), a mutant form that fails to be degraded by furin proteases.

Effects of calcium and of the Vitamin D system on skeletal and calcium homeostasis: lessons from genetic models.

Targeted deletion of genes encoding the 1,25-dihydroxyVitamin D [1,25(OH)(2)D]-synthesizing enzyme, 25 hydroxyVitamin D-1alpha-hydroxylase [1alpha(OH)ase or CYP27B1], and of the nuclear receptor for 1,25(OH)(2)D, the Vitamin D receptor (VDR), have provided useful mouse models of the inherited human diseases, Vitamin D-dependent rickets types I and II. We employed these models and double null mutants to examine the effects of calcium and of the 1,25(OH)(2)D/VDR system on skeletal and calcium homeostasis. Optimal dietary calcium absorption required both 1,25(OH)(2)D and the VDR.

Parathyroid hormone-related peptide is required for increased trabecular bone volume in parathyroid hormone-null mice.

We investigated the relative contributions of PTH and PTHrP to the skeletal phenotype of mice deficient in PTH (PTH(-/-)). PTH(-/-) mice and PTH(-/-) mice lacking one allele encoding PTHrP (PTH(-/-)PTHrP(+/-)) were compared. Both mutants displayed similar biochemical abnormalities of hypoparathyroidism, but skeletal PTHrP mRNA and protein were decreased in PTH(-/-)PTHrP(+/ -) mice.

Vitamin D analogs as therapeutic agents: a clinical study update.

Vitamin D3 is modified by vitamin D3-25-hydroxylase in the liver and 25-hydroxyvitamin D3-1 alpha-hydroxylase in the kidney to form the active metabolite 1 alpha,25-dihydroxyvitamin D3. The binding of 1 alpha,25-dihydroxyvitamin D3 to the vitamin D receptor (VDR), a nuclear receptor, activates VDR to interact with retinoid X receptor (RXR) and form the VDR/RXR/co-factor complex, which binds to vitamin D response elements in the promoter region of target genes to regulate gene transcription. 1 alpha,25-dihydroxyvitamin D3 regulates the homeostasis of calcium and phosphorus, and also controls the expression of parathyroid hormone.

Cartilage abnormalities are associated with abnormal Phex expression and with altered matrix protein and MMP-9 localization in Hyp mice.

X-linked hypophosphatemic rickets (HYP) in humans is caused by mutations in the PHEX gene. This gene mutation is also found in Hyp mice, the murine homologue of the human disease. At present, it is unknown why loss of Phex function leads to cartilage abnormalities in Hyp mice.

Impaired endochondral bone development and osteopenia in Gli2-deficient mice.

Mice homozygous for targeted disruption of the zinc finger domain of Gli2 (Gli2(zfd/zfd)) die at birth with developmental defects in several organ systems including the skeleton. The current studies were undertaken to define the role of Gli2 in endochondral bone development by characterizing the molecular defects in the limbs and vertebrae of Gli2(zfd/zfd) mice. The bones of mutant mice removed by cesarian section at E16.

Signalling by fibroblast growth factor receptor 3 and parathyroid hormone-related peptide coordinate cartilage and bone development.

Bone development is regulated by conserved signalling pathways that are linked to multifunctional growth factors and their high affinity receptors. Parathyroid hormone-related peptide (PTHrP) and fibroblast growth factor receptor 3 (FGFR3) have been shown to play pivotal, and sometimes complementary, roles in the replication, maturation and death of chondrocytes during endochondral bone formation. To gain further insight into how these pathways coordinate cartilage and bone development, we generated mice lacking expression of both PTHrP and FGFR3.

Inactivation of the 25-hydroxyvitamin D 1alpha-hydroxylase and vitamin D receptor demonstrates independent and interdependent effects of calcium and vitamin D on skeletal and mineral homeostasis.

We employed a genetic approach to determine whether deficiency of 1,25-dihydroxyvitamin D (1,25(OH)2D) and deficiency of the vitamin D receptor (VDR) produce the same alterations in skeletal and calcium homeostasis and whether calcium can subserve the skeletal functions of 1,25(OH)2D and the VDR. Mice with targeted deletion of the 25-hydroxyvitamin D 1alpha-hydroxylase (1alpha(OH)ase-/-) gene, the VDR gene, and both genes were exposed to 1) a high calcium intake, which maintained fertility but left mice hypocalcemic; 2) this intake plus three times weekly injections of 1,25(OH)2D3, which normalized calcium in the 1alpha(OH)ase-/- mice only; or 3) a "rescue" diet, which normalized calcium in all mutants. These regimens induced different phenotypic changes, thereby disclosing selective modulation by calcium and the vitamin D system.

Skeletal abnormalities in Pth-null mice are influenced by dietary calcium.

We have examined the role of PTH in the postnatal state in a mouse model of PTH deficiency generated by targeting the Pth gene in embryonic stem cells. Mice homozygous for the ablated allele, when maintained on a normal calcium intake, developed hypocalcemia, hyperphosphatemia, and low circulating 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] levels consistent with primary hypoparathyroidism. Bone turnover was reduced, leading to increased trabecular and cortical bone volume in PTH-deficient mice.

Defective bone mineralization and osteopenia in young adult FGFR3-/- mice.

Mutations that cause constitutive activation of fibroblast growth factor receptor 3 (FGFR3) result in skeletal disorders that are characterized by short-limbed dwarfism and premature closure of cranial sutures. In previous work, it was shown that congenital deficiency of FGFR3 led to skeletal overgrowth. Using a combination of imaging, classic histology and molecular cell biology we now show that young adult FGFR3(-/-) mice are osteopenic due to reduced cortical bone thickness and defective trabecular bone mineralization.