Romosozumab or Alendronate for Fracture Prevention in Women with Osteoporosis.

BACKGROUND: Romosozumab is a monoclonal antibody that binds to and inhibits sclerostin, increases bone formation, and decreases bone resorption. METHODS: We enrolled 4093 postmenopausal women with osteoporosis and a fragility fracture and randomly assigned them in a 1:1 ratio to receive monthly subcutaneous romosozumab (210 mg) or weekly oral alendronate (70 mg) in a blinded fashion for 12 months, followed by open-label alendronate in both groups. The primary end points were the cumulative incidence of new vertebral fracture at 24 months and the cumulative incidence of clinical fracture (nonvertebral and symptomatic vertebral fracture) at the time of the primary analysis (after clinical fractures had been confirmed in ≥330 patients). Secondary end points included the incidences of nonvertebral and hip fracture at the time of the primary analysis. Serious cardiovascular adverse events, osteonecrosis of the jaw, and atypical femoral fractures were adjudicated. RESULTS: Over a period of 24 months, a 48% lower risk of new vertebral fractures was observed in the romosozumab-to-alendronate group (6.2% [127 of 2046 patients]) than in the alendronate-to-alendronate group (11.9% [243 of 2047 patients]) (P<0.001). Clinical fractures occurred in 198 of 2046 patients (9.7%) in the romosozumab-to-alendronate group versus 266 of 2047 patients (13.0%) in the alendronate-to-alendronate group, representing a 27% lower risk with romosozumab (P<0.001). The risk of nonvertebral fractures was lower by 19% in the romosozumab-to-alendronate group than in the alendronate-to-alendronate group (178 of 2046 patients [8.7%] vs. 217 of 2047 patients [10.6%]; P=0.04), and the risk of hip fracture was lower by 38% (41 of 2046 patients [2.0%] vs. 66 of 2047 patients [3.2%]; P=0.02). Overall adverse events and serious adverse events were balanced between the two groups. During year 1, positively adjudicated serious cardiovascular adverse events were observed more often with romosozumab than with alendronate (50 of 2040 patients [2.5%] vs. 38 of 2014 patients [1.9%]). During the open-label alendronate period, adjudicated events of osteonecrosis of the jaw (1 event each in the romosozumab-to-alendronate and alendronate-to-alendronate groups) and atypical femoral fracture (2 events and 4 events, respectively) were observed. CONCLUSIONS: In postmenopausal women with osteoporosis who were at high risk for fracture, romosozumab treatment for 12 months followed by alendronate resulted in a significantly lower risk of fracture than alendronate alone. (Funded by Amgen and others; ARCH ClinicalTrials.gov number, NCT01631214 .).

Parathyroid Hormone-Related Protein (PTHrP): An Emerging Target in Cancer Progression and Metastasis.

PTHrP was first discovered as the most common mediator of malignancy-associated hypercalcemia. Subsequently, the discovery of its ubiquitous expression in normal tissues unraveled its role as a physiological autocrine/paracrine regulator. The significance of PTHrP in cancer is not confined to malignancy-associated hypercalcemia, and sufficient evidence now also supports its role in skeletal metastasis through its modulation of bone turnover. Furthermore, our own studies have recently shown the critical role of PTHrP in breast cancer initiation, growth, and metastasis. More recently, we have provided new evidence that overexpression of PTHrP is associated with higher incidence of brain metastasis and decreased overall survival in triple-negative breast cancer patients. Further mechanistic studies in human and mouse model are necessary to fully understand the role of PTHrP in tumor progression and metastasis.

Defective interplay between mTORC1 activity and endoplasmic reticulum stress-unfolded protein response in uremic vascular calcification.

Vascular calcification increases the risk of cardiovascular disease and death in patients with chronic kidney disease (CKD). Increased activity of mammalian target of rapamycin complex 1 (mTORC1) and endoplasmic reticulum (ER) stress-unfolded protein response (UPR) are independently reported to partake in the pathogenesis of vascular calcification in CKD. However, the association between mTORC1 activity and ER stress-UPR remains unknown. We report here that components of the uremic state [activation of the receptor for advanced glycation end products (RAGE) and hyperphosphatemia] potentiate vascular smooth muscle cell (VSMC) calcification by inducing persistent and exaggerated activity of mTORC1. This gives rise to prolonged and excessive ER stress-UPR as well as attenuated levels of sestrin 1 ( Sesn1) and Sesn3 feeding back to inhibit mTORC1 activity. Activating transcription factor 4 arising from the UPR mediates cell death via expression of CCAAT/enhancer-binding protein (c/EBP) homologous protein (CHOP), impairs the generation of pyrophosphate, a potent inhibitor of mineralization, and potentiates VSMC transdifferentiation to the osteochondrocytic phenotype. Short-term treatment of CKD mice with rapamycin, an inhibitor of mTORC1, or tauroursodeoxycholic acid, a bile acid that restores ER homeostasis, normalized mTORC1 activity, molecular markers of UPR, and calcium content of aortas. Collectively, these data highlight that increased and/or protracted mTORC1 activity arising from the uremic state leads to dysregulated ER stress-UPR and VSMC calcification. Manipulation of the mTORC1-ER stress-UPR pathway opens up new therapeutic strategies for the prevention and treatment of vascular calcification in CKD.

Fibroblast Growth Factor 23 Regulation by Systemic and Local Osteoblast-Synthesized 1,25-Dihydroxyvitamin D.

Circulating levels of fibroblast growth factor 23 (FGF23) increase during the early stages of kidney disease, but the underlying mechanism remains incompletely characterized. We investigated the role of vitamin D metabolites in regulating intact FGF23 production in genetically modified mice without and with adenine-induced uremia. Exogenous calcitriol (1,25-dihydroxyvitamin D) and high circulating levels of calcidiol (25-hydroxyvitamin D) each increased serum FGF23 levels in wild-type mice and in mice with global deficiency of the Cyp27b1 gene encoding 25-hydroxyvitamin D 1-α-hydroxylase, which produces 1,25-hydroxyvitamin D. Compared with wild-type mice, normal, or uremic mice lacking Cyp27b1 had lower levels of serum FGF23, despite having high concentrations of parathyroid hormone, but administration of exogenous 1,25-dihydroxyvitamin D increased FGF23 levels. Furthermore, raising serum calcium levels in Cyp27b1-depleted mice directly increased FGF23 levels and indirectly enhanced the action of ambient vitamin D metabolites via the vitamin D receptor. In chromatin immunoprecipitation assays, 25-hydroxyvitamin D promoted binding of the vitamin D receptor and retinoid X receptor to the promoters of osteoblastic target genes. Conditional osteoblastic deletion of Cyp27b1 caused lower serum FGF23 levels, despite normal circulating levels of vitamin D metabolites. In adenine-induced uremia, only a modest increase in serum FGF23 levels occurred in mice with osteoblastic deletion of Cyp27b1 (12-fold) compared with a large increase (58-fold) in wild-type mice. Therefore, in addition to the direct effect of high circulating concentrations of 25-hydroxyvitamin D, local osteoblastic conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D appears to be an important positive regulator of FGF23 production, particularly in uremia.

Knockdown of parathyroid hormone related protein in smooth muscle cells alters renal hemodynamics but not blood pressure.

Parathyroid hormone-related protein (PTHrP) belongs to vasoactive factors that regulate blood pressure and renal hemodynamics both by reducing vascular tone and raising renin release. PTHrP is expressed in systemic and renal vasculature. Here, we wanted to assess the contribution of vascular smooth muscle cell endogenous PTHrP to the regulation of cardiovascular and renal functions. We generated a mouse strain (SMA-CreERT2/PTHrPL2/L2 or premutant PTHrPSM-/-), which allows temporally controlled, smooth muscle-targeted PTHrP knockdown in adult mice. Tamoxifen treatment induced efficient recombination of PTHrP-floxed alleles and decreased PTHrP expression in vascular and visceral smooth muscle cells of PTHrPSM-/- mice. Blood pressure remained unchanged in PTHrPSM-/- mice, but plasma renin concentration and creatinine clearance were reduced. Renal hemodynamics were further analyzed during clearance measurements in anesthetized mice. Conditional knockdown of PTHrP decreased renal plasma flow and glomerular filtration rate with concomitant reduction in filtration fraction. Similar measurements were repeated during acute saline volume expansion. Saline volume expansion induced a rise in renal plasma flow and reduced filtration fraction; both were blunted in PTHrPSM-/- mice leading to impaired diuresis. These findings show that endogenous vascular smooth muscle PTHrP controls renal hemodynamics under basal conditions, and it is an essential factor in renal vasodilation elicited by saline volume expansion.

PTHrP Regulates Fatty Acid Metabolism via Novel lncRNA in Breast Cancer Initiation and Progression Models.

Parathyroid hormone-related peptide (PTHrP) is the primary cause of malignancy-associated hypercalcemia (MAH). We previously showed that PTHrP ablation, in the MMTV-PyMT murine model of breast cancer (BC) progression, can dramatically prolong tumor latency, slow tumor growth, and prevent metastatic spread. However, the signaling mechanisms using lineage tracing have not yet been carefully analyzed. Here, we generated Pthrpflox/flox; Cre+ mT/mG mice (KO) and Pthrpwt/wt; Cre+ mT/mG tumor mice (WT) to examine the signaling pathways under the control of PTHrP from the early to late stages of tumorigenesis. GFP+ mammary epithelial cells were further enriched for subsequent RNA sequencing (RNAseq) analyses. We observed significant upregulation of cell cycle signaling and fatty acid metabolism in PTHrP WT tumors, which are linked to tumor initiation and progression. Next, we observed that the expression levels of a novel lncRNA, GM50337, along with stearoyl-Coenzyme A desaturase 1 (Scd1) are significantly upregulated in PTHrP WT but not in KO tumors. We further validated a potential human orthologue lncRNA, OLMALINC, together with SCD1 that can be regulated via PTHrP in human BC cell lines. In conclusion, these novel findings could be used to develop targeted strategies for the treatment of BC and its metastatic complications.

Defective postnatal endochondral bone development by chondrocyte-specific targeted expression of parathyroid hormone type 2 receptor.

The human parathyroid hormone type 2 receptor (PTH2R) is activated by PTH and by tuberoinfundibular peptide of 39 residues (TIP39), the latter likely acting as its natural ligand. Although the receptor is expressed at highest levels in the nervous system, we have observed that both PTH2R and TIP39 are expressed in the newborn mouse growth plate, with the receptor localizing in the resting zone and the ligand TIP39 localizing exclusively in prehypertrophic and hypertrophic chondrocytes. To address the role of PTH2R in postnatal skeletal growth and development, Col2a1-hPTH2R (PTH2R-Tg) transgenic mice were generated. The mice were viable and of nearly normal size at birth. Expression of the transgene in the growth plate was limited to chondrocytes. We found that chondrocyte proliferation was decreased, as determined by in vivo BrdU labeling of proliferating chondrocytes and CDK4 and p21 expression in the growth plate of Col2a1-hPTH2R transgenic mice. Similarly, the differentiation and maturation of chondrocytes was delayed, as characterized by decreased Sox9 expression and weaker immunostaining for the chondrocyte differentiation markers collagen type II and type X and proteoglycans. As well, there was altered expression of Gdf5, Wdr5, and ß-catenin, factors implicated in chondrocyte maturation, proliferation, and differentiation.These effects impacted on the process of endochondral ossification, resulting in delayed formation of the secondary ossification center, and diminished trabecular bone volume. The findings substantiate a role for PTH2R signaling in postnatal growth plate development and subsequent bone mass acquisition.

The calcium-sensing receptor complements parathyroid hormone-induced bone turnover in discrete skeletal compartments in mice.

Although the calcium-sensing receptor (CaSR) and parathyroid hormone (PTH) may each exert skeletal effects, it is uncertain how CaSR and PTH interact at the level of bone in primary hyperparathyroidism (PHPT). Therefore, we simulated PHPT with 2 wk of continuous PTH infusion in adult mice with deletion of the PTH gene (Pth(-/-) mice) and with deletion of both PTH and CaSR genes (Pth(-/-)-Casr (-/-) mice) and compared skeletal phenotypes. PTH infusion in Pth(-/-) mice increased cortical bone turnover, augmented cortical porosity, and reduced cortical bone volume, femoral bone mineral density (BMD), and bone mineral content (BMC); these effects were markedly attenuated in PTH-infused Pth(-/-)-Casr(-/-) mice. In the absence of CaSR, the PTH-stimulated expression of receptor activator of nuclear factor-κB ligand and tartrate-resistant acid phosphatase and PTH-stimulated osteoclastogenesis was also reduced. In trabecular bone, PTH-induced increases in bone turnover, trabecular bone volume, and trabecular number were lower in Pth(-/-)-Casr(-/-) mice than in Pth(-/-) mice. PTH-stimulated genetic markers of osteoblast activity were also lower. These results are consistent with a role for CaSR in modulating both PTH-induced bone resorption and PTH-induced bone formation in discrete skeletal compartments.

Burosumab Treatment for Autosomal Recessive Hypophosphatemic Rickets Type 1 (ARHR1).

CONTEXT: Autosomal recessive hypophosphatemic rickets (ARHR) are rare, heritable renal phosphate-wasting disorders that arise from overexpression of the bone-derived phosphaturic hormone fibroblast growth factor 23 (FGF23) leading to impaired bone mineralization (rickets and osteomalacia). Inactivating mutations of Dentin matrix protein 1 (DMP1) give rise to ARHR type 1 (ARHR1). Short stature, prominent bowing of the legs, fractures/pseudofractures, and severe enthesopathy are prominent in this patient population. Traditionally, treatment consists of oral phosphate replacement and the addition of calcitriol but this approach is limited by modest efficacy and potential renal and gastrointestinal side effects. OBJECTIVE: The advent of burosumab (Crysvita), a fully humanized monoclonal antibody to FGF23 for the treatment of X-linked hypophosphatemia and tumor-induced osteomalacia, offers a unique opportunity to evaluate its safety and efficacy in patients with ARHR1. RESULTS: Monthly administration of burosumab to 2 brothers afflicted with the disorder resulted in normalization of serum phosphate, healing of pseudofracture, diminished fatigue, less bone pain, and reduced incapacity arising from the extensive enthesopathy and soft tissue fibrosis/calcification that characterizes this disorder. No adverse effects were reported following burosumab administration. CONCLUSION: The present report highlights the beneficial biochemical and clinical outcomes associated with the use of burosumab in patients with ARHR1.

SCF-SKP2 E3 ubiquitin ligase links mTORC1/ER stress/ISR with YAP activation in murine renal cystogenesis.

The Hippo pathway nuclear effector Yes-associated protein (YAP) potentiates the progression of polycystic kidney disease (PKD) arising from ciliopathies. The mechanisms underlying the increase in YAP expression and transcriptional activity in PKD remain obscure. We observed that in kidneys from mice with juvenile cystic kidney (jck) ciliopathy, the aberrant hyperactivity of mechanistic target of rapamycin complex 1 (mTORC1), driven by ERK1/2 and PI3K/AKT cascades, induced ER proteotoxic stress. To reduce this stress by reprogramming translation, the protein kinase R-like ER kinase-eukaryotic initiation factor 2α (PERK/eIF2α) arm of the integrated stress response (ISR) was activated. PERK-mediated phosphorylation of eIF2α drove the selective translation of activating transcription factor 4 (ATF4), potentiating YAP expression. In parallel, YAP underwent K63-linked polyubiquitination by SCF S-phase kinase-associated protein 2 (SKP2) E3 ubiquitin ligase, a Hippo-independent, nonproteolytic ubiquitination that enhances YAP nuclear trafficking and transcriptional activity in cancer cells. Defective ISR cellular adaptation to ER stress in eIF2α phosphorylation-deficient jck mice further augmented YAP-mediated transcriptional activity and renal cyst growth. Conversely, pharmacological tuning down of ER stress/ISR activity and SKP2 expression in jck mice by administration of tauroursodeoxycholic acid (TUDCA) or tolvaptan impeded these processes. Restoring ER homeostasis and/or interfering with the SKP2-YAP interaction represent potential therapeutic avenues for stemming the progression of renal cystogenesis.

Vitamin D status and response to daily 400 IU vitamin D3 and weekly alendronate 70 mg in men and women with osteoporosis.

BACKGROUND: Suboptimal vitamin D status is common in elderly individuals. However, the extent of vitamin D inadequacy in men and women being treated for osteoporosis in a family practice setting has not been well characterized. OBJECTIVE: To describe the distribution of serum 25-hydroxyvitamin D (25-[OH] D) in Canadian men and postmenopausal women with osteoporosis taking 400 IU or less of vitamin D daily and to evaluate the safety, tolerability, and impact of vitamin D(3) supplementation 400 IU daily taken concurrently with alendronate sodium 70 mg weekly. METHODS: This was a prospective, single-cohort, open-label, multicenter study. Community-dwelling men and postmenopausal women with osteoporosis were recruited at 197 sites across Canada. Patients received vitamin D(3) 400 IU/day supplementation coadministered with alendronate 70 mg/wk for 16 weeks. The primary outcome was the distribution of serum 25-(OH) D at baseline. Secondary outcome measures included changes from baseline in serum 25-(OH) D levels, adherence to study treatments, and incidence of treatment-related adverse events (AEs). RESULTS: Of the 681 patients included in the analysis, 485 (71.2%) completed the study. Patients were predominantly female (83.1%) with a mean (SD) age of 67.6 (10.7) years. At baseline, mean (SD) serum 25-(OH) D concentration was 25.4 (9.9) ng/mL and 68.0% of the patients had inadequate (less than 30 ng/mL) vitamin D status. At week 16, concentrations increased by 35.1% to 31.2 (9.2) ng/mL (p < 0.001) and the proportion of patients with inadequate 25-(OH) D levels was reduced to 47.0%. Adherence to the treatment regimen was high (greater than 95%). Gastrointestinal disorders were the most frequently reported (6.9%) treatment-related AEs. CONCLUSIONS: About two thirds of patients previously diagnosed with osteoporosis have inadequate vitamin D status. A treatment regimen consisting of alendronate 70 mg/wk administered with daily vitamin D(3) 400 IU supplementation significantly increased patients' serum 25-(OH) D levels, but 47% did not achieve optimal levels. These results support both the National Osteoporosis Foundation and Osteoporosis Canada recommendations for higher vitamin D supplement doses (at least 800 IU daily) in osteoporotic patients receiving pharmacologic therapy for osteoporosis and for monitoring their serum 25-(OH) D response.

Fibroblast growth factor 23 overexpression impacts negatively on dentin mineralization and dentinogenesis in mice.

1. Though previous studies have shown that fibroblast growth factor 23 (FGF23) mRNA expression localizes in ameloblasts and odontoblasts in teeth, it is unclear what effect FGF23 overexpression has on dentin mineralization and dentinogenesis. Toward this end, the phenotypes of mandibles and teeth were compared between 6-week-old FGF23 transgenic mice and their wild-type littermates by radiography, microcomputed tomography scanning, histology, histochemistry and immunohistochemistry. 2. The mineral density was reduced in all teeth, including molars and incisors, and in the mandible, and the mineralized tooth volume in incisor and molars, and the mineralized cortical and alveolar bone volume in mandibles were decreased in FGF23 transgenic mice compared with their wild-type littermates. The dental volume, reparative dentin area, the expression of dentin sialoprotein in dentin, and the deposition of type I collagen and osteocalcin in the dental matrix were significantly reduced. However, the predentin volume and the expression of biglycan in dentin were increased in FGF23 transgenic mice compared with their wild-type littermates. 3. The results of the present study show that FGF23 overexpression plays a negative regulatory role on dentin mineralization and dentinogenesis.

Parathyroid hormone related protein (PTHrP) in tumor progression.

Parathyroid hormone-related protein (PTHrP) is widely expressed in fetal and adult tissues and is a key regulator for cellular calcium transport and smooth muscle cell contractility, as well as a crucial control factor in cell proliferation, development and differentiation. PTHrP stimulates or inhibits apoptosis in an autocrine/paracrine and intracrine fashion, and is particularly important for hair follicle and bone development, mammary epithelial development and tooth eruption. PTHrP's dysregulated expression has traditionally been associated with oncogenic pathologies as the major causative agent of malignancy-associated hypercalcemia, but recent evidence revealed a driving role in skeletal metastasis progression. Here, we demonstrate that PTHrP is also closely involved in breast cancer initiation, growth and metastasis through mechanisms separate from its bone turnover action, and we suggest that PTHrP as a facilitator of oncogenes would be a novel target for therapeutic purposes.

Severe growth retardation and early lethality in mice lacking the nuclear localization sequence and C-terminus of PTH-related protein.

Parathyroid hormone (PTH) plays a central role in the regulation of serum calcium and phosphorus homeostasis, while parathyroid hormone-related protein (PTHrP) has important developmental roles. Both peptides signal through the same G protein-coupled receptor, the PTH/PTHrP or PTH type 1 receptor (PTH1R). PTHrP, normally a secreted protein, also contains a nuclear localization signal (NLS) that in vitro imparts functionality to the protein at the level of the nucleus. We investigated this functionality in vivo by introducing a premature termination codon in Pthrp in ES cells and generating mice that express PTHrP (1-84), a truncated form of the protein that is missing the NLS and the C-terminal region of the protein but can still signal through its cell surface receptor. Mice homozygous for the knock-in mutation (Pthrp KI) displayed retarded growth, early senescence, and malnutrition leading postnatally to their rapid demise. Decreased cellular proliferative capacity and increased apoptosis in multiple tissues including bone and bone marrow cells were associated with altered expression and subcellular distribution of the senescence-associated tumor suppressor proteins p16(INK4a) and p21 and the oncogenes Cyclin D, pRb, and Bmi-1. These findings provide in vivo experimental proof that substantiates the biologic relevance of the NLS and C-terminal portion of PTHrP, a polypeptide ligand that signals mainly via a cell surface G protein-coupled receptor.

Role of PTHrP nuclear localization and carboxyl terminus sequences in postnatal spinal cord development.

Parathyroid hormone-related peptide (PTHrP) acts under physiological conditions to regulate normal development of several tissues and organs. The role of PTHrP in spinal cord development has not been characterized. Pthrp knock in (Pthrp KI) mice were genetically modified to produce PTHrP in which there is a deficiency of the nuclear localization sequence (NLS) and C-terminus. Using this genetically modified mouse model, we have characterized its effect on spinal cord development early postnatally. The spinal cords from Pthrp KI mice displayed a significant reduction in its length, weight, and cross-sectional area compared to wild-type controls. Histologically, there was a decreased development of neurons and glial cells that caused decreased cell proliferation and increased apoptosis. The neural stem cells (NSCs) cultures also revealed decreased cell proliferation and differentiation and increased apoptosis. The proposed mechanism of delayed spinal cord development in Pthrp KI mice may be due to alteration in associated pathways in regulation of cell-division cycles and apoptosis. There was significant downregulation of Bmi-1 and upregulation of cyclin-dependent kinase inhibitors p27, p21, and p16 in Pthrp KI animals. We conclude that NLS and C-terminus peptide segments of PTHrP play an important role in inhibiting cell apoptosis and stimulation of cellular proliferation necessary for normal spinal cord development.

TIP39/parathyroid hormone type 2 receptor signaling is a potent inhibitor of chondrocyte proliferation and differentiation.

Tuberoinfundibular peptide of 39 residues (TIP39) is a member of the parathyroid hormone (PTH) family of peptide hormones that exerts its function by interacting with the PTH type 2 receptor (PTH2R). Presently, no known function has been attributed to this signaling pathway in the developing skeleton. We observed that TIP39 and PTH2R were present in the newborn mouse growth plate, with the receptor localizing in the resting zone whereas ligand expression was restricted exclusively in prehypertrophic and hypertrophic chondrocytes. By 8 wk of life, PTH2R, and to a lesser degree TIP39, immunoreactivity was present in articular chondrocytes. We therefore sought to investigate the role of TIP39/PTH2R signaling in chondrocytes by generating stably transfected CFK2 chondrocytic cells overexpressing PTH2R (CFK2R). TIP39 treatment of CFK2R clones in culture inhibited their proliferation by restricting cells at the G(0)/G(1) phase of the cell cycle, coupled with decreased expression and activity of cyclin-dependent kinases Cdk2 and Cdk4, while p21, an inhibitor of Cdks, was upregulated. In addition, TIP39 treatment decreased expression of differentiation markers in these cells associated with marked alterations in extracellular matrix and metalloproteinase expression. Transcription of Sox9, the master regulator of cartilage differentiation, was reduced in TIP39-treated CFK2R clones. Moreover, Sox9 promoter activity, as measured by luciferase reporter assay, was markedly diminished after TIP39 treatment. In summary, our results show that TIP39/PTH2R signaling inhibits proliferation and alters differentiation of chondrocytes by modulating SOX9 expression, thereby substantiating the functional significance of this signaling pathway in chondrocyte biology.

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. Daily administration of the 1-34 amino-terminal fragment of parathyroid hormone (PTH 1-34) to Pthrp+/- mice resulted in profound improvement in all parameters of skeletal microarchitecture, surpassing the improvement observed in treated WT littermates. These findings establish a pivotal role for osteoblast-derived PTH-related protein (PTHrP) as a potent endogenous bone anabolic factor that potentiates bone formation by altering osteoblast recruitment and survival and whose level of expression in the bone microenvironment influences the therapeutic efficacy of exogenous PTH 1-34.

Early lethality in Hyp mice with targeted deletion of Pth gene.

Although increased circulating levels of PTH with mild hypocalcemia has been reported in Hyp mice, hyperparathyroidism in X-linked hypophosphatemic rickets is postulated to arise from the standard use of phosphate salts, which induce chronic stimulation of PTH secretion. In this study, we sought to examine the role of PTH in the metabolic derangements associated with Hyp by generating hemizygous hypophosphatemic (Hyp/Y) mice homozygous for the Pth-null allele (Pth(-/-);Hyp/Y). Early postnatal lethality was observed in the Pth(-/-);Hyp/Y mice. Within the first 6 h, postpartum serum phosphorus increased to levels comparable to those in the Pth(-/-) mice, whereas in Hyp mice, it decreased during the first 48 h after birth. Serum calcium concentration started low after birth and remained reduced in both Pth(-/-);Hyp/Y and Pth(-/-) mice although more profoundly so in the former group, whereas in Hyp/Y mice, the levels were initially lower than but reached wild-type levels by 24 h. Circulating PTH levels in Hyp/Y mice were higher than wild-type levels throughout the first 48 h after birth and continued to be so well into adulthood. Twice-daily administration of PTH 1-34 to Pth(-/-);Hyp/Y newborn mice increased serum calcium levels and prevented their early demise. The findings here indicate that the cause of death in the Pth(-/-);Hyp/Y mice is severe hypocalcemia. A potential role for fibroblast growth factor 23 in promoting secondary hyperparathyroidism by suppressing renal 25-hydroxyvitamin D(3)-1alpha-hydroxylase (Cyp27b1) activity while increasing that of renal 25-hydroxyvitamin D(3) 24-hydroxylase (Cyp24) is proposed. Hyperparathyroidism, therefore, is an integral component in the pathophysiology of Hyp, and likely X-linked hypophosphatemic rickets and serves to prevent severe hypocalcemia in mice and perhaps in patients afflicted with the disorder.

Altered ovarian function affects skeletal homeostasis independent of the action of follicle-stimulating hormone.

Osteoporosis is a leading public health problem. Although a major cause in women is thought to be a decline in estrogen, it has recently been proposed that FSH or follitropin is required for osteoporotic bone loss. We examined the FSH receptor null mouse (FORKO mouse) to determine whether altered ovarian function could induce bone loss independent of FSH action. By 3 months of age, FORKO mice developed age-dependent declines in bone mineral density and trabecular bone volume of the lumbar spine and femur, which could be partly reversed by ovarian transplantation. Bilateral ovariectomy reduced elevated circulating testosterone levels in FORKO mice and decreased bone mass to levels indistinguishable from those in ovariectomized wild-type controls. Androgen receptor blockade and especially aromatase inhibition each produced bone volume reductions in the FORKO mouse. The results indicate that ovarian secretory products, notably estrogen, and peripheral conversion of ovarian androgen to estrogen can alter bone homeostasis independent of any bone resorptive action of FSH.

Parathyroid hormone is essential for normal fetal bone formation.

Parathyroid hormone (PTH) is a potent pharmacologic inducer of new bone formation, but no physiologic anabolic effect of PTH on adult bone has been described. We investigated the role of PTH in fetal skeletal development by comparing newborn mice lacking either PTH, PTH-related peptide (PTHrP), or both peptides. PTH-deficient mice were dysmorphic but viable, whereas mice lacking PTHrP died at birth with dyschondroplasia. PTH-deficient mice uniquely demonstrated diminished cartilage matrix mineralization, decreased neovascularization with reduced expression of angiopoietin-1, and reduced metaphyseal osteoblasts and trabecular bone. Compound mutants displayed the combined cartilaginous and osseous defects of both single mutants. These results indicate that coordinated action of both PTH and PTHrP are required to achieve normal fetal skeletal morphogenesis, and they demonstrate an essential function for PTH at the cartilage-bone interface. The effect of PTH on fetal osteoblasts may be relevant to its postnatal anabolic effects on trabecular bone.