Osteopontin deficiency induces parathyroid hormone enhancement of cortical bone formation

Early Biomarkers of Altered Renal Function and Orthostatic Intolerance During 10-day Bedrest

Exposure to actual or simulated microgravity results in alterations of renal function, fluid redistribution, and bone loss, which is coupled to a rise of urinary calcium excretion. We provided evidence that high calcium delivery to the collecting duct reduces local Aquaporin 2 (AQP2)-mediated water reabsorption under vasopressin action, thus limiting the maximal urinary concentration to reduce calcium saturation. To investigate early renal adaptation into simulated microgravity, we investigated the effects of 10 days of strict bedrest in 10 healthy volunteers. We report here that 10 days of inactivity are associated with a transient, significant decrease (day 5) in vasopressin (copeptin) paralleled by a decrease in AQP2 excretion, consistent with an increased central volume to the heart, resulting in reduced water reabsorption. Moreover, bedrest caused a significant increase in calciuria secondary to bone demineralization paralleled by a decrease in PTH. Urinary osteopontin, a glycoprotein exerting a protective effect on stone formation, was significantly reduced during bedrest. Moreover, a significant increase in adrenomedullin (day 5), a peptide with vasodepressor properties, was observed at day 5, which may contribute to the known reduced orthostatic capacity post-bedrest. We conclude that renal function is altered in simulated microgravity and is associated with an early increase in the risk of stone formation and reduced orthostatic capacity post-bedrest within a few days of inactivity.

Anabolic actions of PTH in murine models: two decades of insights

Parathyroid hormone (PTH) is produced by the parathyroid glands in response to low serum calcium concentrations where it targets bones, kidneys, and indirectly, intestines. The N-terminus of PTH has been investigated for decades for its ability to stimulate bone formation when administered intermittently (iPTH) and is used clinically as an effective anabolic agent for the treatment of osteoporosis. Despite great interest in iPTH and its clinical use, the mechanisms of PTH action remain complicated and not fully defined. More than 70 gene targets in more than 90 murine models have been utilized to better understand PTH anabolic actions. Because murine studies utilized wild-type mice as positive controls, a variety of variables were analyzed to better understand the optimal conditions under which iPTH functions. The greatest responses to iPTH were in male mice, with treatment starting later than 12 weeks of age, a treatment duration lasting 5-6 weeks, and a PTH dose of 30-60 μg/kg/day. This comprehensive study also evaluated these genetic models relative to the bone formative actions with a primary focus on the trabecular compartment revealing trends in critical genes and gene families relevant for PTH anabolic actions. The summation of these data revealed the gene deletions with the greatest increase in trabecular bone volume in response to iPTH. These included PTH and 1-α-hydroxylase (Pth;1α(OH)ase, 62-fold), amphiregulin (Areg, 15.8-fold), and PTH related protein (Pthrp, 10.2-fold). The deletions with the greatest inhibition of the anabolic response include deletions of: proteoglycan 4 (Prg4, -9.7-fold), low-density lipoprotein receptor-related protein 6 (Lrp6, 1.3-fold), and low-density lipoprotein receptor-related protein 5 (Lrp5, -1.0-fold). Anabolic actions of iPTH were broadly affected via multiple and diverse genes. This data provides critical insight for future research and development, as well as application to human therapeutics. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

FAM20C Overview: Classic and Novel Targets, Pathogenic Variants and Raine Syndrome Phenotypes

FAM20C is a gene coding for a protein kinase that targets S-X-E/pS motifs on different phosphoproteins belonging to diverse tissues. Pathogenic variants of FAM20C are responsible for Raine syndrome (RS), initially described as a lethal and congenital osteosclerotic dysplasia characterized by generalized atherosclerosis with periosteal bone formation, characteristic facial dysmorphisms and intracerebral calcifications. The aim of this review is to give an overview of targets and variants of FAM20C as well as RS aspects. We performed a wide phenotypic review focusing on clinical aspects and differences between all lethal (LRS) and non-lethal (NLRS) reported cases, besides the FAM20C pathogenic variant description for each. As new targets of FAM20C kinase have been identified, we reviewed FAM20C targets and their functions in bone and other tissues, with emphasis on novel targets not previously considered. We found the classic lethal and milder non-lethal phenotypes. The milder phenotype is defined by a large spectrum ranging from osteonecrosis to osteosclerosis with additional congenital defects or intellectual disability in some cases. We discuss our current understanding of FAM20C deficiency, its mechanism in RS through classic FAM20C targets in bone tissue and its potential biological relevance through novel targets in non-bone tissues.

Fracture healing in a collagen-induced arthritis rat model: Radiology and histology evidence

This research was designed to investigate the fracture healing pattern in a rheumatoid arthritis (RA) rat model. A mid-shaft femur fracture (RA + F) model and normal fracture (NF) model as control were established. Micro-CT, H&E staining, TB staining, SO staining, tartrate-resistant acid phosphates, and immunohistochemistry test were performed. In the micro-CT images and H&E stains, fracture gaps were evident in the RA + F group 4 and 8 weeks after fracture. In detail, the bone mineral density, the ratio of bone volume to tissue volume, and trabecular thickness of the RA + F group were significantly lower than those of the NF group at all time points. Trabecular number value was significantly lower in the RA + F group 4 weeks after surgery in comparison with that of the NF group. Furthermore, the structure model index test result of the RA + F group was significantly higher than that of the NF group at all time points. TB staining and SO staining test results showed that the NF group had more cartilaginous callus in the earlier stage of bone healing process (4 weeks), and less cartilage callus formation in the later stage (8 weeks) in comparison with that of the RA + F group. Osteoclasts statistics score in the NF group were obviously lower than that of the RA + F group at all time points. MMP-3 and OPN protein levels of the fracture area in the RA + F group were significantly higher than those in the NF group. This study improves the understanding of the bone healing characteristics in patients with RA. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2876-2885, 2018.

Exploring the separation power of mixed-modal resins for purification of recombinant osteopontin from clarified Escherichia coli lysates

Osteopontin (OPN) is a structural protein with potential value in therapeutic and diagnostic applications. Low titer, acidic isoelectric point, and the lack of well-defined secondary and tertiary structure were some of the challenges that complicated purification development of OPN from recombinant Escherichia coli lysates. Reported processes for OPN recovery from recombinant sources use nonorthogonal unit operations and often suffer from low yield. In this work, we expanded the search for an optimal OPN purification method by including mixed-modal resins with both ionic and hydrophobic properties (Capto adhere, HEA HyperCel, and PPA HyperCel). Plate-based high-throughput screening (HTS) platform revealed useful information about the interactions between the three different ligands and OPN as function of pH and ionic strength. The HTS data allowed the selection of OPN adsorption and elution conditions that were tested and optimized in a batch mode. In terms of purification factor and yield, HEA HyperCel performed significantly better than the other two mixed-modal resins. Pairing HEA HyperCel with a strong anion exchange step (Capto Q) resulted in a two-step purification process that achieved 45-fold purification of OPN with a final purity of 95% and 44% overall yield. The orthogonality provided by mixed-modal and ion exchange steps resulted in higher yield in fewer unit operations than reported processes. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2722, 2019.

Effect of parathyroidectomy on osteopontin and undercarboxylated osteocalcin in patients with primary hyperparathyroidism

PURPOSE

Surgical treatment for primary hyperparathyroidism (PHPT) improves bone metabolism. Osteocalcin (OC) and its undercarboxylated form (ucOC) are associated with bone and energy metabolism. Osteopontin (OPN), a multifunctional protein expressed in bone, is involved in resorption, along with β-carboxyl-terminal cross-linking telopeptide of type 1 collagen (β-CTX), and osteoprotegerin (OPG). Our aim was to investigate these biomarkers of bone metabolism in patients with PHPT.

METHODS

We examined 30 individuals with PHPT, in a clinical research facility, before and 1 month following parathyroidectomy. Circulating levels of OC, ucOC, OPN, β-CTX, and OPG were examined as bone biomarkers along with inflammatory markers (e.g., interleukin-6 [IL-6], lipocalin-2), insulin resistance (i.e., homeostasis model assessment for insulin resistance [HOMA-IR]), adiposity (i.e., leptin, adiponectin), PTH, calcium, 25-hydroxyvitamin D, creatinine, and demographics.

RESULTS

Participants (27 females/3 males) were 60 ± 9 (mean±SD) years old. There was a significant reduction of ucOC (7.9 ± 5.1 [median±SIQR] vs. 6.6 ± 3.7 ng/mL, p = 0.022) and OPN (75.4 ± 14.5 vs. 54.5 ± 9.2 ng/mL, p < 0.001) pre- versus post-parathyroidectomy. There were no univariate differences postoperatively for IL-6, HOMA-IR, leptin, or adiponectin. Regression analysis showed that postoperative levels of adiponectin, IL-6, and OPN were significantly associated with ucOC, while adjusting for PTH and albumin corrected calcium levels (model R² = 0.610, p = 0.001). With OPN as the dependent variable, higher adiponectin and lower ucOC were significantly associated with lower OPN levels postoperatively (model R² = 0.505, p = 0.010).

CONCLUSION

The lower 1-month postoperative OPN and ucOC levels in PHPT seem to indicate reduced bone resorption. Decreased ucOC levels may also suggest lower energy demands postoperatively.

Osteopontin -- a promising biomarker for cancer therapy

Osteopontin (OPN), a multifunctional protein, has emerged as a potentially valuable biomarker for diagnosing and treating cancers. Recent research focuses on its involvement in tumor biology including the cell proliferation, survival, angiogenesis, invasion, and metastasis. Understanding the molecular mechanisms and pharmacological effects of OPN in cancer development could lead to new targets for improving cancer diagnosis and treatment. This review explains how the structurally conserved domains of OPN are associated with OPN signaling mediators and CD44, and how the conserved OPN domains determine biological functions. The authors have reviewed representative works of OPN expression in breast cancer and colorectal cancer to elucidate the relationship between OPN and cancer/tumor biology. It has also been shown that the prognostic sensitivity in non-small cell lung cancer, hepatocellular carcinoma, gastric cancer, and ovarian cancer improved compared to the individual marker when OPN was analyzed in conjunction with other markers. The therapeutic approaches based on OPN inhibitors are discussed to illustrate recent research progress. Previous clinical data has indicated that OPN has played a unique role in cancer development, but further investigation is required to understand the underlying mechanism. More clinical trials are also required to examine the applicability and efficacy of OPN inhibitors in cancer therapy.

The role of the SIBLING, Bone Sialoprotein in skeletal biology - Contribution of mouse experimental genetics

Bone Sialoprotein (BSP) is a member of the "Small Integrin-Binding Ligand N-linked Glycoproteins" (SIBLING) extracellular matrix protein family of mineralized tissues. BSP has been less studied than other SIBLING proteins such as Osteopontin (OPN), which is coexpressed with it in several skeletal cell types. Here we review the contribution of genetically engineered mice (BSP gene knockout and overexpression) to the understanding of the role of BSP in the bone organ. The studies made so far highlight the role of BSP in skeletal mineralization, as well as its importance for proper osteoblast and osteoclast differentiation and activity, most prominently in primary/repair bone. The absence of BSP also affects the local environment of the bone tissue, in particular hematopoiesis and vascularization. Interestingly, lack of BSP induces an overexpression of OPN, and the cognate protein could be responsible for some aspects of the BSP gene knockout skeletal phenotype, while replacing BSP for some of its functions. Such interplay between the partly overlapping functions of SIBLING proteins, as well as the network of cross-regulations in which they are involved should now be the focus of further work.

Angiotensin receptor I stimulates osteoprogenitor proliferation through TGFβ-mediated signaling

Clinical studies of large human populations and pharmacological interventions in rodent models have recently suggested that anti-hypertensive drugs that target angiotensin II (Ang II) activity may also reduce loss of bone mineral density. Here, we identified in a genetic screening the Ang II type I receptor (AT1R) as a potential determinant of osteogenic differentiation and, implicitly, bone formation. Silencing of AT1R expression by RNA interference severely impaired the maturation of a multipotent mesenchymal cell line (W20-17) along the osteoblastic lineage. The same effect was also observed after the addition of the AT1R antagonist losartan but not the AT2R inhibitor PD123,319. Additional cell culture assays traced the time of greatest losartan action to the early stages of W20-17 differentiation, namely during cell proliferation. Indeed, addition of Ang II increased proliferation of differentiating W20-17 and primary mesenchymal stem cells and this stimulation was reversed by losartan treatment. Cells treated with losartan also displayed an appreciable decrease of activated (phosphorylated)-Smad2/3 proteins. Moreover, Ang II treatment elevated endogenous transforming growth factor β (TGFβ) expression considerably and in an AT1R-dependent manner. Finally, exogenous TGFβ was able to restore high proliferative activity to W20-17 cells that were treated with both Ang II and losartan. Collectively, these results suggest a novel mechanism of Ang II action in bone metabolism that is mediated by TGFβ and targets proliferation of osteoblast progenitors.

Toddaculin, Isolated from of Toddalia asiatica (L.) Lam., Inhibited Osteoclastogenesis in RAW 264 Cells and Enhanced Osteoblastogenesis in MC3T3-E1 Cells

Osteoporosis with bone loss is widely recognized as a major health problem. Bone homeostasis is maintained by balancing bone formation and bone resorption. The imbalance caused by increased bone resorption over bone formation can lead to various bone-related diseases such as osteoporosis and rheumatoid arthritis. Osteoclasts are the principal cells responsible for bone resorption and the main targets of anti-resorptive therapies. However, excessive inhibition of osteoclast differentiation may lead to inhibition of osteoblast differentiation. Therefore, it is important to screen for new compounds capable of inhibiting bone resorption and enhancing bone formation. Toddalia asiatica (L.) Lam. has been utilized traditionally for medicinal purposes such as the treatment of rheumatism. Currently, the extract is considered to be a good source of pharmacological agents for the treatment of bone-related diseases, but the active compounds have yet to be identified. We investigated whether toddaculin, derived from Toddalia asiatica (L.) Lam., affects both processes by inhibiting bone resorption and enhancing bone formation. Towards this end, we used pre-osteoclastic RAW 264 cells and pre-osteoblastic MC3T3-E1 cells. We found that toddaculin not only inhibited the differentiation of osteoclasts via activation of the NF-κB, ERK 1/2, and p38 MAPK signaling pathways, but it also induced differentiation and mineralization of osteoblasts by regulating differentiation factors. Thus, toddaculin might be beneficial for the prevention and treatment of osteoporosis.

Blocking the expression of both bone sialoprotein (BSP) and osteopontin (OPN) impairs the anabolic action of PTH in mouse calvaria bone

Osteopontin (OPN) and bone sialoprotein (BSP) are coexpressed in osteoblasts and osteoclasts, and display overlapping properties. We used daily injection of parathyroid hormone 1-84 (iPTH) over the calvaria of BSP knockout (-/-) mice to investigate further their functional specificity and redundancy. iPTH stimulated bone formation in both +/+ and -/- mice, increasing to the same degree periosteum, osteoid and total bone thickness. Expression of OPN, osterix, osteocalcin (OCN) and DMP1 was also increased by iPTH in both genotypes. In contrast to +/+, calvaria cell cultures from -/- mice revealed few osteoblast colonies, no mineralization and little expression of OCN, MEPE or DMP1. In contrast, OPN levels were 5× higher in -/- versus +/+ cultures. iPTH increased alkaline phosphatase (ALP) activity in cell cultures of both genotypes, with higher OCN and the induction of mineralization in -/- cultures. siRNA blocking of OPN expression did not alter the anabolic action of the hormone in BSP +/+ calvaria, while it blunted iPTH effects in -/- mice, reduced to a modest increase in periosteum thickness. In -/- (not +/+) cell cultures, siOPN blocked the stimulation by iPTH of ALP activity and OCN expression, as well as the induction of mineralization. Thus, full expression of either OPN or BSP is necessary for the anabolic effect of PTH at least in the ectopic calvaria injection model. This suggests that OPN may compensate for the lack of BSP in the response to this hormonal challenge, and provides evidence of functional overlap between these cognate proteins.

Hypophosphatemic osteomalacia and bone sclerosis caused by a novel homozygous mutation of the FAM20C gene in an elderly man with a mild variant of Raine syndrome

BACKGROUND

Hypophosphatemia and increased serum fibroblast growth factor 23 (FGF23) levels have been reported in young brothers with compound heterozygous mutations for the FAM20C gene; however, rickets was not observed in these cases. We report an adult case of Raine syndrome accompanying hypophosphatemic osteomalacia with a homozygous FAM20C mutation (R408W) associated with increased periosteal bone formation in the long bones and an increase in bone mineral density in the femoral neck.

CASE

The patient, a 61-year-old man, was born from a cousin-to-cousin marriage. A short stature and severe dental demineralization were reported at an elementary school age. Hypophosphatemia was noted inadvertently at 27years old, at which time he started to take an active vitamin D metabolite (alphacalcidol) and phosphate. He also manifested ossification of the posterior longitudinal ligament. On bone biopsy performed at the age of 41years, we found severe osteomalacia surrounding osteocytes, which appeared to be an advanced form of periosteocytic hypomineralized lesions compared to those reported in patients with X-linked hypophosphatemic rickets. Laboratory data at 61years of age revealed markedly increased serum intact-FGF23 levels, which were likely to be the cause of hypophosphatemia and the decreased level of 1,25(OH)2D. We recently identified a homozygous FAM20C mutation, which was R408W, in this patient. When expressed in HEK293 cells, the R408W mutant protein exhibited impaired kinase activity and secretion.

DISCUSSION

Our findings suggest that certain homozygous FAM20C mutations can cause FGF23-related hypophosphatemic osteomalacia and indicate the multiple roles of FAM20C in bone.

Cytoplasmic reactive oxygen species and SOD1 regulate bone mass during mechanical unloading

Oxidative stress contributes to the pathogenesis of age-related diseases as well as bone fragility. Our previous study demonstrated that copper/zinc superoxide dismutase (Sod1)-deficient mice exhibit the induction of intracellular reactive oxygen species (ROS) and bone fragility resulting from low-turnover bone loss and impaired collagen cross-linking (Nojiri et al. J Bone Miner Res. 2011;26:2682-94). Mechanical stress also plays an important role in the maintenance of homeostasis in bone tissue. However, the molecular links between oxidative and mechanical stresses in bone tissue have not been fully elucidated. We herein report that mechanical unloading significantly increased intracellular ROS production and the specific upregulation of Sod1 in bone tissue in a tail-suspension experiment. We also reveal that Sod1 loss exacerbated bone loss via reduced osteoblastic abilities during mechanical unloading. Interestingly, we found that the administration of an antioxidant, vitamin C, significantly attenuated bone loss during unloading. These results indicate that mechanical unloading, in part, regulates bone mass via intracellular ROS generation and the Sod1 expression, suggesting that activating Sod1 may be a preventive strategy for ameliorating mechanical unloading-induced bone loss.

Anabolic action of parathyroid hormone regulated by the β2-adrenergic receptor

Parathyroid hormone (PTH), the major calcium-regulating hormone, and norepinephrine (NE), the principal neurotransmitter of sympathetic nerves, regulate bone remodeling by activating distinct cell-surface G protein-coupled receptors in osteoblasts: the parathyroid hormone type 1 receptor (PTHR) and the β(2)-adrenergic receptor (β(2)AR), respectively. These receptors activate a common cAMP/PKA signal transduction pathway mediated through the stimulatory heterotrimeric G protein. Activation of β(2)AR via the sympathetic nervous system decreases bone formation and increases bone resorption. Conversely, daily injection of PTH (1-34), a regimen known as intermittent (i)PTH treatment, increases bone mass through the stimulation of trabecular and cortical bone formation and decreases fracture incidences in severe cases of osteoporosis. Here, we show that iPTH has no osteoanabolic activity in mice lacking the β(2)AR. β(2)AR deficiency suppressed both iPTH-induced increase in bone formation and resorption. We showed that the lack of β(2)AR blocks expression of iPTH-target genes involved in bone formation and resorption that are regulated by the cAMP/PKA pathway. These data implicate an unexpected functional interaction between PTHR and β(2)AR, two G protein-coupled receptors from distinct families, which control bone formation and PTH anabolism.

Mitogen-activated protein kinase phosphatase 1 regulates bone mass, osteoblast gene expression, and responsiveness to parathyroid hormone

Parathyroid hormone (PTH) signaling via PTH 1 receptor (PTH1R) involves mitogen-activated protein kinase (MAPK) pathways. MAPK phosphatase 1 (MKP1) dephosphorylates and inactivates MAPKs in osteoblasts, the bone-forming cells. We previously showed that PTH1R activation in differentiated osteoblasts upregulates MKP1 and downregulates pERK1/2-MAPK and cyclin D1. In this study, we evaluated the skeletal phenotype of Mkp1 knockout (KO) mice and the effects of PTH in vivo and in vitro. Microcomputed tomography analysis of proximal tibiae and distal femora from 12-week-old Mkp1 KO female mice revealed osteopenic phenotype with significant reduction (8-46%) in bone parameters compared with wild-type (WT) controls. Histomorphometric analysis showed decreased trabecular bone area in KO females. Levels of serum osteocalcin (OCN) were lower and serum tartrate-resistant acid phosphatase 5b (TRAP5b) was higher in KO animals. Treatment of neonatal mice with hPTH (1-34) for 3 weeks showed attenuated anabolic responses in the distal femora of KO mice compared with WT mice. Primary osteoblasts derived from KO mice displayed delayed differentiation determined by alkaline phosphatase activity, and reduced expressions of Ocn and Runx2 genes associated with osteoblast maturation and function. Cells from KO females exhibited attenuated PTH response in mineralized nodule formation in vitro. Remarkably, this observation was correlated with decreased PTH response of matrix Gla protein expression. Expressions of pERK1/2 and cyclin D1 were inhibited dramatically by PTH in differentiated osteoblasts from WT mice but much less in osteoblasts from Mkp1 KO mice. In conclusion, MKP1 is important for bone homeostasis, osteoblast differentiation and skeletal responsiveness to PTH.

Cytoplasmic superoxide causes bone fragility owing to low-turnover osteoporosis and impaired collagen cross-linking

The aging process correlates with the accumulation of cellular and tissue damage caused by oxidative stress. Although previous studies have suggested that oxidative stress plays a pathologic role in the development of bone fragility, little direct evidence has been found. In order to investigate the pathologic significance of oxidative stress in bones, we analyzed the bone tissue of mice deficient in cytoplasmic copper/zinc superoxide dismutase (CuZn-SOD, encoded by the Sod1 gene; Sod1(-/-)). In this study, we showed for the first time that in vivo cytoplasmic superoxide caused a distinct weakness in bone stiffness and decreased BMD, aging-like changes in collagen cross-linking, and transcriptional alterations in the genes associated with osteogenesis. We also showed that the surface areas of osteoblasts and osteoclasts were decreased significantly in the lumbar vertebrae of Sod1(-/-) mice, indicating the occurrence of low-turnover osteopenia. In vitro experiments demonstrated that intracellular oxidative stress induced cell death and reduced the proliferation in primary osteoblasts but not in osteoclasts, indicating that impaired osteoblast viability caused the decrease in osteoblast number and suppressed RANKL/M-CSF osteoclastogenic signaling in bone. Furthermore, treatment with an antioxidant, vitamin C, effectively improved bone fragility and osteoblastic survival. These results imply that intracellular redox imbalance caused by SOD1 deficiency plays a pivotal role in the development and progression of bone fragility both in vivo and in vitro. We herein present a valuable model for investigating the effects of oxidative stress on bone fragility in order to develop suitable therapeutic interventions.

Biomineralization of bone: a fresh view of the roles of non-collagenous proteins

The impact of genetics has dramatically affected our understanding of the functions of non-collagenous proteins. Specifically, mutations and knockouts have defined their cellular spectrum of actions. However, the biochemical mechanisms mediated by non-collagenous proteins in biomineralization remain elusive. It is likely that this understanding will require more focused functional testing at the protein, cell, and tissue level. Although initially viewed as rather redundant and static acidic calcium binding proteins, it is now clear that non-collagenous proteins in mineralizing tissues represent diverse entities capable of forming multiple protein-protein interactions which act in positive and negative ways to regulate the process of bone mineralization. Several new examples from the author's laboratory are provided which illustrate this theme including an apparent activating effect of hydroxyapatite crystals on metalloproteinases. This review emphasizes the view that secreted non-collagenous proteins in mineralizing bone actively participate in the mineralization process and ultimately control where and how much mineral crystal is deposited, as well as determining the quality and biomechanical properties of the mineralized matrix produced.

Osteopontin regulates anabolic effect in human menopausal osteoporosis with intermittent parathyroid hormone treatment

In this pilot study, we demonstrated that women with osteopontin (OPN) over-expression show less resistance to postmenopausal osteoporosis than women with normal OPN levels. We hypothesized that the levels of plasma OPN could be used as a treatment indicator for intermittent parathyroid hormone (PTH)-treated menopausal osteoporosis. We demonstrated that plasma OPN levels could be used as a biomarker for early treatment response.

INTRODUCTION

Animal studies indicate that OPN-deficient mice are resistant to ovariectomy induced osteoporosis. Our pilot study also demonstrated women with OPN over expression may show less resistance to postmenopausal osteoporosis. The role of plasma OPN in PTH1-34-treated osteoporosis remains unclear.

METHODS

From September 2005 to September 2006, 31 menopausal women over 45 years of age with severe osteoporosis were enrolled in our study. Subjects were treated with PTH1-34 subcutaneously at a dose of 20 μg/day. Plasma OPN levels and BMD of the lumbar spine and hip were measured using ELISA and dual-energy X-ray absorptiometry at baseline, 3, 6, and 9 months. Response to the treatment was assessed by the sequential change in bone mineral density and OPN expression using a general linear mixed model.

RESULTS

The plasma OPN decreased sequentially and significantly throughout the 9-month treatment course from 20.75 ± 5.36 to 11.2 ± 4.37 ng/ml (p < 0.001). The sequential improvement in the T-score and Z-score was significant in the lumbar spine but not in the hip area. In the lumbar spine, when the plasma OPN decreased by 1 ng/ml the T-score increased by 0.0406 and the Z-score increased by 0.0572 of lumbar spine.

CONCLUSION

OPN levels are related to the anabolic effect of PTH in human postmenopausal osteoporosis. Plasma OPN levels could be used as a biomarker for early treatment response.

Continuous treatment with a low-dose β-agonist reduces bone mass by increasing bone resorption without suppressing bone formation

The sympathetic nervous system regulates bone remodeling through the β-adrenergic receptor (β-AR). However, the systemic roles of adrenergic actions on bone remodeling through the β-AR are largely unknown. In this study, we examined the dose effect of continuous treatment with isoprenaline, a nonspecific β-AR agonist, on bone remodeling. Male C57BL/6J mice were intrasubcutaneously administrated with four different doses (5, 25, 50, or 100 μg/g daily) of isoprenaline or vehicle using an osmotic pump for 2 weeks. The region of high-turnover cancellous bone was analyzed by microcomputed tomography (μCT). Continuous isoprenaline treatment caused a ~35.7% decline in the femoral cancellous bone volume fraction (BV/TV) at all doses (5-100 μg/g daily). Furthermore, continuous isoprenaline treatment weakened the bone mechanical properties in the trunk of lumbar vertebra 4 (L4). These parameters did not show significant differences between doses. Histomorphometric analysis revealed that isoprenaline doses of 50 μg/g daily or less did not significantly inhibit bone formation parameters, such as bone formation rate (BFR) and mineral surface/bone surface (MS/BS). Only the highest dose (100 μg/g daily) of isoprenaline significantly inhibited BFR and MS/BS. On the other hand, osteoclast number/bone surface (Oc.N/BS) was enhanced approximately 2.4-fold and osteoclast surface/bone surface (Oc.S/BS) was increased 2.0-fold by all doses of continuous isoprenaline treatment. The osteoclast parameters plateaued at the lowest dose (5 μg/g daily) of continuous isoprenaline treatment. These results indicate that chronic stimulation of β-AR with low-dose agonist treatment induces bone loss mainly via enhanced bone resorption.

Mice lacking bone sialoprotein (BSP) lose bone after ovariectomy and display skeletal site-specific response to intermittent PTH treatment

Bone sialoprotein (BSP) belongs to the small integrin-binding ligand, N-linked glycoprotein (SIBLING) family, whose members play multiple and distinct roles in the development, turnover, and mineralization of bone and dentin. The functions of BSP in bone remodeling are not yet well established. We previously showed that BSP knockout (BSP(-/-)) mice exhibit a higher trabecular bone volume, concomitant with lower bone remodeling, than wild-type (BSP(+/+)) mice. To determine whether bone turnover can be stimulated in the absence of BSP, we subjected BSP(+/+) and BSP(-/-) mice to catabolic [ovariectomy (OVX)] or anabolic (intermittent PTH administration) hormonal challenges. BSP(-/-) mice progressively develop hypocalcemia and high serum PTH between 2 and 4 months of age. Fifteen and 30 d after OVX, microtomography analysis showed a significant decrease of trabecular bone volume in tibiae of both genotypes. Histomorphometric parameters of bone formation and resorption were significantly increased by OVX. PTH treatment resulted in an increase of trabecular thickness and both bone formation and resorption parameters at all skeletal sites in both genotypes and a decrease of trabecular bone volume in tibiae of BSP(+/+) but not BSP(-/-) mice. PTH increased cortical thickness and bone area in BSP(+/+) but not BSP(-/-) mice and stimulated the bone formation rate specifically in the endosteum of BSP(+/+) mice and the periosteum of BSP(-/-) mice. PTH enhanced the expression of RANKL, MEPE, and DMP1 in both genotypes but increased OPG and OPN expression only in BSP(-/-) mice. In conclusion, despite the low basal turnover, both catabolic and anabolic challenges increase bone formation and resorption in BSP(-/-) mice, suggesting that compensatory pathways are operative in the skeleton of BSP-deficient mice. Although up-regulation of one or several other SIBLINGs is a possible mechanism, further studies are needed to analyze the interplay and cross-regulation involved in compensating for the absence of BSP.

Increased serum osteopontin is a risk factor for osteoporosis in menopausal women

SUMMARY

Osteopontin (OPN)-deficient mice are resistant to ovariectomy-induced osteoporosis. Therefore, we hypothesized that women with OPN overexpression may show less resistance to postmenopausal osteoporosis. In this study, we first demonstrated that serum OPN levels could be used as a biomarker for the early diagnosis of osteoporosis in postmenopausal women.

INTRODUCTION

Animal studies indicate that OPN-deficient mice are resistant to ovariectomy-induced osteoporosis.

METHODS

From 2004 to 2006, 124 women over the age of 45 were enrolled in a menopausal group, while another 95 women, from 25 to 45 years of age with regular menstruation, were enrolled into a childbearing age group. The serum concentrations of OPN were calculated using the enzyme-link immunosorbent assay method, and bone mineral densities were determined with dual energy X-ray absorptiometry.

RESULTS

Serum OPN levels had a significant positive correlation with age (menopausal group, p < 0.0001) and a negative correlation with body weight, height, hip bone mineral density, and T-scores in the menopausal group. In contrast, there was a positive correlation with the E2 concentration and height, but there was no significant association with the above variables in the childbearing age group. Additionally, high serum OPN levels (>14.7 ng/ml) was a significant risk factor causing menopausal osteoporosis (odds ratio = 2.96, 95% confidence interval, 1.055-8.345).

CONCLUSION

Serum OPN levels could be used as a biomarker for the early diagnosis of osteoporosis in postmenopausal women.

Adipocyte-mononuclear cell interaction, Toll-like receptor 4 activation, and high glucose synergistically up-regulate osteopontin expression via an interleukin 6-mediated mechanism

Although it has been reported that osteopontin, a matrix glycoprotein and proinflammatory cytokine, mediates obesity-induced adipose tissue macrophage infiltration and insulin resistance, it remains unclear how osteopontin is up-regulated in adipose tissue in obese humans and animals. In this study, we incubated U937 mononuclear cells with adipocytes in a transwell system and studied how cell interaction regulated osteopontin expression. Results showed that coculture of U937 cells with adipocytes led to a marked increase in osteopontin production when compared with that released by independent cultures of U937 cells. Moreover, lipopolysaccharide or palmitic acid-induced TLR4 activation and high glucose further augmented the coculture-stimulated osteopontin secretion. Similar observations were made in the coculture of human primary monocytes and adipocytes. Real time PCR studies showed that coculture of U937 cells and adipocytes increased osteopontin mRNA in U937 cells, but not adipocytes, suggesting that adipocyte-derived soluble factor may stimulate osteopontin expression by U937 cells. In our studies to explore the underlying mechanism, we found that the neutralizing antibodies against interleukin (IL)-6 or IL-6 small interfering RNA transfection in adipocytes effectively inhibited coculture-stimulated osteopontin expression, suggesting that IL-6 released by adipocytes plays an essential role in the coculture-stimulated osteopontin expression by U937 cells. In conclusion, this study has demonstrated that cell interaction, TLR4 activation, and high glucose up-regulate osteopontin expression, and adipocyte-derived IL-6 played a major role in the up-regulation.

Angiotensin II type 2 receptor blockade increases bone mass

Renin angiotensin system (RAS) regulates circulating blood volume and blood pressure systemically, whereas RAS also plays a role in the local milieu. Previous in vitro studies suggested that RAS may be involved in the regulation of bone cells. However, it was not known whether molecules involved in RAS are present in bone in vivo. In this study, we examined the presence of RAS components in adult bone and the effects of angiotensin II type 2 (AT2) receptor blocker on bone mass. Immunohistochemistry revealed that AT2 receptor protein was expressed in both osteoblasts and osteoclasts. In addition, renin and angiotensin II-converting enzyme were expressed in bone cells in vivo. Treatment with AT2 receptor blocker significantly enhanced the levels of bone mass, and this effect was based on the enhancement of osteoblastic activity as well as the suppression of osteoclastic activity in vivo. These results indicate that RAS components are present in adult bone and that blockade of AT2 receptor results in alteration in bone mass.

Accentuated osteoclastic response to parathyroid hormone undermines bone mass acquisition in osteonectin-null mice

Matricellular proteins play a unique role in the skeleton as regulators of bone remodeling, and the matricellular protein osteonectin (SPARC, BM-40) is the most abundant non-collagenous protein in bone. In the absence of osteonectin, mice develop progressive low turnover osteopenia, particularly affecting trabecular bone. Polymorphisms in a regulatory region of the osteonectin gene are associated with bone mass in a subset of idiopathic osteoporosis patients, and these polymorphisms likely regulate osteonectin expression. Thus it is important to determine how osteonectin gene dosage affects skeletal function. Moreover, intermittent administration of parathyroid hormone (PTH) (1-34) is the only anabolic therapy approved for the treatment of osteoporosis, and it is critical to understand how modulators of bone remodeling, such as osteonectin, affect skeletal response to anabolic agents. In this study, 10 week old female wild type, osteonectin-haploinsufficient, and osteonectin-null mice (C57Bl/6 genetic background) were given 80 microg/kg body weight/day PTH(1-34) for 4 weeks. Osteonectin gene dosage had a profound effect on bone microarchitecture. The connectivity density of trabecular bone in osteonectin-haploinsufficient mice was substantially decreased compared with that of wild type mice, suggesting compromised mechanical properties. Whereas mice of each genotype had a similar osteoblastic response to PTH treatment, the osteoclastic response was accentuated in osteonectin-haploinsufficient and osteonectin-null mice. Eroded surface and osteoclast number were significantly higher in PTH-treated osteonectin-null mice, as was endosteal area. In vitro studies confirmed that PTH induced the formation of more osteoclast-like cells in marrow from osteonectin-null mice compared with wild type. PTH treated osteonectin-null bone marrow cells expressed more RANKL mRNA compared with wild type. However, the ratio of RANKL:OPG mRNA was somewhat lower in PTH treated osteonectin-null cultures. Increased expression of RANKL in response to PTH could contribute to the accentuated osteoclastic response in osteonectin-/- mice, but other mechanisms are also likely to be involved. The molecular mechanisms by which PTH elicits bone anabolic vs. bone catabolic effects remain poorly understood. Our results imply that osteonectin levels may play a role in modulating the balance of bone formation and resorption in response to PTH.

Altered osteoclast development and function in osteopontin deficient mice

The role of osteopontin in bone resorption was elucidated by studies of mice with knock out of the osteopontin gene generated by a different approach compared to previous models. Thus, a targeting vector with the promoter region as well as exons 1, 2, and 3 of the osteopontin gene was replaced by a loxP-flanked Neo-TK cassette, and this cassette was eliminated through transient expression of Cre recombinase. The recombined ES cells were used to create mice lacking expression of the osteopontin gene. Tissues from these mice were subjected structural and molecular analyses including morphometry and proteomics. The bone of the null mice contained no osteopontin but showed no significant alterations with regard to other bone proteins. The bone volume was normal in young null animals but in the lower metaphysis, the volume and number of osteoclasts were increased. Notably, the volume and length of the osteoclast ruffled border was several folds lower, indicating a lower resorptive capacity. The null mice did not develop the bone loss characteristic for osteoporosis demonstrated in old wild-type female animals. This quantitative study demonstrates a bone phenotype in the osteopontin null mice of all ages. The data provides further evidence for a role of osteopontin in osteoclast activity.

Osteopontin negatively regulates parathyroid hormone receptor signaling in osteoblasts

Systemic hormonal control exerts its effect through the regulation of local target tissues, which in turn regulate upstream signals in a feedback loop. The parathyroid hormone (PTH) axis is a well defined hormonal signaling system that regulates calcium levels and bone metabolism. To understand the interplay between systemic and local signaling in bone, we examined the effects of deficiency of the bone matrix protein osteopontin (OPN) on the systemic effects of PTH specifically within osteoblastic cell lineages. Parathyroid hormone receptor (PPR) transgenic mice expressing a constitutively active form of the receptor (caPPR) specifically in cells of the osteoblast lineage have a high bone mass phenotype. In these mice, OPN deficiency further increased bone mass. This increase was associated with conversion of the major intertrabecular cell population from hematopoietic cells to stromal/osteoblastic cells and parallel elevations in histomorphometric and biochemical parameters of bone formation and resorption. Treatment with small interfering RNA (siRNA) for osteopontin enhanced H223R mutant caPPR-induced cAMP-response element (CRE) activity levels by about 10-fold. Thus, in addition to the well known calcemic feedback system for PTH, local feedback regulation by the bone matrix protein OPN also plays a significant role in the regulation of PTH actions.

Osteopontin

Osteopontin (OPN) is a multifunctional molecule highly expressed in chronic inflammatory and autoimmune diseases, and it is specifically localized in and around inflammatory cells. OPN is a secreted adhesive molecule, and it is thought to aid in the recruitment of monocytes-macrophages and to regulate cytokine production in macrophages, dendritic cells, and T-cells. OPN has been classified as T-helper 1 cytokine and thus believed to exacerbate inflammation in several chronic inflammatory diseases, including atherosclerosis. Besides proinflammatory functions, physiologically OPN is a potent inhibitor of mineralization, it prevents ectopic calcium deposits and is a potent inducible inhibitor of vascular calcification. Clinically, OPN plasma levels have been found associated with various inflammatory diseases, including cardiovascular burden. It is thus imperative to dissect the OPN proinflammatory and anticalcific functions. OPN recruitment functions of inflammatory cells are thought to be mediated through its adhesive domains, especially the arginine-glycine-aspartate (RGD) sequence that interacts with several integrin heterodimers. However, the integrin receptors and intracellular pathways mediating OPN effects on immune cells are not well established. Furthermore, several studies show that OPN is cleaved by at least 2 classes of proteases: thrombin and matrix-metalloproteases (MMPs). Most importantly, at least in vitro , fragments generated by cleavage not only maintain OPN adhesive functions but also expose new active domains that may impart new activities. The role for OPN proteolytic fragments in vivo is almost completely unexplored. We believe that further knowledge of the effects of OPN fragments on cell responses might help in designing therapeutics targeting inflammatory and cardiovascular diseases.

Serum osteopontin, an enhancer of tumor metastasis to bone, promotes B16 melanoma cell migration

Tumor malignancy is associated with several features such as proliferation ability and frequency of metastasis. Since tumor metastasis shortens patients' lifetime, establishment of therapy for anti-metastasis is very important. Osteopontin (OPN), which abundantly expressed in bone matrix, is involved in cell adhesion, migration, extracellular matrix (ECM) invasion and cell proliferation via interaction with its receptor, that is, alphavbeta3 integrin. OPN is believed to be a positive regulator of tumor metastasis in vivo. However, how OPN regulates metastasis is largely unknown. Here, we explore the role of OPN in cell migration. Serum from wild-type mice induced cell migration of B16 melanoma cells, while serum from OPN-deficient mouse suppressed this event. The presence of recombinant OPN significantly enhanced cell migration compared to albumin containing medium. OPN-induced cell migration was suppressed by inhibiting the ERK/MAPK pathway indicating that OPN-induced cell migration depends on this pathway. Overexpression of OPN in these cancer cells per se promoted cell proliferation and tended to increase B16 cell migration suggesting that OPN promotes bone metastasis by playing dual roles both in host microenvironment and in tumor cell itself. In conclusion, the elevated OPN expression in host tissue and tumor cell itself promotes tumor cell migration reading to tumor metastasis, suggesting that neutralization of OPN-induced signal might be effective in suppression of tumor metastasis.

Cnot7-null mice exhibit high bone mass phenotype and modulation of BMP actions

Cnot7 is a recently identified regulator of spermatogenesis in adult mice. Because Cnot7 binds to Tob, a BMP inhibitor shown to be involved in bone metabolism, we examined whether Cnot7 is involved in bone mass regulation by using adult Cnot7 deficient mice. Cnot7-/- mice exhibited a high bone mass phenotype. This was associated with an increase in bone formation rate but not with any alteration in bone resorption parameters. On BMP treatment, Cnot7-/- cells expressed higher levels of alkaline phosphatase compared with control cells. Direct BMP2 injection induced larger bone mass in Cnot7-/- calvaria than control in vivo. These observations revealed that Cnot7 is an endogenous suppressor of bone mass and inhibits BMP actions in osteoblasts.

INTRODUCTION

The molecular mechanisms involved in the determination of bone mass have been gradually understood based on recent analyses. Cnot7 (Ccr4-Not complex 7) is a component of transcriptional Ccr4-Not complex, is conserved from yeast to human, and binds to Tob, but its function in bone is not understood.

MATERIALS AND METHODS

To elucidate the role of involvement of Cnot7 in bone mass determination, we examined the bone of adult male Cnot7-null and heterozygous mice based on microCT analyses, histomorphometry, cell cultures, and in vivo BMP assays.

RESULTS

Cnot7-/- mice showed an increase in bone mass levels by >50% compared with controls. Analyses of the histomorphometric parameters indicated that bone formation activity in Cnot7-/- mice was enhanced, whereas bone resorption activity was not altered. These effects on osteoblasts were cell autonomous because mineralized nodule formation was enhanced in the cultures of bone marrow cells prepared from Cnot7-/- mice. In vitro analyses to elucidate Cnot7 effects revealed that BMP-induced expression of alkaline phosphatase in Cnot7-/- calvaria-derived osteoblastic cells was enhanced compared with controls. Moreover, BMP injection-induced new bone formation in vivo was enhanced in Cnot7-/- mice.

CONCLUSIONS

These observations indicated that Cnot7 is an endogenous suppressor of bone mass in adult mice and inhibits BMP actions.

Lack of Schnurri-2 expression associates with reduced bone remodeling and osteopenia

Regulation of bone remodeling determines the levels of bone mass and its imbalance causes major skeletal diseases such as osteoporosis. A zinc finger protein, Schnurri-2 (SHN-2), was recently demonstrated to regulate bone morphogenetic protein-dependent adipogenesis and lymphogenesis. However, the role of SHN-2 in bone is not known. Here, we investigated the effects of Shn-2 deficiency on bone metabolism and cell function in Shn-2-null mice. Lack of SHN-2 expression reduced bone remodeling by suppressing both osteoblastic bone formation and osteoclastic bone resorption activities in vivo. Shn-2 deficiency suppressed osterix and osteocalcin expression as well as in vitro mineralization. Conversely, Shn-2 overexpression enhanced osteocalcin promoter activity and bone morphogenetic protein-dependent osteoblastic differentiation. Shn-2 deficiency suppressed Nfatc1 and c-fos expression leading to reduction of tartrate-resistant acid phosphatase-positive cell development in vivo as well as in the cultures of bone marrow cells. These studies demonstrate that SHN-2 regulates the activities of critical transcription factors required for normal bone remodeling.

Effects of cyclic vs. daily treatment with human parathyroid hormone (1-34) on murine bone structure and cellular activity

Previously, we demonstrated that the human parathyroid hormone (1-34) fragment (hPTH(1-34)) increased bone strength in proportion to its effects on BMD and cortical bone structure in the murine femur by comparing cyclic vs. daily administration of hPTH(1-34). Both cyclic and daily regimens increased vertebral BMD similarly at 7 weeks. Here, we have examined the effects of daily and cyclic PTH regimens on bone structure and cellular activity by static and dynamic histomorphometry. Twenty-week-old, intact female C57BL/J6 mice were treated with the following regimens (n=7 for each group): daily injection with vehicle for 7 weeks [control]; daily injection with hPTH(1-34) (40 microg/kg/day) for 7 weeks [daily PTH]; and daily injection with hPTH(1-34) (40 microg/kg/day) and vehicle alternating weekly for 7 weeks [cyclic PTH]. At days 9 and 10, and 2 and 3 prior to euthanasia, calcein (10 mg/kg) was injected subcutaneously. At the end of study, the lumbar vertebrae 1-3 and the left femora were excised, cleaned, and processed for histomorphometry. In the lumbar vertebrae, daily and cyclic PTH regimens significantly increased cancellous bone volume (BV/TV), trabecular number, trabecular osteoclast and osteoblast perimeters, trabecular mineral apposition rate (MAR) and bone formation rate (BFR), and periosteal MAR and BFR compared to control, with no significant difference between the two PTH-treated groups. Increased trabecular tunneling was observed in both PTH-treated groups. Both regimens tended to increase vertebral cortical bone formation parameters with the effects at the periosteum site being more marked than those at the endosteum site, resulting in a significant increase in cortical width. In the femur, the effects of cyclic PTH on BV/TV, trabecular width and number, trabecular and endocortical osteoblast and osteoclast perimeters, cortical width, and trabecular and periosteal BFR were less marked than those of daily PTH. A cyclic PTH regimen was as effective as a daily regimen in improving cancellous and cortical bone microarchitecture and cellular activity in the murine vertebra.

Over-expression of TIMP-1 in osteoblasts increases the anabolic response to PTH

Intermittent PTH treatment induces structural changes that affect cancellous bone mass and have led to its indication for the treatment of osteoporosis. PTH is also known to upregulate the expression of matrix metalloproteinases (MMP) in osteoblasts. We wanted to find out whether inhibiting osteoblastic MMPs can affect the anabolic action of PTH in vivo. We had shown previously that mice over-expressing TIMP-1 (tissue inhibitor of MMPs) specifically in osteoblasts display an increase in bone mineral density and bone mass combined with an overall decrease in bone turnover. In the present study, 10-week-old wild-type (WT) and transgenic (TG) mice were treated with PTH at 40 microg/kg/day for 1.5 months. DEXA analysis was performed before and after treatment, and histomorphometric and molecular analysis were carried out at the end of the experiment. Our findings indicate that the transgene boosted the anabolic action of PTH. The femurs of PTH-treated TG mice displayed a greater increase in bone mineral density and trabecular bone volume than treated WT mice. Interestingly, the positive effect of the transgene on the action of PTH resulted from both reduced bone resorption activity and an increase in the bone formation rate. Osteoclastic surfaces that were increased in PTH-treated WT mice remained unchanged in TG mice, suggesting a decrease in osteoclastic differentiation. Histomorphometric data also indicate that PTH administration increased osteoblast activity in TG mice and affected the number of osteoblasts in WT mice. In conclusion, we demonstrate that inhibiting osteoblastic MMPs can potentiate the anabolic effect of PTH by decreasing osteoclast activity and increasing osteoblast activity. Our data also suggest that osteoblastic MMPs have some role in mediating the anabolic effects of PTH in vivo and indicate that inhibitors of MMPs could constitute a new therapy for degenerative diseases.

Osteopontin is associated with nuclear factor κB gene expression during tail-suspension-induced bone loss

Osteoporosis due to unloading-induced bone loss is a critical issue in the modern aging society. Although the mechanisms underlying this phenomenon are largely unknown, osteopontin (OPN) is one of the critical mediators required for unloading-induced bone loss [M. Ishijima, S.R. Rittling, T. Yamashita, K. Tsuji, H. Kurosawa, A. Nifuji, D.T. Denhardt, and M. Noda, Enhancement of osteoclastic bone resorption and suppression of osteoblastic bone formation in response to reduced mechanical stress do not occur in the absence of osteopontin, J Exp Med, 193 (2001) 399-404]. To clarify the molecular bases for OPN actions, we carried out microarray analyses on the genes expressed in the femoral bone marrow cells in wild type and OPN-/- mice. The removal of the mechanical load induced bone loss in wild type, but not in OPN-/- mice, as previously reported. Expression analysis of 9586 cDNAs on a microarray system revealed that OPN deficiency blocked tail-suspension-induced expression of ten genes (group A). This observation was confirmed based on semi-quantitative RT-PCR analyses. On the other hand, expression of four genes (group B) was not altered by tail suspension in wild type but was enhanced in OPN-deficient mice. NF-kappaB p105 subunit gene (Nfkb1) was found in group A and Bax in group B. p53 gene expression was upregulated by tail suspension in wild type mice, but it was no longer observed in OPN-/- mice. These data indicate that OPN acts to mediate mechanical stress signaling upstream to the genes encoding apoptosis-related molecules, and its action is associated with alteration of the genes.

Molecular genetic studies of gene identification for osteoporosis: a 2004 update

This review summarizes comprehensively the most important and representative molecular genetics studies of gene identification for osteoporosis published up to the end of December 2004. It is intended to constitute a sequential update of our previously published review covering the available data up to the end of 2002. Evidence from candidate gene association studies and genome-wide linkage studies in humans, as well as quantitative trait locus mapping animal models are reviewed separately. Studies of transgenic and knockout mice models relevant to osteoporosis are summarized. An important extension of this update is incorporation of functional genomic studies (including DNA microarrays and proteomics) on osteogenesis and osteoporosis, in light of the rapid advances and the promising prospects of the field. Comments are made on the most notable findings and representative studies for their potential influence and implications on our present understanding of genetics of osteoporosis. The format adopted by this review should be ideal for accommodating future new advances and studies.

Aprt/Opn double knockout mice: osteopontin is a modifier of kidney stone disease severity

BACKGROUND

Osteopontin (OPN) is reported to have two distinct functions in kidney disease: Promotion of inflammation at sites of tissue injury, and inhibition of calcium oxalate monohydrate stone formation. However, many of the studies supporting these functions were carried out in animal models of acute renal injury or in cultured cells; thus, the role of OPN in chronic renal disease is not well defined. We examined the role of OPN in adenine phosphoribosyltransferase (Aprt) knockout mice, in which inflammation and formation of 2,8-dihydroxyadenine (DHA) kidney stones are prominent features, by generating Aprt/Opn double knockout mice.

METHODS

We characterized the phenotypes of six- and 12-week-old Aprt-/- Opn-/-, Aprt-/- Opn+/+, Aprt+/+ Opn-/-, and Aprt+/+ Opn+/+ male and female mice using biochemical, histologic, immunohistochemical, and in situ hybridization techniques.

RESULTS

At 6 weeks of age, there was no difference in phenotype between double knockout and Aprt knockout mice. At 12 weeks, there was increased adenine and DHA excretion, renal crystal deposition, and inflammation in double knockout versus Aprt knockout male mice. Double knockout and Aprt knockout female mice at 12 weeks had less pathology than their male counterparts, but kidneys from double knockout females showed more inflammation compared with Aprt knockout females; both genotypes had similar levels of DHA crystal deposition.

CONCLUSION

We conclude that (1) OPN is a major inhibitor of DHA crystal deposition and inflammation in male mice; and (2) OPN is a major modifier of the inflammatory response but not of crystal deposition in female mice. Thus, separate mechanisms appear responsible for the tissue changes seen in DKO males versus females.

Homocysteine attenuates the expression of osteocalcin but enhances osteopontin in MC3T3-E1 preosteoblastic cells

It has been pointed out that very high plasma levels of homocysteine are characteristic of homocystinuria, a rare autosomal recessive disease accompanied by the early onset of generalized osteoporosis. However, it is unclear by which mechanism hyperhomocysteine induces osteoporosis, although it is known to interfere with the formation of cross-links in collagen, an essential process in bone formation. Therefore, we investigated the effect of homcysteine on expression of osteocalcin and osteopontin in MC3T3-E1 preosteoblastic cells. Confluent cells were grown in RPMI 1640 containing 10% fetal calf serum with or without homocysteine in an atmosphere of 95% humidified air, 5% CO2 at 37 degrees C. The secretion of osteocalcin from the cells increased time-dependently until the end of culture (day 34), but 500 microM homocysteine led to an approximately 61% decrease for osteocalcin after 19 days of culture as compared with the control. On the other hand, osteopontin was not inhibited by 500 microM homocysteine but rather activated, and ranged from 134%-209% of the control level in the period from 10 days until the end of culture. From analysis of RT-PCR for mRNA of osteocalcin and osteopontin at the end of the culture, homocysteine levels of 100 and 500 microM significantly increased the expression of osteopontin mRNA with the control (p < 0.05). In contrast, the expression of osteopontin mRNA was suppressed in a dose-dependent manner, showing a mirror image of the effect on osteopontin mRNA. These findings suggest that hyperhomocystenemia appears to be an independent risk factor for osteoporosis by disturbing osteoblast function.

beta-Arrestin2 regulates the differential response of cortical and trabecular bone to intermittent PTH in female mice

Cytoplasmic arrestins regulate PTH signaling in vitro. We show that female beta-arrestin2(-/-) mice have decreased bone mass and altered bone architecture. The effects of intermittent PTH administration on bone microarchitecture differed in beta-arrestin2(-/-) and wildtype mice. These data indicate that arrestin-mediated regulation of intracellular signaling contributes to the differential effects of PTH at endosteal and periosteal bone surfaces.

INTRODUCTION

The effects of PTH differ at endosteal and periosteal surfaces, suggesting that PTH activity in these compartments may depend on some yet unidentified mechanism(s) of regulation. The action of PTH in bone is mediated primarily by intracellular cAMP, and the cytoplasmic molecule beta-arrestin2 plays a central role in this signaling regulation. Thus, we hypothesized that arrestins would modulate the effects of PTH on bone in vivo.

MATERIALS AND METHODS

We used pDXA, muCT, histomorphometry, and serum markers of bone turnover to assess the skeletal response to intermittent PTH (0, 20, 40, or 80 mug/kg/day) in adult female mice null for beta-arrestin2 (beta-arr2(-/-)) and wildtype (WT) littermates (7-11/group).

RESULTS AND CONCLUSIONS

beta-arr2(-/-) mice had significantly lower total body BMD, trabecular bone volume fraction (BV/TV), and femoral cross-sectional area compared with WT. In WT females, PTH increased total body BMD, trabecular bone parameters, and cortical thickness, with a trend toward decreased midfemoral medullary area. In beta-arr2(-/-) mice, PTH not only improved total body BMD, trabecular bone architecture, and cortical thickness, but also dose-dependently increased femoral cross-sectional area and medullary area. Histomorphometry showed that PTH-stimulated periosteal bone formation was 2-fold higher in beta-arr2(-/-) compared with WT. Osteocalcin levels were significantly lower in beta-arr2(-/-) mice, but increased dose-dependently with PTH in both beta-arr2(-/-) and WT. In contrast, whereas the resorption marker TRACP5B increased dose-dependently in WT, 20-80 mug/kg/day of PTH was equipotent with regard to stimulation of TRACP5B in beta-arr2(-/-). In summary, beta-arrestin2 plays an important role in bone mass acquisition and remodeling. In estrogen-replete female mice, the ability of intermittent PTH to stimulate periosteal bone apposition and endosteal resorption is inhibited by arrestins. We therefore infer that arrestin-mediated regulation of intracellular signaling contributes to the differential effects of PTH on cancellous and cortical bone.

Role of matrix Gla protein in parathyroid hormone inhibition of osteoblast mineralization

Parathyroid hormone (PTH) exerts biphasic effects on bone, dependent on the frequency and dose of administration. The catabolic actions of PTH on bone have been associated with continuous treatment, an increase in osteoblast-mediated resorption of bone via osteoclast activation, and inhibition of osteoblast activity and mineralization. Downregulation of differentiation markers and inhibition of mineralization by PTH have been reported in primary calvarial explants and osteoblast cell lines. Using MC3T3-E1 osteoblast-like cells, we have shown that matrix Gla protein (MGP) can be induced by PTH, and that this induction may explain the PTH-mediated inhibition of osteoblast biomineralization. MGP is a known inhibitor of mineralization, and mice deficient in Mgp show severe vascular calcification and premature bone mineralization. This review discusses the role of MGP in mineralization, comparing bone and vascular mineralization. In addition to MGP, the regulation and possible role of osteopontin, another known regulator of osteoblast mineralization, in PTH-mediated regulation of bone and vascular mineralization is discussed.