Effect of treatment for 6 months with human parathyroid hormone (1-34) peptide in ovariectomized cynomolgus monkeys (Macaca fascicularis)

Primary hyperparathyroidism and sarcoma: A case report and literature review

ABSTRACT

The relationship between primary hyperparathyroidism (PHPT) and bone sarcoma is debatable, especially after wider use of teriparatide treatment, concerns have intensified on the issue. Extensive search in English literature revealed 10 cases reported having PHPT and sarcomas. Besides, three cases of bone sarcoma occurring after teriparatide treatment had been reported. Hereby, we report a 51-year-old woman with a prolonged history of PHPT. She was diagnosed with chondrosarcoma 9 years after refusal and lack of treatment for PHPT. She was cured surgically for both chondrosarcoma and parathyroid adenoma at 1-year interval. So far, large cohorts did not show an increase in the incidence of bone sarcomas in PHPT. Several case observations, including the current one, as well as data from in vitro and rat studies, pointed out prolonged parathormone exposure, may be a risk for bone sarcomas. Under these circumstances, a safer attitude on individual basis would be the prevention of prolonged parathormone exposures.

Dose-dependent effects of pharmaceutical treatments on bone matrix properties in ovariectomized rats

As both anabolic and anti-catabolic osteoporosis drugs affect bone formation and resorption processes, they may contribute to bone's overall mechanical behavior by altering the quality of the bone matrix. We used an ovariectomized rat model and a novel fracture mechanics approach to investigate whether treatment with an anabolic (parathyroid hormone) or anti-catabolic (alendronate) osteoporosis drugs will alter the organic and mineral matrix components and consequently cortical bone fracture toughness. Ovariectomized (at 5 months age) rats were treated with either parathyroid hormone or alendronate at low and high doses for 6 months (age 6–12 months). Specifically, treatment groups included untreated ovariectomized controls (n = 9), high-dose alendronate (n = 10), low-dose alendronate (n = 9), high-dose parathyroid hormone (n = 10), and low-dose parathyroid hormone (n = 9). After euthanasia, cortical microbeams from the lateral quadrant were extracted, notched, and tested in 3-point bending to measure fracture toughness. Portions of the bone were used to measure changes in the 1) organic matrix through quantification of advanced glycation end-products (AGEs) and non-collagenous proteins, and 2) mineral matrix through assessment of mineral crystallinity. Compared to the ovariectomized group, rats treated with high doses of parathyroid hormone and alendronate had significantly increased cortical bone fracture toughness, which corresponded primarily to increased non-collagenous proteins while there was no change in AGEs. Additionally, low-dose PTH treatment increased matrix crystallinity and decreased AGE levels. In summary, ovariectomized rats treated with pharmaceutical drugs had increased non-collagenous matrix proteins and improved fracture toughness compared to controls. Further investigation is required for different doses and longer treatment periods.

•

Alendronate increases non-collagenous proteins and improves fracture toughness.

•

Parathyroid hormone also increases collagen maturity and mineral crystallinity.

•

Both treatments minimize accumulation of advanced glycation end-products.

Principles of the activin receptor signaling pathway and its inhibition

This review captures the anabolic and stimulatory effects observed with inhibition of the transforming growth factor β superfamily in muscle, blood, and bone. New medicinal substances that rectify activin, myostatin, and growth differentiation factor 11 signaling give hope to the many whose lives are affected by deterioration of these tissues. The review first covers the origin, structure, and common pathway of activins, myostatin, and growth differentiation factor 11 along with the pharmacodynamics of the new class of molecules designed to oppose the activin receptor signaling pathway. Current terminology surrounding this new class of molecules is inconsistent and does not infer functionality. Adopting inhibitors of the activin receptor signaling pathway (IASPs) as a generic term is proposed because it encapsulates the molecular mechanisms along the pathway trajectory. To conclude, a pragmatic classification of IASPs is presented that integrates functionality and side effects based on the data available from animals and humans. This provides researchers and clinicians with a tool to tailor IASPs therapy according to the need of projects or patients and with respect to side effects.

Cortical Bone Porosity in Rabbit Models of Osteoporosis

Cortical bone porosity is intimately linked with remodeling, is of growing clinical interest, and is increasingly accessible by imaging. Thus, the potential of animal models of osteoporosis (OP) to provide a platform for studying how porosity develops and responds to interventions is tremendous. To date, rabbit models of OP have largely focused on trabecular microarchitecture or bone density; some such as ovariectomy (OVX) have uncertain efficacy and cortical porosity has not been extensively reported. Our primary objective was to characterize tibial cortical porosity in rabbit-based models of OP, including OVX, glucocorticoids (GC), and OVX + GC relative to controls (SHAM). We sought to: (i) test the hypothesis that intracortical remodeling is elevated in these models; (ii) contrast cortical remodeling and porosity in these models with that induced by parathyroid hormone (1-34; PTH); and (iii) contrast trabecular morphology in the proximal tibia across all groups. Evidence that an increase in cortical porosity occurred in all groups was observed, although this was the least robust for GC. Histomorphometric measures supported the hypothesis that remodeling rate was elevated in all groups and also revealed evidence of uncoupling of bone resorption and formation in the GC and OVX + GC groups. For trabecular bone, a pattern of loss was observed for OVX, GC, and OVX + GC groups, whereas the opposite was observed for PTH. Change in trabecular number best explained these patterns. Taken together, the findings indicated rabbit models provide a viable and varied platform for the study of OP and associated changes in cortical remodeling and porosity. Intriguingly, the evidence revealed differing effects on the cortical and trabecular envelopes for the PTH model. © 2020 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR)..

Bone's Material Constituents and their Contribution to Bone Strength in Health, Disease, and Treatment

Type 1 collagen matrix volume, its degree of completeness of its mineralization, the extent of collagen crosslinking and water content, and the non-collagenous proteins like osteopontin and osteocalcin comprise the main constituents of bone's material composition. Each influences material strength and change in different ways during advancing age, health, disease, and drug therapy. These traits are not quantifiable using bone densitometry and their plurality is better captured by the term bone 'qualities' than 'quality'. These qualities are the subject of this manuscript.

Effects of eight-month treatment with ONO-5334, a cathepsin K inhibitor, on bone metabolism, strength and microstructure in ovariectomized cynomolgus monkeys

This study examined the effect of ONO-5334, a cathepsin K inhibitor, on bone turnover, mineral density (BMD), mechanical strength and microstructure in ovariectomized (OVX) cynomolgus monkeys. Vehicle, ONO-5334 (3, 10 or 30 mg/kg) or alendronate (0.5 mg/kg) was orally administered for eight months to sham- and OVX-operated monkeys. ONO-5334 dose-dependently suppressed OVX-induced increase in bone turnover markers (urinary C-terminal cross-linking telopeptide of type I collagen (CTX) and serum osteocalcin). At the dose of 30 mg/kg, ONO-5334 maintained urinary CTX at nearly zero level and kept serum osteocalcin around the level of the sham animals. Marker levels in the alendronate-treated animals were similar to those in the sham animals throughout the study. ONO-5334 dose-dependently reversed the effect of OVX on vertebral BMD as measured by dual-energy X-ray absorptiometry (DXA) with improvement of bone mechanical strength. Both ONO-5334 and alendronate suppressed OVX-induced changes in vertebral microstructure and turnover state. In the femoral neck, peripheral quantitative computed tomography (pQCT) analysis showed that ONO-5334 increased total and cortical BMD. In particular, ONO-5334 significantly increased cortical BMD with improvement of bone mechanical strength. In microstructural analysis, alendronate suppressed OVX-induced increase in femoral mid-shaft osteonal bone formation rate (BFR) to a level below that recorded in the sham group, whereas ONO-5334 at 30 mg/kg did not suppress periosteal, osteonal and endocortical BFR. This finding supports the significant effect of ONO-5334 on cortical BMD and mechanical strength in the femoral neck. The results of this study suggest that ONO-5334 has good therapeutic potential for the treatment of osteoporosis.

A soluble activin receptor Type IIA fusion protein (ACE-011) increases bone mass via a dual anabolic-antiresorptive effect in Cynomolgus monkeys

Activin A belongs to the TGF-beta superfamily and plays an important role in bone metabolism. It was reported that a soluble form of extracellular domain of the activin receptor type IIA (ActRIIA) fused to the Fc domain of murine IgG, an activin antagonist, has an anabolic effect on bone in intact and ovariectomized mice. The present study was designed to examine the skeletal effect of human ActRIIA-IgG1-Fc (ACE-011) in non-human primates. Young adult female Cynomolgus monkeys were given a biweekly subcutaneous injection of either 10mg/kg ACE-011 or vehicle (VEH) for 3months. Treatment effects were evaluated by histomorphometric analysis of the distal femur, femoral midshaft, femoral neck and 12th thoracic vertebrae, by muCT analysis of femoral neck and by biomarkers of bone turnover. Compared to VEH, at the distal femur ACE-011-treated monkeys had significantly increased cancellous bone volume (+93%), bone formation rate per bone surface (+166%) and osteoblast surface (+196%) indicating an anabolic action. Monkeys treated with ACE-011 also had decreased osteoclast surface and number. No differences were observed in parameters of cortical bone at the midshaft of the femur. Similar to distal femur, ACE-011-treated monkeys had significantly greater cancellous bone volume, bone formation rate and osteoblast surface at the femoral neck relative to VEH. A significant increase in bone formation rate and osteoblast surface with a decrease in osteoclast surface was observed in thoracic vertebrae. muCT analysis of femoral neck indicated more plate-like structure in ACE-011-treated monkeys. Monkeys treated with ACE-011 had no effect on serum bone-specific alkaline phosphatase and CTX at the end of the study. These observations demonstrate that ACE-011 is a dual anabolic-antiresorptive compound, improving cancellous bone volume by promoting bone formation and inhibiting bone resorption in non-human primates. Thus, soluble ActRIIA fusion protein may be useful in the prevention and/or treatment of osteoporosis and other diseases involving accelerated bone loss.

Treatment with a soluble receptor for activin improves bone mass and structure in the axial and appendicular skeleton of female cynomolgus macaques (Macaca fascicularis)

A recent study suggests that activin inhibits bone matrix mineralization, whereas treatment of mice with a soluble form of the activin type IIA receptor markedly increases bone mass and strength. To further extend these observations, we determined the skeletal effects of inhibiting activin signaling through the ActRIIA receptor in a large animal model with a hormonal profile and bone metabolism similar to humans. Ten female cynomolgus monkeys (Macaca fascicularis) were divided into two weight-matched groups and treated biweekly, for 3 months, with either a subcutaneous injection 10 mg/kg of a soluble form of the ActRIIA receptor fused with the Fc portion of human IgG(1) (ACE-011) or vehicle (VEH). Bone mineral density (BMD), micro-architecture, compressive mechanical properties, and ash fraction were assessed at the end of the treatment period. BMD was significantly higher in ACE-011 treated individuals compared to VEH: +13% (p=0.003) in the 5th lumbar vertebral body and +15% (p=0.05) in the distal femur. In addition, trabecular volumetric bone density at the distal femur was 72% (p=0.0004) higher than the VEH-treated group. Monkeys treated with ACE-011 also had a significantly higher L5 vertebral body trabecular bone volume (p=0.002) and compressive mechanical properties. Ash fraction of L4 trabecular bone cores did not differ between groups. These results demonstrate that treatment with a soluble form of ActRIIA (ACE-011) enhances bone mass and bone strength in cynomolgus monkeys, and provide strong rationale for exploring the use of ACE-011 to prevent and/or treat skeletal fragility.

Skeletal health: primate model of postmenopausal osteoporosis

Currently, the nonhuman primate is the most widely used large animal model to evaluate the safety and efficacy of new drug entities to treat or prevent estrogen-deficiency-induced bone loss and osteoporosis. Surgical ovariectomy (OVX) induces a state of high bone turnover and rapid bone loss establishing a new steady-state bone mass within 8-9 months. Many systems in the monkey are similar to humans, including skeletal and reproductive physiology and the immune system, making this a plausible model suitable to evaluate the effects of new bone drugs. The long-term sequelae following OVX and withdrawal of monthly exposure to cyclic reproductive hormones in older female monkeys (cynomolgus and rhesus) mimics estrogen depletion and postmenopausal bone loss occurring in women. Characterization of the primate model revealed an apparent limitation to the extent of bone loss. Animals lose bone mass after OVX, but the extent of the bone loss cannot be described as osteoporotic. The small differences between OVX and sham-operated controls in many important bone measurements is overcome by including 15-20 animals per group to provide adequate statistical power. The long-term, at least 16 month, bone safety studies performed to satisfy regulatory guidelines provide an opportunity to study treatment effects for an extended period not covered in shorter-term safety studies. In vivo end-points such as densitometry and biochemical markers translate easily to clinical use, while biomechanical end-points that cannot be measured clinically can be used to predict fracture prevention. To date, the monkey OVX model has been used to support submissions for many new drugs including anabolics, bisphosphonates and selective estrogen receptor modulators. Despite its limitations, the OVX monkey model remains the best characterized of the large animal models of osteopenia and has become integral to osteoporosis drug development.

Treatment with a potent cathepsin K inhibitor preserves cortical and trabecular bone mass in ovariectomized monkeys

The cysteine protease cathepsin K is involved in osteoclast-mediated bone resorption. We evaluated the effect of daily oral dosing of an inhibitor of human cathepsin K (SB-462795 [relacatib]) for 9 months on bone turnover, mass, and architecture in estrogen-deficient cynomolgus monkeys. Ovariectomized animals were treated orally with relacatib at 1, 3, or 10 mg/kg/day, or oral vehicle plus alendronate at 0.05 mg/kg by IV injection once every 2 weeks. The control groups, ovariectomized and sham-ovariectomized animals, received vehicle (all groups n = 20 animals). Samples for biomarker analysis were collected at various times, bone mass changes were evaluated at 6 and 9 months of treatment, and histomorphometric analysis was performed at 9 months. Relacatib significantly reduced urinary N-telopeptide excretion within 1 week of treatment at all dose levels, an effect that was maintained at the highest dose level. At some time points bone formation markers were elevated at the lowest dose of relacatib. Animals treated with relacatib had dose-dependent preservation of areal bone mineral density reaching statistical significance in distal femur. In femur neck there was significant preservation of total volumetric BMD (vBMD) by relacatib. By histomorphometry, relacatib reduced indices of bone resorption and formation at cancellous sites as did alendronate. In cortical bone, osteonal bone formation rate was reduced by alendronate but preserved at low and medium doses of relacatib. Thus, relacatib preserved cortical and cancellous bone mass in ovariectomized monkeys.

Shockwave exerts osteogenic effect on osteoporotic bone in an ovariectomized goat model

Our recent in vitro study showed that extracorporeal shock wave (ESW) stimulated calcium deposition in human periosteal cells. In this study, we hypothesized that the use of ESW could induce new bone formation in osteoporotic bone. Using our established osteoporotic goat model, the calcaneus, distal radius and femoral condyle of the left limb were treated with ESW once per month; the contralateral side served as the control. Bone mineral density (BMD), microarchitecture and dynamic histomorphometric index were evaluated after 9 months. Trabecular BMD of the calcaneus increased significantly by 2.90%. This finding was substantiated by micro-computed tomography findings showing that trabecular bone volume fraction and trabecular thickness of the treated calcaneus were enhanced compared with the contralateral control. However, significant difference could not be detected in the other two weight-bearing skeletal sites. Mineral apposition rates of all ESW-treated regions were also consistently higher than those of the control. These findings suggest that ESW treatment could enhance local BMD by inducing new bone formation, yet the effect was more apparent in non-weight-bearing sites.

Lack of bone neoplasms and persistence of bone efficacy in cynomolgus macaques after long-term treatment with teriparatide [rhPTH(1-34)]

In rats, teriparatide [rhPTH(1-34)] causes marked increases in bone mass and osteosarcoma. In primates, teriparatide causes lesser increases in bone mass, and osteosarcomas have not been reported. Previous studies in primates were not designed to detect bone tumors and did not include a prolonged post-treatment observation period to determine whether tumors would arise after cessation of treatment. Ovariectomized (OVX), skeletally mature, cynomolgus monkeys (n = 30 per group) were given teriparatide for 18 mo at either 0 or 5 microg/kg/d subcutaneously. After 18 mo of treatment, subgroups of six monkeys from both groups were killed and evaluated, whereas all remaining monkeys entered a 3-yr observation period in which they did not receive teriparatide. Surveillance for bone tumors was accomplished with plain film radiographs, visual examination of the skeleton at necropsy, and histologic evaluation of multiple skeletal sites. Quantitative assessments of bone mass, architecture, and strength were also performed. After the 18-mo treatment period, vertebral BMD, BMC, and strength (ultimate load) were increased by 29%, 36%, and 52%, respectively, compared with OVX controls. Proximal femur BMD, BMC, and strength were also increased by 15%, 28% and 33%, respectively. After 3 yr without treatment, no differences in bone mass or strength at the vertebra were observed relative to OVX controls; however, the femoral neck showed significant persistence in stiffness (20%), BMC (14%), and trabecular BV/TV (53%), indicating a retention of teriparatide efficacy at the hip. Radiographs and histology did not identify any bone proliferative lesions or microscopic lesions of osteosarcoma at the end of the treatment or observation period. These data indicate that teriparatide did not induce bone proliferative lesions over a 4.5-yr interval of observation, including 18 mo of treatment and 3 yr of follow-up observation. Bone analyses confirmed that teriparatide caused increases in bone mass and strength, consistent with previous studies. During the withdrawal phase, beneficial effects of teriparatide treatment on the vertebra were lost; however, some of the beneficial effects on the proximal femur persisted for 3 yr after cessation of treatment. Although the lack of bone tumors in this study provides some additional reassurance regarding the safety of teriparatide for the primate skeleton, the small group size and other limitations of this, or any other animal study, limit the ability to draw definitive conclusions regarding the risk of bone tumor developments in patients.

Biomechanical testing in experimental bone interventions--May the power be with you

Total variation in any measured variable, in conjunction with expected treatment effect, defines the minimum sample size (minSS) required to detect the expected effect with statistical confidence should the effect truly exist. A comprehensive literature survey of 3472 original studies was carried out to identify studies with biomechanical testing of whole bones. Total variation in common biomechanical traits and expected treatment effects in typical interventions were statistically determined. According to this survey, total variation in biomechanical traits between different species of experimental animals was similar, justifying the use of rat femur as a model in further analyses. Due to poorer precision, stiffness and energy absorption assessment require substantially larger sample size than breaking load. Due to same reason, minSS for femoral neck compression test is considerably larger than for femoral shaft three-point bending test. For the bending test, minSS to show a 10% treatment effect in the breaking load with 80% statistical power is 11rats/group, while corresponding minSS is 23 for the stiffness, and 53 for the energy absorption. For the femoral neck compression test, minSSs are 16, 51, and 134rats/group, respectively. Among the reviewed studies, the mean sample size was 11animals/group. This underscores the need for considerably larger sample sizes in experimental bone interventions which employ mechanical traits as primary outcome variables. In particular, poor precision and generally small expected treatment effects compromise the utility of stiffness and energy absorption assessments in experimental bone interventions.

Molecular mechanism of osteochondroprogenitor fate determination during bone formation

Osteoblasts and chondrocytes, which derive from a common mesenchymal precursor (osteochondroprogenitor), are involved in bone formation and remodeling in vivo. Determination of osteochondroprogenitor fate is under the control of complex hormonal and local factors converging onto a series of temporospatial dependent transcription regulators. Sox9, together with L-Sox5 and Sox6, of the Sox family is required for chondrogenic differentiation commitment, while Runx2/Cbfa 1, a member of runt family and Osterix/Osx, a novel zinc finger-containing transcription factor play a pivotal role in osteoblast differentiation decision and hypertrophic chondrocyte maturation. Recent in vitro and in vivo evidence suggests beta-catenin, a transcriptional activator in the canonical Wnt pathway, can act as a determinant factor for controlling chondrocyte and osteoblast differentiation. Here we focus on several intensively studied transcription factors and Wnt/beta-catenin signal molecules to illustrate the regulatory mechanism in directing commitment between osteoblast and chondrocyte, which will eventually allow us to properly manipulate the mesenchymal progenitor cell differentiation on bone and regeneration of cartilage tissue engineering.

The effects of combination of alendronate and human parathyroid hormone(1-34) on bone strength are synergistic in the lumbar vertebra and additive in the femur of C57BL/6J mice

A cyclic PTH regimen is as effective as a daily regimen on bone density gain in humans and in improving bone quality in mice. Our previous murine study evaluated the effects of a cyclic PTH regimen in the absence of a bisphosphonate, whereas our human study addressed the effects of a cyclic PTH regimen in the presence of ongoing alendronate (ALN) treatment. Accordingly, the current study examined the effects of cyclic or daily PTH regimens in combination with ALN on bone quality and bone density in mice. Twenty-week-old, female C57BL/6J mice were treated with the following sc injections (n = 10): 1) vehicle for 5 d/wk (control); 2) ALN (20 microg/kg x d) 3 d/wk (ALN); 3) human PTH(1-34) (40 microg/kg x d) 5 d/wk (daily PTH); 4) daily PTH in addition to ALN (daily PTH plus ALN); 5) PTH 5 d/wk and vehicle 5 d/wk alternating weekly (cyclic PTH); 6) cyclic PTH in addition to ALN (cyclic PTH plus ALN); and 7) PTH and ALN alternating weekly (alt PTH and ALN). Bone mineral density was measured weekly by dual-energy x-ray absorptiometry, and at 7 wk, bone markers, bone structure, and bone strength were evaluated by biochemical assays, peripheral quantitative computed tomography and mechanical testing, respectively. At 7 wk, all treatments significantly increased femoral and vertebral bone mineral density. ALN slightly decreased endosteal circumference, whereas PTH increased periosteal circumference, resulting in significant increases in femoral cortical thickness in all groups. PTH and ALN enhanced bone strength synergistically in the lumbar vertebrae and additively in the femur. Combined therapy, however, had no effects on bone markers. The results show that combinations of ALN and PTH, in both daily and cyclic regimens, produce more beneficial effects than treatment with either agent alone, suggesting that the mechanisms of actions of ALN and PTH on bone quality may be complementary.

Effects of daily treatment with parathyroid hormone 1-84 for 16 months on density, architecture and biomechanical properties of cortical bone in adult ovariectomized rhesus monkeys

Treatment with parathyroid hormone 1-84 (PTH) or teriparatide increases osteonal remodeling and decreases bone mineral density (BMD) at cortical (Ct) bone sites but may also increase bone size. Decreases in BMD and increases in size exert opposing effects on bone strength. In adult ovariectomized (OVX) rhesus monkeys, we assessed the effects of daily PTH treatment (5, 10 or 25 microg/kg) for 16 months on BMD at the radial, tibial and femoral diaphyses, and on biomechanical properties (3-point bending) of radial cortical bone and the femoral diaphysis. PTH treatment did not affect areal BMD measured by dual-energy X-ray absorptiometry at the tibial diaphysis but caused a rapid, dose-related decrease at the distal radial diaphysis. Peripheral quantitative computed tomography at the radial and femoral diaphyses confirmed a significant PTH dose-related decrease in volumetric Ct.BMD caused primarily by increased cortical area. Significant increases in cortical thickness were the result of nonsignificant increases in periosteal length and decreases in endocortical length. Histomorphometry revealed increased endocortical bone formation at the tibial diaphysis and rib, higher Haversian remodeling at the rib and increased cortical porosity at the rib and tibia. Biomechanical testing at the femoral diaphysis showed that PTH treatment had no effect on peak load, but significantly decreased stiffness and increased work-to-failure (the energy required to break the bone). Similar changes occurred in radial cortical beams but only stiffness was changed significantly. Thus, PTH treatment of OVX rhesus monkeys decreased BMD and stiffness of cortical bone but did not affect peak load, likely because of increased bone size. However, PTH treatment increased the energy required to break the femur making it more resistant to fracture.

Effects of treatment of ovariectomized adult rhesus monkeys with parathyroid hormone 1-84 for 16 months on trabecular and cortical bone structure and biomechanical properties of the proximal femur

Treatment of monkeys and humans with parathyroid hormone (PTH) 1-84 stimulates skeletal remodeling, which increases trabecular (Tb) bone mineral density (BMD) but decreases cortical (Ct) BMD at locations where these bone types predominate. We report the effects of daily PTH treatment (5, 10, or 25 microg/kg) of ovariectomized (OVX) rhesus monkeys for 16 months on bone structure and biomechanical properties at the proximal femur, a mixed trabecular and cortical bone site. PTH reversed the OVX-induced decrease in BMD measured by dual-energy X-ray absorptiometry at the proximal femur, femoral neck, and distal femur. Peripheral quantitative computed tomography confirmed a significant decrease in Ct.BMD and an increase in Tb.BMD at the total proximal femur and at the proximal and distal femoral metaphyses. The decrease in Ct.BMD resulted primarily from increased area because cortical bone mineral content was unaffected by PTH. Histomorphometry revealed that PTH significantly increased the trabecular bone formation rate (BFR) as well as trabecular bone volume and number. PTH did not affect periosteal or haversian BFR at the femoral neck, but cortical porosity was increased slightly. PTH had no effects on stiffness or peak load measured using a shear test, whereas work-to-failure, the energy required to fracture, was increased significantly. Thus, PTH treatment induced changes in trabecular and cortical bone at the proximal femur that were similar to those occurring at sites where these bone types predominate. Together, the changes had no effect on stiffness or peak load but increased the energy required to break the proximal femur, thereby making it more resistant to fracture.

The effect of matrix bound parathyroid hormone on bone regeneration

INTRODUCTION

Autogenous bone is the most successful bone-grafting material; however, multiple disadvantages continue to drive developments of improved methods for bone regeneration.

AIM

The aim of the present study was to test the hypothesis that an arginine-glycine-aspartic acid (RGD) modified polyethylene glycol-based matrix (PEG) containing covalently bound peptides of the parathyroid hormone (PTH(1-34)) enhances bone regeneration to a degree similar to autogenous bone.

MATERIAL AND METHODS

Six American foxhounds received a total of 48 cylindrical titanium implants placed in the mandible between the first premolar and the second molar. Five, respectively, 7 months following tooth extraction, implants were placed into the center of surgically created defects. This resulted in a circumferential bone defect simulating an alveolar defect with a circular gap of 1.5 mm. Four treatment modalities were randomly allocated to the four defects per side: (1) PEG-matrix containing 20 microg/ml of PTH(1-34), and 350 microg/ml cys-RGD peptide, (2) PEG alone, (3) autogenous bone and (4) empty defects. Histomorphometric analysis was performed 4 and 12 weeks after implantation. The area fraction of newly formed bone was determined within the former defect and the degree of bone-to-implant contact (BIC) was evaluated both in the defect region and in the apical region of the implant. For statistical analysis ANOVA and subsequent pairwise Student's t-test were applied.

RESULTS

Healing was uneventful and all implants were histologically integrated. Histomorphometric analysis after 4 weeks showed an average area fraction of newly formed bone of 41.7+/-1.8% for matrix-PTH, 26.6+/-4.1% for PEG alone, 43.9+/-4.5% for autogenous bone, and 28.9+/-1.5% for empty defects. After 12 weeks, the respective values were 49.4+/-7.0% for matrix-PTH, 39.3+/-5.7% for PEG alone, 50.5+/-3.4% for autogenous bone and 38.7+/-1.9% for empty defects. Statistical analysis after 4 and 12 weeks revealed significantly more newly formed bone in the PTH(1-34) group compared with PEG alone or empty defects, whereas no difference could be detected against autogenous bone. Regarding BIC no significant difference was observed between the four treatment groups neither at 4 nor at 12 weeks.

CONCLUSION

It is concluded that an RGD-modified PEG hydrogel containing PTH(1-34) is an effective matrix system to obtain bone regeneration.

Effects of a one-month treatment with PTH(1-34) on bone formation on cancellous, endocortical, and periosteal surfaces of the human ilium

Using bone histomorphometry, we found that a 1-month treatment with PTH(1-34) [hPTH(1-34)] stimulated new bone formation on cancellous, endocortical, and periosteal bone surfaces. Enhanced bone formation was associated with an increase in osteoblast apoptosis.

INTRODUCTION

The precise mechanisms by which hPTH(1-34) increases bone mass and improves bone structure are unclear. Using bone histomorphometry, we studied the early effects of treating postmenopausal women with osteoporosis with hPTH(1-34).

MATERIALS AND METHODS

Tetracycline-labeled iliac crest bone biopsies were obtained from 27 postmenopausal women with osteoporosis who were treated for 1 month with hPTH(1-34), 50 microg daily subcutaneously. The results were compared with tetracycline-labeled biopsies from a representative control group of 13 postmenopausal women with osteoporosis.

RESULTS

The bone formation rate on the cancellous and endocortical surfaces was higher in hPTH(1-34)-treated women than in control women by factors of 4.5 and 5.0, respectively. We also showed a 4-fold increase in bone formation rate on the periosteal surface, suggesting that hPTH(1-34) has the potential to increase bone diameter in humans. On the cancellous and endocortical surfaces, the increased bone formation rate was primarily caused by stimulation of formation in ongoing remodeling units, with a modest amount of increased formation on previously quiescent surfaces. hPTH(1-34)-stimulated bone formation was associated with an increase in osteoblast apoptosis, which may reflect enhanced turnover of the osteoblast population and may contribute to the anabolic action of hPTH(1-34).

CONCLUSIONS

These findings provide new insight into the cellular basis by which hPTH(1-34) improves cancellous and cortical bone architecture and geometry in patients with osteoporosis.

Treatment of skeletally mature ovariectomized rhesus monkeys with PTH(1-84) for 16 months increases bone formation and density and improves trabecular architecture and biomechanical properties at the lumbar spine

Histomorphometric studies of treatments for osteoporosis in humans are restricted to iliac crest biopsies. We studied the effects of PTH(1-84) treatment at the lumbar spine of skeletally mature ovariectomized rhesus monkeys. PTH increased bone turnover, rapidly normalized BMD, and increased vertebral compressive strength. PTH increased trabecular bone volume primarily by increasing trabecular number by markedly increasing intratrabecular tunneling.

INTRODUCTION

Histomorphometric studies of the anabolic properties of PTH(1-84) (PTH) and related peptides in human bone are restricted to iliac crest biopsies. The ovariectomized (OVX) monkey is an accepted model of human postmenopausal bone loss and was used to study the effects of PTH treatment at clinically relevant skeletal sites.

MATERIALS AND METHODS

Skeletally mature rhesus monkeys were OVX or sham-operated and, after a bone depletion period of 9 months, treated daily for 16 months with PTH (5, 10, or 25 microg/kg). Markers of bone formation (serum osteocalcin) and resorption (urine N-telopeptide [NTX]) and lumbar spine BMD were measured throughout the study. Trabecular architecture and vertebral biomechanical properties were quantified at 16 months.

RESULTS

PTH treatment induced dose-dependent increases in bone turnover but did not increase serum calcium. Osteocalcin was significantly increased above OVX controls by 1 month. NTX was significantly elevated at 1 month with the highest dose, but not until 12 months with the 5 and 10 microg/kg doses. Lumbar spine BMD was 5% lower in OVX than in sham animals when treatment was started. All PTH doses increased BMD rapidly, with sham levels restored by 3-7 months with 10 and 25 microg/kg and by 16 months with 5 microg/kg. PTH treatment increased trabecular bone volume (BV/TV), primarily by increasing trabecular number, and dose-dependently increased bone formation rate (BFR) solely by increasing mineralizing surface. The largest effects on BV/TV and yield load occurred with the 10 microg/kg dose. The highest dose reduced trabecular thickness by markedly increasing intratrabecular tunneling.

CONCLUSIONS

PTH treatment of OVX rhesus monkeys increased bone turnover and increased BV/TV, BMD, and strength at the lumbar spine. All PTH doses were safe, but the 10 microg/kg dose was generally optimal, possibly because the highest dose resulted in too marked a stimulation of bone remodeling.

Primary hyperparathyroidism and malignancy: "studies by nature"

The American Food and Drug Administration approval of parathyroid hormone (PTH) administration for osteoporosis as well as the possibility for its future therapeutic applications requires an examination of the suggested association between PTH and cancer, particularly osteosarcomas. The objective was to evaluate such a connection by collecting observational data from two groups of patients, designated as "studies by nature". Cohort 1: Medical records of all patients with primary hyperparathyroidism that were treated in a referral center during a 12-year period were retrospectively reviewed for malignancy before, at the time or after diagnosis. Cohort 2: Records of patients with osteosarcomas that were treated in referral centers during 15 years were retrospectively reviewed for hyperparathyroidism, as indicated by history or laboratory results. There were 582 patients with primary hyperparathyroidism. While 56 (9.6%) had malignancy, 47 (8%) developed cancer after diagnosis with hyperparathyroidism during 6.1 years of documentation. This rate did not exceed the incidence of developing cancer among the general population. Although thyroid cancer was about 4 times the incidence in the general population, this may be attributed to a high level of detection while work-up, treating and following the parathyroid disease. None had osteosarcoma. None of the 126 patients with osteosarcoma had documentation of primary hyperparathyroidism or had biochemical evidence of hyperparathyroidism. No obvious association was found between primary hyperparathyroidism and cancer. Similarly, there was no demonstrable relationship between osteosarcomas and hyperthyroidism biochemical stigmata. Since PTH may contribute to tumor invasiveness, screening for existing neoplasms, especially prostate and breast, before PTH treatment may be of importance.

Effects of parathyroid hormone on Wnt signaling pathway in bone

The Wnt signaling pathway has recently been demonstrated to play an important role in bone cell function. In previous studies using DNA microarray analyses, we observed a change in some of the molecular components of the canonical Wnt pathway namely, frizzled-1 (FZD-1) and axil, in response to continuous parathyroid hormone (PTH) treatment in rats. In the present study, we further explored other components of the Wnt signaling pathway in rat distal metaphyseal bone in vivo, and rat osteoblastic osteosarcoma cells (UMR 106) in culture. Several Wnt pathway components, including low-density lipoprotein-receptor-related protein 5 (LRP5), LRP6, FZD-1, Dickkopf-1 (Dkk-1), and Kremen-1 (KRM-1), were expressed in bone in vivo and in osteoblasts in vitro. Continuous exposure to PTH (1-38) both in vivo and in vitro upregulated the mRNA expression of LRP6 and FZD-1 and decreased LRP5 and Dkk-1. These effects in UMR 106 cells were associated with an increase in beta-catenin as measured by Western blots and resulted in functional activation (three to six-fold) of a downstream Wnt responsive TBE6-luciferase (TCF/LEF-binding element) reporter gene. Activation of the TBE6-luciferase reporter gene by PTH (1-38) in UMR 106 cells was inhibited by the protein kinase A (PKA) inhibitor, H89. Activation was mimicked by PTH (1-31), PTH-related protein (1-34), and forskolin, but both PTH (3-34) and (7-34) had no effect. These findings suggest that the effect of PTH on the canonical Wnt signaling pathway occurs at least in part via the cAMP-PKA pathway through the differential regulation of the receptor complex proteins (FZD-1/LRP5 or LRP6) and the antagonist (Dkk-1). Taken together, these results reveal a possible role for the Wnt signaling pathway in PTH actions in bone.

Primary hyperparathyroidism and osteosarcoma: examination of a large cohort identifies three cases of fibroblastic osteosarcoma

To study a possible relationship between hyperparathyroidism and osteosarcoma, we reviewed 1234 osteosarcoma patients. In this cohort, only three patients had a diagnosis of both hyperparathyroidism and fibroblastic osteosarcoma. These results indicate that hyperparathyroidism is not more prevalent in patients with osteosarcoma than in the general population. However, the presence of hyperparathyroidism may modify the histologic and cytologic features of osteosarcoma.

INTRODUCTION

The finding of osteosarcoma in rats receiving human PTH(1-34) raised the question of whether hyperparathyroidism might be a risk factor for development of osteosarcoma in humans.

MATERIALS AND METHODS

To study a possible relationship between hyperparathyroidism and osteosarcoma, we reviewed the medical records of 1234 osteosarcoma patients seen at The M.D. Anderson Cancer Center since 1948. Our study focused on clinical, biochemical, radiologic, and histopathologic findings indicative of primary hyperparathyroidism and the features of osteosarcoma.

RESULTS

Of the 1234 cases reviewed, 3 patients had a diagnosis of both primary hyperparathyroidism and osteosarcoma. In two cases, hyperparathyroidism preceded the osteosarcoma, and in one case, both conditions were diagnosed at the same time. In two cases with concomitant hyperparathyroidism and osteosarcoma, features of osteitis fibrocystica were identified. The third patient was treated for hyperparathyroidism 3 years before osteosarcoma was diagnosed. All three patients had histologic features of fibroblastic osteosarcoma, a type that accounts for no more than 20% of osteosarcomas. To assess whether the prevalence of hyperparathyroidism was greater than expected in the normal population, we compared the age- and sex-specific prevalence in our cohort to a population of healthy individuals in Tromso, Norway. This analysis showed no significant differences between the two populations, despite the fact that a higher prevalence of hyperparathyroidism (6.9% versus 1.6%) was noted in the 60- to 69-year-old female osteosarcoma age group.

CONCLUSIONS

Our results indicate that hyperparathyroidism is not more prevalent in affected individuals with osteosarcoma than in the general population. The finding of fibroblastic osteosarcoma in all three patients raises the question of whether coexistent hyperparathyroidism may modify the cytologic and histologic features of the malignancy.

Bone neoplasms in F344 rats given teriparatide [rhPTH(1-34)] are dependent on duration of treatment and dose

A long-term study was conducted in female F344 rats to determine the relative importance of dose, treatment duration, and age at initiation of treatment on the incidence of teriparatide [rhPTH[1-34)]-induced bone proliferative lesions. Treatment groups consisted of different combinations of dose (0, 5, or 30 microg/kg/d), treatment duration (6, 20, or 24 months) and age at initiation of treatment (2 or 6 months of age). The primary endpoints were the incidence of bone neoplasms and effects on bone mass and structure as evaluated by quantitative computed tomography and histomorphometery. Significant increases in the incidence of bone tumors (osteoma, osteoblastoma, and osteosarcoma) occurred in rats treated with 30 microg/kg for 20 or 24 months. No neoplasms were found when the 5 microg/kg treatment was initiated at 6 months of age and continued for either 6 or 20 months (up to 70% of life span). This treatment regimen defined a "no-effect" dose for neoplasm formation that nevertheless resulted in substantial increases in bone mass. These results demonstrate that treatment duration and administered dose are the most important factors in the teriparatide-induced bone tumors in rats.

Raloxifene and teriparatide (hPTH 1-34) have complementary effects on the osteopenic skeleton of ovariectomized rats

The skeletal efficacy of raloxifene (Ral) plus weekly teriparatide [recombinant human parathyroid hormone (1-34), TPTD] combinations relative to each treatment alone or sequentially were evaluated in osteopenic, ovariectomized rats. In the first study, 6-month-old Sprague-Dawley rats were ovariectomized (Ovx) and permitted to lose bone for 1 month before treatment for the following 3 months. Raloxifene (Ral, 1 mg/kg/day orally) was evaluated alone and in combination with TPTD (10 or 30 microg/kg/week) administered weekly by subcutaneous injection. QCT, biomechanical testing, and histomorphometry were used to quantitate skeletal effects. Weekly TPTD alone at either dose had no skeletal effect relative to Ovx. Daily Ral prevented further loss of vertebral bone mineral density (BMD), resulting in BMD that was significantly greater than Ovx, but significantly less than age-matched, sham-Ovx, vehicle controls (sham). The raloxifene plus 30 microg/kg/week TPTD group had vertebral BMD that was significantly greater than Ovx, Ral alone, and both TPTD dose-alone groups. Therefore, the Ral plus TPTD group completely restored bone mass to sham levels. Compression testing of lumbar vertebra L5 confirmed increased strength for both Ral plus TPTD combinations relative to Ovx, with strength not different from sham. Histomorphometry of the proximal tibial metaphysis showed that Ovx significantly increased eroded surface and bone formation compared to sham. Raloxifene treatment restored eroded surface and bone formation rate back to sham levels. Raloxifene plus TPTD at 30 microg/kg/week resulted in a significantly higher mineral appositional rate compared to Ral and sham, which was not different from Ovx and TPTD alone. Raloxifene plus TPTD at both doses had eroded surfaces that were significantly less than Ovx but not different from sham or Ral alone. In a sequential study, 6-month-old Ovx rats were permitted to develop osteopenia for 2 months before a daily TPTD 80 microg/kg/day subcutaneous injection was initiated. Following 2 months of TPTD treatment, animals were either (1) continued on TPTD, (2) discontinued from TPTD, (3) switched to Ral 3 mg/kg/day, oral, or 17 alpha-ethynyl estradiol (EE2) 0.1 mg/kg/day, oral, for another 2 months. Raloxifene and EE2 maintained most of TPTD-induced new bone in Ovx rats by preventing the increase in bone turnover rate after withdrawal of TPTD. Raloxifene also restored the elevated bone formation activity induced by TPTD to the level of sham. These data suggest that Ral and TPTD have complementary interactions in osteopenic, Ovx rats. Raloxifene inhibited bone resorption, and reduced high bone turnover without significantly retarding TPTD stimulation of bone formation activity.

Differential transcriptional effects of PTH and estrogen during anabolic bone formation

The aim of this study was to compare transcriptional regulation in vivo during anabolic bone formation induced by either estradiol (E2) treatment or intermittent parathyroid hormone[1-34] (PTH) therapy. We utilized an ovariectomized (OVX) mouse model of osteoporosis and transcriptional profiling to identify genes upregulated by either high-dose E2 or PTH. Five weeks post-OVX, the mice were administered either E2 and/or PTH, or vehicle for 4 weeks. Femoral bones were analyzed by microCT and histomorphometry to confirm the anabolic effect of each treatment. OVX vehicle-treated control mice lost metaphyseal trabecular bone, with significant decrease in trabecular number, thickness, and connectivity. Both E2 and PTH treatments increased trabecular and cortical bone indices above the level of the sham operated controls, fully restoring both bone volume and bone mineral density (BMD). Moreover, PTH/E2 combination treatment led to significantly greater increase in cancellous bone and BMD than would be expected from the additive effects of the separate treatments. To determine whether PTH and E2 treatments were stimulating similar bone anabolic mechanisms, or were activating distinct signaling pathways, we compared patterns of gene expression using transcriptional profiling after either E2 or PTH treatment. After 4, 11, and 24 days of treatment, total RNA was collected from both the distal femoral metaphysis and diaphysis. Transcriptional profiling was performed using Affymetrix GeneChip probe arrays, comprised of approximately 36,000 full-length mouse genes and EST clusters from the UniGene database. Several markers of osteoblast activity, including c-fos, RANKL, PHEX, and PTHR1, were consistently upregulated by PTH in both skeletal sites. PTH treatment also increased expression of Cathespin K, consistent with the predicted increase in osteoclast activity. E2 treatment upregulated a largely distinct set of genes, including TGFbeta3, and BMP1, as well as several genes critical for cell cycle control, including Cyclin D1 and CDK inhibitor 1A. Overall, comparison of transcriptional profiles suggest that anabolic responses in bone to PTH and high-dose E2 treatment after OVX-induced osteoporosis involve largely distinct patterns of gene regulation, each resulting in restoration of bone mass.

Teriparatide [PTH(1-34)] strengthens the proximal femur of ovariectomized nonhuman primates despite increasing porosity

OVX monkeys treated for 18 months with 1 or 5 microg/kg/d teriparatide [PTH (1-34)] had significantly stronger proximal femora relative to ovariectomized controls. Teriparatide enhancement of cortical area, cortical width, and trabecular bone volume seemed to more than compensate for the dose-dependent increase in cortical porosity. Beneficial effects of teriparatide treatment on the proximal femur persisted beyond the treatment period and may extend to the marrow.

INTRODUCTION

We conducted a detailed quantitative analysis of the effects of teriparatide on the proximal femur of ovariectomized monkeys. Teriparatide increased bone mass, enhanced structural architecture, and strengthened the hip, despite increasing cortical porosity.

MATERIALS AND METHODS

Monkeys were treated with vehicle (sham or OVX controls), 1 microg/kg/day teriparatide [parathyroid hormone (1-34); PTH1], or 5 microg/kg/day teriparatide (PTH5) for 18 months or for 12 months followed by 6 months of treatment withdrawal (PTH1W and PTH5W, respectively). Excised proximal femora were analyzed by microCT, conventional histomorphometry, and biomechanics.

RESULTS AND CONCLUSIONS

The femoral neck showed significant reduction in trabecular bone volume (BV/TV) for OVX compared with sham, whereas PTH1 BV/TV was restored to sham levels and PTH5 BV/TV was greater than sham and OVX. The withdrawal groups had BV/TVs intermediate between sham and OVX. PTH1 had trabecular number (Tb.N) greater than OVX, and PTH5 Tb.N was greater than sham and OVX. The withdrawal groups had Tb.Ns intermediate between sham and OVX. No differences between groups were observed for trabecular orientation or trabecular thickness. Teriparatide dose-dependently increased bone formation rate and activation frequency in the femoral neck. Cellular composition analyses suggested a tendency of ovariectomy to increase adiposity of marrow by 100%, whereas PTH tended to reduce adipocyte number and increase osteoblast number compared with OVX. Analyses of the cortex showed dose-dependent elevation of cortical porosity, which was consistent with enhanced bone turnover with treatment. Cortical porosity was reduced after withdrawal of teriparatide, because PTH1W cortical porosity was lower than OVX, whereas PTH5W cortical porosity was intermediate between sham and OVX. Increased cortical porosity did not weaken the proximal femora. Biomechanics showed that ovariectomy weakened proximal femora compared with sham, but PTH1, PTH5, and PTH1W were stronger than OVX and not different from sham. PTH5W strength was intermediate between sham and OVX. Therefore, teriparatide had beneficial effects on the proximal femur, despite increasing cortical porosity. Cortical porosity did not adversely affect the mechanical integrity of the proximal femora, because enhanced cortical area and trabecular bone volume more than compensated for the porosity. Much of the beneficial effects of teriparatide were retained after 6 months withdrawal from treatment. PTH effects on the femoral neck were not limited to bone but may include inhibition of OVX-stimulated adiposity of the marrow.

Combination of local and systemic parathyroid hormone enhances bone regeneration

Parathyroid hormone is one of the most promising therapeutic agents for osteoporosis, but its use to facilitate bone regeneration in osseous defects is less clear. The purpose of the current study was to determine the effects of combining systematic parathyroid hormone and a local parathyroid hormone gene therapy in a critical-sized osteotomy model. Rats received bilateral femoral osteotomies followed by implantation of a gene-activated matrix encoding parathyroid hormone (1-34) on one side and a control gene-activated matrix on te opposite side. Systematic parathyroid hormone (1-34) or vehicle was injected daily and rats were sacrificed 6 weeks later. Systematic parathyroid hormone increased bone mineral density and bone mineral content measured by dual-energy xray absorptiometry analysis of tibias and vertebrae, and increased serum osteocalcin levels during healing of osteotomies. Furthermore, comparing osteotomy sites that received the same gene-activated matrices as vehicle-injected rats, parathyroid hormone-injected rats showed trends of greater bone areas via histomorphometric and microradiographic analyses and higher osteocalcin messenger ribonucleic acid expression via Northern blot analyses. The combination of systemic and local parathyroid hormone led to higher bone mineral density, bone mineral content, and bone area, a trend for greater radiographic-detected bone area and higher expression of osteocalcin in osteotomy sites when compared with the individual treatment or control groups. Local parathyroid hormone gene therapy enhanced the anabolic effect of systemic parathyroid hormone during osteotomy healing. This study supports the concept of a combined local and systemic approach for enhancing the repair of a fracture at risk for nonunion.

A osteoporose e os distúrbios endócrinos da tireóide e das gônadas

Apesar da dedicação incessante dos pesquisadores no estudo da osteoporose, muito ainda necessita ser elucidado. A deficiência dos esteróides sexuais, principalmente a de estrógeno, é considerada a principal causa de osteoporose, embora existam inúmeros outros fatores envolvidos. O hipertireoidismo, por exemplo, é considerado um dos fatores de risco para indução ou agravamento da osteoporose e tem despertado o interesse para o estudo dos efeitos de T3 e T4 sobre o metabolismo ósseo. Embora o hipotireoidismo e a afuncionalidade das gônadas seja uma associação freqüente na mulher, a hipofunção da tireóide não é considerada fator de risco para a osteoporose da menopausa. Assim, o estudo da inter-relação entre os distúrbios endócrinos, tão comuns na idade avançada, e a osteoporose é fundamental, pois deste conhecimento poderão advir meios de controle e tratamento adequados, bem como a definição da real natureza do distúrbio ósseo. O objetivo desta revisão é apresentar e discutir alguns aspectos da osteoporose e sua inter-relação com os distúrbios endócrinos da tireóide e das gônadas.

New bone formation with teriparatide [human parathyroid hormone-(1-34)] is not retarded by long-term pretreatment with alendronate, estrogen, or raloxifene in ovariectomized rats

With the ready availability of several osteoporosis therapies, teriparatide [human PTH-(1-34)] is likely to be prescribed to postmenopausal women with prior exposure to agents that prevent bone loss, such as bisphosphonates, estrogen, or selective estrogen receptor modulators. Therefore, we evaluated the ability of once daily teriparatide to induce bone formation in ovariectomized (Ovx) rats with extended prior exposure to various antiresorptive agents, such as alendronate (ABP), 17 alpha-ethinyl estradiol (EE), or raloxifene (Ral). Sprague Dawley rats were Ovx and treated with ABP (28 microg/kg, twice weekly), EE (0.1 mg/kg per d), or Ral (1 mg/kg per d) for 10 months before switching to teriparatide 30 microg/kg per d for another 2 months. Analysis of the proximal tibial metaphysis showed that all three antiresorptive agents prevented ovariectomy-induced bone loss after 10 months, but were mechanistically distinct, as shown by histomorphometry. Before teriparatide treatment, ABP strongly suppressed activation frequency and bone formation rate to below levels in other treatment groups, whereas these parameters were not different from sham values for EE or Ral. Trabecular area for ABP, EE, and Ral were greater than that in Ovx controls. However, the trabecular bone effects of ABP were attributed not only to effects on the secondary spongiosa, but also to the preservation of primary spongiosa, which was prevented from remodeling. After 2 months of teriparatide treatment, lumbar vertebra showed relative bone mineral density increases of 18%, 7%, 11%, and 10% for vehicle/teriparatide, ABP/teriparatide, EE/teriparatide, and Ral/teriparatide, respectively, compared with 10 month levels. Histomorphometry showed that trabecular area was increased by 105%, 113%, 36%, and 48% for vehicle/teriparatide, ABP/teriparatide, EE/teriparatide, and Ral/teriparatide, respectively, compared with 10 month levels. Teriparatide enhanced mineralizing surface, mineral apposition rate, and bone formation rate in all groups. Compression testing of vertebra showed that teriparatide improved strength (peak load) and toughness in all groups to a proportionately similar extent compared with 10 month levels. These data showed a surprising ability of the rat skeleton to respond to teriparatide despite extensive pretreatment with ABP, EE, or Ral. Therefore, the mature skeleton of Ovx rats remains highly responsive to the appositional effects of teriparatide regardless of pretreatment status in terms of cancellous bone area or rate of bone turnover.

Bone mineral density in chimpanzees, humans, and Japanese macaques

We performed a comparative study of bone mechanical properties in the radii of chimpanzees (Pan troglodytes), humans (Homo sapiens), and Japanese macaques (Macaca fuscata) using peripheral quantitative computed tomography. We investigated: (1)cortical bone area relative to the total periosteal area (PrA); (2) trabecular bone area relative to PrA; (3) cortical bone density; and (4) trabecular bone density. The cortical bone area index for chimpanzees was almost the same as that of Japanese macaques, whereas the equivalent value in humans was about the two-fifths that of the others. Values for the other three properties were constant among these three catarrhine species. Chimpanzees do not particularly resemble humans, but are more similar to digitigrade macaques in terms of bone properties. The constant trabecular bone area index and trabecular density value in these species may suggest that a certain amount of trabecular bone (20-30% of total bone area at the distal 4% level of the forearm) is necessary to achieve normal bone turnover. The physiological metabolism of bone, including cortical bone density, might be conserved in these catarrhines.

Bone mineral and collagen quality in humeri of ovariectomized cynomolgus monkeys given rhPTH(1-34) for 18 months

A recent study of ovariectomized monkeys, treated with recombinant human parathyroid hormone (rhPTH)(1-34) at 1 or 5 mg/kg/day for 18 months or for 12 months followed by 6 months withdrawal from treatment, showed significant differences in the geometry and histomorphometry of cortical bone of the midshaft humerus. To determine the extent to which the rapid bone turnover and cortical porosity induced by rhPTH(1-34) in ovariectomized monkeys modified mineral content, mineral crystal maturity and collagen maturity (cross-link distribution) in the cortical periosteal and endosteal regions, cross-sections of the cortical bone of the mid-humerus, were examined using Fourier transform infrared imaging (FTIRI). FTIRI analyses demonstrated that rhPTH(1-34) altered bone mineral and collagen properties in a dose-dependent manner. Mineral crystal maturity and collagen cross-link ratio (pyridinoline/dehydro-dihydroxylysinonorleucine) on both endosteal and periosteal surfaces decreased relative to ovariectomized animals, consistent with new bone formation. These changes were partially sustained after withdrawal of the higher dose of rhPTH(1-34), suggesting a prolonged after-effect on bone properties for at least two bone remodeling cycles. In conclusion, treatment of ovariectomized monkeys with rhPTH(1-34) had significant effects on cortical bone mineral-to-matrix ratio, mineral crystal maturity, and collagen cross-link ratio. These were fully reversible when the 1-microg rhPTH(1-34) treatment was withdrawn, but only partially reversed when the 5-microg rhPTH(1-34) dose was withdrawn.

Bone growth stimulators. New tools for treating bone loss and mending fractures

In the new millennium, humans will be traveling to Mars and eventually beyond with skeletons that respond to microgravity by self-destructing. Meanwhile in Earth's aging populations growing numbers of men and many more women are suffering from crippling bone loss. During the first decade after menopause all women suffer an accelerating loss of bone, which in some of them is severe enough to result in "spontaneous" crushing of vertebrae and fracturing of hips by ordinary body movements. This is osteoporosis, which all too often requires prolonged and expensive care, the physical and mental stress of which may even kill the patient. Osteoporosis in postmenopausal women is caused by the loss of estrogen. The slower development of osteoporosis in aging men is also due at least in part to a loss of the estrogen made in ever smaller amounts in bone cells from the declining level of circulating testosterone and is needed for bone maintenance as it is in women. The loss of estrogen increases the generation, longevity, and activity of bone-resorbing osteoclasts. The destructive osteoclast surge can be blocked by estrogens and selective estrogen receptor modulators (SERMs) as well as antiosteoclast agents such as bisphosphonates and calcitonin. But these agents stimulate only a limited amount of bone growth as the unaffected osteoblasts fill in the holes that were dug by the now suppressed osteoclasts. They do not stimulate osteoblasts to make bone--they are antiresorptives not bone anabolic agents. (However, certain estrogen analogs and bisphosphates may stimulate bone growth to some extent by lengthening osteoblast working lives.) To grow new bone and restore bone strength lost in space and on Earth we must know what controls bone growth and destruction. Here we discuss the newest bone controllers and how they might operate. These include leptin from adipocytes and osteoblasts and the statins that are widely used to reduce blood cholesterol and cardiovascular damage. But the main focus of this article is necessarily the currently most promising of the anabolic agents, the potent parathyroid hormone (PTH) and certain of its 31- to 38-aminoacid fragments, which are either in or about to be in clinical trial or in the case of Lilly's Forteo [hPTH-(1-34)] tentatively approved by the Food and Drug Administration for treating osteoporosis and mending fractures.

Use of biochemical markers to monitor changes in bone turnover in cynomolgus monkeys

The ovariectomized old cynomolgus monkey is a recognized model of human osteoporosis, and the same species can be used for the assessment of the efficacy and potential toxicity of agents intended to prevent or treat osteoporosis. Several assays have been developed that can measure the same biochemical markers of bone turnover as are used in human patients for the diagnosis and treatment follow-up of bone-related diseases, including osteoporosis. The aim of the present study was to describe the results obtained with these assays in normal control monkeys, their variations with age and sex, and their sensitivity in monitoring the bone turnover induced by ovariectomy in old skeletally mature cynomolgus monkeys. Seven old cynomolgus monkeys were bilaterally ovariectomized and 13 age-matched monkeys were sham-operated. Bone mineral density and biochemical markers were measured before and at regular intervals after surgery for up to 20 months. Total alkaline phosphatase (total ALP), bone-specific alkaline phosphatase isoenzyme (bone ALP) and osteocalcin (OC) were highly correlated to the decrease in bone mineral density (BMD) induced by ovariectomy. Deoxypyridinoline (DPD) measured by enzyme-linked immunoassay was insensitive to the bone resorption induced by ovariectomy, but cross-linked N-telopeptide (NTX-I) was higher in ovariectomized monkeys than in control monkeys. These results demonstrate that reliable biochemical parameters are available to adequately monitor and provide insight into osteoclastic bone resorption and osteoblastic bone formation, the two components of bone turnover in this animal model, and can thus be used to assess the efficacy and toxicity of potential therapeutic agents.

Parathyroid hormone (hPTH 1-38) stimulates the expression of UBP41, an ubiquitin-specific protease, in bone

Parathyroid hormone (PTH) stimulates bone formation in both animals and humans, and the expression of a number of genes has been implicated in the mediation of this effect. To discover new bone factors that initiate and support this phenomenon, we used differential display reverse transcription polymerase chain reaction (DDRT-PCR) and screened for genes, which are differentially expressed in osteoblast-enriched femoral metaphyseal primary spongiosa of young male rats after a single subcutaneous (s.c.) injection of hPTH (1-38) (8 microg/100 g). We found and cloned one full-length cDNA, which encodes a putative 348 amino acid protein. Sequence analysis of this protein demonstrates a 98, 93.7, and 82.5% identity with mouse, human, and chicken ubiquitin-specific protease UBP41, respectively. Northern blot analysis confirmed that a 3.8-4 kb UBP41 mRNA transcript was rapidly increased 1 h after acute hPTH (1-38) exposure in both metaphyseal (6- to 8-fold) and diaphyseal (3-fold) bone, but returned to control levels by 24 h after exposure. In contrast, continuous exposure to hPTH (1-38), resulted in a rapid and sustained elevation of UBP41 mRNA. PTH (1-31), which stimulates intracellular cAMP, and PTHrP (1-34) both induced UBP41 mRNA expression; whereas PTH analogs (3-34) and (7-34), that do not stimulate cAMP, had no effect on UBP41 expression. UBP41 mRNA expression was also rapidly induced 1 h after injection of PGE2, but returned to the control level by 6 to 24 h. In vitro, UBP41 mRNA is expressed in primary osteoblasts (metaphyseal and diaphyseal derived) and in the osteoblast-like cell lines UMR106, ROS17/2.8, and BALC. PTH (1-38) treatment induced UPB41 expression (3.6- to 13-fold) in both primary cultures of osteoblasts and in UMR106 cells. Further analysis in UMR 106 cells demonstrated that PGE2, forskolin and dibutyryl cAMP increased UBP41 mRNA expression 4-, 4.5-, and 2.4-fold, respectively. Tissue distribution analysis of UBP41 mRNA detected transcripts in brain, heart, skeletal muscle, kidney, liver, and testis. Together, these results demonstrate that UBP41, an ubiquitin-specific protease, is selectively upregulated in bone by the osteotropic agents PTH, PTHrP, and PGE2, possibly via the PKA/cAMP pathway. We speculate that the rapid induction of UBP41 in response to these physiological regulators contributes to the mechanism by which either the structure, activity, half-life or localization of essential proteins are modified to maintain bone homeostasis.

Effects of cerivastatin and parathyroid hormone on the lumbar vertebra of aging male Sprague-Dawley rats

Both men and women lose bone at a late age (aging bone loss). The aim of this study was to determine whether cerivastatin and parathyroid hormone (PTH) can prevent aging bone loss in men. Bone loss in aged male Sprague-Dawley (SD) rats was used as a model for age-related bone loss in men. Nine-month-old male SD rats were divided into six groups: (1) baseline controls (killed at the beginning of the study); (2) age-matched controls; (3) parathyroid hormone (PTH; 80 microg/kg body weight per day for 5 days/week) treated; (4) low-dose cerivastatin (0.2 mg/kg body weight per day) treated; and (5) medium-dose cerivastatin (0.4 mg/kg body weight per day) treated; and (6) high-dose cerivastatin (0.8 mg/kg body weight per day) treated. Groups 2-6 were treated for 23 weeks between the ages of 9 and 15 months and killed at the end of 23 weeks. The fourth lumbar vertebra was analyzed using peripheral quantitative computed tomography (pQCT). It is shown that age-matched controls had decreased cancellous bone mineral content (Cn. BMC) by 19% (p < 0.05) and cancellous bone mineral density Cn. BMD) by 22% (p < 0.01) when compared with baseline controls. All three doses of cerivastatin resulted in lower Cn. BMC and Cn. BMD when compared with age-matched controls, but this decrease was not statistically significant. In the PTH-treated group, Cn. BMC increased by 5% (p < 0.0001) and Cn. BMD increased by 37% (p < 0.0001) when compared with age-matched controls. In age-matched controls, cortical bone mineral content (Ct. BMC) and cortical bone mineral density (Ct. BMD) decreased slightly, but not significantly, when compared with baseline controls. Ct. BMD did not change significantly at any of the three doses in the cerivastatin-treated groups. In the PTH-treated group, Ct. BMC increased by 23% (p < 0.0001) when compared with age-matched controls. We confirmed that male SD rats lose bone with aging in the lumbar vertebra, and it is concluded that cerivastatin, at all doses administered, did not prevent this age-related bone loss. In contrast, PTH prevented age-related bone loss in the vertebra of male SD rats.

The nucleotide sequences of the parathyroid gene in primates (suborder Anthropoidea)

Nucleotide sequences of the parathyroid (PTH) gene of 12 species of primates belonging to suborder Anthropoidea were examined. The PTH gene contains one intron that separates two exons that code the sequence of prepro and PTH, respectively. The intron of the PTH gene in Cebus apella, Callithrix jacchus, and Saguinus oedipus was 102 bp long, whereas a 103-bp intron was observed in the remaining species. Phylogenetic analysis using the nucleotide sequences of PTH revealed that these 12 species of primates of suborder Anthropoidea could be divided into two groups of the infraorder Platyrrhini (C. apella, C. jacchus, and S. oedipus) and the infraorder Catarrhini (Macaca fascicularis, Macaca fuscata, Cercopithecus aethiops, Papio hamadryas, Presbytes obscura, Hylobates lar, Pongo pygmaeus, Pan troglodytes, and Pan paniscus). The latter infraorder could be further subdivided into two subgroups belonging to the superfamily Cercopithecoidea (M. fascicularis, M. fuscata, C. aethiops, P. hamadryas, and P. obscura) and the superfamily Hominoidea (H. lar, P. pygmaeus, P. troglodytes, and P. paniscus). The deduced amino acid sequences of PTH gene between 12 species of nonhuman primates and human revealed no amino acid substitution in mature PTH among orangutans, chimpanzees, and humans. The results indicated that the PTH gene is very conserved among primates, especially between great apes and humans. The apes are the most suitable animals to be used for studying the bone metabolism and applying the knowledge to clinical use in humans.

Catabolic effects of continuous human PTH (1--38) in vivo is associated with sustained stimulation of RANKL and inhibition of osteoprotegerin and gene-associated bone formation

Continuous infusion of PTH in vivo results in active bone resorption. To investigate the molecular basis of the catabolic effect of PTH in vivo, we evaluated the role of OPG and RANKL, which are known to influence osteoclast formation and function. Weanling rats fed a calcium-free diet were parathyroidectomized and infused with PTH via an Alzet pump to examine: 1) the changes of serum-ionized calcium and osteoclast number, 2) the expression of OPG/RANKL mRNA and protein, and 3) the expression of osteoblast phenotype bone formation-associated genes such as osteoblast specific transcription factor, osteocalcin, bone sialoprotein, and type I collagen. PTH (1--38) (0.01--20 microg/100 g) continuous infusion for 1--24 h resulted in a dose-dependent increase in serum-ionized calcium in parathyroidectomized rats and a corresponding dose-dependent increase in osteoclast number, indicating an increased bone resorption. At 20 microg/100 g PTH dose level, serum-ionized calcium was 2.1-fold of the vehicle control and not different from the Sham-parathyroidectomized rats, and osteoclast number was 3-fold of the vehicle control and 1.7-fold of the Sham-parathyroidectomized rats. In the distal femur, RANKL mRNA expression was increased (27-fold) and OPG mRNA expression was decreased (4.6-fold). The changes in RANKL and OPG mRNA levels were rapid (as early as 1 h), dose dependent, and sustained over a 24-h period that was examined. Immunohistochemical evaluation of bone sections confirmed that OPG level was reduced in proximal tibial metaphysis upon PTH infusion. Circulating OPG protein level was also decreased by 32% when compared with the parathyroidectomized control. The expression of genes that mark the osteoblast phenotype was significantly decreased [osteoblast specific transcription factor (2.3-fold), osteocalcin (3-fold), bone sialoprotein (2.8-fold), and type I collagen (5-fold)]. These results suggest that the catabolic effect of PTH infusion in vivo in this well-established resorption model is associated with a reciprocal expression of OPG/RANKL and a co-ordinate decrease in the expression of bone formation-related genes. We propose that the rapid and sustained increase in RANKL and decrease in OPG initiate maintain and favor the cascade of events in the differentiation/recruitment and activation of osteoclasts.

Human parathyroid hormone 1-34 reverses bone loss in ovariectomized mice

The experimental work characterizing the anabolic effect of parathyroid hormone (PTH) in bone has been performed in nonmurine ovariectomized (OVX) animals, mainly rats. A major drawback of these animal models is their inaccessibility to genetic manipulations such as gene knockout and overexpression. Therefore, this study on PTH anabolic activity was carried out in OVX mice that can be manipulated genetically in future studies. Adult Swiss-Webster mice were OVX, and after the fifth postoperative week were treated intermittently with human PTH(1-34) [hPTH(1-34)] or vehicle for 4 weeks. Femoral bones were evaluated by microcomputed tomography (microCT) followed by histomorphometry. A tight correlation was observed between trabecular density (BV/TV) determinations made by both methods. The BV/TV showed >60% loss in the distal metaphysis in 5-week and 9-week post-OVX, non-PTH-treated animals. PTH induced a approximately 35% recovery of this loss and a approximately 40% reversal of the associated decreases in trabecular number (Tb.N) and connectivity. PTH also caused a shift from single to double calcein-labeled trabecular surfaces, a significant enhancement in the mineralizing perimeter and a respective 2- and 3-fold stimulation of the mineral appositional rate (MAR) and bone formation rate (BFR). Diaphyseal endosteal cortical MAR and thickness also were increased with a high correlation between these parameters. These data show that OVX osteoporotic mice respond to PTH by increased osteoblast activity and the consequent restoration of trabecular network. The Swiss-Webster mouse model will be useful in future studies investigating molecular mechanisms involved in the pathogenesis and treatment of osteoporosis, including the mechanisms of action of known and future bone antiresorptive and anabolic agents.

Effects of human parathyroid hormone (1-34), LY333334, on bone mass, remodeling, and mechanical properties of cortical bone during the first remodeling cycle in rabbits

We have previously shown that parathyroid hormone (PTH) increases cortical bone mass and mechanical strength of female rabbits after 140 days of treatment. However, cortical porosity was also shown to increase. If cortical porosity increases prior to the change in geometry, there may be a transient decrease in cortical bone strength that could make the bone more susceptible to fracture in the early phase of treatment. The purpose of this study is to examine the effects of PTH on the remodeling dynamics and mechanical properties of cortical bone in rabbits, which exhibit haversian remodeling, during the first remodeling cycle after the initiation of treatment. Fifty 9-month-old intact female New Zealand white rabbits were randomized into five groups. A baseline control group was killed at the start of the experiment. The two PTH-treated groups were given human PTH(1-34) at 10 microg/kg daily subcutaneously for 35 (P35) or 70 (P70) days. Two respective age-matched control groups (V35, V70) were injected with vehicle. Histomorphometry of the cortical bone in the tibial midshaft showed that, although intracortical activation frequency was significantly increased by PTH at 35 days, there was no significant increase of cortical porosity in the first remodeling cycle (70 days). Moreover, stimulation of cortical surface bone formation in the treated animals led to significantly greater cortical area and greater bone strength in both P35 and P70. We conclude that, although intracortical remodeling increases within the first remodeling period (70 days) in animals treated with 10 microg/kg PTH, the greater cortical area due to acceleration of bone formation on cortical surfaces increases cortical bone strength. There is no mechanical risk during the first remodeling cycle associated with intermittent PTH treatment in animals with normal bone mass.

Treatment with human parathyroid hormone (1-34) for 18 months increases cancellous bone volume and improves trabecular architecture in ovariectomized cynomolgus monkeys (Macaca fascicularis)

A key feature of postmenopausal osteoporosis is the loss of trabecular bone mass and connectivity. The current study focuses on these parameters in the assessment of long-term (12 and 18 months) parathyroid hormone (PTH) therapy and its withdrawal (6 months) in the ovariectomized cynomolgus monkey (Macaca fascicularis), a well-characterized model for bone changes associated with postmenopausal osteoporosis. We used static and dynamic histomorphometric parameters to assess the amount and architecture of cancellous bone in four clinically important sites for osteoporotic fractures, including the lumbar vertebra, femoral neck, distal radius, and iliac crest. Recombinant human PTH(1-34) was administered daily to two groups for 18 months at 1.0 microg/kg per day (n = 19) and 5.0 microg/kg per day (n = 21). To study the effects of PTH withdrawal, two groups were administered PTH(1-34) daily for 12 months at 1.0 microg/kg per day (n = 20) and 5.0 microg/kg per day (n = 20), followed by daily administration of vehicle for 6 months. Sham-ovariectomized and ovariectomized (ovx) groups each received daily injections of vehicle for 18 months. Treatment with PTH had minimal effects on bone formation rates at the timepoints studied, but markedly increased cancellous bone volume relative to ovx monkeys in iliac crest biopsies at 6 and 15 months, as well as in terminal specimens of lumbar vertebrae, femoral neck, and distal radius after 18 months. At all sites, PTH significantly improved trabecular architecture, as evidenced by increased trabecular number (Tb.N) and decreased trabecular separation (Tb.Sp), with no significant change in trabecular thickness (Tb.Th). The mechanism of these structural changes is suggested by qualitative observations of trabecular tunneling observed in the iliac crest and vertebra. Longitudinal tunneling of thickened individual trabeculae is hypothesized to convert them into multiple trabeculae, resulting in a normalization of Tb.Th, but an increase in Tb.N. A significant positive effect on cancellous bone volume was still apparent after a 3-6 month withdrawal period following 12 months of PTH treatment in the iliac crest, vertebra, and femoral neck. Corresponding increases in Tb.N and decreases in Tb.Sp also remained significant after PTH withdrawal at these three sites. The distal radius was relatively insensitive to PTH treatment or its withdrawal, compared with the other bones. In summary, PTH therapy dramatically improved cancellous bone mass and architecture in both axial and appendicular sites.

ADAMTS-1: A cellular disintegrin and metalloprotease with thrombospondin motifs is a target for parathyroid hormone in bone

PTH stimulates bone formation in animals and humans, and the expressions of a number of genes have been implicated in the mediation of this effect. To discover new bone factors that initiate and support this phenomenon we used differential display RT-PCR and screened for genes that are selectively expressed in osteoblast-enriched femoral metaphyseal primary spongiosa of young male rats after a single s.c. injection of human PTH-(1-38) (8 microg/100 g). We show that one of the messenger RNAs that is up-regulated in bone is ADAMTS-1, a new member of the ADAM (A disintegrin and metalloprotease) gene family containing thrombospondin type I motifs. ADAMTS-1 consists of multiple domains common to ADAM family of proteins, including pro-, metalloprotease-like, and disintegrin-like domains. However, unlike other ADAMs, ADAMTS-1 does not possess a transmembrane or cytoplasmic domain and is a secreted protein. Northern blot analysis confirmed that ADAMTS-1 was up-regulated in both metaphyseal (14- to 35-fold) and diaphyseal (4.2-fold) bone 1 h after PTH-(1-38) injection and returned to control levels by 24 h. We also analyzed the regulation of ADAMTS-1 in response to various PTH/PTH-related peptide (PTHrP) analogs and found that PTH-(1-31) and PTHrP-(1-34), which activate the protein kinase A (PKA) pathway, induce ADAMTS-1 expression 1 h after injection, whereas PTH-(3-34) and PTH-(7-34), which do not activate the PKA pathway, did not regulate expression. To investigate the effect of other osteotropic agents, we analyzed ADAMTS-1 expression after a single dose of PGE2 (6 mg/kg) and found that it was up-regulated 1 h after injection and returned to control levels by 6 h. In vitro ADAMTS-1 is expressed in primary osteoblasts and osteoblastic cell lines, but was not detectable in osteoclasts generated from macrophage colony-stimulating factor/receptor activator of NF-kappaB ligand/transforming growth factor-beta1-treated bone marrow cells. Treatment of UMR 106 osteosarcoma cells with PTH, PGE2, forskolin, or (Bu)2cAMP increased ADAMTS-1 expression 7-, 4-, 5-, and 5-fold, respectively. Also, in vitro treatment with 1alpha,25-dihydroxyvitamin D3 increased ADAMTS-1 expression 3-fold. Tissue distribution analysis showed that ADAMTS-1 is expressed at high levels in many tissues, including the heart, lung, liver, skeletal muscle, and kidney. Taken together, these results demonstrate that ADAMTS-1 is specifically up-regulated in bone and osteoblasts by the osteotropic agents PTH, PTHrP, and PGE2 possibly via the cAMP/PKA pathway. We speculate that the rapid and transient increase in ADAMTS-1 expression may contribute to some of the effects of PTH on bone turnover.