Early effects of short-term parathyroid hormone administration on bone mass, mineral content, and strength in female rats

Intermittent parathyroid hormone promotes cementogenesis via ephrinB2-EPHB4 forward signaling

Intermittent parathyroid hormone (PTH) promotes periodontal repair, but the underlying mechanisms remained unclear. Recent studies found that ephrinB2-EPHB4 forward signaling mediated the anabolic effect of PTH in bone homeostasis. Considering the similarities between cementum and bone, we aimed to examine the therapeutic effect of PTH on resorbed roots and explore the role of forward signaling in this process. In vivo experiments showed that intermittent PTH significantly accelerated the regeneration of root resorption and promoted expression of EPHB4 and ephrinB2. When the signaling was blocked, the resorption repair was also delayed. In vitro studies showed that intermittent PTH promoted the expression of EPHB4 and ephrinB2 in OCCM-30 cells. The effects of PTH on the mineralization capacity of OCCM-30 cells was mediated through the ephrinB2-EPHB4 forward signaling. These results support the premise that the anabolic effects of intermittent PTH on the regeneration of root resorption is via the ephrinB2-EPHB4 forward signaling pathway.

Short-term intermittent administration of parathyroid hormone facilitates osteogenesis by different mechanisms in cancellous and cortical bone

Intermittent administration of human parathyroid hormone (1–34)[hPTH(1–34)] induces anabolic action on the bones. To understand the mechanism underlying the early phase of hPTH(1–34)-induced anabolic action, we investigated the expression profiles of osterix and sclerostin after short-term intermittent administration of hPTH(1–34) using immunohistochemistry in adult rats. In the cancellous bone, hPTH(1–34) administration greatly increased the number of osterix-positive cells in the bone marrow on day 1, but the cells gradually decreased on days 3 and 5. Injections of hPTH(1–34) induced no significant changes in the number of sclerostin-positive osteocytes in the cancellous bone. In the cortical bone, intermittent administration of hPTH(1–34) significantly reduced the number of sclerostin-positive osteocytes. The serum sclerostin level was downregulated and the osteocalcin level was upregulated on day 5 after intermittent administration of hPTH(1–34). Intermittent hPTH(1–34) injections increased osteoblast surface, osteoid thickness, and osteoid surface in cancellous bone, but not in cortical bone. This study suggested that the increase in osterix-positive osteoprogenitors in cancellous bone and the decrease in sclerostin-positive osteocytes in cortical bone play important roles in anabolic action on osteogenesis induced by short-term administration of hPTH(1–34).

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We analyzed the effects of hPTH(1–34) injection into rats at early phase.

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hPTH(1–34) injection increased the osterix-positive cells in bone marrow.

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hPTH(1–34) injection decreased sclerostin-positive cells in cortical bone.

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hPTH(1–34) exerts different effects in cancellous and cortical bone.

Hierarchical scaffolds enhance osteogenic differentiation of human Wharton's jelly derived stem cells

Hierarchical structures, constituted by polymeric nano and microfibers, have been considered promising scaffolds for tissue engineering strategies, mainly because they mimic, in some way, the complexity and nanoscale detail observed in real organs. The chondrogenic potential of these scaffolds has been previously demonstrated, but their osteogenic potential is not yet corroborated. In order to assess if a hierarchical structure, with nanoscale details incorporated, is an improved scaffold for bone tissue regeneration, we evaluate cell adhesion, proliferation, and osteogenic differentiation of human Wharton's jelly derived stem cells (hWJSCs), seeded into hierarchical fibrous scaffolds. Biological data corroborates that hierarchical fibrous scaffolds show an enhanced cell entrapment when compared to rapid prototyped scaffolds without nanofibers. Furthermore, upregulation of bone specific genes and calcium phosphate deposition confirms the successful osteogenic differentiation of hWJSCs on these scaffolds. These results support our hypothesis that a scaffold with hierarchical structure, in conjugation with hWJSCs, represents a possible feasible strategy for bone tissue engineering applications.

Detecting early bone changes using in vivo micro-CT in ovariectomized, zoledronic acid-treated, and sham-operated rats

SUMMARY

This study monitored in vivo the effect on bone microarchitecture of initiating antiresorptive treatment with zoledronic acid in rats at 2 weeks following ovariectomy, an early phase at which major degenerative bone changes have been found to occur. The treatment still facilitated the full reversal of cancellous bone loss in rat tibia, highlighting the importance of the time point of initiation of antiresorptive treatment.

INTRODUCTION

Injection of zoledronic acid in rats at time of ovariectomy has been found to fully preserve tibial bone microarchitecture over time, whereas injection at 8 weeks after ovariectomy has shown partial bone recovery. This study investigated the effect on microarchitecture of initiating antiresorptive treatment in the early phase following ovariectomy, at 2 weeks, a time point at which major degenerative changes in the bone have been found to occur.

METHODS

Female Sprague-Dawley rats were divided into ovariectomized group, ovariectomized group treated with zoledronic acid, and sham-operated group. In vivo micro-CT scanning of rat tibiae and morphometric analysis were performed at 0, 2, 4, 8, and 12 weeks after ovariectomy, with zoledronic acid treatment beginning 2 weeks after ovariectomy. Data were first analyzed with repeated measures analysis of variance (longitudinal study design) and then without repeated measures (cross-sectional study design).

RESULTS

The ovariectomized group demonstrated dramatic bone loss, first detected at week 2. Conversely, at week 4, the zoledronic acid-treated group returned microstructural parameters to baseline values. Remarkable increases in bone parameters were found after 6 weeks of treatment and maintained similar to sham group until the end. The longitudinal study design provided earlier detection of bone changes compared to the cross-sectional study design.

CONCLUSIONS

Treatment with zoledronic acid as late as 2 weeks after ovariectomy still facilitates the full reversal of cancellous bone loss in the rat tibia.

Parathyroid Hormone Update

PTH is an exciting new treatment option for postmenopausal women and hypogonadal men who have osteoporosis. As an anabolic agent that affects bone metabolism, it represents an entirely new class of medication for osteoporosis and a novel approach to reducing fracture risk. Numerous clinical trials have demonstrated increases in trabecular and cortical BMD (trabecular more than cortical) in men and women, and reduction in vertebral and nonvertebral fractures in postmenopausal women. Studies suggest that it is safe for use for up to 2 years, but further studies are needed to tes longer intervals of use. Although the combination of PTH and bisphosphonates does not seem to be additive, sequential therapy of PTH followed by bisphosphonate yields maximum gains in BMD compared with combined use or monotherapy with antiresorptive agents. As our knowledge of PTH grows, this is an exciting time for researchers, clinicians, and patients who study, treat, and live with the devastating consequences of progressive osteoporosis.

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

OBJECTIVE

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

METHODS

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

RESULTS

Antiresorptive therapies increase bone strength through decreasing bone turnover. This lower bone turnover results in a higher mean mineralization and decreases the number of active resorption pits within bone at any given time. These resorption pits are speculated to be areas of focal weakness and a higher number of them would, if all other things were equal, result in greater fragility. Parathyroid hormone therapy increases the rate of bone remodeling, which introduces many resorption pits, but this source of strength loss is thought to be compensated by rapid increases in bone mass.

CONCLUSIONS

Both the antiresorptives, particularly bisphosphonates, and the parathyroid hormone therapy increase bone strength; however, the changes that are elicited to achieve this differ significantly.

Impaired bone anabolic response to parathyroid hormone in Fgf2-/- and Fgf2+/- mice

Since parathyroid hormone (PTH) increased FGF2 mRNA and protein expression in osteoblasts, and serum FGF-2 was increased in osteoporotic patients treated with PTH, we assessed whether the anabolic effect of PTH was impaired in Fgf2-/- mice. Eight-week-old Fgf2+/+ and Fgf2-/- male mice were treated with rhPTH 1-34 (80mug/kg) for 4 weeks. Micro-CT and histomorphometry demonstrated that PTH significantly increased parameters of bone formation in femurs from Fgf2+/+ mice but the changes were smaller and not significant in Fgf2-/- mice. IGF-1 was significantly reduced in serum from PTH-treated Fgf2-/- mice. DEXA analysis of femurs from Fgf2+/+, Fgf2+/-, and Fgf2-/- mice treated with rhPTH (160mug/kg) for 10 days showed that PTH significantly increased femoral BMD in Fgf2+/+ by 18%; by only 3% in Fgf2+/- mice and reduced by 3% in Fgf2-/- mice. We conclude that endogenous Fgf2 is important for maximum bone anabolic effect of PTH in mice.

Response of induced bone defects in horses to collagen matrix containing the human parathyroid hormone gene

OBJECTIVE

To determine whether human parathyroid hormone (hPTH) gene in collagen matrix could safely promote bone formation in diaphyseal or subchondral bones of horses.

ANIMALS

8 clinically normal adult horses.

PROCEDURE

Amount, rate, and quality of bone healing for 13 weeks were determined by use of radiography, quantitative computed tomography, and histomorphometric analysis. Diaphyseal cortex and subchondral bone defects of metacarpi were filled with hPTH(1-34) gene-activated matrix (GAM) or remained untreated. Joints were assessed on the basis of circumference, synovial fluid analysis, pain on flexion, lameness, and gross and histologic examination.

RESULTS

Bone volume index was greater for cortical defects treated with hPTH(1-34) GAM, compared with untreated defects. Bone production in cortical defects treated with hPTH(1-34) GAM positively correlated with native bone formation in untreated defects. In contrast, less bone was detected in hPTH(1-34) GAM-treated subchondral bone defects, compared with untreated defects, and histology confirmed poorer healing and residual collagen sponge.

CONCLUSIONS AND CLINICAL RELEVANCE

Use of hPTH(1-34) GAM induced greater total bone, specifically periosteal bone, after 13 weeks of healing in cortical defects of horses. The hPTH(1-34) GAM impeded healing of subchondral bone but was biocompatible with joint tissues. Promotion of periosteal bone formation may be beneficial for healing of cortical fractures in horses, but the delay in onset of bone formation may negate benefits. The hPTH(1-34) GAM used in this study should not be placed in articular subchondral bone defects, but contact with articular surfaces is unlikely to cause short-term adverse effects.

Strontium ranelate: a novel mode of action leading to renewed bone quality

Various bone resorption inhibitors and bone stimulators have been shown to decrease the risk of osteoporotic fractures. However, there is still a need for agents promoting bone formation by inducing positive uncoupling between bone formation and bone resorption. In vitro studies have suggested that strontium ranelate enhances osteoblast cell replication and activity. Simultaneously, strontium ranelate dose-dependently inhibits osteoclast activity. In vivo studies indicate that strontium ranelate stimulates bone formation and inhibits bone resorption and prevents bone loss and/or promotes bone gain. This positive uncoupling between bone formation and bone resorption results in bone gain and improvement in bone geometry and microarchitecture, without affecting the intrinsic bone tissue quality. Thus, all the determinants of bone strength are positively influenced. In conclusion, strontium ranelate, a new treatment of postmenopausal osteoporosis, acts through an innovative mode of action, both stimulating bone formation and inhibiting bone resorption, resulting in the rebalancing of bone turnover in favor of bone formation. Strontium ranelate increases bone mass while preserving the bone mineralization process, resulting in improvement in bone strength and bone quality.

Bone strength and its determinants

Osteoporosis is a disease defined by decreased bone mass and alteration of microarchitecture which results in increased bone fragility and increased risk of fracture. The major complication of osteoporosis, i.e., fracture, is due to a lower bone strength. Thus, any treatment of osteoporosis implies an improvement in bone strength. Bone strength is determined by bone geometry, cortical thickness and porosity, trabecular bone morphology, and intrinsic properties of bony tissue. Bone strength is indirectly estimated by bone mineral density (BMD) using dual-energy X-ray absorptiometry (DXA). Since DXA-measured BMD accounts for 60-70% of the variation in bone strength, some important factors are not captured by DXA in the progression of osteoporosis and the effects of antiosteoporotic treatment. Geometry and trabecular microarchitecture have also to be taken into account. Thus, the assessment of intrinsic mechanical quality of bony tissue should provide a better understanding of the role of tissue quality in determining bone strength. The careful investigation of all the determinants of bone strength (bone tissue included) should be considered in the pathophysiology of osteoporosis and in the mechanisms of action of antiosteoporotic drugs.

Intermittent administration of parathyroid hormone (1-37) improves growth and bone mineral density in uremic rats

BACKGROUND

Parathyroid hormone (PTH) is secreted in a pulsatile fashion. Continuous infusion of PTH(1-84) resulted in a net decrease in trabecular bone volume. Differential effects have been reported following an intermittent application of PTH. We investigated the effects of a continuous infusion and of an intermittent (2 times daily subcutaneously) administration of PTH(1-37) on growth and bone mineral density (BMD) in healthy and uremic rats.

METHODS

Two-stage subtotal nephrectomy was performed on 130 g female Sprague-Dawley rats. PTH(1-37) or solvent was administered through minipumps in sham-operated and uremic rats (60 microg/kg x day for 2 weeks). The effect of intermittent administration was tested with a subcutaneous injection of solvent: 30 microg/kg PTH(1-37) two times per day, 100 pmol calcitriol (C)/kg two times per day, or both. The length (snout-tailtip) and BMD were measured at the start of uremia and at sacrifice. BMD was measured by peripheral quantitative computer tomography at the proximal tibia, 6 and 12 mm distal of the kneejoint space. Femur bone morphology was assessed by x-rays, and calcium content was measured by atomic absorption spectrophotometry.

RESULTS

Length gain was not altered by the continuous infusion of PTH. In contrast, it was significantly increased by intermittent PTH (control solvent 5.35 +/- 0.37 cm vs. control PTH 6.19 +/- 0.47 cm; uremia solvent 4.78 +/- 0.20 cm vs. uremia PTH 6.17 +/- 0.36 cm; P < 0.05). Intermittent PTH but not C increased BMD in uremic rats (Delta total BMD 134 + 13.3 vs. 76.3 +/- 11.5 mg/mL; P < 0.05). X-rays revealed increased bone mass following treatment with PTH but not with C. Uremia decreased bone calcium content (64 +/- 0.3 vs. 73. 3 +/- 2.5 mg/mL), which was normalized by PTH (80 +/- 3.6 mg/mL, P < 0.05) but not by C (69 +/- 1.9 mg/mL).

CONCLUSION

Pulsatile administration of PTH does not adversely affect, but improves longitudinal growth independent of concomitant treatment with C. At the same time PTH increases BMD and the calcium content of bone.

Effects of continuous infusion of PTH on experimental tooth movement in rats

Development of new methods for accelerating orthodontic tooth movement has been strongly desired for shortening of the treatment period. The rate of orthodontic tooth movement is dependent on the rate of bone resorption occurring in the compressed periodontium in the direction of orthodontic force applied to the tooth. In the present study, we examined the effects of continuous infusion of parathyroid hormone (PTH) on tooth movement. Male rats weighing 350-400 g were treated with subcutaneous of vehicle or hPTH(1-84) at 1-10 micrograms/100 g of body weight/day. When the upper right first molar (M1) was moved mesially for 72 h by the insertion of an elastic band between the first and second molars, M1 movement was accelerated by PTH infusion at 10 micrograms. PTH infusion caused a 2- to 3-fold increase in the number of osteoclasts in the compressed periodontium of M1, indicating that such treatment accelerated tooth movement by enhancing bone resorptive activity induced in the compressed periodontium. When M1 was moved mesially by an orthodontic coil spring ligated between upper incisors and M1 for 12 days, PTH(1-84) infusion at 10 micrograms caused a 2-fold increase in the rate of M1 movement. PTH(1-34) infusion at 4 micrograms had an effect comparable to that of PTH(1-84). However, intermittent injection of PTH(1-34) did not accelerate M1 movement. PTH infusion for 13 days did not affect either bone mineral measurements or the serum calcium level. These findings suggest that continuous administration of PTH is applicable to accelerate orthodontic tooth movement.