Transient effects of subcutaneously administered prostaglandin E2 on cancellous and cortical bone in young adult dogs

Significance of mechanical loading in bone fracture healing, bone regeneration, and vascularization

In 1892, J.L. Wolff proposed that bone could respond to mechanical and biophysical stimuli as a dynamic organ. This theory presents a unique opportunity for investigations on bone and its potential to aid in tissue repair. Routine activities such as exercise or machinery application can exert mechanical loads on bone. Previous research has demonstrated that mechanical loading can affect the differentiation and development of mesenchymal tissue. However, the extent to which mechanical stimulation can help repair or generate bone tissue and the related mechanisms remain unclear. Four key cell types in bone tissue, including osteoblasts, osteoclasts, bone lining cells, and osteocytes, play critical roles in responding to mechanical stimuli, while other cell lineages such as myocytes, platelets, fibroblasts, endothelial cells, and chondrocytes also exhibit mechanosensitivity. Mechanical loading can regulate the biological functions of bone tissue through the mechanosensor of bone cells intraosseously, making it a potential target for fracture healing and bone regeneration. This review aims to clarify these issues and explain bone remodeling, structure dynamics, and mechano-transduction processes in response to mechanical loading. Loading of different magnitudes, frequencies, and types, such as dynamic versus static loads, are analyzed to determine the effects of mechanical stimulation on bone tissue structure and cellular function. Finally, the importance of vascularization in nutrient supply for bone healing and regeneration was further discussed.

Activation of Bone Remodeling Compartments in BMP-2-Injected Knees Supports a Local Vascular Mechanism for Arthritis-Related Bone Changes

BACKGROUND

Synovial membrane-derived factors are implicated in arthritis-related bone changes. The route that synovial factors use to access subchondral bone and the mechanisms responsible for these bone changes remain unclear. A safety study involving intra-articular injection of bone morphogenetic protein-2 (BMP-2)/calcium phosphate matrix (CPM) or CPM addresses these issues.

METHODS

Knee joints in 21 monkeys were injected with CPM or 1.5 or 4.5 mg/mL BMP-2/CPM and were evaluated at 1 and 8 weeks. Contralateral joints were injected with saline solution. Knee joints in 4 animals each were injected with 1.5 or 4.5 mg/mL BMP-2/CPM. Contralateral joints were injected with corresponding treatments at 8 weeks. Both joints were evaluated at 16 weeks. Harvested joints were evaluated grossly and with histomorphometry. Knee joints in 3 animals were injected with 125I-labeled BMP-2/CPM and evaluated with scintigraphy and autoradiography at 2 weeks to determine BMP-2 distribution.

RESULTS

All treatments induced transient synovitis and increased capsular vascularization, observed to anastomose with metaphyseal venous sinusoids, but did not damage articular cartilage. Both treatments induced unanticipated activation of vascular-associated trabecular bone remodeling compartments (BRCs) restricted to injected knees. Bone volume increased in BMP-2/CPM-injected knees at 8 and 16 weeks. Scintigraphy demonstrated metaphyseal 125I-labeled BMP-2 localization restricted to injected knees, confirming local rather than systemic BMP-2 release. Autoradiography demonstrated that BMP-2 diffusion through articular cartilage into the metaphysis was blocked by the tidemark. The lack of marrow activation or de novo bone formation, previously reported following metaphyseal BMP-2/CPM administration, confirmed BMP-2 and synovial-derived factors were not free in the marrow. The 125I-labeled BMP-2/CPM, observed within venous sinusoids of injected knees, confirmed the potential for capsular and metaphyseal venous portal communication.

CONCLUSIONS

This study identifies a synovitis-induced venous portal circulation between the joint capsule and the metaphysis as an alternative to systemic circulation and local diffusion for synovial membrane-derived factors to reach subchondral bone. This study also identifies vascular-associated BRCs as a mechanism for arthritis-associated subchondral bone changes and provides additional support for their role in physiological trabecular bone remodeling and/or modeling.

CLINICAL RELEVANCE

Inhibition of synovitis and accompanying abnormal vascularization may limit bone changes associated with arthritis.

Expression of Prostaglandin E2 Receptors in Acquired Middle Ear Cholesteatoma

Objectives

To investigate the expression of prostaglandin E₂ receptor subtypes, E-prostanoid (EP) 1–4 receptors, in acquired cholesteatoma and its possible role in the pathologic process of this disorder.

Methods

Specimens of human acquired cholesteatoma were obtained from 29 patients and 19 skin biopsies of normal external auditory canal were as controls. The mRNA and protein expression of EP receptors was assessed by quantitative real-time polymerase chain reaction, immunohistochemistry and Western blot.

Results

In acquired cholesteatoma, EP1–EP4 receptors were mainly expressed on squamous epithelium and subepithelial infiltrated inflammatory cells. In external auditory canal skin, EP1–EP4 receptors were mainly expressed on squamous epithelium and glandular epithelium. The expression of EP4 receptor on mRNA and protein levels were significant lower in acquired cholesteatoma compared with controls. EP1–EP3 receptors had no significant difference between the experimental and control group.

Conclusion

Low expression of EP4 may play a crucial role in the pathologic process of inflammation reaction and bone destruction in acquired cholesteatoma, but not EP1, EP2, or EP3 receptors.

Intraosseous injection of rhBMP-2/calcium phosphate matrix improves bone structure and strength in the proximal aspect of the femur in chronic ovariectomized nonhuman primates

BACKGROUND

Osteoporosis results in a decrease in bone density, bone quality, and strength throughout the skeleton. Despite systemic therapies, the morbidity and mortality that are associated with hip fractures remain a major consequence of osteoporosis.

METHODS

We used fourteen chronic ovariectomized female cynomolgus monkeys in this study. Six animals received an intraosseous injection of 0.5 mL of 1.5 mg/mL recombinant human bone morphogenetic protein-2/calcium phosphate matrix (rhBMP-2/CPM) into the femoral neck of one femur, and six animals received an intraosseous injection of 0.5 mL of CPM alone into the femoral neck of one femur. The contralateral femur of each of the animals was left untreated. The proximal aspect of each femur was evaluated monthly with use of radiography and at six months with use of peripheral quantitative computed tomography, microcomputed tomography, histological analysis, and mechanical testing. Two additional animals received an intraosseous injection of 0.5 mL of 1.5 mg/mL rhBMP-2/CPM into the femoral neck of one femur. The contralateral femur of each animal was left untreated. Bone formation in the intact specimens from these animals was histologically analyzed at one month in one animal and at three months in the other.

RESULTS

Radiographic evaluation over the six-month study period demonstrated an increase in cortical thickness and density in the rhBMP-2/CPM-treated femora as compared to the findings in the untreated contralateral femora or the femora that had been treated with CPM alone. At six months, the rhBMP-2/CPM-treated femora had decreased cortical density and increased cross-sectional area, cortical thickness, trabecular density, and trabecular volume fraction as compared with the contralateral untreated femora and the femora that had received CPM treatment alone, but the differences between the femora that had been treated with CPM alone and the contralateral untreated femora did not reach significance. Increases in bone structure resulted in a 13.7% ± 7.6% (p = 0.032) increase in the maximum bending force at the femoral neck as compared with that at the femoral neck of the contralateral untreated femora. The maximum bending force at the femoral neck was similar between the femora that had been treated with CPM alone and the contralateral untreated femora. De novo and appositional bone formation was present at one month after treatment in the rhBMP-2/CPM-treated femora.

CONCLUSIONS

This study demonstrates an increase in bone structure and mechanical properties at six months following a single injection of rhBMP-2/CPM into the femoral neck of chronic ovariectomized nonhuman primates.

rhBMP-2/calcium phosphate matrix induces bone formation while limiting transient bone resorption in a nonhuman primate core defect model

BACKGROUND

Transient bone resorption limits the use of recombinant human bone morphogenetic protein-2 (rhBMP-2)/absorbable collagen sponge in metaphyseal bone. The purpose of the present study was to evaluate the efficacy of rhBMP-2/calcium phosphate matrix (CPM) to induce bone formation while limiting transient bone resorption in nonhuman primate core defects.

METHODS

Metaphyseal core defects were created in eighteen cynomolgus monkeys. rhBMP-2 retention was evaluated in the distal part of the radius. Bone formation was evaluated at eight weeks following treatment with 1.5 or 4.5-mg/mL rhBMP-2/CPM, CPM alone, or no treatment in the distal part of the radius, the proximal part of the tibia, and the proximal part of the femur; at twenty-four weeks following treatment with 1.5-mg/mL rhBMP-2/CPM or CPM alone in the proximal part of the tibia; and at one, two, and four weeks following treatment with 1.5-mg/mL rhBMP-2/CPM or no treatment in the distal part of the radius. Bone resorption was evaluated at four weeks following treatment with 1.5, 2.0, 3.0, and 4.5-mg/mL rhBMP-2/CPM or CPM alone in the distal part of the femur. Evaluations were performed with use of scintigraphy, radiographs, histological analysis, and computed tomography.

RESULTS

Seventy-eight percent, 64%, 50%, 35%, and 12% of the rhBMP-2 was retained in the distal part of the radius at one, seven, fourteen, twenty-one, and forty-nine days after surgery. rhBMP-2/CPM increased bone formation within core defects and surrounding trabeculae compared with CPM alone or no treatment at all anatomic locations at eight weeks, and bone formation was ongoing in the rhBMP-2/CPM-treated proximal tibial sites at twenty-four weeks. Bone formation began in the trabeculae surrounding the core defects at one week and was observed adjacent to the resorbing CPM within the core defects and in the surrounding trabecular bone at two and four weeks in the rhBMP-2/CPM-treated distal radial sites. Bone formation was confined to the region immediately surrounding the core defects in the untreated distal radial sites at all time points. Transient bone resorption was only observed in the distal femoral sites treated with 4.5 mg/mL of rhBMP-2/CPM at two weeks.

CONCLUSIONS

Treatment of nonhuman primate metaphyseal core defects with 1.5 to 3.0-mg/mL rhBMP-2/CPM resulted in bone formation without transient bone resorption.

CLINICAL RELEVANCE

rhBMP-2/CPM may be useful to accelerate healing of metaphyseal bone defects in humans.

Effects of long-term administration of carprofen on healing of a tibial osteotomy in dogs

OBJECTIVE

To evaluate effects of long-term administration of carprofen on healing of a tibial osteotomy in dogs.

ANIMALS

12 healthy female Beagles.

PROCEDURES

A mid-diaphyseal transverse osteotomy (stabilized with an intramedullary pin) of the right tibia was performed in each dog. The carprofen group (n = 6 dogs) received carprofen (2.2 mg/kg, PO, q 12 h) for 120 days; the control group (6) received no treatment. Bone healing and change in callus area were assessed radiographically over time. Dogs were euthanized 120 days after surgery, and tibiae were evaluated biomechanically and histologically.

RESULTS

The osteotomy line was not evident in the control group on radiographs obtained 120 days after surgery. In contrast, the osteotomy line was still evident in the carprofen group. Callus area was significantly less in the carprofen group, compared with the area in the control group, at 20, 30, and 60 days after surgery. At 120 days after surgery, stiffness, elastic modulus, and flexural rigidity in the carprofen group were significantly lower than corresponding values in the control group. Furthermore, histologic evaluation revealed that the cartilage area within the callus in the carprofen group was significantly greater than that in the control group.

CONCLUSIONS AND CLINICAL RELEVANCE

Long-term administration of carprofen appeared to inhibit bone healing in dogs that underwent tibial osteotomy. We recommend caution for carprofen administration when treating fractures that have delays in healing associated with a reduction in osteogenesis as well as fractures associated with diseases that predispose animals to delays of osseous repair.

rhBMP-2 induces transient bone resorption followed by bone formation in a nonhuman primate core-defect model

BACKGROUND

Bone resorption preceding bone formation has been reported following the administration of recombinant human bone morphogenetic protein-2 (rhBMP-2) delivered in an absorbable collagen sponge (ACS) in metaphyseal bone. This study characterizes treatment with rhBMP-2/ACS in metaphyseal bone with use of a nonhuman primate core-defect model.

METHODS

Unilateral proximal femoral core defects were treated with 360 microg of rhBMP-2/ACS or ACS alone or were left untreated in seven, five, and five adult male cynomolgus monkeys, respectively. Distal femoral core defects in seven of the above animals were treated with 360 microg of rhBMP-2/ACS in one limb and ACS alone in the contralateral limb. Retention of rhBMP-2 in the proximal part of the femora was determined with use of tracer amounts of (125)I-rhBMP-2 imaged with a gamma camera. The distal part of the femora was evaluated with in vivo computed tomography. Computed tomography and histological evaluation were performed on harvested segments in all animals at twenty-four weeks. The histological response in the proximal and distal parts of the femora containing core defects treated with 360 microg of rhBMP-2/ACS in one limb and ACS alone in the contralateral limb was evaluated at one, two, and four weeks in three animals per time point.

RESULTS

Approximately 39.9%, 24.2%, 3.4%, and 0.5% of the rhBMP-2 was retained in the proximal part of the femora at one, seven, fourteen, and twenty-one days, respectively. The mineral density and trabecular volume fraction of the core defects treated with rhBMP-2/ACS, those treated with ACS alone, and untreated core defects in the proximal part of the femora were 81%, 54%, and 20%, respectively, and 94%, 36%, and 31%, respectively, of the corresponding region in the contralateral limbs at twenty-four weeks. The mineral density and trabecular volume fraction of the region surrounding the core defects treated with rhBMP-2/ACS, those treated with ACS alone, and untreated core defects were 112%, 105%, and 104%, respectively, and 117%, 108%, and 107%, respectively, of the corresponding region in the contralateral limbs. Treatment with rhBMP-2/ACS increased the size of the proximal and distal core defects compared with treatment with ACS alone. Histological evaluation of the rhBMP-2/ACS-treated limbs demonstrated that bone resorption was initiated at one week in association with osteoclasts and receptor activator of nuclear factor-kappaB ligand-positive stained spindle-shaped cells and peaked at two weeks. Bone formation was observed at two weeks and was ongoing at twenty-four weeks.

CONCLUSIONS

Treatment of metaphyseal core defects with rhBMP-2/ACS resulted in bone resorption followed by bone formation in nonhuman primates.

Cytokine Release from Osteoblasts in Response to Different Intensities of Pulsed Electromagnetic Field Stimulation

We use an in-vitro osteoblast cell culture model to investigate the effects of low-frequency (7.5 Hz) pulsed electromagnetic field (PEMF) stimulation on osteoblast population, cytokines (prostaglandin E(2) (PGE(2)), transforming growth factor beta1(TGFbeta1), and alkaline phosphatase (ALP) activity to find the optimal intensity of PEMF for osteoblast growth. The results demonstrate that PEMF can stimulate osteoblast growth, release of TGFbeta1, and, in addition, an increase of ALP activity. The synthesis and release of PGE(2) in the culture medium are reduced with increasing numbers of cells. Higher intensity does not necessarily mean increased osteoblast growth, and the most efficient intensity is about 2 mV/cm in this case. Although the lower intensities of the PEMF are yet to be determined, the results of this study can shed light on the mechanisms of PEMF stimulation on non union fracture therapy and osteoporosis prevention in the future.

Synergistic effect of vitamin K2 and prostaglandin E2 on cancellous bone mass in hypophysectomized young rats

Hypophysectomy (HX) results in cessation of bone growth and cancellous osteopenia in rats. It has been reported that prostaglandin E2 (PGE2) improves cortical and cancellous bone mass in HX rats. The purpose of the present study was to examine whether combined administration of vitamin K2 and PGE2 would have a more beneficial effect on bone than single administration of either alone in HX rats. Forty-three female Sprague-Dawley rats, 6 weeks of age, were randomized by the stratified weight method into five groups: intact controls, HX, HX + vitamin K2 (30 mg/kg, p.o., daily), HX + PGE2 (0.83 mg/kg, i.m., 5 days a week), and HX + vitamin K2 + PGE2. The duration of the experiment was 4 weeks. There was a reduction in cancellous bone volume/total tissue volume (BV/TV) of the proximal tibial metaphysis and a reduction in total tissue area and cortical area (Ct.Ar) of the tibial diaphysis. Vitamin K2 did not affect cancellous BV/TV or Ct.Ar. On the other hand, PGE2 attenuated the loss of cancellous BV/TV in association with higher bone formation rate/bone surface (BFR/BS) and eroded surface (ES)/BS compared with intact controls. PGE2 also increased percent Ct.Ar compared with nontreated HX rats as a result of attenuation of a decrease in periosteal BFR/BS. Vitamin K2 had a synergistic effect with PGE2 on cancellous BV/TV as a result of the suppression of an increase in ES/BS observed by PGE2 treatment. These results suggested that PGE2 had an anabolic action on cancellous and cortical bone and that despite no apparent effect of vitamin K2 on bone, it had a synergistic effect with PGE2 on cancellous bone mass in young HX rats.

Nonsteroidal anti-inflammatory drug-induced fracture nonunion: an inhibition of angiogenesis?

BACKGROUND

Approximately 5% to 10% of fractures may result in delayed union or nonunion. The results of research done over the past three decades have shown that the use of nonsteroidal anti-inflammatory drugs (NSAIDs) has an inhibitory effect on fracture repair, but the exact mechanism of action remains to be elucidated. Cancer research has identified that NSAIDs impede cell proliferation by inhibiting angiogenesis. It is proposed that a similar mechanism occurs in the induction of NSAID-induced nonunions. This hypothesis was investigated in a randomized placebo-controlled trial of the NSAID rofecoxib with use of a murine femoral fracture model.

METHODS

Two hundred and forty mice were randomized to receive either the nonsteroidal anti-inflammatory drug rofecoxib (5 mg/kg orally) in a 0.5% methylcellulose solution (the NSAID group) or the 0.5% methylcellulose solution only (the control group). Two hundred and thirty-five of the 240 mice underwent surgery to induce an open transverse middiaphyseal femoral fracture, which was then treated with use of a custom-made external fixator. Five additional animals underwent sham surgery with no fracture induced. Outcomes measures included radiographic assessment, histologic analysis, biomechanical testing, and use of laser Doppler flowmetry to assess blood flow across the fracture gap.

RESULTS

Radiography revealed similar healing patterns in both groups; however, at the later stages (day 32), the NSAID group had poorer healing. Histological analysis demonstrated that the control animals healed quicker (at days 24 and 32) and had more callus and less fibrous tissue (at days 8 and 32) than the NSAID animals did. Biomechanical testing found that the control animals were stronger at day 32. Both groups exhibited a similar pattern of blood flow; however, the NSAID group exhibited a lower median flow from day 4 onward (significant at days 4, 16, and 24). Positive correlations were demonstrated between both histological and radiographic assessments of healing and increasing blood flow. NSAID-treated animals exhibited lower blood flow and poorer healing by all parameters. Regression analysis, however, demonstrated that the negative effect of NSAIDs on fracture repair is independent of its inhibitory action on blood flow.

CONCLUSIONS

Following the development of a novel method of analyzing functional vascularity across a fracture gap, we have demonstrated that the cyclooxygenase-2 (COX-2) inhibitor rofecoxib has a significant negative effect on blood flow across the fracture gap as well as an inhibiting effect on fracture repair.

CLINICAL RELEVANCE

COX-2 inhibitors are marketed as having low side-effect profiles. We propose that these drugs should be used with caution in all patients following osseous trauma and, in particular, after injuries that may already predispose a fracture to a delayed union due to osseous, vascular, or patient-related factors.

Bone strength: current concepts

Bones serve several mechanical functions, including acoustic amplification in the middle ear, shielding vital organs from trauma, and serving as levers for muscles to contract against. Bone is a multiphase material made up of a tough collagenous matrix intermingled with rigid mineral crystals. The mineral gives bone its stiffness. Without sufficient mineralization, bones will plastically deform under load. Collagen provides toughness to bone making it less brittle so that it better resists fracture. Bone adapts to mechanical stresses largely by changing its size and shape, which are major determinants of its resistance to fracture. Tissue is added in regions of high mechanical stress providing an efficient means for improving bone strength. Experiments have shown that small additions of bone mineral density (BMD) (5-8%) caused by mechanical loading can improve bone strength by over 60% and extend bone fatigue life by 100-fold. Consequently, it is clear that bone tissue possesses a mechanosensing apparatus that directs osteogenesis to where it is most needed for improving bone strength. The biological processes involved in bone mechanotransduction are poorly understood and further investigation of the molecular mechanisms involved might uncover drug targets for osteoporosis. Several pathways are emerging from current research, including membrane ion channels, ATP signaling, second messengers, such as prostaglandins and nitric oxide, insulin-like growth factors, and Wnt signaling.

Prostaglandin E1 increases in vivo and in vitro calcitriol biosynthesis in rabbits

INTRODUCTION

Prostaglandins have an anabolic effect on bone. Possible mediation of this effect is via calcitriol. This study determines in vivo and in vitro effects of PGE(1) on calcitriol synthesis.

METHODOLOGY

In vivo: rabbits received intravenous vehicle or prostaglandin E(1) (50 microg/day) for 20 days before measurements of serum total and ionic calcium, magnesium and phosphorus levels, total and bone-specific alkaline phosphatases, 25(OH)D(3), calcitriol, parathyroid hormone and calcitonin. In vitro: rabbit proximal renal tubules were incubated with 25(OH)D(3) (8 microM) together with PGE(1) (2.82 x 10(-6) M) and the prostaglandin receptor inhibitor AH6809 (10(-4) M) in selected samples. After 5 or 30 min incubation, calcitriol production was measured by radioimmunoassay and data analysed statistically.

RESULTS

In vivo, in groups receiving PGE(1), levels of total Ca, Mg and calcitriol increased significantly and 25 dihydroxyvitamin D(3), parathyroid hormone and calcitonin remained unchanged. In vitro, PGE(1) increased calcitriol biosynthesis and the prostaglandin inhibitor AH6809 reduced calcitriol levels significantly after prolonged incubation.

CONCLUSIONS

In vivo and in vitro results demonstrate that PGE(1) stimulates calcitriol synthesis. This study represent a major advancement in knowledge of bone metabolism.

Bone biomechanical properties in EP4 knockout mice

Among the four prostaglandin E receptor subtypes, EP(4) has been implicated as an important regulator of both bone formation and bone resorption; however, the integrated activities of this receptor on bone biomechanical properties have not been examined previously. This study compared the bone biomechanical properties of EP(4) knockout (KO) transgenic mice to strain-matched wild-type (WT) controls. We examined two groups of adult female mice: WT (n = 12) and EP(4) KO (n = 12). Femurs were tested in three-point bending and the lumbar-4 (L4) vertebral body by compression. Distal femur and vertebral body trabecular bone architecture were quantified using micro-computed tomography. Biomechanical structural parameters (ultimate/yield load, stiffness) were measured and apparent material parameters (ultimate/yield stress, modulus) calculated. Body weights and bone sizes were not different between EP(4) KO and WT mice (P > 0.05, Student's t-test). EP(4) KO mice exhibited reduced structural (ultimate/yield load) and apparent material (ultimate/yield stress) strength in the femoral shaft and vertebral body compared to WT (P < 0.05). Vertebral body stiffness and femoral neck ultimate load (structural strength) were marginally lower in EP(4) KO than that in WT mice (P < 0.1). In addition, EP(4) KO mice have smaller distal femur and vertebral bone volume to total volume (BV/TV) trabecular thickness than WT mice (P < 0.05). These results suggest that the prostaglandin receptor EP(4) has an important role in determining biomechanical competence in the mouse skeleton. Despite similar bone size, the absence of an EP(4) receptor may have removed a necessary link for bone adaptation pathways, which resulted in relatively weaker bone properties.

Histomorphologic evaluation of extracorporeal shock wave therapy of the fourth metatarsal bone and the origin of the suspensory ligament in horses without lameness

OBJECTIVE

To determine via histologic examination and scintigraphy the effect of focused extracorporeal shock wave therapy (ESWT) on normal bone and the bone-ligament interface in horses.

ANIMALS

6 horses without lameness.

PROCEDURE

Origins of the suspensory ligament at the metacarpus (35-mm probe depth) and fourth metatarsal bone (5-mm probe depth) were treated twice (days 0 and 16) with 2,000 shocks (energy flux density, 0.15 mJ/mm2). One forelimb and 1 hind limb were randomly treated, and the contralateral limbs served as nontreated controls. Bone scans were performed on days -1 (before ESWT), 3, 16, and 19. Histomorphologic studies of control and treated tissues were performed on day 30.

RESULTS

ESWT significantly increased the number of osteoblasts but caused no damage to associated soft tissue structures and did not induce cortical microfractures. A significant correlation between osteoblast numbers and radiopharmaceutical uptake was noticed on lateral views of the hind limb on days 3 and 16 and on caudal views of the forelimb on day 3.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that ESWT has the potential to increase osteoblast numbers in horses. The correlation between increased osteoblast numbers and radio-pharmaceutical uptake 3 days and 16 days after the first ESWT suggested that stimulation of osteogenesis occurred soon after ESWT. No damage to bone or the bone-ligament interface should occur at the settings used in this study, and ESWT can therefore be administered safely in horses.

High-dose risedronate treatment partially preserves cancellous bone mass and microarchitecture during long-term disuse

Disuse induces rapid and severe bone loss in larger mammals as a result of greatly elevated osteoclastic resorption. In this study, we tested whether risedronate (RIS), a potent inhibitor of osteoclastic activity, would effectively prevent cancellous bone loss in female beagles (5-7 years old, N = 28) subjected to single forelimb immobilization (IM) for 12 months. Age-matched, non-IM dogs served as controls (Con). Half the animals from each group received RIS 1 mg/kg p.o. daily (Con + RIS, IM + RIS). Remaining dogs received sterile water (Con, IM). Histomorphometry showed that IM caused a dramatic reduction in cancellous bone mass (-71%) of distal 2nd metacarpals, characterized by marked decreases in trabecular width (-51%) and number (-41%), and 4-fold increases in the indices of bone resorption (eroded surface, osteoclast number, and surface). Bone formation indices (calcein-labeled surface, osteoid surface, and bone formation rate) were also significantly higher in IM than in controls. Activation frequency in IM increased about 4-fold beyond control level. RIS treatment reduced, but did not abolish cancellous bone loss due to immobilization. IM animals treated with RIS lost nearly 50% of cancellous bone mass, while trabecular width and number were reduced by 31% and 25%, respectively. In both RIS-treated control and IM animals, overall bone formation parameters (mineralized bone surface fraction and bone formation rate) remained roughly at intact control levels; however, mineral apposition rate relative to intact control was reduced 40% in RIS-treated control and 86% in RIS-treated IM animals. These results indicate that high-dose RIS treatment might suppress osteoblastic function, especially under long-term disuse. Interestingly, bone resorption parameters in RIS-treated IM animals reached levels even higher than in vehicle-treated IM animals; values for eroded surface, osteoclast number, and surface were 84%, 53%, and 83% above vehicle-treated IM values, respectively. Our data indicate that risedronate treatment is partially effective in preventing cancellous bone loss during long-term disuse. Moreover, our results suggest that bisphosphonates can impair the ability of mature osteoclasts to resorb bone, but cannot overcome the strong stimulus for osteoclast recruitment caused by long-term disuse.

Mechanisms by which exercise improves bone strength

Certain exercises can induce osteogenesis and improve bone strength, yet the biological processes involved in bone mechanotransduction are only beginning to be understood. Several pathways are emerging from current research, including calcium signaling associated with membrane ion channels, adenosine triphosphate signaling, second messengers such as prostaglandins and nitric oxide, and signaling involving mitogen-activated protein kinase. One characteristic of the mechanosensing apparatus that has only recently been studied is the important role of desensitization. Experimental protocols that insert "rest" periods to reduce the effects of desensitization can double anabolic responses to mechanical loading. Exercises that reduce desensitization may provide an effective means to build bone strength.

Bone adaptation to load: microdamage as a stimulus for bone remodelling

Mechanical loading in the proximal radius was increased by ulnar osteotomy (Group O), altered by Steinmann pinning (Group P) or unaltered in sham operated controls (Group C) in skeletally mature female sheep, aged 2-4 years. A series of intravenous fluorochromes were given to label bone formation and fuchsin-stained microdamage assessed at intervals of up to 24 weeks. Microcracks were present in all groups and were found in the original cortex near the periosteal surface. No microcracks were found in the new, fibrolamellar bone laid down at periosteal or endosteal surfaces. Mean microcrack length (49 microm, SD 10 microm) did not differ between groups or overtime. Microcrack numerical and surface densities and resorption cavity density peaked in all groups at 6 weeks, consistent with a regional acceleratory phenomenon (RAP), but the peaks were significantly greater in Group O. The density of refilling or secondary osteons peaked at 10 weeks and the mean time required for the formation of an osteon was 7.51 +/- 0.59 weeks. Fatigue-induced microdamage is normally present in bone and is increased due to repetitive loading of the mechanically overloaded radius. The location and timing of microcracks, resorption cavities and secondary osteons are consistent with the activation-resorption-formation remodelling cycle and suggest that microdamage is a stimulus for bone remodelling.

Optimum intensities of ultrasound for PGE(2) secretion and growth of osteoblasts

This study compared the effects of different intensity ultrasound (US) on osteoblasts in the far-field model with effects of the near-field model from the literature, to understand the relations between prostaglandin E(2) (PGE(2)) and osteoblast growth. We used an in vitro model to investigate the effects of 1-MHz, pulsed 1:4, and five different spatial-average temporal-peak intensity (150, 300, 600, 1200 and 2400 mW/cm(2)) US stimulations in far-field exposure (240 mm) on osteoblasts for 15 min. Optimum intensity in this study was 600 mW/cm(2), and cell density and PGE(2) secretion could be significantly stimulated at this intensity. This research may indicate that the growth of osteoblasts by US stimulation was, at least partly, due to increases in the synthesis and secretion of PGE(2). This well-controlled model can lead to further research on the biologic mechanisms for US.

In vitro effects of low-intensity ultrasound stimulation on the bone cells

Mechanical perturbations serve as extracellular signals to a variety of cells, including bone cells. Low-intensity pulsed ultrasound produces significant multifunctional effects that are directly relevant to bone formation and resorption. Ultrasound stimulation has been shown to accelerate bone-defect healing and trabecular bone regeneration. In this study, we use an in vitro bone cell culture model to investigate the effect of low-intensity pulsed ultrasound. The rat alveolar mononuclear cell-calvaria osteoblast coculture system was used in this study. Before treatment, the bone cells were cultured for 3 days to facilitate their attachment and differentiation. Then, ultrasound exposure (frequency = 1 MHz, intensity = 0.068 W/cm(2)) or sham exposure for 20 min per day was applied until the end of the experiment. Half of the culture media were obtained on the 4th, 5th, 6th, 7th, 8th, 9th, and 10th days for the analysis of cytokines and biochemical parameters. At the end of the experiment, cells were fixed and stained for identification and quantification of the osteoblast and osteoclast cells. After low-intensity pulse ultrasound stimulation, the osteoblast cell counts were significantly increased, whereas the osteoclast cell counts were significantly decreased. The total alkaline phosphatase amount in the culture medium was increased after 7 days of ultrasound stimulation, and tumor necrosis factor-alpha in ultrasound-stimulated bone cells was significantly increased after the 7th day of culture and reached 474.77% of the control medium on the 10th day of culture. The results of this study suggest that low-intensity ultrasound treatment may have a stimulatory effect on bone-healing processes.

Effect of administration of lipoprostaglandin E(1) on physical activity and bone resorption in patients with neurogenic intermittent claudication

The effect of the systemic administration of prostaglandin (PG) E(1) on physical activity and bone resorption was examined in patients with intermittent claudication (gait disturbance). Twenty male patients (age, 67.2 +/- 7.8 years; mean +/- SD) with neurogenic intermittent claudication caused by lumbar spinal canal stenosis were included in the study. Lipo-PG E(1) was administered intravenously, at a daily dose of 10 microg, on 3 days a week, for 6 months in all patients. Clinical symptoms, levels of urinary cross-linked N-telopeptides of type I collagen (NTx), and metacarpal cortical bone mineral density (BMD) were assessed before and just after the 6 months of treatment. Subjective symptoms, including leg pain and/or tingling and gait disturbance, and restrictions of the activities of daily living were significantly improved. However, no significant changes were observed in either urinary NTx levels or metacarpal cortical BMD. These findings suggest that the systemic administration of PG E(1) appears to improve subjective symptoms and activities of daily living in elderly male patients with neurogenic intermittent claudication, but does not affect either bone resorption or metacarpal cortical BMD. Short-term systemic administration of PG E(1) and increased physical activity in elderly male patients with gait disturbance caused by lumbar spinal canal stenosis may not affect bone resorption.

The effect of prostaglandin E1 on human bone metabolism: evaluation by biochemical markers for bone turnover

Prostaglandins (PGs) have complex and multiple effects on bone metabolism. Although the osteogenic effect of PGE in humans was initially found in an infant with a congenital heart disease, there have been few reports on the effect of PGE in human in vivo. The aim of this study was to investigate the effect of PGE1 on human bone metabolism, using biochemical bone markers. A total of 18 subjects were treated with PGE1 in lipid microspheres. Six subjects were given 10 microg of lipo-PGE, intravenously daily for 14 days, and twelve subjects were given the same dose twice a week for 7 weeks. Before and after the administration of PGE1, blood and a spot urine was obtained in the morning. Bone formation markers (alkaline phosphatase, osteocalcin, procollagen I carboxy-terminal peptide) did not change. In the subjects with daily administration for 2 weeks, type I collagen pyridinolines crosslinked C-telopeptide (ICTP) increased significantly. In the subjects treated twice a week, free deoxypyridinoline (Dpd) increased significantly. When all subjects were analyzed, bone resorption markers (ICTP and Dpd) increased, but not significantly (p=0.055 for ICTP, p=0.055 for Dpd). Therefore, PGE1 at the dosage used in this study did not increase bone formation but increased bone resorption in humans.

Anabolic effect of prostaglandin E2 on cortical bone of aged male rats comes mainly from modeling-dependent bone gain

In this study, prostaglandin E2 (3 mg/kg per day) was administered to 20-month-old male Wistar rats for 10 and 30 days. Histomorphometric analyses were performed on double-fluorescent-labeled undecalcified tibial shaft sections. Thirty days of prostaglandin E2 (PGE2) administration increased bone formation rate/total bone surface from undetectable levels to 0.6 microm/day at the periosteal surface and from 0.5 to 2.1 microm/day at the endocortical surface. Endocortical osteoid surface area increased from 2% to 67% at day 10 and decreased to 6% at day 30; woven and lamellar bone formation started at day 0, but was most obvious at day 30, resulting in a 12% increase of total bone mass. The red to yellow marrow ratio was 0.2 in pretreatment controls, and increased to 1.6 by day 10 and 2.4 by day 30 with PGE2 administration. Intracortical cavity number and area increased after 10 days of PGE2 treatment, but with forming osteon number and area far exceeding those of resorption cavities at day 30. Endocortical modeling surface/endocortical surface was only 1.5%, and remodeling was 11.1% in pretreatment controls. PGE2 treatment increased modeling to 24.5% in the 10 day group and 93.7% in the 30 day group, whereas remodeling remained unchanged at 10 days, and decreased to 6.2% at 30 days. Osteoprogenitor cells and osteoblasts could not be detected in pretreatment controls, but increased by day 10, and returned almost to control levels by 30 days. Our data indicate that PGE2 induced periosteal and endocortical bone formation mainly by modeling-dependent bone gain, accompanied by increases in intracortical remodeling and red bone marrow, and a transient increase in the osteoprogenitor cells adjacent to the endocortical surface. These findings suggest that 20-month-old male Wistar rats were very responsive to the anabolic action of PGE2 in the tibial shaft, a site consisting mainly of cortical bone and yellow marrow.

Cloning, structural characterization, and chromosomal localization of the gene encoding the human prostaglandin E(2) receptor EP2 subtype

Northern blot analysis of human placental RNA using a probe to the 5' end of the human prostaglandin E(2) (PGE(2)) EP2 receptor subtype coding region revealed the existence of a high abundance, low molecular weight transcript. To investigate the origin of this transcript, and its possible relationship to the human EP2 mRNA, we have cloned and characterized the gene encoding the human PGE(2) EP2 receptor subtype, identified transcriptional initiation and termination sites in two tissues (spleen and thymus), and determined its chromosomal localization. The human EP2 gene consists of two exons separated by a large intron, utilizes a common initiation site in both spleen and thymus at 1113 bp upstream of the translation initiation site, and has 3' transcript termini at 1140 bp and 1149 bp downstream of the translation stop site in spleen and thymus respectively. Southern and fluorescence in situ hybridization analysis demonstrated the human EP2 gene to be a single copy gene located in band 22 of the long arm of chromosome 14 (14q22). Though our initial interest in this gene was to investigate potential differential splicing of the human EP2 gene in placenta, this work demonstrates that the atypical transcript observed in placenta probably arises from a distinct, yet related, gene. Knowledge of the sequence, structure, and transcription events associated with the human EP2 gene will enable a broader understanding of its regulation and potential role in normal physiology and disease.

Bone defect healing enhanced by ultrasound stimulation: an in vitro tissue culture model

Ultrasound has many medical applications. Previous animal and clinical studies have clearly shown a positive effect of ultrasound on the rate of osseous repair. The present in vitro study was designed to elucidate the specific response of bony tissue to ultrasound treatment. Bilateral femora were obtained from 36 mature male Wistar rats. A bone defect was created at the center of each distal metaphysis. The femora were maintained for either 7 or 14 days in in vitro tissue culture and received 15 min of ultrasound stimulation or a sham exposure. The ultrasound intensity used was either 320 or 770 mW/cm2. Healing of the bone defect was evaluated by histomorphological examination and by analysis for the synthesis and secretion of prostaglandin E2. The results showed that ultrasound stimulation can accelerate both defect healing and trabecular bone regeneration. All experimental femoral defects treated with ultrasound healed faster than the untreated cortical defects, but only the defects receiving 770 mW/cm2 reached a level that was significantly different. The healing rate for the 320-mW/cm2 stimulated defects was intermediate between that of the 770-mW/cm2 and sham-exposed defects. With ultrasound stimulation, prostaglandin E2 secretion by the experimental femora decreased significantly. Changes in the prostaglandin synthesis and concentration were found to correspond to changes in the amount of trabecular regeneration and to acceleration of bone healing. This highly controlled and well-studied model of ultrasound stimulation of bone healing in vitro can be used to further examine the biological mechanisms involved.

Mechanical strain stimulates nitric oxide production by rapid activation of endothelial nitric oxide synthase in osteocytes

Previous studies have indicated that physiological levels of dynamic mechanical strain produce rapid increases in nitric oxide (NO) release from rat ulna explants and primary cultures of osteoblast-like cells and embryonic chick osteocytes derived from long bones. To establish the mechanism by which loading-induced NO production may be regulated, we have examined: nitric oxide synthase (NOS) isoform mRNA and protein expression, the effect of mechanical loading in vivo on NOS mRNA expression, and the effect of mechanical strain on NO production by bone cells in culture. Using Northern blot analyses, in situ hybridization, and immunocytochemistry we have established that the predominant NOS isoform expressed in rat long bone periosteal osteoblasts and in a distinct population of cortical bone osteocytes is the endothelial form of NOS (eNOS), with little or no expression of the inducible NOS or neuronal NOS isoforms. In contrast, in non-load-bearing calvariae there are no detectable levels of eNOS in osteocytes and little in osteoblasts. Consistent with these observations, ulnar explants release NO rapidly in response to loading in vitro, presumably through the activation of eNOS, whereas calvarial explants do not. The relative contribution of different bone cells to these rapid increases in strain-induced NO release was established by assessment of medium nitrite (stable NO metabolite) concentration, which showed that purified populations of osteocytes produce significantly greater quantities of NO per cell in response to mechanical strain than osteoblast-like cells derived from the same bones. Using Northern blot hybridization, we have also shown that neither a single nor five consecutive daily periods of in vivo mechanical loading produced any significant effect on different NOS isoform mRNA expression in rat ulnae. In conclusion, our results indicate that eNOS is the prevailing isoform expressed by cells of the osteoblast/osteocyte lineage and that strain produces increases in the activity of eNOS without apparently altering the levels of eNOS mRNA.

Volume-referent bone turnover estimated from the interlabel area fraction after sequential labeling

In the compact bony otic capsule remodeling is low, and bone remodeling units are distributed centrifugally in relation to inner ear tissues and spaces. Fluorochrome-labeled bone remodeling units are scarce, abortive, and tortuous with no uniform direction of movement. This study presents a method for the estimation of volume-referent bone turnover based on measurements of the fractional area between labels after sequential labeling with osteofluorochromes. The applicability of this method is tested against a classical quantification method in undecalcified cortical specimens from the canine humerus, where both methods can be used. The estimate of bone turnover derived from the new sequential labeling in eight dogs is 7.4% (SEM 2.1%) per year and the classic estimate derived from calculations of the formative osteonal area and the formative period yields 6.9% (SEM 2.1%) per year. Agreement is sufficient to justify future measurements of absolute bone turnover in sequentially labeled perilabyrinthine bone.

Regulation of connexin43 expression and function by prostaglandin E2 (PGE2) and parathyroid hormone (PTH) in osteoblastic cells

Connexin43 (Cx43) forms gap junctions that mediate intercellular communication between osteoblasts. We have examined the effects of prostaglandin E2 (PGE2) and parathyroid hormone (PTH) on gap junctional communication in the rat osteogenic sarcoma cells UMR 106-01. Incubation with either PGE2 or PTH rapidly (within 30 min) increased transfer of negatively charged dyes between UMR 106-01 cells. This stimulatory effect lasted for at least 4 h. Both PGE2 and PTH increased steady-state levels of Cx43 mRNA, but only after 2-4 h of incubation. Transfection with a Cx43 gene construct linked to luciferase showed that this effect of PTH was the result of transcriptional upregulation of Cx43 promoter. Stimulation of dye coupling and Cx43 gene transcription were reproduced by forskolin and 8Br-cAMP. Exposure to PGE2 for 30 min increased Cx43 abundance at appositional membranes in UMR 106-01, whereas total Cx43 protein levels increased only after 4-6 h of incubation with either PGE2 or PTH. Inhibition of protein synthesis by cycloheximide did not affect this early stimulation of dye coupling, but it significantly inhibited the sustained effect of PTH and forskolin on cell coupling. In summary, both PTH and PGE2, presumably through cAMP production, enhance gap junctional communication in osteoblastic cell cultures via two mechanisms: initial rapid redistribution of Cx43 to the cell membrane, and later stimulation of Cx43 gene expression. Modulation of intercellular communication represents a novel mechanism by which osteotropic factors regulate the activity of bone forming cells.

Prostaglandin E2 (PGE2) and risedronate was superior to PGE2 alone in maintaining newly added bone in the cortical bone site after withdrawal in older intact rats

The objects of this study were (1) to determine the effects of risedronate (Ris) and prostaglandin E2 (PGE2) alone and in combination, on tibial diaphyses of older intact female rats; and (2) to observe the fate of any extra bone if formed after withdrawal of the treatment. Nine-month-old female Sprague-Dawley rats were treated with 6 mg of PGE2/kg/day, 1 or 5 micrograms of Ris/kg twice a week, or 6 mg of PGE2/kg/day plus 1 or 5 micrograms of Ris/kg twice a week for the first 60 days and followed by vehicle injections for another 60 days. Cross-sections of double fluorescent labeled, undecalcified tibial diaphyses proximal to the tibiofibular junction were processed for histomorphometry. We found that: (1) neither the 1 microgram nor the 5 micrograms of Ris treatment in the 60-day on/60-day off group showed any histomorphometric differences from age-related controls; (2) while the 60 days of PGE2 treatment added extra cortical bone (6%) on the tibial shaft (due to stimulation of periosteal, endocortical, and marrow trabecular bone formation), the new endocortical and most of the new marrow trabecular bone were lost when treatment was withdrawn; however, the new periosteal bone remained; (3) PGE2 with Ris added the same amount of new bone to tibial diaphysis as did PGE2 alone and upon withdrawal, new marrow trabecular bone was lost but new periosteal and endocortical bones were preserved in PGE2 + 1 microgram of Ris on/off group. In contrast, all the new bone was maintained in the PGE2 + 5 micrograms of Ris on/off group; (4) PGE2 + Ris cotreatment failed to block the increase in cortical bone porosity induced by PGE2; and (5) in the PGE2 alone and PGE2 + 1 microgram of Ris on/off groups bone turnover was higher than that in the PGE2 + 5 micrograms of Ris on/off group. These results indicate that on/off treatment with PGE2 and Ris is superior to PGE2 alone in that it forms the same amount of new bone during treatment, but preserves more cortical bone during withdrawal. Depression of bone resorption and turnover were the tissue mechanisms responsible for this protection.

Evaluation of signal transduction mechanisms for the mitogenic effects of prostaglandin E2 in normal human bone cells in vitro

Prostaglandin E2 (PGE2) is one of the most potent stimulators of bone formation in vivo. In these studies, we investigated the mechanism(s) underlying PGE2 effects on human bone formation by evaluating the effects of PGE2 on normal human bone cell (HBC) proliferation in vitro. Cell proliferation of normal HBCs was increased by PGE2 as measured by increased [3H]thymidine incorporation after 18 h and increased cell number after 48 h of treatment. The effect of PGE2 to stimulate cell proliferation was biphasic, with a maximum stimulation between 0.01 and 1.0 nM PGE2 in different experiments. At higher concentrations of PGE2 (0.1 microM), HBC proliferation was inhibited. Signal transduction for PGE2 has been reported to include both protein kinase A (PKA) and protein kinase C (PKC) pathways. In these studies, concentrations of PGE2 which stimulated cell proliferation did not increase cyclic adenosine monophosphate (cAMP) production. However, higher concentrations of PGE2 increased cAMP production (7- to 12-fold at 1-10 microM) and inhibited cell proliferation. Because stimulators of PKC, such as phorbol esters, have been reported to stimulate cell proliferation, the action of PKC inhibitors were tested. Both staurosporine and sangivamysin (PKC inhibitors) totally abrogated the effect of PGE2 to stimulate cell proliferation. Additional studies revealed that PGE2 increased 45Ca uptake in a dose-dependent manner with a peak response occurring between 1 and 10 nM PGE2 concentrations in different experiments. Furthermore, when the calcium channel blocker, verapamil, was added to HBC cultures treated with PGE2, the stimulation of 45Ca uptake and cell proliferation by PGE2 was completely blocked. These data suggest that PGE2 increases cell proliferation through activation of a verapamil-sensitive calcium channel. In conclusion, these data are consistent with a model in which stimulation of HBC proliferation by low doses of PGE2 is mediated by an enhancement of phospholipase C, which results in both an increase in PKC activity and an increase in intracellular calcium influx.

The biochemical pathway mediating the proliferative response of bone cells to a mechanical stimulus

Calvarial bone cells of rats were subjected to either a cyclic biaxial strain of 0.17 per cent (1700 microstrain) or a hydrostatic pressure of 2.5, five, or ten pounds per square inch (17.2, 34.5, or sixty-nine kilopascals). The frequency was held constant at one hertz for both types of mechanical stimulation. When cultured bone cells that had been subjected to a cyclic biaxial strain for two hours were harvested twenty-two hours later, it was found that the level of prostaglandin E2 had increased significantly (p < 0.01) as had cellular proliferation (p < 0.01), as indicated by the incorporation of [3H]-thymidine. The addition to the medium of indomethacin, an inhibitor of prostaglandin synthesis, at a ten-micromolar concentration significantly inhibited (p < 0.01) the increase in prostaglandin E2 synthesis but had no effect on the strain-induced increase in cellular proliferation, as indicated by the incorporation of [3H]-thymidine. Twenty-four hours after exposure to the same cyclic biaxial strain for thirty seconds, other cultured bone cells showed a significant increase in the level of cytoskeletal calmodulin (p < 0.05) and in the DNA content (p < 0.05). N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide (W-7), a calmodulin antagonist, was added to the medium at a one-micromolar concentration, which had been shown to have no effect on the increase in the DNA content of control cells; W-7 completely blocked the increase in the level of cytoskeletal calmodulin and in the DNA content in the cells that were subjected to a cyclic biaxial strain. The bone cells subjected to a hydrostatic pressure showed a dose-dependent increase in the concentration of cytosolic Ca2+, as measured with Fura 2-AM, a fluorescent indicator of intracellular calcium. With a pressure of ten pounds per square inch (sixty-nine kilopascals), the increase in the concentration of cytosolic Ca2+ was nearly eight times greater than that at 2.5 pounds per square inch (17.2 kilopascals) (126 +/- 15.2 compared with 16 +/- 8.0 nanomolar, mean and standard deviation). The addition to the medium of neomycin, an inhibitor of the inositol phosphate cascade, at a ten-millimolar concentration completely blocked the increase in the concentration of cytosolic Ca2+ in these cells; this concentration of neomycin had been shown to have no effect on proliferation in control bone cells. There was also a dose-dependent relationship between the duration of the stimulus and the cellular proliferation. Remarkably, one cycle of pressure at ten pounds per square inch (sixty-nine kilopascals) and a frequency of approximately one hertz produced a 57 per cent increase in the incorporation of [3H]-thymidine at twenty-four hours (p < 0.001). From these findings, we hypothesized that the inositol phosphate cascade-cytosolic Ca(2+)-cytoskeletal calmodulin system plays a dominant role in the signal transduction of a mechanical stimulus into increased proliferation of bone cells, at least under the conditions reported here.

Bone and tissue changes following prostaglandin therapy in neonates

OBJECTIVE

To review case reports describing prostaglandin-induced bone changes and tissue swelling in neonates.

DATA SOURCES

An Index Medicus and bibliographic search of the English-language literature pertaining to prostaglandin-induced bone changes and tissue swelling in neonates.

DATA SYNTHESIS

Short- and long-term prostaglandin infusion in neonates is associated with cortical proliferation throughout the skeletal system. Total resolution of these changes has occurred in seven patients described to date; other reports did not note either the time or the regression of these changes. Skeletal changes may occur within 9 days of initiation of prostaglandin therapy and include widened fontanelles, pretibial and soft-tissue swelling, and swelling of the upper and lower extremities. These reactions may last up to 38 weeks following discontinuation of therapy. Alkaline phosphatase (AP) concentrations have been shown to be increased in four cases, as well as in one retrospective analysis. Although these were not prospective studies, evaluation of AP may provide a means to monitor neonates receiving prostaglandin therapy.

CONCLUSIONS

Pharmacists need to be aware of the potentially serious skeletal changes, encourage dosage titration as soon as possible, and limit the duration of time neonates are exposed to prostaglandin therapy. Monitoring of AP concentrations during therapy with prostaglandins may be beneficial in predicting or preventing further complications.

Intraosseous infusion of prostaglandin E2 prevents disuse-induced bone loss in the tibia

We investigated whether intraosseous injection of prostaglandin E2 would preserve tibial bone mass in the skeletally unloaded limb of a large animal model. Skeletal unloading of one rear limb was produced by unilateral Achilles tenectomy in the goat. Prostaglandin E2 was injected at 0.5 or 1.0 mg (1 ml of volume) twice daily, beginning on day 7 and continuing for 10 days, through an implant that had been surgically placed in the proximal tibial metaphysis. Thirty-five days after surgery, the tibiae were harvested for measurement of static and dynamic bone parameters and mechanical characteristics using transmission ultrasound. Prostaglandin E2 produced a dose-dependent increase in the formation of woven new bone at all bone envelopes. The 1.0 mg dosage prevented and partially reversed the effects of skeletal unloading and added new bone (p < 0.05) compared with the unloaded tibiae. Because prostaglandin E2 increased both bone formation and resorption and the new bone produced was primarily woven bone, the material properties of the tibiae infused with prostaglandin E2 did not increase significantly during the study compared with the unloaded and weight-bearing tibiae.

Effects of prostaglandin E2 and F2 alpha on the skeleton of osteopenic ovariectomized rats

This article contains the histomorphometric evaluation of the effects of prostaglandin F2 alpha (PGF2 alpha) on cancellous bone from the lumbar vertebra and cortical bone from the tibial shaft of ovariectomized, osteopenic rats. These effects were then compared with those of prostaglandin E2 (PGE2). Three-month-old rats were either ovariectomized (ovx) or sham-ovx. Then, either PGF2 alpha or PGE2 in doses of 1 and 3 mg/kg/day was given subcutaneously for 21 days at 150 days post ovx. Histomorphometric analysis was performed separately on both the primary and secondary spongiosae of the fourth lumbar vertebral bodies (LVB) and on tibial shafts. The ovx rats exhibited osteopenia in both primary (-23% to -37%) and secondary (-20%) spongiosae of the LVB, but not in the tibial shafts at 150 and 171 days post ovx. In the LVB, PGE2 in doses of 1 or 3 mg/kg/day for 21 days restored trabecular bone volume to the levels of sham-ovx controls in the primary spongiosa. However, in the secondary spongiosa, the treatments only thickened the trabeculae. The effects of the PGF2 alpha treatment were similar to those of the PGE2 in both the primary and the secondary spongiosae. While both PGF2 alpha and PGE2 treatments stimulated bone formation in the LVB as indicated by the increases in labeled perimeter, tissue and bone area-based bone formation rates, PGE2 is about 10 times more potent than PGF2 alpha in these effects. The PGE2 treatment also elevated activation frequency in the LVB, while the PGF2 alpha treatment did not. The treatments differed in that PGE2 at these dose levels did not alter the eroded surface in the LVB while PGF2 alpha decreased it significantly. Thus, the increase of the ratio of labeled to eroded perimeter in the LVB in PGF2 alpha-treated animals was much more than that in PGE2-treated animals. In the tibial shafts, PGE2 in doses of 1 and 3 mg/kg/day produced new marrow trabeculae in 2 of 6 and 3 of 6 of the ovx rats. However, no new trabecula was found in PGF2 alpha-treated tibial shafts. Higher doses of PGE2 also increased periosteal labeled perimeter, MAR, and BFR/BS, while PGF2 alpha did not produce any significant change in these parameters. Both PGE2 and PGF2 alpha in doses of 1 and 3 mg/kg/day increased the labeled perimeter, MAR and BFR/BS and decreased the eroded perimeter in the endocortical surface. We concluded that both PGF2 alpha and PGE2 in doses of 1 and 3 mg/kg/day for 21 days exhibited anabolic bone effects. The effects were mostly confined to an increase in trabecular volume in the primary spongiosa of the LVB and in the endocortical surface of tibial shafts. The tissue level mechanism behind this appears to be that PGE2 and PGF2 alpha can both stimulate osteoblast recruitment and activity. Overall, we found PGE2 to be more potent than PGF2 alpha at the same dose level at the endocortical surface. Furthermore, new marrow trabecular bone formed only after PGE2 treatment. PGF2 alpha differed from PGE2 by significantly reducing the trabecular eroded surface in ovx rats.

Animal models of cortical porosity

Increased cortical porosity (Ct.Po) has been observed as a consequence of aging, disease (hyperparathyroidism, osteoporosis), and pharmacologic intervention (thyroid hormone, fluoride, parathyroid hormone, prostaglandins). Whether this is permanent or transient in humans has not been established nor has the impact of Ct.Po on bone strength or fracture incidence been determined. There is a need to understand the causes and consequences of increased Ct.Po and how it relates to disease states and osteoporosis therapies. The mechanism of increased Ct.Po is thought to be an increased activation of Haversian remodeling systems accompanied by increased Haversian canal diameter. Increased activation at the endocortical surface and subsequent trabecularization may also contribute to loss of cortical bone. Ct.Po has been observed in several animal models which can be used to further study the phenomenon. Ct.Po is seen in normal dogs and is increased by treatment with PTH and prostaglandins. In the dog it has also been shown that increased Ct.Po is reversible after anabolic therapy is discontinued. Furthermore, in dogs an antiresorptive agent (risedronate) will block PTH-induced increases in Ct.Po (control 1.6% +/- 0.7; PTH 3.0% +/- 1.1, PTH plus risedronate 1.8% +/- 1.2) without interfering with anabolic effects. In rats, increased Ct.Po occurs following treatment with anabolic agents such as PTH, prostaglandin, and insulin-like growth factor 1 (IGF-1). For instance, with IGF-1, Ct.Po increased from 0.4% +/- 0.4 to 5.4% +/- 1.8. Other species in which Ct.Po is observed include primates and ferrets.(ABSTRACT TRUNCATED AT 250 WORDS)

Risedronate treatment does not increase microdamage in the canine femoral neck

Cortical and trabecular bone from the femoral neck of 24 adult female beagle dogs was examined for microdamage following 2 years of treatment with risedronate (NE-58095). Specimens of the femoral neck, sectioned between the femoral head and the intertrochanteric groove, were bulk stained in 1% basic fuchsin in graded alcohols and embedded in methylmethacrylate. Five transverse sections of 100 microns from each specimen were examined for microdamage and measurement of cortical and trabecular area, and three sections from each specimen were measured for calculation of trabecular and cortical bone activation frequency (Ac.f) and bone formation rate (BFR/BV) in the superior and anterior regions of the femoral neck. Although no statistical differences were observed among groups for numerical density or length of microcracks, Kruskal-Wallis analysis showed differences among groups for both cortical and trabecular bone area (p < 0.05). Ac.f was significantly lower in both cortical bone (p < 0.05) and trabecular bone (p < 0.005) of the femoral neck at all dosage levels. No significant difference was observed among groups for trabecular mean wall thickness. The hypothesis that microdamage accumulation increases following reduction in Ac.f was not supported for the canine femoral neck in this experiment. This result could be explained by the fact that microdamage does not accumulate following treatment; that transient increases in microdamage at the beginning of the study period had been repaired; or finally, that the canine femoral neck does not reflect weight-bearing conditions of clinical relevance to humans for assessment of microdamage.

Effect of prostaglandin E2 on mineralization of bone nodules formed by fetal rat calvarial cells

The effects of PGE2 on mineralized bone nodule formation were studied in fetal rat calvarial (RC) cells in vitro. Continuous exposure of RC cells to 3 x 10(-8) M PGE2 induced a twofold increase in mineralized bone nodule formation and a 1.5-fold increase in alkaline phosphatase (ALPase) activity without affecting RC cell growth. These stimulatory effects were evoked by concentrations of 3 x 10(-9)-3 x 10(-6) M PGE2 and the maximal effect was observed with 3 x 10(-8) M PGE2. The in vitro effects of PGE2 were evident when RC cells were exposed to it on days 8-14 and 8-21, which correspond to the post-confluent culture stage, but no effects were observed when the cells were exposed on days 1-7, the growth stage. The ALPase activity was also higher (1.2-1.4-fold) when 3 x 10(-8) M PGE2 was added during the post-confluent stage. In order to determine the effect of PGE2 during the mineralization phase of bone nodules in the presence of a large population of osteoprogenitor cells, RC cells were exposed to dexamethasone for 7 days before PGE2 was added during the post-confluent stage. A significantly higher percentage of nodules mineralized were observed with 3 x 10(-8)-3 x 10(-9) M PGE2 (1.6- and 1.4-fold, respectively), than in control cultures. Analysis of the mineral-related proteins by EDTA extraction of bone nodules followed by electrophoresis and Stains-All staining revealed an increased total amount of osteopontin extracted from the mineralized matrix after PGE2 treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of different magnitudes of tension force on prostaglandin E2 production by human periodontal ligament cells

Periodontal ligament (PDL) cells are known to produce prostaglandin E2 (PGE2) in response to mechanical stress. However, the rate of PGE2 production from PDL cells in response to different magnitudes of tension forces has not been examined. This study, therefore, was undertaken to determine the effect of different magnitudes of tension forces on PGE2 production and inositol trisphosphate (IP3) levels in PDL cells in vitro. Human PDL cells were cultured on flexible-bottomed plates and placed on a Flexercell strain unit. Cells were flexed at six cycles (5-s strain, 5-s relaxation) at six steps of tension force (9, 12, 15, 18, 21, 24% increase in surface area) for 5 days. PGE2 production and IP3 levels were determined by radioimmunoassay. There was a 6- and 25-fold increase in the rate of PGE2 production by cells exposed to low (9%) and high (24%) tension forces, respectively, and these increases were tension force-dependent. Tension force also induced increases in the intracellular levels of IP3 that did not seem to be directly related to the production of PGE2. The different rates of PGE2 production by PDL cells in response to different magnitudes of mechanical stress may be of importance in PDL and alveolar bone metabolism.

Prostaglandin-stimulated second messenger signaling in bone-derived endothelial cells is dependent on confluency in culture

New bone formation is associated with an increase in blood flow by the invasion of capillaries. Endothelial cells that line the capillaries can produce paracrine factors that affect bone growth and development, and in turn, could be affected by products produced by bone cells, in particular the osteoblasts. Since osteoblasts produce prostaglandins E2 and F2 alpha (PGE2, PGF2 alpha), it was investigated if these PGs were agonists to bone-derived endothelial cells (BBE) by assessing changes in cAMP and free cytosolic calcium concentration ([Ca2+]i) second messenger generation. We found that confluent cultures of BBE cells, a clonal endothelial cell line derived from bovine sternal bone, responded to 1 microM PGE2 by an increase in cAMP. PGF2 alpha at the same concentration was less potent in stimulating an increase in cAMP production in confluent BBE cells. Subconfluent cells with a morphology similar to that of fibroblastic cells were not as sensitive to PGE2-stimulated cAMP generation. PGF2 alpha failed to elicit any cAMP production in subconfluent cultures. PGE2 and PGF2 alpha both stimulated an increase in [Ca2+]i concentration in a dose-dependent manner. The potency of PGE2 was similar to that of PGF2 alpha in stimulating an increase in [Ca2+]i. The Ca2+ response was mostly independent of extracellular Ca+, was unchanged even with prior indomethacin treatment, was unaffected by caffeine pretreatment, but was abolished subsequent to thapsigargin pretreatment. The PG-induced increase in [Ca2+]i was also dependent on the confluency of the cells. In a subconfluent state, the responses to PGE2, or PGF2 alpha were either negligible, or only small increases in [Ca2+]i were noted with high concentrations of these two PGs. Consistent, dose-dependent increases in [Ca2+]i were stimulated by these PGs only when the cells were confluent and had a cobblestoned appearance. Since it was previously demonstrated that BBE cells respond to parathyroid hormone (PTH) by the production of cAMP, we tested if bovine PTH(1-34) amide ]bPTH(1-34) also increased [Ca2+]i in these cells. No change in [Ca2+]i was found in response to bPTH (1-34), although bPTH (1-34) stimulated a nine to tenfold increase in cAMP. We conclude that BBE cells respond to PGE2 and PGF2 alpha but not to bPTH(1-34) by an increase in [Ca2+]i probably secondary to stimulation of phospholipase C and that the cAMP and [Ca2+]i second messenger responses in BBE cells are dependent on the state of confluency of the cells.

Induction of vascular endothelial growth factor expression by prostaglandin E2 and E1 in osteoblasts

PGE1 and PGE2 are potent stimulators of bone formation. Osteogenesis is strongly dependent on angiogenesis. Vascular endothelial growth factor (VEFG), a secreted endothelial cell-specific mitogen, has been implicated in physiological and pathological angiogenesis. The aim of this study was to examine the possible role of VEGF in PG stimulation of bone formation. We found that in rat calvaria-derived osteoblast-enriched cells and in the osteoblastic RCT-3 cell line PGE2 and E1 increased VEGF mRNA and protein levels. The increased expression of VEGF mRNA produced by PGE2 was rapid (maximal at 1 h), transient (declined by 3 h), potentiated by cycloheximide, and abolished by actinomycin D. PGE2 had no effect on VEGF mRNA stability, suggesting transcriptional regulation of VEGF expression by PGF2. Rp-cAMP, a cAMP antagonist, suppressed VEGF mRNA induced by PGE2, indicating cAMP mediation. The upregulation of VEGF expression by PGE2 in the preosteoblastic RCT-1 cells was potentiated by treatment with retinoic acid, which induces the differentiation of these cells. The upregulation of VEGF mRNA by PGE2 was inhibited by dexamethasone treatment. In addition, Northern blot analysis showed that VEGF mRNA is expressed in adult rat tibia. In summary, we documented, for the first time, the expression of VEGF in osteoblasts and in bone tissue. Stimulation of VEGF expression by PGs and its suppression by glucocorticoids, which, respectively, stimulate and suppress bone formation, strongly implicate the involvement of VEGF in bone metabolism.

Isolation and characterization of gap junctions in the osteoblastic MC3T3-E1 cell line

Gap junctions are channels connecting cells that function in cell-to-cell communication. Gap junctions are abundant in osteoblastic cells. Membranes enriched for gap junction plaques were obtained by differential centrifugation, followed by treatment of the membranes with potassium iodide and sarkosyl before sucrose density gradient centrifugation. Electron microscopy showed that the preparation was enriched for electron-dense membranes consistent with gap junctions. Coomassie Blue staining of SDS-PAGE preparations revealed a prominent band at approximately 41 kD. Western analysis with a site-directed antibody, CT-360 (D. Laird, California Institute of Technology, Pasadena, CA), to the C-terminal portion of the rat heart connexin 43 molecule was positive in the MC3T3-E1 cell line, a phenotypic osteoblastic cell line derived from normal neonatal mouse calvariae. Western analysis using a monoclonal antibody, R5.21C, to rat liver connexin 32 was negative. Additionally, a prominent band at 59 kD was detected by CT-360 in both gap junction-enriched preparations and cell lysates. Treatment of diluted samples of gap junction-enriched preparations with sulfhydryl reducing agents in combination with detergents resulted in the enhancement and diminution of the 41 and 59 kD bands, respectively. Immunoprecipitation following [35S]methionine/[35S]cysteine labeling revealed a significant band detected at 122 kD in addition to the 41 kD band. To demonstrate functional gap junctions, transfer of lucifer yellow dye to surrounding cells was monitored after microinjection of a target cell. Between passages 10 and 25 in culture, functional cell coupling was found in approximately 70% of injected cells. Coupling was detected within 1-2 minutes after injection. Simultaneous microinjection of the CT-360 antibody with lucifer yellow resulted in the decoupling of cells. In conclusion, (1) MC3T3-E1 cells possess a 41 kD protein that is recognized by connexin 43 antibody to rat heart gap junction; (2) multimers of the MC3T3-E1 gap junctions occur in the preparation; and (3) functional coupling demonstrated by dye transfer may be regulated by region(s) in the C terminus of the connexin molecule.

Improvement of gingival and alveolar bone status in periodontitis-affected hamsters treated with 15-methyl prostaglandin E1

Exogenous PGE1 at pharmacological doses suppresses acute and chronic inflammation manifestations. As periodontitis possesses features of both acute and chronic inflammation, attenuation of periodontal destruction in hamsters was attempted by using 15-M-PGE1, a stable PGE1 analog. The agent was tested at 2 different doses (100 and 150 micrograms/kg/d) and its effects were matched against disease-free and periodontitis-affected animals. No effect was found with the 100 micrograms regimen. In contrast, in the 150 micrograms group, in the gingiva around the first right mandibular molar the pocket epithelium and infiltrated connective tissue (ICT) areas, the mean vascular lumen, the number of PMNLs adherent to blood vessels and infiltrating the ICT were significantly reduced. The number of osteoclasts was markedly diminished as well, but their intrinsic activity was enhanced. Bone formation was totally inhibited in this treatment group. These results indicated that 15-M-PGE1 effectively improved gingival inflammation mostly by reducing edema and PMNL recruitment and controlled alveolar bone destruction by reducing the osteoclast recruitment. From a therapeutic point of view the complete inhibition of formation seems to contraindicate its use.