Restoration of severely depleted femoral trabecular bone in ovariectomized rats by parathyroid hormone-(1-34)

Intermittent administration of parathyroid hormone improves the repairing process of rat calvaria defects: A histomorphometric and radiodensitometric study

Background

The aim of this study was to evaluate the effects of intermittent treatment of parathyroid hormone (PTH (1-34)) on the bone regeneration of critically-sized rat calvarial bone defects.

Material and Methods

Thirty-two male rats were trephined (4mm fullthickness diameter), in the central part of the parietal bones and divided into 2 groups of 16. The PTH group received subcutaneous injections of PTH (1-34) at 40µg/kg, 3 times a week and the control (CTL) group received the vehicle in the same regimen. The rats were sacrificed at 4 weeks post-treatment regimen, the parietal bones were extracted and samples were evaluated through histomorphometry and radiodensitometry.

Results

The histological observations showed that the PTH group presented more “island-like” new bone between the defect margins with fibrous tissues than did the CTL group. The PTH group significantly exhibited greater histologic bone formation than did the CTL group (1.5mm ±0.7; 1.9 mm ± 0.6, p<0.05/ for residual bone defect). The radiodensitometry analysis revealed significant differences among the PTH and CTL groups (2.1 Al eq. ±0.04; 1.8Al eq. ±0.06, p<0.05), demonstrating an increase in bone mineral density. The PTH treatment contributed to the bone formation with a higher amount of mineral and/or fibrous tissue when compared with the CTL group.

Conclusions

The results suggest that it was possible to increase the process of bone regeneration by accelerating the healing process in rat calvarial defects through intermittent administration of the PTH treatment.

Key words: Bone, skull, rats, bone regeneration, bone density.

Parathyroid hormone related to bone regeneration in grafted and nongrafted tooth extraction sockets in rats

OBJECTIVE

The quality and quantity of bone formed in tooth extraction sockets impact implant therapy. Therefore, the establishment of a new approach to enhance bone formation and to minimize bone resorption is important for the success of implant therapy. In this study, we investigated whether intermittent parathyroid hormone (PTH) therapy enhanced bone formation in grafted sockets.

METHODS

Tooth extractions of the maxillary first molars were performed in rats, and the sockets were grafted with xenograft. Intermittent PTH was administered either for 7 days before extractions, for 14 days after extractions, or both. The effect of PTH therapy on bone formation in the grafted sockets was assessed using microcomputed tomography at 14 days after extractions.

RESULTS

PTH therapy for 7 days before extractions was not effective to augment bone fill, whereas PTH therapy for 14 days after operation significantly augmented bone formation in the grafted sockets.

CONCLUSIONS

Intermittent PTH therapy starting right after tooth extractions significantly enhanced bone fill in the grafted sockets, suggesting that PTH therapy can be a strong asset for the success of the ridge preservation procedure.

A naturally occurring rare analog of quercetin promotes peak bone mass achievement and exerts anabolic effect on osteoporotic bone

The effect of quercetin C-glucoside (QCG) on osteoblast function in vitro and bone formation in vivo was investigated. QCG supplementation promoted peak bone mass achievement in growing rats and new bone formation in osteopenic rats. QCG has substantial oral bioavailability. Findings suggest a significant bone anabolic effect of QCG.

INTRODUCTION

Recently, we showed that extracts of Ulmus wallichiana promoted peak bone mass achievement in growing rats and preserved trabecular bone mass and cortical bone strength in ovariectomized (OVx) rats. 3,3',4',5,7-Pentahydroxyflavone-6-C-β-D-glucopyranoside, a QCG, is the most abundant bioactive compound of U. wallichiana extract. We hypothesize that QCG exerts bone anabolic effects by stimulating osteoblast function.

METHODS

Osteoblast cultures were harvested from rat calvaria and bone marrow (BM) to study differentiation and mineralization. In vivo, growing female Sprague Dawley rats and OVx rats with osteopenia were administered QCG (5.0 or 10.0 mg kg(-1) day(-1)) orally for 12 weeks. Efficacy was evaluated by examining changes in bone microarchitecture using histomorphometric and microcomputed tomographic analyses and by determination of new bone formation by fluorescent labeling of bone. Plasma and BM levels of QCG were determined by high-performance liquid chromatography.

RESULTS

QCG was much more potent than quercetin (Q) in stimulating osteoblast differentiation, and the effect of QCG was not mediated by estrogen receptors. In growing rats, QCG increased BM osteoprogenitors, bone mineral density, bone formation rate, and cortical deposition. In osteopenic rats, QCG treatment increased bone formation rate and improved trabecular microarchitecture. Comparison with the sham group (ovary intact) revealed significant restoration of trabecular bone in osteopenic rats treated with QCG. QCG levels in the BM were ~50% of that of the plasma levels.

CONCLUSION

QCG stimulated modeling-directed bone accrual and exerted anabolic effects on osteopenic rats by direct stimulatory effect on osteoprogenitors likely due to substantial QCG delivery at tissue level following oral administration.

Effects of low-dose, intermittent treatment with recombinant human parathyroid hormone (1-34) on chondrogenesis in a model of experimental fracture healing

Recent studies have demonstrated that intermittent administration of parathyroid hormone (PTH) enhances osteogenesis (hard callus formation) and increases mechanical strength in experimental fracture healing. Thus far, however, effects of PTH on chondrogenesis (soft callus formation) during fracture healing have not been fully elucidated. In the present study, we analyzed the underlying molecular mechanism by which exogenous PTH would affect chondrogenesis in a model of experimental fracture healing. Unilateral femoral fractures were produced in 2-month-old Sprague-Dawley rats. Daily subcutaneous injections of 10 microg/kg of recombinant human PTH(1-34) [rhPTH(1-34)] were administered over a 28-day period of fracture healing. Control animals were injected with vehicle solution (normal saline) alone. The results showed that, on day 14 after fracture, cartilage area in the PTH-treated group was significantly increased (1.4-fold) compared with the controls, but this increase was not observed at days 21 and 28. In the early stage of chondrogenesis (days 4-7), cell proliferation, expressed as the rate of proliferating cell nuclear antigen-positive cells, was increased in mesenchymal (chondroprogenitor) cells but not chondrocytes in the PTH-treated group compared with controls. In addition, gene expression of SOX-9 was up-regulated in the PTH-treated group on day 4 (1.4-fold), and this was accompanied by enhanced expression of pro-alpha1 (II) collagen (1.8-fold). After 14 days, there were no significant differences between groups in either cell proliferation or the expression levels of cartilage differentiation-related genes (SOX-9, pro-alpha1 (II) collagen, pro-alpha1 (X) collagen and osteopontin). These results suggest that intermittent treatment with low-dose rhPTH(1-34) induces a larger cartilaginous callus but does not delay chondrocyte differentiation during fracture healing.

Teriparatide (Biosynthetic Human Parathyroid Hormone 1-34): A New Paradigm in the Treatment of Osteoporosis

The ideal treatment of osteoporosis should preferably prevent fractures through normalization of bone mass and bone micro-architecture. Biosynthetic human parathyroid hormone 1-34 (teriparatide) was recently approved in the EU and the USA as the first anabolic treatment of osteoporosis. The effects of teriparatide are mediated by the G-protein-dependent, parathyroid hormone receptor-1 in the cell membrane. The binding of the ligand to the receptor activates adenylate cyclase and a number of phospholipases (A, C, and D) and increases intracellular levels of cAMP and calcium. Intermittent teriparatide increases the number of osteoblasts and bone formation by activation of pre-existing osteoblasts, increased differentiation of lining cells, and reduced osteoblast apoptosis. Anabolic effects of teriparatide on bone have been demonstrated in several species. It increases bone mass, structural integrity, bone diameter, and bone strength. Clinical efficacy was demonstrated in a randomized study comprising 1637 post-menopausal women with osteoporosis showing a 65% and 35% reduction of the relative risk of vertebral and appendicular fractures, respectively, during 18 months of treatment. Moreover, bone mineral density in the lumbar spine and hip increased by 9.7% and 2.6%, respectively. Similar effects on bone mineral density have been reported in men with osteoporosis and in glucocorticoid-induced osteoporosis, however, fracture data are limited in these groups. Direct comparison with alendronate revealed that teriparatide has a more pronounced effect on bone mineral density. Teriparatide should be used in combination with calcium plus vitamin D, and may be combined with hormonal replacement therapy. In contrast, alendronate attenuates the effect of teriparatide. The efficacy of other combinations remains uncertain. After termination of teriparatide, bone mineral density of the lumbar spine is reduced by approximately 2-3% after 2 1/2 years. This decrease is prevented by treatment with bisphosphonates. The most frequent adverse effects with teriparatide are nausea, headache, dizziness, and leg cramps, however, only the latter two differed significantly between the groups receiving teriparatide 20 microg/day and placebo. In the pivotal clinical study, reduced dosage or termination of therapy due to hypercalcaemia was necessary in 3% and 0.2%, respectively. In a rat toxicology study, in which teriparatide was administered in high dosages for an extended period of time, osteosarcoma was seen in a significant number of animals. However, none of the approximately 2800 patients in clinical trials has developed osteosarcoma. Teriparatide constitutes a break-through in the treatment of severe osteoporosis, although a number of issues about the optimal use of teriparatide remains unsettled. The published data provide proof of concept on anabolic therapy which changes several paradigms of bone physiology. Other parathyroid hormone analogues are being investigated in clinical trials and the development of non-peptide, small molecules targeted at the parathyroid hormone receptor may be envisaged.

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

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

Mechanisms for the enhancement of fracture healing in rats treated with intermittent low-dose human parathyroid hormone (1-34)

Recent reports have demonstrated that intermittent treatment with parathyroid hormone (1-34) [PTH(1-34)] increases callus formation and mechanical strength in experimental fracture healing. However, little is known about the optimal dose required for enhancement of fracture repair or the molecular mechanisms by which PTH regulates the healing process. In this study, we analyzed the underlying molecular mechanisms by which PTH affects fracture healing and tested the hypothesis that intermittent low-dose treatment with human PTH(1-34) can increase callus formation and mechanical strength. Unilateral femoral fractures were produced and a daily subcutaneous injection of 10 microg/kg of PTH(1-34) was administered during the entire healing period. Control animals were injected with vehicle solution alone. The results showed that on day 28 and day 42 after fracture, bone mineral content (BMC), bone mineral density (BMD), and ultimate load to failure of the calluses were significantly increased in the PTH-treated group compared with controls (day 28, 61, 46, and 32%; day 42, 119, 74, and 55%, respectively). The number of proliferating cell nuclear antigen (PCNA)-positive subperiosteal osteoprogenitor cells was significantly increased in the calluses of the PTH-treated group on day 2, and TRAP+ multinucleated cells were significantly increased in areas of callus cancellous bone on day 7. The levels of expression of type I collagen (COLlA1), osteonectin (ON), ALP, and osteocalcin (OC) mRNA were increased markedly in the PTH-treated group and accompanied by enhanced expression of insulin-like growth factor (IGF)-I mRNA during the early stages of healing (days 4-7). The increased expression of COL1A1, ON, ALP, and OC mRNA continued during the later stages of healing (days 14-21) despite a lack of up-regulation of IGF-I mRNA. These results suggest that treatment of fractures with intermittent low dose PTH(1-34) enhances callus formation by the early stimulation of proliferation and differentiation of osteoprogenitor cells, increases production of bone matrix proteins, and enhances osteoclastogenesis during the phase of callus remodeling. The resultant effect to increase callus mechanical strength supports the concept that clinical investigations on the ability of injectable low-dose PTH(1-34) to enhance fracture healing are indicated.

OPG and PTH-(1-34) have additive effects on bone density and mechanical strength in osteopenic ovariectomized rats

PTH is a potent bone anabolic factor, and its combination with antiresorptive agents has been proposed as a therapy for osteoporosis. We tested the effects of PTH, alone and in combination with the novel antiresorptive agent OPG, in a rat model of severe osteopenia. Sprague Dawley rats were sham-operated or ovariectomized at 3 months of age. Rats were untreated for 15 months, at which time ovariectomy had caused significant decreases in bone mineral density in the lumbar vertebrae and femur. Rats were then treated for 5.5 months with vehicle (PBS), human PTH-(1-34) (80 microg/kg), rat OPG (10 mg/kg), or OPG plus PTH (all three times per wk, sc). Treatment of ovariectomized rats with OPG or PTH alone increased bone mineral density in the lumbar vertebrae and femur, whereas PTH plus OPG caused significantly greater and more rapid increases than either therapy alone (P < 0.05). OPG significantly reduced osteoclast surface in the lumbar vertebrae and femur (P < 0.05 vs. sham or ovariectomized), but had no effect on osteoblast surface at either site. Ovariectomy significantly decreased the mechanical strength of the lumbar vertebrae and femur. In the lumbar vertebrae, OPG plus PTH was significantly more effective than PTH alone at reversing ovariectomy-induced deficits in stiffness and elastic modulus. These data suggest that OPG plus PTH represent a potentially useful therapeutic option for patients with severe osteoporosis.

Glucose-dependent insulinotropic peptide is an integrative hormone with osteotropic effects

Glucose-dependent insulinotropic peptide (GIP) is a gut-derived hormone known to be important in modulating glucose-induced insulin secretion. In addition, GIP receptors are widely distributed and may have effects on multiple other tissues: fat cells, adrenal glands, endothelium and brain. We have demonstrated recently that GIP also has anabolic effects on bone-derived cells. We now demonstrate that GIP administration prevents the bone loss associated with ovariectomy. We propose that GIP plays a unique role in signaling the bone about nutrient availability, indicating the importance of the gut hormones in directing absorbed nutrients to the bone, and suggesting the concept of an 'entero-osseous axis'. Thus, GIP plays an integrative role helping coordinate efficient and targeted nutrient absorption and distribution.

Bone tissue and its mineralization in aged estrogen-depleted rats after long-term intermittent treatment with parathyroid hormone (PTH) analog SDZ PTS 893 or human PTH(1-34)

Intermittently administered parathyroid hormone (PTH) is a potent bone anabolic agent. We aimed to determine the impact of long-term treatment with PTH on bone structure, dynamics, and mineralization. We ovariectomized (ovx) 1-year-old rats with the exception of a baseline and a sham-operated group. Twelve weeks later, a 36 week treatment with PTH analog SDZ PTS 893 (12.5, 25, 50, 100 microg/kg), human PTH(1-34) (25, 50, 100 microg/kg), or vehicle (ovx, sham) was initiated. Bone dynamics, structure, and mineralization were evaluated in the lumbar spine and in the femoral diaphysis. Cancellous bone turnover was elevated 12 weeks postovariectomy in estrogen-deficient, vehicle-treated animals, but returned to the level of the sham group by 48 weeks. The animals experienced substantial cancellous bone loss associated with a reduction of trabecular number and presented with a partly rod-like trabecular network. After 36 weeks of treatment with SDZ PTS 893 or human PTH(1-34), cancellous bone formation rates and turnover were raised in all treated groups compared with age-matched controls. The mineral apposition rate was increasing with dose. This amplified matrix synthesis led to trabecular thickening, but not to an increase in trabecular number, resulting in a crude, plate-like cancellous network with a high bone volume fraction. Fluorochrome label-based cortical bone dynamics demonstrated that a thick ring of new bone was formed at the endocortex by activation of modeling drifts during treatment. Treatment-induced cortical bone formation was increased with dose at the subperiosteal and endocortical envelopes, but substantially higher at the latter. Intracortical bone turnover was elevated near the endocortex. Bone mineralization was undisturbed in all compartments. The average degree of mineralization was lowered slightly, reflecting the increased portion of new bone formed during treatment. In summary, the main anabolic effect was mediated for both peptides by an increase in bone apposition with dose, persisting throughout treatment that lasted more than one third of the lifespan of the rats, and direct activation of bone-forming surfaces. As a result, a substantial amount of new bone, maintained at elevated turnover and adequate mineralization levels, formed predominantly at compartments exposed to bone marrow.

Daily transient decreases in plasma parathyroid hormone levels induced by the calcimimetic NPS R-568 slows the rate of bone loss but does not increase bone mass in ovariectomized rats

Daily parathyroid hormone (PTH) injections that transiently increase plasma PTH levels within the physiological range increase bone mass in osteopenic, ovariectomized (ovx) rats. This study tested the hypothesis that repeated transient decreases in plasma PTH levels from normal, induced by the daily oral administration of the calcimimetic NPS R-568, would induce an anabolic effect in bone of ovx rats with established osteopenia and/or prevent the rapid bone loss that occurs following ovx. In the reversal study, NPS R-568 was administered orally (10 or 100 micromol/kg) for 30 days to 14-month-old retired breeder rats that were ovx 5 months earlier. NPS R-568 treatment did not increase bone formation rate (BFR) or cancellous bone area (B.Ar) in the proximal tibial metaphysis, or bone mineral density (BMD), at any femoral site. In the prevention study, 3-month-old virgin rats were ovx and given NPS R-568 for the following 28 days. The 10 micromol/kg dose prevented the increase in osteoclast number and 42% of the loss of B.Ar, without affecting the elevated osteoblast populations or BFR. Surprisingly, the 100 micromol/kg dose had fewer protective effects, despite preventing the increase in BFR in both cancellous and cortical bone. Detailed analysis of cancellous bone showed that tendency for a dose-related protection of true cancellous bone occurred, but, while the 10 micromol/kg dose prevented 88% of the loss of calcified cartilage seen in control ovx rats, the 100 micromol/kg dose increased that loss by a further 31%. The mechanism underlying these disparate effects of NPS R-568 on calcified cartilage accumulation in the tibial metaphysis is unclear, but may be related to the different effects that the two doses have on plasma Ca(2+) levels. In conclusion, transient increases in PTH levels above basal, and not simple oscillations in hormone levels below normal, appear necessary for the anabolic properties of endogenous PTH to be manifested in the bones of osteopenic ovx rats.

Effect of intermittent administration of parathyroid hormone on fracture healing in ovariectomized rats

The effects of intermittent administration of parathyroid hormone on fracture healing in ovariectomized rats were examined to evaluate its potential use as a therapeutic agent for osteoporotic fractures. Three months postovariectomy, bilateral tibial shaft fractures were induced and stabilized by intramedullary nailing with Kirschner wires. Saline, 17beta-estradiol (Sigma Chemical Corp, St Louis, Mo), or recombinant human parathyroid hormone (1-84) (Korean Green-Cross Pharm Corp, Seoul, Korea) was given once a day for 30 consecutive days during fracture healing. Fracture healing was assessed by morphometric and mechanical analysis of fracture callus. Intermittent parathyroid hormone administration increased the morphometric and mechanical parameters in a dose-dependent manner. A bone-resorption inhibiting agent, 17beta-estradiol did not offer advantage in terms of fracture healing in ovariectomized rats. Findings suggest intermittent parathyroid hormone administration may benefit osteoporosis and fracture.

Lactam formation increases receptor binding, adenylyl cyclase stimulation and bone growth stimulation by human parathyroid hormone (hPTH)(1-28)NH2

Human parathyroid hormone (1-28)NH2 [hPTH(1-28)NH2] is the smallest of the PTH fragments that can fully stimulate adenylyl cyclase in ROS 17/2 rat osteoblast-like osteosarcoma cells. This fragment has an IC50 of 110 nM for displacing 125I-[Nle8,18,Tyr34]bovine PTH(1-34)NH2 from HKRK B7 porcine kidney cells, which stably express 950,000 human type 1 PTH/PTH-related protein (PTHrP) receptors (PTH1Rs) per cell. It also has an EC50 of 23.9 nM for stimulating adenylyl cyclase in ROS 17/2 cells. Increasing the amphiphilicity of the alpha-helix in the residue 17-28 region by replacing Lys27 with Leu and stabilizing the helix by forming a lactam between Glu22 and Lys26 to produce the [Leu27]cyclo(Glu22-Lys26)hPTH(1-28)NH2 analog dramatically reduced the IC50 for displacing 125I-[Nle8,18,Tyr34]bPTH(1-34)NH2 from hPTH1Rs from 110 to 6 nM and dropped the EC50 for adenylyl cyclase stimulation in ROS 17/2 cells from 23.9 to 9.6 nM. These modifications also increased the osteogenic potency of hPTH(1-28)NH2. Thus, hPTH(1-28)NH2 did not significantly stimulate either femoral or vertebral trabecular bone growth in rats when injected daily at a dose of 5 nmol/100 g body weight for 6 weeks, beginning 2 weeks after ovariectomy (OVX), but it strongly stimulated the growth of trabeculae in the cancellous bone of the distal femurs and L5 vertebrae when injected at 25 nmol/100 g body weight. By contrast [Leu27]cyclo(Glu22-Lys26)hPTH(1-28)NH2 significantly stimulated trabecular bone growth when injected at 5 nmol/100 g of body weight. Thus, these modifications have brought the bone anabolic potency of hPTH(1-28)NH2 considerably closer to the potencies of the larger PTH peptides and analogs.

Effect of raloxifene-analog (LY 117018-Hcl) on the bone marrow of ovariectomized mice

The effects of LY117018-Hcl (Ralox-A) on body metabolism and differentiation of bone marrow cells were studied in ovariectomized (OVX) mice. We used a mouse model in which estrogen depletion was established for a period of three months before treatment. After that period the animals were divided into three experimental groups consisting of sham-operated, OVX, and OVX-Ralox-A-treated mice. The OVX animals received daily treatment of Ralox-A during two time periods (35 and 65 days). After the treatment we measured the serum levels of protein, ion(s), lipid content, liver, and kidney functions. Our findings indicated that a change in hormonal state did not affect basic body metabolism except for causing an increase in triglycerides (TG) in the OVX mice, which was lowered by the Ralox-A. A higher alkaline phosphatase (ALK-P) level was observed in serum of the OVX-Ralox-A-treated mice than in serum of the OVX mice. We investigated the effects of estrogen depletion on the differentiation of hematopoietic and stromal cells that directly affect bone resorption and formation. OVX and OVX-treated mice were compared with the sham group and assessed for the alteration of these cells' differentiation. The proliferation of stromal stem cells was measured by CFU-F assay in vitro. A decrease in CFU-F colonies derived from OVX mice was observed and after the Ralox-A treatment the number of CFU-F reached sham levels. On the contrary, an upregulation of myeloid cells was observed when analyzed by FACS and by granulocyte/macrophage-colony forming unit (G/M-CFU) assay in selective culture conditions. The G/M-CFUs were increased in the OVX mice and were reduced to sham levels after Ralox-A treatment. In this study, we demonstrated cellular changes of stromal and hemopoietic cells in OVX mice and a beneficial Ralox-A effect that protected such cellular changes.

Intermittent parathyroid hormone (1-34) treatment increases callus formation and mechanical strength of healing rat fractures

The influence of intermittent parathyroid hormone (PTH(1-34)) administration on callus formation and mechanical strength of tibial fractures in rats was investigated after 20 and 40 days of healing. A dose of 60 microg of PTH(1-34)/kg/day and 200 microg of PTH(1-34)/kg/day, respectively, was administered during the entire periods of healing, and control animals with fractures were given vehicle. The dose of 200 microg of PTH(1-34)/kg/day increased the ultimate load and the external callus volume of the fractures by 75% and 99%, respectively, after 20 days of healing and by 175% and 72%, respectively, after 40 days of healing. The dose of 60 microg of PTH(1-34)/kg/day did not influence either ultimate load or external callus volume of the fractures after 20 days of healing, but the ultimate load was increased by 132% and the external callus volume was increased by 42% after 40 days of healing. During the healing period, the callus bone mineral content (BMC) increased in all groups. After 40 days of healing, the callus BMC was increased by 108% in the 200 microg of PTH(1-34)/kg/day group and by 76% in the 60 microg of PTH(1-34)/kg/day group. Both doses of PTH(1-34) steadily augmented the contralateral intact tibia BMC (20 days and 40 days: 60 microg of PTH (1-34)/kg/day 9% and 19%, respectively; 200 microg of PTH (1-34)/kg/day 12% and 27%, respectively) and bone mineral density (20 days and 40 days: 60 microg of PTH(1-34)/kg/day 11% and 12%, respectively; 200 microg of PTH(1-34)/kg/day 11% and 15%, respectively).

Skeletal unloading causes resistance of osteoprogenitor cells to parathyroid hormone and to insulin-like growth factor-I

Skeletal unloading decreases bone formation and osteoblast number in vivo and decreases the number and proliferation of bone marrow osteoprogenitor (BMOp) cells in vitro. We tested the ability of parathyroid hormone (PTH) to stimulate BMOp cells in vivo by treating Sprague Dawley rats (n = 32) with intermittent PTH(1-34) (1 h/day at 8 microg/100 g of body weight), or with vehicle via osmotic minipumps during 7 days of normal weight bearing or hind limb unloading. Marrow cells were flushed from the femur and cultured at the same initial density for up to 21 days. PTH treatment of normally loaded rats caused a 2.5-fold increase in the number of BMOp cells, with similar increases in alkaline phosphatase (ALP) activity and mineralization, compared with cultures from vehicle-treated rats. PTH treatment of hind limb unloaded rats failed to stimulate BMOp cell number, ALP activity, or mineralization. Hind limb unloading had no significant effect on PTH receptor mRNA or protein levels in the tibia. Direct in vitro PTH challenge of BMOp cells isolated from normally loaded bone failed to stimulate their proliferation and inhibited their differentiation, suggesting that the in vivo anabolic effect of intermittent PTH on BMOp cells was mediated indirectly by a PTH-induced factor. We hypothesize that this factor is insulin-like growth factor-I (IGF-I), which stimulated the in vitro proliferation and differentiation of BMOp cells isolated from normally loaded bone, but not from unloaded bone. These results suggest that IGF-I mediates the ability of PTH to stimulate BMOp cell proliferation in normally loaded bone, and that BMOp cells in unloaded bone are resistant to the anabolic effect of intermittent PTH therapy due to their resistance to IGF-I.

The role of parathyroid hormone in the pathogenesis, prevention and treatment of postmenopausal osteoporosis

Parathyroid hormone (PTH) is the principal regulator of bone remodeling in the adult skeleton. The acute in vivo effect of PTH is to increase bone resorption, although sustained increases in its circulating levels accelerate both formation and resorption. These divergent effects have focused attention on PTH as a factor contributing to bone loss in some postmenopausal women, as well as interest in its role as therapy for the disease. Sustained increases in PTH are classically seen in primary hyperparathyroidism. While still controversial, increasing evidence suggests that primary hyperparathyroidism is associated with increased rates of bone loss, particularly from cortical sites in the skeleton. It is clear that the remodeling space is increased in primary hyperparathyroidism, and that surgical correction of the disease leads to substantial increases in bone mass in patients with osteoporosis. Recently, secondary hyperparathyroidism has emerged as an important contributor to increased rates of bone turnover and bone loss in postmenopausal women. The etiology of secondary hyperparathyroidism in postmenopausal women is complex, and is probably related to alterations in vitamin D metabolism and tissue responsiveness to 1,25(OH)2vitamin D. PTH has emerged at the forefront of anabolic therapies for the treatment of postmenopausal osteoporosis. When given as a single agent, intermittent daily subcutaneous administration of PTH induces consistent gains in trabecular bone mass with more varying effects on the cortical envelope. However, recent therapeutic trials employing a second agent, most notably estrogen, give hope that this approach may provide the first truly efficacious anabolic therapy for this devastating disease.

Plasma levels of parathyroid hormone that induce anabolic effects in bone of ovariectomized rats can be achieved by stimulation of endogenous hormone secretion

Parathyroid hormone (PTH) administration increases bone mass in normal and osteopenic animals. However, this treatment currently requires the daily injection of large amounts of PTH, and the relationship of these doses to plasma levels of PTH that are achievable physiologically is unknown. We determined in ovariectomized (ovx) rats: 1) the plasma PTH levels that occur after the subcutaneous injection of graded doses of rat PTH, 2) whether similar PTH levels can be achieved by stimulation of endogenous PTH secretion, and 3) whether a plasma PTH profile that is achievable physiologically is anabolic on bone. Injection of 1, 5, or 25 micrograms/kg rat PTH-(1-34) increased plasma PTH by 46, 164, or 520 pg/mL, respectively, above basal levels within 60 min. Infusion of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid for 2 h reduced plasma Ca2+ by 0.36 mmol/L and produced a total plasma PTH response (area under the plasma PTH curve) similar to that with the 5 micrograms/kg rat PTH injection. Then, 1, 5, or 25 micrograms/kg doses of rat PTH-(1-34) were injected subcutaneously daily for 28 days in 19-week-old rats that were ovx 7 weeks earlier. The 5 and 25 micrograms/kg doses significantly increased bone mineral density in the distal femur and trabecular bone area and average trabecular thickness in the proximal tibia. All doses of PTH significantly increased indices of trabecular connectivity and cancellous bone formation, including double-labeled surface, mineralizing surface, and surface-referent bone formation rate. In conclusion, anabolic effects on bone can be achieved with a plasma PTH profile similar to that attained following stimulation of the parathyroid gland by induced hypocalcemia. These data suggest that agents that transiently increase endogenous PTH secretion may represent a novel means to promote anabolic effects in skeletal tissues.

Anabolic effects of estrogen and parathyroid hormone on skeletal tissues: the use of creatine kinase B activity as a response marker

The rapid stimulation of the specific activity of the brain type isozyme of creatine kinase (CK BB) is an almost universal marker of cell stimulation. We have studied its stimulation in skeletal-derived cells and shown that the increase in its activity is closely correlated with the biochemical parameter of cell proliferation - [(3)thymidine incorporation into DNA - and with the morphological parameters of bone growth, increase in thickness of cortical bone and of the number of cells in the proliferating zone of the epiphyseal growth plate. We have used the increase in CK activity to demonstrate sex specific stimulation of diaphyseal bone, exclusively by estrogens in females and by androgens in males, and the dependence of sex steroid stimulation on an adequate level of vitamin D. After finding that parathyroid hormone can act as a mitogen via a phospholipase-C-phosphoinositide turnover pathway, we collaborated with colleagues at the GBF in Braunschweig to find that mid-region fragments of PTH could act exclusively as mitogens, without stimulating cAMP production leading to bone resorption. hPTH (28-48) variants designed to be resistant to proteolysis were efficient in stimulating CK specific activity in vitro and in vivo and increased cortical bone thickness and the number of proliferating epiphyseal cartilage cells in rat long bones. These results are put into an historical context and compared with recent studies, in this short, selective review.

Comparison of the ability of recombinant human parathyroid hormone, rhPTH-(1-84), and hPTH-(1-31)NH2 to stimulate femoral trabecular bone growth in ovariectomized rats

A recombinant human parathyroid hormone, rhPTH-(1-84), which is currently in Phase II clinical trial, and hPTH-(1-31)NH2 (Ostabolin) are promising anabolic agents for treating osteoporosis because they can stimulate cortical and trabecular bone growth in osteopenic, ovariectomized (OVX) rats and in osteoporotic, postmenopausal women when injected subcutaneously and intermittently at low doses. We have now found that, despite their different sizes and signaling properties (rhPTH-(1-84) stimulates adenylyl cyclase and phospholipase C; hPTH-(1-31)NH2 only stimulates adenylyl cyclase), they are equally osteogenic in OVX rats. Thus daily subcutaneous injections of 0.6 nmol/100 g of body weight of rhPTH-(1-84) or hPTH-(1-31)NH2 into 3-month-old OVX rats for 6 weeks starting 2 weeks after OVX equally reduced the otherwise large OVX-triggered loss of femoral trabecular bone. Daily subcutaneous injections of 0. 4 or 0.8 nmol/100 g of body weight of the two agents for 6 weeks also equally increased the mean thickness of the remaining femoral trabeculae in 3-month-old and 1-year-old OVX rats to 20 to 80% above the value in normal animals when started 9 weeks after ovariectomy.

Stimulation of the growth of femoral trabecular bone in ovariectomized rats by the novel parathyroid hormone fragment, hPTH-(1-31)NH2 (Ostabolin)

The human parathyroid hormone, hPTH-(1-84), and its hPTH-(1-34) fragment are promising anabolic agents for treating osteoporosis because they can strongly stimulate the production of biomechanically effective cortical and trabecular bone in osteopenic ovariectomized (OVX) rats and trabecular bone in osteoporotic postmenopausal humans. The ideal PTH fragment for treating osteoporosis would be the smallest and functionally simplest fragment that activates only one signal mechanism and still strongly stimulates trabecular bone growth. A new PTH fragment, hPTH-(1-31)NH2, which only stimulates adenylyl cyclase instead of stimulating both adenylyl cyclase and phospholipase-C as do hPTH-(1-84) and hPTH-(1-34), is this minimum, high-potency anabolic fragment. hPTH-(1-31)NH2 (which we have named Ostabolin) can greatly thicken trabeculae and increase the dry weight and calcium content of trabecular bone in the distal femurs of osteopenic, young, sexually mature OVX Sprague-Dawley rats when injected subcutaneously each day for 6 weeks at doses between 0.4 and 1.6 nmole/100 g of body weight.