PTH analogues and osteoporotic fractures

Parathyroid hormone and trabectedin have differing effects on macrophages and stress fracture repair

Stress fractures occur as a result of repeated mechanical stress on bone and are commonly found in the load-bearing lower extremities. Macrophages are key players in the immune system and play an important role in bone remodeling and fracture healing. However, the role of macrophages in stress fractures has not been adequately addressed. We hypothesize that macrophage infiltration into a stress fracture callus site promotes bone healing. To test this, a unilateral stress fracture induction model was employed in which the murine ulna of four-month-old, C57BL/6 J male mice was repeatedly loaded with a pre-determined force until the bone was displaced a distance below the threshold for complete fracture. Mice were treated daily with parathyroid hormone (PTH, 50 μg/kg/day) starting two days before injury and continued until 24 h before euthanasia either four or six days after injury, or treated with trabectedin (0.15 mg/kg) on the day of stress fracture and euthanized three or seven days after injury. These treatments were used due to their established effects on macrophages. While macrophages have been implicated in the anabolic effects of PTH, trabectedin, an FDA approved chemotherapeutic, compromises macrophage function and reduces bone mass. At three- and four-days post injury, callus macrophage numbers were analyzed histologically. There was a significant increase in macrophages with PTH treatment compared to vehicle in the callus site. By one week of healing, treatments differentially affected the bony callus as analyzed by microcomputed tomography. PTH enhanced callus bone volume. Conversely, callus bone volume was decreased with trabectedin treatment. Interestingly, concurrent treatment with PTH and trabectedin rescued the reduction observed in the callus with trabectedin treatment alone. This study reports on the key involvement of macrophages during stress fracture healing. Given these observed outcomes on macrophage physiology and bone healing, these findings may be important for patients actively receiving either of these FDA-approved therapeutics.

Research progress of targeted therapy regulating Th17/Treg balance in bone immune diseases

Rheumatoid arthritis (RA) and postmenopausal osteoporosis (PMOP) are common bone-immune diseases. The imbalance between helper (Th17) and regulatory T cells (Tregs) produced during differentiation of CD4+ T cells plays a key regulatory role in bone remodelling disorders in RA and PMOP. However, the specific regulatory mechanism of this imbalance in bone remodelling in RA and PMOP has not been clarified. Identifying the regulatory mechanism underlying the Th17/Treg imbalance in RA and PMOP during bone remodelling represents a key factor in the research and development of new drugs for bone immune diseases. In this review, the potential roles of Th17, Treg, and Th17/Treg imbalance in regulating bone remodelling in RA and PMOP have been summarised, and the potential mechanisms by which probiotics, traditional Chinese medicine compounds, and monomers maintain bone remodelling by regulating the Th17/Treg balance are expounded. The maintenance of Th17/Treg balance could be considered as an therapeutic alternative for the treatment of RA and PMOP. This study also summarizes the advantages and disadvantages of conventional treatments and the quality of life and rehabilitation of patients with RA and PMOP. The findings presented her will provide a better understanding of the close relationship between bone immunity and bone remodelling in chronic bone diseases and new ideas for future research, prevention, and treatment of bone immune diseases.

Effectiveness of Weekly Teriparatide Injection in Postmenopausal Patients with Hip Fractures

Background

Teriparatide is an effective anabolic agent used in the treatment of severe osteoporosis. In addition, it is also used to promote fracture healing. The purpose of this double-blind randomized controlled trial was to evaluate the influence of weekly teriparatide administration on bone formation in hip fracture patients.

Methods

The control group (n = 41) was composed of patients treated with normal saline other than teriparatide, and the teriparatide group (n = 51) consisted of patients who received weekly teriparatide. Bone turnover markers, C-terminal telopeptide (CTx) and osteocalcin (OC), were assessed through blood tests at the initial hospital visit and 3-month, 6-month, and 1-year follow-ups. Dual-energy X-ray absorptiometry was performed 5 days postoperatively and at 1-year postoperative follow-up. The degree of fracture union was evaluated by comparing the radiographic union scoring system for hips using Radiographic Union Score for Hip (RUSH) scores between the two groups at 3 months, 6 months, and 1 year after surgery.

Results

Evaluation of the rate of change in bone mineral density over 1 year showed that the lumber bone mineral density increased by more than 7% in the experimental group. The control group did not show a difference between the CTx and OC at 6 months, but the difference between the CTx and OC values was large at 6 months in the experimental group. The mean RUSH score was significantly different between the control group and the experimental group: 12.105 and 15.476, respectively (p = 0.004), at 3 months and 18.571 and 22.389, respectively, at 6 months (p = 0.006).

Conclusions

Weekly use of teriparatide improved fracture healing, bone formation, and clinical outcomes at 1 year after hip fracture surgery by the anabolic window effect.

Antagonizing microRNA-19a/b augments PTH anabolic action and restores bone mass in osteoporosis in mice

Postmenopausal bone loss often leads to osteoporosis and fragility fractures. Bone mass can be increased by the first 34 amino acids of human parathyroid hormone (PTH), parathyroid hormone‐related protein (PTHrP), or by a monoclonal antibody against sclerostin (Scl‐Ab). Here, we show that PTH and Scl‐Ab reduce the expression of microRNA‐19a and microRNA‐19b (miR‐19a/b) in bone. In bones from patients with lower bone mass and from osteoporotic mice, miR‐19a/b expression is elevated, suggesting an inhibitory function in bone remodeling. Indeed, antagonizing miR‐19a/b in vivo increased bone mass without overt cytotoxic effects. We identified TG‐interacting factor 1 (Tgif1) as the target of miR‐19a/b in osteoblasts and essential for the increase in bone mass following miR‐19a/b inhibition. Furthermore, antagonizing miR‐19a/b augments the gain in bone mass by PTH and restores bone loss in mouse models of osteoporosis in a dual mode of action by supporting bone formation and decreasing receptor activator of NF‐κB ligand (RANKL)‐dependent bone resorption. Thus, this study identifies novel mechanisms regulating bone remodeling, which opens opportunities for new therapeutic concepts to treat bone fragility.

Combined Therapy of Yishen Zhuanggu Decoction and Caltrate D600 Alleviates Postmenopausal Osteoporosis by Targeting FoxO3a and Activating the Wnt/β-Catenin Pathway

Background

Postmenopausal osteoporosis (PMO) is the most prevalent metabolic bone disease in women. Yishen Zhuanggu (YSZG) decoction and Caltrate D600 reportedly affects bone formation. This study aimed to investigate the efficacy and mechanism of YSZG decoction combined with Caltrate D600 in PMO treatment.

Methods

Ovariectomy-induced PMO rat model was treated with YSZG or/and Caltrate D600 for 12 weeks. Femur bone mineral density (BMD), osteoporosis-related protein expression, and serum parameters were measured. Pathological features of femur bone tissues were observed using hematoxylin and eosin staining. Serum levels of oxidative stress parameters were measured using corresponding commercial kits. The mRNA and protein expression of FoxO3a, Wnt, and β-catenin was detected using qRT-PCR and western blotting.

Results

The BMD and ultimate load of PMO rats were increased after treatment with YSZG. YSZG treatment promoted the bone trabeculae formation of PMO rats. YSZG treatment also induced bone differentiation and suppress oxidative stress in PMO rats, evidenced by the increased BALP, Runx2, OPG, SOD, and CAT levels, as well as the decreased TRACP 5b, RANKL, ROS, and MDA levels. Additionally, YSZG treatment downregulated the FoxO3a expression and upregulated the levels of Wnt and β-catenin in PMO rats. Caltrate D600 addition showed an auxiliary effect for YSZG.

Conclusion

YSZG decoction exerts the antiosteoporotic effect on PMO by restraining the FoxO3a expression and activating the Wnt/β-catenin pathway, which has an impressive synergistic effect with Caltrate D600.

Zhuang-Gu-Fang Treats Osteoporosis in Ovariectomized Rats by Increasing the Osteogenesis-Related Factors Leptin, Ghrelin, and PYY

Zhuang-Gu-Fang is a Chinese medicinal compound mixture, which is mainly composed of traditional remedies like the Epimedium Herb, Astragalus, and Eucommia among many others. The study is aimed at investigating the therapeutic effect of Zhuang-Gu-Fang in ovariectomized rats. Fifty six-month-old Wistar rats were randomly selected and divided into 5 groups (n = 10), namely, model group, positive group, low-dose Chinese medicine group, medium-dose group, and high-dose group. Another 10 sham operation Wistar rats were taken as a negative control group. After 3 months of intervention, the bone mineral density (BMD), procollagen type I N-peptide (PINP), beta C-terminal cross-linked telopeptides of type I collagen carboxyl-terminal peptide (β-CTX), Leptin, Ghrelin, and Peptide YY (PYY) of each group were measured. Besides, the ultrastructure of bone structure and osteoblasts was also observed by transmission electron microscopy. Western blot method was used to detect the expression levels of Leptin and Ghrelin in bone tissue, and RT-PCR detected the mRNA expression levels of Leptin and Ghrelin. BMD test indicated that Zhuang-Gu-Fang could effectively prevent the loss of tibia bone in ovariectomized rats. Histomorphology analysis showed that Zhuang-Gu-Fang could preserve trabecular bone structure integrity and improve osteoblast ultrastructure. Notably, the study found out that Zhuang-Gu-Fang worked through balancing the bone metabolism via increasing bone formation/resorption ratio. Additionally, Zhuang-Gu-Fang highlighted the recovery effects in multiple levels of osteogenesis- and osteanagenesis-related factors Leptin, Ghrelin, and PYY. Conclusively, the study proved the therapeutic potential of the Zhuang-Gu-Fang for postmenopausal osteoporosis (PMOP) and further revealed that its therapeutic effect was related to the balance of bone metabolism and the recovery effects of bone-related factors Leptin, Ghrelin, and PYY.

Effects of Injected Antiosteoporotic Medication Versus Oral Bisphosphonates on Rates of Repeated Vertebroplasty or Kyphoplasty

PURPOSE

Vertebral compression fractures can recur within a few years after percutaneous vertebroplasty (PVP) or kyphoplasty.

METHODS

We conducted a population-based study using data from the Taiwan National Health Insurance Research Database to investigate the efficacy of various antiosteoporotic treatments in reducing the prevalence rate of repeated PVP or kyphoplasty in patients hospitalized from January 1, 1997, to December 31, 2004. We included patients with vertebral compression fractures after PVP or kyphoplasty who received oral bisphosphonates (OB group; n = 6141) or injected drug therapies (injection group; n = 4308).

FINDINGS

The incidence rate of repeated PVP or kyphoplasty was significantly lower in the I/Z/D (denosumab monotherapy or ibandronate or zoledronate with or without denosumab) group than in the OB group (crude subdistribution hazard ratio [sHR], 0.79; 95% CI, 0.70-0.90; P < 0.05; adjusted sHR, 0.77; 95% CI, 0.68-0.87; P < 0.0001). The analysis revealed a significantly lower incidence rate of repeated PVP or kyphoplasty in the I/Z group compared with that in the OB group (crude sHR, 0.82; 95% CI, 0.72-0.94; P = 0.0038; adjusted sHR, 0.80; 95% CI, 0.70-0.91; P = 0.0011). The denosumab group also exhibited a significantly lower incidence rate of repeated PVP or kyphoplasty than did the OB group (crude sHR, 0.61; 95% CI, 0.46-0.80; P = 0.0005; adjusted sHR, 0.58; 95% CI, 0.44-0.77; P = 0.0001). Although the teriparatide group had higher fracture frequency than did the OB group, the analysis revealed no significant difference between the OB and teriparatide groups with respect to the incidence rate of repeated PVP or kyphoplasty (adjusted sHR, 1.08; 95% CI, 0.92-1.26; P = 0. 3747).

IMPLICATIONS

Injected antiosteoporotic medication was associated with lower rates of repeated vertebroplasty and kyphoplasty than was OB application.

Clinical Application of Teriparatide in Fracture Prevention: A Systematic Review

BACKGROUND

Teriparatide, a 1-34 fragment of parathyroid hormone (PTH) that maintains most of the biological activities of PTH, has been employed since 2002 as an anabolic agent for osteoporotic individuals who are at high risk of fracture. The purpose of the present review is to provide a systematic summary and timely update on treatment with teriparatide for fracture prevention.

METHODS

Electronic databases, including OVID MEDLINE, OVID Embase, and the Cochrane Library, were searched on February 9, 2018, to identify published systematic reviews and meta-analyses addressing treatment with teriparatide for fracture prevention, and A Measurement Tool to Assess Systematic Reviews 2 (AMSTAR 2) was used to assess the quality of included studies.

RESULTS

Seventeen studies were included. Of the 17 eligible studies, 3 were rated as high quality, 3 were rated as moderate quality, 6 were rated as low quality, and 5 were rated as critically low quality. Teriparatide reduced vertebral and overall nonvertebral fractures in osteoporotic patients regardless of the existence of precipitating conditions, including postmenopausal status, glucocorticoid treatment, and chronic kidney disease, as compared with placebo, but not the site-specific nonvertebral fractures of the wrist and hip. Teriparatide did not more effectively reduce fracture risks when compared with other medications, such as bisphosphonates, selective estrogen receptor modulators, RANKL (receptor activator of nuclear factor kappa-beta ligand) inhibitor, or strontium ranelate.

CONCLUSIONS

Teriparatide was safe and was not associated with an increased rate of adverse events when compared with other drugs. Teriparatide was effective for the prevention of vertebral and overall nonvertebral fractures in osteoporotic patients but not for the prevention of site-specific nonvertebral fractures at the wrist and hip.

LEVEL OF EVIDENCE

Therapeutic Level I. See Instructions for Authors for a complete description of levels of evidence.

The efficacy and safety of vertebral fracture prevention therapies in post-menopausal osteoporosis treatment: Which therapies work best? a network meta-analysis

Objectives

Osteoporosis has become an increasing concern for older people as it may potentially lead to osteoporotic fractures. This study is designed to assess the efficacy and safety of ten therapies for post-menopausal women using network meta-analysis.

Methods

We conducted a systematic search in several databases, including PubMed and Embase. A random-effects model was employed and results were assessed by the odds ratio (OR) and corresponding 95% confidence intervals (CI). Furthermore, with respect to each outcome, each intervention was ranked according to the surface under the cumulative ranking curve (SUCRA) value.

Results

With respect to preventing new vertebral fractures (NVF), all ten drugs outperformed placebo, and etidronate proved to be the most effective treatment (OR 0.24, 95% CI 0.14 to 0.39). In addition, zoledronic acid and parathyroid hormone ranked higher compared with the other drugs. With respect to preventing clinical vertebral fractures (CVF), zoledronic acid proved to be the most effective drug (OR = 0.25, 95% CI 0.08 to 0.92), with denosumab as a desirable second option (OR = 0.48, 95% CI 0.22 to 0.96), when both were compared with placebo. As for adverse events (AE) and severe adverse events (SAE), no significant difference was observed. According to SUCRA, etidronate ranked first in preventing CVF; parathyroid hormone and zoledronic acid ranked highly in preventing NVF and CVF. Raloxifene was safe with a high rank in preventing AEs and SAEs though performed unsatisfactorily in efficacy.

Conclusions

This study suggests that, taking efficacy and safety into account, parathyroid hormone and zoledronic acid had the highest probability of satisfactory performance in preventing osteoporotic fractures.

Cite this article: G. Wang, L. Sui, P. Gai, G. Li, X. Qi, X. Jiang. The efficacy and safety of vertebral fracture prevention therapies in post-menopausal osteoporosis treatment: Which therapies work best? a network meta-analysis. Bone Joint Res 2017;6:452–463. DOI: 10.1302/2046-3758.67.BJR-2016-0292.R1.

Selection of animal models for pre-clinical strategies in evaluating the fracture healing, bone graft substitutes and bone tissue regeneration and engineering

In vitro assays can be useful in determining biological mechanism and optimizing scaffold parameters, however translation of the in vitro results to clinics is generally hard. Animal experimentation is a better approximation than in vitro tests, and usage of animal models is often essential in extrapolating the experimental results and translating the information in a human clinical setting. In addition, usage of animal models to study fracture healing is useful to answer questions related to the most effective method to treat humans. There are several factors that should be considered when selecting an animal model. These include availability of the animal, cost, ease of handling and care, size of the animal, acceptability to society, resistance to surgery, infection and disease, biological properties analogous to humans, bone structure and composition, as well as bone modeling and remodeling characteristics. Animal experiments on bone healing have been conducted on small and large animals, including mice, rats, rabbits, dogs, pigs, goats and sheep. This review also describes the molecular events during various steps of fracture healing and explains different means of fracture healing evaluation including biomechanical, histopathological and radiological assessments.

Recombinant human bone morphogenetic protein-type 2 (rhBMP-2) enhances local bone formation in the lumbar spine of osteoporotic sheep

The failure of orthopedic implants in osteoporotic patients is attributed to the lack of sufficient bone stock and regenerative capacity but most treatments for osteoporosis fail to address this issue. rhBMP-2 is known to promote bone formation under normal conditions but has not been used clinically in the osteoporotic condition. Osteoporosis was induced in 19 ewes using ovariectomy, low calcium diet, and steroid injection. After induction, the steroid was withdrawn and pellets containing inert carrier with rhBMP-2 in either slow or fast-release formulation were implanted into the lumbar vertebrae of each animal. After 2, 3, and 6 months the spines were harvested and assessed for changes in BMD and histomorphometric indices. BMD did not change after cessation of steroid treatment. After 2 months BV/TV increased in the vicinity of the pellets containing the fast-release rhBMP-2 and was sustained for the duration of the study. Focal voids surrounding all implants, particularly the slow-release formulation, were observed initially but resolved with time. Increased BV/TV adjacent to rhBMP-2 pellets suggests it could be used for localized treatment of osteoporosis. Refinement of the delivery system and supplementary treatments may be necessary to overcome the initial catabolic effects of rhBMP-2.

Evaluation of the efficacy, safety and pharmacokinetic profile of oral recombinant human parathyroid hormone [rhPTH(1-31)NH(2)] in postmenopausal women with osteoporosis

CONTEXT

Treatment of osteoporosis with subcutaneous (SC) injections of rhPTH(1-34) or rhPTH(1-84) is associated with significant improvements in BMD and reductions in osteoporotic fractures. However, subcutaneous injections can be associated with discomfort and thus deteriorating compliance.

OBJECTIVE

The UGL-OR1001 trial aimed to establish the efficacy and safety parameters of a novel oral tablet formulation of rhPTH(1-31)NH(2) and matching placebo tablets and open-label teriparatide positive control in postmenopausal women with osteoporosis.

DESIGN

24 weeks of randomized, double-blind treatment with once daily doses of 5mg oral treatment or corresponding placebo, or open-label subcutaneous teriparatide.

PATIENTS OR OTHER PARTICIPANTS

Women diagnosed with postmenopausal osteoporosis as detected by lumbar spine DXA, with an exclusion of those with prior treatment with bone active agents.

INTERVENTION(S)

Orally formulated recombinant human PTH(1-31)NH(2) and placebo, or open-label subcutaneous teriparatide as a positive control.

MAIN OUTCOME MEASURE(S)

The primary endpoint was to characterize the percent change from baseline in bone mineral density (BMD) at L1-L4 axial lumbar spine after 24 weeks in the rhPTH(1-31)NH(2) arm. Secondary and exploratory endpoints included safety and tolerability of the oral formulation, measurement of biochemical markers of bone turnover, and evaluation of the PK profile at first and last dose. The study was registered at ClinicalTrials.gov with the identifier: NCT01321723.

RESULTS

The oral tablet formulation of rhPTH(1-31)NH(2) resulted in similar PK profiles at both timepoints with mean C(max) values similar to subcutaneous administration. In the rhPTH(1-31)NH(2) arm, a 2.2% increase in lumbar spine BMD was observed compared to baseline (p<0.001), while no change was observed in the placebo arm. Open-label teriparatide resulted in a 5.1% increase in LS BMD (p<0.001). In the oral PTH study arm, the bone formation marker osteocalcin was increased by 32%, 21% and 23% at Weeks 4, 12 and 24, respectively. There was no significant increase in the level of the bone resorption marker CTx-1.

CONCLUSIONS

In summary, these data demonstrate that enteric-coated oral tablet formulation technology consistently generated robust levels of exposure of rhPTH(1-31)NH(2) leading to induction of bone formation without inducing bone resorption resulting in significantly increased levels of LS BMD. Few adverse events were observed, recommending this orally delivered drug candidate for further development.

Potential role of high mobility group box protein 1 and intermittent PTH (1-34) in periodontal tissue repair following orthodontic tooth movement in rats

OBJECTIVES

Recent studies indicate that high mobility group box protein 1 (HMGB1) can be released by necrotic and damaged cells and functions as an alarmin that is recognized by the innate immune system. Little is known about the role of HMGB1 within the periodontal ligament (PDL). Therefore, we examined HMGB1 expression by PDL cells in vitro and compared the findings to an in vivo model of orthodontically induced tooth root resorption. In addition, we addressed the question of whether a potentially anabolic intermittent administration of parathyroid hormone (iPTH) would modulate the expression of HMGB1.

MATERIALS AND METHODS

In confluent PDL cell cultures, HMGB1 messenger RNA (mRNA) expression was quantified by real-time polymerase chain reaction. In a rat model comprising 25 animals, mechanical loading for 5 days was followed by administration of either iPTH (1-34) systemically or sham injections for up to 56 days. HMGB1 expression was determined by means of immunohistochemistry and histomorphometry.

RESULTS

The in vitro experiments revealed an inhibitory effect of iPTH on basal HMGB1 mRNA expression in confluent PDL cells. In vivo, the mechanical force-induced enhanced HMGB1 protein expression declined time dependently. Intermittent PTH further inhibited HMGB1 expression. The significantly higher basal HMGB1 protein expression in the former compression side was followed by a more pronounced time- and iPTH-dependent decline in the same area.

CONCLUSIONS

These data indicate a major role for HMGB1 in the regulation of PDL wound healing following mechanical load-induced tissue injury.

CLINICAL RELEVANCE

The findings point to the potential benefit of iPTH in the attempt to support these immune-associated reparative processes.

Effect of intermittent PTH(1-34) on human periodontal ligament cells transplanted into immunocompromised mice

Residual periodontal ligament (PDL) cells in the damaged tissue are considered a prerequisite for a successful regeneration of the periodontal architecture with all its components, including gingiva, PDL, cementum, and bone. Among other approaches, current concepts in tissue engineering aim at a hormonal support of the regenerative capacity of PDL cells as well as at a supplementation of lost cells for regeneration. Here, we investigated how far an anabolic, intermittent parathyroid hormone (iPTH) administration would enhance the osteoblastic differentiation of PDL cells and the cellular ability to mineralize the extracellular matrix in an in vivo transplantation model. PDL cells were predifferentiated in a standard osteogenic medium for 3 weeks before subcutaneous transplantation into CD-1 nude mice using gelatin sponges as carrier. Daily injections of 40 μg/kg body weight PTH(1-34) or an equivalent dose of vehicle for 4 weeks were followed by explantation of the specimens and an immunohistochemical analysis of the osteoblastic marker proteins alkaline phosphatase (ALP), osteopontin, and osteocalcin. Signs of biomineralization were visualized by means of alizarin red staining. For verification of the systemic effect of iPTH application, blood serum levels of osteocalcin were determined. The osteogenic medium stimulated the expression of ALP and PTH1-receptor mRNA in the cultures. After transplantation, iPTH resulted in an increased cytoplasmic and extracellular immunoreactivity for all markers investigated. In contrast to only sporadic areas of mineralization under control conditions, several foci of mineralization were observed in the iPTH group. Blood serum levels of osteocalcin were elevated significantly with iPTH. These data indicate that the osteoblastic differentiation of human PDL cells and their ability for biomineralization can be positively influenced by iPTH in vivo. These findings hold out a promising prospect for the support of periodontal regeneration.

Teriparatide - Indications beyond osteoporosis

Osteoporosis is a condition of impaired bone strength that results in an increased risk of fracture. The current and most popular pharmacological options for the treatment of osteoporosis include antiresorptive therapy, in particular, oral bisphosphonates (alendronate, risedronate, ibandronate). Anabolic agents like teriparatide have widened our therapeutic options. They act by directly stimulating bone formation and improving bone mass quantity and quality. Two forms of recombinant human parathyroid hormone (PTH) are available : full-length PTH (PTH 1-84; approved in the EU only) and the 1-34 N-terminal active fragment of PTH (teriparatide, US FDA approved). This review aims to discuss the benefits of teriparatide beyond the currently licensed indications like fracture healing, dental stability, osteonecrosis of jaw, hypoparathyroidism, and hypocalcemia.

Receptor oligomerization in family B1 of G-protein-coupled receptors: focus on BRET investigations and the link between GPCR oligomerization and binding cooperativity

The superfamily of the seven transmembrane G-protein-coupled receptors (7TM/GPCRs) is the largest family of membrane-associated receptors. GPCRs are involved in the pathophysiology of numerous human diseases, and they constitute an estimated 30-40% of all drug targets. During the last two decades, GPCR oligomerization has been extensively studied using methods like bioluminescence resonance energy transfer (BRET) and today, receptor-receptor interactions within the GPCR superfamily is a well-established phenomenon. Evidence of the impact of GPCR oligomerization on, e.g., ligand binding, receptor expression, and signal transduction indicates the physiological and pharmacological importance of these receptor interactions. In contrast to the larger and more thoroughly studied GPCR subfamilies A and C, the B1 subfamily is small and comprises only 15 members, including, e.g., the secretin receptor, the glucagon receptor, and the receptors for parathyroid hormone (PTHR1 and PTHR2). The dysregulation of several family B1 receptors is involved in diseases, such as diabetes, chronic inflammation, and osteoporosis which underlines the pathophysiological importance of this GPCR subfamily. In spite of this, investigation of family B1 receptor oligomerization and especially its pharmacological importance is still at an early stage. Even though GPCR oligomerization is a well-established phenomenon, there is a need for more investigations providing a direct link between these interactions and receptor functionality in family B1 GPCRs. One example of the functional effects of GPCR oligomerization is the facilitation of allosterism including cooperativity in ligand binding to GPCRs. Here, we review the currently available data on family B1 GPCR homo- and heteromerization, mainly based on BRET investigations. Furthermore, we cover the functional influence of oligomerization on ligand binding as well as the link between oligomerization and binding cooperativity.

Osteoporotic fracture and parathyroid hormone

Osteoporosis and age-related bone loss is associated with changes in bone remodeling characterized by decreased bone formation relative to bone resorption, resulting in bone fragility and increased risk of fractures. Stimulating the function of bone-forming osteoblasts, is the preferred pharmacological intervention for osteoporosis. Recombinant parathyroid hormone (PTH), PTH(1-34), is an anabolic agent with proven benefits to bone strength and has been characterized as a potential therapy for skeletal repair. In spite of PTH's clinical use, safety is a major consideration for long-term treatment. Studies have demonstrated that intermittent PTH treatment enhances and accelerates the skeletal repair process via a number of mechanisms. Recent research into the molecular mechanism of PTH action on bone tissue has led to the development of PTH analogs to control osteoporotic fractures. This review summarizes a number of advances made in the field of PTH and bone fracture to combat these injuries in humans and in animal models. The ultimate goal of providing an alternative to PTH, currently the sole anabolic therapy in clinical use, to promote bone formation and improve bone strength in the aging population is yet to be achieved.

Parathyroid hormone analogues in the treatment of osteoporosis

Osteoporosis is characterized by the occurrence of fragility fractures. Over the past years, various treatment options have become available, mostly antiresorptive agents such as bisphosphonates. However, antiresorptive therapy cannot restore bone mass and structure that has been lost due to increased remodeling. In this case, recombinant human parathyroid hormone (PTH) analogues-the full-length PTH(1-84) or the shortened molecule PTH(1-34), which is also known as teriparatide-present the possibility of increasing the formation of new bone substance by virtue of their anabolic effects. The bone formation induced by PTH analogues not only increases BMD or bone mass but also improves the microarchitecture of the skeleton, thereby leading to improved strength of bone and increased mechanical resistance. Controlled trials have shown that both analogues significantly reduce the incidence of vertebral fractures, and PTH(1-34) also reduces the risk of nonvertebral fractures. The need for daily self-injection and the higher cost compared with other forms of treatment limit the widespread use of PTH analogues. Nevertheless, treatment with PTH analogues should be considered in postmenopausal women and men with severe osteoporosis, as well as in patients on established glucocorticoid treatment with a high fracture risk. Concurrent therapy with antiresorptive agents should be avoided, but sequential therapy with these agents might consolidate the beneficial effects on the skeleton.

Bone regeneration: current concepts and future directions

Bone regeneration is a complex, well-orchestrated physiological process of bone formation, which can be seen during normal fracture healing, and is involved in continuous remodelling throughout adult life. However, there are complex clinical conditions in which bone regeneration is required in large quantity, such as for skeletal reconstruction of large bone defects created by trauma, infection, tumour resection and skeletal abnormalities, or cases in which the regenerative process is compromised, including avascular necrosis, atrophic non-unions and osteoporosis. Currently, there is a plethora of different strategies to augment the impaired or 'insufficient' bone-regeneration process, including the 'gold standard' autologous bone graft, free fibula vascularised graft, allograft implantation, and use of growth factors, osteoconductive scaffolds, osteoprogenitor cells and distraction osteogenesis. Improved 'local' strategies in terms of tissue engineering and gene therapy, or even 'systemic' enhancement of bone repair, are under intense investigation, in an effort to overcome the limitations of the current methods, to produce bone-graft substitutes with biomechanical properties that are as identical to normal bone as possible, to accelerate the overall regeneration process, or even to address systemic conditions, such as skeletal disorders and osteoporosis.

Parathyroid hormone regulates osteoblast differentiation in a Wnt/β-catenin-dependent manner

Intermittent parathyroid hormone (PTH) administration shows an anabolic effect on bone. However, the mechanisms are not fully studied. Recent studies suggest that Wnt signaling is involved in PTH-induced bone formation. The current study was to examine if Wnt/β-catenin pathway is required during PTH-induced osteoblast differentiation. Osteoblastic MC3T3-E1 cells were treated with human PTH (1-34) (hPTH [1-34]) and expression levels of osteoblast differentiation markers were detected by real-time PCR. RNA levels of β-catenin, Runx2, Osteocalcin, Alkaline phosphatase, and Bone sialoprotein were significantly up-regulated after treatment with 10(-8) M of hPTH (1-34) for 6 h. Alkaline phosphatase activity and protein expression of β-catenin were also increased after 6 days of intermittent treatment with hPTH (1-34) in MC3T3-E1 cells. hPTH (1-34) significantly enhanced Topflash Luciferase activity after 6 h of treatment. More important, PTH-induced Alkaline phosphatase activity was significantly inhibited by knocking down β-catenin expression in cells using siRNA. Real-time RT-PCR results further showed down regulation of Runx2, Osteocalcin, Alkaline phosphatase, Bone sialoprotein gene expression in β-catenin siRNA transfected cells with/without PTH treatment. These results clearly indicate that PTH stimulates Wnt/β-catenin pathway in MC3T3-E1 cells and osteoblast differentiation markers expression was up-regulated by activation of Wnt/β-catenin signaling. Our study demonstrated that PTH-induced osteoblast differentiation mainly through activation of Wnt/β-catenin pathway in osteoblastic MC3T3-E1 cells.