Stimulate Bone Resorption Research Articles

<b>Jagged1 Stimulates Bone Resorption during Orthodontic Tooth Movement </b>

<b>Jagged1 Stimulates Bone Resorption during Orthodontic Tooth Movement </b>

Erythropoietin Stimulates Bone Resorption Via Direct Activation of the Monocytic Lineage and Via Increased RANKL Production By B Cells and Osteoblasts

The negative effect of hypoxia on bone metabolism is well established but its mechanism of action is not fully understood. Hypoxia triggers the production of erythropoietin (EPO), a hormone most recognized for its hematopoietic function. An increasing number of roles unrelated to red blood cell production have been attributed to EPO, many of which are mediated by non-erythroid cells. In light of the controversy on the effect of increased serum levels of EPO on the skeleton, we investigated here the effect of the hormone on bone metabolism using EPO overexpressing (Tg6), and EPO-administered adult mice. In these models, the increase in EPO levels is similar to its physiologic increase at high altitude without the confounding direct effect of hypoxia on bone cells. Using microcomputed tomography, histology and serum markers we found that high EPO levels result in a severe trabecular bone loss (-32 to -61% decrease in bone density in Tg6 and EPO-injected animals, respectively; p<0.05 versus their respective controls throughout), due to increased bone resorption (+28 to +18% in TRAP5b serum levels) and reduced bone formation rate (-19 to -74%). This bone loss consisted of reduced trabecular number, but not thickness, with no effect on the cortical bone compartment. A similar bone response was observed with high and low doses of EPO, with no difference between intermittent and continuous administration modes. Using flow cytometry analysis and specific cell surface markers, we found that EPO (both overexpression and injection) targets the monocytic lineage by increasing the number of CD115+CD265- monocytes/macrophages by 43 and 57% (p<0.05), CD115+ CD265+ pre-osteoclasts by 2 and 4-fold (p=0.02) and mature osteoclasts (TRAP positive) by 64 and 88% (p<0.05, Figure 1A) in bones, compared to WT and diluent controls, respectively.EPO has direct stimulatory effects in vitro on both macrophages and preosteoclasts thus coupling immune and skeletal systems, Activation of purified BM-derived macrophages with EPO, led to a 33% increase (p=0.01) in phagocytosis of 5T33 multiple myeloma cells. EPO strongly stimulated osteoclastogenesis (TRAP staining, Figure 1B) and pit resorption (measured in calcium-phosphate-coated wells) in a cell-autonomous manner. Furthermore, our data indicate that EPO receptor (EPO-R) signaling in osteoclast precursors involves the Jak2 and PI3K pathways, but is independent of the MAPK/MEK pathway. In addition to the direct effect of EPO on monocyte derived cells, high EPO also led to a 1.6 fold (p=0.03) increase in the transcript levels of the osteoclastogenic cytokine RANKL in total bone marrow. Notably, the major sources of RANKL are B cells and osteoblasts, and our data indicate that both cell types express EPO-R, suggesting that they are direct targets of EPO. Accordingly we found an EPO-related increase in membrane bound RANKL on B cells (1.8-fold, p=0.0003) and osteoblasts (1.16-fold, p=0.001) isolated from bone marrow (Figure 1C). To test whether EPO-R on osteoblasts also mediates the attenuation of bone formation, we treated isolated osteoblasts with EPO. However, EPO treatment in vitro did not inhibit osteoblast differentiation and activity, as demonstrated by RT-qPCR of marker genes and mineralization assays.Taken together, these findings demonstrate that EPO strongly regulates bone mass by stimulating osteoclastogenesis and attenuating bone formation. The osteoclastogenic action of EPO is mediated both directly by EPO-R on the monocytic lineage and indirectly via EPO-R signaling on B cells and osteoblasts. Regarding bone formation, the negative effect of EPO on bone formation rate in vivo was not reproduced in isolated cells, thus implying the involvement of cells beyond the osteoblastic lineage. We also propose a new mechanism for hypoxia-induced bone loss that involves EPO action on monocytic, B cell and osteoblastic lineages. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Osteoporosis increases subsequent risk of gallstone: a nationwide population-based cohort study in Taiwan.

BackgroundOsteopontin (OPN) is a pro-inflammatory cytokine which is expressed in various tissues. It participates in the bone remodeling process and stimulates bone resorption by osteoclasts. It is also a core protein of cholesterol gallstones. We hypothesized osteoporotic patients might have higher risk in developing gallstones and conducted a population-based study to examine the risk of developing gallstone in osteoporotic patients in Taiwan.MethodsA total of 1,638 patients diagnosed with osteoporosis between 2003 and 2005 were identified in the National Health Insurance Research Database. A comparison cohort without osteoporosis (n =6,552) was randomly matched to each osteoporosis patient at a ratio of 4: 1 based on age and sex. A Cox proportional-hazards regression analysis was performed to evaluate the 5-year gallstone-free survival rates for the 2 cohorts.ResultsDuring the 5-year follow-up period, 114 and 311 cases of gallstone occurred in the osteoporosis and comparison cohorts, respectively. After adjusting for the confounders, the Cox regression analysis of the risk of gallstone in the osteoporosis and comparison cohorts yielded a hazard ratio of 1.35 (95% confidence interval: 1.07 - 1.69; p < 0 .01).ConclusionPatients with osteoporosis in Taiwan have a higher risk of developing gallstone than the general population.

Myeloma bone disease: pathogenesis, current treatments and future targets.

Patients with myeloma develop localized and generalized bone loss leading to hypercalcaemia, accelerated osteoporosis, vertebral wedge fractures, other pathological fractures, spinal cord compression and bone pain. Bone loss is mediated by a variety of biological modifiers including osteoclast-activating factors (OAF) and osteoblast (OB) inhibitory factors produced either directly by malignant plasma cells (MPCs) or as a consequence of their interaction with the bone marrow microenvironment (BMM). Raised levels of OAFs such as receptor activator of nuclear factor-kappa B ligand (RANKL), macrophage inflammatory protein 1 alpha, tumour necrosis factor-alpha and interleukin 6 stimulate bone resorption by recruiting additional osteoclasts. Via opposing mechanisms, increases in OB inhibitory factors, such as dickkopf-1 (Dkk-1), soluble frizzled-related protein-3 and hepatocyte growth factor (HGF), suppress bone formation by inhibiting the differentiation and recruitment of OBs. These changes result in an uncoupling of physiological bone remodelling, leading to myeloma bone disease (MBD). Moreover, the altered BMM provides a fertile ground for the growth and survival of MPCs. Current clinical management of MBD is both reactive (to pain and fractures) and preventive, with bisphosphonates (BPs) being the mainstay of pharmacological treatment. However, side effects and uncertainties associated with BPs warrant the search for more targeted treatments for MBD. This review will summarize recent developments in understanding the intimate relationship between MBD and the BMM and the novel ways in which they are being therapeutically targeted. All data included were sourced and referenced from PubMed. The clinical utility of BP therapy is well established. However, there is general acknowledgement that BPs are only partially successful in the treatment of MBD. The number of skeletal events attributable to myeloma are reduced by BPs but not totally eliminated. Furthermore, existing damage is not repaired. It is widely recognized that more effective treatments are needed. There remains controversy concerning the duration of BP therapy. Whether denosumab is a viable alternative to BP therapy is also contested. Many of the new therapeutic strategies discussed are yet to translate to clinical practice and demonstrate equal efficacy or superiority to BP therapy. It also remains controversial whether reported anti-tumour effects of bone-modulating therapies are clinically significant. The potential clinical utility of bone anabolic therapies including agents such as anti-Dkk-1, anti-sclerostin and anti-HGF is becoming increasingly recognized. Further research effectively targeting the mediators of MBD, targeting both bone resorption and bone formation, is urgently needed. This should translate promptly to clinical trials of combination therapy comprising anti-resorptives and bone anabolic therapies to demonstrate efficacy and improved outcomes over BPs.

Cimetidine Reduces Alveolar Bone Loss in Induced Periodontitis in Rat Molars

There is evidence that histamine released during inflammation plays a role in bone metabolism via the H2 receptor, stimulating bone resorption. The purpose of this study is to evaluate whether cimetidine, a histamine H2-receptor antagonist, interferes with the initiation and progression of induced periodontal disease in rat molars. Forty male rats received 100 mg/kg body weight of cimetidine (cimetidine group [CimG]) or saline solution (sham group [SG]). Periodontal disease was induced in the maxillary left first molars (PDSG and PDCimG); maxillary right molars were used as non-ligature controls. After 7, 15, 30, and 50 days, maxillary fragments were embedded in paraffin. The sections were stained with Masson trichrome and hematoxylin and eosin and subjected to the tartrate-resistant acid phosphatase (TRAP) method. The distances between the cemento-enamel junction (CEJ) and alveolar process (AP) crest, as well as between the CEJ and junctional epithelium (JE) level, were measured; the number of inflammatory cells was computed. Receptor activator of nuclear factor kappa B ligand (RANKL) and osteoprotegerin (OPG) immunohistochemistry was carried out, and the RANKL/OPG ratio was calculated. In PDSG and PDCimG, a significant increase (P ≤0.05) was observed in CEJ-AP and CEJ-JE distances. However, the increases in both distances were significantly less in PDCimG compared with PDSG at 15, 30, and 50 days. Numerous TRAP-positive osteoclasts were found in the PDSG and PDCimG. In PDCimG, the volume density of inflammatory cells and the RANKL/OPG ratio were significantly lower (P ≤0.05) than in PDSG. Cimetidine exerts a beneficial effect on periodontal disease in rats, decreasing the RANKL/OPG ratio in gingival connective tissue and reducing alveolar bone resorption.

Instructions for producing a mouse model of glucocorticoid-induced osteoporosis.

Glucocorticoids are effective drugs used for the treatment of inflammatory diseases such as rheumatoid arthritis or asthma. Furthermore, they regulate various physiological processes, including bone remodeling. However, long-term high- and even low-dose glucocorticoid use is associated with a compromised bone quality and an increased fracture risk. At the cellular level, glucocorticoids suppress bone formation and stimulate bone resorption, which leads to loss of bone mass. To investigate the underlying mechanisms and new therapeutic strategies, the in vivo model for glucocorticoid-induced bone loss is widely used. This protocol outlines the common procedure that is currently used for the induction of bone loss in mice using glucocorticoids. It further provides useful hints and highlights possible pitfalls to take into account before starting an experiment.

Osteocyte-derived RANKL is a critical mediator of the increased bone resorption caused by dietary calcium deficiency

Parathyroid hormone (PTH) excess stimulates bone resorption. This effect is associated with increased expression of the osteoclastogenic cytokine receptor activator of nuclear factor κB ligand (RANKL) in bone. However, several different cell types, including bone marrow stromal cells, osteocytes, and T lymphocytes, express both RANKL and the PTH receptor and it is unclear whether RANKL expression by any of these cell types is required for PTH-induced bone loss. Here we have used mice lacking the RANKL gene in osteocytes to determine whether RANKL produced by this cell type is required for the bone loss caused by secondary hyperparathyroidism induced by dietary calcium deficiency in adult mice. Thirty days of dietary calcium deficiency caused bone loss in control mice, but this effect was blunted in mice lacking RANKL in osteocytes. The increase in RANKL expression in bone and the increase in osteoclast number caused by dietary calcium deficiency were also blunted in mice lacking RANKL in osteocytes. These results demonstrate that RANKL produced by osteocytes contributes to the increased bone resorption and the bone loss caused by secondary hyperparathyroidism, strengthening the evidence that osteocytes are an important target cell for hormonal control of bone remodeling.

AB0156 Increased Soluble Semaphorin 4D in Rheumatoid Arthritis

BackgroundRheumatoid arthritis (RA) is an autoimmune and inflammatory disorder of joints, which characterized with periarticular bone resorption. Recently cross-disciplinary field osteoimmunology was established because of reciprocal interactions between the immune...

Parathyroid hormone receptor signaling induces bone resorption in the adult skeleton by directly regulating the RANKL gene in osteocytes.

PTH upregulates the expression of the receptor activator of nuclear factor κB ligand (Rankl) in cells of the osteoblastic lineage, but the precise differentiation stage of the PTH target cell responsible for RANKL-mediated stimulation of bone resorption remains undefined. We report that constitutive activation of PTH receptor signaling only in osteocytes in transgenic mice (DMP1-caPTHR1) was sufficient to increase Rankl expression and bone resorption. Resorption in DMP1-caPTHR1 mice crossed with mice lacking the distal control region regulated by PTH in the Rankl gene (DCR(-/-)) was similar to DMP1-caPTHR1 mice at 1 month of age, but progressively declined to reach values undistinguishable from wild-type (WT) mice at 5 months of age. Moreover, DMP1-caPTHR1 mice exhibited low tissue material density and increased serum alkaline phosphatase activity at 5 month of age, and these indices of high remodeling were partially and totally corrected in compound DMP1-caPTHR1;DCR(-/-) male mice, and less affected in female mice. Rankl expression in bones from DMP1-caPTHR1 mice was elevated at both 1 and 5 months of age, whereas it was high, similar to DMP1-caPTHR1 mice at 1 month, but low, similar to WT levels at 5 months in compound mice. Moreover, PTH increased Rankl and decreased Sost and Opg expression in ex vivo bone organ cultures established from WT mice, but only regulated Sost and Opg expression in cultures from DCR(-/-) mice. PTH also increased RANKL expression in osteocyte-containing primary cultures of calvarial cells, in isolated murine osteocytes, and in WT but not in DCR(-/-) osteocyte-enriched bones. Thus, PTH upregulates Rankl expression in osteocytes in vitro, ex vivo and in vivo, and resorption induced by PTH receptor signaling in the adult skeleton requires direct regulation of the Rankl gene in osteocytes.

Vitamin D activities for health outcomes.

Reports describing significant health risks due to inadequate vitamin D status continue to generate considerable interest amongst the medical and lay communities alike. Recent research on the various molecular activities of the vitamin D system, including the nuclear vitamin D receptor and other receptors for 1,25-dihydroxyvitamin D and vitamin D metabolism, provides evidence that the vitamin D system carries out biological activities across a wide range of tissues similar to other nuclear receptor hormones. This knowledge provides physiological plausibility of the various health benefits claimed to be provided by vitamin D and supports the proposals for conducting clinical trials. The vitamin D system plays critical roles in the maintenance of plasma calcium and phosphate and bone mineral homeostasis. Recent evidence confirms that plasma calcium homeostasis is the critical factor modulating vitamin D activity. Vitamin D activities in the skeleton include stimulation or inhibition of bone resorption and inhibition or stimulation of bone formation. The three major bone cell types, which are osteoblasts, osteocytes and osteoclasts, can all respond to vitamin D via the classical nuclear vitamin D receptor and metabolize 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D to activate the vitamin D receptor and modulate gene expression. Dietary calcium intake interacts with vitamin D metabolism at both the renal and bone tissue levels to direct either a catabolic action on the bone through the endocrine system when calcium intake is inadequate or an anabolic action through a bone autocrine or paracrine system when calcium intake is sufficient.

Vitamin D endocrine system and osteoclasts

Vitamin D was discovered as an anti-rachitic agent preventing a failure in bone mineralization, but it is now established that the active form of vitamin D3 (1α,25(OH)2D3) induces bone resorption. Discovery of the receptor activator of nuclear factor -κB ligand (RANKL) uncovered the molecular mechanism by which 1α,25(OH)2D3 stimulates bone resorption. Treating osteoblastic cells with 1α,25(OH)2D3 stimulates RANKL expression, which in turn induces osteoclastogenesis. Nevertheless, active vitamin D compounds such as calcitriol (1α,25(OH)2D3), alfacalcidol (1α(OH)D3) and eldecalcitol (1α,25-dihydroxy-2β-(3-hydroxypropoxy) vitamin D3) have been used as therapeutic drugs for osteoporosis, as they increase bone mineral density (BMD) in osteoporotic patients. Paradoxically, the increase in BMD is caused by the suppression of bone resorption. Several studies have been performed to elucidate the mechanism by which active vitamin D compounds suppress bone resorption in vivo. Our study showed that daily administration of eldecalcitol to mice suppressed neither the number of osteoclast precursors in the bone marrow nor the number of osteoclasts formed in ex vivo cultures. Eldecalcitol administration suppressed RANKL expression in osteoblasts. This review discusses how the difference between in vitro and in vivo effects of active vitamin D compounds on bone resorption is induced.

Obesity and fracture risk.

Obesity and osteoporosis are two common diseases with an increasing prevalence and a high impact on morbidity and mortality. Obese women have always been considered protected against osteoporosis and osteoporotic fractures. However, several recent studies have challenged the widespread belief that obesity is protective against fracture and have suggested that obesity is a risk factor for certain fractures. Fat and bone are linked by many pathways, which ultimately serve the function of providing a skeleton appropriate to the mass of adipose tissue it is carrying. Leptin, adiponectin, adipocytic estrogens and insulin/amylin are involved in this connection. However, excessive body fat, and particularly abdominal fat, produces inflammatory cytokines which may stimulate bone resorption and reduce bone strength. This review aimed to examine the literature data on the relationships of BMI and fat mass with factures in adult and elderly subjects. Even though the more recent studies have shown conflicting results, there is growing evidence that obesity, and particularly severe obesity, may be related to an increased risk of fracture at different skeletal sites which is partially independent from BMD. Moreover, the relationship between obesity and fracture appears to be markedly influenced by ethnicity, gender and fat distribution. Even though the incidence and the pathogenesis of fracture in obese individuals has not yet been clearly defined, the growing evidence that obesity may be related to an increased risk of fracture has important public health implications and emphasizes the need to develop effective strategies to reduce fracture risk in obese subjects.

Role of nutritional factor menaquinone-7 in bone homeostasis and osteoporosis prevention

Bone homeostasis is mainly regulated by osteoblasts, osteocytes and osteoclasts. Osteoblasts promote bone formation and osteoclasts stimulate bone resorption. Bone loss with aging is due to decreased osteoblastic bone formation and increased osteoclastic bone resorption. Bone loss with various pathophysiologic states leads to osteoporosis. Nutrition and functional food factors may play a role in the prevention of bone loss. Menaquinone-7 (MK-7), a kind of vitamin K 2 , has been shown to reveal stimulatory effects on osteoblastic bone formation and suppressive effects on osteoclastic bone resorption in vitro. MK-7 stimulates protein synthesis (including osteocalcin and other protein molecules) in osteoblastic cells. MK-7 regulates the gene expression of various proteins, which are related to cellular functions in osteblastic and osteoclastic cells. MK-7 suppresses activation of NF-κB signaling pathways in osteoblasts and osteoclasts. Moreover, dietary intake of MK-7 has been shown to reveal potential preventive effects on bone loss in osteoporosis animal models and postmenopausal women. This review will discuss the role of dietary MK-7 in the prevention of osteoporosis.

Effect of chronic metabolic acidosis on bone density and bone architecture in vivo in rats

Chronic metabolic acidosis (CMA) might result in a decrease in vivo in bone mass based on its reported in vitro inhibition of bone mineralization, bone formation, or stimulation of bone resorption, but such data, in the absence of other disorders, have not been reported. CMA also results in negative nitrogen balance, which might decrease skeletal muscle mass. This study analyzed the net in vivo effects of CMA's cellular and physicochemical processes on bone turnover, trabecular and cortical bone density, and bone microarchitecture using both peripheral quantitative computed tomography and μCT. CMA induced by NH4Cl administration (15 mEq/kg body wt/day) in intact and ovariectomized (OVX) rats resulted in stable CMA (mean Δ[HCO3(-)]p = 10 mmol/l). CMA decreased plasma osteocalcin and increased TRAP5b in intact and OVX animals. CMA decreased total volumetric bone mineral density (vBMD) after 6 and 10 wk (week 10: intact normal +2.1 ± 0.9% vs. intact acidosis -3.6 ± 1.2%, P < 0.001), an effect attributable to a decrease in cortical thickness and, thus, cortical bone mass (no significant effect on cancellous vBMD, week 10) attributed to an increase in endosteal bone resorption (nominally increased endosteal circumference). Trabecular bone volume (BV/TV) decreased significantly in both CMA groups at 6 and 10 wk, associated with a decrease in trabecular number. CMA significantly decreased muscle cross-sectional area in the proximal hindlimb at 6 and 10 wk. In conclusion, chronic metabolic acidosis induces a large decrease in cortical bone mass (a prime determinant of bone fragility) in intact and OVX rats and impairs bone microarchitecture characterized by a decrease in trabecular number.

Cervical vertebral erosion caused by bilateral vertebral artery tortuosity, predisposing to spinal, sprain: A medieval case study

Bone resorption within the cervical spine due to vertebral arterial tortuosities is rarely observed in medical practice because the condition often lacks clinical symptoms. Traumatic complications involving the vertebral arteries are relatively common and occasionally very serious, but very few affect bone, appearing only when survival has been sufficiently long for a pseudoaneurysm to form. CT scans and MRI screening, practised increasingly today following traffic and sports accidents, incidentally show that arterial tortuosities that had stimulated bone resorption are relatively frequent. Only rarely do such tortuosities cause nerve compression or trigger orthopaedic problems, while large pseudoaneurysms and congenital absence of a vertebral pedicle may require surgery to stabilize the spine. There are few publications by palaeopathologists reporting such conditions of the cervical vertebrae. This contribution reports a case of a tiered bilateral tortuosity of the vertebral artery dating from the Early Middle Ages; it provides a basis by which to recognize this type of lesion in osteoarchaeology, and it attests to the fact that multiple tortuosities may lead to spinal instability in the form of spine sprain.

Mechanism of bone mass regulation by mechanical stress

Osteocytes establish an extensive intercellular and extracellular communication system via gap junction-coupled cell processes and canaliculi, through which cell processes pass throughout bone, and the communication system is extended to osteoblasts on the bone surface. The lacunocanalicular network formed by osteocytes is thought to be an ideal mechanosensory system and suitable for mechanotransduction, by which mechanical energy is converted into electrical and/or biochemical signals. The function of osteocytes cannot be estimated based on the events caused by osteocyte death, because apoptotic osteocytes are not phagocytosed, undergo secondary necrosis, and trigger a process of repair to replace the damaged bone. The analysis of the mice, in which both intercellular and extracellular communication systems are disturbed, shows that the osteocyte network mildly inhibits bone formation and mildly stimulates bone resorption in physiological condition. In unloaded condition, the functions of the osteocyte network are augmented, and it strongly inhibits bone formation and strongly stimulates bone resorption, at least in part, through the induction of Sost in osteocytes and Rankl in osteoblasts.

Serotonin acts as a novel regulator of interleukin-6 secretion in osteocytes through the activation of the 5-HT2B receptor and the ERK1/2 signalling pathway

Interleukin-6 (IL-6) is a potent stimulator of osteoclastic bone resorption. Osteocyte secretion of IL-6 plays an important role in bone metabolism. Serotonin (5-HT) has recently been reported to regulate bone metabolism. The aim of this study was to evaluate the effect of serotonin on osteocyte expression of IL-6. The requirement for the 5-HT receptor(s) and the role of the extracellular signal-regulated kinase 1/2 (ERK1/2) in serotonin-induced IL-6 synthesis were examined. In this study, real-time PCR and ELISA were used to analyse IL-6 gene and protein expression in serotonin-stimulated MLO-Y4 cells. ERK1/2 pathway activation was determined by Western blot. We found that serotonin significantly activated the ERK1/2 pathway and induced IL-6 mRNA expression and protein synthesis in cultured MLO-Y4 cells. However, these effects were abolished by pre-treatment of MLO-Y4 cells with a 5-HT2B receptor antagonist, RS127445 or the ERK1/2 inhibitor, PD98059. Our results indicate that serotonin stimulates osteocyte secretion of IL-6 and that this effect is associated with activation of 5-HT2B receptor and the ERK1/2 pathway. These findings provide support for a role of serotonin in bone metabolism by indicating serotonin regulates bone remodelling by mediating an inflammatory cytokine.

Smoke carcinogens cause bone loss through the aryl hydrocarbon receptor and induction of Cyp1 enzymes

Smoking is a major risk factor for osteoporosis and fracture, but the mechanism through which smoke causes bone loss remains unclear. Here, we show that the smoke toxins benzo(a)pyrene (BaP) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) interact with the aryl hydrocarbon receptor (Ahr) to induce osteoclastic bone resorption through the activation of cytochrome P450 1a/1b (Cyp1) enzymes. BaP and TCDD enhanced osteoclast formation in bone marrow cell cultures and gavage with BaP stimulated bone resorption and osteoclastogenesis in vivo. The osteoclastogenesis triggered by BaP or RANK-L was reduced in Ahr(-/-) cells, consistent with the high bone mass noted in Ahr(-/-) male mice. The receptor activator of NF-κB ligand (RANK-L) also failed to induce the expression of Cyp1 enzymes in Ahr(-/-) cells. Furthermore, the osteoclastogenesis induced by TCDD was lower in Cyp1a1/1a2(-/-) and Cyp1a1/1a2/1b1(-/-) cultures, indicating that Ahr was upstream of the Cyp enzymes. Likewise, the pharmacological inhibition of the Cyp1 enzymes with tetramethylsilane or proadifen reduced osteoclastogenesis. Finally, deletion of the Cyp1a1, Cyp1a2, and Cyp1b1 in triple knockout mice resulted in reduced bone resorption and recapitulated the high bone mass phenotype of Ahr(-/-) mice. Overall, the data identify the Ahr and Cyp1 enzymes not only in the pathophysiology of smoke-induced osteoporosis, but also as potential targets for selective modulation by new therapeutics.

Zebrafish scales respond differently to in vitro dynamic and static acceleration: Analysis of interaction between osteoblasts and osteoclasts

Zebrafish scales consist of bone-forming osteoblasts, bone-resorbing osteoclasts, and calcified bone matrix. To elucidate the underlying molecular mechanism of the effects induced by dynamic and static acceleration, we investigated the scale osteoblast- and osteoclast-specific marker gene expression involving osteoblast–osteoclast communication molecules. Osteoblasts express RANKL, which binds to the osteoclast surface receptor, RANK, and stimulates bone resorption. OPG, on the other hand, is secreted by osteoblast as a decoy receptor for RANKL, prevents RANKL from binding to RANK and thus prevents bone resorption. Therefore, the RANK–RANKL–OPG pathway contributes to the regulation of osteoclastogenesis by osteoblasts. Semaphorin 4D, in contrast, is expressed on osteoclasts, and binding to its receptor Plexin-B1 on osteoblasts results in suppression of bone formation. In the present study, we found that both dynamic and static acceleration at 3.0× g decreased RANKL/OPG ratio and increased osteoblast-specific functional mRNA such as alkaline phosphatase, while static acceleration increased and dynamic acceleration decreased osteoclast-specific mRNA such as cathepsin K. Static acceleration increased semaphorin 4D mRNA expression, while dynamic acceleration had no effect. The results of the present study indicated that osteoclasts have predominant control over bone metabolism via semaphorin 4D expression induced by static acceleration at 3.0× g.

Cinacalcet attenuates hypercalcemia observed in mice bearing either Rice H-500 Leydig cell or C26-DCT colon tumors

Excessive secretion of parathyroid hormone-related protein (PTHrP) by tumors stimulates bone resorption and increases renal tubular reabsorption of calcium, resulting in hypercalcemia of malignancy. We investigated the ability of cinacalcet, an allosteric modulator of the calcium-sensing receptor, to attenuate hypercalcemia by assessing its effects on blood ionized calcium, serum PTHrP, and calcium-sensing receptor mRNA in mice bearing either Rice H-500 Leydig cell or C26-DCT colon tumors. Cinacalcet effectively decreased hypercalcemia in a dose- and enantiomer-dependent manner; furthermore, cinacalcet normalized phosphorus levels, but did not affect serum PTHrP. Ribonuclease protection assay results demonstrated presence of PTHrP receptor, but not calcium-sensing receptor mRNA in C26-DCT tumors. The mechanism by which cinacalcet lowered serum calcium was investigated in parathyroidectomized rats (i.e., without PTH) made hypercalcemic by PTHrP. Cinacalcet attenuated PTHrP-mediated elevations in blood ionized calcium, which were accompanied by increased plasma calcitonin. Taken together these results suggest that the cinacalcet-mediated decrease in serum calcium is not the result of a direct effect on tumor cells, but rather is the result of increased calcitonin release. In summary, cinacalcet effectively reduced tumor-mediated hypercalcemia and corrected hypophosphatemia in mice. Further investigation of cinacalcet for treatment of hypercalcemia of malignancy is warranted.