Genetic background influences fluoride's effects on osteoclastogenesis

Fluoride-induced apoptosis in non-skeletal tissues of experimental animals: A systematic review and meta-analysis

Different studies have suggested that fluoride can induce apoptosis in non-skeletal tissues, however, evidence from these experimental studies is still controversial. This meta-analysis aims to clarify the mechanism of fluoride-induced apoptosis in non-skeletal tissues of experimental animals. Primary studies which measured apoptosis were identified through exhaustive database searching in PubMed, Embase, Web of Science Core Collection, Scopus, and references of included studies. A random effects model with standardized mean difference (SMD) was used for meta-analyses. The heterogeneity of the studies was evaluated using Higgin's I2 statistics. The risk of bias and publication bias were assessed using the SYRCLE's risk of bias tool and Egger's test, respectively. There was an increase in total apoptotic cells, and the expression of Bax, Bax/Bcl-2 ratio, caspase-3, caspase-8, caspase-9, Cyt c, and p53, and a decrease in the expression of Bcl-2 in the fluoride-treated groups as compared to the control groups. However, there was no evidence of a difference in the expression of APAF-1 in the two groups. The subgroup analysis highlighted the role of the intervention period in modification of the apoptotic effect of fluoride and that the susceptibility and tolerance of different animal species and tissues vary. Meta-regression analysis indicated that the studies' effect size for total apoptotic cells was influenced by animal species and that of Bax by the sample source. The results of this meta-analysis revealed that fluoride causes apoptosis by up-regulating caspase-3, -8, and -9, Cyt c, p53, Bax, and down-regulating Bcl-2 with a concomitant up-regulation of the Bax/Bcl-2 ratio.

Alleviative Effects of Exercise on Bone Remodeling in Fluorosis Mice

Fluorine is widely present in nature in the form of fluoride. Prolonged high-dose fluoride exposure can cause skeletal fluorosis, resulting in osteosclerosis, osteoporosis or osteomalacia. It has been proved that exercise is one of the important factors affecting the health of the bone and promoting bone formation. To investigate the effects of exercise on bone remodeling in fluorosis mice, 120 male 3-week-old ICR mice were randomly divided into four groups: control group (C), exercise group (E), fluoride group (F), fluoride plus exercise group (F + E). After 8-week physical exercise and/or fluoride exposure, we evaluated the content of fluorine, the histopathological structure and microstructure of femur, bone metabolism biochemical indexes and oxidative stress related parameters, and the mRNA and protein levels of genes in BMP-2/Smads and OPG/RANKL/RANK signaling pathways. Our results showed that 100 mg/L NaF exposure increased the accumulation of fluoride in bone, altered histology of bone, and enhanced the activities of ALP and TRACP. Meanwhile, excessive fluoride induced oxidative stress in bone tissue by increasing the content of ROS and MDA, and decreasing the activities of antioxidant enzymes. In addition, the results of qRT-PCR suggested that NaF significantly increased the mRNA expression of BMP-2, Smad-5, Col IA1, Col IA2, OPG, RANKL and RANK, as well as the elevated proteins of OPG, RANKL and RANK. However, these fluoride-induced changes were alleviated after moderate exercise. Taken together, these findings indicated that moderate exercise decreased the toxicity of fluoride by reducing the accumulation of fluorine in the body to relieve the bone damage caused by fluorosis.

Fluoride Alters Signaling Pathways Associated with the Initiation of Dentin Mineralization in Enamel Fluorosis Susceptible Mice

Fluoride can alter the formation of mineralized tissues, including enamel, dentin, and bone. Dentin fluorosis occurs in tandem with enamel fluorosis. However, the pathogenesis of dentin fluorosis and its mechanisms are poorly understood. In this study, we report the effects of fluoride on the initiation of dentin matrix formation and odontoblast function. Mice from two enamel fluorosis susceptible strains (A/J and C57BL/6J) were given either 0 or 50 ppm fluoride in drinking water for 4 weeks. In both mouse strains, there was no overall change in dentin thickness, but fluoride treatment resulted in a significant increase in the thickness of the predentin layer. The lightly mineralized layer (LL), which lies at the border between predentin and fully mineralized dentin and is associated with dentin phosphoprotein (DPP), was absent in fluoride exposed mice. Consistent with a possible reduction of DPP, fluoride-treated mice showed reduced immunostaining for dentin sialoprotein (DSP). Fluoride reduced RUNX2, the transcription regulator of dentin sialophosphoprotein (DSPP), that is cleaved to form both DPP and DSP. In fluoride-treated mouse odontoblasts, the effect of fluoride was further seen in the upstream of RUNX2 as the reduced nuclear translocation of β-catenin and phosphorylated p65/NFκB. In vitro, MD10-F2 pre-odontoblast cells showed inhibition of the Dspp mRNA level in the presence of 10 μM fluoride, and qPCR analysis showed a significantly downregulated level of mRNAs for RUNX2, β-catenin, and Wnt10b. These findings indicate that in mice, systemic exposure to excess fluoride resulted in reduced Wnt/β-catenin signaling in differentiating odontoblasts to downregulate DSPP production via RUNX2.

The Inverted U-Curve Association of Fluoride and Osteoclast Formation in Mice

The effect of fluoride on osteoclasts is still controversial. In the past, researchers thought that the effects of fluoride on osteoclast and osteoblast formation occurred in a dose-dependent pattern. However, our previous in vitro study showed fluoride elicited a notably different effect on osteoclast formation. To further verify the relationship between fluoride and osteoclast formation in vivo, 60 male C57BL/6 mice were randomly divided into three groups: two treatment groups consuming water supplemented with 50 and 100 mg/L of fluoride, and a third control group with nonsupplemented water. Ion selective electrode method analysis was used to detect bone fluoride content, and the effects of fluoride on bone tissue were assessed with hematoxylin and eosin (HE) staining. Additionally, the expression of BGP and ALP were examined by Western blot analysis, and tartrate-resistant acid phosphatase (TRAP) was assessed with immunohistochemistry. Osteoclasts in bone tissue were identified with a combination method of TRAP staining and cell morphology assessment. Results showed increasing expression of BGP among treatment groups as fluoride exposure increased, and ALP expression in the 100 mg/L treatment group was significantly higher than that for both the 50 mg/L treatment and control groups. The number of osteoclasts in the 50 mg/L group was highest amongst the three groups, followed by the 100 mg/L treatment and then by the control group, with the latter showing significantly fewer osteoclasts than in either treatment group. These results suggest that fluoride enhances bone formation at increasing levels of fluoride exposure. However, the inverted U-curve association was found between fluoride exposure and osteoclast formation, with the higher dose of fluoride having slightly reduced osteoclast formation. The results from this study may provide key insights towards understanding the role of osteoclasts in the progression of skeletal fluorosis.

The role of TGFβ receptor 1-smad3 signaling in regulating the osteoclastic mode affected by fluoride

Studies that have focused on the role TGFβ signaling plays in osteoclast activity are gradually increasing; however, literature is rare in terms of fluorosis. The aim of this study is to observe the role the TβR1/Smad3 pathway plays in fluoride regulating cellsosteoclast-like cells that are under the treatment of TGFβ receptor 1 kinase. The RANKL-mediated osteoclast-like cells from RAW264.7 cells were used as osteoclast precursor model. The profile of miRNA expression in fluoride-treated osteoclast-like cells exhibited 303 upregulated miRNAs, 61 downregulated miRNAs, and further drew 37 signaling pathway maps by KEGG and Biocarta pathway enrichment analysis. TGFβ and its downstream effectors were included among them. Osteoclast viability, formation and function were detected via MTT method, bone resorption pit and tartrate-resistant acid phosphatase (TRACP) staining, respectively. Results demonstrated that different doses of fluoride exhibited a biphasic effect on osteoclast cell viability, differentiation, formation and function. It indicated that a low dose of fluoride treatment stimulated them, but high dose inhibited them. SB431542 acted as TβR1 kinase inhibitor and blocked viability, formation and function of osteoclast-like cells regulated by fluoride. The expression of the osteoclast marker, RANK, and TβR1/Smad3 at gene and protein level was analyzed under fluoride with and without SB431542 treatment. Fluoride treatment indicated little effect on the RANK protein expression; however it significantly influenced TRACP expression in osteoclast-like cells. The stimulation of fluoride on the expression of Smad3 gene and phosphorylated Smad3 protein exhibited dose-dependent manner. SB431542 significantly impeded phosphorylation of Smad3 protein and TRACP expression in osteoclast-like cells that were exposed to fluoride. Our work demonstrated that TGFβ signaling played a key role in fluoride regulating osteoclast differentiation, formation and function. It elucidated that TβR1/Smad3 pathway participated in the mechanism of biphasic modulation of osteoclast mode regulated by fluoride.

ClC-7/Ostm1 contribute to the ability of tea polyphenols to maintain bone homeostasis in C57BL/6 mice, protecting against fluorosis

Epidemiological investigations indicate that certain ingredients in tea bricks can antagonize the adverse effects of fluoride. Tea polyphenols (TPs), the most bioactive ingredient in tea bricks, have been demonstrated to be potent bone-supporting agents. ClC‑7 is known to be crucial for osteoclast (OC) bone resorption. Thus, in this study, we investigated the potential protective effects of TPs against fluorosis using a mouse model and explored the underlying mechanisms with particular focus on ClC‑7. A total of 40, healthy, 3‑week‑old male C57BL/6 mice were randomly divided into 4 groups (n=10/group) by weight as follows: distilled water (control group), 100 mg/l fluoridated water (F group), water containing 10 g/l TPs (TP group) and water containing 100 mg/l fluoride and 10 g/l TPs (F + TP group). After 15 weeks, and after the mice were sacrificed, the long bones were removed and bone marrow-derived macrophages were cultured ex vivo in order to perform several experiments. OCs were identified and counted by tartrate‑resistant acid phosphatase (TRAP) staining. The consumption of fluoride resulted in severe fluorosis and in an impaired OC function [impaired bone resorption, and a low mRNA expression of nuclear factor of activated T-cells 1 (NFATc1), ATPase H+ transporting V0 subunit D2 (ATP6v0d2) and osteopetrosis‑associated transmembrane protein 1 (Ostm1)]. In the F + TP group, fluorosis was attenuated and OC function was restored, but not the high bone fluoride content. Compared with the F group, mature OCs in the F + TP group expressed higher mRNA levels of ClC‑7 and Ostm1; the transportation and retaining of Cl‑ was improved, as shown by the fluorescence intensity experiment. On the whole, our findings indicate that TPs mitigate fluorosis in C57BL/6 mice by regulating OC bone resorption. Fluoride inhibits OC resorption by inhibiting ClC‑7 and Ostm1, whereas TPs attenuate this inhibitory effect of fluoride.

Fluoride affects bone repair differently in mice models with distinct bone densities

We grouped mice [strains: C57BL/6J (n=32) and C3H/HeJ (n=32)] to address the influence of bone density on fluoride's (F's) biological effects. These animals received low-fluoride food and water containing 0 (control group) or 50ppm of F for up to 28days. The upper left central incisor was extracted, and the left maxilla was collected at 7, 14, 21, and 28days for histological and histomorphometric analysis to estimate bone neoformation. Our results showed bone neoformation in all of the evaluated groups, with the presence of bone islets invading the center of the alveoli when replacing the existing connective tissue. Curiously, this biological phenomenon was more evident in the C57BL/6J strain. The histomorphometric analysis confirmed the histological findings in relation to the amount of new bone tissue and showed a decrease in C3H/HeJ mice (control group). Altogether, our results showed differential effects of fluoride bone metabolism, confirming a genetic component in susceptibility to the effects of fluoride.

Sodium fluoride induces hypercalcemia resulting from the upregulation of both osteoblastic and osteoclastic activities in goldfish, Carassius auratus

The influence of sodium fluoride (NaF) on calcium metabolism was examined in goldfish (fresh water teleost). At 2days after administration of NaF (500ng/g body weight; 5μg/g body weight) (around 10(-5) to 10(-4)M in goldfish), we indicated that plasma calcium levels upregulated in both doses of NaF-treated goldfish. To examine the mechanism of hypercalcemia by NaF treatments, therefore, direct effects of NaF on osteoblasts and osteoclasts in goldfish were investigated by an original assay system using teleost scale which has osteoblasts, osteoclasts and bone matrix. Alkaline phosphatase activity in the scales increased with the treatment of NaF (10(-6) and 10(-5)M) during 6h of incubation. Also, tartrate-resistant acid phosphatase activity increased after exposure to NaF (10(-5)M) at the 6h of incubation. To investigate the osteoclastic activation, the mRNA expression of osteoclastogenesis related factors were examined. The receptor activator of the nuclear factor-κB ligand (RANKL) which is known as a factor for osteoclastogenesis, increased in the NaF-treated scales after 6h of incubation. The ratio of RANKL/osteoprotegerin (osteoclastogenesis inhibitory factor) significantly increased after 6h of incubation. Resulting from the increase of RANKL mRNA level, the expression of transcription-regulating factors was significantly increased. Furthermore, the expression of functional genes, cathepsin K and matrix metalloproteinase-9 mRNA, was significantly increased. In our knowledge, this is the first report concerning the effects of NaF on osteoblasts and osteoclasts in teleosts. We concluded that NaF influences calcium metabolism via osteoclastic activation in goldfish.

Fluoride Modulates Parathyroid Hormone Secretion in vivo and in vitro

The study objective was to investigate the effects of fluoride on intact parathyroid hormone (iPTH) secretion. Thyro-parathyroid complexes (TPC) from C3H (n = 18) and B6 (n = 18) mice were cultured in Ca²⁺-optimized medium. TPC were treated with 0, 250, or 500 µM NaF for 24 h and secreted iPTH assayed by ELISA. C3H (n = 78) and B6 (n = 78) mice were gavaged once with distilled or fluoride (0.001 mg [F⁻]/g of body weight) water. At serial time points (0.5-96 h) serum iPTH, fluoride, total calcium, phosphorus, and magnesium levels were determined. Expression of genes involved in mineral regulation via the bone-parathyroid-kidney (BPK) axis, such as parathyroid hormone (Pth), calcium-sensing receptor (Casr), vitamin D receptor (Vdr), parathyroid hormone-like hormone (Pthlh), fibroblast growth factor 23 (Fgf23), α-Klotho (αKlotho), fibroblast growth factor receptor 1c (Fgf1rc), tumor necrosis factor 11 (Tnfs11), parathyroid hormone receptor 1 (Pth1r), solute carrier family 34 member 1 (Slc34a1), solute carrier 9 member 3 regulator 1 (Slc9a3r1), chloride channel 5 (Clcn5), and PDZ domain-containing 1 (Pdzk1), was determined in TPC, humeri, and kidneys at 24 h. An in vitro decrease in iPTH was seen in C3H and B6 TPC at 500 µM (p < 0.001). In vivo levels of serum fluoride peaked at 0.5 h in both C3H (p = 0.002) and B6 (p = 0.01). In C3H, iPTH decreased at 24 h (p < 0.0001), returning to baseline at 48 h. In B6, iPTH increased at 12 h (p < 0.001), returning to baseline at 24 h. Serum total calcium, phosphorus, and magnesium levels did not change significantly. Pth, Casr,αKlotho,Fgf1rc,Vdr, and Pthlh were significantly upregulated in C3H TPC compared to B6. In conclusion, the effects of fluoride on TPC in vitro were equivalent between the 2 mouse strains. However, fluoride demonstrated an early strain-dependent effect on iPTH secretion in vivo. Both strains demonstrated differences in the expression of genes involved in the BPK axis, suggesting a possible role in the physiologic handling of fluoride.

Noncoding RNAs: Possible Players in the Development of Fluorosis

Fluorosis is caused by excess of fluoride intake over a long period of time. Aberrant change in the Runt-related transcription factor 2 (RUNX2) mediated signaling cascade is one of the decisive steps during the pathogenesis of fluorosis. Up to date, role of fluoride on the epigenetic alterations is not studied. In the present study, global expression profiling of short noncoding RNAs, in particular miRNAs and snoRNAs, was carried out in sodium fluoride (NaF) treated human osteosarcoma (HOS) cells to understand their possible role in the development of fluorosis. qPCR and in silico hybridization revealed that miR-124 and miR-155 can be directly involved in the transcriptional regulation of Runt-related transcription factor 2 (RUNX2) and receptor activator of nuclear factor κ-B ligand (RANKL) genes. Compared to control, C/D box analysis revealed marked elevation in the number of UG dinucleotides and D-box sequences in NaF exposed HOS cells. Herein, we report miR-124 and miR-155 as the new possible players involved in the development of fluorosis. We show that the alterations in UG dinucleotides and D-box sequences of snoRNAs could be due to NaF exposure.

Bone response to fluoride exposure is influenced by genetics

Genetic factors influence the effects of fluoride (F) on amelogenesis and bone homeostasis but the underlying molecular mechanisms remain undefined. A label-free proteomics approach was employed to identify and evaluate changes in bone protein expression in two mouse strains having different susceptibilities to develop dental fluorosis and to alter bone quality. In vivo bone formation and histomorphometry after F intake were also evaluated and related to the proteome. Resistant 129P3/J and susceptible A/J mice were assigned to three groups given low-F food and water containing 0, 10 or 50 ppmF for 8 weeks. Plasma was evaluated for alkaline phosphatase activity. Femurs, tibiae and lumbar vertebrae were evaluated using micro-CT analysis and mineral apposition rate (MAR) was measured in cortical bone. For quantitative proteomic analysis, bone proteins were extracted and analyzed using liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS), followed by label-free semi-quantitative differential expression analysis. Alterations in several bone proteins were found among the F treatment groups within each mouse strain and between the strains for each F treatment group (ratio ≥1.5 or ≤0.5; p<0.05). Although F treatment had no significant effects on BMD or bone histomorphometry in either strain, MAR was higher in the 50 ppmF 129P3/J mice than in the 50 ppmF A/J mice treated with 50 ppmF showing that F increased bone formation in a strain-specific manner. Also, F exposure was associated with dose-specific and strain-specific alterations in expression of proteins involved in osteogenesis and osteoclastogenesis. In conclusion, our findings confirm a genetic influence in bone response to F exposure and point to several proteins that may act as targets for the differential F responses in this tissue.

Differential effects of fluoride during osteoblasts mineralization in C57BL/6J and C3H/HeJ inbred strains of mice

The behavior of fluoride ions in biological systems has advantages and problems. On one hand, fluoride could be a mitogenic stimulus for osteoblasts. However, high concentrations of this element can cause apoptosis in rat and mouse osteoblasts. Toward an understanding of this effect, we examined the role of sodium fluoride (NaF) in two mouse calvaria osteoblasts during the mineralization process. The animals used were C3H/HeJ (C3) and C57BL/6J (B6) mice. The calvaria cells were cultured for 28 days in the presence of several doses of NaF (0, 5, 10, 25, 50, and 75 μM), and we performed the assays: mineralized nodule measurements, alkaline phosphatase (ALP) activity, determination of type I collagen, and matrix metalloproteinase-2 (MMP-2) activity. The results showed no effects on alkaline phosphatase activity but decreased mineralized nodule formation. In B6 cells, the NaF effect was already seen with 10 μM of NaF and a greater increase of cellular type I collagen, and MMP-2 activity was upregulated after 7 days of NaF exposure. C3 osteoblasts showed a reduction in the mineralization pattern only after 50 μM of NaF with a slight increase of type I collagen and downregulation of MMP-2 activity during the mineralization period. In conclusion, fluoride affects the production and degradation of the extracellular matrix during early onset and probably during the mineralization period. Additionally, the genetic factors may contribute to the variation in cell response to fluoride exposure, and the differences observed between the two strains could be explained by an alteration of the bone matrix metabolism (synthesis and degradation).

Recombinant biglycan promotes bone morphogenetic protein-induced osteogenesis

The aim of this study was to determine the effects of glutathione-S-transferase-fused recombinant biglycan (GST-BGN) on craniofacial bone regeneration. We recently demonstrated a positive effect of tissue-derived BGN on bone morphogenetic protein 2 (BMP-2) function, which is exerted likely via the BGN core protein. Here, we investigated the effects of GST-BGN lacking any posttranslational modifications on BMP-2 function in vitro and in vivo. In the C2C12 cell culture system, BMP-2-induced Smad 1/5/8 phosphorylation and alkaline phosphatase activity were both enhanced by the addition of GST-BGN. For the in vivo effect, we employed a Sprague-Dawley rat mandible defect model utilizing 1 µg (optimal) or 0.1 µg (suboptimal) of BMP-2 combined with 0, 2, 4, or 8 µg of GST-BGN. At 2 weeks post-surgery, newly formed bone was evaluated by microcomputed tomography and histologic analyses. The results revealed that the greatest amounts of bone within the defect were formed in the groups of suboptimal BMP-2 combined with 4 or 8 µg of GST-BGN. Also, bone was well organized versus that formed by the optimal dose of BMP. These results indicate that recombinant BGN is an efficient substrate to promote low-dose BMP-induced osteogenesis.

Effects of salubrinal on development of osteoclasts and osteoblasts from bone marrow-derived cells

Background

Osteoporosis is a skeletal disease leading to an increased risk of bone fracture. Using a mouse osteoporosis model induced by administration of a receptor activator of nuclear factor kappa-B ligand (RANKL), salubrinal was recently reported as a potential therapeutic agent. To evaluate the role of salubrinal in cellular fates as well as migratory and adhesive functions of osteoclast/osteoblast precursors, we examined the development of primary bone marrow-derived cells in the presence and absence of salubrinal. We addressed a question: are salubrinal’s actions more potent to the cells isolated from the osteoporotic mice than those isolated from the control mice?

Methods

Using the RANKL-injected and control mice, bone marrow-derived cells were harvested. Osteoclastogenesis was induced by macrophage-colony stimulating factor and RANKL, while osteoblastogenesis was driven by dexamethasone, ascorbic acid, and β-glycerophosphate.

Results

The results revealed that salubrinal suppressed the numbers of colony forming-unit (CFU)-granulocyte/macrophages and CFU-macrophages, as well as formation of mature osteoclasts in a dosage-dependent manner. Salubrinal also suppressed migration and adhesion of pre-osteoclasts and increased the number of CFU-osteoblasts. Salubrinal was more effective in exerting its effects in the cells isolated from the RANKL-injected mice than the control. Consistent with cellular fates and functions, salubrinal reduced the expression of nuclear factor of activated T cells c1 (NFATc1) as well as tartrate-resistant acid phosphatase.

Conclusions

The results support the notion that salubrinal exhibits significant inhibition of osteoclastogenesis as well as stimulation of osteoblastogenesis in bone marrow-derived cells, and its efficacy is enhanced in the cells harvested from the osteoporotic bone samples.

Fine mapping of dental fluorosis quantitative trait loci in mice

Genetic factors underlie the susceptibility and the resistance to dental fluorosis (DF). The A/J (DF susceptible) and 129P3/J (DF resistant) mouse strains have previously been used to detect quantitative trait loci (QTLs) associated with DF on chromosome (Chr) 2 and Chr 11. In the present study, increased marker density genotyping followed by interval mapping was performed to narrow the QTL intervals and improve the logarithm of the odds (to the base 10) (LOD) scores. Narrower intervals were obtained on Chr 2 where LOD ≥ 6.0 (57-84 cM or ≈ 51 Mb), LOD ≥ 7.0 (62-79 cM or ≈ 32 Mb), and LOD ≥ 8.0 (65-74 cM or ≈ 17 Mb); and on Chr 11 where LOD ≥ 6.0 (18-51 cM or ≈ 53 Mb), LOD ≥ 7.0 (28-48 cM or ≈ 34 Mb), and LOD ≥ 8.0 (31-45 cM or ≈ 22 Mb). Haplotype analysis between A/J and 129P3/J mice further reduced the QTL intervals. Accn1 was selected as a candidate gene based upon its location near the peak LOD score on Chr 11 and distant homology with the Caenorhabditis elegans fluoride-resistance gene, flr1. The severity of DF between Accn1(-/-) and wild-type mice was not significantly different. Hence, the loss of ACCN1 function does not modify DF severity in mice. Narrowing the DF QTL intervals will facilitate additional candidate gene selections and interrogation.

Constitutive activation of p38 MAPK in tumor cells contributes to osteolytic bone lesions in multiple myeloma

Bone destruction is a hallmark of multiple myeloma and affects more than 80% of patients. However, current therapy is unable to completely cure and/or prevent bone lesions. Although it is accepted that myeloma cells mediate bone destruction by inhibition of osteoblasts and activation of osteoclasts, the underlying mechanism is still poorly understood. This study demonstrates that constitutive activation of p38 mitogen-activated protein kinase in myeloma cells is responsible for myeloma-induced osteolysis. Our results show that p38 is constitutively activated in most myeloma cell lines and primary myeloma cells from patients. Myeloma cells with high/detectable p38 activity, but not those with low/undetectable p38 activity, injected into SCID or SCID-hu mice caused bone destruction. Inhibition or knockdown of p38 in human myeloma reduced or prevented myeloma-induced osteolytic bone lesions without affecting tumor growth, survival, or homing to bone. Mechanistic studies showed that myeloma cell p38 activity inhibited osteoblastogenesis and bone formation and activated osteoclastogenesis and bone resorption in myeloma-bearing SCID mice. This study elucidates a novel molecular mechanism—sactivation of p38 signaling in myeloma cells—by which myeloma cells induce osteolytic bone lesions and indicates that targeting myeloma cell p38 may be a viable approach to treating or preventing myeloma bone disease.

Genetic Background Strongly Influences the Bone Phenotype of P2X7 Receptor Knockout Mice

The purinergic P2X7 receptor is expressed by bone cells and has been shown to be important in both bone formation and bone resorption. In this study we investigated the importance of the genetic background of the mouse strains on which the P2X7 knock-out models were based by comparing bone status of a new BALB/cJ P2X7(-/-) strain with a previous one based on the C57BL/6 strain. Female four-month-old mice from both strains were DXA scanned on a PIXImus densitometer; femurs were collected for bone strength measurements and serum for bone marker analysis. Bone-related parameters that were altered only slightly in the B6 P2X7(-/-) became significantly altered in the BALB/cJ P2X7(-/-) when compared to their wild type littermates. The BALB/cJ P2X7(-/-) showed reduced levels of serum C-telopeptide fragment (s-CTX), higher bone mineral density, and increased bone strength compared to the wild type littermates. In conclusion, we have shown that the genetic background of P2X7(-/-) mice strongly influences the bone phenotype of the P2X7(-/-) mice and that P2X7 has a more significant regulatory role in bone remodeling than found in previous studies.

Phenotypic variation of fluoride responses between inbred strains of mice

Excessive systemic exposure to fluoride (F) can lead to disturbances in bone homeostasis and dental enamel development. We have previously shown strain-specific responses to F in the development of dental fluorosis (DF) and in bone formation/mineralization. The current study was undertaken to further investigate F responsive variations in bone metabolism and to determine possible relationships with DF susceptibility. Seven-week-old male mice from FVB/NJ, C57BL/6J, C3H/HeJ, A/J, 129S1/SvImJ, AKR/J, DBA/2J, and BALB/cByJ inbred strains were exposed to NaF (0 or 50 ppm as F(-)) in drinking water for 60 days. Sera were collected for F, Ca, Mg, PO(4), iPTH, sRANKL, and ALP levels. Bone marrow cells were subjected to ex vivo cell culture for osteoclast potential and CFU colony assays (CFU-fibroblast, CFU-osteoblast, CFU-erythrocyte/granulocyte/macrophage/megakaryocyte, CFU-granulocyte/macrophage, CFU-macrophage, and CFU-granulocyte). Femurs and vertebrae were subjected to micro-CT analyses, biomechanical testing, and F, Mg, and Ca content assays. DF was evaluated using quantitative fluorescence and clinical criteria. Strain-specific responses to F were observed for DF, serum studies, ex vivo cell culture studies, and bone quality. Among the strains, there were no patterns or significant correlations between DF severity and the actions of F on bone homeostasis (serum studies, ex vivo assays, or bone quality parameters). The genetic background continues to play a role in the actions of F on tooth enamel development and bone homeostasis. F exposure led to variable phenotypic responses between strains involving dental enamel development and bone metabolism.

Fluoride's effects on the formation of teeth and bones, and the influence of genetics

Fluorides are present in the environment. Excessive systemic exposure to fluorides can lead to disturbances of bone homeostasis (skeletal fluorosis) and enamel development (dental/enamel fluorosis). The severity of dental fluorosis is also dependent upon fluoride dose and the timing and duration of fluoride exposure. Fluoride's actions on bone cells predominate as anabolic effects both in vitro and in vivo. More recently, fluoride has been shown to induce osteoclastogenesis in mice. Fluorides appear to mediate their actions through the MAPK signaling pathway and can lead to changes in gene expression, cell stress, and cell death. Different strains of inbred mice demonstrate differential physiological responses to ingested fluoride. Genetic studies in mice are capable of identifying and characterizing fluoride-responsive genetic variations. Ultimately, this can lead to the identification of at-risk human populations who are susceptible to the unwanted or potentially adverse effects of fluoride action and to the elucidation of fundamental mechanisms by which fluoride affects biomineralization.

Modulation of murine bone marrow-derived CFU-F and CFU-OB by in vivo bisphosphonate and fluoride treatments

OBJECTIVES

Bisphosphonates (BPN) have actions on a variety of cell types including: osteoclasts, osteoblasts, osteocytes, and endothelial cells. The objectives of this report are to review the current state of understanding of the effects of BPNs on orthodontic tooth movement and to provide evidence on BPN's in vivo effects on bone marrow-derived osteoprogenitor cells.

MATERIAL AND METHODS

Mice from the C3H/HeJ (C3H), C57BL/6J (B6), FVB/NJ (FVB), and BALB/cByJ (BALB) strains were treated for 3 weeks with 0, 3, 30, or 150 mcg/kg/week alendronate (ALN) administered subcutaneous alone or in combination with 50 ppm fluoride (F). Bone marrow cells were harvested and subjected to in vitro colony-forming unit fibroblast (CFU-F) and colony-forming unit osteoblasts (CFU-OB) assays.

RESULTS

Baseline differences in CFU-F, CFU-OB/ALP+, and CFU-OB/total were observed among the four strains. Strain-specific responses to ALN and F treatments were observed for CFU-F, CFU-OB/ALP+, and CFU-OB/total. F treatment alone resulted in decreases in CFU-F (p = 0.013), CFU-OB/ALP+ (p = 0.005), and CFU-OB/total (p = 0.003) in the C3H strain. CFU-F (p = 0.036) were decreased by F in the B6 strain. No significant (NS) effects of F were observed for FVB and BALB. ALN treatment resulted in a significant decrease in CFU-F (p = 0.0014) and CFU-OB/total (p = 0.028) in C3H only. ALN treatment had NS effect on CFU-OB/ALP+ in all four strains.

CONCLUSION

Genetic factors appear to play a role in ALN's effects on CFU-F and CFU-OB/total but not on CFU-OB/ALP+.

Fluoride effects on bone formation and mineralization are influenced by genetics

INTRODUCTION

A variation in bone response to fluoride (F(-)) exposure has been attributed to genetic factors. Increasing fluoride doses (0 ppm, 25 ppm, 50 ppm, 100 ppm) for three inbred mouse strains with different susceptibilities to developing dental enamel fluorosis (A/J, a "susceptible" strain; SWR/J, an "intermediate" strain; 129P3/J, a "resistant" strain) had different effects on their cortical and trabecular bone mechanical properties. In this paper, the structural and material properties of the bone were evaluated to explain the previously observed changes in mechanical properties.

MATERIALS AND METHODS

This study assessed the effect of increasing fluoride doses on the bone formation, microarchitecture, mineralization and microhardness of the A/J, SWR/J and 129P3/J mouse strains. Bone microarchitecture was quantified with microcomputed tomography and strut analysis. Bone formation was evaluated by static histomorphometry. Bone mineralization was quantified with backscattered electron (BSE) imaging and powder X-ray diffraction. Microhardness measurements were taken from the vertebral bodies (cortical and trabecular bones) and the cortex of the distal femur.

RESULTS

Fluoride treatment had no significant effect on bone microarchitecture for any of the strains. All three strains demonstrated a significant increase in osteoid formation at the largest fluoride dose. Vertebral body trabecular bone BSE imaging revealed significantly decreased mineralization heterogeneity in the SWR/J strain at 50 ppm and 100 ppm F(-). The trabecular and cortical bone mineralization profiles showed a non-significant shift towards higher mineralization with increasing F(-) dose in the three strains. Powder X-ray diffraction showed significantly smaller crystals for the 129P3/J strain, and increased crystal width with increasing F(-) dose for all strains. There was no effect of F(-) on trabecular and cortical bone microhardness.

CONCLUSION

Fluoride treatment had no significant effect on bone microarchitecture in these three strains. The increased osteoid formation and decreased mineralization heterogeneity support the theory that F(-) delays mineralization of new bone. The increasing crystal width with increasing F(-) dose confirms earlier results and correlates with most of the decreased mechanical properties. An increase in bone F(-) may affect the mineral-organic interfacial bonding and/or bone matrix proteins, interfering with bone crystal growth inhibition on the crystallite faces as well as bonding between the mineral and organic interface. The smaller bone crystallites of the 129P3/J (resistant) strain may indicate a stronger organic/inorganic interface, reducing crystallite growth rate and increasing interfacial mechanical strength.

Fluoride induces endoplasmic reticulum stress and inhibits protein synthesis and secretion

BACKGROUND

Exposure to excessive amounts of fluoride (F(-)) causes dental fluorosis in susceptible individuals; however, the mechanism of F(-)-induced toxicity is unclear. Previously, we have shown that high-dose F(-) activates the unfolded protein response (UPR) in ameloblasts that are responsible for dental enamel formation. The UPR is a signaling pathway responsible for either alleviating endoplasmic reticulum (ER) stress or for inducing apoptosis of the stressed cells.

OBJECTIVES

In this study we determined if low-dose F(-) causes ER stress and activates the UPR, and we also determined whether F(-) interferes with the secretion of proteins from the ER.

METHODS

We stably transfected the ameloblast-derived LS8 cell line with secreted alkaline phosphatase (SEAP) and determined activity and localization of SEAP and F(-)-mediated induction of UPR proteins. Also, incisors from mice given drinking water containing various concentrations of F(-) were examined for eucaryotic initiation factor-2, subunit alpha (eIF2alpha) phosphorylation.

RESULTS

We found that F(-) decreases the extracellular secretion of SEAP in a linear, dose-dependent manner. We also found a corresponding increase in the intracellular accumulation of SEAP after exposure to F(-). These changes are associated with the induction of UPR proteins such as the molecular chaperone BiP and phosphorylation of the UPR sensor PKR-like ER kinase, and its substrate, eIF2alpha. Importantly, F(-)-induced phosphorylation of eIF2alphawas confirmed in vivo.

CONCLUSIONS

These data suggest that F(-) initiates an ER stress response in ameloblasts that interferes with protein synthesis and secretion. Consequently, ameloblast function during enamel development may be impaired, and this may culminate in dental fluorosis.

Detection of dental fluorosis-associated quantitative trait Loci on mouse chromosomes 2 and 11

Systemic exposure to greater than optimal fluoride (F) can lead to dental fluorosis (DF). Parental A/J (DF-susceptible) and 129P3/J (DF-resistant) inbred mice were used for histological studies and to generate F2 progeny. Mice were treated with 0 or 50 ppm F in their drinking water for 60 days. A clinical criterion (modified Thylstrup and Fejerskov categorical scale) was used to assess the severity of DF for each individual F2 animal. Parental strains were subjected to histological examination of maturing enamel. F treatment resulted in accumulation of amelogenins in the maturing enamel of A/J mice. Quantitative trait loci (QTL) detection was performed using phenotypic extreme F2 animals genotyped for 354 single nucleotide polymorphism-based markers distributed throughout the mouse genome followed by chi(2) analysis. Significant evidence of association was observed on chromosomes 2 and 11 for a series of consecutive markers (p < 0.0001). Further analyses were performed to examine whether the phenotypic effects were found in both male and female F2 mice or whether there was evidence for gender-specific effects. Analyses performed using the markers on chromosomes 2 and 11 which were significant in the mixed-gender mice were also significant when analyses were limited to only the male or female mice. The QTL detected on chromosomes 2 and 11 which influence the variation in response to fluorosis have their effect in mice of both genders. Finally, the QTL in both chromosomes appear to have an additive effect.