Effects of different regimens of sodium fluoride treatment for osteoporosis on the structure, remodeling and mineralization of bone

Role of Phosphate in Biomineralization

Inorganic phosphate is a vital constituent of cells and cell membranes, body fluids, and hard tissues. It is a major intracellular divalent anion, participates in many genetic, energy and intermediary metabolic pathways, and is important for bone health. Although we usually think of phosphate mostly in terms of its level in the serum, it is needed for many biological and structural functions of the body. Availability of adequate calcium and inorganic phosphate in the right proportions at the right place is essential for proper acquisition, biomineralization, and maintenance of mass and strength of the skeleton. The three specialized mineralized tissues, bones, teeth, and ossicles, differ from all other tissues in the human body because of their unique ability to mineralize, and the degree and process of mineralization in these tissues also differ to suit the specific functions: locomotion, chewing, and hearing, respectively. Biomineralization is a dynamic, complex, and lifelong process by which precipitations of inorganic calcium and inorganic phosphate divalent ions form biological hard tissues. Understanding the biomineralization process is important for the management of diseases caused by both defective and abnormal mineralization. Hypophosphatemia results in mineralization defects and osteomalacia, and hyperphosphatemia is implicated in abnormal excess calcification and/or ossification, but the exact mechanisms underlying these processes are not fully understood. In this review, we summarize available evidence on the role of phosphate in biomineralization. Other manuscripts in this issue of the journal deal with other relevant aspects of phosphate homeostasis, phosphate signaling and sensing, and disorders resulting from hypo- and hyperphosphatemic states.

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.

Deterioration of teeth and alveolar bone loss due to chronic environmental high-level fluoride and low calcium exposure

OBJECTIVES

Health risks due to chronic exposure to highly fluoridated groundwater could be underestimated because fluoride might not only influence the teeth in an aesthetic manner but also seems to led to dentoalveolar structure changes. Therefore, we studied the tooth and alveolar bone structures of Dorper sheep chronically exposed to very highly fluoridated and low calcium groundwater in the Kalahari Desert in comparison to controls consuming groundwater with low fluoride and normal calcium levels within the World Health Organization (WHO) recommended range.

MATERIALS AND METHODS

Two flocks of Dorper ewes in Namibia were studied. Chemical analyses of water, blood and urine were performed. Mineralized tissue investigations included radiography, HR-pQCT analyses, histomorphometry, energy-dispersive X-ray spectroscopy and X-ray diffraction-analyses.

RESULTS

Fluoride levels were significantly elevated in water, blood and urine samples in the Kalahari group compared to the low fluoride control samples. In addition to high fluoride, low calcium levels were detected in the Kalahari water. Tooth height and mandibular bone quality were significantly decreased in sheep, exposed to very high levels of fluoride and low levels of calcium in drinking water. Particularly, bone volume and cortical thickness of the mandibular bone were significantly reduced in these sheep.

CONCLUSIONS

The current study suggests that chronic environmental fluoride exposure with levels above the recommended limits in combination with low calcium uptake can cause significant attrition of teeth and a significant impaired mandibular bone quality.

CLINICAL RELEVANCE

In the presence of high fluoride and low calcium-associated dental changes, deterioration of the mandibular bone and a potential alveolar bone loss needs to be considered regardless whether other signs of systemic skeletal fluorosis are observed or not.

Effects of thirty elements on bone metabolism

The human skeleton, made of 206 bones, plays vital roles including supporting the body, protecting organs, enabling movement, and storing minerals. Bones are made of organic structures, intimately connected with an inorganic matrix produced by bone cells. Many elements are ubiquitous in our environment, and many impact bone metabolism. Most elements have antagonistic actions depending on concentration. Indeed, some elements are essential, others are deleterious, and many can be both. Several pathways mediate effects of element deficiencies or excesses on bone metabolism. This paper aims to identify all elements that impact bone health and explore the mechanisms by which they act. To date, this is the first time that the effects of thirty minerals on bone metabolism have been summarized.

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.

Surface properties and ion release from fluoride-containing bioactive glasses promote osteoblast differentiation and mineralization in vitro

Bioactive glasses (BG) are suitable for bone regeneration applications as they bond with bone and can be tailored to release therapeutic ions. Fluoride, which is widely recognized to prevent dental caries, is efficacious in promoting bone formation and preventing osteoporosis-related fractures when administered at appropriate doses. To take advantage of these properties, we created BG incorporating increasing levels of fluoride whilst holding their silicate structure constant, and tested their effects on human osteoblasts in vitro. Our results demonstrate that, whilst cell proliferation was highest on low-fluoride-containing BG, markers for differentiation and mineralization were highest on BG with the highest fluoride contents, a likely effect of a combination of surface effects and ion release. Furthermore, osteoblasts exposed to the dissolution products of fluoride-containing BG or early doses of sodium fluoride showed increased alkaline phosphatase activity, a marker for bone mineralization, suggesting that fluoride can direct osteoblast differentiation. Taken together, these results suggest that BG that can release therapeutic levels of fluoride may find use in a range of bone regeneration applications.

Glucocorticoid-induced osteoporosis: treatment update and review

Glucocorticoid-induced osteoporosis (GIO) is a serious consequence of glucocorticoid therapy leading to fractures in 30-50% of patients. A wide range of protective medications have been studied in this condition including calcium, vitamin D, vitamin D analogs, oral and intravenous bisphosphonates, sex hormones, anabolic agents and calcitonin. The mechanism of action, and evidence for these therapies, are reviewed - focusing on important trials and new evidence. Recently published guidelines are also reviewed and compared. Bisphosphonates are currently the recommended first-line therapy for the prevention and treatment of GIO. They have been shown to increase bone mineral density (BMD) at the spine and hip and to decrease the incidence of vertebral fractures (especially in postmenopausal women). Testosterone therapy and female hormone replacement therapy (HRT) have been found to increase lumbar spine BMD in hypogonadal patients on glucocorticoid therapy, but effects on hip BMD have not been consistent and there is no fracture data in the GIO population. Similarly, calcitonin increases lumbar spine BMD but has no proven fracture efficacy. The effect of selective estrogen receptor modulators, the oral contraceptive pill and strontium on GIO is relatively unknown. Parathyroid hormone (PTH 1-34) and zoledronic acid have emerged as exciting new options for the treatment of GIO. Both therapies have been found to result in gains in BMD at the spine and hip that are either noninferior or superior to those seen with oral bisphosphonate therapy. PTH 1-34 has also been found to decrease the incidence of new vertebral fractures and may be an option in high-risk patients established on long-term glucocorticoid therapy.

Skeletal fluorosis due to excessive tea and toothpaste consumption

We describe the case of a 53-year-old woman who presented with a metatarsal fracture and was found to have a bone mineral density (BMD) T-score of +11 in the lumbar spine and +7.6 in the hip. Subsequent investigation revealed very high serum, urine and tissue fluoride levels, associated with excessive tea and toothpaste consumption. The case emphasises the need to exclude fluorosis in individuals with unexpectedly high BMD levels.

Genetic background influences fluoride's effects on osteoclastogenesis

Excessive fluoride (F) can lead to abnormal bone biology. Numerous studies have focused on the anabolic action of F yet little is known regarding any action on osteoclastogenesis. Little is known regarding the influence of an individual's genetic background on the responses of bone cells to F. Four-week old C57BL/6J (B6) and C3H/HeJ (C3H) female mice were treated with NaF in the drinking water (0 ppm, 50 ppm and 100 ppm F ion) for 3 weeks. Bone marrow cells were harvested for osteoclastogenesis and hematopoietic colony-forming cell assays. Sera were analyzed for biochemical and bone markers. Femurs, tibiae, and lumbar vertebrae were subjected to microCT analysis. Tibiae and femurs were subjected to histology and biomechanical testing, respectively. The results demonstrated new actions of F on osteoclastogenesis and hematopoietic cell differentiation. Strain-specific responses were observed. The anabolic action of F was favored in B6 mice exhibiting dose-dependent increases in serum ALP activity (p<0.001); in proximal tibia trabecular and vertebral BMD (tibia at 50&100 ppm, p=0.001; vertebrae at 50 and 100 ppm, p=0.023&0.019, respectively); and decrease in intact PTH and sRANKL (p=0.045 and p<0.001, respectively). F treatment in B6 mice also resulted in increased numbers of CFU-GEMM colonies (p=0.025). Strain-specific accumulations in bone [F] were observed. For C3H mice, dose-dependent increases were observed in osteoclast potential (p<0.001), in situ trabecular osteoclast number (p=0.007), hematopoietic colony forming units (CFU-GEMM: p<0.001, CFU-GM: p=0.006, CFU-M: p<0.001), and serum markers for osteoclastogenesis (intact PTH: p=0.004, RANKL: p=0.022, TRAP5b: p<0.001). A concordant decrease in serum OPG (p=0.005) was also observed. Fluoride treatment had no significant effects on bone morphology, BMD, and serum PYD cross-links in C3H suggesting a lack of significant bone resorption. Mechanical properties were also unaltered in C3H. In conclusion, short term F treatment at physiological levels has strain-specific effects in mice. The expected anabolic effects were observed in B6 and novel actions hallmarked by enhanced osteoclastogenesis shifts in hematopoietic cell differentiation in the C3H strain.

Addition of monofluorophosphate to estrogen therapy in postmenopausal osteoporosis: a randomized controlled trial

INTRODUCTION

Treatment of osteoporosis with high-dose fluoride alone does not reduce fracture risk. We hypothesized that the antifracture efficacy of fluoride could be optimized by its use in low doses combined with an antiresorptive agent.

EXPERIMENTAL SUBJECTS

Subjects included 80 women with postmenopausal osteoporosis who had been taking estrogen for at least 1 yr.

METHODS

Subjects were randomized to receive monofluorophosphate (MFP) (fluoride content of 20 mg/d) or placebo over 4 yr in a double-blind trial.

RESULTS AND DISCUSSION

There were progressive increases in lumbar spine bone density over the duration of the study (MFP, 22%; placebo, 6%; P < 0.0001). In the trabecular bone of L3, these increases were even greater (MFP, 49%; placebo, 2.5%; P < 0.0001). In the proximal femur, there were smaller but significant treatment effects (P = 0.015). Total body scans and their subregions also showed significantly greater increases in the MFP group. Bone formation markers increased significantly in the MFP group at yr 1. Hyperosteoidosis was present in biopsies from five of seven MFP subjects, with osteomalacia in two of seven. The hazards ratio for vertebral fractures was 0.20 (95% confidence interval, 0.05-1.30), and the incidence rate ratio was 0.12 (95% confidence interval, 0.06-0.23; P < 0.01). The hazards ratio for nonvertebral fractures was 3.3 (95% confidence interval, 0.8-12.0).

CONCLUSIONS

We conclude that fluoride at 20 mg/d produces substantial increases in bone mineral density but still interferes with bone mineralization. This indicates that most previous studies with this ion have used toxic doses and that much lower doses should be assessed to find a safe dose window for the use of this powerful anabolic agent.

Pathogenesis and Prevention of Bone Loss in Patients Who Have Kidney Disease and Receive Long-Term Immunosuppression

The coexistence of kidney disease with a need for immunosuppressive therapy leads to the convergence of several threats to bone. These comprise general effects of the primary disease, e.g., inflammatory state, more specific effects of acute renal failure or chronic kidney disease, and effects of therapies. Multisystem inflammatory disease that requires immunosuppression is associated frequently with kidney damage, and any reduction of kidney function that takes the patient into or beyond chronic kidney disease stage 2 for more than a short time is likely to have a negative impact on bone health. Bone mineral density frequently is low and fracture rates are high, although correlations often are poor. Chronic inflammation leads to local and systemic imbalance between bone formation and resorption. Upregulation of NF-kappabeta ligand (RANKL) and variable downregulation of osteoprotegerin are implicated, and bone health may improve in response to treatment of the inflammatory state. Certain immunosuppressive agents, especially glucocorticoids and calcineurin inhibitors, contribute further to bone loss. Antiresorptive agents such as bisphosphonates are used widely and, although able to prevent loss of bone mineral density, have uncertain effects on fracture rates. Augmentation of anabolic activity is desirable but elusive. Synthetic parathyroid hormone is untested but has potential. Manipulation of the RANKL/osteoprotegerin system now is feasible using antibodies to RANKL or synthetic osteoprotegerin. In the future, manipulation of the calcium-sensing receptor using calcimimetic or calcilytic agents may allow the anabolic effects of parathyroid hormone to be harnessed to good effect. With all of these therapies, it will be important to assess response in relation to important clinical end points such as fracture.

Pharmacotherapy of osteoporosis in postmenopausal women: focus on safety

The therapy of osteoporosis has made enormous strides in the last decade. There is now a range of interventions, each with its pros and cons. Calcium and vitamin D supplementation remain the foundation and have few safety issues. Bisphosphonates are widely used, though gastrointestinal tolerance is a problem with some oral preparations. Intravenous administration may circumvent this, although this introduces the smaller problem of acute phase reactions. The side effect profile of hormone replacement therapy (HRT) is still being delineated after 40 years of use, with substantial new information expected in the next few years. This will clarify its place in the medical management of the menopause. Raloxifene appears to have a superior safety profile to HRT, though its efficacy on bone may be less. While none of these options is suitable for everyone, the range of available therapies does mean that most patients can find an intervention that is effective and acceptable.