2,3,7,8-Tetrachlorodibenzo-p-dioxin inhibits differentiation of normal diploid rat osteoblasts in vitro

Polycyclic aromatic hydrocarbons in bone homeostasis

Prolonged exposure to polycyclic aromatic hydrocarbons (PAHs) may result in autoimmune diseases, such as rheumatoid arthritis (RA) and osteoporosis (OP), which are based on an imbalance in bone homeostasis. These diseases are characterized by bone erosion and even a disruption in homeostasis, including in osteoblasts and osteoclasts. Current evidence indicates that multiple factors affect the progression of bone homeostasis, such as genetic susceptibility and epigenetic modifications. However, environmental factors, especially PAHs from various sources, have been shown to play an increasingly prominent role in the progression of bone homeostasis. Hence, it is essential to investigate the effects and pathogenesis of PAHs in bone homeostasis. In this review, recent progress is summarized concerning the effects and mechanisms of PAHs and their ligands and receptors in bone homeostasis. Moreover, strategies based on the effects and mechanisms of PAHs in the regulation of the bone balance and alleviation of bone destruction are also reviewed. We further discuss the future challenges and perspectives regarding the roles of PAHs in autoimmune diseases based on bone homeostasis.

Bone toxicity induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and the retinoid system: A causality analysis anchored in osteoblast gene expression and mouse data

Dioxin exposures impact on bone quality and osteoblast differentiation, as well as retinoic acid metabolism and signaling. In this study we analyzed associations between increased circulating retinol concentrations and altered bone mineral density in a mouse model following oral exposure to 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD). Additionally, effects of TCDD on differentiation marker genes and genes involved with retinoic acid metabolism were analysed in an osteoblast cell model followed by benchmark dose-response analyses of the gene expression data. Study results show that the increased trabecular and decreased cortical bone mineral density in the mouse model following TCDD exposure are associated with increased circulating retinol concentrations. Also, TCDD disrupted the expression of genes involved in osteoblast differentiation and retinoic acid synthesis, degradation, and nuclear translocation in directions compatible with increasing cellular retinoic acid levels. Further evaluation of the obtained results in relation to previously published data by the use of mode-of-action and weight-of-evidence inspired analytical approaches strengthened the evidence that TCDD-induced bone and retinoid system changes are causally related and compatible with an endocrine disruption mode of action.

Bone Disruption and Environmental Pollutants

BACKGROUND

Endocrine Disrupting Chemicals (EDCs) are ubiquitous and may significantly contribute in environmental pollution, thus contaminating humans and wildlife. Environmental pollutants could interfere with bone homeostasis by means of different mechanisms, which include hormonal imbalance, direct osteoblasts toxicity and enanchment of osteoclasts activity, thus leading to osteopenia or osteoporosis. Among these, bisphenols, dioxins, polycyclic aromatic hydrocarbons, polychlorobiphenyls, poly- and perfluoroalkyls, phthalates, parabens, organotins and cadmium may play a role in bone distuption.

METHODS

PubMed/MEDLINE, ISI-web of knowledge and Google scholar databases were searched for medical subject headings terms and free-text word related to the aforementioned classes of chemicals and bone metabolism and remodelling for better clarifying and understanding the main mechanisms of bone disruption.

RESULTS

Several of EDCs act as xenoestrogens. Considering that estrogens play a significant role in regulating bone remodeling, most of these chemicals generate hormonal imbalance with possible detrimental consequences on bone tissue structure and its mechanical and non-mechanical properties.

DISCUSSION

A lot of evidences about bone distruptors came from in vitro studies or animal models, and conduct to equivocal results. In addition, a few data derived form humans and most of these data focused on the impact of EDCs on bone mineral density without considering their influence on long-term fracture risk. Moreover, it should be taken into account that humans are exposed to a mixture of EDCs and the final effect on bone metabolism might be the result of either a synergism or antagonist effects among them. Age of first exposure, cumulative dose exposure over time, and the usually observed non-monotonic dose-response curve for EDCs should be considered as other important variable influencing the final effect on bone metabolism.

CONCLUSION

Taking into account these variables, observational studies are needed to better analyze this issue both for echological purpose and to preserve bone health.

The Role of Aryl-Hydrocarbon Receptor (AhR) in Osteoclast Differentiation and Function

Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that plays a crucial role in bone remodeling through altering the interplay between bone-forming osteoblasts and bone-resorbing osteoclasts. While effects of AhR signaling in osteoblasts are well understood, the role and mechanism of AhR signaling in regulating osteoclastogenesis is not widely understood. AhR, when binding with exogenous ligands (environmental pollutants such as polycylic aryl hydrocarbon (PAH), dioxins) or endogenous ligand indoxyl-sulfate (IS), has dual functions that are mediated by the nature of the binding ligand, binding time, and specific pathways of distinct ligands. In this review, AhR is discussed with a focus on (i) the role of AhR in osteoclast differentiation and function and (ii) the mechanisms of AhR signaling in inhibiting or promoting osteoclastogenesis. These findings facilitate an understanding of the role of AhR in the functional regulation of osteoclasts and in osteoclast-induced bone destructive conditions such as rheumatoid arthritis and cancer.

Molecular and cellular mechanisms linking air pollution and bone damage

Air pollution is the second most important risk factor associated with noncommunicable diseases after smoking. The effects of pollution on health are commonly attributable to particulate matter (PM), a complex mixture of particles suspended in the air. PM can penetrate the lower respiratory tract and has harmful direct and indirect effects on different organs and tissues. Direct effects are caused by the ability of PM components to cross the respiratory membrane and enter the bloodstream; indirect effects are systemic consequences of the local airway response. Recent work suggests that PM is an independent risk factor for low bone mineral density and osteoporosis-related fractures. Osteoporosis is a common age-related disease closely linked to bone fractures, with severe clinical consequences affecting quality of life, morbidity, and mortality. In this review, we discuss potential mechanisms behind the association between outdoor air pollution, especially PM, and bone damage. The discussion features four main mechanisms: 1) several different atmospheric pollutants can induce low-grade systemic inflammation, which affects bone metabolism through a specific effect of cytokines such as TNFα, IL-1β, IL-6, and IL-17 on osteoblast and osteoclast differentiation and function; 2) some pollutants, particularly certain gas and metal compounds, can cause oxidative damage in the airway and bone cells; 3) different groups of pollutants can act as endocrine disruptors when binding to the receptors in bone cells, changing their functioning; and 4) air pollution can directly and indirectly cause vitamin D deficiency. Characterizing these mechanisms will better define the physiopathology of bone damage, and recognizing air pollution as a modifiable risk factor for osteoporosis will inform environmental policies. Such knowledge will also guide the prevention of fractures due to fragility and help reduce health-related costs.

The Protective Effect of α-Tocopherol on the Content of Selected Elements in the Calvaria for Exposed Hens to TCDD in the Early Embryonic Period

This paper focuses on negative effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on element content in male chicken calvaria and α-tocopherol (vitamin E) ability to reduce its toxic potential on bone mineralization in offspring. In the experiment carried out once, a solution containing only DMSO, TCDD, TCDD + α-tocopherol, and exclusively α-tocopherol was administrated. Subsequently, on the 5th day after hatching, the mineral composition of the chicken calvaria was evaluated. The results obtained suggest that the use of α-tocopherol contributes to the maintenance of the concentration of calcium, magnesium, and manganese in the chicken calvaria treated with TCDD in the embryonic period. In turn, vitamin E increases the level of zinc. It has been found that α-tocopherol in chicken embryos has a protective effect against disturbance of level of chosen trace elements in the bones of offspring caused by the TCDD.

Indirect study of the effect of α-tocopherol and acetylsalicylic acid on the mineral composition of bone tissue in the offspring of female rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin: long-term observations

This paper discusses problems related to the influence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the mineral composition of the calvaria in the offspring of female rats. The female rats were administered with a single dose of TCDD and subsequently, after three-weeks, with α-tocopherol or acetylsalicylic acid. The research focused on analysis of the main mineral elements (Ca, Mg, Fe, Zn). The aim of the study was to determine the effect of dioxins and various doses of drugs on bone mineral composition in a six-month observation period. The mineral composition was analyzed using an atomic spectrometry method. Data were statistically analyzed and verified at a significance level of p = 0.05. The use of α-tocopherol normalizes bone resorption and formation disturbed by TCDD, maintaining the content of the studied elements at the physiological level. In turn, administration of acetylsalicylic acid limits the bone resorption process, which affects the element content.

Modulating effect of selected pharmaceuticals on bone in female rats exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

This paper discusses the problems connected with the influence of TCDD on the mechanical properties and structure of the bone tissue in female rats treated with the dioxin. Moreover an analysis of the protective role of tocopherol, acetylsalicylic acid, dexamethazone and levamisol was performed. Rat tibiae were assessed by mechanical testing, and histological and trace element analysis. It was proved that TCDD has negative effect on bone mechanical properties, histological microstructure and trace element content. The results indicate that usage of both steroid and non-steroid anti-inflammatory medicaments, along with tocopherol and levamisol, modulating their activity, can reduce the negative effect of dioxin activity on the bone tissue of female rats intoxicated with TCDD.

Perfluorooctanoic acid induces mitochondrial dysfunction in MC3T3-E1 osteoblast cells

Perfluorooctanoic acid (PFOA), a stable organic perfluorinated compound, is an emerging persistent organic pollutant, found widely in human and wildlife populations. Recent evidence suggests that exposure to environmental toxicants can be associated with higher risks of osteoporosis and fractures. We studied the cellular toxicology of PFOA in MC3T3-E1osteoblast cells. To examine the effect of PFOA, we measured cell viability, reactive oxygen species (ROS), mitochondrial superoxide, and mitochondrial parameters including adenosine triphosphate (ATP) level, mitochondrial membrane potential (MMP), cardiolipin content, and cytochrome c release in MC3T3-E1 cells. Incubating MC3T3-E1 cells in different concentrations of PFOA for 48 h resulted in a concentration-dependent decrease in cell viability and significant inductions of ROS and mitochondrial superoxide. Moreover, PFOA induced MMP collapse, cardiolipin peroxidation, cytochrome c release, and decreased ATP levels, which in turn induced apoptosis or necrosis. When osteoblast differentiation markers were assessed, PFOA treatment caused a significant reduction in alkaline phosphatase activity, collagen synthesis, and mineralization in the cells. In summary, we found an ROS- and mitochondria-mediated pathway for the induction of cell damage by PFOA in MC3T3-E1 cells. Together, our results indicate that mitochondrial toxicity could be a plausible mechanism for the toxic effects of PFOA on osteoblast function.

From the Cover: Embryonic Exposure to TCDD Impacts Osteogenesis of the Axial Skeleton in Japanese medaka, Oryzias latipes

Recent studies from mammalian, fish, and in vitro models have identified bone and cartilage development as sensitive targets for dioxins and other aryl hydrocarbon receptor ligands. In this study, we assess how embryonic 2,3,7,8-tetrachlorochlorodibenzo-p-dioxin (TCDD) exposure impacts axial osteogenesis in Japanese medaka (Oryzias latipes), a vertebrate model of human bone development. Embryos from inbred wild-type Orange-red Hd-dR and 3 transgenic medaka lines (twist:EGFP, osx/sp7:mCherry, col10a1:nlGFP) were exposed to 0.15 nM and 0.3 nM TCDD and reared until 20 dpf. Individuals were stained for mineralized bone and imaged using confocal microscopy to assess skeletal alterations in medial vertebrae in combination with a qualitative spatial analysis of osteoblast and osteoblast progenitor cell populations. Exposure to TCDD resulted in an overall attenuation of vertebral ossification characterized by truncated centra, and reduced neural and hemal arch lengths. Effects on mineralization were consistent with modifications in cell number and cell localization of transgene-labeled osteoblast and osteoblast progenitor cells. Endogenous expression of osteogenic regulators runt-related transcription factor 2 (runx2) and osterix (osx/sp7), and extracellular matrix genes osteopontin (spp1), collagen type I alpha I (col1), collagen type X alpha I (col10a1), and osteocalcin (bglap/osc) was significantly diminished at 20 dpf following TCDD exposure as compared with controls. Through global transcriptomic analysis more than 590 differentially expressed genes were identified and mapped to select pathological states including inflammatory disease, connective tissue disorders, and skeletal and muscular disorders. Taken together, results from this study suggest that TCDD exposure inhibits axial bone formation through dysregulation of osteoblast differentiation. This approach highlights the advantages and sensitivity of using small fish models to investigate how xenobiotic exposure may impact skeletal development.

Benzo[a]pyrene/aryl hydrocarbon receptor signaling inhibits osteoblastic differentiation and collagen synthesis of human periodontal ligament cells

BACKGROUND AND OBJECTIVE

Cigarette smoking has detrimental effects on periodontal tissue, and is known to be a risk factor for periodontal disease, including the loss of alveolar bone and ligament tissue. However, the direct effects of cigarette smoking on periodontal tissue remain unclear. Recently, we demonstrated that benzo[a]pyrene (BaP), which is a prototypic member of polycyclic aryl hydrocarbons and forms part of the content of cigarettes, attenuated the expression of extracellular matrix remodeling-related genes in human periodontal ligament (PDL) cells (HPDLCs). Thus, we aimed to examine the effects of BaP on the osteoblastic differentiation and collagen synthesis of HPDLCs.

MATERIAL AND METHODS

HPDLCs were obtained from healthy molars of three patients, and quantitative reverse transcription-polymerase chain reaction were performed for gene expression analyses of cytochrome P450 1A1 and 1B1, alkaline phosphatase, bone sialoprotein and aryl hydrocarbon receptor (AhR), a receptor for polycyclic aryl hydrocarbons. We have also analyzed the role of the AhR, using 2-methyl-2H-pyrazole-3-carboxylic acid (2-methyl-4-o-tolylazo-phenyl)-amide (CH-223191), which is an AhR antagonist.

RESULTS

The treatment of HPDLCs with BaP reduced mRNA expression of osteogenic genes, alkaline phosphatase activity, mineralization and collagen synthesis. The treatment with CH-223191 subsequently restored the observed suppressive effects of BaP on HPDLCs.

CONCLUSIONS

The present results suggest that BaP exerts inhibitory effects on the maintenance of homeostasis in HPDL tissue, such as osteoblastic differentiation and collagen synthesis of HPDLCs, and that this signaling pathway could be suppressed by preventing the transactivity of AhR. Future studies may unveil a role for the inhibition of AhR as a promising therapeutic agent for periodontal disease caused by cigarette smoking.

New insights to the role of aryl hydrocarbon receptor in bone phenotype and in dioxin-induced modulation of bone microarchitecture and material properties

Bone is a target for high affinity aryl hydrocarbon receptor (AHR) ligands, such as dioxins. Although bone morphology, mineral density and strength are sensitive endpoints of dioxin toxicity, less is known about effects on bone microarchitecture and material properties. This study characterizes TCDD-induced modulations of bone tissue, and the role of AHR in dioxin-induced bone toxicity and for normal bone phenotype. Six AHR-knockout (Ahr(-/-)) and wild-type (Ahr(+/+)) mice of both genders were exposed to TCDD weekly for 10 weeks, at a total dose of 200μg/kgbw. Bones were examined with micro-computed tomography, nanoindentation and biomechanical testing. Serum levels of bone remodeling markers were analyzed, and the expression of genes related to osteogenic differentiation was profiled using PCR array. In Ahr(+/+) mice, TCDD-exposure resulted in harder bone matrix, thinner and more porous cortical bone, and a more compact trabecular bone compartment. Bone remodeling markers and altered expression of a number of osteogenesis related genes indicated imbalanced bone remodeling. Untreated Ahr(-/-) mice displayed a slightly modified bone phenotype as compared with untreated Ahr(+/+) mice, while TCDD exposure caused only a few changes in bones of Ahr(-/-) mice. Part of the effects of both TCDD-exposure and AHR-deficiency were gender dependent. In conclusion, exposure of adult mice to TCDD resulted in harder bone matrix, thinner cortical bone, mechanically weaker bones and most notably, increased trabecular bone volume fraction in Ahr(+/+) mice. AHR is involved in bone development of a normal bone phenotype, and is crucial for manifestation of TCDD-induced bone alterations.

Endocrine disruptors and bone metabolism

Bone microenvironment is a complex dynamic equilibrium between osteoclasts and osteoblasts and is modulated by a wide variety of hormones and osteocyte mediators secreted in response to physiological and pathological conditions. The rate of remodeling involves tight coupling and regulation of both cells population and is regulated by a wide variety of hormones and mediators such as parathyroid hormone, prostaglandins, thyroid hormone, sex steroids, etc. It is also well documented that bone formation is easily influenced by the exposure of osteoblasts and osteoclasts to chemical compounds. Currently, humans and wildlife animals are exposed to various environmental xenoestrogens typically at low doses. These compounds, known as endocrine disruptor chemicals (EDCs), can alter the systemic hormonal regulation of the bone remodeling process and the skeletal formation. This review highlights the effects of the EDCs on mammalian bone turnover and development providing a macro and molecular view of their action.

The effects of the organopollutant PCB 126 on bone density in juvenile diamondback terrapins (Malaclemys terrapin)

Bone is a dynamic tissue with diverse functions including growth, structural support, pH balance and reproduction. These functions may be compromised in the presence of organopollutants that can alter bone properties. We exposed juvenile diamondback terrapins (Malaclemys terrapin) to 3,3',4,4',5-pentachlorobiphenyl (PCB 126), a ubiquitous anthropogenic organochlorine, and measured organic content, apparent bone mineral density (aBMD) using radiography and computed tomography, and quantified bone microstructure using histological preparations of femora. PCB-exposed terrapins were smaller in total size. Skulls of exposed animals had a higher organic content and a skeletal phenotype more typical of younger animals. The femora of exposed individuals had significantly reduced aBMD and significantly more cortical area occupied by non-bone. Because bone is an integral component of physiology, the observed skeletal changes can have far-reaching impacts on feeding and locomotor performance, calcium reserves and ultimately life history traits and reproductive success. Additionally, we caution that measurements of bone morphology, density, and composition from field-collected animals need to account not only for relatedness and age, but also environmental pollutants.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure on bone material properties

Dioxins are known to decrease bone strength, architecture and density. However, their detailed effects on bone material properties are unknown. Here we used nanoindentation methods to characterize the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on nanomechanical behaviour of bone matrix. Pregnant rats were treated with a single intragastric dose of TCDD (1 microg/kg) or vehicle on gestational day 11. Tibias of female offspring were sampled on postnatal day (PND) 35 or 70, scanned at mid-diaphysis with pQCT, and evaluated by three-point bending and nanoindentation. TCDD treatment decreased bone mineralization (p<0.05), tibial length (p<0.01), cross-sectional geometry (p<0.05) and bending strength (p<0.05). Controls showed normal maturation pattern between PND 35 and 70 with decreased plasticity by 5.3% and increased dynamic hardness, storage and complex moduli by 26%, 13% and 12% respectively (p<0.05), while similar maturation was not observed in TCDD-exposed pups. In conclusion, for the first time, we demonstrate retardation of bone matrix maturation process in TCDD-exposed animals. In addition, the study confirms that developmental TCDD exposure has adverse effects on bone size, strength and mineralization. The current results in conjunction with macromechanical behaviour suggest that reduced bone strength caused by TCDD is more associated with the mineralization and altered geometry of bones than with changes at the bone matrix level.

Transgenic mice with a constitutively active aryl hydrocarbon receptor display a gender-specific bone phenotype

Bone tissue homeostasis is governed by hormones, growth factors, and cytokines and can be distorted by environmental pollutants, such as ligands to the aryl hydrocarbon receptor (AhR). A transgenic mouse expressing a constitutively active aryl hydrocarbon receptor (CA-AhR), mimicking continuous low-dose exposure to AhR ligands, was used to explore potential long-term effects of these ligands on bone. The density, content, and dimensions of cortical and trabecular bone, as well as physical properties, were significantly altered in female transgenic mice, while almost no alterations were detected in males. Osteoclast volume density and serum level of C-telopeptide of type I collagen (CTX), reflecting osteoclast activity, were both increased by approximately 60% in female CA-AhR mice, while serum tartrate-resistant acid phosphatase (TRAP) 5b, reflecting osteoclast numbers, was unchanged. Subsequently, the resorption index (CTX/TRAP 5b) was increased by 90%, indicating increased osteoclast activity in female CA-AhR. Moreover, the protein level of the osteoclast collagenase cathepsin K was increased by 40% in bone extracts of female CA-AhR mice. The messenger RNA expression of several osteoclast- and osteoblast-associated genes was altered in female transgenic mice but not in males. Notably, early markers for osteoclast and osteoblast differentiation were normal, while the expression of functional markers of osteoclasts and osteoblasts were reduced. In conclusion, a low continuous activation of the AhR leads to a skeletal phenotype with increased bone resorption associated with more ductile bones in females but not in males. The results indicate the presence of an interaction between the AhR and a female-specific mechanism implicated in inhibition of osteoclast development and function. Female bone tissue appears more susceptible to dioxins and other AhR ligands than male bone tissue.

Dioxins interfere with differentiation of osteoblasts and osteoclasts

We have previously shown that the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) affects bone growth, modelling and mechanical strength in vivo. In this study, we utilized differentiation of bone marrow stem cells to osteoblasts and osteoclasts as a model system to study the effects of TCDD on bones. Stem cells were isolated from bone marrow of femurs and tibias of rats and mice. Progress of osteoblastic differentiation was monitored by measuring mRNA expression levels of differentiation markers from control and TCDD-treated cells using quantitative RT-PCR. TCDD significantly and dose-dependently decreased the mRNA levels of RUNX2, alkaline phosphatase and osteocalcin. Also the activity of alkaline phosphatase was significantly inhibited in both rat and mice cells. In the case of osteoclasts, TCDD decreased the number of TRACP+ multinucleated cells, with corresponding decreases in the number of F-actin rings and the area of resorption. Studies in AHR-knockout mice indicated that TCDD has no effect on the expression of osteoblastic differentiation markers suggesting that TCDD mediates its effects by AHR. Both osteoblastic and osteoclastic effects took place at very low doses of TCDD, as in most cases 100 fM TCDD was enough to significantly affect the differentiation markers. Therefore, differentiation of osteoblasts and osteoclasts from bone marrow stem cells seems to be a very sensitive target for TCDD. Disrupting effects in osteoblastic cells, in addition to disturbed osteoclastogenesis, may thus play a role in adverse effects on bone quality in TCDD exposed animals.

Short-term exposure to dioxin impairs bone tissue in male rats

Chronic and sub-chronic studies in rats have previously shown that dioxin-like compounds impair the bone tissue homeostasis. In the present study, tibiae and serum were analyzed to study possible effects of short term dioxin exposure on rats. Two month old (ca. 200g) male rats were injected with 50microg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) kg(-1) bw and tibiae were excised 5d following the exposure. Bone composition, dimensions and strength were analyzed by pQCT and three-point bending test on tibiae. In addition, detailed bone composition was analyzed by optical emission spectroscopy (ICP-OES) and Fourier transform infrared spectrometry (FTIR). Analysis of the serum bone biomarkers procollagen type-I N-terminal propeptide (PINP) and carboxyterminal cross linking teleopeptide (CTX) were also performed. pQCT-results showed alterations in the metaphysis, with a significant decrease in trabecular bone cross-sectional area (-19%, p<0.05) and a significant increase in total bone mineral density (+7%, p<0.05) in TCDD-exposed rats. Analyses of the bones by ICP-OES and FTIR showed that bones from exposed rats had a higher relative proportion of crystalline phosphate (+13% for a1080 and +11% for a1113, p<0.05) and lower acid phosphate content (-22% for a1145, p<0.05), resembling the composition of more mature bones. Serum analysis showed that the bone formation marker PINP was decreased (-37%, p<0.05) and that the bone resorption marker CTX was increased (+14%, p<0.05) indicating a net loss of bone tissue. In conclusion, 5d of exposure to TCDD was sufficient to negatively affect bone tissue in male rats.

AHR-dependent misregulation of Wnt signaling disrupts tissue regeneration

The origins of molecular toxicology can be traced to understanding the interactions between halogenated aromatic hydrocarbons and the aryl hydrocarbon receptor (AHR). The physiological consequences of activation of the aryl hydrocarbon receptor are diverse, and we are just beginning to understand the importance of the AHR signal transduction pathway in homeostasis and disease. The many downstream targets that mediate these biological responses remain undefined. Studies have exploited the power of the zebrafish model to elucidate the mechanisms by which AHR activation disrupts biological signaling. Recent genomic analysis performed in a zebrafish tissue regeneration model revealed functional cross talk between AHR and the well-established Wnt/beta-catenin signal transduction pathway. This review focuses on the development of the zebrafish model of AHR biology and the application of in vivo toxicogenomics to unravel molecular mechanisms.

Environmental toxicants may modulate osteoblast differentiation by a mechanism involving the aryl hydrocarbon receptor

The AHR mediates many of the toxicological effects of aromatic hydrocarbons. We show that AHR expression in osteoblasts parallels the induction of early bone-specific genes involved in maturation. The AHR may not only mediate the effects of toxicants, but with an as yet unidentified ligand, be involved in the differentiation pathways of osteoblasts.

INTRODUCTION

Metabolic bone diseases arise as a result of an imbalance in bone cell activities. Recent evidence suggests that environmental toxicants may be contributing factors altering these activities. One candidate molecule implicated in mediating the toxic effects of exogenous compounds is the aryl hydrocarbon receptor (AHR).

MATERIALS AND METHODS

Osteoblasts isolated from neonatal rat calvaria were analyzed for AHR expression by quantitative PCR, Western blot, and immunohistochemistry. In addition, AHR activation was evaluated by electromobility gel shift assay and fluorescence microscopy.

RESULTS

Our findings showed AHR expression in mature osteoblasts in vivo. The pattern of AHR expression peaks after alkaline phosphatase and before induction of osteocalcin. We first show that AHR functions as a transactivating receptor in osteoblasts, as evidenced by its ligand-dependent migration to the nucleus and its association with known dioxin response elements. AHR activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) mediated the induction of cytochrome p450 1A1 and cycloxygenase-2 protein levels. This effect could be inhibited by the potent AHR antagonist, 3'4 methoxynitroflavone. Furthermore, lead treatment of osteoblasts upregulates the expression of AHR mRNA and protein levels, supporting a novel mechanism whereby lead in the skeleton may increase the sensitivity of bone cells to toxicant exposure.

CONCLUSIONS

These data imply that the AHR mediates the effects of aromatic toxicants on bone and that AHR expression is regulated during osteoblast differentiation.

Expression of osteoblast marker genes in rat calvarial osteoblast-like cells, and effects of the endocrine disrupters diphenylolpropane, benzophenone-3, resveratrol and silymarin

Compelling evidence indicates that some endocrine disrupters (EDs), acting as selective estrogen-receptor modulators, interfere with osteoblast differentiation and function. Hence, we investigated whether four EDs [bisphenol-A (BSP), benzophenone-3 (BP3), resveratrol and silymarin] affect differentiation and growth of rat calvarial osteoblast-like (ROB) cells in primary in vitro culture. ROB cells were cultured for up 30 days in a medium supplemented with fetal calf serum (FCS), and conventional RT-PCR detected the expression of collagen-1alpha and osteonectin mRNAs through the entire culture period. Real time-PCR demonstrated that at days 2 and 7 of culture the expressions of collagen-1alpha and osteonectin were very low, and underwent a 192- and a 334-fold increase, respectively, at day 21 of culture. In contrast, osteocalcin expression remained unchanged from days 2 to 21 of culture. EIA showed that ROB cells secreted sizeable amounts of osteocalcin and osteopontin between days 13 and 15 of culture. EDs were added at day 13 of culture at concentrations ranging from 10(-10) to 10(-6) M, being the culture medium deprived of FCS, and their effects were tested 48 h later. None of EDs was found to affect osteocalcin and osteopontin secretion from ROB cells, suggesting that their effects were tested at a relatively earlier stage of culture, when ROB cell differentiation into osteoblats is not fully accomplished, and/or the presence of estrogens contained in FCS is needed for EDs to exert their osteoblast-differentiation modulating action. BSP and BP3, but not resveratrol and silymarin, decreased proliferative activity of cultured ROB cells, a cytotoxic effect conceivably independent of their estrogen-receptor modulating activity.

Osteopontin: A rapid and sensitive response to dioxin exposure in the osteoblastic cell line UMR-106

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is an endocrine disrupting environmental pollutant that, among other effects, affects bone tissue. TCDD modulates the transcription of various genes, e.g., CYP1A1, and the present study is a part of a project aiming at developing an in vitro model system for identifying biomarkers specific for dioxin-induced effects in osteoblasts. Osteopontin (OPN) is an adhesion protein, suggested to be important in bone remodeling and our results indicate that TCDD down-regulates the transcription of OPN in the osteoblastic cell line, UMR-106. The present study shows that UMR-106 expresses the AhR and that the expression of CYP1A1 is induced after exposure to TCDD, while down-regulation of OPN is an even more rapid response and a sensitive biomarker to TCDD exposure in this osteoblastic cell line. In conclusion, this osteoblastic cell line may be used as an in vitro model-system for studying dioxin-induced effects on osteoblasts.

Bone resorption by aryl hydrocarbon receptor-expressing osteoclasts is not disturbed by TCDD in short-term cultures

Polychlorinated dibenzo-p-dioxins (PCDDs) are highly toxic environmental contaminants, and 2,3,7,8-tetrachlorobenzo-p-dioxin (TCDD) is the most potent dioxin. Dioxins bind specifically to the cytosolic aryl hydrocarbon receptor (AHR), which is a ligand-activated transcription factor, and a majority of toxic effects of dioxins are mediated via AHR. We have recently demonstrated that TCDD disrupts bone modeling and decreases bone mechanical strength, and that partial resistance to these effects is related to an altered transactivation domain in AHR structure. In order to better understand the effects of dioxins on bone, we studied the presence and precise localization of AHR and also the number and activity of osteoclasts after TCDD treatments. Total RNA was extracted from mixed bone cell population cultures and expression of AHR mRNA was studied using RT-PCR. Bone cells expressed a considerable amount of AHR mRNA. To see which bone cells express AHR, immunostainings were performed in primary rat bone cell cultures, pure human osteoclast cultures and histological sections from AHR knockout and wild type bones. Immunostaining revealed a strong expression of AHR both in osteoclasts and osteoblasts with an especially prominent stain in bone resorbing osteoclasts. Effects of dioxin on primary bone cells were evaluated after TCDD treatment in the pit formation assay. The activity of osteoclasts was not affected measured as the percentage of active osteoclasts and the actual area of resorbed bone. These data indicate that even though TCDD-treated bones show decreased mechanical strength and size, this is not a direct result from increased osteoclastic bone resorption.

Inhibition of osteoclast differentiation by polycyclic aryl hydrocarbons is dependent on cell density and RANKL concentration

We investigated the effect of representative polycyclic aryl hydrocarbons (PAHs), benzo[a]pyrene (BaP), and 7,12-dimethylbenz[a]anthracene (DMBA) on osteoclast differentiation and function by using dispersed cancellous bone derived rabbit osteoclasts and the RAW264.7 cells. These cells differentiate into osteoclasts when exposed to receptor activator of NF-kappaB ligand (RANKL). The rabbit osteoclasts were exposed to 10(-6) to 10(-9)M BaP or DMBA and the tartrate-resistant acid phosphatase (TRAP)-positive cells were counted. The effect of PAHs on osteoclast differentiation in dispersed rabbit osteoclast-containing stromal cell populations was cell density dependent, suggesting that the cell density of stromal cells, osteoclast precursors, and/or mature osteoclasts are factors regulating the effect of PAHs. To investigate the direct effect of BaP on osteoclast differentiation, RAW264.7 cells were exposed to 10(-5) to 10(-6) M BaP. Treatment of RAW264.7 cells cultured with 25 ng/ml soluble RANKL and 10(-5)M BaP for 5 days decreased osteoclast differentiation, TRAP activity levels, and resorption of bone-like substrata. The inhibition was prevented by 10(-6) to 10(-7) M resveratrol, an aryl hydrocarbon receptor (AhR) antagonist, and by higher concentrations of RANKL. To investigate the ability of RANKL to reverse BaP-mediated inhibition, gene expression was determined by RT-PCR. Cytochrome P450 1B1 (CYP1B1) mRNA, one of the genes activated by BaP, was present only in the groups exposed to BaP; the levels of CYP1B1 mRNA decreased in the presence of increasing concentrations of RANKL. These results suggest that the inhibitory effects of PAHs on osteoclastogenesis are direct and likely involve interaction of the RANKL and PAH signaling pathways.

Effects of In Utero and Lactational TCDD Exposure on Bone Development in Differentially Sensitive Rat Lines

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a notorious model compound of highly toxic environmental pollutants, polychlorinated dibenzo-p-dioxins (PCDDs). Their toxic effects are mediated via cytosolic aryl hydrocarbon receptor (AHR). We studied the effects of several dose levels of TCDD on developing rat bone after maternal exposure at different times of gestation and lactation in three differentially sensitive rat lines. Rat lines A, B, and C differ in their sensitivity to TCDD due to mutated AHR (Ahr(hw)) in line A and another TCDD-resistance allele (B(hw)) in line B. Line C rats have no resistance alleles. Offspring were analyzed for bone mineral density and geometry by peripheral quantitative computed tomography (pQCT) and for bone biomechanics by three-point bending at mid-diaphysis of tibia and femur and by axial loading at femoral neck. TCDD treatment resulted in bone defects, mainly in offspring of the most sensitive line C at a maternal dose of 1 microg/kg. They included decreased bone length, cross-sectional area of cortex, and bone mineral density. Mechanical testing revealed significantly reduced bending breaking force and stiffness of tibia, femur, and femoral neck. The effects were exposure time-dependent, and earlier exposure caused more severe defects. Gestational exposure alone was not sufficient, but lactational exposure was required to cause the bone defects. Most of the defects were recovered at the age of 1 year. The results indicate that dioxins affect developing bone by interfering with bone growth and mechanical strength and that the effects are mainly reversible. The dioxin-resistance alleles, Ahr(hw) and B(hw) increase the resistance to these defects.

Estrogen supplementation modulates effects of the endocrine disrupting pollutant PCB126 in rat bone and uterus

The aims of the present study are to compare effects of estrogen depletion (OVX) and estradiol (E2) supplementation on the tissue effects of exposure to the endocrine disrupting organochlorine 3,3',4,4',5-pentachlorobiphenyl (PCB126). For this purpose two highly estrogen-dependent tissues, bone and uterus, were studied. Forty rats exposed to PCB126 (ip) for 3 months (total dose 384 microg/kg body weight (bw)) were randomized in to OVX/sham operation or E2 supplementation (ip, 23 microg/kg, 3 days weekly) per vehicle (corn oil) groups in a 2 x 2 factorial design. Sham operated rats were treated with vehicle, PCB or PCB plus E2 (sham, sham + PCB and sham + PCB + E2, n=10 per group) whereas ovariectomized were treated with vehicle, PCB or PCB plus E2(OVX, OVX + PCB and OVX + PCB + E2, n=10 per group). As control groups served OVX or sham, and OVX + E2 (n=10 in each group). In OVX rats PCB126 + E2 treatment increased trabecular bone volume (TBV) (P<0.01), whilst the opposite was found in sham-operated rats (P<0.01). In OVX animals exposed to PCB126, E2 supplementation decreased the uterine weight and increased the uterine ERbeta mRNA level, whilst no difference was found between the PCB126 and PCB126 + E2 exposed groups in the sham-operated animals. In conclusion, estrogen modulates PCB126 induced effects on trabecular bone, as well as several uterine parameters. These results further support an important role of estrogen on the toxic effects of PCB126 on bone and uterus.

Aryl hydrocarbon receptor-mediated suppression of GH receptor and Janus kinase 2 expression in mice

Differential mRNA display revealed that a cDNA encoding the major urinary protein 2 (MUP2) that belongs to the lipocalin superfamily was absent in livers of mice treated with 3-methylcholanthrene (MC). The expression of MUP2 is known to be stimulated by growth hormone (GH), through the GH receptor (GHR), Janus kinase 2 (JAK2) and signal transducer and activator of transcription 5 (STAT5) signal transduction pathway. Since MC is an aryl hydrocarbon receptor (AhR) ligand, the effects of MC treatment on the expression of GHR, JAK2 or STAT5 in the livers of wild-type or AhR-null mice were examined. The result indicated that the expression of GHR and JAK2 mRNA was greatly decreased by MC in wild-type mice but not in AhR-null mice. In addition, the binding activity of STAT5 bound to STAT5-binding element was reduced after MC treatment in wild-type mice but not in AhR-null mice. Based on these results, we conclude that the suppression of MUP2 mRNA expression by MC is caused by the AhR-mediated disruption of the GH signaling pathway. Possible mechanism(s) by which exposure to aromatic hydrocarbons causes a decrease in the body weight of mice, which has been referred to as wasting syndrome, will also be discussed.

Abnormal bone composition in female juvenile American alligators from a pesticide-polluted lake (Lake Apopka, Florida)

Reproductive disorders have been found in pesticide-exposed alligators living in Lake Apopka, Florida (USA). These disorders have been hypothesized to be caused by exposure to endocrine- disruptive estrogen-like contaminants. The aim of this study was to expand our analysis beyond previous studies by investigating whether bone tissue, known to be affected by sex steroid hormones, is a potential target of endocrine disruptors. Long bones from 16 juvenile female alligators from Lake Apopka (pesticide-contaminated lake) and Lake Woodruff (control lake) were evaluated by peripheral quantitative computed tomography. We observed significant differences in bone composition, with female alligators from the contaminated lake having greater trabecular bone mineral density (BMD), total BMD, and trabecular mineral content compared with females from the control lake (p < 0.05). Increased trabecular and total BMD measurements suggest that juvenile female alligators from Lake Apopka were exposed to contaminants that created an internal environment more estrogenic than that normally observed. This estrogenic environment could be caused by both natural and anthropogenic compounds. Effects on BMD indicate interference with bone homeostasis. We hypothesize that contaminants present in the lake inhibit the natural and continuous resorption of bone tissue, resulting in increased bone mass. Although this is the only study performed to date examining effects of environmental estrogenic compounds on alligator bones, it supports previous laboratory-based studies in rodents. Further, this study is important in demonstrating that the alterations in morphology and physiology induced in free-ranging individuals living in environments contaminated with endocrine-active compounds are not limited to a few systems or tissues; rather, effects can be observed in many tissues affected by these hormones.

Role of mediator in transcriptional activation by the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AHR) binds many aromatic hydrocarbon compounds and mediates their carcinogenesis. We demonstrate that the endogenous AHR physically associates with the endogenous TRAP/DRIP/ARC/Mediator complex in a ligand-dependent manner. The Med220 subunit, which is known to interact with several nuclear hormone receptors through its LXXLL motifs, potentiates AHR-dependent reporter gene activity in an LXXLL-independent manner. Depletion of Med220 substantially reduces endogenous AHR-mediated transcription from the mouse cytochrome P4501A1 gene (CYP1A1). Both Med220 and CDK8 (another subunit of TRAP/DRIP/ARC/Mediator) are recruited to the CYP1A1 enhancer in a TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin)-dependent fashion in vivo, and Med220 LXXLL motifs are not required. Med220 rapidly and persistently associates with the enhancer but not the promoter of the CYP1A1 gene after TCDD treatment with similar kinetics as AHR and the coactivators p300 and p/CIP. Our findings demonstrate a novel role for Med220 in AHR-regulated transcription that differs mechanistically from its role in transcriptional regulation by other previously studied transcription factors.

Inhibition of osteoclast formation by 3-methylcholanthrene, a ligand for arylhydrocarbon receptor: suppression of osteoclast differentiation factor in osteogenic cells

We investigated the effects of 3-methylcholanthrene (3MC), a ligand for arylhydrocarbon receptor (AhR), on osteoclastogenesis. Osteoclast-like cells, in cocultures with mouse spleen cells and clonal osteogenic stromal ST2 cells, are formed from spleen cells by a combination of the receptor activator of nuclear factor-kappaB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) produced by ST2 cells in response to 1alpha,25(OH)(2) Vitamin D(3). 3MC dose-dependently inhibited the formation of mono- and multinuclear osteoclast-like cells. However, 3MC did not inhibit the formation of osteoclast-like cells from mouse spleen cells which was supported by the exogenous soluble RANKL and M-CSF. 3MC did not affect the formation of an actin ring and pits on slices of dentine by osteoclast-like cells, both of which are typical indices of osteoclast activity. These results suggest that 3MC affects osteoclast-supporting cells such as ST2 cells but not osteoclast precursor cells and mature osteoclastic cells. When we measured the expression levels of RANKL mRNA in ST2 cells, 3MC dose-dependently decreased the level of this mRNA. However, 3MC did not affect levels of mRNAs for osteoprotegerin (OPG), M-CSF, and the receptor of 1alpha,25(OH)(2) Vitamin D(3) in ST2 cells. Furthermore, soluble RANKL was able to counteract the inhibitory effect of 3MC on the formation of osteoclast-like cells. Our findings indicate that 3MC inhibits osteoclastogenesis via the inhibition of RANKL expression in osteoblastic cells.

Incidence of hospitalized osteoporotic fractures in cohorts with high dietary intake of persistent organochlorine compounds

OBJECTIVES

Environmental factors might be of importance for the dramatically increasing incidence of osteoporotic fractures. Persistent organochlorine compounds (POCs) have, in animal studies, impaired normal bone metabolism and resulted in increased bone fragility. The goal of this study was to assess whether a high dietary intake of POCs through fatty fish from the Baltic Sea may result in an increased incidence of osteoporotic fractures.

METHODS

Fishermen and their wives from the Swedish east coast on the Baltic Sea (exposed) and west coasts (unexposed) constituted the study base. Information on vital status and hospitalization of persons with fractures from 1987 to 1996, classified according to the ninth revision of the International Classification of Diseases, was retrieved for 17,823 subjects through register linkages. The impact of coastal stretch on fracture incidence was assessed by Poisson regression models, with age and calendar year also being taken into account.

RESULTS

There was a significantly increased incidence rate ratio (IRR: 2.29, 95% confidence intervals (CI): 1.23-4.28) for vertebral fractures among east-coast women, and a similar, but non-significant, similar tendency in men (IRR 1.45, 95% CI 0.74-2.84). Such cohort differences were not seen for any other fracture type.

CONCLUSIONS

The results give some indirect support for the notion that a high dietary intake of POCs through fatty fish from the Baltic Sea might be a risk factor for vertebral fractures, but it cannot be excluded that confounding from differences in smoking habits might explain part of the observed effects. To clarify this, detailed individual information on exposure and potential confounders has to be ascertained.

3-Methylcholanthrene, which binds to the arylhydrocarbon receptor, inhibits proliferation and differentiation of osteoblasts in vitro and ossification in vivo

3-Methylcholanthrene (3MC) is a ligand for arylhydrocarbon receptor (AhR), which binds dioxin. We examined the effects of 3MC on the proliferation and differentiation of osteoblasts using cultures of rat calvarial osteoblast-like cells (ROB cells) and mouse calvarial clonal preosteoblastic cells (MC3T3-E1 cells). Analysis by RT-PCR revealed that the mRNAs for AhR and AhR nuclear translocators were expressed in both ROB and MC3T3-E1 cells. Cell proliferation and the synthesis of DNA by ROB cells and MC3T3-E1 cells were markedly inhibited on exposure of cells to 3MC. Furthermore, 3MC reduced the activity of alkaline phosphatase and the rate of deposition of calcium by cells. The level of expression of mRNA for osteocalcin, which is a marker of osteoblastic differentiation, was also depressed by 3MC. Moreover, when 3MC (1 mg/kg body weight) was administered sc to pregnant mice at 10.5, 12.5, and 14.5 d post coitus, fetuses examined subsequently at 15.5 or 17.5 d post coitus revealed evidence of inhibition of appropriate calcification of bones. The treated metacarpals showed no subperiosteal bone matrix histologically. Our findings indicate that 3MC might have critical effects on the formation of bone both in vivo and in vitro.

Polycyclic aromatic hydrocarbons present in cigarette smoke cause bone loss in an ovariectomized rat model

A number of epidemiological studies have suggested that cigarette smoking is a risk factor for osteoporosis. Benzo(a)pyrene (BaP) and 7,12-dimethylbenz(a)anthracene (DMBA) are polycyclic aromatic hydrocarbons (PAHs) found in the tar fraction of cigarette smoke, as well as in car exhaust and furnace gases. We hypothesized that BaP and DMBA are responsible, through interaction with the aryl hydrocarbon receptor (AhR), for the bone loss and fragility seen in smoking-related osteoporosis. In this study four groups of 9-month-old Sprague-Dawley rats were examined. An intact group served as controls. A second control was the ovariectomized (ovx) group. The third group (ovx + E(2)) were ovariectomized and also given a continuous basal dose of estrogen by implanted estrogen pellet (0.085 mg of 17beta-estradiol per rat). The fourth group (ovx + E(2) + BaP/DMBA) was ovariectomized with an estradiol pellet, and received subcutaneous injections of 250 microg/kg of BaP/DMBA weekly for 15 weeks. The loss of ovarian function allowed the study of a direct effect of BaP/DMBA on bone while the concomitant estrogen repletion prevented ovx-related bone loss. Dual-energy X-ray absorptiometry (DEXA), histomorphometry, image analysis, and mechanical testing were used to determine the effect of the treatments on bone. The DEXA results showed a significant (p < 0.05) decrease in bone mineral density compared with intact controls with both ovx alone and with ovx + E(2) + BaP/DMBA treatment. The ovx + E(2) rats were similar to the intact controls. The osteoid parameters showed a significant increase (p < 0.05) with BaP/DMBA addition vs. intact controls, mimicking the ovx rats. The ovx + E(2) rats had osteoid parameters comparable to those of intact rats. Bone connectivity was decreased in the ovx and ovx + E(2) + BaP/DMBA animals. Connectivity of the ovx + E(2) rats was comparable to that of intact animals. A decrease in failure force was seen in three-point bending for the ovx + E(2) + BaP/DMBA group and in vertebral compression in both the ovx and ovx + E(2) + BaP/DMBA groups vs. intact controls. The mechanical properties of the ovx + E(2) rats were similar to those of intact rats. These results demonstrate that BaP/DMBA causes a loss of bone mass and bone strength, possibly through an increase in bone turnover. This is the first in vivo study linking environmental toxicants, found in the tar fraction of cigarette smoke and in urban air pollution, to loss of bone mass and strength in estrogen-replete ovx rats.

Aryl hydrocarbon receptor is required for prevention of blood clotting and for the development of vasculature and bone in the embryos of medaka fish, Oryzias latipes

The aryl hydrocarbon receptor (AHR) is a member of ligand-activated transcription factors and conserved among vertebrates. To investigate the role of AHR in fish development, medaka embryos were treated with agonist (2,3,7,8-tetrachlorodibenzo-p-dioxin), antagonists (alpha-naphthoflavone and resveratrol), and inhibitor (piperonyl butoxide) of cytochromes (Cyts) P450 encoded by a battery of target genes. These embryos were found to have similar abnormal phenotypes. Among the most consistent phenotypes were blood clotting and malformation of bone that were associated with vascular damages. These results thus indicate that control of AHR is important for proper development of fish embryos. AHR may control levels of Cyts P450 that are responsible for synthesis and metabolism of a toxic compound that caused the abnormal phenotypes. Complementary DNA fragments encoding AHR homologs were cloned from medaka embryos. AHR-specific mRNA was ubiquitously expressed in embryos and adult tissues.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on bone in two rat strains with different aryl hydrocarbon receptor structures

Polychlorinated dibenzo-p-dioxins (PCDDs) are highly toxic environmental contaminants, and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most potent dioxin. Here, we studied the effects of TCDD on bone. Two rat strains, Han/Wistar (H/W) and Long-Evans (L-E), were used because they exhibit a 1000-fold sensitivity difference in acute lethality of TCDD, which difference is related to the aryl hydrocarbon receptor (AHR). TCDD inhibited the tibial growth dose dependently, the effect being manifested at lower doses in the more sensitive L-E strain. In H/W rats the effect of TCDD was seen only at the high dose of 170 microg/kg (p < 0.05), whereas in the sensitive L-E rats a significant reduction of bone growth was already seen at 1.7 microg/kg (p < 0.01). This reduction was caused by the smaller tibial size because the diaphyseal bone mineral density (BMD) did not change. The three-point bending breaking force of the tibia was significantly reduced in H/W rats at 170 microg/kg (p < 0.05), but tibial stiffness was lower already at the dose of 17 microg/kg (p < 0.05). In the sensitive L-E strain, both breaking force and stiffness were reduced at the dose of 17 microg/kg (p < 0.001). These results indicate that TCDD dose-dependently interferes with bone growth, modeling, and mechanical strength. The altered transactivation domain of AHR is associated with a lower sensitivity of bone to TCDD in H/W rats, suggesting that AHR plays a role in modulating the effects of dioxins on bone.

Effects of dioxin and estrogen on collagenase-3 in UMR 106-01 osteosarcoma cells

Since estrogen is important in preventing osteoporosis in postmenopausal women and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is an estrogen antagonist in reproductive tissues, we investigated the effects of 17beta-estradiol (E2) and TCDD on collagenase-3 secretion using parathyroid hormone (PTH)-stimulated UMR 106-01 cells, a rat osteoblastic osteosarcoma cell line. Whereas E2 or TCDD had no effect on UMR cells in the absence of PTH, cells grown in the presence of 10(-7) M PTH, which induces a dramatic 30-fold increase in collagenase-3 secretion, surprisingly demonstrated a further stimulation of collagenase-3 secretion in the presence of TCDD or E2. However, the potentiating response was biphasic; i.e., at higher concentrations of E2 or TCDD, there was no enhancement of the PTH effect. PTH induces multiple effects on UMR cells, including inducing collagenase-3 mRNA transcription and regulating its extracellular abundance through a specific receptor and endocytosis. Thus, we investigated the ability of TCDD or E2 to stimulate the induction of collagenase-3 mRNA using Northern analysis. As previously reported, PTH dose dependently induced collagenase-3 mRNA after 4 h of treatment. There was little effect of TCDD or E2 on PTH-induced levels of collagenase-3 mRNA. These data could not account for the final effects on secreted collagenase-3. We postulated that low concentrations of E2 and TCDD may downregulate the collagenase-3 endocytotic two-step receptor-mediated process that includes the LDL-receptor-related protein to enhance the effects of PTH. However, this was not the case. Therefore, we conclude that low concentrations of TCDD and estrogen alter translation or secretion of PTH-stimulated collagenase-3.

Change of bone tissue composition and impaired bone strength in rats exposed to 3,3',4,4',5-pentachlorobiphenyl (PCB126)

The aim of this study was to compare effects of estrogen depletion (ovariectomy) and exposure to 3,3',4,4',5-pentachlorobiphenyl (PCB126) on bone strength and bone tissue composition in the rat. Half of the rats were ovariectomized (n=20) and the remainder were sham-operated. Ten of the ovariectomized rats and ten of the sham operated were exposed to PCB126 (ip injections) for 3 months (total dose, 384 microgram/kg bodyweight), while those remaining received the vehicle. The humerus and femur were used for analysis of torsional strength and biochemical studies, respectively. Both sham-operated and ovariectomized animals showed a significantly shorter bone length, lower water content and a decreased torsional stiffness when exposed to PCB126. Sham-operated rats exposed to PCB126 had lower maximum torque when compared with sham operated controls. The PCB126-exposed rats also exhibited a significantly lower collagen concentration, but showed a higher pyridinoline concentration of cortical bone. PCB126 exposure decreased the hepatic level of vitamin A but increased vitamin A levels in serum and kidneys. Ovariectomy per se increased bone length and organic content and decreased the inorganic content significantly, but did not affect any of the tested biomechanical parameters. In conclusion, this study showed that the common environmental pollutant PCB126 impaired bone strength and altered bone composition. It is hypothesized that these effects might partly be explained by PCB-induced retinoid disturbances.

Effects of the antiestrogenic environmental pollutant 3,3',4,4', 5-pentachlorobiphenyl (PCB #126) in rat bone and uterus: diverging effects in ovariectomized and intact animals

The purpose of this study was to compare effects on rat bone and uterus of estrogen depletion and exposure to the coplanar PCB-congener 3,3',4,4',5-pentachlorobiphenyl (PCB #126) which exhibits anti-estrogenic properties. Half of the rats were ovariectomized (n = 20) and the other half were sham-operated. Ten of the ovariectomized rats and ten of the sham operated were exposed to PCB #126 (ip injections) for 3 months (total dose: 384 microgram/kg body wt). The remaining control rats were injected with corn oil (vehicle). The rats were killed and the tibiae and uteri were dissected. The left tibia was used for measurements of weight, length, and bone mineral density and the right for histomorphometrical analysis. The uteri were analyzed with respect to estrogen receptor content. PCB #126 exposure did not affect bone mineral density or trabecular bone volume of tibia in sham-operated rats. In ovariectomized rats PCB #126 exposure resulted in a decreased length and an increased bone mineral density of tibia. An obvious PCB #126 induced increase in osteoid surface was observed in sham-operated rats. The cortical thickness and the organic content of the tibia were also increased in these rats. In estrogen deprived tissue like the uteri of ovariectomized rats, PCB #126 showed weak estrogen agonistic activity. The observed effects of PCB #126 on bone and uterine tissues differed between ovariectomized and sham-operated rats.