Environmental Toxicants May Modulate Osteoblast Differentiation by a Mechanism Involving the Aryl Hydrocarbon Receptor

Abstract

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, 3Ј4 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.

Environmental contaminants

“…Other studies relating to the skeleton have reported the harmful effects of environmental toxicants to be also mediated by AhR. 101 These results are in line with the effects of BPA on AhR expression in the testis, as discussed above, suggesting that these effects may be coordinately regulated across different organs in the mammalian body. In a more recent study, Oury et al reported on an intriguing connection between the skelton and the testis; 102 these authors showed that osteocalcin directly affects the production of testosterone by Leydig cells in the testis which is needed for many aspects of reproductive function, including BTB maintenance and germ cell survival.…”

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

“…Most dioxin-induced effects are mediated via the aryl hydrocarbon receptor (AHR), which is a ligandactivated transcription factor. AHR is expressed in osteoblasts (Gierthy et al, 1994;Ilvesaro et al, 2005;Ryan et al, 2007) and osteoclasts (Ilvesaro et al, 2005), and in differentiating osteoblasts the expression peaks after the matrix maturation stage and before the initiation of mineralization (Ryan et al, 2007). Low concentrations of TCDD have been recently shown to interfere with the differentiation of both osteoblasts and osteoclasts in vitro (Korkalainen et al, 2009), and thereby bone formation and remodelling.…”

“…Low concentrations of TCDD have been recently shown to interfere with the differentiation of both osteoblasts and osteoclasts in vitro (Korkalainen et al, 2009), and thereby bone formation and remodelling. In differentiating osteoblasts, TCDD decreases the expression of RUNX2, alkaline phosphatase and osteocalcin (Korkalainen et al, 2009;Ryan et al, 2007), whereas in osteoclasts it decreases the number and resorbing activity of the cells (Korkalainen et al, 2009).…”

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

“…Developmental exposure to TCDD in rats adversely affects the structural and mechanical properties of bone (Miettinen et al, 2005), and numerous in vitro studies have confirmed TCDD-mediated inhibition of osteoblast differentiation in murine cell models (Gierthy et al, 1994;Korkalainen et al, 2009;Ryan et al, 2007;Singh et al, 2000;Yu et al, 2014). In humans, elevated exposure to polychlorinated dibenzo-p-dioxins and furans during breastfeeding has been strongly associated with hypomineralization of teeth (Alaluusua et al, 1996).…”