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.


BACKGROUND
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).


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.
Ryan
Elizabeth P
EP
Department of Environmental Medicine, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642, USA.
Holz
Jonathan D
JD
Mulcahey
Mary
M
Sheu
Tzong-Jen
TJ
Gasiewicz
Thomas A
TA
Puzas
J Edward
JE
D000469:Alkaline Phosphatase / Q000378:metabolism; D000818:Animals; D002454:Cell Differentiation / Q000187:drug effects*; D002478:Cells, Cultured; D051546:Cyclooxygenase 2 / Q000378:metabolism; D019363:Cytochrome P-450 CYP1A1 / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D015386:Hazardous Substances / Q000494:pharmacology*; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D010006:Osteoblasts / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D000072317:Polychlorinated Dibenzodioxins / Q000494:pharmacology*; D051381:Rats; D018336:Receptors, Aryl Hydrocarbon / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation / Q000235:genetics; D015854:Up-Regulation
D015386:Hazardous Substances; D000072317:Polychlorinated Dibenzodioxins; D018336:Receptors, Aryl Hydrocarbon; D019363:Cytochrome P-450 CYP1A1; D051546:Cyclooxygenase 2; D000469:Alkaline Phosphatase