Roles of Aryl Hydrocarbon Receptor in Aromatase-Dependent Cell Proliferation in Human Osteoblasts

Abstract

Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and its expression is influenced by environmental compounds, such as 3-methylcholanthrene (3-MC) and β-naphthoflavone (β-NF). AhR and its downstream genes, such as CYP1A1, are considered to play a pivotal role in xenobiotic responses. AhR signaling has also been proposed to mediate osteogenesis in experimental animals, but its details have remained unclear. Therefore, in this study, we examined the possible roles of AhR in human bone. Immunohistochemical analysis revealed that AhR was detected in both osteoblasts and osteoclasts. We then screened AhR-target genes using a microarray analysis in human osteoblastic hFOB cells. Results of microarray and subsequent PCR analysis did reveal that estrogen metabolizing and synthesizing enzymes, such as CYP1B1 and aromatase, were increased by 3-MC in hFOB and osteosarcoma cell line, MG-63. The subsequent antibody cytokine analysis also demonstrated that interleukin-1β and -6 expression was increased by 3-MC and β-NF in hFOB cells and these interleukins were well known to induce aromatase. We then examined the cell proliferation rate of hFOB and MG-63 cells co-treated with 3-MC and testosterone as an aromatase substrate. The status of cell proliferation in both hFOB and MG-63 cells was stimulated by 3-MC and testosterone treatment, which was also inhibited by an estrogen blocker, aromatase inhibitor, or AhR antagonist. These findings indicated that AhR could regulate estrogen synthesis and metabolism in bone tissues through cytokine/aromatase signaling.

Maintenance of bone mass despite estrogen depletion in female common marmoset monkeys (Callithrix jacchus)

“…It is also possible that maintenance of bone mass in marmosets is regulated, at least in part, by potentially very low levels of estrogen that remain after ovx or by extra-ovarian estrogen (Kenealy et al, 2013;Terasawa, 2018) perhaps from bone (Miki et al, 2017), adipose tissue (DiSilvestro et al, 2014), or the hypothalamus (Guerriero, Keen, Millar, & Terasawa, 2012). Extra-ovarian estrogen can exert negative feedback regulation of gonadotropin release in female marmosets (Kraynak et al, 2017) suggesting that it might be involved in the bone preservation we observed.…”

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin increases the activation of aryl hydrocarbon receptor and is associated with the aggressiveness of osteosarcoma MG-63 osteoblast-like cells

“…The MG-63 cells are the intermediate cell type in terms of differences in extracellular matrix formation. Recently, Miki et al ( 26 ) reported AhR influenced by environmental compounds, such as 3-methylcholanthrene (3-MC) and β-naphthoflavone (β-NF), could regulate estrogen synthesis and metabolism in bone tissues through cytokine/aromatase signaling based on a hFOB cell and MG-63 cell line model. These increments of AhR-target genes in MG-63 cells were inhibited by co-treatment of AhR antagonist, CH-223191.…”

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

“…AhR is well-known to be expressed in bone cells, including OB and OC [ 12 , 24 ]. The activation of AhR by a representative uremic toxin and putative endogenous ligand indoxyl-sulfate (IS) affects osteoclastogenesis in a dose-dependent, exposure duration-dependent manner.…”