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Mechanistic insight into the effects of Aryl Hydrocarbon Receptor activation on osteogenic differentiation
Chawon Yun1, Joseph A Weiner1, Danielle S Chun1
1Northwestern University Department of Orthopaedic Surgery, Chicago, IL, USA.
Abstract:
While inhibition of bone healing and increased rates of pseudarthrosis are known adverse outcomes associated with cigarette smoking, the underlying mechanisms by which this occurs are not well understood. Recent work has implicated the Aryl Hydrocarbon Receptor (Ahr) as one mediator of the anti-osteogenic effects of cigarette smoke (CS), which contains numerous toxic ligands for the Ahr. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) is a high-affinity Ahr ligand frequently used to evaluate Ahr pathway activation. The purpose of this study was to elucidate the downstream mechanisms of dioxin action on bone regeneration and investigate Ahr antagonism as a potential therapeutic approach to mitigate the effects of dioxin on bone. Markers of osteogenic activity and differentiation were assessed in primary rat bone marrow stromal cells (BMSC) after exposure to dioxin, Ahr antagonists, or antagonist + dioxin. Four Ahr antagonists were evaluated: α-Naphthoflavone (ANF), resveratrol (Res), 3,3'-Diindolylmethane (DIM), and luteolin (Lut). Our results demonstrate that dioxin inhibited ALP activity, migratory capacity, and matrix mineralization, whereas co-treatment with each of the antagonists mitigated these effects. Dioxin also inhibited BMSC chemotaxis, while co-treatment with several antagonists partially rescued this effect. RNA and protein expression studies found that dioxin down-regulated numerous pro-osteogenic targets, whereas co-treatment with Ahr antagonists prevented these dioxin-induced expression changes to varying degrees. Our results suggest that dioxin adversely affects bone regeneration in a myriad of ways, many of which appear to be mediated by the Ahr. Our work suggests that the Ahr should be investigated as a therapeutic target to combat the adverse effects of CS on bone healing.
Insights
Cigarette smoke impairs bone healing by activating the Aryl Hydrocarbon Receptor (Ahr). Blocking Ahr with antagonists can mitigate these negative effects on bone regeneration, suggesting Ahr as a therapeutic target.
Area of Science:
- Bone regeneration
- Cellular signaling
- Toxicology
Background:
- Cigarette smoking is linked to poor bone healing and non-union (pseudarthrosis).
- The Aryl Hydrocarbon Receptor (Ahr) is implicated in mediating the anti-osteogenic effects of cigarette smoke components.
- 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) is a potent Ahr ligand used to study Ahr pathway activation.
Purpose of the Study:
- To understand how dioxin affects bone regeneration mechanisms via the Ahr pathway.
- To explore Ahr antagonism as a potential therapy against dioxin's detrimental effects on bone healing.
Main Methods:
- Primary rat bone marrow stromal cells (BMSC) were treated with dioxin and/or Ahr antagonists (ANF, Res, DIM, Lut).
- Osteogenic activity, differentiation markers, cell migration, and chemotaxis were assessed.
- RNA and protein expression of osteogenic targets were analyzed.
Main Results:
- Dioxin inhibited BMSC alkaline phosphatase (ALP) activity, migration, and matrix mineralization.
- Ahr antagonists partially or fully reversed dioxin's inhibitory effects on osteogenic activity and migration.
- Dioxin down-regulated pro-osteogenic gene and protein expression, which was prevented by Ahr antagonists.
Conclusions:
- Dioxin negatively impacts multiple aspects of bone regeneration, largely mediated by the Ahr pathway.
- Ahr antagonists show promise in counteracting the adverse effects of dioxin on bone healing.
- Targeting the Ahr pathway could be a therapeutic strategy to combat smoking-induced bone healing complications.