Corally López Vega
Husam Bensreti
Colby Gross
Dima Alhamad
Alok Tripathi
Shabiha Sultana
Christopher L. Yearwood
Jennifer Dorn
Caihong Dai
Wendy B. Bollag
Carlos M. Isales
Meghan E. McGee-Lawrence

Universidad Central del Caribe
Augusta University Medical College

Introduction

Osteoporosis remains a critical public health issue, particularly in patients exposed to glucocorticoids. The glucocorticoid receptor (GR) and aryl hydrocarbon receptor (AhR) pathways are central to bone physiology. Literature suggests that activated GR can influence AhR expression and vice versa; however, the implications of this interaction on osteoblast-lineage cells and bone health remain unclear. This study investigates the crosstalk between ligand-activated GR and AhR signaling, hypothesizing that their interaction functionally regulates bone homeostasis.

Methods

Mice with loxp sites flanking exon 2 of the GR gene were crossed with Osx-Cre+ mice to generate GR-conditional knockout (GR-CKO; GRfl/fl: Osx-Cre+) mice and wild-type (GR-WT; GRfl/fl: Osx–Cre–) littermates. At 6 months, mice received intraperitoneal injections of the AhR ligand Kynurenine (KYN) 5 days per week for 8 weeks. Body composition and bone mineral density (BMD) were measured using DXA scans at baseline and 4 weeks. Osteoblast progenitor ST2 cells were treated with GR (dexamethasone) and AhR (FICZ) ligands, followed by RT-qPCR to analyze GR and AhR target genes (Hsd11b1 and CYP1A1, respectively). Data were analyzed using t-tests or two-way ANOVA.

Results

In vitro RT-qPCR showed that ligand-activated GR inhibited AhR target gene expression (p=0.034). KYN treatment reduced whole-body BMD in GR-WT females (p<0.05) but not in GR-CKO females. No significant effects on femoral BMD were observed. Female GR-CKO mice treated with KYN also showed a trend toward reduced fat mass (p=0.09).

Conclusions

This study reveals functional crosstalk between GR and AhR pathways, with ligand-activated GR suppressing AhR signaling. The differing effects of KYN on BMD in GR-WT and GR-CKO mice highlight the complexity of GR-AhR interactions, suggesting potential therapeutic targets for bone-related disorders. Future studies will further investigate these interactions at protein and genomic levels.

IACUC Approval

Protocol #: 2023-101

Acknowledgments

Research was supported by the PARIS program (T35 AG067577), NIA P01-AG036675 (Project 4), and NIA R01 R01AG067510.

Keywords

Glucocorticoid receptor (GR), Aryl hydrocarbon receptor (AhR), Bone mineral density (BMD), Osteoblasts, Crosstalk