Background
Glucocorticoid-induced osteoporosis (GIOP) is the most common secondary osteoporosis. Patients with GIOP are susceptible to fractures and the subsequent delayed bone union or nonunion. Thus, effective drugs and targets need to be explored. In this regard, the present study aims to reveal the possible mechanism of the anti-GIOP effect of all-trans retinoic acid (ATRA).
Conclusion
ATRA can reverse DEX-inhibited osteogenic differentiation both in vitro and in vivo, which may be closely related to the downregulation of DEX-promoted Serpina3n. Hence, ATRA may be viewed as a novel therapeutic agent, and Serpina3n may act as a new target for GIOP.
Methods
Bone morphogenetic protein 9 (BMP9)-transfected mesenchymal stem cells (MSCs) were used as an in vitro osteogenic model to deduce the relationship between ATRA and dexamethasone (DEX). The osteogenic markers runt-related transcription factor 2 (RUNX2), alkaline phosphatase (ALP), and osteopontin were detected using real-time quantitative polymerase chain reaction, Western blot, and immunofluorescent staining assay. ALP activities and matrix mineralization were evaluated using ALP staining and Alizarin Red S staining assay, respectively. The novel genes associated with ATRA and DEX were detected using RNA sequencing (RNA-seq). The binding of the protein-DNA complex was validated using chromatin immunoprecipitation (ChIP) assay. Rat GIOP models were constructed using intraperitoneal injection of dexamethasone at a dose of 1 mg/kg, while ATRA intragastric administration was applied to prevent and treat GIOP. These effects were evaluated based on the serum detection of the osteogenic markers osteocalcin and tartrate-resistant acid phosphatase 5b, histological staining, and micro-computed tomography analysis.
Results
ATRA enhanced BMP9-induced ALP, RUNX2 expressions, ALP activities, and matrix mineralization in mouse embryonic fibroblasts as well as C3H10T1/2 and C2C12 cells, while a high concentration of DEX attenuated these markers. When DEX was combined with ATRA, the latter reversed DEX-inhibited ALP activities and osteogenic markers. In vivo analysis showed that ATRA reversed DEX-inhibited bone volume, bone trabecular number, and thickness. During the reversal process of ATRA, the expression of retinoic acid receptor beta (RARβ) was elevated. RARβ inhibitor Le135 partly blocked the reversal effect of ATRA. Meanwhile, RNA-seq demonstrated that serine protease inhibitor, clade A, member 3N (Serpina3n) was remarkably upregulated by DEX but downregulated when combined with ATRA. Overexpression of Serpina3n attenuated ATRA-promoted osteogenic differentiation, whereas knockdown of Serpina3n blocked DEX-inhibited osteogenic differentiation. Furthermore, ChIP assay revealed that RARβ can regulate the expression of Serpina3n.