Abstract
Glucocorticoid osteoporosis is a serious side effect of long term glucocorticoid uptake and it is caused by osteoblast apoptosis and imbalance in the major bone remodeling pathway RANK/RANKL/OPG. The impact of glucocorticoid on the maintenance of RANK/RANKL/OPG is well explored; dexamethasone was shown to disturb the ratio between OPG and RANKL level by decreasing the expression level of OPG and increasing level of RANKL. Here, were aimed to decipher whether glucocorticoid receptor directly influences RANKL promoter activity and its transcriptional regulation. We demonstrate that overexpression of glucocorticoid receptor (GR) NR3C1 increased RANKL promoter activity in human osteosarcoma, cervical cancer (2-fold) and adenocarcinoma cells (4.5-fold). Mutational analysis revealed that +352 site in the RANKL promoter is functional glucocorticoid responsive element (GRE) since the effect of GR on RANKL promoter activity was diminished by mutation at this site. Overexpression of NR3C1 upregulated RANKL mRNA expression 1.5-fold in human A549 and HOS cells. On the other hand silencing of NR3C1 caused slight decrease in RANKL mRNA level, suggesting that NR3C1 directly accounts for RANKL transcriptional regulation. Using electrophoretic mobility shift assay we demonstrate that NR3C1 binds to the proximal RANKL promoter region. Our study provides evidences that NR3C1 directly upregulates RANKL transcription in human cell lines and connects the missing link in the mechanism of RANK/RANKL/OPG imbalance of glucocorticoid induced osteoporosis.