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Glucocorticoids decrease vitamin D receptor number and gene expression in human osteosarcoma cells
M Godschalk1, J R Levy, R W Downs
1Department of Internal Medicine, McGuire VA Medical Center, Richmond, Virginia.
Abstract:
The mechanisms by which glucocorticoids (GC) inhibit some actions of vitamin D [1,25-(OH)2D3] are not well understood, but there is growing evidence that GC alter vitamin D receptor (VDR) number. We studied the effects of dexamethasone (DEX) on VDR number and mRNA in the human osteosarcoma cell line, MG-63. The effects of DEX on 1,25-(OH)2D3 binding were examined by incubating confluent cells overnight in media without or with 10(-6) M DEX. DEX decreased VDR number (B max) by approximately 70% (110 versus 32 fmol/mg cellular protein, p less than 0.001) without significantly changing the apparent affinity (K'D) of 1,25-(OH)2D3 for its receptor (3.8 versus 2.2 x 10(-10) M, p greater than 0.05). Overnight incubation of MG-63 cells with DEX produced a time- and dose-responsive decrease in VDR mRNA compared to untreated controls (p less than 0.01). To determine the mechanism of the DEX-mediated decrease in VDR mRNA, the effect of DEX on VDR mRNA stability was studied. We found that the half-life for the VDR mRNA was approximately 5.7 h and was not significantly changed when the cells were incubated with DEX (approximately 6.3 h). We conclude that DEX decreases both VDR number and mRNA in MG-63 osteosarcoma cells. Since the half-life of VDR mRNA was not significantly modified by dexamethasone, glucocorticoids appear to decrease VDR mRNA by inhibiting VDR gene transcription or by affecting the processing of VDR mRNA.
Insights
Glucocorticoids like dexamethasone reduce vitamin D receptor (VDR) number and mRNA levels in osteosarcoma cells. This decrease in VDR is likely due to reduced gene transcription or mRNA processing, not altered mRNA stability.
Area of Science:
- Endocrinology
- Molecular Biology
- Cell Biology
Background:
- Glucocorticoids (GC) are known to inhibit some vitamin D actions.
- Evidence suggests GC influence vitamin D receptor (VDR) levels.
- The precise mechanisms of GC inhibition on vitamin D are not fully understood.
Purpose of the Study:
- To investigate the effects of dexamethasone (DEX) on VDR number and mRNA in MG-63 human osteosarcoma cells.
- To elucidate the mechanism by which DEX alters VDR expression.
Main Methods:
- MG-63 cells were treated with DEX.
- VDR number and binding affinity for 1,25-(OH)2D3 were measured.
- VDR mRNA levels and stability were assessed following DEX treatment.
Main Results:
- DEX significantly decreased VDR number by approximately 70% without altering 1,25-(OH)2D3 binding affinity.
- DEX treatment resulted in a time- and dose-dependent reduction in VDR mRNA.
- DEX did not significantly affect the half-life of VDR mRNA, indicating stability was maintained.
Conclusions:
- Dexamethasone reduces both VDR number and VDR mRNA in MG-63 cells.
- The observed decrease in VDR mRNA is likely mediated by inhibited VDR gene transcription or altered mRNA processing.
- Glucocorticoids impact vitamin D signaling pathways by modulating VDR expression.