Tumor necrosis factor α decreases 1,25-dihydroxyvitamin D3 receptors in osteoblastic ROS 17/2.8 cells

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Abstract
Bone remodeling is a complex process regulated by systemic hormones, local cytokines, and growth factors. One cytokine, tumor necrosis factor α (TNF-α), is known to have potent inhibitory effects on osteoblast matrix protein production and to stimulate osteoclast recruitment. We have previously shown that TNF-α inhibits 1,25-(OH)2D3-stimulated synthesis of bone gla protein (BGP), an abundant and osteoblast-specific matrix constituent. We hypothesized that the mechanism of TNF-α action included inhibition of intracellular 1,25-(OH)2D3 receptor (VDR) number or function. To test this, the osteoblastic cell line ROS 17/2.8 was cultured in the presence or absence of TNF-α (100 ng/ml), and binding of [3H]1,25-(OH)2D3 to 0.3 M KCl extracts of cytosol was measured by equilibrium assay. Specific [3H]1,25-(OH)2D3 binding decreased 70%, 25 h after addition of TNF-α. The decrease in [3H]1,25-(OH)2D3 binding was seen by 18 h, was sustained throughout the 72 h culture period, and was greater in low-density cultures. Scatchard analysis confirmed that TNF-α (100 ng/ml for 24 h) caused a decrease in the number of binding sites without change in VDR affinity. Northern analysis with a VDR riboprobe revealed that the decrease in VDR occurred without a change in the 4.4 kb steady-state VDR mRNA [VDR/cyclophilin mRNA signal ratio: control, 2.25; TNF-α, 2.24 (24 h),2.17 (40 h), n = 2 flasks/time point]. These results suggest that TNF-α action on osteoblastic cells includes an inhibitory effect on VDR number at a point distal to the synthesis of VDR mRNA.
Funding Information
  • NIH Training (2T32DK07298-11)