Abstract
Fluid flow plays an important role in load-induced bone remodeling. However, the molecular mechanism of flow-induced signal transduction in osteoblasts remains unclear. In endothelial cells, fluid flow alters activation of NF-kappaB resulting in changes in expression of cell adhesion molecules. To test the hypothesis that fluid flow alters NF-kappaB activation and expression of cell adhesion molecules in osteoblastic cells, we examined the effect of oscillating fluid flow (OFF) on tumor necrosis factor (TNF)-alpha-induced NF-kappaB activation in rat osteoblast-like UMR106 cells. We found that OFF inhibits NF-kappaB-DNA binding activities, especially TNF-alpha-induced p50-p65 heterodimer NF-kappaB activation and TNF-alpha-induced intercellular adhesion molecule-1 mRNA expression. The inhibitory effects of OFF on both TNF-alpha-induced NF-kappaB activation and intercellular adhesion molecule-1 mRNA expression were shear stress-dependent and also increased with OFF exposure duration, indicating that OFF has potent effects on mechanotransduction pathways. OFF also inhibited TNF-alpha-induced IkappaBalpha degradation and TNF-alpha-induced IkappaB kinase (IKK) activity in a shear stress-dependent manner. These results demonstrate that IKK is an initial target molecule for OFF effects on osteoblastic cells. Thus, OFF inhibits TNF-alpha-induced IKK activation, leading to a decrease in phosphorylation and degradation of inhibitory IkappaBalpha, which in turn results in the decrease of TNF-alpha-induced NF-kappaB activation and potentially the transcription of target genes.
Highlights
Skeletal systems are maintained by continuous bone remodeling
To test the hypothesis that fluid flow alters NF-B activation and expression of cell adhesion molecules in osteoblastic cells, we examined the effect of oscillating fluid flow (OFF) on tumor necrosis factor (TNF)-␣-induced NF-B activation in rat osteoblast-like UMR106 cells
We previously demonstrated that Tumor necrosis factor (TNF)-␣ induces the expression of intracellular adhesion molecule (ICAM)-1 through transcription factor NF-B activation in osteoblasts, leading to the promotion of bone resorption [17, 18]
Summary
Skeletal systems are maintained by continuous bone remodeling. Mechanical loading, as well as a number of biochemical factors, regulates this bone remodeling. The inhibitory effects of OFF on both TNF-␣-induced NF-B activation and intercellular adhesion molecule-1 mRNA expression were shear stress-dependent and increased with OFF exposure duration, indicating that OFF has potent effects on mechanotransduction pathways. We previously demonstrated that TNF-␣ induces the expression of intracellular adhesion molecule (ICAM)-1 through transcription factor NF-B activation in osteoblasts, leading to the promotion of bone resorption [17, 18].
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