Nitric Oxide is Involved in the Down-regulation of SOST Expression Induced by Mechanical Loading

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Mechanical stimulation reduces sclerostin expression in rodents. However, few data are available about the effect of physical stimuli in human systems. Recently we observed that the demethylating agent AzadC induces SOST expression in bone cells. This allowed us in this study to explore the effect of mechanical loading on SOST expression by subjecting AzadC-treated human bone cells to pulsating fluid flow (PFF). PFF significantly decreased the AzadC-induced expression of SOST. This effect persisted for at least 24 h, and in fact SOST expression was lower at 24 h after PFF treatment than at 1 h after PFF treatment (PFF/static ratio 0.47 ± 0.04 vs. 0.63 ± 0.03 respectively, p = 0.03). The PFF-induced decrease in SOST expression was not due to a change in the methylation profile of the SOST promoter. However, PFF stimulated nitric oxide (NO) synthesis, which appeared essential for the PFF effect on SOST expression. In fact, the NO synthase inhibitor 1400 W prevented the effect of PFF on SOST expression. Moreover, the NO-donor SNAP decreased SOST mRNA in bone organ cultures. The conditioned medium (CM) of cells subjected to PFF induced a 38 ± 4 % decrease in SOST expression (p = 0.03) in static cultures and diminished the transcriptional activity of reporter vectors with the cloned SOST promoter (Static-CM: 1.47 ± 0.10 vs. 0.78 ± 0.09, p = 0.02). This is consistent with a PFF-induced secretion of factors that modulate SOST. Our results suggest that NO and other soluble factors are involved in the inhibition of SOST expression by PFF.