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Abstract
Osteoblasts play a pivotal role in load-driven bone formation by activating Wnt signaling through a signal from osteocytes as a mechanosensor. Osteoblasts are also sensitive to mechanical stimulation, but the role of RhoA, a small GTPase involved in the regulation of cytoskeleton adhesion complexes, in mechanotransduction of osteoblasts is not completely understood. Using MC3T3-E1 osteoblast-like cells under 1 hr flow treatment at 10 dyn/cm2, we examined a hypothesis that RhoA signaling mediates the cellular responses to flow-induced shear stress. To test the hypothesis, we conducted genome-wide pathway analysis and evaluated the role of RhoA in molecular signaling. Activity of RhoA was determined with a RhoA biosensor, which determined the activation state of RhoA based on a fluorescence resonance energy transfer between CFP and YFP fluorophores. A pathway analysis indicated that flow treatment activated phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signaling as well as a circadian regulatory pathway. Western blot analysis revealed that in response to flow treatment phosphorylation of Akt in PI3K signaling and phosphorylation of p38 and ERK1/2 in MAPK signaling were induced. FRET measurement showed that RhoA was activated by flow treatment, and an inhibitor to a Rho kinase significantly reduced flow-induced phosphorylation of p38, ERK1/2, and Akt as well as flow-driven elevation of the mRNA levels of osteopontin and cyclooxygenase-2. Collectively, the result demonstrates that in response to 1 hr flow treatment to MC3T3-E1 cells at 10 dyn/cm2, RhoA plays a critical role in activating PI3K and MAPK signaling as well as modulating the circadian regulatory pathway.
Acknowledgment
This study was supported by NIH (AR052144 to HY).
Declaration of interest
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.