Fluid Shear Stress Suppresses Osteoclast Differentiation in RAW264.7 Cells through Extracellular Signal-Regulated Kinase 5 (ERK5) Signaling Pathway.

BACKGROUND Although extracellular signal-regulated kinase 5 (ERK5) is known to be critical for osteoclast differentiation, there are few studies on how fluid shear stress (FSS) regulates osteoclast differentiation through the ERK5 signaling pathway. We examined the expression of nuclear factor of activated T cells c1 (NFATc1) in RAW264.7 cells and its downstream factors, including cathepsin K (CTSK), tartrate-resistant acid phosphatase (TRAP), matrix metalloproteinases-9 (MMP-9) and their relationship with ERK5. MATERIAL AND METHODS RAW264.7 cells were treated with RANKL, XMD8-92 (ERK5 inhibitor), and then loaded onto 12 dyn/cm² FSS for 4 days. Endpoints measured were osteoclast differentiation, bone resorption, and TRAP activity. Cell viability was detected by using the Cell Counting Kit-8 (CCK-8) assay. Western blot was used to analyze protein expression of phosphorylated-ERK5 (p-ERK5), NFATc1, CTSK, TRAP, and MMP-9. RESULTS FSS inhibited osteoclast differentiation and expression of NFATc1, CTSK, TRAP, and MMP-9; cell viability was not affected. ERK5 expression increased by FSS but not by RANKL, and it was blocked by XMD8-92. Furthermore, FSS suppressed osteoclast differentiation in RAW264.7 cells through ERK5 pathway. CONCLUSIONS Our findings demonstrated that FSS inhibited osteoclast differentiation in RAW264.7 cells via the ERK5 pathway through reduced NFATc1 expression and its downstream factors MMP-9, CTSK, and TRAP.