Effect of fluid flow-induced shear stress on osteoclast formation induced by osteocyte

To study the role of osteocyte in bone remodeling due to mechanical loading in vitro.

MLO-Y4 osteocyte-like cells were exposed to fluid flow-induced shear stress(12dyn/cm(2))for 0, 1, 2, 4, 6, 12, and 24 hours. Osteocyte exposed to shear stress at different time points were used in co-culture system for 9 days, and then the cells were stained with tartrate-resistant acid phosphatase on the 9(th) day and the amount of positively stained osteoclasts were counted and compared. The expressions of osteoprotegerin (OPG) and receptor activator of nuclear factor-kappa (RANKL) were detected by semi-quantitative reverse transcription polymerase chain reaction (semi-quantitative RT-PCR).

Compared with bone cells without stimulation with fluid flow-induced shear stress, the amount of osteocytes significantly decreased at all time points after the application of fluid flow-induced shear stress (all P<0.05). The OPG expression at mRNA levels was significantly up-regulated in the first 12 hours (P<0.001), the RANKL mRNA expression was significantly down-regulated in the first 4 hours (P<0.05), and the RANKL/OPG ratio significantly decreased within 12 hours (P<0.01). However, all these indicators showed no significant difference at 24 hours when compared with the pre-stimulation level.

Osteocytes may act as mechanosensors that are able to inhibit bone resorption after mechanical loading; however, such effect shows certain adaptation ability to shear stress as time goes.