Effects of cyclic tension stress on the apoptosis of osteoclasts in vitro.

The aim of the present study was to investigate the effect of cyclic tension stress on osteoclast apoptosis in vitro using murine RAW264.7 cells treated with receptor activator of nuclear factor-κB. Using the EF3200 mechanical testing instrument with BioDynamic bioreactor system, cultured osteoclasts which were seeded in a silicone rubber membrane load carrier, were loaded with periodic cyclic stretch microstrain. The induced osteoclasts were subjected to 0, 5, 10 and 15% stretch microstrain for 1 h daily for three days. The number of tartrate-resistant acid phosphatase-positive osteoclasts and the resorption area were assessed. Osteoclast apoptosis was detected by the Annexin V-fluorescein isothiocyanate (FITC)/propidium iodide binding assay. The mRNA expression of Bcl-2, Bax, caspase-3 and cytochrome c was detected following force loading using reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analysis. Compared with the cells under no cyclic tension stress, the number of osteoclasts and the resorption area were increased in the cells under 10 and 15% stretch microstrain. The Annexin V binding assay showed that the early apoptosis rate of the 5, 10 and 15% stretch microstrain groups was decreased compared with that of the control group. RT-qPCR results showed that the Bcl-2/Bax ratio was significantly increased in the cells subjected to 5, 10 and 15% stretch microstrain compared with that in the control cells (P<0.05), while the expression of cytochrome c in the 10 and 15% stretch microstrain groups was decreased significantly (P<0.05). No significant difference was observed between the cytochrome c expression of the 5% stretch microstrain group and that of the control group (P>0.05). The expression of caspase-3 in the 5, 10 and 15% stretch microstrain groups was decreased significantly compared with that in the control group (P<0.05). These data suggest that cyclic tension stress can inhibit apoptosis in osteoclasts, possibly by increasing the Bcl-2/Bax ratio, inhibiting the activity of caspase-3 and downregulating the expression of cytochrome c.