Rac1 knock-down impairs growth and induces apoptosis in the osteoblast cell line OP9. Photomicrograph of OP9 cells infected with nontargeting virus pLKO.1 (SCR; A) or infected with Rac1-shRNA 88 (pLKO.1 vector) demonstrating cytoskeletal elongation (B arrowheads). (C) Apoptosis (annexin V⁺ cells by flow cytometry) was induced by Rac1shRNA (88 and 92: apoptosis, 24.4% ± 2.6% and 17.4% ± 3.3%, respectively), but not with nontargeting hairpin (SCR, apoptosis 11.7% ± 0.2%, P < .01 vs sh88, P = .05 vs sh92). (D) Reduction in S-phase cell determined by bromodeoxyuridine incorporation caused by Rac1shRNA (88 and 92: S-phase 1.1% ± 0.9% and 1.9% ± 2.0%, respectively), but not nontargeting (SCR: S-phase 9.6% ± 4.0%, P < .05 for comparison with 88 and 92) control. (E) Representative image of bromodeoxyuridine incorporation in OP9 infected with nontargeting (SCR) control hairpin compared with Rac1shRNA 88 (F). All values are means ± SD. n = 3 per condition for each experiment.