Molecular mechanism of cell apoptosis by paclitaxel and pirarubicin in a human osteosarcoma cell line. Paclitaxel and pirarubicin exhibit cytotoxic and antitumor activities. However, little is known about the apoptosis-inducing effects of paclitaxel and pirarubicin on human osteosarcoma MG-63 cells.The effects of paclitaxel and pirarubicin on cell cycle arrest and apoptosis were studied in MG-63 cells using flow cytometry. PCNA, Bcl-2, Bax, cyclin D1 and cyclin E expression was assessed by Western blotting.Paclitaxel and pirarubicin caused G2/M and G0/G1 cell cycle arrest in MG-63 cells, respectively. Apoptosis of MG-63 cells mediated by paclitaxel was dependent on treatment duration. Interestingly, in cells treated with pirarubicin, apoptosis was related to treatment duration at concentrations of 10(2)-10(3) nM, whereas the effect of treatment duration was less marked at concentrations >10(4)-10(5) nM. Furthermore, paclitaxel and pirarubicin suppressed the expression of PCNA, cyclin D1, cyclin E and Bcl-2, and increased Bax expression.These results suggest that the G2/M or G0/G1 cell cycle arrest and apoptosis induced by paclitaxel and pirarubicin are Bcl-2/Bax dependent, suggesting favorable effects of combination therapy with paclitaxel and pirarubicin in the treatment of osteosarcoma.(c) 2010 S. Karger AG, Basel.