[Mechanism of G2/M cell cycle arrest before apoptosis in leukemia cell line HL-60 induced by proteasome inhibitor MG132].

BACKGROUND & OBJECTIVE: Proteasome inhibitor is a kind of potential anti-tumor drug,it can induce apoptosis in various tumor cells. This study was designed to investigate the molecular mechanism of apoptosis and G(2)/M arrest in leukemia cell line HL-60 induced by proteasome inhibitor MG132 (Z-Leu-Leu-Leu-CHO). METHODS: Apoptosis in HL-60 cells was observed under fluorescent microscope, flow cytometry and immunoblot were used to analyze cell apoptosis, cell cycle arrest, and the mechanisms. RESULTS: MG132 (2 micromol/L)induced apoptosis in HL-60 cells after 24-h treatment. Meanwhile, HL-60 cells were arrested at G(2)/M phase before apoptosis after induced by MG132. The percentage of G(2)/M phase in MG132-treated HL-60 cells at 12 h was 63.42+/-2.02,while that in untreated cells was 7.29+/-3.01 (P< 0.01). The percentage of apoptosis in MG132-treated HL-60 cells at 24 h was 16.67+/-1.48, while untreated cells had no death (P< 0.01). Compared to the treatment with MG132 only, caffeine (2 mmol/L) exposure can reduce G(2)/M arrest and apoptosis in MG132-treated HL-60 cells. Expression of cyclin-dependent kinase inhibitor p21waf/cip1 up-regulated after treated with MG132 for 3 h, but no p53 or p27 detected. CONCLUSIONS: Proteasome inhibitor MG132 can induce G2/M arrest before the apoptosis appeared in HL-60 cells. The obvious up-regulation of p21 indicated that it is p21(waf/cip1), but not p53 or p53-related proteins,that involved in the regulation of G(2)/M arrest and subsequent apoptosis induced by MG132 in HL-60 cells.