OBJECTIVETo overcome the drug-resistance of tumor cells is one of the methods of improving the therapeutic results. Histone deacetylase inhibitors (HDACIs) is a novel class of chemotherapeutic agents which can induce apoptosis of tumor cells. Valproic acid (VPA) is a common drug used in the treatment of epilepsy. It has been shown that VPA has a marked HDACIs effect at the pharmaceutical level, and can induce the differentiation and apoptosis of transformed cells. But the mechanism of its effect has not been clarified. The aim of this study was to investigate the mechanism of VPA in inducing the apoptosis of leukemic cells at molecular level.

METHODSThe cell lines U937, Jurkat clone E6 - 1 (Jurkat) and BALL-1 were cultured in RPMI 1640 medium containing 20% calf serum, then divided into three groups (control group, 1 mmol/L VPA group and 1 mmol/L VPA + 1 micromol/L Pan-caspase inhibitor zVAD-fmk group). At 72 hours after the treatment, the cells were double stained with Aunexin and PI (propidium iodide) and then were analyzed with the flow cytometry (FCM) to detect apoptosis. Before and after treatment with VPA the mean fluorescence index (MFI) of Bcl-2, Bax, Bcl-xl and the levels of caspase 8, 9 and 3 were also detected with the FCM. The changes of P(44/42) mitogen activating protein kinase (MAPK) and phosphorylated P(44/42) MAPK were determined by Western blotting.

RESULTSSeventy-two hours after the treatment, 1 mmol/L VPA induced apoptosis of U937 and Jurkat. The apoptotic rate of U937 was (75.78 +/- 4.20)% and that of Jurkat was (53.50 +/- 5.87)% (P < 0.01, vs. control group); zVAD-fmk could fully inhibit the apoptosis of U937, and the apoptotic rate was (2.89 +/- 0.36)%; while it could partly inhibit the apoptosis of Jurkat, and the apoptotic rate was (15.38 +/- 1.40)% (P < 0.01). 1 mmol/L VPA could not induce the apoptosis of BALL-1 which had a high expression level of Bcl-2. The MFI of Bcl-2, Bax and Bcl-xl in these three cell lines did not change significantly with VPA (P > 0.05). After treatment with VPA, the level of caspase 3 in U937 increased from (14.09 +/- 1.19)% to (32.30 +/- 2.47)%, and caspase 8 from (4.58 +/- 1.41)% to (86.47 +/- 3.26)% (P < 0.01), but there was no significant change in caspase 9 [(13.25 +/- 3.11)% and (10.95 +/- 1.30)%]. In Jurkat, the level of caspase 3 increased from (12.01 +/- 1.63)% to (35.56 +/- 0.27)%, and caspase 9 from (13.89 +/- 1.71)% to (75.89 +/- 4.08)% (P < 0.01 for both); no significant change was observed for caspase 8 [(5.94 +/- 1.38)% and (5.44 +/- 0.72)%]. In BALL-1, there was a slight decline in caspase 3 (P < 0.05). With the effect of VPA, levels of P(44/42) MAPK and phosphorylated P(44/42) MAPK decreased in all three cell lines (P < 0.01).

CONCLUSIONVPA could induce apoptosis of U937 through the activation of caspase 3 and 8; and it induced the apoptosis of Jurkat involving the activation of caspase 3 and 9. P(44/42) MAPK pathway also plays an important role in this course. VPA induced apoptosis of these cell lines without the alteration of Bcl-2, Bax and Bcl-xl. High level of Bcl-2 could antagonize the effect of VPA.