[Role of pro-apoptotic protein Bim in mouse thymocytes apoptosis induced by dexamethasone]

To explore the role of pro-apoptotic protein Bim in the mouse thymocyte apoptosis induced by dexamethasone (DEX).

1 micromol/L of dexamethasone was used to induce the apoptosis of BALB/c mouse thymocyte cultured in vitro. The thymocyte apoptosis was detected by transmission electron microscope, Annexin V-FITC/PI flow cytometry and DNA agarose gel electrophoresis. The RT-PCR method was applied to the semi-quantitative analysis of Bim mRNA expression induced by 1 micromol/L DEX at different time point and by different final concentration dexamethasone used for 1 h.

(1) With 1 micromol/L DEX used to cell incubation for 4 h, the mouse thymocyte showed the morphological characteristics of apoptosis and the typical DNA ladder on electrophoresis gel. The analyses of Annexin V-FITC /PI flow cytometry showed that the thymocyte apoptosis rates at different time point (2 h, 4 h, 8 h) were significantly higher in dexamethasone treated group than in control group. (2) With 1 micromol/L DEX used to cell incubation for 1/2 hr, the expressions of three Bim mRNAs in mouse thymocyte began to increase and the BimL expression was predominant among three mRNA isoforms. The peak values of BimL and BimEL mRNA expression appeared at 1 h and 11 h which were earlier to dexamethasone treated group than control group, and the BimL and BimEL expression levels at different time point (except at 0 h and 24 h) were significantly increased (P < 0.05). The peak value of BimS mRNA expression of dexamethasone treated group was 4 hours earlier than that of control group, but there was no significant difference between two groups (P > 0. 05). (3) When the mouse thymocyte was incubated with various concentration (0, 10(-2),10(-1), 1, 10 micromol/L) DEX for 1 h, there was no significant percentage difference occurring between living and apoptotic cell. While the total of Bim mRNA expression was increasing in a concentration-dependent manner, the mRNA increase of BimL isoform was still the most predominant among three mRNA isoforms.

The pro-apoptotic protein Bim plays an important role in mouse thymocyte apoptosis induced by dexamethasone, and the BimL mRNA is the predominant isoform in this process. The increase of Bim mRNA expression appears in very early stage of thymocyte apoptosis and both time- and dose-dependent.