BimL displacing Bcl-xL promotes bax translocation during TNFα-induced apoptosis

Bcl-2 family proteins are implicated as essential regulators in tumor necrosis factor-alpha (TNF alpha)-induced apoptosis. Bim(L), a BH3-only member of Bcl-2 family, can directly or indirectly activate the proapoptotic Bax and the subsequent mitochondrial apoptotic pathway. However, the molecular mechanism of Bim(L) activating Bax activation during TNF alpha-induced apoptosis is not fully understood. In this study, the role of Bim(L) in Bax activation during TNF alpha-induced apoptosis was investigated in differentiated PC12 and MCF7 cells, with real-time single-cell analysis. The experimental results show that Bax translocated to mitochondria and cytochrome c (Cyt c) released from mitochondria after TNF alpha treatment. Furthermore, SP600125 (specific inhibitor of JNK) could inhibit the Cyt c release from mitochondria. Co-immunoprecipitation results show that, the interaction between Bcl-x(L) and Bax decreased after TNF alpha treatment, while that between Bcl-x(L) and Bim(L) increased. Bax did not co-immunoprecipitate with Bim(L) before or after the TNF alpha treatment. In addition, the increased interaction between Bim(L) and Bcl-x(L) was dynamically monitored by using fluorescence resonance energy transfer (FRET) technique. Most importantly, there was no evidence of Bim(L) redistribution to mitochondria until cell apoptosis. By comprehensively analyzing these data, it is concluded that Bim(L) displaces Bcl-x(L) in the mitochondria and promotes Bax translocation during TNF alpha-induced apoptosis.