Differential regulation of nuclear and mitochondrial Bcl-2 in T cell apoptosis.
Activated T cells require anti-apoptotic cytokines for their survival. The anti-apoptotic effects of these factors are mediated by their influence on the balance of expression and localisation of pro- and anti-apoptotic members of the Bcl-2 family. Among the anti-apoptotic Bcl-2 family members, the expression level of Bcl-2 itself and its interaction with the pro-apoptotic protein Bim are now regarded as crucial for the regulation of survival in activated T cells. We studied the changes in Bcl-2 levels and its subcellular distribution in relation to mitochondrial depolarisation and caspase activation in survival factor deprived T cells. Intriguingly, the total Bcl-2 level appeared to remain stable, even after caspase 3 activation indicated entry into the execution phase of apoptosis. However, cell fractionation experiments showed that while the dominant nuclear pool of Bcl-2 remained stable during apoptosis, the level of the smaller mitochondrial pool was rapidly downregulated. Signals induced by anti-apoptotic cytokines continuously replenish the mitochondrial pool, but nuclear Bcl-2 is independent of such signals. Mitochondrial Bcl-2 is lost rapidly by a caspase independent mechanism in the absence of survival factors, in contrast only a small proportion of the nuclear pool of Bcl-2 is lost during the execution phase and this loss is a caspase dependent process. We conclude that these two intracellular pools of Bcl-2 are regulated through different mechanisms and only the cytokine-mediated regulation of the mitochondrial pool is relevant to the control of the initiation of apoptosis.
Scheel-Toellner
Dagmar
D
MRC Centre of Immune Regulation, Division of Immunity and Infection, Institute of Biomedical Research, University of Birmingham, Birmingham, UK. d.scheel@bham.ac.uk
Raza
Karim
K
Assi
Lakhvir
L
Pilling
Darrell
D
Ross
Emma J
EJ
Lee
Wing Yiu
WY
Curnow
S John
SJ
Buckley
Christopher D
CD
Akbar
Arne N
AN
Lord
Janet M
JM
Salmon
Mike
M
D017209:Apoptosis; D015496:CD4-Positive T-Lymphocytes / Q000166:cytology / Q000378:metabolism*; D053148:Caspase 3 / Q000378:metabolism; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism*; D006801:Humans; D007376:Interleukin-2 / Q000378:metabolism*; D008213:Lymphocyte Activation; D008928:Mitochondria / Q000378:metabolism*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*
D007376:Interleukin-2; D019253:Proto-Oncogene Proteins c-bcl-2; D053148:Caspase 3