Spatial differences in active caspase-8 defines its role in T-cell activation versus cell death.
Caspase-8, a cysteine-protease, initiates apoptosis when activated by death receptors. Caspase-8 is also essential for initiating T lymphocyte proliferation following T-cell antigen receptor (TCR) signaling. Given these disparate functions of caspase-8, we sought to determine whether this represented only a difference in the magnitude of caspase-8 activation, or different intracellular locations of active caspase-8. We demonstrate by high-resolution multicolor confocal laser scanning microscopy an aggregation of active caspase-8 within membrane lipid rafts in T cells stimulated with anti-CD3. This suggests that following TCR stimulation active caspase-8 physically interacts with lipid raft proteins, possibly to form a signaling platform. In contrast, Fas stimulation of T cells resulted in a much more profound activation of caspase-8 that was exclusively cytosolic. These confocal microscopic findings were confirmed using discontinuous sucrose gradient ultracentrifugation to isolate lipid raft versus cytosolic components. This sequestration model of caspase-8 activation was further supported by the observation that a classic caspase-8 substrate, BID, was not cleaved in CD3-stimulated T cells, but was cleaved after Fas engagement. Our data support a model that the location of active caspase-8 may profoundly influence its functional capacity as a regulator of either cell cycling or cell death.
Koenig
A
A
Department of Medicine, Immunobiology Program, The University of Vermont College of Medicine, Burlington, VT 05405, USA. akoenig@uvm.edu
Russell
J Q
JQ
Rodgers
W A
WA
Budd
R C
RC
D017252:CD3 Complex / Q000378:metabolism; D053181:Caspase 8 / Q000378:metabolism*; D016923:Cell Death; D053938:DNA Fragmentation; D004789:Enzyme Activation; D006801:Humans; D019169:Jurkat Cells; D007700:Kinetics; D008213:Lymphocyte Activation / Q000276:immunology*; D021962:Membrane Microdomains / Q000201:enzymology; D021381:Protein Transport; D011948:Receptors, Antigen, T-Cell / Q000276:immunology; D013601:T-Lymphocytes / Q000166:cytology* / Q000201:enzymology* / Q000276:immunology; D019014:fas Receptor / Q000378:metabolism
D017252:CD3 Complex; C491901:FAS protein, human; D011948:Receptors, Antigen, T-Cell; D019014:fas Receptor; D053181:Caspase 8