Effects of Bcl-2 levels on Fas signaling-induced caspase-3 activation: molecular genetic tests of computational model predictions

J Immunol. 2005 Jul 15;175(2):985-95. doi: 10.4049/jimmunol.175.2.985.


Fas-induced apoptosis is a critical process for normal immune system development and function. Although many molecular components in the Fas signaling pathway have been identified, a systematic understanding of how they work together to determine network dynamics and apoptosis itself has remained elusive. To address this, we generated a computational model for interpreting and predicting effects of pathway component properties. The model integrates current information concerning the signaling network downstream of Fas activation, through both type I and type II pathways, until activation of caspase-3. Unknown parameter values in the model were estimated using experimental data obtained from human Jurkat T cells. To elucidate critical signaling network properties, we examined the effects of altering the level of Bcl-2 on the kinetics of caspase-3 activation, using both overexpression and knockdown in the model and experimentally. Overexpression was used to distinguish among alternative hypotheses for inhibitory binding interactions of Bcl-2 with various components in the mitochondrial pathway. In comparing model simulations with experimental results, we find the best agreement when Bcl-2 blocks the release of cytochrome c by binding to both Bax and truncated Bid instead of Bax, truncated Bid, or Bid alone. Moreover, although Bcl-2 overexpression strongly reduces caspase-3 activation, Bcl-2 knockdown has a negligible effect, demonstrating a general model finding that varying the expression levels of signal molecules frequently has asymmetric effects on the outcome. Finally, we demonstrate that the relative dominance of type I vs type II pathways can be switched by varying particular signaling component levels without changing network structure.

Publication types

  • Research Support, Non-U.S. Gov't
  • Validation Study

MeSH terms

  • Apoptosis / genetics
  • Apoptosis / immunology
  • BH3 Interacting Domain Death Agonist Protein
  • Carrier Proteins / metabolism
  • Caspase 3
  • Caspase Inhibitors
  • Caspases / metabolism*
  • Cell Line
  • Cell Line, Transformed
  • Computational Biology / methods*
  • Computer Simulation
  • Enzyme Activation / immunology
  • Fas Ligand Protein
  • Humans
  • Jurkat Cells
  • Membrane Glycoproteins / metabolism
  • Membrane Proteins / metabolism
  • Mitochondria / enzymology
  • Mitochondria / immunology
  • Mitochondria / metabolism
  • Models, Immunological*
  • Predictive Value of Tests
  • Protein Binding / genetics
  • Protein Binding / immunology
  • Proto-Oncogene Proteins c-bcl-2 / biosynthesis
  • Proto-Oncogene Proteins c-bcl-2 / deficiency
  • Proto-Oncogene Proteins c-bcl-2 / genetics
  • Proto-Oncogene Proteins c-bcl-2 / metabolism
  • Proto-Oncogene Proteins c-bcl-2 / physiology*
  • RNA Interference / immunology
  • Sensitivity and Specificity
  • Signal Transduction / genetics
  • Signal Transduction / immunology*
  • Transduction, Genetic / methods*
  • Up-Regulation / immunology*
  • bcl-2 Homologous Antagonist-Killer Protein
  • bcl-2-Associated X Protein
  • fas Receptor / physiology*


  • BAK1 protein, human
  • BAX protein, human
  • BH3 Interacting Domain Death Agonist Protein
  • BID protein, human
  • Carrier Proteins
  • Caspase Inhibitors
  • FASLG protein, human
  • Fas Ligand Protein
  • Membrane Glycoproteins
  • Membrane Proteins
  • Proto-Oncogene Proteins c-bcl-2
  • bcl-2 Homologous Antagonist-Killer Protein
  • bcl-2-Associated X Protein
  • fas Receptor
  • CASP3 protein, human
  • Caspase 3
  • Caspases