Fig. 6. Stability of procaspase-8 cleavage products and quantitative description of cell death kinetics based on a tBID threshold model.

(A) Half-life for caspase-8 intermediates. Lines show means, and areas indicate SDs for the best 1% best of n = 1000 cis/trans model fits, simulated at [CD95L] = 500 ng/ml. (B) Derivation of a cell death model. The cis/trans model was extended to describe cell death by assuming that a cell dies once the tBID concentration exceeds a certain threshold. The model was trained using CD95-HeLa cell data, then simulations were run to predict the response of the HeLa wt cells (after reducing the median CD95R number by 10-fold to account for the reduced number of receptors in HeLa wt compared to CD95-HeLa cells), and finally the amount of cell death of the HeLa wt cells under various conditions was compared to the results of the simulations. (C) Experimental values (squares) and predictions (lines and shaded areas) of median cell death times for HeLa wt and CD95-HeLa cells. For validation, model fits to CD95-HeLa cells were used to predict the kinetics of HeLa wt apoptosis. Error bars represent SDs between cell death time medians in different fields of view in a Lab-Tek chamber (solid lines: means; shaded areas: SDs; best 1% of n = 1000 fits). (D) Coefficients of variation for cell death times in the range of 0.5 to 0.6 for HeLa wt cells and in the range of 0.1 to 0.2 for CD95-HeLa cells (squares with error bars estimated by bootstrapping) and from cis/trans model simulations, based on fits to CD95-HeLa data. Lines and shaded areas correspond to SDs as in (C). Predicted coefficients of variation (VarK) were calculated at ligand concentrations at which at least 50% of the simulated cells underwent apoptosis during t_exp (t_apt < t_exp, limits marked by “V”). VarK estimates including cells undergoing apoptosis later than t_exp are shown as a dotted line.