  Apoptosis, or genetically programmed cell death, is a crucial cellular
process that maintains the balance between life and death in cells. The precise
molecular mechanism of apoptosis signaling and how these two pathways are
differentially activated under distinct apoptotic stimuli is poorly understood.
We developed a Monte Carlo-based stochastic simulation model that can
characterize distinct signaling behaviors in the two major pathways of
apoptotic signaling using a novel probability distribution-based approach.
Specifically, we show that for a weak death signal, such as low levels of death
ligand Fas (CD95) binding or under stress conditions, the type 2 mitochondrial
pathway dominates apoptotic signaling. Our results also show signaling in the
type 2 pathway is stochastic, where the population average over many cells does
not capture the cell-to-cell fluctuations in the time course (~1 - 10 hours) of
downstream caspase-3 activation. On the contrary, the probability distribution
of caspase-3 activation for the mitochondrial pathway shows a distinct bimodal
behavior that can be used to characterize the stochastic signaling in type 2
apoptosis. Interestingly, such stochastic fluctuations in apoptosis signaling
happen even in the presence of large numbers of signaling molecules. In a
fluctuating environment, such stochasticity in the timecourse of caspase-3
activation may be an adaptive mechanism for allowing a competing survival
signal to win over a weak death trigger before the critical cell fate decision
is made and thus minimizes the risk of pathologies.
