Cell-to-Cell Variability in PI3K Protein Level Regulates PI3K-AKT Pathway Activity in Cell Populations

Background: Cell-to-cell variability in populations has been widely observed in mammalian cells. This heterogeneity can result from random stochastic events or can be deliberately maintained through regulatory processes. In the latter case, heterogeneity should confer a selective advantage that benefits the entire population. Results: Using multicolor flow cytometry, we have uncovered robust heterogeneity in phosphoinositide 3-kinase (PI3K) activity in MCF10A cell populations, which had been previously masked by techniques that only measure population averages. We show that AKT activity is bimodal in response to EGF stimulation and correlates with PI3K protein level, such that only cells with high PI3K protein can activate AKT. We further show that heterogeneity in PI3K protein levels is invariably maintained in cell populations through a degradation/resynthesis cycle that can be regulated by cell density. Conclusions: Given that the PI3K pathway is one of the most frequently upregulated pathways in cancer, we propose that heterogeneity in PI3K activity is beneficial to normal tissues by restricting PI3K activation to only a subset of cells. This may serve to protect the population as a whole from over-activating the pathway, which can lead to cellular senescence or cancer. Consistent with this, we show that oncogenic mutations in p110 alpha (H1047R and E545K) partially evade this negative regulation, resulting in increased AKT activity in the population.