Even genetically identical cells can show differential expression or activation of particular proteins, differences blurred by techniques that assess protein abundance or activity in large populations of cells. Yuan et al. used multicolor flow cytometry to monitor activity of the protein kinase AKT in individual cells in populations of the MCF10A human breast cancer cell line and found that stimulation with epidermal growth factor (EGF) led to a bimodal response. About 30% of the cells showed AKT activation (assessed by phosphorylation on Ser473) in response to EGF, whereas AKT remained unphosphorylated in the other 70%. Although the bimodal distribution of phosphorylated AKT did not reflect differences in AKT abundance, experiments using a tagged form of the p110α catalytic subunit of phosphoinositide 3-kinase (PI3K) revealed a bimodal distribution of p110α (mirrored by that of the endogenous p85α subunit) and showed that AKT activity correlated with PI3K abundance. Flow cytometry analysis revealed that, although population abundance of p110α was unchanged after EGF stimulation (as also seen by Western blot), there were dynamic changes in p110α abundance in those cells that contained phosphorylated AKT and a shift in the relative numbers of cells with low, medium, or high levels of p110α. Pharmacological analysis indicated that these changes in p110α abundance depended on a cycle of proteasomal degradation and resynthesis. Analyses of MCF10A cells expressing tagged forms of wild-type p110α or p110α with either of two oncogenic mutations showed a greater fraction of p110αhigh cells in the mutant populations, as well as increased AKT activity. Intriguingly, in clonal lines derived from either wild-type or mutant cells, the typical bimodal profile of p110α abundance and AKT activity was reversed, so that the majority of cells were p110αhigh (with phosphorylated AKT). Postulating that abundant p110α is crucial for colony formation, the authors determined that, following sequential passages, this distribution generally reverted toward the profile in which the majority of cells were p110αlow. Clonal populations derived from mutant cells reverted more slowly, and those that failed to revert showed phenotypes consistent with senescence. In exponentially growing populations, the p110αhigh population decreased with cell density and in cells with multiple contacts. These findings were echoed in BRCA-driven mouse mammary tumors, where cells on tumor edges were enriched for phosphorylated AKT. The authors conclude that heterogeneity in AKT activity in cell populations depends on PI3K abundance and propose that restriction of PI3K activity to a subpopulation of cells may be beneficial to the population as a whole.
T. L. Yuan, G. Wulf, L. Burga, L. C. Cantley, Cell-to-cell variability in PI3K protein level regulates PI3K-AKT pathway activity in cell populations. Curr. Biol. 21, 173–183 (2011). [PubMed]