Systems-level consequences of low RAF abundance for EGFR-ERK signaling.

RAFs initiate the cascade leading to activation of the extracellular signal-regulated kinases (ERKs). In a substantial fraction of cancer cells, RAFs are the least abundant pathway proteins between receptor tyrosine kinases and ERKs. In some cases, active RAF kinases are present at the plasma membrane at just hundreds of copies per cell, but the consequences of such limited RAF abundance are unclear. By developing continuum and stochastic computational models of the epidermal growth factor receptor (EGFR)-ERK pathway, we showed that low RAF abundance creates signaling bottlenecks between receptor tyrosine kinases and ERK with a potential for stochastic RAF dynamics that can propagate especially to low-abundance downstream pathway proteins. RAF bottlenecks were also predicted to impede ERK activation by oncogenic RAS mutants. Advanced parameter sensitivity and sloppiness analyses identified RAS activation and RAS-RAF interactions as strong determinants of signaling in low-RAF settings and revealed an efficient model fitting approach. This work provides quantitative insight into a common, but unexplored, regime for EGFR-ERK signaling and a systematic approach to develop and characterize dynamic models of receptor-mediated signaling.