Oncogenic and Wild-type Ras Play Divergent Roles in the Regulation of Mitogen-Activated Protein Kinase Signaling

H-Ras, K-Ras, and N-Ras regulate cellular growth and survival and are often activated by somatic mutation in human tumors. Although oncogenic lesions occur in a single Ras isoform within individual tumors, it is unclear whether the remaining wild-type isoforms play supporting roles in tumor growth. Here, we show that oncogenic and wild-type Ras isoforms play independent and nonredundant roles within the cell. Oncogenic Ras regulates basal effector pathway signaling, whereas wild-type Ras mediates signaling downstream of activated receptor tyrosine kinases (RTK). We show that both are necessary for exponential growth of Ras-mutant cell lines. Furthermore, we show that oncogenic Ras desensitizes signaling from EGF receptor (EGFR). Depletion of oncogenic Ras with siRNA oligonucleotides relieves this negative feedback, leading to the hyperactivation of EGFR and wild-type Ras signaling. Consistent with this model, combining oncogenic Ras depletion with EGFR inhibition potently increases cell death.