UCSF

Ras mutations drive approximately one third of human cancers by aberrant regulation of the mitogen-activated protein kinase (MAPK) signaling cascade and other effector pathways. While oncogenic Ras mutations are commonly viewed as sufficient for constitutive GTP-loading, it is unclear whether these mutant forms display differences in reliance on upstream and downstream growth signals for activity. Use of mouse embryonic fibroblasts (MEFs) devoid of the H-, N- and KRAS alleles, and rescued by single Ras isoforms, has provided a model for comparison of the requirements for growth factor signaling amongst oncogenic Ras mutants. In MEFs expressing KRAS G12C, G12V or G12D, growth factors increase GTP loading, in a Sos-dependent manner. In contrast, Q61L and Q61R mutants show little, if any, increase in GTP-loading. These differences reflect differences in intrinsic GTPase rates. Growth factors activate phospholipase C (PLC) in addition to SOS, resulting in PKC activation and mobilization of calcium. PKC increases GTP-loading on G12 mutant KRAS proteins through SOS1 and activates MAPK signaling downstream from Ras. However, different KRAS mutants vary dramatically in their requirement for PLC signaling to activate the MAPK pathway. Downstream of PLC, the calcium-regulated chloride channel TMEM16A (ANO1) completes an autocrine EGFR feedback loop that modulates the MAPK response to calcium flux in a KRAS mutation-dependent manner. The discovery that specific Ras mutants may be more susceptible to EGFR and PLC inhibition or downstream modulation of calcium signaling may provide a potent therapeutic strategy for certain Ras-driven cancers.