Abstract PR04: Small molecule-mediated activation of Ras elicits inhibition of MAPK and PI3K signaling though pathway feedback

Oncogenic mutation or hyper-activation of Ras results in aberrant cellular signaling and is responsible for approximately 30% of all human tumors. Therefore, Ras is an outstanding candidate for therapeutic inhibition. However, the discovery of potent inhibitors has been challenging. Our laboratory has recently discovered small molecules that perturb Son of Sevenless (SOS) -catalyzed nucleotide exchange on Ras (Burns et al. PNAS 2014). Here we describe experiments conducted to determine the mechanism of compound action. SOS activator compound 4 (C4) induces signaling flux through the MAPK pathway in response to elevated Ras-GTP levels, reminiscent of epidermal growth factor signaling. We used mass spectrometry to show that in response to C4 stimulation, phospho-modifications on SOS1 increased at ERK phosphorylation consensus sequences. We hypothesize that these phospho-modifications can cause delocalization of SOS and membrane bound Ras, and we are currently using proximity-ligation to assess whether the interaction between SOS1 and Ras is affected by C4 treatment. Cross-talk between the MAPK and PI3K signaling pathways may be responsible for the inhibition of phospho-AKT Ser473 levels in response to MAPK signaling flux, or vice versa. Here we show, using MEK and PI3K inhibitor pre-treatments, that events in both pathways are independent after C4 treatment. As both SOS1 and PI3K bind to Ras at a similar site, we are currently testing the hypothesis that C4 disrupts the interaction between Ras and PI3K, whilst allowing signaling flux through the MAPK pathway. Overall, we have discovered small-molecule activators of SOS that modulate Ras-GTP levels, resulting in signaling flux through the MAPK pathway and eventual downstream inhibition of both the MAPK and PI3K pathways in cancer cells. This abstract is also being presented as Poster A15. Citation Format: Jennifer E. Howes, Denis T. Akan, Bethany M. Alicie, Alex G. Waterson, Olivia W. Rossanese, Stephen W. Fesik. Small molecule-mediated activation of Ras elicits inhibition of MAPK and PI3K signaling though pathway feedback. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr PR04.