Abstract 865: Small molecule-mediated modulation of Ras elicits inhibition of phospho ERK signaling through negative feedback on SOS1

Abstract

Abstract Oncogenic mutation of Ras is responsible for more than 30% of all human tumors. Therefore, pharmacological modulation of Ras has attracted great interest as a therapeutic strategy. Our laboratory has recently discovered small molecules that activate Son of Sevenless (SOS)-catalyzed nucleotide exchange on Ras and paradoxically inhibit downstream signaling. Here we describe how pharmacologically targeting SOS1 induces biphasic modulation of Ras-GTP and downstream ERK levels. We consistently observed this phenotype in a variety of cell lines expressing different Ras mutant isoforms, and using multiple chemotypes of compound. In elucidating the molecular mechanism, we observed that compound treatment caused an increase in phosphorylation at ERK consensus motifs on SOS1 that was not observed with the expression of a non-phosphorylatable S1178A SOS1 mutant, or after pre-treatment with an ERK inhibitor. Phosphorylation at S1178 on SOS1 is known to inhibit the association between SOS1 and GRB2 and disrupt SOS1 membrane localization. Consistent with this, we show that wild-type SOS1 and GRB2 dissociated in a time dependent fashion in response to compound treatment, and conversely, this interaction was enhanced with the expression of a S1178A SOS1 mutant. Pre-treatment with an ERK inhibitor prevented compound-induced inhibition of the association between SOS1 and GRB2. Furthermore, in cells expressing either S1178A SOS1 or a constitutively membrane-bound CAAX box tagged SOS1 mutant, we observed elevated Ras-GTP levels over time in response to compound, as compared to the biphasic changes in Ras-GTP exhibited in cells expressing wild-type SOS1. These results show that small molecule agonists of SOS1 cause paradoxical inhibition of Ras-GTP and MAPK signaling in Ras mutant cancer cells, through negative feedback on SOS1. Overall, these compounds provide us with the unique opportunity to better understand the biological functions of SOS and Ras in cancer cells, and may aid in the discovery of small molecules to treat Ras-driven tumors. Citation Format: Jennifer E. Howes, Denis T. Akan, Michael C. Burns, Qi Sun, Andrew J. Little, DeMarco V. Camper, Jason R. Abbott, Jason Phan, Taekyu Lee, Olivia W. Rossanese, Alex G. Waterson, Stephen W. Fesik. Small molecule-mediated modulation of Ras elicits inhibition of phospho ERK signaling through negative feedback on SOS1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 865.