Abstract LB-96: Sensitivity to combined MEK and mTORC1/2 inhibition in colorectal cancer is dictated by p53 mutational status. .
Authors: García-García Celina , Rivas Martín A. , Ibrahim Yasir H. , Calvo María Teresa , Grueso Judit , Antón Pilar , Aura Claudia , Jessen Katti , Palmer Héctor , Tabernero Josep , Scaltriti Maurizio , Baselga José , Serra Violeta 
Published: August 2014

Abstract
Activating mutations in RAS and PI3K pathways confer resistance to currently used anti-EGFR-based therapies in colorectal cancer (CRC). Although dual inhibition of MEK and PI3K/Akt has proven some efficacy in CRC preclinical models with aberrations in both pathways, little is known about other possible alterations that can limit the efficacy of this combination and preclude patient selection. Here, we explored whether p53 mutations influence the response to combined MEK and PI3K pathway inhibition by analyzing the antiproliferative activity of the investigational dual mTORC1/2 inhibitor MLN0128, and the MEK inhibitor PD0325901, in three p53 wild-type and three p53 mutated CRC cell lines with concomitant KRAS/BRAF and PIK3CA mutations. Expectedly, PD0325901 and MLN0128 inhibited ERK and mTOR signaling, respectively, across all cell-lines. However, the treatment combination selectively enhanced apoptosis only in p53 wild-type cell lines. Overexpression of mutant p53 in p53 wild-type CRC cells prevented the proapoptotic effects of combined PD0325901 and MLN0128, underscoring the causative role of mutant p53 in limiting the sensitivity to this pharmacological approach. Similar results were observed in vivo, where the inhibition of MEK and mTORC1/2 reduced tumor growth in a higher extent than single agents in p53 wild-type xenograft models, but not in p53 mutant ones. p53 can inhibit cell apoptosis and cycle progression partly by increasing the expression of the cyclin-dependent kinase inhibitor p21, which, in turn, is regulated by MEK-ERK-cMyc pathway. Accordingly, we observed a decrease of cMyc and an increase of p21 levels following MEK inhibition, which could exert an anti-apoptotic function in wild-type p53 CRC. As expected, the combined treatment with the mTORC1/2 inhibitor prevented the PD0325901-induced p21 expression and resulted in enhanced apoptosis. Knockdown of p21 in p53 wild-type cells also sensitized CRC cells to MEK inhibitor-induced apoptosis, confirming the antiapoptotic role of p21 upon PD0325901 treatment and supporting the hypothesis that p21-downregulation mediated by MLN0128 is needed to achieve cell death following dual MEK and mTOR inhibition. In summary, our data suggest that the presence of wild-type p53 is a determinant of sensitivity to the combined suppression of MEK and PI3K-pathways in CRC.
Citation Format: Celina García-García, Martín A. Rivas, Yasir H. Ibrahim, María Teresa Calvo, Judit Grueso, Pilar Antón, Claudia Aura, Katti Jessen, Héctor Palmer, Josep Tabernero, Maurizio Scaltriti, José Baselga, Violeta Serra. Sensitivity to combined MEK and mTORC1/2 inhibition in colorectal cancer is dictated by p53 mutational status. . [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr LB-96. doi:10.1158/1538-7445.AM2013-LB-96