Abstract A54: Requirement for interaction of PI 3-kinase p110alpha with Ras in lung tumor maintenance
Authors: Sheridan Clare , Castellano Esther , Thin May Zaw , Murillo Miguel , Lassailly Francois , Stamp Gordon , Downward Julian 
Published: August 2014

Abstract
Ras proteins are one of the most prevalent oncogenic drivers of human cancer and lung, pancreatic and colon cancers frequently harbor mutated forms of the KRAS gene. The oncogenic role of Ras is executed via direct interaction with, and activation of, a number of downstream effectors including PI 3-kinases, Raf kinases, RalGDS and Tiam1. Previously our laboratory has demonstrated that disruption of PI 3-kinase activation by oncogenic K-Ras, via mutation of two key residues (T208D and K227A) in the Ras binding domain of p110alpha ( p110alpha-RBD), prevents tumor development in a mouse model of lung cancer. As tumor maintenance is clinically of more significance than tumor initiation to the treatment of pre-existing human cancers, we have subsequently developed an inducible mouse model to investigate whether interaction of Ras and PI 3-kinase is required for the maintenance of established tumors. For this we utilised K-Ras LA2 mice which spontaneously develop K-Ras mutant lung tumors. We have also introduced one allele of Pik3ca mutated in the Ras binding domain, one allele of Pik3ca WT flanked by lox-p sites, and a separate allele of inducible Cre-ER recombinase. Adult mice harboring lung tumors were then treated with tamoxifen to facilitate Cre mediated removal of the floxed allele and exclusive expression of p110alpha-RBD in established tumors. Using MicroCT scanning to monitor individual tumor growth, we have found that interruption of Ras PI 3-kinase signalling induces partial tumor regression, prevents further tumor growth and results in long-term stabilization of lung tumors. Expression of the p110alpha-RBD mutant decreased downstream signalling and reduced overall tumor burden. Orthotopic transplantation of tumor cells into WT mice with intact PI 3-kinase signaling revealed that the tumor cells are intrinsically sensitive to p110alpha-RBD expression. We also compared the effect of p110alpha-RBD expression with complete removal of p110alpha and found that inhibition of tumor growth occurred to a similar extent indicating that p110alpha signalling in these tumors is dependent upon interaction with Ras. Furthermore, combination of p110alpha-RBD expression with inhibition of the MAPK pathway promoted extensive tumor regression and dramatically reduced tumor burden. This effect was also observed with co-treatment with PI 3-kinase inhibitors and Mek inhibitors. This study demonstrates that the interaction of Ras and PI 3-kinase continues to play an important role in the maintenance of established tumors and in tumor progression. Our results indicate that the PI3K pathway is a viable therapeutic target in Ras mutant tumors, particularly in combination with other therapeutics such as Mek inhibitors.
Citation Format: Clare Sheridan, Esther Castellano, May Zaw Thin, Miguel Murillo, Francois Lassailly, Gordon Stamp, Julian Downward. Requirement for interaction of PI 3-kinase p110alpha with Ras in lung tumor maintenance. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr A54.