Abstract 2321: Rapid induction of apoptosis by PI3K inhibitors is dependent upon their transient inhibition of RAS signaling.

Abstract

Abstract Hyperactivation of the PI3K/AKT/mTOR pathway in tumors causes feedback inhibition throughout the signaling network, including inhibition of receptor tyrosine kinase (RTK) expression and function. Exposure of these tumors to mTOR or AKT inhibitors relieves this feedback and reactivates RTK signaling. RTK reactivation promotes PI3K activity and attenuates the antitumor effects of the drugs. We have now characterized the details of PI3K-induced feedback using a selective class 1 PI3K inhibitor and compared its effects with those elicited by a selective allosteric inhibitor of AKT. In breast cancer cell lines with dysregulation of PI3K signaling, both drugs cause marked inhibition of AKT phosphorylation and relief of feedback inhibition of HER3, IGF1, and insulin RTKs. Whereas both inhibitors cause tumor cells to undergo growth arrest, only the PI3K inhibitor causes a rapid induction of apoptosis within 2 hours of drug addition. As opposed to the PI3K inhibitor, the AKT inhibitor induces PI3K activity and a set of substrates that are inhibited by the PI3K inhibitor. We therefore sought to identify processes that are selectively inhibited by the PI3K inhibitor to explain its selective induction of apoptosis. AKT inhibition causes the induction of ERK phosphorylation in these cells. In contrast, we found that PI3K inhibition causes rapid inhibition of RAS-GTP levels, RAF activation and ERK phosphorylation in tumor cells with wild type RAS, but not those with mutant RAS. Inhibition of RAS is transient, recovering 2-3 hours after drug addition. We determined that adding MEK inhibitors to AKT inhibitors is similarly associated with a rapid induction of apoptosis within 2 hours. Moreover, addition of EGF to tumor cells co-treated with PI3K inhibitors prevents ERK inhibition and apoptosis, whereas addition of MEK inhibitors to the PI3K inhibitor, EGF treated cells restores the apoptotic phenotype. We conclude that PI3K is upstream of both RAS and AKT in RAS wild type tumors. Inhibitors of PI3K cause sustained inhibition of AKT and transient inhibition of RAS with both effects required for rapid induction of apoptosis. These data suggest that pulsatile inhibition of PI3K may maximize the therapeutic efficacy of this approach. Citation Format: Marie Will, Weiyi Toy, Zhan Yao, Poulikos Poulikakos, Vanessa Rodrik-Outmezguine, Claudia Schneider, Juliane Paul, Jose Baselga, Ningshu Liu, Neal Rosen, Sarat Chandarlapaty. Rapid induction of apoptosis by PI3K inhibitors is dependent upon their transient inhibition of RAS signaling. [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 2321. doi:10.1158/1538-7445.AM2013-2321