Abstract LB-012: Inhibition of PI3K induces paracrine factors, which promote growth and survival of human breast cancer cells
Authors: Young Christian D. , Koch James P. , Cook Rebecca S. , Arteaga Carlos L. 
Published: August 2015

Phosphoinositide 3-kinase (PI3K) is aberrantly activated in many human cancers. To blunt the mitogenic action of this oncogenic pathway, PI3K inhibitors are currently in clinical development. However, inhibition of PI3K results in feedback activation of receptor tyrosine kinases (RTKs) and cap-independent translation of pro-survival proteins, thus diminishing the net antitumor effect of PI3K inhibitors. Therefore, characterization of pathways potentially activated by PI3K inhibitors is necessary in order to identify drug combinations which will better eradicate tumors. We demonstrate herein that inhibition of PI3K in breast cancer cells resulted in the increased expression of EGFR ligands and activation of EGFR/ERK signaling. FoxO transcription factors are repressed by PI3K and previous studies have shown that inhibition of PI3K results in nuclear localization of FoxO and activation of FoxO-mediated transcription of RTKs and IGF-I/II. However, RNAi-mediated knockdown of FoxO3A only partially attenuated the activation of EGFR induced by PI3K inhibition. Using a panel of transcription factor luciferase reporters, we identified 10 transcription factors (including FoxO) which are activated upon PI3K inhibition in three breast cancer cell lines from different intrinsic subtypes (MCF7, BT20 and SUM159). In addition, the serum-free media conditioned by MCF7 or BT20 cells in the presence of the pan-PI3K inhibitor BKM120 induced the survival and proliferation of recipient cancer cells which otherwise undergo apoptosis in serum-free conditions. Thus, we hypothesized that inhibition of PI3K results in

activation of paracrine factors, including EGFR ligands, which may promote the survival of a heterogeneous tumor cell population. Indeed, evaluation of media conditioned by MCF7 and BT20 cells with cytokine/growth factor antibody arrays demonstrated over 100 proteins to be induced by PI3K inhibition, including ligands for receptors in the EGF, FGF, cytokine, chemokine and TGFβ families. We are currently performing SILAC-based mass spectrometry profiling of media conditioned by breast cancer cells ± PI3K inhibitors to determine modulation of secreted factors upon inhibition of PI3K. We are also

determining whether inhibition of PI3K results in increased ADAM protease activity, inducing the shedding of EGFR ligands from the cell membrane. The results of these experiments will identify extracellular factors activated by the inhibition of PI3K and suggest which pathways need to be simultaneously inhibited to maximize the clinical activity of PI3K inhibitors. Further, these secreted proteins may serve as circulating pharmacodynamics biomarkers indicative of effective blockade of PI3K in patients.

Citation Format: Christian D. Young, James P. Koch, Rebecca S. Cook, Carlos L. Arteaga. Inhibition of PI3K induces paracrine factors, which promote growth and survival of human breast cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-012. doi:10.1158/1538-7445.AM2015-LB-012