Abstract 1650: Targeting of compensatory signaling pathways identified in a synthetic lethal screen leads to synergistic growth inhibition in bladder cancer cells

Abstract

Abstract Despite promising preclinical results, monotherapies that target deregulated signal transduction pathways often are insufficient to slow or shrink tumors in human patients. We hypothesize that multiple pathways within the cellular signaling network are altered in cancer cells, and that these pathways are acting in a redundant or compensatory manner to provide resistance against drugs targeting a single protein. A treatment plan consisting of multiple small molecules that inhibit these compensatory and redundant pathways should overcome this resistance. To discover pathways that act in a redundant or compensatory manner, we performed a screen that allows us to examine the cellular effects of blocking pathways both individually and in combination. We found that the combination of the epidermal growth factor receptor (EGFR) inhibitor lapatinib in combination with the phosphoinositol-3-kinase (PI3K) inhibitor LY294002 produced synergistic growth inhibition in UMUC-6 bladder cancer cells. In order to confirm that this effect was due to inhibition of the putative targets of these compounds, we substituted structurally distinct EGFR and PI3K inhibitors for lapatinib and LY294002 and obtained similar results. Cell cycle analysis of cells treated with this combination revealed an arrest in the G1 phase, which could explain the synergistic inhibition observed in the screen. In order to find a molecular mechanism for this synergistic effect, we performed Western blotting analysis of the cells treated with each drug individually and in combination. We found that treatment with LY294002 caused an increase in levels of phosphorylated SHC, and ERK, indicating that receptor tyrosine kinase signaling, and possibly EGFR signaling is activated to compensate for PI3K pathway inhibition. Inhibition of ERK activity with PD325901 produced the same effect as EGFR inhibition, confirming the role of ERK as a compensatory signaling molecule for PI3K inhibition. Parallel RNAi experiments against SHC are currently ongoing. These data suggest that synthetic lethal screening with a library of targeted inhibitors of signaling proteins can be used to search for combinatorial effects and functionally identify compensatory and redundant relationships between different signaling components. This work paves the way for the development of rational combinatorial therapies for the effective treatment of bladder cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1650.