Abstract 3320: Plasticity between the cancer stem-like and non cancer stem-like states is regulated by the PI3K/Akt/β-catenin/CBP pathway

Introduction: Cancer stem-like cells (CSC) - a subpopulation of self-renewing, tumorigenic, drug resistant tumor cells - are thought to promote cancer formation, therapy resistance and disease progression. Recently, we and others reported that the CSC phenotype may not represent a static pre-existing seed population; rather, CSC subpopulations can arise through direct conversion of non-CSC to the CSC phenotype. Here we show that PI3K/Akt/β-catenin/CBP signaling plays a key role in mediating this important phenotypic plasticity. Methods: As previously reported, fluorescence activated cell sorting (FACS) and Hoechst dye exclusion were applied to cancer cell lines in order to isolate side populations (SP) of cells enriched for CSC properties (high tumorigenicity and drug resistance) versus non-side-populations (NSP) of cells lacking these properties. GFP+ NSP were re-combined with GFP− SP to enable tracking of the two phenotypes in culture over time. The cells were treated with pharmacological and siRNA inhibitors targeting members of the PI3K/Akt/β-catenin/CBP pathway, and their effects on SP size and NSP-to-SP conversion were measured. Results: NSP-to-SP conversion was significantly reduced by exposure to LY294002, a PI3K/Akt inhibitor, and the same effect was recapitulated by direct siRNA knockdown of Akt. Further downstream, BIO (6-bromoindirubin-3′-oxime) was used to inhibit GSK-3β, a protein that normally potentiates β-catenin degradation. Exposure to BIO increased both the overall levels and the nuclear localization of β-catenin and significantly increased SP size. Conversely, siRNA knockdown of β-catenin signficantly decreased SP size. Still further downstream, two β-catenin transcription co-factors, CBP and P300, exerted opposite effects on the CSC phenotype: Blocking β-catenin-CBP interaction with the specific inhibitor ICG001 significantly decreased SP size and sphere formation, whereas blocking β-catenin-P300 interaction with the specific inhibitor IQ-1 significantly increased SP size. Moreover, ICG001 inhibition of β-catenin-CBP interaction abrogated the SP expansion caused by upstream BIO inhibition of GSK-3β. ICG001 did not affect the nuclear localization of β-catenin, suggesting that CBP indeed cooperates with β-catenin further downstream to transcriptionally impact NSP-to-SP conversion. Conclusion: Cellular plasticity between the non-CSC and CSC phenotypes has considerable therapeutic ramifications because it implies that the drug-resistant, aggressive CSC subpopulation may be replenished from the larger non-CSC population. Our present studies identify PI3K/Akt/β-catenin/CBP signaling as playing a key role in mediating this phenotypic plasticity. Therefore, therapeutic targeting of this pathway may be uniquely effective at overcoming the therapy resistance and disease progression attributed to the CSC phenotype. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3320. doi:1538-7445.AM2012-3320