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Par6 links PI3K to the TGFbeta receptor complex: potential implications for PI3K targeting in advanced TGFbeta-dependent breast cancer

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Title: Par6 links PI3K to the TGFbeta receptor complex: potential implications for PI3K targeting in advanced TGFbeta-dependent breast cancer
Author: Delroba, Alham
Department: Department of Biomedical Sciences
Program: Biomedical Sciences
Advisor: Viloria-Petit, Alicia
Abstract: Transforming growth factor β (TGFβ) signalling promotes breast cancer metastasis, but also inhibits tumour growth during the early stages of cancer by promoting apoptosis. We have previously identified Par6, a key modulator of apical-basal polarity, as a critical mediator of TGFβ- induced apoptosis in early cancerous mammary cells. Our preliminary findings in the laboratory suggest that Par6-mediated cell death requires its activation by TGFβ and correlates with a reduction in the activity of phosphatidylinositol 3-kinase (PI3K). Based on these observations we hypothesize that a Par6-PI3K signalling axis functions within a cell polarity-associated signalling network that is crucial in determining cell death in response to TGFβ, and propose that PI3K inhibition could potentially rescue TGFβ-dependent apoptosis in advanced breast cancer cells. To address this hypothesis, we first aimed at demonstrating a physical interaction between Par6 and PI3K. We performed Par6 and p85 (PI3K regulatory subunit) immunoprecipitation and immunoblotting on lysates obtained from metastatic mammary EMT6 cells overexpressing exogenous Flag-tagged wild type Par6 or a dominant negative mutant of Par6 that cannot be phosphorylated on S345 in response to TGFβ. Our results indicate that Par6 and p85 interact within a complex that also contains TGFβ receptor I and the small GTPase Cdc42, which were identified as their common binding partners. Furthermore, this Par6-p85 interaction is independent of phosphorylation at S345 and independent of TGFβ receptor I activity. The PAR6-p85 interaction was also confirmed endogenously in advanced human breast cancer cells, MDA-MB-231 and T47D. Additionally, we found that inhibition of PI3K activity by using a selective inhibitor GDC-0941 results in an apoptotic response which is cell-type specific. While it induces cell death in T47D cells, it has no effect in MDA-MB-231 cells. Importantly, our preliminary data from PI3K targeting experiments indicates that cells expressing a mutant overactive PIK3CA, which also have relatively high endogenous levels of PAR6 and TGFβ signalling, such as T47D, are susceptible to PI3K inhibition. TGFβ signalling further increases the apoptotic response to PI3K inhibitor compounds in these cells. These findings support the notion that TGFβ signalling might, under specific molecular settings, be used as a complementary predictor of susceptibility of breast cancer cells to PI3K inhibitors.
Date: 2016-12-20
Rights: Attribution-NonCommercial-NoDerivs 2.5 Canada
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Attribution-NonCommercial-NoDerivs 2.5 Canada Except where otherwise noted, this item's license is described as Attribution-NonCommercial-NoDerivs 2.5 Canada