Abstract 909: Hsp90 inhibitor AUY922 abrogates mTOR inhibitor AZD8055-induced up-regulation of HER2/HER3 and potentiates antiproliferative activity in human breast cancer cells.
Authors: Chen Simeng , Guo Chenliang , Ding Jian , Meng Linghua 
Published: August 2014

Abstract
Mammalian target of rapamycin (mTOR) has been validated as a promising target for cancer therapy. Several mTOR kinase inhibitors displaying potent antitumor activity due to their capability to inhibit both mTORC1 and mTORC2 have entered clinical trials. It has been recently reported that long term inhibition of mTOR led to activation of upstream receptor tyrosine kinases and Akt, which may attenuate the efficacy of mTOR kinase inhibitors. As a number of receptor tyrosine kinases and AKT are client proteins of Hsp90, we hypothesized that combination of inhibitors targeting mTOR and Hsp90 respectively would potentiate the anticancer activity. By calculating the Combination Index, mTOR kinase inhibitor AZD8055 and Hsp90 inhibitor AUY922 displayed synergistic effect against a panel of human breast cancer cells irrespective their ER, HER2 or PI3K status. The combination of sub-optimal concentration of these two drugs exerted significant antiproliferative activity, which is associated with enhanced cell cycle arrest but not induction of apoptosis. Down-regulation of either raptor or rictor by specific shRNA reinforced the antitumor activity of AUY922, indicating restraint of both mTORC1 and mTORC2 contribute to the synergistic activity of AZD8055 with AUY922. In T47D cells,long time exposure to AZD8055 led to restoration of phosphorylated AKT at T308 and GSK3β at S9, indicating reactivation of AKT, which was abrogated by AUY922 co-treatment. Moreover, phosphorylation of AKT at S473 was more significantly inhibited when cells were co-treated with AZD8055 and AUY922 than single agent treatment. We further revealed that AUY922 prevented feedback activation of PI3K upon AZD8055 treatment. As PI3K is not a client protein of Hsp90, block of PI3K activation by AUY922 might be due to down-regulation of upstream tyrosine kinase receptors. Accordingly, though the mRNA levels of EGFR, HER2, HER3 and HER4 increased after either single agent treatment or co-treatment, AUY922 not only inhibited the expression of tested tyrosine kinases, but more importantly, abrogated activation of HER2/HER3 induced by AZD8055 .
In summary, the combination of catalytic mTOR inhibitor and Hsp90 inhibitor demonstrated synergistic activity against breast cancer cells, which is due to inhibition of tyrosine kinases expression and abrogation of feedback activation of HER2/HER3. As both AZD8055 and AUY922 are in clinical trials for breast cancer therapy, this study established a mechanistic rationale for a combination approach using AZD8255 and AUY922 in the treatment of breast cancer.
Citation Format: Simeng Chen, Chenliang Guo, Jian Ding, Linghua Meng. Hsp90 inhibitor AUY922 abrogates mTOR inhibitor AZD8055-induced up-regulation of HER2/HER3 and potentiates antiproliferative activity in human breast cancer cells. [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 909. doi:10.1158/1538-7445.AM2013-909