Disruption of the RICTOR/mTORC2 complex enhances the response of head and neck squamous cell carcinoma cells to PI3K inhibition.

Phosphoinositide 3-kinase (PI3K) is aberrantly activated in head and neck squamous cell carcinomas (HNSCC) and plays a pivotal role in tumorigenesis by driving Akt signaling, leading to cell survival and proliferation. Phosphorylation of Akt Thr308 by PI3K-PDK1 and Akt Ser473 by mammalian target of rapamycin complex 2 (mTORC2) activates Akt. Targeted inhibition of PI3K is a major area of preclinical and clinical investigation as it reduces Akt Thr308 phosphorylation, suppressing downstream mTORC1 activity. However, inhibition of mTORC1 releases feedback inhibition of mTORC2, resulting in a resurgence of Akt activation mediated by mTORC2. While the role of PI3K-activated Akt signaling is well established in HNSCC, the significance of mTORC2-driven Akt signaling has not been thoroughly examined. Here we explore the expression and function of mTORC2 and its obligate subunit RICTOR in HNSCC primary tumors and cell lines. We find RICTOR to be overexpressed in a subset of HNSCC tumors, including those with PIK3CA or EGFR gene amplifications. Whereas overexpression of RICTOR reduced susceptibility of HNSCC tumor cells to PI3K inhibition, genetic ablation of RICTOR using CRISPR/Cas9 sensitized cells to PI3K inhibition, as well as to EGFR inhibition and cisplatin treatment. Further, mTORC2 disruption led to reduced viability and colony forming abilities of HNSCC cells relative to their parental lines and induced loss of both activating Akt phosphorylation modifications (Thr308 and Ser473). Taken together, our findings establish RICTOR/mTORC2 as a critical oncogenic complex in HNSCC and rationalize the development of an mTORC2-specific inhibitor for use in HNSCC, either combined with agents already under investigation, or as an independent therapy.

Ruicci KM ，Plantinga P ，Pinto N ，Khan MI ，Stecho W ，Dhaliwal SS ，Yoo J ，Fung K ，MacNeil D ，Mymryk JS ，Barrett JW ，Howlett CJ ，Nichols AC ... -  《-》

Down-regulation of Rictor enhances cell sensitivity to PI3K inhibitor LY294002 by blocking mTORC2-medicated phosphorylation of Akt/PRAS40 in esophageal squamous cell carcinoma.

PI3K/Akt/mTOR signaling pathway plays a vital role in regulating cell survival, differentiation, metabolism and migration, which is frequently hyperactive in a number of cancers, including esophageal squamous cell carcinoma (ESCC). As the core subunit of mTORC2, Rictor is shown to be amplified in ESCC patients' tissues and plays an important role in regulation of Akt. The objective of this study is to evaluate the effects of Rictor knockdown on cell sensitivity to PI3K inhibitor LY294002 in ESCC cells and ESCC xenografts as well as its mechanisms. We found LY294002 obviously restrained cell proliferation in dose-dependent and time-dependent manners by inhibiting PI3K/Akt/mTOR/p70S6K signaling pathway, whereas triggered mTORC2-medicated phosphorylation of Akt (Ser473)/PRAS40 (Thr246) in ECa109 and EC9706 cells. Stable knockdown of Rictor by shRNA enhanced the inhibitory effects of LY294002 on cell proliferative, migration and colony formation, as well as promoted its effects on cell cycle arrest and cell apoptosis in vitro. Furthermore, stable knockdown of Rictor enhanced the antitumor effects of LY294002 by inhibiting tumor growth and promoting cell apoptosis in vivo. Mechanistic assay revealed that knockdown of Rictor could attenuate LY294002-induced phosphorylation of Akt (Ser473)/PRAS40 (Thr246). Our results provide rationale that combined inhibition of Rictor/mTORC2 and PI3K for the treatment of ESCC.

Hou G ，Zhao Q ，Zhang M ，Fan T ，Liu M ，Shi X ，Ren Y ，Wang Y ，Zhou J ，Lu Z ... -  《-》

A positive feedback loop involving EGFR/Akt/mTORC1 and IKK/NF-kB regulates head and neck squamous cell carcinoma proliferation.

Li Z ，Yang Z ，Passaniti A ，Lapidus RG ，Liu X ，Cullen KJ ，Dan HC ... -  《Oncotarget》

Differential modulation of PI3K/Akt/mTOR activity by EGFR inhibitors: A rationale for co-targeting EGFR and PI3K in cisplatin-resistant HNSCC.

To find a new strategy to treat cisplatin-resistant head and neck squamous cell carcinoma (HNSCC), we investigated the effects of EGFR inhibitors on the PI3K/Akt/mTOR pathway and determined the efficacy of EGFR inhibitors in combination with PI3K inhibitors to suppress cell proliferation in cisplatin-resistant-HNSCC. The cisplatin-resistant HNSCC cell lines were treated with four FDA approved EGFR inhibitors, which included Gefitinb or Erlotinib alone, or in combination with the pan-PI3K inhibitor, BKM120. Phosphorylation and total protein levels of cells were assessed by Western blot analysis. Cell proliferation was examined by MTS assay. Apoptosis was analyzed by flow cytometry. Cisplatin-resistant HNSCC cells were also resistant to EGFR inhibitors. However, a combination of EGFR inhibitors with PI3K inhibitor BKM120 dramatically improved the efficacy of EGFR inhibitors to inhibit cell proliferation and induce apoptosis. Furthermore, treatment with EGFR inhibitors differentially affected the phosphorylation of Akt and mTOR, which included partial inhibition, no inhibition, and induction. A combination of EGFR inhibitors and BKM120 completely blocked phosphorylation of EGFR, Akt, and S6K (an mTOR target). Our data provided a rationale for EGFR inhibitors in combination with PI3K inhibitors to treat cisplatin-resistant HNSCC.

Liao J ，Yang Z ，Azarbarzin S ，Cullen KJ ，Dan H ... -  《-》

Two distinct mTORC2-dependent pathways converge on Rac1 to drive breast cancer metastasis.

Morrison Joly M ，Williams MM ，Hicks DJ ，Jones B ，Sanchez V ，Young CD ，Sarbassov DD ，Muller WJ ，Brantley-Sieders D ，Cook RS ... -  《-》