Significant attention has been given to molecular target therapies in pancreatic cancer due to the limited survival benefits provided by standard chemotherapy. The phosphatidylinositol 3-kinase (PI3K)/Akt/mTORC1/S6K pathway which is aberrantly stimulated in pancreatic ductal adenocarcinoma (PDAC) has emerged as a target for therapy. Inhibitors of PI3K and/or mTOR (PI3K/TOR-KIs) are been developed to target this pathway. However, mTORC1/S6K also mediates negative feedback loops that attenuate upstream signaling. Suppression of these feedback loops opposes the growth-suppressive effects of mTOR inhibitors and leads to drug resistance. Here, we demonstrate that treatment of a panel of pancreatic ductal adenocarcinoma (PDAC) cells including PANC-1, MiaPaCa-2, AsPC-1 and BxPC-3 with the dual PI3K/mTOR kinase inhibitor (PI3K/TOR-KI) NPV-BEZ23 induces a novel negative feedback loop leading to increased activity of MEK/ERK pathway in PDAC cells. Exposure of the above mentioned PDAC cells to NPV-BEZ235 potently blocked mTORC1 activation mTORC1/S6K activation (scored by S6 phosphorylation at Ser240/244), mTORC1/4E-BP1 (assayed by 4E-BP1 phosphorylation at Thr37/46) and mTORC2-mediated Akt phosphorylation at Ser473, in a concentration-dependent manner. Strikingly, NPV-BEZ235 markedly enhanced the MEK/ERK pathway in a dose-dependent manner. Maximal ERK over-activation coincided with complete inhibition of phosphorylation of Akt and 4E-BP1. Other PI3K/TOR-KIs, including PKI-587 and GDC-0980 also induced ERK over-activation. The MEK inhibitors PD0325901 or Trametinib prevented ERK over-activation induced by PI3K/TOR-KIs at least acutely. The combination of NPV-BEZ235 and PD0325901 caused a more pronounced inhibition of cell growth than that produced by each inhibitor individually. Mechanistic studies assessing PI3K activity in single PDAC cells using a PIP3 fluorescent reporter indicated that NPV-BEZ235, PKI-587 and GDC-0980 act through a PI3K-independent pathway. Doses of PI3K/TOR-KIs that enhanced MEK/ERK activation coincided with those that inhibited mTORC2-mediated Akt phosphorylation on Ser473, suggesting a role of mTORC2. Knockdown of Rictor markedly increased baseline levels of ERK phosphorylation and treatment with NVP-BEZ235 did not produce further enhancement of ERK activation. These results imply that Rictor or mTORC2 exerts feedback inhibition of the MEK/ERK pathway in pancreatic cancer cells. We propose that dual PI3K/mTOR inhibitors suppress a novel negative feedback loop mediated by mTORC2 thereby leading to enhanced MEK/ERK pathway activity in pancreatic cancer cells. The therapeutic effectiveness of PI3K/TOR-KIs inhibitors in PDAC and possibly other malignancies can be diminished by activation of MEK/ERK that opposes their anti-proliferative effects and leads to drug resistance.

Note: This abstract was not presented at the meeting.

Citation Format: Heloisa P. Soares, Ming Ming, Michelle Mellon, Steven H. Young, Liang Han, James Sinnet-Smith, Enrique Rozengurt. Targeting PI3K/mTOR leads to MEK/ERK over-activation in pancreatic cancer through suppression of mTORC2. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 670. doi:10.1158/1538-7445.AM2015-670