Abstract 427: Targeted inhibition of the mTOR complex components, RAPTOR and RICTOR, attenuates progression of colorectal cancer

Abstract

Abstract Colorectal cancer (CRC) is the second leading cause of cancer deaths in the USA. The mammalian target of rapamycin (mTOR) acts downstream of PI3K to regulate cell growth, proliferation and survival. The mTOR kinase nucleates two distinct complexes, mTORC1 and mTORC2. The mTOR-RAPTOR complex (mTORC1) regulates translation initiation through the effectors S6K and 4E-BP1, while the mTOR-RICTOR complex (mTORC2) regulates the actin cytoskeleton and activates Akt, a key regulator of cell survival. Rapamycin inhibits the kinase activity of mTORC1; prolonged rapamycin treatment also inhibits mTORC2 assembly and Akt activation in certain cells. The purpose of the present study was to determine the effect of targeting mTORC1 and mTORC2 upon progression of CRCs. METHODS. HCT116 and SW480 human colon cancer cells were treated with rapamycin. Cells were also transfected with shRNA directed against RAPTOR or RICTOR. Effects on migration (measured by wound healing assay) and invasion (measured by transwell matrigel assay) were analyzed. Changes in actin cytoskeleton and cell-cell contacts were determined by immunofluorescence microscopy. RESULTS. (i) Immunohistochemical analysis showed that the mTORC1 and mTORC2 components, mTOR, RAPTOR and RICTOR, are overexpressed in primary CRCs and matched liver metastases compared to normal colonic tissue. (ii) Treatment with rapamycin significantly decreased the migration and invasion of HCT116 and SW480 cells. (iii) Stable shRNA-mediated knockdown of RAPTOR and RICTOR significantly decreased the migration and invasion of HCT116 and SW480 cells. (iv) Stable shRNA-mediated knockdown of RAPTOR and RICTOR leads to mesenchymal-to-epithelial transition, as evidenced by increased E-cadherin and decreased vimentin levels. (v) Stable shRNA-mediated knockdown of RAPTOR and RICTOR leads to characteristic rearrangements in actin cytoskeleton as well as increased cell-cell contact. CONCLUSIONS. The mTORC1 and mTORC2 components, mTOR, RAPTOR and RICTOR, are overexpressed in both primary CRCs as well as matched liver metastases. Although the mTOR signaling axis is primarily associated with growth of normal and cancer cells, we demonstrate that stable inhibition of both mTORC1 and mTORC2 decreases migration and invasion capabilities of CRC cells as well as causing a mesenchymal-to-epithelial change in phenotype. The mechanism for this decrease in migration and invasion may be via modulation of actin cytoskeletal rearrangement and cell-cell contact. Thus, targeted inhibition of mTOR signaling represents a novel therapeutic strategy to attenuate the progression and metastasis of CRCs. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 427.