mTORC1 and mTORC2 regulate EMT, motility and metastasis of colorectal cancer via RhoA and Rac1 signaling pathways

Activation of phosphatidylinositol 3-kinase (PI3K)/Akt signaling is associated with growth and progression of colorectal cancer (CRC). We have previously shown that the mammalian target of rapamycin (mTOR) kinase, a downstream effector of PI3K/Akt signaling, regulates tumorigenesis of CRC. However, the contribution of mTOR and its interaction partners towards regulating CRC progression and metastasis remains poorly understood. We found that increased expression of mTOR, Raptor and Rictor mRNA was noted with advanced stages of CRC suggesting that mTOR signaling may be associated with CRC progression and metastasis. mTOR, Raptor and Rictor protein levels were also significantly elevated in primary CRCs (stage IV) and their matched distant metastasis compared to normal colon. Inhibition of mTOR signaling, using rapamycin or stable inhibition of mTORC1 (Raptor) and mTORC2 (Rictor), attenuated migration and invasion of CRCs. Furthermore, knockdown of mTORC1 and mTORC2 induced a mesenchymal-epithelial transition and enhanced chemosensitivity of CRCs to oxaliplatin. We observed increased cell-cell contact as well as decreased actin cytoskeletal remodeling concomitant with decreased activation of the small GTPases, RhoA and Rac1, upon inhibition of both mTORC1 and mTORC2. Finally, establishment of CRC metastasis in vivo was completely abolished with targeted inhibition of mTORC1 and mTORC2 irrespective of the site of colonization. Our findings support a role for elevated mTORC1 and mTORC2 activity in regulating EMT, motility and metastasis of CRCs via RhoA and Rac1 signaling. These findings provide the rationale for including mTOR kinase inhibitors, which inhibit both mTORC1 and mTORC2, as part of the therapeutic regimen for CRC patients.