mTORC2/AKT/HSF1/HuR Constitute A Feed-Forward Loop Regulating Rictor Expression And Tumor Growth In Glioblastoma

Abstract

Overexpression of Rictor has been demonstrated to result in increased mTORC2 nucleation and activity leading to tumor growth and increased invasive characteristics in glioblastoma multiforme (GBM). However the mechanisms regulating Rictor expression in these tumors is not clearly understood. In this report, we demonstrate that Rictor is regulated at the level of mRNA translation via HSF1-induced HuR activity. HuR is shown to directly bind the 3' UTR of the Rictor transcript and enhance translational efficiency. Moreover, we demonstrate that mTORC2/AKT signaling activates HSF1 resulting in a feed-forward cascade in which continued mTORC2 activity is able to drive Rictor expression. RNAi-mediated blockade of AKT, HSF1 or HuR is sufficient to downregulate Rictor and inhibit GBM growth and invasive characteristics in vitro and suppresses xenograft growth in mice. We further demonstrate that constitutive overexpression of HuR is able to maintain Rictor expression under conditions of AKT or HSF1 loss. In an additional level of regulation, miR-218, a known Rictor targeting miRNA is shown to be subject to mTORC2/STAT3-mediated repression. The expression of these components is also examined in patient GBM samples and correlative associations between the relative expression of these factors support the presence of these signaling relationships in GBM. These data support a role for a feed-forward loop mechanism by which mTORC2 activity stimulates Rictor translational efficiency and suppresses miR-218 resulting in enhanced mTORC2 activity in these tumors.