mTORC2/AKT/HSF1/HuR constitute a feed-forward loop regulating Rictor expression and tumor growth in glioblastoma

Oncogene. 2018 Feb 8;37(6):732-743. doi: 10.1038/onc.2017.360. Epub 2017 Oct 23.

Abstract

Overexpression of Rictor has been demonstrated to result in increased mechanistic target of rapamycin C2 (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 heat-shock transcription factor 1 (HSF1)-induced HuR activity. HuR is shown to directly bind the 3' untranslated region 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 suppress xenograft growth in mice. Modulation of AKT or HSF1 activity via the ectopic expression of mutant alleles support the ability of AKT to activate HSF1 and demonstrate continued HSF1/HuR/Rictor signaling in the context of AKT knockdown. We further show that constitutive overexpression of HuR is able to maintain Rictor expression under conditions of AKT or HSF1 loss. 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 via an AKT/HSF1/HuR signaling cascade resulting in enhanced mTORC2 activity in these tumors.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Apoptosis
  • Biomarkers, Tumor / genetics
  • Biomarkers, Tumor / metabolism*
  • Brain Neoplasms / genetics
  • Brain Neoplasms / metabolism
  • Brain Neoplasms / pathology
  • Cell Proliferation
  • ELAV-Like Protein 1 / genetics
  • ELAV-Like Protein 1 / metabolism*
  • Female
  • Follow-Up Studies
  • Gene Expression Regulation, Neoplastic
  • Glioblastoma / genetics
  • Glioblastoma / metabolism
  • Glioblastoma / pathology*
  • Heat Shock Transcription Factors / genetics
  • Heat Shock Transcription Factors / metabolism*
  • Humans
  • Mechanistic Target of Rapamycin Complex 2 / genetics
  • Mechanistic Target of Rapamycin Complex 2 / metabolism*
  • Mice
  • Mice, SCID
  • Phosphorylation
  • Prognosis
  • Proto-Oncogene Proteins c-akt / genetics
  • Proto-Oncogene Proteins c-akt / metabolism*
  • Rapamycin-Insensitive Companion of mTOR Protein / genetics
  • Rapamycin-Insensitive Companion of mTOR Protein / metabolism*
  • Signal Transduction
  • Tumor Cells, Cultured
  • Xenograft Model Antitumor Assays

Substances

  • Biomarkers, Tumor
  • ELAV-Like Protein 1
  • ELAVL1 protein, human
  • HSF1 protein, human
  • Heat Shock Transcription Factors
  • RICTOR protein, human
  • Rapamycin-Insensitive Companion of mTOR Protein
  • AKT1 protein, human
  • Mechanistic Target of Rapamycin Complex 2
  • Proto-Oncogene Proteins c-akt