Mechanistic Target of Rapamycin (mTOR) Inhibition Synergizes with Reduced Internal Ribosome Entry Site (IRES)-mediated Translation of Cyclin D1 and c-MYC mRNAs to Treat Glioblastoma

J Biol Chem. 2016 Jul 1;291(27):14146-14159. doi: 10.1074/jbc.M116.726927. Epub 2016 May 11.

Abstract

Our previous work has demonstrated an intrinsic mRNA-specific protein synthesis salvage pathway operative in glioblastoma (GBM) tumor cells that is resistant to mechanistic target of rapamycin (mTOR) inhibitors. The activation of this internal ribosome entry site (IRES)-dependent mRNA translation initiation pathway results in continued translation of critical transcripts involved in cell cycle progression in the face of global eIF-4E-mediated translation inhibition. Recently we identified compound 11 (C11), a small molecule capable of inhibiting c-MYC IRES translation as a consequence of blocking the interaction of a requisite c-MYC IRES trans-acting factor, heterogeneous nuclear ribonucleoprotein A1, with its IRES. Here we demonstrate that C11 also blocks cyclin D1 IRES-dependent initiation and demonstrates synergistic anti-GBM properties when combined with the mechanistic target of rapamycin kinase inhibitor PP242. The structure-activity relationship of C11 was investigated and resulted in the identification of IRES-J007, which displayed improved IRES-dependent initiation blockade and synergistic anti-GBM effects with PP242. Mechanistic studies with C11 and IRES-J007 revealed binding of the inhibitors within the UP1 fragment of heterogeneous nuclear ribonucleoprotein A1, and docking analysis suggested a small pocket within close proximity to RRM2 as the potential binding site. We further demonstrate that co-therapy with IRES-J007 and PP242 significantly reduces tumor growth of GBM xenografts in mice and that combined inhibitor treatments markedly reduce the mRNA translational state of cyclin D1 and c-MYC transcripts in these tumors. These data support the combined use of IRES-J007 and PP242 to achieve synergistic antitumor responses in GBM.

Keywords: IRES; ITAF; Myc (c-Myc); cyclin D1; glioblastoma; heterogeneous nuclear ribonucleoprotein (hnRNP); mRNA translation; mechanistic target of rapamycin (mTOR); translation.

Publication types

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

MeSH terms

  • Animals
  • Brain Neoplasms / metabolism
  • Brain Neoplasms / therapy*
  • Cell Line, Tumor
  • Cyclin D1 / genetics*
  • Female
  • Genes, myc*
  • Glioblastoma / metabolism
  • Glioblastoma / therapy*
  • Heterografts
  • Humans
  • Internal Ribosome Entry Sites*
  • Mice
  • Protein Biosynthesis*
  • RNA, Messenger / genetics*
  • TOR Serine-Threonine Kinases / antagonists & inhibitors*

Substances

  • Internal Ribosome Entry Sites
  • RNA, Messenger
  • Cyclin D1
  • MTOR protein, human
  • TOR Serine-Threonine Kinases

Associated data

  • PDB/1HA1