Abstract 1137: Imp2 controls oxidative phosphorylation in glioblastoma cancer stem cells

Abstract

Abstract Insulin-like growth factor 2 mRNA binding protein 2 (Imp2, IGF2BP2) is expressed during embryonic development in the brain and associated with several types of cancer. In preliminary studies we assessed Imp2 expression in normal brain and glial tumors. Immunohistochemistry revealed that Imp2 is present in 75% cases of the most malignant glial tumor, glioblastoma multiforme (GBM), whereas it is absent in low grade gliomas and in normal brain. GBM harbor a subpopulation of undifferentiated, slowly proliferating cells that correspond to the current definition of cancer stem cells (CSC). Analysis of these cells selected based on the CSC-associated CD133 marker expression, revealed that they express higher levels of Imp2 than their CD133-negative counterparts. Primary cultures of gliospheres that allow for CSC enrichment, also expressed high Imp2 levels. Imp2 depletion by shRNA significantly decreased GBM CSC ability to form spheres in single cell clonogenic assays and diminished their tumorigenic potential, suggesting that Imp2 is important for CSC maintenance. To investigate the role of Imp2 in CSC biology, Imp2 ribonucleoprotein complexes were purified and both Imp2-bound mRNA and proteins were analyzed, by microarray and mass spectrometry, respectively. Interestingly, both Imp2-bound RNA and Imp2-associated proteins were implicated in mitochondrial metabolism, in particular with oxidative phosphorylation (OxPhos). Involvement of Imp2 in mitochondrial metabolism was therefore tested. Depletion of Imp2 in GBM CSC impaired OxPhos, but did not affect other mitochondrial processes. Moreover, Imp2 knock-down decreased cellular ATP levels, suggesting that OxPhos is the main energy producing pathway in GBM CSC. Accordingly, inhibition of respiratory complex I with rotenone mimicked the Imp2 knock-down phenotype, as it diminished proliferation, sphere formation, oxygen consumption and ATP levels. In contrast, inhibition of anaerobic glycolysis, a pathway associated with cancer bioenergetics, did not affect GBM CSC. The mechanism by which Imp2 controls OxPhos may involve its protein interactors, among which we found three subunits of respiratory complex I, responsible for initiation of complex assembly. Imp2 depletion affects activity and assembly of respiratory complex I, suggesting that the interaction between Imp2 and these subunits is crucial for electron entry into the OxPhos chain. In addition, Imp2 may control cellular respiration by providing mRNA delivery. Purification of mitochondria-bound polyribosomes revealed that overexpression of Imp2 increases the amount of Imp2-bound mRNA in this fraction. These data imply that Imp2 delivers nuclear-encoded mRNA onto ribosomes attached to the mitochondrial surface, where co-translational insertion into mitochondrial membranes can occur. Our results have uncovered a novel function of the oncofetal protein Imp2 related to metabolic requirements of GBM CSC. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1137. doi:1538-7445.AM2012-1137