PgmNr D1320: Understanding the mechanism of RIOK2 function in Glioblastoma.

Authors:
A. S. Chen; R. D. Read


Institutes
Emory University, Atlanta, GA.


Keyword: cancer

Abstract:

Glioblastoma multiforme (GBM), a tumor derived from glia and glial progenitor cells, is the most aggressive and prevalent form of primary brain cancer and is incurable. Amplification, mutation, and/or overexpression of the EGFR receptor tyrosine kinase and activating mutations in components of the PI3K pathway are common in GBM tumors, although the pathways that act downstream of EGFR and PI3K to drive tumorigenesis remain poorly understood. To better understand the underlying biology of tumorigenesis, we use a Drosophila melanogaster GBM model in which malignant neoplastic tumors arise from glial progenitor cells overexpressing activated oncogenic versions of EGFR and PI3K. This Drosophila model was used in kinome-wide genetic screens that identified Right-Open-Reading-Frame-2 (RIOK2), an atypical serine-threonine kinase, as a possible driver of EGFR-PI3K-dependent GBM. Subsequent studies in patient-derived GBM cell cultures suggest that RIOK2 promotes GBM cell proliferation and survival in response to oncogenic EGFR and PI3K signaling, and that, interestingly, these effects are dependent on RIOK2 catalytic kinase activity. Our data strongly implicate RIOK2 in driving tumorgenesis; however, little is understood about RIOK2 function or downstream targets. Preliminary immunoprecipitation experiments of RIOK2 from patient-derived GBM cell cultures coupled with proteomics identified several RIOK2 binding proteins and potential substrates. We conducted subsequent experiments in Drosophila since Drosophila possess orthologs for all of the RIOK2 binding proteins, and the functions of Drosophila orthologs of many of these proteins are evolutionary conserved. Using our Drosophila GBM model, we conducted a blind screen of all of our candidate RIOK2 substrates, and we discovered that RNAi against several mRNA-binding proteins drastically reduced aberrant glial cell proliferation and invasion similar to RIOK2 knock-down. These RNA binding proteins are involved in stabilizing and promoting the translation of their target mRNAs, several of which are known drivers of GBM that are overexpressed in response to oncogenic EGFR and PI3K signaling. Preliminary analysis of human GBM cells showed that, upon RIOK2 knockdown, protein levels were reduced for several of these target mRNAs. Based on our preliminary results all together, we hypothesize that RIOK2 drives tumorigenesis by modulating the activity of RNA-binding proteins involved in stabilizing and promoting the translation of their target mRNAs, and that this promotes the translation of target mRNAs that drive tumor cell proliferation and survival. Our study uses Drosophila as a model organism to better understand the mechanisms of GBM tumorigenesis and to gain insight into the biology of novel pathways that drive GBM.