SC-21 * TRIM3, A HUMAN ORTHOLOG OF DROSOPHILA BRAT, MAINTAINS ASYMMETRIC CELL DIVISION OF GLIOMA STEM CELLS BY REGULATING c-MYC AND NOTCH1

Abstract

Glioma stem cells, capable of self-renewal and multipotent differentiation, influence neoplastic growth by a dynamic balance between symmetric and asymmetric cell division. Growth is favored by a shift to symmetric division in which greater numbers of self-renewing cells emerge, as opposed to asymmetric division, which generates one stem and one differentiated cell. Drosophila Brain Tumor (Brat) protein is a critical driver of asymmetric division and differentiation, and neuroblast cells devoid of Brat undergo symmetric cell division to generate self-renewing cells that result in the brat mutant phenotype. Using a brat RNAi driven by the neuroblast specific promoter inscuteable in Drosophila model, we demonstrated accumulation of proliferative neuroblasts using inscuteable driven RFP. brat RNAi driven with eyeless promoter in Drosophila eye also induced eye overgrowth. Reduced Brat in these settings was associated with upregulation of Notch protein, suggesting a suppressive role on Notch signaling by Brat. Brat also has been shown to promote asymmetric cell division through post-transcriptional suppression of dMyc, although detailed mechanisms are lacking. We have shown that TRIM3, a human ortholog of Drosophila brat, is deleted in more than 25% of glioblastomas (GBMs). We have demonstrated that expression of TRIM3 in GBM-derived neurospheres significantly reduces proliferation, neurosphere formation, and the expression of stem cell markers CD133, NESTIN and NANOG. In GBM stem cells, expression of TRIM3 leads to a greater percentage undergoing asymmetric cell division. As with Brat in Drosophila, TRIM3 suppresses c-MYC expression and activity. We provide evidence that TRIM3, an E3 ubiquitin ligase, binds with and ubiquitinates c-MYC protein, resulting in its degradation. Subsequent attenuation of c-MYC expression inhibits proliferation and self-renewal. Altogether, our current data support a critical role of TRIM3 in maintaining asymmetric cell division of glioma stem cells by regulating c-MYC and NOTCH1.