RRAD Promotes EGFR-Mediated STAT3 Activation and Induces Temozolomide Resistance of Malignant Glioblastoma

Abstract

Glioblastoma multiforme (GBM) is an extremely aggressive brain cancer with a median survival of less than 2 years. GBM is characterized by abnormal activation of receptor tyrosine kinase and constitutively activated STAT3. Although EGFR phosphorylation and STAT3 activation are essential for the maintenance of GBM cancer stem cells, the molecular mechanism underlying endosome-mediated STAT3 activation is not fully understood. In the current study, we showed that GTP-binding protein RRAD (RAS associated with diabetes, RAD) physically associates with EGFR, and EEA1, enhancing the stability and endosome-associated nuclear translocation of EGFR. Functionally, RRAD contributes to the activation of STAT3 and expression of the stem cell factors OCT4, NANOG, and SOX2, thereby enhancing self-renewing ability, tumor sphere formation, EMT, and in vivo tumorigenesis. Most importantly, RRAD contributes to poor survival in patients with GBM. RRAD expression is correlated with temozolomide resistance, and, conversely, depletion of RRAD leads to sensitization of highly temozolomide-resistant GBM cells. Our data collectively support a novel function of RRAD in STAT3 activation and provide evidence that RRAD acts as a positive regulator in the EGFR signaling pathway. These results demonstrate a critical role for RRAD in GBM tumorigenesis and provide a rationale for the development of pharmacologic inhibitors of RRAD in GBM. Mol Cancer Ther; 13(12); 3049-61. Ó2014 AACR.

Alterations in Cell Motility, Proliferation, and Metabolism in Novel Models of Acquired Temozolomide Resistant Glioblastoma

“…Acquisition of TMZ resistance was associated with a marked increase in the number of intracellular and extracellular vesicles in our models. The increase in early endosome marker EEA1 expression is interesting, as others have shown EEA1 to interact directly with RRAD, STAT3, and EGFR to regulate EGFR's subcellular localization and resistance to TMZ 25 . Clinically, higher EEA1 mRNA expression is associated with poor survival.…”

Sulforaphane reverses chemo-resistance to temozolomide in glioblastoma cells by NF-κB-dependent pathway downregulating MGMT expression

“…A major advance in GBM therapy as an advantageous treatment is the involvement of DNA alkylating agent temozolomide (TMZ) (4). TMZ, an orally administered alkylating agent with relatively low toxicity, exerts the antitumor activity by interfering with DNA replication.…”

Dual functionalized brain-targeting nanoinhibitors restrain temozolomide-resistant glioma via attenuating EGFR and MET signaling pathways

“…It was a major characteristic in GBM that RTKs, including IGFR, HGFR (MET), FGFR, VEGFR and the EGFR family, were abnormally activated 6 . Upon activation by ligands, RTK signals would be transduced through two major downstream pathways, Ras/MAPK/ERK and Ras/PI3K/AKT 8 , involved in the regulation of cancer chemoresistance 10,11 . EGFR is a member of the Erb family of RTKs regulating EGFR/NEAT1/EZH2 axis, and is critical for glioma cell growth and invasion 52 .…”

“…The coactivation of receptor tyrosine kinase (RTK) proteins, including insulin-like growth factor receptor (IGFR), hepatocyte growth factor receptor (HGFR; also known as MET), fibroblast growth factor receptor (FGFR), vascular endothelial growth factor receptor (VEGFR) and the EGFR family 6 , acts as an important mechanism by which cancer cells develop acquired chemoresistance 7 . As RTK members, the epidermal growth factor receptor (EGFR), mesenchymal-epithelial transition factor (MET) and two major downstream pathways (Ras/MAPK/ERK and Ras/PI3K/ AKT) 8 trigger uncontrolled cell growth, angiogenesis, and metastasis and chemoresistance [9][10][11] . Crosstalk between EGFR and MET contributes to the poor efficacy in the clinical treatment and malignant progresses such as alternative DNA damage repair [12][13][14][15][16] .…”