Critical functions of RhoB in support of glioblastoma tumorigenesis.

RhoB and its close homologs, RhoA and RhoC, belong to the Rho small GTPase family. Similar to the related Ras GTPases, these proteins are also molecular switches that regulate downstream effector molecules via cycling between GTP- and GDP-bound states. Rho GTPases are crucial regulators of actin dynamics and play a central role in mediating signal transduction from extracellular stimuli that target cytoskeleton.1 Regulation of the actin cytoskeleton by Rho GTPases has diverse impacts on cell mobility, vesicle trafficking, and cell-cycle progression. Although genes encoding Rho GTPases are rarely mutated in human cancers, changes in expression of Rho GTPases are extensively documented. RhoA and RhoC are upregulated in a number of cancer types with potential roles in proliferation, survival, and metastasis.2,3 In contrast, evidence for the role of RhoB in tumorigenesis remains controversial. Early studies suggest that RhoB may promote Ras-induced transformation.4,5 However, immunohistochemical stainings show that expression of RhoB inversely correlates with tumor grade and invasiveness in gastric cancer, head and neck squamous carcinoma, and lung cancer,6–9 suggesting potential tumor suppressive functions of RhoB in these cancers. Knockout of RhoB in mice does not affect normal biological functions but increases susceptibility to carcinogen-induced skin tumorigenesis.10 Nonetheless, emerging evidence suggests potential roles of RhoB in supporting tumorigenic functions. For example, RhoB expression is upregulated in T-acute lymphoblastic leukemia cells in comparison with normal T cells.11 High RhoB expression is found to associate with poor survival in lung adenocarcinoma.12 Depletion of RhoB in lung cancer cells impairs metastatic potential and increases sensitivity to chemoradiotherapy.12 In addition, activation of RhoB appears to protect cancer cells from radiation-induced apoptosis and mitotic cell death.13–15 RhoB induction by nongenotoxic stresses also appears to improve cellular survival, potentially via activation of NF-κB.16,17 These observations collectively suggest that the roles of RhoB in human cancer are context dependent. In glioma, the expression patterns and functions of RhoB remain largely unknown. Forget et al have analyzed expression of RhoA, RhoB, and Rac1 by immunoblotting tumor homogenates derived from patients with gliomas of different stages.18 Their results indicated reduced protein levels of all 3 of these GTPases in tumors relative to normal brain tissues, despite several prior studies suggesting upregulation of RhoA and Rac1 in several cancer types.19 Furthermore, expression of the Rho GTPases inversely correlated with tumor grade, with glioblastoma exhibiting the lowest expression.18 However, data derived from The Cancer Genome Atlas (TCGA) and the Repository for Molecular Brain Neoplasia Data (REMBRANDT) do not suggest significant changes of RhoB at DNA or mRNA levels in glioma. Other studies have suggested possible tumorigenic functions of RhoB in glioblastoma. In the U87 glioblastoma cell line, RhoB is activated following hypoxic stimulus and is required for stabilization of the hypoxia-inducible factor-1α, which is a key promoter of glioblastoma pathogenesis.20 The same group further demonstrated that the αvβ3 and αvβ5 integrin-focal adhesion kinase pathways regulate glioblastoma response to hypoxia via activation of RhoB,21 suggesting that RhoB regulates a mechanistic link between extracellular matrix and hypoxic response. In addition, the same integrins also regulate glioblastoma response to ionizing radiation through activation of RhoB.22 Increased expression of αvβ3 integrin, the positive RhoB regulator, is associated with poor prognosis in glioblastoma.23 These results suggest potential oncogenic activities of RhoB in glioblastoma. In the present study, we investigated the expression and functions of RhoB in a panel of glioblastoma surgical specimens and primary xenograft lines. Our results demonstrated important functions of RhoB in proliferation and survival of glioblastoma cells and in key signal transduction pathway including p53 and STAT3.