The duality of epidermal growth factor receptor (EGFR) signaling and neural stem cell phenotype: cell enhancer or cell transformer?

Recruitment of neural stem cells (NSCs) represents an elegant strategy for replacing adult central nervous system (CNS) cells lost to injury or disease. However, except in the rostral migratory stream to the olfactory bulb, the adult CNS harbors a relatively non permissive environment for motility of neural stem cells. This opens the possibility of therapeutic enhancement of NSC motility towards sites of CNS injury or disease. The Epidermal Growth Factor Receptor (EGFR) is involved in the activation of a number of downstream pathways that regulate the phenotype of progenitor cells. Activated EGFR tyrosine kinase activity enhances NSC migration, proliferation, and survival. However, EGFR signaling is also known to play a role in the most malignant and highly invasive of human tumors, glioblastoma multiforme (GBM). Recent evidence supports the theory that GBM derives from a 'cancer stem cell' and that EGFR signals are commonly altered in these precursor cells. This article will review the role of EGFR signaling as it relates to neural stem cell motility and invasion. The duality of altered EGFR signaling in neural progenitor cells is discussed and opportunities for enhancing the recruitment of adult progenitors, and consequences of altering EGFR signaling in progenitor cells will be highlighted.     

Evaluation of Tyrosine Kinase Inhibitor Combinations for Glioblastoma Therapy

Glioblastoma multiforme (GBM) is the most common intracranial cancer but despite recent advances in therapy the overall survival remains about 20 months. Whole genome exon sequencing studies implicate mutations in the receptor tyrosine kinase pathways (RTK) for driving tumor growth in over 80% of GBMs. In spite of various RTKs being mutated or altered in the majority of GBMs, clinical studies have not been able to demonstrate efficacy of molecular targeted therapies using tyrosine kinase inhibitors in GBMs. Activation of multiple downstream signaling pathways has been implicated as a possible means by which inhibition of a single RTK has been ineffective in GBM. In this study, we sought a combination of approved drugs that would inhibit in vitro and in vivo growth of GBM oncospheres. A combination consisting of gefitinib and sunitinib acted synergistically in inhibiting growth of GBM oncospheres in vitro. Sunitinib was the only RTK inhibitor that could induce apoptosis in GBM cells. However, the in vivo efficacy testing of the gefitinib and sunitinib combination in an EGFR amplified/ PTEN wild type GBM xenograft model revealed that gefitinib alone could significantly improve survival in animals whereas sunitinib did not show any survival benefit. Subsequent testing of the same drug combination in a different syngeneic glioma model that lacked EGFR amplification but was more susceptible to sunitinib in vitro demonstrated no survival benefit when treated with gefitinib or sunitinib or the gefitinib and sunitinib combination. Although a modest survival benefit was obtained in one of two animal models with EGFR amplification due to gefitinib alone, the addition of sunitinib, to test our best in vitro combination therapy, did not translate to any additional in vivo benefit. Improved targeted therapies, with drug properties favorable to intracranial tumors, are likely required to form effective drug combinations for GBM.     

Epidermal Growth Factor (EGF)-enhanced Vascular Cell Adhesion Molecule-1 (VCAM-1) Expression Promotes Macrophage and Glioblastoma Cell Interaction and Tumor Cell Invasion

Activated EGF receptor (EGFR) signaling plays an instrumental role in glioblastoma (GBM) progression. However, how EGFR activation regulates the tumor microenvironment to promote GBM cell invasion remains to be clarified. Here, we demonstrate that the levels of EGFR activation in tumor cells correlated with the levels of macrophage infiltration in human GBM specimens. This was supported by our observation that EGFR activation enhanced the interaction between macrophages and GBM cells. In addition, EGF treatment induced up-regulation of vascular cell adhesion molecule-1 (VCAM-1) expression in a PKCϵ- and NF-κB-dependent manner. Depletion of VCAM-1 interrupted the binding of macrophages to GBM cells and inhibited EGF-induced and macrophage-promoted GBM cell invasion. These results demonstrate an instrumental role for EGF-induced up-regulation of VCAM-1 expression in EGFR activation-promoted macrophage-tumor cell interaction and tumor cell invasion and indicate that VCAM-1 is a potential molecular target for improving cancer therapy.     

NEK2 enhances malignancies of glioblastoma via NIK/NF-κB pathway

Glioblastoma (GBM) is one of the most lethal primary brain tumor with a poor median survival less than 15 months. Despite the development of the clinical strategies over the decades, the outcomes for GBM patients remain dismal due to the strong proliferation and invasion ability and the acquired resistance to radiotherapy and chemotherapy. Therefore, developing new biomarkers and therapeutic strategies targeting GBM is in urgent need. In this study, gene expression datasets and relevant clinical information were extracted from public cancers/glioma datasets, including TCGA, GRAVENDEEL, REMBRANDT, and GILL datasets. Differentially expressed genes were analyzed and NEK2 was picked as a candidate gene for subsequent validation. Human tissue samples and corresponding data were collected from our center and detected by immunohistochemistry analysis. Molecular biological assays and in vivo xenograft transplantation were performed to confirm the bioinformatic findings. High-throughput RNA sequencing, followed by KEGG analysis, GSEA analysis and GO analysis were conducted to identify potential signaling pathways related to NEK2 expression. Subsequent mechanism assays were used to verify the relationship between NEK2 and NF-κB signaling. Overall, we identified that NEK2 is significantly upregulated in GBM and the higher expression of NEK2 exhibited a poorer prognosis. Functionally, NEK2 knockdown attenuated cell proliferation, migration, invasion, and tumorigenesis of GBM while NEK2 overexpression promoted the GBM progression. Furthermore, High-throughput RNA sequencing and bioinformatics analysis indicated that NEK2 was positively related to the NF-κB signaling pathway in GBM. Mechanically, NEK2 activated the noncanonical NF-κB signaling pathway by phosphorylating NIK and increasing the activity and stability of NIK. In conclusion, NEK2 promoted the progression of GBM through activation of noncanonical NF-κB signaling, indicating that NEK2- NF-κB axis could be a potential drug target for GBM.Subject terms: CNS cancer, Nuclear receptors     

EGFR and c-Met Cross Talk in Glioblastoma and Its Regulation by Human Cord Blood Stem Cells

Receptor tyrosine kinases (RTK) and their ligands control critical biologic processes, such as cell proliferation, migration, and differentiation. Aberrant expression of these receptor kinases in tumor cells alters multiple downstream signaling cascades that ultimately drive the malignant phenotype by enhancing tumor cell proliferation, invasion, metastasis, and angiogenesis. As observed in human glioblastoma (hGBM) and other cancers, this dysregulation of RTK networks correlates with poor patient survival. Epidermal growth factor receptor (EGFR) and c-Met, two well-known receptor kinases, are coexpressed in multiple cancers including hGBM, corroborating that their downstream signaling pathways enhance a malignant phenotype. The integration of c-Met and EGFR signaling in cancer cells indicates that treatment regimens designed to target both receptor pathways simultaneously could prove effective, though resistance to tyrosine kinase inhibitors continues to be a substantial obstacle. In the present study, we analyzed the antitumor efficacy of EGFR inhibitors erlotinib and gefitinib and c-Met inhibitor PHA-665752, along with their respective small hairpin RNAs (shRNAs) alone or in combination with human umbilical cord blood stem cells (hUCBSCs), in glioma cell lines and in animal xenograft models. We also measured the effect of dual inhibition of EGFR/c-Met pathways on invasion and wound healing. Combination treatments of hUCBSC with tyrosine kinase inhibitors significantly inhibited invasion and wound healing in U251 and 5310 cell lines, thereby indicating the role of hUCBSC in inhibition of RTK-driven cell behavior. Further, the EGFR and c-Met localization in glioma cells and hGBM clinical specimens indicated that a possible cross talk exists between EGFR and c-Met signaling pathway.     

MALT1 is a potential therapeutic target in glioblastoma and plays a crucial role in EGFR‐induced NF‐κB activation

Glioblastoma multiforme (GBM) is the most common malignant tumour in the adult brain and hard to treat. Nuclear factor κB (NF‐κB) signalling has a crucial role in the tumorigenesis of GBM. EGFR signalling is an important driver of NF‐κB activation in GBM; however, the correlation between EGFR and the NF‐κB pathway remains unclear. In this study, we investigated the role of mucosa‐associated lymphoma antigen 1 (MALT1) in glioma progression and evaluated the anti‐tumour activity and effectiveness of MI‐2, a MALT1 inhibitor in a pre‐clinical GBM model. We identified a paracaspase MALT1 that is involved in EGFR‐induced NF‐kB activation in GBM. MALT1 deficiency or inhibition significantly affected the proliferation, survival, migration and invasion of GBM cells both in vitro and in vivo. Moreover, MALT1 inhibition caused G1 cell cycle arrest by regulating multiple cell cycle–associated proteins. Mechanistically, MALTI inhibition blocks the degradation of IκBα and prevents the nuclear accumulation of the NF‐κB p65 subunit in GBM cells. This study found that MALT1, a key signal transduction cascade, can mediate EGFR‐induced NF‐kB activation in GBM and may be potentially used as a novel therapeutic target for GBM.     

Microenvironmental Stiffness Enhances Glioma Cell Proliferation by Stimulating Epidermal Growth Factor Receptor Signaling

The aggressive and rapidly lethal brain tumor glioblastoma (GBM) is associated with profound tissue stiffening and genomic lesions in key members of the epidermal growth factor receptor (EGFR) pathway. Previous studies from our laboratory have shown that increasing microenvironmental stiffness in culture can strongly enhance glioma cell behaviors relevant to tumor progression, including proliferation, yet it has remained unclear whether stiffness and EGFR regulate proliferation through common or independent signaling mechanisms. Here we test the hypothesis that microenvironmental stiffness regulates cell cycle progression and proliferation in GBM tumor cells by altering EGFR-dependent signaling. We began by performing an unbiased reverse phase protein array screen, which revealed that stiffness modulates expression and phosphorylation of a broad range of signals relevant to proliferation, including members of the EGFR pathway. We subsequently found that culturing human GBM tumor cells on progressively stiffer culture substrates both dramatically increases proliferation and facilitates passage through the G1/S checkpoint of the cell cycle, consistent with an EGFR-dependent process. Western Blots showed that increasing microenvironmental stiffness enhances the expression and phosphorylation of EGFR and its downstream effector Akt. Pharmacological loss-of-function studies revealed that the stiffness-sensitivity of proliferation is strongly blunted by inhibition of EGFR, Akt, or PI3 kinase. Finally, we observed that stiffness strongly regulates EGFR clustering, with phosphorylated EGFR condensing into vinculin-positive focal adhesions on stiff substrates and dispersing as microenvironmental stiffness falls to physiological levels. Our findings collectively support a model in which tissue stiffening promotes GBM proliferation by spatially and biochemically amplifying EGFR signaling.     

Activation of EGFR promotes squamous carcinoma SCC10A cell migration and invasion via inducing EMT-like phenotype change and MMP-9-mediated degradation of E-cadherin

EGFR is a potent stimulator of invasion and metastasis in head and neck squamous cell carcinomas (HNSCC). However, the mechanism by which EGFR may stimulate tumor cell invasion and metastasis still need to be elucidated. In this study, we showed that activation of EGFR by EGF in HNSCC cell line SCC10A enhanced cell migration and invasion, and induced loss of epitheloid phenotype in parallel with downregulation of E-cadherin and upregulation of N-cadherin and vimentin, indicating that EGFR promoted SCC10A cell migration and invasion possibly by an epithelial to mesenchymal transition (EMT)-like phenotype change. Interestingly, activation of EGFR by EGF induced production of matrix metalloproteinase-9 (MMP-9) and soluble E-cadherin (sE-cad), and knockdown of MMP-9 by siRNA inhibited sE-cad production induced by EGF in SCC10A. Moreover, both MMP-9 knockdown and E-cadherin overexpression inhibited cell migration and invasion induced by EGF in SCC10A. The results indicate that EGFR activation promoted cell migration and invasion through inducing MMP-9-mediated degradation of E-cadherin into sE-cad. Pharmacologic inhibition of EGFR, MEK, and PI3K kinase activity in SCC10A reduced phosphorylated levels of ERK-1/2 and AKT, production of MMP-9 and sE-cad, cell migration and invasion, and expressional changes of EMT markers (E-cadherin and N-cadherin) induced by EGF, indicating that EGFR activation promotes cell migration and invasion via ERK-1/2 and PI3K-regulated MMP-9/E-cadherin signaling pathways. Taken together, the data suggest that EGFR activation promotes HNSCC SCC10A cell migration and invasion by inducing EMT-like phenotype change and MMP-9-mediated degradation of E-cadherin into sE-cad related to activation of ERK-1/2 and PI3K signaling pathways.     

Acridine yellow G blocks glioblastoma growth via dual inhibition of epidermal growth factor receptor and protein kinase C kinases

Amplification of the epidermal growth factor receptor (EGFR), frequently expressed as a constitutively active deletion mutant (EGFRvIII), occurs commonly in glioblastoma multiformes (GBM). However, blockade of EGFR is therapeutically disappointing for gliomas with PTEN deletion. To search for small molecules treating this aggressive cancer, we have established a cell-based screening and successfully identified acridine yellow G that preferentially blocks cell proliferation of the most malignant U87MG/EGFRvIII cells over the less malignant U87MG/PTEN cells. Oral administration of this compound markedly diminishes the brain tumor volumes in both subcutaneous and intracranial models. It directly inhibits EGFR and PKCs with IC(50) values of ~7.5 and 5 μM, respectively. It dually inhibits EGFR and PKCs, resulting in a blockade of mammalian target of rapamycin signaling and cell cycle arrest in the G(1) phase, which leads to activation of apoptosis in the tumors. Hence, combinatorial inhibition of EGFR and PKCs might provide proof of concept in developing therapeutic agents for treating malignant glioma and other human cancers.     

Maintenance of EGFR and EGFRvIII expressions in an in vivo and in vitro model of human glioblastoma multiforme

Glioblastoma multiforme (GBM) is the most common, and most aggressive primary brain tumor among adults. A vast majority of the tumors express high levels of the epidermal growth factor receptor (EGFR) as a consequence of gene amplification. Furthermore, gene amplification is often associated with mutation of EGFR, and the constitutive activated deletion variant EGFRvIII is the most common EGFR mutation found in GBM. Activated EGFR signaling, through overexpression and/or mutation, is involved in increased tumorigenic potential. As such, EGFR is an attractive target for GBM therapy. However, clinical studies with EGFR inhibitors have shown inconsistent results, and as such, further knowledge regarding the role of EGFR and EGFRvIII in GBM is needed. For this, an appropriate in vivo/in vitro tumor model is required. Here, we report the establishment of an experimental GBM model in which the expressions of EGFR and EGFRvIII are maintained both in xenograft tumors growing subcutaneously on mice and in cell cultures established in stem cell conditions. With this model it will be possible to further study the role of EGFR and EGFRvIII, and response to targeted therapy, in GBM.     

Induction of autophagy enhances apoptotic cell death via epidermal growth factor receptor inhibition by canertinib in cervical cancer cells

BackgroundIt has been known epidermal growth factor receptor (EGFR) frequently overexpressed in cervical cancer. High levels of EGFR expression in their tumors leads to a poor prognosis and inhibition frequently induces autophagy in cancer cells. This study aimed to investigate whether EGFR inhibition by canertinib induces autophagy and this induction influence the effect of Palladium (Pd) (II) complex and 5-fluorouracil (5-FU) especially in nontoxic doses.MethodsCytotoxicity was evaluated by using SRB assay. Apoptosis, autophagy, and EGFR key markers were determined by flow cytometry, fluorescence staining, and immunoblotting. Colony formation, invasion, and wound healing assays were performed to investigate cell proliferation, invasion, and migration, respectively.ResultsBlocking EGFR by the pan-ErbB tyrosine kinase inhibitor canertinib inhibited cell growth of HeLa cervical cancer cells in combination with Pd(II) complex and 5-FU. Combination of canertinib and Pd(II) complex promotes autophagy and apoptosis of HeLa cancer cells via blockade of the PI3K/AKT and MAPK/ERK pathway, which leads to cervical cancer cell death. ROS accumulation and DNA damage were increased after combinatorial treatment which causes depolarization of the mitochondrial inner membrane and leads to apoptotic cell death. Canertinib combined with Pd(II) complex leads to inhibition of migration and invasion.ConclusionInhibition of EGFR signaling by canertinib in combination with Pd(II) complex promotes apoptosis and autophagy via blockade of the PI3K/AKT and MAPK/ERK.General significanceThe cytotoxic activity of Pd(II) complex and 5-FU on HeLa cells is mediated by EGFR inhibition and autophagy induction, leading to activation of mitochondrial apoptotic cell death.     

RhoJ facilitates angiogenesis in glioblastoma via JNK/VEGFR2 mediated activation of PAK and ERK signaling pathways

Glioblastoma (GBM) is a highly vascularized malignant tumor that depends on new blood vessel formation. Small molecules targeting the angiogenic process may be an effective anti-GBM therapeutic strategy. We previously demonstrated that RhoJ promoted the progression and invasion of GBM. RhoJ has also been shown to be expressed in endothelial cells and plays an important role in regulating endothelial cell migration and tumor angiogenesis. Therefore, we aimed to evaluate the role and mechanism of actions of RhoJ in GBM angiogenesis. We analyzed the expression of RhoJ in different grade gliomas and investigated its role in GBM angiogenesis in vivo and in vitro. Furtherly, RNA sequencing (RNA-seq), Western blotting and immunofluorescence were performed to identify the molecular mechanism of RhoJ in regulating endothelial cell behavior and GBM angiogenesis. Here, we found that silencing RhoJ resulted in inhibition of HUVEC cell migration and blood vessel formation. Overexpression of RhoJ promoted the expression of CD31, EpCAM and moesin, suggesting RhoJ facilitated angiogenesis and the malignant progression of GBM. RNA-seq data showed that VEGF/TNF signaling pathway positively regulated RhoJ. The expression levels of RhoJ was upregulated with the stimulation of VEGF, and reduced by the treatment of JNK inhibitor SP600125. It was also found that the activity of PAK-BRAF-ERK was down-regulated upon RhoJ and JNK knockdown. In conclusion, these results suggested that RhoJ plays an essential role in regulating GBM angiogenesis through the JNK/VEGFR2-PAK-ERK signaling pathway and there might exist a VEGF-JNK/ERK-VEGF circuitry. Thus, RhoJ may be a candidate therapeutic target for anti-angiogenesis treatment in GBM.     

Mannose Phosphate Isomerase Regulates Fibroblast Growth Factor Receptor Family Signaling and Glioma Radiosensitivity

Asparagine-linked glycosylation is an endoplasmic reticulum co- and post- translational modification that enables the transit and function of receptor tyrosine kinase (RTK) glycoproteins. To gain insight into the regulatory role of glycosylation enzymes on RTK function, we investigated shRNA and siRNA knockdown of mannose phosphate isomerase (MPI), an enzyme required for mature glycan precursor biosynthesis. Loss of MPI activity reduced phosphorylation of FGFR family receptors in U-251 and SKMG-3 malignant glioma cell lines and also resulted in significant decreases in FRS2, Akt, and MAPK signaling. However, MPI knockdown did not affect ligand-induced activation or signaling of EGFR or MET RTKs, suggesting that FGFRs are more susceptible to MPI inhibition. The reductions in FGFR signaling were not caused by loss of FGF ligands or receptors, but instead were caused by interference with receptor dimerization. Investigations into the cellular consequences of MPI knockdown showed that cellular programs driven by FGFR signaling, and integral to the clinical progression of malignant glioma, were impaired. In addition to a blockade of cellular migration, MPI knockdown also significantly reduced glioma cell clonogenic survival following ionizing radiation. Therefore our results suggest that targeted inhibition of enzymes required for cell surface receptor glycosylation can be manipulated to produce discrete and limited consequences for critical client glycoproteins expressed by tumor cells. Furthermore, this work identifies MPI as a potential enzymatic target for disrupting cell surface receptor-dependent survival signaling and as a novel approach for therapeutic radiosensitization.     

Ganoderic acid A holds promising cytotoxicity on human glioblastoma mediated by incurring apoptosis and autophagy and inactivating PI3K/AKT signaling pathway

Ganoderic acid A (GA‐A), recognized as a lanostanetriterpene isolated from Ganoderma lucidum, demonstrates an efficient antitumor activity in multiple cancers. To date, it is unclear whether and how GA‐A functions on human glioblastoma (GBM). To unravel the functional significance of GA‐A on human glioblastoma (GBM), the cell‐counting kit‐8 and transwell assays were used to detect proliferation, migration, and invasion of human GBM cell after GA‐A treatment. Then, we utilized the flow cytometry and western blot to further evaluate the effect of GA‐A on GBM cell. Further, activities of autophagy and PI3K/AKT signaling were assessed by Western blot assay. We found that GA‐A significantly inhibited proliferation, migration, and invasion of GBM cell. Additionally, GA‐A markedly triggered cell apoptosis, which incarnated an elevation trend in apoptotic percentage, simultaneously, an increased level of proapoptosis protein (Bax and active caspase‐3) and a decreased level of antiapoptosis protein (Bcl‐2), induced by GA‐A treatment. Meanwhile, levels of two well‐known autophagy markers (beclin 1 and LC3 II) increased while another autophagic substrate (P‐62) was reduced. Moreover, the expressions levels of phosphorylated AKT, mTOR, p‐P70S6K, and cyclin D1 in the PI3K/AKT pathway were significantly reduced, which revealed GA‐A repressed the activation of PI3K/AKT signaling pathway. Collectively, these results indicate that GA‐A may encourage U251 cell growth and invasion/migration inhibition, apoptosis, and autophagy through the inactivation of PI3K/AKT signaling pathway in human GBM. Hence, GA‐A may be a potent antitumorigenic agent for human GBM in future clinical practice.     

GBP5 drives malignancy of glioblastoma via the Src/ERK1/2/MMP3 pathway

Guanylate binding proteins (GBPs), a family of interferon-inducible large GTPase, play a pivotal role in cell-autonomous immunity and tumor malignant transformation. Glioblastoma (GBM) is the most prevalent and lethal primary brain tumor in adults. Here we show that GBP5 was highly expressed in GBM cell lines and in clinical samples, especially in the mesenchymal subtype. The expression levels of GBP5 were negatively correlated with the prognosis of GBM patients. Overexpression of GBP5 promoted the proliferation, migration, and invasion of GBM cells in vitro and in vivo. In contrast, silencing GBP5 by RNA interference exhibited the opposite effects. Consequently, targeting GBP5 in GBM cells resulted in impaired tumor growth and prolonged survival time of mice with GBM tumors. We further identified that the Src/ERK1/2/MMP3 axis was essential for GBP5-promoted GBM aggressiveness. These findings suggest that GBP5 may represent a novel target for GBM intervention.Subject terms: CNS cancer, Oncogenes     

Decrease of circARID1A retards glioblastoma invasion by modulating miR-370-3p/ TGFBR2 pathway

Increasing evidence suggests that circular RNAs (circRNAs) are involved in regulating tumor biological activity. Glioblastoma (GBM) is one of the most lethal diseases characterized by highly aggressive proliferative and invasive behaviors. We aimed to explore how circRNAs influenced GBM biological activity. By circRNA array analysis we found that circARID1A was significantly up-regulated in GBM. Next, we found that circARID1A was up-regulated in GBM tissues and cell lines. Interfering with circARID1A inhibited the migration and invasion of a human GBM cell line U87. By performing dual-luciferase reporter assays, RNA pull-down and fluorescent in situ hybridization (FISH), we determined that circARID1A directly bound to miR-370-3p. Moreover, we confirmed that transforming growth factor beta receptor 2 (TGFBR2) was the target gene of miR-370-3p by performing RNA pull-down, dual-luciferase reporter assays and western blotting. Further experiments verified that circARID1A promoted GBM cell migration and invasion by modulating miR-370-3p/ TGFBR2 pathway. In addition, we demonstrated that silencing circARID1A restrain the growth of GBM in vivo. Finally, we showed that circARID1A was abundant in GBM cell derived exosomes. In conclusion, circARID1A participated in regulating migration and invasion of GBM via modulation of miR-370-3p/ TGFBR2 and thus may be a potential serum biomarker of GBM.     

EGFR inhibition in glioma cells modulates Rho signaling to inhibit cell motility and invasion and cooperates with temozolomide to reduce cell growth

Enforced EGFR activation upon gene amplification and/or mutation is a common hallmark of malignant glioma. Small molecule EGFR tyrosine kinase inhibitors, such as erlotinib (Tarceva), have shown some activity in a subset of glioma patients in recent trials, although the reported data on the cellular basis of glioma cell responsiveness to these compounds have been contradictory. Here we have used a panel of human glioma cell lines, including cells with amplified or mutant EGFR, to further characterize the cellular effects of EGFR inhibition with erlotinib. Dose-response and cellular growth assays indicate that erlotinib reduces cell proliferation in all tested cell lines without inducing cytotoxic effects. Flow cytometric analyses confirm that EGFR inhibition does not induce apoptosis in glioma cells, leading to cell cycle arrest in G(1). Interestingly, erlotinib also prevents spontaneous multicellular tumour spheroid growth in U87MG cells and cooperates with sub-optimal doses of temozolomide (TMZ) to reduce multicellular tumour spheroid growth. This cooperation appears to be schedule-dependent, since pre-treatment with erlotinib protects against TMZ-induced cytotoxicity whereas concomitant treatment results in a cooperative effect. Cell cycle arrest in erlotinib-treated cells is associated with an inhibition of ERK and Akt signaling, resulting in cyclin D1 downregulation, an increase in p27(kip1) levels and pRB hypophosphorylation. Interestingly, EGFR inhibition also perturbs Rho GTPase signaling and cellular morphology, leading to Rho/ROCK-dependent formation of actin stress fibres and the inhibition of glioma cell motility and invasion.