Differential sensitivity of Glioma stem cells to Aurora kinase A inhibitors: implications for stem cell mitosis and centrosome dynamics

Glioma stem-cell-like cells are considered to be responsible for treatment resistance and tumour recurrence following chemo-radiation in glioblastoma patients, but specific targets by which to kill the cancer stem cell population remain elusive. A characteristic feature of stem cells is their ability to undergo both symmetric and asymmetric cell divisions. In this study we have analysed specific features of glioma stem cell mitosis. We found that glioma stem cells appear to be highly prone to undergo aberrant cell division and polyploidization. Moreover, we discovered a pronounced change in the dynamic of mitotic centrosome maturation in these cells. Accordingly, glioma stem cell survival appeared to be strongly dependent on Aurora A activity. Unlike differentiated cells, glioma stem cells responded to moderate Aurora A inhibition with spindle defects, polyploidization and a dramatic increase in cellular senescence, and were selectively sensitive to Aurora A and Plk1 inhibitor treatment. Our study proposes inhibition of centrosomal kinases as a novel strategy to selectively target glioma stem cells

Hypoxia-inducible factor 1 alpha is required for the tumourigenic and aggressive phenotype associated with Rab25 expression in ovarian cancer

The small GTPase Rab25 has been functionally linked to tumour progression and aggressiveness in ovarian cancer and promotes invasion in three-dimensional environments. This type of migration has been shown to require the expression of the hypoxia-inducible factor 1 alpha (HIF-1α). In this report we demonstrate that Rab25 regulates HIF-1α protein expression in an oxygen independent manner in a panel of cancer cell lines. Regulation of HIF-1α protein expression by Rab25 did not require transcriptional upregulation, but was dependent on de novo protein synthesis through the Erbb2/ERK1/2 and p70S6K/mTOR pathways. Rab25 expression induced HIF-1 transcriptional activity, increased cisplatin resistance, and conferred intraperitoneal growth to the A2780 cell line in immunocompromised mice. Targeting HIF1 activity by silencing HIF-1β re-sensitised cells to cisplatin in vitro and reduced tumour formation of A2780-Rab25 expressing cells in vivo in a mouse ovarian peritoneal carcinomatosis model. Similar effects on cisplatin resistance in vitro and intraperitoneal tumourigenesis in vivo were obtained after HIF1b knockdown in the ovarian cancer cell line SKOV3, which expresses endogenous Rab25 and HIF-1α at atmospheric oxygen concentrations. Our results suggest that Rab25 tumourigenic potential and chemoresistance relies on HIF1 activity in aggressive and metastatic ovarian cancer. Targeting HIF-1 activity may potentially be effective either alone or in combination with standard chemotherapy for aggressive metastatic ovarian cancer

A novel 3D human glioblastoma cell culture system for modeling drug and radiation responses

Background. Glioblastoma (GBM) is the most common primary brain tumor, with dismal prognosis. The failure of drug–radiation combinations with promising preclinical data to translate into effective clinical treatments may relate to the use of simplified 2-dimensional in vitro GBM cultures. Methods. We developed a customized 3D GBM culture system based on a polystyrene scaffold (Alvetex) that recapitulates key histological features of GBM and compared it with conventional 2D cultures with respect to their response to radiation and to molecular targeted agents for which clinical data are available. Results. In 3 patient-derived GBM lines, no difference in radiation sensitivity was observed between 2D and 3D cultures, as measured by clonogenic survival. Three different molecular targeted agents, for which robust clinical data are available were evaluated in 2D and 3D conditions: (i) temozolomide, which improves overall survival and is standard of care for GBM, exhibited statistically significant effects on clonogenic survival in both patient-derived cell lines when evaluated in the 3D model compared with only one cell line in 2D cells; (ii) bevacizumab, which has been shown to increase progression-free survival when added to standard chemoradiation in phase III clinical trials, exhibited marked radiosensitizing activity in our 3D model but had no effect on 2D cells; and (iii) erlotinib, which had no efficacy in clinical trials, displayed no activity in our 3D GBM model, but radiosensitized 2D cells. Conclusions. Our 3D model reliably predicted clinical efficacy, strongly supporting its clinical relevance and potential value in preclinical evaluation of drug–radiation combinations for GBM

Regulation of RNA polymerase III transcription by the oncoproteins ErbB2/Neu, c-Myc and Id2

For cells to proliferate, a certain size needs to be reached, which is mainly determined by the rate of translation. Thus, the constituents of the translational apparatus play an essential role in cell growth and proliferation. Because some of the products transcribed by pol III transcription are constituents of the translational apparatus, the rate of pol III transcription will affect cell growth rate. Mitogenic stimulation induces pol III activity [1], while differentiation represses pol III transcription [2]. The retinoblastoma protein (RB) has been implicated in repression of pol III transcription by inhibiting TFIIIB activity through binding of its hypophosphory-lated form [3,4]. The oncoproteins ErbB2/neu, c-Myc and Id2 have been shown to block RB function by inducing its phosphorylation (ErbB2 and c-Myc) [5,6], or by binding and sequestrating it (Id2) [7,8]. These oncoproteins have been shown to be involved in the upregulation of cell proliferation, growth and the repression of differentiation, processes in which pol III transcription displays similar regulation. For this reason, their involvement in the regulation of pol III output was investigated. The data presented here demonstrate that all three oncoproteins upregulate pol III-transcribed genes. Although these oncoproteins can inhibit RB function, only ErbB2/neu seems to induce pol III activity disrupting TFIIIB-RB complexes. In addition, consitutive activation of ErbB2/neu also produces several other changes that can affect pol III transcription, including protein modifications of the TFIIIB subunits TBP and Brf, as well as induction of TBP, the pol III subunit BN51 and the transcription factor c-Myc. Furthermore, it was demonstrated that by inhibiting expression of c-Myc, the stimulation of pol III activity by ErbB2/neu could be overriden. Together these results suggest that constitutive activation of ErbB2/neu increases pol III transcription through several mechanisms, though induction of c- Myc appears to play an essential role in this upregulation. On the other hand, the activation of pol III transcription by c-Myc and H2 is through an RB-independent mechanism, since their overexpression in the absence of the entire RB family, still produced pol III upregulation. Instead, they activate directly pol III transcription. Chromatin immunoprecipitation revealed that both endogenous c-Myc and Id2 are present at class III genes in cultured mammalian cells. c-Myc and Id2 induction of pol III activity appears to be through direct mechanisms by interaction of these proteins with TFIIIB. In the case of c-Myc, coimmunoprecipitations demonstrated that it interacts with TFIIIB at physiological ratios. Furthermore, both endogenous c-Myc as well as endogenous Id2 were found to cofractionate with samples containing TFIIIB activity prepared on several columns, such as MonoQ gradients. Prom these results it can be concluded that endogenous c-Myc binds stably and specifically to TFIIIB. Further analysis has to be performed for to confirm the interaction between Id2 with TFIIIB. In summary, this work has identified three oncogene products that can increase pol III transcriptional output. These findings have important implications for tumour development in a range of tissue types

Cucurbit [7] uril encapsulated cisplatin overcomes resistance to cisplatin induced by Rab25 overexpression in an intraperitoneal ovarian cancer model

Background Ovarian cancer is the most fatal of gynaecological malignancies, usually detected at a late stage with intraperitoneal dissemination. Appropriate preclinical models are needed that recapitulate both the histopathological and molecular features of human ovarian cancer for drug-efficacy analysis. Methods Longitudinal studies comparing cisplatin performance either alone or in a novel cisplatin-based delivery-system, cucurbit[7]uril-encapsulated cisplatin (cisplatin@CB[7]) were performed on subcutaneous (s.c.) and intraperitoneal (i.p.) xenografts using the human ovarian cancer cell line A2780 stably expressing the small GTPase Rab25, which allows A2780 intraperitoneal growth; and luciferase, to allow tumour load measurement by non-invasive bioluminescent imaging. Results Rab25 expression induced cisplatin resistance compared to the parental cell line as assessed by the MTT assay in vitro. These findings did not translate in vivo, where cisplatin resistance was determined by the microenvironment. Subcutaneous xenografts of either parental A2780 or cisplatin-resistant Rab25-expressing A2780 cells presented similar responses to cisplatin treatment. In contrast, increased cisplatin resistance was only detected in i.p. tumours. Treatment of the cisplatin-resistant i.p. model with the novel cisplatin@CB[7] delivery system resulted in a substantial reduction of i.p. tumour load and increased necrosis. Conclusions Poor clinical performance of novel chemotherapeutics might reflect inappropriate preclinical models. Here we present an ovarian i.p. model that recapitulates the histopathological and chemoresistant features of the clinical disease. In addition, we demonstrate that the novel cisplatin-delivery system, cisplatin@CB[7] may have utility in the treatment of drug-resistant ovarian human cancers

Cholesterol protects glioblastoma cells against premature mitosis

Glioblastoma is the most common malignant primary tumour with a dismal prognosis. So far, no inhibitors targeting frequently altered pathways in GBM have improved patient survival. Premature entry to mitosis by small molecules that promote cancer cells to bypass the cell-cycle checkpoints have shown potent anti-tumour activity in vitro in a variety of cancers including GBM. We have reproduced these cytotoxic effects in our patient-derived GBM cell lines with the small molecule ME-344 and the Wee1-specific inhibitor AZ1775 both by by clonogenic survival and cell viability assays (EC50 values ranging from 0.003–0.02 μM and 0.2–0.4 μM, respectively). ME-344 and AZ1775 triggered profound morphological and cell cycle effects including mitotic induction, arrest and mitotic catastrophe. Bioinformatic analysis of global mRNA expression of our GBM cell lines stratified by ME-344 sensitivity showed a correlation between high ABCA1 and low cholesterol pathway gene expression with high sensitivity, and vice-versa. Cholesterol is a main component of membranes and is critical for cell growth and mitosis progression. GBM cells rely on cholesterol for survival. Due to the unique metabolic environment of the brain where a nearly unlimited supply of cholesterol is provided by astrocytes, targeting cellular activities regulated by cholesterol might lose their anti-tumour activity. Here we report that cholesterol confers cytoprotection to AZ1775 and ME344 in all GBM cell lines tested. These results suggest that cholesterol can override premature mitotic anti-tumour activity, indicating that mitotic induction and cholesterol inhibition might be a better therapeutic strategy for GBM than either treatment alone

Inhibitors of poly(ADP-ribose) glycohydrolase (PARG) exhibit single agent therapeutic activity and sensitize glioblastoma cells to ionizing radiation

Objective The lack of an effective therapy for glioblastoma (GBM) largely results from the intrinsic resistance of GBM cells. The radiosensitizing activity of inhibitors of poly(ADP-ribose) polymerases (PARPs) highlights the important role of poly(ADP-ribose) (PAR) in the DNA damage response. In contrast to PARPs, inhibition of poly(ADP-ribose) glycohydrolase (PARG), the enzyme responsible for degrading PAR chains, has shown single agent therapeutic activity in non-glioma cancer cells. This work aims to validate the therapeutic potential of PARG inhibitors (PARGi) in GBM. Results Baseline PAR levels were found to vary between different primary and commercial GBM cells, with PARylation increasing upon exposure of cells to ionizing radiation (IR), as expected. Target engagement of a novel PARGi, PDD00017273, was confirmed by the accumulation of nuclear PAR in treated cells. Inhibitor specificity was demonstrated using an inactive control compound and by combining PARGi with the PARP inhibitor olaparib, which blocked the effect. Single agent treatment with PARGi reduced the clonogenic survival of GBM cells in a concentration-dependent manner. Importantly, PARGi also sensitized GBM cells to IR (sensitizer enhancement ratios, SER, ≥ 1.40) Conclusion In contrast to PARP inhibitors, novel PARGi exhibit single agent activity against a panel of GBM cell lines, and also show robust radiosensitizing activity. PARGi therefore have therapeutic potential in this cancer of unmet need

Dynamic telomerase gene suppression via network effects of GSK3 inhibition

<b>Background</b>: Telomerase controls telomere homeostasis and cell immortality and is a promising anti-cancer target, but few small molecule telomerase inhibitors have been developed. Reactivated transcription of the catalytic subunit hTERT in cancer cells controls telomerase expression. Better understanding of upstream pathways is critical for effective anti-telomerase therapeutics and may reveal new targets to inhibit hTERT expression. <b>Methodology/Principal Findings</b>: In a focused promoter screen, several GSK3 inhibitors suppressed hTERT reporter activity. GSK3 inhibition using 6-bromoindirubin-3′-oxime suppressed hTERT expression, telomerase activity and telomere length in several cancer cell lines and growth and hTERT expression in ovarian cancer xenografts. Microarray analysis, network modelling and oligonucleotide binding assays suggested that multiple transcription factors were affected. Extensive remodelling involving Sp1, STAT3, c-Myc, NFκB, and p53 occurred at the endogenous hTERT promoter. RNAi screening of the hTERT promoter revealed multiple kinase genes which affect the hTERT promoter, potentially acting through these factors. Prolonged inhibitor treatments caused dynamic expression both of hTERT and of c-Jun, p53, STAT3, AR and c-Myc. <b>Conclusions/Significance</b>: Our results indicate that GSK3 activates hTERT expression in cancer cells and contributes to telomere length homeostasis. GSK3 inhibition is a clinical strategy for several chronic diseases. These results imply that it may also be useful in cancer therapy. However, the complex network effects we show here have implications for either setting

Cucurbit[7]uril encapsulated cisplatin overcomes cisplatin resistance via a pharmacokinetic effect

The cucurbit[n]uril (CB[n]) family of macrocycles has been shown to have potential in drug delivery where they are able to provide physical and chemical stability to drugs, improve drug solubility, control drug release and mask the taste of drugs. Cisplatin is a small molecule platinum-based anticancer drug that has severe dose-limiting side-effects. Cisplatin forms a host–guest complex with cucurbit[7]uril (cisplatin@CB[7]) with the platinum atom and both chlorido ligands located inside the macrocycle, with binding stabilised by four hydrogen bonds (2.15–2.44 A ˚ ). Whilst CB[7] has no effect on the in vitro cytotoxicity of cisplatin in the human ovarian carcinoma cell line A2780 and its cisplatin-resistant sub-lines A2780/cp70 and MCP1, there is a significant effect on in vivo cytotoxicity using human tumour xenografts. Cisplatin@CB[7] is just as effective on A2780 tumours compared with free cisplatin, and in the cisplatin-resistant A2780/cp70 tumours cisplatin@CB[7] markedly slows tumour growth. The ability of cisplatin@CB[7] to overcome resistance in vivo appears to be a pharmacokinetic effect. Whilst the peak plasma level and tissue distribution are the same for cisplatin@CB[7] and free cisplatin, the total concentration of circulating cisplatin@CB[7] over a period of 24 hours is significantly higher than for free cisplatin when administered at the equivalent dose. The results provide the first example of overcoming drug resistance via a purely pharmacokinetic effect rather than drug design or better tumour targeting, and demonstrate that in vitro assays are no longer as important in screening advanced systems of drug delivery

Regulation of DNA double strand break repair by EGF and VEGF signalling reveals Akt to be a critical therapeutic target in glioblastoma

Glioblastoma (GBM) is currently incurable. Its radioresistance has been attributed to a subpopulation of cells termed ‘GBM stem-like cells’ characterised by multipotentiality and tumorigenicity. The discrepancy between pre-clinical and clinical effects of molecular targeted agents on radiosensitivity indicates that 2D in vitro models of GBM do not recapitulate the clinical scenario. In a 3D model developed in our laboratory, EGFR inhibitors failed to enhance radiosensitivity recapitulating their lack of efficacy in the clinic, contrasting with their radiosensitising activity in 2D cultures. Conversely, inhibition of VEGF signalling caused significant radiosensitisation of 3D cultures but had no effect in 2D conditions. The critical role of the DNA damage response in mediating these effects is illustrated by the consistent correlation between radiosensitivity, unrepaired double-strand breaks (γH2AX foci), mitotic catastrophe and micronuclei in both 2D and 3D models. Further investigation revealed unrepaired DSB to be associated with delayed resolution of phosphorylated DNA-PKcs nuclear foci and reduced formation of Rad51 foci. Hence in 2D conditions, EGFR signalling appeared to promote efficient non-homologous end-joining (NHEJ) repair, while in 3D conditions this process was dependent on VEGF signalling. Motivated by previous reports that radiation induced EGFR signalling promotes NHEJ via Akt mediated phosphorylation of DNA-PKcs, we investigated the role of Akt signalling in 2D and 3D systems. Specific inhibition of Akt using the small molecule inhibitor MK-2206 or Akti knockdown increased radiation sensitivity in both 2D and 3D models to a similar extent as EGFR or VEGF inhibition respectively. In keeping with this, phosphorylation of Akt was EGFR dependent in 2D GSC cultures but VEGF dependent in the 3D model. MK-2206 induced radiosensitivity was correlated with increased unrepaired DSBs and extended mouse survival in an U87MGLuc2 orthotopic model compared to radiation only. Our data identify Akt as a promising therapeutic target in combination with radiation for GBM