SummaryPatients with glioblastoma die from local relapse despite surgery and high-dose radiotherapy. Resistance to radiotherapy is thought to be due to efficient DNA double-strand break (DSB) repair in stem-like cells able to survive DNA damage and repopulate the tumor. We used clinical samples and patient-derived glioblastoma stem cells (GSCs) to confirm that the DSB repair protein RAD51 is highly expressed in GSCs, which are reliant on RAD51-dependent DSB repair after radiation. RAD51 expression and RAD51 foci numbers fall when these cells move toward astrocytic differentiation. In GSCs, the small-molecule RAD51 inhibitors RI-1 and B02 prevent RAD51 focus formation, reduce DNA DSB repair, and cause significant radiosensitization. We further demonstrate that treatment with these agents combined with radiation promotes loss of stem cells defined by SOX2 expression. This indicates that RAD51-dependent repair represents an effective and specific target in GSCs.

For several well-documented reasons, it has been challenging to develop artificial small molecule inhibitors of protein/protein complexes. Such reagents are of particular interest for transcription factor complexes given links between their misregulation and disease. Here we report parallel approaches to identify regulators of a hypoxia signaling transcription factor complex, involving the ARNT subunit of the HIF (Hypoxia Inducible Factor) activator and the TACC3 (Transforming Acidic Coiled Coil Containing Protein 3) coactivator. In one route, we used in vitro NMR and biochemical screening to identify small molecules that selectively bind within the ARNT PAS (Per-ARNT-Sim) domain that recruits TACC3, identifying KG-548 as an ARNT/TACC3 disruptor. A parallel, cell-based screening approach previously implicated the small molecule KHS101 as an inhibitor of TACC3 signaling. Here, we show that KHS101 works indirectly on HIF complex formation by destabilizing both TACC3 and the HIF component HIF-1α. Overall, our data identify small molecule regulators for this important complex and highlight the utility of pursuing parallel strategies to develop protein/protein inhibitors.

Pharmacological inhibition of uncontrolled cell growth with small-molecule inhibitors is a potential strategy for treating glioblastoma multiforme (GBM), the most malignant primary brain cancer. We showed that the synthetic small-molecule KHS101 promoted tumor cell death in diverse GBM cell models, independent of their tumor subtype, and without affecting the viability of noncancerous brain cell lines. KHS101 exerted cytotoxic effects by disrupting the mitochondrial chaperone heat shock protein family D member 1 (HSPD1). In GBM cells, KHS101 promoted aggregation of proteins regulating mitochondrial integrity and energy metabolism. Mitochondrial bioenergetic capacity and glycolytic activity were selectively impaired in KHS101-treated GBM cells. In two intracranial patient-derived xenograft tumor models in mice, systemic administration of KHS101 reduced tumor growth and increased survival without discernible side effects. These findings suggest that targeting of HSPD1-dependent metabolic pathways might be an effective strategy for treating GBM.

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Historically, recombinant membrane protein production has been a major challenge meaning that many fewer membrane protein structures have been published than those of soluble proteins. However, there has been a recent, almost exponential increase in the number of membrane protein structures being deposited in the Protein Data Bank. This suggests that empirical methods are now available that can ensure the required protein supply for these difficult targets. This review focuses on methods that are available for protein production in yeast, which is an important source of recombinant eukaryotic membrane proteins. We provide an overview of approaches to optimize the expression plasmid, host cell and culture conditions, as well as the extraction and purification of functional protein for crystallization trials in preparation for structural studies.

Receptor-mediated activation of phosphatidylcholine phosphatidohydrolase or phospholipase D (PLD) was studied in Chinese hamster ovary (CHO) cells expressing the cholecystokinin-A (CCK-A) receptor. Cells were labelled with [3H]myristic acid for 24 h and PLD-catalysed [3H]phosphatidylethanol formation was measured in the presence of 1% (v/v) ethanol. Cholecystokinin-(26-33)-peptide amide (CCK8) increased PLD activity both time- and dose-dependently. Maximal activation of protein kinase C (PKC) with 1 microM PMA or sustained elevation of the cytosolic free Ca2+ concentration ([Ca2+]i) with 1 microM thapsigargin increased PLD activity to 50% and 70% of the maximal value obtained with CCK8 respectively. The stimulatory effects of CCK8, PMA and thapsigargin were abolished in cells in which PKC was downregulated or inhibited by chelerythrine. PMA/Ca2+-stimulated PLD activity was absent in a homogenate of PKC-downregulated cells but could be restored upon addition of purified rat brain PKC. CCK8-induced PLD activation was inhibited by 90% in the absence of external Ca2+, demonstrating that receptor-mediated activation of PKC in itself does not significantly add to PLD activation but requires a sustained increase in [Ca2+]i. Taken together, the results presented demonstrate that, in CHO-CCK-A cells, receptor-mediated PLD activation is completely dependent on PKC, but that the extent to which PLD becomes activated depends largely, if not entirely, on the magnitude and duration of the agonist-induced increase in [Ca2+]i.

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A cell population with stem cell like characteristics is thought to underlie treatment resistance and local recurrence in glioma. In this work we investigate the role of the DNA repair protein RAD51 in these cells and the impact of inhibiting homologous recombination repair on radiation resistance in vitro and in vivo. We used a model of inducible differentiation of patient derived glioma cells to investigate repair protein levels, repair foci formation and kinetics in clonogenic, stem-like populations and their differentiated counterparts. We examined co-expression of stem cell markers with RAD51 protein at whole population level using western blotting, immunocytochemistry and RT-PCR in cultured cells and immunohistochemistry in tumor material. Single cell expression was analysed using the Fluidigm C1 platform. We examined the effect of two specific inhibitors of RAD51 (B02, RI-1) on the same cell pairs in vitro and used the γH2AX assay to assess differences in repair kinetics. We used subcutaneous models of glioma to evaluate the effect of one of these agents (RI-1) on tumour growth delay with and without fractionated radiation doses in vivo. Primary glioma stem cells expressed high levels of RAD51 protein and exhibited high numbers of foci per nucleus following radiation exposure. Levels of both protein and repair foci fell after cells were transferred to differentiating conditions (serum+BMP4), as did expression of stem cell markers (NESTIN, SOX2). Single cell analysis showed a highly significant association between SOX2 and RAD51 expression (P< 0.0001). A similar association between RAD51 expression and stem cell marker expression was demonstrated in a series of glioblastoma specimens. When primary glioma cultures were treated with B02, RI-1 or siRNA targeting RAD51 in vitro, significant radiosensitisation was achieved with dose modifying factors ∼1.4 after single radiation doses. Delayed repair of DNA double strand breaks was also apparent, with a higher proportion of γH2AX foci unresolved in treated cells at 24 hours post irradiation. We also demonstrated significantly enhanced tumor growth delay in vivo when RI-1 was combined with fractionated radiation therapy (5Gy x 3) in a subcutaneous model. These data suggest that RAD51 dependent DNA repair by homologous recombination represents a specific target in the stem-like fraction of glioblastoma and inhibiting this pathway is a promising approach to radiosensitisation. Citation Format: Henry King, Helen Payne, Tim Brend, Anjana Patel, Alex Wright, Teklu Englu, Lucy Stead, Heiko Wurdak, Susan C. Short. Radioresistance in glioma stem cells driven by Rad51 dependent homologous recombination repair. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3303. doi:10.1158/1538-7445.AM2015-3303