Targeting de novo lipid synthesis induces lipotoxicity and impairs DNA damage repair in glioblastoma mouse models

Deregulated de novo lipid synthesis (DNLS) is a potential druggable vulnerability in glioblastoma (GBM), a highly lethal and incurable cancer. Yet the molecular mechanisms that determine susceptibility to DNLS-targeted therapies remain unknown, and the lack of brain-penetrant inhibitors of DNLS has prevented their clinical evaluation as GBM therapeutics. Here, we report that YTX-7739, a clinical-stage inhibitor of stearoyl CoA desaturase (SCD), triggers lipotoxicity in patient-derived GBM stem-like cells (GSCs) and inhibits fatty acid desaturation in GSCs orthotopically implanted in mice. When administered as a single agent, or in combination with temozolomide (TMZ), YTX-7739 showed therapeutic efficacy in orthotopic GSC mouse models owing to its lipotoxicity and ability to impair DNA damage repair. Leveraging genetic, pharmacological, and physiological manipulation of key signaling nodes in gliomagenesis complemented with shotgun lipidomics, we show that aberrant MEK/ERK signaling and its repression of the energy sensor AMP-activated protein kinase (AMPK) primarily drive therapeutic vulnerability to SCD and other DNLS inhibitors. Conversely, AMPK activation mitigates lipotoxicity and renders GSCs resistant to the loss of DNLS, both in culture and in vivo, by decreasing the saturation state of phospholipids and diverting toxic lipids into lipid droplets. Together, our findings reveal mechanisms of metabolic plasticity in GSCs and provide a framework for the rational integration of DNLS-targeted GBM therapies.

TMET-25. MEK/ERK AND AMPK SIGNALING DICTATE VULNERABILITY TO FATTY ACID DESATURATION INHIBITION IN GLIOBLASTOMA

Lipid metabolism is altered in many cancers. Glioma stem cells (GSCs), a subpopulation that contributes to molecular heterogeneity and therapeutic resistance of glioblastoma (GBM), are highly dependent on de novo lipid synthesis (DNLS) for energy production and proliferation. Although DNLS is a promising therapeutic target in GBM and other tumors, the molecular mechanisms that determine the dependence on DNLS remain largely unknown. In this study, we aimed to define the genetic drivers of vulnerability to DNLS inhibitors in GSCs and to explore the molecular mechanisms dictating resistance to a brain-penetrant inhibitor of stearoyl-CoA desaturase (SCD), a key enzyme involved in DNLS. Using an astrocyte model of gliomagenesis, we discovered that activation of RAS/MEK/ERK promotes vulnerability to DNLS inhibitors, whereas defective MEK/ERK activity drives resistance to SCD inhibition. Also, we identified the energy sensor AMPK as a downstream target of MEK/ERK and discovered an adaptive metabolic reprograming of GSCs dictated by AMPK activation. Shotgun lipidomics revealed that constitutive AMPK activation protects from SCD inhibition-induced lipotoxicity and endoplasmic reticulum stress and preserves the ability of transformed cells to form brain tumors in mice. Mechanistically, we found that AMPKs ability to circumvent lipotoxicity was achieved by altering the lipid composition and saturation state of cell membranes, and channeling potentially lipotoxic fatty acids into lipid droplets (i.e., lipid storage). These results strongly suggest that the efficacy of DNLS-targeted therapy can be dictated by the MEK/ERK and AMPK signaling status of GSCs. These data also suggest that MEK/ERK and AMPK signaling could be used as predictive biomarkers to understand which GBM patients are likely to respond to DNLS-targeted therapy. In conclusion, our data uncover an unexpected mechanism by which GSCs avoid a metabolic vulnerability and provides a rationale for integrating DNLS-targeted therapies for GBM treatment.

TMET-02. THERAPEUTIC EFFICACY OF A NOVEL BRAIN PENETRANT SCD INHIBITOR ON BREAST CANCER BRAIN METASTASES

Brain metastases are a significant therapeutic challenge and are associated with high morbidity and mortality. Recent evidence suggests that brain metastases have increased fatty acid synthesis compared to extracranial tumors. The enzyme stearoyl-CoA desaturase (SCD), which converts saturated long-chain fatty acids into monounsaturated fatty acids, is often upregulated in breast cancer brain metastasis (BCBM) and is considered a promising therapeutic target. However, the lack of brain penetrant SCD inhibitors limited their use in patients. This study aimed to investigate the therapeutic efficacy of a clinical-stage brain penetrant inhibitor of SCD to treat BCBM. We found that pharmacologic inhibition of SCD in multiple breast cancer cell lines induced significant cytotoxicity, mainly by apoptosis. Gas chromatography/mass spectrometry confirmed that the fatty acid desaturation index, which measures SCD activity, was significantly lower after treatment. SCD inhibition triggered endoplasmic reticulum (ER) stress and increased DNA damage due to a higher production of reactive oxygen species. Further, SCD inhibition decreased DNA damage repair by inhibiting homology-directed repair in vitro and in a mouse model of breast cancer. Notably, inhibition of SCD led to a significant decrease in tumor growth in a subcutaneous mouse model of breast cancer and a significant increase in overall survival in a BCBM mouse model. Cytotoxicity was enhanced when this SCD inhibitor was combined with either a DNA damage-inducing agent (temozolomide or radiation) or with a PARP inhibitor (Niraparib). Finally, we observed that combination therapy of SCD inhibition and Niraparib significantly increased overall survival in a BCBM mouse model. Our results suggest that inhibition of SCD alone or in combination with a PARP inhibitor is a promising therapeutic strategy to treat BCBM.

CSIG-17. SCD5 PROTECTS GLIOBLASTOMA STEM CELLS FROM DEATH AND DIFFERENTIATION BY MODULATING INTRACELLULAR LIPID COMPOSITION

Glioblastoma (GBM) is the most common malignant brain cancer in adults, enriched in a small subpopulation of glioma stem cells (GSC), which can drive tumor recurrence and therapeutic resistance. Considerable evidence suggests that the endogenous levels of unsaturated fatty acids (FA) are crucial regulators of GSCs survival and self-renewal. Stearoyl-CoA desaturase-1 (SCD-1) is the most abundant desaturase in humans. We have previously shown that SCD1 activity is required for GSCs self-renewal and brain tumor initiation. However, SCD1 orthologous isoform, SCD5, has been poorly characterized and its potential role in GBM has not been previously reported. We have observed that SCD5 is highly enriched in GSC both at the mRNA and protein levels. Genetic downregulation of SCD5 in GSCs led to a remarkable decrease in stem cell markers, impaired cell viability and the ability to form neurospheres. Further, the downregulation of SCD5 in GSCs orthotopically implanted in mice resulted in delayed tumor growth and extended overall survival. Shotgun lipidomics in GSCs after either SCD1 or SCD5 knock-down revealed a largely distinctive lipidome profile, highlighting the divergent role of these two isoforms in GBM lipid metabolism. Surprisingly, lipidomics analysis showed that both SCD1 and SCD5 are required to synthesize a variety of lipid species involved in receptor tyrosine kinase (RTKs) and GPCRs signal transduction, directly linking FA synthesis with the oncogenic signaling. We confirmed these results by immunoblot analysis. Using specific tagging and immunofluorescence analysis, we observed that, despite a spatial overlap in SCD1 and SCD5 expression, SCD5 is uniquely present in some subcellular locations. This suggests that different functions of these isoforms could be related to different subcellular localization. Altogether, our results underscore a novel function of SCD isoforms in GSCs metabolism and highlight SCD5 as a potential therapeutic target for GBM.

Multiplexed bioluminescence-mediated tracking of DNA double-strand break repairs in vitro and in vivo

The dynamics of DNA double-strand break (DSB) repairs including homology-directed repair and nonhomologous end joining play an important role in diseases and therapies. However, investigating DSB repair is typically a low-throughput and cross-sectional process, requiring disruption of cells and organisms for subsequent nuclease-, sequencing- or reporter-based assays. In this protocol, we provide instructions for establishing a bioluminescent repair reporter system using engineered Gaussia and Vargula luciferases for noninvasive tracking of homology-directed repair and nonhomologous end joining, respectively, induced by SceI meganuclease, SpCas9 or SpCas9 D10A nickase-mediated editing. We also describe complementation with orthogonal DSB repair assays and omics analyses to validate the reporter readouts. The bioluminescent repair reporter system provides longitudinal and rapid readout (~seconds per sample) to accurately and efficiently measure the efficacy of genome-editing tools and small-molecule modulators on DSB repair. This protocol takes ~2-4 weeks to establish, and as little as 2 h to complete the assay. The entire bioluminescent repair reporter procedure can be performed by one person with standard molecular biology expertise and equipment. However, orthogonal DNA repair assays would require a specialized facility that performs Sanger sequencing or next-generation sequencing.

DDRE-11. TARGETING FATTY ACID BIOSYNTHESIS IN GLIOBLASTOMA

We recently provided evidence that endoplasmic reticulum (ER) stress promotes fatty acid (FA) biosynthesis in glioblastoma (GBM) cancer stem cells (GSCs). We determined that Stearoyl CoA Desaturase 1 (SCD), a key FA desaturase, is essential for regulating ER homeostasis in GSCs, and showed that these cells are highly susceptible to pharmacological perturbation of SCD activity. An impaired SCD activity leads to the toxic accumulation of saturated FA and activates cell death signaling mediated by the ER sensor Inositol-requiring enzyme 1 (IRE1). This in turn promotes an IRE1-mediated mRNA decay of key DNA damage repair genes and impairs the ability of GSCs to repair DNA damage caused by radiation or chemotherapy. Consequently, combining SCD inhibition with temozolomide (TMZ) leads to major cytotoxicity both in TMZ-sensitive, and TMZ-resistant patient-derived GBM cells. Pharmacological inhibition of SCD delivered through the nasal route in mice, had a remarkable therapeutic benefit in patient-derived orthotopic GSCs mouse models, yet the modest brain permeability of the currently available SCD inhibitors precludes their clinical translation. To overcome this challenge, we have recently acquired a first-in-class, clinically relevant SCD inhibitor. This compound has undergone extensive pharmacokinetic and pharmacodynamic studies which confirmed brain permeability, efficacy, and safety in small animals and non-human primates. We show that the combination of this SCD inhibitor with TMZ is effective both in cultured GSCs, and in preclinical GSCs orthotopic mouse models. Our results support the clinical investigation of this new class of SCD inhibitors, in combination with TMZ, in patients diagnosed with GBM.

Intranasal delivery of experimental compounds in orthotopic brain tumor mouse models

Effective therapeutics for malignant primary brain tumors, such as glioblastomas (GBMs), are urgently needed. To facilitate and expedite early-phase GBM therapeutic development, we describe a protocol that allows the intranasal delivery of experimental compounds in GBM orthotopic mouse models. Compounds delivered through this route can bypass the blood-brain barrier and thus help validate effective therapeutic targets for GBMs.

For complete details on the use and execution of this protocol, please refer to Pinkham et al. (2019).

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Using patient-derived tumor cells to study and image GBM tumor growth in mice

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Delivery of therapeutics via the intranasal route to bypass the blood-brain barrier

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This delivery method can expedite early-phase testing of GBM therapeutics

A multiplexed bioluminescent reporter for sensitive and non-invasive tracking of DNA double strand break repair dynamics in vitro and in vivo

Tracking DNA double strand break (DSB) repair is paramount for the understanding and therapeutic development of various diseases including cancers. Herein, we describe a multiplexed bioluminescent repair reporter (BLRR) for non-invasive monitoring of DSB repair pathways in living cells and animals. The BLRR approach employs secreted Gaussia and Vargula luciferases to simultaneously detect homology-directed repair (HDR) and non-homologous end joining (NHEJ), respectively. BLRR data are consistent with next-generation sequencing results for reporting HDR (R2 = 0.9722) and NHEJ (R2 = 0.919) events. Moreover, BLRR analysis allows longitudinal tracking of HDR and NHEJ activities in cells, and enables detection of DSB repairs in xenografted tumours in vivo. Using the BLRR system, we observed a significant difference in the efficiency of CRISPR/Cas9-mediated editing with guide RNAs only 1-10 bp apart. Moreover, BLRR analysis detected altered dynamics for DSB repair induced by small-molecule modulators. Finally, we discovered HDR-suppressing functions of anticancer cardiac glycosides in human glioblastomas and glioma cancer stem-like cells via inhibition of DNA repair protein RAD51 homolog 1 (RAD51). The BLRR method provides a highly sensitive platform to simultaneously and longitudinally track HDR and NHEJ dynamics that is sufficiently versatile for elucidating the physiology and therapeutic development of DSB repair.

Obtusaquinone: A Cysteine-Modifying Compound That Targets Keap1 for Degradation

We have previously identified the natural product obtusaquinone (OBT) as a potent antineoplastic agent with promising in vivo activity in glioblastoma and breast cancer through the activation of oxidative stress; however, the molecular properties of this compound remained elusive. We used a multidisciplinary approach comprising medicinal chemistry, quantitative mass spectrometry-based proteomics, functional studies in cancer cells, and pharmacokinetic analysis, as well as mouse xenograft models to develop and validate novel OBT analogs and characterize the molecular mechanism of action of OBT. We show here that OBT binds to cysteine residues with a particular affinity to cysteine-rich Keap1, a member of the CUL3 ubiquitin ligase complex. This binding promotes an overall stress response and results in ubiquitination and proteasomal degradation of Keap1 and downstream activation of the Nrf2 pathway. Using positron emission tomography (PET) imaging with the PET-tracer 2-[¹⁸F]fluoro-2-deoxy-d-glucose (FDG), we confirm that OBT is able to penetrate the brain and functionally target brain tumors. Finally, we show that an OBT analog with improved pharmacological properties, including enhanced potency, stability, and solubility, retains the antineoplastic properties in a xenograft mouse model.

Stearoyl CoA Desaturase Is Essential for Regulation of Endoplasmic Reticulum Homeostasis and Tumor Growth in Glioblastoma Cancer Stem Cells

Inherent plasticity and various survival cues allow glioblastoma stem-like cells (GSCs) to survive and proliferate under intrinsic and extrinsic stress conditions. Here, we report that GSCs depend on the adaptive activation of ER stress and subsequent activation of lipogenesis and particularly stearoyl CoA desaturase (SCD1), which promotes ER homeostasis, cytoprotection, and tumor initiation. Pharmacological targeting of SCD1 is particularly toxic due to the accumulation of saturated fatty acids, which exacerbates ER stress, triggers apoptosis, impairs RAD51-mediated DNA repair, and achieves a remarkable therapeutic outcome with 25%-100% cure rate in xenograft mouse models. Mechanistically, divergent cell fates under varying levels of ER stress are primarily controlled by the ER sensor IRE1, which either promotes SCD1 transcriptional activation or converts to apoptotic signaling when SCD1 activity is impaired. Taken together, the dependence of GSCs on fatty acid desaturation presents an exploitable vulnerability to target glioblastoma.

Dissecting inherent intratumor heterogeneity in patient-derived glioblastoma culture models

Background

Molecular profile of glioblastoma multiforme (GBM) revealed 4 subtypes, 2 of which, proneural and mesenchymal, have been predominantly observed, with the latter displaying a more aggressive phenotype and increased therapeutic resistance. Single-cell RNA sequencing revealed that multiple subtypes actually reside within the same tumor, suggesting cellular heterogeneity in GBM. Further, plasticity between these 2 subtypes is observed during tumor recurrence and in response to radiation therapy.

Methods

Patient-derived GBM stemlike cells were cultured as neurospheres. These cells were differentiated in serum by attaching to the culture dishes. The "floating" cells that were not attached/differentiated were harvested from the conditioned medium. The characteristics of these cells were studied with limiting dilution assays and immunofluorescence staining. Cell growth and nuclear factor-kappaB (NFkB) activation were monitored using bioluminescent assays as well as quantitative polymerase chain reaction and western blotting. In vivo tumorigenesis was evaluated in orthotopic xenograft models using bioluminescence imaging.

Results

Patient-derived GBM stemlike cells undergo differentiation by attaching to the culture dish in serum-containing medium. We observed that a small subset of these cells escape this adhesion/differentiation and grow as floating cells. These cells displayed enhanced cancer stem cell properties with a molecular and phenotypic mesenchymal signature, including resistance to radiation and targeted therapies, a more aggressive tumor formation, and NFkB activation.

Conclusion

Our results endorse inherent intratumor molecular subtype heterogeneity in glioblastoma and provide a valuable approach to study phenotypic plasticity, which could be applied to find novel therapeutic strategies to eradicate this aggressive tumor and can be extended to other cancer types.

Intracranial AAV-sTRAIL combined with lanatoside C prolongs survival in an orthotopic xenograft mouse model of invasive glioblastoma

Glioblastoma (GBM) is the most common malignant brain tumor in adults. We designed an adeno-associated virus (AAV) vector for intracranial delivery of secreted, soluble tumor necrosis factor-related apoptosis-inducing ligand (sTRAIL) to GBM tumors in mice and combined it with the TRAIL-sensitizing cardiac glycoside, lanatoside C (lan C). We applied this combined therapy to two different GBM models using human U87 glioma cells and primary patient-derived GBM neural spheres in culture and in orthotopic GBM xenograft models in mice. In U87 cells, conditioned medium from AAV2-sTRAIL expressing cells combined with lan C induced 80% cell death. Similarly, lan C sensitized primary GBM spheres to sTRAIL causing over 90% cell death. In mice bearing intracranial U87 tumors treated with AAVrh.8-sTRAIL, administration of lan C caused a decrease in tumor-associated Fluc signal, while tumor size increased within days of stopping the treatment. Another round of lan C treatment re-sensitized GBM tumor to sTRAIL-induced cell death. AAVrh.8-sTRAIL treatment alone and combined with lanatoside C resulted in a significant decrease in tumor growth and longer survival of mice bearing orthotopic invasive GBM brain tumors. In summary, AAV-sTRAIL combined with lanatoside C induced cell death in U87 glioma cells and patient-derived GBM neural spheres in culture and in vivo leading to an increased in overall mice survival.

Systemic anticancer neural stem cells in combination with a cardiac glycoside for glioblastoma therapy

The tumor-tropic properties of neural stem cells (NSCs) have been shown to serve as a novel strategy to deliver therapeutic genes to tumors. Recently, we have reported that the cardiac glycoside lanatoside C (Lan C) sensitizes glioma cells to the anticancer agent tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Here, we engineered an FDA-approved human NSC line to synthesize and secrete TRAIL and the Gaussia luciferase (Gluc) blood reporter. We showed that upon systemic injection, these cells selectively migrate toward tumors in the mice brain across the blood-brain barrier, target invasive glioma stem-like cells, and induce tumor regression when combined with Lan C. Gluc blood assay revealed that 30% of NSCs survived 1 day postsystemic injection and around 0.5% of these cells remained viable after 5 weeks in glioma-bearing mice. This study demonstrates the potential of systemic injection of NSCs to deliver anticancer agents, such as TRAIL, which yields glioma regression when combined with Lan C.

Cell-based bioluminescence screening assays

Drug screening is an essential and widely used technique for drug discovery in various biomedical fields notably in oncology. Here we describe a functional screening assay based on the bioluminescence detection of a secreted luciferase for monitoring cell viability of cancer cells in a high-throughput format. This assay allows the screening of large libraries comprising thousands of compounds and the identification of potential anticancer molecules in a rapid, facile, and cost-effective manner.

Bioluminescence imaging: basics and practical limitations

Over the last three decades, imaging has been a thriving field with continuous egression of more reliable and highly sophisticated tools and techniques allowing better understanding of biological processes in living organisms. This field continues to expand and its applications broaden to encompass limitless applications in various biomedical research areas. It is however, of utmost importance to understand the capabilities and limitations of this technique as new challenges and hurdles continue to arise. This chapter describes the general properties of bioluminescence imaging and commonly used reporters while underlining the challenges and limitations with these modalities.

Abstract A254: Systemic injection of human neural stem cells expressing anti-cancer agent targets invasive gliomas and induces tumor regression in combination with a cardiac glycoside

Glioblastoma multiforme (GBM) is the most common and most aggressive malignant form of brain tumors in human. The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is regarded as an anticancer agent, however, a considerable number of tumor types, including GBM, are resistant to TRAIL. Also, TRAIL therapeutic use in vivo is limited by a short half-life in plasma due to a rapid clearance by the kidney and a poor penetration across the blood-brain barrier. The tumor-tropic properties of neural stem cells (NSCs) could serve as a novel strategy to deliver therapeutic genes to tumors in the brain. Furthermore, we have previously reported that the cardiac glycoside lanatoside C sensitizes GBM cells to TRAIL-induced apoptosis. In this study, we engineered human neural stem cell to synthesize and secrete an active form of TRAIL (NSC-sTRAIL) as well as the secreted Gaussia luciferase (Gluc) as a blood marker for cell proliferation. We showed that these cells selectively migrate toward GBM stem-like cells (GSCs) in a transwell assay. However, GSCs’ growth was only decreased by 11.6%. When this co-culture was treated with lanatoside C, GSCs’ growth was inhibited by 81.5% in a caspase-mediated manner. We then tested this combined therapy in an invasive orthotopic model using primary GSC neurospheres expressing firefly luciferase. Upon systemic injection, Gluc blood assay showed that at the next day at least 30% of NSC-sTRAIL, and after 5 weeks up to 0.5% of these cells survived. Histological analysis showed that the NSC-sTRAIL cells crossed the blood-brain barrier and migrated toward invasive GSC tumors. Bioluminescence imaging showed that tumors in mice treated with NSC-sTRAIL cells in combination with lanatoside C resulted in significant tumor regression and mouse survival elongation as compared to single therapy. Taken together, this study demonstrated that systemic injection of neural stem cells targets invasive brain tumors and could be used to deliver an anticancer agent such as sTRAIL which yields significant tumor regression when used in combination with lanatoside C.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):A254.

Citation Format: Jian Teng, Seyedali Hejazi, Christian E. Badr, Bakhos A. Tannous. Systemic injection of human neural stem cells expressing anti-cancer agent targets invasive gliomas and induces tumor regression in combination with a cardiac glycoside. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr A254.

Targeting cancer cells with the natural compound obtusaquinone

BACKGROUND

Tumor cells present high levels of oxidative stress. Cancer therapeutics exploiting such biochemical changes by increasing reactive oxygen species (ROS) production or decreasing intracellular ROS scavengers could provide a powerful treatment strategy.

METHODS

To test the effect of our compound, obtusaquinone (OBT), we used several cell viability assays on seven different glioblastoma (GBM) cell lines and primary cells and on 12 different cell lines representing various cancer types in culture as well as on subcutaneous (n = 7 mice per group) and two intracranial GBM (n = 6-8 mice per group) and breast cancer (n = 6 mice per group) tumor models in vivo. Immunoblotting, immunostaining, flow cytometry, and biochemical assays were used to investigate the OBT mechanism of action. Histopathological analysis (n = 2 mice per group) and blood chemistry (n = 2 mice per group) were used to test for any compound-related toxicity. Statistical tests were two-sided.

RESULTS

OBT induced rapid increase in intracellular ROS levels, downregulation of cellular glutathione levels and increase in its oxidized form, and activation of cellular stress pathways and DNA damage, subsequently leading to apoptosis. Oxidative stress is believed to be the main mechanism through which this compounds targets cancer cells. OBT was well tolerated in mice, slowed tumor growth, and statistically prolonged survival in GBM tumor models. The ratio of median survival in U251 intracranial model in OBT vs control was 1.367 (95% confidence interval [CI] of ratio = 1.031 to 1.367, P = .008). Tumor growth inhibition was also observed in a mouse breast cancer model (average tumor volume per mouse, OBT vs control: 36.3 vs 200.4mm(3), difference = 164.1mm(3), 95% CI =72.6 to 255.6mm(3), P = .005).

CONCLUSIONS

Given its properties and efficacy in cancer killing, our results suggest that OBT is a promising cancer therapeutic.

Bioluminescence imaging: progress and applications

Application of bioluminescence imaging has increased tremendously in the past decade and has significantly contributed to core conceptual advances in biomedical research. This technology provides valuable means for monitoring of different biological processes in immunology, oncology, virology and neuroscience. In this review, we discuss current trends in bioluminescence and its application in different fields with an emphasis on cancer research.

Lanatoside C sensitizes glioblastoma cells to tumor necrosis factor-related apoptosis-inducing ligand and induces an alternative cell death pathway

Human glioblastoma (GBM) cells are notorious for their resistance to apoptosis-inducing therapeutics. We have identified lanatoside C as a sensitizer of GBM cells to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced cell death partly by upregulation of the death receptor 5. We show that lanatoside C sensitizes GBM cells to TRAIL-induced apoptosis in a GBM xenograft model in vivo. Lanatoside C on its own serves as a therapeutic agent against GBM by activating a caspase-independent cell death pathway. Cells treated with lanatoside C showed necrotic cell morphology with absence of caspase activation, low mitochondrial membrane potential, and early intracellular ATP depletion. In conclusion, lanatoside C sensitizes GBM cells to TRAIL-induced cell death and mitigates apoptosis resistance of glioblastoma cells by inducing an alternative cell death pathway. To our knowledge, this is one of the first examples of use of caspase-independent cell death inducers to trigger tumor regression in vivo. Activation of such mechanism may be a useful strategy to counter resistance of cancer cells to apoptosis.

Functional drug screening assay reveals potential glioma therapeutics

Here we describe a novel functional screening assay based on bioluminescence monitoring of the naturally secreted Gaussia luciferase (Gluc) in the conditioned medium of cultured cells. Using this assay, we identified small-molecule drugs that sensitized brain tumor cells to the tumor necrosis factor-related apoptosis-inducing ligand-induced cell death. Human glioblastoma multiforme cells were engineered by gene transfer to express Gluc as a reporter for cell viability, which can be monitored over time by bioluminescence measurements using a plate luminometer. We have optimized the Gluc assay for screening and validated it using the National Institute of Neurological Disorders and Stroke (NINDS) custom collection II library consisting of 1,040 drugs and bioactive compounds, most of which are Food and Drug Administration-approved and are able to cross the blood-brain barrier. We found that the cardiac glycosides family sensitized glioblastoma multiforme cells to the tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis. In conclusion, the Gluc secretion assay is a robust tool for functional drug screening and can be applied to many different fields including cancer.

Suicidal gene therapy in an NF-κB-controlled tumor environment as monitored by a secreted blood reporter

The nuclear factor-κB (NF-κB) is known to be activated in many cancer types including lung, ovarian, astrocytomas, melanoma, prostate as well as glioblastoma, and has been shown to correlate with disease progression. We have cloned a novel NF-κB-based reporter system (five tandem repeats of NF-κB responsive genomic element (NF; 14 bp each)) to drive the expression cassette for both a fusion between the yeast cytosine deaminase and uracil phosphoribosyltransferase (CU) as a therapeutic gene and the secreted Gaussia luciferase (Gluc) as a blood reporter, separated by an internal ribosomal entry site (NF-CU-IGluc). We showed that malignant tumor cells have high expression of Gluc, which correlates to high activation of NF-κB. When NF-κB was further activated by tumor necrosis factor-α in these cells, we observed up to 10-fold increase in Gluc levels and therefore transgene expression in human glioma cells served to greatly enhance the sensitization of these cells to the prodrug, 5-fluorocytosine both in cultured cells and in vivo subcutaneous tumor xenograft model. This inducible system provides a tool to enhance the expression of imaging and therapeutic genes for cancer therapy.

Real-time monitoring of nuclear factor kappaB activity in cultured cells and in animal models

Nuclear factor kappaB (NF-kappaB) is a transcription factor that plays a major role in many human disorders, including immune diseases and cancer. We designed a reporter system based on NF-kappaB responsive promoter elements driving expression of the secreted Gaussia princeps luciferase (Gluc). We show that this bioluminescent reporter is a highly sensitive tool for noninvasive monitoring of the kinetics of NF-kappaB activation and inhibition over time, both in conditioned medium of cultured cells and in the blood and urine of animals. NF-kappaB activation was successfully monitored in real time in endothelial cells in response to tumor angiogenic signaling, as well as in monocytes in response to inflammation. Further, we demonstrated dual blood monitoring of both NF-kappaB activation during tumor development as correlated to tumor formation using the NF-kappaB Gluc reporter, as well as the secreted alkaline phosphatase reporter. This NF-kappaB reporter system provides a powerful tool for monitoring NF-kappaB activity in real time in vitro and in vivo.

Real-Time Monitoring of Nuclear Factor κB Activity in Cultured Cells and in Animal Models

Nuclear factor κB (NF-κB) is a transcription factor that plays a major role in many human disorders, including immune diseases and cancer. We designed a reporter system based on NF-κB responsive promoter elements driving expression of the secreted Gaussia princeps luciferase (Gluc). We show that this bioluminescent reporter is a highly sensitive tool for noninvasive monitoring of the kinetics of NF-κB activation and inhibition over time, both in conditioned medium of cultured cells and in the blood and urine of animals. NF-κB activation was successfully monitored in real time in endothelial cells in response to tumor angiogenic signaling, as well as in monocytes in response to inflammation. Further, we demonstrated dual blood monitoring of both NF-κB activation during tumor development as correlated to tumor formation using the NF-κB Gluc reporter, as well as the secreted alkaline phosphatase reporter. This NF-κB reporter system provides a powerful tool for monitoring NF-κB activity in real time in vitro and in vivo.

A highly sensitive assay for monitoring the secretory pathway and ER stress

BACKGROUND

The secretory pathway is a critical index of the capacity of cells to incorporate proteins into cellular membranes and secrete proteins into the extracellular space. Importantly it is disrupted in response to stress to the endoplasmic reticulum that can be induced by a variety of factors, including expression of mutant proteins and physiologic stress. Activation of the ER stress response is critical in the etiology of a number of diseases, such as diabetes and neurodegeneration, as well as cancer. We have developed a highly sensitive assay to monitor processing of proteins through the secretory pathway and endoplasmic reticulum (ER) stress in real-time based on the naturally secreted Gaussia luciferase (Gluc).

METHODOLOGY/PRINCIPLE FINDINGS

An expression cassette for Gluc was delivered to cells, and its secretion was monitored by measuring luciferase activity in the conditioned medium. Gluc secretion was decreased down to 90% when these cells were treated with drugs that interfere with the secretory pathway at different steps. Fusing Gluc to a fluorescent protein allowed quantitation and visualization of the secretory pathway in real-time. Expression of this reporter protein did not itself elicit an ER stress response in cells; however, Gluc proved very sensitive at sensing this type of stress, which is associated with a temporary decrease in processing of proteins through the secretory pathway. The Gluc secretion assay was over 20,000-fold more sensitive as compared to the secreted alkaline phosphatase (SEAP), a well established assay for monitoring of protein processing and ER stress in mammalian cells.

CONCLUSIONS/SIGNIFICANCE

The Gluc assay provides a fast, quantitative and sensitive technique to monitor the secretory pathway and ER stress and its compatibility with high throughput screening will allow discovery of drugs for treatment of conditions in which the ER stress is generally induced.