Ketone body β-hydroxybutyrate (BHB) preserves mitochondrial bioenergetics

The ketogenic diet is an emerging therapeutic approach for refractory epilepsy, as well as certain rare and neurodegenerative disorders. The main ketone body, β-hydroxybutyrate (BHB), is the primary energy substrate endogenously produced in a ketogenic diet, however, mechanisms of its therapeutic actions remain unknown. Here, we studied the effects of BHB on mitochondrial energetics, both in non-stimulated conditions and during glutamate-mediated hyperexcitation. We found that glutamate-induced hyperexcitation stimulated mitochondrial respiration in cultured cortical neurons, and that this response was greater in cultures supplemented with BHB than with glucose. BHB enabled a stronger and more sustained maximal uncoupled respiration, indicating that BHB enables neurons to respond more efficiently to increased energy demands such as induced during hyperexcitation. We found that cytosolic Ca2+ was required for BHB-mediated enhancement of mitochondrial function, and that this enhancement was independent of the mitochondrial glutamate-aspartate carrier, Aralar/AGC1. Our results suggest that BHB exerts its protective effects against hyperexcitation by enhancing mitochondrial function through a Ca2+-dependent, but Aralar/AGC1-independent stimulation of mitochondrial respiration.

Comparative analysis of deeply phenotyped GBM cohorts of 'short-term' and 'long-term' survivors

BACKGROUND

Glioblastoma (GBM) is an aggressive brain cancer that typically results in death in the first 15 months after diagnosis. There have been limited advances in finding new treatments for GBM. In this study, we investigated molecular differences between patients with extremely short (≤ 9 months, Short term survivors, STS) and long survival (≥ 36 months, Long term survivors, LTS).

METHODS

Patients were selected from an in-house cohort (GLIOTRAIN-cohort), using defined inclusion criteria (Karnofsky score > 70; age < 70 years old; Stupp protocol as first line treatment, IDH wild type), and a multi-omic analysis of LTS and STS GBM samples was performed.

RESULTS

Transcriptomic analysis of tumour samples identified cilium gene signatures as enriched in LTS. Moreover, Immunohistochemical analysis confirmed the presence of cilia in the tumours of LTS. Notably, reverse phase protein array analysis (RPPA) demonstrated increased phosphorylated GAB1 (Y627), SRC (Y527), BCL2 (S70) and RAF (S338) protein expression in STS compared to LTS. Next, we identified 25 unique master regulators (MR) and 13 transcription factors (TFs) belonging to ontologies of integrin signalling and cell cycle to be upregulated in STS.

CONCLUSION

Overall, comparison of STS and LTS GBM patients, identifies novel biomarkers and potential actionable therapeutic targets for the management of GBM.

Fluorescence Time-lapse Imaging of Entosis Using Tetramethylrhodamine Methyl Ester Staining

Entosis is a process where a living cell launches an invasion into another living cell's cytoplasm. These inner cells can survive inside outer cells for a long period of time, can undergo cell division, or can be released. However, the fate of most inner cells is lysosomal degradation by entotic cell death. Entosis can be detected by imaging a combination of membrane, cytoplasmic, nuclear, and lysosomal staining in the cells. Here, we provide a protocol for detecting entosis events and measuring the kinetics of entotic cell death by time-lapse imaging using tetramethylrhodamine methyl ester (TMRM) staining. This protocol was validated in: J Cell Biol (2021), DOI: 10.1083/jcb.202010030.

Effect of TP53 deficiency and KRAS signaling on the bioenergetics of colon cancer cells in response to different substrates: A single cell study

Metabolic reprogramming is a hallmark of cancer. Somatic mutations in genes involved in oncogenic signaling pathways, including KRAS and TP53, rewire the metabolic machinery in cancer cells. We here set out to determine, at the single cell level, metabolic signatures in human colon cancer cells engineered to express combinations of activating KRAS gene mutations and TP53 gene deletions. Specifically, we explored how somatic mutations in these genes and substrate availability (lactate, glucose, substrate deprivation) from the extracellular microenvironment affect bioenergetic parameters, including cellular ATP, NADH and mitochondrial membrane potential dynamics. Employing cytosolic and mitochondrial FRET-based ATP probes, fluorescent NADH sensors, and the membrane-permeant cationic fluorescent probe TMRM in HCT-116 cells as a model system, we observed that TP53 deletion and KRAS mutations drive a shift in metabolic signatures enabling lactate to become an efficient metabolite to replenish both ATP and NADH following nutrient deprivation. Intriguingly, cytosolic, mitochondrial and overall cellular ATP measurements revealed that, in WT KRAS cells, TP53 deficiency leads to an enhanced ATP production in the presence of extracellular lactate and glucose, and to the greatest increase in ATP following a starvation period. On the other hand, oncogenic KRAS in TP53-deficient cells reversed the alterations in cellular ATP levels. Moreover, cell population measurements of mitochondrial and glycolytic metabolism using a Seahorse analyzer demonstrated that WT KRAS TP53-silenced cells display an increase of the basal respiration and tightly-coupled mitochondria, in the presence of glucose as substrate, compared to TP53 competent cells. Furthermore, cells possessing oncogenic KRAS, independently of TP53 status, showed less pronounced mitochondrial membrane potential changes in response to metabolic nutrients. Furthermore, analysis of cytosolic and mitochondrial NADH levels revealed that the simultaneous presence of TP53 deletion and oncogenic KRAS showed the most pronounced alteration in cytosolic and mitochondrial NADH during metabolic stress. In conclusion, our findings demonstrate how activating KRAS mutation and loss of TP53 remodel cancer metabolism and lead to alterations in bioenergetics under metabolic stress conditions by modulating cellular ATP production, NADH oxidation, mitochondrial respiration and function.

AMPK-regulated miRNA-210-3p is activated during ischaemic neuronal injury and modulates PI3K-p70S6K signalling

Progressive neuronal injury following ischaemic stroke is associated with glutamate-induced depolarization, energetic stress and activation of AMP-activated protein kinase (AMPK). We here identify a molecular signature associated with neuronal AMPK activation, as a critical regulator of cellular response to energetic stress following ischaemia. We report a robust induction of microRNA miR-210-3p both in vitro in primary cortical neurons in response to acute AMPK activation and following ischaemic stroke in vivo. Bioinformatics and reverse phase protein array analysis of neuronal protein expression changes in vivo following administration of a miR-210-3p mimic revealed altered expression of phosphatase and tensin homolog (PTEN), 3-phosphoinositide-dependent protein kinase 1 (PDK1), ribosomal protein S6 kinase (p70S6K) and ribosomal protein S6 (RPS6) signalling in response to increasing miR-210-3p. In vivo, we observed a corresponding reduction in p70S6K activity following ischaemic stroke. Utilizing models of glutamate receptor over-activation in primary neurons, we demonstrated that induction of miR-210-3p was accompanied by sustained suppression of p70S6K activity and that this effect was reversed by miR-210-3p inhibition. Collectively, these results provide new molecular insight into the regulation of cell signalling during ischaemic injury, and suggest a novel mechanism whereby AMPK regulates miR-210-3p to control p70S6K activity in ischaemic stroke and excitotoxic injury.

TRAIL signaling promotes entosis in colorectal cancer

Entosis is a form of nonphagocytic cell-in-cell (CIC) interaction where a living cell enters into another. Tumors show evidence of entosis; however, factors controlling entosis remain to be elucidated. Here, we find that besides inducing apoptosis, TRAIL signaling is a potent activator of entosis in colon cancer cells. Initiation of both apoptosis and entosis requires TRAIL receptors DR4 and DR5; however, induction of apoptosis and entosis diverges at caspase-8 as its structural presence is sufficient for induction of entosis but not apoptosis. Although apoptosis and entosis are morphologically and biochemically distinct, knockout of Bax and Bak, or inhibition of caspases, also inhibits entotic cell death and promotes survival and release of inner cells. Analysis of colorectal cancer tumors reveals a significant association between TRAIL signaling and CIC structures. Finally, the presence of CIC structures in the invasive front regions of colorectal tumors shows a strong correlation with adverse patient prognosis.

Mitochondrial Carrier Homolog 2 Functionally Co-operates With BH3 Interacting-Domain Death Agonist in Promoting Ca2+-Induced Neuronal Injury

The BH3 interacting-domain death agonist (BID) is a pro-apoptotic member of the Bcl-2 protein family. While proteolytic processing of BID links death receptor-induced apoptosis to the mitochondrial apoptosis pathway, we previously showed that full length BID also translocates to mitochondria during Ca²⁺-induced neuronal cell death. Moreover, mitochondrial carrier homolog 2 (MTCH2) was identified as a mitochondrial protein that interacts with BID during cell death. We started our studies by investigating the effect of Mtch2 silencing in a well-established model of Ca²⁺-induced mitochondrial permeability transition pore opening in non-neuronal HCT116 cells. We found that silencing of Mtch2 inhibited mitochondrial swelling and the associated decrease in mitochondrial energetics, suggesting a pro-death function for MTCH2 during Ca²⁺-induced injury. Next, we explored the role of BID and MTCH2 in mediating Ca²⁺-induced injury in primary cortical neurons triggered by prolonged activation of NMDA glutamate receptors. Analysis of intracellular Ca²⁺ transients, using time-lapse confocal microscopy, revealed that neurons lacking Bid showed markedly reduced Ca²⁺ levels during the NMDA excitation period. These Ca²⁺ transients were further decreased when Mtch2 was also silenced. Collectively, our data suggest that BID and MTCH2 functionally interact to promote Ca²⁺-induced neuronal injury.

Transcriptional CDK Inhibitors CYC065 and THZ1 Induce Apoptosis in Glioma Stem Cells Derived from Recurrent GBM

Glioma stem cells (GSCs) are tumour initiating cells which contribute to treatment resistance, temozolomide (TMZ) chemotherapy and radiotherapy, in glioblastoma (GBM), the most aggressive adult brain tumour. A major contributor to the uncontrolled tumour cell proliferation in GBM is the hyper activation of cyclin-dependent kinases (CDKs). Due to resistance to standard of care, GBMs relapse in almost all patients. Targeting GSCs using transcriptional CDK inhibitors, CYC065 and THZ1 is a potential novel treatment to prevent relapse of the tumour. TCGA-GBM data analysis has shown that the GSC markers, CD133 and CD44 were significantly upregulated in GBM patient tumours compared to non-tumour tissue. CD133 and CD44 stem cell markers were also expressed in gliomaspheres derived from recurrent GBM tumours. Light Sheet Florescence Microscopy (LSFM) further revealed heterogeneous expression of these GSC markers in gliomaspheres. Gliomaspheres from recurrent tumours were highly sensitive to transcriptional CDK inhibitors, CYC065 and THZ1 and underwent apoptosis while being resistant to TMZ. Apoptotic cell death in GSC subpopulations and non-stem tumour cells resulted in sphere disruption. Collectively, our study highlights the potential of these novel CKIs to induce cell death in GSCs from recurrent tumours, warranting further clinical investigation.

Multiple screening approaches reveal HDAC6 as a novel regulator of glycolytic metabolism in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is a subtype of breast cancer without a targeted form of therapy. Unfortunately, up to 70% of patients with TNBC develop resistance to treatment. A known contributor to chemoresistance is dysfunctional mitochondrial apoptosis signaling. We set up a phenotypic small-molecule screen to reveal vulnerabilities in TNBC cells that were independent of mitochondrial apoptosis. Using a functional genetic approach, we identified that a "hit" compound, BAS-2, had a potentially similar mechanism of action to histone deacetylase inhibitors (HDAC). An in vitro HDAC inhibitor assay confirmed that the compound selectively inhibited HDAC6. Using state-of-the-art acetylome mass spectrometry, we identified glycolytic substrates of HDAC6 in TNBC cells. We confirmed that inhibition or knockout of HDAC6 reduced glycolytic metabolism both in vitro and in vivo. Through a series of unbiased screening approaches, we have identified a previously unidentified role for HDAC6 in regulating glycolytic metabolism.

Molecular subtype-specific responses of colon cancer cells to the SMAC mimetic Birinapant

Colorectal cancer is a molecularly heterogeneous disease. Responses to genotoxic chemotherapy in the adjuvant or palliative setting vary greatly between patients, and colorectal cancer cells often resist chemotherapy by evading apoptosis. Antagonists of an inhibitor of apoptosis proteins (IAPs) can restore defective apoptosis signaling by degrading cIAP1 and cIAP2 proteins and by inhibition of XIAP. Due to the multiple molecular mechanisms-of-action of these targets, responses to IAP antagonist may differ between molecularly distinct colon cancer cells. In this study, responses to the IAP antagonist Birinapant and oxaliplatin/5-fluorouracil (5-FU) were investigated in 14 colon cancer cell lines, representing the consensus molecular subtypes (CMS). Treatment with Birinapant alone did not result in a substantial increase in apoptotic cells in this cell line panel. Annexin-V/PI assays quantified by flow cytometry and high-content screening showed that Birinapant increased responses of CMS1 and partially CMS3 cell lines to oxaliplatin/5-FU, whereas CMS2 cells were not effectively sensitized. FRET-based imaging of caspase-8 and -3 activation validated these differences at the single-cell level, with CMS1 cells displaying sustained activation of caspase-8-like activity during Birinapant and oxaliplatin/5-FU co-treatment, ultimately activating the intrinsic mitochondrial apoptosis pathway. In CMS2 cell lines, Birinapant exhibited synergistic effects in combination with TNFα, suggesting that Birinapant can restore extrinsic apoptosis signaling in the context of inflammatory signals in this subtype. To explore this further, we co-cultured CMS2 and CMS1 colon cancer cells with peripheral blood mononuclear cells. We observed increased cell death during Birinapant single treatment in these co-cultures, which was abrogated by anti-TNFα-neutralizing antibodies. Collectively, our study demonstrates that IAP inhibition is a promising modulator of response to oxaliplatin/5-FU in colorectal cancers of the CMS1 subtype, and may show promise as in the CMS2 subtype, suggesting that molecular subtyping may aid as a patient stratification tool for IAP antagonists in this disease.

ER stress signaling has an activating transcription factor 6α (ATF6)-dependent "off-switch"

In response to an accumulation of unfolded proteins in the endoplasmic reticulum (ER) lumen, three ER transmembrane signaling proteins, inositol-requiring enzyme 1 (IRE1), PRKR-like ER kinase (PERK), and activating transcription factor 6α (ATF6α), are activated. These proteins initiate a signaling and transcriptional network termed the unfolded protein response (UPR), which re-establishes cellular proteostasis. When this restoration fails, however, cells undergo apoptosis. To investigate cross-talk between these different UPR enzymes, here we developed a high-content live cell screening platform to image fluorescent UPR-reporter cell lines derived from human SH-SY5Y neuroblastoma cells in which different ER stress signaling proteins were silenced through lentivirus-delivered shRNA constructs. We observed that loss of ATF6 expression results in uncontrolled IRE1-reporter activity and increases X box-binding protein 1 (XBP1) splicing. Transient increases in both IRE1 mRNA and IRE1 protein levels were observed in response to ER stress, suggesting that IRE1 up-regulation is a general feature of ER stress signaling and was further increased in cells lacking ATF6 expression. Moreover, overexpression of the transcriptionally active N-terminal domain of ATF6 reversed the increases in IRE1 levels. Furthermore, inhibition of IRE1 kinase activity or of downstream JNK activity prevented an increase in IRE1 levels during ER stress, suggesting that IRE1 transcription is regulated through a positive feed-forward loop. Collectively, our results indicate that from the moment of activation, IRE1 signaling during ER stress has an ATF6-dependent "off-switch."

BCL2 and BCL(X)L selective inhibitors decrease mitochondrial ATP production in breast cancer cells and are synthetically lethal when combined with 2-deoxy-D-glucose

Cancer cells display differences regarding their engagement of glycolytic vs. mitochondrial oxidative phosphorylation (OXPHOS) pathway. Triple negative breast cancer, an aggressive form of breast cancer, is characterized by elevated glycolysis, while estrogen receptor positive breast cancer cells rely predominantly on OXPHOS. BCL2 proteins control the process of mitochondrial outer membrane permeabilization during apoptosis, but also regulate cellular bioenergetics. Because BCL2 proteins are overexpressed in breast cancer and targetable by selective antagonists, we here analysed the effect of BCL2 and BCL(X)L selective inhibitors, Venetoclax and WEHI-539, on mitochondrial bioenergetics and cell death. Employing single cell imaging using a FRET-based mitochondrial ATP sensor, we found that MCF7 breast cancer cells supplied with mitochondrial substrates reduced their mitochondrial ATP production when treated with Venetoclax or WEHI-539 at concentrations that per se did not induce cell death. Treatments with lower concentrations of both inhibitors also reduced the length of the mitochondrial network and the dynamics, as evaluated by quantitative confocal microscopy. We next tested the hypothesis that mitochondrial ATP production inhibition with BCL2 or BCL(X)L antagonists was synthetically lethal when combined with glycolysis inhibition. Treatment with 2-deoxy-D-glucose in combination with Venetoclax or WEHI-539 synergistically reduced the cellular bioenergetics of ER+ and TNBC breast cancer cells and abolished their clonogenic potential. Synthetic lethality was also observed when cultures were grown in 3D spheres. Our findings demonstrate that BCL2 antagonists exert potent effects on cancer metabolism independent of cell death-inducing effects, and demonstrate a synthetic lethality when these are applied in combination with glycolysis inhibitors.

Systems modeling accurately predicts responses to genotoxic agents and their synergism with BCL-2 inhibitors in triple negative breast cancer cells

Triple negative breast cancer (TNBC) is an aggressive form of breast cancer which accounts for 15-20% of this disease and is currently treated with genotoxic chemotherapy. The BCL2 (B-cell lymphoma 2) family of proteins controls the process of mitochondrial outer membrane permeabilization (MOMP), which is required for the activation of the mitochondrial apoptosis pathway in response to genotoxic agents. We previously developed a deterministic systems model of BCL2 protein interactions, DR_MOMP that calculates the sensitivity of cells to undergo mitochondrial apoptosis. Here we determined whether DR_MOMP predicts responses of TNBC cells to genotoxic agents and the re-sensitization of resistant cells by BCL2 inhibitors. Using absolute protein levels of BAX, BAK, BCL2, BCL(X)L and MCL1 as input for DR_MOMP, we found a strong correlation between model predictions and responses of a panel of TNBC cells to 24 and 48 h cisplatin (R2 = 0.96 and 0.95, respectively) and paclitaxel treatments (R2 = 0.94 and 0.95, respectively). This outperformed single protein correlations (best performer BCL(X)L with R2 of 0.69 and 0.50 for cisplatin and paclitaxel treatments, respectively) and BCL2 proteins ratio (R2 of 0.50 for cisplatin and 0.49 for paclitaxel). Next we performed synergy studies using the BCL2 selective antagonist Venetoclax /ABT199, the BCL(X)L selective antagonist WEHI-539, or the MCL1 selective antagonist A-1210477 in combination with cisplatin. In silico predictions by DR_MOMP revealed substantial differences in treatment responses of BCL(X)L, BCL2 or MCL1 inhibitors combinations with cisplatin that were successfully validated in cell lines. Our findings provide evidence that DR_MOMP predicts responses of TNBC cells to genotoxic therapy, and can aid in the choice of the optimal BCL2 protein antagonist for combination treatments of resistant cells.

Defining external factors that determine neuronal survival, apoptosis and necrosis during excitotoxic injury using a high content screening imaging platform

Cell death induced by excessive glutamate receptor overactivation, excitotoxicity, has been implicated in several acute and chronic neurological disorders. While numerous studies have demonstrated the contribution of biochemically and genetically activated cell death pathways in excitotoxic injury, the factors mediating passive, excitotoxic necrosis are less thoroughly investigated. To address this question, we developed a high content screening (HCS) based assay to collect high volumes of quantitative cellular imaging data and elucidated the effects of intrinsic and external factors on excitotoxic necrosis and apoptosis. The analysis workflow consisted of robust nuclei segmentation, tracking and a classification algorithm, which enabled automated analysis of large amounts of data to identify and quantify viable, apoptotic and necrotic neuronal populations. We show that mouse cerebellar granule neurons plated at low or high density underwent significantly increased necrosis compared to neurons seeded at medium density. Increased extracellular Ca²⁺ sensitized neurons to glutamate-induced excitotoxicity, but surprisingly potentiated cell death mainly through apoptosis. We also demonstrate that inhibition of various cell death signaling pathways (including inhibition of calpain, PARP and AMPK activation) primarily reduced excitotoxic apoptosis. Excitotoxic necrosis instead increased with low extracellular glucose availability. Our study is the first of its kind to establish and implement a HCS based assay to investigate the contribution of external and intrinsic factors to excitotoxic apoptosis and necrosis.

Abstract 1012: Systems analysis of colon cancer cell metabolism rewired by p53 and KRAS mutations

Introduction. Somatic mutations in proto-oncogenes and tumour-suppressor genes contribute to rewire the already deregulated metabolic network in cancer cells, resulting in uncontrolled proliferation and oncogenesis. In this study, we set out to establish a dynamic mathematical model of bioenergetics and to exploit it to explore the multifaceted cross-talk between bioenergetics, somatic gene mutations in KRAS and p53, and cell proliferation and survival.

Model Development. We have developed an ordinary differential equations-based model of central carbon metabolism in cancer cells which includes glycolysis, pentose phosphate pathway, citric acid cycle and respiratory chain, based on our previous work and published models. The model describes how nutrients (glucose, glutamine, lactate, pyruvate, serine and glycine) from the extracellular micro-environment affect bioenergetics parameters. The resulting model predictions are linked to cell proliferation via a heuristic function. Enzymatic activities regulated by p53 and KRAS mutations were obtained by mining publically available datasets and their regulation by the mutational status was modelled by adapting the corresponding kinetic parameters. To estimate the kinetic parameters, model simulation outputs were fitted to a portfolio of experimental data both generated de novo in house and gathered from the literature in HCT-116 colon cancer cells. Experiments were performed on parental HCT-116 (p53 competent; harbouring a KRAS mutation on exon 2 of codon G13) and three derived mutant cell lines covering all four combinations of p53 and KRAS mutational status to isolate their relative and joint effect on bioenergetics signatures. HCT-116-derived cell lines included: p53 proficient cells with the KRAS allelic mutation silenced by homologous recombination in the presence or absence of p53 knockout by lentiviral shRNA.

Results. The model was calibrated against ATP concentrations measured via single-cell microscopy (ATeam probe and TMRM dye) following pharmacological inhibition of respiratory chain complexes (rotenone, sodium azide and oligomycin) as a function of nutrients availability (glucose, lactate, pyruvate). Modelling results revealed that p53 and KRAS mutations drive a shift in metabolic signatures and L-lactate emerged as a pivotal metabolite to stratify the different phenotypes. Systems analysis revealed that in KRAS mutated cells p53 deficiency leads to an increase in glucose uptake and flux through the pentose phosphate pathway and a decrease in lactate production. Indeed, p53 deficient HCT-116 cells showed a decrease in extracellular lactate with respect to p53 proficient cells in validation experiments.

Conclusions. The computational model developed can be used to benchmark mechanistic hypotheses by which tumour suppressors and/or oncogenic mutations rewire metabolism and to identify putative targets for therapeutic intervention.

Citation Format: Manuela Salvucci, Robert O’Byrne, Natalia Niewidok, Séan Kilbride, Caoimhín G. Concannon, Heiko Düssmann, Heinrich H. Huber, Jochen HM Prehn. Systems analysis of colon cancer cell metabolism rewired by p53 and KRAS mutations. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1012.

Abstract 4224: Multiplexing single cell time lapse imaging of intra-cellular oxygen in cancer cells

Oxygen plays an important role in aerobic energy metabolism and signal transduction. The detection of intracellular or subcellular molecular oxygen (O2) levels is important for the understanding of cell physiology and mitochondrial energy metabolism, and the alterations correlated with cancer. Detection of O2 has recently been improved significantly with the development of live cell time lapse microscopy compatible oxygen sensitive probes.

We here utilized nanoparticle probe MitoImageTM-MM2 with embedded poly(9,9-dioctylfluorene) (PFO) as an O2-independent component and Pt(II)-5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)-porphyrin (PtTFPP) to perform time lapse ratiometric measurements of intra-cellular oxygen levels. Loading of HeLa cervical cancer cells with MM2 enables us to image kinetics of intracellular O2 concentration in response to the mitochondrial respiratory chain inhibitors Antimycin A (complex III) or sodium azide (complex IV). These changes were detected in respiration-supporting medium equilibrated with 5% (53 μM) and 20% (225 μM)O2. Also exposing cells to varying extracellular oxygen concentrations (21 - 225 μM) showed strong dependence of the intracellular oxygen levels. The mitochondrial membrane potential was monitored by simultaneous imaging of Tetra-Methyl-Rhodamine-Methyl ester (TMRM) in the same sample. Multiplexing with GFP finally allowed us to monitor intracellular O2 during the apoptotic signaling process of mitochondrial outer membrane permeabilisation (MOMP) in HeLa expressing cytochrome-c-eGFP and stained with TMRM.

In conclusion, the MitoImageTM-MM2 nanoparticle probe enables single-cell, time-lapse imaging of alterations in intracellular oxygen levels in cancer cells, and can be employed in multiplex assays to investigate alterations in intracellular oxygen levels relative to protein dynamics and mitochondrial function.

Keywords: Intracellular oxygen; phosphorescent oxygen-sensitive nanoparticles; HeLa cells; MitoImage ratiometric probes; confocal microscopy.

Citation Format: Heiko Düssmann, Sergio Perez, Ujval Anil Kumar, Dmitri B. Papkovsky, Jochen HM Prehn. Multiplexing single cell time lapse imaging of intra-cellular oxygen in cancer cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4224.

Versatile Conjugated Polymer Nanoparticles for High-Resolution O2 Imaging in Cells and 3D Tissue Models

High brightness, chemical and photostability, tunable characteristics, and spectral and surface properties are important attributes for nanoparticle probes designed for live cell imaging. We describe a class of nanoparticles for high-resolution imaging of O2 that consists of a substituted conjugated polymer (polyfluorene or poly(fluorene-alt-benzothiadiazole)) acting as a FRET antenna and a fluorescent reference with covalently bound phosphorescent metalloporphyrin (PtTFPP, PtTPTBPF). The nanoparticles prepared from such copolymers by precipitation method display stability, enhanced (>5-10 times) brightness under one- and two-photon excitation, compatibility with ratiometric and lifetime-based imaging modes, and low toxicity for cells. Their cell-staining properties can be modulated with positively and negatively charged groups grafted to the backbone. The "zwitter-ionic" nanoparticles show high cell-staining efficiency, while their cell entry mechanisms differ for the different 3D models. When injected in the bloodstream, the cationic and anionic nanoparticles show similar distribution in vivo. These features and tunable properties make the conjugated polymer based phosphorescent nanoparticles a versatile tool for quantitative O2 imaging with a broad range of cell and 3D tissue models.

Imaging of single cell responses to ER stress indicates that the relative dynamics of IRE1/XBP1 and PERK/ATF4 signalling rather than a switch between signalling branches determine cell survival

An accumulation of misfolded proteins in the endoplasmic reticulum (ER) triggers the unfolded protein response (UPR) mediated via the activation of three transmembrane proteins IRE1, PERK and ATF6. Signalling through these proteins is aimed at enhancing the ER folding capacity and reducing the folding load. If these processes fail to re-establish protein homeostasis within the ER, then cell death prevails via apoptosis. How the shift from pro-survival to pro-apoptotic signalling is regulated remains unclear with both IRE1 and PERK signalling associated with pro-survival as well as pro-apoptotic signalling. To investigate the temporal activation of IRE1 and PERK in live cells and their relationship to cellular fate, we devised single cell reporters for both ER stress signalling branches. SH-SY5Y neural cells stably expressing these fluorescent protein reporter constructs to monitor IRE1-splicing activity and PERK-mediated ATF4-translation were imaged using single cell and high content time lapse live cell microscopy. We could correlate an early onset and attenuation of XBP1 splicing in the IRE1-reporter cells as cytoprotective. Indeed, silencing of IRE1 expression using shRNA inhibited splicing of XBP1 resulting in an early onset of cell death. In contrast, in the PERK-reporter cells, we observed that a slow rate of ATF4-translation and late re-initiation of general translation coincided with cells which were resistant to ER stress-induced cell death. Interestingly, whereas silencing of PERK did not affect overall levels of cell death in response to ER stress, it did increase sensitivity to ER stressors at early time points following treatment. Our results suggest that apoptosis activation in response to ER stress is not caused by a preferential activation of a single UPR branch, or by a switch from one branch to the other. Rather, our data indicated that the relative timing of IRE1 and PERK signalling determines the shift from cell survival to apoptosis.

Latrepirdine is a potent activator of AMP-activated protein kinase and reduces neuronal excitability

Latrepirdine/Dimebon is a small-molecule compound with attributed neurocognitive-enhancing activities, which has recently been tested in clinical trials for the treatment of Alzheimer's and Huntington's disease. Latrepirdine has been suggested to be a neuroprotective agent that increases mitochondrial function, however the molecular mechanisms underlying these activities have remained elusive. We here demonstrate that latrepirdine, at (sub)nanomolar concentrations (0.1 nM), activates the energy sensor AMP-activated protein kinase (AMPK). Treatment of primary neurons with latrepirdine increased intracellular ATP levels and glucose transporter 3 translocation to the plasma membrane. Latrepirdine also increased mitochondrial uptake of the voltage-sensitive probe TMRM. Gene silencing of AMPKα or its upstream kinases, LKB1 and CaMKKβ, inhibited this effect. However, studies using the plasma membrane potential indicator DisBAC2(3) demonstrated that the effects of latrepirdine on TMRM uptake were largely mediated by plasma membrane hyperpolarization, precluding a purely 'mitochondrial' mechanism of action. In line with a stabilizing effect of latrepirdine on plasma membrane potential, pretreatment with latrepirdine reduced spontaneous Ca(2+) oscillations as well as glutamate-induced Ca(2+) increases in primary neurons, and protected neurons against glutamate toxicity. In conclusion, our experiments demonstrate that latrepirdine is a potent activator of AMPK, and suggest that one of the main pharmacological activities of latrepirdine is a reduction in neuronal excitability.

Single cell imaging of the heat shock response during proteasome inhibitor-induced apoptosis in colon cancer cells suggests that magnitude and length rather than time of onset determines resistance to apoptosis

Targeting the proteasome is a valuable approach for cancer therapy, potentially limited by pro-survival pathways induced in parallel to cell death. Whether these pro-survival pathways are activated in all cells, show different activation kinetics in sensitive versus resistant cells, or interact functionally with cell death pathways is unknown. We monitored activation of the heat shock response (HSR), a key survival pathway induced by proteasome inhibition, relative to apoptosis activation in HCT116 colon cancer cells expressing green fluorescent protein (GFP) under the control of the Hsp70 promoter. Single cell and high content time-lapse imaging of epoxomicin treatment revealed that neither basal activity, nor the time of onset of the HSR differed between resistant and sensitive populations. However, resistant cells had significantly higher and prolonged reporter activity than those that succumbed to cell death. p53 deficiency protected against cell death but failed to modulate the HSR. In contrast, inhibition of the HSR significantly increased the cytotoxicity of epoxomicin. Our data provide novel insights into the kinetics and heterogeneity of HSR during proteasome inhibition, suggesting that the HSR modulates cell death signaling unidirectionally.

AMP-activated protein kinase (AMPK)-induced preconditioning in primary cortical neurons involves activation of MCL-1

Neuronal preconditioning is a phenomenon where a previous exposure to a sub-lethal stress stimulus increases the resistance of neurons towards a second, normally lethal stress stimulus. Activation of the energy stress sensor, AMP-activated protein kinase (AMPK) has been shown to contribute to the protective effects of ischaemic and mitochondrial uncoupling-induced preconditioning in neurons, however, the molecular basis of AMPK-mediated preconditioning has been less well characterized. We investigated the effect of AMPK preconditioning using 5-aminoimidazole-4-carboxamide riboside (AICAR) in a model of NMDA-mediated excitotoxic injury in primary mouse cortical neurons. Activation of AMPK with low concentrations of AICAR (0.1 mM for 2 h) induced a transient increase in AMPK phosphorylation, protecting neurons against NMDA-induced excitotoxicity. Analysing potential targets of AMPK activation, demonstrated a marked increase in mRNA expression and protein levels of the anti-apoptotic BCL-2 family protein myeloid cell leukaemia sequence 1 (MCL-1) in AICAR-preconditioned neurons. Interestingly, over-expression of MCL-1 protected neurons against NMDA-induced excitotoxicity while MCL-1 gene silencing abolished the effect of AICAR preconditioning. Monitored intracellular Ca²⁺ levels during NMDA excitation revealed that MCL-1 over-expressing neurons exhibited improved bioenergetics and markedly reduced Ca²⁺ elevations, suggesting a potential mechanism through which MCL-1 confers neuroprotection. This study identifies MCL-1 as a key effector of AMPK-induced preconditioning in neurons.

Proteasome inhibition can induce an autophagy-dependent apical activation of caspase-8

Antiapoptotic Bcl-2 family proteins are often highly expressed in chemotherapy-resistant cancers and impair mitochondrial outer membrane permeabilisation (MOMP), an important requirement for caspase activation via the intrinsic apoptosis pathway. Interestingly, although Bcl-2 overexpression in HeLa cervical cancer cells abrogated caspase processing in response to intrinsic apoptosis induction by staurosporine, tunicamycin or etoposide, residual caspase processing was observed following proteasome inhibition by bortezomib ([(1R)-3-methyl-1-({(2S)-3-phenyl-2-[(pyrazin-2-ylcarbonyl)amino]propanoyl}amino)butyl]boronic acid), epoxomicin (N-acetyl-N-methyl-lisoleucyl-L-isoleucyl-N-[(1S)-3-methyl-1-[[(2R)-2-methyloxiranyl]carbonyl]butyl]-L-threoninamide) or MG-132 (N-(benzyloxycarbonyl)leucinylleucinylleucinal). Similar responses were found in Bcl-2-overexpressing H460 NSCLC cells and Bax/Bak-deficient mouse embyronic fibroblasts. Mild caspase processing resulted in low DEVDase activities, which were MOMP independent and persisted for long periods without evoking immediate cell death. Surprisingly, depletion of caspase-3 and experiments in caspase-7-depleted MCF-7-Bcl-2 cells indicated that the DEVDase activity did not originate from effector caspases. Instead, Fas-associated death domain (FADD)-dependent caspase-8 activation was the major contributor to the slow, incomplete substrate cleavage. Caspase-8 activation was independent of death ligands, but required the induction of autophagy and the presence of Atg5. Depletion of XIAP or addition of XIAP-antagonising peptides resulted in a switch towards efficient apoptosis execution, suggesting that the requirement for MOMP was bypassed by activating the caspase-8/caspase-3 axis. Combination treatments of proteasome inhibitors and XIAP antagonists therefore represent a promising strategy to eliminate highly resistant cancer cells, which overexpress antiapoptotic Bcl-2 family members.

ALISSA: an automated live-cell imaging system for signal transduction analyses

Probe photobleaching and a specimen's sensitivity to phototoxicity severely limit the number of possible excitation cycles in time-lapse fluorescent microscopy experiments. Consequently, when a study of cellular processes requires measurements over hours or days, temporal resolution is limited, and spontaneous or rapid events may be missed, thus limiting conclusions about transduction events. We have developed ALISSA, a design framework and reference implementation for an automated live-cell imaging system for signal transduction analysis. It allows an adaptation of image modalities and laser resources tailored to the biological process, and thereby extends temporal resolution from minutes to seconds. The system employs online image analysis to detect cellular events that are then used to exercise microscope control. It consists of a reusable image analysis software for cell segmentation, tracking, and time series extraction, and a measurement-specific process control software that can be easily adapted to various biological settings. We have applied the ALISSA framework to the analysis of apoptosis as a demonstration case for slow onset and rapid execution signaling. The demonstration provides a clear proof-of-concept for ALISSA, and offers guidelines for its application in a broad spectrum of signal transduction studies.

Single-cell quantification of Bax activation and mathematical modelling suggest pore formation on minimal mitochondrial Bax accumulation

Mitochondrial outer membrane permeabilisation (MOMP) during apoptosis is triggered by the activation and oligomerisation of Bax and Bak, but a quantification of these processes in individual cells has not yet been performed. Single-cell imaging of Bax translocation and oligomerisation in Bax-deficient DU-145 cells expressing CFP-Bax and YFP-Bax revealed that both processes started only minutes before or concomitantly with MOMP, with the majority of Bax translocation and oligomerisation occurring downstream of MOMP. Quantification of YFP-Bax concentrations at mitochondria revealed an increase of only 1.8 + or - 1.5% at MOMP onset. This was increased to 11.2 + or - 3.6% in bak-silenced cells. These data suggested that Bax activation exceeded by far the quantities required for MOMP induction, and that minimal Bax or Bak activation may be sufficient to trigger rapid pore formation. In a cellular automaton modelling approach that incorporated the quantities and movement probabilities of Bax and its inhibitors, activators and enablers in the mitochondrial membrane, we could re-model rapid pore formation kinetics at submaximal Bax activation.

Mitochondrial and plasma membrane potential of cultured cerebellar neurons during glutamate-induced necrosis, apoptosis, and tolerance

A failure of mitochondrial bioenergetics has been shown to be closely associated with the onset of apoptotic and necrotic neuronal injury. Here, we developed an automated computational model that interprets the single-cell fluorescence for tetramethylrhodamine methyl ester (TMRM) as a consequence of changes in either delta psi(m) or delta psi(p), thus allowing for the characterization of responses for populations of single cells and subsequent statistical analysis. Necrotic injury triggered by prolonged glutamate excitation resulted in a rapid monophasic or biphasic loss of delta psi(m) that was closely associated with a loss of delta psi(p) and a rapid decrease in neuronal NADPH and ATP levels. Delayed apoptotic injury, induced by transient glutamate excitation, resulted in a small, reversible decrease in TMRM fluorescence, followed by a sustained hyperpolarization of delta psi(m) as confirmed using the delta psi(p)-sensitive anionic probe DiBAC2(3). This hyperpolarization of delta psi(m) was closely associated with a significant increase in neuronal glucose uptake, NADPH availability, and ATP levels. Statistical analysis of the changes in delta psi(m) or delta psi(p) at a single-cell level revealed two major correlations; those neurons displaying a more pronounced depolarization of delta psi(p) during the initial phase of glutamate excitation entered apoptosis more rapidly, and neurons that displayed a more pronounced hyperpolarization of delta psi(m) after glutamate excitation survived longer. Indeed, those neurons that were tolerant to transient glutamate excitation (18%) showed the most significant increases in delta psi(m). Our results indicate that a hyperpolarization of delta psi(m) is associated with increased glucose uptake, NADPH availability, and survival responses during excitotoxic injury.

The Myosin IXb motor activity targets the myosin IXb RhoGAP domain as cargo to sites of actin polymerization

Myosin IXb (Myo9b) is a single-headed processive myosin that exhibits Rho GTPase-activating protein (RhoGAP) activity in its tail region. Using live cell imaging, we determined that Myo9b is recruited to extending lamellipodia, ruffles, and filopodia, the regions of active actin polymerization. A functional motor domain was both necessary and sufficient for targeting Myo9b to these regions. The head domains of class IX myosins comprise a large insertion in loop2. Deletion of the large Myo9b head loop 2 insertion abrogated the enrichment in extending lamellipodia and ruffles, but enhanced significantly the enrichment at the tips of filopodia and retraction fibers. The enrichment in the tips of filopodia and retraction fibers depended on four lysine residues C-terminal to the loop 2 insertion and the tail region. Fluorescence recovery after photobleaching and photoactivation experiments in lamellipodia revealed that the dynamics of Myo9b was comparable to that of actin. The exchange rates depended on the Myo9b motor region and motor activity, and they were also dependent on the turnover of F-actin. These results demonstrate that Myo9b functions as a motorized RhoGAP molecule in regions of actin polymerization and identify Myo9b head sequences important for in vivo motor properties.

APOPTO-CELL a simulation tool and interactive database for analyzing cellular susceptibility to apoptosis

We have developed a web service that provides a comprehensive analysis of the susceptibility of cells to undergo apoptosis in response to an activation of the mitochondrial apoptotic pathway. Based on ordinary differential equations, (pre-determined) protein concentrations and release kinetics of mitochondrial pro-apoptotic factors, a network of 52 reactions and 19 reaction partners can be employed as a tool to display temporal protein profiles, to identify key regulatory proteins and to determine critical threshold concentrations required for the execution of apoptosis in HeLa cancer cells or other cell types. The web service also provides an interactive database function for the deposition of cell-type-specific quantitative data. In addition, the web service provides an output that can be compared directly to experimental results obtained from real-time single-cell experiments, making this a widely applicable systems biology tool for apoptosis and cancer researchers.

AVAILABILITY

http://systemsbiology.rcsi.ie/apopto-cell.html.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Real-time single cell analysis of Smac/DIABLO release during apoptosis

We examined the temporal and causal relationship between Smac/DIABLO release, cytochrome c (cyt-c) release, and caspase activation at the single cell level during apoptosis. Cells treated with the broad-spectrum caspase inhibitor z-VAD-fmk, caspase-3 (Casp-3)-deficient MCF-7 cells, as well as Bax-deficient DU-145 cells released Smac/DIABLO and cyt-c in response to proapoptotic agents. Real-time confocal imaging of MCF-7 cells stably expressing Smac/DIABLO-yellow fluorescent protein (YFP) revealed that the average duration of Smac/DIABLO-YFP release was greater than that of cyt-c-green fluorescent protein (GFP). However, there was no significant difference in the time to the onset of release, and both cyt-c-GFP and Smac/DIABLO-YFP release coincided with mitochondrial membrane potential depolarization. We also observed no significant differences in the Smac/DIABLO-YFP release kinetics when z-VAD-fmk-sensitive caspases were inhibited or Casp-3 was reintroduced. Simultaneous measurement of DEVDase activation and Smac/DIABLO-YFP release demonstrated that DEVDase activation occurred within 10 min of release, even in the absence of Casp-3.

Novel benzylidene-9(10H)-anthracenones as highly active antimicrotubule agents. Synthesis, antiproliferative activity, and inhibition of tubulin polymerization

A novel series of 10-benzylidene-9(10H)-anthracenones and 10-(phenylmethyl)-9(10H)-anthracenones were synthesized and evaluated for antiproliferative activity in an assay based on K562 leukemia cells. The 3-hydroxy-4-methoxybenzylidene analogue 9h was found to be the most active compound (IC(50) K562: 20 nM). Structure-activity relationships are also considered. The highly active compound 9h and the 2,4-dimethoxy-3-hydroxybenzylidene analogue 9l were tested against five tumor cell lines using the XTT assay, including multidrug resistant phenotypes. Induction of cell death in a variety of tumor cell lines was determined in a monolayer assay using propidium iodide. Noteworthy, all compounds within the series induced elongations in K562 cells similar to vinblastine-treated cells. The effect of the lead compound 9h on K562 cell growth was associated with cell cycle arrest in G2/M. Concentrations for 50% KB/HeLa cells arrested in G2/M after treatment with 9h and 9l were determined and found to be in the range of 0.2 microM. Additionally, we monitored the dose dependent caspase-3-like protease activity in K562 cells and MCF-7/Casp-3 cells treated with 9h, indicating induction of apoptosis. Western blotting analysis demonstrated that 9h caused a shift in tubulin concentration from the polymerized state found in the cell pellet to the unpolymerized state found in the cell supernatant. Seven compounds strongly inhibited tubulin polymerization with activities higher or comparable to those of the reference compounds such as colchicine, podophyllotoxin, and nocodazole. In general, the antiproliferative activity correlated with inhibition of tubulin polymerization. The most active compounds strongly displaced [(3)H]colchicine from its binding site in the tubulin, yielding IC(50) values 3- to 4-fold lower than that of colchicine. The novel benzylidene-9(10H)-anthracenones described in the present study constitute an interesting group of highly active and easily accessible antimitotic agents that inhibit tubulin polymerization.

Mitochondrial membrane permeabilization and superoxide production during apoptosis. A single-cell analysis

The temporal relationship between mitochondrial membrane permeabilization and reactive oxygen species production during apoptosis remains unknown. We analyzed the rate of superoxide production of human breast carcinoma cells expressing a cytochrome c-green fluorescent protein (cyt-c-GFP) fusion protein at the single-cell level during apoptosis. In cells treated with the proapoptotic agents staurosporine (3 microm) or tumor necrosis factor-alpha (100 ng/ml), the release of cyt-c-GFP was individually set for each cell, and the majority of the fusion protein was released in less than 10 min. Prior to the release of the fusion protein, cells demonstrated a constant rate of superoxide production determined with the probe hydroethidine. After the release was completed, the superoxide concentration increased rapidly to a level more than 3-fold above baseline. Treatment with the broad spectrum caspase inhibitor z-Val-Ala-Asp(O-methyl)-fluoromethylketone (z-VAD-fmk; 200 microm) did not alter the kinetics of the cyt-c-GFP release but significantly reduced superoxide concentration after the release of cyt-c-GFP. Interestingly, treatment with z-VAD-fmk also reduced the increase in superoxide concentration in response to menadione in the absence of mitochondrial cyt-c-GFP release. Mitochondrial depolarization with the protonophore carbonyl cyanide p-trifluoromethoxy-phenylhydrazone per se did not trigger cyt-c-GFP release or an increase in superoxide production. Our data suggest that mitochondria increase their superoxide production during apoptosis directly after the quantitative release of soluble intermembrane proteins and demonstrate novel antioxidative effects of the commonly used caspase inhibitor z-VAD-fmk.

Outer mitochondrial membrane permeabilization during apoptosis triggers caspase-independent mitochondrial and caspase-dependent plasma membrane potential depolarization: a single-cell analysis

Little is known about the temporal relationship between mitochondrial and plasma membrane potential changes and outer mitochondrial membrane permeabilization during apoptosis. Confocal imaging of breast carcinoma and HeLa cells stably transfected with cytochrome-C-GFP demonstrated that mitochondria rapidly depolarized after the release of the fusion protein into the cytosol. Of note, mitochondria did not completely depolarize but established a new steady-state level that could be further dissipated by treatment with the protonophore carbonyl cyanide p-trifluoromethoxy-phenylhydrazone. Treatment with the F(O)F(1)-ATP-synthase inhibitor oligomycin likewise induced a collapse of this steady-state level, suggesting that F(O)F(1)-ATP-synthase reversal maintained mitochondrial potential after outer mitochondrial membrane permeabilization. Treatment with a broad spectrum caspase inhibitor failed to inhibit the partial depolarization of mitochondria during apoptosis, yet potently abolished the activation of effector caspases detected by fluorescence resonance energy transfer analysis in the same experiment. Interestingly, the onset of mitochondrial depolarization was always coupled with a depolarization of the plasma membrane potential. This was associated with the degradation of the regulatory Na(+)/K(+)-ATPase beta-subunit, and both events were blocked by caspase inhibition. Our results demonstrate that outer mitochondrial membrane permeabilization coordinates the depolarization of both membrane potentials during apoptosis.

The death associated protein (DAP) kinase homologue Dlk/ZIP kinase induces p19ARF- and p53-independent apoptosis

Dlk/ZIP kinase is one of five members of the death associated protein (DAP) kinase family. DAP kinase is able to induce apoptosis in a p19ARF/p53-dependent manner. We elucidated the potential role of the p19ARF/p53 pathway in Dlk/ZIP kinase-triggered cell death. Overexpression of a constitutively pro-apoptotic form of Dlk/ZIP kinase induced apoptosis in rat fibroblast cells which express wild-type p19ARF and p53. Cell death was characterised by apoptotic membrane blebbing, mitochondrial depolarisation, cytochrome c release and activation of caspase-3. However, Dlk/ZIP kinase-triggered cell death was also observed in p19ARF-deficient and p53-deficient mouse fibroblast cells. Quantitative analysis revealed that the status of p53 had no major influence on cellular susceptibility to Dlk/ZIP kinase-triggered cell death. Loss of p53 did not prevent Dlk/ZIP kinase-induced mitochondrial membrane depolarisation and release of cytochrome c. Furthermore, overexpression of Dlk/ZIP kinase did not lead to an increased expression of pro-apoptotic p53 target genes in either cell line. These data suggest that Dlk/ZIP kinase is able to trigger the mitochondrial apoptosis pathway independent of the p19ARF/p53 signalling pathway.

Apoptosis induced by disruption of the actin cytoskeleton is mediated via activation of CD95 (Fas/APO-1)

Activation of the death receptor CD95 by its ligand or by UV radiation is associated with receptor clustering. The mechanism underlying this clustering is mostly unclear. Here we show that although disruption of the actin cytoskeleton by cytochalasin B (CyB) itself induces moderate apoptosis, it enhances apoptosis in HeLa cells induced either by UV radiation or an agonistic anti-CD95 antibody. CyB augments UV-induced apoptosis independently of UV-mediated DNA damage, since induction of DNA repair by exogenous DNA repair enzymes did not alter its enhancing effect. Inhibition of caspase-8, the most upstream caspase in CD95 signaling, blocked the apoptotic effect of CyB and the enhancing effect on UV- and CD95-induced apoptosis. Confocal laser scanning microscopy revealed that (i) CyB induces CD95 clustering, (ii) enhances UV-induced CD95 clustering, and (iii) CD95 clusters colocalize with disrupted actin filaments, suggesting a link between receptor clustering and actin rearrangement. Disruption of CD95 signaling by a dominant negative mutant of the signaling protein FADD protected from CyB-induced apoptosis and prevented the UV-enhancing effect. Accordingly, both the apoptotic and the enhancing effect of CyB was reduced in epidermal cells obtained from CD95 deficient mice (lpr) when compared to wild-type mice. These data suggest that disruption of the cytoskeleton causes apoptosis via activation of CD95 and enhances UV-induced apoptosis, possibly via aiding receptor clustering.

Dominant-negative suppression of HNF-1 alpha results in mitochondrial dysfunction, INS-1 cell apoptosis, and increased sensitivity to ceramide-, but not to high glucose-induced cell death

Maturity onset diabetes of the young (MODY) 3 is a monogenic form of diabetes caused by mutations in the transcription factor hepatocyte nuclear factor (HNF)-1 alpha. We investigated the involvement of apoptotic events in INS-1 insulinoma cells overexpressing wild-type HNF-1 alpha (WT-HNF-1 alpha) or a dominant-negative mutant (DN-HNF-1 alpha) under control of a doxycycline-dependent transcriptional activator. Forty-eight h after induction of DN-HNF-1 alpha, INS-1 cells activated caspase-3 and underwent apoptotic cell death, while cells overexpressing WT-HNF-1 alpha remained viable. Mitochondrial cytochrome c release and activation of caspase-9 accompanied DN-HNF-1 alpha-induced apoptosis, suggesting the involvement of the mitochondrial apoptosis pathway. Activation of caspases was preceded by mitochondrial hyperpolarization and decreased expression of the anti-apoptotic protein Bcl-xL. Transient overexpression of Bcl-xL was sufficient to rescue INS-1 cells from DN-HNF-1 alpha-induced apoptosis. Both WT- and DN-HNF-1 alpha-expressing cells demonstrated similar increases in apoptosis when cultured at high glucose (25 mm). In contrast, induction of DN-HNF-1 alpha highly sensitized cells to ceramide toxicity. In cells cultured at low glucose, DN-HNF-1 alpha induction also caused up-regulation of the cell cycle inhibitor p27(KIP1). Therefore, our data indicate that increased sensitivity to the mitochondrial apoptosis pathway and decreased cell proliferation may account for the progressive loss of beta-cell function seen in MODY 3 subjects.

Na(+)/H(+) exchange subtype 1 inhibition during extracellular acidification and hypoxia in glioma cells

Lactacidosis is a common feature of ischaemic brain tissue, but its role in ischaemic neuropathology is still not fully understood. Na(+)/H(+) exchange, a mechanism involved in the regulation of intracellular pH (pH(i)), is activated by low pH(i). The role of Na(+)/H(+) exchange subtype 1 was investigated during extracellular acidification and subsequent pH recovery in the absence and presence of (4-isopropyl-3-methylsulphonyl-benzoyl)-guanidine methanesulfonate (HOE642, Cariporid), a new selective and powerful inhibitor of the Na(+)/H(+) exchanger subtype 1 (NHE-1). It was compared for normoxia and hypoxia in two glioma cell lines (C6 and F98). pH(i) was monitored by fluorescence spectroscopy using the intracellularly trapped pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Alterations in glial cell metabolism were characterized using high-resolution (1)H, (13)C and (31)P NMR spectroscopy of perchloric acid extracts. NHE-1 contributed to glial pH regulation, especially at pathologically low pH(i) values. NHE-1 inhibition with HOE642 during acidification caused exacerbated metabolic disorders which were prolonged during extracellular pH recovery. However, NHE-1 inhibition during hypoxia protected the energy state of glial cells.

Multiple kinetics of mitochondrial cytochrome c release in drug-induced apoptosis

We investigated cytochrome c release kinetics in response to three apoptosis-inducing agents (tumor necrosis factor-alpha, staurosporine, and valinomycin) in MCF-7/Casp-3 cells stably transfected with enhanced green fluorescent protein (EGFP)-tagged cytochrome c. All three agents induced significant caspase activation in the cultures determined by monitoring the cleavage of fluorigenic caspase substrates in extracts from drug-treated MCF-7/Casp-3 cells, albeit the valinomycin-induced activation was less pronounced. Time-lapse confocal microscopy showed that tumor necrosis factor-alpha and staurosporine caused rapid, one- or multiple-step release of cytochrome c-EGFP from mitochondria. In contrast, valinomycin-induced cytochrome c-EGFP release occurred slowly over several hours. Unlike staurosporine, the valinomycin-induced cytochrome c release was not associated with translocation of the proapoptotic Bax protein to the mitochondria, and was not accompanied by co-release of the proapoptotic Smac protein. Immunoprecipitation experiments revealed that cytochrome c was also released out of the cell into the extracellular space before loss of plasma membrane integrity. Our data indicate the existence of multiple kinetics of cytochrome c release in drug-induced apoptosis.

Dissipation of potassium and proton gradients inhibits mitochondrial hyperpolarization and cytochrome c release during neural apoptosis

Exposure of rat hippocampal neurons or human D283 medulloblastoma cells to the apoptosis-inducing kinase inhibitor staurosporine induced rapid cytochrome c release from mitochondria and activation of the executioner caspase-3. Measurements of cellular tetramethylrhodamine ethyl ester fluorescence and subsequent simulation of fluorescence changes based on Nernst calculations of fluorescence in the extracellular, cytoplasmic, and mitochondrial compartments revealed that the release of cytochrome c was preceded by mitochondrial hyperpolarization. Overexpression of the anti-apoptotic protein Bcl-xL, but not pharmacological blockade of outward potassium currents, inhibited staurosporine-induced hyperpolarization and apoptosis. Dissipation of mitochondrial potassium and proton gradients by valinomycin or carbonyl cyanide p-trifluoromethoxy-phenylhydrazone also potently inhibited staurosporine-induced hyperpolarization, cytochrome c release, and caspase activation. This effect was not attributable to changes in cellular ATP levels. Prolonged exposure to valinomycin induced significant matrix swelling, and per se also caused release of cytochrome c from mitochondria. In contrast to staurosporine, however, valinomycin-induced cytochrome c release and cell death were not associated with caspase-3 activation and insensitive to Bcl-xL overexpression. Our data suggest two distinct mechanisms for mitochondrial cytochrome c release: (1) active cytochrome c release associated with early mitochondrial hyperpolarization, leading to neuronal apoptosis, and (2) passive cytochrome c release secondary to mitochondrial depolarization and matrix swelling.