Abstract B098: GPX4 is a broadly shared gene vulnerability among residual tumors

The heterogeneity of tumor cells underlies acquired drug resistance to a variety of therapeutic approaches. The advent of next-generation sequencing has facilitated a wave of studies identifying genetic mutations, which may be preexisting or acquired during treatment, that drive drug resistance and tumor relapse. However, it has recently become clear that non-mutational mechanisms of drug resistance, such as cell state switching from an epithelial to mesenchymal state, can also play an important role in the process of acquired drug resistance. Non-mutational drug resistance is relatively poorly understood and represents fertile ground for the discovery of novel therapeutic targets. Drug-tolerant “persister” cells are an experimental model of non-mutational cancer drug resistance in which small fractions (<5%) of cells within cancer cell lines survive cytotoxic drug exposure despite lacking resistance-conferring mutations. These residual surviving persister cells occupy a reversible quiescent state with a unique chromatin landscape. Persister cells regrow and become resensitized to drug, reminiscent of clinical observations of secondary responses from retreatment after a drug holiday. Persister cells also eventually obtain genetic mutations and reenter the cell cycle after weeks or months of continuous drug exposure, modeling the process of acquisition of resistance-conferring genetic mutations in patients during treatment. Here, we report on our efforts to identify a widely shared gene vulnerability in persister cells that transcends tissue lineage, genetic mutation background, and drug treatment regimens. Through a functional genomics approach entailing RNA-seq, pathway analysis, and a focused chemical inhibitor screen, we have identified a gene, glutathione peroxidase 4 (GPX4), that is specifically essential to persister cells. When GPX4 is chemically inhibited or genetically ablated, persister cells across a wide range of tissue lineages undergo ferroptosis–a recently discovered mechanism of non-apoptotic caspase-independent cell death. Ferroptosis occurs when lipid peroxides accumulate in cells, and as the only human enzyme capable of scavenging lipid peroxides, GPX4 plays a key role in preventing ferroptosis. Compared to drug-naïve parental cells or nontransformed normal cells, persister cells are strongly differentially sensitive to GPX4 inhibition and ferroptosis. This sensitivity is the result of a disabled antioxidant program in persister cells marked by a global downregulation of antioxidant genes including Nrf2 targets, and decreased levels of reducing cofactors glutathione and NADPH. As a first step toward raising GPX4 as a promising preclinical drug target in vivo, we also show that targeting GPX4 in residual melanoma xenograft tumors prevents tumor relapse. Therefore, GPX4 is an extremely promising drug target that may be exploited to prevent tumor relapse across a wide spectrum of tumor types and drug treatments.

Citation Format: Matthew J. Hangauer, Vasanthi S. Viswanathan, Matthew J. Ryan, Dhruv Bole, John K. Eaton, Alexandre Matov, Jacqueline Galeas, Harshil D. Dhruv, Michael E. Berens, Stuart L. Schreiber, Frank McCormick, Michael T. McManus. GPX4 is a broadly shared gene vulnerability among residual tumors [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B098.

Drug-tolerant persister cancer cells are vulnerable to GPX4 inhibition

Acquired drug resistance prevents cancer therapies from achieving stable and complete responses.¹ Emerging evidence implicates a key role for nonmutational drug resistance mechanisms underlying the survival of residual cancer “persister” cells.^2-4 The persister cell pool constitutes a reservoir from which drug-resistant tumours may emerge. Targeting persister cells therefore presents a therapeutic opportunity to impede tumour relapse.⁵ In an earlier report, we found that cancer cells in a high mesenchymal therapy-resistant cell state are dependent on the lipid hydroperoxidase GPX4 for survival.⁶ Here, we describe the discovery that a similar therapy-resistant cell state underlies the behavior of persister cells derived from a wide range of cancers and drug treatments. Consequently, we show that persister cells acquire a dependency on GPX4. We demonstrate that loss of GPX4 function results in selective persister cell ferroptotic death in vitro and prevents tumour relapse in vivo. These findings support targeting GPX4 as a therapeutic strategy to prevent acquired drug resistance.

Abstract 243: Investigating microtubule growth dynamics in patient-derived metastatic prostate cancer cells

Advanced prostate cancer (PC) is treated primarily by means of chemotherapy with one of the clinically approved microtubule (MT)-targeting agents (MTAs): paclitaxel, docetaxel and cabazitaxel. The mechanism of action of the MTAs on the cellular level is to bind to MTs and inhibit MT-dependent intracellular pathways, ultimately leading to cell death. In PC, MTAs are the only chemotherapy class shown to improve survival, however, there is high variability of response to MTA chemotherapy among patients as well as emergence of resistance to drug action which hampers the clinical efficacy. Clinicians need additional information to match individual patient tumors to the most effective drugs or refrain from treating patients that will not respond. However, the precise mechanism of action or resistance development to MTAs is not understood, which represents a critical gap in our knowledge to select the best treatment. Our hypothesis is that there are inherent differences in tumor MT dynamics among individual patients which dictates their response to treatment with specific MTAs. To this end, we envision that a systematic characterization of PC MT dynamics and their response to MTAs will allow chemotherapy customization and prolong survival. Specifically, our approach consists of the analysis of metastatic castrate resistant PC (mCRPC) organoids (Gao D. et al, 2014) derived from bone (acetabulum, vertebrae), soft tissue (salvage prostatectomy, lymph nodes), pleural effusion and circulating tumor cells biopsy samples obtained from metastatic patients who have received hormonal therapy and the MTA docetaxel or hormonal therapy alone or no treatment. We apply a quantitative single-cell image analysis algorithm (ClusterTrack) to measure a panel of twelve distinct MT dynamics parameters (a MT dynamics signature). Preliminary analysis suggests differences between different mCRPC organoids. MT tracking results indicate a correlation of decreased MT growth rate with increased Op18/stathmin expression. In this context, we hypothesize that differential expression of MT regulating genes affects specific parameters of MT dynamics, thus determining the sensitivity to MTA treatment.

Citation Format: Alexandre Matov, Johan de Rooij, Jay Gatlin, Julia Rohrberg, Nikta Ahmad, Rahul Aggarwal, Jeff Simko, Andrei Goga, Charles Ryan, Torsten Wittmann. Investigating microtubule growth dynamics in patient-derived metastatic prostate 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 243.

Abstract 3673: High-density lipoprotein-like nanoparticles target SR-B1 and inhibit the cellular uptake of melanoma-cell derived exosomes

Exosomes play a crucial role in the progression of cancer through the transport of a variety molecular cargo, including proteins, lipids, and nucleic acids, to and from cells as a means of intercellular communication. Unraveling mechanisms of exosome-cell interactions may open avenues for studying cellular communication and lead to new therapies. Cellular exosome uptake depends on cholesterol-rich membrane microdomains called lipid rafts. Non-specific depletion of lipid raft cholesterol reduces cellular exosome uptake; however, to our knowledge, no targeted mechanism of inhibiting cellular exosome uptake has been reported. Scavenger receptor type B-1 (SR-B1) localizes to lipid rafts, and is a high-affinity receptor for cholesterol-rich high-density lipoproteins (HDL). SR-B1 is an intriguing therapeutic target because it is upregulated in many different cancers due to the high need for cholesterol of rapidly dividing cancer cells. Therefore, we hypothesized that specific targeting of SR-B1 and modulation of cholesterol flux through this receptor with biomimetic HDL-like nanoparticles (HDL NPs) would disrupt cellular exosome uptake. As a model, we explored exosomes derived from melanoma cells as they have been shown to promote angiogenesis and immunosuppression both crucial events in melanoma progression. Melanoma exosomes have also been shown to actively prepare metastatic sites, creating a suitable microenvironment allowing for the development of metastasis. Because of this, targeting exosomes and intercellular signaling could be beneficial for the treatment of metastatic melanoma. Using a variety of techniques including confocal microscopy, flow cytometry and automated image analysis, data demonstrate that HDL NPs specifically target SR-B1 in lipid rafts in melanoma cells and modulate cholesterol flux through this receptor. This leads to a clustering of SR-B1 at the cell membrane and potent inhibition of the cellular uptake of melanoma cell-derived exosomes.

Citation Format: Michael P. Plebanek, Alexandre Matov, Kannan Mautharasan, Jesse Gatlin, C. Shad Thaxton. High-density lipoprotein-like nanoparticles target SR-B1 and inhibit the cellular uptake of melanoma-cell derived exosomes. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3673. doi:10.1158/1538-7445.AM2015-3673

Abstract 1158: Microtubule perturbation regulates remodeling of tumor microenvironment by modulating the composition of tumor cell secretome

Microtubules (MTs) represent one of the most effective targets in cancer chemotherapy. However, MT-targeting drugs, such as the taxanes, often fail in the metastatic setting and mechanisms underlying drug resistance are poorly elucidated. An important process during tumor development and metastasis involves the dynamic remodeling of the 3D microenvironment surrounding the tumor cells which enables them to successfully proliferate and metastasize. We hypothesized that tumor cell MTs may regulate the cellular signaling process that controls tumor-mediated remodeling of the microenvironment by paracrine activation of fibroblasts. To assess the role of MT dynamics in the secretion of factors from the tumor cells that mediate communication with the microenvironment, we collected conditioned medium (CM) from untreated or paclitaxel (PTX)-pre-treated metastatic breast cancer MDA-MB-231 cells and applied it to HMF3S human mammary fibroblasts. We observed that CM from untreated 231 cells resulted in fibroblast activation as evidenced by their increased directional 3D cell motility towards a serum gradient. CM from PTX-pretreated 231 cells, at concentrations that suppress MT dynamics, but not from the PTX-resistant β-tubulin mutation 231 clone, 231K20T cells, did not result in activation of fibroblasts' 3D motility, suggesting that MT-mediated secretion of soluble factor(s) (secretome) underlies fibroblast activation. Mass-spectrometry analysis of CM derived from untreated or PTX-pretreated 231 cells identified proteins whose secretion was diminished following suppression of MT dynamics and which are involved in cell motility, cell-cell communication, or ECM remodeling, such as TGFβ, CTGF, c-Met, fibronectin, and lysyl oxidase 2. Treatment of 231-derived CM with an anti-TGFβ blocking antibody diminished HMF3S fibroblast motility in 3D, suggesting that the presence of TGFβ in the tumor cell secretome mediated the activation of tumor associated fibroblasts. To further probe the MT-dependent regulation of TGFβ secretion we showed that PTX treatment prevented trafficking of intracellular TGFβ to the cell surface in 231 cells, likely underlying its reduced secretion. Confocal reflectance and two-photon microscopic imaging revealed that CM treatment of HMF3S fibroblasts induced significant collagen remodeling comparable to treatment with exogenous TGFβ alone. Our findings point to a novel role of interphase MTs in tumor cell secretome and raise the possibility that MTs may regulate secretion of distinct soluble factors in different types of tumor cells, which then mediate the communication between tumor cell compartment and the tumor microenvironment. These novel insights have significant implications for the mechanism of action and resistance to MT inhibitors.

Citation Format: Katsuhiro Kita, Andy Tran, Lewis M. Brown, Duane C. Hassane, Shawn Carey, Alexandre Matov, Cynthia A. Reinhart-King, Paraskevi Giannakakou. Microtubule perturbation regulates remodeling of tumor microenvironment by modulating the composition of tumor cell secretome. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1158. doi:10.1158/1538-7445.AM2014-1158

Abstract 923: Analysis of microtubule perturbations and androgen receptor localization in circulating tumor cells from castration resistant prostate cancer patients as predictive biomarkers of clinical response to docetaxel chemotherapy

Although the recent availability of novel treatment for castration resistant prostate cancer (CRPC) has shown improvements in overall survival, CRPC is incurable and lethal disease. It has been recognized that tumor progression despite castrate levels of androgens is still associated with active signaling from the androgen receptor (AR) which affects gene transcription following its nuclear accumulation. Taxanes bind to and stabilize cellular microtubules (MTs) perturbing the fine and intricate organization of the microtubule network thus, impairing MT-based cell processes like cell division and intracellular trafficking. Our recent work showed that AR translocation from the cytoplasm to the nucleus is MT-dependent and that taxane treatment sequesters AR into the cytoplasm inhibiting the receptors transcriptional activity. While this novel mechanism of taxane activity may explain the clinical efficacy of taxanes in CRPC, clinically we still fail to understand the molecular basis of patient response or resistance to taxanes. We hypothesized that engagement of the MT-AR pathway by the taxanes can be used as read-out of effective drug-target engagement in cancer cells and can be used as biomarker predictive of taxane activity in CRPC patients.

To assess the predictive role of these biomarkers, we are currently conducting a prospective clinical study in which we isolate circulating tumor cells (CTCs) from the peripheral blood of CRPC patient undergoing docetaxel treatment. We have enrolled 50 of the 80 total patients, from which we isolated CTCs before (at baseline) and during docetaxel treatment (at day 8 of first cycle of docetaxel and upon relapse) using two different approaches: the EpCAM-based immunomagnetic enrichment (CellSearch) and the ficolling technique to isolate unenriched PBMCs inclusive of CTCs. To avoid issues with leucocyte contamination we subject the EpCAM captured cells are to staining for DAPI , PSMA, CD-45, AR and Tubulin and image them by confocal microscopy. CTCs are defined as nucleated, PSMA+, CD-45- cells. The unenriched PBMC population is also subjected to the same multiplex confocal microscopy protocol. Microtubule network morphology and AR subcellular localization in then assessed in CTCs and each biomarker alone and in combination is correlated with clinical response to docetaxel treatment defined by PCWG2 recommendation. So far we have collected and analyzed baseline and cycle 1-Day 8 samples from all 50 patients and relapse samples from 31 patients.

Determination of effective drug-target engagement on C1D8 may allow early detection of molecular response to treatment, or lack of molecular response, which will ultimately allow for chemotherapy customization for the individual patient.

Citation Format: Shinsuke Tasaki, Matthew Sung, Alexandre Matov, Giuseppe Galletti, Elan Diamond, Neil Bander, Kathy Zhou, Scott Tagawa, David Nanus, Paraskevi Giannakakou. Analysis of microtubule perturbations and androgen receptor localization in circulating tumor cells from castration resistant prostate cancer patients as predictive biomarkers of clinical response to docetaxel chemotherapy. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 923. doi:10.1158/1538-7445.AM2014-923

Androgen receptor splice variants determine taxane sensitivity in prostate cancer

Prostate cancer growth depends on androgen receptor signaling. Androgen ablation therapy induces expression of constitutively active androgen receptor splice variants that drive disease progression. Taxanes are a standard of care therapy in castration-resistant prostate cancer (CRPC); however, mechanisms underlying the clinical activity of taxanes are poorly understood. Recent work suggests that the microtubule network of prostate cells is critical for androgen receptor nuclear translocation and activity. In this study, we used a set of androgen receptor deletion mutants to identify the microtubule-binding domain of the androgen receptor, which encompasses the DNA binding domain plus hinge region. We report that two clinically relevant androgen receptor splice variants, ARv567 and ARv7, differentially associate with microtubules and dynein motor protein, thereby resulting in differential taxane sensitivity in vitro and in vivo. ARv7, which lacks the hinge region, did not co-sediment with microtubules or coprecipitate with dynein motor protein, unlike ARv567. Mechanistic investigations revealed that the nuclear accumulation and transcriptional activity of ARv7 was unaffected by taxane treatment. In contrast, the microtubule-interacting splice variant ARv567 was sensitive to taxane-induced microtubule stabilization. In ARv567-expressing LuCap86.2 tumor xenografts, docetaxel treatment was highly efficacious, whereas ARv7-expressing LuCap23.1 tumor xenografts displayed docetaxel resistance. Our results suggest that androgen receptor variants that accumulate in CRPC cells utilize distinct pathways of nuclear import that affect the antitumor efficacy of taxanes, suggesting a mechanistic rationale to customize treatments for patients with CRPC, which might improve outcomes.

Abstract 2888: Computational analysis of microtubule dynamics for personalized cancer therapy

Metastatic prostate cancer (PC) is treated primarily by means of taxane-based chemotherapy with one of the clinically used taxanes: paclitaxel, docetaxel and cabazitaxel. At the cellular level, the taxanes bind microtubules (MTs), inhibit MT dynamics and alter the spatial organization of the MT network. Thereby the taxanes inhibit the intracellular trafficking of transcription factors critical for tumor survival. In PC, taxanes are the only chemotherapy class shown to improve survival, however, the emergence of clinical taxane resistance hampers their clinical efficacy. In addition, patients resistant to one taxane often respond to another, yet, currently we do not have the ability to match individual patients to a specific taxane. In cells, different taxanes have differential effects on microtubule dynamics which may ultimately pre-determine their efficacy for each individual patient. Thus, we hypothesized that the particular pattern of dynamic behavior of the MT cytoskeleton in individual patients could be exploited therapeutically. Therefore, we looked into dissecting the concrete effects of each of the taxanes for several PC cells lines. Preliminary unpublished data have revealed that PC cells expressing the TMPRSS2-ERG fusion protein exhibit taxane resistance. In addition, we have recently shown that the androgen receptor (AR) binds MTs in order to traffic to the nucleus and that taxane-mediated inhibition of AR nuclear accumulation correlates with patient clinical response to taxane therapy. Moreover, we have shown that the two clinically relevant AR splice variants, ARv567 and ARv7, show differential response to taxane therapy. Therefore, we set out to investigate the hypothesis that ERG fusion and/or AR variants might modulate endogenous microtubule dynamics in a way that determines cellular response to taxane treatment. Xenografts PC models expressing the ARv7 variant exhibit reduced sensitivity to docetaxel treatment. In addition, docetaxel's ability to induce MT stabilization is significantly impaired in ERG+ cells. We have recently established isogenic PC cell line series with inducible ERG (DU145 ERG+ and ERG-), as well as M12 cells without endogenous AR that stably express, wt-AR, ARv567 and ARv7. We then tested in a systematic way endogenous MT dynamics using live cell confocal microscopy of labeled MT tips following EGFP-EB1 lentiviral transduction. We use computer vision algorithms to obtain statistically representative results for the effects of ERG or AR on microtubule homeostasis following treatment with each of the three taxanes. We measure changes in MT behavior as statistically significant shifts in different parameters of MT dynamics measured from >20,000 MTs for each condition. Our preliminary results revealed that the presence of ERG fusion a correlation between MT dynamics and AR variant expression in PC cell lines.

Citation Format: Alexandre Matov, Dragi Kimovski, Giuseppe Galletti, Nancy Chan, Benet Pera, William T. Harkcom, David S. Rickman, Paraskevi Giannakakou. Computational analysis of microtubule dynamics for personalized cancer therapy. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2888. doi:10.1158/1538-7445.AM2013-2888

Abstract 3492: Using CTCs to interrogate mechanisms of taxane resistance in the prospective TAXYNERGY clinical trial in prostate cancer

Prostate cancer progression into castration-resistant prostate cancer (CRPC) is driven by continued androgen receptor (AR) signaling despite surgical and chemical androgen ablation. The taxanes represent the only class of chemotherapy that improves overall survival in CRPC patients. Despite their success, CRPC patients do progress on taxane treatment rendering taxane-resistant tumors. The molecular mechanisms underlying clinical taxane resistance in CRPC have not been well elucidated due to the lack of available tumor tissue to study. Circulating tumor cells (CTCs) represent a liquid biopsy of the original tumor and isolation of them can lead to their molecular characterization potentially revealing predictive biomarkers for taxane sensitivity or resistance. Here, we use a geometrically enhanced differential immunocapture (GEDI) microfluidic device that couples an anti-prostate specific membrane antigen (PSMA) antibody with optimized 3D geometry to capture and isolate live CTCs from whole blood of CRPC patients. The GEDI-microfluidic device was shown to have a 2-400 fold higher sensitivity for CTC capture than the FDA-approved CellSearch® system. We have previously shown that CRPC patient CTCs can be used to derive functional information that correlates to clinical response to taxane chemotherapy, namely AR subcellular localization status. We have developed a suite of other functional assays that can be performed on live GEDI-captured CTCs that enable their molecular characterization and allow us to test specific mechanistic hypotheses based on our extensive preclinical data. Included, and herein described, are the determination of AR subcellular localization, extent of effective drug-target engagement assessed by microtubule bundling, identification of RNA species relevant to the mechanism of taxane resistance and computer vision algorithms that will allow for enriched and automated analysis of high-volume image sets of GEDI-captured CTCs. In addition, we will be testing the hypothesis that distinct AR splice variants may affect patient sensitivity to taxane-based chemotherapy. This suite of assays are being rigorously applied in a phase II clinical trial in which chemotherapy-naïve CRPC patients will be initially treated with either docetaxel or cabazitaxel and clinically evaluated for an early switch to the other taxane following disease progression. This prospective, randomized, multi-site clinical trial will enroll 100 CRPC patients within one year. Patients will be followed until relapse and each patient will have 15 independent GEDI assays performed across five time points from baseline to chemotherapy crossover to relapse. The depth of coverage this suite of assays provides will offer unique insights for potential mechanisms of clinical taxane resistance and predictive biomarkers for taxane sensitivity in CRPC patient CTCs.

Citation Format: Matthew S. Sung, Ada Gjyrezi, Guang Yu Lee, Alexandre Matov, Giuseppe Galletti, Matthew Loftus, Yusef Syed, Timothy Lannin, Atanas Hristov, Christopher Mason, Scott Tagawa, Brian Kirby, David Nanus, Paraskevi Giannakakou. Using CTCs to interrogate mechanisms of taxane resistance in the prospective TAXYNERGY clinical trial in prostate cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3492. doi:10.1158/1538-7445.AM2013-3492

Abstract 4081: Taxane sensitivity in prostate cancer is determined by androgen receptor splice variants

Although its pivotal role in prostate cancer care, androgen deprivation therapy (ADT) is not curative as many patients progress to a castration-resistant stage (CRPC) which remains driven by pathologic reactivation of androgen receptor (AR). Taxanes represent the only chemotherapy class that improves survival in CRPC and as such Docetaxel (DTX) and Cabazitaxel (CTX) are standard of care.

Recent evidence revealed the presence of several AR splice variants lacking the ligand binding domain and thus constitutively active in CRPC. Previous data from our lab showed that taxanes impair the ligand-induced nuclear accumulation and transcriptional activity of full-length AR (AR-FL) in taxane treated human prostate cancer cells and CRPC patients. Indeed we showed that AR cytoplasmic sequestration in patient's circulating tumour cells (CTCs) was significantly correlated with clinical response to taxane chemotherapy. This is due to AR binding microtubules (MTs) and using them for nuclear accumulation supported by the dynein motor protein. However, the effect of taxane on the nuclear accumulation and activity of the AR variants is unknown. Thus, we set out to investigate the mechanism underlying AR variant nuclear accumulation and the impact of taxane treatment on AR variant function focusing on the two clinically relevant AR splice variants ARv567 and AR-V7.

A microtubule co-sedimentation assay revealed that ARv567 is associated with MTs, in contrast to AR-V7. Serial AR mutagenesis demonstrated that the MT binding domain comprises a region aa 559 - 663 which is missing in AR-V7. Dynamitin overexpression inhibited the nuclear accumulation of AR-FL and ARv567 but had no effect on AR-V7, suggesting that AR-V7’s nuclear translocation is independent of dynein-based MT transport. To examine the impact of taxane treatment on variant activity we microinjected GFP-tagged AR-FL, or AR variants into the nucleus of PC3 cells and monitored the dynamics of AR nuclear translocation using live-cell confocal microscopy. Moreover we stably transfected M12 cells with AR-FL, AR-V7 and AR-v567 and assessed AR's nuclear translocation in response to R1881 and in the presence or absence of DTX. Our data showed that taxanes significantly inhibited the nuclear accumulation and activity of AR-FL and ARv567 but not that of AR-V7 and that this effect is androgen independent. We are currently elucidating the effect of taxanes on AR variant transcriptional activity using luciferase reporter and specific genes expression assays. Taken together our data revealed that functional MTs are required for AR's nuclear transport in a ligand independent manner, as in the case of ARv567. However, this does not apply to AR-V7 which is not under MT control and thus insensitive to taxane treatment. These data suggest that impairment of AR variant expression might be predictive of clinical taxane sensitivity in CRPC.

Citation Format: Luigi Portella, Maria Thadani-Mulero, Alexandre Matov, David M. Nanus, Stephen R. Plymate, Paraskevi Giannakakou. Taxane sensitivity in prostate cancer is determined by androgen receptor splice variants. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4081. doi:10.1158/1538-7445.AM2013-4081

Abstract LB-59: ERG induces taxane resistance in castration-resistant prostate cancer

Taxanes are the only chemotherapies to prolong survival for patients with metastatic castration-resistant prostate cancer (CRPC), and both docetaxel (doc) and cabazitaxel (cab) are FDA approved for this indication. Despite the initial efficacy of taxanes in treating CRPC, all patients ultimately fail due to the development of drug resistance. In this study we show that ERG over-expression, which occurs in roughly 50% of prostate cancers, is associated with both doc and cab resistance using in vitro and in vivo models of CRPC. Using either tetracycline-inducible or constitutively active expression systems in multiple prostate cancer cell lines, we found that ERG over-expression leads to at least a 10 fold increase in IC50 doses of doc or cab based on cell viability assays and inhibition of taxane-induced apoptosis. Similarly, ERG over-expressing prostate cancer xenografts were reproducibly more resistant to cab compared to the cab-sensitive control xenografts. Based on a biochemical and cell biological characterization, we observed that ERG binds directly to tubulin in the cytoplasm of prostate cancer cells, as well as in circulating tumor cells from patients treated with doc, and prevents taxane-induced microtubule stabilization. ERG over-expression affects several parameters of microtubule dynamics and inhibits effective drug-target engagement of doc or cab with tubulin, either directly via its tubulin interaction or indirectly by affecting transcription of genes that modulate microtubule homeostasis. In addition, ERG induces and binds to clusterin, a molecular chaperone also associated with taxane resistance. Clusterin knock-down with siRNA abrogates ERG-induced taxane resistance. Altogether, this data suggests that ERG, an oncogenic transcription factor, plays a novel role beyond regulating gene expression and functions outside the nucleus to cooperate with clusterin and tubulin in promoting taxane resistance. Determining ERG status in patient tumors may aid in patient selection for doc or cab therapy and/or influence co-targeting approaches using the clusterin antisense inhibitor OGX-011.

Citation Format: Giuseppe Galletti, Himisha Beltran, Alexandre Matov, Jacqueline Fontugne, Juan Miguel Mosquera, Ladan Fazli, Scott Tagawa, David Nanus, Martin Gleave, Mark Rubin, Paraskevi Giannakakou, David S. Rickman. ERG induces taxane resistance in castration-resistant prostate cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr LB-59. doi:10.1158/1538-7445.AM2013-LB-59

plusTipTracker: Quantitative image analysis software for the measurement of microtubule dynamics

Here we introduce plusTipTracker, a Matlab-based open source software package that combines automated tracking, data analysis, and visualization tools for movies of fluorescently-labeled microtubule (MT) plus end binding proteins (+TIPs). Although +TIPs mark only phases of MT growth, the plusTipTracker software allows inference of additional MT dynamics, including phases of pause and shrinkage, by linking collinear, sequential growth tracks. The algorithm underlying the reconstruction of full MT trajectories relies on the spatially and temporally global tracking framework described in Jaqaman et al. (2008). Post-processing of track populations yields a wealth of quantitative phenotypic information about MT network architecture that can be explored using several visualization modalities and bioinformatics tools included in plusTipTracker. Graphical user interfaces enable novice Matlab users to track thousands of MTs in minutes. In this paper, we describe the algorithms used by plusTipTracker and show how the package can be used to study regional differences in the relative proportion of MT subpopulations within a single cell. The strategy of grouping +TIP growth tracks for the analysis of MT dynamics has been introduced before (Matov et al., 2010). The numerical methods and analytical functionality incorporated in plusTipTracker substantially advance this previous work in terms of flexibility and robustness. To illustrate the enhanced performance of the new software we thus compare computer-assembled +TIP-marked trajectories to manually-traced MT trajectories from the same movie used in Matov et al. (2010).

Optimal-Flow Minimum-Cost Correspondence Assignment in Particle Flow Tracking

A diversity of tracking problems exists in which cohorts of densely packed particles move in an organized fashion, however the stability of individual particles within the cohort is low. Moreover, the flows of cohorts can regionally overlap. Together, these conditions yield a complex tracking scenario that can not be addressed by optical flow techniques that assume piecewise coherent flows, or by multiparticle tracking techniques that suffer from the local ambiguity in particle assignment. Here, we propose a graph-based assignment of particles in three consecutive frames to recover from image sequences the instantaneous organized motion of groups of particles, i.e. flows. The algorithm makes no a priori assumptions on the fraction of particles participating in organized movement, as this number continuously alters with the evolution of the flow fields in time. Graph-based assignment methods generally maximize the number of acceptable particles assignments between consecutive frames and only then minimize the association cost. In dense and unstable particle flow fields this approach produces many false positives. The here proposed approach avoids this via solution of a multi-objective optimization problem in which the number of assignments is maximized while their total association cost is minimized at the same time. The method is validated on standard benchmark data for particle tracking. In addition, we demonstrate its application to live cell microscopy where several large molecular populations with different behaviors are tracked.

Quantitative image analysis identifies pVHL as a key regulator of microtubule dynamic instability

The product of the von Hippel-Lindau tumor suppressor gene stabilizes microtubules by inhibiting GTPase activity.

Von Hippel-Lindau (VHL) tumor suppressor gene mutations predispose carriers to kidney cancer. The protein pVHL has been shown to interact with microtubules (MTs), which is critical to cilia maintenance and mitotic spindle orientation. However, the function for pVHL in the regulation of MT dynamics is unknown. We tracked MT growth via the plus end marker EB3 (end-binding protein 3)-GFP and inferred additional parameters of MT dynamics indirectly by spatiotemporal grouping of growth tracks from live cell imaging. Our data establish pVHL as a near-optimal MT-stabilizing protein: it attenuates tubulin turnover, both during MT growth and shrinkage, inhibits catastrophe, and enhances rescue frequencies. These functions are mediated, in part, by inhibition of tubulin guanosine triphosphatase activity in vitro and at MT plus ends and along the MT lattice in vivo. Mutants connected to the VHL cancer syndrome are differentially compromised in these activities. Thus, single cell–level analysis of pVHL MT regulatory function allows new predictions for genotype to phenotype associations that deviate from the coarser clinically defined mutant classifications.

Directly probing the mechanical properties of the spindle and its matrix

Several recent models for spindle length regulation propose an elastic pole to pole spindle matrix that is sufficiently strong to bear or antagonize forces generated by microtubules and microtubule motors. We tested this hypothesis using microneedles to skewer metaphase spindles in Xenopus laevis egg extracts. Microneedle tips inserted into a spindle just outside the metaphase plate resulted in spindle movement along the interpolar axis at a velocity slightly slower than microtubule poleward flux, bringing the nearest pole toward the needle. Spindle velocity decreased near the pole, which often split apart slowly, eventually letting the spindle move completely off the needle. When two needles were inserted on either side of the metaphase plate and rapidly moved apart, there was minimal spindle deformation until they reached the poles. In contrast, needle separation in the equatorial direction rapidly increased spindle width as constant length spindle fibers pulled the poles together. These observations indicate that an isotropic spindle matrix does not make a significant mechanical contribution to metaphase spindle length determination.

GSK3beta phosphorylation modulates CLASP-microtubule association and lamella microtubule attachment

Polarity of the microtubule (MT) cytoskeleton is essential for many cell functions. Cytoplasmic linker–associated proteins (CLASPs) are MT-associated proteins thought to organize intracellular MTs and display a unique spatiotemporal regulation. In migrating epithelial cells, CLASPs track MT plus ends in the cell body but bind along MTs in the lamella. In this study, we demonstrate that glycogen synthase kinase 3β (GSK3β) directly phosphorylates CLASPs at multiple sites in the domain required for MT plus end tracking. Although complete phosphorylation disrupts both plus end tracking and association along lamella MTs, we show that partial phosphorylation of the identified GSK3β motifs determines whether CLASPs track plus ends or associate along MTs. In addition, we find that expression of constitutively active GSK3β destabilizes lamella MTs by disrupting lateral MT interactions with the cell cortex. GSK3β-induced lamella MT destabilization was partially rescued by expression of CLASP2 with mutated phosphorylation sites. This indicates that CLASP-mediated stabilization of peripheral MTs, which likely occurs in the vicinity of focal adhesions, may be regulated by local GSK3β inactivation.

Spindle fusion requires dynein-mediated sliding of oppositely oriented microtubules

BACKGROUND

Bipolar spindle assembly is critical for achieving accurate segregation of chromosomes. In the absence of centrosomes, meiotic spindles achieve bipolarity by a combination of chromosome-initiated microtubule nucleation and stabilization and motor-driven organization of microtubules. Once assembled, the spindle structure is maintained on a relatively long time scale despite the high turnover of the microtubules that comprise it. To study the underlying mechanisms responsible for spindle assembly and steady-state maintenance, we used microneedle manipulation of preassembled spindles in Xenopus egg extracts.

RESULTS

When two meiotic spindles were brought close enough together, they interacted, creating an interconnected microtubule structure with supernumerary poles. Without exception, the perturbed system eventually re-established bipolarity, forming a single spindle of normal shape and size. Bipolar spindle fusion was blocked when cytoplasmic dynein function was perturbed, suggesting a critical role for the motor in this process. The fusion of Eg5-inhibited monopoles also required dynein function but only occurred if the initial interpolar separation was less than twice the microtubule radius of a typical monopole.

CONCLUSIONS

Our experiments uniquely illustrate the architectural plasticity of the spindle and reveal a robust ability of the system to attain a bipolar morphology. We hypothesize that a major mechanism driving spindle fusion is dynein-mediated sliding of oppositely oriented microtubules, a novel function for the motor, and posit that this same mechanism might also be involved in normal spindle assembly and homeostasis.

Periodic patterns of actin turnover in lamellipodia and lamellae of migrating epithelial cells analyzed by quantitative Fluorescent Speckle Microscopy

We measured actin turnover in lamellipodia and lamellae of migrating cells, using quantitative Fluorescent Speckle Microscopy. Lamellae disassembled at low rates from the front to the back. However, the dominant feature in their turnover was a spatially random pattern of periodic polymerization and depolymerization moving with the retrograde flow. Power spectra contained frequencies between 0.5 and 1 cycle/min. The spectra remained unchanged when applying Latrunculin A and Jasplakinolide in low doses, except that additional frequencies occurred beyond 1 cycle/min. Whereas Latrunculin did not change the rate of mean disassembly, Jasplakinolide halted it completely, indicating that the steady state and the dynamics of actin turnover are differentially affected by pharmacological agents. Lamellipodia assembled in recurring bursts at the leading edge and disassembled approximately 2.5 microm behind. Events of polymerization correlated spatially and temporally with transient formation of Arp2/3 clusters. In lamellae, Arp2/3 accumulation and polymerization correlated only spatially, suggesting an Arp2/3-independent mechanism for filament nucleation. To acquire these data we had to enhance the resolution of quantitative Fluorescent Speckle Microscopy to the submicron level. Several algorithmic advances in speckle image processing are described enabling the analysis of kinetic and kinematic activities of polymer networks at the level of single speckles.

Signal analysis of total internal reflection fluorescent speckle microscopy (TIR-FSM) and wide-field epi-fluorescence FSM of the actin cytoskeleton and focal adhesions in living cells

Fluorescent speckle microscopy (FSM) uses low levels of fluorescent proteins to create fluorescent speckles on cytoskeletal polymers in high-resolution fluorescence images of living cells. The dynamics of speckles over time encode subunit turnover and motion of the cytoskeletal polymers. We sought to improve on current FSM technology by first expanding it to study the dynamics of a non-polymeric macromolecular assembly, using focal adhesions as a test case, and second, to exploit for FSM the high contrast afforded by total internal reflection fluorescence microscopy (TIR-FM). Here, we first demonstrate that low levels of expression of a green fluorescent protein (GFP) conjugate of the focal adhesion protein, vinculin, results in clusters of fluorescent vinculin speckles on the ventral cell surface, which by immunofluorescence labelling of total vinculin correspond to sparse labelling of dense focal adhesion structures. This demonstrates that the FSM principle can be applied to study focal adhesions. We then use both GFP-vinculin expression and microinjected fluorescently labelled purified actin to compare quantitatively the speckle signal in FSM images of focal adhesions and the actin cytoskeleton in living cells by TIR-FM and wide-field epifluorescence microscopy. We use quantitative FSM image analysis software to define two new parameters for analysing FSM signal features that we can extract automatically: speckle modulation and speckle detectability. Our analysis shows that TIR-FSM affords major improvements in these parameters compared with wide-field epifluorescence FSM. Finally, we find that use of a crippled eukaryotic expression promoter for driving low-level GFP-fusion protein expression is a useful tool for FSM imaging. When used in time-lapse mode, TIR-FSM of actin and GFP-conjugated focal adhesion proteins will allow quantification of molecular dynamics within interesting macromolecular assemblies at the ventral surface of living cells.