Detection of phenotype-specific therapeutic vulnerabilities in breast cells using a CRISPR loss-of-function screen

Cellular phenotype plasticity between the epithelial and mesenchymal states has been linked to metastasis and heterogeneous responses to cancer therapy, and remains a challenge for the treatment of triple-negative breast cancer (TNBC). Here, we used isogenic human breast epithelial cell lines, D492 and D492M, representing the epithelial and mesenchymal phenotypes, respectively. We employed a CRISPR-Cas9 loss-of-function screen targeting a 2240-gene 'druggable genome' to identify phenotype-specific vulnerabilities. Cells with the epithelial phenotype were more vulnerable to the loss of genes related to EGFR-RAS-MAPK signaling, while the mesenchymal-like cells had increased sensitivity to knockout of G₂ -M cell cycle regulators. Furthermore, we discovered knockouts that sensitize to the mTOR inhibitor everolimus and the chemotherapeutic drug fluorouracil in a phenotype-specific manner. Specifically, loss of EGFR and fatty acid synthase (FASN) increased the effectiveness of the drugs in the epithelial and mesenchymal phenotypes, respectively. These phenotype-associated genetic vulnerabilities were confirmed using targeted inhibitors of EGFR (gefitinib), G₂ -M transition (STLC), and FASN (Fasnall). In conclusion, a CRISPR-Cas9 loss-of-function screen enables the identification of phenotype-specific genetic vulnerabilities that can pinpoint actionable targets and promising therapeutic combinations.

R2* Relaxation Affects Pharmacokinetic Analysis of Dynamic Contrast-Enhanced MRI in Cancer and Underestimates Treatment Response at 7 T

Effective transverse relaxivity of gadolinium-based contrast agents is often neglected in dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). Here, we assess time and tissue dependence of R2* enhancement and its impact on pharmacokinetic parameter quantification and treatment monitoring. Multiecho DCE-MRI was performed at 7 T on mice bearing subcutaneous TOV-21G human ovarian cancer xenografts (n = 8) and on the transgenic adenocarcinoma of the mouse prostate (TRAMP) model (n = 7). Subsequently, the TOV-21G tumor-bearing mice were treated with bevacizumab and rescanned 2 days later. Pharmacokinetic analysis (extended Tofts model) was performed using either the first echo signal only (standard single-echo DCE-MRI) or the estimated signal at TE = 0 derived from exponential fitting of R2* relaxation (R2*-corrected). Neglecting R2* enhancement causes underestimation of Gd-DOTA concentration (peak enhancement underestimated by 9.4%-16% in TOV-21G tumors and 13%-20% in TRAMP prostates). Median K^trans and v_e were underestimated in every mouse (TOV-21G K^trans: 11%-19%, TOV-21G v_e: 5.3%-8.9%; TRAMP K^trans: 8.6%-19%, TRAMP v_e: 12%-21%). Bevacizumab treatment reduced K^trans in all TOV-21G tumors after 48 hours. Treatment effect was significantly greater in all tumors after R2* correction (median change of -0.050 min^-1 in R2*-corrected K^trans vs. -0.037 min^-1 in uncorrected K^trans). R2* enhancement in DCE-MRI is both time- and tissue-dependent and may not be negligible at 7 T in tissue with high K^trans. This has consequences for the use of K^trans and other DCE-MRI parameters as biomarkers, because treatment effect size can be underestimated when R2* enhancement is neglected.

Glutamine to proline conversion is associated with response to glutaminase inhibition in breast cancer

INTRODUCTION

Glutaminase inhibitors target cancer cells by blocking the conversion of glutamine to glutamate, thereby potentially interfering with anaplerosis and synthesis of amino acids and glutathione. The drug CB-839 has shown promising effects in preclinical experiments and is currently undergoing clinical trials in several human malignancies, including triple-negative breast cancer (TNBC). However, response to glutaminase inhibitors is variable and there is a need for identification of predictive response biomarkers. The aim of this study was to determine how glutamine is utilized in two patient-derived xenograft (PDX) models of breast cancer representing luminal-like/ER+ (MAS98.06) and basal-like/triple-negative (MAS98.12) breast cancer and to explore the metabolic effects of CB-839 treatment.

EXPERIMENTAL

MAS98.06 and MAS98.12 PDX mice received CB-839 (200 mg/kg) or drug vehicle two times daily p.o. for up to 28 days (n = 5 per group), and the effect on tumor growth was evaluated. Expression of 60 genes and seven glutaminolysis key enzymes were determined using gene expression microarray analysis and immunohistochemistry (IHC), respectively, in untreated tumors. Uptake and conversion of glutamine were determined in the PDX models using HR MAS MRS after i.v. infusion of [5-¹³C] glutamine when the models had received CB-839 (200 mg/kg) or vehicle for 2 days (n = 5 per group).

RESULTS

Tumor growth measurements showed that CB-839 significantly inhibited tumor growth in MAS98.06 tumors, but not in MAS98.12 tumors. Gene expression and IHC analysis indicated a higher proline synthesis from glutamine in untreated MAS98.06 tumors. This was confirmed by HR MAS MRS of untreated tumors demonstrating that MAS98.06 used glutamine to produce proline, glutamate, and alanine, and MAS98.12 to produce glutamate and lactate. In both models, treatment with CB-839 resulted in accumulation of glutamine. In addition, CB-839 caused depletion of alanine, proline, and glutamate ([1-13C] glutamate) in the MAS98.06 model.

CONCLUSION

Our findings indicate that TNBCs may not be universally sensitive to glutaminase inhibitors. The major difference in the metabolic fate of glutamine between responding MAS98.06 xenografts and non-responding MAS98.12 xenografts is the utilization of glutamine for production of proline. We therefore suggest that addiction to proline synthesis from glutamine is associated with response to CB-839 in breast cancer. The effect of glutaminase inhibition in two breast cancer patient-derived xenograft (PDX) models. ¹³C HR MAS MRS analysis of tumor tissue from CB-839-treated and untreated models receiving ¹³C-labeled glutamine ([5-¹³C] Gln) shows that the glutaminase inhibitor CB-839 is causing an accumulation of glutamine (arrow up) in two PDX models representing luminal-like breast cancer (MAS98.06) and basal-like breast cancer (MAS98.12). In MAS98.06 tumors, CB-839 is in addition causing depletion of proline ([5-¹³C] Pro), alanine ([1-¹³C] Ala), and glutamate ([1-¹³C] Glu), which could explain why CB-839 causes tumor growth inhibition in MAS98.06 tumors, but not in MAS98.12 tumors.

NMR-based metabolomics of biofluids in cancer

This review describes the current status of NMR-based metabolomics of biofluids with respect to cancer risk assessment, detection, disease characterization, prognosis, and treatment monitoring. While the metabolism of cancer cells is altered compared with that of non-proliferating cells, the metabolome of blood and urine reflects the entire organism. We conclude that many studies show impressive associations between biofluid metabolomics and cancer progression, but translation to clinical practice is currently hindered by lack of validation, difficulties in biological interpretation, and non-standardized analytical procedures.

APIM-peptide targeting PCNA improves the efficacy of docetaxel treatment in the TRAMP mouse model of prostate cancer

Docetaxel is the chemotherapeutic choice for metastatic hormone-refractory prostate cancer, however, it only marginally improves the survival rate. The purpose of the present study was to examine if a peptide targeting the cellular scaffold protein PCNA could improve docetaxel's efficacy. We found that docetaxel given in combination with a cell penetrating peptide containing the AlkB homolog 2 PCNA interacting motif (APIM-peptide), reduced the prostate volume and limited prostate cancer regrowth in vivo in the immunocompetent transgenic adenocarcinoma model of prostate cancer (TRAMP). In accordance with this, we found that the APIM-peptide enhanced the efficacy of docetaxel in vitro. Gene expression analysis on prostate cancer cell lines indicated that the combination of docetaxel and APIM-peptide alters expression of genes involved in cellular signaling, apoptosis, and prostate cancer development. These changes were not detected in single agent treated cells. Our results suggest that targeting PCNA and thereby affecting multiple cellular pathways simultaneously has the potential to improve docetaxel therapy of advanced prostate cancer.

Inhibition of O-GlcNAc transferase activity reprograms prostate cancer cell metabolism

Metabolic networks are highly connected and complex, but a single enzyme, O-GlcNAc transferase (OGT) can sense the availability of metabolites and also modify target proteins. We show that inhibition of OGT activity inhibits the proliferation of prostate cancer cells, leads to sustained loss of c-MYC and suppresses the expression of CDK1, elevated expression of which predicts prostate cancer recurrence (p=0.00179). Metabolic profiling revealed decreased glucose consumption and lactate production after OGT inhibition. This decreased glycolytic activity specifically sensitized prostate cancer cells, but not cells representing normal prostate epithelium, to inhibitors of oxidative phosphorylation (rotenone and metformin). Intra-cellular alanine was depleted upon OGT inhibitor treatment. OGT inhibitor increased the expression and activity of alanine aminotransferase (GPT2), an enzyme that can be targeted with a clinically approved drug, cycloserine. Simultaneous inhibition of OGT and GPT2 inhibited cell viability and growth rate, and additionally activated a cell death response. These combinatorial effects were predominantly seen in prostate cancer cells, but not in a cell-line derived from normal prostate epithelium. Combinatorial treatments were confirmed with two inhibitors against both OGT and GPT2. Taken together, here we report the reprogramming of energy metabolism upon inhibition of OGT activity, and identify synergistically lethal combinations that are prostate cancer cell specific.

Non-Invasive Prostate Cancer Characterization with Diffusion-Weighted MRI: Insight from In silico Studies of a Transgenic Mouse Model

Diffusion-weighted magnetic resonance imaging (DWI) enables non-invasive, quantitative staging of prostate cancer via measurement of the apparent diffusion coefficient (ADC) of water within tissues. In cancer, more advanced disease is often characterized by higher cellular density (cellularity), which is generally accepted to correspond to a lower measured ADC. A quantitative relationship between tissue structure and in vivo measurements of ADC has yet to be determined for prostate cancer. In this study, we establish a theoretical framework for relating ADC measurements with tissue cellularity and the proportion of space occupied by prostate lumina, both of which are estimated through automatic image processing of whole-slide digital histology samples taken from a cohort of six healthy mice and nine transgenic adenocarcinoma of the mouse prostate (TRAMP) mice. We demonstrate that a significant inverse relationship exists between ADC and tissue cellularity that is well characterized by our model, and that a decrease of the luminal space within the prostate is associated with a decrease in ADC and more aggressive tumor subtype. The parameters estimated from our model in this mouse cohort predict the diffusion coefficient of water within the prostate-tissue to be 2.18 × 10-3 mm2/s (95% CI: 1.90, 2.55). This value is significantly lower than the diffusion coefficient of free water at body temperature suggesting that the presence of organelles and macromolecules within tissues can drastically hinder the random motion of water molecules within prostate tissue. We validate the assumptions made by our model using novel in silico analysis of whole-slide histology to provide the simulated ADC (sADC); this is demonstrated to have a significant positive correlation with in vivo measured ADC (r2 = 0.55) in our mouse population. The estimation of the structural properties of prostate tissue is vital for predicting and staging cancer aggressiveness, but prostate tissue biopsies are painful, invasive, and are prone to complications such as sepsis. The developments made in this study provide the possibility of estimating the structural properties of prostate tissue via non-invasive virtual biopsies from MRI, minimizing the need for multiple tissue biopsies and allowing sequential measurements to be made for prostate cancer monitoring.

Abstract 4412: Metabolic reprogramming in EMT - targeting regulatory nodes in mesenchymal cells

To combat cancer we have to avoid development of resistant and metastatic disease. Breast cancer cells can switch from an epithelial to mesenchymal phenotype through a process called epithelial to mesenchymal transition/EMT. Emerging evidence suggests that this process is vital to avoid treatment pressure and to gain metastatic capacity. Furthermore, recent literature shows that metabolic reprogramming is an essential attribute of cellular plasticity. Metabolic targeting could therefore be an attractive possibility to prevent development of resistance and metastatic dissemination. Here we tried to understand the metabolic phenotype of EMT and the mechanisms linking the metabolic programs to cellular plasticity. We also aimed to unravel compensatory metabolic pathways and use the metabolic inhibitors to sensitize breast cancer cells to conventional therapy.

To that end we have investigated the metabolic signature of the D492 EMT cell model. The D492 cell line, established from human breast epithelial progenitor cells, has retained stem cell characteristics and has the ability to undergo EMT upon stromal (endothelial) influence, forming the mesenchymal D492M cells. Thus, D492 cell system has preserved the natural flexibility of breast epithelial progenitor cells, and constitutes a unique platform to unravel the factors responsible for stromal cell-induced cellular plasticity.

We show that metabolic reprogramming is essential for induction of the mesenchymal phenotype using metabolomic profiling. Using Ultra performance liquid chromatography Mass Spectrometry and gene expression profiling we have created genome-scale metabolic models of D492 and D492M. Our data show that glycolytic flux and oxidative phosphorylation is higher in D492, however, D492M cells rely more on amino acid anaplerosis and fatty acid oxidation to fuel the TCA cycle. Glutamine and glucose tracing using NMR will give further insight into the difference in metabolism between the two cell lines.

We have used these data to find metabolic targets that lock the cells in the epithelial state or identify the means to induce lethality in the mesenchymal cells.

Using the metabolic rewiring of EMT in the D492 cell model we can understand the mechanisms responsible for treatment resistance, identify compensatory metabolic pathways during treatment and find metabolic inhibitors that will sensitize BC cells to conventional therapy.

Citation Format: Bylgja Hilmarsdottir, Skarphedinn Halldorsson, Maria T. Grinde, Anna Barkovskaya, Solveig Pettersen, Thorarinn Gudjonsson, Siver A. Moestue, Ottar Rolfsson, Gunhild M. Mælandsmo. Metabolic reprogramming in EMT - targeting regulatory nodes in mesenchymal cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4412. doi:10.1158/1538-7445.AM2017-4412

Pharmacokinetics of Perfluorobutane after Intra-Venous Bolus Injection of Sonazoid in Healthy Chinese Volunteers

Sonazoid is an ultrasound contrast agent based on microbubbles (MB) containing perfluorobutane (PFB) gas. Sonazoid is approved in Japan, Korea and Norway for contrast-enhanced ultrasonography of focal liver lesions and focal breast lesions (Japan only). The objective of this study was to determine the pharmacokinetics (PKs) and safety of Sonazoid in Chinese healthy volunteers (HVs) and to evaluate the potential for ethnic differences in PKs between Chinese and Caucasian HVs. Sonazoid was administered as an intra-venous bolus injection at the clinical dose of 0.12 μL or 0.60 μL MB/kg body weight to two groups of eight Chinese HVs. Expired air and blood samples were collected and analyzed using a validated gas chromatographic tandem mass spectrometry method, and the main PK parameters were calculated. The highest PFB concentrations in blood were observed shortly after intra-venous administration of Sonazoid, and elimination of PFB was rapid. In the 0.12 μL MB/kg body weight cohort, PFB concentrations above the limit of quantification were observed for only 10 to 15 min post-injection. In the 0.60 μL MB/kg body weight cohort, PFB concentrations above the limit of quantification were observed for 60 min post-injection, and the shape of the elimination curve suggested a biphasic elimination profile. The maximum observed concentration (C_max) values of PFB in blood were 2.3 ± 1.1 and 19.1 ± 9.2 ng/g for the 0.12 and 0.60 μL MB/kg body weight dose groups (mean ± standard deviation). Area under the curve values were 10.1 ± 2.7 and 90.1 ± 38.3 ng × min/g for the 0.12 and 0.60 μL MB/kg body weight dose groups. C_max values of PFB in exhaled air were 0.35 ± 0.2 and 2.4 ± 0.7 ng/mL for the 0.12 and 0.60 μL MB/kg body weight dose groups. Assessment of laboratory parameters, vital signs, oxygen saturation and electrocardiograms revealed no changes indicative of a concern. The PK profile and safety data generated in the Chinese HVs were comparable to previous data for Caucasian HVs.

Identification of Metastasis-Associated Metabolic Profiles of Tumors by (1)H-HR-MAS-MRS

Tumors develop an abnormal microenvironment during growth, and similar to the metastatic phenotype, the metabolic phenotype of cancer cells is tightly linked to characteristics of the tumor microenvironment (TME). In this study, we explored relationships between metabolic profile, metastatic propensity, and hypoxia in experimental tumors in an attempt to identify metastasis-associated metabolic profiles. Two human melanoma xenograft lines (A-07, R-18) showing different TMEs were used as cancer models. Metabolic profile was assessed by proton high resolution magic angle spinning magnetic resonance spectroscopy (¹H-HR-MAS-MRS). Tumor hypoxia was detected in immunostained histological preparations by using pimonidazole as a hypoxia marker. Twenty-four samples from 10 A-07 tumors and 28 samples from 10 R-18 tumors were analyzed. Metastasis was associated with hypoxia in both A-07 and R-18 tumors, and ¹H-HR-MAS-MRS discriminated between tissue samples with and tissue samples without hypoxic regions in both models, primarily because hypoxia was associated with high lactate resonance peaks in A-07 tumors and with low lactate resonance peaks in R-18 tumors. Similarly, metastatic and non-metastatic R-18 tumors showed significantly different metabolic profiles, but not metastatic and non-metastatic A-07 tumors, probably because some samples from the metastatic A-07 tumors were derived from tumor regions without hypoxic tissue. This study suggests that ¹H-HR-MAS-MRS may be a valuable tool for evaluating the role of hypoxia and lactate in tumor metastasis as well as for identification of metastasis-associated metabolic profiles.

Abstract 3737: Inhibition of O-GlcNAc transferase in tamoxifen resistant breast cancer cells

O-linked N-acetyl-glucosamine transferase (OGT) is an enzyme that catalyzes addition of the O-GlcNAc modification to a wide range of intracellular proteins. The O-GlcNAc modification is a product of the hexosamine biosynthetic pathway, which requires glucose and glutamine as substrates. Uptake of both of these nutrients is often up-regulated in cancer, which in turn leads to an increase in the total protein O-GlcNAcylation. Increased OGT expression has also been reported in most cancer types, including the most frequently diagnosed cancer in women, breast cancer. Many of the breast cancers rely on estrogen receptor alpha (ERα) for proliferation and have shown a strong response to the ERα inhibition, most commonly achieved by treatment with tamoxifen. However, while efficient, prolonged exposure to tamoxifen commonly causes resistance and relapse of the disease. It is therefore vital to uncover mechanisms which contribute to the resistance in order to develop adequate treatment strategy for these patients.

Here, we have investigated the effect of targeting OGT in an isogenic pair of ERα-positive tamoxifen-sensitive MCF7 and tamoxifen-resistant TAMR breast cancer cell lines. OGT inhibition decreased viability and triggered cell death in both cell lines. These responses were associated with over 50% reduction in ERα expression in both MCF7 and TAMR cells. Reduced O-GlcNAcylation has previously been reported to induce endoplasmic reticulum stress and activation of transcription factor C/EBP homologous protein (CHOP), which promotes cell death. Targeting OGT resulted in a strong increase of CHOP expression, which appeared more prominent in the TAMR cells. Finally, targeting OGT induced a very pronounced cell cycle arrest in the G2/M phase in the TAMR cells, while the MCF7 cell lined showed a very modest response.

Taken together, these results indicate that targeting OGT leads to a differential response in the tamoxifen-sensitive and resistant breast cancer cells. Currently, we are using an expanded panel of tamoxifen-resistant cell lines to perform expression microarrays, metabolic flux assays and DNA damage response analysis in order to uncover the cause of the differential response to OGT targeting. This may help us identify potential therapeutic combinations that might be suitable for treatment of tamoxifen-resistant cancers.

Citation Format: Anna Barkovskaya, Lina Prasmickaite, Ian G. Mills, Gunhild M. Mælandsmo, Siver A. Moestue, Harri M. Itkonen. Inhibition of O-GlcNAc transferase in tamoxifen resistant breast 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 3737.

Diffusion-weighted MRI for early detection and characterization of prostate cancer in the transgenic adenocarcinoma of the mouse prostate model

PURPOSE

To improve early diagnosis of prostate cancer to aid clinical decision-making. Diffusion-weighted magnetic resonance imaging (DW-MRI) is sensitive to water diffusion throughout tissues, which correlates with Gleason score, a histological measure of prostate cancer aggressiveness. In this study the ability of DW-MRI to detect prostate cancer onset and development was evaluated in transgenic adenocarcinoma of the mouse prostate (TRAMP) mice.

MATERIALS AND METHODS

T2 -weighted and DW-MRI were acquired using a 7T MR scanner, 200 mm bore diameter; 10 TRAMP and 6 C57BL/6 control mice were scanned every 4 weeks from 8 weeks of age until sacrifice at 28-30 weeks. After sacrifice, the genitourinary tract was excised and sectioned for histological analysis. Histology slides registered with DW-MR images allowed for validation of DW-MR images and the apparent diffusion coefficient (ADC) as tools for cancer detection and disease stratification. An automated early assessment tool based on ADC threshold values was developed to aid cancer detection and progression monitoring.

RESULTS

The ADC differentiated between control prostate ((1.86 ± 0.20) × 10(-3) mm(2) /s) and normal TRAMP prostate ((1.38 ± 0.10) × 10(-3) mm(2) /s) (P = 0.0001), between TRAMP prostate and well-differentiated cancer ((0.93 ± 0.18) × 10(-3) mm(2) /s) (P = 0.0006), and between well-differentiated cancer and poorly differentiated cancer ((0.63 ± 0.06) × 10(-3) mm(2) /s) (P = 0.02).

CONCLUSION

DW-MRI is a tool for early detection of cancer, and discrimination between cancer stages in the TRAMP model. The incorporation of DW-MRI-based prostate cancer stratification and monitoring could increase the accuracy of preclinical trials using TRAMP mice.

Abstract A36: Protein expression analysis of intratumor heterogeneity in a luminal-like breast cancer xenograft

Estrogen receptor is a key driver in breast cancer and is expressed in about 75% of breast tumors. ER positive tumors are susceptible to endocrine therapies; however, the major obstacle for curative treatment is recurrence due to resistance to anti-estrogens. Endocrine therapies may induce a selective pressure promoting growth of estrogen independent cell subclones. Our aim was to reveal molecular changes occurring in tumors in response to anti-estrogen treatment, and to identify subpopulations of cells able to withstand anti-estrogen treatment.

A luminal-like estrogen-dependent orthotopically growing xenograft model was treated with fulvestrant, or exposed to estrogen deprivation. The effect of ER-signaling inhibition was analyzed using quantitative mass spectrometry (MS) -based proteomic analysis and high resolution magic angle spinning magnetic resonance spectroscopy (HR MAS MRS). Cell surface marker expression (CD24 and SSEA-4) was monitored by flow cytometry, allowing detailed comparison of protein expression between intratumor cell subpopulations.

We found that both modes of anti-estrogen therapy restrained tumor growth and induced expression of enzymes involved in TCA cycle, oxidative phosphorylation and fatty acid beta-oxidation. This was accompanied by changes in levels of specific metabolites indicative of a possible reprogramming of cell metabolism and utilization of oxidative phosphorylation in preference to aerobic glycolysis (decrease in Warburg effect). Furthermore, anti-estrogen treatment seemed to have selective effects on intratumor cell subpopulations, specified by expression of the markers CD24 and SSEA-4. More specifically, highly tumorigenic CD24low/SSEA-4low (dbl. low) cells were eliminated and the seemingly more benign CD24high/SSEA-4high (dbl. high) cells were enriched in the residual tumor. When comparing the proteome in dbl. low verus dbl. high cells sorted from untreated tumors, metabolism was one of the most differentially enriched processes. Enzymes involved in glycolysis, TCA cycle, respiratory electron transport chain and fatty acid were more abundant in the dbl. high subpopulation.

These results suggest that cancer cells may reprogram their metabolism in response to anti-estrogen therapy to support a less estrogen-dependent phenotype. Moreover, subpopulations of cells with different metabolism may exist within the growing tumor, and these may respond differently to anti-estrogen treatment.

Citation Format: Nirma Skrbo, Ufuk Kirik, Alexandr Kristian, Paolo Cifani, Linn Antberg, Siver A. Moestue, Olav Engebraaten, Gunhild M. Mælandsmo, Kristin Andersen, Peter James, Therese Sørlie. Protein expression analysis of intratumor heterogeneity in a luminal-like breast cancer xenograft. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr A36.

Impact of Freezing Delay Time on Tissue Samples for Metabolomic Studies

Introduction

Metabolic profiling of intact tumor tissue by high-resolution magic angle spinning (HR MAS) MR spectroscopy (MRS) provides important biological information possibly useful for clinical diagnosis and development of novel treatment strategies. However, generation of high-quality data requires that sample handling from surgical resection until analysis is performed using systematically validated procedures. In this study, we investigated the effect of postsurgical freezing delay time on global metabolic profiles and stability of individual metabolites in intact tumor tissue.

Materials and methods

Tumor tissue samples collected from two patient-derived breast cancer xenograft models (n = 3 for each model) were divided into pieces that were snap-frozen in liquid nitrogen at 0, 15, 30, 60, 90, and 120 min after surgical removal. In addition, one sample was analyzed immediately, representing the metabolic profile of fresh tissue exposed neither to liquid nitrogen nor to room temperature. We also evaluated the metabolic effect of prolonged spinning during the HR MAS experiments in biopsies from breast cancer patients (n = 14). All samples were analyzed by proton HR MAS MRS on a Bruker Avance DRX600 spectrometer, and changes in metabolic profiles were evaluated using multivariate analysis and linear mixed modeling.

Results

Multivariate analysis showed that the metabolic differences between the two breast cancer models were more prominent than variation caused by freezing delay time. No significant changes in levels of individual metabolites were observed in samples frozen within 30 min of resection. After this time point, levels of choline increased, whereas ascorbate, creatine, and glutathione (GS) levels decreased. Freezing had a significant effect on several metabolites but is an essential procedure for research and biobank purposes. Furthermore, four metabolites (glucose, glycine, glycerophosphocholine, and choline) were affected by prolonged HR MAS experiment time possibly caused by physical release of metabolites caused by spinning or due to structural degradation processes.

Conclusion

The MR metabolic profiles of tumor samples are reproducible and robust to variation in postsurgical freezing delay up to 30 min.

Metabolic reprogramming supports the invasive phenotype in malignant melanoma

Invasiveness is a hallmark of aggressive cancer like malignant melanoma, and factors involved in acquisition or maintenance of an invasive phenotype are attractive targets for therapy. We investigated melanoma phenotype modulation induced by the metastasis-promoting microenvironmental protein S100A4, focusing on the relationship between enhanced cellular motility, dedifferentiation and metabolic changes. In poorly motile, well-differentiated Melmet 5 cells, S100A4 stimulated migration, invasion and simultaneously down-regulated differentiation genes and modulated expression of metabolism genes. Metabolic studies confirmed suppressed mitochondrial respiration and activated glycolytic flux in the S100A4 stimulated cells, indicating a metabolic switch toward aerobic glycolysis, known as the Warburg effect. Reversal of the glycolytic switch by dichloracetate induced apoptosis and reduced cell growth, particularly in the S100A4 stimulated cells. This implies that cells with stimulated invasiveness get survival benefit from the glycolytic switch and, therefore, become more vulnerable to glycolysis inhibition. In conclusion, our data indicate that transition to the invasive phenotype in melanoma involves dedifferentiation and metabolic reprogramming from mitochondrial oxidation to glycolysis, which facilitates survival of the invasive cancer cells. Therapeutic strategies targeting the metabolic reprogramming may therefore be effective against the invasive phenotype.

MRI reveals the in vivo cellular and vascular response to BEZ235 in ovarian cancer xenografts with different PI3-kinase pathway activity

BACKGROUND

The phosphoinositide-3 kinase (PI3K) pathway is an attractive therapeutic target. However, difficulty in predicting therapeutic response limits the clinical implementation of PI3K inhibitors. This study evaluates the utility of clinically relevant magnetic resonance imaging (MRI) biomarkers for noninvasively assessing the in vivo response to the dual PI3K/mTOR inhibitor BEZ235 in two ovarian cancer models with differential PI3K pathway activity.

METHODS

The PI3K signalling activity of TOV-21G and TOV-112D human ovarian cancer cells was investigated in vitro. Cellular and vascular response of the xenografts to BEZ235 treatment (65 mg kg(-1), 3 days) was assessed in vivo using diffusion-weighted (DW) and dynamic contrast-enhanced (DCE)-MRI. Micro-computed tomography was performed to investigate changes in vascular morphology.

RESULTS

The TOV-21G cells showed higher PI3K signalling activity than TOV-112D cells in vitro and in vivo. Treated TOV-21G xenografts decreased in volume and DW-MRI revealed an increased water diffusivity that was not found in TOV-112D xenografts. Treatment-induced improvement in vascular functionality was detected with DCE-MRI in both models. Changes in vascular morphology were not found.

CONCLUSIONS

Our results suggest that DW- and DCE-MRI can detect cellular and vascular response to PI3K/mTOR inhibition in vivo. However, only DW-MRI could discriminate between a strong and weak response to BEZ235.

In vivo ³¹P magnetic resonance spectroscopic imaging (MRSI) for metabolic profiling of human breast cancer xenografts

PURPOSE

To study cancer associated with abnormal metabolism of phospholipids, of which several have been proposed as biomarkers for malignancy or to monitor response to anticancer therapy. We explored 3D (31) P magnetic resonance spectroscopic imaging (MRSI) at high magnetic field for in vivo assessment of individual phospholipids in two patient-derived breast cancer xenografts representing good and poor prognosis (luminal- and basal-like tumors).

MATERIALS AND METHODS

Metabolic profiles from luminal-like and basal-like xenograft tumors were obtained in vivo using 3D (31) P MRSI at 11.7T and from tissue extracts in vitro at 14.1T. Gene expression analysis was performed in order to support metabolic differences between the two xenografts.

RESULTS

In vivo (31) P MR spectra were obtained in which the prominent resonances from phospholipid metabolites were detected at a high signal-to-noise ratio (SNR >7.5). Metabolic profiles obtained in vivo were in agreement with those obtained in vitro and could be used to discriminate between the two xenograft models, based on the levels of phosphocholine, phosphoethanolamine, glycerophosphocholine, and glycerophosphoethanolamine. The differences in phospholipid metabolite concentration could partly be explained by gene expression profiles.

CONCLUSION

Noninvasive metabolic profiling by 3D (31) P MRSI can discriminate between subtypes of breast cancer based on different concentrations of choline- and ethanolamine-containing phospholipids.

Interplay of choline metabolites and genes in patient-derived breast cancer xenografts

Introduction

Dysregulated choline metabolism is a well-known feature of breast cancer, but the underlying mechanisms are not fully understood. In this study, the metabolomic and transcriptomic characteristics of a large panel of human breast cancer xenograft models were mapped, with focus on choline metabolism.

Methods

Tumor specimens from 34 patient-derived xenograft models were collected and divided in two. One part was examined using high-resolution magic angle spinning (HR-MAS) MR spectroscopy while another part was analyzed using gene expression microarrays. Expression data of genes encoding proteins in the choline metabolism pathway were analyzed and correlated to the levels of choline (Cho), phosphocholine (PCho) and glycerophosphocholine (GPC) using Pearson’s correlation analysis. For comparison purposes, metabolic and gene expression data were collected from human breast tumors belonging to corresponding molecular subgroups.

Results

Most of the xenograft models were classified as basal-like (N = 19) or luminal B (N = 7). These two subgroups showed significantly different choline metabolic and gene expression profiles. The luminal B xenografts were characterized by a high PCho/GPC ratio while the basal-like xenografts were characterized by highly variable PCho/GPC ratio. Also, Cho, PCho and GPC levels were correlated to expression of several genes encoding proteins in the choline metabolism pathway, including choline kinase alpha (CHKA) and glycerophosphodiester phosphodiesterase domain containing 5 (GDPD5). These characteristics were similar to those found in human tumor samples.

Conclusion

The higher PCho/GPC ratio found in luminal B compared with most basal-like breast cancer xenograft models and human tissue samples do not correspond to results observed from in vitro studies. It is likely that microenvironmental factors play a role in the in vivo regulation of choline metabolism. Cho, PCho and GPC were correlated to different choline pathway-encoding genes in luminal B compared with basal-like xenografts, suggesting that regulation of choline metabolism may vary between different breast cancer subgroups. The concordance between the metabolic and gene expression profiles from xenograft models with breast cancer tissue samples from patients indicates that these xenografts are representative models of human breast cancer and represent relevant models to study tumor metabolism in vivo.

Abstract P6-04-08: Cytosolic phospholipase A2 (cPLA2) as a therapeutic target in basal-like breast cancer

Introduction: Basal-like breast cancer is frequently associated with triple negative phenotype, and there is a need for novel therapeutic strategies for this patient population. Inhibitors of cytosolic phospholipase A2 (cPLA2) have been suggested to block both MAPK and PI3K signalling and have a high potential for activity in basal-like breast cancer (Lin 1993, Wen 2013). In this study, we compared the expression of PLA2G4A between luminal B and basal-like breast cancer, both in patient-derived xenograft models (PDX) and human cancer tissue. In addition, we studied the effect of the novel cPLA2 inhibitor AVX235 on tumor growth in a basal-like PDX model.

Materials and methods: Tumor tissue specimens were obtained from PDX models (n = 26) and a clinical breast cancer biobank (n = 32). Gene expression analysis was carried out on Agilent 8×60K microarrays. The expression of 54 genes directly involved in choline metabolism was examined. Differential expression of choline genes between basal-like and luminal B tumors was calculated by subtraction of log2 expression values. Mice carrying bilateral MAS98.12 basal-like xenografts (Bergamaschi 2009) were treated with the cPLA2 inhibitor AVX235 (30 mg/kg i.p. daily for 7 days then every second day for 14 days, n = 6) or drug-free vehicle (control group, n = 6).

Results: The PDX models were subtyped into basal-like (n = 19) and luminal B (n = 7) subtypes based on the expression of 500 intrinsic genes [Sørlie 2003]. For the human cancer tissue, there were 18 basal-like and 14 luminal B tumors. There was a significant correlation between differential choline gene expression in basal-like vs luminal B tumors in PDX models and human cancer tissue (p<1.3*10−12). Both in PDX models (p<0.04) and human cancer tissue (p<0.0003), PLA2G4A was significantly higher expressed in basal-like than luminal B tumors. Treatment with AVX235 markedly reduced the growth rate of MAS98.12 xenografts compared to controls. After 19 treatmment days, the mean tumor volume (normalised to volume at start of treatment) in the treatment group was 36% of the tumor volume in the control group, the difference being statistically significant (p = 0.023). No signs of treatment-related adverse effects were observed.

Conclusion: PLA2G4A is higher expressed in basal-like than in luminal B breast cancer. Treatment with the cPLA2 inhibitor AVX235 significantly inhibits tumor growth. These data suggest that cPLA2 inhibitors may be of particular value in treatment of basal-like breast cancer.

References: Lin LL et al: cPLA2 is phosphorylated and activated by MAP kinase. Cell 1993; 72; 269-278. Wen ZH et al: Critical role of arachidonic acid-activated mTOR signaling in breast carcinogenesis and angiogenesis. Oncogene 2013; 32; 160-170. Bergamaschi A et al: Molecular profiling and characterization of luminal-like and basal-like in vivo breast cancer xenograft models. Mol Oncol 2009; 3; 469-482. Sørlie T et al: Repeated observation of breast tumor subtypes in independent gene expression data sets. Proc Natl Acad Sci USA 2003; 100; 8418-8423.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P6-04-08.

Quantitative (31)P HR-MAS MR spectroscopy for detection of response to PI3K/mTOR inhibition in breast cancer xenografts

PURPOSE

Phospholipid metabolites are of importance in cancer studies, and have been suggested as candidate metabolic biomarkers for response to targeted anticancer drugs. The purpose of this study was to develop a phosphorus ((31) P) high resolution magic angle spinning magnetic resonance spectroscopy protocol for quantification of phosphorylated metabolites in intact cancer tissue.

METHODS

(31) P spectra were acquired on a 14.1 T spectrometer with a triplet (1) H/(13) C/(31) P MAS probe. Quantification of metabolites was performed using the PULCON principle. Basal-like and luminal-like breast cancer xenografts were treated with the dual PI3K/mTOR inhibitor BEZ235, and the impact of treatment on the concentration of phosphocholine, glycerophosphocholine, phosphoethanolamine and glycerophosphoethanolamine was evaluated.

RESULTS

In basal-like xenografts, BEZ235 treatment induced a significant decrease in phosphoethanolamine (-25.6%, P = 0.01) whilst phosphocholine (16.5%, P = 0.02) and glycerophosphocholine (37.3%, P < 0.001) were significantly increased. The metabolic changes could partially be explained by increased levels of phospholipase A2 group 4A (PLA2G4A).

CONCLUSION

(31) P high resolution magic angle spinning magnetic resonance spectroscopy is a useful method for quantitative assessment of metabolic responses to PI3K inhibition. Using the PULCON principle for quantification, the levels of phosphocholine, glycerophosphocholine, phosphoethanolamine, and glycerophosphoethanolamine could be evaluated with high precision and accuracy.

Low-molecular contrast agent dynamic contrast-enhanced (DCE)-MRI and diffusion-weighted (DW)-MRI in early assessment of bevacizumab treatment in breast cancer xenografts

PURPOSE

To investigate the effect of bevacizumab treatment on vascular architecture and function in two xenograft models with different angiogenic properties using diffusion-weighted magnetic resonance imaging (DW-MRI) and dynamic contrast-enhanced MRI (DCE-MRI).

MATERIALS AND METHODS

Mice carrying basal-like (MAS98.12) or luminal-like (MAS98.06) orthotopic breast cancer xenografts were treated with bevacizumab (5 mg/kg), doxorubicin (8 mg/kg), or both drugs in combination. DW-MRI and DCE-MRI were performed before and 3 days after treatment using a Bruker 7T preclinical scanner. Mean microvessel density (MVD) and proliferating microvessel density (pMVD) in the tumors were determined for evaluation of vascular response to bevacizumab treatment.

RESULTS

No changes in DCE-MRI or DW-MRI parameters were observed in untreated controls during the experiment period. DW-MRI showed increased apparent diffusion coefficient (ADC) values in all treatment groups in both basal-like and luminal-like xenografts. DCE-MRI showed increased contrast agent uptake, particularly in central regions of the tumors, after bevacizumab/combination treatment in both xenograft models. This was accompanied by decreased MVD and pMVD in basal-like xenografts. Doxorubicin treatment had no effect on DCE-MRI parameters in any of the xenograft models.

CONCLUSION

Both DW-MRI and DCE-MRI demonstrated an early response to bevacizumab treatment in the xenograft tumors. Increased contrast agent uptake and reduced MVD/pMVD is consistent with a normalization of vascular function.

Intravascular targets for molecular contrast-enhanced ultrasound imaging

Molecular targeting of contrast agents for ultrasound imaging is emerging as a new medical imaging modality. It combines advances in ultrasound technology with principles of molecular imaging, thereby allowing non-invasive assessment of biological processes in vivo. Preclinical studies have shown that microbubbles, which provide contrast during ultrasound imaging, can be targeted to specific molecular markers. These microbubbles accumulate in tissue with target (over) expression, thereby significantly increasing the ultrasound signal. This concept offers safe and low-cost imaging with high spatial resolution and sensitivity. It is therefore considered to have great potential in cancer imaging, and early-phase clinical trials are ongoing. In this review, we summarize the current literature on targets that have been successfully imaged in preclinical models using molecularly targeted ultrasound contrast agents. Based on preclinical experience, we discuss the potential clinical utility of targeted microbubbles.

13C high-resolution-magic angle spinning MRS reveals differences in glucose metabolism between two breast cancer xenograft models with different gene expression patterns

Tumor cells have increased glycolytic activity, and glucose is mainly used to form lactate and alanine, even when high concentrations of oxygen are present (Warburg effect). The purpose of the present study was to investigate glucose metabolism in two xenograft models representing basal-like and luminal-like breast cancer using (13) C high-resolution-magic angle spinning (HR-MAS) MRS and gene expression analysis. Tumor tissue was collected from two groups for each model: untreated mice (n=19) and a group of mice (n=16) that received an injection of [1-(13) C]-glucose 10 or 15 min before harvesting the tissue. (13) C HR-MAS MRS was performed on the tumor samples and differences in the glucose/alanine (Glc/Ala), glucose/lactate (Glc/Lac) and alanine/lactate (Ala/Lac) ratios between the models were studied. The expression of glycolytic genes was studied using tumor tissue from the same models. In the natural abundance MR spectra, a significantly lower Glc/Ala and Glc/Lac ratio (p<0.001) was observed in the luminal-like model compared with the basal-like model. In the labeled samples, the predominant glucose metabolites were lactate and alanine. Significantly lower Glc/Ala and Glc/Lac ratios were observed in the luminal-like model (p<0.05). Most genes contributing to glycolysis were expressed at higher levels in the luminal-like model (fdr<0.001). The lower Glc/Ala and Glc/Lac ratios and higher glycolytic gene expression observed in the luminal-like model indicates that the transformation of glucose to lactate and alanine occurred faster in this model than in the basal-like model, which has a growth rate several times faster than that of the luminal-like model. The results from the present study suggest that the tumor growth rate is not necessarily a determinant of glycolytic activity.

MRS and MRSI guidance in molecular medicine: targeting and monitoring of choline and glucose metabolism in cancer

MRS and MRSI are valuable tools for the detection of metabolic changes in tumors. The currently emerging era of molecular medicine, which is shaped by molecularly targeted anticancer therapies combined with molecular imaging of the effects of such therapies, requires powerful imaging technologies that are able to detect molecular information. MRS and MRSI are such technologies that are able to detect metabolites arising from glucose and choline metabolism in noninvasive in vivo settings and at higher resolution in tissue samples. The roles played by MRS and MRSI in the diagnosis of different types of cancer, as well as in the early monitoring of the tumor response to traditional chemotherapies, are reviewed. The emerging roles of MRS and MRSI in the development and detection of novel targeted anticancer therapies that target oncogenic signaling pathways or markers in choline or glucose metabolism are discussed.

Distinct choline metabolic profiles are associated with differences in gene expression for basal-like and luminal-like breast cancer xenograft models

Background

Increased concentrations of choline-containing compounds are frequently observed in breast carcinomas, and may serve as biomarkers for both diagnostic and treatment monitoring purposes. However, underlying mechanisms for the abnormal choline metabolism are poorly understood.

Methods

The concentrations of choline-derived metabolites were determined in xenografted primary human breast carcinomas, representing basal-like and luminal-like subtypes. Quantification of metabolites in fresh frozen tissue was performed using high-resolution magic angle spinning magnetic resonance spectroscopy (HR MAS MRS).

The expression of genes involved in phosphatidylcholine (PtdCho) metabolism was retrieved from whole genome expression microarray analyses.

The metabolite profiles from xenografts were compared with profiles from human breast cancer, sampled from patients with estrogen/progesterone receptor positive (ER+/PgR+) or triple negative (ER-/PgR-/HER2-) breast cancer.

Results

In basal-like xenografts, glycerophosphocholine (GPC) concentrations were higher than phosphocholine (PCho) concentrations, whereas this pattern was reversed in luminal-like xenografts. These differences may be explained by lower choline kinase (CHKA, CHKB) expression as well as higher PtdCho degradation mediated by higher expression of phospholipase A2 group 4A (PLA2G4A) and phospholipase B1 (PLB1) in the basal-like model. The glycine concentration was higher in the basal-like model. Although glycine could be derived from energy metabolism pathways, the gene expression data suggested a metabolic shift from PtdCho synthesis to glycine formation in basal-like xenografts. In agreement with results from the xenograft models, tissue samples from triple negative breast carcinomas had higher GPC/PCho ratio than samples from ER+/PgR+ carcinomas, suggesting that the choline metabolism in the experimental models is representative for luminal-like and basal-like human breast cancer.

Conclusions

The differences in choline metabolite concentrations corresponded well with differences in gene expression, demonstrating distinct metabolic profiles in the xenograft models representing basal-like and luminal-like breast cancer. The same characteristics of choline metabolite profiles were also observed in patient material from ER+/PgR+ and triple-negative breast cancer, suggesting that the xenografts are relevant model systems for studies of choline metabolism in luminal-like and basal-like breast cancer.