Characterizing the efficacy of cancer therapeutics in patient-derived xenograft models of metastatic breast cancer

The Transformative Role of 3D Culture Models in Triple-Negative Breast Cancer Research

Advancements in cell culturing techniques have allowed the development of three-dimensional (3D) cell culture models sourced directly from patients' tissues and tumors, faithfully replicating the native tissue environment. These models provide a more clinically relevant platform for studying disease progression and treatment responses compared to traditional two-dimensional (2D) models. Patient-derived organoids (PDOs) and patient-derived xenograft organoids (PDXOs) emerge as innovative 3D cancer models capable of accurately mimicking the tumor's unique features, enhancing our understanding of tumor complexities, and predicting clinical outcomes. Triple-negative breast cancer (TNBC) poses significant clinical challenges due to its aggressive nature, propensity for early metastasis, and limited treatment options. TNBC PDOs and PDXOs have significantly contributed to the comprehension of TNBC, providing novel insights into its underlying mechanism and identifying potential therapeutic targets. This review explores the transformative role of various 3D cancer models in elucidating TNBC pathogenesis and guiding novel therapeutic strategies. It also provides an overview of diverse 3D cell culture models, derived from cell lines and tumors, highlighting their advantages and culturing challenges. Finally, it delves into live-cell imaging techniques, endpoint assays, and alternative cell culture media and methodologies, such as scaffold-free and scaffold-based systems, essential for advancing 3D cancer model research and development.

Breast cancer brain metastasis: from etiology to state-of-the-art modeling

Currently, breast carcinoma is the most common form of malignancy and the main cause of cancer mortality in women worldwide. The metastasis of cancer cells from the primary tumor site to other organs in the body, notably the lungs, bones, brain, and liver, is what causes breast cancer to ultimately be fatal. Brain metastases occur in as many as 30% of patients with advanced breast cancer, and the 1-year survival rate of these patients is around 20%. Many researchers have focused on brain metastasis, but due to its complexities, many aspects of this process are still relatively unclear. To develop and test novel therapies for this fatal condition, pre-clinical models are required that can mimic the biological processes involved in breast cancer brain metastasis (BCBM). The application of many breakthroughs in the area of tissue engineering has resulted in the development of scaffold or matrix-based culture methods that more accurately imitate the original extracellular matrix (ECM) of metastatic tumors. Furthermore, specific cell lines are now being used to create three-dimensional (3D) cultures that can be used to model metastasis. These 3D cultures satisfy the requirement for in vitro methodologies that allow for a more accurate investigation of the molecular pathways as well as a more in-depth examination of the effects of the medication being tested. In this review, we talk about the latest advances in modeling BCBM using cell lines, animals, and tissue engineering methods.

Predictors of success in establishing orthotopic patient-derived xenograft models of triple negative breast cancer

Patient-derived xenograft (PDX) models of breast cancer are an effective discovery platform and tool for preclinical pharmacologic testing and biomarker identification. We established orthotopic PDX models of triple negative breast cancer (TNBC) from the primary breast tumors of patients prior to and following neoadjuvant chemotherapy (NACT) while they were enrolled in the ARTEMIS trial (NCT02276443). Serial biopsies were obtained from patients prior to treatment (pre-NACT), from poorly responsive disease after four cycles of Adriamycin and cyclophosphamide (AC, mid-NACT), and in cases of AC-resistance, after a 3-month course of different experimental therapies and/or additional chemotherapy (post-NACT). Our study cohort includes a total of 269 fine needle aspirates (FNAs) from 217 women, generating a total of 62 PDX models (overall success-rate = 23%). Success of PDX engraftment was generally higher from those cancers that proved to be treatment-resistant, whether poorly responsive to AC as determined by ultrasound measurements mid-NACT (p = 0.063), RCB II/III status after NACT (p = 0.046), or metastatic relapse within 2 years of surgery (p = 0.008). TNBC molecular subtype determined from gene expression microarrays of pre-NACT tumors revealed no significant association with PDX engraftment rate (p = 0.877). Finally, we developed a statistical model predictive of PDX engraftment using percent Ki67 positive cells in the patient's diagnostic biopsy, positive lymph node status at diagnosis, and low volumetric reduction of the patient's tumor following AC treatment. This novel bank of 62 PDX models of TNBC provides a valuable resource for biomarker discovery and preclinical therapeutic trials aimed at improving neoadjuvant response rates for patients with TNBC.

Transcriptomic changes underlying EGFR inhibitor resistance in human and mouse models of basal-like breast cancer

The goals of this study were to identify transcriptomic changes that arise in basal-like breast cancer cells during the development of resistance to epidermal growth factor receptor inhibitors (EGFRi) and to identify drugs that are cytotoxic once EGFRi resistance occurs. Human patient-derived xenografts (PDXs) were grown in immunodeficient mice and treated with a set of EGFRi; the EGFRi erlotinib was selected for more expansive in vivo studies. Single-cell RNA sequencing was performed on mammary tumors from the basal-like PDX WHIM2 that was treated with vehicle or erlotinib for 9 weeks. The PDX was then subjected to long-term erlotinib treatment in vivo. Through serial passaging, an erlotinib-resistant subline of WHIM2 was generated. Bulk RNA-sequencing was performed on parental and erlotinib-resistant tumors. In vitro high-throughput drug screening with > 500 clinically used compounds was performed on parental and erlotinib-resistant cells. Previously published bulk gene expression microarray data from MMTV-Wnt1 tumors were contrasted with the WHIM2 PDX data. Erlotinib effectively inhibited WHIM2 tumor growth for approximately 4 weeks. Compared to untreated cells, single-cell RNA sequencing revealed that a greater proportion of erlotinib-treated cells were in the G1 phase of the cell cycle. Comparison of WHIM2 and MMTV-Wnt1 gene expression data revealed a set of 38 overlapping genes that were differentially expressed in the erlotinib-resistant WHIM2 and MMTV-Wnt1 tumors. Comparison of all three data types revealed five genes that were upregulated across all erlotinib-resistant samples: IL19, KLK7, LCN2, SAA1, and SAA2. Of these five genes, LCN2 was most abundantly expressed in triple-negative breast cancers, and its knockdown restored erlotinib sensitivity in vitro. Despite transcriptomic differences, parental and erlotinib-resistant WHIM2 displayed similar responses to the majority of drugs assessed for cytotoxicity in vitro. This study identified transcriptomic changes arising in erlotinib-resistant basal-like breast cancer. These data could be used to identify a biomarker or develop a gene signature predictive of patient response to EGFRi. Future studies should explore the predictive capacity of these gene signatures as well as how LCN2 contributes to the development of EGFRi resistance.

Genomic screening reveals ubiquitin-like modifier activating enzyme 1 as a potent and druggable target in c-MYC-high triple negative breast cancer models

Triple negative breast cancer (TNBC) accounts for over 30% of all breast cancer (BC)-related deaths, despite accounting for only 10% to 15% of total BC cases. Targeted therapy development has largely stalled in TNBC, underlined by a lack of traditionally druggable addictions like receptor tyrosine kinases (RTKs). Here, through full genome CRISPR/Cas9 screening of TNBC models, we have uncovered the sensitivity of TNBCs to the depletion of the ubiquitin-like modifier activating enzyme 1 (UBA1). Targeting UBA1 with the first-in-class UBA1 inhibitor TAK-243 induced unresolvable endoplasmic reticulum (ER)-stress and activating transcription factor 4 (ATF4)-mediated upregulation of proapoptotic NOXA, leading to cell death. c-MYC expression correlates with TAK-243 sensitivity and cooperates with TAK-243 to induce a stress response and cell death. Importantly, there was an order of magnitude greater sensitivity of TNBC lines to TAK-243 compared to normal tissue-derived cells. In five patient derived xenograft models (PDXs) of TNBC, TAK-243 therapy led to tumor inhibition or frank tumor regression. Moreover, in an intracardiac metastatic model of TNBC, TAK-243 markedly reduced metastatic burden, indicating UBA1 is a potential new target in TNBC expressing high levels of c-MYC.

In Vivo Modeling of Human Breast Cancer Using Cell Line and Patient-Derived Xenografts

Historically, human breast cancer has been modeled largely in vitro using long-established cell lines primarily in two-dimensional culture, but also in three-dimensional cultures of varying cellular and molecular complexities. A subset of cell line models has also been used in vivo as cell line-derived xenografts (CDX). While outstanding for conducting detailed molecular analysis of regulatory mechanisms that may function in vivo, results of drug response studies using long-established cell lines have largely failed to translate clinically. In an attempt to address this shortcoming, many laboratories have succeeded in developing clinically annotated patient-derived xenograft (PDX) models of human cancers, including breast, in a variety of host systems. While immunocompromised mice are the predominant host, the immunocompromised rat and pig, zebrafish, as well as the chicken egg chorioallantoic membrane (CAM) have also emerged as potential host platforms to help address perceived shortcomings of immunocompromised mice. With any modeling platform, the two main issues to be resolved are criteria for "credentialing" the models as valid models to represent human cancer, and utility with respect to the ability to generate clinically relevant translational research data. Such data are beginning to emerge, particularly with the activities of PDX consortia such as the NCI PDXNet Program, EuroPDX, and the International Breast Cancer Consortium, as well as a host of pharmaceutical companies and contract research organizations (CRO). This review focuses primarily on these important aspects of PDX-related research, with a focus on breast cancer.

Preclinical Models of Brain Metastases in Breast Cancer

Breast cancer remains a leading cause of mortality among women worldwide. Brain metastases confer extremely poor prognosis due to a lack of understanding of their specific biology, unique physiologic and anatomic features of the brain, and limited treatment strategies. A major roadblock in advancing the treatment of breast cancer brain metastases (BCBM) is the scarcity of representative experimental preclinical models. Current models are predominantly based on the use of animal xenograft models with immortalized breast cancer cell lines that poorly capture the disease’s heterogeneity. Recent years have witnessed the development of patient-derived in vitro and in vivo breast cancer culturing systems that more closely recapitulate the biology from individual patients. These advances led to the development of modern patient-tissue-based experimental models for BCBM. The success of preclinical models is also based on the imaging technologies used to detect metastases. Advances in animal brain imaging, including cellular MRI and multimodality imaging, allow sensitive and specific detection of brain metastases and monitoring treatment responses. These imaging technologies, together with novel translational breast cancer models based on patient-derived cancer tissues, represent a unique opportunity to advance our understanding of brain metastases biology and develop novel treatment approaches. This review discusses the state-of-the-art knowledge in preclinical models of this disease.

Exploitation of sulfated glycosaminoglycan status for precision medicine of Triplatin in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is a subtype of breast cancer lacking targetable biomarkers. TNBC is known to be most aggressive, and when metastatic is often drug resistant and uncurable. Biomarkers predicting response to therapy improve treatment decisions and allow personalized approaches for TNBC patients. This study explores sulfated glycosaminoglycan (sGAG) levels as a predictor of TNBC response to platinum therapy. sGAG levels were quantified in three distinct TNBC tumor models including cell line-derived, patient-derived xenograft (PDX) tumors, and isogenic models deficient in sGAG biosynthesis. The in vivo antitumor efficacy of Triplatin, a sGAG-directed platinum agent, was compared in these models to the clinical platinum agent, carboplatin. We determined that >40% of TNBC PDX tissue microarray samples have high levels of sGAGs. The in vivo accumulation of Triplatin in tumors as well as antitumor efficacy of Triplatin positively correlated with sGAG levels on tumor cells, whereas carboplatin followed the opposite trend. In carboplatin-resistant tumor models expressing high levels of sGAGs, Triplatin decreased primary tumor growth, reduced lung metastases, and inhibited metastatic growth in lungs, liver, and ovaries. sGAG levels served as a predictor of Triplatin sensitivity in TNBC. Triplatin may be particularly beneficial in treating patients with chemotherapy-resistant tumors who have evidence of residual disease after standard neoadjuvant chemotherapy. More effective neoadjuvant and adjuvant treatment will likely improve clinical outcome of TNBC.

A method for the efficient iron-labeling of patient-derived xenograft cells and cellular imaging validation

There is momentum towards implementing patient-derived xenograft models (PDX) in cancer research to reflect the histopathology, tumor behavior, and metastatic properties observed in the original tumor. To study PDX cells preclinically, we used both bioluminescence imaging (BLI) to evaluate cell viability and magnetic particle imaging (MPI), an emerging imaging technology to allow for detection and quantification of iron nanoparticles. The goal of this study was to develop the first successful iron labeling method of breast cancer cells derived from patient brain metsastases and validate this method with imaging during tumor development. The overall workflow of this labeling method is as follows: adherent and non-adherent luciferase expressing human breast cancer PDX cells (F2-7) are dissociated and concurrently labeled after incubation with micron-sized iron oxide particles (MPIO; 25 μg Fe/ml), with labeling validated by cellular imaging with MPI and BLI. In this study, NOD/SCID/ILIIrg^-/- (n = 5) mice Received injections of 1 × 10⁶ iron-labeled F2-7 cells into the fourth mammary fat pad (MFP). BLI was performed longitudinally to day 49 and MPI was performed up to day 28. In vivo BLI revealed that signal increased over time with tumor development. MPI revealed decreasing signal in the tumors over time. Here, we demonstrate the first application of MPI to monitor the growth of a PDX MFP tumor and the first successful labeling of PDX cells with iron oxide particles. Imaging of PDX cells provides a powerful system to better develop personalized therapies targeting breast cancer brain metastasis.

Identification of nuclear export inhibitor-based combination therapies in preclinical models of triple-negative breast cancer

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High-throughput drug screening reveals promising therapeutic candidates for TNBC.

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KPT-330, an XPO1 inhibitor, and GSK2126458 exhibit synergism in preclinical models of TNBC.

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XPO1 is overexpressed in basal-like breast tumors.

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XPO1 expression is associated with PIK3CA, MTOR, and MKI67 expression at the single-cell level.

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XPO1 overexpression in basal-like patients is associated with greater rates of metastases.

An estimated 284,000 Americans will be diagnosed with breast cancer in 2021. Of these individuals, 15–20% have basal-like triple-negative breast cancer (TNBC), which is known to be highly metastatic. Chemotherapy is standard of care for TNBC patients, but chemoresistance is a common clinical problem. There is currently a lack of alternative, targeted treatment strategies for TNBC; this study sought to identify novel therapeutic combinations to treat basal-like TNBCs. For these studies, four human basal-like TNBC cell lines were utilized to determine the cytotoxicity profile of 1363 clinically-used drugs. Ten promising therapeutic candidates were identified, and synergism studies were performed in vitro. Two drug combinations that included KPT-330, an XPO1 inhibitor, were synergistic in all four cell lines. In vivo testing of four basal-like patient-derived xenografts (PDX) identified one combination, KPT-330 and GSK2126458 (a PI3K/mTOR inhibitor), that decreased tumor burden in mice significantly more than monotherapy with either single agent. Bulk and single-cell RNA-sequencing, immunohistochemistry, and analysis of published genomic datasets found that XPO1 was abundantly expressed in human basal-like TNBC cell lines, PDXs, and patient tumor samples. Within basal-like PDXs, XPO1 overexpression was associated with increased proliferation at the cellular level. Within patient datasets, XPO1 overexpression was correlated with greater rates of metastasis in patients with basal-like tumors. These studies identify a promising potential new combination therapy for patients with basal-like breast cancer.

Prolactin Drives a Dynamic STAT5A/HDAC6/HMGN2 Cis-Regulatory Landscape Exploitable in ER+ Breast Cancer

The hormone prolactin has been implicated in breast cancer pathogenesis and regulates chromatin engagement by the transcription factor, STAT5A. STAT5A is known to inducibly bind promoters and cis-regulatory elements genome-wide, though the mechanisms by which it exerts specificity and regulation of target gene expression remain enigmatic. We previously identified HDAC6 and HMGN2 as cofactors that facilitate prolactin-induced, STAT5A-mediated gene expression. Here, multicondition STAT5A, HDAC6, and HMGN2 chromatin immunoprecipitation and sequencing with parallel condition RNA-seq are utilized to reveal the cis-regulatory landscape and cofactor dynamics underlying prolactin-stimulated gene expression in breast cancer. We find that prolactin-regulated genes are significantly enriched for cis-regulatory elements bound by HDAC6 and HMGN2, and that inducible STAT5A binding at enhancers, rather than promoters, conveys specificity for prolactin-regulated genes. The selective HDAC6 inhibitor, ACY-241, blocks prolactin-induced STAT5A chromatin engagement at cis-regulatory elements as well as a significant proportion of prolactin-stimulated gene expression. We identify functional pathways known to contribute to the development and/or progression of breast cancer that are activated by prolactin and inhibited by ACY-241. Additionally, we find that the DNA sequences underlying shared STAT5A and HDAC6 binding sites at enhancers are differentially enriched for estrogen response elements (ESR1 and ESR2 motifs) relative to enhancers bound by STAT5A alone. Gene set enrichment analysis identifies significant overlap of ERα-regulated genes with genes regulated by prolactin, particularly prolactin-regulated genes with promoters or enhancers co-occupied by both STAT5A and HDAC6. Lastly, the therapeutic efficacy of ACY-241 is demonstrated in in vitro and in vivo breast cancer models, where we identify synergistic ACY-241 drug combinations and observe differential sensitivity of ER+ models relative to ER- models.

Chromatin conformation capture (Hi-C) sequencing of patient-derived xenografts: analysis guidelines

BACKGROUND

Sequencing of patient-derived xenograft (PDX) mouse models allows investigation of the molecular mechanisms of human tumor samples engrafted in a mouse host. Thus, both human and mouse genetic material is sequenced. Several methods have been developed to remove mouse sequencing reads from RNA-seq or exome sequencing PDX data and improve the downstream signal. However, for more recent chromatin conformation capture technologies (Hi-C), the effect of mouse reads remains undefined.

RESULTS

We evaluated the effect of mouse read removal on the quality of Hi-C data using in silico created PDX Hi-C data with 10% and 30% mouse reads. Additionally, we generated 2 experimental PDX Hi-C datasets using different library preparation strategies. We evaluated 3 alignment strategies (Direct, Xenome, Combined) and 3 pipelines (Juicer, HiC-Pro, HiCExplorer) on Hi-C data quality.

CONCLUSIONS

Removal of mouse reads had little-to-no effect on data quality as compared with the results obtained with the Direct alignment strategy. Juicer extracted more valid chromatin interactions for Hi-C matrices, regardless of the mouse read removal strategy. However, the pipeline effect was minimal, while the library preparation strategy had the largest effect on all quality metrics. Together, our study presents comprehensive guidelines on PDX Hi-C data processing.

Pharmacologic profiling of patient-derived xenograft models of primary treatment-naïve triple-negative breast cancer

Triple-negative breast cancer (TNBC) accounts for 15-20% of breast cancer cases in the United States, lacks targeted therapeutic options, and is associated with a 40-80% risk of recurrence. Thus, identifying actionable targets in treatment-naïve and chemoresistant TNBC is a critical unmet medical need. To address this need, we performed high-throughput drug viability screens on human tumor cells isolated from 16 patient-derived xenograft models of treatment-naïve primary TNBC. The models span a range of TNBC subtypes and exhibit a diverse set of putative driver mutations, thus providing a unique patient-derived, molecularly annotated pharmacologic resource that is reflective of TNBC. We identified therapeutically actionable targets including kinesin spindle protein (KSP). The KSP inhibitor targets the mitotic spindle through mechanisms independent of microtubule stability and showed efficacy in models that were resistant to microtubule inhibitors used as part of the current standard of care for TNBC. We also observed subtype selectivity of Prima-1^Met, which showed higher levels of efficacy in the mesenchymal subtype. Coupling pharmacologic data with genomic and transcriptomic information, we showed that Prima-1^Met activity was independent of its canonical target, mutant p53, and was better associated with glutathione metabolism, providing an alternate molecularly defined biomarker for this drug.

The Roles of Autophagy and Senescence in the Tumor Cell Response to Radiation

Radiation is a critical pillar in cancer therapeutics, exerting its anti-tumor DNA-damaging effects through various direct and indirect mechanisms. Radiation has served as an effective mode of treatment for a number of cancer types, providing both curative and palliative treatment; however, resistance to therapy persists as a fundamental limitation. While cancer cell death is the ideal outcome of any anti-tumor treatment, radiation induces several responses, including apoptotic cell death, mitotic catastrophe, autophagy and senescence, where autophagy and senescence may promote cell survival. In most cases, autophagy, a conventionally cytoprotective mechanism, is a "first" responder to damage incurred from chemotherapy and radiation treatment. The paradigm developed on the premise that autophagy is cytoprotective in nature has provided the rationale for current clinical trials designed with the goal of radiosensitizing cancer cells through the use of autophagy inhibitors; however, these have failed to produce consistent results. Delving further into pre-clinical studies, autophagy has actually been shown to take diverse, sometimes opposing, forms, such as acting in a cytotoxic or nonprotective fashion, which may be partially responsible for the inconsistency of clinical outcomes. Furthermore, autophagy can have both pro- and anti-tumorigenic effects, while also having an important immune modulatory function. Senescence often occurs in tandem with autophagy, which is also the case with radiation. Radiation-induced senescence is frequently followed by a phase of proliferative recovery in a subset of cells and has been proposed as a tumor dormancy model, which can contribute to resistance to therapy and possibly also disease recurrence. Senescence induction is often accompanied by a unique secretory phenotype that can either promote or suppress immune functions, depending on the expression profile of cytokines and chemokines. Novel therapeutics selectively cytotoxic to senescent cells (senolytics) may prove to prolong remission by delaying disease recurrence in patients. Accurate assessment of primary responses to radiation may provide potential targets that can be manipulated for therapeutic benefit to sensitize cancer cells to radiotherapy, while sparing normal tissue.

Orthotopic Implantation Achieves Better Engraftment and Faster Growth Than Subcutaneous Implantation in Breast Cancer Patient-Derived Xenografts

Patient-Derived Xenograft (PDX) is now accepted as a murine model that better mimics human cancer when compared to a conventional cancer cell-line inoculation model. Some claim the advantage of orthotopic site implantation of patient tumor (OS) over ectopic implantation into the subcutaneous space (SQ); however, there has been no study that describes a head-to-head comparison of oncological differences between these two models to date. We hypothesize that OS tumors re-transplant and grow better than SQ tumors and are therefore a better model to evaluate tumor aggressiveness. Breast cancer PDXs were generated using the tumors derived from 11 patients into NOD scid gamma (NSG) mice. We used six ER(+)HER2(-) tumors and five triple negative (TN) tumors for a total of 11 tumors. Five PDX lines grew for an overall engraftment rate of 45%. We present our OS implantation method in detail. The re-transplantation rate of TN tumors in each transplant site was significantly higher in OS when compared to SQ tumors (70.1% vs. 32.1%, p < 0.01). OS tumors grow significantly faster than SQ tumors. Similarly, OS tumors demonstrated significantly more mitotic figures and Ki-67 positive cells than SQ tumors. The tumor re-transplantation rate significantly increased by the second and third generations with the OS method. The time from implantation to development of a palpable tumor dramatically decreased after the first passage. PDX of ER(+) tumors demonstrated significantly lower engraftment rates and slower tumor growth than TN tumors, which remarkably improved by the first passage. Orthotopically implanted PDX tumors showed better re-transplantation rates, greater tumor size, and more significant growth compared to the subcutaneously implanted model.

Identification of synergistic drug combinations using breast cancer patient-derived xenografts

Compared with other breast cancer subtypes, triple-negative breast cancer (TNBC) is associated with relatively poor outcomes due to its metastatic propensity, frequent failure to respond to chemotherapy, and lack of alternative, targeted treatment options, despite decades of major research efforts. Our studies sought to identify promising targeted therapeutic candidates for TNBC through in vitro screening of 1,363 drugs in patient-derived xenograft (PDX) models. Using this approach, we generated a dataset that can be used to assess and compare responses of various breast cancer PDXs to many different drugs. Through a series of further drug screening assays and two-drug combination testing, we identified that the combination of afatinib (epidermal growth factor receptor (EGFR) inhibitor) and YM155 (inhibitor of baculoviral inhibitor of apoptosis repeat-containing 5 (BIRC5; survivin) expression) is synergistically cytotoxic across multiple models of basal-like TNBC and reduces PDX mammary tumor growth in vivo. We found that YM155 reduces EGFR expression in TNBC cells, shedding light on its potential mechanism of synergism with afatinib. Both EGFR and BIRC5 are highly expressed in basal-like PDXs, cell lines, and patients, and high expression of both genes reduces metastasis-free survival, suggesting that co-targeting of these proteins holds promise for potential clinical success in TNBC.

The utility of the "Glowing Head" mouse for breast cancer metastasis research

The expression of cellular reporters to label cancer cells, such as green fluorescent protein (GFP) and luciferase, can stimulate immune responses and effect tumor growth. Recently, a mouse model that expresses GFP and luciferase in the anterior pituitary gland was generated to tolerize mice to these proteins; the "Glowing Head" mouse. Mice were obtained from a commercial vendor, bred, and then used for tumor growth and metastasis studies. The transgene expression of luciferase was assessed within tumor-naïve mice as well as mice with mammary tumors or metastases. Tumor-free mice with white fur, compared to black fur, allowed for stronger luciferase transgene expression to be observed in the pituitary, sternum, and femur. Growth of four different luciferase-expressing mouse cancer cell lines readily occurred in the mammary gland. Though sternum expression of the luciferase transgene occurred in cancer-free mice, growth or death of luciferase positive cancer cells in the lung could be observed. Liver metastases seeded by portal vein injections of luciferase positive cancer cell lines were completely distinct from luciferase transgene expression. Though lung and brain metastasis studies have limitations, the Glowing Head mouse can be useful to inhibit immune system rejection of luciferase or GFP expressing cancer cells. This mouse model is most beneficial for studies of mammary tumors and liver metastases.

Quantitative assessment of breast cancer liver metastasis expansion with patient-derived xenografts

Advanced breast cancer often spreads to the bone, brain, liver, and lungs. The survival time of a patient with breast cancer liver metastasis is often less than 9 months without treatment. Experimental model systems often focus on the lung as a site of metastatic relapse, and therefore, there is less of an understanding of the biological processes that occur during expansive liver metastasis growth. In these studies, 14 genetically distinct breast cancer patient-derived xenografts (PDXs) were characterized for growth in the liver after portal vein injection of cancer cells. Growth in the liver occurred in 12 of 14 models, and the relative growth rate across the PDXs was overall similar to growth in the mammary gland. Pathological and immunohistochemical analyses revealed that the proliferation rates of metastases were relatively similar as the metastases expanded until the tumors became necrotic, and then slightly lower proliferation rates were observed. There were influxes of macrophages and neutrophils as the metastases increased in size, suggesting these innate immune cells may result in differential responses to therapeutics in micrometastases compared to macrometastases. The development and characterization of these models is important as future studies can utilize this information to determine if targeted therapies can slow the progression of metastatic disease at different stages in the liver.

Drug resistance profiling of a new triple negative breast cancer patient-derived xenograft model

Background

Triple-negative breast cancer (TNBC) represents an aggressive subtype with limited therapeutic options. Experimental preclinical models that recapitulate their tumors of origin can accelerate target identification, thereby potentially improving therapeutic efficacy. Patient-derived xenografts (PDXs), due to their genomic and transcriptomic fidelity to the tumors from which they are derived, are poised to improve the preclinical testing of drug-target combinations in translational models. Despite the previous development of breast and TNBC PDX models, those derived from patients with demonstrated health-disparities are lacking.

Methods

We use an aggressive TNBC PDX model propagated in SCID/Beige mice that was established from an African-American woman, TU-BcX-2 K1, and assess its metastatic potential and drug sensitivities under distinct in vitro conditions. Cellular derivatives of the primary tumor or the PDX were grown in 2D culture conditions or grown in mammospheres 3D culture. Flow cytometry and fluorescence staining was used to quantify cancer stem cell-like populations. qRT-PCR was used to describe the mesenchymal gene signature of the tumor. The sensitivity of TU-BcX-2 K1-derived cells to anti-neoplastic oncology drugs was compared in adherent cells and mammospheres. Drug response was evaluated using a live/dead staining kit and crystal violet staining.

Results

TU-BcX-2 K1 has a low propensity for metastasis, reflects a mesenchymal state, and contains a large burden of cancer stem cells. We show that TU-BcX-2 K1 cells have differential responses to cytotoxic and targeted therapies in 2D compared to 3D culture conditions insofar as several drug classes conferred sensitivity in 2D but not in 3D culture, or cells grown as mammospheres.

Conclusions

Here we introduce a new TNBC PDX model and demonstrate the differences in evaluating drug sensitivity in adherent cells compared to mammosphere, or suspension, culture.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5401-2) contains supplementary material, which is available to authorized users.

Separation of breast cancer and organ microenvironment transcriptomes in metastases

Background

The seed and soil hypothesis was proposed over a century ago to describe why cancer cells (seeds) grow in certain organs (soil). Since then, the genetic properties that define the cancer cells have been heavily investigated; however, genomic mediators within the organ microenvironment that mediate successful metastatic growth are less understood. These studies sought to identify cancer- and organ-specific genomic programs that mediate metastasis.

Methods

In these studies, a set of 14 human breast cancer patient-derived xenograft (PDX) metastasis models was developed and then tested for metastatic tropism with two approaches: spontaneous metastases from mammary tumors and intravenous injection of PDX cells. The transcriptomes of the cancer cells when growing as tumors or metastases were separated from the transcriptomes of the microenvironment via species-specific separation of the genomes. Drug treatment of PDX spheroids was performed to determine if genes activated in metastases may identify targetable mediators of viability.

Results

The experimental approaches that generated metastases in PDX models were identified. RNA sequencing of 134 tumors, metastases, and normal non-metastatic organs identified cancer- and organ-specific genomic properties that mediated metastasis. A common genomic response of the liver microenvironment was found to occur in reaction to the invading PDX cells. Genes within the cancer cells were found to be either transiently regulated by the microenvironment or permanently altered due to clonal selection of metastatic sublines. Gene Set Enrichment Analyses identified more than 400 gene signatures that were commonly activated in metastases across basal-like PDXs. A Src signaling signature was found to be extensively upregulated in metastases, and Src inhibitors were found to be cytotoxic to PDX spheroids.

Conclusions

These studies identified that during the growth of breast cancer metastases, there were genomic changes that occurred within both the cancer cells and the organ microenvironment. We hypothesize that pathways upregulated in metastases are mediators of viability and that simultaneously targeting changes within different cancer cell pathways and/or different tissue compartments may be needed for inhibition of disease progression.

Electronic supplementary material

The online version of this article (10.1186/s13058-019-1123-2) contains supplementary material, which is available to authorized users.

Meeting report: Metastasis Research Society (MRS) 17th Biennial conference and associated Young Investigator Satellite Meeting (YISM) on cancer metastasis

The Metastasis Research Society (MRS) 17th Biennial conference on metastasis was held on the 1st to the 5th of August 2018 at Princeton University, NJ, USA. The meeting was held around themes addressing notable aspects of the understanding and treatment of metastasis and metastatic disease covering basic, translational, and clinical research. Importantly, the meeting was largely supported by our patient advocate partners including Susan G. Komen for the Cure, Theresa's Research Foundation and METAvivor. There were a total of 85 presentations from invited and selected speakers spread across the main congress and presentations from the preceding Young Investigator Satellite Meeting. Presentations are summarized in this report by session topic.

Live Cell Mass Accumulation Measurement Non-Invasively Predicts Carboplatin Sensitivity in Triple-Negative Breast Cancer Patient-Derived Xenografts

Prompt and repeated assessments of tumor sensitivity to available therapeutics could reduce patient morbidity and mortality by quickly identifying therapeutic resistance and optimizing treatment regimens. Analysis of changes in cancer cell biomass has shown promise in assessing drug sensitivity and fulfilling these requirements. However, a major limitation of previous studies in solid tumors, which comprise 90% of cancers, is the use of cancer cell lines rather than freshly isolated tumor material. As a result, existing biomass protocols are not obviously extensible to real patient tumors owing to potential artifacts that would be generated by the removal of cells from their microenvironment and the deleterious effects of excision and purification. In this present work, we show that simple excision of human triple-negative breast cancer (TNBC) tumors growing in immunodeficient mouse, patient-derived xenograft (PDX) models, followed by enzymatic disaggregation into single cell suspension, is enabling for rapid and accurate biomass accumulation-based predictions of in vivo sensitivity to the chemotherapeutic drug carboplatin. We successfully correlate in vitro biomass results with in vivo treatment results in three TNBC PDX models that have differential sensitivity to this drug. With a maximum turnaround time of 40 h from tumor excision to useable results and a fully-automated analysis pipeline, the assay described here has significant potential for translation to clinical practice.

Increasing intratumor C/EBP-β LIP and nitric oxide levels overcome resistance to doxorubicin in triple negative breast cancer

BACKGROUND

Triple negative breast cancer (TNBC) easily develops resistance to the first-line drug doxorubicin, because of the high levels of the drug efflux transporter P-glycoprotein (Pgp) and the activation of pro-survival pathways dependent on endoplasmic reticulum (ER). Interfering with these mechanisms may overcome the resistance to doxorubicin, a still unmet need in TNBC.

METHODS

We analyzed a panel of human and murine breast cancer cells for their resistance to doxorubicin, Pgp expression, lysosome and proteasome activity, nitrite production, ER-dependent cell death and immunogenic cell death parameters. We evaluated the efficacy of genetic (C/EBP-β LIP induction) and pharmacological strategies (lysosome and proteasome inhibitors), in restoring the ER-dependent and immunogenic-dependent cell death induced by doxorubicin, in vitro and in syngeneic mice bearing chemoresistant TNBC. The results were analyzed by one-way analysis of variance test.

RESULTS

We found that TNBC cells characterized by high levels of Pgp and resistance to doxorubicin, had low induction of the ER-dependent pro-apoptotic factor C/EBP-β LIP upon doxorubicin treatment and high activities of lysosome and proteasome that constitutively destroyed LIP. The combination of chloroquine and bortezomib restored doxorubicin sensitivity by activating multiple and interconnected mechanisms. First, chloroquine and bortezomib prevented C/EBP-β LIP degradation and activated LIP-dependent CHOP/TRB3/caspase 3 axis in response to doxorubicin. Second, C/EBP-β LIP down-regulated Pgp and up-regulated calreticulin that triggered the dendritic cell (DC)-mediated phagocytosis of tumor cell, followed by the activation of anti-tumor CD8⁺T-lymphocytes upon doxorubicin treatment. Third, chloroquine and bortezomib increased the endogenous production of nitric oxide that further induced C/EBP-β LIP and inhibited Pgp activity, enhancing doxorubicin's cytotoxicity. In orthotopic models of resistant TNBC, intratumor C/EBP-β LIP induction - achieved by a specific expression vector or by chloroquine and bortezomib - effectively reduced tumor growth and Pgp expression, increased intra-tumor apoptosis and anti-tumor immune-infiltrate, rescuing the efficacy of doxorubicin.

CONCLUSIONS

We suggest that preventing C/EBP-β LIP degradation by lysosome and proteasome inhibitors triggers multiple virtuous circuitries that restore ER-dependent apoptosis, down-regulate Pgp and re-activate the DC/CD8⁺T-lymphocytes response against TNBC. Lysosome and proteasome inhibitors associated with doxorubicin may overcome the resistance to the drug in TNBC.