Patient-Derived Models of Cancer in the NCI PDMC Consortium: Selection, Pitfalls, and Practical Recommendations

For over a century, early researchers sought to study biological organisms in a laboratory setting, leading to the generation of both in vitro and in vivo model systems. Patient-derived models of cancer (PDMCs) have more recently come to the forefront of preclinical cancer models and are even finding their way into clinical practice as part of functional precision medicine programs. The PDMC Consortium, supported by the Division of Cancer Biology in the National Cancer Institute of the National Institutes of Health, seeks to understand the biological principles that govern the various PDMC behaviors, particularly in response to perturbagens, such as cancer therapeutics. Based on collective experience from the consortium groups, we provide insight regarding PDMCs established both in vitro and in vivo, with a focus on practical matters related to developing and maintaining key cancer models through a series of vignettes. Although every model has the potential to offer valuable insights, the choice of the right model should be guided by the research question. However, recognizing the inherent constraints in each model is crucial. Our objective here is to delineate the strengths and limitations of each model as established by individual vignettes. Further advances in PDMCs and the development of novel model systems will enable us to better understand human biology and improve the study of human pathology in the lab.

Micelle-Formulated Juglone Effectively Targets Pancreatic Cancer and Remodels the Tumor Microenvironment

Pancreatic cancer remains a formidable challenge due to limited treatment options and its aggressive nature. In recent years, the naturally occurring anticancer compound juglone has emerged as a potential therapeutic candidate, showing promising results in inhibiting tumor growth and inducing cancer cell apoptosis. However, concerns over its toxicity have hampered juglone's clinical application. To address this issue, we have explored the use of polymeric micelles as a delivery system for juglone in pancreatic cancer treatment. These micelles, formulated using Poloxamer 407 and D-α-Tocopherol polyethylene glycol 1000 succinate, offer an innovative solution to enhance juglone's therapeutic potential while minimizing toxicity. In-vitro studies have demonstrated that micelle-formulated juglone (JM) effectively decreases proliferation and migration and increases apoptosis in pancreatic cancer cell lines. Importantly, in-vivo, JM exhibited no toxicity, allowing for increased dosing frequency compared to free drug administration. In mice, JM significantly reduced tumor growth in subcutaneous xenograft and orthotopic pancreatic cancer models. Beyond its direct antitumor effects, JM treatment also influenced the tumor microenvironment. In immunocompetent mice, JM increased immune cell infiltration and decreased stromal deposition and activation markers, suggesting an immunomodulatory role. To understand JM's mechanism of action, we conducted RNA sequencing and subsequent differential expression analysis on tumors that were treated with JM. The administration of JM treatment reduced the expression levels of the oncogenic protein MYC, thereby emphasizing its potential as a focused, therapeutic intervention. In conclusion, the polymeric micelles-mediated delivery of juglone holds excellent promise in pancreatic cancer therapy. This approach offers improved drug delivery, reduced toxicity, and enhanced therapeutic efficacy.

MYC Deregulation and PTEN Loss Model Tumor and Stromal Heterogeneity of Aggressive Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) patients have a poor prognosis and few treatment options. Mouse models of TNBC are important for development of new therapies, however, few mouse models represent the complexity of TNBC. Here, we develop a female TNBC murine model by mimicking two common TNBC mutations with high co-occurrence: amplification of the oncogene MYC and deletion of the tumor suppressor PTEN. This Myc;Ptenfl model develops heterogeneous triple-negative mammary tumors that display histological and molecular features commonly found in human TNBC. Our research involves deep molecular and spatial analyses on Myc;Ptenfl tumors including bulk and single-cell RNA-sequencing, and multiplex tissue-imaging. Through comparison with human TNBC, we demonstrate that this genetic mouse model develops mammary tumors with differential survival and therapeutic responses that closely resemble the inter- and intra-tumoral and microenvironmental heterogeneity of human TNBC, providing a pre-clinical tool for assessing the spectrum of patient TNBC biology and drug response.

Abstract PR012: A novel, immune-competent, Myc-dependent murine model of rapid metastatic recurrence of pancreatic cancer after resection

Pancreatic ductal adenocarcinoma (PDAc) remains a resistant malignancy with dismal outcomes. Early diagnosis, systemic treatment, and complete resection are interdependently essential in improving survival. But even with these interventions, 20-30% of patients will experience a metastatic recurrence within six months of surgery. This “rapid recurrence” (rrPDAc) is devastating and poorly understood, contributing to the nihilism surrounding pancreatic cancer. Overlapping etiologies of these metastatic lesions are possible. They include occult synchronous metastases, as well as disseminated metachronous lesions, both of which we hypothesize may be affected by systemic and microenvironmental changes that occur due to surgical intervention. In human rrPDAc primary tumors, we have identified increased expression of Myc-targets and differences in elements of the tumor immune microenvironment when compared to long-term non-recurrers, in the absence of substantial clinical differences. We describe a novel mouse model of immune competent, surgically resected human PDAc that models rapid recurrence compared to control mice. Our lab has developed an inducible, p48-Cre-recombinase driven LSL-KrasG12D/+ LSL-ROSA-MYC+/+ mouse model that reproducibly develops ADM to PanIN to PDAc lesions that highly recapitulate human carcinogenesis. Lesions lose Smad4 expression progressively, a feature associated with metastatic phenotypes of human PDAc, and also metastasize to the liver. Considering the role MYC plays in regulation of the immune microenvironment, we hypothesized that lines derived from this model would perform well in a model of rapid recurrence. We derived several cell lines from these tumors and implanted them orthotopically in syngeneic mice, monitoring tumor development over fourteen days with ultrasound. Mice were then subjected to takedown (‘pre-op’, n = 5), anesthesia-only controls (n = 17), sham surgical incision (n = 11), and distal pancreatectomy (n = 14). No micro-metastases were identified in livers of the ‘preop’ controls. Mice were tracked via twice-weekly trans-abdominal ultrasound. Surgically resected and sham surgery mice developed metastases a median of 10 days earlier than controls (p = 0.008), suggesting that surgical intervention perturbs the development of metastatic lesions. Furthermore, we have demonstrated that circulating tumor cells may be isolated from the portal venous drainage of these mice, allowing for a novel resource in studying pre-, intra-, and metastatic-compartments of tumor. This model will allow for investigation into rrPDAc and the role surgery may play in exacerbation of metastasis.

Citation Format: Patrick J. Worth, Isabel A. English, Jackie M. Phipps, Jason M. Link, Ellen M. Langer, Motoyuki Tsuda, Colin Daniel, Carl Pelz, Jonathan Brody, Brett C. Sheppard, Rosalie C. Sears. A novel, immune-competent, Myc-dependent murine model of rapid metastatic recurrence of pancreatic cancer after resection [abstract]. In: Proceedings of the AACR Special Conference: Cancer Metastasis; 2022 Nov 14-17; Portland, OR. Philadelphia (PA): AACR; Cancer Res 2022;83(2 Suppl_2):Abstract nr PR012.

Abstract C056: The prolyl isomerase PIN1 controls fibroblast state plasticity to impact pancreatic cancer

Pancreatic cancer associated fibroblasts (CAFs) have been described to play multiple, often conflicting, roles to support or restrict tumor growth. Recent work suggests that heterogeneous differentiation states of fibroblasts contribute to these diverse functions and that reprogramming of fibroblast states can influence tumor outcomes. The impact of distinct fibroblast subpopulations on heterogeneous tumor development and growth, however, remain incompletely understood. PIN1 is a phosphorylation-directed prolyl isomerase that alters the conformation and, therefore, the function of many proteins. PIN1 is overexpressed in cancer and contributes to cancer cell-intrinsic pro-tumorigenic behaviors including cellular proliferation and migration. While its pro-tumor functions have generated interest in therapeutic targeting of PIN1 for cancer treatment, the direct effects of PIN1 inhibition on tumor-associated stromal phenotypes are poorly understood. We have found that PIN1 loss or inhibition results in decreased tumor growth in vivo in transplant or genetically engineered mouse models. Moreover, in syngeneic orthotopic allograft models in which loss of PIN1 is restricted to the tumor host, we observed that KPC tumor cells still have decreased growth, suggesting a critical role for PIN1 in the tumor microenvironment. We observed that PIN1 loss or inhibition in vivo was accompanied by decreased expression of alpha-SMA, a marker of myofibroblast-like CAFs, and have identified a role for PIN1 in the phenotypic and epigenetic response to TGF-beta, a major driver of the myCAF state. PIN1low pancreatic stellate cells or CAFs display altered cell morphology, decreased proliferation, decreased ECM deposition, as well as altered paracrine signaling to cancer cells and other stromal cells. We are currently using 2D co-cultures, heterotypic 3D bioprinted tissues, and in vivo mouse models to interrogate the molecular mechanisms by which PIN1 controls fibroblast phenotypes and functional impact of altering fibroblast state on tumor phenotypes and outcomes. In addition, we are defining PIN1-dependent mechanisms of crosstalk between neoplastic and non-neoplastic cells and are investigating the requirements for specific fibroblast states to support in vivo growth of heterogeneous pancreatic cancer cells.

Citation Format: Ellen M. Langer, Vidhi Shah, Amy Farrell, Colin Daniel, Xiaoyan Wang, Kevin MacPherson, Brittany L. Allen-Petersen, Motoyuki Tsuda, Mara Sherman, Andrew Adey, Rosalie C. Sears. The prolyl isomerase PIN1 controls fibroblast state plasticity to impact pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr C056.

Abstract A067: Myc drives phenotypic heterogeneity, metastasis, and therapy resistance in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAc) ranks among the top three most aggressive cancers in the United States and is projected to increase in incidence over the next few years. Standard of care treatment for PDAc consists of a cocktail of harsh chemotherapies, which have improved overall survival by only a few percentage points—to a 5-year survival rate of 11%. One commonly deregulated pathway in PDAc is c-MYC (MYC), a potent transcription factor. MYC plays an important role in tumor progression and its deregulation has been correlated with tumor aggressiveness and therapeutic resistance in PDAc and other cancers. Recently, oncogenic MYC expression has been shown to regulate elements of the tumor microenvironment (TME) in mouse models of multiple cancers. In PDAc, MYC’s expression has been linked to a desmoplastic immune suppressive TME, yet the specific mechanism has yet to be described. Here, we present a novel genetically engineered mouse model (GEMM) of PDAc that can be used to better model the disease and to interrogate questions of how MYC regulates the tumor immune and stromal microenvironments. Our KMCERT2 model has inducible Cre-driven expression of both mutant Kras and low deregulated Myc in the pancreas. We show that deregulated MYC cooperates with KRASG12D in the adult pancreas to drive PDAc, and our model recapitulates inter- and intra-tumoral heterogeneity seen within clinical PDAc populations. Currently, a majority of murine studies of PDAc are performed using an embryonic KrasG12D- and p53 loss/mutant-driven PDAc model (KPC). RNA- and DNA sequencing on both microdissected autochthonous tumor specimens and KMCERT2 tumor-derived cell lines was conducted to further understand the mechanisms underlying our observed phenotypes. In contrast to the KPC model, our inducible KMCERT2 model of PDAc displays genetic changes, such as CDKN2A and SMAD4 loss, comparable to human disease. Interestingly, multiplexed immunohistochemistry analysis of immune cell composition of spontaneous KMCERT2 tumors compared to the commonly used KPC shows an increased density of antigen presenting cells (APCs) within MYC-driven tumors. Human PDAc is often resistant to standard of care therapies such as gemcitabine and FOLFIRINOX. Orthotopic therapeutic studies using our KMCERT2 tumor-derived cell lines demonstrate a similar resistance to these therapies, allowing us to use this model to better understand the mechanisms leading to therapeutic resistance and to test new therapies. In addition, we find consistent metastasis to the liver in both spontaneous and orthotopic transplant settings. Together, this work investigates the role of deregulated MYC expression in metastatic behavior, immune phenotypes, and therapeutic response in murine PDAc. It also provides both spontaneous and orthotopic mouse models of PDAc that recapitulate the heterogeneous and highly metastatic nature of the human disease, allowing for important therapeutic testing opportunities.

Citation Format: Isabel A. English, Patrick J. Worth, Amy S. Farrell, Brittany L. Allen-Petersen, Vidhi Shah, Courtney Betts, Carl Pelz, Xiaoyan Wang, Colin J. Daniel, Mary C Thoma, Lisa M Coussens, Ellen M Langer, Rosalie C Sears. Myc drives phenotypic heterogeneity, metastasis, and therapy resistance in pancreatic ductal adenocarcinoma [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr A067.

Abstract PO-061: Myc drives phenotypic heterogeneity, metastasis, and therapy resistance in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) ranks among the top three most aggressive cancers in the United States and is projected to increase in incidence over the next few years. Standard of care treatment for PDAC consists of a cocktail of harsh chemotherapies, which have improved overall survival by only a few percentage points – to a 5-year survival rate of 10%. One commonly deregulated pathway in PDAC is c-MYC (MYC), a potent transcription factor. MYC plays an important role in tumor progression and its deregulation has been correlated with tumor aggressiveness and therapeutic resistance in PDAC and other cancers. Recently, oncogenic MYC expression has been shown to regulate elements of the tumor microenvironment (TME) in mouse models of multiple cancers. In PDAC, MYC’s expression has been linked to a desmoplastic immune suppressive TME, yet the specific mechanism has yet to be described. Here, in order to better model the disease and to interrogate questions of how MYC regulates the tumor immune and stromal microenvironment, we have generated a novel genetically engineered mouse model (GEMM) of PDAC. Our model (KMCERT2) has inducible Cre-driven expression of both mutant Kras and low deregulated Myc in the pancreas. We have found that deregulated MYC cooperates with KRASG12D in the adult pancreas to drive PDAC in our inducible KMCERT2 mouse model and that our model recapitulates inter- and intra-tumoral heterogeneity seen within clinical PDAC populations as well as consistent metastasis to liver in both spontaneous and orthotopic transplant settings. Currently, a majority of murine studies of PDAC are performed using an embryonic KrasG12D- and p53 loss/mutant-driven PDAC model (KPC). In contrast to the KPC model, our inducible KMCERT2 model of PDAC displays genetic changes, such as CDKN2A and SMAD4 loss, comparable to human disease. Interestingly, multiplexed immunohistochemistry analysis of immune cell composition of spontaneous KMCERT2 tumors compared to the commonly used KPC shows an increased density of antigen presenting cells (APCs) within MYC-driven tumors. Human PDAC is often resistant to standard of care therapies such as gemcitabine and FOLFIRINOX. Orthotopic therapeutic studies using our KMCERT2 cell lines demonstrate a similar resistance to these therapies. To further understand the mechanisms underlying our observed phenotypes, we have conducted RNAseq and DNA sequencing on both microdissected autochthonous tumor specimens and KMCERT2 tumor-derived cell lines. Together, this work investigates the role of deregulated MYC expression in metastatic behavior, immune phenotypes, and therapeutic response in murine PDAC. It also provides both spontaneous and orthotopic mouse models of PDAC that recapitulate the heterogeneous and highly metastatic nature of the human disease, allowing for important therapeutic testing opportunities.

Citation Format: Isabel A. English, Patrick J. Worth, Amy T. Farrell, Brittany L. Allen-Petersen, Vidhi Shah, Courtney Betts, Xiaoyan Wang, Colin J. Daniel, Mary C. Thoma, Lisa M. Coussens, Ellen M. Langer, Rosalie C. Sears. Myc drives phenotypic heterogeneity, metastasis, and therapy resistance in pancreatic ductal adenocarcinoma [abstract]. In: Proceedings of the AACR Virtual Special Conference on Pancreatic Cancer; 2021 Sep 29-30. Philadelphia (PA): AACR; Cancer Res 2021;81(22 Suppl):Abstract nr PO-061.

Abstract PO-113: The prolyl isomerase PIN1 plays a critical role in fibroblast differentiation states to support pancreatic cancer

PIN1 is a phosphorylation-directed prolyl isomerase that alters the conformation and, therefore, the function of many proteins. PIN1 overexpression in cancer contributes to cancer cell-intrinsic phenotypes including cellular proliferation and migration. While its pro-tumor functions have generated interest in therapeutic targeting of PIN1 for cancer treatment, the effects of PIN1 inhibition on tumor-associated stromal phenotypes have not yet been studied. We assessed pancreatic cancer xenografts and genetically engineered p48-Cre; LSL-KrasG12D; p53R172H (KPC) mice that were treated with small molecule PIN1 inhibitors or crossed into a full body PIN1 knockout (Pin1−/−), and found that PIN1 inhibition or loss decreased tumor growth and extended overall survival. To interrogate a direct role for PIN1 in the stroma, we orthotopically injected a KPC cell line into syngeneic Pin1+/+ or Pin1−/− hosts and found dramatic reduction of tumor cell growth in Pin1−/− hosts. Further analysis of the Pin1−/− tumor microenvironment revealed decreased expression of alpha-SMA, a marker of myofibroblastic cancer associated fibroblasts (myCAFs), as well as decreased ECM deposition and/or organization. Pancreatic stellate cells (PSCs) activated in the tumor microenvironment play a major role in the deposition of ECM and secrete growth factors to support tumor cell proliferation and survival. We, therefore, interrogated the role of PIN1 in PSCs. We found that loss of PIN1 in PSCs inhibits TGF-beta-induced stellate cell activation into a myofibroblast phenotype. Single cell ATAC-seq analysis demonstrated that a subset of TGF-beta responsive changes to chromatin accessibility are impaired in the absence of PIN1, and suggests that specific transcription factor families may play a role in the PIN1-dependent response to TGF-beta. Further analysis of PSCs or CAFs with PIN1 loss indicated that, at baseline, these cells express gene programs consistent with the recently described antigen presenting CAFs (apCAFs). Finally, in addition to changes in cellular state and plasticity, we found that loss of PIN1 alters PSC secretion of paracrine factors that support oncogenic phenotypes. For example, PSCs with loss of PIN1 have reduced expression of HGF and increased expression of VEGF, resulting in altered cancer cell and vascular phenotypes. This work establishes a role for PIN1 in regulating fibroblast function and suggests that targeting PIN1 in cancer will have a broad anti-tumor effect. Our ongoing work continues to use 2D co-cultures, heterotypic 3D bioprinted tissues, and in vivo mouse models to interrogate the precise mechanisms by which PIN1 controls fibroblast phenotypes and impact of these changes on tumor phenotypes and outcomes.

Citation Format: Ellen M. Langer, Isabel A. English, Vidhi Shah, Kevin MacPherson, Kayleigh M. Kresse, Brittany L. Allen-Petersen, Colin J. Daniel, Mara H. Sherman, Andrew Adey, Rosalie C. Sears. The prolyl isomerase PIN1 plays a critical role in fibroblast differentiation states to support pancreatic cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on Pancreatic Cancer; 2021 Sep 29-30. Philadelphia (PA): AACR; Cancer Res 2021;81(22 Suppl):Abstract nr PO-113.

Sulfopin is a covalent inhibitor of Pin1 that blocks Myc-driven tumors in vivo

The peptidyl-prolyl isomerase, Pin1, is exploited in cancer to activate oncogenes and inactivate tumor suppressors. However, despite considerable efforts, Pin1 has remained an elusive drug target. Here, we screened an electrophilic fragment library to identify covalent inhibitors targeting Pin1's active site Cys113, leading to the development of Sulfopin, a nanomolar Pin1 inhibitor. Sulfopin is highly selective, as validated by two independent chemoproteomics methods, achieves potent cellular and in vivo target engagement and phenocopies Pin1 genetic knockout. Pin1 inhibition had only a modest effect on cancer cell line viability. Nevertheless, Sulfopin induced downregulation of c-Myc target genes, reduced tumor progression and conferred survival benefit in murine and zebrafish models of MYCN-driven neuroblastoma, and in a murine model of pancreatic cancer. Our results demonstrate that Sulfopin is a chemical probe suitable for assessment of Pin1-dependent pharmacology in cells and in vivo, and that Pin1 warrants further investigation as a potential cancer drug target.

Abstract LT012: The prolyl isomerase PIN1 plays a critical role in fibroblast plasticity to impact pancreatic cancer

PIN1 is a phosphorylation-directed prolyl isomerase that alters the conformation and, therefore, the function of many proteins. Due to its role in activation and stabilization of many oncogenes, we hypothesized that targeting PIN1 in pancreatic ductal adenocarcinoma (PDA) would slow tumor growth. We tested this hypothesis in vitro and in vivo with PIN1 inhibitors and/or genetic model systems. Pancreatic cancer cell lines knocked down for PIN1 or treated with PIN1 inhibitors showed decreased proliferation, invasion, and anchorage independent growth compared to control lines. Consistent with these in vitro results, treatment of pancreatic cancer xenografts or genetically engineered p48-Cre; LSL-KrasG12D; p53R172H (KPC) mice with PIN1 inhibitors decreased tumor growth and extended overall survival. Similar results were seen in KPC mice that were crossed into a full body PIN1 knockout (PIN1−/−). Further analysis of KPC PIN1−/− tumors revealed not only reduced size of pancreatic tumors, but also decreased alpha-SMA expression and decreased ECM deposition in the stroma surrounding the tumors. PDA is characterized by a dense, desmoplastic tumor stroma that contributes to tumor growth, metastasis, and therapeutic resistance. Pancreatic stellate cells (PSCs) that are activated in the tumor microenvironment play a major role in the deposition of ECM and secrete growth factors to support tumor cell proliferation and survival. To interrogate a direct role for PIN1 in the stroma, we first orthotopically injected a KPC cell line into syngeneic PIN1+/+ or PIN1−/− mice and found dramatic reduction of tumor cell growth in PIN1−/− hosts. Next, we analyzed PSCs in vitro and found that loss of PIN1 reduces their proliferation and alters their secretion of paracrine factors that support oncogenic phenotypes. For example, PSCs with loss of PIN1 have reduced expression of HGF and increased expression of SPINT1 and SPINT2, inhibitors of HGF activation. Conditioned media from control PSCs, but not from PSCs lacking PIN1 expression, activates the MET receptor on cancer cell lines, resulting in altered cancer cell phenotypes. In addition, we show that loss of PIN1 in PSCs inhibits TGF-beta induced stellate cells activation into a myofibroblast phenotype. Single cell ATAC-seq analysis demonstrated that a subset of TGF-beta responsive chromatin changes are impaired in the absence of PIN1. Our ongoing work utilizes 2D co-cultures, heterotypic 3D bioprinted tissues, and in vivo mouse models to interrogate the mechanisms by which fibroblast phenotypes and the tumor-stromal crosstalk is impacted by PIN1.

Citation Format: Ellen M. Langer, Isabel A. English, Kayleigh M. Kresse, Kevin MacPherson, Brittany L. Allen-Petersen, Colin J. Daniel, Andrew Adey, Rosalie C. Sears. The prolyl isomerase PIN1 plays a critical role in fibroblast plasticity to impact pancreatic cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on the Evolving Tumor Microenvironment in Cancer Progression: Mechanisms and Emerging Therapeutic Opportunities; in association with the Tumor Microenvironment (TME) Working Group; 2021 Jan 11-12. Philadelphia (PA): AACR; Cancer Res 2021;81(5 Suppl):Abstract nr LT012.

Abstract IA-06: Heterotypic 3D bioprinted tissues to study pancreatic cancer

Crosstalk between tumor cells and the tumor microenvironment contributes critically to tumor initiation, tumor progression, and therapeutic response. The tumor microenvironment (TME) is composed of multiple cell types (e.g. fibroblast, vascular, and immune cells) that are temporally and spatially dynamic. Stromal cells in the TME respond not only to the growing tumor cells, but are also affected by concurrent changes in the accessibility and composition of the extracellular matrix (ECM), signaling molecules, nutrients and oxygen. Together, the interactions of tumor cells and stromal cells regulates the tumor phenotype and, ultimately, affects patient outcomes. As more is understood about the dynamic relationship of multiple cell types in the tumor microenvironment, it has become clear that better models of human cancer are needed. In order to understand the impact of heterotypic crosstalk on tumor behaviors such as initiation, progression, and response to therapies, we recently developed heterotypic, scaffold-free tissue models of advanced cancer using an extrusion-based bioprinter system. Multiple cell types including cancer cells, fibroblasts, endothelial cells, mesenchymal stem cells, and immune cells can be incorporated into bioprinted tissues with defined spatial architecture, and the system is compatible with patient-derived cells. Within these structures, cells exhibit a tissue-like cellular density, deposit ECM, and self-organize to form complex structures, as illustrated by formation of nascent endothelial networks. Cell intrinsic, extrinsic, and spatial phenotypes, including cell survival, cell proliferation, differentiation state, ECM deposition, and cellular migration, can be assessed within these tissues following exposure to extrinsic signals or therapies. Our current work is focused in three main areas. First, we aim to understand the signaling between tumor and stromal cell types in early cancer development that contributes to malignant progression. Second, we are working to understand the influence of fibroblast heterogeneity on tumor cell phenotypes, using genetic or non-genetic perturbagens to alter fibroblast activation. Finally, we are building tissues comprising patient-derived cells to understand how the microenvironment influences therapeutic response. Together, our work demonstrates that bioprinted tumor tissues recapitulate many aspects of in vivo neoplastic tissues, and provides a manipulable model system to interrogate molecular mechanisms of tumor development, progression, and treatment response.

Citation Format: Ellen M Langer, Brittany L Allen-Petersen, Isabel A English, Jason M Link, Rosalie C Sears. Heterotypic 3D bioprinted tissues to study pancreatic cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on Pancreatic Cancer; 2020 Sep 29-30. Philadelphia (PA): AACR; Cancer Res 2020;80(22 Suppl):Abstract nr IA-06.

Altering MYC phosphorylation in the epidermis increases the stem cell population and contributes to the development, progression, and metastasis of squamous cell carcinoma

cMYC (MYC) is a potent oncoprotein that is subject to post-translational modifications that affect its stability and activity. Here, we show that Serine 62 phosphorylation, which increases MYC stability and oncogenic activity, is elevated while Threonine 58 phosphorylation, which targets MYC for degradation, is decreased in squamous cell carcinoma (SCC). The oncogenic role of MYC in the development of SCC is unclear since studies have shown in normal skin that wild-type MYC overexpression can drive loss of stem cells and epidermal differentiation. To investigate whether and how altered MYC phosphorylation might affect SCC development, progression, and metastasis, we generated mice with inducible expression of MYC^WT or MYC^T58A in the basal layer of the skin epidermis. In the T58A mutant, MYC is stabilized with constitutive S62 phosphorylation. When challenged with DMBA/TPA-mediated carcinogenesis, MYC^T58A mice had accelerated development of papillomas, increased conversion to malignant lesions, and increased metastasis as compared to MYC^WT mice. In addition, MYC^T58A-driven SCC displayed stem cell gene expression not observed with MYC^WT, including increased expression of Lgr6, Sox2, and CD34. In support of MYC^T58A enhancing stem cell phenotypes, its expression was associated with an increased number of BrdU long-term label-retaining cells, increased CD34 expression in hair follicles, and increased colony formation from neonatal keratinocytes. Together, these results indicate that altering MYC phosphorylation changes its oncogenic activity—instead of diminishing establishment and/or maintenance of epidermal stem cell populations like wild-type MYC, pS62-MYC enhances these populations and, under carcinogenic conditions, pS62-MYC expression results in aggressive tumor phenotypes.

Abstract A27: Heterotypic 3D bioprinted tissues to interrogate tumor-microenvironment crosstalk in cancer

The tumor microenvironment is composed of multiple cell types (e.g., fibroblast, vascular, and immune cells), extracellular matrix (ECM) proteins, and signaling molecules that critically influence tumor cell phenotypes. The study of isolated tumor cells in culture has contributed to the discovery of oncogenes and tumor suppressors that regulate oncogenic phenotypes. As tumors develop, however, cancer cells secrete factors that functionally reprogram the genetically “normal” stromal cells in their environment. These tumor-associated stromal cells then further alter the microenvironment, providing tumor-stromal cell and tumor-ECM interactions that critically regulate tumor behaviors such as early tumor initiation, metastatic progression, and response to therapies. We recently developed heterotypic, scaffold-free tissue models of advanced cancer using an extrusion-based bioprinter system. Multiple cell types including cancer cells, fibroblasts, bone marrow-derived mesenchymal stem cells, and endothelial cells can be incorporated into bioprinted tissues with defined architecture. We found that cells within these structures exhibit a tissue-like cellular density, proliferate, deposit ECM, migrate, and respond to extrinsic signals. Cells within these tissues self-organize to form complex structures, as indicated by nascent endothelial networks, and respond to extrinsic signals. We assessed intrinsic, extrinsic, and spatial tumorigenic phenotypes including cell survival, cell proliferation, differentiation state, ECM deposition, and cellular migration within these tissues in response to extrinsic signals or therapies. Together, this work demonstrates that bioprinted tumor tissue models recapitulate many aspects of in vivo neoplastic tissues. We are continuing to use 3D bioprinted tumor tissues, leveraging the manipulable nature of these models to interrogate the role of distinct tumor microenvironments on tumor cell phenotypes including proliferation, migration, and response to therapies. In pancreatic cancer models, we have printed tissues containing pancreatic stellate cells with and without knockdown of the prolyl isomerase PIN1, which we show inhibits stellate cell activation. We show that loss of PIN1 in the stellate cells surrounding the tumor has effects not only on activation markers of fibroblasts, but also on tumor, endothelial, and immune phenotypes within the tissues. Further work is focused on characterizing the signals that drive these changes.

Citation Format: Ellen M. Langer, Isabel A. English, Megan A. Turnidge, Zinab O. Doha, Rosalie C. Sears. Heterotypic 3D bioprinted tissues to interrogate tumor-microenvironment crosstalk in cancer [abstract]. In: Proceedings of the AACR Special Conference on the Evolving Landscape of Cancer Modeling; 2020 Mar 2-5; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2020;80(11 Suppl):Abstract nr A27.

PIN1 Provides Dynamic Control of MYC in Response to Extrinsic Signals

PIN1 is a phosphorylation-directed member of the peptidyl-prolyl cis/trans isomerase (PPIase) family that facilitates conformational changes in phosphorylated targets such as c-MYC (MYC). Following signaling events that mediate phosphorylation of MYC at Serine 62, PIN1 establishes structurally distinct pools of MYC through its trans-cis and cis-trans isomerization activity at Proline 63. Through these isomerization steps, PIN1 functionally regulates MYC's stability, the molecular timing of its DNA binding and transcriptional activity, and its subnuclear localization. Recently, our group showed that Serine 62 phosphorylated MYC can associate with the inner basket of the nuclear pore (NP) in a PIN1-dependent manner. The poised euchromatin at the NP basket enables rapid cellular response to environmental signals and cell stress, and PIN1-mediated trafficking of MYC calibrates this response. In this perspective, we describe the molecular aspects of PIN1 target recognition and PIN1's function in the context of its temporal and spatial regulation of MYC.

Modeling differentiation-state transitions linked to therapeutic escape in triple-negative breast cancer

Drug resistance in breast cancer cell populations has been shown to arise through phenotypic transition of cancer cells to a drug-tolerant state, for example through epithelial-to-mesenchymal transition or transition to a cancer stem cell state. However, many breast tumors are a heterogeneous mixture of cell types with numerous epigenetic states in addition to stem-like and mesenchymal phenotypes, and the dynamic behavior of this heterogeneous mixture in response to drug treatment is not well-understood. Recently, we showed that plasticity between differentiation states, as identified with intracellular markers such as cytokeratins, is linked to resistance to specific targeted therapeutics. Understanding the dynamics of differentiation-state transitions in this context could facilitate the development of more effective treatments for cancers that exhibit phenotypic heterogeneity and plasticity. In this work, we develop computational models of a drug-treated, phenotypically heterogeneous triple-negative breast cancer (TNBC) cell line to elucidate the feasibility of differentiation-state transition as a mechanism for therapeutic escape in this tumor subtype. Specifically, we use modeling to predict the changes in differentiation-state transitions that underlie specific therapy-induced changes in differentiation-state marker expression that we recently observed in the HCC1143 cell line. We report several statistically significant therapy-induced changes in transition rates between basal, luminal, mesenchymal, and non-basal/non-luminal/non-mesenchymal differentiation states in HCC1143 cell populations. Moreover, we validate model predictions on cell division and cell death empirically, and we test our models on an independent data set. Overall, we demonstrate that changes in differentiation-state transition rates induced by targeted therapy can provoke distinct differentiation-state aggregations of drug-resistant cells, which may be fundamental to the design of improved therapeutic regimens for cancers with phenotypic heterogeneity.

Modeling Tumor Phenotypes In Vitro with Three-Dimensional Bioprinting

The tumor microenvironment plays a critical role in tumor growth, progression, and therapeutic resistance, but interrogating the role of specific tumor-stromal interactions on tumorigenic phenotypes is challenging within in vivo tissues. Here, we tested whether three-dimensional (3D) bioprinting could improve in vitro models by incorporating multiple cell types into scaffold-free tumor tissues with defined architecture. We generated tumor tissues from distinct subtypes of breast or pancreatic cancer in relevant microenvironments and demonstrate that this technique can model patient-specific tumors by using primary patient tissue. We assess intrinsic, extrinsic, and spatial tumorigenic phenotypes in bioprinted tissues and find that cellular proliferation, extracellular matrix deposition, and cellular migration are altered in response to extrinsic signals or therapies. Together, this work demonstrates that multi-cell-type bioprinted tissues can recapitulate aspects of in vivo neoplastic tissues and provide a manipulable system for the interrogation of multiple tumorigenic endpoints in the context of distinct tumor microenvironments.

ZEB1-repressed microRNAs inhibit autocrine signaling that promotes vascular mimicry of breast cancer cells

During normal tumor growth and in response to some therapies, tumor cells experience acute or chronic deprivation of nutrients and oxygen and induce tumor vascularization. While this occurs predominately through sprouting angiogenesis, tumor cells have also been shown to directly contribute to vessel formation through vascular mimicry (VM) and/or endothelial transdifferentiation. The extrinsic and intrinsic mechanisms underlying tumor cell adoption of endothelial phenotypes, however, are not well understood. Here we show that serum withdrawal induces mesenchymal breast cancer cells to undergo VM and that knockdown of the epithelial-to-mesenchymal transition (EMT) regulator, Zinc finger E-box binding homeobox 1 (ZEB1), or overexpression of the ZEB1-repressed microRNAs (miRNAs), miR-200c, miR-183, miR-96 and miR-182 inhibits this process. We find that secreted proteins Fibronectin 1 (FN1) and serine protease inhibitor (serpin) family E member 2 (SERPINE2) are essential for VM in this system. These secreted factors are upregulated in mesenchymal cells in response to serum withdrawal, and overexpression of VM-inhibiting miRNAs abrogates this upregulation. Intriguingly, the receptors for these secreted proteins, low-density lipoprotein receptor-related protein 1 (LRP1) and Integrin beta 1 (ITGB1), are also targets of the VM-inhibiting miRNAs, suggesting that autocrine signaling stimulating VM is regulated by ZEB1-repressed miRNA clusters. Together, these data provide mechanistic insight into the regulation of VM and suggest that miRNAs repressed during EMT, in addition to suppressing migratory and stem-like properties of tumor cells, also inhibit endothelial phenotypes of breast cancer cells adopted in response to a nutrient-deficient microenvironment.

The tumor suppressor phosphatase PP2A-B56α regulates stemness and promotes the initiation of malignancies in a novel murine model

Protein phosphatase 2A (PP2A) is a ubiquitously expressed Serine-Threonine phosphatase mediating 30–50% of protein phosphatase activity. PP2A functions as a heterotrimeric complex, with the B subunits directing target specificity to regulate the activity of many key pathways that control cellular phenotypes. PP2A-B56α has been shown to play a tumor suppressor role and to negatively control c-MYC stability and activity. Loss of B56α promotes cellular transformation, likely at least in part through its regulation of c-MYC. Here we report generation of a B56α hypomorph mouse with very low B56α expression that we used to study the physiologic activity of the PP2A-B56α phosphatase. The predominant phenotype we observed in mice with B56α deficiency in the whole body was spontaneous skin lesion formation with hyperproliferation of the epidermis, hair follicles and sebaceous glands. Increased levels of c-MYC phosphorylation on Serine62 and c-MYC activity were observed in the skin lesions of the B56α^hm/hm mice. B56α deficiency was found to increase the number of skin stem cells, and consistent with this, papilloma initiation was accelerated in a carcinogenesis model. Further analysis of additional tissues revealed increased inflammation in spleen, liver, lung, and intestinal lymph nodes as well as in the skin lesions, resembling elevated extramedullary hematopoiesis phenotypes in the B56α^hm/hm mice. We also observed an increase in the clonogenicity of bone marrow stem cells in B56α^hm/hm mice. Overall, this model suggests that B56α is important for stem cells to maintain homeostasis and that B56α loss leading to increased activity of important oncogenes, including c-MYC, can result in aberrant cell growth and increased stem cells that can contribute to the initiation of malignancy.

The impact of chromosomal translocation locus and fusion oncogene coding sequence in synovial sarcomagenesis

Synovial sarcomas are aggressive soft-tissue malignancies that express chromosomal translocation-generated fusion genes, SS18-SSX1 or SS18-SSX2 in most cases. Here, we report a mouse sarcoma model expressing SS18-SSX1, complementing our prior model expressing SS18-SSX2. Exome sequencing identified no recurrent secondary mutations in tumors of either genotype. Most of the few mutations identified in single tumors were present in genes that were minimally or not expressed in any of the tumors. Chromosome 6, either entirely or around the fusion gene expression locus, demonstrated a copy number gain in a majority of tumors of both genotypes. Thus, by fusion oncogene coding sequence alone, SS18-SSX1 and SS18-SSX2 can each drive comparable synovial sarcomagenesis, independent from other genetic drivers. SS18-SSX1 and SS18-SSX2 tumor transcriptomes demonstrated very few consistent differences overall. In direct tumorigenesis comparisons, SS18-SSX2 was slightly more sarcomagenic than SS18-SSX1, but equivalent in its generation of biphasic histologic features. Meta-analysis of human synovial sarcoma patient series identified two tumor-gentoype-phenotype correlations that were not modeled by the mice, namely a scarcity of male hosts and biphasic histologic features among SS18-SSX2 tumors. Re-analysis of human SS18-SSX1 and SS18-SSX2 tumor transcriptomes demonstrated very few consistent differences, but highlighted increased native SSX2 expression in SS18-SSX1 tumors. This suggests that the translocated locus may drive genotype-phenotype differences more than the coding sequence of the fusion gene created. Two possible roles for native SSX2 in synovial sarcomagenesis are explored. Thus, even specific partial failures of mouse genetic modeling can be instructive to human tumor biology.

Abstract B51: Modeling the intrinsic and extrinsic influences on breast cancer phenotypic heterogeneity using mouse models and three-dimensional bioprinting

Breast cancer cells exhibit intertumoral and intratumoral heterogeneity due to both tumor cell intrinsic and extrinsic influence. We have been modeling phenotypic heterogeneity in breast cancer using multiple model systems to interrogate the causes of this heterogeneity as well as its effects on therapeutic resistance. Toward this end, we generated a novel, genetically-engineered mouse model of triple negative breast cancer driven by loss of PTEN in combination with low-level deregulated c-Myc expression. Expression of deregulated Myc significantly accelerates tumorigenesis in this model. Histologic and global gene expression analyses reveal that this model generates predominately two phenotypic groups within triple negative breast cancer. One group is composed of fibrotic tumors that have increased ECM deposition as well as increased alpha-Smooth Muscle Actin (SMA) and Fibroblast Associated Protein (FAP) expression in the stroma. These tumors express a Claudin-low molecular signature and have high p-SMAD3 expression. The second group is composed of adenocarcinomas that have less stromal involvement and decreased ECM deposition. In preliminary studies, the more fibrotic tumors exhibit increased resistance to targeted therapeutics. To better understand if this resistance is due to intrinsic or extrinsic influence, we are generating primary cultures of both the tumor and stromal cells from these mice. Through additional in vitro and in vivo studies, we will interrogate the mechanisms of therapeutic resistance. To build upon this approach, we are also utilizing three-dimensional bioprinting to model the heterogeneity of tumor/stroma interactions with human cell lines or primary cultures. For this model, we surround breast cancer cells with stromal cells of the tumor microenvironment, including fibroblasts and endothelial cells. As the structures mature over the course of 1-3 weeks, the cells within these tissues self-organize and respond to extrinsic signals. In this system, we can assess the contribution of distinct cell types to the overall histology of the tissues as well as interrogate the mechanisms driving specific phenotypes by manipulating either the tumor or stromal cells prior to printing. Together, analysis of both the mouse models and bioprinted human tissues will help reveal nodes of crosstalk between breast tumor cells and cells within the microenvironment that affect baseline phenotypic heterogeneity and/or therapeutic efficacy.

Citation Format: Ellen M. Langer, Xiaoyan Wang, Juan Liang, Brittany L. Allen-Petersen, Nicholas D. Kendsersky, Tyler Risom, Carl Pelz, Rosalie C. Sears. Modeling the intrinsic and extrinsic influences on breast cancer phenotypic heterogeneity using mouse models and three-dimensional bioprinting. [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 B51.

Snail promotes the cell-autonomous generation of Flk1(+) endothelial cells through the repression of the microRNA-200 family

Expression of the transcription factor Snail is required for normal vasculogenesis in the developing mouse embryo. In addition, tumors expressing Snail have been associated with a more malignant phenotype, with both increased invasive properties and angiogenesis. Although the relationship between Snail and vasculogenesis has been noted, no mechanistic analysis has been elucidated. Here, we show that in addition to inducing an epithelial mesenchymal transition, Snail promotes the cell-autonomous induction of Flk1(+) endothelial cells in an early subset of differentiating mouse embryonic stem (ES) cells. Cells that become Flk1+ in response to Snail have a transcriptional profile specific to Gata6+primitive endoderm, but not the early Nanog+epiblast. We further show that Snail's ability to promote Flk1(+) endothelium depends on fibroblast growth factor signaling as well as the repression of the microRNA-200 (miR-200) family, which directly targets the 3' UTRs of Flk1 and Ets1. Together, our results show that Snail is capable of inducing Flk1+ lineage commitment in a subset of differentiating ES cells through the down-regulation of the miR-200 family. We hypothesize that this mechanism of Snail-induced vasculogenesis may be conserved in both the early developing embryo and malignant cancers.

Snail and the microRNA-200 family act in opposition to regulate epithelial-to-mesenchymal transition and germ layer fate restriction in differentiating ESCs

The reprogramming of somatic cells to inducible pluripotent stem cells requires a mesenchymal-to-epithelial transition. While differentiating ESCs can undergo the reverse process or epithelial-to-mesenchymal transition (EMT), little is known about the role of EMT in ESC differentiation and fate commitment. Here, we show that Snail homolog 1 (Snail) is expressed during ESC differentiation and is capable of inducing EMT on day 2 of ESC differentiation. Induction of EMT by Snail promotes mesoderm commitment while repressing markers of the primitive ectoderm and epiblast. Snail's impact on differentiation can be partly explained through its regulation of a number of ESC-associated microRNAs, including the microRNA-200 (miR-200) family. The miR-200 family is normally expressed in ESCs but is downregulated in a Wnt-dependent manner during EMT. Maintenance of miR-200 expression stalls differentiating ESCs at the epiblast-like stem cell (EpiSC) stage. Consistent with a role for activin in maintaining the EpiSC state, we find that inhibition of activin signaling decreases miR-200 expression and allows EMT to proceed with a bias toward neuroectoderm commitment. Furthermore, miR-200 requires activin to efficiently maintain cells at the epiblast stage. Together, these findings demonstrate that Snail and miR-200 act in opposition to regulate EMT and exit from the EpiSC stage toward induction of germ layer fates. By modulating expression levels of Snail, activin, and miR-200, we are able to control the order in which cells undergo EMT and transition out of the EpiSC state. Stem Cells 2011;29:764–776

Mesp1 coordinately regulates cardiovascular fate restriction and epithelial-mesenchymal transition in differentiating ESCs

Wnt signaling is required for development of mesoderm-derived lineages and expression of transcription factors associated with the primitive streak. In a functional screen, we examined the mesoderm-inducing capacity of transcription factors whose expression was Wnt-dependent in differentiating ESCs. In contrast to many inactive factors, we found that mesoderm posterior 1 (Mesp1) promoted mesoderm development independently of Wnt signaling. Transient Mesp1 expression in ESCs promotes changes associated with epithelial-mesenchymal transition (EMT) and induction of Snai1, consistent with a role in gastrulation. Mesp1 expression also restricted the potential fates derived from ESCs, generating mesoderm progenitors with cardiovascular, but not hematopoietic, potential. Thus, in addition to its effects on EMT, Mesp1 may be capable of generating the recently identified multipotent cardiovascular progenitor from ESCs in vitro.

Ajuba LIM proteins are snail/slug corepressors required for neural crest development in Xenopus

Snail family transcriptional repressors regulate epithelial mesenchymal transitions during physiological and pathological processes. A conserved SNAG repression domain present in all vertebrate Snail proteins is necessary for repressor complex assembly. Here, we identify the Ajuba family of LIM proteins as functional corepressors of the Snail family via an interaction with the SNAG domain. Ajuba LIM proteins interact with Snail in the nucleus on endogenous E-cadherin promoters and contribute to Snail-dependent repression of E-cadherin. Using Xenopus neural crest as a model of in vivo Snail- or Slug-induced EMT, we demonstrate that Ajuba LIM proteins contribute to neural crest development as Snail/Slug corepressors and are required for in vivo Snail/Slug function. Because Ajuba LIM proteins are also components of adherens junctions and contribute to their assembly or stability, their functional interaction with Snail proteins in the nucleus suggests that Ajuba LIM proteins are important regulators of epithelia dynamics communicating surface events with nuclear responses.

The Ajuba LIM domain protein is a corepressor for SNAG domain mediated repression and participates in nucleocytoplasmic Shuttling

The SNAG repression domain is comprised of a highly conserved 21-amino acid sequence, is named for its presence in the Snail/growth factor independence-1 class of zinc finger transcription factors, and is present in a variety of proto-oncogenic transcription factors and developmental regulators. The prototype SNAG domain containing oncogene, growth factor independence-1, is responsible for the development of T cell thymomas. The SNAIL proteins also encode the SNAG domain and play key roles in epithelial mesenchymal differentiation events during development and metastasis. Significantly, these oncogenic functions require a functional SNAG domain. The molecular mechanisms of SNAG domain-mediated transcriptional repression are largely unknown. Using a yeast two-hybrid strategy, we identified Ajuba, a multiple LIM domain protein that can function as a corepressor for the SNAG domain. Ajuba interacts with the SNAG domain in vitro and in vivo, colocalizes with it, and enhances SNAG-mediated transcriptional repression. Ajuba shuttles between the cytoplasm and the nucleus and may form a novel intracellular signaling system. Using an integrated reporter gene combined with chromatin immunoprecipitation, we observed rapid, SNAG-dependent assembly of a multiprotein complex that included Ajuba, SNAG, and histone modifications consistent with the repressed state. Thus, SNAG domain proteins may bind Ajuba, trapping it in the nucleus where it functions as an adapter or molecular scaffold for the assembly of macromolecular repression complexes at target promoters.

Bcl-xL deamidation is a critical switch in the regulation of the response to DNA damage

The therapeutic value of DNA-damaging antineoplastic agents is dependent upon their ability to induce tumor cell apoptosis while sparing most normal tissues. Here, we show that a component of the apoptotic response to these agents in several different types of tumor cells is the deamidation of two asparagines in the unstructured loop of Bcl-xL, and we demonstrate that deamidation of these asparagines imports susceptibility to apoptosis by disrupting the ability of Bcl-xL to block the proapoptotic activity of BH3 domain-only proteins. Conversely, Bcl-xL deamidation is actively suppressed in fibroblasts, and suppression of deamidation is an essential component of their resistance to DNA damage-induced apoptosis. Our results suggest that the regulation of Bcl-xL deamidation has a critical role in the tumor-specific activity of DNA-damaging antineoplastic agents.

The applications of the BrdUrd-technique for the estimation of cycling S-phase cells in human renal cell carcinoma

After testing the BrdUrd technique on experimental tumour cell lines, we applied the technique to human renal cell carcinoma in vitro. We compared the results with the data acquired after FCM analysis and 3H-thymidine treatment. In contrast to BrdUrd the 3H-thymidine uptake seemed to be limited in suspended cells. FCM data represented the DNA distribution of cells. BrdUrd labelling on the other hand detected DNA synthesizing cells. Only both methods in parallel were able to discriminate between proliferating cells and resting cells with an S-phase equivalent DNA content.

Cycling S-phase cells in animal and spontaneous tumours. I. Comparison of the BrdUrd and 3H-thymidine techniques and flow cytometry for the estimation of S-phase frequency

Evaluation of the proliferative activities of cell populations has mainly been restricted to the use of autoradiography and flow cytometric measurements. The introduction of a new BrdUrd specific antibody makes it possible to determine exactly the DNA synthesizing cells. The BrdUrd technique is safe with respect to handling and the results are obtained within five hours. The suitability of the BrdUrd labelling procedure has been studied in different cell lines and compared with 3H-thymidine autoradiography and flow cytometry.