Low-grade and high-grade mammary carcinomas in WAP-T transgenic mice are independent entities distinguished by Met expression

Basal-like mammary carcinomas stimulate cancer stem cell properties through AXL-signaling to induce chemotherapy resistance

Basal-like breast cancer (BLBC) is the most aggressive and heterogeneous breast cancer (BC) subtype. Conventional chemotherapies represent next to surgery the most frequently employed treatment options. Unfortunately, resistant tumor phenotypes often develop, resulting in therapeutic failure. To identify the early events occurring upon the first drug application and initiating chemotherapy resistance in BLBC, we leveraged the WAP-T syngeneic mammary carcinoma mouse model and we developed a strategy combining magnetic-activated cell sorting (MACS)-based tumor cell enrichment with high-throughput transcriptome analyses. We discovered that chemotherapy induced a massive gene expression reprogramming toward stemness acquisition to tolerate and survive the cytotoxic treatment in vitro and in vivo. Retransplantation experiments revealed that one single cycle of cytotoxic drug combination therapy (Cyclophosphamide, Adriamycin and 5-Fluorouracil) suffices to induce resistant tumor cell phenotypes in vivo. We identified Axl and its ligand Pros1 as highly induced genes driving cancer stem cell (CSC) properties upon chemotherapy in vivo and in vitro. Furthermore, from our analysis of BLBC patient datasets, we found that AXL expression is also strongly correlated with CSC-gene signatures, a poor response to conventional therapies and worse survival outcomes in those patients. Finally, we demonstrate that AXL inhibition sensitized BLBC-cells to cytotoxic treatment in vitro. Together, our data support AXL as a promising therapeutic target to optimize the efficiency of conventional cytotoxic therapies in BLBC.

ROBO3s: a novel ROBO3 short isoform promoting breast cancer aggressiveness

Basal-like breast cancer (BLBC) is a highly aggressive breast cancer subtype frequently associated with poor prognosis. Due to the scarcity of targeted treatment options, conventional cytotoxic chemotherapies frequently remain the standard of care. Unfortunately, their efficacy is limited as BLBC malignancies rapidly develop resistant phenotypes. Using transcriptomic and proteomic approaches in human and murine BLBC cells, we aimed to elucidate the molecular mechanisms underlying the acquisition of aggressive and chemotherapy-resistant phenotypes in these mammary tumors. Specifically, we identified and characterized a novel short isoform of Roundabout Guidance Receptor 3 (ROBO3s), upregulated in BLBC in response to chemotherapy and encoding for a protein variant lacking the transmembrane domain. We established an important role for the ROBO3s isoform, mediating cancer stem cell properties by stimulating the Hippo-YAP signaling pathway, and thus driving resistance of BLBC cells to cytotoxic drugs. By uncovering the conservation of ROBO3s expression across multiple cancer types, as well as its association with reduced BLBC-patient survival, we emphasize its potential as a prognostic marker and identify a novel attractive target for anti-cancer drug development.

Decreased PRC2 activity supports the survival of basal-like breast cancer cells to cytotoxic treatments

Breast cancer (BC) is the most common cancer occurring in women but also rarely develops in men. Recent advances in early diagnosis and development of targeted therapies have greatly improved the survival rate of BC patients. However, the basal-like BC subtype (BLBC), largely overlapping with the triple-negative BC subtype (TNBC), lacks such drug targets and conventional cytotoxic chemotherapies often remain the only treatment option. Thus, the development of resistance to cytotoxic therapies has fatal consequences. To assess the involvement of epigenetic mechanisms and their therapeutic potential increasing cytotoxic drug efficiency, we combined high-throughput RNA- and ChIP-sequencing analyses in BLBC cells. Tumor cells surviving chemotherapy upregulated transcriptional programs of epithelial-to-mesenchymal transition (EMT) and stemness. To our surprise, the same cells showed a pronounced reduction of polycomb repressive complex 2 (PRC2) activity via downregulation of its subunits Ezh2, Suz12, Rbbp7 and Mtf2. Mechanistically, loss of PRC2 activity leads to the de-repression of a set of genes through an epigenetic switch from repressive H3K27me3 to activating H3K27ac mark at regulatory regions. We identified Nfatc1 as an upregulated gene upon loss of PRC2 activity and directly implicated in the transcriptional changes happening upon survival to chemotherapy. Blocking NFATc1 activation reduced epithelial-to-mesenchymal transition, aggressiveness, and therapy resistance of BLBC cells. Our data demonstrate a previously unknown function of PRC2 maintaining low Nfatc1 expression levels and thereby repressing aggressiveness and therapy resistance in BLBC.

CEACAM1 controls the EMT switch in murine mammary carcinoma in vitro and in vivo

We analyzed the molecular basis for carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1)-controlled inhibition of epithelial-mesenchymal transition (EMT) in a mouse model for mammary adenocarcinoma (WAP-T mice). We demonstrate that silencing of CEACAM1 in WAP-T tumor-derived G-2 cells induces epithelial-mesenchymal plasticity (EMP), as evidenced by typical changes of gene expression, morphology and increased invasion. In contrast, reintroduction of CEACAM1 into G-2 cells reversed up-regulation of genes imposing mesenchymal transition, as well as cellular invasion. We identified the Wnt-pathway as target for CEACAM1-mediated repression of EMT. Importantly, β-catenin phosphorylation status and transcriptional activity strongly depend on CEACAM1 expression: CEACAM1^high G-2 cells displayed enhanced phosphorylation of β-catenin at S33/S37/T41 and decreased phosphorylation at Y86, thereby inhibiting canonical Wnt/β-catenin signaling. We identified Src-homology 2 domain-containing phosphatase 2 (SHP-2) as a critical binding partner of CEACAM1 that could modulate β-catenin Y86 phosphorylation. Hence, CEACAM1 serves as a scaffold that controls membrane proximal β-catenin signaling. In vivo, mammary tumors of WAP-T/CEACAM1^null mice displayed increased nuclear translocation of β-catenin and a dramatically enhanced metastasis rate compared to WAP-T mice. Hence, CEACAM1 controls EMT in vitro and in vivo by site-specific regulation of β-catenin phosphorylation. Survival analyses of human mammary carcinoma patients corroborated these data, indicating that CEACAM1 is a prognostic marker for breast cancer survival.

T-cell epitope strength in WAP-T mouse mammary carcinomas is an important determinant in PD1/PD-L1 immune checkpoint blockade therapy

Using the SV40 transgenic WAP-T/WAP-T_NP mouse models for mammary carcinomas, we compared the response to immune checkpoint blockade therapy in tumor mice expressing either SV40 T-antigen containing the LCMV NP-epitope (T-Ag_NP in WAP-T_NP mice), or the unmodified T-antigen (T-Ag in WAP-T mice). Specifically, we asked, whether the presence of the highly immunogenic NP-epitope in T-Ag_NP influences this response in comparison to the weakly immunogenic T-cell epitopes of T-Ag in WAP-T tumor mice. Treatment of WAP-T_NP tumor mice with either anti-PD1 or anti-PD-L1 antibodies led to tumor regression, with anti-PD-L1 treatment being more effective. However, tumors had fully re-appeared after 21 days, indicating that CTL exhaustion had been rapidly re-established. Surprisingly, the same treatment applied to WAP-T tumor mice resulted in a significantly prolonged period of tumor regression. We provide evidence that in contrast to the weak antigenic stimuli exerted by T-cell epitopes of T-Ag, the strong antigenic stimulus of the NP-epitope in T-Ag_NP has a dual effect: (i) a rapid generation of active NP-specific CTLs, accompanied (ii) by accelerated CTL exhaustion. Our data support the hypothesis that the immunogenicity of tumor antigen T-cell epitopes strongly influences the success of immune checkpoint blockade therapy.

Immunotherapy of WAP-TNP mice with early stage mammary gland tumors

The SV40 transgenic BALB/c mouse based WAP-T/WAP-T_NP model for triple-negative breast cancer allows the analysis of parameters influencing immunotherapeutic approaches. Except for WAP-T_NP tumors expressing the immune-dominant LCMV NP-epitope within SV40 T-antigen (T-Ag_NP) which is not expressed by T-Ag of WAP-T tumors, the tumors are extremely similar. Comparative anti-PD1/PD-L1 immunotherapy of WAP-T and WAP-T_NP mice supported the hypothesis that the immunogenicity of tumor antigen T-cell epitopes strongly influences the success of immune checkpoint blockade therapy, with highly immunogenic T-cell epitopes favoring rapid CTL exhaustion. Here we analyzed the immune response in NP8 mice during early times of tumor development. LCMV infection of lactating NP8 mice induced lifelong tumor protection by memory CTLs. Immunization with LCMV after involution and appearance of T-Ag_NP expressing parity-induced tumor progenitor cells could not cure the mice, as memory CTLs became exhausted. However, immunization significantly prolonged the time of tumor outgrowth. Elimination of exhausted CTLs and of immunosuppressive cells by sub-lethal γ-irradiation, followed by adoptive transfer of NP-epitope specific CTLs into NP8 tumor mice with early lesions, completely prevented tumor outgrowth, when lymphocytes obtained after injection of weakly immunogenic NP8 tumor-derived cells into BALB/c mice were transferred. Transfer of lymphocytes obtained after infection of BALB/c mice with highly immunogenic LCMV into such mice delayed tumor outgrowth for a significant period, but could not prevent it. We conclude that eliminating exhausted CTLs and immune-suppressive cells followed by transfer or generation of low-avidity tumor antigen-specific CTLs might be a promising approach for curative tumor immunotherapy.

An inducible transgenic mouse breast cancer model for the analysis of tumor antigen specific CD8+ T-cell responses

In Simian virus 40 (SV40) transgenic BALB/c WAP-T mice tumor development and progression is driven by SV40 tumor antigens encoded by inducible transgenes. WAP-T mice constitute a well characterized mouse model for breast cancer with strong similarities to the corresponding human disease. BALB/c mice mount only a weak cellular immune response against SV40 T-antigen (T-Ag). For studying tumor antigen specific CD8⁺ T-cell responses against transgene expressing cells, we created WAP-T_NP mice, in which the transgene additionally codes for the NP_118–126-epitope contained within the nucleoprotein of lymphocytic choriomeningitis virus (LCMV), the immune-dominant T-cell epitope in BALB/c mice. We then investigated in WAP-T_NP mice the immune responses against SV40 tumor antigens and the NP-epitope within the chimeric T-Ag/NP protein (T-Ag_NP). Analysis of the immune-reactivity against T-Ag in WAP-T and of T-Ag_NP in WAP-T_NP mice revealed that, in contrast to wild type (wt) BALB/c mice, WAP-T and WAP-T_NP mice were non-reactive against T-Ag. However, like wtBALB/c mice, WAP-T as well as WAP-T_NP mice were highly reactive against the immune-dominant LCMV NP-epitope, thereby allowing the analysis of NP-epitope specific cellular immune responses in WAP-T_NP mice. LCMV infection of WAP-T_NP mice induced a strong, LCMV NP-epitope specific CD8⁺ T-cell response, which was able to specifically eliminate T-Ag_NP expressing mammary epithelial cells both prior to tumor formation (i.e. in cells of lactating mammary glands), as well as in invasive tumors. Elimination of tumor cells, however, was only transient, even after repeated LCMV infections. Further studies showed that already non-infected WAP-T_NP tumor mice contained LCMV NP-epitope specific CD8⁺ T-cells, albeit with strongly reduced, though measurable activity. Functional impairment of these ‘endogenous’ NP-epitope specific T-cells seems to be caused by expression of the programmed death-1 protein (PD1), as anti-PD1 treatment of splenocytes from WAP-T_NP tumor mice restored their activity. These characteristics are similar to those found in many tumor patients and render WAP-T_NP mice a suitable model for analyzing parameters to overcome the blockade of immune checkpoints in tumor patients.

Whole-exome DNA sequence analysis of Brca2- and Trp53-deficient mouse mammary gland tumours

Germline mutations in the tumour suppressor BRCA2 predispose to breast, ovarian and a number of other human cancers. Brca2-deficient mouse models are used for preclinical studies but the pattern of genomic alterations in these tumours has not yet been described in detail. We have performed whole-exome DNA sequencing analysis of mouse mammary tumours from Blg-Cre Brca2(f/f) Trp53(f/f) animals, a model of BRCA2-deficient human cancer. We also used the sequencing data to estimate DNA copy number alterations in these tumours and identified a recurrent copy number gain in Met, which has been found amplified in other mouse mammary cancer models. Through a comparative genomic analysis, we identified several mouse Blg-Cre Brca2(f/f) Trp53(f/f) mammary tumour somatic mutations in genes that are also mutated in human cancer, but few of these genes have been found frequently mutated in human breast cancer. A more detailed analysis of these somatic mutations revealed a set of genes that are mutated in human BRCA2 mutant breast and ovarian tumours and that are also mutated in mouse Brca2-null, Trp53-null mammary tumours. Finally, a DNA deletion surrounded by microhomology signature found in human BRCA1/2-deficient cancers was not common in the genome of these mouse tumours. Although a useful model, there are some differences in the genomic landscape of tumours arising in Blg-Cre Brca2(f/f) Trp53(f/f) mice compared to human BRCA-mutated breast cancers. Therefore, this needs to be taken into account in the use of this model.

Epithelial-mesenchymal plasticity is a decisive feature for the metastatic outgrowth of disseminated WAP-T mouse mammary carcinoma cells

Background

Experimental analysis of the metastatic cascade requires suitable model systems which allow tracing of disseminated tumor cells and the identification of factors leading to metastatic outgrowth in distant organs. Such models, especially models using immune-competent mice, are rather scarce. We here analyze tumor cell dissemination and metastasis in an immune-competent transplantable mouse mammary tumor model, based on the SV40 transgenic WAP-T mouse mammary carcinoma model.

Methods

We orthotopically transplanted into immune-competent WAP-T mice two tumor cell lines (H8N8, moderately metastatic, and G-2, non-metastatic), developed from primary WAP-T tumors. G-2 and H8N8 cells exhibit stem cell characteristics, form homeostatic, heterotypic tumor cell systems in vitro, and closely mimic endogenous primary tumors after orthotopic transplantation into syngeneic, immune-competent WAP-T mice. Tumor cell transgene-specific PCR allows monitoring of tumor cell dissemination into distinct organs, and immunohistochemistry for SV40 T-antigen tracing of single disseminated tumor cells (DTC).

Results

While only H8N8 cell-derived tumors developed metastases, tumors induced with both cell lines disseminated into a variety of organs with similar efficiency and similar organ distribution. H8N8 metastases arose only in lungs, indicating that organ-specific metastatic outgrowth depends on the ability of DTC to re-establish a tumor cell system rather than on invasion per se. Resection of small tumors (0.5 cm³) prevented metastasis of H8N8-derived tumors, most likely due to the rather short half-life of DTC, and thus to shorter exposure of the mice to DTC. In experimental metastasis by tail vein injection, G-2 and H8N8 cells both were able to form lung metastases with similar efficiency. However, after injection of sorted “mesenchymal” and “epithelial” G-2 cell subpopulations, only the “epithelial” subpopulation formed lung metastases.

Conclusions

We demonstrate the utility of our mouse model to analyze factors influencing tumor cell dissemination and metastasis. We suggest that the different metastatic capacity of G-2 and H8N8 cells is due to their different degrees of epithelial-mesenchymal plasticity (EMP), and thus the ability of the respective disseminated cells to revert from a “mesenchymal” to an “epithelial” differentiation state.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1165-5) contains supplementary material, which is available to authorized users.

Chemotherapy of WAP-T mouse mammary carcinomas aggravates tumor phenotype and enhances tumor cell dissemination

In this study, the effects of the standard chemotherapy, cyclophosphamide/adriamycin/5-fluorouracil (CAF) on tumor growth, dissemination and recurrence after orthotopic implantation of murine G-2 cells were analyzed in the syngeneic immunocompetent whey acidic protein-T mouse model (Wegwitz et al., PLoS One 2010; 5:e12103; Schulze-Garg et al., Oncogene 2000; 19:1028-37). Single-dose CAF treatment reduced tumor size significantly, but was not able to eradicate all tumor cells, as recurrent tumor growth was observed 4 weeks after CAF treatment. Nine days after CAF treatment, residual tumors showed features of regressive alterations and were composed of mesenchymal-like tumor cells, infiltrating immune cells and some tumor-associated fibroblasts with an intense deposition of collagen. Recurrent tumors were characterized by coagulative necrosis and less tumor cell differentiation compared with untreated tumors, suggesting a more aggressive tumor phenotype. In support, tumor cell dissemination was strongly enhanced in mice that had developed recurrent tumors in comparison with untreated controls, although only few disseminated tumor cells could be detected in various organs 9 days after CAF application. In vitro experiments revealed that CAF treatment of G-2 cells eliminates the vast majority of epithelial tumor cells, whereas tumor cells with a mesenchymal phenotype survive. These results together with the in vivo findings suggest that tumor cells that underwent epithelial-mesenchymal transition and/or exhibit stem-cell-like properties are difficult to eliminate using one round of CAF chemotherapy. The model system described here provides a valuable tool for the characterization of the effects of chemotherapeutic regimens on recurrent tumor growth and on tumor cell dissemination, thereby enabling the development and preclinical evaluation of novel therapeutic strategies to target mammary carcinomas.

Mutant p53 promotes epithelial-mesenchymal plasticity and enhances metastasis in mammary carcinomas of WAP-T mice

To study the postulated mutant p53 (mutp53) "gain of function" effects in mammary tumor development, progression and metastasis, we crossed SV40 transgenic WAP-T mice with mutant p53 transgenic WAP-mutp53 mice. Compared to tumors in monotransgenic WAP-T mice, tumors in bitransgenic WAP-T x WAP-mutp53 mice showed higher tumor grading, enhanced vascularization, and significantly increased metastasis. Bitransgenic tumors revealed a gene signature associated with the oncogenic epithelial-mesenchymal transition pathway (EMT gene signature). In cultures of WAP-T tumor-derived G-2 cancer cells, which are comprised of subpopulations displaying "mesenchymal" and "epithelial" phenotypes, this EMT gene signature was associated with the "mesenchymal" compartment. Furthermore, ectopic expression of mutp53 in G-2 cells sufficed to induce a strong EMT phenotype. In contrast to these in vitro effects, monotransgenic and bitransgenic tumors were phenotypically similar suggesting that in vivo the tumor cell phenotype might be under control of the tumor microenvironment. In support, orthotopic transplantation of G-2 cells as well as of G-2 cells expressing ectopic mutp53 into syngeneic mice resulted in tumors with a predominantly epithelial phenotype, closely similar to that of endogenous primary tumors. We conclude that induction of an EMT gene signature by mutp53 in bitransgenic tumors primarily promotes tumor cell plasticity, that is, the probability of tumor cells to undergo EMT processes under appropriate stimuli, thereby possibly increasing their potential to disseminate and metastasize.

Aberrant Proliferation of Differentiating Alveolar Cells Induces Hyperplasia in Resting Mammary Glands of SV40-TAg Transgenic Mice

WAP-T1 transgenic mice express SV40-TAg under control of the whey acidic protein (WAP) promoter, which directs activity of this strong viral oncogene to luminal cells of the mammary gland. Resting uniparous WAP-T1 glands develop hyperplasia composed of TAg positive cells prior to appearance of advanced tumor stages. We show that cells in hyperplasia display markers of alveolar differentiation, suggesting that TAg targets differentiating cells of the alveolar compartment. The glands show significant expression of Elf5 and milk genes (Lalba, Csn2, and Wap). TAg expressing cells largely co-stain with antibodies to Elf5, lack the epithelial marker Sca1, and are hormone receptor negative. High expression levels of Elf5 but not of milk genes are also seen in resting glands of normal BALB/c mice. This indicates that expression of Elf5 in resting WAP-T1 glands is not specifically induced by TAg. CK6a positive luminal cells lack TAg. These cells co-express the markers prominin-1, CK6a, and Sca1, and are positive for hormone receptors. These hormone sensitive cells localize to ducts and seem not to be targeted by TAg. Despite reaching an advanced stage in alveolar differentiation, the cells in hyperplasia do not exit the cell cycle. Thus, expression of TAg in conjunction with regular morphogenetic processes of alveologenesis seem to provide the basis for a hormone independent, unscheduled proliferation of differentiating cells in resting glands of WAP-T1 transgenic mice, leading to the formation of hyperplastic lesions.

SV40 TAg mouse models of cancer

The discovery of a number of viruses with the ability to induce tumours in animals and transform human cells has vastly impacted cancer research. Much of what is known about tumorigenesis today regarding tumour drivers and tumour suppressors has been discovered through experiments using viruses. The SV40 virus has proven extremely successful in generating transgenic models of many human cancer types and this review provides an overview of these models and seeks to give evidence as to their relevance in this modern era of personalised medicine and technological advancements.