Molecular mechanisms regulating epithelial-to-mesenchymal transition and therapy sensitivity
in breast cancer and glioblastoma
Liang, Yuanke
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CHAPTER
1
General introduction Breast cancer
In 2018 there were about 2.1 million newly diagnosed breast cancer cases worldwide [1]. Among females breast cancer is the most commonly diagnosed cancer with a 24.3% in-cidence rate and is the main cause of cancer death (15% mortality rate) [1]. It accounts for < 1% of all cancer cases in men and the female-to-male ratio is approximately 100:1. Surgery is the mainstay treatment for non-metastatic breast cancers. However, more exten-sive disease requires various therapeutic approaches including endocrine therapy, radiation therapy, chemotherapy and targeted therapy such as HER2 monoclonal antibodies, tyrosine kinase inhibitors, PARP inhibitors, mTOR inhibitors and cyclin-dependent kinase inhibitors. The selected treatment depends on both tumor biology characteristics and clinical-related factors as described in more detail below.
Breast cancer is a complex disease having very distinct clinical, morphological and molecular entities. Clinically, this heterogeneous disease is categorized into three basic therapeutic groups defined by expression of the estrogen receptor (ER), progesterone receptor (PR) and the epidermal growth factor receptor ErbB2/Her2 (Her2 positive). The Luminal A subtype (ER+ and/or PR+, HER2−, low Ki67) is the most numerous and diverse with clinical charac-teristics of slow-growth, less aggressive and low recurrence rates making this breast cancer subtype having the best prognosis. The Luminal B subtype (ER+ and/or PR+, HER2+ or HER2− with high Ki67) is characterized by high proliferation rates and has worse prognosis than Lu-minal A subtype. For LuLu-minal breast cancer, endocrine therapy is the mainstay for treatment that includes tamoxifen, aromatase inhibitors and fulvestrant. Metastatic Luminal breast cancer may develop resistance to standard hormonal therapies and additional novel tar-geted strategies are available such as CDK4/6 inhibitors, mTOR inhibitors, PI3K inhibitors and Histone Deacetylase (HDAC) inhibitors. HER2 overexpressing subtype (HER2 amplified, ER−, PR−) tends to grow and disseminate faster than other subtypes and patients are often diagnosed at high grade and node positivity. For treating HER2+ breast cancer, anti-HER2 monoclonal antibodies such as trastuzumab and pertuzumab are available, but also tyrosine kinase inhibitor (TKI) inhibitors and PI3K/Akt/mTOR inhibitors are being used for treatment. Triple negative breast cancer (TNBC) lacks expression of ER, PR and HER2, and usually pres-ents at younger age displaying a high histologic grade and higher rates of distant recurrence after surgery. Standard chemotherapy remains the mainstay of treatment for TNBC but re-sistance limits efficacy of this treatment. This together with absence of effective targeted therapy results in an especially poor prognosis for this patients with this subtype [2].
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The luminal-A subtype, the most diagnosed breast cancer subtype, originates from the lu-minal epithelium of small mammary ducts and expresses lulu-minal/epithelial markers such as E-cadherin, ERα, PR and luminal-associated transcription factors GATA3 and FOXA1 [3]. Luminal-A breast cancer is sensitive to ER-directed endocrine therapy and has a favorable prognosis [4]. Estrogens are known to bind specific nuclear receptors, the ERs, that regulate normal growth and development of the mammary tree. ERα expression is regarded as a favorable prognostic indicator in breast cancer [5, 6]. The dependency of luminal A cancers on ERα forms the rationale for ‘hormonal’ therapies involving antiestrogens or CYP19/aro-matase inhibitors [7].
Endocrine therapy, the mainstay of treatment in ER+ breast cancers, aims to block ER func-tioning with either tamoxifen or by depleting estrogen availability by inhibiting its synthesis using aromatase inhibitors (AIs). AIs are used in postmenopausal women and gonads activ-ity is impaired in premenopausal women by ovariectomy. These therapeutic strategies are implemented both for early and metastatic breast cancer. However, not all patients benefit from tamoxifen endocrine therapy. Despite initial responses about 30% of ER+ patients ulti-mately display local recurrence and distant metastases resulting in reduced survival [8-10]. A number of studies have suggested that the mechanisms conferring tamoxifen resistance include the modification or loss of ERα expression [11], deregulation of signal transduction pathways, aberrant expression of specific driver proteins and abnormality in tamoxifen met-abolic activity [12, 13].
Chemotherapy and resistance in triple negative breast cancer
TNBC represents an aggressive receptor-negative subtype that constitutes 12%–18% of breast cancer patients [14]. For women presenting with TNBC, endocrine therapies such as tamoxifen, aromatase inhibitors or anti-Her2 therapy like Trastazumab and Lapatinib, are not efficacious. With no effective specific targeted therapy readily available for TNBC, these patients have a high risk of relapse and a poorer overall survival (OS) compared to luminal subtypes [15].
The standard treatment of TNBC patients is neoadjuvant chemotherapy with taxanes (mi-totic inhibitors) and anthracyclines (DNA intercalators). Platinum agents have seen renewed interest in TNBC since women with BRCA1 mutations have a high rate of response to cispla-tin [16, 17]. However, TNBC patients with advanced disease typically respond poorly to cur-rent chemotherapeutics and patients that respond initially well to chemotherapy develop in about 30%–50% resistance leading to poor overall survival [14, 18].
It has been demonstrated that several mechanisms play an important role in chemoresis-tance of TNBC, including presence of ATP-binding cassette (ABC) transporters, mutations in
DNA repair enzymes such as topoisomerase II and DNA mismatch repair enzymes, aberrant activation of NF-κB or PI3K/AKT signaling pathways, alterations in genes involved in apopto-sis such as p53, but also the presence of cancer stem cells (CSCs) and various signals origi-nating from the tumor microenvironment (TME). In addition, recently, increasing evidence has indicated that activation of the epithelial-to-mesenchymal transition (EMT) program contributes critically to the development of drug resistance in a variety of cancer types, thereby permitting clinical relapse [19]. Taken together, molecular pathways as targets or as predictors of response to chemotherapy remain to be further identified.
EMT and metastasis
The vast majority of breast cancer-related deaths involve metastatic disease [1, 3]. For gen-erating distant metastasis tumor cells at the primary site need to undergo a sequence of events. Tumor cells must invade, disseminate through blood vessels or lymphatics, seed and colonize at distant organ sites to form macrometastases leading to secondary tumors [20]. One of the pivotal processes that induces tumor metastasis is conversion of epithelial cells to a mesenchymal phenotype through a molecular program known as EMT [3, 21]. The EMT program is involved in normal embryogenesis and in various pathological processes, including tissue fibrosis, wound healing and carcinoma progression. In tumors, EMT com-prises a series of intricate biological and biochemical changes that include loss of cell–cell junctions and apical-basal polarity, acquisition of cell motility and invasion potential linked with cytoskeletal alterations, altered cell–matrix adhesion and ability to reorganize and degrade extracellular matrix (ECM). Together this results in loss of epithelial- and gain of mesenchymal-like phenotypic characteristics, such as an elongated, spindle-shaped mor-phology and a high degree of motility [19, 22, 23]. Various signaling pathways implicated in the induction of EMT have been identified such as TGFβ–SMAD signaling, the canonical or non-canonical Wnt pathway, growth factor–receptor tyrosine kinase signaling and ECM–in-tegrin signaling pathways [24-26]. EMT-inducing signals converge on several transcription factors, including ZEB1, ZEB2, Snail, Slug and Twist [27-29]. In addition, other transcription factors, such as YAP/TAZ, Notch1, and SOX4, also have critical roles in EMT induction [30-32]. EMT can be monitored by suppression of genes associated with an epithelial phenotype like E-cadherin, and upregulation of mesenchymal markers, such as fibronectin, N-cadherin, and vimentin[19].
EMT is reversible. Cancer cells that have undergone EMT and traveled to distant organs of the body must have a mechanism that allows them to infiltrate tissues and settle down to form a secondary tumor [33]. Mesenchymal to epithelial transition (MET), the opposite of EMT, has been proposed as an important process for establishment of the metastatic neo-plasm [34]. EMT is thought to be crucial for the initial transformation from benign to inva-sive carcinoma, whereas MET is important for the later stages of metastasis. For instance,
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Chao et al. found that 62% of metastatic lesions in breast cancer patients had increased E-cadherin expression compared to the primary tumor [35]. However, whether MET is req-uisite for tumor metastasis is still controversial. Somarelli et al. generated MET reporter mo-dels and revealed that metastasis occurred via both MET-dependent and MET-independent mechanisms in carcinosarcomas and in prostate cancer, respectively [36]. Thus, a nuanced view on role of EMT/MET in cancer metastasis remains to be determined.
Notch family
The Notch signaling pathway is evolutionary conserved in multicellular organisms and is important at various stages during development. Notch was originally discovered through mutation analyses in fruit fly Drosophila melanogaster with notched wings more than 100 years ago. In 1980s, Michael Young’s and Spyros Artavanis-Tsakonas’ research groups suc-cessfully cloned the Notch receptor and were able to attribute the wing-notching phenotype to gene haplo-insufficiency [37, 38]. In mammals, there are four different Notch receptors (Notch1-4), and five ligands, delta-like ligand 1 (DLL1), delta-like ligand 3 (DLL3), delta-like ligand 4 (DLL4), Jagged-1 (JAG1) and Jagged-2 (JAG2) [39].
Notch receptors are produced in the endoplasmic reticulum (ER) and processed by furin-like convertase (S1 cleavage) in the Golgi compartment during trafficking to the cell mem-brane. When Notch receptors interact with ligand by cell-to-cell contact, receptor–ligand engagement triggers a second NECD (Notch Extracellular Domain) cleavage (S2 cleavage) by a metalloproteinase ADAM (known as Kuzbanian in Drosophila melanogaster). After that, γ-secretase cleaves Notch within its transmembrane domain at site 3 (S3 cleavage) to re-lease various forms of the Notch Intracellular Domain (NICD). Rere-leased NICD translocates to the nucleus, where it forms a transcriptional complex with the DNA binding factor RBPJ (also known as CSL), and Mastermind (Mam) and transcriptional co-activators to drive the expression of Notch target genes [40].
The Notch signaling cascade is critical for cell proliferation, differentiation, development and tissue homeostasis [40, 41]. Deregulated Notch signaling has been implicated in tumorigen-esis and metastasis of breast cancers and the development of therapeutic agents that target the key steps in the Notch signaling pathway may provide strategies to inhibit tumor growth [42]. Notch-1 and Notch-4 are responsible for tumorigenesis and Notch-2 was identified as a tumor suppressor gene in various studies [43-46]. Harrison et al. showed that Notch4 and Notch1 signaling enhances breast cancer stem cell (CSC) activity, and inhibition of Notch4 or Notch1 reduces tumor formation in vivo [47]. The role of Notch3 in breast cancer is contro-versial. Bouras et al. found that Notch3 levels are specifically elevated in mouse mammary luminal progenitor and epithelial cells, but not in mammary stem cells [48]; similar results were also reported by transcriptome analysis [49]. In transformed breast cells, Notch3 is
re-quired for breast luminal filling by inhibiting apoptosis [50]. In transgenic mice, Notch3 pro-motes lobular-alveolar epithelial cell expansion and leads to tumor formation [51]. Pasquale
et al. reported that an IL-6 autocrine loop could induce long-term enhancement of the
ag-gressive features of breast cancer cells by sustaining upregulation of the Notch-3/CA-IX axis [52]. However, Wang et al. found that Notch3 knockdown promotes breast tumor growth by inducing IL6 expression and activation of STAT3 [53]. In addition, Cui et al. demonstrated that Notch3 functions as a tumor suppressor by controlling cellular senescence, inhibiting breast cancer cell proliferation by up-regulating p21 [54] involving the induction of CDH1 ex-pression [55]. Overall, the mechanistic details underlying Notch family roles in breast cancer and how broadly applicable they are to cancer remain to be ascertained.
CD146/MCAM
Melanoma cell adhesion molecule (MCAM) also named cluster of differentiation 146 (CD146) or Mucin 18 (MUC18) was first discovered by J.P. Jonhson in 1987 and identified as a marker of metastasizing melanoma. It is an integral membrane glycoprotein belonging to the immunoglobulin (Ig) superfamily originally discovered in metastatic melanoma and associated to a poor prognosis [56, 57].
As a cell adhesion molecule, CD146 locates on the cell surface and is involved in the binding to other cells or to the ECM involving homotypic or heterotypic protein interactions to facil-itate inter- and extra-cellular interactions in response to physiological signals. In addition to proposed CD146-CD146 interactions, currently several ligands for CD146 have been identi-fied including Laminin-411 that was shown to facilitate T cells entry into the central nervous system (CNS) [58].
Subsequent studies revealed that CD146 is highly expressed in many cancers such as breast cancer, gastric cancer, non-small cell lung cancer and prostate cancer [59]. In addition, CD146 is regarded as a marker of endothelial and pericyte cells playing a crucial role in vas-cular development including tumor angiogenesis [60, 61]. It has been reported that CD146 is higher expressed in tumor vessels compared with normal blood vessels. CD146 was iden-tified as a component of the VEGF signalosome being a co-receptor of vascular endothelial growth factor receptor-2 (VEGFR-2) able to promote tumor angiogenesis [62, 63]. Recently, CD146 was discovered to be a mesenchymal marker and a unique EMT inducer in breast cancer and a predictor of poor prognosis [64, 65]. What’s more, CD146 can activate PI3K/ AKT, NF-κB pathway and inhibitor of DNA binding 1 (ID1), depicting mechanisms by which CD146 contributes to tumor metastasis. In small-cell lung cancer, CD146 has been reported to mediate chemoresistance via regulating the PI3K/AKT/SOX2 pathway [66]. Thus, better understanding the functions and regulatory mechanisms of CD146 are crucial in order to exploit its potential as therapeutic target in cancer.
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Glioblastoma
In this thesis we also performed studies in highly malignant brain tumors, glioblastoma (GBM). GBM is the most common and lethal primary malignant brain tumor among tumors of the CNS, comprising for 47.7% of cases (United State statistical data) [67]. GBM is more common in older adults (median age of 65 years) and the average age-adjusted incidence rate is 3.21 per 100,000 people (United State statistical data) [67]. Primary GBM is charac-terized by rapid, aggressive growth and there are no curative treatment options currently available. Despite maximal initial resection followed by radiation and chemotherapy, about 70% of GBM patients will experience disease progression within one year of diagnosis [68], with 40.2% survival at 1-year and just 5.6% survival at 5-year; median survival with standard of care is approximately 14 to 15 months [67].
Classification
The 2016 World Health Organization (WHO) classification of CNS tumors includes both molecular markers along with histological features to identify and classify different sub-types of diffuse glioma. Normally, malignant gliomas are graded between I and IV based on pathological characteristics including cellularity, mitotic activity, nuclear atypia, micro-vascular proliferation and necrosis [69]. GBM are classified as WHO grade IV tumors. Cur-rently, the Isocitrate Dehydrogenase (IDH) gene mutational status and methylation status of the MGMT promoter are used as prognostic markers in GBM [70]. GBMs are divided into two categories: IDH-wildtype and IDH-mutant. Despite their similar histology, IDH-mutant GBM patients are generally younger and have a somewhat better prognosis than those with IDH-wildtype [71, 72]. In addition, tumor transcriptome profiling has been used for interro-gating pathway functionality and phenotype-based patient classification. In order to under-stand the biology of GBM, The Cancer Genome Atlas Consortium (TCGA) used nearly 600 GBMs to perform high-dimensional profiling and molecular classification [73-77]. Common mutations in genes such as IDH1,TP53, PTEN, and EGFR, as well as the frequent and concur-rent presence of abnormalities in the p53, RB, and receptor tyrosine kinase pathways were identified. Furthermore, transcriptional profiling has resulted in classification into four GBM subtypes, proneural (PN), neural (NE), classical (CL), and mesenchymal (MES), of which PN and MES groups appeared most distinct [77].
The CL subtype was defined by a combination of chromosome 7 amplification with chro-mosome 10 loss in 100% of cases. In addition, epidermal growth factor receptor (EGFR) amplification was increased in 97% of CL tumors with an associated homozygous deletion of cyclin-dependent kinase inhibitor 2A (CDKN2A). The MES subtype is often accompanied by NF1 and PTEN mutations and exhibits elevated mesenchymal marker expressions like YKL40, MET as well as NF-κB. The PN subtype exhibits IDH1 mutations and the highest rate
of platelet-derived growth factor receptor alpha (PDGFRA) amplifications. The NE subtype was identified based on the expression of neural markers [77]. Tumors with a MES subtype are more aggressive, resistant to treatment and are associated with a poor outcome, where-as PN GBM relate to a more favorable prognosis [77-79].
Aggressiveness
GBM is the most aggressive brain tumor. Unlike most other cancers, in which distant metas-tasis is the leading cause of death, GBM rarely metastasizes outside the CNS [80]. Instead, GBM tumor recurrence occurs in the brain, frequently adjacent to the site of the original tumor and the high rate of recurrence is the main cause of death. In clinical surgical treat-ment, extensive and complete resection of GBM is impossible to achieve since these tumors always infiltrate to the eloquent areas of the brain, including the important functional areas that control speech, senses and motor function. For CT imaging, GBM tumor cells can be de-tected up to 6 cm from abnormal areas on CT [81]. These invasive tumor cells invariably re-main within the surrounding brain, leading to tumor recurrence and disease progress [82]. EMT is known to play a pivotal role in tumor aggressiveness. Similarly, MES GBM was re-ported to have the highest degree of invasiveness and radioresistance associated with worst outcome. Foxm1, ZEB1, WNT/β-catenin, YAP/TAZ, C/EBP-β and STAT were identified as im-portant regulators of mesenchymal transition (MT) in GBM [83-86]. The strong invasive be-havior of GBM is not only an inherent property of GBM cells, but is also highly regulated by the TME, particularly by resident macrophages named microglia and hypoxic conditions. For example, immune cells and microglia can produce TGF-β, which induces ZEB1 thus promot-ing MT and aggressiveness in GBM cells [87]. In addition, a hypoxic microenvironment can induce tumor cell migration and invasion by promoting a MT mediated by HIF1α-ZEB1 axis [88].
Glioblastoma stem cells
CSCs, also known as tumor-initiating cells, maintain self-renew potential, high tumorigenic-ity and display often multilineage differentiation capactumorigenic-ity thus contributing to cellular het-erogeneity within tumors [89-91]. CSCs in malignant gliomas, called glioblastoma stem cells (GSCs) often have the potential to differentiate into cells resembling neurons, astrocytes and oligodendrocytes that are normally produced from neuronal stem cells. GSCs are also characterized by self-renewal potential and increased tumor forming ability, and were also linked with immune evasion and trans-differentiation ability into vascular cells contributing to angiogenesis in GBM [92-94].
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Although the functional criteria defining GSCs have been relatively well established, the molecular characteristics of these cells haven’t been fully understood. CD133, a member of prominin family of pentaspan membrane glycoproteins, has been widely investigated and often, but not always, linked to self-renewal and the tumorigenic potential of stem/pro-genitor cells in human GBM [95]. Additional markers for GSCs have been identified such as SOX2, OCT4, BMI-1, CD15, Musashi-1, NANOG, L1CAM [96-101]. However, like CD133 none of these markers are universal and the intracellular localization of some markers make them less suitable candidates for both GSC isolation and targeted therapeutic purposes [102]. Even though conventional treatment for GBM may eliminate the majority of tumor cells, recurrences always occur that has been attributed to the presence of therapy resistant GSCs [103-105]. Evidence has suggested that GSCs are strongly preserved and regulated by the microenvironment of the niche where the GSCs reside, providing multiple regulatory mechanisms that contribute to the chemo- and radioresistance [106]. Vascular and necrot-ic/hypoxic niches may be the functional and specialized microenvironment which regulate GSCs self-renewal and support their expansion and spread, further promoting brain tumor growth and invasion [107-109].
Therapeutic resistance
Diagnosed GBM patients require a multidisciplinary treatment including maximal surgical resection, followed by concurrent treatment with radiation and temozolomide (TMZ), an oral alkylating chemotherapy agent. However, despite this multimodular approach tumors regenerate likely by failure to eradicate resistant and highly tumorigenic GSCs. For example, the poor radiotherapy (RT) responses of GBM patients have been associated with GSCs [103]. RT induces tumor cell death through damaging biomolecules, such as proteins and DNA, resulting in halting cell cycle progression and subsequent cell necrosis or apoptosis. The success or failure of radiotherapy is determined by repair of DNA damage, redistribu-tion of the cell cycle, repopularedistribu-tion of tumors and reoxygenaredistribu-tion of hypoxic tumor areas [110]. Compared with non-stem cells, high radioresistance of GSCs has been correlated with hyperactivation of the DNA damage response (DDR) [101],especially with regard to double-strand break (DSB) repair [111]. Several kinases orchestrate the DDR including ATR, ATM, CHK1, CHK2, WEE1, together with the p53 pathway. Shafiq et al. reported that the expression of ATM, ATR, CHK1, and PARP1 are increased in GSCs and contribute to radio-resistance, which can be counteracted by combined inhibition of cell-cycle checkpoint and DNA repair targets providing a promising strategy to overcome radioresistance of GSC [112]. Zhang et al. revealed the association between EMT and radioresistance. Ionizing radiation hyperactivated ATM and upregulated ZEB1 that directly interacted with USP7 and stabilized CHK1, thereby promoting homologous recombination-dependent DNA repair and resis-tance to radiation. Interestingly, Cheng et al. reported that CD146 protein was up-regulated in cervical cancer cells after radiation, but its involvement was not further elucidated [113].
Overall, the role of CD146 in EMT, radiation response and aggressiveness in GBM remains largely unexplored.
Aim of the thesis
The defining feature of tumor malignancy entails the aggressive ability to invade adjacent tissues and metastasize to distant sites, which is the main cause of cancer-related deaths. Chemo- and radiotherapy are still the most common treatments for cancer, however, suc-cess is usually limited by the development of drug resistance. Thus, targeting specific mol-ecules involved in tumor cell aggressiveness and therapeutic resistance may offer potential strategies for improving the prognosis in cancer. The main aim of the research presented in this thesis is to explore the involvement and regulatory mechanism of EMT in relation to tumor aggressiveness and therapeutic resistance in breast cancer and GBM.
Outline of the thesis
In the first part of the thesis we focus on breast cancer. Tamoxifen resistance presents a prominent clinical challenge in endocrine therapy for hormone sensitive breast cancer. However, the underlying mechanisms that contribute to tamoxifen resistance are not fully understood. In order to identify mechanisms of tamoxifen resistance, in Chapter 2, we es-tablished a tamoxifen resistant MCF-7 cell line model (MCF-7-Tam-R) by continuously incu-bating MCF-7 cells with 4-OH-tamoxifen. We next investigated whether acquisition of the tamoxifen resistance was accompanied by EMT and found that CD146/MCAM, a unique EMT inducer, was significantly up-regulated in MCF-7-Tam-R cells. The regulatory mech-anism of CD146 in promoting tamoxifen resistance in breast cancer cells was further ex-plored. Finally, we examined the association of CD146 expression in the prognosis of breast cancer patients, particularly in the subgroup only treated with tamoxifen.
In breast cancer, the main cause of deaths results from metastasis at distant organs. In
Chapter 3, we aimed to explore the role of Notch3, a poorly studied Notch family member,
in EMT and metastasis of breast cancer. Analyzing the expression association among Notch, ERα, and EMT markers in breast cancer cell lines and patient tissues, we revealed a positive relationship of Notch3 with ERα. We validated the functions of Notch3 in breast cancer cells by using in vitro and in vivo models. Further we delineate the role of a Notch3/ERα axis in maintaining the luminal phenotype and inhibiting tumorigenesis and metastases in breast cancer. Since the regulation of Notch3 is largely unclear, in chapter 4, we performed bioinformatic analysis to identify putative microRNAs targeting Notch3. Mechanistic studies identified miR-221 /222 as regulators of Notch3 in breast cancer cells.
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Notch3 and GATA binding protein 3 (GATA-3) have been individually shown to maintain a luminal phenotype, associated with ER status and inhibition of EMT in breast cancers. These properties prompted us to investigate whether there is a correlation between Notch3 and GATA-3. In chapter 5, we identified GATA3 as down-stream target of Notch3 in breast cancer. Taken together, in chapter 3 to chapter 5 we identify a novel mechanism whereby a miRNA/ Notch3/GATA3/ERα pathway regulates EMT and metastasis in breast cancer.
Resistance to chemotherapy of TNBC continues to be a critical issue in the clinic. In chapter
6, the importance of Notch1 in regulating chemoresistance in breast cancer is explored. We
generated a cisplatin resistant TNBC cell line model to explore the mechanism of chemore-sistance in TNBC. We found that the expression of Notch1 and CD146 in cisplatin-resistant MDA-MB-231 cells are significantly higher than wild-type counterparts. In addition, the ex-pression of Notch1 and CD146 in TNBC patients’ survival prognosis were analyzed, partic-ularly in those treated with chemotherapy. Finally, we discovered that Notch1, CD146 and EMT regulate chemoresistance of TNBC cells.
The role of CD146 in the malignant behavior of GBM is largely unknown. In Chapter 7 we studied the possible function of CD146 in different malignant properties of GBM and an-alyzed underlying mechanisms. We examined CD146 expression in TCGA database and in patient-derived GBM neurosphere models, enriched for GSCs. CD146 appears elevated in GBM compared with normal brain tissue and exhibited various expression levels in different GBM neurospheres. Using ectopic CD146/GFP fusion protein overexpression model and a CD146 CRISPR/Cas9 knockout model, we were able to identify roles of CD146 in multiple tu-morigenic processes of GBM such as mesenchymal characteristics, stemness and radioresis-tance. Interestingly, the Hippo/YAP pathway was identified as a novel downstream effector of CD146 function in GBM.
To conclude, in chapter 8, the main results obtained and described in this thesis are sum-marized and future perspectives are discussed. Finally, summaries in Chinese and Dutch are provided in chapters 9 and 10.
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