University of Groningen
Functions of the C/EBPβ isoforms in breast cancer
Sterken, Britt
DOI:
10.33612/diss.172465560
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Publication date: 2021
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Sterken, B. (2021). Functions of the C/EBPβ isoforms in breast cancer. University of Groningen. https://doi.org/10.33612/diss.172465560
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Aim of the thesis
Breast cancer is the most commonly diagnosed cancer type in women, with its survival outcome being strongly dependent on the stage of the breast cancer at the time of diagnosis. This thesis focuses on the investigation of the role of the transcription factor CCAAT/enhancer binding protein (C/EBP) β in breast cancer development and metastasis. The transcription factor C/EBPβ regulates cell proliferation and differentiation in a variety of tissues. The transactivation capacity of C/EBPβ is largely determined by the ratio between the isoforms C/EBPβ-LAP and C/EBPβ-LIP. The deregulated expression of LIP and LAP has been found to contribute to ageing and oncogenesis. Overexpression of LIP in the mammary epithelium predisposes to hyperplasia and neoplasia and high LIP expression has been found in ER-, PR- human breast cancer tissues, which is often associated with poor clinical outcome due to the inefficiency of compounds targeting the hormone receptors. However, much remains unknown about the functions of the C/EBPβ-isoforms in different types of breast cancer and breast cancer progression. The aim of this thesis is to investigate the functions of the C/EBPβ-isoforms in breast cancer development and metastasis. The first part of the thesis investigates the role of C/EBPβ in triple negative breast cancer, and the influence of the C/EBPβ-isoforms on cell migration and invasion. By performing transcriptome analysis we aim to identify downstream mediators of C/EBPβ involved in breast cancer development and metastasis. To investigate the role of the isoforms of C/EBPβ in breast cancer development and metastasis in vivo, we generate LIP- and LAP-specific p53-deficient genetically engineered mouse models. Previously, we demonstrated that the translation into the truncated isoforms of C/EBPα (p30) and C/EBPβ (LIP) is stimulated by mTORC1 signalling and that C/EBPβ-LIP deficient mice show improved metabolic health, increased lifespan and reduced cancer incidence, resembling the phenotype of animals with reduced mTORC1 signalling. Here, we describe the generation of a C/EBPα-p30 deficient model, to investigate the effects of reduced C/EBPα-p30 expression on metabolism, ageing and oncogenesis.
Outline of the thesis
Chapter II reviews the existing literature surrounding the function(s) of C/EBPβ and its isoforms in breast cancer development, metastasis and the microenvironment. We describe how the expression of the C/EBPβ-isoforms is regulated by translational control and how these isoforms are involved in differentiation, proliferation, and oncogenesis in different tissues. Specifically, we focus on the existing literature reporting the functions of the C/EBPβ-isoforms in HER2 (human epidermal growth factor receptor 2) positive breast cancer and upcoming studies identifying C/EBPβ as a potential regulator in triple-negative breast cancer and the cancer microenvironment.
In Chapter III we describe how the C/EBPβ-isoforms regulate breast cancer cell migration and invasion. We observe a high expression of LIP and a high LIP/LAP ratio in cell lines derived from triple-negative breast cancer. Exogenous expression of LAP in triple-negative breast cancer cell lines reduces cell migration and invasion while exogenous expression of LIP boosts the migration of untransformed breast epithelial cells. This migratory phenotype is accompanied by a mixed regulation of epithelial and mesenchymal markers by C/EBPβ, where C/EBPβ-LAP upregulates various pro-oncogenic extracellular matrix genes. Chapter IV describes the generation of LIP- and LAP-specific p53-deficient genetically engineered mouse models for breast cancer. We describe how the conditional CAG-promoter driven expression of C/EBPβ-LIP or C/EBPβ-LAP from the Col1a1-locus is epigenetically repressed in p53-deficient mammary tumour development. We show that integration and initial overexpression of LIP and LAP is obtained in mammary epithelial cells but is epigenetically repressed over time. Analysis of p53-deficient tumours reveals high endogenous expression of C/EBPβ with variation in the LIP/LAP ratio, however, the LIP/LAP ratio does not correlate with tumour onset.
Chapter V describes the generation of the p30-deficient CebpaΔuORF mouse model.
To study the effects of p30-depletion, we mutated the start codon of the uORF by CRISPR/Cas9 gene targeting and homology directed repair (HDR) in zygotes.
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Initial analyses reveal successful ablation of the p30 protein and no differences in Mendelian ratios and weight in the offspring. With this study, we aim to study the C/EBPα isoform-specific effects on metabolism, haematopoiesis and cancer development.
Chapter VI summarises and concludes the chapters described above and proposes ideas for future research.
