Browsing by Author "Bischoff, Annabell"

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Open Access
A global microRNA screen identifies regulators of the ErbB receptor signaling network
(2015) Bischoff, Annabell; Bayerlová, Michaela; Strotbek, Michaela; Schmid, Simone; Beissbarth, Tim; Olayioye, Monilola A.
Background: The growth factor heregulin (HRG) potently stimulates epithelial cell survival and proliferation through the binding of its cognate receptor ErbB3 (also known as HER3). ErbB3-dependent signal transmission relies on the dimerization partner ErbB2, a receptor tyrosine kinase that is frequently overexpressed and/or amplified in breast cancer cells. Substantial evidence suggests that deregulated ErbB3 expression also contributes to the transformed phenotype of breast cancer cells. Results: By genome-wide screening, we identify 43 microRNAs (miRNAs) that specifically impact HRG-induced activation of the PI3K-Akt pathway. Bioinformatic analysis combined with experimental validation reveals a highly connected molecular miRNA-gene interaction network particularly for the negative screen hits. For selected miRNAs, namely miR-149, miR-148b, miR-326, and miR-520a-3p, we demonstrate the simultaneous downregulation of the ErbB3 receptor and multiple downstream signaling molecules, explaining their efficient dampening of HRG responses and ascribing to these miRNAs potential context-dependent tumor suppressive functions. Conclusions: Given the contribution of HRG signaling and the PI3K-Akt pathway in particular to tumorigenesis, this study not only provides mechanistic insight into the function of miRNAs but also has implications for future clinical applications.
Open Access
miRNAs in control of oncogenic signaling in breast cancer cells
(2014) Bischoff, Annabell; Monilola Olayioye (Prof. Dr. )
microRNAs (miRNAs) are 17-nt to 24-nt short non-coding RNAs that have emerged as criti-cal regulators of gene expression in almost all forms of life. miRNAs act by partial comple-mentary binding usually within the 3’-untranslated region (3’UTR) of the mRNA target result-ing in translational repression and/or mRNA degradation. Microarray and proteomic experi-ments have demonstrated the impact of a single miRNA on fine-tuning expression of a hun-dred of targets affecting a multitude of biological processes such as development, prolifera-tion and apoptosis. Deregulation of miRNA function is also implicated in various diseases including the development of cancer. Furthermore, recent miRNA profiling studies conducted on different tumor types have identified sets of miRNAs that have altered expression in tumor and normal tissue, making them attractive targets for therapeutic intervention or as diagnostic markers. Nevertheless, target identification and detailed knowledge of miRNA functions is the key for the correct selection of miRNAs causally involved in the specific disease process. This thesis focuses especially on the role of miRNAs in two processes that are of major in-terest to breast cancer research; the ErbB2/ErbB3/Akt signaling pathway and cancer cell motility. Prolonged ErbB2/ErbB3/Akt signaling is frequently reported in various cancers and enables the cell to bypass targeting therapies as it favors cell survival. In the case of breast cancer, this is particularly achieved by increased ErbB2/ErbB3 receptor activation. In order to investigate the extent by which miRNAs modulate the ErbB receptor signaling pathway, we performed a genome-wide screen in the breast cancer cell line MCF7 based on Akt phos-phorylation as a read-out. We identified 43 miRNAs that specifically regulate heregulin (HRG)-induced Akt activation, either positively or negatively, and revealed the complexity of coordinated miRNA-target interactions within the ErbB signaling pathway. We further validat-ed four miRNAs, miR-149, miR-148b, miR-326, and miR-520a-3p, with potential tumor sup-pressive function as novel regulators of ErbB3 transcript and protein levels. But also the ex-pression levels of other key components within the ErbB/Akt pathway were affected either on the protein or mRNA level, like Erk1/2 and PIK3CA. The selected miRNAs further efficiently blocked ErbB signaling and HRG-dependent proliferation, supporting their tumor-suppressive role. In the second part, we focused on the role of the screen hit miR-149 in regulating cancer cell motility, a process that is of pivotal importance for the formation of metastasis especially in later stages of breast cancer. Clinical data revealed that miR-149 is downregulated in various tumor types, including the basal-like breast cancer subtype. Using the basal-like MDA-MB-231 cell line as a model system and a combination of biochemical and cellular as-says, we showed that miR-149 interfered with signaling downstream of integrin receptors at multiple levels, impairing Rac activation and efficiently blocking cell migration and invasion both in vitro and in vivo. Furthermore, Rap1a, Rap1b and Vav2 were identified as potential targets of miR-149. In addition, we confirmed the observed phenotype in other basal-like breast cancer cell lines, therefore providing evidence that miR-149 has broader tumor-suppressive function. Taken together, this study demonstrates a new role for several miRNAs in regulating cancer-associated pathways and further broadens our knowledge of their functions.