Combinatorial targeting of PI3K and MAPK pathways by miR-564 to inhibit proliferation and invasion in breast cancer

Breast cancer is among most common malignant tumors worldwide and one of the deadliest cancer types among women. Like other cancer types, dysregulation of signaling pathways is the major cause of uncontrolled cell proliferation, inhibition of apoptosis and eventually metastasis of breast cancer. PI3K and MAPK signaling pathways are among top most deregulated pathways promoting proliferation and invasion in cancer. Clinically approved kinase inhibitors targeting main regulators of these pathways have limited success due to cross-talks between these cascades and creating potential bypass mechanisms. MiRNAs (miRNAs) are 18-22 nucleotides long small non-coding RNAs, functioning by targeting one or more genes simultaneously. The extensive studies on miRNAs showed that they are highly associated with the control of cancer-related processes such as proliferation, migration and invasion. In this thesis, my aim was to identify a potential miRNA functioning as a dual inhibitor of both PI3K and MAPK pathways regulating oncogenic processes in breast cancer. Our previous miRNA mimic screen with reverse phase protein array (RPPA) has been reanalyzed regarding PI3K, MAPK and cell cycle protein regulations. Among top 50 candidate miRNAs, miR-564 was shown to act as a dual inhibitor of PI3K and MAPK pathways in breast cancer cells and inhibiting proliferation through G1 cell cycle arrest. Furthermore, I showed that miR-564 reduces migration and invasion of aggressive breast cancer cells via blocking epithelial-mesenchymal transition (EMT). Direct targeting of AKT2, GNA12, GYS1 and SRF genes in combination may play role in miR-564 being a dual inhibitor of these pathways. Moreover, both high miR-564 expression and low expression of miR-564 target genes were shown to be associated with reduced invasiveness of tumors as well as distant relapse-free survival of breast cancer patients. Overall, I showed that, in addition to being a dual inhibitor of PI3K and MAPK pathways by combinatorial targeting network of genes, miR-564 is a prognostic marker for breast cancer and a promising druggable target.