Breast cancer is the most common cancer in women and a leading cause of cancer mortality worldwide, mainly due to metastatic progression. Although in the last years cancer has become more curable, the path to eradicate it, is still long and convolute. Understanding molecular mechanisms that induce and support cancer progression and aggressiveness is crucial to find new targets and drugs to treat it. High Mobility Group A1 (HMGA1) protein is an oncofetal architectural chromatin factor that promotes neoplastic transformation and progression but its role in breast cancer aggressiveness is still unclear. The aim of this thesis was to unravel the involvement of HMGA1 in this disease highlighting which pathways are intersected. Thus, working on basal-like breast cancer subtype, we demonstrated that HMGA1 plays a crucial role in conferring metastatic traits. Indeed, HMGA1 silencing reduces migration and invasion properties in vitro and metastasis formation in vivo with a concomitant mesenchymal to epithelial transition and decreases stem cell properties and self-renewal activity. We performed microarray analysis in cells expressing or depleted for HMGA1 and we identified a specific 130-HMGA1 gene signature associated with poor prognosis. This analysis allowed us to identify pathways, controlled by HMGA1 and known to be involved in aggressiveness traits, such as Wnt/beta-catenin and Notch. Moreover, among the genes present in the 130-HMGA1 gene signature we deepen the relationship between HMGA1 and CCNE2, one of the gene most correlated with clinical outcome. We showed that CCNE2 acts downstream HMGA1 to regulate the migration and invasion proprieties of basal-like breast cancer cells. Moreover, we demonstrated that CCNE2 action is mediated by the oncogene YAP, the downstream effector of the Hippo pathway. Indeed, knock down of both HMGA1 and CCNE2 impaired nuclear localization and activity of YAP, acting upstream of the Hippo pathway core kinases MST1/2 and LATS1/2. Moreover, in breast cancer patients, HMGA1 and CCNE2 expression was associated with YAP/TAZ signature further supporting this connection. Because CDKs are the main partners of CCNE2, we blocked their activity using CDK inhibitors in order to impair HMGA1-CCNE2-YAP axis and we demonstrated a decrease in cell migration and an induction of translocation of YAP from nucleus to cytoplasm. Therefore, this thesis highlights the involvement of HMGA1 in breast cancer metastasis through the interplay with different pathways. In particular, we identified for the first time a role of HMGA1 in regulating YAP through the modulation of the Hippo pathway.
