Abstract
Distant metastases are the leading cause of breast cancer related death. It is important to understand how the metastatic process is regulated in order to develop new therapies that would oppose metastasis and improve cancer outcome. Initiation of metastasis requires cell invasion and escape from the primary tumor into the vasculature followed by colonization of secondary sites. Tumor invasion and intravasation are enabled by the epithelial mesenchymal transition (EMT), a process in which epithelial cells lose polarity and intracellular adhesion and acquire motility and invasiveness. This thesis work has revealed novel mechanisms whereby cancer metastasis is regulated via induction of EMT. First, this thesis work provides a novel mechanism whereby VEGFA promotes metastasis through activation of EMT. VEGFA is best known as an angiogenic agent, but it also promotes cancer invasion and metastasis through mechanisms that are not fully understood. Our prior work showed VEGFA mediates cancer stem cell (CSC) expansion via induction of SOX2, a key stem cell driver in breast cancer models. Here, we showed VEGFA rapidly upregulates SOX2, leading to SNAI2 induction and EMT. Sox2 downregulates miR-452, a novel metastasis suppressor, that we show directly targets the SNAI2 3’ UTR. VEGFA induction of SOX2 leads to loss of miR452 and upregulation of Slug, driving EMT and metastasis in breast cancer models. Second, we also uncovered a new mechanism whereby T157 and T198-phosphorylated, deregulated p27 contributes to cancer metastasis. In normal cells, p27 regulates cell cycle and functions as an atypical tumor suppressor. While p27 is rarely completely lost, it is frequently deregulated through either excess degradation or through key C-terminal phosphorylations in human cancers. We and others have previously identified an oncogenic role for p27 in motility, invasion and metastasis resulting from these C-terminal phosphorylations. Here we showed that p27CK-DD induced EMT and enhanced metastatic potential of breast cancer cell lines. Knockdown of p27 in highly metastatic EMT-transformed cell lines with high levels of p27pT157pT198 (p27pTpT) reverted EMT and impaired metastatic potential. Mechanistically, we provides evidence for p27 as a transcriptional co-regulator of cJun. We showed that C-terminally phophorylated p27 binds and activates cJun, and forms a complex with cJun at an enhancer region upstream of the TGF-β2 gene to induce TGF-β2 and EMT. My Ph.D. data identify novel pathways in which Sox2, upregulated by VEGFA, contributes to activation of EMT and metastasis through Slug. Furthermore, this thesis work reveals an oncogenic function of p27 to promote tumor progression through EMT via cJun-mediated TGF-β2 induction. Since treatment of metastasis is the final therapeutic frontier, it is hoped that mechanistic insights into acquisition of metastatic potential through EMT may ultimately generate new strategies for opposing metastasis and improving outcome of cancer.