Function of Long Noncoding RNAs in Breast Cancer

Breast cancer is a disease that will be diagnosed in about 1 in 10 women throughout their lifetime. The majority of breast cancers are originated from the epithelial cells of the mammary ducts, and this occurrence can be due to several factors including hereditary and acquired mutation. There are several major breast cancer subtypes, including estrogen receptor-α (ERα)-positive, HER2-enriched and triple-negative (TNBC). Patients diagnosed with ER+ tumors are generally treated with estrogen blockers (e.g., tamoxifen, letrozole and fulvestrant). Patients with HER2+ tumors are commonly administered with drugs that block HER2 signaling (e.g., trastuzumab) or inhibit HER2’s tyrosine kinase activity (e.g., lapatinib). For patients with TNBC, chemotherapies such as taxanes and anthracyclines are standard of care therapies. However, for each breast cancer subtype, a significant number of patients develop resistance to these therapies and eventually die from metastasis, a process which accounts for ~90% of breast cancer mortality. Currently, metastatic breast cancer is incurable, and the short median survival of 3 years for patients with metastatic breast cancer has not significantly changed in over 20 years. Therefore identification of new molecules that are involved in breast cancer metastasis and development of more precisely targeted therapeutic strategies are urgently needed to improve the clinical outcome for this disease. The transforming growth factor pathway beta (TGFβ) pathway has been show to play a key role in metastasis through induction of epithelial-mesenchymal transition (EMT), cell migration and invasion. Over more than a decade, this pathway has been studied across several cancers and it is now better established that it has context-dependent tumor suppressive and oncogenic qualities. In the early stages of breast cancer, TGFβ pathway is a suppressor of benign and early stage tumor growth. However, as disease progresses and corresponding levels of TGFβ ligands become elevated, a “switch” will take place and promote oncogenic phenotypes like EMT and cancer cell stemness which drive metastasis. Long noncoding RNAs (lncRNAs) are an emerging subclass of RNA molecules in cancer biology. LncRNAs are \u3e200nt and can influence target gene expression locally in “cis”, or along a distant chromosome in “trans”, through various mechanisms and interactions with other biological molecules. The contribution of TGFβ-regulated lncRNAs to associated phenotypes like EMT and cancer cell stemness has not been very well studied. The aim of this doctoral dissertation is to address the functional and mechanistic roles of lncRNAs in these processes. Using a well-established TGFβ-induced EMT model (e.g., mouse mammary epithelial cell NMuMG treated with TGFβ, we have identified 3 conserved lncRNAs (lncRNA-HIT, WDFY3-AS2 and TIL) that are significantly upregulated upon TGFβ-induced EMT. They all mediate TGFβ-induced EMT, cell migration and invasion. Overexpression of these lncRNAs is frequently detected during the breast cancer progression and is associated with high grade and late stage of breast cancer as well as metastatic lesion. We have also demonstrated that lncRNA-HIT positively regulates HOXA13 through “cis” mechanism and that WDFY3-AS2 induces WDFY3 and STAT3 expression at mRNA level by direct interaction with hnRNP-R. Interestingly, TIL stimulates C-MYC protein but not mRNA expression by promoting Akt phosphorylation of NF90 leading to its translation from the nucleus to cytosol where NF90 binds to C-MYC mRNA and enhances C-MYC translation. Importantly, we have shown that knockdown of lncRNA-HIT and WDFY3-AS2 significantly reduces breast cancer growth and lung metastasis in orthotopic breast cancer model. These findings indicate that these TGF-induced lncRNAs play critical role in EMT, metastasis, and are relevant in human patient tumors. Therefore, it is important to consider utilizing these molecules for clinical applications like diagnosis, monitoring recurrence, predicting a response to therapy, and even as a direct target for therapeutic intervention