Identifying Novel Inhibitors of Epithelial to Mesenchymal Transition (EMT) for Targeting Pancreatic Cancer Metastasis and Cancer Stem Cells

Abstract

Pancreatic cancer is the fourth leading cause of cancer-related death in the United States, and it is expected to become the second-leading cause of cancer-related death by 2030. Currently, there are no early detection tests and most patients with localized disease have no recognizable symptoms. As a result, more than half of the patients with this disease are diagnosed at a stage where metastases have developed, for whom the overall 5-year survival is only 2%. Moreover, these tumors are highly enriched with a cancer stem cell (CSC) population (~1%), which is highly resistant to chemotherapeutic drugs, and therefore escapes chemotherapy and promotes tumor recurrence. Recent evidence suggests that epithelial to mesenchymal transition (EMT) is associated with metastasis, generation of CSCs, and treatment resistance in solid tumors including pancreatic cancer. Therefore, compounds inhibiting EMT hold the potential to reverse drug-resistance or inhibit metastasis and CSCs, and therefore could provide better treatment outcome for patients with pancreatic cancer. The overall goal of this dissertation is to investigate novel EMT inhibitors for targeting pancreatic cancer metastasis and CSCs. First we demonstrated in preclinical models that treatment with pharmacological doses of ascorbate resulted in inhibition of EMT, metastasis and CSCs. In addition, ascorbate decreased the expression of HDAC6, and inhibited activity of Sirt-2 by depleting NAD+ levels resulting in robustly increased α-tubulin acetylation in pancreatic cancer cells. Ascorbate mediated tubulin acetylation promoted α-tubulin polymerization and stabilization, mimicking the cellular outcomes of paclitaxel to inhibit cancer cell metastasis. Next, we investigated novel derivatives of the histone deacetylase (HDAC) inhibitors SAHA and MS-275, which are known EMT inhibitors. In an effort to increase efficacy and reduce toxicities of the HDAC inhibitors, we found that the novel synthetic compounds St-1 and St-3 potently inhibited pancreatic cancer cell proliferation and CSCs. St-1 has exhibited similar potency in HDAC inhibition compared to the parent compounds (SAHA and MS-275). Surprisingly, St-3 acted via totally different mechanisms from SAHA and MS-275. St-3 exhibited anti-tumor effects by blocking the interaction of human antigen R (HuR) with its target mRNAs. Finally, we established and performed a high throughput screening approach to identify inhibitiors of cancer cell EMT. 1-(benzylsulfonyl) indoline (BSI) was found to be a novel EMT inhibitor. BSI significantly inhibited pancreatic cancer cell migration invasion and CSCs. To enhance the efficacy of BSI, several analogues of BSI were tested for their activities on EMT and CSC inhibition. However, BSI analogues failed to show superior anti-migration or anti-CSC activities compare to BSI. In conclusion, this dissertation resulted in the identification of several novel EMT inhibitors, which can be tested further in preclinical studies for their anti-tumor efficacy.