Date of Award

2022

Document Type

Campus Access Dissertation

Degree Name

Doctor of Philosophy in Biochemistry and Molecular Biology (PhD)

Administrative Home Department

Department of Biological Sciences

Advisor 1

Xiaohu (Mark) Tang

Committee Member 1

Guiliang Tang

Committee Member 2

Marina Tanasova

Committee Member 3

Zhiying Shan

Abstract

Pancreatic cancer is estimated to be the second deadliest cancer by the mortality rate in USA by 2030. Almost 96 percent of pancreatic cancer patients die in 5 years after initial diagnosis due to limited effective therapeutic options. New targeted therapies should be explored to better combat pancreatic cancer in a more selective way. Cysteine acts as a limiting factor for synthesis of glutathione (GSH), which is a cofactor to the glutathione peroxidase GPX4, and keeps reactive oxygen species (ROS) in check. Previous studies have proposed cysteine depletion as a potential therapy strategy to treat many cancers. We observed that cell vulnerability to cysteine depletion also exists in pancreatic cancer but in a subtype-dependent manner. We observed that the epithelial-type PDAC showed resistance to cysteine depletion, while the mesenchymal-type was vulnerable, indicating a correlation between the state of epithelial to mesenchymal transition (EMT) and cysteine dependence. Next, we found that the expression of Sepp1correlates with cysteine dependence. Knockdown of Sepp1 rendered the epithelial-type cancer cells sensitive to cysteine depletion. Since Sepp1 is a selenium transporter, we found selenium addition can protect mesenchymal-type PDAC cells from cysteine depletion. Further investigation revealed that GSH remains depleted in selenium-treated cells, suggesting that the protective role of selenium is independent of the cysteine-GSH-GPX4 axis. The thioredoxin system (Trx) is considered as a redundant pathway to the GPX4 axis to clean ROS up in cells. Thioredoxin reductase TrxRs are also selenoproteins and potentially disturbed by the selenium presence. We found that the protection by selenium addition is dependent on the Trx system. Furthermore, the mitochondrial thioredoxin reductase (TrxR2) specifically contributes to this protection. Subsequently, inhibition of the Thioredoxin (Trx) system in combination with cysteine depletion can overcome the erastin-resistance in the epithelial-type PDAC cells. In addition, we found that Sepp1 modulates the Trx system on both transcriptional and translational levels. Overall, our study suggests that pancreatic cancer exhibits differential cysteine dependence in a tumor-subtype dependent manner. The EMT marker can be used to differentiate cysteine dependence in PDAC. Sepp1 or selenium addition can protect cells from cysteine depletion mediated ferroptosis via the Trx pathway. Our study also suggests that targeting of both GPX4 and Trx pathways can be a potential adjuvant-therapy option to overcome the erastin-resistance in pancreatic cancer.

Available for download on Sunday, January 01, 2023

Share

COinS