DOI
https://doi.org/10.25772/1YVQ-D607
Author ORCID Identifier
https://orcid.org/0000-0002-2183-3170
Defense Date
2021
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Pharmacology & Toxicology
First Advisor
David A. Gewirtz
Abstract
Cancer-associated deaths account for the second-highest mortality rates in the United States. Primary modalities of treatment often include surgery, radiation, and chemotherapy, and may also incorporate targeted therapy and immunotherapy. However, resistance to these treatments remains high, resulting in disease reoccurrence and poor survival rates. While apoptosis or cell death of tumor cells is the ideal outcome for anti-cancer therapy, this is often not the case, and in fact cancer cells may upregulate several pathways, such as autophagy and senescence, as a means to undergo alternative cell fate and evade apoptotic cell death. An essential tumor suppressor gene, TP53, regulates all three of these processes, apoptosis, autophagy, and senescence, and loss of function or mutated TP53 is often implicated in early tumorigenesis and reduced sensitivity to antineoplastic therapy. To assess the effects of p53 status on the functionality of autophagy and cellular responses to radiation and chemotherapy, we utilized a pair of isogenic non-small cell lung cancer cells (NSCLC) expressing wild type p53 (H460wt) or lacking p53 expression generated using CRISPR/Cas9 editing (H460crp53). Exposure to the DNA-damaging agents, cisplatin and radiotherapy, revealed differential sensitivity between H460wt and H460crp53 cells, in which H460crp53 cells were significantly less sensitive to cisplatin and radiation exposure compared to their wild-type counterpart. In response to radiotherapy, apoptosis was induced to similar extents in both cell lines, while autophagy interference identified a nonprotective function of autophagy in response in both cell lines, regardless of p53 status. Rather, the differential radiosensitivity exhibited between H460wt and H460crp53 cells was attributed to differences in senescence induction, where H460wt cells demonstrated a significantly greater extent of senescence induction. Of particular interest was the finding that when the same set of isogenic cell lines was exposed to cisplatin, the cells exhibited a similar extent of senescence induction over time; however, autophagy inhibition revealed two different functional forms of autophagy: nonprotective autophagy in H460wt cells and cytoprotective autophagy in H460crp53 cells. Blockade of cytoprotective autophagy in H460crp53 exposed to cisplatin was sufficient to restore sensitivity and apoptosis induction to a similar extent as in the H460wt cells, further confirming the existence of an autophagic switch and the role of cytoprotective autophagy in the initial resistance to cytotoxic therapy. Finally, given concomitant activation of both autophagy and senescence in response to chemotherapy and radiation, we also examined the relationship between these two processes. At least in the case of nonprotective autophagy, autophagy inhibition did not interfere with senescence induction or proliferative recovery from growth arrest, indicating these two processes may be dissociated when autophagy is nonprotective in function. Taken together, cancer chemotherapy and radiotherapy activate a number of cellular mechanisms, such as autophagy and senescence, and not solely apoptotic cell death; consequently, further analysis and screening are warranted prior to therapeutic administration of autophagy inhibitors to patients. While autophagy seems to be an attractive therapeutic target under its cytoprotective function, autophagy can in fact play multiple functions and switch functional responses. These studies demonstrate that autophagy is contextual in nature and may, in part, depend on the therapeutic modality utilized and the p53 status of the tumor cells.
Rights
© Nipa H Patel
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
5-7-2021