Author ORCID Identifier

https://orcid.org/0000-0003-2938-8785

Year of Publication

2021

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Medicine

Department/School/Program

Toxicology and Cancer Biology

First Advisor

Dr. Chengfeng Yang

Second Advisor

Dr. Zhishan Wang

Abstract

Cadmium (Cd) is a ubiquitous pollutant in the environment and a known carcinogen for lung cancer. Cd has been shown to act as a weak mutagen, which suggests that it may exert tumorigenic effect through non-genotoxic ways, such as epigenetic mechanisms. The goal of this project is to investigate the mechanisms of Cd carcinogenesis focusing on the role of lncRNA dysregulations. The Cd-exposed cells formed significantly more colonies in soft agar, displayed cancer stem cell (CSC)-like property and formed tumors in nude mice. Mechanistically, the lncRNA microarray analysis revealed that chronic Cd exposure dysregulates lncRNA expressions. Q-PCR analysis confirmed the significant upregulation of the oncogenic lncRNA DUXAP10 level in Cd-transformed cells. Knockdown of DUXAP10 in Cd-transformed cells significantly reduced their CSC-like property. Further mechanistic studies showed that DUXAP10 activates the Hedgehog pathway to promote Cd-induced CSC-like property. Furthermore, it was determined that chronic Cd exposure upregulates DUXAP10 expression by inducing Pax6 expression.

In addition to oncogenic lncRNA upregulation, Cd exposure was also found to downregulate the expression of a tumor suppressive lncRNA MEG3 in Cd-transformed cells. Meanwhile, the levels of DNMTs in Cd-transformed cells were found significantly elevated. Bisulfite-sequencing study revealed that the differentially methylated region (DMR) upstream of MEG3 is hypermethylated in Cd-transformed cells, indicating that the promoted DNMTs activity contributed to downregulation of MEG3. Stably expressing MEG3 in Cd-transformed cells decreased cell proliferation and induced CSC-like property. Further studies showed that MEG3 inhibits cell transformation by limiting cell proliferation and inducing apoptosis. Mechanistic studies revealed that MEG3 reduced cell proliferation by regulating the levels of cell cycle proteins and induced apoptosis by inhibiting the level of Bcl-xL. These findings suggest that dysregulations of lncRNAs play important roles in Cd carcinogenesis.

As well as epigenetic dysregulations, we also determined the effect of genetic factor contributing to lung cancer. Enhanced EGFR signaling contributes to 60% of NSCLC cases. However, there is an unmet need to solve acquired resistance to tyrosine kinase inhibitors and low response rate for immunotherapy in lung cancer patients. This study was performed to investigate the role of SOCS3 in EGFR mutation-driven lung cancer and to explore the potential of its regulatory axis as therapeutic target for the development of novel approach. In our transgenic mouse model, overexpression of SOCS3 significantly inhibited tumor formation with mutated EGFR. Further investigation for the underlying mechanism revealed that SOCS3 downregulates YAP protein, which further suppressed Bcl-2 family proteins. External YAP inhibitor was shown to efficiently inhibit the growth of tumor organoids. In vivo studies demonstrated that SOCS3 downregulating YAP leads to less immunosuppressive tumor microenvironment. Lastly, SOCS3 was often found to be silenced in cancers. To mimic this circumstance, the therapeutic efficacy of utilizing external YAP inhibitor combined with anti-PD-L1 was assessed and showed promising outcomes. These results suggest the critical role of SOCS3 as a biomarker for the oncolytic immune environment and provide a novel insight for improving lung cancer immunotherapy.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2021.356

Funding Information

This study was supported by the National Institute of Environmental Sciences grants to Dr. Chengfeng Yang (R01ES026151; R01ES029496; R01ES029942) in 2016 through 2020.

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