Graduation Year

2016

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Biology (Cell Biology, Microbiology, Molecular Biology)

Major Professor

Srikumar Chellappan, Ph.D.

Committee Member

W. Douglas Cress, Ph.D.

Committee Member

Eric Haura, M.D.

Committee Member

Conor Lynch, Ph.D.

Keywords

Smoking-related lung cancer, Sox2, alpha7, E2F1, Yap1

Abstract

Lung cancer is the leading cause of cancer-related death in both men and women, nationally and internationally and kills more people each year than breast, prostate, and colon cancers combined. Non-small cell lung carcinoma (NSCLC) is the most common histological subtype of lung cancer, and accounts for 85% of all cases. Cigarette smoking is the single greatest risk factor for lung cancer, and is correlated with 80-90% of all lung cancer deaths. Nicotine, the addictive component of tobacco smoke, is not a carcinogen and cannot initiate tumors itself; however, it is known to act as a tumor promoter, by enhancing the proliferation, migration, and invasion of cells in vitro, thus accelerating tumor growth and metastasis in vivo. Nicotine exerts is tumor promoting effects primarily by binding to, and activation of, nicotinic acetylcholine receptors (nAChRs), specifically the α7 subunit of nAChRs. While α7 nAChR is expressed in a wide array of cells, how its expression is regulated is not fully understood. Here we sought to elucidate the transcriptional regulation of α7 nAChR in NSCLC cells. We report that α7 nAChR expression is induced by nicotine in an autoregulatory feedforward loop, and that the α7 gene promoter is differentially regulated by E2F1 and STAT1 transcription factors at an overlapping binding site suggesting a competitive interplay. Depletion of E2F1 resulted in a reduced ability of nicotine to induce α7 nAChR, while depletion of STAT1 resulted in enhanced induction, suggesting that nicotine might use these two transcription factors to modulate the expression of α7 nAChR in a very precise fashion. More recently, nicotine has been implicated in promoting self-renewal of stem-like side-population cells from lung cancers. Cancer stem-like cells have been implicated in tumor initiation as well as the maintenance, drug resistance, dormancy, recurrence, and metastasis of various tumor types. We had previously shown that the embryonic stem cell transcription factor, Sox2, is indispensable for self-renewal of stem-like cells from lung adenocarcinoma cell lines; hence we sought to determine whether nicotine enhances stemness of lung cancer stem-like cells through Sox2. We find that nicotine can induce the expression of Sox2 at the transcriptional level and this occurs through a nAChR-Src-Yap1-E2F1 signaling axis. Over recent years, electronic cigarettes (e-cigarettes) have emerged as healthy alternatives to traditional cigarette smoking as they do not contain tobacco; however, they do still contain nicotine. Our studies show that e-cigarette components can enhance tumor promoting properties of NSCLC cells similar to that observed with nicotine alone, and find that they can induce expression of Sox2 and mesenchymal markers as well as enhance migration and stemness of NSCLC cells. Taken together, these studies reveal novel molecular mechanisms by which exposure to nicotine, via cigarette smoke or e-cigarettes, could alter the oncogenic potential of NSCLC cells.

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