Lung cancer remains one of the leading causes of cancer-related death worldwide and approximately 80% of lung cancers are non-small cell lung carcinomas (NSCLC). The incidence of lung cancer is highly correlated with cigarette smoking and nicotine is the addictive component of tobacco smoke. Nicotine can promote proliferation, migration, and invasion of cells in vitro and promote tumor growth and metastasis in vivo through activation of nicotine acetylcholine receptors. Recently, nicotine is implicated in promoting self-renewal of cancer stem cells in lung cancer. Cancer stem cells (CSC) are a subset of tumor cells that has the ability to self-renew and generate tumor heterogeneity leading to tumor progression and invasion. This study was done to evaluate the expression of CSC markers SOX-2 and OCT-4 in NSCLC and their regulation by nicotine.
Tissue samples were collected from 40 histopathologically confirmed cases of NSCLC and expression of SOX-2 and OCT-4 was evaluated using immunohistochemistry. The mean immunoreactive score (staining intensity x proportion of positive cells) was calculated and compared between tumor tissue and adjacent normal lung tissue. The immunohistochemical expressions of SOX-2 and OCT-4 were also correlated with clinicopathological parameters such as age, gender, tobacco habit and histopathological grade of the tumor. Experiments were conducted using A549 human lung adenocarcinoma cell lines, where cells were serum starved for 24 hrs and stimulated with 2 μM nicotine. The induction of SOX-2 was examined using immunofluroscence microscopy.
The mean immunoreactive score of SOX-2 and OCT-4 was higher in tumor tissues compared to adjacent normal tissue in patients with NSCLC Higher expression of CSC markers was noticed in tobacco habituates (p = 0.001) and in poorly differentiated tumors (p = 0.01). SOX-2 was found to be induced after 21 hours of nicotine stimulation in A549 lung cancer cells.
Our findings shed light on novel molecular mechanisms underlying the pathophysiology of smoking- related lung cancer in the context of cancer stem cell populations, and reveal new pathways involved that could potentially be exploited therapeutically.
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All authors have declared no conflicts of interest.