Recently, the SOX2 gene has been reported to be amplified in human lung
squamous cell carcinomas. However, its roles in human
lung adenocarcinomas are still elusive. In this study, we analyzed the functions of SOX2 in
cancer stem-like cells (CSCs)/
cancer-initiating cells (CICs) derived from human
lung adenocarcinoma. Human lung CSCs/CICs were isolated as higher tumorigenic side population (SP) cells using
Hoechst 33342 dye from several
lung cancer cell lines. Four of nine
lung cancer cell lines were positive for SP cells (LHK2, 1-87, A549, Lc817). The ratios of SP cells ranged from 0.4% for Lc817 to 2.8% for LHK2. To analyze the molecular aspects of SP cells, we performed microarray screening and RT-PCR analysis, and isolated SOX2 as one of a SP cell-specific gene. SOX2 was expressed predominantly in LHK2 and 1-87 SP cells, and was also expressed in several other
cancer cell lines. The expression of SOX2
protein in primary human
lung cancer tissues were also confirmed by immunohistochemical staining, and SOX2 was detected in more than 80% of primary
lung cancer tissues. To address SOX2 molecular functions, we established a SOX2-overexpressed LHK2 and A549 cell line (LHK2-SOX2 and A549-SOX2). LHK2-SOX2 cells showed higher rates of SP cells and higher expression of POU5F1 compared with control cells. LHK2-SOX2 and A549-SOX2 cells showed relatively higher tumorigenicity than control cells. On the other hand, SOX2
mRNA knockdown of LHK2 SP cells by gene-specific
siRNA completely abrogated tumorigenicity in vivo. These observations indicate that SOX2 has a role in maintenance of stemness and tumorigenicity of human
lung adenocarcinoma CSCs/CICs and is a potential target for treatment.