The emerging role of SOX2 in cell proliferation and survival and its crosstalk with oncogenic signaling in lung cancer.

Tumor cells have long been observed to share several biological characteristics with normal stem/progenitor cells; however, the oncogenic mechanisms underlying the lung stem/progenitor cell signaling remain elusive. Here, we report that SOX2, a self-renewal factor in lung stem/progenitor cells, is highly expressed in a subclass of lung cancer cells, the proliferation, survival, and chemoresistance of which are dependent on SOX2 signaling. Overexpression of SOX2 promotes oncogenic phenotypes in lung cancer cells; knockdown of SOX2 attenuated cell proliferation. We observed that SOX2 increased the expression of epidermal growth factor receptor (EGFR), and EGFR activation further upregulated SOX2 levels, forming a positive feedback loop. SOX2 expression promoted chemoresistance, and silencing of SOX2 perturbed mitochondrial function, causing marked apoptosis and autophagy. SOX2 induced BCL2L1, the ectopic expression of which rescued the effects of SOX2 silencing on apoptosis, autophagy, and mitochondrial function. SOX2 promoted tumor formation, along with increased cell proliferation in a xenograft mouse model. SOX2 expression is associated with poor prognosis in lung cancer patients; moreover, SOX2, EGFR, and BCL2L1 expression levels were significantly correlated in lung tumors. Our findings support the emerging role of SOX2 in cell proliferation and survival by eliciting oncogenic EGFR and BCL2L1 signaling with potential applications as a prognosis marker and a therapeutic target in lung cancer.

CD13 (aminopeptidase N) can associate with tumor-associated antigen L6 and enhance the motility of human lung cancer cells.

Cancer metastasis is a multiple-step process that involves the regulated interaction of diverse cellular proteins. We recently reported that the expression of tumor-associated antigen L6 (TAL6) promoted the invasiveness of lung cancer cells and was inversely correlated with disease-free survival of squamous lung carcinoma patients. We now report that CD13 (aminopeptidase N) can associate with TAL6 and can enhance cancer cell migration. CD13 was shown by coimmunoprecipitation to associate in vitro with TAL6 on several cancer cell lines and to associate in vivo by antibody-mediated copatching immunofluorescence. CD13 was selectively expressed on highly invasive CL1-5 lung cancer cells as compared to poorly invasive CL1-0 lung cancer cells. The role of CD13 aminopeptidase activity in regulating cell motility was investigated with chemical inhibitors, specific antibodies and a catalytically inactive CD13 protein. Inhibition of CD13 aminopeptidase activity by nontoxic concentrations of leuhistin modestly decreased the migration of CL1-5 cells. In contrast, binding of CD13 by specific antibodies significantly reduced both the migration and the invasion of CL1-5 cells. Poorly invasive CL1-0 cells that stably expressed CD13 displayed significantly (p < or = 0.0005) enhanced cell migration (300% of control). Expression of an enzymatically inactive CD13 mutant on CL1-0 cells also significantly (p < or = 0.0005) enhanced cell migration (200% of control). Our results show that TAL6 and CD13 can form a complex on lung cancer cells, that these molecules can modulate cell migration and invasion and that the influence of CD13 on cell motility did not strictly depend on its aminopeptidase activity.

Tumor-associated antigen L6 and the invasion of human lung cancer cells.

Metastasis is a coordinated process that depends on the interaction of cancer cells with the tumor microenvironment. Members of the transmembrane-4 superfamily (TM4SF) of surface proteins have been implicated in the regulation of cancer cell metastasis, and the expression of several TM4SF members on tumor cells is inversely correlated with patient prognosis. The tumor-associated antigen L6 (TAL6), a distant member of the TM4SF, is expressed on most epithelial cell carcinomas and is a target for antibody-mediated therapy. We examined whether TAL6 may play a role in cancer metastasis by using an established series of human lung carcinoma cell lines (CL1-0 to CL1-5) that exhibit increasing invasiveness in vitro and in vivo. We found that TAL6 expression correlated with the in vitro invasiveness of CL lung carcinoma cells (r(2) = 0.98) and human carcinoma cells (r(2) = 0.69). Forced expression of TAL6 on CL1-0 lung carcinoma cells significantly increased their in vitro invasiveness and decreased the survival of SCID mice in an experimental metastasis model. Specific antibody against TAL6 (monoclonal antibody L6) significantly reduced the migration and invasiveness of CL1-5 lung carcinoma cells. The effects of monoclonal antibody L6 on CL1-5 invasion required clustering of TAL6 on the cell surface. Real-time reverse transcription-PCR of lung cancer specimens showed that increased expression of TAL6 was significantly associated with early postoperative relapse (P = 0.034) and shorter survival (P = 0.025) in squamous cell lung cancer patients. Thus, TAL6 appears to be involved in cancer invasion and metastasis.