Background. DEAD/H (Asp-Glu-Ala-Asp/His) box polypeptide 3, X-linked (DDX3X) is a member of the DEAD-box family of ATP-dependent RNA helicases. DEAD-box helicases perform multiple functions, including RNA splicing, mRNA export, transcriptional and translational regulation, RNA decay, and ribosome biogenesis. Using two-dimensional electrophoretic and MS/MS spectrometric analyses, we demonstrated that DDX3X is one of the proteins that are specifically expressed in CD133+ B16 melanoma cells. These cells possess cancer stem cell (CSC)-like properties, such as high tumorigenicity, spheroid formation ability, and anchorage independent growth. DDX3X is evolutionarily well conserved from yeast to humans, suggesting that it is essential for cell survival. In humans, DDX3X deletion or dysfunction results in genetically related primary amenorrhea and impaired female fertility. Although originally DDX3X was reported to suppress growth by modulating the p21waf/cip1 gene expression, recent studies have shown direct correlations between DDX3X and oncogenesis. Furthermore, DDX3X overexpression in breast cancer cells has been shown to facilitate β-catenin signal transduction and induce epithelial mesenchymal transition (EMT), a known feature of CSCs. However, the mechanism by which DDX3X plays an oncogenic role to induce CSC features remains unclear.
Results. Immunoblotting analyses revealed that all of the examined cancer cells, including lung cancer, colon cancer, and breast cancer cells, strongly expressed DDX3X, whereas normal human epidermal keratinocytes, human microvascular endothelial cells, and normal human bronchial epithelial cells faintly expressed DDX3X. Furthermore, putative CSC marker-positive cancer cells, such as 87.5, HCT116, and MCF7, strongly expressed DDX3X. PC9 cells, which are lung adenocarcinoma cells harboring EGFR exon 19 deletion, showed relatively weak expression of DDX3X and did not express CD133 or other CSC markers. Parental PC9 cells were ALDH−, CD44low, and E-cadherinmid and proliferated in an anchorage dependent manner. Overexpression of DDX3X in PC9 cells led to increase in the number of ALDH+ CD44high E-cadherinhigh and ALDH+ CD44low E-cadherin− cancer cells, both of which proliferated in an anchorage independent manner. Moreover, PC9 cells that overexpressed DDX3X (PC9DDX3X) acquired EGFR-TKI resistance. In the presence of erlotinib at 100 times the concentration at IC50, apoptosis was not induced in PC9DDX3X.
Conclusion. DDX3X expression is correlated with CSC-like properties in human cancer cells. DDX3X plays a role in the EGFR-TKI resistance mechanism, and it could be used as a possible target molecule to modulate EGFR-TKI resistance in lung cancer cells harboring EGFR-activating mutation.
Citation Format: Koichiro Nozaki, Hiroshi Kagamu, Satoshi Shoji, Natsue Igarashi, Msaaki Okajima, Satoru Miura, Satoshi Watanabe, Hirohisa Yoshizawa, Ichiei Narita. DEAD/H (Asp-Glu-Ala-Asp/His) box polypeptide 3, X-linked plays oncogenic roles to induce cancer stem cell-like properties and resistance to EGFR-TKI. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3716. doi:10.1158/1538-7445.AM2013-3716