Cell Death and Survival Let-7c Governs the Acquisition of Chemo- or Radioresistance and Epithelial-to-Mesenchymal Transition Phenotypes in Docetaxel-Resistant Lung Adenocarcinoma

MicroRNA (miRNA) expression and functions have been reported to contribute to phenotypic features of tumor cells. Although targets and functional roles for many miRNAs have been described in lung adenocarcinoma (LAD), their pathophysiologic roles in phenotypes of chemoresistant LAD cells are still largely unclear. Previously, docetaxel (DTX)-resistant LAD cell lines (SPC-A1/DTX and H1299/DTX) were established by our laboratory and displayed chemoor radioresistance and mesenchymal features with enhanced invasiveness and motility. Unbiased miRNA profiling indicated that let-7c (MIRLET7C) was significantly downregulated in SPC-A1/DTX cells. Ectopic let-7c expression increased the in vitro and in vivo chemoor radiosensitivity of DTX-resistant LAD cells through enhanced apoptosis, reversal of epithelial-tomesenchymal phenotypes, and inhibition of in vivo metastatic potential via inactivation of Akt phosphorylation, whereas a let-7c inhibitor decreased the chemoor radiosensitivity of parental cells. Further investigation suggested that let-7c significantly reduced the luciferase activity of a Bcl-xL 3'-UTR-based reporter, concordant with reduced Bcl-xL protein levels. Additionally, siRNA-mediated Bcl-xL knockdown mimicked the same effects of let-7c precursor, and enforced Bcl-xL expression partially rescued the effects of let-7c precursor in DTX-resistant LAD cells. Furthermore, we found that Bcl-xL was significantly upregulated in DTX-nonresponding LAD tissues, and its expression was inversely correlated with let-7c expression. This study suggests an important role for let-7c in the molecular etiology of chemoresistant lung adenocarcinoma. Mol Cancer Res; 11(7); 699–713. 2013 AACR. Introduction Lung adenocarcinoma (LAD) is the most common type of lung cancer,making up 30% to 40%of all non–small cell lung carcinoma (NSCLC) cases (1). Currently, the treatment of advanced LAD may include chemotherapy, radiotherapy, molecular-targeted therapy, or a combination of these (2). However, due to development of resistance to chemoor radiotherapy, the prognosis of patients with LAD still remains poor. Chemotherapyor radiotherapyinduced epithelial-to-mesenchymal transition (EMT) in tumor cells has been linked to chemoor radiotherapeutic resistance (3–5). Therefore, a better understanding of the nature of chemoradiotherapy resistance and the molecular mechanisms that govern chemotherapy-induced EMT in LAD could lead to exploration of novel molecular targets. miRNAs are a class of noncoding RNAs, which have been reported to be involved in the regulation of many biologic processes including embryonic development and tumorigenesis (6–11). Many miRNAs have been reported to be associated with lung cancer cell survival and proliferation (12). However, there have been few studies focusing on the role of miRNAs in the chemoradioresistance and EMT of lung cancer. Previously, we have identified miRNA expression profiles in docetaxel (DTX)-resistant LAD cell line (SPC-A1/DTX) and found that Let-7c was significantly downregulated (13). Recently, the associations between Let-7c and lung cancer are reported by other research groups. Navarro' and colleagues reported that members of the lethal7 (let-7) family were downregulated both in embryonic lung tissue and in lung tumors and low levels of let-7c were associated with absence ofmetastases and early-stageNSCLC (14).Nagayama and colleagues identified 51 genomic regions Authors' Affiliations: Department of Medical Oncology, Jinling Hospital, School of Medicine, Nanjing University; and Department of Biochemistry and Molecular Biology, Nanjing Medical University, Nanjing, PR China Note: Supplementary data for this article are available at Molecular Cancer Research Online (http://mcr.aacrjournals.org/). S.-Y. Cui, J.-Y. Huang, and Y.-T. Chen should be regarded as joint first authors for their equal contributions. Corresponding Authors: Rui Wang and Long-Bang Chen, Department of MedicalOncology, JinlingHospital, School ofMedicine,NanjingUniversity, 315ZhongshanEastRoad, Nanjing, Jiangsu210002, PRChina. Phone: 8625-80860072; Fax: 86-25-80860072; E-mail: [email protected]; and Long-Bang Chen, [email protected]. doi: 10.1158/1541-7786.MCR-13-0019-T 2013 American Association for Cancer Research. Molecular Cancer Research www.aacrjournals.org 699 on June 19, 2017. © 2013 American Association for Cancer Research. mcr.aacrjournals.org Downloaded from Published OnlineFirst April 5, 2013; DOI: 10.1158/1541-7786.MCR-13-0019-T