At present, lung cancer has been a great threat to both men and women, which caused about 1.5 million deaths in global annually [1, 2]. The main variables linked with the incidence and progression of lung cancer cover smoking, hormonal imbalance, various epigenetic and genetic mutations, and high exposure to environmental pollutants [2, 3]. On the basis of histological features, lung cancer can be divided into small cell (SCLC) and non-small cell lung cancers (NSCLC), and NSCLC accounted for more than 85% of patients with lung cancer . NSCLC has several predominant subtypes such as adenocarcinoma, squamous-cell carcinoma and large-cell carcinoma, which primarily evolve from the epithelial cell lining. Compared to small cell carcinoma, NSCLCs are insensitive to chemotherapy . In addition, high recurrence rate, early formation of micro-metastases, and some problems in detection and diagnosis also lead to poor prognosis and low 5-year survival rate (less than 20% in total) in all lung cancers [2, 6]. To enhance the detection, diagnosis and treatment of lung cancer, many methods have been improved or developed, including various imaging technologies (such as X-ray, computed tomography, and positron emission tomography), nanoparticle systems, biomarkers and treat targets. Among them, the identification of biomarkers was considered as a promising way to improve cancer diagnosis and detection. Recently, food and drug administration in US has approved several biomarkers in the detection and diagnosis of lung cancer. In 2004, carcinoembryonic antigen as a protein biomarker has been approved for lung cancer diagnosis. Unfortunately, it has poor effect and it has better to be combined with other biomarkers (such as cytokeratin fragment-21) [7, 8]. Thus, more biomarkers were necessary for more accurate detection of lung cancer, particularly early stage.
In clinic, the biomarkers used for lung cancer detection, include tumor infiltrating lymphocytes, cell-free DNA (cfDNA), autoantibodies, microRNAs (miRNAs) and so on [9–12]. Among them, miRNAs are considered as important players in many biological processes and linked with pathological parameters of various cancers, such as ovarian cancer, liver cancer and colorectal cancer, implying their potential as cancer biomarkers [13–15]. In lung cancer, some studies disclosed that many miRNAs displayed 92–100% specificity and 75–85% sensitivity in differentiation between lung cancer and normal samples [2, 16]. Moreover, expression regulation of miRNAs probably was linked with the occurrence and progression of lung cancer. For instance, miR-4735-3p and miR-183-5p were over-expressed in NSCLC tissues and cell lines, while miR-143 and miR-520a-3p were down-regulated in lung cancer [17–20]. Nevertheless, the regulatory mechanism of miRNAs is still exploring in lung cancer and other cancers. It was generally accepted that the miRNAs possibly regulated expression of target genes in tumors by blocking or degrading mRNA molecules of target genes [21, 22]. More works need to be done to deeply understand their mechanism, especially NSCLC.
MicroRNA-665 (miR-665), an exosomal miRNA, has been found that it is associated with different diseases, like congestive heart failure , anesthesia-related cognitive dysfunction , inflammatory bowel disease , and cancer . Interestingly, miR-665 showed different functions for different diseases. For examples, miR-665 upregulation promoted cell apoptosis and colitis in inflammatory bowel disease through suppressing the endoplasmic reticulum stress components XBP1 and ORMDL3; while its high-expression could promote tumor growth and cell proliferation in hepatocellular carcinoma by increasing the expression of proteins in the MAPK/ERK pathway [25, 26]. However, its effects on NSCLC have not been reported. The key purpose of this work was to explore miR-665 roles and possible mechanism in NSCLC. Firstly, quantitative real time PCR (q RT-PCR) was performed to measure the expression level of miR-665 in NSCLC tissues and cell lines. Then, we constructed cells that could stably over-express and low-express miR-665 by cell transfection, and then studied its functions on cell proliferation, migration and apoptosis. Finally, prediction software was used to obtain a potential target of miR-665 and further experiments were carried out to verify it.