Although tremendous progress has been made in the understanding of mechanisms implicated in lung tumorigenesis, the five-year relative survival rate of lung cancer patients is still unsatisfactory, especially in patients with distant metastasis (only about 5%) [13]. The major obstacle for lung cancer treatment is late-stage diagnosis, which leaves very limited therapeutic options with little success rate [14]. Reliable new prognostic biomarkers for lung cancer would have great significance.
LncRNAs are a class of versatile RNAs involved in tumor initiation, progression and metastasis at the epigenetic, transcriptional, and post-transcriptional levels [15, 16]. Increasing evidence shows that lncRNAs are aberrantly expressed in lung cancers and closely related to the clinical outcome in lung cancer patients. For instance, MALAT1, one of the identified cancer-related lncRNAs, was reported to be closely associated with distant metastasis in NSCLC patients [17]. LncRNA regulator of reprogramming (linc-ROR) had a higher expression in NCSLC tissues than adjacent non-tumor tissues, and this elevated linc-ROR expression positively correlated to advanced TNM stage and lower five-year overall survival [18]. Also, lncRNA bladder cancer associated transcript 1 (BLACAT1) was upregulated in both NCSLC tissues and cells, with its elevated expression facilitating the proliferation and invasion of NCSLC cells [19].
Although many lncRNAs have been discovered, their exact function in cancers and the underlying mechanisms still require deeper study. Here, we investigated the role that LINC00483, a functional lncRNA discovered in 2017, plays in the most common lung cancer: lung adenocarcinoma. LINC00483 was found to be upregulated in LUAD tissues and cells. This pattern was also reported for colorectal and gastric cancer cells in previous studies [7, 20]. Elevated LINC00483 expression positively correlated to shorter survival time, advanced TNM stage, larger tumors and positive lymph node metastasis. An integrated bioinformatics analysis showed that LINC00483 has prognostic power in endometrial carcinoma [21].
Malignant proliferation of cancer cells plays a critical role in the development and progression of cancers [22]. LINC00483 knockdown led to an obvious inhibition of LUAD cell proliferation in this study. It could also suppress tumor growth in vivo.
The consequences of abnormal cell migration include tumor formation, while invasive cancer cells can permeate into nearby tissues and further trigger distant metastasis [23, 24]. Approximately 90% of LUAD death results from distant metastasis of cancer cells to other organs.
EMT is a crucial process by which epithelial cells acquire the invasiveness of mesenchymal cells, which facilitates cancer invasiveness and metastasis [25, 26]. In our study, LINC00483 knockdown suppressed the migration and invasion of LUAD cells and this was accompanied with changes in the expression level of EMT-related markers: Snails and N-cadherin were downregulated while E-cadherin expression was elevated. Our findings were highly consistent with those of a previous study that showed that LINC00483 silencing inhibited EMT by interacting with HOXA10 in LUAD [27]. These results indicate that LINC00483 promotes the proliferation and invasion of LUAD cells and may further facilitate cancer metastasis.
LncRNAs can “talk to” microRNAs according to the “competitive endogenous RNA (ceRNA)” hypothesis. In this study, we discovered that the microRNA miR-204-3p directly interacts with LINC00483. LINC00483 is mainly expressed in cytoplasm where it acts as a sponge of miR-204-3p, as validated using the luciferase reporter assay. Furthermore, RNA immunoprecipitation with Ago2 revealed that LINC00483 and miR-204-3p are highly enriched in LUAD cells. The expression of LINC00483 negatively correlated to that of miR-204-3p in both LUAD tissues and cells. MiR-204-3p was downregulated in tumor tissues, and overexpression of miR-204-3p inhibited proliferation, migration and invasion while promoting apoptosis in several cancers [9, 28, 29]. In particular, the inhibition of proliferation and invasion caused by LINC00483 silencing was abolished after miR-204-3p inhibition. This was in accordance with the anti-tumor effect reported in previous studies [9, 30].
All these results suggest that LINC00483 exerts its tumor-promoting function by regulating miR-204-3p. This new regulatory axis may provide novel therapeutic target for LUAD treatment.
In addition, we validated that ETS1 is a downstream target gene of miR-204-3p and that ETS1 expression positively correlates to LINC00483 level. ETS1 is upregulated in cancer cells and is linked to poor clinical outcome in patients, so it may serve as a diagnostic marker [11, 31, 32]. ETS1 also facilitated the acquisition of invasiveness, drug resistance and neo-angiogenesis in cancer cells [11].
Our results showed that LINC00483 promoted LUAD progression by sponging miR-204-3p and further restoring ETS1. This gives further information about this new regulatory axis for LUAD development.
Although solid work has been performed to investigate the role of LINC00483 in lung adenocarcinoma, our study still has limitations. We did not investigate the impact of LINC00483 on tumor metastasis in the mouse model, and the promotion of EMT mediated by LINC00483 also needs more experimental proof.