1 Introduction

Nasopharyngeal carcinoma (NPC) is a unique head and neck cancer with highly malignant phenotype. Recently, with the wide application of imagological examination, image quality has been improved. However, about 20% of NPC patients still have local–regional recurrence and distant metastasis, which affects the therapeutic efficacy of patients with NPC. Therefore, the invasion and metastasis mechanisms of in NPC needs to be elucidated.

The invasion and metastasis of tumor is an orderly and complex biological process. The mechanism involves in the the activation and inhibition of related signaling pathways by promoting or suppressing corresponding function. Currently, few drugs are developed for targeted NPC therapy, and more than 70% of treatment failures are due to metastasis. Therefore, strengthening the research on invasion and metastasis-related molecules, and providing a scientific basis for the screening of biomarkers, which would be helpful for evaluating the risk and identifying therapeutic targets [1].

In recent years, the molecular mechanism of invasion and metastasis of NPC has been expanded from coding RNAs to some non-coding RNAs (ncRNAs) with important regulatory functions. These studies elucidate the molecular process of invasion and metastasis at multiple levels. Herein, we summarized the molecular mechanisms reported in our previous investigations.

2 ncRNAs

2.1 miRNA

MicroRNAs (miRNAs) belong to ncRNAs that play an important role in cellular biological process. Most miRNA genes exist in a single-stranded form consisting of about 22 nucleotides (nt). Targeting 3’ untranslated regions (3’ UTR) of specific mRNAs is the main function of miRNAs [2].

We put forward the specific roles and mechanisms of miR-140 in NPC for the first time, which was markedly down-regulated in NPC tissues. Our research proved that miR-140 regulates the cell migration and invasion in NPC cell lines by negatively targeting CXCR4 (Table 1, number 1). miR-449b-3p, as a tumor suppressor gene, is down-expressed in NPC samples and inhibited cell invasion and migration (Table 1, number 2). EMT is regarded as the classic signal pathway of metastasis mechanism. miR-449b-3p regulated the EMT related proteins expression, which reflected the EMT related phenotype changes in NPC. Subsequently, the miR-449b-3p/ADAM17/NF-κB axis was identified. Our studies also explored other miRNAs that could regulate invasion or metastasis in different pathways. MiR-432 acts by binding to the 3′-UTR of E2F3 directly in NPC (Table 1, number 3). Our study indicated that miR-34a regulates TGF-β/SMAD4 signal axis (Table 1, number 4). Other signal axes, such as miR-154-5p/KIF14, miR-203a-3p/LASP1, miR-98/STAT3, miR-122/ PI3K/AKT, and miR-184/ Notch2 (Table 1, number 5–9) were also explored. In summary, miRNAs could be considered as tumor suppressors or promoters that target certain cancer-related genes in malignant NPC. Through the bioinformatics data platform under the background of big data our center built, we explored a new precision diagnosis strategy suitable for the characteristics of Chinese NPC patients. Should high-promising EBV miRNA biomarkers being discovered, it may benefit minimally invasive detection of NPC in the future.

Table 1 The related articles reported in this manuscript of our previous research
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2.2 lncRNA

Long non-coding RNAs (lncRNAs) are involved in the regulation of gene expression and attract great attention in various fields. However, the molecular mechanism of lncRNA involved in invasion and metastasis in NPC remains unclear. The expression of ZNRD1-AS1, a zinc ribbon domain containing 1 antisense 1, is upregulated in NPC cells, tissues and other cancers [3, 4]. ZNRD1-AS1 is overexpressed in 40 NPC tumor samples and 5 normal nasopharyngeal tissues from our clinical center. Moreover, they are positively correlated with advanced TNM stage. And we also demonstrate that ZNRD1-AS1 promotes the invasion and metastasis via the miR-335–ROCK1 axis in NPsC (Table 1, number 10).lncRNA-LUADT1 promotes invasion and metastasis through Hippo/YAP signaling pathway, and it is regulated by miR-1207-5p, showing a competitive relationship. This lnRNA-miRNA-gene-regulated axis strengthens the theory that ncRNA deeply affected the biological process of invasion and metastasis (Table 1, number 11). LncRNA-UCA1/miR-145/ADAM17 (Table 1, number 12–13) is also involved in metastatic mechanisms, and the lncRNA-UCA1 act as a molecular sponge in this pathway. Another lncRNA, lncRNA01133, mediates the biological function of NPC cells by binding to YBX1 (Table 1, number 14). We also explored the lncRNA-CYTOR which led to the promotion of NPC by inducing the up-regulation of ANXA2 by binding to miR‐613 competitively, thus (Table 1, number 15). The impact of lncRNAn326322 on NPC was firstly reported by our studies till now (Table 1, number 16). Owing to the lack of an open reading frame, these lncRNAs participate in signaling pathways through the sponge function [5]. The competing endogenous RNAs (ceRNA) hypothesis confirms that lncRNA can regulate sponge microRNAs through post-transcriptional ways, thus affecting the function of microRNAs and increasing the expression of target gene [6,7,8].

2.3 circRNA

Circular RNAs (circRNAs) are widely expressed in a nonlinear state in eukaryotic cells. They have a closed ring structure, which is usually looped by the exon or intron of the host gene. Their expression of circRNAs is relatively stable and they are difficult to degrade by exonuclease, which can be used as an ideal clinical diagnostic marker. Moreover, circRNA could serve as a molecular sponge to adsorb miRNAs, and affect gene expression by regulating splicing and translation. The expression of circFOXM1 is higher in NPC cell lines and is mainly located in the cytoplasm. Consistent with previous studies, we found that circRNAs could act as a “microRNA sponge” to regulate downstream gene expression (Table 1, number 17). Research work demonstrated the sponge function of circFOXM1 and the downstream miR-136-5p-SMAD2 signal axis. Furthermore, advanced clinical stage was associated with this circFOXM1 signal axis, which indicated that circFOXM1 might be a possible target in clinical NPC strategy. CircRNA, hsa-circ-0046263, could sponge miR-133a-5p and upregulate the expression of the downstream targeting gene IGFBP3. Besides, it promotes migration, invasion, and tumor metastasis of NPC cells (Table 1, number 18). Interestingly, IGFBP3 was also a downstream targeted gene that crossed over other signal pathways and mechanisms which were described below.

We elaborated the evolution model of NPC from multiple levels, such as circRNA expression microarray from five pairs of NPC tissue samples. This further supported the spatio-temporal heterogeneity of NPC, and the identified biomarkers could be used as a target in the preparation of reagents, kits or tests for the assessment of NPC metastasis. They can also be applied in drugs screening for NPC metastasis.

3 Coding RNAs

The activation of the Wnt/β-catenin signaling pathway play a role in ZNF488-induced EMT transition (Table 1, number 19). As an oncogene, ZNF488 is important in invasion and tumorigenesis in NPC. HMGA2, which is highly expressed during embryogenesis, is a member of the high-mobility group family (Table 1, number 20). It also has carcinogenicity in the metastatic process due to the TGFβ/Smad3 signaling pathway. This biomarker may greatly contribute to solve the radioresistance and promote radiotherapy sensitivity. It can be considered as a radiosensitization target and become an effective method for the treatment of patients with radiotherapy resistance.

4 N6-Methyladenosine

N6-methyladenosine (m6A) is the most abundant post-transcriptional modification in mRNA [9]. The main regulators of m6A are “writers, erasers, and readers” [10]. The methyltransferase complex is also called writers, while METTL3 is the only catalytic subunits. The “erasers” refer to FTO and ALKBH5, and the “readers” mainly includes YTH domain-family and IGF2BP family. These proteins contribute to the dynamic and reversible process of m6A modifications. IGF2BP3, an m6A readers, remains an unidentified mechanism in NPC. We investigated the basic expression and biological functions of IGF2BP3 in NPC. We first identified an oncogenic axis consisting of MYC-IGF2BP3-KPNA2 (Table 1, number 21). IGF2BP3 is highly expressed in tumor tissues and is related to the advanced stage, which has been reflected in vitro. We demonstrated that the genetic alteration of IGF2BP3 was due to MYC which acted as a transcription factor binding to the promoter region of IGF2BP3 and taking over the activation of mRNA amplification. Given that IGF2BP3 had the characteristics of RBP KPNA2, it was positively expressed in NPC tissues. Our data illustrated that the IGF2BP3 affected stability of KPNA2 mRNA in an m6A-dependent manner. This hypothesis was also verified in vivo. In conclusion, IGF2BP3 played a critical role in NPC progression by affecting the mRNA stability of KPNA2.

LncRNA EPB41L4A-AS2 is also involved in invasion and metastasis of NPC. lnRNA EPB41L4A-AS2 inhibited the metastasis of NPC by binding to YBX. Simultaneously lncRNA EPB41L4A-AS2 reduced the stability of snail mRNA and reverse EMT via the miR-107-LATS2 axis at the post-transcriptional level (Table 1, number 22).

5 Conclusion

Our research group dedicates to exploring the mechanisms of non-coding RNAs in the invasion and metastasis in NPC. Besides, we discovered and explored other molecules, such as super-lncRNAs and ubiquitinates. We elaborated on the clinical and therapeutic implications of putative biomarkers that for clinical therapy of invasion and metastasis in NPC. A molecular stratification system containing miRNA, circRNA and other biomarkers was established, which is valuable for the establishment of accurate diagnosis system of NPC in our center.