Non-small cell lung cancer (NSCLC) is one of the leading causes of cancer-related deaths worldwide (1). Although great advances have been made in surgery, chemotherapy, and immunotherapy, the 5-year survival rate of patients with NSCLC is still only about 15% due to the high rate of distant metastasis and recurrence (2, 3). Therefore, the invasion and the metastasis of cancer cells are serious challenges in the treatment of NSCLC. In-depth understanding of the potential mechanisms of the occurrence and development of NSCLC is of great significance in order to improve the effect of clinical treatment.

Long non-coding RNA (lncRNA) is a transcript consisting of more than 200 nucleotides in length (4). It is well known that lncRNA can regulate the expressions of many genes and participate in the development of tumors (5). Metastasis-associated lung adenocarcinoma transcript-1 (MALAT1) was initially found to be overexpressed in early NSCLC, which is a type of non-coding ribonucleic acid (6). Although there have been many studies on MALAT1 in the past, the specific molecular mechanism of MALAT1 regulation of NSCLC is still not very clear (7). In the past decade, more and more studies have found that MALAT1 can regulate its downstream target molecules by directly binding to microRNA (miRNA), thus playing an important role in the cell proliferation, metastasis, invasion, and treatment of drug resistance in NSCLC (8–11). In this review, we first briefly introduce the basic properties and biological functions of MALAT1, focus on the molecular mechanism of MALAT1 as an miRNA sponge in the occurrence and the development of NSCLC, and highlight the application and potential prospect of MALAT1 in molecular biological markers and targeted therapy in NSCLC.

MALAT1 is also termed nuclear enriched abundant transcript 2 (NEAT2) (12). The structure and biogenesis of its genes are located in human chromosome 11q13 and mouse chromosome 19qA (13, 14). The MALAT1 transcript is about 7 kb in humans and 6.7 kb in mice (12, 15). Previously, MALAT1 was named because of its clinical significance in predicting the metastasis and survival of early NSCLC, but a subsequent study showed that MALAT1 is widely expressed in normal tissues and is extremely abundant and widely conserved in 33 species of mammals (6, 16), which indicates that MALAT1 may have potentially important biological functions (17).

Different from the typical mechanism of cleavage and polyadenylation, the MALAT1 3′ end lacks the structure of poly(A) tail (18). With the cleavage of ribonuclease (RNase P), the primary transcript of MALAT1 forms a mature transcript of 7 kb and a small transcript fragment at the 3′ end ( Figure 1 ) (18). The mature transcript is mainly located in nuclear bodies known as nuclear speckles, which are subnuclear structures enriched with RNA processing factors and poly(A)⁺ RNAs and involved in posttranscriptional regulation of gene expression (19, 20). Its 3′ end is highly conserved and forms a unique triple-helix structure that can protect it from the damage of 3′–5′ exonucleases, which is beneficial to the stability of MALAT1 (21, 22). The small transcript fragment is bound by ribonuclease Z (RNase Z) and further cleaved and modified by the CCA-adding enzyme to produce a 61-nt-long lncRNA called MALAT1-associated small cytoplasmic RNA (mascRNA), then folds into the transfer RNA (tRNA) cloverleaf structure and is exported to the cytoplasm ( Figure 1 ) (18). MALAT1 located in nuclear speckles can regulate other physiological and pathological processes such as embryonic development, tumor progression, cardiovascular remodeling, and tissue inflammation mainly by affecting gene transcription, interfering with messenger RNA (mRNA) cleavage, regulating epigenetic changes, or acting as a competitive endogenous RNA (23–28). There are few reports on the role of mascRNA, which may participate in cardiovascular innate immunity by affecting fas ligand (FASLG), tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), etc. (29) It may also be part of the molecular mechanism of function in cancer to regulate the glutaminyl-tRNA synthetase (QARS) protein levels and promote global protein translation and cell proliferation (30).

Previous studies have found that MALAT1 can participate in the regulation of biological function through the following main mechanisms ( Figure 2 ): 1) affecting the gene transcription. MALAT1 can recruit Sp1, a transcription factor, in multiple myeloma. Sp1 can activate and promote the secretion of growth factor TGF-β by binding to the prompter of latent transforming growth factor beta binding protein 3 (LTBP3) (31). MALAT1 can promote the transcription of telomeric repeat-binding factor 2 (TRF2) by recruiting RNApol II, P300, and CRUPT to bind to the promoter region of TRF2, which promotes the growth of liver cancer stem cells (32). 2) Affecting the alternative splicing of pre-mRNAs. MALAT1 is identified as a nuclear-retained regulatory RNA that can interact with the serine- and arginine-rich (SR) protein splicing factors such as SRSF1, SRSF2, and SRSF3, affect the distribution of splicing factors in nuclear speckle domains, and regulate alternative splicing of pre-mRNAs (33). Additionally, MALAT1 can promote ovarian cancer progression by regulating the splicing factor RBFOX2-mediated alternative splicing (34). Furthermore, MALAT1 can induct the oncogenic splicing factor SRSF1 and modulate the alterative splicing of SK61 in hepatocellular carcinoma (35). 3) Regulating protein activity. MALAT1 can competitively bind to SFPQ leading to PTBP2 release from the SFPQ/PTBP2 complex, which enhances the function of PTBP2 in promoting tumor cell proliferation and migration (36). 4) Mediating epigenetic changes. Malat1 can cause the trimethylation of histone 3 lysine 9 (H3K9me3) by recruiting the suppressor of variegation 3–9 homolog 1 (Suv39h1) to MyoD-binding loci. This trimethylation suppresses the transcriptional activity of MyoD, which represses myoblast differentiation (37). In addition, the overexpression of MALAT1 could increase the expression of acetyl-H4 histone in the IQ motif-containing GTPase-activating protein 1 (IQGAP1) promoter, which may promote the proliferation and invasion of thyroid cancer cells (38). 5) Promoting the nuclear and cytoplasmic translocation of cellular proteins. MALAT1 retains the serine/arginine-rich proteins SF2/ASF from the cytoplasm to the nucleus, thus promoting the development of gastric cancer cells (39). MALAT1 can bind to an abundant nuclear factor heterogeneous nuclear ribonucleoprotein C (hnRNPC) protein, which could transfer from the nucleus to the cytoplasm during cell division, to assist its translocation (40). (6) Acting as an endogenous miRNA sponge. MiRNAs play an important role in cell proliferation, differentiation, apoptosis, and development. Recent evidence suggests that other RNAs such as lncRNA can also compete with mRNAs by sponging miRNAs (41). Among these lncRNAs, MALAT1 is one of the most studied RNAs involved in various molecular processes such as endogenous miRNA sponging (42). Here, we will focus on the potential function of MALAT1 as a miRNA sponge in NSCLC ( Table 1 ).

Although there are many methods for the diagnosis of NSCLC, these may not fully meet the needs of early diagnosis of the cancer. MALAT1 is a relatively stable RNA transcript with a half-life of 9–12 h, which may be due to its triple-helix structure at the 3′-end (21, 22, 60). This characteristic of having a long half-life makes MALAT1 easy to detect in tumor tissues and body fluids. Research has shown that MALAT1 can be used as a biomarker for the diagnosis of many kinds of malignant tumors (61–63). Especially in NSCLC, the high expression of MALAT1 was significantly correlated with tumor node metastasis (TNM) stage, vascular invasion, pathological differentiation, and recurrence (64). Further studies have shown that the overexpression of MALAT1 was significantly related to the prognosis of lung squamous cell carcinoma, which is one type of NSCLC (65). Moreover, different expression levels of MALAT1 in peripheral blood were observed between cancer patients and healthy controls (66).

Rong et al. found that the levels of MALAT1 in serum exosomes were higher in patients with NSCLC, suggesting that exosome-derived MALAT1 may also reflect the biological changes of NSCLC cells (49). Zhang et al. found that the expression of MALAT1 in serum exosomes of NSCLC patients was upregulated and that the level of exosomal MALAT1 was positively correlated with tumor stage and lymph node metastasis (67). The above data suggest that MALAT1 in exosomes may also be used as a serum-based tumor biomarker to diagnose and predict NSCLC. Liquid biopsy provides the opportunity of detecting and monitoring cancer in various body fluids by detecting free circulating tumor cells, circulating tumor DNA fragments, circulating RNA, and exosomes (68). Its advantage lies in that it can reduce the harm of biopsy through noninvasive sampling and has important significance for the early diagnosis of cancer, but the low expression level of MALAT1 in blood makes sensitive analysis difficult (66). Although some progress has been made in the detection of MALAT1 in blood with traditional RT-PCR, the procedure is complicated, the amount of serum required is large, and the equipment is expensive. A recent study by Chen et al. showed that the detection of the levels of MALAT1 in blood was more rapid, sensitive, and inexpensive when using a novel ultrasensitive screen-printed carbon electrode (SPCE)-based electrochemical biosensor that uses a Au nanocluster (NC)/multi-walled carbon nanotube (MWCNT)–NH₂ nanostructure (69). This new methodology for the detection of MALAT1 will increase its applicability to clinical diagnosis of NSCLC.

In addition, the expression level of MALAT1 can also be used as a biomarker of chemosensitivity in different cancers (43, 70–72). Resistance to multiple drugs is the main cause of chemotherapy failure in patients with lung cancer (73). Studies have shown that MALAT1 is also involved in the drug resistance of NSCLC. For example, Fang et al. found that the expression of MALAT1 was upregulated in DDP-resistant A549 cells. MALAT1 upregulated MRP1 and MDR1 by activating STAT3, thus reducing the sensitivity to DDP in vitro and in vivo (74). NSCLC patients carrying epidermal growth factor receptor (EGFR) mutations initially respond to EGFR tyrosine kinase inhibitors (EGFR-TKIs) such as gefitinib, but gradually developed acquired drug resistance (75, 76). It was found that the overexpression of MALAT1 could eliminate not only the inhibitory effect of polyphyllin I (PPI) on the activity of gefitinib-resistant NSCLC cells but also the apoptosis induced by PPI, while MALAT1 gene knockout could enhance the inhibition and apoptosis induced by PPI (77). These data suggest that MALAT1 may represent a candidate biomarker and therapeutic target for chemotherapy drug resistance.

Due to the enrichment and high expression of MALAT1 in the nucleus, its effect on traditional shRNAs or siRNAs may not be ideal and prone to off-target effects (78). The application of antisense oligonucleotides (ASOs) is a valuable method to antagonize MALAT1. ASOs, which are small RNA/DNA-based oligonucleotides capable of crossing cell membranes and binding to the target RNA in the nucleus and cytoplasm, are divided into two main categories: mixmeRs and gapmeRs (79, 80). Gutschner et al. found that MALAT1 could be targeted with second-generation ASOs, thus leading to the drastic reduction of lung cancer metastasis in a pulmonary metastatic model in vivo (78). Moreover, the same investigators achieved functional knockout of MALAT1 through zinc finger nuclease (ZFN)-mediated site-specific integration of RNA destabilizing elements into the human genome, which showed efficient silencing of the highly abundant MALAT1 in human lung cancer cells (78).

As an important and highly conserved lncRNA, MALAT1 has been widely studied, especially its role in tumorigenesis, metastasis, drug resistance, and clinical prognosis (81–83). However, the specific role of MALAT1 in the occurrence and development of NSCLC has not been fully elucidated. Based on the basic biological properties of MALAT1, more and more studies have shown that it can be used as a bait for miRNA to share miRNA response elements (MREs) with mRNAs, which indirectly affects the expression of some specific downstream genes, thus promoting the proliferation, invasion, apoptosis, drug resistance, and tumor growth of NSCLC. In general, MALAT1 is mostly known to be enriched in nuclear speckles, and we also agree that cytoplasmic P-bodies are the localizing site of the RNA-induced silencing complex (RISC) effector proteins Ago1–4 and the functional site of miRNA-mediated gene silencing (84). The vast majority of researchers used to apply bioinformatics program such as ChipBase, LncRNAdb, and StarBase to predict the interaction between MALAT1 and miRNA in previous research on MALAT1 as a miRNA sponge in NSCLC. Subsequently, they verified the direct interaction using luciferase reporter, RNA immunoprecipitation (RIP), and MALAT1 pull-down assays. However, there was little focus on the sites (cytoplasm or nucleus) where these interactions occur. On the contrary, Jin et al. demonstrated that MALAT1 and miR-1914-3p are abundant and stable in the cellular cytoplasm using RNA fluorescence in situ hybridization assay and confirmed that MALAT1 directly binds miR-1914-3p using luciferase reporter assay, RIP for argonaute 2 (Ago2) in A549 cells, and RNA pull-down assay (54). Additionally, Leucci et al. showed that miR-9 targets MALAT1 for degradation in the nucleus by directly binding to two miRNA binding sites (85). Furthermore, Wu et al. found that Ago2 was expressed both in the nucleus and cytoplasm of sw480 cells (86). Moreover, Gagnon et al. reported that 75% of the miRNAs in the cytoplasm could shuttle into the nucleus and then bind to nuclear Ago2 (87). These studies showed that the distribution of MALAT1 or miRNA is not limited to the nucleus or cytoplasm. Hence, we wondered whether MALAT1 or miRNA might be involved in some cases with nucleoplasmic translocation. Additionally, the locations of MALAT1 and various miRNA interactions in NSCLC cells need to be further verified and explored.

Taken together, based on the literature, some miRNAs such as miR-142-3p and miR-101-3p can target MALAT1 for degradation, thereby negatively inhibiting the lever of MALAT1 in NSCLC (44, 53). On the contrary, MALAT1 can also act as a miRNA sponge by sequestering the target miRNAs and affecting downstream gene expression, and the expression level of MALAT1 was negatively correlated with the expressions of miRNAs in NSCLC (53). Whether miRNA is degraded or recycled remains to be investigated. It also has been reported that MALAT1 and some miRNAs were more abundant in the Ago2 pellet than in the immunoglobulin G (IgG) pellet by conducting an RIP assay, which suggested that MALAT1 might be a target of miRNA through an Ago2-dependent manner.

Intriguingly, there is an exosome-derived MALAT1 in the serum of NSCLC patients, and the expression of MALAT1 in exosomes is highly correlated with the TNM stage and lymphatic metastasis of NSCLC. However, at present, the mechanism of MALAT1 in the exosomes of NSCLC patients remains in the preliminary research stage and needs to be further clarified. It is interesting to note that, due to the enrichment and high expression of MALAT1 in the nucleus, the specific mechanism of MALAT1 packing into exosomes that are rarely reported remains to be explored in the future, although it is common for lncRNA as a cargo to be loaded into exosomes. Moreover, MALAT1 may be a key actor in the hallmark of resisting cell death as it can decrease the levels of cleaved CASP3 in NSCLCs, which leads to escaping apoptosis (77, 88). On the contrary, whether MALAT1 detection in serum due to cell death may involve complex mechanisms needs to be further studied.

In addition, MALAT1 knockout mice did not cause obvious phenotype in development, gene expression, and physiological function, which is not consistent with MALAT1 being involved in the occurrence and development of NSCLC in vitro, so this also needs to be further explored (89). In-depth understanding of the function and regulatory mechanism of MALAT1 in NSCLC may provide a new breakthrough for the diagnosis and targeted therapy of NSCLC in the future.

QZ wrote the manuscript and designed the figures. DC constructed the topic. LL, JZ, and YC provided scientific suggestions and participated in manuscript preparation. DX and YY provided guidance and revised this manuscript. All authors contributed to the article and approved the submitted version.

The study was supported by grants provided by The National Natural Science Foundation of China (81871709), the Natural Science Foundation of Suzhou (KJXW2017063), Natural Science Foundation of Zhangjiagang (ZKS2022).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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