Differential expression and clinical significance of long non-coding RNAs in the development and progression of lung adenocarcinoma

With the development of gene testing technology, we have found many different genes, and lncRNA is one of them. LncRNAs refer to a non-protein coding RNA molecule with a length of more than 200bp, which is one of the focuses of research on human malignant diseases such as LUAD. LncRNAs act as an oncogene or inhibitor to regulate the occurrence and progression of tumors. The differential expression of LncRNAs promotes or inhibits the progression of lung adenocarcinoma by affecting cell proliferation, metastasis, invasion, and apoptosis, thus affecting the prognosis and survival rate of patients. Therefore, LncRNAs can be used as a potential target for diagnosis and treatment of cancer. The early diagnosis of the disease was made through the detection of tumor markers. Because lung adenocarcinoma is not easy to diagnose in the early stage and tumor markers are easy to ignore, LncRNAs play an important role in the diagnosis and treatment of lung adenocarcinoma. The main purpose of this article is to summarize the known effects of LncRNAs on lung adenocarcinoma, the effect of differential expression of LncRNAs on the progression of lung adenocarcinoma, and related signal transduction pathways. And to provide a new idea for the future research of lung adenocarcinoma-related LncRNAs.


Introduction
In the past, much of the non-protein-coding portion of the human genome has historically been regarded as useless DNA.Through examination, we found different classes of non-coding RNAs.Non-coding RNAs are broadly categorized based on their size: small non-coding RNAs represent noncoding transcripts less than 200 nucleotides in length, such as miRNAs, siRNAs, and piRNAs (1).Among the various types of non-protein-coding transcripts, a class referred to as long noncoding RNAs (lncRNAs) has attracted increasing attention.LncRNAs are defined as transcripts of more than 200 nucleotides that are not translated into proteins (2).LncRNAs are often master regulators of gene expression and exert their functions through transcriptional, posttranscriptional, and epigenetic gene-regulatory mechanisms (3).For the past few years, LncRNAs have become one of the hotspots of tumor research, and the dysfunction of LncRNAs is considered to be one of the main factors leading to a variety of diseases, including cancer, cardiovascular disease, fibrosis, obesity, etc. (4).LncRNAs are involved in oncogenes and tumor suppressor genes by regulating target genes or signal pathways at epigenetic, transcriptional and post-transcriptional levels (5).
Increasing studies demonstrated the involvement of thousands of LncRNAs in almost all life processes and played an indispensable regulatory role.Studies have found that there are two broad mechanisms by which LncRNAs exert their function.One mode of function involves LncRNA associations with promoters and regulatory regions of genes, where they recruit transcriptionallyactivating or repressive protein complexes that are either directly involved in transcriptional initiation, or alternatively chromatinremodeling complexes that once recruited by LncRNAs render chromatin transcriptionally repressive.Another mode of function involves LncRNAs that act by sequestration of regulatory factors.One example is that of "sponge" LncRNAs (ceRNAs) that bind microRNAs, thus protecting their target mRNAs from the effects of microRNAs.Since microRNAs typically repress translation or destabilize mRNAs, LncRNA sponge action typically increases protein production.A second example of sequestration-acting LncRNAs is that of decoy LncRNAs, which sequester gene regulatory proteins (2).Some LncRNAs were spliced into mRNAlike PolyA tail and promoter structures, which could be dynamically expressed and spliced in different ways during tissue and organ differentiation, thus showing time and space specificity.
Meantime, about LncRNA functions, many studies have found that LncRNAs have multiple biological functions: they are involved in chromatin modification, genomic imprinting, intranuclear transportation, and other regulations.Protecting protein-coding genes in multiple modes; containing highly efficient key sequences, and more sensitive structural and functional constraints than proteins; having conserved secondary structure, splicing form, and precise subcellular localization; having cis and trans regulation.The function of LncRNAs cannot be determined by its simple classification but is usually determined by its localization, sequence, and/or secondary structure.
In LncRNA expression, further development of genomic technologies has proved that LncRNAs are important in complex organisms by regulating chromatin remodeling and transcriptional regulation, etc., and participate in the pluripotency and differentiation of stem cells, which correlate to the progression of a variety of human diseases like cancer and neurological disorders.Different LncRNAs play different roles in cellular processes.Some LncRNAs participate in the imprinting process, affecting the single allelic expression of genes.Some LncRNAs perform epigenetic modification by introducing chromatin remodeling complexes into specific chromatin sites (6).

LUAD overview
Lung cancer is the second most common malignancy in the world and the death rate is increasing year by year (7,8).Lung cancer is mainly divided into small cell (SCLC) and non-small cell (NSCLC).Approximately 80-85% of lung patients are diagnosed with non-small cell lung cancer (NSCLC) in pathological diagnosis, of which lung adenocarcinoma (LUAD) is the most common subtype (9,10).Lung adenocarcinoma (LUAD) is the most common histologic type of lung cancer with a high mortality rate.LUAD is one of the deadliest cancers in the world.The main treatment for LUAD is surgery, but the prognosis is poor due to Histological heterogeneity (11).LUADs originate from cells that secrete surfactant components.Morphological patterns of LUADs include acinar, papillary, solid, micropapillary, and invasive mucinous types.Lepidic components or pure lepidic patterns are found in noninvasive forms, previously classified as bronchoalveolar carcinoma, which can be associated with invasive mucinous or acinar LUAD (12).As for clinicopathological features of lung cancer, the most common is smoking.However, LUAD can occur in both smokers and non-smokers.LUADs are the most common type of lung cancer seen in non-smokers.It is more common in women than in men and is more likely to occur in younger people than other types of lung cancer and to present at more advanced stages of the disease (13).
Early symptoms of lung cancer are relatively benign, and this contributes to low detection rates in early disease stages.And because the lung adenocarcinoma is not sensitive to the treatment of anticancer drugs, the drug resistance increases gradually, and the drug resistance is not obvious.Therefore, the metastatic rate of advanced lung cancer is high and the overall prognosis is poor (14).For the past few years, despite the advancement of the treatment for LUAD, it still has some limitations on the whole.The early diagnosis of LUAD is difficult, and it tends to relapse and metastasis, leading to poor prognosis for LUAD patients.Although great progress has been made in diagnosis and treatment recently, the long-term clinical prognosis and overall survival of lung adenocarcinoma (LUAD) patients remain unsatisfactory (15).The inadequate comprehension of LUADrelated biological mechanisms limits the improvement in therapeutic efficacies.Thus, it is urgent to further elucidate the involved mechanisms of cancer progression to improve the clinical outcome of LUAD patients (16).

The mechanism of LncRNAs in LUAD
With the development of next-generation sequencing technology, the in-depth study of non-coding genes has attracted more and more attention from researchers.Many studies have shown that the treatment of tumors is related to LncRNAs.Aberrantly expressed LncRNAs play pivotal roles in the progression of LUAD.In the meantime, the aberrant expression and mutation patterns are tightly related to tumor proliferation, apoptosis, invasion, metastasis, TNM stage, drug resistance, lymphatic metastasis, and shorter prognosis.For example, Zhang et al (17) found that the up-regulate of CRNDE was significantly correlated with poor differentiation, TNM stage, lymph node metastasis, radiotherapy response, and significantly shorter overall survival.In addition to the abnormal expression of LncRNAs has an impact on lung adenocarcinoma, some LncRNAs can also act as oncogenes to interact with miRNA, thus acting with certain proteins to affect the function of lung adenocarcinoma cells.MicroRNAs (miRNAs) are conserved, non-coding short RNA molecules.MiRNAs can regulate numerous cellular functions such as cell growth, migration, invasion, and EMT (18).Significant evidence shows that abnormal regulation of miRNAs is related to the malignant progression of many cancers, and miRNAs are potential therapeutic targets for lung cancer (19).Yang et al (14) found that LncRNA PCAT6 can act as a molecular sponge for miR-545-3p and that this promotes LUAD development.

The differential expression of lncRNA in LUAD
Long non-coding RNAs (LncRNAs) are a kind of transcriptional RNA containing over 200 nucleotides, which are encoded by the genome and lack protein-coding function.It is related to a variety of cellular processes, including the regulation of gene expression (including epigenetics, pre-transcription, and posttranscription), tissue metabolism, cell growth, differentiation, and development (20).The abnormal expression of LncRNAs is significant in multiple cancers.
To distinguish the published LncRNAs biomarkers for diagnosis or prognosis in lung adenocarcinoma patients, entry terms "(LncRNA OR long no-coding RNA) AND (lung adenocarcinoma OR LUAD) AND (high expression OR low expression)" were firstly seek in the NCBI PubMed database.Finally,62 up-regulated LncRNAs and 15 downregulated LncRNAs in lung adenocarcinoma were identified from these studies, as shown in Figure 1.

High-level expressed lncRNA in LUAD and clinical significance
During the data retrieval, we found that 62 articles demonstrated the up-regulation of LncRNAs.When analyzing these data, we found that the up-regulation of LncRNAs is often related to the poor prognosis of lung adenocarcinoma, but also affects the function of lung adenocarcinoma cells.At the same time, we also found that the clinicopathology of most LncRNAs is related to tumor size, TNM stage, and lymph node metastasis.The data are shown in Table 1.

Low-level expressed lncRNA in LUAD and clinical significance
We also analyzed the down-regulation of LncRNAs.In the retrieved data, we found that most of the down-regulation was related to the poor prognosis of patients with lung adenocarcinoma, but the down-regulation of LncRNAs suggested a better prognosis.For example, Liu et al. (21) found that LncRNA SGMS1-AS1 was down-regulated in LUAD tissue as well as cells, which was related to good prognosis of patients with lung adenocarcinoma.The same down-regulation has an effect on cell function.The data are shown in Table 2.

High-level expression and low-level expressed LncRNA in LUAD and clinical significance of LncRNA
In the summary, we found that a LncRNA is expressed differently through different pathways in lung adenocarcinoma.It can be either high expression or low expression.However, both high and low expression lead to poor prognosis in patients with lung adenocarcinoma.This lncRNA is SNHG7.Pei et al. ( 22) found that LncRNA SNHG7 was found to be down-regulated in LUAD tissues Screening of differentially expressed lncRNA in lung adenocarcinoma.

The regulation of signaling pathway that lncRNA participated in LUAD
A large number of studies have proved that LncRNAs are an important regulator of gene regulation, and they can play a key role in a variety of biological functions and disease processes.The LncRNAmediated gene expression involves various mechanisms, such as regulation of transcription, translation, protein modification, and the formation of RNA-protein or protein-protein complexes (24).Cellular signal transduction plays a key role in various cellular processes that respond to intracellular or extracellular stimuli.Many signaling pathways have been found in cells, which are usually essential for cascading biological responses and gene expression (24).Most studies have shown that LncRNAs are involved in various pathways, and the common signal pathways are P53, Notch, JATT, STAT, PI3K, Akt, Wnt/b-catenin (beta-catenin), and so on.

JAK/STAT signaling pathway
Han et al (25) found that LncRNA ZFPM2-AS1 negative regulation of ZFPM2 expression and positive regulation of PD-L1 expression via the JAK-STAT and AKT pathways.Xiao et al. (26) found that LncRNA LOC389641 is a potential oncogene that acts by influencing MET, EGFR, and STAT3, thereby regulating multiple cell survival/death signals including cell proliferation, colony formation, cell invasion, apoptosis, and autophagy.Wu et al. (27) found that LncRNA LEISA promotes the progression of lung adenocarcinoma via enhancing interaction between STAT3 and IL-6 promoters, as shown in Figure 2.

PI3K/Akt/mTOR signaling pathway
Zhang et al. (23) found that LncRNA SNHG7 activates the PI3K/AKT pathway through CUL4A and PTEN, thus enhancing the resistance of LUAD cells to docetaxel resistance.Qin et al. (28) found that LncRNA MIR31HG expression increases gefitinib resistance in non-small cell lung cancer cell lines through the EGFR/PI3K/AKT signaling pathway.Zhang et al. (29) found that overexpression of LncRNA HOTTIP promotes proliferation and drug resistance of lung adenocarcinoma by regulating the AKT signaling pathway.Lu et al. (30) found that LncRNA DANCR contributes to lung adenocarcinoma progression by sponging miR-496 to modulate mTOR expression.as shown in Figure 3.

Wnt/b-catenin (beta-catenin) signaling pathway
Han et al. (31) found that LncRNA UPLA1 was mainly located in the nucleus using FISH assay and that it promoted Wnt/bcatenin signaling by binding to desmoplakin using RNA pulldown assay and mass spectrometry.Pei et al. (22) found that LncRNA SNHG7 interacted with microRNA mir-181 and sequentially upregulated cbx7.cbx7, which suppresses the Wnt/b-catenin pathway in LUAD, was found to be a direct target of mir-181.Yang et al. (32) found that STAT1-induced upregulation of LINC00467 promoted LUAD progression by epigenetically silencing DKK1 to activate the Wnt/b-catenin signaling pathway.Wan et al. (33) found that LINC00491 promotes MTSS1 degradation via the ubiquitinproteasome degradation through the Wnt/b-Catenin-signaling pathway, and promotes LUAD proliferation and metastasis.Zhu et al. (34) found that LINC00669 promotes lung adenocarcinoma growth by stimulating the Wnt/b-catenin signaling pathway, as shown in Figure 4.

P53 signaling pathway
Wu et al. (35) found that LncRNA CASC2/miR-21/p53 forms a positive feedback loop to mediate cell proliferation and apoptosis in LUAD, which may provide new insight into the pathological mechanisms of LUAD.As shown in Figure 5.

TGF-3/SMAD signaling pathway
Wang et al. (36) found that LncRNA MIR99AHG acted as a sponge of miR-136-5p to inhibit miR-136-5p expression, upregulated the expression of USP4, and enhanced the stability of ACE2 protein, thereby inhibiting EMT in pulmonary fibrosis.Jiang et al. (37) found that LncRNA HCP5 is induced by TGF-b and is directly regulated by SMAD3, which, in turn, is a positive regulator of the TGF-b/SMAD signaling via the miR-203/SNAI axis.This study demonstrates that overexpression of HCP5 in LUAD is important for promoting EMT and metastasis and may be a potential therapeutic target for LUAD treatment (37).Zhang et al. (38) found that LINC01977, a cancer-testis LncRNA, was hijacked by SE, which promoted proliferation and invasion both in vitro and in vivo.Activating SMAD3 to bind the promoter and the SE of LINC01977, which up-regulated LINC01977 expression.
LINC01977 also promoted malignancy via the canonical TGF-b/ SMAD3 pathway.as shown in Figure 6.

Notch signaling pathway
Guo et al. (39) found that LncRNA LOC107985872 promotes lung adenocarcinoma progression via the notch1 signaling pathway with exposure to traffic-originated PM2.5 organic extract.Zhang et al. (40) found that ZEB1-induced LINC01123 mediates LUAD development via miR-449b-5p/NOTCH1 axis-activated NOTCH1 LncRNA is involved in tumor cell proliferation, migration, invasion, apoptosis and other processes of lung adenocarcinoma through PI3K/Akt/mTOR signaling pathway.LncRNA is involved in the processes of lung adenocarcinoma cell proliferation, apoptosis, metastasis, invasion and EMT through the regulation of JAK/STAT signaling pathway.

ATR/CHK1 signaling pathway
Jiang et al. (42) found that LncRNA ZBED5-AS1 enhanced the metastatic potential of LUAD cells through the EMT pathway by activating ZNF146/ATR/CHK1 axis.as shown in Figure 8.

FIGURE 5
LncRNA is involved in the processes of lung cancer cell proliferation, apoptosis through the regulation of P53 signaling pathway.(57) found that LINC00968 inhibits the tumorigenesis and metastasis of lung adenocarcinoma via serving as a ceRNA against miR-9-5p and increasing CPEB3.as shown in Figure 10.

Conclusions
Lung cancer ranks second among health problems and is the major cause of cancer-associated mortality worldwide.LUAD is considered to LncRNA regulates metastasis of LUAD cells via regulation the ATRR/CHK1 signaling pathway.LncRNA regulates cell proliferation, migration, invasion and apoptosis via regulation the miRNA-3p signaling axis.
be the most common subtype in nonsmokers.The prevalence of LUAD is rapidly increasing with the development of anti-smoking movements.Although headway has been made in cancer treatment, the overall survival of LUAD remains disappointing due to a lack of reliable early prognostic indicators (65).LncRNAs, over the last decade, have been recognized as a diverse class of macromolecules in terms of function and mechanism.Many LncRNAs have demonstrated functional activity in a wide range of cancers.While the LncRNAs transcriptome is far from complete, we now have an appreciation of their diversity due to cell-type specificity.This characteristic can aid in the development of early detection methods and targeted therapies for multiple cancer types.Alongside these immediate applications, understanding the mechanism(s) by which these transcripts are regulated will shed light on the etiology of cancer development, allowing clinicians to implement better treatment strategies and improve overall survival rates.Given the effect of differential expression of LncRNAs on the progression of lung adenocarcinoma, we looked up a large number of literature and analyzed it.We found that the high and low expression of LncRNAs can promote or inhibit lung adenocarcinoma cells, mainly affecting tumor proliferation, migration, apoptosis, and invasion.At the same time, for the treatment of anticancer drugs, LncRNAs will affect the drug resistance of patients with lung adenocarcinoma to anticancer drugs.At present, lung adenocarcinoma is one of the major cancers  threatening the lives of Chinese people.We need to pay a lot of attention to the study of lung adenocarcinoma to improve the prognosis and survival rate of patients.It has important clinical significance for the detection of tumor markers of lung adenocarcinoma and the improvement of early detection, early diagnosis, and early treatment of patients with lung adenocarcinoma.
The main purpose of this paper is to summarize the effects of LncRNAs on the occurrence and development of lung adenocarcinoma and to provide a new idea for future research on lung adenocarcinoma-related LncRNAs.About the study of LncRNAs in lung adenocarcinoma, there are still many deficiencies in our research, which will be further studied in the future.

FIGURE 7
FIGURE 7LncRNA regulates cell functions and EMT of LUAD cells via regulation the Notch signaling pathway.

FIGURE 6
FIGURE 6LncRNA regulates tumor motility, invasiveness and other functions of LUAD cells via regulation the TGF-3/SMAD signaling pathway.

FIGURE 10 LINC
FIGURE 10LINC regulates cell functions and EMT via regulation the miRNA-5p signaling axis.

FIGURE 11 LncRNAs
FIGURE 11    LncRNAs regulates tumor motility and functions of LUAD cells via regulation the miRNA-5p signaling axis.

TABLE 1
Biological function and clinical significance of up-regulated lncRNA in lung adenocarcinoma.

TABLE 2
Biological function and clinical significance of down-regulated lncRNA in lung adenocarcinoma.