The Emerging Landscape of Long Non-Coding RNAs in Colorectal Cancer Metastasis

Colorectal cancer (CRC) is one of the most common gastrointestinal cancers, with extremely high rates of morbidity and mortality. The main cause of death in CRC is distant metastasis; it affects patient prognosis and survival and is one of the key challenges in the treatment of CRC. Long non-coding RNAs (lncRNAs) are a group of non-coding RNA molecules with more than 200 nucleotides. Abnormal lncRNA expression is closely related to the occurrence and progression of several diseases, including cancer. Recent studies have shown that numerous lncRNAs play pivotal roles in the CRC metastasis, and reversing the expression of these lncRNAs through artificial means can reduce the malignant phenotype of metastatic CRC to some extent. This review summarizes the major mechanisms of lncRNAs in CRC metastasis and proposes lncRNAs as potential therapeutic targets for CRC and molecular markers for early diagnosis.


INTRODUCTION
Colorectal cancer (CRC) is currently the third most common malignant tumor worldwide. Approximately 1.8 million new cases and nearly 900,000 deaths are reported worldwide each year. The high incidence and high mortality of CRC are serious threats to human health (1,2). The occurrence and development of CRC is a complex process that involves exogenous and endogenous factors, such as Signaling molecules, homeostasis, microenvironment, diet, and lifestyle, which play an important role in the CRC pathogenesis (3,4). In recent years, the molecular pathological epidemiology (MPE) has showed that the diet and lifestyle are closely related to the tumorigenesis. For example, smoking, eating red and processed meat, excess alcohol intake, and certain drugs (e.g., aspirin) have been confirmed to be related to the occurrence and development of CRC (5). With the rapid progress in clinical treatment, the 5-year survival rates of patients with CRC has improved significantly. However, the treatment outcomes in patients with metastatic CRC are still not ideal, and the 5-year survival rate in such patients is only~12% (6)(7)(8). Metastasis of CRC is an important factor leading to the CRC recurrence and death. Therefore, elucidating the molecular mechanism of CRC metastasis and identifying molecular markers related to metastasis are critical for improving the treatment outcomes of CRC.
Long non-coding RNAs (lncRNAs) are non-coding RNA molecules that are greater than 200 nucleotides in length. Most of them are transcribed by RNA polymerase II and share similarities with messenger RNAs (mRNAs), although they lack coding ability (9). lncRNAs can be divided into five categories according to their positional relationship with protein-coding genes: sense, antisense, bidirectional, interintron, and intergenic lncRNAs (10). Accumulating evidence strongly suggests that lncRNAs are an important class of molecules that regulate genomic processes. The long nucleotide chain of lncRNAs can either form a complex spatial structure and interact with protein factors, or provide a large binding site for the concurrent binding of several molecules that collectively participate in X-chromosome silencing, genomic imprinting, epigenetic regulation, transcriptional activation or interference, nuclear and cytoplasmic trafficking, mRNA splicing and degradation, and genomic imprinting, among others (11). Since lncRNAs play important roles in various aspects of gene expression, the relationship between lncRNAs and tumors has become the focus area of current research. A variety of lncRNAs have been shown to promote or suppress tumorigenesis in different cancers. For instance, Zhuang et al. (12) found that lncRNA GClnc1 promotes the proliferation and invasion of bladder cancer by activating MYC expression. LncRNA PVT1 plays a carcinogenic role in prostate cancer and is a potential diagnostic biomarker (13). In CRC, researchers have found numerous differentially expressed lncRNAs and confirmed their important roles in regulating CRC cell proliferation, apoptosis, invasion, and metastasis as well as sensitivity to radiotherapy and chemotherapy (14). For instance, the HOXB-AS3 peptide encoded by lncRNA HOXB-AS3 has been shown to inhibit the growth of CRC (15). Accumulated evidence indicates that lncRNAs are important markers of CRC metastasis. Yue et al. observed that lncRNA CYTOR can promote the CRC metastasis via the Wnt/b-catenin signaling pathway (16). Therefore, lncRNAs are potential therapeutic targets for CRC.

CHARACTERISTICS AND ROLES OF LNCRNAS
Generally, non-coding RNAs can be divided into long-chain and short-chain non-coding RNAs based on their lengths (17). The first long non-coding RNA transcript sequence discovered in eukaryotes has a length of more than 200 nt and an mRNA-like structure. After splicing, a 7mC cap is usually added at the 5′end of the lncRNA sequence, and a polyA tail is sometimes added to the 3′end (18,19). Studies have shown that for some lncRNAs, corresponding DNA regions are located between genes or introns, some overlap with protein-coding genes, while some lncRNAs encode a small number of functional short peptides (20,21). While the primary structure of an lncRNA is its nucleotide sequence, its functional activity depends on base pairing but it is less conserved than its higher-order structure (22,23). The secondary and tertiary structures of lncRNAs determine their functions. The secondary structures mainly include double helices and hairpins, whereas the tertiary structures are more diverse, such as sarcin-ricin loops. The lower conservation of its primary structure is balanced by these higher-order structures (24)(25)(26).
The main modes of action reported for lncRNAs include: ① interfering with mRNA cleavage by forming complementary double-stranded RNA (27), ② altering the activity of a specific protein through direct binding (28), ③ changing the cytoplasmic localization of a specific protein through direct binding (29), ④ altering the expression of target genes by inhibiting RNA polymerase II, or through chromatin remodeling and histone modification (30), ⑤ interfering with target gene expression by initiating transcription from the promoter region of proteincoding genes (31), ⑥ forming double-stranded RNAs with the transcripts of protein-coding genes and producing endogenous siRNAs through the action of Dicer (32), ⑦ acting as a structural component by forming a nucleic acid-protein complex (33), and ⑧ acting as the precursor of a small RNAs (such as a miRNAs or piRNAs). LncRNAs are mostly expressed in the nucleus and their expression levels are lower compared to those of mRNAs (34). However, lncRNAs are intricately involved in the regulation of various biological activities owing to their tissue-specific expression, and they can also affect disease processes (35). LncRNAs can also regulate the expression of important genes at multiple levels via epigenetic regulation and by modulating transcription, post-transcriptional processes, translation, and protein modification either as an initially transcribed RNA or a mature spliced RNA. Moreover, lncRNAs play important roles in physiological processes including development, tissue differentiation, reproduction, and immunity as well as in the formation and development of tumors.

MECHANISM OF LNCRNA ACTION IN CRC METASTASIS
Tumor metastasis is the process wherein malignant cells detach from the primary tumor site and are translocated through the circulatory system to secondary tissues or organs, where they colonize and form secondary tumors (36). Tumor invasion and metastasis are complex, dynamic processes that typically involve changes in the tumor microenvironment, epithelialmesenchymal transition (EMT), hypoxia, and angiogenesis among other mechanisms (37). Accumulating studies have shown that lncRNAs regulate CRC metastasis mainly by regulating key factors that simultaneously affect multiple signaling pathways that are closely related to tumor metastasis. In other cases, lncRNAs can sponge miRNAs to regulate the expression of target genes. lncRNAs can also bind directly to proteins to induce the protein degradation via affecting their phosphorylation or ubiquitination. Tumor invasion and metastasis affect patient prognosis and survival and are important causes of tumor-related death; hence, blocking these processes remains a critical challenge in cancer treatment (38).
Several studies have reported that the Wnt/b-catenin signaling pathway is closely related to CRC metastasis. Yue et al. (16) observed that lncRNA CYTOR, which is highly expressed in CRC, forms a positive feed forward loop with bcatenin and participates in the regulation of colon cancer metastasis. In this process, cell receptors bind to cytoplasmic b-catenin and block b-catenin phosphorylation catalyzed by casein kinase 1 (CK1), leading to the accumulation of bcatenin and its nuclear transport. Subsequently, the b-catenin/ TCF complex activates the expression of cell receptor encoding genes, thereby forming a positive feed forward loop. LncRNA SLCO4A1-AS1 inhibits the interaction of b-catenin with GSKb, inhibits b-catenin phosphorylation, and improves b-catenin stability, ultimately promoting the proliferation, migration, and invasion of CRC cells (82). Wu et al. (83) showed that lncRNA JMJD2C promotes CRC metastasis by enhancing the b-catenin signaling pathway and participating in the regulation of histone methylation at the MALAT1 promoter. In addition to directly participating in b-catenin signaling pathway transduction, lncRNAs can also play indirect regulatory roles in this signaling pathway. Research has shown that NEAT1 indirectly activates the Wnt/b-catenin signaling pathway through DDX5, and therefore, exerts its carcinogenic effects are mediated by DDX5 (53).
The PI3K/AKT signaling pathway also plays a key role in CRC metastasis, and several lncRNAs have been shown to modulate this pathway. Song et al. (84) found that the expression of the lncRNA, PlncRNA-1, was significantly higher in CRC tissues, and PlncRNA-1 knockout significantly reduced the spread, migration, and invasion of CRC cells. Further functional analysis showed that PlncRNA-1 affects the growth and metastasis of CRC mainly through the PI3K/AKT signaling pathway. The lncRNA SNHG6 inhibits ETS1 expression by directly targeting its 3′-untranslated region (UTR) and  (81) inhibiting the expression of phosphoinositide 3-kinase (PI3K)/ protein kinase B (AKT)/rapamycin mechanical target (mTOR) to activate the CRC invasion (85). In addition, Wang et al. (86) found that lncRNA AB073614 promotes the proliferation and metastasis of CRC cells mainly through the PI3K/AKT signaling pathway. The lncRNA ST3Gal6 antisense 1 (ST3Gal6-AS1) is derived from the promoter region of gene encoding sialyltransferase ST3Gal6, and it mediates a-2,3 sialylation through the ST3Gal6-AS1/ST3Gal6 axis, thereby regulating PI3K/Akt signaling and leading to the nuclear translocation of Foxo1 in CRC cells (87). Several other signaling pathways have been confirmed to play important roles in CRC metastasis. Functional analysis has shown that the lncRNA FEZF1-AS1, which is upregulated in CRC tissues, can bind to pyruvate kinase 2 (PKM2) protein and improve its stability. Higher cytoplasmic levels of PKM2 promote pyruvate kinase activity and lactate production (aerobic glycolysis), whereas higher nuclear levels of PKM2, induced by FEZF1-AS1, activate STAT3 signaling, which promotes the proliferation and metastasis of CRC cells (88). Zhou et al. (78) found that lncRNA-cCSC1 can modulate the characteristics of CRC stem cells by activating the Hedgehog signaling pathway and thus, plays an important role in CRC metastasis.
The migration and invasion of tumor cells require cytoskeletal rearrangement. Tang et al. (89) reported that lncRNAs can directly regulate the cytoskeleton in a variety of tumors and can alter the cytoskeleton via Rho/ROCK signaling during tumor migration. The lncRNA EPB41L4A-AS1 is overexpressed in CRC tissues and may affect proliferation, invasion, and migration by activating the Rho/ROCK-related protein kinase signaling pathway. Therefore, EPB41L4A-AS1 could be used as a new biomarker for the diagnosis and targeted treatment of CRC (90). Further, Tang et al. (91) studied the specific role of lncRNA-SLCO4A1-AS1 in CRC and found that its effects on cell proliferation, migration, and invasion were mainly associated with regulating the EGFR/ MAPK pathway. Studies have shown that 1a, 25-(OH)2D and vitamin D receptor (VDR) in CRC cells stimulate MEG3 expression by directly binding to the promoter of lncRNA MEG3; MEG3 acts as a tumor suppressor by regulating clusterin activity. Therefore, the VDR/lncRNA MEG3/clusterin signaling pathway is a potential therapeutic target and prognostic biomarker for CRC patients (92).

LncRNAs Regulate CRC Metastasis Through Sponging miRNA
In recent years, several studies have shown that since lncRNAs contain several introns, they can sponge miRNAs to form competing endogenous RNA (ceRNA) networks. LncRNAs are transported to target cells via circulation, bind to intracellular miRNAs, sponge them, and limit their ability to interfere with the translation of their target mRNAs; a process important for cancer cell proliferation, invasion, migration, and apoptosis. Thus, the ability to sponge miRNAs is an important mechanism by which lncRNAs regulate CRC metastasis ( Figure 1).
Yan et al. (93) reported the lncRNA LINC00668, which is encoded on chromosome 18p11.31, as a newly discovered lncRNA associated with cancers. LINC00668 is upregulated in CRC cancer tissues and cells and studies have shown that LINC00668 can bind to miR-188-5p in CRC cells. Therefore, LINC00668 may play a carcinogenic role in CRC by sponging miR-188-5p and upregulating USP 47 expression. Shan et al. (94) found that lncRNA SNHG7 regulates GALT1 levels by activating miR-216b and plays a carcinogenic role in CRC development. Xu et al. (95) reported that MIR17HG promotes CRC by inducing NF-kB/RELA expression and competitively sponging miR-375. LncRNA-SNHG5 has been shown to affect the proliferation, metastasis, and migration of CRC cells by regulating miR-132-3p/CREB5 (96). LncRNA-CRNDE modulates CRC progression and chemotherapy resistance by regulating the expression level of miR-181a-5p and the activity of the Wnt/b-catenin signaling pathway (49). LncRNA HNF1A-AS1, which is upregulated in colon cancer tissues, is closely related to clinical staging, vascular invasion, lymph node metastasis, and distant metastasis. In addition, HNF1A-AS1 regulates the expression of miRNA-34a by acting as a ceRNA, thereby inhibiting the miR-34a/SIRT1/p53 feedback loop and activating the Wnt signaling pathway to promote the development of colon cancer (97). LncRNA MIR4435-2HG was first found in lung cancer tissues where it functions as a ceRNA and sponges miR-206 to upregulate the expression of YAP 1. MIR4435-2HG promotes the CRC growth and metastasis via the miR-206/YAP 1 axis (98). A functional analysis by Yang et al. (99) showed that knocking out lncRNA-FTX significantly inhibited the proliferation, migration, and invasion of CRC cells. Further analysis showed that FTX could directly interact with miR-215 and inhibit its expression, thereby inhibiting the metastasis of CRC. In CRC cells, the expression of lncRNA TUG1 is abnormally high, whereas the expression of miR-600 is downregulated in CRC tissues, cell lines, and metastatic tissues. Moreover, TUG1 inhibits the migration, invasion, and EMT of CRC cells by competing with miR-600 (100).
Li et al. (101) revealed the previously unrecognized role of the lncRNA ZDHHC8P1/miR-34a regulatory axis in regulating the progression and metastasis of CRC and proposed a viable approach to treat late-stage metastatic CRC patients. LncRNA SNHG1 expression is upregulated in human CRC tissues. In the cytoplasm, SNHG1 sponges miR-154-5p, thereby reducing its ability to inhibit the expression of cyclin D2 (CCND2). In the nucleus, SNHG1 directly interacts with polycomb repressive complex 2 (PRC2) and modulates histone methylation at the promoters of Kruppel-like factor 2 (KLF2) and cyclin-dependent kinase inhibitor 2B (CDKN2B) (102). In vivo and in vitro experiments by Zhuang et al. (103) showed that lncRNA MALAT1 promotes CRC metastasis mainly via the lncRNA MALAT1/miR-106b-5p/SLAIN2 axis. LncRNA TTTY15 expression is abnormally upregulated in CRC tissues and it functions as a ceRNA by sponging miR-29a-3p to regulate the expression of the target gene DVL3, which affects the proliferation and metastasis of CRC (104). The results of in vivo and in vitro experiments have shown that a novel oncogenic lncRNA, RP11-757G1.5, which is overexpressed in CRC tissues, regulates the expression of YAP1 by sponging miR-139-5p and inhibiting its activity, thereby promoting the metastasis and invasion in CRC (105).

LncRNAs Regulate CRC Metastasis Through Protein Binding
Similar to molecular chaperones, lncRNAs bind directly to transcription factors and form RNA-protein-DNA ternary complexes that regulate the transcription of downstream target genes involved in the CRC metastasis ( Figure 2). LncRNAs act by two main mechanisms, which occur in different parts of the cells. In the nucleus, lncRNAs can coordinate with or antagonize transcription factors, thereby regulating the transcription of metastasis-related genes. In the cytoplasm, lncRNAs can bind to proteins and alter their post-translational modifications to induce the protein degradation; when these proteins are relevant to cancer, these effects can impact tumor metastasis.
The lncRNA SATB2-AS1 is specifically downregulated in CRC tissues. A mechanistic analysis showed that SATB2-AS1 binds directly to WDR5 and GADD45A and cis-activates SATB2 transcription by modulating histone H3 lysine 4 trimethylation (H3K4me3) and DNA demethylation in the SATB2 promoter region (106). A study by Wu et al. (107) showed that in intestinal cancer cells, the lncRNA RP11/hnRNPA2B1 (protein)/mRNA complex accelerated the degradation of Siah1 and Fbxo45 mRNAs, both of which encode ubiquitin E3 ligases, thereby preventing the proteasomal degradation of Zeb1, a transcription factor associated with EMT. This post-translational upregulation of Zeb1 is critical to RP11-induced dissemination of intestinal cancer cells. The lncRNA CPS1-IT can block hypoxia-induced autophagy by inhibiting HIF-1a levels, thereby preventing EMT and metastasis in CRC (108). Recent studies have shown that lncRNA RPPH1 can interact with b-III tubulin (TUBB3) to prevent its ubiquitination, which induces EMT and promotes CRC metastasis (109). The lncRNA LUCAT1 was shown to promote the proliferation, apoptosis, migration, and invasion of CRC cells in vitro and in vivo. Analysis showed that LUCAT1 binds to UBA52, which encodes ubiquitin, and the 60S ribosomal protein L40 (RPL40). By binding to UBA52, LUCAT1 targets the ribosomal protein L40/MDM2/p53 pathway to promote tumorigenesis and induce CRC cell cycle arrest and apoptosis (78). The lncRNA SNHG14, which is highly expressed in CRC, promotes CRC cell proliferation, motility, and EMT in vitro. SNHG14 promotes CRC progression by inhibiting EPHA7mediated negative regulation through a process dependent on the transcription factor EZH2. SNHG14 enhances the stability of EZH2 mRNA by interacting with the RNA-binding protein FUS and sponging miR-186-5p, thereby mitigating miR-186-5pinduced silencing and increasing EZH2 expression in CRC (110). Ding et al. (111) found that the combination of lncRNA CRNDE and EZH2, a key component of PRC2, inhibited the expression of two downstream target genes dual-specific phosphatase 5 (DUSP5) and CDKN1A, which play important roles in CRC proliferation and metastasis. LINC01413 binds to hnRNP-K and induces nuclear translocation of YAP1 (associated protein 1) TAZ, thus regulating the expression of ZEB1 in CRC cells and promoting cancer metastasis (112). Zhang et al. (54) found that upregulation of lncRNA CASC11 in CRC is correlated with CRC growth and metastasis and that it exerts its effects by interacting with hnRNP-K protein and activating the Wnt/b-   (113) demonstrated that knockout of the lncRNA ROR gene significantly increased the protein levels of p53 and its target genes, whereas the overexpression of ROR exerted the opposite effect. Thus, we conclude that the level of p53 protein is negatively correlated with ROR, and ROR may p a r t i c i p a t e i n t h e C R C p r o g r e s s i o n v i a t h e p 5 3 signaling pathway.

CLINICAL SIGNIFICANCE OF LNCRNAS IN CRC METASTASIS
Several studies have revealed that lncRNAs exert important biological effects in the CRC metastasis. Thus, the most practical application of lncRNAs is that they can be used as markers for early diagnosis of CRC metastasis. To improve the convenience and speed of CRC diagnosis, the differentially expressed lncRNAs can be detected in metastatic and nonmetastatic samples (such as blood or urine). In addition, some lncRNAs closely correlate with the sensitivity to radiotherapy and chemotherapy, which may help to design novel therapies with better efficacy for the clinical treatment of metastatic CRC. One challenge associated with existing diagnostic biomarkers of CRC is that they lack sufficient sensitivity and specificity, which can lead to false positive or false negative results. In recent years, several studies have shown that some lncRNAs can be detected in the blood, urine, serum, and other body fluids of patients with cancer (114). These lncRNAs could be used as biomarkers for the early diagnosis of cancer and prediction of patient prognosis (  74,77,78,82,84,87,88,91,94,. For example, lncRNA RP11-296E3.2, which is highly expressed in metastatic CRC, is associated with short overall survival (OS). In terms of its sensitivity and specificity of diagnosing CRC metastasis, RP11-296E3.2 was superior to CEA in plasma (113). Xu et al. (222) found that the plasma levels of four lncRNAs, ZFAS1, SNHG11, LINC00909, and LINC00654, were significantly lower in postoperative CRC samples than in preoperative samples. The combination of these four lncRNAs showed high diagnostic performance for early CRC. Studies have shown that lncRNA TINCR can affect the PI3K/Akt/mTOR signaling pathway by sponging miR-7-5p and playing a role in promoting CRC. In addition, compared with healthy controls, plasma levels of lncRNA TINCR were significantly elevated in CRC patients, which suggests its potential for the detecting early CRC (154). A correlation analysis by Pan et al. (223) showed that in patients with early CRC, plasma levels of lncRNA PVT1 are significantly higher than those of CEA, suggesting that PVT1 has great potential as a marker for the diagnosis of early CRC. A decrease in lncRNA-ATB expression significantly affects the progression of colon cancer by altering the expression of epithelial markers such as E-cad. A related clinical analysis showed that the level of plasma lncRNA-ATB was significantly increased in colon cancer patients at 1 month after surgery, suggesting that it may be useful for the early diagnosis of CRC (213). Ye et al. (80) observed that the level of lnc-GNAT1-1 in the plasma of CRC patients is related to tumor node metastasis (TNM) staging, while the receiver operating characteristic curve (ROC) showed that plasma lnc-GNAT1-1 has a moderate to good diagnostic efficiency for CRC.
LncRNAs have been shown to play roles in lymph node metastasis, lung metastasis, bone metastasis, and brain metastasis associated with several cancers (224). LncRNA CCAT2 is highly expressed in CRC and its expression is closely related to TNM stage as CCAT2 levels are increased from stages I to IV. High CCAT2 expression is closely associated with poor cell differentiation and depth of tumor invasion, lymph node metastasis, distant metastasis, vascular infiltration, and advanced TNM staging, and may be associated with increased liver metastasis (190). LINC00858 expression levels are significantly higher in CRC tissues than in adjacent tissues, and high LINC00858 expression is related to TNM staging, lymph node metastasis, and histological grade. Silencing of LINC00858 inhibits the proliferation, migration, and invasion of CRC cells and induces apoptosis (150). The expression level of MFI2-AS1 are closely related to tumor histological grade, lymphatic and distant metastasis, TNM staging, and vascular infiltration (225). High expression of lncRNA BANCR in CRC is associated with lymph node metastasis and the OS of patients with high BANCR expression is shorter (76). Chen et al. (226) divided 115 CRC patients into two groups based on the median lncRNA XIST expression level and an analysis of these groups showed that XIST expression was closely correlated with tumor size, histological grade, distant metastasis, and TNM staging. Similarly, the expression of lncRNA SNHG3 was significantly upregulated in CRC tissues, and SNHG3 expression was positively correlated with the advanced clinical stage and distant metastasis (118).
LncRNAs are an important group of molecules in the human transcriptome. LncRNAs play important roles not only in several physiological processes but also in various disease processes including cancer development and metastasis. Many lncRNAs are tumor specific and their expression can alter sensitivity to radiotherapy and chemotherapy. Therefore, they are expected to be useful as new therapeutic targets (227). LncRNA MALAT1, which was first found to be differentially expressed in patients with non-small cell lung cancer, is also significantly overexpressed in CRC. Low MALAT1 expression can inhibit the progression and metastasis of CRC and increase the sensitivity of cancer cells to 5-FU. This provides a new direction for designing novel therapeutic regimens for metastatic CRC (228). In addition, MALAT1 was found to be significantly upregulated in CRC tissues and cells treated with oxaliplatin. It promotes anti-oxidative response mainly via the miR-324-3p/ADAM17 axis and enhances sensitivity to oxaliplatin (229). In an experiment designed to select lncRNAs related to oxaliplatin resistance, Sun et al. (230) observed that the  lncRNAs CRNDE, H19, UCA1, and HOTAIR affect the sensitivity to oxaliplatin. High expression of HOTAIR is associated with advanced tumor nodules and metastatic stages and poor prognosis of CRC. Peng et al. (231) observed that downregulation of lncRNA POU5F1P4 reduced the sensitivity of metastatic CRC cells to cetuximab, and could be a potential new treatment for metastatic CRC. Wang et al. (232) showed that the LINC00473 expression level was significantly higher in a group of drug-resistant patients than that in non-drug-resistant patients and knockdown of LINC00473 restored paclitaxelinduced cytotoxicity, inhibited cell viability and colony formation, induced apoptosis, and weakened the ability of tumor cells to migrate or invade.

DISCUSSION
The CRC metastasis is induced by a variety of factors in vivo and in vitro. Among the in vivo factors, changes in the tumor cell adhesion to surrounding cells and extracellular matrix, EMT, and the dysregulation of various motor proteins promote the CRC metastasis. Several signaling pathways such as Wnt/b-Catenin and PI3K/AKT signaling pathway play important roles in the CRC metastasis. LncRNAs also act as ceRNA to regulate the expression of downstream target genes or components of CRC metastasis-associated signaling pathways to impact CRC metastasis. Epidemiological studies have shown that CRC metastasis is closely related to several in vitro factors. For example, tea polyphenols (TPs) can exert anti-inflammatory, anti-oxidant, or pro-oxidant effects to promote apoptosis and act at multiple levels to inhibit CRC growth and metastasis (233). Nicotine upregulates the expression of UCA1 and HIF-1a in CRC cells and promotes the proliferation and metastasis of CRC cells (234). In addition, individuals with a family history of colorectal cancer and inflammatory bowel disease are more likely to develop colorectal cancer than individuals without such a family history of these diseases (4). Exploring the relationship among diet, lifestyle, and the risk of CRC occurrence and metastasis from the perspective of molecular epidemiology, and clarifying the critical exposure duration will help us better understand how these factors affect CRC occurrence and pathogenesis. Understanding the occurrence and development of the disease can help further to understand the clinical outcome (235). Elucidating the effects of in vivo factors, exploring the mechanism specific to colorectal cancer metastasis, identifying the important molecules involved in CRC pathogenesis will help the early clinical diagnosis and optimal treatment of CRC patients.
Few methods are available for CRC screening and most of the biomarkers used to diagnose CRC, such as CA199, are differentially expressed in many cancers. Therefore, CRC diagnosis lacks specificity and sensitivity. Mounting evidence has shown that abnormal expression of lncRNAs in human tissues and serum holds potential for early diagnosis and predicting patient prognosis. For example, expression of DANCR was lower in serum samples of postoperative patients than in patients with recurrence; moreover, serum DANCR expression significantly correlated with TNM staging (236).
Research has significantly advanced our understanding of the mechanisms underlying CRC and the therapeutic outcomes have been improved significantly. However, in metastatic CRC, the treatment outcomes, and mortality rate remain unsatisfactory. Therefore, there is an urgent need to find new therapeutic targets for metastatic CRC. Animal-based studies have shown that lncRNAs play important roles in metastatic CRC and can be used as potential targets for clinical treatment. Upon lncRNA-RI silencing, CRC cells show stronger radiosensitivity, making it a potential therapeutic target for metastatic CRC (237). Wu et al. (238) established a mouse xenograft model and observed that loss of lncRNA PVT1 and overexpression of miR-16-5p can minimize tumor volume. Through the lncRNA PVT1-miR-16-5p/VEGFA/VEGFR1/ AKT axis, lncRNA PVT1 is directly involved in the progression of CRC and is a potential target for CRC treatment. Animal experiments by Yao et al. (239) showed that MIR600HG can inhibit tumor formation. Compared with lncRNA MIR600HG alone, combination therapy with MIR600HG and oxaliplatin significantly inhibited CRC stem cell metastasis and tumor growth.
Although lncRNAs have shown great potential in clinical applications, following gaps remain in lncRNA research. 1) The specific mechanisms underlying the effects of various lncRNAs in CRC remain unclear, highlighting the need for further research on the occurrence and development of CRC. 2) In terms of their utility as CRC biomarkers, the heterogeneity of lncRNA expression may make it difficult to achieve an accurate diagnosis. 3) Only a few animal experiments have been carried out to confirm treatment outcomes. Thus, limited data make it difficult to confirm the reliability of lncRNAs as diagnostic and therapeutic markers. Therefore, it is imperative to further explore the relationships between lncRNAs and CRC so that a solid foundation can be laid for their future use in CRC diagnosis and treatment. Nonetheless, research on lncRNAs in human cancers is expected to lead to major breakthroughs in terms of early diagnosis, risk detection, and treatment in the near future.