Theoretical perspectives and clinical applications of non-coding RNA in lung cancer metastasis: a systematic review

Abstract

Lung cancer is one of the deadliest malignancies worldwide, with distant metastasis being a major cause of death. However, the specific mechanisms of lung cancer metastasis remain unclear. NcRNAs, a widely present type of non-coding RNAs in the body, constitute about 98% of the human genome, lacking protein-coding capacity but involved in various cellular processes such as proliferation, apoptosis, invasion, and migration. Studies have shown that ncRNAs play a crucial role in the metastasis of lung cancer, although research in this area is limited. This review summarizes the biological origins and functions of ncRNAs, their specific roles and mechanisms in lung cancer metastasis, and discusses their potential for early screening and therapeutic applications in lung cancer. Furthermore, it outlines the challenges in translating basic advancements of ncRNAs in lung cancer metastasis into clinical practice.

Introduction

Lung cancer is a highly aggressive malignancy with the highest morbidity and mortality of any cancer worldwide posing a great threat to human health [1]. According to the histological characteristics of cancer cells, lung cancer can mainly be divided into two types: small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC). NSCLC, the most common subtype, accounts for 85–90% of all lung cancer types. NSCLC consists of various histological subtypes, including lung adenocarcinoma (LUAD), lung squamous cell carcinoma (LUSC), and large cell carcinoma [2]. Despite significant progress in our understanding of the cancer biology of lung cancer, the application of diagnostic biomarkers, and improvements in treatment methods over the past decade, the incidence and mortality rates of lung cancer remain high [3].

Metastasis is one of the biological characteristics of malignant tumors, and the ability of tumors to metastasize is crucial for their outcome and prognosis [4]. Statistics show that cancer, as the second leading cause of death worldwide, is responsible for 90% of cancer deaths due to metastasis [5]. Tumor metastasis is a multistep cascade process, where cells that successfully metastasize attain novel phenotypic traits while discarding others [6]. Cancer cells achieve metastasis through several mechanisms: On one hand, tumors can induce angiogenesis with tumor-associated macrophages and various angiogenic factors [7]. Once cancer cells invade the circulation, they interact with platelets, leading to direct and indirect "education" processes. This interaction alters platelets' adhesion molecules, glycoproteins, nucleic acids, proteins, and various receptors, thereby promoting tumor growth and dissemination [8]. At some point while circulating, cancer cells exit the bloodstream endothelium and enter target tissues, a process referred to as extravasation. The timing of this process varies depending on the tumor cells [9]. Subsequently, cancer cells settle in organ microenvironments conducive to their growth and adaptation (Fig. 1). In the case of lung cancer, the most common sites of metastasis are the brain, bones, lymph nodes, and liver [10]. Unfortunately, the mechanisms of lung cancer metastasis are not yet fully understood. Current well-studied mechanisms include the regulation of epithelial-mesenchymal transition (EMT) in lung cancer cells to control the metastatic process [11], the overexpression of the NFIB gene in lung cancer [12], and the regulation of the tumor microenvironment (TME) [13, 14]. In these processes, EMT is particularly important. In recent years, ncRNAs have attracted considerable attention due to their unique regulatory mechanisms. Their roles in regulating cell proliferation and apoptosis in lung cancer have been revealed, and their importance in the metastasis of lung cancer is increasingly evident [15]. ncRNAs have become one of the focal points in lung cancer metastasis research.

Fig. 1.

Mechanism of Lung Cancer Metastasis. Tumors induce angiogenesis through tumor-associated macrophages and various vasoactive factors. Additionally, tumors can recruit endothelial progenitor cells to induce angiogenesis. Due to the lack of tight cell junctions between lymphatic endothelial cells, lymphatic vessels are more permeable than blood vessels. After invading the circulation, cancer cells preferentially bind to platelets. The interaction between platelets and cancer cells facilitates their extravasation into target tissues, where they colonize and thrive in the microenvironment that supports their growth

ncRNAs account for approximately 98% of the human genome and lack protein-coding capacity [16]. They can be classified into various types based on their length, structure, and location. The most extensively studied types currently include long non-coding RNAs (lncRNAs), microRNAs (miRNAs), circular RNAs (circRNAs), piwi-interacting RNAs (piRNAs), among others [17]. With the advancement of whole-genome sequencing technology, an increasing number of ncRNAs have been discovered. In the past, ncRNAs were referred to as "transcriptional noise" or "junk transcripts" with no biological function. However, a large body of evidence in recent years has shown that ncRNAs can regulate various cellular biological processes through multiple mechanisms. For example, In chronic lymphocytic leukemia (CLL), miR-146b-5p inhibits the growth of CLL cells by downregulating the IL-23 receptor complex [18]. Additionally, miR-205 has been shown to regulate the progression of prostate cancer by inhibiting de novo cholesterol synthesis through the suppression of squalene epoxidase expression [19]. Studies have found that ncRNAs play a significant role in the metastasis of lung cancer. However, cancer metastasis severely impacts the treatment and prognosis of lung cancer patients, and ncRNAs provide a new target for regulating cancer metastasis [20, 21].

In recent years, as understanding of ncRNAs has deepened, their influence and specific mechanisms in the process of lung cancer metastasis have also been elucidated [22]. Mechanistically, ncRNAs operate through multiple pathways, such as (1) acting as ceRNAs; (2) regulating transcription; (3) serving as modulators of protein–protein interactions; and (4) regulating the translation of peptides or proteins. Based on these mechanisms, ncRNAs can influence lung cancer cell metastasis. The following brief overview will discuss miRNAs, lncRNAs, and circRNAs related to lung cancer metastasis.

miRNAs

miRNAs' biological origin

miRNAs are small, conserved ncRNAs, typically 18–25 nucleotides long, that regulate gene expression at the post-transcriptional level. They are first transcribed by RNA polymerase II into primary miRNA transcripts (pri-miRNA), which are processed in the nucleus into precursor miRNAs (pre-miRNA). These precursors are then transported to the cytoplasm by exportin-5 and RAN-GTP, where the DICER ribonuclease further processes them into mature miRNAs (Fig. 2) [23]. In the final stage, these mature miRNAs are incorporated into the RNA-induced silencing complex (RISC), which is guided by the miRNA strand to bind complementary sequences in target mRNAs. This interaction results in either translational repression or mRNA degradation, effectively regulating gene expression [24]. In 2002, the first evidence linking miRNAs to cancer was demonstrated when Dr. Croce found downregulation of the miRNA genes miR15 and miR16 in CLL [25]. Currently, increasing evidence suggests that miRNAs play a crucial role in cancer. Mechanistically, specific miRNAs regulate gene expression by binding complementary sequences in the 3' untranslated region (3'-UTR) of target mRNAs. Through their regulation of gene expression and transcription, miRNAs participate in various biological processes, including tumorigenesis and metastasis [26, 27]. Lung cancer metastasis is a leading cause of patient mortality, and miRNAs are involved in this process. Therefore, understanding these mechanisms is crucial for the treatment and early diagnosis of lung cancer.

Fig. 2.

Biogenesis and Functional Effects of ncRNAs. DNA nucleotides are transcribed into primary miRNA transcripts by RNA polymerase II. These are then processed into precursor miRNA within the nucleus. Subsequently, pre-miRNAs are transported to the cytoplasm via exportin-5 and RAN-GTP. In the cytoplasm, they are further processed by the DICER endonuclease into mature miRNAs, which then return to the nucleus to exert their functions. lncRNAs are transcribed by RNA polymerase II from promoter regions of independent genes, producing intronic lncRNA, intergenic lncRNA (lincRNA), and natural antisense transcripts. These lncRNAs exit the nucleus to act as competing endogenous RNAs or interact with proteins. circRNAs are produced by back-splicing of pre-mRNA, forming a closed loop by covalently linking the 5’-head to the 3’-tail. circRNAs can also function as ceRNAs or interact with proteins

Relationship between miRNAs and lung cancer metastasis

Genetic association

The expression of oncogenes, activation of tumor suppressor genes, and mutations in related oncogenes all play significant roles in the development and progression of cancer. In lung cancer, certain gene expression levels are crucial for the process of lung cancer cell metastasis. Therefore, studying these genes can further elucidate the process of lung cancer metastasis and provide new targets for the treatment, prognosis, and monitoring of lung cancer. ncRNAs can directly target these relevant genes, regulate their expression, and thereby control the process of lung cancer cell metastasis. Research related to miRNAs in this regard is relatively extensive.

Research has found that certain miRNAs play dual roles in regulating lung cancer progression. For instance, GATA6 is a transcription factor crucial for lung morphogenesis and is associated with cancer metastasis and prognosis. However, in NSCLC, the expression of GATA6 is increased in the early stages compared to normal tissue, which may be related to oncogenes. In high-grade NSCLC, GATA6 expression decreases, and this reduction can enhance metastatic capability [28]. TSPAN12 is a member of the tetraspanin family. In hepatocellular carcinoma, downregulation of TSPAN12 expression can inhibit the growth of primary tumors [29]. However, the opposite effect is observed in lung cancer [30]. miR-196b-5p directly targets sites on these genes, negatively regulating their expression, thereby promoting cancer cell migration, proliferation, and cell cycle regulation (Table 1) [31]. Additionally, miR-196b-5p is significantly upregulated in lung cancer cells, directly targeting the tumor suppressor gene FAS and signal transducer and activator of transcription 3 (STAT3), thus regulating the cell cycle to promote cancer cell proliferation [32]. There are many miRNAs that regulate the proliferation, invasion, metastasis, and apoptosis of lung cancer cells. For example, miR-4293 promotes tumor cell proliferation and metastasis while inhibiting apoptosis. Its target mRNA, decapping enzyme 2 (DCP2), is suppressed by miR-4293. In normal cells, DCP2 directly or indirectly interacts with WFDC2P to negatively regulate its expression, inhibiting cancer development. However, in tumor cells, miR-4293 downregulates DCP2 levels, leading to overexpression of WFDC2P, which promotes cancer cell proliferation and suppresses apoptosis (Table 1) [33]. Similarly, hypoxia-inducible factor-1α (HIF-1α) directly binds to miR-200c and downregulates its expression, suppressing lung cancer cell migration (Table 1). miR-1275 is highly upregulated in LUAD tissues. It directly targets the Wnt/β-catenin and Notch pathways to promote carcinogenesis. HIF-1α regulates miR-1275, making it a potential therapeutic target for LUAD (Table 1) [34]. Studying miRNAs and their related genes can provide new therapeutic targets for lung cancer treatment. For example, upstream stimulating factor 1 (USF1) is a target gene of miR-210-3p. USF1 binds to its downstream target PCGF3, and miR-210-3p inhibits USF1-mediated expression of PCGF3, thereby promoting lung cancer metastasis (Table 1) [35]. Conversely, miRNA-99b-5p has opposite effects. It is downregulated in NSCLC cell lines, and FZDB has been identified as a direct target of miRNA-99b-5p. This inhibition suppresses cell proliferation, migration, and invasion (Table 1) [36].

Table 1.

Roles of miRNAs in Regulating Lung Cancer Metastasis and Their Impact on the Disease

miRNA	Expression	Target gene and/or signaling	Influencing Processes	Impact on Diseases
miR-1	Downregulation	CXCR4/FOXM1	Inhibition of Angiogenesis	Improvement
miR-1246	Upregulation	GSK-3β/β-catenin	Promotion of EMT-induced Cell Detachment, Enhanced Cell Invasion and Migration Capabilities	Deterioration
miR-1275	Upregulation	Wnt/β-catenin、Notch	Promotion of Cell Migration, Proliferation, and Invasion	Deterioration
miR-135b	Upregulation	CYLD/NF-κB	Promotion of NSCLC Cell Proliferation, Migration, Invasion, Anti-apoptosis, and Angiogenesis	Deterioration
miR-141	Upregulation	GAX	Promotion of Lung Cancer Cell Migration, Invasion, and Angiogenesis	Deterioration
miR-186	Downregulation	Dicer1	Inhibition of Cell Migration, Invasion, and EMT	Improvement
miR-1915-3p	Downregulation	SET	Inhibition of Cell Migration, Invasion, and EMT	Improvement
miR-196a	Upregulation	ANXA1	Promotion of Lung Cancer Cell Migration and Invasion	Deterioration
miR-196b-5p	Upregulation	GATA6、TSPAN12	Promotion of Lung Cancer Cell Migration, Proliferation, and Cell Cycle	Deterioration
miR-200	Downregulation	Jag1、Jag2	Inhibition of Lung Cancer Cell Metastasis	Improvement
miR-200c	Downregulation	HIF-1α	Inhibition of Lung Cancer Cell Migratio	Improvement
miR-20b	Upregulation	Wnt/β-catenin	Promotion of NSCLC Cell Proliferation, Migration, and Invasion	Deterioration
miR-210-3p	Upregulation	USF1	Promotion of Lung Cancer Cell Migration	Deterioration
miR-27b	Downregulation	Snail1	Inhibition of NSCLC Tumor Cell Proliferation, Migration, and EMT	Improvement
miR-33a-5p	Upregulation	Twist1/Wnt/β-Catenin	Promotion of EMT and Cancer Cell Invasion	Deterioration
miR-4293	Upregulation	DCP2/WFDC21P/STAT3	Promotion of Lung Cancer Cell Proliferation and Migration	Deterioration
miR-495 and miR-543	Downregulation	PAK3	Inhibition of Lung Cancer Cell Metastasis	Improvement
miR-590-5p	Downregulation	STAT3 and Its Downstream Factors	Inhibition of NSCLC Cell Viability, Proliferation, Colony Formation, Migration, and Invasion, and Induction of Apoptosis	Improvement
miR-596-3p	Downregulation	YAP1、IL-8	Inhibition of Brain Metastasis in NSCLC	Improvement
miR-7-5p	Downregulation	TGF-β2	Inhibition of NSCLC Cell Migration and Invasion	Improvement
miR-937-3p	Upregulation	SOX11/PI3K/AKT	Promotion of LUAD Cell Angiogenesis, Invasion, and Metastasis	Deterioration
miR-98-5p	Downregulation	TGFBR1	Inhibiting EMT Process to Suppress Proliferation and Metastasis of NSCLC Cells	Improvement
miRNA-99b-5p	Downregulation	FZDB	Inhibition of Cell Proliferation, Migration, and Invasion	Improvement

Regulation of EMT

The extent of EMT significantly influences the degree of malignancy in tumor cells in tumor cells. Transforming growth factor beta receptor 1 (TGFBR1) is a direct target of miR-98-5p, and it is involved in various biological processes such as cell motility, differentiation, adhesion, division, and apoptosis. TGFBR1 can mediate EMT and play a role in tumor invasion. Mechanistically, miR-98-5p directly targets TGFBR1 to inhibit the EMT process in tumor cells, thereby suppressing the proliferation and metastasis of NSCLC cells (Table 1) [37]. SET has been identified as an oncogene in various cancers such as hepatocellular carcinoma, pancreatic cancer, glioblastoma, and lung cancer. SET is significantly expressed in lung cancer tissues and regulates the EMT process in tumor cells. Additionally, it is a direct target of miR-1915-3p. miR-1915-3p directly binds to the 3' untranslated region of SET, exerting inhibitory effects on cell migration, invasion, and EMT (Table 1) [38]. Dicer1 is a well-conserved RNase III ribonuclease that previous studies have shown to play a crucial role in the development and progression of tumors. In colorectal cancer, Dicer1 deficiency induces EMT and enhances the metastatic capability of cancer cells [39]. Simultaneously, Dicer1 is a direct target of miR-186. miR-186 binds to Dicer1 and reduces its expression, thereby regulating processes such as invasion, proliferation, migration, and EMT in lung cancer cells (Table 1) [40]. Contrary to mechanisms that inhibit tumor cell development by suppressing the EMT process, there are also miRNAs that promote tumor cells toward a malignant direction by enhancing EMT. For instance, β-Catenin is a key protein in the Wnt/β-Catenin signaling cascade involved in processes such as embryonic development, stem cell maintenance, tumor initiation, and metastasis. Glycogen synthase kinase-3β (GSK-3β) negatively regulates the Wnt/β-Catenin signaling pathway. miR-1246 targets GSK-3β/β-catenin to regulate the Wnt/β-Catenin pathway, promoting EMT, cell detachment, increased invasion, and metastasis capability (Table 1) [41]. Previous studies have demonstrated that miR-33a-5p and Twist1 can induce EMT in cancer cells. In lung cancer tissues, lncRNA JPX is upregulated and promotes cancer cell growth both in vivo and in vitro. Mechanistically, the lncRNA JPX/miR-33a-5p/Twist1 axis activates the Wnt/β-Catenin pathway to induce EMT and cancer cell invasion (Table 1, 2) [42].

Table 2.

Roles of lncRNAs in Regulating Lung Cancer Metastasis and Their Impact on the Disease

lncRNA	Expression	Target gene and/or signaling	Influencing Processes	Impact on Diseases
DUBR	Downregulation	ZBTB11	Inhibition of LUAD Cell Migration and Invasion	Improvement
FEZF1-AS1	Upregulation	miR-516b-5p/ITGA11	Promotion of Cell Proliferation, Migration, and Invasion, Inhibition of Cell Cycle Arrest	Deterioration
HOXC-AS3	Upregulation	YBX1	Promotion of NSCLC Cell Proliferation, Migration, and Invasion, and Tumor Growth and Metastasis	Deterioration
LCAT1	Upregulation	IGF2BP2	Promotion of Lung Cancer Cell Growth, Survival, and Migration	Deterioration
LINC00301	Upregulation	FOXC1	Promotion of NSCLC Cell Growth and Migration	Deterioration
LINC00662	Upregulation	miR-320d/E2F1	Promotion of NSCLC Cell Proliferation, Invasion, and Migration, and Inhibition of Apoptosis and Cell Cycle Arrest	Deterioration
LINC00969	Upregulation	NLRP3	Inhibition of heat shock protein deposition in lung cancer promotes the development of acquired erlotinib resistance	Deterioration
LINC01140	Upregulation	miR-33a-5p、iR-33b-5p/c-Myc;miR-377-3p、miR-155-5p/PD-L1	Promotion of Lung Cancer Cell Proliferation, Migration, and Invasion, and Reduction of DDP-Induced Apoptosis	Deterioration
LINC01234	Upregulation	LSD1、EZH2、miR-27b-3p、miR-340-5p	Promotion of NSCLC Cell Migration and Invasion	Deterioration
LINC01503	Upregulation	miR-342-3p/LASP1	Promotion of NSCLC Proliferation, Migration, and Invasion	Deterioration
LINC01561	Upregulation	miR-760/SHCBP1	Promotion of NSCLC Cell Proliferation, Migration, and Invasion, Inhibition of Apoptosis	Deterioration
lncPCAT1	Upregulation	SOX2	Promoting Tumor Progression to Advanced Stages	Deterioration
lncRNA FTX	Downregulation	miR-200a-3p/FOXA2	Inhibition of NSCLC Proliferation and Metastasis	Improvement
LncRNA JPX	Upregulation	Wnt/β-catenin	Promotion of Lung Cancer Cell Proliferation, Migration, and Invasion	Deterioration
NNT-AS1	Upregulation	miR-22/FOXM1	Promotion of LUSC Cell Migration and Invasion, Inhibition of Cell Apoptosis	Deterioration
OGFRP1	Upregulation	miR-4640-5p/eIF5A	Promotion of NSCLC Cell Proliferation, Migration, and Invasion	Deterioration
RFX3-AS1	Upregulation	miR-577/STAT3	Promotion of NSCLC Cell Proliferation, Migration, Invasion, and Epithelial-Mesenchymal Transition (EMT), Inhibition of Cell Apoptosis	Deterioration
SNHG17	Upregulation	miR449a/TGIF2	Promotion of NSCLC Cell Proliferation, Migration, Invasion, and Epithelial-Mesenchymal Transition	Deterioration
SNHG18	Upregulation	miR-211-5p/BRD4	Promotion of NSCLC Cell Proliferation and Invasion	Deterioration
TDRG1	Upregulation	miR-214-5p/KLF5	Promotion of NSCLC Cell Growth, Migration, and Invasion, Inhibition of Cell Apoptosis	Deterioration
THOR	Upregulation	IGF2BP1	Promotion of NSCLC Cell Growth, Proliferation, Migration, and Invasion, Inhibition of Cell Apoptosis	Deterioration
UCC	Upregulation	miR-143-3p	Promotion of NSCLC Cell Proliferation and Migration	Deterioration

Involved in angiogenesis

As is well known, angiogenesis plays a decisive role in the development of tumors, and existing research has demonstrated that miRNAs are important regulatory factors in modulating tumor angiogenesis, invasion, growth, and metastasis processes. For example, miR-135b affects multiple biological processes in NSCLC cells and significantly impacts lung cancer cell metastasis. The NF-κB signaling pathway plays a crucial role in processes such as tumor cell proliferation, migration, invasion, anti-apoptosis, and angiogenesis. miR-135b binds to the 3' untranslated region (UTR) of the Cylindromatosis (CYLD) gene mutation, activating the NF-κB signaling pathway. Furthermore, IL-6/STAT3 can elevate miR-135b levels, with STAT3 directly binding to the miR-135b promoter, forming a new positive feedback loop in the IL-6/STAT3/miR-135b/NF-κB signaling pathway that regulates the corresponding biological processes (Table 1) [43]. miR-141, via exosomes, targets the specificity protein homolog (GAX), promoting vascularization and malignant progression in lung cancer. In-depth research into this discovery offers new possibilities for the treatment of lung cancer (Table 1) [44]. miR-937-3p plays a role in various diseases, such as colorectal, breast, and liver cancers, where it is highly expressed, promoting tumor development and serving as a prognostic marker for patients. SOX11 functions as a tumor suppressor gene and is also a direct target of miR-937-3p. miR-937-3p expression is regulated by oncogenic factors, primarily MYC, which upregulates its expression. Subsequently, miR-937-3p targets SOX11, activating the PI3K/AKT pathway and enhancing vascularization in LUAD (Table 1) [45]. Another participant in angiogenesis is miR-1. CXCR4 is a well-characterized chemokine receptor that regulates cancer metastasis, including in SCLC. Its downstream target FOXM1, a transcription factor of the Forkhead box family, plays crucial roles in biological mechanisms such as cell cycle regulation, angiogenesis, cell proliferation, DNA damage repair, and apoptosis. Mechanistically, miR-1 negatively regulates both CXCR4 and FOXM1 expression through the CXCR4/FOXM1 axis, inhibiting angiogenesis and suppressing the growth, proliferation, and metastatic potential of cancer cells (Table 1) [46].

As tumor suppressors

The dual role of miRNAs includes both promoting the malignant progression of cancer cells and inhibiting this process. Research has now demonstrated that miRNAs can act as tumor suppressors by inhibiting the malignant progression of tumors.

Currently, research on circulating miRNAs has become a hotspot. Studying circulating miRNAs can monitor the occurrence and development of cancer, as well as predict and monitor cancer prognosis. For example, circulating miR-590-5p has been identified as a relevant target for predicting lung cancer progression. miR-590-5p directly targets STAT3, negatively regulating its expression, and thereby modulating downstream factors such as cyclin D1, c-Myc, Vimentin, and β-catenin synthesis. This regulation influences processes like the cell cycle and EMT, ultimately inhibiting the malignant behavior of tumor cells (Table 1) [47]. Lung cancer brain metastasis is a serious clinical issue and relatively common. YAP1 and IL-8 are two important genes in cancer cell invasion and metastasis. Research has found that miR-596-3p is highly expressed in primary tumor cells but low in metastatic cancer cells. Studies have shown that miR-596-3p targets YAP1 and blocks YAP1-induced MMP2 expression, thereby inhibiting endothelial cell migration. Additionally, miR-596-3p suppresses IL-8 secretion, reducing blood–brain barrier permeability, and making it more difficult for metastatic cancer cells to pass through the blood–brain barrier. Therefore, based on these mechanisms, miR-596-3p can inhibit lung cancer metastasis (Table 1) [48]. As is well known, EMT is a central process in cancer metastasis, characterized by loss of apical-basal polarity, increased front-rear polarity, reduced cell adhesion, transition from epithelial to mesenchymal phenotype, and acquisition of mesenchymal trait, associated with embryogenesis, tissue regeneration, and cancer progression [49]. Thus, regulating EMT can potentially modulate cancer metastasis. Snail1, an evolutionarily conserved zinc-finger transcription factor, can induce EMT in various tissues. miR-27b functions as a tumor suppressor, exhibiting low expression in NSCLC cells. Research indicates that miR-27b directly targets Snail1, inhibiting its ability and thereby disrupting EMT (Table 1) [50].

Invasion-metastasis cascade is a unique cancer behavior where certain miRNAs can regulate this process, playing a role in suppressing tumor development. Smad4 acts as a tumor suppressor and is a central intracellular mediator of TGF-β signaling, its inactivation being implicated in various cancers. PAK3 has been identified as a downstream effector molecule of Smad4; Smad4 directly binds PAK3 to mediate cancer cell metastasis. miR-495 and miR-543 also directly bind to the 3’UTR of PAK3, attenuating the metastatic potential of lung cancer cells both in vivo and in vitro (Table 1) [51]. Changes in the TME influence cancer cell metastasis; uncontrolled proliferation of cancer cells can lead to hypoxia and, in severe cases, acidosis—a chronic and severe TME that induces lung cancer metastasis by inhibiting miR-7-5p. TGF-β2 is a target of miR-7-5p; overexpression of miR-7-5p reduces TGF-β2 levels, weakening lung cancer metastasis potential and regulating TME (Table 1) [52]. According to reports, highly metastatic LUAD is associated with a TME characterized by cancer-associated fibroblasts (CAFs) expansion and original metastatic and fibrosis-associated TME. miR-200 acts as an inhibitor of lung cancer cell metastasis; its overexpression reduces the expression of Notch ligands Jag1 and Jag2 in fibroblasts, inhibiting the proliferation of adjacent tumor-associated fibroblasts and thereby weakening the metastatic potential of lung cancer cells (Table 1) [53].

lncRNAs

lncRNAs' biological origin

lncRNAs, a subclass of ncRNAs over 200 nucleotides in length, are found in the nucleus or cytoplasm and regulate gene expression on multiple levels. Historically, the earliest discovered lncRNAs were H19 and XIST, initially mistaken as mRNAs [54]. lncRNAs are transcribed by RNA polymerase II from the promoter regions of independent genes and can be classified into three main types: Intronic lncRNA, long intergenic non-protein coding RNAs(lincRNAs), and Natural antisense transcript (NAT) (Fig. 2) [13]. Emerging research indicates that lncRNAs expression is associated with pathological responses in various diseases, including cancer [55]. Growing evidence shows that lncRNAs influence cancer progression by serving as ceRNAs or miRNA sponges, engaging in signaling pathways, and interacting with proteins.

Relationship between lncRNAs and lung cancer metastasis

Acting as ceRNAs

Acting as ceRNAs or “sponging” miRNAs is one of the most characteristic mechanisms of lncRNAs. ceRNAs regulate downstream mRNA expression by binding shared miRNAs, participating in the development of various tumors [56]. Typically, miRNAs bind to the 3'-untranslated region (3'-UTR) of target gene mRNA to regulate gene expression. lncRNAs can bind miRNAs, leading to miRNA sponging, where miRNAs are unable to inhibit the translation of target mRNA or induce mRNA degradation once sequestered [57]. Therefore, lncRNAs often function as ceRNAs to sequester various types of miRNAs and regulate downstream target genes at the post-transcriptional level.

Nicotinamide nucleotide transhydrogenase antisense RNA 1 (NNT-AS1) is a newly discovered lncRNA that exhibits abnormal expression in tumors and is implicated in the pathogenesis and oncogenesis of human cancers such as colorectal cancer and cervical cancer [58, 59]. NNT-AS1 is upregulated in LUSC tissues and cell lines, where it acts as a sponge for miR-22, leading to increased expression of FOXM1. This promotes migration and invasion of LUSC cells while inhibiting apoptosis (Table 2) [60]. Another example is the FEZ family zinc finger-1 antisense RNA 1 (FEZF1-AS1), which is highly expressed in colorectal cancer, gastric cancer, and ovarian cancer [61, 62]. In LUAD, FEZF1-AS1 expression is upregulated and mediated by FEZF1. Further research has shown that FEZF1-AS1 in NSCLC can upregulate ITGA11 expression by binding to miR-516b-5p, thereby promoting proliferation, migration, and invasion of NSCLC cells while inhibiting cell cycle arrest (Table 2) [63]. Regulatory Factor X3 antisense RNA 1 (RFX3-AS1) is the antisense transcript of the RFX coding gene located on chromosome 9p24.2. In NSCLC tissues and cells, upregulated RFX3-AS1 binds and downregulates miR-577, thereby activating STAT3. This promotes proliferation, migration, invasion, and EMT of NSCLC cells while inhibiting apoptosis (Table 2) [64].

lincRNAs represent another type of lncRNAs. It has been reported that LINC01140 is highly expressed in lung cancer tissues and cells, and its levels correlate with patient survival rates. Mechanistically, LINC01140 acts as a miRNA sponge by directly binding miR-33a-5p and miR-33b-5p, leading to upregulation of c-Myc expression. Additionally, it directly inhibits miR-377-3p and miR-155-5p, resulting in the upregulation of PD-L1 expression, thereby enhancing the proliferation, migration, and invasion capabilities of lung cancer cells (Table 2) [65]. Another LINC00662, exhibits increased expression in plasma-derived exosomes from NSCLC patients. This exosome-derived LINC00662 acts as a sponge for miR-320d in NSCLC cells, thereby enhancing E2F1 expression. This promotes the proliferation, invasion, and migration of NSCLC cells while inhibiting apoptosis and cell cycle arrest (Table 2) [66]. LINC01503 is a super-enhancer-driven lncRNA overexpressed in NSCLC samples and cell lines. It upregulates LASP1 expression by binding to miR-342-3p, thereby promoting the proliferation, migration, and invasion of NSCLC (Table 2) [67]. Additionally, research has elucidated that in NSCLC, SOX2 activates LINC01561, which then competitively binds to miR-760 to upregulate SHCBP1 expression. This enhances the proliferation, migration, and invasion of NSCLC cells while inhibiting apoptosis (Table 2) [68].

In addition to AS-lncRNAs and lincRNAs, various other lncRNAs play roles in lung cancer by acting as ceRNAs. For example, in NSCLC, Testis Development Related Gene 1 (lncRNA TDRG1) interacts with miR-214-5p to upregulate KLF5 expression, exerting oncogenic effects (Table 2) [69]. Opioid growth factor receptor pseudogene 1 (OGFRP1) has been recently found to be highly expressed in NSCLC tissues compared to normal lung tissues. OGFRP1 interacts directly with miR-4640-5p to upregulate eIF5A expression, promoting proliferation, migration, and invasion of NSCLC cells (Table 2) [70]. LINC01140 is highly expressed in human LUAD tissues and cell lines, targeting miR-377-3p and miR-155-5p to negatively regulate their expression. This leads to high expression of the downstream effector PD-L1, contributing to cancer cell proliferation, migration, invasion, and immune evasion (Table 2) [65]. Additionally, small nucleolar RNA host gene 18 (SNHG18), an independent prognostic factor for NSCLC, is associated with lymph node metastasis and reduced overall survival rates in patients. Mechanistically, SNHG18 promotes metastasis in NSCLC cells by inhibiting miR-211-5p and inducing BRD4 expression (Table 2) [71].

Protein binding

lncRNAs mediate their functions through another common mechanism, which involves interacting with proteins. lncRNAs can serve as protein baits to recruit or sequester proteins or act as scaffolds that coordinate or complex different proteins together [72]. The interaction between lncRNAs and proteins can influence lncRNA processing, modification, stability, and function, and their dysregulation is implicated in various human cancers, potentially contributing to tumor development and progression [73].

LCAT1 is a newly discovered lncRNA that is overexpressed in lung cancer and correlates with poor prognosis in lung cancer patients. Biologically, LCAT1 promotes the growth and metastasis of lung cancer cells both in vitro and in vivo [74]. Mechanistically, LCAT1 physically interacts with IGF2BP2 (insulin-like growth factor 2 mRNA-binding protein 2, an m6A reader protein), stabilizing IGF2BP2 by protecting it from autophagic degradation. This stabilization further enhances the translation of CDC6 mRNA through m6A modification, thereby promoting the growth, survival, and migration of lung cancer cells (Table 2) [75]. Similarly, research has identified a highly conserved non-coding RNA that binds to IGF2BP1, named THOR (Testis-associated Highly-conserved Oncogenic Long non-coding RNA), which is highly expressed in various cancer tissues. THOR directly interacts with IGF2BP1 (Insulin-like Growth Factor 2 mRNA-binding Protein 1), promoting the stability of several key oncogenic mRNAs, including IGF2, MYC, CTNNB1, and PTEN, enhancing their biological functions in cancer (Table 2) [76]. In NSCLC, their interaction enhances cell growth and vitality while suppressing apoptosis, thereby promoting the malignant progression of NSCLC [77]. However, whether its potential molecular mechanisms are similar to those of LCAT1 mentioned earlier remains unclear and requires further study. HOXC-AS3 is a NAT of the HOXC10 gene, implicated in various cancers [78]. Studies have reported increased expression of HOXC-AS3 in lung cancer cells [79]. Subsequently, another study specifically elucidated that in NSCLC, HOXC-AS3 stabilizes YBX1 expression by directly binding to YBX1, thereby inhibiting MDM2-mediated ubiquitination. YBX1, in turn, interacts with the HOXC8 promoter to enhance its transcription, promoting proliferation, migration, and invasion of NSCLC cells, as well as tumor growth and metastasis (Table 2) [80].

LINC00301 is highly expressed in NSCLC cells, suggesting its association with the malignant progression of lung cancer. FOXC1 is a transcription factor, and experimental evidence has shown that LINC00301 is a direct target of FOXC1 (Table 2). Mechanistically, FOXC1 directly interacts with LINC00301 to upregulate its expression. This leads to the recruitment of the downstream effector EZH2 and mediates H3K27me2 at the EAF3 promoter, suppressing the transcription of EAF2 (a tumor suppressor). Consequently, this cascade promotes changes in downstream factors that facilitate NSCLC cell growth and migration [81]. Additionally, Prostate Cancer-Associated Transcript 1 (PCAT1), initially identified as a prostate-specific regulator of cell proliferation, is also dysregulated in NSCLC cells and correlates with NSCLC progression. Mechanistically, lncPCAT1 binds to the SOX2 promoter, accelerating SOX2 transcription. Overexpression of SOX2 is positively correlated with advanced tumor occurrence, drug resistance, and poor prognosis, and it regulates various biological processes including apoptosis and the cell cycle (Table 2) [82]. Similarly, Long Intergenic Non-Protein Coding RNA 1234 (LINC01234), which is highly expressed in NSCLC cells, can interact with RNA-binding proteins (such as ZEH2, SUZ12, and LSD1) to regulate transcriptional levels of gene expression (Table 2) [83].

As tumor suppressors

Although extensive research indicates that lncRNAs often promote malignant progression in cancer, a minority of lncRNAs are known to function as tumor suppressors. Its mechanism may involve regulating intracellular oxidative phosphorylation, interacting with other molecules to affect signaling pathways related to tumor cell proliferation and migration, among others.

Reportedly, DUBR (also known as LINC00883) is associated with various cancers, downregulated in recurrent neuroblastoma cell lines, and correlates with poor prognosis [84]. Significant downregulation of DUBR in LUAD similarly correlates with adverse outcomes. Specifically, DUBR upregulates ZBTB11 expression, attenuating cellular oxidative phosphorylation and inhibiting migration and invasion of LUAD cells (Table 2). In LUAD, overexpressed c-Myc binds to the promoter of DUBR, thereby suppressing its transcription and preventing its tumor-suppressive role [85]. Another lncRNA FTX (five prime to Xist), initially discovered at the Xist gene locus [86], has been found to inhibit the proliferation and migration of liver cancer cells by physically interacting with miR-374a and MCM2. Higher levels of FTX expression are associated with longer patient survival [87]. In NSCLC, FTX interacts with miR-200a-3p to promote FOXA2 expression, inhibiting proliferation and migration of NSCLC cells, suggesting FTX may act as a tumor suppressor in NSCLC progression (Table 2) [88].

circRNA

circRNAs' biological origin

CircRNAs are a type of highly conserved non-coding RNA composed solely of intron sequences, widely present in eukaryotic cells. With the development of bioinformatics, a large number of circRNAs have been identified. CircRNAs typically range in length from 200 to 3000 nucleotides and are generated by back-splicing of pre-mRNA. Unlike linear RNAs, they lack a 5’-cap and 3’-poly(A) tail structure; instead, their 5’-end is covalently linked to the 3’-end, forming a closed loop (Fig. 2). Due to this unique structure, circRNAs exhibit high stability [89].

It has been demonstrated that circRNAs primarily function through the following five mechanisms: (1) Acting as ceRNAs or miRNA sponges. For example, circ-PITX1 functions as a ceRNA by sponging miR-30e-5p, enhancing ITGA6 expression and promoting NSCLC development (Table 3). Similarly, Circ-0008003 acts as a sponge for miR-488, upregulating ZNF281 levels in vivo, thereby accelerating the occurrence and development of NSCLC (Table 3) [90, 91]. Another example of functioning as miRNA sponges are hsa_circ_0030998, which targets miR-558 and downregulates its expression, inhibiting malignant phenotypes such as proliferation, migration, and invasion of lung cancer cells, thus slowing down lung cancer progression (Table 3) [92]. In contrast, circBIRC6 and circMET sponge miR-145 and oncogenic miR-145-5p, respectively, promote NSCLC cell progression (Table 3) [93, 94]. (2) Transcriptional regulation. For example, upregulation of HuR under hypoxic conditions can regulate the expression of genes ATG7 and ATG16L1, enhancing autophagosome formation [95]. (3) Regulation of protein–protein interactions. circRNAs can act as regulators involved in gene toxicity stress, hypoxia, and heat shock stress regulation [96]. Circ-Foxo3 acts as a scaffold and binds to both proteins p53 and MDM2, regulating MDM2 ubiquitination. This enhances MDM2-dependent p53 ubiquitination, leading to rapid degradation of p53. Additionally, it protects Foxo3 from ubiquitination and degradation, promoting apoptosis in cancer cell lines [97]. (4) Translation of peptides and proteins. In recent years, studies have found that ncRNAs can also encode peptides or proteins. Peptides or proteins encoded by ncRNAs can significantly inhibit tumor metabolism reprogramming, stabilize oncogenic proteins, and regulate processes such as EMT, demonstrating potent anti-tumor functions [98]. Research has shown that circRNAs have the potential to encode proteins and can undergo translation, a process initiated by N6-methyladenosine (m6A) [99].

Table 3.

Roles of circRNAs in Regulating Lung Cancer Metastasis and Their Impact on the Disease

circRNA	Expression	Target gene and/or signaling	Influencing Processes	Impact on Diseases
Circ_0003028	Upregulation	miR-1298-5p/GOT2	Promotion of LC Cell Proliferation and Migration	Deterioration
Circ_0006988	Upregulation	miR-491-5p/MAP3K3	Promotion of LC Cell Proliferation and Migration	Deterioration
Circ-0000284	Upregulation	miR-377/PD-L1	Promotion of NSCLC Migration and Invasion	Deterioration
Circ-0001875	Upregulation	miR-31-5p/SP1	Promotion of NSCLC Cell Proliferation and Migration	Deterioration
Circ-0008003	Upregulation	miR-488/ZNF281	Promotion of NSCLC Cell Invasion and Proliferation	Deterioration
circ-0010235	Upregulation	miR-636/PD-L1	Promoting Lung Cancer Progression and Immune Evasion	Deterioration
Circ-0020123	Upregulation	miR-590-5p/THBS2	Promotion of NSCLC Proliferation, Invasion, and Migration, Inhibition of Apoptosis	Deterioration
Circ-100565	Upregulation	miR-506-3p/HMGA2	Promotion of NSCLC Cell Proliferation, Migration, and Invasion	Deterioration
CircBIRC6	Upregulation	miR-145	Promotion of NSCLC Cell Colony Formation, Migration, and Invasion, Inhibition of Apoptosis	Deterioration
Circ-BIRC6	Upregulation	miR-4491/Wnt2B/β-catenin	Promotion of NSCLC Cell Migration and Invasion	Deterioration
CircCCDC66	Upregulation	miR-33a-5p/KPNA4	Promotion of NSCLC Cell Proliferation, Migration, and Invasion, Inhibition of Cell Apoptosis	Deterioration
Circ-CCS	Upregulation	miR-383/E2F7	Promotion of Lung Cancer Cell Growth and Migration, Inhibition of Cell Apoptosis	Deterioration
CircFGFR1	Upregulation	miR-381-3p/CXCR4	Promoting Lung Cancer Progression	Deterioration
CircGLIS3	Upregulation	miR-644a	Promotion of NSCLC Cell Proliferation, Migration, and Invasion, Inhibition of Cell Apoptosis, Facilitation of NSCLC Progression and Tumor Growth	Deterioration
CircMET	Upregulation	miR-145-5p/CXCL3	Promotion of NSCLC Cell Proliferation and Migration	Deterioration
Circ-PITX1	Upregulation	miR-30e-5p/ITGA6	Promotion of NSCLC Cell Proliferation, Migration, Invasion, EMT, Inhibition of Apoptosis	Deterioration
CircSATB2	Upregulation	miR-326/FSCN1	Promotion of NSCLC Cell Proliferation, Migration, and Invasion, Induction of Abnormal Proliferation in Normal Human Bronchial Epithelial Cells	Deterioration
CircSCAP	Downregulation	SF3A3	Inhibition of NSCLC Cell Proliferation, Migration, Promotion of Apoptosis	Improvement
CircSHKBP1	Upregulation	miR-1294/PKM2	Promotion of NSCLC Cell Proliferation, Migration, and Invasion	Deterioration
CircSOX13	Upregulation	miR-3194-3p	Promotion of NSCLC Proliferation, Invasion, and Migration, Inhibition of Apoptosis	Deterioration
CircVMP1	Upregulation	miR-524-5p/METTL3	Promotion of NSCLC Cell Proliferation, Sphere Formation, Migration, Invasion, Drug Resistance, and Inhibition of Apoptosis	Deterioration
CircZNF609	Upregulation	miR-142-3p/GNB2	Promotion of LC Cell Proliferation and Migration	Deterioration
Hsa_circ_0001666	Upregulation	miR-1184, miR-548I/AGO1	Promotion of NSCLC Cell Migration and Invasion	Deterioration
Hsa_circ_0003222	Upregulation	miR-527/PHF21B	Promotion of NSCLC Cell Proliferation, Migration, Invasion	Deterioration
Hsa_circ_0030998	Downregulation	miR-558	Inhibition of Lung Cancer Cell Proliferation, Migration, and Invasion	Improvement
Hsa_circ_0129047	Downregulation	miR-1206/BMPR2	Inhibition of LAC Cell Viability, Proliferation, Adhesion, Migration, and Invasion, Promotion of Apoptosis	Improvement

Relationship between circRNAs and lung cancer metastasis

Acting as ceRNAs

Apart from lncRNAs, circRNAs also function as ceRNAs. ceRNA is a class of RNA molecules that regulate miRNA activity by competing for the same miRNA binding sites, thereby affecting the miRNA-mediated regulation of target genes. The competing endogenous RNA (ceRNA) network refers to the regulatory interactions between different types of RNA molecules (such as mRNA, lncRNA, and circRNA) that compete for binding to shared microRNAs (miRNAs). By sequestering miRNAs, these RNA molecules can regulate the availability of miRNAs to target mRNAs, thereby modulating gene expression. This dynamic network plays a crucial role in various biological processes, including tumor progression and metastasis, by influencing the expression of oncogenes and tumor suppressors. CircRNAs regulate downstream gene expression through this mechanism, thereby influencing tumor biological processes. For example, circ-BIRC6 targets miR-217 and directly binds to it to enhance the expression of amyloid-beta precursor protein-binding protein 2, accelerating NSCLC cell progression [100]. Bone morphogenetic protein is a member of the transforming growth factor β signaling molecule superfamily, involved in bone formation within cartilage, and its ligand is encoded by bone morphogenetic protein receptor type 2 (BMPR2) protein. This process is somewhat related to tumor occurrence and development. Studies have found that hsa_circ_0129047 is downregulated in LUAD patients, while miR-1206 is overexpressed, leading to BMPR2 downregulation and enhanced migration and invasion of cancer cells (Table 3) [101]. Research has found that CircHIPK3 is widely expressed in NSCLC cell lines. As a sponge for miR-124, CircHIPK3 overexpression significantly increases colony formation in NSCLC cell lines. The increased colony formation suggests that cancer cells can effectively overcome therapeutic obstacles, promoting metastasis and recurrence [102]. CircRNAs that promote the malignant progression of lung cancer cells also include hsa_circ_0010235 and circVMP1, which sponge miR-433-3p and miR-524-5p, respectively (Table 3). The former upregulates levels of TOR signaling pathway regulator-like (TIPRL), while the latter enhances expression of methyltransferase complex catalytic subunit (METTL3) and SRY-box transcription factor (SOX2). Both contribute to promoting tumor cell growth and proliferation [103, 104]. ZNF281 is a novel oncogenic protein documented in colorectal and pancreatic cancers, where its high expression in cancer cells enhances metastatic potential and reduces patient survival rates. ZNF281 is a direct target of miR-488, and circ-0008003 acts as a sponge for miR-488, upregulating ZNF281 expression to promote oncogenesis in NSCLC (Table 3) [91]. Similarly, the latter acts as a sponge for miR-527, upregulating the expression of PHF21B (a gene that can promote stem cell-like characteristics in prostate cancer cell lines and contains predicted binding sites for miR-527) to exert the same effect (Table 3) [105]. CircGLIS3 targets miR-644a directly, negatively regulating its expression and promoting cell proliferation (Table 3) [106]. Similarly, circ-100565 regulates the miR-506-3p/HMGA2 axis to enhance NSCLC cell proliferation (Table 3) [107].

Impact on metabolic regulation

Many studies have found that circRNAs can also indirectly regulate the expression of downstream proteins by binding to specific miRNAs, thereby promoting tumor cell growth and metastasis. Mitogen-activated protein kinase kinase 3 (MAP3K3) is a critical regulator in the MAPK signaling pathway, playing an essential role in mediating cellular responses to various stimuli. Specifically, MAP3K3 activates downstream MAP2Ks, which in turn phosphorylate and activate MAPKs such as JNK, p38, and ERK [108]. These MAPKs are well-known for their roles in regulating cell survival, proliferation, and stress responses. Thus, MAP3K3 indirectly supports cell survival and proliferation by activating these MAPK signaling cascades. For example, in ovarian cancer, lncRNA CTBP1-DT promotes the growth and progression of high-grade serous ovarian carcinoma (HGSOC) both in vitro and in vivo by competitively binding to miR-188-5p, thereby protecting MAP3K3 from degradation [109]. This suggests that MAP3K3 plays a potential role in supporting tumorigenesis. Similarly, Circ_0006988 regulates the miR-491-5p/MAP3K3 pathway, sponging miR-491-5p to promote angiogenesis and facilitate cancer cell metastasis (Table 3) [110]. Glutamic-oxaloacetic transaminase 2 (GOT2) is a key enzyme in tumor cell amino acid metabolism associated with NSCLC progression. During this process, circ_0003028 sponges miR-1298-5p, upregulating GOT2 expression to enhance angiogenesis and promote malignancy (Table 3) [111]. CircSATB2, a 1004nt circular RNA, involves the SATB2 gene cluster associated with normal neurodevelopment but lacks protein-coding ability. Studies indicate that FSCN1 correlates with tumor cell proliferation, invasion, and migration. CircSATB2 binds directly to miR-326 via exosomes (natural carriers in vivo), downregulating miR-326 expression. This dysregulates FSCN1 expression, elevating intracellular FSCN1 levels and promoting tumor progression towards unfavorable outcomes (Table 3). Additionally, circSATB2 induces abnormal proliferation of normal epithelial cells, contributing to cancer initiation [112]. miR-4491 has been identified as a tumor suppressor in NSCLC. Animal experiments have shown that the depletion of circ-BIRC6 inhibits tumor growth in xenograft mouse models. Circ-BIRC6 acts as a sponge for miR-4491, suppressing its expression and promoting cancer cell proliferation. Knockdown of circ-BIRC6 also inhibits Wnt2B/β-catenin expression, thus promoting tumor growth (Table 3). Therefore, circ-BIRC6 is proposed as a novel target for lung cancer research [113]. Thrombospondin-2 (THBS2) is a secreted protein known to be highly expressed in cervical cancer [114], colorectal cancer [115], and NSCLC [116]. In NSCLC, THBS2 acts as a downstream factor of miR-590-5p, promoting tumor cell proliferation and inhibiting apoptosis. Circ-0020123 functions as a sponge for miR-590-5p; its overexpression regulates the miR-590-5p/THBS2 axis to prevent cell death (Table 3) [117]. Similarly, circSOX13 operates through competitive binding to miR-3194-3p, mediating the expression of microtubule-associated protein RP/EB family member 1 and exerting its oncogenic role (Table 3) [118]. Nuclear protein subunit alpha-4 (KPNA4) has been identified as a direct target of miR-33a-5p; circCCDC66 has been implicated in colon cancer (Table 3) [119] and is upregulated in NSCLC. In NSCLC, circCCDC66 upregulation acts as a sponge for miR-33a-5p, enhancing KPNA4 expression to inhibit cancer cell apoptosis and accelerate the malignant progression of NSCLC cells (Table 3) [120]. Tumor cells proliferate and differentiate continuously, leading to central hypoxia in tumor tissues due to rapid cell division and inadequate blood supply. Under these conditions, tumor cells derive energy primarily from glycolysis. PKM2 is a rate-limiting enzyme in glycolysis that promotes cancer progression by acting as a coactivator of HIF-1α and regulating metabolic reprogramming in NSCLC cells. CircSHKBP1 is upregulated in NSCLC and promotes PKM2 expression by sponging miR-1294 to suppress its expression, thereby facilitating malignant tumor cell progression (Table 3) [121]. CircZNF609, a protein-coding ncRNA, in lung cancer adsorbs miR-142-3p and regulates the miR-142-3p/GNB2 axis (GNB2, full name G protein subunit beta 2), thereby promoting tumor cell proliferation and migration (Table 3) [122]. Furthermore, studies have reported that two endogenous circRNAs, hsa_circ_0001666 and hsa_circ_0003222, also play significant roles in promoting cancer cell proliferation. The former sponges miR-1184 and miR-548I to alleviate miRNA suppression of the target gene AGO1 (also known as EIF2C1, a member of the AGO protein family) (Table 3). It also activates the phosphatidylinositol 3-kinase (PI3K)/AKT/mTOR signaling pathway to promote cell invasion [123].

Promoting immune evasion

Tumor immune escape refers to the phenomenon where tumor cells evade recognition and attack by the immune system through various mechanisms, allowing for unlimited growth and metastasis. It is a crucial strategy for tumor survival and development [124]. Unlike metastasis mechanisms, circRNAs help tumors escape immune surveillance by regulating immune cell functions and the expression of immune checkpoints. The programmed death ligand-1/programmed death-1 (PD-L1/PD-1) signaling pathway plays a significant role in tumor immune suppression by inhibiting T lymphocyte activation and enhancing tumor cell tolerance, thus facilitating tumor immune escape. Circ-0010235 is overexpressed in lung cancer and promotes cancer cell proliferation, invasion, and immune escape. MiR-636 is a target of circ-0010235, and PD-L1 is a direct target of miR-636. Mechanistically, circ-0010235 promotes lung cancer development and immune escape by regulating the miR-636/PD-L1 axis (Table 3) [125]. Fibroblast growth factor receptor 1 (FGFR1) is an oncogene known to participate in multiple mechanisms that promote NSCLC cell migration, invasion, and immune escape. CircFGFR1 directly binds to miR-381-3p and upregulates its expression, promoting the expression of miR-381-3p's target gene, C-X-C motif chemokine receptor 4 (CXCR4), thereby exerting oncogenic effects (Table 3) [126]. CXCL3 is known to be an oncogene in various cancers. The circMET oncogene mediates lung cancer cell motility, invasion, and metastasis. CircMET is widely expressed in lung cancer cells and acts as an oncogenic initiator. Mechanistically, circMET sponges miR-145-5p, leading to upregulation of CXCL3 expression, which could be a potential novel target for lung cancer therapy (Table 3) [127]. Examples abound of circRNAs acting as miRNA sponges. For instance, circ-001687 enhances zeb1 expression by sequestering miR-326, inducing PD-1/PD-L1 pathway-dependent immune evasion, and promoting apoptosis of CD8 + T cells [128]. Circ-0000284 adsorbs miR-377 to upregulate PD-L1 expression, facilitating cancer cell infiltration and invasion (Table 3) [129].

As tumor suppressors

In circRNAs, many also play roles in inhibiting tumor growth, proliferation, migration, and infiltration. For example, splicing factor 3a subunit 3 (SF3A3) regulates the selective splicing of MDM4 (an important p53 regulator). Research has shown that SF3A3 downregulation can inhibit malignancy in NSCLC cells by activating the p53 signaling pathway. Upstream in this process, circSCAP plays a regulatory role. Mechanistically, circSCAP directly binds to SF3A3, inducing its ubiquitination and degradation, thereby activating the downstream p53 signaling pathway to promote cell apoptosis and delay cancer progression (Table 3) [130].

Others

In addition to the aforementioned mechanisms, circRNAs can also bind to proteins and interact to regulate processes related to lung cancer metastasis. For example, Specificity protein 1 (SP1) is a transcription factor involved in cancer cell proliferation and metastasis. In lung cancer, circ-0001875 inhibits miR-31-5P expression and acts as a sponge to bind miR-31-5p in its seed region, thereby upregulating SP1 levels (Table 3). EMT is a critical step in cancer cell metastasis, with E-cadherin, N-cadherin, and p-Smad2 serving as markers of EMT. Overexpression of circ-0001875 and SP1 can decrease E-cadherin and increase N-cadherin and p-Smad2 levels. From these findings, it is evident that the circ-0001875/miR-31-5p/SP1 axis regulates the EMT process through the TGFβ/Smad2 signaling pathway. If tumor cells grow too quickly for new blood vessels to keep pace, tumor cell hypoxia can result. Concurrently, HIF-1α levels increase due to hypoxia, and elevated HIF-1α binds with SP1 to promote circ-0001875 expression thus can regulate the malignant behavior of lung cancer cells [131]. CircRNAs regulate a wide range of pathways. In addition to promoting angiogenesis and supporting immune evasion as mentioned above, they also include metabolic reprogramming. For example, circMTO1 promotes glycolysis and lactate accumulation in tumor cells by regulating the PI3K/Akt signaling pathway, supporting the metabolic adaptability and growth of tumors [132].

Clinical significance

As previously mentioned, miRNA binding to target genes is crucial for regulating lung cancer progression and metastasis. For example, miR-21 can promote the growth and migration of lung cancer cells as an oncogene. It has been proven that miR-21 inhibitors can suppress the development of lung cancer [133]. In contrast, miR-34a is one of the most common miRNAs with antitumor effects, inducing cell cycle arrest and apoptosis [134]. The development of miR-21 inhibitors or miR-34a mimics has demonstrated the feasibility of miRNA-based therapies, highlighting the potential of translating basic research into clinical applications for lung cancer treatment.

Currently, ncRNAs have been shown to be involved in various biological processes of cells, and in recent years, they have been proposed as therapeutic and early diagnosis targets for lung cancer. miR-20b can downregulate the expression of adenomatous polyposis coli (APC), which is a key negative regulator of the Wnt/β-catenin signaling pathway. miR-20b is upregulated in NSCLC, leading to the activation of the Wnt/β-catenin signaling pathway and promoting cell proliferation. Therefore, inhibiting the expression of miR-20b can help reverse this process (Table 1) [135]. Circ-CCS enhances the expression of E2F7 by sponging miR-383, thereby promoting the growth and metastasis of lung cancer cells and inhibiting apoptosis to facilitate lung cancer progression (Table 3) [136]. Additionally, existing research has demonstrated that ncRNAs target a broad spectrum of genes involved in different carcinogenic pathways, making them useful for combating drug resistance caused by chemotherapy, radiotherapy, and other corresponding treatments. EDU experimental results indicate that knocking out circSOX13 inhibits DNA replication in A549/DDP and H1299/DDP cells.Furthermore, knockout of circSOX13 significantly reduces migration and invasion of A549/DDP and H1299/DDP cells while promoting apoptosis. These results indicate that circSOX13 promotes platinum resistance in NSCLC [118]. In erlotinib-resistant NSCLC, the expression of miR-34a is lower, while the levels of Axl, Gas6, and their downstream signaling proteins are significantly higher in PC9-Gef and HCC827-Gef cells compared to their parental cells. This suggests that miR-34a could serve as a prognostic marker or a potential therapeutic target for NSCLC [137]. Deletion of CricVMP1 inhibits DDP resistance and the proliferation, sphere formation, migration, and invasion of NSCLC cells, and promotes apoptosis in DDP-resistant NSCLC cells [104]. Recent studies have revealed that miRNAs, including miR-21 and miR-155, significantly contribute to immune escape in NSCLC. These miRNAs modulate immune checkpoint molecules like PD-L1, and suppress anti-tumor immune responses by influencing T cell activity and tumor-associated macrophages (TAMs), thereby enhancing immune evasion. This also provides potential for immunotherapy. Currently, the delivery methods for miRNA therapy include the use of nanoparticle-based delivery systems and systemic administration [138]. However, since miRNAs are involved in various physiological and pathological processes, their alteration may lead to some unforeseen side effects, such as inflammatory responses, changes in immune tolerance, cell damage, organ toxicity, and off-target effects. Off-target effects typically occur because miRNA binding is not completely specific; they may interact with multiple non-target genes, resulting in undesirable side effects [139]. Therefore, the design of personalized treatment plans is of great importance in reducing side effects and off-target effects. It is well known that smoking is closely associated with lung cancer, and experimental studies have shown that cigarette smoke extract (CSE) reduces miR-217 levels and increases lncRNA MALA1 levels in human bronchial epithelial cells undergoing EMT. Exogenous administration of miR-217 mimics significantly inhibits CSE-induced expression of lncRNA MALA1 [140]. lncRNAs have been demonstrated as regulators of gene expression, playing crucial roles in regulating EMT in lung cancer cells. For instance, LINC01089 is significantly downregulated in NSCLC tissues. Mechanistically, LINC01089 acts as a sponge for miR-27a, regulating its expression in NSCLC. Similarly, LINC inhibits Wnt/β-catenin-EMT pathways and upregulates SFRP1 expression by sequestering miR-27a to suppress EMT in NSCLC [141]. lncRNA-LINP1 is underexpressed in lung cancer cells, and it prevents further cancer progression by inhibiting the EMT process. TGF-β, a common cytokine, induces EMT in lung cancer cells and is a critical factor in tumor cell migration, invasion, and acquisition of stemness. TGF-β regulates the levels of lncRNA-LINP1 to alter its inhibition of EMT [142]. Lately, targeting ncRNAs have emerged as a very promising tool for the treatment of various diseases including several types of cancer, infections, cardiovascular, and respiratory diseases [143]. LINC00969 inhibits pyroptosis in lung cancer by epigenetically repressing NLRP3 at the transcriptional and post-transcriptional levels, thereby promoting acquired erlotinib resistance [144]. It holds potential as a novel biomarker and therapeutic target. However, research in this area remains limited and requires further attention.

Currently, the primary method for diagnosing tumors is through primary tumor biopsies. However, research suggests that primary tumor biopsies may not reflect the heterogeneity of the disease [145]. To overcome these challenges, the concept of "liquid biopsy" has been proposed. This involves detecting circulating tumor cells, circulating tumor DNA, cell-free ncRNAs (primarily miRNAs), tumor-induced platelets, extracellular vesicles, etc., in the blood to determine whether a tumor has occurred or if it has infiltrated or metastasized [146]. The dysregulation of ncRNAs is associated with various biological processes, and they have been proposed as biomarkers for early screening, early diagnosis, and prognosis. For example, the upregulation of lncRNA DUBR in LUAD increases the expression of ZBTB11, which inhibits cancer cell invasion and migration by attenuating cellular oxidative phosphorylation [147]. The expression patterns of ncRNAs in the serum of cancer patients vary, and ncRNAs are more stable in serum, plasma, and FFPE-preserved samples compared to mRNA. Additionally, ncRNAs are proposed for early screening and diagnosis of lung cancer due to their potential to predict cancer risk in high-risk populations and relatively lower experimental costs [148]. lncRNA UCC is highly expressed in NSCLC tissues and cell lines, suggesting that UCC may be a potential novel oncogenic lncRNA or a new early diagnostic biomarker for NSCLC (Table 2) [149]. Similarly, studies have shown that lncRNA SNHG17 promotes NSCLC cell proliferation, migration, invasion, and EMT by regulating the miR449a/TGIF2 axis. SNHG17 is identified as a novel oncogenic lncRNA and can also be used for early diagnosis of lung cancer (Table 2) [150]. Additionally, due to the distinct metabolism of lung cancer cells compared to normal cells, the levels of ncRNAs in the body undergo changes, making them proposed for monitoring lung cancer treatment effectiveness. miR-196a is overexpressed in lung cancer and promotes migration and invasion by downregulating ANXA1 expression and increasing CCL2 secretion from CAFs. Monitoring miR-196a levels can provide insights into the progression of lung cancer (Table 1) [151].

Exosomes play a significant role in monitoring tumor cell progression. Exosomes are small extracellular vesicles with a diameter of 30–150 nm [152], which evolve into multivesicular bodies through late endosomal budding [153]. During this process, bioactive factors such as DNA, RNA, and proteins are encapsulated within them [154]. RNA, notably including ncRNAs, is the predominant substance within exosomes, capable of reflecting tumor progression and dynamic processes of tumor cells. Exosomes are released into the external environment and enter the peripheral bloodstream, where their protective nature ensures that ncRNAs are not degraded, thereby maintaining blood flow stability. Consequently, circulating exosomal RNA is emerging as a hotspot for early cancer monitoring and patient prognosis assessment. For instance, lncRNA RP5-977B1 is upregulated in NSCLC and is contained within exosomes, enhancing its stability. This makes it a novel minimally invasive biomarker for the diagnosis and prognosis of NSCLC [155]. In NSCLC patients, exosomal miR-34c-3p is significantly underexpressed. Exosomes transport low levels of miR-34c-3p into the cytoplasm, where it upregulates integrin α2β1 levels, promoting invasion and migration of NSCLC cells. Based on this, miR-34c-3p serves as a diagnostic and prognostic biomarker for NSCLC [156]. CircRNAs are considered crucial regulatory factors in various cancers. CircDNER can be transported into the cytoplasm via exosomes to exert its biological functions. Mechanistically, circDNER acts as a sponge for miR-139-5p, thereby upregulating ITGBB expression. Overexpression of miR-139-5p reverses the malignant behaviors mediated by circDNER-induced PTX-resistant cells, whereas increased ITGBB promotes these behaviors. Monitoring exosomal circDNER provides insights into cancer progression [157]. Similarly, miR-3180-3p is significantly reduced in NSCLC cell lines and patient tissues. Treatment with exosomal miR-3180-3p significantly inhibits the proliferation and migration of NSCLC cells, which correlates with levels of the FOXP4 protein. Mechanistically, exosomal miR-3180-3p suppresses NSCLC proliferation and migration by downregulating FOXP4 [158]. In summary, traditional methods, including tissue biopsy, circulating tumor cells, and cell-free DNA detection, exhibit limitations such as invasiveness, sample heterogeneity, low sensitivity, low specificity, and instability. In comparison, circulating exosomal RNA detection offers unique advantages. Therefore, studying exosomal vesicles in vivo for tumor monitoring is an emerging and promising field for the future.

Prospects and challenges

According to research, lung cancer is one of the leading causes of death worldwide, with distant metastasis being the primary cause of mortality. Due to the lack of effective diagnostic tools, many lung cancer patients are diagnosed at advanced stages, contributing to high incidence and mortality rates. Advances in biotechnologies such as bioinformatics analysis, high-throughput sequencing, genome editing, mouse models, and drug chemistry have gradually revealed the functions and properties of ncRNAs. Studies have shown that ncRNAs participate in various biological processes of lung cancer metastasis, including cell proliferation, apoptosis, angiogenesis, and invasion. This review discusses the specific nature and functions of ncRNAs, along with the latest research progress on their involvement in lung cancer metastasis. However, there is still limited and relatively shallow research on ncRNAs in lung cancer. Therefore, further understanding of the specific regulatory mechanisms of ncRNAs and their related signaling pathways in lung cancer could provide insights into the mechanisms underlying lung cancer metastasis progression.

ncRNAs have intricate relationships with lung cancer in many aspects, which should be further investigated in the future. This research not only contributes to a better understanding of ncRNA biology but also helps elucidate the relevance of ncRNAs to lung cancer. piRNAs are a class of non-coding small RNAs widely expressed in germ cells, interacting with PIWI proteins. Mechanistically, piRNAs regulate SPOCD1 expression, participate in DNA methylation of transposon elements, and also have the ability to promote gene DNA methylation encoding transposition-related proteins [159]. Additionally, numerous studies have found close associations between piRNAs and tumor cells [160]. Bone remodeling is a dynamic process between osteoblast-mediated bone formation and osteoclast-mediated bone resorption, crucial for postmenopausal osteoporosis. In animal experiments, piRNA-63049 expression significantly increased in postmenopausal rats, and knocking out piRNA-63049 promoted osteogenesis in bone marrow stromal cells through the Wnt2b/β-catenin signaling pathway. Administering a piRNA-63049 antagonist to ovariectomized rats alleviated bone loss by enhancing bone formation [161]. piRNAs' relationship with tumor cells manifests in regulating tumor cell metastasis. For instance, in cervical cancer tissues and cells, high expression of piRNA-14633 promotes cancer cell proliferation, invasion, and migration. Additionally, elevated expression of piRNA-14633 enhances m6A RNA methylation levels and stabilizes mRNA of methyltransferase-like protein 14 [162]. Diffuse large B-cell lymphoma (DLBCL) is a subtype of the most common malignant lymphoma. piRNAs have been shown to be epigenetic effectors, and in DLBCL, high expression of piRNA-30473 supports an aggressive cancer cell phenotype. Suppressing piRNA-30473 expression inhibits cancer cell proliferation, induces cell cycle arrest, and thereby regulates tumor proliferation, invasion, and migration [163]. In summary, the role of ncRNAs in lung cancer metastasis is worthy of investigation.

Despite the exciting progress made in ncRNA research mentioned above, several challenges persist in their application to lung cancer research: (1) Due to variations in the length and patterns of ncRNAs, and the complex roles of ncRNA modifiers in different tumors, it is challenging to select specific targets from numerous candidate targets for study. Consequently, the specific mechanisms underlying certain biological processes in lung cancer cells remain unclear. For instance, while it is known that ncRNAs can regulate angiogenesis, there is limited literature explaining the specific mechanisms driving this process. Therefore, research on using ncRNAs for targeted therapy in lung cancer is still in its early stages. (2) The heterogeneity of the TME complicates the communication of information between cancer tissues and normal tissues, as well as the application of ncRNAs in clinical cancer treatment. Issues such as low transfection efficiency, off-target effects, RNA instability, and short half-lives due to degradation further hinder their application. Additionally, ncRNAs can be regulated differently among immune cells, tumor cells, and their cancerous cells. Currently, various combinations of carriers are being used to address these challenges, with existing carriers mainly including nanoparticles, ncRNA modifications, and oncolytic adenoviruses.(3) ncRNAs can target hundreds of mRNAs, thus ncRNA-induced therapies inevitably pose almost unavoidable and unpredictable side effects.(4) There is a lack of repeatability between studies. Differences in technologies, such as RNA purification procedures, measurement platforms, and statistical methods, can lead to variability between research results, making replication difficult. Therefore, research in this area is far from complete and requires further exploration.

This review comprehensively reviews recent advances in ncRNA research related to lung cancer metastasis, providing a brief understanding of the specific mechanisms and pathways of ncRNAs in lung cancer metastasis. It underscores the significant clinical importance of ncRNAs in the context of lung cancer applications.

Conclusion

There is a new understanding of the functions and roles of ncRNAs in the process of lung cancer metastasis. In this review, we summarize the roles, specific mechanisms of different types of ncRNAs in the metastasis of lung cancer. This paper highlights that ncRNAs are involved in many physiological and pathological processes, particularly in the development and progression of tumors. This indicates that ncRNAs have intricate relationships with lung cancer, especially in the context of distant metastasis, and also opens new directions for the treatment, early screening, and diagnosis of lung cancer.

Abbreviations

SCLC

Small cell lung cancer

NSCLC

Non-small cell lung cancer

LUAD

Lung adenocarcinoma

LUSC

Lung squamous cell carcinoma

EMT

Epithelial-mesenchymal transition

TME

Tumor microenvironment

lncRNAs

Long non-coding RNAs

miRNAs

MicroRNAs

circRNAs

Circular RNAs

piRNAs

Piwi-interacting RNAs

CLL

Chronic lymphocytic leukemia

FSCN1

Fascin actin-bundling protein 1

STAT3

Signal transducer and Activator of transcription 3

TGFBR1

Transforming growth factor beta receptor 1

DCP2

Decapping enzyme 2

NIPBL

Nipped-B-like protein

HIF-1α

Hypoxia-inducible factor-1α

USF1

Upstream stimulating factor 1

CAFs

Cancer-associated fibroblasts

lincRNAs

Long intergenic non-protein coding RNAs

NAT

Natural antisense transcript

ceRNAs

Competitive endogenous RNAs

NNT-AS1

Nicotinamide nucleotide transhydrogenase antisense RNA 1

FEZF1-AS1

FEZ family zinc finger-1 antisense RNA 1

OGFRP1

Opioid growth factor receptor pseudogene 1

SNHG18

Small nucleolar RNA host gene 18

GOT2

Glutamic-oxaloacetic transaminase 2

THBS2

Thrombospondin-2

KPNA4

Nuclear protein subunit alpha-4

PD-L1/PD-1

The programmed death ligand-1/programmed death-1

SF3A3

Splicing factor 3a subunit 3

SP1

Specificity protein 1

CSE

Cigarette smoke extract

DLBCL

Diffuse large B-cell lymphoma

MAP3K3

Mitogen-activated protein kinase kinase 3

APC

Adenomatous polyposis coli

Author contributions

K.J. conceived the idea; J.Y. and Y.L. performed the literature search and drafted the manuscript. Besides, J.Y. and Y.L. drew the figures in the article.; Z.Y. and Z.W. revised the manuscript; The author(s) read and approved the final manuscript.

Funding

Not applicable.

Data availability 

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

No ethics approval was required for this review that did not involve patients or patient data.

Consent for publication

All authors consent to publication.

Competing interests

The authors declare no competing interests.