Long non-coding RNA MALAT1: A crucial factor in fibrotic diseases.

Conceptualization, L.R; writing—original draft preparation, L.R., W.P., and J.F.; writing—review and editing, L.T. and J.F.; funding acquisition, L.T and J.F. All authors have read and agreed to the published version of the manuscript.

Fibrotic diseases, characterized by dysregulated ECM deposition and tissue scarring in various organs, such as the liver, lungs, kidneys, and heart, pose a significant global health challenge. MALAT1 is a key molecular driver of fibrosis across multiple organ systems, modulating various cellular processes, such as EMT, fibroblast activation, and inflammatory signaling. This section describes the known roles of MALAT1 in fibrosis, highlighting its conserved action mechanisms, including miRNA sponging, epigenetic regulation, signaling pathway modulation, and therapeutic potential. Fibrosis is the ultimate pathological outcome of several chronic inflammatory diseases. It is marked by the excessive accumulation of fibrous connective tissues, including ECM components, such as collagen and fibronectin. This build-up occurs in and around inflamed and damaged tissues, potentially leading to permanent scarring, organ dysfunction, and even death. 52 , 53 Fibrosis is mainly characterized by the abnormal formation and remodeling of fibrous connective tissue in response to injury. It is associated with many organ injuries and failures (e.g., cirrhosis, pulmonary fibrosis [PF], and cardiac fibrosis) and responsible for 45% of deaths in industrialized countries. 54 Over one-third of the annual mortality rate in industrialized countries is due to fibrotic diseases of various organs, including the heart, blood vessels, lungs, kidneys, and liver. 55 The fibrotic responses consist of (1) responses triggered by a primary organ lesion, (2) activation of effector cells (fibroblasts) induced by inflammatory mediators, and (3) a constitutive phase of the ECM. 56 Acute inflammatory responses play a significant role in the initiation of fibrosis in several organ systems. For example, in bleomycin-induced PF and carbon-tetrachloride-induced hepatic fibrosis, transient exposure to drugs induces apoptosis of epithelial cells and necrosis of hepatocytes, respectively, and activates an inflammatory wound-healing response that leads to the transient overexpression of ECM components in the affected tissues. 57 , 58 Fibrosis significantly disrupts the normal physiological functions of organs. Onset and progression of hepatic fibrosis results in the destruction of the liver structure due to fibrous scarring, which further causes the loss of liver cells and liver dysfunction, ultimately leading to liver failure. 55 PF progression causes significant lung impairment, even death in some cases. 59 Currently, no specific drugs are available for fibrotic disease treatment. TGF-β1 is a master regulator of fibrosis, driving ECM accumulation via Smad-dependent and non-Smad pathways. The canonical Smad pathway involves TGF-β1 binding to its receptor and activating the phosphorylation of Smad2/3, which complexes with Smad4 to transcribe the pro-fibrotic genes (e.g., collagen I/III). 60 Non-Smad pathways, including the mitogen-activated protein kinase and phosphoinositide 3-kinase/AKT pathways, synergistically enhance fibroblast activation and ECM synthesis. 61 TGF-β1 also promotes myofibroblast differentiation, exacerbating tissue remodeling and fibrosis in organs, such as the liver, lungs, and kidneys. Dysregulation of these pathways underlies pathological fibrogenesis. 62 LncRNAs play regulatory roles in the expression of genes and proteins during liver fibrosis progression. Overexpression of specific genes, including H19 , maternally expressed gene 3, growth-arrest-specific transcript 5, Gm5091 , NR_002155.1 , and hypoxia-inducible factor (HIF)-1α-antisense RNA 1, inhibits liver fibrosis progression. 63 In contrast, upregulation of the levels of nuclear paraspeckle assembly transcript 1, HOX transcript antisense RNA, and liver-enriched fibrosis-associated lncRNA 1 facilitates liver fibrosis. EB1-antisense RNA 1 exacerbates PF by modulating miR-141-3p expression. 64 Long intergenic ncRNA 1700020I14Rik targets the miR-34a-5p/sirtuin (Sirt)-1/HIF-1α axis, thereby inhibiting cell proliferation and fibrosis in diabetic nephropathy (DN). 65 Growth-arrest-specific transcript 5 regulates cardiac fibroblast activity and myocardial fibrosis by targeting the miR-21/phosphatase and tensin homolog/matrix metalloproteinase-2 axis. 66 MALAT1 plays crucial regulatory roles in the pathogenesis of fibrosis. The following sections outline the roles of MALAT1 in organ fibrosis. Liver exhibits great regeneration capacity; however, persistent damage leads to fibrosis, hardening, and even hepatocellular carcinoma. 67 Liver fibrosis is a response to chronic liver injury caused by various factors, such as alcohol consumption, non-alcoholic steatohepatitis (NASH), viral hepatitis (hepatitis B [hepatitis B virus] and hepatitis C), autoimmune hepatitis, non-alcoholic fatty liver disease, and cholestatic liver disease. Hepatic stellate cells (HSCs) comprise 5%–8% of hepatocytes and are essential for hepatocyte regeneration and liver fibrosis development. 68 , 69 Recently, several studies have demonstrated the link between lncRNAs and development of liver fibrosis and highlighted the potential value of lncRNAs in the diagnosis, prognosis, and treatment of liver fibrosis. 70 Wang et al. revealed enhanced MALAT1 expression co-localized with increased expression levels of fibrosis markers (type I collagen and alpha-smooth muscle actin [α-SMA]) and Wnt/β-linker signaling proteins (β-linker, cytosolic protein D1, and c-myc) in fibrotic liver tissues. Similar to TGF-β1, extracellular vesicles (EVs) isolated from the plasma of patients with hepatic fibrosis and high MALAT1 levels upregulated the expression levels of fibrotic markers and enhanced the Wnt/β-catenin signaling pathway in HSCs. When MALAT1 expression was blocked by MALAT1 -specific siRNA, the effects of both EVs and TGF-β1 on HSCs were abolished. This indicates that MALAT1 within EVs is crucial for mediating the pro-fibrotic effects of EVs on HSCs, and its function is parallel to that of TGF-β1 in promoting HSC activation and fibrosis-related signaling. 71 Reduced MALAT1 expression results in the deacetylation of Smad3 and subsequent inactivation of HSCs. 72 During HSC activation, TGF-β1 initiates a signaling pathway through Smad proteins, leading to collagen production. Yu et al. discovered that, in a carbon-tetrachloride-induced mouse model of hepatic fibrosis, activated HSCs significantly upregulate the levels of MALAT1 and Ras-associated C3 botulinum toxin substrate 1 (Rac1), a target of miR-101b , with MALAT1 acting as ceRNA to enhance the expression of Rac1, thereby promoting HSC proliferation and activation. 73 Dai et al. extracted MALAT1 from human hepatocytes treated with arsenite and revealed that MALAT1 binds to miRNA-26b . This interaction promotes the activation of LX-2 cells, a key event facilitating the progression of liver fibrosis. 74 Researchers have reported that MALAT1 adsorbs miR-181a in myeloma cells. 75 Moreover, miR-181a inhibits hepatic fibrosis progression by rescuing the aberrant inflammatory cascade and preventing HSC hyperproliferation via inhibition of TLR4/NF-κB signaling. 76 , 77 Wang et al. knocked down or overexpressed MALAT1 and inhibitors and mimics of miR-181a-5p to assess the effects of MALAT1 and miR-181a-5p on LX-2 cell activation and inflammation. They confirmed that TGF-β1-induced increase in viability, proliferation, migration, adhesion, and collagen production of activated LX-2 cells and lipopolysaccharide (LPS)-induced inflammation were rescued after MALAT1 knockdown or miR-181a-5p mimic transfection. 78 SIRT1 contributes to apoptosis and reverses activated LX-2 cells. SIRT1 protein levels are significantly reduced in the fibrotic livers of mice treated with carbon tetrachloride. TGF-β1-induced activation of LX-2 cells significantly inhibits SIRT1 protein expression. However, RNA interference (RNAi)-mediated inhibition of MALAT1 reduces the myofibroblast marker levels and restores the SIRT1 protein levels. 79 NASH is a severe clinical manifestation of non-alcoholic fatty liver disease (NAFLD) with marked hepatic inflammation and liver fibrosis. 80 Studies on MALAT1 in patients with NASH with lobular inflammation and advanced fibrosis have shed light on its differential expression in diseased liver tissues. Functional enrichment analysis implicates MALAT1 in various signaling pathways, including the TGF-β1, TNF, insulin resistance, and ECM maintenance pathways. MALAT1 is highly expressed in fibrotic liver tissues and associated with the chemokine C-X-C motif chemokine ligand 5 (CXCL5). MALAT1 is possibly involved in hepatic fibrosis via mechanisms involving inflammatory chemokines. 81 LncRNA MALAT1 plays a driving role in liver fibrosis ( Table 1 ); therefore, targeting MALAT1 and reducing its expression are promising therapeutic strategies for liver fibrosis. Table 1 Role of MALAT1 in liver fibrosis Model Expression Function Role Reference Liver fibrosis Up MALAT1 ↑→Wnt/β-catenin↑, α-SMA↑, Col1α1↑→HSCs activate↑ facilitator of fibrotic processes Wang et al. 71 ; Jiang et al. 72 Up MALAT1 ↑→miR-101b↓→Rac1↑→HSCs activate↑proliferative↑ facilitator of fibrotic processes Yu et al. 73 Up MALAT1 binding to miRNA-26b→the activation of LX-2 facilitator of fibrotic processes Dai et al. 74 Up MALAT1 ↑→miR-181a↓→TLR4/NF-κB signaling↑→HSCs proliferative↑ facilitator of fibrotic processes Jiang et al. 76 Up MALAT1 ↑, miR-181a-5p↑→LX -2 viability↑, proliferation↑, migration↑, adhesion↑, collagen production↑ facilitator of fibrotic processes Wang et al. 78 Up MALAT1 ↑→SIRT1↓→HSCs activate↑ facilitator of fibrotic processes Wu et al. 79 Role of MALAT1 in liver fibrosis PF is a fatal lung disease pathologically characterized by the damage and abnormal proliferation of alveolar epithelial cells, ECM deposition, and proliferation and activation of fibroblasts. 82 PF, often referred to as a “tumor-like disease,” exhibits a high mortality rate and averages a survival period of 2.8 years. 59 Several diseases cause PF. The most prevalent form of PF is idiopathic PF. Idiopathic PF represents a progressive chronic fibrotic lung disease. Replacement of healthy tissues with an altered ECM and disruption of the alveolar structure decrease the lung compliance and disrupt gas exchange, ultimately causing respiratory failure and even death. 83 EMT is the process by which epithelial cells lose intercellular contact and transform into mesenchymal cells. Several transcription factors, including Snail, Zeb, and Twist, regulate EMT, which plays a key role in lung fibrosis. 84 Silicosis is an incurable and irreversible interstitial pulmonary fibrotic disease. It is typically caused by occupational exposure to silica dust. Yan et al. validated the reduction in miR-503 levels in fibrotic mouse lung tissues, human bronchial epithelial cells, and A549 human lung adenocarcinoma cells following silica exposure. Upregulated MALAT1 functioned as a ceRNA by directly binding to miR-503 , decreasing miR-503 expression and counteracting its role in inhibiting silica-induced EMT in PF. 85 Macrophages play a crucial role in PF development. MALAT1 levels are upregulated in pro-fibrotic alveolar macrophages treated with LPS and downregulated in cells treated with IL-4. Moreover, MALAT1 knockdown attenuates the LPS-induced M1 macrophage activation but enhances the differentiation and pro-fibrotic phenotype of IL-4-activated M2 macrophages. 86 MALAT1 exerts predominantly negative regulatory effects on PF, as indicated in Table 2 . Inhalation-based siRNA delivery is a promising strategy to treat various lung-injury diseases, including PF. 94 Further exploration of MALAT1 regulatory mechanisms can reveal novel therapeutic targets for PF. Table 2 Role of MALAT1 in other fibrotic diseases Model Expression Function Role Reference Pulmonary fibrosis up MALAT1 ↑→miR-503↓→cell EMT↑ facilitator of fibrotic processes Yan et al. 85 up MALAT1 ↑ in LPS-induced macrophages→M1 Macrophages activate↑→cell fibrosis↑ facilitator of fibrotic processes Cui et al. 86 down MALAT1 ↓ in IL-4-induced macrophages→M2 Macrophages activate↑→cell fibrosis↑ Inhibitors of the fibrotic process Renal fibrosis up MALAT1 ↑→miR-145/FAK pathway↑→cell fibrosis↑ facilitator of fibrotic processes Liu et al. 87 Renal interstitial fibrosis up MALAT1 ↑→ miR-124-3p↓→ITGB1↑, E-cad↓→RIF↑ facilitator of fibrotic processes Xia et al. 88 Diabetic nephropathy fibrosis up MALAT1 ↑→miR-145↓→ZEB2↓→cell EMT, fibrosis↑ facilitator of fibrotic processes Liu et al. 89 up MALAT1 ↑→miR-2355-3p↓→IL6ST↓→ fibrosis↑, cell damage↑ facilitator of fibrotic processes Huang et al. 90 Myocardial fibrosis up MALAT1 ↑→miR-145↓→TGF-β↑, α-SMA↑→cell proliferative↑ facilitator of fibrotic processes Huang et al. 91 Diabetic cardiomyopathy fibrosis up MALAT1 ↑→miR-141↓→TGF-β1/Smads signaling↑→cell fibrosis↑ facilitator of fibrotic processes Che et al. 92 Endometriosis up MALAT1 ↑→miR-141-3p↓→NLRP3↑, IL-β↑→TGF-β1/Smads signaling↑→cell fibrosis↑ facilitator of fibrotic processes Zhu et al. 93 Role of MALAT1 in other fibrotic diseases Chronic kidney disease affects over 10% of the global population and is a significant risk factor for end-stage kidney disease (ESKD). 95 , 96 Renal interstitial fibrosis (RIF) often acts as the ultimate common pathway for most renal diseases leading to ESKD. 97 , 98 RIF represents a wound-healing response, 99 characterized by EMT, massive fibroblast activation, and ECM deposition in renal tubular epithelial cells, 100 leading to the formation of renal scar tissues and promoting ESKD. 101 Liu et al. reported that m 6 A-induced lncRNA MALAT1 aggravates renal fibrogenesis in obstructive nephropathy via the miR-145/FAK pathway. 87 LncRNA MALAT1 and integrin subunit beta 1 (ITGB1) are significantly overexpressed, whereas miR-124-3p levels are downregulated in TGF-β1-induced HK2/NRK-49F cells and unilateral ureteral obstruction mouse model. However, lncRNA MALAT1 silencing notably reduces the protein levels of ITGB1, a marker associated with fibrosis, and increases those of E-calmodulin. MALAT1 promotes RIF in vitro and in vivo via the miR-124-3p/ITGB1 signaling pathway. 88 DN, a significant complication in individuals with diabetes, is the leading cause of death. Fibrosis is caused by changes in cell cycle and growth during DN and excessive ECM synthesis and accumulation. 102 For the first time, Liu et al. showed that MALAT1 and miR-145 regulate high glucose-induced renal tubal EMT and fibrosis. Mechanistically, MALAT1 acts as a sponge RNA for miR-145 to deregulate the expression of the target gene, zinc finger E-box-binding homeobox 2, and induce EMT and fibrosis. 89 In a previous study, kidney tissues of diabetic rats showed MALAT1 upregulation, and mercury treatment increased cell proliferation and invasion in the treated cells. MALAT1 overexpression also increased the protein levels of collagen types I and IV, fibronectin, and laminin in HK-2 cells. However, MALAT1 knockdown had the opposite effect. MALAT1 also enhances renal fibrosis and HG-induced cellular damage in the HK-2 cells of diabetic rats via the miR-2355-3p/IL6 cytokine family signal transducer axis. 90 QiHuang YiShen granules, a traditional Chinese herbal medicine formula, significantly reduce renal collagen fiber deposition and lncRNA MALAT1 levels, upregulate the foot cell marker protein nephrin levels, and downregulate the levels of junctional proteins, ferroptosis suppressor protein-1, and mesenchymal cell markers, thereby attenuating DN by inhibiting MALAT1 expression. 103 MALAT1 modulates renal fibrosis by adsorbing miRNAs or directly acting on specific signaling pathways ( Table 2 ). Therefore, use of pharmacological synergism and downregulation of MALAT1 are viable treatment options for renal fibrosis in clinical settings, as evidenced by the findings of the above-mentioned studies. Myocardial infarction (MI) is the primary cause of human morbidity and mortality worldwide. 104 MI leads to cardiomyocyte death, followed by cardiac fibrosis. This is characterized by increased cardiac fibroblast activity and ECM production in the myocardium. 105 TGF-β1 and angiotensin II (Ang-II) actively regulate myocardial fibrosis by inducing myofibroblast transdifferentiation and enhancing ECM protein expression. 106 MALAT1 levels are elevated, whereas miR-145 levels are reduced in hearts affected by MI and Ang-II-treated cardiac fibroblasts. However, MALAT1 depletion reverses the inhibition of miR-145 expression. Furthermore, MALAT1 knockdown enhances the cardiac function in MI and inhibits fibroblast proliferation, collagen production, and α-SMA expression in Ang-II-treated cardiac fibroblasts. Overall, MALAT1 promotes myocardial fibrosis and impairs the cardiac function following MI by regulating the TGF-β1 activity via miR-145 . 91 Diabetic cardiomyopathy (DCM) is a common cardiovascular complication of diabetes that is a major contributor to heart failure in people with diabetes. 107 In patients with diabetes mellitus (DM) and animal models of DM, myocardial fibrosis is a prominent pathological process in DCM. 108 , 109 Melatonin attenuates cardiac fibrosis in diabetic mice by inhibiting the NLR family pyrin domain-containing 3 (NLRP3) inflammatory vesicle activation and TGF-β1/Smad signaling mediated by MALAT1 / miR-141 , with MALAT1 playing a key role. 92 si- MALAT1 decreases inflammation and collagen accumulation in cardiomyocytes via the Hippo–YAP pathway and CREB in high-glucose cardiac fibroblasts and DCM mice. 110 MALAT1 levels in mouse skeletal muscle cells decrease significantly with age, and MALAT1 silencing increases the TGF-β1 levels in skeletal muscle cells in vitro and in vivo . 111 Xu et al. indicated that lnc - MALAT1 played pivotal roles in NLRP3-induced episclerosis and fibrosis in endometriosis by sponging miR-141-3p , highlighting it as a novel therapeutic target for endometriosis. 112 MALAT1 knockdown attenuates the fibrotic responses in human endometrial stromal cells via the miR-22-3p/TGFβR1/Smad2/3 pathway. 93 In summary, MALAT1 acts as a multifunctional regulator in fibrotic diseases, driving pathological remodeling across organs through conserved molecular mechanisms, such as miRNA sponging (e.g., miR-181a , miR-503 , and miR-124-3p ) and epigenetic and transcriptional regulation (as shown in Figure 2 ). Additionally, MALAT1 interacts with the TGF-β1 signaling pathway, acting as a molecular sponge for specific miRNAs targeting the TGF-β1 pathway components. By adsorbing these miRNAs, MALAT1 increases the TGF-β1-related gene levels. MALAT1 can upregulate the expression of TGF-β1 or enhance the cellular sensitivity to TGF-β1 stimulation, thereby promoting the activation of the Smad pathway and subsequent fibrotic responses. Its upregulation in hepatic, pulmonary, renal, and cardiac fibrosis promotes fibroblast activation, EMT, and ECM accumulation, and its clinical correlations further underscore its potential as a diagnostic biomarker. Gene therapy aimed at blocking MALAT1 can alleviate fibrosis and restore the normal physiological functions in the affected organs. Figure 2 The roles of MALAT1 in various organ fibrosis processes MALAT1 affects organ fibrosis, including hepatic, pulmonary, renal, and myocardial fibrosis, either by adsorbing miRNAs or by directly acting on signaling pathways. The roles of MALAT1 in various organ fibrosis processes MALAT1 affects organ fibrosis, including hepatic, pulmonary, renal, and myocardial fibrosis, either by adsorbing miRNAs or by directly acting on signaling pathways.

As shown in Tables 1 and 2 , MALAT1 plays critical roles in various physiological and pathological processes, particularly organ fibrosis. The above-discussed studies highlight the diagnostic, prognostic, and therapeutic potentials of this lncRNA. Therefore, further evaluation of MALAT1 detection and targeting strategies is essential for the diagnosis and treatment of fibrotic diseases. Its elevated levels in fibrotic tissues and body fluids make MALAT1 a promising diagnostic biomarker for the early detection and classification of fibrotic diseases. Although traditional molecular bioassays, such as reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and fluorescence in situ hybridization (FISH), can detect MALAT1 in cells and tissues, accurate quantification of this lncRNA at the single-cell level remains challenging, hindering disease diagnosis and treatment. Yang et al. developed a DNAzyme-driven fluorescent light-up aptasensor for the label-free detection of MALAT1 . They developed advanced fluorescent light-up aptasensors for the simultaneous quantification of multiple lncRNAs, including MALAT1 , 113 in living cells and breast tissues through DNAzyme-mediated cleavage reactions and transcription-driven synthesis of light-up aptamers. Through multiple cyclic enzymatic repair amplifications, Zhao et al. introduced a mix-and-read assay for the sensitive and rapid detection of MALAT1 at the single-cell level, offering an efficient solution for breast cancer diagnosis. 114 MALAT1 plays significant roles in the occurrence and progression fibrosis in various tissues, similar to its roles in cancer. It regulates fibroblast activation, ECM component production, and key signaling pathways in fibrosis. Breast cancer typically involves the fibrosis-related phenotype of breast tissue hardening. 115 Based on these studies on MALAT1 detection in breast cancer, suitable methods to detect MALAT1 in fibrotic tissues can be developed using similar principles and techniques. In addition to its direct roles, various modifications, such as methylation, are crucial for MALAT1 functions. Therefore, detection of these modifications shows significant clinical value in disease research. For example, N 6 -methyladenosine (m 6 A) modification of MALAT1 exacerbates renal fibrogenesis in obstructive nephropathy. 87 A TadA8.20-assisted m 6 A RNA imaging single-base resolution method has been developed to precisely visualize and quantify m 6 A modifications at specific RNA sites within MALAT1 in breast cancer cell lines. 116 The clustered regularly interspaced short palindromic repeats (CRISPR) technology has revolutionized genetics and genomics, showing advanced applications beyond gene editing. Particularly, CRISPR technology exhibits substantial potential for the detection of lncRNAs, including MALAT1 . A previous study developed a strategy based on CRISPR/Cas12a and xenonucleic acid probes for the sensitive detection of site-specific m 6 A modifications in MALAT1 . 117 With the advancement of artificial intelligence, use of deep learning methods for specific lncRNA detection represents a novel approach for early disease diagnosis. 118 Such emerging strategies can significantly enhance the accurate detection and quantification of MALAT1 , contributing to the development of new diagnostic approaches for fibrotic diseases. MALAT1 is a fibrosis biomarker. Accurate and specific diagnostic tools are necessary to detect its differential expression in fibrotic tissues. MALAT1 can be detected by methods less invasive than some existing methods, such as biopsy, using simple blood and urine samples, which are more convenient for patients. However, such detection methods lack standardization. Therefore, determination of an optimal critical value is essential to ensure the accuracy and reliability of detection. Increasing evidence suggests that organ fibrosis not only marks the end stage of chronic injury or inflammation but also creates a microenvironment that is conducive to tumorigenesis. 119 For instance, liver fibrosis can progress to cirrhosis, which is a well-established risk factor for hepatocellular carcinoma 120 ; similarly, idiopathic pulmonary fibrosis is associated with a significantly increased risk of lung cancer. 121 Additionally, several clinical and experimental studies have shown that stromal fibrosis is often present in areas surrounding both benign and malignant lesions in the breast. High-density breast tissue, which is largely composed of fibrous stroma, is one of the strongest risk factors for developing breast cancer. 122 These observations indicate a close relationship between fibrosis and carcinogenesis. As we mentioned above, MALAT1 has been identified as upregulated in various fibrotic diseases (such as liver, lung, kidney fibrosis, etc.) as well as in multiple cancer types. Functionally, MALAT1 can regulate cell proliferation, migration, apoptosis, and extracellular matrix metabolism—processes fundamental to both fibrosis and cancer development. Notably, MALAT1 expression often rises in the stage of fibrosis and correlates with tumor progression, suggesting its value as an early diagnostic and prognostic biomarker. Therefore, the early detection of altered expression levels of biomarkers such as MALAT1 could enable the identification of individuals at high risk for progression from fibrosis to cancer, thereby facilitating timely intervention and potentially halting or reversing disease progression. LncRNA-targeted nucleic acid drugs represent a rapidly growing research area in molecular medicine. Several nucleic-acid-based approaches and agents, including siRNAs, antisense oligonucleotides (ASOs), and CRISPR/Cas techniques, have been developed to target lncRNAs. siRNAs induce RNAi, leading to MALAT1 transcripts degradation and subsequent decrease in its expression. MALAT1 -specific siRNAs attenuate TGF-β1 during HSC activation and initiates key signaling pathways via Smad proteins, reducing α-SMA and collagen production, ultimately alleviating liver fibrosis. 71 RNAi-mediated inhibition of MALAT1 also decreases the myofibroblast marker levels and restores the SIRT1 protein levels, leading to apoptosis and reversal of activated LX-2 cells, thereby promoting the regression of liver fibrosis. 79 MALAT1 elimination upregulates the miR-503 levels, thereby inhibiting silica-induced EMT in PF. 79 MALAT1 knockdown notably reduces the protein levels of ITGB1, a fibrosis-related marker, and increases those of E-calmodulin in HK2/NRK-49F cells, 88 thereby alleviating renal fibrosis. MALAT1 knockdown enhances the cardiac function in MI and simultaneously inhibits fibroblast proliferation, collagen production, and α-SMA expression in cardiac fibroblasts. 91 Although siRNAs facilitate the potent and specific knockdown of lncRNAs, they are susceptible to degradation in bloods, necessitating the use of effective delivery systems such as lipid nanoparticles, or conjugation with targeting ligands. Lennox and Behlke reported that nuclear lncRNAs are effectively suppressed using ASOs, whereas cytoplasmic lncRNAs are effectively targeted by siRNAs. 123 As MALAT1 predominantly localizes to the nucleus, 123 ASOs are promising nucleic acid drugs to specifically target MALAT1 . ASOs are short, synthetic single-stranded oligonucleotides binding to the target RNA molecules via complementary base pairing. This interaction leads to the RNase-H-mediated degradation of the target RNA or induces steric hindrance, preventing the target RNA from interacting with its molecular partners. A previous study achieved the sustained gene knockdown of MALAT1 by designing and administering an anti- MALAT1 ASO to the mouse eye via intracameral or intravitreal injection. 124 This demonstrates the significant potential of ASOs to treat eye diseases by specifically targeting MALAT1 . Wheeler et al. reported that systemically administered ASOs effectively cause MALAT1 knockdown in muscle tissues, suggesting a novel therapeutic avenue for myotonic dystrophy. 125 Other studies have used ASOs to target MALAT1 in different cancers including lung and 126 breast cancers, 127 which have been comprehensively reviewed by Zhou. 128 As specific modifications can enhance the stability, efficacy, and tissue specificity of ASOs, different modifications have been examined to optimize ASO performance. Particularly, phosphorothioate (PS)-modified ASOs have been extensively investigated. These molecules are typically synthesized by replacing one non-bridging oxygen atom in the phosphodiester backbone with sulfur. This PS modification enhances the ASO stability against nuclease-mediated degradation. 129 Miller et al. 129 revealed stabilin-1 and -2 as specific receptors for the cellular internalization of PS-modified ASOs upon systemic administration. Additionally, 2′-O-methoxyethyl (2′-MOE) and constrained ethyl bicyclic nucleic acid (cEt) modifications are widely used to modify ASOs. Specifically, 2′-MOE ASOs incorporate a 2′-MOE group into the sugar moiety of the nucleotide, whereas cEt ASOs integrate cEt modifications into their structure. Hung et al. 130 evaluated the effects of a 20-mer ASO gapmer modified with 2′-MOE and another 16-mer ASO containing cEt, which targeted MALAT1 in different tissues, via systemic antisense drug administration in mice and non-human primates. They found that ASOs with cEt chemistry exhibited better performance than those with the 2′-MOE modification. Other modifications of MALAT1 ASOs, including guanidine-bridged nucleic acids, 131 cholesterol-conjugated heteroduplex oligonucleotides, 132 and gapmer-locked nucleic acids, 127 , 133 have also been investigated. Collectively, these modifications enhance the therapeutic potential of MALAT1 ASOs by improving their stability, binding affinity, organ-specific delivery, and overall efficacy ( Table 3 ) summarizes the applications of ASOs, including modified ASOs targeting MALAT1 in various organs, for various diseases. MALAT1 -targeted ASOs specifically bind to MALAT1 transcripts, inducing their degradation or inhibiting their functions. MALAT1 downregulation disrupts the fibrotic signaling cascade, inhibiting fibroblast activation and fibrosis progression. Table 3 Available strategies targeting MALAT1 Candidates Targeting organ/diseases Targeting process affected/Checked Reference ASO muscle systemic administration of ASO for hereditary degenerative disease muscular dystrophy type 1 Wheeler et al. 125 lung cancer ASO targets MALAT1 to prevent lung cancer metastasis Gutschner et al. 126 breast cancer systemic knockdown of MALAT1 using ASOs in the mouse mammary carcinoma model leads to significantly slower tumor growth and a marked reduction in metastasis preparation of lipid nanoparticles for delivery of ASOs targeting breast cancer cells effectively knocked down MALAT1 in a mouse model Arun et al. 127 ; Sarkar et al. 134 liver, kidney, muscle, lung, adipose, adrenal gland, and peripheral nerve tissue in both mice and non-human primates constrained ethyl bicyclic nucleic acid gapmer ASOs were more efficacious than the ones with 2′-O-methoxy ethyl in all tissues examined Hung et al. 130 skeletal muscle target knockdown (KD) activity of MALAT1 antis-ASO containing guanidinium-bridged nucleic acid (GuNA) shows potent target KD activity comparable to LNA ASO in skeletal-muscle-derived cell lines Sasaki et al. 131 mouse quadriceps and cardiac muscles oligonucleotide transport vector (OTV) to reach ASOs across the mammalian blood-brain barrier to enable targeted knockdown of MALAT1 Barker et al. 135 mouse eye intracameral or intravitreal injection of an anti- MALAT1 ASO in the mouse eye, leading to long-lasting gene knockdown of MALAT1 Hu et al. 124 Chol-HDO brain the cholesterol-conjugated heteroduplex oligonucleotide (Chol-HDO) is a double-stranded complex; it comprises an ASO and its complementary strand with a cholesterol ligand. It effectively achieves MALAT1 knockdown in mouse brains following systemic injection. Goto et al. 132 two locked nucleic acid (LNA) oligonucleotides L15 and PS-L15 ASO human colorectal carcinoma, breast cancer L15 and PS-L15 cause RNA-LNA-RNA to form a triple helix with MALAT1 , significantly reducing its level Kumar et al. 133 paeoniae radix alba ulcerative colitis Acts on the MALAT1 /HIF-1α pathway to attenuate dextrose-sodium-sulfate-induced ulcerative colitis in rats Shivakumar et al. 136 lentinus edodes mycelia polysaccharide heat shock in HUVECs effectively reduces the expression of MALAT1 , inhibiting AGE-induced heat shock in HUVECs Gu et al. 137 dexmedetomidine nerve damage Dexmedetomidine elevated MALAT1 alleviating the neurological injury of rats with rat middle cerebral artery occlusion and promoted the viability of neurons Liu et al. 138 norepinephrine neurons Norepinephrine up-regulates the expression of MALAT1 in HT22 cells Wang et al. 139 quercetin MCF7 cells quercetin strongly binds to the MALAT1 triplex and downregulates its level Rakheja et al. 140 DPFp8 – a diphenylfuran-based small molecule, as an excellent binder to MALAT1 triple helix with highly affinity and selectivity Donlic et al. 141 Compound M5 multiple myeloma compound M5, a diazepine-indene scaffold, destabilizes the MALAT1 triple structure and exerts an antiproliferative effect in an in vitro model of multiple myeloma Qin et al. 142 Propofol cerebral ischemia/reperfusion injury Propofol pre-treatment markedly suppressed MALAT1 expression in ischemia/reperfusion injury exhibiting a protective role PPF pretreatment suppressed MALAT1 expressions ameliorating myocardial ischemia-reperfusion injury Hu et al. 143 Betulinic acid hepatocellular carcinoma betulinic acid downregulated MALAT1 expression inducing apoptosis of hepatocellular carcinoma Zhu et al. 144 Ginsenoside Rg1 spinal cord injury Rg1 can induce MALAT1 expression to activate the Lamc1/PI3K/AKT signaling pathway by sponging with miR-124-3p, activating astrocytes to promote the repair of spinal cord injury. Zhu et al. 144 ; Li et al. 145 L15, LNA; PS-L15, phosphorothioate LNA. Available strategies targeting MALAT1 L15, LNA; PS-L15, phosphorothioate LNA. ASO modification is closely associated with fibrosis treatment. Unmodified ASOs are prone to degradation and exhibit poor uptake. Chemical structural modifications, such as thiophosphorylation, enhance the stability of ASOs, improve their target cell delivery, 146 increase their binding capacity to MALAT1 , and assist in inhibiting the pro-fibrotic pathway mediated by MALAT1 . Furthermore, chemical modifications to ASOs—such as backbone modifications (phosphorothioate), sugar modifications (2 ′ -O-methyl[2 ′ -OMe], 2 ′ -O-methoxyethyl [2 ′ -MOE]), and conjugations (e.g., with N-acetylgalactosamine [GalNAc] for liver targeting)—significantly influence their biodistribution, stability, and uptake by specific organs. For example, GalNAc-conjugated ASOs are very efficiently taken up by hepatocytes through the asialoglycoprotein receptor, making the liver the most successful target for ASO delivery currently. 147 For the treatment of cystic fibrosis in the lung, aerosolized ASOs, including phosphorothioate (PS)-, 2 ′ -OMe-, or 2 ′ -MOE-modified ASOs, represent a highly efficient method for intratracheal delivery in clinical trials. 148 Otherwise, ASO or siRNA drug delivery for organ targeting still faces numerous challenges. The liver is the most readily targetable organ due to high asialoglycoprotein receptor expression, but organs like the lung and kidney lack highly specific receptor-mediated delivery systems, requiring reliance on physical delivery (e.g., nebulization) or novel ligand conjugation (e.g., receptor ligands targeting renal tubular epithelial cells)—approaches with higher technical complexity. 147 ASO accumulation in vivo may cause cumulative toxicity, such as deposition in renal excretory organs. Although siRNAs share similar mechanisms with ASOs, they are more susceptible to nuclease degradation and depend on delivery systems LNPs, which may induce inflammatory responses or lipid toxicity. 149 Fibrotic lesions, often accompanied by inflammation, vascular abnormalities, and matrix sclerosis, can hinder ASO diffusion and penetration, necessitating the development of intelligent delivery systems responsive to the microenvironment (e.g., low pH, high matrix density). 150 MALAT1 possesses an unusual 3′-terminal structural motif, known as the element for nuclear expression (ENE). This motif forms a triple helix via interactions between the U-rich hairpin and A-rich 3′ tail of the transcript. The ENE motif protects MALAT1 from degradation, leading to the excess accumulation of MALAT1 transcripts in the nucleus. 13 Donlic et al. 141 constructed a small-molecule library derived from the RNA-binding scaffold, diphenylfuran (DPF), screened it against diverse nucleic acid structures, and provided the first evidence that small molecules can selectively target the MALAT1 triple helix. Subsequently, they identified DPFp8, a small DPF-based molecule, as an excellent binder of the MALAT1 triple helix with high affinity and selectivity. 141 Based on the mouse triple helix within MALAT1 , which shares approximately 90% homology with the human MALAT1 ENE triplex, Abulwerdi et al. 151 used a small-molecule microarray strategy to identify two ligands specifically binding to this structure. Both ligands reduced the MALAT1 RNA levels in vitro and in vivo . RmsdXNA, a machine learning model, has been developed to predict the root-mean-square deviation of the ligand docking poses in nucleic acid complexes. This model is used to identify the potential ligands effectively binding to MALAT1 . 152 Future studies should further investigate the effects of these ligands in suitable experiments. Phytochemicals paclitaxel and resveratrol are used to treat of various diseases. Paclitaxel inhibits sepsis-mediated acute lung injury by attenuating the TLR-4/NF-κB pathway. 153 It also inhibits MALAT1 , thereby increasing miR-370-3p expression and decreasing the high-mobility group box 1 expression. Paclitaxel attenuates LPS-induced acute kidney injury. 154 Resveratrol mitigates methamphetamine-induced alterations in alveolar epithelial permeability and apoptosis, thereby attenuating the progression of chronic lung injury. 155 MALAT1 acts as a target of miR-223p , which potentially reduces its activity. The pyrin structural domain of the NOD-like receptor family, including NLRP3, acts as a downstream target of miR-22-3p . 156 The Chinese herbal formula Beng Huang Yi Sheng Granules exert protective effects against DN by modulating MALAT1 expression. 103 Several other chemical compounds derived from plants, including quercetin, 140 betulinic acid, 157 and ginsenoside Rg1, 144 effectively modulate MALAT1 expression. These findings are summarized in Table 3 . Interestingly, several anesthetics exert regulatory effects on MALAT1 . Dexmedetomidine attenuates pulmonary edema, alveolar septal rupture, and inflammatory cell and erythrocyte infiltration in the lung tissues of acute lung injury model rats. Additionally, it upregulates MALAT1 , which subsequently inhibits endoplasmic reticulum stress and promotes cell apoptosis. 145 Propofol suppresses MALAT1 expression in ischemia/reperfusion injury, thereby exerting protective effects. 143 , 158 These findings suggest that anesthesia influences MALAT1 expression. Natural hormones also influence MALAT1 expression. Norepinephrine, a neurotransmitter and hormone, upregulates MALAT1 levels in HT22 cells. 139 Melatonin, a hormone regulating the circadian rhythm, attenuates cardiac fibrosis in diabetic mice by inhibiting MALAT1/miR-141-mediated TGF-β1/Smad signaling. 92 Therefore, small-molecule drugs exhibit great potential for targeting MALAT1 -associated pathways in fibrosis. MALAT1 is intricately involved in multiple key signaling cascades driving fibrotic processes, such as the TGF-β/Smad pathways. Elucidation of the mechanisms by which small-molecule drugs inhibit MALAT1 will facilitate the development of new therapeutic strategies for fibrotic diseases. MALAT1 has garnered widespread attention for organ fibrosis detection and development of effective therapeutic strategies. Nucleic acid drugs targeting this lncRNA, such as ASOs, siRNAs, and targeted small-molecule drugs, hold great promise for MALAT1 -related disease treatment. However, such therapeutic approaches face significant challenges in ensuring organ-specific delivery, preventing off-target effects, maintaining the drug stability, and inhibiting immune activation. Nevertheless, advancements in nucleic acid and small-molecule drugs and delivery methods have accelerated the clinical translation of these therapies. Although preclinical studies have shown encouraging results, further clinical trials are essential to comprehensively evaluate the safety, efficacy, and long-term effects of MALAT1 -targeted nucleic acids and small-molecule drugs.

LncRNAs have recently attracted attention in liver disease research. As a lncRNA family member, MALAT1 plays crucial roles in various physiological and pathological processes. During development, high MALAT1 levels in various tissues contribute to essential regulatory functions in neural development, skeletal muscle formation, and angiogenesis. However, it plays predominantly harmful roles in the pathogenesis of organ fibrosis. For example, MALAT1 activates HSCs through various mechanisms, including Wnt/β-catenin signaling regulation and miRNA interactions, to promote liver fibrosis progression. Additionally, it influences crucial processes, such as EMT and macrophage polarization, to exacerbate lung fibrosis. It also exacerbates fibrosis in renal and myocardial fibrosis and endometriosis-associated fibrotic conditions by regulating specific miRNA-target gene axes ( Figure 3 ). Figure 3 MALAT1 promotes the development of liver fibrosis by adsorbing miRNA to drive the TGF-β1/smad axis MALAT1 promotes the development of liver fibrosis by adsorbing miRNA to drive the TGF-β1/smad axis MALAT1 exhibits significant potential for the diagnosis, prognosis, and treatment of fibrotic diseases. Its high levels in fibrotic tissues and body fluids make it a promising biomarker for fibrotic diseases. Advancements in assay technologies have facilitated the precise quantification of MALAT1 at the single-cell level and detection of any modifications, contributing to early disease diagnosis and treatment. Among therapeutic interventions, nucleic acid drugs, such as siRNAs and ASOs, and small-molecule drugs exhibit the capacity to target MALAT1 . Use of siRNAs to reduce MALAT1 expression via RNAi is also a promising therapeutic approach. Additionally, various chemical modifications enhance the stability of ASOs, facilitating the efficient degradation or suppression of MALAT1 . Small-molecule drugs targeting the unique structure of MALAT1 and those derived from natural compounds also modulate MALAT1 expression (listed in Table 3 ). Clinical translation of MALAT1 -targeted therapies is limited by several challenges, including issues related to organ-specific drug delivery that hinder effective organ targeting, off-target effects leading to deleterious side effects, low drug stability affecting the therapeutic activity, and potential drug-induced immune activation. Nevertheless, recent advances in nucleic acid and small-molecule drug design and delivery show promise for overcoming these challenges. However, more in-depth preclinical and clinical studies comprehensively evaluating the safety, efficacy, and long-term effects of MALAT1 -targeted drugs are necessary to facilitate the development of novel therapeutic strategies for fibrotic diseases.

Approximately 75% of the human genome produces transcripts, however, only 2% encodes proteins. Most of these transcripts are non-coding RNAs (ncRNAs), such as microRNAs (miRNAs) and long non-coding RNAs (lncRNAs). 1 LncRNAs are a class of RNA transcripts longer than 200 nt. Over 60,000 lncRNAs have been estimated in humans to date, with this number continuously expanding. 2 LncRNAs play an important role in regulating tumorigenesis and development. Bioinformatics analyses have revealed that lncRNAs significantly influence the tumor pathology, including cell proliferation, migration, invasion, epithelial-mesenchymal transition (EMT), apoptosis, and resistance to anticancer drugs. 3 , 4 , 5 , 6 , 7 LncRNAs regulate mRNA expression by sponging the target miRNAs, acting as competitive endogenous RNAs (ceRNAs) in the regulatory network implicated in cancer development, apoptosis, and drug resistance. 8 , 9 , 10 Metastasis-associated lung adenocarcinoma transcript 1 ( MALAT1 ), also known as nuclear enriched abundant transcript 2, is an lncRNA greater than 8,000 nt in length on chromosome 11q13. It is mainly localized to the nucleus and highly conserved in mammals. 11 RNA polymerase II transcribes the MALAT1 transcript. The primary MALAT1 transcript is approximately 8 kb in length in humans and 6.7 kb in mice. As shown in Figure 1 , ribonuclease (RNase) P and Z act on MALAT1 to produce one large fragment of approximately 6.7 kb and one small fragment of 61 nt, known as the MALAT1 -associated small cytoplasmic RNA (mascRNA). 12 The large fragments and mature transcripts of MALAT1 are highly stable due to a distinctive triple helix structure at the 3′-end, which confers resistance to nucleic acid exonucleases. 13 Natural antisense transcript TALAM1 maintains high MALAT1 levels through a feedforward positive regulatory loop, further increasing the MALAT1 stability. 14 MALAT1 remains stable for up to 16 h in human B cells and approximately 9–12 h in cancer cells. 15 , 16 Additionally, large fragments and mature MALAT1 transcripts stay in the nucleus and localize to the nuclear speckles. 12 When RNase P cleaves the 5′-leader sequence of MALAT1 , RNase Z removes the 3′-tail sequence, which is followed by the addition of cytosine-cytosine-adenine (CCA) via CCA addition enzymes to form small mascRNA fragments. In contrast to that in pre-transfer RNA processing and maturation, the product is an abundant stable 61 nt mascRNA with CCA at the 3′-end. This mascRNA is identical to the upper part of the tRNA fold, including coaxially stacked T-stem loops and receptor stems, and also exhibits a characteristic flat “elbow” feature consisting of tertiary base pairs between the T and D loops. 17 Mature MALAT1 and mascRNA function intracellularly to promote cell proliferation, migration, and invasion. In vitro experiments have demonstrated that these two molecules drive tumorigenesis and metastasis in mouse tumor models. 18 Following the completion of its function in driving MALAT1 maturation in the nucleus, mascRNA is exported to the cytoplasm, where it performs additional functions, including promotion of translation, regulation of innate immunity, and maintenance of macrophage functions. To maintain macrophage functions, mascRNA negatively regulates Toll-like receptor (TLR)-mediated MyD88-dependent pro-inflammatory signaling and positively regulates TRIF-dependent interferon signaling by promoting the degradation of the tumor necrosis factor (TNF)-receptor-associated factor 6 for ubiquitination. 19 A previous study reported the induction of antiviral immunity genes and cellular resistance to coxsackievirus B3 infection via recombinant mascRNA expression in cardiomyocytes. 20 However, most studies on the physiological and pathological functions of MALAT1 have focused on its larger fragments, with only a few exploring the functional significance of mascRNA. Additionally, MALAT1 transcripts undergo various epitopic transcriptional changes, including m 6 A, pseudouridylation, and 5-methylcytosine modifications. 21 , 22 , 23 Addition of m 6 A at position A2577 destabilizes the hairpin stem of MALAT1 , making it susceptible to RNA-binding proteins (e.g., heterogeneous nuclear ribonucleoprotein C) and disrupting its functions. 24 Figure 1 The processing mechanisms and functional roles of the lncRNA MALAT1 . MALAT1 biogenesis The 3′ end of the MALAT1 primary transcript is produced by the tRNA biogenesis factor, which produces two types of RNAs: the nuclear-retained long noncoding MALAT1 RNA and the tRNA-like small RNA (mascRNA). RNase P recognizes specific secondary structures near the 3′ end of the MALAT1 transcript and produces the mature MALAT1 3′ end and the 5′ end of the mascRNA. The tRNA-like small RNA is then cleaved by RNase Z for CCA addition and exported to the cytoplasm. The mature MALAT1 transcript is localized to nuclear speckles. MALAT1 transcripts perform biological roles in the nucleus (including transcription and splicing of precursor RNAs), while mascRNAs exit the nucleus to carry out their functions in the cytoplasm (including enhancement of translation, regulation of immunity, and maintenance of macrophage function). The processing mechanisms and functional roles of the lncRNA MALAT1 . MALAT1 biogenesis The 3′ end of the MALAT1 primary transcript is produced by the tRNA biogenesis factor, which produces two types of RNAs: the nuclear-retained long noncoding MALAT1 RNA and the tRNA-like small RNA (mascRNA). RNase P recognizes specific secondary structures near the 3′ end of the MALAT1 transcript and produces the mature MALAT1 3′ end and the 5′ end of the mascRNA. The tRNA-like small RNA is then cleaved by RNase Z for CCA addition and exported to the cytoplasm. The mature MALAT1 transcript is localized to nuclear speckles. MALAT1 transcripts perform biological roles in the nucleus (including transcription and splicing of precursor RNAs), while mascRNAs exit the nucleus to carry out their functions in the cytoplasm (including enhancement of translation, regulation of immunity, and maintenance of macrophage function). MALAT1 was discovered in early non-small-cell lung cancer and used as a diagnostic marker for it thereafter. 25 It regulates biological processes by directly or indirectly affecting transcription or regulating the splicing of alternative precursor mRNAs. 26 It binds to several other precursor mRNA splicing factors, including the serine- and arginine-rich splicing factor 1, SON1, and heterogeneous nuclear ribonucleoproteins C and H1, enriched in the nuclear speckle. 27 , 28 , 29 MALAT1 is associated with the phosphorylation of serine- and arginine-rich splicing factors, affecting their speckle localization and roles in selective pre-mRNA splicing. 27 MALAT1 also performs specific functions at the transcriptional level. For instance, in vivo cross-linking experiments have revealed that MALAT1 binds to the chromatin of actively transcribed genes, regulating their expression at the transcriptional level. 30 MALAT1 binds to various transcription factors and transcriptional co-activators, including the latent transforming growth factor (TGF)-β binding protein-3, forkhead box O1, PC2, and high-mobility group AT-hook 2. 31 , 32 , 33 , 34 In addition to its roles in splicing and transcription, MALAT1 functions as a ceRNA or miRNA sponge, adsorbing miRNAs and exerting its biological functions under diverse conditions. It promotes osteosarcoma progression via RET upregulation by sponging miR-129-5p and subsequently activating the protein kinase B (AKT). However, MALAT1 silencing reduces the growth and proliferation of osteosarcoma cell. 35 , 36 In colorectal cancer, MALAT1 induces metastasis and proliferation by sponging miR-26a and miR-26b , thereby promoting fucosylation through fucosyltransferase 4 and activating the phosphoinositide 3-kinase/AKT pathways. 37 In summary, MALAT1 plays important roles in the biological processes of gene transcription and RNA stabilization, thereby affecting the physiological functions of cells. By regulating various cell biological processes, MALAT1 serves as a key player in various physiological and pathological processes, including tissue inflammation, embryonic implantation, cardiovascular remodeling, and tumor progression. 38 Many studies have revealed the elevated levels of MALAT1 in various tumor types and its effects on cancer cell growth, migration, invasion, and apoptosis, as reviewed by Zhao and Li. 39 , 40

The authors declare no conflicts of interest.

MALAT1 expression levels are elevated in the brain tissues, particularly in the highly active regions of the human neocortex. It regulates synaptic density in hippocampal neurons by controlling neuroligin 1 and synaptic cell adhesion molecule 1. 41 Considering its essential roles in neuronal development, MALAT1 depletion alters the expression of genes related to synapse and dendrite growth. 42 MALAT1 was initially identified in the hippocampal and Purkinje cells between postnatal day 0 (P0) and P7 in mice, with its levels reaching a peak on P28. 41 MALAT1 expression is possibly associated with skeletal myogenesis. In vitro , myogenesis experiments have shown the sustained increase in MALAT1 levels in actively proliferating primary human and rat adult myoblasts (C2C12) during fusion and differentiation. 42 MALAT1 levels in mouse skeletal muscle cells are reduced by increasing the levels of a potent negative regulator of skeletal myogenesis, a muscle growth inhibitor. 42 , 43 The specific silencing of MALAT1 by siRNA small interfering RNA (siRNA) significantly inhibited skeletal muscle cell proliferation, preventing cell growth in the G0/G1 phase compared to that in the skeletal muscle cells transfected with the blank siRNA. 43 In one study, expression level of myostatin, a key target of muscle growth inhibitors, was reduced by 17% following the partial depletion of MALAT1 . 44 MALTA1 plays a significant role in the regulation of angiogenesis and formation of new blood vessels. Michalik et al. demonstrated that neonatal retinal vascularization is delayed in MALAT1 knockout mice on P5 compared to that in the wild-type controls. 45 In vitro studies were used to substantiate the hypothesis that de-silencing of MALAT1 by siRNA or pharmacological inhibition induces a shift in human vascular endothelial cells from a proliferative state to a migratory state, which leads to aberrant vascular sprouting. 45 Generally, MALAT1 plays fundamental regulatory roles in typical physiological and pathological processes, including cancer, neural development, skeletal muscle formation, and angiogenesis, by regulating cell proliferation. Cell proliferation and extracellular matrix (ECM) remodeling drive cancer progression and fibrosis, sharing key mechanisms. In cancer, specific signaling pathways (e.g., TGF-β and Wnt/β-catenin pathways) activate fibroblasts, promoting ECM deposition and invasiveness. 46 Fibrosis involves chronic inflammation (interleukin [IL]-6 and TNF-α) inducing fibroblast activation via signal transducer and activator of transcription 3/nuclear factor (NF) κB, thereby stiffening the tissues. 47 ECM remodeling enhances cancer cell mechanosensitivity via integrin-focal adhesion kinase signaling. 48 The TGF-β superfamily exhibits dual roles, promoting EMT/cancer-associated fibroblast activation in cancer and collagen cross-linking, 49 , 50 reinforcing its pathobiological relevance. 50 During fibrosis, abnormal angiogenesis and vascular remodeling are key features that worsen the tissue structure and promote fibrosis progression via microenvironment regulation. In fibrotic diseases (e.g., pulmonary, hepatic, and renal fibrosis), dysregulation of pro-angiogenic (vascular endothelial growth factor [VEGF] and fibroblast growth factor 2) and anti-angiogenic (thrombospondin-1) factors drives pathological angiogenesis. VEGF activates VEGF receptor-2 to induce endothelial proliferation/migration, permeability, and fibrotic basement membrane precursor deposition. 51 Based on the commonalities between cancer and fibrosis mentioned above, as well as the key vascular regulatory roles of MALAT1 in the process of fibrosis, it can be concluded that the specific roles played by MALAT1 in fibrotic diseases cannot be ignored. MALAT1 plays pivotal roles in the pathogenesis of organ fibrosis, including hepatic, pulmonary, renal, and myocardial fibrosis. In this review, we systematically discuss the roles of MALAT1 in various physiological processes and diseases, especially organ fibrosis. This review highlights the potential of MALAT1 as a biomarker, driver, prognostic indicator, and therapeutic target for organ fibrosis.

This research was funded by the 10.13039/501100001809 Natural Science Foundation of China (No. 32271363 ), Sichuan Science and Technology Program (No. 2025ZNSFSC0744), Health Commission of Sichuan Province Medical Science and Technology Program (No. 24QNMP040) and the Scientific Research Foundation of 10.13039/501100014895 Southwest Medical University (No. 2023QN036 ).