Unlike sncRNAs, linear lncRNAs are transcripts longer than 200 nucleotides and share several common features with mRNAs, such as the transcription by RNA polymerase II, the 5′capping, the splicing, and the polyadenylation. lncRNAs are less expressed than mRNAs and they have tissue and developmental-stage-specific expression profiles
[11]. In detail, lncRNAs constitute a heterogeneous large group of ncRNAs subdivisible, according to their genomic biogenesis, into five categories: (a)
intronic lncRNAs, which originate from intronic regions within protein-coding genes; (b)
intergenic lncRNAs, whose transcription occurs at the intergenic region level, between two protein-coding genes; (c)
enhancer lncRNAs, whose transcription occurs in the enhancer sites of the genome; (d)
bidirectional lncRNAs, which are transcribed bidirectionally in the promoter-regions of protein-coding genes; and (e)
antisense lncRNAs, which originate from overlapping protein-coding genes and from which they are transcribed following the antisense direction
[11]. Consistently with their heterogeneous biogenesis, the linear lncRNAs, whose heterogeneity appears to mirror their crucial regulatory functions in cancerogenesis processes, are very heterogeneous
[11]. In this sense, linear lncRNAs can guide, after selectively binding, specific proteins on the cellular surface, and drive the release of the substance in the opportune target cell (“guide lncRNAs”). Moreover, they can act as molecular decoys to bind and facilitate the degradation of targeted proteins or other RNAs (“decoy lncRNAs”). Furthermore, they can serve as scaffolds (“scaffold lncRNAs”) functioning as a pivotal platform to assemble different molecular components (other RNAs and proteins). Lastly, by interacting with transcription factors or chromatin-modifying enzymes, they can tissue-specifically regulate the expression of targeted genes
[11].
A particular emerging class of ncRNAs with a specific morphology is represented by the circular RNAs (CircRNAs), covalently closed RNA molecules modulating gene expression by acting as transcriptional regulators, miRNA sponges, and protein templates
[12]. Due to their pleiotropic modes of action, circRNAs have been proposed as diagnostic and prognostic HCC markers and their potential role is reported in detail in the dedicated subsection of this paragraph.
2. Role of Linear Long Non-Coding RNA Dysregulation in Hepatocellular Carcinoma Progression
In recent years, it has been shown that lncRNAs can modulate different stages of liver diseases, affecting immune responses, liver regeneration, and redox signaling
[13][14].
The dysregulation of lncRNA patterns promotes the worsening of liver pathologies, liver outgrowth, and oxidative stress, which eventually result in the initiation and progression of HCC through different signaling pathways
[14]. Multiple lncRNA profiling studies on whole-genome transcriptome sequencing platforms have highlighted the altered expression pattern of lncRNAs in human HCC, suggesting a significant difference in the global expression profile between tumors and nontumor cells
[14].
Abnormal cell proliferation is identified by cell cycle disturbances that result in uncontrolled cell division.
lncRNAs are involved in different phases of the cell cycle by directly and indirectly influencing the activities of several Cyclin-dependent kinases (CDK)
[15].
For instance, the interaction of
Lnc-UCID with DHX9 (DExH-Box Helicase 9) determines CDK6 enhanced expression, the promotion of G0/G1 to S phase transition, and, ultimately, HCC cell proliferation
[16].
A central regulator of cell cycle progression is the E2F transcription factor family, whose expression and functions are tightly linked with the occurrence and development of various malignant tumors
[17]. In this regard, many lncRNAs exhibit involvement in HCC progression due to the regulation of E2F family members, like E2F1 and E2F2. For instance, lncRNA
CASC11 enhances the stability of E2F1 mRNA by recruiting EIF4A3, which leads to the upregulation of E2F1 and, ultimately, promotes hepatocarcinogenesis and sponging mir-296–5p in HCC cells. It consequently activates the Wnt/β-catenin pathway by enhancing SOX12 and promoting HCC cell proliferation and progression
[18].
Among scaffold lncRNAs,
HULC lncRNA has been well-characterized as an oncogenic molecule, singularly upregulated in human HCC
[19].
HULC lncRNA is implicated in HCC progression mainly by functioning as a molecular decoy of miR-107, which usually represses the E2F1 transcription factor
[20]. Lu and colleagues demonstrated HULC’s capability to disable miR-107 functioning in the HCC microenvironment and, consequently, promote the activation of spongiosine kinase 1 with the stimulation of angiogenesis metastatic cell dissemination processes
[20]. Consistent with this, Zhu and colleagues reported the capability of HULC to downregulate miR-29.
HOTAIR displays its role in hepatoma cells at the epigenetic level. Indeed, it was shown to be able to interact and guide polycomb group complex 2 (PRC2) in the repression of specific target genes via the installation of transcriptional repressive histone 3 lysine 27 trimethylation (H3K27me3)
[21]. It has been described by Fu and others that
HOTAIR epigenetically silences miR-218-2 on chromosome 5, with the intervention of PRC2 to suppress P14 and P16 (tumor suppressor genes) signaling in the HepG2 cell line
[22].
3. Etiology-Specific Dysregulated Linear lncRNAs in Hepatocellular Carcinoma Progression
Recently, several findings supporting the implication of lncRNAs in the pathogenesis of NAFLD-related HCC were published.
The dysregulation of miR-155 expression represents a relevant pathogenetic moment, contributing to the onset of HCC in the early stage of cancerogenesis in the NAFLD context
[23][24]. In line with this, Yuan et al. revealed miR-155 as a target of the lncRNA
CASC2: in particular,
CASC2 was found to be downregulated in HCC tissue samples and was revealed to act as a molecular sponge inhibiting the expression of miR-155
[25].
Moreover, Wang et al. reported that the overexpression of lncRNA
NEAT1 was positively correlated with the upregulation of acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS), both involved in NAFLD pathogenesis
[26]. Conversely, treatment with si-
NEAT1 lentivirus reduced the triglyceride and cholesterol levels in the animal model, further supporting the involvement of
NEAT1 in NAFLD progression. Additionally, in 2019, Si-si Jin and colleagues demonstrated how
NEAT1 affects miR-506 expression in BRL3A cells treated with free fatty acids (FFA)
[27]. Hence,
NEAT1 was upregulated while miR-506 was downregulated in the progression of NAFLD; furthermore,
NEAT1 and miR-506 were proven to regulate fibrosis, inflammatory response, and lipid metabolism
[27].
Another lncRNA that seems to exert a critical role in NAFLD-related HCC is
MALAT1, whose expression was found to be elevated in liver tissue of NAFLD patients and HepG2 cells treated with 1 mM of FFA as an NAFLD in vitro model
[28]. The knockdown of
MALAT1 upregulated the expression of PPAR and reduced CD36 levels, thus reversing fatty-acid-induced lipid accumulation in HepG2 cells
[28].
MALAT1 is also a crucial oncogene involved in the upregulation of Serine/arginine-rich splicing factor 1 (SRSF1) and the activation of the Wnt pathway, thus promoting HCC growth and development in liver tumors of the HCC mouse model
[29]. Altogether, these findings highlighted a possible involvement of
MALAT1 as a new potential therapeutic target.
LncRNAs also exhibit a crucial role in infection-related HCC.
For instance, the integration of HBV into a normally silenced region of chromosome 8p11.21 produces a novel chimeric
HBx-LINE1 lncRNA, which exerts its function HBx-LINE1 by activating the Wnt signaling pathway and promoting hepatic injury by acting as a decoy to sequester liver-specific miR-122
[30]. Finally, high levels of
HBx-LINE1 have been found in HBV-related HCCs, showing the importance of
HBx-LINE1 as an independent prognostic factor in this context
[30]. Moreover, HBV protein X (HBx) is highly carcinogenic, and 90% of HBx transgenic mice develop HCC
[31]. HBx proteins seem to be implicated in the activation of several lncRNAs in HBV-related HCCs, such as DBH-AS1, lncRNA-UCA1, and HULC in HCC cell lines
[32].
A profiling expression study by Zhang et al. demonstrated an altered lncRNA expression in a series of patients with different stages of HCV-associated liver disease
[33].
Linc01419 was found to be highly expressed in early-stage HCCs compared with pre-malignant dysplastic nodules
[33]. In addition, another lncRNA,
K021443, was found to be overexpressed in advanced HCC
[33].
Additionally,
Linc01152 overexpression determines an increased HCC cell proliferation and tumor formation in nude mice, probably due to the activation of the transcription of IL-23 (interleukin 23), which induces the STAT3 pathway. Strikingly, linc01152 is described as being downregulated in all viral-hepatitis-related HCC through an unknown mechanism
[33][34].
4. Circular RNAs in Liver Cancer Progression: Novel Diagnostic/Prognostic Markers?
In the following parallel-natured pathogenetic–clinical section, by focusing particularly on circRNAs correlated with HCC genesis and progression, we present an overview of ncRNAs that represent potentially useful novel tools in the diagnosis of HCC, as well as ncRNAs that can predict individual outcomes. In the last decade, also considering their well-demonstrated key regulator role in cancer-related molecular pathways, the world of circRNAs has been deeply investigated. In terms of downregulation or overexpression, a growing number of emerging studies provide evidence of several dysregulated circRNAs creating a complex pathomolecular network in the HCC context.
Preliminary observational research revealed higher has_circ_0005075 expression levels in HCC tissues compared with corresponding nontumorous counterparts. In addition, has_circ_0005075 expression was significantly upregulated in tumoral specimens, and, relevantly, the correlation analysis including several clinicopathological parameters of HCC patients demonstrated the relationship between has_circ_0005075 expression levels and HCC tumor size. However, despite these interesting results, its function remains to be clarified
[35]. For this purpose, a subsequent brilliant in vitro study by Li et al. showed the association of has_circ_0005075 HCC overexpression with increased numbers of proliferative, migrated, and invasive SMMC-7721 cells, as well as the capability of this circRNA to promote HCC progression through the downregulation of miR-431
[36].
Consistent with this, Pan et al., by evaluating miR-431 expression levels in 95 HCC cases, highlighted that this miRNA was markedly downregulated in the HCC samples compared with corresponding adjacent nontumoral tissues
[37].
However, even though the correlation analysis provided evidence for the relationship of miR-431 downexpression with multiple malignant characteristics, including lymph node metastasis, the HCC diagnostic accuracy of the assessment of miR-431 expression levels appeared not likewise elevated [Area under the Receiving Operator (ROC) Curve (AUC): 0.66]
[37].
Moreover, in a recent in vitro–in vivo study, Sun et al. brilliantly investigated the function and regulation of circ_0038718 in HCC, suggesting its crucial role in fueling liver cancer progression
[38]. The researchers highlighted high expression levels of circ_0038718 in cell lines and HCC specimens and correlated this result with worsened prognosis in HCC patients. In vitro, circ_0038718 knockdown reduced HCC advancement mechanisms as cancer cell proliferation and cellular invasion
[38]. Mechanistically, this molecule may act as the sponge of TS miR-139-3p, and, in support of this, the inhibition of miR-139-3p abrogated the regulatory effect of circ-0038718 in HCC cells, suggesting miR139 as a circ_0038718-specific target
[38]. In the same vein, a group of researchers had previously tried to identify differentially expressed human miRNAs between HCC and normal liver tissues by evaluating the miRNA expression profiles of 375 HCC and 50 normal liver tissues. The researchers shed light on has-miR-139-5p as a potential discriminant marker, and, further, the ROC curve analysis suggested that the survival prediction model developed based on tumor stage and hsa-miR-139-5p expression levels exhibited good performance in predicting the 3-year overall survival (OS) of HCC patients. By combining these findings, the encouraging results proposed circ_0038718 and miR-139 as crucial regulators of liver cancer progression and promising reciprocally influenced prognostic indicators, while also providing potential therapeutic targets for HCC treatment
[26][38].
Despite the progressively increasing evidence on circRNA- and miRNA-related implicated in the progression of HCC, some circRNAs continue to be “orphans” of specific miRNAs, and vice versa, suggesting that several molecular mechanisms have yet to be elucidated. For instance, although the TS miR-122 represents the most frequently detected miRNA in the liver and its crucial role in cancer suppression has been largely demonstrated, circRNAs specifically targeting and regulating this molecule expression and functioning have not been identified
[39]. Of relevance, miR-122 appears also closely related to the prognosis of HCC: decreased miR-122 levels have been associated with cell proliferation, invasion, and metastasis mechanisms, and several targets of miR-122 have been implicated in tumorigenesis, including ADAM10, cyclin G1, SRF, Wnt1, and IGF1R
[40]. Conversely, circ_0128298 represents a circRNA orphan for relative-targeted miRNAs. Due to its capability of promoting proliferation and metastasis by not-fully clarified molecular pathways, and considering its significant upregulation in liver cancer samples compared with those of peritumorous tissues, this molecule has been proposed as another relevant circRNA associated with HCC progression
[41]. If on one side, it has been demonstrated to have only a moderate HCC diagnostic accuracy [AUC: 0.66; sensitivity: 0.71; specificity: 0.81], on the other, this circRNA tissue expression relevantly correlated with the OS
[41].
However, the current goal in this context appears to approach circRNA biological effects by identifying specific circRNA signatures. In this sense, a recently published meta-analysis including a total of eight studies highlighted, through adequate ROC curve analysis, an elevated diagnostic accuracy [AUC: 0.86; sensitivity: 0.78; specificity:0.80] for the specific circRNAs expression profile [downregulated circRNAs: hsa_circ_0003570, circZKSCAN1, hsa_circ_0001649, hsa_circ_0004018 tissue levels, and hsa_circ_0001445 serum levels; overexpressed circRNAs: hsa_circ_0005075, hsa_circ_0091582, and hsa_circ_0128298 tissue levels] in confirming HCC
[42]. In addition to the potential diagnostic role, survival analyses also revealed a relevant prognostic implication by showing that abnormally expressed circRNAs were intimately associated with tumor size, serum alpha fetal protein level, differentiation grade, microvascular invasion, and metastasis in patients with HCC, as well as that the downregulated circRNA expression signature correlated perfectly with HCC survival [hazard ratio (HR): 0.42], whereas the HCC cases with high circRNA levels had significantly poorer prognoses than those of patients with low circRNA levels (HR: 2.22). In the same vein, J. Cao et al. more recently explored lncRNA expression levels in HCC and para-carcinoma tissue to filter out 19 specific differentially expressed lncRNAs, demonstrating for this 19-lncRNA signature a good diagnostic accuracy in diagnosis and prognosis prediction in patients with HCC (AUC > 0.70)
[43].
Altogether, these encouraging findings suggest that ncRNA expression signatures could be considered a potential tool for the diagnosis and prognosis determination of HCC.
5. Etiology-Specific Dysregulated Circular RNAs in Hepatocellular Carcinoma Progression
In addition to the above-presented evidence composing a clinical–pathogenetic heterogeneous background, identifying an association between definite ncRNAs and specific etiological HCC agents, in the optic of HCC-patient-tailored management, constitutes a current research challenge and would certainly represent a medical breakthrough.
In the context of viral hepatitis, the detailed molecular mechanisms by which HBV contributes to the development of HCC remain largely unknown, and specific aberrantly expressed circRNAs might be involved in the pathogenesis of HBV-associated liver cancer by regulating several tumor-related molecules or pathways
[44]. In this regard, a total of 13,124 circRNAs were recently found dysregulated in HBV-related HCC, and notably, the circRNA–miRNA interaction network analysis revealed that 6020 circRNAs were predicted to target 1654 miRNAs
[44].
In particular, the downregulation of
circRNA_10156 has been demonstrated to suppress liver cancer cell proliferation, suggesting this molecule is a pro-tumorigenic element
[44]. In the same study, the researchers clarified the pathogenetic role of this ncRNA and the relatively targeted miRNA, highlighting that circRNA_10156 may act as a molecular sponge of the oncomiR miR-149-3p, which serves a crucial role in tumor development
[44]. Consistently, they demonstrated that the depletion of circRNA_10156 upregulated miR-149-3p, reduced Akt1 serine/threonine-protein kinase expression, and suppressed liver cancer cell proliferation
[44].
In the last few years, in parallel to viral hepatitis scenarios, given also the epidemiological data, NAFLD-related HCC represents, likewise, a research field where the role of circRNAs has been intensively investigated
[45]. In this regard,
circRNA CDR1-AS has been shown to be upregulated in HCC tissues with simultaneous downregulation of miR-7. This circRNA has been proposed to act as a molecular sponge of TS miR-7; consistently, CDR1-AS-determined decreased miR-7 expression promotes HCC progression facilitating the invasion/proliferation mechanisms of cancer cells
[45][46][47].
In the same vein,
circRNA_0067934 can promote progression events associated with HCC worsening as the invasion and metastasis-related mechanisms via the β-catenin/Wnt signaling pathway in the NAFLD setting. In support of this, the enhanced expression of this molecule has been found in HCC tissues compared with peritumoral healthy specimens and its inhibition suppresses the invasion and metastasis of liver cancer
[45][48].
In contrast with circRNA_CDR1-AS and circRNA_0067934,
circRNA MTO1, by acting as the oncomiR miR-9 sponge, has been associated with anti-cancer advancement mechanisms and its upregulation was proposed to negatively regulate HCC progression
[45][49].
All these findings demonstrate how a better understanding of the molecular etiological-specific mechanisms of HCC carcinogenesis would contribute to the development of more effective molecular targeted interventions for the primary prevention and treatment of HCC.
Table 1 summarizes lncRNA dysregulated pathways influencing etiologically specific HCC progression mechanisms.
Table 1. Major long non-coding RNAs associated with hepatocellular carcinoma (HCC) progression in Non-alcoholic Fatty Liver and viral hepatitis with relative mechanisms of action.
|
|
Molecules’ Name |
Biogenesis/ Expression Status |
Influence on Molecular Pathways Promoting HCC Progression Mechanisms |
microRNA(s) Targeted |
Non-alcoholic Fatty Liver Disease (NAFLD)—HCC-related context |
Linear Long Non-coding RNAs |
NEAT1 |
Overexpression [26][27] |
Promotion of NAFLD-related fibrosis worsening and HCC cell proliferation by sponging miR-506 [normally down-regulating acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) expression] [26]. |
miR-506 [27] |
MALAT1 |
Overexpression [28] |
Upregulation of the splicing factor “SRSF1” and activation of the β-catenin/Wnt pathway [28]. |
Undefined/NA in this context |
CASC2 |
Downregulation [25] |
Regulation of cell proliferation and dysregulation by sponging the oncoMiR miR-155 [25] |
miR-155 [25] |
Circular Long-Non-coding RNAs |
circRNA CDR1-AS |
Overexpression [47] |
Promotion of invasion/proliferation mechanisms of liver cancer cells by sponging miR-7 (normally inhibiting spindle checkpoint protein “SPC24”) [47]. |
miR-7 [46] |
circRNA_0067934 |
Overexpression [48] |
Promotion of invasion and metastasis-related mechanisms by sponging miR-1234 (normally regulating the β-catenin/Wnt signaling pathway) [48]. |
miR-1234 [48] |
circRNA MTO1 |
Downregulation [49] |
Promotion of HCC progression by sponging the oncomiRNA “miR-9” [49]. |
miR-9 [49] |
Viral-hepatitis—HCC-related context |
|
HBx-LINE1 lncRNA |
Integration of HBV DNA in ch.8p11.21 [31] |
In HBV context: activation of the Wnt signaling pathway; acting as a decoy to sequester miR-122 [30][31]. |
miR-122 [30] |
HULC |
Overexpression induced by HBx protein [19][32] |
In HBV context: activation of sphingosine-kinase-1-mediated angiogenesis, functioning as a molecular decoy of miR-107 (normally repressing the expression of the transcription factor E2F1) [32]. |
miR-107 [20] |
Linear Long Non-coding RNAs |
Linc01419 |
Overexpression [33] |
In HCV context:
- ○
-
Promotion of cell proliferation and metastasis by sponging miR-485-5p (normally downregulating “LSM4” expression) [50].
- ○
-
Promotion of cell proliferation and metastasis by enhancing NDRG1 promoter activity [51].
- ○
-
Promotion of cell proliferation by recruiting XRCC5 and regulating its phosphorylation to repair DNA damage [52].
- ○
-
Promotion of proliferation by targeting EZH2-regulated RECK expression [53].
|
miR-485-5p [50] |
LncAK021443 |
Overexpression [54] |
In HCV context: promotion of cell proliferation, invasion, and metastasis by repressing epithelial-mesenchymal transition (EMT) [33][55] |
Unidentified/NA in this context |
Linc01152 |
Overexpression [56] |
In HBV context: increases cell proliferation by activating the STAT3 pathway [56]. |
Unidentified |
Viral-hepatitis—HCC-related context |
Circular Long-Non-coding RNAs |
circRNA_10156 |
Overexpression [44] |
In HBV context: promotes cell proliferation by sponging miR-149-3p (usually down-regulating the AKT1/mTOR pathway) [44]. |
miR-149-3p [44] |
circ-RNF13 [circ_0067717] |
Downregulation [57] |
In HBV context: promotes HCC progression by sponging miR-424-5p (usually regulating TGFβ-induced factor homeobox 2 (TGIF2) [57]. |
miR-424-5p [57] |
circ_0027089 |
Overexpression [58] |
In HBV contexts: acts as an oncogene and promotes the development of HBV-related HCC by regulating nucleus accumbens associated protein 1 (NACC1) via competitively targeting miR-136-5p [59]. |
miR-136-5p [59] |