Hepatocellular carcinoma (HCC) is an invasive type of cancer that leads to over 700,000 deaths annually worldwide (1). This tragedy occurs due to lack of effective diagnostic tools as well as efficient therapeutic approaches (2, 3). Although surgical removal, liver transplants, and chemotherapeutic agents are the most commonly used treatments for HCC, they are ineffective in many cases due to multifocal development and distant metastasis caused by late diagnosis (4, 5). Approximately 70% of patients with HCC are diagnosed late, therefore, even if these therapeutic strategies are used in early stages, they are unable to improve the 5-year overall survival rate over 70% (6, 7). As a result, advances in diagnostic and therapeutic approaches must be expanded to reduce HCC mortality.

Non-coding RNAs (ncRNAs) are RNAs derived from the human genome but not translated into protein (8). NcRNAs regulate gene expression through several pathways, such as interactions with epigenetic factors to silence genes and interactions with transcription factors to inhibit or promote target gene expression (9). Based on abundant evidence, ncRNAs play an important role in the initiation and progression of various cancers (10, 11). Cancer cells release many of these ncRNAs into the blood or urine, so they can be used as a diagnostic factor. These ncRNAs are divided into four main types, including microRNA (miRNA), long non-coding RNA (lncRNA), circular RNA (circRNA) and PIWI interacting RNA (piRNA) (12). CircRNA have a unique and stable structure that comprises covalently closed loops (13). They are crucial in the development of several malignancies, including colorectal cancer (14), cholangiocarcinoma (15), bladder cancer (16), and HCC (17), through the regulation of cancer cell proliferation, migration and metastasis, and chemoresistance (18). Most important functions of circRNA for regulating target gene expression are miRNA regulation and effect on the signaling pathways such as Wnt/β-catenin (19).

The Wnt family includes a group of proteins that play important roles in a various cellular functions, such as organ development, stem cell renewal, and the cell viability (20). The Wnt Cascade is divided into different branches, including the canonical Wnt/β-catenin (Wnt/β-catenin dependent pathway) and the non-canonical Wnt/β-catenin pathway (β-catenin-independent pathway). The latter is further divided into the Wnt/calcium and planar cell polarity (PCP) pathways that have an effect on cancer progression and propagation (21). Wnt/β-catenin is involved in numerous physiological mechanisms, such as cell expansion and differentiation, cell death, migration, and tissue homeostasis (22). Abnormalities in Wnt signaling have been linked to several cancers, including gastrointestinal cancers, especially colorectal cancer, leukemia, melanoma, breast cancer and also in a significant subset of HCC (23, 24).

Several studies have found that circRNAs have the potential to be used as biomarkers in tissue samples for cancers diagnosis, with noticeable clinical applications for predicting HCC prognosis. This could potentially lead to improved therapeutic strategies (25, 26). For instance, Peisi kou et al, revealed that expression of Circular RNA hsa_circ_0078602 was meaningfully decreased in 79 samples of HCC patients when compared to controlled non-tumoral tissues (P=0.015). Kaplan-Meier survival analysis indicated that lower expression of hsa_circ_0078602 is associated with poor prognosis in HCC cases (27). In contrast, another study on HCC tissue samples from 59 patients found that circ_0000267 is associated with poor prognosis and promotes proliferation, migration and infiltration of cancer cells through sponging miR‐646 (28).

Our theory is that circRNAs could become a novel approach for detecting and treating HCC in the future. To support this idea, we conducted a review of recent studies that investigate the impact of circRNAs on HCC development, with a particular focus on how they affect miRNA regulation and the Wnt/β-catenin signaling pathway.

The Wnt pathway comprises two types of signal transduction pathways (STPs): canonical (Wnt/β-catenin) and non-canonical (planar cell pole (PCP) and Wnt/calcium (Wnt/Ca2+)) (29, 30) as shown in ( Figure 1 ). Many studies have investigated the role of Wnt signaling in the tumorigenesis, with an focus on signaling that relies on β-catenin (31). In the absence of Wnt, a complex of β-catenin degradation proteins, including Axis inhibition protein (AXIN), adenomatous polyposis coli (APC), and glycogen synthase kinase 3-beta (GSK-3-beta), is formed. This complex breaks down β-catenin through phosphorylation at both serine and threonine residues. When Wnt is present, it binds to one of the ten Frizzled (FZD) receptors, leading to the formation of a receptor complex containing Wnt, FZD, lipoprotein receptor-related protein (LRP), Disheveled (DVL), and AXIN. Inside this dynamic complex, DVL is phosphorylated and eventually blocks GSK-3-beta, which makes β-catenin less phosphorylated and prevents its proteolytic degradation (32). β-catenin is then stored in the cytoplasm and can be translocated to the nucleus, where it interacts with members of the TCF/LEF family of gene-activating transcription factors, such as CREB-binding protein (CBP) and p300. c-Myc and cyclin D1 among the many target genes (33). Receptor tyrosine kinase-like orphan receptor 2 (ROR2), a transmembrane orphan receptor tyrosine kinase required for non-canonical Wnt signaling, can also function as canonical signaling through FZD2 interaction (34). Therefore, ROR2 and other Wnt-binding receptors like the receptor tyrosine kinase RYK may play a regulatory role in β-catenin-independent Wnt signaling. Wnt ligands that activate the two non-canonical Wnt pathways include Wnt4, Wnt5a, Wnt7a, Wnt11, and Wnt16. Wnt11. For instance, stimulates the PCP signaling pathway, which in turn activates the small GTPases RAS homologue gene-family member A (RhoA) and Ras-related C3 botulinum toxin substrate 1 (RAC1) (35). This causes the stress kinases Jun N-terminal kinase (JNK) and Rho-associated coiled-coil-containing protein kinase 1 (ROCK) to be activated, altering cell adherence and migration. Wnt5a is the most well-known Ca2⁺-dependent Wnt signaling inducer, triggering STP via DVL-3 and phospholipases (36). When this pathway is activated, there is a temporary increase in cytoplasmic free Ca2⁺, which can stimulate the PKC family, calcium calmodulin mediated kinase II (CaMKII), and the calcineurin phosphatase. In addition to Ca2⁺-dependent Wnt STP, tumor cells have been found to have a novel non-canonical Wnt STP mediated by FYN tyrosine kinase and STAT3 (37). Wnt proteins are controversial in that they may interact with receptors, control the Wnt STP and Wnt-related elements. For instance, non-canonical Wnt signaling inhibits canonical Wnt activity via a variety of pathways, including PKC-alpha, CaMKII-Transforming growth factor β-Activated Kinase (TAK)1, Nemo-like Kinase (NLK), Siah2 E3 ubiquitin ligase, and calcineurin-NFAT. Non-canonical Wnt ligands have also been shown to activate the canonical pathway and further complicate Wnt signaling, making modification in cancer therapies challenging (38).

According to the circular structure and lack of 3′-end and 5′-end of circRNAs, they have higher stability than linear RNAs and resist degradation by RNAase R (39, 40). CircRNAs play an essential role in various cellular processes and gene expression via regulating RNA polymerase II function and participating in mRNA processing (41). Thousands of genes have been found to produce circRNAs in mammals. Unlike linear RNAs, circRNAs are generated through the non-sequential back-splicing (or exon skipping) of protein-coding RNA precursors catalyzed by spliceosomes or ribozymes (42). CircRNAs may arise from different gene parts such as exons, introns, intergenic sites, antisense, and untranslated regions (UTRs) (43) and are divided into five major types based on the origin of their gene: Exonic circRNA (the most abundant type), intronic circular RNA, Exon-intron circular RNA, Intragenic circular RNAs, and Antisense circular RNAs (39). The two most widely accepted models for exonic-circRNA biogenesis are lariat-induced circularization and intron pairings (44). Non-adjacent exons of a pre-mRNA become closer together in the lariat-driven circularization model by developing an intermediate lariat structure containing multiple exons and introns. Exonic circRNA is produced after intron separation by connecting the downstream exon (splice donor) and upstream exon (splice receptor) (45). In some cases, during lariat-driven circularization, introns are not spliced out, resulting in the formation of exon-intron circRNA (46). The intron pairing-driven circularization is based on the complementary pairing of Alu repeats on the intron ends, followed by pre-mRNA reverse splicing, which results in the formation of exonic and exon-intron circRNA (43). The functions of circRNA generally are: regulating gene splicing and transcription, regulating protein translation and function, sponging miRNA, and interacting with RNA-binding proteins (RBP) (47, 48). Localization of circRNA can also determine its function, as nuclear circRNAs mainly regulate transcriptional processes, and cytoplasmic circRNA mainly regulates the functions of miRNA and protein. Even though most circRNA are ncRNAs, recent studies revealed that some cytoplasmic circRNA can be translated to peptides (49). Expression of circRNA is different in various cell types and tissues; for instance, circRNA is widely distributed in the brain and is involved in brain development and function (50, 51). As circRNAs play a pivotal role in regulating many cellular processes, aberrant expression of circRNA leads to impaired cellular function. Several investigations have revealed that circRNA is overexpressed in a variety of human diseases, including cardiovascular disease, neurologic disease, autoimmune disease, and cancers (52). CircRNA can exert tumorigenesis or tumor suppressor effects through various mechanisms, such as regulating cell proliferation and apoptosis. Therefore, circRNA can be considered a diagnostic marker and therapeutic target in several diseases, including different types of cancers.

Given the severity and high recurrence rate of HCC, the prognosis remains poor. As a result, effective diagnostic and prognostic biomarkers are required. Several biomarkers have been suggested for HCC diagnosis and prognosis, including alpha-fetoprotein (AFP), Lens culinaris-agglutinin-reactive fraction of AFP (AFP-L3), protein induced by vitamin K absence orantagonist-II (PIVKA-II), vascular endothelial growth factor (VEGF), hypoxia-inducible factor (HIF), and others (53). On the other hand, circRNAs are increasingly linked to HCC progression and migration, despite their unknown function (54). Shang et al. discovered that hsa_circ_0005075 is overexpressed in HCC tissues and correlates with tumor size (55). Similarly, Qin et al. discovered that hsa_circ_0001649 was down expressed in HCC cells, indicating that it could be used as a prognostic and diagnostic biomarker (56). Recent studies have linked high ciRS-7 and AFP levels in HCC to hepatic microvascular invasion (57). CircRNAs could also communicate with miRNAs and act as miRNA sponges, influencing HCC progression by controlling downstream of a desired gene (58). Hsa_circ_0001649 and hsa_circ_0005075 contain potential miR-182 and miR-93 binding sites, respectively (59). MiR-182 and miR-93 have been identified as possible biomarkers for HCC prognosis and diagnosis. Furthermore, mounting evidence suggests that Wnt-related circRNAs have diagnostic biomarker potential and are linked to tumor development (60). In the following sections, we discuss about Wnt signaling pathways and their relationship with HCC as well as the role of the circRNA/Wnt axis in the HCC progression.

CircRNAs have been demonstrated to modulate cell proliferation, migration, and apoptosis by regulating gene transcription or by activating or inactivating signaling pathways associated with cancer (60). The most circRNAs function as miRNA sponges, activating or inhibiting the Wnt signaling pathway (61). As research advances, more circRNA-Wnt pathway interactions will be discovered. CircRNAs can influence cancer development and advancement through direct or indirect interactions with the Wnt pathway. Wnt-related circRNA abnormal activation has been linked to cancer cell proliferation, progression, invasion, and metastasis in tumors of the gastrointestinal and respiratory tracts, the central nervous system, the musculoskeletal system, and the endocrine system (60). The relevant studies suggest that this abnormal expression could be used as prognostic and diagnostic biomarkers, as well as treatment targets of cancer. These findings add to our knowledge of the fundamental processes that lead to cancer initiation and development. As a result, an overview of the most recent research on the circRNA-Wnt/β-catenin pathway in HCC and its regulatory framework for developing new treatments was conducted ( Figure 2 and Table 1 ). The next section will discuss the expression, characteristics, activities, and processes of circRNAs/Wnt axis in HCC.

Clinicians have believed that treatment choices require early prognostic data. A growing body of evidence suggests that Wnt-related circRNAs could be valuable for prognosis (60). In several malignancies, these circRNAs have a strong correlation with 5-year overall survival, disease-, recurrence-, and progression-free survival. According to Li et al., there was a negative correlation between the overexpression of circCCT3 and the colorectal cancer patient’s chance of disease-free survival (119). It has also been demonstrated that lower recurrence-free survival is associated with reduced circZKSCAN1 expression in HCC patients (70). Furthermore, increased level of circ_0109046 in endometrial cancer patients can predict a worse 5-year overall survival (120).Based on studies, it has been determined that circ_0067934 and circZFR are potential prognostic biomarkers in patients with HCC (63, 106). Such research has crucial consequences for determining prognosis and selecting treatments. Wnt-related circRNAs are also linked to other important prognostic variables. For instance, low levels of circMTO1 can predict late TNM stage and lymphatic metastases in colorectal cancer (CRC) (121). Thus, targeting Wnt-related circRNA expression could be a beneficial cancer therapeutic strategy. The most circRNAs function as miRNA sponges, activating or inactivating the Wnt pathway. It is also possible to control the target miRNAs of Wnt-related circRNAs. In CRC, for example, injection of MiR-582 significantly reversed the cellular mechanisms governed by circ_0009361 (122). CircRNA_NEK6 acts as a sponge for miR-370-3p, stimulating the FZD8/Wnt axis to promote the development of thyroid carcinoma (123). Circ_0121582 acts as a sponge of miR-224 in AML to overexpress GSK3β and trigger the Wnt/β-catenin pathway (124). Circ-SFMBT2 regulates the miR-1276/CTNNB1/Wnt/β-catenin axis, contributing to gastric cancer formation and progression. Knocking down circ_SMAD4 could inhibit the development of gastric cancer by decreasing cell proliferation (125). In addition, circ-ZNF124 silence has been shown to suppress NSCLC phenotypes (125). However, the current challenge is to identify targeted therapeutics capable of continually modulating circRNA expression and delivering its impact. This necessitates an in-depth knowledge of the biology and role of Wnt-related circRNAs.

NcRNAs are play a promising role in regulating different types of cancers includes HCC through the regulation of gene expression and modulation of biological pathways. In this context, targeting ncRNAs, include circRNAs, represents a novel method in cancer treatment and control drug resistance with advantages compared to conventional therapeutic strategies, such as specificity and potential reduction in toxicity (126, 127). For instance, Yi Li et al. proposed a potential approach to target circ_0000098 in HCC through a targeted drug delivery strategy using platelet-coated particles entrapped with doxorubicin and short hairpin RNA against circ_0000098. This was injected into mouse HCC models, and the results demonstrated remarkable sensitivity of HCC cells to doxorubicin and a reduction in drug resistance (127).

The Wnt signaling pathway is required for several physiological processes, including cell differentiation, motility, expansion, and muscular tissue formation. Both circRNA and the Wnt signaling pathway play important roles in cancer initiation and development. Developing evidence suggests that the circRNA/Wnt axis regulates tumor advancement, invasion, and metastasis by modulating the expression of cancer-associated genes. CircRNAs that are involved in the Wnt pathway typically perform their function by sponging miRNAs, which can be used as diagnostic markers. Wnt-related circRNAs may potentially be useful as biomarkers in the therapy of HCC. To manage tumor growth, researchers are attempting to either upregulate tumor-promoting circRNAs or downregulate tumor-suppressor circRNAs. Further research is needed to confirm the interactions and related mechanisms between suppressive miRNAs and corresponding circRNAs involved in the Wnt pathway for the diagnosis, prognosis, and treatment of HCC.

The authors confirm contributions to this paper as follows,

AM, conceptualization, investigation, database searching, writing, and review manuscript. FZ, ST, HR, MM, YG, NH, SH, SG, RM, BG, and ZA, data collection, writing, draft preparation, and figure design. MA and HM contributed in the project administration, editing, validation, coordinated the study, and accurately evaluated the manuscript. All authors contributed to the article and approved the submitted version.