As the most important intracellular nuclear transcription factor, the nuclear factor kappa B (NF-κB) promotes the transcription of genes with κB binding sites that are responsible for the manipulation of multiple biological processes, such as inflammation, immune response, and apoptosis (1). Recently, it has been reported that constitutive activation of the NF-κB signaling is observed in hepatocellular carcinoma (HCC) (2, 3). Additionally, accumulating evidence has shown that NF-κB plays a critical role in the transcriptional regulation of genes that concern diverse pathological aspects of HCC with respect to cell transformation, proliferation, survival, invasion, metastasis and drug resistance (4–6). HCC that accounts for 80%-90% of all primary liver cancers is ranked as the second leading cause of cancer-related deaths and the fifth most common human cancer around the world (7–10). Therefore, targeting NF-κB signaling pathway warrants future research, which may contribute to novel HCC-specific diagnostic and therapeutic strategies.

NF-κB is assembled into a heterodimeric or homodimeric complex by different subunits of the Rel family, which consists of five members, including RelA (p65), RelB, C-Rel, NF-κB1 (p50/p105), and NF-κB2 (p52/p100) (11). Under physiological conditions, these subunits are associated with the inhibitor of κB (IκB), whose function is to effectively sequester NF-κB in the cytoplasm. When cells are stimulated by a cascade of signaling events such as stress, bacteria, viruses or cytokines, NF-κB becomes rapidly activated, then translocates into the nucleus where it binds to the κB elements of gene promoters or enhancers, thereby triggering transcription of target genes (12). Typically, there are two different pathways that mediate NF-κB activation, including a canonical and a noncanonical pathway. In the canonical pathway, the key event is the release of NF-κB from the NF-κB/IκB trimer. In response to specific stimuli, NF-κB-bound IκB is phosphorylated at Ser32 and Ser36 residues via the IκB kinase (IKK) complex formed by two catalytic subunits (IKK1/2, a.k.a. IKKα and IKKβ) and the scaffold/adaptor protein NF-κB essential modulator (NEMO; also known as IKKγ) (13). Phosphorylated IκB subsequently quickly undergoes polyubiquitination through the SCF-β-TrCP complex followed by 26S proteasome-mediated degradation, allowing the nucleus entry of NF-κB (14–16). The noncanonical NF-κB pathway, which is usually activated following the induction of members of the tumour necrosis factor receptor (TNFR) superfamily, mainly relies on NF-κB-induced kinase (NIK) and IKKα subunits to induce phosphorylation of NF-κB precursor protein p100 at Ser866 and Ser870 residues (14, 17). Phosphorylation targets p100 for subsequent partial processing to form the mature NF-κB p52 subunit through the ubiquitin-proteasome pathway, which then binds to RelB to form a p52-RelB heterodimer with transcriptional activity (17).

Apart from the common pathways described above, recent evidence indicates that non-coding RNAs (ncRNAs) act as vital regulatory roles in the NF-κB signaling by diverse mechanisms. Despite a lack of protein-coding potential, ncRNAs serve as pivotal functional components or regulatory molecules for genetic expression (18). Generally, ncRNAs can be categorized into housekeeping ncRNAs and regulatory ncRNAs in term of their discrepancy in expression levels and functional features. The former that are profusely and omnipresently expressed in cells are necessary for cells to survive, the latter usually participate in genetic expression at epigenetic, transcriptional, and post-transcriptional levels (19). Among regulatory ncRNAs, the regulation of NF-κB signaling in HCC by microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) has been relatively well characterized, while other regulatory ncRNAs, for instance, small interfering RNAs (siRNAs), PIWI interacting RNAs (piRNAs) as well as circular RNAs (circRNAs), have been rarely reported to regulate NF-κB signaling of HCC. Herein, we put a particularly focus on up-to-date findings regarding the role of ncRNAs in NF-κB signaling of HCC ( Figure 1 ), then discuss the potential significance of ncRNAs in overcoming the obstacle of NF-κB-associated chemoresistance in HCC, finally future research directions and challenges are addressed.

miRNAs are a distinct kind of evolutionarily-conserved and endogenous ncRNAs of 19–25 nt in length (20). The predominant function of miRNAs includes either accelerated degradation or reduced translation of target messenger RNAs (mRNAs), which can be achieved by the conjugation of a miRNA to the 3 ‘untranslated region (UTR) of the target mRNA (21–23). miRNAs have recently acquired considerable attention in the research of hepatic carcinoma (24). Overwhelming evidence has emerged that miRNAs are involved in the malignant biological activity of HCC by playing the part of either oncogenic or tumor suppressor factors (25), some of which have been reported directly or indirectly to regulate NF-κB pathway and/or NF-κB activity to mediate HCC development ( Table 1 and Figure 1 ).

By convention, ncRNAs with the minimum size limit of 200 nt are defined as lncRNAs (76). Classification of lncRNAs is still at its infancy due to few structural, functional or mechanistic features common to all mammalian lncRNAs. Here, lncRNAs are categorized according to their modes of action and functions. Generally, the potential modes of action of lncRNAs depend on their subcellular localization. Functions of lncRNAs within the nucleus of the cell include transcriptional regulation, enhancer-associated ncRNAs, epigenetic regulation and regulation of nuclear architecture; Cytoplasmic functions of lncRNAs include targeting mRNAs for degradation by a process called Staufen 1 (STAU1)-mediated decay, maintaining mRNA stability and functioning as miRNA sponges (18). To date, lncRNAs have been revealed as essential regulators in HCC (77). Interestingly, some lncRNAs have been identified to implicate in hepatocarcinogenesis by modulating the NF-κB signaling ( Table 2 and Figure 1 ).

With the emergence of new drugs and the standardization of chemotherapy regimens, chemotherapy has become one of the most important modes of cancer treatment besides surgery, which has improved the survival rate and time of tumor patients to a certain extent. However, chemotherapy has its own limitations, such as high toxicity, immunosuppression, and primary and/or secondary resistance of tumor cells(chemoresistance). Of the three limitations listed, chemoresistance poses the greatest obstacle to the effective treatment of HCC patients using chemotherapy (91). Therefore, in order to improve the efficacy of chemotherapy and overcome chemoresistance, the mechanisms of chemoresistance and the molecular regulatory networks implicated in HCC still need to be further studied. It has been detected that NF-κB signaling is frequently activated in HCC, which is closely related to the onset of chemoresistance in this setting (92, 93). In addition to the induction of NF-κB signaling by extracellular ligand/cell-surface receptors interactions, chemotherapy-induced DNA damage can also activate NF-κB, leading to the transcription of numerous NF-κB-activated anti-apoptotic genes, the desensitization of cells to apoptosis, and further promotion of cancer progression (94, 95). Over the past years, the regulatory function of ncRNAs in hepatocarcinogenesis and chemoresistance has attracted extensive attention (10). As some reports show promising data, targeting the NF-κB pathway by ncRNAs seems to improve chemosensitivity of patients with HCC to chemotherapeutic agents.

Paclitaxel is one of the most widely used chemotherapy drugs, employed in the treatment of various malignant tumors (96–98), including HCC. However, chemoresistance of paclitaxel often occurs in patients with HCC, with NF-κB signaling being implicated in the mechanisms of paclitaxel-specific chemoresistance. As mentioned above, ncRNAs are of great use in improving the efficacy and chemosensitivity by targeting NF-κB signaling. For example, knocking down the expression of miR-16 increased the chemoresistance of HCC cell lines to paclitaxel through the NF-κB signaling, and the restoration of miR-16 expression effectively reversed chemoresistance of HCC by targeting IKKβ (30). Doxorubicin, another chemotherapeutic agent widely used against various malignant tumors, mainly interferes with the function of DNA topoisomerase II-α and breaks DNA double-strand to induce apoptosis of tumor cells (99). In HCC, chemoresistance to doxorubicin is another clinical problem yet to be solved. Doxorubicin was found to dramatically elevate phosphorylation level of p65, leading to the activation of numerous anti-apoptotic genes in HCC cells. However, restoring the expression of miR-26b dramatically blocked the nuclear translocation of NF-κB, further decreasing the occurrence of NF-κB-mediated chemoresistance of HCC cells to doxorubicin (29). Another study has proved that the over-expression of lncRNA 00607 enhances the sensitization of HCC cells to doxorubicin and other chemotherapeutic drugs via NF-κB p65/p53 signaling axis (81). LncRNA PDIA3P1 was an oncogenic lncRNAs and its presence was also associated with chemoresistance of HCC to doxorubicin, which protected HCC cells from doxorubicin-induced apoptosis through NF-κB activation. Therefore, inhibition of PDIA3P1 was a useful method to restore the chemosensitivity of HCC to doxorubicin (55). Cisplatin is a platinum-containing anticancer drug, which functions to facilitate the apoptosis of cancer cells through the interference in DNA repair mechanisms and the induction of DNA damage (100). Recently, a study uncovered that the chemosensitivity of HCC to cisplatin was correlated with the dysregulation of miR-1180. The higher the expression of miR-1180, the more severe the extent of chemoresistance. In terms of mechanism, high expression of miR-1180 facilitated the downregulation of NF-κB-negative regulators which in turn caused the NF-κB-mediated chemoresistance of HCC cells to cisplatin (50). Sorafenib is a first-line chemotherapy agent for advanced HCC patients (101). As an oral multikinase inhibitor, sorafenib plays an anti-cancer role by inhibiting cell proliferation and angiogenesis (102). Unfortunately, most HCC patients are prone to develop chemoresistance to sorafenib during treatment and ultimately gain poor clinical outcomes (102). Given the central role of sorafenib in HCC therapy, it is urgent to further study the exact mechanisms of sorafenib resistance in HCC so as to improve chemosensitivity of HCC to sorafenib. Recent studies have revealed that the activation of NF‐κB is identified as a crucial molecule leading to sorafenib resistance in HCC (102–104). Meanwhile, ncRNAs have been considered to be vital regulators in sorafenib resistance of HCC (105). Notably, certain ncRNAs that are discussed in this review have been confirmed to be involved in sorafenib resistance in HCC, such as miR-124, NEAT1, SNHG16 and so on. Therefore, subsequent studies will need to focus on understanding whether these ncRNAs are implicated in the development of NF‐κB-mediated sorafenib resistance in HCC.

HCC can be in part ascribed to NF-κB signaling activation, the aberrant activation of which is linked with initiation, progression, metastasis, and drug resistance of HCC. As discussed in this review, small and long ncRNAs have emerged as promising molecules for regulating NF-κB signaling, and the restoration or inhibition of ncRNAs expression levels has shown high therapeutic potential in HCC. Generally, there are two therapeutic strategies that target ncRNAs in HCC. The first method aims to restore the tumor suppressor activity of ncRNAs that are lost or downregulated in HCC by using synthetic ncRNA molecules with same function, such as ncRNA mimics or ncRNAs expression vectors. The second approach aims to block the oncogenic activity of ncRNAs that are abnormally overexpressed in HCC. Both strategies can be applied to miRNAs, but in the case of lncRNAs, blocking their function is more reasonable than restoring the biological activity of these transcripts (106). CircRNAs have become a latest research hotspot in the field of ncRNAs, and their potential clinical value has been widely studied. Compared with linear RNAs, circRNAs are highly stable and covalently closed loop transcripts without 5′ caps and 3′ tails (107, 108). They are widely found in a variety of eukaryotes with extremely significant biological functions (109). With the development of high-throughput sequencing techniques, numerous circRNAs have been discovered to be correlated with the occurrence and development of various diseases, which especially exert an important influence on the pathogenesis, diagnosis, treatment and prognosis of tumors (109, 110). Recent evidence indicates that circRNAs might involve in the pathogenesis of HCC and exert their regulatory roles in HCC mainly by sponging miRNAs (109). Although there have been few reports on the crosstalk between circRNAs and NF-κB in HCC, some circRNAs have been discovered to be able to sponge the miRNAs that are reported herein which can mediate NF-κB signaling in HCC. For example, circZNF609-miR-342-3p (111), circPTGR-miR-129-5p (112) and circHIPK3-miR-124 (113) pathways have been discovered in HCC recently, suggesting that these circRNAs that function as ceRNAs might mediate HCC progression though regulating NF-κB signaling. Moreover, circRNAs have also been discovered to have complex roles in mediating NF-κB signaling, which contributes to the development of colorectal cancer (114), ovarian cancer (115), breast cancer (116) and other cancers (117, 118). This evidence further confirms that circRNA-NF-κB pathway may serve as a novel future research direction in HCC.

Although ncRNAs represent promising targets for human cancer therapeutic interventions, several issues still need to be addressed. First, the relationship between the dysregulation of ncRNAs and HCC remains unclear based on the current studies, further in-depth research is needed to ascertain whether the dysregulation of ncRNAs leads to the occurrence and development of HCC or whether the development of HCC causes the abnormal expression of ncRNAs in the first place. Second, the tumor microenvironment of HCC is a complex system composed of many cell subsets. It will be quite crucial to elucidate the cellular sources of the abnormally expressed ncRNAs in the tumor microenvironment of HCC, and define the mechanisms employed by those ncRNAs in the initiation and development of HCC. Future research will need to focus on understanding the origins, action targets, traits and functions of ncRNAs, particularly identifying the characteristics of deterministic ncRNAs in HCC and the specific action targets of these ncRNAs through large-scale and comprehensive analytical studies. These findings will help to develop potential diagnostic, therapeutic, and prognostic approaches for HCC. Finally, the development of ncRNA-based anti-HCC therapies is still in its infancy, therefore, more attention should be paid to the multi-targets, off-target, instability and other defects in the research and clinical application of ncRNA mimics and antagonists, so as to strengthen anti-HCC therapeutic efficacy and reduce side effects.

YZ wrote the manuscript. JS prepared figures. SL made tables. MZ and YL determined the topic of the manuscript and participated in its coordination and modification. All authors contributed to the article and approved the submitted version.

The study was supported by Grants from the National Nature Science Foundation of China, No. U20A20348, the National Nature Science Foundation of China, No. 81871646, the Scientific and Technological Innovation Leading Talents of “Ten Thousand Talents Plan” of Zhejiang Province, No. 2020R52010.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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