Cold-inducible RNA-binding protein (CIRBP; also called CIRP and hnRNP A18) was identified as a cold-shock protein and an RNA-binding protein (RBP) expressed following a variety of stressors, such as hypoxia, cold shock and UV radiation (1-3). In total, two major CRIBP transcripts are expressed in cells through N⁶-methyladenosine modification-mediated alternative splicing (2,4-6). The large isoform of CIRBP (CIRBP-L) contains 297 amino acids and another short one (CIRBP-S) encodes 172 amino acids (Fig. 1). CIRBP is translated in the nucleus and migrates to the cytoplasm following stimulation (1,7). CIRBP contains an RNA-recognition motif (RRM) in the N-terminal domain and an arginine-rich motif (RGG) in the C-terminal region (1); it interacts with the 5′ or 3′-UTR of partner mRNAs through its RRM and regulates its expression post-transcriptionally (1,8). The RGG domain of CIRBP induces the protein-protein interaction, thereby modulating the protein-RNA interaction. Therefore, it is likely that CIRBP acts as a chaperone protein to interact and support RNA structure, assembly and transport of various proteins (9).

Moreover, CIRBP participates in multiple cellular signaling pathways as a crucial regulator. In the apoptosis pathway, mild hypothermia can protect cells from death in part through CIRBP, which activates the MAPK and NF-κB pathways (3). This indicates that CIRBP functions as a regulator of cell viability by activating survival signaling. Under mild hypothermia and UV radiation, CIRBP upregulates the expression of thioredoxin (TRX), which protects cells from oxidative damage by sequestering reactive oxygen species (ROS) (10,11). These findings indicate that CIRBP can induce anti-senescence signaling through TRX-mediated antioxidant activity. In addition, CIRBP is involved in various biological processes, including DNA repair, circadian clock regulation, telomere integrity, nutrient deficiency, inflammatory response signaling and cardiac electrophysiology (12-18). Furthermore, CIRBP is also involved in various human diseases, including sepsis, Alzheimer's disease and pancreatitis (19-24).

In recent years, numerous studies have suggested the involvement of CIRBP in several forms of human cancer. In the present review, the roles of CIRBP and its target mRNAs in cancer are summarized, and its potential as a therapeutic target is evaluated.

RBPs not only serve important roles in multiple physiological signaling pathways, but also act as important regulators of cancer genesis and progression. Several studies have reported that RBPs influence cancer progression by acting as either oncogenes or tumor suppressors (25,26). In order for normal cells to develop into cancer cells, they must go through a multi-step process to acquire the hallmarks of cancer. Hallmarks of cancer have been previously described and updated with newly identified characteristics of cancer (27). In the present review, the role of CIRBP in human cancers was summarized based on the hallmarks of cancer. Similar to other RBPs, CIRBP has a promotive or inhibitory regulatory effect on carcinogenesis, depending on the cancer subtype (Table I).

CIRBP is commonly overexpressed in a number of cancer tissues and cell lines. It acts as an oncogene by increasing the stability and translation of cancer-associated mRNA targets. However, several studies have also suggested the potential of CIRBP as a tumor suppressor by modulating the stability of target mRNAs (Fig. 1; Table II). CIRBP can bind the 5′ and 3′-UTRs of mRNAs, as well as poly U sequences at the 3′-ends (68). It has been suggested that its binding is important for the translation of interacting mRNAs by regulating polyadenylation and 3′-end cleavage (7,37). In the context of stress-induced regulation, abnormal upregulation of CIRBP promotes hypoxia inducible factor (HIF)-1α expression (29). Due to stabilization of the HIF-1α mRNA transcript, increased HIF-1α can bind to the promoter region of prostaglandin I2 synthase, a tumor suppressor, resulting in its downregulation (29) and an increase in the growth and invasion of cancer cells. An in vitro study demonstrated that CIRBP can also increase the mRNA stability of cyclin E1 in breast cancer (69). Responding to DNA damage, CIRBP can bind to the 3′-UTRs of TRX, replication protein A2 and ATR serine/threonine kinase mRNAs and increase their translational efficiencies (7,10,70). A recent study reported that, in luminal breast cancer, CIRBP is upregulated and enhances oncogenic properties by downregulating the CST3 mRNA expression levels (31). Notably, CIRBP can also enhance telomere maintenance by upregulating TERT mRNA levels (40). In most human cancer cells, active telomerase is upregulated, highlighting the importance of TERT expression and telomerase activity in promoting cancer progression (71,72). Other CIRBP-mediated regulatory effects have also been reported in human cancers. For example, CIRBP can increase phosphorylation of ribosomal protein S6, and eukaryotic translation initiation factor 4E-binding protein1, a protein that regulates the elongation phases of translation (73). In addition, CIRBP can promote cell proliferation by upregulating cyclin D1 and downregulating p27 via ERK signaling (33). Within the MAPK pathway, ERK signaling is involved in various human diseases, including inflammatory-related diseases and cancer (74,75). Additionally, CIRBP reduces phosphorylation of p27 by interacting with DYRK1B and inhibiting its binding to p27 in mouse germ cells (32). CIRBP also interferes with the phosphorylation of cyclin D1 by DYRK1B, thereby stabilizing cyclin D1 and ultimately increasing proliferation (32). Conversely, another study showed that CIRBP had an anti-proliferative function by binding to the 5′-UTR of p27 and increasing p27 expression in mouse embryonic fibroblasts (37).

The association between cancer and inflammation has been reported in numerous studies. In chronic airway inflammation disease, CIRBP upregulates mucin-5AC, which is associated with pulmonary disease via NF-κB/TLR4 signaling (76). In a CAC mouse model, CIRBP depletion reduced the level of inflammation markers, such as TNF-α and IL-23, and consequently decreased the susceptibility to CAC development (52). CIRBP can induce ROS accumulation by increasing the expression of inflammatory cytokines (IL-6 and IL-1β) in liver-specific macrophages. Conversely, CIRBP-knockout mice exhibited a decreased level of inflammatory cytokines with attenuated ROS accumulation (53). Together, these studies suggest that CIRBP may function as a tumor promoter or tumor suppressor by modulating the expression of inflammatory mediators.

Applicable prognostic cancer biomarkers in cancer are crucial for better tumor prediction and treatment planning. Several studies have shown the potential of RBPs as prognostic markers for various types of cancer, such as gastric or breast cancer (77,78). Consequently, databases such as TCGA (https://portal.gdc.cancer.gov/) and GEO (https://www.ncbi.nlm.nih.gov/geo/) containing the expression level of CIRBP in samples from patients with cancer were selected, and the potential of CIRBP as a prognostic marker in human cancers was presented (Table III).

A recent study indicated that CIRBP is methylated in the plasma of non-small cell lung carcinoma (NSCLC) with occult lymph node metastasis. RNA sequencing data obtained from The Cancer Genome Atlas (TCGA) also revealed that the mRNA expression levels of CIRBP are higher in metastatic tissues compared with primary breast tumor samples (79). Similar to previous RNA sequencing data, CIRBP is upregulated in invasive ductal carcinoma (59) and in patient with brain metastases with a high recurrence rate (60). These studies suggest that CIRBP can promote cancer metastasis. Conversely, CIRBP is inversely correlated with lymph node invasion and distant metastasis in nasopharyngeal carcinoma (56). Additionally, CIRBP is differentially upregulated in non-triple negative breast cancer (TNBC) compared with metastasis-related TNBC (63). Although the evidence of CIRBP involvement in metastasis is still incomplete, CIRBP may potentially represent a crucial component of the metastatic process.

To overcome low survival rate of patients with metastatic cancer, it is necessary to identify the biomarkers for early diagnosis before metastasis to distant organs. Recently, genomic profiling analysis using Gene Expression Omnibus and TCGA datasets revealed that high expression levels of CIRBP are correlated with good prognosis in patients with early-stage NSCLC with low metastasis (80). Stratification according to TNM classification revealed that a higher CIRBP expression level is frequently detected in T1-T2, M0 and I-II tumors compared with T3-T4, M1 and III-IV nasopharyngeal carcinoma tissues, respectively (56). Likewise, gene expression profiles based on microarrays have demonstrated that CIRBP is significantly upregulated in benign tumors compared with malignant ovarian cancers (81). Conversely, CIRBP is significantly associated with histological classification, clinical stages and lymph node metastasis in OSCC samples (55). Although it is important to classify the subtypes of breast cancer, there is currently no good parameter to distinguish invasive breast carcinoma (IBC) from ductal carcinoma in situ (DCIS) (82). By screening autoantibodies using protein microarrays with DCIS and IBC samples, CIRBP was identified as an autoantibody signature that could discriminate DCIS from IBC. This result indicates that CIRBP may represent a novel prognostic marker in breast cancer (83). CIRBP is also a splicing factor (SF), which are important factors in cancer progression (84,85). By comparing RNA expression levels of various SFs between primary cancer and their metastatic counterparts from TCGA, it was found that CIRBP expression is higher in metastatic tissues compared with original tumors (79). Along with SF, alternative splicing events (ASEs) are also responsible for cancer development and progression (86,87). RNA sequencing and ASE-related datasets of breast cancer samples obtained from TCGA revealed that CIRBP, may serve as a predictor for survival in prognostic-related ASE (59). Together, these results suggest that CIRBP may function as a prognostic marker in a number of cancer types.

The use of cytotoxic drugs is the main treatment method for advanced and aggressive cancers, and cancers without specific therapeutic targets. However, resistance to cytotoxic chemotherapy and drug side effects are major barriers to attaining a complete response (88). Several studies have reported that resistance to chemotherapy is enhanced by secretory molecules that can promote the repair signaling coordinated by TLR4 (89,90). CIRBP can trigger the secretion of TNF-α through the activation of TLR4 and NF-κB in macrophages. Several studies have also reported that CIRBP can mediate inflammatory signaling via regulation of TLR4 signaling (76,91). Based on these results, CIRBP-derived oligopeptides or neutralizing antibodies were demonstrated to ameliorate sepsis-mediated injury of the lung and kidney (15,92,93). These CIRBP antagonists can block the interaction of extracellular CIRBP with TLR4/myeloid differentiation 2 receptor complex to inhibit the downstream signaling (15).

The circadian clock is an important molecular mechanism for the maintenance of homeostasis and its imbalance facilitates tumor progression (94). In various cancer types, circadian genes are associated with chemoresistance and cancer progression (95,96). Thus, there is a novel approach that indirectly or directly targets circadian clock genes to remove cancer and improve survival rates (97,98). Several studies have suggested that CIRBP can be used in cancer treatment by regulating circadian genes (13,68). Chemotherapeutic drugs can induce apoptosis, necrosis and autophagy in cancerous tissues (99,100). As CIRBP exerts a protective role in apoptosis in neurons and cardiac cells, combined therapy of cytotoxic drugs with anti-CIRBP therapeutics may improve the response efficacy and survival rate in patients with neuronal and cardiac abnormalities (101,102).

Small molecules that complement biologics, such as antibodies, have advantages of cost effectiveness and cell permeability for applications in cancer therapy. Several chemical probes targeting specific RBPs have been shown to be able to function as selective inhibitors by modulating RBP-target mRNA interactions (103-107). Recently, it has been reported that a probe can interfere with CIRBP-RNA associations, inhibit cytotoxic T-lymphocyte protein-4 and TRX expression, and suppress the progression of various cancer types without side effects (108). Further studies are needed to apply these CIRBP antagonists for cancer therapy in the future.

The present review summarized recent findings about the roles of CIRBP in cancer development, metastasis and cancer therapy (Fig. 2). During cancer proliferation and metastasis, the function of CIRBP appears to be driven primarily by promoting the stability and translation of target mRNAs. Conversely, certain studies have demonstrated that CIRBP serves as a tumor suppressor in cancer progression by modulating the multiple steps of cell proliferation. These controversial roles of CIRBP in human cancers may originate from the alternative splicing of the CIRBP transcript (2,4-6). Differentially expressed splicing variants may interact and modulate the different target mRNAs, depending on cancer subtypes or cell contexts. To understand the exact role of CIRBP in cancers, target mRNAs of each splicing isoform should be identified and the regulatory mechanism analyzed in human cancers. Clinical studies have shown that CIRBP may represent a prognostic marker of cancer progression. Although numerous studies have reported roles of CIRBP in cancer biology, further detailed studies are required to elucidate the exact role of CIRBP in human cancers and to evaluate the potential of the application of CIRBP-targeted cancer therapy.