The RNF protein family contains a large number of members that are associated with several types of digestive system tumors, such as colorectal cancer and hepatocellular carcinoma (24,25). RNFs also play a vital role in the occurrence of GC. RNF6 encodes a 685-amino-acid protein with a coiled-coil domain at the N-terminus and a RING-H2 finger at the C-terminus (26). RNF38 shares a similar structure with RNF6 (27), and previous studies have shown that RNF6 and RNF38 are overexpressed in GC and regulate GC cell growth. Both RNF6 and RNF38 induce polyubiquitination of tyrosine-protein phosphatase non-receptor type 6 and subsequently enhance STAT3 signaling, which promotes the proliferation of GC cells (Fig. 1) (15,16).

RNF26, which is located in the endoplasmic reticulum, is a polypeptide of 433 amino acids with an N-terminal leucine zipper domain and a C-terminal RING finger domain (28,29). The expression level of RNF26 is upregulated in several types of human cancer cell lines, including HL-60, HeLa S3, SW480 and MKN7 cells (28). As the substrate protein of RNF26, the mediator of interferon regulatory factor 3 can be ubiquitinated and regulate the innate antiviral response (30). However, the functional mechanism of RNF26 in GC has not been fully elucidated.

Similar to RNF6, RNF26 and RNF38, RNF185 also acts as an oncogene in GC, but with distinct subcellular localization and a distinct mechanism of action. RNF185 localizes to the mitochondria, contains a C3HC4-type RING domain and two transmembrane domains (31). PRA1 family protein 3 (JWA) is a multifunctional cytoskeleton-binding protein induced by all-trans retinoic acid. The function of JWA involves enhancing intracellular defenses against H₂O₂-induced oxidative stress and reducing cell apoptosis (32). RN F185 can downregulate the expression of JWA and promote GC cell migration (Fig. 1) (33). Generally, higher expression of RNF185 is associated with a worse prognosis of GC.

RNF43 and zinc/RING finger 3 (ZNRF3) are homologous proteins that have antagonistic effects in combination with leucine-rich repeat-containing G-protein-coupled receptor 5 (Lgr5) (34). The Lgr5 protein, a member of the G-protein coupled receptor family of proteins, was identified as a novel gastrointestinal stem cell marker (35). Moreover, Lgr5-positive gastric stem cells are cancer-initiating cells able to drive GC cell self-renewal, which contributes to malignant progression (35–37). RNF43 negatively regulates the Wnt/β-catenin pathway by recognizing Lgr5 and markedly downregulating the expression of Lgr5 protein (34). A previous study demonstrated that ZNRF3 acts as a tumor suppressor by downregulating the expression levels of β-catenin and transcription factor 4 protein (38). RNF43/ZNRF3 mediates the ubiquitylation of seven transmembrane domains of frizzled receptors and subsequently inhibits the proliferation of GC cells (39). A few large-scale genomic analyses have reported RNF43 mutations in different cancer types, including GC (40). Whole-genome sequencing revealed that RNF43 is mutated in 4.8% of microsatellite-stable and 54.6% of microsatellite-unstable tumors (41). The mutational landscape of RNF43 may provide a new approach to facilitate genome-guided personalized therapy in GC. Overall, RNF43/ZNRF3 is a tumor suppressor and a potential therapeutic target for GC. Therefore, five members of the RNF subfamily participate in regulating the development of GC, where RNF6, RNF26, RNF38 and RNF185 function as carcinogenic factors, while RNF43/ZNRF3 acts as a tumor suppressor.

MARCH8 is a member of the MARCH subfamily. A recent study identified 11 E3 ligases that contained the RING-CH domain and were named the MARCH1-11 subfamily (42). The structure of MARCH8 contains the RING-CH domain and transmembrane domains. JWA is a downstream protein of RNF185 ligase (33); it promotes the ubiquitination of death receptor 4 by increasing the expression of MARCH8 in GC cells, thereby reducing tumor necrosis factor-related apoptosis-inducing ligand-mediated apoptosis (Fig. 1) (43). Moreover, MARCH8 induces the apoptosis of GC cell lines by inactivating the PI3K and β-catenin/STAT3 signaling pathways (44). In summary, MARCH8 is a tumor suppressor in GC.

TRIM proteins form a subfamily that regulates multiple cellular processes. TRIMs share a common N-terminal tripartite domain, a RING domain, one or two B-boxes and coiled-coil domains (45). To date, three E3 ligases have been shown to be involved in the development and metastasis of GC. They are TRIM28 (also known as KAP1 or TIF1-β), TRIM29 and TRIM59 (46–49). TRIM28 is a universal transcriptional corepressor (50). The overexpression of TRIM28 causes peritoneal carcinomatosis and poor prognosis in GC (47). Similar to TRIM28, the expression of TRIM29 is also upregulated in GC; the high expression of TRIM29 mRNA may be an independent predictor of lymph node metastasis and depth of invasion (48). TRIM59 has been reported in several human tumors and acts on diverse signaling pathways, such as the focal adhesion kinase/AKT/matrix metalloproteinases pathway in epithelial ovarian cancer (51), the PI3K/AKT/mTOR pathway in human cholangiocarcinoma (52), the Wnt/β-catenin signaling pathway in neuroblastoma (53), the NF-κB pathway in non-small cell lung cancer (54) and the p53 signaling pathway in GC (49). The p53 signaling pathway can be negatively regulated by the E3 ligase TRIM59, which enhances the ubiquitination and subsequent degradation of p53 (Fig. 2). TRIM59 might promote gastric carcinogenesis through this mechanism (53). In summary, TRIM28, TRIM29 and TRIM59 play oncogenic roles in gastric tumorigenesis, but only the regulatory mechanism of TRIM59 has been elucidated.

PIRH2 E3 ligases are crucial negative regulators of p53 (Fig. 2). In addition to p53, many other proteins, such as p63, p73, c-Myc, p27, DNA polymerase Eta and checkpoint kinase 2, can be ubiquitylated by PIRH2 (55). PIRH2 plays an important role in the regulation of many types of tumors, such as glioma, lung cancer and breast cancer (56–58). In GC, PIRH2 is a key E3 ligase of p53 ubiquitination, and silencing of PIRH2 causes p53 protein accumulation (59). This study also demonstrates that a microRNA (miR)-100-RNF144B-PIRH2-p53-dependent pathway might be a novel mechanism of ubiquitin-mediated p53 degradation in GC cells (59).

CBL proteins contain two highly conserved domains: The N-terminal tyrosine kinase-binding domain and the C3HC4 RING finger domain (60). The mammalian CBL family consists of Cbl, Cbl-b and c-Cbl ligases (61). The three members have similar functions, perhaps due to the specific structure. The c-Cbl protein was the first CBL family protein discovered, followed by Cbl-b and Cbl (62). Previous studies have confirmed that all three CBL proteins are closely associated with GC. In 2000, a study reported that the c-Cbl protein was frequently tyrosine phosphorylated in a tumor-specific manner in human GC tissues (63). Subsequently, c-Cbl was found to be involved in stomach carcinogenesis by connecting with the EGFR system (64).

Cbl-b has a significant impact on the prognosis and drug sensitivity of GC, and a previous study showed that Cbl-b is an oncogene (64). However, subsequent studies provide different perspectives, in which Cbl-b is found to enhance the sensitivity to 5-fluorouracil and cetuximab in GC cells through the ubiquitination pathway (65,66). Other studies have also indicated that Cbl-b inhibits tumor metastasis and growth in multiple drug-resistant (MDR) gastric and breast cancer cells, as well as increasing the sensitivity of MDR cells to anticancer drugs (67–69). These findings provide a new research direction for the chemotherapy and targeted therapy of GC.

Cbl, in conjunction with the EGFR system, might be related to gastric carcinogenesis and metastasis (70). In summary, Cbl and c-Cbl are oncogenes in GC, whereas Cbl-b can act as an oncogene or tumor suppressor, and only its regulatory mechanism has been well clarified among the three members.

Previous studies have reported that the aberrant methylation of CHFR promotes the development of GC (71,72). As an E3 ligase, CHFR contains an N-terminal forkhead-associated domain, a central RING domain and a C-terminal cysteine-rich region (73). PARP-1 may be a substrate by CHFR for ubiquitination and degradation in GC; this process leads to cell cycle arrest before entering mitosis and inhibits the proliferation of GC cells (74). Thus, CHFR functions as a tumor suppressor in GC.

These two E3 ligases are rarely mentioned in GC. MKRN1 was first identified owing to its interaction with human telomerase reverse transcriptase (TERT) and modulation of telomere length homeostasis (75). The MKRN1-mediated ubiquitination of tumor suppressor ARF (p14ARF) was described in 2011 (76). MKRN1 induces the ubiquitination and degradation of p14ARF and downregulated p14ARF expression (Fig. 2) (76). Furthermore, MKRN1 overexpression was associated with well-differentiated gastric carcinoma, whereas p14ARF overexpression was associated with poorly differentiated gastric carcinoma. FIGC, a novel FGF-induced ubiquitin-protein ligase, consists of an N-terminal RING finger module and proline-rich region at the C-terminus. Only one study has shown that FIGC probably functions as an E3 ligase and is implicated in carcinogenesis through the dysregulation of fibroblast growth modulator (77). In brief, further research is needed to confirm the mechanism of these two E3 ligases in GC.

Dimeric RING domain E3 ligases can be classified into homodimers and heterodimers. MDM2, a heterodimeric RING ligase, was originally identified as a ubiquitin ligase E3 that promotes the degradation of tumor suppressor p53 (Fig. 2) (78). Subsequent studies suggest that MDM2 can ubiquitinate and degrade multiple signaling molecules in GC, including p53, forkhead box protein O3A (FOXO3A) and runt-related transcription factor 3 (RUNX3) (78–81). Human hTERT can interact with MDM2 and dramatically increase the ubiquitination of FOXO3A, resulting in the invasion of GC cells (79). RUNX3 is known as a tumor suppressor (82). MDM2 ligases can recognize Lys94 and Lys148 of RUNX3 and decrease the expression levels of RUNX3 (80). Overall, MDM2 acts as a carcinogenic factor in GC by affecting different signaling proteins.

CHIP includes a C-terminal U-box domain and an N-terminal tetratricopeptide repeat domain, which have E3 ubiquitin ligase activity and interact with the molecular chaperones Hsc70-Hsp70 and Hsp90, respectively (83). CHIP is characterized as a homologous dimeric RING ligase and antioncogene in human cancer (15,84). The U-box domain of CHIP facilitates TNF receptor-associated factor 2 ubiquitination for degradation and then inactivates NF-κB signaling (Fig. 1). A previous study also showed that CHIP expression prevents the angiogenesis and metastasis of GC (85). Above all, CHIP overexpression is correlated with good prognosis in GC patients, and targeting CHIP may be a new approach in GC therapy.

The CRL1 complex comprises SKP1, CUL1, RING box1 (RBX1) and a member of the F-box protein family (86). The F-box protein family can be further categorized into three subclasses: i) FBXW; ii) F-box and leucine-rich repeat (FBXL); and iii) F-boxes containing other domain motifs (FBXO) proteins. Each subunit of SCF has unique features as follows: i) CUL1 serves as a rigid molecular scaffold protein; ii) RBX1 contains a RING finger domain for the recruitment of E2 enzyme; iii) SKP1 functions as an adaptor; iv) F-box proteins act as a substrate-determining component (86,87), many of them function as E3 ligase and will be discussed in detail below.

The WD repeat domain comprises a 44–60 residue sequence that typically contains the GH dipeptide 11–24 residues from its N-terminus and the WD dipeptide at the C-terminus (88). This class of E3 ligases mainly recognizes proteins involved in cell cycle regulation and tumorigenesis, thereby regulating cancerous growth. FBXW7 and β-transducin repeat-containing protein (β-TRCP) are directly correlated with the progression of GC in the form of E3 ligases (89,90).

FBXW7 is a well-characterized SCF in GC and facilitates the destruction of oncogenic proteins, such as c-Myc, transforming protein RhoA (RhoA), transcription activator BRG1 (Brg1) and zinc-finger protein GFI-1 (GFI1) (90–93). These targeted proteins all govern gastric tumorigenesis; for example, Brg1 promotes the metastasis of GC (92), RhoA has been implicated in gastric tumorigenesis (Fig. 1) (94), and GFI1 promotes GC cell proliferation as an oncoprotein (93). FBXW7 is also regulated by several upstream signaling molecules. Previous studies indicated that microRNAs and long noncoding RNAs are involved in the occurrence and development of GC by altering the expression of FBXW7 (95,96). Therefore, FBXW7 is a complex tumor suppressor in GC because of its involvement in numerous upstream and downstream signals.

β-TRCP has two distinct isoforms, β-TRCP1 and β-TRCP2, which share similar biochemical properties (97). It was reported that β-TRCP1 and β-TRCP2 were predominantly expressed in the stomach and the small intestine, respectively. A previous study showed that β-TRCP2 might promote gastric carcinogenesis through the activation of the Wnt signaling pathway (98). Moreover, β-TRCPs participate in the regulation of the AKT signaling pathway (Fig. 3) and epidermal growth factor receptor/glycogen synthase kinase-3β/FOXP3 axis through the ubiquitination of PH domain leucine-rich repeat-containing protein phosphatase 1 and FOX3, respectively (99,100). One previous study has reported that β-TRCP can serve as a tumor suppressor or oncoprotein in the etiology of a variety of cancers depending on the type of tumor tissue (99). Nevertheless, β-TRCP might serve as an oncoprotein in GC.

FBXL proteins contain leucine-rich repeat sequences. FBXL2 and FBXL5 exhibit similar characteristics in GC (101,102). FBXL2 promotes the ubiquitination and degradation of FOXM1, which then inhibits GC proliferation (101). Similarly, FBXL5 can also suppress GC cell migration by the ubiquitination-mediated destruction of Cortactin and Snail1 (102,103). FBXO31, a member of the third class of the F-box protein family, can also target Snail1 for its ubiquitylation and degradation (104). Hence, FBXL2, FBXL5 and FBXO31 exert tumor inhibitory roles in GC.

CUL1 is a member of the CUL family and acts as a scaffold protein of the SCF ubiquitin E3 ligase (105). CUL1 is modified by the ubiquitin-like protein NEDD8 and enhances the activity of SCF ligases to p27 (106). Several studies have shown that diverse types of human malignant tumors are related to CUL1. CUL1 can facilitate cell proliferation in osteosarcoma, breast cancer, prostate cancer and lung cancer in vitro and in vivo (107–110). In addition, the co-expression of CUL1 and CUL2 induces the initiation of carcinogenesis in colorectal cancer by arresting p53-positive colon cells in the G1 phase of the cell cycle (111). Before these findings, immunohistochemistry results suggested that high expression levels of CUL1 were detected in 60% of all GC tissues, in a study of 792 patients (112). Further in vitro studies showed that increased CUL1 expression was correlated with poor patient survival by decreasing p27 expression (112) (Fig. 2). Therefore, CUL1, an oncoprotein, can be regarded as a prognostic biomarker of GC.

Von Hippel-Lindau disease was first considered to be a heritable cancer syndrome characterized by retinal and neuronal hemangioblastoma owing to a mutation in the VHL gene (113). Although the pVHL protein itself displays no enzymatic activity, pVHL functions as a substrate recognition subunit in CRL2 E3 ligases after binding with the elongin and CUL proteins (114). pVHL is a tumor suppressor in renal cell carcinoma (RCC) (115,116). Serine/threonine-protein kinase NEK8 is a serine/threonine-specific protein kinase family member that serves a role in the progression of mitosis and carcinogenesis (117). NEK8 is reported to be a novel target of pVHL in the regulation of RCC and GC (118,119); pVHL promotes NEK8 protein degradation via the proteasome-ubiquitin pathway in GC cells (119). In summary, the E3 ligase pVHL targets NEK8 and inhibits the proliferation, colony formation and migration of GC (119).

COP1 protein structure comprises an N-terminal RING finger region, a coiled-coil domain and seven WD40 repeats at its C-terminus (120). COP1 is well studied in plants, but it has been little studied in humans. COP1 is known as a central repressor in the light signaling pathway and forms a complex with suppressor of PHYA-1 (COP1-SPA1) (121). This complex interacts with the CUL4-DDB1 ligase, which belongs to the CRL4 family, to form CUL4-DDB1^COP1-SPA1 ligases in plants (122). However, the catalytic mechanism of COP1 has not been fully elucidated in humans. Previous studies revealed that the ubiquitin ligase COP1 promoted the progression of multiple cancer types in vitro, including GC, by downregulating the expression of p53 (Fig. 2) (123–125). The role of COP1 in GC is controversial (123–126). One previous study indicates that low COP1 expression resulted in poorer prognoses in patients with GCs (126); however, more studies have suggested that COP1 functions as an oncogene (123–125).

In summary, there are eight multi-subunit RING domain E3 ligases associated with GC, including six SCFs, one CRL2 ligase and one CRL4 ligase. Among these eight CRLs, β-TRCP, CUL1 and COP1 are oncoproteins, whereas FBXW7, FBXL2, FBXL5, FBXO31 and pVHL act as tumor suppressors.

UBR5 is the only member of Other HECT ligase family that serves a role in regulating GC cell growth (140). Structurally, UBR5 is composed of an N-terminal ubiquitin-associated domain, two nuclear localization signals, a ubiquitin recognin box domain, a C-terminal poly(A)-binding domain and a HECT domain at its far C-terminus (141). A previous study showed that UBR5 gene mutations occur in 27.8% of GC and 23.3% of colorectal cancer (142). Gastrokine 1, a gastric tumor suppressor, can be downregulated by UBR5 E3 ligase (Fig. 1), and the overexpression of UBR5 is associated with poor overall and disease-free survival (140). Thus, UBR5 may serve as a carcinogenic agent and a prognostic factor in GC.