Introduction
Gastric cancer (GC) is a widespread disease and a significant threat to human health (Bray et al.
2018; Smyth et al.
2020). According to the GLOBOCAN 2020 report, the incidence of GC ranked fifth and its mortality ranked fourth among all types of tumors (Smyth et al.
2020; Ferlay
2020. Accessed on September 30, 2021). In 2020, the global incidence of new cases exceeded one million (1,089,103), leading to 768,793 fatalities (Ferlay
2020. Accessed September 30, 2021). In recent years, the overall incidence and mortality of GC have shown a stable decrease, attributed to advancements in early diagnosis, standardization of treatment methods including surgery, neoadjuvant therapy, and targeted therapy (Yonemura et al.
2016; Tan
2019; Kawazoe et al.
2020; Sexton et al.
2020). Despite significant progress being achieved in the last decade, the long-term survival of patients with advanced GC still remains unsatisfactory. The 5-year survival rate following radical gastrectomy and chemotherapy for patients with GC varies from 30 to 50% (Cats et al.
2018; Al-Batran et al.
2019). Consequently, there is an urgent need to identify more precise and efficacious therapeutic targets.
The ubiquitin-conjugating enzyme E2 (E2) plays a crucial role in the ubiquitin–proteasome pathway by facilitating the transfer of ubiquitin to the substrate protein. As an E2 enzyme, UBE2L3 facilitates the ubiquitination process by catalyzing a variety of substrate proteins in conjunction with E1s and E3s. UBE2L3 is situated on chromosome 22 q11.2-13.1 and codes for 153 amino acid residues (Clague et al.
2015). An increasing body of research has demonstrated the involvement of UBE2L3 in the pathogenesis of various diseases through the regulation of protein stability. These diseases include systemic lupus erythematosus (SLE) (Kim et al.
2020; Mauro et al.
2023), hepatitis B virus (HBV) infection (Zhou et al.
2019), Alzheimer’s disease (AD) (Wei et al.
2019), ischemic stroke (IS) (Wei et al.
2019), celiac disease (CeD) (Fernandez-Jimenez and Bilbao
2019), and rheumatoid arthritis (RA) (Zeng et al.
2021). Furthermore, it has been observed that UBE2L3 exhibits abnormally high expression in various human tumor types, indicating a potential role for UBE2L3 as an oncogene. UBE2L3 has been demonstrated to facilitate the migration of cervical cancer cells (Weinberg et al. 2020; Yi et al.
2020a,
b). Furthermore, several studies have suggested that the depletion of UBE2L3 inhibits the proliferation and induces apoptosis of hepatocellular carcinoma cells (Tao et al.
2020). Additionally, UBE2L3 in promoting the migration and invasion of lung cancer cells (Ma et al.
2022), and can be used as a potential response biomarker to enhance the efficacy of HSP90 inhibitors against lung adenocarcinoma (Marrugal et al.
2023). Another investigation demonstrated that UBE2L3 facilitated the cellular malignant characteristics of oral squamous cell carcinoma, such as proliferation, invasion, migration, and in vivo tumor growth (Cui et al.
2022). Nevertheless, the specific function of UBE2L3 in gastric cancer remains unclear.
This study aimed to examine the clinical significance of UBE2L3 expression in human GC tissues, explore the relationship between UBE2L3 expression and clinicopathological features, and assess its prognostic value using a multi-experimental approach.
Materials and methods
The analysis of gene expression data
Tissue specimens
Human tissue specimens were obtained from GC patients who underwent surgical excision at the Second Hospital of Lanzhou University between December 2016 and June 2017. The informed consent was signed by all patients, and the study protocol received approval from the Ethics Committee of the Second Hospital of Lanzhou University. The study included 125 patients with primary gastric cancer, with 11 patients excluded due to less than 5 years of follow-up. The cohort consisted of 114 patients, comprising 87 men and 27 women, all of whom did not undergo chemotherapy or radiotherapy before the surgical procedure, and all completed a 5-year follow-up period. All 114 specimens underwent confirmation through pathological examination and TNM staging.
Cell culture
The six cell lines, namely GES-1, MKN45, MKN28, N87, AGS, and HGC27, were acquired from the Digestive Tumor Laboratory of Gansu Province. The six cell lines were cultured in RPMI 1640 medium (Gibco, USA) supplemented with 10% fetal bovine serum (FBS, Gibco, USA), and maintained in a humidified incubator at 37 °C with a 5% CO2 atmosphere.
Lentiviral transfection
In the MKN45, MKN28, and AGS cell lines, the experimental group consisted of cells transfected with lentivirus obtained from GenePharma, China, while the negative control group comprised cells transfected with an empty lentiviral vector. Cell transfection was conducted using Invitrogen following the manufacturer’s protocol. Cells were cultured in six-well plates (Corning, USA) until reaching a cell density of 20–30%. Subsequently, the cells were harvested 18 h after transfection with lentivirus. After 72 h of continuous culture, the cells were subjected to puromycin screening for an additional 72 h. The transfection efficiency was assessed by observing green fluorescent protein expression under a microscope, and it was found that the fluorescence efficiency exceeded 80%, indicating a successful transfection.
Cell viability assay
In this study, the cell proliferation potential of GC cells was assessed using the Cell Counting Kit-8 (CCK-8) assay kit (Solarbio, China). The cells (MKN45, MKN28, AGS) were seeded at a density of 1000 cells per well in 96-well plates (Corning, USA). The medium in each well was removed at different time intervals (0, 24, 48, 72, and 96 h). Subsequently, 10 μL of the prepared CCK-8 reagent was introduced into each well and incubated for 1 h at 37 °C in a light-sheltered incubator. Following incubation, the absorbance value (OD) was assessed at a wavelength of 450 nm.
The cells were cultured at a density of 500 cells per well in 6-well plates. The cells were cultured without disturbance for a period of two weeks to facilitate the formation of proliferating colonies. Subsequently, the proliferating cells underwent two gentle washes with phosphate-buffered saline (PBS) to eliminate any non-adherent cells. Subsequently, the cells were treated with paraformaldehyde for 15 min and then subjected to staining with 0.1% crystal violet (Solarbio, China) for an additional 15 min. The number of colonies was determined using Image J software.
Cell invasion assay
The cells that were treated with trypsin were subsequently placed in the upper chamber of the transwell system (Corning, USA) that was coated with Matrigel (BD Bioscience). The cell count was 1 × 105 cells per upper chamber. The cells underwent two gentle washes with PBS following 18–72 h of incubation. Subsequently, the cells underwent fixation and staining using the identical procedure employed in the colony formation assay. The number of invasive cells was visualized and quantified.
Apoptosis assay
Apoptosis in GC cells was evaluated using flow cytometry. To analyze apoptosis, the apoptosis rate of adherent cells was assessed using the AnnexinV-APC/7-ADD apoptosis kit (Multisciences, China). The GC cell samples were suspended again in a binding buffer. 5 μl of Annexin V-APC and 10 μl of 7-AAD were added to each tube, followed by a 5-min incubation at room temperature in the dark. At the conclusion of the incubation period, the samples underwent processing using flow cytometry (Canto BD).
Western blot analysis
Proteins were initially extracted using RIPA buffer containing a protease inhibitor cocktail (Beyotime), and the quantification of protein levels was determined using the BCA assay (Beyotime). Following the isolation of proteins, the entire protein content was separated using 10% SDS-PAGE and subsequently transferred to PVDF membranes for Western blot analysis. Subsequently, the membranes were blocked with 5% skim milk for 1 h, followed by sequential incubation with primary antibodies overnight and secondary antibodies for 1 h. To analyze and document the protein bands, the membrane underwent imaging using the enhancement chemiluminescence reagent (Biosharp, China) and a chemiluminescence imager (Tanon 4600, China). The antibodies used for Western blot analysis were as follows: anti-UBE2L3 (dilution 1:10,000, Abcam, USA) and anti-GAPDH (dilution 1:5000, ImmunoWay Biotechnology, USA).
Quantitative real-time PCR (qRT-PCR)
The total RNA was isolated using AG RNAex Pro Reagent (Accurate Biology, China), followed by RNA conversion to cDNA, and subsequently subjected to quantitative real-time PCR using a CFX96 Touch qRT-PCR system (Bio-Rad). GAPDH was employed as an internal control for mRNA analysis. The values of the target genes were determined using the 2
−ΔΔCt method. The primer sequences have been documented in Table
1.
Table 1
Primer sequences in qRT-PCR
UBE2L3-forward | 5ʹ-GAAGCCAGCAACCAAAACCG-3ʹ |
UBE2L3-reverse | 5ʹ-TTCAGCTAGGTCAGCCCGAA-3ʹ |
GAPDH-forward | 5ʹ-GCACCGTCAAGGCTGAGAAC-3ʹ |
GAPDH-reverse | 5ʹ-TGGTGAAGACGCCAGTGGA-3ʹ |
Subcutaneous xenograft model in nude mice
In this investigation, two cohorts of stably transfected MKN45 cells (1 × 106 cells) were subcutaneously administered to female Balb/c nude mice aged 5–6 weeks old, obtained from Chengdu Yaokang Biotechnology Co., LTD. Tumor dimensions were assessed at 3-day intervals over a period of 21 days. Subsequently, the mice were euthanized following inhalation of ether anesthesia, and the xenograft tumors were removed, weighed, and then stored in a refrigerator at − 80 ℃ after resection for future experiments.
Immunohistochemical and scoring method
The immunohistochemical method was used to detect the expression level of UBE2L3 protein in human tissues and tumor tissues of nude mice. Tissue microarrays were created using gastric cancer (GC) tissues and adjacent normal tissues from 125 GC patients. Each sample was represented by two cores with a diameter of 1mm. Subsequently, the tumor tissue samples in nude mice were fixed, embedded in paraffin, and sectioned into 5 µm slices. Subsequently, the sections underwent antigen retrieval, followed by overnight incubation at 4 °C with anti-UBE2L3 antibody (1:500, Abcam, USA) and anti-Ki67 antibody (1:500, Servicebio, China). Following incubation with secondary antibodies, the sections underwent staining with 3,3ʹ-diaminobenzidine, and subsequent incubation with hematoxylin. Two pathologists assessed UBE2L3 immunostaining based on the percentage of stained cells and the intensity of staining in each section. The scoring system was determined by the proportion of stained cells in the positive area (0 = 0–5%, 1 = 6–25%, 2 = 26–50%, 3 = 51–75%, 4 = 76–100%) and the intensity of staining (0 = no staining, 1 = weak staining, 2 = moderate staining, 3 = strong staining). The immunohistochemical score is calculated by multiplying the score of the staining area by the score of the staining intensity. The samples were subsequently categorized as negative (-, 0 points), weakly positive (+ , 1–4 points), moderately positive (+ + , 5–8 points), and strongly positive (+ + +, 9–12 points).
Statistical analysis
The study results were analyzed using GraphPad Prism (version 8.0, USA). The study utilized a Student’s t-test to compare two groups and a one-way analysis of variance (ANOVA) to compare multiple groups. The results of the study were reported as mean ± standard deviation (SD). P values less than 0.05 were deemed to be statistically significant. The chi-square test was employed to assess the correlation between UBE2L3 expression and clinicopathological data. Survival analysis was evaluated using the log-rank test.
Conclusion
The human genome contains an estimated 40 genes that encode E2s. The genes were categorized into four classes and exhibited a conserved domain that included catalytic Cysteine residues (Stewart et al.
2016; Welsh et al.
2022). In recent years, there has been increasing research interest in the role of E2s in tumors. UBE2T, functioning as an E2 enzyme, has been identified as a crucial regulator of tumor advancement in various types of cancer, including gastric cancer (Yu et al.
2021), hepatocellular carcinoma (Sun et al.
2020), pancreatic cancer (Jiang et al.
2023), lung adenocarcinoma (Zhu et al.
2021), retinoblastoma (Xu et al.
2022), and glioblastoma (Huang et al.
2020). Furthermore, Qi et al. reported that the overexpression of UbcH5c was linked to a negative prognosis in pancreatic cancer (Qi et al.
2022). Another study has confirmed that UBE2J1 inhibits the progression of colorectal cancer by enhancing the ubiquitination and degradation of RPS3 (Wang et al.
2023). Furthermore, UBE2M and UBE2F, functioning as E2 enzymes, facilitate the transfer of NEDD8 to NEDD8 E3 ligase (Zheng et al.
2021). Several studies have demonstrated their overexpression in various types of cancer, such as hepatocellular carcinoma, breast cancer, lung adenocarcinoma, osteosarcoma, and esophageal squamous cell carcinoma (Zhou et al.
2017; Heo et al.
2020; Wang et al.
2020; Zheng et al.
2021). Moreover, an additional E2 enzyme, UBE2C, has been found to facilitate the proliferation and viability of lung carcinoma cells harboring Kras mutations. UBE2C was also found to be essential for the development of Krasg12d-induced lung tumors (Zhang et al.
2023). Regarding UBE2L3, its biological functions have been reported in hepatocellular carcinoma, cervical cancer, and lung adenocarcinoma (Tao et al.
2020; Yi et al.
2020a,
b; Marrugal et al.
2023).
The study revealed that the expression levels of UBE2L3 mRNA were significantly elevated in the majority of gastric cancer tissues compared to adjacent non-tumor gastric tissues, as determined through analysis of publicly available databases. Furthermore, we verified the increased expression of UBE2L3 in 125 GC tissues compared to adjacent tissue through immunohistochemistry analysis. Significantly, it was observed that elevated UBE2L3 expression was associated with the degree of differentiation, TNM classification, and prognosis in patients with GC. Furthermore, out of the 125 patients, 23 had metastasis. The expression level of UBE2L3 in GC tissues was significantly higher than that in adjacent normal tissues, both in patients with and without metastasis. Subsequently, the biological function of UBE2L3 in gastric cancer was validated through a series of laboratory methods. The findings indicated that UBE2L3 facilitated the proliferation, clone formation, and invasion of GC cell lines, while suppressing the apoptosis of GC cell lines. In vivo experiments, UBE2L3 was found to promote tumor progression. The aforementioned findings indicate that UBE2L3 functions as an oncogene in GC.
To date, research on the molecular mechanisms of UBE2L3 in diseases has primarily concentrated on the following areas. First, UBE2L3 is involved in the regulation of the NF-κB signaling pathway. It has been reported that UBE2L3 significantly enhances NF-κB activation induced by TLR7 in SLE patients through its interaction with LUBAC (Mauro et al.
2023). Taehyeung Kim and colleagues observed an up-regulation of UBE2L3 and a down-regulation of TNFAIP3 in the context of NF-κB activation, which subsequently led to the induction of inflammatory factors, suggesting a synergistic role (Kim et al.
2020). Second, the process of p53 polyubiquitination mediated by UBE2L3 has been found to decrease p53 stability during the progression of cervical cancer (Yi et al.
2020a,
b). Third, UBE2L3 was involved in the process of lysophagy and the ubiquitination of lysosomes in conjunction with UBE2N (Shima et al.
2023). Finally, the suppression of UBE2L3 resulted in an elevation of non-homologous end joining and a reduction in homologous recombination during double-strand break repair, achieved through the regulation of 53BP1 protein levels (Han et al.
2014; Pozo et al.
2017). Nevertheless, further research is required to elucidate the mechanism through which UBE2L3 facilitates tumor initiation and progression in gastric cancer.
Recent clinical applications have confirmed the effectiveness of HER2 in targeted therapy for GC; however, the potential audience for this treatment remains limited. In the future, the treatment of gastric cancer is expected to involve a combination of targeted therapies and a gradual shift toward personalized treatment approaches. The identification of UBE2L3 and the prospect of further comprehensive investigation suggest the potential for significant advancements in targeted therapy for gastric cancer.
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