METTL3-mediated N6-methyladenosine modification of STAT5A promotes gastric cancer progression by regulating KLF4.

N6-methyladenosine (m6A) is the predominant post-transcriptional RNA modification in eukaryotes and plays a pivotal regulatory role in various aspects of RNA fate determination, such as mRNA stability, alternative splicing, and translation. Dysregulation of the critical m6A methyltransferase METTL3 is implicated in tumorigenesis and development. Here, this work showed that METTL3 is upregulated in gastric cancer tissues and is associated with poor prognosis. METTL3 methylates the A2318 site within the coding sequence (CDS) region of STAT5A. IGF2BP2 recognizes and binds METTL3-mediated m6A modification of STAT5A through its GXXG motif in the KH3 and KH4 domains, leading to increased stability of STAT5A mRNA. In addition, both METTL3 and IGF2BP2 are positively correlated with STAT5A in human gastric cancer tissue samples. Helicobacter pylori infection increased the expression level of METTL3 in gastric cancer cells, thereby leading to the upregulation of STAT5A. Functional studies indicated that STAT5A overexpression markedly enhances the proliferation and migration of GC cells, whereas STAT5A knockdown has inhibitory effects. Further nude mouse experiments showed that STAT5A knockdown effectively inhibits the growth and metastasis of gastric cancer in vivo. Moreover, as a transcription factor, STAT5A represses KLF4 transcription by binding to its promoter region. The overexpression of KLF4 can counteract the oncogenic impact of STAT5A. Overall, this study highlights the crucial role of m6A in gastric cancer and provides potential therapeutic targets for gastric cancer.

Ubiquitin-specific Protease 35 Promotes Gastric Cancer Metastasis by Increasing the Stability of Snail1.

Deubiquitinase (DUB) dysregulation is closely associated with multiple diseases, including tumors. In this study, we used data from The Cancer Genome Atlas and Gene Expression Omnibus databases to analyze the expression of 51 ubiquitin-specific proteases (USPs) in gastric cancer (GC) tissues and adjacent non-neoplastic tissues. The Kaplan-Meier Plotter database was used to analyze the association of the differentially expressed USPs with the overall survival of patients with GC. The results showed that five USPs (USP5, USP10, USP13, USP21, and USP35) were highly expressed in GC tissues and were associated with poor prognosis in patients with GC. Because the epithelial-mesenchymal transition enables epithelial cells to acquire mesenchymal features and contributes to poor prognosis, we investigated whether these USPs had regulatory effects on the key epithelial-mesenchymal transition transcription factor Snail1. Our results showed that USP35 exhibited the most significant regulation on Snail1. Overexpression of USP35 increased and its knockdown decreased Snail1 protein levels. Mechanistically, USP35 interacted with Snail1 and removed its polyubiquitinated chain, thereby increasing its stability. Furthermore, USP35 promoted the invasion and migration of GC cells depending on its DUB activity. USP35 knockdown exhibited the opposite effect. Snail1 depletion partially abrogated the biological effects of USP35. Experiments using nude mouse tail vein injections indicated that wild-type USP35, but not the catalytically inactive USP35-C450A mutant, dramatically enhanced cell colonization and tumorigenesis in the lungs of mice. In addition, USP35 positively correlated with Snail1 expression in clinical GC tissues. Helicobacter pylori infection increased USP35 and Snail1 expression levels. Altogether, we found that USP35 can deubiquitinate Snail1 and increase its expression, thereby contributing to the malignant progression of GC. Therefore, USP35 may serve as a viable target for GC treatment.

USP13 deubiquitinates and stabilizes cyclin D1 to promote gastric cancer cell cycle progression and cell proliferation.

The reversible post-translational modifications of protein ubiquitination and deubiquitination play a crucial regulatory role in cellular homeostasis. Deubiquitinases (DUBs) are responsible for the removal of ubiquitin from the protein substrates. The dysregulation of the DUBs may give rise to the occurrence and development of tumors. In this study, we investigated the gastric cancer (GC) data from the TCGA and GEO databases and found that ubiquitin-specific protease USP13 was significantly up-regulated in GC samples. The higher expression of USP13 was associated with the worse prognosis and shorter overall survival (OS) of GC patients. Enforced expression of USP13 in GC cells promoted the cell cycle progression and cell proliferation in an enzymatically dependent manner. Conversely, suppression of USP13 led to GC cell cycle arrest in G1 phase and the inhibition of cell proliferation. Nude mouse experiments indicated that depletion of USP13 in GC cells dramatically suppressed tumor growth in vivo. Mechanistically, USP13 physically bound to the N-terminal domain of cyclin D1 and removed its K48- but not K63-linked polyubiquitination chain, thereby stabilizing and increasing cyclin D1. Furthermore, re-expression of cyclin D1 partially reversed the cell cycle arrest and cell proliferation inhibition induced by USP13 depletion in GC cells. Additionally, USP13 protein abundance was positively correlated with the protein level of cyclin D1 in human GC tissues. Taken together, our data demonstrate that USP13 deubiquitinates and stabilizes cyclin D1, thereby promoting cell cycle progression and cell proliferation in GC. These findings suggest that USP13 might be a promising therapeutic target for the treatment of GC.

Pseudophosphatase STYX is induced by Helicobacter pylori and promotes gastric cancer progression by inhibiting FBXO31 function.

Gastric cancer (GC) is one of the most common malignancies in the world and ranks third in terms of cancer-related deaths. The catalytically inactive pseudophosphatase STYX (serine/threonine/tyrosine interacting protein) is a member of the protein tyrosine phosphatase family. It has been recently reported that STYX functions as a potential oncogene in different types of cancers. However, the potential role and regulatory mechanism of STYX in GC remains unknown. In this study, we find that STYX is highly expressed in GC tissues compared with adjacent noncancerous tissues and closely correlates with the prognosis of GC patients. STYX overexpression facilitates the proliferation and migration in GC cells, whereas STYX knockdown has the opposite effects. Nude mice experiments indicate that STYX knockdown in GC cells dramatically suppresses the tumor growth and lung metastasis in vivo. Mechanically, our results suggest that STYX interacts with the F-box protein FBXO31 and disrupts the degradation function of FBXO31 to its target proteins CyclinD1 and Snail1, thereby increasing the level of CyclinD1 and Snail1 in GC. STYX-mediated biological changes can be reversed by the co-expression of STYX and FBXO31 in GC cells. In addition, transcription factor c-Jun can enhance the expression of STYX in GC. The expression of STYX can also be induced by Helicobacter pylori (H. pylori) infection in c-Jun-dependent manner. Together, our present study suggests that STYX plays an oncogenic role in GC by inhibiting FBXO31 function and represents a potential therapeutic target and prognostic biomarker in GC.

Circular RNA hsa_circ_0004872 inhibits gastric cancer progression via the miR-224/Smad4/ADAR1 successive regulatory circuit.

BACKGROUND: Emerging evidence has shown that circular RNAs (circRNAs) play a crucial regulatory role in the occurrence and development of cancer. Exploring the roles and mechanisms of circRNAs in tumorigenesis and progression may help to identify new diagnostic markers and therapeutic targets. In the present study, we investigated the role and regulatory mechanism of hsa_circ_0004872 in gastric cancer (GC). METHODS: qRT-PCR was used to determine the expression of hsa_circ_0004872 in GC tissues and cells. EdU, CCK-8, transwell and scratch wound healing assays were used to assess the role of hsa_circ_0004872 in GC cell proliferation, invasion and migration, respectively. Subcutaneous and tail vein tumor injections in nude mice were used to assess the role of hsa_circ_0004872 in vivo. RIP assay, biotin-coupled probe pull-down assay, FISH and luciferase reporter assay were performed to confirm the relationship between hsa_circ_0004872 and the identified miRNA. ChIP assay, luciferase reporter assay and western blot were used to determine the direct binding of Smad4 to the promoter of the ADAR1 gene. RESULTS: In this study, we found that hsa_circ_0004872 was dramatically downregulated in GC tissues compared with adjacent noncancerous tissues. The expression level of hsa_circ_0004872 was associated with tumor size and local lymph node metastasis. Enforced expression of hsa_circ_0004872 inhibited the proliferation, invasion and migration of GC cells, whereas knockdown of hsa_circ_0004872 had the opposite effects. Nude mice experiments showed that ectopic expression of hsa_circ_0004872 dramatically inhibited tumor growth and metastasis in vivo. Moreover, we demonstrated that hsa_circ_0004872 acted as a "molecular sponge" for miR-224 to upregulate the expression of the miR-224 downstream targets p21 and Smad4. Importantly, we found that the RNA-editing enzyme ADAR1 inhibited hsa_circ_0004872 expression and further led to the upregulation of miR-224. Smad4, the downstream target of miR-224, could further affect hsa_circ_0004872 levels by directly binding to the promoter region of ADAR1 to inhibit ADAR1 expression. CONCLUSIONS: Our findings showed that hsa_circ_0004872 acted as a tumor suppressor in GC by forming a negative regulatory loop consisting of hsa_circ_0004872/miR-224/Smad4/ADAR1. Thus, hsa_circ_0004872 may serve as a potential biomarker and therapeutic target for GC.

NF-κB/miR-223-3p/ARID1A axis is involved in Helicobacter pylori CagA-induced gastric carcinogenesis and progression.

Infection with Helicobacter pylori (H. pylori) and the resulting gastric inflammation is regarded as the strongest risk factor for gastric carcinogenesis and progression. NF-κB plays an important role in linking H. pylori-mediated inflammation to cancer. However, the underlying mechanisms are poorly understood. In this study, we find that H. pylori infection induces miR-223-3p expression in H. pylori CagA-dependent manner. NF-κB stimulates miR-223-3p expression via directly binding to the promoter of miR-223-3p and is required for H. pylori CagA-mediated upregulation of miR-223-3p. miR-223-3p promotes the proliferation and migration of gastric cancer cells by directly targeting ARID1A and decreasing its expression. Furthermore, miR-223-3p/ARID1A axis is involved in CagA-induced cell proliferation and migration. In the clinical setting, the level of miR-223-3p is upregulated, while ARID1A is downregulated significantly in human gastric cancer tissues compared with the corresponding noncancerous tissues. The expression level of miR-223-3p is significantly higher in H. pylori-positive gastric cancer tissues than that in H. pylori-negative tissues. Moreover, a negative correlation between miR-223-3p and ARID1A expression is found in the gastric cancer tissues. Taken together, our findings suggested NF-κB/miR-223-3p/ARID1A axis may link the process of H. pylori-induced chronic inflammation to gastric cancer, thereby providing a new insight into the mechanism underlying H. pylori-associated gastric diseases.

FBXO31 Suppresses Gastric Cancer EMT by Targeting Snail1 for Proteasomal Degradation.

The F-box protein FBXO31, a component of the Skp1/Cul1/F-box (SCF) E3 ubiquitin ligase complex, plays an important regulatory role in neuronal development, stress response, and tumorigenesis. Our recent report indicates that FBXO31 functions as a tumor suppressor in gastric cancer, and the loss of FBXO31 protein is associated with a higher malignant phenotype and poorer prognosis. However, little is known about the underlying mechanism. In this study, FBXO31 inhibits gastric cancer progression by suppressing the epithelial-mesenchymal transition (EMT). FBXO31 overexpression decreases, whereas its inhibition increases, the protein level of the EMT transcription factor Snail1 (SNAI1), respectively. Further evidence demonstrates that FBXO31 interacts with Snail1 and mediates the ubiquitin- and proteasome-dependent degradation of Snail1 in gastric cancer. The F-box domain of FBXO31 and the phosphorylation of Snail1 are necessary for the molecular interaction between FBXO31 and Snail1. Mouse modeling experiments reveal that FBXO31 overexpression inhibits in vivo colonization of gastric cancer cells. Furthermore, a highly significant negative correlation between FBXO31 and Snail1 is validated in human gastric cancer clinical specimens. Taken together, these findings identify Snail1 as a new target protein of FBXO31 in gastric cancer and substantiate a novel regulatory role of FBXO31 on gastric cancer progression and metastasis.Implication: These findings demonstrate that FBXO31 exerts the tumor-inhibitory role in gastric cancer by ubiquitin-mediated degradation of Snail1, which represents a viable strategy of FBXO31 activators in the prevention and therapy of gastric cancer. Mol Cancer Res; 16(2); 286-95. ©2017 AACR.