Stomach cancer is a worldwide health problem. Annually, more than one million people worldwide are diagnosed with stomach cancer (Thrift and El-Serag 2020). Despite the decline in global morbidity and mortality over the past 50 years, gastric cancer remains the third leading cause of cancer death (Fock 2014). Therefore, understanding the underlying mechanisms involved in the initiation and development of gastric cancer is critical to improving the current detection and treatment of gastric cancer. Scientific studies of most cancers show that LncRNA expression in cancerous tissue differs from that in adjacent normal tissue (Yang et al. 2014). In recent years, scientists have attempted to use this type of difference in lncRNA expression as a diagnostic, prognostic, and therapeutic biomarker. In addition, lncRNAs could be a valuable clue for identifying mechanisms involved in cancer (Iaccarino and Klapper 2021).
Small nucleolar RNAs (snoRNAs)play an important role in rRNA processing as guide RNAs for posttranscriptional modification of ribosomal RNA (Bratkovič and Rogelj 2011). Most snoRNAs are located in the intron of protein-coding and non-protein-coding genes (Dieci et al. 2009). Genes containing snoRNAs are referred to as small nucleolar RNA host genes (SNHGs). Several SNHGs that act as oncogenes or tumor suppressors in various cancers have been identified in genome-wide association studies of tumor samples. Currently, there are 22 members of the SNHG family (SNHG1 to SNHG22) that regulate proliferation, apoptosis, invasion, and migration in various cancers (Qin et al. 2020). The importance of SNHGs in the pathogenesis of various cancers has been documented. For example, SNHG3 and SNHG9 have been reported to promote cell proliferation and tumor progression in breast and ovarian cancers(Li et al. 2020; Chen et al. 2021). In addition, knockdown of SNHG4 in lung cancer suppresses cell proliferation, invasion, and migration and induces apoptosis(Wang and Quan 2021), meanwhile several SNHG genes such as SNHG1, SNHG2, SNHG4, SNHG5, SNHG6, SNHG7, SNHG12, SNHG14, SNHG15, SNHG16, SNHG17, and SNHG20 were upregulated in GC (Yang et al. 2019). A previous study by Wang et al confirmed that the expression of SNHG4 in gastric cancer was significantly higher in cancer tissues than in normal tissues. Their study also showed that increased expression of SNHG4 affected cancer development, including proliferation and invasion, and accelerated the occurrence of EMT (Wang et al. 2021). However, there were no studies on the significant correlation between SNHG4 expression in gastric cancer and histological grade, stage, tumor size, metastasis status, and survival rate. In this study, we performed a comprehensive bioinformatics investigation of SNHG4 using the TCGA dataset. The result showed that SNHG4 was significantly upregulated in GC tumor tissue. We performed validation at the experimental level by examining SNHG4 expression in GC tumor tissues and comparing them with normal tissues using the RT-PCR technique, and SNHG4 expression was found to be upregulated in tumor tissues. The increased expression was associated with advanced stages, high tumor grade, larger tumor size and lymph node metastasis. This result confirms the role of SNHG4 in the initiation and development of GC. For further investigation, we examined survival analysis using the Kaplan-Meier plotter database. The results showed that overexpression of SNHG4 correlated with poor OS and PPS. This finding suggests that SNHG4 plays an oncogenic role in the pathogenesis of gastric cancer and that the role of SNHG4 in metastasis risk and tumor progression is more important than that of cancer genesis. The ROC curve analysis showed that SNHG4 has reasonable diagnostic potential for distinguishing cancer stages and probably metastasis in gastric cancer.
Cancer enrichment analysis of SNHG4-related genes was performed using WebGestalt. based on WebGestalt among the top target miRNA related to SNHG4, miR-490 is downregulated in human GC tissues, and it has been reported that increased miR-490 expression (in GC cell lines) decreases cell viability, migration, invasion, and increased apoptosis(Yu et al. 2019). Our results indicate that E2F1 is a transcription factor associated with SNHG4. Several studies on E2F1 in gastric cancer have shown that E2F1 is upregulated in GC and accelerates the development of tumorigenesis (Guo et al. 2010). The activity of E2F1 is regulated by Rb (retinoblastoma protein). Hypo phosphorylated Rb inactivates DNA binding and transcriptional activity of E2F1. Rb hyperphosphorylation caused by CDK activity leads to the release of Rb from E2F1, and the free E2F1 regulates the transcription of various genes involved in the cell cycle (Harbour and Dean 2000).
Based on GO and KEGG pathway enrichment analysis, SNHG4 was found to be primarily involved in oncogenic pathways and SNHG4 can promote tumorigenesis and progression in GC. Significant GO terms related to the biological process of SNHG4-associated genes included cell-matrix adhesion regulation, cell-substrate adhesion regulation, and cell-matrix adhesion. The role of these biological processes in cancer progression and tumor metastasis is well known (Lock et al. 2008). KEGG enrichment analysis of the genes that showed the most significant correlation with SNHG4 revealed that SNHG4 may play a role in signaling pathways involved in prostate cancer, bladder cancer, microRNAs in cancer, pancreatic cancer, non-small cell lung cancer, and gastric cancer. Among the proteins interacting with SNHG4, ELAVL1 and IGF2BP2 had the highest number of SNHG4 target sites and might be downstream targets of SNHG4. Interestingly, a search on GeneCards for the signaling pathway involving these proteins revealed that PI3K-Akt-mTOR and ERK-MAPK could be the possible signaling pathways. Both signaling pathways have been described as primarily important cell signaling pathways during tumorigenesis and tumor progression, so SNHG4 may play a role in regulating the signaling pathways through its interaction with IGF2BP2 and ELAVL1.