With the improvements in surgical techniques and the progress in traditional radiotherapy and chemotherapy as well as the implementation of neoadjuvant therapy, the 5-year survival rate of early gastric cancer is more than 95%. However, the low rate of early diagnosis means that most patients are diagnosed with advanced disease and therefore miss the best time to undergo surgery (26). Previous studies have shown that gastric cancer biomarkers include HER2, E-cadherin, fibroblast growth factor receptor, mammalian rapamycin target and hepatocyte growth factor receptor, as well as microRNAs, long noncoding RNAs, matrix metalloproteinases, PD-L1, TP53 and microsatellite instability (27). In this study, we found a relationship between ASF1B and gastric cancer, indicating that ASF1B can be used as a new therapeutic target for gastric cancer.
Current studies have shown that ASF1 is a H3-H4 histone chaperone and participates in DNA replication and repair as well as transcriptional regulation (12, 13). The prognostic factors in gastric cancer include cell cycle regulatory factors, microsatellite instability (MSI), apoptosis regulatory factors, DNA repair proteins and so on(28). The cyclin kinase inhibitor P27KiP1 can slow or even arrest cell division. A study showed that low expression of P27KiP1 is related to poor prognosis in patients with gastric cancer(29). MSI is mainly caused by gene mutation. Recent mutation analysis of gastric cancer shows that there are 37 obviously mutated genes, such as TP53, KRAS, and PIK3, and that the main cause of high mutagenicity is defective DNA mismatch repair(30, 31). Similarly, BRCA1 is a tumour suppressor protein that plays an important role in a variety of cellular processes, including DNA damage repair, cell cycle checkpoint control, transcriptional regulation, chromatin remodelling and apoptosis(32–36). The regulation of p53 by BRCA1 can best explain the regulation of transcription factor activity by BRCA1. The interaction between BRCA1 and p53 leads to increased transcription of p53 response promoters, such as p21 and Bax, and induces apoptosis in cancer cells(37).
BRCA1 induces apoptosis by regulating the expression of PIG3 through p53, and the expression of PIG3 is related to better OS in breast cancer patients(38). Because ASF1B and BRCA1 have similar functions in cells and TIMER online analysis showed that BRCA1 expression is highly positively correlated with ASF1B expression, we speculated that ASF1B may play a similar role in gastric cancer. In this study, through bioinformatics multiplatform analysis, it was found that the expression level of ASF1B in gastric cancer tissues was higher than that in normal tissues and that patients with high ASF1B expression had good prognoses, which were related to tumour size and depth of invasion. Cox regression analysis showed that the expression level of ASF1B was an independent risk factor for the prognosis of patients with colorectal cancer. Therefore, we thought that ASF1B may be a molecular marker to predict the development and prognosis of gastric cancer. Then, we investigated the biological function of ASF1B in gastric cancer. Highly expressed genes associated with ASF1B in the module defined by WGCNA were selected for KEGG and GO enrichment analysis, and the results showed that these genes were enriched in the DNA damage repair, cell cycle, transcriptional regulation, chromatin remodelling and p53 signalling pathways. In addition, GSEA of ASF1B was performed, and the results revealed that ASF1B was involved in the above pathways, consistent with our previous speculation. In vitro, an ASF1B knockout cell line was generated. The results showed that low expression of ASF1B promoted the malignant behaviour of gastric cancer by promoting the proliferation and migration of gastric cancer cells.
Activation of the p53 pathway occurs through regulation of apoptosis, cell cycle arrest, differentiation and senescence(39). P53 controls apoptosis through the intrinsic pathway and activates factors that initiate caspase-mediated apoptosis(40). Bcl2 and its family members play an important role in the mitochondrial-dependent endogenous apoptotic pathway. Bcl-2 and Bax cooperate to change the mitochondrial membrane potential and permeability, thus initiating the release of regulatory proteins that activate caspases(41). We found that the apoptotic capacity was decreased, the expression of p53 and Bax was inhibited, and the expression of Bcl-2 was upregulated in the cell line with low ASF1B expression, suggesting that ASF1B may exert an antitumour effect by promoting apoptosis through the Bax/Bcl-2-p53 axis.