Advances in Research of PRMT7, DVL3 and Gastric Cancer

Xuemei WANG, Yu CHENG, Jiemin QI

Department of Pathology, Chengde Medical College, Chengde 067000, China

Abstract Protein arginine methyltransferase 7 (PRMT7) is closely related to the formation of lung cancer, breast cancer and other malignant tumors, and it may be a potential target gene for malignant tumor therapy. Dishevelleds (DVLs) are key factors involved in cell proliferation, invasion and metastasis. Among the dishevelleds, the dishevelled segment polarity protein 3 (DVL3) is a key protein in the Wnt signaling pathway, and its abnormal expression plays an important role in the occurrence and development of malignant tumors. This paper reviewed the advances in research of PRMT7 and DVL3 in gastric cancer.

Key words Gastric cancer, Histone modifications, Protein Arginine Methyltransferase 7 (PRMT7), Dishevelled segment polarity protein 3 (DVL3)

1 Introduction

The gastric cancer is a malignant tumor, and its occurrence and development are caused by abnormal cell proliferation, apoptosis, invasion and migration. Gastric cancer is an important cause of human death. In recent years, the research of epigenetics in tumors has become more and more hot. A large amount of evidence has confirmed that post-translational modifications of histones in epigenetics are related to the occurrence and development of malignant tumors, among which methylation modification is particularly important[1-2]. Different sites and different degrees of methylation modification of histones can affect the expression and function of different genes, resulting in gene transcription activation or silencing. The most common histone methylated amino acids are lysine and arginine, and the modifications occur at the N-terminus of the lysine side chain and the N-terminus of the arginine guanidine group, respectively[3]. The family of protein arginine methyltransferases (PRMTs) is widely involved in important biological processes such as cell signal transduction, gene transcription, chromatin remodeling, stem cell fate determination and cell proliferation[4]. Dishevelled segment polarity protein 3 (DVL3) is a newl substrate of PRMT7 that can be post-translationally modified by arginine methylationinvitroandinvivo. Arginine methylation is an important switch in the Wnt signaling pathway. As an important regulator, it regulates the Wnt signaling pathway and related target genes in the pathway. There is often abnormal expression of methylated arginine transferase in malignant tumors[5]. Among the PRMTs family, PRMT7 has been relatively hot in research in recent years and has received high attention. In this study, we intended to explore the expression of PRMT7 and DVL3 in gastric cancer and to analyze their clinical significance.

2 PRMT7 and gastric cancer

2.1 Structure of PRMT7Mirandaetal.[6]discovered PRMT7 in Chinese hamster ovary cells for the first time, and named it protein arginine methyltransferase based on the characteristics of its catalytic activity. PRMT7 encodes a total of 692 amino acids, located on human chromosome 16q22.1, with a molecular weight of about 78KDa, and can be distributed in the cytoplasm and nucleus. The difference between PRMT7 and its family members is that in the encoded 692 amino acids, it contains two core domains: the N-terminal core domain (amino acids 1-378) and the C-terminal core domain (amino acids 379-692). The N-terminal domain is a necessary condition for the enzymatic activity of PRMT7, the sequence homology between the two is very low, and the N-terminal repeats are involved in the process of methyl transfer. Because the C-terminal sequence is conserved due to its conservative structure compared with the N-terminal, it can play a regulatory function[7]. The existing studies have shown that PRMT7 can methylate four core histones, namely H2B, H2A, H3, and H4. It is highly specific to the N-terminal gene sequence of H2B histones, followed by H4. Using top-down mass spectrometry detection technology, it was detected that H2B Arg-29, Arg-31, Arg-33 and H4 Arg-17, Arg-19 in the substrate recognition motif (RXR motif) are prone to single occurrence methylation. H2B Arg-31 is the primary target of PRMT7. When two arginine residues coexisted and there was only one residue between them, the activity of PRMT7 was significantly increased[8]. PRMT7 can methylate histone H2A Arg-3 site, respond to DNA damage, and inhibit the transcription of DNA polymerase. PRMT7 participates in the process of cell proliferation through methylation modification of histones downstream of the TGF-β signaling pathway[9-10].

2.2 PRMT7-related signal transduction pathwayAt present, it has been found that PRMT7 can mediate four signaling pathways in the regulation of malignant tumors. (i) Wnt signaling pathway. Histone methylation is essential for the correct expression of the genome. Methylated arginine is not only a regulator of DVL3 translocation to the cell membrane, but also a regulator of Wnt signaling activation. The deletion of PRMT7 can lead to the dysregulation of Wnt signaling, which in turn affects the expression levels of its downstream target genes such asc-myc[11]. In normal mature cells, the Wnt signaling pathway is turned off, while abnormal activation often leads to the occurrence of tumors. (ii) Epithelial-mesenchymal transition (EMT). PRMT7 inhibits the expression of E-cadherin by binding to the proximal promoter of epithelial cadherin (E-cadherin). Down-regulation of E-cadherin expression is the most important indicator of EMT. In cancer cells, E-cadherin is generally underexpressed. Silencing PRMT7 can restore E-cadherin expression and reduce cell migration and invasion, while overexpression of PRMT7 mediates the occurrence of EMT, and can promote the invasion and migration ability of cancer cells, thereby causing tumor invasion and metastasis[12]. (iii) PI3K/AKT/mTOR signaling pathway. PI3KP85, AKT123 and mTOR are key proteins in this signaling pathway and play an important role in tumor cell proliferation and development. Through qRT-PCR and Western Blot experiments in liver cancer, it was found that the phosphorylation levels of the three key proteins were significantly decreased after overexpression of PRMT7. The results showed that the occurrence of tumors was closely related to the abnormal activation of PI3K/AKT/mTOR signaling pathway[13]. (iv) Hedgehog (HH) signaling pathway. Glioma-associated oncogene 2 (GLI2) is a major negative regulator of the HH signaling pathway. PRMT7 induces GLI2 expression by activating the HH signaling pathway and enhances the activity of GLI2, thereby inhibiting cellular senescence. The cellular senescence mechanism mediated by PRMT7 is involved in occurrence of tumors, and this signaling pathway plays an important role in cell development and homeostasis[14].

2.3 Role of PRMT7 in gastric cancer and other tumorsMore and more scholars believe that the occurrence of tumor is inseparable from histone modification. The results of histone modifications leading to transcriptional activation or repression depends on factors such as which residues are modified and the type of modification. Methylation changes important processes of gene expression and cell inactivation, such as: regulation of cell cycle, cell adhesion, invasion and migration of cancer cells, apoptosis and proliferation, and regulation of signaling pathways, all of these are all related to histone methylation[15]. Cheng Dezhietal.[16]believed that in non-small cell lung cancer, PRMT7 expression in cancer tissue is higher than that in normal tissue. They verified by Transwell invasion assay and colony formation assay, and found that overexpression of PRMT7 can promote the invasion ability and colony formation of lung cancer A549 and SPC-A1 cells, showing the effect of promoting tumor growth. Related studies reported that PRMT7 also showed a high expression state in breast cancer. From the perspective of the action mecahnism, it is because PRMT7 undergoes auto-methylation on the COOH-terminal domain R531, which in turn binds to transcription factors, the methylated PRMT7 is recruited to the promoter E-cadherin, which counteracts the transcriptional repression marker H4R3me2s, leading to EMT. Automethylation of PRMT7 enhances the invasive and metastatic ability of cancer cells, thereby promoting breast cancer metastasis[17]. According to findings of Ge Shuangetal.[13], PRMT7 is down-regulated in liver cancer and plays a negative regulatory role in the growth and proliferation of cancer cells and the process of tumor invasion and metastasis. After PRMT7 was up-regulated, the phosphorylation levels of AKT and mTOR were reduced, the PI3K/AKT/mTOR signaling pathway was inhibited, the ability of cancer cells to invade and migrate was reduced, and cell apoptosis was reduced. The expression of PRMT7 in liver cancer was opposite to that in lung cancer and breast cancer. So far, there is no relevant literature on PRMT7 and gastric cancer, thus it is necessary to make further study on how it plays a role in gastric cancer.

3 DVL3 and gastric cancer

3.1 Structure and biological function of DVL3Dishevelled (DVL) is also known as segmented polar protein. The DVL family originated from multicellular organisms and was gradually discovered in mammals as a core component of the Wnt signaling pathway[18]. The full length ofDVL3gene is 2 063 bp, located on chromosome 3q27, and its molecular weight is about 78 KDa. It is a new methylation substrate of PRMT7. Its protein structure analysis showed that DVL3 has three conserved domains. (i) N-terminal DIX domain. This domain contains 80 amino acids, and a single DIX domain consists of 5 β chains and 1 α-helical chain, and it plays the function of polymerizing DVL3. (ii) Central PDZ domain. This domain contains 90 amino acids and consists of 6 β chains and 2 α-helical chains. This domain binds directly to transmembrane proteins (Frizzled, FZD) to transfer signals from membrane receptors to downstream target genes. The PDZ domain is an ideal drug target in the Wnt signaling pathway. (iii) C-terminal DEP. This domain is located in the COOH end region of DVL, contains 80 amino acids, and is composed of 4 β chains and 3 α-helix chains. It is the most critical domain and regulates cell signal transduction[19]. The polymerization of DVL3 is closely related to the ability to activate Wnt/β-catenin signaling. DVL3 is recruited by FZDs through its PDZ domain and polymerized to the cell membrane through its DIX domain, promoting the phosphorylation of downstream proteins in the signaling pathway and affecting their expression levels, which lays a solid foundation for understanding how DVL3 plays a role in tumor development.

3.2 DVL3-mediated related signaling pathwaysThere are three related signaling pathways mediated by DVL3. (i) Wnt/β-catenin signal transduction pathway, also known as the classical Wnt signal transduction pathway. The regulation of β-catenin stability is the core mechanism of this pathway regulation. When cells are stimulated by Wnt signaling, extracellular Wnt protein binds to FZD receptor and low-density lipoprotein receptor-related protein 5/6 (LRP-5/6) to activate Wnt/β-catenin signaling transduction guide way. Induced activation and recruitment of DVL3 at FZD receptors make β-catenin fail to exert its normal biological functions. Abnormal recruitment of DVL3 can activate Wnt target genes, resulting in abnormal cell proliferation and resistance to apoptosis, which in turn accelerates tumor formation. Abnormal activation of Wnt/β-catenin signaling pathway plays an important role in the occurrence, development, proliferation and migration of cancer[20]. (ii) Wnt/Ca2+signal transduction pathway. This pathway is centered on the increase of intracellular Ca2+concentration. DVL3 and Wnt5a are important components of the Wnt/Ca2+signaling pathway. Wnt5a is a pro-oncogene involved in cancer invasion and metastasis. DVL3 is essential for Wnt5a to activate the Wnt/Ca2+pathway. Overexpression of DVL3 can abolish the ability of Wnt5a to stimulate Ca2+mobilization, thereby playing a role in regulating gene expression[21]. (iii) Notch signaling pathway. DVL3 is a member of the Notch signaling pathway. Silencing the expression of DVL3 can activate the Notch signaling pathway and its downstream target genescyclinD3,c-myc,etc., which are involved in tumor initiation and progression[22].

3.3 Role of DVL3 in gastric cancer and other tumorsThere are certain differences in the expression of DVL3 in different types of cancer. DVL3 can participate in multiple processes such as tumor cell proliferation, infiltration, and metastasis. Wei Qiangetal.[23]found that DVL3 was overexpressed in non-small cell lung cancer, and the expression level in cancer tissue was significantly higher than that in adjacent normal tissue by immunohistochemical SP method. Transfected lung cancer A549 and QG56 cells can significantly enhance the invasive ability of DVL3 and promote cancer metastasis. Velázquezetal.[24]used RT-qPCR for detection, and found that DVL3 was highly expressed in colon cancer, and played a stabilizing role through its DIX domain, preventing DVL3 from being degraded by proteases and promoting tumor formation. In esophageal squamous cell carcinoma tissue, the expression of DVL3 was significantly increased in cancer tissue, and played an important role in the invasion and metastasis of esophageal carcinoma. It was verified by flow cytometry that silencing DVL3 could promote the apoptosis of cancer cells, and overexpression of DVL3 promoted the occurrence of esophageal cancer. The abnormal expression of DVL3 is closely related to the early diagnosis, prognosis and treatment of esophageal cancer[25]. In rectal cancer, long non-coding RNA (TTTY15) can enhance the expression of DVL3 by regulating miR-29a-3p, thereby promoting the proliferation of cancer cells and inducing the occurrence of rectal cancer[26]. Relevant studies have found that DVL3 also expresses the invasive and metastatic abilities of cancer cells to varying degrees in prostate cancer[27], liver cancer[28], and breast cancer[29]. DVL3 is rarely reported in gastric cancer, and there is only one relevant literature at present. DVL3 is an important regulator of Wnt signaling pathway and it was detected by IHC, qRT-PCR and Western Blot that DVL3 was highly expressed in gastric cancer tissues. Studies have shown that silencing DVL3 inhibits tumor formation. Overexpression of DVL3 can enhance the proliferation and invasion of gastric cancer cells[30].

4 Conclusions and prospects

PRMT7 is closely related to the formation and progression of tumors, which has been confirmed in a variety of tumors, but no relevant literature has been reported in gastric cancer. As a key protein in the Wnt signaling pathway, DVL3 is involved in the infiltration, proliferation and invasion and metastasis of gastric cancer cells, and it plays an important role in the biological process of gastric cancer. At present, there is still no report on the correlation between PRMT7 and DVL3 in gastric cancer. Whether PRMT7 can affect the Wnt pathway by methylating DVL3 to play a role in gastric cancer is still unknown. The regulatory mechanism of the two needs to be further studied. With the continuous advance of bioinformatics technology and the continuous development of science and technology, it is necessary to clearly understand the mechanism of action of PRMT7 and DVL3 in tumors. This is conducive to the development of highly specific, new and effective gastric cancer drugs. It is expected to provide new clues for the early diagnosis of gastric cancer, improve the diagnosis rate of gastric cancer, reduce the mortality rate, and also provide a solid and reliable theoretical basis for the effective treatment of gastric cancer.