GASTRIC CARCINOMA AND ETIOLOGY

The second leading cause of death among all malignancies and the world’s fourth most frequent cancer is gastric carcinoma. Gastric cancer is a heterogeneous disease with the origin of familial and sporadic disease; gastric cancer initiation and progression are caused by various factors such as genetic, epigenetic, and environmental risk factors.1,2

Epigenetic alterations have been described as being correlated with the diagnosis and treatment of gastric cancer patients. More than 90% of all gastric cancers are related to Helicobacter pylori infection, and about 10% of gastric cancers are familial. Sporadic gastric cancer develops in a  model of chronic inflammation, intestinal metaplasia, dysplasia, and eventually adenocarcinoma. Sporadic gastric cancer is defined by typical epigenetic modifications but limited genetic alterations across the stages.3 Epigenetic alterations determine cell behavior by modulating the transcriptional availability of diverse parts of the genome through DNA methylation, chromatin modification, histone modifications, and non-coding RNAs (e.g., micro RNAs, long non-coding RNAs, and small interfering RNAs).4 Chromatin remodeling and DNA methylation are 2 crucial epigenetic alterations; chromatin remodeling takes place through histone modifications (principally on the N terminal tails) that finally affect the interplay of DNA and chromatin-modifying proteins, whereas DNA methylation is a chemical alteration in the DNA sequence that frequently occurs in CpG dinucleotides via cytosine methylation.5,6

Helicobacter pylori is a type I carcinogen and an overpowering etiological factor for gastric cancer. H. pylori is well known to stimulate hypermethylation and silencing of various tumor-suppressor genes in H. pyloriinfected non-cancerous and cancerous gastric mucosa. Hypermethylation  of tumor-suppressor genes such as CDKN2A, MHL1, MGMT, PTEN, and RUNX3 has been related to the initiation of tumorigenesis and gastric cancer progression.7,8 WW domain-containing oxidoreductase (WWOX) is a tumor-suppressor gene, which was associated with H. pylori, and has  been found to be decreased in gastric cancer. Also, H. pylori-infected gastric cancer cell lines demonstrated hypermethylation of WWOX, which  was linked with H. pylori-induced increased expression of DNMT1 and DNMT3.9

EPIGENETIC ALTERATION IN GASTRIC CANCER

DNA methyltransferases (DNMTs) catalyze DNA methylation, and epigenetic studies proposed that DNA methylation interacts with histone modifications, resulting in the change in gene expression.10 The expression of enhancer of zeste homolog 2 (EZH2) as a histone methyltransferase has been reported in a few types of cancer. The multiple crucial biological effects of EZH2 have been identified including signal transduction, DNA repair, cell division, and autophagy (Figure 1).11-14

Enhanced  EZH2  expression  is  a  biomarker  of  metastasis  in  various  cancers, such as prostate, breast, hepatocellular carcinomas, colorectal cancer, and lung cancer. Enhancer of zeste homolog 2 histone methyltransferase  has  been  suggested  as  a  new  therapeutic  target.  More recently, overexpression of EZH2 has been revealed in gastric cancer; nevertheless,  the  clinical  importance  and  molecular  mechanisms  of  EZH2 controlling gastric cancer progression are well-known. Enhancer of zeste homolog 2 is a member of the polycomb repressive complex 2  (PRC2),  which  catalyzes  trimethylation  of  histone  H3  lysine  27  (H3K27me3) and initiates carcinogenesis via epigenetically silencing tumor-suppressor genes.10,14-16

ENHANCER OF ZESTE HOMOLOG 2 AND SIGNALING PATHWAYS INVOLVED IN GASTRIC CANCER

Bone morphogenetic proteins (BMPs) are a member of transforming growth factor-beta and function through the BMP-SMAD (Suppressor of Mothers against Decapentaplegic) pathway. For the first time, BMPs were found to be proteins that stimulate ectopic bone generation and initiate  the  development  of  gastrointestinal  organs.  Bone  morphogenetic proteins have crucial roles in cell proliferation and apoptosis in numerous  cancers  such  as  gastric  cancer.  Many  studies  have  shown  that BMP2 modulates the expression of EZH2. However, as a tumor-suppressor  gene,  BMP2  might  suppress  the  EZH2  expression  and H3K27me3 through a new SMAD-BMP pathway, leading to a reduc-tion in CDK2, CDK4, and CDK6 and enhancement in P21 and P16.17

Phosphatase and tensin homolog (PTEN) is a straight target of EZH2. Most significantly, EZH2 can bind to the PTEN locus, decrease PTEN expression, and activate the Akt pathway. Besides, the EZH2–PTEN axis upregulates vimentin and decreases E-cadherin, thus finally leading to the acquisition of the epith elial –mese nchym al transition in gastric cancer cells.18

Inflammation plays a significant role in gastric cancer progression, and anomalous IL-6/STAT3 signaling pathways in cancer cells have been identified as an important mechanism for the onset and development of cancer. Interleukin-6 stimulates the signal transducer and activator of transcription 3 (STAT3), resulting in epigenetic abnormality during tumorigenesis, especially in the expression of the EZH2 histone meth-yltransferase enzyme. EZH2 binds to STAT3 and methylates it, causing increased STAT3 activity by enhancing STAT3 tyrosine phosphoryla-tion. An  EZH2  inhibitor  reverses  the  inhibiting  of  polycomb  target genes  and  reduces  STAT3  activity.  Consequently,  targeting  STAT3,  p-STAT3, and EZH2 might be a sensitive medicine and new therapy for gastric cancer patients.18,19

NON-CODING RNAS AND ENHANCER OF ZESTE HOMOLOG 2 IN GASTRIC CANCER

MicroRNAs (miRNAs) are small RNAs (about 22 nt) that have a sig-nificant role in modulating gene expression in tumor samples and are interesting as helpful biomarkers for early phase prognosis and diagnosis  of  cancer.  Various  microRNAs  have  been  linked  with  gastric cancer and its prognosis. In cohort studies, miR-221, miR-376c, and miR-744 were defined as being talented in distinguishing gastric cancer patients.20,21

Chromobox protein homolog 7 (CBX7), an element of the polycomb group family, is linked to the regulation of cancer cell proliferation, progression, and metastasis. The CBX7 overexpression is reported in gastric tumors. AKT-NF-κB-miR-21 signaling axis has been indicated as a critical effector of CBX7 in gastric cancer stem cell initiation and expansion.21 The studies showed that miR-625-3p, miR-217, and miR-137 were closely correlated with lymph node metastasis, remote metas-tasis, and poor prognosis in gastric cancer patients. They also showed that  miR-625-3p,  miR-217,  and  miR-137  suppressed  the  malignant  progression of gastric cancer via EZH2 regulation.22-24

Long non-coding RNAs (lncRNAs) are non-coding RNAs that have no  protein-coding  potential  and  are  longer  than  200  nt.  Long  non-coding RNAs have been identified as critical regulators of biological activities  and  various  cancers  through  their  interplay  with  multiple  biomolecules. For example, C8orf76 as an oncogene directly targets lncRNA-DUSP5P1  for  its  transcriptional  stimulation,  consequently  activating  the  MAPK/ERK  signaling  pathway  and  initiating  gastric tumorigenesis. Recent studies have investigated Linc01503 in gastric cancer and have indicated that it is closely associated with the overall survival of gastric cancer patients. Linc01503 could promote gastric cancer cell invasion via activating the wnt signaling pathway and interacting with early growth response protein 1 (EGR1). Furthermore, the knockdown of linc01503 resulted in the induction of apoptosis and the G1/G0 phase stopping in gastric cancer. Also, these studies reported that the interaction of EZH2 with lysine (K)-specific demethylase 1A (LSD1)  resulted  in  the  silencing  of  cyclindependent  kinase  inhibitor  1A  (CDKN1A)  and  dual-specificity  phosphatase  5  (DUSP5)  and mediated cell cycle progression and tumorigenesis in gastric cancer. Consequentially,  the  EGR1/ linc0 1503/ EZH2/ DUSP5 /CDKN 1A  axis  may be an appealing target for gastric cancer therapeutics.25,26

Another study showed that LINC00337 as an oncogene was increased in  the  gastric  cancer  cells,  and  the  overexpression  of  LINC00337  defines  the  poor  clinical  outcome.  The  in  vitro  and  in  vivo  studies showed that reducing LINC00337 suppressed invasion, proliferation, and tumorigenesis. It was also reported that LINC00337 epigenetically inhibited  p21  through  EZH2-mediated  blocking.27 Small nucleolar RNA host gene 6 (SNHG6) has an important role in the progression of gastric cancers. Knockdown of lncRNA SNHG6 suppresses gastric cancer development through overexpression of p21. Also, the inhibition of SNHG6 and the increase of p21 have resulted in the activation of the JNK pathway and repression of EZH2. Thus, SNHG6 plays a significant role in gastric cancer progression.28

A  new  study  shows  that  LINC00460  as  an  independent  prognostic  marker is upregulated in gastric cancer tissues compared to normal tissues.  Investigation  of  the  function  of  LINC00460  has  shown  that  LINC00460  initiated  gastric  cancer  proliferation  via  the  decrease  in cyclin G2, which is modulated by LSD1 and EZH2 proteins.29

ENHANCER OF ZESTE HOMOLOG 2 INHIBITORS IN GASTRIC CANCER

More studies have shown the role of EZH2 in cancer pathophysiology. RNA-based silencing such as siRNAs and shRNAs have been widely utilized to inhibit and modulate EZH2 signaling in cancer experimental research. Pharmacological and biological inhibition of EZH2 has been an area for enhancement of research efforts. Enhancer of zeste homolog  2  inhibitors  include  GSK926,  GSK343,  3-deazaneplanocin  A (DZNep), valemetostat, and tazemetostat (E7438/EPZ6438).

The strategies to suppress EZH2 contain inhibition of double EZH1/EZH2 and inhibitors that disrupt the PRC2 structure. Other strategies for the degradation of EZH2 are attempts to deliver specific drugs and siRNAs together with nanoparticles.30-32 Cisplatin and oxaliplatin are a member of platinum drugs and the first-line chemotherapy for gastric cancer. Cisplatin resistance is an important strategy in gastric cancer chemotherapy. The inhibition of EZH2 by siRNA improves cisplatin resistance in AGS gastric cancer cells.33 Another  study  showed  that that  miR-625-3p,  miR-217,  and  miR-137  suppressed  the  malignant  progression of gastric cancer via EZH2 regulation.22-24

Long non-coding RNAs (lncRNAs) are non-coding RNAs that have no  protein-coding  potential  and  are  longer  than  200  nt.  Long  non-coding RNAs have been identified as critical regulators of biological activities  and  various  cancers  through  their  interplay  with  multiple  biomolecules. For example, C8orf76 as an oncogene directly targets lncRNA-DUSP5P1  for  its  transcriptional  stimulation,  consequently  activating  the  MAPK/ERK  signaling  pathway  and  initiating  gastric tumorigenesis. Recent studies have investigated Linc01503 in gastric cancer and have indicated that it is closely associated with the overall survival of gastric cancer patients. Linc01503 could promote gastric cancer cell invasion via activating the wnt signaling pathway and interacting with early growth response protein 1 (EGR1). Furthermore, the knockdown of linc01503 resulted in the induction of apoptosis and the G1/G0 phase stopping in gastric cancer. Also, these studies reported that the interaction of EZH2 with lysine (K)-specific demethylase 1A (LSD1)  resulted  in  the  silencing  of  cyclin-dependent  kinase  inhibitor  1A  (CDKN1A)  and  dual-specificity  phosphatase  5  (DUSP5)  and mediated cell cycle progression and tumorigenesis in gastric cancer. Consequentially,  the  EGR1/ linc0 1503/ EZH2/ DUSP5 /CDKN 1A  axis  may be an appealing target for gastric cancer therapeutics.25,26

Another study showed that LINC00337 as an oncogene was increased in  the  gastric  cancer  cells,  and  the  overexpression  of  LINC00337  defines  the  poor  clinical  outcome.  The  in  vitro  and  in  vivo  studies showed that reducing LINC00337 suppressed invasion, proliferation, and tumorigenesis. It was also reported that LINC00337 epigenetically inhibited  p21  through  EZH2-mediated  blocking.27 Small nucleolar RNA host gene 6 (SNHG6) has an important role in the progression of gastric cancers. Knockdown of lncRNA SNHG6 suppresses gastric cancer development through overexpression of p21. Also, the inhibition of SNHG6 and the increase of p21 have resulted in the activation of the JNK pathway and repression of EZH2. Thus, SNHG6 plays a significant role in gastric cancer progression.28

A  new  study  shows  that  LINC00460  as  an  independent  prognostic  marker is upregulated in gastric cancer tissues compared to normal tissues.  Investigation  of  the  function  of  LINC00460  has  shown  that  LINC00460  initiated  gastric  cancer  proliferation  via  the  decrease  in cyclin G2, which is modulated by LSD1 and EZH2 proteins.29

ENHANCER OF ZESTE HOMOLOG 2 INHIBITORS IN GASTRIC CANCER

More studies have shown the role of EZH2 in cancer pathophysiology. RNA-based silencing such as siRNAs and shRNAs have been widely utilized to inhibit and modulate EZH2 signaling in cancer experimental research. Pharmacological and biological inhibition of EZH2 has been an area for enhancement of research efforts. Enhancer of zeste homolog  2  inhibitors  include  GSK926,  GSK343,  3-deazaneplanocin  A (DZNep), valemetostat, and tazemetostat (E7438/EPZ6438).

The strategies to suppress EZH2 contain inhibition of double EZH1/EZH2 and inhibitors that disrupt the PRC2 structure. Other strategies for the degradation of EZH2 are attempts to deliver specific drugs and siRNAs together with nanoparticles.30-32 Cisplatin and oxaliplatin are a member of platinum drugs and the first-line chemotherapy for gastric cancer. Cisplatin resistance is an important strategy in gastric cancer chemotherapy. The inhibition of EZH2 by siRNA improves cisplatin resistance in AGS gastric cancer cells.33 Another  study  showed  that DZNep as a histone methyltransferase inhibitor reverses cisplatin resis-tance of gastric cancer cells through the inhibition of HIF-1α, Wnt/β-catenin signaling molecules, and EZH2 protein.34  Nevertheless,  the  knockdown of lncRNA HOXD cluster antisense RNA 1 (HOXD-AS1) has  reversed  cisplatin  resistance  through  the  epigenetic  silencing  of  programmed cell death 4 via binding to EZH2 in gastric cancer cells.35

CONCLUSION

Enhanced  EZH2  expression  is  a  biomarker  of  metastasis  in  various  cancers, such as prostate, breast, hepatocellular carcinomas, colorectal cancer, lung cancer, and gastric cancer. Nevertheless, the clinical importance  and  molecular  mechanisms  of  EZH2  controlling  gastric  cancer progression are well known. This review provided roles of the dysregulation of EZH2 in gastric cancer and possible mechanisms for the suppression of EZH2 activity.