The present study focuses on the overexpression of RHOA, CXADR, CSNK1A1, CCND2, DVL2, FZD8, and NFACT1 genes belonging to the WNT signaling pathway. Expression rates were compared with GC, IM patients and control group. In addition, effects of HP on the expression of these genes in IM patients have been evaluated.
RHOA (Ras homolog gene family member A), a key step along the WNT signaling pathway, is a member of RHO family, which is considered to play a role in the development of various cancers and active in cellular signal transmission over GTPase (Maeda et al. 2015) [17]. It plays a role in events such as cell cycle, polarization, and migration. RHOA GTPase activation may be altered in the tumor tissue due to mutation. Consequently, GTPase-influenced G1-S phase of the cell cycle may gain pace or may slow down. A RHOA mutation is obtained in GC by cell culture assays. In these mutations, RhoA-specific siRNA, which is active in the G1-S domain of cell division, is detected (Zhang et al. 2009)[18].This represents another significant evidence for the importance of RHOA in cell proliferation (Karlsson et al. 2009) [19].Pan et al (2004), in a study in which they evaluated RHO family expressions in a cell culture assay, demonstrated that high RHOA expression was associated with poor-differentiated GC and TNM classification increased with higher expression rates[20]. Ushiku et al (2015) studied 87 patients with GC and found signet-ring cell appearance in 18 out of 22 RHOA-mutant patients. The patients frequently presented a mixed and diffuse involvement. Intestinal type GC was a rare clinical picture in RHOA-mutant patients. RHOA mutation was not considered a significant prognostic parameter[21]. In another analysis by Kakiuchi et al (2014) on 87 patients with diffuse GC, RHOA mutation was detected in 25.3% (22/87) of the patients[22]. A study by Wang et al (2014) presented similar findings as well. They found no RHOA mutation in the patients with intestinal type GC, whereas 14.3% (14/98) of the diffuse type GC cases had RHOA mutation[23]. Lastly, Röcken et al (2016) retrospectively examined the pathological preparations of 415 patients to evaluate the clinical and pathological situation of RHOA-mutant cases. Cases present poor-differentiated and largely distal GC. Interestingly, mutant cases had a lower T grade and did not present distant metastasis[24]. In another analysis by Chang et al (2016), the role of RHOA pathway dysregulation in GC and the feasibility of clinically targeting RHOA for GC therapy were systematically assessed. As a result, they demonstrated that RHOA gene in the WNT signaling pathway was described as both a potential therapeutic target and a biomarker for the Asian GC[12].
The WNT signaling pathway gets activated via canonical and non-canonical ways. In the non-canonical way, the signal is transmitted to RHOA/RHOU/RAC/CDC42, JNK, PKC, NFAT, and NLK cascade via Frizzled (FZD) receptors and PTK7/ROR2/RYK co-receptors. LRPs (low-density lipoprotein-related receptor) play a role in the canonical way, whereby FZD receptors upregulate MYC, CCND1, FGF20, JAG1, WISP1, and DKK1 genes with LRP5/LRP6 co-receptors (Katoh et al. 2007; Katoh et al. 2009; Joiner et al. 2013) [25–27]. LRP5 and LRP6 are currently being researched for other diseases as well as carcinogenesis. Their overexpression or mutation has been demonstrated as a factor in many diseases (Joiner et al. 2013; Ettenberg et al. 2010) [27, 28]. The literature on adenocarcinomas is limited. A study by Liu et al. on advanced-stage GC cases found that the presence of mutation was inversely proportionated with disease-free and median survival in patients with LRP5 mutation (Liu et al. 2014) [29].
FZD is a part of WNT-FZD-LRP5-LRP6 component and functions as a receptor along the WNT signaling pathway. It activates the Beta-catenin complex. The triggering of this signaling pathway results in the activation of disheveled (DVL) proteins, inhibition of GSK-3 kinase, nuclear accumulation of beta-catenin, and activation of WNT target genes (Ueno et al. 2013) [30].. There is limited knowledge on the relation between GC and FZD gene, which presents an increased expression in different types of GC. FZD receptors were evaluated for GC in an analysis conducted by Kirikoshi et al (2001). They cloned FZD1-10of the FZD family and checked expression profiles of patients with GC. The analysis showed increased FZD2 and FZD8 in 4 out of 10 patients. The study emphasized that FZD2 and FZD8 might serve as a potential biomarker for GC. No association was detected between the expression of FZD8 and HP (Kirikoshi et al. 2001) [31].
Disheveled Segment Polarity Protein 2 (DVL2) is a major step along the WNT signaling pathway. It binds to the cytoplasmic C-terminal of the FZDreceptor and ensures the downward transmission of the signal. It plays a role in both canonical and non-canonical WNT signaling pathways. DVL2 binds to the Axin-APCcomplex that downregulates the Beta-catenin signaling pathway and activates the pathway (Gnad et al. 2010; Katoh 2005; Schwarz-Romond et al. 2007) [32–34]. A study by Metcalfe et al (2010) found overexpressed DVL2 in colorectal adenocarcinoma (CRC) cells. Based on the idea that the inhibition of DVL2 by siRNA reduces the activity of Beta-catenin signaling pathway, it was thought that DVL2 activation in patients with CRC led to a hyper-activated Beta-catenin signaling pathway. An analysis conducted on 24 patients with CRC detected that the DVL2 level in 1/3 of CRC cells was higher than that of the normal tissue on the tumor surgical border. The study proceeded by increasing the number of subjects to 393. Patients were divided into the groups of normal mucosa, hyperplastic, adenomatous polyp, and CRC. When Beta-catenin-positive nuclei were ordered from normal tissue to the carcinomatous tissue, the expression rate was observed to increase within the cell groups. This increased expression level was also the case for Axin and DVL2, as well. The preservation of this rate across different tumor stages suggests the upregulation of DVL2 and Axin genes in patients with CRC in response to APC loss. With a WNT simulation, the same study also demonstrated that the upregulation of DVL2 might also occur post-transcriptionally. This led the authors to evaluate DVL2 in APC-mutant rats. A low level of DVL2 was found proportionated to reduced tumor burden; and shortened small intestine, as well as reduced number and diameter of crypts were observed in DVL2 reduction [35]. A study by Chong et al (2014) found that DVL2 and ETS1 proteins were more specific to diffuse and mixed type GC than the intestinal type. Both genes display a methylation similar to the normal tissue and intestinal type GC, whereas the methylation rate was high in the mixed and diffuse type GC. These findings suggest that abnormal methylation starts from the non-carcinomatous normal tissue in the mixed and diffuse type GC [36].
CyclinD2 (CCND2) is a member of the cyclin family. CCND2 regulates CDK kinase and forms a complex with CDK4 and CDK6. CCND2 is the regulator of this complex. It enables cell cycle to proceed from G1 to S phase. CCND2 also participates in the phosphorylation of RB (Retinoblastoma-associated protein), which is a tumor suppressor protein (Truong et al. 2015) [37]. Therefore, the activation of CDK/Cyclin complex may gain importance in carcinogenesis (Myant et al. 2011) [38]. In a study conducted on rats by Cole et al (2010), authors suggested that the inhibition of WNT signaling pathway could prove to be a treatment alternative for patients with CRC. The study demonstrated that Cyclin D/CDK4-6 complex increased in the intestinal tissues of APC-negative rats. In APC-positive rats, however, CyclinD2 level was low in the intestinal tissue. The most important findings were as follows: CyclinD2 did not affect normal enterocyte development and CDK4/6 inhibition suppressed the adenomatous tissue in knockout rats[39]. A retrospective study conducted by Shi et al (2015) on patients with GC demonstrated significantly increased CyclinD2 (CCND2) in the tumorous tissue compared to the normal tissue (P < 0.001). The same analysis also detected that increased CCND2 expression was associated with the depth of invasion, lymph node positivity and distant metastasis, and advanced TNM staging. Authors suggested that increased CCND2 expression could be a prognostic factor[40].
Two casein proteins were purified: Casein kinase (CK) proteins CD1 and CD2. Casein kinase 1 (CK1) is a monomeric protein of a serine and threonine structure. It takes part in tasks such as DNA repair, cell division, nuclear settlement, membrane transport, etc. CK1-alpha is thought to phosphorylate beta-catenin and played a role in the degradation process when it is not bound to the WNT receptor (FZD). When CK1 binds to the WNT receptor, it phosphorylates Ck1-epsilon intracellular signaling protein, while DVL and CK1-gamma phosphorylate LRP5/6 co-receptors (Wang et al. 2010) [41]. As to its impact on tumorigenesis, studies on CSNK1A1 present divergent and contradictory values.
CXADR (Coxsackie virus and adeno virus receptor) gene codes type 1 membrane receptor protein for the adeno virus C sub-group and Coxsackie B virus. Bowles et al. mapped the functional CXADR gene at chromosome 21 and the pseudo-CXADR gene at chromosomes 15, 18, and 21. It was found at 21q11 location by a FISH analysis (Bowles et al. 1999) [42]. Gamma-delta T-cells in the epithelial tissue play a critical role for defense against environmental factors such as infection, trauma and malignancy. The CXADR gene, which is included in the present study, has been previously isolated in pancreatic and small intestine tissues and demonstrated to be active in cell proliferation through cytokines, which suggests that this gene could be active in GIS adenocarcinomas.
Studies on NFACT1 gene, which is a part of this study, are largely conducted osteocytes. There is no study available on its relationship with adenocarcinomas; however, it has been demonstrated that HP causes the inhibition of NFACT, increases VacA secretion, and consequently leads to the vacuolization of epithelial cells. This intensifies the process whereby HP becomes chronic (Gebert et al. 2003) [43]. Although there is no relevant study in the literature and our findings contradict it, there are some data available to claim that HP becomes chronic by inhibiting NFACT, which suggests that this gene may be associated with adenocarcinoma.
A review of our findings
We classified the findings of genes in two groups. The first group included the genes RHOA, LRP5, FZD8, DVL2, and CCND2. Similar to the reports in the literature, RHOA, LRP5, FZD8, DVL2, and CCND2 were found significantly overexpressed compared to the patients in the control group. Again, similar to the findings reported in the literature, we detected that these genes were significantly overexpressed in diffuse type GC patients compared with both the control group and patients without a diffuse involvement. Patients with a distant metastasis had a significantly higher overexpression of RHOA, LRP5, FZD8, DVL2, and CCND2 than non-metastatic patients. This finding contradicted previous reports for RHOA. Compared with survival, exitus patients presented an overexpression of RHOA, LRP5, FZD8, DVL2, and CCND2, although the difference was not significant. As part of the present study, we checked RHOA, LRP5, FZD8, DVL2, and CCND2 in patients with IM and found that they were significantly overexpressed with respect to the control group. HP-positive and/or HP-negative patients had a significantly higher overexpression of RHOA, LRP5, FZD8, DVL2, and CCND2. Patients with HP-positive IM showed significantly overexpressed RHOA, LRP5, FZD8, DVL2, and CCND2 than the HP-negative ones. A comparison with GC cases revealed no significant difference from the patients with IM in terms of gene expression (p > 0.05). RHOA, LRP5, FZD8, DVL2, and CCND2 were overexpressed in HP-positive and HP-negative patients. The overexpression was higher in the former group than the latter.
The second group included the genes CSNK1A1, CXADR, and NFACT. CSNK1A1 showed no significant overexpression in GC or IM patient groups compared with the control group. Contrary to previous reports on patients with adenocarcinoma, the present study did not detect a significantly overexpressed CXADR in GC or IM patient groups than the control group. Although our findings do not support it, previous reports claiming that HP becomes chronic by causing NFACT inhibition suggest that this gene could be associated with adenocarcinoma.