RNA Binding Protein QKI Inhibits Gastric Cancer Development Through Post-transcriptionally Repressing COX2Expression

Gastric cancer is a prevalent malignant tumor of the digestive system. In ourcountry, the mortality of gastric cancer ranks the first among all of of thedifferent tumors. During the development of gastric cancer, inflammatorystimulation plays a very important role. Deciphering the molecular events ofinflammation in the cancer progression is critical for the diagnosis and treatmentof gastric cancer.The occurrence of gastric cancer is a multiple factors and there are manymolecular abnormalities involved. The process of the precancerous lesion ofgastric cancer could be divided into chronic gastritis, intestinal metaplasia, gastriculcer, and other inflammation related steps. In this regard, continuousinflammation is the most important stimulus for the transformation from normalgastric epithelium cells to malignant cells.Cyclooxygenase (COX) is the key enzyme of arachidonic acid metabolism,which catalyzes arachidonic acid into prostaglandins. There are two subtypes ofCOX, COX1and COX2respctively. COX1expresses across the whole body.COX2doesn’t express at the basal condition, but could be induced in a variety ofpathologicalor physiological conditions. In Gastric cancer, the excessive COX2expression is mainly found in the body of the stomach and a few in thecardia.The expression level of COX2positively relates with the size of the tumorand tumor metastasis. Patients with lymph node metastasis show higher COX2mRNA expression than those without lymph node metastasis. The mechanism is probably due to COX2and its products PGE2can promote vascular endothelialgrowth factor expression and thus angiogenesis induction. From all above we canconclude that COX2is critical for the development of gastric cancer. Study ofregulatory mechanisim of COX2is essential to understand the pathogenesis ofgastric cancer.There are two main COX2transcripts: COX2(4.6KB) and COX2(2.8KB).COX2(4.6KB) is the main transcript with2hours half-life. So it is easy to decay.Under inflammatory conditions, COX2elevated both at transcriptional level andpost-transcriptional level. In post-transcriptional level, RNA-binding proteinHuR interacts with the3’UTR of4.6KB transcripts, leading to enhanced COX2mRNA stability.RNA-binding protein QKI belongs to STAR family, which is highly conservedacross different species. QKI is involved in the process of myelin formation.Besides, QKI is also involved in angiogenesis, cell apoptosis, cell growth,morphogenesis, etc. QKI can recognize the mRNA sequence with specificcharacteristics. It is believed that QKI may bind and regulate more than1480mRNAs. We have found that QKI is a potential tumor-suppressor gene. In thecolon cancer, QKI can inhibit β-catenin activation and thus inhibit Wnt signalingpathways, leading to inhibition of cell proliferation and promotion of the colonepithelial cell differentiation. In the intestinal epithelium cells, QKI was alsoregulated by E2F1at the same time and in turn extending G1phase through anegative feedback mechanism. So during the initiation and the development ofgastric cancer, QKI might also play a tumor-suppressor Role. From the reports inthe literatures, q26.6where QKI localized is heterozygous missing in gatriccancer with a certain proportion of, impling that QKI may be a tumor suppressorgene. Bioinformatics analysis finds that the3’UTR of COX2have multiple quakingresponse components (QRE) sequence5’-A (C/A) UAA-3’, suggesting thatCOX2may be a QKI target. In view of the QKI suppressor functions in coloncancer and the loss of heterozygous in gastric cancer, we speculate in the gastriccancer COX2is affected by the QKI. Based on the above assumptions, our studyproduces the following results.1) RNA-binding protein QKI expression decreased in tumor cells andclinical tissue: we compared the expression changes of QKI in clinicalsamples. Then immunohistochemical and Western Blotting shows that inthe gastric cancer tissue, QKI-5,6expression were significantly reduced,compared with normal tissue.Real time PCR and Western Blottingexperiments showed higher QKI expression in normal gastric epitheliumcell lines GES than that in the gastric cancer cell lines too. These resultssuggest that QKI may be a tumor suppressor gene.2) QKI expression decreased in gastric cancer due to high promotermethylation: In silico analysis found that QKI promoter is rich in CpG.QKI promoter methylation degree in tumor samples and gastric cancer celllines were significantly higher than normal cells and tissues. QKIexpression can be recovered partially by the methylation enzyme inhibitors5-aza-dC in SGC-7901cells where QKI promoter is highly methylated.All of these suggest thatQKI promoter methylation is an importantreasonfor reduced QKI expression in the gastric cancer.3) QKI can repress gastric cancer cell proliferation and migration: inorder to test the tumor suppressor genes function of QKI in gastric cancer,we overexpressed QKI-5, QKI-6in gastric cancer cell lines. Through MTT,Brdu labeling and tanswell experiments, we found that QKI can suppress gastric canecer cellsproliferation and migration.4) QKI inhibited gastric cancer proliferation and migration throughnegatively control of COX2: Real time PCR, ELISA experiments in thegastric cancer cell lines AGS, MKN45further showed that overexpressQKI-5,6all can inhibit COX2expression. In contrast, RNA interference ofQKI increased COX2expression. COX23’UTR Luciferease assay showedthat QKI can regulate COX2by interacts with the mRNA.To sum up, we first found QKI may be a tumor-suppressor gene in gastriccancer. Reduced QKI expression, which is caused by promoter methylation, mayinfluence the development of gastric cancer. Molecularly, QKI negativelycontrols COX2expression by interact with COX23’UTR. Our reaserch mayprovide new ideas and strategies for clinical diagnosis and treatment of gastriccancer.