Gastric cancer is the fifth most commonly diagnosed cancer and the third leading cause of cancer deaths in men and women worldwide . It is a complex, heterogeneous, and multifactorial disease with different phenotypes . Most gastric cancers are adenocarcinomas that can be histologically divided into intestinal, diffuse, mixed, and indeterminate subtypes and vary in morphology, epidemiology, progression pattern, genetics, and clinical type. The most common subtype is intestinal, which occurs in approximately 54% of cases. Under normal conditions, CDX2 expression is restricted to the intestine. However, it is ectopically expressed in IM lesions of the stomach and intestinal-type gastric carcinoma [1−15]. Several studies have reported that CDX2 acts as a tumor suppressor in gastric cancer [10, 12, 15, 28−38]. However, others consider CDX2 an oncogene in gastric cancer [1, 2, 16−27]. These contradictory findings imply that CDX2 plays a complex regulatory role in gastric cancer. It was reported that Reg IV promoted tumor cell proliferation, metastasis, and migration and inhibited apoptosis in gastric cancer [43−46]. Collectively, although CDX2 and Reg IV are involved in the development and progression of gastric cancer, the regulatory correlation between CDX2 and Reg IV in gastric cancer remains unclear.
Results from this study demonstrated the following: First, there was a significantly positive correlation between the expression of CDX2 and Reg IV at the mRNA and protein levels in gastric cancer tissues. Previous studies have shown that CDX2 expression is associated with Reg IV expression [48, 52]. Yasui et al. reported that all of the Reg IV positive cells were also positive for CDX2 in both IM and gastric cancer; however, CDX2 positive cells were not consistently positive for Reg IV. This indicates that CDX2 might be an upstream regulator of Reg IV expression . Another recent study showed that Reg IV expression was observed in all CDX2 positive cases of ovarian mucinous cancer . Our study further demonstrated that CDX2 expression was positively correlated with Reg IV expression in gastric cancer tissues. Second, CDX2 upregulated Reg IV expression in gastric cancer cells. Previous studies demonstrated that Reg IV expression was induced by glioma-associated oncogene homolog 1 (GLI1) in pancreatic cancer  or by growth factors including transforming growth factor-α (TGF-α), epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), and hepatocyte growth factor (HGF) in colon cancer . In the present study, Reg IV expression was also positively regulated by CDX2 in gastric cancer cells. Consistent with our findings, previous studies reported that Reg IV expression was induced by CDX2 overexpression in OMC-3 ovarian mucinous carcinoma cells and HT-29 colon cancer cells [49, 52] and suppressed by CDX2 siRNAs in HSC-39 gastric cancer cells . Third, Reg IV had no effect on CDX2 expression in gastric cancer cells. SOX9 is a transcription factor which belongs to the SOX family and was induced by Reg IV in MKN-28, MKN-45, and AGS gastric cancer cells [47, 48]. CDX2 is also a transcription factor; however, silencing and overexpression of Reg IV did not affect CDX2 expression in AGS and MKN-45 cells. Fourth, CDX2 promoted migration and invasion of gastric cancer cells. The same research group has shown that CDX2 siRNA significantly inhibited cell proliferation, caused G0/G1 phase cell cycle arrest, induced apoptosis, and decreased migration and invasion in MGC-803 cells , whereas CDX2 overexpression produced results similar to those of CDX2 siRNA in MGC-803 cells , suggesting that CDX2 may play a dual role in the regulation of cell growth and death in MGC-803 cells. However, another study showed that CDX2 overexpression suppressed cell migration and invasion in MGC-803 cells and that CDX2 silencing promoted migration and invasion in NCI-N87 cells , indicating that CDX2 may inhibit migration and invasion of gastric cancer cells. Moreover, another report showed that ectopic expression of CDX2 reduced migration and invasion in MKN-45 cells , which is contrary to our findings in MKN-45 cells. The inconsistency in these results may be due to the different gastric cancer cell lines used in the studies. We previously found that Reg IV overexpression upregulated SOX9 and promoted migration and invasiveness of tumor cells, while Reg IV silencing produced opposite results. In addition, SOX9 silencing inhibited the migration and invasion in MKN-45 and AGS cells, demonstrating that Reg IV may promote invasion and migration of MKN-45 and AGS cells through upregulation of SOX9 in AGS and MKN-45 cells . In this study, CDX2 overexpression upregulated Reg IV expression and enhanced migration and invasion and CDX2 silencing downregulated Reg IV expression and suppressed migration and invasion in AGS and MKN-45 cells. These results demonstrate that CDX2 may promote cell migration and invasion through upregulation of Reg IV/SOX9 signaling in AGS and MKN-45 cells. Nevertheless, our and other studies also showed that silencing of SOX9 upregulated Reg IV protein expression in AGS, MKN-45, and MKN-74 cells [47, 48]. Furthermore, a previous report concluded that SOX9 also inhibited CDX2 protein expression both in intestinal adenocarcinoma cells in vitro and in a nude mouse xenograft tumor model . Thus, we speculate that the aberrant expression of SOX9 may trigger negative feedback regulation on CDX2 and Reg IV in gastric cancer cells. Specifically, the ectopic expression of CDX2 may increase Reg IV and SOX9 levels, leading to the enhancement of migration and invasion. When SOX9 is increased to a certain level, it may conversely suppress the expression of CDX2 and Reg IV, resulting in a reduction of migration and invasion of gastric cancer cells. Further studies are needed to confirm these hypotheses.