Esophageal cancer is one of the common malignant tumors in the digestive system. The incidence rate of esophageal cancer in China is relatively high, such as in Fu Jian and other provinces. The incidence and death rate of esophageal cancer in China accounts for more than 50% of the world's total, which brings a great burden to China's medical industry and economic development .
This study is the first to explore the expression of CUEDC2 in esophageal squamous cell carcinoma tissues and cancer adjacent tissues. Double immunofluorescence method was also the first used to explore the differences in the expression of CUEDC2 and CD68-labeled macrophages in esophageal cancer tissues, and it was believed that the expression of CUEDC2 protein might be involved in the occurrence and progression of esophageal squamous cell carcinoma. Now the results are discussed as follows.
CUEDC2 is involved in various pathophysiological processes, such as the regulation of cell cycle and inflammatory response, as well as tumor occurrence and development [10, 36]. CUEDC2 can modulate the inflammatory response by inhibiting the activation of NF-κB and STAT3 signaling pathways . CUEDC2 can regulate the process of mitosis as well. When exposed to ultraviolet radiation, there occurs the degradation of CUEDC2 which may increase the instability of genes in the cell cycle, thus exerting its role in the development of tumors . Current research demonstrates that During the survival and proliferation of tumor cells, CUEDC2 can show its influence in the aerobic glycolysis of cells, and increase the production of lactic acid and the uptake of glucose by tumor cells during the process, so as to provide energy for tumor cells .
The inactivation of tumor suppressor genes and the activation of proto-oncogenes can participate in the proliferation and differentiation of tumor cells.
In this study, 9 cases (9/50 = 18%) of 50 esophageal squamous cell carcinoma tissues showed high expression of CUEDC2, while 41 cases (41/50 = 82%) showed low expression. There were 11 cases (11/20 = 55%) with high expression of CUEDC2 in the para-cancerous tissues of 20 cases of esophageal squamous cell carcinoma, and 9 cases (9/20 = 45%) with low expression, the difference was statistically significant. Imaginably,this result indicates that with the progress of the disease, the expression of CUEDC2 in ESCC is lower than that in normal tissues, that is to say, the expression of CUEDC2 protein may play a role as an oncogene in the occurrence and development of ESCC. However, how CUEDC2 acts on ESCC and the specific mechanism need to be further studied.
Till now, there appear to be no reports on the expression of CUEDC2 protein in ESCC patients and its relationship between the general clinical and pathological data. We found that CUEDC2 protein expression and esophageal squamous cell carcinoma of the patient's age and gender had no significant correlation, and may be due to women's groups were of less samples (5 cases in this group, including 0 women patients appeared CUEDC2 high expression). Taken together, in future follow-up experiments we have to further increase the sample size to enhance the experimental conclusion more persuasive. Meanwhile, the relationship between CUEDC2 and esophageal squamous cell carcinoma patients' smoking status, tumor size and location, differentiation degree, invasion depth, lymph node metastasis and tumor grade were analyzed. No significant correlation was found between CUEDC2 expression and ESCC in this study, which may be attributed to the limited sample size included in this study.
Interestingly, by analyzing the relationship between CUEDC2 and tumor size of esophageal squamous cell carcinoma, it was acknowledged that when the tumor diameter was < 5 cm, the low expression of CUEDC2 was found in 32 cases (32/50 = 64.00%); whereas, if the diameter was ≥ 5 cm, and the low expression was found in 9 cases (9/50 = 18.00%). That is to say, although there is no significant difference between the expression of CUEDC2 in this study and the tumor size, we may come to a conclusion that the smaller the tumor diameter is, the higher proportion of the low expression of CUEDC2, which also simultaneously renders some implications for the further study on the expression of CUEDC2 in ESCC.
Therefore, it cannot be concluded that the expression of CUEDC2 protein has nothing to do with the general clinical data and pathological classification of patients with esophageal squamous cell carcinoma.
Through the postoperative follow-up survival conditions in patients with esophageal squamous cell carcinomas and usage of Kaplan Meier-method of single factor regression analysis, we found that CUEDC2 protein expression and tumor grade have an impact on the survival prognosis of esophageal squamous carcinoma, and the lower expression of CUEDC2 corresponds to lower survival rate, which provides a new basis for the prognosis prediction of patients with esophageal squamous carcinoma assessment. Besides, what cannot be ignored is that CUEDC2 may serve as a potential tumor suppressor gene in the development of esophageal cancer.
Infectious diseases and chronic inflammatory disease account for about 25% of cancer-inducing factors. As we said before, the macrophages not only play an important role in the immune system but also, they are indispensable parts of the inflammatory response. CD68, as one of the markers of tumor tissue macrophages, its expression may impact on immunosuppression, tumor invasion, inflammation and other mechanisms involved in tumor progression .
In this study, it was identified that CD68 was relatively highly expressed in esophageal squamous cell carcinoma tissue, which was consistent with the study of Rongling Xie. Meanwhile, the infiltration density of CD68-labeled macrophages in esophageal squamous cell carcinoma tissue was relatively increased, which means that CD68-labeled macrophages may participate in the disease process of esophageal squamous cell carcinoma. In immunofluorescence, it showed that CUEDC2 was relatively low in esophageal carcinoma tissues, which was basically consistent with the results of immunohistochemistry in this study. Statistical analysis of the Wilcoxon sign rank sum test showed that there were significant differences in the expression of CD68 and CUEDC2 in esophageal cancer tissues, and the differences were statistically significant. Combined with immunofluorescence double standard atlas, we are the first to discover the phenomenon that the green light of CD68 markers and the red light of CUEDC2 markers, which resulted in yellow fluorescence, suggesting there were positioning phenomenon. Besides, there may be a potential interaction between both proteins, participating in the development of esophageal cancer, but the specific area of effect and mechanism needs further research.
Chen yuan et al.  found that CUEDC2 expression was increased when monocytes differentiated into macrophages. As mentioned in the introduction, when inflammatory stimulation is experienced in the mouse model of inflammatory bowel disease, the knockout of CUEDC2 will significantly increase the expression of inflammatory factors such as interleukin-6, and meanwhile cause the continuous activation of macrophages, which may induce tumor formation. However, when infiltrated into macrophages of colon cancer, the expression of CUEDC2 is significantly reduced or hardly expressed, which means that CUEDC2 can participate in the regulation of macrophage-induced inflammatory response and tumor progression.
In conclusion, CUEDC2 has a low expression in esophageal cancer and is possibly capable of regulating inflammatory responses mediated by macrophages. Furthermore, CD68, relatively higher expression in esophageal cancer, can be involved in the inflammatory response function in tumor progression. Finally, the double immunofluorescence test method has shown that there were co-localization phenomenon, supporting the assumption that there were potential interactions between the expression of CUEDC2 and CD68 labeled macrophages in esophageal cancer which indicates their possible roles of providing new exploration evidence for the pathogenesis of esophageal cancer in "inflammation and cancer" chain . However, it remains to be verified.
In this study, only semi-quantitative analysis of esophageal squamous cell carcinoma tissues was conducted from the perspectives of immunohistochemistry and immunofluorescence, and the expression of CUEDC2 was not completely quantified by PCR and other experiments, which could be improved to make the experimental conclusions more convincing. Further study on the interaction between CUEDC2 and CD68 will be helpful to elucidate the pathogenesis of esophageal cancer and is expected to be a new target for the treatment and diagnosis of esophageal cancer.