Up-regulation Of Gp78 Is Associated With Down-regulation Of KAI1 Gene In Gastric Carcinoma And Hepatocellular Carcinoma

Background and objectiveHepatocellular carcinoma and gastric carcinoma are common malignant tumors in digestive system, and their incidences are increasing in year by year in China. Tumor invasion and metastasis are common cause of death. Balance between oncogenes and antioncogenes, and alternations of related metastatic genes are involved in tumor invasion and metastasis. The transfer process includes tumor cell movement, adhesion, formation of new blood vessels, and matrix degradation.Ubiquitin proteasome pathway (UPP) comprises a series of related enzymes, such as ubiquitin, ubiquitin-activating enzyme(E1), ubiquitin-conjugating enzymes(E2), ubiquitin-ligating enzymes (E3), and 20S and 26S proteasomes. 26 S is assembled by complex 20 S and19 S. Among these, E3, also named gp78, is closely correlated with the genesis and development of many malignant tumors. Further, its role potential as a pharmacological target has been well investigated.KAI1 gene is located on human chromosome 11p11.2, and its full length cDNA is about 2.4 kb. It was previously proved to be a suppressor for metastasis of prostatic carcinoma. It encodes KAI1 protein with a molecular weight of 29,610 Da. In fact, people have found that the protein construction of KAI1 was identical to that of CD82. KAI1 is a member of transmembrane 4 superfamily (TM4SF). It has four hydrophobic transmembrane regions and a large extracellular hydrophilic region, and the latter contains three potential N-glycosylation sites. It was confirmed that KAI1 could affect metastasis and invasion of tumor cell via influencing cell-cell, matrix-matrix interactions. A large number of studies have shown that KAI1 protein was involved in the reaction of cell -cell and cell-matrix. Also, it can combine to adhesion receptors of cell surface, thereby block adhesion receptors and suppress tumor cell adhesion, inhibiting tumor cell metastasis and protecting organs from injury of endogenous and exogenous substances.Gp78 is also known as tumor autocrine motility factor receptor which is also a transmembrane protein, closely related to tumor metastasis. Studies have showed that gp78 was an ubiquitin protein ligase (E3) based ring finger structure of endoplasmic reticulum (ER). Gp78 can specifically recruit MmUBC7, and MmUBC7 , an ubiquitin-protein ligase (E2), and initiates endoplasmic reticulum (ER) associated degradation through a specific region. More importantly, gp78 can also mediate degradation of CD3δ, whereas CD3δis a most characteristic substrate in ER-associated degradation. On the contrary, the stability of CD3δis greatly enhanced when gp78 is lack of a compact ring and multiple transmembrane regions. Thus, gp78 is thought to be a potential junction for ubiquitination, ER degradation, as well as tumor metastasis. Meanwhile, it suggested that gp78 was associated with prognosis of cancer. In study of the correlation between KAI1 and gp78, it was found that gp78 could target to KAI1, thereby decreased KAI1 expression and increased KAI1 degradation, which might enhance the metastasis potency of sarcoma cell line.Actually, systemic study of gp78 and KAI1 in digestive tract tumors is rare so far. In this study, we investigated the clinical significance and correlation of gp78 and KAI1 expression in the genesis and development of hepatoma and gastric carcinoma via determination of the expressions of gp78 and KAI1 in the digestive tract tumors . Futher more, we successfully established hepatoma cell metastatic model in nude mice and then used immunohistochemistry and Western blot to study the expressions of gp78 and KAI1 in liver tissue. Hopefully, we intend to preliminarily clarify the possible mechanisms of gp78 and KAI1 in digestive tract tumors and provide some clinical and experimental evidences for those two genes that might be potential molecules for clinical diagnosis, prognosis, and pharmacological targets of digestive tract tumors..Methods(i) Hepatocellular carcinoma and gastric carcinoma tissue samples were collected. Immunohistochemistry and Western blot were employed to detect the expressions of gp78 and KAI1 in the carcinoma tissues.(ii) Hepatoma cell metastatic mouse model was successfully established. Immunohistochemistry and Western blot were used to assay the expression levels of KAI1 and gp78 proteins in live tissue of the nude mice.(iii) All the data were expressed as mean±standard deviation. SPSS10.0 statistical software was used for analysis. The difference between two groups was examined by T test orχ2 test. Paired-sample t test was used to compare the difference between cancer and normal tissue. The correlation between gp78 and KAI1 was analyzed by spearman correlation analysis. A p value less than 0.05 was considered significant.Results(i) Western blot and immunohistochemistry results suggested that the expression of gp78 was up-regulated in human hepatocellular carcinoma and gastric carcinoma, whereas the expression of KAI1 was significantly down-regulated in these digestive tract tumors.(ii) Similarly, enhanced gp78 and reduced KAI1 expressions were also found in the live tissue in nude mice of hepatoma cell metastasis model.(iii) There was a negative correlation between gp78 and KAI1 in hepatocellular carcinoma and gastric carcinoma.Conclusion(i) Gp78 and KAI1 were differentially expressed in human hepatocellular carcinoma and gastric carcinoma. The gp78 was significantly up-regulated and the KAI1 was remarkably down-regulated.(ii) The expression of gp78 was negatively correlated with KAI1 in cancer tissue compared with normal tissue.(iii) Gp78 could significantly down-regulate the expression of KAI1, and overexpression of gp78 resulted in increased degradation of tumor suppressor KAI1, which might be a possible mechanism that was responsible for invasion and metastasis of digestive tract tumors.(iv) In the nude mice liver tissue in situ, expression of gp78 protein was significantly increased, whereas the expression of KAI1 was remarkably decreased. Similarly, in the liver metastasis tissue, gp78 protein level was significantly elevated, while the level of KAI1 was remarkably reduced, which may suggest that KAI1 and gp78 were involved in the invasion and metastasis process of hepatocellular carcinoma. KAI1 degradation might be caused by gp78 overexpression and then that led to the metastasis occurrence of in situ hepatocellular carcinoma.