The Role Of Cav-1 On The Transformation, Apoptosis And N-glycans Branch Formation Of Mouse Hepatoma Cells

Caveolin-1 is a major structural protein of caveolae and plays important roles in signal transduction, cellular transformation and tumor metastasis. Currently, the exact functions of Cav-1in tumor remain controversial. Cav-1may act as a tumor suppressor in most (but not all) cancer cells, and Cav-1also acts as a tumor promoter in certain tumor cell types. Thus, the Cav-1gene has been regarded as both a tumor suppressor gene and an oncogene. However, the exact role of Cav-1in hepatoma remains unknown. Therefore, an understanding of the molecular mechanisms involved in tumor cell transformation and progression could be helpful in developing more effective treatments for hepatoma.In previous study, we have reported that Cav-1was highly expressed in mouse hepatoma cells with lymphatic metastasis potential, and could increase their invasive ability by up-regulating CD147 glycosylation level. This suggested that Cav-1expression might be correlated with the malign- nant phenotypes of hepatoma cells. However, the role of Cav-1in cell transformation and apoptosis has not been reported. Here,the relationship between Cav-1expression and malignant phenotypes was studied through overexpression of Cav-1in Cav-1-negative mouse hepatoma Hepa1-6 cells and gene-specific suppression of Cav-1expression in Cav-1-positive Hca-F cells.In this study, exogenous expression of Cav-1in Hepa1-6 cells increased their transformation potential both in vitro and in vivo, and prevented ActD- induced apoptosis via the activation of survivin-mediated survival pathway. By contrast, down-regulation of Cav-1in Hca-F cells using small inter- fering RNA significantly attenuated cell transformation ability, and induced apoptosis and enhanced cell sensitivity to ActD. These results suggest that Cav-1could play an active role in mediating cell transformation and survival as well as cell invasion, and thus acts as a tumor promoter in hepatoma cells. Moreover, Cav-1might be a potential target for gene and drug treatment of hepatoma.In addition, to further explore the exact roles of Cav-1and the possible utility of Cav-1downregulation for hepatoma therapy, we used RNAi technique to knockdown the expression of Cav-1in H22 hepatoma cells. In the present study, we have examined the phenotypic changes, including transformation capability and apoptotic potential, resulting from the reduction of Cav-1expression in transfected H22 cells both in vitro and in vivo. The results showed that Cav-1-siRNA could effectively inhibit the expression of Cav-1in H22 cells, and attenuate the transformation and survival capability of H22 cells in vitro and in vivo. Thus, Cav-1gene can be regarded as a very good target gene in genetic therapy for hepatoma and the use of Cav-1-siRNA deserves further investigations as a novel approach to cancer therapy.CD147, also known as matrix metalloproteinase inducer (EMMPRIN), is a highly glycosylated immunoglobulin superfamily transmembrane pro- tein. As a result of heterogeneous N-glycosylation, CD147 exists in both highly glycosylated form, HG-CD147 (40–60 kDa) and lowly glycosylated form, LG-CD147 (32 kDa). CD147 was enriched on the surface of many malignant tumor cells, and can induce matrix metalloproteinase (MMP) production, thereby affect tumor cell invasion and metastasis.Many studies have shown that the malignant phenotypes of tumor cells are correlated with the exce- ssive formation of cell surfaceβ1, 6GlcNAc branching structure. However, the effect of Cav-1on N-glycoproteinβ1, 6GlcNAc branch structure of mouse hepatoma cells is rarely reported.Our previous study showed that the CD147 expression level in Hca-F cells was significantly higher than that in Hca-P and Hepa1-6 cells. Cav-1expression resulted in the transformation from LG-CD147 to HG-CD147. However, there are few reports on the sugar chain structure of CD147 in different lymphatic metastasis potential of mouse hepatoma cell lines. The mechanism of the effect of Cav-1 on the CD147 glycosylation remains undetermined. In this study, we found that Cav-1silencing could inhibitβ1, 6GlcNAc branching formation, and decrease the invasive and metastasis ability of mouse hepatoma H22 cells. Conversely, Cav-1overexpression could upregulateβ1, 6GlcNAc branching formation, and increase the migratory and invasive ability of mouse hepatoma cells.In conclusion, Cav-1plays an active role in mediating cell transformation and survival, and is therefore regarded as an oncogene in hepatoma cells. Cav-1is involved in the hepatoma cell apoptosis through regulation of survivin-mediated survival pathway. Cav-1is involved in the hepatoma cell invasion and metastasis through regulation of N-glycanβ1, 6GlcNAc branching formation. These findings may implicate, at least partially, that Cav-1could represent an appropriate target for drug treatment of hepatoma.