| Summary: | 碩士 === 國立陽明大學 === 醫學生物技術研究所 === 88 === Abstract
Osteopontin (OPN) is a highly phosphorylated and glycosylated secreted protein found in many body fluids, notably plasma, urine, bile, milk and in the extracellular matrix of mineralized tissues. OPN has a variety of function including the initiation of bone calcification process, regulation of calcium salts solubility, pathological calcification of urinary stones and atherosclerotic plaques, and early resistance to bacterial infection. Recently, expression of OPN has been shown to be linked to tumorigenesis and metastasis in experimental animals, cell cultures, and human cancer. Precisely what role OPN plays in these processes is unclear. The role of OPN is further complicated and dampened by the observation that OPN mRNA is detected in tumor infiltrating macrophages rather than in tumor cells. In human, most investigations were focused on breast cancer. So far, there is no report of the OPN expression in hepatocellular carcinoma (HCC).
Using differential display method, we identified a cDNA fragement that was overexpressed in liver cancer and identical to the cDNA of OPN. In this study of OPN mRNA expression in HCC, our results are as follows.
(1). There are two forms of OPN mRNA in liver and cancer. The long form (OPN-a) was predominantly overexpressed in tumors and some nontumor livers, not the small one that lacks exon 5 . The exon 5 of long form contains that encodes 14 a.a. and possesses an SSEE amino acid sequence. The two serine residues can be phosphorylated by kinase.
(2). The overexpression of OPN mRNA was found in 56 out of 70 primary unifocal HCCs, 4 out of 5 hepatoblastomas, 6 out of 7 cholangiocellular carcinomas, 3 out of 6 recurrent HCCs, 6 out of 6 metastatic colorectal carcinomas (CRC). In contrast, none of the 6 nontumor livers bearing metastatic CRC overexpressed OPN mRNA.
(3). For clinicopathological correlation, the 70 patients with unifocal primary HCC were selected for analysis. HCC with OPN mRNA overexpression was more often of II-III grade (84.7% versus 57.1%, P=0.029) and invasive tumor (83.9% versus 50.0%, P=0.029). Moreover, HCC with overexpression of OPN mRNA had a lower three-year survival (26.9% versus 61.5%, P=0.025). We also found that OPN mRNA overexpression tended to occur in HCC positive for chronic HBV infection (76.8% versus 50%, P=0.094) and serum AFP elevation (50.0% versus 21.4%, P=0.073).
(4). OPN mRNA overexpression in the nontumor liver correlated with higher frequency of tumor recurrence (72.7% versus 32.4%, P=0.017).
(5). Using RNA in situ with purified riboprobe we were able to detect OPN mRNA expression in tumor cells and not in the infiltrating macrophages and lymphocytes.
(6). The overexpression of OPN mRNA in the tumor was related to hypomethylation of the gene.
In conclusion, OPN mRNA was often overexpressed in HCC and the overexpression was at least partly related to hypomethylation. We demonstrated that OPN mRNA was overexpressed in tumor cells and the overexpression in HCC correlated with poor differentiation, tumor invasion, and unfavorable progression. Whether the OPN expression in the nontumor liver could be a potential marker in predicting tumor recurrence in HCC warrants further clarification.
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