| Summary: | 博士 === 國立臺灣大學 === 臨床醫學研究所 === 88 === The importance of cell adhesion in the progression of tumor malignancy has been widely investigated. It has been demonstrated that the ability of cancer cells to infiltrate surrounding tissues and subsequently to detach and migrate may be related to alterations in the adhesiveness between cancer cells and their surroundings. It has also been shown that cellular morphogenesis and differentiation are highly influenced by a family of adhesion molecules. In order to investigate the adhesion property of intercellular junction, we examined the distribution of E-cadherin and its three associated proteins, α-, β-, and γ-catenin in two different systems: one is the cancerous tissue removed from the patients, and the other is the human thyroid cancer cell line.
In this study, we examined the expression of E-cadherin, α-, β-, and γ-catenin in normal and neoplastic thyroid tissue by immunofluorescence microscopy and Western blot analysis. In normal thyroid tissue as well as in nodular goiter, staining for E-cadherin, α-, β-, and γ-catenin was seen mainly at the lateral surface of epithelial cells in the follicle. The presence of these molecules was also confirmed by Western blotting. In the follicular adenoma samples, all of which were found to have characteristic distribution for E-cadherin and β-catenin while 1/4 of which were shown not to express α-catenin or γ-catenin, respectively. In follicular carcinoma tissue, the presence of E-cadherin and α-catenin was convincing, however, the results of β- and γ-catenin were highly variable: β-catenin was absent in most follicular carcinomas (8/10) while γ-catenin was absent in some follicular carcinomas (3/10). From these results, we concluded that the major difference in the adhesion property between follicular adenoma and follicular carcinoma is the lack of expression of β-catenin in the latter. This indicates that β-catenin, but not E-cadherin, can be used as a molecular marker in the preoperative diagnosis of benign/malignant thyroid tumor. Therefore we suggest that immunocytochemical examination of β-catenin expression should be included in the routine thyroid biopsy for reference purposes.
Due to limited samples of cancerous tissue from surgical specimen and difficulty to obtain a monolayer cell for staining, we examined the distribution of cadherin-catenin complex in a Chinese human thyroid cancer cell line (CGTH W-2). In normal thyroid smears, E-cadherin, α-, β-, and γ-catenin were found to localize at the intercellular junction by immunofluorescence staining, and appeared as proteins of 135, 102, 90 and 80 kD on Western blots. In thyroid cancer cell line (CGTH W-2 cells) no E-cadherin and γ-catenin were detected by either immunofluorescence or Western blotting. α- and β-catenin were present, however, they were diffusely distributed in the cytoplasm of most cells. Our present data suggest that the loss of cell adhesiveness in these cancer cells may be due to the impaired assembly of the cadherin-catenin complex at cell-cell junction. This may explain the observation that these cells are able to induce metastatic cancer in mice. It should be noted that, despite of its major cytoplasmic distribution, β-catenin was also found at cell-cell junction in 10% of the cells, where it co-localized with α-actinin. In these cells, some of the adherens junction components, β-catenin, α-actinin, vinculin and actin bundles, are all correctly localized in the absence of E-cadherin, γ-catenin and functional α-catenin. Thus we speculate that there should be proteins that are functionally-related to E-cadherin and α-catenin to maintain the partial integrity of adherens junction complex.
On the other hand, we were interested in exploring the possibility of employing hyperthermia as an alternative in the treatment of thyroid cancer. Therefore, we studied the effects of high-temperature treatment (41-43℃) on cell morphology, proliferation, cytoskeleton organization, focal adhesion complex, and integrin-based signal transduction pathway in human thyroid cancer cell line. Incubation at the critical temperature (42-43℃) resulted in the inhibition of cell proliferation and irreversible changes in cell morphology. Immunofluorescence staining on the cytoskeletal structures indicated that (43℃) hyperthermia -induced disassembly of cytoskeleton resulted in cell shrinkage and its detachment from the substratum. In the time course study of immunofluorescence staining for focal adhesion proteins, we showed that heat treatment first disrupted integrin, followed by the disappearance of focal adhesion kinase and the dephosphorylation of focal adhesion complex, finally resulted in the disassembly of vinculin and actin bundles. Loss of integrin may also explain the observed growth inhibition at the critical temperature. Since integrin is also involved in the adhesion-induced phosphorylation of mitogen- activated protein (MAP) kinase which regulates gene expression and cell cycle progression, its disruption is very likely to interfere with the signaling pathway and result in the inhibition of cell proliferation. The lack of cell adhesion may also halt the cell cycle through the reduced expression of cyclin A, which is a key regulator in cell cycle progression.
Our studies indicated that cell-cell adhesion became decreased during the development of thyroid cancer. Further investigation on the precise roles of adhesion molecules in thyroid cancer progression is necessary, as it shall open an avenue in the diagnosis of thyroid cancer at the molecular level. The application of hyperthermia in treatment of thyroid cancer needs further exploration.
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