| Summary: | 博士 === 國立陽明大學 === 臨床醫學研究所 === 93 === Thymosin β-4 (Tβ-4), a small peptide originally isolated from calf thymus, modulates the formation of F-actin microfilaments by sequestering the monomeric G-actin. Recent studies have shown that overexpression of the Tβ-4 gene occurs not only in many human carcinomas but also in the highly metastatic melanomas and fibrosarcomas. However, little is known about the specific growth advantages acquired by different tumors from this genetic abnormality.
To address above questions, Tβ-4-overexpressing human colon carcinoma (SW480) cells were established by stable transfection and their phenotypic changes were monitored. We found both the morphology and the cortical actin cytoskeleton of SW480 cells were altered by Tβ-4 overexpression. Moreover, both cellular level and that distributed over the intercellular junctions of the E-cadherin were decreased in the Tβ-4 overexpressers, which were accompanied by a 2-fold increase in their saturation densities as well as an increased motility. Meanwhile, these cells also exhibited an increased ability to form colonies in soft agar, which was accompanied by an enhanced focal contact. Interestingly, a dramatic increase of growth rate was detected in the Tβ-4 overexpressers, which might be attributed to an accelerated proliferation induced by c-Myc that was activated by nuclear β-catenin.
Cell-matrix and cell-cell adhesive interactions play important roles in the normal organization and stabilization of the cell layer in epithelial tissue. Alterations in the expression and function of these adhesion systems that cause a switch to a migratory phenotype in tumor invasion and metastasis are critical for the malignant conversion of epithelial cells. Since the aforementioned adhesive systems are dramatically altered by enforced Tβ-4 expression, we therefore examined the invasion capability of Tβ-4-overexpressing SW480 cells and the expression levels of Tβ-4 as well as several proteins that participate in different stages of tumor progression in matched samples of human primary colorectal adenocarcinoma and liver metastases from several patients. A marked increase on the invasiveness in Tβ-4-overexpressing SW480 cells with increased levels and activity of matrix metalloproteinase-7 (MMP-7) was observed. Furthermore, the levels of Fas as well as the susceptibility to Fas ligand-mediated apoptosis in Tβ-4-overexpressing cells were significantly decreased. Interestingly, the levels of Tβ-4 mRNA, β-catenin, c-Myc, and MMP-7 in metastatic liver lesions were relatively higher, whereas the levels of E-cadherin and Fas were significantly lower than those in the matched primary colorectal tumors.
In summary, our data suggest that the drastic growth property and motility changes of the SW480 cells overexpressing Tβ-4 gene are due mainly to a deregulated cell-cell adhesion arisen from the downregulation of E-cadherin, plus uncontrolled cell proliferation owing to the upregulation of β-catenin, both resulted probably from a breakdown of actin microfilaments caused by the overexpression of this G-actin sequestering peptide. These results suggest that upregulation of Tβ-4, by promoting the disruption of cell-cell adhesion and a consequential activation of the β-catenin signaling, could be a key event in the acquisition of growth advantages as well as invasive phenotypes in human colorectal carcinomas.
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