| Summary: | 碩士 === 國立陽明大學 === 生物藥學研究所 === 97 === Thymosin β4 (Tβ4) is a 5 kDa intracellular G-actin sequestering peptide which regulates F-actin formation. Previous studies have shown that overexpression of Tβ4 gene occurs not only in human colorectal carcinomas (CRC) but also in their liver metastases. In fact, increased migration and invasion were found in several Tβ4-overexpressing stable clones derived from SW480 human colon cancer cells. Although the latter could be attributed in part to an upregulation of MMP-7 in these cells, the underlying mechanism for the former is largely unknown at present.
In this study, transwell migration assay was used to analyze the influence of Tβ4 upregulation on the migration ability of SW480 cells. Our data showed that Tβ4-overexpressing stable clones Tb3 and Tb4 migrated better than the parental SW480 and vector-transfected (BK2) cells. Accordingly, knockdown of Tβ4 expression in Tb3 and Tb4 cells by its shRNA reduced their migration ability in a dose-dependent manner. Interestingly, even though higher levels of active Rac1 were found in Tb3 and Tb4 cells, treating them with a Rac1 inhibitor, NSC23766, did not diminish their migration, suggesting that guanine nucleotide exchange factors (GEFs) Trio and Tiam1 are not responsible for Rac1 activation in these cells. Surprisingly, higher RNA and protein levels of IQGAP1, a positive regulator of Rac1, were found in Tb3 and Tb4 cells whose knockdown resulted in a drastic reduction in the motility of these cells. Furthermore, co-immunoprecipitation assay showed that IQGAP1 could form a complex with integrin-linked kinase (ILK), and the latter has previously been shown to be upregulated in Tb3 and Tb4 cells. Together, our data suggest that Tβ4 can facilitate the migration of SW480 cells by activating Rac1 via upregulating IQGAP1 expression.
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