AIM: To study the effect of NS-398, a selective cyclooxygenase2 (COX-2) inhibitor, on invasion of colon cancer cell line HT-29 in vitro and to explore its mechanisms.METHODS: Invasive behaviors of the malignant colon cancer cell line HT-29 were investigated in this study.Expressions of COX-2 and CD44v6 in HT-29 cells were detected by flow cytometry. Cellular survival rate was determined by MTT assay. The invasive capacity was quantified by a modified Boyden chamber model. Alterations of cytoskeleton component F-actin were observed by confocal laser scanning microscope.RESULTS: Flow cytometry analysis showed that COX-2was highly expressed in HT-29 cells. The invasive capability of HT-29 cells could be greatly inhibited by NS-398 at the experimental concentrations of 0.1, 1.0 and 10 μmol/L with an inhibitory rate of 22.74%, 42.35% and 58.61% (P＜0.01),respectively. MTT assay showed that NS-398 at the experimental concentrations had no significant influence on cellular viability, indicating that such anti-invasive effects had no relationship with cytotoxicity. F-actin was mainly distributed around nuclei forming annular structure in HT-29cells. After exposure to NS-398 of 10 μmol/L, the annular structure around nuclei disappeared and the fluorescence intensity of F-actin decreased obviously. Treatment with NS-398 could down-regulate the expression of CD44v6 as well.CONCLUSION: NS-398 has anti-invasive effects on colon cancer HT-29 cells in vitro, which may be mediated by a novel mechanism of disruption of cytoskeleton. Downregulation of CD44v6 expression may be related to alterations of cytoskeleton.