Objective:This study is to determine the effect of the natural product parthenolide,a sesquiterpene lactone isolated from extracts of the herb Tanacetum parthenium,on the proliferation of vascular smooth muscle cells(VSMCs) . Methods:Rat aortic VSMCs were isolated and cultured in vitro,and treated with different concentrations of parthenolide(10,20 and 30 μmol/L) . [3H]thymidine incorporation was used as an index of cell proliferation. Cell cycle progression and distribution were determined by flow cytometric analysis. Furthermore,the expression of several regulatory proteins relevant to VSMC proliferation including IκBα,cyclooxygenase-2(Cox-2) ,p21,and p27 was examined to investigate the potential molecular mechanism. Results:Treatment with parthenolide significantly decreased the [3H]thymidine incorporation into DNA by 30%～56% relative to control values in a dose-dependent manner(P<0.05) . Addition of parthenolide also increased cell population at G0/G1 phase by 19.2%～65.7%(P<0.05) and decreased cell population at S phase by 50.7%～84.8%(P<0.05) ,which is consistent with its stimulatory effects on p21 and p27. In addition,parthenolide also increased IκBα expression and reduced Cox-2 expression in a time-dependent manner. Conclusion:Our results show that parthenolide significantly inhibits the VSMC proliferation by inducing G0/G1 cell cycle arrest. IκBα and Cox-2 are likely involved in such inhibitory effect of parthenolide on VSMC proliferation. These findings warrant further investigation on potential therapeutic implications of parthenolide on VSMC proliferation in vivo. Objective:This study is to determine the effect of the natural product parthenolide,a sesquiterpene lactone isolated from extracts of the herb Tanacetum parthenium, on the proliferation of vascular smooth muscle cells(VSMCs). Methods:Rat aortic VSMCs were isolated and cultured in vitro ,and treated with different concentrations of parthenolide(10,20 and 30 μmol/L) . [3H]thymidine incorporation was used as an index of cell proliferation. Cell cycle progression and distribution were determined by flow cytometric analysis. The results showed that treatment with parthenolide significantly decreased the frequency of VSMC proliferation including IκBα, cyclooxygenase-2 (Cox-2), p21, and p27 was examined to investigate the potential molecular mechanism. [3H]thymidine incorporation into DNA by 30% ~56% relative to control values ​​in a dose-dependent manner (P<0.05). Addition of parthenolide also increased cell population at G0/G1 phase by 19.2% to 65.7% (P< 0.05) and decreased cell population at S phase by 50.7%-84.8% (P<0.05), which is consistent with its stimulatory effects on p21 and p27. In addition, parthenolide also increased IκBα expression and reduced Cox-2 expression in a time -dependent manner. Conclusion: Our results show that parthenolide significantly inhibits the VSMC proliferation by inducing G0/G1 cell cycle arrest. IκBα and Cox-2 are easily involved in such inhibitory effect of parthenolide on VSMC proliferation. These findings warrant further investigation on potential Therapeutic implications of parthenolide on VSMC proliferation in vivo.