Objective      To determine the effect of down-regulating insulin-induced transcription activator protein 1 (AP-1) on the expression regulation of alpha smooth muscle actin (SM-α) and cell proliferation in rat vascular smooth muscle cells (VSMCs).       Methods      Flow cytometry was employed to analyze the cell cycle of VSMCs after treatment of 0

25

50

100 and 125 nmol/L insulin for 24 h. Plasmid vector encoding AP-1 gene was designed and then transfected into VSMCs. Meanwhile there were 4 groups of VSMCs

that is

control

insulin treatment

AP-1 expression vector

and insulin+ AP-1 expression vector group. The expression of AP-1 and SM-α at protein and mRNA levels was detected by Western blotting and RT-PCR respectively.       Results      Flow cytometry results showed that after stimulating of insulin at different concentrations (0

25

50

100 and 125 nmol/L)

the percentage of the cells at S phase was risen to a peak and then declined after rising [(23.49±1.17)%

(28.08±0.68)%

(30.44±1.77)%

(47.47±0.88)% and (46.23±1.65)%

P0.05]. The peak of the amount of S phase cells were the VSMCs treated by 100 nmol/L insulin [(47.47±0.88)%]. Western blotting and RT-PCR showed that insulin down-regulated the expression of AP-1 (0.46±0.14 and 0.47±0.09) (P0.05) and suppressed the expression of SM-α (0.32±0.03 and 0.41±0.03) (P0.05) by stimulating VSMCs.       Conclusion      Insulin suppresses the expression of SM-α by down-regulating the expression of AP-1 to induce the proliferation of VSMCs.