Abstract: AimTo study the inhibitory effect of dexamethasone and indomethacin on matrix metalloproteinase-9 (MMP-9) and investigate their mechanism of MMP-9 inhibition on the level of protein and mRNA. MethodsU937 cells were cultured in RPMI 1640 medium with 10% fetal calf serum (FCS). After U937 cells were exposed to 1×10^-8 mol·L^-1 phorbol 12-myristate 13-acetate (PMA) for 24 hours without FCS, the activity of MMP-9 in the supernatant was analyzed by gelatin zymography. The MMP-9 protein secreted from U937 in serum-free conditional media was detected by Western blot using special polyclonal antibodies. The mRNA expression of MMP-9 was investigated by RT-PCR. ResultsGelatin zymography showed that MMP-9 activity in U937 cells supernatant increased significantly after exposed to 1×10^-8 mol·L^-1 PMA for 24 hours without FCS. Dexamethasone at 1×10^-5, 1×10^-7, 1×10-9 mol·L^-1 and indomethacin at 1×10^-5, 1×10^-6, 1×10^-7 mol·L^-1 can inhibit this increase. Western blot showed that MMP-9 secretion from U937 cells in serum-free conditional media was increased under the same stimulating condition, Dexamethasone and indomethacin can inhibit MMP-9 secretion in U937 cells stimulated by 1×10^-8 mol·L^-1 PMA and the inhibitory effect was increased as their concentration increased. RT-PCR showed that MMP-9 mRNA expression of U937 cells was also inhibited and the inhibitory effect was increased as their concentration increased. ConclusionInhibition of MMP-9 activity may be one of the anti-inflammatory mechanisms of dexamethasone and indomethacin; the inhibition of MMP-9 activity is coherent with the inhibition of MMP-9 secretion and mRNA expression.