Objective      To investigate the effect of norepinephrine (NE) on the expression of matrix metalloproteinase-9 (MMP-9) in human macrophages

and to explore its pro-inflammatory and pro-atherosclerotic mechanisms.       Methods      Murine U937 macrophages were cultured and pretreated wi

th antioxidant NAC (10 mmol/L) and NAD(P)H oxidase inhibitor DPI (10

-5

mol/L) for 1 h. Then the cells were treated with different concentrations (10

-10

10

-9

10

-8

and 10

-7

mol/L) of NE at different time points (0

1

3

6

12 and 24 h). MMP-9 mRNA level was analyzed by RT-PCR. MMP-9 protein expression was measured by Western blotting. MMP-9 protein level in cell supernatant was detected by ELISA. Intracellular reactive oxygen species (ROS) generation was observed with fluorescence probes

and intracellular MMP-9 mRNA and protein expressions were detected by RT-PCR and ELISA.       Results      The mRNA expression level (4.65 times that of the control group in the 10

-7

mol/L NE group

P0.01) and protein expression level (4.14 times that of the control group in the 10

-7

mol/L NE group

P0.01) of MMP-9 in macrophages were increased by NE in a dose- and time-dependent manner. The levels of ROS were 2.18

3.22

3.64 and 4.50 times that of the control group in the 10

-10

10

-9

10

-8

and 10

-7

mol/L NE groups

respectively (P0.01). NAC and DPI significantly antagonized the effect of NE on MMP-9 mRNA (DPI group was 75.2% that of the control group

P0.05; NAC group was 50% that of the control group

P0.01) and protein expressions (DPI group was 69.8% that of the control group

P0.05; NAC group was 67.1% that of the control group

P0.05).       Conclusion      NE upregulates MMP-9 expression in U937 cells in a dose- and time-dependent manner

and induces ROS secretion via NAD(P)H oxidase

which mediates the pro-inflammatory effect of NE to promote the formation and development of atherosclerosis.