Combined stimulation with cyclic stretching and hypoxia increases production of matrix metalloproteinase-9 and cytokines by macrophages

Macrophages in the vessel wall of advanced abdominal aortic aneurysms (AAAs) are subjected to cyclic stretching and hypoxia because of pulsatile blood flow and intraluminal thrombi, respectively. It is possible that these conditions induce abnormal changes in macrophage functions, such as increased production of matrix metalloproteinase-2 (MMP-2), MMP-9, and inflammatory cytokines, leading to weakening of the aortic wall through excessive extracellular matrix disruption. Here we show the effects of cyclic stretching and hypoxia on the production of MMP-9 and inflammatory cytokines by macrophages. Gelatin zymography revealed that MMP-9 production by macrophages was significantly increased by 5% and 10% cyclic stretching under hypoxia (2.2% O ₂). Using enzyme-linked immunosorbent assay, we also evaluated the production of 12 different inflammatory cytokines and found that there was a tendency toward higher expressions of interleukin-8 and tumor necrosis factor-α by macrophages subjected to 10% cyclic stretching under normoxia and hypoxia. Next, we evaluated apoptosis of smooth muscle cells (SMCs) in medium conditioned by macrophages cultured under the 2 conditions described above. SMC apoptosis increased significantly when exposed to media harvested from macrophages subjected to 10% cyclic stretching under normoxia and hypoxia. On the basis of these results, we believe that macrophages produce cytokines that induce SMC apoptosis. Our results suggest that the combination of cyclic stretching and hypoxia stimulates MMP-9 and cytokine production in macrophages, which may result in weakening of AAA walls.