Matrix metalloproteinase 8 (neutrophil collagenase) in the pathogenesis of abdominal aortic aneurysm

Background: Loss of elastin is the initiating event in abdominal aortic aneurysm (AAA) formation, whereas loss of collagen is required for continued expansion. The elastolytic matrix metalloproteinases (MMPs) 2 and 9 are well described, but the source of excessive collagenolysis remains undefined. The aim of this study was to determine the expression of MMP-8, a potent type I collagenase, in normal aorta and AAA. 

Methods: Infrarenal aortic biopsies were taken from 40 AAA and ten age-matched normal aortas. The concentrations of MMP-8 protein and its inhibitors, tissue inhibitor of metalloproteinase (TIMP) 1 and TIMP-2, were quantified by enzyme-linked immunosorbent assay. Immunohistochemistry was used to localize MMP-8 expression. 

Results: MMP-8 concentrations were significantly raised in AAA compared with normal aorta (active MMP-8: 4.5 versus 0.5 ng per mg protein, P < 0.001; total MMP-8: 16.6 versus 2.8 ng per mg protein, P < 0.001). Levels of TIMP-1 and TIMP-2 were significantly lower in AAA than in normal aortic samples (TIMP-1: 142.2 versus 302.8 ng per mg protein; P = 0.010; TIMP-2: 9.2 versus 33.1 ng per mg protein, P < 0.001). Immunohistochemistry localized MMP-8 to mesenchymal cells within the adventitia of the aortic wall. 

Conclusion: The high concentration of MMP-8 in aortic aneurysms represents a potent pathway for collagen degradation, and hence aneurysm formation and expansion.