MMP9 and triple-A syndrome: Angiotensin II activates the angiotensin II receptor type 1 (AT1) to promote a series of molecular changes implicated in AAA formation including up-regulation of pro-inflammatory cytokines and proteins, such as osteopontin (OPN), osteoprotegerin (OPG), transforming growth factor-beta1 (TGF-β1), and matrix metalloproteinase-9 (MMP-9) [29–31].