MMP9 and triple-A syndrome: However, Chew, Conte & Khalil (2004) and Chew, Orshal & Khalil (2003, 2004) found that elastase and matrix metalloproteinase (MMP, mainly MMP2 and MMP9) both promote early dilation of AAA by inhibiting vascular smooth muscle contraction through the mechanism of Ca2+ entry inhibition of vascular smooth muscle.