Also, an in vitro study revealed that recombinant human OPG stimulates the activity of MMP-9 in aortic VSMCs and thereby limits cell proliferation and survival; it was also shown to induce IL-6 secretion and MMP-2/9 activity in monocytes, which play an even more clear role than OPG in AAA formation. This evidence concerns the gene MMP2 and triple-A syndrome.