Activation of inducible nitric oxide synthase increases MMP-2 and MMP-9 levels in ApoE-knockout mice.

Objectives: Atheroma with reduced collagen content becomes fragile, but the underlying mechanisms have not been established. We investigated the influence of inducible nitric oxide synthase (iNOS) induction upon matrix metalloproteinases (MMP)s and collagen content in atheroma.

Methods And Results: ApoE-/- x iNOS-/- double knockout and ApoE-/- x iNOS+/+ mice were fed a high-cholesterol diet for 15 weeks. Large atheromatous lesions of comparable size appeared in the roof of the aorta in both strains. Induction of iNOS mRNA was observed only in the atheroma of the ApoE-/-/iNOS+/+ mice. Collagen content was sparse and fat droplets were increased. Gelatin zymography and in situ zymography showed that pro- and active forms of MMP-2 and MMP-9 were more strongly expressed in ApoE-/-/iNOS+/+ than in ApoE-/-/iNOS-/- mice, nitrotyrosine and MMP-9 were co-expressed in the atheroma.

Conclusion: We conclude that induction of iNOS in atheroma of high-cholesterol-fed ApoE-/-/iNOS+/+ mice leads to increased production and activation of MMPs, with a subsequent decrease in collagen content, affording fragile plaque.