Background— We hypothesized that collagenolytic activity produced by activated macrophages contributes to collagen loss and the subsequent instability of atheromatous lesions, a common trigger of acute coronary syndromes. However, no direct in vivo evidence links collagenases with the regulation of collagen content in atherosclerotic plaques.

Methods and Results— To test the hypothesis that collagenases influence the structure of atheromata, we examined collagen accumulation in atherosclerotic lesions of apolipoprotein E-deficient mice (apoE^−/−) that express collagenase-resistant collagen-I (Col^R/R/apoE^−/−, n=12) or wild-type collagen-expressing mice (Col^+/+/apoE^−/−, n=12). Aortic atheromata of both groups had similar sizes and numbers of macrophages, a major source of collagenases. However, aortic intimas from Col^R/R/apoE^−/− mice contained fewer smooth muscle cells, a source of collagen, probably because of decreased migration or proliferation or increased cell death. Despite reduced numbers of smooth muscle cells, atheromata of Col^R/R/apoE^−/− mice contained significantly more intimal collagen than did those of Col^+/+/apoE^−/− mice.

Conclusion— These results establish that collagenase action regulates plaque collagen turnover and smooth muscle cell accumulation.