Transgenic Overexpression of Macrophage Matrix Metalloproteinase-9 Exacerbates Age-related Cardiac Hypertrophy, Vessel Rarefaction, Inflammation, and Fibrosis

Advancing age is an independent risk factor for cardiovascular disease. Matrix metalloproteinase-9 (MMP-9) is secreted by macrophages and robustly increases in the left ventricle (LV) with age. The present study investigated the effect of MMP-9 overexpression in macrophages on cardiac aging. We compared 16- to 21-mo-old C57BL/6J wild-type (WT) and transgenic (TG) male and female mice ( = 15-20/group). MMP-9 overexpression amplified the hypertrophic response to aging, as evidenced by increased LV wall thickness and myocyte cross-sectional areas ( < 0.05 for both). MMP-9 overexpression reduced LV expression of the angiogenesis-related factors ICAM-1, integrins α3 and β3, platelet/endothelial cell adhesion molecule-1, thrombospondin-1, tenascin-c, and versican (all < 0.05). Concomitantly, the number of vessels in the TG was lower than WT LV ( < 0.05). This led to a mismatch in the muscle-to-vessel ratio and resulted in increased cardiac inflammation. Out of 84 inflammatory genes analyzed, 16 genes increased in the TG compared with WT (all < 0.05). Of the elevated genes, 14 were proinflammatory genes. The increase in cardiac inflammation resulted in greater accumulation of interstitial collagen in TG ( < 0.05). Fractional shortening was similar between groups, indicating that global cardiac function was still preserved at this age. In conclusion, overexpression of MMP-9 in macrophages resulted in exacerbated cardiac hypertrophy in the setting of vessel rarefaction, which resulted in enhanced inflammation and fibrosis to augment the cardiac-aging phenotype. Our results provide evidence that macrophage-derived MMP-9 may be a therapeutic target in elderly subjects. The present study was the first to use mice with transgenic overexpression of matrix metalloproteinase-9 (MMP-9) in macrophages to examine the effects of macrophage-derived MMP-9 on cardiac aging. We found that an elevation in macrophage-derived MMP-9 induced a greater age-dependent cardiac hypertrophy and vessel rarefaction phenotype, which enhanced cardiac inflammation and fibrosis.