Are They Antiatherogenic but Proaneurysmal?
The matrix metalloproteinases (MMPs) are a family of enzymes (25 identified to date) that have in common the ability to degrade many molecules of the extracellular matrix. MMP activity can be inhibited by the endogenous tissue inhibitors of metalloproteinases (TIMPs 1 through 4), and the net proteolytic activity within a tissue is a function of the balance of MMPs/TIMPs.1 Numerous studies have shown that MMPs and TIMPs are expressed during vascular remodeling in the pathological conditions of atherosclerosis, restenosis, and aneurysm formation.2 Despite this burgeoning knowledge, we are still hampered by an incomplete understanding of the scope and the consequences of MMP/TIMP involvement in the pathogenesis of vascular disease.
In atherosclerosis and restenosis, MMPs are produced by the major cell types inhabiting the plaque, vascular smooth muscle cells (SMCs), and leukocytes of the monocyte/macrophage and lymphocytic lineages. MMP-1, -2, -3, -9, -12, and -13 have been detected in plaques, along with the TIMP-1, -2, and -4.2 MMPs produced by SMCs clear a path for migration from media to intima by digesting the extracellular matrix, and SMC migration can be inhibited by administration of nonselective MMP inhibitors3–5⇓⇓ or transfection of the genes for TIMP-1 or TIMP-2 into the injured vessel wall.6,7⇓ Macrophages produce abundant amounts of MMPs, which are used to invade through the endothelium and into the atherosclerotic plaque.8 MMPs are colocalized with macrophages in the core and shoulders of established plaques, areas that are very susceptible to the complications of erosion and rupture.9 MMPs are also expressed by inflammatory cells found in abdominal aortic aneurysms,10 and experimental studies using rat and mouse models point to a causal role for the MMPs in the pathogenesis of aneurysm.11,12⇓
A great deal of effort in vascular biology has centered on the hypothesis that inhibiting MMP activity will reduce plaque volume by inhibiting the migration of SMCs and macrophages into the plaque and prevent the later complications of plaque rupture and aneurysm formation. However, the mechanisms of MMP action in complex models of atherosclerosis are largely unknown. With the advent of transgenic technology, better models of atherosclerosis have been developed, including the cholesterol-fed ApoE-null mouse, which is characterized by elevated circulating lipoproteins, and the development of lipid-rich plaques containing inflammatory macrophages and lymphocytes.13
In an article published in this issue of Circulation Research, Silence et al14 have uncovered dual roles of MMPs in the ApoE-null mouse model of atherosclerosis. Surprisingly, they found that deletion of the TIMP-1 gene resulted in reduction of plaque size in the ApoE-null mouse. TIMP-1 inhibits the activity of many MMPs, including the collagenases, gelatinases, and stromelysins. In the absence of the TIMP-1 gene, there was an increase in the number of macrophages present in aortic intimal lesions. The authors postulate that increased MMP activity (predominantly MMP-2), which colocalized with the macrophages, resulted in collagen degradation, thereby reducing plaque size. The reduction in plaque size was evident despite increased lipid accumulation in the lesions of the ApoE-null:TIMP-1-null mice. Unfortunately, the potential for aneurysm formation was substantially elevated in these mice, as evidenced by an increase in the frequency of disruptions in the internal elastic lamina.
The results presented here seemingly contradict a central dogma in atherosclerosis—that increased MMP activity leads to the formation of a thicker neointima. However, we must remember that most of the earlier studies with MMP inhibitors used experimental models where SMC migration was the main, if not the only, determinant of intimal lesion formation. By contrast, a growing body of experimental evidence from murine atherosclerosis models supports the postulate that increased macrophage-derived MMP activity may limit plaque progression. For example, plaque size and collagen content were greater in mice with double knockout of the MMP-3 and ApoE genes, compared to ApoE-null littermate controls.15 Consistent with this, plaque size, lipid deposition, and collagen content were reduced in ApoE-null mice that overexpressed MMP-1 in the macrophages.16 Taken together with results from the present study, this suggests that the activity of several plaque MMPs may actually be antiatherogenic. In this light, it is interesting to note that polymorphisms in the human MMP-3 promoter leading to decreased expression of this gene have been correlated with an increased incidence of atherosclerosis.17 Further work will be necessary to determine the full spectrum of anti- or proatherogenic activities of the many MMPs that are expressed in atherosclerosis. Another caveat is that the previous studies using knockout mice address only plaque progression. By contrast, increasing circulating TIMP-1 levels in the ApoE-null mouse induced the regression of pre-established lesions.18 Thus, the effects of altering the balance of MMPs/TIMPs may differ during the time course of lesion development.
Although the reduction in plaque size seen in the absence of TIMP-1 is potentially beneficial, deletion of TIMP-1 leads to increased degradation of the aortic elastic lamellae and thus may predispose to aneurysmal dilation and rupture. This is probably due to the increase in the amounts and the diversity of the MMPs produced by macrophages and/or to increased MMP activation by reactive oxygen species, nitric oxide, and peroxynitrite formation.19 In this context, it is interesting to speculate that aneurysm formation may be a case of MMP-mediated outward vessel remodeling gone bad.20 In the future, it will be important to investigate the potential for plaque rupture in the ApoE-null:TIMP-1-null mouse model. Rosenfeld et al21 have reported a significant incidence of intraplaque hemorrhage and plaque rupture at very late times during lesion development in the ApoE-null mouse model. In the absence of TIMP-1, the increased clearance of collagen may accelerate the destabilization of the atherosclerotic plaque, leaving it vulnerable to rupture.
Finally, it is important to remember that MMPs and TIMPs have functions beyond their roles in matrix degradation. For example, TIMP-1 is a growth factor for several cell types.1 In addition, MMPs degrade components of the extracellular matrix that stimulate cell growth and migration, such as collagen and osteopontin.23–25⇓⇓ MMPs also disrupt cell-cell interactions by cleaving cadherins or disrupting matrix-integrin associations, leading to apoptosis.1 Any of these mechanisms could be pertinent to atheroma or aneurysm formation, because they lead to a reduction in cell number.
In conclusion, the data presented in this interesting study highlights the importance of assessing all the potential mechanisms of MMP/TIMP action in the most appropriate experimental models. Clearly the use of transgenic mouse models is extremely informative, but also very complicated. We must take great care to interpret the data in terms of the cell types present in human atherosclerosis, available knowledge of the diversity of MMP/TIMP functions, and the time course of expression and activity of these enzymes. In this way, we can design selective therapies to be administered at the appropriate time and location to prevent the progression and complications of atherosclerosis.
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
- ↵Galis ZS, Khatri JJ. Matrix metalloproteinases in vascular remodeling and atherogenesis: the good, the bad, and the ugly. Circ Res. 2002; 90: 251–262.
- ↵Bendeck MP, Irvin C, Reidy MA. Inhibition of matrix metalloproteinase activity inhibits smooth muscle cell migration but not neointimal thickening after arterial injury. Circ Res. 1996; 78: 38–43.
- ↵Zempo N, Koyama N, Kenagy RD, Lea HJ, Clowes A. Regulation of vascular smooth muscle cell migration and proliferation in vitro and in injured rat arteries by a synthetic matrix metalloproteinase inhibitor. Arterioscler Thromb Vasc Biol. 1996; 16: 28–33.
- ↵Forough R, Koyama N, Hasenstab D, Lea H, Clowes M, Nikkari ST, Clowes AW. Overexpression of tissue inhibitor of matrix metalloproteinase-1 inhibits vascular smooth muscle cell functions in vitro and in vivo. Circ Res. 1996; 79: 812–820.
- ↵Galis ZS, Sukhova GK, Kranzhofer R, Clark S, Libby P. Macrophage foam cells from experimental atheroma constitutively produce matrix-degrading proteinases. Proc Natl Acad Sci U S A. 1995; 92: 402–406.
- ↵Silence J, Collen D. Lijnen HR. Reduced atherosclerotic plaque but enhanced aneurysm formation in mice with inactivation of the tissue inhibitor of matrix metalloproteinase-1 (TIMP-1) gene. Circ Res. 2002; 90: 897–903.
- ↵Silence J, Lupu F, Collen D, Lijnen HR. Persistence of atherosclerotic plaque but reduced aneurysm formation in mice with stromelysin-1 (MMP-3) gene inactivation. Arterioscler Thromb Vasc Biol. 2001; 21: 1440–1445.
- ↵Ye S, Eriksson P, Hamsten A, Kurkinen M, Humphries SE, Henney AM. Progression of coronary atherosclerosis is associated with a common genetic variant of the human stromelysin-1 promoter which results in reduced gene expression. J Biol Chem. 1996; 271: 13055–13060.
- ↵Rouis M, Adamy C, Duverger N, Lesnik P, Horellou P, Moreau M, Emmanuel F, Caillaud JM, Laplaud PM, Dachet C, Chapman MJ. Adenovirus-mediated overexpression of tissue inhibitor of metalloproteinase-1 reduces atherosclerotic lesions in apolipoprotein E- deficient mice. Circulation. 1999; 100: 533–540.
- ↵Pasterkamp G, de Kleijn DP, Borst C. Arterial remodeling in atherosclerosis, restenosis and after alteration of blood flow: potential mechanisms and clinical implications. Cardiovasc Res. 2000; 45: 843–852.
- ↵Rosenfeld ME, Polinsky P, Virmani R, Kauser K, Rubanyi G, Schwartz SM. Advanced atherosclerotic lesions in the innominate artery of the ApoE knockout mouse. Arterioscler Thromb Vasc Biol. 2000; 20: 2587–2592.
- Deleted in proof.
- ↵Liaw L, Almeida M, Hart CE, Schwartz SM, Giachelli CM. Osteopontin promotes vascular cell adhesion and spreading and is chemotactic for smooth muscle cells in vitro. Circ Res. 1994; 74: 214–224.