In the ARIC study, TIMP-1 and MMP-1 levels were tested, and no association with incident CAD was found. The results of animal and other human studies into the possible role of different MMPs and TIMPs in CAD have been inconsistent. Although many have shown an association between MMPs or TIMPs and CAD,14–18
found no connection. Similarly, there has been inconsistent association between the various members of the MMP or TIMP families and CAD—for example, a 2002 study20
showed an atheroprotective counter-regulatory function of TIMP-1, whereas a more recent study4
raised the question of whether TIMP-1 is atherogenic. When plasma levels of MMP-2, MMP-3, MMP-9, TIMP-1, and TIMP-2 were measured in 53 men who had premature stable CAD,16
MMP-9 and TIMP-1 were significantly higher and MMP-3 and TIMP-2 were significantly lower in the CAD patients than in the control group of 133 age-matched men. Similarly, MMP-9 and TIMP-1 were increased and MMP-2 and TIMP-2 were decreased in 200 men who had premature CAD, in comparison with a control group of 201 age-matched men.17
In other studies,4,19,21
TIMP-1 has been associated with all-cause mortality, myocardial infarction, and acute coronary syndromes. An elevated TIMP-1 level was associated with the presence of carotid plaque in 238 men who were known to be free of CAD, in a multivariate model that was adjusted for age, BMI, smoking, total cholesterol and triglycerides, C-reactive protein (CRP) level, diabetes mellitus, systolic blood pressure, and heart rate (odds ratio; 2.89; 95% CI, 1.12–7.47; P
<0.01). Although TIMP-1 was also associated with common carotid artery intima–media thickness in a univariate analysis,22
the association was not present in a multivariate analysis.22
On the other hand, in yet another report,23
MMP-9 and TIMP-2, but not TIMP-1, were elevated in 204 patients who had stable CAD, in comparison with a control group.
Investigations into the role of MMPs and TIMPs in the pathobiology of atherosclerosis have also produced varying results. Increased expression of interstitial collagenase (MMP-1) has been described more often in vulnerable atherosclerotic plaques than in lesion-free areas of the vessels.24
showed that CRP (an inflammatory marker that is associated with CAD) augmented MMP-1 and MMP-10 mRNA expression in human umbilical-vein endothelial cells, and that MMP-1 and MMP-10 were significantly elevated in persons whose CRP levels were greater than 3 mg/L.
Overexpression of TIMP-1 by adenovirus-mediated gene transfer has been shown to inhibit smooth-muscle-cell migration and neointimal formation in human saphenous veins,26
and the adenovirus-mediated overexpression of TIMP-1 in atherosclerosis-susceptible apolipoprotein E-deficient mice significantly reduced atherosclerotic lesions.27
However, the investigators of that murine model also reported28
that in apolipoprotein E-deficient mice that were overexpressing TIMP-1, the formation of aneurysms was decreased, but the development of atherosclerosis was not. Similarly, MMP-1 and MMP-3 levels were reported to be higher in aneurysmal human atherosclerotic plaques than in occlusive lesions, whereas TIMP-1 was associated with calcification, a marker of plaque stability.29
In another apolipoprotein E-deficient murine model, in which the mice were maintained on a high-fat diet, transfection with TIMP-2 but not with TIMP-1 resulted in the inhibition of atherosclerotic plaque development and of plaque destabilization in the brachial artery. The authors concluded that TIMP-2 (and not TIMP-1) was an effective inhibitor of plaque growth, and that this inhibition may be secondary to the effect of TIMP-2 on the behaviors of smooth muscle cells and macrophages.30
Hence, the available human and animal data on the potential associations of MMPs and TIMPs with CAD have not been conclusive. Our study has the advantages of 1) being a population-based study that provides information on a middle-aged U.S. population rather than on a high-risk patient cohort that has established atherosclerosis, and 2) having a longer follow-up than that in other investigations. The lack of association of TIMP-1 and MMP-1 with incident CAD in the current study provides important additional information on the unclear relationship between MMPs or TIMPs and CAD. Perhaps the balance or the ratio between MMPs and TIMPs is more important in determining the contribution of these markers to atherosclerosis. Matrix metalloproteinases are known to have a role in degrading collagen,1
and TIMPs, as described, act as counter-regulatory enzymes. Therefore, the TIMP level may increase only when the MMP level also increases. In our study, there was no difference between the case and comparison groups with respect to detectable MMP levels, and perhaps this also accounted for there being no difference in the TIMP-1 levels.
Matrix metalloproteinases and TIMPs have been associated with established risk factors for atherosclerosis.31–34
The Framingham Heart Study investigators31
reported that TIMP-1 was associated with all major cardiovascular risk factors and hypothesized that the risk factors influence vascular and cardiac remodeling via extracellular matrix degradation. A report from another study32
described significantly higher MMP-9 and TIMP-1 levels in patients who had hypertension than in control-group patients, and that MMP-9 levels decreased and TIMP-1 levels increased after antihypertensive treatment. In our investigation, the prevalence of hypertension was not significantly different between our 2 groups; however, there were significant differences in serum cholesterol levels and diabetes mellitus (adjusted for in the various models). In our comparison group, TIMP-1 level was associated with some markers, such as fibrinogen and von Willebrand factor. Further evaluation will be required to understand better whether MMPs and TIMPs are primarily involved in atherogenesis and plaque rupture, whether they are primarily responsive to other traditional risk factors, and what their effects are on the vasculature and on the inflammatory process that promotes atherosclerosis development.
Several limitations of our study should be noted. The assay with which we analyzed MMP-1 detected only levels greater than 1.7 ng/mL, and most of our patients had levels of less than 1.7 ng/mL; therefore, ratios of MMP to TIMP could not be determined. Only MMP-1 and TIMP-1 were measured, and not the other metalloproteinases (including MMP-9) and their inhibitors. The tested samples were stored at −70 °C until they were analyzed, and the protracted storage may have led to the degradation or modification of MMP-1 or TIMP-1; however, any effect would have been similar for both the case and comparison groups. Finally, we examined only plasma levels of metalloproteinases, which may not correlate with tissue levels in arterial plaques.
Despite these limitations, the data from our analyses suggest that plasma levels of MMP-1 and TIMP-1 were not predictive of incident CAD events in the ARIC population of asymptomatic middle-aged adults who had no atherosclerosis upon enrollment. The exact role of MMP-1 and TIMP-1 in CAD will need better characterization.