Using a case–cohort design, we selected 216 individuals who had incident coronary artery disease (case group) and 225 individuals from a cohort random sample (comparison group) from participants enrolled in the Atherosclerosis Risk in Communities study. We measured plasma levels of MMP-1 and TIMP-1, traditional risk factors, and other markers of inflammation.

We found no significant difference in TIMP-1 levels between the case group (827.8 ± 23.8 ng/mL) and the comparison group (819.31 ± 16.1 ng/mL) (P=0.77), and no significant difference in the frequency of MMP-1 levels that were dichotomized at the minimum detectable value of 1.7 ng/mL (P=0.49). In models adjusted for age, sex, race, body mass index, hypertension, diabetes, total cholesterol, high-density lipoprotein cholesterol, triglycerides, fibrinogen, von Willebrand factor, and white blood cell count, the hazard-rate ratio for incident coronary artery disease was 1.14 (95% confidence interval, 0.63–2.04; P=0.67) for individuals whose TIMP-1 levels were above, versus at or below the mean, and 1.17 (95% confidence interval, 0.63–2.19; P=0.62) for individuals whose MMP-1 levels were above 1.7 ng/mL.

We conclude that TIMP-1 and MMP-1 levels in plasma were not predictive of incident coronary artery disease in a case–cohort random sample of the Atherosclerosis Risk in Communities study, a population study of asymptomatic middle-aged adults who had no prevalent atherosclerosis upon enrollment.

Components of the inflammatory cascade that is thought to be associated with atherogenesis and plaque rupture have been identified during the last 2 decades and have been examined as risk factors for atherosclerotic vascular disease. Among these are matrix metalloproteinases (MMPs), a family of enzymes that play an important role in the degradation of collagen and other extracellular matrix macromolecules.^1,2 By degrading the insoluble fibrillar components of the fibrous cap,³ MMPs are also thought to have a strong influence on plaque rupture. The activity of MMP is tightly regulated by various endogenous inhibitors, including another family of proteins—tissue inhibitors of matrix metalloproteinases (TIMPs).^2,4,5

Inflammatory cytokines induce MMP expression in endothelial cells, smooth muscle cells, and macrophages. It has been proposed that an imbalance between MMPs and their respective tissue inhibitors constitutes a causal factor for cardiovascular disease.

Although results of research studies have suggested a role for MMP-1 and TIMP-1 in the pathophysiology of atherosclerosis, relatively few prospective epidemiologic studies have investigated the relationship between circulating plasma MMP-1 and TIMP-1 levels and the occurrence of coronary artery disease (CAD). We hypothesized that elevated levels of MMP-1 and TIMP-1 are associated with incident CAD, and we sought to discover a possible relationship between levels of these substances and incident CAD in the Atherosclerosis Risk in Communities (ARIC) study, a prospective study of atherosclerosis and cardiovascular disease incidence.

TABLE I. Baseline Characteristics of the Case and Comparison (Cohort Random Sample) Groups

We used Cox proportional hazards models to evaluate TIMP-1 that was dichotomized at the population mean (>856.9 ng/mL vs ≤856.9 ng/mL) and to evaluate MMP-1 that was dichotomized at the minimum detectable level (>1.7 ng/mL vs ≤1.7 ng/mL) (Table II). The method of Barlow¹³ was used to adjust for the sampling approach in the Cox proportional hazards modeling. For the case group, the follow-up interval was defined as the time between visit 2 of the ARIC study and the date of the 1st CAD event, the date of death, the date of last contact, or 31 December 2001, whichever occurred first. For the comparison group from the cohort random sample, follow-up was from visit 2 of the ARIC study until the date of death, the date of last contact, or 31 December 2001, whichever occurred first. The established cardiovascular risk factors that we evaluated as potential confounders in the 3 Cox models included age, sex, race, BMI, hypertension, diabetes mellitus, von Willebrand factor, and white blood cell count, and levels of total cholesterol, high-density lipoprotein cholesterol (HDL-C), triglycerides, and fibrinogen. Covariates were evaluated for statistical significance by use of the Wald χ² statistic; P <0.05 was considered statistically significant.

TABLE II. Relationship between Coronary Artery Disease Case Status and Dichotomized MMP-1 and TIMP-1 Measurements

Using weighted t tests, we evaluated possible associations by considering the comparison group's mean levels of various variables. These variables included age, BMI, waist-to-hip ratio, white blood cell count, fibrinogen, von Willebrand factor, and levels of total cholesterol, triglycerides, low-density lipoprotein cholesterol (LDL-C), and HDL-C. We associated these with TIMP-1 that was dichotomized at its mean (856.9 ng/mL) and for MMP-1 that was dichotomized at 1.7 ng/mL, the minimum level of detection (Table III).

TABLE III. Associations in Groups Dichotomized by TIMP-1 and MMP-1 in the Cohort Random Sample

Individuals with incident CAD (case group) were more likely than those in the comparison group from the cohort random sample to have diabetes mellitus, higher waist-to-hip ratios, higher white blood cell counts, higher levels of total cholesterol, LDL-C, and triglycerides, and lower levels of HDL-C. However, no significant differences were noted between the groups with respect to measurements of hemostasis, such as fibrinogen and von Willebrand factor. There were no significant differences between the groups in MMP-1 levels >1.7 ng/mL (P=0.49) or in mean TIMP-1 levels (case group, 827.8 ± 23.8 ng/mL vs comparison group, 819.31 ± 16.1 ng/mL; P=0.77).

When dichotomized at the mean (>856.9 vs ≤856.9 ng/mL), TIMP-1 levels were not associated with incidence of CAD in the various analytical models (Table II). For TIMP-1 dichotomized at the mean, the hazard-rate ratios were 1.25 (95% confidence interval [CI], 0.80–1.95; P=0.33) in a model adjusted for age, sex, and race; 1.03 (95% CI, 0.6–1.77; P=0.91) in a model adjusted for age, sex, race, BMI, hypertension, diabetes mellitus, total cholesterol, and HDL-C; and 1.14 (95% CI, 0.63–2.04; P=0.67) in a model adjusted for age, sex, race, BMI, hypertension, diabetes mellitus, total cholesterol, HDL-C, triglycerides, fibrinogen, von Willebrand factor, and white blood cell count. When evaluated as a dichotomous variable (levels >1.7 ng/mL vs levels ≤1.7 ng/mL), MMP-1 level was also not significantly associated with incident CAD in the respective models, with hazard-rate ratios of 1.20 (95% CI, 0.77–1.88; P=0.43), 0.92 (95% CI, 0.52–1.65; P= 0.78), and 1.17 (95% CI, 0.63–2.19; P= 0.67) (Table II).

When potential associations of TIMP-1 with various variables were examined in the comparison group from the cohort random sample, participants with a TIMP-1 level above the mean were older (P <0.01) and had a higher BMI (P=0.01), a higher waist-to-hip ratio (P <0.01), significantly higher levels of fibrinogen (P=0.04) and von Willebrand factor (P=0.03), and significantly lower HDL-C levels (P <0.01) (Table III). There were no differences in the prevalence of diabetes mellitus or hypertension, smoking, or BMI between individuals in the comparison group who had TIMP-1 levels above the mean, versus below. On the other hand, when MMP-1 levels were dichotomized at 1.7 ng/mL, there were no significant differences between comparison-group individuals who had MMP-1 levels >1.7 ng/mL versus ≤1.7 ng/mL (Table III).

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 another study¹⁹ 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 Another investigation²⁵ 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.

The authors thank the staff and participants of the ARIC study for their important contributions, and Joanna Brooks, BA, for her editorial assistance.