Tissue Inhibitor of Metalloproteinase-1: Actions beyond Matrix Metalloproteinase Inhibition

Matrix metalloproteinases (MMPs) are a family of zinc-dependent enzymes that includes 25 current members. MMPs degrade the structural components of the extracellular matrix and process bioactive molecules including cytokines, chemokines and growth factors [1]. MMP activity is tightly controlled by the endogenous tissue inhibitors of metalloproteinases (TIMPs) comprising 4 homologous members (TIMP-1, TIMP-2, TIMP-3 and TIMP-4). The dynamic balance between MMPs and TIMPs controls extracellular matrix turnover and maintains tissue homeostasis. Alterations in the balance between MMPs and TIMPs are implicated in the pathogenesis of cardiovascular disease [2].In this issue of Cardiology, Wang et al. [3] investigated whether plasma MMP-9 and TIMP-1 levels as well as the MMP-9/TIMP-1 ratio could be used as biomarkers to predict future cardiovascular events in Chinese patients with mild to moderate coronary artery stenosis. Immediately after diagnosis, plasma levels of MMP-9 and TIMP-1 were measured, and the patients were followed for a median of 64 months. Patients with median TIMP-1 levels >37.6 ng/ml had an increased incidence of major adverse cardiac events (MACEs) during the follow-up period, supported by the results of multivariate Cox proportional hazards analysis after adjustment of covariates. In contrast, no correlation was observed between MMP-9 or the MMP-9/TIMP-1 ratio and the incidence of MACEs. Therefore, TIMP-1 may serve as a promising biomarker to predict the outcomes of patients with mild to moderate coronary artery lesions. TIMP-1, a 28-kDa glycoprotein, is produced by cardiac myocytes and fibroblasts in the normal heart [2]. TIMP-1 binds with active MMPs in a noncovalent 1:1 stoichiometric relation to inhibit MMP activity. Due to neutrophil infiltration, TIMP-1 mRNA levels increase by as early as 6 h after myocardial infarction (MI). TIMP-1 deficiency accelerates remodeling and dilation of the left ventricle (LV), and this effect could be rescued by MMP inhibition [4]. TIMP-1 overexpression reduces atherosclerotic lesions in apolipoprotein E-deficient mice, accompanied by a reduction in the MMP-2, MMP-3 and MMP-13 content [5]. Combined, these studies define that inhibiting MMP activity mediates TIMP-1-dependent favorable effects.Independent of inhibiting MMP activity, TIMP-1 also modulates a broad range of biological processes, including cell growth, proliferation, apoptosis, migration and angiogenesis, by binding to unidentified receptors and inducing specific signaling cascades [6]. In vitro, TIMP-1 displays growth-promoting activity in a wide range of cells, mediated via the receptor tyrosine kinase/mitogen-activated protein kinase signaling pathway [7,8]. It is TIMP-1, and not an MMP inhibitor, that induces proliferation of aortic smooth muscle cells involving the phosphoinositide 3-kinase pathway [9]. TIMP-1 directly induces smooth-muscle actin expression and stimulates Smad-3 phosphorylation in cardiac fibroblasts as well as inhibiting apoptosis in myocytes [10]. Of note, TIMP-1 suppresses microvascular endothelial cell migration via both MMP-dependent and MMP-independent mechanisms [11]. The MMP-independent roles of TIMP-1 may, at least partially, explain why TIMP-1, but not MMP-9, independently predicted the risk of MACEs in the study by Wang et al. [3].The findings that TIMP-1 acts as an independent predictor of MACEs are supported in previous studies. The Framingham Heart Study revealed that plasma TIMP-1 levels are higher in men than women, and increase with age, body mass index and the total/high-density lipoprotein cholesterol ratio [12]. Subjects who smoke or have diabetes have higher plasma TIMP-1 levels. More importantly, after adjusting for age, sex and height, plasma TIMP-1 is positively correlated with LV mass, wall thickness, end-systolic diameter and the risk of having an increased LV end-diastolic diameter or wall thickness, but it is negatively associated with fractional shortening [12]. Lubos et al. [13 ]report that higher mean concentrations of TIMP-1 predispose patients with suspected coronary artery disease to a greater chance of MACEs independently of sex and age. Kelly et al. [14 ]have shown that plasma TIMP-1 levels are positively associated with the occurrence of MACEs in patients presenting with acute MI. TIMP-1 concentrations increase along with the quartiles of the GRACE (Global Registry of Acute Coronary Events) score, and a combination of TIMP-1 and GRACE score displays a greater area under the receiver operator characteristic curve [14]. This indicates that TIMP-1 offers information in addition to GRACE score for assessing prognosis.MMP-9, one of the most studied MMPs, is involved in cardiac aging and multiple cardiovascular diseases such as MI, atherosclerosis and hypertension [1,15]. During cardiac aging, circulating and cardiac MMP-9 increase, and MMP-9 deletion abolishes age-induced diastolic dysfunction, which may be mediated by facilitating anti-inflammatory M2 macrophage polarization and inhibiting collagen deposition [15,16]. MMP-9 expression substantially increases after MI, and MMP-9 deletion attenuates cardiac dilation and dysfunction in both young (8-10 weeks) and old (11-36 months) mice, indicating a detrimental role for MMP-9 [17,18]. The potential mechanisms are strongly associated with promoting M2 macrophage polarization and suppressing inflammation [18]. However, transgenic overexpression of MMP-9 in macrophages also shows a beneficial impact on cardiac remodeling and function after MI, signifying the biphasic functions of MMP-9 [19]. Therefore, in the MI setting, MMP-9 may assert various roles, depending on its cellular source and spatiotemporal expression. In atherosclerosis, MMP-9 mainly derives from macrophage-derived foam cells, smooth-muscle cells and endothelial cells, and is positively correlated with increased plaque vulnerability and cardiovascular mortality [20]. MMP-9 activity is induced at a very early stage of hypertension development, promoting collagen breakdown and arterial destruction. Hypertensive patients have higher serum levels of MMP-9; this is positively associated with aortic stiffness [21]. Due to the critical roles MMP-9 plays in cardiovascular disease, evaluating MMP-9 as a biomarker has attracted much attention. Postmortem examination revealed that patients with infarct rupture after MI had higher cardiac MMP-9 activity and increased inflammatory cell numbers when compared to nonrupture patients [22]. This suggests that inflammatory cells generate MMP-9, which destroys collagen to impair infarct formation. In a clinical study of 1,127 patients with coronary artery disease, the median concentrations of plasma MMP-9 were significantly higher in the patients who subsequently experienced a fatal cardiovascular event than in those who did not [23]. Patients with the highest quartile of MMP-9 had the highest mortality rates during the follow-up period. After adjustment for clinical and therapeutic confounders, the MMP-9 levels were independently associated with a high rate of mortality. In another study, patients with acute coronary syndrome were found to have higher levels of MMP-9 in serum (median 4,000 pg/ml) than patients with stable angina pectoris (900 pg/ml) and healthy controls (87 pg/ml), and the patients presenting acute coronary syndrome with higher MMP-9 levels had poor outcomes (recurrent ischemic attacks, congestive heart failure or death) [24]. An MMP-9 cut-off value of 3,100 pg/ml has been proposed to distinguish MI from unstable angina, while the best prognostic utility has been set at 4,700 pg/ml, indicating good potential for MMP-9 as a marker for diagnosis and prognosis. Of interest, the study by Wang et al. [3 ]did not find a correlation between plasma MMP-9 concentrations and future cardiovascular events in the patient cohort. This discrepancy could be partially explained by the differences in inclusion criteria, the race of the patient and other confounding factors. For example, Blankenberg et al. [23] enrolled German patients with a stenosis of >30% for their study whereas Wang et al. [3] enrolled Chinese patients with a stenosis of 20-70% in a major coronary artery. Further investigation to determine why these different cohorts relied differently on MMP-9 could shed further mechanistic insight into its role in cardiovascular disease. A proteomics approach would be ideal for this investigation.This study has several minor limitations that should be taken into consideration. First, TIMP-1 and MMP-9 levels were measured at one time point only, i.e. right after diagnosis. Multiple time point measurements during the follow-up period would be better for understanding the correlation between TIMP-1 and MMP-9 with MACEs in patients at specific times along the pathology continuum. Second, large-scale, multicenter clinical trials are warranted to validate the findings in larger populations and to better understand if different populations use TIMP-1 and MMP-9 differently.In conclusion, Wang et al. [3 ]demonstrated that TIMP-1, but not MMP-9, can serve as an independent biomarker to predict MACEs in Chinese patients with mild to moderate coronary artery stenosis. We acknowledge the support of the American Heart Association 15SDG22930009 (Y.M.), the National Institutes of Health HHSN 268201000036C (N01-HV-00244), HL075360 and GM114833 (M.L.L.), P01HL051971 and P20GM104357 and the Biomedical Laboratory Research and Development Service of the Veterans Affairs Office of Research and Development Award 5I01BX000505 (M.L.L.).No conflicts of interest were declared.