• Cardiac function
• contractile proteins
• inflammation
• myocardial ischaemia and infarction (IHD)
• oxidative stress

In the past three decades, percutaneous coronary intervention has become the standard of treatment in acute coronary syndromes. However, reopening of acutely occluded coronary arteries paradoxically causes reperfusion injury characterised by a localised, acute inflammatory response and myocardial damage. Because of the lack of good plasma biomarkers, reperfusion-induced myocardial damage and its contribution to final infarct size is difficult to evaluate.

Biomarkers play an important role in diagnosis, prognosis and risk evaluation in the treatment of cardiovascular diseases. Ideal biomarkers should not only have a good sensitivity and dynamic range, but should also correlate well to the therapeutic response. There is increasing evidence that matrix metalloproteinase 2 (MMP-2) may be such a biomarker based on recent clarification of its pathophysiological significance in acute myocardial infarction.

Matrix metalloproteinases (MMP) are a family of zinc-dependent endopeptidases that are synthesised as zymogens. Of particular interest with regard to the heart are the gelatinases MMP-2 and MMP-9, as the former is found in cardiac myocytes,1 cardiac fibroblasts2 and endocardial cells,3 whereas the latter is found in activated neutrophils within infarct tissue. MMP activities are regulated by a family of endogenous protease inhibitors, the tissue inhibitors of metalloproteinases (TIMP-1–4). MMP were initially recognised for their ability to effect tissue remodelling by proteolysis of several proteins which make up the extracellular matrix. Such remodelling processes take course over periods of weeks to months. More recently, however, a number of studies have demonstrated that MMP-2 plays a key role in acute processes in heart muscle with a timescale of seconds to hours during reperfusion following experimental ischaemia in the heart.

We have shown that MMP-2 is directly involved in acute myocardial stunning injury seen during reperfusion following ischaemia in isolated rat hearts.4 The sudden reintroduction of molecular oxygen to ischaemic tissue during reperfusion re-energises mitochondria and reactivates the electron transport chain, causing a rapid (peak within 30 s) and large increase in the biosynthesis of reactive oxygen species including peroxynitrite,5 the toxic reaction product of nitric oxide and superoxide, which rapidly activates MMP-2.6 MMP-2 activation by reactive oxygen species is probably a very early response of the myocyte to enhanced oxidative stress at a time when the cell is still reversibly injured. Once activated within the myocyte, MMP-2 proteolyses a variety of sarcomeric proteins including troponin I,7 myosin light chain-1,8 α-actinin9 and titin.10 We have also demonstrated that intracoronary infusion of peroxynitrite stimulates the activation and subsequent release of MMP-2 from the myocardium, which precedes the depression of mechanical function and mimics the effects of stunning injury.11 Peroxynitrite-induced injury could also be attenuated by the administration of MMP inhibitors.11 These studies provide evidence that MMP-2 activation and release is a primary effector of acute cardiac mechanical dysfunction.

During ischaemia–reperfusion injury, the release of MMP-2 from heart muscle is probably a means to abrogate intracellular proteolytic stress and thus the extent of injury.12 The release of MMP-2 peaked within 1–5 min of reperfusion and its release was enhanced with the increasing duration of ischaemia.4 Moreover, the activation of MMP-2 was associated with impaired mechanical function of the heart, because two chemically unrelated inhibitors of MMP activity (doxycycline and o-phenanthroline), a MMP-2 neutralising antibody, were able to improve the functional recovery of ischaemic–reperfused hearts.4 At the end of 30 min of reperfusion there was a clear-cut depletion of MMP-2 activity in the myocardium, showing that its rapid activation and release of MMP-2 from the heart is a direct consequence of this injury. These observations imply that MMP-2 may be an early, sensitive and treatment-responsive biomarker for acute ischaemia–reperfusion injury in acute coronary syndromes.

We then showed that there is intracellular co-localisation of MMP-2 with troponin I, the regulatory element of contractile proteins within the sarcomere of the cardiac myocyte.7 Biochemical assay also showed that troponin I is indeed an excellent substrate for MMP-2. Rapid proteolytic cleavage of troponin I by activated MMP-2 may be a significant source of troponin I fragments in plasma. Inhibition of MMP activity also prevented troponin I degradation while improving the recovery of mechanical function in ischaemic–reperfused hearts.7 Because troponin I is currently considered to be the ‘gold standard’ biomarker in the clinical setting for assessing myocardial damage, the establishment of a direct correlation between myocardial MMP-2 and serum troponin I would further support the notion that MMP-2 may serve as a biomarker for ischaemic cardiac disease.

The majority of studies on the pathophysiological significance of MMP-2 in ischaemic cardiac disease were generated in rodent models. Although MMP-2 is a highly conserved protein across mammals (eg, its amino acid sequence in mice shares 97% homology with human MMP-2) potential interspecies differences still need to be considered in clinical studies. We therefore studied the changes in MMP-2, MMP-9 and TIMP during acute myocardial ischaemia–reperfusion injury in 15 patients undergoing coronary artery bypass graft surgery with cardiopulmonary bypass.13 Within 10 min of reperfusion (aortic cross-clamp release) there was a marked increase in MMP-2 and MMP-9 activities in both cardiac biopsies and plasma from these patients. Given the important role of intracellular MMP-2 in oxidative stress and reperfusion injury in the heart, it is very important at this stage to promote clinical trials examining the role of MMP-2 in cardiovascular disease.

A report by Nilsson et al14 published in Heart (see page 31) made encouraging progress in defining the diagnostic value of plasma MMP-2 as a biomarker for ischaemic heart injury. The study demonstrated a consistent correlation between the level of plasma MMP-2 with final infarct size and ventricular dysfunction in ST-segment elevation acute myocardial infarction (STEMI) patients. Fifty-eight STEMI patients originally enrolled in the FIRE trial who received primary percutaneous coronary intervention were closely monitored for infarct size, left ventricular dysfunction and remodelling by cardiac MRI at 5 days and 4 months after STEMI. The baseline and 12-h levels of plasma MMP-2 consistently and highly significantly correlated with outcome measures of infarct size and left ventricular dysfunction both at 5 days and at 4 months. In contrast, TIMP-2 and peak levels of TIMP-1 taken at the same time show only a weak correlation with infarct size and left ventricular dysfunction, whereas MMP-9 and myeloperoxidase did not correlate at all. It is of interest in this study that the plasma MMP-2 level also correlates well with the blood level of its intracellular cleavage substrate, troponin I. That is the first double-blind, randomised, placebo controlled, multicentre trial focused on temporal changes of MMP and TIMP in the plasma of STEMI patients undergoing primary percutaneous coronary intervention. These robust and convincing results shed light on the use of early plasma MMP-2 monitoring to assess the extent of myocardial damage following percutaneous coronary intervention and to guide the clinical stratification of management for reducing the consequences of ischaemic–reperfusion injury.

Cardiac biomarkers play an important clinical role in the diagnosis, prognosis, monitoring, risk stratification and therapeutic selection of patients with acute cardiovascular diseases. Numerous biomarkers have been studied thus far for acute coronary syndromes including creatine kinase, creatine kinase myocardial type, myoglobin, GDP-15, H-FABP, sLOX-1, myeloperoxidase, cardiac troponin and many more. Many of them possess differential specificity, sensitivity and diagnostic values. Among them, cardiac troponin has evolved to be the ‘gold standard’ to assess acute coronary syndrome patients. The strong prognostic value of the cardiac troponin family (troponin I and troponin T) was validated by different clinical trials.15 However, they still have some limitations in specificity because some other pathological circumstances such as chronic renal failure, inflammatory diseases, cardiomyopathy, congestive heart failure, pulmonary embolism, rhabdomyolysis, sepsis and left ventricular hypertrophy also show elevated troponin in the absence of myocardial ischaemia.16 The assessment of troponin may also be influenced by a variety of factors such as serum heterophile and autoantibodies, which could contribute to false positive results and the presence of epitopes from truncated troponin C could also mask the detection antibody. MMP-2, as a biomarker upstream of the biological pathway of cardiac troponin I may surpass these flaws. Most importantly, the study by Nilsson et al14 provides more evidence that suggests a clear relationship between the extent of injury, ie, infarct size and plasma levels of MMP-2.

In order to ultimately define the possible use of plasma MMP-2 in guiding therapy decisions for individual patients, this will have to be thoroughly evaluated by larger multicentre clinical studies before its acceptance into routine clinical use. With further elucidation of the pathophysiological role of different cardiac biomarkers, more strategic and combined use of cardiac biomarkers may significantly contribute in the future to improve and personalise the therapy of ischaemic heart diseases.