Matrix metalloproteinase-2 activation: critical to myocardial contractile dysfunction following ischaemia–reperfusion

This editorial refers to ‘Myocardial MMP-2 contributes to SERCA2a proteolysis during cardiac ischaemia-reperfusion injury’, by A. Roczkowsky et al., pp. 1021–1031. Prolonged myocardial ischaemia can lead to cardiomyocyte necrosis and irreversible contractile dysfunction. Apart from timely reperfusion, the extent of myocardial contractile impairment can also potentially be limited by post- (or pre-) conditioning stimuli, as well as restriction of ischaemia–reperfusion injury (IRI). While the existence of IRI is beyond doubt, its pathogenesis is still uncertain, but includes components of excessive reactive oxygen species generation, inflammation, intracellular calcium overload, impaired energetics, and injury to subcellular organelles, particularly mitochondria.1,2 No particular ‘key biochemical pathway’ has emerged thus far. However, at least in animal models, it appears that complex interactions occur between a number of IRI-activated reactions and the extent of both short- and long-term myocardial dysfunction. Although processes such as the precipitation of oxidative or nitrosative stress and associated mitochondrial dysfunction are important to the development of cellular necrosis in IRI, there is strong evidence that intracellular calcium overload also plays a critical role.1,2 Previous studies have suggested that disordered intracellular calcium kinetics reflects not only impairment of diastolic calcium efflux across the sarcolemma, but to a greater extent, failure of the reuptake processes which control calcium sequestration by the sarcoplasmic reticulum (SR).2,3 Nevertheless, the accumulated evidence suggests that intracellular calcium overload functions as a reverberative influence, both in the context of evolving IRI and in the prolongation of its impact on myocardial contraction and relaxation.4 Thus, despite the fact that calcium overload represents a secondary, rather than initiating, pathophysiological development in the context of IRI, it still represents a potential therapeutic target. Cardiac sarco/endoplasmic reticulum calcium-dependent ATPase 2a (SERCA2a) represents a fundamental controlling mechanism for the process of calcium reuptake into the SR. Although the immediate primary impact of SERCA2a activity is calcium sequestration within the SR and consequent myocardial relaxation, the resultant repletion of calcium stores also controls the capacity for subsequent myocardial contraction. Data from heterozygous SERCA2a knockout mice5 show greater infarct size and worsened myocardial function post-reperfusion. In other models of heart failure, pharmacological strategies6 or gene therapy7 aiming to restore SERCA2a activity have yielded promising results. Given the central role of intracellular calcium overload on the impact of IRI, it is important to understand as fully as possible the mechanisms underlying alterations in the control of calcium handling. In this regard, one immediate question arises: is SERCA2a activity impaired during IRI, and, if so, what modulates this impairment? Over the last 20 years, Schulz et al.8 in Edmonton have extensively investigated the roles of oxidative and nitrosative stress on IRI and discovered that peroxynitrite (ONOO−), generated during reperfusion, induced myocardial injury mainly through activation of matrix metalloproteinase-2 (MMP-2),9 a zinc-containing proteinase. This research group also found that, apart from extracellular matrix proteins, MMP-2 also targeted intracellular proteins including troponin I, explaining the proteolytic degradation of troponin I as a component of IRI.9 In the context of reducing IRI and preventing heart failure, pharmacological inhibition of MMPs, for example with doxycycline7,8 has yielded largely positive results in animal models. In spite of this, a short-term randomized controlled clinical trial involving a non-selective MMP inhibitor failed to reduce left ventricular remodelling or improve clinical outcomes in patients with acute myocardial ischaemia.10 More recent research has therefore been focusing on designing compounds that are selective MMP-2 inhibitors. Further insight is shed on the importance of the ONOO−/MMP-2 cascade within the SR, specifically in the context of IRI. Roczkowsky et al.11 have shown that in isolated perfused normal working rat hearts, selective MMP-2 inhibition resulted in improved contractility post-IR, along with reduced generation of a 70 kDa cleavage product of SERCA2a. Further in vitro experiments, incubating SR-enriched cardiac microsomes with exogenous MMP-2, also led to SERCA2a cleavage. Therefore, these data represent strong evidence that MMP-2 functions within the SR as an inactivator of SERCA2a, and that this inactivation contributes to early systolic dysfunction post-reperfusion. In many aspects, therefore, the current study adds to the previous work of Schulz et al. to highlight the central and multifold importance of the ONOO−/MMP-2 cascade in the pathogenesis of IRI-associated myocardial stunning and incremental cell death. It is also possible that these findings may be relevant to other forms of cardiotoxicity, such as anthracycline-induced dysfunction.12 Interestingly, while there is no absolute claim of exclusivity of this cascade as a controlling mechanism, the schema for the operation of this cascade which can be constructed from the accumulated data suggests little biochemical redundancy, so that it can be postulated that effective limitation of either ONOO− generation and/or MMP-2 activation might well lead to improved outcomes following emergency reperfusion of infarct-related arteries (Figure 1). Focused schematic concept of the potential role of the ONOO−/MMP-2 cascade in the induction of impairment of myocardial systolic function during I/R injury. Note that ONOO− also induces activation of the DNA-repair/energy ‘sink’ enzyme poly(ADP-ribose) polymerase-1 (PARP-1). However, a number of important issues remain to be addressed in order to translate these findings to the clinical setting. For example, there is residual uncertainty as to the practicality of ensuring the very rapid inhibition, post-onset of ischaemia, of a biochemical cascade which appears to operate predominantly via the very early and transient generation of ONOO− following reperfusion. Certainly, this kinetic obstacle cannot easily be overcome in the clinical setting. There are also some residual uncertainties regarding the pivotal role of ONOO− generation/MMP-2 activation in IRI, given that complete inhibition of MMP-2 is not required for reducing infarct size13 and the considerable evidence of importance of the various pro-survival biochemical cascades. That being said, a recent randomized controlled trial involving early use of N-acetylcysteine (which inter alia lowers nitrosative stress) as an adjunct to mechanical reperfusion showed substantial improvement of myocardial salvage,14 potentially via reducing IRI. Of course, inhibition of the ONOO−/MMP-2 cascade on a chronic basis might be more feasible as a prophylactic treatment against myocardial injury, for example during high-risk coronary revascularization. Even in a prophylactic setting, the issue of whether it is most effective to limit ONOO− formation, MMP-2 activation or both would remain important. Finally, it is likely that specifically targeting intracellular MMP-2 and its degradation of SERCA2a in an acute setting would be particularly challenging, in the context of the need to achieve early limitation of SERCA2a degradation. Whatever these residual challenges are, the currently published study of Roczkowsky et al. has refocused our understanding of IRI, and therefore increased the range of our potential therapeutic options. Conflict of interest: none declared. C.-R.C. was supported by a postdoctoral fellowship jointly funded by the National Health and Medical Researh Council of Australia (APP1162356) and National Heart Foundation of Australia (102126). The opinions expressed in this article are not necessarily those of the Editors of Cardiovascular Research or of the European Society of Cardiology.