Circulating HtrA2 as a novel biomarker for mitochondrial induced cardiomyocyte apoptosis and ischemia-reperfusion injury in ST-segment elevation myocardial infarction
Introduction
The extent of myocardial damage in patients with acute ST-segment elevation myocardial infarction (STEMI) depends on both the ischemic time to reperfusion as well as an injury induced by ischemia-reperfusion (I/R) resulting in a complex cascade of cellular myocardial and coronary microvascular reactions [1], [2]. Coronary microvascular damage and myocardial I/R injury are connected, but the causal relationship is unclear.
In the coronary vasculature I/R injury causes microvascular dysfunction, increased permeability, microembolization, impaired vasomotion and stasis leading to capillary destruction [1]. Despite restoration of epicardial blood flow through percutaneous coronary intervention (PCI) impaired myocardial blood flow is often clinically observed. The no-reflow phenomenon is the most severe form of microvascular I/R injury [3]. Despite the progress in PCI, one third of STEMI patients still fail to show complete reperfusion due to I/R injury and no-reflow [4], [5]. These patients exhibit poorer outcomes [6], [7]. Failure of adequate ST segment resolution usually reflects no-reflow [8].
Cardiomyocyte necrosis is a result of uncoordinated pathophysiological mechanisms during ischemia, which is accelerated during I/R injury [9].
Cellular calcium overload, digestion of the cytoskeleton and sarcolemma by calpains and reactive oxygen species (ROS) all contribute to necrotic cell death [1]. The loss of cardiomyocytes is not only a consequence of necrosis but also of regulated cell death (apoptosis). Opening of the mitochondrial permeability transition pore (MPTP) is pivotal for the initiation of both forms [1]. Ischemia causes necrosis, whereas apoptosis is thought to be the main mechanism of myocardial cell death in I/R injury. However, this has only been demonstrated in a murine model of myocardial infarction and I/R injury [10]. Overall, the quantitative contribution of apoptosis and necrosis to myocardial cell death is not clear but mitochondria are known to play a decisive role in both processes [1].
Mitochondria function as central regulators of apoptosis [11]. Excessive reactive oxygen species (ROS), Bcl-2 proteins and dysregulated calcium promote mitochondrial membrane permeabilisation during reperfusion and cause the release of apoptotic factors into the cytosol [12]. As a consequence of I/R injury in the heart, the high-temperature requirement protein A2 (HtrA2) is released from the mitochondria intermembrane space of cardiomyocytes to the cytosol together with cytochrome c, whereupon it induces protease activity-dependent apoptosis mediated via caspases [13], [14]. In aging rats the expression and leakage of HtrA2 is increased and enhances I/R injury [15]. Cytosolic HtrA2 has also demonstrated functional importance in rat hearts, whereby inhibition of HtrA2 using UCF-101 resulted in amelioration of heart dysfunction following I/R injury in vivo [16]. In a recent publication cardiac specific overexpression of HtrA2 in a mouse model induced myocardial apoptosis and cardiac dysfunction [17]. Cytochrome c is currently a promising biomarker for I/R injury [18], [19]. To our knowledge, it is currently not known whether HtrA2 is also released into the circulation. Hence, circulating HtrA2 might also show potential as a novel biomarker for mitochondrial-induced I/R injury and consequent cellular apoptosis. If validated, HtrA2 might have a number of utilities, including being able to measure the efficacy of I/R targeted therapeutics and also allowing the identification of patients with I/R injury after STEMI, giving a better idea of prognosis, thereby directing further treatment. Establishing a valid marker for I/R injury is particularly interesting because currently validated biomarkers in myocardial infarction, such as troponins and CK-MB, are markers of myocardial cell lysis, but they are not able to specifically assess myocardial damage induced by mitochondrial-associated I/R injury and apoptosis.
The aim of this study was to determine if HtrA2 is detectable and elevated in the serum of patients with STEMI after PCI, compared to patients with non–ST-segment elevation myocardial infarction (NSTEMI), stable coronary artery disease (CAD) and a control group who had CAD excluded by coronary angiogram. STEMI is known to be associated with severe ischemia and when PCI is instituted, there is abrupt reperfusion, theoretically resulting in greater I/R injury compared to the other groups studied in this report. We also assessed if any correlation exists between an increase in HtrA2 and other important clinical variables. We further hypothesized that HtrA2 is able to predict reperfusion injury assessed via measurement of ST-segment resolution or lack thereof on electrocardiogram [5], [8], [20].
Section snippets
Patient population
In this single-center prospective study, peripheral venous blood was obtained consecutively from 37 STEMI patients, 20 NSTEMI patients, 17 patients with stable CAD and 9 patients with CAD excluded who presented to the emergency department with cardiac-suspected chest pain between September 2013 and August 2015.
Patients with STEMI as evidenced by ST-elevation > 0.1 mV in at least two contiguous leads were included in this study.
Major exclusion criteria included previous severe heart failure (i.e.
Patient characteristics
The average age across the entire patient population was 67 years. The STEMI population was composed of 78% men, in comparison to 75% men in NSTEMIs, 71% in the stable CAD group and 44% men in the CAD excluded group. Table 1 presents the baseline characteristics across all patient groups. Significant differences were found in the number of patients with diagnosed diabetes mellitus and dyslipidemia, with patients within the stable CAD group displaying a higher prevalence of both conditions. There
Discussion
In this study we investigated for the first time if HtrA2 is detectable and elevated in the serum of patients with STEMI after PCI compared to patients with NSTEMI, stable CAD and a control group who had CAD excluded by coronary angiogram, in order to validate HtrA2 as a potentially novel biomarker for mitochondrial reperfusion-associated cardiomyocyte apoptosis and severe I/R injury. We assessed if HtrA2 correlates with clinical variables and markers of cell lysis (CK, CK-MB and cTnT). We
Limitations
The results of this study support our hypothesis. However, our study has some limitations. The clinical characteristics were obtained retrospectively and hence our results may be subject to bias and confounders. Furthermore, this study was limited by the relatively small sample size of patients with STEMI and not powered to identify significant HtrA2 levels. This may also explain why HtrA2 levels did not reach statistical significance in its association with reperfusion injury assessed by ECG.
Conclusions
To the best of our knowledge, we have described for the first time significantly increased serum levels of HtrA2 in patients with STEMI compared to NSTEMI and stable CAD. HtrA2 correlated significantly with the peak levels of cell lysis biomarkers (CK and cTnT). HtrA2 serum levels also demonstrated a trend in being correlated to adequacy of reperfusion post-PCI assessed via measurement of ST-segment resolution, and exhibited superior sensitivity and specificity for predicting reperfusion injury
Conflict of interest
The authors report no relationships that could be construed as a conflict of interest.
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This author takes responsibility for all aspects of the reliability and freedom from bias of the data presented and their discussed interpretation.