In the proteomic investigation of the association of thrombus and inflammation, we identified 50 upregulated proteins and 421 downregulated proteins in the MVO group. The upregulated proteins were significantly enriched in O2/CO2 exchange in erythrocytes, neutrophil degranulation, platelet activation, complement and coagulation cascades and acute inflammatory response. Protein‒protein interaction analysis revealed the central role of SLC4A1 and SLC2A1, which are both involved in the coagulation and inflammation response. Moreover, drug screening discovered 4 drug candidates for MVO treatment: D-64131, TC-1, SB-431542 and alvespimycin. Our study was the first to evaluate inflammation comprehensively in STEMI with MVO using high throughput proteomics, and confirmed the role of inflammation as a mediator between thrombosis and MVO.
Thrombosis-inflammation paradigm in STEMI
In STEMI patients with high thrombus burden, thrombectomy during primary PCI did not further reduce CV death, MI, cardiogenic shock, or Class IV heart failure compared with PCI alone, but did significantly increase stroke [13, 14]. Additionally, antithrombotic regimens cannot completely prevent thrombotic events [15, 16]. In contrast, the highly dynamic and orchestrated inflammatory response is rapidly mounted to the injured area, which, paradoxically, can further exacerbate myocardial injury [17, 18]. This implies that a therapeutic gap due to a third, not yet adequately addressed mechanism, inflammation, has received considerable attention owing to the coronavirus disease COVID-19 pandemic [19, 20]. Therefore, an emerging concept is of particular interest to target thromboinflammation—that is, the aberrant and excessive activation of immunothrombosis—contributes to preventing thrombotic complications in CVD [10]. From Table 1, the statistics of baseline clinical characteristics, we found that the CRP value, a downstream marker of IL-6, and the ratio of neutrophils/RBCs were higher in the MVO + group than in the MVO- group, although the number of inflammatory cells in circulation was comparable, which indicated that the degree of polarization of inflammatory cells and the pathophysiology mechanisms in the microenvironment of thrombosis may vary in the two groups.
SLC4A1 and SLC2A1 may be mediators between inflammation and thrombosis
Previous studies have suggested that MVO by cardiac magnetic resonance imaging (CMR) is a major contributor to final infarct size and provides independent prognostic value [21]. Mechanistically, microvascular dysfunction is primarily caused by endothelial swelling, coronary microembolization of atherosclerotic debris, cardiomyocyte swelling, vasomotion dysfunction, aggregation of leukocytes, platelets and erythrocytes, and capillary destruction and hemorrhage [9, 22]. Therefore, we hypothesize that some factors relating to immune thrombosis are directly or indirectly involved in MVO formation. From the 4D high-throughput proteomics data of coronary thrombi, we identified that the enriched terms included neutrophil degranulation, platelet activation, complement and coagulation cascades, acute inflammatory response, and O2/CO2 exchange in erythrocytes. The functional enrichment results strongly suggested that coagulation and inflammation processes were involved in the pathogenesis of MVO.
In addition, protein‒protein interaction analysis revealed the central role of SLC4A1 and SLC2A1. The SLC4A1 gene located on chromosome 17q21-q22 encodes two proteins: the longer erythrocyte SLC4A1 protein and the shorter kidney SLC4A1 protein. It is a transmembrane glycoprotein involved in anion exchange as Na+-independent CI-/HCO3-exchanges [23]. It is widely expressed in erythrocytes, kidney, heart and colon [24]. SLC4A1 has been previously reported to be related to thromboembolism disease and diabetes mellitus [25, 26]. In addition, SLC4A1 was also associated with inflammation [5] and oxidative stress [24]. However, the effect of SLC4A1 in myocardial infarction has not been reported. Solute carrier family 2 member 1 (SLC2A1, also known as glucose transporter 1 (GLUT1)) is the major inducible glucose transporter for glucose uptake to fuel the glycolytic pathway and works in almost all mammalian cells, especially in activated leukocytes [27]. Neutrophils are the first leukocytes to massively invade the myocardium after AMI, and their inflammatory function relies greatly on glycolysis [28]. In response to immunological stimulation, neutrophil increase the surface expression of SLC2A1 for glucose uptake, as an energy source for neutrophil extracellular trap (NET) formation [29, 30]. Moreover, glucose metabolism transformation allows for NADPH production, thereby fueling NADPH oxidase to produce superoxide and NET release [31]. Collectively, inhibition of SLC4A1, SLC2A1 or both may attenuate the formation and release of NETs, alleviating reperfusion injury and MVO formation.
Clinical impact and future research.
Traditional risk assessment tools are only modestly prognostic in people with known cardiovascular disease, or elderly individuals without confirmed cardiovascular disease [32]. The insensitivity to improvements in traditional risk factors and imaging measurements is a problem for both clinical trials and medical practice [33]. Individualizing residual cardiovascular risk assessment enables precisive allocation and monitoring of the benefit of cardioprotective therapies [34]. Inflammation biomarkers may have potential in screening, diagnostic, and prognostic purposes for STEMI with MVO [35, 36]. Several inflammation-related proteins in our study deserve further investigation, either a biomarker for screening or as potential therapeutic targets in preclinical practice. Furthermore, we screened potential therapeutic drugs for treating MVO. Considering that increased systemic inflammation is a precursor of STEMI and the progression of recovery, which is supported by our results, it would be worthwhile to investigate whether early modulation of inflammatory blood cells prevents or delays the onset of cardiovascular frailty.
Limitations.
Our study does have some limitations. It is important to note that these results were obtained with aspirated material obtained during PCI, and it is unclear to what extent anticoagulant drug therapies applied prior to angioplasty could modify the thrombus proteome. On the other hand, thrombus aspiration according to the Class 2B guidelines, as our cohort was from a single research center, its relatively low number of enrolled patients might potentially introduce selection bias and residual confounding. The differentially expressed proteins need to be further validated in a larger cohort of patients. Finally, to determine the prognostic value of the identified biomarkers, the patient follow-up data should be further considered for the occurrence of major adverse cardiovascular events.