This sub-study investigates Olink Cardiovascular III panel protein biomarkers in patients with stable angina who were apparent responders to cardiac shock wave therapy. The purpose was to explore the potential link of protein candidates with the expected activation of angiogenesis mechanism induced by shear endothelial stress. We identified 28 serum proteins that showed clear associations with clinically significant improvement in exercise tolerance, myocardial contractility and perfusion after CSWT. The discriminated proteins according to network analysis are involved in biological pathways promoting angiogenesis: cell adhesion, immune and inflammation response, tissue remodeling, proteolysis and catabolic process.
A recent review summarized potential plasma and serum proteomic biomarkers in cardiovascular disease which are associated with inflammation, wound healing and coagulation, proteolysis and extracellular matrix organization, handling of cholesterol and low-density lipoproteins [12]. Currently, there is a lack of clinical studies investigating angiogenesis process in humans after revascularization procedures. Some studies showed raised levels of circulating VEGF after percutaneous intervention [13, 14] and coronary artery bypass grafting [15]. Most protein candidates were tested in oncology, and most pathophysiological mechanisms are associated with tumor angiogenesis and progression. Only a few of these biomarkers were tested in patients with myocardial ischemia.
A cardiac shockwave therapy is a regenerative treatment method that utilizes non-invasive application of low-intensity shock waves to stimulate angiogenesis. Several studies demonstrated that the application of low-intensity shockwaves (SW) might induce the release of endothelial nitric oxide synthase (eNOS), vascular endothelial growth factor (VEGF), and proliferating cell antinuclear antigen (PCNA) [16, 17, 18, 19]. There are some data that CSWT induces recruitment of progenitor cells to the ischemic zones [20, 21], ameliorates left ventricular remodeling after myocardial infarction [20, 22]. The ultrasound-mediated stimulation of endothelial cells culture increased proliferation rates, and enhanced migration and sprouting in the 3-D spheroid cultures [23].
The regulation of angiogenesis involves several growth factors, cytokines, signaling cascades and cellular processes that are triggered in response to either an inflammatory or ischemic stimulus. Inflammatory and ischemia-driven angiogenesis are regulated via two different yet slightly overlapping pathways [24], which are promoted by circulating endothelial progenitor cells (EPCs) [25], VEGF [26, 27] and involve cell migration, proliferation and tubulogenesis [28] to form new blood vessels. EPCs have also been shown to incorporate into tubules and participate in angiogenesis directly [29].
The Olink panel contains 92 protein candidates potentially important for the pathogenesis of cardiovascular diseases and inflammation. Based on widely used public-access bioinformatic databases, including Uniport, Human Protein Atlas, Gene Ontology and DisGeNET, the vendor classified biomarkers according to biological process, disease area and tissue expression into 17 biological pathways, including angiogenesis, blood vessel morphogenesis, catabolic process, cell adhesion, chemotaxis, coagulation, heart development, immune, inflammatory response, mitogen-activated protein kinase (MAPK) cascade, platelet activation, proteolysis, regulation of blood pressure, response to hypoxia, to peptide hormone, wound healing and other gene ontology terms [30].
Cell adhesion mechanism
We determined a significant increase in 9 biomarkers levels that mediates cell adhesion (Table 3 and Fig. 4).
CD166 antigen (ALCAM) enables the isolation and differentiation of progenitor cells into cardiomyocytes, endothelial and smooth muscle cells. A rat model of myocardial infarction demonstrated that ALCAM improves angiogenesis by activating AKT-MAPK signaling, thus being included in tissue recovery and cardiac function restoration [31].
Ephrin type-B receptor 4 (EPHB4) maintains vascular integrity and heart homeostasis [32].
Overexpression of integrin beta-2 precursor (ITGB2) enhances the homing and engraftment of transplanted adipose-derived stem cells to infarcted myocardium through ICAM-1/ITGB2 interactions, augments angiogenesis, and improves myocardial perfusion [33].
Aminopeptidase N (APN) is expressed in vascular endothelial cells and is involved in capillary tube formation [34].
The data on remaining three proteins - contactin-1 (CNTN-1), spondin 1 (SPON1) and epidermal growth factor receptor (EGFR) - activation in ischemic myocardium are limited. They are known to have a role in tumor angiogenesis (EGFR [35]) and in the development of the central nervous system through its adhesion functions and promoting axonal growth (CNTN-1 and SPON1[36, 37]).
Immune and inflammatory response
Seven biomarkers, that appeared to react to CSWT, reflect immune and inflammatory response. Tartrate-resistant acid phosphatase 5 (TR-AP), a marker of systemic inflammatory burden, is secreted by activated macrophages [38]. TR-AP promotes inflammation-induced angiogenesis [39], while osteopontin (OPN) acts through regulating the VEGF pathway [40]. This could explain the relationship of increase of TR-AP level with prolonged exercise duration in our patients.
Hemoglobin scavenger receptor (CD163) is exclusively expressed on macrophages, where it acts as a receptor for haemoglobin [41] and stimulates muscle regeneration after ischaemic injury [42].
The intercellular adhesion molecule 2 (ICAM-2) is expressed at the endothelial junctions, regulates angiogenesis via cell migration and Rac activation during tube formation [43].
Myoloblastin (PRTN3) is a protease secreted by myeloid cells and allocated on the cell surface of neutrophils and endothelial cells. PRTN3 is involved in inflammatory processes through MMP activation and potentially in tumor invasion [44].
A recent in vivo study showed that neutrophil-derived myeloperoxidase (MPO) activates proteins that drive post-ischemic angiogenesis [45].
Animal studies have shown that treatment with IL-18 binding protein (IL-18BP) improves postischemic myocardial dysfunction by reducing IL-18 levels [46].
Tissue remodeling
We identified an increase of 5 biomarkers that participate in tissue remodeling. Growth differentiation factor-15 (GDF15) participates in tissue repair by exerting antiapoptotic and anti-inflammatory effects whilst maintaining vascular endothelial functions [47].
Collagen type 1 alpha 1 (COL1A1) is related to diabetic kidney disease [48] and might play role in the development of diabetic retinopathy [49] and cancer progression [50]. Of note, we found an association of elevated COL1A1 levels with improved myocardial contractility.
Trefoil factor 3 (TFF3) is the driving factor of cancer, which is involved in the proliferation, invasion, resistance to apoptosis, and angiogenesis of cancer cells [51].
Insulin-like growth factor-binding protein-2 (IGFBP-2) promotes angiogenesis by activating VEGF gene expression [52]. IGFBP-2 has also been identified as a pro-angiogenic factor in cancer studies [53, 54]. IGFBP-2 can augment tube formation in HUVECs [53], also it can be excreted by cancer cells, mediates activation of IGF-1R (insulin-like growth factor receptor 1) on endothelial cells and promoting their recruitment for metastatic angiogenesis [54].
Caspase 3 (CASP-3) is a marker of apoptosis. Activation of CASP-3 is considered a deleterious event in different pathophysiologic processes, such as degenerative diseases, ischemia, inflammation and cancer. Nevertheless, all nonapoptotic roles of CASP-3 rely on its proteolytic activity [55]. Recently, CASP-3 was described as a mediator of neoangiogenesis in dying cells after X-ray therapy [56].
Proteolysis and catabolic process
Cathepsin B (CTSB) is essential in regulating cell death, inflammatory response and angiogenesis [57].
Kallikrein kinase 6 (KLK 6), carboxypeptidase A1 (CPA1) and carboxypeptidase B1 (CPB1) have not previously been tested in patients with cardiovascular diseases. Proteases are involved in the pathogenesis of cancer, inflammatory diseases, disorders of the musculoskeletal system, and immunogenesis [58].
Paraoxonase 3 (PON3) protects cells and tissues from oxidative stress by reducing reactive oxygen species [59] and abrogates the development of atherosclerosis [60]. Interestingly, among others this protein showed an association with improvement in exercise capacity of study patients.
The physiological role of bleomycin hydrolase (BLMH) is unknown. However, it has been suggested that BLM hydrolase may play a part in inflammation by regulating the secretion of chemokines [61].
Fatty acid binding protein 4 (FABP4) regulates glucose and lipid homeostasis as well as inflammation through its actions in adipocytes and macrophages. FABP4 has been shown to be expressed specifically within capillaries and small veins of various tissues. In vitro study has demonstrated that FABP4 is required for vascular endothelial growth factor signaling and cell proliferation, associated with angiogenesis [62].
Network analysis of biomarkers
The extended network analysis revealed eleven biomarkers that likely are involved in several biological processes such as cell adhesion and proteolysis and catabolic process (ANPEP and SPON1), cell adhesion and tissue remodeling (CASP3, TFF3 and IGFBP-2), proteolysis and catabolic process and immune and inflammatory response (KLK6 and PON3), immune and inflammatory response and tissue remodeling (SPP1 and TR-AP), immune and inflammatory response and cell adhesion (CNTN1 and ITGB2), Table 2, Fig. 4.