Secretome Analysis of Cardiomyocytes Identifies PCSK6 (Proprotein Convertase Subtilisin/Kexin Type 6) as a Novel Player in Cardiac Remodeling After Myocardial Infarction

BACKGROUND:Acute occlusion of a coronary artery results in swift tissue necrosis. Bordering areas of the infarcted myocardium may also experience impaired blood supply and reduced oxygen delivery leading to altered metabolic and mechanical processes. While transcriptional changes in hypoxic cardiomyocytes are well-studied, little is known about the proteins that are actively secreted from these cells. METHODS:We established a novel secretome analysis of cardiomyocytes by combining stable isotope labeling and click chemistry with subsequent mass spectrometry analysis. Further functional validation experiments included ELISA measurement of human samples, murine LAD ligation and adeno-associated virus (AAV) 9-mediated in vivo overexpression in mice. RESULTS:The presented approach is feasible for the analysis of the secretome of primary cardiomyocytes without serum starvation. 1026 proteins were identified to be secreted within 24 hours, indicating a 5-fold increase in detection compared to former approaches. Among them, a variety of proteins have so far not been explored in the context of cardiovascular pathologies.One of the most strongly upregulated secreted factors upon hypoxia was proprotein convertase subtilisin/kexin type 6 (PCSK6). Validation experiments revealed an increase of PCSK6 on mRNA and protein level in hypoxic cardiomyocytes. PCSK6 expression was elevated in hearts of mice following 3 days of ligation of the left anterior descending artery, a finding confirmed by immunohistochemistry. ELISA measurements in human serum also indicate distinct kinetics for PCSK6 in patients suffering from acute myocardial infarction, with a peak on day 3 post- infarction. Transfer of PCSK6-depleted cardiomyocyte secretome resulted in decreased expression of collagen I and III in fibroblasts compared to control treated cells, and siRNA mediated knockdown of PCSK6 in cardiomyocytes impacted transforming growth factor-² activation and mothers against decapentaplegic homolog 3 (SMAD3) translocation in fibroblasts. An adeno-associated virus (AAV) 9-mediated, cardiomyocyte-specific overexpression of PCSK6 in mice resulted in increased collagen expression and cardiac fibrosis as well as decreased left ventricular function after myocardial infarction. CONCLUSIONS:A novel mass spectrometry-based approach allows the investigation of the secretome of primary cardiomyocytes. Analysis of hypoxia-induced secretion led to the identification of PCSK6 to be crucially involved in