Abstract
Introduction: Elevated circulating levels of solubilised urokinase plasminogen activator receptor (uPAR) predict adverse cardiovascular outcomes; however, its role in cardiac pathophysiology is unclear. uPAR is a cell-surface receptor involved in extracellular matrix remodelling, fibrinolysis, and integrin-mediated signalling in non-cardiac systems. This study examined (i) myocardial uptake of circulating soluble uPAR, (ii) the expression of uPAR in failing human heart tissue, and (iii) its capacity to interact with integrin subunits.
Methods: suPAR concentrations across organ sites were measured using suPARnostic ELISA (ViroGates) in patients undergoing clinically indicated cardiac catheterization (n=45) to evaluate organ-specific extraction. uPAR protein expression was analyzed by Western blotting in paired atrial and ventricular tissues from explanted failing human hearts (n=12) (Uvitec Nine-alliance). In-house polyclonal antibodies targeting distinct uPAR domains enabled the detection of full-length and cleaved isoforms. In vitro co-incubation of recombinant uPAR with candidate integrin heterodimers was performed at 37°C for 4 h and analyzed by SDS-PAGE.
Results: Regional blood sampling revealed reduced suPAR concentrations in the coronary sinus (p=0.01) and renal vein (p<0.0001) compared to their respective femoral arteries, suggesting renal and cardiac clearance/uptake of circulating uPAR. Immunoblotting confirmed uPAR expression in both atrial and ventricular tissues, with bands between ~35-55 kDa corresponding to full-length and truncated isoforms. In vitro binding assays demonstrated a direct interaction between uPAR and αVβ3 integrin, implicating a defined epitope of uPAR in integrin binding. In-house antibodies against this uPAR integrin-binding epitope confirmed its expression in human cardiac tissue, supporting its in situ expression.
Conclusion: We provide novel evidence that uPAR is expressed in failing human myocardium and contains integrin-binding domains in situ. The observed myocardial uptake of circulating suPAR, combined with the presence of integrin-binding isoforms, suggests that uPAR may participate in signal transduction pathways involved in cardiac regulation. Further investigations into uPAR expression in normal and diseased hearts, as well as the functionality of uPAR-integrin pathways, may reveal its role in heart failure.