Fibroblast circSamd4 promotes cardiac fibrosis via activating plasminogen activator inhibitor-1

Cardiac fibrosis remains an unresolved clinical issue in patients with heart diseases. CircRNAs have emerged as potential targets for treatment of heart diseases. Exploring the functional circRNAs in fibroblast activation is one of the ways to develop innovative drugs for the treatment of cardiac fibrosis. This study aimed to screen for fibroblast-related circRNAs in cardiac fibrosis and elucidate their roles and underlying mechanisms. By screening for fibrosis-responsible circular RNAs (circRNAs), we identified a highly conserved circRNA, circular RNA Sterile alpha motif domain containing 4 (circSamd4), that drives cardiac fibrosis. circSamd4 is prominently expressed in cardiac fibroblasts (CFs) and is upregulated in the fibrotic hearts of humans and mice. Fibroblast-specific silencing of circSamd4 reduced cardiac fibroblast activation and alleviates cardiac fibrosis. Conversely, overexpression of circSamd4 in fibroblasts exacerbates cardiac fibrosis and rescues cardiac function. Bioinformatics and functional analyses revealed that circSamd4 regulates the plasminogen activation. Plasminogen activator inhibitor-1 (PAI-1, encoded by Serpine1) is a key effector of plasminogen activation and redox homeostasis and contributes to fibrotic diseases. Here, PAI-1 serves as a leading functional downstream factor of circSamd4 because PAI-1 is highly expressed in cardiac fibroblasts and contributes to circSamd4 functions in regulating fibroblast activation and cardiac fibrosis. Mechanistically, circSamd4 functions as a sponge for miR-1894-3p to trigger Serpine1 expression and subsequent fibroblast activation, and cardiac fibrosis. Therefore, we identified a fibroblast-specific circSamd4-miR-1894-3p-Serpine1 axis driving fibroblast activation and cardiac fibrosis. Adeno-associated virus (AAV)-mediated knockdown of circSamd4 or Serpine1 alleviated cardiac fibrosis and cardiac dysfunction. These findings suggest that circSamd4 and Serpine1 are promising therapeutic targets for inhibiting cardiac fibrosis.