Oxytocin suppresses endothelial-to-mesenchymal transition to mitigate cardiac fibrosis via the eNOS/cGMP/PKG pathway.

Cardiac fibrosis is a major pathological feature that contributes to the development and progression of heart failure. Endothelial-to-mesenchymal transition (EndMT) is increasingly recognized as an important driver of cardiac fibrosis; however, its regulatory mechanisms remain incompletely understood. Oxytocin (OT) has been reported to exert cardioprotective effects, but whether it modulates EndMT and the underlying signaling pathways requires further investigation. To address this question, a rat model of isoproterenol (ISO)-induced cardiac fibrosis was established. Cardiac structure and function were assessed by echocardiography and histological staining. In vitro, human cardiac microvascular endothelial cells (hCMECs) were stimulated with TGF-β and IL-1β to induce EndMT. Molecular mechanisms were investigated using Western blotting, immunofluorescence, nitric oxide (NO) and cGMP assays, together with pharmacological inhibition and rescue experiments targeting the eNOS/cGMP/PKG signaling pathway. OT markedly attenuated ISO-induced cardiac fibrosis and ameliorated cardiac dysfunction in rats. Moreover, OT suppressed EndMT, as reflected by increased expression of endothelial markers together with reduced expression of mesenchymal markers. Mechanistically, OT enhanced eNOS phosphorylation, increased NO production, and activated downstream cGMP/PKG signaling. Notably, inhibition of cGMP signaling abolished the protective effects of OT, whereas activation of cGMP/PKG rescued EndMT suppression in NOS3-knockdown cells, demonstrating a functional interaction within this pathway. Collectively, these findings indicate that OT mitigates cardiac fibrosis, at least in part, through inhibition of EndMT mediated by activation of the eNOS/cGMP/PKG signaling pathway. This study provides new mechanistic insights into OT-mediated cardioprotection and highlights EndMT and the eNOS/cGMP/PKG axis as potential therapeutic targets for cardiac fibrosis.