Background: Apelin is a novel peptide which along with its receptor, APJ, mediates apelinergic signaling. Apelinergic signaling plays a critical role in cardiovascular homeostasis, including regulation of blood pressure. While exogenous apelin blocks Angiotensin II (AngII) mediated nuclear signaling, the role and regulation of endogenous apelin in hypertension (HTN) remains obscure. We hypothesize that apelinergic pathway is downregulated in HTN, which is primarily mediated by aberrant AngII signaling.
Approach: To test our hypothesis we utilized two mouse models of HTN, including AngII infusion and oral administration of N (ω)-nitro-l-arginine methyl ester (L-NAME). Blood pressure was monitored via noninvasive tail-cuff device. To determine the signaling involved we investigated the effect of AngII on apelinergic pathway in vitro.
Results: Cardiac apelin was decreased significantly in both murine models of HTN. Downregulated apelin also corresponded with increased deposition of collagen, and up-regulation of senescence markers including PAI-1. Meanwhile, APJ levels were unaffected in both these hypertensive models. In our in vitro studies AngII downregulated apelin expression in human aortic endothelial cells (HAECs) and human cardiac fibroblasts (HCFs). Furthermore, our studies in AngII infused mice and in HCFs highlight the role of TGF-β-pSMAD signaling, independent of MEK involvement, in AngII induced apelin downregulation.
Conclusion and Significance: Our studies demonstrate that aberrant AngII signaling downregulates apelin in HTN. This downregulation involves canonical Tgf-β1 signaling and affects apelin transcription. Importantly, we propose that AngII mediates its hypertensive pathology by decreasing apelinergic regulation. Since exogenous apelin blocks AngII signaling, further knowledge and negation of AngII induced apelin downregulation could result in the development of novel anti-hypertensive therapies.