The role of beta arrestin 1 in vascular tone regulation of pulmonary arteries

Abstract

Vascular tone regulation is facilitated by a complex interplay of various constricting and relaxing mechanisms. Excessive vasoconstriction is a prominent symptom of multiple vascular diseases and target of numerous pharmacotherapeutic treatments. Pulmonary arterial hypertension (PAH) is a severe pulmonary vascular disease characterized by excessive vasoconstriction of pulmonary arteries (PAs) and adverse vascular remodeling.^1,2 The NO-sGC signaling axis is an important vasorelaxant signaling pathway in tone regulation of PAs. Beta arrestins are well-known for their regulation of GPCR signaling as well as their function as scaffolds in numerous signaling pathways.³ The motivation of this work is to unravel the role of beta arrestins in vascular tone regulation in the lung. Two studies revealed that beta Arr2 modulates murine bronchorelaxation by desensitization of beta-adrenergic receptors in airway smooth muscle cells.^4,5 So far, little is known about the role of beta Arr1 in vascular tone regulation, especially in PAH. This work tries to fill this gap and characterizes the role of beta Arr1 in PA tone regulation and in PAH.<br /> Force measurement experiments in murine PA rings revealed an impaired vasorelaxation upon application of the NO donor SNP in the absence of beta Arr1. To unravel the mechanism of impaired NO-mediated vasorelaxation in beta Arr1-/- mice, relevant signaling molecules in the NO-sGC signaling pathway were tested. Inhibition of phosphodiesterase 5 (PDE5) by Sildenafil, as well as direct activation of the protein kinase G by 8-pCPT-cGMP and adenylyl cyclase by Forskolin revealed no difference in beta Arr1-/- PA vasorelaxation. A contribution of endothelial cells was excluded by removal of the PA endothelium. Conclusively, modulation of other relevant molecules of the NO-sGC signaling pathway remained unaffected suggesting a direct impact of the sGC enzyme on impaired vasorelaxation in absence of beta Arr1.<br /> Analysis of mRNA and protein expression revealed unaltered sGC levels despite absence of beta Arr1 in PAs indicating a functional defect rather than a downregulation of sGC. Measurements of cGMP production confirmed an impaired NO-dependent sGC activation in HEK293 cells lacking beta Arr1. The sGC enzyme contains a NO-binding heme group and removal of the heme group or oxidation of the ferrous ion (Fe2+) abolishes any NO-mediated activity.^6,7 Characterization of sGC heme states was performed in force measurement experiments by application of sGC modulators with different sGC activation mechanisms: application of the sGC stimulator BAY41-2271 indicates an impaired function of NO-sensitive ferrous (Fe2+) sGC in absence of beta Arr1. Administration of the sGC activator BAY58-2667 in the same setting suggested that heme-independent sGC remains unaffected. Therefore, this work reveals an impaired heme-dependent sGC activation in absence of beta Arr1. Immunoprecipitation experiments in HEK293 cells highlights a direct interaction between beta Arr1 and the sGC, as well as between beta Arr1 and the heme iron reductase Cyb5r3. Finally, beta Arr1-/- mice had increased right ventricular systolic pressure, pulmonary vascular remodeling and right heart hypertrophy already under normoxic conditions, which were further increased upon chronic hypoxia. These experiments indicated an important role of beta Arr1 for the prevention of pulmonary hypertension even under normoxic conditions in vivo.<br /> In summary, this work uncovered an important role of beta Arr1 in NO-dependent vasorelaxation of murine PAs. We provided evidence that beta Arr1 is relevant for heme-dependent sGC activation in PAs with physiological and pathophysiological relevance. In addition, significantly impaired sGC stimulability and beta Arr1-sGC interaction in HEK293 cells suggest a global beta Arr1 effect beyond PAs and thus expand current knowledge about the beta Arr1 functions.<br /> <br /> References<br /> 1. Thenappan, T., Ormiston, M. L., Ryan, J. J. & Archer, S. L. Pulmonary arterial hypertension: pathogenesis and clinical management. BMJ (Clinical research ed.) 360, j5492; 10.1136/bmj.j5492 (2018).<br /> 2. Prins, K. W. & Thenappan, T. World Health Organization Group I Pulmonary Hypertension: Epidemiology and Pathophysiology. Cardiology clinics 34, 363–374; 10.1016/j.ccl.2016.04.001 (2016).<br /> 3. DeWire, S. M., Ahn, S., Lefkowitz, R. J. & Shenoy, S. K. Beta-arrestins and cell signaling. Annual review of physiology 69, 483–510; 10.1146/annurev.physiol.69.022405.154749 (2007).<br /> 4. Pera, T. et al. 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