New Approaches Targeting the Renin-Angiotensin System: Inhibition of Brain Aminopeptidase A, ACE2 Ubiquitination, and Angiotensinogen

Abstract

Despite the availability of various therapeutic classes of antihypertensive drugs, hypertension remains poorly controlled, in part because of poor adherence. Hence, there is a need for the development of antihypertensive drugs acting on new targets to improve control of blood pressure. This review discusses novel insights (including the data of recent clinical trials) with regard to interference with the renin-angiotensin system, focusing on the enzymes aminopeptidase A and angiotensin-converting enzyme 2 (ACE2) in the brain, as well as the substrate of renin- angiotensinogen-in the liver. It raises the possibility that centrally acting amino peptidase A inhibitors (eg, firibastat), preventing the conversion of angiotensin II to angiotensin III in the brain, might be particularly useful in African Americans and patients with obesity. Firibastat additionally upregulates brain ACE2, allowing the conversion of angiotensin II to its protective metabolite angiotensin-(1-7). Furthermore, antisense oligonucleotides or small interfering ribonucleic acids suppress hepatic angiotensinogen for weeks to months after 1 injection and thus could potentially overcome adherence issues. Finally, interference with ACE2 ubiquitination is emerging as a future option for the treatment of neurogenic hypertension, given that ubiquitination resistance might upregulate ACE2 activity.

Figure 1.

Panel A. Pharmacological and structural properties of the aminopeptidase A (APA) inhibitors EC33 and NI929, and their prodrugs, RB150/firibastat, NI956 and QGC606.^{, ,} The S1 subsite is visualized in the 3D model of human APA after molecular docking of EC33 (orange). Molecular docking of NI929 (blue) in the 3D model of human APA is also shown. Panel B. Mode of action of the APA inhibitor prodrug, RB150/firibastat, on the control of blood pressure in hypertensive rats, modified from Marc et al. and Hmazzou et al. Angiotensin (Ang) III controls blood pressure via 1) an increase in the release of arginine-vasopressin from the posterior pituitary, 2) activation of sympathetic premotor neuron activity at the level of the rostral ventrolateral medulla (RVLM), and 3) baroreflex inhibition at the level of the solitary tract (NTS). APA converts Ang II into Ang III. Firibastat is composed of 2 EC33 molecules linked by a disulfide bridge. This allows passage of the blood-brain barrier, after which reductases cleave firibastat into 2 EC33 molecules. APA inhibition simultaneously activates ACE2. This enzyme converts Ang II into Ang-(1–7), which activates the Mas receptor, thereby potentially contributing to the blood pressure-lowering effect. APit, anterior pituitary; PPit, posterior pituitary; PVN, paraventricular nucleus; SFO, subfornical organ; SON, supraoptic nucleus. The panels have been generated based upon data in earlier references.^{, ,}

Figure 2.

E3 Ligases, DUB and other agents regulating ACE2 ubiquitination. ACE2 ubiquitination leads to internalization and degradation in lysosomes and the proteasome. Several E3 ligases, miRNA, LPS and vitamin C have been reported to promote ACE2 ubiquitination while deubiquitinases, Apelin 13, long non-coding RNA and MG132 oppose this post translational modification. With the exception of Nedd4-2, the role of the renin-angiotensin system in these processes has not been elucidated. Abbreviations: AT₁R, angiotensin II type 1 receptor; AT₂R, angiotensin II type 2 receptor; MasR, Mas receptor; Ub, ubiquitin; Vit C, vitamin C.

Figure 3.

ACE2 ubiquitination in neurogenic hypertension: a working model. ACE2–5R expression in the BNST blocks Ang II-mediated ACE2 ubiquitination by Nedd4-2, thus stabilizing ACE2 compensatory activity in GABAergic neurons and reinforcing the inhibitory tone to PVN glutamatergic neurons. Stimulation of GABA-A receptors in the PVN reduces the excitatory activity of these pre-sympathetic neurons, leading to a reduction of sympathetic drive to end organs, ultimately resulting in a reduction of blood pressure. Abbreviations: ACE2–5R, ubiquitination resistant ACE2; AT₁R, angiotensin II type 1 receptor; BNST, bed nucleus of the stria terminalis; GABA-AR, GABA-A receptor; PVN, paraventricular nucleus; RVLM, rostral ventrolateral medulla; Ub, ubiquitin.

Figure 4.

Overview of angiotensinogen (AGT) suppression using antisense oligonucleotides (ASO) or small interference RNA (siRNA). siRNAs, delivered as duplexes, enter the cell and are incorporated into RNAi silencing complex (RISC) in the cytoplasm. The RISC complex with the active guide strand binds the complementary sequence within the target mRNA, resulting in Argonaut 2–mediated cleavage and subsequent angiotensinogen mRNA degradation. Trivalent N-acetylgalactosamine (GalNAc)-conjugation allows high-affinity binding to the asialoglycoprotein receptor. This receptor is highly expressed on hepatocytes and binding results in rapid endocytosis. ASOs are single-stranded oligonucleotides that find their way alone. Liver-targeted angiotensinogen suppression by siRNA or ASO suppresses angiotensin II formation in blood, heart and kidney, thereby lowering blood pressure, and exerting renoprotection and cardioprotection in multiple hypertension/chronic kidney damage models, including the spontaneously hypertensive rat, the 5/6^th nephrectomy rat, the deoxycorticosterone acetate-salt rat and the diabetic TGR(mRen2)27 rat.