The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target.

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

Role of endothelial dysfunction in the severity of COVID­19 infection (Review).

COVID­19 patients with severe infection have been observed to have elevated auto­antibodies (AAs) against angiotensin II receptor type 1 (AT1R) and endothelin (ET) 1 receptor type A (ETAR), compared with healthy controls and patients with favorable (mild) infection. AT1R and ETAR are G protein­coupled receptors, located on vascular smooth muscle cells, fibroblasts, immune and endothelial cells, and are activated by angiotensin II (Ang II) and ET1 respectively. AAs that are specific for these receptors have a functional role similar to the natural ligands, but with a more prolonged vasoconstrictive effect. They also induce the production of fibroblast collagen, the release of reactive oxygen species and the secretion of proinflammatory cytokines (including IL­6, IL­8 and TNF­α) by immune cells. Despite the presence of AAs in severe COVID­19 infected patients, their contribution and implication in the severity of the disease is still not well understood and further studies are warranted. The present review described the major vascular homeostasis systems [ET and renin­angiotensin­aldosterone system (RAAS)], the vital regulative role of nitric oxide, the AAs, and finally the administration of angiotensin II receptor blockers (ARBs), so as to provide more insight into the interplay that exists among these components and their contribution to the severity, prognosis and possible treatment of COVID­19.

RAAS Blockade and COVID-19: Mechanistic Modeling of Mas and AT1 Receptor Occupancy as Indicators of Pro-Inflammatory and Anti-Inflammatory Balance.

ACE inhibitors (ACEis) and angiotensin receptor blockers (ARBs) are standard-of-care treatments for hypertension and diabetes, common comorbidities among hospitalized patients with coronavirus disease 2019 (COVID-19). Their use in the setting of COVID-19 has been heavily debated due to potential interactions with ACE2, an enzyme that links the pro-inflammatory and anti-inflammatory arms of the renin angiotensin system, but also the entryway by which severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) invades cells. ACE2 expression is altered by age, hypertension, diabetes, and the virus itself. This study integrated available information about the renin angiotensin aldosterone system (RAAS) and effects of SARS-CoV-2 and its comorbidities on ACE2 into a mechanistic mathematical model and aimed to quantitatively predict effects of ACEi/ARBs on the RAAS pro-inflammatory/anti-inflammatory balance. RAAS blockade prior to SARS-CoV-2 infection is predicted to increase the mas-AT1 receptor occupancy ratio up to 20-fold, indicating that in patients already taking an ACEi/ARB before infection, the anti-inflammatory arm is already elevated while the pro-inflammatory arm is suppressed. Predicted pro-inflammatory shifts in the mas-AT1 ratio due to ACE2 downregulation by SARS-CoV-2 were small relative to anti-inflammatory shifts induced by ACEi/ARB. Predicted effects of changes in ACE2 expression with comorbidities of diabetes, hypertension, or aging on mas-AT1 occupancy ratio were also relatively small. Last, predicted changes in the angiotensin (Ang(1-7)) production rate with ACEi/ARB therapy, comorbidities, or infection were all small relative to exogenous Ang(1-7) infusion rates shown experimentally to protect against acute lung injury, suggesting that any changes in the ACE2-Ang(1-7)-mas arm may not be large enough to play a major role in COVID-19 pathophysiology.

Functional Complexes of Angiotensin-Converting Enzyme 2 and Renin-Angiotensin System Receptors: Expression in Adult but Not Fetal Lung Tissue.

Angiotensin-converting enzyme 2 (ACE2) is a membrane peptidase and a component of the renin-angiotensin system (RAS) that has been found in cells of all organs, including the lungs. While ACE2 has been identified as the receptor for severe acute respiratory syndrome (SARS) coronaviruses, the mechanism underlying cell entry remains unknown. Human immunodeficiency virus infects target cells via CXC chemokine receptor 4 (CXCR4)-mediated endocytosis. Furthermore, CXCR4 interacts with dipeptidyl peptidase-4 (CD26/DPPIV), an enzyme that cleaves CXCL12/SDF-1, which is the chemokine that activates this receptor. By analogy, we hypothesized that ACE2 might also be capable of interactions with RAS-associated G-protein coupled receptors. Using resonance energy transfer and cAMP and mitogen-activated protein kinase signaling assays, we found that human ACE2 interacts with RAS-related receptors, namely the angiotensin II type 1 receptor (AT1R), the angiotensin II type 2 receptor (AT2R), and the MAS1 oncogene receptor (MasR). Although these interactions lead to minor alterations of signal transduction, ligand binding to AT1R and AT2R, but not to MasR, resulted in the upregulation of ACE2 cell surface expression. Proximity ligation assays performed in situ revealed macromolecular complexes containing ACE2 and AT1R, AT2R or MasR in adult but not fetal mouse lung tissue. These findings highlight the relevance of RAS in SARS-CoV-2 infection and the role of ACE2-containing complexes as potential therapeutic targets.

Comprehensive analysis of drugs to treat SARS­CoV­2 infection: Mechanistic insights into current COVID­19 therapies (Review).

The major impact produced by the severe acute respiratory syndrome coronavirus 2 (SARS­CoV­2) focused many researchers attention to find treatments that can suppress transmission or ameliorate the disease. Despite the very fast and large flow of scientific data on possible treatment solutions, none have yet demonstrated unequivocal clinical utility against coronavirus disease 2019 (COVID­19). This work represents an exhaustive and critical review of all available data on potential treatments for COVID­19, highlighting their mechanistic characteristics and the strategy development rationale. Drug repurposing, also known as drug repositioning, and target based methods are the most used strategies to advance therapeutic solutions into clinical practice. Current in silico, in vitro and in vivo evidence regarding proposed treatments are summarized providing strong support for future research efforts.

Withaferin A: a potential therapeutic agent against COVID-19 infection.

The outbreak and continued spread of the novel coronavirus disease 2019 (COVID-19) is a preeminent global health threat that has resulted in the infection of over 11.5 million people worldwide. In addition, the pandemic has claimed the lives of over 530,000 people worldwide. Age and the presence of underlying comorbid conditions have been found to be key determinants of patient mortality. One such comorbidity is the presence of an oncological malignancy, with cancer patients exhibiting an approximate two-fold increase in mortality rate. Due to a lack of data, no consensus has been reached about the best practices for the diagnosis and treatment of cancer patients. Interestingly, two independent research groups have discovered that Withaferin A (WFA), a steroidal lactone with anti-inflammatory and anti-tumorigenic properties, may bind to the viral spike (S-) protein of SARS-CoV-2. Further, preliminary data from our research group has demonstrated that WFA does not alter expression of ACE2 in the lungs of tumor-bearing female mice. Downregulation of ACE2 has recently been demonstrated to increase the severity of COVID-19. Therefore, WFA demonstrates real potential as a therapeutic agent to treat or prevent the spread of COVID-19 due to the reported interference in viral S-protein to host receptor binding and its lack of effect on ACE2 expression in the lungs.

SARS-CoV-2 as a Factor to Disbalance the Renin-Angiotensin System: A Suspect in the Case of Exacerbated IL-6 Production.

Fever in infections correlates with inflammation, macrophage infiltration into the affected organ, macrophage activation, and release of cytokines involved in immune response, hematopoiesis, and homeostatic processes. Angiotensin-converting enzyme 2 (ACE2) is the canonical cell surface receptor for SARS-CoV-2. ACE2 together with angiotensin receptor types 1 and 2 and ACE2 are components of the renin-angiotensin system (RAS). Exacerbated production of cytokines, mainly IL-6, points to macrophages as key to understand differential COVID-19 severity. SARS-CoV-2 may modulate macrophage-mediated inflammation events by altering the balance between angiotensin II, which activates angiotensin receptor types 1 and 2, and angiotensin 1-7 and alamandine, which activate MAS proto-oncogene and MAS-related D receptors, respectively. In addition to macrophages, lung cells express RAS components; also, some lung cells are able to produce IL-6. Addressing how SARS-CoV-2 unbalances RAS functionality via ACE2 will help design therapies to attenuate a COVID-19-related cytokine storm.

SARS-CoV-2 Infections and ACE2: Clinical Outcomes Linked With Increased Morbidity and Mortality in Individuals With Diabetes.

Individuals with diabetes suffering from coronavirus disease 2019 (COVID-19) exhibit increased morbidity and mortality compared with individuals without diabetes. In this Perspective, we critically evaluate and argue that this is due to a dysregulated renin-angiotensin system (RAS). Previously, we have shown that loss of angiotensin-I converting enzyme 2 (ACE2) promotes the ACE/angiotensin-II (Ang-II)/angiotensin type 1 receptor (AT1R) axis, a deleterious arm of RAS, unleashing its detrimental effects in diabetes. As suggested by the recent reports regarding the pathogenesis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), upon entry into the host, this virus binds to the extracellular domain of ACE2 in nasal, lung, and gut epithelial cells through its spike glycoprotein subunit S1. We put forth the hypothesis that during this process, reduced ACE2 could result in clinical deterioration in COVID-19 patients with diabetes via aggravating Ang-II-dependent pathways and partly driving not only lung but also bone marrow and gastrointestinal pathology. In addition to systemic RAS, the pathophysiological response of the local RAS within the intestinal epithelium involves mechanisms distinct from that of RAS in the lung; however, both lung and gut are impacted by diabetes-induced bone marrow dysfunction. Careful targeting of the systemic and tissue RAS may optimize clinical outcomes in subjects with diabetes infected with SARS-CoV-2.

The renin-angiotensin system: An integrated view of lung disease and coagulopathy in COVID-19 and therapeutic implications.

The renin-angiotensin system (RAS) has long been appreciated as a major regulator of blood pressure, but has more recently been recognized as a mechanism for modulating inflammation as well. While there has been concern in COVID-19 patients over the use of drugs that target this system, the RAS has not been explored fully as a druggable target. The abbreviated description of the RAS suggests that its dysregulation may be at the center of COVID-19.

JAK Inhibition as a New Treatment Strategy for Patients with COVID-19.

After the advent of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the outbreak of coronavirus disease 2019 (COVID-19) commenced across the world. Understanding the Immunopathogenesis of COVID-19 is essential for interrupting viral infectivity and preventing aberrant immune responses before a vaccine can be developed. In this review, we provide the latest insights into the roles of angiotensin-converting enzyme II (ACE2) and Ang II receptor-1 (AT1-R) in this disease. Novel therapeutic strategies, including recombinant ACE2, ACE inhibitors, AT1-R blockers, and Ang 1-7 peptides, may prevent or reduce viruses-induced pulmonary, cardiac, and renal injuries. However, more studies are needed to clarify the efficacy of these therapeutics. Furthermore, considering the common role of the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway in AT1-R expressed on peripheral tissues and cytokine receptors on the surface of immune cells, potential targeting of this pathway using JAK inhibitors (JAKinibs) is suggested as a promising approach in patients with COVID-19 who are admitted to hospitals. In addition to antiviral therapy, potential ACE2- and AT1-R-inhibiting strategies, and other supportive care, we suggest other potential JAKinibs and novel anti-inflammatory combination therapies that affect the JAK-STAT pathway in patients with COVID-19. Since the combination of MTX and baricitinib leads to outstanding clinical outcomes, the addition of baricitinib to MTX might be a potential strategy.