Evidence for Angiotensin II as a Naturally Existing Suppressor for the Guanylyl Cyclase A Receptor and Cyclic GMP Generation.

The natriuretic peptide system (NPS) and renin-angiotensin-aldosterone system (RAAS) function oppositely at multiple levels. While it has long been suspected that angiotensin II (ANGII) may directly suppress NPS activity, no clear evidence to date supports this notion. This study was designed to systematically investigate ANGII-NPS interaction in humans, in vivo, and in vitro. Circulating atrial, b-type, and c-type natriuretic peptides (ANP, BNP, CNP), cyclic guanosine monophosphate (cGMP), and ANGII were simultaneously investigated in 128 human subjects. Prompted hypothesis was validated in vivo to determine the influence of ANGII on ANP actions. The underlying mechanisms were further explored via in vitro approaches. In humans, ANGII demonstrated an inverse relationship with ANP, BNP, and cGMP. In regression models predicting cGMP, adding ANGII levels and the interaction term between ANGII and natriuretic peptides increased the predictive accuracy of the base models constructed with either ANP or BNP, but not CNP. Importantly, stratified correlation analysis further revealed a positive association between cGMP and ANP or BNP only in subjects with low, but not high, ANGII levels. In rats, co-infusion of ANGII even at a physiological dose attenuated cGMP generation mediated by ANP infusion. In vitro, we found the suppressive effect of ANGII on ANP-stimulated cGMP requires the presence of ANGII type-1 (AT1) receptor and mechanistically involves protein kinase C (PKC), as this suppression can be substantially rescued by either valsartan (AT1 blocker) or Go6983 (PKC inhibitor). Using surface plasmon resonance (SPR), we showed ANGII has low binding affinity to the guanylyl cyclase A (GC-A) receptor compared to ANP or BNP. Our study reveals ANGII is a natural suppressor for the cGMP-generating action of GC-A via AT1/PKC dependent manner and highlights the importance of dual-targeting RAAS and NPS in maximizing beneficial properties of natriuretic peptides in cardiovascular protection.

Does the Naked Emperor Parable Apply to Current Perceptions of the Contribution of Renin Angiotensin System Inhibition in Hypertension?

PURPOSE OF REVIEW: To address contemporary hypertension challenges, a critical reexamination of therapeutic accomplishments using angiotensin converting enzyme inhibitors and angiotensin II receptor blockers, and a greater appreciation of evidence-based shortcomings from randomized clinical trials are fundamental in accelerating future progress. RECENT FINDINGS: Medications targeting angiotensin II mechanism of action are essential for managing primary hypertension, type 2 diabetes, heart failure, and chronic kidney disease. While the ability of angiotensin converting enzyme inhibitors and angiotensin II receptor blockers to control blood pressure is undisputed, practitioners, hypertension specialists, and researchers hold low awareness of these drugs' limitations in preventing or reducing the risk of cardiovascular events. Biases in interpreting gained knowledge from data obtained in randomized clinical trials include a pervasive emphasis on using relative risk reduction over absolute risk reduction. Furthermore, recommendations for clinical practice in international hypertension guidelines fail to address the significance of a residual risk several orders of magnitude greater than the benefits. We analyze the limitations of the clinical trials that have led to current recommended treatment guidelines. We define and quantify the magnitude of the residual risk in published hypertension trials and explore how activation of alternate compensatory bioprocessing components within the renin angiotensin system bypass the ability of angiotensin converting enzyme inhibitors and angiotensin II receptor blockers to achieve a significant reduction in total and cardiovascular deaths. We complete this presentation by outlining the current incipient but promising potential of immunotherapy to block angiotensin II pathology alone or possibly in combination with other antihypertensive drugs. A full appreciation of the magnitude of the residual risk associated with current renin angiotensin system-based therapies constitutes a vital underpinning for seeking new molecular approaches to halt or even reverse the cardiovascular complications of primary hypertension and encourage investigating a new generation of ACE inhibitors and ARBs with increased capacity to reach the intracellular compartments at which Ang II can be generated.

Commentary on: Renin-angiotensin system overactivation in perivascular adipose tissue contributes to vascular dysfunction in heart failure.

We comment on the publication of a paper in which Brazilian investigators evaluate the anticontractile response of perivascular adipose tissue (PVAT) in experimental heart failure (HF) induced in rats by occlusion of a coronary artery.

Atrial appendage angiotensin-converting enzyme-2, aging and cardiac surgical patients: a platform for understanding aging-related coronavirus disease-2019 vulnerabilities.

PURPOSE OF REVIEW: Hospitalizations for COVID-19 dramatically increase with age. This is likely because of increases in fragility across biological repair systems and a weakened immune system, including loss of the cardiorenal protective arm of the renin--angiotensin system (RAS), composed of angiotensin-converting enzyme-2 (ACE2)/angiotensin-(1--7) [Ang-(1--7)] and its actions through the Mas receptor. The purpose of this review is to explore how cardiac ACE2 changes with age, cardiac diseases, comorbid conditions and pharmaceutical regimens in order to shed light on a potential hormonal unbalance facilitating SARs-CoV-2 vulnerabilities in older adults. RECENT FINDINGS: Increased ACE2 gene expression has been reported in human hearts with myocardial infarction, cardiac remodeling and heart failure. We also found ACE2 mRNA in atrial appendage tissue from cardiac surgical patients to be positively associated with age, elevated by certain comorbid conditions (e.g. COPD and previous stroke) and increased in conjunction with patients' chronic use of antithrombotic agents and thiazide diuretics but not drugs that block the renin--angiotensin system. SUMMARY: Cardiac ACE2 may have bifunctional roles in COVID-19 as ACE2 not only mediates cellular susceptibility to SARS-CoV-2 infection but also protects the heart via the ACE2/Ang-(1--7) pathway. Linking tissue ACE2 from cardiac surgery patients to their comorbid conditions and medical regimens provides a unique latform to address the influence that altered expression of the ACE2/Ang-(1-7)/Mas receptor axis might have on SARs-CoV-2 vulnerability in older adults.

Twenty years of progress in angiotensin converting enzyme 2 and its link to SARS-CoV-2 disease.

The virulence of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and the aggressive nature of the disease has transformed the universal pace of research in the desperate attempt to seek effective therapies to halt the morbidity and mortality of this pandemic. The rapid sequencing of the SARS-CoV-2 virus facilitated identification of the receptor for angiotensin converting enzyme 2 (ACE2) as the high affinity binding site that allows virus endocytosis. Parallel evidence that coronavirus disease 2019 (COVID-19) disease evolution shows greater lethality in patients with antecedent cardiovascular disease, diabetes, or even obesity questioned the potential unfavorable contribution of angiotensin converting enzyme (ACE) inhibitors or angiotensin II (Ang II) receptor blockers as facilitators of adverse outcomes due to the ability of these therapies to augment the transcription of Ace2 with consequent increase in protein formation and enzymatic activity. We review, here, the specific studies that support a role of these agents in altering the expression and activity of ACE2 and underscore that the robustness of the experimental data is associated with weak clinical long-term studies of the existence of a similar regulation of tissue or plasma ACE2 in human subjects.

Multifunctional Role of Chymase in Acute and Chronic Tissue Injury and Remodeling.

Chymase is the most efficient Ang II (angiotensin II)-forming enzyme in the human body and has been implicated in a wide variety of human diseases that also implicate its many other protease actions. Largely thought to be the product of mast cells, the identification of other cellular sources including cardiac fibroblasts and vascular endothelial cells demonstrates a more widely dispersed production and distribution system in various tissues. Furthermore, newly emerging evidence for its intracellular presence in cardiomyocytes and smooth muscle cells opens an entirely new compartment of chymase-mediated actions that were previously thought to be limited to the extracellular space. This review illustrates how these multiple chymase-mediated mechanisms of action can explain the residual risk in clinical trials of cardiovascular disease using conventional renin-angiotensin system blockade.

Differential Expression of the Angiotensin-(1-12)/Chymase Axis in Human Atrial Tissue.

BACKGROUND: Heart chymase rather than angiotensin (Ang)-converting enzyme has higher specificity for Ang I conversion into Ang II in humans. A new pathway for direct cardiac Ang II generation has been revealed through the demonstration that Ang-(1-12) is cleaved by chymase to generate Ang II directly. Herein, we address whether Ang-(1-12), chymase messenger RNA (mRNA), and activity levels can be differentiated in human atrial tissue from normal and diseased hearts and if these measures associate with various pathologic heart conditions. MATERIALS AND METHODS: Atrial appendages were collected from 11 nonfailing donor hearts and 111 patients undergoing heart surgery for the correction of valvular heart disease, resistant atrial fibrillation, or ischemic heart disease. Chymase mRNA was analyzed by real-time polymerase chain reaction and enzymatic activity by high-performance liquid chromatography using Ang-(1-12) as the substrate. Ang-(1-12) levels were determined by immunohistochemical staining. RESULTS: Chymase gene transcripts, chymase activity, and immunoreactive Ang-(1-12) expression levels were higher in left atrial tissue compared with right atrial tissue, irrespective of cardiac disease. In addition, left atrial chymase mRNA expression was significantly higher in stroke versus nonstroke patients and in cardiac surgery patients who had a history of postoperative atrial fibrillation versus nonatrial fibrillation. Correlation analysis showed that left atrial chymase mRNA was positively related to left atrial enlargement, as determined by echocardiography. CONCLUSIONS: As Ang-(1-12) expression and chymase gene transcripts and enzymatic activity levels were positively linked to left atrial size in patients with left ventricular heart disease, an important alternate Ang II forming pathway, via Ang-(1-12) and chymase, in maladaptive atrial and ventricular remodeling in humans is uncovered.

Is Sex a Determinant of COVID-19 Infection? Truth or Myth?

PURPOSE OF REVIEW: Angiotensin-converting enzyme 2 (ACE2), a specific high-affinity angiotensin II-hydrolytic enzyme, is the vector that facilitates cellular entry of SARS-CoV-1 and the novel SARS-CoV-2 coronavirus. SARS-CoV-2, which crossed species barriers to infect humans, is highly contagious and associated with high lethality due to multi-organ failure, mostly in older patients with other co-morbidities. RECENT FINDINGS: Accumulating clinical evidence demonstrates that the intensity of the infection and its complications are more prominent in men. It has been postulated that potential functional modulation of ACE2 by estrogen may explain the sex difference in morbidity and mortality. We review here the evidence regarding the role of estrogenic hormones in ACE2 expression and regulation, with the intent of bringing to the forefront potential mechanisms that may explain sex differences in SARS-CoV-2 infection and COVID-19 outcomes, assist in management of COVID-19, and uncover new therapeutic strategies.

Mast cell peptidases (carboxypeptidase A and chymase)-mediated hydrolysis of human angiotensin-(1-12) substrate.

Angiotensin processing peptidases (carboxypeptidase A (CPA) and chymase) are stored in cardiac mast cell (MC) secretory granules in large quantity and are co-released into the extracellular environment after activation/degranulation. In the human heart, chymase is primarily responsible for angiotensin II (Ang II) generation from the alternate substrate angiotensin-(1-12) (Ang-(1-12)). We investigated the individual and combined hydrolytic specificity of CPA and chymase enzymes (1:1 and 1:⅓ ratio) in the processing of the human Ang-(1-12) (hAng-(1-12)) substrate. To determine the Km and Vmax, the CPA and recombinant human chymase (rhChymase) enzymes were incubated with increasing concentrations of hAng-(1-12) substrate (0-300 µM). We found that CPA alone sequentially metabolized hAng-(1-12) substrate into angiotensin-(1-9) (Ang-(1-9), 53%), Ang II (22%) and angiotensin-(1-7) (Ang-(1-7), 11%) during a 15 min incubation. In the presence of rhChymase alone, 125I-hAng-(1-12) was directly metabolized into Ang II (89%) and no further hydrolysis of Ang II was detected. In the presence of both CPA + rhChymase enzymes (1:1 or 1:⅓ ratio), the amount of Ang II formation from 125I-hAng-(1-12) within a 5 min incubation period were 68% or 65%, respectively. In the presence of both (CPA + rhChymase), small amounts of Ang-(1-9) and Ang-(1-7) were generated from 125I-hAng-(1-12). The Km and Vmax values were 150 ±â€¯5 µM and 384 ±â€¯23 nM/min/mg of CPA and 40 ±â€¯9 µM and 116 ±â€¯20 nM/min/mg of rhChymase. The catalytic efficiency (Vmax/Km ratio) was higher for rhChymase/hAng-(1-12) compared to CPA/hAng-(1-12). Compared to CPA, chymase has a much higher affinity to hydrolyze the hAng-(1-12) substrate directly into Ang II. In addition, Ang II and Ang-(1-7) are the end products of chymase and CPA, respectively. Overall, our findings suggest that the Ang II generation from hAng-(1-12) is primarily mediated by chymase rather than CPA.

NLRP3 inhibition improves heart function in GPER knockout mice.

The molecular mechanisms of postmenopausal heart diseases in women may involve the loss of estrogen-deactivation of its membrane receptor, G-protein coupled estrogen receptor (GPER), and subsequent activation of the cardiac NLRP3 inflammasome, a component of the innate immune system. To study the potential effects of cardiac GPER on NLRP3-mediated inflammatory pathways, we characterized changes in innate immunity gene transcripts in hearts from 6-month-old cardiomyocyte-specific GPER knockout (KO) mice and their GPER-intact wild type (WT) littermates using RT2 Profiler™ real-time PCR array. GPER deletion in cardiomyocytes decreased %fractional shortening (%FS) and myocardial relaxation (e'), and increased the early mitral inflow filling velocity-to-early mitral annular descent velocity ratio (E/e'), determined by echocardiography, and increased the mRNA levels of atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP), determined by real-time PCR. Of the 84 inflammasome-related genes tested, 9 genes were upregulated, including NLRP3 and IL-18, while 1 gene, IL-12a, was downregulated in GPER KO when compared to WT. The importance of NLRP3 upregulation in GPER KO-induced heart failure was further confirmed by an in vivo study showing that, compared to vehicle-treated KO mice, 8 weeks of treatment with a NLRP3 inhibitor, MCC950 (10 mg/kg, i.p., 3 times per week), significantly limited hypertrophic remodeling, defined by reductions in heart weight/body weight, and improved systolic and diastolic functional indices, including increases in %FS and e', and decreases E/e' (P < 0.05). Both ANF and BNP mRNA levels were also significantly reduced by chronic MCC950 treatment. The findings from this study point toward a new understanding for the increased occurrence of heart diseases in women following loss or absence of estrogenic protection and GPER activation that involves cardiac NLRP3 inflammatory pathways.

G-Protein-Coupled Estrogen Receptor Agonist G1 Improves Diastolic Function and Attenuates Cardiac Renin-Angiotensin System Activation in Estrogen-Deficient Hypertensive Rats.

This study was aimed to clarify differences in how specific agonists of the 3 estrogen receptors (ERs) influence diastolic function and the renin-angiotensin system (RAS) after ovariectomy (OVX) in 24 female spontaneously hypertensive rat (SHR) undergoing bilateral OVX at 12 weeks of age. Eight weeks after surgery, rats were randomized (n = 6/group) to receive equipotent, daily treatments of one of the ER agonists (ERα agonist, propyl pyrazole trisphenol 94 µg/kg; ERß agonist, diarylpropionitrile 58 µg/kg; G-protein-coupled estrogen receptor [GPER] agonist, G1 100 µg/kg), or vehicle (peanut oil). After 4 weeks of treatment, left ventricular function/structure and systemic/intracardiac pressure measurements were obtained by echocardiography and a fluid-filled catheter attached to a pressure transducer, respectively. Selective ER agonist treatment with G1 or propyl pyrazole trisphenol led to improvements in diastolic function after estrogen loss when compared with vehicle-treated OVX rats. Although mean arterial blood pressure was not overtly different among groups, chronic G1, but not the other ER ligands, enhanced the in vitro vasorelaxant responsiveness to acetylcholine in aortic rings. These favorable effects of G1 were further linked to reductions in cardiac angiotensin-converting enzyme activity, AT1R protein expression, and Ang II immunoreactivity. Activation of ERß had no effect on cardiac function and did not alter components of the canonical cardiac RAS in comparison with vehicle-treated OVX SHR. These data imply that of the 3 ERs, GPER has a unique role in preserving diastolic function and favorably modulating the cardiac RAS independent of arterial pressure. Specifically, if GPER is pharmacologically activated, it could provide a therapeutic opportunity to limit the development and/or progression of diastolic dysfunction in hypertensive women after estrogen loss.

Blunting of estrogen modulation of cardiac cellular chymase/RAS activity and function in SHR.

The relatively low efficacy of ACE-inhibitors in the treatment of heart failure in women after estrogen loss may be due to their inability to reach the intracellular sites at which angiotensin (Ang) II is generated and/or the existence of cell-specific mechanisms in which ACE is not the essential processing pathway for Ang II formation. We compared the metabolic pathway for Ang II formation in freshly isolated myocytes (CMs) and non-myocytes (NCMs) in cardiac membranes extracted from hearts of gonadal-intact and ovariectomized (OVX) adult WKY and SHR rats. Plasma Ang II levels were higher in WKY vs. SHR (strain effect: WKY: 62 ± 6 pg/ml vs. SHR: 42 ± 9 pg/ml; p < 0.01), independent of OVX. The enzymatic activities of chymase, ACE, and ACE2 were higher in NCMs versus CMs, irrespective of whether assays were performed in cardiac membranes from WKY or SHR or in the presence or absence of OVX. E2 depletion increased chymase activity, but not ACE activity, in both CMs and NCMs. Moreover, cardiac myocyte chymase activity associated with diastolic function in WKYs and cardiac structure in SHRs while no relevant functional and structural relationships between the classic enzymatic pathway of Ang II formation by ACE or the counter-regulatory Ang-(1-7) forming path from Ang II via ACE2 were apparent. The significance of these novel findings is that targeted cell-specific chymase rather than ACE inhibition may have a greater benefit in the management of HF in women after menopause.

Cardiomyocyte-specific deletion of the G protein-coupled estrogen receptor (GPER) leads to left ventricular dysfunction and adverse remodeling: A sex-specific gene profiling analysis.

Activation of G protein-coupled estrogen receptor (GPER) by its agonist, G1, protects the heart from stressors such as pressure-overload, ischemia, a high-salt diet, estrogen loss, and aging, in various male and female animal models. Due to nonspecific effects of G1, the exact functions of cardiac GPER cannot be concluded from studies using systemic G1 administration. Moreover, global knockdown of GPER affects glucose homeostasis, blood pressure, and many other cardiovascular-related systems, thereby confounding interpretation of its direct cardiac actions. We generated a cardiomyocyte-specific GPER knockout (KO) mouse model to specifically investigate the functions of GPER in cardiomyocytes. Compared to wild type mice, cardiomyocyte-specific GPER KO mice exhibited adverse alterations in cardiac structure and impaired systolic and diastolic function, as measured by echocardiography. Gene deletion effects on left ventricular dimensions were more profound in male KO mice compared to female KO mice. Analysis of DNA microarray data from isolated cardiomyocytes of wild type and KO mice revealed sex-based differences in gene expression profiles affecting multiple transcriptional networks. Gene Set Enrichment Analysis (GSEA) revealed that mitochondrial genes are enriched in GPER KO females, whereas inflammatory response genes are enriched in GPER KO males, compared to their wild type counterparts of the same sex. The cardiomyocyte-specific GPER KO mouse model provides us with a powerful tool to study the functions of GPER in cardiomyocytes. The gene expression profiles of the GPER KO mice provide foundational information for further study of the mechanisms underlying sex-specific cardioprotection by GPER.

Role of mineralocorticoid receptor antagonists in cardiovascular disease.

Aldosterone exerts its best known sodium homeostasis actions by controlling sodium excretion at the level of the distal tubules via activation of the apical epithelial sodium channel and the basolateral Na(+)/K(+)ATPase pump. Recently, this mineralocorticoid hormone has been demonstrated to act on the heart and blood vessels. Excess release of aldosterone in relation to the salt status induces both genomic and nongenomic effects that by promoting endothelial dysfunction, and vascular and cardiorenal adverse remodeling, contribute to the target organ damage found in hypertension, heart failure, myocardial infarction, and chronic renal failure. Mineralocorticoid receptor blockers have been shown to be highly effective in resistant hypertension and to slow down heart failure progression, and in experimental animals, the development of atherosclerosis. Blockade of the action of aldosterone and potentially other mineralocorticoid steroids has been increasingly demonstrated to be an extremely beneficial therapy in different forms of cardiovascular disease. This review provides a summary of the knowledge that exists on aldosterone actions in the cardiovascular system and, in providing the translational impact of this knowledge to the clinical arena, illustrates how much more needs to be achieved in exploring the use of mineralocorticoid receptor blockers in less advanced stages of heart, renal, and vascular disease.

Renin angiotensin aldosterone inhibition in the treatment of cardiovascular disease.

A collective century of discoveries establishes the importance of the renin angiotensin aldosterone system in maintaining blood pressure, fluid volume and electrolyte homeostasis via autocrine, paracrine and endocrine signaling. While research continues to yield new functions of angiotensin II and angiotensin-(1-7), the gap between basic research and clinical application of these new findings is widening. As data accumulates on the efficacy of angiotensin converting enzyme inhibitors and angiotensin II receptor blockers as drugs of fundamental importance in the treatment of cardiovascular and renal disorders, it is becoming apparent that the achieved clinical benefits is suboptimal and surprisingly no different than what can be achieved with other therapeutic interventions. We discuss this issue and summarize new pathways and mechanisms effecting the synthesis and actions of angiotensin II. The presence of renin-independent non-canonical pathways for angiotensin II production are largely unaffected by agents inhibiting renin angiotensin system activity. Hence, new efforts should be directed to develop drugs that can effectively block the synthesis and/or action of intracellular angiotensin II. Improved drug penetration into cardiac or renal sites of disease, inhibiting chymase the primary angiotensin II forming enzyme in the human heart, and/or inhibiting angiotensinogen synthesis would all be more effective strategies to inhibit the system. Additionally, given the role of angiotensin II in the maintenance of renal homeostatic mechanisms, any new inhibitor should possess greater selectivity of targeting pathogenic angiotensin II signaling processes and thereby limit inappropriate inhibition.

Novel Cardiac Intracrine Mechanisms Based on Ang-(1-12)/Chymase Axis Require a Revision of Therapeutic Approaches in Human Heart Disease.

PURPOSE OF THE REVIEW: Drugs targeting the renin-angiotensin system (RAS), namely angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers, are the most commonly prescribed drugs for patients with or at risk for cardiovascular events. However, new treatment strategies aimed at mitigating the rise of the heart failure pandemic are warranted because clinical trials show that RAS blockers have limited benefits in halting disease progression. The main goal of this review is to put forward the concept of an intracrine RAS signaling through the novel angiotensin-(1-12)/chymase axis as the main source of deleterious angiotensin II (Ang II) in cardiac maladaptive remodeling leading to heart failure (HF). RECENT FINDINGS: Expanding traditional knowledge, Ang II can be produced in tissues independently from the circulatory renin-angiotensin system. In the heart, angiotensin-(1-12) [Ang-(1-12)], a recently discovered derivative of angiotensinogen, is a precursor of Ang II, and chymase rather than ACE is the main enzyme contributing to the direct production of Ang II from Ang-(1-12). The Ang-(1-12)/chymase axis is an independent intracrine pathway accounting for the trophic, contractile, and pro-arrhythmic Ang II actions in the human heart. Ang-(1-12) expression and chymase activity have been found elevated in the left atrial appendage of heart disease subjects, suggesting a pivotal role of this axis in the progression of HF. Recent meta-analysis of large clinical trials on the use of ACE inhibitors and angiotensin receptor blockers in cardiovascular disease has demonstrated an imbalance between patients that significantly benefit from these therapeutic agents and those that remain at risk for heart disease progression. Looking to find an explanation, detailed investigation on the RAS has unveiled a previously unrecognized complexity of substrates and enzymes in tissues ultimately associated with the production of Ang II that may explain the shortcomings of ACE inhibition and angiotensin receptor blockade. Discovery of the Ang-(1-12)/chymase axis in human hearts, capable of producing Ang II independently from the circulatory RAS, has led to the notion that a tissue-delimited RAS signaling in an intracrine fashion may account for the deleterious effects of Ang II in the heart, contributing to the transition from maladaptive cardiac remodeling to heart failure. Targeting intracellular RAS signaling may improve current therapies aimed at reducing the burden of heart failure.

Increased fibroblast chymase production mediates procollagen autophagic digestion in volume overload.

BACKGROUND: Previous work has identified mast cells as the major source of chymase largely associated with a profibrotic phenotype. We recently reported increased fibroblast autophagic procollagen degradation in a rat model of pure volume overload (VO). Here we demonstrate a connection between increased fibroblast chymase production and autophagic digestion of procollagen in the pure VO of aortocaval fistula (ACF) in the rat. METHODS AND RESULTS: Isolated LV fibroblasts taken from 4 and 12week ACF Sprague-Dawley rats have significant increases in chymase mRNA and chymase activity. Increased intracellular chymase protein is documented by immunocytochemistry in the ACF fibroblasts compared to cells obtained from age-matched sham rats. To implicate VO as a stimulus for chymase production, we show that isolated adult rat LV fibroblasts subjected to 24h of 20% cyclical stretch induces chymase mRNA and protein production. Exogenous chymase treatment of control isolated adult cardiac fibroblasts demonstrates that chymase is internalized through a dynamin-dependent mechanism. Chymase treatment leads to an increased formation of autophagic vacuoles, LC3-II production, autophagic flux, resulting in increased procollagen degradation. Chymase inhibitor treatment reduces cyclical stretch-induced autophagy in isolated cardiac fibroblasts, demonstrating chymase's role in autophagy induction. CONCLUSION: In a pure VO model, chymase produced in adult cardiac fibroblasts leads to autophagic degradation of newly synthesized intracellular procollagen I, suggesting a new role of chymase in extracellular matrix degradation.

Cardiac angiotensin-(1-12) expression and systemic hypertension in rats expressing the human angiotensinogen gene.

Angiotensin-(1-12) [ANG-(1-12)] is processed into ANG II by chymase in rodent and human heart tissue. Differences in the amino acid sequence of rat and human ANG-(1-12) render the human angiotensinogen (hAGT) protein refractory to cleavage by renin. We used transgenic rats harboring the hAGT gene [TGR(hAGT)L1623] to assess the non-renin-dependent effects of increased hAGT expression on heart function and arterial pressure. Compared with Sprague-Dawley (SD) control rats (n= 11), male homozygous TGR(hAGT)L1623 (n= 9) demonstrated sustained daytime and nighttime hypertension associated with no changes in heart rate but increased heart rate lability. Increased heart weight/tibial length ratio and echocardiographic indexes of cardiac hypertrophy were associated with modest reduction of systolic function in hAGT rats. Robust human ANG-(1-12) immunofluorescence within myocytes of TGR(hAGT)L1623 rats was associated with a fourfold increase in cardiac ANG II content. Chymase enzymatic activity, using the rat or human ANG-(1-12) as a substrate, was not different in the cardiac tissue of SD and hAGT rats. Since both cardiac angiotensin-converting enzyme (ACE) and ACE2 activities were not different among the two strains, the changes in cardiac structure and function, blood pressure, and left ventricular ANG II content might be a product of an increased cardiac expression of ANG II generated through a non-renin-dependent mechanism. The data also underscore the existence in the rat of alternate enzymes capable of acting on hAGT protein. Homozygous transgenic rats expressing the hAGT gene represent a novel tool to investigate the contribution of human relevant renin-independent cardiac ANG II formation and function.

Intracrine angiotensin II functions originate from noncanonical pathways in the human heart.

Although it is well-known that excess renin angiotensin system (RAS) activity contributes to the pathophysiology of cardiac and vascular disease, tissue-based expression of RAS genes has given rise to the possibility that intracellularly produced angiotensin II (Ang II) may be a critical contributor to disease processes. An extended form of angiotensin I (Ang I), the dodecapeptide angiotensin-(1-12) [Ang-(1-12)], that generates Ang II directly from chymase, particularly in the human heart, reinforces the possibility that an alternative noncanonical renin independent pathway for Ang II formation may be important in explaining the mechanisms by which the hormone contributes to adverse cardiac and vascular remodeling. This review summarizes the work that has been done in evaluating the functional significance of Ang-(1-12) and how this substrate generated from angiotensinogen by a yet to be identified enzyme enhances knowledge about Ang II pathological actions.