Kandarp H. Shah

A Modern Understanding of the Traditional and Nontraditional Biological Functions of Angiotensin-Converting Enzyme

Angiotensin-converting enzyme (ACE) is a zinc-dependent peptidase responsible for converting angiotensin I into the vasoconstrictor angiotensin II. However, ACE is a relatively nonspecific peptidase that is capable of cleaving a wide range of substrates. Because of this, ACE and its peptide substrates and products affect many physiologic processes, including blood pressure control, hematopoiesis, reproduction, renal development, renal function, and the immune response. The defining feature of ACE is that it is composed of two homologous and independently catalytic domains, the result of an ancient gene duplication, and ACE-like genes are widely distributed in nature. The two ACE catalytic domains contribute to the wide substrate diversity of ACE and, by extension, the physiologic impact of the enzyme. Several studies suggest that the two catalytic domains have different biologic functions. Recently, the X-ray crystal structure of ACE has elucidated some of the structural differences between the two ACE domains. This is important now that ACE domain-specific inhibitors have been synthesized and characterized. Once widely available, these reagents will undoubtedly be powerful tools for probing the physiologic actions of each ACE domain. In turn, this knowledge should allow clinicians to envision new therapies for diseases not currently treated with ACE inhibitors.

Microglia Participate in Neurogenic Regulation of Hypertension

Hypertension is associated with neuroinflammation and increased sympathetic tone. Interference with neuroinflammation by an anti-inflammatory reagent or overexpression of interleukin-10 in the brain was found to attenuate hypertension. However, the cellular mechanism of neuroinflammation, as well as its impact on neurogenic regulation of blood pressure, is unclear. Here, we found that hypertension, induced by either angiotensin II or L-NG-nitro-l-arginine methyl ester, is accompanied by microglial activation as manifested by microgliosis and proinflammatory cytokine upregulation. Targeted depletion of microglia significantly attenuated neuroinflammation, glutamate receptor expression in the paraventricular nucleus, plasma vasopressin level, kidney norepinephrine concentration, and blood pressure. Furthermore, when microglia were preactivated and transferred into the brains of normotensive mice, there was a significantly prolonged pressor response to intracerebroventricular injection of angiotensin II, and inactivation of microglia eliminated these effects. These data demonstrate that microglia, the resident immune cells in the brain, are the major cellular factors in mediating neuroinflammation and modulating neuronal excitation, which contributes to the elevated blood pressure.

Renal Angiotensin-Converting Enzyme Is Essential for the Hypertension Induced by Nitric Oxide Synthesis Inhibition

The kidney is an important source of angiotensin-converting enzyme (ACE) in many species, including humans. However, the specific effects of local ACE on renal function and, by extension, BP control are not completely understood. We previously showed that mice lacking renal ACE, are resistant to the hypertension induced by angiotensin II infusion. Here, we examined the responses of these mice to the low-systemic angiotensin II hypertensive model of nitric oxide synthesis inhibition with L-NAME. In contrast to wild-type mice, mice without renal ACE did not develop hypertension, had lower renal angiotensin II levels, and enhanced natriuresis in response to L-NAME. During L-NAME treatment, the absence of renal ACE was associated with blunted GFR responses; greater reductions in abundance of proximal tubule Na(+)/H(+) exchanger 3, Na(+)/Pi co-transporter 2, phosphorylated Na(+)/K(+)/Cl(-) cotransporter, and phosphorylated Na(+)/Cl(-) cotransporter; and greater reductions in abundance and processing of the γ isoform of the epithelial Na(+) channel. In summary, the presence of ACE in renal tissue facilitates angiotensin II accumulation, GFR reductions, and changes in the expression levels and post-translational modification of sodium transporters that are obligatory for sodium retention and hypertension in response to nitric oxide synthesis inhibition.

Myeloid Suppressor Cells Accumulate and Regulate Blood Pressure in Hypertension

RATIONALE Chronic inflammation is a major contributor to the progressive pathology of hypertension, and T-cell activation is required for the genesis of hypertension. However, the precise role of myeloid cells in this process is unclear. OBJECTIVE To characterize and understand the role of peripheral myeloid cells in the development of hypertension. METHODS AND RESULTS We examined myeloid cells in the periphery of hypertensive mice and found that increased numbers of CD11b(+)Gr1(+) myeloid cells in blood and the spleen are a characteristic of 3 murine models of experimental hypertension (angiotensin II, L-NG-nitroarginine methyl ester, and high salt). These cells express surface markers and transcription factors associated with immaturity and immunosuppression. Also, they produce hydrogen peroxide to suppress T-cell activation. These are characteristics of myeloid-derived suppressor cells (MDSCs). Depletion of hypertensive MDSCs increased blood pressure and renal inflammation. In contrast, adoptive transfer of wild-type MDSCs to hypertensive mice reduced blood pressure, whereas the transfer of nicotinamide adenine dinucleotide phosphate oxidase 2-deficient MDSCs did not. CONCLUSION The accumulation of MDSCs is a characteristic of experimental models of hypertension. MDSCs limit inflammation and the increase of blood pressure through the production of hydrogen peroxide.

Angiotensin-converting enzyme overexpression in myelomonocytes prevents Alzheimer’s-like cognitive decline

Cognitive decline in patients with Alzheimers disease (AD) is associated with elevated brain levels of amyloid β protein (Aβ), particularly neurotoxic Aβ(1-42). Angiotensin-converting enzyme (ACE) can degrade Aβ(1-42), and ACE overexpression in myelomonocytic cells enhances their immune function. To examine the effect of targeted ACE overexpression on AD, we crossed ACE(10/10) mice, which overexpress ACE in myelomonocytes using the c-fms promoter, with the transgenic APP(SWE)/PS1(ΔE9) mouse model of AD (AD⁺). Evaluation of brain tissue from these AD⁺ACE(10/10) mice at 7 and 13 months revealed that levels of both soluble and insoluble brain Aβ(1-42) were reduced compared with those in AD⁺ mice. Furthermore, both plaque burden and astrogliosis were drastically reduced. Administration of the ACE inhibitor ramipril increased Aβ levels in AD⁺ACE(10/10) mice compared with the levels induced by the ACE-independent vasodilator hydralazine. Overall, AD⁺ACE(10/10) mice had less brain-infiltrating cells, consistent with reduced AD-associated pathology, though ACE-overexpressing macrophages were abundant around and engulfing Aβ plaques. At 11 and 12 months of age, the AD⁺ACE(10/WT) and AD⁺ACE(10/10) mice were virtually equivalent to non-AD mice in cognitive ability, as assessed by maze-based behavioral tests. Our data demonstrate that an enhanced immune response, coupled with increased myelomonocytic expression of catalytically active ACE, prevents cognitive decline in a murine model of AD.

Rediscovering ACE: Novel insights into the many roles of the angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) is best known for the catalytic conversion of angiotensin I to angiotensin II. However, the use of gene-targeting techniques has led to mouse models highlighting many other biochemical properties and actions of this enzyme. This review discusses recent studies examining the functional significance of ACE tissue-specific expression and the presence in ACE of two independent catalytic sites with distinct substrates and biological effects. It is these features which explain why ACE makes important contributions to many different physiological processes including renal development, blood pressure control, inflammation, and immunity.

Renal Generation of Angiotensin II and the Pathogenesis of Hypertension

The existence of a complete and functional renin-angiotensin system along the nephron is widely recognized. However, its precise role in blood pressure control and, by extension, hypertension is still uncertain. While most investigators agree that overexpressing RAS components along the nephron results in hypertension, two important issues remain: whether the local RAS works as a separate entity or represents an extension of the systemic RAS and whether locally generated angiotensin II has specific renal effects on blood pressure that are distinct from systemic angiotensin II. This review addresses these issues while emphasizing the unique role of local angiotensin II in the response of the kidney to hypertensive stimuli and the induction of hypertension.

Nontraditional Roles of Angiotensin-Converting Enzyme

Many articles have described the biochemistry of the renin-angiotensin system (RAS). Simply put, renin and angiotensin-converting enzyme (ACE) produce angiotensin II, which affects the brain (increased thirst), the gut (increased salt absorption), the adrenals (aldosterone production), the kidneys (salt and water retention), the heart (increased cardiac output), and vascular smooth muscle (vasoconstriction). Inhibitors of ACE and the angiotensin II type 1 receptor are now widely used to reduce blood pressure and to treat hypertension, heart failure, and renal disease.1 The effectiveness of these drugs and abundant data from animal models show that the RAS is the bodys central mechanism for regulating blood pressure. Modern studies of the RAS have substantially broadened the known roles of this system by showing that angiotensin II influences cellular proliferation and the inflammatory response. Although this wider interpretation of the RAS encompasses more than control of blood pressure, it still focuses on the effects of angiotensin II in the context of vascular injury. This increased understanding of the RAS inevitably leads to the question of whether, even now, we fully understand all of the physiological roles of the RAS. Here, we argue that the answer is no, that nature uses the components of the RAS and, in particular, ACE for a wide variety of physiological tasks. ### ACE Is Necessary for Renal Development and Normal Male Fertility In understanding the function of the RAS, a powerful tool is the ability to genetically alter mouse genes. Knockout studies of angiotensinogen, renin, ACE, or the angiotensin II type 1 receptor have confirmed the important role of these proteins in regulating blood pressure, but they also underline the wider physiological role for the RAS.2 For example, adult mice lacking ACE show marked abnormalities of renal development characterized by hypertrophy of small arteries, interstitial fibrosis, atrophy of the inner medulla and renal papilla, and dilation of calyces. Renal …

Increased Angiotensin II–Induced Hypertension and Inflammatory Cytokines in Mice Lacking Angiotensin-Converting Enzyme N Domain Activity

—Angiotensin-converting enzyme (ACE) is composed of the N- and C-terminal catalytic domains. To study the role of the ACE domains in the inflammatory response, N-knockout (KO) and C-KO mice, models lacking 1 of the 2 ACE domains, were analyzed during angiotensin II–induced hypertension. At 2 weeks, N-KO mice have systolic blood pressures that averaged 173±4.6 mm Hg, which is more than 25 mm Hg higher than the blood pressures observed in wild-type or C-KO mice (146±3.2 and 147±4.2 mm Hg). After 3 weeks, blood pressure differences between N-KO, C-KO, and wild-type were even more pronounced. Macrophages from N-KO mice have increased expression of tumor necrosis factor &agr; after stimulation with either lipopolysaccharide (about 4-fold) or angiotensin II (about 2-fold), as compared with C-KO or wild-type mice. Inhibition of the enzyme prolyl oligopeptidase, responsible for the formation of acetyl-SerAspLysPro and other peptides, eliminated the blood pressure difference and the difference in tumor necrosis factor &agr; expression between angiotensin II–treated N-KO and wild-type mice. However, this appears independent of acetyl-SerAspLysPro. These data establish significant differences in the inflammatory response as a function of ACE N- or C-domain catalytic activity. They also indicate a novel role of prolyl oligopeptidase in the cytokine regulation and in the blood pressure response to experimental hypertension.

The intrarenal generation of angiotensin II is required for experimental hypertension

Hypertension is a major risk factor for cardiovascular disease. While the cause of hypertension is multifactorial, renal dysregulation of salt and water excretion is a major factor. All components of the renin-angiotensin system are produced locally in the kidney, suggesting that intrarenal generation of angiotensin II plays a key role in blood pressure regulation. Here, we show that two mouse models lacking renal angiotensin converting enzyme (ACE) are protected against angiotensin II and l-NAME induced hypertension. In response to hypertensive stimuli, mice lacking renal ACE do not produce renal angiotensin II. These studies indicate that the intrarenal renin-angiotensin system works as an entity separate from systemic angiotensin II generation. Renal ACE appears necessary for experimental hypertension.