Angiotensin II receptors: localization of type I and type II in rat epididymides of different developmental stages

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.

Regulation of Male Fertility by the Renin-Angiotensin System

The renin-angiotensin system (RAS) is a peptidic system known mainly for its roles in the maintenance of blood pressure and electrolyte and fluid homeostasis. However, several tissues and cells have been described to possess an intrinsic RAS that acts locally through different paracrine and autocrine mechanisms. In the male reproductive system, several components of this system have been observed in various organs and tissues, such as the testes, spermatozoa and seminal fluid. Some functions attributed to this local RAS are maintenance of seminal plasma electrolytes, regulation of steroidogenesis and spermatogenesis, and sperm functions. However, their specific actions in these locations are not fully understood. Therefore, a deep knowledge of the functions of the RAS at both the testicular and seminal levels could clarify its roles in male infertility and sperm physiology, and the different RAS elements could be used to design tools enabling the diagnosis and/or treatment of male infertility.

Renin Angiotensin System, COVID-19 and Male Fertility: Any Risk for Conceiving?

The current knowledge concerning the connection between severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the renin-angiotensin system (RAS) system in the male reproductive apparatus is still limited, so dedicated studies are urgently required. Concerns about the male fertility consequences of SARS-CoV-2 infection have started to emerge, since epidemiologic studies observed that this coronavirus affects male patients more frequently and with increased severity, possibly because of the hormone-regulated expression of angiotensin-converting enzyme 2 (ACE2) receptor. A disturbance in fertility is also expected based on studies of the previous SARS-CoV infection, which targets the same ACE2 receptor when entering the host cells. In addition, bioinformatics analyses reveal the abundant expression of ACE2 receptor in the male reproductive tissues, particularly in the testis. It has been proposed that pharmacological intervention favoring the angiotensin-(1-7)/ACE2/Mas receptor pathway and increasing ACE2 expression and activity could greatly prevent inflammatory lesions in this area. Finally, in laboratories performing assisted reproductive technologies it is recommended that more attention should be paid not only to sperm quality but also to safety aspects. Data about the potential infectivity of seminal fluid are in fact conflicting and do not exclude risks for both personnel and patients. The potential infectivity of SARS-CoV-2 in reproductive male tissues should be strongly considered and further investigated for the proper management of in vitro fertilization procedures.

Seminal levels of angiotensin II and angiotensin II type 2 receptor expression on spermatozoa in varicocele patients: Relation to fertility status

Several theories were proposed to explain the pathophysiology of varicocele-related infertility seen in some patients. Our aim was to study the levels of angiotensin II in semen and angiotensin II type 2 receptor expression on spermatozoa in varicocele patients in relation to their fertility status and to evaluate the influence of varicocelectomy on their levels in infertile varicocele patients. Thirty fertile and 30 infertile varicocele patients and 30 healthy controls were subjected to measurement of reproductive hormones, semen analysis, measurement of seminal angiotensin II and evaluation of angiotensin II type 2 receptor expression on spermatozoa. Infertile varicocele patients underwent varicocelectomy and were re-evaluated for the same parameters after the operation. Sperm concentration, morphology, progressive motility, seminal angiotensin II and angiotensin II type 2 receptor expression were significantly lower in infertile varicocele patients compared with the other groups. Post-operative values showed significant increase in the studied parameters compared with the pre-operative values but not to other two groups. A significant positive correlation between angiotensin II type 2 receptor expression and progressive motility was detected in all studied groups. In conclusion, dysregulation of angiotensin II and angiotensin II type 2 receptor in varicocele patients may be involved in varicocele-related infertility.

Effect of angiotensin AT1 receptor antagonist on D-galactosamine-induced acute liver injury

1. Acute liver injury is a severe disease in which metabolic homeostasis is affected. The presence of liver cell death triggers a cascade of inflammatory responses leading to various degrees of liver damage. The pathophysiology of liver injury is complex, involving an interplay between parenchymal and non-parenchymal cells. 2. There is increasing evidence for a role of the local renin-angiotensin system (RAS) in liver cell death, inflammatory response and liver regeneration. It has been shown that the local RAS plays an important regulatory role in a variety of tissues. In experimental hepatic fibrogenesis, the angiotensin AT(1) receptor (AT(1)R) blocker losartan has been shown to be able to attenuate transforming growth factor-b1 activity and collagen gene expression. 3. In the present study, using a D-galactosamine (GalN)-induced liver failure rat, AT(1)R were localized around the centrilobular region, which was not evident in normal liver. Blood tests showed an elevation of total bilirubin and alanine aminotransferase. Furthermore, there was an increase in tissue-specific inhibitor of metalloproteinase (TIMP)-1 protein in the liver. Losartan treatment was able to reduce all these parameters. Levels of TIMP-1 protein were reduced by 1.5- and 1.56-fold on Days 1 and 3, respectively (both P < 0.05), in the losartan-treated group relative to the GalN-treated group. The survival rate of the losartan-treated group was significantly higher than that of the GalN-treated group (5 day survival 85 vs 42.5%, respectively; P < 0.05). 4. In conclusion, the AT(1)R blocker losartan suppresses GalN-induced liver injury. This may indicate that AT(1)R blockers may have therapeutic potential in the treatment of acute liver injury.

Role of the AT1A receptor in the CO2-induced stimulation of HCO3- reabsorption by renal proximal tubules

The proximal tubule (PT) is major site for the reabsorption of filtered HCO(3)(-). Previous work on the rabbit PT showed that 1) increases in basolateral (BL) CO(2) concentration ([CO(2)](BL)) raise the HCO(3)(-) reabsorption rate (J(HCO(3))), and 2) the increase that luminal angiotensin II (ANG II) produces in J(HCO(3)) is greatest at 0% [CO(2)](BL) and falls to nearly zero at 20%. Here, we investigate the role of angiotensin receptors in the [CO(2)](BL) dependence of J(HCO(3)) in isolated perfused PTs. We found that, in rabbit S2 PT segments, luminal 10(-8) M saralasin (peptide antagonist of ANG II receptors), lowers baseline J(HCO(3)) (5% CO(2)) to the value normally seen at 0% in the absence of inhibitors and eliminates the J(HCO(3)) response to changes in [CO(2)](BL). However, basolateral 10(-8) M saralasin has no effect. As with saralasin, luminal 10(-8) M candesartan (AT(1) antagonist) reduces baseline J(HCO(3)) and eliminates the [CO(2)](BL) dependence of J(HCO(3)). Luminal 10(-7) M PD 123319 (AT(2) antagonist) has no effect. Finally, we compared PTs from wild-type and AT(1A)-null mice of the same genetic background. Knocking out AT(1A) modestly lowers baseline J(HCO(3)) and, like luminal saralasin or candesartan in rabbits, eliminates the J(HCO(3)) response to changes in [CO(2)](BL). Our accumulated evidence suggests that ANG II endogenous to the PT binds to the apical AT(1A) receptor and that this interaction is critical for both baseline J(HCO(3)) and its response to changes in [CO(2)](BL). Neither apical AT(2) receptors nor basolateral ANG II receptors are involved in these processes.

Physiology of Local Renin-Angiotensin Systems

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

Vasectomy-dependent dysregulation of a local renin-angiotensin system in the epididymis of the cynomolgus monkey (Macaca fascicularis)

The mammalian epididymis is a fundamental organ for sperm cell maturation; it allows mammals to acquire their fertilizing ability. We have previously shown that during obstruction in cases of vasectomy, gene expression profiles were modified in human and cynomolgus monkey epididymides. Paracrine factors thus appear to be key elements in local gene expression along the epididymis. Local renin-angiotensin systems (RAS) have been described in many other organs as paracrine regulators of gene expression. This work demonstrates the presence of a local RAS in the epididymis of the cynomolgus monkey and investigates the vasectomy-dependent changes occurring in this system. After unilateral vasectomy in 4 monkeys (two for 3 days and two others for 7 days), the presence of two major components of the RAS (ie, angiotensinogen [ANG] and the type 1 receptor to angiotensin II [AT-I]) was evaluated in the vasectomized and the normal controlateral epididymides of each monkey. We also show by in situ hybridization that the principal cells of the epididymis express ANG and AT-I mRNAs and immunohistochemistry permitted to verify the co-localization of the AT-I protein and mRNA. Quantitative comparisons of individual variations in the mRNA and protein profiles for ANG and AT-I revealed that vasectomy altered the RAS expression profiles in an individual manner, thus confirming its role as a local system. This study provides a good basis for further investigation of the possible implications of the RAS in the physiology of the epididymis. Furthermore, the individual dependent modifications are in accordance with the very fluctuating results obtained in the fertility status of human patients undergoing a vasectomy reversal. The variations observed in the RAS expression profiles may be a good model to study the causes of the overall epididymal gene expression dysregulation that follows vasectomy and potentially affects fertility.

Synthesis and secretion of angiotensin II by the prostate gland in vitro

The renin angiotensin system has been shown to have tissue-related functions that are distinct from its systemic roles. We showed that angiotensin II type 1 (AT1) receptors are present in mammalian sperm, and angiotensin II stimulates sperm motility and capacitation. In addition, angiotensin II is present in human seminal plasma at concentrations higher than found in blood. In testing the possibility that the prostate may be the source of seminal plasma angiotensin II, mRNA coding for angiotensinogen, (pro)renin, and angiotensin-converting enzyme were identified by RT-PCR in rat and human prostate and in prostate LNCaP cells, as well as the angiotensin receptors types 1 and 2 (AT1 and AT2) in human tissues and AT1 in rat. In human tissue, immunocytochemistry showed cellular colocalization of renin with the AT1 receptor in secretory epithelial cells. Confirmation of the capacity of the prostate to secrete angiotensin II was shown by the detection of immunoreactive angiotensin in media removed from rat prostate organ cultures and LNCaP cells. Rat prostate angiotensin secretion was enhanced by dihydrotestosterone, but LNCaP angiotensin was stimulated by estradiol. This stimulation was blocked by tamoxifen. Rat prostate AT1 receptor expression was much greater in prepuberal than in postpuberal rats but was not affected by a low-sodium diet. It was, however, significantly enhanced by captopril pretreatment. These findings all suggest the independence of prostate and systemic renin angiotensin system regulation. The data presented here suggest that the prostate may be a source of the secreted angiotensin II found in seminal plasma.

Expression of platelet-derived growth factor (PDGF) in the epididymis and analysis of the epididymal development in PDGF-A, PDGF-B, and PDGF receptor beta deficient mice

The platelet-derived growth factor (PDGF) family of ligands and receptors play a pivotal role in the development of various organs. The critical importance of the PDGF-mediated signaling during embryonic development and adult physiology of the kidney and the common mesonephric origin of the epididymis and kidney prompted us to investigate the immunohistochemical localization of PDGF A- and B-chain and PDGF receptor (PDGFR) alpha- and beta-subunit in rat and mouse epididymis, the expression profiles of the corresponding mRNAs, and the consequences of a loss-of-function mutation at the PDGF-A, PDGF-B, and PDGFR-beta loci on mouse epididymis phenotypic appearance. Prenatally, PDGF-A and PDGFR-alpha immunohistochemical staining was seen in both species, whereas PDGF-B and PDGFR-beta were absent. The cellular localization of PDGF-A within the epithelium and the alpha-receptor in the mesenchyme in either mouse or rat before birth suggests that the PDGF-A/PDGFR-alpha system might be involved in the epididymal epithelial-mesenchymal interaction during the fetal period of life. Postnatally, PDGF A- and B-ligand and PDGFR alpha- and beta-subunit were confined in the epithelium. The identity of PDGF and PDGFR proteins were further confirmed by immunoblotting. In line with the immunohistochemical studies, PDGF-A and PDGFR-alpha mRNAs were seen by reverse transcription-polymerase chain reaction in rat and mouse tissue before birth, whereas PDGF-B and PDGFR-beta were almost not detectable. During the first days of life, PDGF-B and PDGFR-beta genes started to appear, and the overall trend in mRNA expression throughout postnatal development showed that the transcripts levels for PDGF-A, PDGF-B, PDGFR-beta, and PDGFR-alpha were constant with the only exception of a progressive decrease of PDGFR-alpha in adult rats. The PDGF-A null mutation strongly influenced the epididymal phenotype starting from puberty; only fetal PDGF-B and PDGFR-beta -/- mice were available, and no differences were seen in the epididymis of these animals, compared with wild-type littermates. Taken together, these data indicate that the PDGF system is highly expressed in the epididymis and suggest that PDGF could be involved in the maintenance of morphological structure and functional control of this organ.

Isoforms of angiotensin I-converting enzyme in the development and differentiation of human testis and epididymis

Angiotensin I-converting enzyme (ACE; CD143, Kininase II, EC 3.4.15.1) is known to be crucial for male fertility in animal models. We therefore studied its testicular (tACE) and somatic (sACE) isoforms in foetal and adult human testis and epididymis using monoclonal antibodies and cRNA probes. During spermatogenesis, tACE was found only in differentiating germ cells and was the only isoform within the seminiferous tubules of adult men. Although tACE mRNA was present in spermatocytes, tACE protein was initially found in post-meiotic step 3 spermatids and increased markedly during further differentiation. The enzyme was strictly confined to the adluminal membrane site of elongating spermatids and was localized at the neck and midpiece region of released and ejaculated spermatozoa. In contrast, sACE was expressed heterogeneously in Leydig cells and endothelial cells of the testicular interstitium, and homogeneously along the luminal surface of epithelial cells lining the ductuli efferents, corpus and cauda of epididymis, and vas deferens. The cell- and site-restricted pattern of sACE corresponded to that found in foetal tissues except an additional and transient expression of sACE in foetal germ cells and foetal Sertoli cells. Our study documents for the first time in humans the regulation and unique cellular distribution of ACE isoforms during the ontogenesis of the lower male genital tract.

Testicular hormonal regulation of the renin-angiotensin system in the rat epididymis

Evidence for the existence of an intrinsic angiotensin system based on locally formed angiotensinogen as a precursor for angiotensin production has been demonstrated in the rat epididymis. The data strongly support the presence of an epididymal renin-angiotensin system (RAS) which may be important for epididymal and sperm functions. In the present study, the effects of castration and testicular hormonal replacement on the expression of RAS components from the rat epididymis are investigated at the gene and protein levels. Results from northern blot and western blot analyses consistently showed that the expression of angiotensinogen mRNA and protein was apparently abolished by castration whereas their expression was completely restored to control levels when the castrated rats were hormonally replaced with either testosterone alone or with combined testosterone and estradiol. Northern blot did not detect any signal for angiotensinogen mRNA while western blot could detect a weak signal for angiotensinogen protein when the castrated rats were replaced with estradiol alone. Renin could be detected neither in control, castrated nor hormonally replaced rats. Moreover, the expression of angiotensin II receptor, type I (AT1) was almost abolished by castration as demonstrated by northern blot and reverse transcription-polymerase chain reaction. These data indicate that the expression of RAS by the rat epididymis at the levels of its precursor angiotensinogen and its receptor AT1, is subject to the regulation of testicular hormones and its expression appears to be predominantly testosterone-dependent.

Angiotensinogen expression by rat epididymis: evidence for an intrinsic, angiotensin-generating system

Previous studies have suggested the presence of an intrinsic renin-angiotensin system (RAS) in the rat epididymis with functions in epididymal activity and sperm maturation. In the present study, the localization and expression of angiotensinogen, the component of the RAS which is indispensable for intracellular angiotensin generation, were investigated by immunochemistry, hybridization histochemistry and by reverse-transcriptase polymerase chain reaction (RT-PCR). Western blot analysis of protein from the epididymis confirmed the presence of angiotensinogen with the expected molecular mass of about 60 kDa, in agreement with results from other tissues. Immunocytochemistry showed the regional localization of immunoreactivity for angiotensinogen in the rat epididymis. In situ hybridization histochemistry further demonstrated the expression of angiotensinogen mRNA by the epididymal epithelium in a region-specific manner along the length of the rat epididymis. RT-PCR confirmed that the rat epididymis expresses angiotensinogen mRNA. On the other hand, mRNA of renin was not detected in the rat epididymis using Northern blot and RT-PCR analyses. The present study strongly supports the existence of an intrinsic, angiotensin-generating system based on locally formed angiotensinogen as a precursor for angiotensin production. This epididymal RAS may have paracrine or autocrine roles in mediating the epididymal and sperm functions.

The AT2 receptor: fact, fancy and fantasy

The angiotensin AT2 receptor subtype was recently cloned and pharmacologically characterized but its function still remains elusive and controversial. It is a member of the G-protein coupled receptor superfamily with a minimal sequence homology with the AT1 receptor, responsible for the known effect of angiotensin II. The AT2 receptor displays a totally different signaling mechanisms from the AT1 receptor and involves various phosphatases. It is expressed at low density in adult tissues but up-regulated in pathological circumstances. Clearly, the AT2 receptor has antiproliferative properties and therefore opposes the growth promoting effect linked to the AT1 receptor stimulation. It is also reported that the AT2 receptor regulates ionic fluxes, affects differentiation and nerve regeneration, has anti-angiogenic and anti-fibrotic properties and stimulates apoptosis. However, the results, although suggestive, are sometimes equivocal. Obviously, the AT2 receptor plays a role in the pathogenesis and remodeling of cardiovascular and renal diseases. A more extensive knowledge of the AT2 receptor could therefore contribute to the understanding of the clincial beneficial effects of the AT1 receptor antagonists.

Angiotensin II: a reproductive hormone too?

It has long been known that angiotensin II (Ang II) can affect reproductive tissues such as the uterus. However, the existence of a local renin-angiotensin system (RAS) in female as well as male reproductive tissues is a relatively recent observation. Of great interest is the discovery that all components of the RAS are present in the ovary, that the ovary secretes components of the RAS into the bloodstream, and that the ovary itself is responsive to Ang II. Recent studies suggest that the primary role of Ang II in the ovary is to cause atresia in non-ovulatory follicles; however, there is also compelling data to suggest that Ang II facilitates ovulation. Male reproductive structures also contain all of the components of the RAS, gonadotropins regulate the activity of these components, and these tissues have Ang II receptors. Of great interest is the expression of testis-specific angiotensin-converting enzyme (ACE), which is located on germ cells. Recent studies using gene knock-out techniques indicate that testis ACE plays an important role in male fertility. However, the overall significance of the RAS for normal reproductive function remains questionable. There is now a body of evidence implicating the RAS in pathophysiologies associated with reproductive function, which gives rise to the possibility that drugs acting on the RAS might ameliorate some of these disorders. Considerable work remains to determine the role of Ang II in reproductive functions.

Differential gene expression of angiotensin II receptor subtypes in the epididymides of mature and immature rats

Previous studies have suggested that epididymal and sperm functions are subject to control by a local renin-angiotensin II system (RAS) in the rat epididymis. Type-1 angiotensin II receptor, AT1 and type-2 receptor, AT2 were localized in epididymal epithelium, indicating that RAS may act in a paracrine or autocrine fashion to regulate fluid secretion, probably through the basally placed membrane-bound AT1 protein as revealed by immunocytochemical and electrophysiological studies. In the present work, the expression of the angiotensin II receptor subtypes in the rat epididymis was showed by western blot analysis and reverse-transcription polymerase chain reaction (RT-PCR) using specific primers for the angiotensin II receptor subtypes. Western blot analysis showed the expression of AT1 receptor in the rat epididymis. Results from RT-PCR, using specific primers based on the corresponding angiotensin II receptor subtype genes for AT1a, AT1b and AT2 , demonstrated the differential expression of mRNAs from these receptor subtypes in the epididymides of mature and immature rats. Both the genes for AT1a and AT1b, but not that for AT2, are predominantly expressed in the epididymides of mature rat. In contrast, only AT1a and AT2 were highly expressed in the epididymides of immature rat. These results suggest that the expression of type-1 and type-2 angiotensin II receptor subtypes are developmentally regulated. Type-1 subtype may play a role in regulation of electrolyte and fluid transport in mature rat whereas type-2 subtype may be important in growth and development in the immature rat.

Immunohistochemical localization of angiotensin II in the mouse pancreas

Previous studies have suggested the presence of a tissue renin-angiotensin II system in the pancreas. These studies were based on the observation of several key components of the renin-angiotensin II system using molecular biological, biochemical and pharmacological approaches. In the present study, angiotensin II was localized immunohistochemically in the mouse pancreas using an indirect immunoperoxidase-staining technique. The results showed that angiotensin II-like immunoreactivity was localized predominantly in the endothelial cells of pancreatic blood vessels and the epithelial cells of pancreatic ducts from a subgroup of the vessels and ducts. Compared with those found in the pancreatic blood vessels and ductal system, a less pronounced immunoreactivity for angiotensin II was also observed in the acinar cells and in the smooth muscle layers overlying the pancreatic ducts as well as the blood vessels. However, no angiotensin II-like immunoreactivity was detected in the islet cells. Taken together with previous findings, the present results suggest a local angiotensin II-forming system in the mouse pancreas, which may be a significant autocrine or paracrine modulator of diverse pancreatic functions, including regulation of pancreatic blood flow and pancreatic anion secretion.