Changes in skin angiotensin II receptors in rats during wound healing

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.

Balance and circumstance: The renin angiotensin system in wound healing and fibrosis

The tissue renin angiotensin system (tRAS) is a locally-acting master-modulator of tissue homeostasis and regeneration. Through these abilities, it is emerging as an attractive target for therapies aiming to restore tissue homeostasis in conditions associated with disturbed wound healing. The tRAS can be divided into two axes - one being pro-inflammatory and pro-fibrotic and one being anti-inflammatory and anti-fibrotic. However, the division of the axes is fuzzy and imperfect as the axes are codependent and the outcome of tRAS activation is determined by the context. Although the tRAS is a local system it shares its key enzymes, ligands and receptors with the systemic RAS and is consequently also targeted by repurposing of drugs developed against the systemic RAS to manage hypertension. With a focus on the skin we will here discuss the tRAS, its involvement in physiological and pathological wound healing, and the therapeutic aptitude of its targeting to treat chronic wounds and fibrosis.

Early intervention by Captopril does not improve wound healing of partial thickness burn wounds in a rat model

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Inhibition of the Renin Angiotensin System does not influence the inflammatory reaction in the burn wounds.

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Inhibition of the Renin Angiotensin System early during burn wound healing does not improve the process.

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Inhibition of the Renin Angiotensin System early during burn wound healing does not reduce scar formation.

The Renin Angiotensin System is involved in fibrotic pathologies in various organs such as heart, kidney and liver. Inhibition of this system by angiotensin converting enzyme antagonists, such as Captopril, has been shown beneficial effects on these pathologies. Captopril reduced the inflammatory reaction but also directly influenced the fibrotic process.

Prolonged and excessive inflammatory response is a major cause of hypertrophic scar formation in burns. We therefore evaluated the effect of Captopril on the healing of partial thickness burn wounds in a rat model.

Partial thickness contact burns were inflicted on the dorsum of the rats. The rats received either systemic or local treatment with Captopril. The inflammatory reaction and wound healing (scar) parameters were investigated and compared to control animals.

In this study we could not detect positive effects of either administration route with Captopril on the inflammatory reaction, nor on wound healing parameters. The local treatment showed reduced wound closure in comparison to the systemic treatment and the control group.

Early Captopril treatment of burn wounds did not show the beneficial effects that were reported for fibrotic disorders in other tissues. To influence the fibrotic response Captopril treatment at a later time point, e.g. during the remodeling phase, might still have beneficial effects.

Angiotensin receptors in Dupuytren's tissue: Implications for the pharnnacological treatment of Dupuytren's disease

The role of Angiotensin II as a pro-fibrotic mediator has been established in models of cardiac, hepatic and renal fibrosis. The administration of Angiotensin-Converting Enzyme (ACE) – inhibitors to these models results in a reduction in the myofibroblast population and collagen synthesis. In rodent excisionat wound-healing experiments, an ACE inhibitor reduced the rate of wound contraction, collagen deposition and angiogenesis. Using immunohistochemistry, the presence of Angiotensin I receptors was identified within tissue samples from patients with Dupuytren's disease. These were found to be co-localised with areas of myofibrobtast expression. This co-localisation has implications for the potential of pharmacological regulation of Dupuytren's disease. Further research is necessary to confirm whether the use of ACE-inhibitors can modulate this disease process, which until now has not been responsive to safe, effective pharmacological treatment.

Mechanisms of Cachexia in Chronic Disease States

Sarcopenia and cachexia are muscle wasting syndromes associated with aging and with many chronic diseases, such as congestive heart failure (CHF), diabetes, cancer, chronic obstructive pulmonary disease and chronic kidney disease (CKD). While mechanisms are complex, these conditions are often accompanied by elevated angiotensin II (Ang II). Patients with advanced CHF or CKD often have increased Ang II levels and cachexia, and angiotensin-converting enzyme inhibitor treatment improves weight loss. It was found that Ang II infusion in rodents leads to skeletal muscle wasting. Ang II increases cytokines and circulating hormones, such as tumor necrosis factor-α, interleukin-6, serum amyloid-A and glucocorticoids, which regulate muscle protein synthesis and degradation. Ang II-induced muscle wasting is caused by alterations in insulin-like growth factor-1 signaling, enhanced muscle protein breakdown via the ubiquitin-proteasome system and decreased appetite resulting from the downregulation of hypothalamic orexigenic neuropeptides, such as Npy and orexin. Ang II also inhibits 5' adenosine monophosphate-activated protein kinase activity and disrupts normal energy balance via the activation of 5' adenosine monophosphate-activated protein kinase phosphatase PP2Cα. Furthermore, Ang II inhibits skeletal muscle stem (satellite) cell proliferation, leading to lowered muscle regenerative capacity. Distinct satellite cell angiotensin receptor subtypes have different effects on different stages of differentiation and are critical for the regulation of muscle regeneration. These data suggest that the renin-angiotensin system plays a critical role in mechanisms underlying cachexia in chronic disease states, and it is a promising target for the treatment of muscle atrophy in patients with diseases such as CHF and CKD.

NorLeu3-Angiotensin (1-7) [DSC127] as a Therapy for the Healing of Diabetic Foot Ulcers

Significance: Diabetes is a disorder that is well known to delay wound repair resulting in the formation of colonized chronic wounds. Over their lifetime, diabetic patients have a 25% incidence of foot ulcers (DFUs), which contribute to increased risk of morbidity, including osteomyelitis and amputations, and increased burden to the healthcare system. Recent Advances: The only active product approved for the treatment of diabetic ulcers, Regranex^®, is not widely used due to minimal proven efficacy and recent warnings added to the Instructions for Use. A novel topical agent that accelerates healing and increases the proportion of fully healed DFUs, DSC127 [aclerastide; active ingredient, NorLeu³-angiotensin (1-7) (NorLeu³-A(1-7))], is recruiting patients in Phase III clinical trials (NCT01830348 and NCT01849965). NorLeu³-A(1-7) is an analog of the naturally occurring peptide, angiotensin 1-7. The mechanisms of action include induction of progenitor proliferation, accelerated vascularization, collagen deposition, and re-epithelialization. Critical Issues: Current modalities for the treatment of DFUs include strict offloading, bandaging, debridement and, on a limited basis, application of Regranex. Novel potent therapies are needed to combat this significant burden to the diabetic patient and the healthcare system. Future Direction: Preclinical and clinical research shows that DSC127 is highly effective in the closure of diabetic wounds and is superior to Regranex in animal studies. Clinical development of DSC127 as a topical agent for the healing of DFU is underway. Further investigation into the mechanisms by which this product accelerates healing is warranted.

NorLeu3-A(1-7) stimulation of diabetic foot ulcer healing: results of a randomized, parallel-group, double-blind, placebo-controlled phase 2 clinical trial

This randomized, double-blind, placebo-controlled Phase 2 clinical trial explored NorLeu(3)-A(1-7) (DSC127) safety and healing efficacy in diabetic foot ulcers. Patients with chronic, noninfected, neuropathic, or neuroischemic plantar Wagner Grade 1 or 2 foot ulcers (n = 172) were screened for nonhealing. Subjects were randomized to receive 4 weeks' once-daily topical treatment with 0.03% DSC127 (n = 26), 0.01% DSC127 (n = 27), or Placebo (n = 24), followed by 20 weeks' standard of care. DSC127 was assessed for safety (including laboratory values and adverse events), primary efficacy (% ulcers completely epithelialized at Week 12), and durability of effect. Baseline, demography, and safety parameters were compared between intent-to-treat groups and were comparable. Dose-response curves for DSC127 effect on % area reduction from baseline at Week 12 (40% placebo; 67% 0.01% DSC127; 80% 0.03% DSC127) and 24 (23% placebo; 53% 0.01% DSC127; 95% 0.03% DSC127) followed a log-linear pattern for both intent-to-treat and per-protocol populations. Covariate analysis compared reduction in ulcer area, depth, and volume from baseline; reductions in the 0.03% DSC127 group were greater at Weeks 12 and 24. Placebo-treated ulcers healed in a median 22 weeks vs. 8.5 weeks for 0.03%DSC127 (p = 0.04). This study provides preliminary evidence that DSC127 is safe and effective in accelerating the healing of diabetic foot ulcers.

The Expression of MTUS1/ATIP and Its Major Isoforms, ATIP1 and ATIP3, in Human Prostate Cancer

Angiotensin II (Ang II), the main effector of the renin angiotensin system, acts upon two distinct transmembrane receptors, the Ang II type 1 and the type 2 (AT₂-) receptor, to induce promotion and inhibition of ERK2 phosphorylation. The AT₂-receptor, through an interaction with its putative signaling partner MTUS1/ATIP (AT₂-receptor interacting protein), inhibits the mitogenic effects of EGF in prostate cancer cell lines representing both early and late stage disease. This is the first report on the expression of ATIP in normal and malignant human prostatic biopsies. The expression of ATIP and its major isoforms, ATIP1 and ATIP3, in normal prostatic cells and three prostate cancer cell lines was examined using QPCR and immunohistochemistry. Human biopsies containing benign prostatic hyperplasia (BPH), high grade prostatic intraepithelial neoplasia (HGPIN) and well, moderately and poorly differentiated prostate cancer were also examined. Overall, ATIP1 and ATIP3 mRNA expression was increased in malignant compared to normal tissues and cell lines. ATIP immunostaining was low or absent in both the basal and columnar epithelial cell layers surrounding BPH acini; however, it was observed in high concentration in neoplastic epithelial cells of HGPIN and was clearly evident in cytoplasms of malignant cells in all prostate cancer grades. ATIP immunostaining was also identified in the cytoplasms of LNCaP and PC3 prostate cancer cells. As the AT₂-receptor/ATIP inhibitory signaling pathway exists in malignant cells in all grades of prostate cancer, enhancement of this pathway may be a therapeutic target even after the development of androgen-independence.

Accelerated healing of diabetic wounds by NorLeu(3)-angiotensin (1-7)

INTRODUCTION

Diabetes is a disorder that is well known to delay wound repair resulting in the formation of colonized, chronic wounds. The resultant ulcers contribute to increased risk of morbidity, including osteomyelitis and amputations, and increased burden to the healthcare system.

AREAS COVERED

The only active product approved for the treatment of diabetic ulcers, Regranex, has been shown to reduce amputation risk, but is not widely used due to minimal proven efficacy and recent warnings added to the Instructions for Use. This review provides an overview of the development of NorLeu(3)-angiotensin (1-7) (NorLeu(3)-A(1-7)) as an active agent for the treatment of diabetic wounds. NorLeu(3)-A(1-7) is an analog of the naturally occurring peptide, angiotensin 1-7. The mechanisms of action include induction of progenitor proliferation and accelerated vascularization, collagen deposition and re-epithelialization.

EXPERT OPINION

Research to date has shown that NorLeu(3)-A(1-7) is highly effective in the closure of diabetic wounds and is superior to Regranex in animal studies. Further clinical development of this product as a topical agent for the healing of chronic wounds and investigation into the mechanisms by which this product accelerates healing are warranted.

Role of the renin-angiotensin system in prostate cancer

Prostate cancer is highly prevalent in Western society, and its early stages can be controlled by androgen ablation therapy. However, the cancer eventually regresses to an androgen-independent state for which there is no effective treatment. The renin-angiotensin system (RAS), in particular the octapeptide angiotensin II, is now recognised to have important effects on growth factor signalling and cell growth in addition to its well known actions on blood pressure, fluid homeostasis and electrolyte balance. All components of the RAS have been recently identified in the prostate, consistent with the expression of a local RAS system in this tissue. This review focuses on the role of the RAS in the prostate, and the possibility that this pathway may be a potential therapeutic target for the treatment of prostate cancer and other prostatic diseases.

Angiotensin II enhances EGF receptor expression levels via ROS formation in HaCaT cells

BACKGROUND

Recent work has shown a novel function of angiotensin II (Ang II) in skin wound healing in which reactive oxygen species might be involved. As Ang II is known to increase superoxide production by activating NADPH oxidase in some non-phagocytic cells, we hypothesized that the produced superoxide by NADPH activation could contribute to the regulation of epidermal growth factor receptor (EGFR) in keratinocytes.

OBJECTIVE

We examined whether Ang II could generate superoxide and enhance EGFR expression levels in HaCaT cells.

METHODS

Superoxide formation was assessed by using hydroethidine. EGFR expression levels were examined by Western blotting.

RESULTS

Ang II (1-100 microM) increased the superoxide formation. Ang II (1-100 microM) resulted in a dose-dependent increase in cell proliferation in HaCaT cells. Heparin-binding epidermal growth factor activated the EGFR at 5-10 min. Although Ang II did not activate the EGFR, the expression levels of EGFR protein were increased in HaCaT cells treated with Ang II (1 microM) at 6h. Apocynin, a NADPH oxidase inhibitor, decreased the expression levels of EGFR. Xanthine/xanthine oxidase system, an exogenous superoxide generating system, enhanced the EGFR protein expression. Although Ang II did not affect the nitric oxide (NO) production, a NO synthase inhibitor N(omega)-nitro-l-arginine methyl ester suppressed the Ang II-induced EGFR expression levels in HaCaT cells. Thus, constitutive NO is required for the Ang II-induced EGFR expression in HaCaT cells.

CONCLUSION

These results suggest that Ang II enhances the cell proliferation and EGFR expression via superoxide production under the regulation of NO in HaCaT cells, implying that Ang II may regulate the proliferation, differentiation and tumorigenesis of the epidermis by harmonizing the superoxide and NO production.

Potential cellular and molecular causes of hypertrophic scar formation

A scar is an expected result of wound healing. However, in some individuals, and particularly in burn victims, the wound healing processes may lead to a fibrotic hypertrophic scar, which is raised, red, inflexible and responsible for serious functional and cosmetic problems. It seems that a wide array of subsequent processes are involved in hypertrophic scar formation, like an affected haemostasis, exaggerated inflammation, prolonged reepithelialization, overabundant extracellular matrix production, augmented neovascularization, atypical extracellular matrix remodeling and reduced apoptosis. Platelets, macrophages, T-lymphocytes, mast cells, Langerhans cells and keratinocytes are directly and indirectly involved in the activation of fibroblasts, which in turn produce excess extracellular matrix. Following the chronology of normal wound healing, we unravel, clarify and reorganize the complex molecular and cellular key processes that may be responsible for hypertrophic scars. It remains unclear whether these processes are a cause or a consequence of unusual scar tissue formation, but raising evidence exists that immunological responses early following wounding play an important role. Therefore, when developing preventive treatment modalities, one should aim to put the early affected wound healing processes back on track as quickly as possible.

Lessons from ONTARGET

The recently published results of the ONTARGET trial shed a new light on the cardiovascular protection of patients at high risk of a cardiovascular event. Despite a number of trials looking at the efficacy of Angiotensin Converting Enzyme inhibitors (ACEis) or Angiotensin Receptor Blockers (ARBs) in the prevention of cardiovascular events in patients with specific high risk profiles, the question of the equivalence of ACEis and ARBs remained unanswered. The ONTARGET trial has shown that telmisartan 80 mg administered for a median duration of 4.5 years to patients at high risk of developing a major cardiovascular event, is equally effective to ramipril 10 mg. In addition, telmisartan was slightly better tolerated. The comparator ramipril has been chosen as it is currently the gold standard ACEi since the results of the HOPE study, in terms of the composite outcome of cardiovascular death, myocardial infarction and stroke. Moreover, ONTARGET is the first trial to test the hypothesis of superiority of adding an ARB (telmisartan 80 mg) to an ACEi (ramipril 10 mg) over the ACEi ramipril monotherapy in cardiovascular protection of the same broad range of high-risk patients. Surprisingly, despite a more pronounced blood pressure lowering, the combination of the two agents did not lead to an additional decrease in the number of events, but had significantly more side-effects compared to ramipril monotherapy. ONTARGET is a landmark study, performed according to the highest statistical and clinical standards, providing compelling evidence and clear answers to two important clinical questions.

Angiotensin II Captopril Cotreatment Augments Angiogenesis in Abdominal Skin Flap in Rats

The effect of captopril, angiotensin-converting enzyme inhibitor, on angiogenesis in several reports remained unclear. Its effect on neovascularization in rat abdominal skin flaps was investigated. Flap elevation, based on the right superficial inferior epigastric artery was performed with or without the administration of captopril (10 mg/kg/d), Ang II (100 microg/kg/d), or captopril and Ang II cotreatment. Mean arterial pressure (MAP), microangiography, capillary density measurement, necrosis area determination, laser Doppler flowmetry (LDF), AT1 and vascular endothelial growth factor (VEGF) immunostaining were used to evaluate the effects of captopril and the interaction between captopril and Ang II on the angiogenesis. Ang II and captopril cotreatment improved angiogenesis more than Ang II or captopril alone. The reduction of necrosis, enhancement of vascular network formation, capillary density, VEGF immunostaining, and local blood flow were evident in the cotreated group. We suggest that Ang II and captopril cotreatment improves ischemia-induced angiogenesis and increased viability and vascularity of skin flap in rats.

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.

Angiotensin II regulates phosphoinositide 3 kinase/Akt cascade via a negative crosstalk between AT1 and AT2 receptors in skin fibroblasts of human hypertrophic scars

Angiotensin II (Ang II) stimulation has been shown to regulate proliferation of skin fibroblasts and production of extracellular matrix, which are very important process in skin wound healing and scarring; however, the signaling pathways involved in this process, especially in humans, are less explored. In the present study, we used skin fibroblasts of human hypertrophic scar, which expressed both AT1 and AT2 receptors, and observed that Ang II increased Akt phosphorylation and phosphoinositide 3 kinase (PI 3-K) activity. In addition, the Ang II-induced Akt phosphorylation was blocked by wortmannin, a PI 3-K inhibitor. This Ang II-activated PI 3-K/Akt cascade was markedly inhibited by valsartan, an AT(1) receptor-specific blocker, whereas it was enhanced by PD123319, an AT(2) receptor antagonist. On the other hand, the Ang II- or EGF-induced activation of PI 3-K/Akt was strongly attenuated by AG1478, an inhibitor of epidermal growth factor (EGF) receptor kinase. Moreover, Ang II stimulated tyrosine phosphorylation of EGF receptor and p85alpha subunit of PI 3-K accompanied by an increase in their association, which was inhibited by valsartan, and enhanced by PD123319. The Ang II-induced transactivation of EGF receptor resulted in activation of extracellular signal-regulated kinase (ERK) that was also inhibited by valsartan, and enhanced by PD123319. Taken together, our results showed that AT(1) receptor-mediated activation of PI 3-K/Akt cascades occurs at least partially via the transactivation of EGF receptor, which is under a negative control by AT(2) receptor in hypertrophic scar fibroblasts. These findings contribute to understanding the molecular mechanism of human hypertrophic scar formation.

A novel function of angiotensin II in skin wound healing. Induction of fibroblast and keratinocyte migration by angiotensin II via heparin-binding epidermal growth factor (EGF)-like growth factor-mediated EGF receptor transactivation

The role of angiotensin II (Ang II) in the control of systemic blood pressure and volume homeostasis is well known and has been extensively studied. Recently, Ang II was suggested to also have a function in skin wound healing. In the present study, the in vivo function of Ang II in skin wound healing was investigated using Ang II type 1 receptor (AT1R) knock-out mice. Wound healing in these mice was found to be markedly delayed. Keratinocytes and fibroblasts play important roles in wound healing, and thus the effect of Ang II on the migration of these cells was examined. Ang II stimulated keratinocyte and fibroblast migration in a dose-dependent manner. It has been reported that G protein-coupled receptor (GPCR) activation induces epidermal growth factor (EGF) receptor (EGFR) transactivation through the shedding of heparin-binding EGF-like growth factor (HB-EGF). As AT1R is a GPCR, it was hypothesized that Ang II-induced keratinocyte and fibroblast migration is mediated by EGFR transactivation. Ang II induced EGFR phosphorylation, which was inhibited by an AT1R antagonist, HB-EGF neutralizing antibody, and an HB-EGF antagonist in both keratinocytes and in fibroblasts. Moreover, Ang II-induced migration of keratinocytes and fibroblasts was also prevented by these inhibitors. Taken together, these findings clearly demonstrate, for the first time, that Ang II plays an important role in skin wound healing and that it functions by accelerating keratinocyte and fibroblast migration in a process mediated by HB-EGF shedding.

Differential expression of angiotensin receptors in human cutaneous wound healing

BACKGROUND

Angiotensin AT1 and AT2 receptors are expressed in human skin. Furthermore, AT2 receptors have been reported to be upregulated during tissue repair and remodelling in various noncutaneous human tissues.

OBJECTIVES

Detection of alterations in angiotensin II receptor expression during wound healing in human skin.

METHODS

Three models were employed. (i) Primary human keratinocytes were razor scraped in culture flasks and alterations in the expression of angiotensin receptor mRNA determined by semiquantitative reverse transcription-polymerase chain reaction for 1-12 h thereafter. (ii) Early wound healing (48 h after cutting) was studied in punch biopsies from human skin ex vivo by means of immunohistochemical staining using polyclonal antibodies against the AT1 or AT2 receptor. (iii) In vivo wound healing was studied in sections of human cutaneous scars by immunohistochemistry to determine receptor expression early (2 days) and late (2 weeks-3 months) after surgery.

RESULTS

In all experimental settings, an upregulation of both receptor subtypes was noticed after wounding. Immunohistochemically stained skin sections showed a stronger expression of AT2 than of AT1 receptors within the area of scarring. Enhanced receptor expression was detectable as early as 24 h after injury and lasted for up to 3 months.

CONCLUSIONS

From these data, we conclude that angiotensin AT1 and AT2 receptors are upregulated in human cutaneous wounds, giving further support to the concept that angiotensin II plays a role even at an early stage during cutaneous wound healing.

Dynamic expression of the angiotensin II type 2 receptor and duodenal mucosal alkaline secretion in the Sprague-Dawley rat

Activation of angiotensin II type 2 receptors (AT2R) has been shown to stimulate duodenal mucosal alkaline secretion (DMAS) in Sprague-Dawley rats (S-D). This finding could not be confirmed in another line of S-D, and the present study investigates whether the level of AT2R expression determines the response to the AT2R agonist CGP42112A. DMAS was measured in anaesthetized rats using in situ pH-stat titration. Real-time PCR and Western blot were used to assess AT1R and AT2R RNA and protein expression, respectively. CGP42112A (0.1 microg kg(-1)min(-1) I.V.) elicited a 45% net increase in DMAS in the previous S-D line studied, whereas no change occurred in the new S-D line. Luminal administration of prostaglandin E2 (10(-5) M) increased DMAS similarly in both S-D lines. AT2R protein expression was significantly higher in tissue from the previous line compared to the new line. Individual AT1R to AT2R ratios (RNA and protein) were significantly higher in the new line compared to the previous S-D line. In the new S-D line intravenous infusion of angiotensin II (Ang II; 10 microg kg(-1) h(-1)) over 120 min significantly lowered the duodenal AT1aR to AT2R RNA ratio. Prolonged Ang II infusion over 240 min increased AT2R protein expression and evoked a 42% stimulatory response in DMAS to CGP42112A. The level of local AT2R expression determines the effect of the AT2R agonist CGP42112A on rat duodenal mucosal alkaline secretion. AT2R expression should be confirmed before interpreting the experimental effects of pharmacological interferences with this receptor.

New selective AT2 receptor ligands encompassing a gamma-turn mimetic replacing the amino acid residues 4-5 of angiotensin II act as agonists

New benzodiazepine-based gamma-turn mimetics with one or two amino acid side chains were synthesized. The gamma-turn mimetics were incorporated into angiotensin II (Ang II) replacing the Val(3)-Tyr(4)-Ile(5) or Tyr(4)-Ile(5) peptide segments. All of the resulting pseudopeptides displayed high AT(2)/AT(1) receptor selectivity and exhibited AT(2) receptor affinity in the low nanomolar range. Molecular modeling was used to investigate whether the compounds binding to the AT(2) receptor could position important structural elements in common areas. A previously described benzodiazepine-based gamma-turn mimetic with high affinity for the AT(2) receptor was also included in the modeling. It was found that the molecules, although being structurally quite different, could adopt the same binding mode/interaction pattern in agreement with the model hypothesis. The pseudopeptides selected for agonist studies were shown to act as AT(2) receptor agonists being able to induce outgrowth of neurite cells, stimulate p42/p44(mapk), and suppress proliferation of PC12 cells.

Neuroprotection with angiotensin receptor antagonists: a review of the evidence and potential mechanisms

The peptide hormone angiotensin (A)-II, the major effector peptide of the renin-angiotensin system (RAS), is well established to play a pivotal role in the systemic regulation of blood pressure, fluid, and electrolyte homeostasis. Recent biochemical and neurophysiologic studies have documented local intrinsic angiotensin-generating systems in organs and tissues such as the brain, retina, bone marrow, liver, and pancreas. The locally generated angiotensin peptides have multiple and novel actions including stimulating cell growth and anti-proliferative and/or antiapoptotic actions. In the mammalian brain, all components of the RAS are present including angiotensin receptor subtypes 1 (AT(1)) and 2 (AT(2)). A-II exerts most of its well defined physiologic and pathophysiologic actions, including those on the central and peripheral nervous system, through its AT(1) receptor subtype. While the AT(1) receptor is responsible for the classical effects of A-II, it has been found that the AT(2) receptor is linked to totally different signalling mechanisms and this has revealed hitherto unknown functions of A-II. AT(2) receptors are expressed at low density in many healthy adult tissues, but are upregulated in a variety of human diseases. This receptor not only contributes to stroke-related pathologic mechanisms (e.g. hypertension, atherothrombosis, and cardiac hypertrophy) but may also be involved in post-ischemic damage to the brain. It has been reported that the AT(2) receptor regulates several functions of nerve cells, e.g. ionic fluxes, cell differentiation, and neuronal tissue regeneration, and also modulates programmed cell death. In this article, we review the experimental evidence supporting the notion that blockade of brain AT(1) receptors can be beneficial with respect to stroke incidence and outcome. We further delineate how AT(2) receptors could be involved in neuronal regeneration following brain injury such as stroke or CNS trauma. The current review is focussed on some of the new functions arising from the locally formed A-II with particular attention to its emerging neuroprotective role in the brain.

Effects of angiotensin II receptor signaling during skin wound healing

The tissue angiotensin (Ang) system, which acts independently of the circulating renin Ang system, is supposed to play an important role in tissue repair in the heart and kidney. In the skin, the role of the system for wound healing has remained to be ascertained. Our study demonstrated that oral administration of selective AngII type-1 receptor (AT(1)) blocker suppressed keratinocyte re-epithelization and angiogenesis during skin wound healing in rats. Immunoprecipitation and Western blot analysis indicated the existence of AT(1) and AngII type-2 receptor (AT(2)) in cultured keratinocytes and myofibroblasts. In a bromodeoxyuridine incorporation study, induction of AT(1) signaling enhanced the incorporation into keratinocytes and myofibroblasts. Wound healing migration assays revealed that induction of AT(1) signaling accelerated keratinocyte re-epithelization and myofibroblasts recovering. In these experiments, induction of AT(2) signaling acted vice versa. Taken together, our study suggests that skin wound healing is regulated by balance of opposing signals between AT(1) and AT(2).

Are angiotensin receptor blockers neuroprotective?

Stroke is one of the leading causes of invalidism and death in the industrialized world. Among others, the renin- angiotensin system (RAS) has been implicated in the pathogenesis and outcome of ischemic events, including stroke. Angiotensin II (Ang II), the major effector peptide of the RAS, exerts most of its well-defined physiologic and pathophysiologic actions, including those on the central and peripheral nervous system, through its Ang II type 1 (AT1) receptor subtype. This receptor not only contributes to stroke-related pathologic mechanisms (eg, hypertension, atherothrombosis, and cardiac hypertrophy) but also may be involved in postischemic damage to the brain. However, it has also been demonstrated that Ang II, via its AT2 receptor subtype, accelerates neuronal tissue regeneration after injury. In this article, we review the experimental evidence supporting the notion that blockade of brain AT1 receptors can be beneficial with respect to stroke incidence and outcome. We further delineate how AT2 receptors could be involved in neuronal regeneration following brain injury, such as stroke. In doing so, we also attempt to shed some light on the mechanisms by which AT1 receptor blockers, which leave the AT2 receptor unopposed, might exert protective actions in brain ischemia.

A selective AT2 receptor ligand with a gamma-turn-like mimetic replacing the amino acid residues 4-5 of angiotensin II

Three angiotensin II (Ang II) analogues encompassing a benzodiazepine-based gamma-turn-like scaffold have been synthesized. Evaluation of the compounds in a radioligand binding assay showed that they had no affinity to the rat liver AT(1) receptor. However, one of the compounds displayed considerable affinity to the pig uterus AT(2) receptor (K(i) = 3.0 nM) while the other two lacked affinity to this receptor. It was hypothesized that the reason for the inactivity of one of these analogues to the AT(2) receptor was that the guanidino group of the Arg(2) residue and/or the N-terminal end of the pseudopeptide could not interact optimally with the receptor. To investigate this hypothesis, a conformational analysis was performed and a comparison was carried out with the monocyclic methylenedithioether analogue cyclo(S-CH(2)-S)[Cys(3,5)]Ang II which is known to bind with high affinity to the AT(2) receptor (K(i) = 0.62 nM). This comparison showed that, in the compounds with high AT(2) receptor affinity, the guanidino group of the Arg(2) residue and the N-terminal end could access common regions of space that were not accessible to the inactive compound. To examine the importance of the guanidino group for binding, the Arg side chain was removed by substituting Arg(2) for Ala(2) in the analogue having the high affinity. This analogue lacked affinity to AT(2) receptors, which supports the role of the guanidino group in receptor binding.

Regulation of collagen synthesis in mouse skin fibroblasts by distinct angiotensin II receptor subtypes

We examined the possibility of whether angiotensin (Ang) II type 1 (AT1) and type 2 (AT2) receptor stimulation differentially regulates collagen production in mouse skin fibroblasts. Both AT1 and AT2 receptors were expressed in neonatal skin fibroblasts prepared from wild-type mice to a similar degree, and the AT1a receptor was exclusively expressed as opposed to the AT1b receptor. In wild-type fibroblasts, Ang II increased collagen synthesis accompanied by an increase in expression of tissue inhibitor of metalloproteinase (TIMP)-1, and these increases were inhibited by valsartan, an AT1 receptor blocker, but augmented by PD123319, an AT2 receptor antagonist. Ang II decreased basal and IGF-I-induced collagen production and inhibited TIMP-1 expression in neonatal skin fibroblasts prepared from AT1a knockout (KO) mice. These Ang II-mediated inhibitory effects on collagen production and TIMP-1 expression observed in AT1a KO fibroblasts were attenuated by the addition of PD123319 or a tyrosine phosphatase inhibitor, sodium orthovanadate, but not affected by a serine/threonine phosphatase inhibitor, okadaic acid. Moreover, we demonstrated that transfection of a catalytically inactive, dominant negative SHP-1 (Src homology 2-containing protein-tyrosine phosphatase-1) mutant inhibited the Ang II-mediated inhibitory effect on both collagen synthesis and TIMP-1 expression in AT1a KO fibroblasts. These results suggest that AT1a receptor stimulation increases collagen production in skin fibroblasts at least in part due to the inhibition of collagen degradation via the increase in TIMP-1 expression, whereas AT2 receptor stimulation exerts inhibitory effects on TIMP-1 expression, which is mediated at least partially by the activation of SHP-1, thereby possibly inhibiting collagen production.

Angiotensin AT2 receptors: cardiovascular hope or hype?

British Journal of Pharmacology (2003) 140, 809–824. doi:10.1038/sj.bjp.0705448

Can angiotensin-converting enzyme inhibitors reverse atherosclerosis?

Angiotensin II, a potent vasoconstrictor, is mainly present in the vascular endothelium. Multiple studies have confirmed that angiotensin-converting enzyme (ACE) inhibitors, which block the formation of angiotensin II, lower blood pressure and also improve heart failure. These agents not only have beneficial hemodynamic effects but also bestow additional benefits on vascular function and prevent clinical cardiovascular events in patients at risk for coronary artery disease. These latter benefits may represent effects of ACE inhibitors on local endocrine pathways, inflammatory processes, and atherosclerosis taking place within the arterial wall. Current evidence suggests that, although ACE inhibitors may not substantially reverse atherosclerotic plaque already present, they may slow the progression of such atherosclerotic lesions. In addition, by modulating inflammatory pathways within and adjacent to the atherosclerotic lesion, they may stabilize an unstable plaque and therefore decrease the risk of plaque rupture and its complications.

The role of the renin-angiotensin system in the development of cardiovascular disease

A direct, continuous, and independent relation between blood pressure and the incidence of various cardiovascular events, such as stroke and myocardial infarction, is now well accepted. The increase in risk can be attributed to structural and functional changes in target organs. Central to many of these pathophysiologic processes is the renin-angiotensin system (RAS), specifically, angiotensin II. Binding of angiotensin II to angiotensin II type-1 (AT(1)) receptors produces acute vasoconstriction, leading to an increase in blood pressure. AT(1) receptor activation also contributes independently to chronic disease pathology by promoting vascular growth and proliferation, and endothelial dysfunction. These negative consequences of angiotensin II are partly counteracted by angiotensin II type-2 (AT(2)) receptor stimulation, which has favorable effects on tissue growth and repair processes. Thus, the use of selective AT(1) receptor antagonists in the treatment of hypertension has a 2-fold rationale: (1) selective AT(1) receptor blockade targets the final common pathway for all major detrimental cardiovascular actions of angiotensin II, and (2) circulating angiotensin II levels (which increase during AT(1) receptor antagonist treatment) will be free to act only at unopposed AT(2) receptors, potentially providing additional end-organ protection. Angiotensin-converting enzyme (ACE) inhibitors interrupt the RAS by preventing the conversion of angiotensin I to angiotensin II. They also increase plasma levels of bradykinin, which possesses vasodilatory and tissue-protective properties. The combination of an AT(1) receptor antagonist with an ACE inhibitor represents an appealing therapeutic strategy, because it should produce more complete blockade of the RAS, while preserving the beneficial effects mediated by AT(2) receptor stimulation and increased bradykinin levels.

Angiotensin and its AT2 receptor: new insights into an old system

The AT2 receptor represents a true receptor, but signals and functions in unexpected ways compared to the respective features of the 'classical' AT1 receptor. Moreover, some of the actions of the AT2 receptor are even directly opposed to those of the AT1 receptor, especially concerning the growth- and differentiation-modulating actions of ANG II. The regulation of the AT2 receptor itself by its agonist, as well as by growth factors during ontogenesis, and its acknowledged effects on the regulation of cell growth, differentiation and apoptosis, points towards a role of a program modulator in embryonic development and regeneration.

Development of angiotensin (1-7) as an agent to accelerate dermal repair

Angiotensin II has been shown to be a potent agent in the acceleration of wound repair. Angiotensin (1-7), a fragment of angiotensin II that is not hypertensive, was found to be comparable to angiotensin II in accelerating dermal healing. This activity was evaluated in four models: rat and diabetic mouse full-thickness excisional wounds; rat random flap; and guinea pig partial thickness thermal injury. In all models, angiotensin (1-7) was comparable to angiotensin II. Angiotensin (1-7) accelerated the closure of wounds in diabetic mice and rats. In diabetic mice the resultant tissue at day 25 after injury was more comparable to normal tissue than the fibrotic scar observed in placebo-treated wounds. In the random flap model, angiotensin (1-7) was comparable to angiotensin II in maintaining flap viability (approximately 82%) and flap survival (40%). Finally, angiotensin (1-7) increased proliferation in the hair follicles at the edge of the wound and site of thermal injury, and the number of patent blood vessels on day 7 after partial thickness thermal injury. These data may be partially explained by the effect of angiotensin II and angiotensin (1-7) on keratinocyte proliferation. While platelet-derived growth factor had no effect on keratinocyte proliferation, angiotensin II and angiotensin (1-7) significantly increased keratinocyte proliferation. These data show that angiotensin(1-7) is comparable to angiotensin II in accelerating skin repair. Furthermore, the hypertensive and wound healing effects can be separated within the family of angiotensin peptides.

Reversible inactivation of AT(2) angiotensin II receptor from cysteine-disulfide bond exchange

Dithiothreitol (DTT) treatment of angiotensin II (Ang II) type 2 (AT(2)) receptor potentiates ligand binding, but the underlying mechanism is not known. Two disulfide bonds proposed in the extracellular domain were examined in this report. Based on the analysis of ligand affinity of cysteine (Cys, C) to alanine (Ala, A) substitution mutants, we provide evidence that Cys(35)-Cys(290) and Cys(117)-Cys(195) disulfide bonds are formed in the wild-type AT(2) receptor. Disruption of the highly conserved Cys(117)-Cys(195) disulfide bond linking the second and third extracellular segments leads to inactivation of the receptor. The Cys(35)-Cys(290) bond is highly sensitive to DTT. Its breakage results in an increased binding affinity for both Ang II and the AT(2) receptor-specific antagonist PD123319. Surprisingly, in the single Cys mutants, C35A and C290A, a labile population of receptors is produced which can be re-folded to high-affinity state by DTT treatment. These results suggest that the free -SH group of Cys(35) or Cys(290) competes with the disulfide bond formation between Cys(117) and Cys(195). This Cys-disulfide bond exchange results in production of the inactive population of the mutant receptors through formation of a non-native disulfide bond.

Wound healing: captopril, an angiogenesis inhibitor, and Staphylococcus aureus peptidoglycan

BACKGROUND

Captopril, an angiotensin-converting enzyme inhibitor, used for treating hypertension and heart failure, inhibits angiogenesis in the corneas of rats in response to basic fibroblast growth factor, slows the growth of experimental tumors in rats, and leads to the regression of Kaposi's sarcoma. Because angiogenesis is key to wound healing, we hypothesized that captopril would impair wound healing. We hypothesized also that because local application at operation of Staphylococcus aureus peptidoglycan (SaPG) increases angiogenesis and accelerates wound healing in rats, SaPG would prevent or ameliorate the postulated captopril-impaired wound healing.

MATERIALS AND METHODS

In each experiment, rats were divided randomly into two groups: one drinking tap water, and the other, tap water containing 0.5 mg captopril/ml. All ate chow and drank ad libitum, pre-operatively (4-12 days) and postoperatively (7 days). In experiments 1 and 2, bilateral paravertebral 5.5-cm skin incisions were made aseptically (intraperitoneal sodium pentobarbital), and closed with interrupted No. 35 stainless-steel sutures. On one side, the wound was immediately inoculated with 157 microliter pyrogen-free isotonic saline and on the other side the wound was inoculated with 157 microliter saline containing 4.7 mg SaPG (860 microgram SaPG/cm incision). In the third experiment, polyvinyl alcohol (PVA) sponges (16-17 mg dry wt each) containing either 50 microliter saline or 0.5 mg SaPG in 50 microliter saline were implanted subcutaneously, two on each side, via 1-cm incisions closed with a single suture. In the fourth experiment, 5.5-cm bilateral skin incisions and subcutaneous implantation of PVA sponges were done as described but all sites were instilled with saline only. All rats were euthanized (CO(2) asphyxia) 7 days postoperatively.

RESULTS

Wound breaking strength (WBS) of the saline-treated incisions was significantly higher (P < 0.001) in captopril-treated rats than in controls (172 +/- 13 g vs 105 +/- 6 g) in experiment 1 and higher, but not significantly in captopril-treated rats in experiment 2 (153 +/- 8 g vs 114 +/- 6 g) (PNS). SaPG inoculation of the incisions increased WBS significantly in both control and captopril-treated rats: 187 +/- 11 g vs 105 +/- 6 g (P < 0.001) and 283 +/- 16 g vs 172 +/- 13 g (P < 0.001), respectively, in experiment 1, and 217 +/- 13 g vs 114 +/- 6 g (P < 0.0001) (controls) and 266 +/- 17 g vs 153 +/- 8 g (captopril-treated rats) (P < 0.0001) in experiment 2. In experiment 3, subcutaneous PVA saline-inoculated sponge reparative tissue hydroxyproline (OHP) content was similar in control and captopril-treated rats, and SaPG inoculation increased reparative tissue OHP significantly in both groups: 2458 +/- 218 microgram/100 mg dry sponge vs 3869 +/- 230 microgram/100 mg (P < 0.001) (controls) and 2489 +/- 166 microgram/100 mg vs 4176 +/- 418 microgram/100 mg (P < 0.001) (captopril-treated rats). Histologically, angiogenesis and reparative tissue collagen were similar in control and captopril-treated rats, in both saline-inoculated and SaPG-inoculated sponges. In experiment 4 (all incisions and subcutaneous PVA sponges were saline-inoculated), there was no significant difference in WBS between control and captopril-treated rats (107 +/- 6 g vs 96 +/- 5 g, NS). PVA sponge reparative tissue OHP was significantly higher in captopril-treated rats: 3698 +/- 170 microgram/100 mg dry sponge vs 2534 +/- 100 microgram/100 mg (P < 0.0001).

CONCLUSION

Unexpectedly, in four experiments, captopril did not inhibit WBS or PVA sponge reparative tissue angiogenesis or collagen accumulation; in fact, WBS was increased significantly in one of three experiments, and PVA sponge reparative tissue OHP was increased significantly in one of two experiments. Also, captopril did not interfere with the wound healing-accelerating effect of SaPG.

The angiotensin II type 2 receptor: an enigma with multiple variations

Since it was discovered ten years ago, the angiotensin II (ANG II) type 2 (AT(2)) receptor has been an enigma. This receptor binds ANG II with a high affinity but is not responsible for mediating any of the classical physiological actions of this peptide, all of which involve the ANG II type 1 (AT(1)) receptor. Furthermore, the AT(2) receptor exhibits dramatic differences in biochemical and functional properties and in patterns of expression compared with the AT(1) receptor. During the past decade, much information has been gathered about the AT(2) receptor, and the steadily increasing number of publications indicates a growing interest in this new and independent area of research. A number of studies suggest a role of AT(2) receptors in brain, renal, and cardiovascular functions and in the processes of apoptosis and tissue regeneration. Despite these advances, nothing stands out as the major singular function of these receptors. The study of AT(2) receptors has reached a crossroads, and innovative approaches must be considered so that unifying mechanisms as to the function of these unique receptors can be put forward. In this review we will discuss the advances that have been made in understanding the biology of the AT(2) receptor. Furthermore, we will consider how these discoveries, along with newer experimental approaches, may eventually lead to the elusive physiological and pathophysiological functions of these receptors.

The angiotensin type 2 receptor: variations on an enigmatic theme

Since its discovery and molecular characterization, the angiotensin AT2.receptor has been enigmatic with respect to signalling pathways and function. Evidence now emerges that angiotensin II exerts actions through the AT2 receptor which are directly opposed to those mediated by the AT1 receptor. This can be exemplified e.g. by mutually antagonizing effects on cell growth. Upregulated by the endogenous agonist itself, as well as by several growth- and differentiating factors in development and tissue injury, the AT2 receptor appears to act as a modulator of complex biological programmes involved in embryonic development, cell differentiation, tissue protection and regeneration, as well as in programmed cell death. Research on the AT2 receptor has thus unveiled hitherto unknown functions of the renin-angiotensin system extending far beyond the classical role of this old hormonal system in cardiovascular control.

The renin-angiotensin system and its receptors

The renin-angiotensin system (RAS) plays an important role in blood pressure control and in water and salt homeostasis. It is involved in the pathophysiology of hypertension and structural alterations of the vasculature, kidney, and heart, including neointima formation, nephrosclerosis, postinfarction remodeling, and cardiac left ventricular hypertrophy (LVH). Recently, an increased knowledge of the effector peptides of the RAS, their receptors, and their respective functions has led to a new principle of treatment for hypertension: the inhibition of angiotensin (Ang) II via angiotensin-converting enzyme inhibitors or Ang II-receptor antagonists. In this review, the Ang receptors AT1 and AT2 and the potential roles of shorter angiotensin fragments, including Ang III(2-8), Ang IV(3-8), and Ang(1-7), are discussed.

The angiotensin II type 2 (AT2) receptor promotes axonal regeneration in the optic nerve of adult rats

The renin-angiotensin system (RAS) has been traditionally linked to blood pressure and volume regulation mediated through the angiotensin II (ANG II) type 1 (AT1) receptor. Here we report that ANG II via its ANG II type 2 (AT2) receptor promotes the axonal elongation of postnatal rat retinal explants (postnatal day 11) and dorsal root ganglia neurons in vitro, and, moreover, axonal regeneration of retinal ganglion cells after optic nerve crush in vivo. In retinal explants, ANG II (10(-7)-10(-5) M) induced neurite elongation via its AT2 receptor, since the effects were mimicked by the AT2 receptor agonist CGP 42112 (10(-5) M) and were entirely abolished by costimulation with the AT2 receptor antagonist PD 123177 (10(-5) M), but not by the AT1 receptor antagonist losartan (10(-5) M). To investigate whether ANG II is able to promote axonal regeneration in vivo, we performed optic nerve crush experiments in the adult rats. After ANG II treatment (0.6 nmol), an increased number of growth-associated protein (GAP)-43-positive fibers was detected and the regenerating fibers regularly crossed the lesion site (1.6 mm). Cotreatment with the AT2 receptor antagonist PD 123177 (6 nmol), but not with the AT1 receptor antagonist losartan (6 nmol), completely abolished the ANG II-induced axonal regeneration, providing for the first time direct evidence for receptor-specific neurotrophic action of ANG II in the central nervous system of adult mammals and revealing a hitherto unknown function of the RAS.

Sciatic nerve transection evokes lasting up-regulation of angiotensin AT2 and AT1 receptor mRNA in adult rat dorsal root ganglia and sciatic nerves

The angiotensin AT2 receptor is involved in tissue repair and cellular stress responses in non-neuronal cells. We have previously observed that the AT2 receptor-induced neurite formation in PC12W cells is paralleled by a reduced neurofilament M expression as it occurs in nerve fiber regeneration. Here we show that transection and crush of sciatic nerve fibers of adult rats results in dramatic changes of AT2, AT1a and AT1b receptor mRNA in dorsal root ganglion neurons (DRGs) and in sciatic nerves 3, 14 and 28 days after axotomy and crush. The expression patterns were determined by reverse transcription polymerase chain reaction (RT-PCR) assay, and the specificity of amplification products was verified by Southern blot hybridization. Whereas axotomy evoked a transient increase of AT2 receptor mRNA by more than 1000% after 3 days in proximal and after 14 days in distal sciatic nerve stumps (510%), the maximum expression in DRGs was observed after 14 days (1100%). Sciatic nerve crush resulted in a time-dependent up-regulation of AT2 receptor mRNA in sciatic nerve segments coinciding with the successful regeneration of nerve fibers. In sciatic nerves, AT1a and AT1b receptor mRNA levels were increased within different time-courses and to different extents with a maximum expression of 570%. In contrast to AT1a receptor mRNAs, AT1b receptor mRNA levels were increased in DRGs by maximally 800%. These results suggest that AT2 and AT1 receptor-mediated pathways are involved in Schwann cell-mediated myelination and in neuroregenerative responses of DRGs.

Alterations of angiotensin II Receptor levels in sutured wounds in rat skin

Angiotensin II receptor levels have been shown to vary with postoperative time in tissue harvested from full-thickness dermal excisional wounds on adult rats. This study examined the expression of AII receptors in a sutured wound model. Two full-thickness incisional wounds were made in the dorsal skin of adult Sprague-Dawley rats and sutured immediately under general anesthesia. The wound tissues were harvested at 0, 0.5, 1, 2, 4, 24 h and on days 2, 3, 4, 5, 7, and 10 after the wounding. The levels of 125I-Sar1.Ile8-AII bound to membrane preparations of the wound tissues decreased at early time points (from 0.5 to 4 h), increased from day 1 to day 7, and returned to nonsurgical levels by day 10. Competitive binding studies showed that the receptors were predominantly of the AT1 receptor subtype. These results suggest that an immediate and transient reduction in AII receptor expression occurred after wounding, followed by an increase in the number of AII receptors that was maintained for 5 to 7 days postoperatively. Because these data are consistent with those observed after excisional wounding, temporal changes in AII receptor expression may be integral to the process of wound healing.

Comparative studies of the angiogenic activity of vasoactive intestinal peptide, endothelins-1 and -3 and angiotensin II in a rat sponge model

1 The angiogenic activity of four vasoactive peptides with a range of vasodilator and vasoconstrictor properties, i.e. vasoactive intestinal peptide (VIP), endothelin-1, endothelin-3 and angiotensin II, were investigated in a rat sponge model. Neovascularization was assessed by the 133Xe clearance technique and confirmed by histological studies. 2 Daily doses of the vasodilator peptide, VIP (1000 pmol), caused intense neovascularization, but a lower dose (10 pmol) produced no apparent effect. However, the lower dose of VIP, when given with a subthreshold dose of interleukin-1 alpha (0.3 pmol), produced an angiogenic response similar to that seen with the higher dose of VIP. The neovascular response induced by co-administration of VIP and interleukin-1 alpha was inhibited by simultaneous administration of 100 pmol VIP (10-28), a specific VIP receptor antagonist. 3 In contrast, daily doses of 10, 100 or 1000 pmol endothelin-3 (a mixed vasoconstrictor and vasodilator with more marked vasodilator activity) or of 100 or 1000 pmol endothelin-1 (also with mixed activity but with much more pronounced vasoconstrictor response) produced no apparent effect on sponge-induced angiogenesis. 4 The vasoconstrictor peptide, angiotensin II, in daily doses of 1000 pmol, caused an intense neovascularization like VIP but lower doses of angiotensin II (10 or 100 pmol) produced no apparent effect. The lowest dose of angiotensin II (10 pmol) when administered with the subthreshold dose of interleukin-1 alpha (0.3 pmol) had no effect on the basal neovascular response in the sponges. The angiotensin II-induced neovascular response was inhibited by co-administration of 100 nmol of the specific AT1 receptor antagonist, losartan, but not by the AT2 receptor antagonist, PD 123319. 5 These data show that VIP and angiotensin II possess angiogenic activity. However, endothelin-1 and endothelin-3 had no activity at the doses used. Thus the angiogenic response is not related to local vasoconstriction or vasodilatation in the sponges. The blockade of VIP- and angiotensin II-induced angiogenesis at the receptor level suggests that receptor modulation could provide a strategy for the management of angiogenic diseases.

Angiotensin-converting enzyme and wound healing in diverse tissues of the rat

Autoradiographic binding density of angiotensin-converting enzyme (ACE), an indirect measure of ACE activity, is markedly increased at sites of fibrous tissue that appear in the injured heart. This includes myocardial infarction (MI) caused by left coronary artery ligation; endocardial fibrosis of the interventricular septum and perivascular fibrosis of intramyocardial coronary arterioles of the right ventricle, each of which appear remote to MI; and pericardial fibrosis after pericardiotomy (without MI). Expressed in fibroblast-like cells found at each site of tissue repair, ACE may be common to tissue repair in the rat heart, irrespective of the etiologic basis of injury. To address this hypothesis and to determine whether this also applies to other tissues (skin and kidney), the present study was undertaken. ACE binding density was measured by quantitative in vitro autoradiography (125I-351A) in injured rat heart, skin, and kidney. Experimental observations included foreign-body fibrosis after placement of silk ligature in skin or myocardium, endomyocardial myocyte necrosis and fibrosis that accompanied isoproterenol administration (1 mg/kg sc x 2 days), and embolic infarction of the kidney as a result of mural thrombus of the left ventricle that appeared after anterior MI. Fibrosis was identified by collagen-specific staining with picrosirius red. Hematoxylin-eosin staining and immunohistochemical labeling with alpha-smooth muscle actin (alpha-SMA) antibody were used to address cell morphology and phenotype, respectively. We found (1) endomyocardial fibrosis 2 weeks after isoproterenol; (2) fibrosis surrounding silk suture in heart and skin 1 week after placement; (3) renal infarction 1 week after left coronary artery ligation; (4) numerous fibroblast-like cells containing alpha-SMA, as well as macrophages, at sites of repair in all tissues studied; and (5) markedly increased ACE binding density at each of these sites. Thus ACE is integral to tissue repair in the heart, skin, and kidney of the rat, irrespective of the etiologic basis of injury. At these sites ACE may serve to regulate local concentrations of substances involved in tissue repair.

The renin-angiotensin-system in the skin. Evidence for its presence and possible functional implications

Among the several hormonal systems regulating body functions, the renin-angiotensin-system has long been considered a classical endocrine system with angiotensin II, its effector hormone, being synthesized in and subsequently distributed by the circulation to act on its numerous, mainly renal and cardiovascular target organs throughout the body. Angiotensin II has long been regarded to be primarily responsible for the regulation of blood-pressure and of volume- and electrolyte-homeostasis. Recent evidence suggests that it also affects cellular proliferation and differentiation via the so-called local or tissue-renin-angiotensin-systems. Such trophic actions have already been observed in tissues not belonging to the renal or cardiovascular systems such as cultured cells of neuronal origin. Evidence for a rôle of angiotensin II in the skin is so far scanty and mainly based on the demonstration of angiotensin receptors on cultured human keratinocytes and in subcutaneous tissue of rats. Although almost every single component of the renin-angiotensin-system has already been identified in skin of one or another species, comprehensive data regarding the skin renin-angiotensin-system as a whole within one particular species, especially in man, are still lacking. The present manuscript reviews novel recent data regarding the renin-angiotensin-system particularly in skin, and it discusses a possible functional rôle of the cutaneous renin-angiotensin-system on the basis of these findings.

A specific template-assembled peptidic agonist for the angiotensin II receptor subtype 2 (AT2) and its effect on inferior olivary neurones

We synthesized a molecule composed of two angiotensin II 4-8 pentapeptide fragments attached to a carrier molecule (TA), according to the template-assembled synthetic proteins concept. This molecule was investigated for receptor binding on angiotensin type-1 and type-2 receptors (AT1 and AT2) and its biological activity was determined by iontophoretic experiments on neurones of the inferior olive (ION) that express only AT2 receptors. TA binds exclusively to the AT2 receptor and mediates an agonistic angiotensin-II effect on the ION. TA is the first agonist available to study the direct stimulation of AT2 receptors.