Analysis of functional domains of angiotensin II type 2 receptor involved in apoptosis

Activation of angiotensin II type 2 receptor leads to preservation of primary cilia in tubular cells during renal ischaemia-reperfusion injury

Ischaemia-reperfusion (IR)-associated acute kidney injury (AKI) is a severe clinical condition that lacks effective pharmacological treatments. Our recent research revealed that pretreatment with the angiotensin II type 2 receptor (AT2R) agonist C21 alleviates kidney damage during IR. Primary cilia are organelles crucial for regulation of epithelial cell homeostasis, which are significantly affected by IR injury. This study aimed to evaluate the impact of AT2R activation on cilia integrity during IR and to identify pathways involved in the nephroprotective effect of C21. Rats were subjected to 40 min of unilateral ischaemia followed by 24 h of reperfusion. Immunofluorescence analysis of the kidneys showed that the nephroprotective effect of C21 was associated with preservation of cilia integrity in tubular cells. AT2R agonists increased α-tubulin acetylation in primary cilia in tubular cells in vivo and in a cell model. Analysis of ERK phosphorylation indicated that AT2R activation led to diminished activation of ERK1/2 in tubular cells. Similar to AT2R agonists, inhibitors of α-tubulin deacetylase HDAC6 or inhibitors of ERK activation ameliorated IR-induced cell death and preserved cilia integrity. Immunofluorescence analysis of tubular cells revealed significant ERK localization at primary cilia and demonstrated that ERK inhibition increased cilia levels of acetylated α-tubulin. Overall, our findings demonstrate that C21 elicits a preconditioning effect that enhances cilia stability in renal tubular cells, thereby preserving their integrity when exposed to IR injury. Furthermore, our results indicate that this effect might be mediated by AT2R-induced inhibition of ERK activation. These findings offer potential insights for the development of pharmacological interventions to mitigate IR-associated AKI. KEY POINTS: The AT2R agonist C21 prevents primary cilia shortening and tubular cell deciliation during renal ischaemia-reperfusion. AT2R activation inhibits ERK1/2 in renal tubular cells. Both AT2R agonists and ERK1/2 inhibitors increase alpha-tubulin acetylation at the primary cilium in tubular cells. AT2R activation, ERK1/2 inhibition or inhibition of alpha-tubulin deacetylation elicit protective effects in tubular cells subjected to ischaemia-reperfusion injury.

Unraveling the Relevance of Tissue-Specific Decellularized Extracellular Matrix Hydrogels for Vocal Fold Regenerative Biomaterials: A Comprehensive Proteomic and In Vitro Study

Decellularized extracellular matrix (dECM) is a promising material for tissue engineering applications. Tissue-specific dECM is often seen as a favorable material that recapitulates a native-like microenvironment for cellular remodeling. However, the minute quantity of dECM derivable from small organs like the vocal fold (VF) hampers manufacturing scalability. Small intestinal submucosa (SIS), a commercial product with proven regenerative capacity, may be a viable option for VF applications. This study aims to compare dECM hydrogels derived from SIS or VF tissue with respect to protein content and functionality using mass spectrometry-based proteomics and in vitro studies. Proteomic analysis reveals that VF and SIS dECM share 75% of core matrisome proteins. Although VF dECM proteins have greater overlap with native VF, SIS dECM shows less cross-sample variability. Following decellularization, significant reductions of soluble collagen (61%), elastin (81%), and hyaluronan (44%) are noted in VF dECM. SIS dECM contains comparable elastin and hyaluronan but 67% greater soluble collagen than VF dECM. Cells deposit more neo-collagen on SIS than VF-dECM hydrogels, whereas neo-elastin (~50 μg/scaffold) and neo-hyaluronan (~ 6 μg/scaffold) are comparable between the two hydrogels. Overall, SIS dECM possesses reasonably similar proteomic profile and regenerative capacity to VF dECM. SIS dECM is considered a promising alternative for dECM-derived biomaterials for VF regeneration.

Delineating the conformational landscape and intrinsic properties of the angiotensin II type 2 receptor using a computational study

As a key regulator for the renin-angiotensin system, a class A G protein-coupled receptor (GPCR), AngII type 2 receptor (AT₂R), plays a pivotal role in the homeostasis of the cardiovascular system. Compared with other GPCRs, AT₂R has a unique antagonist-bound conformation and its mechanism is still an enigma. Here, we applied combined dynamic and evolutional approaches to investigate the conformational space and intrinsic properties of AT₂R. With molecular dynamic simulations, Markov State Models, and statistics coupled analysis, we captured the conformational landscape of AT₂R and identified its uniquity from both dynamical and evolutional viewpoints. A cryptic pocket was also discovered in the intermediate state during conformation transitions. These findings offer a deeper understanding of the AT₂R mechanism at an atomic level and provide hints for the design of novel AT₂R modulators.

The anti-fibrotic actions of relaxin are mediated through AT2 R-associated protein phosphatases via RXFP1-AT2 R functional crosstalk in human cardiac myofibroblasts

Fibrosis is a hallmark of several cardiovascular diseases. The relaxin family peptide receptor 1 (RXFP1) agonist, relaxin, has rapidly occurring anti-fibrotic actions which are mediated through RXFP1 and angiotensin II receptor crosstalk on renal and cardiac myofibroblasts. Here, we investigated whether this would allow relaxin to indirectly activate angiotensin II type 2 receptor (AT₂ R)-specific signal transduction in primary human cardiac myofibroblasts (HCMFs). The anti-fibrotic effects of recombinant human relaxin (RLX; 16.8 nM) or the AT₂ R-agonist, Compound 21 (C21; 1 μM), were evaluated in TGF-β1-stimulated HCMFs, in the absence or presence of an RXFP1 antagonist (1 μM) or AT₂ R antagonist (0.1 μM) to confirm RXFP1-AT₂ R crosstalk. Competition binding for RXFP1 was determined. Western blotting was performed to determine which AT₂ R-specific protein phosphatases were expressed by HCMFs; then, the anti-fibrotic effects of RLX and/or C21 were evaluated in the absence or presence of pharmacological inhibition (NSC95397 (1 μM) for MKP-1; okadaic acid (10 nM) for PP2A) or siRNA-knockdown of these phosphatases after 72 hours. The RLX- or C21-induced increase in ERK1/2 and nNOS phosphorylation, and decrease in α-SMA (myofibroblast differentiation) and collagen-I expression by HCMFs was abrogated by pharmacological blockade of RXFP1 or the AT₂ R, confirming RXFP1-AT₂ R crosstalk in these cells. HCMFs were found to express AT₂ R-dependent MKP-1 and PP2A phosphatases, while pharmacological blockade or siRNA-knockdown of either phosphatase also abolished RLX and/or C21 signal transduction in HCMFs (all P < .05 vs RLX or C21 alone). These findings demonstrated that RLX can indirectly activate AT₂ R-dependent phosphatase activity in HCMFs by signaling through RXFP1-AT₂ R crosstalk, which have important therapeutic implications for its anti-fibrotic actions.

Significance of intranuclear angiotensin-II type 2 receptor in oral squamous cell carcinoma

The renin-angiotensin system (RAS) is implicated in the maintenance of blood pressure and in many other biological processes including tumorigenesis and metastasis formation. Angiotensin-II (A-II) type 2 receptor (AGTR2) seems to be involved in different types of cancer; its role, however, is still unclear. Here, we investigated the role of RAS, and specifically that of AGTR2, in oral squamous cell carcinoma (OSCC) progression. AGTR2 has opposite effect on vasodilation and blood pressure compared to AGTR1. In 23 OSCCs, we found that the AGTR1/AGTR2 mRNA ratio was inversely associated with disease progression, while nuclear AGTR2 positivity was associated with disease progression. In the human OSCC cell lines HSC3 and HSC4, AGTR1 was associated with proliferation and invasion, while AGTR2 was associated with anti-apoptosis and anti-oxidative stress. Levels of nuclear AGTR2 confirmed by subcellular fractionation increased in hypoxic and hyperglycemic conditions, in which apoptosis and oxidative stress were suppressed and the redox status altered to reduction. Accumulation of nuclear AGTR2 by inhibition of extranuclear transportation decreased apoptosis and increased proliferation and invasion in HSC3 cells. Intratumoral angiotensin-II (but not serum angiotensin-II) levels were associated with stage and nuclear AGTR2 positivity. In OSCC cell lines, intracellular angiotensin-II was produced by themselves. Notably, losartan, an angiotensin receptor blocker, inhibited intracellular angiotensin-II production and AGTR2 nuclear localization to enhance the antitumoral effect of 5-FU in an OSCC tumor model. While the precise role of nuclear AGTR2 requires further examination, these data suggest that the intracellular angiotensin system might be a significant target for OSCC.

Angiotensin II Type 2 Receptor and Receptor Mas Are Colocalized and Functionally Interdependent in Obese Zucker Rat Kidney

The actions of angiotensin II type 2 receptor (AT₂R) and the receptor Mas (MasR) are complex but show similar pronatriuretic function; particularly, AT₂R expression and natriuretic function are enhanced in obese/diabetic rat kidney. In light of some reports suggesting a potential positive interaction between these receptors, we tested hypothesis that renal AT₂R and MasR physically interact and are interdependent to stimulate cell signaling and promote natriuresis in obese rats. We found that infusion of AT₂R agonist C21 in obese Zucker rats (OZR) increased urine flow and urinary Na excretion which were attenuated by simultaneous infusion of the AT₂R antagonist PD123319 or the MasR antagonist A-779. Similarly, infusion of MasR agonist Ang-(1-7) in OZR increased urine flow and urinary Na excretion, which were attenuated by simultaneous infusion of A-779 or PD123319. Experiment in isolated renal proximal tubules of OZR revealed that both the agonists C21 and Ang-(1-7) stimulated NO which was blocked by either of the receptor antagonists. Dual labeling of AT₂R and MasR in OZR kidney sections and human proximal tubule epithelial cells showed that AT₂R and MasR are colocalized. The AT₂R also coimmunoprecipitated with MasR in cortical homogenate of OZR. Immunoblotting of cortical homogenate cross-linked with zero-length oxidative (sulfhydryl groups) cross-linker cupric-phenanthroline revealed a shift of AT₂R and MasR bands upward with overlapping migration for their complexes which were sensitive to the reducing β-mercaptoethanol, suggesting involvement of -SH groups in cross-linking. Collectively, the study reveals that AT₂R and MasR are colocalized and functionally interdependent in terms of stimulating NO and promoting diuretic/natriuretic response.

Classical Renin-Angiotensin system in kidney physiology

The renin-angiotensin system has powerful effects in control of the blood pressure and sodium homeostasis. These actions are coordinated through integrated actions in the kidney, cardiovascular system and the central nervous system. Along with its impact on blood pressure, the renin-angiotensin system also influences a range of processes from inflammation and immune responses to longevity. Here, we review the actions of the "classical" renin-angiotensin system, whereby the substrate protein angiotensinogen is processed in a two-step reaction by renin and angiotensin converting enzyme, resulting in the sequential generation of angiotensin I and angiotensin II, the major biologically active renin-angiotensin system peptide, which exerts its actions via type 1 and type 2 angiotensin receptors. In recent years, several new enzymes, peptides, and receptors related to the renin-angiotensin system have been identified, manifesting a complexity that was previously unappreciated. While the functions of these alternative pathways will be reviewed elsewhere in this journal, our focus here is on the physiological role of components of the "classical" renin-angiotensin system, with an emphasis on new developments and modern concepts.

Regulation of angiotensin II receptors beyond the classical pathway

The RAS (renin–angiotensin system) plays a role not only in the cardiovascular system, including blood pressure regulation, but also in the central nervous system. AngII (angiotensin II) binds two major receptors: the AT1 receptor (AngII type 1 receptor) and AT2 receptor (AngII type 2 receptor). It has been recognized that AT2 receptor activation not only opposes AT1 receptor actions, but also has unique effects beyond inhibitory cross-talk with AT1 receptor signalling. Novel pathways beyond the classical actions of RAS, the ACE (angiotensin-converting enzyme)/AngII/AT1 receptor axis, have been highlighted: the ACE2/Ang-(1–7) [angiotensin-(1–7)]/Mas receptor axis as a new opposing axis against the ACE/AngII/AT1 receptor axis, novel AngII-receptor-interacting proteins and various AngII-receptor-activation mechanisms including dimer formation. ATRAP (AT1-receptor-associated protein) and ATIP (AT2-receptor-interacting protein) are well-characterized AngII-receptor-associated proteins. These proteins could regulate the functions of AngII receptors and thereby influence various pathophysiological states. Moreover, the possible cross-talk between PPAR (peroxisome-proliferator-activated receptor)-γ and AngII receptor subtypes is an intriguing issue to be addressed in order to understand the roles of RAS in the metabolic syndrome, and interestingly some ARBs (AT1-receptor blockers) have been reported to have an AT1-receptor-blocking action with a partial PPAR-γ agonistic effect. These emerging concepts concerning the regulation of AngII receptors are discussed in the present review.

Regulation of AT1R expression through HuR by insulin

Angiotensin II type 1 receptor (AT1R) has a pathophysiological role in hypertension, atherosclerosis and heart failure. Type 2 diabetes is hyperinsulinemic state and a major risk factor for atherosclerosis and hypertension. It is known that hyperinsulinemia upregulates AT1R expression post-transcriptionally by increasing the half-life of AT1R mRNA, but little is known about the mechanism of this effect. In the present study, we first identified AT1R 3′-UTR as a mediator of insulin effect. Using 3′-UTR as a bait, we identified through analysis of insulin-stimulated cell lysates by affinity purification and mass spectrometry HuR as an insulin-regulated AT1R mRNA binding protein. By ribonucleoprotein immunoprecipitation, we found HuR binding to AT1R to be increased by insulin. Overexpression of HuR leads to increased AT1R expression in a 3′-UTR-dependent manner. Both insulin and HuR overexpression stabilize AT1R 3′-UTR and their responsive element within 3′-UTR are located within the same region. Cell fractionation demonstrated that insulin induced HuR translocation from nucleus to cytoplasm increased HuR binding to cytoplasmic AT1R 3′-UTR. Consistent with HuR translocation playing a mechanistic role in HuR effect, a reduction in the cytoplasmic levels of HuR either by silencing of HuR expression or by inhibition of HuR translocation into cytoplasm attenuated insulin response. These results show that HuR translocation to cytoplasm is enhanced by insulin leading to AT1R upregulation through HuR-mediated stabilization of AT1R mRNA.

Expression of the vasoactive proteins AT1, AT2, and ANP by pregnancy-induced mouse uterine natural killer cells

Angiotensin II receptor type 1 (AT1) activation leads to vasoconstriction and type 2 receptor (AT2) leads to vasodilation. Atrial natriuretic peptide (ANP) antagonizes the effects of AT1. In human and murine pregnancies, uterine natural killer (uNK) cells closely associate with decidual blood vessels. Protein localization of AT1, AT2, and ANP to mouse uNK cells was examined between gestation days (gds) 6 and 12, the interval of uNK cell expansion. Percentages of uNK cells expressing AT1 or AT2 changed between gd6 and gd10. Atrial natriuretic peptide did not localize to uNK cells at gd6 or 8, but did colocalize to uNK cells at gd10 and 12, times immediately after spiral arterial modification. This is the first report of AT1, AT2, and ANP expression in uterine immune cells. Expression of these molecules suggests that uNK cells have the potential to contribute to the changes in blood pressure that occur between days 5 and 12 of pregnancy in mice.

Regulation of macrophage nitric oxide production by the protein tyrosine phosphatase Src homology 2 domain phosphotyrosine phosphatase 1 (SHP-1)

Nitric oxide (NO) is a potent molecule involved in the cytotoxic effects mediated by macrophages (MØ) against microorganisms. We previously reported that Src homology 2 domain phosphotyrosine phosphatase 1 (SHP-1)-deficient cells generate a greater amount of NO than wild-type cells in response to interferon-gamma (IFN-gamma). We also reported that the Leishmania-induced MØ SHP-1 activity is needed for the survival of the parasite within phagocytes through the attenuation of NO-dependent and NO-independent mechanisms. In the present study, we investigated the role of SHP-1 in regulating key signalling molecules important in MØ NO generation. Janus tyrosine kinase 2 (JAK2), mitogen-activated extracellular signal-regulated protein kinase kinase (MEK), extracellular signal-regulated kinases 1 and 2 (Erk1/Erk2) mitogen-activated protein kinases, p38 and stress-activated mitogen-activated protein kinases/c-Jun NH(2)-terminal kinase (SAPK/JNK) were examined in immortalized bone marrow-derived MØ (BMDM) from both SHP-1-deficient motheaten mice (me-3) and their respective littermates (LM-1). The results indicated that Erk1/Erk2 and SAPK/JNK are the main kinases regulated by SHP-1 because the absence of SHP-1 caused an increase in their phosphorylation. Moreover, only Apigenin, the specific inhibitor of Erk1/Erk2, was able to block IFN-gamma-induced inducible nitric oxide synthase (iNOS) transcription and translation in me-3 cells. Transcription factor analyses revealed that in the absence of SHP-1, activator protein-1 (AP-1) was activated. The activation of AP-1, and not nuclear factor-kappaB (NF-kappaB) or signal transducer and activator of transcription-1 alpha (STAT-1 alpha), may explain the enhanced NO generation in SHP-1-deficient cells. These observations emphasize the involvement of the MAPKs Erk1/Erk2 and SAPK/JNK in NO generation via AP-1 activation. Collectively, our findings suggest that SHP-1 plays a pivotal role in the negative regulation of signalling events leading to iNOS expression and NO generation. Furthermore, our observations underline the importance of SHP-1-mediated negative regulation in maintaining NO homeostasis and thus preventing the abnormal generation of NO that can be detrimental to the host.

Posttranscriptional regulation of angiotensin II type 1 receptor expression by glyceraldehyde 3-phosphate dehydrogenase

Regulation of angiotensin II type 1 receptor (AT1R) has a pathophysiological role in hypertension, atherosclerosis and heart failure. We started from an observation that the 3′-untranslated region (3′-UTR) of AT1R mRNA suppressed AT1R translation. Using affinity purification for the separation of 3′-UTR-binding proteins and mass spectrometry for their identification, we describe glyceraldehyde 3-phosphate dehydrogenase (GAPDH) as an AT1R 3′-UTR-binding protein. RNA electrophoretic mobility shift analysis with purified GAPDH further demonstrated a direct interaction with the 3′-UTR while GAPDH immunoprecipitation confirmed this interaction with endogenous AT1R mRNA. GAPDH-binding site was mapped to 1–100 of 3′-UTR. GAPDH-bound target mRNAs were identified by expression array hybridization. Analysis of secondary structures shared among GAPDH targets led to the identification of a RNA motif rich in adenines and uracils. Silencing of GAPDH increased the expression of both endogenous and transfected AT1R. Similarly, a decrease in GAPDH expression by H₂O₂ led to an increased level of AT1R expression. Consistent with GAPDH having a central role in H₂O₂-mediated AT1R regulation, both the deletion of GAPDH-binding site and GAPDH overexpression attenuated the effect of H₂O₂ on AT1R mRNA. Taken together, GAPDH is a translational suppressor of AT1R and mediates the effect of H₂O₂ on AT1R mRNA.

Sharpening the edges of understanding the structure/function of the LPA1 receptor: expression in cancer and mechanisms of regulation

Since the molecular cloning of the vzg-1/Edg-2/LPA1 gene, studies have attempted to characterize LPA1 receptor functionality into a single categorical role, different from the other Edg-family LPA receptors. The desire to categorize LPA1 function has highlighted its complexity and demonstrated that the LPA1 receptor does not have one absolute function throughout every system. The central nervous system is highly enriched in the LPA1 receptor, suggesting an integral role in neuronal processes. Metastatic and invasive breast cancer also appears to have LPA-mediated LPA1 receptor functions that enhance phenotypes associated with tumorigenesis. LPA1 possesses a number of motifs conserved among G protein-coupled receptors (GPCRs): a DRY-like motif, a PDZ domain, Ser/Thr predicted sites of phosphorylation, a di-leucine motif, double cysteines in the tail and conserved residues that stabilize structure and determine ligand binding. The third intracellular loop of the LPA1 receptor may be the crux of receptor signaling and attenuation with phosphorylation of Thr-236 potentially a key determinant of basal LPA1 signaling. Mutagenesis data supports the notion that Thr-236 regulates this process since mutating Thr-236 to Ala-236 increased basal and LPA-mediated serum response factor (SRF) signaling activity and Lys-236 further increased this basal signaling. Here we describe progress on defining the major functions of the LPA1 receptor, discuss a context dependent dualistic role as both a negative regulator in cancer and a proto-oncogene, outline its structural components at the molecular amino acid level and present mutagenesis data on the third intracellular loop of the receptor.

Angiotensin II Signaling in Vascular Physiology and Pathophysiology

Initially recognized as a physiologic regulator of blood pressure and body fluid homeostasis, angiotensin (Ang) II has now been shown in innumerable experiments and clinical studies to contribute to the development and maintenance of cardiovascular disease. Dissection of its signaling mechanisms over the past decades has led to the discovery of several novel concepts, such as tissue-specific metabolism of Ang peptides. Identification and cloning of the various receptors through which Ang II acts on almost all tissues has led to the development of specific pharmacologic inhibitors with proven clinical benefit in patients with cardiovascular disorders. Work on the G-protein-coupled Ang II Type 1 receptor has demonstrated that different receptors interact through oligomerization, compartmentalization, and transactivation, and may explain how Ang II can activate G-protein-independent pathways. Unraveling the downstream effects of Ang II in specific cell types corroborates the importance of the cellular redox state on certain signaling pathways. Finally, the effects of Ang II on cell function and phenotype, such as the expression of inflammatory cytokines and receptors promoting the recruitment of inflammatory cells into vascular tissues, have indicated its role in local inflammation as a general pathogenetic basis of cardiovascular disease. The recognition of Ang II as a contributor to such fundamental pathophysiologic mechanisms, which are believed to be a common pathway for diverse cardiovascular risk factors like hypertension and diabetes, has greatly advanced our knowledge of pathologic signaling in vascular tissues and may help to eventually define novel targets for pharmacologic interventions.

Emerging Concepts of Regulation of Angiotensin II Receptors

Angiotensin (Ang) II exerts its important physiological functions through 2 distinct receptor subtypes, type 1 (AT 1 ) and type 2 (AT 2 ) receptors. Recently, new evidence has accumulated showing the existence of several novel receptor interacting proteins and various angiotensin II receptor activation mechanisms beyond the classical actions of receptors for Ang II. These associated proteins could contribute not only to Ang II receptors’ functions, but also to influencing pathophysiological states. Receptor dimerization of Ang II receptors such as homodimer, heterodimer, and complex formation with other G protein-coupled receptors has also been focused on as a new mechanism of their activation or inactivation. Moreover, ligand-independent receptor activation systems such as mechanical stretch for the AT 1 receptor have also been revealed. These emerging concepts of regulation of Ang II receptors and a new insight into future drug discovery are discussed in this review.

A possible anti-inflammatory role of angiotensin II type 2 receptor in immune-mediated glomerulonephritis during type 1 receptor blockade

We previously reported that angiotensin II type 1 receptor (AT1R) blockade attenuates renal inflammation/fibrogenesis in immune-mediated glomerulonephritis via angiotensin II type 2 receptor (AT2R). In the present study, further in vivo experiments revealed that AT2R was expressed in tubular epithelial cells of nephritic kidneys in mice, and feedback activation of the renin-angiotensin system during AT1R blockade significantly reduced p-ERK, but not intranuclear nuclear factor-kappaB, levels via AT2R. This led to reduction in mRNA levels of the proinflammatory mediator monocyte chemoattractant protein-1 and overall interstitial inflammation and subsequent fibrogenesis. Specific blockade of ERK expression in tubular epithelium by anti-sense oligodeoxynucleotides also attenuated interstitial inflammation, mimicking the anti-inflammatory action of AT2R in nephritic kidneys. Alternatively, we succeeded in confirming such an AT(2)R function by demonstrating that AT1R blockade did not confer renoprotection in nephritic, AT2R gene-deficient mice. Additional in vitro experiments revealed that AT2R activation did not affect nuclear factor-kappaB activation by tumor necrosis factor-alpha in cultured tubular epithelial cells, although it inhibited ERK phosphorylation, which reduced monocyte chemoattractant protein-1 mRNA levels. These results suggest that feedback activation of AT2Rs in tubular epithelium of nephritic kidneys plays an important role in attenuating interstitial inflammation.

Lipopolysaccharide-activated SHP-1-deficient motheaten microglia release increased nitric oxide, TNF-alpha, and IL-1beta

Accumulating evidence suggests a deleterious role for activated microglia in facilitating neuronal death by producing neurocytotoxic substances during injury, infection, or neurodegenerative diseases. After cochlear ablation, abnormal microglial activation accompanied by increased neuronal loss within the auditory brainstem occurs in motheaten (me/me) mice deficient in the protein tyrosine phosphatase SHP-1. To determine whether abnormally activated microglia contribute to neuronal death in me/me mice, primary microglial cultures from me/me and wild-type mouse cortices were stimulated by the bacterial endotoxin lipopolysaccharide (LPS) to evaluate the secretion of the neurotoxic mediators nitric oxide (NO), tumor necrosis factor-alpha (TNF-alpha), and interleukin-1beta (IL-1beta). Me/me microglia release significantly greater amounts of all three mediators compared with wild-type microglia. However, the increased release of these compounds in microglia lacking SHP-1 does not appear to occur through activation of extracellular signal-regulated kinase (ERK), p38 kinase subgroups of mitogen-activated protein (MAP) kinases, or increases in NF-kappaB-inducing kinase (NIK). These results suggest that abnormal microglial activation and release of neurotoxic compounds may potentiate neuronal death in deafferented cells and can thus potentiate neurodegeneration in the me/me brainstem. Our data also indicate that SHP-1 is engaged in signaling pathways in LPS-activated microglia, but not through regulation of the ERK and p38 MAP kinases.

Regulation of Transport of the Angiotensin AT2 Receptor by a Novel Membrane-Associated Golgi Protein

Objective— Synthesis and maturation of G protein–coupled receptors are complex events that require an intricate combination of processes including protein folding, posttranslational modifications, and transport through distinct cellular compartments. Little is known concerning the regulation of G protein–coupled receptor transport from the endoplasmic reticulum to the cell surface.

Methods and Results— Here we show that the cytoplasmatic carboxy-terminal of the angiotensin AT2 receptor (AT2R) acts independently as an endoplasmic reticulum–export signal. Using a yeast two-hybrid system, we identified a Golgi membrane–associated protein termed ATBP50 (for AT2R binding protein of 50 kDa) that binds to this motif. We also cloned ATBP60 and ATBP135 encoded by the same gene as ATBP50 that mapped to chromosomes 8p21.3. Downregulation of ATBP50 using siRNA leads to retention of AT2R in inner compartments, reduced cell surface expression, and decreased antiproliferative effects of the receptor.

Conclusion— Our results indicate that ATBP50 regulates the transport of the AT2R to cell membrane by binding to a specific motif within its cytoplasmic carboxy-terminal and thereby enabling the antiproliferative effects of the receptor.

Trans-inactivation of receptor tyrosine kinases by novel angiotensin II AT2 receptor-interacting protein, ATIP

Negative regulation of mitogenic pathways is a fundamental process that remains poorly characterized. The angiotensin II AT2 receptor is a rare example of a 7-transmembrane domain receptor that negatively cross-talks with receptor tyrosine kinases to inhibit cell growth. In the present study, we report the molecular cloning of a novel protein, ATIP1 (AT2-interacting protein), which interacts with the C-terminal tail of the AT2 receptor, but not with those of other receptors such as angiotensin AT1, bradykinin BK2, and adrenergic beta(2) receptor. ATIP1 defines a family of at least four members that possess the same domain of interaction with the AT2 receptor, contain a large coiled-coil region, and are able to dimerize. Ectopic expression of ATIP1 in eukaryotic cells leads to inhibition of insulin, basic fibroblast growth factor, and epidermal growth factor-induced ERK2 activation and DNA synthesis, and attenuates insulin receptor autophosphorylation, in the same way as the AT2 receptor. The inhibitory effect of ATIP1 requires expression, but not ligand activation, of the AT2 receptor and is further increased in the presence of Ang II, indicating that ATIP1 cooperates with AT2 to transinactivate receptor tyrosine kinases. Our findings therefore identify ATIP1 as a novel early component of growth inhibitory signaling cascade.

Regulation of Inhibitory Protein-κB and Monocyte Chemoattractant Protein-1 by Angiotensin II Type 2 Receptor-Activated Src Homology Protein Tyrosine Phosphatase-1 in Fetal Vascular Smooth Muscle Cells

In the present study we examined the effects of angiotensin II (Ang II) type 2 (AT(2)) receptor stimulation on AT(1) receptor-mediated monocyte chemoattractant protein-1 (MCP-1) expression and the possible mechanisms of AT(2) receptor-mediated signaling in cultured rat fetal vascular smooth muscle cells, which express both AT(1) and AT(2) receptors. Ang II stimulation induced MCP-1 mRNA expression as well as an increase in nuclear factor-kappaB (NF-kappaB) binding to the corresponding cis DNA element of the MCP-1 promoter region and a decrease in the cytosolic inhibitory protein-kappaB (IkappaB) protein level via AT(1) receptor stimulation, whereas stimulation of the AT(2) receptor decreased Ang II-induced MCP-1 expression, NF-kappaB DNA binding, and IkappaB degradation, suggesting that activation of the AT(2) receptor attenuated AT(1) receptor-mediated MCP-1 expression via a decrease in NF-kappaB DNA binding and an increase in IkappaB stability. Moreover, we demonstrated that AT(2) receptor stimulation attenuated TNFalpha-mediated NF-kappaB activation and MCP-1 expression. A tyrosine phosphatase inhibitor, orthovanadate, attenuated the AT(2) receptor-mediated increase in IkappaB protein. Moreover, we observed that two IkappaB subunits (IkappaBalpha and IkappaBbeta) were tyrosine-phosphorylated after Ang II stimulation. Transfection of a dominant-negative Src homology protein tyrosine phosphatase-1 mutant into vascular smooth muscle cells inhibited the AT(2) receptor-mediated increase in IkappaB, leading to a significant increase in AT(1) receptor-induced NF-kappaB activation and MCP-1 expression. Taken together, our results demonstrated that AT(2) receptor stimulation attenuated MCP-1 expression via IkappaB stabilization, and Src homology protein tyrosine phosphatase-1 might play a critical role in the transcriptional regulation of MCP-1 expression through the control of IkappaB protein stability.

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 II blocks nicotine-mediated neuroprotection against beta-amyloid (1-42) via activation of the tyrosine phosphatase SHP-1

We showed recently that nicotine activates the growth-promoting enzyme Janus kinase 2 (JAK2) in PC12 cells and that preincubation of these cells with the JAK2-specific inhibitor AG-490 blocked the nicotine-induced neuroprotection against beta-amyloid (1-42) [Abeta (1-42)]. These results provided direct evidence for linkage between JAK2 and the alpha7 nicotinic acetylcholine receptor-induced neuroprotection in PC12 cells. We also showed that preincubation with angiotensin II (Ang II), functioning via the angiotensin II type 2 (AT2) receptor, blocked both the nicotine-induced activation of JAK2 and its neuroprotection against Abeta (1-42). Recently growth-inhibitory effects of the AT2 receptor have been reported to be mediated by the activation of protein tyrosine phosphatases (PTPases) and that AT2 receptor stimulation is associated with a rapid activation of the PTPase SHP-1 (the cytoplasmic tyrosine phosphatase that contains Src homology 2 domains), a negative regulator of JAK2 signaling. Therefore, the potential biological significance of AT2 receptor-induced effects on both the nicotine-induced activation of JAK2 and its neuroprotection against Abeta (1-42) led us to investigate whether SHP-1 activation could be involved in this process. We found that Ang II induced the activation of SHP-1 and that an antisense against SHP-1 not only augmented the nicotine-induced tyrosine phosphorylation of JAK2 but also blocked the Ang II neutralization of the nicotine-induced neuroprotection. These results demonstrate that nicotine-induced tyrosine phosphorylation of JAK2 and neuroprotection against Abeta (1-42) in PC12 cells are blocked by Ang II via AT2 receptor-induced activation of SHP-1.

Angiotensin AT2 receptors: cardiovascular hope or hype?

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

Angiotensin II modulates nitric oxide-induced cardiac fibroblast apoptosis by activation of AKT/PKB

Previously we found that interleukin-1beta (IL-1beta)-activated inducible nitric oxide (NO) synthase (iNOS) expression and that NO production can trigger cardiac fibroblast (CFb) apoptosis. Here, we provide evidence that angiotensin II (ANG II) significantly attenuated IL-1beta-induced iNOS expression and NO production in CFbs while simultaneously decreasing apoptotic frequency. The anti-apoptotic effect of ANG II was abolished when cells were pretreated with the specific ANG II type 1 receptor (AT1) antagonist losartan, but not by the AT2 antagonist DP-123319. Furthermore, ANG II also protected CFbs from apoptosis induced by the NO donor diethylenetriamine NONOate and this effect was associated with phosphorylation of Akt/protein kinase B at Ser473. The effects of ANG II on Akt phosphorylation and NO donor-induced CFb apoptosis were abrogated when cells were preincubated with the specific phosphatidylinositol 3-kinase inhibitors wortmannin or LY-294002. These data demonstrate that ANG II protection of CFbs from IL-1beta-induced apoptosis is associated with downregulation of iNOS expression and requires an intact phosphatidylinositol 3-kinase-Akt survival signal pathway. The findings suggest that ANG II and NO may play a role in regulating the cell population size by their countervailing influences on cardiac fibroblast viability.

Ceramide in apoptosis: an overview and current perspectives

Recent years have witnessed significant advances in the understanding of the role of ceramide in apoptosis. This review summarizes these recent findings and discusses insights from studies of ceramide metabolism, topology, and effector actions. The recent identification of several genes for enzymes of ceramide metabolism, the development of mass spectrometric methods for ceramide analysis, and the increasing molecular and pharmacological tools to probe ceramide metabolism and function promise an accelerated phase in defining the molecular and biochemical details of the role of ceramide in apoptosis.

Identification of the region of AT2 receptor needed for inhibition of the AT1 receptor-mediated inositol 1,4,5-triphosphate generation

Increase in the intracellular inositol triphosphate (IP3) levels in Xenopus oocytes in response to expression and activation of rat angiotensin II (Ang II) receptor AT1 was inhibited by co-expression of rat AT2 receptor. To identify which region of the AT2 was involved in this inhibition, ability of three AT2 mutants to abolish this inhibition was analyzed. Deletion of the C-terminus of the AT2 did not abolish this inhibition. Replacing Ile249 in the third intracellular loop (3rd ICL) of the AT2 with proline, corresponding amino acid in the AT1, in the mutant M6, resulted in slightly reduced affinity to [125I]Ang II (K(d)=0.259 nM), however, did not abolish the inhibition. In contrast, replacing eight more amino acids in the 3rd ICL of the AT2 (at positions 241-244, 250-251 and 255-256) with that of the AT1 in the mutant M8, not only increased the affinity of the AT2 receptor to [125I]Ang II (K(d)=0.038 nM) but also abolished AT2-mediated inhibition. Interestingly, activation of the M8 by Ang II binding also resulted in increase in the intracellular IP(3) levels in oocytes. These results imply that the region of the 3rd ICL of AT2 spanning amino acids 241-256 is sufficient for the AT2-mediated inhibition of AT1-stimulated IP3 generation. Moreover, these nine mutations are also sufficient to render the AT2 with the ability to activate phospholipase C.

Gbeta gamma -independent constitutive association of Galpha s with SHP-1 and angiotensin II receptor AT2 is essential in AT2-mediated ITIM-independent activation of SHP-1

Conventional mode of activation of SH2 domain-containing phosphatase 1 (SHP-1) by a single transmembrane (TM) inhibitory receptor such as killer cell inhibitory receptor, Fcgamma receptor type IIb1, and paired Ig-like receptors of inhibitory types requires tyrosine phosphorylation of immunoreceptor tyrosine-based inhibitory (ITIM) motifs in the cytoplasmic domains of the inhibitory receptors. Contrary to this paradigm, AT(2), a G protein-coupled 7TM receptor that does not undergo tyrosine phosphorylation in response to angiotensin II (Ang II) stimulation, also activates SHP-1. Here we show that SHP-1 constitutively and physically associates with AT(2) receptor in transfected COS-7 cells. On stimulation by Ang II, SHP-1 becomes activated and dissociated from AT(2) receptor, independent of pertussis toxin. Cotransfection of transducin G(betagamma) inhibits SHP-1/AT(2) association and the SHP-1 activation, whereas cotransfection of C-terminal of beta-adrenergic receptor kinase, which abrogates G(betagamma) signaling, facilitates SHP-1 activation. Surprisingly, SHP-1/AT(2) association and the SHP-1 activation requires the presence of G(alphas) as shown by differential coimmunoprecipitation, dominant negative G(alphas), constitutively active G(alphas), and G(alpha) peptides. A mutant AT(2) receptor D141A-R142L that is inactive in G(alpha) protein activation constitutively associates with SHP-1 and activates it. Together, these results indicate that G(alphas) alone, rather than exclusively in the form of G(alphabetagamma) heterotrimer may facilitate signal transduction for G protein-coupled receptors, suggesting a novel mechanism distinct from the classic paradigm of heterotrimeric G proteins. The AT(2)-mediated ITIM-independent activation of SHP-1 that is distinct from the conventional mode of activation, may represent a general paradigm for activation of SHP-1/2-class tyrosine phosphatases by G protein-coupled receptors.

Angiotensin II subtype 2 receptor activation inhibits insulin-induced phosphoinositide 3-kinase and Akt and induces apoptosis in PC12W cells

In the present study, we identified novel negative cross-talk between the angiotensin II subtype 2 (AT2) receptor and insulin receptor signaling in the regulation of phosphoinositide 3-kinase (PI3K), Akt, and apoptosis in rat pheochromocytoma cell line, PC12W cells, which exclusively express AT2 receptor. We demonstrated that insulin-mediated insulin receptor substrate (IRS)-2-associated PI3K activity was inhibited by AT2 receptor stimulation, whereas IRS-1-associated PI3K activity was not significantly influenced. AT2 receptor stimulation did not change insulin-induced tyrosine phosphorylation of IRS-2 or its association with the p85alpha subunit of PI3K, but led to a significant reduction of insulin-induced p85alpha phosphorylation. AT2 receptor stimulation increased the association of a protein tyrosine phosphatase, SHP-1, with IRS-2. Moreover, we demonstrated that AT2 receptor stimulation inhibited insulin-induced Akt phosphorylation and that insulin-mediated antiapoptotic effect was also blocked by AT2 receptor activation. Overexpression of a catalytically inactive dominant negative SHP-1 markedly attenuated the AT2 receptor- mediated inhibition of IRS-2-associated PI3K activity, Akt phosphorylation, and antiapoptotic effect induced by insulin. Taken together, these results indicate that AT2 receptor-mediated activation of SHP-1 and the consequent inhibition IRS-2-associated PI3K activity contributed at least partly to the inhibition of Akt phosphorylation, thereby inducing apoptosis.

Role of C-terminal cytoplasmic domain of the AT2 receptor in ligand binding and signaling

A stop codon at position 322 was introduced to generate a truncated, C-terminal-deleted AT2 receptor. Expression studies in Xenopus oocytes showed that C-terminal-deleted AT2 had reduced affinity to [(125)I]angiotensin II (K(d)=1.7 nM) and enhanced binding of the AT2-specific peptidic ligand [(125)I]CGP42112A (K(d)=0.097 nM). AT2 activation by angiotensin II resulted in reduction of cGMP levels in oocytes and this reduction was further enhanced by C-terminal deletion, implying that the C-terminus may have a negative effect on the AT2-mediated cGMP reduction. Moreover, interaction of the AT2 with the ATP-binding domain of the human ErbB3 receptor in yeast two-hybrid assay was abolished by C-terminal deletion. In summary, the C-terminal cytoplasmic tail of AT2 modulates its ligand binding and signaling properties.

Alu DNA polymorphism in ACE gene is protective for age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of blindness in the elderly. We report an association between an Alu polymorphism in the angiotensin-converting enzyme (ACE) gene with the dry/atrophic form of AMD. Using the polymerase chain reaction (PCR) on genomic DNA isolated from patients with AMD (n=173), and an age-matched control population (n=189), we amplified a region polymorphic for an Alu element insertion in the ACE gene. The Alu(+/+) genotype occurred 4.5 times more frequently in the control population than the dry/atrophic AMD patient population, (p=0.004). The predominance of the Alu(+/+) genotype within the unaffected control group represents a protective insertion with respect to the human ocular disease, dry/atrophic AMD. This is the first demonstration of an Alu element insertion exerting protective effects against a known human disease.

Tumor necrosis factor-alpha inhibits growth factor-mediated cell proliferation through SHP-1 activation in endothelial cells

Src homology 2-containing protein-tyrosine phosphatase 1 (SHP-1) is known to regulate signal transduction through the dephosphorylation of tyrosine kinases. In this study, we addressed the role of SHP-1 under tumor necrosis factor-alpha (TNF-alpha) stimulation in endothelial cells. The addition of recombinant vascular endothelial growth factor (50 ng/mL) or epidermal growth factor (50 ng/mL) significantly increased thymidine incorporation and c-fos promoter activity, whereas TNF-alpha (5 ng/mL) attenuated these effects in human or bovine aortic endothelial cells. In bovine aortic endothelial cells, we confirmed endogenous SHP-1 expression and that TNF-alpha activated SHP-1. Importantly, overexpression of dominant-negative SHP-1 attenuated the effect of TNF-alpha on thymidine incorporation and c-fos promoter activity. In addition, TNF-alpha attenuated vascular endothelial growth factor- and epidermal growth factor-induced extracellular signal-regulated kinase phosphorylation, whereas overexpression of dominant-negative SHP-1 prevented this inhibitory effect of TNF-alpha. Taken together, our results suggested that TNF-alpha inhibited growth factor-mediated cell proliferation through SHP-1 activation.

Estrogen activates phosphatases and antagonizes growth-promoting effect of angiotensin II

Accumulating evidence suggests that estrogen exerts cardioprotective effects and protects against neointima formation in response to vascular injury in vivo, whereas angiotensin (Ang) II stimulation via the Ang II type 1 (AT(1)) receptor exaggerates vascular injury. We postulate that estrogen treatment antagonizes the AT(1) receptor-mediated growth-promoting effects in vascular smooth muscle cells (VSMCs). The present in vitro study was designed to explore this possibility and to establish the cellular mechanism whereby estrogen attenuates the growth of VSMCs. Primary cultures of VSMCs derived from male adult Sprague-Dawley rats express exclusively AT(1) receptors. Treatment with Ang II enhanced proliferation of VSMC and c-fos expression, whereas 17beta-estradiol (E2) attenuated these vasotrophic effects of Ang II. We also demonstrated that E2 attenuated AT(1) receptor-mediated extracellular signal-regulated kinase activation and that this effect of E2 was restored by pretreatment with vanadate or okadaic acid. Moreover, we demonstrated that E2 enhanced SHP-1 activity, rapidly reaching a peak after 3 minutes of E2 stimulation, whereas E2 transactivated mitogen-activated protein kinase phosphatase-1 expression, showing a peak after 60 minutes of E2 treatment. SHP-1 activation was not influenced by actinomycin D treatment, whereas E2-mediated mitogen-activated protein kinase phosphatase-1 expression was attenuated. Taken together, our results suggest a novel mechanism of vasoprotection by which estrogen antagonizes the effect of the AT(1) receptor via the activation and induction of phosphatases through nongenomic as well as genomic signaling.

Regulation of MAP kinase activity by peptide receptor signalling pathway: paradigms of multiplicity

G protein-coupled receptors (GPCRs) can stimulate the mitogen-activated protein kinase (MAPK) cascade and thereby induce cellular proliferation like receptor tyrosine kinases (RTKs). Work over the past 5 years has established several models which reduce the links of G(i)-, G(q)-, and G(s)-coupled receptors to MAPK on few principle pathways. They include (i) Ras-dependent activation of MAPK via transactivation of RTKs such as the epidermal growth factor receptor (EGFR), (ii) Ras-independent MAPK activation via protein kinase C (PKC) that converges with the RTK signalling at the level of Raf, and (iii) activation as well as inactivation of MAPK via the cAMP/protein kinase A (PKA) pathway in dependency on the type of Raf. Most of these generalizing hypotheses are founded on experimental data obtained from expression studies and using a limited set of individual receptors. This review will compare these models with pathways to MAPK found for a great variety of peptide hormone and neuropeptide receptor subtypes in various cells. It becomes evident that under endogenous conditions, the transactivation pathway is less dominant as postulated, whereas pathways involving isoforms of PKC and, especially, phosphoinositide 3-kinase (PI-3K) appear to play a more important role as assumed so far. Highly cell-specific and unusual connections of signalling proteins towards MAPK, in particular tumour cells, might provide points of attacks for new therapeutic concepts.

The angiotensin II AT2 receptor is an AT1 receptor antagonist

The vasopressor angiotensin II activates AT(1) and AT(2) receptors. Most of the known in vivo effects of angiotensin II are mediated by AT(1) receptors while the biological functions of AT(2) receptors are less clear. We report here that the AT(2) receptor binds directly to the AT(1) receptor and thereby antagonizes the function of the AT(1) receptor. The AT(1)-specific antagonism of the AT(2) receptor was independent of AT(2) receptor activation and signaling, and it was effective on different cells and on human myometrial biopsies with AT(1)/AT(2) receptor expression. Thus, the AT(2) receptor is the first identified example of a G-protein-coupled receptor which acts as a receptor-specific antagonist.

Angiotensin II type 2 receptor inhibits epidermal growth factor receptor transactivation by increasing association of SHP-1 tyrosine phosphatase

Angiotensin (Ang) II has 2 major receptor isoforms, Ang type 1 (AT(1)) and Ang type (AT(2)). AT(1) transphosphorylates epidermal growth factor receptor (EGFR) to activate extracellular signal-regulated kinase (ERK). Although AT(2) was shown to inactivate ERK, the action of AT(2) on EGFR activation remains undefined. Using AT(2)-overexpressing vascular smooth muscle cells from AT(2) transgenic mice, we studied these undefined actions of AT(2). Maximal ERK activity induced by Ang II was increased 1.9- and 2.2-fold by AT(2) inhibition, which was abolished by orthovanadate but not okadaic acid or pertussis toxin. AT(2) inhibited AT(1)-mediated EGFR tyrosine phosphorylation by 63%. The activity of SHP-1 tyrosine phosphatase was significantly upregulated 1 minute after AT(2) stimulation and association of SHP-1 with EGFR was increased, whereas AT(2) failed to tyrosine phosphorylate SHP-1. Stable overexpression of SHP-1-dominant negative mutant completely abolished AT(2)-mediated inhibition of EGFR and ERK activation. AT(1)-mediated c-fos mRNA accumulation was attenuated by 48% by AT(2) stimulation. Induction of fibronectin gene containing an AP-1 responsive element in its 5'-flanking region was decreased by 37% after AT(2) stimulation, corresponding to the results of gel mobility assay with the AP-1 sequence of fibronectin as a probe. These findings suggested that AT(2) inhibits ERK activity by inducing SHP-1 activity, leading to decreases in AP-1 activity and AP-1-regulated gene expression, in which EGFR dephosphorylation plays an important role via association of SHP-1.

Beneficial effects on skeletal muscle of the angiotensin II type 1 receptor blocker irbesartan in experimental heart failure

BACKGROUND

In congestive heart failure (CHF), skeletal muscle shows increased expression of fast myosin heavy chains (MHC) and fibers, muscle atrophy, increased fatigability, and decreased endurance. Atrophy is secondary to myocyte apoptosis, which is probably triggered by tumor necrosis factor-alpha (TNFalpha). Angiotensin II receptors are thought to play a role in controlling apoptosis. We tested the hypothesis that angiotensin II receptor blockade could prevent skeletal muscle apoptosis in rats with CHF.

METHODS AND RESULTS

CHF was induced by injecting 36 rats with 30 mg/kg monocrotaline. Ten additional animals were injected with saline and acted as controls. After 2 weeks, 18 of the 36 rats with CHF were treated with 7 mg. kg(-1). d(-1) irbesartan through osmotic minipumps, and 10 of the 36 rats were treated with 2 mg. kg(-1). d(-1) nifedipine in drinking water. After 2 additional weeks, rats were killed. Tibialis anterior cross-sectional area, MHC composition, myocyte apoptosis, Bcl-2, pro-caspase 3, and activated caspases 3 and 9 were determined, as were plasma levels of TNFalpha and angiotensin II. Myocyte apoptosis and muscle atrophy were significantly decreased with irbesartan compared with untreated CHF rats. Irbesartan-treated rats had fewer cells labeled positively with terminal deoxynucleotidal transferase-mediated dUTP nick-end labeling and fewer caspases; however, they also had increased Bcl-2 levels and muscle fiber cross-sectional areas. The MHC pattern in irbesartan-treated animals was similar to that in controls. Nifedipine animals behaved like the untreated CHF animals. Angiotensin II was increased 3- to 4-fold in all CHF rats (treated and untreated). TNFalpha levels were decreased in irbesartan-treated rats but not in nifedipine-treated rats.

CONCLUSIONS

Angiotensin II receptor blockade can protect from the development of apoptosis-dependent atrophy and from changes in MHCS: The reduction of TNFalpha may play a role in this process.

Effect of angiotensin II type 2 receptor on tyrosine kinase Pyk2 and c-Jun NH2-terminal kinase via SHP-1 tyrosine phosphatase activity: evidence from vascular-targeted transgenic mice of AT2 receptor

Angiotensin II (Ang II) has two major receptor isoforms, AT1 and AT2. AT1 transphosphorylates Ca(2+)-sensitive tyrosine kinase Pyk2 to activate c-Jun NH2-terminal kinase (JNK). Although AT2 inactivates extracellular signal-regulated kinase (ERK) via tyrosine phosphatases (PTP), the action of AT2 on Pyk2 and JNK remains undefined. Using AT2-overexpressing vascular smooth muscle cells (AT2-VSMC) from AT2-transgenic mice, we studied these undefined actions of AT2. AT1-mediated JNK activity was increased 2.2-fold by AT2 inhibition, which was abolished by orthovanadate. AT2 did not affect AT1-mediated Pyk2 phosphorylation, but attenuated c-Jun mRNA accumulation by 32%. The activity of src-homology 2 domain-containing PTP (SHP-1) was significantly upregulated 1 min after AT2 stimulation. Stable overexpression of SHP-1 dominant negative mutant in AT2-VSMC completely abolished AT2-mediated inhibition of JNK activation and c-Jun expression. These findings suggest that AT2 inhibits JNK activity by affecting the downstream signal of Pyk2 in a SHP-1-dependent manner, leading to a decrease in c-Jun expression.

Angiotensin II type 2 receptors: signalling and pathophysiological role

The signalling mechanisms and biological significance of the angiotensin II type 2 receptor have long been unknown. In recent years, studies, first in cell culture models but now increasingly also in vivo, have shed some light on the molecular events occurring after a stimulation of the receptor with its ligand as well as on its physiological effects and its significance for pathophysiological processes. There is increasing evidence that the angiotensin II type 2 receptor is involved in different pathophysiological processes, such as myocardial infarction, heart and kidney failure, and stroke.

Functional reconstitution of the angiotensin II type 2 receptor and G(i) activation

On the basis of the patterns of conserved amino acid sequence, the angiotensin II type 2 (AT(2)) receptor belongs to the family of serpentine receptors, which relay signals from extracellular stimuli to heterotrimeric G proteins. However, the AT(2) receptor signal transduction mechanisms are poorly understood. We have measured AT(2)-triggered activation of purified heterotrimeric proteins in urea-extracted membranes from cultured COS-7 cells expressing the recombinant receptor. This procedure removes contaminating GTP-binding proteins without inactivating the serpentine receptor. Binding studies using [(125)I] angiotensin (Ang) II revealed a single binding site with a K(d)=0.45 and a capacity of 627 fmol/mg protein in the extracted membranes. The AT(2) receptor caused a rapid activation of alpha(i) and alpha(o) but not of alpha(q) and alpha(s), as measured by radioactive guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) binding. Activation required the presence of activated receptors, betagamma, and alpha subunits. As a first step aimed at developing an in vitro assay to examine AT(2) receptor pharmacology, we tested a battery of Ang II-related ligands for their ability to promote AT(1) or AT(2) receptor-catalyzed G(i) activation. Two proteolytic fragments of Ang II, Ang III and Ang1-7, also promoted activation of alpha(i) through the AT(2) receptor. Furthermore, we found that [Sar(1),Ala(8)]Ang II is an antagonist for both AT(1) and AT(2) receptors and that CPG42112 behaves as a partial agonist for the AT(2) receptor. In combination with previous observations, these results show that the AT(2) receptor is fully capable of activating G(i) and provides a new tool for exploring AT(2) receptor pharmacology and interactions with G-protein trimers.

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.