B2 kinin receptor-mediated internalization of bradykinin in DDT1 MF-2 smooth muscle cells is paralleled by sequestration of the occupied receptors

Role of Endothelial G Protein-Coupled Receptor Kinase 2 in Angioedema

Excessive BK (bradykinin) stimulation is responsible for the exaggerated permeabilization of the endothelium in angioedema. However, the molecular mechanisms underlying these responses have not been investigated. BK receptors are Gq-protein-coupled receptors phosphorylated by GRK2 (G protein-coupled receptor kinase 2) with a hitherto unknown biological and pathophysiological significance. In the present study, we sought to identify the functional role of GRK2 in angioedema through the regulation of BK signaling. We found that the accumulation of cytosolic Ca²⁺ in endothelial cells induced by BK was sensitive to GRK2 activity, as it was significantly augmented by inhibiting the kinase. Accordingly, permeabilization and NO production induced by BK were enhanced, as well. In vivo, mice with reduced GRK2 levels in the endothelium (Tie2-CRE/GRK2^fl+/fl-) exhibited an increased response to BK in terms of vascular permeability and extravasation. Finally, patients with reduced GRK2 levels displayed a severe phenotype of angioedema. Taken together, these findings establish GRK2 as a novel pivotal regulator of BK signaling with an essential role in the pathophysiology of vascular permeability and angioedema.

Unveiling the participation of avian kinin ornithokinin and its receptors in the chicken inflammatory response

Vasoactive peptides are key early mediators of inflammation released through activation of different enzymatic systems. The mammalian kinin-kallikrein (K-KLK) system produces bradykinin (BK) through proteolytic cleavage of a kininogen precursor by enzymes named kallikreins. BK acts through specific ubiquitous G-protein coupled receptors (B1R and B2R) to participate in physiological processes and inflammatory responses, such as activation of mononuclear phagocytes. In chickens, the BK-like nonapeptide ornithokinin (OK) has been shown to promote intracellular calcium increase in embryonic fibroblasts and to be vasodilatory in vivo. Also, one of its receptors (B2R) was already cloned. However, the participation of chicken K-KLK system components in the inflammatory response remains unknown and was therefore investigated. We first showed that B1R, B2R and kininogen 1 (KNG1) are expressed in unstimulated chicken tissues and macrophages. We next showed that chicken B1R and B2R are expressed at transcript and protein levels in chicken macrophages and are upregulated by E. coli LPS or avian pathogenic E. coli (APEC) infection. Interestingly, exogenous OK induced internalization and degradation of OK receptors protein, notably B2R. Also, OK induced intracellular calcium increase and potentiated zymosan-induced ROS production and Dextran-FITC endocytosis by chicken macrophages. Exogenous OK itself did not promote APEC killing and had no pro-inflammatory effect. However, when combined with LPS or APEC, OK upregulated cytokine/chemokine gene expression and NO production by chicken macrophages. This effect was not blocked by canonical non-peptide B1R or B2R receptor antagonists but was GPCR- and PI3K/Akt-dependent. In vivo, pulmonary colibacillosis led to upregulation of OK receptors expression in chicken lungs and liver. Also, colibacillosis led to significant upregulation of OK precursor KNG1 expression in liver and in cultured hepatocytes (LMH). We therefore provide hitherto unknown information on how OK and its receptors are involved in inflammation and infection in chickens.

Bradykinin promotes neuron-generating division of neural progenitor cells through ERK activation

During brain development, cells proliferate, migrate and differentiate in highly accurate patterns. In this context, published results indicate that bradykinin functions in neural fate determination, favoring neurogenesis and migration. However, mechanisms underlying bradykinin function are yet to be explored. Our findings indicate a previously unidentified role for bradykinin action in inducing neuron-generating division in vitro and in vivo, given that bradykinin lengthened the G1-phase of the neural progenitor cells (NPC) cycle and increased TIS21 (also known as PC3 and BTG2) expression in hippocampus from newborn mice. This role, triggered by activation of the kinin-B2 receptor, was conditioned by ERK1/2 activation. Moreover, immunohistochemistry analysis of hippocampal dentate gyrus showed that the percentage of Ki67(+) cells markedly increased in bradykinin-treated mice, and ERK1/2 inhibition affected this neurogenic response. The progress of neurogenesis depended on sustained ERK phosphorylation and resulted in ERK1/2 translocation to the nucleus in NPCs and PC12 cells, changing expression of genes such as Hes1 and Ngn2 (also known as Neurog2). In agreement with the function of ERK in integrating signaling pathways, effects of bradykinin in stimulating neurogenesis were reversed following removal of protein kinase C (PKC)-mediated sustained phosphorylation.

Vasoinhibins regulate the inner and outer blood-retinal barrier and limit retinal oxidative stress

Vasoinhibins are prolactin fragments present in the retina, where they have been shown to prevent the hypervasopermeability associated with diabetes. Enhanced bradykinin (BK) production contributes to the increased transport through the blood-retina barrier (BRB) in diabetes. Here, we studied if vasoinhibins regulate BRB permeability by targeting the vascular endothelium and retinal pigment epithelium (RPE) components of this barrier. Intravitreal injection of BK in male rats increased BRB permeability. Vasoinhibins prevented this effect, as did the B2 receptor antagonist Hoe-140. BK induced a transient decrease in mouse retinal and brain capillary endothelial monolayer resistance that was blocked by vasoinhibins. Both vasoinhibins and the nitric oxide (NO) synthase inhibitor L-NAME, but not the antioxidant N-acetyl cysteine (NAC), blocked the transient decrease in bovine umbilical vein endothelial cell (BUVEC) monolayer resistance induced by BK; this block was reversed by the NO donor DETANONOate. Vasoinhibins also prevented the BK-induced actin cytoskeleton redistribution, as did L-NAME. BK transiently decreased human RPE (ARPE-19) cell monolayer resistance, and this effect was blocked by vasoinhibins, L-NAME, and NAC. DETANONOate reverted the blocking effect of vasoinhibins. Similar to BK, the radical initiator Luperox induced a reduction in ARPE-19 cell monolayer resistance, which was prevented by vasoinhibins. These effects on RPE resistance coincided with actin cytoskeleton redistribution. Intravitreal injection of vasoinhibins reduced the levels of reactive oxygen species (ROS) in retinas of streptozotocin-induced diabetic rats, particularly in the RPE and capillary-containing layers. Thus, vasoinhibins reduce BRB permeability by targeting both its main inner and outer components through NO- and ROS-dependent pathways, offering potential treatment strategies against diabetic retinopathies.

Signaling through a G Protein-coupled receptor and its corresponding G protein follows a stoichiometrically limited model

The bradykinin receptor is a G protein-coupled receptor (GPCR) that is coupled to the Galpha(q) family of heterotrimeric G proteins. In general, a GPCR can exert intracellular signals either by transiently associating with multiple diffusing G protein subunits or by activating a G protein that is stably bound to the receptor, thus generating a signal that is limited by the stoichiometry of the complex. Here we have distinguished between these models by monitoring the association of type 2 bradykinin receptor (B(2)R) and the Galpha(q)/Gbetagamma heterotrimer in living human embryonic kidney 293 cells expressing fluorescent-tagged proteins. Stable B(2)R-Galpha(q) x Gbetagamma complexes are observed in resting cells by fluorescence resonance energy transfer from either Galpha(q)-eCFP or eCFP-Gbetagamma to B(2)R-eYFP. Stimulating the cells with bradykinin causes detachment of B(2)R from the G protein subunits as the receptor internalizes into early endosomes, with a corresponding elimination of B(2)R-G protein fluorescence resonance energy transfer because Galpha(q) and its associated Gbetagamma remain on the plasma membrane. Single point and scanning fluorescence correlation spectroscopy measurements show that a portion of B(2)R molecules diffuses with a mobility corresponding to dimers or small oligomers, whereas a second fraction diffuses in higher order molecular assemblies. Our studies support a model in which receptors are pre-coupled with their corresponding G proteins in the basal state of cells thereby limiting the response to an external signal to a defined stoichiometry that allows for a rapid and directed cellular response.

Kinins Promote B2Receptor Endocytosis and Delay Constitutive B1Receptor Endocytosis

Upon sustained insult, kinins are released and many kinin responses, such as inflammatory pain, adapt from a B2 receptor (B2R) type in the acute phase to a B1 receptor (B1R) type in the chronic phase. In this study, we show that kinins modulate receptor endocytosis to rapidly decrease B2R and increase B1R on the cell surface. B2Rs, which require agonist for activity, are stable plasma membrane components without agonist but recruit beta-arrestin 2, internalize in a clathrin-dependent manner, and recycle rapidly upon agonist treatment. In contrast, B1Rs, which are inducible and constitutively active, constitutively internalize without agonist via a clathrin-dependent pathway, do not recruit beta-arrestin 2, bind G protein-coupled receptor sorting protein, and target lysosomes for degradation. Agonist delays B1R endocytosis, thus transiently stabilizing the receptor. Most of the receptor trafficking phenotypes are transplantable from one receptor to the other through exchange of the C-terminal receptor tails, indicating that the tails contain epitopes that are important for the binding of protein partners that participate in the endocytic and postendocytic receptor choices. It is noteworthy that the agonist delay of B1R endocytosis is not transplanted to the B2R via the B1R tail, suggesting that this property of the B1R requires another domain. These events provide a rapid kinin-dependent mechanism for 1) regulating the constitutive B1R activity and 2) shifting the balance of accessible receptors in favor of B1R.

International union of pharmacology. XLV. Classification of the kinin receptor family: from molecular mechanisms to pathophysiological consequences

Kinins are proinflammatory peptides that mediate numerous vascular and pain responses to tissue injury. Two pharmacologically distinct kinin receptor subtypes have been identified and characterized for these peptides, which are named B1 and B2 and belong to the rhodopsin family of G protein-coupled receptors. The B2 receptor mediates the action of bradykinin (BK) and lysyl-bradykinin (Lys-BK), the first set of bioactive kinins formed in response to injury from kininogen precursors through the actions of plasma and tissue kallikreins, whereas the B(1) receptor mediates the action of des-Arg9-BK and Lys-des-Arg9-BK, the second set of bioactive kinins formed through the actions of carboxypeptidases on BK and Lys-BK, respectively. The B2 receptor is ubiquitous and constitutively expressed, whereas the B1 receptor is expressed at a very low level in healthy tissues but induced following injury by various proinflammatory cytokines such as interleukin-1beta. Both receptors act through G alpha(q) to stimulate phospholipase C beta followed by phosphoinositide hydrolysis and intracellular free Ca2+ mobilization and through G alpha(i) to inhibit adenylate cyclase and stimulate the mitogen-activated protein kinase pathways. The use of mice lacking each receptor gene and various specific peptidic and nonpeptidic antagonists have implicated both B1 and B2 receptors as potential therapeutic targets in several pathophysiological events related to inflammation such as pain, sepsis, allergic asthma, rhinitis, and edema, as well as diabetes and cancer. This review is a comprehensive presentation of our current understanding of these receptors in terms of molecular and cell biology, physiology, pharmacology, and involvement in human disease and drug development.

Spontaneous Formation of a Proteolytic B1 and B2 Bradykinin Receptor Complex with Enhanced Signaling Capacity

B1 bradykinin receptor (B1R) induction is critical in the adaptation of the kinin-mediated inflammatory response from a B2 bradykinin receptor (B2R) subtype to a B1R subtype that occurs during chronic insult. Here, we show that B1R spontaneously forms a proteolytic plasma membrane complex with B2R along with increased receptor signaling capacity. Co-expression of hemagglutinin-tagged B2R with FLAG-tagged B1R in HEK293 cells resulted in degradation of B2R as determined by the diminution of the intact 65-kDa B2R species and the appearance of proteolytic B2R products at 30-40 kDa and by the reduction in B2R bradykinin binding sites. On the other hand, the 35-kDa B1R remained intact. Receptor co-expression also led to an increase in constitutive and agonist-stimulated receptor signaling. Selective immunoprecipitation with epitope-specific antibodies revealed a spontaneously formed heterologous receptor complex, which was composed of the intact 35-kDa B1R and the B2R degradation products. Cellular fractionation, cell surface biotinylation, and immunoelectron microscopy showed that B2R.B1R complexes were present on the cell surface. This is the first evidence that a heterologous G protein-coupled receptor complex in the plasma membrane is linked to proteolytic degradation of a participating receptor, and this mechanism may contribute to the adaptation of the kinin response from a B2 type to a B1 type during chronic insult.

Downregulation of bradykinin B2 receptor in human fibroblasts during prolonged agonist exposure

Sustained activation of G protein-coupled receptors results in an attenuation of cellular responses, a phenomenon termed desensitization. Whereas mechanisms for rapid desensitization of ligand-receptor-G protein-effector systems are relatively well characterized, much less is known about long-term adaptation processes that occur in the continuous presence of an agonist. Here we have studied the fate of endogenously expressed bradykinin B(2) receptors on human fibroblasts during prolonged agonist treatment. Stimulation with bradykinin for up to 24 h resulted in a 50% reduction of surface binding sites that was paralleled by a similar decrease of total B(2) receptor protein followed by Western blotting using monoclonal antibodies to the B(2) receptor. Whereas B(2) receptor mRNA levels did not change during 24 h of agonist treatment, B(2) receptor de novo synthesis was attenuated by 35-50%, indicating translational control of B(2) receptor levels. Furthermore, the half-life of B(2) receptor protein was shortened by 20-40% as shown by (35)S-labeled pulse-chase and immunoprecipitation experiments. This study demonstrates that bradykinin B(2) receptor expression during long-term agonist treatment is primarily regulated on the (post)translational level, i.e., by attenuation of de novo synthesis and by reduction of receptor stability.

Facilitation of drug entry into the CNS via transient permeation of blood brain barrier: laboratory and preliminary clinical evidence from bradykinin receptor agonist, Cereport

One novel approach of transporting drugs into the central nervous system (CNS) involves the activation of receptors on the endothelial cells comprising the blood brain barrier (BBB). Recently the selective B(2) bradykinin receptor agonist, Cereport (also called RMP-7), has been shown to transiently increase permeability of the BBB. Although initially developed to increase the permeability of the vasculature feeding glioma, recent studies have demonstrated that Cereport also increases the delivery of pharmacological agents across the normal (i.e. nontumor) BBB. In this review paper, we discuss evidence of enhanced CNS delivery of carboplatin, loperamide, and cyclosporin-A, which are accompanied by enhanced chemotherapeutic, analgesic and neuroprotective effects, respectively. These observations suggest feasibility of Cereport as an adjunct therapy to pharmacological treatments that require drug availability in the CNS to exert therapeutic efficacy. Because many potential drugs for CNS disorders normally do not cross the BBB, Cereport-induced transient permeation of BBB stands as an efficacious strategy for enhancing pharmacotherapy.

Activation of bradykinin B1 receptor by ACE inhibitors

ACE or kininase II inhibitors are very important, widely used therapeutic agents for the treatment of a variety of diseases. Although they inhibit ACE, thus, angiotensin II release and bradykinin (BK) inactivation, this inhibition alone does not suffice to explain their successful application in medical practice. Enalaprilat and other ACE inhibitors at nanomolar concentrations activate the BK B1 receptor directly in the absence of ACE and the peptide ligands, des-Arg-kinins. The inhibitors activate at the Zn-binding pentameric consensus sequence HEXXH (195 -199) of B1, a motif also present in the active centers of ACE but absent from the BK B2 receptor. ACE inhibitors, when activating the B1 receptor, elevate intracellular calcium [Ca2+]i and release NO from cultured cells. Activation by ACE inhibitor was abolished by Ca-EDTA, a B1 receptor antagonist, by a synthetic undecapeptide representing the 192-202 sequence in the B1 receptor, and by site-directed mutagenesis of H195 to A. With the exception of the B1 receptor blocker, these agents and the mutation did not affect the actions of the peptide ligand des-Arg10-Lys1-BK. Ischemia and inflammatory cytokines induce B1 receptors and elevate its expression. Direct activation of the B1 receptor by ACE inhibitors can contribute to their therapeutic efficacy, for example, by releasing NO in vascular beds, or to some of their side effects.

Determination of bradykinin B2 receptor in vivo phosphorylation sites and their role in receptor function

Reversible phosphorylation plays important roles in G protein-coupled receptor signaling, desensitization, and endocytosis, yet the precise location and role of in vivo phosphorylation sites is unknown for most receptors. Using metabolic 32P labeling and phosphopeptide sequencing we provide a complete phosphorylation map of the human bradykinin B2 receptor in its native cellular environment. We identified three serine residues, Ser(339), Ser(346), and Ser(348), at the C-terminal tail as principal phosphorylation sites. Constitutive phosphorylation occurs at Ser(348), while ligand-induced phosphorylation is found at Ser(339) and Ser(346)/Ser(348) that could be executed by several G protein-coupled receptor kinases. In addition, we found a protein kinase C-dependent phosphorylation of Ser(346) that was mutually exclusive with the basal phosphorylation at Ser(348) and therefore may be implicated in differential regulation of B2 receptor activation. Functional analysis of receptor mutants revealed that a low phosphorylation stoichiometry is sufficient to initiate receptor sequestration while a clustered phosphorylation around Ser(346) is necessary for desensitization of the B2 receptor-induced phospholipase C activation. This was further supported by the specifically reduced Ser(346)/Ser(348) phosphorylation observed upon stimulation with a nondesensitizing B2 receptor agonist. The differential usage of clustered phosphoacceptor sites points to distinct roles of multiple kinases in controlling G protein-coupled receptor function.

Kinin receptors in pain and inflammation

Kinins are among the most potent autacoids involved in inflammatory, vascular and pain processes. These short-lived peptides, including bradykinin, kallidin and T-kinin, are generated during tissue injury and noxious stimulation. However, emerging evidence also suggests that kinins are stored in neuronal elements of the central nervous system (CNS) where they are thought to play a role as neuromediators in various cerebral functions, particularly in the control of nociceptive information. Kinins exert their biological effects through the activation of two transmembrane G-protein-coupled receptors, denoted bradykinin B(1) and B(2). Whereas the B(2) receptor is constitutive and activated by the parent molecules, the B(1) receptor is generally underexpressed in normal tissues and is activated by kinins deprived of the C-terminal Arg (des-Arg(9)-kinins). The induction and increased expression of B(1) receptor occur following tissue injury or after treatment with bacterial endotoxins or cytokines such as interleukin-1 beta and tumor necrosis factor-alpha. This review summarizes the most recent data from various animal models which convey support for a role of B(2) receptors in the acute phase of the inflammatory and pain response, and for a role of B(1) receptors in the chronic phase of the response. The B(1) receptor may exert a strategic role in inflammatory diseases with an immune component (diabetes, asthma, rheumatoid arthritis and multiple sclerosis). New information is provided regarding the role of sensory mechanisms subserving spinal hyperalgesia and intrapleural neutrophil migration that occur upon B(1) receptor activation in streptozotocin-treated rats, a model of insulin-dependent diabetes mellitus in which the B(1) receptor seems to be rapidly overexpressed. Although it is widely accepted that the blockade of kinin receptors with specific antagonists could be of benefit in the treatment of somatic and visceral inflammation and pain, recent molecular and functional evidence suggests that the activation of B(1) receptors with an agonist may afford a novel therapeutic approach in the CNS inflammatory demyelinating disorder encountered in multiple sclerosis by reducing immune cell infiltration (T-lymphocytes) into the brain. Hence, the B(1) receptor may exert either a protective or detrimental effect depending on the inflammatory disease. This dual function of the B(1) receptor deserves to be investigated further.

Agonist-promoted trafficking of human bradykinin receptors: arrestin- and dynamin-independent sequestration of the B2 receptor and bradykinin in HEK293 cells

In this study, we analysed the agonist-promoted trafficking of human B(2) (B(2)R) and B(1) (B(1)R) bradykinin (BK) receptors using wild-type and green fluorescent protein (GFP)-tagged receptors in HEK293 cells. B(2)R was sequestered to a major extent upon exposure to BK, as determined by the loss of cell-surface B(2)R using radioligand binding and by imaging of B(2)R-GFP using laser-scanning confocal fluorescence microscopy. Concurrent BK sequestration was revealed by the appearance of acid-resistant specific BK receptor binding. The same techniques showed that B(1)R was sequestered to a considerably lesser extent upon binding of des-Arg(10)-kallidin. B(2)R sequestration was rapid (half-life approximately 5 min) and reached a steady-state level that was significantly lower than that of BK sequestration. B(2)R sequestration was minimally inhibited by K44A dynamin (22.4+/-3.7%), and was insensitive to arrestin-(319-418), which are dominant-negative mutants of dynamin I and beta-arrestin respectively. Furthermore, the B(2)R-mediated sequestration of BK was completely insensitive to both mutants, as was the association of BK with a caveolae-enriched fraction of the cells. On the other hand, agonist-promoted sequestration of the beta(2)-adrenergic receptor was dramatically inhibited by K44A dynamin (81.2+/-16.3%) and by arrestin-(319-418) (36.9+/-4.4%). Our results show that B(2)R is sequestered to a significantly greater extent than is B(1)R upon agonist treatment in HEK293 cells. Furthermore, B(2)R appears to be recycled in the process of sequestering BK, and this process occurs in a dynamin- and beta-arrestin-independent manner and, at least in part, involves caveolae.

G-protein-coupled receptors as targets for gene transfer vectors using natural small-molecule ligands

Gene therapy for cystic fibrosis (CF) has focused on correcting electrolyte transport in airway epithelia. However, success has been limited by the failure of vectors to attach and enter into airway epithelia, and may require redirecting vectors to targets on the apical membrane of airway cells that mediate these functions. The G-protein-coupled P2Y2 receptor (P2Y2-R) is abundantly expressed on the airway lumenal surface and internalizes into coated pits upon agonist activation. We tested whether a small-molecule-agonist (UTP) could direct vectors to P2Y2-R and mediate attachment, internalization, and gene transfer. Fluorescein-UTP studies demonstrated that P2Y2-R agonists internalized with their receptor, and biotinylated UTP (BUTP) mediated P2Y2-R-specific internalization of fluorescently labeled streptavidin (SAF) or SAF conjugated to biotinylated Cy3 adenoviral-vector (BCAV). BUTP conjugated to BCAV mediated P2Y2-R-specific gene transfer in (1) adenoviral-resistant A9 and polarized MDCK cells by means of heterologous P2Y2-R, and (2) well-differentiated human airway epithelial cells by means of endogenous P2Y2-R. Targeting vectors with small-molecule-ligands to apical membrane G-protein-coupled receptors may be a feasible approach for successful CF gene therapy.

Replacement of the transmembrane anchor in angiotensin I-converting enzyme (ACE) with a glycosylphosphatidylinositol tail affects activation of the B2 bradykinin receptor by ACE inhibitors

To investigate further the relationship of angiotensin I-converting enzyme (ACE) inhibitors to activation of the B(2) bradykinin (BK) receptor, we transfected Chinese hamster ovary cells to stably express the human receptor and either wild-type ACE (WT-ACE), an ACE construct with most of the cytosolic portion deleted (Cyt-del-ACE), or ACE with a glycosylphosphatidylinositol (GPI) anchor replacing the transmembrane and cytosolic domains (GPI-ACE). BK or its ACE-resistant analogue were the agonists. All activities (arachidonic acid release and calcium mobilization) were blocked by the B(2) antagonist HOE 140. B(2) was desensitized by repeated administration of BK but resensitized to agonist by ACE inhibitors in the cells expressing both B(2) and either WT-ACE or Cyt-del-ACE. In GPI-ACE expressing cells, the B(2) receptor was still activated by the agonists, but ACE inhibitors did not resensitize. Pretreatment with filipin returned the sensitivity to inhibitors. In immunocytochemistry, GPI-ACE showed patchy, uneven distribution on the plasma membrane that was restored by filipin. Thus, ACE inhibitors were inactive as long as GPI-ACE was sequestered in cholesterol-rich membrane domains. WT-ACE and B(2) receptor in Chinese hamster ovary cells co-immunoprecipitated with antibody to receptor, suggesting an interaction on the cell membrane. ACE inhibitors augment BK effects on receptors indirectly only when enzyme and receptor molecules are sterically close, possibly forming a heterodimer.

Potentiation of the vascular response to kinins by inhibition of myocardial kininases

Inhibitors of angiotensin I-converting enzyme (ACE) are very efficacious in the potentiation of the actions of bradykinin (BK) and are able to provoke a B(2) receptor-mediated vasodilation even after desensitization of this receptor. Because this activity cannot be easily explained only by an inhibition of kinin degradation, direct interactions of ACE inhibitors with the B(2) receptor or its signal transduction have been hypothesized. To clarify the significance of degradation-independent potentiation, we studied the vasodilatory effects of BK and 2 degradation-resistant B(2) receptor agonists in the isolated rat heart, a model in which ACE and aminopeptidase P (APP) contribute equally to the degradation of BK. Coronary vasodilation to BK and to a peptidic (B6014) and a nonpeptidic (FR190997) degradation-resistant B(2) agonist was assessed in the presence or absence of the ACE inhibitor ramiprilat, the APP inhibitor mercaptoethanol, or both. Ramiprilat or mercaptoethanol induced leftward shifts in the BK dose-response curve (EC(50)=3.4 nmol/L) by a factor of 4.6 or 4.9, respectively. Combined inhibition of ACE and APP reduced the EC(50) of BK to 0.18 nmol/L (ie, by a factor of 19) but potentiated the activity of B6014 (EC(50)=1.9 nmol/L) only weakly without altering that of FR190997 (EC(50)=0.34 nmol/L). Desensitization of B(2) receptors was induced by the administration of BK (0.2 micromol/L) or FR190997 (0.1 micromol/L) for 30 minutes; the vascular reactivity to ramiprilat or increasing doses of BK was tested thereafter. After desensitization with BK, but not FR190997, an additional application of ramiprilat provoked a B(2) receptor-mediated vasodilation. High BK concentrations were still effective at the desensitized receptor. The process of desensitization was not altered by ramiprilat. These results show that in this model, all potentiating actions of ACE inhibitors on kinin-induced vasodilation are exclusively related to the reduction in BK breakdown and are equivalently provoked by APP inhibition. The desensitization of B(2) receptors is overcome by increasing BK concentrations, either directly or through the inhibition of ACE. These observations do not suggest any direct interactions of ACE inhibitors with the B(2) receptor or its signal transduction but point to a very high activity of BK degradation in the vicinity of the B(2) receptor in combination with a stimulation-dependent reduction in receptor affinity.

Potentiation of Bradykinin Actions by ACE Inhibitors

Angiotensin I-converting enzyme (kininase II; ACE) inhibitors, antibodies to ACE and slowly cleaved substrates of ACE potentiate the effect of bradykinin and its analogs on their B2 receptors independently of blocking peptide metabolism. ACE inhibitors also resensitized the receptors desensitized by the ligand (tachyphylaxis). The studies were performed on isolated organs and cells co-transfected with the receptor and the enzyme or constitutively expressing them. This enhancement of the effect of B2 ligands is attributed to a crosstalk between the enzyme and the receptor, and not to a direct action on the receptors. It might reflect some of the local activities of ACE inhibitors.

Autoregulation of bradykinin receptors: agonists in the presence of interleukin-1beta shift the repertoire of receptor subtypes from B2 to B1 in human lung fibroblasts

Elevated formation of bradykinin (BK) and Lys-BK or kallidin (KD) and their carboxypeptidase metabolites desArg(9)BK and desArg(10)KD is evident at sites of inflammation. Moreover, B2 receptors (B2R), which mediate the action of BK and KD, participates in the acute stage of the inflammatory and pain response, whereas B1 receptors (B1R), through which desArg(9)BK and desArg(10)KD act, partake in the chronic stage. We hypothesized that kinins autoregulate B2R and B1R expression in favor of B1R. Incubation of IMR-90 cells with BK (100 nM) led to a loss (89%) of B2R with a half-life (T(1/2)) of 7.0 min. Concomitantly, BK increased B1R (2- to 3-fold) with a T(1/2) of 120 min. DesArg(10)KD (100 nM) had no effect on B2R but increased B1R (3- to 4-fold) with the same rate as BK. Interleukin-1beta (IL-1beta; 500 pg/ml) also increased B1R (4- to 6-fold). Although both desArg(10)KD and BK increased the level of IL-1beta mRNA, IL-1beta receptor antagonist inhibited the increase in B1R only in response to BK. DesArg(10)KD and BK synergistically increased B1R (9-fold), which was further increased by inclusion of IL-1beta (36-fold). Therefore, kinin metabolism and kinin-stimulated production of cytokines may play a pivotal role in shifting the repertoire of kinin receptor subtypes in favor of B1R during inflammation.

Pharmacology and cardiovascular implications of the kinin-kallikrein system

Kinins are peptide hormones that can exert a significant influence on the regulation of blood pressure and vascular tone due to their vasodilatatory, natriuretic and growth modulating activity. Their cardiovascular involvement in physiological and pathophysiological situations has been studied intensively since inhibitors for angiotensin I-converting enzyme and selective receptor antagonists have become available for pharmacologically potentiating or inhibiting kinin-mediated reactions. Molecular biological analysis and the establishment of genetically modified animal models have also allowed newer information to be acquired on this subject. In this review, the components and cardiovascularly relevant mechanisms of the kinin-kallikrein system shall be described. Organ-specific effects concerning the kidneys, the vascular system, the heart and nervous tissue shall also be illustrated. On this issue, the physiological functions and pathophysiological implications of the kinin-kallikrein system should be clearly distinguished from the many, mostly endothelium-mediated protective effects which occur during ACE inhibition due to the potentiation of kinin effects. Finally, a view shall also be cast upon newly discovered targets of action, which could be exploited for therapeutically altering the kinin-kallikrein system.

Enhancement of bradykinin and resensitization of its B2 receptor

We studied the enhancement of the effects of bradykinin B2 receptor agonists by agents that react with active centers of angiotensin-converting enzyme (ACE) independent of enzymatic inactivation. The potentiation and the desensitization and resensitization of B2 receptor were assessed by measuring [3H]arachidonic acid release and [Ca2+]i mobilization in Chinese hamster ovary cells transfected to express human ACE and B2 receptor, or in endothelial cells with constitutively expressed ACE and receptor. Administration of bradykinin or its ACE-resistant analogue desensitized the receptor, but it was resensitized (arachidonic acid release or [Ca2+]i mobilization) by agents such as enalaprilat (1 micromol/L). Enalaprilat was inactive in the absence of ACE expression. La3+ (100 micromol/L) inhibited the apparent resensitization, probably by blocking the entry of extracellular calcium. Enalaprilat resensitized the receptor via ACE to release arachidonic acid by bradykinin at a lower concentration (5 nmol/L) than required to mobilize [Ca2+]i (1 micromol/L). Monoclonal antibodies inhibiting the ACE N-domain active center and polyclonal antiserum potentiated bradykinin. The snake venom peptide BPP5a and metabolites of angiotensin and bradykinin (angiotensin-[1-9], angiotensin-[1-7], bradykinin-[1-8]; 1 micromol/L) enhanced arachidonic acid release by bradykinin. Angiotensin-(1-9) and -(1-7) also resensitized the receptor. Enalaprilat potentiated the bradykinin effect in cells expressing a mutant ACE with a single N-domain active site. Agents that reacted with a single active site, on the N-domain or on the C-domain, potentiated bradykinin not by blocking its inactivation but by inducing crosstalk between ACE and the receptor. Enalaprilat enhanced signaling via ACE by Galphai in lower concentration than by Galphaq-coupled receptor.

Agonist-induced redistribution of bradykinin B2 receptor in caveolae

Redistribution of receptors within the plasma membrane as well as between the plasma membrane and various cell compartments presents an important way of regulating the cellular responsiveness to their cognate agonists. We have applied immunocytochemical methods to localize the bradykinin B2 receptor and to examine its agonist induced redistribution in A431 cells. In situ labeling with antibodies to ectodomain-2 of the receptor which do not interfere with bradykinin binding of the receptor showed a random distribution of the B2 receptor on the plasma membrane. Stimulation of cells with 20 nM bradykinin markedly reduced the accessibility of the antibody to its corresponding epitope in non-permeabilized cells. Immuno-electron microscopy revealed the presence of receptors in membrane-near vesicles that are surrounded by an electron-transparent halo. Fluorescence microscopic double labeling co-localized the B2 receptor protein with caveolin-1 by a convergent pattern of punctate staining. At the ultrastructural level the B2 receptor protein was found in vesicles that bear the immunolabel of caveolin-1 and display the morphological characteristics of caveolae. We conclude that stimulation of B2 receptors results in their redistribution and sequestration in caveolae, an event that is likely to be implicated in receptor signaling and/or desensitization. The localization of B2 receptors in endosome-like structures after prolonged exposure to bradykinin might indicate that the internalization through caveolae may communicate with other endocytotic pathways of A431 cells.

Influence of the cytosolic carboxyl termini of human B1 and B2 kinin receptors on receptor sequestration, ligand internalization, and signal transduction

To determine the role of the cytoplasmic carboxyl termini of human B1 and B2 kinin receptors (B1KR and B2KR, respectively) in the internalization of their respective ligands, des-Arg10-kallidin and bradykinin (BK), both wild type receptors, as well as truncated B2KRs, a mutated B2KR, and chimeric receptors were stably expressed in Chinese hamster ovary cells. Incubation of [3H]BK at 37 degrees C with cells expressing wild type B2KR resulted in pronounced and rapid ligand internalization ( approximately 80% after 10 min). By contrast, incubation of 3H-labeled des-Arg10-kallidin with cells expressing B1KR resulted in a modest, slow internalization of the ligand (<20% after 10 min). Replacement, from Cys324, of the cytoplasmic carboxyl terminus of the B2KR with that of the B1KR from Cys330 (both Cys residues are putative palmitoylation sites) greatly reduced ligand internalization ( approximately 40% after 10 min) without significantly altering Kd or ligand-induced signal activation. By marked contrast, the corresponding replacement, of the sequence from Cys330 of the cytoplasmic carboxyl terminus of the B1KR with the segment of the B2KR, led to a striking increase of ligand internalization ( approximately 75% within 10 min) without altering Kd or ligand-induced signal activation. Truncation of the B2KR to within three amino acids of Cys324 (truncation at Gly327) led to strongly reduced ligand internalization ( approximately 40% after 10 min). Truncation of the B2KR up to Lys315 almost completely abolished internalization of [3H]BK (10% after 10 min). This additional reduction is apparently not caused by the loss of the potential palmitoylation site at Cys324, since a B2KR with a point mutation of Cys324 to Ala internalized [3H]BK as rapidly as the wild type B2KR. From these results we conclude that the cytoplasmic carboxyl terminus of the human B2KR contains sequences that are necessary and sufficient to permit rapid ligand-induced sequestration of human kinin receptors and internalization of their agonists.

Negative cooperativity in the human bradykinin B2 receptor

A human kidney bradykinin (BK) B2 receptor cDNA was transfected in CHO-K1 cells to establish cell lines that express stably and at high density a receptor exhibiting B2 receptor properties in terms of coupling to cell signaling effectors, desensitization, and internalization. A cell line with a density of 1.3 x 10(6) receptors/cell allowed us to carry out a detailed study of BK-receptor interaction over a wide range of BK concentrations. A model assuming that BK binds to two receptor affinity states (depending on guanine nucleotide-sensitive coupling) was not sufficient to account for the kinetics of BK binding. Equilibrium kinetic analysis and studies of the effects of receptor occupancy by agonists or antagonists on the kinetics of BK-receptor complex dissociation revealed features typical of negative cooperative binding. The negative cooperativity phenomenon was also observed in isolated membranes in both the presence and absence of guanine nucleotide. Thus, following the interaction with BK, B2 receptor molecules likely interact with each other, resulting in an acceleration of bound ligand dissociation and a decrease in the apparent affinity of the receptor for BK. This phenomenon can participate in the desensitization process.

Potentiation of the actions of bradykinin by angiotensin I-converting enzyme inhibitors. The role of expressed human bradykinin B2 receptors and angiotensin I-converting enzyme in CHO cells

Part of the beneficial effects of angiotensin I-converting enzyme (ACE) inhibitors are due to augmenting the actions of bradykinin (BK). We studied this effect of enalaprilat on the binding of [3H]BK to Chinese hamster ovary (CHO) cells stably transfected to express the human BK B2 receptor alone (CHO-3B) or in combination with ACE (CHO-15AB). In CHO-15AB cells, enalaprilat (1 mumol/L) increased the total number of low-affinity [3H]BK binding sites on the cells at 37 degrees C, but not at 4 degrees C, from 18.4 +/- 4.3 to 40.3 +/- 11.9 fmol/10(6) cells (P < .05; Kd, 2.3 +/- 0.8 and 5.9 +/- 1.3 nmol/L; n = 4). Enalaprilat preserved a portion of the receptors in high-affinity conformation (Kd, 0.17 +/- 0.08 nmol/L; 8.1 +/- 0.9 fmol/10(6) cells). Enalaprilat decreased the IC50 of [Hyp3-Tyr(Me)8]BK, the BK analogue more resistant to ACE, from 3.2 +/- 0.8 to 0.41 +/- 0.16 nmol/L (P < .05, n = 3). The biphasic displacement curve of the binding of [3H]BK also suggested the presence of high-affinity BK binding sites. Enalaprilat (5 nmol to 1 mumol/L) potentiated the release of [3H]arachidonic acid and the liberation of inositol 1,4,5-trisphosphate (IP3) induced by BK and [Hyp3-Tyr(Me)8]BK. Moreover, enalaprilat (1 mumol/L) completely and immediately restored the response of the B2 receptor, desensitized by the agonist (1 mumol/L [Hyp3-Tyr(Me)8]BK); this effect was blocked by the antagonist, HOE 140. Finally, enalaprilat, but not the prodrug enalapril, decreased internalization of the receptor from 70 +/- 9% to 45 +/- 9% (P < .05, n = 7). In CHO-3B cells, enalaprilat was ineffective. ACE inhibitors in the presence of both the B2 receptor and ACE enhance BK binding, protect high-affinity receptors, block receptor desensitization, and decrease internalization, thereby potentiating BK beyond blocking its hydrolysis.

[3H]bradykinin receptor-binding, receptor-recycling, and receptor-internalization of the B2 bradykinin receptor in the murine osteoblast-like cell line MC3T3-E1

Bradykinin (BK) has been demonstrated to induce inositol phosphate production, release of intracellular Ca2+, and prostaglandin E2 (PGE2) synthesis in the murine osteoblast-like cell line MC3T3-E1. Because cellular response to BK is a function of receptor affinity, receptor coupling, and receptor recycling, we investigated kinetic properties, specificity, and regulation at the BK-receptor level on intact, BK-sensitive MC3T3-E1 cells. Our results clearly demonstrate the existence of a single category of binding sites for [3H]BK (kD =366+/-98 pM; Bmax =45.3+/-6.6 fmol/mg of protein). Displacement studies with various BK analogs gave a rank order compatible with a B2 BK-receptor type (BK > Lys-BK > [Hyp3]-BK > Met-Lys-BK > HOE140 > Tyr-BK > Tyr8-BK > D-Arg, [Hyp3, Thi5,8, D-Phe7]-BK > [D-Phe7]-BK > des-Arg9-BK > des-Arg9, [Leu8]-BK = angiotensin II). No atypic high-affinity binding sites for the B1 receptor agonist des-Arg9-BK could be observed. Prestimulation of MC3T3-E1 cells with BK resulted in the disappearance of accessible B2 receptors at the cell surface by internalization. Postexposure of BK-pretreated cells to ligand-free medium resulted in almost complete receptor restoration within 30 minutes, exhibiting an intermediate state of two categories of binding sites (kD1 =444+/-37 pM, Bmax1 =9.2+/-0.3 fmol/mg of protein and kD2 =2.7+/-0.28 pM, Bmax2 =24.2+/-0.2 fmol/mg of protein), probably representing coupled and uncoupled B2 receptors. Prolonged stimulation with BK (2.5-5 h) also revealed the temporal occurrence of two categories of binding sites after 2.5 h (kD1 =228+/-3.5 pM; Bmax1 =15.6+/-0.6 fmol/mg of protein; kD2 =2.7+/-0.25 nM; Bmax2 =40.7+/-1.5 fmol/mg of protein), whereas low-affinity binding sites disappeared after 5 h.

Bradykinin sequesters B2 bradykinin receptors and the receptor-coupled Galpha subunits Galphaq and Galphai in caveolae in DDT1 MF-2 smooth muscle cells

In this report, we show that the vasoactive peptide agonist bradykinin (BK) when bound to B2 BK receptors on DDT1 MF-2 smooth muscle cells promotes the recruitment and sequestration of the occupied receptors and the receptor-coupled G-protein alpha subunits Galphaq and Galphai in caveolae. Association of ligand receptor complexes and Galpha subunits with caveolae was indicated by their co-enrichment on density gradients with caveolin, a marker protein for caveolae. Caveolin and Galpha subunits were monitored by immunoblotting, whereas receptors were monitored as ligand receptor complexes formed by labeling receptors with the agonist BK or the antagonist NPC17731 prior to cell disruption and caveolae enrichment. These complexes were detected with radioligand and by immunoblotting with BK antibodies. A direct interaction of Galpha subunits with caveolin was also indicated by their co-immunoprecipitation. Immunoelectron microscopy revealed that the enriched caveolin, Galpha subunits, and BK receptor complexes were present in structures of 0.1-0.2 microm. At 4 degrees C, BK and NPC17731 receptor complexes were detected in caveolae, and both complexes were sensitive to acid washing prior to cell disruption and caveolae enrichment. Elevation of the temperature to 37 degrees C increased the amount of BK receptor complexes in caveolae with a maximal response at 10 min (continuous labeling) or 20 min (single-round labeling), and the complexes became acid-resistant. These conditions also increased the amount of Galphaq and Galphai in caveolae with a maximal response at 5-10 min. In contrast, the NPC17731 receptor complexes remained acid-sensitive and dissociated at this temperature, and antagonists did not increase the amount of Galpha subunits in caveolae. These results show that some agonists that act through G-protein-coupled receptors promote the association of their receptors and receptor-coupled Galpha subunits with caveolae.

Stable expression of the human kinin B1 receptor in Chinese hamster ovary cells. Characterization of ligand binding and effector pathways

To delineate ligand binding and functional characteristics of the human B1 kinin receptor, a stable clone of Chinese hamster ovary cells expressing a single class of binding sites for [3H]des-Arg10-lysylbradykinin with a Kd of 0.3 nM and a Bmax of 38 fmol/mg protein ( approximately 40,000 receptors/cell) was isolated. Studies with peptide analogs showed that a lysine residue at position 1 (based on the lysylbradykinin sequence) of ligands was essential for high affinity binding to the human B1 receptor. In marked contrast to cloned Chinese hamster ovary cells expressing the human kinin B2 receptor, which internalized approximately 80% of the ligand within 5 min upon exposure to 2 nM [3H]bradykinin, exposure of cells expressing the B1 receptor to 1 nM [3H]des-Arg10-lysylbradykinin resulted in minimal ligand internalization. Stimulation of the B1 receptor led to inositol phosphate generation and transient increases in intracellular calcium, confirming coupling to phospholipase C, while immunoprecipitation of photoaffinity-labeled G-proteins from membranes indicated specific coupling of the receptor to Galphaq/11 and Galphai1,2. The B1, unlike the B2, receptor does not desensitize (as demonstrated by continuous phosphoinositide hydrolysis), enhancing the potential role of this receptor during inflammatory events.

Ligand-induced phosphorylation/dephosphorylation of the endogenous bradykinin B2 receptor from human fibroblasts

We have studied the ligand-induced phosphorylation/dephosphorylation of the bradykinin B2 receptor endogenously expressed in human HF-15 fibroblasts. An antiserum (AS346) to a synthetic peptide (CRS36), derived from the extreme carboxyl terminus of the human B2 receptor, precipitated the receptor from solubilized membranes of HF-15 cells that had been labeled with [32P]orthophosphate. A low basal level of B2 receptor phosphorylation was found in the absence of a ligand. Stimulation of the cells with the B2 receptor agonists bradykinin, [Lys0,Hyp3]bradykinin, kallidin, and T-kinin resulted in a rapid and efficient phosphorylation of the receptor. The B2 receptor antagonist HOE140 and the B1 receptor agonist des-Arg9-bradykinin failed to induce significant phosphorylation of the B2 receptor. Phosphoamino acid analysis revealed that the B2 receptor is phosphorylated on serine and threonine, but not on tyrosine residues. The ligand-induced phosphorylation of the receptor was concentration-dependent, with an apparent EC50 of 33 nM, and peaked at 1 min after challenge. The kinin-stimulated phosphorylation of the B2 receptor was rapid and transient and paralleled the kinetics of desensitization/resensitization of the receptor as followed by [Ca2+]i release and radioligand binding assay, respectively. The ligand-induced phosphorylation of the B2 receptor was independent of the protein kinase C pathway. In vitro experiments suggest betaARK1 (beta-adrenergic receptor kinase) as a candidate kinase that could mediate the homologous B2 receptor phosphorylation. Inhibitors of protein phosphatases 1 and 2A effectively blocked the dephosphorylation, but did not affect the internalization of the B2 receptor, whereas inhibitors of receptor internalization delayed its dephosphorylation. These finding point to a role of ligand-induced phosphorylation in the desensitization and redistribution of the bradykinin receptor in human fibroblasts.

Cyclic GMP regulates activation of phosphoinositidase C by bradykinin in sensory neurons

Prior exposure of cultured neonatal rat dorsal root ganglion (DRG) neurons to bradykinin resulted in marked attenuation of bradykinin-induced activation of phosphoinositidase C (PIC). The (logconcentration)-response curve for bradykinin-induced [3H]inositol trisphosphate ([3H]IP3) formation was shifted to the right and the maximum response was reduced. Bradykinin increases cyclic GMP (cGMP) in DRG neurons [Burgess, Mullaney, McNeill, Coote, Minhas and Wood (1989) J. Neurochem. 53, 1212-1218] and treatment of the neurons with dibutyryl cGMP (dbcGMP) had a similar, inhibitory, effect on bradykinin-induced [3H]IP3 formation. NG-Nitro-L-arginine (LNNA) blocked bradykinin-induced formation of cGMP. It prevented the functional uncoupling induced by pretreatment with bradykinin, but not the inhibitory effect of dbcGMP on [3H]IP3 formation. The ability of LNNA to prevent desensitization was reversed by excess L-arginine, indicating that its actions were mediated through inhibition of nitric oxide synthase. In addition to functional desensitization, exposure to bradykinin reduced the number of cell-surface receptors detected with [3H]bradykinin, without affecting its KD value for the remaining sites. In contrast to bradykinin, pretreatment with dbcGMP had no effect on either the KD or B(max) for [3H]bradykinin binding. This implies that the inhibitory effect of dbcGMP was down-stream from the binding of bradykinin to its receptor and upstream of IP3 formation. The lack of effect of dbcGMP on [3H]bradykinin binding suggests that the decrease in receptor number induced by bradykinin was mediated by a different mechanism and was not a key factor in the rapid phase of desensitization in these cells.

Kinin-mediated activation of endothelial no formation: possible role during myocardial ischemia

Endothelial cells produce a variety of factors involved in the control of vascular tone, platelet activation and cell growth, one of the most important being nitric oxide (NO). Although continuously produced in response to fluid shear stress, the release of NO from these cells can be enhanced further by humoral stimuli, such as bradykinin. This is the result of a chain of complex intracellular events involving changes in Ca2+, pH and protein phosphorylation. Endothelial cells are also capable of synthesizing bradykinin from an endogenous source, the release of which is markedly enhanced under hypoxic conditions. The finding that ACE inhibitors promote the local accumulation of the peptide and increase its efficacy at the receptor level may partly explain the potent anti-ischemic and cardioprotective effects of these drugs.