Phorbol ester and neomycin dissociate bradykinin receptor-mediated arachidonic acid release and polyphosphoinositide hydrolysis in Madin-Darby canine kidney cells. Evidence that bradykinin mediates noninterdependent activation of phospholipases A2 and C

Early reduction of SARS-CoV-2-replication in bronchial epithelium by kinin B2 receptor antagonism

Abstract

SARS-CoV-2 has evolved to enter the host via the ACE2 receptor which is part of the kinin-kallikrein pathway. This complex pathway is only poorly understood in context of immune regulation but critical to control infection. This study examines SARS-CoV-2-infection and epithelial mechanisms of the kinin-kallikrein-system at the kinin B₂ receptor level in SARS-CoV-2-infection that is of direct translational relevance. From acute SARS-CoV-2-positive study participants and -negative controls, transcriptomes of nasal curettages were analyzed. Primary airway epithelial cells (NHBEs) were infected with SARS-CoV-2 and treated with the approved B₂R-antagonist icatibant. SARS-CoV-2 RNA RT-qPCR, cytotoxicity assays, plaque assays, and transcriptome analyses were performed. The treatment effect was further studied in a murine airway inflammation model in vivo. Here, we report a broad and strong upregulation of kallikreins and the kinin B₂ receptor (B₂R) in the nasal mucosa of acutely symptomatic SARS-CoV-2-positive study participants. A B₂R-antagonist impeded SARS-CoV-2 replication and spread in NHBEs, as determined in plaque assays on Vero-E6 cells. B₂R-antagonism reduced the expression of SARS-CoV-2 entry receptor ACE2, G protein–coupled receptor signaling, and ion transport in vitro and in a murine airway inflammation in vivo model. In summary, this study provides evidence that treatment with B₂R-antagonists protects airway epithelial cells from SARS-CoV-2 by inhibiting its replication and spread, through the reduction of ACE2 levels and the interference with several cellular signaling processes. Future clinical studies need to shed light on the airway protection potential of approved B₂R-antagonists, like icatibant, in the treatment of early-stage COVID-19.

Graphical Abstract

Key messages

Induction of kinin B₂ receptor in the nose of SARS-CoV-2-positive patients.

Treatment with B₂R-antagonist protects airway epithelial cells from SARS-CoV-2.

B₂R-antagonist reduces ACE2 levels in vivo and ex vivo.

Protection by B₂R-antagonist is mediated by inhibiting viral replication and spread.

Supplementary information

The online version contains supplementary material available at 10.1007/s00109-022-02182-7.

Bradykinin/B2 receptor activation regulates renin in M-1 cells via protein kinase C and nitric oxide

In the collecting duct (CD), the interactions of renin angiotensin system (RAS) and kallikrein-kinin system (KKS) modulate Na⁺ reabsorption, volume homeostasis, and blood pressure. In this study, we used a mouse kidney cortical CD cell line (M-1 cells) to test the hypothesis that in the CD, the activation of bradykinin B₂ receptor (B₂R) increases renin synthesis and release. Physiological concentrations of bradykinin (BK) treatment of M-1 cells increased renin mRNA and prorenin and renin protein contents in a dose-dependent manner and increased threefold renin content in the cell culture media. These effects were mediated by protein kinase C (PKC) independently of protein kinase A (PKA) because B₂R antagonism with Icatibant and PKC inhibition with calphostin C, prevented these responses, but PKA inhibition with H89 did not modify the effects elicited by the B₂R activation. BK-dependent stimulation of renin gene expression in CD cells also involved nitric oxide (NO) pathway because increased cGMP levels and inhibition of NO synthase with L-NAME prevented it. Complementary renin immunohistochemical studies performed in kidneys from mice with conventional B₂R knockout and conditional B₂R knockout in the CD, showed marked decreased renin immunoreactivity in CD, regardless of the renin presence in juxtaglomerular cells in the knockout mice. These results indicate that the activation of B₂R increases renin synthesis and release by the CD cells through PKC stimulation and NO release, which support further the interactions between the RAS and KKS.

Targeting the 'Janus face' of the B2-bradykinin receptor

INTRODUCTION

Kinins are main active mediators of the kallikrein-kinin system (KKS) via bradykinin type 1 inducible (B1R) and type 2 constitutive (B2R) receptors. B2R mediates most physiological bradykinin (BK) responses, including vasodilation, natriuresis, NO, prostaglandins release.

AREAS COVERED

The article summarizes knowledge on kinins, B2R signaling and biological functions; highlights crosstalks between B2R and renin-angiotensin system (RAS). The double role (Janus face) in physiopathology, namely the beneficial protection of the endothelium, which forms the basis for the therapeutical utilization of B2 receptor agonists, on the one side, and the involvement of B2R in inflammation or infection diseases and in pain mechanisms, which justifies the use of B2R antagonists, on the other side, is extensively analyzed.

EXPERT OPINION

For decades, the B2R has been unconsciously activated during angiotensin-converting enzyme inhibitor (ACEI) or angiotensin receptor blocker (ARB) treatments. Whether direct B2R targeting with stable agonists could bring additional therapeutic benefit to RAS inhibition should be investigated. Efficacy, established in experimental models, should be confirmed by translational studies in cardiovascular pathologies, glaucoma, Duchenne cardiopathy and during brain cancer therapy. The other face of B2R is targeted by antagonists already approved to treat hereditary angioedema. The use of antagonists could be extended to other angioedema and efficacy tested against acute pain and inflammatory diseases.

Cysteinyl leukotrienes mediate the response of submucosal ganglia from rat colon to bradykinin

The aim of the present study was to find out the mechanism by which the inflammatory mediator, bradykinin, induces an increase of the cytosolic Ca(2+) concentration ([Ca(2+)](i)) in enteric neurons. For this purpose, ganglia in the isolated submucosa from rat colon were loaded with the Ca(2+)-sensitive dye, fura-2, and were exposed to bradykinin (2·10(-8)mol/l). Under control conditions, the kinin evoked a transient increase in [Ca(2+)](i). Preincubation with quinacrine or arachidonyltrifluoromethylketone (AACOCF(3)), i.e. blockers of cytosolic phospholipase A(2), prevented the raise of [Ca(2+)](i). This inhibition was mimicked by 5,8,11,14-eicosatetrayonic acid (ETYA), an inhibitor of cyclooxygenases as well as lipoxygenases, and by BWA4C, a selective inhibitor of lipoxygenases, whereas indomethacin was ineffective, suggesting the mediation of the kinin response by a lipoxygenase metabolite. Indeed, a leukotriene, leukotriene D(4) (LTD(4)), mimicked the effect of bradykinin. The LTD(4) receptor blocker, MK-571, inhibited the increase in [Ca(2+)](i) evoked by LTD(4) and by bradykinin. Consequently, bradykinin receptors in submucosal ganglia from rat colon are coupled to a stimulation of phospholipase A(2), the release of arachidonic acid and the production of LTD(4), which seems to be finally responsible for the change in the cytosolic Ca(2+) concentration.

SHP-1, a novel peptide isolated from seahorse inhibits collagen release through the suppression of collagenases 1 and 3, nitric oxide products regulated by NF-kappaB/p38 kinase

Considerable efforts have been taken to identify natural peptides as potential bioactive substances. In this study, novel peptide (SHP-1) derived from seahorse (Hippocampus, Syngnathidae) hydrolysate was explored for its inhibitory effects on collagen release in arthritis with the investigation of its underlying mechanism of action. The efficacy of SHP-1 was determined on cartilage protective effects such as inhibition of collagen and GAG release. SHP-1 was able to suppress not only the expression of collagenases 1 and 3, but also the production of NO via down-regulation of iNOS. However, it presented an irrelevant effect on the level of GAG release in chondrocytic and osteoblastic cells. Inhibition of collagen release by SHP-1 is associated with restraining the phosphorylation of NF-kappaB and p38 kinase cascade. Therefore, it could be suggested that SHP-1 has a potential to be used in arthritis treatment.

Early activation of bradykinin B2 receptor aggravates reactive oxygen species generation and renal damage in ischemia/reperfusion injury

The kallikrein/kinin system is beneficial in ischemia/reperfusion injury in heart, controversial in brain, but detrimental in lung, liver, and intestine. We examined the role of the kallikrein/kinin system in acute ischemia/reperfusion renal injury induced by 40 min occlusion of the renal artery followed by reperfusion. Rats were infused with tissue kallikrein protein 5 days before (pretreated group) or after (treated group) ischemia. Two days later, the pretreated group exhibited the worst renal dysfunction, followed by the treated group, then the control group. Kallikrein increased tubular necrosis and inflammatory cell infiltration with generation of more tumor necrosis factor-alpha and monocyte chemoattractant protein-1. Reactive oxygen species (ROS), malondialdehyde, and reduced/oxidized glutathione measurement revealed that the oxidative stress was augmented by kallikrein administration in both ischemic and reperfusion phases. The groups with more ROS generation also had more apoptotic renal cells. The deleterious effects of kallikrein on ischemia/reperfusion injury were reversed by cotreatment with bradykinin B2 receptor (B2R) antagonist, but not B1 receptor antagonist, and were not associated with hemodynamic changes. We conclude that early activation of B2R augmented ROS generation in ischemia/reperfusion renal injury, resulting in subsequent apoptosis, inflammation, and tissue damage. This finding suggests the potential application of B2R antagonists in acute ischemic renal disease associated with bradykinin activation.

Long polyamines act as cofactors in PIP2 activation of inward rectifier potassium (Kir2.1) channels

Phosphatidylinosital-4,5-bisphosphate (PIP₂) acts as an essential factor regulating the activity of all Kir channels. In most Kir members, the dependence on PIP₂ is modulated by other factors, such as protein kinases (in Kir1), G_βγ (in Kir3), and the sulfonylurea receptor (in Kir6). So far, however, no regulator has been identified in Kir2 channels. Here we show that polyamines, which cause inward rectification by selectively blocking outward current, also regulate the interaction of PIP₂ with Kir2.1 channels to maintain channel availability. Using spermine and diamines as polyamine analogs, we demonstrate that both spontaneous and PIP₂ antibody–induced rundown of Kir2.1 channels in excised inside-out patches was markedly slowed by long polyamines; in contrast, polyamines with shorter chain length were ineffective. In K188Q mutant channels, which have a low PIP₂ affinity, application PIP₂ (10 μM) was unable to activate channel activity in the absence of polyamines, but markedly activated channels in the presence of long diamines. Using neomycin as a measure of PIP₂ affinity, we found that long polyamines were capable of strengthening either the wild type or K188Q channels' interaction with PIP₂. The negatively charged D172 residue inside the transmembrane pore region was critical for the shift of channel–PIP₂ binding affinity by long polyamines. Sustained pore block by polyamines was neither sufficient nor necessary for this effect. We conclude that long polyamines serve a dual role as both blockers and coactivators (with PIP₂) of Kir2.1 channels.

Adenosine induces ATP release via an inositol 1,4,5-trisphosphate signaling pathway in MDCK cells

ATP is released into extracellular space as an autocrine/paracrine molecule by mechanical stress and pharmacological-receptor activation. Released ATP is partly metabolized by ectoenzymes to adenosine. In the present study, we found that adenosine causes ATP release in Madin-Darby canine kidney cells. This release was completely inhibited by CPT (an A1 receptor antagonist), U-73122 (a phospholipase C inhibitor), 2-APB (an inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) receptor blocker), thapsigargin (a Ca2+-ATPase inhibitor), and BAPTA/AM (an intracellular Ca2+ chelator), but not by DMPX (an A2 receptor antagonist). However, forskolin, epinephrine, and isoproterenol, inducers of cAMP accumulation, failed to release ATP. Adenosine increased intracellular Ca2+ concentrations that were strongly blocked by CPT, U-73122, 2-APB, and thapsigargin. Moreover, adenosine enhanced accumulations of Ins(1,4,5)P3 that were significantly reduced by U-73122 and CPT. These data suggest that adenosine induces the release of ATP by activating an Ins(1,4,5)P3 sensitive-Ca2+ pathway through the stimulation of A1 receptors.

Bradykinin B2 receptor antagonist FR173657 ameliorates small bowel ischemia-reperfusion injury in dogs

Bradykinin mediates acute inflammation by increasing microvascular permeability, vasodilation, leukocyte migration and accumulation, and the production of arachidonic acid via phospholipase A2 activation. Arachidonic acid metabolites, or eicosanoids, are potent modulators of biological functions, particularly inflammation. Bradykinin exerts its inflammatory effects via the bradykinin B2 receptor. The aim of this study was to evaluate the effect of a bradykinin B2 receptor antagonist, FR173657 (FR), on intestinal ischemia-reperfusion (I/R) injury. Twenty-eight mongrel dogs were divided into four groups (n = 7 per group). Group I underwent I/R alone, Group II underwent I/R and received FR treatment, Group III was sham operated, and Group IV was sham operated and received FR treatment. The FR treatment consisted of FR continuously from 30 min prior to ischemia to 2 hr after reperfusion. In the I/R procedure, the superior mesenteric artery (SMA) and vein were clamped for 2 hr and then released to permit reperfusion for 12 hr. The intramucosal pH (pHi), SMA blood flow, and mucosal tissue blood flow were measured during the reperfusion period. The serum thromboxane B2 and 6-keto-prostaglandin F1alpha levels were determined, and tissue samples were examined histologically. Results showed that tissue blood flow, pHi, and SMA blood flow after reperfusion were maintained in Group II in comparison with Group I. Histopathological examination showed less severe mucosal damage after reperfusion in Group II than in Group I. The serum thromboxane B2 and 6-keto-prostagland in F1alpha levels were significantly lower in Group II than in Group I (P < 0.05). We conclude that FR treatment appears to have clear protective effects on small bowel I/R injury by inhibiting the release of eicosanoids.

Effects of a bradykinin B(2) receptor antagonist on ischemia-reperfusion injury in a canine lung transplantation model

BACKGROUND

This study investigated the effects of a bradykinin B(2) receptor antagonist, FR173657 (FR), on ischemia-reperfusion (I/R) injury in a canine lung transplantation model.

METHODS

Eighteen pairs of weight-matched dogs were randomly divided into 3 groups. Six pairs were assigned to the FR(D+R) group, in which FR (100 nmol/kg/h) was administered to the transplant donor continuously beginning 30 minutes before ischemia until the onset of ischemia, and FR was administered to the transplant recipient beginning 30 minutes before reperfusion and continuing for 2 hours after reperfusion. Another 6 pairs of dogs were assigned to the FR(R) group, in which FR was administered only to the recipient in the same manner as in the FR(D+R) group. The other pairs were assigned to the control group, in which vehicle alone was administered. Orthotopic left lung transplantation was performed after 12-hour cold storage in Euro-Collins solution. Fifteen minutes after reperfusion, the right pulmonary artery and the right stem bronchus were ligated. The animals were measured for 4 hours after reperfusion for left pulmonary vascular resistance (L-PVR), cardiac output (CO), arterial oxygen pressure (PaO(2)) and alveolar-arterial oxygen pressure difference (A-aD(O(2))). Lung specimens were harvested for measurement of the wet-to-dry lung weight ratio (WDR), histopathologic studies and polymorphonuclear neutrophil (PMN) count.

RESULTS

Compared with the control group, PaO(2), A-aDO(2), L-PVR and CO were all significantly (p < 0.05) improved and WDR significantly (p < 0.05) lower in both the FR(D+R) and FR(R) groups. Moreover, in the FR-treated groups, histologic tissue edema was mild, and PMN infiltration was significantly (p < 0.05) reduced.

CONCLUSIONS

The bradykinin B(2) receptor antagonist, FR173657, ameliorates I/R injury in lung grafts, indicating that protection of lung grafts can be achieved by the administration of FR solely to the transplant recipient.

Activation of phospholipase C increases intramembrane electric fields in N1E-115 neuroblastoma cells

We imaged the intramembrane potential (a combination of transmembrane, surface, and dipole potential) on N1E-115 neuroblastoma cells with a voltage-sensitive dye. After activation of the B(2) bradykinin receptor, the electric field sensed by the dye increased by an amount equivalent to a depolarization of 83 mV. The increase in intramembrane potential was blocked by the phospholipase C (PLC) inhibitors U-73122 and neomycin, and was invariably accompanied by a transient rise of [Ca(2+)](i). A depolarized inner surface potential, as the membrane loses negative charges via phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolysis, and an increase in the dipole potential, as PIP(2) is hydrolyzed to 1,2-diacylglycerol (DAG), can each account for a small portion of the change in intramembrane potential. The primary contribution to the measured change in intramembrane potential may arise from an increased dipole potential, as DAG molecules are generated from hydrolysis of other phospholipids. We found bradykinin produced an inhibition of a M-type voltage-dependent K(+) current (I(K(M))). This inhibition was also blocked by the PLC inhibitors and had similar kinetics as the bradykinin-induced modulation of intramembrane potential. Our results suggest that the change in the local intramembrane potential induced by bradykinin may play a role in mediating the I(K(M)) inhibition.

Adenophostin A and imipramine are two activators of the olfactory inositol 1,4,5-trisphosphate-gated channel in fish olfatory cilia

Binding of an odorant to its receptor activates the cAMP-dependent pathway, and also leads to inositol 1,4,5-trisphosphate (InsP(3)) production. This induces opening of a plasma membrane channel in olfactory receptor cells (ORCs). We investigated single-channel properties of this channel in the presence of a phospholipase C (PLC) activator (imipramine) and of a potent activator of the InsP(3)/Ca(2+) release channel (adenophostin A) by reconstituting carp olfactory cilia into planar lipid bilayers. In the presence of 53 mM barium as a charge carrier, the addition of 50 microM imipramine induced a current of 1.53+/-0.05 pA at 0 mV. There were two different mean open times (6.0+/-0.6 ms and 49.6+/-6.4 ms). The I/ V curve displayed a slope conductance of 50+/-2 pS. Channel activity was transient and was blocked by neomycin (50 microM). These observations suggest that imipramine may activate the olfactory InsP(3)-gated channel through PLC. Using the same ionic conditions, the application of 0.5 microM adenophostin A triggered a current of 1.47+/-0.04 pA at 0 mV. The I/ V curve displayed a slope conductance of 60+/-2 pS. This channel showed only a single mean open time (15.0+/-0.3 ms) and was strongly inhibited by ruthenium red (30 microM) and heparin (10 microg/mL). These results indicate that adenophostin A and imipramine may act on the ciliary InsP(3)-gated channel and are potentially valuable pharmacological tools for studying olfactory transduction mechanisms.

Effects of a bradykinin B(2) receptor antagonist, FR173657, on pulmonary ischemia-reperfusion injury in dogs

BACKGROUND

This study investigated the effects of a bradykinin B(2) receptor antagonist, FR173657 (FR), on pulmonary ischemia-reperfusion (I/R) injury.

METHODS

Twenty-four mongrel dogs were divided into four groups (n = 6 each). In Groups I, II and III, FR doses of 33, 100 and 300 nmol/kg per hour, respectively, were administered continuously beginning 30 minutes before ischemia and continuing for 2 hours after reperfusion. In Group IV, vehicle alone was administered. Warm ischemia was induced for 3 hours by clamping the left pulmonary artery and veins. Simultaneously, the left stem bronchus was bisected and then anastomosed before reperfusion. Fifteen minutes after reperfusion, the right pulmonary artery and bronchus were ligated. Left pulmonary vascular resistance (L-PVR), cardiac output (CO), arterial oxygen pressure (PaO(2)) and the alveolar - arterial oxygen pressure difference (A-aDO2) were measured for 4 hours after reperfusion. Lung tissue was harvested for wet-to-dry weight ratio (WDR) measurements, histopathologic studies and polymorphonuclear neutrophil (PMN) counts. Serum thromboxane (TX) B(2), 6-keto-prostaglandin (PG) F(1alpha) and leukotriene (LT) B(4) levels were also measured.

RESULTS

PaO(2), A-aDO2, L-PVR and CO were significantly (p < 0.05) improved and WDR was significantly (p < 0.05) lower in Groups II and III than in Group IV. Histologic tissue edema was mild, and PMN infiltration was significantly (p < 0.05) reduced in Groups I, II and III compared with Group IV. TXB(2) levels were significantly (p < 0.05) lower in Group II than in Group IV, whereas 6-keto-PGF(1alpha) levels were not significantly different. LTB(4) levels were significantly (p < 0.05) lower in Groups II and III than in Group IV.

CONCLUSIONS

FR appears to have a protective effect on pulmonary I/R injury stemming from the inhibition of eicosanoid release.

alpha(1)-adrenergic receptors mediate LH-releasing hormone secretion through phospholipases C and A(2) in immortalized hypothalamic neurons

Norepinephrine has long been known to stimulate the pulsatile and preovulatory release of LH-releasing hormone (LHRH). In vivo and in vitro studies indicate that these effects are mediated primarily through alpha(1)-adrenergic receptors (alpha(1)-ARs). With the immortalized hypothalamic LHRH neurons, we have found that alpha(1)-adrenergic agents directly stimulate the secretion of LHRH in a dose-dependent manner. Ligand binding and RNA studies demonstrate that the GT1 cells contain both alpha(1A)- and alpha(1B)-ARs. Competition binding experiments show that approximately 75% of the binding is due to alpha(1B)-ARs; the remainder is made up of alpha(1A)-ARs. Receptor activation leads to stimulation of PLC. PLC beta 1 and PLC beta 3 are expressed in GT1 neurons, and these PLCs are probably responsible for the release of diacylglycerol and IP as well as the increase in intracellular calcium. The mobilization of cytoplasmic calcium is sufficient to stimulate cytosolic PLA(2) (cPLA(2)) and release arachidonic acid. A dissection of the contributions of the phospholipases to LHRH secretion suggests that cPLA(2) acts downstream of PLC and that it significantly augments the PLC-stimulated LHRH secretory response. Inasmuch as the alpha(1)-ARs are known to play a critical role in LHRH physiology, we propose that both PLC and cPLA(2) are critical in regulating and amplifying LHRH release.

COX-2 inhibition prevents insulin-dependent diabetes in low-dose streptozotocin-treated mice

Insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease believed to be caused by an inflammatory process in the pancreas leading to selective destruction of the beta cells. Inducible cyclooxygenase (COX-2) is expressed under inflammatory conditions and its product prostaglandin E(2) (PGE(2)) is an important inflammation mediator. We report here that administration of the selective COX-2 inhibitor NS-398 prevents the onset of diabetes in mice brought on by multiple low-doses of streptozotocin (STZ). Histological observations indicated that STZ-mediated destruction of beta cells was prevented by NS-398 treatment. Delayed (day 3) administration of NS-398 was also protective in this model. No protective effect was observed when NS-398 was administered prior to a high, toxic dose of STZ. These results demonstrate the critical importance of COX-2 activity in autoimmune destruction of beta cells, and point to the fact that COX-2 inhibition can potentially develop into a preventive therapy against IDDM.

Asymmetric distribution of muscarinic acetylcholine receptors in Madin-Darby canine kidney cells

We have characterized the muscarinic ACh receptors (mAChRs) expressed in Madin- Darby canine kidney (MDCK) strain II epithelial cells. Binding studies with the membrane-impermeable antagonist N-[(3)H]methylscopolamine demonstrated that mAChRs are approximately 2.5 times more abundant on the basolateral than on the apical surface. Apical, but not basolateral, mAChRs inhibited forskolin-stimulated adenylyl cyclase activity in response to the agonist carbachol. Neither apical nor basolateral mAChRs exhibited detectable carbachol-stimulated phospholipase C activity. Carbachol application to the apical or the basolateral membrane resulted in a threefold increase in intracellular Ca(2+) concentration, which was completely inhibited by pertussis toxin on the apical side and partially inhibited on the basolateral side. RT-PCR analysis showed that MDCK cells express the M(4) and M(5) receptor mRNAs. These data suggest that M(4) receptors reside on the apical and basolateral membranes of polarized MDCK strain II cells and that the M(5) receptor may reside in the basolateral membrane of a subset of cells.

Thapsigargin-insensitive calcium pools in vascular smooth muscle cells

Since sarcoplasmic Ca2+-ATPase may play an important role for the regulation of cytosolic free calcium concentration ([Ca2+]i) and may be altered in primary hypertension, the effects of thapsigargin and bradykinin on intracellular calcium pools in cultured vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats of the Münster strain (SHR) and normotensive Wistar-Kyoto (WKY) rats were investigated. VSMC were cultured on glass cover slips and [Ca2+]i was measured using the fluorescent dye fura2. To exclude transplasmamembrane calcium influx all experiments were performed in a calcium free medium. Thapsigargin, a selective inhibitor of the sarcoplasmic Ca2+-ATPase, and bradykinin, that is known to induce inositol trisphosphate release, dose dependently caused an increase of [Ca2+]i by emptying intracellular Ca2+ stores. The peak increase of [Ca2+]i after addition of saturation doses of thapsigargin (1 micromol/L) was not significantly different in the two strains (SHR: 69 +/- 11 nmol/L, n=24; WKY: 58 +/- 12 nmol/L, n=20; mean +/- SEM). When 10 micromol/L bradykinin was added after depletion of the thapsigargin-sensitive pools, still a release of [Ca2+]i could be observed. The bradykinin-induced [Ca2+]i increase was similar in the absence and presence of thapsigargin in VSMC from SHR (62 +/- 12 nmol/L, n=20; vs 52 +/- 18 nmol/L, n=22). In contrast, in the VSMC from WKY a significant reduction of the bradykinin induced [Ca2+]i-increase could be observed after the depletion of the thapsigargin sensitive calcium pools (70 +/- 8 nmol/L, n=21, vs. 33 +/- 7, n=20; p<0.002). It is concluded that bradykinin releases calcium from a pool that is not refilled by the common, thapsigargin-sensitive Ca2+-ATPase. In contrast to VSMC from normotensive WKY, in VSMC from spontaneously hypertensive rats thapsigargin and bradykinin sensitive pools may be regulated separately.

Inhibition of phospholipase A2-mediated arachidonic acid release by cyclic AMP defines a negative feedback loop for P2Y receptor activation in Madin-Darby canine kidney D1 cells

In Madin-Darby canine kidney D1 cells extracellular nucleotides activate P2Y receptors that couple to several signal transduction pathways, including stimulation of multiple phospholipases and adenylyl cyclase. For one class of P2Y receptors, P2Y2 receptors, this stimulation of adenylyl cyclase and increase in cAMP occurs via the conversion of phospholipase A2 (PLA2)-generated arachidonic acid (AA) to prostaglandins (e.g. PGE2). These prostaglandins then stimulate adenylyl cyclase activity, presumably via activation of prostanoid receptors. In the current study we show that agents that increase cellular cAMP levels (including PGE2, forskolin, and the beta-adrenergic agonist isoproterenol) can inhibit P2Y receptor-promoted AA release. The protein kinase A (PKA) inhibitor H89 blocks this effect, suggesting that this feedback inhibition occurs via activation of PKA. Studies with PGE2 indicate that inhibition of AA release is attributable to inhibition of mitogen-activated protein kinase activity and in turn of P2Y receptor stimulated PLA2 activity. Although cAMP/PKA-mediated inhibition occurs for P2Y receptor-promoted AA release, we did not find such inhibition for epinephrine (alpha1-adrenergic) or bradykinin-mediated AA release. Taken together, these results indicate that negative feedback regulation via cAMP/PKA-mediated inhibition of mitogen-activated protein kinase occurs for some, but not all, classes of receptors that promote PLA2 activation and AA release. We speculate that receptor-selective feedback inhibition occurs because PLA2 activation by different receptors in Madin-Darby canine kidney D1 cells involves the utilization of different signaling components that are differentially sensitive to increases in cAMP or, alternatively, because of compartmentation of signaling components.

Activation of muscarinic receptors in porcine airway smooth muscle elicits a transient increase in phospholipase D activity

Phospholipase D (PLD) is a phosphodiesterase that catalyses hydrolysis of phosphatidylcholine to produce phosphatidic acid and choline. In the presence of ethanol, PLD also catalyses the formation of phosphatidylethanol, which is a unique characteristic of this enzyme. Muscarinic receptor-induced changes in the activity of PLD were investigated in porcine tracheal smooth muscle by measuring the formation of [3H]phosphatidic acid ([3H]PA) and [3H]phosphatidylethanol ([3H]PEth) after labeling the muscle strips with [3H]palmitic acid. The cholinergic receptor agonist acetylcholine (Ach) significantly but transiently increased formation of both [3H]PA and [3H]PEth in a concentration-dependent manner (>105-400% vs. controls in the presence of 10(-6) to 10(-4) M Ach) when pretreated with 100 mM ethanol. The Ach receptor-mediated increase in PLD activity was inhibited by atropine (10(-6) M), indicating that activation of PLD occurred via muscarinic receptors. Activation of protein kinase C (PKC) by phorbol-12-myristate-13-acetate (PMA) increased PLD activity that was effectively blocked by the PKC inhibitors calphostin C (10(-8) to 10(-6) M) and GFX (10(-8) to 10(-6) M). Ach-induced increases in PLD activity were also significantly, but incompletely, inhibited by both GFX and calphostin C. From the present data, we conclude that in tracheal smooth muscle, muscarinic acetylcholine receptor-induced PLD activation is transient in nature and coupled to these receptors via PKC. However, PKC activation is not solely responsible for Ach-induced activation of PLD in porcine tracheal smooth muscle.

Two B1 and B2 bradykinin receptor antagonists fail to inhibit the Ca2+ response elicited by bradykinin in human skin fibroblasts

The elevation of intracellular [Ca2+] induced by bradykinin (Bk) was monitored with fura-2 fluorescence in human skin fibroblasts. Neither [des-Arg10][Leu9]kallidin nor D-Arg[Hyp3,Thi5,D-Tic7,Oic8]bradykinin (HOE140) inhibited the Ca2+ response stimulated by Bk. Moreover, each behaved as a partial agonist causing the elevation of intracellular [Ca2+].

Synergy between cyclo-oxygenase-2 induction and arachidonic acid supply in vivo: consequences for nonsteroidal antiinflammatory drug efficacy

Prostanoids produced via the action of cyclo-oxygenase-2 (COX-2) appear central to many inflammatory conditions. Here we show in LPS-treated rats, however, that COX-2 induction alone does not greatly increase prostanoid production in vivo. For this, a second, arachidonic acid liberating stimulus is also required. Thus, only after intravenous injection of bradykinin or exogenous arachidonic acid was a marked increase in prostanoid formation seen. There is, therefore, synergy between proinflammatory mediators: both induction of COX-2 protein and an increase in the supply of arachidonic acid are required to greatly enhance prostanoid production. Second, we show that supplying arachidonic acid to increase prostanoid production reduces the effectiveness of both currently used nonsteroidal antiinflammatory drugs (NSAIDs) (diclofenac) and novel COX-2-selective inhibitors (NS-398, celecoxib) as inhibitors of COX-2 activity. Our data lead to two important conclusions. First, increased prostanoid production in inflammation is a two-component response: increased COX-2 expression and increased arachidonic acid supply. Second, the supply of arachidonic acid to COX-2 determines the effectiveness of NSAIDs. NSAIDs and selective COX-2 inhibitors, therefore, will generally be less effective at more inflamed sites, providing a rationale for the very high doses of NSAIDs required in human conditions such as rheumatoid arthritis.--Hamilton, L. C., Tomlinson, A. M., Mitchell, J. A., Warner, T. D. Synergy between cyclo-oxygenase-2 induction and arachidonic acid supply in vivo: consequences for nonsteroidal antiinflammatory drug efficacy.

Bradykinin-evoked Ca2+ mobilization in Madin Darby canine kidney cells

We studied the mechanisms underlying the bradykinin-evoked changes in intracellular calcium concentration ([Ca2+]i) in Madin Darby canine kidney (MDCK) cells. Bradykinin evoked a [Ca2+]i transient in a dose-dependent manner, measured by fura-2 fluorimetry and digital video imaging. The transient consisted of a rise and a decay and [Ca2+]i returned to baseline without oscillations. External Ca2+ influx occurred, as demonstrated by Mn2+ quench and external Ca2+ removal measurements. Bradykinin acted by stimulating bradykinin B2 receptors as evidenced by blockade by D-arginyl-L-arginlyl-L-prolyl-trans-4-hydroxy-L-prolylglycyl -3-(2-thienyl)-L-alanyl-L-seryl-D-1,2,3,4-tetrahydro-3-isoquinolineca rbonyl-L-(2alpha,3beta,7alphabeta)-octahydro-1 H-indole-2-carbonyl-L-arginine (HOE 140) but not by D-arginyl-L-arginlyl-L-prolyl-trans-4-hydroxy-L-proylglycyl- 3-(2-thienyl)-L-alanyl-L-seryl-D-1,2,3,4-tetrahydro-3-isoquinolinecar bonyl-L-(2alpha,3beta,7alphabeta)-octahydro-1 H-indole-2-carbonyl ([Des-Arg]HOE 140). The [Ca2+]i signal was abolished by 1-(6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1 H-pyrrole-2,5-dione (U73122) and partially inhibited by neomycin, implying mediation by phospholipase C. The transient was initiated by a release of Ca2+ from internal stores since it was abolished by pretreatment with thapsigargin or cyclopiazonic acid. The mobilization of the internal Ca2+ store subsequently triggered a 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1 H-imidazole hydrochloride (SKF 96365)-insensitive Ca2+ entry. Pretreatment with carbonylcyanide m-chlorophynylhydrozone and gly-phe-beta-naphthylamide did not alter the transient, thus excluding the participation of mitochondria and lysosomes. Efflux via Ca2+ pumps contributed to the decay of the transient. Efflux via Na+/Ca2+ exchange or sequestration by mitochondria and lysosomes was insignificant. The transient was blunted by the protein kinase C activator phorbol 12-myristate 13-acetate, and was enhanced by the protein kinase C inhibitors sphingosine and chelerythrine, the protein kinase A inhibitor 2,5-di-(t-butyl)-1,4-hydroquinone, N-[2-(p-bromocinnamylamino)ethyl]5-isoquinolinesulfonamide (H-89), the agent 8-(diethylamino)octyl 3,4,5-trimethoxybenzoate (TMB-8), and agents that elevated levels of 3',5'-cyclic guanosine monophosphate. The transient did not heterologously desensitize with that evoked by ATP, ADP or UTP.

Pharmacological comparison of UTP- and thapsigargin-induced arachidonic acid release in mouse RAW 264.7 macrophages

1. Although stimulation of mouse RAW 264.7 macrophages by UTP elicits a rapid increase in intracellular free Ca2+ ([Ca2+]i), phosphoinositide (PI) turnover, and arachidonic acid (AA) release, the causal relationship between these signalling pathways is still unclear. In the present study, we investigated the involvement of phosphoinositide-dependent phospholipase C (PI-PLC) activation, Ca2+ increase and protein kinase activation in UTP-induced AA release. The effects of stimulating RAW 264.7 cells with thapsigargin, which cannot activate the inositol phosphate (IP) cascade, but results in the release of sequestered Ca2+ and an influx of extracellular Ca2+, was compared with the effects of UTP stimulation to elucidate the multiple regulatory pathways for cPLA2 activation. 2. In RAW 264.7 cells UTP (100 microM) and thapsigargin (1 microM) caused 2 and 1.2 fold increases, respectively, in [3H]-AA release. The release of [3H]-AA following treatment with UTP and thapsigargin were non-additive, totally abolished in the Ca2+-free buffer, BAPTA (30 microM)-containing buffer or in the presence of the cPLA2 inhibitor MAFP (50 microM), and inhibited by pretreatment of cells with pertussis toxin (100 ng ml(-1)) or 4-bromophenacyl bromide (100 microM). By contrast, aristolochic acid (an inhibitor of sPLA2) had no effect on UTP and thapsigargin responses. 3. U73122 (10 microM) and neomycin (3 mM), inhibitors of PI-PLC, inhibited UTP-induced IP formation (88% and 83% inhibition, respectively) and AA release (76% and 58%, respectively), accompanied by a decrease in the [Ca2+]i rise. 4. Wortmannin attenuated the IP response of UTP in a concentration-dependent manner (over the range 10 nM-3 microM), and reduced the UTP-induced AA release in parallel. RHC 80267 (30 microM), a specific diacylglycerol lipase inhibitor, had no effect on UTP-induced AA release. 5. Short-term treatment with PMA (1 microM) inhibited the UTP-stimulated accumulation of IP and increase in [Ca2+]i, but had no effect on the release of AA. In contrast, the AA release caused by thapsigargin was increased by PMA. 6. The role of PKC in UTP- and thapsigargin-mediated AA release was shown by the blockade of these effects by staurosporine (1 microM), Ro 31-8220 (10 microM), Go 6976 (1 microM) and the down-regulation of PKC. 7. Following treatment of cells with SK&F 96365 (30 microM), thapsigargin-, but not UTP-, induced Ca2+ influx, and the accompanying AA release, were down-regulated. 8. Neither PD 98059 (100 microM), MEK a inhibitor, nor genistein (100 microM), a tyrosine kinase inhibitor, had any effect on the AA responses induced by UTP and thapsigargin. 9. We conclude that UTP-induced cPLA2 activity depends on the activation of PI-PLC and the sustained elevation of intracellular Ca2+, which is essential for the activation of cPLA2 by UTP and thapsigargin. The [Ca2+]i-dependent AA release that follows treatment with both stimuli was potentiated by the activity of protein kinase C (PKC). A pertussis toxin-sensitive pathway downstream of the increase in [Ca2+]i was also shown to be involved in AA release.

Drugs affecting phospholipase C-mediated signal transduction block the olfactory cyclic nucleotide-gated current of adult zebrafish

Amino acid and bile salt odorants are detected by zebrafish with relatively independent odorant receptors, but the transduction cascade(s) subsequently activated by these odorants remains unknown. Electro-olfactogram recording methods were used to determine the effects of two drugs, reported to affect phospholipase C (PLC)/inositol tripohsphate (IP3)-mediated olfactory transduction in other vertebrate species, on amino acid and bile salt-evoked responses. At the appropriate concentrations, either an IP3-gated channel blocker, ruthenium red (0.01-0.1 microM), or a PLC inhibitor, neomycin (50 microM), reduced amino-acid-evoked responses to a significantly greater extent than bile salt-evoked responses. Excised patch recording techniques were used to measure the affects of these drugs on second-messenger-activated currents. Ruthenium red and neomycin are both effective blockers of the olfactory cyclic nucleotide-gated (CNG) current. Both drugs blocked the CNG channel in a voltage-dependent and reversible manner. No IP3-activated currents could be recorded. The differential effects of ruthenium red and neomycin on odor-evoked responses suggest the activation of multiple transduction cascades. The nonspecific actions of these drugs on odor-activated transduction pathways and our inability to record an IP3-activated current do not permit the conclusion that zebrafish, like other fish species, use a PLC/IP3-mediated transduction cascade in the detection of odorants.

Stimulation of phospholipase D via alpha1-adrenergic receptors in Madin-Darby canine kidney cells is independent of PKCalpha and -epsilon activation

We have demonstrated previously that protein kinase Calpha (PKCalpha) plays a key role in regulating phospholipase D (PLD) activation by nucleotides and the phorbol ester phorbol-12-myristate-13-acetate in Madin-Darby canine kidney (MDCK-D1) cells. In the current work, we investigated PLD activation in MDCK-D1 cells triggered by the adrenergic receptor agonist epinephrine and its mechanism of activation. Epinephrine, acting through the alpha1-adrenergic receptor subtype, promoted transient translocation of PKCalpha and more prolonged translocation of PKCepsilon to the membrane fraction, indicating activation of these two isoforms. In addition, epinephrine promoted activation of PLD, as shown by a sustained accumulation of phosphatidylethanol. All of these events were blocked by pretreatment of cells with the alpha1-adrenergic antagonist prazosin. D609, an inhibitor of phosphatidylcholine hydrolysis, blocked translocation of PKCalpha and PKCepsilon but did not inhibit PLD activation. Unlike results with PMA, or with the P2 purinergic receptor agonist ATP, epinephrine-stimulated PLD activity was not inhibited in MDCK-D1 cells in which PKCalpha expression is attenuated by an antisense cDNA construct or in cells in which PKC activity was inhibited by 1 microM GF 109203X. However, PLD activation by epinephrine was abolished by concomitant incubation of cells with the calcium chelator EGTA. These data, together with previous results, are consistent with the hypothesis that in MDCK-D1 cells, epinephrine acting on alpha1-adrenergic receptors, promotes a rapid increase in cytosolic Ca2+ that promotes activation of PLD through an as-yet poorly defined mechanism. The data demonstrate that different types of G protein-linked receptors that activate PLD can mediate this activation in either a PKC activation-dependent or -independent manner within a single cell type.

Bradykinin-stimulated cPLA2 phosphorylation is protein kinase C dependent in rabbit CCD cells

We have used an established cell line of rabbit cortical collecting duct (RCCD) epithelial cells representing a mixed population of principal and intercalated cell types to determine which phospholipase A2 (PLA2) enzyme therein is responsible for bradykinin (BK)-stimulated arachidonic acid (AA) release and how its activation is regulated. BK-stimulated AA release was reduced 92% by arachidonyl trifluoromethyl ketone, an inhibitor of cytosolic PLA2 (cPLA2). Examination of PLA2 activity in vitro demonstrated that BK stimulation resulted in a greater than twofold increase in PLA2 activity and that this activity was dithiothreitol insensitive and was inhibited by an antibody directed against cPLA2. To determine a possible role for protein kinase C (PKC) in the BK-mediated activation of cPLA2, we used the PKC-specific inhibitor Ro31-8220 and examined its effects on AA release, cPLA2 activity, and phosphorylation. Ro31-8220 reduced BK-stimulated AA release and cPLA2 activity by 51 and 58%, respectively. cPLA2 activity stimulated by phorbol ester [phorbol 12-myristate 13-acetate (PMA)] displayed a similar degree of activation and was associated with an increase in serine phosphorylation identical to that caused by BK. The phosphorylation-induced activation of this enzyme was confirmed by the phosphatase-mediated reversal of both BK- and PMA-stimulated cPLA2 activity. In addition, we have also found that PMA stimulation did not cause a synergistic potentiation of BK-stimulated AA release as did calcium ionophore. This occurred despite membrane PKC activity increasing 93% in response to PMA vs. 42% in response to BK. These data, taken together, indicate that cPLA2 is the enzyme responsible for BK-mediated AA release, and, moreover, they indicate that PKC is involved in the onset responses of cPLA2 to BK.

Bradykinin-stimulated arachidonic acid release from MDCK cells is not protein kinase C dependent

Bradykinin (BK)-induced release of arachidonic acid (AA) from Madin-Darby canine kidney (MDCK) D1 cells was investigated. Phorbol 12-myristate 13-acetate (PMA) caused a synergistic increase in BK- and A-23187-induced release of AA but alone had no effect on this release. Inhibition of protein kinase C (PKC) with bisindolmaleimide I (BIS) abolished the synergistic effects of PMA but did not affect AA release caused by BK or A-23187 alone. Downregulation of PKC with 100 nM PMA resulted in a reduction of AA release induced by BK or A-23187 addition, which corresponded to a decrease in cytoplasmic phospholipase A2 (cPLA2) activity as measured in cell extracts. Although Western blotting revealed no differences in cPLA2 expression as a result of PMA treatment, phosphorylation of the enzyme, as assessed by phosphoserine content, was significantly reduced in PKC-depleted cells. These results imply that, with PKC downregulation, subsequent BK stimulation results in a Ca(2+)-dependent translocation of a less phosphorylated, less active form of cPLA2. Any stimulation of PKC by BK addition did not appear as a significant event in onset responses leading to AA release. On the other hand, inhibition of the mitogen-activated protein kinase (MAPK) cascade with the MAPK kinase inhibitor, PD-98059, significantly decreased BK-induced release of AA, a finding that, with our other results, points to the existence of a PKC-independent route for stimulation of MAPK and the propagation of onset responses.

Prostaglandin H synthase: protein synthesis-independent regulation in bovine aortic endothelial cells

The objective of the present study was to examine whether prostaglandin H synthase (PGHS) can be regulated by pathways independent of de novo synthesis of PGHS. Incubation of bovine aortic endothelial cells (BAEC) for as short as 5 min with NaF (40 mM) resulted in a 60% increase in PGHS activity. PGHS activity induced by NaF was unaffected by either 10 microM cycloheximide or 1 microM actinomycin D. Aspirin (25 microM) completely inhibited resting PGHS activity, and NaF did not induce further stimulation. NS-398 (500 nM), a specific PGHS-2 inhibitor, was ineffective. Basic fibroblast growth factor (bFGF) induced a significant increase in PGHS activity within 30 min and was insensitive to cycloheximide. The levels of PGHS-1 and PGHS-2 proteins, as measured by Western blots, were not affected by NaF or bFGF. The tyrosine kinase inhibitor genistein attenuated PGHS activity that was induced by NaF and bFGF, whereas the tyrosine phosphatase inhibitor, sodium orthovanadate, augmented these responses. The G protein activators 5'-guanylyl imidodiphosphate and guanosine 5'-O-(3-thiotriphosphate) inhibited both resting and NaF-induced PGHS activities. These results suggest-that, in BAEC, PGHS-1 activity can be regulated by tyrosine kinase and/or G proteins, independently of de novo protein synthesis.

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.

Localization of the bradykinin B2 receptor in uterus, bladder and Madin-Darby canine kidney cells

Kinins are biologically active peptides that act through specific receptors, B1 and B2. Here we describe the localization of the bradykinin B2 receptor in Madin-Darby canine kidney cells and in the uterus and urinary bladder of rat or human origin. We discuss the suitability of anti-peptide antibodies to assess the tissue distribution of bradykinin B2 receptors.

Endothelin-3 attenuates the cyclic GMP responses to C-type natriuretic peptide in cultured mouse astrocytes

The effect of endothelin-3 (ET-3) on cyclic GMP (cGMP) responses to C-type natriuretic peptide (CNP) was studied in primary cultures of mouse astrocytes. Attenuation of CNP-stimulated cGMP formation by ET-3 was time-dependent, with maximum inhibition achieved at 30 min of preincubation. ET-3 suppressed cGMP production in response to 10 nM CNP in a dose-dependent fashion, with an IC50 of 0.04 nM and a maximal inhibitory concentration of 1 microM, which led to a 66% reduction of the cGMP increment from 45.0 +/- 4.2 pmol/mg protein to 15.4 +/- 2.6 pmol/mg protein. ET-1, ET-2, and ET-3 were equipotent in suppressing the CNP-induced cGMP response, suggesting that this effect was mediated by ETB receptors. Staurosporine, Ro 31-8220, calcium-free medium, nifedipine, verapamil, lanthanum, thapsigargin, BAPTA, W7, calmidazolium, U-73122, neomycin, quinacrine, wortmannin, herbimycin-A, okadaic acid, and sodium orthovanadate failed to block the effect of ET-3. Cycloheximide (100 microM), however, partially but significantly reversed the inhibitory effect of ET-3 on CNP-induced cGMP from 48.2 to 73.3% of the control value. The results support the premise that ET-3 and CNP interact within the central nervous system. The data also suggest that cGMP accumulation in mouse astrocytes is mediated by activation of certain kinases through as yet undefined mechanisms and not by protein kinase C, increased intracellular calcium, or other second messenger pathways such as phospholipases A2, C, D, tyrosine kinase, or protein phosphatases.

P2-purinoceptors utilize multiple signalling pathways in MDCK-D1 cells

1. Madin-Darby canine kidney (MDCK) cells are a widely used model system for the study of epithelial cells. We have utilized a clonal variant, MDCK-D1, to examine signalling by P2-purinoceptors. 2. Several lines of evidence that lead us to conclude that MDCK-D1 cells co-express P2a- and P2y-purinoceptors and that both subtypes are linked to the release of arachidonic acid and metabolites (AA) include: (a) relative potency of nucleotide analogues in promoting AA release; (b) blockade by the antagonist suramin of response to the P2Y-selective agonist, 2-methylthio ATP (2-MT-ATP), but not to the P2a-selective agonist, UTP; and (c) additivity of response to 2-MT-ATP and UTP. AA release is a consequence of activation of phospholipase A2 (PLA2), most likely the 85 kDa cytosolic PLA2. 3. Treatment of MDCK-D1 cells with ATP, but not UTP, increases inositol 1,4,5-trisphosphate formation while both UTP and ATP increase phosphatidylcholine hydrolysis, ATP, UTP, and 2-MT-ATP can also stimulate phospholipase D activity. 4. Purine nucleotides increase cellular cAMP levels in MDCK-D1 cells in a manner that depends, at least in part, on activation of cyclooxygenase, since cAMP generation stimulated by ATP or UTP is inhibited by treatment of cells with indomethacin. Because cyclooxygenase-derived PGE2 can bind to prostaglandin receptors and stimulate synthesis of cAMP, nucleotides may raise cAMP in an autocrine or paracrine fashion. 5. Taken together, these results indicate that MDCK-D1 cells co-express P2a and P2y-purinoceptors and that these receptors utilize several mechanisms to regulate cell function, including activation of multiple phospholipases and autocrine/paracrine action of products.

Mechanisms of prostanoid synthesis in human synovial cells: cytokine-peptide synergism

Bradykinin (BK)2 and interleukin-1 (IL-1) interact synergistically to stimulate prostaglandin synthesis in human synovial fibroblast-like cells. The effect of BK is rapid and correlates with its capacity to elevate cytosolic levels of calcium ([Ca2+]i), while IL-1's effect is slow and s dependent upon de novo protein synthesis. The mechanism of this synergistic interaction was investigated. In the basal state, high levels of arachidonic acid (AA) were spontaneously released from synovial cells but near absent levels of cyclooxygenase activity prevented metabolism of AA to prostanoid. BK was a potent stimulus for elevating AA, but not prostaglandins, above basal levels. IL-1, in contrast, increased prostaglandins but not AA, above basal levels. IL-1 treatment was not associated with a loss or redistribution of AA among phospholipid classes. These results are consistent with high basal phospholipase activity in synovial cells and demonstrate the ability of BK, presumably via its ability to raise [Ca2+]i, to further elevate this activity(ies). Metabolism of AA to prostanoid is minimal in resting and BK-stimulated synovial cells, however, without the concomitant induction of cyclooxygenase activity by IL-1. These studies clarify the different, but synergistic, mechanisms of action of a peptide and cytokine in stimulating prostanoid synthesis in synovial cells. In addition, these data extend the results of previous investigations in demonstrating that basal phospholipase activity provides sufficient AA substrate for IL-1 induced prostanoid synthesis without invoking the concomitant induction of phospholipase activity by IL-1.

Role of PLA2, PLC, and PLD in bradykinin-induced release of arachidonic acid in MDCK cells

The role of cytosolic phospholipase A2 (cPLA2), phosphatidylcholine-specific phospholipase C (PC-PLC) and phospholipase D (PLD) in the bradykinin (BK)-stimulated release of arachidonic acid (AA) was examined in Madin-Darby canine kidney (MDCK) cells. Release of AA, phosphorylcholine, choline, and phosphatidic acid (PA) or the transphosphatidylation product, phosphatidylethanol, was detected after 1 min of BK stimulation. A role for PC-PLC was confirmed with D609, which reduced BK-stimulated AA by 70%. Ethanol (EtOH), which blunts PA formation, diminished BK-stimulated AA release by 50%. Together, D609 and EtOH inhibited this release almost completely. Evidence indicated that diacylglycerol and PA can enhance PLA2 activity when added to cytosol extracts. The enzyme responsible for AA release was characterized as cPLA2, since PLA2 activity assayed in cell extracts was largely inhibited by an antibody to this enzyme. The membrane fraction PLA2 activity increased significantly in BK-stimulated cells. We conclude that BK signaling in MDCK cells is mediated by the lipid products of PC-PLC and PLD, increasing cPLA2 activity, possibly by causing perturbations in the bilayer structure of its substrate, by a direct effect on the enzyme or by activation of protein kinases such as protein kinase C.

Thromboxane A2 mediates the stimulation of inositol 1,4,5-trisphosphate production and intracellular calcium mobilization by bradykinin in neonatal rat ventricular cardiomyocytes

Bradykinin is a mediator of the protection of myocardium by angiotensin I-converting enzyme/kininase II inhibitors. We reported that the activation of B2 bradykinin receptors in neonatal rat cardiac myocytes in primary culture was followed by hydrolysis of phosphatidylinositol 4,5-bisphosphate and formation of inositol 1,4,5-trisphosphate (IP3). Here we examine the regulation of IP3 formation stimulated by bradykinin. Activation of myocytes with 1 mu/L bradykinin increased IP3 production from 117 +/- 8.3 to 1011 +/- 48.6 pmol/mg protein. Treatment of the cells with 10 mu/L indomethacin or 1 mu/L dexamethasone partially blocked this bradykinin-induced response. Moreover, either U73122, a phospholipase C inhibitor, or (p-amylcinnamoyl) anthranilic acid, a phospholipase A2 inhibitor, blunted the IP3 response to bradykinin. Because thromboxane A2 stimulates inositol bisphosphate metabolism in guinea pig atria, we also investigated the effect of the thromboxane A2 receptor antagonist BM 13177 (1 mu/L), which strongly attenuated the stimulated IP3 production. Since thromboxane A2 appears to partly mediate the IP3 response to bradykinin, we examined the effect of the stable thromboxane A2 mimetic U46619. Control cultures were stimulated more by U46619 than by bradykinin (1629 +/- 14.5 versus 1011 +/- 48.6 pmol IP3/mg protein). This property of U46619 was selectively antagonized by BM 13177. Inhibition of either phospholipase C or phospholipase A2 blunted the IP3 response to U46619. Short-term (30 minutes) activation of protein kinase C with phorbol 12-myristate 13-acetate (10 pmol/L to 1 mu/L) attenuated the IP3 accumulation in response to bradykinin; the effect of phorbol 12-myristate 13-acetate was reversed with 1 mu/L staurosporine, a protein kinase C inhibitor. Treatment with 1 microgram/mL cholera toxin or pertussis toxin for 4 hours amplified the IP3 response to 10 nmol/L bradykinin from 570 +/- 20.0 to 1150 +/- 51.3 and to 1016.7 +/- 21.9 pmol/mg protein. Bradykinin mobilized 9.4% of intracellular calcium stores in cardiomyocytes as assessed by chlortetracycline-based fluorometry, and this effect of bradykinin was blocked by BM 13177 or the B2 bradykinin receptor blocker Hoe 140 by more than 70%. In functional studies, bradykinin (1 mu/L) increased by 12% the twitch contractile force of neonatal rat ventricular strips paced at threshold intensity, but this was unaffected by BM 13177. In conclusion, in cardiomyocytes, bradykinin enhances IP3 production mostly via phospholipase A2 stimulation and thromboxane A2 formation. This prostanoid in turn stimulates its receptor and activates phospholipase C, which then splits phosphatidylinositol 4,5-bisphosphate into IP3 and diacylglycerol. The effect of bradykinin on phospholipase C, via thromboxane A2, is negatively regulated by protein kinase C activation.

Differential involvement of calcium channels and protein kinase-C activity in GnRH-induced phospholipase-C, -A2 and -D activation in a gonadotrope cell line (alpha T3-1)

The mode of action of GnRH on pituitary gonadotropes involves metabolism of phospholipids, protein kinase-C (PKC) and voltage sensitive Ca2+ channels (VSCC) activation. We have studied the differential role of PKC and VSCC on the coupling of the GnRH receptor with phospholipases-C (PLC), -A2 (PLA2) and -D (PLD) activities in a gonadotrope cell line (alpha T3-1), by measuring the production of inositol phosphates (IPs), arachidonic acid (AA) and phosphatidylethanol (PEt) respectively. We demonstrated that in these cells GnRH stimulated through a specific receptor, IPs formation, a rapid and sustained diacylglycerol generation, consequently AA release and a delayed PEt production in a dose-dependent manner. In contrast to GnRH-induced PLC activity, the PLA2 and PLD stimulation by the neuropeptide involved Ca2+ mobilization via VSCC activation. BAY-K8644 a VSCC agonist significantly potentiated, while the VSCC antagonist nitrendipine markedly inhibited GnRH-induced AA release and PEt production. TPA, a phorbol ester which induced a rapid and important redistribution of PKC, although unable to elicit PLC or PLA2 stimulation, specifically provoked PLD activation in a PKC-dependent but Ca(2+)-independent manner. The PKC stimulation by TPA significantly inhibited the GnRH-stimulated IPs and AA formation, while it potentiated the GnRH-evoked PEt production. This negative feed-back of PKC on GnRH-Induced PLC and PLA2 activities was reversed when PKC was either down regulated after long TPA treatments or inhibited by the PKC inhibitors, staurosporine or GF109203X. The GnRH-induced PEt formation was markedly diminished in PKC depleted cells or after PKC inhibition. Under such conditions, both agonist and antagonist of VSCC became less effective in modulating the remaining GnRH-evoked PEt formation. These results suggest that PKC, in coordination with Ca2+, plays a key role in regulating the cross-talk between the multiple phospholipases implicated in the GnRH signal transduction.

Integrin alpha v beta 3 and phospholipase C regulate prostacyclin formation of endothelial cells caused by ancrod-generated fibrin

Ancrod-generated fibrin has been shown to stimulate prostacyclin synthesis of human umbilical vein endothelial cells (Chang et al., 1994, Biochem. Biophys. Res. Commun. 203, 1920). We further investigated its mechanism of action. The increment of 6-keto prostaglandin F1 alpha stimulated by ancrod-generated fibrin was almost completely inhibited when endothelial cells were either pretreated with 50 microM 8-(N,N'-diethylamino)octyl-3,4,5- trimethoxybenzoate (TMB-8) or preloaded with 15 microM 1,2-bis(2- aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). 6-Keto prostaglandin F1 alpha production during 2 and 10 h incubation period was also inhibited by 1.2 mM ethyleneglycol-bis-(beta-aminoethylether)-N,N,N',N'-tetraace tic acid (EGTA) (41 +/- 12 and 53 +/- 17% inhibition, respectively). Further, ancrod-generated fibrin caused a rapid-onset increase in [3h]inositol monophosphate (IP1) formation in endothelial cells. This increase in IP1 was significantly inhibited by 1 mM Gly-Pro-Arg-Pro, 1 mM neomycin or 100 ng/ml pertussis toxin. At the same time, neomycin and pertussis toxin also significantly inhibited 6-keto prostaglandin F1 alpha synthesis of endothelial cells stimulated by ancrod-generated fibrin. Additionally, the increment of IP1 production as well as prostacyclin production were significantly inhibited by monoclonal antibodies directed against alpha v beta 3. These results suggest that intra- and extra-cellular Ca2+ participate in prostacyclin synthesis stimulated by ancrod-generated fibrin. Ancrod-generated fibrin stimulates pertussis toxin-sensitive G-protein regulated phosphoinositide breakdown, which is responsible for prostacyclin synthesis. This augmentation in prostacyclin synthesis and phosphoinositide breakdown caused by ancrod-generated fibrin area, at least in part, mediated by fibrin binding to integrin alpha v beta 3 on endothelial cells.

Regulation of the renal Na-HCO3 cotransporter: V. mechanism of the inhibitory effect of parathyroid hormone

PTH administration decreases proximal HCO3 reabsorption and inhibits the brush border Na-H antiporter. We studied the effect of PTH on the renal Na-HCO3 cotransporter and examined whether this effect is mediated through the adenylate cyclase/cyclic AMP system or through the phospholipase A pathway. We studied the effect of PTH [1-34] on the Na-HCO3 cotransporter activity in rabbit renal basolateral membranes incubated with 50 microM ATP by measuring the 22Na uptake in the presence of HCO3 and gluconate. Na-HCO3 cotransporter activity (expressed in nmol/mg protein/3 seconds) was taken as the difference in 22Na uptake in the presence of HCO3 and gluconate. PTH (10(-10) M) completely inhibited Na-HCO3 cotransporter activity from 1.23 +/- 0.14 to -0.58 +/- 0.23, P < 0.001. This effect of PTH to inhibit the Na-HCO3 cotransporter was prevented by the polyclonal antibody against G alpha s indicating that PTH acts through G alpha s protein. Because G alpha s stimulates adenylate cyclase/cyclic AMP system, we examined the effect of PTH in the presence and in the absence of the adenylate cyclase inhibitor, dideoxyadenosine (DDA). DDA alone (10(-4) M) stimulated the Na-HCO3 cotransporter activity. In the presence of DDA, the net inhibitory effect of PTH was the same magnitude as that of control, suggesting the existence of other pathways for the effect of PTH on the cotransporter. Calmodulin inhibition also partially prevented the effect of PTH. To determine whether the inhibitory effect of PTH is mediated at least in part, through phospholipase A, we first examined the effect of PTH on arachidonic acid release and then measured the Na-HCO3 cotransporter activity in presence and in absence of arachidonic acid or eicosatetraynoic acid (ETA), an inhibitor of arachidonic acid metabolism. PTH significantly increased the release of arachidonic acid by isolated proximal tubule cells and arachidonic acid inhibited the Na-HCO3 cotransporter in basolateral membranes. ETA (3 microM) partially prevented the inhibitory effect of PTH. In cultured proximal tubule cells, PTH inhibited the HCO3-dependent 22Na uptake and ethoxyresorufin, an inhibitor of cytochrome P-450, blocked the inhibitory effect of PTH on the cotransporter. These results demonstrate that PTH inhibits the renal Na-HCO3 cotransporter through multiple mechanisms, that are mediated through G proteins, G alpha s and GP, and CaM-KII.

P2 purinergic receptor agonists enhance cAMP production in Madin-Darby canine kidney epithelial cells via an autocrine/paracrine mechanism

Mechanisms of cross-talk between different classes of signaling molecules are inadequately understood. We have used clonal Madin-Darby canine kidney (MDCK-D1) epithelial cells as a model system to investigate the effects of extracellular nucleotides (e.g. ATP, UTP), which promote increase in activity of several phospholipases, on cAMP production. In contrast to observations in some other cell systems, ATP and UTP, acting via P2 purinergic receptors, stimulated cAMP production in MDCK-D1 cells. At maximally effective concentrations, ATP and UTP were not additive with the beta-adrenergic receptor agonist isoproterenol, but were synergistic with forskolin in increasing cAMP production, indicating that G alpha s is activated by these nucleotides. Additionally, we found that (a) nucleotide-induced increases in cAMP were blocked by indomethacin, a cyclooxygenase inhibitor, (b) arachidonic acid increased cellular cAMP levels in an indomethacin-sensitive fashion, and (c) PGE2, the major metabolite of arachidonic acid, stimulated cAMP formation. Overall, our results suggest a mechanism by which extracellular nucleotides stimulate release of arachidonic acid which is metabolized to PGE2 which, in turn, acts in an autocrine/paracrine fashion via prostaglandin receptors to activate G alpha s and increase cAMP. Based on the ability of extracellular nucleotides to stimulate the formation and release of prostaglandins in MDCK-D1 epithelial and other cells, we hypothesize that receptor-mediated prostaglandin release may be a general mechanism that regulates cAMP formation in many types of cells.

Regulation of eicosanoid synthesis in fibroblasts from inflamed gallbladders

Gallbladder cell cultures obtained from rabbits subjected to sham or 72 h of bile duct ligation (72 h BDL, cholecystitis model) were incubated with calcium ionophore (A23187), dibutyryl cAMP (cAMP), and phorbol 12,13-diacetate (phorbol) to determine the intracellular signal transduction mechanisms responsible for increased inflamed gallbladder eicosanoid synthesis. Incubation of sham and 72 h BDL cell cultures with A23187 or phorbol significantly increased, whereas cAMP decreased, release of 6-keto-PGF1 alpha, PGE2, thromboxane B2 (measured by enzyme immunoassay) in a dose-related manner. Seventy-two-hour BDL cell cultures contained a specific 2-fold increased level of prostacyclin synthase compared to sham cell cultures which was not altered by preincubation with A23187, phorbol or cAMP. These findings suggest that increased PGI2 release in the sham and inflamed cell cultures following A23187 and phorbol stimulation was mediated in part via the inositol triphosphate pathway and protein kinase C activation and was not associated with altered cyclooxygenase or prostacyclin synthase content.

The morphologic change of endothelial cells by ancrod-generated fibrin is triggered by alpha v beta 3 integrin binding and the subsequent activation of a G-protein coupled phospholipase C

The mechanism of morphologic change of human cultured umbilical vein endothelial cells (HUVECs) caused by fibrin was investigated. Ancrod, a thrombin-like enzyme, did not cause morphologic alteration of HUVEC by itself at concentrations ranging from 0.01 to 10 U/ml. However, when 0.02 U/ml of ancrod was added to cultured HUVEC monolayers in the presence of citrated plasma, it caused pronounced morphologic change of HUVEC after 6-10 h incubation period. Gly-Pro-Arg-Pro (4 mg/ml), an inhibitor of fibrin polymerization, prevented the morphologic alteration, indicating that the morphologic alteration was caused by the polymerized fibrin. The morphologic change of HUVEC caused by ancrod-generated fibrin was not observed in the presence of an intracellular calcium mobilization inhibitor TMB-8 (50 microM), and the morphologic alteration was also less pronounced with BAPTA(15 microM)-loaded HUVECs and HUVECs pretreated with EGTA (1.2 mM). Ancrod (in Medium 199) itself did not stimulate phosphoinositide breakdown of HUVEC. However, when ancrod was present in plasma, it caused an increase of [3H]IP1 of HUVECs preloaded with [3H]myoinositol. This IP1 increment was inhibited by Gly-Pro-Arg-Pro. The increase of IP1 was significantly inhibited by the pretreatment of monoclonal antibodies 23C6 and 7E3 directed against alpha v beta 3 integrin. Neomycin (1 mM) and pertussis toxin (100 ng/ml), but not aspirin or mepacrine, blocked this enhanced phosphoinositide breakdown. The morphologic change was also prevented by the monoclonal antibodies, 23C6 and 7E3. These results suggest that both intra- and extra-cellular calcium participate in the event of morphologic change of HUVEC caused by ancrod-generated fibrin, and the morphologic change is mediated, at least in part, by fibrin binding to integrin alpha v beta 3 on HUVECs, causing the subsequent activation of the endogenous G-protein coupled phospholipase C.

Regulation of bradykinin-stimulated phospholipase C and arachidonic acid release by protein kinase A in MDCK-D1 cells

Regulation of phospholipases C (PLC) and arachidonic acid (AA) release by cAMP-dependent protein kinase (PKA) was investigated in MDCK-D1 cells. Bradykinin (BDK) was used to stimulate PLC and AA release, while arginine vasopressin (AVP), forskolin (FSK), isobutylmethylxanthine (IBMX) were used to increase cAMP levels and stimulate PKA. When cells were preincubated for 20 min with 10 microM FSK + 0.5 mM IBMX, and subsequently treated with 1 microM BDK or control medium for 40 min, the basal and BDK-stimulated PLC activity, measured as accumulated labelled inositol phosphate (InsP) after 40 min and inositol trisphosphate (InsP3) after 10 s, were significantly inhibited. In a parallel manner, FSK + IBMX also significantly decreased both basal and BDK-stimulated diacylglycerol (DAG) production. The basal and BDK-enhanced AA release into the media was also significantly inhibited by pretreatment with FSK + IBMX. In parallel experiments, H-89, a specific inhibitor of PKA, was preincubated for 60 min prior to addition of BDK and this resulted in a reversal of FSK+IBMX-induced inhibition of basal and BDK-stimulated PLC activity and AA release. An inhibitor of inositide-hydrolysing PLC, U73122, (1 microM) was also found to blunt BDK-stimulated PLC activity and BDK-enhanced AA release which indicated that stimulation of AA release by the nonapeptide was second to PLC activation. The ionophore, A23187, (10 microM) greatly stimulated AA release and to a much lesser extent, PLC activity. Its effect on AA release however was not blocked by inhibiting protein kinase C (PKC) with staurosporine (SSP) and consequently did not notably involve the PLC-PKC cascade. Activation of PKA with FSK + IBMX was found to significantly inhibit the enhancement of AA release by ionophore. With 12-tetradecanoyl-phorbol-13-acetate (TPA) also present there was a synergistic increase in the A23187-stimulated AA release and activation of PKA under such conditions inhibited AA release to a similar extent though the synergistic effect remained. The results strongly suggest a role for PKA in the regulation of PLC activity and AA release in MDCK-D1 cells. Control of AA release by PKA, is mediated both by mechanisms which involve blunting of PLC activity and mechanisms which are downstream from the PLC-PKC cascade.

Targeted disruption of a B2 bradykinin receptor gene in mice eliminates bradykinin action in smooth muscle and neurons

Mice that are homozygous for the targeted disruption of the gene encoding the B2 bradykinin receptor have been generated. The gene disruption results in a deletion of the entire coding sequence for the B2 receptor. The disruption of the B2 receptor gene has been confirmed by genetic, biochemical, and pharmacological analyses. Mice that are homozygous for the disruption of the B2 receptor gene are fertile and indistinguishable from their littermates by visual inspection. Bradykinin fails to produce responses in pharmacological preparations from ileum, uterus, and the superior cervical ganglia from these mice. Therefore, expression of a single gene appears to be responsible for conferring responsiveness to bradykinin in these tissues.

Characterization of [hydroxyproline9]luteinizing hormone-releasing hormone and its smallest precursor forms in immortalized luteinizing hormone-releasing hormone-secreting neurons (GT1-7), and evaluation of their mode of action on pituitary cells

[Hydroxyproline9]luteinizing hormone-releasing hormone ([Hyp9]LHRH), an endogenous hydroxylated post-translational product of the LHRH sequence, has been isolated from mammalian hypothalamus. Using the LHRH-hypothalamic cell line (GT1-7) of fetal origin, we attempted to define the substrates available for the hydroxylation process during LHRH synthesis and to characterize immunologically the [Hyp9]LHRH and pro-[Hyp9]LHRH forms with anti-LHRH antibodies of different specificities after separation by HPLC. Their biological activity and mode of action were evaluated and compared to that of LHRH and LHRH intermediate precursors in normal pituitary cells and in a gonanodotrope cell line alpha T3-1. immunoreactivity was progressively increased in cells and media during cell culture. [Hyp9]LHRH and its two smallest precursor forms ([Hyp9]LHRH-(Gly11) and -(11-13)) were detected in cells and in media. They were simultaneously detected with the homologous LHRH molecular forms indicating that the hydroxylation occurs early in the processing of pro-LHRH. [Hyp9]LHRH-like molecules were more abundant than LHRH forms in media. This predominant release may thus represent a physiological process occurring during fetal life. Free acid forms of both decapeptides were detected only in cells. Furthermore, the results obtained suggest that conversion of Gln1 in pyroGlu1 occurs before or during processing into the hydroxylated or non-hydroxylated LHRH intermediate (11-13)-precursors. The biosynthetic pathway is thus common for both decapeptides and it is not altered by the hydroxylation process. LHRH and [Hyp9]LHRH shared the same receptor for their biological activity, as assessed by measuring luteinizing hormone release and activation of phospholipase C and A2. [Hyp9]LHRH was, however, less potent than LHRH.