Apical-basolateral membrane asymmetry in canine cortical collecting tubule cells. Bradykinin, arginine vasopressin, prostaglandin E2 interrelationships

Involvement of H+-gradient dependent transporter in PGE2 release from A549 cells

The purpose of this study was to identify the transporter involved in the release of prostaglandin E₂ (PGE₂). In the present study, transport assays were conducted using membrane vesicles prepared from human lung adenocarcinoma A549 cells, thus enabling identification of the novel exporter present in A549 cells. PGE₂ transport into A549 vesicles was higher in the presence of a proton (H⁺)-gradient, thus suggesting the involvement of PGE₂H⁺ symporter in PGE₂ transport. Results from our experiments showed enhanced PGE₂ release in A549 cells in the presence of H⁺-gradient ([H⁺]_{extracellular} < [H⁺]_{intracellular}). Moreover, in vesicular transport assays, H⁺-gradient-dependent transport of PGE₂ did not show saturation up to 500 μM PGE_2, and 10 mM aromatic monocarboxylic acids (acetylsalicylic acid, salicylic acid, and p-nitrobenzoic acid) significantly inhibited PGE₂ transport by 62-70%. These results suggest, the involvement of monocarboxylate transporters in the H⁺-gradient-dependent PGE₂ export.

A seven-step plan for becoming a moderately rich and famous biochemist

Omega-6 polyunsaturated fatty acids were identified as essential nutrients in 1930. Their essentiality is largely due to their function as prostaglandin (PG) precursors. I spent most of my career in biochemistry determining how PG biosynthesis is regulated. PGs are lipid mediators formed in response to certain circulating hormones and cytokines. PGs act near their sites of synthesis to signal neighboring cells to coordinate their responses (e.g. when platelets interact with blood vessels). The committed step in PG synthesis is the conversion of a 20-carbon omega-6 fatty acid called arachidonic acid to prostaglandin endoperoxide H₂ (PGH₂). Depending on the tissue and the hormone or cytokine stimulus, this reaction is catalyzed by either cyclooxygenase-1 or cyclooxygenase-2 (COX-1 or COX-2). Once formed, PGH₂ is converted, again depending on the context, to one of several downstream PG subtypes that act via specific G protein-coupled receptors. Nonsteroidal anti-inflammatory drugs (e.g. aspirin, ibuprofen, and naproxen) block PG synthesis by inhibiting COX-1 and COX-2. COX-2 is also inhibited by COX-2-selective inhibitors. Inhibition of COX-1 by low-dose aspirin prevents thrombosis. COX-2 inhibition reduces inflammation and pain. Investigating the mysteries of COXs anchored my scientific career. I attribute my successes to the great good fortune of having been surrounded by people who helped me make the most of my talents. I have written this reflection in a light-hearted fashion as a self-help essay, while highlighting the people and factors that most impacted me during my upbringing and then during my maturation and evolution as a biochemist.

Renal β-intercalated cells maintain body fluid and electrolyte balance

Inactivation of the B1 proton pump subunit (ATP6V1B1) in intercalated cells (ICs) leads to type I distal renal tubular acidosis (dRTA), a disease associated with salt- and potassium-losing nephropathy. Here we show that mice deficient in ATP6V1B1 (Atp6v1b1-/- mice) displayed renal loss of NaCl, K+, and water, causing hypovolemia, hypokalemia, and polyuria. We demonstrated that NaCl loss originated from the cortical collecting duct, where activity of both the epithelial sodium channel (ENaC) and the pendrin/Na(+)-driven chloride/bicarbonate exchanger (pendrin/NDCBE) transport system was impaired. ENaC was appropriately increased in the medullary collecting duct, suggesting a localized inhibition in the cortex. We detected high urinary prostaglandin E2 (PGE2) and ATP levels in Atp6v1b1-/- mice. Inhibition of PGE2 synthesis in vivo restored ENaC protein levels specifically in the cortex. It also normalized protein levels of the large conductance calcium-activated potassium channel and the water channel aquaporin 2, and improved polyuria and hypokalemia in mutant mice. Furthermore, pharmacological inactivation of the proton pump in β-ICs induced release of PGE2 through activation of calcium-coupled purinergic receptors. In the present study, we identified ATP-triggered PGE2 paracrine signaling originating from β-ICs as a mechanism in the development of the hydroelectrolytic imbalance associated with dRTA. Our data indicate that in addition to principal cells, ICs are also critical in maintaining sodium balance and, hence, normal vascular volume and blood pressure.

3-Acrylamide-4-aryloxyindoles: synthesis, biological evaluation and metabolic stability of potent and selective EP3 receptor antagonists

A series of potent and selective EP(3) receptor antagonists are described. Utilizing a pharmacophore model developed for the EP(3) receptor, a series of 3,4-disubstituted indoles were found to be efficient ligands for this target. These compounds showed high selectivity over IP, FP and other EP receptors. An optimized molecule 7c featured a sound profile and potency in the functional rat and human platelet aggregation assays.

3,4-Disubstituted indole acylsulfonamides: a novel series of potent and selective human EP3 receptor antagonists

A series of potent and selective EP(3) receptor antagonists are described. Utilizing a pharmacophore model developed for the EP(3) receptor, a series of 3,4-disubstituted indoles were shown to be high affinity ligands for this target. These compounds showed high selectivity over IP, FP and other EP receptors and are potent antagonists in functional assays.

Dietary salt induces transcription of the prostaglandin transporter gene in renal collecting ducts

Prostaglandin E(2) (PGE(2)) plays an important role in maintaining body fluid homeostasis by activating its receptors on the renal collecting duct (CD) to stimulate renal Na(+) and water excretion. The PG carrier prostaglandin transporter (PGT) is expressed on the CD apical membrane, where it mediates PG reuptake as part of the termination of autocrine PG signaling. Here we tested the hypothesis that dietary salt loading regulates PGT gene transcription in renal CDs. We placed green fluorescence protein (GFP) under control of 3.3 kb of the mouse PGT promoter and injected this construct into the pronuclei of fertilized FVB mouse eggs. Four of thirty-eight offspring were GFP positive by genotyping. We extensively characterized one (no. 29) PGT-GFP transgenic mouse line. On microscopic examination, GFP was expressed in CDs as determined by their expression of aquaporin-2. We fed mice a low (0.03% NaCl)-, normal (0.3% NaCl)-, or high-salt (3% NaCl) diet for 2 wk and quantified CD GFP expression. The average number of GFP-positive CD cells per microscopic section varied directly with dietary salt intake. Compared with mice on the control (0.3% sodium) diet, mice on a low-sodium (0.03%) diet had reduced numbers of GFP-positive cells (71% of control, P < 0.001), whereas mice on a high-sodium (3%) diet had increased numbers of GFP-positive cells (139% of control, P < 0.001). This increase in apparent CD PGT transcription resulted in a 51-55% increase (P < 0.001) in whole kidney PGT mRNA levels as determined by real-time PCR. The regulation of PG signal termination via reuptake represents a new pathway for controlling renal Na(+) balance.

Transport of prostaglandin E1 across the blood-brain barrier in rats

The transport of prostaglandin E(1) (PGE(1)) across the blood-brain barrier (BBB) was characterized using an in-situ rat brain perfusion technique. The uptake of [(3)H]PGE(1) was not affected by shortchain monocarboxylic acids (butyric acid and valeric acid). On the other hand, uptake of [(3)H]PGE(1) was significantly inhibited by medium-chain monocarboxylic acids such as hexanoic acid, enanthic acid and octanoic acid. These medium-chain monocarboxylic acids showed a more potent inhibitory effect on [(3)H]PGE(1) uptake with increasing number of carbon atoms. In contrast, there was no decrease in [(3)H]PGE(1) transport by any dicarboxylic acids with 5-8 carbon atoms. Valproic acid decreased [(3)H]PGE(1) uptake, whereas p-aminohippuric acid, a substrate for the organic anion transporter family, did not inhibit [(3)H]PGE(1) transport. Bromocresol green, an inhibitor of prostaglandin transporter (PGT), strongly decreased [(3)H]PGE(1) transport across the BBB. In addition, digoxin and taurocholate, substrates for organic anion transporting polypeptide subtype 2 (Oatp2), significantly inhibited [(3)H]PGE(1) uptake. RT-PCR analysis revealed that PGT mRNA and Oatp2 mRNA are expressed in a capillary-rich fraction from rat brain. Thus, it is suggested that PGE(1) transport across the BBB is mediated by some specific transport systems, possibly by the members of the Oatp family.

Bradykinin regulates cyclooxygenase-2 in rat renal thick ascending limb cells

Cyclooxygenase-2 (COX-2) is constitutively expressed in a subset of thick ascending limb cells in the cortex and medulla and increases when the renin-angiotensin and kallikrein-kinin systems are activated. Although the contribution of angiotensin II to the regulation of COX-2 is known, the effects of bradykinin on COX-2 expression have not been determined in this nephron segment. We evaluated expression of B2 bradykinin receptors in thick ascending limb cells containing COX-2 and the effect of bradykinin on COX-2 expression in primary cultured medullary thick ascending cells. The presence of B2 receptors was studied in renal sections by immunohistochemistry with antibodies against B2, COX-2, and Tamm-Horsfall glycoprotein. B2 receptors were detected on the apical and basolateral portion of the thick ascending cells. These cells also contained COX-2, suggesting that COX-2 expression may be regulated via B2 receptor. Incubation of cultured medullary thick ascending cells with bradykinin (10(-7) to 10(-5) mol/L) induced a significant increase on COX-2 protein expression. Maximal expression of COX-2 was observed 4 hours after exposure to bradykinin (10(-7) mol/L), effect abolished by a B2 receptor antagonist (HOE-140; 10(-6) mol/L). Prostaglandin E2 production increased when these cells were challenged with bradykinin for 4 hours, indicating that COX-2 was enzymatically active. We have demonstrated (1) the presence of B2 receptors in thick ascending limb cells expressing COX-2 and (2) the stimulatory effect of bradykinin on COX-2 protein expression, via B2 receptors, in cultured medullary thick ascending cells. We suggest that bradykinin can affect ion transport in the thick ascending limb via a COX-2-mediated mechanism.

PGE2 stimulates Cl- secretion in murine M-1 cortical collecting duct cells in an autocrine manner

Prostaglandin E2 (PGE2) is thought to be an important modulator of renal ion and water transport, but its effects remain complex and incompletely understood. Here we examined the effects of PGE2 on transepithelial ion transport of M-1 mouse cortical collecting duct cells using short-circuit current (ISC) measurements. Basolateral addition of PGE2 (1 microM) produced a transient peak increase in ISC of 6.3+/-0.8 microA cm(-2) (n=11), followed by a sustained plateau. The PGE2-evoked response was preserved in the presence of 100 micro M apical amiloride with an average peak increase of 10.6+/-1.0 microA cm(-2) (n=23). However, it was greatly diminished in both the presence of apical diphenylamine-2-carboxylic acid (DPC, 1 mM) and the absence of extracellular Cl-, indicating that Cl- secretion had been stimulated. Basolateral PGE2 induced a concentration dependent response, with an EC50 of about 8 nM. Apical addition of PGE2 elicited an ISC response similar to that observed with basolateral PGE2. Furthermore, apical exposure to arachidonic acid (AA) produced a similar increase in ISC, which could be prevented by the cyclooxygenase inhibitor indomethacin, while AA failed to exert an additional effect in the presence of PGE2. Using RT-PCR, we confirmed the expression of the PGE2 (EP) receptor subtypes EP1, EP3 and EP4 but not of EP2 in cultured M-1 CCD cells. We conclude that M-1 cells express functional cyclooxygenase activity and can generate PGE2 which acts in an autocrine manner, causing Cl- secretion.

Renal kallikrein-kinin system damage and salt sensitivity: insights from experimental models

The importance of tubulointerstitial injury in the pathophysiology of human essential hypertension, and particularly salt sensitivity, is increasingly recognized. Since the renal kallikrein-kinin system (KKS) is located in the tubulointerstitial region of the kidney it is reasonable to expect that injury to this area, whatever the cause, may impair KKS production and compromise its role in blood pressure regulation. In this review we discuss evidence of injury in the renal kallikrein-producing structures in three different experimental models characterized by prominent tubulointerstitial lesions: subtotal nephrectomy; inhibition of nitric oxide synthase; and overload proteinuria. These three experimental models have in common the development of important tubulointerstitial damage and salt-sensitive hypertension expressed after the initial injury has ceased. In these three models, reduced KKS activity may contribute to the establishment of a pathophysiologic state characterized by unopposed hyperactivity of the renin-angiotensin system, resulting in salt retention.

Functional role of the NPxxY motif in internalization of the type 2 vasopressin receptor in LLC-PK1 cells

Interaction of the type 2 vasopressin receptor (V2R) with hormone causes desensitization and internalization. To study the role of the V2R NPxxY motif (which is involved in the clathrin-mediated endocytosis of several other receptors) in this process, we expressed FLAG-tagged wild-type V2R and a Y325F mutant V2R in LLC-PK1a epithelial cells that have low levels of endogenous V2R. Both proteins had a similar apical (35%) and basolateral (65%) membrane distribution. Substitution of Tyr325 with Phe325 prevented ligand-induced internalization of V2R determined by [3H]AVP binding and immunofluorescence but did not prevent ligand binding or signal transduction via adenylyl cyclase. Desensitization and resensitization of the V2R-Y325F mutation occurred independently of internalization. The involvement of clathrin in V2R downregulation was also shown by immunogold electron microscopy. We conclude that the NPxxY motif of the V2R is critically involved in receptor downregulation via clathrin-mediated internalization. However, this motif is not essential for the apical/basolateral sorting and polarized distribution of the V2R in LLC-PK1a cells or for adenylyl cyclase-mediated signal transduction.

The human multidrug resistance protein MRP4 functions as a prostaglandin efflux transporter and is inhibited by nonsteroidal antiinflammatory drugs

Prostaglandins are involved in a wide variety of physiological and pathophysiological processes, but the mechanism of prostaglandin release from cells is not completely understood. Although poorly membrane permeable, prostaglandins are believed to exit cells by passive diffusion. We have investigated the interaction between prostaglandins and members of the ATP-binding cassette (ABC) transporter ABCC [multidrug resistance protein (MRP)] family of membrane export pumps. In inside-out membrane vesicles derived from insect cells or HEK293 cells, MRP4 catalyzed the time- and ATP-dependent uptake of prostaglandin E1 (PGE1) and PGE2. In contrast, MRP1, MRP2, MRP3, and MRP5 did not transport PGE1 or PGE2. The MRP4-mediated transport of PGE1 and PGE2 displayed saturation kinetics, with Km values of 2.1 and 3.4 microM, respectively. Further studies showed that PGF1alpha, PGF2alpha, PGA1, and thromboxane B2 were high-affinity inhibitors (and therefore presumably substrates) of MRP4. Furthermore, several nonsteroidal antiinflammatory drugs were potent inhibitors of MRP4 at concentrations that did not inhibit MRP1. In cells expressing the prostaglandin transporter PGT, the steady-state accumulation of PGE1 and PGE2 was reduced proportional to MRP4 expression. Inhibition of MRP4 by an MRP4-specific RNA interference construct or by indomethacin reversed this accumulation deficit. Together, these data suggest that MRP4 can release prostaglandins from cells, and that, in addition to inhibiting prostaglandin synthesis, some nonsteroidal antiinflammatory drugs might also act by inhibiting this release.

The renal kallikrein-kinin system: its role as a safety valve for excess sodium intake, and its attenuation as a possible etiologic factor in salt-sensitive hypertension

The distal tubules of the kidney express the full set of the components of the kallikrein-kinin system, which works independently from the plasma kallikrein-kinin system. Studies on the role of the renal kallikrein-kinin system, using congenitally kininogen-deficient Brown-Norway Katholiek rats and also bradykinin B2 receptor knockout mice, revealed that this system starts to function and to induce natriuresis and diuresis when sodium accumulates in the body as a result of excess sodium intake or aldosterone release, for example, by angiotensin II. Thus, it can be hypothesized that the system works as a safety valve for sodium accumulation. The large numbers of studies on hypertensive animal models and on essential hypertensive patients, particularly those with salt sensitivity, indicate a tendency toward the reduced excretion of urinary kallikrein, although this reduction is modified by potassium intake and impaired renal function. We hypothesize that the reduced excretion of the renal kallikrein may be attributable to a genetic defect of factor(s) in renal kallikrein secretion process and may cause salt-sensitive hypertension after salt intake.

Prostaglandin transport

Newly synthesized prostaglandins (PGs) efflux from cells by simple diffusion, driven by pH and the membrane potential. Metabolic clearance requires energy-dependent uptake across the plasma membrane, followed by cytoplasmic oxidation. Several PG carriers have been cloned and characterized. PGT is broadly expressed in cyclooxygenase (COX)-positive cells, appears to be a lactate/PG exchanger, and is coordinately regulated with COX. By analogy with neurotransmitter release and re-uptake, PGT may regulate pericellular PG levels via re-uptake. PGT may also direct PGs towards and/or away from specific sets of PG receptors. Other members of the OATP transporter family also catalyze PG uptake; these are variably expressed and have variable affinities for PGs. The OATs are alpha-ketoglutarate/organic anion exchangers that accept PGs; these probably represent the uptake step in renal and hepatic PG degradation and excretion. Finally, certain glutathione-conjugated leukotrienes and PGs are actively extruded from cells by the MRPs; these may also play a role in metabolic clearance of PGs.

Inhibition of interleukin-1beta-induced COX-2 and EP3 gene expression by sodium salicylate enhances pancreatic islet beta-cell function

Previous work has suggested that functional interrelationships may exist between inhibition of insulin secretion by interleukin (IL)-1beta and the endogenous synthesis of prostaglandin E(2) (PGE(2)) in the pancreatic islet. These studies were performed to ascertain the relative abundance of E prostaglandin (EP) receptor mRNAs in tissues that are major targets, or major degradative sites, of insulin; to identify which EP receptor type mediates PGE(2) inhibition of insulin secretion in pancreatic islets; and to examine possible sites of action through which sodium salicylate might affect IL-1beta/PGE(2) interactions. Real-time fluorescence-based RT-PCR indicated that EP3 is the most abundant EP receptor type in islets, liver, kidney, and epididymal fat. EP3 mRNA is the least, whereas EP2 mRNA is the most, abundant type in skeletal muscle. Misoprostol, an EP3 agonist, inhibited glucose-induced insulin secretion from islets, an event that was prevented by preincubation with pertussis toxin, by decreasing cAMP. Electromobility shift assays demonstrated that sodium salicylate inhibits IL-1beta-induced nuclear factor-kappaB (NF-kappaB) activation. Sodium salicylate also prevented IL-1beta from inducing EP3 and cyclooxygenase (COX)-2 gene expression in islets and thereby prevented IL-1beta from inhibiting glucose-induced insulin secretion. These findings indicate that the sites of action through which sodium salicylate inhibits these negative effects of IL-1beta on beta-cell function include activation of NF-kappaB as well as generation of PGE(2) by COX-2.

The spinal phospholipase-cyclooxygenase-prostanoid cascade in nociceptive processing

Intrathecal phospholipase A2 (PLA2) and cyclooxygenase-2 (COX-2), but not COX-1, inhibitors attenuate facilitated pain states generated by peripheral injury/inflammation and by direct activation of spinal glutamate and substance P receptors. These results are consistent with the constitutive expression of PLA2 and COX-2 in spinal cord, the spinal release of prostaglandins by persistent afferent input, and the effects of prostaglandins on spinal excitability. Whereas the acute actions of COX-2 inhibitors are clearly mediated by constitutively expressed spinal COX-2, studies of spinal COX-2 expression indicate that it is upregulated by neural input and circulating cytokines. Given the intrathecal potency of COX-2 inhibitors, the comparable efficacy of intrathecal versus systemic COX-2 inhibitors in hyperalgesic states not associated with inflammation, and the onset of antihyperalgesic activity prior to COX-2 upregulation, it is argued that a principal antihyperalgesic mechanism of COX-2 inhibitors lies with modulation of constitutive COX-2 present at the spinal level.

Functional characterization of basolateral and luminal dopamine receptors in rabbit CCD

Previous studies reported the existence of both D1- and D2-like receptors in the cortical collecting duct (CCD). However, especially with regard to natriuresis, it remains controversial. In the present study, rabbit CCD was perfused to characterize the receptor subtypes responsible for the tubular actions. Basolateral dopamine (DA) induced a dose-dependent depolarization of transepithelial voltage. Basolateral domperidone, a D2-like receptor antagonist, abolished depolarization, whereas SKF-81297, a D1-like receptor agonist, showed no significant change. In addition, bromocriptine, a D2-like receptor agonist, also caused depolarization, whereas SKF-81297, a D1-like receptor agonist, did not depolarize significantly. Moreover, RBI-257, a D4-specific antagonist, reversed the basolateral DA-induced depolarization. In contrast to the basolateral side, luminal DA caused depolarization via a D1-like receptor; however the change was less than that for basolateral DA. For further evaluation, 22Na+ flux (J(Na)) was measured to confirm the effect of DA on Na+ transport. Basolateral DA also caused a suppression of J(Na), and this reaction was abolished by domperidone. These results suggested that the basolateral D2-like receptor is mainly responsible for the natriuretic action of DA in rabbit CCD.

Immunohistochemical localization of prostaglandin EP3 receptor in the rat nervous system

The prostaglandin EP3 receptor (EP3R) subtype is believed to mediate large portions of diverse physiologic actions of prostaglandin E2 in the nervous system. However, the distribution of EP3R protein has not yet been unveiled in the peripheral or central nervous systems. The authors raised a polyclonal antibody against an amino-terminal portion of rat EP3R that recognized specifically the receptor protein. In this study, immunoblotting analysis with this antibody showed several immunoreactive bands with different molecular weights in rat brain extracts and in membrane fractions of recombinant EP3R-expressing culture cells, and treatment with N-glycosidase shifted those immunoreactive bands to an apparently single band with a lower molecular weight, suggesting that EP3R proteins are modified posttranslationally with carbohydrate moieties of various sizes. The authors performed immunohistochemical investigation of EP3R in the rat brain, spinal cord, and peripheral ganglia by using the antibody. EP3R-like immunoreactivity was observed in many and discrete regions of the rostrocaudal axis of the nervous system. The signals were particularly strong in the anterior, intralaminar, and midline thalamic nuclear groups; the median preoptic nucleus; the medial mammillary nucleus; the superior colliculus; the periaqueductal gray; the lateral parabrachial nucleus; the nucleus of the solitary tract; and laminae I and II of the medullary and spinal dorsal horns. Sensory ganglia, such as the trigeminal, dorsal root, and nodose ganglia, contained many immunopositive neurons. Neuronal cells in the locus coeruleus and raphe nuclei exhibited EP3R-like immunoreactivity. This suggests that EP3R plays regulatory roles in the noradrenergic and serotonergic monoamine systems. Autonomic preganglionic nuclei, such as the dorsal motor nucleus of the vagus nerve, the spinal intermediolateral nucleus, and the sacral parasympathetic nucleus, also contained neuronal cell bodies with the immunoreactivity, implying modulatory functions of EP3R in the central autonomic nervous system. The characteristic distribution of EP3R provides valuable information on the mechanisms for various physiologic actions of prostaglandin E2 in the central and peripheral nervous systems.

Different membrane targeting of prostaglandin EP3 receptor isoforms dependent on their carboxy-terminal tail structures

Mouse prostaglandin EP3 receptor consists of three isoforms, EP3alpha, beta and gamma, with different carboxy-terminal tails. To assess the role of their carboxy-terminal tails in membrane targeting, we examined subcellular localization of myc-tagged EP3 isoforms expressed in MDCK cells. Two isoforms, EP3alpha and EP3beta, were localized in the intracellular compartment but not in the plasma membrane, while the EP3gamma isoform was found in the lateral plasma membrane and in part in the intracellular compartment. Mutant EP3 receptor lacking the carboxy-terminal tail was localized in the intracellular compartment but not in the plasma membrane. Thus, EP3 isoforms differ in subcellular targeting, and the carboxy-terminal tails play an important role in determination of the membrane targeting of EP3 receptor.

Prostanoid receptors: structures, properties, and functions

Prostanoids are the cyclooxygenase metabolites of arachidonic acid and include prostaglandin (PG) D(2), PGE(2), PGF(2alpha), PGI(2), and thromboxne A(2). They are synthesized and released upon cell stimulation and act on cells in the vicinity of their synthesis to exert their actions. Receptors mediating the actions of prostanoids were recently identified and cloned. They are G protein-coupled receptors with seven transmembrane domains. There are eight types and subtypes of prostanoid receptors that are encoded by different genes but as a whole constitute a subfamily in the superfamily of the rhodopsin-type receptors. Each of the receptors was expressed in cultured cells, and its ligand-binding properties and signal transduction pathways were characterized. Moreover, domains and amino acid residues conferring the specificities of ligand binding and signal transduction are being clarified. Information also is accumulating as to the distribution of these receptors in the body. It is also becoming clear for some types of receptors how expression of their genes is regulated. Furthermore, the gene for each of the eight types of prostanoid receptor has been disrupted, and mice deficient in each type of receptor are being examined to identify and assess the roles played by each receptor under various physiological and pathophysiological conditions. In this article, we summarize these findings and attempt to give an overview of the current status of research on the prostanoid receptors.

Are salivary glands cell lines in culture a good model for purinergic receptors in salivary glands?

A major obstacle in studying the physiological and biochemical processes of salivary secretion is the lack of a good ductal cell line model. HSY, an immortalised cell line originating from human parotid gland intercalated ducts, provides a possible model for purinergic mechanisms in ductal cells. Unlike the biphasic dose response to ATP of isolated submandibular ductal cells, the rise in [Ca2+]i in HSY cells shows single Michaelis-Menten kinetics with an apparent Ka of 0.8 microM. Pre-incubation with thapsigargin inhibited the ATP induced [Ca2+]i rise. Both ATP (10 microM) and carbachol (100 microM) increased IP3 production. Intercalated duct cells may differentiate into acinar or ductal cells in response to appropriate stimuli from extracellular matrix We therefore attempted to induce a duct-like phenotype in the striated duct-derived HSY cells by growing them on microcarrier beads coated with type I collagen. In Ca-containing medium cells grown on all substrates showed similar responses to ATP. In contrast, in Ca-free medium, [Ca2+]i rose only slightly in cells grown on beads relative to those on glass. This probably resulted from reduced IP3 production. Carbachol also induced a much smaller increase in [Ca2+]i and less IP3 production in cells grown on Cytodex-3. The HSY response to purinergic stimuli by an increase in [Ca2+]i and IP3 means that they can be used to study the metabotropic purinergic pathway. The impairment in the HSY responses grown on Cytodex-3 can be used to probe phosposinositol signal transduction in salivary cells.

Molecular mechanisms of prostaglandin transport

Despite the fact that prostaglandins (PGs) have low intrinsic permeabilities across the plasma membrane, they must cross it twice: first upon release from the cytosol into the blood, and again upon cellular uptake prior to oxidation. Until recently, there were no cloned carriers that transported PGs. PGT is a broadly-expressed, 12-membrane-spanning domain integral membrane protein. When heterologously expressed in HeLa cells or Xenopus oocytes, it catalyzes the rapid, specific, and high-affinity uptake of PGE2, PGF2 alpha, PGD2, 8-iso-PGF2 alpha, and thromboxane B2. Functional studies indicate that PGT transports its substrate as the charged anion. The PGT substrate specificity and inhibitor profile match remarkably well with earlier in situ studies on the metabolic clearance of PGs by rat lung. Because PGT expression is especially high in this tissue, it is likely that PGT mediates the membrane step in PG clearance by the pulmonary circulation. Evidence is presented that PGT may play additional roles in other tissues and that there may be additional PG transporters yet to be identified molecularly.

Two isoforms of prostaglandin EP3 receptor exhibiting constitutive activity and agonist-dependent activity in Rho-mediated stress fiber formation

We have cloned two isoforms of the mouse prostaglandin E receptor EP3 subtype, EP3alpha and EP3beta, with different carboxyl-terminal tails, produced through alternative splicing. To determine the functional differences between the two isoforms, we examined the role of the isoforms in regulation of the actin cytoskeleton using Mardin-Darby canine kidney cells expressing these isoforms. The EP3alpha isoform constitutively induced stress fiber formation, independent of an agonist, while the EP3beta isoform agonist-dependently induced stress fiber formation. Pertussis toxin did not prevent stress fiber formation. This signaling pathway is mediated by Rho, because C3 transferase microinjection inhibited stress fiber formation. Therefore, the physiological significance of these isoforms of the EP3 receptor may lie in their different agonist dependency in Rho-mediated stress fiber formation via a pertussis toxin-insensitive G protein.

Functional interaction of the carboxylic acid group of agonists and the arginine residue of the seventh transmembrane domain of prostaglandin E receptor EP3 subtype

Prostaglandin (PG) E2 binds to PGE receptor EP3 subtype and induces Gi activity. To assess the role of the interaction of the carboxylic acid group of agonists and its putative binding site, Arg-309 in the seventh transmembrane domain of EP3alpha receptor, in receptor activation, we have mutated the positively charged Arg-309 to the polar but uncharged Gln (EP3alpha-R309Q) and Asn (EP3alpha-R309N), and to the non-polar Leu (EP3alpha-R309L). Wild-type, EP3alpha-R309Q and EP3alpha-R309N receptors showed high-affinity binding for PGE2, but the EP3alpha-R309L receptor showed very-low-affinity binding. Guanosine 5'-[gamma-thio]triphosphate increased the PGE2 binding to the wild-type receptor, decreased the binding to EP3alpha-R309Q and EP3alpha-R309N receptors, but did not affect that to the EP3alpha-R309L receptor. Furthermore we examined the Gi activities of two types of EP3 agonist, TEI-3356 with a negatively charged carboxylic acid, and TEI-4343, a methyl ester of TEI-3356 with an uncharged but polar group, towards those receptors. Both agonists inhibited the forskolin-stimulated cAMP formation in wild-type, EP3alpha-R309Q and EP3alpha-R309N receptors in the same concentration-dependent manner, but the agonists showed a very low inhibition of EP3alpha-R309L receptor. These findings demonstrate that the hydrogen-bonding interaction of EP3 agonists and residue 309 in the seventh transmembrane domain of the EP3alpha receptor is sufficient for the functional activation of the EP3alpha receptor.

Epidermal growth factor receptor activation induces nuclear targeting of cyclooxygenase-2, basolateral release of prostaglandins, and mitogenesis in polarizing colon cancer cells

Nonsteroidal antiinflammatory drugs reduce the risk of colon cancer, possibly via cyclooxygenase (COX) inhibition. The growth factor-inducible COX-2, which is overexpressed in neoplastic colonic tissue, is an attractive target to mediate this effect. Herein we have exploited the ability of a human colon cancer cell line, HCA-7 Colony 29, to polarize when cultured on Transwell (Costar) filters to study COX-2 production and the vectorial release of prostaglandins (PGs). Administration of type alpha transforming growth factor to the basolateral compartment, in which the epidermal growth factor receptor (EGFR) resides, results in a marked induction of COX-2 immunoreactivity at the base of the cells and the unexpected appearance of COX-2 in the nucleus. The increase in COX-2 protein is associated with a dose- and time-dependent increase in PG levels in the basolateral, but not apical, medium. Amphiregulin is the most abundantly expressed EGFR ligand in these cells, and the protein is present at the basolateral surface. EGFR blockade reduces baseline COX-2 immunoreactivity, PG levels, and mitogenesis in a concentration-dependent manner. Two specific COX-2 inhibitors, SC-58125 and NS 398, also, in a dose-dependent manner, attenuate baseline and type alpha transforming growth factor-stimulated mitogenesis, although PG levels are decreased > 90% at all concentrations of inhibitor tested. These findings show that activation of the EGFR stimulates COX-2 production and its translocation to the nucleus, vectorial release of PGs, and mitogenesis in polarized HCA-7 Colony 29 cells.

Endogenously produced prostanoids stimulate calcium reabsorption in the rabbit cortical collecting system

1. The influence of endogenously produced prostanoids on active transepithelial Ca2+ transport and cAMP formation was investigated in immunodissected rabbit kidney connecting and cortical collecting tubule cells grown to confluency on permeable supports. 2. The cyclo-oxygenase inhibitor indomethacin dose-dependently (IC50 = 18 nM) reduced the net apical-to-basolateral Ca2+ transport by 57%. Inhibition was reversed in medium obtained from monolayers incubated in the absence of indomethacin. 3. HPLC analysis following incubation with 14C-labelled arachidonic acid revealed the presence of a wide variety of radiolabelled prostanoids in both the apical and basolateral media. These findings are compatible with the endogenous production and subsequent release of stimulatory prostanoids. 4. The inhibitory action of indomethacin was reversed by the addition of the prostanoids PGE1, PGE2 and PGA2, but not PGD2, PGF2 alpha, the stable PGI2 analogue cicaprost or the thromboxane A2 mimetic U-46619. PGE2 stimulated transepithelial Ca2+ transport dose dependently (EC50 = 3 nM), irrespective of the compartment of which it was added. The stimulatory effect of PGE2 was paralleled by increased cAMP formation, suggesting the apical and basolateral presence of stimulatory prostanoid receptors EP2 and/or EP4. 5. Sulprostone, an analogue selective for EP1 and EP3 receptors, inhibited transepithelial Ca2+ transport in indomethacin-treated monolayers only when applied basolaterally, suggesting the exclusive presence of inhibitory EP receptors on the basolateral membrane. 6. The percentage by which parathyroid hormone and arginine vasopressin increased both transepithelial Ca2+ transport and cAMP formation was dramatically increased in indomethacin-inhibited cells as compared with control cells, demonstrating that indomethacin unmasks the actions of these hormones to their full extent.

Prostaglandin E receptor EP3gamma isoform, with mostly full constitutive Gi activity and agonist-dependent Gs activity

We recently demonstrated that two exclusively Gi-coupled isoforms of the mouse EP3 receptor, EP3alpha and beta, with different carboxyl-terminal tails, differed in agonist-independent constitutive Gi activity, and the carboxyl-terminal tail-truncated receptor showed full constitutive activity (Hasegawa, H., Negishi, M., and Ichikawa, A. (1996) J. Biol. Chem. 271, 1857-1860). Here we further examined Gi and Gs activities of the third isoform, EP3gamma, coupled to both Gi and Gs. The My receptor showed mostly full constitutive Gi activity and agonist-dependent Gs activity. The truncated receptor also showed agonist-dependent Gs activity, but the level was lower than that of the EP3gamma receptor. Thus, the carboxyl-terminal tail would differentially regulate Gi and Gs activities of the EP3 receptor.

The role of prostaglandin receptors in regulating cerebral blood flow in the perinatal period

Prostaglandins exert significant effects on the range of cerebral blood flow autoregulation. However, the newborn exhibits a narrow cerebral blood flow autoregulatory range compared to the adult, and this apparently contributes to the susceptibility of the newborn to major perinatal complications such as intraventricular cerebral haemorrhage. Reduced vasoconstriction in response to prostaglandins due to the fewer prostaglandin receptors, especially for PGE2 (EP) and PGF2 alpha (FP), seems to contribute in part to the narrower range of cerebral blood flow autoregulation in the newborn. Evidence suggests that high levels of prostaglandins in the perinatal period are responsible for the down-regulation of neurovascular EP and FP receptors. We review the pharmacology of prostaglandin receptors, in particular PGE2 and PGF2 alpha receptors, their ontogeny on the neural vasculature, the perinatal regulation of their expression, and how these changes relate to the control of neural blood flow autoregulation.

Effects of angiotensin-converting enzyme and neutral endopeptidase inhibitors: influence of bradykinin

These experiments compare the effects of a neutral endopeptidase inhibitor, retrothiorphan, 1-[(1-mercaptomethyl-2-phenyl)ethyl]amino-1-oxopropanoic acid, a converting enzyme inhibitor, enalaprilat, and the combination of the two inhibitors on changes in blood pressure and renal function induced by exogenous and endogenous bradykinin in deoxycorticosterone acetate (DOCA)-salt rats. Enalaprilat potentiated the exogenous bradykinin-induced hypotensive responses while retrothiorphan potentiated the effects on urinary cyclic-GMP (cGMP) and bradykinin. The combination potentiated the exogenous bradykinin-induced hypotensive effects and the bradykinin-induced urinary excretion of cGMP, bradykinin and prostaglandin. The bradykinin B2 receptor antagonist, Hoe 140, had no effect on the enalaprilat- and retrothiorphan-induced changes in blood pressure and renal function. In conclusion, while angiotensin-converting enzyme and neutral endopeptidase are involved in the vascular and renal catabolism of exogenous bradykinin, the effects of the peptidase inhibitors do not appear to depend on the protection of endogenous bradykinin under acute conditions in DOCA-salt rats.

Two isoforms of the prostaglandin E receptor EP3 subtype different in agonist-independent constitutive activity

We previously identified two isoforms of the mouse prostaglandin E receptor EP3 subtype, EP3 alpha and EP3 beta, with different carboxyl-terminal tails, produced through alternative splicing and showing different efficiency in inhibition of adenylate cyclase (Sugimoto, Y., Negishi, M., Hayashi, Y., Namba, T., Honda, A., Watabe, A., Hirata, M., Narumiya, S., and Ichikawa, A. (1993) J. Biol. Chem. 268, 2712-2718). To assess the role of the carboxyl-terminal tails in the G protein coupling properties of the EP3 receptor, we examined the Gi activities of EP3 alpha, EP3 beta, and the mutant receptor, in which the carboxyl-terminal tail was truncated at the splicing site. The EP3 alpha receptor showed marked agonist-independent constitutive inhibition of adenylate cyclase, while EP3 beta receptor had no agonist-independent inhibition. On the other hand, the truncated receptor showed only agonist-independent constitutive inhibition. The constitutive activity of these receptors on the stimulation of GTPase activity of Gi was also observed. Thus, alternative splicing produced two isoforms with different carboxyl-terminal tails and with different constitutive activity, and the truncation of the carboxyl-terminal tail caused full constitutive activity.

Identification and characterization of a prostaglandin transporter

Carrier-mediated prostaglandin transport has been postulated to occur in many tissues. On the basis of sequence homology, the protein of unknown function encoded by the rat matrin F/G complementary DNA was predicted to be an organic anion transporter. Expression of the matrin F/G complementary DNA in HeLa cells or Xenopus oocytes conferred the property of specific transport of prostaglandins. The tissue distribution of matrin F/G messenger RNA and the sensitivity of matrin F/G-induced prostaglandin transport to inhibitors were similar to those of endogenous prostaglandin transport. The protein encoded by the matrin F/G complementary DNA is thus preferably called PGT because it is likely to function as a prostaglandin transporter.

Effect of inhibiting renal kallikrein on prostaglandin E2, water, and sodium excretion

To test the hypothesis that renal kinins act as natriuretic and diuretic hormones, we examined the effect of inhibiting glandular kallikrein on renal function in normotensive unanesthetized rats during normal sodium intake. To inhibit kallikrein at both the luminal and basolateral sides of the distal nephron, we used Fab fragments of monoclonal antibodies to rat urinary kallikrein (Fab-kallikrein). Fab fragments have advantages over intact IgG: they are filtered through the glomerulus and reach the lumen of the distal nephron, where kallikrein is localized and urinary kinins are released. Furthermore, the Fab fragment-antigen complex does not activate the complement system, avoiding the side effects associated with intact antibodies. Fab-kallikrein effectively blocked generation of kinins in the nephron lumen, decreasing urinary kininogenase activity (kallikrein) by 74% to 85% and kinin excretion by 76% to 79%. Fab-kallikrein induced a 30% decrease in urine volume and a 20% to 40% decrease in urinary sodium excretion but did not alter blood pressure, glomerular filtration rate, or renal blood flow. Although urinary prostaglandin E2 excretion also tended to decrease, this change was slower and of lesser magnitude than those of kinin and kininogenase excretion and did not attain statistical significance after Bonferroni's correction. In controls injected with either vehicle or Fab fragments of monoclonal antibodies to ricin (a vegetable protein not present in mammals), none of these parameters decreased significantly. We conclude that renal kinins participate in the short-term regulation of water and sodium excretion in normotensive unanesthetized rats, acting as diuretic and natriuretic hormones.(ABSTRACT TRUNCATED AT 250 WORDS)

Synergistic action of vasopressin and aldosterone on basolateral Na(+)-K(+)-ATPase in the cortical collecting duct

The respective effects of aldosterone and arginine vasopressin (AVP) were examined on the number of active Na(+)-K(+)-ATPase and their pumping activity in nonperfused microdissected mouse cortical collecting tubules (CCD) by measuring specific 3H-ouabain binding and ouabain-sensitive 86Rb uptake. In adrenalectomized (ADX) animals, incubation of CCD with AVP (10(-8) M for 5 min) had no effect on the number of pumps. In contrast, in ADX animals replete with aldosterone, AVP induced a approximately equal to 40% increase in the number of pumps. This was accompanied by a approximately equal to 60-65% increase in ouabain-sensitive Rb uptake. AVP effect was dose-dependent (10(-10)-10(-8) M) and was reproduced by dDAVP, forskolin and 8-Br cAMP, indicating a V2 pathway. It was inhibited by amiloride 10(-5) M, and did not occur in CCD incubated in hyperosmotic solution, suggesting that the signal was transmitted via apical sodium entry and cell swelling. Finally, the AVP-dependent increase in the number of pumps was rapid (within 5 min) and transient (< 25 min). These results demonstrate that, in the CCD, aldosterone and AVP act synergistically to increase not only the apical sodium entry but also the basolateral Na(+)-K(+)-ATPase transport capacity: AVP allows a rapid recruitment and/or activation of an aldosterone-dependent pool of latent Na(+)-K(+)-ATPase.

Induction of alkalinization in cultured renal cells (MDCK line) by prostaglandin E2

The effect of prostaglandin E2 (PGE2), on the intracellular pH (pHi) in BCECF-loaded Madin Darby Canine Kidney (MDCK) cells was investigated. PGE2 elevated the pHi. Under resting conditions, pHi of MDCK cells suspended in PBS at pH 7.4 was 7.11 +/- 0.08; PGE2 increased pHi with an EC50 of 0.16 microM. PGF2 alpha elicited a similar response to PGE2, with an EC50 of 0.24 microM. Amiloride (0.4 mM) reversed the response to PGE2 (control 7.18 +/- 0.05; PGE2 7.26 +/- 0.05; after amiloride 7.18 +/- 0.05). In MDCK cells exposed to a Na(+)-free solution, alkalinization induced by this eicosanoid was blocked (Ringer-choline 7.16 +/- 0.03; PGE2 7.16 +/- 0.02). PGE2 increased by 100% the rate of recovery after an acidification pulse with ammonium chloride. In the presence of Ringer-HCO3- (pH 7.4), there was a delay in the maximal response to this prostaglandin (PBS 2.2 +/- 0.27, Ringer-bicarbonate 3.4 +/- 0.55 min) and the pHi increment was less marked than in PBS (0.09 pH units in HCO3- versus 0.16 pH units in PBS; P < 0.001). This effect of PGE2 was not blocked by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (1.0 mM). PMA (100 nM), activator of protein kinase C, mimicked the response to PGE2, suggesting the participation of this kinase on the effect of the prostanoid. As expected, two inhibitors of protein kinase C, staurosporine and sphingosine, abolished the response to PGE2. Staurosporine (0.10 microM), an inhibitor of protein kinase C, blocked the response to PGE2 (control 7.02 +/- 0.04; PGE2 and staurosporine 7.03 +/- 0.04, n = 9, not significant). Sphingosine, another inhibitor of protein kinase C, also blocked the response to PGE2. Two analogues of cAMP did not modify the pHi. In summary, PGE2 induced an intracellular alkalinization via stimulation of a Na+/H+ exchanger, with the participation of protein kinase C, in MDCK cells.

Inhibition of prostaglandin E2-responsive adenylyl cyclase in embryonal human kidney 293 cells by phorbol esters

A clonal primary embryonal human kidney cell line, 293, increased cAMP production in response to prostaglandin E2 (PGE2) (0.02-2 microM). The purpose of this study was to show the effects of tumor-promoting phorbol esters (e.g., 4 beta-phorbol 12-myristate 13-acetate, PMA) on PGE2-stimulated cAMP production. Pretreatment with PMA (0.2-200 nM) for 30 min markedly reduced PGE2-stimulated cAMP production in the presence of 0.5 mM isobutylmethylxanthine. The reduction by PMA was dose- and time-dependent. PMA seems to attenuate the increase in cAMP accumulation elicited by PGE2 primarily, if not entirely, by inhibiting adenylyl cyclase activity, since we were unable to demonstrate an effect of PMA on the degradation half-life of cAMP in intact 293 cells. The action of PMA had some specificity for the agonist used; thus, PMA inhibited PGE2-activated adenylyl cyclase but had no effect on the forskolin-activated enzyme. Co-pretreatment with PMA and H-7, an inhibitor of protein kinase C (PKC), partially prevented the PMA-induced attenuation of the PGE2-stimulated cAMP accumulation, and 1-oleoyl-2-acetylglycerol, a synthetic diacylglycerol analog, partially mimicked the PMA action. Thus, PMA appeared to decrease cAMP production by a PKC-mediated mechanism, inhibiting adenylyl cyclase activity at a point other than the catalytic subunit of the enzyme in the kidney 293 cell line.

TEI-3356, a highly selective agonist for the prostaglandin EP3 receptor

Recently, we cloned cDNAs for the three mouse PGE receptor subtypes, EP1, EP2 and EP3, and the prostacyclin receptor, and established cells that stably express each receptor. We examined the selectivity of TEI-3356, an isocarbacyclin analogue, compared with other EP agonists, sulprostone and misoprostol, using Chinese hamster ovary cells expressing each cloned receptor. TEI-3356 selectively displaced the [3H]PGE2 binding to EP3-expressing cell membranes, but showed very low affinity for both EP1 and EP2. Although TEI-3356 is an isocarbacyclin analogue, it showed low affinity for the prostacyclin receptor. On the other hand, sulprostone strongly displaced the [3H]PGE2 binding to EP1 and EP3, but not to EP2. Misoprostol weakly bound to the three subtypes without selectivity. TEI-3356 decreased the forskolin-induced cAMP formation in a concentration-dependent manner in the EP3-expressing cells, the half-maximal concentration for the inhibition being similar to that of sulprostone but lower than that of PGE2. These results demonstrate that TEI-3356 is a potent and highly selective agonist for the EP3 receptor.