Activation of the osmo-sensitive chloride conductance involves P21rho and is accompanied by a transient reorganization of the F-actin cytoskeleton

Hypo-osmotic stimulation of human Intestine 407 cells rapidly activated compensatory CL- and K+ conductances that limited excessive cell swelling and, finally, restored the original cell volume. Osmotic cell swelling was accompanied by a rapid and transient reorganization of the F-actin cytoskeleton, affecting both stress fibers as well as apical ruffles. In addition, an increase in total cellular F-actin was observed. Pretreatment of the cells with recombinant Clostridium botulinum C3 exoenzyme, but not with mutant enzyme (C3-E173Q) devoid of ADP-ribosyltransferase activity, greatly reduced the activation of the osmo-sensitive anion efflux, suggesting a role for the ras-related GTPase p21rho. In contrast, introducing dominant negative N17-p21rac into the cells did not affect the volume-sensitive efflux. Cell swelling-induced reorganization of F-actin coincided with a transient, C3 exoenzyme-sensitive tyrosine phosphorylation of p125 focal adhesion kinase (p125FAK) as well as with an increase in phosphatidylinositol-3-kinase (PtdIns-3-kinase) activity. Pretreatment of the cells with wortmannin, a specific inhibitor of PtdIns-3-kinase, largely inhibited the volume-sensitive ion efflux. Taken together, our results indicate the involvement of a p21rho signaling cascade and actin filaments in the activation of volume-sensitive chloride channels.

ROCK-I and ROCK-II, two isoforms of Rho-associated coiled-coil forming protein serine/threonine kinase in mice

We recently identified a novel human protein kinase, p160 ROCK, as a putative downstream target of the small GTPase Rho. Using the human ROCK cDNA as a probe, we isolated cDNA of two distinct, highly related sequences from mouse libraries. One encoded a mouse counterpart of human ROCK (ROCK-I), and the other encoded a novel ROCK-related kinase (ROCK-II). Like ROCK/ROCK-I, ROCK-II also bound to GTP-Rho selectively. ROCK-I mRNA was ubiquitously expressed except in the brain and muscle, whereas ROCK-II mRNA was expressed abundantly in the brain, muscle, heart, lung and placenta. These results suggest that at least two ROCK isoforms are present in a single species and play distinct roles in Rho-mediated signalling pathways.

The small GTPase Rho: cellular functions and signal transduction

Rho, a Ras homologue of small GTPase, is present from yeast to mammals. It shuttles between the active GTP-bound form and the inactive GDP-bound form and works as a switch in stimulus-evoked cell adhesion and motility, enhancement of contractile responses, and cytokinesis. In these actions, Rho directs the reorganization of the actin cytoskeleton at a specific time and at a specific site in the cell. It also activates serum response factor possibly via a kinase cascade and mediates a growth signal to nuclei. Two signalling processes are known to lead to Rho activation: one is activation of certain types of G-protein-coupled receptors such as lysophosphatidic acid receptor, and the other is activation of other small GTPases including Ras, CDC42, and Rac. Molecules catalyzing the GDP-GTP exchange of Rho, Rho guanine nucleotide exchange factors (Rho GEF), and those catalyzing the acceleration of GTP hydrolysis, Rho GTPase activating proteins (Rho GAP), were identified as Dbl- and Bcr-containing molecules, respectively. In addition, a molecule inhibiting guanine nucleotide exchange of Rho, Rho guanine nucleotide dissociation inhibitor (Rho-GDI), was isolated and characterized. More recently, putative Rho targets possibly mediating various Rho actions have been identified by their selective interaction with GTP-bound Rho. They include lipid kinases such as phosphatidyl-inositol-5-kinase and protein serine/threonine kinases such as PKN and p160ROCK. A model of the molecular mechanism of action of Rho constructed on the basis of these findings is presented. There are, however, still many unclarified links between cell stimulation, Rho activation and final Rho actions.

Involvement of rho p21 in cyclic strain-induced tyrosine phosphorylation of focal adhesion kinase (pp125FAK), morphological changes and migration of endothelial cells

The molecular mechanisms by which endothelial cells sense and respond to physical forces remain to be elucidated. Recently we reported that cyclic strain-induced morphological change and migration of EC were regulated by the tyrosine phosphorylation of focal adhesion kinase (pp125FAK) and paxillin. The aim of the present study was to clarify the role of the small GTP-binding protein rho p21 in EC exposed to cyclic strain. Bovine aortic endothelial cells (EC) were subjected to 10% average strain at 60 cycle/min. Clostridium botulinum C3 transferase (C3) was used as a specific inhibitor of rho p21. Preincubation of EC with C3 inhibited ADP-ribosylation of rho (94%) and inhibited the morphological change, reorganization of actin filaments, and migration induced by cyclic strain. Moreover, C3 inhibited the cyclic strain-induced tyrosine phosphorylation of pp125FAK and paxillin. These results demonstrate that rho downregulates the tyrosine phosphorylation of pp125FAK and paxillin and can modulate the morphological changes and migration induced by cyclic strain.

Rhotekin, a new putative target for Rho bearing homology to a serine/threonine kinase, PKN, and rhophilin in the rho-binding domain

Using a mouse embryo cDNA library, we conducted a two-hybrid screening to identify new partners for the small GTPase Rho. One clone obtained by this procedure contained a novel cDNA of 291 base pairs and interacted strongly with RhoA and RhoC, weakly with RhoB, and not at all with Rac1 and Cdc42Hs. Full-length cDNAs were then isolated from a mouse brain library. While multiple splicing variants were common, we identified three cDNAs with an identical open reading frame encoding a 61-kDa protein that we named rhotekin (from the Japanese "teki," meaning target). The N-terminal part of rhotekin, encoded by the initial cDNA and produced in bacteria as a glutathione S-transferase fusion protein, exhibited in vitro binding to 35S-labeled guanosine 5'-3-O-(thio)triphosphate-bound Rho, but not to Rac1 or Cdc42Hs in ligand overlay assays. In addition, this peptide inhibited both endogenous and GTPase-activating protein-stimulated Rho GTPase activity. The amino acid sequence of this region shares approximately 30% identity with the Rho-binding domains of rhophilin and a serine/threonine kinase, PKN, two other Rho target proteins that we recently identified (Watanabe, G., Saito, Y., Madaule, P., Ishizaki, T., Fujisawa, K., Morii, N., Mukai, H., Ono, Y., Kakizuka, A., and Narumiya, S. (1996) Science 271, 645-648). Thus, not only is rhotekin a novel partner for Rho, but it also belongs to a wide family of proteins that bear a consensus Rho-binding sequence at the N terminus. To our knowledge, this is the first conserved sequence for Rho effectors, and we have termed this region Rho effector motif class 1.

Gene expression of the human prostaglandin E receptor EP4 subtype: differential regulation in monocytoid and lymphoid lineage cells by phorbol ester

We isolated a cDNA clone encoding the human prostaglandin (PG) E receptor EP4 subtype and examined the gene expression in human blood cells. Northern blot analysis revealed that the EP4 gene is expressed at a high level in peripheral blood mononuclear cells, and at lower levels in cultured human blood cell lines, THP-1 and U937 (monocytoid cell lines), MOLT-4 and Jurkat (T-cell lines), and Raji (B-cell line). To examine regulation of the EP4 gene expression in the immune system, we studied the effects of phorbol 12-myristate 13-acetate (PMA) on these cell lines. Gene expression was upregulated in THP-1, U937, and Raji cells by PMA, and was downregulated in MOLT-4 and Jurkat cells. In THP-1 cells the effects of PMA were further analyzed, and the upregulation of the EP4 gene was shown to be followed by an increase in PGE2 binding sites and in PGE2-induced cAMP accumulation. In the striking contrast, other PGE receptor subtypes (EP1, EP2 and EP3) and other prostanoid receptors (IP and DP) were shown not to be upregulated by PMA. Therefore, this is the first demonstration of a highly specific upregulation of the EP4 subtype in THP-1 cells treated with PMA, suggesting the importance of the EP4 subtype in the immune system. In the present study we also clarified that EP4 gene expression is regulated differently among human monocytoid and lymphoid lineage cells, thus leading to the better understanding of the regulatory mechanisms for the human EP4 gene expression in the immune system.

Expanded polyglutamine in the Machado-Joseph disease protein induces cell death in vitro and in vivo

Recently, we identified a novel gene, MJD1, which contains an expanded CAG triplet repeat in Machado-Joseph disease. Here we report the induction of apoptosis in cultured cells expressing a portion of the MJD1 gene that includes the expanded CAG repeats. Cell death occurs only when the CAG repeat is translated into polyglutamine residues, which apparently precipitate in large covalently modified forms. We also created ataxic transgenic mice by expressing the expanded polyglutamine stretch in Purkinje cells. Our results demonstrate the potential involvement of the expanded polyglutamine as the common aetiological agent for inherited neurodegenerative diseases with CAG expansions.

Characterization of the interaction between RhoA and the amino-terminal region of PKN

The yeast two-hybrid system and in vitro binding assay were carried out to characterize the interaction between PKN and a small GTP-binding protein, RhoA. It was revealed that the region corresponding to the amino acid residues 33-111 in the amino-terminal region of PKN was sufficient to confer the ability to associate with RhoA. Each synthetic peptide fragment corresponding to the amino acid residues 74-93 and 94-113 of PKN inhibited the interaction between PKN and RhoA in the in vitro binding assay, suggesting that this region is important in the association with RhoA. The endogenous and the GAP-stimulated GTPase activity of RhoA was inhibited by the interaction with PKN, suggesting the presence of a regulatory mechanism that sustains the GTP-bound active form of RhoA.

Protection from anti-TCR/CD3-induced apoptosis in immature thymocytes by a signal through thymic shared antigen-1/stem cell antigen-2

During T cell development in the thymus, the expression of thymic shared antigen-1 (TSA-1)/stem cell antigen-2 (Sca-2), a glycosylphosphatidylinositol (GPI)-anchored differentiation antigen, is developmentally regulated. The expression level of TSA-1 is the highest in most immature CD4- CD8- thymocytes, high in CD4+ CD8+ thymocytes, but barely detectable in mature CD4+ CD8- or CD4- CD8- thymocytes and peripheral T cells. We have previously shown that surface TSA-1 expression in peripheral T cells is induced upon activation and that anti-TSA-1 mAb inhibits the T cell receptor (TCR) signaling pathway in activated T cells. In the present study, we have analyzed a role of TSA-1 in thymic selection events, especially in TCR-mediated apoptosis. In in vitro experiments, anti-TSA-1 blocked anti-CD3-induced cell death of T cell hybridomas. When anti-TSA-1 was injected into newborn mice in vivo together with anti-CD3 epsilon or anti-TCR-beta, TCR/CD3-mediated apoptosis of thymocytes was almost completely blocked. The blockade of apoptosis was defined by the inhibition of, first, the decrease in total number of thymocytes; second, the decrease in percentages of CD4+ CD8+ thymocytes; and third, the induction of DNA fragmentation. However, anti-TSA-1 did not block either steroid- or radiation-induced apoptosis, indicating that a signal via TSA-1 does not inhibit a common pathway of thymocyte apoptosis. Since TCR-mediated apoptosis is pivotal in thymic ontogeny, these results suggest that TSA-1/Sca-2 is an important cell surface molecule regulating the fate of a developing T cell.

Prostaglandin E receptor subtypes in mouse osteoblastic cell line

PGE2 is one of the key molecules in the osteoblast. It is the major prostanoid in the bone, and its production is under the control of both systemic and local factors. PGE2 has been reported to have multiple actions in the osteoblast, such as growth promotion and cell differentiation. To better understand the action of PGE2 in the osteoblast, we determined the PGE receptor subtypes in MC3T3-E1, an osteoblastic cell line derived from the normal mouse calvaria. Northern blot analysis revealed that EP1 and EP4 subtypes are expressed in MC3T3-E1. In contrast, EP3 subtype was not detected by either Northern blot analysis or RT-PCR. The contribution of each subtype was evaluated by studying the effects of subtype-specific analogs on osteoblastic function at confluency and 5 days after confluency. An EP1 agonist, 17-phenyl-omega-trinor PGE2, increased DNA synthesis and decreased alkaline phosphatase activity. 11-Deoxy-PGE1, and EP2 and EP4 agonist, decreased DNA synthesis and increased alkaline phosphatase activity at both stages. Butaprost, an EP2-selective agonist, showed effects similar to those of 11-deoxy-PGE1 only at confluency. Another and more differentiated osteoblastic marker, osteocalcin production, was detectable and was stimulated by 11-deoxy-PGE1 only 5 days after confluency. The exposure of these cells to EP1 agonist changed the cell shape to a more fibroblastic appearance. These results indicate that EP1, EP4, and probably EP2 are present in MC3T3-E1 cells; EP1 promotes cell growth, and EP2 and EP4 mediate differentiation of the osteoblast. Furthermore, the decreased response to EP2-specific agonist 5 days after confluency suggests that the expression of PGE receptor subtype is dependent on the stage of osteoblastic differentiation. This is the first report to determine PGE receptor subtypes in the bone.

Roles of insulin, guanosine 5'-[gamma-thio]triphosphate and phorbol 12-myristate 13-acetate in signalling pathways of GLUT4 translocation

Insulin, guanosine 5'-[gamma-thio]triphosphate (GTP[S] and phorbol 12-myristate 13-acetate (PMA) trigger the translocation of Gl UT4 (type 4 glucose transporter; insulin-sensitive glucose transporter) from an intracellular pool to the cell surface. We have developed a highly sensitive and quantitative method to detect GLUT4 immunologically on the surface of intact 3T3-L1 adipocytes and Chinese hamster ovary (CHO) cells, using c-myc epitope-tagged GLUT4 (GLUT4myc). We examined the roles of insulin, GTP[S] and PMA in the signalling pathways of GLUT4 translocation in the CHO cell system. Among small molecular GTP-binding proteins, ras, rab3D, rad and rho seem to be candidates as signal transmitters of insulin-stimulated GLUT4 translocation. Overexpression of wild-type H-ras and the dominant negative mutant H-rass17N in our cell system respectively enhanced and blocked insulin-stimulated activation of mitogen-activated protein kinase, but did not affect insulin-stimulated GLUT4 translocation. Overexpression of rab3D or rad in the cells did not affect GLUT4 translocation triggered by insulin, GTP[S] or PMA. Treatment with Botulinum C3 exoenzyme, a specific inhibitor of rho, had no effect on GLUT4 translocation induced by insulin, GTP[S] or PMA. Therefore these small molecular GTP-binding proteins are not likely to be involved in GLUT4 translocation. In addition, insulin, GTP[S] and PMA apparently stimulate GLUT4 translocation through independent pathways.

The small GTP-binding protein Rho binds to and activates a 160 kDa Ser/Thr protein kinase homologous to myotonic dystrophy kinase

The small GTP-binding protein Rho functions as a molecular switch in the formation of focal adhesions and stress fibers, cytokinesis and transcriptional activation. The biochemical mechanism underlying these actions remains unknown. Using a ligand overlay assay, we purified a 160 kDa platelet protein that bound specifically to GTP-bound Rho. This protein, p160, underwent autophosphorylation at its serine and threonine residues and showed the kinase activity to exogenous substrates. Both activities were enhanced by the addition of GTP-bound Rho. A cDNA encoding p160 coded for a 1354 amino acid protein. This protein has a Ser/Thr kinase domain in its N-terminus, followed by a coiled-coil structure approximately 600 amino acids long, and a cysteine-rich zinc finger-like motif and a pleckstrin homology region in the C-terminus. The N-terminus region including a kinase domain and a part of coiled-coil structure showed strong homology to myotonic dystrophy kinase over 500 residues. When co-expressed with RhoA in COS cells, p160 was co-precipitated with the expressed Rho and its kinase activity was activated, indicating that p160 can associate physically and functionally with Rho both in vitro and in vivo.

Membrane depolarization by activation of prostaglandin E receptor EP3 subtype of putative serotonergic neurons in the dorsal raphe nucleus of the rat

A whole-cell current-clamp study using a thin slice preparation of the rat brain was carried out to elucidate the function of prostaglandin E (PGE) receptor EP3 subtype in the dorsal raphe nucleus (DR), where mRNA of this subtype is highly expressed. Bath application of PGE2 or M&B 28767, an EP3 agonist, depolarized the membrane of the large DR neurons in a concentration-dependent manner between 10(-9) and 10(-6) M. These neurons showed hyperpolarization of membrane potential to 10 or 50 microM serotonin. Neither an EP2 receptor agonist, butaprost, an EP2/EP4 receptor agonist, 11-deoxy-PGE1, nor an EP1 receptor agonist, 17-phenyl-PGE2, had any effect on large DR neurons between 10(-9) and 10(-6) M. The M&B 28767-induced depolarization was observed in a Ca(2+)-free, high Mg2+ (5 mM) solution containing 0.3 microM tetrodotoxin, and occurred equally well when intracellular Cl- was replaced by gluconate. These results suggest that direct agonist-activation of EP3 receptor depolarizes the membrane by a cationic conductance, leading to excitation of DR neurons, and indicate a physiological implication that EP3 receptor may modulate the serotonergic inhibition of neuronal activities.

Mapping of the genes encoding mouse prostaglandin D, E, and F and prostacyclin receptors

Prostaglandins and prostacyclin are metabolites of arachidonic acid and exert a variety of actions to maintain local homeostasis in the body. Their actions are mediated by cell surface receptors specific to the respective ligands. Using a panel of interspecific back-cross mice, we have mapped the prostaglandin D receptor gene (Ptgdr), prostaglandin E receptor subtype EP(1) gene (Ptgerepl), prostaglandin F receptor gene (Ptgfr), and prostacyclin receptor gene (Ptgir). Ptgdr mapped to proximal Chr 14, Ptgfr mapped to distal Chr 3, Ptgerepl mapped to middle Chr 8, and Ptgir mapped to proximal Chr 7.

Two thromboxane A2 receptor isoforms in human platelets. Opposite coupling to adenylyl cyclase with different sensitivity to Arg60 to Leu mutation

Thromboxane A2 (TXA2) receptor is a key molecule in hemostasis as its abnormality leads to bleeding disorders. Two isoforms of the human TXA2 receptor have been cloned; one from placenta and the other from endothelium, here referred to as TXR alpha and TXR beta, respectively. These isoforms differ only in their carboxyl-terminal tails. We report that both isoforms are present in human platelets. The two isoforms expressed in cultured cells show similar ligand binding characteristics and phospholipase C (PLC) activation but oppositely regulate adenylyl cyclase activity; TXR alpha activates adenylyl cyclase, while TXR beta inhibits it. The Arg60 to Leu mutant of TXR alpha, which has been shown to impair PLC activation (Hirata, T., A. Kakizuka, F. Ushikubi, I. Fuse, M. Okuma, and S. Narumiya. 1994. J. Clin. Invest. 94: 1662-1667), also impairs adenylyl cyclase stimulation, whereas that of TXR beta retains its activity to inhibit adenylyl cyclase. These findings suggest that the pathway linked to adenylyl cyclase inhibition might be involved in some of the TXA2-induced platelet responses such as shape change and phospholipase A2 activation which remain unaffected in the patients with this mutation.

Protein kinase N (PKN) and PKN-related protein rhophilin as targets of small GTPase Rho

The Rho guanosine 5'-triphosphatase (GTPase) cycles between the active guanosine triphosphate (GTP)-bound form and the inactive guanosine diphosphate-bound form and regulates cell adhesion and cytokinesis, but how it exerts these actions is unknown. The yeast two-hybrid system was used to clone a complementary DNA for a protein (designated Rhophilin) that specifically bound to GTP-Rho. The Rho-binding domain of this protein has 40 percent identity with a putative regulatory domain of a protein kinase, PKN. PKN itself bound to GTP-Rho and was activated by this binding both in vitro and in vivo. This study indicates that a serine-threonine protein kinase is a Rho effector and presents an amino acid sequence motif for binding to GTP-Rho that may be shared by a family of Rho target proteins.

Molecular characterization of a dominantly inherited bleeding disorder with impaired platelet responses to thromboxane A2

Thromboxane A2 (TXA2) is a major arachidonic acid metabolite of platelets and induces platelet functions by binding to specific receptors on the membrane. We have found patients with hemostatic defects due to impaired platelet responses to TXA2, and molecular characterization of the patients has been carried out. Platelets from these two unrelated patients showed impaired aggregation responses to TXA2 and its analogues despite the normal response to thrombin. Although the patients' platelets exhibited normal binding activities to TXA2 analogues, they showed decreased GTPase activity and second messenger formation when stimulated by STA2, a stable TXA2 agonist. To understand the molecular basis of this abnormality, we determined the cDNA sequence of the TXA2 receptor by reverse transcription-polymerase chain reaction (RT-PCR) from the patient's platelet RNA, and identified a single amino acid substitution (Arg60 for Leu) in the first cytoplasmic loop of the receptor. This mutation was found in both isoforms of the platelet TXA2 receptor which we have recently found: TXR alpha with the same structure as the placental TXA2 receptor and TXR beta with the same structure as the endothelial TXA2 receptor, and was detected exclusively in affected members of two unrelated families with the disorder. The mutant TXR alpha and TXR beta expressed in COS-m6 cells showed decreased agonist-induced phospholipase C activation despite their normal ligand binding affinities. These results suggest that the Arg60 for Leu mutation is responsible for the disorder and imply a critical role for the first cytoplasmic loop in the interaction of the TXA2 receptor with the G protein.

A novel partner for the GTP-bound forms of rho and rac

Using the yeast two hybrid system and overlay assays we identified a putative rholrac effector, citron, which interacts with the GTP-bound forms of rho and rac1, but not with cdc42. Extensive homologies to known proteins were not observed. This 183 kDa protein contains a C6H2 zinc finger, a PH domain, and a long coiled-coil forming region including 4 leucine zippers and the rholrac binding site. We recently identified three others putative rho effectors characterized by a common rho binding motif. Citron does not share this motif and displays a distinctive protein organization, thus defining a separate class of rho partners.

In situ hybridization studies of prostacyclin receptor mRNA expression in various mouse organs

1. Expression of prostacyclin receptor (IP receptor) mRNA was examined in various mouse organs, and the cells expressing IP receptor mRNA were identified by in situ hybridization studies. Co-localization of mRNA for the IP receptor with that for preprotachykinin A (PPTA), a precursor protein for substance P, with mRNA for the prostaglandin E receptor subtypes (EP1, EP3 and EP4), and with renin mRNA, was examined by double in situ hybridization studies in the dorsal root ganglion and kidney, respectively. 2. IP receptor mRNA was expressed in the thymus and spleen. Expression in the thymus was found exclusively in the medulla, where mature thymocytes expressed transcripts for the IP receptor. Expression in the spleen was found as scattered signals over the white pulp and as punctate signals in the red pulp. The former was found in splenic lymphocytes and the latter in megakaryocytes. 3. IP receptor mRNA was also expressed in the vascular tissues of various organs such as the aorta, coronary arteries, pulmonary arteries and the cerebral arteries, where its expression was confined to smooth muscle cells. No expression was found in veins. In the kidney, IP receptor mRNA was detected in the interlobular arteries and glomerular arterioles but not in the juxtaglomerular (JG) cells which were labelled with the renin mRNA probe. 4. IP receptor mRNA was expressed in about 40% of the neurones in the dorsal root ganglion. Both small- and large-sized neurones were labelled but no labelling was found in the glia. Expression of PPTA mRNA was found in about 30% of total neurones. About 70% of these neurones expressed IP receptor mRNA, and about half of the IP receptor-positive neurones expressed PPTA mRNA. In addition to IP mRNA, mRNAs for EP1, EP3 and EP4 receptors were expressed in about 30%, 50% and 20%, respectively, of the dorsal root ganglion neurones. About 25%, 41% and 24% of the IP receptor-positive neurons co-expressed the EP1, EP3 and EP4 receptor, respectively. 5. These results not only verified IP receptor expression in various cells and tissues known to be sensitive to prostacyclin, but also revealed its expression in other systems, which urges the study of the actions of prostacyclin in these tissues. They also indicated that the actions of prostacyclin on blood vessels and platelets are mediated by the same type of receptor. Absence of IP receptor mRNA in the JG cells suggests that the action of prostacyclin on renin release may be indirect.

Gene expression of prostacyclin receptor in the hypertrophied heart of spontaneously hypertensive rats

1. Prostacyclin elicits potent vasodilation and inhibition of platelet aggregation through binding to its membrane receptor. The impairment of prostacyclin receptor activity is implicated in various human cardiovascular diseases. We recently succeeded in molecular cloning of cDNA for the mouse, rat, and human prostacyclin receptors. 2. In the present study, we examined the mRNA expression of the prostacyclin receptor in various rat tissues, and further investigated its gene expression in the hypertrophied cardiac ventricles of stroke-prone spontaneously hypertensive rats (SHRSP). 3. In rat tissues, a single RNA band of approximately 3.7 kb was detected by northern blotting analysis using rat prostacyclin receptor cDNA as a probe. In adult Wistar rats, abundant mRNA expression was observed in the aorta, lung and spleen. Substantial amounts of transcript were expressed in the heart, pancreas, thymus and stomach. In contrast, no mRNA expression was detected in the brain. 4. We further examined the mRNA expression of the prostacyclin receptor in the ventricles of 21 week old SHRSP. The ventricles of SHRSP showed remarkable hypertrophy, compared with those of age-matched Wistar-Kyoto (WKY) rats. The expression of prostacyclin receptor mRNA in the hypertrophied ventricles of SHRSP was almost equivalent to that in the ventricles of WKY. 5. The present study revealed the gene expression of the prostacyclin receptor in various rat tissues, and further demonstrated the receptor mRNA expression in hypertensive cardiac hypertrophy. The present study will give a clue to investigate the clinical implication of prostacyclin and its receptor.

Molecular cloning and expression of multiple isoforms of human prostaglandin E receptor EP3 subtype generated by alternative messenger RNA splicing: multiple second messenger systems and tissue-specific distributions

Five distinct cDNA clones encoding four different isoforms of human prostaglandin (PG) E receptor EP3 subtype were isolated from a human kidney cDNA library. Two cDNA clones differed only in their 3'-untranslated regions. The four isoforms, tentatively named EP3-I, EP3-II, EP3-III, and EP3-IV, which were generated by alternative mRNA splicing, had identical amino acid sequences except for their different carboxyl-terminal tails. Transfection experiments revealed that all the four isoforms show high binding affinities to PGE2, PGE1, and M&B28767, an EP3-specific agonist, whereas their downstream signaling pathways are divergent. M&B28767 increased cAMP concentrations in cells expressing EP3-II and EP3-IV, whereas it inhibited forskolin-induced cAMP accumulations in cells expressing all EP3 isoforms. M&B28767 also stimulated phosphoinositide turnover in cells expressing EP3-I and EP3-II. Northern blot analysis revealed that the EP3 gene is expressed in a wide variety of human tissues. The human EP3 mRNA was present most abundantly in the kidney, pancreas, and uterus. A substantial expression was also detected in the heart, liver, skeletal muscle, small intestine, colon, prostate, ovary, and testis. Furthermore, reverse transcription-polymerase chain reaction analysis demonstrated tissue-specific expressions of the five different EP3 mRNA species. The present study suggests the presence of the multiple systems of PGE2/EP3 isoforms and leads to the better understanding of its physiological and pathophysiological implications in humans.

Synergistic activation of rat brain phospholipase D by ADP-ribosylation factor and rhoA p21, and its inhibition by Clostridium botulinum C3 exoenzyme

An activator of rat brain phospholipase D (PLD) that is distinct from the already identified PLD activator, ADP-ribosylation factor (ARF), was partially purified from bovine brain cytosol by a series of chromatographic steps. The partially purified preparation contained a 22-kDa substrate for Clostridium botulinum C3 exoenzyme ADP-ribosyltransferase, which strongly reacted with anti-rhoA p21 antibody, but not with anti-rac1 p21 or anti-cdc42Hs p21 antibody. Treatment of the partially purified PLD-activating factor with both C3 exoenzyme and NAD significantly inhibited the PLD-stimulating activity. These results suggest that rhoA p21 is, at least in part, responsible for the PLD-stimulating activity in the preparation. Recombinant isoprenylated rhoA p21 expressed in and purified from Sf9 cells activated rat brain PLD in a concentration- and GTP gamma S (guanosine 5'-O-(3-thiotriphosphate))-dependent manner. In contrast, recombinant non-isoprenylated rhoA p21 (fused to glutathione S-transferase) expressed in Escherichia coli failed to activate the PLD. This difference cannot be explained by a lower affinity of non-isoprenylated rhoA p21 for GTP gamma S, as the rates of [35S]GTP gamma S binding were very similar for both recombinant preparations and the GTP gamma S-bound form of non-isoprenylated rhoA p21 did not induce PLD activation. Interestingly, recombinant isoprenylated rhoA p21 and ARF synergistically activated rat brain PLD; a similar pattern was seen with the partially purified PLD-activating factor. The synergistic activation was inhibited by C3 exoenzyme-catalyzed ADP-ribosylation of recombinant isoprenylated rhoA p21 in a NAD-dependent manner. Inhibition correlated with the extent of ADP-ribosylation. These findings suggest that rhoA p21 regulates rat brain PLD in concert with ARF, and that isoprenylation of rhoA p21 is essential for PLD regulation in vitro.