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Microsomal prostaglandin E synthase-1 and blood pressure regulation

2007; Elsevier BV; Volume: 72; Issue: 3 Linguagem: Inglês

10.1038/sj.ki.5002326

1523-1755

Tianxin Yang,

Eicosanoids and Hypertension Pharmacology

Prostaglandin E (PGE)2 is a major arachidonic acid metabolite in a wide variety of tissues and is implicated in the control of inflammatory as well as physiological responses. At least three major forms of PGE synthase (PGES) have recently been cloned and characterized: membrane-associated PGES (mPGES)-1, mPGES-2, and cytosolic PGES (cPGES). Among them, mPGES-1 is highly inducible by cytokine and is critically involved in pain and inflammatory responses. Emerging evidence suggests that mPGES-1 may also participate in blood pressure (BP) regulation through an impact on renal and vascular functions. Within the kidney, mPGES-1 predominates in the distal nephron where its expression is highly inducible by salt loading. Mice lacking mPGES-1 exhibit blunted natriuretic response paralleled with remarkably suppressed nitric oxide production, leading to salt-sensitive hypertension. These mice also exhibit an exaggerated hypertensive response to angiotensin II infusion. Together, these results suggest that mPGES-1 may be an important physiological regulator of BP. Prostaglandin E (PGE)2 is a major arachidonic acid metabolite in a wide variety of tissues and is implicated in the control of inflammatory as well as physiological responses. At least three major forms of PGE synthase (PGES) have recently been cloned and characterized: membrane-associated PGES (mPGES)-1, mPGES-2, and cytosolic PGES (cPGES). Among them, mPGES-1 is highly inducible by cytokine and is critically involved in pain and inflammatory responses. Emerging evidence suggests that mPGES-1 may also participate in blood pressure (BP) regulation through an impact on renal and vascular functions. Within the kidney, mPGES-1 predominates in the distal nephron where its expression is highly inducible by salt loading. Mice lacking mPGES-1 exhibit blunted natriuretic response paralleled with remarkably suppressed nitric oxide production, leading to salt-sensitive hypertension. These mice also exhibit an exaggerated hypertensive response to angiotensin II infusion. Together, these results suggest that mPGES-1 may be an important physiological regulator of BP. The E series of prostaglandins (PGEs) are the major products of arachidonic acid metabolism and are implicated in a wide variety of physiological and pathological processes. PGE2 is a major mediator of inflammation and is also a key player in the control of various physiological functions.1.Breyer M.D. Breyer R.M. G protein-coupled prostanoid receptors and the kidney.Annu Rev Physiol. 2001; 63: 579-605Crossref PubMed Scopus (185) Google Scholar The biosynthesis of PGE2 requires three sequential steps of the cyclooxygenase (COX) pathway: the release of arachidonic acid from membrane glycerophospholipids by phospholipase A2, conversion of arachidonic acid into the unstable intermediate PGH2 by COX-1 or -2, and isomerization of PGH2 to PGE2 by PE synthase (PGES).2.Smith W.L. Garavito R.M. DeWitt D.L. Prostaglandin endoperoxide H synthases (cyclooxygenases)-1 and -2.J Biol Chem. 1996; 271: 33157-33160Crossref PubMed Scopus (1799) Google Scholar,3.Murakami M. Kudo I. Prostaglandin E synthase: a novel drug target for inflammation and cancer.Curr Pharm Des. 2006; 12: 943-954Crossref PubMed Scopus (123) Google Scholar The activity of PGES has been detected in both cytosolic and microsomal fractions of various cells. To date, at least three major forms of PGES have been cloned and characterized.3.Murakami M. Kudo I. Prostaglandin E synthase: a novel drug target for inflammation and cancer.Curr Pharm Des. 2006; 12: 943-954Crossref PubMed Scopus (123) Google Scholar They are designated as membrane-associated PGES (mPGES)-1, mPGES-2, and cytosolic PGES (cPGES). These isoforms of PGES represent products of three distinct genes with marked differences in enzymatic properties, modes of expression, and cellular and subcellular localizations, suggesting distinct functions.3.Murakami M. Kudo I. Prostaglandin E synthase: a novel drug target for inflammation and cancer.Curr Pharm Des. 2006; 12: 943-954Crossref PubMed Scopus (123) Google Scholar The first PGES (now referred to as mPGES-1) was cloned from human cells as microsomal glutathione S-transferase 1-like 1 whose PGE synthesis activity was dependent on glutathione.4.Jakobsson P.J. Thoren S. Morgenstern R. Samuelsson B. Identification of human prostaglandin E synthase: a microsomal, glutathione-dependent, inducible enzyme, constituting a potential novel drug target.Proc Natl Acad Sci USA. 1999; 96: 7220-7225Crossref PubMed Scopus (867) Google Scholar This was followed by the isolation of the mouse, rat, and zebrafish homologs.5.Murakami M. Naraba H. Tanioka T. et al.Regulation of prostaglandin E2 biosynthesis by inducible membrane-associated prostaglandin E2 synthase that acts in concert with cyclooxygenase-2.J Biol Chem. 2000; 275: 32783-32792Crossref PubMed Scopus (832) Google Scholar, 6.Mancini J.A. Blood K. Guay J. et al.Cloning, expression, and up-regulation of inducible rat prostaglandin e synthase during lipopolysaccharide-induced pyresis and adjuvant-induced arthritis.J Biol Chem. 2001; 276: 4469-4475Crossref PubMed Scopus (215) Google Scholar, 7.Yamagata K. Matsumura K. Inoue W. et al.Coexpression of microsomal-type prostaglandin E synthase with cyclooxygenase-2 in brain endothelial cells of rats during endotoxin-induced fever.J Neurosci. 2001; 21: 2669-2677Crossref PubMed Google Scholar, 8.Pini B. Grosser T. Lawson J.A. et al.Prostaglandin E synthases in zebrafish.Arterioscler Thromb Vasc Biol. 2005; 25: 315-320Crossref PubMed Scopus (39) Google Scholar mPGES-1 is a member of the membrane-associated proteins in eicosanoids and glutathione metabolism superfamily, which also contains 5-lipoxygenase-activating protein and leukotrane C4 synthase.6.Mancini J.A. Blood K. Guay J. et al.Cloning, expression, and up-regulation of inducible rat prostaglandin e synthase during lipopolysaccharide-induced pyresis and adjuvant-induced arthritis.J Biol Chem. 2001; 276: 4469-4475Crossref PubMed Scopus (215) Google Scholar The enzyme is a 16-kDa membrane-bound protein encoded by a 2.0-kb transcript. Like COX-2, mPGES-1 expression in inflammatory cells is highly inducible by proinflammatory stimuli. Several recent studies employing mPGES-1-deficient mice demonstrate a major role of mPGES-1 in pain and inflammatory responses.9.Trebino C.E. Stock J.L. Gibbons C.P. et al.Impaired inflammatory and pain responses in mice lacking an inducible prostaglandin E synthase.Proc Natl Acad Sci USA. 2003; 100: 9044-9049Crossref PubMed Scopus (484) Google Scholar,10.Kamei D. Yamakawa K. Takegoshi Y. et al.Reduced pain hypersensitivity and inflammation in mice lacking microsomal prostaglandin e synthase-1.J Biol Chem. 2004; 279: 33684-33695Crossref PubMed Scopus (262) Google Scholar By contrast, the physiological function of this enzyme has been studied in much less detail. Recent evidence from our group suggests that mPGES-1 may play an important role in facilitating renal salt excretion as well as in modulating the response to angiotensin (Ang) II.11.Jia Z. Zhang A. Zhang H. et al.Deletion of microsomal prostaglandin E synthase-1 increases sensitivity to salt loading and angiotensin II infusion.Circ Res. 2006; 99: 1243-1251Crossref PubMed Scopus (85) Google Scholar mPGES-2 is a second mPGES that was originally purified from bovine heart and subsequently cloned.12.Watanabe K. Kurihara K. Suzuki T. Purification and characterization of membrane-bound prostaglandin E synthase from bovine heart.Biochim Biophys Acta. 1999; 1439: 406-414Crossref PubMed Scopus (96) Google Scholar,13.Tanikawa N. Ohmiya Y. Ohkubo H. et al.Identification and characterization of a novel type of membrane-associated prostaglandin E synthase.Biochem Biophys Res Commun. 2002; 291: 884-889Crossref PubMed Scopus (270) Google Scholar It is a 43-kDa protein containing a unique N-terminal hydrophobic domain, which determines the association with Golgi membrane. This enzyme is constitutively expressed in various cells and is considered to be functionally coupled with COX-1 and -2. cPGES was originally identified from rat brain as a cytosolic glutathione-dependent PGES. It was identical to p23, which is a steroid hormone receptor/hsp90-associated protein.14.Tanioka T. Nakatani Y. Semmyo N. et al.Molecular identification of cytosolic prostaglandin E2 synthase that is functionally coupled with cyclooxygenase-1 in immediate prostaglandin E2 biosynthesis.J Biol Chem. 2000; 275: 32775-32782Crossref PubMed Scopus (613) Google Scholar The enzyme was constitutively expressed in the cytosol in a wide variety of cells and tissues and was unaltered by proinflammatory stimuli except that it was induced in rat brain following lipopolysaccharide treatment. The fact that cPGES and COX-1 are colocalized to the cytosol and also share the same constitutive properties favors a functional coupling between the two enzymes. mPGES-1 is abundantly expressed in the kidney with a substantially higher level in the renal medulla than in renal cortex,15.Schneider A. Zhang Y. Zhang M. et al.Membrane-associated PGE synthase-1 (mPGES-1) is coexpressed with both COX-1 and COX-2 in the kidney.Kidney Int. 2004; 65: 1205-1213Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar a pattern similar to COX-1 and -2.16.Jensen B.L. Kurtz A. Differential regulation of renal cyclooxygenase mRNA by dietary salt intake.Kidney Int. 1997; 52: 1242-1249Abstract Full Text PDF PubMed Scopus (116) Google Scholar A significant number of published studies using immunohistochemistry, in situ hybridization, and reverse transcriptase-polymerase chain reaction on microdissected nephron segments all consistently documented the predominant expression of mPGES-1 in the whole collecting duct (CD) system of mouse,17.Guan Y. Zhang Y. Schneider A. et al.Urogenital distribution of a mouse membrane-associated prostaglandin E(2) synthase.Am J Physiol Renal Physiol. 2001; 281: F1173-1177Crossref PubMed Scopus (48) Google Scholar rat,18.Vitzthum H. Abt I. Einhellig S. Kurtz A. Gene expression of prostanoid forming enzymes along the rat nephron.Kidney Int. 2002; 62: 1570-1581Abstract Full Text Full Text PDF PubMed Scopus (57) Google Scholar and rabbit.19.Fuson A.L. Komlosi P. Unlap T.M. et al.Immunolocalization of a microsomal prostaglandin E synthase in rabbit kidney.Am J Physiol Renal Physiol. 2003; 285: F558-564Crossref PubMed Scopus (46) Google Scholar Within the CD, the immunoreactive labeling was restricted to the principle but not intercalated cells.19.Fuson A.L. Komlosi P. Unlap T.M. et al.Immunolocalization of a microsomal prostaglandin E synthase in rabbit kidney.Am J Physiol Renal Physiol. 2003; 285: F558-564Crossref PubMed Scopus (46) Google Scholar This corresponds to the observations that the CD possesses the greatest rate of PGE2 synthesis of any segment along the nephron and that there are well-established inhibitory effects of PGE2 on Na+ and water transport in the CD.20.Bonvalet J.P. Pradelles P. Farman N. Segmental synthesis and actions of prostaglandins along the nephron.Am J Physiol. 1987; 253: F377-F387PubMed Google Scholar Double labeling studies using in situ hybridization and immunostaining demonstrated that mPGES-1 predominantly colocalizes with COX-1 in the CD, suggesting a functional coupling between the two enzymes in the distal nephron. In line with this notion, mPGES-1- and COX-1-deficient mice both develop hypertension when fed a high-salt diet.11.Jia Z. Zhang A. Zhang H. et al.Deletion of microsomal prostaglandin E synthase-1 increases sensitivity to salt loading and angiotensin II infusion.Circ Res. 2006; 99: 1243-1251Crossref PubMed Scopus (85) Google Scholar,21.Ye W. Zhang H. Hillas E. et al.Expression and function of COX isoforms in renal medulla: evidence for regulation of salt sensitivity and blood pressure.Am J Physiol Renal Physiol. 2006; 290: F542-549Crossref PubMed Scopus (57) Google Scholar A low level of mPGES-1 expression was inconsistently found in renal medullary interstitial cells11.Jia Z. Zhang A. Zhang H. et al.Deletion of microsomal prostaglandin E synthase-1 increases sensitivity to salt loading and angiotensin II infusion.Circ Res. 2006; 99: 1243-1251Crossref PubMed Scopus (85) Google Scholar, 15.Schneider A. Zhang Y. Zhang M. et al.Membrane-associated PGE synthase-1 (mPGES-1) is coexpressed with both COX-1 and COX-2 in the kidney.Kidney Int. 2004; 65: 1205-1213Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar, 19.Fuson A.L. Komlosi P. Unlap T.M. et al.Immunolocalization of a microsomal prostaglandin E synthase in rabbit kidney.Am J Physiol Renal Physiol. 2003; 285: F558-564Crossref PubMed Scopus (46) Google Scholar where COX-2 is abundantly expressed. This makes less likely that mPGES-1 couples with COX-2-mediating PGE2 synthesis in renal medullary interstitial cells. This statement is not necessarily inconsistent with the notion that COX-2-derived prostanoids may be important regulators of BP during high salt intake.21.Ye W. Zhang H. Hillas E. et al.Expression and function of COX isoforms in renal medulla: evidence for regulation of salt sensitivity and blood pressure.Am J Physiol Renal Physiol. 2006; 290: F542-549Crossref PubMed Scopus (57) Google Scholar It is possible that COX-2 may be linked with other PGES isoforms (e.g., cPGES or mPGES-2) than mPGES-1 to sustain PGE2 synthesis in renal medullary interstitial cells in response to high salt loading. Emerging evidence suggests that mPGES-1 expression in the CD is further upregulated in response to chronic salt loading.11.Jia Z. Zhang A. Zhang H. et al.Deletion of microsomal prostaglandin E synthase-1 increases sensitivity to salt loading and angiotensin II infusion.Circ Res. 2006; 99: 1243-1251Crossref PubMed Scopus (85) Google Scholar In this study, the total abundance of mPGES-1 protein, as determined by immunoblotting, significantly increased in both cortex and the inner medulla following a 7-day high-salt diet. The increased expression was restricted to the CD as determined by immunostaining. In primary CD cells, hypertonic NaCl exerted a direct stimulatory effect on mPGES-1 expression. The ion-substitution experiments demonstrated that the stimulation was mediated by extracellular Cl- and not Na+. In general, these results suggest a physiological role of mPGES-1 in the CD. The macula densa is a specialized cell type critically important for transducing the signal from the luminal electrolyte concentration to the afferent arterioles. There is mounting evidence that COX-2-derived PGE2 participates in the signaling transduction in the macula densa. Therefore, attempts have been made to link this phenomenon with mPGES-1. Subsequently, mPGES-1 expression was detected in the macula densa in the rabbit kidney by both immunohistochemistry and in situ hybridization despite some variations in the expression levels in different reports.15.Schneider A. Zhang Y. Zhang M. et al.Membrane-associated PGE synthase-1 (mPGES-1) is coexpressed with both COX-1 and COX-2 in the kidney.Kidney Int. 2004; 65: 1205-1213Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar,19.Fuson A.L. Komlosi P. Unlap T.M. et al.Immunolocalization of a microsomal prostaglandin E synthase in rabbit kidney.Am J Physiol Renal Physiol. 2003; 285: F558-564Crossref PubMed Scopus (46) Google Scholar Furthermore, mPGES-1 expression in the macula densa was responsive to the known stimuli for COX-2, including salt depletion and Ang-converting enzyme inhibition.19.Fuson A.L. Komlosi P. Unlap T.M. et al.Immunolocalization of a microsomal prostaglandin E synthase in rabbit kidney.Am J Physiol Renal Physiol. 2003; 285: F558-564Crossref PubMed Scopus (46) Google Scholar In general agreement with this finding, mPGES expression was induced in parallel with COX-2 in the macula densa in children with Bartter's syndrome.22.Komhoff M. Reinalter S.C. Grone H.J. Seyberth H.W. Induction of microsomal prostaglandin E2 synthase in the macula densa in children with hypokalemic salt-losing tubulopathies.Pediatr Res. 2004; 55: 261-266Crossref PubMed Scopus (21) Google Scholar Together, these studies suggest a possibility that mPGES-1 may functionally couple with COX-2 in the macula densa control of renin secretion. However, this notion is inconsistent with several other studies, which failed to detect mPGES-1 expression in the macula densa. In this regard, neither immunostaining nor in situ hybridization studies were able to detect the signal in the macula densa in the mouse kidney.11.Jia Z. Zhang A. Zhang H. et al.Deletion of microsomal prostaglandin E synthase-1 increases sensitivity to salt loading and angiotensin II infusion.Circ Res. 2006; 99: 1243-1251Crossref PubMed Scopus (85) Google Scholar,17.Guan Y. Zhang Y. Schneider A. et al.Urogenital distribution of a mouse membrane-associated prostaglandin E(2) synthase.Am J Physiol Renal Physiol. 2001; 281: F1173-1177Crossref PubMed Scopus (48) Google Scholar mPGES-1 mRNA was also undetectable in microdissected glomerulus with attachment of the macula densa from Sprague–Dawley rats although this preparation contained abundant signals of COX-2 and Na+-K+-2Cl- cotransporter.18.Vitzthum H. Abt I. Einhellig S. Kurtz A. Gene expression of prostanoid forming enzymes along the rat nephron.Kidney Int. 2002; 62: 1570-1581Abstract Full Text Full Text PDF PubMed Scopus (57) Google Scholar It is unclear whether differences in species or sensitivities of the detection systems could account for the disparities among different reports. This calls for a need for further clarification of the expression and function of mPGES-1 in the macula densa cells. In contrast to the detailed analysis of mPGES-1 expression in the kidney, the expression of the other two isoforms of PGES is relatively less studied. By in situ hybridization, mPGES-2 mRNA was detected in substantially higher levels in the renal cortex than in the renal medulla in the mouse kidney and by immunohistochemistry mPGES-2 protein was ubiquitously expressed along the nephron except that it was detected at much lower levels in the glomeruli.23.Yang G. Chen L. Zhang Y. et al.Expression of mouse membrane-associated prostaglandin E(2) synthase-2 (mPGES-2) along the urogenital tract.Biochim Biophys Acta. 2006; 1761: 1459-1468Crossref PubMed Scopus (26) Google Scholar The renal expression of cPGES was studied only by in situ hybridization and the signal was diffusely expressed in all nephron segments.24.Zhang Y. Schneider A. Rao R. et al.Genomic structure and genitourinary expression of mouse cytosolic prostaglandin E(2) synthase gene.Biochim Biophys Acta. 2003; 1634: 15-23Crossref PubMed Scopus (18) Google Scholar The diffused expression patterns of mPGES-2 and cPGES are apparently different from the localized distribution of COX-1 and -2 in the kidney. Therefore, the functional roles of mPGES-2 and cPGES in the kidney still remain elusive. PGE2 is an important autocrine/paracrine factor that contributes to sodium balance and BP control through mechanisms that primarily involve the regulation of renal excretory function and vasculature tone.1.Breyer M.D. Breyer R.M. G protein-coupled prostanoid receptors and the kidney.Annu Rev Physiol. 2001; 63: 579-605Crossref PubMed Scopus (185) Google Scholar Recently, mPGES-1 has been identified in the renal distal nephron and the vasculature. These findings raise a possibility that mPGES-1 may participate in BP regulation through an influence on renal and vascular functions. To examine this possibility, mPGES-1 -/- mice were placed on a 7-day high-salt diet plus saline as drinking fluid and mean arterial pressure was monitored by telemetry. Following chronic salt loading, the -/- mice developed more severe hypertension as compared to the +/+ controls (Figure 1a). This result is consistent with the observation that the -/- mice exhibited an impaired ability to excrete a sodium load in both acute and chronic settings.11.Jia Z. Zhang A. Zhang H. et al.Deletion of microsomal prostaglandin E synthase-1 increases sensitivity to salt loading and angiotensin II infusion.Circ Res. 2006; 99: 1243-1251Crossref PubMed Scopus (85) Google Scholar It is highly possible that the abnormal salt handling accounts for the hypertensive phenotype. Given the nature of systemic deletion of the mPGES-1 gene, the site of action of mPGES-1 still cannot be determined. However, several lines of evidence point to the distal nephron. First, the induction of mPGES-1 in response to high salt loading was restricted in the entire CD system but not other nephron segments. Second, the CD is well known to be a major site of both production and action of PGE2. These findings indicate an autocrine mode of action of mPGES-1 in the distal nephron. mPGES-1 may also act in a paracrine manner in which PGE2 produced from the CD may modulate the transport function in the thick ascending limb and may also augment local blood flow by dilating the smooth muscle cells of vasa recta. Our results, however, are somewhat inconsistent with the report by Cheng et al.25.Cheng Y. Wang M. Yu Y. et al.Cyclooxygenases, microsomal prostaglandin E synthase-1, and cardiovascular function.J Clin Invest. 2006; 116: 1391-1399Crossref PubMed Scopus (289) Google Scholar who were unable to observe an elevated BP in mPGES-1 knockout mice on a high-salt diet. It is unclear whether differences in experimental protocols or genetic backgrounds may account for the disparities between the two studies. In line with the notion that PGE2 functions as a natriuretic factor in the distal nephron, chronic salt loading is reported to increase renal synthesis of PGE2. In the 1970s, an acute saline infusion was shown to increase prostaglandin-like materials in renal venous blood and medulla of canine kidney.26.Shimizu K. Yamamoto M. Yoshitoshi Y. Effects of saline infusion on prostaglandin-like materials in renal venous blood and medulla of canine kidney.Jpn Heart J. 1973; 14: 140-145Crossref PubMed Scopus (11) Google Scholar In the 1980s, a significant number of studies in rodents quite consistently showed that chronic salt loading elevated urinary excretion and renal medullary production of PGE2.21.Ye W. Zhang H. Hillas E. et al.Expression and function of COX isoforms in renal medulla: evidence for regulation of salt sensitivity and blood pressure.Am J Physiol Renal Physiol. 2006; 290: F542-549Crossref PubMed Scopus (57) Google Scholar, 27.Limas C. Goldman P. Limas C.J. Iwai J. Effect of salt on prostaglandin metabolism in hypertension-prone and - resistant Dahl rats.Hypertension. 1981; 3: 219-224Crossref PubMed Scopus (43) Google Scholar, 28.Limas C. Limas C.J. Up-regulation of renal prostaglandin receptors in genetic salt-dependent hypertension.Hypertension. 1986; 8: 566-571Crossref PubMed Scopus (14) Google Scholar The source and mechanisms of increased renal synthesis of PGE2 during high salt intake have puzzled the field for more than 30 years. There is even a debate that increased urinary PGE2 excretion during high salt loading might be related to increased urine flow. This issue has been partially resolved by the experiments employing mPGES-1 -/- mice which exhibit a remarkable blockade of high salt-induced urinary PGE2 excretion.11.Jia Z. Zhang A. Zhang H. et al.Deletion of microsomal prostaglandin E synthase-1 increases sensitivity to salt loading and angiotensin II infusion.Circ Res. 2006; 99: 1243-1251Crossref PubMed Scopus (85) Google Scholar The almost exclusive localization of high salt-induced mPGES-1 expression in the CD indicates that the increased renal PGE2 synthesis may occur primarily in the distal nephron via mPGES-1. The biological action of PGE2 is mediated by G protein-coupled E-prostanoid receptors designated EP1, EP2, EP3, and EP4.29.Breyer M.D. Breyer R.M. Prostaglandin E receptors and the kidney.Am J Physiol Renal Physiol. 2000; 279: F12-23PubMed Google Scholar These four subtypes of EP receptor couple to distinct signaling pathways. In general, the EP1 receptor signals via intracellular Ca2+ and phosphatidylinositol–bisphosphate hydrolysis, whereas the EP2 and EP4 receptors are coupled to Gs and signal by intracellular cyclic adenosine monophosphate. The EP3 receptor is more complex in that it exists in multiple slice isoforms which display differential coupling to distinct signaling mechanisms via Gi (inhibition of cyclic adenosine monophosphate formation), Gs (stimulation of cyclic adenosine monophosphate formation), and Gq (stimulation of intracellular Ca2+ release). Specific EP receptors mediating the natriuretic action of mPGES-1-derived PGE2 still remains elusive. The EP1 receptor mRNA is detected predominantly in the CD where activation of this receptor inhibits sodium and water reabsorption via a Ca2+-coupled mechanism. This evidence favors the EP1 as a candidate EP receptor responsible for the well-documented natriuretic and diuretic effects of PGE2. However, this notion is somewhat incompatible with a recent finding that mice lacking the EP1 exhibit reduced but not enhanced urine-concentrating capability.30.Kennedy C.R. Xiong H. Rahal S. Vanderluit J. et al.Urine Concentrating defect in prostaglandin EP1-deficient mice.Am J Physiol Renal Physiol. 2006; 292: F868-F875Crossref PubMed Scopus (31) Google Scholar The EP4 receptor is primarily expressed in the glomerulus and according to some, but not all, studies is found in the CD. Despite evidence for a potential role of EP4 receptor in furosemide-induced natriuretic and diuretic responses,31.Nusing R.M. Treude A. Weissenberger C. et al.Dominant role of prostaglandin E2 EP4 receptor in furosemide-induced salt-losing tubulopathy: a model for hyperprostaglandin E syndrome/antenatal Bartter syndrome.J Am Soc Nephrol. 2005; 16: 2354-2362Crossref PubMed Scopus (43) Google Scholar a specific role of this receptor in the CD remains elusive. The EP3 receptor is abundantly expressed in the renal medulla and the CD and mice lacking the EP3 exhibit normal urinary concentrating function. Together, the net in vivo contribution of these three EP receptors to the control of sodium balance and BP is unclear and awaits analysis of the respective EP knockout mice challenged with salt loading. The EP2 predominates in the vasculature but not the CD, and mice lacking this receptor develop salt-sensitive hypertension, a phenotype similar to mPGES-1 knockout mice. These results suggest that the EP2 may in part mediate the natriuretic and antihypertensive action of mPGES-1-derived PGE2. The results also indicate a possible paracrine mode of action of PGE2 in which mPGES-1-derived PGE2 is released from the CD and acts on the EP2 in the blood vessels. Further evidence in support of the role of mPGES-1 in BP regulation came from the observation that mPGES-1 -/- mice develop an exaggerated hypertensive response to chronic Ang II infusion as compared with wild-type controls.11.Jia Z. Zhang A. Zhang H. et al.Deletion of microsomal prostaglandin E synthase-1 increases sensitivity to salt loading and angiotensin II infusion.Circ Res. 2006; 99: 1243-1251Crossref PubMed Scopus (85) Google Scholar This finding may indicate the vascular action of mPGES-1. In line with this notion, this enzyme has been detected in the vasculature both in vivo and in vitro despite some discrepancies regarding its presence in vascular smooth muscle versus endothelial cells. As compared with the well-recognized vasodilatory action of prostacyclin, the vascular effects of PGE2 have not been well appreciated. This is partially due to the fact that PGE2 can be either vasodilatory or vasoconstrictory depending on the activation of distinct EP receptors and the net in vivo effect of PGE2 might become unpredictable. The hypertensive phenotype of mPGES-1 -/- mice in the setting of Ang II infusion represents an in vivo evidence for the overall vasodilatory and antihypertensive function of endogenous PGE2. Within the kidney, the renal medulla has the greatest capacity for synthesis of PGs and nitric oxide (NO), the two important regulators of distal tubular fluid reabsorption. In line with this notion, the two isoforms of COX (COX-1 and -2) and the three isoforms of nitric oxide synthase (NOS) (neuronal NOS, inducible NOS, and endothelial NOS) are all predominantly expressed in the renal medulla where most of these enzymes except COX-1 can be induced by a high-salt diet. Functional studies indicate that renal medullary PGs21.Ye W. Zhang H. Hillas E. et al.Expression and function of COX isoforms in renal medulla: evidence for regulation of salt sensitivity and blood pressure.Am J Physiol Renal Physiol. 2006; 290: F542-549Crossref PubMed Scopus (57) Google Scholar,32.Zewde T. Mattson D.L. Inhibition of cyclooxygenase-2 in the rat renal medulla leads to sodium-sensitive hypertension.Hypertension. 2004; 44: 424-428Crossref PubMed Scopus (79) Google Scholar and NO33.Mattson D.L. Roman R.J. Cowley Jr, A.W. Role of nitric oxide in renal papillary blood flow and sodium excretion.Hypertension. 1992; 19: 766-769Crossref PubMed Scopus (201) Google Scholar,34.Mattson D.L. Lu S. Nakanishi K. et al.Effect of chronic renal medullary nitric oxide inhibition on blood pressure.Am J Physiol. 1994; 266: H1918-1926PubMed Google Scholar exert similar natriuretic and diuretic effects to stabilize BP during salt loading. The interaction has also been demonstrated in renal hemodynamic responses to bradykinin, norepinephrine, and Ang II.35.Rodriguez F. Llinas M.T. Moreno C. et al.Role of cyclooxygenase-2-derived metabolites and NO in renal response to bradykinin.Hypertension. 2001; 37: 129-134Crossref PubMed Scopus (14) Google Scholar,36.Lopez R. Llinas M.T. Roig F. et al.Role of nitric oxide and cyclooxygenase-2 in regulating the renal hemodynamic response to norepinephrine.Am J Physiol Regul Integr Comp Physiol. 2003; 284: R488-493Crossref PubMed Scopus (19) Google Scholar If operation of a renal medullary depressor–natriuretic mechanism relies on the action of both PGs and NO, the question arises as to how the actions of these two autocrine/paracrine factors are coordinated. Existing literature indicates a mutual interaction between the two pathways. It is well documented that NO exerts a direct stimulatory effect on PGE2 synthesis in a variety of cell types mostly through a COX-2-dependent mechanism. NO can activate COX-2 at levels of gene expression as well as enzyme activity. On the other hand, evidence demonstrates that PGE2 stimulates NO formation, mostly in nonrenal cells. In this regard, PGE2 augments neuronal NOS and endothelial NOS activities at post-translational levels in the spinal cord37.Matsumura S. Abe T. Mabuchi T. et al.Rho-kinase mediates spinal nitric oxide formation by prostaglandin E2 via EP3 subtype.Biochem Biophys Res Commun. 2005; 338: 550-557Crossref PubMed Scopus (13) Google Scholar and in cultured human vein endothelial cells,38.Namkoong S. Lee S.J. Kim C.K. et al.Prostaglandin E2 stimulates angiogenesis by activating the nitric oxide/cGMP pathway in human umbilical vein endothelial cells.Exp Mol Med. 2005; 37: 588-600Crossref PubMed Scopus (96) Google Scholar respectively. The mutual interaction of PGE2 and NO has been observed in renal cells as well, especially in the CD cells where NO stimulates COX-2 expression and PGE2 release through mitogen-activated protein kinases39.Yang T. Zhang A. Pasumarthy A. et al.Nitric oxide stimulates COX-2 expression in cultured collecting duct cells through MAP kinases and superoxide but not cGMP.Am J Physiol Renal Physiol. 2006; 291: F891-F895Crossref PubMed Scopus (38) Google Scholar,40.Cheng H.F. Zhang M.Z. Harris R.C. Nitric oxide stimulates cyclooxygenase-2 in cultured cTAL cells through a p38-dependent pathway.Am J Physiol Renal Physiol. 2006; 290: F1391-F1397Crossref PubMed Scopus (26) Google Scholar while PGE2 enhances cytokine-induced NO formation probably through cyclic adenosine monophosphate.41.Huang C.N. Liu K.L. Cheng C.H. et al.PGE2 enhances cytokine-elicited nitric oxide production in mouse cortical collecting duct cells.Nitric Oxide. 2005; 12: 150-158Crossref PubMed Scopus (8) Google Scholar These in vitro studies indicate that the two signaling pathways might be mutually stimulatory in the distal nephron as in other nonrenal cells. A breakthrough has been made in understanding the in vivo relationship between PGE2 and NO using mPGES-1 -/- mice.11.Jia Z. Zhang A. Zhang H. et al.Deletion of microsomal prostaglandin E synthase-1 increases sensitivity to salt loading and angiotensin II infusion.Circ Res. 2006; 99: 1243-1251Crossref PubMed Scopus (85) Google Scholar As expected, chronic high salt loading gradually and significantly increased urinary nitrate/nitrite excretion in mPGES-1 +/+ mice with a maximal increase of 15-fold on day 7. In a sharp contrast, this increase was not seen in mPGES-1 -/- mice and urinary nitrate/nitrite excretion in these animals was even suppressed to undetectable levels after 3 days of high salt loading (Figure 1b). This result was further confirmed by measurement of urinary cyclic guanosine monophosphate (Figure 1c). Together, these findings reveal a linear relationship between PGE2 and NO in the kidney at least in the setting of chronic salt loading. The precise location for PGE2-dependent NO production under this condition is unknown but evidence points to the distal nephron. In this regard, the induction of renal mPGES-1 expression in response to high salt loading was restricted to the CD. Furthermore, high salt-induced expression of neuronal NOS, a major NO-producing enzyme in the CD was almost completely abolished in mPGES-1 -/- mice. These findings indicate that PGE2 may regulate sodium and water transport in the CD via NO (Figure 2). The EP receptor subtype responsible for stimulation of NO formation in the CD still remains elusive. Among the four EP receptor subtypes, the EP1 is well known to mediate the tubular effects of PGE2 via elevation of intracellular Ca2+. Whether or not the EP1-elicited calcium signaling in the CD is linked to NO is unclear. In summary, mPGES-1 has an established role in pain and inflammatory responses and emerging evidence suggests that this enzyme is also involved in the maintenance of normal BP. mPGES-1 is predominantly expressed in the distal nephron and is further upregulated by a high-salt diet, representing a major source of high salt-induced PGE2 synthesis. This enzyme helps stabilize BP by facilitating renal sodium excretion through a mechanism involving stimulation of NO synthesis. These results provide a new insight into the interaction of PGE2 and NO in BP regulation. This work was supported by NIH Grants HL079453, DK066592, DK069460, and DK064981 and Merit Review (to T Yang).