Osmoregulation of endothelial nitric-oxide synthase gene expression in inner medullary collecting duct cells. Role in activation of the type A natriuretic peptide receptor

Generation and characterization of a conditional eNOS knock out mouse model for cell-specific reactivation of eNOS in gain-of-function studies

Nitric oxide (NO) produced by endothelial nitric oxide synthase (eNOS) in the vessel wall regulates blood pressure and cardiovascular hemodynamics. In this study, we generated conditional eNOS knock out (KO) mice characterized by a duplicated/inverted exon 2 flanked with two pairs of loxP regions (eNOSinv/inv); a Cre-recombinase activity induces cell-specific reactivation of eNOS, as a result of a flipping of the inverted exon 2 (eNOSfl). This work aimed to test the efficiency of the Cre-mediated cell-specific recombination and the resulting eNOS expression/function. As proof of concept, we crossed eNOSinv/inv mice with DeleterCrepos (DelCrepos) mice, expressing Cre recombinase in all cells. We generated heterozygous eNOSfl/inv or homozygous eNOSfl/fl mice, and eNOSinv/inv littermate mice. We found that both eNOSfl/fl and eNOSfl/inv mice express eNOS and the overall expression level depends on the number of mutated alleles, while eNOSinv/inv mice did not show any eNOS expression. Vascular endothelial function was restored in eNOSfl/fl and eNOSfl/inv mice, as determined by ACh-dependent vasodilation of aortic rings. Cre-dependent reactivation of eNOS in eNOSfl/fl and eNOSfl/inv mice rescued eNOSinv/inv (phenotypically global eNOS KO) mice from hypertension. These findings demonstrate that eNOS expression is restored in eNOSfl/fl mice at comparable physiological levels of WT mice, and its functional activity is independent on the number of the reactivated alleles. Therefore, eNOSinv/inv mice are a useful model for studying the effects of conditional reactivation of eNOS and gene dosage effects in specific cells for gain-of-function studies.

Lack of urea transporters, UT-A1 and UT-A3, increases nitric oxide accumulation to dampen medullary sodium reabsorption through ENaC

Although the role of urea in urine concentration is known, the effect of urea handling by the urea transporters (UTs), UT-A1 and UT-A3, on sodium balance remains elusive. Serum and urinary sodium concentration is similar between wild-type mice (WT) and UT-A3 null (UT-A3 KO) mice; however, mice lacking both UT-A1 and UT-A3 (UT-A1/A3 KO) have significantly lower serum sodium and higher urinary sodium. Protein expression of renal sodium transporters is unchanged among all three genotypes. WT, UT-A3 KO, and UT-A1/A3 KO acutely respond to hydrochlorothiazide and furosemide; however, UT-A1/A3 KO fail to show a diuretic or natriuretic response following amiloride administration, indicating that baseline epithelial Na+ channel (ENaC) activity is impaired. UT-A1/A3 KO have more ENaC at the apical membrane than WT mice, and single-channel analysis of ENaC in split-open inner medullary collecting duct (IMCD) isolated in saline shows that ENaC channel density and open probability is higher in UT-A1/A3 KO than WT. UT-A1/A3 KO excrete more urinary nitric oxide (NO), a paracrine inhibitor of ENaC, and inner medullary nitric oxide synthase 1 mRNA expression is ~40-fold higher than WT. Because endogenous NO is unstable, ENaC activity was reassessed in split-open IMCD with the NO donor PAPA NONOate [1-propanamine-3-(2-hydroxy-2-nitroso-1-propylhydrazine)], and ENaC activity was almost abolished in UT-A1/A3 KO. In summary, loss of both UT-A1 and UT-A3 (but not UT-A3 alone) causes elevated medullary NO production and salt wasting. NO inhibition of ENaC, despite elevated apical accumulation of ENaC in UT-A1/A3 KO IMCD, appears to be the main contributor to natriuresis in UT-A1/A3 KO mice.

Collecting duct-specific knockout of nitric oxide synthase 3 impairs water excretion in a sex-dependent manner

Nitric oxide (NO) inhibits collecting duct (CD) Na⁺ and water reabsorption. Mice with CD-specific knockout (KO) of NO synthase 1 (NOS1) have salt-sensitive hypertension. In contrast, the role of NOS3 in CD salt and water reabsorption is unknown. Mice with CD NOS3 KO were generated with loxP-flanked exons 9-12 (encodes the calmodulin binding site) of the NOS3 gene and the aquaporin-2 promoter-Cre transgene. There were no differences between control and CD NOS3 KO mice, irrespective of sex, in food intake, water intake, urine volume, urinary Na⁺ or K⁺ excretion, plasma renin concentration, blood pressure, or pulse during 7 days of normal (0.3%), high (3.17%), or low (0.03%) Na⁺ intake. Blood pressure was similar between genotypes during DOCA-high salt. CD NOS3 KO did not alter urine volume or urine osmolality after water deprivation. In contrast, CD NOS3 KO male, but not female, mice had lower urine volume and higher urine osmolality over the course of 7 days of water loading compared with control mice. Male, but not female, CD NOS3 KO mice had reduced urinary nitrite+nitrate excretion compared with controls after 7 days of water loading. Urine AVP and AVP-stimulated cAMP accumulation in isolated inner medullary CD were similar between genotypes. Western analysis did not reveal a significant effect of CD NOS3 KO on renal aquaporin expression. In summary, these data suggest that CD NOS3 may be involved in the diuretic response to a water load in a sex-specific manner; the mechanism of this effect remains to be determined.

Transcriptomic analysis reveals specific osmoregulatory adaptive responses in gill mitochondria-rich cells and pavement cells of the Japanese eel

BACKGROUND

Homeostasis of ions and water is important for the maintenance of cellular functions. The regulation of the homeostasis is particularly important in euryhaline fish that migrate between freshwater (FW) and seawater (SW) environments. The fish gill, the major tissue that forms an interface separating the extracellular fluids and external water environment, has an effective transport system to maintain and regulate a constant body osmolality. In fish gills, the two major epithelial cells, pavement cells (PVCs) and mitochondria-rich cells (MRCs), are known to play key and complementary roles in ion transport at the interface. Discovering the robust mechanisms underlying the two cell types' response to osmotic stress would benefit our understanding of the fundamental mechanism allowing PVCs and MRCs to handle osmotic stress. Owing to the limited genomic data available on estuarine species, existing knowledge in this area is slim. In this study, transcriptome analyses were conducted using PVCs and MRCs isolated from Japanese eels adapted to FW or SW environments to provide a genome-wide molecular study to unravel the fundamental processes at work.

RESULTS

The study identified more than 12,000 transcripts in the gill cells. Interestingly, remarkable differential expressed genes (DEGs) were identified in PVCs (970 transcripts) instead of MRCs (400 transcripts) in gills of fish adapted to FW or SW. Since PVCs cover more than 90 % of the gill epithelial surface, the greater change in gene expression patterns in PVCs in response to external osmolality is anticipated. In the integrity pathway analysis, 19 common biological functions were identified in PVCs and MRCs. In the enriched signaling pathways analysis, most pathways differed between PVCs and MRCs; 14 enriched pathways were identified in PVCs and 12 in MRCs. The results suggest that the osmoregulatory responses in PVCs and MRCs are cell-type specific, which supports the complementary functions of the cells in osmoregulation.

CONCLUSIONS

This is the first study to provide transcriptomic analysis of PVCs and MRCs in gills of eels adapted to FW or SW environments. It describes the cell-type specific transcriptomic network in different tonicity. The findings consolidate the known osmoregulatory pathways and provide molecular insight in osmoregulation. The presented data will be useful for researchers to select their targets for further studies.

Hypotonicity-induced reduction of aquaporin-2 transcription in mpkCCD cells is independent of the tonicity responsive element, vasopressin, and cAMP

The syndrome of inappropriate antidiuretic hormone secretion is characterized by excessive water uptake and hyponatremia. The extent of hyponatremia, however, is less than anticipated, which is ascribed to a defense mechanism, the vasopressin-escape, and is suggested to involve a tonicity-determined down-regulation of the water channel aquaporin-2 (AQP2). The underlying mechanism, however, is poorly understood. To study this, we used the mouse cortical collecting duct (mpkCCD) cell line. MpkCCD cells, transfected with an AQP2-promoter luciferase construct showed a reduced and increased AQP2 abundance and transcription following culture in hypotonic and hypertonic medium, respectively. This depended on tonicity rather than osmolality and occurred independently of the vasopressin analog dDAVP, cAMP levels, or protein kinase A activity. Although prostaglandins and nitric oxide reduced AQP2 abundance, inhibition of their synthesis did not influence tonicity-induced AQP2 transcription. Also, cells in which the cAMP or tonicity-responsive element (CRE/TonE) in the AQP2-promoter were mutated showed a similar response to hypotonicity. Instead, the tonicity-responsive elements were pin-pointed to nucleotides -283 to -252 and -157 to -126 bp. In conclusion, our data indicate that hypotonicity reduces AQP2 abundance and transcription, which occurs independently of vasopressin, cAMP, and the known TonE and CRE in the AQP2-promoter. Increased prostaglandin and nitric oxide, as found in vivo, may contribute to reduced AQP2 in vasopressin-escape, but do not mediate the effect of hypotonicity on AQP2 transcription. Our data suggest that two novel segments (-283 to -252 and -157 to -126 bp) in the AQP2-promoter mediate the hypotonicity-induced AQP2 down-regulation during vasopressin-escape.

Upregulation of ANP and NPR-C mRNA in the kidney and heart of eNOS knockout mice

OBJECTIVES

The aim of the present studywas to examine the question of whether the atrial natriuretic peptide (ANP) system is altered by endothelial nitric-oxide synthase (eNOS).

METHODS

Male eNOS-deficient mice (eNOS-/-) and wild type control mice (eNOS+/+, C57B1/6J) were used. Blood pressure was measured in anesthetized mice by tail cuff plethysmography and renal function was measured. Expression of ANP, natriuretic peptide receptor (NPR)-A, NPR-C, and tonicity-responsive enhancer binding protein (TonEBP) mRNA was determined by real-time PCR. Localization of (125)I-ANP binding sites was measured using in vitro autoradiography.

RESULTS

In eNOS-/- mice, systolic blood pressure increased and left ventricular hypertrophy was observed. Urine volume and osmolarity did not change. Expression of ANP markedly increased in the heart and kidney of eNOS-/- mice. Expression of NPR-A and NPR-C increased in the heart and tended to increase in the kidney of eNOS-/- mice. In the renal medulla in particular, increased expression of NPR-C was more prominent. Expression of TonEBP mRNA was markedly decreased in the renal medulla, but not in the renal cortex. Maximum binding capacity (B(max)) of ANP and C-ANP increased in the renal medulla in eNOS-/- mice.

CONCLUSION

These results suggest that the eNOS-NO system may be partly involved in regulation of ANP, NPR-A, -C, and TonEBP mRNA expression in the kidney.

Ligand-mediated endocytosis and intracellular sequestration of guanylyl cyclase/natriuretic peptide receptors: role of GDAY motif

The guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), also referred to as GC-A, is a single polypeptide molecule having a critical function in blood pressure regulation and cardiovascular homeostasis. GC-A/NPRA, which resides in the plasma membrane, consists of an extracellular ligand-binding domain, a single transmembrane domain, and an intracellular cytoplasmic region containing a protein kinase-like homology domain (KHD) and a guanylyl cyclase (GC) catalytic domain. After binding with atrial and brain natriuretic peptides (ANP and BNP), GC-A/NPRA is internalized and sequestered into intracellular compartments. Therefore, GC-A/NPRA is a dynamic cellular macromolecule that traverses different subcellular compartments through its lifetime. This review describes the roles of short-signal sequences in the internalization, trafficking, and intracellular redistribution of GC-A/NPRA from cell surface to cell interior. Evidence indicates that, after internalization, the ligand-receptor complexes dissociate inside the cell and a population of GC-A/NPRA recycles back to the plasma membrane. Subsequently, the disassociated ligands are degraded in the lysosomes. However, a small percentage of the ligand escapes the lysosomal degradative pathway, and is released intact into culture medium. Using pharmacologic and molecular perturbants, emphasis has been placed on the cellular regulation and processing of ligand-bound GC-A/NPRA in terms of receptor trafficking and down-regulation in intact cells. The discussion is concluded by examining the functions of short-signal sequence motifs in the cellular life-cycle of GC-A/NPRA, including endocytosis, trafficking, metabolic processing, inactivation, and/or down-regulation in model cell systems.

Osmoregulation of natriuretic peptide receptors in bromoethylamine-treated rat kidney

Extracellular osmolarity is known as an important factor for the regulation of natriuretic peptide receptors (NPRs). We investigated the intra-renal osmoregulation of NPRs using renal medullectomized rats with bromoethylamine hydrobromide (BEA, 200mg/kg). The administration of BEA caused the decreased food intake and body weight. Water intake was decreased on the first day and then increased from the second day. Urine volume was persistently increased from the first day and free water clearance was also increased from the second day. Urinary excretions of sodium and potassium were decreased on the second day and then recovered to control level. Plasma levels of atrial natriuretic peptide (ANP) and Dendroaspis natriuretic peptide (DNP) in BEA-treated rats were not different from control rats. The inactive renin was increased. The maximum binding capacities of (125)I-ANP as well as (125)I-DNP decreased in glomeruli and medulla of BEA-treated rat kidneys but the binding affinity was not changed. In renal cortex, the gene expressions of ANP, NPR-A, and NPR-B were not changed but that of NPR-C decreased. In renal medulla, the gene expressions of NPR-A, -B, and -C decreased without change in ANP mRNA. Both renal medullary osmolarity and sodium concentration by BEA treatment were lower than those in control kidney. The cGMP concentrations in renal medulla and urine in BEA-treated rats were higher than those in control rats. These results suggest that the increased cGMP production may be partly involved in the decrease in NPRs mRNA expression and their binding capacities by BEA-induced medullectomy.

Tonicity-dependent induction of Sgk1 expression has a potential role in dehydration-induced natriuresis in rodents

In various mammalian species, including humans, water restriction leads to an acute increase in urinary sodium excretion. This process, known as dehydration natriuresis, helps prevent further accentuation of hypernatremia and the accompanying rise in extracellular tonicity. Serum- and glucocorticoid-inducible kinase (Sgk1), which is expressed in the renal medulla, is regulated by extracellular tonicity. However, the mechanism of its regulation and the physiological role of hypertonicity-induced SGK1 gene expression remain unclear. Here, we identified a tonicity-responsive enhancer (TonE) upstream of the rat Sgk1 transcriptional start site. The transcription factor NFAT5 associated with TonE in a tonicity-dependent fashion in cultured rat renal medullary cells, and selective blockade of NFAT5 activity resulted in suppression of the osmotic induction of the Sgk1 promoter. In vivo, water restriction of rats or mice led to increased urine osmolality, increased Sgk1 expression, increased expression of the type A natriuretic peptide receptor (NPR-A), and dehydration natriuresis. In cultured rat renal medullary cells, siRNA-mediated Sgk1 knockdown blocked the osmotic induction of natriuretic peptide receptor 1 (Npr1) gene expression. Furthermore, Npr1-/- mice were resistant to dehydration natriuresis, which suggests that Sgk1-dependent activation of the NPR-A pathway may contribute to this response. Collectively, these findings define a specific mechanistic pathway for the osmotic regulation of Sgk1 gene expression and suggest that Sgk1 may play an important role in promoting the physiological response of the kidney to elevations in extracellular tonicity.

Nitric oxide decreases expression of osmoprotective genes via direct inhibition of TonEBP transcriptional activity

During antidiuresis, renal medullary cells adapt to the hyperosmotic interstitial environment by increased expression of osmoprotective genes, which is driven by a common transcriptional activator, tonicity-responsive enhancer binding protein (TonEBP). Because nitric oxide (NO) is abundantly produced in the renal medulla, the present studies addressed the effect of NO on expression of osmoprotective genes and TonEBP activation in MDCK cells. Several structurally unrelated NO donors blunted tonicity-induced up-regulation of TonEBP target genes involved in intracellular accumulation of organic osmolytes. These effects were mediated by reduced transcriptional activity of TonEBP, as assessed by tonicity-responsive elements- and aldose reductase promoter-driven reporter constructs. Neither total TonEBP abundance nor nuclear translocation of TonEBP was affected by NO. Furthermore, 8-bromo-cGMP and peroxynitrite failed to reproduce the inhibitory effect of NO, indicating that NO acts directly on TonEBP rather than through classical NO signaling pathways. In support of this notion, electrophoretic mobility shift assays showed reduced binding of TonEBP to its target sequence in nuclear extracts prepared from MDCK cells treated with NO in vivo and in nuclear extracts exposed to NO in vitro. Furthermore, immunoprecipitation of S-nitrosylated proteins and the biotin-switch method identified TonEBP as a target for S-nitrosylation, which correlates with reduced DNA binding and transcriptional activity. These observations disclose a novel direct inhibitory effect of NO on TonEBP, a phenomenon that may be relevant for regulation of osmoprotective genes in the renal medulla.

Cellular Response to Hyperosmotic Stresses

Cells in the renal inner medulla are normally exposed to extraordinarily high levels of NaCl and urea. The osmotic stress causes numerous perturbations because of the hypertonic effect of high NaCl and the direct denaturation of cellular macromolecules by high urea. High NaCl and urea elevate reactive oxygen species, cause cytoskeletal rearrangement, inhibit DNA replication and transcription, inhibit translation, depolarize mitochondria, and damage DNA and proteins. Nevertheless, cells can accommodate by changes that include accumulation of organic osmolytes and increased expression of heat shock proteins. Failure to accommodate results in cell death by apoptosis. Although the adapted cells survive and function, many of the original perturbations persist, and even contribute to signaling the adaptive responses. This review addresses both the perturbing effects of high NaCl and urea and the adaptive responses. We speculate on the sensors of osmolality and document the multiple pathways that signal activation of the transcription factor TonEBP/OREBP, which directs many aspects of adaptation. The facts that numerous cellular functions are altered by hyperosmolality and remain so, even after adaptation, indicate that both the effects of hyperosmolality and adaptation to it involve profound alterations of the state of the cells.

Proteomic identification of processes and pathways characteristic of osmoregulatory tissues in spiny dogfish shark (Squalus acanthias)

We used dogfish shark (Squalus acanthias) as a model for proteome analysis of six different tissues to evaluate tissue-specific protein expression on a global scale and to deduce specific functions and the relatedness of multiple tissues from their proteomes. Proteomes of heart, brain, kidney, intestine, gill, and rectal gland were separated by two-dimensional gel electrophoresis (2DGE), gel images were matched using Delta 2D software and then evaluated for tissue-specific proteins. Sixty-one proteins (4%) were found to be in only a single type of tissue and 535 proteins (36%) were equally abundant in all six tissues. Relatedness between tissues was assessed based on tissue-specific expression patterns of all 1465 consistently resolved protein spots. This analysis revealed that tissues with osmoregulatory function (kidney, intestine, gill, rectal gland) were more similar in their overall proteomes than non-osmoregulatory tissues (heart, brain). Sixty-one proteins were identified by MALDI-TOF/TOF mass spectrometry and biological functions characteristic of osmoregulatory tissues were derived from gene ontology and molecular pathway analysis. Our data demonstrate that the molecular machinery for energy and urea metabolism and the Rho-GTPase/cytoskeleton pathway are enriched in osmoregulatory tissues of sharks. Our work provides a strong rationale for further study of the contribution of these mechanisms to the osmoregulation of marine sharks.

Endothelin-1 stimulates NO production and inhibits cAMP accumulation in rat inner medullary collecting duct through independent pathways

Endothelin-1 (ET-1) inhibition of vasopressin (AVP)-stimulated cAMP accumulation in the collecting duct has been hypothesized to be mediated, at least in part, by nitric oxide (NO). To examine this, the effect of ET-1 on NO production by acutely isolated rat inner medullary collecting duct (IMCD) cell suspensions and the role of NO in mediating ET-1 effects on AVP-stimulated cAMP accumulation were studied. ET-1 dose dependently (first evident at 100 pM ET-1) increased IMCD NO production as determined by DAF-FM fluorescence. ET(B) receptor (BQ-788), but not ET(A) receptor (BQ-123), antagonism blocked this effect. Nonspecific NO synthase (NOS) inhibitors [N(G)-nitro-L-arginine methyl ester (L-NAME) or N(G)-monomethyl-L-arginine] or NOS-1 inhibitors (SMTC or VNIO) inhibited the ET-1 response, whereas NOS-2 or NOS-3 inhibitors (L-NAA or 1400W) were ineffective. ET-1 also increased cGMP accumulation. ET-1 caused a 35% reduction in AVP-stimulated cAMP levels; however, this response was not affected by L-NAME or SMTC. The addition of L-arginine, NADPH, tetrahydrobiopterin, or tempol (to reduce superoxide-dependent conversion of NO to peroxynitrate) did not affect the response. NO donors (SNAP or spermine NONOate), at concentrations that stimulated DAF-FM fluorescence and increased cGMP levels, did not alter AVP-stimulated cAMP accumulation in the IMCD cell suspensions. In conclusion, ET-1 stimulates IMCD NO production through activation of the ET(B) receptor and NOS-1. However, neither ET-1-mediated NO production nor NO donors inhibit AVP-stimulated cAMP accumulation, indicating that NO does not mediate ET-1 inhibition of cAMP production by the IMCD.

Posttranscriptional mechanisms contribute to osmotic regulation of ANG type 1 receptors in cultured rat renomedullary interstitial cells

Previously, we showed that ANG II receptors in cultured rat renomedullary interstitial cells (RMICs) are osmotically regulated (19). The current study examined the mechanisms underlying this osmotic regulation in RMICs cultured in isoosmotic (300 mosmol/kgH2O) and hyperosmotic (600 mosmol/kgH2O) conditions. Radioligand competition analysis coupled with RNase protection assays (RPA) and ligand-mediated receptor internalization studies revealed that RMICs primarily express the type 1a angiotensin receptor (AT(1a)R). When cultured under hyperosmotic conditions, the density (B(max)) of AT1R in RMIC membranes decreased by 31% [B(max) (pmol/mg protein): 300 mosmol/kgH2O, 6.44 +/- 0.46 vs. 600 mosmol/kgH2O, 4.42 +/- 0.37, n = 8, P < 0.01], under conditions in which no detectable changes in AT(1a)R mRNA expression or in the kinetics of ligand-mediated AT1R internalization were observed. RNA electromobility shift assays showed that RNA protein complex (RPC) formation between RMIC cytosolic RNA binding proteins and the 5' leader sequence (5'LS) of the AT(1a)R was increased 1.5-fold under hyperosmotic conditions [5'LS RPC (arbitrary units): 300 mosmol/kgH2O, 0.79 +/- 0.08 vs. 600 mosmol/kgH2O, 1.17 +/- 0.07, n = 4, P < 0.01]. These results suggest that the downregulation of AT(1a)R expression in RMICs cultured under hyperosmotic conditions is regulated at the posttranscriptional level by RNA binding proteins that interact within the 5'LS of the AT(1a)R mRNA. The downregulation of AT(1a)R expression under hyperosmotic conditions may be an important mechanism by which the activity of ANG II is regulated in the hyperosmotic renal medulla.

Regulation of guanylyl cyclase/natriuretic peptide receptor-A gene expression

Natriuretic peptide receptor-A (NPRA) is the biological receptor of the peptide hormones atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP). The level and activity of this receptor determines the biological effects of ANP and BNP in different tissues mainly directed towards the maintenance of salt and water homeostasis. The core transcriptional machinery of the TATA-less Npr1 gene, which encodes NPRA, consists of three SP1 binding sites and the inverted CCAAT box. This promoter region of Npr1 gene has been shown to contain several putative binding sites for the known transcription factors, but the functional significance of most of these regulatory sequences is yet to be elucidated. The present review discusses the current knowledge of the functional significance of the promoter region of Npr1 gene and its transcriptional regulation by a number of factors including different hormones, growth factors, changes in extracellular osmolarity, and certain physiological and patho-physiological conditions.

Recent advances in the regulation of nitric oxide in the kidney

Nitric oxide (NO) plays important roles in the regulation of renal function and the long-term control of blood pressure. New roles of NO have been proposed recently in diabetes, nephrotoxicity, and pregnancy. NO derived from all 3 NOS isoforms contributes to the overall regulation of kidney function, and recent advances in our understanding of their regulation have been made lately. In this regard, substrate and cofactor availability play important roles in regulating nitric oxide synthase (NOS) activity not only by limiting enzyme activity but also by influencing the coupling of NOS with its cofactors, tetrahydrobiopterin and NADPH. Protein-protein interactions are now recognized to be important negative and positive regulators of NOS. Phosphorylation is another component of the mechanism whereby NOS is activated or deactivated. Increased NOS expression can also influence enzyme activity; however, the degree of expression does not always correlate with enzyme activity because increased NO levels can result in inhibition of NOS. Finally, other potential regulators of NOS such as endogenous L-arginine analogs may also be important. In this article, we summarize recent advances in the regulation of activity and expression of the NOS isoforms within the kidney.

Antioxidative effect of p38 mitogen-activated protein kinase inhibitor in the kidney of hypertensive rat

OBJECTIVE

Nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase is regulated by angiotensin II, interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha via p38 mitogen-activated protein kinase (MAPK). We hypothesized that p38 MAPK inhibitor, FR167653, may suppress NAD(P)H oxidase and its oxygen radical production and ameliorate renal damage in Dahl salt-sensitive rats with heart failure (DSHF).

METHODS

DSHF rats were fed with 8% NaCl diet from 6 to 18 weeks old. Eleven-week-old DSHF rats received either vehicle or FR167653 (2 mg/kg per day) for 7 weeks and the renal NAD(P)H oxidase p47phox and nitric oxide synthase (NOS), superoxide production and renal damage were evaluated in comparison with the control Dahl salt-resistant rat fed with 8% NaCl diet.

RESULTS

In the kidney of DSHF rat, phosphorylated p38 MAPK was enhanced with an increased IL-1beta and TNF-alpha production compared with control rats. Treatment with FR167653 significantly suppressed p38 MAPK, IL-1beta and TNF-alpha. Renal NAD(P)H oxidase p47phox expression and superoxide production were significantly increased in the DSHF rats and treatment with FR167653 suppressed NAD(P)H oxidase expression and reduced superoxide formation. Renal endothelial and inducible NOS were reduced in DSHF rats compared with control rats, but FR167653 increased NOS and NO production in the kidney. Proteinuria, glomerulosclerosis and interstitial macrophage migration via intercellular adhesion molecule-1 (ICAM-1) were enhanced in DSHF rat and they were ameliorated by FR167653.

CONCLUSION

The inhibition of p38 MAPK by FR167653 reduced renal IL-1beta and TNF-alpha production and ameliorated renal damage in hypertensive rat via suppression of NAD(P)H oxidase and enhanced NO bioavailability.

1,25 dihydroxyvitamin d amplifies type a natriuretic peptide receptor expression and activity in target cells

1,25 dihydroxyvitamin D (VD) has been shown to exert a number of beneficial effects on cardiovascular function, including reduction in BP and inhibition of cardiac hypertrophy. In an effort to identify a possible mechanistic link between VD and these salutary effects, the role of VD in controlling the activity and expression of the type A natriuretic peptide receptor (NPR-A), a receptor that signals reductions in BP and suppression of cellular growth in the myocardium and vascular wall, was investigated. VD, as well as the nonhypercalcemic analogue RO-25-6760, increased NPR-A-dependent cyclic guanosine monophosphate production and NPR-A gene expression in cultured rat aortic smooth muscle cells. The increase in NPR-A expression was associated with an increase in NPR-A gene promoter activity that was critically dependent on the presence of a functional VD receptor response element located approximately 495 bp upstream from the transcription start site of the gene. This element was associated with the VD receptor/retinoid X receptor complex in vitro. Mutation of this element resulted in complete elimination of the VD-dependent induction of the NPR-A gene promoter but did not affect osmotic stimulation of the promoter. Treatment of rats with RO-25-6760 for 7 d increased the atrial natriuretic peptide-dependent excretion of sodium and cyclic guanosine monophosphate without affecting mean arterial BP or plasma calcium levels. This was associated with a twofold increase in NPR-A mRNA levels in the inner medulla. Amplification of NPR-A activity represents a plausible mechanism to account for at least some of the beneficial effects that VD exerts on cardiovascular function.

Sgk1 mediates osmotic induction of NPR-A gene in rat inner medullary collecting duct cells

We have shown previously that increased extracellular osmolality stimulates expression and promoter activity of the type A natriuretic peptide receptor (NPR-A) gene in rat inner medullary collecting duct (IMCD) cells through a mechanism that involves activation of p38 mitogen-activated protein kinase (MAPK). The serum and glucocorticoid inducible kinase (Sgk) is thought to participate in the regulation of sodium handling in distal tubular segments. We sought to determine whether this kinase might be involved in the osmotic stimulation of NPR-A gene promoter activity. Exposure of cultured IMCD cells to an additional 75 mmol/L NaCl in culture media (final osmolality 475 mosm/kg) resulted in an approximately 4-fold increase in Sgk1 protein levels after 7 hours. The Sgk1 induction was almost completely inhibited by the p38 MAPK inhibitor SB203580, indicating that NaCl activates Sgk1 through the p38 MAPK pathway. Transient transfection of a mouse Sgk1 expression vector along with a -1590 NPR-A luciferase reporter resulted in an approximately 3-fold increment in reporter activity, which was significantly reduced by cotransfection with a kinase-dead Sgk1 mutant. The NaCl-dependent induction was partially blocked (approximately 40% inhibition) by cotransfection of the kinase-dead Sgk1 mutant. Neither Sgk1 nor the kinase-dead mutant had any effect on endothelial nitric oxide synthase (eNOS) promoter activity, and the Sgk1 mutant and 8-bromo-cyclic guanosine monophosphate were, to some degree, additive in reducing osmotically stimulated NPR-A promoter activity. Collectively, these data imply that Sgk1 operates over an eNOS-independent, p38 MAPK-dependent pathway in mediating osmotic induction of the NPR-A gene promoter.

Endothelin inhibits NPR-A and stimulates eNOS gene expression in rat IMCD cells

We have shown in previous studies that high extracellular tonicity is associated with increased expression of the type A natriuretic peptide receptor (NPR-A) and reduced expression of the endothelial NO synthase (eNOS) gene in cultured rat inner-medullary collecting duct cells. The vasoactive peptide endothelin has been shown to be avidly expressed in this nephron segment, and to be subject to osmotic regulation. We asked whether endothelin might play a role in the control of basal or osmotically regulated NPR-A or eNOS gene expression in these cells. Although exogenous endothelin had little or no effect on basal expression of eNOS mRNA or protein or NPR-A gene expression, both the type A (BQ610) and type B (IRL1038) endothelin receptor antagonists proved capable of reducing eNOS mRNA and protein expression, and increasing levels of the NPR-A mRNA. Increased extracellular tonicity reduced endothelin mRNA accumulation in these cells (approximately 15% of control levels); however, exogenous endothelin failed to normalize osmotically increased NPR-A activity or expression, or osmotically suppressed eNOS expression. Collectively, these data demonstrate the presence of a number of independent but highly interactive local regulatory networks governing fluid and electrolyte handling in this distal nephron segment.