Therapeutic antibody targeting natriuretic peptide receptor 1 inhibits gastric cancer growth via BCL-2-mediated intrinsic apoptosis

Despite recent advances in the development of therapeutic antibodies, the prognosis of unresectable or metastatic gastric cancer (GC) remains poor. Here, we searched for genes involved in the malignant phenotype of GC and investigated the potential of one candidate gene to serve as a novel therapeutic target. Analysis of transcriptome datasets of GC identified natriuretic peptide receptor 1 (NPR1), a plasma membrane protein, as a potential target. We employed a panel of human GC cell lines and gene-specific small interfering RNA-mediated NPR1 silencing to investigate the roles of NPR1 in malignancy-associated functions and intracellular signaling pathways. We generated an anti-NPR1 polyclonal antibody and examined its efficacy in a mouse xenograft model of GC peritoneal dissemination. Associations between NPR1 expression in GC tissue and clinicopathological factors were also evaluated. NPR1 mRNA was significantly upregulated in several GC cell lines compared with normal epithelial cells. NPR1 silencing attenuated GC cell proliferation, invasion, and migration, and additionally induced the intrinsic apoptosis pathway associated with mitochondrial dysfunction and caspase activation via downregulation of BCL-2. Administration of anti-NPR1 antibody significantly reduced the number and volume of GC peritoneal tumors in xenografted mice. High expression of NPR1 mRNA in clinical GC specimens was associated with a significantly higher rate of postoperative recurrence and poorer prognosis. NPR1 regulates the intrinsic apoptosis pathway and plays an important role in promoting the GC malignant phenotype. Inhibition of NPR1 with antibodies may have potential as a novel therapeutic modality for unresectable or metastatic GC.

Small atrial natriuretic peptide analogs: design, synthesis, and structural requirements for guanylate cyclase activation

Structure/activity studies on atrial natriuretic peptide ANP (1-28) have highlighted three portions of the native molecule as necessary for its biological responses. We have linked these three regions and excised the remaining segments to produce a family of small analogues (less than half the size of the parent) which demonstrate the full range of ANP's actions. Importantly, these compounds act at both major types of ANP receptor. Two critical modifications lead to more potent analogues; both involve expanding the cyclic portion of the molecule. Further optimization of one of these modified structures leads to A68828, a full ANP agonist which shows promise as a preventative agent against acute renal failure.

Progressive cardiac hypertrophy and dysfunction in atrial natriuretic peptide receptor (GC-A) deficient mice

OBJECTIVE

To investigate how permanent inhibition of guanylyl cyclase A receptor (GC-A) affects cardiac function.

METHODS

Hearts of GC-A-/- and corresponding wild type mice (GC-A+/+) were characterised by histological, western blotting, and northern blotting analyses. Cardiac function was evaluated in isolated, working heart preparations.

RESULTS

At 4 months of age, GC-A-/- mice had global cardiac hypertrophy (about a 40% increase in cardiac weight) without interstitial fibrosis. Examination of heart function found a significant delay in the time of relaxation; all other parameters of cardiac contractility were similar to those in wild type mice. At 12 months, the hypertrophic changes were much more severe (about a 61% increase in cardiac weight), together with a shift in cardiac gene expression (enhanced concentrations of atrial natriuretic peptide (3.8-fold), B type natriuretic peptide (2-fold), beta myosin heavy chain (1.6-fold) and alpha skeletal actin (1.7-fold) mRNA), increased expression of cytoskeletal tubulin and desmin (by 29.6% and 25.6%, respectively), and pronounced interstitial fibrosis. These changes were associated with significantly impaired cardiac contractility (+dP/dt decreased by about 10%) and relaxation (-dP/dt decreased by 21%), as well as depressed contractile responses to pressure load (all p < 0.05).

CONCLUSIONS

Chronic hypertension in GC-A-/- mice is associated with progressive cardiac changes--namely, initially compensated cardiomyocyte hypertrophy, which is complicated by interstitial fibrosis and impaired cardiac contractility at later stages.

Identification of the atrial natriuretic factor-RICreceptor subtype (B-clone) in cultured rat aortic smooth muscle cells

The present report demonstrates the presence in cultured rat aortic smooth muscle cells of a natriuretic factor receptor subtype with a specificity typical of the ANF-R1C (B-clone) receptor subtype. To prove the existence of this receptor subtype in this cell line we show that pCNP-(82-103) is the most potent activator of the intrinsic guanylate cyclase activity, and that [125I]pCNP-(82-103) binds to a specific receptor subtype which is insensitive to the ANF-R2 specific ligand, C-ANF. The investigation of its binding characteristics show the rank potency order of the natriuretic factors in competing for pCNP binding to be pCNP greater than pBNP greater than rANF. Furthermore it was possible to covalently photolabel this receptor subtype with underivatized]125I]pCNP and show that it is composed of a single subunit of 130 kDa with very high specificity for pCNP.

Molecular and cellular physiology of the dissociation of atrial natriuretic peptide from guanylyl cyclase a receptors

Guanylyl cyclase subtype A (GCA) is the main receptor that mediates the effects of atrial natriuretic peptide (ANP) in the regulation of plasma volume and blood pressure. The dynamics of the dissociation of ANP from GCA were investigated in cultured Chinese hamster ovary (CHO) cells stably transfected with wild-type (WT) or mutant GCA receptors. The rate of dissociation of specifically bound (125)I-ANP-(1-28) from intact CHOGCAWT cells at 37 degrees C was extremely rapid (K(off) = 0.49 +/- 0.02 min(-1)), whereas in isolated membranes prepared from these cells, the dissociation at 37 degrees C was >10-fold slower (K(off) = 0.035 +/- 0.006 min(-1)). The dissociation of ANP from CHOGCAWT cells showed remarkable temperature dependence. Between 22 and 37 degrees C, K(off) increased approximately 8 times, whereas between 4 and 22 degrees C, it increased only 1.5 times. Total deletion of the cytoplasmic domain or of the catalytic guanylyl cyclase sequence within this domain abolished ANP-induced increases in cGMP, dramatically slowed receptor-ligand dissociation by at least 10-fold, and abolished the temperature dependence of the dissociation of ANP. Deletion of the kinase-like domain led to maximal constitutive activation of guanylyl cyclase, markedly decreased K(off) to 0.064 +/- 0.006 min(-1), and also abolished the temperature dependence of dissociation. Substitution of Ser(506) by Ala and particularly the double substitution of Gly(505) and Ser(506) by Ala within the kinase-like domain markedly reduced ANP-induced increases in cGMP, whereas K(off) decreased modestly (albeit significantly) to 0.36 +/- 0.03 and 0.24 +/- 0.02 min(-1), respectively. As a whole, the results demonstrate for the first time that temperature per se or ATP alone cannot account for rapid GCA receptor-ligand dissociation under physiological conditions and suggest that ligand dissociation is modulated in part by the interaction of still unidentified cytosolic factors with the cytoplasmic domain of GCA.

Apparent B-type natriuretic peptide selectivity in the kidney due to differential processing

Two natriuretic peptides, atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP), are found principally in the heart. In preliminary experiments with mouse kidney cells or slices, we found mouse BNP1-45 much more potent than ANP1-28 in causing elevations of cGMP (>50-fold). The guanylyl cyclase-A (GC-A) receptor has been suggested to represent the primary means by which both peptides signal. In cultured cells overexpressing GC-A, BNP and ANP were almost equivalent in potency, suggesting that a receptor unique for BNP exists in the kidney. However, in mice lacking the GC-A gene, neither BNP nor ANP significantly elevated cGMP in kidney slices. Phosphoramidon, a neutral endopeptidase inhibitor, shifted the apparent potency of ANP to values equivalent to that of BNP, suggesting these kidney cell/slices rapidly degrade ANP but not BNP. Mass spectroscopic analysis confirmed that ANP is rapidly cleaved at the first cysteine of the disulfide ring, whereas BNP is particularly stable to such cleavage. Other tissues (heart, aorta) failed to significantly degrade ANP or BNP, and therefore the kidney-specific degradation of ANP provides a mechanism for preferential regulation of kidney function by BNP independent of peripheral ANP concentration.

Expression of GC-C, a receptor-guanylate cyclase, and its endogenous ligands uroguanylin and guanylin along the rostrocaudal axis of the intestine

Members of the receptor-guanylate cyclase (rGC) family possess an intracellular catalytic domain that is regulated by an extracellular receptor domain. GC-C, an intestinally expressed rGC, was initially cloned by homology as an orphan receptor. The search for its ligands has yielded three candidates: STa (a bacterial toxin that causes traveler's diarrhea) and the endogenous peptides uroguanylin and guanylin. Here, by performing Northern and Western blots, and by measuring [125I]STa binding and STa-dependent elevation of cGMP levels, we investigate whether the distribution of GC-C matches that of its endogenous ligands in the rat intestine. We establish that 1) uroguanylin is essentially restricted to small bowel; 2) guanylin is very low in proximal small bowel, increasing to prominent levels in distal small bowel and throughout colon; 3) GC-C messenger RNA and STa-binding sites are uniformly expressed throughout the intestine; and 4) GC-C-mediated cGMP synthesis peaks at the proximal and distal extremes of the intestine (duodenum and colon), but is nearly absent in the middle (ileum). These observations suggest that GC-C's activity may be posttranslationally regulated, demonstrate that the distribution of GC-C is appropriate to mediate the actions of both uroguanylin and guanylin, and help to refine current hypotheses about the physiological role(s) of these peptides.

C-Type natriuretic peptide/guanylate cyclase B system in rat osteogenic ROB-C26 cells and its down-regulation by dexamethazone

There is recent evidence that natriuretic peptides are important regulators of bone and cartilage, although they were originally identified as the cardiac hormones causing natriuresis and hypotension. Three members of natriuretic peptide family are known: atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). The biologically active receptors for these peptides are particulate guanylate cyclases; the two known types are GC-A and GC-B. ANP and BNP have high affinities for GC-A, and CNP is the preferred ligand for GC-B. In this paper we report the results of our study of the expression and possible role(s) of natriuretic peptides in the ROB-C26 cell, which is an osteogenic cell line with multiple potentials for differentiating into myoblast, osteoblast, and adipocyte. ROB-C26 cells produced cGMP in response to natriuretic peptides at both their basal state and after enhanced differentiation into osteoblast which was induced by bone morphogenetic protein [(BMP)-2]. CNP was far more potent than ANP in cGMP production. In contrast, enhanced differentiation into adipocyte by dexamethasone resulted in the marked decrease in their responsiveness to natriuretic peptides. Although the messages for GC-A and GC-B were demonstrated by Northern blot analysis at both the basal stage and after BMP treatment, they were down-regulated after dexamethasone treatment. The presence of CNP was shown by RT-PCR and immunohistochemistry in ROB-C26 cells. C3H10T1/2, which is another and more primitive mesenchymal cell line, also produced cGMP in response to CNP, and less potently to ANP. Culturing ROB-C26 cells with CNP or 8-bromo cGMP decreased [(3)H]thymidine uptake and slightly increased the message for alkaline phosphatase, which is a marker for osteoblast differentiation. These results suggest that the CNP/GC-B system is preferentially expressed in the cells of osteogenic lineage and their expression is down-regulated with differentiation into adipocyte lineage. The CNP/GC-B system is likely to be an autocrine/paracrine regulator of osteoblast growth and differentiation.

Reciprocal antagonism coordinates C-type natriuretic peptide and mitogen-signaling pathways in fibroblasts

The fibroblast, a cell central to effective wound remodeling, not only contains various growth factor receptors but also high activities of a guanylyl cyclase receptor (GC-B). Here we demonstrate that marked elevations of cyclic GMP induced by C-type natriuretic peptide (CNP), the ligand of GC-B, blocks activation of the mitogen-activated protein kinase cascade in fibroblasts. We also show that platelet-derived growth factor, fibroblast growth factor, serum, or Na3VO4 rapidly (within 5 min) and extensively (up to 85% inhibition) disrupt CNP-dependent elevations of cyclic GMP. In addition, the mitogens also lower cyclic GMP concentrations (50% decrease) in cells not treated with CNP. Cytoplasmic forms of guanylyl cyclase, in contrast to the CNP-stimulated pathway, are not antagonized by the various mitogens. The effects of the mitogens on cellular cyclic GMP are fully explained by a direct and stable inactivation of GC-B. Homogenates obtained from fibroblasts treated with or without the various mitogens contain equivalent amounts of GC-B protein, but both ligand-dependent and ligand-independent activity are markedly (up to 90% inhibition of CNP-dependent activity) decreased after mitogen addition. The stable inactivation is correlated with the dephosphorylation of phosphoserine and phosphothreonine residues of the cyclase receptor. These results not only establish a specific and reciprocal antagonistic relationship between mitogen-activated and GC-B-regulated signaling pathways in the fibroblast but also suggest that one of the earliest events following mitogen activation of a fibroblast is an interruption of cyclic GMP production from this receptor.

The heart communicates with the kidney exclusively through the guanylyl cyclase-A receptor: acute handling of sodium and water in response to volume expansion

Disruption of guanylyl cyclase-A (GC-A) results in mice displaying an elevated blood pressure, which is not altered by high or low dietary salt. However, atrial natriuretic peptide (ANP), a proposed ligand for GC-A, has been suggested as critical for the maintenance of normal blood pressure during high salt intake. In this report, we show that infusion of ANP results in substantial natriuresis and diuresis in wild-type mice but fails to cause significant changes in sodium excretion or urine output in GC-A-deficient mice. ANP, therefore, appears to signal through GC-A in the kidney. Other natriuretic/diuretic factors could be released from the heart. Therefore, acute volume expansion was used as a means to cause release of granules from the atrium of the heart. That granule release occurred was confirmed by measurements of plasma ANP concentrations, which were markedly elevated in both wild-type and GC-A-null mice. After volume expansion, urine output as well as urinary sodium and cyclic GMP excretion increased rapidly and markedly in wild-type mice, but the rapid increases were abolished in GC-A-deficient animals. These results strongly suggest that natriuretic/diuretic factors released from the heart function exclusively through GC-A.

The guanylyl cyclase-A receptor transduces an atrial natriuretic peptide/ATP activation signal in the absence of other proteins

Attempts to activate partially purified preparations of the guanylyl cyclase-A (GC-A) receptor with atrial natriuretic peptide (ANP) have previously failed, leading to speculation that essential cofactors are lost during purification procedures. The receptor was modified to contain the FLAG epitope (DYKDDDDK), expressed in Sf9 cells, and purified to apparent homogeneity (4.3 mumol cyclic GMP formed/min/mg protein; 5.8 mmol 125I-ANP binding site/mg protein) by a combination of immunoaffinity, Q-Sepharose FF, and wheat germ agglutinin batch chromatography. High initial protein/detergent ratios, the presence of glycerol (40%), and the inclusion of protein phosphatase inhibitors in all buffers resulted in the purification of a receptor that continued to transduce the ANP/ATP activation signal. Both native and purified GC-A contained a single class of high affinity ANP binding sites (Kd = 60 pM) and an equivalent EC50 for ATP (0.3 mM). Positive cooperativity as a function of MnGTP was retained during purification. Thus, GC-A is capable of transducing a ligand binding signal in the absence of other proteins.

Salt-resistant hypertension in mice lacking the guanylyl cyclase-A receptor for atrial natriuretic peptide

Around half of all humans with essential hypertension are resistant to salt (blood pressure does not change by more than 5 mm Hg when salt intake is high), and although various inbred strains of rats display salt-insensitive elevated blood pressure, a gene defect to account for the phenotype has not been described. Atrial natriuretic peptide (ANP) is released from the heart in response to atrial stretch and is thought to mediate its natriuretic and vaso-relaxant effects through the guanylyl cyclase-A receptor (GC-A). Here we report that disruption of the GC-A gene results in chronic elevations of blood pressure in mice on a normal salt diet. Unexpectedly, the blood pressure remains elevated and unchanged in response to either minimal or high salt diets. Aldosterone and ANP concentrations are not affected by the genotype. Therefore, mutations in the GC-A gene could explain some salt-resistant forms of essential hypertension and, coupled with previous work, further suggest that the GC-A signaling pathway dominates at the level of peripheral resistance, where it can operate independently of ANP.

Dominant negative mutations of the guanylyl cyclase-A receptor. Extracellular domain deletion and catalytic domain point mutations

Guanylyl cyclase-A (GC-A), a receptor for A-type natriuretic peptide (ANP), contains an extracellular ligand-binding domain, a single transmembrane domain, and intracellular protein kinase-like and cyclase catalytic domains. Expression of the putative cyclase catalytic region (HCAT) resulted in the formation of an active enzyme that migrated as a homodimer on gel filtration columns; treatment with sodium trichloroacetate caused dissociation of the dimer and a loss of cyclase activity. Co-transfection of HCAT and full-length GC-A led to elevated basal intact cell cGMP concentrations and increased cell homogenate guanylyl cyclase activity. However, atrial natriuretic peptide-induced elevations of cGMP and cyclase activity were inhibited by the introduction of HCAT. Alanine scanning mutagenesis of highly conserved residues within HCAT identified one mutation (D893A) that destroyed enzyme activity but not the ability of the mutant subunit to form homodimers. The mutant subunit inhibited the cyclase activity of wild-type HCAT (approximately 70%) as well as that of full-length GC-A (approximately 85%) in co-expression studies where the amount of wild-type HCAT or full-length GC-A was not altered. Unlike co-transfection with wild-type HCAT, co-transfection of HCA-TD893A and GC-A did not result in elevated basal intact cell cGMP concentrations. For the first time we describe deletion and point mutations within the plasma membrane family of guanylyl cyclase receptors that result in the formation of effective dominant negative proteins.

Determination of the solution structure of the peptide hormone guanylin: observation of a novel form of topological stereoisomerism

Guanylin is a 15 amino acid mammalian hormone containing two disulfide bonds. Guanylin shares sequence similarity with the bacterial heat-stable enterotoxin (STa) and is capable of binding to and stimulating the STa guanylyl cyclase receptor. Biologically active peptides have been prepared by two methods: (1) enzymatic treatment of a 99 residue proprotein (denoted proguanylin) expressed in Escherichia coli and (2) solid-phase chemical synthesis. Although both sources yield material that is pure by high-performance liquid chromatography and mass spectrometry, analysis by nuclear magnetic resonance (NMR) indicates that peptides from both sources contain two conformationally distinct species present in a 1:1 ratio. The chemical shift differences between the two species are large, allowing unambiguous sequential NMR assignments to be made for both sets of resonances. Exchange between the two forms was not observed even at 70 degrees C. Structural restraints have been generated from nuclear Overhauser effects and scalar coupling constants and used to calculate structures for both forms using distance geometry and restrained energy minimization. The resulting structures for the first isoform are well defined (root-mean-square deviation from the average structure for backbone atoms of 0.47 A) and adopt a right-handed spiral conformation, similar to that observed for heat stable enterotoxin. The second isoform is less well defined (root-mean-square deviation from the average structure for backbone atoms of 1.07 A) but clearly adopts a very different fold consisting of a left-hand spiral. The differences in structure suggest that the two forms may have very different affinities toward the STa receptor. The observation of such isomerism has important implications for the common practice of introducing multiple disulfide bonds into small peptides to limit conformational flexibility and enhance bioactivity.

Seminal plasma factors that cause large elevations in cellular cyclic GMP are C-type natriuretic peptides

Seminal plasma (porcine) was shown to elevate cyclic GMP in Balb/3T3 cells (235-fold), NIH/3T3, Rat-2, but not in human T84 cells. Seminal plasma and C-type natriuretic peptide (CNP) markedly elevated cyclic GMP of 293 cells stably transfected with the guanylyl cyclase-B (GC-B) receptor, but failed to elevate cyclic GMP concentrations of 293 cells stably transfected with the guanylyl cyclase-A receptor. The seminal plasma activity was analyzed by reverse-phase high performance liquid chromatography; two major fractions were obtained and the amino acid sequence of one matched that of proCNP-103. For the second peak of activity, sequence could not be obtained but the purified material bound to antibody specific for CNP. CNP concentrations in seminal plasma and seminal vesicle fluid were shown to be 2,000- and 100,000-fold greater than those found in porcine brain, respectively. Significant amounts of immunoreactive CNP also were detected in tracheal mucosa and uterus suggesting a general signaling role for the peptide. That seminal plasma CNP serves a function during fertilization was suggested by finding expression of GC-B mRNA in the uterus/oviduct and CNP-stimulatable GC-B in the intact uterus. Therefore, some or all of the GC-B receptor appears to exist on the apical membrane of uterine epithelial cells.

Atrial natriuretic peptide-induced inhibition of aldosterone secretion: a quest for mediator(s)

Atrial natriuretic peptide (ANP) inhibits aldosterone secretion evoked by its physiological secretagogues by a mechanism(s) likely to involve intracellular messengers. When one examines the results of various investigations so far, this premise, although not definitive yet, seems to be supported. Therefore a brief perspective on the cellular messengers of the various secretagogues is provided before the inquiry into the possible mechanism of action of ANP. The receptors of ANP in the adrenal cells have been identified and characterized. ANP inhibits adenylate cyclase in various tissues through an inhibitory G protein, which appears to explain in part the inhibitory effect of ANP on adrenocorticotropin-induced aldosterone secretion. However, there could be other possible effects of ANP as discussed. ANP probably inhibits aldosterone secretion evoked by angiotensin II and potassium by interfering with the appropriate changes in calcium flux and cell calcium concentration, concomitants of stimulation by these secretagogues. The potential modes of these effects are probed. The role of guanosine 3',5'-cyclic monophosphate, which is increased by receptor activation of guanylate cyclase by ANP and is thought to play a major role in the biological effects of ANP in some other tissues, remains controversial in the aldosterone-lowering effect of ANP, and this is also discussed extensively in this review.

Ligand-independent oligomerization of natriuretic peptide receptors. Identification of heteromeric receptors and a dominant negative mutant

Activation of many single-transmembrane receptors requires ligand-induced receptor oligomerization. We have examined the oligomerization of the atrial natriuretic peptide receptor, NPR-A, using epitope-tagged receptor in a co-immunoprecipitation assay. Unlike other single-transmembrane receptors, NPR-A oligomerized in a ligand-independent fashion. Extracellular receptor sequences were both necessary and sufficient for oligomer formation. NPR-A was also able to oligomerize with the related natriuretic peptide receptor, NPR-B. A truncated NPR-A lacking most of the cytoplasmic domain blocked activation of the full-length receptor, presumably through formation of an inactive heteromer. These results indicate that oligomerization of this single-transmembrane receptor is important for the transduction of a conformational change across the plasma membrane but are not consistent with models in which natriuretic peptide receptor oligomerization serves merely to bring intracellular domains together.

The role of the C-terminal region of rat brain natriuretic peptide in receptor selectivity

Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) have different C-terminal tail structures compared with the rather conservative ring structures which consist of 17 amino acid residues. To examine the different effects of the tail structures of ANP and BNP on their interaction with receptors, we synthesized several peptide analogs and measured their biological actions in three different assay systems. Deletion of the C-terminal tail from rat BNP did not effect the vasorelaxation activity against rat aorta, but it promoted cGMP production in cultured rat aortic smooth muscle cells (RASMC). Deletion of the C-terminal tail from rat ANP diminished both vasorelaxant and cGMP producing activities. In a binding competition assay with RASMC and [125I]rat ANP-(1-28), the competition activities of both ANP and BNP were greatly reduced by C-terminal deletion. In addition, we obtained agonists with novel receptor selectivity.

Atrial natriuretic peptide inhibits the production and secretion of endothelin from cultured endothelial cells. Mediation through the C receptor

Atrial natriuretic peptide (ANP) and endothelin-1 (ET-1) are vasoactive peptides produced in cells of the cardiovascular system. We examined the effects of ANP on ET-1 transcription, production (translation), and secretion in cultured bovine aortic endothelial cells (BAEC). ANP and C-ANP 4-23 (a specific ligand for the C or non-guanylate cyclase receptor) equipotently inhibited the synthesis of prepro-ET-1 and ET-1 proteins in BAEC by at least 50%. Both of these forms of ANP and another C receptor specific ligand, nanopiperazine ANP (11-15)-NH2, inhibited ET-1 secretion by as much as 55%. LY 83583, an inhibitor of ANP-induced cGMP generation, failed to reverse the ANP-induced inhibition of ET-1 secretion. This further indicated that the guanylate cyclase-linked B receptor is not involved. The decreased ET-1 secretion caused by C-ANP 4-23 was reversed by 8-bromo-cAMP or amiloride, which prevents ANP-induced inhibition of cAMP. We also found that ANP and C-ANP 4-23 augmented ET-1 mRNA levels in BAEC by prolonging the mRNA half-life. ANP or cycloheximide comparably inhibited ET-1 translation while increasing ET-1 mRNA levels, suggesting that the two events are related. These results indicate that ANP inhibits ET-1 protein production and secretion while stabilizing the ET-1 mRNA. The effects of ANP are mediated through the C receptor and are probably the result of ANP inhibiting the generation of cAMP. These findings suggest a potentially important new function for this receptor to mediate, in part, the interactions of ANP and ET in the vasculature.

Atrial natriuretic peptide receptors in human endometrial stromal cells

Atrial natriuretic peptide (ANP) has been shown to affect water and ion transport and specific ANP binding has been identified in several secretory tissues. ANP commonly acts via stimulation of membrane-bound particulate guanylate cyclase with the production of cyclic guanosine monophosphate (cGMP). We questioned whether ANP played a role in the complex cyclic transformation of the endometrium into a secretory tissue, and whether its action was cGMP mediated. Endometrium was obtained by biopsy in regularly menstruating women and stromal cells were isolated and cultured for use in this study. ANP competitive binding assays were performed using 125I-labeled ANP (0.1 nmol/L) and increasing concentrations of unlabeled ANP (0-1000 nmol/L). Optimal binding was obtained after 3-h incubation at 4 C and binding characteristics, including dissociation constant and binding site quantity, were estimated by Scatchard analysis. Specific, high affinity (dissociation constant, 0.078 +/- 0.004 nmol/L) and low capacity (4,877 +/- 1,951 binding sites/cell) ANP binding was identified, with nonspecific binding representing less than or equal to 16% of total binding. Evaluation of ANP-stimulated cyclic nucleotide production revealed an increase in cGMP production, with a 7-fold increase at 1000 nmol/L ANP, and no effect on cAMP production. In conclusion, we have identified specific high affinity receptors for ANP in human endometrial cells, suggesting a role for ANP in endometrial cell function and/or development mediated via cGMP production. We propose that ANP may affect local salt and water metabolism, may be involved in the secretory evolution of glandular and stromal cells, and may further facilitate endometrial development via modulation of local vascular tone and endothelial permeability.

Dephosphorylation of the guanylyl cyclase-A receptor causes desensitization

Atrial natriuretic peptide (ANP) binds to the guanylyl cyclase-A (GC-A) receptor found in tissues such as the kidney and adrenal gland, resulting in marked elevations of the intracellular signaling molecule, cGMP. Here, GC-A is shown to exist as a phosphoprotein when expressed in human embryonic 293 cells. The 32P is principally associated with phosphoserine, with only trace amounts of phosphothreonine. The addition of ANP causes a time-dependent dephosphorylation of the receptor, as well as desensitization, which is not due to an ANP-mediated decrease in the amount of receptor protein. The mobility of GC-A on sodium dodecyl sulfate-polyacrylamide gel electrophoresis increases after treatment of cells with ANP, and protein phosphatase 2A induces the same mobility shift. The protein phosphatase also catalyzes dephosphorylation of GC-A, and this is directly correlated with decreases in ANP-stimulatable guanylyl cyclase activity. Okadaic acid, an inhibitor of protein phosphatase 2A, blocks both the dephosphorylation and the desensitization. Therefore, in contrast to many other cell surface receptors, GC-A is desensitized by ligand-induced dephosphorylation.

A new member of the natriuretic peptide family is present in the venom of the green mamba (Dendroaspis angusticeps)

This paper describes the purification, sequence, and biological properties of a 38-amino acid residue peptide from the venom of Dendroaspis angusticeps which shared important sequence homologies with natriuretic peptides. Dendroaspis natriuretic peptide (DNP) relaxed aortic strips that had been contracted by 40 mM KCl with a potency (K0.5 = 20 nM) similar to that of atrial natriuretic peptide (ANP) and larger than that of C type natriuretic peptide (CNP). The relaxing actions of ANP and DNP (both at 100 nM) were mutually exclusive. Bovine aortic endothelial cells responded to ANP (K0.5 = 3 nM) and DNP (K0.5 = 3 nM) but not to CNP by a large activation of guanylate cyclase. Rat aortic myocytes showed larger cGMP responses to CNP (K0.5 = 10 nM) than to ANP or DNP (K0.5 = 100 nM). Finally, DNP completely prevented the specific 125I-ANP binding to clearance receptors in cultured aortic myocytes with a potency (Kd = 10 nM) that was less than that of ANP (Kd = 0.3 nM). It is concluded that DNP is a new member of the family of natriuretic peptides and that it recognizes ANPA receptors and clearance, ANPc receptors, but not CNP-specific ANPB receptors.

Characterization of ANF-R2 receptor-mediated inhibition of adenylate cyclase

We have characterized the ANF-R2 receptor-mediated inhibition of adenylate cyclase with respect to its modulation by several regulators. ANF (99-126) inhibits adenylate cyclase activity only in the presence of guanine nucleotides. The maximal inhibition (approximately 45%) was observed in the presence of 10-30 microM GTP gamma S, and at higher concentrations, the inhibitory effect of ANF was completely abolished. ANF-mediated inhibition was not dependent on the presence of monovalent cations, however Na+ enhanced the degree of inhibition by about 60%, whereas K+ and Li+ suppressed the extent of inhibition by about 50%. On the other hand, divalent cation, such as Mn2+ decreased the degree of inhibition in a concentration dependent manner, with an apparent Ki of about 0.7 mM, and at 2 mM; the inhibition was completely abolished. In addition, proteolytic digestion of the membranes with trypsin (40 ng/ml) resulted in the attenuation of ANF-mediated inhibition of adenylate cyclase. Other membrane disrupting agents such as neuraminidase and phospholipase A2 treatments also inhibited completely, the ANF-mediated inhibition of enzyme activity. N-Ethylmaleimide (NEM), phorbol ester and Ca(2+)-phospholipid dependent protein kinase (C-kinase) which have been shown to interact with inhibitory guanine nucleotide regulating protein (Gi) also resulted in the attenuation of ANF-mediated inhibition of adenylate cyclase activity. These results indicate that in addition to the Gi, the phospholipids and glycoproteins may also play an important role in the expression of ANF-R2 receptor-mediated inhibition of adenylate cyclase.

Renal receptors for atrial and C-type natriuretic peptides in the rat

Receptors for alpha-atrial natriuretic peptide (alpha-ANP) and C-type natriuretic peptide [CNP-(1-22)] were quantified in kidneys from adult Wistar rats by in vitro autoradiography. 125I-labeled alpha-ANP (100 pM) bound reversibly to glomeruli, outer medullary vasa recta, and inner medulla with an apparent dissociation constant (Kd) of 3–6 nM. The presence of 10 microM des-[Gln18,Ser19,Gly20,Leu21,Gly22]ANP-(4– 23) (C-ANP), a specific ligand of the ANPR-C subtype of alpha-ANP receptor, inhibited approximately 50% of the glomerular binding of 125I-alpha-ANP, and this moiety of glomerular binding was also inhibited by CNP-(1–22) with an apparent inhibitory constant (Ki) of 10.47 +/- 7.59 nM. C-ANP and CNP-(1–22) showed little affinity for the medullary binding sites of alpha-ANP. 125I-[Tyr0]CNP-(1–22) (110 pM) bound solely to glomeruli and was competitively displaced by increasing concentrations of [Tyr0]CNP-(1–22) with an apparent Kd of 1.42 +/- 0.48 nM. Binding of increasing concentrations (25 pM to 1 nM) of 125I-[Tyr0]CNP-(1–22) in the presence or absence of 1 microM [Tyr0]CNP-(1–22) also demonstrated a high affinity (Kd of 0.41 +/- 0.07 nM) for the glomerular binding of 125I-[Tyr0]CNP-(1–22). Bound 125I-[Tyr0]CNP-(1–22) could be displaced by excess alpha-ANP and excess CNP-(1–22), both with high affinities. The glomerular binding of 125I-[Tyr0]CNP-(1–22) was also prevented by 10 microM C-ANP. Guanosine 3',5'-cyclic monophosphate produced by isolated glomeruli was measured by radioimmunoassay.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal receptors and effects of atrial natriuretic factor in compensatory renal hypertrophy

In the present study we investigated the in vivo and in vitro renal responsiveness to ANF, and the adaptation of ANF receptors in compensatory renal hypertrophy in the rat. One week after left nephrectomy (UNx), plasma levels of immunoreactive ANF, blood pressure (MAP), hematocrit (Hct), and urine flow rate (V) were unaltered compared to control (C) rats. Baseline GFR and potassium excretion (UKV) were significantly higher, and sodium excretion (UNaV) tended to be elevated in UNx rats. Administered ANF led to similar dose-related decreases in MAP and increases in Hct in UNx and C rats. However, at each dose of infused ANF, absolute values and the increase in GFR and UNaV were higher in UNx than in C rats. Hypertrophied (H) kidneys were removed from UNx and perfused in vitro to determine distribution and density of ANF receptors, responsiveness to ANF, and receptor-mediated organ clearance of 125I-ANF1-28. The density of ANF receptors in cortex, outer medulla, and papilla of H kidneys was not significantly different from that in C kidneys. In H isolated kidneys, ANF led to dose-related increases in GFR, V, UNaV, and UKV that were indistinguishable (P greater than 0.05) from those in C kidneys. Receptor-mediated organ clearance of 125I-ANF1-28 in isolated H kidneys was 2.8 +/- .02 ml/min, a value not significantly different (P greater than 0.05) from that in C kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)

Homology modelling of integrin EF-hands. Evidence for widespread use of a conserved cation-binding site

Integrin alpha-subunits contain three or four peptide sequences that are similar to the EF-hand, a 13-residue bivalent cation-binding motif found in calmodulin and parvalbumin. The integrin sequences differ from classical EF-hands in that they lack a co-ordinating residue at position 12. One hypothesis to explain integrin-ligand binding is that aspartate-containing recognition sequences in integrin ligands, which bind at or near to the EF-hand-like sequences, may take the place of the missing residue and co-ordinate directly to the bound cation. In this report, homology modelling of integrin EF-hand-like sequences has been performed using the X-ray structure of calmodulin as a template in order to assess the functional activity of the integrin sequences. In the calmodulin-integrin hybrid structures, integrin EF-hand-like sequences were able to retain cations whereas control sequences did not. Structural analyses demonstrated that the integrin sequences in the hybrid proteins closely resembled conventional EF-hands. The integrin sequences are therefore highly likely to bind Ca2+ ions in vivo, a prerequisite for the ligand-binding model. Database searching with a matrix derived from known integrin EF-hand-like sequences has been used to identify other proteins containing the integrin EF-hand-like motif. Annexin V (anchorin CII), atrial natriuretic peptide receptors and the 70 kDa heat-shock protein were identified by the matrix; the functions of these proteins are known from previous studies to be bivalent cation-dependent. These findings suggest that the integrin EF-hand-like sequence may be a more common motif than originally thought.

Phenotype-related alteration in expression of natriuretic peptide receptors in aortic smooth muscle cells

To elucidate the physiological and pathophysiological roles of the natriuretic peptide family in vascular smooth muscle cells, in which the natriuretic peptide family is implicated in growth inhibition as well as vasorelaxation, we have examined the phenotype-related expression of three kinds of natriuretic peptide receptors in rat aortic smooth muscle cells. The expression of natriuretic peptide receptors at the mRNA level was studied by Northern blot hybridization, and the expression at the protein level was determined by the cGMP production method and receptor binding assay. In intact aortic media, atrial natriuretic peptide (ANP)-A receptor mRNA and ANP-B receptor mRNA were detected, and the potency of cGMP production by ANP was at least two orders of magnitude stronger than that by C-type natriuretic peptide. Clearance receptor mRNA was undetectable, and only a small amount of the clearance receptor was detected by the binding assay in intact aortic media. By contrast, in cultured aortic smooth muscle cells at the first, fifth, and 17th passages, the ANP-B receptor mRNA level markedly increased; meanwhile, the expression of the ANP-A receptor mRNA became undetectable. C-type natriuretic peptide was one order of magnitude more potent than ANP in cGMP production in cultured aortic smooth muscle cells. The clearance receptor density and its mRNA level increased tremendously in these cultured cells. These results demonstrate that the marked phenotype-related alteration occurs in the expression of natriuretic peptide receptors in rat aortic smooth muscle cells.

Inhibitory effects of ATP and other nucleotides on atrial natriuretic peptide (ANP) binding to R1-type ANP receptors in human neuroblastoma NB-OK-1 cell membranes

ATP dose-dependently inhibited rat 125I-ANP-(99-126) binding to membranes from the human neuroblastoma cell line NB-OK-1 by increasing the KD value for the hormone without altering the Bmax value. After a 20 min preincubation with 37.5 pM 125I-ANP-(99-126) and 0.5 mM ATP, followed by the addition of 0.3 microM unlabelled ANP-(99-126), the proportion of rapidly dissociating receptors was 4-times higher than in the absence of ATP. The other nucleotides ADP, AMP, AMP-PNP, ATP gamma S, GTP, GDP, GMP, GMP-PNP and GTP gamma S were also inhibitory but with a lower potency and/or efficacy. Binding equilibrium data were satisfactorily simulated by a computer program based on partially competitive binding of ANP-(99-126) and the nucleotides, and this, together with the data on dissociation kinetics, strongly suggests that several nucleotides, when added at concentrations up to 1 mM, form a ternary ANP-receptor-nucleotide complex.

Conservation of the kinaselike regulatory domain is essential for activation of the natriuretic peptide receptor guanylyl cyclases

The natriuretic peptide receptors, NPR-A and NPR-B, are two members of the newly described class of receptor guanylyl cyclases. The kinaselike domain of these proteins is an important regulator of the guanylyl cyclase activity. To begin to understand the molecular nature of this type of regulation, we made complete and partial deletions of the kinase domain in NPR-A and NPR-B. We also made chimeric proteins in which the kinase domains of NPR-A and NPR-B were exchanged or replaced with kinase domains from structurally similar proteins. Complete deletion of the kinase homology domain in NPR-A and NPR-B resulted in constitutive activation of the guanylyl cyclase. Various partial deletions of this region produced proteins that had no ability to activate the enzyme with or without hormone stimulation. The kinase homology domain can be exchanged between the two subtypes with no effect on regulation. However, structurally similar kinaselike domains, such as from the epidermal growth factor receptor or from the heat-stable enterotoxin receptor, another member of the receptor guanylyl cyclase family, were not able to regulate the guanylyl cyclase activity correctly. These findings suggest that the kinaselike domain of NPR-A and NPR-B requires strict sequence conservation to maintain proper regulation of their guanylyl cyclase activity.

A ring-reversed analog of atrial natriuretic peptide retains receptor binding, guanylate cyclase stimulation

We have prepared an atrial natriuretic peptide analog, ANP[13-27][1-12], in which the connectivity of the disulfide-linked ring has been reversed by formally cleaving the ring and cyclizing the N- and C-terminal tails. This analog, which retains many of the spatial relationships of the native molecule, binds to both ANP-A and ANP-C receptor subtypes, and triggers the production of cyclic-GMP by ANP-A. ANP-C binding of ANP[13-27][1- 12] is roughly equipotent to that of ANP itself, although the ring cleavage falls within the putative ANP-C binding domain. ANP[13-27][1-8], a truncated analog in which much of this binding domain has been removed, surprisingly maintains a high affinity for ANP-C; however, this peptide has lost the ability to activate the ANP-A-linked guanylate cyclase.

Dynamics of atrial natriuretic factor-guanylate cyclase receptors and receptor-ligand complexes in cultured glomerular mesangial and renomedullary interstitial cells

The dynamics of the guanylate cyclase receptor of atrial natriuretic factor (GCA-ANF receptor) were investigated in cultured glomerular mesangial and renomedullary interstitial cells from the rat. In these cells, the GCA-ANF receptor did not mediate internalization and lysosomal hydrolysis of 125I-ANF1-28 and did not undergo ligand-induced endocytosis. Glomerular mesangial cells were able, however, to mediate internalization and lysosomal hydrolysis of 125I-ANF1-28 via clearance ANF (C-ANF) receptors and to promote rapid receptor-mediated internalization and lysosomal hydrolysis of 125I-(Sar1) angiotensin II. Radioligand specifically bound to surface GCA-ANF receptors was rapidly dissociated at 37 degrees C (k(off) greater than 0.8 min-1), with a Q10(30-37 degrees C) greater than 6. The dissociation was markedly slower at subphysiological temperatures (Q10(4-30 degrees C), 2-3) or in the presence of 0.5 mM amiloride. The results demonstrate that the GCA-ANF receptor, contrary to C-ANF receptors and most other polypeptide hormone receptors, is a membrane resident protein that does not mediate internalization and lysosomal hydrolysis of ligand. The termination of the interaction of ANF with GCA-ANF receptors results from a physiological process that leads to rapid dissociation of receptor-ligand complexes. The unique dynamics of GCA-ANF receptor-ligand complexes are likely to contribute importantly to stimulus-response homeostasis of ANF.

Characterization of natriuretic peptide receptors in cultured cells

To elucidate physiological and clinical implications of the natriuretic peptide family, the expression of receptors for natriuretic peptides has been examined in cultured cells (a rat pheochromocytoma cell line [PC12], bovine endothelial cells, rat aortic smooth muscle cells, human mesangial cells, and a porcine kidney epithelial cell line [LLC-PK1]) by Northern blot analysis and cyclic GMP production method for the ANP-A and ANP-B receptors and by Northern blot analysis and binding assay for the clearance receptor. The ANP-A receptor was predominantly expressed in PC12 cells, bovine endothelial cells, and LLC-PK1 cells but was barely expressed in rat aortic smooth muscle cells and human mesangial cells. By contrast, the ANP-B receptor was the major subtype of the biologically active receptors in rat aortic smooth muscle cells and human mesangial cells. Only a small amount of the ANP-B receptor was detected in PC12 cells, bovine endothelial cells, and LLC-PK1 cells. The clearance receptor was abundantly expressed in rat aortic smooth muscle cells and human mesangial cells and was also present in bovine endothelial cells, but it was undetectable in PC12 cells and LLC-PK1 cells. These results demonstrate that the expression of three natriuretic peptide receptors varies from cell to cell, which is relevant to cell- or tissue-specific action of the natriuretic peptide family.

Atrial natriuretic peptide and urinary cyclic guanosine monophosphate in patients with chronic heart failure

Circulating concentrations of human atrial natriuretic peptide (hANP) are elevated in patients with heart failure; however, the natriuretic effect of hANP is blunted in these patients. In this study, the relationship between urinary cGMP, the second messenger for the natriuretic effect of hANP in vivo, and endogenous hANP was examined in six patients with heart failure and four normal subjects. In addition, right heart catheterization for the determination of central hemodynamics was performed in the heart failure patients. The heart failure patients were in New York Heart Association Classes II to IV and were receiving no medications at the time of the study. Supine plasma hANP and urinary cGMP concentrations were determined on two occasions in each subject, as were right and left atrial pressures in the heart failure patients. At the time of study, the patients were in positive sodium balance, and control subjects were in normal sodium balance. Plasma hANP and urinary cGMP excretion rates were elevated in heart failure patients as compared with those in controls: hANP, 139.0 +/- 42.0 versus 22.0 +/- 6.1 pg/mL (P less than 0.05); urinary cGMP, 1.14 +/- 0.31 versus 0.35 +/- 0.05 nmol/min (P less than 0.05). In heart failure patients, right atrial pressure correlated positively with plasma hANP (r = 0.96; P less than 0.01) and urinary cGMP concentrations (r = 0.93; P less than 0.05) and the excretion rate (r = 0.92; P less than 0.05). Moreover, plasma hANP was strongly correlated with urinary cGMP concentration (r = 0.91; P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide clearance receptors in trout: effects of receptor inhibition in vivo

Inactivation of circulating atrial natriuretic peptides (ANP) by specialized clearance (C) receptors has been characterized in mammals but has not been examined in fish. In the present study arterial blood pressure, urine flow, and urine electrolytes were measured in chronically cannulated rainbow trout, Oncorhynchus mykiss, during infusion of the specific C receptor inhibitor, SC-46542. C receptor inhibition decreased blood pressure and pulse pressure, increased heart rate and urine flow, but did not affect urinary electrolyte concentrations. These responses are consistent with those produced by exogenous ANP administration and indicate that: (1) trout possess C-type receptors capable of ANP inactivation, and (2) ANP-like molecules are continuously released and metabolized by trout in vivo. Phosphoramidon, an inhibitor of neutral endopeptidase, did not enhance the SC-46542 response, indicating that C receptors predominate in ANP inactivation in these fish.

Clearance of brain natriuretic peptide in patients with chronic heart failure: indirect evidence for a neutral endopeptidase mechanism but against an atrial natriuretic peptide clearance receptor mechanism

1. Brain natriuretic peptide is a new natriuretic hormone with striking similarity to atrial natriuretic peptide, but there are no previous data concerning its clearance in man. Two pathways of clearance for atrial natriuretic peptide are recognized: degradation by neutral endopeptidase and binding to atrial natriuretic peptide clearance receptors. We have examined the effect of candoxatril, an inhibitor of neutral endopeptidase (dose range 10-200 mg), and the effect of an infusion of a pharmacological dose [45 micrograms (90 micrograms in two patients)] of synthetic human atrial natriuretic peptide on plasma human brain natriuretic peptide-like immunoreactivity levels in seven patients with mild to moderate chronic heart failure. 2. Plasma human brain natriuretic peptide-like immunoreactivity levels were elevated in all patients (mean +/- SEM 22.0 +/- 6.2 pmol/l) compared with healthy control subjects (1.3 +/- 0.2 pmol/l, n = 11). 3. In all patients, candoxatril increased both plasma atrial natriuretic peptide (P less than 0.05) and plasma human brain natriuretic peptide-like immunoreactivity (P less than 0.05) levels. 4. By contrast, an exogenous infusion of atrial natriuretic peptide had no effect on plasma human brain natriuretic peptide-like immunoreactivity levels despite increasing the plasma atrial natriuretic peptide concentration to 424 +/- 74 pmol/l, which is a level of atrial natriuretic peptide which would have 'swamped' all atrial natriuretic peptide clearance receptors. 5. We have therefore shown that plasma human brain natriuretic peptide-like immunoreactivity levels in chronic heart failure are increased by a neutral endopeptidase inhibitor, but are unchanged by an exogenous infusion of atrial natriuretic peptide.(ABSTRACT TRUNCATED AT 250 WORDS)

Highly efficient photoaffinity labeling of the hormone binding domain of atrial natriuretic factor receptor

A high-efficiency photoaffinity derivative of atrial natriuretic factor (ANF) was developed for studying the peptide binding domain of the receptor protein and for better characterization of this receptor in tissues with a low density of binding sites. The position of the photosensitive residue was chosen on the basis of a molecular conformational model and on structure-activity relationship studies which both indicate that the carboxy-terminal end of the peptide is part of a hydrophobic pole likely to interact deeply within the ANF binding pocket of the receptor. Selection of the photoreactive residue p-benzoylphenylalanine (BPA) as a substitute for arginine in position 125 of the peptide sequence led to a photoaffinity derivative with a high (63%) efficiency of covalent incorporation to the receptor protein. This derivative (BPA-ANF) has a 10-fold lower affinity when compared with ANF, but it is a full agonist in stimulating cGMP production and inhibiting aldosterone secretion in bovine adrenal zona glomerulosa. Photoaffinity labeling with BPA-ANF specifically identifies ANF-R1 and ANF-R2 receptor proteins with a 10-fold higher efficiency than with azido derivatives of ANF or with cross-linking agents. This new ANF derivative therefore appears to be useful for studying ANF receptors in tissues with low levels of expression, for locating receptor following cellular internalization, and for tagging proteolytic fragments of the receptor amenable to amino acid microsequencing.

Expression of receptors for atrial natriuretic peptide on the murine bone marrow-derived stromal cells

Atrial natriuretic peptide (ANP) receptors were identified on both murine bone marrow-derived stromal cell lines A-3 and ALC and primary cultured cells using [125I]ANP binding assays and Northern blot analyses. The binding of [125I] ANP to the stromal cells was rapid, saturable, and of high affinity. The dissociation constants between ANP and its receptors on these cells showed no difference among cell types, while maximal binding capacity values were different among cell types. Competitive inhibition of [125I]ANP binding with C-atrial natriuretic factor, specific for ANP clearance receptor (ANPR-C), revealed that most of [125I]ANP-binding sites corresponded to ANPR-C. Northern blotting data corroborated that bone marrow-derived stromal cells expressed ANPR-C. However, in ALC cells, ANP biological receptors (either ANPR-A or ANPR-B), the mol wt of which is approximately 130K, were detected, and cGMP was accumulated after stimulation with ANP. On the other hand, in another stromal cell clone, A-3 cells, the expression of biological receptor was not detected in the affinity cross-linking and competitive inhibition experiments using [125I]ANP. However, A-3 cells accumulated cGMP by responding to ANPR-B-specific ligand, C-type natriuretic peptide. These results suggest that ALC cells equally express ANPR-A and ANPR-B, while A-3 cells express ANPR-B dominantly. Although the physiological roles of these receptors in the bone marrow is still not resolved, ANP is expected to play a role in the regulation of stromal cell functions in bone marrow.

Atrial natriuretic factor: its (patho)physiological significance in humans

The first human studies using relatively high-doses of ANF revealed similar effects as observed in the preceding animal reports, including effects on systemic vasculature (blood pressure fall, decrease in intravascular volume), renal vasculature (rise in GFR, fall in renal blood flow), renal electrolyte excretion (rises in many electrolytes), and changes in release of a number of different hormones. Whether all these changes are the result of direct ANF effects or secondary to a (single) primary event of the hormone remains to be determined. Certainly, it has been proven that more physiological doses of ANF fail to induce short-term changes in many of these parameters leaving only a rise in hematocrit, natriuresis and an inhibition of the RAAS as important detectable ANF effects in humans. This leads us to hypothesize that ANF is a "natriuretic" hormone with physiological significance. The primary function in humans is to regulate sodium homeostasis in response to changes in intravascular volume (cardiac atrial stretch). Induction of excess renal sodium excretion and extracellular volume shift appear to be the effector mechanisms. The exact mechanism of the natriuresis in humans still needs to be resolved. It appears however, that possibly a small rise in GFR, a reduction in proximal and distal tubular sodium reabsorption, as well as an ensuing medullary washout, are of importance. The pathophysiological role of ANF in human disease is unclear. One may find elevated plasma irANF levels and/or decreased responses to exogenous ANF in some disease states. Whether these findings are secondary to the disease state rather than the cause of the disease remains to be resolved. Therapeutic applications for ANF, or drugs that intervene in its production or receptor-binding, seem to be multiple. Most important could be the antihypertensive effect, although areas such as congestive heart failure, renal failure, liver cirrhosis and the nephrotic syndrome cannot be excluded. Although the data that have been gathered to date allowed us to draw some careful conclusions as to the (patho)physiological role of ANF, the exact place of ANF in sodium homeostatic control must still be better defined. To achieve this, we will need more carefully designed low-dose ANF infusion, as well as ANF-breakdown inhibitor studies. Even more promising, however, is the potential area of studies open to us when ANF-receptor (ant)agonists become available for human use.

Atrial natriuretic peptide receptor subtypes in rat neuronal and astrocyte glial cultures

We have compared the levels and subtypes of atrial natriuretic peptide (ANP) receptors in astrocyte glial and neuronal cultures prepared from the hypothalamus and brain stem of 1-day-old rats. Astrocyte glial cultures contain approximately twice the number of ANP receptors, as measured by 125I-ANP specific binding, compared with neuronal cultures. Rat ANP-(99-126), rat brain natriuretic peptide (BNP32), C-type natriuretic peptide (CNP-22), atriopeptin I, and [des-Gln18,Ser19,Gly20,Leu21, Gly22]atrial natriuretic factor-(4-23)-NH2[C-ANF-(4-23)] all competed strongly for 125I-ANP binding in both culture types, with inhibitory constant values ranging from 0.47 to 8.07 nM. The presence of ANP-C receptors (clearance type) in both cell types is indicated from the strong competition of 125I-ANP specific binding by C-ANF-(4-23). The potency profiles for stimulation of guanosine 3',5'-cyclic monophosphate levels by these peptides were ANP = BNP much greater than CNP-22 greater than atriopeptin I in astrocyte glia and CNP-22 much greater than BNP32 greater than ANP greater than atriopeptin I in neuronal cultures. These results indicate that both types of culture contain guanylate cyclase-coupled ANP receptors, with astrocytes containing predominantly the ANP-A subtype and neurons predominantly the ANP-B subtype.

Modulation by NaCl of atrial natriuretic peptide receptor levels and cyclic GMP responsiveness to atrial natriuretic peptide of cultured vascular endothelial cells

Type C atrial natriuretic peptide (ANP) receptor levels in cultured vascular endothelial cells were found to be very sensitive to NaCl and shown to be inversely related to the magnitude of ANP-induced cGMP response of the cells. Endothelial cells from bovine carotid artery were subcultured in Eagle's minimum essential medium supplemented with 10% fetal bovine serum (MEM-FBS) and in MEM-FBS plus 25 and 50 mM NaCl. Determination, after several passages, of ANP receptor levels in these cells by 125I-ANP binding assay and affinity labeling revealed a marked reduction in the number of type C receptor in the NaCl-treated cells, whereas type A receptor density was not affected. RNase protection assay to estimate the levels of type C receptor mRNA indicated that the reduction occurred at a pre-translational level. In spite of the decrease in type C receptor number and no significant change in type A receptor (i.e. particulate guanylate cyclase) levels, cGMP response of the NaCl-treated cells to ANP was greatly exaggerated; this sensitization was also observed in membrane preparations. Simple masking of type C ANP receptor with C-ANF (des-[Gln18,Ser19,Gly20,Leu21,Gly22]ANP), a ring-deleted ANP analog, did not produce any sensitization of the cGMP response to ANP; therefore, the above phenomenon cannot simply be explained by the clearance function of the type C receptor. Although whether the type C receptor depletion is directly related to the sensitization of the type A receptor/cyclase is not known, the phenomenon reported and characterized here will serve as a useful basis for elucidating ANP receptor regulation and activation.

Atrial natriuretic peptide in portal vein-ligated rats: alterations in cardiac production, plasma level and glomerular receptor density and affinity

The atrial natriuretic peptide hormonal system is altered to a variable degree in patients with cirrhosis. Portal pressure and portal-systemic shunting are also varied in cirrhosis. We used a portal vein-ligated rat model with predictable portal hypertension to study the effects of portal hypertension alone on the atrial natriuretic peptide hormonal system. Sham-operated rats were used as controls. Mean portal pressure was significantly increased in portal vein-ligated rats (portal vein-ligated rats, 21.7 +/- 0.74 cm H2O; sham-operated rats, 13.7 +/- 0.47 cm H2O; p less than 0.0001). Plasma atrial natriuretic peptide decreased 50% in the portal vein-ligated rats (p less than 0.0001). Atrial natriuretic peptide messenger RNA level was decreased by 40% to 60% in the left and right atria and in the ventricles of portal vein-ligated rats (p less than 0.05 for each chamber). Only one class of glomerular binding site was identified by competitive binding studies. The atrial natriuretic peptide glomerular receptor density increased in the portal vein-ligated rats (portal vein-ligated rats, 1,660 +/- 393; sham-operated 725 +/- 147 fmol/mg protein, p less than 0.02), whereas affinity decreased (portal vein-ligated, 1.69 +/- 0.49; sham-operated, 0.55 +/- 0.12 nmol/L, p less than 0.02). No difference was seen in the amount of cyclic GMP generated by atrial natriuretic peptide stimulation in isolated glomeruli from portal vein-ligated and sham-operated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell surface receptors and ectoenzymes in mesangial cells

Mesangial cells possess a variety of receptors for hormones and autacoids. They are also equipped with ectoenzymes whose function may be to control the availability of autacoids and hormones at their receptor sites. Several examples are considered. Receptors for angiotensin II (AII) are present both on murine and human mesangial cells. One single group of receptors has been demonstrated in each of these preparations. Mesangial cell AII receptors are linked to phospholipase C via a G protein. They belong to the AT1 subtype because (125I)AII is displaced from its binding sites preferentially by AT1 antagonists such as DUP 753 and EXP 3,174, whereas AT2 antagonists are much less potent. AT1 antagonists suppress the biological effects of AII in mesangial cells, including the stimulation of intracellular calcium concentration and the increase of prostaglandin synthesis and of (3H)leucine incorporation. Mesangial cells also have receptors for atrial natriuretic factor, but the distribution between B receptors with guanylate cyclase activity and clearance (C) receptors varies with the species. Both types are present in murine mesangial cells, whereas only C receptors are found in human mesangial cells. In contrast, human epithelial cells possess both B and C receptors. Ecto-5'-nucleotidase activity results in the production of adenosine, which acts on mesangial cells through A1 and A2 receptors. This enzyme is markedly induced in rat mesangial cells by interleukin-1, whose effect is mediated in part by prostaglandin E2 and cAMP. Various other cAMP-stimulating agents also induce 5'-nucleotidase expression in rat mesangial cells. Ectopeptidases are present in all glomerular cell types but essentially in epithelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic factor in experimental cirrhosis in rats

Atrial natriuretic factor (ANF) is a cardiac hormone with potent natriuretic, diuretic, and vasorelaxant properties. Although abnormalities in ANF release, plasma level, and renal receptors have been described in humans and/or animals with cirrhosis and ascites, little is known about the ANF hormonal system in cirrhosis without ascites. The aim of this study was to examine the ANF hormonal system in an animal model of cirrhosis to determine whether compensated cirrhosis is associated with changes in the ANF hormonal system. Pair-fed rats were studied 5-7 weeks after either sham surgery or bile duct ligation. Bile duct-ligated (BDL) rats had elevated portal pressure and cirrhosis but did not have ascites. ANF messenger RNA levels were increased onefold in the atria of BDL rats. Sham-operated and BDL rats had similar plasma ANF levels. Competitive binding inhibition studies of isolated glomeruli showed a single class of receptors in sham-operated and BDL rats. The equilibrium dissociation constant was similar in sham-operated (0.51 nmol/L) and BDL (0.63 nmol/L) rats. Glomerular ANF receptor density increased significantly in BDL rats. Cyclic guanosine monophosphate generation in isolated glomeruli in response to 100 nmol/L ANF decreased slightly but not significantly in BDL rats. It was concluded that the ANF hormonal system is altered in cirrhosis without ascites; atrial messenger RNA level and glomerular ANF receptor density are increased.