Endothelin-3 reduces C-type natriuretic peptide-induced cyclic GMP formation in C6 glioma cells

Phosphorylation-Dependent Regulation of Guanylyl Cyclase (GC)-A and Other Membrane GC Receptors

Receptor guanylyl cyclases (GCs) are single membrane spanning, multidomain enzymes, that synthesize cGMP in response to natriuretic peptides or other ligands. They are evolutionarily conserved from sea urchins to humans and regulate diverse physiologies. Most family members are phosphorylated on 4 to 7 conserved serines or threonines at the beginning of their kinase homology domains. This review describes studies that demonstrate that phosphorylation and dephosphorylation are required for activation and inactivation of these enzymes, respectively. Phosphorylation sites in GC-A, GC-B, GC-E, and sea urchin receptors are discussed, as are mutant receptors that mimic the dephosphorylated inactive or phosphorylated active forms of GC-A and GC-B, respectively. A salt bridge model is described that explains why phosphorylation is required for enzyme activation. Potential kinases, phosphatases, and ATP regulation of GC receptors are also discussed. Critically, knock-in mice with glutamate substitutions for receptor phosphorylation sites are described. The inability of opposing signaling pathways to inhibit cGMP synthesis in mice where GC-A or GC-B cannot be dephosphorylated demonstrates the necessity of receptor dephosphorylation in vivo. Cardiac hypertrophy, oocyte meiosis, long-bone growth/achondroplasia, and bone density are regulated by GC phosphorylation, but additional processes are likely to be identified in the future.

Sensitivity of the Natriuretic Peptide/cGMP System to Hyperammonaemia in Rat C6 Glioma Cells and GPNT Brain Endothelial Cells

C-type natriuretic peptide (CNP) is the major natriuretic peptide of the central nervous system and acts via its selective guanylyl cyclase-B (GC-B) receptor to regulate cGMP production in neurons, astrocytes and endothelial cells. CNP is implicated in the regulation of neurogenesis, axonal bifurcation, as well as learning and memory. Several neurological disorders result in toxic concentrations of ammonia (hyperammonaemia), which can adversely affect astrocyte function. However, the relationship between CNP and hyperammonaemia is poorly understood. Here, we examine the molecular and pharmacological control of CNP in rat C6 glioma cells and rat GPNT brain endothelial cells, under conditions of hyperammonaemia. Concentration-dependent inhibition of C6 glioma cell proliferation by hyperammonaemia was unaffected by CNP co-treatment. Furthermore, hyperammonaemia pre-treatment (for 1 h and 24 h) caused a significant inhibition in subsequent CNP-stimulated cGMP accumulation in both C6 and GPNT cells, whereas nitric-oxide-dependent cGMP accumulation was not affected. CNP-stimulated cGMP efflux from C6 glioma cells was significantly reduced under conditions of hyperammonaemia, potentially via a mechanism involving changed in phosphodiesterase expression. Hyperammonaemia-stimulated ROS production was unaffected by CNP but enhanced by a nitric oxide donor in C6 cells. Extracellular vesicle production from C6 cells was enhanced by hyperammonaemia, and these vesicles caused impaired CNP-stimulated cGMP signalling in GPNT cells. Collectively, these data demonstrate functional interaction between CNP signalling and hyperammonaemia in C6 glioma and GPNT cells, but the exact mechanisms remain to be established.

Mutations in Tyr808 reveal a potential auto-inhibitory mechanism of guanylate cyclase-B regulation

In this study, Tyr⁸⁰⁸ in GC-B (guanylate cyclase-B), a receptor of the CNP (C-type natriuretic peptide), has been shown to be a critical regulator of GC-B activity. In searching for phosphorylation sites that could account for suppression of GC-B activity by S1P (sphingosine-1-phosphate), mutations were introduced into several candidate serine/threonine and tyrosine residues. Although no novel phosphorylation sites that influenced the suppression of GC-B were identified, experiments revealed that mutations in Tyr⁸⁰⁸ markedly enhanced GC-B activity. CNP-stimulated activities of the Y808F and Y808A mutants were greater than 30-fold and 70-fold higher, respectively, than that of WT (wild-type) GC-B. The Y808E and Y808S mutants were constitutively active, expressing 270-fold higher activity without CNP stimulation than WT GC-B. Those mutations also influenced the sensitivity of GC-B to a variety of inhibitors, including S1P, Na₃VO₄ and PMA. Y808A, Y808E and Y808S mutations markedly weakened S1P- and Na₃VO₄-dependent suppression of GC-B activity, whereas Y808E and Y808S mutations rather elevated cGMP production. Tyr⁸⁰⁸ is conserved in all membrane-bound GCs and located in the niche domain showing sequence similarity to a partial fragment of the HNOBA (haem nitric oxide binding associated) domain, which is found in soluble GC and in bacterial haem-binding kinases. This finding provides new insight into the activation mechanism of GCs.

Phosphorylation-dependent regulation of the guanylyl cyclase-linked natriuretic peptide receptors

Natriuretic peptides are a family of hormones/paracrine factors that regulate blood pressure, cardiovascular homeostasis and bone growth. The mammalian family consists of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP). A family of three cell surface receptors mediates their physiologic effects. Two are receptor guanylyl cyclases known as NPR-A/GC-A and NPR-B/GC-B. Peptide binding to these enzymes stimulates the synthesis of the intracellular second messenger, cGMP, whereas a third receptor, NPR-C, lacks enzymatic activity and functions primarily as a clearance receptor. Here, we provide a brief review of how various desensitizing agents and/or conditions inhibit NPR-A and NPR-B by decreasing their phosphorylation state.

Lysophosphatidic acid inhibits C-type natriuretic peptide activation of guanylyl cyclase-B

C-type natriuretic peptide (CNP), found in endothelial cells, chondrocytes, and neurons, binds its cognate transmembrane receptor, natriuretic peptide receptor-B (NPR-B/GC-B), and stimulates the synthesis of the intracellular signaling molecule, cGMP. The known physiologic consequences of this binding event are vasorelaxation, inhibition of cell proliferation, and the stimulation of long bone growth. Here we report that 10% fetal bovine serum markedly reduced CNP-dependent cGMP elevations in NIH3T3 fibroblast. The purified serum components platelet-derived growth factor and lysophosphatidic acid (LPA) mimicked the effect of serum on CNP-dependent cGMP elevations, but the latter factor resulted in the most dramatic reductions. The LPA-dependent inhibition was rapid and dose dependent, having t(1/2) and IC(50) values of approximately 5 min and 3.0 micro M LPA, respectively. The decreased cGMP concentrations resulted from reduced CNP-dependent NPR-B guanylyl cyclase activity that did not require losses in receptor protein or activation of protein kinase C, indicating a previously undescribed desensitization pathway. These data suggest that NPR-B is repressed by LPA and that one mechanism by which LPA exerts its effects is through the heterologous desensitization of the CNP/NPR-B/cGMP pathway. We hypothesize that cross-talk between the LPA and CNP signaling pathway maximizes the response of fibroblasts in the wound-healing process.

Vasopressin-dependent inhibition of the C-type natriuretic peptide receptor, NPR-B/GC-B, requires elevated intracellular calcium concentrations

Natriuretic peptides bind their cognate cell surface guanylyl cyclase receptors and elevate intracellular cGMP concentrations. In vascular smooth muscle cells, this results in the activation of the type I cGMP-dependent protein kinase and vasorelaxation. In contrast, pressor hormones like arginine-vasopressin, angiotensin II, and endothelin bind serpentine receptors that interact with G(q) and activate phospholipase Cbeta. The products of this enzyme, diacylglycerol and inositol trisphosphate, activate the conventional and novel forms of protein kinase C (PKC) and elevate intracellular calcium concentrations, respectively. The latter response results in vasoconstriction, which opposes the actions of natriuretic peptides. Previous reports have shown that pressor hormones inhibit natriuretic peptide receptors NPR-A or NPR-B in a variety of different cell types. Although the mechanism for this inhibition remains unknown, it has been universally accepted that PKC is an obligatory component of this pathway primarily because pharmacologic activators of PKC mimic the inhibitory effects of these hormones. Here, we show that in A10 vascular smooth muscle cells, neither chronic PKC down-regulation nor specific PKC inhibitors block the AVP-dependent desensitization of NPR-B even though both processes block PKC-dependent desensitization. In contrast, the cell-permeable calcium chelator, BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester), abrogates the AVP-dependent desensitization of NPR-B, and ionomycin, a calcium ionophore, mimics the AVP effect. These data show that the inositol trisphosphate/calcium arm of the phospholipase C pathway mediates the desensitization of a natriuretic peptide receptor in A10 cells. In addition, we report that CNP attenuates AVP-dependent elevations in intracellular calcium concentrations. Together, these data reveal a dominant role for intracellular calcium in the reciprocal regulation of these two important vasoactive signaling systems.

Activation of protein kinase C stimulates the dephosphorylation of natriuretic peptide receptor-B at a single serine residue: a possible mechanism of heterologous desensitization

The binding of atrial natriuretic peptide and C-type natriuretic peptide (CNP) to the guanylyl cyclase-linked natriuretic peptide receptors A and B (NPR-A and -B), respectively, stimulates increases in intracellular cGMP concentrations. The vasoactive peptides vasopressin, angiotensin II, and endothelin inhibit natriuretic peptide-dependent cGMP elevations by activating protein kinase C (PKC). Recently, we identified six in vivo phosphorylation sites for NPR-A and five sites for NPR-B and demonstrated that the phosphorylation of these sites is required for ligand-dependent receptor activation. Here, we show that phorbol 12-myristate 13-acetate, a direct activator of PKC, causes the dephosphorylation and desensitization of NPR-B. In contrast to the CNP-dependent desensitization process, which results in coordinate dephosphorylation of all five sites in the receptor, phorbol 12-myristate 13-acetate treatment causes the dephosphorylation of only one site, which we have identified as Ser(523). The conversion of this residue to alanine or glutamate did not reduce the amount of mature receptor protein as indicated by detergent-dependent guanylyl cyclase activities or Western blot analysis but completely blocked the ability of PKC to induce the dephosphorylation and desensitization of NPR-B. Thus, in contrast to previous reports suggesting that PKC directly phosphorylates and inhibits guanylyl cyclase-linked natriuretic peptide receptors, we show that PKC-dependent dephosphorylation of NPR-B at Ser(523) provides a possible molecular explanation for how pressor hormones inhibit CNP signaling.