Receptor phosphorylation does not mediate cross talk between muscarinic M(3) and bradykinin B(2) receptors

Astrocyte D-serine modulates the activation of neuronal NOS leading to the development of mechanical allodynia in peripheral neuropathy

Spinal D-serine plays an important role in nociception via an increase in phosphorylation of the N-Methyl-D-aspartate (NMDA) receptor GluN1 subunit (pGluN1). However, the cellular mechanisms underlying this process have not been elucidated. Here, we investigate the possible role of neuronal nitric oxide synthase (nNOS) in the D-serine-induced potentiation of NMDA receptor function and the induction of neuropathic pain in a chronic constriction injury (CCI) model. Intrathecal administration of the serine racemase inhibitor, L-serine O-sulfate potassium salt (LSOS) or the D-serine degrading enzyme, D-amino acid oxidase (DAAO) on post-operative days 0-3 significantly reduced the CCI-induced increase in nitric oxide (NO) levels and nicotinamide adenine dinucleotide phosphate-diaphorase staining in lumbar dorsal horn neurons, as well as the CCI-induced decrease in phosphorylation (Ser847) of nNOS (pnNOS) on day 3 post-CCI surgery. LSOS or DAAO administration suppressed the CCI-induced development of mechanical allodynia and protein kinase C (PKC)-dependent (Ser896) phosphorylation of GluN1 on day 3 post-surgery, which were reversed by the co-administration of the NO donor, 3-morpholinosydnonimine hydrochloride (SIN-1). In naïve mice, exogenous D-serine increased NO levels via decreases in pnNOS. D-serine-induced increases in mechanical hypersensitivity, NO levels, PKC-dependent pGluN1, and NMDA-induced spontaneous nociception were reduced by pretreatment with the nNOS inhibitor, 7-nitroindazole or with the NMDA receptor antagonists, 7-chlorokynurenic acid and MK-801. Collectively, we show that spinal D-serine modulates nNOS activity and concomitant NO production leading to increases in PKC-dependent pGluN1 and ultimately contributing to the induction of mechanical allodynia following peripheral nerve injury.

Vascular Kinin B1 and B2 Receptors Determine Endothelial Dysfunction through Neuronal Nitric Oxide Synthase

B₁- and B₂-kinin receptors are G protein-coupled receptors that play an important role in the vascular function. Therefore, the present study was designed to evaluate the participation of kinin receptors in the acetylcholine (ACh)-induced vascular relaxation, focusing on the protein-protein interaction involving kinin receptors with endothelial and neuronal nitric oxide synthases (eNOS and nNOS). Vascular reactivity, nitric oxide (NO·) and reactive oxygen species (ROS) generation, co-immunoprecipitation were assessed in thoracic aorta from male wild-type (WT), B₁- (B₁R^−/−), B₂- (B₂R^−/−) knockout mice. Some vascular reactivity experiments were also performed in a double kinin receptors knockout mice (B₁B₂R^−/−). For pharmacological studies, selective B₁- and B₂-kinin receptors antagonists, NOS inhibitors and superoxide dismutase (SOD) mimetic were used. First, we show that B₁- and B₂-kinin receptors form heteromers with nNOS and eNOS in thoracic aorta. To investigate the functionality of these protein-protein interactions, we took advantage of pharmacological tools and knockout mice. Importantly, our results show that kinin receptors regulate ACh-induced relaxation via nNOS signaling in thoracic aorta with no changes in NO· donor-induced relaxation. Interestingly, B₁B₂R^−/− presented similar level of vascular dysfunction as found in B₁R^−/− or B₂R^−/− mice. In accordance, aortic rings from B₁R^−/− or B₂R^−/− mice exhibit decreased NO· bioavailability and increased superoxide generation compared to WT mice, suggesting the involvement of excessive ROS generation in the endothelial dysfunction of B₁R^−/− and B₂R^−/− mice. Alongside, we show that impaired endothelial vasorelaxation induced by ACh in B₁R^−/− or B₂R^−/− mice was rescued by the SOD mimetic compound. Taken together, our findings show that B₁- and B₂-kinin receptors regulate the endothelium-dependent vasodilation of ACh through nNOS activity and indicate that molecular disturbance of short-range interaction between B₁- and B₂-kinin receptors with nNOS might be involved in the oxidative pathogenesis of endothelial dysfunction.

Connexin 43 hemichannels contribute to cytoplasmic Ca2+ oscillations by providing a bimodal Ca2+-dependent Ca2+ entry pathway

Many cellular functions are driven by changes in the intracellular Ca(2+) concentration ([Ca(2+)](i)) that are highly organized in time and space. Ca(2+) oscillations are particularly important in this respect and are based on positive and negative [Ca(2+)](i) feedback on inositol 1,4,5-trisphosphate receptors (InsP(3)Rs). Connexin hemichannels are Ca(2+)-permeable plasma membrane channels that are also controlled by [Ca(2+)](i). We aimed to investigate how hemichannels may contribute to Ca(2+) oscillations. Madin-Darby canine kidney cells expressing connexin-32 (Cx32) and Cx43 were exposed to bradykinin (BK) or ATP to induce Ca(2+) oscillations. BK-induced oscillations were rapidly (minutes) and reversibly inhibited by the connexin-mimetic peptides (32)Gap27/(43)Gap26, whereas ATP-induced oscillations were unaffected. Furthermore, these peptides inhibited the BK-triggered release of calcein, a hemichannel-permeable dye. BK-induced oscillations, but not those induced by ATP, were dependent on extracellular Ca(2+). Alleviating the negative feedback of [Ca(2+)](i) on InsP(3)Rs using cytochrome c inhibited BK- and ATP-induced oscillations. Cx32 and Cx43 hemichannels are activated by <500 nm [Ca(2+)](i) but inhibited by higher concentrations and CT9 peptide (last 9 amino acids of the Cx43 C terminus) removes this high [Ca(2+)](i) inhibition. Unlike interfering with the bell-shaped dependence of InsP(3)Rs to [Ca(2+)](i), CT9 peptide prevented BK-induced oscillations but not those triggered by ATP. Collectively, these data indicate that connexin hemichannels contribute to BK-induced oscillations by allowing Ca(2+) entry during the rising phase of the Ca(2+) spikes and by providing an OFF mechanism during the falling phase of the spikes. Hemichannels were not sufficient to ignite oscillations by themselves; however, their contribution was crucial as hemichannel inhibition stopped the oscillations.

Receptor oligomerization: a pivotal mechanism for regulating chemokine function

Since the first reports on chemokine function, much information has been generated on the implications of these molecules in numerous physiological and pathological processes, as well as on the signaling events activated through their binding to receptors. Despite these extensive studies, no chemokine-related drugs have yet been approved for use in patients with inflammatory or autoimmune diseases. This discrepancy between efforts and results has forced a re-evaluation of the chemokine field. We have explored chemokine receptor conformations at the cell surface and found that, as is the case for other G protein-coupled receptors, chemokine receptors are not isolated entities that are activated following ligand binding; rather, they are found as dimers and/or higher order oligomers at the cell surface, even in the absence of ligands. These complexes form organized arrays that can be modified by receptor expression and ligand levels, indicating that they are dynamic structures. The way in which these receptor complexes are stabilized modulates ligand binding, as well as their pharmacological properties and the signaling events activated. These conformations thus represent a mechanism that increases the broad variety of chemokine functions. Understanding these receptor interactions and their dynamics at the cell surface is thus critical for influencing chemokine function and could open up new possibilities for drug design.

Adenosine A(1) receptor agonist N(6)-cyclohexyl-adenosine induced phosphorylation of delta opioid receptor and desensitization of its signaling

AIM

To define the effect of adenosine A(1) receptor (A(1)R) on delta opioid receptor (DOR)-mediated signal transduction.

METHODS

CHO cells stably expressing HA-tagged A(1)R and DOR-CFP fusion protein were used. The localization of receptors was observed using confocal microscope. DOR-mediated inhibition of adenylyl cyclase was measured using cyclic AMP assay. Western blots were employed to detect the phosphorylation of Akt and the DOR. The effect of A(1)R agonist N(6)-cyclohexyladenosine (CHA) on DOR down-regulation was assessed using radioligand binding assay.

RESULTS

CHA 1 micromol/L time-dependently attenuated DOR agonist [D-Pen(2,5)]enkephalin (DPDPE)-induced inhibition of intracellular cAMP accumulation with a t(1/2)=2.56 (2.09-3.31) h. Pretreatment with 1 micromol/L CHA for 24 h caused a right shift of the dose-response curve of DPDPE-mediated inhibition of cAMP accumulation, with a significant increase in EC(50) but no change in E(max). Pretreatment with 1 micromol/L CHA for 1 h also induced a significant attenuation of DPDPE-stimulated phosphorylation of Akt. Moreover, CHA time-dependently phosphorylated DOR (Ser363), and this effect was inhibited by A(1)R antagonist 1,3-Dipropyl-8-cyclopentylxanthine (DPCPX) but not by DOR antagonist naloxone. However, CHA failed to produce the down-regulation of DOR, as neither receptor affinity (K(d)) nor receptor density (B(max)) of DOR showed significant change after chronic CHA exposure.

CONCLUSION

Activation of A(1)R by its agonist caused heterologous desensitization of DOR-mediated inhibition of intracellular cAMP accumulation and phosphorylation of Akt. Activation of A(1)R by its agonist also induced heterologous phosphorylation but not down-regulation of DOR.

The Gbeta5-RGS7 complex selectively inhibits muscarinic M3 receptor signaling via the interaction between the third intracellular loop of the receptor and the DEP domain of RGS7

Regulators of G protein signaling (RGS) make up a diverse family primarily known as GTPase-activating proteins (GAPs) for heterotrimeric G proteins. In addition to the RGS domain, which is responsible for GAP activity, most RGS proteins contain other distinct structural motifs. For example, members of the R7 family of RGS proteins contain a DEP, GGL, and novel DHEX domain and are obligatory dimers with G protein beta subunit Gbeta5. Here we show that the Gbeta5-RGS7 complex can inhibit Ca2+ mobilization elicited by muscarinic acetylcholine receptor type 3 (M3R), but not by other Gq-coupled receptors such as M1, M5, histamine H1, and GNRH receptors. The isolated DEP domain of RGS7 is sufficient for the inhibition of M3R signaling, whereas the deletion of the DEP domain renders the Gbeta5-RGS7 complex ineffective. Deletion of a portion of the third intracellular loop allowed the receptor (M3R-short) to signal but rendered it insensitive to the effect of the Gbeta5-RGS7 complex. Accordingly, the recombinant DEP domain bound in vitro to the GST-fused i3 loop of the M3R. These results identify a novel molecular mechanism that can impart receptor subtype selectivity on signal transduction via Gq-coupled muscarinic receptors.

A role for inflammatory mediators in heterologous desensitization of CysLT1 receptor in human monocytes

Cysteinyl-leukotrienes (cysteinyl-LT) are rapidly generated at sites of inflammation and, in addition to their role in asthma, rhinitis, and other immune disorders, are increasingly regarded as significant inflammatory factors in cancer, gastrointestinal, cardiovascular diseases. We recently demonstrated that in monocyte/macrophage-like U937 cells, extracellular nucleotides heterologously desensitize CysLT(1) receptor (CysLT(1)R)-induced Ca(2+) transients. Given that monocytes express a number of other inflammatory and chemoattractant receptors, this study was aimed at characterizing transregulation between these different stimuli. We demonstrate that in U937 cells and in primary human monocytes, a series of inflammatory mediators activating G(i)-coupled receptor (FPR1, BLT(1)) desensitize CysLT(1)R-induced Ca(2+) response unidirectionally through activation of PKC. Conversely, PAF-R, exclusively coupled to G(q), cross-desensitizes CysLT(1)R without the apparent involvement of any kinase. Interestingly, G(s)-coupled receptors (beta(2)AR, H(1/2)R, EP(2/4)R) are also able to desensitize CysLT(1)R response through activation of PKA. Heterologous desensitization seems to affect mostly the G(i)-mediated signaling of the CysLT(1)R. The hierarchy of desensitization among agonists may be important for leukocyte signal processing at the site of inflammation. Considering that monocytes/macrophages are likely to be the major source of cysteinyl-LT in many immunological and inflammatory processes, shedding light on how their receptors are regulated will certainly help to better understand the role of these cells in orchestrating this complex network of integrated signals.

Effect of telmisartan on expression of protein kinase C-alpha in kidneys of diabetic mice

AIM

To investigate the effects of angiotensin receptor blocker (ARB) telmisartan on the expression and distribution of protein kinase C (PKC)-alpha in the kidneys of diabetic mice.

METHODS

Diabetic mice were induced with streptozotocin and a group of them were randomly selected for treatment with telmisartan. After 6 weeks, the expression and localization of PKC-alpha in the renal cortex, and the outer and inner medulla were assessed by immunohistochemistry and semiquantitative Western blotting. In addition, expressions of PKC-alpha, transforming growth factor-beta1 (TGF-beta1), and vascular endothelial growth factor (VEGF) in glomeruli were measured by semiquantitative immunohistochemistry.

RESULTS

Diabetic and normal mice showed similar distributions of PKC-alpha in the kidneys. The expression of PKC-alpha was found in glomeruli, epithelial cells of proximal tubules, and medullary-collecting duct, while not in the medullary and cortical thick ascending limb, and was different in the epithelial cells of proximal tubules of diabetic nephropathy (DN) mice, PKC-alpha was mostly translocated from the basement membrane to the apical membrane, whereas it was largely translocated from the apical membrane to the basement membrane in epithelial cells of the inner medullary-collecting duct. Western blotting detected increased expression of PKC-alpha in the renal cortex and outer medulla, but not in the inner medulla of DN mice. Enhanced expressions of PKC-alpha, TGF-beta1, and VEGF were shown in the glomeruli of DN mice, where PKC-alpha exhibited a correlation to VEGF, but no correlation to TGF-beta1. ARB telmisartan attenuated alterations of PKC-alpha as mentioned earlier in the DN mice.

CONCLUSION

Our findings suggest that PKC-alpha may play a role in the pathogenesis of DN, and that the nephroprotective effects of ARB telmisartan may be partly associated with its influence on PKC-alpha.

Nitric oxide synthase inhibitors modulate nerve growth factor-mediated regulation of amyloid precursor protein expression in PC12 cells

Nerve growth factor (NGF) can regulate nitric oxide synthase (NOS) expression and nitric oxide (NO) can modulate NGF-mediated neurotrophic responses. In this study, the role of NO in NGF-stimulated amyloid precursor protein (APP) levels was studied. PC12 cells were treated with either the non-selective NOS inhibitor N(omega)-nitro-L-arginine methylester (L-NAME) or the inducible NOS selective inhibitor s-methylisothiourea (S-MIU), and the effect on NGF-mediated increases in APP expression was determined. NGF significantly increased total APP protein levels following 96 h of treatment and this increase was prevented in cells pre-treated with S-MIU. Pre-treatment of cells with actinomycin D also blocked this NGF-mediated induction of APP, indicating de novo protein synthesis is necessary. Treatment with NGF increased APP promoter activity; however, this increase was only partially inhibited by pre-treatment with S-MIU and was increased in the presence of L-NAME. This suggests that NO may be modulating other aspects of APP expression in addition to transcription. Inhibition of NGF signaling pathways was also investigated using inhibitors of mitogen-activated protein (MAP) kinase (U0126), Akt (LY294002) and protein kinase C (PKC; U73122 and bisindolylmaleimide 1 (BIS-1)) activation. Inhibition of each of these pathways prevented NGF-mediated increases in APP protein expression; however, only BIS-1 attenuated NGF-mediated increases in promoter activation. This study indicates that NO is involved in the NGF-mediated regulation of APP, in part at the level of APP transcription and could involve the modulation of NGF signal transduction pathways.

Thiol-related genes in diabetic complications: a novel protective role for endogenous thioredoxin 2

OBJECTIVE

Our laboratory and others have found that deficiencies in cellular thiols may be importantly involved in the development of diabetic complications. However, the role for specific thiol-related genes in diabetic complications is unclear.

METHODS AND RESULTS

We began the present study by systematically determining the expression level of 11 thiol-related genes in three tissues from rats with streptozotocin-induced diabetes. Several thiol-related genes were found to exhibit diabetes-associated, time-dependent differential expression. Thioredoxin 2, a mitochondrion-specific thioredoxin whose role in diabetes was unknown, was suppressed in the aorta from rats with two weeks of diabetes. When thioredoxin 2 expression in human umbilical vein endothelial cells was knocked-down by small interfering RNA, high-ambient glucose-elicited substantial injurious effects (n=5 to 9, P<0.05), including increases in cytosolic cytochrome c (by 2.2+/-0.6-fold), lipid peroxidation (by 40+/-8%), fibronectin expression (by 35+/-7%), and oxidized glutathione, and decreases in endothelial nitric oxide synthase expression (by 79+/-15%), basal accumulation of nitrite/nitrate (by 68+/-16%), total free thiols (by 42+/-8%), and glutathione (by 6+/-1%). In the absence of thioredoxin 2 knockdown, high-ambient glucose did not have significant effects on any of these measurements. The effect of thioredoxin 2 knockdown appeared to be associated with increases in glucose consumption and glucose transporter 1 expression.

CONCLUSIONS

These results provided the first expression profile of thiol-related genes in a model of diabetes and demonstrated a novel role for endogenous thioredoxin 2 in protecting cells against high ambient glucose.

Modulation of Na+ transport and epithelial sodium channel expression by protein kinase C in rat alveolar epithelial cells

Although the amiloride-sensitive epithelial sodium channel (ENaC) plays an important role in the modulation of alveolar liquid clearance, the precise mechanism of its regulation in alveolar epithelial cells is still under investigation. Protein kinase C (PKC) has been shown to alter ENaC expression and activity in renal epithelial cells, but much less is known about its role in alveolar epithelial cells. The objective of this study was to determine whether PKC activation modulates ENaC expression and transepithelial Na+ transport in cultured rat alveolar epithelial cells. Alveolar type II cells were isolated and cultured for 3 to 4 d before they were stimulated with phorbol 12-myristate 13-acetate (PMA 100 nmol/L) for 4 to 24 h. PMA treatment significantly decreased alpha, beta, and gammaENaC expression in a time-dependent manner, whereas an inactive form of phorbol ester had no apparent effect. This inhibitory action was seen with only 5-min exposure to PMA, which suggested that PKC activation was very important for the reduction of alphaENaC expression. The PKC inhibitors bisindolylmaleimide at 2 micromol/L and Gö6976 at 2 micromol/L diminished the PMA-induced suppression of alphaENaC expression, while rottlerin at 1 micromol/L had no effect. PMA elicited a decrease in total and amiloride-sensitive current across alveolar epithelial cell monolayers. This decline in amiloride-sensitive current was not blocked by PKC inhibitors except for a partial inhibition with bisindolylmaleimide. PMA induced a decrease in rubidium uptake, indicating potential Na+-K+-ATPase inhibition. However, since ouabain-sensitive current in apically permeabilized epithelial cells was similar in PMA-treated and control cells, the inhibition was most probably related to reduced Na+ entry at the apical surface of the cells. We conclude that PKC activation modulates ENaC expression and probably ENaC activity in alveolar epithelial cells. Ca2+-dependent PKC is potentially involved in this response.

Mechanisms underlying regulation of a barium-sensitive K+ conductance by ATP in single proximal tubule cells isolated from frog kidney

K(+) channels play an important role in pump-leak coupling and volume regulation in the renal proximal tubule. Previous experiments have identified a barium-sensitive K(+) conductance (G(Ba)) in proximal tubule cells isolated from frog kidneys. In this paper we examine the regulation of G(Ba) by ATP. G(Ba) was measured in single cells isolated from frog kidney using the whole-cell patch-clamp technique. G(Ba) was activated by 2 mM: intracellular ATP. This activation was enhanced by inhibition of protein kinase C and attenuated by inhibition of protein kinase A, indicating reciprocal regulation by these kinases. Activation by ATP was reduced in the presence of a hypertonic bath solution, suggesting that cell swelling was required. However, after activation to steady-state, G(Ba )was not sensitive to cell-volume changes. Hypotonic shock-induced volume regulation was inhibited by barium and quinidine, inhibitors of G(Ba). The effect of maximal inhibitory concentrations of barium and quinidine on volume regulation was similar and addition of both blockers together did not augment the inhibitory response. G(Ba) was also activated by ADP, via a mechanism dependent on the presence of Mg(2+). However, the responses to ADP and ATP were not additive, suggesting that these nucleotides may share a common mechanism of activation. The regulation of G(Ba) by ATP was biphasic, with a half-maximal activating concentration of 0.89 mM and a half maximal inhibitory concentration of 6.71 mM. The sensitivity to nucleotides suggests that G(Ba) may be regulated by the metabolic state of the cell. Furthermore, the sensitivity to solution osmolality, coupled with the blocker profile of inhibition of volume regulation, suggests that G(Ba) could play a role in volume regulation.

Parathyroid hormone-mediated regulation of Na+-K+-ATPase requires ERK-dependent translocation of protein kinase Calpha

Parathyroid hormone (PTH) inhibits Na+-K+-ATPase activity by serine phosphorylation of the alpha1 subunit through protein kinase C (PKC)- and extracellular signal-regulated kinase (ERK)-dependent pathways. Based on previous studies we postulated that PTH regulates sodium pump activity through isoform-specific PKC-dependent activation of ERK. In the present work utilizing opossum kidney cells, a model of renal proximal tubule, PTH stimulated membrane translocation of PKCalpha by 102 +/- 16% and PKCbetaI by 41 +/- 7% but had no effect on PKCbetaII and PKCzeta. Both PKCalpha and PKCbetaI phosphorylated the Na+-K+-ATPase alpha1 subunit in vitro. PTH increased the activity of PKCalpha but not PKCbetaI. Coimmunoprecipitation assays demonstrated that treatment with PTH enhanced the association between Na+-K+-ATPase alpha1 subunit and PKCalpha, whereas the association between Na+-K+-ATPase alpha1 subunit and PKCbetaI remained unchanged. A PKCalpha inhibitory peptide blocked PTH-stimulated serine phosphorylation of the Na+-K+-ATPase alpha1 subunit and inhibition of Na+-K+-ATPase activity. Pharmacologic inhibition of MEK-1 blocked PTH-stimulated translocation of PKCalpha, whereas transfection of constitutively active MEK-1 cDNA induced translocation of PKCalpha and increased phosphorylation of the Na+-K+-ATPase alpha1 subunit. In contrast, PTH-stimulated ERK activation was not inhibited by pretreatment with the PKCalpha inhibitory peptide. Inhibition of PKCalpha expression by siRNA did not inhibit PTH-mediated ERK activation but significantly reduced PTH-mediated phosphorylation of the Na+-K+-ATPase alpha1 subunit. Pharmacologic inhibition of phosphoinositide 3-kinase blocked PTH-stimulated ERK activation, translocation of PKCalpha, and phosphorylation of the Na+-K+-ATPase alpha1 subunit. We conclude that PTH stimulates Na+-K+-ATPase phosphorylation and decreases the activity of Na+-K+-ATPase by ERK-dependent activation of PKCalpha.

Heterodimerization and cross-desensitization between the mu-opioid receptor and the chemokine CCR5 receptor

Cross-desensitization between micro-opioid receptor agonists and CC chemokines was shown to occur in immune cells and in the central nervous system. However, these cells do not permit examination of potential mechanisms at cellular levels due to low levels and mixed populations of receptors. In this study, we investigated possible interactions and biochemical mechanisms of cross-desensitization between the mu-opioid and chemokine CCR5 receptors coexpressed in Chinese hamster ovary (CHO) cells. Hemagglutinin (HA)-tagged micro-opioid receptor coimmunoprecipitated with FLAG (Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys)-tagged chemokine receptor CCR5 in cells expressing the two receptors, but not in a mixture of cells transfected with one of the two receptors, indicating that the two receptors form heterodimers. Treatment with the mu-opioid receptor agonist DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin), the chemokine RANTES (Regulated on Activation, Normal T cell-Expressed and -Secreted) (CCL5), or both, did not affect the level of coimmunoprecipitation. DAMGO and RANTES (CCL5) induced chemotaxis in CHO cells coexpressing both receptors, and preincubation with either DAMGO or RANTES (CCL5) profoundly inhibited chemotaxis caused by the other. DAMGO pretreatment enhanced phosphorylation of the chemokine CCR5 receptor and reduced RANTES (CCL5)-promoted [35S]GTP gamma S binding. Conversely, RANTES (CCL5) preincubation slightly increased phosphorylation of the mu-opioid receptor and significantly reduced DAMGO-induced [35S]GTP gamma S binding. These results indicate that activation of either receptor affected G protein coupling of the other, likely due to enhanced phosphorylation of the receptor. Heterodimerization between the two receptors may contribute to the observed cross-desensitization.

Non-visual GRKs: are we seeing the whole picture?

G-protein-coupled receptor kinases (GRKs) comprise a family of seven mammalian serine/threonine protein kinases that phosphorylate and regulate agonist-occupied or constitutively active G-protein-coupled receptors (GPCRs). Studies of the details and consequences of these mechanisms have focused heavily on the original beta-adrenoceptor kinase (beta-ARK) family (GRK2 and GRK3) and, in particular, on phosphorylation-dependent recruitment of adaptor proteins such as the beta-arrestins. However, recent work has indicated roles for the other, non-visual GRKs (GRK4, GRK5 and GRK6) and has revealed potential phosphorylation-independent regulation of GPCRs by GRK2 and GRK3. In this article, we review this newer information and attempt to put it into context with GRKs as physiological regulators that could be appropriate targets for future pharmacological intervention.

Specificity of g protein-coupled receptor kinase 6-mediated phosphorylation and regulation of single-cell m3 muscarinic acetylcholine receptor signaling

Previously we have shown that G protein-coupled receptor kinase (GRK) 6 plays a major role in the regulation of the human M3 muscarinic acetylcholine receptor (M3 mAChR) in the human neuroblastoma SH-SY5Y. However, 30-fold overexpression of the catalytically inactive, dominant-negative K215RGRK6 produced only a 50% suppression of M3 mAChR phosphorylation and desensitization. Here, we have attempted to determine whether other endogenous kinases play a role in the regulation of M3 mAChR signaling. In contrast to the clear attenuating effect of K215RGRK6 expression on M3 mAChR regulation, dominant-negative forms of GRKs (K220RGRK2, K220RGRK3, K215RGRK5) and casein kinase 1alpha (K46RCK1alpha) were without effect. In addition, inhibition of a variety of second-messenger-regulated kinases and the tyrosine kinase Src also had no effect upon agonist-stimulated M3 mAChR regulation. To investigate further the desensitization process we have followed changes in inositol 1,4,5-trisphosphate in single SHSY5Y cells using the pleckstrin homology domain of PLCdelta1 tagged with green fluorescent protein (eGFP-PHPLCdelta1). Stimulation of cells with approximate EC50 concentrations of agonist before and after a desensitizing period of agonist exposure resulted in a marked attenuation of the latter response. Altered GRK6 activity, through overexpression of wild-type GRK6 or K215RGRK6, enhanced or reduced the degree of M3 mAChR desensitization, respectively. Taken together, our data indicate that M3 mAChR desensitization is mediated by GRK6 in human SH-SY5Y cells, and we show that receptor desensitization of phospholipase C signaling can be monitored in 'real-time' in single, living cells.

Roles of superoxide, peroxynitrite, and protein kinase C in the development of tolerance to nitroglycerin

A current hypothesis states that tolerance to nitroglycerin (GTN) involves increased formation of superoxide (O2*-). Studies showing that inhibitors of protein kinase C (PKC) prevent tolerance to GTN suggest the involvement of PKC activation, which can also increase O2*-. We examined the roles of O2*-, peroxynitrite (ONOO-), and PKC activation in GTN tolerance. Pre-exposure of rat aortic rings to GTN (5 x 10(-4) M) for 2 h caused tolerance to the vasodilating effect of GTN, as evidenced by a substantial rightward shift of GTN concentration-relaxation curves. This shift was reduced by treatment of the rings with the antioxidants uric acid, vitamin C, or tempol or the PKC inhibitor chelerythrine. We also found that O2*- generation via xanthine/xanthine oxidase in the bath induced tolerance to GTN. However, responses to nitroprusside were not affected. In vivo tolerance produced in rats by 3-day i.v. infusion of GTN was also almost completely prevented by coinfusion of tempol. In bovine aortic endothelial cells (EC), addition of GTN produced a marked increase in tyrosine nitrosylation, indicating increased ONOO- formation. This action was blocked by prior treatment with uric acid, superoxide dismutase, NG-nitro-L-arginine methyl ester, or chelerythrine. We also demonstrated that GTN translocates the alpha- and epsilonPKC isoforms in EC. However, PKCzeta was not affected by GTN treatment. In conclusion, tolerance to GTN involves enhanced production of O2*- and ONOO- and activation of NO synthase. Furthermore, sustained activation of alpha- and epsilonPKC isozymes in EC by GTN may play a role in development of tolerance.

Downregulation of bradykinin B2 receptor in human fibroblasts during prolonged agonist exposure

Sustained activation of G protein-coupled receptors results in an attenuation of cellular responses, a phenomenon termed desensitization. Whereas mechanisms for rapid desensitization of ligand-receptor-G protein-effector systems are relatively well characterized, much less is known about long-term adaptation processes that occur in the continuous presence of an agonist. Here we have studied the fate of endogenously expressed bradykinin B(2) receptors on human fibroblasts during prolonged agonist treatment. Stimulation with bradykinin for up to 24 h resulted in a 50% reduction of surface binding sites that was paralleled by a similar decrease of total B(2) receptor protein followed by Western blotting using monoclonal antibodies to the B(2) receptor. Whereas B(2) receptor mRNA levels did not change during 24 h of agonist treatment, B(2) receptor de novo synthesis was attenuated by 35-50%, indicating translational control of B(2) receptor levels. Furthermore, the half-life of B(2) receptor protein was shortened by 20-40% as shown by (35)S-labeled pulse-chase and immunoprecipitation experiments. This study demonstrates that bradykinin B(2) receptor expression during long-term agonist treatment is primarily regulated on the (post)translational level, i.e., by attenuation of de novo synthesis and by reduction of receptor stability.

High-salt diet increases sensitivity to NO and eNOS expression but not NO production in THALs

L-Arginine inhibits thick ascending limb (THAL) NaCl absorption by activating endothelial NO synthase (eNOS) and increasing NO production. Inhibition of renal NO production combined with a high-salt diet produces hypertension, and the THAL has been implicated in salt-sensitive hypertension. We hypothesized that a high-salt diet enhances the inhibitory action of L-arginine on NaCl absorption by THALs because of increased eNOS expression and NO production. To test this, we used isolated THALs from rats on a normal-salt (NS) or high-salt diet (HS) for 7 to 10 days. L-Arginine (1 mmol/L) decreased chloride absorption by 56+/-10% in THALs from rats on a HS diet, but only 29+/-3% in THALs from rats on a NS diet. eNOS expression in isolated THALs from rats on a HS diet was increased by 3.9-fold compared with NS (P<0.03). However, L-arginine increased NO levels to the same extent in THALs from both groups, as measured with DAF-2 DA or a NO-sensitive electrode. To determine whether a HS diet increases the sensitivity of the THAL to NO, we tested the effects of the NO donor spermine NONOate on chloride absorption. In THALs from rats on a HS diet, 1 and 5 micromol/L spermine NONOate reduced chloride absorption by 35+/-5% and 58+/-6%, respectively. In contrast, these same concentrations of spermine NONOate reduced chloride absorption by 4+/-4% (P<0.03 versus HS diet) and 43+/-9% in THALs from rats on a NS diet. We conclude that a HS diet enhances the effect of NO in the THAL. L-Arginine-stimulated NO production was not enhanced by a HS diet, despite increased eNOS protein.

Xenobiotic export pumps, endothelin signaling, and tubular nephrotoxicants--a case of molecular hijacking

This article is a review on recent studies in intact renal proximal tubules that link tubular nephrotoxicants with endothelin (ET) regulation of xenobiotic export pump function. The data show that transport on p-glycoprotein and Mrp2 decreases rapidly when ET signals through an ET(B) receptor, NO synthase (NOS), and protein kinase C (PKC). Surprisingly, nephrotoxicants, such as radiocontrast agents, aminoglycoside antibiotics, and heavy metal salts, "hijack" this signaling pathway, causing ET release from the tubules, hormone binding to its receptor, activation of NOS and PKC, and reduced xenobiotic transport. These findings suggest a new common mechanism by which nephrotoxicants may act to disrupt renal tubular function.

Endogenous G protein-coupled receptor kinase 6 Regulates M3 muscarinic acetylcholine receptor phosphorylation and desensitization in human SH-SY5Y neuroblastoma cells

We have previously shown that overexpression of G protein-coupled receptor kinase 6 (GRK6) enhanced the phosphorylation and desensitization of the endogenously expressed M(3) muscarinic acetylcholine (mACh) receptor in human SH-SY5Y neuroblastoma cells. In this study we have examined the potential role of endogenous GRK6 in the regulation of M(3) mACh receptor by blocking its action through the introduction of a kinase-dead, dominant-negative GRK6 ((K215R)GRK6). (K215R)GRK6 expression inhibited methacholine-stimulated M(3) mACh receptor phosphorylation by 50% compared with plasmid transfected control cells. Guanosine-5'-O-(3-[(35)S]thio)triphosphate binding and immunoprecipitation studies, conducted after agonist pretreatment (3 min), indicated that M(3) mACh receptor-G alpha(q/11) uncoupling was attenuated by 50% in cells expressing (K215R)GRK6 when compared with control cells. In contrast, expression of the related dominant-negative kinase (K215R)GRK5 had no effect on M(3) mACh receptor phosphorylation or uncoupling. Time course studies also showed that agonist-stimulated [(3)H]inositol phosphate accumulations were more sustained in cells expressing (K215R)GRK6 compared with control and (K215R)GRK5-expressing cells, whereas (K215R)GRK6 expression had no effect on the phospholipase C response to direct stimulation of G proteins with AlF(4)(-). The ability of (K215R)GRK6 to inhibit agonist-mediated M(3) mACh receptor phosphorylation and G protein uncoupling suggests that endogenous GRK6 mediates, at least in part, M(3) mACh receptor desensitization in the SH-SY5Y cell line.

Mechanisms underlying the contraction induced by bradykinin in the guinea pig epithelium-denuded trachea

This study investigates some of the mechanisms by which bradykinin (BK) triggers contraction of epithelium-denuded strips of guinea pig trachea (GPT). Cumulative or single additions of BK, T-BK, L-BK, or ML-BK in the presence of captopril (30 microM) produced graded GPT contractions with the following rank order of potency (EC50 level): T-BK (31.3 nM) > BK (40.0 nM) > L-BK (56.0 nM) > ML-BK (77.0 nM). BK-induced contraction (100 nM) in GPT was completely inhibited by either HOE 140 or NPC 17731 with mean IC50 values of 17 and 217 nM, respectively. Addition of BK (100 nM) at 30 min intervals, induced progressive tachyphylaxis, which was complete after 4 h. The tachyphylaxis induced by BK was unaffected by L-NOARG (nitric oxide synthase inhibitor, 100 microM) or valeryl salicylate (a cyclooxygenase-1 (COX-1) inhibitor, 30 microM), but was prevented by a low concentration of indomethacin, diclofenac (non-selective COX inhibitors, 3 nM each) or by NS 398 (a COX-2 inhibitor, 10 nM). Furthermore, higher concentrations of indomethacin, diclofenac, phenidone (a lypooxygenase (LOX) and COX inhibitor), or NS 398, caused graded inhibition of BK-induced contraction, with mean IC50 values of 0.28, 0.08, 46.37, and 0.15 microM, respectively. Together, these results suggest that BK-induced contraction in GPT involves activation of B2 receptors and release of prostanoids from COX-2 pathway. Furthermore, the tachyphylaxis induced by BK was insensitive to the nitric oxide and COX-1 inhibitors, but was prevented by non-selective and selective COX-2 inhibitors, indicating a mediation via COX-2-derived arachidonic acid metabolites.

Homologous and heterologous uncoupling of muscarinic M(3) and alpha(1B) adrenoceptors to Galpha(q/11) in SH-SY5Y human neuroblastoma cells

1. The present study employed a [(35)S]-GTPgammaS binding protocol in conjunction with immunoprecipitation (IP) of the Galpha subunits to investigate the desensitization of G(q/11)-coupled receptors at the level of the G-protein activation. Membranes from SH-SY5Y cells expressing the recombinant human alpha(1B)-adrenoceptor (alpha(1B)-AR) (and endogenously expressing the M(3) muscarinic acetylcholine receptor (M(3)-AChR)) exhibited G(q/11) activation in a concentration-dependent manner in response to noradrenaline or methacholine. 2. Pre-treatment of intact cells with agonist prior to membrane preparation and use in the [(35)S]-GTPgammaS IP assay demonstrated that both receptors were homologously desensitized by pre-treatment with agonist since the G(q/11) activation in response to a secondary challenge with agonist was markedly reduced. Stimulation of alpha(1B)-AR was effective at heterologously desensitizing the M(3)-AChR. The PKC inhibitor, Ro-31-8220 (10 microM) was ineffective at preventing the agonist-mediated receptor desensitization. 3. [(32)P]P(i)-labelled cells allowed the detection of increases in receptor phosphorylation. Phorbol 12,13 dibutyrate (PDBu) (1 microM) was effective at producing a Ro-31-8220 (10 microM)-sensitive, detectable increase in alpha(1B)-AR but not M(3)-AChR phosphorylation. Noradrenaline (30 microM) stimulated alpha(1B)-AR phosphorylation, which could be partially inhibited by Ro-31-8220 (10 microM). The phosphorylation of M(3)-AChR was increased by methacholine (100 microM) incubation and this effect appeared to be insensitive to Ro-31-8220 (10 microM). 4. These findings demonstrate that [(35)S]-GTPgammaS-Galpha-subunit IP can be used to estimate receptor desensitization as a decline in receptor-G-protein coupling. Both the alpha(1B)-AR and M(3)-AChR undergo rapid homologous desensitization that is associated with an increase in receptor phosphorylation. The heterologous desensitization of M(3)-AChR produced by alpha(1B)-AR stimulation is not associated with a detectable increase in M(3)-AChR phosphorylation, suggesting that receptor phosphorylation is not necessarily a prerequisite for desensitization.

Interaction of protein kinase C and cAMP-dependent pathways in the phosphorylation of the Na,K-ATPase

To test the hypothesis that there is cross-talk between the protein kinase C (PKC) and protein kinase A (PKA) pathways in the regulation of the Na,K-ATPase, we measured its phosphorylation in mammalian cell cultures. Phosphorylation of the PKC site, Ser-18, appeared to be due to the activation of the alpha isoform of the kinase. In NRK-52E and L6 cells, this phosphorylation was reduced by prior activation of a cAMP-dependent signaling pathway with forskolin. In principle this would be consistent with direct interaction between the two phosphorylation sites, but further investigation suggested a more indirect mechanism. First, phosphorylation of Ser-938, the PKA site, could not be detected despite the presence of active PKA. Second, there was a major reduction in the phosphorylation of unrelated phosphoproteins as a consequence of elevation of cAMP, suggesting generalized reduction of kinase activity or activation of phosphatase activity. In NRK-52E and L6, phosphorylation of the Na, K-ATPase at Ser-18 paralleled this global change. In C6 cells, in contrast, there was no cAMP effect on Na,K-ATPase phosphorylation at Ser-18 and no global cAMP effect on other phosphoproteins. The cross-talk is evidently mediated by events occurring at the cellular level.

Phosphorylation and regulation of a Gq/11-coupled receptor by casein kinase 1alpha

Agonist-mediated receptor phosphorylation by one or more of the members of the G-protein receptor kinase (GRK) family is an established model for G-protein-coupled receptor (GPCR) phosphorylation resulting in receptor desensitization. Our recent studies have, however, suggested that an alternative route to GPCR phosphorylation may be an operation involving casein kinase 1alpha (CK1alpha). In the current study we investigate the involvement of CK1alpha in the phosphorylation of the human m3-muscarinic receptor in intact cells. We show that expression of a catalytically inactive mutant of CK1alpha, designed to act in a dominant negative manner, inhibits agonist-mediated receptor phosphorylation by approximately 40% in COS-7 and HEK-293 cells. Furthermore, we present evidence that a peptide corresponding to the third intracellular loop of the m3-muscarinic receptor (Ser(345)-Leu(463)) is an inhibitor of CK1alpha due to its ability to both act as a pseudo-substrate for CK1alpha and form a high affinity complex with CK1alpha. Expression of this peptide was able to reduce both basal and agonist-mediated m3-muscarinic receptor phosphorylation in intact cells. These results support the notion that CK1alpha is able to mediate GPCR phosphorylation in an agonist-dependent manner and that this may provide a novel mechanism for GPCR phosphorylation. The functional role of phosphorylation was investigated using a mutant of the m3-muscarinic receptor that showed an approximately 80% reduction in agonist-mediated phosphorylation. Surprisingly, this mutant underwent agonist-mediated desensitization suggesting that, unlike many GPCRs, desensitization of the m3-muscarinic receptor is not mediated by receptor phosphorylation. The inositol (1,4, 5)-trisphosphate response did, however, appear to be dramatically potentiated in the phosphorylation-deficient mutant indicating that phosphorylation may instead control the magnitude of the initial inositol phosphate response.