Lipid rafts keep NADPH oxidase in the inactive state in human renal proximal tubule cells

HK-2 human renal proximal tubule cells as a model for G protein-coupled receptor kinase type 4-mediated dopamine 1 receptor uncoupling

HK-2 human renal proximal tubule cells (RPTC) are commonly used in the in vitro study of "normal" RPTCs. We discovered recently that HK-2 cells are uncoupled from dopamine 1 receptor (D(1)R) adenylyl cyclase (AC) stimulation. We hypothesized that G protein-coupled receptor kinase type 4 (GRK4) single nucleotide polymorphisms may be responsible for the D(1)R/AC uncoupling in HK-2. This hypothesis was tested by genotyping GRK4 single nucleotide polymorphisms, measuring D(1)-like receptor agonist (fenoldopam)-stimulated cAMP accumulation, quantifying D(1)R inhibition of sodium transport, and testing the ability of GRK4 small interfering RNA to reverse the D(1)R/AC uncoupling. We compared HK-2 with 2 normally coupled human RPTC cell lines and 2 uncoupled RPTC cell lines. The HK-2 cell line was found to have 4 of 6 potential GRK4 single nucleotide polymorphisms known to uncouple the D(1)R from AC (namely, R65L, A142V, and A486V). AC response to fenoldopam stimulation was increased in the 2 normally coupled human RPTC cell lines (FEN: 2.02+/-0.05-fold and 2.33+/-0.19-fold over control; P<0.001; n=4) but not in the 2 uncoupled or HK-2 cell lines. GRK4 small interfering RNA rescued the fenoldopam-mediated AC stimulation in the uncoupled cells, including HK-2. The expected fenoldopam-mediated inhibition of sodium hydrogen exchanger type 3 was absent in HK-2 (n=6) and uncoupled RPTC cell lines (n=6) but was observed in the 2 normally coupled human RPTC cell lines (-25.41+/-4.7% and -27.36+/-2.70%; P<0.001; n=6), which express wild-type GRK4. Despite the fact that HK-2 cells retain many functional characteristics of RPTCs, they are not normal from the perspective of dopaminergic function.

Chlamydial infection of monocytes stimulates IL-1beta secretion through activation of the NLRP3 inflammasome

Chlamydia trachomatis infections represent the leading cause of bacterial sexually-transmitted disease in the United States and can cause serious tissue damage leading to infertility and ectopic pregnancies in women. Inflammation and hence the innate immune response to chlamydial infection contributes significantly to tissue damage, particularly by secreting proinflammatory cytokines such as interleukin (IL)-1beta from monocytes, macrophages and dendritic cells. Here we demonstrate that C. trachomatis or Chlamydia muridarum infection of a monocytic cell line leads to caspase-1 activation and IL-1beta secretion through a process requiring the NLRP3 inflammasome. Thus, secretion of IL-1beta decreased significantly when cells were depleted of NLRP3 or treated with the anti-inflammatory inhibitors parthenolide or Bay 11-7082, which inhibit inflammasomes and the transcription factor NF-kappaB. As for other infections causing NRLP3 inflammasome assembly, caspase-1 activation in monocytes is triggered by potassium efflux and reactive oxygen species production. However, anti-oxidants inhibited IL-1beta secretion only partially. Atypically for a bacterial infection, caspase-1 activation during chlamydial infection also involves partially the spleen tyrosine kinase (Syk), which is usually associated with a pathogen recognition receptor for fungal pathogens. Secretion of IL-1beta during infection by many bacteria requires both microbial products from the pathogen and an exogenous danger signal, but chlamydial infection provides both the pathogen-associated molecular patterns and danger signals necessary for IL-1beta synthesis and its secretion from human monocytes. Use of inhibitors that target the inflammasome in animals should therefore dampen inflammation during chlamydial infection.

Dopamine and G protein-coupled receptor kinase 4 in the kidney: role in blood pressure regulation

Complex interactions between genes and environment result in a sodium-induced elevation in blood pressure (salt sensitivity) and/or hypertension that lead to significant morbidity and mortality affecting up to 25% of the middle-aged adult population worldwide. Determining the etiology of genetic and/or environmentally-induced high blood pressure has been difficult because of the many interacting systems involved. Two main pathways have been implicated as principal determinants of blood pressure since they are located in the kidney (the key organ responsible for blood pressure regulation), and have profound effects on sodium balance: the dopaminergic and renin-angiotensin systems. These systems counteract or modulate each other, in concert with a host of intracellular second messenger pathways to regulate sodium and water balance. In particular, the G protein-coupled receptor kinase type 4 (GRK4) appears to play a key role in regulating dopaminergic-mediated natriuresis. Constitutively activated GRK4 gene variants (R65L, A142V, and A486V), by themselves or by their interaction with other genes involved in blood pressure regulation, are associated with essential hypertension and/or salt-sensitive hypertension in several ethnic groups. GRK4γ 142Vtransgenic mice are hypertensive on normal salt intake while GRK4γ 486V transgenic mice develop hypertension only with an increase in salt intake. GRK4 gene variants have been shown to hyperphosphorylate, desensitize, and internalize two members of the dopamine receptor family, the D(1) (D(1)R) and D(3) (D(3)R) dopamine receptors, but also increase the expression of a key receptor of the renin-angiotensin system, the angiotensin type 1 receptor (AT(1)R). Knowledge of the numerous blood pressure regulatory pathways involving angiotensin and dopamine may provide new therapeutic approaches to the pharmacological regulation of sodium excretion and ultimately blood pressure control.

Caveolin-1 and dopamine-mediated internalization of NaKATPase in human renal proximal tubule cells

In moderate sodium-replete states, dopamine 1-like receptors (D1R/D5R) are responsible for regulating >50% of renal sodium excretion. This is partly mediated by internalization and inactivation of NaKATPase, when associated with adapter protein 2. We used dopaminergic stimulation via fenoldopam (D1-like receptor agonist) to study the interaction among D1-like receptors, caveolin-1 (CAV1), and the G protein-coupled receptor kinase type 4 in cultured human renal proximal tubule cells (RPTCs). We compared 2 groups of RPTCs, 1 of cell lines that were isolated from normal subjects (nRPTCs) and a second group of cell lines that have D1-like receptors that are uncoupled (uncoupled RPTCs) from adenylyl cyclase second messengers. In nRPTCs, fenoldopam increased the plasma membrane expression of D1R (10.0-fold) and CAV1 (1.3-fold) and markedly decreased G protein-coupled receptor kinase type 4 by 94+/-8%; no effects were seen in uncoupled RPTCs. Fenoldopam also increased the association of adapter protein 2 and NaKATPase by 53+/-9% in nRPTCs but not in uncoupled RPTCs. When CAV1 expression was reduced by 86.0+/-8.5% using small interfering RNA, restimulation of the D1-like receptors with fenoldopam in nRPTCs resulted in only a 7+/-9% increase in association between adapter protein 2 and NaKATPase. Basal CAV1 expression and association with G protein-coupled receptor kinase type 4 was decreased in uncoupled RPTCs (58+/-5% decrease in association) relative to nRPTCs. We conclude that the scaffolding protein CAV1 is necessary for the association of D1-like receptors with G protein-coupled receptor kinase type 4 and the adapter protein 2-associated reduction in plasma membrane NaKATPase.

G protein-coupled receptor kinase 4 (GRK4) regulates the phosphorylation and function of the dopamine D3 receptor

During conditions of moderate sodium excess, the dopaminergic system regulates blood pressure and water and electrolyte balance by engendering natriuresis. Dopamine exerts its effects on dopamine receptors, including the dopamine D(3) receptor. G protein-coupled receptor kinase 4 (GRK4), whose gene locus (4p16.3) is linked to essential hypertension, desensitizes the D(1) receptor, another dopamine receptor. This study evaluated the role of GRK4 on D(3) receptor function in human proximal tubule cells. D(3) receptor co-segregated in lipid rafts and co-immunoprecipitated and co-localized in human proximal tubule cells and in proximal and distal tubules and glomeruli of kidneys of Wistar Kyoto rats. Bimolecular fluorescence complementation and confocal microscopy revealed that agonist activation of the receptor initiated the interaction between D(3) receptor and GRK4 at the cell membrane and promoted it intracellularly, presumably en route to endosomal trafficking. Of the four GRK4 splice variants, GRK4-gamma and GRK4-alpha mediated a 3- and 2-fold increase in the phosphorylation of agonist-activated D(3) receptor, respectively. Inhibition of GRK activity with heparin or knockdown of GRK4 expression via RNA interference completely abolished p44/42 phosphorylation and mitogenesis induced by D(3) receptor stimulation. These data demonstrate that GRK4, specifically the GRK4-gamma and GRK4-alpha isoforms, phosphorylates the D(3) receptor and is crucial for its signaling in human proximal tubule cells.

Lipid rafts and caveolae and their role in compartmentation of redox signaling

Membrane (lipid) rafts and caveolae, a subset of rafts, are cellular domains that concentrate plasma membrane proteins and lipids involved in the regulation of cell function. In addition to providing signaling platforms for G-protein-coupled receptors and certain tyrosine kinase receptors, rafts/caveolae can influence redox signaling. This review discusses molecular characteristics of and methods to study rafts/caveolae, determinants that contribute to the localization of molecules in these entities, an overview of signaling molecules that show such localization, and the contribution of rafts/caveolae to redox signaling. Of particular note is the evidence that endothelial nitric oxide synthase (eNOS), NADPH oxygenase, and heme oxygenase, along with other less well-studied redox systems, localize in rafts and caveolae. The precise basis for this localization and the contribution of raft/caveolae-localized redox components to physiology and disease are important issues for future studies.

Oxidative stress and hypertension

This review has summarized some of the data supporting a role of ROS and oxidant stress in the genesis of hypertension. There is evidence that hypertensive stimuli, such as high salt and angiotensin II, promote the production of ROS in the brain, the kidney, and the vasculature and that each of these sites contributes either to hypertension or to the untoward sequelae of this disease. Although the NADPH oxidase in these various organs is a predominant source, other enzymes likely contribute to ROS production and signaling in these tissues. A major clinical challenge is that the routinely used antioxidants are ineffective in preventing or treating cardiovascular disease and hypertension. This is likely because these drugs are either ineffective or act in a non-targeted fashion, such that they remove not only injurious ROS Fig. 5. Proposed role of T cells in the genesis of hypertension and the role of the NADPH oxidase in multiple cells/organs in modulating this effect. In this scenario, angiotensin II stimulates an NADPH oxidase in the CVOs of the brain, increasing sympathetic outflow. Sympathetic nerve terminals in lymph nodes activate T cells, and angiotensin II also directly activates T cells. These stimuli also activate expression of homing signals in the vessel and likely the kidney, which attract T cells to these organs. T cells release cytokines that stimulate the vessel and kidney NADPH oxidases, promoting vasoconstriction and sodium retention. SFO, subfornical organ. 630 Harrison & Gongora but also those involved in normal cell signaling. A potentially important and relatively new direction is the concept that inflammatory cells such as T cells contribute to hypertension. Future studies are needed to understand the interaction of T cells with the CNS, the kidney, and the vasculature and how this might be interrupted to provide therapeutic benefit.

D1-like receptors regulate NADPH oxidase activity and subunit expression in lipid raft microdomains of renal proximal tubule cells

NADPH oxidase (Nox)-dependent reactive oxygen species production is implicated in the pathogenesis of cardiovascular diseases, including hypertension. We tested the hypothesis that oxidase subunits are differentially regulated in renal proximal tubules from normotensive and spontaneously hypertensive rats. Basal Nox2 and Nox4, but not Rac1, in immortalized renal proximal tubule cells and brush border membranes were greater in hypertensive than in normotensive rats. However, more Rac1 was expressed in lipid rafts in cells from hypertensive rats than in cells from normotensive rats; the converse was observed with Nox4, whereas Nox2 expression was similar. The D(1)-like receptor agonist fenoldopam decreased Nox2 and Rac1 protein in lipid rafts to a greater extent in hypertensive than in normotensive rats. Basal oxidase activity was 3-fold higher in hypertensive than in normotensive rats but was inhibited to a greater extent by fenoldopam in normotensive (58+/-3.3%) than in hypertensive rats (31+/-5.2%; P<0.05; n=6 per group). Fenoldopam decreased the amount of Nox2 that coimmunoprecipitated with p67(phox) in cells from normotensive rats. D(1)-like receptors may decrease oxidase activity by disrupting the distribution and assembly of oxidase subunits in cell membrane microdomains. The cholesterol-depleting reagent methyl-beta-cyclodextrin decreased oxidase activity and cholesterol content to a greater extent in hypertensive than in normotensive rats. The greater basal levels of Nox2 and Nox4 in cell membranes and Nox2 and Rac1 in lipid rafts in hypertensive rats than in normotensive rats may explain the increased basal oxidase activity in hypertensive rats.

Dopamine and angiotensin as renal counterregulatory systems controlling sodium balance

PURPOSE OF REVIEW

To review the recent evidence demonstrating how the renal dopaminergic and angiotensin systems control renal electrolyte balance through various receptor-mediated pathways with counterregulatory interactions.

RECENT FINDINGS

Stimulation of the renal rennin-angiotensin system results in increased sodium reabsorption, whereas the opposite is true for stimulation of the renal dopaminergic system. An underactive renal dopaminergic system has been associated with increased sodium reabsorption and hypertension. Recent findings indicate novel cell surface receptor-mediated mechanisms by which these two renal endocrine systems directly counterregulate each other. Each of the dopamine receptors (D1R through D5R) have been implicated in dopamine-mediated natriuresis, in addition to counterregulating the angiotensin type 1 R. Dopamine D1-like (D1R and D5R) stimulation has also been found to induce an AT2 receptor- dependent natriuresis. Recently, it has also been discovered that reactive oxygen species can play a role in inactivating the D1 receptor and activating the angiotensin type 1 R.

SUMMARY

Current therapeutic interventions for hypertension predominantly involve correction of an overactive rennin-angiotensin aldosterone system. Recent evidence suggests that stimulation of the renal dopaminergic system and possibly activation of AT2 receptors, as well as decreasing reactive oxygen species, may provide additional therapeutic approaches.