Identification of a prostaglandin-responsive element in the Na,K-ATPase beta 1 promoter that is regulated by cAMP and Ca2+. Evidence for an interactive role of cAMP regulatory element-binding protein and Sp1

Mechanism of Na-K-ATPase Inhibition by PGE2 in Intestinal Epithelial Cells

The primary means of intestinal absorption of nutrients by villus cells is via Na-dependent nutrient co-transporters located in the brush border membrane (BBM). These secondary active co-transport processes require a favorable transcellular Na gradient that is provided by Na-K-ATPase. In chronic enteritis, malabsorption of essential nutrients is partially due to inhibition of villus Na-K-ATPase activity mediated by specific immune inflammatory mediators that are known to be elevated in the inflamed mucosa. However, how Prostaglandin E2 (PGE2), a specific mediator of nutrient malabsorption in the villus BBM, may mediate the inhibition of Na-K-ATPase is not known. Therefore, this study aimed to determine the effect of PGE2 on Na-K-ATPase in villus cells and define its mechanism of action. In vitro, in IEC-18 cells, PGE2 treatment significantly reduced Na-K-ATPase activity, accompanied by a significant increase in the intracellular levels of cyclic Adenosine Monophosphate (cAMP). The treatment with cAMP analog 8-Bromo-cAMP mimicked the PGE2-mediated effect on Na-K-ATPase activity, while Rp-cAMP (PKA inhibitor) pretreatment reversed the same. The mechanism of inhibition of PGE2 was secondary to a transcriptional reduction in the Na-K-ATPase α1 and β1 subunit genes, which was reversed by the Rp-cAMP pretreatment. Thus, the PGE2-mediated activation of the PKA pathway mediates the transcriptional inhibition of Na-K-ATPase activity in vitro.

Exclusive Temporal Stimulation of IL-10 Expression in LPS-Stimulated Mouse Macrophages by cAMP Inducers and Type I Interferons

Expression of the key anti-inflammatory cytokine IL-10 in lipopolysaccharide (LPS)-stimulated macrophages is mediated by a delayed autocrine/paracrine loop of type I interferons (IFN) to ensure timely attenuation of inflammation. We have previously shown that cAMP synergizes with early IL-10 expression by LPS, but is unable to amplify the late type I IFN-dependent activity. We now examined the mechanism of this synergistic transcription in mouse macrophages at the promoter level, and explored the crosstalk between type I IFN signaling and cAMP, using the β-adrenergic receptor agonist, isoproterenol, as a cAMP inducer. We show that silencing of the type I IFN receptor enables isoproterenol to synergize with LPS also at the late phase, implying that autocrine type I IFN activity hinders synergistic augmentation of LPS-stimulated IL-10 expression by cAMP at the late phase. Furthermore, IL-10 expression in LPS-stimulated macrophages is exclusively stimulated by either IFNα or isoproterenol. We identified a set of two proximate and inter-dependent cAMP response element (CRE) sites that cooperatively regulate early IL-10 transcription in response to isoproterenol-stimulated CREB and that further synergize with a constitutive Sp1 site. At the late phase, up-regulation of Sp1 activity by LPS-stimulated type I IFN is correlated with loss of function of the CRE sites, suggesting a mechanism for the loss of synergism when LPS-stimulated macrophages switch to type I IFN-dependent IL-10 expression. This report delineates the molecular mechanism of cAMP-accelerated IL-10 transcription in LPS-stimulated murine macrophages that can limit inflammation at its onset.

Salt Inducible Kinase Signaling Networks: Implications for Acute Kidney Injury and Therapeutic Potential

A number of signal transduction pathways are activated during Acute Kidney Injury (AKI). Of particular interest is the Salt Inducible Kinase (SIK) signaling network, and its effects on the Renal Proximal Tubule (RPT), one of the primary targets of injury in AKI. The SIK1 network is activated in the RPT following an increase in intracellular Na⁺ (Na⁺_in), resulting in an increase in Na,K-ATPase activity, in addition to the phosphorylation of Class IIa Histone Deacetylases (HDACs). In addition, activated SIKs repress transcriptional regulation mediated by the interaction between cAMP Regulatory Element Binding Protein (CREB) and CREB Regulated Transcriptional Coactivators (CRTCs). Through their transcriptional effects, members of the SIK family regulate a number of metabolic processes, including such cellular processes regulated during AKI as fatty acid metabolism and mitochondrial biogenesis. SIKs are involved in regulating a number of other cellular events which occur during AKI, including apoptosis, the Epithelial to Mesenchymal Transition (EMT), and cell division. Recently, the different SIK kinase isoforms have emerged as promising drug targets, more than 20 new SIK2 inhibitors and activators having been identified by MALDI-TOF screening assays. Their implementation in the future should prove to be important in such renal disease states as AKI.

Gene Level Regulation of Na,K-ATPase in the Renal Proximal Tubule Is Controlled by Two Independent but Interacting Regulatory Mechanisms Involving Salt Inducible Kinase 1 and CREB-Regulated Transcriptional Coactivators

For many years, studies concerning the regulation of Na,K-ATPase were restricted to acute regulatory mechanisms, which affected the phosphorylation of Na,K-ATPase, and thus its retention on the plasma membrane. However, in recent years, this focus has changed. Na,K-ATPase has been established as a signal transducer, which becomes part of a signaling complex as a consequence of ouabain binding. Na,K-ATPase within this signaling complex is localized in caveolae, where Na,K-ATPase has also been observed to regulate Inositol 1,4,5-Trisphosphate Receptor (IP3R)-mediated calcium release. This latter association has been implicated as playing a role in signaling by G Protein Coupled Receptors (GPCRs). Here, the consequences of signaling by renal effectors that act via such GPCRs are reviewed, including their regulatory effects on Na,K-ATPase gene expression in the renal proximal tubule (RPT). Two major types of gene regulation entail signaling by Salt Inducible Kinase 1 (SIK1). On one hand, SIK1 acts so as to block signaling via cAMP Response Element (CRE) Binding Protein (CREB) Regulated Transcriptional Coactivators (CRTCs) and on the other hand, SIK1 acts so as to stimulate signaling via the Myocyte Enhancer Factor 2 (MEF2)/nuclear factor of activated T cell (NFAT) regulated genes. Ultimate consequences of these pathways include regulatory effects which alter the rate of transcription of the Na,K-ATPase β1 subunit gene atp1b1 by CREB, as well as by MEF2/NFAT.

Inhibition of D4 Dopamine Receptors on Insulin Receptor Expression and Effect in Renal Proximal Tubule Cells

BACKGROUND

Ion transport in the renal proximal tubule (RPT), which is increased in essential hypertension, is regulated by numerous hormones and humoral factors, including insulin and dopamine. Activation of dopamine receptor inhibits sodium reabsorption, whereas activation of insulin receptor increases sodium reabsorption in RPTs, and hyperinsulinemic animals and patients have defective renal dopaminergic system. We presume that there is an inhibition of D4 receptor on insulin receptor expression and effect, and the regulation is lost in spontaneously hypertensive rats (SHRs).

METHODS AND RESULTS

Insulin receptor expression was determined by immunoblotting, and Na(+)-K(+)-ATPase activity was detected in both Wistar-Kyoto (WKY) and SHR RPT cells. Stimulation of D4 receptor with PD168077 decreased expression of insulin receptors, which was blocked in the presence of the calcium-channel blocker, nicardipine (10(-6) mol/L per 24 hours), in cell culture medium without calcium or in the presence of inositol 1,4,5-trisphosphate (IP3) receptor blocker (2-aminoethyl diphenylborinate [2-ADB]; 10(-6) mol/L per 24 hours), indicating that extracellular calcium entry and calcium release from the endoplasmic reticulum were involved in the signal pathway. Stimulation of the insulin receptor stimulated Na(+)-K(+)-ATPase activity, whereas pretreatment with PD168077 for 24 hours decreased the inhibitory effects of insulin receptor on Na(+)-K(+)-ATPase activity in WKY cells. However, in SHR cells, inhibition of D4 receptor on insulin receptor expression and effect were lost.

CONCLUSIONS

Activation of D4 receptor inhibits insulin receptor expression in RPT cells from WKY rats. The aberrant inhibition of D4 receptor on insulin receptor expression and effect might be involved in the pathogenesis of essential hypertension.

Renal proximal tubule Na,K-ATPase is controlled by CREB-regulated transcriptional coactivators as well as salt-inducible kinase 1

Sodium reabsorption by the kidney is regulated by locally produced natriuretic and anti-natriuretic factors, including dopamine and norepinephrine, respectively. Previous studies indicated that signaling events initiated by these natriuretic and anti-natriuretic factors achieve their effects by altering the phosphorylation of Na,K-ATPase in the renal proximal tubule, and that protein kinase A (PKA) and calcium-mediated signaling pathways are involved. The same signaling pathways also control the transcription of the Na,K-ATPase β subunit gene atp1b1 in renal proximal tubule cells. In this report, evidence is presented that (1) both the recently discovered cAMP-regulated transcriptional coactivators (CRTCs) and salt-inducible kinase 1 (SIK1) contribute to the transcriptional regulation of atp1b1 in renal proximal tubule (RPT) cells and (2) renal effectors, including norepinephrine, dopamine, prostaglandins, and sodium, play a role. Exogenously expressed CRTCs stimulate atp1b1 transcription. Evidence for a role of endogenous CRTCs includes the loss of transcriptional regulation of atp1b1 by a dominant-negative CRTC, as well as by a CREB mutant, with an altered CRTC binding site. In a number of experimental systems, SIK phosphorylates CRTCs, which are then sequestered in the cytoplasm, preventing their nuclear effects. Consistent with such a role of SIK in primary RPT cells, atp1b1 transcription increased in the presence of a dominant-negative SIK1, and in addition, regulation by dopamine, norepinephrine, and monensin was disrupted by a dominant-negative SIK1. These latter observations can be explained if SIK1 is phosphorylated and inactivated in the presence of these renal effectors. Our results support the hypothesis that Na,K-ATPase in the renal proximal tubule is regulated at the transcriptional level via SIK1 and CRTCs by renal effectors, in addition to the previously reported control of the phosphorylation of Na,K-ATPase.

Transcriptional regulators of Na,K-ATPase subunits

The Na,K-ATPase classically serves as an ion pump creating an electrochemical gradient across the plasma membrane that is essential for transepithelial transport, nutrient uptake and membrane potential. In addition, Na,K-ATPase also functions as a receptor, a signal transducer and a cell adhesion molecule. With such diverse roles, it is understandable that the Na,K-ATPase subunits, the catalytic α-subunit, the β-subunit and the FXYD proteins, are controlled extensively during development and to accommodate physiological needs. The spatial and temporal expression of Na,K-ATPase is partially regulated at the transcriptional level. Numerous transcription factors, hormones, growth factors, lipids, and extracellular stimuli modulate the transcription of the Na,K-ATPase subunits. Moreover, epigenetic mechanisms also contribute to the regulation of Na,K-ATPase expression. With the ever growing knowledge about diseases associated with the malfunction of Na,K-ATPase, this review aims at summarizing the best-characterized transcription regulators that modulate Na,K-ATPase subunit levels. As abnormal expression of Na,K-ATPase subunits has been observed in many carcinoma, we will also discuss transcription factors that are associated with epithelial-mesenchymal transition, a crucial step in the progression of many tumors to malignant disease.

The cell- and tissue-specific transcription mechanism of the TATA-less syntaxin 1A gene

Syntaxin 1A (Stx1a) plays an important role in regulation of neuronal synaptic function. To clarify the mechanism of basic transcriptional regulation and neuron-specific transcription of Stx1a we cloned the Stx1a gene from rat, in which knowledge of the expression profile was accumulated, and elucidated that Stx1a consisting of 10 exons, possesses multiple transcription initiation sites and a 204-bp core promoter region (CPR) essential for transcription in PC12 cells. The TATA-less, conserved, GC-rich CPR has 2 specific protein (SP) sites that bind SP1 and are responsible for 65% of promoter activity. The endogenous CPR, including 23 CpG sites, is not methylated in PC12 cells, which express Stx1a and fetal rat skin keratinocyte (FRSK) cells, which do not, although an exogenous methylated CPR suppresses reporter activity in both lines. Trichostatin A (TSA) and class I histone deacetylase (HDAC) inhibitors, but not 5-azacytidine, induce Stx1a in FRSK cells. Acetylated histone H3 only associates to the CPR in FRSK cells after TSA addition, whereas the high acetylated histone H3-CPR association in PC12 cells was unchanged following treatment. HDAC inhibitor induction of Stx1a was negated by mithramycin A and deletion/mutation of 2 SP sites. HDAC1, HDAC2, and HDAC8 detach from the CPR when treated with TSA in FRSK cells and are associated with the CPR in lungs, and acetylated histone H3 associates to this region in the brain. In the first study characterizing a syntaxin promoter, we show that association of SP1 and acetylated histone H3 to CPR is important for Stx1a transcription and that HDAC1, HDAC2, and HDAC8 decide cell/tissue specificity in a suppressive manner.

Antagonism of the prostaglandin E2 EP1 receptor in MDCK cells increases growth through activation of Akt and the epidermal growth factor receptor

The actions of prostaglandin E2 (PGE2) in the kidney are mediated by G protein-coupled E-prostanoid (EP) receptors, which affect renal growth and function. This report examines the role of EP receptors in mediating the effects of PGE2 on Madin-Darby canine kidney (MDCK) cell growth. The results indicate that activation of Gs-coupled EP2 and EP4 by PGE2 results in increased growth, while EP1 activation is growth inhibitory. Indeed, two EP1 antagonists (ONO-8711 and SC51089) stimulate, rather than inhibit, MDCK cell growth, an effect that is lost following an EP1 knockdown. Similar observations were made with M1 collecting duct and rabbit kidney proximal tubule cells. ONO-8711 even stimulates growth in the absence of exogenous PGE2, an effect that is prevented by ibuprofen (indicating a dependence upon endogenous PGE2). The involvement of Akt was indicated by the observation that 1) ONO-8711 and SC51089 increase Akt phosphorylation, and 2) MK2206, an Akt inhibitor, prevents the increased growth caused by ONO-8711. The involvement of the EGF receptor (EGFR) was indicated by 1) the increased phosphorylation of the EGFR caused by SC51089 and 2) the loss of the growth-stimulatory effect of ONO-8711 and SC51089 caused by the EGFR kinase inhibitor AG1478. The growth-stimulatory effect of ONO-8711 was lost following an EGFR knockdown, and transduction of MDCK cells with a dominant negative EGFR. These results support the hypothesis that 1) signaling via the EP1 receptor involves Akt as well as the EGFR, and 2), EP1 receptor pharmacology may be employed to prevent the aberrant growth associated with a number of renal diseases.

Soluble Adenylyl Cyclase in Vascular Endothelium

The Ca 2+ - and bicarbonate-activated soluble adenylyl cyclase (sAC) has been identified recently as an important mediator of aldosterone signaling in the kidney. Nuclear sAC has been reported to stimulate cAMP response element–binding protein 1 phosphorylation via protein kinase A, suggesting an alternative cAMP pathway in the nucleus. In this study, we analyzed the sAC as a potential modulator of endothelial stiffness in the vascular endothelium. We determined the contribution of sAC to cAMP response element–mediated transcriptional activation in vascular endothelial cells and kidney collecting duct cells. Inhibition of sAC by the specific inhibitor KH7 significantly reduced cAMP response element–mediated promoter activity and affected cAMP response element–binding protein 1 phosphorylation. Furthermore, KH7 and anti-sAC small interfering RNA significantly decreased mRNA and protein levels of epithelial sodium channel-α and Na + /K + -ATPase-α. Using atomic force microscopy, a nano-technique that measures stiffness and deformability of living cells, we detected significant endothelial cell softening after sAC inhibition. Our results suggest that the sAC is a regulator of gene expression involved in aldosterone signaling and an important regulator of endothelial stiffness. Additional studies are warranted to investigate the protective action of sAC inhibitors in humans for potential clinical use.

Targeting of renal proximal tubule Na,K-ATPase by salt-inducible kinase

The renal proximal tubule (RPT) is a central locale for Na+ reabsorption, and blood pressure regulation. Na+ reabsorption in the RPT depends upon the Na,K-ATPase, which is controlled by a complex regulatory network, including Salt-Inducible Protein Kinase (SIK). SIKs are recently discovered members of the AMP-activated Protein Kinase (AMPK) family, which regulate salt homeostasis and metabolism in a number of tissues. In the RPT, SIK interacts with the Na,K-ATPase in the basolateral membrane (BM), regulating both the activity and level of Na,K-ATPase in the BM. Thus, Na,K-ATPase activity can be rapidly adjusted in response to changes in Na+ balance. Long-term changes in Na+ intake affect the state of SIK phosphorylation, and as a consequence the phosphorylation of TORCs, Transducers of Regulated CREB (cAMP Regulatory Element Binding Protein). Once phosphorylated, TORCs enter the nucleus, and activate transcription of the ATP1B1 gene encoding for the Na,K-ATPase beta subunit.

Changes in Na+/K+-ATPase activity, unsaturated fatty acids and metallothioneins in gills of the shore crab Carcinus aestuarii after dilute seawater acclimation

We have assessed the activity of Na(+)/K(+)-ATPase, cAMP, free fatty acids (FFA) and metallothionein (MT) in the posterior gills of the brackish water shore crab Carcinus aestuarii during acclimation to 10 ppt dilute seawater (DSW). Following 3-18 days acclimation in DSW specific activity of Na(+)/K(+)-ATPase in native gill homogenates and partially purified membrane vesicles was progressively increased, from 1.7- to 3.9-fold. After short-term acclimation of crabs in DSW with added sucrose to make media isosmotic with the haemolymph the specific Na(+)/K(+)-ATPase activity in homogenates was not increased, relative to SW enzyme activity. Moreover, hyposmotic conditions led to depletion of cAMP in gills. In partially purified membrane vesicles isolated from posterior gills, fatty acids with compositions 16:0, 18:0, 18:1, 20:4 and 20:5 dominated in both SW- and DSW-acclimated Carcinus. During a year in which the metabolic activity of crabs was increased, the arachidonic/linoleic acids ratio (ARA/LA) for DSW-acclimated crabs was markedly increased relative to that in SW. Increased Na(+)+K(+)-ATPase activity under hyposmotic stress may be modulated at least partially by the changed proportion of fatty acids in the purified membranes of posterior gills. Long-term acclimation of shore crabs to DSW resulted in a 2.6-fold increase in cytosolic metallothionein (MT) content in posterior gills over those in SW crabs. Assuming an antioxidant role of MT associated with intracellular zinc partitioning, the observed MT induction in posterior gills may be considered an adaptive response of C. aestuarii to hyposmotic stress.

Role of Sp1 in Transcription of Human ATP2A2 Gene in Keratinocytes

The ATP2A2 gene encodes Ca2+-dependent ATPase, the dysfunction of which causes Darier disease. In this study, we analyzed the promoter structure of the human ATP2A2 gene using primary normal human keratinocytes (NHK). Reporter assays showed that deletion of -550/-529, -488/-472, -390/-362, or -42/-21 resulted in a significant decrease in human ATP2A2 promoter activity. Electrophoretic mobility shift assay (EMSA) showed that Sp1 is a transcription factor that binds to the -550/-529 and -488/-472 regions of the promoter. Chromatin immunoprecipitation (ChIP) assay demonstrated that Sp1, but not Sp3, binds to the promoter region of the ATP2A2 gene in NHK cells in vivo. Knockdown of Sp1 expression by small interfering RNA resulted in a marked reduction in ATP2A2 promoter activity and ATP2A2 mRNA levels in NHK, suggesting that Sp1 positively transactivates the ATP2A2 promoter in NHK. This is early evidence demonstrating that Sp1 plays an important and positive role in ATP2A2 gene expression in NHK in vivo and in vitro.

Regulation of Na,K-ATPase alpha1 subunit gene transcription in response to low K(+): role of CRE/ATF- and GC box-binding proteins

Na,K-ATPase expression is upregulated in mammalian cells as a consequence of persistent inhibition of Na,K-ATPase enzymatic activity by low external K(+). We previously demonstrated that exposure of neonatal rat cardiac myocytes to low K(+) increased Na,K-ATPase alpha1 subunit mRNA content and promoter activity. In this work, we utilized transient transfection studies with rat Na,K-ATPase alpha1 subunit 5'-flanking region deletion plasmids to identify DNA sequences required for low K(+)-mediated stimulation of alpha1 subunit promoter expression in cardiac myocytes. Maximal low K(+)-responsiveness of the alpha1 promoter was found to be dependent on nucleotides from -102 to -62 and a downstream region from +53 to +261. Further analysis of the upstream low K(+)-responsive region using mutant constructs revealed that a CRE/ATF site at -70 to -63 and a GC box motif at -57 to -48 were both required for the effect of low K(+) on alpha1 subunit gene transcription. Electrophoretic mobility shift assays revealed that low K(+) increased binding of transcription factors to the GC box and, to a lesser extent, to the CRE/ATF site. Western blot analysis demonstrated that exposure of cardiac myocytes to low K(+) resulted in increased nuclear content of Sp1, Sp3 and CREB-1. Finally, a selective increase in phosphorylation of Sp1 was found in nuclear extracts from low K(+)-treated cells. We conclude that low K(+)-mediated upregulation of Na,K-ATPase alpha1 subunit gene expression in neonatal rat cardiac myocytes is dependent, in part, on CRE/ATF- and GC box-binding transcription factors.

Prostaglandins regulate transcription by means of prostaglandin response elements located in the promoters of mammalian Na,K-ATPase beta 1 subunit genes

Prostaglandins are potent products of arachidonic acid metabolism that play significant roles in regulating ion transport in the kidney. In the Madin Darby canine kidney (MDCK) cell line prostaglandin E(1) (PGE(1)) stimulates the activity of the Na,K-ATPase and regulates transcription. Transient transfection studies conducted in MDCK cells with a human Na,K-ATPase beta1 subunit promoter/luciferase construct, pHbeta1-1141 Luc, showed a PGE(1) stimulation. The PGE(1) stimulation was inhibited by the PGE receptor antagonists SC19220 and AH6809, indicating the involvement of EP1 receptors (coupled to phospholipase C) and EP2 receptors (coupled to adenylate cyclase), respectively. A prostaglandin-regulatory element (PGRE) within the beta1 subunit promoter (-110 to -92, AGTCCCTGC) is sufficient to elicit a PGE(1) stimulation in a heterologous promoter (in pLUC-MCS). Studies with promoter mutants indicated that in addition to the PGRE, an adjacent Sp1 site was also essential for regulation by PGE(1). Consistent with the involvement of Sp1 are the results of DNA affinity precipitation studies, which indicate that Sp1 as well as CREB, and Sp3 all bind to the PGRE. The involvement of this PGRE in transcriptional regulation of the Na,K-ATPase beta1 gene was examined in a number of species. Only human and chimpanzee promoters possessed an identical PGRE site, unlike dog, rat, and mouse, which possessed Sp1 sites in similar locations. Two alternative PGREs were subsequently identified. The sequence of the one of these PGREs (TGACCTTC, -445 to -438) was conserved throughout all species examined, suggesting its physiologic significance.

Divergent signaling pathways mediate induction of Na,K-ATPase α1 and β1 subunit gene transcription by low potassium

Prolonged inhibition of Na,K-ATPase enzymatic activity by exposure of a variety of mammalian cells to low external K+ yields a subsequent adaptive up-regulation of Na,K-ATPase expression. The aim of this study was to examine the intracellular signal transduction system that is responsible for mediating increased Na,K-ATPase subunit gene expression in primary cultures of neonatal rat cardiac myocytes. In this work, we show long-term inhibition of Na,K-ATPase function with 0.6 mM K+ resulted in hypertrophy of cardiac myocytes and augmentation of Na,K-ATPase alpha1 and beta1 subunit gene expression. Transient transfection experiments in neonatal rat cardiac myocytes demonstrated that low K+ induction of alpha1 and beta1 gene transcription was dependent on intracellular Ca2+ and activation of calcineurin. Based on effects of pharmacological inhibitors, protein kinase A (PKA), extracellular signal-regulated kinase 1/2 (ERK1/2) and histone deacetylase were found to be unique downstream components in the low K+ signal transduction pathway leading to increased alpha1 subunit promoter activity. Similarly, low K+-induced beta1 subunit gene transcription was dependent on activation of protein kinase C (PKC), c-Jun-N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). These findings indicate that persistent inhibition of Na,K-ATPase activity with low external K+ activates overlapping and Na,K-ATPase subunit gene-specific signaling pathways in cardiac myocytes.

Regulation of the Na-K-ATPase beta(1)-subunit promoter by multiple prostaglandin-responsive elements

Renal prostaglandins modulate the activity of a number of the transport systems in the kidney, including the Na-K-ATPase. Not only do prostaglandins have acute affects on renal Na-K-ATPase, but in addition prostaglandins have chronic affects, which include regulation at the transcriptional level. Previously, we have presented evidence that one such prostaglandin, PGE(1), stimulates the transcription of the human Na-K-ATPase beta(1)-subunit gene in Madin-Darby canine kidney cells via cAMP- and Ca(2+)-mediated pathways (Taub M, Borsick M, Geisel J, Matlhagela K, Rajkhowa T, and Allen C. Exp Cell Res 299: 1-14, 2004; Matlhagela K, Borsick M, Rajkhowa T, and Taub M. J Biol Chem 280: 334-346, 2005). Evidence was presented indicating that PGE(1) stimulation was mediated through the binding of cAMP-regulatory element binding protein (CREB) to a prostaglandin-responsive element (PGRE) as well as Sp1 binding to an adjacent Sp1 site. In this report, we present evidence from EMSAs and DNA affinity precipitation studies that another PGRE present in the Na-K-ATPase beta(1)-subunit promoter similarly binds CREB and Sp1. The evidence that indicates a requirement for CREB as well as Sp1 for gene activation through both PGREs (PGRE1 and PGRE3) includes studies with a dominant negative CREB (KCREB), Drosophila SL2 cells, and PGRE mutants. The results of these studies are indicative of a synergism between Sp1 and CREB in mediating regulation by PGRE3; while regulation occurring through PGRE1 also involves Sp1 and CREB, the mechanism appears to be distinct.

Involvement of EP1 and EP2 receptors in the regulation of the Na,K-ATPase by prostaglandins in MDCK cells

Prostaglandins are key regulators of ion transport in the kidney. In MDCK cells, which model distal tubule cells, the transcription of the Na,K-ATPase beta1 subunit is regulated by PGE1 and PGE2. To identify the EP receptors that mediate transcriptional regulation, transient transfection studies are conducted using the human beta1promoter/luciferase construct, pHbeta1-1141 Luc. The involvement of EP1 and EP2 receptors is indicated by studies with the EP1 selective agonist 17-phenyl trinor PGE2, and the EP2 selective agonist butaprost (which stimulate), as well as by studies with the antagonists SC-51089 (EP1 specific) and AH 6809 (EP1 and EP2 specific). Consistent with the involvement of Gs coupled EP2 receptors, is that the PGE1 stimulation is inhibited by the PKAI expression vector (encoding the protein kinase A (PKA) inhibitory protein), as well as by the myristolated PKA inhibitory peptide PKI. In addition to this evidence (for the involvement of EP2 receptors), evidence for the involvement of EP1 receptors in the PGE1 mediated stimulation of Na,K-ATPase beta subunit gene transcription includes the stimulatory effect of 17-phenyl trinor PGE2, as well as the inhibitory effects of SC-51089. Also consistent with the involvement of Gq coupled EP1 receptors, the PGE1 stimulation is inhibited by the PKCI vector (encoding the PKC inhibitory domain), the PKC inhibitor Go 6976, thapsigargin, as well as the calmodulin antagonists W7 and W13.

COX-2 expression in the guinea pig cochlea is partly altered by moderate sound exposure

The cyclooxygenase-2 isoform (COX-2) was found recently to be constitutively expressed in the guinea pig inner ear. To gain knowledge about its role in sound perception, alterations in the COX-2 level of moderate noise-stimulated cochleae were determined. Staining intensities were quantified in different regions using an immunohistochemical staining procedure and computer-assisted system. After 70 dB and 90 dB noise exposure for 1 h at 8000 Hz, COX-2 downregulation was observed in the organ of Corti, which was most prominent in Deiters' cells near Hensen cells and outer hair cells. In pillar cells, COX-2 levels were only slightly reduced after 70 dB but strongly diminished after 90 dB exposure. In Hensen cells, COX-2 was downregulated after 70 dB stimulation, revealing a decreasing COX-2 content from the third to the first turn of the cochlea and a homogeneously reduced enzyme expression in all three turns after 90 dB. The COX-2 content in inner hair cells was nearly identical to unexposed cochleae after 70 dB exposure but significantly reduced after 90 dB stimulation. In spiral ganglion cells, stria vascularis, spiral ligament and limbus, COX-2 expression was unchanged after 70 dB and 90 dB. We suggest that alterations in COX-2 expression might contribute to diminished sensitivity at the cochlea after noise exposure to reduce subsequent noise distress, termed sound conditioning.