Extracellular ATP and UTP activation of phospholipase D is mediated by protein kinase C-epsilon in rat renal mesangial cells

Isolation of Arabidopsis extracellular ATP binding proteins by affinity proteomics and identification of PHOSPHOLIPASE C-LIKE 1 as an extracellular protein essential for fumonisin B1 toxicity

ATP is secreted to the extracellular matrix, where it activates plasma membrane receptors for controlling plant growth and stress-adaptive processes. DOES NOT RESPOND TO NUCLEOTIDES 1 (DORN1), was the first plant ATP receptor to be identified but key downstream proteins remain sought after. Here, we identified 120 proteins secreted by Arabidopsis cell cultures and screened them for putative stress-responsive proteins using ATP-affinity purification. We report three Arabidopsis proteins isolated by ATP-affinity: PEROXIDASE 52, SUBTILASE-LIKE SERINE PROTEASE 1.7 and PHOSPHOLIPASE C-LIKE 1. In wild-type Arabidopsis, the expression of genes encoding all three proteins responded to fumonisin B1, a cell death-activating mycotoxin. The expression of PEROXIDASE 52 and PHOSPHOLIPASE C-LIKE 1 was altered in fumonisin B1-resistant salicylic acid induction-deficient (sid2) mutants. Exposure to fumonisin B1 suppressed PHOSPHOLIPASE C-LIKE 1 expression in sid2 mutants, suggesting that the inactivation of this gene might provide mycotoxin tolerance. Accordingly, gene knockout mutants of PHOSPHOLIPASE C-LIKE 1 were resistant to fumonisin B1-induced death. The activation of PHOSPHOLIPASE C-LIKE 1 gene expression by exogenous ATP was not blocked in dorn1 loss-of-function mutants, indicating that DORN1 is not required. Furthermore, exogenous ATP rescued both the wild type and the dorn1 mutants from fumonisin-B1 toxicity, suggesting that different ATP receptor(s) are operational in this process. Our results point to the existence of additional plant ATP receptor(s) and provide crucial downstream targets for use in designing screens to identify these receptors. Finally, PHOSPHOLIPASE C-LIKE 1 serves as a convergence point for fumonisin B1 and extracellular ATP signalling, and functions in the Arabidopsis stress response to fumonisin B1.

Post-column infused internal standard assisted lipidomics profiling strategy and its application on phosphatidylcholine research

Various lipidomics studies have revealed the potential of using phospholipids as disease biomarkers for conditions such as Alzheimer's disease, cancer, and sepsis. Establishing accurate quantification methods for targeted phospholipid analysis is important for making these potential markers more clinically relevant. Although a stable isotope labelled-internal standard method can provide good quantification accuracy for endogenous metabolite quantification, there are limited isotope labelled phosphatidylcholines (PCs) commercially available. For this reason, this study proposed a postcolumn infused-internal standard (PCI-IS) method for the accurate quantification of PCs. To demonstrate the quantification accuracy of the PCI-IS method combined with the matrix normalization factor (MNF), 2 LPCs and 6 PCs have been quantified in the human plasma specimens, and the results showed that the PCI-IS combined with MNF method can provide quantification results as accurate as those of the standard addition method (SAM) but without the need for the labor-intensive SAM procedure. We additionally applied the PCI-IS method for improving the PC profiling accuracy, and the results indicated that the biased estimation of the PC composition caused by the MEs can be resolved by PCI-IS correction. Finally, the method was applied to investigate drug resistance in lung cancer cells. Decreased levels of PCs in drug resistant cells disclose the potential role of PCs in drug resistance. We anticipate that the PCI-IS strategy could help quantitative lipidomics move forward and further contribute to various clinical and biomedical studies.

Extracellular Nucleotides and P2 Receptors in Renal Function

The understanding of the nucleotide/P2 receptor system in the regulation of renal hemodynamics and transport function has grown exponentially over the last 20 yr. This review attempts to integrate the available data while also identifying areas of missing information. First, the determinants of nucleotide concentrations in the interstitial and tubular fluids of the kidney are described, including mechanisms of cellular release of nucleotides and their extracellular breakdown. Then the renal cell membrane expression of P2X and P2Y receptors is discussed in the context of their effects on renal vascular and tubular functions. Attention is paid to effects on the cortical vasculature and intraglomerular structures, autoregulation of renal blood flow, tubuloglomerular feedback, and the control of medullary blood flow. The role of the nucleotide/P2 receptor system in the autocrine/paracrine regulation of sodium and fluid transport in the tubular and collecting duct system is outlined together with its role in integrative sodium and fluid homeostasis and blood pressure control. The final section summarizes the rapidly growing evidence indicating a prominent role of the extracellular nucleotide/P2 receptor system in the pathophysiology of the kidney and aims to identify potential therapeutic opportunities, including hypertension, lithium-induced nephropathy, polycystic kidney disease, and kidney inflammation. We are only beginning to unravel the distinct physiological and pathophysiological influences of the extracellular nucleotide/P2 receptor system and the associated therapeutic perspectives.

Increased Uridine Adenosine Tetraphosphate Concentrations in Plasma of Juvenile Hypertensives

BACKGROUND

Uridine adenosine tetraphosphate (Up4A) was been recently characterized as a potent vasoconstrictor. Up4A occurs in plasma from healthy subjects at concentrations sufficient to cause strong vasoconstrictive effects. In this study, Up4A concentrations in plasma from juvenile hypertensives and normotensives were determined.

METHODS AND RESULTS

Up4A was purified to homogeneity by preparative reverse phase high performance liquid-chromatography (HPLC), affinity chromatography HPLC, and analytic reverse phase HPLC from deproteinized plasma of juvenile hypertensives and normotensives. Mean total plasma Up4A concentration was significantly increased in juvenile hypertensives compared with juvenile normotensives (33.0+/-25.4 versus 3.7+/-0.9 nmol/L; mean+/-SEM, n=40 and 38, respectively; P<0.005). Accordingly, Up4A showed a significant association with juvenile hypertension (OR for ln(Up4A): 1.82; 95% CI 1.12, 2.95). Plasma Up4A concentrations correlated with left ventricular mass (Kendall-tau correlation coefficient 0.220, n=40; P<0.05) and intima media wall thickness (Kendall-tau correlation coefficient 0.296, n=40; P<0.05) in the hypertensives. Because the increased intima media thickness may be related to proliferative effects of Up4A, we studied the effects of Up4A on human vascular smooth muscle cell proliferation. The maximum proliferative effect of Up4A was 80.0+/-24.0% % above control (P<0.01). The proliferative effect of Up4A on smooth muscle cells is cell cycle-dependent, involving stimulation of S phase entry.

CONCLUSION

Circulating levels of Up4A are strongly associated with juvenile hypertension. The endothelium-derived vasoconstrictor Up4A may contribute to the early development of primary hypertension and is moreover an important risk factor of juvenile hypertension.

ATP potentiates interleukin-1 beta-induced MMP-9 expression in mesangial cells via recruitment of the ELAV protein HuR

Renal mesangial cells express high levels of matrix metalloproteinase 9 (MMP-9) in response to inflammatory cytokines such as interleukin (IL)-1 beta. We demonstrate here that the stable ATP analog adenosine 5'-O-(thiotriphosphate) (ATP gamma S) potently amplifies the cytokine-induced gelatinolytic content of mesangial cells mainly by an increase in the MMP-9 steady-state mRNA level. A Luciferase reporter gene containing 1.3 kb of the MMP-9 5'-promoter region showed weak responses to ATP gamma S but conferred a strong ATP-dependent increase in Luciferase activity when under the additional control of the 3'-untranslated region of MMP-9. By in vitro degradation assay and actinomycin D experiments we found that ATP gamma S potently delayed the decay of MMP-9 mRNA. Gel-shift and supershift assays demonstrated that three AU-rich elements (AREs) present in the 3'-untranslated region of MMP-9 are constitutively bound by complexes containing the mRNA stabilizing factor HuR. The RNA binding of these complexes was markedly increased by ATP gamma S. Mutation of each ARE element strongly impaired the RNA binding of the HuR containing complexes. Reporter gene assays revealed that mutation of one ARE did not affect the stimulatory effects by ATP gamma S, but mutation of all three ARE motifs caused a loss of ATP-dependent increase in luciferase activity without affecting IL-1 beta-inducibility. By confocal microscopy we demonstrate that ATP gamma S increased the nucleo cytoplasmic shuttling of HuR and caused an increase in the cytosolic HuR level as shown by cell fractionation experiments. Together, our results indicate that the amplification of MMP-9 expression by extracellular ATP is triggered through mechanisms that likely involve a HuR-dependent rise in MMP-9 mRNA stability.

P2 receptor antagonist PPADS inhibits mesangial cell proliferation in experimental mesangial proliferative glomerulonephritis

BACKGROUND

Although extracellular nucleotides have been shown to confer mitogenic effects in cultured rat mesangial cells through activation of purinergic P2 receptors (P2Y receptors), thus far the in vivo relevance of these findings is unclear. Virtually all cells and in particular the dense granules of platelets contain high levels of nucleotides that are released upon cell injury or platelet aggregation. In experimental mesangial proliferative glomerulonephritis in the rat (anti-Thy1 model), mesangiolysis and glomerular platelet aggregation are followed by a pronounced mesangial cell (MC) proliferative response leading to glomerular hypercellularity. Therefore, we examined the role of extracellular nucleotides and their corresponding receptors in nucleotide-stimulated cultured mesangial cells and in inflammatory glomerular disease using the P2 receptor antagonist PPADS.

METHODS

The effects of PPADS on nucleotide- or fetal calf serum (FCS)-stimulated proliferation of cultured MC were measured by cell counting and [3H]thymidine incorporation assay. After induction of the anti-Thy1 model, rats received injections of the P2-receptor antagonist PPADS at different doses (15, 30, 60 mg/kg BW). Proliferating mesangial and non-mesangial cells, mesangial cell activation, matrix accumulation, influx of inflammatory cells, mesangiolysis, microaneurysm formation, and renal functional parameters were assessed during anti-Thy1 disease. P2Y-mRNA and protein expression was assessed using RT-PCR and real time PCR, Northern blot analysis, in situ hybridization, and immunohistochemistry.

RESULTS

In cultured mesangial cells, PPADS inhibited nucleotide, but not FCS-stimulated proliferation in a dose-dependent manner. In the anti-Thy1 model, PPADS specifically and dose-dependently reduced early (day 3), but not late (day 8), glomerular mesangial cell proliferation as well as phenotypic activation of the mesangium and slightly matrix expansion. While no consistent effect was obtained in regard to the degree of mesangiolysis, influx of inflammatory cells, proteinuria or blood pressure, PPADS treatment increased serum creatinine and urea in anti-Thy1 rats. P2Y receptor expression (P2Y2 and P2Y6) was detected in cultured MC and isolated glomeruli, and demonstrated a transient marked increase during anti-Thy1 disease.

CONCLUSION

These data strongly suggest an in vivo role for extracellular nucleotides in mediating early MC proliferation after MC injury.

Regulation of phospholipase D in human placental trophoblasts by the P(2) purinergic receptor

Phospholipase D (PLD) is present in human placental tissue. Since purinergic receptor agonists activate PLD in many different cell types, we evaluated the purinergic activation of the enzyme in cultured trophoblasts from the placenta. We found that P(2) receptor agonists stimulate PLD. The preferred ligand for P(2X7) (P(2Z)) receptor subtype, BzBz-ATP (10(-3)M ), induced the enzyme more than ten times over basal (unstimulated) activity, while ATP caused a much smaller increase. ATPgammaS, ADP and UTP were even less effective, compared to BzBz-ATP or ATP. AMP and alpha,beta-methyl-ATP, a P(2X) agonist that is uniquely inactive on the P(2X7) subtype, had no effect. This represents the first suggestion of the presence of the P(2X7) type of receptor in human trophoblasts that was directly confirmed by immunoblot detection. The action of BzBz-ATP was dependent upon the presence of calcium in the culture medium and was inhibited by high (5m M ) Mg(++) concentration. P(2X7) receptor subtype specific antagonists, ATP-2',3'-dialdehyde (o-ATP), CBB and the broad specificity P(2) inhibitor PPADS inhibited the effect of BzBz-ATP. Pertussis toxin treatment did not inhibit the effect. Down-regulation of cPKC/nPKC isoforms by prolonged PMA treatment (36 h, 10(-7)M ) prevented the stimulation of PLD by P(2) agonists or the calcium ionophore A-23187. PLA(2) inhibitors did not block the effect of BzBz-ATP. The possibility for a calcium influx related interdependence of PLC and PLD was evaluated. For PLC activation, UTP and ATP surpassed BzBz-ATP, while ionophore did not elevate PLC (assessed by IP(3) measurements). This suggested the predominance of a P(2Y2) receptor in the whole cell in gross activation of PLC. PLD was affected with a reversed order of potency. These results and the dependence of PLD on PKC activity implies that a restricted, membrane localized calcium flux activates PKC and in turn, mediates the P(2X7) dependent stimulation of PLD. This may have implications for physiologic regulation of trophoblast function.

Ionic currents activated via purinergic receptors in the cumulus cell-enclosed mouse oocyte

Several chemical signals synthesized in the ovary, including neurotransmitters, have been proposed to serve as regulators of folliculogenesis, however, their mechanisms of action have not been completely elucidated. Here, electrophysiological and molecular biology techniques were used to study responses generated via purinergic stimulation in cultured mouse cumulus cell-enclosed oocytes (CEOs). Application of extracellular ATP elicited depolarizing responses in CEOs. Using the voltage clamp technique by impaling oocytes with two microelectrodes, we determined that these responses were mainly due to activation of two distinct ionic currents. The first corresponded to the opening of Ca2+-dependent Cl- channels (I(Cl(Ca))) and the second to the opening of Ca2+-independent channels that are permeable to Na+ (I(c+)). The potency order for different nucleotides (50 micro M) was UTP > ATP > 2meS-ATP > ADP, and alpha,betame-ATP and adenosine were found to be inactive. Suramin (100 micro M) blocked the response elicited by ATP or UTP. In addition, voltage dependent K+ currents activated by depolarization of CEOs were characterized. All CEO ionic currents recorded from the oocyte were completely inhibited by octanol (1 mM), a gap junction blocker. Thus, purinergic responses and K+ currents originate mainly in the membrane of cumulus cells. Transcripts of the purinergic receptor P2Y2 subtype were amplified by polymerase chain reaction from the cDNA of granulosa cells or cumulus cells. This study shows that P2Y2 receptors are expressed in CEOs, and that their stimulation opens at least two different types of ion channels. Both the ion channels and the receptors seemed to be located in the cumulus cells, which transmit their corresponding electrical signals to the oocyte via gap junction channels.

Extracellular ATP and UTP activate the protein kinase B/Akt cascade via the P2Y(2) purinoceptor in renal mesangial cells

Extracellular nucleotides can activate a common purinoceptor mediating various cell responses. In this study we report that stimulation of rat mesangial cells with ATP and UTP leads to a rapid activation of the protein kinase B/Akt (PKB) pathway. Time-course studies reveal a rapid and transient phosphorylation of both Ser(473) and Thr(308) of PKB with a maximal effect after 5 min of stimulation. The response is concentration-dependent with a maximal effect at 30 microM of ATP and UTP. Western blot analysis of mesangial cells reveals the expression of the isoenzymes PKB-alpha and PKB-gamma, but not the PKB-beta. ATP and UTP also activate the upstream located PI 3-kinase-dependent kinase. Furthermore, the ATP- and UTP-induced PKB phosphorylation is abolished by two inhibitors of the PI 3-kinase. In addition, suramin, a putative P2Y(2) receptor antagonist, and pertussis toxin, an inhibitor of G(i)/G(o) activation, markedly block ATP- and UTP-induced PKB phosphorylation. A series of ATP and UTP analogues were tested for their ability to stimulate PKB phosphorylation. UTP, ATP and gamma-thio-ATP are the only compounds capable of activating PKB. Stress-induced apoptosis of mesangial cells is reduced by the stable ATP analogue, gamma-thio-ATP, and this inhibitory effect is reversed in the presence of LY 294002. In summary, these results demonstrate that extracellular nucleotides are able to activate the PI 3-kinase/PDK/PKB cascade via the P2Y(2)-receptor and a pertussis toxin-sensitive G(i) protein. Moreover, in mesangial cells this cascade may have an important role in the antiapoptotic response but not in the mitogenic or inflammatory response produced by extracellular nucleotides.

P2 receptors in regulation of renal microvascular function

In the last 10-15 years, interest in the physiological role of P2 receptors has grown rapidly. Cellular, tissue, and organ responses to P2 receptor activation have been described in numerous in vivo and in vitro models. The purpose of this review is to provide an update of the recent advances made in determining the involvement of P2 receptors in the control of renal hemodynamics and the renal microcirculation. Special attention will be paid to work published in the last 5-6 years directed at understanding the role of P2 receptors in the physiological control of renal microvascular function. Several investigators have begun to evaluate the effects of P2 receptor activation on renal microvascular function across several species. In vivo and in vitro evidence consistently supports the hypothesis that P2 receptor activation by locally released extracellular nucleotides influences microvascular function. Extracellular nucleotides selectively influence preglomerular resistance without having an effect on postglomerular tone. P2 receptor inactivation blocks autoregulatory behavior whereas responsiveness to other vasoconstrictor agonists is retained. P2 receptor stimulation activates multiple intracellular signal transduction pathways in preglomerular smooth muscle cells and mesangial cells. Renal microvascular cells and mesangial cells express multiple subtypes of P2 receptors; however, the specific role each plays in regulating vascular and mesangial cell function remains unclear. Accordingly, the results of studies performed to date provide strong support for the hypothesis that P2 receptors are important contributors to the physiological regulation of renal microvascular and/or glomerular function.

Renal microvascular effects of P2 receptor stimulation

1. The field of extracellular nucleotides and purinoceptors has undergone a resurgence of interest and enthusiasm in the past decade. More and more investigators are probing the physiological and pathophysiological roles of P2 receptors in virtually every organ system, including the kidney. 2. With this renewed interest has come a new appreciation for the roles extracellular adenine nucleotides can play in regulating or modulating renal function. In the past 5 years, investigators have provided compelling evidence that extracellular nucleotides, working through activation of P2 purinoceptors, have a significant impact on renal microvascular function, mesangial cell function and on renal epithelial transport. 3. Evidence has been uncovered that implicates P2 receptor activation in mediating renal microvascular autoregulatory behaviour. Locally released ATP has a direct paracrine and/or autocrine effect modulating renal epithelial transporters and tubular epithelial channels to influence tubular fluid composition. 4. While the specific roles of extracellular nucleotides and their receptors in the kidney have not been absolutely identified, it now appears clear that endogenously released ATP may play a significant role in regulating kidney function. 5. The purpose of the present review is to update our current understanding of the effect of P2 receptor activation on renal microvascular function and to detail the signal transduction mechanisms known to be involved.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Potentiation of cytokine induction of group IIA phospholipase A(2) in rat mesangial cells by ATP and adenosine via the A2A adenosine receptor

1. In rat mesangial cells extracellular nucleotides were found to increase arachidonic acid release by a cytosolic phospholipase A(2) through the P2Y(2) purinergic receptor. 2. In this study we investigated the effects of ATP and UTP on interleukin-1ss (IL-1ss)-induced mRNA expression and activity of group IIA phospholipase A(2) (sPLA(2)-IIA) in rat mesangial cells. 3. Treatment of cells for 24 h with extracellular ATP potentiated IL-1ss-stimulated sPLA(2)-IIA induction, whereas UTP had no effect. 4. We obtained the following evidence that the P2Y(2) receptor is not involved in the potentiation of sPLA(2)-IIA induction: (i) ATP-gamma-S had no enhancing effect; (ii) suramin, a P(2) receptor antagonist, did not inhibit ATP-mediated potentiation; (iii) inhibition of degradation of extracellular nucleotides by the 5'-ectonucleotidase inhibitor AOPCP did not enhance sPLA(2)-IIA induction and (iv) adenosine deaminase treatment completely abolished the ATP-mediated potentiation of sPLA(2)-IIA induction. 5. In contrast, treatment of mesangial cells with adenosine or the A2A receptor agonist CGS 21680 mimicked the effects of ATP in enhancing IL-1ss-stimulated sPLA(2)-IIA induction, whereas the specific A2A receptor antagonist ZM 241385 completely abolished the potentiating effect of ATP or adenosine. 6. The protein kinase A inhibitor Rp-8-Br-cyclic AMPS dose-dependently inhibited the enhancing effect of ATP or adenosine indicating the participation of an adenosine receptor-mediated cyclic AMP-dependent signalling pathway. 7. These data indicate that ATP mediates proinflammatory long-term effects in rat mesangial cells via its degradation product adenosine through the A2A receptor resulting in potentiation of sPLA(2)-IIA induction.

The D2s dopamine receptor stimulates phospholipase D activity: a novel signaling pathway for dopamine

The D2 dopamine receptor isoforms signal to a variety of cellular effectors in both the central nervous system and periphery. Two alternative splice forms of the D2 dopamine receptor exist, the D2s (short) and D2l (long), which has an insertion of 29 amino acids in the third intracellular loop (). In cells of the anterior lobe of the pituitary, D2 dopamine receptors (both forms) are present on lactotroph cells coupled to the inhibition of adenylyl cyclase, activation of voltage-gated calcium channels, and inhibition of potassium channels. We describe here a novel signaling pathway for the D2s, which is the activation of phospholipase D (PLD). GH4C1 cells, a clonal line derived from a rat pituitary tumor, were stably transfected with the gene encoding the D2s, generating GH4-121 cells. Treatment of GH4-121 cells with a dopaminergic agonist resulted in activation of PLD in both a dose-dependent and time-dependent manner. This signaling pathway was not inhibited by prior treatment of cells with pertussis toxin at concentrations that ablate other D2s receptor signaling in this cell line. The stimulation of PLD activity by D2s appeared to correlate with the presence of a specific protein kinase C isoform, PKCepsilon. The D2s stimulation of PLD activity was blocked by preincubation of cells with C3 exoenzyme, indicating that the stimulation of PLD may involve Rho family members. The stimulation of PLD by dopaminergic agonists took place in the absence of any detectable stimulation of phosphoinositide metabolism.

Growth hormones reverse desensitization of P2Y(2) receptors in rat mesangial cells

Rat glomerular mesangial cells (GMC) express P2Y(2) purinoceptors and respond to nucleotide stimuli with a transient increase in the cytosolic Ca(2+) concentration and the receptors desensitize upon repeated stimulation with nucleotide. We demonstrate that there is a cross-talk from the signaling of tyrosine kinase to P2Y(2) receptors. For most cells repeated applications of ATP completely abolished the response, as did activation of PKC with 500 nM PMA. In contrast, preincubation with the PKC inhibitor chelerythrine (100 nM) prevented desensitization. Desensitization after application of ATP was reversed by subsequent incubation with PDGF-BB (50 ng/ml) or insulin (660 mU/ml). We conclude that the desensitization is caused by phosphorylation due to PKC and is under the control of growth factors. The findings support the hypothesis that growth hormones potentiate nucleotides as proinflammatory mediators and we hypothesize that they have bearing on the hyperfiltration seen in diabetes.

Extracellular nucleotides activate the p38-stress-activated protein kinase cascade in glomerular mesangial cells

1. Extracellular ATP and UTP have been reported to activate a nucleotide receptor (P2Y2-receptor) that mediates arachidonic acid release with subsequent prostaglandin formation, a reaction critically depending on the activity of a cytosolic phospholipase A2. In addition, extracellular nucleotides trigger activation of the classical mitogen-activated protein kinase (MAPK) cascade and cell proliferation as well as of the stress-activated protein kinase (SAPK) cascade. 2. In this study, we report that ATP and UTP are also able to activate the p38-MAPK pathway as measured by phosphorylation of the p38-MAPK and its upstream activators MKK3/6, as well as phosphorylation of the transcription factor ATF2 in a immunocomplex-kinase assay. 3. Time courses reveal that ATP and UTP induce a rapid and transient activation of the p38-MAPK activity with a maximal activation after 5 min of stimulation which declined to control levels over the next 20 min. 4. A series of ATP and UPT analogues were tested for their ability to stimulate p38-MAPK activity. UTP and ATP were very effective analogues to activate p38-MAPK, whereas ADP and gamma-thio-ATP had only moderate activating effects. 2-Methyl-thio-ATP, beta gamma-imido-ATP, AMP, adenosine and UDP had no significant effects of p38-MAPK activity. In addition, the extracellular nucleotide-mediated effect on p38-MAPK was almost completely blocked by 1 mM of suramin, a putative P2-purinoceptor antagonist. 5. In summary, these results demonstrate for the first time that extracellular nucleotides are able to activate the MKK3/6- p38-MAPK cascade most likely via the P2Y2-receptor. Moreover, this finding implies that all three MAPK subtypes are signalling candidates for extracellular nucleotide-stimulated cell responses.

ATP-stimulated Ca2+ influx and phospholipase D activities of a rat brain-derived type-2 astrocyte cell line, RBA-2, are mediated through P2X7 receptors

This study characterizes and examines the P2 receptor-mediated signal transduction pathway of a rat brain-derived type 2 astrocyte cell line, RBA-2. ATP induced Ca2+ influx and activated phospholipase D (PLD). The ATP-stimulated Ca2+ influx was inhibited by pretreating cells with P2 receptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), in a concentration-dependent manner. The agonist 2'- and 3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP) stimulated the largest increases in intracellular Ca2+ concentrations ([Ca2+]i); ATP, 2-methylthioadenosine triphosphate tetrasodium, and ATPgammaS were much less effective, whereas UTP, ADP, alpha,beta-methylene-ATP, and beta,gamma-methylene-ATP were ineffective. Furthermore, removal of extracellular Mg2+ enhanced the ATP- and BzATP-stimulated increases in [Ca2+]i. BzATP stimulated PLD in a concentration- and time-dependent manner that could be abolished by removal of extracellular Ca2+ and was inhibited by suramin, PPADS, and oxidized ATP. In addition, PLD activities were activated by the Ca2+ mobilization agent, ionomycin, in an extracellular Ca2+ concentration-dependent manner. Both staurosporine and prolonged phorbol ester treatment inhibited BzATP-stimulated PLD activity. Taken together, these data indicate that activation of the P2X7 receptors induces Ca2+ influx and stimulates a Ca2+-dependent PLD in RBA-2 astrocytes. Furthermore, protein kinase C regulates this PLD.

Mitogenic signaling by ATP/P2Y purinergic receptors in astrocytes: involvement of a calcium-independent protein kinase C, extracellular signal-regulated protein kinase pathway distinct from the phosphatidylinositol-specific phospholipase C/calcium pathway

Activation of ATP/P2Y purinergic receptors stimulates proliferation of astrocytes, but the mitogenic signaling pathway linked to these G-protein-coupled receptors is unknown. We have investigated the role of extracellular signal-regulated protein kinase (ERK) in P2Y receptor-stimulated mitogenic signaling as well as the pathway that couples P2Y receptors to ERK. Downregulation of protein kinase C (PKC) in primary cultures of rat cerebral cortical astrocytes greatly reduced the ability of extracellular ATP to stimulate ERK. Because occupancy of P2Y receptors also leads to inositol phosphate formation, calcium mobilization, and PKC activation, we explored the possibility that signaling from P2Y receptors to ERK is mediated by a phosphatidylinositol-specific phospholipase C (PI-PLC)/calcium pathway. However, neither inhibition of PI-PLC nor chelation of calcium significantly reduced ATP-stimulated ERK activity. Moreover, a preferential inhibitor of calcium-dependent PKC isoforms, Gö 6976, was significantly less effective in blocking ATP-stimulated ERK activity than GF102903X, an inhibitor of both calcium-dependent and -independent PKC isoforms. Furthermore, ATP stimulated a rapid translocation of PKCdelta, a calcium-independent PKC isoform, but not PKCgamma, a calcium-dependent PKC isoform. ATP also stimulated a rapid increase in choline, and inhibition of phosphatidylcholine hydrolysis blocked ATP-evoked ERK activation. These results indicate that P2Y receptors in astrocytes are coupled independently to PI-PLC/calcium and ERK pathways and suggest that signaling from P2Y receptors to ERK involves a calcium-independent PKC isoform and hydrolysis of phosphatidylcholine by phospholipase D. In addition, we found that inhibition of ERK activation blocked extracellular ATP-stimulated DNA synthesis, thereby indicating that the ERK pathway mediates mitogenic signaling by P2Y receptors.

Mitogenic signaling by ATP/P2Y purinergic receptors in astrocytes: involvement of a calcium-independent protein kinase C, extracellular signal-regulated protein kinase pathway distinct from the phosphatidylinositol-specific phospholipase C/calcium pathway

Activation of ATP/P2Y purinergic receptors stimulates proliferation of astrocytes, but the mitogenic signaling pathway linked to these G-protein-coupled receptors is unknown. We have investigated the role of extracellular signal-regulated protein kinase (ERK) in P2Y receptor-stimulated mitogenic signaling as well as the pathway that couples P2Y receptors to ERK. Downregulation of protein kinase C (PKC) in primary cultures of rat cerebral cortical astrocytes greatly reduced the ability of extracellular ATP to stimulate ERK. Because occupancy of P2Y receptors also leads to inositol phosphate formation, calcium mobilization, and PKC activation, we explored the possibility that signaling from P2Y receptors to ERK is mediated by a phosphatidylinositol-specific phospholipase C (PI-PLC)/calcium pathway. However, neither inhibition of PI-PLC nor chelation of calcium significantly reduced ATP-stimulated ERK activity. Moreover, a preferential inhibitor of calcium-dependent PKC isoforms, Gö 6976, was significantly less effective in blocking ATP-stimulated ERK activity than GF102903X, an inhibitor of both calcium-dependent and -independent PKC isoforms. Furthermore, ATP stimulated a rapid translocation of PKCdelta, a calcium-independent PKC isoform, but not PKCgamma, a calcium-dependent PKC isoform. ATP also stimulated a rapid increase in choline, and inhibition of phosphatidylcholine hydrolysis blocked ATP-evoked ERK activation. These results indicate that P2Y receptors in astrocytes are coupled independently to PI-PLC/calcium and ERK pathways and suggest that signaling from P2Y receptors to ERK involves a calcium-independent PKC isoform and hydrolysis of phosphatidylcholine by phospholipase D. In addition, we found that inhibition of ERK activation blocked extracellular ATP-stimulated DNA synthesis, thereby indicating that the ERK pathway mediates mitogenic signaling by P2Y receptors.

1,25-dihydroxyvitamin D3 and TPA activate phospholipase D in Caco-2 cells: role of PKC-alpha

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] and 12-O-tetradecanoylphorbol 13-acetate (TPA) both activated phospholipase D (PLD) in Caco-2 cells. GF-109203x, an inhibitor of protein kinase C (PKC) isoforms, inhibited this activation by both of these agonists. 1,25(OH)2D3 activated PKC-alpha, but not PKC-beta1, -betaII, -delta, or -zeta, whereas TPA activated PKC-alpha, -beta1, and -delta. Chronic treatment with TPA (1 microM, 24 h) significantly reduced the expression of PKC-alpha, -betaI, and -delta and markedly reduced the ability of 1,25(OH)2D3 or TPA to acutely stimulate PLD. Removal of Ca2+ from the medium, as well as preincubation of cells with Gö-6976, an inhibitor of Ca2+-dependent PKC isoforms, significantly reduced the stimulation of PLD by 1,25(OH)2D3 or TPA. Treatment with 12-deoxyphorbol-13-phenylacetate-20-acetate, which specifically activates PKC-betaI and -betaII, however, failed to stimulate PLD. In addition, the activation of PLD by 1,25(OH)2D3 or TPA was markedly reduced or accentuated in stably transfected cells with inhibited or amplified PKC-alpha expression, respectively. Taken together, these observations indicate that PKC-alpha is intimately involved in the stimulation of PLD in Caco-2 cells by 1,25(OH)2D3 or TPA.

Ca2+ response of rat mesangial cells to ATP analogues

The aim of this investigation was to characterise the effects of ATP analogues and UTP on the single cell intracellular Ca2+ concentration ([Ca2+]i) in cultured rat mesangial cells. Typically, there were two phases in the Ca2+ response to the agonists, an initial fast transient peak and a subsequent slower decline, or plateau, phase. For the peak amplitude in [Ca2+]i the agonists had about equal effect. But when taking in consideration the percentage of responding cells and the integrated Ca2+ response over 1 min, the order of efficacy of nucleotide agonists (100 microM) was UTP = ATP > ATPgammaS > ADP = 2MeS-ATP (2-methylthio-ATP). Adenosine, AMP and beta,gamma-Me-ATP (100 microM) had no effect. Suramine (100 microM) and reactive blue (50 microM) decreased the number of responding cells. Removing Ca2+ from the bath diminished neither the peak in [Ca2+]i nor the percentage of responding cells, but the average [Ca2+]i increase in 1 min was significantly reduced. The results indicate that P2Y2 receptors are present in rat mesangial cells but it cannot be excluded that there are receptors distinct from P2Y2 which also mediate a rise in [Ca2+]i.

Selective ceramide binding to protein kinase C-alpha and -delta isoenzymes in renal mesangial cells

Ceramide is an important lipid second messenger produced by sphingolipid metabolism in cells exposed to a limited number of agonists and in turn triggers several cell responses in a protein kinase C (PKC)-dependent manner. Stimulation of mesangial cells with a radioiodinated photoaffinity labeling analogue of ceramide, (N-[3-[[[2-(125I)iodo-4-[3-(trifluoromethyl)-3H-diazirin-3-yl]benz yl] oxy]carbonyl]propanoyl]-D-erythro-sphingosine) ([125I]-TID-ceramide), defines PKC-alpha and PKC-delta as direct targets of ceramide. No binding of ceramide to PKC-epsilon and PKC-zeta could be detected. Moreover, TID-ceramide selectively binds to recombinant PKC-alpha and -delta but not to PKC-epsilon and -zeta isoenzymes. In vitro kinase activity assays reveal that only the binding of ceramide to PKC-alpha is accompanied by an increase in kinase activity. In contrast, there is no change in in vitro kinase activity of the other isoforms tested, i.e., PKC-delta, -epsilon, and -zeta, toward any of the conventional substrates tested. However, it is noteworthy that PKC-delta shows a decreased autophosphorylation upon ceramide binding. In vivo, activation of PKC-alpha by ceramide is monitored by a delayed translocation of the isoform from the cytosol to the membrane fraction, detectable after 1 h of stimulation. In contrast, neither PKC-delta, nor -epsilon nor -zeta is redistributed by ceramide. One functional cell response mediated by PKC-alpha in mesangial cells is a negative feedback regulation of ligand-stimulated phosphoinositide hydrolysis. When cells are pretreated with ceramide, ATP-induced inositol trisphosphate formation is time-dependently reduced. A maximal inhibition is observed after 2 h of ceramide exposure. In summary, these results suggest that ceramide selectively interacts with the alpha- and delta-isoforms of PKC in mesangial cells. Whereas PKC-alpha is activated with pronounced inhibition of hormone-stimulated phosphoinositide signaling, PKC-delta displays a decrease in its autophosphorylation, suggesting a negative role of ceramide binding on PKC-delta activity.

Tamoxifen induces selective membrane association of protein kinase C epsilon in MCF-7 human breast cancer cells

Tamoxifen, a synthetic antiestrogen, is known for its antitumoral action in vivo; however, it is well accepted that many tamoxifen effects are elicited via estrogen receptor-independent routes. Previously, we reported that tamoxifen induces PKC translocation in fibroblasts. In the present study, we investigated the influence of tamoxifen, and several triphenylethylene derivatives, on protein kinase C (PKC) in MCF-7 human breast cancer cells. As measured by Western blot analysis, tamoxifen elicited isozyme-specific membrane association of PKC-epsilon, which was time-dependent (as early as 5 min post-treatment) and dose-dependent (5.0-20 microM). Tamoxifen did not influence translocation of alpha, beta, gamma, delta or zeta PKC isoforms. Structure-activity relationship studies demonstrated chemical requirements for PKC-epsilon translocation, with tamoxifen, 3-OH-tamoxifen and clomiphene being active. Compounds without the basic amino side chain, such as triphenylethylene, or minus a phenyl group, such as N,N-dimethyl-2-[(4-phenylmethyl)phenoxy]ethanamine, were not active. In vitro cell growth assays showed a correlation between agent-induced PKC-epsilon translocation and inhibition of cell growth. Exposure of cells to clomiphene resulted in apoptosis. Since PKC-epsilon has been associated with cell differentiation and cellular growth-related processes, the antiproliferative influence of tamoxifen on MCF-7 cells may be related to the interaction with PKC-epsilon.

Glomerular mesangial cells: electrophysiology and regulation of contraction

Mesangial cells are smooth muscle-like pericytes that abut and surround the filtration capillaries within the glomerulus. Studies of the fine ultrastructure of the glomerulus show that the mesangial cell and the capillary basement membrane form a biomechanical unit capable of regulating filtration surface area as well as intraglomerular blood volume. Structural and functional studies suggest that mesangial cells regulate filtration rate in both a static and dynamic fashion. Mesangial excitability enables a homeostatic intraglomerular stretch reflex that integrates an increase in filtration pressure with a reduction in capillary surface area. In addition, mesangial tone is regulated by diverse vasoactive hormones. Agonists, such as angiotensin II, contract mesangial cells through a signal transduction pathway that releases intracellular stores of Ca2+, which subsequently activate nonselective cation channels and Cl- channels to depolarize the plasma membrane. The change in membrane potential activates voltage-gated Ca2+ channels, allowing Ca2+ cell entry and further activation of depolarizing conductances. Contraction and entry of cell Ca2+ are inhibited only when Ca2+-activated K+ channels (BK(Ca)) are activated and the membrane is hyperpolarized toward the K+ equilibrium potential. The mesangial BK(Ca) is a weak regulator of contraction in unstimulated cells; however, the gain of the feedback is increased by atrial natriuretic peptide, nitric oxide, and the second messenger cGMP, which activates protein kinase G and decreases both the voltage and Ca2+ activation thresholds of BK(Ca) independent of sensitivity. This enables BK(Ca) to more effectively counter membrane depolarization and voltage-gated Ca2+ influx. After hyperpolarizing the membrane, BK(Ca) rapidly inactivates because of dephosphorylation by protein phosphatase 2A. Regulation of ion channels has been linked casually to hyperfiltration during early stages of diabetes mellitus. Determining the signaling pathways controlling the electrophysiology of glomerular mesangial cells is important for understanding how glomerular filtration rate is regulated in health and disease.

Extracellular ATP: a growth factor for vascular smooth muscle cells

1. Extracellular adenosine triphosphate (ATP) is mitogenic for vascular smooth muscle cells (VSMC) and stimulates several events that are important for cell proliferation: DNA synthesis, protein synthesis, increase of cell number, immediate early genes, cell-cycle progression, and tyrosine phosphorylation. 2. Receptor characterization indicates mitogenic effects of both P2U and P2Y receptors. The P2X receptor is lost in cultured VSMC and is not involved. Several related biological substances such as UTP, ITP, GTP, AP4A, ADP, and UDP are also mitogenic. 3. Signal transduction is mediated via Gq-proteins, phospholipase C beta, phospholipase D, diacyl glycerol, protein kinase C alpha, delta, Raf-1, MEK, and MAPK. 4. ATP acts synergistically with polypeptide growth factors (PDGF, bFGF, IGF-1, EGF, insulin) and growth factors acting via G-protein-coupled receptors (noradrenaline, neuropeptide Y, 5-hydroxytryptamine, angiotensin II, endothelin-1). 5. The mitogenic effects have been demonstrated in rat, porcine, and bovine VSMC and cells from human coronary arteries, aorta, and subcutaneous arteries and veins. 6. The trophic effects on VSMC and the abundant sources for extracellular ATP in the vessel wall make a pathophysiological role probable in the development of atherosclerosis, neointima-formation after angioplasty, and possibly hypertension.

Diadenosine polyphosphate-mediated activation of phospholipase D in isolated rat liver cells

Diadenosine polyphosphates (ApnAs) can, through interaction with appropriate purinoceptors, affect a range of cellular activities. Ap4A, the most prominent naturally occurring diadenosine polyphosphate, stimulates alterations in intracellular calcium homeostasis and subsequent activation of glycogen breakdown in isolated liver cells. Here we show that Ap4A, and other naturally occurring diadenosine polyphosphates, also stimulates phospholipase D (PLD) activity in isolated rat liver cells. The characteristics of Ap4A-mediated activation of PLD are similar to those for the activation of PLD by extracellular ATP. These results are discussed in the context of the relation between diadenosine polyphosphate- and adenine mononucleotide-mediated cellular signalling processes.

Phosphatidylcholine-specific phospholipase C regulates thapsigargin-induced calcium influx in human lymphocytes

The involvement of phosphatidylcholine-specific phospholipase C (PC-PLC) and D (PC-PLD) in the regulation of the thapsigargin-induced Ca2+ increase was investigated. Pretreatment of human lymphocytes with the PC-PLC inhibitors D609 or U73122 enhanced the thapsigargin-induced Ca2+ influx. By contrast, no effect was observed in the presence of phospholipase D inhibitor butanol. Addition of exogenous PC-PLC but not PC-PLD to lymphocytes prestimulated with thapsigargin led to a decrease of intracellular Ca2+. In addition, thapsigargin was shown to release diacylglycerol (DAG) from cellular phosphatidylcholine pools. The thapsigargin-induced DAG formation was inhibited by U73122 and D609 but not by butanol. Moreover, no formation of the PC-PLD activity marker phosphatidylbutanol was detected. Thapsigargin-induced DAG formation was dependent on the Ca2+ entry, as it was abolished in the absence of extracellular Ca2+ or in the presence of Ni2+. Further investigations demonstrated that the inhibition of the cellular DAG target, protein kinase C (PKC), enhanced thapsigargin-induced Ca2+ increase, whereas direct PKC activation had an inhibitory effect. Taken together, our results reveal the involvement of PC-PLC in the regulation of the thapsigargin-induced Ca2+ increase and point to the existence of a physiologic feedback mechanism activated by Ca2+ influx and acting via consecutive activation of PC-PLC and PKC to limit the rise of intracellular Ca2+.

Uridine nucleotide receptors and their ligands: structural, physiological, and pathophysiological aspects, with special emphasis on the nervous system

This review presents data on metabotropic uridine nucleotide receptor subtypes (P2UR) activated by UTP, sometimes also by UDP and/or ATP. Some chemical details of receptor subtypes and ligand interactions are described. Ligand-activated P2UR subtypes may couple to different second messengers, yet little is known about the nature of the coupling G-proteins. Data evaluating UTP as a physiological ligand include UTP origin, release and metabolism and illuminate especially roles for P2UR in the nervous system. No evidence shows UTP as a synaptic transmitter; sympathetic neurons may, however, carry P2UR allowing UTP-stimulation of norepinephrine release. UTP and derivatives act as therapeutic agents in several diseases involving mutated genes of transepithelial conductance regulators, including cystic fibrosis. This focuses interest to the synthesis of new compounds. Further, therapeutically used pyrimidine and pyrimidine analogues are suspected to have CNS-pathological effects. The presently scarce information in these areas strongly underlines the need for and importance of intense research on the suspected pyrimidine derivative triggered pathology as well as on the role of P2UR receptors in physiology and pathophysiology.

Regulation of filtration rate by glomerular mesangial cells in health and diabetic renal disease

The rate of renal filtration is in large part responsible for volume and electrolyte balance in an organism. Integral components of the renal glomerulus are the mesangial cells (MCs), excitable renal pericytes that regulate the glomerular filtration rate by modulating the surface area of the capillaries. Similar to vascular smooth muscle, the signal transduction pathways and ion selective channels regulating isotonic and isometric contraction of MCs are dependent on the voltage-gated Ca influx. During the response to contractile agonists, both Cl and nonselective cation channels play critical roles to depolarize the membrane potential and activate Ca channels. The relaxation pathways involve a negative-feedback mechanism that counteracts mesangial contraction by regulating voltage-dependent Ca signaling. Part of the feedback response involves the activation of plasmalemmal K channels, which hyperpolarize the membrane potential and inhibit voltage-gated Ca entry. This calcium- and voltage-activated feedback K (BKCa) channel shares biophysical, pharmacologic, and molecular properties with the BKCa channels identified in brain and muscle, and with the sio gene product as expressed in Xenopus laevis oocytes. Systemic hormones, such as atrial natriuretic peptide, and paracrine factors, such as nitric oxide (NO), use guanosine 3',5'-cyclic monophosphate (GMP) as a second messenger and enhance the gain in this feedback system by decreasing the voltage and Ca activation thresholds for BKCa. Diabetes mellitus is often associated with high rates of glomerular filtration, mesangial expansion, and secretory abnormalities of the basement membrane. NO-mediated increases in negative-feedback regulation of mesangial tone may attribute, in part, to the pathology of hyperfiltration. Stimulation of inducible nitric oxide synthetase in glomerular MCs by inflammatory cytokines is a possible positive-feedback pathway that contributes to further glomerular destruction. In addition, high ambient glucose, through modulation of BKCa activity, facilitates MC relaxation and thus propagates hyperfiltration. Since cellular arachidonic acid is metabolically linked to extracellular glucose, this fatty acid is a possible mediator of the pathologic actions of hyperglycemia. Clarification of the signal transduction pathways and ionic mechanisms regulating the normal and dysfunctional tones of MCs is essential for rational clinical management of glomerular disease and critical to understanding fluid and electrolyte homeostasis.

Inhibition of GTP gamma S-dependent phospholipase D and Rho membrane association by calphostin is independent of protein kinase C catalytic activity

We studied the relationships between the activation of phospholipase D (PLD) by guanine nucleotides and phorbol esters in permeabilized U937 promonocytes and in solubilized extracts prepared from U937 cell membranes. Treatment of permeabilized cells with phorbol myristate acetate (PMA) strongly potentiated GTP gamma S-dependent PLD activity at free Ca2+ < 100 nM. In the absence of GTP gamma S, PMA stimulated only minor PLD activity. This suggested synergistic interaction between regulatory G-proteins and a protein kinase C (PKC) family kinase. The potential role of PKC was evaluated by testing two mechanistically distinct PKC inhibitors, bisindolylmaleimide (BIM) and calphostin. BIM inhibits PKC enzymes via competition with ATP for binding to the catalytic domain, while calphostin competes with PMA or diglyceride for binding to the regulatory domain. The ability of PMA to potentiate the GTP gamma S-dependent PLD was not inhibited by BIM. In contrast, calphostin strongly inhibited the GTP gamma S-dependent PLD activity, both in the presence and absence of PMA as a potentiating agent. Calphostin also produced complete inhibition of a GTP gamma S-dependent PLD activity, present in solubilized membrane extracts, which was assayed using phospholipid vesicles of defined composition. Treatment of reconstituted membrane/cytosol mixtures with calphostin also produced complete inhibition of the GTP gamma S-induced translocation of Rho A from cytosol to membrane. In contrast to its effects on the U937 cell PLD, calphostin did not inhibit the activity of purified PLD from cabbage. These results suggest that the assembly of active RhoA/PLD signaling complexes on membranes involves a phorbol ester/calphostin-binding protein, but is not dependent on PKC-type catalytic activity.

Tamoxifen activates cellular phospholipase C and D and elicits protein kinase C translocation

The antiestrogen tamoxifen is widely used for endocrine therapy of breast cancer; however, the mechanisms of estrogen receptor-independent interactions of tamoxifen remain ill defined. Here we examine the effect of tamoxifen on the initial steps of cell signal transduction. To this end, phospholipid metabolism and protein kinase C (PKC) translocation were assessed in CCD986SK human mammary fibroblasts treated with tamoxifen. The addition of tamoxifen resulted in dose-dependent and time-dependent increases in the cellular second messengers phosphatidate (PA) and diacylglycerol (DG). On addition of ethanol to the medium, tamoxifen induced the formation of phosphatidylethanol, demonstrating that tamoxifen activates phospholipase D (PLD). Cellular DG also increased in the presence of ethanol, showing that tamoxifen also activates phospholipase C (PLC). In cells prelabeled with choline and ethanolamine, tamoxifen caused increases in choline, phosphorylcholine, ethanolamine and phosphorylethanolamine. Structure-activity relationship studies for activation of PLD revealed that tamoxifen was the most effective, whereas 4-hydroxy tamoxifen was nearly devoid of activity. Phorbol diesters also activated PLD, but estrogen had no influence. Pretreatment of cells with phorbol dibutyrate (PKC down-regulation protocol) blocked phorbol diester- and tamoxifen-induced PLD activity. Exposure of cells to the PKC inhibitor GF 109203X diminished tamoxifen-induced PLD activity. Addition of tamoxifen to cultures elicited selective membrane association of PKC epsilon. We conclude that tamoxifen exerts considerable extra-nuclear influence at the transmembrane signaling level. These events may contribute to effects beyond the scope of estrogen receptor-dependent actions.

Stimulation by extracellular ATP and UTP of the stress-activated protein kinase cascade in rat renal mesangial cells

1. Extracellular adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) have been shown to activate a nucleotide receptor (P2U receptor) in rat mesangial cells that mediates phosphoinositide and phosphatidylcholine hydrolysis by phospholipases C and D, respectively. This is followed by an increased activity of the mitogen-activated protein kinase cascade and cell proliferation. Here we show that ATP and UTP potently stimulate the stress-activated protein kinase pathway and phosphorylation of the transcription factor c-Jun. 2. Both nucleotides stimulated a rapid (within 5 min) and concentration-dependent activation of stress-activated protein kinases as measured by the phosphorylation of c-Jun in a solid phase kinase assay. 3. When added at 100 microM the rank order of potency of a series of nucleotide analogues for stimulation of c-Jun phosphorylation was UTP > ATP = UDP = ATP gamma S > 2-methylthio-ATP > beta gamma-imido-ATP = ADP > AMP = UMP = adenosine = uridine. Activation of stress-activated protein kinase activity by ATP and UTP was dose-dependently attenuated by suramin. 4. Down-regulation of protein kinase C-alpha, -delta and -epsilon isoenzymes by 24 h treatment of the cells with 12-O-tetradecanoylphorbol 13-acetate did not inhibit ATP- and UTP-induced activation of c-Jun phosphorylation. Furthermore, the specific protein kinase C inhibitors, CGP 41251 and Ro 31-8220, did not inhibit nucleotide-stimulated c-Jun phosphorylation, suggesting that protein kinase C is not involved in ATP- and UTP-triggered stress-activated protein kinase activation. 5. Pretreatment of the cells with pertussis toxin or the tyrosine kinase inhibitor, genistein, strongly attenuated ATP- and UTP-induced c-Jun phosphorylation. Furthermore, N-acetyl-cysteine completely blocked the activation of stress-activated protein kinase in response to extracellular nucleotide stimulation. 6. In summary, these results suggest that ATP and UTP trigger the activation of the stress-activated protein kinase module in mesangial cells by a pathway independent of protein kinase C but requiring a pertussis toxin-sensitive G-protein and tyrosine kinase activation.

Extracellular nucleotides as signalling molecules for renal mesangial cells

1. Glomerular diseases frequently cause chronic renal failure which ultimately requires dialysis and kidney transplantation. The events leading to destruction of the glomerular filtration apparatus include injury of glomerular cells, aggregation of thrombocytes and infiltration of immune cells into the glomerulus. 2. Nucleotides (e.g. ATP and UTP) are present in all glomerular cell types as well as in thrombocytes. The release of nucleotides into the extracellular space occurs after damage of glomerular cells and aggregation of thrombocytes. Several in vitro and in vivo findings indicate that extracellular nucleotides may play a role as pro-inflammatory mediators in glomerulonephritis. 3. A hallmark finding in kidney biopsies from patients with glomerulonephritis is proliferation of glomerular mesangial cells. Cell culture studies demonstrated that extracellular ATP (10-300 microM) stimulated growth of mesangial cells. The mitogenic effect of ATP was potentiated in the presence of multiple growth factors. 4. Nucleotide-induced signalling in mesangial cells included an increase of intracellular calcium, activation of phosphatidylinositol-specific phospholipase C and phospholipase D, inhibition of adenylylcyclase, stimulation of mitogen-activated protein kinase and increased expression of the immediate early genes, c-fos, c-jun and Egr-1. 5. In previous studies of experimental mesangioproliferative glomerulonephritis, exogenously given ADP beta S and ATP gamma S have been shown to aggravate the course of the disease, while 2-chloroadenosine had beneficial effects. 6. Taken together, these findings support the concept that nucleotides may function as proinflammatory mediators in glomerulonephritis while adenosine may have antiinflammatory effects.

Regulatory functions of protein kinase C isoenzymes in purinoceptor signalling in mesangial cells

1. The expression and functional roles of protein kinase C (PKC) isoenzymes in purinoceptor signalling have been examined in rat renal mesangial cells. 2. It is observed that rat mesangial cells express four PKC isoenzymes, PKC-alpha, -delta, -epsilon and zeta, as determined by Western blot analysis. No PKC-beta, -gamma, -eta, -theta, or -mu isoforms were detected. 3. By using specific PKC inhibitors and down-regulation experiments we provide evidence that PKC alpha acts as a negative feedback regulator of ATP- and UTP-stimulated phosphoinositide turnover, whereas PKC epsilon triggers arachidonic acid release and subsequent prostaglandin synthesis and stimulates a phosphatidylcholine-hydrolysing phospholipase D. Moreover, PKC delta may activate the mitogen-activated protein kinase cascade and thus promote mesangial cell proliferation in response to extracellular ATP and UTP. 4. In summary our data identify mesangial cells in culture as an excellently characterized cell culture system with well-defined functions of PKC isoenzymes. Functional identification of PKC isoenzymes involved in specific cell responses is one of the most promising steps towards understanding of molecular mechanisms of cell regulation and identifies new targets for drug development.

Extracellular ATP increases cytosolic calcium in cultured rat renal arterial smooth muscle cells

1. Experiments were conducted on cultured renal arterial smooth muscle cells to determine the ability of extracellular ATP to alter cytosolic calcium concentration and to determine the mechanisms by which this effect occurs. 2. ATP (100 mumol/L) caused the fluorescence ratio of fura-2 to increase from a control value of 1.06 +/- 0.05 to 2.06 +/- 0.13 (P < 0.01) before stabilizing at a sustained level of 1.35 +/- 0.04 (n = 8; P < 0.05). 3. Removal of extracellular calcium from the bathing medium resulted in an attenuation of the initial response to 100 mumol/L ATP with cell fluorescence increasing from 1.16 +/- 0.18 to 1.44 +/- 0.18 ratio units (n = 5). Furthermore, the initial increase in fluorescence ratio rapidly declined to 1.02 +/- 0.06, indicating that an influx of extracellular calcium is required to sustain the increase in fura-2 fluorescence. 4. Depletion of intracellular calcium pools with thapsigargin prevented the increase in fura-2 fluorescence evoked by ATP. 5. These data suggest that ATP-mediated increases in cytosolic calcium in cultured renal arterial smooth muscle cells involve calcium release from the thapsigargin-sensitive, intracellular pool in conjunction with calcium influx from the extracellular medium.

Biphasic effect of ATP on neutrophil functions mediated by P2U and adenosine A2A receptors

In agreement with previous findings, the oxidative burst induced by fMet-Leu-Phe (fMLP) in polymorphonuclear neutrophils was enhanced by adenosine triphosphate (ATP) and uridine 5'-triphosphate (UTP), although these nucleotides were inactive as agonists per se. However, the enhancement by ATP was rapidly reversed to an inhibition by prolonged incubation. Adenosine diphosphate (ADP) was always inhibitory. Inhibition mediated by ATP coincided with its conversion into ADP, adenosine monophosphate (AMP), and adenosine. In addition, the inhibitory effects of ATP and ADP on the oxidative burst were virtually abolished by 9-chloro-2-(2-furanyl)-5,6-dihydro-[1,2,4]-triazolo[1,5] quinazolin-5-imine monomethanesulfonate (CGS 15943), a nonselective adenosine receptor antagonist, whereas the priming effect of ATP was antagonized by adenosine. Both ATP and UTP showed a similar potency in activating elastase release, intracellular inositol-(1,4,5)-triphosphate (IP3) formation and an increase in cytosolic calcium. Neither ATP nor UTP affected the initial rise in cytosolic calcium induced by fMLP, but did enhance the secondary calcium increase. When added simultaneously with fMLP, ADP and adenosine abolished this second calcium peak without influencing the first. These results indicate that purine and pyrimidine nucleotides acting on P2U-like receptors, which are coupled to the IP3 pathway, can increase calcium, release elastase, and enhance the oxidative burst induced by chemo-kines. Adenosine formed by hydrolysis from ATP and ADP, by contrast, reduces the oxidative bursts and the influx of extracellular calcium induced by fMLP.

Stimulation by the nucleotides, ATP and UTP of mitogen-activated protein kinase in EAhy 926 endothelial cells

1. We have investigated the characteristics of activation of the 42kDa isoform of mitogen-activated protein (MAP) kinase in response to various nucleotides in the endothelial cell line EAhy 926. 2. Adenosine 5'-triphosphate (ATP) in the concentration range 0.1-100 microM stimulated the rapid and transient tyrosine phosphorylation and activation of the 42 kDa isoform of MAP kinase in EAhy 926 endothelial cells which peaked at 2 min and returned to basal values by 60 min. ATP also stimulated a similar response in primary cultured bovine aortic endothelial cells. 3. Uridine 5' triphosphate (UTP) also stimulated the 42 kDa isoform of MAP kinase with similar potency to ATP (EC50 values 5.1 +/- 0.2 microM for UTP; 2.9 +/- 0.8 microM for ATP), whilst the selective P2Y-purinoceptor agonist, 2-methylthioATP (2-meSATP) was without effect up to concentrations of 100 microM. In bovine aortic endothelial cells however, UTP and 2-meSATP both stimulated MAP kinase. 4. Pretreatment of cells for 24 h with 12-O tetradecanoyl phorbol 13-acetate resulted in the loss of the alpha and epsilon isoforms of protein kinase C (PKC) and virtual abolition of nucleotide-stimulated MAP kinase activity (> 90% inhibition). 5. Preincubation for 30 min with the PKC inhibitor, Ro-31 8220 (10 microM) reduced MAP-kinase activation at 2 min but potentiated the response at 60 min. 6. Removal of extracellular calcium in the presence of EGTA reduced the MAP kinase activation in response to UTP by approximately 30-50%. 7. Pretreatment with pertussis toxin (18 h, 50 ng ml-1) did not significantly affect the UTP-mediated activation of pp42 MAP kinase. 8. These results show that in the EAhy 926 endothelial cell line, nucleotides stimulate activation of MAP kinase in a protein kinase C-dependent manner through interaction with a P2U-purinoceptor.

P2 purinergic receptor agonists enhance cAMP production in Madin-Darby canine kidney epithelial cells via an autocrine/paracrine mechanism

Mechanisms of cross-talk between different classes of signaling molecules are inadequately understood. We have used clonal Madin-Darby canine kidney (MDCK-D1) epithelial cells as a model system to investigate the effects of extracellular nucleotides (e.g. ATP, UTP), which promote increase in activity of several phospholipases, on cAMP production. In contrast to observations in some other cell systems, ATP and UTP, acting via P2 purinergic receptors, stimulated cAMP production in MDCK-D1 cells. At maximally effective concentrations, ATP and UTP were not additive with the beta-adrenergic receptor agonist isoproterenol, but were synergistic with forskolin in increasing cAMP production, indicating that G alpha s is activated by these nucleotides. Additionally, we found that (a) nucleotide-induced increases in cAMP were blocked by indomethacin, a cyclooxygenase inhibitor, (b) arachidonic acid increased cellular cAMP levels in an indomethacin-sensitive fashion, and (c) PGE2, the major metabolite of arachidonic acid, stimulated cAMP formation. Overall, our results suggest a mechanism by which extracellular nucleotides stimulate release of arachidonic acid which is metabolized to PGE2 which, in turn, acts in an autocrine/paracrine fashion via prostaglandin receptors to activate G alpha s and increase cAMP. Based on the ability of extracellular nucleotides to stimulate the formation and release of prostaglandins in MDCK-D1 epithelial and other cells, we hypothesize that receptor-mediated prostaglandin release may be a general mechanism that regulates cAMP formation in many types of cells.

Phospholipase D activation by P2Z-purinoceptor agonists in human lymphocytes is dependent on bivalent cation influx

The role of bivalent cations in ATP-stimulated phospholipase D (PLD) activity was investigated in human leukaemic lymphocytes. Cells were labelled with [3H]oleic acid and incubated with extracellular ATP or benzoylbenzoic ATP in the presence of 1 mM Ca2+ and butanol, and PLD activity was assayed by the accumulation of [3H]phosphatidylbutanol ([3H]PBut). ATP stimulated PLD activity in a dose-dependent manner, and the inhibitory effects of suramin, oxidized ATP and extracellular Mg2+ suggested that the effect of ATP was mediated by P2Z purinoceptors known to be present on lymphocytes. Thapsigargin increased cytosolic [Ca2+] but did not stimulate PLD activity, whereas preloading cells with a Ca2+ chelator reduced cytosolic [Ca2+] and, paradoxically, potentiated ATP-stimulated [3H]PBut accumulation. ATP-stimulated [3H]PBut formation was supported by both Ba2+ and Sr2+ when they were substituted for extracellular Ca2+. Addition of EGTA to block bivalent cation influx inhibited the majority of ATP-stimulated PLD activity. Furthermore ATP-stimulated PLD activity showed a linear relationship to extracellular [Ba2+], and ATP-induced 133Ba2+ influx also had a linear dependence on extracellular [Ba2+]. These results suggest that ATP stimulates PLD activity in direct proportion to the influx of bivalent cations through the P2Z-purinoceptor ion channel and that this PLD activity is insensitive to changes in bulk cytosolic [Ca2+].

Muscarinic regulation of phospholipase D and its role in arachidonic acid release in rat submandibular acinar cells

The characteristics of muscarinic cholinergic-induced phospholipase D (PLD) activation, and the involvement of the enzyme in the release of arachidonic acid were examined in rat submandibular acinar cells. Carbachol produced a dose-related activation of PLD to around fivefold control values at 100 microM agonist concentration. This was associated with the appearance of free choline, phosphatidic acid and arachidonic acid, indicating that the PLD substrate was phosphatidylcholine. The response to carbachol was inhibited by 60% by U73122, a blocker of a phospholipase C (PLC) specific to phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], suggesting that the cleavage of phosphatidylcholine by PLD was, at least in part, secondary to agonist-coupled hydrolysis of PtdIns(4,5)P2 by PLC. Consistent with this, PLD was also activated to levels comparable to those induced by carbachol, by the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), and the Ca2+ mobilizer, thapsigargin, two agents that respectively mimic the activation of protein kinase C (PKC) by diacylglycerol and the elevation of cytosolic Ca2+ by inositol 1,4,5-triphosphate [Ins(1,4,5)P3] in the phosphoinositide effect. The cell-permeant Ca2+ chelator 1,2-bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester (BAPTA/AM) abolished the thapsigargin-induced activation of PLD and inhibited the responses of PLD to carbachol and TPA by 60%. The PKC inhibitor, Ro-31-8220, also inhibited the activation of PLD by carbacol and TPA to a level of approximately double control values, but had no effect on the thapsigargin-induced elevation of PLD. A role for both the PKC-associated and Ca(2+)-mobilizing arms of the PtdIns(4,5)P2-PLC pathway in PLD regulation is thus suggested. Pretreatment of cells with the phosphatidate phosphohydrolase blocker, propranolol, significantly enhanced the carbachol-induced elevation of phosphatidic acid, but decreased agonist-stimulated production of diacylglycerol and arachidonic acid, indicating that phosphatidlycholine was the likely source of arachidonic acid. We therefore propose that, in submandibular mucous acinar cells, muscarinic activation of the PtdIns(4,5)P2-PLC pathway regulates phosphatidylcholine-specific PLD through both the PKC- and Ca(2+)-mobilizing arms of the phosphoinositide response, and that diacylglycerol, derived from phosphatidylcholine via phosphatidic acid, is a source of free arachidonic acid.

Bradykinin-stimulated phosphatidylcholine hydrolysis in airway smooth muscle: the role of Ca2+ and protein kinase C

The regulation of phosphatidylcholine (PtdCho) hydrolysis by Ca2+ and protein kinase C (PKC) was measured in [3H]palmitate-labelled cultured guinea-pig airway smooth-muscle cells as phosphatidylbutanol ([3H]PtdBut) and phosphatidate ([3H]PtdOH) formation in the presence of butanol. The former is a direct measure of phospholipase D (PLD) activity, whereas the latter, in airway smooth muscle, is indicative of net PtdCho-specific phospholipase C (PLC)-like/diacylglycerol (DG) kinase activity. Bradykinin-stimulated responses exhibited a requirement for extracellular Ca2+ influx, since they were inhibited in the presence of EGTA. This influx was independent of voltage-operated channels, since the L-type channel blocker nifedipine (up to 10 microM) was without effect on bradykinin-stimulated responses. In support of this, membrane depolarization with KCl (30 mM) failed to elicit either response. However, bradykinin-stimulated formation of both [3H]PtdBut and [3H]PtdOH was partially inhibited by 100 microM SKF96365. Ionomycin, a Ca2+ ionophore, induced PtdCho hydrolysis to a greater extent than bradykinin, also in an extracellular-Ca(2+)-dependent manner. Thapsigargin-induced emptying of intracellular Ca2+ pools elicited the formation of both [3H]PtdBut and [3H]PtdOH and displayed a requirement for extracellular Ca2+. Bradykinin-stimulated PtdCho-specific PLC-like/DG kinase pathway and PLD responses were unaffected by thapsigargin pretreatment, thereby questioning the role of Ins(1,4,5)P3/Ins(1,3,4,5)P4-dependent Ca2+ stores in the receptor stimulation of these activities in airway smooth-muscle cells. In this regard, we have previously demonstrated that the bradykinin-stimulated PtdCho-specific PLD and PLC-like activities can occur under conditions of apparent complete blockade of bradykinin-stimulated Ins(1,4,5)P3 formation by receptor antagonist in guinea-pig airway smooth muscle. The PKC inhibitor, Ro31-8220, selectively blocked both bradykinin- and ionomycin-stimulated PLD activity in a concentration-dependent manner (IC50 approx. 1 microM), but was without effect on bradykinin-stimulated PtdCho-PLC-like/DG kinase-derived PtdOH formation. In contrast, an inhibitor of PtdCho-PLC, D609, selectively blocked the formation of [3H]PtdOH in the presence of butanol (PtdCho-PLC-like/DG kinase activity), but not [3H]PtdBut formation. In conclusion, PtdCho hydrolysis appears to occur via two distinguishable routes which both require extracellular Ca2+, whereas only the PLD route is regulated by PKC.

Differential regulation of phospholipase D and phospholipase C by protein kinase C-beta and -delta in liver macrophages

We have studied activation of phospholipase (PL) C and PLD in liver macrophages labelled with [3H]arachidonic acid. Zymosan, phorbol 12-myristate 13-acetate (PMA), A23187 and fluoride but not arachidonic acid or lipopolysaccharide (LPS) induce an activation of PLD ([3H]phosphatidylethanol (PEt) accumulation). An activation of PLC ([3H]diacylglycerol (DAG) accumulation) is measured with zymosan, PMA and fluoride but not with A23187, LPS or arachidonic acid whereas inositol phosphates are formed with zymosan, only. Removal of extracellular calcium reduces the formation of [3H]PEt and [3H]DAG while pretreatment of the cells with dexamethasone reduces [3H]PEt formation, only. PMA- and zymosan-induced activation of PLD and PMA-induced activation of PLC both seem to be mediated by protein kinase (PK) C-beta whereas zymosan-induced activation of PLC is negatively controlled by PKC-delta. We could furthermore present evidence that the release of [3H]arachidonic acid in these cells occurs independent of an activation of PLD.

Review: Ca(2+)-mobilizing receptors for ATP and UTP

Extracellular nucleotides are potent Ca2+ mobilizing agents. A variety of receptors for extracellular ATP are recognised. Some are involved in fast neuronal transmission and operate as ligand-gated ion channels. Others are involved in the paracrine or autocrine modulation of cell function. Many receptors of this type are coupled to phosphoinositide-specific phospholipase C and, in some cases, other phospholipases. One of these receptors (P2z), however, also appears to operate, at least in part, as a ligand-gated ion channel. Pharmacological data suggest that one nucleotide receptor subtype (currently designated P2U) responds selectively to either a purine nucleotide, ATP, or a pyrimidine nucleotide, UTP. According to an alternative view, ATP and UTP recognise distinct receptors. Because of the diversity of receptors for extracellular nucleotides this may be the case in some cells. Nevertheless, a G-protein coupled receptor that confers both ATP and UTP sensitivity has been cloned, expressed in cultured cell lines and sequenced. This receptor appears to have two ligand binding domains that may partially overlap. The nature of this overlap is discussed and a simple model presented. Activation of the receptor protein via one or other ligand binding domain may underlie some of the more subtle differences between the effects of ATP and UTP.

5-Hydroxytryptamine 5-HT1B receptors inhibiting cyclic AMP accumulation in rat renal mesangial cells

A clonal cell line derived from rat renal mesangial cells was shown to express endogenous 5-hydroxytryptamine (serotonin, 5-HT) receptors that mediate inhibition of cyclic AMP accumulation. These receptors were characterized as being of the 5-HT1B receptor subtype. 5-HT1 receptor agonists inhibited forskolin-stimulated cyclic AMP accumulation in rat renal mesangial cells (60-70% maximal inhibition) with the following rank order of potency (mean pEC50 values +/- SEM, n > or = 3): ergotamine (9.58 +/- 0.51) > RU 24969 (8.67 +/- 0.23) > or = 5-CT (8.42 +/- 0.06) > or = CP 93129 (8.15 +/- 0.27) > 5-HT (7.75 +/- 0.11) > sumatriptan (6.29 +/- 0.30) > 8-OH-DPAT (4.32 +/- 0.15). 5-HT2 and 5-HT4 receptor agonists were without effect. 5-HT-induced inhibition of cyclic AMP accumulation was abolished by a pre-treatment of the cells with pertussis toxin. (-)Propranolol was a partial agonist (27% maximal inhibition, pEC50 7.19 +/- 0.24, n = 3); when used as an antagonist at 1 microM, it shifted the concentration-response curve of 5-HT to the right (pKB 7.22 +/- 0.35, n = 3). Methiothepin was a competitive antagonist of 5-HT (pA2 8.04 +/- 0.10, Schild slope 0.87 +/- 0.21, n = 3). Rauwolscine (10 microM) had no antagonist activity. There was a significant correlation (r = 0.98, P = 0.0001) between the cyclic AMP data obtained in rat mesangial cells and 5-HT1B binding data reported in rat brain cortex. The same pattern of responses was observed in early passages of primary cultures of rat mesangial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of nucleotide receptors inhibits M-type K current [IK(M)] in neuroblastoma x glioma hybrid cells

A phospholipase-C-linked nucleotide receptor, sensitive to both uridine and adenosine triphosphate (UTP and ATP) has been cloned from NG108-15 neuroblastoma x glioma hybrid cells. We have tested whether activation of this receptor could inhibit the voltage-dependent K+ current [IK(M) or "M-current"] in NG108-15 cells recorded using whole-cell patch-clamp methods. Both UTP and ATP inhibited IK(M) by 44% and 42%, respectively, at 100 microM. Mean IC50 values were: UTP, 0.77 +/- 0.27 microM; ATP, 1.81 +/- 0.82 microM. The order of nucleotide and nucleoside activity at 100 microM was: UTP = ATP > ATP [gamma S] = ITP > 2-MeSATP > ADP = GTP >> AMP-CPP, adenosine, where ATP[gamma S] is adenosine 5'-O-(3-thiotriphosphate), ITP is inosine 5'-triphosphate, 2-MeSATP is 2-methylthio ATP and AMP-CPP is alpha, beta methylene ATP. This rank order accords with their activities at the cloned P2U receptor. Effects were not inhibited by suramin (up to 500 microM) or by pre-incubation for 12 h in 500 ng.ml-1 Pertussis toxin. Inhibition of IK(M) was frequently preceded by a transient outward current, probably a Ca(2+)-activated K+ current, responding to Ca2+ mobilization. No effect on the delayed rectifier K+ current was observed. These observations match those expected from stimulating other phospholipase-C-linked receptors in NG108-15 cells.

Stimulation by extracellular ATP and UTP of the mitogen-activated protein kinase cascade and proliferation of rat renal mesangial cells

1. Extracellular ATP and UTP have been reported to activate a nucleotide receptor that mediates phosphoinositide and phosphatidylcholine hydrolysis by phospholipases C and D, respectively. Here we report that ATP and UTP potently stimulate mesangial cell proliferation. 2. Both nucleotides stimulate phosphorylation and activation of mitogen-activated protein kinase and a biphasic phosphorylation of the up-stream mitogen-activated protein kinase kinase. 3. When added at 100 microM, ATP gamma S, UTP and ATP were the most potent activators of mitogen-activated protein kinase. beta gamma-imido-ATP was somewhat less active and ADP and 2-methylthio-ATP caused a weak induction of enzyme activity. Activation of mitogen-activated protein kinase by both ATP and UTP is dose-dependently attenuated by the P2-receptor antagonist, suramin. 4. The protein kinase C activator 12-0-tetradecanoylphorbol 13-acetate, but not the biologically inactive 4 alpha-phorbol 12,13-didecanoate, increased mitogen-activated protein kinase activity in mesangial cells, suggesting that protein kinase C may mediate nucleotide-induced stimulation of mitogen-activated protein kinase. 5. Down-regulation of protein kinase C -alpha and -delta isoenzymes by 4 h or 8 h treatment with phorbol ester partially inhibited ATP- and UTP-triggered mitogen-activated protein kinase activation. Moreover, a 24 h treatment of mesangial cells with phorbol ester, a regimen that also causes depletion of protein kinase C-epsilon did not further reduce the level of mitogen-activated protein kinase stimulation. 6. The specific protein kinase C inhibitor, CGP 41251, which displayed a selectivity for the Ca2+-dependent isoenzymes, as compared to the Ca2+-independent isoenzymes did not inhibit nucleotide stimulated mitogen-activated protein kinase phosphorylation, thus implicating the involvement of a Ca2+-independent protein kinase C isoform.7. In summary, these results suggest that ATP and UTP trigger the activation of the mitogen-activated protein kinase signalling cascade in mesangial cells and this may be responsible for the potent mitogenic activity of both nucleotides.

Possible role of protein kinase C-epsilon isoenzyme in inhibition of interleukin 1 beta induction of nitric oxide synthase in rat renal mesangial cells

In cultured glomerular mesangial cells, interleukin 1 beta (IL-1 beta) has been shown to induce a dose- and time-dependent accumulation of nitrite, a stable metabolite of nitric oxide (NO). In parallel, increased levels of mRNA of an inducible macrophage-type of nitric oxide synthase (iNOS) were observed after incubating mesangial cells with IL-1 beta. Here we report that addition of the biologically active phorbol esters, phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate (PDBu), dose-dependently inhibited the IL-1 beta-stimulated increase in iNOS mRNA levels and nitrite production. In contrast, the biologically inactive phorbol ester 4 alpha-phorbol 12,13-didecanoate, had no effect on cytokine induction of iNOS and nitrite formation. Incubation of mesangial cells with PMA or PDBu alone, in the absence of IL-1 beta, did not trigger any iNOS expression. Time-course studies indicated that phorbol ester needs to be added for only 1 h in order to maximally inhibit cytokine-induced nitrite production. Down-regulation of protein kinase C (PKC)-alpha and -delta isoenzymes by 8 h PMA or PDBu treatment before stimulation with IL-1 beta still resulted in full inhibition of iNOS induction. In contrast, a 24 h treatment of mesangial cells with PMA or PDBu, a regimen that also causes depletion of PKC-epsilon, abolished inhibition of IL-1 beta-induced iNOS expression and nitrite production. In addition, the selective PKC inhibitor calphostin C potentiated IL-1 beta induction of iNOS activity. In summary these data suggest that IL-1 beta induction of iNOS expression is tonically suppressed by PKC and the epsilon-isoenzyme is the most likely candidate mediating this effect.

Comparison of different tumour promoters and bryostatin 1 on protein kinase C activation and down-regulation in rat renal mesangial cells

The effects of a series of protein kinase C (PKC) activators with different spectra of biological activities and reportedly different patterns of PKC isoenzyme activation were examined in renal mesangial cells. Treatment of mesangial cells with the tumor promoters phorbol 12-myristate 13-acetate (PMA), debromoaplysiatoxin, dihydroteleocidin and thymeleatoxin, as well as with the marine natural product bryostatin 1, caused translocation and at least partial down-regulation of the PKC-alpha, -delta and -epsilon isoenzymes as assessed by immunoblot analysis. Bryostatin 1 mediates a faster depletion of PKC-alpha isoform than any of the other PKC activators. Thymeleatoxin, which has been reported to selectively activate PKC-alpha, -beta and -gamma, but not PKC-delta or -epsilon isoenzymes in vitro, turned out to exert the most potent effect on PKC-delta and -epsilon in mesangial cells and down-regulated these isotypes within 8-24 hr. None of the compounds tested affected cellular distribution or amount of PKC-zeta in mesangial cells. Thus, all of the PKC activators tested are able to translocate and down-regulate three of the four PKC isoenzymes present in mesangial cells although with different kinetics. All PKC activators stimulated a phospholipase A2-mediated arachidonic acid release, a phospholipase D-mediated phosphatidylcholine hydrolysis, a comparable small proliferative response and an inhibition of phospholipase C-mediated inositol trisphosphate generation. These results suggest: (i) that the PKC activators investigated in this study do not display any type of isotype-specificity that could be used to selectively activate or down-regulate PKC isoenzymes in intact cell-systems; (ii) that thymeleatoxin has a different isoenzyme selectivity in intact cells as compared to in vitro enzyme inhibition data; and (iii) PKC-zeta is resistant to all PKC activators investigated in this study.