Activation of phospholipase D in porcine tracheal smooth muscle: role of phosphatidylinositol 3-kinase and RhoA activation

Membrane trafficking factors are involved in the hypotonic activation of the taurine channel in the little skate (Raja erinacea) red blood cell

In response to volume expansion, red blood cells of the little skate (Raja erinacea) initially swell and then release small organic compounds and osmotically obligated water in what is called a regulatory volume decrease (RVD) to restore cell volume. One of the major intracellular solutes lost during this process is the non-metabolized beta amino acid taurine. This hypoosmotic-induced increase in cell taurine permeability requires the anion exchanger, skAE1. The abundance of this transporter increases on the surface plasma membrane by a process of exocytosis. The second-messenger pathways involved in exocytosis of skAE1 were investigated with the use of inhibitors which affect membrane trafficking. Hypoosmotic-stimulated taurine uptake was significantly decreased by 42% with wortmannin, a phosphatidylinositol 3-kinase (PI3 kinase) inhibitor. Additional evidence for the involvement of PI3K was obtained with a second inhibitor, LY294002, which decreased the hypoosmotic-stimulated taurine uptake by 28%. The state of actin is also involved, as the actin filament depolymerizer latrunculin B decreased hypoosmotic-stimulated taurine uptake by approximately 40%. Although hypoosmotic conditions did not stimulate changes in the distribution of actin between filamentous and globular forms, latrunculin stimulated a decrease in filamentous actin and increase in globular actin in both isoosmotic and hypoosmotic conditions. Disruptors of other potential cytoskeletal factors (myosin, kinesin, dynein, and microtubules) did not affect taurine uptake. The present results suggest that the exocytosis of skAE1 stimulated by hyposmotic-induced cell volume expansion requires activation of PI3 kinase and is regulated by the state of actin filaments.

Regulation of acetylcholine-induced phosphorylation of PLD1 in porcine tracheal smooth muscle

The muscarinic agonist, acetylcholine (ACh), stimulates phospholipase D (PLD) activity in tracheal smooth muscle cells. Direct activation of protein kinase C (PKC) by phorbol-12-myristate-13-acetate (PMA) also stimulates PLD in this tissue. Activation of ACh-induced PLD was inhibited by the tyrosine kinase inhibitor genistein in a concentration-dependent manner. Presently known isoforms of PLD, PLD1 and PLD2, were identified in tracheal smooth muscle and their activation-induced phosphorylation status studied. Both ACh and PMA increased phosphorylation of PLD1 that was significantly blocked by genistein or the PKC inhibitor calphostin C. PLD2 phosphorylation was not detected in the present experiments. Western blots probed with an anti-phosphotyrosine antibody indicate that PLD1 in this tissue is phosphorylated on tyrosine residues after ACh or PMA stimulation. Tyrosine phosphorylation of PLD1 was blocked by genistein and calphostin C. No tyrosine residues were phosphorylated on PLD2. Taken together, these results demonstrate that porcine tracheal smooth muscle cells express both isoforms PLD1 and PLD2. However, on muscarinic activation only PLD1 in this tissue is phosphorylated by PKC via a tyrosine-kinase-dependent pathway.

Reactive oxygen species as mediators of signal transduction in ischemic preconditioning

Ischemic preconditioning (IPC) is a most powerful endogenous mechanism for myocardial protection against ischemia/reperfusion injury. It is now apparent that reactive oxygen species (ROS) generated in the mitochondrial respiratory chain act as a trigger of IPC. ROS mediate signal transduction in the early phase of IPC through the posttranslational modification of redox-sensitive proteins. ROS-mediated activation of Src tyrosine kinases serves a scaffold for interaction of proteins recruited by G protein-coupled receptors and growth factor receptors that is necessary for amplification of cardioprotective signal transduction. Protein kinase C (PKC) plays a central role in this signaling cascade. A crucial target of PKC is the mitochondrial ATP-sensitive potassium channel, which acts as a trigger and a mediator of IPC. Mitogen-activated protein (MAP) kinases (extracellular signal-regulated kinase, p38 MAP kinase, and c-Jun NH(2)-terminal kinase) are thought to exist downstream of the Src-PKC signaling module, although the role of MAP kinases in IPC remains undetermined. The late phase of IPC is mediated by cardioprotective gene expression. This mechanism involves redox-sensitive activation of transcription factors through PKC and tyrosine kinase signal transduction pathways that are in common with the early phase of IPC. The effector proteins then act against myocardial necrosis and stunning presumably through alleviation of oxidative stress and Ca(2+) overload. Elucidation of IPC-mediated complex signaling processes will help in the development of more effective pharmacological approaches for prevention of myocardial ischemia/reperfusion injury.

Phosphatidylinositol 3-kinase modulates vascular smooth muscle contraction by calcium and myosin light chain phosphorylation-independent and -dependent pathways

Regulation of smooth muscle contraction involves a number of signaling mechanisms that include both kinase and phosphatase reactions. The goal of the present study was to determine the role of one such kinase, phosphatidylinositol (PI)3-kinase, in vascular smooth muscle excitation-contraction coupling. Using intact medial strips of the swine carotid artery, we found that inhibition of PI3-kinase by LY-294002 resulted in a concentration-dependent decrease in the contractile response to both agonist stimulation and membrane depolarization-dependent contractions and a decrease in Ca(2+)-dependent myosin light chain (MLC) phosphorylation, the primary step in the initiation of smooth muscle contraction. Inhibition of PI3-kinase also depressed phorbol dibutyrate-induced contractions, which are not dependent on either Ca(2+) or MLC phosphorylation but are dependent on protein kinase C. To determine the Ca(2+)-dependent site of action of PI3-kinase, we determined the effect of several inhibitors of calcium metabolism on LY-294002-dependent inhibition of contraction. These inhibitors included nifedipine, SK&F-96365, and caffeine. Only SK&F-96365 blocked the LY-294002-dependent inhibition of contraction. Interestingly, all compounds blocked the LY-294002-dependent inhibition of MLC phosphorylation. Our results suggest that activation of PI3-kinase is involved in a Ca(2+)- and MLC phosphorylation-independent pathway for contraction likely to involve protein kinase C. In addition, our results also suggest that activation of PI3-kinase is involved in Ca(2+)-dependent signaling at the level of receptor-operated calcium channels.

AngII induces transient phospholipase D activity in the H295R glomerulosa cell model

In this report we demonstrate that in human adrenocortical carcinoma NCI H295R cells, a model for adrenal glomerulosa cells, PLD was activated both by AngII and protein kinase C (PKC)-activating phorbol 12-myristate 13-acetate (PMA). However, while PMA triggered sustained PLD activation, AngII induced transient PLD activation, in contrast to results in bovine glomerulosa cells in primary culture. Despite the transient effect of AngII on PLD activity, PLD-derived lipid signals were required for maximal AngII-elicited aldosterone secretion. AngII-induced PLD activation was inhibited by PKC inhibitors, but not by tyrosine kinase or calcium/calmodulin-dependent kinase inhibitors or a calmodulin antagonist. Both AngII- and PMA-stimulated PLD activity was enhanced by phosphoinositide 3-kinase (PI3K) inhibitors. Akt, a downstream protein kinase activated by the products of PI3K, was constitutively active in H295R cells, and this activity was blocked by PI3K inhibitors. These results suggested that in H295R adrenocortical carcinoma cells, AngII-induced PLD activation was promoted by PKC and inhibited by the constitutively active PI3K pathway.

Regulation of parathyroid hormone-stimulated phospholipase D in UMR-106 cells by calcium, MAP kinase, and small G proteins

Signaling intermediates for PTH and phorbol activation of PLD in UMR-106 cells were determined. Calcium was required, and the effects of PTH, phorbol, and calcium were dependent on p42/44 MAP kinase and small G proteins, specifically RhoA, acting through Rho kinase.

INTRODUCTION

Phospholipase D (PLD) plays a key signaling role in numerous cellular processes. PLD-stimulated hydrolysis of phosphatidylcholine (PC) generates phosphatidic acid, a source of diacylglycerol (DAG). We previously reported that parathyroid hormone (PTH) stimulates PLD activity in UMR-106 osteoblastic cells by a protein kinase C (PKC)-independent mechanism. The current study investigated the roles of calcium, MAP kinase, and small G proteins in PTH- and phorbol-12,13-dibutyrate (PDBu)-stimulated transphosphatidylation of ethanol, a reaction catalyzed by PLD.

METHODS

UMR-106 cells were labeled with 3H-palmitic and treated in the presence of ethanol. Phosphatidylethanol was separated by thin-layer chromatography and detected by autoradiography, and the bands were scraped and counted. Statistical significance of the responses from three to nine replicates was determined by ANOVA and Tukey's post-test.

RESULTS AND CONCLUSIONS

PTH and PDBu effects were attenuated by EGTA, BAPTA, nifedipine, and dantrolene, whereas ionomycin or 2X calcium increased basal PLD activity. PTH activated p42/p44 MAP kinase, and the effects of PTH, PDBu, and ionomycin on PLD, but not on calcium influx, were prevented by the MEK inhibitors PD98059 and U0126. Small G proteins were shown to be involved in the effects of PTH, PDBu, and ionomycin on PLD. Inhibition of ARF by brefeldin prevented the PLD activation by all three agonists. A nonselective Rho/Rac/cdc-42 inhibitor, Clostridium difficile toxin B, also inhibited the effects of all three agonists on PLD. More selective inhibition of RhoA with a dominant negative RhoA construct or by inhibiting geranylgeranyltransferase I antagonized the effects of PTH, PDBu, and ionomycin, as did inhibiting the downstream kinase, Rho kinase. The current results reveal the importance of calcium, MAP kinase, and small G proteins in PTH and PDBu stimulation of PLD activity in UMR-106 cells.

Phosphatidic acid production, required for cholecystokinin octapeptide-stimulated amylase secretion from pancreatic acinar AR42J cells, is regulated by a wortmannin-sensitive process

To investigate the role of phospholipids in exocytotic secretory events, we utilized rat pancreatic acinar AR42J cells that secreted amylase in response to cholecystokinin octapeptide (CCK-8). Wortmannin, an inhibitor of phosphoinositide 3-kinase (PI3K), was found to inhibit the secretion in a dose-dependent manner. When changes in cell membrane phospholipids were investigated before and after CCK-8 stimulation using [32P]orthophosphoric acid-labeled AR42J cells, we observed a rapid increase in phosphatidic acid (PtdOH) levels right after stimulation, which was not observed in non-stimulated cells. The increase, however, was suppressed by wortmannin pre-treatment, which also inhibited amylase secretion. Changes in other major phospholipids were not significant. These results indicate that CCK-8 induces amylase secretion through PI3K-regulated production of PtdOH in cell membranes.