Enhancing effect of wortmannin on muscarinic stimulation of phospholipase D in rat pheochromocytoma PC12 cells

Neurodegenerative effects of azithromycin in differentiated PC12 cells

Azithromycin is a widely used macrolide antibiotic with sustained and high tissue penetration and intracellular accumulation. While short-term exposure to low-dose azithromycin is usually well tolerated, prolonged treatment can lead to unwanted neurological effects like paresthesia and hearing loss. However, the mechanism causing neurodegeneration is still unknown. Here, we show that even low therapeutically relevant azithromycin concentrations like 1µg/ml decreased cell viability by 15% and induced neurite loss of 47% after 96h in differentiated PC12 cells, which are a well-established model system for neuronal cells. When higher concentrations were used, the drug-induced effects occurred earlier and were more pronounced. Thereby, azithromycin altered tropomyosin-related kinase A (TrkA) signaling and attenuated protein kinase B (Akt) activity, which subsequently induced autophagy. Simultaneously, the antibiotic impaired lysosomal functions by blocking the autophagic flux, and this concurrence reduced cell viability. In good agreement with reversible effects observed in patients, PC12 cells could completely recover if azithromycin was removed after 24h. In addition, the detrimental effects of azithromycin were limited to differentiated cells, as confirmed in the human neuronal model cell line SH-SY5Y. Thus, azithromycin alters cell surface receptor signaling and autophagy in neuronal cells, but does not automatically induce irreversible damage when used in low concentrations and for a short time.

Involvement of phosphatidylinositol 3'-kinase in stem-cell-factor-induced phospholipase D activation and arachidonic acid release

We have shown previously that the stem cell factor (SCF) receptor undergoes phosphorylation on serine residues following ligand stimulation, and that this phopshorylation is dependent mainly on the activity of protein kinase C (PKC). In the present study, we have further investigated the molecular mechanisms behind SCF-stimulated activation of PKC, and found that SCF does not activate phosphatidylinositol-specific phospholipase C. In contrast, phospholipase D (PLD) is activated in response to SCF in a dose-dependent manner. Activation of PLD was not inhibited by calphostin C, an inhibitor of PKC. On the other hand, inhibitors of phosphatidylinositol PtdIns 3'-kinase (PtdIns 3'-kinase), i.e. wortmannin and LY294002, inhibited SCF-induced PLD activation. Moreover, a mutant SCF receptor in which Tyr721, which is responsible for activation of PtdIns 3'-kinase, is mutated to a phenylalanine residue was unable to mediate activation of PLD. Thus, PtdIns 3'-kinase appears to be essential for SCF-induced PLD activation. Furthermore, we demonstrate that phosphatidic acid (PtdH), generated through the action of PLD in response to SCF, is metabolized to diacylglycerol by dephosphorylation. Diacylglycerol can then activate PKC, and, moreover, after deacylation by a diacylglycerol lipase, yield arachidonic acid, an important second messenger in cell signaling.

The enhancement by wortmannin of protein kinase C-dependent activation of phospholipase D in vascular endothelial cells

Phosphatidic acid generation by phospholipase D (PLD) activation has been implicated in agonist- and oxidant-mediated endothelial cell signal transduction. We examined the effect of wortmannin on PLD activation in pulmonary artery endothelial and smooth muscle cells in culture. Pretreatment of bovine pulmonary artery endothelial cells (BPAECs) with wortmannin potentiated TPA- (100 nM), ATP- (100 microM), and bradykinin- (1 microM) induced [32P]PEt formation, an index of PLD activation. However, wortmannin by itself had no effect on PLD activity. The potentiating effect of wortmannin on TPA-induced PLD activation was dose- (1-10 microM) and time-dependent (5-30 min) and was inhibited by bisindoylmalemide, an inhibitor of protein kinase C (PKC). Furthermore, down-regulation of PKC by prolonged treatment with TPA (100 nM, 18 h) attenuated the wortmannin effect. This effect of wortmannin was specific for TPA- or agonist-induced PLD activation as no potentiation of [32P]PEt formation was observed with H2O2 (1 mM) or ionomycin (1 microM). The effect of wortmannin was not due to activation of PKC alpha as determined by western blot analysis of PKC alpha in the cytosol and membrane fractions. Also, genistein, an inhibitor of tyrosine kinases, did not attenuate the wortmannin-mediated potentiation of PLD thereby suggesting non-involvement of protein tyrosine phosphorylation. These results indicate that wortmannin potentiates PKC-dependent stimulation of PLD in vascular endothelial cells.

Novel functions of phosphatidylinositol 3-kinase in terminally differentiated cells

Importance of phosphatidylinositol 3-kinase (PI 3-kinase) in signalling pathways leading to growth stimulation has already been reviewed in this journal and others. Evidence has now been accumulating that PI 3-kinase is involved in transmission of activation signals in terminally differentiated cells, especially signals starting from receptors which have no intrinsic tyrosine kinase domain. The pioneer works showed the presence of PI 3-kinase activity and the accumulation of the reaction products of PI 3-kinase correlated with the cell responses. However, these studies were done in only limited cell responses such as respiratory burst in neutrophils and degranulation in platelets. Recent finding of a potent and selective inhibitor of PI 3-kinase, wortmannin, reported from three independent groups including us, gave a new and powerful tool not only to confirm the suggested functions but also to reveal new functions of PI 3-kinase such as histamine release from antigen-stimulated mast cells/basophils and glucose uptake in insulin-stimulated adipocytes. Nearly one hundred papers which describe the action of wortmannin on various cells have been reported during one year after the publication of the discovery of wortmannin as PI 3-kinase inhibitor, suggesting possible involvement of the enzyme in the diverse cell responses besides cell proliferation.

Signalling pathways as targets for anticancer drug development

Intracellular signalling pathways mediating the effects of oncogenes on cell growth and transformation offer novel targets for the development of anticancer drugs. With this approach, it may be sufficient to target a component of the signalling pathway activated by the oncogene rather than the oncogene product itself. In this review, the abilities of some antiproliferative drugs to inhibit signalling targets are considered. There are some anticancer drugs already in clinical trial that may act by inhibiting signalling targets, as well as drugs in preclinical development. Some problems that may be encountered in developing this new class of anticancer drugs are discussed.

Wortmannin is a potent phosphatidylinositol 3-kinase inhibitor: the role of phosphatidylinositol 3,4,5-trisphosphate in neutrophil responses

Phosphatidylinositol 3,4,5-trisphosphate (PtdInsP3) is rapidly produced upon exposure of neutrophils to the chemoattractant N-formylmethionyl-leucylphenylalanine (fMLP), and has been proposed to act as a second messenger mediating actin polymerization and respiratory-burst activity. Here we present evidence that wortmannin, a known inhibitor of respiratory-burst activity, acts on PtdIns 3-kinase, the enzyme producing PtdInsP3 from PtdIns(4,5)P2. Pretreatment of 32P-labelled human neutrophils with 100 nM wortmannin totally abolished fMLP-mediated PtdInsP3 production, raised PtdInsP2 levels, and did not affect cellular PtdInsP and PtdIns contents. The inhibitory effect on PtdInsP3 formation in intact cells was dose-dependent, with an IC50 of approximately 5 nM. Similar results were obtained with PtdIns 3-kinase immunoprecipitated by antibodies against the p85 regulatory subunit: wortmannin totally inhibited PtdIns3P production in immunoprecipitates at concentrations of 10-100 nM (IC50 approximately 1 nM). These results illustrate the direct and specific inhibition of PtdIns 3-kinase by wortmannin. Since agonist-mediated respiratory-burst activation is most sensitive to wortmannin (IC50 = 12 nM), this suggests that agonist-mediated PtdInsP3 formation is indispensable for this cell response. Neutrophils pretreated with wortmannin develop oscillatory changes in F-actin content, but actin polymerization in response to fMLP is not inhibited. This, and the absence of PtdInsP3 under these conditions, are in agreement with a modulatory role for PtdInsP3 in cytoskeletal rearrangements, but imply that PtdInsP3 production is not a primary event triggering elongation of actin filaments in neutrophils.

Phospholipase D-catalyzed hydrolysis of phosphatidylcholine provides the choline precursor for acetylcholine synthesis in a human neuronal cell line

To identify the metabolic pathway that generates choline (Cho) for acetylcholine (AcCho) from its storage pool in membrane phosphatidylcholine (PtdCho), human neuronal cells (LA-N-2) were radioisotopically labeled with 1-O-hexadecyl-2-hydroxy-sn-glycero(3)phospho[14C]choline. The compound was efficiently taken up by the cells and metabolically labelled PtdCho, Cho, AcCho, and phosphocholine pools. In pulse-chase experiments, the specific radioactivities of the metabolites of 1-O-hexadecyl-2-hydroxy-sn-glycero(3)-phospho[14C]choline indicated that it was rapidly acylated to Ptd-Cho and then hydrolyzed first to free Cho and not to phosphocholine or glycerophosphocholine. This Cho was subsequently converted to AcCho and to phosphocholine. In the absence of exogenous Cho, at least 15% of the total cellular AcCho pool was synthesized by this pathway in 1 h. The data demonstrate that the liberation of the free Cho precursor for AcCho synthesis from PtdCho can be accomplished in a one-step process, indicating the involvement of a phospholipase D-type enzyme. In the presence of hemicholinium-3, which inhibits Cho transport, the amount of intracellular [14C]Cho metabolites that accumulated during the chase period was higher than in control cells, indicating that PtdCho hydrolysis liberated Cho directly into the cytoplasm. These data show that cholinergic cells are characterized by an intracellular pathway, catalyzed by a phospholipase D, that generates Cho for AcCho synthesis from PtdCho. Abnormalities in the regulation of this pathway may contribute to selective vulnerability of cholinergic neurons in certain neurodegenerative diseases, e.g., Alzheimer disease.