Phorbol ester stimulation of phosphatidylcholine synthesis in four cultured neural cell lines: correlations with expression of protein kinase C isoforms

Melatonin induces neuritogenesis at early stages in N1E-115 cells through actin rearrangements via activation of protein kinase C and Rho-associated kinase

Melatonin increases neurite formation in N1E-115 cells through microtubule enlargement elicited by calmodulin antagonism and vimentin intermediate filament reorganization caused by protein kinase C (PKC) activation. Microfilament rearrangement is also a necessary process in growth cone formation during neurite outgrowth. In this work, we studied the effect of melatonin on microfilament rearrangements present at early stages of neurite formation and the possible participation of PKC and the Rho-associated kinase (ROCK), which is a downstream kinase in the PKC signaling pathway. The results showed that 1 nm melatonin increased both the number of cells with filopodia and with long neurites. Similar results were obtained with the PKC activator phorbol 12-myristate 13-acetate (PMA). Both melatonin and PMA increased the quantity of filamentous actin. In contrast, the PKC inhibitor bisindolylmaleimide abolished microfilament organization elicited by either melatonin or PMA, while the Rho inhibitor C3, or the ROCK inhibitor Y27632, abolished the bipolar neurite morphology of N1E-115 cells. Instead, these inhibitors prompted neurite ramification. ROCK activity measured in whole cell extracts and in N1E-115 cells was increased in the presence of melatonin and PMA. The results indicate that melatonin increases the number of cells with immature neurites and suggest that these neurites can be susceptible to differentiation by incoming extracellular signals. Data also indicate that PKC and ROCK are involved at initial stages of neurite formation in the mechanism by which melatonin recruits cells for later differentiation.

The MARCKS family of phospholipid binding proteins: regulation of phospholipase D and other cellular components

Myristoylated alanine-rich C kinase substrate (MARCKS) and MARCKS-related protein (MRP) are essential proteins that are implicated in coordination of membrane-cytoskeletal signalling events, such as cell adhesion, migration, secretion, and phagocytosis in a variety of cell types. The most prominent structural feature of MARCKS and MRP is a central basic effector domain (ED) that binds F-actin, Ca2+-calmodulin, and acidic phospholipids; phosphorylation of key serine residues within the ED by protein kinase C (PKC) prevents the above interactions. While the precise roles of MARCKS and MRP have not been established, recent attention has focussed on the high affinity of the MARCKS ED for phosphatidylinositol 4,5-bisphosphate (PIP2), and a model has emerged in which calmodulin- or PKC-mediated regulation of these proteins at specific membrane sites could in turn control spatial availability of PIP2. The present review summarizes recent progress in this area and discusses how the above model might explain a role for MARCKS and MRP in activation of phospholipase D and other PIP2-dependent cellular processes.

Cell membranes and apoptosis: role of cardiolipin, phosphatidylcholine, and anticancer lipid analogues

The apoptotic program utilizes cellular membranes to transduce and generate operative signals. Lipids are major components of cellular membranes and have the potential to control the effectiveness of the signal by directing it to the proper location, being a source of new signals or as mediators in the response. These possible lipid functions are illustrated in the present review, focussing on the role that two different phospholipids, cardiolipin and phosphatidyl choline, play in apoptosis. Mitochondria have a central role in apoptosis, and many important aspects of the process mediated by this organelle converge through its distinctive lipid cardiolipin. Specifically, changes in cardiolipin metabolism have been detected in early steps of the death program and it is postulated (i) to mediate recruitment of pro apoptotic proteins like Bid to the mitochondria surface and (ii) to actively participate in the release of proteins relevant for the execution phase of apoptosis, like cytochrome c. Unlike the organelle specific distribution of cardiolipin, phosphatidylcholine is widely distributed among all organelles of the cell. The importance of phosphatidylcholine in apoptosis has been approached mainly through the study of the mode of action of (i) phosphatidylcholine anticancer analogues such as edelfosine and (ii) molecules that alter phosphatidylcholine metabolism, such as farnesol. The contribution of phosphatidylcholine metabolism to the apoptotic program is discussed, analyzing the experimental evidence available and pointing out some controversies in the proposed mechanisms of action.

Effect of D609 on phosphatidylcholine metabolism in the nuclei of LA-N-1 neuroblastoma cells: a key role for diacylglycerol

In our previous studies, TPA treatment of LA-N-1 cells stimulated the production of diacylglycerol in nuclei, probably through the activation of a phospholipase C. Stimulation of the synthesis of nuclear phosphatidylcholine by the activation of CTP:phosphocholine cytidylyltransferase was also observed. The present data show that both effects were inhibited by the pretreatment of the cells with D609, a selective phosphatidylcholine-phospholipase C inhibitor, indicating that the diacylglycerol produced through the hydrolysis of phosphatidylcholine in the nuclei is reutilized for the synthesis of nuclear phosphatidylcholine and is required for the activation of CTP:phosphocholine cytidylyltransferase.

Phosphatidylcholine metabolism in nuclei of phorbol ester-activated LA-N-1 neuroblastoma cells

The agonist stimulation of a variety of cells results in the induction of specific lipid metabolism in nuclear membranes, supporting the hypothesis of an important role of the lipids in nuclear signal transduction. While the existence of a phosphatidylinositol cycle has been reported in cellular nuclei, little attention has been given to the metabolism of phosphatidylcholine in nuclear signaling. In the present study the metabolism of phosphatidylcholine in the nuclei of neuroblastoma cells LA-N-1 was investigated. The incubation of LA-N-1 nuclei with radioactive choline, phosphocholine or CDP-choline led to the production of labelled phosphatidylcholine. The incorporation of choline and phosphocholine but not CDP-choline was enhanced in nuclei of TPA treated cells. Moreover the presence of choline kinase, phosphocholine cytidylyltransferase and phosphocholine transferase activities were detected in the nuclei and the TPA treatment of the cells stimulated the activity of the phosphocholine cytidylyltransferase. When cells prelabelled with [3H]palmitic acid were stimulated with TPA in the presence of ethanol, an increase of labelled diacylglycerol and phosphatidylethanol in the nuclei was observed. Similarly, an increase of labelled diacylglycerol and phosphatidic acid but not of phosphatidylethanol occurred in [3H]palmitic acid prelabelled nuclei stimulated with TPA in the presence of ethanol. However the production of phosphatidylethanol was observed when the nuclei were treated with TPA in the presence of ATP and GTPgammaS. The stimulation of [3H]choline prelabelled nuclei with TPA also generated the release of free choline and phosphocholine. The results indicate the presence of PLD and probably PLC activities in LA-N-1 nuclei and the involvement of phosphatidylcholine in the production of nuclear lipid second messengers upon TPA stimulation of LA-N-1 cells. The correlation of the disappearance of phosphatidylcholine, the production of diacylglycerol and phosphatidic acid with the stimulation of phosphatidylcholine synthesis in nuclei of TPA treated LA-N-1 suggests the existence of a phosphatidylcholine cycle in these nuclei.

Surfactant secretagogue activation of protein kinase C isoforms in cultured rat type II cells

Several lung surfactant secretagogues are known to activate protein kinase C (PKC) in type II cells. Such agents include 12-O-tetradecanoylphorbol 13-acetate (TPA) and cell-permeable diacylglycerols that directly activate PKC. Other agents include ATP and UTP, which act at P2Y(2) receptors coupled to phosphoinositide-specific phospholipase C, activation of which leads to formation of diacylglycerols and consequent activation of PKC. Activation of PKC is associated with redistribution of enzyme from a cytosolic to a membrane fraction of the cell. We examined the PKC isomers that are translocated by ATP, UTP, TPA, and dioctanoylglycerol in cultured type II cells isolated from adult rats. PKC isoforms were identified by Western blotting using isoform-specific antibodies. Treatment of type II cells with ATP, UTP, TPA, and dioctanoylglycerol resulted in a significant redistribution of PKC-mu from cytosol to membrane. TPA and dioctanoylglycerol also activated PKC-alpha, -betaI, -betaII, -delta, and -eta, but those isoforms were not activated by ATP or UTP. The effects of TPA and dioctanoylglycerol on PKC-mu were more pronounced than those of the P2Y(2) agonists, and the effect of TPA was also more rapid than that of ATP. The data show that direct activators and agents that generate endogenous diacylglycerols have different PKC activation patterns. Because it is activated by different types of secretagogues, PKC-mu may have an important role in the physiological regulation of surfactant secretion.

Lipopolysaccharide induction of MARCKS-related protein and cytokine secretion are differentially impaired in microglia from LPS-nonresponsive (C3H/HeJ) mice

Many events involved in activation of microglia and leukocytes by lipopolysaccharide (LPS) are mediated by protein kinase C (PKC), and we have recently demonstrated that a major PKC substrate, MARCKS-related protein (MRP), is selectively induced by LPS in murine microglia. In microglia from LPS-nonresponsive (C3H/HeJ) mice, induction of MRP and secretion of CSF-1 required much higher LPS concentrations (> or = 100 ng/ml) than in normal (C3H/OuJ) microglia (< or = 10 ng/ml). By contrast, TNF alpha production was not significantly increased in C3H/HeJ microglia even at 1 microgram LPS/ml. Microglia expressed PKC isoforms alpha, beta, delta, and zeta (but not gamma and epsilon); PKC isoform levels were similar in both normal and C3H/HeJ microglia and no significant change in response to LPS was noted. Our results indicate that LPS alters PKC substrate (rather than kinase) expression, and that the Lpsd mutation in C3H/HeJ mice differentially affects regulation of several gene products implicated in microglial function.

Phospholipase D activity is altered in X-linked adrenoleukodystrophy heterozygous carriers, but not in hemizygous patients

Abnormalities in levels of choline and its metabolites have been reported in the lesions of brains of X-linked adrenoleukodystrophy (X-ALD) patients. We have examined the turnover of the major choline-containing phospholipid, phosphatidylcholine (PtdCho), in fibroblasts from hemizygous X-ALD, heterozygous X-ALD, Zellweger syndrome (ZW), and male and female control individuals to assess possible alterations in PtdCho metabolism mediated by activation of protein kinase C (PKC). Hydrolysis of PtdCho by phospholipase D (PLD) and resynthesis of PtdCho from labeled choline were stimulated 2- to 4-fold by PKC activation with the phorbol ester, 4beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA), in all cells except those from heterozygous X-ALD individuals. No differences in quantity or intracellular distribution of PKC activity, PKC isoforms by Western blot analysis, or of the PKC substrate, myristoylated alanine-rich C kinase substrate (MARCKS), were apparent in any of the cells. Thus, altered PtdCho metabolism was not directly linked to either of these inherited defects that result in abnormal peroxisomal functions. Further, altered responsiveness of PLD in X-ALD heterozygotes was independent of changes in PKC and MARCKS.

PKC isoforms and other signaling proteins involved in surfactant secretion in developing rat type II cells

We previously reported that there is a developmental increase in surfactant secretion in response to P2Y2 purinoceptor agonists. UTP does not stimulate secretion in type II cells from 1- or 2-day-old rats; there is a small response to UTP in cells from 4-day-old animals, and the response increases with increasing age thereafter. Second messenger formation in response to P2Y2 agonists has a similar developmental pattern. We have investigated whether the failure to respond to P2Y2 agonists is due to a deficiency in the P2Y2 receptor or in downstream signaling factors. We compared type II cells from adult and 1- to 2-day-old rats with respect to expression of the P2Y2 receptor gene and the levels of phospholipase C-beta (PLC-beta) and protein kinase C (PKC) isomers and of the alpha-subunit of the GTP-binding protein Gq. We measured gene expression by reverse transcriptase-polymerase chain reaction and protein levels by immunoblotting. We identified PKC-alpha, -betaI, -betaII, -delta, -eta, -zeta, -theta, and -mu, PLC-beta3, and Gqalpha in adult and newborn type II cells. PKC-epsilon, -gamma, and -lambda and PLC-beta1, -beta2, and -beta4 were not present in adult or newborn type II cells. Expression of the P2Y2 receptor gene was essentially the same in newborn and adult cells. However, the levels of PKC-alpha, -betaI, -betaII, and -zeta in newborn type II cells were only 43-57% those of adult cells. The level of PKC-theta also tended to be lower in the newborn cells. There was little difference between newborn and adult type II cells in the levels of PKC-delta, -eta, and -mu, PLC-beta3, and Gqalpha. These data suggest that the lack of response of early newborn type II cells to P2Y2 agonists is not due to a lack of expression of the receptor gene but possibly to insufficient amounts of one or more of the alpha, betaI, betaII, or zeta PKC isoforms.

Overexpression of myristoylated alanine-rich C-kinase substrate enhances activation of phospholipase D by protein kinase C in SK-N-MC human neuroblastoma cells

Signal transduction can involve the activation of protein kinase C (PKC) and the subsequent phosphorylation of protein substrates, including myristoylated alanine-rich C kinase substrate (MARCKS). Previously we showed that stimulation of phosphatidylcholine (PtdCho) synthesis by PMA in SK-N-MC human neuroblastoma cells required overexpression of MARCKS, whereas PKCalpha alone was insufficient. We have now investigated the role of MARCKS in PMA-stimulated PtdCho hydrolysis by phospholipase D (PLD). Overexpression of MARCKS enhanced PLD activity 1.3-2.5-fold compared with vector controls in unstimulated cells, and 3-4-fold in cells stimulated with 100 nM PMA. PMA-stimulated PLD activity was blocked by the PKC inhibitor bisindolylmaleimide. Activation of PLD by PMA was linear with time to 60 min, whereas stimulation of PtdCho synthesis by PMA in clones overexpressing MARCKS was observed after a 15 min time lag, suggesting that the hydrolysis of PtdCho by PLD preceded synthesis. The formation of phosphatidylbutanol by PLD was greatest when PtdCho was the predominantly labelled phospholipid, indicating that PtdCho was the preferred, but not the only, phospholipid substrate for PLD. Cells overexpressing MARCKS had 2-fold higher levels of PKCalpha than in vector control cells analysed by Western blot analysis; levels of PKCbeta and PLD were similar in all clones. The loss of both MARCKS and PKCalpha expression at higher subcultures of the clones was paralleled by the loss of stimulation of PLD activity and PtdCho synthesis by PMA. Our results show that MARCKS is an essential link in the PKC-mediated activation of PtdCho-specific PLD in these cells and that the stimulation of PtdCho synthesis by PMA is a secondary response.

Diacylglycerol molecular species in plasma membrane and microsomes change transiently with endothelin-1 treatment of glioma cells

Agonist-induced intracellular signal transduction often involves activation of protein kinase C by diacylglycerol (DAG) released from membrane phospholipids by phospholipases. Using either DAG kinase or HPLC assays to quantitatively determine DAG mass, we observed a time-dependent increase in DAG accumulation upon incubation of rat C6 glioma cells with 200 nM endothelin-1 (ET-1). Total cell DAG rapidly increased by 25-35% from a basal level of 4.5 +/- 0.3 nmol/mg protein during one min of ET-1 treatment and remained constant or slightly decreased between 1 and 2 min. Thereafter, DAG increased to a maximum (1.6-fold above basal) by 5-10 min. and remained elevated to 30 min. Resolution of DAG molecular species by HPLC after incubation of cells with ET-1 revealed that accumulation of DAG species differed in total cell lysate and subcellular compartments. In plasma membrane, major DAG species increased at 1 min. followed by a decrease at 10 min. whereas in microsomes DAG species did not change at 1 min. and decreased at 10 min. Although phospholipid sources of DAG species were not identified specifically, there was preferential hydrolysis of molecular species of phospholipid for DAG production. We propose that molecular species of DAG produced at the plasma membrane may be transferred to the endoplasmic reticulum so that phospholipid resynthesis can replenish molecular species initially utilized in signal transduction.

Involvement of phospholipase D and protein kinase C in phorbol ester and fatty acid stimulated turnover of phosphatidylcholine and phosphatidylethanolamine in neural cells

Hydrolysis of phosphatidylcholine (PtdCho) can provide lipid second messengers involved in sustained signal transduction. Four neural-derived cell lines (C6 rat glioma; N1E-115 mouse and SK-N-MC and SK-N-SH human neuroblastoma) express different protein kinase C (PKC) isoforms and differentially respond to 4beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA)-stimulation of PtdCho synthesis. We examined involvement of PLD and PKC in the hydrolysis and resynthesis of PtdCho and phosphatidylethanolamine stimulated by beta-TPA, bryostatin (a non-phorbol PKC activator) and oleic acid (18:1n-9) in the four cell lines. beta-TPA or bryostatin produced similar enhancement of [3H]Cho incorporation, loss of stimulated synthesis after down regulation of PKC, and activation of PLD. In C6 cells, staurosporine (STS) and bis-indolylmaleimide (BIM) only partially inhibited basal and beta-TPA-stimulated PLD activity measured as choline or ethanolamine release; phosphatidylbutanol formation after prelabeling with [9,10-3H]18:1n-9, [9,10-3H]myristic acid (14:0), [1-14C]eicosapentaenoic acid (20:5n-3) or 1-O-[alkyl-1', 2-3H]-sn-glyceryl-3-phosphorylcholine gave similar results. STS at >200 nM activated PLD in the presence or absence of beta-TPA. In SK-N-SH cells where PtdCho synthesis was stimulated by beta-TPA or bryostatin, no effect of these agents on PLD was observed. 18:1n-9 stimulated PtdCho synthesis and, to a lesser extent, hydrolysis by PLD both with and without beta-TPA present. Fatty acids had no effect on PKC activities and down regulation of PKC with beta-TPA enhanced fatty acid stimulation of PtdCho synthesis. Thus, activation of PLD hydrolysis preceding resynthesis is involved in the stimulatory effects of beta-TPA on PtdCho synthesis in some but not all of these neural derived cells. Further, PLD hydrolysis of PtdCho and PtdEtn appear to have differing aspects of regulation. Fatty acid regulation of PtdCho synthesis occurs independent of PKC activation. Accordingly, regulation of membrane phospholipid degradation and resynthesis in association with lipid second messenger generation can involve a complex interplay of PLD, PKC, and fatty acids. (c) 1998 Elsevier Science B.V.

Inhibitors of actin polymerization and calmodulin binding enhance protein kinase C-induced translocation of MARCKS in C6 glioma cells

MARCKS (myristoylated alanine-rich C-kinase substrate) is known to interact with calmodulin, actin filaments, and anionic phospholipids at a central basic domain which is also the site of phosphorylation by protein kinase C (PKC). In the present study, cytochalasin D (CD) and calmodulin antagonists were used to examine the influence of F-actin and calmodulin on membrane interaction of MARCKS in C6 glioma cells. CD treatment for 1 h disrupted F-actin filaments, increased membrane bound immunoreactive MARCKS (from 51% to 62% of total), yet markedly enhanced the amount of MARCKS translocated to the cytosolic fraction in response to the phorbol ester 4beta-12-O-tetradecanoylphorbol 13-acetate. In contrast, CD treatment had no effect on phorbol ester-stimulated phosphorylation of MARCKS or on translocation of PKC alpha to the membrane fraction. Staurosporine also increased membrane association of MARCKS in a PKC-independent manner, as no change in MARCKS phosphorylation was noted and bis-indolylmaleimide (a more specific PKC inhibitor) did not alter MARCKS distribution. Staurosporine inhibited the phorbol ester-induced translocation of MARCKS but not of PKC alpha in both CD pretreated and untreated cells. Calmodulin antagonists (trifluoperazine, calmidazolium) had little effect on the cellular distribution or phosphorylation of MARCKS, but were synergistic with phorbol ester in translocating MARCKS from the membrane without a further increase in its phosphorylation. We conclude that cytoskeletal integrity is not required for phosphorylation and translocation of MARCKS in response to activated PKC, but that interaction with both F-actin and calmodulin might serve to independently modulate PKC-regulated localization and function of MARCKS at cellular membranes.

Phospholipase D hydrolysis of plasmalogen and diacyl ethanolamine phosphoglycerides by protein kinase C dependent and independent mechanisms

Ethanolamine phosphoglycerides (EPG) are potential sources of lipid second messengers in signal transduction pathways. We investigated EPG turnover, including both 1-alkenyl-2-acyl- (plasmalogen) and diacyl-classes, in response to stimulation of protein kinase C (PKC) by phorbol ester (4 beta-12-O-tetradecanoylphorbol-13-acetate (TPA)) in cultured C6 rat glioma cells. Release of ethanolamine to the medium from EPG prelabeled with [14C]ethanolamine indicated that initial (< 60 min) TPA-stimulated hydrolysis of EPG was predominantly by phospholipase D (PLD). Effects of TPA on PLD activity specifically with EPG was confirmed using trans-phosphatidylation by incubating cells prelabeled with [14C]eicosapentaenoic acid (20:5n-3) with 100 nM TPA and 1% butanol. Analysis of acid-labile phosphatidylbutanol and remaining EPG showed utilization of both plasmalogen and non-plasmalogen EPG. Staurosporine (STS) inhibited PKC at 200-500 nM but stimulated PLD activity 2-fold at > or = 1 microM. However, STS did not eliminate all TPA-stimulated PLD activity, even when PKC was > 98% inhibited. Bis-indolylmaleimide (BIM) fully inhibited PKC activity but had no independent effects on PLD and did not completely inhibit TPA- or bryostatin-stimulated PLD activity. Down-regulation of PKC by chronic exposure to TPA eliminated stimulation of PLD by TPA but not by STS. Thus, PLD hydrolysis of both plasmalogen and diacyl-EPG is a source of potential lipid second messengers in C6 glioma cells. PLD is stimulated by activation of PKC and by PKC-independent action of STS. Further, the possibility that TPA may also elicit responses through a mechanism independent of PKC activity is suggested.

Protein kinase C isoforms and growth, differentiation and phosphatidylcholine turnover in human neuroblastoma cells

Neuroblastoma and glioma cells differentially express isoforms of protein kinase C (PKC) and myristoylated PKC substrates (e.g. MARCKS). Correlation with metabolism of membrane phospholipids suggests that PKC-alpha and MARCKS may be required to mediate phosphatidylcholine turnover stimulated by phorbol ester (beta-TPA). To evaluate relationships to neural cell differentiation, SK-N-SH human neuroblastoma cells were treated with 20 nM beta-TPA. In beta-TPA-treated cells, growth arrest and differentiation occurred (neurite extension; 40-60% decrease in cell number, total protein and RNA). By day 4, mRNA for PKC-alpha and MARCKS increased and, after an initial decrease, PKC-alpha protein also increased. At day 4, phosphatidylcholine synthesis was 3-5 fold greater than in control cells. In contrast, C6 glioma cells treated with beta-TPA showed no growth arrest, decreased PKC-alpha protein (< 20%) and lower phosphatidylcholine synthesis. Thus, induced differentiation of human neuroblastoma cells involved increased expression of PKC-alpha and MARCKS and synthesis of phosphatidylcholine, consistent with involvement of PKC-alpha and MARCKS in regulation of phosphatidylcholine turnover during neurite growth.