Stimulation of phosphatidylcholine breakdown and diacylglycerol production by growth factors in Swiss-3T3 cells

Real-time changes in 1H and 31P NMR spectra of malignant human mammary epithelial cells during treatment with the anti-inflammatory agent indomethacin

Choline metabolites in malignant human mammary epithelial cells (HMECs) are significantly altered compared to normal HMECs. (1)H NMR studies of cell extracts have shown that treatment of malignant HMECs with a nonsteroidal anti-inflammatory agent, indomethacin, results in a distribution of choline compounds more typical of nonmalignant HMECs. To follow the time course of these changes, in this study real-time monitoring of choline compounds of malignant MDA-MB-231 cells was performed during treatment with indomethacin. The contribution of changes in intra- and extracellular pH to changes in choline compounds was also examined. Changes in water-soluble choline phospholipid metabolites, such as phosphocholine (PC), glycerophosphocholine (GPC), and total choline, as well as intracellular pH, were monitored by (31)P and diffusion-weighted (1)H NMR spectroscopy of living cells using an NMR-compatible perfusion system. An accumulation of GPC and a decrease of PC, resulting in an increased [GPC]/[PC] ratio, were detected within 2 hr of treatment with 200 microM indomethacin. Since a decreased [GPC]/[PC] ratio is associated with increased malignancy, these data demonstrate that nonspecific cyclooxygenase inhibition by indomethacin alters the choline metabolite profile of malignant cells towards a less malignant phenotype. These changes were not related to alterations of intra- or extracellular pH.

Final report on the safety assessment of Lecithin and Hydrogenated Lecithin

Lecithin is a naturally occurring mixture of the diglycerides of stearic, palmitic, and oleic acids, linked to the choline ester of phosphoric acid, commonly called phosphatidylcholine. Hydrogenated Lecithin is the product of controlled hydrogenation of Lecithin. Bilayers of these phospholipids in water may form liposomes, a spherical structure in which the acyl chains are inside and not exposed to the aqueous phase. Lecithin and Hydrogenated Lecithin are used in a large number of cosmetic formulations as skin conditioning agents-miscellaneous and as surfactant-emulsifying agents. Hydrogenated Lecithin is also used as a suspending agent-nonsurfactant. Historical data on concentration of use of Lecithin reveals that 0.1% to 1.0% is the concentration range most frequently seen, with concentrations up to 50% reported for two moisturizing products. A solution of 65% Lecithin is currently reported to be used at concentrations up to 3% in cosmetics. Nonocclusive application of Lecithin-containing liposomes to murine skin resulted in 30% penetration to the subdermis. In piglet skin, the same application resulted in 99% accumulating in the stratum corneum. In general, liposomes are considered effective in capturing other compounds inside their spherical structure and delivering any such captured compound through the skin barrier. As a result, caution should be exhibited in formulating cosmetic products that contain these ingredients in combination with other ingredients whose safety is based on their lack of absorption or where dermal absorption is a concern. Lecithin is virtually nontoxic in acute oral studies, short-term oral studies, and subchronic dermal studies in animals. Lecithin is not a reproductive toxicant, nor is it mutagenic in several assays. In an oral carcinogenicity study, brain neoplasms were found in mice exposed to Lecithin. In a subcutaneous carcinogenicity study, no neoplasms were found in mice and rats exposed to Lecithin. Adverse reactions to Lecithin in a metered-dose inhaler have been reported. Lecithin and Hydrogenated Lecithin were generally nonirritating and nonsensitizing in animal and human skin. Based on the available data, Lecithin and Hydrogenated Lecithin are safe as used in rinse-off cosmetic products; they may be safely used in leave-on products at concentrations up to 15%, the highest concentration tested in clinical irritation and sensitization studies; but the safety of use could not be substantiated in cosmetic products likely to be inhaled. Because of the possibility of formation of nitrosamines, these ingredients should not be used in cosmetic products in which N-nitroso compounds may be formed.

Phospholipase D and its product, phosphatidic acid, mediate agonist-dependent raf-1 translocation to the plasma membrane and the activation of the mitogen-activated protein kinase pathway

The primary known function of phospholipase D (PLD) is to generate phosphatidic acid (PA) via the hydrolysis of phosphatidylcholine. However, the functional role of PA is not well understood. We report here evidence that links the activation of PLD by insulin and the subsequent generation of PA to the activation of the Raf-1-mitogen-activated protein kinase (MAPK) cascade. Brefeldin A (BFA), an inhibitor of the activation of ADP-ribosylation factor proteins, inhibited insulin-dependent production of PA and MAPK phosphorylation. The addition of PA reversed the inhibition of MAPK activation by BFA. Overexpression of a catalytically inactive variant of PLD2, but not PLD1, blocked insulin-dependent activation of PLD and phosphorylation of MAPK. Real time imaging analysis showed that insulin induced Raf-1 translocation to cell membranes by a process that was inhibited by BFA. PA addition reversed the effects of BFA on Raf-1 translocation. However, PA did not activate Raf-1 in vitro or in vivo, suggesting that the primary function of PA is to enhance the recruitment of Raf-1 to the plasma membrane where other factors may activate it. Finally, we found that the recruitment of Raf-1 to the plasma membrane was transient, but Raf-1 remained bound to endocytic vesicles.

Diacylglycerol--when is it an intracellular messenger?

Distinct, structurally different forms of sn-1,2-diacylglycerol are found in cells, these are polyunsaturated, mono- or di-unsaturated and saturated. The pathways that generate or metabolise sn-1, 2-diacylglycerol are reviewed. The evidence that it is the polyunsaturated forms of sn-1,2-diacylglycerol, but the more saturated forms of phosphatidate which function as intracellular signals is considered.

ADP-ribosylation factor proteins mediate agonist-induced activation of phospholipase D

The role of small G proteins of the ADP-ribosylation factor (ARF) and Rho families on the activation of phospholipase D (PLD) by platelet-derived growth factor (PDGF) and phorbol esters (PMA) has been investigated. The activation of PLD by PDGF and PMA was blocked by brefeldin A (BFA), an inhibitor of ARF activation, but not by Clostridium botulinum C3 exotoxin, an inhibitor of the activity of Rho. PDGF and PMA, in the presence of GTPgammaS, promoted the association of ARF and RhoA with cell membranes. Cells depleted of ARF and Rho by digitonin permeabilization showed a significant reduction of the activity of phospholipase D. Recombinant ARF was sufficient to restore agonist-induced PLD activity to digitonin-permeabilized, cytoplasm-depleted cells. In contrast, isoprenylated recombinant RhoA had no effects in this reconstitution assay. HIRcB cells were transiently transfected with wild-type and dominant-negative mutants of ARF1 and ARF6. Neither wt-ARF1 nor wt-ARF6 had any effects on agonist-dependent PLD activity. However, dominant-negative ARF1 and ARF6 mutants blocked the stimulation of PLD by PDGF but only partially inhibited the effects of PMA. These results demonstrate that ARF rather than Rho proteins mediate the activation of PLD by PDGF and phorbol esters in HIRcB fibroblasts.

Stimulation of phosphatidylcholine turnover by beta-phorbol ester and diacylglycerol in the primordial human placenta: the suggested role of phospholipase D activation

The effects of 4beta-phorbol-12-myristate-13-acetate (PMA) and 1,2-(sn)-dioctanoylglycerol (DOCG) on the phosphatidylcholine (PC) turnover (defined as degradation to diacylglycerol followed by PC resynthesis) and on the activity of PC-specific phospholipase D were investigated in placental mince incubated with various radiolabelled precursors in vitro. Experiments with [32P]phosphate indicated that 1 microM PMA and 125-250 microM DOCG were the lowest concentrations that led to maximal and selective stimulation of PC labelling. Moreover, PMA and DOCG acted along different time courses: PMA enhanced labelling after 60 min incubation, with a lag period of at least 30 min, whereas DOCG stimulated PC labelling after only 30 min with no further increase in the next 30 min. The following findings suggest that increased labelling of PC with [32P]phosphate in PMA-treated tissue reflects an increased rate of PC turnover: (1) the effects of PMA and DOCG were additive and PMA did not have any effect on the labelling of PC(DOCG) indicating that it stimulated PC labelling even if it did not activate CTP:choline cytidylyl transferase, the regulatory enzyme of PC synthesis de novo; (2) PMA did not increase the labelling of PC from [3H]glycerol or [3H]glucose ruling out a PMA-promoted availability of glycolytic and/or lipolytic intermediates for PC formation; and (3) the PMA effect was attended by an increased labelling of phosphatidic acid whereas there was no change in the labelling of lyso-PC, indicating the activation of phospholipase D. Experiments in which the transphosphatidylation reaction between [3H]myristic acid-labelled PC and ethanol was used to estimate phospholipase D activity showed 2.4-fold and 1.4-1.8-fold activations by PMA and DOCG, respectively, with no additivity noted. These results suggest that PMA stimulates PC turnover in the early human placenta via the activation of phospholipase D. Rapid metabolic conversion decreases the capacity of DOCG to accelerate PC-turnover and to activate phospholipase D. The early DOCG-induced stimulation of PC labelling with [32P]phosphate is attributed mainly to its known activating effect on CTP: choline cytidylyl transferase.

ARF proteins mediate insulin-dependent activation of phospholipase D

BACKGROUND

ADP-ribosylation factors (ARFs) have been shown to activate phospholipase D (PLD), an enzyme modulated by extracellular signals, including several growth factors and, in particular, insulin. We have tested the hypothesis that ARF proteins are involved specifically in insulin-induced activation of PLD.

RESULTS

We found that in membranes obtained from HIRcB cells, a cell line derived from Rat-1 fibroblasts that overexpresses normal human insulin receptors, binding of the GTP analogue GTPgammaS to purified bovine or recombinant ARF was enhanced in the presence of insulin. Membranes obtained from cells that overexpressed a mutated, nonfunctional insulin receptor failed to stimulate ARF activation. Insulin promoted the association of ARF proteins with membranes in the presence of GTPgammaS in permeabilized cells. Insulin activated PLD in permeabilized HIRcB cells by a process that required GTPgammaS and ARF. Azido-gamma[32P]-GTP labelling of immunoprecipitated receptors revealed the presence of a unique 19 kD band; ARF proteins are approximately this size, and analysis using specific monoclonal antibodies demonstrated that ARF proteins coimmunoprecipitated with the insulin receptor. Coimmunoprecipitation of ARF with the receptor was inhibited by guanine nucleotides and stimulated by insulin. No evidence of the coprecipitation of ARF with mutant receptors could be obtained using azido-gamma[32P]-GTP or anti-ARF antibodies.

CONCLUSIONS

The activation of ARF proteins is stimulated by insulin and this process plays an important role in insulin-mediated regulation of PLD.

Interaction of urokinase-type plasminogen activator with its receptor rapidly induces activation of glucose transporters

The interaction of urokinase-type plasminogen activator (u-PA) or of u-PA amino-terminal fragment (u-PA-ATF) with the cell surface receptor (u-PAR) was found to stimulate an increase of glucose uptake in many cell lines, ranging from normal and transformed human fibroblasts, mouse fibroblasts transfected with human u-PAR, and cells of epidermal origin. Such increase of glucose uptake reached a peak within 5-10 min, depending on the cell line, and occurred through the facilitative glucose transporters (GLUTs), since it was inhibited by cytochalasin B. Each cell line showed a specific mosaic of glucose transporter isoforms, GLUT2 being the most widespread and GLUT1 the most abundant, when present. u-PAR stimulation was followed by translocation of GLUT1 from the microsomal to the membrane compartment, as shown by both immunoblotting and immunofluorescence of sonicated plasma membrane sheets and by activation of GLUT2 on the cell surface. Both translocation and activation resulted inhibitable by protein-tyrosine kinase inhibitors and independent of downregulation of protein kinase C (PKC). The increase of intracellular glucose was followed by neosynthesis of diacylglycerol (DAG) from glucose, as previously shown. Such neosynthesis was completely inhibited by impairment of facilitative GLUT transport by cytochalasin B. DAG neosynthesis was followed by activation of PKC, whose activity translocated into the intracellular compartment (PKM), where it probably phosphorylates substrates required for u-PAR-dependent chemotaxis. Our data show that u-PAR-mediated signal transduction, related with u-PA-induced chemotaxis, involves activation of tyrosine kinase-dependent glucose transporters, leading to increased de novo DAG synthesis from glucose, eventually resulting in activation of PKC.

Alpha 1-adrenergic stimulation differentially regulates ether-linked diacylglycerols in airway epithelial cells from normal and cystic fibrosis patients

Alpha 1-adrenergic stimulation of human airway epithelial cells induces a transient increase in polyphosphoinositide turnover coincident with augmented Na+Cl-(K+) cotransport activity. This activation of airway epithelial cells also results in a biphasic elevation of diacylglycerols. To better understand the significance of these distinct diacylglycerol pools, we now characterize the mass of ether- and ester-linked diacylglycerol species. We demonstrate that the relative mass of ether-linked diacylglycerols is reduced in airway epithelium from cystic fibrosis patients in the presence or absence of alpha 1-adrenergic stimulation. This reduction in ether-linked diacylglycerol mass may represent a compensatory mechanism to help maintain normal chloride influx in cystic fibrosis patients.

Regulation of phospholipase D by tyrosine kinases

Activation of phospholipase D (PLD) represents part of an important signalling pathway in mammalian cells. Phospholipase D catalyzed hydrolysis of phospholipids generates phosphatidic acid (PA) which is subsequently metabolized to lyso-PA (LPA) or diacylglycerol (DAG). While DAG is an endogenous activator of protein kinase C (PKC), PA and LPA have been recognized as second messengers as well. Activation of PLD in response to an external stimulus may involve PKC, Ca2+, G-proteins and/or tyrosine kinases. In this review, we will address the role of protein tyrosine phosphorylation in growth factor-, agonist- and oxidant-mediated activation of PLD. Furthermore, a possible link between PKC, Ca2+, G-proteins and tyrosine kinases is discussed to indicate the complexity involved in the regulation of PLD in mammalian cells.

Regulation of phospholipase D by protein kinase C

In nearly all mammalian cells and tissues examined, protein kinase C (PKC) has been shown to serve as a major regulator of a phosphatidylcholine-specific phospholipase D (PLD) activity. At least 12 distinct isoforms of PKC have been described so far; of these enzymes only the alpha- and beta-isoforms were found to regulate PLD activity. While the mechanism of this regulation has remained unknown, available evidence suggests that both phosphorylating and non-phosphorylating mechanisms may be involved. A phosphatidylcholine-specific PLD activity was recently purified from pig lung, but its possible regulation by PKC has not been reported yet. Several cell types and tissues appear to express additional forms of PLD which can hydrolyze either phosphatidylethanolamine or phosphatidylinositol. It has also been reported that at least one form of PLD can be activated by oncogenes, but not by PKC activators. Similar to activated PKC, some of the primary and secondary products of PLD-mediated phospholipid hydrolysis, including phosphatidic acid, 1,2-diacylglycerol, choline phosphate and ethanolamine, also exhibit mitogenic/co-mitogenic effects in cultured cells. Furthermore, both the PLD and PKC systems have been implicated in the regulation of vesicle transport and exocytosis. Recently the PLD enzyme has been cloned and the tools of molecular biology to study its biological roles will soon be available. Using specific inhibitors of growth regulating signals and vesicle transport, so far no convincing evidence has been reported to support the role of PLD in the mediation of any of the above cellular effects of activated PKC.

Stimulation of phospholipase D by epidermal growth factor requires protein kinase C activation in Swiss 3T3 cells

The proposal that epidermal growth factor (EGF) activates phospholipase D (PLD) by a mechanism(s) not involving phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) hydrolysis was examined in Swiss 3T3 fibroblasts. EGF, basic fibroblast growth factor (bFGF), bombesin, and platelet-derived growth factor (PDGF) activated PLD as measured by transphosphatidylation of butanol to phosphatidylbutanol. The increase in inositol phosphates induced by bFGF, EGF, or bombesin was significantly enhanced by Ro-31-8220, an inhibitor of protein kinase C (PKC), suggesting that PtdIns(4,5)P2-hydrolyzing phospholipase is coupled to the receptors for these agonists but that the response is down-regulated by PKC. Activation of PLD by EGF was inhibited dose dependently by the PKC inhibitors bis-indolylmaleimide and Ro-31-8220, which also inhibited the effects of bFGF, bombesin, and PDGF. Down-regulation of PKC by prolonged treatment with 4 beta-phorbol 12-myristate 13-acetate also abolished EGF- and PDGF-stimulated phosphatidylbutanol formation. EGF and bombesin induced biphasic translocations of PKC delta and epsilon to the membrane that were detectable at 15 s. In the presence of Ro-31-8220, translocation of PKC alpha became evident, and membrane association of the delta- and epsilon-isozymes was enhanced and/or sustained in response to the two agonists. The inhibitor also enhanced EGF-stimulated [3H]diacylglycerol formation in cells preincubated with [3H]arachidonic acid, which labeled predominantly phosphatidylinositol, but inhibited [3H]diacylglycerol production in cells preincubated with [3H]myristic acid, which labeled mainly phosphatidylcholine. These data support the conclusion that EGF can stimulate diacylglycerol formation from PtdIns(4,5)P2 and that PKC performs the dual role of down-regulating this response as well as mediating phosphatidylcholine hydrolysis. In summary, all of the results of the study indicate that PLD activation by EGF is downstream of PtdIns(4,5)P2-hydrolyzing phospholipase and is dependent upon subsequent PKC activation.

Generation of phosphorylcholine as an essential event in the activation of Raf-1 and MAP-kinases in growth factors-induced mitogenic stimulation

Cell proliferation is regulated by an appropriate combination of intracellular signals involving activation of kinases and the generation of phospholipid metabolites. We report here that growth factors induce a biphasic generation of phosphorylcholine (PCho) in quiescent NIH 3T3 cells, resulting in an early and transient increase at 100 s and a larger and sustained increase after 3 h of stimulation. Generation of PCho at both early and late times of growth factors stimulation results from the consecutive activation of phospholipase D (PLD) and choline kinase (ChoK). Production of PCho by specific growth factors seems an essential requirement for the early signals associated to activation of Raf-1 and MAP kinases, since blockage of choline kinase completely inhibited activation of Raf-1 and MAP kinases by PDGF or FGF. Both the transient early increase and the late sustained increase in PCho are required for the induction of DNA-synthesis, besides completion of the activation of the serine/threonine kinases cascade. Thus, our results strongly suggest that generation of PCho by the PLD/choline kinase pathway is one of the critical steps in regulating cell growth in NIH 3T3 stimulated by growth factors.

Insulin-stimulated hydrolysis of phosphatidylcholine by phospholipase C and phospholipase D in cultured rat hepatocytes

We have investigated the mechanism of action by which insulin increases phosphatidate (PA) and diacylglycerol (DAG) levels in cultured rat hepatocytes. Insulin initially stimulated phosphatidylcholine-dependent phospholipase D (PC-PLD) with a significant increase in both PA and intracellular as well as extracellular choline. The involvement of phospholipase D was confirmed by the formation of PC-derived phosphatidylethanol in the presence of ethanol. The DAG increase appeared to be biphasic. Only the early phase of DAG production was inhibited by propranolol, an inhibitor of the phosphatidate phosphatase (PAP) responsible for the conversion of PA into DAG, suggesting that initially the DAG increase is due to the PLD-PAP pathway. The delayed DAG increase was in parallel with increased intracellular and extracellular phosphocholine and probably derived directly from PC-PLC activity. Experiments performed in the presence of 1 microM phorbol 12-myristate 13-acetate (PMA) indicated that protein kinase C (PKC) mediated the insulin effect on PC-PLC, but not on PC-PLD. These findings were confirmed using the PKC inhibitors calphostin, H7 and staurosporine. The dual activation of these phospholipases with a biphasic elevation of DAG levels and activation of specific PKC isoenzymes could be necessary to elicit both early and delayed effects of insulin.

Vitamin E suppresses diacylglycerol (DAG) level in thrombin-stimulated endothelial cells through an increase of DAG kinase activity

The present study has examined the role of vitamin E, a natural lipid antioxidant, in the production of diacylglycerol (DAG) and phosphatidic acid (PA) in thrombin-stimulated human endothelial cells. Cells were labelled with [3H]myristate and the incorporation and distribution of [3H]myristate into cellular lipids was not affected by vitamin E. However, in response to thrombin stimulation, considerably more PA and less DAG were formed in cells enriched with vitamin E. The time-course of thrombin stimulation indicated that vitamin E attenuated the accumulation of sustained DAG levels with a concomitant increase in PA. Direct determination of DAG mass further confirmed that vitamin E suppresses the accumulation of DAG induced by thrombin. In the presence of ethanol, the formation of [3H]phosphatidylethanol (PEt) in [3H]myristate-labelled cells stimulated by thrombin was unaffected by vitamin E enrichment. DL-Propranolol, a PA phosphohydrolase inhibitor, caused an accumulation of PA, without affecting DAG formation in either vitamin E-treated and untreated cells. This indicated that the increase in PA and decrease in DAG in vitamin E-treated cells was not due to a stimulation of phospholipase D or an inhibition of PA phosphohydrolase. Determination of inositol phosphates formation in response to thrombin showed that the change of DAG levels elicited by vitamin E was independent of phospholipase C-induced hydrolysis of inositol phospholipids. In contrast, analysis of DAG kinase activity revealed that vitamin E enrichment enhanced the activity of the enzyme in both basal and thrombin-stimulated cells. Taken together, these data indicated that vitamin E caused an increased conversion of DAG to PA by activating DAG kinase activity without causing any change in the activities of phospholipase D, PA phosphohydrolase or phospholipase C.

Phosphatidylcholine breakdown and signal transduction

PC hydrolysis by PLA2, PLC or PLD is a widespread response elicited by most growth factors, cytokines, neurotransmitters, hormones and other extracellular signals. The mechanisms can involve G-proteins, PKC, Ca2+ and tyrosine kinase activities. Although an agonist-responsive cytosolic PLA2 has been purified, cloned and sequenced, the agonist-responsive form(s) of PC-PLC has not been identified and no form of PC-PLD has been purified or cloned. Regulation of PLA2 by Ca2+ and MAPK is well established and involves membrane translocation and phosphorylation, respectively. PKC regulation of the enzyme in intact cells is probably mediated by MAPK. The question of G-protein control of PLA2 remains controversial since the nature of the G-protein is unknown and it is not established that its interaction with the enzyme is direct or not. Growth factor regulation of PLA2 involves tyrosine kinase activity, but not necessarily PKC. It may be mediated by MAPK. The physiological significance of PLA2 activation is undoubtedly related to the release of AA for eicosanoid production, but the LPC formed may have actions also. There is much evidence that PKC regulates PC-PLC and PC-PLD and this is probably a major mechanism by which agonists that promote PI hydrolysis secondarily activate PC hydrolysis. Since no agonist-responsive forms of either phospholipase have been isolated, it is not clear that PKC exerts its effects directly on the enzymes. Although it is assumed that a phosphorylation mechanism is involved, this may not be the case, and regulation may be by protein-protein interactions. G-protein control of PC-PLD is well-established, although, again, it has not been demonstrated that this is direct, and the nature of the G-protein(s) involved is unknown. In some cell types, there is evidence of the participation of a soluble protein, which may be a low Mr GTP-binding protein. What role this plays in the activation of PC-PLD is obscure. Agonist activation of PC hydrolysis in cells is usually Ca(2+)-dependent, but the step at which Ca2+ is involved is unclear, since PC-PLD and PC-PLC per se are not influenced by physiological concentrations of the ion. Most growth factors promote PC hydrolysis and this is mainly due to activation of PKC as a result of PI breakdown. However, in some cases, PC breakdown occurs in the absence of PI hydrolysis, implying another mechanism that does not involve PI-derived DAG.(ABSTRACT TRUNCATED AT 400 WORDS)

Prostaglandin F2 alpha-stimulated phospholipase D activation in osteoblast-like MC3T3-E1 cells: involvement in sustained 1,2-diacylglycerol production

In [3H]myristic acid-labelled osteoblast-like MC3T3-E1 cells, prostaglandin F2 alpha (PGF2 alpha)-induced PLD activity was assessed by measuring the [3H]phosphatidylethanol (PEt) formation in the presence of ethanol. Inhibition of the increase in intracellular Ca2+ concentration ([Ca2+]i) by U73122, an inhibitor of phosphoinositide-specific phospholipase C (PI-PLC), or chelation of extracellular Ca2+ with EGTA or of intracellular Ca2+ with BAPTA, suppressed PGF2 alpha-induced phospholipase D (PLD) activation. Neither protein kinase C (PKC) inhibitors nor PKC down-regulation with phorbol 12-myristate 13-acetate affected PGF2 alpha-induced [3H]PEt formation. In permeabilized cells, guanosine 5'-[gamma-thio]triphosphate enhanced PGF2 alpha 's potency in [3H]PEt formation in the presence of Ca2+. The pretreatment of intact cells with pertussis toxin failed to inhibit PGF2 alpha-induced [3H]PEt formation. PGF2 alpha caused a biphasic production of [3H]1,2-diacylglycerol ([3H]1,2-DAG) in [3H]glycerol-labelled cells. The initial transient phase was decreased by U73122, whereas the late sustained phase was decreased by ethanol and the phosphatidic acid phosphohydrolase inhibitor, propranolol. From these results, it was suggested that PGF2 alpha-induced PLD activation was mediated by the dual control of the [Ca2+]i increase due to PI-PLC activation and activation of pertussis-toxin-insensitive G-protein, but not mediated by PKC, and also that PLD activation was involved in the late sustained 1,2-DAG generation in MC3T3-E1 cells.

Evidence that inositol phospholipids, but not choline phospholipids, are a potential source of growth-regulating second messenger molecules in HL60 leukaemia cells

Diacylglycerol is a potent second messenger which is generated via the cleavage of inositol- or choline-containing phospholipids and is involved in the transduction of proliferative signals. We have previously obtained evidence that the constitutive breakdown of inositol lipids may contribute to signalling the continuous proliferation of HL60 leukaemia cells (Porfiri E., Hoffbrand A. V. & Wickremasinghe R. G. (1991) Blood 78, 1069-1077). In order to assess the role of choline lipids as potential sources of growth-regulating second messengers, we have studied the pathways of constitutive breakdown of radiolabelled phosphatidylcholine in intact HL60 cells. Neither exponentially growing HL60 cells nor HL60 cells which had been induced to cease proliferation by treatment with dimethyl-sulphoxide degraded choline lipids via phospholipase C- or phospholipase D-catalysed pathways. Both pathways were, however, activated by phorbol myristate acetate irrespective of proliferation status. The data here suggest that, unlike inositol lipids, choline lipids are not a source of second messenger molecules with potential roles in the regulation of HL60 cell proliferation.

Distinct mechanisms of phospholipase D activation and attenuation utilized by different mitogens in NIH-3T3 fibroblasts

The activation of phospholipase D (PLD) by platelet-derived growth factor (PDGF), prostaglandin F2 alpha and 12-O-tetradecanoylphorbol 13-acetate (TPA) was studied in NIH-3T3 fibroblasts. PLD activation was determined by measuring the production of both [3H]phosphatidic acid and [3H]phosphatidylpropanol (products of the PLD-catalyzed hydrolysis and transphosphatidylation reactions, respectively), in cells that were metabolically pre-labeled with [3H]oleic acid. All mitogens caused a rapid (within 2 min) activation of PLD. Activation of PLD by prostaglandin F2 alpha and PDGF was transient and declined to near basal levels by 15 min and 55 min, respectively. In contrast, TPA-induced activation of PLD was sustained for at least 60 min of incubation. A combination of maximally effective concentrations of PDGF and TPA stimulated PLD activity in a non-additive manner, while the effect of prostaglandin F2 alpha was additional to that of either PDGF or TPA. The protein kinase inhibitor staurosporine inhibited PLD activation by PDGF or TPA with almost identical dose/response curves. In contrast, staurosporine potentiated prostaglandin-F2 alpha-induced PLD activation. The specific protein kinase C inhibitor GF109203X (a bisindolylmaleimide) inhibited PLD activation by prostaglandin F2 alpha and PDGF at concentrations higher than those required for inhibition of PLD activation induced by TPA. Depletion of cellular protein kinase C abolished PLD activation by all three mitogens without affecting in vitro activity of membrane-bound PLD. The distinct kinetics of PLD activation and its differential susceptibility to protein kinase inhibitors suggest the existence of agonist-specific activation and/or inactivation mechanisms. The results indicate also that protein kinase C participates in the mechanism of PLD activation via PDGF, while the effect of prostaglandin F2 alpha involves a pathway independent of protein kinase C.

Phosphomonoester is associated with proliferation in human breast cancer: a 31P MRS study

Phospholipid metabolism of human breast cancer was studied by 31P magnetic resonance spectroscopy (MRS). In vivo localised 31P MR spectra were obtained from the tumour alone using phase modulated rotating frame imaging. For 31 tumours, median (range) phosphomonoester (PME) to ATP ratio was 1.48 (0.57-3.78) and phosphodiester (PDE) to ATP ratio was 1.65 (0.44-3.89). DNA index and S phase fraction (SPF) were measured by flow cytometry of paraffin embedded tissue. Twelve (39%) tumours were diploid and 19 aneuploid. Median (range) SPF for 29 assessable tumours was 5.3% (0.6-28%), with significantly greater median SPF for aneuploid tumours (9.3%) than diploid (3.8%, P = 0.007). There was a significant association between PME/ATP and SPF (P = 0.03) due to a significant correlation for aneuploid tumours (P = 0.01). High resolution 31P MRS of extracts from 18 tumours (including seven studied in vivo) demonstrated that the PME peak consists predominantly of phosphoethanolamine (PE) with a smaller contribution from phosphocholine (PC) (median (range) PE/PC: 3.02 (1.13-5.09)). Changes in PME/ATP were observed for two tumours where tamoxifen stablized disease and may be consistent with the cytostatic effects of this drug.

Cooperative effects of ethanol and protein kinase C activators on phospholipase-D-mediated hydrolysis of phosphatidylethanolamine in NIH 3T3 fibroblasts

In a previous study, ethanol was shown to enhance the stimulatory effect of phorbol 12-myristate 13-acetate (PMA), a prominent activator of protein kinase C (PKC), on phospholipase-D (PLD)-mediated hydrolysis of phosphatidylethanolamine (PtdEtn) in NIH 3T3 fibroblasts (Kiss et al. (1991) Eur. J. Biochem. 197, 785-790). Here, the mechanism and possible significance of ethanol-stimulated PtdEtn hydrolysis was further studied. In [14C]ethanolamine-labeled NIH 3T3 fibroblasts, 10 mM ethanol enhanced PMA-induced hydrolysis of PtdEtn 1.5-2.0-fold during a 2.5-15-min incubation period. Other alcohols, including glycerol, methanol, and 1-propanol, also enhanced PMA-induced PtdEtn hydrolysis. Of the other PLD activators tested, ethanol potentiated the PKC-dependent stimulatory effect of bombesin but failed to alter the apparently PKC-independent stimulatory effect of serum. Pretreatment of [14C]ethanolamine-labeled fibroblasts with 200 mM ethanol for 20 min resulted in increased (approx. 2-fold) hydrolysis of [14C]PtdEtn in isolated membranes. In membranes from ethanol-treated, but not from untreated, cells, PMA further enhanced (approx. 1.5-fold) the production of [14C]ethanolamine. Ethanol exerted none of the above stimulatory effects on phosphatidylcholine hydrolysis. These results suggest that the specific stimulatory action of ethanol on PLD-mediated PtdEtn hydrolysis can occur in vivo and may involve increased binding of a regulatory PKC-isoform to membranes.

The long-term combined stimulatory effects of ethanol and phorbol ester on phosphatidylethanolamine hydrolysis are mediated by a phospholipase C and prevented by overexpressed alpha-protein kinase C in fibroblasts

The protein kinase C (PKC) activator 12-O-tetradecanoylphorbol 13-acetate (TPA) has been shown to potentiate the stimulatory effect of ethanol on the hydrolysis of phosphatidylethanolamine (PtdEtn) in NIH 3T3 fibroblasts. Following an initial 20-min period, the main product of PtdEtn degradation in cells treated with TPA plus ethanol was ethanolamine phosphate. Here, we have examined the regulatory role of PKC and the possible catalytic role of phospholipase C in the formation of ethanolamine phosphate. TPA, bryostatin, and bombesin, direct or indirect activators of PKC, had similar potentiating effects on ethanol-induced formation of [14C]ethanolamine phosphate from [14C]PtdEtn in [14C]ethanolamine-prelabelled NIH 3T3 fibroblasts. At lower concentrations of ethanol (40-80 mM), significant stimulation of ethanolamine phosphate formation required longer treatments (2 h or longer). The combined effects of TPA (100 nM) and ethanol (50-200 mM) on ethanolamine phosphate formation were not inhibited by the PKC inhibitors staurosporine or 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7). In contrast, these inhibitors significantly inhibited TPA-induced formation of ethanolamine, catalyzed by a phospholipase-D-type enzyme. In membranes isolated from TPA+ethanol-treated cells, enhanced formation of ethanolamine phosphate was maintained for at least 20 min. Down-regulation of PKC by prolonged (24-h) treatment of NIH 3T3 fibroblasts by 300 nM TPA enhanced, while overexpression of alpha-PKC in Balb/c fibroblasts diminished, the stimulatory effect of ethanol on the formation of ethanolamine phosphate. Finally, addition of the protein phosphatase inhibitor okadaic acid (2 microM) to fibroblasts inhibited TPA+ethanol-induced formation of ethanolamine phosphate. These results suggest that alpha-PKC-mediated protein phosphorylation may negatively regulate PtdEtn hydrolysis and that the potentiating effect of TPA may result, at least partly, from increased degradation of this PKC isoform.

Differential effects of platelet-derived growth factor, serum and bombesin on phospholipase D-mediated hydrolysis of phosphatidylethanolamine in NIH 3T3 fibroblasts

In previous studies, activators of protein kinase C, sphingosine, ATP and various oncogenes were each found to enhance phospholipase D-mediated hydrolysis of phosphatidylethanolamine (PtdEtn) in NIH 3T3 fibroblasts. Here I examined possible stimulation of PtdEtn hydrolysis by various growth-stimulatory agents, including serum, bombesin, platelet-derived growth factor (PDGF), fibroblast growth factor (FGF) and insulin. Treatment of NIH 3T3 fibroblasts, prelabelled with [14C]Etn or [32P]PtdEtn, with PDGF-BB resulted in enhanced formation of [14C]Etn or [32P]phosphatidic acid from the respective labelled cellular pools of PtdEtn. A maximal effect (approximately 3-fold stimulation) on PtdEtn hydrolysis was obtained with 50 ng of PDGF/ml after 5 min of treatment. Phosphatidylcholine (PtdCho) was also hydrolysed, although less extensively than PtdEtn, in PDGF-stimulated cells. PDGF-stimulate hydrolysis of both PtdEtn and PtdCho was prevented by prolonged (30 h) treatment of cells with 400 nM-phorbol 12-myristate 13-acetate (PMA). Similar to PDGF, fetal calf serum (1-10%) also stimulated PtdEtn hydrolysis. However, in contrast to PDGF, the effect of serum on PtdEtn hydrolysis (i) was not diminished by pretreatment with PMA, and (ii) was synergistic with that of PMA after a 1 h incubation. Compared with PDGF and serum, bombesin had less effect on PtdEtn hydrolysis, while FGF and insulin had no effects at all. In contrast to PDGF or serum, bombesin inhibited the effect of PMA on PtdEtn hydrolysis.

Glucocorticoid regulation of phospholipid turnover and protein kinase C activity in mouse hepatoma 22 cells

Glucocorticoids induce growth inhibition in certain sensitive hepatoma cells. To investigate how glucocorticoids interact with growth-factor-dependent pathways, we studied the effects of dexamethasone (Dex) on the DNA synthesis, protein kinase C (PKC) activity and phospholipid turnover in mouse hepatoma 22 cells. Dex was found to reduce DNA synthesis in slowly growing hepatoma cells, whereas exponentially growing cells were Dex-insensitive. Direct measurements of PKC activity in the hormone-sensitive hepatoma 22 cells showed a rapid inhibition (within 30 min) when treated with Dex. Dex addition to hormone-sensitive but not to hormone-insensitive hepatoma 22 cells for 30 min caused a significant decrease of 32P-incorporation into the major cellular phospholipids: phosphatidylcholine, phosphatidylglycerol and phosphoinositides. At the same time, the analysis of the correlation between changes in PKC activity and phospholipid turnover showed that synthesis of phosphatidylcholine and phosphatidylglycerol was under positive control of PKC activity. The data suggest that suppression of phospholipid turnover in hormone-sensitive hepatoma 22 cells is one of the early events caused by glucocorticoids, whereas the decrease of PKC activity induced by the hormone is mediated, probably, via changes in phospholipid metabolism.

Hepatocyte growth factor (HGF) mediates the sustained formation of 1,2-diacylglycerol via phosphatidylcholine-phospholipase C in cultured rat hepatocytes

The addition of hepatocyte growth factor (HGF) to rat hepatocytes in primary culture resulted in the formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and 1,2-diacylglycerol (DG) by a phosphoinositide-specific phospholipase C (PI-PLC). DG showed a biphasic increase; the first phase, corresponding with the peak of Ins(1,4,5)P3 and a second larger and prolonged phase. The HGF stimulates the phosphatidylcholine (PC)-derived prolonged DG formation by a phospholipase C pathway (PC-PLC) but not by a phospholipase D pathway. HGF also was found to elicit [Ca2+] oscillations which may be associated with the prolonged DG production from PC via the PC-PLC phospholipase C pathway.

The regulation of phospholipase D activity and its role in sn-1,2-diradylglycerol formation in bombesin- and phorbol 12-myristate 13-acetate-stimulated Swiss 3T3 cells

Addition of the phorbol ester phorbol 12-myristate 13-acetate (PMA) to quiescent Swiss 3T3 cells resulted in a sustained increase in sn-1,2-diradylglycerol (DG) mass and [3H]DG in [3H]palmitate-labelled cells where phosphatidylcholine was the major labelled phospholipid. This occurred in the absence of inositol phosphate accumulation. In [3H]palmitate-labelled cells both bombesin and PMA stimulated the formation of phosphatidylbutanol ([3H]PtdBut) in the presence of 0.3% (v/v) butan-1-ol. The kinetics of [3H]PtdBut formation were consistent with phospholipase D (PLD) activation preceding sustained DG formation. The inclusion of butan-1-ol inhibited 70% of PMA-stimulated DG formation but only 30% of the bombesin response. The ability of bombesin and PMA to stimulate the accumulation of [3H]PtdBut was completely abolished in Swiss 3T3 cells which had been pre-treated with 400 nM-PMA for 48 h to down-regulate protein kinase C activity. PMA-stimulated [3H]PtdBut formation was inhibited by 90% by the protein kinase C inhibitor Ro-31-8220 (10 microM), but bombesin-stimulated PtdBut accumulation was inhibited by at most 50% by the same concentration of inhibitor. Cyclic AMP-elevating agents, i.e. forskolin, dibutyryl cyclic AMP and isobutylmethylxanthine, did not inhibit bombesin stimulation of PLD activity. Bombesin-stimulated PLD activity was inhibited by 50% by buffering of the extracellular Ca2+ concentration to 150 nM, but combination of this treatment with Ro-31-8220 addition was less than additive. Ionophore A23187 alone was able to stimulate PLD activity, but this response was inhibited 50% by Ro-31-8220. Thapsigargin was unable to stimulate PLD activity and had no modulatory effect upon bombesin-stimulated PLD activity at any agonist concentration. The results are discussed in terms of the role of PLD in DG generation and the regulation of PLD activity both by bombesin and by PMA.

Mitogenic signal transduction in normal and transformed 32D hematopoietic cells

We studied mitogenic signal transduction in normal and oncogene-transformed 32D cells, a murine hematopoietic cell line that is normally dependent on interleukin-3 (IL3) for proliferation and survival. The formation of second messengers was measured in normal cells stimulated with IL3, and in cells transfected with foreign growth factor receptor genes and stimulated with appropriate growth factors. We also measured the steady-state level of second messengers in 32D cells transformed by erbB, abl, and src oncogenes which abrogate growth factor requirement. We found that IL3 stimulated the formation of diacylglycerol independently of inositol lipid turnover, but concomitantly with increased turnover of phosphatidylcholine. Epidermal growth factor (EGF), and platelet-derived growth factor (PDGF) stimulated the 'classical' turnover of inositol lipids with formation of diacylglycerol and calcium-mobilizing inositol phosphates. Colony stimulating factor-1 triggered inositol lipid turnover, although to a much lower extent than EGF and PDGF. Transformed cells showed elevated levels of diacylglycerol together with increased turnover of phosphoinositides and phosphatidylcholine. Taken together these results indicate that different growth factors and oncoproteins associate with multiple signalling pathways in 32D cells.

Multiple sources of sn-1,2-diacylglycerol in platelet-derived-growth-factor-stimulated Swiss 3T3 fibroblasts. Evidence for activation of phosphoinositidase C and phosphatidylcholine-specific phospholipase D

Platelet-derived growth factor (PDGF) stimulated sn-1,2-diacylglycerol (DAG) mass formation in Swiss 3T3 fibroblasts with a lag time of some 30 s. The response was biphasic, with the second phase being sustained over time. PDGF also stimulated the formation of Ins(1,4,5)P3 with a similar lag time to the DAG response, suggesting that DAG is derived from PtdIns(4,5)P2 hydrolysis at this time point. PDGF-stimulated phosphatidylcholine (PtdCho) hydrolysis in Swiss 3T3 fibroblasts, as measured by the formation of water-soluble choline metabolites and phosphatidylbutanol (PtdBut) accumulation, was by a phospholipase D (PLD)-catalysed pathway which was kinetically downstream of initial PtdIns(4,5)P2 hydrolysis. Accumulation of PtdBut increased up to 15 min, suggesting that PLD activity is not rapidly densitized in response to PDGF. The kinetics of PtdCho hydrolysis closely paralleled the second phase of DAG formation, strongly suggesting that during prolonged stimulation periods PtdCho is a major source of DAG in these cells. However, since PtdIns(4,5)P2 breakdown was also prolonged, PDGF-stimulated DAG may be derived from both phospholipids. Down-regulation of protein kinase C (PKC), by pre-treatment with phorbol 12-myristate 13-acetate, abolished both [3H]choline and [3H]PtdBut formation, suggesting that PLD-catalysed PtdCho hydrolysis may be dependent on PKC activation, supporting its dependence on prior PtdIns(4,5)P2 hydrolysis.

Angiotensin receptor-mediated stimulation of diacylglycerol production in pulmonary artery endothelial cells

The stimulatory effects of angiotensin (Ang) I, Ang II, and Ang III on production of diacylglycerol (DAG), a second messenger, were examined in porcine pulmonary artery endothelial cells. Ang I, Ang II, and Ang III provoked rapid increases in [3H]glycerol labeling of DAG. The stimulatory effect on DAG production was maximal after 1 and 5 min. Pretreatment of cells with angiotensin-converting enzyme activity inhibitors prevented the stimulatory effect of Ang I on DAG production, indicating that Ang II but not Ang I is responsible for increased DAG production. The stimulatory effects of Ang II and Ang III on DAG production were concentration dependent and were maximal at a 10-nM concentration of both Ang II and Ang III. Data from further experiments revealed that the Ang II- and Ang III-elicited formation of DAG is derived from the coordinated hydrolysis of membrane phosphatidylinositol and phosphatidylcholine by phospholipase C- and phospholipase D-catalyzed pathways. The angiotensin analogue [Sar1 Ile8] Ang II, an Ang II receptor antagonist, blocked the hydrolysis of phosphatidylinositol and phosphatidylcholine and thus the increased production of DAG by Ang II and Ang III. These results indicate that Ang II- and Ang III-induced stimulation of DAG production in pulmonary artery endothelial cells involves multiple pathways of phospholipid hydrolysis and is mediated by angiotensin receptors.

Antigen-induced biphasic diacylglycerol formation in RBL-2H3 cells: the late sustained phase due to phosphatidylcholine hydrolysis is dependent on protein kinase C

Exposure to antigen (Ag) caused a biphasic 1,2-diacylglycerol (DG) production in [3H]myristic acid-labeled RBL-2H3 cells; the early, small transient phase and the second large sustained phase. The accumulation of phosphatidic acid (PA) or phosphatidylethanol (PEt) in the presence of ethanol was paralleled by the second-phase DG generation. Ag-induced formation of phosphocholine and choline in [3H]choline-labeled cells suggested the hydrolysis of phosphatidylcholine (PC) by phospholipases C and D. Treatment with phorbol myristate (PMA) or A23187 caused increases in [3H]DG and water-soluble [3H]choline metabolites. In protein kinase C (PKC) down-regulated cells, PEt formation was markedly reduced. In these cells DG production induced by Ag and A23187 was largely suppressed, thus indicating that PKC would play an important regulatory role for PC hydrolysis. However, because the A23187 treatment showed significant accumulation of water-soluble choline metabolites in PKC down-regulated cells, an increase in intracellular Ca2+ is another factor regulating PC hydrolysis. Taken together, these results may indicate that PC hydrolysis in response to Ag is dependent on PKC and Ca2+.

Hydrolysis of phosphatidylcholine by phospholipase D is a common response to mitogens which stimulate inositol lipid hydrolysis in Swiss 3T3 fibroblasts

The stimulated hydrolysis of inositol lipids and phosphatidylcholine (PtdCho) by bombesin, [Arg8]vasopressin ([Arg8]Vp) and prostaglandin F2 alpha (PGF2 alpha) was analysed in Swiss 3T3 cells pre-labelled to isotopic equilibrium with either [methyl-3H]choline, myo-[2-3H]inositol or [9,10 (n)-3H]palmitic acid. All three agonists activated the phospholipase D-catalysed hydrolysis of PtdCho as determined by the release of [3H]choline (Cho) and the formation of [3H]phosphatidylbutanol (PtdBut). The release of [3H]choline by each agonist exhibited similar sensitivity to prolonged pre-exposure to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). The release of [3H]choline exhibited the same dose dependency as the production of total inositol phosphates for each mitogen suggesting that the two responses might be mediated through identical receptors. Acute pre-treatment with TPA allowed the dissociation of inositol lipid hydrolysis from PtdCho breakdown, since it inhibited inositol phosphate accumulation but stimulated choline generation. The loss of mitogen stimulated choline release in cells pre-treated with the phorbol ester for 48 h was not due to loss of stimulated inositol phosphate production which was reproducibly enhanced in these 'down-regulated' cells.

Thrombin induces a biphasic 1,2-diacylglycerol production in human platelets

The 1,2-diacylglycerol (DAG) mass content was measured in thrombin-stimulated human platelets. Thrombin stimulates a biphasic accumulation of DAG, with an early phase reaching a peak at 10 s and a later phase reaching a peak at 2-3 min. The time course of first-phase DAG production corresponded well to that of Ins(1,4,5)P3 formation, which was rapid and transient. The second phase of DAG accumulation occurred after the level of Ins(1,4,5)P3 returned to nearly basal. Thrombin stimulated the decrease in PtdIns and phosphatidylcholine contents. The source of second-phase DAG was examined in platelets prelabelled with three radioactive fatty acids, i.e. arachidonic, palmitic and myristic. Thrombin stimulated the increase in radioactivity of DAG with decline of PtdIns in platelets labelled with [3H]arachidonic acid or [3H]palmitic acid, in which PtdIns was considerably labelled. In contrast, significant accumulation of [3H]DAG was not observed in [3H]myristic acid-labelled platelets, in which PtdIns was poorly labelled. In platelets prelabelled with [3H]inositol, an increase in InsP in response to thrombin was seen for more than 5 min. In contrast, upon stimulation, significant increases in [3H]phosphocholine and [3H]choline were not observed in [methyl-3H]choline-labelled platelets. Thrombin induced a small production of phosphatidylethanol, when ethanol was present during stimulation. However, the formation of DAG and phosphatidic acid was not significantly affected by ethanol. These results suggest that thrombin stimulates a biphasic accumulation of DAG, initially from PtdInsP2 and later from PtdIns in human platelets.

Transduction of the bradykinin response in human fibroblasts: prolonged elevation of diacylglycerol level and its correlation with protein kinase C activation

Stimulation of quiescent human fibroblasts with the peptide mitogen bradykinin (BK) led to a biphasic elevation in cellular 1,2-diacylglycerol (DAG), as estimated by either measurement of total DAG mass or [3H]arachidonate incorporation. A rapid initial transient that peaked 15 s after BK addition was followed by a decline to near basal levels then a second rise to a plateau phase during which DAG levels remained elevated for less than or equal to 45 min. The source of the initial DAG transient appeared to be primarily polyphosphoinositides as these phospholipids were rapidly hydrolyzed after BK addition. This transient correlates well temporally with previous observations of the kinetics of inositol trisphosphate accumulation and intracellular free [Ca2+] observed in the same cells. Cultures preincubated with [3H]myristic acid incorporated label predominantly into the phosphatidylcholine (PC) pool. Subsequent addition of BK under these conditions caused only a relatively slow accumulation of [3H]DAG to a plateau level, without an initial transient. Together with the observation that PC was found to decrease upon BK stimulation, these observations suggest that the late phase of DAG accumulation may involve breakdown of other phospholipids including PC. To investigate the consequences of DAG elevation we examined the phosphorylation of an acidic 80 kDa protein, whose phosphorylation is solely dependent on the activation of protein kinase C (PK-C). The 80 kDa fibroblast protein could be immunoprecipitated by an antibody to bovine brain "myristoylated and alanine-rich C-kinase substrate" (MARCKS) and phosphopeptide maps of brain and fibroblast MARCKS were similar. Stimulation of [32P]-prelabeled fibroblasts with serum, BK, vasopressin, or 12-O-tetradecanoyl phorbol acetate, but not epidermal growth factor or calcium ionophores, resulted in the rapid phosphorylation of MARCKS. With BK or serum this phosphorylation showed an initial transient peak at less than 1 min then rose again to a plateau level that was sustained for less than or equal to 45 min. Removal of BK resulted in a rapid decline in MARCKS phosphorylation. These studies show that the biphasic DAG signal in BK-stimulated human fibroblasts correlates well with the state of activation of PK-C. However, the persistent activation of PK-C does not appear to require continued high levels of Ca2+.

Effects of insulin and phospholipase C in control and denervated rat skeletal muscle

Phospholipase C (PLC), an enzyme that increases endogenous 1,2-diacylglycerol (DAG), caused dose-dependent stimulation of 2-deoxy-D-glucose (2-DG) uptake in rat soleus muscles; the maximal effect was less than that of insulin. In denervated muscles the effect of insulin on 2-DG uptake was markedly reduced, whereas the response to PLC was identical to that of control muscles. Both PLC and insulin stimulated glucose incorporation into glycogen in control but not in denervated solei. Amino acid transport was unaffected by PLC; however, the enzyme completely inhibited the stimulation of amino acid transport by insulin. PLC did not activate the insulin receptor tyrosine kinase but decreased activation of the receptor by insulin in vivo. Basal muscle DAG content increased after denervation. Incubation with PLC markedly increased DAG in control and in denervated muscle. Insulin increased total DAG mass less than PLC in control muscles and did not affect DAG in denervated muscles. In media without added Ca2+, PLC stimulation of DAG production was impaired, and 2-DG uptake was unresponsive to PLC. The data are consistent with, but do not prove, that a subpopulation of DAGs may participate in insulin-mediated stimulation of glucose transport. They also suggest that the denervation-induced insulin resistance of glucose transport may reflect impaired generation of certain DAGs involved in the signaling cascade.

Diacylglycerol production induced by growth hormone in Ob1771 preadipocytes arises from phosphatidylcholine breakdown

Growth Hormone has recently been shown to stimulate the formation of diacylglycerol in Ob1771 mouse preadipocyte cells without increasing inositol lipid turnover. Addition of growth hormone to Ob1771 cells prelabelled with [3H]glycerol or [3H]choline led to a rapid, transient and stoechiometric formation of labelled diacylglycerol and phosphocholine, respectively. In contrast, no change was observed in the level of choline and phosphatidic acid whereas the release of water-soluble metabolites in [3H]ethanolamine prelabelled cells exposed to growth hormone was hardly detectable. Stimulation by growth hormone of cells prelabelled with (2-palmitoyl 9, 10 [3H])phosphatidylcholine also induced the production of labelled diacyglycerol. Pertussis toxin abolished both diacylglycerol and phosphocholine formation induced by growth hormone. It is concluded that growth hormone mediates diacylglycerol production in Ob1771 cells by means of phosphatidylcholine breakdown involving a phospholipase C which is likely coupled to the growth hormone receptor via a pertussis toxin-sensitive G-protein.

Stimulation of the hydrolysis of phosphatidylinositol 4,5-bisphosphate and phosphatidylcholine by endothelin, a complete mitogen for Rat-1 fibroblasts

The mitogenic activity of endothelin and its ability to stimulate PtdIns(4,5)P2 and phosphatidylcholine turnover in Rat-1 fibroblasts was studied. Stimulated incorporation of [3H]thymidine occurred in the absence of any other added growth factors. The endothelins stimulated rapid generation of both Ins(1,4,5)P3 and choline. Endothelin-1 and endothelin-2 were equipotent in stimulating both responses, but endothelin-3 was less potent. Endothelin-1-stimulated Ins(1,4,5)P3 generation reached a maximum at 5 s and then declined; however, the response was long-lived, with a 4.5-fold elevation over basal still observed after 15 min. Endothelin-stimulated choline generation was observed with no increase in choline phosphate; indeed, the apparent level of this metabolite fell after 30 min of stimulation, presumably due to the observed stimulation of phosphatidylcholine synthesis. The endothelin-stimulated increase in choline generation was abolished in cells where protein kinase C was down-regulated. However, endothelin-stimulated choline generation was greater than that observed in response to a protein kinase C-activating phorbol ester, raising the possibility that the peptide activates phospholipase D by both protein kinase C-dependent and -independent mechanisms.