Stimulated phosphatidylcholine hydrolysis as a signal transduction pathway in mitogenesis

Advanced Meditation and Vegan Diet Increased Acylglycines and Reduced Lipids Associated with Improved Health: A Prospective Longitudinal Study

Introduction: Samyama is an Isha Yoga 8-day residential meditation/yoga retreat combined with 60 days of preparation with vegan diet. We showed earlier Samyama retreat was associated with lower systemic inflammation and favorable lipid profiles along with other physical and mental health benefits. There is no mechanistic study on the impact of an advanced meditative process on multiple blood lipids and their implications on meditation-related improved physical and mental wellbeing. Methods: Sixty-four Samyama participants on vegan diet had blood sampled immediately before and immediately after the 8-day retreat for lipidomic analysis. The complex plasma lipidome was characterized using high-resolution mass spectrometric analysis and tandem mass spectrometry. Results: Pre- and post-Samyama blood samples of 64 Samyama participants were analyzed. Acylglycines (acetyl, propionyl, butyryl, and valeryl) were increased in the plasma post-Samyama compared with pre-Samyama (p < 0.001). Levels of glycerophosphocholines, glycerophosphoethanolamines, di-unsaturated ethanolamine plasmalogens, cholesterol esters, acylcarnitines, and acylgylcerines (triacylglycerols and diacylglycerols) decreased after the Samyama meditation. Plasma levels of glycerophosphoserines or glycerophosphoinositols were unchanged. Conclusion: An 8-day advanced meditation retreat resulted in increased acylglycines, an endocannabinoid-like fatty acid amide associated with increased cellular anandamide levels, anti-inflammation, analgesia, and vascular relaxation. Other serum lipid levels, including some that are associated with increased risk of atherosclerosis, were reduced following the Samyama program. ClinicalTrials.gov Registration: Identifier: NCT04366544.

Bacterial protein toxins and lipids: role in toxin targeting and activity

All bacterial toxins, which globally are hydrophilic proteins, interact first with their target cells by recognizing a surface receptor, which is either a lipid or a lipid derivative, or another compound but in a lipid environment. Intracellular active toxins follow various trafficking pathways, the sorting of which is greatly dependent on the nature of the receptor, notably lipidic receptor or receptor embedded into a distinct environment such as lipid microdomains. Numerous other toxins act locally on cell membrane. Indeed, phospholipase activity is a common mechanism shared by several membrane-damaging toxins. In addition, many toxins active intracellularly or on cell membrane modulate host cell phospholipid pathways. Unusually, a few bacterial toxins require a lipid post-translational modification to be active. Thereby, lipids are obligate partners of bacterial toxins.

Effect of S-adenosylmethionine on Age-induced Hepatocyte Damage in Old Wistar Rats

Aging is accompanied by changes in the morphology and physiology of organs and tissues, such as the liver. This process might be due to the accumulation of oxidative damage induced by reactive oxygen (ROS) and reactive nitrogen species (RNS). Hepatocytes are very rich in mitochondria and have a high respiratory rate, so they are exposed to large amounts of ROS and permanent oxidative stress. S-Adenosylmethionine (SAMe) is an endogenous metabolite that has shown to exert protective effects on different experimental pathological models in which free radicals are involved. The aim of this study was to investigate the effect of SAMe on age-induced damage in hepatocytes. For this purpose, male and female Wistar rats of 18 and 2 months of age were used. Cells were isolated and, after incubation in the presence or in the absence of SAMe, different parameters were measured. Aging induced a significant increase in nitric oxide, carbon monoxide and cGMP, and a reduction in reduced glutathione, ATP and phosphatidylcholine synthesis, as well as in methionine- adenosyl-transferase and methyl-transferase activities. Incubation of old cells with SAMe prevented all these age-related changes, reaching values in some of the parameters similar to those found in young animals. In conclusion, SAMe seems to have beneficial effects against age-induced damage in hepatocytes.

Effect of melatonin administration on parameters related to oxidative damage in hepatocytes isolated from old Wistar rats

Aging induces changes in several organs and tissues, such as the liver, and this process might be due to oxidative damage caused by free radicals and inflammatory mediators. Melatonin is a secretory product with well-known antioxidant properties. The aim of this study was to investigate the effect of melatonin administration on age-induced alterations in hepatocytes. Twenty-two-month old male Wistar rats were treated with oral melatonin for 10 wk. At the end of the treatment, hepatocytes were isolated and cultured, and different parameters were measured in both cells and medium. Aging induced a significant increase in lipid peroxidation, nitric oxide, carbon monoxide and cyclic guanosyl-monophosphate, as well as a reduction in adenosine triphosphate content and phosphatidylcholine synthesis when compared to young animals. Melatonin administration significantly ameliorated all these age-related changes in males. Melatonin administration seems to exert beneficial effects against age-induced changes in hepatocytes.

Effect of recombinant human growth hormone on age-related hepatocyte changes in old male and female Wistar rats

Aging induces changes in several organs, such as the liver, and this process might be due to damage caused by free radicals and inflammatory mediators. The growth hormone (GH)/insulin-like growth factor-1 (IGF-1) axis shows a reduction with age, and this fact could be associated with some age-related changes. The aim of this study was to investigate the effect of GH administration on age-induced alterations in hepatocytes. Two and twenty two month-old male and female Wistar rats were used. Old rats were treated with human recombinant GH for 10 wk. At the end of the treatment, hepatocytes were isolated from the liver and cultured, and different parameters were measured in cells and medium. Plasma IGF-1 was also measured. Aging significantly decreased plasma IGF-1 in males. In females, plasma IGF-1 was also reduced, but not significantly. GH treatment restored plasma IGF-1 levels to values similar to young males. Aging was associated with a significant increase in lipid peroxidation (LPO), nitric oxide (NO), carbon monoxide (CO) and cyclic guanosyl-monophosphate (cGMP), as well as a reduction in adenosyl triphosphate (ATP) and phosphatidylcholine (PC) synthesis. GH administration partially prevented all these changes in males. In females, some of the parameters were significantly improved by GH (ATP, CO, cGMP), while others showed a tendency to improvement, although differences did not reach significance. In conclusion, GH administration could exert beneficial effects against age-related changes in hepatocytes, mainly in males.

Electrical activity regulates AChR gene expression via JNK, PKCzeta and Sp1 in skeletal chick muscle

Electrical activity of myotubes represses nicotinic acetylcholine receptor (AChR) gene expression. This effect is mimicked by okadaic acid and blocked by tetrodotoxin (TTX) or staurosporine in cultured myocytes [Altiok et al., EMBO J. 16 (1997) 717-725]. In this study, we investigated the mechanism of this repression. We show that addition of exogenous phospholipase D (PLD) and C inhibits AChR expression in a manner which parallels that of okadaic acid. Furthermore, okadaic acid caused an increase of the threonine phosphorylation of protein kinase Czeta (PKCzeta) and activator of transcription factor (ATF2) and a decrease of the phosphorylation of Sp1. All these effects were reversed by staurosporine, and TTX also abolished ATF2 phosphorylation. These data reveal a possible involvement of PLD, c-jun N-terminal kinase, PKCzeta and Sp1 in the repression of AChR genes by electrical activity.

Role of diacylglycerol (DAG) in hormonal induction of S phase in hepatocytes: the DAG-dependent protein kinase C pathway is not activated by epidermal growth factor (EGF), but is involved in mediating the enhancement of responsiveness to EGF by vasopressin, angiotensin II, and norepinephrine

The role of diacylglycerol (DAG) in hormonal induction of S phase was investigated in primary cultures of rat hepatocytes. In this model, several agonists that bind to G protein-coupled receptors act as comitogens when added to the cells soon after plating (i.e., in Go/early Gl phase), while the cells are most responsive to the mitogenic effect of epidermal growth factor (EGF) at 24-48 h of culturing (i.e., mid/late Gl). It was found that the cellular concentration of DAG rose markedly and progressively during the first 24 h of culturing. Exposure of the hepatocytes at 3 h to alpha1-adrenergic stimulation (norepinephrine with timolol), vasopressin, or angiotensin II further increased this rise, producing a sustained increase in the DAG level. Norepinephrine, which was the most efficient comitogen, produced the most prolonged DAG elevation. In contrast, no significant increase of DAG was found in response to EGF, neither at 3 nor at 24 h, using concentrations that markedly stimulated the ERK subgroup of the mitogen-activated protein kinases (MAPK) and DNA synthesis. Addition of Bacillus cereus phosphatidylcholine-specific phospholipase C (PC-PLC) strongly elevated DAG, while Streptomyces phospholipase D (PLD) increased phosphatidic acid (PA) but not DAG. B. cereus PC-PLC and the protein kinase C (PKC) activator tetradecanoyl phorbol-acetate (TPA), like norepinephrine, vasopressin, and angiotensin II, stimulated MAPK and enhanced the stimulatory effect of EGF on DNA synthesis. The PKC inhibitor GF109203X did not diminish the effect of EGF on MAPK or DNA synthesis, but strongly inhibited the effects of norepinephrine, vasopressin, angiotensin II, TPA and B. cereus PC-PLC on MAPK and almost abolished the enhancement by these agents of EGF-stimulated DNA synthesis. These results suggest that although generation of DAG is not a direct downstream response mediating the effects of the EGF receptor in hepatocytes, a sustained elevation of DAG with activation of PKC markedly increases the responsiveness to EGF. Mechanisms involving DAG and PKC seem to play a role in the comitogenic effects of various agents that bind to G protein-coupled receptors and activate the cells early in Gl, such as norepinephrine, angiotensin II, and vasopressin.

Antagonistic Roles for Phospholipase D Activities in B Cell Signaling: While the Antigen Receptors Transduce Mitogenic Signals Via a Novel Phospholipase D Activity, Phosphatidylcholine-Phospholipase D Mediates Antiproliferative Signals

Cross-linking of the Ag receptors on B cells induces DNA synthesis and proliferation. Butanol trap experiments suggest that one or more phospholipase D activities play a key role in this process. Although phosphatidylcholine-phospholipase D has been shown to play a central role in the transduction of proliferative responses for a wide variety of calcium-mobilizing receptors, we show that the Ag receptors are not coupled to this phospholipase. In addition, phosphatidylcholine-phospholipase D is not stimulated under conditions that mimic T cell-dependent B cell activation. In contrast, ATP, which inhibits surface Ig (sIg)-mediated DNA synthesis in murine B cells via P2-purinoceptors, activates phosphatidylcholine-phospholipase D. Phosphatidylcholine-phospholipase D is therefore associated with antiproliferative signal transduction in mature B cells, but it does not transduce early signals associated with sIg-mediated growth arrest or apoptosis in immature B cells. Mitogenic stimulation of sIg is, however, coupled to a novel nonphosphatidylcholine-hydrolyzing phospholipase D activity. The resultant sIg-generated phosphatidic acid, unlike the phosphatidylcholine-derived phosphatidic acid generated via the purinoceptors, is converted to diacylglycerol. These data provide the first evidence that while the novel sIg-coupled phospholipase D and resultant diacylglycerol generation may play a role in B cell survival and proliferation, phosphatidylcholine-phospholipase D may transduce, via phosphatidic acid, negative immunomodulatory signals in mature B lymphocytes.

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.

Distinct phospholipase C-regulated signalling pathways in Swiss 3T3 fibroblasts induce the rapid generation of the same polyunsaturated diacylglycerols

Prostaglandin F2alpha, platelet-derived growth factor (PDGF) and calcium ionophore A23187 stimulated the rapid (within 25 s) generation of polyunsaturated 1,2-diacylglycerol (DAG) species, in particular 18:0/20:3n-9, 18:0/20:4n-6 and 18:0/20:5n-3, in Swiss 3T3 fibroblasts. This was followed by a second sustained phase characterised by saturated, monounsaturated and diunsaturated DAG species derived, at least partially, from a phospholipase D/phosphatidate phosphohydrolase-linked pathway. This could be directly activated by phorbol ester. Assay of rat brain protein kinase C (PKC) in lipid vesicles showed that first phase, polyunsaturated-enriched DAG isolated from Swiss 3T3 cells was a more potent activator of kinase activity compared to that achieved with DAG from control or 5 min stimulated cells. Thus activation of distinct members of the phospholipase C family leads to the rapid and almost identical generation of polyunsaturated DAG species which are capable of preferentially activating protein kinase C (PKC).

Basic fibroblast growth factor-stimulated arachidonic acid release in rat pancreatic acini: sequential action of tyrosine kinase, phospholipase C, protein kinase C and diacylglycerol lipase

This study was performed to evaluate the effect of human recombinant basic fibroblast growth factor on arachidonic acid release from rat pancreatic acini and to determine the cellular mechanism involved. From enzymatic assays, basic fibroblast growth factor did not significantly stimulate phospholipase A2 activity, whereas it significantly increased diacylglycerol lipase activity. Validity of phospholipase A2 or diacylglycerol lipase inhibitors was confirmed by their ability to inhibit phospholipase A2 or diacylglycerol lipase activities. Basic fibroblast growth factor increased intracellular accumulation and extracellular release of arachidonic acid from metabolically labelled acinar cells in a concentration- and time-dependent manner. This effect was maximal with 50 pM basic fibroblast growth factor and became significant after a 5-min incubation period. The protein tyrosine kinase inhibitor, 0.5 mM genistein, inhibited arachidonic acid release in basic fibroblast growth factor-stimulated acini, whereas 100 microM vanadate, a protein tyrosine phosphatase inhibitor, enhanced arachidonic acid release. Two phospholipase A2 inhibitors, mepacrine and aristolochic acid, failed to attenuate basic fibroblast growth factor-stimulated arachidonic acid release. A diacylglycerol lipase inhibitor RHC 80267 at 150 microM and 50 microM completely inhibited 50 pM basic fibroblast growth factor-induced intracellular accumulation and extracellular release of arachidonic acid, respectively. Furthermore, basic fibroblast growth factor stimulated arachidonic acid release was also inhibited by 10 microM U73122 and by 100 nM staurosporine, phospholipase C and protein kinase C respective inhibitors. Wortmannin, an inhibitor of basic fibroblast growth factor-stimulated phospholipase D, did not affect arachidonic acid release. 100 nM 4 beta-phorbol 12-myristate 13-acetate also increased arachidonic acid release, an effect also inhibited by staurosporine. Taken together, these data demonstrate activation of diacylglycerol lipase and arachidonic acid release in pancreatic acini upon stimulation by basic fibroblast growth factor, and strongly indicate that arachidonic acid release in response to basic fibroblast growth factor depends upon the sequential action of tyrosine kinase, phospholipase C, protein kinase C and diacylglycerol lipase but not from phospholipase A2 not phospholipase D activation.

Growth-promoting effects of Ca(2+)-mobilizing agents in hepatocytes: lack of correlation between the acute activation of phosphoinositide-specific phospholipase C and the stimulation of DNA synthesis by angiotensin II, vasopressin, norepinephrine, and prostaglandin F2 alpha

Although several hormones that promote hepatocyte proliferation also activate phosphoinositide-specific phospholipase C (PI-PLC) and mobilize Ca2+, the role of PI-PLC in the growth-stimulating effect of these agents is not clear. We have investigated this issue further, by exposing freshly isolated adult rat hepatocytes to vasopressin, angiotensin II, norepinephrine (in the presence of the beta-adrenoceptor blocker timolol) or PGF2 alpha, and examined both acute responses and the subsequent DNA synthesis when the cells were grown in monolayer culture. All the agonists elevated the level of inositol 1,4,5-trisphosphate (InsP3) and enhanced the DNA synthesis, amplifying the response to epidermal growth factor (EGF), and this comitogenic effect could be exerted by a single exposure of the cells 24 h prior to the addition of EGF. The acute activation of PI-PLC, measured as the early rise (peak 15-60 s) in InsP3, was 8-10-fold with vasopressin or angiotensin II, 3-4-fold with norepinephrine, and approximately 2-fold with PGF2 alpha. For all the agonists, a rise in cytosolic free Ca2+ in 100% of the cells and a maximal increase in glycogen phosphorylase activity were evoked at concentrations that approximately doubled the level of InsP3. However, the growth-stimulatory effects of these agonists showed a different order of efficacy as compared to the activation of PI-PLC; in terms of the maximal stimulation of DNA synthesis, the effects were: norepinephrine approximately PGF2 alpha > angiotensin II > vasopressin. Also, norepinephrine, PGF2 alpha, and angiotensin II, but not vasopressin, further enhanced the DNA synthesis when their concentrations were increased above those yielding maximal elevation of InsP3. In experiments where vasopressin and angiotensin II were combined, their effects on the DNA synthesis were additive while the InsP3 responses were not. The results show that the extent of the initial activation of PI-PLC is not the determinant for the magnitude of the growth effects of Ca(2+)-mobilizing hormones in hepatocytes. This suggests either (a) that the proliferative response to these agents is determined by the activity of PI-PLC at a later time, or its integral over an extended part of the prereplicative period, rather than by the acute activation, or (b) that additional, PI-PLC-independent, mechanisms are required.

The co-mitogenic combination of transforming growth factor beta 1 and bombesin protects protein kinase C-delta from late-phase down-regulation, despite synergy in diacylglycerol accumulation

Bombesin induces the down-regulation of protein kinase C-delta (PKC-delta) and PKC-epsilon in Swiss 3T3 cells. Simultaneous addition of transforming growth factor beta 1 (TGF beta 1) selectively blocks PKC-delta down-regulation at mid-S-phase, whereas PKC-epsilon levels continue to decline. Northern blot analysis shows that PKC-epsilon levels could be controlled in part at the level of transcription; PKC-delta mRNA levels remained constant at these later times. Bombesin induces a sustained elevation of some species of diacylglycerol (DAG), consistent with the observed loss of PKC-delta and PKC-epsilon. Interestingly, the combination of bombesin and TGF-beta 1 produces an even greater DAG response. Flow cytometric analysis demonstrates that bombesin induces only 15% of the cells to enter the cell cycle, in contrast to the combination of TGF beta 1 plus bombesin which induces 75-80% of the cells to progress through the cycle. The protection of PKC-delta from down-regulation under conditions of sustained DAG elevation correlates with the mitogenic response and implies that the down-regulation process itself is regulated. Consistent with this, it is demonstrated that bombesin plus TGF beta 1 protects PKC-delta from phorbol ester-induced down-regulation.

Dual pathways for carbamylcholine-stimulated arachidonic acid release in rat pancreatic acini

Recent studies suggested the involvement of arachidonic acid in the mediation of pancreatic amylase release. However, an effect of carbamylcholine on arachidonic acid release has not yet been reported in the exocrine pancreas. This study was performed to evaluate the effect of carbamylcholine on arachidonic acid release and determine the underlying intracellular mechanisms. From enzymatic assays, phospholipase A(2) and diacylglycerol lipase were activated by carbamylcholine and these activations were inhibited by the phospholipase A(2) inhibitors, mepacrine and aristolochic acid, and by the diacylglycerol lipase inhibitor RHC 80267. Carbamylcholine also increased arachidonic acid release in a concentration-dependent manner. Both phospholipase A(2) and diacylglycerol inhibitors partially inhibited carbamylcholine-stimulated arachidonic acid release. The phospholipase C inhibitor U73122 and the protein kinase C inhibitor staurosporine also caused partial inhibition. Arachidonic acid release by carbamylcholine was suppressed by the simultaneous addition of RHC 80267 with either phospholipase A(2) inhibitors. Our data demonstrate that phospholipase A(2) and diacylglycerol lipase are activated and arachidonic acid is released in pancreatic acini by carbamylcholine. Dual pathways are responsible for carbamylcholine-induced arachidonic acid release. One such pathway involves the sequential action of phospholipase C, protein kinase C and diacylglycerol lipase, whereas the other involves phospholipase A(2) activation.

Diradylglycerol formation in cholecystokinin-stimulated rabbit pancreatic acini. Assessment of precursor phospholipids by means of molecular species analysis

The aim of the present study was to assess the origin of the 1,2-diradylglycerols produced during prolonged hormonal stimulation of rabbit pancreatic acini by comparison of their relative molecular species composition with that of the major acinar phospholipids. Both phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) consisted of 1,2-diacyl as well as 1-alk-1-2-acyl species. In contrast, phosphatidylinositol (PtdIns), phosphatidylserine and phosphatidic acid existed only in the 1,2-diacyl form. Acinar cells did not contain detectable amounts of 1-alkyl-2-acyl phospholipids. Similarly, the acinar 1,2-diradylglycerol fraction consisted of 1,2-diacylglycerols and 1-alk-1-enyl-2-acylglycerols. Mass 1,2-diradylglycerol measurements revealed that prolonged stimulation with cholecystokinin resulted in a marked and sustained increase in acinar 1,2-diradylglycerol content. Based on the relative amounts of the 1,2-diacyl species present in both the 1,2-diradylglycerol fraction and the individual phospholipids, it is calculated that under control conditions 60% of the 1,2-diacylglycerols originate from PtdCho and 40% from PtdIns, whereas under stimulatory conditions 53% is calculated to be derived from PtdCho, 46% from PtdIns and 1% from PtdEtn. Likewise, it is calculated that in control as well as stimulated acini 100% of the 1-alk-l-enyl-2-acylglycerols originate from plasmenylcholine. Further evidence in favour of the idea that at least a considerable part of the 1,2-diacylglycerols produced during prolonged hormonal stimulation originate from inositolphospholipids is provided by the observation that labeling of phosphatidylinositol 4,5-bisphosphate with inorganic phosphate reached isotopic equilibrium markedly faster under stimulatory conditions as compared to the control situation, which is in agreement with an elevated turnover rate. The data presented support the idea that PtdCho and inositolphospholipids are the major precursors in basal and stimulated 1,2-diradylglycerol production in rabbit pancreatic acini.

HDL3 stimulates multiple signaling pathways in human skin fibroblasts

The influence of HDL3 on phospholipid breakdown was examined in human skin fibroblasts. HDL3 elicited phosphatidylcholine (PC) and phosphatidylinositol (PI) turnover and activated multiple phospholipases. In [14C]lyso-PC-labeled or [14C]choline (Cho)-labeled cells, a biphasic activation of PC-specific phospholipase D (PLD) with peak maxima 30 to 60 seconds and 5 to 7 minutes after stimulation with 20 micrograms/mL HDL3 was shown by (1) a 1.5- to 3-fold increase in Cho release, and (3) transphosphatidylation of PC to phosphatidylbutanol in the presence of 0.3% butanol. Activation of PC-specific PLD was paralleled by an activation of PC-specific phospholipase C (PLC). A significant increase in [14C]diacylglycerol (DG) was seen from 2 minutes after stimulation onward and remained for at least 2 hours. By means of butanol, the PA-phosphohydrolase (PPH) inhibitor propranolol, and the PC-PLC inhibitor D609, we demonstrated that the initial PC-derived DG formation occurred primarily by a coupled PLD/PPH pathway and that a major part of the sustained DG formation was derived directly from PC by PC-PLC. By down-regulating protein kinase C (PKC) we demonstrated that PKC activates PC-PLC and desensitizes PC-PLD at no longer incubation times. The sustained PC hydrolysis as well as HDL3-mediated PI turnover and PC resynthesis was observed on stimulation with 5 to 75 micrograms/mL HDL3, whereas the rapid activation of PC-PLD/PPH was detected only on stimulation with HDL3 at concentrations of between 10 and 75 micrograms/mL. Only the latter response could be mimicked by apolipoprotein A-I and apolipoprotein A-II proteoliposomes, and only this response was inducible by cholesterol loading. The HDL3-mediated second-messenger responses were inhibited by modification of HDL3 by tetranitromethane and could not be mimicked by protein-free liposomes. These data suggest that HDL3-induced cell signaling in human skin fibroblasts is mediated by specific protein-receptor interaction and that more than one agonist activity may be involved.

Diacylglycerol elevations in control platelets are unaccompanied by pleckstrin phosphorylation. Implications for the role of diacylglycerol in platelet activation

Several laboratories have reported that diacylglycerol levels in human platelets (approximately 100 pmol/10(9) platelets) increased severalfold in response to 0.5-1 U/ml thrombin. We report here fluctuations in diacylglycerol mass in control platelets, the magnitude of which were 60-90% of that measured in platelets treated with 0.2-0.5 U/ml of thrombin. These control platelets were not activated by such criteria as absence of aggregation, secretion, phosphatidic acid production and phosphorylation of the protein kinase C substrate, pleckstrin. Thrombin treatment evoked all of the above responses. Analysis of the diacylglycerol molecular species by reverse-phase HPLC of the dimethylated, phosphorylated derivatives showed that all of the molecular species that were present in control platelets were also present in thrombin-treated platelets. Most of the species appeared to fluctuate at random in control platelets with the exception of 1-stearoyl-2-arachidonoyl-sn-glycerol which was more or less stable and increased severalfold over control values only upon thrombin treatment. Furthermore, only this species accumulated as [32P]phosphorylated PtdOH in thrombin-treated platelets prelabelled with [32P]Pi. Our findings show that, in platelets, elevation of diacylglycerol molecular species other than the 1-stearoyl-2-arachidonoyl species occurs, but these changes are not necessarily linked to activation of protein kinase C as measured by pleckstrin phosphorylation which was observed only upon elevation of 1-stearoyl-2-arachidonoyl-sn-glycerol.

Map kinase activation in Swiss 3T3 cells stimulated with gastrin-releasing peptide is associated with increased phosphorylation of a 78,000 M(r) protein immunoprecipitated by anti-raf kinase anti-serum

Addition of 10 nM gastrin-releasing peptide (GRP) to Swiss 3T3 fibroblasts induced a transient (1-2 min) tyrosine phosphorylation and activation of mitogen-activated protein kinase (MAP kinase). Increased activity of 42,000 M(r) MAP kinase was detected with immunochemical methods; however, in situ kinase detection on renaturated SDS-polyacrylamide electrophoresis gel revealed activation of both 42,000 and 44,000 M(r) MAP kinase species. Furthermore, stimulation of 32P-labelled cells with 10 nM GRP for 2 min resulted in an increased phosphorylation of a protein with an approximate molecular mass of 78,000 M(r) in anti-raf kinase and anti-MAP kinase kinase immunoprecipitates of cytosolic extracts from 32P-labelled cells. The presented data demonstrated that GRP induces MAP kinase activation in Swiss 3T3 fibroblasts, and furthermore suggest a role for raf kinase in this process.

Epidermal growth factor stimulates distinct diradylglycerol species generation in Swiss 3T3 fibroblasts: evidence for a potential phosphatidylcholine-specific phospholipase C-catalysed pathway

Stimulation of 3T3 fibroblasts with epidermal growth factor (EGF) results in an increase in 1,2-diacylglycerol (DAG) mass which is maximal at 25 s, declining at 1 min and returning to basal levels by 30 min. No changes in alkylacylglycerol or alkenylacylglycerol were detected. Three species account for most of this mass increase: 18:0/20:5,n-3, 18:0/20:4,n-6 and 18:0/20:3,n-9. These species are characteristic of the phosphoinositides; however, previous work failed to detect any EGF-stimulated rise in inositol phosphates in these cells [Cook and Wakelam (1992) Biochem. J. 285, 247-253]. This ruled out phosphoinositide hydrolysis by phospholipase C, but raised the possibility of phospholipase D/phosphatidate phosphohydrolase-catalysed hydrolysis of phosphatidylinositol. The inclusion of butanol in the incubation medium failed to block the diacylglycerol changes, indicating that the phospholipase D pathway is not involved and that DAG must be derived from another source, probably via phospholipase C-catalysed hydrolysis of a phosphatidylcholine pool that is particularly rich in these species. The tyrosine kinase inhibitor ST-271 almost abolished the elevation in 18:0/20:5,n-3, 18:0/20:4, n-6 and 18:0/20:3,n-9 at 25 s, but only reduced the rise in total DAG mass by about 50%. The protein kinase C (PKC) inhibitor Ro-31-8220 increased DAG levels at all time points but had no effect on the species profiles. This provides additional evidence for PKC-mediated regulation of cell-surface EGF receptors, since the inhibition of PKC would increase the availability and/or ligand binding affinity of receptors at the plasma membrane and hence increase and prolong the response to EGF.

Rapid desensitization and resensitization of bombesin-stimulated phospholipase D activity in Swiss 3T3 cells

The kinetics of bombesin-stimulated phospholipase D (PLD) activity were examined in Swiss 3T3 fibroblasts. The stimulated activity was found to rapidly desensitize, being completely absent after 40 s. This activity then quickly, but incompletely, resensitized, with PLD being detectable after a 4.5 min wash of the desensitized cells and 75-80% of the activity being recovered after 10 min. The desensitization was dose-dependent; however, the half-maximal stimulatory concentration of bombesin was an order of magnitude lower than that required for bombesin-stimulated second messenger generation and the KD for bombesin receptor binding. This suggested that desensitization was stimulated by a 'downstream' effect, but experiments have ruled out changes in protein kinase C activity and Ca2+ concentration. Binding experiments suggested that part of the desensitization is due to receptor internalization, and the requirement for an extracellular agonist for resensitization implies that receptor recycling plays a role. Over an extended time course, cycles of desensitization and resensitization of bombesin-stimulated PLD activity were apparent which may be relevant to mitogenic signalling. These studies add further evidence for a second messenger pathway of PLD activation, and the disparity between the kinetics of diacylglycerol generation and PLD activation supports the possibility that phosphatidic acid may have a messenger role in stimulated cells.

A role for protein kinase C-epsilon in angiotensin II stimulation of phospholipase D in rat renal mesangial cells

The role of Ca2+ and protein kinase C (PKC) in the regulation of phosphatidylcholine-hydrolyzing phospholipase D (PLD) was investigated in angiotensin II-stimulated mesangial cells. Elevation of cytosolic free Ca2+ by the calcium ionophore, A23187, or the Ca(2+)-ATPase inhibitor, thapsigargin, slightly increased PLD-stimulated phosphatidylethanol formation. However, chelation of cytosolic Ca2+ with high concentrations of quin 2 did not attenuate angiotensin II-induced phosphatidylethanol production, thus suggesting that Ca2+ is not crucially involved in agonist-stimulated PLD activation. Stimulation of PKC by phorbol esters increased PLD activity in mesangial cells. Down-regulation of PKC-alpha and -delta isoenzymes by 8 h phorbol ester treatment still resulted in full PLD activation. In contrast, a 24 h treatment of mesangial cells with phorbol ester, a regimen that also causes depletion of PKC-epsilon, abolished angiotensin II-evoked phosphatidylethanol formation. In addition, the selective PKC inhibitor, calphostin C, attenuated hormone-induced PLD activity. In summary, these data suggest that angiotensin II stimulation of phospholipase D appears to involve the PKC-epsilon isoenzyme, activated by DAG derived from phosphoinositide hydrolysis.

Diabetes induces selective alterations in the expression of protein kinase C isoforms in hepatocytes

Membrane and cytosol fractions from hepatocytes of both normal and streptozotocin-induced diabetic animals were probed with a panel of polyclonal anti-peptide antisera in order to identify protein kinase C (PKC) isoforms. Immunoreactive species were noted with antisera specific for alpha (approximately 81 kDa), beta-II (approximately 82 kDA), epsilon (approximately 95 kDa) and epsilon (approximately 79 kDa). In addition, a species migrating with an apparent size of approximately 94 kDa was also detected in cytosol fractions using an antiserum specific for PKC-alpha. Each of these species was specifically displaced when the PKC-isoform specific peptide was included in the immunodetection system. No immunoreactive species consistent with the presence of the beta-I, gamma, delta and eta isoforms of protein kinase C was observed. Induction of diabetes using streptozotocin invoked selective alterations in the expression of PKC isoforms which were reversed upon insulin therapy. In the cytosol fraction, marked increases of approximately 3-fold occurred in levels of the beta-II isoform and the approximately 90 kDa (upper) form of PKC-alpha, with no apparent/little change in the levels of the approximately 81 kDa (lower) form of PKC-alpha and those of PKC-zeta. Diabetes induction also appeared to have elicited the translocation of PKC-beta-II and the approximately 81 kDa (lower) form of PKC-alpha to the membrane fraction where immunoreactivity for these species was now apparent. The level of PKC-epsilon, which was noted only in membrane fractions, was also increased upon induction of diabetes. It is suggested that the selective alterations in the expression of PKC isoforms occurring upon streptozotocin-induced diabetes may lead to altered cellular functioning and underly defects in inhibitory G-protein functioning and insulin action which characterise this animal model of diabetes.

Bombesin stimulates distinct time-dependent changes in the sn-1,2-diradylglycerol molecular species profile from Swiss 3T3 fibroblasts as analysed by 3,5-dinitrobenzoyl derivatization and h.p.l.c. separation

We have developed procedures for the analysis of endogenous diradylglycerol (DRG) molecular species using derivatization with 3,5-dinitrobenzoyl chloride. The introduction of this strong chromatophore enabled us to separate less than 1 nmol of DRG into its three classes (diacylglycerol, alkylacylglycerol and alkenylacylglycerol) using a combination of h.p.l.c. and t.l.c. followed by reversed-phase h.p.l.c. to resolve these classes into their component molecular species. When applied to Swiss 3T3 mouse fibroblasts stimulated with bombesin for 25 s, 5 min or 30 min, subtle time-dependent changes in the DRG patterns were observed, with only certain polyunsaturated 1,2-diacyglycerol species [18:0/20:3(n-9), 18:0/20:4(n-6), 18:0/20:4(n-3), 18:0/20:5(n-3), 18:1(n-9)/20:3(n-9), 18:1(n-9)/20:4(n-6), 16:0/22:6(n-3), 18:0/20:3(n-6) and 16:0/20:5(n-3)] showing significant agonist-stimulated increases. The amounts of the first six species were all raised at 25 s, whereas all except the latter two were elevated at 5 min. By 30 min these last species were also increased but 18:0/20:3(n-9) had returned to basal levels. Overall DRG levels, as measured by total molecular-species peak area, remained effectively constant. No changes in the amount or species profile of 1-alkyl-2-acylglycerol were observed. Comparison of these species with the acyl-chain structure of phospholipids supports the idea that inositol lipids could be the source of DRG at early stimulation times, but phosphatidylcholine appears to be a phospholipase substrate at all times. These results indicate sequential activation of several phospholipases with different substrate specificities and/or access to different phospholipid pools. They also suggest that only polyunsaturated DRGs act as second messengers and that changes in the relative amounts of these species may trigger activation of different proteins and/or isoforms (e.g. the different isoforms of protein kinase C).

Activation of phospholipase D by alpha-thrombin or epidermal growth factor contributes to the formation of phosphatidic acid, but not to observed increases in 1,2-diacylglycerol

The receptor-mediated activation of a phosphatidylcholine-hydrolysing phospholipase D (PLD) has recently been described. We investigated the effect of alpha-thrombin and epidermal growth factor (EGF) on cellular PLD activity in order to determine the role of this enzyme in mitogen-induced increases in phosphatidic acid and sn-1,2-diacylglycerol. In the presence of ethanol, stimulation of [3H]myristic acid-labelled quiescent IIC9 cells with alpha-thrombin or EGF resulted in a rapid increase in radiolabelled phosphatidyl-ethanol which reached a plateau at 1 min, indicating the rapid and transient activation of PLD. We observed a concomitant decrease in the mitogen-stimulated increase of radiolabelled phosphatidic acid. In contrast, ethanol did not significantly effect the elevation of sn-1,2-diacylglycerol levels stimulated by alpha-thrombin or EGF as determined by measurement of sn-1,2-diacylglycerol mass or the appearance of [3H]1,2-diacylglycerol. A novel lipid, detected by two-dimensional t.l.c. analysis, was generated in [3H]myristic acid-labelled cells stimulated with alpha-thrombin, but not EGF, in the presence of ethanol. Treatment in vitro of cellular lipids isolated from [3H]myristic acid-labelled cultures with PLD in the presence of ethanol also resulted in the generation of this novel lipid species, supporting the role of this enzyme in its production. These data indicate that in quiescent IIC9 cells: (a) alpha-thrombin or EGF rapidly and transiently activates a PLD; (b) although this activation is responsible for part of the mitogen-induced increases in phosphatidic acid, it does not contribute to induced increases in sn-1,2-diacylglycerol; and (c) activation of this enzyme appears to be involved in the formation of a novel lipid generated in response to alpha-thrombin, but not EGF, in IIC9 fibroblasts.

Epidermal growth factor increases sn-1,2-diacylglycerol levels and activates phospholipase D-catalysed phosphatidylcholine breakdown in Swiss 3T3 cells in the absence of inositol-lipid hydrolysis

Addition of epidermal growth factor (EGF) to quiescent Swiss 3T3 cells resulted in a sustained increase in cellular diacylglycerol (DG) content in the absence of inositol-lipid hydrolysis. In the presence of non-cytotoxic concentrations of butan-1-ol, EGF stimulated the formation of phosphatidylbutanol, indicating that the EGF receptor was able to couple to the activation of phospholipase D (PLD). EGF-stimulated release of choline from Swiss 3T3 cells suggested that the major substrate for this PLD was phosphatidylcholine. Unlike bombesin-stimulated PLD activity, the response to EGF was not inhibited by a selective protein kinase C (PKC) inhibitor (Ro-31-8220), suggesting that it was not dependent on PKC activation. Pre-treatment of Swiss 3T3 cells with the EGF-receptor tyrosine kinase inhibitor AG18 selectively inhibited EGF-stimulated PLD activity; bombesin-stimulated PLD activity was unaffected. Butan-1-ol inhibited phorbol ester- and bombesin-stimulated DG formation suggesting a role for a coupled PLD/phosphatidate phosphohydrolase pathway; in contrast, EGF-stimulated DG formation was unaffected.

v-Src induces elevated levels of diglyceride by stimulation of phosphatidylcholine hydrolysis

When Rat-1 cells bearing the ts LA29 mutant of Rous sarcoma virus (Rat1 LA29) are shifted from restrictive to permissive temperature, the pp60v-Src tyrosine kinase is activated and there is an increase in the cellular level of sn1,2-diacylglycerol (DRG) within 30 min which is not accompanied by increased inositol phospholipid hydrolysis. Temperature shift also increases the hydrolysis of phosphatidylcholine (PC), as determined by an increase in the generation of water soluble choline metabolites. Transphosphatidylation studies have shown that this occurs at least in part via a phospholipase D (PLD) catalysed pathway.