Interleukin-1 and endothelin stimulate distinct species of diglycerides that differentially regulate protein kinase C in mesangial cells

Bioactive Ether Lipids: Primordial Modulators of Cellular Signaling

The primacy of lipids as essential components of cellular membranes is conserved across taxonomic domains. In addition to this crucial role as a semi-permeable barrier, lipids are also increasingly recognized as important signaling molecules with diverse functional mechanisms ranging from cell surface receptor binding to the intracellular regulation of enzymatic cascades. In this review, we focus on ether lipids, an ancient family of lipids having ether-linked structures that chemically differ from their more prevalent acyl relatives. In particular, we examine ether lipid biosynthesis in the peroxisome of mammalian cells, the roles of selected glycerolipids and glycerophospholipids in signal transduction in both prokaryotes and eukaryotes, and finally, the potential therapeutic contributions of synthetic ether lipids to the treatment of cancer.

Plasmalogens, platelet-activating factor and beyond - Ether lipids in signaling and neurodegeneration

Glycerol-based ether lipids including ether phospholipids form a specialized branch of lipids that in mammals require peroxisomes for their biosynthesis. They are major components of biological membranes and one particular subgroup, the plasmalogens, is widely regarded as a cellular antioxidant. Their vast potential to influence signal transduction pathways is less well known. Here, we summarize the literature showing associations with essential signaling cascades for a wide variety of ether lipids, including platelet-activating factor, alkylglycerols, ether-linked lysophosphatidic acid and plasmalogen-derived polyunsaturated fatty acids. The available experimental evidence demonstrates links to several common players like protein kinase C, peroxisome proliferator-activated receptors or mitogen-activated protein kinases. Furthermore, ether lipid levels have repeatedly been connected to some of the most abundant neurological diseases, particularly Alzheimer's disease and more recently also neurodevelopmental disorders like autism. Thus, we critically discuss the potential role of these compounds in the etiology and pathophysiology of these diseases with an emphasis on signaling processes. Finally, we review the emerging interest in plasmalogens as treatment target in neurological diseases, assessing available data and highlighting future perspectives. Although many aspects of ether lipid involvement in cellular signaling identified in vitro still have to be confirmed in vivo, the compiled data show many intriguing properties and contributions of these lipids to health and disease that will trigger further research.

Ether lipid metabolism by AADACL1 regulates platelet function and thrombosis

We previously reported the discovery of a novel lipid deacetylase in platelets, arylacetamide deacetylase-like 1 (AADACL1/NCEH1), and that its inhibition impairs agonist-induced platelet aggregation, Rap1 GTP loading, protein kinase C (PKC) activation, and ex vivo thrombus growth. However, precise mechanisms by which AADACL1 impacts platelet signaling and function in vivo are currently unknown. Here, we demonstrate that AADACL1 regulates the accumulation of ether lipids that impact PKC signaling networks crucial for platelet activation in vitro and in vivo. Human platelets treated with the AADACL1 inhibitor JW480 or the AADACL1 substrate 1-O-hexadecyl-2-acetyl-sn-glycerol (HAG) exhibited decreased platelet aggregation, granule secretion, Ca2+ flux, and PKC phosphorylation. Decreased aggregation and secretion were rescued by exogenous adenosine 5'-diphosphate, indicating that AADACL1 likely functions to induce dense granule secretion. Experiments with P2Y12-/- and CalDAG GEFI-/- mice revealed that the P2Y12 pathway is the predominate target of HAG-mediated inhibition of platelet aggregation. HAG itself displayed weak agonist properties and likely mediates its inhibitory effects via conversion to a phosphorylated metabolite, HAGP, which directly interacted with the C1a domains of 2 distinct PKC isoforms and blocked PKC kinase activity in vitro. Finally, AADACL1 inhibition in rats reduced platelet aggregation, protected against FeCl3-induced arterial thrombosis, and delayed tail bleeding time. In summary, our data support a model whereby AADACL1 inhibition shifts the platelet ether lipidome to an inhibitory axis of HAGP accumulation that impairs PKC activation, granule secretion, and recruitment of platelets to sites of vascular damage.

Deoxysphingolipids and ether-linked diacylglycerols accumulate in the tissues of aged mice

Background

Senescence is a well-known risk factor for several diseases, such as neurodegenerative disorders. Therefore, studies exploring the mechanisms underlying aging are expected to guide the discovery of novel drug targets and biomarkers for these diseases. However, a comprehensive overview of the metabolic and lipidomic changes in healthy aging mammals is lacking. To understand the changes of metabolism with aging, especially lipid metabolism, we analyzed the metabolomes and lipidomes of the cerebral cortex, liver, femoral muscle, and epididymal fat in young and aged mice.

Results

Two-dimensional cluster analysis revealed clear separation between the metabolite profiles of the aged and young groups. Deoxydihydroceramide (doxDHCer), deoxyceramide (doxCer), and ether-linked diacylglycerol (DAG) levels were elevated during aging.

Conclusion

This is the first report of age-related variations in deoxysphingolipid and ether-linked DAG levels in mice. DoxCer, doxDHCer, and ether-linked DAGs may be associated with senescence in mammalian tissues.

Electronic supplementary material

The online version of this article (10.1186/s13578-019-0324-9) contains supplementary material, which is available to authorized users.

Ether-linked diglycerides inhibit vascular smooth muscle cell growth via decreased MAPK and PI3K/Akt signaling

Diglycerides (DGs) are phospholipid-derived second messengers that regulate PKC-dependent signaling pathways. Distinct species of DGs are generated from inflammatory cytokines and growth factors. Growth factors increase diacyl- but not ether-linked DG species, whereas inflammatory cytokines predominately generate alkyl, acyl- and alkenyl, acyl-linked DG species in rat mesenchymal cells. These DG species have been shown to differentially regulate protein kinase C (PKC) isotypes. Ester-linked diacylglycerols activate PKC-epsilon and cellular proliferation in contrast to ether-linked DGs, which lead to growth arrest through the inactivation of PKC-epsilon. It is now hypothesized that ether-linked DGs inhibit mitogenesis through the inactivation of ERK and/or Akt signaling cascades. We demonstrate that cell-permeable ether-linked DGs reduce vascular smooth muscle cell growth by inhibiting platelet-derived growth factor-stimulated ERK in a PKC-epsilon-dependent manner. This inhibition is specific to the ERK pathway, since ether-linked DGs do not affect growth factor-induced activation of other family members of the MAPKs, including p38 MAPK and c-Jun NH(2)-terminal kinases. We also demonstrate that ether-linked DGs reduce prosurvival phosphatidylinositol 3-kinase (PI3K)/Akt signaling, independent of PKC-epsilon, by diminishing an interaction between the subunits of PI3K and not by affecting protein phosphatase 2A or lipid (phosphatase and tensin homologue deleted in chromosome 10) phosphatases. Taken together, our studies identify ether-linked DGs as potential adjuvant therapies to limit vascular smooth muscle migration and mitogenesis in atherosclerotic and restenotic models.

Metabolic and molecular aspects of ethanolamine phospholipid biosynthesis: the role of CTP:phosphoethanolamine cytidylyltransferase (Pcyt2)

The CDP-ethanolamine branch of the Kennedy pathway is the major route for the formation of ethanolamine-derived phospholipids, including diacyl phosphatidylethanolamine and alkenylacyl phosphatidylethanolamine derivatives, known as plasmalogens. Ethanolamine phospholipids are essential structural components of the cell membranes and play regulatory roles in cell division, cell signaling, activation, autophagy, and phagocytosis. The physiological importance of plasmalogens has not been not fully elucidated, although they are known for their antioxidant properties and deficiencies in a number of inherited peroxisomal disorders. This review highlights important aspects of ethanolamine phospholipid metabolism and reports current molecular information on 1 of the regulatory enzymes in their synthesis, CTP:phosphoethanolamine cytidylyltransferase (Pcyt2). Pcyt2 is encoded by a single, nonredundant gene in animal species that could be alternatively spliced into 2 potential protein products. We describe properties of the mouse and human Pcyt2 genes and their regulatory promoters and provide molecular evidence for the existence of 2 distinct Pcyt2 proteins. The goal is to obtain more insight into Pcyt2 catalytic function and regulation to facilitate a better understanding of the production of ethanolamine phospholipids via the CDP-ethanolamine branch of the Kennedy pathway.

Effects of Blocking the Renin-Angiotensin System on Expression and Translocation of Protein Kinase C Isoforms in the Kidney of Diabetic Rats

BACKGROUND

High glucose and angiotensin II (Ang II) can activate protein kinase C (PKC) in diabetes mellitus. However, it is not clear which isoform of PKC is activated by glucose or Ang II. Our study focused on the effects of angiotensin blockade, using the angiotensin-converting enzyme inhibitor fosinopril, the Ang II receptor blocker irbesartan and their combination, on the expression and translocation of PKC isoforms alpha and betaII in the renal cortex and medulla in diabetes.

METHODS

Hyperglycemia was induced with streptozotocin and diabetic rats were randomized to 4 groups: diabetic control, irbesartan group (40 mg/kg daily), fosinopril group (40 mg/kg daily) and combination group (irbesartan plus fosinopril, 20 mg/kg daily, respectively); age-matched normal rats served as normal control. After 4 weeks, expression and translocation of PKC-alpha and -betaII in the renal cortex and medulla were assessed by immunohistochemistry and Western immunoblotting.

RESULTS

The expression of PKC-alpha in the membrane and cytosol fractions from the renal cortex was significantly higher in diabetic rats (276.83 +/- 32.44% in membrane, 149.04 +/- 23.42% in cytosol) than that in normal ones. The expression of PKC-betaII in the renal cortex of diabetic rats decreased significantly in the membrane (50.00 +/- 11.68%, p < 0.05) and remained unchanged in the cytosol (94.51 +/- 11.69%, p > 0.05) compared with normal controls. Treatment with irbesartan, fosinopril and their combination partially corrected the abnormalities mentioned above. For the expression of PKC-alpha and -betaII in the medulla, no difference was detected among the 5 groups.

CONCLUSION

The renin-angiotensin system was implicated in the pathogenesis of diabetic nephropathy by regulating the activation of PKC isoforms.

Signaling events mediating activation of brain ethanolamine plasmalogen hydrolysis by ceramide

Ceramide is a lipid second messenger that acts on multiple-target enzymes, some of which are involved in other signal-transduction systems. We have previously demonstrated that endogenous ceramide modifies the metabolism of brain ethanolamine plasmalogens. The mechanism involved was studied. On the basis of measurements of breakdown products, specific inhibitor effects, and previous findings, we suggest that a plasmalogen-selective phospholipase A2 is the ceramide target. Arachidonate-rich pools of the diacylphosphatidylethanolamine subclass were also affected by ceramide, but the most affected were plasmalogens. Concomitantly with production of free arachidonate, increased 1-O-arachidonoyl ceramide formation was observed. Quinacrine (phospholipase A2 inhibitor) and 1-O-octadecyl-2-O-methyl-rac-glycerol-3-phosphocholine (CoA-independent transacylase inhibitor) prevented all of these ceramide-elicited effects. Therefore, phospholipase and transacylase activities are tightly coupled. Okadaic acid (phosphatase 2A inhibitor) and PD 98059 (mitogen-activated protein kinase inhibitor) modified basal levels of ceramide and sphingomyelinase-induced accumulation of ceramide, respectively. Therefore, they provided no evidence to determine whether there is a sensitive enzyme downstream of ceramide. The evidence shows that there are serine-dependent and thiol-dependent enzymes downstream of ceramide generation. Furthermore, experiments with Ac-DEVD-CMK (caspase-3 specific inhibitor) have led us to conclude that caspase-3 is downstream of ceramide in activating the brain plasmalogen-selective phospholipase A2.

Inhibitory actions of ceramide upon PKC-epsilon/ERK interactions

We have previously shown that interleukin-1 receptor-generated ceramide induces growth arrest in smooth muscle pericytes by inhibiting an upstream kinase in the extracellular signal-regulated kinase (ERK) cascade. Here, we now report the mechanism by which ceramide inhibits ERK activity. Ceramide renders the human embryonic kidney 293 cells (HEK 293) resistant to the mitogenic actions of growth factors and activators of protein kinase C (PKC). A role for PKC to mediate ceramide inhibition of growth factor-induced ERK activity and mitogenesis is suggested, as exogenous ceramide directly inhibits both immunoprecipitated and recombinant PKC-epsilon activities. To confirm that PKC-epsilon is necessary for ceramide-inhibited ERK activity, HEK 293 cells were transfected with a dominant-negative mutant of PKC-epsilon (DeltaPKC-epsilon). These transfected cells respond to insulin-like growth factor I (IGF-I) with a significantly decreased ERK activity that is not further reduced by ceramide treatment. Coimmunoprecipitation studies reveal that the treatment with IGF-I induces the association of ERK with PKC-epsilon but not with PKC-zeta. Ceramide treatment significantly inhibits the IGF-I-induced PKC-epsilon interaction with bioactive phosphorylated ERK. Ceramide also inhibits IGF-I-induced PKC-epsilon association with Raf-1, an upstream kinase of ERK. Together, these studies demonstrate that ceramide exerts anti-mitogenic actions by limiting the ability of PKC-epsilon to form a signaling complex with Raf-1 and ERK.

Ceramide directly activates protein kinase C zeta to regulate a stress-activated protein kinase signaling complex

We have previously shown that interleukin 1 (IL-1)-receptor-generated ceramide induces growth arrest in smooth muscle pericytes by activating an upstream kinase in the stress-activated protein kinase (SAPK) cascade. We now report the mechanism by which ceramide activates the SAPK signaling pathway in human embryonic kidney cells (HEK-293). We demonstrate that ceramide activation of protein kinase C zeta (PKCzeta) mediates SAPK signal complex formation and subsequent growth suppression. Ceramide directly activates both immunoprecipitated and recombinant human PKCzeta in vitro. Additionally, ceramide activates SAPK activity, which is blocked with a dominant-negative mutant of PKCzeta. Co-immunoprecipitation studies reveal that ceramide induces the association of SAPK with PKCzeta, but not with PKCepsilon. In addition, ceramide treatment induces PKCzeta association with phosphorylated SEK and MEKK1, elements of the SAPK signaling complex. The biological role of ceramide to induce cell cycle arrest is mimicked by overexpression of a constitutively active PKCzeta. Together, these studies demonstrate that ceramide induces cell cycle arrest by enhancing the ability of PKCzeta to form a signaling complex with MEKK1, SEK, and SAPK.

PKC signaling in CF/T43 cell line: regulation of NKCC1 by PKC-delta isotype

Cystic fibrosis (CF) airway epithelial cells have a reduced mass of ether-linked diacylglycerols which might alter protein kinase C (PKC)-regulated Cl secretion. PKC regulation of basolateral Na-K-2Cl cotransport (NKCC1) was investigated in CF nasal polyp epithelial cells and a CF/T43 cell line to ascertain whether PKC signaling was altered in CF. NKCC1 was detected as bumetanide-sensitive (86)Rb influx. Methoxamine, a alpha(1)-adrenergic agonist, increased PKC activity in cytosol and a particulate fraction for a prolonged time period, as predicted from previous studies on the generation of diglycerides induced with methoxamine. Short-term stimulation of CF/T43 cells for 40 s promoted a shift in PKC-delta and -zeta to a particulate fraction, increased activity of immune complexes of cytosolic PKC-delta and of particulate PKC-zeta and increased activity of NKCC1. Pretreatment with antisense oligonucleotide to PKC-delta blocked methoxamine-stimulated PKC-delta activity, reduced PKC-delta mass by 61.4%, and prevented methoxamine-stimulated activity of NKCC1. Sense and missense oligonucleotide to PKC-delta and antisense oligonucleotide to PKC-zeta did not alter expression of PKC-delta or the effects of methoxamine. These results demonstrate that PKC-delta-dependent activation of NKCC1 is preserved in CF cells and suggest that regulation of NKCC1 is independent of low ether-linked diglyceride mass.

Pro-inflammatory cytokines stimulate mitogen-activated protein kinase subfamilies, increase phosphorylation of c-Jun and ATF2 and upregulate c-Jun protein in neonatal rat ventricular myocytes

Pro-inflammatory cytokines may be important in the pathophysiological responses of the heart. We investigated the activation of the three mitogen-activated protein kinase (MAPK) subfamilies ¿c-Jun N-terminal kinases (JNKs), p38-MAPKs and extracellularly-responsive kinases (ERKs) by interleukin-1 beta (IL-1 beta) or tumour necrosis factor alpha (TNF alpha) in primary cultures of myocytes isolated from neonatal rat ventricles. Both cytokines stimulated a rapid (maximal within 10 min) increase in JNK activity. Although activation of JNKs by IL-1 beta was transient returning to control values within 1 h, the response to TNF alpha was sustained. IL-1 beta and TNF alpha also stimulated p38-MAPK phosphorylation, but the response to IL-1 beta was consistently greater than TNF alpha. Both cytokines activated ERKs, but to a lesser degree than that induced by phorbol esters. The transcription factors, c-Jun and ATF2, are phosphorylated by the MAPKs and are implicated in the upregulation of c-Jun. IL-1 beta and TNF alpha stimulated the phosphorylation of c-Jun and ATF2. However, IL-1 beta induced a greater increase in c-Jun protein. Inhibitors of protein kinase C (PKC) (Ro318220, GF109203X) and the ERK cascade (PD98059) attenuated the increase in c-Jun induced by IL-1 beta, but LY294002 (an inhibitor of phosphatidylinositol 3' kinase) and SB203580 (an inhibitor of p38-MAPK, which also inhibits certain JNK isoforms) had no effect. These data illustrate that some of the pathological effects of IL-1 beta and TNF alpha may be mediated through the MAPK cascades, and that the ERK cascade, rather than JNKs or p38-MAPKs, are implicated in the upregulation of c-Jun by IL-1 beta.

15-HETE-substituted diglycerides selectively regulate PKC isotypes in human tracheal epithelial cells

Human tracheal epithelial (TE) cells selectively incorporate their major lipoxygenase product, 15-hydroxyeicosatetraenoic acid (15-HETE), into the sn-2 position of phosphatidylinositol (PI) (S. E. Alpert and R. W. Walenga. Am. J. Respir. Cell Mol. Biol. 8: 273-281, 1993). Here we investigated whether 15-HETE-PI is a substrate for receptor-mediated generation of 15-HETE-substituted diglycerides (DGs) and whether these 15-HETE-DGs directly activate and/or alter conventional diacylglycerol-induced activation of protein kinase C (PKC) isotypes in these cells. Primary human TE monolayers incubated with 0.5 microM 15-[3H]-HETE or 15-[14C]HETE for 1-2 h were stimulated with 1 nM to 1 microM platelet-activating factor (PAF) for 30 s to 6 min, and the radiolabel in the medium, cellular phospholipids, and neutral lipids was assessed by high-performance liquid and thin-layer chromatography. PAF mobilized radiolabel from PI in a dose-dependent manner (22 +/- 5% decrease after 1 microM PAF) without a concomitant release of free intra- or extracellular 15-HETE. 14C-labeled DGs were present in unstimulated TE monolayers incubated with 15-[14C]HETE, and the major 14C band, identified as sn-1,2-15-[14C]HETE-DG, increased transiently in response to PAF. Western blots of freshly isolated and cultured human TE cells revealed PKC isotypes alpha, betaI, betaII, delta, epsilon, and zeta. In vitro, cell-generated sn-1, 2-15-[14C]HETE-DG selectively activated immunoprecipitated PKC-alpha and inhibited diacylglycerol-induced activation of PKC-alpha, -delta, -betaI, and -betaII. Our observations indicate that 15-HETE-DGs can modulate the activity of PKC isotypes in human TE cells and suggest an intracellular autocrine role for 15-HETE in human airway epithelia.

Interleukin-2 causes an increase in saturated/monounsaturated phosphatidic acid derived from 1,2-diacylglycerol and 1-O-alkyl-2-acylglycerol

Phosphatidic acid generation through activation of diacylglycerol kinase alpha has been implicated in interleukin-2-dependent T-lymphocyte proliferation. To investigate this lipid signaling in more detail, we characterized the molecular structures of the diradylglycerols and phosphatidic acids in the murine CTLL-2 T-cell line under both basal and stimulated conditions. In resting cells, 1,2-diacylglycerol and 1-O-alkyl-2-acylglycerol subtypes represented 44 and 55% of total diradylglycerol, respectively, and both showed a highly saturated profile containing primarily 16:0 and 18:1 fatty acids. 1-O-Alk-1'-enyl-2-acylglycerol represented 1-2% of total diradylglycerol. Interleukin-2 stimulation did not alter the molecular species profiles, however, it did selectively reduce total 1-O-alkyl-2-acylglycerol by over 50% at 15 min while only causing a 10% drop in 1,2-diacylglycerol. When radiolabeled CTLL-2 cells were challenged with interleukin-2, no change in the cellular content of phosphatidylcholine nor phosphatidylethanolamine was observed thereby ruling out phospholipase C activity as the source of diradylglycerol. In addition, interleukin-2 failed to stimulate de novo synthesis of diradylglycerol. Structural analysis revealed approximately equal amounts of 1,2-diacyl phosphatidic acid and 1-O-alkyl-2-acyl phosphatidic acid under resting conditions, both containing only saturated and monounsaturated fatty acids. After acute (2 and 15 min) interleukin-2 stimulation the total phosphatidic acid mass increased, almost entirely through the formation of 1-O-alkyl-2-acyl species. In vitro assays revealed that both 1,2-diacylglycerol and 1-O-alkyl-2-acylglycerol were substrates for 1,2-diacylglycerol kinase alpha, the major isoform in CTLL-2 cells, and that the lipid kinase activity was almost totally inhibited by R59949. In conclusion, this investigation shows that, in CTLL-2 cells, 1,2-diacylglycerol kinase alpha specifically phosphorylates a pre-existing pool of 1-O-alkyl-2-acylglycerol to form the intracellular messenger 1-O-alkyl-2-acyl phosphatidic acid.

Cellular activation triggered by the autosomal dominant polycystic kidney disease gene product PKD2

Autosomal dominant polycystic kidney disease (ADPKD) is caused by germ line mutations in at least three ADPKD genes. Two recently isolated ADPKD genes, PKD1 and PKD2, encode integral membrane proteins of unknown function. We found that PKD2 upregulated AP-1-dependent transcription in human embryonic kidney 293T cells. The PKD2-mediated AP-1 activity was dependent upon activation of the mitogen-activated protein kinases p38 and JNK1 and protein kinase C (PKC) epsilon, a calcium-independent PKC isozyme. Staurosporine, but not the calcium chelator BAPTA [1,2-bis(o-aminophenoxy)ethane-N,N,N', N'-tetraacetate], inhibited PKD2-mediated signaling, consistent with the involvement of a calcium-independent PKC isozyme. Coexpression of PKD2 with the interacting C terminus of PKD1 dramatically augmented PKD2-mediated AP-1 activation. The synergistic signaling between PKD1 and PKD2 involved the activation of two distinct PKC isozymes, PKC alpha and PKC epsilon, respectively. Our findings are consistent with others that support a functional connection between PKD1 and PKD2 involving multiple signaling pathways that converge to induce AP-1 activity, a transcription factor that regulates different cellular programs such as proliferation, differentiation, and apoptosis. Activation of these signaling cascades may promote the full maturation of developing tubular epithelial cells, while inactivation of these signaling cascades may impair terminal differentiation and facilitate the development of renal tubular cysts.

Release of glycosylphosphatidylinositol-anchored carboxypeptidase M by phosphatidylinositol-specific phospholipase C upregulates enzyme synthesis

Carboxypeptidase M (CPM), a glycosylphosphatidylinositol (GPI)-anchored membrane protein, remained at a constant level in confluent Madin Darby canine kidney (MDCK) cells but was continually released into the medium in soluble form. The released CPM contained ethanolamine, indicating liberation by a phospholipase. Treatment of MDCK cells with 0.01 U/ml phosphatidylinositol-specific phospholipase C for 6 h led to a 5.5-fold increase in soluble CPM, yet the activity in cells remained constant, resulting in a 30% increase in total activity. The increase was due to new protein synthesis as evidenced by inhibition with 0.2 microM cycloheximide and a 63% increase in [35S]methionine incorporation into newly synthesized CPM. MDCK cells treated with 1-alkyl-2-acyl-glycerol, the diglyceride component of mammalian glycosylphosphatidylinositol anchors, exhibited a 36% increase in CPM activity, but diacylglycerols or phorbol esters were ineffective. Thus, release of GPI-anchored CPM can generate a diglyceride signal to replenish and maintain constant levels on the cell surface.

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).

High-resolution separation and quantification of neutral lipid and phospholipid species in mammalian cells and sera by multi-one-dimensional thin-layer chromatography

An improvement of current methods is needed for simple, rapid, and precise quantification of cellular lipids, including rare species of biologically active cellular lipids, such as phosphatidic acid (PA) and diradylglycerol (DG). In addition, further analysis of hydrolyzed acyl chains from these species by methods such as gas chromatography requires complete separations. Methods have been developed for the quantification of neutral lipids and several phospholipids extracted from mammalian cells and sera. Lipid masses were determined for the major classes of the neutral, nonpolar lipids, and of the phospholipids. The lipid classes were separated by a multistep thin-layer chromatography (TLC) procedure in different solvent systems, a method which we have designated as multi-one-dimensional thin-layer chromatography (MOD-TLC). Resolved lipid bands were visualized by the lipophilic dye primulin (direct yellow 59) and scanned by an automated laser-fluorescence detector. The mass of each band was determined by comparing band intensities of unknown samples to dilution curves of authentic standards. With modifications in solvent mixtures and length of separation times, the majority of biological lipids could be resolved and quantified with MOD-TLC methods. Since the detection method is nondestructive, purified lipids could then be recovered by scraping the visualized bands and extracting the lipids from the silica. The structural identities of the recovered lipids were confirmed by fast-atom bombardment and electrospray mass spectrometry. Extracted lipids were also hydrolyzed to release acyl chains and acyl chain species were determined in comparison to authentic standards by gas chromatography. PA and DG levels in ECV.304 cells were found to be 4. 6 and 3.3%, respectively, of PC levels, with a PA/DG ratio of 1.4, which is in accord with published experience using other methods and different cell types. PA in human serum was detected at 0.6% of PC, indicating the sensitivity of the technique. In contrast to two-dimensional thin-layer chromatography, which allows for good resolution of some lipid species, but cannot be used to analyze more than a single experimental point per plate, MOD-TLC allows for direct comparative analysis of multiple samples on a single TLC plate, while still providing good resolution for the quantification of most major classes of lipid species.

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

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

Interleukin-1-induced ether-linked diglycerides inhibit calcium-insensitive protein kinase C isotypes. Implications for growth senescence

It is hypothesized that inflammatory cytokines and vasoactive peptides stimulate distinct species of diglycerides that differentially regulate protein kinase C isotypes. In published data, we demonstrated that interleukin-1, in contrast to endothelin, selectively generates ether-linked diglyceride species (alkyl, acyl- and alkenyl, acylglycerols) in rat mesangial cells, a smooth muscle-like pericyte in the glomerulus. We now demonstrate both in intact cell and in cell-free preparations that these interleukin-1 receptor-generated ether-linked diglycerides inhibit immunoprecipitated protein kinase C delta and epsilon but not zeta activity. Neither interleukin-1 nor endothelin affect de novo protein expression of these protein kinase C isotypes. As down-regulation of calcium-insensitive protein kinase C isotypes has been linked to antimitogenic activity, we investigated growth arrest as a functional correlate for IL-1-generated ether-linked diglycerides. Cell-permeable ether-linked diglycerides mimic the effects of interleukin-1 to induce a growth-arrested state in both G-protein-linked receptor- and tyrosine kinase receptor-stimulated mesangial cells. This signaling mechanism implicates cytokine receptor-induced ether-linked diglycerides as second messengers that inhibit the bioactivity of calcium-insensitive protein kinase C isotypes resulting in growth arrest.

Opposite, binary regulatory pathways involved in IL-1-mediated stromelysin gene expression in rat mesangial cells

Glomerular mesangial cells express matrix metalloproteinase sromelysin in response to the proinflammatory cytokine IL-1 beta. The present study was conducted to identify intracellular machinery involved in this IL-1 action, especially focusing on the role of the TPA response element (TRE) located in the 5'-flanking region of the stromelysin gene. Using transient transfection with a pTRE-LacZ reporter plasmid, we detected no obvious up-regulation of TRE activity in rat mesangial cells following the IL-1 stimulation. However, the basal activity of TRE was found to be essential to the stromelysin induction, since (i) mesangial cells stably expressing a transdominant negative mutant of c-Jun, which effectively suppressed both basal and inducible TRE activity, exhibited the blunted expression of stromelysin in response to IL-1 beta, whereas (ii) transfection with a c-fos antisense gene, which suppressed only the inducible TRE activity, did not affect the stromelysin induction. To seek cooperative pathways required for the IL-1 action, we next focused on protein kinases, the potential regulators of the stromelysin gene. Stimulation of mesangial cells with a protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate (PMA), induced the stromelysin transcript without affecting TRE activity. Depletion of intracellular PKC by high-dose PMA or inhibition of PKC activity with calphostin C suppressed the stromelysin induction by IL-1 beta, suggesting the crucial contribution of a PKC-mediated, but TRE-independent pathway. In contrast, either cAMP inducer forskolin or dibutyryl cAMP suppressed the IL-1-mediated stromelysin expression. An inhibitor of cAMP-dependent protein kinase A (PKA), HA1004, enhanced the IL-1 effect in a dose-dependent manner. Unexpectedly, the inhibitory action of PKA was not through cAMP response element (CRE) but through TRE, because (i) activation of CRE was not induced by IL-1 beta, and (ii) cAMP-mediated activation of PKA suppressed the basal TRE activity. These findings elucidated the unique, binary regulation of stromelysin by IL-1 beta; that is, IL-1 up-regulated the transcript via the PKC-dependent pathway under the cooperation with constitutively active TRE, and this stimulatory effect was in part counterbalanced by the IL-1-inducible PKA which down-regulated the basal TRE activity.

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 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.