Tricyclodecan-9-yl-xanthogenate (D609) mechanism of actions: a mini-review of literature

Upstream and downstream pathways of diacylglycerol kinase : Novel phosphatidylinositol turnover-independent signal transduction pathways

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol (DG) to produce phosphatidic acid (PA). Mammalian DGK comprise ten isozymes (α-κ) that regulate a wide variety of physiological and pathological events. Recently, we revealed that DGK isozymes use saturated fatty acid (SFA)/monosaturated fatty acid (MUFA)-containing and docosahexaenoic acid (22:6)-containing DG species, but not phosphatidylinositol (PI) turnover-derived 18:0/20:4-DG. For example, DGKδ, which is involved in the pathogenesis of type 2 diabetes, preferentially uses SFA/MUFA-containing DG species, such as 16:0/16:0- and 16:0/18:1-DG species, in high glucose-stimulated skeletal muscle cells. Moreover, DGKδ, which destabilizes the serotonin transporter (SERT) and regulates the serotonergic system in the brain, primarily generates 18:0/22:6-PA. Furthermore, 16:0/16:0-PA is produced by DGKζ in Neuro-2a cells during neuronal differentiation. We searched for SFA/MUFA-PA- and 18:0/22:6-PA-selective binding proteins (candidate downstream targets of DGKδ) and found that SFA/MUFA-PA binds to and activates the creatine kinase muscle type, an energy-metabolizing enzyme, and that 18:0/22:6-PA interacts with and activates Praja-1, an E3 ubiquitin ligase acting on SERT, and synaptojanin-1, a key player in the synaptic vesicle cycle. Next, we searched for SFA/MUFA-DG-generating enzymes upstream of DGKδ. We found that sphingomyelin synthase (SMS)1, SMS2, and SMS-related protein (SMSr) commonly act as phosphatidylcholine (PC)-phospholipase C (PLC) and phosphatidylethanolamine (PE)-PLC, generating SFA/MUFA-DG species, in addition to SMS and ceramide phosphoethanolamine synthase. Moreover, the orphan phosphatase PHOSPHO1 showed PC- and PE-PLC activities that produced SFA/MUFA-DG. Although PC- and PE-PLC activities were first described 70-35 years ago, their proteins and genes were not identified for a long time. We found that DGKδ interacts with SMSr and PHOSPHO1, and that DGKζ binds to SMS1 and SMSr. Taken together, these results strongly suggest that there are previously unrecognized signal transduction pathways that include DGK isozymes and generate and utilize SFA/MUFA-DG/PA or 18:0/22:6-DG/PA but not PI-turnover-derived 18:0/20:4-DG/PA.

Extracellular Vesicle Inhibitors Enhance Cholix-Induced Cell Death via Regulation of the JNK-Dependent Pathway

Vibrio cholerae is an important foodborne pathogen. Cholix cytotoxin (Cholix), produced by V. cholerae, is a novel eukaryotic elongation factor 2 (eEF2) adenosine diphosphate ribosyltransferase that causes host cell death by inhibiting protein synthesis. However, the role of Cholix in the infectious diseases caused by V. cholerae remains unclear. Some bacterial cytotoxins are carried by host extracellular vesicles (EVs) and transferred to other cells. In this study, we investigated the effects of EV inhibitors and EV-regulating proteins on Cholix-induced hepatocyte death. We observed that Cholix-induced cell death was significantly enhanced in the presence of EV inhibitors (e.g., dimethyl amiloride, and desipramine) and Rab27a-knockdown cells, but it did not involve a sphingomyelin-dependent pathway. RNA sequencing analysis revealed that desipramine, imipramine, and EV inhibitors promoted the Cholix-activated c-Jun NH2-terminal kinase (JNK) pathway. Furthermore, JNK inhibition decreased desipramine-enhanced Cholix-induced poly (ADP-ribose) polymerase (PARP) cleavage. In addition, suppression of Apaf-1 by small interfering RNA further enhanced Cholix-induced PARP cleavage by desipramine. We identified a novel function of desipramine in which the stimulated JNK pathway promoted a mitochondria-independent cell death pathway by Cholix.

Subcellular imaging of lipids and sugars using genetically encoded proximity sensors

Live cell imaging of lipids and other metabolites is a long-standing challenge in cell biology. Bioorthogonal labeling tools allow for the conjugation of fluorophores to several phospholipid classes, but cannot discern their trafficking between adjacent organelles or asymmetry across individual membrane leaflets. Here we present fluorogen-activating coincidence sensing (FACES), a chemogenetic tool capable of quantitatively imaging subcellular lipid pools and reporting their transbilayer orientation in living cells. FACES combines bioorthogonal chemistry with genetically encoded fluorogen-activating proteins (FAPs) for reversible proximity sensing of conjugated molecules. We first validate this approach for quantifying discrete phosphatidylcholine pools in the ER and mitochondria that are trafficked by lipid transfer proteins. We then show that transmembrane domain-containing FAPs can be used to reveal the membrane asymmetry of multiple lipid classes that are generated in the trans-Golgi network. Lastly, we demonstrate that FACES is a generalizable tool for subcellular bioorthogonal imaging by measuring changes in mitochondrial N -acetylhexosamine levels. These results demonstrate the use of fluorogenic tags for spatially-defined molecular imaging.

Reactions of 1,3-Diphenyl Cyclopentadiene with α-Aryldiazo Ketones to Enable C-H Insertions versus [4 + 2]-Cycloadditions via Au Catalyst and P(C6F5)3 Additive, Respectively

Two distinct reaction chemoselectivities were reported for the reactions of α-aryldiazo ketone with 1,3-diphenylcyclopentadiene using gold catalyst and phosphine additives, respectively. In the presence of gold catalyst, α-aryldiazo ketone forms gold carbenes initially that are trapped with this 1,3-disubstituted cyclopentadiene to afford C-H insertion products. In the presence of P(C6F5)3 additive, α-aryldiazo ketone forms diarylketenes initially at elevated temperature, which are further stabilized by P(C6F5)3 to secure their entity before proceeding to unprecedented [4C + 2C] cycloadditions.

Human sphingomyelin synthase 1 generates diacylglycerol in the presence and absence of ceramide via multiple enzymatic activities

Sphingomyelin (SM) synthase 1 (SMS1), which is involved in lipodystrophy, deafness, and thrombasthenia, generates diacylglycerol (DG) and SM using phosphatidylcholine (PC) and ceramide as substrates. Here, we found that SMS1 possesses DG-generating activities via hydrolysis of PC and phosphatidylethanolamine (PE) in the absence of ceramide and ceramide phosphoethanolamine synthase (CPES) activity. In the presence of the same concentration (4.7 mol%) of PC and ceramide, the amounts of DG produced by SMS and PC-phospholipase C (PLC) activities of SMS1 were approximately 65% and 35% of total DG production, respectively. PC-PLC activity showed substrate selectivity for saturated and/or monounsaturated fatty acid-containing PC species. A PC-PLC/SMS inhibitor, D609, inhibited only SMS activity. Mn2+ inhibited only PC-PLC activity. Intriguingly, DG attenuated SMS/CPES activities. Our study indicates that SMS1 is a unique enzyme with PC-PLC/PE-PLC/SMS/CPES activities.

Sphingomyelin synthase-related protein SMSr is a phosphatidylethanolamine phospholipase C that promotes nonalcoholic fatty liver disease

Sphingomyelin synthase (SMS)-related protein (SMSr) is a phosphatidylethanolamine phospholipase C (PE-PLC) that is conserved and ubiquitous in mammals. However, its biological function is still not clear. We previously observed that SMS1 deficiency-mediated glucosylceramide accumulation caused nonalcoholic fatty liver diseases (NAFLD), including nonalcoholic steatohepatitis (NASH) and liver fibrosis. Here, first, we evaluated high-fat diet/fructose-induced NAFLD in Smsr KO and WT mice. Second, we evaluated whether SMSr deficiency can reverse SMS1 deficiency-mediated NAFLD, using Sms1/Sms2 double and Sms1/Sms2/Smsr triple KO mice. We found that SMSr/PE-PLC deficiency attenuated high-fat diet/fructose-induced fatty liver and NASH, and attenuated glucosylceramide accumulation-induced NASH, fibrosis, and tumor formation. Further, we found that SMSr/PE-PLC deficiency reduced the expression of many inflammatory cytokines and fibrosis-related factors, and PE supplementation in vitro or in vivo mimicked the condition of SMSr/PE-PLC deficiency. Furthermore, we demonstrated that SMSr/PE-PLC deficiency or PE supplementation effectively prevented membrane-bound β-catenin transfer to the nucleus, thereby preventing tumor-related gene expression. Finally, we observed that patients with NASH had higher SMSr protein levels in the liver, lower plasma PE levels, and lower plasma PE/phosphatidylcholine ratios, and that human plasma PE levels are negatively associated with tumor necrosis factor-α and transforming growth factor β1 levels. In conclusion, SMSr/PE-PLC deficiency causes PE accumulation, which can attenuate fatty liver, NASH, and fibrosis. These results suggest that SMSr/PE-PLC inhibition therapy may mitigate NAFLD.

Targeting the Sphingolipid Rheostat in Gliomas

Gliomas are highly aggressive cancer types that are in urgent need of novel drugs and targeted therapies. Treatment protocols have not improved in over a decade, and glioma patient survival remains among the worst of all cancer types. As a result, cancer metabolism research has served as an innovative approach to identifying novel glioma targets and improving our understanding of brain tumors. Recent research has uncovered a unique metabolic vulnerability in the sphingolipid pathways of gliomas that possess the IDH1 mutation. Sphingolipids are a family of lipid signaling molecules that play a variety of second messenger functions in cellular regulation. The two primary metabolites, sphingosine-1-phosphate (S1P) and ceramide, maintain a rheostat balance and play opposing roles in cell survival and proliferation. Altering the rheostat such that the pro-apoptotic signaling of the ceramides outweighs the pro-survival S1P signaling in glioma cells diminishes the hallmarks of cancer and enhances tumor cell death. Throughout this review, we discuss the sphingolipid pathway and identify the enzymes that can be most effectively targeted to alter the sphingolipid rheostat and enhance apoptosis in gliomas. We discuss each pathway’s steps based on their site of occurrence in the organelles and postulate novel targets that can effectively exploit this vulnerability.

Sphingolipids and Lymphomas: A Double-Edged Sword

Lymphomas are a highly heterogeneous group of hematological neoplasms. Given their ethiopathogenic complexity, their classification and management can become difficult tasks; therefore, new approaches are continuously being sought. Metabolic reprogramming at the lipid level is a hot topic in cancer research, and sphingolipidomics has gained particular focus in this area due to the bioactive nature of molecules such as sphingoid bases, sphingosine-1-phosphate, ceramides, sphingomyelin, cerebrosides, globosides, and gangliosides. Sphingolipid metabolism has become especially exciting because they are involved in virtually every cellular process through an extremely intricate metabolic web; in fact, no two sphingolipids share the same fate. Unsurprisingly, a disruption at this level is a recurrent mechanism in lymphomagenesis, dissemination, and chemoresistance, which means potential biomarkers and therapeutical targets might be hiding within these pathways. Many comprehensive reviews describing their role in cancer exist, but because most research has been conducted in solid malignancies, evidence in lymphomagenesis is somewhat limited. In this review, we summarize key aspects of sphingolipid biochemistry and discuss their known impact in cancer biology, with a particular focus on lymphomas and possible therapeutical strategies against them.

Tricyclodecan-9-yl-Xanthogenate (D609): Mechanism of Action and Pharmacological Applications

Tricyclodecan-9-yl xanthogenate (D609) is a synthetic tricyclic compound possessing a xanthate group. This xanthogenate compound is known for its diverse pharmacological properties. Over the last three decades, many studies have reported the biological activities of D609, including antioxidant, antiapoptotic, anticholinergic, anti-tumor, anti-inflammatory, anti-viral, anti-proliferative, and neuroprotective activities. Its mechanism of action is extensively attributed to its ability to cause the competitive inhibition of phosphatidylcholine (PC)-specific phospholipase C (PC-PLC) and sphingomyelin synthase (SMS). The inhibition of PCPLC or SMS affects secondary messengers with a lipidic nature, i.e., 1,2-diacylglycerol (DAG) and ceramide. Various in vitro/in vivo studies suggest that PCPLC and SMS inhibition regulate the cell cycle, block cellular proliferation, and induce differentiation. D609 acts as a pro-inflammatory cytokine antagonist and diminishes Aβ-stimulated toxicity. PCPLC enzymatic activity essentially requires Zn2+, and D609 might act as a potential chelator of Zn2+, thereby blocking PCPLC enzymatic activity. D609 also demonstrates promising results in reducing atherosclerotic plaque formation, post-stroke cerebral infarction, and cancer progression. The present compilation provides a comprehensive mechanistic insight into D609, including its chemistry, mechanism of action, and regulation of various pharmacological activities.

D609 inhibition of phosphatidylcholine-specific phospholipase C attenuates prolonged insulin stimulation-mediated GLUT4 downregulation in 3T3-L1 adipocytes

Glucose uptake is stimulated by insulin via stimulation of glucose transporter 4 (GLUT4) translocation to the plasma membrane from intracellular compartments in adipose tissue and muscles. Insulin stimulation for prolonged periods depletes GLUT4 protein, particularly in highly insulin-responsive GLUT4 storage vesicles. This depletion mainly occurs via H₂O₂-mediated retromer inhibition. However, the post-receptor mechanism of insulin activation of oxidative stress remains unknown. Here, we show that phosphatidylcholine-specific phospholipase C (PC-PLC) plays an important role in insulin-mediated downregulation of GLUT4. In the study, 3T3-L1 adipocytes were exposed to a PC-PLC inhibitor, tricyclodecan-9-yl-xanthogenate (D609), for 30 min prior to the stimulation with 500 nM insulin for 4 h, weakening the depletion of GLUT4. D609 also prevents insulin-driven H₂O₂ generation in 3T3-L1 adipocytes. Exogenous PC-PLC and its product, phosphocholine (PCho), also caused GLUT4 depletion and promoted H₂O₂ generation in 3T3-L1 adipocytes. Furthermore, insulin-mediated the increase in the cellular membrane PC-PLC activity was observed in Amplex Red assays. These results suggested that PC-PLC plays an important role in insulin-mediated downregulation of GLUT4 and that PCho may serve as a signaling molecule.

RNAseq of TGF-β receptor type I kinase-dependent genes in oral fibroblast exposed to milk

Background

Milk is a rich source of natural growth factors that may support oral tissue homeostasis and wound healing. We had shown earlier that blocking TGF-β receptor type I kinase with the inhibitor SB431542 abolished the expression of IL11 and other genes in human gingival fibroblasts exposed to the aqueous fraction of milk. Our aim was to identify the entire signature of TGF-β receptor type I kinase-dependent genes regulated by the aqueous fraction of human milk.

Result

RNAseq revealed 99 genes being strongly regulated by milk requiring activation of the SB431542-dependent TGF-β receptor type I kinase. Among the SB431542-dependent genes is IL11 but also cadherins, claudins, collagens, potassium channels, keratins, solute carrier family proteins, transcription factors, transmembrane proteins, tumor necrosis factor ligand superfamily members, and tetraspanin family members. When focusing on our candidate gene, we could identify D609 to suppress IL11 expression, independent of phospholipase C, sphinosine-1 phosphate synthesis, and Smad-3 phosphorylation and its nuclear translocation. In contrast, genistein and blocking phosphoinositide 3-kinases by wortmannin and LY294002 increased the milk-induced IL11 expression in gingival fibroblasts.

Conclusion

Taken together, our data revealed TGF-β receptor type I kinase signaling to cause major changes of the genetic signature of gingival fibroblasts exposed to aqueous fraction of human milk.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-021-01913-5.

Sphingomyelin synthases 1 and 2 exhibit phosphatidylcholine phospholipase C activity

Many studies have confirmed the enzymatic activity of a mammalian phosphatidylcholine (PC) phospholipase C (PLC) (PC-PLC), which produces diacylglycerol (DAG) and phosphocholine through the hydrolysis of PC in the absence of ceramide. However, the protein(s) responsible for this activity have never yet been identified. Based on the fact that tricyclodecan-9-yl-potassium xanthate can inhibit both PC-PLC and sphingomyelin synthase (SMS) activities, and SMS1 and SMS2 have a conserved catalytic domain that could mediate a nucleophilic attack on the phosphodiester bond of PC, we hypothesized that both SMS1 and SMS2 might have PC-PLC activity. In the present study, we found that purified recombinant SMS1 and SMS2 but not SMS-related protein have PC-PLC activity. Moreover, we prepared liver-specific Sms1/global Sms2 double-KO mice. We found that liver PC-PLC activity was significantly reduced and steady-state levels of PC and DAG in the liver were regulated by the deficiency, in comparison with control mice. Using adenovirus, we expressed Sms1 and Sms2 genes in the liver of the double-KO mice, respectively, and found that expressed SMS1 and SMS2 can hydrolyze PC to produce DAG and phosphocholine. Thus, SMS1 and SMS2 exhibit PC-PLC activity in vitro and in vivo.

Amyloid Beta-Peptide 25-35 (Aβ25-35) Induces Cytotoxicity via Multiple Mechanisms: Roles of the Inhibition of Glucosylceramide Synthase by Aβ25-35 and Its Protection by D609

Sphingolipids (SLs), such as ceramide, glucosylceramide (GlcCer), and sphingomyelin, play important roles in the normal development/functions of the brain and peripheral tissues. Disruption of SL homeostasis in cells/organelles, specifically up-regulation of ceramide, is involved in multiple diseases including Alzheimer's disease (AD). One of the pathological features of AD is aggregates of amyloid beta (Aβ) peptides, and SLs regulate both the formation/aggregation of Aβ and Aβ-induced cellular responses. Up-regulation of ceramide levels via de novo and salvage synthesis pathways is reported in Aβ-treated cells and brains with AD; however, the effects of Aβ on ceramide decomposition pathways have not been elucidated. Thus, we investigated the effects of the 25-35-amino acid Aβ peptide (Aβ_25-35), the fundamental cytotoxic domain of Aβ, on SL metabolism in cells treated with the fluorescent nitrobenzo-2-oxa-1,3-diazole-labeled C6-ceramide (NBD-ceramide). Aβ_25-35 treatment reduced the formation of NBD-GlcCer mediated by GlcCer synthase (GCS) without affecting the formation of NBD-sphingomyelin or NBD-ceramide-1-phosphate, and reduced cell viability. Aβ_25-35-induced responses decreased in cells treated with D609, a putative inhibitor of sphingomyelin synthases. Aβ_25-35-induced cytotoxicity significantly increased in GCS-knockout cells and pharmacological inhibition of GCS alone demonstrated cytotoxicity. Our study revealed that Aβ_25-35-induced cytotoxicity is at least partially mediated by the inhibition of GCS activity.

α-Xanthylmethyl Ketones from α-Diazo ketones

A simple and efficient method to obtain α-xanthylmethyl ketones from α-diazo ketones is described. The reaction proceeds through a protonation/nucleophilic substitution sequence in the presence of p-toluenesulfonic acid and potassium ethyl xanthogenate as the nucleophile. As α-diazo ketones can be readily synthesized from ubiquitous carboxylic acids, a broad variety of xanthates can be obtained, including examples from naturally occurring substrates.

New Molecular Targets for Antidepressant Drugs

Major depressive disorder (MDD) is a common and severe mental disorder that is usually recurrent and has a high risk of suicide. This disorder manifests not only with psychological symptoms but also multiple changes throughout the body, including increased risks of obesity, diabetes, and cardiovascular disease. Peripheral markers of oxidative stress and inflammation are elevated. MDD is therefore best described as a multisystem whole-body disease. Pharmacological treatment with antidepressants usually requires several weeks before the desired effects manifest. Previous theories of depression, such as the monoamine or neurogenesis hypotheses, do not explain these characteristics well. In recent years, new mechanisms of action have been discovered for long-standing antidepressants that also shed new light on depression, including the sphingolipid system and the receptor for brain-derived neurotrophic factor (BDNF).

Sphingomyelin synthase related protein is a mammalian phosphatidylethanolamine phospholipase C

Sphingomyelin synthase related protein (SMSr) has no SM synthase activity but has ceramide phosphorylethanolamine (CPE) synthase activity in vitro. Although SMSr is ubiquitously expressed in all tested tissues, the CPE levels in most mammalian tissues or cells are extremely low or undetectable. Therefore, SMSr seems not to be a functional CPE synthase in vivo and its real biological function needs to be elucidated. In this study, we utilized purified recombinant SMSr and adenovirus-mediated SMSr in vivo expression to show that SMSr has phosphatidylethanolamine phospholipases C (PE-PLC) activity, i.e., it can generate DAG through PE hydrolysis in the absence of ceramide. Further, we found that SMSr has no phosphatidylcholine (PC)-PLC, phosphatidylserine (PS)-PLC, phosphatidylglycerol (PG)-PLC, and phosphatidic phosphatase (PAP) activities, indicating that SMSr-mediated PE-PLC activity has specificity. We conclude that SMSr is a mammalian PE-PLC. Importantly, SMSr can regulate steady state levels of PE in vivo, and it should be a new tool for PE-related biological study.

Sphingomyelin synthase-related protein generates diacylglycerol via the hydrolysis of glycerophospholipids in the absence of ceramide

Diacylglycerol (DG) is a well-established lipid second messenger. Sphingomyelin synthase (SMS)-related protein (SMSr) produces DG and ceramide phosphoethanolamine (CPE) by the transfer of phosphoethanolamine from phosphatidylethanolamine (PE) to ceramide. We previously reported that human SMSr overexpressed in COS-7 cells significantly increased DG levels, particularly saturated and/or monounsaturated fatty acid-containing DG molecular species, and provided DG to DG kinase (DGK) δ, which regulates various pathophysiological events, including epidermal growth factor-dependent cell proliferation, type 2 diabetes, and obsessive-compulsive disorder. However, mammalian SMSr puzzlingly produces only trace amounts of CPE/DG. To clarify this discrepancy, we highly purified SMSr and examined its activities other than CPE synthase. Intriguingly, purified SMSr showed a DG-generating activity via hydrolysis of PE, phosphatidic acid (PA), phosphatidylinositol (PI), and phosphatidylcholine (PC) in the absence of ceramide. DG generation through the PA phosphatase (PAP) activity of SMSr was approximately 300-fold higher than that with PE and ceramide. SMSr hydrolyzed PI ten times stronger than PI(4,5)bisphosphate (PI(4,5)P₂). The PAP and PC-phospholipase C (PLC) activities of SMSr were inhibited by propranolol, a PAP inhibitor, and by D609, an SMS/PC-PLC inhibitor. Moreover, SMSr showed substrate selectivity for saturated and/or monounsaturated fatty acid-containing PA molecular species, but not arachidonic-acid-containing PA, which is exclusively generated in the PI(4,5)P₂ cycle. We confirmed that SMSr expressed in COS-7 cells showed PAP and PI-PLC activities. Taken together, our study indicated that SMSr possesses previously unrecognized enzyme activities, PAP and PI/PE/PC-PLC, and constitutes a novel DG/PA signaling pathway together with DGKδ, which is independent of the PI(4,5)P₂ cycle.

Functional role of phosphatidylcholine-specific phospholipase C in regulating leukotriene synthesis and degranulation in human eosinophils

Phosphatidylinositol-specific phospholipase C (PI-PLC) and cytosolic phospholipase A₂ (cPLA₂) regulate both eosinophil degranulation and leukotriene (LT) synthesis via PI-PLC-mediated calcium influx and cPLA₂ activation. Phosphatidylcholine-specific phospholipase C (PC-PLC) likely plays a key role in cellular signaling, including the eosinophilic allergic inflammatory response. This study examined the role of PC-PLC in eosinophil LT synthesis and degranulation using tricyclodecan-9-yl-xanthogenate (D609), a PC-specific PLC inhibitor. D609 inhibited N-formyl-met-leu-phe + cytochalasin B (fMLP/B)-induced arachidonic acid (AA) release and leukotriene C₄ (LTC₄) secretion. However, at concentrations that blocked both AA release and LTC₄ secretion, D609 had no significant inhibitory effect on stimulated cPLA₂ activity. D609 also partially blocked fMLP/B-induced calcium influx, indicating that inhibition of AA release and LTC₄ secretion by D609 is due to inhibition of calcium-mediated cPLA₂ translocation to intracellular membranes, not inhibition of cPLA₂ activity. In addition, D609 inhibited fMLP/B-stimulated eosinophil peroxidase release, indicating that PC-PLC regulates fMLP/B-induced eosinophil degranulation by increasing the intracellular calcium concentration ([Ca²⁺]_i). Overall, our results showed that PC-PLC is critical for fMLP/B-stimulated eosinophil LT synthesis and degranulation. In addition, degranulation requires calcium influx, while PC-PLC regulates LTC₄ synthesis through calcium-mediated cPLA₂ activation.

Epigallocatechin-3-gallate mobilizes intracellular Ca2+ in prostate cancer cells through combined Ca2+ entry and Ca2+-induced Ca2+ release

AIMS

To elucidate the mechanism by which (-)-epigallocatechin-3-gallate (EGCG) mediates intracellular Ca²⁺ increase in androgen-independent prostate cancer (PCa) cells.

MAIN METHODS

Following exposure to different doses of EGCG, viability of DU145 and PC3 PCa cells was evaluated by MTT assay and the intracellular Ca²⁺ dynamics by the fluorescent Ca²⁺ chelator Fura-2. The expression of different channels was investigated by qPCR analysis and sulfhydryl bonds by Ellman's assay.

KEY FINDINGS

EGCG inhibited DU145 and PC3 proliferation with IC₅₀ = 46 and 56 μM, respectively, and induced dose-dependent peaks of internal Ca²⁺ that were dependent on extracellular Ca²⁺. The expression of TRPC4 and TRPC6 channels was revealed by qPCR in PC3 cells, but lack of effect by modulators and blockers ruled out an exclusive role for these, as well as for voltage-dependent T-type Ca²⁺ channels. Application of dithiothreitol and catalase and sulfhydryl (SH) measurements showed that EGCG-induced Ca²⁺ rise depends on SH oxidation, while the effect of EGTA, dantrolene, and the PLC inhibitor U73122 suggested that EGCG-induced Ca²⁺ influx acts as a trigger for Ca²⁺-induced Ca²⁺ release, involving both ryanodine and IP₃ receptors. Different from EGCG, ATP caused a rapid Ca²⁺ increase, which was independent of external Ca²⁺, but sensitive to U73122.

SIGNIFICANCE

EGCG induces an internal Ca²⁺ increase in PCa cells by a multi-step mechanism. As dysregulation of cytosolic Ca²⁺ is directly linked to apoptosis in PCa cells, these data confirm the possibility of using EGCG as a synergistic adjuvant in combined therapies for recalcitrant malignancies like androgen-independent PCa.

Sphingolipids Modulate Secretion of Glycosylphosphatidylinositol-Anchored Plasmodesmata Proteins and Callose Deposition

Plasma membranes encapsulated in the symplasmic nanochannels of plasmodesmata (PD) contain abundant lipid rafts, which are enriched with sphingolipids (SLs) and sterols. Reduction of sterols has highlighted the role played by lipid raft integrity in the intercellular trafficking of glycosylphosphatidylinositol (GPI)-anchored PD proteins, particularly in affecting callose enhancement. The presence of callose at PD is strongly attributed to the regulation of callose accumulation and callose degradation by callose synthases and β-1,3-glucanases (BGs), respectively. SLs are implicated in signaling and membrane protein trafficking; however, the underlying processes linking SL composition to the control of symplasmic apertures remain unknown. The wide variety of SLs in plants prompted us to investigate which SL molecules are important for regulating symplasmic apertures in Arabidopsis (Arabidopsis thaliana). We introduced several potential SL pathway inhibitors and genetically modified SL contents using two independent SL pathway mutants. We were able to modulate callose deposition to control symplasmic connectivity through perturbations of SL metabolism. Alteration in glucosylhydroxyceramides or related SL composition particularly disturbed the secretory machinery for the GPI-anchored PdBG2 protein, resulting in an overaccumulation of callose. Moreover, our results revealed that SL-enriched lipid rafts link symplasmic channeling to PD callose homeostasis by controlling the targeting of GPI-anchored PdBG2. This study elevates our understanding of the molecular linkage underlying intracellular trafficking and precise targeting of GPI-anchored PD proteins incorporating glucosyl SLs.

Fibulin-1 Integrates Subendothelial Extracellular Matrices and Contributes to Anatomical Closure of the Ductus Arteriosus

OBJECTIVE

The ductus arteriosus (DA) is a fetal artery connecting the aorta and pulmonary arteries. Progressive matrix remodeling, that is, intimal thickening (IT), occurs in the subendothelial region of DA to bring anatomic DA closure. IT is comprised of multiple ECMs (extracellular matrices) and migrated smooth muscle cells (SMCs). Because glycoprotein fibulin-1 binds to multiple ECMs and regulates morphogenesis during development, we investigated the role of fibulin-1 in DA closure. Approach and Results: Fibulin-1-deficient (Fbln1^-/-) mice exhibited patent DA with hypoplastic IT. An unbiased transcriptome analysis revealed that EP4 (prostaglandin E receptor 4) stimulation markedly increased fibulin-1 in DA-SMCs via phospholipase C-NFκB (nuclear factor κB) signaling pathways. Fluorescence-activated cell sorting (FACS) analysis demonstrated that fibulin-1 binding protein versican was derived from DA-endothelial cells (ECs). We examined the effect of fibulin-1 on directional migration toward ECs in association with versican by using cocultured DA-SMCs and ECs. EP4 stimulation promoted directional DA-SMC migration toward ECs, which was attenuated by either silencing fibulin-1 or versican. Immunofluorescence demonstrated that fibulin-1 and versican V0/V1 were coexpressed at the IT of wild-type DA, whereas 30% of versican-deleted mice lacking a hyaluronan binding site displayed patent DA. Fibulin-1 expression was attenuated in the EP4-deficient mouse (Ptger4^-/-) DA, which exhibits patent DA with hypoplastic IT, and fibulin-1 protein administration restored IT formation. In human DA, fibulin-1 and versican were abundantly expressed in SMCs and ECs, respectively.

CONCLUSIONS

Fibulin-1 contributes to DA closure by forming an environment favoring directional SMC migration toward the subendothelial region, at least, in part, in combination with EC-derived versican and its binding partner hyaluronan.

Gold(I)-Catalyzed Cycloisomerization of 3-Alkoxyl-1,6-diynes: A Facile Access to Bicyclo[2.2.1]hept-5-en-2-ones

A novel gold‐catalyzed cycloisomerization of 1,6‐diynes was achieved, providing an atom‐economic approach to a diverse set of bicyclo[2.2.1]hept‐5‐en‐2‐ones in moderate to good yields. With unsymmetrical starting materials with two different internal alkynyl substituents, to some extent, the regioselectivity could be controlled by both electronic and steric factors. This unprecedented reactivity pattern may inspire new and unconventional strategies for the preparation of bridged ring systems.

Diacylglycerol kinase and phospholipase D inhibitors alter the cellular lipidome and endosomal sorting towards the Golgi apparatus

The membrane lipids diacylglycerol (DAG) and phosphatidic acid (PA) are important second messengers that can regulate membrane transport by recruiting proteins to the membrane and by altering biophysical membrane properties. DAG and PA are involved in the transport from the Golgi apparatus to endosomes, and we have here investigated whether changes in these lipids might be important for regulation of transport to the Golgi using the protein toxin ricin. Modulation of DAG and PA levels using DAG kinase (DGK) and phospholipase D (PLD) inhibitors gave a strong increase in retrograde ricin transport, but had little impact on ricin recycling or degradation. Inhibitor treatment strongly affected the endosome morphology, increasing endosomal tubulation and size. Furthermore, ricin was present in these tubular structures together with proteins known to regulate retrograde transport. Using siRNA to knock down different isoforms of PLD and DGK, we found that several isoforms of PLD and DGK are involved in regulating ricin transport to the Golgi. Finally, by performing lipidomic analysis we found that the DGK inhibitor gave a weak, but expected, increase in DAG levels, while the PLD inhibitor gave a strong and unexpected increase in DAG levels, showing that it is important to perform lipidomic analysis when using inhibitors of lipid metabolism.

Antineoplastic Agents Targeting Sphingolipid Pathways

Emerging studies in the enigmatic area of bioactive lipids have made many exciting new discoveries in recent years. Once thought to play a strictly structural role in cellular function, it has since been determined that sphingolipids and their metabolites perform a vast variety of cellular functions beyond what was previously believed. Of utmost importance is their role in cellular signaling, for it is now well understood that select sphingolipids serve as bioactive molecules that play critical roles in both cancer cell death and survival, as well as other cellular responses such as chronic inflammation, protection from intestinal pathogens, and intrinsic protection from intestinal contents, each of which are associated with oncogenesis. Importantly, it has been demonstrated time and time again that many different tumors display dysregulation of sphingolipid metabolism, and the exact profile of said dysregulation has been proven to be useful in determining not only the presence of a tumor, but also the susceptibility to various chemotherapeutic drugs, as well as the metastasizing characteristics of the malignancies. Since these discoveries surfaced it has become apparent that the understanding of sphingolipid metabolism and profile will likely become of great importance in the clinic for both chemotherapy and diagnostics of cancer. The goal of this paper is to provide a comprehensive review of the current state of chemotherapeutic agents that target sphingolipid metabolism that are undergoing clinical trials. Additionally, we will formulate questions involving the use of sphingolipid metabolism as chemotherapeutic targets in need of further research.

D609 protects retinal pigmented epithelium as a potential therapy for age-related macular degeneration

Accumulated oxidative damage may lead to irreversible retinal pigmented epithelium (RPE) cell death, which is considered to be the primary cause of dry age-related macular degeneration (AMD), leading to blindness in the elderly. However, an effective therapy for this disease is lacking. Here, we described a robust high-content screening procedure with a library of 814 protective compounds and found that D609 strongly protected RPE cells from sodium iodate (SI)-induced oxidative cell death and prolonged their healthy survival. D609 effectively attenuated excessive reactive oxygen species (ROS) and prevented severe mitochondrial loss due to oxidative stress in the RPE cells. Surprisingly, the potent antioxidative effects of D609 were not achieved through its own reducibility but were primarily dependent on its ability to increase the expression of metallothionein. The injection of this small water-soluble molecule also showed an explicit protective effect of the RPE layer in an SI-induced AMD mouse model. These findings suggested that D609 could serve as a novel antioxidative protector of RPE cells both in vitro and in vivo and unveiled a novel antioxidative mechanism of D609, which may ultimately have clinical applications for the treatment of AMD.

Potential therapeutic targets for atherosclerosis in sphingolipid metabolism

Sphingolipids, such as sphingomyelins, ceramides, glycosphingolipids, and sphingosine-1-phosphates (S1P) are a large group of structurally and functionally diverse molecules. Some specific species are found associated with atherogenesis and provide novel therapeutic targets. Herein, we briefly review how sphingolipids are implicated in the progression of atherosclerosis and related diseases, and then we discuss the potential therapy options by targetting several key enzymes in sphingolipid metabolism.

Glutathione-mimetic D609 alleviates memory deficits and reduces amyloid-β deposition in an AβPP/PS1 transgenic mouse model

Excessive extracellular deposition of amyloid-β-peptide (Aβ) in the brain is a pathological hallmark of Alzheimer’s disease (AD). Oxidative stress is associated with the onset and progression of AD and contributes to Aβ generation. Tricyclodecan-9-yl-xanthogenate (D609) is a glutathione (GSH)-mimetic compound. Although the antioxidant properties of D609 have been well-studied, its potential therapeutic significance on AD remains unclear. In the present study, we used a mouse model of AD to investigate the effects and the mechanism of action of D609 on AD. We found that D609 treatment significantly improved the spatial learning and alleviated the memory decline in the mice harboring amyloid precursor protein (APP) and presenilin-1 (PS1) double mutations (AβPP/PS1 mice). D609 treatment also increased GSH level, GSH and oxidative glutathione ratio, and superoxide dismutase activity, whereas decreased malondialdehyde and protein carbonyl levels, suggesting that D609 alleviated oxidative stress in AβPP/PS1 mice. In addition, D609 reduced β-secretase 1 level and decreased amyloidogenic processing of AβPP, consequently reducing Aβ deposition in the mice. Thus, our findings suggest that D609 might produce beneficial effects on the prevention and treatment of AD.

Codetermination of Sphingomyelin and Cholesterol in Cellular Plasma Membrane in Sphingomyelin-Depletion-Induced Cholesterol Efflux

Quantification of multiple lipids with different contents in plasma membrane in single cells is significant, but challenging, for investigating lipid interactions and the role of dominant protein transporters. In this paper, comonitoring the alteration of low-content sphingomyelin (SM) and high-content cholesterol in plasma membrane of one living cell is realized by use of luminol electrochemiluminescence (ECL) for the first time. Concentrations of SM as low as 0.5 μM are detected, which permits the measurement of low-content membrane SM in single cells. More membrane cholesterol is observed in individual cells after depletion of membrane SM, providing direct evidence about SM-depletion-induced cholesterol efflux. The upregulation of ATP-binding cassette transporters A1 (ABCA1) and G1 (ABCG1) in SM-depleted cells induces a further increase in membrane cholesterol. These results imply that higher expression of ABCA1/G1 promotes cholesterol trafficking, which offers additional information to solve the debate about ABC transporters in cholesterol efflux. Moreover, the established approach offers a special strategy to investigate the correlation of membrane lipids and the role of transporters in cholesterol trafficking.

Sphingolipids and lipid rafts: Novel concepts and methods of analysis

About twenty years ago, the functional lipid raft model of the plasma membrane was published. It took into account decades of research showing that cellular membranes are not just homogenous mixtures of lipids and proteins. Lateral anisotropy leads to assembly of membrane domains with specific lipid and protein composition regulating vesicular traffic, cell polarity, and cell signaling pathways in a plethora of biological processes. However, what appeared to be a clearly defined entity of clustered raft lipids and proteins became increasingly fluid over the years, and many of the fundamental questions about biogenesis and structure of lipid rafts remained unanswered. Experimental obstacles in visualizing lipids and their interactions hampered progress in understanding just how big rafts are, where and when they are formed, and with which proteins raft lipids interact. In recent years, we have begun to answer some of these questions and sphingolipids may take center stage in re-defining the meaning and functional significance of lipid rafts. In addition to the archetypical cholesterol-sphingomyelin raft with liquid ordered (Lo) phase and the liquid-disordered (Ld) non-raft regions of cellular membranes, a third type of microdomains termed ceramide-rich platforms (CRPs) with gel-like structure has been identified. CRPs are "ceramide rafts" that may offer some fresh view on the membrane mesostructure and answer several critical questions for our understanding of lipid rafts.

Host lipidome analysis during rhinovirus replication in HBECs identifies potential therapeutic targets

In patients with asthma or chronic obstructive pulmonary disease, rhinovirus (RV) infections can provoke acute worsening of disease, and limited treatment options exist. Viral replication in the host cell induces significant remodeling of intracellular membranes, but few studies have explored this mechanistically or as a therapeutic opportunity. We performed unbiased lipidomic analysis on human bronchial epithelial cells infected over a 6 h period with the RV-A1b strain of RV to determine changes in 493 distinct lipid species. Through pathway and network analysis, we identified temporal changes in the apparent activities of a number of lipid metabolizing and signaling enzymes. In particular, analysis highlighted FA synthesis and ceramide metabolism as potential anti-rhinoviral targets. To validate the importance of these enzymes in viral replication, we explored the effects of commercially available enzyme inhibitors upon RV-A1b infection and replication. Ceranib-1, D609, and C75 were the most potent inhibitors, which confirmed that FAS and ceramidase are potential inhibitory targets in rhinoviral infections. More broadly, this study demonstrates the potential of lipidomics and pathway analysis to identify novel targets to treat human disorders.

Xanthates: Metabolism by Flavoprotein-Containing Monooxygenases and Antimycobacterial Activity

Ethionamide (ETH) plays a central role in the treatment of tuberculosis in patients resistant to the first-line drugs. The ETH, thioamide, and thiourea class of antituberculosis agents are prodrugs that are oxidatively converted to their active S-oxides by the mycobacterial flavin-dependent monooxygenase (EtaA) of Mycobacterium tuberculosis, thus initiating the chain of reactions that result in inhibition of mycolic acid biosynthesis and cell lysis. As part of a search for new lead candidates, we report here that several xanthates are oxidized by purified EtaA to S-oxide metabolites (perxanthates), which are implicated in the antimycobacterial activity of these compounds. This process, which is analogous to that responsible for activation of ETH, is also catalyzed by human flavoprotein monooxygenase 3. EtaA was not inhibited in a time-dependent manner during the reaction. Xanthates with longer alkyl chains were oxidized more efficiently. EtaA oxidized octyl-xanthate (K_m = 5 µM; V_max = 1.023 nmolP/min; k_cat = 5.2 molP/min/molE) more efficiently than ETH (194 µM; 1.46 nmolP/min; 7.73 nmolP/min/molE, respectively). Furthermore, the in vitro antimycobacterial activity of four xanthates against M. tuberculosis H37Hv was higher (minimum inhibitory concentration of around 1 µM) than that of ETH (12 µM).

Sphingomyelin Metabolism Is a Regulator of K-Ras Function

K-Ras must localize to the plasma membrane (PM) for biological activity. We show here that multiple acid sphingomyelinase (ASM) inhibitors, including tricyclic antidepressants, mislocalized phosphatidylserine (PtdSer) and K-RasG12V from the PM, resulting in abrogation of K-RasG12V signaling and potent, selective growth inhibition of mutant K-Ras-transformed cancer cells. Concordantly, in nude mice, the ASM inhibitor fendiline decreased the rate of growth of oncogenic K-Ras-expressing MiaPaCa-2 tumors but had no effect on the growth of the wild-type K-Ras-expressing BxPC-3 tumors. ASM inhibitors also inhibited activated LET-60 (a K-Ras ortholog) signaling in Caenorhabditis elegans, as evidenced by suppression of the induced multivulva phenotype. Using RNA interference against C. elegans genes encoding other enzymes in the sphingomyelin (SM) biosynthetic pathway, we identified 14 enzymes whose knockdown strongly or moderately suppressed the LET-60 multivulva phenotype. In mammalian cells, pharmacological agents that target these enzymes all depleted PtdSer from the PM and caused K-RasG12V mislocalization. These effects correlated with changes in SM levels or subcellular distribution. Selected compounds, including sphingosine kinase inhibitors, potently inhibited the proliferation of oncogenic K-Ras-expressing pancreatic cancer cells. In conclusion, these results show that normal SM metabolism is critical for K-Ras function, which may present therapeutic options for the treatment of K-Ras-driven cancers.

Phospholipases: An Overview

Phospholipases are lipolytic enzymes that hydrolyze phospholipid substrates at specific ester bonds. Phospholipases are widespread in nature and play very diverse roles from aggression in snake venom to signal transduction, lipid mediator production, and metabolite digestion in humans. Phospholipases vary considerably in structure, function, regulation, and mode of action. Tremendous advances in understanding the structure and function of phospholipases have occurred in the last decades. This introductory chapter is aimed at providing a general framework of the current understanding of phospholipases and a discussion of their mechanisms of action and emerging biological functions.

Where do substrates of diacylglycerol kinases come from? Diacylglycerol kinases utilize diacylglycerol species supplied from phosphatidylinositol turnover-independent pathways

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol (DG) to produce phosphatidic acid (PA). Mammalian DGK comprises ten isozymes (α-κ) and regulates a wide variety of physiological and pathological events, such as cancer, type II diabetes, neuronal disorders and immune responses. DG and PA consist of various molecular species that have different acyl chains at the sn-1 and sn-2 positions, and consequently, mammalian cells contain at least 50 structurally distinct DG/PA species. Because DGK is one of the components of phosphatidylinositol (PI) turnover, the generally accepted dogma is that all DGK isozymes utilize 18:0/20:4-DG derived from PI turnover. We recently established a specific liquid chromatography-mass spectrometry method to analyze which PA species were generated by DGK isozymes in a cell stimulation-dependent manner. Interestingly, we determined that DGKδ, which is closely related to the pathogenesis of type II diabetes, preferentially utilized 14:0/16:0-, 14:0/16:1-, 16:0/16:0-, 16:0/16:1-, 16:0/18:0- and 16:0/18:1-DG species (X:Y = the total number of carbon atoms: the total number of double bonds) supplied from the phosphatidylcholine-specific phospholipase C pathway, but not 18:0/20:4-DG, in high glucose-stimulated C2C12 myoblasts. Moreover, DGKα mainly consumed 14:0/16:0-, 16:0/18:1-, 18:0/18:1- and 18:1/18:1-DG species during cell proliferation in AKI melanoma cells. Furthermore, we found that 16:0/16:0-PA was specifically produced by DGKζ in Neuro-2a cells during retinoic acid- and serum starvation-induced neuronal differentiation. These results indicate that DGK isozymes utilize a variety of DG molecular species derived from PI turnover-independent pathways as substrates in different stimuli and cells. DGK isozymes phosphorylate various DG species to generate various PA species. It was revealed that the modes of activation of conventional and novel protein kinase isoforms by DG molecular species varied considerably. However, PA species-selective binding proteins have not been found to date. Therefore, we next attempted to identify PA species-selective binding proteins from the mouse brain and identified α-synuclein, which has causal links to Parkinson's disease. Intriguingly, we determined that among phospholipids, including several PA species (16:0/16:0-PA, 16:0/18:1-PA, 18:1/18:1-PA, 18:0/18:0-PA and 18:0/20:4-PA); 18:1/18:1-PA was the most strongly bound PA to α-synuclein. Moreover, 18:1/18:1-PA strongly enhanced secondary structural changes from the random coil form to the α-helix form and generated a multimeric and proteinase K-resistant α-synuclein protein. In contrast with the dogma described above, our recent studies strongly suggest that PI turnover-derived DG species and also various DG species derived from PI turnover-independent pathways are utilized by DGK isozymes. DG species supplied from distinct pathways may be utilized by DGK isozymes based on different stimuli present in different types of cells, and individual PA molecular species would have specific targets and exert their own physiological functions.

Phosphatidylcholine-specific phospholipase C inhibition down- regulates CXCR4 expression and interferes with proliferation, invasion and glycolysis in glioma cells

Background

The chemokine receptor CXCR4 plays a crucial role in tumors, including glioblastoma multiforme (GBM), the most aggressive glioma.

Phosphatidylcholine-specific phospholipase C (PC-PLC), a catabolic enzyme of PC metabolism, is involved in several aspects of cancer biology and its inhibition down-modulates the expression of growth factor membrane receptors interfering with their signaling pathways.

In the present work we investigated the possible interplay between CXCR4 and PC-PLC in GBM cells.

Methods

Confocal microscopy, immunoprecipitation, western blot analyses, and the evaluation of migration and invasion potential were performed on U87MG cells after PC-PLC inhibition with the xanthate D609. The intracellular metabolome was investigated by magnetic resonance spectroscopy; lactate levels and lactate dehydrogenase (LDH) activity were analyzed by colorimetric assay.

Results

Our studies demonstrated that CXCR4 and PC-PLC co-localize and are associated on U87MG cell membrane. D609 reduced CXCR4 expression, cell proliferation and invasion, interfering with AKT and EGFR activation and expression. Metabolic analyses showed a decrease in intracellular lactate concentration together with a decrement in LDH activity.

Conclusions

Our data suggest that inhibition of PC-PLC could represent a new molecular approach in glioma biology not only for its ability in modulating cell metabolism, glioma growth and motility, but also for its inhibitory effect on crucial molecules involved in cancer progression.

Lysophosphatidic acid-induced itch is mediated by signalling of LPA5 receptor, phospholipase D and TRPA1/TRPV1

KEY POINTS

Lysophosphatidic acid (LPA) is an itch mediator, but not a pain mediator by a cheek injection model. Dorsal root ganglion neurons directly respond to LPA depending on transient receptor potential ankyrin 1 (TRPA1) and vanilloid 1 (TRPV1). LPA-induced itch-related behaviours are decreased in TRPA1-knockout (KO), TRPV1KO or TRPA1TRPV1 double KO mice. TRPA1 and TRPV1 channels are activated by intracellular LPA, but not by extracellular LPA following LPA₅ receptor activation with an activity of Ca²⁺ -independent phospholipase A₂ and phospholipase D. Intracellular LPA interaction sites of TRPA1 are KK672-673 and KR977-978 (K: lysine, R: arginine).

ABSTRACT

Intractable and continuous itch sensations often accompany diseases such as atopic dermatitis, neurogenic lesions, uremia and cholestasis. Lysophosphatidic acid (LPA) is an itch mediator found in cholestatic itch patients and it induces acute itch and pain in experimental rodent models. However, the molecular mechanism by which LPA activates peripheral sensory neurons remains unknown. In this study, we used a cheek injection method in mice to reveal that LPA induced itch-related behaviours but not pain-related behaviours. The LPA-induced itch behaviour and cellular effects were dependent on transient receptor potential ankyrin 1 (TRPA1) and vanilloid 1 (TRPV1), which are important for itch signal transduction. We also found that, among the six LPA receptors, the LPA₅ receptor had the greatest involvement in itching. Furthermore, we demonstrated that phospholipase D (PLD) plays a critical role downstream of LPA₅ and that LPA directly and intracellularly activates TRPA1 and TRPV1. These results suggest a unique mechanism by which cytoplasmic LPA produced de novo could activate TRPA1 and TRPV1. We conclude that LPA-induced itch is mediated by LPA₅ , PLD, TRPA1 and TRPV1 signalling, and thus targeting TRPA1, TRPV1 or PLD could be effective for cholestatic itch interventions.

A Role for Ceramides, but Not Sphingomyelins, as Antagonists of Insulin Signaling and Mitochondrial Metabolism in C2C12 Myotubes

The accumulation of sphingolipids in obesity leads to impairments in insulin sensitivity and mitochondrial metabolism, but the precise species driving these defects is unclear. We have modeled these obesity-induced effects in cultured C2C12 myotubes, using BSA-conjugated palmitate to increase synthesis of endogenous sphingolipids and to inhibit insulin signaling and oxidative phosphorylation. Palmitate (a) induced the accumulation of sphingomyelin (SM) precursors such as sphinganine, dihydroceramide, and ceramide; (b) inhibited insulin stimulation of a central modulator of anabolic metabolism, Akt/PKB; (c) inhibited insulin-stimulated glycogen synthesis; and (d) decreased oxygen consumption and ATP synthesis. Under these conditions, palmitate failed to alter levels of SMs, which are the most abundant sphingolipids, suggesting that they are not the primary intermediates accounting for the deleterious palmitate effects. Treating cells with a pharmacological inhibitor of SM synthase or using CRISPR to knock out the Sms2 gene recapitulated the palmitate effects by inducing the accumulation of SM precursors and impairing insulin signaling and mitochondrial metabolism. To profile the sphingolipids that accumulate in obesity, we performed lipidomics on quadriceps muscles from obese mice with impaired glucose tolerance. Like the cultured myotubes, these tissues accumulated ceramides but not SMs. Collectively, these data suggest that SM precursors such as ceramides, rather than SMs, are likely nutritional antagonists of metabolic function in skeletal muscle.

Cell lipid metabolism modulators 2-bromopalmitate, D609, monensin, U18666A and probucol shift discoidal HDL formation to the smaller-sized particles: implications for the mechanism of HDL assembly

ATP-binding cassette transporter A1 (ABCA1) mediates formation of disc-shaped high-density lipoprotein (HDL) from cell lipid and lipid-free apolipoprotein A-I (apo A-I). Discoidal HDL particles are heterogeneous in physicochemical characteristics for reasons that are understood incompletely. Discoidal lipoprotein particles similar in characteristics and heterogeneity to cell-formed discoidal HDL can be reconstituted from purified lipids and apo A-I by cell-free, physicochemical methods. The heterogeneity of reconstituted HDL (rHDL) is sensitive to the lipid composition of the starting lipid/apo A-I mixture. To determine whether the heterogeneity of cell-formed HDL is similarly sensitive to changes in cell lipids, we investigated four compounds that have well-established effects on cell lipid metabolism and ABCA1-mediated cell cholesterol efflux. 2-Bromopalmitate, D609, monensin and U18666A decreased formation of the larger-sized, but dramatically increased formation of the smaller-sized HDL. 2-Bromopalmitate did not appear to affect ABCA1 activity, subcellular localization or oligomerization, but induced dissolution of the cholesterol-phospholipid complexes in the plasma membrane. Arachidonic and linoleic acids shifted HDL formation to the smaller-sized species. Tangier disease mutations and inhibitors of ABCA1 activity wheat germ agglutinin and AG 490 reduced formation of both larger-sized and smaller-sized HDL. The effect of probucol was similar to the effect of 2-bromopalmitate. Taking rHDL formation as a paradigm, we propose that ABCA1 mutations and activity inhibitors reduce the amount of cell lipid available for HDL formation, and the compounds in the 2-bromopalmitate group and the polyunsaturated fatty acids change cell lipid composition from one that favors formation of the larger-sized HDL particles to one that favors formation of the smaller-sized species.

An asymmetric approach to bicyclo[2.2.1]heptane-1-carboxylates via a formal [4 + 2] cycloaddition reaction enabled by organocatalysis

An organocatalytic formal [4 + 2] cycloaddition reaction has been realized that permits rapid access to a wide range of bicyclo[2.2.1]heptane-1-carboxylates in a highly enantioselective manner from simple starting materials under mild and operationally simple conditions.