Saturated fatty acid- and/or monounsaturated fatty acid-containing phosphatidic acids selectively interact with heat shock protein 27.

Diacylglycerol kinase (DGK) α, which is a key enzyme in the progression of cancer and, in contrast, in T-cell activity attenuation, preferentially produces saturated fatty acid (SFA)- and/or monounsaturated fatty acid (MUFA)-containing phosphatidic acids (PAs), such as 16:0/16:0-, 16:0/18:0-, and 16:1/16:1-PA, in melanoma cells. In the present study, we searched for the target proteins of 16:0/16:0-PA in melanoma cells and identified heat shock protein (HSP) 27, which acts as a molecular chaperone and contributes to cancer progression. HSP27 more strongly interacted with PA than other phospholipids, including phosphatidylcholine, phosphatidylserine, phosphatidylglycerol, cardiolipin, phosphatidylinositol, phosphatidylinositol 4-monophosphate, and phosphatidylinositol 4,5-bisphosphate. Moreover, HSP27 is more preferentially bound to SFA- and/or MUFA-containing PAs, including 16:0/16:0- and 16:0/18:1-PAs, than PUFA-containing PAs, including 18:0/20:4- and 18:0/22:6-PA. Furthermore, HSP27 and constitutively active DGKα expressed in COS-7 cells colocalized in a DGK activity-dependent manner. Notably, 16:0/16:0-PA, but not phosphatidylcholine or 16:0/16:0-phosphatidylserine, induced oligomer dissociation of HSP27, which enhances its chaperone activity. Intriguingly, HSP27 protein was barely detectable in Jurkat T cells, while the protein band was intensely detected in AKI melanoma cells. Taken together, these results strongly suggest that SFA- and/or MUFA-containing PAs produced by DGKα selectively target HSP27 and regulate its cancer-progressive function in melanoma cells but not in T cells.

Downregulation of diacylglycerol kinase delta contributes to hyperglycemia-induced insulin resistance.

Type 2 (non-insulin-dependent) diabetes mellitus is a progressive metabolic disorder arising from genetic and environmental factors that impair beta cell function and insulin action in peripheral tissues. We identified reduced diacylglycerol kinase delta (DGKdelta) expression and DGK activity in skeletal muscle from type 2 diabetic patients. In diabetic animals, reduced DGKdelta protein and DGK kinase activity were restored upon correction of glycemia. DGKdelta haploinsufficiency increased diacylglycerol content, reduced peripheral insulin sensitivity, insulin signaling, and glucose transport, and led to age-dependent obesity. Metabolic flexibility, evident by the transition between lipid and carbohydrate utilization during fasted and fed conditions, was impaired in DGKdelta haploinsufficient mice. We reveal a previously unrecognized role for DGKdelta in contributing to hyperglycemia-induced peripheral insulin resistance and thereby exacerbating the severity of type 2 diabetes. DGKdelta deficiency causes peripheral insulin resistance and metabolic inflexibility. These defects in glucose and energy homeostasis contribute to mild obesity later in life.

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.

Docosahexaenoic acid-containing phosphatidic acid interacts with clathrin coat assembly protein AP180 and regulates its interaction with clathrin.

The clathrin coat assembly protein AP180 drives endocytosis, which is crucial for numerous physiological events, such as the internalization and recycling of receptors, uptake of neurotransmitters and entry of viruses, including SARS-CoV-2, by interacting with clathrin. Moreover, dysfunction of AP180 underlies the pathogenesis of Alzheimer's disease. Therefore, it is important to understand the mechanisms of assembly and, especially, disassembly of AP180/clathrin-containing cages. Here, we identified AP180 as a novel phosphatidic acid (PA)-binding protein from the mouse brain. Intriguingly, liposome binding assays using various phospholipids and PA species revealed that AP180 most strongly bound to 1-stearoyl-2-docosahexaenoyl-PA (18:0/22:6-PA) to a comparable extent as phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), which is known to associate with AP180. An AP180 N-terminal homology domain (1-289 aa) interacted with 18:0/22:6-PA, and a lysine-rich motif (K38-K39-K40) was essential for binding. The 18:0/22:6-PA in liposomes in 100 nm diameter showed strong AP180-binding activity at neutral pH. Notably, 18:0/22:6-PA significantly attenuated the interaction of AP180 with clathrin. However, PI(4,5)P2 did not show such an effect. Taken together, these results indicate the novel mechanism by which 18:0/22:6-PA selectively regulates the disassembly of AP180/clathrin-containing cages.

Diacylglycerol kinase η regulates cell proliferation and its levels are elevated by glucocorticoids in undifferentiated neuroblastoma cells.

Knockout mice of diacylglycerol kinase (DGK) η, which has been repeatedly suggested to be associated with bipolar disorder (BPD) by genome-wide association studies, exhibited abnormal behaviors similar to the manic phase of BPD. Chronic stress is also linked to changes in mood symptoms, including BPD. In the present study, we analyzed the effects of the glucocorticoid stress hormones, triamcinolone acetonide (TAA) and dexamethasone (DEX), on DGKη protein levels in neuroblastoma cell lines, Neuro-2a and SH-SY5Y. The protein levels of DGKη were significantly increased in the undifferentiated Neuro-2a and SH-SY5Y cells by TAA and DEX, but not in the differentiated neuroblastoma cells. To assess the functions of DGKη in undifferentiated neuroblastoma cells, we established DGKη-deficient SH-SY5Y cells using the clustered regularly interspaced palindromic repeat/caspase 9 system. Notably, proliferation of DGKη-deficient SH-SY5Y cells was markedly attenuated, concomitant with the decrease in levels of phosphorylated extracellular signal-regulated kinase. Taken together, these results suggest that DGKη levels are increased in undifferentiated neuroblastoma cells by glucocorticoid stress hormones and regulate cell proliferation.

Combination therapy for hepatocellular carcinoma with diacylglycerol kinase alpha inhibition and anti-programmed cell death-1 ligand blockade.

Activation of diacylglycerol kinase alpha (DGKα) augments proliferation and suppresses apoptosis of cancer cells and induces T lymphocyte anergy. We investigated the dual effects of DGKα inhibition on tumorigenesis and anti-tumor immunity with the aim of establishing a novel therapeutic strategy for cancer. We examined the effects of a DGKα inhibitor (DGKAI) on liver cancer cell proliferation and cytokine production by immune cells in vitro and on tumorigenesis and host immunity in a hepatocellular carcinoma (HCC) mouse model. Oral DGKAI significantly suppressed tumor growth and prolonged survival in model mice. Tumor infiltration of T cells and dendritic cells was also enhanced in mice treated with DGKAI, and the production of cytokines and cytotoxic molecules by CD4+ and CD8+ T cells was increased. Depletion of CD8+ T cells reduced the effect of DGKAI. Furthermore, interferon-γ stimulation augmented the expression of programmed cell death-1 ligand (PD-L1) on cancer cells, and DGKAI plus an anti-PD-L1 antibody strongly suppressed the tumor growth. These results suggest that DGKα inhibition may be a promising new treatment strategy for HCC.

Membrane morphology determines diacylglycerol kinase α substrate acyl chain specificity.

Diacylglycerol kinases catalyze the ATP-dependent phosphorylation of diacylglycerol (DAG) to produce phosphatidic acid (PA). In humans, the alpha isoform (DGKα) has emerged as a potential target in the treatment of cancer due to its anti-tumor and pro-immune responses. However, its mechanism of action at a molecular level is not fully understood. In this work, a systematic investigation of the role played by the membrane in the regulation of the enzymatic properties of human DGKα is presented. By using a cell-free system with purified DGKα and model membranes of variable physical and chemical properties, it is shown that membrane physical properties determine human DGKα substrate acyl chain specificity. In model membranes with a flat morphology; DGKα presents high enzymatic activity, but it is not able to differentiate DAG molecular species. Furthermore, DGKα enzymatic properties are insensitive to membrane intrinsic curvature. However, in the presence of model membranes with altered morphology, specifically the presence of physically curved membrane structures, DGKα bears substrate acyl chain specificity for palmitic acid-containing DAG. The present results identify changes in membrane morphology as one possible mechanism for the depletion of specific pools of DAG as well as the production of specific pools of PA by DGKα, adding an extra layer of regulation on the interconversion of these two potent lipid-signaling molecules. It is proposed that the interplay between membrane physical (shape) and chemical (lipid composition) properties guarantee a fine-tuned signal transduction system dependent on the levels and molecular species of DAG and PA.

Diacylglycerol kinase δ functions as a proliferation suppressor in pancreatic ß-cells.

Although an aberrant reduction in pancreatic ß-cell mass contributes to the pathogenesis of diabetes, the mechanism underlying the regulation of ß-cell mass is poorly understood. Here, we show that diacylglycerol kinase δ (DGKδ) is a key enzyme in the regulation of ß-cell mass. DGKδ expression was detected in the nucleus of ß-cells. We developed ß-cell-specific DGKδ knockout (ßDGKδ KO) mice, which showed lower blood glucose, higher plasma insulin levels, and better glucose tolerance compared to control mice. Moreover, an increased number of small islets and Ki-67-positive islet cells, as well as elevated cyclin B1 expression in the islets, were detected in the pancreas of ßDGKδ KO mice. DGKδ knockdown in the ß-cell line MIN6 induced significant increases in bromodeoxyuridine (BrdU) incorporation and cyclin B1 expression. Finally, we confirmed that streptozotocin-induced hyperglycemia and ß-cell loss were alleviated in ßDGKδ KO mice. Thus, suppressing the expression or enzymatic activity of DGKδ that functions as a suppressor of ß-cell proliferation could be a novel therapeutic approach to increase ß-cell mass for the treatment of diabetes.

Diacylglycerol kinase δ and sphingomyelin synthase-related protein functionally interact via their sterile α motif domains.

The δ isozyme of diacylglycerol kinase (DGKδ) plays critical roles in lipid signaling by converting diacylglycerol (DG) to phosphatidic acid (PA). We previously demonstrated that DGKδ preferably phosphorylates palmitic acid (16:0)- and/or palmitoleic acid (16:1)-containing DG molecular species, but not arachidonic acid (20:4)-containing DG species, which are recognized as DGK substrates derived from phosphatidylinositol turnover, in high glucose-stimulated myoblasts. However, little is known about the origin of these DG molecular species. DGKδ and two DG-generating enzymes, sphingomyelin synthase (SMS) 1 and SMS-related protein (SMSr), contain a sterile α motif domain (SAMD). In this study, we found that SMSr-SAMD, but not SMS1-SAMD, co-immunoprecipitates with DGKδ-SAMD. Full-length DGKδ co-precipitated with full-length SMSr more strongly than with SMS1. However, SAMD-deleted variants of SMSr and DGKδ interacted only weakly with full-length DGKδ and SMSr, respectively. These results strongly suggested that DGKδ interacts with SMSr through their respective SAMDs. To determine the functional outcomes of the relationship between DGKδ and SMSr, we used LC-MS/MS to investigate whether overexpression of DGKδ and/or SMSr in COS-7 cells alters the levels of PA species. We found that SMSr overexpression significantly enhances the production of 16:0- or 16:1-containing PA species such as 14:0/16:0-, 16:0/16:0-, 16:0/18:1-, and/or 16:1/18:1-PA in DGKδ-overexpressing COS-7 cells. Moreover, SMSr enhanced DGKδ activity via their SAMDs in vitro Taken together, these results strongly suggest that SMSr is a candidate DG-providing enzyme upstream of DGKδ and that the two enzymes represent a new pathway independent of phosphatidylinositol turnover.

Combined inhibition/silencing of diacylglycerol kinase α and ζ simultaneously and synergistically enhances interleukin-2 production in T cells and induces cell death of melanoma cells.

The α-isozyme of diacylglycerol kinase (DGK) enhances cancer cell proliferation and, conversely, it promotes the nonresponsive immune state known as T-cell anergy. Moreover, a DGKα-selective inhibitor, CU-3, induced cell death in cancer-derived cells and simultaneously enhanced T-cell interleukin-2 production. In addition to DGKα, DGKζ is also known to induce T-cell anergy. In the present study, we examined whether combined inhibition/silencing of DGKα and DGKζ synergistically enhanced T-cell activity. Combined treatment with CU-3 or DGKα-small interfering RNA (siRNA) and DGKζ-siRNA more potently enhanced T-cell receptor-crosslink-dependent interleukin-2 production in Jurkat T cells than treatment with either alone. Intriguingly, in addition to activating T cells, dual inhibition/silencing of DGKα and DGKζ synergistically reduced viability and increased caspase 3/7 activity in AKI melanoma cells. Taken together, these results indicate that combined inhibition/silencing of DGKα and DGKζ simultaneously and synergistically enhances interleukin-2 production in T cells and induces cell death in melanoma. Therefore, dual inhibition/silencing of these DGK isozymes represents an ideal therapy that potently attenuates cancer cell proliferation and simultaneously enhances immune responses that impact anticancer immunity.

Palmitic acid- and/or palmitoleic acid-containing phosphatidic acids are generated by diacylglycerol kinase α in starved Jurkat T cells.

Diacylglycerol kinase (DGK) α enhances the proliferation of melanoma and hepatocellular carcinoma cells whereas, in contrast, DGKα induces a nonproliferative state in T cells. We previously found that DGKα produces palmitic acid (16:0)-containing PA species, such as 16:0/16:0- and 16:0/18:0-PA, in melanoma cells under serum-starved (nonproliferative) conditions. In the present study, we identified the PA species generated by DGKα in T cells under serum-starved (nonproliferative) conditions. We found that serum starvation markedly increased the levels of many PA species, such as 14:1/16:1-, 14:0/16:1-, 14:0/16:0-, 16:1/16:2-, 16:1/16:1-, 16:0/16:1-, 16:0/16:0-, 16:1/18:2-, 16:1/18:1-, 16:0/18:1-, 16:0/18:0-, 18:1/18:2-, 18:1/18:1- and 18:0/18:1-PA, in Jurkat T cells. In lysates from serum-starved Jurkat T cells, DGKα activity, which was Ca2+-dependent and sensitive to a DGKα-specific inhibitor (CU-3), was substantially increased, indicating its activation. Moreover, CU-3 (1-10 µM) significantly reduced the amounts of palmitic acid- and/or palmitoleic acid (16:1)-containing PA species, such as 14:1/16:1-, 14:0/16:1-, 14:0/16:0-, 16:1/16:2-, 16:1/16:1-, 16:0/16:1-, 16:0/16:0-, 16:0/18:1- and 16:0/18:0-PA, which were increased by serum starvation. These results indicate that DGKα generates different PA species in starved melanoma cells (palmitic acid-containing PA species) and T cells (palmitic acid- and/or palmitoleic acid (16:1)-containing PA species). Therefore, the differences in the PA molecular species may account for the opposing functions of DGKα in melanoma and T cells.

New Era of Diacylglycerol Kinase, Phosphatidic Acid and Phosphatidic Acid-Binding Protein.

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol (DG) to generate phosphatidic acid (PA). Mammalian DGK consists of ten isozymes (α-κ) and governs a wide range of physiological and pathological events, including immune responses, neuronal networking, bipolar disorder, obsessive-compulsive disorder, fragile X syndrome, cancer, and type 2 diabetes. DG and PA comprise diverse molecular species that have different acyl chains at the sn-1 and sn-2 positions. Because the DGK activity is essential for phosphatidylinositol turnover, which exclusively produces 1-stearoyl-2-arachidonoyl-DG, it has been generally thought that all DGK isozymes utilize the DG species derived from the turnover. However, it was recently revealed that DGK isozymes, except for DGKε, phosphorylate diverse DG species, which are not derived from phosphatidylinositol turnover. In addition, various PA-binding proteins (PABPs), which have different selectivities for PA species, were recently found. These results suggest that DGK-PA-PABP axes can potentially construct a large and complex signaling network and play physiologically and pathologically important roles in addition to DGK-dependent attenuation of DG-DG-binding protein axes. For example, 1-stearoyl-2-docosahexaenoyl-PA produced by DGKδ interacts with and activates Praja-1, the E3 ubiquitin ligase acting on the serotonin transporter, which is a target of drugs for obsessive-compulsive and major depressive disorders, in the brain. This article reviews recent research progress on PA species produced by DGK isozymes, the selective binding of PABPs to PA species and a phosphatidylinositol turnover-independent DG supply pathway.

Diacylglycerol kinase α-selective inhibitors induce apoptosis and reduce viability of melanoma and several other cancer cell lines.

Diacylglycerol (DG) kinase (DGK), which phosphorylates DG to generate phosphatidic acid (PA), consists of ten isozymes (α-к). Recently, we identified a novel small molecule inhibitor, CU-3, that selectively inhibits the activity of the α isozyme. In addition, we newly obtained Compound A, which selectively and strongly inhibits type I DGKs (α, ß, and γ). In the present study, we demonstrated that both CU-3 and Compound A induced apoptosis (caspase 3/7 activity and DNA fragmentation) and viability reduction of AKI melanoma cells. Liquid chromatography-mass spectrometry revealed that the production of 32:0- and 34:0-PA species was commonly attenuated by CU-3 and Compound A, suggesting that lower levels of these PA molecular species are involved in the apoptosis induction and viability reduction of AKI cells. We determined the effects of the DGKα inhibitors on several other cancer cell lines derived from refractory cancers. In addition to melanoma, the DGKα inhibitors enhanced caspase 3/7 activity and reduced the viability of hepatocellular carcinoma, glioblastoma, and pancreatic cancer cells, but not breast adenocarcinoma cells. Interestingly, Western blot analysis indicated that the DGKα expression levels were positively correlated with the sensitivity to the DGK inhibitors. Because both CU-3 and Compound A induced interleukin-2 production by T cells, it is believed that these two compounds can enhance cancer immunity. Taken together, our results suggest that DGKα inhibitors are promising anticancer drugs.

Creatine kinase muscle type specifically interacts with saturated fatty acid- and/or monounsaturated fatty acid-containing phosphatidic acids.

Diacylglycerol kinase (DGK) δ, which is a key enzyme in the pathogenesis of type 2 diabetes (T2D), preferentially generates saturated fatty acid (SFA)- and/or monounsaturated fatty acid (MUFA)-containing phosphatidic acids (PAs) such as 16:0/16:0-PA and 16:0/18:1-PA, but not polyunsaturated fatty acid (PUFA)-containing PAs, in glucose-stimulated myoblast cells. Here, we searched for the target proteins of 16:0/16:0-PA in the mouse skeletal muscle and identified an energy metabolizing enzyme, creatine kinase muscle type (CKM), which is correlated with T2D. CKM bound to 16:0/16:0-PA with the highest affinity (dissociation constant: 2.0 µM) among all the PA-binding proteins reported thus far. Intriguingly, CKM preferentially interacted with SFA- and/or MUFA-containing PAs, but not with PUFA-containing PAs. Notably, CKM exclusively interacted with PA, whereas the protein did not bind to other lipids such as diacylglycerol, phosphatidylserine, phosphatidylglycerol, phosphatidylinositol, phosphatidylinositol (3,4,5)-trisphosphate and cardiolipin. Taken together, these results demonstrate that CKM is a very unique PA-binding protein that possesses exceedingly high affinity for PA, exceptional preference for SFA/MUFA-PA and extremely high specificity to PA and suggest that SFA/MUFA-PAs produced by DGKδ are novel regulators of CKM function.

Abnormalities of the serotonergic system in diacylglycerol kinase δ-deficient mouse brain.

We previously reported that brain-specific diacylglycerol kinase (DGK) δ-knockout (KO) mice showed obsessive-compulsive disorder (OCD)-like behaviors, which were alleviated by a serotonin (5-HT) transporter (SERT) inhibitor. However, the molecular mechanisms causing the OCD-like abnormal behaviors remain unclear. In the present study, we found that DGKδ deficiency increased SERT protein levels in the mouse cerebral cortex. Moreover, DGKδ interacted and co-localized with SERT. Furthermore, DGKδ-KO decreased tryptophan hydroxylase-2 expression and increased monoamine oxidase-A expression. Indeed, the amount of 5-HT in the cerebral cortex was significantly decreased in DGKδ-KO mice. These data strongly suggest that OCD-like behaviors in the DGKδ-KO mice are caused by comprehensive and composite serotonergic hypofunction.

Diacylglycerol kinase ß induces filopodium formation via its C1, catalytic and carboxy-terminal domains and interacts with the Rac1-GTPase-activating protein, ß2-chimaerin.

The ß-isoform of diacylglycerol kinase (DGK) localizes predominantly to neurons and induces neurite outgrowth and spine formation. However, the detailed molecular mechanisms underlying the functions of DGKß remain elusive. During the course of studies on other DGK isozymes, we unexpectedly found that the overexpression of wild-type DGKß in COS-7 cells markedly induced filopodium formation. Because filopodium formation is closely related to neurite outgrowth and spine formation, we constructed various DGKß mutants and compared their abilities to induce filopodium formation in order to elucidate the structure-function relationships of DGKß. We found that the C-terminal, C1 and catalytic domains and catalytic activity were indispensable for filopodium formation, but the recoverin homology domain and EF-hand motifs were not. Moreover, the extent of plasma membrane localization and F-actin colocalization were positively correlated with filopodium formation. Intriguingly, DGKß selectively interacted and colocalized at the plasma membrane with a Rac1-GTPase-activating protein, ß2-chimaerin, which is an inducer of filopodia; it also interacted, to lesser extent, with α2-chimaerin, but not with α1- or ß1-chimaerin. Moreover, DGKß enhanced the plasma membrane localization of ß2-chimaerin. These results suggest that DGKß plays an important role in neurite outgrowth and spine formation in neurons via its ability to induce filopodium formation.

Chronic administration of myristic acid improves hyperglycaemia in the Nagoya-Shibata-Yasuda mouse model of congenital type 2 diabetes.

AIMS/HYPOTHESIS: Previously, we demonstrated that myristic acid (14:0) increases levels of diacylglycerol kinase (DGK) δ, a key enzyme involved in type 2 diabetes exacerbation, and enhances glucose uptake in C2C12 myotube cells. Moreover, results from a population-based cohort study suggest that consumption of high-fat dairy products, which contain high amounts of myristic acid, is associated with a lower risk of developing type 2 diabetes. Taken together, we hypothesised that intake of myristic acid reduces type 2 diabetes risk in vivo. The aim of this study was to examine the glucose-lowering effect of myristic acid in Nagoya-Shibata-Yasuda (NSY) mice, a spontaneous model for studying obesity-related type 2 diabetes. METHODS: Male NSY mice were orally administered vehicle (n = 9), 300 mg/kg of myristic acid (n = 14) or 300 mg/kg of palmitic acid (16:0) (n = 9) every other day from 4 weeks of age. Glucose and insulin tolerance tests were performed at weeks 18, 24 and 30, and weeks 20 and 26, respectively. DGKδ levels were measured in skeletal muscle from 32-36-week-old NSY mice via western blot. RESULTS: Chronic oral administration of myristic acid ameliorated glucose tolerance (24-28% decrease in blood glucose levels during glucose tolerance tests) and reduced insulin-responsive blood glucose levels (~20% decrease) in male NSY mice compared with vehicle and palmitic acid groups at 24-30 weeks of age (the age at which the severity of type 2 diabetes is exacerbated in NSY mice). Myristic acid also attenuated the increase in body weight seen in NSY mice. Furthermore, the fatty acid increased DGKδ levels (~1.6-fold) in skeletal muscle of NSY mice. CONCLUSIONS/INTERPRETATION: These results suggest that the chronic oral administration of myristic acid improves hyperglycaemia by decreasing insulin-responsive glucose levels and reducing body weight, and that the fatty acid accounts for the diabetes protective properties of high-fat dairy products. Myristic acid is a potential candidate for the prevention and treatment of type 2 diabetes mellitus and its related diseases.

The Pleckstrin Homology Domain of Diacylglycerol Kinase η Strongly and Selectively Binds to Phosphatidylinositol 4,5-Bisphosphate.

Type II diacylglycerol kinase (DGK) isozymes (δ, η, and κ) have a pleckstrin homology domain (PH) at their N termini. Here, we investigated the lipid binding properties of the PHs of type II DGK isozymes using protein-lipid overlay and liposome binding assays. The PH of DGKη showed the most pronounced binding activity to phosphatidylinositol (PI) 4,5-bisphosphate (PI(4,5)P2) among the various glycero- and sphingolipids including PI 3,4,5-trisphosphate, PI 3,4-bisphosphate, PI 3-phosphate, PI 4-phosphate, and PI 5-phosphate. Moreover, the PI(4,5)P2binding activity of the DGKη-PH was significantly stronger than that of other type II DGK isozymes. Notably, compared with the PH of phospholipase C (PLC) δ1, which is generally utilized as a cellular PI(4,5)P2- probe, the DGKη-PH is equal to or superior than the PLCδ1-PH in terms of affinity and selectivity for PI(4,5)P2 Furthermore, in COS-7 cells, GFP-fused wild-type DGKη1 and its PH partly translocated from the cytoplasm to the plasma membrane where the PLCδ1-PH was co-localized in response to hyperosmotic stress in an inositol 5-phosphatase-sensitive manner, whereas a PH deletion mutant did not. Moreover, K74A and R85A mutants of DGKη-PH, which lack the conserved basic amino acids thought to ligate PI(4,5)P2, were indeed unable to bind to PI(4,5)P2and co-localize with the PLCδ1-PH even in osmotically shocked cells. Overexpression of wild-type DGKη1 enhanced EGF-dependent phosphorylation of ERK, whereas either K74A or R85A mutant did not. Taken together, these results indicate that the DGKη-PH preferentially interacts with PI(4,5)P2and has crucial roles in regulating the subcellular localization and physiological function of DGKη. Moreover, the DGKη-PH could serve as an excellent cellular sensor for PI(4,5)P2.

Distinct 1-monoacylglycerol and 2-monoacylglycerol kinase activities of diacylglycerol kinase isozymes.

Diacylglycerol kinase (DGK) consists of ten isozymes and is involved in a wide variety of patho-physiological events. However, the enzymological properties of DGKs have not been fully understood. In this study, we performed a comprehensive analysis on the 1-monoacylglycerol kinase (MGK) and 2-MGK activities of ten DGK isozymes. We revealed that type I (α, ß and γ), type II (δ, η and κ) and type III (ε) DGKs have 7.9-19.2% 2-MGK activity compared to their DGK activities, whereas their 1-MGK activities were <3.0%. Both the 1-MGK and 2-MGK activities of the type IV DGKs (ζ and ι) were <1% relative to their DGK activities. Intriguingly, type V DGKθ has approximately 6% 1-MGK activity and <2% 2-MGK activity compared to its DGK activity. Purified DGKθ exhibited the same results, indicating that its 1-MGK activity is intrinsic. Therefore, DGK isozymes are categorized into three types with respect to their 1-MGK and 2-MGK activities: those having (1) 2-MGK activity relatively stronger than their 1-MGK activity (types I-III), (2) only negligible 1-MGK and 2-MGK activities (type IV), and (3) 1-MGK activity stronger than its 2-MGK activity (type V). The 1-MGK activity of DGKθ and the 2-MGK activity of DGKα were stronger than those of the acylglycerol kinase reported as 1-MGK and 2-MGK to date. The presence or absence of 1-MGK and 2-MGK activities may be essential to the patho-physiological functions of each DGK isozyme.