Diacylglycerol kinases: Why so many of them?

Polyunsaturated fatty acid-containing phosphatidic acids selectively interact with L-lactate dehydrogenase A and induce its secondary structural change and inactivation

Phosphatidic acid (PA) consists of various molecular species that have different fatty acyl chains at the sn-1 and sn-2 positions; and consequently, mammalian cells contain at least 50 structurally distinct PA molecular species. However, the different roles of each PA species are poorly understood. In the present study, we attempted to identify dipalmitoyl (16:0/16:0)-PA-binding proteins from mouse skeletal muscle using liposome precipitation and tandem mass spectrometry analysis. We identified L-lactate dehydrogenase (LDH) A, which catalyzes conversion of pyruvate to lactate and is a key checkpoint of anaerobic glycolysis critical for tumor growth, as a 16:0/16:0-PA-binding protein. LDHA did not substantially associate with other phospholipids, such as phosphatidylcholine, phosphatidylserine, phosphatidylglycerol, phosphatidylinositol, phosphoinositides and cardiolipin at physiological pH (7.4), indicating that LDHA specifically bound to PA. Interestingly, 18:0/18:0-, 18:0/20:4- and 18:0/22:6-PA also interacted with LDHA, and their binding activities were stronger than 16:0/16:0-PA at pH 7.4. Moreover, circular dichroism spectrometry showed that 18:0/20:4- and 18:0/22:6-PA, but not 16:0/16:0- or 18:0/18:0-PA, significantly reduced the α-helical structure of LDHA. Furthermore, 18:0/20:4- and 18:0/22:6-PA attenuated LDH activity. Taken together, we demonstrated for the first time that LDHA is a PA-binding protein and is a unique PA-binding protein that is structurally and functionally controlled by associating with 18:0/20:4- and 18:0/22:6-PA.

Epitope Mapping of the Anti-Diacylglycerol Kinase Monoclonal Antibody DhMab-4 for Immunohistochemical Analysis

Diacylglycerol kinase (DGK) plays a pivotal role in intracellular signaling pathways in mammals. Activated G protein-coupled receptor activates phospholipase C (PLC) through heterotrimeric G protein, following which PLC hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP₂) into diacylglycerol (DG) and inositol 1,4,5-trisphosphate (IP₃). DGK catalyzes DG phosphorylation to produce phosphatidic acid. DG and phosphatidic acid function as second messengers and their intracellular concentrations are regulated by DGK; therefore, DGK plays an important role in regulating many biological processes. There are ten DGK isozymes, of which DGKη is classified as a type II DGK. Reports have shown that DGKη is associated with several diseases; for example, it is highly expressed in the hippocampus and cerebellum and is a key element in bipolar disorder. Although a DGKη-specific monoclonal antibody (mAb) is necessary to reveal the association between the expression of DGKη and diseases, an anti-DGKη mAb for immunohistochemistry has not yet been established. In this study, we established a specific anti-human DGKη (hDGKη) mAb, DhMab-4 (mouse IgG_2b, kappa). DhMab-4 strongly stained Purkinje cells of human cerebellum in immunohistochemistry analysis. For epitope mapping of DhMab-4, we produced deletion or point mutants of hDGKη and performed western blotting to determine the binding epitope of DhMab-4. DhMab-4 reacted with dN745 mutant but not with dN750 mutant, indicating that the N-terminus of the DhMab-4 epitope is located between amino acids 745 and 750. More detailed analysis using point mutants demonstrated that five mutants, that is, D747A, P748A, F749A, G750A, and T752A, were not detected by DhMab-4. These results indicate that Asp747, Pro748, Phe749, Gly750, and Thr752 are important for DhMab-4 binding to hDGKη.

Cellular phosphatidic acid sensor, α-synuclein N-terminal domain, detects endogenous phosphatidic acid in macrophagic phagosomes and neuronal growth cones

Phosphatidic acid (PA) is the simplest phospholipid and is involved in the regulation of various cellular events. Recently, we developed a new PA sensor, the N-terminal region of α-synuclein (α-Syn-N). However, whether α-Syn-N can sense physiologically produced, endogenous PA remains unclear. We first established an inactive PA sensor (α-Syn-N-KQ) as a negative control by replacing all eleven lysine residues with glutamine residues. Using confocal microscopy, we next verified that α-Syn-N, but not α-Syn-N-KQ, detected PA in macrophagic phagosomes in which PA is known to be enriched, further indicating that α-Syn-N can be used as a reliable PA sensor in cells. Finally, because PA generated during neuronal differentiation is critical for neurite outgrowth, we investigated the subcellular distribution of PA using α-Syn-N. We found that α-Syn-N, but not α-Syn-N-KQ, accumulated at the peripheral regions (close to the plasma membrane) of neuronal growth cones. Experiments using a phospholipase D (PLD) inhibitor strongly suggested that PA in the peripheral regions of the growth cone was primarily produced by PLD. Our findings provide a reliable sensor of endogenous PA and novel insights into the distribution of PA during neuronal differentiation.

DGKA Provides Platinum Resistance in Ovarian Cancer Through Activation of c-JUN-WEE1 Signaling

PURPOSE

Although platinum compounds are the first-line treatment for ovarian cancer, the majority of patients relapse and develop resistance to treatment. However, the mechanism underlying resistance is unclear. The goal of our study is to decipher the mechanism by which a metabolic kinase, diacylglycerol kinase alpha (DGKA), confers platinum resistance in ovarian cancer.

EXPERIMENTAL DESIGN

Metabolic kinase RNAi synthetic lethal screening was used to identify a cisplatin resistance driver in ovarian cancer. DGKA variants were used to demonstrate the need for DGKA activity in cisplatin resistance. Phospho-proteomic and genomic screens were performed to identify downstream effectors of DGKA. Therapeutic efficacy of targeting DGKA was confirmed and clinical relevance of DGKA signaling was validated using ovarian cancer patient-derived tumors that had different responses to platinum-based therapy.

RESULTS

We found that platinum resistance was mediated by DGKA and its product, phosphatidic acid (PA), in ovarian cancer. Proteomic and genomic screens revealed that DGKA activates the transcription factor c-JUN and consequently enhances expression of a cell-cycle regulator, WEE1. Mechanistically, PA facilitates c-JUN N-terminal kinase recruitment to c-JUN and its nuclear localization, leading to c-JUN activation upon cisplatin exposure. Pharmacologic inhibition of DGKA sensitized ovarian cancer cells to cisplatin treatment and DGKA-c-JUN-WEE1 signaling positively correlated with platinum resistance in tumors derived from patients with ovarian cancer.

CONCLUSIONS

Our study demonstrates how the DGKA-derived lipid messenger, PA, contributes to cisplatin resistance by intertwining with kinase and transcription networks, and provides preclinical evidence for targeting DGKA as a new strategy in ovarian cancer treatment to battle cisplatin resistance.

DGKζ depletion attenuates HIF-1α induction and SIRT1 expression, but enhances TAK1-mediated AMPKα phosphorylation under hypoxia

Cells cope with environmental changes through various mechanisms. Pathways involving HIF-1, SIRT1, and AMPK play major roles in energy homeostasis under stress conditions. Diacylglycerol kinase (DGK) constitutes an enzyme family that catalyzes conversion of diacylglycerol to phosphatidic acid. We reported earlier that energy depletion such as ischemia induces proteasomal degradation of DGKζ before cell death, suggesting involvement of DGKζ in energy homeostasis. This study examines how DGKζ depletion affects the regulation of HIF-1α, SIRT1, and AMPKα. Under hypoxia DGKζ depletion attenuates HIF-1α induction and SIRT1 expression, which might render cells vulnerable to energy stress. However, DGKζ depletion engenders enhanced AMPKα phosphorylation by upstream kinase TAK1 and an increase in intracellular ATP levels. Results suggest that DGKζ exerts a suppressive effect on TAK1 activity in the AMPK activation mechanism, and that DGKζ depletion might engender dysregulation of the AMPK-mediated energy sensor system.

DGKγ Knock-Out Mice Show Impairments in Cerebellar Motor Coordination, LTD, and the Dendritic Development of Purkinje Cells through the Activation of PKCγ

Diacylglycerol kinase γ (DGKγ) regulates protein kinase C (PKC) activity by converting DG to phosphatidic acid (PA). DGKγ directly interacts with PKCγ and is phosphorylated by PKCγ, resulting in the upregulation of lipid kinase activity. PKC dysfunction impairs motor coordination, indicating that the regulation of PKC activity is important for motor coordination. DGKγ and PKC are abundantly expressed in cerebellar Purkinje cells. However, the physiological role of DGKγ has not been elucidated. Therefore, we developed DGKγ knock-out (KO) mice and tested their cerebellar motor coordination. In DGKγ KO mice, cerebellar motor coordination and long-term depression (LTD) were impaired, and the dendrites of Purkinje cells from DGKγ KO mice were significantly retracted. Interestingly, treatment with the cPKC inhibitor Gö6976 (Gö) rescued the dendritic retraction of primary cultured Purkinje cells from DGKγ KO mice. In contrast, treatment with the PKC activator 12-o-tetradecanoylphorbol 13-acetate (TPA) reduced morphologic alterations in the dendrites of Purkinje cells from wild-type (WT) mice. In addition, we confirmed the upregulation of PKCγ activity in the cerebellum of DGKγ KO mice and rescued impaired LTD in DGKγ KO mice with a PKCγ-specific inhibitor. Furthermore, impairment of motor coordination observed in DGKγ KO mice was rescued in tm1c mice with DGKγ reexpression induced by the FLP-flippase recognition target (FRT) recombination system. These results indicate that DGKγ is involved in cerebellar LTD and the dendritic development of Purkinje cells through the regulation of PKCγ activity, and thus contributes to cerebellar motor coordination.

A PI(4,5)P2-derived "gasoline engine model" for the sustained B cell receptor activation

To efficiently initiate activation responses against rare ligands in the microenvironment, lymphocytes employ sophisticated mechanisms involving signaling amplification. Recently, a signaling amplification mechanism initiated from phosphatidylinositol (PI) 4, 5-biphosphate [PI(4,5)P2] hydrolysis and synthesis for sustained B cell activation has been reported. Antigen and B cell receptor (BCR) recognition triggered the prompt reduction of PI(4,5)P2 density within the BCR microclusters, which led to the positive feedback for the synthesis of PI(4,5)P2 outside of the BCR microclusters. At single molecule level, the diffusion of PI(4,5)P2 was slow, allowing for the maintenance of a PI(4,5)P2 density gradient between the inside and outside of the BCR microclusters and the persistent supply of PI(4,5)P2 from outside to inside of the BCR microclusters. Here, we review studies that have contributed to uncovering the molecular mechanisms of PI(4,5)P2-derived signaling amplification model. Based on these studies, we proposed a "gasoline engine model" in which the activation of B cell signaling inside the microclusters is similar to the working principle of burning gasoline within the engine chamber of a gasoline engine. We also discuss the evidences showing the potential universality of this model and future prospects.

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.

Structure activity relationship studies on Amb639752: toward the identification of a common pharmacophoric structure for DGKα inhibitors

A series of analogues of Amb639752, a novel diacylglycerol kinase (DGK) inhibitor recently discovered by us via virtual screening, have been tested. The compounds were evaluated as DGK inhibitors on α, θ, and ζ isoforms, and as antagonists on serotonin receptors. From these assays emerged two novel compounds, namely 11 and 20, which with an IC₅₀ respectively of 1.6 and 1.8 µM are the most potent inhibitors of DGKα discovered to date. Both compounds demonstrated the ability to restore apoptosis in a cellular model of X-linked lymphoproliferative disease as well as the capacity to reduce the migration of cancer cells, suggesting their potential utility in preventing metastasis. Finally, relying on experimental biological data, molecular modelling studies allow us to set a three-point pharmacophore model for DGK inhibitors.

Diacylglycerol Kinase ζ Regulates Macrophage Responses in Juvenile Arthritis and Cytokine Storm Syndrome Mouse Models

Dysregulation of monocyte and macrophage responses are often observed in children with systemic juvenile idiopathic arthritis (sJIA) and cytokine storm syndrome (CSS), a potentially fatal complication of chronic rheumatic diseases. Both conditions are associated with activation of TLR signaling in monocyte and macrophage lineage cells, leading to overwhelming inflammatory responses. Despite the importance of TLR engagement in activating proinflammatory macrophages, relatively little is known about activation of intrinsic negative regulatory pathways to attenuate excessive inflammatory responses. In this study, we demonstrate that loss of diacylglycerol (DAG) kinase (Dgk) ζ, an enzyme which converts DAG into phosphatidic acid, limits inflammatory cytokine production in an arthritic mouse model dependent on TLR2 signaling and in a CSS mouse model dependent on TLR9 signaling. In vitro, Dgkζ deficiency results in reduced production of TNF-α, IL-6, and IL-1β and in limited M1 macrophage polarization. Mechanistically, Dgkζ deficiency decreases STAT1 and STAT3 phosphorylation. Moreover, Dgkζ levels are increased in macrophages derived from mice with CSS or exposed to plasma from sJIA patients with active disease. Our data suggest that Dgkζ induction in arthritic conditions perpetuates systemic inflammatory responses mediated by macrophages and highlight a potential role of Dgkζ-DAG/phosphatidic acid axis as a modulator of inflammatory cytokine production in sJIA and CSS.

mTORC1 is involved in DGKβ-induced neurite outgrowth and spinogenesis

Diacylglycerol kinase β (DGKβ) is an enzyme converting DG to phosphatidic acid (PA) and is specifically expressed in neurons, especially those in the cerebral cortex, hippocampus and striatum. We previously reported that DGKβ induces neurite outgrowth and spinogenesis, contributing to higher brain function including emotion and memory, and plasma membrane localization of DGKβ via the C1 domain and a cluster of basic amino acids at the C-terminus is necessary for its function. To clarify the mechanisms involved in neuronal development by DGKβ, we investigated whether DGKβ activity induces neurite outgrowth using human neuroblastoma SH-SY5Y cells. DGKβ induced neurite outgrowth by activation of mammalian target of rapamycin complex 1 (mTORC1) through a kinase-dependent pathway. In addition, in primary cultured cortical and hippocampal neurons, inhibition of mTORC1 abolished DGKβ induced-neurite outgrowth, branching and spinogenesis. These results indicated that DGKβ induces neurite outgrowth and spinogenesis by activating mTORC1 in a kinase-dependent pathway.

Characterization of α-synuclein N-terminal domain as a novel cellular phosphatidic acid sensor

Tracking the localization and dynamics of the intracellular bioactive lipid phosphatidic acid (PA) is important for understanding diverse biological phenomena. Although several PA sensors have been developed, better ones are still needed for comprehensive PA detection in cells. We recently found that α-synuclein (α-Syn) selectively and strongly bound to PA in vitro. Here, we revealed that the N-terminal region of α-Syn (α-Syn-N) specifically bound to PA, with a dissociation constant of 6.6 μm. α-Syn-N colocalized with PA-producing enzymes, diacylglycerol kinase (DGK) β at the plasma membrane (PM), myristoylated DGKζ at the Golgi apparatus, phorbol ester-stimulated DGKγ at the PM, and phospholipase D2 at the PM and Golgi but not with the phosphatidylinositol-4,5-bisphosphate-producing enzyme in COS-7 cells. However, α-Syn-N failed to colocalize with them in the presence of their inhibitors and/or their inactive mutants. These results indicate that α-Syn-N specifically binds to cellular PA and can be applied as an excellent PA sensor.

Analytical Method for Diacylglycerol Kinase ζ Activity in Cells Using Protein Myristoylation and Liquid Chromatography-Tandem Mass Spectrometry

Specific inhibitors of diacylglycerol kinase (DGK) ζ can be promising anticancer medications via the activation of cancer immunity. Although the detection of cellular activities of target enzymes is essential for drug screening in addition to in vitro assays, it is difficult to detect the activity of DGKζ in cells. In the present study, we generated AcGFP-DGKζ cDNA with a consensus N-myristoylation sequence at the 5' end (Myr-AcGFP-DGKζ) to target DGKζ to membranes. Using liquid chromatography (LC)-tandem mass spectrometry (MS/MS) (LC-MS/MS), we showed that Myr-AcGFP-DGKζ, but not AcGFP-DGKζ without the myristoylation sequence, substantially augmented the levels of several phosphatidic acid (PtdOH) species. In contrast to Myr-AcGFP-DGKζ, its inactive mutant did not exhibit an increase in PtdOH production, indicating that the increase in PtdOH production was DGK activity-dependent. This method will be useful in chemical compound selection for the development of drugs targeting DGKζ and can be applicable to various soluble (nonmembrane bound) lipid-metabolizing enzymes, including other DGK isozymes.

Mice lacking DGKε show increased beige adipogenesis in visceral white adipose tissue after long-term high fat diet in a COX-2- dependent manner

Adipose tissue is a central site for energy storage in the form of triglyceride (TG). Under excess energy conditions, TG is synthesized by acylation of diacylglycerol (DG), whereas TG is broken down into DG and free fatty acid, which provide energy for mitochondrial lipid oxidation when needed. In this regard, DG is not merely an intermediate metabolite for TG metabolism; it also serves as a signaling molecule. DG kinase (DGK) phosphorylates DG to produce phosphatidic acid (PA). Consequently, DGK plays a pivotal role in the control of lipid metabolism and signal transduction pathway. Recently, a report has described that DGKε-knockout (KO) mice show expansion of epididymal white adipose tissue (WAT) together with the impairment of glucose clearance after short-term (40 days) high fat diet (HFD) feeding, an early presymptomatic phase of obesity in wild-type animals. Nevertheless, no report describes an investigation of their phenotype under long-term HFD feeding conditions. Remarkably, results obtained during long-term HFD feeding show that WAT mass is decreased significantly and that the blood glucose profile in response to glucose challenge is improved in DGKε-KO mice compared with wild-type, which contrast sharply against the phenotype shown for short-term HFD feeding. Morphological examination reveals that cyclooxygenase-2 (COX-2) expression and clusters of uncoupling protein 1 (UCP1)-positive multilocular brown-like ("beige") adipocyte are induced in DGKε-deficient WAT after long-term HFD feeding, suggesting that beige adipocytes facilitate energy expenditure during prolonged HFD feeding. Administration of celecoxib, a selective inhibitor of COX-2, abolishes the appearance of UCP1-positive beige adipocytes in DGKε-KO mice. These findings suggest that DGKε deficiency promotes visceral WAT remodeling in a COX-2-dependent manner under long-term HFD feeding conditions.

Role of Diacylglycerol Kinases in Glucose and Energy Homeostasis

Diacylglycerol kinases (DGKs) catalyze a reaction that converts diacylglycerol (DAG) to phosphatidic acid (PA). DAG and PA act as intermediates of de novo lipid synthesis, cellular membrane constituents, and signaling molecules. DGK isoforms regulate a variety of intracellular processes by terminating DAG signaling and activating PA-mediated pathways. The ten DGK isoforms are unique, not only structurally, but also in tissue-specific expression profiles, subcellular localization, regulatory mechanisms, and DAG preferences, suggesting isoform-specific functions. DAG accumulation has been associated with insulin resistance; however, this concept is challenged by opposing roles of DGK isoforms in the development of type 2 diabetes and obesity despite elevated DAG levels. This review focuses on the tissue- and isoform-specific role of DGK in glucose and energy homeostasis.

Microarray analysis of gene expression in the diacylglycerol kinase η knockout mouse brain

We have revealed that diacylglycerol kinase η (DGKη)-knockout (KO) mice display bipolar disorder (BPD) remedy-sensitive mania-like behaviors. However, the molecular mechanisms causing the mania-like abnormal behaviors remain unclear. In the present study, microarray analysis was performed to determine global changes in gene expression in the DGKη-KO mouse brain. We found that the DGKη-KO brain had 43 differentially expressed genes and the following five affected biological pathways: "neuroactive ligand-receptor interaction", "transcription by RNA polymerase II", "cytosolic calcium ion concentration", "Jak-STAT signaling pathway" and "ERK1/2 cascade". Interestingly, mRNA levels of prolactin and growth hormone, which are augmented in BPD patients and model animals, were most strongly increased. Notably, all five biological pathways include at least one gene among prolactin, growth hormone, forkhead box P3, glucagon-like peptide 1 receptor and interleukin 1β, which were previously implicated in BPD. Consistent with the microarray data, phosphorylated ERK1/2 levels were decreased in the DGKη-KO brain. Microarray analysis showed that the expression levels of several glycerolipid metabolism-related genes were also changed. Liquid chromatography-mass spectrometry revealed that several polyunsaturated fatty acid (PUFA)-containing phosphatidic acid (PA) molecular species were significantly decreased as a result of DGKη deficiency, suggesting that the decrease affects PUFA metabolism. Intriguingly, the PUFA-containing lysoPA species were markedly decreased in DGKη-KO mouse blood. Taken together, our study provides not only key broad knowledge to gain novel insights into the underlying mechanisms for the mania-like behaviors but also information for developing BPD diagnostics.

Dual effect of reduced type I diacylglycerol kinase activity on insulin secretion from MIN6 β-cells

The role of type I diacylglycerol kinases (DGKs) in the regulation of insulin secretion was investigated in MIN6 β-cells. In intracellular Ca²⁺ concentration ([Ca²⁺]_i) measurement experiments, 1 μM R59949, a type I DGK inhibitor, and 10 μM DiC₈, a diacylglycerol (DAG) analog, amplified 22.2 mM glucose-induced [Ca²⁺]_i oscillations in a protein kinase C (PKC)-dependent manner, whereas 10 μM R59949 and 100 μM DiC₈ decreased [Ca²⁺]_i independent of PKC. High concentrations of R59949 and DiC₈ attenuated voltage-dependent Ca²⁺ channel currents. According to these results, 22.2 mM glucose-stimulated insulin secretion (GSIS) was potentiated by 1 μM R59949 but suppressed by 10 μM of the same. The DGKα inhibitor R59022 showed a similar dual effect. Conversely, DiC₈ at 10 and 100 μM potentiated GSIS, although 100 μM DiC₈ decreased [Ca²⁺]_i. These results suggest that DAG accumulated through declined type I DGK activity shows a dual effect on insulin secretion depending on the degree of accumulation; a mild DAG accumulation induces a PKC-dependent stimulatory effect on insulin secretion, whereas an excessive DAG accumulation suppresses it in a PKC-independent manner, possibly via attenuation of VDCC activity.

Establishment and Epitope Mapping of Anti-Diacylglycerol Kinase α Monoclonal Antibody DaMab-8 for Immunohistochemical Analyses

Diacylglycerol kinase (DGK) converts diacylglycerol (DG) into phosphatidic acid (PA). DGKα, 1 of the 10 DGK isozymes, is involved in T cell function. In the present study, we describe a specific monoclonal antibody DaMab-8 (mouse IgG₁, kappa) against DGKα, which is extremely useful for performing immunohistochemical analysis for T cells in oropharyngeal squamous cell carcinomas. Furthermore, we characterized the binding epitope of DaMab-8 using Western blotting and found that the sites Asn610, Leu611, Trp612, Gly613, Asp614, His619, Tyr623, and Gly624 of DGKα are important for facilitating the DaMab-8 binding to the DGKα protein. Thus, DaMab-8 could be advantageous for immunohistochemical analyses toward clarifying the distribution of DGKα-expressing T cells in every pathophysiological tissue.

Myristic acid specifically stabilizes diacylglycerol kinase δ protein in C2C12 skeletal muscle cells

Decreased levels of the δ isozyme of diacylglycerol kinase (DGK) in skeletal muscle attenuate glucose uptake and, consequently, are critical for the pathogenesis of type 2 diabetes. We recently found that free myristic acid (14:0), but not free palmitic acid (16:0), increased the DGKδ protein levels and enhanced glucose uptake in C2C12 myotube cells. However, it has been unclear how myristic acid regulates the level of DGKδ2 protein. In the present study, we characterized the myristic acid-dependent increase of DGKδ protein. A cycloheximide chase assay demonstrated that myristic acid, but not palmitic acid, markedly stabilized DGKδ protein. Moreover, other DGK isozymes, DGKη and ζ, as well as glucose uptake-related proteins, such as protein kinase C (PKC) α, PKCζ, Akt and glycogen synthase kinase 3β, failed to be stabilized by myristic acid. Furthermore, DGKδ was not stabilized in cultured hepatocellular carcinoma cells, pancreas carcinoma cells or neuroblastoma cells, and only a moderate stabilizing effect was observed in embryonic kidney cells. A proteasome inhibitor and a lysosome inhibitor, MG132 and chloroquine, respectively, partly inhibited DGKδ degradation, suggesting that myristic acid prevents, at least in part, the degradation of DGKδ by the ubiquitin-proteasome system and the autophagy-lysosome pathway. Overall, these results strongly suggest that myristic acid attenuates DGKδ protein degradation in skeletal muscle cells and that this attenuation is fatty acid-, protein- and cell line-specific. These new findings provide novel insights into the molecular mechanisms of the pathogenesis of type 2 diabetes mellitus.

Expression and localization of diacylglycerol kinase ζ in guinea pig cochlea and its functional implication under noise-exposure stress conditions

Cochlear hair cells are essential for the mechanotransduction of hearing. Sensorineural hearing loss can be irreversible because hair cells have a minimal ability to repair or regenerate themselves once damaged. In order to develop therapeutic interventions to prevent hair cell loss, it is necessary to understand the signaling pathway operating in cochlear hair cells and its alteration upon damage. Diacylglycerol kinase (DGK) regulates intracellular signal transduction through phosphorylation of lipidic second messenger diacylglycerol. We have previously reported characteristic expression and localization patterns of DGKs in various organs under pathophysiological conditions. Nevertheless, little is known about morphological and functional aspects of this enzyme family in the cochlea. First RT-PCR analysis reveals predominant mRNA expression of DGKα, DGKε and DGKζ. Immunohistochemical analysis shows that DGKζ localizes to the nuclei of inner hair cells (IHCs), outer hair cells (OHCs), supporting cells and spiral ganglion neurons in guinea pig cochlea under normal conditions. It is well known that loud noise exposure induces cochlear damage, thereby resulting in hair cell loss. In particular, OHCs are highly vulnerable to noise exposure than IHCs. We found that after 1 week of noise exposure DGKζ translocates from the nucleus to the cytoplasm in damage-sensitive OHCs and gradually disappears thereafter. In sharp contrast, DGKζ remains to the nucleus in damage-resistant IHCs. These results suggest that DGKζ cytoplasmic translocation is well correlated with cellular damage under noise-exposure stress conditions and is involved in delayed cell death in cochlear outer hair cells.

Expression, Purification, and Characterization of Human Diacylglycerol Kinase ζ

Diacylglycerol kinase ζ (DGKζ) phosphorylates diacylglycerol (DG) to generate phosphatidic acid. The dysfunction of DGKζ has been linked to several diseases, such as cardiac hypertrophy, ischemia, and seizures. Moreover, much attention has been paid to DGKζ, together with DGKα, as a potential target for cancer immunotherapy. However, DGKζ has never been purified and, thus, neither its enzymatic properties nor its structure has yet been reported, hindering our understanding of the catalytic mechanism of DGKζ and the development of a reasonable structure-based drug design. In the present study, we generated a full-length DGKζ using a baculovirus-insect cell expression system for enzymological and structural studies. Full-length DGKζ remained soluble and was purified to near homogeneity as a monomer with yields suitable for protein crystallization (0.63 mg/1 L culture). Enzymatic characterization showed that the purified DGKζ is in a fully functional state. The K _m values for adenosine triphosphate (ATP) and DG were 0.05 mM and 1.5 mol %, respectively, and the EC50 for the activator phosphatidylserine was 8.6 mol %, indicating that its affinity for ATP is moderately higher than those of DGKα and DGKε, and its affinities for DG and phosphatidylserine are comparable to those of DGKα/DGKε. We further confirmed that the purified enzyme could be concentrated without any significant aggregation. Circular dichroism revealed that DGKζ is comprised of 25% α-helices and 18% β-strands. This is the first successful purification and characterization of the enzymatic and conformational properties of DGKζ. The purification of DGKζ allows detailed analyses of this important enzyme and will advance our understanding of DGKζ-related diseases and therapies.

Crystal structure and calcium-induced conformational changes of diacylglycerol kinase α EF-hand domains

Diacylglycerol kinases (DGKs) are multi-domain lipid kinases that phosphorylate diacylglycerol into phosphatidic acid, modulating the levels of these key signaling lipids. Recently, increasing attention has been paid to DGKα isozyme as a potential target for cancer immunotherapy. We have previously shown that DGKα is positively regulated by Ca²⁺ binding to its N-terminal EF-hand domains (DGKα-EF). However, little progress has been made for the structural biology of mammalian DGKs and the molecular mechanism underlying the Ca²⁺ -triggered activation remains unclear. Here we report the first crystal structure of Ca²⁺ -bound DGKα-EF and analyze the structural changes upon binding to Ca²⁺ . DGKα-EF adopts a canonical EF-hand fold, but unexpectedly, has an additional α-helix (often called a ligand mimic [LM] helix), which is packed into the hydrophobic core. Biophysical and biochemical analyses reveal that DGKα-EF adopts a protease-susceptible "open" conformation without Ca²⁺ that tends to form a dimer. Cooperative binding of two Ca²⁺ ions dissociates the dimer into a well-folded monomer, which resists to proteolysis. Taken together, our results provide experimental evidence that Ca²⁺ binding induces substantial conformational changes in DGKα-EF, which likely regulates intra-molecular interactions responsible for the activation of DGKα and suggest a possible role of the LM helix for the Ca²⁺ -induced conformational changes. SIGNIFICANCE STATEMENT: Diacylglycerol kinases (DGKs), which modulates the levels of two lipid second messengers, diacylglycerol and phosphatidic acid, is still structurally enigmatic enzymes since its first identification in 1959. We here present the first crystal structure of EF-hand domains of diacylglycerol kinase α in its Ca²⁺ bound form and characterize Ca²⁺ -induced conformational changes, which likely regulates intra-molecular interactions. Our study paves the way for future studies to understand the structural basis of DGK isozymes.

DGKZ Acts as a Potential Oncogene in Osteosarcoma Proliferation Through Its Possible Interaction With ERK1/2 and MYC Pathway

Osteosarcoma (OS) is one of the most common primary bone tumors in children and young adults. The majority of osteosarcoma patients have limited alternative therapeutic options and metastatic patients generally have a poor prognosis. Thus, it is important to explore novel effective therapeutic targets in the treatment of osteosarcoma. Diacylglycerol kinase zeta (DGKZ) is a recently identified gene potentially associated with certain human carcinogenesis. However, the role of DGKZ in proliferation of osteosarcoma is still unclear. In this study, DGKZ's expression was firstly investigated in OS tumor samples and correlated with poor outcome in OS patients. Silence of DGKZ by shRNA hampered osteosarcoma cell growth and promoted cell apoptosis in vitro. In vivo, DGKZ's knockout also suppressed xenograft tumor proliferation as determined by bioluminescence imaging and weight/volume measurements. Meanwhile, Affymetrix GeneChip and Ingenuity Pathway Analysis (IPA) revealed that DGKZ knockdown resulted in a decreased activity of MYC pathway, and several target genes expression in MYC pathway were altered, including CCND1, CDKN2B, CDK6, PCNA, and EGR1. Furthermore, immunoprecipitation coupled with mass spectrometry (IP-MS) analysis was used to identify proteins that interacted with DGKZ in OS cells and revealed ERK1/2, a key MYC-interactor, to associate with DGKZ. Together, our study demonstrated that DGKZ might act as an oncogene in osteosarcoma via its possible interaction with ERK1/2 and MYC pathway.

Identification of a novel DGKα inhibitor for XLP-1 therapy by virtual screening

As part of an effort to identify druggable diacylglycerol kinase alpha (DGKα) inhibitors, we used an insilico approach based on chemical homology with the two commercially available DGKα inhibitors R59022 and R59949. Ritanserin and compound AMB639752 emerged from the screening of 127 compounds, showing an inhibitory activity superior to the two commercial inhibitors, being furthermore specific for the alpha isoform of diacylglycerol kinase. Interestingly, AMB639752 was also devoid of serotoninergic activity. The ability of both ritanserin and AMB639752, by inhibiting DGKα in intact cells, to restore restimulation induced cell death (RICD) in SAP deficient lymphocytes was also tested. Both compounds restored RICD at concentrations lower than the two previously available inhibitors, indicating their potential use for the treatment of X-Iinked lymphoproliferative disease 1 (XLP-1), a rare genetic disorder in which DGKα activity is deregulated.

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, β, 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.

Diacylglycerol kinase γ predicts prognosis and functions as a tumor suppressor by negatively regulating glucose transporter 1 in hepatocellular carcinoma

Diacylglycerol kinases (DGK) are a family of enzymes catalyzing the transformation of diacylglycerol into phosphatidic acid, which have been recognized as key regulators in cell signaling pathways. The role of DGKγ in human malignancies has seldom been studied. In this study, we investigated the role of DGKγ in hepatocellular carcinoma (HCC). We found that DGKγ was down-regulated in HCC tumor tissues and cell lines as compared to that in non-tumor tissues. The prognostic value of DGKγ expression was evaluated by Cox regression and Kaplan-Meier analyses. Lower DGKγ expression in tumor tissues was an independent prognostic factor for poor post-surgical overall survival. By using HDACs inhibitors treatment and ChIP-PCR, we discovered that histone H3 and H4 deacetylation mainly contributed to the downregulation of DGKγ expression. Functional studies revealed that ectopic expression of DGKγ inhibited cell proliferation and cell migration in HCC cells. Mechanism studies showed that DGKγ overexpression led to down regulation of GLUT1 protein level and AMPK activity, which result in glucose uptake suppression as well as lactate and ATP production declination. The decrease of GLUT1 level could be partially rescued by treatments with either DGK inhibitor and lysosome inhibitor, indicating DGKγ may down-regulate GLUT1 through its kinase activity and lysosome degradation process. Together, this study demonstrated that DGKγ plays a tumor suppressor role in HCC by negatively regulating GLUT1. DGKγ could be a novel prognostic indicator and therapeutic target for HCC.

Chemoproteomic Discovery of a Ritanserin-Targeted Kinase Network Mediating Apoptotic Cell Death of Lung Tumor Cells

Ritanserin was tested in the clinic as a serotonin receptor inverse agonist but recently emerged as a novel kinase inhibitor with potential applications in cancer. Here, we discovered that ritanserin induced apoptotic cell death of non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC) cells via a serotonin-independent mechanism. We used quantitative chemical proteomics to reveal a ritanserin-dependent kinase network that includes key mediators of lipid [diacylglycerol kinase α, phosphatidylinositol 4-kinase β] and protein [feline encephalitis virus-related kinase, rapidly accelerated fibrosarcoma (RAF)] signaling, metabolism [eukaryotic elongation factor 2 kinase, eukaryotic translation initiation factor 2-α kinase 4], and DNA damage response [tousled-like kinase 2] to broadly kill lung tumor cell types. Whereas ritanserin exhibited polypharmacology in NSCLC proteomes, this compound showed unexpected specificity for c-RAF in the SCLC subtype, with negligible activity against other kinases mediating mitogen-activated protein kinase signaling. Here we show that ritanserin blocks c-RAF but not B-RAF activation of established oncogenic signaling pathways in live cells, providing evidence in support of c-RAF as a key target mediating its anticancer activity. Given the role of c-RAF activation in RAS-mutated cancers resistant to clinical B-RAF inhibitors, our findings may have implications in overcoming resistance mechanisms associated with c-RAF biology. The unique target landscape combined with acceptable safety profiles in humans provides new opportunities for repositioning ritanserin in cancer.

Of local translation control and lipid signaling in neurons

Fine-tuned regulation of new proteins synthesis is key to the fast adaptation of cells to their changing environment and their response to external cues. Protein synthesis regulation is particularly refined and important in the case of highly polarized cells like neurons where translation occurs in the subcellular dendritic compartment to produce long-lasting changes that enable the formation, strengthening and weakening of inter-neuronal connection, constituting synaptic plasticity. The changes in local synaptic proteome of neurons underlie several aspects of synaptic plasticity and new protein synthesis is necessary for long-term memory formation. Details of how neuronal translation is locally controlled only start to be unraveled. A generally accepted view is that mRNAs are transported in a repressed state and are translated locally upon externally cued triggering signaling cascades that derepress or activate translation machinery at specific sites. Some important yet poorly considered intermediates in these cascades of events are signaling lipids such as diacylglycerol and its balancing partner phosphatidic acid. A link between these signaling lipids and the most common inherited cause of intellectual disability, Fragile X syndrome, is emphasizing the important role of these secondary messages in synaptically controlled translation.

Expression and purification of human diacylglycerol kinase α from baculovirus-infected insect cells for structural studies

Diacylglycerol kinases (DGKs) are lipid kinases that modulate the levels of lipid second messengers, diacylglycerol and phosphatidic acid. Recently, increasing attention has been paid to its α isozyme (DGKα) as a potential target for cancer immunotherapy. DGKα consists of the N-terminal regulatory domains including EF-hand motifs and C1 domains, and the C-terminal catalytic domain (DGKα-CD). To date, however, no structures of mammalian DGKs including their CDs have yet been reported, impeding our understanding on the catalytic mechanism of DGKs and the rational structure-based drug design. Here we attempted to produce DGKα-CD or a full-length DGKα using bacterial and baculovirus-insect cell expression system for structural studies. While several DGKα-CD constructs produced using both bacterial and insect cells formed insoluble or soluble aggregates, the full-length DGKα expressed in insect cells remained soluble and was purified to near homogeneity as a monomer with yields (1.3 mg/mL per one L cell culture) feasible for protein crystallization. Following enzymatic characterization showed that the purified DGKα is in fully functional state. We further demonstrated that the purified enzyme could be concentrated without any significant aggregation, and characterized its secondary structure by circular dichroism. Taken together, these results suggest that the presence of N-terminal regulatory domains suppress protein aggregation likely via their intramolecular interactions with DGKα-CD, and demonstrate that the baculovirus-insect cell expression of the full-length form of DGKα, not DGKα-CD alone, represents a promising approach to produce protein sample for structural studies of DGKα. Thus, our study will encourage future efforts to determine the crystal structure of DGK, which has not been determined since it was first identified in 1959.

Diacylglycerol kinase δ controls down-regulation of cyclin D1 for C2C12 myogenic differentiation

Diacylglycerol kinase (DGK) is a lipid-metabolizing enzyme that phosphorylates diacylglycerol (DG) to produce phosphatidic acid (PA). DGKδ is highly expressed in the skeletal muscle, and a decrease in DGKδ expression increases the severity of type 2 diabetes. However, the role of DGKδ in myogenic differentiation is still unknown. The present study demonstrated that DGKδ expression was down-regulated in the early stage of C2C12 myogenic differentiation almost concurrently with a decrease in cyclin D1 expression. The knockdown of DGKδ by DGKδ-specific siRNAs significantly increased the levels of cyclin D1 expression at 48 h after C2C12 myogenic differentiation. In contrast, at the same time, the knockdown of DGKδ decreased the levels of myogenin expression and the number of myosin heavy chain (MHC)-positive cells. These results indicate that DGKδ regulates the early differentiation of C2C12 myoblasts via controlling the down-regulation of cyclin D1 expression. Moreover, the suppression of DGKδ expression increased the phosphorylation levels of conventional and novel protein kinase Cs (cnPKCs). Furthermore, DGKδ suppression increased the levels of cyclin D1 and phospho-cnPKCs even at the first 24 h of myogenic differentiation. These results suggest that DGKδ controls the down-regulation of cyclin D1 expression by attenuating the PKC signaling pathway for C2C12 myogenic differentiation.

Diacylglycerol kinase synthesized by commensal Lactobacillus reuteri diminishes protein kinase C phosphorylation and histamine-mediated signaling in the mammalian intestinal epithelium

Lactobacillus reuteri 6475 (Lr) of the human microbiome synthesizes histamine and can suppress inflammation via type 2 histamine receptor (H2R) activation in the mammalian intestine. Gut microbes such as Lr promote H2R signaling and may suppress H1R proinflammatory signaling pathways in parallel by unknown mechanisms. In this study, we identified a soluble bacterial enzyme known as diacylglycerol kinase (Dgk) from Lr that is secreted into the extracellular milieu and presumably into the intestinal lumen. DgK diminishes diacylglycerol (DAG) quantities in mammalian cells by promoting its metabolic conversion and causing reduced protein kinase C phosphorylation (pPKC) as a net effect in mammalian cells. We demonstrated that histamine synthesized by gut microbes (Lr) activates both mammalian H1R and H2R, but Lr-derived Dgk suppresses the H1R signaling pathway. Phospho-PKC and IκBα were diminished within the intestinal epithelium of mice and humans treated by wild-type (WT) Lr, but pPKC and IκBα were not decreased in treatment with ΔdgkA Lr. Mucosal IL-6 and systemic interleukin (IL)-1α, eotaxin, and granulocyte colony-stimulating factor (G-CSF) were suppressed in WT Lr, but not in ΔdgkA Lr colonized mice. Collectively, the commensal microbe Lr may act as a "microbial antihistamine" by suppressing intestinal H1R-mediated proinflammatory responses via diminished pPKC-mediated mammalian cell signaling.

Deconstructing Lipid Kinase Inhibitors by Chemical Proteomics

Diacylglycerol kinases (DGKs) regulate lipid metabolism and cell signaling through ATP-dependent phosphorylation of diacylglycerol to biosynthesize phosphatidic acid. Selective chemical probes for studying DGKs are currently lacking and are needed to annotate isoform-specific functions of these elusive lipid kinases. Previously, we explored fragment-based approaches to discover a core fragment of DGK-α (DGKα) inhibitors responsible for selective binding to the DGKα active site. Here, we utilize quantitative chemical proteomics to deconstruct widely used DGKα inhibitors to identify structural regions mediating off-target activity. We tested the activity of a fragment (RLM001) derived from a nucleotide-like region found in the DGKα inhibitors R59022 and ritanserin and discovered that RLM001 mimics ATP in its ability to broadly compete at ATP-binding sites of DGKα as well as >60 native ATP-binding proteins (kinases and ATPases) detected in cell proteomes. Equipotent inhibition of activity-based probe labeling by RLM001 supports a contiguous ligand-binding site composed of C1, DAGKc, and DAGKa domains in the DGKα active site. Given the lack of available crystal structures of DGKs, our studies highlight the utility of chemical proteomics in revealing active-site features of lipid kinases to enable development of inhibitors with enhanced selectivity against the human proteome.

DGKζ ablation engenders upregulation of p53 level in the spleen upon whole-body ionizing radiation

The tumor suppressor gene product p53, which coordinates the cellular response to various stresses, is subject to tight regulation by a complex network of signal transduction. The DGK family metabolizes lipidic second messenger diacylglycerol to produce phosphatidic acid. Our earlier studies showed that one isozyme, DGKζ, is involved in the regulatory mechanism of p53. In a cellular model of doxorubicin-induced DNA damage, overexpression of wild-type DGKζ suppresses p53 protein induction and reduces apoptosis, whereas knockdown of DGKζ upregulates p53 protein level and promotes apoptosis. Further examination reveals that DGKζ facilitates p53 degradation via ubiquitin-proteasome system in the cytoplasm. However, it remains undetermined whether the regulatory mechanism of DGKζ on p53 function found in cell-based experiments is also functional at the animal level. This study was conducted to elucidate this point using an experiment with DGKζ-KO mice under DNA damage induced by whole-body ionizing radiation. Our results reveal that p53 protein is induced robustly in the spleen of DGKζ-KO mice upon exposure to ionizing radiation, thereby promoting apoptosis in this organ. Taken together, the results demonstrate that DGKζ plays a sentinel role in p53 expression at the cellular and organismal levels after DNA damaging stress conditions.

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.

DGKζ deficiency protects against peripheral insulin resistance and improves energy metabolism

Diacylglycerol kinases (DGKs) regulate the balance between diacylglycerol (DAG) and phosphatidic acid. DGKζ is highly abundant in skeletal muscle and induces fiber hypertrophy. We hypothesized that DGKζ influences functional and metabolic adaptations in skeletal muscle and whole-body fuel utilization. DAG content was increased in skeletal muscle and adipose tissue, but unaltered in liver of DGKζ KO mice. Linear growth, body weight, fat mass, and lean mass were reduced in DGKζ KO versus wild-type mice. Conversely, male DGKζ KO and wild-type mice displayed a similar robust increase in plantaris weight after functional overload, suggesting that DGKζ is dispensable for muscle hypertrophy. Although glucose tolerance was similar, insulin levels were reduced in high-fat diet (HFD)-fed DGKζ KO versus wild-type mice. Submaximal insulin-stimulated glucose transport and p-Akt Ser⁴⁷³ were increased, suggesting enhanced skeletal muscle insulin sensitivity. Energy homeostasis was altered in DGKζ KO mice, as evidenced by an elevated respiratory exchange ratio, independent of altered physical activity or food intake. In conclusion, DGKζ deficiency increases tissue DAG content and leads to modest growth retardation, reduced adiposity, and protection against insulin resistance. DGKζ plays a role in the control of growth and metabolic processes, further highlighting specialized functions of DGK isoforms in type 2 diabetes pathophysiology.