Fatty acids inhibit growth-factor-induced diacylglycerol kinase alpha activation in vascular smooth-muscle cells

Diacylglycerol Kinase Alpha in Radiation-Induced Fibrosis: Potential as a Predictive Marker or Therapeutic Target

Radiotherapy is an efficient tool in cancer treatment, but it brings along the risk of side effects such as fibrosis in the irradiated healthy tissue thus limiting tumor control and impairing quality of life of cancer survivors. Knowledge on radiation-related fibrosis risk and therapeutic options is still limited and requires further research. Recent studies demonstrated that epigenetic regulation of diacylglycerol kinase alpha (DGKA) is associated with radiation-induced fibrosis. However, the specific mechanisms are still unknown. In this review, we scrutinized the role of DGKA in the radiation response and in further cellular functions to show the potential of DGKA as a predictive marker or a novel target in fibrosis treatment. DGKA was reported to participate in immune response, lipid signaling, exosome production, and migration as well as cell proliferation, all processes which are suggested to be critical steps in fibrogenesis. Most of these functions are based on the conversion of diacylglycerol (DAG) to phosphatidic acid (PA) at plasma membranes, but DGKA might have also other, yet not well-known functions in the nucleus. Current evidence summarized here underlines that DGKA activation may play a central role in fibrosis formation post-irradiation and shows a potential of direct DGKA inhibitors or epigenetic modulators to attenuate pro-fibrotic reactions, thus providing novel therapeutic choices.

The effect of palmitic acid on inflammatory response in macrophages: an overview of molecular mechanisms

Palmitic acid is a saturated fatty acid whose blood concentration is elevated in obese patients. This causes inflammatory responses, where toll-like receptors (TLR), TLR2 and TLR4, play an important role. Nevertheless, palmitic acid is not only a TLR agonist. In the cell, this fatty acid is converted into phospholipids, diacylglycerol and ceramides. They trigger the activation of various signaling pathways that are common for LPS-mediated TLR4 activation. In particular, metabolic products of palmitic acid affect the activation of various PKCs, ER stress and cause an increase in ROS generation. Thanks to this, palmitic acid also strengthens the TLR4-induced signaling. In this review, we discuss the mechanisms of inflammatory response induced by palmitic acid. In particular, we focus on describing its effect on ER stress and IRE1α, and the mechanisms of NF-κB activation. We also present the mechanisms of inflammasome NLRP3 activation and the effect of palmitic acid on enhanced inflammatory response by increasing the expression of FABP4/aP2. Finally, we focus on the consequences of inflammatory responses, in particular, the effect of TNF-α, IL-1β and IL-6 on insulin resistance. Due to the high importance of macrophages and the production of proinflammatory cytokines by them, this work mainly focuses on these cells.

Diacylglycerol kinase regulates tyrosinase expression and function in human melanocytes

Diacylglycerol increases the melanin content of human melanocytes in vitro and increases the pigmentation of guinea pig skin in vivo, but the mechanism(s) underlying those effects remain unknown. In this study, we characterized the role of diacylglycerol kinase (DGK), which phosphorylates diacylglycerol to generate phosphatidic acid, in the regulation of pigmentation. Ten isoforms of DGK have been identified, and we show that DGKζ is the most abundant isoform expressed by human melanocytic cells. Melanin content, tyrosinase activity and tyrosinase protein levels were significantly reduced by a DGK inhibitor, but tyrosinase and MITF mRNA levels were not changed by that inhibition, and there were no effects on the expression of other melanogenesis-related proteins. Isoform-specific siRNAs showed that knockdown of DGKζ decreased melanin content and tyrosinase expression in melanocytic cells. Over-expression of DGKζ increased tyrosinase protein levels, but did not increase tyrosinase mRNA levels. Glycosidase digestion revealed that inhibition of DGK reduced only the mature form of tyrosinase and the decrease of tyrosinase resulting from DGK inhibition could be blocked partially by protease inhibitors. These results suggest that DGK regulates melanogenesis via modulation of the post-translational processing of tyrosinase, which may be related with the protein degradation machinery.

Synthesis, tautomeric structure and antimicrobial activity of 3-arylhydrazono-4-phenyl-[1,2,4]-triazepino[2,3-a]quinazoline-2,7(1H)-diones

A simple strategy for the synthesis of the hitherto unreported 3-arylazo-4-phenyl-[1,2,4]triazepino[2,3-H]quinazoline-2,7(1H)-diones is described. Spectral data indicated that the studied compounds exist predominantly in the hydrazone tautomeric form. The antimicrobial activity of the newly synthesized compounds was also evaluated. The results indicated that some of these compounds have moderate activity towards bacteria.

Impact of α-lipoic acid on liver peroxisome proliferator-activated receptor-α, vascular remodeling, and oxidative stress in insulin-resistant rats

This study sought to determine the impact of α-lipoic acid (LA) on superoxide anion (O(2)(•-)) production and peroxisome proliferator-activated receptor-α (PPARα) expression in liver tissue, plasma free fatty acids (FFA), and aortic remodeling in a rat model of insulin resistance. Sprague-Dawley rats (50-75 g) were given either tap water or a drinking solution containing 10% D-glucose for 14 weeks, combined with a diet with or without LA supplement. O(2)(•-) production was measured by lucigenin chemiluminescence, and PPAR-α expression by Western blotting. Cross-sectional area (CSA) of the aortic media and lumen and number of smooth muscle cells (SMC) were determined histologically. Glucose increased systolic blood pressure (SBP), plasma levels of glucose and insulin, and insulin resistance (HOMA index). All of these effects were attenuated by LA. Whereas glucose had no effect on liver PPAR-α protein level, it decreased plasma FFA. LA decreased the aortic and liver O(2)(•-) production, body weight, and plasma FFA levels in control and glucose-treated rats. Liver PPAR-α protein levels were increased by LA, and negatively correlated with plasma FFA. Medial CSA was reduced in all glucose-treated rats, and positively correlated with plasma FFA but not with SBP or aortic O(2)(•-) production. Glucose also reduced aortic lumen area, so that the media-to-lumen ratio remained unchanged. The ability of LA to lower plasma FFA appears to be mediated, in part, by increased hepatic PPAR-α expression, which may positively affect insulin resistance. Glucose-fed rats may serve as a unique model of aortic atrophic remodeling in hypertension and early metabolic syndrome.

Diacylglycerol Kinase Inhibition and Vascular Function

Diacylglycerol kinases (DGKs), a family of lipid kinases, convert diacylglycerol (DG) to phosphatidic acid (PA). Acting as a second messenger, DG activates protein kinase C (PKC). PA, a signaling lipid, regulates diverse functions involved in physiological responses. Since DGK modulates two lipid second messengers, DG and PA, regulation of DGK could induce related cellular responses. Currently, there are 10 mammalian isoforms of DGK that are categorized into five groups based on their structural features. These diverse isoforms of DGK are considered to activate distinct cellular functions according to extracellular stimuli. Each DGK isoform is thought to play various roles inside the cell, depending on its subcellular localization (nuclear, ER, Golgi complex or cytoplasm). In vascular smooth muscle, vasoconstrictors such as angiotensin II, endothelin-1 and norepinephrine stimulate contraction by increasing inositol trisphosphate (IP(3)), calcium, DG and PKC activity. Inhibition of DGK could increase DG availability and decrease PA levels, as well as alter intracellular responses, including calcium-mediated and PKC-mediated vascular contraction. The purpose of this review is to demonstrate a role of DGK in vascular function. Selective inhibition of DGK isoforms may represent a novel therapeutic approach in vascular dysfunction.

Molecular and signaling mechanisms of atherosclerosis in insulin resistance

Although the prevalence of cardiovascular complications is increased in insulin-resistant individuals, the underlying causes of this link have been elusive. Recent work suggests that several intracellular signal transduction pathways are inappropriately activated by hyperinsulinemia, hyperglycemia, increased free fatty acids, dyslipidemia, various inflammatory cytokines and adipokines--factors that are increased in insulin resistance. Once activated, substantial cross talk occurs between these pathways, especially a self-reinforcing cascade of vascular inflammation and cell dysfunction, greatly increasing the risk and severity of atherosclerosis in the insulin-resistant individual. We review several key cell-signalling pathways, describe how they are activated in they insulin-resistant state and the damage they induce, and discusses possible therapeutic approaches to limit vascular damage.

Inflammatory effects of very low-density lipoprotein and fatty acids

High plasma triacylglycerol (triglyceride, TG) levels is a risk factor for atherosclerosis. Very large lipoproteins, such as chylomicrons, alone are not considered atherogenic, but TG-rich remnant lipoproteins can penetrate into the vascular wall. Importantly, accumulating evidence suggests that all TG-rich lipoproteins stimulate cytokine expression in circulating monocytes. Very low-density lipoprotein (VLDL) stimulates monocyte adhesion to endothelial cells and expression of inflammatory genes in macrophages. Furthermore, fatty acids released from large lipoproteins can stimulate both vascular cells and circulating monocytes. It is likely that fatty acids released from TG-rich lipoproteins contribute to atherogenesis, but the role of fatty acids in ischemic heart disease is not as direct as that of cholesterol. Fatty acids influence plasma lipoprotein levels and either stimulate or suppress numerous cellular functions relevant to atherogenesis. While certain n-3 fatty acids are good for health, most other medium- to long-chain fatty acids appear to promote inflammation in cell culture studies and need to be studied further. Nevertheless, the existing evidence supports the general conclusion that TG-rich lipoproteins and fatty acids greatly accelerate the progression of atherosclerosis. This may be because of their inflammatory effects.

Novel diacylglycerol kinase inhibitor selectively suppressed an U46619-induced enhancement of mouse portal vein contraction under high glucose conditions

1. Diacylglycerol kinase (DG kinase) is a key enzyme in vascular contraction; however, alterations of the regulatory mechanisms in vascular dysfunction are poorly understood. In this study, the effect of a novel DG kinase inhibitor, stemphone, on vascular contraction was investigated. 2. The conventional DG kinase inhibitor, 6-[2-(4-[(4-fluorophenyl)phenyl-methylene]-1-piperidinyl)ethyl]-7-methyl-5H-thiazolo [3,2-alpha] pyrimidine-5-one (R59022) (0.1-30 microm), inhibited thromboxane A(2) analogue 9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F(2alpha) (U46619)-induced sustained contractions in mouse aorta and porcine coronary artery in a dose-dependent manner. Treatment with stemphone did not affect contractions in these tissues. However, stemphone significantly inhibited (>0.3 microm) U46619-induced spontaneous phasic contraction in mouse portal vein. This inhibitory effect was not detected following R59022 treatment in portal vein. Therefore, stemphone demonstrated selectivity in terms of portal vein contraction. 3. Under high glucose (22.2 mm) conditions, U46619-induced contraction was enhanced in these three types of vascular tissue. Inhibitory effects of R59022 were attenuated under these conditions; however, effects of stemphone were observed. These results indicated that stemphone could inhibit portal vein contraction under high glucose conditions, for example, diabetes. These data suggested the possibility that DG kinase may be a target of hyperportal pressure. 4. Total mass of DG was enhanced under high glucose conditions. DG was derived from incorporated glucose via de novo synthesis in the absence of phospholipase C pathway mediation. This enhanced DG under high glucose conditions activated a calcium-independent protein kinase C (PKC). This PKC was associated with calcium-independent DG kinase activation. Treatment with stemphone also inhibited calcium-independent DG kinase. These signal transduction pathways were distinguishable from a DG-PKC pathway under normal glucose conditions. 5. The present investigation suggested that stemphone selectively inhibited overcontraction of portal vein induced by high glucose levels. This phenomenon was attributable to inhibition of calcium-independent DG kinase activation that occurred under high glucose conditions mediated by both DG synthesized from glucose and calcium-independent PKC activation.

Oleate, not ligands of the receptor for advanced glycation end-products, promotes proliferation of human arterial smooth muscle cells

AIMS/HYPOTHESIS

Diabetes accelerates cardiovascular disease caused by atherosclerosis. Accordingly, diabetes accelerates atherosclerotic lesion progression and increases arterial smooth muscle cell proliferation. We hypothesized that diabetes can exert growth-promoting effects on smooth muscle cells via increased advanced glycation end-products or by dyslipidaemia.

METHODS

Primary human arterial smooth muscle cells were stimulated with advanced glycation end-products, other ligands of the receptor for advanced glycation end-products or fatty acids common in triglycerides. Cell proliferation was measured as DNA synthesis, cell cycle distribution and cell number. Effects of oleate on cellular phospholipids, diacylglycerol, triglycerides and cholesterol esters were analyzed by thin-layer chromatography, and oleate accumulation into diacylglycerol was confirmed by gas chromatography.

RESULTS

Human arterial smooth muscle cells express the receptor for advanced glycation end-products, but its ligands N(epsilon)-(carboxymethyl)lysine-modified proteins, methylglyoxal-modified proteins, S100B polypeptide and amyloid-beta (1-40) peptide, exert no mitogenic action. Instead, oleate, one of the most common fatty acids in triglycerides, enhances platelet-derived growth factor-BB-mediated proliferation and oleate-containing 1,2-diacylglycerol formation in smooth muscle cells. This mitogenic effect of oleate depends on phospholipase D activity and is associated with an increased formation of oleate-enriched 1,2-diacylglycerol.

CONCLUSION/INTERPRETATION

Oleate, not ligands of the receptor for advanced glycation end-products, acts as an enhancer of human smooth muscle cell proliferation. Thus, lipid abnormalities, rather than hyperglycaemia, could be a major factor promoting proliferation of smooth muscle cells in atherosclerotic lesions.

Site-directed mutagenesis of the active site of diacylglycerol kinase alpha: calcium and phosphatidylserine stimulate enzyme activity via distinct mechanisms

Diacylglycerol kinases (DAGKs) catalyse ATP-dependent phosphorylation of sn-1,2-diacylglycerol that arises during stimulated phosphatidylinositol turnover. DAGKa is activated in vitro by Ca2+ and by acidic phospholipids. The regulatory region of DAGKa includes an N-terminal RVH motif and EF hands that mediate Ca2+-dependent activation. DAGKa also contains tandem C1 protein kinase C homology domains. We utilized yeast, Saccharomyces cerevisiae, which lacks an endogenous DAGK, to express DAGKa and to determine the enzymic activities of different mutant forms of pig DAGKa in vitro. Six aspartate residues conserved in all DAGKs were individually examined by site-directed mutagenesis. Five of these aspartate residues reside in conserved blocks that correspond to sequences in the catalytic site of phosphofructokinases. Mutation of D434 (Asp434) or D650 abolished all DAGKa activity, whereas substitution of one among D465, D497, D529 and D697 decreased the activity to 6% or less of that for wild-type DAGKa. Roles of homologous residues in phosphofructokinases suggested that the N-terminal half of the DAGK catalytic domain binds Mg-ATP and the C-terminal half binds diacylglycerol. A DAGKa mutant with its entire regulatory region deleted showed a much decreased activity that was not activated by Ca2+, but still exhibited PS (phosphatidylserine)-dependent activation. Moreover, mutations of aspartate residues at the catalytic domain had differential effects on activation by Ca2+ and PS. These results indicate that Ca2+ and PS stimulate DAGKa via distinct mechanisms.

Protein kinase C changes in diabetes: is the concept relevant to neuropathy?

Protein kinase C (PKC) comprises a superfamily of isoenzymes, many of which are activated by 1,2-diacylglycerol (DAG) in the presence of phosphatidylserine. In order to be capable of DAG activation, PKC must first undergo a series of phosphorylation at three conserved sites. PKC isoforms phosphorylate a wide variety of intracellular target proteins and have multiple functions in signal transduction-mediated cellular regulation. An elevation in DAG levels and an increase in composite PKC activity and/or certain isoforms occurs in several nonneural tissues from diabetic animals, including the vasculature. The ability of isoform-specific PKC inhibitors to antagonize diabetes-induced abnormalities has implicated altered PKC beta activity in the onset of several diabetic complications, In contrast to many other tissues, DAG levels fall in diabetic nerve and a consistent pattern of change in PKC activity has not been observed. Treatments that alter PKC activity affect nerve Na+, K+-ATPase activity, but the mechanism involved is not well understood, Inhibition of PKC beta in diabetic rats appears to correct reduced nerve blood flow and decreased nerve conduction velocity. These and other findings indicate that changes in the neurovasculature exert adverse effects during the pathogenesis of diabetic neuropathy. Still unresolved is a clear-cut role for PKC in the development of abnormalities in neural cell metabolism. Further progress will depend on a more complete understanding of the functions of individual PKC isoforms in nerve. Future investigation could focus profitably on biochemical processes in nerve cells that modulate PKC activity and that are altered in diabetes, such as vascular endothelial growth factor levels and production of reactive oxygen species arising from oxidative stress.