Export of mitochondrially synthesized lysophosphatidic acid

Role of acylglycerol kinase in LPA-induced IL-8 secretion and transactivation of epidermal growth factor-receptor in human bronchial epithelial cells

LPA (lysophosphatidic acid) is a potent bioactive phospholipid, which regulates a number of diverse cellular responses through G protein-coupled LPA receptors. Intracellular LPA is generated by the phosphorylation of monoacylglycerol by acylglycerol kinase (AGK); however, the role of intracellular LPA in signaling and cellular responses remains to be elucidated. Here, we investigated signaling pathways of IL-8 secretion mediated by AGK and intracellular LPA in human bronchial epithelial cells (HBEpCs). Expression of AGK in HBEpCs was detected by real-time PCR, and overexpressed AGK was mainly localized in mitochondria as determined by immunofluorescence and confocal microscopy. Overexpression of lentiviral AGK wild type increased intracellular LPA production ( approximately 1.8-fold), enhanced LPA-mediated IL-8 secretion, and stimulated tyrosine phosphorylation epidermal growth factor-receptor (EGF-R). Furthermore, downregulation of native AGK by AGK small interfering RNA decreased intracellular LPA levels ( approximately 2-fold) and attenuated LPA-induced p38 MAPK, JNK, and NF-kappaB activation, tyrosine phosphorylation of EGF-R, and IL-8 secretion. These results suggest that native AGK regulates LPA-mediated IL-8 secretion involving MAPKs, NF-kappaB, and transactivation of EGF-R. Thus AGK may play an important role in innate immunity and airway remodeling during inflammation.

Critical role of acylglycerol kinase in epidermal growth factor-induced mitogenesis of prostate cancer cells

The bioactive phospholipids, LPA (lysophosphatidic acid) and PA (phosphatidic acid), regulate pivotal processes related to the pathogenesis of cancer. Recently, we cloned a novel type of lipid kinase that phosphorylates monoacylglycerols (such as 2-arachidonoylglycerol, an endogenous cannabinoid receptor ligand) and diacylglycerols, to form LPA and PA, respectively. This AGK (acylglycerol kinase) is highly expressed in prostate cancer cell lines and the results reviewed here suggest that AGK might be a critical player in the initiation and progression of prostate cancer. Intriguingly, down-regulation of endogenous AGK inhibited EGF (epidermal growth factor), but not LPA-induced ERK1/2 (extracellular-signal-regulated kinase 1/2) activation and progression through the S-phase of the cell cycle. In this review, we will summarize the evidence demonstrating that AGK amplifies EGF growth signalling pathways that play an important role in the pathophysiology of prostate cancer. Because LPA has long been implicated as an autocrine and paracrine growth stimulatory factor for prostate cancer cells, the identification of this novel lipid kinase that regulates its production could provide new and useful targets for preventive or therapeutic measures.

A novel acylglycerol kinase that produces lysophosphatidic acid modulates cross talk with EGFR in prostate cancer cells

The bioactive phospholipids, lysophosphatidic acid (LPA) and phosphatidic acid (PA), regulate pivotal processes related to the pathogenesis of cancer. Here, we report characterization of a novel lipid kinase, designated acylglycerol kinase (AGK), that phosphorylates monoacylglycerol and diacylglycerol to form LPA and PA, respectively. Confocal microscopy and subcellular fractionation suggest that AGK is localized to the mitochondria. AGK expression was up-regulated in prostate cancers compared with normal prostate tissues from the same patient. Expression of AGK in PC-3 prostate cancer cells markedly increased formation and secretion of LPA. This increase resulted in concomitant transactivation of the EGF receptor and sustained activation of extracellular signal related kinase (ERK) 1/2, culminating in enhanced cell proliferation. AGK expression also increased migratory responses. Conversely, down-regulating expression of endogenous AGK inhibited EGF- but not LPA-induced ERK1/2 activation and progression through the S phase of the cell cycle. Hence, AGK can amplify EGF signaling pathways and may play an important role in the pathophysiology of prostate cancer.

Phosphorylation of Rat Liver Mitochondrial Glycerol-3-phosphate Acyltransferase by Casein Kinase 2

We have previously shown rat liver mitochondrial glycerol-3-phosphate acyltransferase (mtGAT), which catalyzes the first step in de novo glycerolipid biosynthesis, is stimulated by casein kinase 2 (CK2) and that a phosphorylated protein of approximately 85 kDa is present in CK2-treated mitochondria. In this paper, we have identified the (32)P-labeled 85-kDa protein as mtGAT. We have also investigated whether the phosphorylation of mtGAT is because of CK2. Mitochondria were treated with CK2 and [gamma-(32)P]GTP as the phosphate donor. Autoradiography, Western blot, and immunoprecipitation results showed mtGAT was phosphorylated by CK2. Next, we incubated mitochondria with CK2 and either ATP or GTP, in the presence of heparin, a known inhibitor of CK2. Heparin inhibited CK2-induced stimulation of mtGAT activity; this inhibition resulted in decreased (32)P-labeling of mtGAT. Additionally, mitochondria were treated with CK2 and [gamma-(32)P]ATP in the presence of staurosporine (a serine/threonine protein kinase inhibitor), genistein (a tyrosine kinase inhibitor), and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB, a CK2 inhibitor). Only DRB, the CK2 inhibitor, greatly reduced the amount of (32)P-incorporation into mtGAT by CK2. Finally, isolated mitochondrial outer membrane was incubated with cytosol in the presence of [gamma-(32)P]GTP; (32)P-labeled mtGAT was detected. Collectively, these data suggest that CK2 phosphorylates mtGAT. The impact of our results in the regulation of mtGAT and other anabolic processes is discussed.

The biochemistry of hypo- and hyperlipidemic fatty acid derivatives: metabolism and metabolic effects

A selection of amphipatic hyper- and hypolipidemic fatty acid derivatives (fibrates, thia- and branched chain fatty acids) are reviewed. They are probably all ligands for the peroxisome proliferation activation receptor (PPARalpha) which has a low selectivity for its ligands. These compounds give hyper- or hypolipidemic responses depending on their ability to inhibit or stimulate mitochondrial fatty acid oxidation in the liver. The hypolipidemic response is explained by the following metabolic effects: Lipoprotein lipase is induced in liver where it is normally not expressed. Apolipoprotein CIII is downregulated. These two effects in liver lead to a facilitated (re)uptake of chylomicrons and VLDL, thus creating a direct transport of fatty acids from the gut to the liver. Fatty acid metabolizing enzymes in the liver (CPT-I and II, peroxisomal and mitochondrial beta-oxidation enzymes, enzymes of ketogenesis, and omega-oxidation enzymes) are induced and create an increased capacity for fatty acid oxidation. The increased oxidation of fatty acids "drains" fatty acids from the body, reduces VLDL formation, and ultimately explains the antiadiposity and improved insulin sensitivity observed after administration of peroxisome proliferators.

Physiological and nutritional regulation of enzymes of triacylglycerol synthesis

Although triacylglycerol stores play the critical role in an organism's ability to withstand fuel deprivation and are strongly associated with such disorders as diabetes, obesity, and atherosclerotic heart disease, information concerning the enzymes of triacylglycerol synthesis, their regulation by hormones, nutrients, and physiological conditions, their mechanisms of action, and the roles of specific isoforms has been limited by a lack of cloned cDNAs and purified proteins. Fortunately, molecular tools for several key enzymes in the synthetic pathway are becoming available. This review summarizes recent studies of these enzymes, their regulation under varying physiological conditions, their purported roles in synthesis of triacylglycerol and related glycerolipids, the possible functions of different isoenzymes, and the evidence for specialized cellular pools of triacylglycerol and glycerolipid intermediates.

Phosphatidic acid, a key intermediate in lipid metabolism

Phosphatidic acid (PtdOH) is a key intermediate in glycerolipid biosynthesis. Two different pathways are known for de novo formation of this compound, namely (a) the Gro3P (glycerol 3-phosphate) pathway, and (b) the GrnP (dihydroxyacetone phosphate) pathway. Whereas the former route of PtdOH synthesis is present in bacteria and all types of eukaryotes, the GrnP pathway is restricted to yeast and mammalian cells. In this review article, we describe the enzymes catalyzing de novo formation of PtdOH, their properties and their occurrence in different cell types and organelles. Much attention has recently been paid to the subcellular localization of enzymes involved in the biosynthesis of PtdOH. In all eukaryotic cells, microsomes (ER) harbour the complete set of enzymes catalyzing these pathways and are thus the usual organelle for PtdOH formation. In contrast, the contribution of mitochondria to PtdOH synthesis is restricted to certain enzymes and depends on the cell type. In addition, chloroplasts of plants, lipid particles of the yeast, and peroxisomes of mammalian cells are significantly involved in PtdOH biosynthesis. Redundant systems of acyltransferases, the interplay of organelles, regulation of the pathway on the compartmental level, and finally the contribution of alternative pathways (phosphorylation of diacylglycerol and cleavage of phospholipids by phospholipases) to PtdOH biosynthesis appear to be required for the balanced formation of this important lipid intermediate. Dysfunction of enzymes involved in PtdOH synthesis can result in severe defects of various cellular processes. In this context, the possible physiological role(s) of PtdOH and its related metabolites, lysophosphatidic acid and diacylglycerol, will be discussed.

Phosphatidic acid synthesis in mitochondria. Topography of formation and transmembrane migration

The topography of formation and migration of phosphatidic acid (PA) in the transverse plane of rat liver mitochondrial outer membrane (MOM) were investigated. Isolated mitochondria and microsomes, incubated with sn-glycerol 3-phosphate and an immobilized substrate palmitoyl-CoA-agarose, synthesized both lyso-PA and PA. The mitochondrial and microsomal acylation of glycerophosphate with palmitoyl-CoA-agarose was 80-100% of the values obtained in the presence of free palmitoyl-CoA. In another series of experiments, both free polymyxin B and polymyxin B-agarose stimulated mitochondrial glycerophosphate acyltransferase activity approximately 2-fold. When PA loaded mitochondria were treated with liver fatty acid binding protein, a fifth of the phospholipid left the mitochondria. The amount of exportable PA reduced with the increase in the time of incubation. In another approach, PA-loaded mitochondria were treated with phospholipase A(2). The amount of phospholipase A(2)-sensitive PA reduced when the incubation time was increased. Taken together, the results suggest that lysophosphatidic acid (LPA) and PA are synthesized on the outer surface of the MOM and that PA moves to the inner membrane presumably for cardiolipin formation.

Transfer and subsequent metabolism of lysolipids studied by immobilizing subcellular compartments in alginate beads

The transfer and subsequent metabolism of lysophosphatidylcholine between subcellular compartments were studied in vitro by embedding membranes in alginate beads. After several experiments to validate the process, it was demonstrated that lysophosphatidylcholine was transferred from microsomes embedded in alginate beads to immobilized chloroplasts, that this transfer involved the partition of this molecule, and that the imported lysophosphatidylcholine was further used as substrate for phosphatidylcholine biosynthesis. More generally, the technique used makes it possible to avoid any cross-contamination between compartments, to evidence a transfer of molecules, and to study the metabolism of the imported molecules in the acceptor compartment.

On the relation between a stearoyl-specific transacylase from bovine testis membranes and a copurifying acyltransferase

Bovine testis membranes contain a coenzyme A-dependent transacylase that can catalyze the preferential transfer of stearoyl groups from phosphoglycerides to sn-2-acyl molecular species of lysophosphatidic acid and lysophosphatidylinositol [Itabe et al., (1992) J. Biol. Chem. 267, 15319-15325]. We have now purified this enzyme 1000-fold and shown that it copurifies with an acyltransferase. The purified transacylase can use phosphatidic acid, phosphatidylinositol, or phosphatidylinositol-4-phosphate as an acyl donor and catalyzes the transfer of stearoyl groups in preference to palmitoyl groups or oleoyl groups. In contrast, the purified acyltransferase uses acyl-coenzyme A as an acyl donor and shows no such preference for stearoyl group transfer. Furthermore, phosphatidylinositol-4, 5-bisphosphate inhibits the two enzymes to different extents and by different mechanisms. Nevertheless, the enzymes are similar in several respects: they use the same acyl acceptors and, when assayed together, compete for the acyl acceptor, sn-2-oleoyl lysophosphatidic acid; they lose activity in parallel unless stabilized by the addition of an anionic phosphoglyceride or stearoyl-coenzyme A; and they show similar sensitivities to heat and pH. One way to explain these results is to postulate that the transacylase reaction occurs in two successive steps: a stearoyl-specific first step in which a stearoyl group is transferred from an sn-1-stearoyl-2-acyl phosphoglyceride to coenzyme A, and a relatively non-acyl-chain-specific second step in which a stearoyl group is transferred from stearoyl-coenzyme A to an sn-2-acyl lysophosphoglyceride. According to this line of reasoning, the transacylase assay that we have used measures the net effect of both steps, whereas the acyltransferase assay measures only the effect of the second step.

Mammalian mitochondrial glycerol-3-phosphate acyltransferase

Glycerol-3-phosphate acyltransferase (GPAT) is the first committed, and presumed to be a rate-limiting, step in glycerophospholipid biosynthesis. There are two isoforms of GPAT, a mitochondrial and a microsomal form. Mitochondrial GPAT has recently been purified and its gene has been cloned and expressed in baculovirus-infected cells. The GPAT activity was reconstituted using the purified enzyme and various phospholipids. Mitochondrial GPAT prefers saturated fatty acyl-CoA as a substrate. This preference may contribute to the observed asymmetric distribution of saturated and unsaturated fatty acids at the sn-1 and sn-2 positions of cellular glycerophospholipids. A region of homology to various acyltransferases that may be important for catalysis or fatty acyl-CoA binding is present in mitochondrial GPAT. Mitochondrial GPAT is upregulated at the transcriptional level by refeeding a high carbohydrate, fat-free diet to previously fasted mice and by insulin administration to diabetic animals, whereas microsomal GPAT activity is largely unaffected by these treatments.

Increased hepatic beta-oxidation of docosahexaenoic acid, elongation of eicosapentaenoic acid, and acylation of lysophosphatidate in rats fed a docosahexaenoic acid-enriched diet

Rats were fed a diet supplemented with corn oil (n-3 deficient), soy oil, or a mixture containing 8% 22:6n-3 ethyl ester for 6 wk. The hepatic capacities for the beta-oxidation and synthesis of 22:6n-3, in addition to the acylation of lysophosphatidate, were tested in vitro. In rats that were fed a 22:6n-3-enriched diet, both the beta-oxidation of 22:6n-3 and elongation of 20:5n-3 were enhanced compared to those in rats fed the other diets. Acylation of lysophosphatidate was also enhanced in rats fed a 22:6n-3-enriched diet, while the rate of dephosphorylation of phosphatidate was not changed. The amount of 22:6n-3 in the liver was much less than that consumed in a docosahexaenoic acid-enriched diet. These results suggest that a significant amount of dietary 22:6n-3 was degraded via beta-oxidation, and that a portion of the retroconverted 20:5n-3 was recycled for the synthesis of 22:6n-3. The recycling of 20:5n-3 might contribute to the low level of 22:6n-3 in rats fed an n-3-deficient diet.

Mitochondrial phosphatidate is converted to triacylglycerol in rat hepatocytes

Phosphatidate is formed in both the endoplasmic reticulum and the outer mitochondrial membrane in rat liver. To investigate whether the phosphatidate synthesized in mitochondria can be converted to triacylglycerol in vivo, two experimental approaches were employed. (i) [3H]Phosphatidate-labeled mitochondria were enclosed in plasma membrane vesicles and these fused, in the presence of inactivated Sendai virus and calcium ions, to hepatocytes in monolayer culture. The recovery of radioactivity in various cell-associated lipids was measured. (ii) Mitochondrial phosphatidate was labeled with [14C]palmitate in hepatocytes which had been permeabilized with lysophosphatidylcholine and in which the microsomal glycerolphosphate acyltransferase had been inhibited with N-ethylmaleimide. The recovery of radioactivity in various lipids after incubation with particle free supernatant was measured. Evidence was obtained from both these experimental approaches that mitochondrial phosphatidate can be converted to triacylglycerol in rat hepatocytes. The results are discussed in relation to the role of mitochondrial phosphatidate in liver lipid metabolism.

A lysophosphatidic acid-binding cytosolic protein stimulates mitochondrial glycerophosphate acyltransferase

Rat liver cytosolic fraction caused up to five fold stimulation of mitochondrial glycerophosphate acyltransferase apparently by removing the lysophosphatidic acid formed by the acyltransferase. When mitochondria were incubated with palmityl-CoA, [2-3H]-sn-glycerol 3-phosphate and the cytosolic fraction and the supernatant fluid of the incubated mixture was passed through a Sephadex G-100 column, labeled lysophosphatidic acid eluted in three peaks with Mrs (i) 60-70 kDa, (ii) 10-20 kDa, and (iii) less than 5 kDa. Proteins, responsible for binding of lysophosphatidic acid in peaks (i) and (ii), were purified to near homogeneity as judged by electrophoretic analysis. The lysophosphatidic acid binding protein in peak (i) appears to be serum albumin and peak (iii) represents largely unbound lysophosphatidic acid. The 15 kDa protein, purified from peak (ii), bound lysophosphatidic acid, stimulated the acyltransferase and export of lysophosphatidic acid from mitochondria.