Acyl-trafficking in membrane phospholipid fatty acid turnover: the transfer of fatty acid from the acyl-L-carnitine pool to membrane phospholipids in intact human erythrocytes

Factors influencing the eicosanoids synthesis in vivo

External factors activate a sequence of reactions involving the reception, transduction, and transmission of signals to effector cells. There are two main phases of the body's reaction to harmful factors: the first aims to neutralize the harmful factor, while in the second the inflammatory process is reduced in size and resolved. Secondary messengers such as eicosanoids are active in both phases. The discovery of lipoxins and epi-lipoxins demonstrated that not all arachidonic acid (AA) derivatives have proinflammatory activity. It was also revealed that metabolites of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) such as resolvins, protectins, and maresins also take part in the resolution of inflammation. Knowledge of the above properties has stimulated several clinical trials on the influence of EPA and DHA supplementation on various diseases. However, the equivocal results of those trials prevent the formulation of guidelines on EPA and DHA supplementation. Prescription drugs are among the substances with the strongest influence on the profile and quantity of the synthesized eicosanoids. The lack of knowledge about their influence on the conversion of EPA and DHA into eicosanoids may lead to erroneous conclusions from clinical trials.

Carnitine and acylcarnitines: pharmacokinetic, pharmacological and clinical aspects

L-Carnitine (levocarnitine) is a naturally occurring compound found in all mammalian species. The most important biological function of L-carnitine is in the transport of fatty acids into the mitochondria for subsequent β-oxidation, a process which results in the esterification of L-carnitine to form acylcarnitine derivatives. As such, the endogenous carnitine pool is comprised of L-carnitine and various short-, medium- and long-chain acylcarnitines. The physiological importance of L-carnitine and its obligatory role in the mitochondrial metabolism of fatty acids has been clearly established; however, more recently, additional functions of the carnitine system have been described, including the removal of excess acyl groups from the body and the modulation of intracellular coenzyme A (CoA) homeostasis. In light of this, acylcarnitines cannot simply be considered by-products of the enzymatic carnitine transfer system, but provide indirect evidence of altered mitochondrial metabolism. Consequently, examination of the contribution of L-carnitine and acylcarnitines to the endogenous carnitine pool (i.e. carnitine pool composition) is critical in order to adequately characterize metabolic status. The concentrations of L-carnitine and its esters are maintained within relatively narrow limits for normal biological functioning in their pivotal roles in fatty acid oxidation and maintenance of free CoA availability. The homeostasis of carnitine is multifaceted with concentrations achieved and maintained by a combination of oral absorption, de novo biosynthesis, carrier-mediated distribution into tissues and extensive, but saturable, renal tubular reabsorption. Various disorders of carnitine insufficiency have been described but ultimately all result in impaired entry of fatty acids into the mitochondria and consequently disturbed lipid oxidation. Given the sensitivity of acylcarnitine concentrations and the relative carnitine pool composition in reflecting the intramitochondrial acyl-CoA to free CoA ratio (and, hence, any disturbances in mitochondrial metabolism), the relative contribution of L-carnitine and acylcarnitines within the total carnitine pool is therefore considered critical in the identification of mitochondria dysfunction. Although there is considerable research in the literature focused on disorders of carnitine insufficiency, relatively few have examined relative carnitine pool composition in these conditions; consequently, the complexity of these disorders may not be fully understood. Similarly, although important studies have been conducted establishing the pharmacokinetics of exogenous carnitine and short-chain carnitine esters in healthy volunteers, few studies have examined carnitine pharmacokinetics in patient groups. Furthermore, the impact of L-carnitine administration on the kinetics of acylcarnitines has not been established. Given the importance of L-carnitine as well as acylcarnitines in maintaining normal mitochondrial function, this review seeks to examine previous research associated with the homeostasis and pharmacokinetics of L-carnitine and its esters, and highlight potential areas of future research.

Expression patterns of the organic cation/carnitine transporter family in adult murine brain

Organic cation/carnitine transporters transport carnitine, drugs, and xenobiotics (e.g. choline, acetylcarnitine, betaine, valproic acid), and are expressed in muscle, heart, blood vessels, kidney, gut, etc.

OBJECTIVE

To characterize expression patterns of mOctn1, -2 and -3 in murine brain.

METHODS

We applied our transporter-specific antibodies to mOctn1, -2 and -3, followed by 2 0 antibody and DAB peroxidase detection to serial adult murine brain sections counterstained with hematoxylin.

RESULTS

All three transporters showed strong expression in the external plexiform layer of the olfactory bulb and in olfactory nerve, the molecular layer and neuronal processes of input fibres extending vertically in motor cortex, in the dendritic arborization of the cornu ammonis and dendate gyrus (hippocampus), neuronal processes in the arcuate nucleus (hypothalamus), choroid plexus cells, and neuronal cell bodies and dendrites of cranial nerve nuclei V and VII. In the cerebellum, all three transporters were strongly expressed in dendritic processes of Purkinje cells, but Octn1 and -2 were expressed more strongly than Octn3 in Purkinje cell bodies. In spinal cord, Octn1, -2 and -3 were prominent in axons and dendritic end-arborizations of spinal cord neurons in both ascending and descending white matter tracts, whereas Octn3 was also strongly expressed in grey matter, specifically in anterior horn cell bodies. Octn3 was weakly expressed in glomerular layer neuronal cell bodies of olfactory bulb.

CONCLUSIONS

hOCTN2 deficiency presents with carnitine-responsive cardiomyopathy, myopathy and hypoglycemic, hypoketotic coma with strokes, seizures and delays. In mouse, Octn1, -2 and -3 are expressed in many regions throughout the central nervous system with a pattern suggestive of roles in modulating cerebral bioenergetics and in acetylcholine generation for neurotransmission in olfactory, satiety, limbic, memory, motor and sensory functions. This distribution may play a role in the pattern of neurological injury that occurs in hOCTN2 deficiency during catabolic episodes of hypoglycemic, hypoketotic encephalopathy and which may manifest with cognitive impairment, hypotonia and seizures.

A novel brain-expressed protein related to carnitine palmitoyltransferase I

Malonyl-CoenzymeA acts as a fuel sensor, being both an intermediate of fatty acid synthesis and an inhibitor of the two known isoforms of carnitine palmitoyltransferase I (CPT I), which control mitochondrial fatty acid oxidation. We describe here a novel CPT1 family member whose mRNA is present predominantly in brain and testis. Chromosomal locations and genome organization are reported for the mouse and human genes. The protein sequence contains all the residues known to be important for both carnitine acyltransferase activity and malonyl-CoA binding in other family members. Yeast expressed protein has no detectable catalytic activity with several different acyl-CoA esters that are good substrates for other carnitine acyltransferases, including the liver isoform of CPT I, which is also expressed in brain; however, it displays high-affinity malonyl-CoA binding. Thus this new CPT I related protein may be specialized for the metabolism of a distinct class of fatty acids involved in brain function.

Inhibition of platelet-activating factor synthesis in human neutrophils and platelets by propionyl-L-carnitine

Propionyl-L-carnitine (PrC) has been shown to exert beneficial effects in the treatment of myocardial and peripheral ischemia in man. These conditions are associated with the activation of circulating neutrophils and platelets. To determine whether PrC could affect the synthesis of lipid mediators known to influence neutrophil and platelet functions, we explored the effects of PrC on the synthesis of platelet-activating factor (PAF) and arachidonic acid (AA) metabolites. Preincubation (90 min) of human neutrophils with PrC (0.1-100 microM) inhibited the synthesis of PAF and of a PAF analog (1-alkyl-1'enyl-2-acetyl-sn-glycero-3-phosphoethanolamine: AEGPE) induced in vitro by the calcium ionophore A23187. In contrast, concentrations of PrC up to 100 microM did not influence the uptake of exogenous AA or the A23187-induced release of AA and eicosanoids from neutrophils in vitro. PrC (1 microM) also inhibited PAF synthesis from human platelets stimulated in vitro with thrombin, but had no effect on thrombin-induced aggregation. Oral administration of PrC (2 g/day for two weeks) to five normal volunteers resulted in a significant inhibition of PAF and AEGPE synthesis by neutrophils stimulated with A23187 ex vivo, with no effect on AA or eicosanoid release. These data indicate that PrC selectively inhibits in vitro and ex vivo PAF synthesis from human neutrophils and platelets without influencing AA metabolism or eicosanoid release. This effect of PrC might represent an additional mechanism by which this molecule can exert protective effects in tissue ischemia and in other inflammatory diseases associated with neutrophil and platelet activation.

Effects of L-carnitine on leukocyte function and viability in hemodialysis patients: A double-blind randomized trial

Excess morbidity and mortality among long-term hemodialysis patients because of infectious complications is partly caused by an impairment of cellular immune defense. We hypothesized this impairment is related to an abnormal carnitine metabolism also present in these patients. In a double-blind, randomized, placebo-controlled trial, we investigated the effect of L-carnitine on phagocytic function and viability of blood leukocytes in 17 patients undergoing maintenance hemodialysis. After an observation period of 1 month, the patients received either 10 mg/kg of L-carnitine or placebo intravenously at the end of each hemodialysis session over a period of 4 months. Leukocyte oxidative metabolism was measured by means of luminol-enhanced chemiluminescence and superoxide generation after stimulation with Staphylococcus aureus or phorbol myristate acetate. Killing capacity and phagocytosis of radiolabeled staphylococci were determined. A lactate dehydrogenase (LDH) release test was applied to assess cell viability. We were unable to show an effect of L-carnitine on phagocytic function and viability in vivo. Several clinical parameters were observed during the trial. No statistically significant differences concerning dialysis-related morbidity, anemia, or reduction of blood urea nitrogen and creatinine levels were detected. Additionally, we tested the effect of L-carnitine on phagocytic function after in vitro incubation of blood leukocytes, which also showed no changes. LDH release was decreased, indicating an improved viability of these cells. The latter results were found after in vitro incubation of cells, but could not be confirmed in vivo. In summary, we could not show beneficial effects of L-carnitine administration in hemodialysis patients for the dosage and duration of treatment stated, either on phagocytic function and viability or on the clinical and biochemical parameters observed.

Estrogen and tamoxifen metabolites protect smooth muscle cell membrane phospholipids against peroxidation and inhibit cell growth

The goal of this study was to test the hypothesis that antioxidant estrogens, by a mechanism independent of the estrogen receptor, protect phospholipids residing in the plasma membrane of vascular smooth muscle cells from peroxidation and peroxidation-induced cell growth and migration. Peroxidation of membrane phospholipids was assessed by HPLC analysis of phospholipids extracted from rat aortic vascular smooth muscle cells prelabeled with cis-parinaric acid (a fatty acid that is susceptible to peroxidation, which quenches its fluorescent properties). Incubation of cells for 2 hours with the peroxyl radical donor 2,2'-azobis-2,4-dimethylvaleronitrile (AMVN) caused peroxidation of all measured membrane phospholipids. This effect was attenuated by pretreating cells for 15 minutes with 50 to 5000 ng/mL of 2-hydroxyestradiol (strong antioxidant but weak estrogen-receptor ligand) or 4-hydroxytamoxifen (strong antioxidant and potent estrogen-receptor ligand), but not by estrone or droloxifene (both weak antioxidants but potent estrogen-receptor ligands). Moreover, pretreatment of cells for 20 hours with physiological concentrations (0.3 ng/mL) of 2-hydroxyestradiol or pharmacologically relevant concentrations of 4-hydroxytamoxifen (40 ng/mL) also decreased AMVN-induced phospholipid peroxidation. Both 2-hydroxyestradiol and 4-hydroxytamoxifen were as effective as 2,2,5, 7,8-pentamethyl-6-hydrochromane (an antioxidant homolog of vitamin E) in attenuating AMVN-induced peroxidation of membrane phospholipids. Also, physiological concentrations of 2-hydroxyestradiol, but not estrone, and pharmacologically relevant concentrations of 4-hydroxytamoxifen attenuated AMVM-induced DNA synthesis, cell proliferation, and cell migration. These studies demonstrate in vascular smooth muscle cells that antioxidant estrogens via a non-estrogen receptor-dependent mechanism attenuate peroxidation of membrane phospholipids and peroxidation-induced cell growth and migration.

Paraquat-induced phosphatidylserine oxidation and apoptosis are independent of activation of PLA2

Paraquat is a pneumotoxin that causes lung injury by enhancing oxidative stress; however, the cellular responses to these redox events are undefined. We previously showed that paraquat produced selective peroxidation of phosphatidylserine that preceded apoptosis in 32D cells. We now report that the phospholipase A2 (PLA2) inhibitor quinacrine can attenuate phosphatidylserine oxidation and also block paraquat-induced apoptosis. Therefore, we investigated the potential for PLA2 to mediate apoptosis after paraquat. We found that, in contrast to quinacrine, the PLA2 inhibitors manoalide, aristolochic acid, and arachidonyl trifluoromethylketone failed to prevent paraquat-induced apoptosis. Moreover, no evidence of PLA2 activation was observed within 7 h after paraquat exposure. Finally, quinacrine failed to inhibit basal and 4-bromo-A-23187-induced release of [3H]arachidonic acid at concentrations that protected paraquat-induced apoptosis. We conclude that paraquat-induced phosphatidylserine oxidation and apoptosis occurred in the absence of PLA2 activation and that quinacrine protected phosphatidylserine and cell viability after paraquat in a PLA2-independent manner.

Amphotericin B as an intracellular antioxidant: protection against 2,2'-azobis(2,4-dimethylvaleronitrile)-induced peroxidation of membrane phospholipids in rat aortic smooth muscle cells

The antifungal activity of amphotericin B (AmB) and its side-effects (e.g. nephrotoxicity and hemolytic action) are suggested to be associated with its prooxidant effects in target cells. To test this hypothesis, we have undertaken studies to examine the role of AmB in oxidative stress in cultured rat aortic smooth muscle cells (SMC) incubated in the absence or in the presence of a lipid-soluble azo-initiator of peroxyl radicals, 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN). No changes in the pattern of membrane phospholipids could be detected by two-dimensional high performance thin-layer chromatography (HPTLC) after oxidative stress induced by AMVN in which the cells remained viable, as judged by trypan blue exclusion. To improve the sensitivity of detection of oxidative stress in the cells, cis-parinaric acid (PnA) was incorporated biosynthetically into the membrane phospholipids [using PnA-human serum albumin (hSA) complex]. Incubation of the cells under aerobic conditions in the presence of up to 10 microM AmB showed no significant change in the pattern of PnA-labeled phospholipids, suggesting that AmB was not affecting the oxidative state of the cells. In contrast, treatment with AMVN (0.5 mM, incubation in the dark for 2 hr at 37 degrees--conditions in which the viability of the cells was maintained) caused a significant reduction of all fluorescently labeled phospholipid fractions separated by HPLC. When PnA-labeled cells were subjected to oxidative stress by incubation with 0.5 mM AMVN in the presence of AmB, the loss of fluorescent phospholipids was reduced in a concentration-dependent manner over a concentration range of 0.25 to 10 microM. Thus, AmB does not produce any prooxidant effect but rather acts as an intracellular antioxidant.

Overexpression of metallothionein decreases sensitivity of pulmonary endothelial cells to oxidant injury

Metallothionein (MT) is a low-molecular-weight cysteine-rich protein with extensive metal binding capacity and potential nonenzymatic antioxidant activity. Despite the sensitivity of vascular endothelium to either heavy metal toxicity or oxidative stress, little is known regarding the role of MT in endothelial cells. Accordingly, we determined the sensitivity of cultured sheep pulmonary artery endothelial cells (SPAEC) that overexpressed MT to tert-butyl hydroperoxide (t-BOOH), hyperoxia, or 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN; peroxyl radical generator). Nontoxic doses of 10 microM Cd increased MT levels from 0.21 +/- 0.03 to 2.07 +/- 0.24 microg/mg and resulted in resistance to t-BOOH and hyperoxia as determined by reduction of Alamar blue or [3H]serotonin transport, respectively. SPAEC stably transfected with plasmids containing either mouse or human cDNA for MT were resistant to both t-BOOH and hyperoxia. In addition, we examined transition metal-independent, noncytotoxic AMVN-induced lipid peroxidation after metabolic incorporation of the oxidant-sensitive fluorescent fatty acid cis-parinaric acid into phospholipids and high-performance liquid chromatography separation. SPAEC that overexpressed MT after gene transfer completely inhibited peroxyl oxidation of phosphatidylserine, phosphatidylcholine, and sphingomyelin (but not phosphatidylethanolamine) noted in wild-type SPAEC. These data show for the first time that MT can 1) protect pulmonary artery endothelium against a diverse array of prooxidant stimuli and 2) directly intercept peroxyl radicals in a metal-independent fashion, thereby preventing lipid peroxidation in intact cells.

Reduced carnitine palmitoyl transferase activity and altered acyl-trafficking in red blood cells from hemodialysis patients

We measured carnitine palmitoyl transferase activity, free carnitine, and long chain acyl carnitine levels in erythrocytes from 15 uremic patients and 25 controls. Carnitine palmitoyl transferase levels in patients were significantly lower than in controls. The levels of free carnitine and long chain acyl carnitines as well as the long chain acyl carnitine/free carnitine ratio were significantly higher in patients than in controls. Our results suggest that hemodialysis causes alteration in the acyl-trafficking in red blood cells membrane.

Erythrocyte membrane reacylation in juvenile neuronal ceroid-lipofuscinosis: measurement of membrane-bound carnitine palmitoyl transferase, acyl-CoA synthetase, and lysophospholipid: acyl-CoA acyltransferase activities

In order to study the biochemical mechanisms responsible for the membrane fatty acid deficiency in juvenile neuronal ceroid-lipofuscinosis, we have analyzed the reacylation pathway in isolated erythrocyte membranes in 5 patients. We studied membrane carnitine palmitoyl transferase, and developed a combined assay to study acyl-CoA synthetase and lysophospholipid acyl-CoA acyltransferase activities. There were no significant differences between control and patient membranes, suggesting that abnormalities in these 3 putative candidate enzymes are not responsible for the disease.

The variation of carnitine content in human blood cells during disease--a study in bacterial infection and inflammatory bowel disease

Carnitine in erythrocytes and leucocytes represents a small but essential part of the cellular carnitine pool. It was the objective of this study to document the changes of blood cell carnitine concentrations in disease entities with an enhanced cellular metabolism during acute and chronic inflammation. The plasma, erythrocyte, lymphocyte, granulocyte and thrombocyte carnitine concentrations were determined in 23 patients (11.0 +/- 8.8 years) with bacterial infections and nine patients (17.5 +/- 2.4 years) with Crohn disease and compared to 20 healthy controls (27.0 +/- 10.6 years). In patients with bacterial infections the granulocyte carnitine concentrations (126.4 +/- 73.5 nmoles/10(6) cells) were higher (P < 0.001) than in controls (37.9 +/- 22.8 nmoles/10(6) cells). In patients with Crohn disease the lymphocyte carnitine concentrations (169.4 +/- 108.2 nmoles/10(6) cells) were increased (P < 0.001) when compared to controls (48.1 +/- 18.3 nmoles/10(6) cells). The plasma carnitine concentrations were decreased (P < 0.05) in both patient groups, whereas they were increased (P < 0.05) in the patients' erythrocytes. The carnitine concentrations in thrombocytes did not differ significantly within the individual groups.

CONCLUSION

Lymphocyte and granulocyte carnitine concentrations may reflect their enhanced metabolic state during either immunoglobulin formation or phagocytosis.

Effect of L-carnitine and acylcarnitine derivatives on the proliferation and monoclonal antibody production of mouse hybridoma cells in culture

The effect of L-carnitine (Cn) on cell growth metabolism and antibody production rates was investigated using the murine hybridoma cell line, Mark3, in batch and fed harvest cultures. Two acylcarnitine derivatives were also tested: palmitoyl L-Cn and acetyl-DL-Cn. The addition of 20 microM L-Cn to cultures of Mark3 hybridoma cells that had been adapted to L-Cn significantly stimulated monoclonal antibody (mAb) production without affecting cell growth. In contrast, mAb secretion slightly decreased when L-Cn was added to culture of cells that had not been adapted to L-Cn. Palmitoyl L-Cn also stimulated mAb production by adapted cells, whereas the acetyl-DL-Cn, reduced mAb secretion. The presence of L-Cn in the medium did not affect glucose consumption or lactate production, but the metabolism of some amino acids was altered. The medium concentrations of valine, leucine, isoleucine and lysine were enhanced from 22% to 41% according to amino acids, whereas those of alanine, glycine and proline decreased. The mechanism by which L-carnitine affects mAb production and the metabolism of some amino acids is unknown. This effect is likely to be indirect, since there was no net entry of L-[3H]carnitine into the cells.