Accelerated maximal velocity of the red blood cell Na+/K+ pump in hyperlipidemia is related to increase in 1-palmitoyl,2-arachidonoyl-plasmalogen phosphatidylethanolamine

Composition of plasmalogens in serum lipoproteins from patients with non-alcoholic steatohepatitis and their susceptibility to oxidation

BACKGROUND

Plasmalogens are ether phospholipids (PL) with an alkenyl group including vinyl ether bound at the sn-1 position and a polyunsaturated fatty acid bound at the sn-2 position, and are susceptible to oxidation. To date, there are no reports on the relationship between plasmalogen in serum lipoproteins and non-alcoholic steatohepatitis (NASH), caused by multiple factors including oxidative stress. Here, we have investigated the distribution of plasmalogens in serum lipoproteins isolated from NASH patients and healthy volunteers.

METHODS

Serum lipoproteins were separated by gel-filtration chromatography, and analyzed for ethanolamine and choline plasmalogens using liquid chromatography-mass spectrometry.

RESULTS

Both plasmalogen levels were higher in HDL than in VLDL or LDL. The plasmalogens/PL ratio was significantly lower in NASH than controls, for all lipoprotein fractions. Ethanolamine plasmalogens containing 20:4 and 22:6 at the sn-2 position and choline plasmalogens containing 16:0 at the sn-1 position were predominant in each group. In oxidation test using LDL from healthy serum, both types of plasmalogens were decreased during the early stages of oxidation.

CONCLUSION

Plasmalogens could be a potential biomarker for evaluating the early stages of oxidation in NASH.

Plasmalogens inhibit APP processing by directly affecting γ-secretase activity in Alzheimer's disease

Lipids play an important role as risk or protective factors in Alzheimer's disease (AD). Previously it has been shown that plasmalogens, the major brain phospholipids, are altered in AD. However, it remained unclear whether plasmalogens themselves are able to modulate amyloid precursor protein (APP) processing or if the reduced plasmalogen level is a consequence of AD. Here we identify the plasmalogens which are altered in human AD postmortem brains and investigate their impact on APP processing resulting in Aβ production. All tested plasmalogen species showed a reduction in γ-secretase activity whereas β- and α-secretase activity mainly remained unchanged. Plasmalogens directly affected γ-secretase activity, protein and RNA level of the secretases were unaffected, pointing towards a direct influence of plasmalogens on γ-secretase activity. Plasmalogens were also able to decrease γ-secretase activity in human postmortem AD brains emphasizing the impact of plasmalogens in AD. In summary our findings show that decreased plasmalogen levels are not only a consequence of AD but that plasmalogens also decrease APP processing by directly affecting γ-secretase activity, resulting in a vicious cycle: Aβ reduces plasmalogen levels and reduced plasmalogen levels directly increase γ-secretase activity leading to an even stronger production of Aβ peptides.

Membrane plasmalogen composition and cellular cholesterol regulation: a structure activity study

Background

Disrupted cholesterol regulation leading to increased circulating and membrane cholesterol levels is implicated in many age-related chronic diseases such as cardiovascular disease (CVD), Alzheimer's disease (AD), and cancer. In vitro and ex vivo cellular plasmalogen deficiency models have been shown to exhibit impaired intra- and extra-cellular processing of cholesterol. Furthermore, depleted brain plasmalogens have been implicated in AD and serum plasmalogen deficiencies have been linked to AD, CVD, and cancer.

Results

Using plasmalogen deficient (NRel-4) and plasmalogen sufficient (HEK293) cells we investigated the effect of species-dependent plasmalogen restoration/augmentation on membrane cholesterol processing. The results of these studies indicate that the esterification of cholesterol is dependent upon the amount of polyunsaturated fatty acid (PUFA)-containing ethanolamine plasmalogen (PlsEtn) present in the membrane. We further elucidate that the concentration-dependent increase in esterified cholesterol observed with PUFA-PlsEtn was due to a concentration-dependent increase in sterol-O-acyltransferase-1 (SOAT1) levels, an observation not reproduced by 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase inhibition.

Conclusion

The present study describes a novel mechanism of cholesterol regulation that is consistent with clinical and epidemiological studies of cholesterol, aging and disease. Specifically, the present study describes how selective membrane PUFA-PlsEtn enhancement can be achieved using 1-alkyl-2-PUFA glycerols and through this action reduce levels of total and free cholesterol in cells.

Eicosanoid signalling pathways in the heart

Myocardial phospholipids serve as primary reservoirs of arachidonic acid (AA), which is liberated through the rate-determining hydrolytic action of cardiac phospholipases A2 (PLA2s). A predominant PLA2 in myocardium is calcium-independent phospholipase A2beta (iPLA2beta), which, through its calmodulin (CaM) and ATP-binding domains, is regulated by alterations in local cellular Ca2+ concentrations and cardiac bioenergetic status, respectively. Importantly, iPLA2beta has been demonstrated to be activated by ischaemia through elevation of the concentration of myocardial fatty acyl-CoA, which abrogates Ca2+/CaM-mediated inhibition of iPLA2beta. AA released by PLA2-catalysed hydrolysis of phospholipids serves as a precursor for eicosanoids generated by pathways dependent on cyclooxygenases (COX), lipoxygenases (LOX), and cytochromes P450 (CYP). Eicosanoids initiate and propagate diverse signalling cascades, primarily through their interaction with cellular receptors and ion channels. However, during pathologic states such as ischaemia or congestive heart failure, eicosanoids contribute to multiple maladaptive changes including inflammation, alterations of cellular growth programmes, and activation of multiple transcriptional events leading to the deleterious sequelae of these pathologic states. This review summarizes the central roles of myocardial PLA(2)s in eicosanoid signalling in the heart, the major COX, LOX, and CYP pathways of eicosanoid generation in the myocardium, and the effects of important eicosanoids on receptor-, ion channel-, and transcription-mediated processes that facilitate cardiac hypertrophy, mediate ischaemic preconditioning, and precipitate arrhythmogenesis in response to pathologic stimuli.

Relationships between membrane lipids and ion transport in red blood cells of Dahl rats

Distinct changes of membrane lipid content could contribute to the abnormalities of ion transport that take part in the development of salt hypertension in Dahl rats. The relationships between lipid content and particular ion transport systems were studied in red blood cells (RBC) of Dahl rats kept on low- and high-salt diets for 5 weeks since weaning. Dahl salt-sensitive (SS/Jr) rats on high-salt diet had increased blood pressure, levels of plasma triacylglycerols and total plasma cholesterol compared to salt-resistant (SR/Jr) rats. Furthermore, RBC of SS/Jr rats differed from SR/Jr ones by increased content of total membrane phospholipids, but membrane cholesterol was not changed significantly. SS/Jr rats had higher RBC intracellular Na+ (Na(i)+) content and enhanced bumetanide-sensitive Rb+ uptake. RBC membrane content of cholesterol and phospholipids correlated positively with RBC Na(i)+ content, with the activity of Na+-K+ pump and Na+-K+-2Cl- cotransport and also with Rb+ leak. The content of phosphatidylserines plus phosphatidylinositols was positively associated with RBC Na(i)+ content, with the activity of Na+-K+ pump and Na+-K+-2Cl- cotransport and with Rb+ leak. The content of sphingomyelins was positively related to Na+-K+-2Cl- cotransport activity and negatively to ouabain-sensitive Rb+-K+ exchange. We can conclude that observed relationships between ion transport and the membrane content of cholesterol and/or sphingomyelins, which are known to regulate membrane fluidity, might participate in the pathogenesis of salt hypertension in Dahl rats.

Altered Na+-K+ pump activity and plasma lipids in salt-hypertensive Dahl rats: relationship to Atp1a1 gene

A genetic variant of the gene for the alpha(1)-isoform of Na(+)-K(+)-ATPase (Atp1a1) was suggested to be involved in the pathogenesis of salt hypertension in Dahl rats through altered Na(+):K(+) coupling ratio. We studied Na(+)-K(+) pump activity in erythrocytes of Dahl salt-sensitive (SS/Jr) rats in relation to plasma lipids and blood pressure (BP) and the linkage of polymorphic microsatellite marker D2Arb18 (located within intron 1 and exon 2 of Atp1a1 gene) with various phenotypes in 130 SS/Jr x SR/Jr F(2) rats. Salt-hypertensive SS/Jr rats had higher erythrocyte Na(+) content, enhanced ouabain-sensitive (OS) Na(+) and Rb(+) transport, and higher Na(+):Rb(+) coupling ratio of the Na(+)-K(+) pump. BP of F(2) hybrids correlated with erythrocyte Na(+) content, OS Na(+) extrusion, and OS Na(+):Rb(+) coupling ratio, but not with OS Rb(+) uptake. In F(2) hybrids there was a significant association indicating suggestive linkage (P < 0.005, LOD score 2.5) of an intragenic marker D2Arb18 with pulse pressure but not with mean arterial pressure or any parameter of Na(+)-K(+) pump activity (including its Na(+):Rb(+) coupling ratio). In contrast, plasma cholesterol, which was elevated in salt-hypertensive Dahl rats and which correlated with BP in F(2) hybrids, was also positively associated with OS Na(+) extrusion. The abnormal Na(+):K(+) stoichiometry of the Na(+)-K(+) pump is a consequence of elevated erythrocyte Na(+) content and suppressed OS Rb(+):K(+) exchange. In conclusion, abnormal cholesterol metabolism but not the Atp1a1 gene locus might represent an important factor for both high BP and altered Na(+)-K(+) pump function in salt-hypertensive Dahl rats.

Plasmalogens: workhorse lipids of membranes in normal and injured neurons and glia

Plasmalogens are unique glycerophospholipids because they have an enol ether double bond at the sn-1 position of the glycerol backbone. They are found in all mammalian tissues, with ethanolamine plasmalogens 10-fold higher than choline plasmalogens except in muscles. The enol ether double bond at the sn-1 position makes plasmalogens more susceptible to oxidative stress than the corresponding ester-bonded glycerophospholipids. Plasmalogens are not only structural membrane components and a reservoir for second messengers but may also be involved in membrane fusion, ion transport, and cholesterol efflux. Plasmalogens may also act as antioxidants, thus protecting cells from oxidative stress. Receptor-mediated degradation of plasmalogens by plasmalogen-selective phospholipase A2 results in the generation of arachidonic acid, eicosanoids, and platelet activating factor. Low levels of these metabolites have trophic effects, but at high concentration they are cytotoxic and may be involved in allergic response, inflammation, and trauma. Levels of plasmalogens are decreased in several neurological disorders including Alzheimer's disease, ischemia, and spinal cord trauma. This may be due to the stimulation of plasmalogen-selective phospholipase A2. A deficiency of plasmalogens in peroxisomal disorders and Niemann-Pick type C disease indicates that this deficiency may be due to the decreased activity of plasmalogen synthesizing enzymes that occur in peroxisomes.

Chronic dietary n-3 polyunsaturated fatty acids deficiency affects the fatty acid composition of plasmenylethanolamine and phosphatidylethanolamine differently in rat frontal cortex, striatum, and cerebellum

As chronic consumption of a diet devoid of n-3 fatty acid induced modification of neurotransmission pathways in the frontal cortex of rats, plasmalogen alteration could occur in this area. Because of the propensity to facilitate membrane fusion, plasmenylethanolamine (PmE), a major plasmalogen of brain, may be involved in synaptic transmission. Female rats were fed diet containing peanut oil [(n-3)-deficient diet] through two generations. Two weeks before mating, half of the female rats of the second generation received a diet containing peanut oil and rapeseed oil (control group). The distribution and acyl composition of major phospholipids, phosphatidylethanolamine and PmE, were measured in the frontal cortex, striatum, and cerebellum of the male progeny of the two groups at 60 d of age. The n-3 polyunsaturated fatty acid (PUFA) deficiency had no effect on the distribution of phospholipids in all brain regions but affected their acyl composition differently. The level of 22:6n-3 was significantly lower and compensated for by higher levels of n-6 fatty acids in all regions and phospholipids studied. However, docosahexaenoic acid, being more concentrated in the PmE of frontal cortex, is also more decreased in the n-3-deficient rats compared to the striatum. By contrast, striatum PmE has retained more 22:6n-3 than PmE of the other regions. In addition, the increase of n-6 PUFA was significantly lower in frontal cortex PmE compared to the striatum and cerebellum PmE. In association with altered neurotransmission observed in frontal cortex of n-3-deficient rats, our results suggest that frontal cortex PmE might be more affected in chronically alpha-linolenic-deficient rats. However, by retaining 22:6n-3, striatum PmE could be most resilient.

Ether lipids in biomembranes

Plasmalogens (1-O-1'-alkenyl-2-acylglycerophospholipids) and to a lesser extent the 1-O-alkyl analogs are ubiquitous and in some cases major constituents of mammalian cellular membranes and of anaerobic bacteria. In archaebacteria polar lipids of the cell envelope are either diphytanylglycerolipids or bipolar macrocyclic tetraether lipids capable of forming covalently linked 'bilayers'. Information on the possible role of ether lipids as membrane constituents has been obtained from studies on the biophysical properties of model membranes consisting of these lipids. In addition, effects of modified ether lipid content on properties of biological membranes have been investigated using microorganisms or mammalian cells which carry genetic defects in ether lipid biosynthesis. Differential utilization of ether glycerophospholipids by specific phospholipases might play a role in the generation of lipid mediators that are involved in signal transduction. A possible function of plasmalogens as antioxidants has been demonstrated with cultured cells and might play a role in serum lipoproteins. Synthetic ether lipid analogs exert cytostatic effects, most likely by interfering with membrane structure and by specific interaction with components of signal transmission pathways, such as phospholipase C and protein kinase C.