Acyltransferase activities in adult rat type II pneumocyte-derived subcellular fractions

Quantifying Functional Impairment of ABCA3 Variants Associated with Interstitial Lung Disease

ATP-binding cassette subfamily A member 3 (ABCA3) is a lipid transporter within alveolar type II cells. Patients with bi-allelic variants in ABCA3 may suffer from a variable severity of interstitial lung disease. We characterized and quantified ABCA3 variants' overall lipid transport function by assessing the in vitro impairment of its intracellular trafficking and pumping activity. We expressed the results relative to the wild type, integrated the quantitative readouts from eight different assays and used newly generated data combined with previous results to correlate the variants' function and clinical phenotype. We differentiated normal (within 1 normalized standard deviation (nSD) of the wild-type mean), impaired (within 1 to 3 nSD) and defective (beyond 3 nSD) variants. The transport of phosphatidylcholine from the recycling pathway into ABCA3⁺ vesicles proved sensitive to the variants' dysfunction. The sum of the quantitated trafficking and pumping predicted a clinical outcome. More than an approximately 50% loss of function was associated with considerable morbidity and mortality. The in vitro quantification of ABCA3 function enables detailed variant characterization, substantially improves the phenotype prediction of genetic variants and possibly supports future treatment decisions.

LPCAT1 regulates surfactant phospholipid synthesis and is required for transitioning to air breathing in mice

Respiratory distress syndrome (RDS), which is the leading cause of death in premature infants, is caused by surfactant deficiency. The most critical and abundant phospholipid in pulmonary surfactant is saturated phosphatidylcholine (SatPC), which is synthesized in alveolar type II cells de novo or by the deacylation-reacylation of existing phosphatidylcholine species. We recently cloned and partially characterized a mouse enzyme with characteristics of a lung lysophosphatidylcholine acyltransferase (LPCAT1) that we predicted would be involved in surfactant synthesis. Here, we describe our studies investigating whether LPCAT1 is required for pulmonary surfactant homeostasis. To address this issue, we generated mice bearing a hypomorphic allele of Lpcat1 (referred to herein as Lpcat1GT/GT mice) using a genetrap strategy. Newborn Lpcat1GT/GT mice showed varying perinatal mortality from respiratory failure, with affected animals demonstrating hallmarks of respiratory distress such as atelectasis and hyaline membranes. Lpcat1 mRNA levels were reduced in newborn Lpcat1GT/GT mice and directly correlated with SatPC content, LPCAT1 activity, and survival. Surfactant isolated from dead Lpcat1GT/GT mice failed to reduce minimum surface tension to wild-type levels. Collectively, these data demonstrate that full LPCAT1 activity is required to achieve the levels of SatPC essential for the transition to air breathing.

Lysophospholipid acyltransferases: novel potential regulators of the inflammatory response and target for new drug discovery

Molecular and biochemical analyses of membrane phospholipids have revealed that, in addition to their physico-chemical properties, the metabolites of phospholipids play a crucial role in the recognition, signalling and responses of cells to a variety of stimuli. Such responses are mediated in large part by the removal and/or addition of different acyl chains to provide different phospholipid molecular species. The reacylation reactions, catalysed by specific acyltransferases control phospholipid composition and the availability of the important mediators free arachidonic acid and lysophospholipids. Lysophospholipid acyltransferases are therefore key control points for cellular responses to a variety of stimuli including inflammation. Regulation or manipulation of lysophospholipid acyltransferases may thus provide important mechanisms for novel anti-inflammatory therapies. This review will highlight mammalian lysophospholipid acyltransferases with particular reference to the potential role of lysophosphatidylcholine acyltransferase and its substrates in sepsis and other inflammatory conditions and as a potential target for novel anti-inflammatory therapies.

Identification and characterization of a lysophosphatidylcholine acyltransferase in alveolar type II cells

Pulmonary surfactant is a complex of lipids and proteins produced and secreted by alveolar type II cells that provides the low surface tension at the air-liquid interface. The phospholipid most responsible for providing the low surface tension in the lung is dipalmitoylphosphatidylcholine. Dipalmitoylphosphatidylcholine is synthesized in large part by phosphatidylcholine (PC) remodeling, and a lysophosphatidylcholine (lysoPC) acyltransferase is thought to play a critical role in its synthesis. However, this acyltransferase has not yet been identified. We have cloned full-length rat and mouse cDNAs coding for a lysoPC acyltransferase (LPCAT). LPCAT encodes a 535-aa protein of approximately 59 kDa that contains a transmembrane domain and a putative acyltransferase domain. When transfected into COS-7 cells and HEK293 cells, LPCAT significantly increased lysoPC acyltransferase activity. LPCAT preferred lysoPC as a substrate over lysoPA, lysoPI, lysoPS, lysoPE, or lysoPG and prefers palmitoyl-CoA to oleoyl-CoA as the acyl donor. This LPCAT was preferentially expressed in the lung, specifically within alveolar type II cells. Expression in the fetal lung and in rat type II cells correlated with the expression of the surfactant proteins. LPCAT expression in fetal lung explants was sensitive to dexamethasone and FGFs. KGF was a potent stimulator of LPCAT expression in cultured adult type II cells. We hypothesize that LPCAT plays a critical role in regulating surfactant phospholipid biosynthesis and suggest that understanding the regulation of LPCAT will offer important insight into surfactant phospholipid biosynthesis.

Lysophosphatidylethanolamine acyltransferase activity is elevated during cardiac cell differentiation

We examined if elevation in lysophosphatidylethanolamine acyltransferase activity was associated with elevation in phosphatidylethanolamine content during differentiation of P19 teratocarcinoma cells into cardiac myocytes. P19 cells were induced to undergo differentiation into cardiac myocytes by the addition of 1% dimethylsulfoxide to the medium. Immunofluorescence microscopy revealed the presence of striated myosin at 8 days post-dimethylsulfoxide addition confirming differentiation into cardiac cells. The content of phosphatidylethanolamine was increased 2.1-fold (P<0.05) in differentiated cells compared to undifferentiated cells, whereas the content of phosphatidylcholine was reduced 29% (P<0.05). There were no alterations in the pool sizes of other phospholipids, including cardiolipin. The relative abundance of fatty acids in phospholipids of P19 cells was 18:1 > 18:0 > 16:1 = 18:2 > 16:0 = 14:0 > 20:4 and differentiation did not affect the relative amounts of these fatty acids within individual phospholipids. When cells were incubated with [1,3-(3)H]glycerol, radioactivity incorporated into phosphatidylethanolamine was elevated 5.8-fold, whereas radioactivity incorporated into phosphatidylcholine was unaltered. Ethanolaminephosphotransferase, cholinephosphotransferase and membrane CTP:phosphocholine cytidylyltransferase activities were elevated in differentiated cells compared to undifferentiated cells, whereas membrane and cytosolic phospholipase A2 activities were unaltered. Lysophosphatidylethanolamine acyltransferase activities were elevated 2.4-fold (P<0.05). Lysophosphatidylcholine acyltransferase, monolysocardiolipin acyltransferase, acyl-Coenzyme A synthetase and acyl-Coenzyme A hydrolase activities were unaltered in differentiated cells compared to undifferentiated cells. We postulate that during cardiac cell differentiation, the observed elevation in lysophosphatidylethanolamine acyltransferase activity accompanies the elevation in phosphatidylethanolamine mass, possibly to maintain the fatty acyl composition of this phospholipid within the membrane.

Substrate specificity of acyl-CoA:Lysophospholipid acyltransferase (LAT) from pig spleen

The present investigation was undertaken to gain insights into the nature of both substrate binding sites of acyl-CoA:lysophospholipid acyltransferase (LAT) which could be potentially useful for the identification and purification of this specific acyltransferase. Therefore, we have investigated the specificity of LAT from crude membranes of pig spleen toward various 1-palmitoyl-glycerophospholipids and 1-acyl-glycerophosphocholines (1-acyl-GPC). The enzyme showed the highest specificity toward 1-acyl-GPC and was able to distinguish between the acyl-chain length of the 1-acyl group within the 1-acyl-GPC molecule. We found preferential reactivity in the order C10:0 < C12:0 << C14:0, C18:0, C16:0 < C18:1 of 1-acyl-GPC. Lysophosphatidic acid or 1-O-alkyl-GPC were only poor substrates for the enzyme. In competition studies we could show that palmitic acid, oleic acid, arachidonic acid, and palmitoyl-CoA competitively inhibited LAT activity, whereas the coenzyme A failed to inhibit LAT enzyme activity in a concentration-dependent manner. We concluded that the ligand acyl-CoA is bound via its acyl chain. The finding that palmitoyl-CoA was a poor substrate as well as an inhibitor was the basis for protein purification. When palmitoyl-CoA-agarose was used as matrix for affinity chromatography, LAT enzyme activity was bound and eluted by high salt concentrations yielding an estimated 10-fold purification of the solubilized LAT enzyme.

Studies on the formation of dipalmitoyl species of phosphatidylcholine and phosphatidylethanolamine in pulmonary type II cells

Endogenous content of and incorporation of labelled glycerol into alkenylacyl-, alkylacyl- and diacyl-glycerol, -glycerol-3-phosphocholine and -glycero-3-phosphoethanolamine of pulmonary type II cells were measured. On prolonged incubation of type II cells with labelled glycerol, the proportion of label incorporated into the diacyl subclass of these glycerolipids increased and the proportion of label incorporated into the ether lipids declined. Endogenous phosphatidylcholine (PtdCho) of type II cells contained 38.4% of the dipalmitoyl species, but endogenous phosphatidylethanolamine (PtdEtn) only 2.5%. In contrast, similar proportions of labelled glycerol were incorporated into dipalmitoyl-PtdCho and -PtdEtn after short-time incubation but, with prolonged incubation time the proportion of labelled dipalmitoyl-PtdCho increased from 11.3 to 18.8%, whereas that of dipalmitoyl-PtdEtn did not change significantly. Type II cell membranes were found to exhibit cofactor-independent and CoA-mediated transacylations of [1-14C]palmitoyl-lyso-PtdCho and -lyso-PtdEtn. The distribution of label among the palmitic acid-containing species of PtdCho and PtdEtn formed by both transacylation activities was determined. Cofactor-independent and CoA-mediated transacylation showed a strong selectivity for palmitate and arachidonate and a strong discrimination against oleate. The amount (nmol) of dipalmitoyl-PtdEtn formed by both transacylation activities after short-time incubation (2 min) decreased with prolonged incubation time (60 min). In contrast, the nmol of dipalmitoyl-PtdCho formed by cofactor-independent transacylation remains nearly the same after short-time and longer incubation. The nmol of dipalmitoyl-PtdCho formed by CoA-mediated transacylation increased strongly in the same time interval. Beside synthesis de novo via the CDP-choline pathway and reacylation of lyso-PtdCho with palmitoyl-CoA, the CoA-mediated transacylation of lyso-PtdCho may be an effective pathway for the formation of dipalmitoyl-PtdCho in pulmonary type II cells.

Chemical mechanism of lysophosphatidylcholine: lysophosphatidylcholine acyltransferase from rabbit lung. pH-dependence of kinetic parameters

Lysophosphatidylcholine: lysophosphatidylcholine acyltransferase is an enzyme that catalyses two reactions: hydrolysis of lysophosphatidylcholine and transacylation between two molecules of lysophosphatidylcholine to give disaturated phosphatidylcholine. Following the kinetic model previously proposed for this enzyme [Martín, Pérez-Gil, Acebal & Arche (1990) Biochem. J. 266, 47-53], the values of essential pK values in free enzyme and substrate-enzyme complexes have now been determined. The chemical mechanism of catalysis was dependent on the deprotonation of a histidine residue with pK about 5.7. This result was supported by the perturbation of pK values by addition of organic solvent. Very high and exothermic enthalpy of ionization was measured, indicating that a conformational re-arrangement in the enzyme accompanies the ionization of the essential histidine residue. These results, as well as the results from previous studies, enabled the proposal of a chemical mechanism for the enzymic reactions catalysed by lysophosphatidylcholine: lysophosphatidylcholine acyltransferase from rabbit lung.

Theoretical approach to the steady-state kinetics of a bi-substrate acyl-transfer enzyme reaction that follows a hydrolysable-acyl-enzyme-based mechanism. Application to the study of lysophosphatidylcholine:lysophosphatidylcholine acyltransferase from rabbit lung

A kinetic model is proposed for catalysis by an enzyme that has several special characteristics: (i) it catalyses an acyl-transfer bi-substrate reaction between two identical molecules of substrate, (ii) the substrate is an amphiphilic molecule that can be present in two physical forms, namely monomers and micelles, and (iii) the reaction progresses through an acyl-enzyme-based mechanism and the covalent intermediate can react also with water to yield a secondary hydrolytic reaction. The theoretical kinetic equations for both reactions were deduced according to steady-state assumptions and the theoretical plots were predicted. The experimental kinetics of lysophosphatidylcholine:lysophosphatidylcholine acyltransferase from rabbit lung fitted the proposed equations with great accuracy. Also, kinetics of inhibition by products behaved as expected. It was concluded that the competition between two nucleophiles for the covalent acyl-enzyme intermediate, and not a different enzyme action depending on the physical state of the substrate, is responsible for the differences in kinetic pattern for the two activities of the enzyme. This conclusion, together with the fact that the kinetic equation for the transacylation is quadratic, generates a 'hysteretic' pattern that can provide the basis of self-regulatory properties for enzymes to which this model could be applied.

Activity of long chain acyl-CoA synthetase in isolated alveolar type II cells

Fatty-acyl-CoA synthetase activity was determined in rat alveolar type II cells. Compared to whole-lung homogenate, the enzyme specific activity with palmitic acid was 3.6-fold higher in isolated type II alveolar cells. The enzyme in rat alveolar type II cells did not discriminate among various fatty acids, suggesting that supply of fatty acids rather than specificity might be an important factor for their activation in these cells.

Evidence for a high activity of sphingomyelin biosynthesis by phosphocholine transfer from phosphatidylcholine to ceramides in lung lamellar bodies

Biosynthesis of sphingomyelin from ceramides was investigated in lung subcellular fractions by incubating a lyophilized mixture of albumin and subcellular fraction (0.1-0.2 mg of protein) coated with [acyl-14C]-ceramide and phosphatidyl[methyl-3H]choline in Tris-buffer. The lamellar-body-rich fraction exhibited the highest specific activity for sphingomyelin biosynthesis measured by 14C incorporation into sphingomyelins or by [3H]phosphocholine transfer from phosphatidylcholines. Plasma membranes formed the next most active fraction, followed by the 'smooth' and, then, the 'rough' endoplasmic reticulum. Sphingomyelin biosynthesis by lamellar bodies was optimum at pH 7.4 and was inhibited by sphingomyelins formed. Slight inhibitory effects were also observed with Mn2+, Ca2+ and lysophosphatidylcholine. Phosphocholine transfer from CDPcholine was not observed under the reaction conditions employed. Ceramide conversion and phosphocholine transfer increased with ceramide concentration to reach a maximum at about 0.06 mM. The highest conversion rate was observed when 18:1 ceramide was used as an acceptor. When 1-palmitoyl-2-oleoylphosphatidylcholine was the phosphocholine donor, the overall biosynthesis of sphingomyelin was much higher than when using dipalmitoylphosphatidylcholine. These results suggest the possible involvement of the studied reaction in the control of the degree of saturation of the surfactant phosphatidylcholine.

Altered phospholipid secretion in type II pneumocytes isolated from streptozotocin-diabetic rats

To study the effect of diabetes on pulmonary surfactant secretion, type II pneumocytes from adult streptozotocin-induced diabetic rats were placed in short-term culture. As opposed to a linear secretory rate by control type II cells, the secretory rate of type II cells from diabetic animals was biphasic reaching a minimum at 1.5 h. When exogenous surfactant containing radioactive phosphatidylcholine was added to the incubation media for 1.5 h, the cells from diabetic animals incorporated more exogenous phosphatidylcholine into lamellar bodies than control cells. This suggests that in the type II cell from diabetic animals, the rate of reutilization is greater than the rate of secretion until 1.5 h, at which time the rate of secretion becomes greater. The altered secretory pattern was reversed by in vivo insulin treatment 30 min prior to killing but not by the addition of insulin to the incubation media. When challenged by isoproterenol, a beta-adrenergic agonist, the secretory pattern of cells from diabetic animals was biphasic as observed with basal secretion; however, secretion was stimulated 30% as opposed to 100% increase in control cells. These data suggest that basal and stimulated secretion are altered in the cultured type II cell from diabetic animals and restored by in vivo but not in vitro insulin treatment.

Involvement of acyl exchange between acyl-CoA and phosphatidylcholine in the remodelling of phosphatidylcholine in microsomal preparations of rat lung

Microsomal membrane preparations from rat lung catalyse the incorporation of radioactive linolenic acid from [14C]linolenoyl-CoA into position 2 of sn-phosphatidylcholine. The incorporation was stimulated by bovine serum albumin and free CoA. Free fatty acids in the incubation mixtures were not utilised in the incorporation into complex lipids. Fatty acids were transferred to the acyl-CoA pool during the incorporation of linolenic acid into phosphatidylcholine. An increase in lysophosphatidylcholine occurred in incubations containing both bovine serum albumin and free CoA and in the absence of acyl-CoA. The results were consistent with an acyl-CoA: lysophosphatidylcholine acyltransferase operating in both a forwards and backwards direction and thus catalysing the acyl exchange between acyl-CoA and position 2 of sn-phosphatidylcholine. In incubations with mixed species of acyl-CoAs, palmitic acid was the major fatty acid substrate transferred to phosphatidylcholine in acyl exchange, whereas this acid was completely selected against in the acylation of added lysophosphatidylcholine. The selectivity for palmitoyl-CoA was particularly enhanced when the mixed acyl-CoA substrate was presented to the microsomes in molar concentrations equivalent to the molar ratios of the fatty acids in position 2 of sn-phosphatidylcholine. During acyl exchange, the predominant fatty acid transferred to phosphatidylcholine from acyl-CoA was palmitic acid, whereas arachidonic acid was particularly selected for in the reverse reaction from phosphatidylcholine to acyl-CoA. A hypothesis is presented to explain the differential selectivity for acyl species between the forward and backward reactions of the acyltransferase that is based upon different affinities of the enzyme for substrates at high and low concentrations of acyl donor. Acyl exchange between acyl-CoA and phosphatidylcholine offers, therefore, a possible mechanism for the acyl-remodelling of phosphatidylcholine for the production of lung surfactant.