ALCAT1 is a polyglycerophospholipid acyltransferase potently regulated by adenine nucleotide and thyroid status

The LCLAT1/LYCAT acyltransferase is required for EGF-mediated phosphatidylinositol-3,4,5-trisphosphate generation and Akt signaling

Receptor tyrosine kinases such as EGF receptor (EGFR) stimulate phosphoinositide 3 kinases to convert phosphatidylinositol-4,5-bisphosophate [PtdIns(4,5)P2] into phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P3]. PtdIns(3,4,5)P3 then remodels actin and gene expression, and boosts cell survival and proliferation. PtdIns(3,4,5)P3 partly achieves these functions by triggering activation of the kinase Akt, which phosphorylates targets like Tsc2 and GSK3β. Consequently, unchecked upregulation of PtdIns(3,4,5)P3-Akt signaling promotes tumor progression. Interestingly, 50-70% of PtdIns and PtdInsPs have stearate and arachidonate at sn-1 and sn-2 positions of glycerol, respectively, forming a species known as 38:4-PtdIns/PtdInsPs. LCLAT1 and MBOAT7 acyltransferases partly enrich PtdIns in this acyl format. We previously showed that disruption of LCLAT1 lowered PtdIns(4,5)P2 levels and perturbed endocytosis and endocytic trafficking. However, the role of LCLAT1 in receptor tyrosine kinase and PtdIns(3,4,5)P3 signaling was not explored. Here, we show that LCLAT1 silencing in MDA-MB-231 and ARPE-19 cells abated the levels of PtdIns(3,4,5)P3 in response to EGF signaling. Importantly, LCLAT1-silenced cells were also impaired for EGF-driven and insulin-driven Akt activation and downstream signaling. Thus, our work provides first evidence that the LCLAT1 acyltransferase is required for receptor tyrosine kinase signaling.

Phospholipid Acyltransferases: Characterization and Involvement of the Enzymes in Metabolic and Cancer Diseases

This review delves into the enzymatic processes governing the initial stages of glycerophospholipid (phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine) and triacylglycerol synthesis. The key enzymes under scrutiny include GPAT and AGPAT. Additionally, as most AGPATs exhibit LPLAT activity, enzymes participating in the Lands cycle with similar functions are also covered. The review begins by discussing the properties of these enzymes, emphasizing their specificity in enzymatic reactions, notably the incorporation of polyunsaturated fatty acids (PUFAs) such as arachidonic acid and docosahexaenoic acid (DHA) into phospholipids. The paper sheds light on the intricate involvement of these enzymes in various diseases, including obesity, insulin resistance, and cancer. To underscore the relevance of these enzymes in cancer processes, a bioinformatics analysis was conducted. The expression levels of the described enzymes were correlated with the overall survival of patients across 33 different types of cancer using the GEPIA portal. This review further explores the potential therapeutic implications of inhibiting these enzymes in the treatment of metabolic diseases and cancer. By elucidating the intricate enzymatic pathways involved in lipid synthesis and their impact on various pathological conditions, this paper contributes to a comprehensive understanding of these processes and their potential as therapeutic targets.

An upstream open reading frame (5'-uORF) links oxidative stress to translational control of ALCAT1 through phosphorylation of eIF2α

Acyl-CoA:lysocardiolipin acyltransferase 1 (ALCAT1) is an enzyme that promotes mitochondrial dysfunction by catalyzing pathological remodeling of cardiolipin. Upregulation of ALCAT1 protein expression by oxidative stress is implicated in the pathogenesis of age-related metabolic diseases, but the underlying molecular mechanisms remain elusive. In this study, we identified a highly conserved upstream open reading frame (uORF) at the 5'-untranslated region (5'-UTR) of ALCAT1 mRNA as a key regulator of ALCAT1 expression in response to oxidative stress. We show that the uORF serves as a decoy that prevents translation initiation of ALCAT1 under homeostatic condition. The inhibitory activity of the uORF on ALCAT1 mRNA translation is mitigated by oxidative stress but not ER stress, which requires the phosphorylation of eukaryotic translation initiation factor 2α (eIF2α). Consequently, ablation of uORF or eIF2α phosphorylation at Ser51 renders ALCAT1 protein expression unresponsive to induction by oxidative stress. Taken together, our data show that the uORF links oxidative stress to translation control of ALCAT1 mRNAs through phosphorylation of eIF2α at Ser51.

A tail of their own: regulation of cardiolipin and phosphatidylinositol fatty acyl profile by the acyltransferase LCLAT1

Cardiolipin and phosphatidylinositol along with the latter's phosphorylated derivative phosphoinositides, control a wide range of cellular functions from signal transduction, membrane traffic, mitochondrial function, cytoskeletal dynamics, and cell metabolism. An emerging dimension to these lipids is the specificity of their fatty acyl chains that is remarkably distinct from that of other glycerophospholipids. Cardiolipin and phosphatidylinositol undergo acyl remodeling involving the sequential actions of phospholipase A to hydrolyze acyl chains and key acyltransferases that re-acylate with specific acyl groups. LCLAT1 (also known as LYCAT, AGPAT8, LPLAT6, or ALCAT1) is an acyltransferase that contributes to specific acyl profiles for phosphatidylinositol, phosphoinositides, and cardiolipin. As such, perturbations of LCLAT1 lead to alterations in cardiolipin-dependent phenomena such as mitochondrial respiration and dynamics and phosphoinositide-dependent processes such as endocytic membrane traffic and receptor signaling. Here we examine the biochemical and cellular actions of LCLAT1, as well as the contribution of this acyltransferase to the development and specific diseases.

Systematic crosstalk in plasmalogen and diacyl lipid biosynthesis for their differential yet concerted molecular functions in the cell

Plasmalogen is a major phospholipid of mammalian cell membranes. Recently it is becoming evident that the sn-1 vinyl-ether linkage in plasmalogen, contrasting to the ester linkage in the counterpart diacyl glycerophospholipid, yields differential molecular characteristics for these lipids especially related to hydrocarbon-chain order, so as to concertedly regulate biological membrane processes. A role played by NMR in gaining information in this respect, ranging from molecular to tissue levels, draws particular attention. We note here that a broad range of enzymes in de novo synthesis pathway of plasmalogen commonly constitute that of diacyl glycerophospholipid. This fact forms the basis for systematic crosstalk that not only controls a quantitative balance between these lipids, but also senses a defect causing loss of lipid in either pathway for compensation by increase of the counterpart lipid. However, this inherent counterbalancing mechanism paradoxically amplifies imbalance in differential effects of these lipids in a diseased state on membrane processes. While sharing of enzymes has been recognized, it is now possible to overview the crosstalk with growing information for specific enzymes involved. The overview provides a fundamental clue to consider cell and tissue type-dependent schemes in regulating membrane processes by plasmalogen and diacyl glycerophospholipid in health and disease.

Cardioprotection Attributed to Aerobic Exercise-Mediated Inhibition of ALCAT1 and Oxidative Stress-Induced Apoptosis in MI Rats

Cardiolipin (CL) plays a pivotal role in mitochondria-mediated apoptosis. Acyl-CoA: lysocardiolipin acyltransferase 1 (ALCAT1) can accelerate CL reactive oxygen production and cause mitochondrial damage. Although we have demonstrated that aerobic exercise significantly reduced ALCAT1 levels in MI mice, what is the temporal characteristic of ALCAT1 after MI? Little is known. Based on this, the effect of exercise on ALCAT1 in MI rats needs to be further verified. Therefore, this paper aimed to characterize ALCAT1 expression, and investigate the possible impact of exercise on ALCAT1 and its role in fibrosis, antioxidant capacity, and apoptosis in MI rats. Our results indicated that the potential utility of MI increased ALCAT1 expression within 1–6 h of MI, and serum CK and CKMB had significant effects in MI at 24 h, while LDH exerted an effect five days after MI. Furthermore, ALCAT1 expression was upregulated, oxidative capacity and excessive apoptosis were enhanced, and cardiac function was decreased after MI, and aerobic exercise can reverse these changes. These findings revealed a previously unknown endogenous cardiac injury factor, ALCAT1, and demonstrated that ALCAT1 damaged the heart of MI rats, and aerobic exercise reduced ALCAT1 expression, oxidative stress, and apoptosis after MI-induced cardiac injury in rats.

In Search of the Holy Grail: Toward a Unified Hypothesis on Mitochondrial Dysfunction in Age-Related Diseases

Cardiolipin (CL) is a mitochondrial signature phospholipid that plays a pivotal role in mitochondrial dynamics, membrane structure, oxidative phosphorylation, mtDNA bioenergetics, and mitophagy. The depletion or abnormal acyl composition of CL causes mitochondrial dysfunction, which is implicated in the pathogenesis of aging and age-related disorders. However, the molecular mechanisms by which mitochondrial dysfunction causes age-related diseases remain poorly understood. Recent development in the field has identified acyl-CoA:lysocardiolipin acyltransferase 1 (ALCAT1), an acyltransferase upregulated by oxidative stress, as a key enzyme that promotes mitochondrial dysfunction in age-related diseases. ALCAT1 catalyzes CL remodeling with very-long-chain polyunsaturated fatty acids, such as docosahexaenoic acid (DHA). Enrichment of DHA renders CL highly sensitive to oxidative damage by reactive oxygen species (ROS). Oxidized CL becomes a new source of ROS in the form of lipid peroxides, leading to a vicious cycle of oxidative stress, CL depletion, and mitochondrial dysfunction. Consequently, ablation or the pharmacological inhibition of ALCAT1 have been shown to mitigate obesity, type 2 diabetes, heart failure, cardiomyopathy, fatty liver diseases, neurodegenerative diseases, and cancer. The findings suggest that age-related disorders are one disease (aging) manifested by different mitochondrion-sensitive tissues, and therefore should be treated as one disease. This review will discuss a unified hypothesis on CL remodeling by ALCAT1 as the common denominator of mitochondrial dysfunction, linking mitochondrial dysfunction to the development of age-related diseases.

Current Knowledge on the Role of Cardiolipin Remodeling in the Context of Lipid Oxidation and Barth Syndrome

Barth syndrome (BTHS, OMIM 302060) is a genetic disorder caused by variants of the TAFAZZIN gene (G 4.5, OMIM 300394). This debilitating disorder is characterized by cardio- and skeletal myopathy, exercise intolerance, and neutropenia. TAFAZZIN is a transacylase that catalyzes the second step in the cardiolipin (CL) remodeling pathway, preferentially converting saturated CL species into unsaturated CLs that are susceptible to oxidation. As a hallmark mitochondrial membrane lipid, CL has been shown to be essential in a myriad of pathways, including oxidative phosphorylation, the electron transport chain, intermediary metabolism, and intrinsic apoptosis. The pathological severity of BTHS varies substantially from one patient to another, even in individuals bearing the same TAFAZZIN variant. The physiological modifier(s) leading to this disparity, along with the exact molecular mechanism linking CL to the various pathologies, remain largely unknown. Elevated levels of reactive oxygen species (ROS) have been identified in numerous BTHS models, ranging from yeast to human cell lines, suggesting that cellular ROS accumulation may participate in the pathogenesis of BTHS. Although the exact mechanism of how oxidative stress leads to pathogenesis is unknown, it is likely that CL oxidation plays an important role. In this review, we outline what is known about CL oxidation and provide a new perspective linking the functional relevance of CL remodeling and oxidation to ROS mitigation in the context of BTHS.

The lipid environment modulates cardiolipin and phospholipid constitution in wild type and tafazzin-deficient cells

Deficiency of the transacylase tafazzin due to loss of function variants in the X-chromosomal TAFAZZIN gene causes Barth syndrome (BTHS) with severe neonatal or infantile cardiomyopathy, neutropenia, myopathy, and short stature. The condition is characterized by drastic changes in the composition of cardiolipins, a mitochondria-specific class of phospholipids. Studies examining the impact of tafazzin deficiency on the metabolism of other phospholipids have so far generated inhomogeneous and partly conflicting results. Recent studies showed that the cardiolipin composition in cells and different murine tissues is highly dependent on the surrounding lipid environment. In order to study the relevance of different lipid states and tafazzin function for cardiolipin and phospholipid homeostasis we conducted systematic modulation experiments in a CRISPR/Cas9 knock-out model for BTHS. We found that-irrespective of tafazzin function-the composition of cardiolipins strongly depends on the nutritionally available lipid pool. Tafazzin deficiency causes a consistent shift towards cardiolipin species with more saturated and shorter acyl chains. Interestingly, the typical biochemical BTHS phenotype in phospholipid profiles of HEK 293T TAZ knock-out cells strongly depends on the cellular lipid context. In response to altered nutritional lipid compositions, we measured more pronounced changes on phospholipids that were largely masked under standard cell culturing conditions, therewith giving a possible explanation for the conflicting results reported so far on BTHS lipid phenotypes.

Update and nomenclature proposal for mammalian lysophospholipid acyltransferases which create membrane phospholipid diversity

The diversity of glycerophospholipid species in cellular membranes is immense and affects various biological functions. Glycerol-3-phosphate acyltransferases (GPATs) and lysophospholipid acyltransferases (LPLATs), in concert with phospholipase A_1/2s enzymes, contribute to this diversity via selective esterification of fatty acyl chains at the sn-1 or sn-2 positions of membrane phospholipids. These enzymes are conserved across all kingdoms, and in mammals four GPATs of the 1-acylglycerol-3-phosphate O-acyltransferase (AGPAT) family and at least 14 LPLATs, either of the AGPAT or the membrane-bound O-acyltransferase (MBOAT) families, have been identified. Here we provide an overview of the biochemical and biological activities of these mammalian enzymes, including their predicted structures, involvements in human diseases, and essential physiological roles as revealed by gene-deficient mice. Recently, the nomenclature used to refer to these enzymes has generated some confusion due to the use of multiple names to refer to the same enzyme and instances of the same name being used to refer to completely different enzymes. Thus, this review proposes a more uniform LPLAT enzyme nomenclature, as well as providing an update of recent advances made in the study of LPLATs, continuing from our JBC mini review in 2009.

Cardiolipin remodeling by ALCAT1 links hypoxia to coronary artery disease by promoting mitochondrial dysfunction

Cardiolipin is a mitochondrial signature phospholipid that plays a pivotal role in maintaining cardiac health. A loss of tetralinoleoyl cardiolipin (TLCL), the predominant cardiolipin species in the healthy mammalian heart, is implicated in the pathogenesis of coronary heart disease (CHD) through poorly defined mechanisms. Here, we identified acyl-coenzyme A:lysocardiolipin acyltransferase-1 (ALCAT1) as the missing link between hypoxia and CHD in an animal model of myocardial infarction (MI). ALCAT1 is an acyltransferase that promotes mitochondrial dysfunction in aging-related diseases by catalyzing pathological remodeling of cardiolipin. In support of a causative role of ALCAT1 in CHD, we showed that ALCAT1 expression was potently upregulated by MI, linking myocardial hypoxia to oxidative stress, TLCL depletion, and mitochondrial dysfunction. Accordingly, ablation of the ALCAT1 gene or pharmacological inhibition of the ALCAT1 enzyme by Dafaglitapin (Dafa), a potent and highly specific ALCAT1 inhibitor, not only restored TLCL levels but also mitochondrial respiration by attenuating signal transduction pathways mediated by hypoxia-inducible factor 1α (HIF-1α). Consequently, ablation or pharmacological inhibition of ALCAT1 by Dafa effectively mitigated CHD and its underlying pathogenesis, including dilated cardiomyopathy, left ventricle dysfunction, myocardial inflammation, fibrosis, and apoptosis. Together, the findings have provided the first proof-of-concept studies for targeting ALCAT1 as an effective treatment for CHD.

Lysocardiolipin acyltransferase regulates NSCLC cell proliferation and migration by modulating mitochondrial dynamics

Lysocardiolipin acyltransferase (LYCAT), a cardiolipin (CL)-remodeling enzyme, is crucial for maintaining normal mitochondrial function and vascular development. Despite the well-characterized role for LYCAT in the regulation of mitochondrial dynamics, its involvement in lung cancer, if any, remains incompletely understood. In this study, in silico analysis of TCGA lung cancer data sets revealed a significant increase in LYCAT expression, which was later corroborated in human lung cancer tissues and immortalized lung cancer cell lines via indirect immunofluorescence and immunoblotting, respectively. Stable knockdown of LYCAT in NSCLC cell lines not only reduced CL and increased monolyso-CL levels but also reduced in vivo tumor growth, as determined by xenograft studies in athymic nude mice. Furthermore, blocking LYCAT activity using a LYCAT mimetic peptide attenuated cell migration, suggesting a novel role for LYCAT activity in promoting NSCLC. Mechanistically, the pro-proliferative effects of LYCAT were mediated by an increase in mitochondrial fusion and a G₁/S cell cycle transition, both of which are linked to increased cell proliferation. Taken together, these results demonstrate a novel role for LYCAT in promoting NSCLC and suggest that targeting LYCAT expression or activity in NSCLC may provide new avenues for the therapeutic treatment of lung cancer.

Prediabetes Induced by Fructose-Enriched Diet Influences Cardiac Lipidome and Proteome and Leads to Deterioration of Cardiac Function prior to the Development of Excessive Oxidative Stress and Cell Damage

Prediabetes is a condition affecting more than 35% of the population. In some forms, excessive carbohydrate intake (primarily refined sugar) plays a prominent role. Prediabetes is a symptomless, mostly unrecognized disease which increases cardiovascular risk. In our work, we examined the effect of a fructose-enriched diet on cardiac function and lipidome as well as proteome of cardiac muscle. Male Wistar rats were divided into two groups. The control group received a normal diet while the fructose-fed group received 60% fructose-supplemented chow for 24 weeks. Fasting blood glucose measurement and oral glucose tolerance test (OGTT) showed slightly but significantly elevated values due to fructose feeding indicating development of a prediabetic condition. Both echocardiography and isolated working heart perfusion performed at the end of the feeding protocol demonstrated diastolic cardiac dysfunction in the fructose-fed group. Mass spectrometry-based, high-performance lipidomic and proteomic analyses were executed from cardiac tissue. The lipidomic analysis revealed complex rearrangement of the whole lipidome with special emphasis on defects in cardiolipin remodeling. The proteomic analysis showed significant changes in 75 cardiac proteins due to fructose feeding including mitochondria-, apoptosis-, and oxidative stress-related proteins. Nevertheless, just very weak or no signs of apoptosis induction and oxidative stress were detected in the hearts of fructose-fed rats. Our results suggest that fructose feeding induces marked alterations in the cardiac lipidome, especially in cardiolipin remodeling, which leads to mitochondrial dysfunction and impaired cardiac function. However, at the same time, several adaptive responses are induced at the proteome level in order to maintain a homeostatic balance. These findings demonstrate that even very early stages of prediabetes can impair cardiac function and can result in significant changes in the lipidome and proteome of the heart prior to the development of excessive oxidative stress and cell damage.

Characterization of Lipid Profiles after Dietary Intake of Polyunsaturated Fatty Acids Using Integrated Untargeted and Targeted Lipidomics

Illuminating the comprehensive lipid profiles after dietary supplementation of polyunsaturated fatty acids (PUFAs) is crucial to revealing the tissue distribution of PUFAs in living organisms, as well as to providing novel insights into lipid metabolism. Here, we performed lipidomic analyses on mouse plasma and nine tissues, including the liver, kidney, brain, white adipose, heart, lung, small intestine, skeletal muscle, and spleen, with the dietary intake conditions of arachidonic acid (ARA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) as the ethyl ester form. We incorporated targeted and untargeted approaches for profiling oxylipins and complex lipids such as glycerol (phospho) lipids, sphingolipids, and sterols, respectively, which led to the characterization of 1026 lipid molecules from the mouse tissues. The lipidomic analysis indicated that the intake of PUFAs strongly impacted the lipid profiles of metabolic organs such as the liver and kidney, while causing less impact on the brain. Moreover, we revealed a unique lipid modulation in most tissues, where phospholipids containing linoleic acid were significantly decreased in mice on the ARA-supplemented diet, and bis(monoacylglycero)phosphate (BMP) selectively incorporated DHA over ARA and EPA. We comprehensively studied the lipid profiles after dietary intake of PUFAs, which gives insight into lipid metabolism and nutrition research on PUFA supplementation.

Patent highlights October-November 2018

A snapshot of noteworthy recent developments in the patent literature of relevance to pharmaceutical and medical research and development.

An accurate and versatile method for determining the acyl group-introducing position of lysophospholipid acyltransferases

Lysophospholipid acyltransferases (LPLATs) incorporate a fatty acid into the hydroxyl group of lysophospholipids (LPLs) and are critical for determining the fatty acid composition of phospholipids. Previous studies have focused mainly on their molecular identification and their substrate specificity regarding the polar head groups and acyl-CoAs. However, little is known about the positional specificity of the hydroxyl group of the glycerol backbone (sn-2 or sn-1) at which LPLATs introduce a fatty acid. This is mainly due to the instability of LPLs used as an acceptor, especially for LPLs with a fatty acid at the sn-2 position of the glycerol backbone (sn-2-LPLs), which are essential for the enzymatic assay to determine the positional specificity. In this study, we established a method to determine the positional specificity of LPLAT by preparing stable sn-2-LPLs in combination with PLA₂ digestion, and applied the method for determining the positional specificity of several LPLATs including LPCAT1, LYCAT and LPCAT3. We found that LPCAT1 introduced palmitic acid both at the sn-1 and sn-2 positions of palmitoyl-LPC, while LYCAT and LPCAT3 specifically introduced stearic acid at the sn-1 position of LPG and arachidonic acid at the sn-2 position of LPC, respectively. The present method for evaluating the positional specificity could also be used for biochemical characterization of other LPLATs.

Mitochondrial Metabolism in Aging Heart

Altered mitochondrial metabolism is the underlying basis for the increased sensitivity in the aged heart to stress. The aged heart exhibits impaired metabolic flexibility, with a decreased capacity to oxidize fatty acids and enhanced dependence on glucose metabolism. Aging impairs mitochondrial oxidative phosphorylation, with a greater role played by the mitochondria located between the myofibrils, the interfibrillar mitochondria. With aging, there is a decrease in activity of complexes III and IV, which account for the decrease in respiration. Furthermore, aging decreases mitochondrial content among the myofibrils. The end result is that in the interfibrillar area, there is ≈50% decrease in mitochondrial function, affecting all substrates. The defective mitochondria persist in the aged heart, leading to enhanced oxidant production and oxidative injury and the activation of oxidant signaling for cell death. Aging defects in mitochondria represent new therapeutic targets, whether by manipulation of the mitochondrial proteome, modulation of electron transport, activation of biogenesis or mitophagy, or the regulation of mitochondrial fission and fusion. These mechanisms provide new ways to attenuate cardiac disease in elders by preemptive treatment of age-related defects, in contrast to the treatment of disease-induced dysfunction.

Acquired deficiency of tafazzin in the adult heart: Impact on mitochondrial function and response to cardiac injury

The content and composition of cardiolipin (CL) is critical for preservation of mitochondrial oxidative phosphorylation (OXPHOS) and inner membrane integrity. Tafazzin (Taz) is an enzyme responsible for remodeling of immature CL containing mixed acyl groups into the mature tetralinoleyl form (C18:2)4-CL. We hypothesized that acquired defects in Taz in the mature heart would impact remodeling of CL and augment cardiac injury. The role of acquired Taz deficiency was studied using the inducible Taz knockdown (TazKD) mouse. Taz-specific shRNA is induced by doxycycline (DOX). One day of DOX intake decreased Taz mRNA in the heart to 20% vs. DOX-treated WT. Knockdown was initiated at an adult age and was stable during long term feeding. CL phenotype was assessed by (C18:2)4-CL content and was reduced 40% vs. WT at two months of DOX. TazKD showed increased production of reactive oxygen species and increased susceptibility to permeability transition pore opening at baseline. However, OXPHOS measured using the rate of oxygen consumption was unchanged in the setting of acquired Taz deficiency. Infarct size, measured in isolated buffer-perfused Langendorff hearts following 25min. Stop flow ischemia and 60min. Reperfusion was not altered in TazKD hearts. Thus, impaired Taz-function with onset at adult age does not enhance susceptibility to ischemia-reperfusion injury.

LPS impairs oxygen utilization in epithelia by triggering degradation of the mitochondrial enzyme Alcat1

Cardiolipin (also known as PDL6) is an indispensable lipid required for mitochondrial respiration that is generated through de novo synthesis and remodeling. Here, the cardiolipin remodeling enzyme, acyl-CoA:lysocardiolipin-acyltransferase-1 (Alcat1; SwissProt ID, Q6UWP7) is destabilized in epithelia by lipopolysaccharide (LPS) impairing mitochondrial function. Exposure to LPS selectively decreased levels of carbon 20 (C20)-containing cardiolipin molecular species, whereas the content of C18 or C16 species was not significantly altered, consistent with decreased levels of Alcat1. Alcat1 is a labile protein that is lysosomally degraded by the ubiquitin E3 ligase Skp-Cullin-F-box containing the Fbxo28 subunit (SCF-Fbxo28) that targets Alcat1 for monoubiquitylation at residue K183. Interestingly, K183 is also an acetylation-acceptor site, and acetylation conferred stability to the enzyme. Histone deacetylase 2 (HDAC2) interacted with Alcat1, and expression of a plasmid encoding HDAC2 or treatment of cells with LPS deacetylated and destabilized Alcat1, whereas treatment of cells with a pan-HDAC inhibitor increased Alcat1 levels. Alcat1 degradation was partially abrogated in LPS-treated cells that had been silenced for HDAC2 or treated with MLN4924, an inhibitor of Cullin-RING E3 ubiquitin ligases. Thus, LPS increases HDAC2-mediated Alcat1 deacetylation and facilitates SCF-Fbxo28-mediated disposal of Alcat1, thus impairing mitochondrial integrity.

Disorders of phospholipid metabolism: an emerging class of mitochondrial disease due to defects in nuclear genes

The human nuclear and mitochondrial genomes co-exist within each cell. While the mitochondrial genome encodes for a limited number of proteins, transfer RNAs, and ribosomal RNAs, the vast majority of mitochondrial proteins are encoded in the nuclear genome. Of the multitude of mitochondrial disorders known to date, only a fifth are maternally inherited. The recent characterization of the mitochondrial proteome therefore serves as an important step toward delineating the nosology of a large spectrum of phenotypically heterogeneous diseases. Following the identification of the first nuclear gene defect to underlie a mitochondrial disorder, a plenitude of genetic variants that provoke mitochondrial pathophysiology have been molecularly elucidated and classified into six categories that impact: (1) oxidative phosphorylation (subunits and assembly factors); (2) mitochondrial DNA maintenance and expression; (3) mitochondrial protein import and assembly; (4) mitochondrial quality control (chaperones and proteases); (5) iron–sulfur cluster homeostasis; and (6) mitochondrial dynamics (fission and fusion). Here, we propose that an additional class of genetic variant be included in the classification schema to acknowledge the role of genetic defects in phospholipid biosynthesis, remodeling, and metabolism in mitochondrial pathophysiology. This seventh class includes a small but notable group of nuclear-encoded proteins whose dysfunction impacts normal mitochondrial phospholipid metabolism. The resulting human disorders present with a diverse array of pathologic consequences that reflect the variety of functions that phospholipids have in mitochondria and highlight the important role of proper membrane homeostasis in mitochondrial biology.

Cardiolipin remodeling: a regulatory hub for modulating cardiolipin metabolism and function

Cardiolipin (CL), the signature phospholipid of mitochondria, is involved in a plethora of cellular processes and is crucial for mitochondrial function and architecture. The de novo synthesis of CL in the mitochondria is followed by a unique remodeling process, in which CL undergoes cycles of deacylation and reacylation. Specific fatty acyl composition is acquired during this process, and remodeled CL contains predominantly unsaturated fatty acids. The importance of CL remodeling is underscored by the life-threatening genetic disorder Barth syndrome (BTHS), caused by mutations in tafazzin, which reacylates monolysocardiolipin (MLCL) generated from the deacylation of CL. Just as CL-deficient yeast mutants have been instrumental in elucidating functions of this lipid, the recently characterized CL-phospholipase mutant cld1Δ and the tafazzin mutant taz1Δ are powerful tools to understand the functions of CL remodeling. In this review, we discuss recent advances in understanding the role of CL in mitochondria with specific focus on the enigmatic functions of CL remodeling.

Glycerophosphate/Acylglycerophosphate acyltransferases

Acyl-CoA:glycerol-3-phosphate acyltransferase (GPAT) and acyl-CoA: 1-acyl-glycerol-3-phosphate acyltransferase (AGPAT) are involved in the de novo synthesis of triacylglycerol (TAG) and glycerophospholipids. Many enzymes belonging to the GPAT/AGPAT family have recently been identified and their physiological or pathophysiological roles have been proposed. The roles of GPAT/AGPAT in the synthesis of TAG and obesity-related diseases were revealed through the identification of causative genes of these diseases or analyses of genetically manipulated animals. Recent studies have suggested that some isoforms of GPAT/AGPAT family enzymes are involved in the fatty acid remodeling of phospholipids. The enzymology of GPAT/AGPAT and their physiological/pathological roles in the metabolism of glycerolipids have been described and discussed in this review.

Diversity and function of membrane glycerophospholipids generated by the remodeling pathway in mammalian cells

Cellular membranes are composed of numerous kinds of glycerophospholipids with different combinations of polar heads at the sn-3 position and acyl moieties at the sn-1 and sn-2 positions, respectively. The glycerophospholipid compositions of different cell types, organelles, and inner/outer plasma membrane leaflets are quite diverse. The acyl moieties of glycerophospholipids synthesized in the de novo pathway are subsequently remodeled by the action of phospholipases and lysophospholipid acyltransferases. This remodeling cycle contributes to the generation of membrane glycerophospholipid diversity and the production of lipid mediators such as fatty acid derivatives and lysophospholipids. Furthermore, specific glycerophospholipid transporters are also important to organize a unique glycerophospholipid composition in each organelle. Recent progress in this field contributes to understanding how and why membrane glycerophospholipid diversity is organized and maintained.

The functions of cardiolipin in cellular metabolism-potential modifiers of the Barth syndrome phenotype

The phospholipid cardiolipin (CL) plays a role in many cellular functions and signaling pathways both inside and outside of mitochondria. This review focuses on the role of CL in energy metabolism. Many reactions of electron transport and oxidative phosphorylation, the transport of metabolites required for these processes, and the stabilization of electron transport chain supercomplexes require CL. Recent studies indicate that CL is required for the synthesis of iron-sulfur (Fe-S) co-factors, which are essential for numerous metabolic pathways. Activation of carnitine shuttle enzymes that are required for fatty acid metabolism is CL dependent. The presence of substantial amounts of CL in the peroxisomal membrane suggests that CL may be required for peroxisomal functions. Understanding the role of CL in energy metabolism may identify physiological modifiers that exacerbate the loss of CL and underlie the variation in symptoms observed in Barth syndrome, a genetic disorder of CL metabolism.

Unremodeled and remodeled cardiolipin are functionally indistinguishable in yeast

After biosynthesis, an evolutionarily conserved acyl chain remodeling process generates a final highly homogeneous and yet tissue-specific molecular form of the mitochondrial lipid cardiolipin. Hence, cardiolipin molecules in different organisms, and even different tissues within the same organism, contain a distinct collection of attached acyl chains. This observation is the basis for the widely accepted paradigm that the acyl chain composition of cardiolipin is matched to the unique mitochondrial demands of a tissue. For this hypothesis to be correct, cardiolipin molecules with different acyl chain compositions should have distinct functional capacities, and cardiolipin that has been remodeled should promote cardiolipin-dependent mitochondrial processes better than its unremodeled form. However, functional disparities between different molecular forms of cardiolipin have never been established. Here, we interrogate this simple but crucial prediction utilizing the best available model to do so, Saccharomyces cerevisiae. Specifically, we compare the ability of unremodeled and remodeled cardiolipin, which differ markedly in their acyl chain composition, to support mitochondrial activities known to require cardiolipin. Surprisingly, defined changes in the acyl chain composition of cardiolipin do not alter either mitochondrial morphology or oxidative phosphorylation. Importantly, preventing cardiolipin remodeling initiation in yeast lacking TAZ1, an ortholog of the causative gene in Barth syndrome, ameliorates mitochondrial dysfunction. Thus, our data do not support the prevailing hypothesis that unremodeled cardiolipin is functionally distinct from remodeled cardiolipin, at least for the functions examined, suggesting alternative physiological roles for this conserved pathway.

Exosomes as new vesicular lipid transporters involved in cell-cell communication and various pathophysiologies

Exosomes are nanovesicles that have emerged as a new intercellular communication system between an intracellular compartment of a donor cell towards the periphery or an internal compartment of a recipient cell. The bioactivity of exosomes resides not only in their protein and RNA contents but also in their lipidic molecules. Exosomes display original lipids organized in a bilayer membrane and along with the lipid carriers such as fatty acid binding proteins that they contain, exosomes transport bioactive lipids. Exosomes can vectorize lipids such as eicosanoids, fatty acids, and cholesterol, and their lipid composition can be modified by in-vitro manipulation. They also contain lipid related enzymes so that they can constitute an autonomous unit of production of various bioactive lipids. Exosomes can circulate between proximal or distal cells and their fate can be regulated in part by lipidic molecules. Compared to their parental cells, exosomes are enriched in cholesterol and sphingomyelin and their accumulation in cells might modulate recipient cell homeostasis. Exosome release from cells appears to be a general biological process. They have been reported in all biological fluids from which they can be recovered and can be monitors of specific pathophysiological situations. Thus, the lipid content of circulating exosomes could be useful biomarkers of lipid related diseases. Since the first lipid analysis of exosomes ten years ago detailed knowledge of exosomal lipids has accumulated. The role of lipids in exosome fate and bioactivity and how they constitute an additional lipid transport system are considered in this review.

Label-free quantitative proteomic analysis of right ventricular remodeling in infant Tetralogy of Fallot patients

Tetralogy of Fallot (TOF) results in chronic progressive right ventricular (RV) pressure overload and shunt hypoxemia. We investigated the global changes in the proteome of RV among infant patients with and without TOF to gain an insight into early RV remodeling. One hundred and thirty-six differentially expressed proteins were identified using label-free LC-ESI-MS/MS analysis. Western blot results revealed that the expression of 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 2 (PFKFB2) increased significantly in TOF patients; and levels of lysocardiolipin acyltransferase 1 (LCLAT1), lumican (LUM), and versican (VCAN) decreased significantly. QRT-PCR analysis showed that levels of PFKFB2 mRNA were markedly increased, but those of LCLAT1 and LUM were significantly decreased. VCAN mRNA showed no significant change in response to pathophysiology of TOF. The results of immunohistochemical staining were similar to those of Western blot analysis. Results of the proteomic analysis indicated that the level of glycolysis-related proteins had increased and levels of lipid-metabolism-related proteins had decreased. ECM proteins were found to be more down-regulated in TOF in the present study than in previous reports. Taken together, our findings may provide clues to both the metabolic inflexibility and ECM remodeling during the early RV remodeling, which occur in response to chronic hypoxia and long-term pressure overload in TOF patients.

Emerging roles of cardiolipin remodeling in mitochondrial dysfunction associated with diabetes, obesity, and cardiovascular diseases

Cardiolipin (CL) is a phospholipid exclusively localized in inner mitochondrial membrane where it is required for oxidative phosphorylation, ATP synthesis, and mitochondrial bioenergetics. The biological functions of CL are thought to depend on its acyl chain composition which is dominated by linoleic acids in metabolically active tissues. This unique feature is not derived from the de novo biosynthesis of CL, rather from a remodeling process that involves in phospholipases and transacylase/acyltransferase. The remodeling process is also believed to be responsible for generation of CL species that causes oxidative stress and mitochondrial dysfunction. CL is highly sensitive to oxidative damages by reactive oxygen species (ROS) due to its high content in polyunsaturated fatty acids and location near the site of ROS production. Consequently, pathological remodeling of CL has been implicated in the etiology of mitochondrial dysfunction commonly associated with diabetes, obesity, heart failure, neurodegeneration, and aging that are characterized by oxidative stress, CL deficiency, and abnormal CL species. This review summarizes recent progresses in molecular, enzymatic, lipidomic, and metabolic studies that support a critical regulatory role of pathological CL remodeling as a missing link between oxidative stress and mitochondrial dysfunction in metabolic diseases and aging.

Cardiolipin-mediated cellular signaling

This review focuses on recent studies showing that cardiolipin (CL), a unique mitochondrial phospholipid, regulates many cellular functions and signaling pathways, both inside and outside the mitochondria. Inside the mitochondria, CL is a critical target of mitochondrial generated reactive oxygen species (ROS) and regulates signaling events related to apoptosis and aging. CL deficiency causes perturbation of signaling pathways outside the mitochondria, including the PKC-Slt2 cell integrity pathway and the high osmolarity glycerol (HOG) pathway, and is a key player in the cross-talk between the mitochondria and the vacuole. Understanding these connections may shed light on the pathology of Barth syndrome, a disorder of CL remodeling.

Cardiolipin remodeling and the function of tafazzin

Cardiolipin, the specific phospholipid of mitochondria, is involved in the biogenesis, the dynamics, and the supramolecular organization of mitochondrial membranes. Cardiolipin acquires a characteristic composition of fatty acids by post-synthetic remodeling, a process that is crucial for cardiolipin homeostasis and function. The remodeling of cardiolipin depends on the activity of tafazzin, a non-specific phospholipid-lysophospholipid transacylase. This review article discusses recent findings that suggest a novel function of tafazzin in mitochondrial membranes. By shuffling fatty acids between molecular species, tafazzin transforms the lipid composition and by doing so supports changes in the membrane conformation, specifically the generation of membrane curvature. Tafazzin activity is critical for the differentiation of cardiomyocytes, in which the characteristic cristae-rich morphology of cardiac mitochondria evolves. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Human trifunctional protein alpha links cardiolipin remodeling to beta-oxidation

Cardiolipin (CL) is a mitochondrial membrane phospholipid which plays a key role in apoptosis and supports mitochondrial respiratory chain complexes involved in the generation of ATP. In order to facilitate its role CL must be remodeled with appropriate fatty acids. We previously identified a human monolysocardiolipin acyltransferase activity which remodels CL via acylation of monolysocardiolipin (MLCL) to CL and was identical to the alpha subunit of trifunctional protein (αTFP) lacking the first 227 amino acids. Full length αTFP is an enzyme that plays a prominent role in mitochondrial β-oxidation, and in this study we assessed the role, if any, which this metabolic enzyme plays in the remodeling of CL. Purified human recombinant αTFP exhibited acyl-CoA acyltransferase activity in the acylation of MLCL to CL with linoleoyl-CoA, oleoyl-CoA and palmitoyl-CoA as substrates. Expression of αTFP increased radioactive linoleate or oleate or palmitate incorporation into CL in HeLa cells. Expression of αTFP in Barth Syndrome lymphoblasts, which exhibit reduced tetralinoleoyl-CL, elevated linoleoyl-CoA acylation of MLCL to CL in vitro, increased mitochondrial respiratory Complex proteins and increased linoleate-containing species of CL. Knock down of αTFP in Barth Syndrome lymphoblasts resulted in greater accumulation of MLCL than those with normal αTFP levels. The results clearly indicate that the human αTFP exhibits MLCL acyltransferase activity for the resynthesis of CL from MLCL and directly links an enzyme of mitochondrial β-oxidation to CL remodeling.

Ablation of ALCAT1 mitigates hypertrophic cardiomyopathy through effects on oxidative stress and mitophagy

Oxidative stress causes mitochondrial dysfunction and heart failure through unknown mechanisms. Cardiolipin (CL), a mitochondrial membrane phospholipid required for oxidative phosphorylation, plays a pivotal role in cardiac function. The onset of age-related heart diseases is characterized by aberrant CL acyl composition that is highly sensitive to oxidative damage, leading to CL peroxidation and mitochondrial dysfunction. Here we report a key role of ALCAT1, a lysocardiolipin acyltransferase that catalyzes the synthesis of CL with a high peroxidation index, in mitochondrial dysfunction associated with hypertrophic cardiomyopathy. We show that ALCAT1 expression was potently upregulated by the onset of hyperthyroid cardiomyopathy, leading to oxidative stress and mitochondrial dysfunction. Accordingly, overexpression of ALCAT1 in H9c2 cardiac cells caused severe oxidative stress, lipid peroxidation, and mitochondrial DNA (mtDNA) depletion. Conversely, ablation of ALCAT1 prevented the onset of T4-induced cardiomyopathy and cardiac dysfunction. ALCAT1 deficiency also mitigated oxidative stress, insulin resistance, and mitochondrial dysfunction by improving mitochondrial quality control through upregulation of PINK1, a mitochondrial GTPase required for mitochondrial autophagy. Together, these findings implicate a key role of ALCAT1 as the missing link between oxidative stress and mitochondrial dysfunction in the etiology of age-related heart diseases.

Acyl coenzyme A thioesterase Them5/Acot15 is involved in cardiolipin remodeling and fatty liver development

Acyl coenzyme A (acyl-CoA) thioesterases hydrolyze thioester bonds in acyl-CoA metabolites. The majority of mammalian thioesterases are α/β-hydrolases and have been studied extensively. A second class of Hotdog-fold enzymes has been less well described. Here, we present a structural and functional analysis of a new mammalian mitochondrial thioesterase, Them5. Them5 and its paralog, Them4, adopt the classical Hotdog-fold structure and form homodimers in crystals. In vitro, Them5 shows strong thioesterase activity with long-chain acyl-CoAs. Loss of Them5 specifically alters the remodeling process of the mitochondrial phospholipid cardiolipin. Them5(-/-) mice show deregulation of lipid metabolism and the development of fatty liver, exacerbated by a high-fat diet. Consequently, mitochondrial morphology is affected, and functions such as respiration and β-oxidation are impaired. The novel mitochondrial acyl-CoA thioesterase Them5 has a critical and specific role in the cardiolipin remodeling process, connecting it to the development of fatty liver and related conditions.

Comparison of cardiolipins from Drosophila strains with mutations in putative remodeling enzymes

Cardiolipin is a dimeric phospholipid with a characteristic acyl composition that is generated by fatty acid remodeling after de novo synthesis. Several enzymes have been proposed to participate in acyl remodeling of cardiolipin. In order to compare the effect of these enzymes, we determined the pattern of cardiolipin molecular species in Drosophila strains with specific enzyme deletions, using MALDI-TOF mass spectrometry with internal standards. We established the linear range of the method for cardiolipin quantification, determined the relative signal intensities of several cardiolipin standards, and demonstrated satisfying signal-to-noise ratios in cardiolipin spectra from a single fly. Our data demonstrate changes in the cardiolipin composition during the Drosophila life cycle. Comparison of cardiolipin spectra, using vector algebra, showed that inactivation of tafazzin had a large effect on the molecular composition of cardiolipin, inactivation of calcium-independent phospholipase A(2) had a small effect, whereas inactivation of acyl-CoA:lysocardiolipin-acyltransferase and of the trifunctional enzyme did not affect the cardiolipin composition.

LYCAT, a homologue of C. elegans acl-8, acl-9, and acl-10, determines the fatty acid composition of phosphatidylinositol in mice

Mammalian phosphatidylinositol (PI) has a unique fatty acid composition in that 1-stearoyl-2-arachidonoyl species is predominant. This fatty acid composition is formed through fatty acid remodeling by sequential deacylation and reacylation. We recently identified three Caenorhabditis elegans acyltransferases (ACL-8, ACL-9, and ACL-10) that incorporate stearic acid into the sn-1 position of PI. Mammalian LYCAT, which is the closest homolog of ACL-8, ACL-9, and ACL-10, was originally identified as a lysocardiolipin acyltransferase by an in vitro assay and was subsequently reported to possess acyltransferase activity toward various anionic lysophospholipids. However, the in vivo role of mammalian LYCAT in phospholipid fatty acid metabolism has not been well elucidated. In this study, we generated LYCAT-deficient mice and demonstrated that LYCAT determined the fatty acid composition of PI in vivo. LYCAT-deficient mice were outwardly healthy and fertile. In the mice, stearoyl-CoA acyltransferase activity toward the sn-1 position of PI was reduced, and the fatty acid composition of PI, but not those of other major phospholipids, was altered. Furthermore, expression of mouse LYCAT rescued the phenotype of C. elegans acl-8 acl-9 acl-10 triple mutants. Our data indicate that LYCAT is a determinant of PI molecular species and its function is conserved in C. elegans and mammals.

The complexity of cardiolipin in health and disease

Cardiolipin, the signature phospholipid of mitochondria, is a lipid dimer that is important for a diverse range of mitochondrial activities beyond the process of ATP production. Thus not surprisingly, derangements in cardiolipin metabolism are now appreciated to contribute to an assortment of pathological conditions. A comprehensive inventory of enzymes involved in cardiolipin biosynthesis and remodeling was just recently obtained. Post-biosynthesis, the acyl chain composition of cardiolipin is modified by up to three distinct remodeling enzymes that produce either a homogeneous tissue-specific mature form of cardiolipin or alternatively 'bad' cardiolipin that has been linked to mitochondrial dysfunction. In this review, we initially focus on the newly identified players in cardiolipin metabolism and then shift our attention to how changes in cardiolipin metabolism contribute to human disease.

The complexity of cardiolipin in health and disease

Cardiolipin, the signature phospholipid of mitochondria, is a lipid dimer that is important for a diverse range of mitochondrial activities beyond the process of ATP production. Thus not surprisingly, derangements in cardiolipin metabolism are now appreciated to contribute to an assortment of pathological conditions. A comprehensive inventory of enzymes involved in cardiolipin biosynthesis and remodeling was just recently obtained. Post-biosynthesis, the acyl chain composition of cardiolipin is modified by up to three distinct remodeling enzymes that produce either a homogeneous tissue-specific mature form of cardiolipin or alternatively 'bad' cardiolipin that has been linked to mitochondrial dysfunction. In this review, we initially focus on the newly identified players in cardiolipin metabolism and then shift our attention to how changes in cardiolipin metabolism contribute to human disease.

Cardiolipin remodeling by ALCAT1 links oxidative stress and mitochondrial dysfunction to obesity

Oxidative stress causes mitochondrial dysfunction and metabolic complications through unknown mechanisms. Cardiolipin (CL) is a key mitochondrial phospholipid required for oxidative phosphorylation. Oxidative damage to CL from pathological remodeling is implicated in the etiology of mitochondrial dysfunction commonly associated with diabetes, obesity, and other metabolic diseases. Here, we show that ALCAT1, a lyso-CL acyltransferase upregulated by oxidative stress and diet-induced obesity (DIO), catalyzes the synthesis of CL species that are highly sensitive to oxidative damage, leading to mitochondrial dysfunction, ROS production, and insulin resistance. These metabolic disorders were reminiscent of those observed in type 2 diabetes and were reversed by rosiglitazone treatment. Consequently, ALCAT1 deficiency prevented the onset of DIO and significantly improved mitochondrial complex I activity, lipid oxidation, and insulin signaling in ALCAT1(-/-) mice. Collectively, these findings identify a key role of ALCAT1 in regulating CL remodeling, mitochondrial dysfunction, and susceptibility to DIO.

Intracellular phospholipase A1 and acyltransferase, which are involved in Caenorhabditis elegans stem cell divisions, determine the sn-1 fatty acyl chain of phosphatidylinositol

Phosphatidylinositol (PI), an important constituent of membranes, contains stearic acid as the major fatty acid at the sn-1 position. This fatty acid is thought to be incorporated into PI through fatty acid remodeling by sequential deacylation and reacylation. However, the genes responsible for the reaction are unknown, and consequently, the physiological significance of the sn-1 fatty acid remains to be elucidated. Here, we identified acl-8, -9, and -10, which are closely related to each other, and ipla-1 as strong candidates for genes involved in fatty acid remodeling at the sn-1 position of PI. In both ipla-1 mutants and acl-8 acl-9 acl-10 triple mutants of Caenorhabditis elegans, the stearic acid content of PI is reduced, and asymmetric division of stem cell-like epithelial cells is defective. The defects in asymmetric division of these mutants are suppressed by a mutation of the same genes involved in intracellular retrograde transport, suggesting that ipla-1 and acl genes act in the same pathway. IPLA-1 and ACL-10 have phospholipase A(1) and acyltransferase activity, respectively, both of which recognize the sn-1 position of PI as their substrate. We propose that the sn-1 fatty acid of PI is determined by ipla-1 and acl-8, -9, -10 and crucial for asymmetric divisions.

Dynamic simulation of cardiolipin remodeling: greasing the wheels for an interpretative approach to lipidomics

Cardiolipin is a class of mitochondrial specific phospholipid, which is intricately involved in mitochondrial functionality. Differences in cardiolipin species exist in a variety of tissues and diseases. It has been demonstrated that the cardiolipin profile is a key modulator of the functions of many mitochondrial proteins. However, the chemical mechanism(s) leading to normal and/or pathological distribution of cardiolipin species remain elusive. Herein, we describe a novel approach for investigating the molecular mechanism of cardiolipin remodeling through a dynamic simulation. This approach applied data from shotgun lipidomic analyses of the heart, liver, brain, and lung mitochondrial lipidomes to model cardiolipin remodeling, including relative content, regiospecificity, and isomeric composition of cardiolipin species. Generated cardiolipin profiles were nearly identical to those determined by shotgun lipidomics. Importantly, the simulated isomeric compositions of cardiolipin species were further substantiated through product ion analysis. Finally, unique enzymatic activities involved in cardiolipin remodeling were assessed from the parameters used in the dynamic simulation of cardiolipin profiles. Collectively, we described, verified, and demonstrated a novel approach by integrating both lipidomic analysis and dynamic simulation to study cardiolipin biology. We believe this study provides a foundation to investigate cardiolipin metabolism and bioenergetic homeostasis in normal and disease states.

Identification of the human mitochondrial linoleoyl-coenzyme A monolysocardiolipin acyltransferase (MLCL AT-1)

Here we report the identification of a previously uncharacterized human protein as the human monolysocardiolipin acyltransferase-1 (MLCL AT-1). Pig liver mitochondria were treated with n-butyl alcohol followed by Q-Sepharose chromatography, preparative gel electrophoresis, cytidine diphosphate-1,2-diacyl-sn-glycerol-Sepharose chromatography, and finally monolysocardiolipin-adriamycin-agarose affinity chromatography. Elution with either monolysocardiolipin or linoleoyl coenzyme A revealed a major band at 74 kDa with high specific activity (2,300 pmol/min/mg) for the acylation of monolysocardiolipin to cardiolipin using [1-(14)C]linoleoyl coenzyme A as substrate. Matrix-assisted laser desorption ionization time-of-flight-mass spectrometry analysis followed by search of the Mascot protein data base revealed peptide matches consistent with a 59-kDa protein identified as unknown human protein (GenBank(TM) protein accession number AAX93141; nucleotide accession number AC011742.3). The purified human recombinant MLCL AT-1 protein utilized linoleoyl coenzyme A > oleoyl coenzyme A > palmitoyl coenzyme A for the specific acylation of monolysocardiolipin to cardiolipin. Expression of MLCL AT-1 in HeLa cells increased mitochondrial monolysocardiolipin acyltransferase activity and [1-(14)C]linoleic acid incorporated into cardiolipin, whereas RNA interference knockdown of MLCL AT-1 in HeLa cells resulted in reduction in enzyme activity and [1-(14)C]linoleic acid incorporated into cardiolipin. In contrast, expression of MLCL AT-1 in HeLa cells did not alter [1-(14)C]oleic or [1-(14)C]palmitate incorporation into cardiolipin indicating in vivo specificity for the remodeling of cardiolipin with linoleate. Finally, expression of MLCL AT-1 in Barth syndrome lymphoblasts, which exhibit cardiolipin levels 20% that of normal lymphoblasts, increased mitochondrial monolysocardiolipin acyltransferase activity, [1-(14)C]linoleic acid incorporation into cardiolipin, cardiolipin mass, and succinate dehydrogenase (mitochondrial complex II) activity compared with mock-transfected Barth syndrome lymphoblasts. The results identify MLCL AT-1 as a human mitochondrial monolysocardiolipin acyltransferase involved in the remodeling of cardiolipin.