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Phosphatidic Acid, Lysophosphatidic Acid and Related Lipids

Phosphatidic acid or 1,2-diacyl-sn-glycero-3-phosphate is a key intermediate in the biosynthesis both of other glycerophospholipids and of triacylglycerols. It is structurally one of the simplest of the phospholipids and was long thought to be important only as a precursor of other lipids, but it is now known to have many other functions, both in animals and plants. Moreover, its metabolite lysophosphatidic acid is recognized as a key signalling molecule with a myriad of biological effects mediated through specific receptors.

1.  Phosphatidic Acid – Occurrence and Biosynthesis

Phosphatidic acid is not an abundant lipid constituent of any living organism to my knowledge, but it is extremely important as an intermediate in the biosynthesis of triacylglycerols and phospholipids and as a signalling molecule. Indeed, it is often over-estimated in tissues as it can arise by inadvertent enzymatic hydrolysis during inappropriate storage or extraction conditions during analysis. It is the simplest diacyl-glycerophospholipid, and the only one with a phosphomonoester as the head group. The molecule is acidic and carries a negative charge, i.e. it is an anionic lipid.

Structural formula of phosphatidic acid

There are at least four important biosynthetic pathways for phosphatidic acid biosynthesis in different organelles under various stimuli, and possibly resulting in the formation of different molecular species. The main pathway involves sequential acylation of sn-glycerol-3-phosphate, derived from catabolism of glucose, by acyl-coA derivatives of fatty acids as illustrated (see also the web page on biosynthesis of triacylglycerols). First, one acyltransferases catalyses the acylation of position sn-1 to form lysophosphatidic acid (1-acyl-sn-glycerol-3-phosphate), and then a second specific acyltransferase catalyses the acylation of position sn-2 to yield phosphatidic acid.

Biosynthesis of phosphatidic acid

In mammals, the glycerol-3-phosphate acyltransferase that catalyses the first step exists in four isoforms, two in the mitochondrial outer membrane (designated GPAT1 and 2) and two in the endoplasmic reticulum (GPAT3 and 4). All are membrane-bound enzymes, which are believed to span the membranes, but many questions remain regarding the regulation and function of the different isoforms. Similarly, at least three acyl-CoA:lysophosphatidic acid acyltransferases (LPAAT or LPAT or AGPAT1, 2 and 3) in the endoplasmic reticulum are known that catalyse the second step, with a further two (LPAT4 and 5) on the outer mitochondrial membrane. Human LPAT1 showed higher activity with 14:0-, 16:0- and 18:2-CoAs, while LPAT2 prefers 20:4-CoA and LPAT3 produces phosphatidic acid containing docosahexaenoic acid (22:6(n-3)); the last is especially important in retina and testes. The activity in the endoplasmic reticulum predominates in adipose tissue, but the mitochondrial forms are believed to be responsible for half the activity in liver. As there is traffic of phosphatidic acid between the mitochondria and endoplasmic reticulum for remodelling or for synthesis of other lipids, the relative contributions of the two can be difficult to assess. In plants, this pathway exists both in plastids and at the endoplasmic reticulum with multiple isoforms of the two acyltransferases.

In bacteria, two families of enzymes are responsible for acylation of position sn-1 of glycerol-3-phosphate. One present in Escherichia coli, for example, utilizes the acyl-acyl carrier protein (acyl-ACP) products of fatty acid synthesis as acyl donors as well as acyl-CoA derived from exogenous fatty acids. The second set of enzymes makes use of the unique acyl donors, acyl-phosphates derived in part from acyl-ACP, and is present in a wider range of bacteria. Acylation of position sn-2 is performed by a further family of enzymes that uses acyl-ACP as the acyl donor, although some bacterial species may use acyl-CoA also.

In animals, a second biosynthetic pathway utilizes dihydroxyacetone phosphate (DHAP) as the primary precursor and the peroxisomal enzyme, DHAP acyltransferase, which produces acyl-DHAP. This intermediate is converted to lysophosphatidic acid in a NADPH-dependent reaction catalysed by acyl-DHAP reductase, and this is in turn acylated to form phosphatidic acid. This pathway is of particular importance in the biosynthesis of ether lipids.

Biosynthesis of phosphatidic acid via dihydroxyacetone phosphate

A third important route to phosphatidic acid is via hydrolysis of other phospholipids, but especially phosphatidylcholine, by the enzyme phospholipase D (or by a family or related enzymes of this kind). The enzyme is readily available for study in plants, where the special functions of phosphatidic acid have long been known (see below), but it is now recognized that phospholipase D is present in bacteria, yeasts and most animal cells. In the last, it exists in two main isoforms with differing specificities and cellular locations; PLD1 is found mainly in the Golgi-lysosome continuum, while PLD2 is present mainly in the plasma membrane. They are phosphoproteins, the activity of which is regulated by kinases and phosphatases and by binding to phosphatidylinositol-4,5-bisphosphate. In mitochondria, a distinctive enzyme of this type utilizes cardiolipin as substrate. The mechanism involves the use of water as the nucleophile to catalyse the hydrolysis of phosphodiester bonds in phospholipids. Phospholipase D activity is dependent on and regulated by neurotransmitters, hormones, small monomeric GTPases and lipids.

Generation of phosphatidic acid by the action of phospholipase D

In addition to its function in generating phosphatidic acid mainly for signalling purposes but also for the maintenance of membrane composition, phospholipase D is involved in intracellular protein trafficking, cytoskeletal dynamics, cell migration and cell proliferation, partly through protein-protein interactions; it is considered to be important in inflammation and in cancer growth and metastasis as a downstream transcriptional target of proteins involved in the pathophysiology of these diseases. It also has an unusual activity as a guanine nucleotide exchange factor. By a transphosphatidylation reaction with ethanol, it generates phosphatidylethanol, a useful biomarker for ethanol consumption in humans.

Under some conditions, phosphatidic acid can be generated from 1,2-diacyl-sn-glycerols by the action of diacylglycerol kinases (see our web page on diacylglycerols). Such enzymes appear to be ubiquitous in nature, although those in bacteria and yeast are structurally different from the mammalian enzymes. Diacylglycerol kinases, of which at least ten isoforms exist with different sub-cellular locations and functions in animals, use ATP as the phosphate donor. Aside from producing phosphatidic acid for phospholipid production or signalling, these enzymes may attenuate the signalling effects of diacylglycerols. For example, diacylglycerol kinases can contribute to cellular asymmetry and control the polarity of cells by regulating the gradients in diacylglycerol and phosphatidic acid concentrations.

Biosynthesis of phosphatidic acid by diacylglycerol kinases

Phosphatidic acid metabolismAnother possible route to phosphatidic acid production for signalling purposes is via acylation of lysophosphatidic acid, which is also an important signalling lipid as discussed below. In addition, this pathway may be important in membranes, where the protein endophilin has LPAT activity and is believed to generate phosphatidic acid from lysophosphatidic acid in order to alter the curvature of the membrane bilayer.

To summarize, phosphatidic acid generated via 1-acyl-sn-glycerol-3-phosphate is the main route to other glycerolipids, while other pathways may be more important for generating the lipid for signalling functions. Control of its concentration in membranes, especially in the endoplasmic reticulum, is therefore of great importance, and a transcriptional repressor 'Opi1', which binds specifically to phosphatidic acid in membranes, is an important regulatory factor. The subsequent steps in the utilization of phosphatidic acid in the biosynthesis of triacylglycerols and of the various glycerophospholipids are described in separate documents of this website.

Thus, hydrolysis of phosphatidic acid by phosphatidate phosphatase enzymes (including lipins 1, 2 and 3) is the source of sn-1,2-diacylglycerols (DG), which are the precursors for the biosynthesis of triacylglycerols (TAG), phosphatidylcholine (PC) and phosphatidylethanolamine (PE) via the so-called Kennedy pathway (also of monogalactosyldiacylglycerols in plants). Via reaction with cytidine triphosphate, phosphatidic acid is the precursor of cytidine diphosphate diacylglycerol, which is the key intermediate in the synthesis of phosphatidylglycerol (PG), and thence of cardiolipin (CL), and of phosphatidylinositol (PI) and phosphatidylserine (PS). Depending on the organism and other factors, phosphatidylserine can be a precursor for phosphatidylethanolamine, while the latter can give rise to phosphatidylcholine by way of mono- and dimethyl-phosphatidylethanolamine intermediates.

While the fatty acid composition of phosphatidic acid can resemble that of the eventual products, the latter are generally much altered by re-modelling after synthesis via deacylation-reacylation reactions (the Lands' cycle - see the web page on phosphatidylcholine).

2.  Phosphatidic Acid - Biological Functions in Animals

In addition to its role as an intermediate in lipid biosynthesis, phosphatidic acid generated by the action of phospholipase D and by diacylglycerol kinases may have signalling functions as a second messenger, although it is not certain whether all the activities suggested by studies in vitro operate in vivo. Nonetheless, phosphatidic acid has been implicated in many aspects of animal cell biochemistry and physiology.

Some of the observed effects may be explained simply by the physical properties of phosphatidic acid, which has a propensity to form a hexagonal II phase, especially in the presence of calcium ions. Thus, hydrolysis of phosphatidylcholine, a cylindrical non-fusogenic lipid, converts it into cone-shaped phosphatidic acid, which promotes negative membrane curvature and fusion of membranes. It differs from other anionic phospholipids in that its small anionic phosphomonoester head group lies very close to the hydrophobic interior of the lipid bilayer. In model systems, phosphatidic acid can effect membrane fusion, probably because of its ability to form non-bilayer phases. For example, the phosphatidic acid biosynthesis is believed to favour intraluminal budding of endosomal membranes with the formation of exosomes.

Also of relevance in this context is its overall negative charge, and it is not always clear whether some of the observed biological effects are specific to phosphatidic acid or simply to negatively charged phospholipids in general. In contrast to phosphoinositide-interacting proteins, which have defined structural folds, the binding motifs of effector proteins with phosphatidic acid are not highly conserved. However, it has been demonstrated that the positively charged lysine and arginine residues on proteins can bind with some specificity to phosphatidic acid through hydrogen bonding with the phosphate group thus distinguishing it from other phospholipids. An ‘electrostatic-hydrogen bond switch model’ has been proposed in which the head group of phosphatidic acid forms a hydrogen bond to a basic amino acid residue, leading to de-protonation of the head group, increasing its negative charge from -1 to -2 and thus enabling stronger interactions with further basic residues and tight docking with the membrane interacting protein. In this way, phosphatidic acid may tether certain proteins to membranes.

Scottish thistleOne key target of the lipid is mTOR, a serine/threonine protein kinase that regulates cell growth, proliferation, motility and survival, together with protein synthesis and transcription, by integrating both nutrient and growth factor signals. Phosphatidic acid appears to regulate membrane trafficking events,and it is involved in activation of the enzyme NADPH oxidase, which operates as part of the defence mechanism against infection and tissue damage during inflammation. It may have a role in promoting phospholipase A2 activity, and it appears to function in vesicle formation and transport within the cell. By binding to targeted proteins, including protein kinases, protein phosphatases and G-proteins, it may increase or inhibit their activities. For example in yeast, phosphatidic acid in the endoplasmic reticulum binds directly to a specific transcriptional repressor to keep it inactive outside the nucleus; when the lipid precursor inositol is added, this phosphatidic acid is rapidly depleted, releasing the transcriptional factor so that it can be translocated to the nucleus where it is able to repress target genes. The overall effect is a mechanism to control phospholipid synthesis. In addition, phosphatidic acid regulates many aspects of phosphoinositide function. For example, the murine phosphatidylinositol 4-phosphate 5-kinase, the main enzyme generating the lipid second messenger phosphatidylinositol-4,5-bisphosphate, does not appear to function unless phosphatidic acid is bound to it; this lipid, generated by the action of phospholipase D, recruits the enzyme to the membrane and induces a conformational change that regulates its activity. In many cell types, vesicle trafficking, secretion and endocytosis may also require phosphatidic acid derived by the action of phospholipase D.

In relation to signalling activities, it should be noted that phosphatidic acid can be metabolized to sn-1,2-diacylglycerols or to lysophosphatidic acid (see next section), both of which have distinctive signalling functions in their own right. Conversely, both of these compounds can be in effect be de-activated by conversion back to phosphatidic acid.

Phospholipase D isoforms and phosphatidic acid have been implicated in a variety of pathologies including neurodegenerative diseases, blood disorders, late-onset Alzheimer's disease and cancer.

3.  Phosphatidic Acid - Biological Functions in Plants

In addition to its role as one of the central molecules in lipid biosynthesis, phosphatidic acid facilitates the transport of lipids across plant membranes, and it is also the key plant lipid second messenger, which is rapidly and transiently generated in response to many different biotic and abiotic stresses In contrast to animal metabolism, the diacylglycerol signalling pathway is believed to be relatively insignificant in plants.

The main source of phosphatidic acid for these purposes is the action of phospholipase D (PLD) on membrane phospholipids, such as phosphatidylcholine and phosphatidylethanolamine. Plants contain a large number of related enzymes of this type, 12 in Arabidopsis and 17 in rice, in comparison with two in humans and one in yeast, and individual iso-enzymes may elicit specific responses. In the former, the isoforms are grouped into six classes, based on the genic architecture, sequence similarities, domain structures and biochemical properties. These depend mainly on their lipid-binding domains, with some homologous to the human and yeast enzymes and with most containing a characteristic ‘C2’ (calcium- and lipid-binding) domain. The most widespread of these is PLDα, which does not require binding to phosphatidylinositol 4,5-bisphosphate, in contrast to other PLD isoforms and the mammalian enzyme, but millimolar levels of Ca2+ are necessary. Studies with fluorescent biosensors suggest that phosphatidic acid accumulates in the subapical region of the cytosolic leaflet of the plasma membrane.

Phosphatidic acid can also be produced by the sequential action of phospholipase C and diacylglycerol kinase on membrane inositol phospholipids, with diacylglycerols as an intermediate (there are 7 isoenzymes in A. thaliana). One difference from animal metabolism is that diacylglycerol pyrophosphate can be synthesised from phosphatidic acid in plants (see below).

Scottish thistlePhosphatidic acid is required to bind to and activate the monogalactosyldiacylglycerol synthase (MGDG1), located in the inner envelope membrane of the chloroplast. Phospholipase D activity and the phosphatidic acid produced have long been recognized as of importance during germination and senescence, and they have an essential role in the response to stress damage and pathogen attack, both in higher plants and in green algae. A high content of phosphatidic acid induced by phospholipase D action during wounding or senescence brings about a loss of the membrane bilayer phase, as a consequence of the conical shape of this negatively charged phospholipid in comparison to the cylindrical shape of structural phospholipids. This change in ionization properties has crucial effects upon lipid-protein interactions, and has been termed "the electrostatic-hydrogen bond switch model". In addition, phosphatidic acid is important in the response to other forms of stress, including osmotic stress (salinity or drought), cold and oxidation. Although much remains to be learned of the mechanism by which it exerts its effects, it is believed to promote the response to the hormone abscisic acid. In addition, phosphatidic acid may interact with salicylic acid to mediate defence responses.

In plants, phosphatidic acid is involved in many different cell responses induced by hormones, stress and developmental processes. In relation to cellular signalling, it often acts in concert with phosphatidylinositol 4,5-bisphosphate by binding to specific proteins rather than acting via a receptor. As in mammalian cells, targets for such signalling include protein kinases and phosphatases in addition to proteins involved in membrane trafficking and the organization of the cytoskeleton. It can both activate or inhibit enzymes. If the target protein is soluble, binding to phosphatidic acid can cause the protein to be sequestered into a membrane with effects upon downstream targets. For example, it is involved in promoting the growth of pollen-tubes and root hairs, decreasing peroxide-induced cell death, and mediating the signalling processes that lead to responses to ethylene and again to the plant hormone abscisic acid. Thus in the 'model' plant Arabidopsis, phosphatidic acid interacts with a protein phosphatase to signal the closure of stomata promoted by abscisic acid; it interacts also with a further enzyme to mediate the inhibition of stomatal opening effected by abscisic acid. Together these reactions constitute a signalling pathway that regulates water loss from plants.

It is noteworthy that phosphatidic acid production can be initiated by opposing stress factors, such as cold and heat, or by hormones that are considered to be antagonistic, such as abscisic acid and salicylic acid. It is possible that phosphatidic acid molecules synthesised by the two main pathways differ in composition and cellular distributions and so may produce different responses, but this is an open question. Certainly during low temperature stress, phosphatidic acid is generated by the action of diacylglycerol kinase. It also seems likely that these differing activities are controlled by the cellular environment where the lipid is produced and by the availability of target proteins.

As in animals, phosphatidic acid is catabolized and its signalling functions are terminated by lipid phosphate phosphatases and phosphatidic acid hydrolases (lipins), and by acyl-hydrolases and lipoxygenases with the production of fatty acids and other small molecules, which are subsequently absorbed and recycled.

4.  Lysophosphatidic Acid

Structural formula of lysophosphatidic acidLysophosphatidic acid or 1-acyl-sn-glycerol-3-phosphate differs structurally from phosphatidic acid in having only one mole of fatty acid per mole of lipid. As such, it is one of the simplest possible glycerophospholipids. Although it is present at very low levels only in animal tissues, it is extremely important biologically, influencing many biochemical processes, activities that seem to be shared by the 1-O-alkyl- and alkenyl-ether forms. As it lacks one of the fatty acids in comparison to phosphatidic acid, it is a much more hydrophilic molecule, while the additional hydroxyl group strengthens hydrogen bonding within membranes, properties that may be important for its function in cells. While lysophosphatidic acid is a key biosynthetic precursor of phosphatidic acid, there is particular interest in its role as an intercellular lipid mediator with growth factor-like activities. For example, it is rapidly produced and released from activated platelets to influence target cells. It exists in the form of a number of different molecular species, i.e. esterified to 16:0 to 20:4 fatty acids, and some of these have distinct biological properties.

Biosynthesis: The most important source is the activity of an enzyme with lysophospholipase D-like activity and known as ‘autotaxin’ on lysophosphatidylcholine (200 μM in plasma) to yield lysophosphatidic acid in an albumin-bound form, although it is water-soluble because of its small size. This is more abundant in serum (1-5 μM) than in plasma (100 nM), where it accounts for much of the biological activity. Autotaxin is a member of the nucleotide pyrophosphatase/phosphodiesterase family and is also present in cerebrospinal and seminal fluids and many other tissues including cancer cell lines from which it was first isolated and characterized. Indeed, the name derives from the finding that it promoted chemotaxis on melanoma cells in an autocrine fashion. It binds to target cells via integrin and heparan sulfate proteoglycans and this may assist the delivery of lysophosphatidic acid to its receptors. Genetic deletion of the enzyme in mice results in aberrant vascular and neuronal development and soon leads to death of the embryos. However, overexpression of autotaxin causes physical defects also and is eventually lethal to embryos.

Biosynthesis of lysophosphatidic acid

While autotaxin is the primary source of extracellular lysophosphatidic acid, it is now established that lysophosphatidic acid is produced intracellularly by a wide variety of cell types by various mechanisms with phosphatidic acid as the primary precursor; it is produced and acts near its site of synthesis, i.e. it is an autacoid. Hydrolysis of phosphatidic acid by a phospholipase A2 (PLA2) is the main mechanism in platelets, but other cellular enzymes involved include a phosphatidic acid-selective phospholipase A1 (PLA1) producing sn-2-acyl-lysophosphatidic acid, a monoacylglycerol kinase and glycerol-3-phosphate acyltransferase. On the other hand, it is possible that most of the lysophosphatidic acid produced intracellularly is used for synthesis of other phospholipids rather than for signalling purposes.

Function: Although lysophospholipids are relatively small molecules, they carry a high content of information through the nature of the phosphate head group, the positional distribution of the fatty acids on the glycerol moiety, the presence of ether or ester linkages to the glycerol backbone, and the chain-length and degree and position of saturation of the fatty acyl chains. This informational content leads to selectivity in the functional relationship with cell receptors. As most mammalian cells express receptors for lysophosphatidic acid, this lipid may initiate signalling in the cells in which it is produced, as well as affecting neighbouring cells. Characterization of cloned lysophosphatidic acid receptors in combination with strategies of molecular genetics has allowed determination of both signalling and biological effects that are dependent on receptor mechanisms. At least six G protein-coupled receptors that are specific for lysophosphatidic acid have now been identified in vertebrates, each found in particular organs and coupled to at least one or more of the four heterotrimeric Gα proteins and designated LPAR1 to LPAR6. These vary appreciably amino acid sequences but are classified into two subgroups, the EDG (LPAR1-3) and P2Y (LPAR4-6) families, with differing tissue distributions. Experimental activation of these receptors has shown that a range of downstream signalling cascades mediate lysophosphatidic acid signalling. These include activation of protein kinases, adenyl cyclase and phospholipase C, release of arachidonic acid, and much more. There is evidence that lysophosphatidic acid is involved in cell survival in some circumstances, and in programmed cell death in others.

Scottish thistleSignalling by lysophosphatidic acid has regulatory functions in the mammalian reproductive system, both male and female, facilitating oocyte maturation and spermatogenesis through the action of the receptors LPAR1 to LPAR3. There is also evidence that the lipid is involved in brain development, through its activity in neural progenitor cells, neurons and glia, and in vascular remodelling. In the central nervous system, these receptors are thought to play a central role in both triggering and maintaining neuropathic pain by mechanisms that may involve demyelination of damaged nerves. Lysophosphatidic acid has been found in saliva in significant amounts, and it has been suggested that it is involved in wound healing in the upper digestive organs such as the mouth, pharynx, and oesophagus. When applied topically to skin wounds, it has similar effects probably by stimulating proliferation of new cells to seal the wound. Receptor LPAR6 together with the phospholipase A1 is required for the development of hair follicles, and this receptor is also involved in the regulation of endothelial blood-brain barrier function. In bone cells, lysophosphatidic acid is important for in bone homeostasis and repair.

Disease: There is particular interest in the activity of lysophosphatidic acid in various disease states and cancer especially. For example, a finding that lysophosphatidic acid is markedly elevated in the plasma and peritoneal fluid (ascites) of ovarian cancer patients compared to healthy controls may be especially significant. In particular, elevated plasma levels were found in patients in the first stage of ovarian cancer, suggesting that it may represent a useful marker for the early detection of the disease. Lysophosphatidic acid is believed to stimulate the expression of genes for many different enzymes that lead to the proliferation of ovarian cancer cells (via receptor LPAR2 while LPAR4 has opposing effects), and it may induce cell migration.

As lysophosphatidic acid has growth-factor-like activities for many cell types that induce cell proliferation and migration, changes in cellular shape and increasing of endothelial permeability, it is perhaps not surprising that it is relevant to tumor biology. Treatment of various cancer cell types with lysophosphatidic acid promotes the expression and release of interleukin 8 (IL-8), which is a potent angiogenic factor, and thus it has a critical role in the growth and spread of cancers by enhancing the availability of nutrients and oxygen. There is evidence that signalling by lysophosphatidic acid is causally linked to hyperactive lipogenesis in cancer. For example, it activates the sterol regulatory element-binding protein (SREBP) together with the fatty acid synthase and AMP-activated protein kinase–ACC lipogenic cascades leading to elevated synthesis of lipids de novo. Increased autotaxin expression has been demonstrated in a many different cancer cell lines, and the expression of many of the surface receptors for lysophosphatidic acid in cancer cells is aberrant. Cancer cells must evade the immune system during metastasis, and lysophosphatidic acid facilitates this process by inhibiting the activation of T cells. Therefore, lysophosphatidic acid metabolism is a target of the pharmaceutical industry in the search for new drugs for cancer therapy, aided by a knowledge of the crystal structures of three of the receptors.

Problems with lysophosphatidic acid signalling together with changes in autotaxin expression are believed to be factors in metabolic and inflammatory disorders including obesity, insulin resistance, and cardiovascular disease. For example, under certain conditions, it can become athero- and thrombogenic and might aggravate cardiovascular disease. As oxidized low-density lipoproteins promote the production of lysophosphatidic acid, its content in atherosclerotic plaques is high suggesting that it might serve as a biomarker for cardiovascular disease. Indeed, lysophosphatidic acid promotes pro-inflammatory events that lead to the development of atheroma as well encouraging progression of the disease. By mediating platelet aggregation, it could lead to arterial thrombus formation. There is evidence that it is also involved in such inflammatory diseases as rheumatoid arthritis and multiple sclerosis, and that it contributes to neurological disorders, hepatic diseases, asthma, fibrosis and bone malfunction. Drugs that interact with the lysophosphatidic acid receptors are reported to be effective in attenuating symptoms of several diseases in animal models, and three have passed phase I and II clinical trials for idiopathic pulmonary fibrosis and systemic sclerosis. Drugs that target autotaxin production and catabolism of lysophosphatidic acid are also in development.

Related lipids: The sphingolipid analogue, sphingosine-1-phosphate, shows a similar range of activities to lysophosphatidic acid and the two lipids are often discussed together in the same contexts. Acute leukemia cells produce methyl-lysophosphatidic acids (the polar head-group is methylated). As these act as antigens to which a specific group of human T cells react strongly, it is possible that they might be a target for the immunotherapy of hematological malignancies. Other lysophospholipids are known to have distinctive biological functions (see separate web pages).

Catabolism: Deactivation of lysophosphatidic acid is accomplished by dephosphorylation to produce monoacylglycerols by a family of three lipid phosphate phosphatases, which also de-phosphorylate sphingosine-1-phosphate, phosphatidic acid and ceramide 1-phosphate in a non-specific manner. These are integral membrane proteins with the active site in the plasma membrane facing the extracellular environment, enabling them to access and hydrolyse extracellular lysophosphatidic acid and other phospholipids. Lysophosphatidic acid can be converted back to phosphatidic acid by a membrane-bound O-acyltransferase (MBOAT2) specific for lysophosphatidic acid (and lysophosphatidylethanolamine) with a preference for oleoyl-CoA as substrate.

5.  Cyclic Phosphatidic Acid

Structural formula of cyclic phosphatidic acidCyclic phosphatidic acid (sometimes termed ‘cyclic lysophosphatidic acid or cPA’) was isolated originally from a slime mould, but has now been detected in a wide range of organisms including humans, especially in the brain but also bound to albumin in serum (at a concentration of 10-7M, or a tenth that of lysophosphatidic acid, and most abundant in tissues subject to injury). It has a cyclic phosphate at the sn-2 and sn-3 positions of the glycerol carbons, and this structure is absolutely necessary for its biological activity. In human serum, the main molecular species contains palmitic acid, though 18:0, 18:1, and 18:2 are also present. An unusual plasmenylcyclic phosphatidic acid has been isolated from the intestinal bacterium Bifidobacterium longum subs. infantis.

Studies of the biosynthesis of cyclic phosphatidic acid in fetal bovine serum suggest that it is the product of the human enzyme autotaxin, the serum lysophospholipase D that produces lysophosphatidic acid (see above), or by the action of phospholipase D2 after appropriate stimulation. These enzymes appears to produce cyclic phosphatidic acid in serum by an intramolecular transphosphatidylation reaction (this also occurs by the action of a phospholipase D in the venom of the brown spider on lysophosphatidylcholine). However, it can be formed artefactually by the addition of strong acid to serum.

While cyclic phosphatidic acid may have some similar signalling functions to lysophosphatidic acid per se in that it binds to some of the same receptors, it also has some quite distinct activities in animal tissues. For example, cyclic phosphatidic acid is known to be a specific inhibitor of DNA polymerase alpha. It has an appreciable effect on the inhibition of cancer cell invasion and metastasis, a finding that is currently attracting great pharmacological interest; derivatives of cPA, in which the sn-2 or sn-3 oxygen of the glycerol backbone is replaced by a methylene group ('2 carba- or 3 carba-cPA') are stable analogues that are being tested for this purpose. In addition, it inhibits the platelet aggregation induced by lysophosphatidic acid, possibly by inhibiting autotaxin. In the central nervous system, it can enhance cell survival and neurite extension in neurons, while exerting neuroprotective effects against apoptosis. It has beneficial effects in an animal model of multiple sclerosis, it attenuates neuropathic pain and it relieves the symptoms of osteoarthritis. Cyclic phosphatidic acid is a high-affinity and specific ligand for the nuclear receptor PPARγ, which is involved in the regulation of adipogenesis, glucose homoeostasis and processes related to type 2 diabetes.

6.  Pyrophosphatidic Acid

Structural formula of pyrophosphatidic acidPyrophosphatidic acid or sn-1,2-diacylglycero-3-pyrophosphate is an unusual and little known phospholipid that was first identified as a minor component in yeasts, and is also known to be present in mushrooms and higher plants as a product of the enzyme phosphatidic acid kinase, which is present in all plant tissues but especially the plasma membrane.

It is rapidly metabolized back to phosphatidic acid by a specific phosphatase and thence to diacylglycerols, and it may have a function in the phospholipase D signalling cascade in plants, perhaps by attenuating the effects of phosphatidic acid. Pyrophosphatidic acid is barely detectable in non-stimulated plant cells but its concentration increases very rapidly in response to stress situations, including osmotic stress and attack by pathogens. Such findings add to the belief that it is an important signalling molecule in plants under stress, especially in relation to abscisic acid responses. In yeasts, it may have a role in the regulation of the synthesis and metabolism of phospholipids, especially phosphatidylserine.

7.  Analysis

Phosphatidic acid and related lipids are not the easiest to analyse. On adsorption chromatography, retention times tend to be variable and may be dependent to some extent on the nature of the cations associated with the acidic lipids, but two-dimensional TLC can give good results. Phosphatidic acid, bis(monoacyl)glycerophosphate and pyrophosphatidic acid are never easy to distinguish, but modern liquid chromatography-mass spectrometric methods appear to be the answer.

Recommended Reading

Lipid listings Credits/disclaimer Updated: July 24th, 2019 Author: William W. Christie LipidWeb icon