Identification of functional domains of rat intestinal phospholipase B/lipase. Its cDNA cloning, expression, and tissue distribution

Phospholipases in neuronal function: A role in learning and memory?

Despite the human brain being made of nearly 60% fat, the vast majority of studies on the mechanisms of neuronal communication which underpin cognition, memory and learning, primarily focus on proteins and/or (epi)genetic mechanisms. Phospholipids are the main component of all cellular membranes and function as substrates for numerous phospholipid-modifying enzymes, including phospholipases, which release free fatty acids (FFAs) and other lipid metabolites that can alter the intrinsic properties of the membranes, recruit and activate critical proteins, and act as lipid signalling molecules. Here, we will review brain specific phospholipases, their roles in membrane remodelling, neuronal function, learning and memory, as well as their disease implications. In particular, we will highlight key roles of unsaturated FFAs, particularly arachidonic acid, in neurotransmitter release, neuroinflammation and memory. In light of recent findings, we will also discuss the emerging role of phospholipase A₁ and the creation of saturated FFAs in the brain.

Pancreatic and mucosal enzymes in choline phospholipid digestion

The digestion of choline phospholipids is important for choline homeostasis, lipid signaling, postprandial lipid and energy metabolism, and interaction with intestinal bacteria. The digestion is mediated by the combined action of pancreatic and mucosal enzymes. In the proximal small intestine, hydrolysis of phosphatidylcholine (PC) to 1-lyso-PC and free fatty acid (FFA) by the pancreatic phospholipase A₂ IB coincides with the digestion of the dietary triacylglycerols by lipases, but part of the PC digestion is extended and must be mediated by other enzymes as the jejunoileal brush-border phospholipase B/lipase and mucosal secreted phospholipase A₂ X. Absorbed 1-lyso-PC is partitioned in the mucosal cells between degradation and reacylation into chyle PC. Reutilization of choline for hepatic bile PC synthesis, and the reacylation of 1-lyso-PC into chylomicron PC by the lyso-PC-acyl-CoA-acyltransferase 3 are important features of choline recycling and postprandial lipid metabolism. The role of mucosal enzymes is emphasized by sphingomyelin (SM) being sequentially hydrolyzed by brush-border alkaline sphingomyelinase (alk-SMase) and neutral ceramidase to sphingosine and FFA, which are well absorbed. Ceramide and sphingosine-1-phosphate are generated and are both metabolic intermediates and important lipid messengers. Alk-SMase has anti-inflammatory effects that counteract gut inflammation and tumorigenesis. These may be mediated by multiple mechanisms including generation of sphingolipid metabolites and suppression of autotaxin induction and lyso-phosphatidic acid formation. Here we summarize current knowledge on the roles of pancreatic and mucosal enzymes in PC and SM digestion, and its implications in intestinal and liver diseases, bacterial choline metabolism in the gut, and cholesterol absorption.

Intestinal phospholipid and lysophospholipid metabolism in cardiometabolic disease

PURPOSE OF REVIEW

Phospholipids are major constituents in the intestinal lumen after meal consumption. This article highlights current literature suggesting the contributory role of intestinal phospholipid metabolism toward cardiometabolic disease manifestation.

RECENT FINDINGS

Group 1b phospholipase A2 (PLA2g1b) catalyzes phospholipid hydrolysis in the intestinal lumen. The digestive product lysophospholipid, particularly lysophosphatidylcholine (LPC), has a direct role in mediating chylomicron assembly and secretion. The LPC in the digestive tract is further catabolized into lysophosphatidic acid and choline via autotaxin-mediated and autotaxin-independent mechanisms. The LPC and lysophosphatidic acid absorbed through the digestive tract and transported to the plasma directly promote systemic inflammation and cell dysfunction, leading to increased risk of cardiovascular disease and obesity/diabetes. The choline moiety generated in the digestive tract can also be used by gut bacteria to generate trimethylamine, which is subsequently transported to the liver and oxidized into trimethylamine-N-oxide that also enhances atherosclerosis and cardiovascular abnormalities.

SUMMARY

Products of phospholipid metabolism in the intestine through PLA2g1b and autotaxin-mediated pathways directly contribute to cardiometabolic diseases through multiple mechanisms. The implication of these studies is that therapeutic inhibition of PLA2g1b and autotaxin in the digestive tract may be a viable approach for cardiovascular and metabolic disease intervention.

Role of Sphingolipids in Infant Gut Health and Immunity

Sphingomyelin (SM), glycosphingolipids, and gangliosides are important polar lipids in the milk fat globule membrane but are not found in standard milk replacement formulas. Because digestion and absorption of SM and glycosphingolipids generate the bioactive metabolites ceramide, sphingosine, and sphingosine-1-phosphate (S1P), and because intact gangliosides may have beneficial effects in the gut, this may be important for gut integrity and immune maturation in the neonate. The brush border enzymes that hydrolyze milk SM, alkaline sphingomyelinase (nucleotide phosphodiesterase pyrophosphatase 7), and neutral ceramidase are expressed at birth in both term and preterm infants. Released sphingosine is absorbed, phosphorylated to S1P, and converted to palmitic acid via S1P-lyase in the gut mucosa. Hypothetically, S1P also may be released from absorptive cells and exert important paracrine actions favoring epithelial integrity and renewal, as well as immune function, including secretory IgA production and migration of T lymphocyte subpopulations. Gluco-, galacto-, and lactosylceramide are hydrolyzed to ceramide by lactase-phlorizin hydrolase, which also hydrolyzes lactose. Gangliosides may adhere to the brush border and is internalized, modified, and possibly transported into blood, and may exert protective functions by their interactions with bacteria, bacterial toxins, and the brush border.

Forty five years with membrane phospholipids, phospholipases and lipid mediators: A historical perspective

Phospholipases play a key role in the metabolism of phospholipids and in cell signaling. They are also a very useful tool to explore phospholipid structure and metabolism as well as membrane organization. They are at the center of this review, covering a period starting in 1971 and focused on a number of subjects in which my colleagues and I have been involved. Those include determination of phospholipid asymmetry in the blood platelet membrane, biosynthesis of lysophosphatidic acid, biochemistry of platelet-activating factor, first attempts to define the role of phosphoinositides in cell signaling, and identification of novel digestive (phospho)lipases such as pancreatic lipase-related protein 2 (PLRP2) or phospholipase B. Besides recalling some of our contributions to those various fields, this review makes an appraisal of the impressive and often unexpected evolution of those various aspects of membrane phospholipids and lipid mediators. It is also the occasion to propose some new working hypotheses.

Lipid Regulation of Acrosome Exocytosis

Lipids are critical regulators of mammalian sperm function, first helping prevent premature acrosome exocytosis, then enabling sperm to become competent to fertilize at the right place/time through the process of capacitation, and ultimately triggering acrosome exocytosis. Yet because they do not fit neatly into the "DNA--RNA-protein" synthetic pathway, they are understudied and poorly understood. Here, we focus on three lipids or lipid classes-cholesterol, phospholipids, and the ganglioside G(M1)--in context of the modern paradigm of acrosome exocytosis. We describe how these various- species are precisely segregated into membrane macrodomains and microdomains, simultaneously preventing premature exocytosis while acting as foci for organizing regulatory and effector molecules that will enable exocytosis. Although the mechanisms responsible for these domains are poorly defined, there is substantial evidence for their composition and functions. We present diverse ways that lipids and lipid modifications regulate capacitation and acrosome exocytosis, describing in more detail how removal of cholesterol plays a master regulatory role in enabling exocytosis through at least two complementary pathways. First, cholesterol efflux leads to proteolytic activation of phospholipase B, which cleaves both phospholipid tails. The resultant changes in membrane curvature provide a mechanism for the point fusions now known to occur far before a sperm physically interacts with the zona pellucida. Cholesterol efflux also enables G(M1) to regulate the voltage-dependent cation channel, Ca(V)2.3, triggering focal calcium transients required for acrosome exocytosis in response to subsequent whole-cell calcium rises. We close with a model integrating functions for lipids in regulating acrosome exocytosis.

Egg phospholipids and cardiovascular health

Eggs are a major source of phospholipids (PL) in the Western diet. Dietary PL have emerged as a potential source of bioactive lipids that may have widespread effects on pathways related to inflammation, cholesterol metabolism, and high-density lipoprotein (HDL) function. Based on pre-clinical studies, egg phosphatidylcholine (PC) and sphingomyelin appear to regulate cholesterol absorption and inflammation. In clinical studies, egg PL intake is associated with beneficial changes in biomarkers related to HDL reverse cholesterol transport. Recently, egg PC was shown to be a substrate for the generation of trimethylamine N-oxide (TMAO), a gut microbe-dependent metabolite associated with increased cardiovascular disease (CVD) risk. More research is warranted to examine potential serum TMAO responses with chronic egg ingestion and in different populations, such as diabetics. In this review, the recent basic science, clinical, and epidemiological findings examining egg PL intake and risk of CVD are summarized.

The Secretion and Action of Brush Border Enzymes in the Mammalian Small Intestine

Microvilli are conventionally regarded as an extension of the small intestinal absorptive surface, but they are also, as latterly discovered, a launching pad for brush border digestive enzymes. Recent work has demonstrated that motor elements of the microvillus cytoskeleton operate to displace the apical membrane toward the apex of the microvillus, where it vesiculates and is shed into the periapical space. Catalytically active brush border digestive enzymes remain incorporated within the membranes of these vesicles, which shifts the site of BB digestion from the surface of the enterocyte to the periapical space. This process enables nutrient hydrolysis to occur adjacent to the membrane in a pre-absorptive step. The characterization of BB digestive enzymes is influenced by the way in which these enzymes are anchored to the apical membranes of microvilli, their subsequent shedding in membrane vesicles, and their differing susceptibilities to cleavage from the component membranes. In addition, the presence of active intracellular components of these enzymes complicates their quantitative assay and the elucidation of their dynamics. This review summarizes the ontogeny and regulation of BB digestive enzymes and what is known of their kinetics and their action in the peripheral and axial regions of the small intestinal lumen.

Membrane rafts regulate phospholipase B activation in murine sperm

It is intuitive that fertilization—the start of life—involves communication between a sperm cell and an egg. It has been known that to become able to fertilize an egg, a sperm must first communicate with stimuli in the female tract. For example, sterol removal from the plasma membrane is required for sperm to undergo membrane fusion during acrosome exocytosis (AE). However, how membrane lipid changes were transduced into initiation of AE remained unclear. Recently, we found that sperm phospholipase B (PLB) is activated in response to sterol removal and released into the extracellular fluid by proteolytic cleavage. The resultant active PLB fragment can stimulate initiation of AE without other physiological stimulation. These results provide a possible mechanism for how AE is triggered, a critical question given recent data from others that show that AE is induced prior to contact with the egg’s extracellular covering, the zona pellucida.

Extracellular lipid metabolism influences the survival of ovarian cancer cells

Lysophosphatidic acid (LPA) is an extracellular lipid mediator consisting of a fatty acid and a phosphate group linked to the glycerol backbone. Here, we show that 1-oleoyl- and 1-palmitoyl-LPA, but not 1-stearoyl- or alkyl-LPA, enhance HNOA ovarian cancer cell survival. Other lysophospholipids with oleic or lauric acid, but not stearic acid, also induce the survival effects. HNOA cells have the lipase activities that cleave LPA to generate fatty acid. Oleic acid stimulates HNOA cell survival via increased glucose utilization. Our findings suggest that extracellular lysolipid metabolism might play an important role in HNOA cell growth.

Phospholipase B is activated in response to sterol removal and stimulates acrosome exocytosis in murine sperm

Despite a strict requirement for sterol removal for sperm to undergo acrosome exocytosis (AE), the mechanisms by which changes in membrane sterols are transduced into changes in sperm fertilization competence are poorly understood. We have previously shown in live murine sperm that the plasma membrane overlying the acrosome (APM) contains several types of microdomains known as membrane rafts. When characterizing the membrane raft-associated proteomes, we identified phospholipase B (PLB), a calcium-independent enzyme exhibiting multiple activities. Here, we show that sperm surface PLB is activated in response to sterol removal. Both biochemical activity assays and immunoblots of subcellular fractions of sperm incubated with the sterol acceptor 2-hydroxypropyl-β-cyclodextrin (2-OHCD) confirmed the release of an active PLB fragment. Specific protease inhibitors prevented PLB activation, revealing a mechanistic requirement for proteolytic cleavage. Competitive inhibitors of PLB reduced the ability of sperm both to undergo AE and to fertilize oocytes in vitro, suggesting an important role in fertilization. This was reinforced by our finding that incubation either with protein concentrate released from 2-OHCD-treated sperm or with recombinant PLB peptide corresponding to the catalytic domain was able to induce AE in the absence of other stimuli. Together, these results lead us to propose a novel mechanism by which sterol removal promotes membrane fusogenicity and AE, helping confer fertilization competence. Importantly, this mechanism provides a basis for the newly emerging model of AE in which membrane fusions occur during capacitation/transit through the cumulus, prior to any physical contact between the sperm and the oocyte's zona pellucida.

Comparison of lysophospholipid levels in rat feces with those in a standard chow

Although lysophospholipids have attracted much attention due to their diverse physiological activities through their specific receptors, little is known about their metabolic fates in mammalian digestive systems after their ingestion as a minor food component. In this study, we analyzed five lysophospholipids in lipid extracts of a standard rat chow and feces of rats fed the chow by two-dimensional thin layer chromatography and liquid chromatography-tandem mass spectrometry. The most abundant lysophospholipid in the rat chow was lysophosphatidylcholine followed by lysophosphatidylethanolamine, lysophosphatidic acid (LPA), lysophosphatidylinositol and lysophosphatidylserine (LPS) in an increasing order, but their concentrations were very low in rat feces. Among the molecular species of LPS in the chow, only saturated species were detected in the feces in significant amounts. In addition, several molecular species of LPA remained in the feces in variable portions (saturated > monounsaturated > polyunsaturated). These results suggest that a portion of ingested LPA and LPS reach the rat large intestine, affecting physiological colon functions.

Differential expression of intestinal genes in opossums with high and low responses to dietary cholesterol

High and low responding opossums (Monodelphis domestica) differ in their plasma very low density lipoprotein and low density lipoprotein (VLDL+LDL) cholesterol concentrations when they consume a high cholesterol diet, which is due in part to absorption of a higher percentage of dietary cholesterol in high responders. We compared the expression of a set of genes that influence cholesterol absorption in high and low responders fed a basal or a high cholesterol and low fat (HCLF) diet. Up-regulation of the ABCG5, ABCG8, and IBABP genes by the HCLF diet in high and low responders may reduce cholesterol absorption to maintain cholesterol homeostasis. Differences in expression of the phospholipase genes (PLA2 and PLB) and phospholipase activity were associated with differences in cholesterol absorption when opossums were fed cholesterol-enriched diets. Higher PLA2 and PLB mRNA levels and higher phospholipase activity may increase cholesterol absorption in high responders by enhancing the release of cholesterol from bile salt micelles for uptake by intestinal cells.

The identification of a phospholipase B precursor in human neutrophils

A phospholipase B (PLB) precursor was purified from normal human granulocytes using Sephadex G-75, Mono-S cation-exchange and hydroxyapatite columns. The molecular mass of the protein was estimated to be approximately 130 kDa by gel filtration and 22 and 42 kDa by SDS/PAGE. Tryptic peptide and sequence analyses by MALDI-TOF and tandem mass spectrometry (MS/MS) identified the protein as a FLJ22662 (Homo sapiens) gene product, a homologue of the amoeba Dictyostelium discoideum PLB. The native protein needed modifications to acquire deacylation activity against phospholipids including phosphatidylcholine, phosphatidylinositol, phosphatidylethanolamine and lysophospholipids. Enzyme activity was associated with fragments derived from the 42 kDa fragment. The enzyme revealed a PLB nature by removing fatty acids from both the sn-1 and sn-2 positions of phospholipids. The enzyme is active at a broad pH range with an optimum of 7.4. Immunoblotting of neutrophil postnuclear supernatant using antibodies against the 42 kDa fragment detected a band at a molecular mass of 42 kDa, indicating a neutrophil origin of the novel PLB precursor. The existence of the PLB precursor in neutrophils and its enzymatic activity against phospholipids suggest a role in the defence against invading microorganisms and in the generation of lipid mediators of inflammation.

Biology of secretory phospholipase A2

Introduction

The secretory phospholipase A₂ (sPLA₂) family provides a seemingly endless array of potential biological functions that is only beginning to be appreciated. In humans, this family comprises 9 different members that vary in their tissue distribution, hydrolytic activity, and phospholipid substrate specificity. Through their lipase activity, these enzymes trigger various cell-signaling events to regulate cellular functions, directly kill bacteria, or modulate inflammatory responses. In addition, some sPLA₂’s are high affinity ligands for cellular receptors.

Objective

This review merely scratches the surface of some of the actions of sPLA₂s in innate immunity, inflammation, and atherosclerosis. The goal is to provide an overview of recent findings involving sPLA₂s and to point to potential pathophysiologic mechanisms that may become targets for therapy.

Biochemistry and physiology of mammalian secreted phospholipases A2

Phospholipases A(2) (PLA2s) are esterases that hydrolyze the sn-2 ester of glycerophospholipids and constitute one of the largest families of lipid hydrolyzing enzymes. The mammalian genome contains 10 enzymatically active secreted PLA2s (sPLA2s) and two sPLA2-related proteins devoid of lipolytic enzymatic activity. In addition to the well-established functions of one of these enzymes in digestion of dietary phospholipids and another in host defense against bacterial infections, accumulating evidence shows that some of these sPLA2s are involved in arachidonic acid release from cellular phospholipids for the biosynthesis of eicosanoids, especially during inflammation. More speculative results suggest the involvement of one or more sPLA2s in promoting atherosclerosis and cancer. In addition, the mammalian genome encodes several types of sPLA2-binding proteins, and mounting evidence shows that sPLA2s may have functions related to binding to cellular target proteins in a manner independent of their lipolytic enzymatic activity.

Phospholipase A2 and phospholipase B activities in fungi

As saprophytes or disease causing microorganisms, fungi acquire nutrients from dead organic material or living host organisms. Lipids as structural components of cell membranes and storage compartments play an important role as energy-rich food source. In recent years, it also has become clear that lipids have a wide range of bioactive properties including signal transduction and cell to cell communication. Thus, it is not surprising that fungi possess a broad range of hydrolytic enzymes that attack neutral lipids and phospholipids. Especially during infection of a mammalian host, phospholipase A(2) (PLA(2)) enzymes released by fungi could play important roles not only for nutrient acquisition and tissue invasion, but for intricate modulation of the host's immune response. Sequencing of fungal genomes has revealed a wide range of genes encoding PLA(2) activities in fungi. We are just beginning to become aware of the significance these enzymes could have for the fungal cells and their interaction with the host.

Cryptococcal phospholipases: a novel lysophospholipase discovered in the pathogenic fungus Cryptococcus gattii

The pathogenic fungus Cryptococcus neoformans produces an extracellular PLB1 (phospholipase B1), shown previously to be a virulence factor. A novel phospholipase (LPL1) with only LPL (lysophospholipase) and LPTA (transacylase) activities has now been characterized in C. gattii, and found to be a 66-kDa glycoprotein (by SDS/PAGE), with a native molecular mass of 670 kDa. The pI was 6.3, and it was active at high temperatures (to 70 degrees C), as well as at both acidic and neutral pH values. It was stimulated by calcium and palmitoyl carnitine at pH 7.0, but not at pH 5.0, and palmitoyl lysophosphatidylcholine was the preferred substrate. Sequencing indicated that LPL1 is a novel cryptococcal lysophospholipase, and not the gene product of CnLYSO1 or PLB1. A protein with only LPL and LPTA activities was subsequently isolated from two strains of C. neoformans var. grubii. A PLB1 enzyme was isolated from both C. gattii and a highly virulent strain of C. neoformans var. grubii (H99). In both cases, all three enzyme activities (PLB, LPL and LPTA) were present in one 95-120 kDa glycoprotein (by SDS/PAGE) with pI 3.9-4.3. Characterization of PLB1 from C. gattii showed that it differed from that of C. neoformans in its larger native mass (275 kDa), high PLB activity relative to LPL and LPTA, and preference for saturated lipid substrates. Differences in the properties between the secreted phospholipases of the two cryptococcal species could contribute to phenotypic differences that determine their respective environmental niches and different clinical manifestations.

Identification of phospholipase B from Dictyostelium discoideum reveals a new lipase family present in mammals, flies and nematodes, but not yeast

The social amoeba Dictyostelium discoideum exhibits high activities of phospholipase and lysophospholipase [Ferber, Munder, Fischer and Gerisch (1970) Eur. J. Biochem. 14, 253-257]. We assayed Dictyostelium lysates to demonstrate the presence of a highly active phospholipase B (PLB) enzyme that removed both fatty-acid chains from phosphatidylcholine and produced the water-soluble glycerophosphorylcholine. We purified the PLB activity from Dictyostelium cytosol using standard agarose media (size exclusion and ion exchange), and combined this with an affinity purification step using myristoylated ARF1 (ADP-ribosylation factor 1), a protein which has a single fatty acid at its N-terminus. Two proteins co-purified (48 kDa and 65 kDa), and the 48 kDa protein was digested with trypsin, peptide fragments were separated by reverse-phase chromatography, and the resultant peptides were sequenced by Edman degradation. From the peptide sequences obtained, database searches revealed a gene which encodes a protein of 65 kDa with unknown function. The 48 kDa protein therefore appears to be a fragment of the full-length 65 kDa product. Expression of the gene in Escherichia coli confirmed that it encodes a PLB. Characterization of its substrate specificity indicated that, in addition to phosphatidylcholine deacylation, the enzyme also hydrolysed phosphatidylinositol and phosphatidylethanolamine. The PLB identified in the present study is not related to existing PLBs found in bacteria, fungi or mammals. There are, however, genes similar to Dictyostelium PLB in mammals, flies, worms and Giardia, but not in yeast. We therefore have identified a novel family of intracellular PLBs.

Adipose-specific expression, phosphorylation of Ser794 in insulin receptor substrate-1, and activation in diabetic animals of salt-inducible kinase-2

Salt-inducible kinase (SIK), first cloned from the adrenal glands of rats fed a high salt diet, is a serine/threonine protein kinase belonging to an AMP-activated protein kinase family. Induced in Y1 cells at an early stage of ACTH stimulation, it regulated the initial steps of steroidogenesis. Here we report the identification of its isoform SIK2. When a green fluorescent protein-fused SIK2 was expressed in 3T3-L1 preadipocytes, it was mostly present in the cytoplasm. When coexpressed in cAMP-responsive element-reporter assay systems, SIK2 could repress the cAMP-responsive element-dependent transcription, although the degree of repression seemed weaker than that by SIK1. SIK2 was specifically expressed in adipose tissues. When 3T3-L1 cells were treated with the adipose differentiation mixture, SIK2 mRNA was induced within 1 h, the time of induction almost coinciding with that of c/EBPbeta mRNA. Coexpressed with human insulin receptor substrate-1 (IRS-1) in COS cells, SIK2 could phosphorylate Ser(794) of human IRS-1. Adenovirus-mediated overexpression of SIK2 in adipocytes elevated the level of phosphorylation at Ser(789), the mouse equivalent of human Ser(794). Moreover, the activity and content of SIK2 were elevated in white adipose tissues of db/db diabetic mice. These results suggest that highly expressed SIK2 in insulin-stimulated adipocytes phosphorylates Ser(794) of IRS-1 and, as a result, might modulate the efficiency of insulin signal transduction, eventually causing the insulin resistance in diabetic animals.

The phospholipase B homolog Plb1 is a mediator of osmotic stress response and of nutrient-dependent repression of sexual differentiation in the fission yeast Schizosaccharomyces pombe

Although phospholipase B (PLB) enzymes have been described in eukaryotes from yeasts to mammals, their biological functions are poorly understood. Here we describe the characterization of plb1, one of five genes predicted to encode PLB homologs in the fission yeast, Schizosaccharomyces pombe. The plb1 gene is dispensable under normal growth conditions but required for viability in high-osmolarity media and for normal osmotic stress-induced gene expression. Unlike mutants defective in function for the stress-activated MAP kinase Spc1, plb1Delta cells are not hypersensitive to oxidative or temperature stresses, nor do they undergo a G2-specific arrest in response to osmotic stress. In addition to defects in osmotic stress response, plb1Delta cells exhibit a cold-sensitive defect in nutrient-mediated mating repression, a phenotype reminiscent of mutants in the cyclic AMP (cAMP) pathway. We show that, like plb1Delta cells, mutants in the cAMP pathway are defective for growth in high-osmolarity media, demonstrating a previously unrecognized role for the cAMP pathway in osmotic stress response. Furthermore, we show that gain-of function in the cAMP pathway can rescue the osmosensitive growth defect of plb1Delta cells, suggesting that the cAMP pathway is a potential downstream target of the actions of Plb1 in S. pombe.

GDE1/MIR16 is a glycerophosphoinositol phosphodiesterase regulated by stimulation of G protein-coupled receptors

Previously we identified MIR16 (membrane interacting protein of RGS16) as an integral membrane glycoprotein that interacts with regulator of G protein signaling proteins and shares significant sequence homology with bacterial glycerophosphodiester phosphodiesterases (GDEs), suggesting that it is a putative mammalian GDE. Here we show that MIR16 belongs to a large, evolutionarily conserved family of GDEs with a characteristic putative catalytic domain that shares a common motif (amino acids 92-116) with the catalytic domains of mammalian phosphoinositide phospholipases C. Expression of wild-type MIR16 (renamed GDE1), but not two catalytic domain mutants (E97A/D99A and H112A), leads to a dramatic increase in glycerophosphoinositol phosphodiesterase (GPI-PDE) activity in HEK 293T cells. Analysis of substrate specificity shows that GDE1/MIR16 selectively hydrolyzes GPI over glycerophosphocholine. The GPI-PDE activity of GDE1/MIR16 expressed in HEK 293T cells can be regulated by stimulation of G protein-coupled, alpha/beta-adrenergic, and lysophospholipid receptors. Membrane topology studies suggest a model in which the catalytic GDE domain faces the lumenextracellular space and the C terminus faces the cytoplasm. Our results suggest that by serving as a PDE for GPI with its activity regulated by G protein signaling, GDE1/MIR16 provides a link between phosphoinositide metabolism and G protein signal transduction.

Carboxyl ester lipase: structure-function relationship and physiological role in lipoprotein metabolism and atherosclerosis

Carboxyl ester lipase (CEL), previously named cholesterol esterase or bile salt-stimulated (or dependent) lipase, is a lipolytic enzyme capable of hydrolyzing cholesteryl esters, tri-, di-, and mono-acylglycerols, phospholipids, lysophospholipids, and ceramide. The active site catalytic triad of serine-histidine-aspartate is centrally located within the enzyme structure and is partially covered by a surface loop. The carboxyl terminus of the protein regulates enzymatic activity by forming hydrogen bonds with the surface loop to partially shield the active site. Bile salt binding to the loop domain frees the active site for accessibility by water-insoluble substrates. CEL is synthesized primarily in the pancreas and lactating mammary gland, but the enzyme is also expressed in liver, macrophages, and in the vessel wall. In the gastrointestinal tract, CEL serves as a compensatory protein to other lipolytic enzymes for complete digestion and absorption of lipid nutrients. Importantly, CEL also participates in chylomicron assembly and secretion, in a mechanism mediated through its ceramide hydrolytic activity. Cell culture studies suggest a role for CEL in lipoprotein metabolism and oxidized LDL-induced atherosclerosis. Thus, this enzyme, which has a wide substrate reactivity and diffuse anatomic distribution, may have multiple functions in lipid and lipoprotein metabolism, and atherosclerosis.

Guinea pig phospholipase B, identification of the catalytic serine and the proregion involved in its processing and enzymatic activity

Guinea pig phospholipase B (GPPLB) is a glycosylated ectoenzyme of intestinal brush border membrane. It displays a broad substrate specificity and is activated by trypsin cleavage. The primary sequence contains four tandem repeat domains (I to IV) and several serines in lipase consensus sequences. We used site-directed mutagenesis to demonstrate that only the serine 399 present in repeat II is responsible for the various enzymatic activities of GPPLB. Furthermore, we characterized for the first time the retinyl esterase activity of the enzyme. We also constructed and expressed in COS-7 cells, an NH(2)-terminal repeat I deletion mutant which was detected at a very low level by immunoblot. However, confocal microscopy study showed a strong intracellular accumulation with a weak membrane expression of the mutated protein, indicating a role of the NH(2)-terminal repeat I in the processing of GPPLB. Nevertheless, the Western blot-detected protein presented a glycosylation and trypsin sensitivity patterns similar to wild type PLB. The mutant is also fully active without trypsin treatment, in contrast to native enzyme. Thus, we propose a structural model for GPPLB, in which the repeat I constitutes a lid covering the active site and impairing enzymatic activity, its removal by trypsin leading to an active protein.

Phospholipase A2 enzymes

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to liberate arachidonic acid (AA), a precursor of eicosanoids including prostaglandins and leukotrienes. The same reaction also produces lysophosholipids, which represent another class of lipid mediators. So far, at least 19 enzymes that possess PLA2 activity have been identified and cloned in mammals. The secretory PLA2 (sPLA2) family, in which 10 isozymes have been identified, consists of low-molecular weight, Ca2+-requiring secretory enzymes that have been implicated in a number of biological processes, such as modification of eicosanoid generation, inflammation, and host defense. The cytosolic PLA2 (cPLA2) family consists of three enzymes, among which cPLA2alpha has been paid much attention by researchers as an essential component of the initiation of AA metabolism. The activation of cPLA2alpha is tightly regulated by Ca2+ and phosphorylation. The Ca2+-independent PLA2 (iPLA2) family contains two enzymes and may play a major role in phospholipid remodeling. The platelet-activating factor (PAF) acetylhydrolase (PAF-AH) family contains four enzymes that exhibit unique substrate specificity toward PAF and/or oxidized phospholipids. Degradation of these bioactive phospholipids by PAF-AHs may lead to the termination of inflammatory reaction and atherosclerosis.

Human epidermis is a novel site of phospholipase B expression

Phospholipase B (PLB) is an enzyme that displays both phospholipase A(2) and lysophospholipase activities. Analysis of human epidermis homogenates indicated the presence of a 97 kDa PLB protein, as well as a phospholipase A(2) activity, both being enriched in the soluble fraction. Immunolabelling and in situ hybridization experiments showed that this enzyme is expressed in the different layers of epidermis with an accumulation at the dermo-epidermis junction. RT-PCR data indicated that PLB is specifically expressed in natural and reconstructed epidermis. By 3'-RACE-PCR and screening of human genome databases, we obtained a 3600 bp cDNA coding for human PLB highly homologous to already described intestinal brush border PLBs. These data led us to conclude that the soluble PLB corresponds to a proteolytic cleavage of the membrane anchored protein. Altogether, our results provide the first characterization of human PLB which should play an important role in epidermal barrier function.

Ca2+-independent phospholipase A2 activity in apical plasma membranes from the rat parotid gland

An apical-enriched plasma membrane fraction (A-PM) was prepared from rat parotid gland by Mn2+ precipitation. In this fraction, phosphatidylcholine (PC) labelled at the sn-2 position was mainly decomposed into two labelled compounds (free fatty acid and 1,2-diacylglycerol) under Ca2+-free conditions. Studies using double-labelled PC and 2,3-diphosphoglycerate (as a phospholipase D inhibitor) showed that they were produced through different pathways: free fatty acid was released by phospholipase A2 (PLA2) while 1,2-diacylglycerol may be produced by sequential action of phospholipase D and phosphatidate phosphatase. The PLA2 in A-PM did not require Ca2+ for its activity and was highly activated by Triton X-100 and ATP. The inhibitor of the well-documented Ca2+-independent PLA2, bromoenol lactone, did not inhibit the PLA2 activity in A-PM. Although PLA2 activity was detected in other subcellular fractions, the highest specific activity was in A-PM. Its distribution among various fractions was roughly similar to that of the marker enzyme of apical plasma membranes. These findings suggested that Ca2+-independent PLA2 activity is present in apical plasma membranes from rat parotid gland. In addition, to clarify the involvement of the PLA2 in exocytosis, the fusion of exogenous PLA2-treated membranes with secretory granules was examined by fluorescence dequenching assay. This study clearly demonstrated the facilitation of fusion by PLA2 treatment, which suggests some involvement of apical PLA2 in saliva secretion.

Salt-inducible kinase is involved in the ACTH/cAMP-dependent protein kinase signaling in Y1 mouse adrenocortical tumor cells

The involvement of salt-inducible kinase, a recently cloned protein serine/threonine kinase, in adrenal steroidogenesis was investigated. When Y1 mouse adrenocortical tumor cells were stimulated by ACTH, the cellular content of salt-inducible kinase mRNA, protein, and enzyme activity changed rapidly. Its level reached the highest point in 1-2 h and returned to the initial level after 8 h. The mRNA levels of cholesterol side-chain cleavage cytochrome P450 and steroidogenic acute regulatory protein, on the other hand, began to rise after a few hours, reaching the highest levels after 8 h. The salt-inducible kinase mRNA level in ACTH-, forskolin-, or 8-bromo-cAMP-treated Kin-7 cells, mutant Y1 with less cAMP-dependent PKA activity, remained low. However, Kin-7 cells, when transfected with a PKA expression vector, expressed salt-inducible kinase mRNA. Y1 cells that overexpressed salt-inducible kinase were isolated, and the mRNA levels of steroidogenic genes in these cells were compared with those in the parent Y1. The level of cholesterol side-chain cleavage cytochrome P450 mRNA in the salt-inducible kinase-overexpressing cells was markedly low compared with that in the parent, while the levels of Ad4BP/steroidogenic factor-1-, ACTH receptor-, and steroidogenic acute regulatory protein-mRNAs in the former were similar to those in the latter. The ACTH-dependent expression of cholesterol side-chain cleavage cytochrome P450- and steroidogenic acute regulatory protein-mRNAs in the salt-inducible kinase-overexpressing cells was significantly repressed. The promoter activity of the cholesterol side-chain cleavage cytochrome P450 gene was assayed by using Y1 cells transfected with a human cholesterol side-chain cleavage cytochrome P450 promoter-linked reporter gene. Addition of forskolin to the culture medium enhanced the cholesterol side-chain cleavage cytochrome P450 promoter activity, but the forskolin-dependently activated promoter activity was inhibited when the cells were transfected with a salt-inducible kinase expression vector. This inhibition did not occur when the cells were transfected with a salt-inducible kinase (K56M) vector that encoded an inactive kinase. The salt-inducible kinase's inhibitory effect was also observed when nonsteroidogenic, nonAd4BP/steroidogenic factor-1 -expressing, NIH3T3 cells were used for the promoter assays. These results suggested that salt-inducible kinase might play an important role(s) in the cAMP-dependent, but Ad4BP/steroidogenic factor-1-independent, gene expression of cholesterol side-chain cleavage cytochrome P450 in adrenocortical cells.

Identification of essential residues for catalysis of rat intestinal phospholipase B/lipase

Intestinal brush border membrane-associated phospholipase B/lipase (PLB/LIP) consists of four tandem homologous domains (repeats 1 through 4) and a COOH-terminal membrane binding domain, and repeat 2 is the catalytic domain that catalyzes phospholipase A2, lysophospholipase, and lipase activities. We examined the structural basis of the catalysis of PLB/LIP with this unique substrate specificity by site-directed mutagenesis of recombinant repeat 2 enzyme. Ser414 and Ser459 within the active serine-containing consensus sequence G-X-S-X-G in the best-established lipase family were dispensable for activity. In contrast, substitution of Ala for Ser404 almost completely inactivated the three lipolytic activities of PLB/LIP, even though the gross conformation was not altered as determined by CD spectroscopy. Notably, this Ser is located within the conserved G-D-S-L sequence on the NH2-terminal side in lipolytic enzymes of another group proposed recently. Furthermore, mutagenesis and CD spectroscopic analyses suggested that Asp518 and His659, lying within conserved short stretches in the latter group of lipolytic enzymes, were essential for activity. These three essential residues are conserved in the known PLB/LIP enzymes, suggesting that they form the catalytic triad in the active site. These results indicate that PLB/LIP represents a distinct class of the lipase family. PLB/LIP is the first mammalian member of that family. Repeat 2 is equipped with the triad, but not the other repeats, accounting for why only repeat 2 is the catalytic domain. Replacing Thr406 with Gly, matching the enzyme's sequence to the lipase consensus sequence exactly, led to a great decrease in secretion and accumulation of inactive enzyme in the cells, suggesting a role of Thr406 in the structural stability.

Enterophilins, a new family of leucine zipper proteins bearing a b30.2 domain and associated with enterocyte differentiation

Enterocyte terminal differentiation occurs at the crypt-villus junction through the transcriptional activation of cell-specific genes, many of which code for proteins of the brush border membrane such as intestinal alkaline phosphatase, sucrase-isomaltase, or the microvillar structural protein villin. Several studies have shown that this sharp increase in specific mRNA levels is intimately associated with arrest of cell proliferation. We isolated several clones from a guinea pig intestine cDNA library. They encode new proteins characterized by an original structure associating a carboxyl-terminal B30.2/RFP-like domain and a long leucine zipper at the amino terminus. The first member of this novel gene family codes for a 65-kDa protein termed enterophilin-1, which is specifically expressed in enterocytes before their final differentiation. Enterophilin-1 is the most abundant in the small intestine but is still present in significant amounts in colonic enterocytes. In Caco-2 cells, a similar 65-kDa protein was recognized by a specific anti-enterophilin-1 antibody, and its expression was positively correlated with cell differentiation status. In addition, transfection of HT-29 cells with enterophilin-1 full-length cDNA slightly inhibited cell growth and promoted an increase in alkaline phosphatase activity. Taken together, these data identify enterophilins as a new family of proteins associated with enterocyte differentiation.

Compensatory phospholipid digestion is required for cholesterol absorption in pancreatic phospholipase A(2)-deficient mice

BACKGROUND AND AIMS

Numerous studies have suggested phospholipid inhibition of dietary cholesterol absorption through the gastrointestinal tract. This study addressed the importance of luminal phospholipid hydrolysis in this process.

METHODS

The effect of phospholipase inhibition on cholesterol transport from intestinal lumen to the lymphatics was evaluated in lymph fistula rats. Cholesterol and phospholipid absorption efficiency in intact animals was evaluated in control and phospholipase A(2) (PLA2) gene-targeted mice.

RESULTS

The PLA2 inhibitor FPL 67047XX retarded cholesterol absorption in a lymph fistula rat model. Under basal chow-fed dietary conditions, cholesterol absorption efficiency from a single bolus meal, and plasma lipid levels, were similar among PLA2+/+, PLA2+/-, and PLA2-/- mice. Interestingly, the nonhydrolyzable phospholipid dioleoyl ether phosphatidylcholine suppressed cholesterol absorption by 10% to 18% in mice without regard to their PLA2 genotype. When 1-palmitoyl-2-[(14)C]oleoyl-phosphatidylcholine was used as the substrate, the radiolabeled phospholipid was found to be hydrolyzed and absorbed with equal efficiency in PLA2+/+, PLA2+/-, and PLA2-/- mice.

CONCLUSIONS

These results suggested that although phospholipid digestion in the intestinal lumen is a prerequisite for efficient cholesterol absorption, additional enzyme(s) can compensate for pancreatic PLA2 in catalyzing phospholipid digestion and facilitating cholesterol absorption in PLA2 knockout mice.

Increased intestinal phospholipase A(2) activity catalyzed by phospholipase B/lipase in WBN/Kob rats with pancreatic insufficiency

Male WBN/Kob rats derived from the Wistar strain spontaneously develop chronic pancreatitis as late as 3 months old. To assess the degree of disease severity, we compared the lipolytic enzyme levels in pancreas of 2-, 4-, and 6-month-old WBN/Kob rats fed isocaloric no fat (NF) and high fat (HF, 57% of total calories) diets and its pathology. Diet treatment did not significantly affect lipase and group Ib phospholipase A(2) (PLA(2)) levels in the pancreas at all ages. Development of chronic pancreatitis at the age of 4 and 6 months was consistent with the tendency of decreasing group Ib PLA(2) specific content determined by enzyme immunoassay and lipase activity, and the decreased number of group Ib PLA(2)-positive acinar cells. Pancreatic lipase and group Ib PLA(2) levels of 4-month-old WBN/Kob rats were significantly lower than those of control Wistar rats at age 4 months irrespective of diet. This allowed us to adopt 4-month-old WBN/Kob rats as a model of pancreatic insufficiency, which could be a useful tool to examine the role of gastrointestinal enzymes in lipid digestion. Ca(2+)-independent PLA(2) activity of brush border membrane-associated phospholipase B/lipase (PLB/LIP) in ileal mucosa increased significantly in 4-month-old WBN/Kob rats while its content and transcript levels remained constant, suggesting its activation at the enzyme level. In WBN/Kob rats fed the HF diet at age 4 months, PLA(2) activity catalyzed by PLB/LIP in the proximal ileal mucosa was four times the total PLA(2) activity in the intestinal lumen. These results indicate that PLB/LIP compensates for the depletion of pancreatic lipolytic enzymes in WBN/Kob rats with pancreas insufficiency.

New developments in phospholipase A2

Some of the most recent data concerning various phospholipases A2, with special emphasis on secretory, cytosolic, and calcium-independent phospholipases A2 are summarized. Besides their contribution to the production of proinflammatory lipid mediators, the involvement of these enzymes in key cell responses such as apoptosis or tumor cell metastatic potential is also discussed, taking advantage of transgenic models based on gene invalidation by homologous recombination. The possible role of secretory and cytosolic platelet-activating factor acetyl hydrolases is also briefly mentioned. Finally, the ectopic expression in epididymis of an intestinal phospholipase B opens some novel issues as to the possible function of phospholipases in reproduction.

Purification and characterization of a catalytic domain of rat intestinal phospholipase B/lipase associated with brush border membranes

A brush border membrane-associated phospholipase B/lipase was solubilized from the distal two-thirds of rat small intestine by autolysis during storage at -35 degrees C over 1 month, and then the enzyme was purified to homogeneity and characterized enzymatically and structurally. The purified enzyme exhibited broad substrate specificity including esterase, phospholipase A2, lysophospholipase, and lipase activities. SDS-gel electrophoretic and reverse-phase high performance liquid chromatographic analyses demonstrated that a single enzyme catalyzes these activities. It preferred hydrolysis at the sn-2 position of diacylphospholipid and diacylglycerol without strict stereoselectivity, whereas it apparently exhibited no positional specificity toward triacylglycerol. Diisopropyl fluorophosphate, an irreversible inhibitor of serine esterases and lipases inhibited purified enzyme. When the position of enzyme on SDS-gel electrophoresis under the non-reducing conditions was determined by assaying the activity eluted from sliced gels, brush border membrane-associated enzyme corresponded to a approximately 150-kDa protein; autolysis gave a 35-kDa product, in agreement with the results of immunoblot analysis. The purified 35-kDa enzyme consisted of a 14-kDa peptide and a glycosylated 21-kDa peptide. Their NH2-terminal amino acid sequences were determined and found in the second repeat of 161-kDa phospholipase B/lipase with 4-fold tandem repeats of approximately 38 kDa each, which we cloned and sequenced in the accompanying paper (Takemori, H., Zolotaryov, F., Ting, L., Urbain, T., Komatsubara, T., Hatano, O., Okamoto, M., and Tojo, H. (1988) J. Biol. Chem. 273, 2222-2231). These results indicate that the purified enzyme is the catalytic domain derived from the second repeat of brush border membrane-associated phospholipase B/lipase.