Alan Bohrer

Formation of Lithiated Adducts of Glycerophosphocholine Lipids Facilitates their Identification by Electrospray Ionization Tandem Mass Spectrometry

Electrospray ionization (ESI) tandem mass spectrometry (MS) has simplified analysis of phospholipid mixtures, and, in negative ion mode, permits structural identification of picomole amounts of phospholipid species. Collisionally activated dissociation (CAD) of phospholipid anions yields negative ion tandem mass spectra that contain fragment ions representing the fatty acid substituents as carboxylate anions. Glycerophosphocholine (GPC) lipids contain a quaternary nitrogen moiety and more readily form cationic adducts than anionic species, and positive ion tandem mass spectra of protonated GPC species contain no abundant ions that identify fatty acid substituents. We report here that lithiated adducts of GPC species are readily formed by adding lithium hydroxide to the solution in which phospholipid mixtures are infused into the ESI source. CAD of [MLi+] ions of GPC species yields tandem mass spectra that contain prominent ions representing losses of the fatty acid substituents. These ions and their relative abundances can be used to assign the identities and positions of the fatty acid substituents of GPC species. Tandem mass spectrometric scans monitoring neutral losses of the head-group or of fatty acid substituents from lithiated adducts can be used to identify GPC species in tissue phospholipid mixtures. Similar scans monitoring parents of specific product ions can also be used to identify the fatty acid substituents of GPC species, and this facilitates identification of distinct isobaric contributors to ions observed in the ESI/MS total ion current.

Insulin resistance in HIV protease inhibitor-associated diabetes

BACKGROUND Fasting hyperglycemia has been associated with HIV protease inhibitor (PI) therapy. OBJECTIVE To determine whether absolute insulin deficiency or insulin resistance with relative insulin deficiency and an elevated body mass index (BMI) contribute to HIV PI-associated diabetes. DESIGN Cross-sectional evaluation. PATIENTS 8 healthy seronegative men, 10 nondiabetic HIV-positive patients naive to PI, 15 nondiabetic HIV-positive patients receiving PI (BMI = 26 kg/m2), 6 nondiabetic HIV-positive patients receiving PI (BMI = 31 kg/m2), and 8 HIV-positive patients with diabetes receiving PI (BMI = 34 kg/m2). All patients on PI received indinavir. MEASUREMENTS Fasting concentrations of glucoregulatory hormones. Direct effects of indinavir (20 microM) on rat pancreatic beta-cell function in vitro. RESULTS In hyperglycemic HIV-positive subjects, circulating concentrations of insulin, C-peptide, proinsulin, glucagon, and the proinsulin/insulin ratio were increased when compared with those of the other 4 groups (p < .05). Morning fasting serum cortisol concentrations were not different among the 5 groups. Glutamic acid decarboxylase (GAD) antibody titers were uncommon in all groups. High BMI was not always associated with diabetes. In vitro, indinavir did not inhibit proinsulin to insulin conversion or impair glucose-induced secretion of insulin and C-peptide from rat beta-cells. CONCLUSIONS The pathogenesis of HIV PI-associated diabetes involves peripheral insulin resistance with insulin deficiency relative to hyperglucagonemia and a high BMI. Pancreatic beta-cell function was not impaired by indinavir. HIV PI-associated diabetes mirrors that of non-insulin-dependent diabetes mellitus and impaired insulin action in the periphery.

Electrospray ionization tandem mass spectrometric analysis of sulfatide.: Determination of fragmentation patterns and characterization of molecular species expressed in brain and in pancreatic islets

The sphingolipid sulfatide is a component of myelin and some non-neuronal cells. Antibodies to sulfatide occur in some patients with autoimmune neuropathies and in patients with insulin-dependent diabetes mellitus (IDDM) caused by immunologic destruction of insulin-secreting pancreatic islet beta-cells. Distinct sulfatide molecular species may differ in immunogenicity, and facile means to identify sulfatide species in islets and other tissues obtainable in only small amounts could be useful. Electrospray ionization mass spectrometry (ESI/MS) permits structural determination of small quantities of phospholipids and is applied here to sulfatide analysis. We find that sulfatide standards are readily analyzed by negative ion ESI/MS, and tandem mass spectra of individual species exhibit some ions common to all species and other ions that reflect distinct fatty acid substituents in different sulfatide molecules. A signature ion cluster resulting from cleavage directed by the alpha-hydroxy group of sulfatide species with a hydroxylated fatty acid substituent identifies such species. Sulfatide profiles in tissue lipid extracts can be obtained by ESI/MS/MS scanning for common sulfatide ions and for ions reflecting fatty acid substituents. Islets are demonstrated to contain sulfatide and to exhibit a profile of species different from that of brain.

Studies of the Role of Group VI Phospholipase A2 in Fatty Acid Incorporation, Phospholipid Remodeling, Lysophosphatidylcholine Generation, and Secretagogue-induced Arachidonic Acid Release in Pancreatic Islets and Insulinoma Cells

An 84-kDa group VI phospholipase A2 (iPLA2) that does not require Ca2+ for catalysis has been cloned from Chinese hamster ovary cells, murine P388D1 cells, and pancreatic islet β-cells. A housekeeping role for iPLA2 in generating lysophosphatidylcholine (LPC) acceptors for arachidonic acid incorporation into phosphatidylcholine (PC) has been proposed because iPLA2 inhibition reduces LPC levels and suppresses arachidonate incorporation and phospholipid remodeling in P388D1 cells. Because islet β-cell phospholipids are enriched in arachidonate, we have examined the role of iPLA2 in arachidonate incorporation into islets and INS-1 insulinoma cells. Inhibition of iPLA2 with a bromoenol lactone (BEL) suicide substrate did not suppress and generally enhanced [3H]arachidonate incorporation into these cells in the presence or absence of extracellular calcium at varied time points and BEL concentrations. Arachidonate incorporation into islet phospholipids involved deacylation-reacylation and not de novo synthesis, as indicated by experiments with varied extracellular glucose concentrations and by examining [14C]glucose incorporation into phospholipids. BEL also inhibited islet cytosolic phosphatidate phosphohydrolase (PAPH), but the PAPH inhibitor propranolol did not affect arachidonate incorporation into islet or INS-1 cell phospholipids. Inhibition of islet iPLA2 did not alter the phospholipid head-group classes into which [3H]arachidonate was initially incorporated or its subsequent transfer from PC to other lipids. Electrospray ionization mass spectrometric measurements indicated that inhibition of INS-1 cell iPLA2 accelerated arachidonate incorporation into PC and that inhibition of islet iPLA2 reduced LPC levels by 25%, suggesting that LPC mass does not limit arachidonate incorporation into islet PC. Gas chromatography/mass spectrometry measurements indicated that BEL but not propranolol suppressed insulin secretagogue-induced hydrolysis of arachidonate from islet phospholipids. In islets and INS-1 cells, iPLA2 is thus not required for arachidonate incorporation or phospholipid remodeling and may play other roles in these cells.

Isotope Dilution Mass Spectrometric Measurements Indicate That Arachidonylethanolamide, the Proposed Endogenous Ligand of the Cannabinoid Receptor, Accumulates in Rat Brain Tissue Post Mortem but Is Contained at Low Levels in or Is Absent from Fresh Tissue

Arachidonylethanolamide (AEA) isolated from porcine brain binds to cannabinoid receptors, mimics cannabinoid pharmacologic effects, and has been proposed as an endogenous cannabinoid receptor ligand. Demonstration of co-distribution of AEA and cannabinoid receptors in various brain regions could provide supportive evidence for this role. We have performed isotope dilution mass spectrometric measurements of AEA and have demonstrated AEA production by rat tissue homogenates in vitro from exogenous arachidonate and ethanolamine. No detectable endogenous AEA (<3.5 pmol/g of tissue) was observed in fresh rat brain, whether or not inhibitors of AEA hydrolysis were present during tissue processing. AEA (>1 nmol/g) was produced during saponification of brain phospholipid extracts. This appears not to reflect hydrolysis of N-arachidonylethanolamine phospholipid precursors of AEA, because Streptomyces chromfucsis phospholipase D, which is active against NAPE, failed to generate AEA from brain phospholipids despite substantial conversion of phospholipids to phosphatidic acid. Such experiments suggested that the abundance of N-arachidonylethanolamine phospholipid in fresh rat brain may be less than 1 in 106 phospholipid molecules. AEA generated during saponification of tissue phospholipids appears to arise from base-catalyzed aminolysis of arachidonate-containing glycerolipids, because AEA was produced from synthetic (1-stearoyl, 2-arachidonoyl)-phosphatidylethanolamine under saponification conditions, and the amount produced increased 300-fold when free ethanolamine was included in the hydrolysis solution. Although AEA was not detectable (<0.17 pmol/mg of protein) in fresh rat brain, AEA accumulated post mortem to levels of 126 pmol/mg of brain protein. These findings do not exclude the possibility that AEA is rapidly synthesized and degraded locally in vivo, but they indicate that the AEA content of fresh rat brain and of NAPE precursors from which AEA might be derived are exceedingly low and that AEA can be produced artifactually from biological materials.

Insulin Secretory Responses and Phospholipid Composition of Pancreatic Islets from Mice That Do Not Express Group VIA Phospholipase A2 and Effects of Metabolic Stress on Glucose Homeostasis

Studies involving pharmacologic or molecular biologic manipulation of Group VIA phospholipase A2 (iPLA2β) activity in pancreatic islets and insulinoma cells suggest that iPLA2β participates in insulin secretion. It has also been suggested that iPLA2β is a housekeeping enzyme that regulates cell 2-lysophosphatidylcholine (LPC) levels and arachidonate incorporation into phosphatidylcholine (PC). We have generated iPLA2β-null mice by homologous recombination and have reported that they exhibit reduced male fertility and defective motility of spermatozoa. Here we report that pancreatic islets from iPLA2β-null mice have impaired insulin secretory responses to d-glucose and forskolin. Electrospray ionization mass spectrometric analyses indicate that the abundance of arachidonate-containing PC species of islets, brain, and other tissues from iPLA2β-null mice is virtually identical to that of wild-type mice, and no iPLA2β mRNA was observed in any tissue from iPLA2β-null mice at any age. Despite the insulin secretory abnormalities of isolated islets, fasting and fed blood glucose concentrations of iPLA2β-null and wild-type mice are essentially identical under normal circumstances, but iPLA2β-null mice develop more severe hyperglycemia than wild-type mice after administration of multiple low doses of the β-cell toxin streptozotocin, suggesting an impaired islet secretory reserve. A high fat diet also induces more severe glucose intolerance in iPLA2β-null mice than in wild-type mice, but PLA2β-null mice have greater responsiveness to exogenous insulin than do wild-type mice fed a high fat diet. These and previous findings thus indicate that iPLA2β-null mice exhibit phenotypic abnormalities in pancreatic islets in addition to testes and macrophages.

Calcium-independent Phospholipase A2 (iPLA2β)-mediated Ceramide Generation Plays a Key Role in the Cross-talk between the Endoplasmic Reticulum (ER) and Mitochondria during ER Stress-induced Insulin-secreting Cell Apoptosis

Endoplasmic reticulum (ER) stress induces INS-1 cell apoptosis by a pathway involving Ca2+-independent phospholipase A2 (iPLA2β)-mediated ceramide generation, but the mechanism by which iPLA2β and ceramides contribute to apoptosis is not well understood. We report here that both caspase-12 and caspase-3 are activated in INS-1 cells following induction of ER stress with thapsigargin, but only caspase-3 cleavage is amplified in iPLA2β overexpressing INS-1 cells (OE), relative to empty vector-transfected cells, and is suppressed by iPLA2β inhibition. ER stress also led to the release of cytochrome c and Smac and, unexpectedly, their accumulation in the cytosol is amplified in OE cells. These findings raise the likelihood that iPLA2β participates in ER stress-induced apoptosis by activating the intrinsic apoptotic pathway. Consistent with this possibility, we find that ER stress promotes iPLA2β accumulation in the mitochondria, opening of mitochondrial permeability transition pore, and loss in mitochondrial membrane potential (ΔΨ) in INS-1 cells and that these changes are amplified in OE cells. ER stress also led to greater ceramide generation in ER and mitochondria fractions of OE cells. Exposure to ceramide alone induces loss in ΔΨ and apoptosis and these are suppressed by forskolin. ER stress-induced mitochondrial dysfunction and apoptosis are also inhibited by forskolin, as well as by inactivation of iPLA2β or NSMase, suggesting that iPLA2β-mediated generation of ceramides via sphingomyelin hydrolysis during ER stress affect the mitochondria. In support, inhibition of iPLA2β or NSMase prevents cytochrome c release. Collectively, our findings indicate that the iPLA2β-ceramide axis plays a critical role in activating the mitochondrial apoptotic pathway in insulin-secreting cells during ER stress.

Effects of Stable Suppression of Group VIA Phospholipase A2 Expression on Phospholipid Content and Composition, Insulin Secretion, and Proliferation of INS-1 Insulinoma Cells

Studies involving pharmacologic inhibition or transient reduction of Group VIA phospholipase A2 (iPLA2β) expression have suggested that it is a housekeeping enzyme that regulates cell 2-lysophosphatidylcholine (LPC) levels, rates of arachidonate incorporation into phospholipids, and degradation of excess phosphatidylcholine (PC). In insulin-secreting islet β-cells and some other cells, in contrast, iPLA2β signaling functions have been proposed. Using retroviral vectors, we prepared clonal INS-1 β-cell lines in which iPLA2β expression is stably suppressed by small interfering RNA. Two such iPLA2β knockdown (iPLA2β-KD) cell lines express less than 20% of the iPLA2β of control INS-1 cell lines. The iPLA2β-KD INS-1 cells exhibit impaired insulin secretory responses and reduced proliferation rates. Electrospray ionization mass spectrometric analyses of PC and LPC species that accumulate in INS-1 cells cultured with arachidonic acid suggest that 18:0/20:4-glycerophosphocholine (GPC) synthesis involves sn-2 remodeling to yield 16:0/20:4-GPC and then sn-1 remodeling via a 1-lyso/20:4-GPC intermediate. Electrospray ionization mass spectrometric analyses also indicate that the PC and LPC content and composition of iPLA2β-KD and control INS-1 cells are nearly identical, as are the rates of arachidonate incorporation into PC and the composition and remodeling of other phospholipid classes. These findings indicate that iPLA2β plays signaling or effector roles in β-cell secretion and proliferation but that stable suppression of its expression does not affect β-cell GPC lipid content or composition even under conditions in which LPC is being actively consumed by conversion to PC. This calls into question the generality of proposed housekeeping functions for iPLA2β in PC homeostasis and remodeling.

Interleukin-1 Enhances Pancreatic Islet Arachidonic Acid 12-Lipoxygenase Product Generation by Increasing Substrate Availability through a Nitric Oxide-dependent Mechanism

Interleukin-1 (IL-1) impairs insulin secretion from pancreatic islets and may contribute to the pathogenesis of insulin-dependent diabetes mellitus. IL-1 increases islet expression of nitric oxide (NO) synthase, and the resultant overproduction of NO participates in inhibition of insulin secretion because NO synthase inhibitors, e.g.NG-monomethyl-arginine (NMMA), prevent this inhibition. While exploring effects of IL-1 on islet arachidonic acid metabolism, we found that IL-1 increases islet production of the 12-lipoxygenase product 12-hydroxyeicosatetraenoic acid 12-(HETE). This effect requires NO production and is prevented by NMMA. Exploration of the mechanism of this effect indicates that it involves increased availabilty of the substrate arachidonic acid rather than enhanced expression of 12-lipoxygenase. Evidence supporting this conclusion includes the facts that IL-1 does not increase islet 12-lipoxygenase protein or mRNA levels and does not enhance islet conversion of exogenous arachidonate to 12-HETE. Mass spectrometric stereochemical analyses nonetheless indicate that 12-HETE produced by IL-1-treated islets consists only of the S-enantiomer and thus arises from enzyme action. IL-1 does enhance release of nonesterified arachidonate from islets, as measured by isotope dilution mass spectrometry, and this effect is suppressed by NMMA and mimicked by the NO-releasing compound 3-morpholinosydnonimine. Although IL-1 increases neither islet phospholipase A2 (PLA2) activities nor mRNA levels for cytosolic or secretory PLA2, a suicide substrate which inhibits an islet Ca2+-independent PLA2 prevents enhancement of islet arachidonate release by IL-1. IL-1 also impairs esterification of [3H8]arachidonate into islet phospholipids, and this effect is prevented by NMMA and mimicked by the mitochondrial ATP-synthase inhibitor oligomycin. Experiments with exogenous substrates indicate that NMMA does not inhibit and that the NO-releasing compound does not activate islet 12-lipoxygenase or PLA2 activities. These results indicate that a novel action of NO is to increase levels of nonesterified arachidonic acid in islets.

Attenuated Free Cholesterol Loading-induced Apoptosis but Preserved Phospholipid Composition of Peritoneal Macrophages from Mice That Do Not Express Group VIA Phospholipase A2

Mouse macrophages undergo ER stress and apoptosis upon free cholesterol loading (FCL). We recently generated iPLA2β-null mice, and here we demonstrate that iPLA2β-null macrophages have reduced sensitivity to FCL-induced apoptosis, although they and wild-type (WT) cells exhibit similar increases in the transcriptional regulator CHOP. iPLA2β-null macrophages are also less sensitive to apoptosis induced by the sarcoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin and the scavenger receptor A ligand fucoidan, and restoring iPLA2βexpression with recombinant adenovirus increases apoptosis toward WT levels. WT and iPLA2β-null macrophages incorporate [3H]arachidonic acid ([3H]AA]) into glycerophosphocholine lipids equally rapidly and exhibit identical zymosan-induced, cPLA2α-catalyzed [3H]AA release. In contrast, although WT macrophages exhibit robust [3H]AA release upon FCL, this is attenuated in iPLA2β-null macrophages and increases toward WT levels upon restoring iPLA2β expression. Recent reports indicate that iPLA2β modulates mitochondrial cytochrome c release, and we find that thapsigargin and fucoidan induce mitochondrial phospholipid loss and cytochrome c release into WT macrophage cytosol and that these events are blunted in iPLA2β-null cells. Immunoblotting studies indicate that iPLA2β associates with mitochondria in macrophages subjected to ER stress. AA incorporation into glycerophosphocholine lipids is unimpaired in iPLA2β-null macrophages upon electrospray ionization-tandem mass spectrometry analyses, and their complex lipid composition is similar to WT cells. These findings suggest that iPLA2β participates in ER stress-induced macrophage apoptosis caused by FCL or thapsigargin but that deletion of iPLA2β does not impair macrophage arachidonate incorporation or phospholipid composition.