Calcium signals and phospholipid methylation in eukaryotic cells

The reliability of biomedical science: A case history of a maturing experimental field

There is much discussion in the media and some of the scientific literature of how many of the conclusions from scientific research should be doubted. These critiques often focus on studies, typically in non-experimental spheres of biomedical and social sciences - that search large datasets for novel correlations, with a risk that inappropriate statistical evaluations might yield dubious conclusions. By contrast, results from experimental biological research can often be interpreted largely without statistical analysis. Typically: novel observation(s) are reported, and an explanatory hypothesis is offered; multiple labs undertake experiments to test the hypothesis; interpretation of the results may refute the hypothesis, support it or provoke its modification; the test/revise sequence is reiterated many times; and the field moves forward. I illustrate this experimental/non-experimental dichotomy by examining the contrasting recent histories of: (a) our remarkable and growing understanding of how several inositol-containing phospholipids contribute to the lives of eukaryote cells; and (b) the difficulty of achieving any agreed mechanistic understanding of why consuming dietary supplements of inositol is clinically beneficial in some metabolic diseases.

Endoplasmic reticulum stress and calcium imbalance are involved in cadmium-induced lipid aberrancy in Saccharomyces cerevisiae

The endoplasmic reticulum is the key organelle which controls protein folding, lipid biogenesis, and calcium (Ca(2+)) homeostasis. Cd exposure in Saccharomyces cerevisiae activated the unfolded protein response and was confirmed by the increased Kar2p expression. Cd exposure in wild-type (WT) cells increased PC levels and the PC biosynthetic genes. Deletion of the two phospholipid methyltransferases CHO2 and OPI3 modulated PC, TAG levels and the lipid droplets with cadmium exposure. Interestingly, we noticed an increase in the calcium levels upon Cd exposure in the mutant cells. This study concluded that Cd interrupted calcium homeostasis-induced lipid dysregulation leading to ER stress.

Protective effect of S-adenosyl-l-methionine on liver damage induced by biliary obstruction in rats: a histological, ultrastructural and biochemical approach

In human and experimental CCl4-liver damage, S-adenosyl-l-methionine-synthetase and/or the intrahepatic content of S-adenosyl-l-methionine, are diminished and in human cirrhosis phospholipid methyltransferase is markedly reduced. Therefore the aim of this study was to investigate the effect of S-adenosyl-l-methionine administration on liver damage induced by 15-day bile duct ligation. Liver damage was analyzed by histological, ultrastructural and biochemical techniques. Biliary obstruction produced an increase in collagen content, dilation of the bile canaliculi and disorganization of mitochondria. These effects were not observed in the bile-duct-ligated group receiving S-adenosyl-l-methionine. Biochemical results showed that bile duct ligation increased serum bilirubins, and alkaline phosphatase and gamma-glutamyl transpeptidase activities. These effects were prevented significantly by S-adenosyl-l-methionine. On the other hand, glycogen content in the liver was depleted while lipid peroxidation was increased by biliary obstruction, S-adenosyl-l-methionine administration prevented these effects. In the bile-duct-ligated group, hepatocyte and erythrocyte plasma membrane Na+/K+ and Ca(2+)-ATPase were lower than in the control group (p < 0.05). Administration of S-adenosyl-l-methionine preserved ATPase activities. The exogenous S-adenosyl-l-methionine supply is probably responsible for restoring transmethylation lost in liver diseases.

Exogenous gangliosides may affect methylation mechanisms in neuronal cell cultures

Primary neurons in culture from chick embryo cerebral hemispheres were treated with a mixture of gangliosides added to the growth medium (final concentration: 10(-5)M and 10(-8)M) from the 3rd to the 6th day in vitro. Under these conditions methylation processes measured with [3H] and [35S] methionine and [3H]ethanolamine as precursors showed an increased methylation of [3H]ethanolamine containing phospholipids, a correspondent increased conversion of these compounds to [3H]choline containing phospholipids, and a general increased methylation of trichloroacetic acid precipitable macromolecules containing labeled methionine. A small increase in protein synthesis was observed after incubation of neurons with [3H]- and [35S]methionine. This was confirmed after electrophoretic separation of a protein extract with increased 3H- and 35S-labeling in protein bands with moecular weights between 50 and 60 KDaltons. A protein band of about 55 KDaltons appeared to be preferentially labelled when [3H] methionine was the precursor. The treatment with gangliosides increased the incorporation of [methyl-3H] label after incubation of neurons with [3H] methionine, into total DNA and decreased that of total RNA. The treatment of neurons in culture with exogenous gangliosides hence affects differently methylation processes, a finding which may confirm the involvement of gangliosides on the intracellular mediation of neuronal information mechanisms.

Inhibition by staurosporine of mitogen-induced calcium mobilisation in human T lymphoblasts

Addition of monoclonal antibodies (UCHT1) directed against the antigen receptor/CD3 complex or the addition of the mitogens concanavalin A or phytohemagglutinin to human T lymphoblasts induced an elevation of the cytosolic free calcium concentration [( Ca2+]i) as monitored by the fluorescence of the intracellular fura-2. The rise in [Ca2+]i induced by these agents was effectively inhibited by staurosporine, an agent which inhibits protein kinase (PK)C and PKA. Conversely the PKC activator 12-O-tetradecanoyl phorbol-13-acetate decreased the delay and accelerated the rate of elevation of [Ca2+]i induced by the mitogens and UCHT1. Increasing the concentration of cAMP or cGMP in the T lymphoblasts had no effect on agonist-induced elevation of [Ca2+]i. Our data indicate that PKC can exert a positive feedback on the level of cytosolic Ca2+ in T lymphocytes, in contrast to what has been observed with other cells.

Protein methylation inhibits Na+-Ca2+ exchange activity in cardiac sarcolemmal vesicles

We have examined the effect of membrane methylation on the Na+-Ca2+ exchange activity of canine cardiac sarcolemmal vesicles using S-adenosyl-L-methionine as methyl donor. Methylation leads to approximately 40% inhibition of the initial rate of Nai+-dependent Ca2+ uptake. The inhibition is due to a lowering of the Vmax for the reaction. The inhibition is not due to an effect on membrane permeability and is blocked by S-adenosyl-L-homocysteine, an inhibitor of methylation reactions. The following experiments indicated that inhibition of Na+-Ca2+ exchange was due to methylation of membrane protein and not due to methylated phosphatidylethanolamine (PE) compounds (i.e., phosphatidyl-N-monomethylethanolamine (PMME) or phosphatidyl-N,N'-dimethylethanolamine (PDME]: (1) We solubilized sarcolemma and reconstituted activity into vesicles containing no PE. The inhibition by S-adenosyl-L-methionine was not diminished in this environment. (2) We reconstituted sarcolemma into vesicles containing PMME or PDME. These methylated lipid components had no effect on Na+-Ca2+ exchange activity. (3) We verified that many membrane proteins, probably including the exchanger, become methylated.

S-adenosyl-L-methionine modulates Na+ + K+-ATPase activity in rat colonic basolateral membranes

Rat colonic basolateral membranes were incubated with S-adenosyl-L-[methyl-3H]methionine (0.3 mM) at 37 degrees C for 2 h at pH 9.0. This resulted in an increase in the specific activity of Na+ + K+-ATPase by 60%. Kinetic parameter analysis revealed a 2-fold increase in the Vmax. of this enzymatic activity, whereas the Km for ATP was unchanged. The methylation inhibitor S-adenosyl-L-homocysteine (2 mM) significantly reduced these S-adenosyl-L-methionine-stimulated increases in specific activity and the Vmax. of Na+ + K+-ATPase. S-Adenosyl-L-methionine treatment of basolateral membranes was also found to significantly increase the fluidity of these preparations, as assessed by steady-state fluorescence polarization techniques using the fluorophore 1,6-diphenyl-1,3,5-hexatriene; S-adenosyl-L-homocysteine (2 mM) again markedly reduced this S-adenosyl-L-methionine-induced increase in fluidity. While transmethylation reactions involving phospholipids, non-polar lipids and proteins were all found to exist in rat colonic basolateral membranes, based on a number of observations, the results of the present studies suggest that transmethylation of membrane phospholipids, but not membrane non-polar lipids or proteins, influenced the fluidity of basolateral membranes which, in turn, modified Na+ + K+-ATPase activity in these membranes.

Phosphoinositide metabolism and the calcium response to concanavalin A in S49 T-lymphoma cells. A comparison with thymocytes

Comparisons were made between transformed S49 T-lymphoma cells and normal murine thymocytes in their polyphosphoinositides, inositol polyphosphates and cytosolic free calcium concentrations ([Ca2+]i), and the effects of the T-cell mitogen concanavalin A (Con A) on these properties. 1. The ratios of the polyphosphoinositides to phosphatidylinositol in both exponential-phase S49 cells and mitogen-stimulated thymocytes (G1 phase) were greater than in quiescent (G0-phase) thymocytes. 2. In response to Con A, the amount of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) in S49 cells decreased slightly (17% in 30 min), and this was sufficient to account for the small amounts of inositol phosphates that accumulated. In contrast, it has been shown previously that Con A stimulates a rapid resynthesis of PtdInsP2 in thymocytes and the amounts of inositol phosphates released rapidly exceed the steady-state amount of the PtdInsP2 precursor [Taylor, Metcalfe, Hesketh, Smith & Moore (1984) Nature (London) 312, 462-465]. 3. The [Ca2+]i did not differ significantly in S49 cells and thymocytes before the addition of Con A, and the increases in [Ca2+]i in response to Con A were similar in both types of cell. 4. The [Ca2+]i increase in response to Con A was inhibited by similar concentrations of intracellular cyclic AMP (2-10 microM) in S49 cells and thymocytes, suggesting that similar regulatory mechanisms act on this response in both types of cell. The data demonstrate that the basal [Ca2+]i and phosphoinositide metabolism is similar in both the normal cells and their transformed counterparts. In addition, they suggest that the activated Con A receptors generate very similar signals in the two cell types, and that any perturbations of primary signal transduction to the secondary phosphoinositide and [Ca2+]i responses in the S49 phenotype are quantitative rather than qualitative.

Identification and partial characterization of phospholipid methylation in rat small-intestinal brush-border membranes

An earlier study (Biochim. Biophys. Acta 46 (1961) 205-216) failed to detect the enzymatic synthesis of phosphatidylcholine (PC) from phosphatidylethanolamine (PE) via a transmethylation pathway in rat small-intestinal microsomal membranes. This pathway was therefore assumed to be absent from this organ. Recently, however, in our laboratory it has been demonstrated that this pathway for the synthesis of phosphatidylcholine is present in rat colonic brush-border and basolateral membranes. It was therefore of interest to examine whether phospholipid methylation activity was present in rat small-intestinal brush-border membranes. The results of the present experiments demonstrate for the first time that this pathway for the synthesis of phosphatidylcholine exists in these plasma membranes. Evidence to support the enzymatic nature of this reaction include: loss of activity by heat denaturation and at 0 degree C, significant inhibition by S-adenosyl-L-homocysteine and saturation kinetics. The predominant product of this brush-border membrane phospholipid methyltransferase is phosphatidyl-N-monomethylethanolamine. This enzymatic activity has an apparent Km for S-adenosyl-L-methionine of 40 microM, a Vmax of 8.4 pmol/mg protein per 5 min, and a pH optimum of 8.0.

Inhibition of histamine release in vitro by a blocking factor from human serum: comparison with the iron binding proteins transferrin and lactoferrin

Recently, we described the presence of a blocking factor (BF) in rat serum, which inhibited the histamine release from rat mast cells in vivo and in vitro. The blocking activity was demonstrated in human serum as well. Qualitative analysis of the purified preparations demonstrated a major component with an apparent molecular weight of 70,000 daltons. In human serum the blocking factor was identified as transferrin (TF) by serological and biochemical methods. BF (DEAE-peak 1) and the iron binding proteins transferrin and lactoferrin (LF) are shown to inhibit the histamine release in vitro. The dose response curves reveal that inhibition by these proteins is dependent on their degree of iron saturation. Furthermore, unlike lactoferrin, the effects of transferrin and BF (DEAE-peak 1) follow the same pattern. Their mechanism of action remains to be elucidated.

IgE-dependent activation of mast cells is not associated with enhanced phospholipid methylation

Enhanced phospholipid methylation has been suggested to be an obligatory event in activation-secretion coupling in mast cells. However, we have consistently failed to demonstrate increased [3H]methyl incorporation into either whole-lipid extracts or separated lipids to accompany IgE-dependent induction of histamine release from rat peritoneal mast cells despite variation of the experimental protocol, animals and animal diet. Although the presence of 3-deazaadenosine and homocysteine thiolactone reduced IgE-dependent histamine release, treatment with these compounds elevated mast cell cyclic AMP levels. We conclude that IgE-dependent activation of mast cells is not associated with enhanced phospholipid methylation and that inhibition of histamine release by methylation inhibitors may be due to other effects of the drugs.

Regulation of Na+-H+ exchange by transmethylation reactions in rat colonic brush-border membranes

Incubation of rat colonic brush-border membrane vesicles with 200 microM S-adenosyl-L-[Me-3H]methionine resulted in the labeling of both membrane phospholipids and proteins. This labeling was decreased approximately 50% by the methylation inhibitor S-adenosyl-L-homocysteine (2 mM). Utilizing the pH-sensitive fluorescent dye, acridine orange, as a means of determining Na+-H+ exchange, S-adenosyl-L-methionine (200 microM) significantly increased sodium-stimulated proton efflux in these vesicles at all concentrations of sodium (2.5-50 mM) tested. Examination of the kinetic parameters for sodium-stimulated proton efflux in the presence and absence of 200 microM S-adenosyl-L-methionine revealed that the methyl donor increased the Vmax for this exchange mechanism (expressed in arbitrary fluorescence units) by approx. 36% but did not influence its Km for sodium. S-Adenosyl-L-homocysteine (2 mM) inhibited S-adenosyl-L-methionine-mediated stimulation of this exchange process. The results demonstrate that methylation of membrane phospholipids and/or proteins can modulate Na+-H+ exchange in rat colonic brush-border membrane vesicles.

Synthesis of phosphatidylcholine by two distinct methyltransferases in rat colonic brush-border membranes: evidence for extrinsic and intrinsic membrane activities

The enzymatic synthesis of phosphatidylcholine from phosphatidylethanolamine via a transmethylation pathway has not been shown to occur in the small intestine and has been assumed to be absent from the entire gut. The existence of this pathway, however, has not been investigated in the large intestine. Utilizing a recently developed method for the isolation of brush-border membranes from rat colonocytes, the present studies were designed to determine whether phospholipid methylation activity was present in the large intestine. The results demonstrate that this pathway for synthesis of phosphatidylcholine exists in rat colonic plasma membranes and involves at least two distinct methyltransferases. The predominant product of the first enzyme (methyltransferase I) is phosphatidyl-N-monomethylethanolamine; phosphatidylcholine and phosphatidyl-N-monomethylethanolamine are the principal products of the second enzyme (methyltransferase II). Methyltransferase I has an apparent Km for S-adenosyl-L-methionine of 100.0 microM and a pH optimum of 8.0, while methyltransferase II has an apparent Km of 0.3 microM and a pH optimum of 6.0. Additional evidence to support the presence of two distinct enzymes includes the differential effects of ATP, Triton X-100, trypsin treatment, and temperature on their activities.

Antigen-stimulated metabolism of inositol phospholipids in the cloned murine mast-cell line MC9

Cells of the murine mast-cell clone MC9 grown in suspension culture were sensitized with an anti-DNP (dinitrophenol) IgE and subsequently prelabelled by incubating with [32P]Pi. Stimulation of these cells with DNP-BSA (bovine serum albumin) caused marked decreases in [32P]polyphosphoinositides (but not [32P]phosphatidylinositol) with concomitant appearance of [32P]phosphatidic acid. Whereas phosphatidylinositol monophosphate levels returned to baseline values after prolonged stimulation, phosphatidylinositol bisphosphate levels remained depressed. Stimulation of sensitized MC9 cells with DNP-BSA increased rates of incorporation of [32P]Pi into other phospholipids in the order: phosphatidylcholine greater than phosphatidylinositol greater than phosphatidylethanolamine. In sensitized cells prelabelled with [3H]inositol, release of inositol monophosphate, inositol bisphosphate and inositol trisphosphate, was observed after stimulation with DNP-BSA. When Li+ was added to inhibit the phosphatase activity that hydrolysed the phosphomonoester bonds in the sugar phosphates, greater increases were observed in all three inositol phosphates, particularly in inositol trisphosphate. The IgE-stimulated release of inositol trisphosphate was independent of the presence of extracellular Ca2+. In addition, the Ca2+ ionophore A23187 caused neither the decrease in [32P]polyphosphoinositides nor the stimulation of the release of inositol phosphates. These results demonstrate that stimulation of the MC9 cell via its receptor for IgE causes increased phospholipid turnover, with effects on polyphosphoinositides predominating. These data support the hypothesis that hapten cross-bridging of IgE receptors stimulates phospholipase C activity, which may be an early event in stimulus-secretion coupling of mast cells. The results with the Ca2+ ionophore A23187 indicate that an increase in intracellular Ca2+ alone is not sufficient for activation of this enzyme.

Developmental changes in the activity of phosphatidylethanolamine N-methyltransferases in rat brain

The activity of phosphatidylethanolamine N-methyltransferase (PeMT), an enzymic system that catalyses the synthesis of phosphatidylcholine (PtdCho) via sequential methylation of phosphatidylethanolamine (PtdEtn) using S-adenosylmethionine (AdoMet) as a methyl donor, was examined in brain homogenates from rats of various ages. The data thus obtained were consistent with the existence of two distinct enzyme activities within this enzyme system, i.e. one catalysing the methylation of PtdEtn [to form phosphatidyl-N-monomethylethanolamine (PtdMeEtn)], and the other catalysing the methylations of PtdMeEtn and phosphatidyl-NN-dimethylethanolamine (PtdMe2Etn) (to form PtdMe2Etn and PtdCho, respectively). PeMT (PtdEtn-methylating) activity per g of brain was 4-fold higher in neonatal than in adult brains. The enzyme activity in adult brains exhibited Michaelis-Menten kinetics for AdoMet, and its affinity for AdoMet was high (apparent Km 1.6 microM). In neonatal brain the relationships between AdoMet concentrations and PtdMeEtn formation were more complex: a sigmoidal component (with a Hill coefficient of 2.7), requiring 90 microM-AdoMet for half-saturation predominated over the high-affinity component (similar to that of the adult brain). PeMT (PtdMe2Etn-methylating) activity per g of brain increased 2-fold between the 5th and the 20th postnatal days and remained constant thereafter; it was higher than that of PeMT (PtdEtn-methylating) activity at all ages studied, and its affinity for AdoMet was low (apparent Km 99 microM). No sexual dimorphism in brain PeMT activity was observed at any age. We conclude that PeMT (PtdEtn-methylating) catalyses the rate-limiting step in PtdCho synthesis in rat brain, and that PtdCho formation via this pathway may be greatest during the neonatal period.

The effect of polar head group substitution on phospholipid methylation and the beta-adrenergic response in C6 glial cells

The membranes of intact C6 cells were enriched with phosphatidyldimethylethanolamine or phosphatidylmonomethylethanolamine. These cells showed enhanced rates of phospholipid methylation but this was not accompanied by an increased beta-adrenergic response. We conclude that phospholipid methylation is not coupled to the activation of adenylate cyclase in the beta-adrenergic response of C6 glial cells.

Chemical methylation of phosphatidylethanolamine by S-adenosylmethionine

The chemical methylation of phosphatidylethanolamine (PE) by S-adenosyl methionine (SAM) is most active when carried out at alkaline pH's. Phosphatidylmonomethylethanolamine (PMME) and phosphatidyldimethylethanolamine are less effective reactants. The PE present in the microsomal and myelin membrane can serve as an acceptor in this reaction. Thin layer chromatography indicates the formation of the expected products.

Mitogens increase phosphorylation of phosphoinositides in thymocytes

In many cell systems the interaction of ligands with their receptors causes rapid breakdown and resynthesis of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). Recent work has focused on the role of the degradation products of PtdIns(4,5)P2 as intermediates in the activation of cell function and growth: inositol trisphosphate (InsP3) can release Ca2+ from intracellular stores and diacylglycerol is thought to activate protein kinase C. This enzyme is also activated by phorbol esters (for example, 12-O-tetradecanoyl phorbol 13-acetate, TPA) and this is assumed to account for the pleiotropic effects of TPA on cell function and growth. Mouse thymocytes are not mitogenically stimulated by TPA alone, but it is a potent co-mitogen in combination with either concanavalin A (Con A) or A23187 (A. N. Corps and J.C.M., unpublished observations). Here we show that mitogenic concentrations of TPA, A23187 and Con A each cause an increase in the net phosphorylation of phosphatidylinositol (PtdIns) to PtdIns(4,5)P2 in mouse thymocytes. This is consistent with simulation by the mitogens of the same phosphoinositide phosphorylations in intact cells as recently demonstrated for the isolated products of the src and ros viral oncogenes in a cell-free system.